1

Fall 2015

COMP 46055605Computer Networks

Donghyun (David) KimDepartment of Mathematics and PhysicsNorth Carolina Central University

Some slides are in courtesy of J Kurose and K Ross

Chapter 5

Link Layer and LANs

2

Chapter 5 The Data Link LayerChapter goals bull understand principles behind data link

layer servicesbull error detection correctionbull sharing a broadcast channel multiple ac-

cessbull link layer addressingbull reliable data transfer flow control done

bull instantiation and implementation of various link layer technologies

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

3

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

4

Link Layer IntroductionSome terminologybull hosts and routers are nodesbull communication channels that

connect adjacent nodes along communication path are links

bull wired linksbull wireless linksbull LANsbull layer-2 packet is a frame encapsu-

lates datagram

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

data-link layer has responsibility of transferring datagram from one node to adjacent node over a link

5

Link layer contextbull datagram transferred

by different link pro-tocols over different linksbull eg Ethernet on first

link frame relay on in-termediate links 80211 on last link

bull each link protocol provides different servicesbull eg may or may not

provide rdt over linkFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

transportation analogybull trip from Princeton to

Lausannebull limo Princeton to JFKbull plane JFK to Genevabull train Geneva to Lausanne

bull tourist = datagrambull transport segment =

communication linkbull transportation mode =

link layer protocolbull travel agent =

routing algorithm

6

Link Layer Servicesbull framing link access

bull encapsulate datagram into frame adding header trailer

bull channel access if shared mediumbull ldquoMACrdquo addresses used in frame headers to identify

source dest bull different from IP address

bull reliable delivery between adjacent nodesbull we learned how to do this already (chapter 3)bull seldom used on low bit-error link (fiber some

twisted pair)bull wireless links high error rates

bull Q why both link-level and end-end reliabilityFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

7

Link Layer Services (more)bull flow control

bull pacing between adjacent sending and receiving nodes

bull error detection bull errors caused by signal attenuation noise bull receiver detects presence of errors

bull signals sender for retransmission or drops frame

bull error correction bull receiver identifies and corrects bit error(s) without re-

sorting to retransmission

bull half-duplex and full-duplexbull with half duplex nodes at both ends of link can

transmit but not at same timeFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

8

Where is the link layer imple-mentedbull in each and every hostbull link layer implemented

in ldquoadaptorrdquo (aka net-work interface card NIC)bull Ethernet card PCMCI

card 80211 cardbull implements link physical

layer

bull attaches into hostrsquos sys-tem buses

bull combination of hard-ware software firmware

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

controller

physicaltransmission

cpu memory

host bus (eg PCI)

network adaptercard

host schematic

applicationtransportnetwork

link

linkphysical

9

Adaptors Communicating

bull sending sidebull encapsulates datagram

in framebull adds error checking bits

rdt flow control etc

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

bull receiving sidebull looks for errors rdt flow

control etcbull extracts datagram

passes to upper layer at receiving side

controller controller

sending host receiving host

datagram datagram

datagram

frame

10

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

11

Error DetectionEDC= Error Detection and Correction bits (redundancy)D = Data protected by error checking may include header fields

bull Error detection not 100 reliablebull protocol may miss some errors but rarelybull larger EDC field yields better detection and correction

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

otherwise

12

Parity Checking

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Single Bit ParityDetect single bit errors

Two Dimensional Bit ParityDetect and correct single bit errors

0 0

13

Internet checksum (re-view)

Senderbull treat segment contents

as sequence of 16-bit integers

bull checksum addition (1rsquos complement sum) of segment contents

bull sender puts checksum value into UDP check-sum field

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Receiverbull compute checksum of

received segmentbull check if computed

checksum equals check-sum field valuebull NO - error detectedbull YES - no error detected

But maybe errors nonethe-less

Goal detect ldquoerrorsrdquo (eg flipped bits) in trans-mitted packet (note used at transport layer only

14

Checksumming Cyclic Redun-dancy Checkbull view data bits D as a binary numberbull choose r+1 bit pattern (generator) G bull goal choose r CRC bits R such that

bull ltDRgt exactly divisible by G (modulo 2) bull receiver knows G divides ltDRgt by G If non-zero remain-

der error detectedbull can detect all burst errors less than r+1 bits

bull widely used in practice (Ethernet 80211 WiFi ATM)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

15

Checksumming Cyclic Redun-dancy Checkbull 1011 XOR 0101 = 1110 bull 1001 XOR 1101 = 0100

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

16

CRC Example

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

WantD2r XOR R = nG

equivalentlyD2r = nG XOR R

equivalently if we divide D2r by

G want remainder R

R = remainder[ ]D2r

G

17

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

18

Multiple Access Links and Proto-colsTwo types of ldquolinksrdquobull point-to-point

bull PPP for dial-up accessbull point-to-point link between Ethernet switch and host

bull broadcast (shared wire or medium)bull old-fashioned Ethernetbull upstream HFC (Hybrid Fiber-Coaxial)bull 80211 wireless LAN

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

shared wire (eg cabled Ethernet)

shared RF (eg 80211 WiFi)

shared RF(satellite)

humans at acocktail party

(shared air acoustical)

19

Multiple Access protocolsbull single shared broadcast channel bull two or more simultaneous transmis-

sions by nodes interference bull collision if a node receives two or more

signals at the same time

multiple access protocolbull distributed algorithm that deter-

mines how nodes share channel ie determine when node can transmit

bull communication about channel shar-ing must use channel itself bull no out-of-band channel for coordination

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

20

Ideal Multiple Access ProtocolBroadcast channel of rate R bps1 when one node wants to transmit it

can send at rate R2 when M nodes want to transmit each

can send at average rate RM3 fully decentralized

bull no special node to coordinate transmissionsbull no synchronization of clocks slots

4 simple

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

21

MAC Protocols a taxonomyThree broad classesbull Channel Partitioning

bull divide channel into smaller ldquopiecesrdquo (time slots frequency code)

bull allocate piece to node for exclusive use

bull Random Accessbull channel not divided allow collisionsbull ldquorecoverrdquo from collisions

bull ldquoTaking turnsrdquobull nodes take turns but nodes with more to send

can take longer turnsFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

22

Channel Partitioning MAC proto-cols TDMATDMA time division multiple access bull access to channel in rounds bull each station gets fixed length slot

(length = pkt trans time) in each round bull unused slots go idle bull example 6-station LAN 134 have pkt

slots 256 idle

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

1 3 4 1 3 4

6-slotframe

23

Channel Partitioning MAC proto-cols FDMAFDMA frequency division multiple access bull channel spectrum divided into frequency bandsbull each station assigned fixed frequency bandbull unused transmission time in frequency bands go idle bull example 6-station LAN 134 have pkt frequency

bands 256 idle

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

freq

uency

bands time

FDM cable

24

Random Access Protocolsbull When node has packet to send

bull transmit at full channel data rate Rbull no a priori coordination among nodes

bull two or more transmitting nodes ldquocollisionrdquobull random access MAC protocol specifies

bull how to detect collisionsbull how to recover from collisions (eg via delayed retrans-

missions)

bull Examples of random access MAC protocolsbull slotted ALOHAbull ALOHAbull CSMA CSMACD CSMACA

bull (CS Carrier Sensing MA Multiple AccessCD Collision Detection CA Collision Avoidance)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

25

Slotted ALOHAAssumptionsbull all frames same sizebull time divided into

equal size slots (time to transmit 1 frame)

bull nodes start to transmit only slot beginning

bull nodes are synchro-nized

bull if 2 or more nodes transmit in slot all nodes detect collision

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Operationbull when node obtains

fresh frame trans-mits in next slotbull if no collision node

can send new frame in next slot

bull if collision node re-transmits frame in each subsequent slot with prob p until success

26

Slotted ALOHA

Prosbull single active node can

continuously transmit at full rate of channel

bull highly decentralized only slots in nodes need to be in sync

bull simpleFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Consbull collisions wasting slotsbull idle slotsbull nodes may be able to

detect collision in less than time to transmit packet

bull clock synchronization

27

Slotted Aloha Efficiency

bull suppose N nodes with many frames to send each transmits in slot with probability p

bull prob that given node has success in a slot = p(1-p)N-1

bull prob that any node has a success = Np(1-p)N-1

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

bull max efficiency find p that max-imizes Np(1-p)N-1

bull for many nodes take limit of Np(1- p)N-1 as N goes to infinity gives

Max efficiency = 1e = 37

Efficiency long-run fraction of successful slots (many nodes all with many frames to send)

At best channelused for useful transmissions 37of time

28

Pure (unslotted) ALOHAbull unslotted Aloha simpler no synchro-

nizationbull when frame first arrives

bull transmit immediately

bull collision probability increasesbull frame sent at t0 collides with other frames sent in

[t0-1t0+1]

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

29

Pure Aloha EfficiencyP(success by given node) = P(node transmits)

P(no other node transmits in [t0-1t0]

P(no other node transmits in [t0t0+1]

= p (1-p)N-1 (1-p)N-1

= p (1-p)2(N-1)

hellip choosing optimum p and then letting

= 1(2e) = 18

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

N

even worse than slotted Aloha

30

CSMA (Carrier Sense Multiple Access)CSMA listen before transmitIf channel sensed idle transmit entire framebull If channel sensed busy defer transmis-

sion

bull human analogy donrsquot interrupt others

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

31

CSMA Collisionscollisions can still occurbull propagation delay means

two nodes may not hear each otherrsquos transmission

collisionbull entire packet transmis-

sion time wasted

notebull role of distance amp propa-

gation delay in determin-ing collision probability

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

spatial layout of nodes

32

CSMACD (Collision Detec-tion)CSMACD carrier sensing deferral as in CSMA

bull collisions detected within short timebull colliding transmissions aborted reducing channel

wastage

bull collision detection bull easy in wired LANs measure signal strengths

compare transmitted received signalsbull difficult in wireless LANs received signal strength

overwhelmed by local transmission strength

bull human analogy the polite conversationalist

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

33

CSMACD collision detection

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

34

ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols

bull share channel efficiently and fairly at high loadbull inefficient at low load delay in channel access

1N bandwidth allocated even if only 1 active node

Random access MAC protocolsbull efficient at low load single node can fully uti-

lize channelbull high load collision overhead

ldquotaking turnsrdquo protocolsbull look for best of both worlds

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

35

ldquoTaking Turnsrdquo MAC protocolsPolling bull master node ldquoinvitesrdquo

slave nodes to transmit in turn

bull typically used with ldquodumbrdquo slave devices

bull concernsbull polling overhead bull latencybull single point of failure

(master)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

T

data

(nothingto send)

T

36

ldquoTaking Turnsrdquo MAC protocolsToken passingbull control token

passed from one node to next se-quentially

bull token messagebull concerns

bull token overhead bull latencybull single point of failure

(token)Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

master

slaves

poll

data

data

37

Summary of MAC protocolsbull channel partitioning by time frequency or

codebull Time Division Frequency Division

bull random access (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire)

hard in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211

bull taking turnsbull polling from central site token passing

bull Bluetooth FDDI IBM Token Ring Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

38

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

39

MAC Addresses and ARPbull 32-bit IP address

bull network-layer addressbull used to get datagram to destination IP subnet

bull MAC (or LAN or physical or Ethernet) ad-dress bull function get frame from one interface to an-

other physically-connected interface (same network)

bull 48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software set-

table

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

40

LAN Addresses and ARP

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Each adapter on LAN has unique LAN address

Broadcast address =FF-FF-FF-FF-FF-FF

= adapter

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired orwireless)

41

LAN Address (more)bull MAC address allocation administered by IEEEbull manufacturer buys portion of MAC address

space (to assure uniqueness)bull analogy

(a) MAC address like Social Security Num-ber

(b) IP address like postal addressbull MAC flat address portability

bull can move LAN card from one LAN to another

bull IP hierarchical address NOT portablebull address depends on IP subnet to which node is at-

tachedFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

42

ARP Address Resolution Proto-colbull Each IP node (host

router) on LAN has ARP table

bull ARP table IPMAC address mappings for some LAN nodeslt IP address MAC address

TTLgt

bull TTL (Time To Live) time after which ad-dress mapping will be forgotten (typically 20 min)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Question how to determineMAC address of Bknowing Brsquos IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN

137196723

137196778

137196714

137196788

43

ARP protocol Same LAN (net-work)bull A wants to send datagram

to B and Brsquos MAC address not in Arsquos ARP table

bull A broadcasts ARP query packet containing Bs IP address bull dest MAC address = FF-FF-FF-

FF-FF-FFbull all machines on LAN receive

ARP query

bull B receives ARP packet replies to A with its (Bs) MAC addressbull frame sent to Arsquos MAC ad-

dress (unicast)Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

bull A caches (saves) IP-to-MAC address pair in its ARP table until infor-mation becomes old (times out) bull soft state information

that times out (goes away) unless refreshed

bull ARP is ldquoplug-and-playrdquobull nodes create their ARP

tables without interven-tion from net administra-tor

44

Addressing routing to another LAN

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

R

1A-23-F9-CD-06-9B

222222222220111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

walkthrough send datagram from A to B via R assume A knows Brsquos IP address

bull two ARP tables in router R one for each IP network (LAN)

45

bull A creates IP datagram with source A destination B bull A uses ARP to get Rrsquos MAC address for 111111111110bull A creates link-layer frame with Rs MAC address as dest

frame contains A-to-B IP datagrambull Arsquos NIC sends frame bull Rrsquos NIC receives frame bull R removes IP datagram from Ethernet frame sees its des-

tined to Bbull R uses ARP to get Brsquos MAC address bull R creates frame containing A-to-B IP datagram sends to B

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

This is a really importantexample ndash make sure youunderstand

R

1A-23-F9-CD-06-9B

222222222220

111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

46

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

47

Ethernetldquodominantrdquo wired LAN technology bull cheap $20 for NICbull first widely used LAN technologybull simpler cheaper than token LANs and ATMbull kept up with speed race 10 Mbps ndash 10 Gbps

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Metcalfersquos Ethernetsketch

48

Star Topologybull bus topology popular through mid 90s

bull all nodes in same collision domain (can collide with each other)

bull today star topology prevailsbull active switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes do

not collide with each other)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

switch

bus coaxial cable star

49

Ethernet Frame Structurebull Sending adapter encapsulates IP datagram

(or other network layer protocol packet) in Ethernet frame

Preamble bull 7 bytes with pattern 10101010 followed by

one byte with pattern 10101011bull used to synchronize receiver sender clock

ratesFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

50

Ethernet Frame Structure (more)bull Addresses 6 bytes

bull if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

bull otherwise adapter discards frame

bull Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)

bull CRC checked at receiver if error is detected frame is dropped

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51

Ethernet Unreliable connection-lessbull connectionless No handshaking be-

tween sending and receiving NICs bull unreliable receiving NIC doesnrsquot send

acks or nacks to sending NICbull stream of datagrams passed to network

layer can have gaps (missing datagrams)bull gaps will be filled if app is using TCPbull otherwise app will see gaps

bull Ethernetrsquos MAC protocol unslotted CSMACD

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

52

Ethernet CSMACD algorithmbull 1 NIC receives data-

gram from network layer creates frame

bull 2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

bull 3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

bull 4 If NIC detects an-other transmission while transmitting aborts and sends jam signal

bull 5 After aborting NIC enters exponential backoff after mth colli-sion NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

53

Ethernetrsquos CSMACD (more)bull Jam Signal make

sure all other transmitters are aware of collision 48 bits

bull Bit time 1 micro sec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Exponential Backoff bull Goal adapt retransmission

attempts to estimated cur-rent load

bull heavy load random wait will be longer

bull first collision choose K from 01 delay is K 512 bit transmission times

bull after second collision choose K from 0123hellip

bull after ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

54

CSMACD efficiencybull Tprop = max prop delay between 2 nodes in

LANbull ttrans = time to transmit max-size frame

bull efficiency goes to 1 bull as tprop goes to 0

bull as ttrans goes to infinity

bull better performance than ALOHA and simple cheap decentralized

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

transprop ttefficiency

51

1

55

8023 Ethernet Standards Link amp Physical Layersbull many different Ethernet standards

bull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps

1Gbps 10G bpsbull different physical layer media fiber cable

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

applicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twisterpair) physical layer

56

Manchester encodingbull used in 10BaseTbull each bit has a transitionbull allows clocks in sending and receiving nodes to syn-

chronize to each otherbull no need for a centralized global clock among nodes

bull Hey this is physical-layer stuff

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

57

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

58

Hubshellip physical-layer (ldquodumbrdquo) repeaters

bull bits coming in one link go out all other links at same rate

bull all nodes connected to hub can collide with one another

bull no frame bufferingbull no CSMACD at hub host NICs detect colli-

sions

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

twisted pair

hub

59

Switchbull link-layer device smarter than hubs take

active rolebull store forward Ethernet framesbull examine incoming framersquos MAC address se-

lectively forward frame to one-or-more out-going links when frame is to be forwarded on segment uses CSMACD to access segment

bull transparentbull hosts are unaware of presence of switches

bull plug-and-play self-learning (HOW)bull switches do not need to be configured

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

60

Switch allows multiple simulta-neous transmissionsbull hosts have dedicated direct

connection to switchbull switches buffer packetsbull Ethernet protocol used on

each incoming link but no collisions full duplexbull each link is its own collision

domain

bull switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions bull not possible with dumb hub

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 23

45

6

61

Switch Tablebull Q how does switch know

that Arsquo reachable via inter-face 4 Brsquo reachable via inter-face 5

bull A each switch has a switch table each entrybull (MAC address of host interface

to reach host time stamp)

bull looks like a routing tablebull Q how are entries created

maintained in switch table bull something like a routing proto-

colFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

switch with six interfaces(123456)

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

62

Switch self-learningbull switch learns which

hosts can be reached through which inter-facesbull when frame received

switch ldquolearnsrdquo location of sender incoming LAN segment

bull records senderlocation pair in switch table

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

63

Switch frame filteringforward-ingWhen frame received

1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination then if dest on segment from which frame arrived then drop the frame else

forward the frame on interface indicated else flood

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

forward on all but the interface on which the frame arrived

64

Self-learning forwarding exam-ple

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

frame destination unknownflood

Arsquo A

destination A location known

Arsquo 4 60

selective send

65

Interconnecting switchesbull switches can be connected together

bull Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3

bull A self learning (works exactly the same as in single-switch case)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

B

S1

C D

E

FS2

S4

S3

H

I

G

66

Self-learning multi-switch exam-plebull Suppose C sends frame to I I responds to

C

bull Q show switch tables and packet forward-ing in S1 S2 S3 S4

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

B

S1

CD

E F

S2

S4

S3

H

I

G

12

1

2 3

3

1 2 3 12 3

444

67

Institutional network

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

to externalnetwork

router

IP subnet

mail server

web server

68

Switches vs Routersbull both store-and-forward devices

bull routers network layer devices (examine network layer head-ers)

bull switches are link layer devices

bull routers maintain routing tables implement routing al-gorithms

bull switches maintain switch tables implement filtering learning algorithms

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

69

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

• Fall 2015 COMP 46055605 Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link layer context
• Link Layer Services
• Link Layer Services (more)
• Where is the link layer implemented
• Adaptors Communicating
• Link Layer (2)
• Error Detection
• Parity Checking
• Internet checksum (review)
• Checksumming Cyclic Redundancy Check
• Checksumming Cyclic Redundancy Check (2)
• CRC Example
• Link Layer (3)
• Multiple Access Links and Protocols
• Multiple Access protocols
• Ideal Multiple Access Protocol
• MAC Protocols a taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD collision detection
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Link Layer (4)
• MAC Addresses and ARP
• LAN Addresses and ARP
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP protocol Same LAN (network)
• Addressing routing to another LAN
• Slide 45
• Link Layer (5)
• Ethernet
• Star Topology
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester encoding
• Link Layer (6)
• Hubs
• Switch
• Switch allows multiple simultaneous transmissions
• Switch Table
• Switch self-learning
• Switch frame filteringforwarding
• Self-learning forwarding example
• Interconnecting switches
• Self-learning multi-switch example
• Institutional network
• Switches vs Routers
• Link Layer (7)

2

Chapter 5 The Data Link LayerChapter goals bull understand principles behind data link

layer servicesbull error detection correctionbull sharing a broadcast channel multiple ac-

cessbull link layer addressingbull reliable data transfer flow control done

bull instantiation and implementation of various link layer technologies

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

3

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

4

Link Layer IntroductionSome terminologybull hosts and routers are nodesbull communication channels that

connect adjacent nodes along communication path are links

bull wired linksbull wireless linksbull LANsbull layer-2 packet is a frame encapsu-

lates datagram

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

data-link layer has responsibility of transferring datagram from one node to adjacent node over a link

5

Link layer contextbull datagram transferred

by different link pro-tocols over different linksbull eg Ethernet on first

link frame relay on in-termediate links 80211 on last link

bull each link protocol provides different servicesbull eg may or may not

provide rdt over linkFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

transportation analogybull trip from Princeton to

Lausannebull limo Princeton to JFKbull plane JFK to Genevabull train Geneva to Lausanne

bull tourist = datagrambull transport segment =

communication linkbull transportation mode =

link layer protocolbull travel agent =

routing algorithm

6

Link Layer Servicesbull framing link access

bull encapsulate datagram into frame adding header trailer

bull channel access if shared mediumbull ldquoMACrdquo addresses used in frame headers to identify

source dest bull different from IP address

bull reliable delivery between adjacent nodesbull we learned how to do this already (chapter 3)bull seldom used on low bit-error link (fiber some

twisted pair)bull wireless links high error rates

bull Q why both link-level and end-end reliabilityFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

7

Link Layer Services (more)bull flow control

bull pacing between adjacent sending and receiving nodes

bull error detection bull errors caused by signal attenuation noise bull receiver detects presence of errors

bull signals sender for retransmission or drops frame

bull error correction bull receiver identifies and corrects bit error(s) without re-

sorting to retransmission

bull half-duplex and full-duplexbull with half duplex nodes at both ends of link can

transmit but not at same timeFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

8

Where is the link layer imple-mentedbull in each and every hostbull link layer implemented

in ldquoadaptorrdquo (aka net-work interface card NIC)bull Ethernet card PCMCI

card 80211 cardbull implements link physical

layer

bull attaches into hostrsquos sys-tem buses

bull combination of hard-ware software firmware

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

controller

physicaltransmission

cpu memory

host bus (eg PCI)

network adaptercard

host schematic

applicationtransportnetwork

link

linkphysical

9

Adaptors Communicating

bull sending sidebull encapsulates datagram

in framebull adds error checking bits

rdt flow control etc

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

bull receiving sidebull looks for errors rdt flow

control etcbull extracts datagram

passes to upper layer at receiving side

controller controller

sending host receiving host

datagram datagram

datagram

frame

10

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

11

Error DetectionEDC= Error Detection and Correction bits (redundancy)D = Data protected by error checking may include header fields

bull Error detection not 100 reliablebull protocol may miss some errors but rarelybull larger EDC field yields better detection and correction

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

otherwise

12

Parity Checking

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Single Bit ParityDetect single bit errors

Two Dimensional Bit ParityDetect and correct single bit errors

0 0

13

Internet checksum (re-view)

Senderbull treat segment contents

as sequence of 16-bit integers

bull checksum addition (1rsquos complement sum) of segment contents

bull sender puts checksum value into UDP check-sum field

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Receiverbull compute checksum of

received segmentbull check if computed

checksum equals check-sum field valuebull NO - error detectedbull YES - no error detected

But maybe errors nonethe-less

Goal detect ldquoerrorsrdquo (eg flipped bits) in trans-mitted packet (note used at transport layer only

14

Checksumming Cyclic Redun-dancy Checkbull view data bits D as a binary numberbull choose r+1 bit pattern (generator) G bull goal choose r CRC bits R such that

bull ltDRgt exactly divisible by G (modulo 2) bull receiver knows G divides ltDRgt by G If non-zero remain-

der error detectedbull can detect all burst errors less than r+1 bits

bull widely used in practice (Ethernet 80211 WiFi ATM)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

15

Checksumming Cyclic Redun-dancy Checkbull 1011 XOR 0101 = 1110 bull 1001 XOR 1101 = 0100

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

16

CRC Example

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

WantD2r XOR R = nG

equivalentlyD2r = nG XOR R

equivalently if we divide D2r by

G want remainder R

R = remainder[ ]D2r

G

17

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

18

Multiple Access Links and Proto-colsTwo types of ldquolinksrdquobull point-to-point

bull PPP for dial-up accessbull point-to-point link between Ethernet switch and host

bull broadcast (shared wire or medium)bull old-fashioned Ethernetbull upstream HFC (Hybrid Fiber-Coaxial)bull 80211 wireless LAN

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

shared wire (eg cabled Ethernet)

shared RF (eg 80211 WiFi)

shared RF(satellite)

humans at acocktail party

(shared air acoustical)

19

Multiple Access protocolsbull single shared broadcast channel bull two or more simultaneous transmis-

sions by nodes interference bull collision if a node receives two or more

signals at the same time

multiple access protocolbull distributed algorithm that deter-

mines how nodes share channel ie determine when node can transmit

bull communication about channel shar-ing must use channel itself bull no out-of-band channel for coordination

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

20

Ideal Multiple Access ProtocolBroadcast channel of rate R bps1 when one node wants to transmit it

can send at rate R2 when M nodes want to transmit each

can send at average rate RM3 fully decentralized

bull no special node to coordinate transmissionsbull no synchronization of clocks slots

4 simple

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

21

MAC Protocols a taxonomyThree broad classesbull Channel Partitioning

bull divide channel into smaller ldquopiecesrdquo (time slots frequency code)

bull allocate piece to node for exclusive use

bull Random Accessbull channel not divided allow collisionsbull ldquorecoverrdquo from collisions

bull ldquoTaking turnsrdquobull nodes take turns but nodes with more to send

can take longer turnsFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

22

Channel Partitioning MAC proto-cols TDMATDMA time division multiple access bull access to channel in rounds bull each station gets fixed length slot

(length = pkt trans time) in each round bull unused slots go idle bull example 6-station LAN 134 have pkt

slots 256 idle

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

1 3 4 1 3 4

6-slotframe

23

Channel Partitioning MAC proto-cols FDMAFDMA frequency division multiple access bull channel spectrum divided into frequency bandsbull each station assigned fixed frequency bandbull unused transmission time in frequency bands go idle bull example 6-station LAN 134 have pkt frequency

bands 256 idle

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

freq

uency

bands time

FDM cable

24

Random Access Protocolsbull When node has packet to send

bull transmit at full channel data rate Rbull no a priori coordination among nodes

bull two or more transmitting nodes ldquocollisionrdquobull random access MAC protocol specifies

bull how to detect collisionsbull how to recover from collisions (eg via delayed retrans-

missions)

bull Examples of random access MAC protocolsbull slotted ALOHAbull ALOHAbull CSMA CSMACD CSMACA

bull (CS Carrier Sensing MA Multiple AccessCD Collision Detection CA Collision Avoidance)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

25

Slotted ALOHAAssumptionsbull all frames same sizebull time divided into

equal size slots (time to transmit 1 frame)

bull nodes start to transmit only slot beginning

bull nodes are synchro-nized

bull if 2 or more nodes transmit in slot all nodes detect collision

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Operationbull when node obtains

fresh frame trans-mits in next slotbull if no collision node

can send new frame in next slot

bull if collision node re-transmits frame in each subsequent slot with prob p until success

26

Slotted ALOHA

Prosbull single active node can

continuously transmit at full rate of channel

bull highly decentralized only slots in nodes need to be in sync

bull simpleFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Consbull collisions wasting slotsbull idle slotsbull nodes may be able to

detect collision in less than time to transmit packet

bull clock synchronization

27

Slotted Aloha Efficiency

bull suppose N nodes with many frames to send each transmits in slot with probability p

bull prob that given node has success in a slot = p(1-p)N-1

bull prob that any node has a success = Np(1-p)N-1

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

bull max efficiency find p that max-imizes Np(1-p)N-1

bull for many nodes take limit of Np(1- p)N-1 as N goes to infinity gives

Max efficiency = 1e = 37

Efficiency long-run fraction of successful slots (many nodes all with many frames to send)

At best channelused for useful transmissions 37of time

28

Pure (unslotted) ALOHAbull unslotted Aloha simpler no synchro-

nizationbull when frame first arrives

bull transmit immediately

bull collision probability increasesbull frame sent at t0 collides with other frames sent in

[t0-1t0+1]

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

29

Pure Aloha EfficiencyP(success by given node) = P(node transmits)

P(no other node transmits in [t0-1t0]

P(no other node transmits in [t0t0+1]

= p (1-p)N-1 (1-p)N-1

= p (1-p)2(N-1)

hellip choosing optimum p and then letting

= 1(2e) = 18

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

N

even worse than slotted Aloha

30

CSMA (Carrier Sense Multiple Access)CSMA listen before transmitIf channel sensed idle transmit entire framebull If channel sensed busy defer transmis-

sion

bull human analogy donrsquot interrupt others

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

31

CSMA Collisionscollisions can still occurbull propagation delay means

two nodes may not hear each otherrsquos transmission

collisionbull entire packet transmis-

sion time wasted

notebull role of distance amp propa-

gation delay in determin-ing collision probability

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

spatial layout of nodes

32

CSMACD (Collision Detec-tion)CSMACD carrier sensing deferral as in CSMA

bull collisions detected within short timebull colliding transmissions aborted reducing channel

wastage

bull collision detection bull easy in wired LANs measure signal strengths

compare transmitted received signalsbull difficult in wireless LANs received signal strength

overwhelmed by local transmission strength

bull human analogy the polite conversationalist

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

33

CSMACD collision detection

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

34

ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols

bull share channel efficiently and fairly at high loadbull inefficient at low load delay in channel access

1N bandwidth allocated even if only 1 active node

Random access MAC protocolsbull efficient at low load single node can fully uti-

lize channelbull high load collision overhead

ldquotaking turnsrdquo protocolsbull look for best of both worlds

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

35

ldquoTaking Turnsrdquo MAC protocolsPolling bull master node ldquoinvitesrdquo

slave nodes to transmit in turn

bull typically used with ldquodumbrdquo slave devices

bull concernsbull polling overhead bull latencybull single point of failure

(master)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

T

data

(nothingto send)

T

36

ldquoTaking Turnsrdquo MAC protocolsToken passingbull control token

passed from one node to next se-quentially

bull token messagebull concerns

bull token overhead bull latencybull single point of failure

(token)Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

master

slaves

poll

data

data

37

Summary of MAC protocolsbull channel partitioning by time frequency or

codebull Time Division Frequency Division

bull random access (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire)

hard in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211

bull taking turnsbull polling from central site token passing

bull Bluetooth FDDI IBM Token Ring Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

38

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

39

MAC Addresses and ARPbull 32-bit IP address

bull network-layer addressbull used to get datagram to destination IP subnet

bull MAC (or LAN or physical or Ethernet) ad-dress bull function get frame from one interface to an-

other physically-connected interface (same network)

bull 48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software set-

table

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

40

LAN Addresses and ARP

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Each adapter on LAN has unique LAN address

Broadcast address =FF-FF-FF-FF-FF-FF

= adapter

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired orwireless)

41

LAN Address (more)bull MAC address allocation administered by IEEEbull manufacturer buys portion of MAC address

space (to assure uniqueness)bull analogy

(a) MAC address like Social Security Num-ber

(b) IP address like postal addressbull MAC flat address portability

bull can move LAN card from one LAN to another

bull IP hierarchical address NOT portablebull address depends on IP subnet to which node is at-

tachedFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

42

ARP Address Resolution Proto-colbull Each IP node (host

router) on LAN has ARP table

bull ARP table IPMAC address mappings for some LAN nodeslt IP address MAC address

TTLgt

bull TTL (Time To Live) time after which ad-dress mapping will be forgotten (typically 20 min)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Question how to determineMAC address of Bknowing Brsquos IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN

137196723

137196778

137196714

137196788

43

ARP protocol Same LAN (net-work)bull A wants to send datagram

to B and Brsquos MAC address not in Arsquos ARP table

bull A broadcasts ARP query packet containing Bs IP address bull dest MAC address = FF-FF-FF-

FF-FF-FFbull all machines on LAN receive

ARP query

bull B receives ARP packet replies to A with its (Bs) MAC addressbull frame sent to Arsquos MAC ad-

dress (unicast)Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

bull A caches (saves) IP-to-MAC address pair in its ARP table until infor-mation becomes old (times out) bull soft state information

that times out (goes away) unless refreshed

bull ARP is ldquoplug-and-playrdquobull nodes create their ARP

tables without interven-tion from net administra-tor

44

Addressing routing to another LAN

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

R

1A-23-F9-CD-06-9B

222222222220111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

walkthrough send datagram from A to B via R assume A knows Brsquos IP address

bull two ARP tables in router R one for each IP network (LAN)

45

bull A creates IP datagram with source A destination B bull A uses ARP to get Rrsquos MAC address for 111111111110bull A creates link-layer frame with Rs MAC address as dest

frame contains A-to-B IP datagrambull Arsquos NIC sends frame bull Rrsquos NIC receives frame bull R removes IP datagram from Ethernet frame sees its des-

tined to Bbull R uses ARP to get Brsquos MAC address bull R creates frame containing A-to-B IP datagram sends to B

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

This is a really importantexample ndash make sure youunderstand

R

1A-23-F9-CD-06-9B

222222222220

111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

46

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

47

Ethernetldquodominantrdquo wired LAN technology bull cheap $20 for NICbull first widely used LAN technologybull simpler cheaper than token LANs and ATMbull kept up with speed race 10 Mbps ndash 10 Gbps

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Metcalfersquos Ethernetsketch

48

Star Topologybull bus topology popular through mid 90s

bull all nodes in same collision domain (can collide with each other)

bull today star topology prevailsbull active switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes do

not collide with each other)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

switch

bus coaxial cable star

49

Ethernet Frame Structurebull Sending adapter encapsulates IP datagram

(or other network layer protocol packet) in Ethernet frame

Preamble bull 7 bytes with pattern 10101010 followed by

one byte with pattern 10101011bull used to synchronize receiver sender clock

ratesFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

50

Ethernet Frame Structure (more)bull Addresses 6 bytes

bull if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

bull otherwise adapter discards frame

bull Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)

bull CRC checked at receiver if error is detected frame is dropped

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51

Ethernet Unreliable connection-lessbull connectionless No handshaking be-

tween sending and receiving NICs bull unreliable receiving NIC doesnrsquot send

acks or nacks to sending NICbull stream of datagrams passed to network

layer can have gaps (missing datagrams)bull gaps will be filled if app is using TCPbull otherwise app will see gaps

bull Ethernetrsquos MAC protocol unslotted CSMACD

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

52

Ethernet CSMACD algorithmbull 1 NIC receives data-

gram from network layer creates frame

bull 2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

bull 3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

bull 4 If NIC detects an-other transmission while transmitting aborts and sends jam signal

bull 5 After aborting NIC enters exponential backoff after mth colli-sion NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

53

Ethernetrsquos CSMACD (more)bull Jam Signal make

sure all other transmitters are aware of collision 48 bits

bull Bit time 1 micro sec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Exponential Backoff bull Goal adapt retransmission

attempts to estimated cur-rent load

bull heavy load random wait will be longer

bull first collision choose K from 01 delay is K 512 bit transmission times

bull after second collision choose K from 0123hellip

bull after ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

54

CSMACD efficiencybull Tprop = max prop delay between 2 nodes in

LANbull ttrans = time to transmit max-size frame

bull efficiency goes to 1 bull as tprop goes to 0

bull as ttrans goes to infinity

bull better performance than ALOHA and simple cheap decentralized

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

transprop ttefficiency

51

1

55

8023 Ethernet Standards Link amp Physical Layersbull many different Ethernet standards

bull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps

1Gbps 10G bpsbull different physical layer media fiber cable

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

applicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twisterpair) physical layer

56

Manchester encodingbull used in 10BaseTbull each bit has a transitionbull allows clocks in sending and receiving nodes to syn-

chronize to each otherbull no need for a centralized global clock among nodes

bull Hey this is physical-layer stuff

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

57

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

58

Hubshellip physical-layer (ldquodumbrdquo) repeaters

bull bits coming in one link go out all other links at same rate

bull all nodes connected to hub can collide with one another

bull no frame bufferingbull no CSMACD at hub host NICs detect colli-

sions

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

twisted pair

hub

59

Switchbull link-layer device smarter than hubs take

active rolebull store forward Ethernet framesbull examine incoming framersquos MAC address se-

lectively forward frame to one-or-more out-going links when frame is to be forwarded on segment uses CSMACD to access segment

bull transparentbull hosts are unaware of presence of switches

bull plug-and-play self-learning (HOW)bull switches do not need to be configured

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

60

Switch allows multiple simulta-neous transmissionsbull hosts have dedicated direct

connection to switchbull switches buffer packetsbull Ethernet protocol used on

each incoming link but no collisions full duplexbull each link is its own collision

domain

bull switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions bull not possible with dumb hub

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 23

45

6

61

Switch Tablebull Q how does switch know

that Arsquo reachable via inter-face 4 Brsquo reachable via inter-face 5

bull A each switch has a switch table each entrybull (MAC address of host interface

to reach host time stamp)

bull looks like a routing tablebull Q how are entries created

maintained in switch table bull something like a routing proto-

colFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

switch with six interfaces(123456)

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

62

Switch self-learningbull switch learns which

hosts can be reached through which inter-facesbull when frame received

switch ldquolearnsrdquo location of sender incoming LAN segment

bull records senderlocation pair in switch table

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

63

Switch frame filteringforward-ingWhen frame received

1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination then if dest on segment from which frame arrived then drop the frame else

forward the frame on interface indicated else flood

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

forward on all but the interface on which the frame arrived

64

Self-learning forwarding exam-ple

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

frame destination unknownflood

Arsquo A

destination A location known

Arsquo 4 60

selective send

65

Interconnecting switchesbull switches can be connected together

bull Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3

bull A self learning (works exactly the same as in single-switch case)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

B

S1

C D

E

FS2

S4

S3

H

I

G

66

Self-learning multi-switch exam-plebull Suppose C sends frame to I I responds to

C

bull Q show switch tables and packet forward-ing in S1 S2 S3 S4

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

B

S1

CD

E F

S2

S4

S3

H

I

G

12

1

2 3

3

1 2 3 12 3

444

67

Institutional network

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

to externalnetwork

router

IP subnet

mail server

web server

68

Switches vs Routersbull both store-and-forward devices

bull routers network layer devices (examine network layer head-ers)

bull switches are link layer devices

bull routers maintain routing tables implement routing al-gorithms

bull switches maintain switch tables implement filtering learning algorithms

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

69

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

• Fall 2015 COMP 46055605 Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link layer context
• Link Layer Services
• Link Layer Services (more)
• Where is the link layer implemented
• Adaptors Communicating
• Link Layer (2)
• Error Detection
• Parity Checking
• Internet checksum (review)
• Checksumming Cyclic Redundancy Check
• Checksumming Cyclic Redundancy Check (2)
• CRC Example
• Link Layer (3)
• Multiple Access Links and Protocols
• Multiple Access protocols
• Ideal Multiple Access Protocol
• MAC Protocols a taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD collision detection
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Link Layer (4)
• MAC Addresses and ARP
• LAN Addresses and ARP
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP protocol Same LAN (network)
• Addressing routing to another LAN
• Slide 45
• Link Layer (5)
• Ethernet
• Star Topology
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester encoding
• Link Layer (6)
• Hubs
• Switch
• Switch allows multiple simultaneous transmissions
• Switch Table
• Switch self-learning
• Switch frame filteringforwarding
• Self-learning forwarding example
• Interconnecting switches
• Self-learning multi-switch example
• Institutional network
• Switches vs Routers
• Link Layer (7)

3

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

4

Link Layer IntroductionSome terminologybull hosts and routers are nodesbull communication channels that

connect adjacent nodes along communication path are links

bull wired linksbull wireless linksbull LANsbull layer-2 packet is a frame encapsu-

lates datagram

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

data-link layer has responsibility of transferring datagram from one node to adjacent node over a link

5

Link layer contextbull datagram transferred

by different link pro-tocols over different linksbull eg Ethernet on first

link frame relay on in-termediate links 80211 on last link

bull each link protocol provides different servicesbull eg may or may not

provide rdt over linkFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

transportation analogybull trip from Princeton to

Lausannebull limo Princeton to JFKbull plane JFK to Genevabull train Geneva to Lausanne

bull tourist = datagrambull transport segment =

communication linkbull transportation mode =

link layer protocolbull travel agent =

routing algorithm

6

Link Layer Servicesbull framing link access

bull encapsulate datagram into frame adding header trailer

bull channel access if shared mediumbull ldquoMACrdquo addresses used in frame headers to identify

source dest bull different from IP address

bull reliable delivery between adjacent nodesbull we learned how to do this already (chapter 3)bull seldom used on low bit-error link (fiber some

twisted pair)bull wireless links high error rates

bull Q why both link-level and end-end reliabilityFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

7

Link Layer Services (more)bull flow control

bull pacing between adjacent sending and receiving nodes

bull error detection bull errors caused by signal attenuation noise bull receiver detects presence of errors

bull signals sender for retransmission or drops frame

bull error correction bull receiver identifies and corrects bit error(s) without re-

sorting to retransmission

bull half-duplex and full-duplexbull with half duplex nodes at both ends of link can

transmit but not at same timeFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

8

Where is the link layer imple-mentedbull in each and every hostbull link layer implemented

in ldquoadaptorrdquo (aka net-work interface card NIC)bull Ethernet card PCMCI

card 80211 cardbull implements link physical

layer

bull attaches into hostrsquos sys-tem buses

bull combination of hard-ware software firmware

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

controller

physicaltransmission

cpu memory

host bus (eg PCI)

network adaptercard

host schematic

applicationtransportnetwork

link

linkphysical

9

Adaptors Communicating

bull sending sidebull encapsulates datagram

in framebull adds error checking bits

rdt flow control etc

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

bull receiving sidebull looks for errors rdt flow

control etcbull extracts datagram

passes to upper layer at receiving side

controller controller

sending host receiving host

datagram datagram

datagram

frame

10

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

11

Error DetectionEDC= Error Detection and Correction bits (redundancy)D = Data protected by error checking may include header fields

bull Error detection not 100 reliablebull protocol may miss some errors but rarelybull larger EDC field yields better detection and correction

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

otherwise

12

Parity Checking

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Single Bit ParityDetect single bit errors

Two Dimensional Bit ParityDetect and correct single bit errors

0 0

13

Internet checksum (re-view)

Senderbull treat segment contents

as sequence of 16-bit integers

bull checksum addition (1rsquos complement sum) of segment contents

bull sender puts checksum value into UDP check-sum field

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Receiverbull compute checksum of

received segmentbull check if computed

checksum equals check-sum field valuebull NO - error detectedbull YES - no error detected

But maybe errors nonethe-less

Goal detect ldquoerrorsrdquo (eg flipped bits) in trans-mitted packet (note used at transport layer only

14

Checksumming Cyclic Redun-dancy Checkbull view data bits D as a binary numberbull choose r+1 bit pattern (generator) G bull goal choose r CRC bits R such that

bull ltDRgt exactly divisible by G (modulo 2) bull receiver knows G divides ltDRgt by G If non-zero remain-

der error detectedbull can detect all burst errors less than r+1 bits

bull widely used in practice (Ethernet 80211 WiFi ATM)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

15

Checksumming Cyclic Redun-dancy Checkbull 1011 XOR 0101 = 1110 bull 1001 XOR 1101 = 0100

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

16

CRC Example

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

WantD2r XOR R = nG

equivalentlyD2r = nG XOR R

equivalently if we divide D2r by

G want remainder R

R = remainder[ ]D2r

G

17

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

18

Multiple Access Links and Proto-colsTwo types of ldquolinksrdquobull point-to-point

bull PPP for dial-up accessbull point-to-point link between Ethernet switch and host

bull broadcast (shared wire or medium)bull old-fashioned Ethernetbull upstream HFC (Hybrid Fiber-Coaxial)bull 80211 wireless LAN

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

shared wire (eg cabled Ethernet)

shared RF (eg 80211 WiFi)

shared RF(satellite)

humans at acocktail party

(shared air acoustical)

19

Multiple Access protocolsbull single shared broadcast channel bull two or more simultaneous transmis-

sions by nodes interference bull collision if a node receives two or more

signals at the same time

multiple access protocolbull distributed algorithm that deter-

mines how nodes share channel ie determine when node can transmit

bull communication about channel shar-ing must use channel itself bull no out-of-band channel for coordination

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

20

Ideal Multiple Access ProtocolBroadcast channel of rate R bps1 when one node wants to transmit it

can send at rate R2 when M nodes want to transmit each

can send at average rate RM3 fully decentralized

bull no special node to coordinate transmissionsbull no synchronization of clocks slots

4 simple

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

21

MAC Protocols a taxonomyThree broad classesbull Channel Partitioning

bull divide channel into smaller ldquopiecesrdquo (time slots frequency code)

bull allocate piece to node for exclusive use

bull Random Accessbull channel not divided allow collisionsbull ldquorecoverrdquo from collisions

bull ldquoTaking turnsrdquobull nodes take turns but nodes with more to send

can take longer turnsFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

22

Channel Partitioning MAC proto-cols TDMATDMA time division multiple access bull access to channel in rounds bull each station gets fixed length slot

(length = pkt trans time) in each round bull unused slots go idle bull example 6-station LAN 134 have pkt

slots 256 idle

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

1 3 4 1 3 4

6-slotframe

23

Channel Partitioning MAC proto-cols FDMAFDMA frequency division multiple access bull channel spectrum divided into frequency bandsbull each station assigned fixed frequency bandbull unused transmission time in frequency bands go idle bull example 6-station LAN 134 have pkt frequency

bands 256 idle

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

freq

uency

bands time

FDM cable

24

Random Access Protocolsbull When node has packet to send

bull transmit at full channel data rate Rbull no a priori coordination among nodes

bull two or more transmitting nodes ldquocollisionrdquobull random access MAC protocol specifies

bull how to detect collisionsbull how to recover from collisions (eg via delayed retrans-

missions)

bull Examples of random access MAC protocolsbull slotted ALOHAbull ALOHAbull CSMA CSMACD CSMACA

bull (CS Carrier Sensing MA Multiple AccessCD Collision Detection CA Collision Avoidance)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

25

Slotted ALOHAAssumptionsbull all frames same sizebull time divided into

equal size slots (time to transmit 1 frame)

bull nodes start to transmit only slot beginning

bull nodes are synchro-nized

bull if 2 or more nodes transmit in slot all nodes detect collision

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Operationbull when node obtains

fresh frame trans-mits in next slotbull if no collision node

can send new frame in next slot

bull if collision node re-transmits frame in each subsequent slot with prob p until success

26

Slotted ALOHA

Prosbull single active node can

continuously transmit at full rate of channel

bull highly decentralized only slots in nodes need to be in sync

bull simpleFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Consbull collisions wasting slotsbull idle slotsbull nodes may be able to

detect collision in less than time to transmit packet

bull clock synchronization

27

Slotted Aloha Efficiency

bull suppose N nodes with many frames to send each transmits in slot with probability p

bull prob that given node has success in a slot = p(1-p)N-1

bull prob that any node has a success = Np(1-p)N-1

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

bull max efficiency find p that max-imizes Np(1-p)N-1

bull for many nodes take limit of Np(1- p)N-1 as N goes to infinity gives

Max efficiency = 1e = 37

Efficiency long-run fraction of successful slots (many nodes all with many frames to send)

At best channelused for useful transmissions 37of time

28

Pure (unslotted) ALOHAbull unslotted Aloha simpler no synchro-

nizationbull when frame first arrives

bull transmit immediately

bull collision probability increasesbull frame sent at t0 collides with other frames sent in

[t0-1t0+1]

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

29

Pure Aloha EfficiencyP(success by given node) = P(node transmits)

P(no other node transmits in [t0-1t0]

P(no other node transmits in [t0t0+1]

= p (1-p)N-1 (1-p)N-1

= p (1-p)2(N-1)

hellip choosing optimum p and then letting

= 1(2e) = 18

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

N

even worse than slotted Aloha

30

CSMA (Carrier Sense Multiple Access)CSMA listen before transmitIf channel sensed idle transmit entire framebull If channel sensed busy defer transmis-

sion

bull human analogy donrsquot interrupt others

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

31

CSMA Collisionscollisions can still occurbull propagation delay means

two nodes may not hear each otherrsquos transmission

collisionbull entire packet transmis-

sion time wasted

notebull role of distance amp propa-

gation delay in determin-ing collision probability

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

spatial layout of nodes

32

CSMACD (Collision Detec-tion)CSMACD carrier sensing deferral as in CSMA

bull collisions detected within short timebull colliding transmissions aborted reducing channel

wastage

bull collision detection bull easy in wired LANs measure signal strengths

compare transmitted received signalsbull difficult in wireless LANs received signal strength

overwhelmed by local transmission strength

bull human analogy the polite conversationalist

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

33

CSMACD collision detection

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

34

ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols

bull share channel efficiently and fairly at high loadbull inefficient at low load delay in channel access

1N bandwidth allocated even if only 1 active node

Random access MAC protocolsbull efficient at low load single node can fully uti-

lize channelbull high load collision overhead

ldquotaking turnsrdquo protocolsbull look for best of both worlds

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

35

ldquoTaking Turnsrdquo MAC protocolsPolling bull master node ldquoinvitesrdquo

slave nodes to transmit in turn

bull typically used with ldquodumbrdquo slave devices

bull concernsbull polling overhead bull latencybull single point of failure

(master)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

T

data

(nothingto send)

T

36

ldquoTaking Turnsrdquo MAC protocolsToken passingbull control token

passed from one node to next se-quentially

bull token messagebull concerns

bull token overhead bull latencybull single point of failure

(token)Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

master

slaves

poll

data

data

37

Summary of MAC protocolsbull channel partitioning by time frequency or

codebull Time Division Frequency Division

bull random access (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire)

hard in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211

bull taking turnsbull polling from central site token passing

bull Bluetooth FDDI IBM Token Ring Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

38

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

39

MAC Addresses and ARPbull 32-bit IP address

bull network-layer addressbull used to get datagram to destination IP subnet

bull MAC (or LAN or physical or Ethernet) ad-dress bull function get frame from one interface to an-

other physically-connected interface (same network)

bull 48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software set-

table

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

40

LAN Addresses and ARP

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Each adapter on LAN has unique LAN address

Broadcast address =FF-FF-FF-FF-FF-FF

= adapter

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired orwireless)

41

LAN Address (more)bull MAC address allocation administered by IEEEbull manufacturer buys portion of MAC address

space (to assure uniqueness)bull analogy

(a) MAC address like Social Security Num-ber

(b) IP address like postal addressbull MAC flat address portability

bull can move LAN card from one LAN to another

bull IP hierarchical address NOT portablebull address depends on IP subnet to which node is at-

tachedFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

42

ARP Address Resolution Proto-colbull Each IP node (host

router) on LAN has ARP table

bull ARP table IPMAC address mappings for some LAN nodeslt IP address MAC address

TTLgt

bull TTL (Time To Live) time after which ad-dress mapping will be forgotten (typically 20 min)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Question how to determineMAC address of Bknowing Brsquos IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN

137196723

137196778

137196714

137196788

43

ARP protocol Same LAN (net-work)bull A wants to send datagram

to B and Brsquos MAC address not in Arsquos ARP table

bull A broadcasts ARP query packet containing Bs IP address bull dest MAC address = FF-FF-FF-

FF-FF-FFbull all machines on LAN receive

ARP query

bull B receives ARP packet replies to A with its (Bs) MAC addressbull frame sent to Arsquos MAC ad-

dress (unicast)Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

bull A caches (saves) IP-to-MAC address pair in its ARP table until infor-mation becomes old (times out) bull soft state information

that times out (goes away) unless refreshed

bull ARP is ldquoplug-and-playrdquobull nodes create their ARP

tables without interven-tion from net administra-tor

44

Addressing routing to another LAN

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

R

1A-23-F9-CD-06-9B

222222222220111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

walkthrough send datagram from A to B via R assume A knows Brsquos IP address

bull two ARP tables in router R one for each IP network (LAN)

45

bull A creates IP datagram with source A destination B bull A uses ARP to get Rrsquos MAC address for 111111111110bull A creates link-layer frame with Rs MAC address as dest

frame contains A-to-B IP datagrambull Arsquos NIC sends frame bull Rrsquos NIC receives frame bull R removes IP datagram from Ethernet frame sees its des-

tined to Bbull R uses ARP to get Brsquos MAC address bull R creates frame containing A-to-B IP datagram sends to B

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

This is a really importantexample ndash make sure youunderstand

R

1A-23-F9-CD-06-9B

222222222220

111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

46

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

47

Ethernetldquodominantrdquo wired LAN technology bull cheap $20 for NICbull first widely used LAN technologybull simpler cheaper than token LANs and ATMbull kept up with speed race 10 Mbps ndash 10 Gbps

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Metcalfersquos Ethernetsketch

48

Star Topologybull bus topology popular through mid 90s

bull all nodes in same collision domain (can collide with each other)

bull today star topology prevailsbull active switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes do

not collide with each other)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

switch

bus coaxial cable star

49

Ethernet Frame Structurebull Sending adapter encapsulates IP datagram

(or other network layer protocol packet) in Ethernet frame

Preamble bull 7 bytes with pattern 10101010 followed by

one byte with pattern 10101011bull used to synchronize receiver sender clock

ratesFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

50

Ethernet Frame Structure (more)bull Addresses 6 bytes

bull if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

bull otherwise adapter discards frame

bull Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)

bull CRC checked at receiver if error is detected frame is dropped

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51

Ethernet Unreliable connection-lessbull connectionless No handshaking be-

tween sending and receiving NICs bull unreliable receiving NIC doesnrsquot send

acks or nacks to sending NICbull stream of datagrams passed to network

layer can have gaps (missing datagrams)bull gaps will be filled if app is using TCPbull otherwise app will see gaps

bull Ethernetrsquos MAC protocol unslotted CSMACD

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

52

Ethernet CSMACD algorithmbull 1 NIC receives data-

gram from network layer creates frame

bull 2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

bull 3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

bull 4 If NIC detects an-other transmission while transmitting aborts and sends jam signal

bull 5 After aborting NIC enters exponential backoff after mth colli-sion NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

53

Ethernetrsquos CSMACD (more)bull Jam Signal make

sure all other transmitters are aware of collision 48 bits

bull Bit time 1 micro sec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Exponential Backoff bull Goal adapt retransmission

attempts to estimated cur-rent load

bull heavy load random wait will be longer

bull first collision choose K from 01 delay is K 512 bit transmission times

bull after second collision choose K from 0123hellip

bull after ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

54

CSMACD efficiencybull Tprop = max prop delay between 2 nodes in

LANbull ttrans = time to transmit max-size frame

bull efficiency goes to 1 bull as tprop goes to 0

bull as ttrans goes to infinity

bull better performance than ALOHA and simple cheap decentralized

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

transprop ttefficiency

51

1

55

8023 Ethernet Standards Link amp Physical Layersbull many different Ethernet standards

bull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps

1Gbps 10G bpsbull different physical layer media fiber cable

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

applicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twisterpair) physical layer

56

Manchester encodingbull used in 10BaseTbull each bit has a transitionbull allows clocks in sending and receiving nodes to syn-

chronize to each otherbull no need for a centralized global clock among nodes

bull Hey this is physical-layer stuff

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

57

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

58

Hubshellip physical-layer (ldquodumbrdquo) repeaters

bull bits coming in one link go out all other links at same rate

bull all nodes connected to hub can collide with one another

bull no frame bufferingbull no CSMACD at hub host NICs detect colli-

sions

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

twisted pair

hub

59

Switchbull link-layer device smarter than hubs take

active rolebull store forward Ethernet framesbull examine incoming framersquos MAC address se-

lectively forward frame to one-or-more out-going links when frame is to be forwarded on segment uses CSMACD to access segment

bull transparentbull hosts are unaware of presence of switches

bull plug-and-play self-learning (HOW)bull switches do not need to be configured

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

60

Switch allows multiple simulta-neous transmissionsbull hosts have dedicated direct

connection to switchbull switches buffer packetsbull Ethernet protocol used on

each incoming link but no collisions full duplexbull each link is its own collision

domain

bull switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions bull not possible with dumb hub

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 23

45

6

61

Switch Tablebull Q how does switch know

that Arsquo reachable via inter-face 4 Brsquo reachable via inter-face 5

bull A each switch has a switch table each entrybull (MAC address of host interface

to reach host time stamp)

bull looks like a routing tablebull Q how are entries created

maintained in switch table bull something like a routing proto-

colFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

switch with six interfaces(123456)

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

62

Switch self-learningbull switch learns which

hosts can be reached through which inter-facesbull when frame received

switch ldquolearnsrdquo location of sender incoming LAN segment

bull records senderlocation pair in switch table

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

63

Switch frame filteringforward-ingWhen frame received

1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination then if dest on segment from which frame arrived then drop the frame else

forward the frame on interface indicated else flood

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

forward on all but the interface on which the frame arrived

64

Self-learning forwarding exam-ple

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

frame destination unknownflood

Arsquo A

destination A location known

Arsquo 4 60

selective send

65

Interconnecting switchesbull switches can be connected together

bull Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3

bull A self learning (works exactly the same as in single-switch case)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

B

S1

C D

E

FS2

S4

S3

H

I

G

66

Self-learning multi-switch exam-plebull Suppose C sends frame to I I responds to

C

bull Q show switch tables and packet forward-ing in S1 S2 S3 S4

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

B

S1

CD

E F

S2

S4

S3

H

I

G

12

1

2 3

3

1 2 3 12 3

444

67

Institutional network

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

to externalnetwork

router

IP subnet

mail server

web server

68

Switches vs Routersbull both store-and-forward devices

bull routers network layer devices (examine network layer head-ers)

bull switches are link layer devices

bull routers maintain routing tables implement routing al-gorithms

bull switches maintain switch tables implement filtering learning algorithms

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

69

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

• Fall 2015 COMP 46055605 Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link layer context
• Link Layer Services
• Link Layer Services (more)
• Where is the link layer implemented
• Adaptors Communicating
• Link Layer (2)
• Error Detection
• Parity Checking
• Internet checksum (review)
• Checksumming Cyclic Redundancy Check
• Checksumming Cyclic Redundancy Check (2)
• CRC Example
• Link Layer (3)
• Multiple Access Links and Protocols
• Multiple Access protocols
• Ideal Multiple Access Protocol
• MAC Protocols a taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD collision detection
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Link Layer (4)
• MAC Addresses and ARP
• LAN Addresses and ARP
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP protocol Same LAN (network)
• Addressing routing to another LAN
• Slide 45
• Link Layer (5)
• Ethernet
• Star Topology
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester encoding
• Link Layer (6)
• Hubs
• Switch
• Switch allows multiple simultaneous transmissions
• Switch Table
• Switch self-learning
• Switch frame filteringforwarding
• Self-learning forwarding example
• Interconnecting switches
• Self-learning multi-switch example
• Institutional network
• Switches vs Routers
• Link Layer (7)

4

Link Layer IntroductionSome terminologybull hosts and routers are nodesbull communication channels that

connect adjacent nodes along communication path are links

bull wired linksbull wireless linksbull LANsbull layer-2 packet is a frame encapsu-

lates datagram

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

data-link layer has responsibility of transferring datagram from one node to adjacent node over a link

5

Link layer contextbull datagram transferred

by different link pro-tocols over different linksbull eg Ethernet on first

link frame relay on in-termediate links 80211 on last link

bull each link protocol provides different servicesbull eg may or may not

provide rdt over linkFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

transportation analogybull trip from Princeton to

Lausannebull limo Princeton to JFKbull plane JFK to Genevabull train Geneva to Lausanne

bull tourist = datagrambull transport segment =

communication linkbull transportation mode =

link layer protocolbull travel agent =

routing algorithm

6

Link Layer Servicesbull framing link access

bull encapsulate datagram into frame adding header trailer

bull channel access if shared mediumbull ldquoMACrdquo addresses used in frame headers to identify

source dest bull different from IP address

bull reliable delivery between adjacent nodesbull we learned how to do this already (chapter 3)bull seldom used on low bit-error link (fiber some

twisted pair)bull wireless links high error rates

bull Q why both link-level and end-end reliabilityFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

7

Link Layer Services (more)bull flow control

bull pacing between adjacent sending and receiving nodes

bull error detection bull errors caused by signal attenuation noise bull receiver detects presence of errors

bull signals sender for retransmission or drops frame

bull error correction bull receiver identifies and corrects bit error(s) without re-

sorting to retransmission

bull half-duplex and full-duplexbull with half duplex nodes at both ends of link can

transmit but not at same timeFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

8

Where is the link layer imple-mentedbull in each and every hostbull link layer implemented

in ldquoadaptorrdquo (aka net-work interface card NIC)bull Ethernet card PCMCI

card 80211 cardbull implements link physical

layer

bull attaches into hostrsquos sys-tem buses

bull combination of hard-ware software firmware

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

controller

physicaltransmission

cpu memory

host bus (eg PCI)

network adaptercard

host schematic

applicationtransportnetwork

link

linkphysical

9

Adaptors Communicating

bull sending sidebull encapsulates datagram

in framebull adds error checking bits

rdt flow control etc

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

bull receiving sidebull looks for errors rdt flow

control etcbull extracts datagram

passes to upper layer at receiving side

controller controller

sending host receiving host

datagram datagram

datagram

frame

10

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

11

Error DetectionEDC= Error Detection and Correction bits (redundancy)D = Data protected by error checking may include header fields

bull Error detection not 100 reliablebull protocol may miss some errors but rarelybull larger EDC field yields better detection and correction

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

otherwise

12

Parity Checking

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Single Bit ParityDetect single bit errors

Two Dimensional Bit ParityDetect and correct single bit errors

0 0

13

Internet checksum (re-view)

Senderbull treat segment contents

as sequence of 16-bit integers

bull checksum addition (1rsquos complement sum) of segment contents

bull sender puts checksum value into UDP check-sum field

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Receiverbull compute checksum of

received segmentbull check if computed

checksum equals check-sum field valuebull NO - error detectedbull YES - no error detected

But maybe errors nonethe-less

Goal detect ldquoerrorsrdquo (eg flipped bits) in trans-mitted packet (note used at transport layer only

14

Checksumming Cyclic Redun-dancy Checkbull view data bits D as a binary numberbull choose r+1 bit pattern (generator) G bull goal choose r CRC bits R such that

bull ltDRgt exactly divisible by G (modulo 2) bull receiver knows G divides ltDRgt by G If non-zero remain-

der error detectedbull can detect all burst errors less than r+1 bits

bull widely used in practice (Ethernet 80211 WiFi ATM)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

15

Checksumming Cyclic Redun-dancy Checkbull 1011 XOR 0101 = 1110 bull 1001 XOR 1101 = 0100

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

16

CRC Example

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

WantD2r XOR R = nG

equivalentlyD2r = nG XOR R

equivalently if we divide D2r by

G want remainder R

R = remainder[ ]D2r

G

17

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

18

Multiple Access Links and Proto-colsTwo types of ldquolinksrdquobull point-to-point

bull PPP for dial-up accessbull point-to-point link between Ethernet switch and host

bull broadcast (shared wire or medium)bull old-fashioned Ethernetbull upstream HFC (Hybrid Fiber-Coaxial)bull 80211 wireless LAN

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

shared wire (eg cabled Ethernet)

shared RF (eg 80211 WiFi)

shared RF(satellite)

humans at acocktail party

(shared air acoustical)

19

Multiple Access protocolsbull single shared broadcast channel bull two or more simultaneous transmis-

sions by nodes interference bull collision if a node receives two or more

signals at the same time

multiple access protocolbull distributed algorithm that deter-

mines how nodes share channel ie determine when node can transmit

bull communication about channel shar-ing must use channel itself bull no out-of-band channel for coordination

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

20

Ideal Multiple Access ProtocolBroadcast channel of rate R bps1 when one node wants to transmit it

can send at rate R2 when M nodes want to transmit each

can send at average rate RM3 fully decentralized

bull no special node to coordinate transmissionsbull no synchronization of clocks slots

4 simple

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

21

MAC Protocols a taxonomyThree broad classesbull Channel Partitioning

bull divide channel into smaller ldquopiecesrdquo (time slots frequency code)

bull allocate piece to node for exclusive use

bull Random Accessbull channel not divided allow collisionsbull ldquorecoverrdquo from collisions

bull ldquoTaking turnsrdquobull nodes take turns but nodes with more to send

can take longer turnsFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

22

Channel Partitioning MAC proto-cols TDMATDMA time division multiple access bull access to channel in rounds bull each station gets fixed length slot

(length = pkt trans time) in each round bull unused slots go idle bull example 6-station LAN 134 have pkt

slots 256 idle

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

1 3 4 1 3 4

6-slotframe

23

Channel Partitioning MAC proto-cols FDMAFDMA frequency division multiple access bull channel spectrum divided into frequency bandsbull each station assigned fixed frequency bandbull unused transmission time in frequency bands go idle bull example 6-station LAN 134 have pkt frequency

bands 256 idle

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

freq

uency

bands time

FDM cable

24

Random Access Protocolsbull When node has packet to send

bull transmit at full channel data rate Rbull no a priori coordination among nodes

bull two or more transmitting nodes ldquocollisionrdquobull random access MAC protocol specifies

bull how to detect collisionsbull how to recover from collisions (eg via delayed retrans-

missions)

bull Examples of random access MAC protocolsbull slotted ALOHAbull ALOHAbull CSMA CSMACD CSMACA

bull (CS Carrier Sensing MA Multiple AccessCD Collision Detection CA Collision Avoidance)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

25

Slotted ALOHAAssumptionsbull all frames same sizebull time divided into

equal size slots (time to transmit 1 frame)

bull nodes start to transmit only slot beginning

bull nodes are synchro-nized

bull if 2 or more nodes transmit in slot all nodes detect collision

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Operationbull when node obtains

fresh frame trans-mits in next slotbull if no collision node

can send new frame in next slot

bull if collision node re-transmits frame in each subsequent slot with prob p until success

26

Slotted ALOHA

Prosbull single active node can

continuously transmit at full rate of channel

bull highly decentralized only slots in nodes need to be in sync

bull simpleFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Consbull collisions wasting slotsbull idle slotsbull nodes may be able to

detect collision in less than time to transmit packet

bull clock synchronization

27

Slotted Aloha Efficiency

bull suppose N nodes with many frames to send each transmits in slot with probability p

bull prob that given node has success in a slot = p(1-p)N-1

bull prob that any node has a success = Np(1-p)N-1

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

bull max efficiency find p that max-imizes Np(1-p)N-1

bull for many nodes take limit of Np(1- p)N-1 as N goes to infinity gives

Max efficiency = 1e = 37

Efficiency long-run fraction of successful slots (many nodes all with many frames to send)

At best channelused for useful transmissions 37of time

28

Pure (unslotted) ALOHAbull unslotted Aloha simpler no synchro-

nizationbull when frame first arrives

bull transmit immediately

bull collision probability increasesbull frame sent at t0 collides with other frames sent in

[t0-1t0+1]

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

29

Pure Aloha EfficiencyP(success by given node) = P(node transmits)

P(no other node transmits in [t0-1t0]

P(no other node transmits in [t0t0+1]

= p (1-p)N-1 (1-p)N-1

= p (1-p)2(N-1)

hellip choosing optimum p and then letting

= 1(2e) = 18

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

N

even worse than slotted Aloha

30

CSMA (Carrier Sense Multiple Access)CSMA listen before transmitIf channel sensed idle transmit entire framebull If channel sensed busy defer transmis-

sion

bull human analogy donrsquot interrupt others

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

31

CSMA Collisionscollisions can still occurbull propagation delay means

two nodes may not hear each otherrsquos transmission

collisionbull entire packet transmis-

sion time wasted

notebull role of distance amp propa-

gation delay in determin-ing collision probability

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

spatial layout of nodes

32

CSMACD (Collision Detec-tion)CSMACD carrier sensing deferral as in CSMA

bull collisions detected within short timebull colliding transmissions aborted reducing channel

wastage

bull collision detection bull easy in wired LANs measure signal strengths

compare transmitted received signalsbull difficult in wireless LANs received signal strength

overwhelmed by local transmission strength

bull human analogy the polite conversationalist

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

33

CSMACD collision detection

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

34

ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols

bull share channel efficiently and fairly at high loadbull inefficient at low load delay in channel access

1N bandwidth allocated even if only 1 active node

Random access MAC protocolsbull efficient at low load single node can fully uti-

lize channelbull high load collision overhead

ldquotaking turnsrdquo protocolsbull look for best of both worlds

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

35

ldquoTaking Turnsrdquo MAC protocolsPolling bull master node ldquoinvitesrdquo

slave nodes to transmit in turn

bull typically used with ldquodumbrdquo slave devices

bull concernsbull polling overhead bull latencybull single point of failure

(master)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

T

data

(nothingto send)

T

36

ldquoTaking Turnsrdquo MAC protocolsToken passingbull control token

passed from one node to next se-quentially

bull token messagebull concerns

bull token overhead bull latencybull single point of failure

(token)Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

master

slaves

poll

data

data

37

Summary of MAC protocolsbull channel partitioning by time frequency or

codebull Time Division Frequency Division

bull random access (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire)

hard in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211

bull taking turnsbull polling from central site token passing

bull Bluetooth FDDI IBM Token Ring Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

38

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

39

MAC Addresses and ARPbull 32-bit IP address

bull network-layer addressbull used to get datagram to destination IP subnet

bull MAC (or LAN or physical or Ethernet) ad-dress bull function get frame from one interface to an-

other physically-connected interface (same network)

bull 48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software set-

table

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

40

LAN Addresses and ARP

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Each adapter on LAN has unique LAN address

Broadcast address =FF-FF-FF-FF-FF-FF

= adapter

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired orwireless)

41

LAN Address (more)bull MAC address allocation administered by IEEEbull manufacturer buys portion of MAC address

space (to assure uniqueness)bull analogy

(a) MAC address like Social Security Num-ber

(b) IP address like postal addressbull MAC flat address portability

bull can move LAN card from one LAN to another

bull IP hierarchical address NOT portablebull address depends on IP subnet to which node is at-

tachedFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

42

ARP Address Resolution Proto-colbull Each IP node (host

router) on LAN has ARP table

bull ARP table IPMAC address mappings for some LAN nodeslt IP address MAC address

TTLgt

bull TTL (Time To Live) time after which ad-dress mapping will be forgotten (typically 20 min)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Question how to determineMAC address of Bknowing Brsquos IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN

137196723

137196778

137196714

137196788

43

ARP protocol Same LAN (net-work)bull A wants to send datagram

to B and Brsquos MAC address not in Arsquos ARP table

bull A broadcasts ARP query packet containing Bs IP address bull dest MAC address = FF-FF-FF-

FF-FF-FFbull all machines on LAN receive

ARP query

bull B receives ARP packet replies to A with its (Bs) MAC addressbull frame sent to Arsquos MAC ad-

dress (unicast)Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

bull A caches (saves) IP-to-MAC address pair in its ARP table until infor-mation becomes old (times out) bull soft state information

that times out (goes away) unless refreshed

bull ARP is ldquoplug-and-playrdquobull nodes create their ARP

tables without interven-tion from net administra-tor

44

Addressing routing to another LAN

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

R

1A-23-F9-CD-06-9B

222222222220111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

walkthrough send datagram from A to B via R assume A knows Brsquos IP address

bull two ARP tables in router R one for each IP network (LAN)

45

bull A creates IP datagram with source A destination B bull A uses ARP to get Rrsquos MAC address for 111111111110bull A creates link-layer frame with Rs MAC address as dest

frame contains A-to-B IP datagrambull Arsquos NIC sends frame bull Rrsquos NIC receives frame bull R removes IP datagram from Ethernet frame sees its des-

tined to Bbull R uses ARP to get Brsquos MAC address bull R creates frame containing A-to-B IP datagram sends to B

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

This is a really importantexample ndash make sure youunderstand

R

1A-23-F9-CD-06-9B

222222222220

111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

46

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

47

Ethernetldquodominantrdquo wired LAN technology bull cheap $20 for NICbull first widely used LAN technologybull simpler cheaper than token LANs and ATMbull kept up with speed race 10 Mbps ndash 10 Gbps

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Metcalfersquos Ethernetsketch

48

Star Topologybull bus topology popular through mid 90s

bull all nodes in same collision domain (can collide with each other)

bull today star topology prevailsbull active switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes do

not collide with each other)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

switch

bus coaxial cable star

49

Ethernet Frame Structurebull Sending adapter encapsulates IP datagram

(or other network layer protocol packet) in Ethernet frame

Preamble bull 7 bytes with pattern 10101010 followed by

one byte with pattern 10101011bull used to synchronize receiver sender clock

ratesFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

50

Ethernet Frame Structure (more)bull Addresses 6 bytes

bull if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

bull otherwise adapter discards frame

bull Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)

bull CRC checked at receiver if error is detected frame is dropped

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51

Ethernet Unreliable connection-lessbull connectionless No handshaking be-

tween sending and receiving NICs bull unreliable receiving NIC doesnrsquot send

acks or nacks to sending NICbull stream of datagrams passed to network

layer can have gaps (missing datagrams)bull gaps will be filled if app is using TCPbull otherwise app will see gaps

bull Ethernetrsquos MAC protocol unslotted CSMACD

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

52

Ethernet CSMACD algorithmbull 1 NIC receives data-

gram from network layer creates frame

bull 2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

bull 3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

bull 4 If NIC detects an-other transmission while transmitting aborts and sends jam signal

bull 5 After aborting NIC enters exponential backoff after mth colli-sion NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

53

Ethernetrsquos CSMACD (more)bull Jam Signal make

sure all other transmitters are aware of collision 48 bits

bull Bit time 1 micro sec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Exponential Backoff bull Goal adapt retransmission

attempts to estimated cur-rent load

bull heavy load random wait will be longer

bull first collision choose K from 01 delay is K 512 bit transmission times

bull after second collision choose K from 0123hellip

bull after ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

54

CSMACD efficiencybull Tprop = max prop delay between 2 nodes in

LANbull ttrans = time to transmit max-size frame

bull efficiency goes to 1 bull as tprop goes to 0

bull as ttrans goes to infinity

bull better performance than ALOHA and simple cheap decentralized

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

transprop ttefficiency

51

1

55

8023 Ethernet Standards Link amp Physical Layersbull many different Ethernet standards

bull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps

1Gbps 10G bpsbull different physical layer media fiber cable

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

applicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twisterpair) physical layer

56

Manchester encodingbull used in 10BaseTbull each bit has a transitionbull allows clocks in sending and receiving nodes to syn-

chronize to each otherbull no need for a centralized global clock among nodes

bull Hey this is physical-layer stuff

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

57

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

58

Hubshellip physical-layer (ldquodumbrdquo) repeaters

bull bits coming in one link go out all other links at same rate

bull all nodes connected to hub can collide with one another

bull no frame bufferingbull no CSMACD at hub host NICs detect colli-

sions

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

twisted pair

hub

59

Switchbull link-layer device smarter than hubs take

active rolebull store forward Ethernet framesbull examine incoming framersquos MAC address se-

lectively forward frame to one-or-more out-going links when frame is to be forwarded on segment uses CSMACD to access segment

bull transparentbull hosts are unaware of presence of switches

bull plug-and-play self-learning (HOW)bull switches do not need to be configured

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

60

Switch allows multiple simulta-neous transmissionsbull hosts have dedicated direct

connection to switchbull switches buffer packetsbull Ethernet protocol used on

each incoming link but no collisions full duplexbull each link is its own collision

domain

bull switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions bull not possible with dumb hub

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 23

45

6

61

Switch Tablebull Q how does switch know

that Arsquo reachable via inter-face 4 Brsquo reachable via inter-face 5

bull A each switch has a switch table each entrybull (MAC address of host interface

to reach host time stamp)

bull looks like a routing tablebull Q how are entries created

maintained in switch table bull something like a routing proto-

colFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

switch with six interfaces(123456)

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

62

Switch self-learningbull switch learns which

hosts can be reached through which inter-facesbull when frame received

switch ldquolearnsrdquo location of sender incoming LAN segment

bull records senderlocation pair in switch table

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

63

Switch frame filteringforward-ingWhen frame received

1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination then if dest on segment from which frame arrived then drop the frame else

forward the frame on interface indicated else flood

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

forward on all but the interface on which the frame arrived

64

Self-learning forwarding exam-ple

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

frame destination unknownflood

Arsquo A

destination A location known

Arsquo 4 60

selective send

65

Interconnecting switchesbull switches can be connected together

bull Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3

bull A self learning (works exactly the same as in single-switch case)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

B

S1

C D

E

FS2

S4

S3

H

I

G

66

Self-learning multi-switch exam-plebull Suppose C sends frame to I I responds to

C

bull Q show switch tables and packet forward-ing in S1 S2 S3 S4

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

B

S1

CD

E F

S2

S4

S3

H

I

G

12

1

2 3

3

1 2 3 12 3

444

67

Institutional network

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

to externalnetwork

router

IP subnet

mail server

web server

68

Switches vs Routersbull both store-and-forward devices

bull routers network layer devices (examine network layer head-ers)

bull switches are link layer devices

bull routers maintain routing tables implement routing al-gorithms

bull switches maintain switch tables implement filtering learning algorithms

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

69

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

• Fall 2015 COMP 46055605 Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link layer context
• Link Layer Services
• Link Layer Services (more)
• Where is the link layer implemented
• Adaptors Communicating
• Link Layer (2)
• Error Detection
• Parity Checking
• Internet checksum (review)
• Checksumming Cyclic Redundancy Check
• Checksumming Cyclic Redundancy Check (2)
• CRC Example
• Link Layer (3)
• Multiple Access Links and Protocols
• Multiple Access protocols
• Ideal Multiple Access Protocol
• MAC Protocols a taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD collision detection
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Link Layer (4)
• MAC Addresses and ARP
• LAN Addresses and ARP
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP protocol Same LAN (network)
• Addressing routing to another LAN
• Slide 45
• Link Layer (5)
• Ethernet
• Star Topology
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester encoding
• Link Layer (6)
• Hubs
• Switch
• Switch allows multiple simultaneous transmissions
• Switch Table
• Switch self-learning
• Switch frame filteringforwarding
• Self-learning forwarding example
• Interconnecting switches
• Self-learning multi-switch example
• Institutional network
• Switches vs Routers
• Link Layer (7)

5

Link layer contextbull datagram transferred

by different link pro-tocols over different linksbull eg Ethernet on first

link frame relay on in-termediate links 80211 on last link

bull each link protocol provides different servicesbull eg may or may not

provide rdt over linkFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

transportation analogybull trip from Princeton to

Lausannebull limo Princeton to JFKbull plane JFK to Genevabull train Geneva to Lausanne

bull tourist = datagrambull transport segment =

communication linkbull transportation mode =

link layer protocolbull travel agent =

routing algorithm

6

Link Layer Servicesbull framing link access

bull encapsulate datagram into frame adding header trailer

bull channel access if shared mediumbull ldquoMACrdquo addresses used in frame headers to identify

source dest bull different from IP address

bull reliable delivery between adjacent nodesbull we learned how to do this already (chapter 3)bull seldom used on low bit-error link (fiber some

twisted pair)bull wireless links high error rates

bull Q why both link-level and end-end reliabilityFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

7

Link Layer Services (more)bull flow control

bull pacing between adjacent sending and receiving nodes

bull error detection bull errors caused by signal attenuation noise bull receiver detects presence of errors

bull signals sender for retransmission or drops frame

bull error correction bull receiver identifies and corrects bit error(s) without re-

sorting to retransmission

bull half-duplex and full-duplexbull with half duplex nodes at both ends of link can

transmit but not at same timeFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

8

Where is the link layer imple-mentedbull in each and every hostbull link layer implemented

in ldquoadaptorrdquo (aka net-work interface card NIC)bull Ethernet card PCMCI

card 80211 cardbull implements link physical

layer

bull attaches into hostrsquos sys-tem buses

bull combination of hard-ware software firmware

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

controller

physicaltransmission

cpu memory

host bus (eg PCI)

network adaptercard

host schematic

applicationtransportnetwork

link

linkphysical

9

Adaptors Communicating

bull sending sidebull encapsulates datagram

in framebull adds error checking bits

rdt flow control etc

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

bull receiving sidebull looks for errors rdt flow

control etcbull extracts datagram

passes to upper layer at receiving side

controller controller

sending host receiving host

datagram datagram

datagram

frame

10

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

11

Error DetectionEDC= Error Detection and Correction bits (redundancy)D = Data protected by error checking may include header fields

bull Error detection not 100 reliablebull protocol may miss some errors but rarelybull larger EDC field yields better detection and correction

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

otherwise

12

Parity Checking

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Single Bit ParityDetect single bit errors

Two Dimensional Bit ParityDetect and correct single bit errors

0 0

13

Internet checksum (re-view)

Senderbull treat segment contents

as sequence of 16-bit integers

bull checksum addition (1rsquos complement sum) of segment contents

bull sender puts checksum value into UDP check-sum field

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Receiverbull compute checksum of

received segmentbull check if computed

checksum equals check-sum field valuebull NO - error detectedbull YES - no error detected

But maybe errors nonethe-less

Goal detect ldquoerrorsrdquo (eg flipped bits) in trans-mitted packet (note used at transport layer only

14

Checksumming Cyclic Redun-dancy Checkbull view data bits D as a binary numberbull choose r+1 bit pattern (generator) G bull goal choose r CRC bits R such that

bull ltDRgt exactly divisible by G (modulo 2) bull receiver knows G divides ltDRgt by G If non-zero remain-

der error detectedbull can detect all burst errors less than r+1 bits

bull widely used in practice (Ethernet 80211 WiFi ATM)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

15

Checksumming Cyclic Redun-dancy Checkbull 1011 XOR 0101 = 1110 bull 1001 XOR 1101 = 0100

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

16

CRC Example

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

WantD2r XOR R = nG

equivalentlyD2r = nG XOR R

equivalently if we divide D2r by

G want remainder R

R = remainder[ ]D2r

G

17

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

18

Multiple Access Links and Proto-colsTwo types of ldquolinksrdquobull point-to-point

bull PPP for dial-up accessbull point-to-point link between Ethernet switch and host

bull broadcast (shared wire or medium)bull old-fashioned Ethernetbull upstream HFC (Hybrid Fiber-Coaxial)bull 80211 wireless LAN

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

shared wire (eg cabled Ethernet)

shared RF (eg 80211 WiFi)

shared RF(satellite)

humans at acocktail party

(shared air acoustical)

19

Multiple Access protocolsbull single shared broadcast channel bull two or more simultaneous transmis-

sions by nodes interference bull collision if a node receives two or more

signals at the same time

multiple access protocolbull distributed algorithm that deter-

mines how nodes share channel ie determine when node can transmit

bull communication about channel shar-ing must use channel itself bull no out-of-band channel for coordination

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

20

Ideal Multiple Access ProtocolBroadcast channel of rate R bps1 when one node wants to transmit it

can send at rate R2 when M nodes want to transmit each

can send at average rate RM3 fully decentralized

bull no special node to coordinate transmissionsbull no synchronization of clocks slots

4 simple

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

21

MAC Protocols a taxonomyThree broad classesbull Channel Partitioning

bull divide channel into smaller ldquopiecesrdquo (time slots frequency code)

bull allocate piece to node for exclusive use

bull Random Accessbull channel not divided allow collisionsbull ldquorecoverrdquo from collisions

bull ldquoTaking turnsrdquobull nodes take turns but nodes with more to send

can take longer turnsFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

22

Channel Partitioning MAC proto-cols TDMATDMA time division multiple access bull access to channel in rounds bull each station gets fixed length slot

(length = pkt trans time) in each round bull unused slots go idle bull example 6-station LAN 134 have pkt

slots 256 idle

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

1 3 4 1 3 4

6-slotframe

23

Channel Partitioning MAC proto-cols FDMAFDMA frequency division multiple access bull channel spectrum divided into frequency bandsbull each station assigned fixed frequency bandbull unused transmission time in frequency bands go idle bull example 6-station LAN 134 have pkt frequency

bands 256 idle

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

freq

uency

bands time

FDM cable

24

Random Access Protocolsbull When node has packet to send

bull transmit at full channel data rate Rbull no a priori coordination among nodes

bull two or more transmitting nodes ldquocollisionrdquobull random access MAC protocol specifies

bull how to detect collisionsbull how to recover from collisions (eg via delayed retrans-

missions)

bull Examples of random access MAC protocolsbull slotted ALOHAbull ALOHAbull CSMA CSMACD CSMACA

bull (CS Carrier Sensing MA Multiple AccessCD Collision Detection CA Collision Avoidance)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

25

Slotted ALOHAAssumptionsbull all frames same sizebull time divided into

equal size slots (time to transmit 1 frame)

bull nodes start to transmit only slot beginning

bull nodes are synchro-nized

bull if 2 or more nodes transmit in slot all nodes detect collision

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Operationbull when node obtains

fresh frame trans-mits in next slotbull if no collision node

can send new frame in next slot

bull if collision node re-transmits frame in each subsequent slot with prob p until success

26

Slotted ALOHA

Prosbull single active node can

continuously transmit at full rate of channel

bull highly decentralized only slots in nodes need to be in sync

bull simpleFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Consbull collisions wasting slotsbull idle slotsbull nodes may be able to

detect collision in less than time to transmit packet

bull clock synchronization

27

Slotted Aloha Efficiency

bull suppose N nodes with many frames to send each transmits in slot with probability p

bull prob that given node has success in a slot = p(1-p)N-1

bull prob that any node has a success = Np(1-p)N-1

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

bull max efficiency find p that max-imizes Np(1-p)N-1

bull for many nodes take limit of Np(1- p)N-1 as N goes to infinity gives

Max efficiency = 1e = 37

Efficiency long-run fraction of successful slots (many nodes all with many frames to send)

At best channelused for useful transmissions 37of time

28

Pure (unslotted) ALOHAbull unslotted Aloha simpler no synchro-

nizationbull when frame first arrives

bull transmit immediately

bull collision probability increasesbull frame sent at t0 collides with other frames sent in

[t0-1t0+1]

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

29

Pure Aloha EfficiencyP(success by given node) = P(node transmits)

P(no other node transmits in [t0-1t0]

P(no other node transmits in [t0t0+1]

= p (1-p)N-1 (1-p)N-1

= p (1-p)2(N-1)

hellip choosing optimum p and then letting

= 1(2e) = 18

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

N

even worse than slotted Aloha

30

CSMA (Carrier Sense Multiple Access)CSMA listen before transmitIf channel sensed idle transmit entire framebull If channel sensed busy defer transmis-

sion

bull human analogy donrsquot interrupt others

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

31

CSMA Collisionscollisions can still occurbull propagation delay means

two nodes may not hear each otherrsquos transmission

collisionbull entire packet transmis-

sion time wasted

notebull role of distance amp propa-

gation delay in determin-ing collision probability

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

spatial layout of nodes

32

CSMACD (Collision Detec-tion)CSMACD carrier sensing deferral as in CSMA

bull collisions detected within short timebull colliding transmissions aborted reducing channel

wastage

bull collision detection bull easy in wired LANs measure signal strengths

compare transmitted received signalsbull difficult in wireless LANs received signal strength

overwhelmed by local transmission strength

bull human analogy the polite conversationalist

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

33

CSMACD collision detection

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

34

ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols

bull share channel efficiently and fairly at high loadbull inefficient at low load delay in channel access

1N bandwidth allocated even if only 1 active node

Random access MAC protocolsbull efficient at low load single node can fully uti-

lize channelbull high load collision overhead

ldquotaking turnsrdquo protocolsbull look for best of both worlds

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

35

ldquoTaking Turnsrdquo MAC protocolsPolling bull master node ldquoinvitesrdquo

slave nodes to transmit in turn

bull typically used with ldquodumbrdquo slave devices

bull concernsbull polling overhead bull latencybull single point of failure

(master)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

T

data

(nothingto send)

T

36

ldquoTaking Turnsrdquo MAC protocolsToken passingbull control token

passed from one node to next se-quentially

bull token messagebull concerns

bull token overhead bull latencybull single point of failure

(token)Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

master

slaves

poll

data

data

37

Summary of MAC protocolsbull channel partitioning by time frequency or

codebull Time Division Frequency Division

bull random access (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire)

hard in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211

bull taking turnsbull polling from central site token passing

bull Bluetooth FDDI IBM Token Ring Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

38

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

39

MAC Addresses and ARPbull 32-bit IP address

bull network-layer addressbull used to get datagram to destination IP subnet

bull MAC (or LAN or physical or Ethernet) ad-dress bull function get frame from one interface to an-

other physically-connected interface (same network)

bull 48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software set-

table

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

40

LAN Addresses and ARP

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Each adapter on LAN has unique LAN address

Broadcast address =FF-FF-FF-FF-FF-FF

= adapter

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired orwireless)

41

LAN Address (more)bull MAC address allocation administered by IEEEbull manufacturer buys portion of MAC address

space (to assure uniqueness)bull analogy

(a) MAC address like Social Security Num-ber

(b) IP address like postal addressbull MAC flat address portability

bull can move LAN card from one LAN to another

bull IP hierarchical address NOT portablebull address depends on IP subnet to which node is at-

tachedFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

42

ARP Address Resolution Proto-colbull Each IP node (host

router) on LAN has ARP table

bull ARP table IPMAC address mappings for some LAN nodeslt IP address MAC address

TTLgt

bull TTL (Time To Live) time after which ad-dress mapping will be forgotten (typically 20 min)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Question how to determineMAC address of Bknowing Brsquos IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN

137196723

137196778

137196714

137196788

43

ARP protocol Same LAN (net-work)bull A wants to send datagram

to B and Brsquos MAC address not in Arsquos ARP table

bull A broadcasts ARP query packet containing Bs IP address bull dest MAC address = FF-FF-FF-

FF-FF-FFbull all machines on LAN receive

ARP query

bull B receives ARP packet replies to A with its (Bs) MAC addressbull frame sent to Arsquos MAC ad-

dress (unicast)Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

bull A caches (saves) IP-to-MAC address pair in its ARP table until infor-mation becomes old (times out) bull soft state information

that times out (goes away) unless refreshed

bull ARP is ldquoplug-and-playrdquobull nodes create their ARP

tables without interven-tion from net administra-tor

44

Addressing routing to another LAN

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

R

1A-23-F9-CD-06-9B

222222222220111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

walkthrough send datagram from A to B via R assume A knows Brsquos IP address

bull two ARP tables in router R one for each IP network (LAN)

45

bull A creates IP datagram with source A destination B bull A uses ARP to get Rrsquos MAC address for 111111111110bull A creates link-layer frame with Rs MAC address as dest

frame contains A-to-B IP datagrambull Arsquos NIC sends frame bull Rrsquos NIC receives frame bull R removes IP datagram from Ethernet frame sees its des-

tined to Bbull R uses ARP to get Brsquos MAC address bull R creates frame containing A-to-B IP datagram sends to B

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

This is a really importantexample ndash make sure youunderstand

R

1A-23-F9-CD-06-9B

222222222220

111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

46

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

47

Ethernetldquodominantrdquo wired LAN technology bull cheap $20 for NICbull first widely used LAN technologybull simpler cheaper than token LANs and ATMbull kept up with speed race 10 Mbps ndash 10 Gbps

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Metcalfersquos Ethernetsketch

48

Star Topologybull bus topology popular through mid 90s

bull all nodes in same collision domain (can collide with each other)

bull today star topology prevailsbull active switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes do

not collide with each other)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

switch

bus coaxial cable star

49

Ethernet Frame Structurebull Sending adapter encapsulates IP datagram

(or other network layer protocol packet) in Ethernet frame

Preamble bull 7 bytes with pattern 10101010 followed by

one byte with pattern 10101011bull used to synchronize receiver sender clock

ratesFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

50

Ethernet Frame Structure (more)bull Addresses 6 bytes

bull if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

bull otherwise adapter discards frame

bull Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)

bull CRC checked at receiver if error is detected frame is dropped

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51

Ethernet Unreliable connection-lessbull connectionless No handshaking be-

tween sending and receiving NICs bull unreliable receiving NIC doesnrsquot send

acks or nacks to sending NICbull stream of datagrams passed to network

layer can have gaps (missing datagrams)bull gaps will be filled if app is using TCPbull otherwise app will see gaps

bull Ethernetrsquos MAC protocol unslotted CSMACD

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

52

Ethernet CSMACD algorithmbull 1 NIC receives data-

gram from network layer creates frame

bull 2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

bull 3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

bull 4 If NIC detects an-other transmission while transmitting aborts and sends jam signal

bull 5 After aborting NIC enters exponential backoff after mth colli-sion NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

53

Ethernetrsquos CSMACD (more)bull Jam Signal make

sure all other transmitters are aware of collision 48 bits

bull Bit time 1 micro sec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Exponential Backoff bull Goal adapt retransmission

attempts to estimated cur-rent load

bull heavy load random wait will be longer

bull first collision choose K from 01 delay is K 512 bit transmission times

bull after second collision choose K from 0123hellip

bull after ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

54

CSMACD efficiencybull Tprop = max prop delay between 2 nodes in

LANbull ttrans = time to transmit max-size frame

bull efficiency goes to 1 bull as tprop goes to 0

bull as ttrans goes to infinity

bull better performance than ALOHA and simple cheap decentralized

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

transprop ttefficiency

51

1

55

8023 Ethernet Standards Link amp Physical Layersbull many different Ethernet standards

bull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps

1Gbps 10G bpsbull different physical layer media fiber cable

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

applicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twisterpair) physical layer

56

Manchester encodingbull used in 10BaseTbull each bit has a transitionbull allows clocks in sending and receiving nodes to syn-

chronize to each otherbull no need for a centralized global clock among nodes

bull Hey this is physical-layer stuff

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

57

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

58

Hubshellip physical-layer (ldquodumbrdquo) repeaters

bull bits coming in one link go out all other links at same rate

bull all nodes connected to hub can collide with one another

bull no frame bufferingbull no CSMACD at hub host NICs detect colli-

sions

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

twisted pair

hub

59

Switchbull link-layer device smarter than hubs take

active rolebull store forward Ethernet framesbull examine incoming framersquos MAC address se-

lectively forward frame to one-or-more out-going links when frame is to be forwarded on segment uses CSMACD to access segment

bull transparentbull hosts are unaware of presence of switches

bull plug-and-play self-learning (HOW)bull switches do not need to be configured

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

60

Switch allows multiple simulta-neous transmissionsbull hosts have dedicated direct

connection to switchbull switches buffer packetsbull Ethernet protocol used on

each incoming link but no collisions full duplexbull each link is its own collision

domain

bull switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions bull not possible with dumb hub

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 23

45

6

61

Switch Tablebull Q how does switch know

that Arsquo reachable via inter-face 4 Brsquo reachable via inter-face 5

bull A each switch has a switch table each entrybull (MAC address of host interface

to reach host time stamp)

bull looks like a routing tablebull Q how are entries created

maintained in switch table bull something like a routing proto-

colFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

switch with six interfaces(123456)

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

62

Switch self-learningbull switch learns which

hosts can be reached through which inter-facesbull when frame received

switch ldquolearnsrdquo location of sender incoming LAN segment

bull records senderlocation pair in switch table

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

63

Switch frame filteringforward-ingWhen frame received

1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination then if dest on segment from which frame arrived then drop the frame else

forward the frame on interface indicated else flood

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

forward on all but the interface on which the frame arrived

64

Self-learning forwarding exam-ple

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

frame destination unknownflood

Arsquo A

destination A location known

Arsquo 4 60

selective send

65

Interconnecting switchesbull switches can be connected together

bull Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3

bull A self learning (works exactly the same as in single-switch case)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

B

S1

C D

E

FS2

S4

S3

H

I

G

66

Self-learning multi-switch exam-plebull Suppose C sends frame to I I responds to

C

bull Q show switch tables and packet forward-ing in S1 S2 S3 S4

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

B

S1

CD

E F

S2

S4

S3

H

I

G

12

1

2 3

3

1 2 3 12 3

444

67

Institutional network

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

to externalnetwork

router

IP subnet

mail server

web server

68

Switches vs Routersbull both store-and-forward devices

bull routers network layer devices (examine network layer head-ers)

bull switches are link layer devices

bull routers maintain routing tables implement routing al-gorithms

bull switches maintain switch tables implement filtering learning algorithms

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

69

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

• Fall 2015 COMP 46055605 Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link layer context
• Link Layer Services
• Link Layer Services (more)
• Where is the link layer implemented
• Adaptors Communicating
• Link Layer (2)
• Error Detection
• Parity Checking
• Internet checksum (review)
• Checksumming Cyclic Redundancy Check
• Checksumming Cyclic Redundancy Check (2)
• CRC Example
• Link Layer (3)
• Multiple Access Links and Protocols
• Multiple Access protocols
• Ideal Multiple Access Protocol
• MAC Protocols a taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD collision detection
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Link Layer (4)
• MAC Addresses and ARP
• LAN Addresses and ARP
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP protocol Same LAN (network)
• Addressing routing to another LAN
• Slide 45
• Link Layer (5)
• Ethernet
• Star Topology
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester encoding
• Link Layer (6)
• Hubs
• Switch
• Switch allows multiple simultaneous transmissions
• Switch Table
• Switch self-learning
• Switch frame filteringforwarding
• Self-learning forwarding example
• Interconnecting switches
• Self-learning multi-switch example
• Institutional network
• Switches vs Routers
• Link Layer (7)

6

Link Layer Servicesbull framing link access

bull encapsulate datagram into frame adding header trailer

bull channel access if shared mediumbull ldquoMACrdquo addresses used in frame headers to identify

source dest bull different from IP address

bull reliable delivery between adjacent nodesbull we learned how to do this already (chapter 3)bull seldom used on low bit-error link (fiber some

twisted pair)bull wireless links high error rates

bull Q why both link-level and end-end reliabilityFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

7

Link Layer Services (more)bull flow control

bull pacing between adjacent sending and receiving nodes

bull error detection bull errors caused by signal attenuation noise bull receiver detects presence of errors

bull signals sender for retransmission or drops frame

bull error correction bull receiver identifies and corrects bit error(s) without re-

sorting to retransmission

bull half-duplex and full-duplexbull with half duplex nodes at both ends of link can

transmit but not at same timeFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

8

Where is the link layer imple-mentedbull in each and every hostbull link layer implemented

in ldquoadaptorrdquo (aka net-work interface card NIC)bull Ethernet card PCMCI

card 80211 cardbull implements link physical

layer

bull attaches into hostrsquos sys-tem buses

bull combination of hard-ware software firmware

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

controller

physicaltransmission

cpu memory

host bus (eg PCI)

network adaptercard

host schematic

applicationtransportnetwork

link

linkphysical

9

Adaptors Communicating

bull sending sidebull encapsulates datagram

in framebull adds error checking bits

rdt flow control etc

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

bull receiving sidebull looks for errors rdt flow

control etcbull extracts datagram

passes to upper layer at receiving side

controller controller

sending host receiving host

datagram datagram

datagram

frame

10

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

11

Error DetectionEDC= Error Detection and Correction bits (redundancy)D = Data protected by error checking may include header fields

bull Error detection not 100 reliablebull protocol may miss some errors but rarelybull larger EDC field yields better detection and correction

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

otherwise

12

Parity Checking

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Single Bit ParityDetect single bit errors

Two Dimensional Bit ParityDetect and correct single bit errors

0 0

13

Internet checksum (re-view)

Senderbull treat segment contents

as sequence of 16-bit integers

bull checksum addition (1rsquos complement sum) of segment contents

bull sender puts checksum value into UDP check-sum field

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Receiverbull compute checksum of

received segmentbull check if computed

checksum equals check-sum field valuebull NO - error detectedbull YES - no error detected

But maybe errors nonethe-less

Goal detect ldquoerrorsrdquo (eg flipped bits) in trans-mitted packet (note used at transport layer only

14

Checksumming Cyclic Redun-dancy Checkbull view data bits D as a binary numberbull choose r+1 bit pattern (generator) G bull goal choose r CRC bits R such that

bull ltDRgt exactly divisible by G (modulo 2) bull receiver knows G divides ltDRgt by G If non-zero remain-

der error detectedbull can detect all burst errors less than r+1 bits

bull widely used in practice (Ethernet 80211 WiFi ATM)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

15

Checksumming Cyclic Redun-dancy Checkbull 1011 XOR 0101 = 1110 bull 1001 XOR 1101 = 0100

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

16

CRC Example

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

WantD2r XOR R = nG

equivalentlyD2r = nG XOR R

equivalently if we divide D2r by

G want remainder R

R = remainder[ ]D2r

G

17

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

18

Multiple Access Links and Proto-colsTwo types of ldquolinksrdquobull point-to-point

bull PPP for dial-up accessbull point-to-point link between Ethernet switch and host

bull broadcast (shared wire or medium)bull old-fashioned Ethernetbull upstream HFC (Hybrid Fiber-Coaxial)bull 80211 wireless LAN

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

shared wire (eg cabled Ethernet)

shared RF (eg 80211 WiFi)

shared RF(satellite)

humans at acocktail party

(shared air acoustical)

19

Multiple Access protocolsbull single shared broadcast channel bull two or more simultaneous transmis-

sions by nodes interference bull collision if a node receives two or more

signals at the same time

multiple access protocolbull distributed algorithm that deter-

mines how nodes share channel ie determine when node can transmit

bull communication about channel shar-ing must use channel itself bull no out-of-band channel for coordination

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

20

Ideal Multiple Access ProtocolBroadcast channel of rate R bps1 when one node wants to transmit it

can send at rate R2 when M nodes want to transmit each

can send at average rate RM3 fully decentralized

bull no special node to coordinate transmissionsbull no synchronization of clocks slots

4 simple

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

21

MAC Protocols a taxonomyThree broad classesbull Channel Partitioning

bull divide channel into smaller ldquopiecesrdquo (time slots frequency code)

bull allocate piece to node for exclusive use

bull Random Accessbull channel not divided allow collisionsbull ldquorecoverrdquo from collisions

bull ldquoTaking turnsrdquobull nodes take turns but nodes with more to send

can take longer turnsFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

22

Channel Partitioning MAC proto-cols TDMATDMA time division multiple access bull access to channel in rounds bull each station gets fixed length slot

(length = pkt trans time) in each round bull unused slots go idle bull example 6-station LAN 134 have pkt

slots 256 idle

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

1 3 4 1 3 4

6-slotframe

23

Channel Partitioning MAC proto-cols FDMAFDMA frequency division multiple access bull channel spectrum divided into frequency bandsbull each station assigned fixed frequency bandbull unused transmission time in frequency bands go idle bull example 6-station LAN 134 have pkt frequency

bands 256 idle

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

freq

uency

bands time

FDM cable

24

Random Access Protocolsbull When node has packet to send

bull transmit at full channel data rate Rbull no a priori coordination among nodes

bull two or more transmitting nodes ldquocollisionrdquobull random access MAC protocol specifies

bull how to detect collisionsbull how to recover from collisions (eg via delayed retrans-

missions)

bull Examples of random access MAC protocolsbull slotted ALOHAbull ALOHAbull CSMA CSMACD CSMACA

bull (CS Carrier Sensing MA Multiple AccessCD Collision Detection CA Collision Avoidance)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

25

Slotted ALOHAAssumptionsbull all frames same sizebull time divided into

equal size slots (time to transmit 1 frame)

bull nodes start to transmit only slot beginning

bull nodes are synchro-nized

bull if 2 or more nodes transmit in slot all nodes detect collision

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Operationbull when node obtains

fresh frame trans-mits in next slotbull if no collision node

can send new frame in next slot

bull if collision node re-transmits frame in each subsequent slot with prob p until success

26

Slotted ALOHA

Prosbull single active node can

continuously transmit at full rate of channel

bull highly decentralized only slots in nodes need to be in sync

bull simpleFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Consbull collisions wasting slotsbull idle slotsbull nodes may be able to

detect collision in less than time to transmit packet

bull clock synchronization

27

Slotted Aloha Efficiency

bull suppose N nodes with many frames to send each transmits in slot with probability p

bull prob that given node has success in a slot = p(1-p)N-1

bull prob that any node has a success = Np(1-p)N-1

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

bull max efficiency find p that max-imizes Np(1-p)N-1

bull for many nodes take limit of Np(1- p)N-1 as N goes to infinity gives

Max efficiency = 1e = 37

Efficiency long-run fraction of successful slots (many nodes all with many frames to send)

At best channelused for useful transmissions 37of time

28

Pure (unslotted) ALOHAbull unslotted Aloha simpler no synchro-

nizationbull when frame first arrives

bull transmit immediately

bull collision probability increasesbull frame sent at t0 collides with other frames sent in

[t0-1t0+1]

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

29

Pure Aloha EfficiencyP(success by given node) = P(node transmits)

P(no other node transmits in [t0-1t0]

P(no other node transmits in [t0t0+1]

= p (1-p)N-1 (1-p)N-1

= p (1-p)2(N-1)

hellip choosing optimum p and then letting

= 1(2e) = 18

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

N

even worse than slotted Aloha

30

CSMA (Carrier Sense Multiple Access)CSMA listen before transmitIf channel sensed idle transmit entire framebull If channel sensed busy defer transmis-

sion

bull human analogy donrsquot interrupt others

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

31

CSMA Collisionscollisions can still occurbull propagation delay means

two nodes may not hear each otherrsquos transmission

collisionbull entire packet transmis-

sion time wasted

notebull role of distance amp propa-

gation delay in determin-ing collision probability

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

spatial layout of nodes

32

CSMACD (Collision Detec-tion)CSMACD carrier sensing deferral as in CSMA

bull collisions detected within short timebull colliding transmissions aborted reducing channel

wastage

bull collision detection bull easy in wired LANs measure signal strengths

compare transmitted received signalsbull difficult in wireless LANs received signal strength

overwhelmed by local transmission strength

bull human analogy the polite conversationalist

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

33

CSMACD collision detection

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

34

ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols

bull share channel efficiently and fairly at high loadbull inefficient at low load delay in channel access

1N bandwidth allocated even if only 1 active node

Random access MAC protocolsbull efficient at low load single node can fully uti-

lize channelbull high load collision overhead

ldquotaking turnsrdquo protocolsbull look for best of both worlds

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

35

ldquoTaking Turnsrdquo MAC protocolsPolling bull master node ldquoinvitesrdquo

slave nodes to transmit in turn

bull typically used with ldquodumbrdquo slave devices

bull concernsbull polling overhead bull latencybull single point of failure

(master)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

T

data

(nothingto send)

T

36

ldquoTaking Turnsrdquo MAC protocolsToken passingbull control token

passed from one node to next se-quentially

bull token messagebull concerns

bull token overhead bull latencybull single point of failure

(token)Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

master

slaves

poll

data

data

37

Summary of MAC protocolsbull channel partitioning by time frequency or

codebull Time Division Frequency Division

bull random access (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire)

hard in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211

bull taking turnsbull polling from central site token passing

bull Bluetooth FDDI IBM Token Ring Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

38

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

39

MAC Addresses and ARPbull 32-bit IP address

bull network-layer addressbull used to get datagram to destination IP subnet

bull MAC (or LAN or physical or Ethernet) ad-dress bull function get frame from one interface to an-

other physically-connected interface (same network)

bull 48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software set-

table

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

40

LAN Addresses and ARP

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Each adapter on LAN has unique LAN address

Broadcast address =FF-FF-FF-FF-FF-FF

= adapter

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired orwireless)

41

LAN Address (more)bull MAC address allocation administered by IEEEbull manufacturer buys portion of MAC address

space (to assure uniqueness)bull analogy

(a) MAC address like Social Security Num-ber

(b) IP address like postal addressbull MAC flat address portability

bull can move LAN card from one LAN to another

bull IP hierarchical address NOT portablebull address depends on IP subnet to which node is at-

tachedFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

42

ARP Address Resolution Proto-colbull Each IP node (host

router) on LAN has ARP table

bull ARP table IPMAC address mappings for some LAN nodeslt IP address MAC address

TTLgt

bull TTL (Time To Live) time after which ad-dress mapping will be forgotten (typically 20 min)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Question how to determineMAC address of Bknowing Brsquos IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN

137196723

137196778

137196714

137196788

43

ARP protocol Same LAN (net-work)bull A wants to send datagram

to B and Brsquos MAC address not in Arsquos ARP table

bull A broadcasts ARP query packet containing Bs IP address bull dest MAC address = FF-FF-FF-

FF-FF-FFbull all machines on LAN receive

ARP query

bull B receives ARP packet replies to A with its (Bs) MAC addressbull frame sent to Arsquos MAC ad-

dress (unicast)Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

bull A caches (saves) IP-to-MAC address pair in its ARP table until infor-mation becomes old (times out) bull soft state information

that times out (goes away) unless refreshed

bull ARP is ldquoplug-and-playrdquobull nodes create their ARP

tables without interven-tion from net administra-tor

44

Addressing routing to another LAN

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

R

1A-23-F9-CD-06-9B

222222222220111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

walkthrough send datagram from A to B via R assume A knows Brsquos IP address

bull two ARP tables in router R one for each IP network (LAN)

45

bull A creates IP datagram with source A destination B bull A uses ARP to get Rrsquos MAC address for 111111111110bull A creates link-layer frame with Rs MAC address as dest

frame contains A-to-B IP datagrambull Arsquos NIC sends frame bull Rrsquos NIC receives frame bull R removes IP datagram from Ethernet frame sees its des-

tined to Bbull R uses ARP to get Brsquos MAC address bull R creates frame containing A-to-B IP datagram sends to B

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

This is a really importantexample ndash make sure youunderstand

R

1A-23-F9-CD-06-9B

222222222220

111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

46

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

47

Ethernetldquodominantrdquo wired LAN technology bull cheap $20 for NICbull first widely used LAN technologybull simpler cheaper than token LANs and ATMbull kept up with speed race 10 Mbps ndash 10 Gbps

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Metcalfersquos Ethernetsketch

48

Star Topologybull bus topology popular through mid 90s

bull all nodes in same collision domain (can collide with each other)

bull today star topology prevailsbull active switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes do

not collide with each other)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

switch

bus coaxial cable star

49

Ethernet Frame Structurebull Sending adapter encapsulates IP datagram

(or other network layer protocol packet) in Ethernet frame

Preamble bull 7 bytes with pattern 10101010 followed by

one byte with pattern 10101011bull used to synchronize receiver sender clock

ratesFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

50

Ethernet Frame Structure (more)bull Addresses 6 bytes

bull if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

bull otherwise adapter discards frame

bull Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)

bull CRC checked at receiver if error is detected frame is dropped

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51

Ethernet Unreliable connection-lessbull connectionless No handshaking be-

tween sending and receiving NICs bull unreliable receiving NIC doesnrsquot send

acks or nacks to sending NICbull stream of datagrams passed to network

layer can have gaps (missing datagrams)bull gaps will be filled if app is using TCPbull otherwise app will see gaps

bull Ethernetrsquos MAC protocol unslotted CSMACD

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

52

Ethernet CSMACD algorithmbull 1 NIC receives data-

gram from network layer creates frame

bull 2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

bull 3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

bull 4 If NIC detects an-other transmission while transmitting aborts and sends jam signal

bull 5 After aborting NIC enters exponential backoff after mth colli-sion NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

53

Ethernetrsquos CSMACD (more)bull Jam Signal make

sure all other transmitters are aware of collision 48 bits

bull Bit time 1 micro sec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Exponential Backoff bull Goal adapt retransmission

attempts to estimated cur-rent load

bull heavy load random wait will be longer

bull first collision choose K from 01 delay is K 512 bit transmission times

bull after second collision choose K from 0123hellip

bull after ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

54

CSMACD efficiencybull Tprop = max prop delay between 2 nodes in

LANbull ttrans = time to transmit max-size frame

bull efficiency goes to 1 bull as tprop goes to 0

bull as ttrans goes to infinity

bull better performance than ALOHA and simple cheap decentralized

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

transprop ttefficiency

51

1

55

8023 Ethernet Standards Link amp Physical Layersbull many different Ethernet standards

bull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps

1Gbps 10G bpsbull different physical layer media fiber cable

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

applicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twisterpair) physical layer

56

Manchester encodingbull used in 10BaseTbull each bit has a transitionbull allows clocks in sending and receiving nodes to syn-

chronize to each otherbull no need for a centralized global clock among nodes

bull Hey this is physical-layer stuff

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

57

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

58

Hubshellip physical-layer (ldquodumbrdquo) repeaters

bull bits coming in one link go out all other links at same rate

bull all nodes connected to hub can collide with one another

bull no frame bufferingbull no CSMACD at hub host NICs detect colli-

sions

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

twisted pair

hub

59

Switchbull link-layer device smarter than hubs take

active rolebull store forward Ethernet framesbull examine incoming framersquos MAC address se-

lectively forward frame to one-or-more out-going links when frame is to be forwarded on segment uses CSMACD to access segment

bull transparentbull hosts are unaware of presence of switches

bull plug-and-play self-learning (HOW)bull switches do not need to be configured

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

60

Switch allows multiple simulta-neous transmissionsbull hosts have dedicated direct

connection to switchbull switches buffer packetsbull Ethernet protocol used on

each incoming link but no collisions full duplexbull each link is its own collision

domain

bull switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions bull not possible with dumb hub

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 23

45

6

61

Switch Tablebull Q how does switch know

that Arsquo reachable via inter-face 4 Brsquo reachable via inter-face 5

bull A each switch has a switch table each entrybull (MAC address of host interface

to reach host time stamp)

bull looks like a routing tablebull Q how are entries created

maintained in switch table bull something like a routing proto-

colFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

switch with six interfaces(123456)

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

62

Switch self-learningbull switch learns which

hosts can be reached through which inter-facesbull when frame received

switch ldquolearnsrdquo location of sender incoming LAN segment

bull records senderlocation pair in switch table

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

63

Switch frame filteringforward-ingWhen frame received

1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination then if dest on segment from which frame arrived then drop the frame else

forward the frame on interface indicated else flood

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

forward on all but the interface on which the frame arrived

64

Self-learning forwarding exam-ple

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

frame destination unknownflood

Arsquo A

destination A location known

Arsquo 4 60

selective send

65

Interconnecting switchesbull switches can be connected together

bull Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3

bull A self learning (works exactly the same as in single-switch case)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

B

S1

C D

E

FS2

S4

S3

H

I

G

66

Self-learning multi-switch exam-plebull Suppose C sends frame to I I responds to

C

bull Q show switch tables and packet forward-ing in S1 S2 S3 S4

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

B

S1

CD

E F

S2

S4

S3

H

I

G

12

1

2 3

3

1 2 3 12 3

444

67

Institutional network

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

to externalnetwork

router

IP subnet

mail server

web server

68

Switches vs Routersbull both store-and-forward devices

bull routers network layer devices (examine network layer head-ers)

bull switches are link layer devices

bull routers maintain routing tables implement routing al-gorithms

bull switches maintain switch tables implement filtering learning algorithms

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

69

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

• Fall 2015 COMP 46055605 Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link layer context
• Link Layer Services
• Link Layer Services (more)
• Where is the link layer implemented
• Adaptors Communicating
• Link Layer (2)
• Error Detection
• Parity Checking
• Internet checksum (review)
• Checksumming Cyclic Redundancy Check
• Checksumming Cyclic Redundancy Check (2)
• CRC Example
• Link Layer (3)
• Multiple Access Links and Protocols
• Multiple Access protocols
• Ideal Multiple Access Protocol
• MAC Protocols a taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD collision detection
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Link Layer (4)
• MAC Addresses and ARP
• LAN Addresses and ARP
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP protocol Same LAN (network)
• Addressing routing to another LAN
• Slide 45
• Link Layer (5)
• Ethernet
• Star Topology
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester encoding
• Link Layer (6)
• Hubs
• Switch
• Switch allows multiple simultaneous transmissions
• Switch Table
• Switch self-learning
• Switch frame filteringforwarding
• Self-learning forwarding example
• Interconnecting switches
• Self-learning multi-switch example
• Institutional network
• Switches vs Routers
• Link Layer (7)

7

Link Layer Services (more)bull flow control

bull pacing between adjacent sending and receiving nodes

bull error detection bull errors caused by signal attenuation noise bull receiver detects presence of errors

bull signals sender for retransmission or drops frame

bull error correction bull receiver identifies and corrects bit error(s) without re-

sorting to retransmission

bull half-duplex and full-duplexbull with half duplex nodes at both ends of link can

transmit but not at same timeFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

8

Where is the link layer imple-mentedbull in each and every hostbull link layer implemented

in ldquoadaptorrdquo (aka net-work interface card NIC)bull Ethernet card PCMCI

card 80211 cardbull implements link physical

layer

bull attaches into hostrsquos sys-tem buses

bull combination of hard-ware software firmware

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

controller

physicaltransmission

cpu memory

host bus (eg PCI)

network adaptercard

host schematic

applicationtransportnetwork

link

linkphysical

9

Adaptors Communicating

bull sending sidebull encapsulates datagram

in framebull adds error checking bits

rdt flow control etc

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

bull receiving sidebull looks for errors rdt flow

control etcbull extracts datagram

passes to upper layer at receiving side

controller controller

sending host receiving host

datagram datagram

datagram

frame

10

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

11

Error DetectionEDC= Error Detection and Correction bits (redundancy)D = Data protected by error checking may include header fields

bull Error detection not 100 reliablebull protocol may miss some errors but rarelybull larger EDC field yields better detection and correction

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

otherwise

12

Parity Checking

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Single Bit ParityDetect single bit errors

Two Dimensional Bit ParityDetect and correct single bit errors

0 0

13

Internet checksum (re-view)

Senderbull treat segment contents

as sequence of 16-bit integers

bull checksum addition (1rsquos complement sum) of segment contents

bull sender puts checksum value into UDP check-sum field

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Receiverbull compute checksum of

received segmentbull check if computed

checksum equals check-sum field valuebull NO - error detectedbull YES - no error detected

But maybe errors nonethe-less

Goal detect ldquoerrorsrdquo (eg flipped bits) in trans-mitted packet (note used at transport layer only

14

Checksumming Cyclic Redun-dancy Checkbull view data bits D as a binary numberbull choose r+1 bit pattern (generator) G bull goal choose r CRC bits R such that

bull ltDRgt exactly divisible by G (modulo 2) bull receiver knows G divides ltDRgt by G If non-zero remain-

der error detectedbull can detect all burst errors less than r+1 bits

bull widely used in practice (Ethernet 80211 WiFi ATM)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

15

Checksumming Cyclic Redun-dancy Checkbull 1011 XOR 0101 = 1110 bull 1001 XOR 1101 = 0100

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

16

CRC Example

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

WantD2r XOR R = nG

equivalentlyD2r = nG XOR R

equivalently if we divide D2r by

G want remainder R

R = remainder[ ]D2r

G

17

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

18

Multiple Access Links and Proto-colsTwo types of ldquolinksrdquobull point-to-point

bull PPP for dial-up accessbull point-to-point link between Ethernet switch and host

bull broadcast (shared wire or medium)bull old-fashioned Ethernetbull upstream HFC (Hybrid Fiber-Coaxial)bull 80211 wireless LAN

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

shared wire (eg cabled Ethernet)

shared RF (eg 80211 WiFi)

shared RF(satellite)

humans at acocktail party

(shared air acoustical)

19

Multiple Access protocolsbull single shared broadcast channel bull two or more simultaneous transmis-

sions by nodes interference bull collision if a node receives two or more

signals at the same time

multiple access protocolbull distributed algorithm that deter-

mines how nodes share channel ie determine when node can transmit

bull communication about channel shar-ing must use channel itself bull no out-of-band channel for coordination

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

20

Ideal Multiple Access ProtocolBroadcast channel of rate R bps1 when one node wants to transmit it

can send at rate R2 when M nodes want to transmit each

can send at average rate RM3 fully decentralized

bull no special node to coordinate transmissionsbull no synchronization of clocks slots

4 simple

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

21

MAC Protocols a taxonomyThree broad classesbull Channel Partitioning

bull divide channel into smaller ldquopiecesrdquo (time slots frequency code)

bull allocate piece to node for exclusive use

bull Random Accessbull channel not divided allow collisionsbull ldquorecoverrdquo from collisions

bull ldquoTaking turnsrdquobull nodes take turns but nodes with more to send

can take longer turnsFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

22

Channel Partitioning MAC proto-cols TDMATDMA time division multiple access bull access to channel in rounds bull each station gets fixed length slot

(length = pkt trans time) in each round bull unused slots go idle bull example 6-station LAN 134 have pkt

slots 256 idle

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

1 3 4 1 3 4

6-slotframe

23

Channel Partitioning MAC proto-cols FDMAFDMA frequency division multiple access bull channel spectrum divided into frequency bandsbull each station assigned fixed frequency bandbull unused transmission time in frequency bands go idle bull example 6-station LAN 134 have pkt frequency

bands 256 idle

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

freq

uency

bands time

FDM cable

24

Random Access Protocolsbull When node has packet to send

bull transmit at full channel data rate Rbull no a priori coordination among nodes

bull two or more transmitting nodes ldquocollisionrdquobull random access MAC protocol specifies

bull how to detect collisionsbull how to recover from collisions (eg via delayed retrans-

missions)

bull Examples of random access MAC protocolsbull slotted ALOHAbull ALOHAbull CSMA CSMACD CSMACA

bull (CS Carrier Sensing MA Multiple AccessCD Collision Detection CA Collision Avoidance)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

25

Slotted ALOHAAssumptionsbull all frames same sizebull time divided into

equal size slots (time to transmit 1 frame)

bull nodes start to transmit only slot beginning

bull nodes are synchro-nized

bull if 2 or more nodes transmit in slot all nodes detect collision

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Operationbull when node obtains

fresh frame trans-mits in next slotbull if no collision node

can send new frame in next slot

bull if collision node re-transmits frame in each subsequent slot with prob p until success

26

Slotted ALOHA

Prosbull single active node can

continuously transmit at full rate of channel

bull highly decentralized only slots in nodes need to be in sync

bull simpleFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Consbull collisions wasting slotsbull idle slotsbull nodes may be able to

detect collision in less than time to transmit packet

bull clock synchronization

27

Slotted Aloha Efficiency

bull suppose N nodes with many frames to send each transmits in slot with probability p

bull prob that given node has success in a slot = p(1-p)N-1

bull prob that any node has a success = Np(1-p)N-1

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

bull max efficiency find p that max-imizes Np(1-p)N-1

bull for many nodes take limit of Np(1- p)N-1 as N goes to infinity gives

Max efficiency = 1e = 37

Efficiency long-run fraction of successful slots (many nodes all with many frames to send)

At best channelused for useful transmissions 37of time

28

Pure (unslotted) ALOHAbull unslotted Aloha simpler no synchro-

nizationbull when frame first arrives

bull transmit immediately

bull collision probability increasesbull frame sent at t0 collides with other frames sent in

[t0-1t0+1]

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

29

Pure Aloha EfficiencyP(success by given node) = P(node transmits)

P(no other node transmits in [t0-1t0]

P(no other node transmits in [t0t0+1]

= p (1-p)N-1 (1-p)N-1

= p (1-p)2(N-1)

hellip choosing optimum p and then letting

= 1(2e) = 18

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

N

even worse than slotted Aloha

30

CSMA (Carrier Sense Multiple Access)CSMA listen before transmitIf channel sensed idle transmit entire framebull If channel sensed busy defer transmis-

sion

bull human analogy donrsquot interrupt others

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

31

CSMA Collisionscollisions can still occurbull propagation delay means

two nodes may not hear each otherrsquos transmission

collisionbull entire packet transmis-

sion time wasted

notebull role of distance amp propa-

gation delay in determin-ing collision probability

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

spatial layout of nodes

32

CSMACD (Collision Detec-tion)CSMACD carrier sensing deferral as in CSMA

bull collisions detected within short timebull colliding transmissions aborted reducing channel

wastage

bull collision detection bull easy in wired LANs measure signal strengths

compare transmitted received signalsbull difficult in wireless LANs received signal strength

overwhelmed by local transmission strength

bull human analogy the polite conversationalist

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

33

CSMACD collision detection

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

34

ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols

bull share channel efficiently and fairly at high loadbull inefficient at low load delay in channel access

1N bandwidth allocated even if only 1 active node

Random access MAC protocolsbull efficient at low load single node can fully uti-

lize channelbull high load collision overhead

ldquotaking turnsrdquo protocolsbull look for best of both worlds

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

35

ldquoTaking Turnsrdquo MAC protocolsPolling bull master node ldquoinvitesrdquo

slave nodes to transmit in turn

bull typically used with ldquodumbrdquo slave devices

bull concernsbull polling overhead bull latencybull single point of failure

(master)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

T

data

(nothingto send)

T

36

ldquoTaking Turnsrdquo MAC protocolsToken passingbull control token

passed from one node to next se-quentially

bull token messagebull concerns

bull token overhead bull latencybull single point of failure

(token)Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

master

slaves

poll

data

data

37

Summary of MAC protocolsbull channel partitioning by time frequency or

codebull Time Division Frequency Division

bull random access (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire)

hard in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211

bull taking turnsbull polling from central site token passing

bull Bluetooth FDDI IBM Token Ring Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

38

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

39

MAC Addresses and ARPbull 32-bit IP address

bull network-layer addressbull used to get datagram to destination IP subnet

bull MAC (or LAN or physical or Ethernet) ad-dress bull function get frame from one interface to an-

other physically-connected interface (same network)

bull 48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software set-

table

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

40

LAN Addresses and ARP

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Each adapter on LAN has unique LAN address

Broadcast address =FF-FF-FF-FF-FF-FF

= adapter

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired orwireless)

41

LAN Address (more)bull MAC address allocation administered by IEEEbull manufacturer buys portion of MAC address

space (to assure uniqueness)bull analogy

(a) MAC address like Social Security Num-ber

(b) IP address like postal addressbull MAC flat address portability

bull can move LAN card from one LAN to another

bull IP hierarchical address NOT portablebull address depends on IP subnet to which node is at-

tachedFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

42

ARP Address Resolution Proto-colbull Each IP node (host

router) on LAN has ARP table

bull ARP table IPMAC address mappings for some LAN nodeslt IP address MAC address

TTLgt

bull TTL (Time To Live) time after which ad-dress mapping will be forgotten (typically 20 min)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Question how to determineMAC address of Bknowing Brsquos IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN

137196723

137196778

137196714

137196788

43

ARP protocol Same LAN (net-work)bull A wants to send datagram

to B and Brsquos MAC address not in Arsquos ARP table

bull A broadcasts ARP query packet containing Bs IP address bull dest MAC address = FF-FF-FF-

FF-FF-FFbull all machines on LAN receive

ARP query

bull B receives ARP packet replies to A with its (Bs) MAC addressbull frame sent to Arsquos MAC ad-

dress (unicast)Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

bull A caches (saves) IP-to-MAC address pair in its ARP table until infor-mation becomes old (times out) bull soft state information

that times out (goes away) unless refreshed

bull ARP is ldquoplug-and-playrdquobull nodes create their ARP

tables without interven-tion from net administra-tor

44

Addressing routing to another LAN

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

R

1A-23-F9-CD-06-9B

222222222220111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

walkthrough send datagram from A to B via R assume A knows Brsquos IP address

bull two ARP tables in router R one for each IP network (LAN)

45

bull A creates IP datagram with source A destination B bull A uses ARP to get Rrsquos MAC address for 111111111110bull A creates link-layer frame with Rs MAC address as dest

frame contains A-to-B IP datagrambull Arsquos NIC sends frame bull Rrsquos NIC receives frame bull R removes IP datagram from Ethernet frame sees its des-

tined to Bbull R uses ARP to get Brsquos MAC address bull R creates frame containing A-to-B IP datagram sends to B

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

This is a really importantexample ndash make sure youunderstand

R

1A-23-F9-CD-06-9B

222222222220

111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

46

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

47

Ethernetldquodominantrdquo wired LAN technology bull cheap $20 for NICbull first widely used LAN technologybull simpler cheaper than token LANs and ATMbull kept up with speed race 10 Mbps ndash 10 Gbps

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Metcalfersquos Ethernetsketch

48

Star Topologybull bus topology popular through mid 90s

bull all nodes in same collision domain (can collide with each other)

bull today star topology prevailsbull active switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes do

not collide with each other)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

switch

bus coaxial cable star

49

Ethernet Frame Structurebull Sending adapter encapsulates IP datagram

(or other network layer protocol packet) in Ethernet frame

Preamble bull 7 bytes with pattern 10101010 followed by

one byte with pattern 10101011bull used to synchronize receiver sender clock

ratesFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

50

Ethernet Frame Structure (more)bull Addresses 6 bytes

bull if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

bull otherwise adapter discards frame

bull Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)

bull CRC checked at receiver if error is detected frame is dropped

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51

Ethernet Unreliable connection-lessbull connectionless No handshaking be-

tween sending and receiving NICs bull unreliable receiving NIC doesnrsquot send

acks or nacks to sending NICbull stream of datagrams passed to network

layer can have gaps (missing datagrams)bull gaps will be filled if app is using TCPbull otherwise app will see gaps

bull Ethernetrsquos MAC protocol unslotted CSMACD

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

52

Ethernet CSMACD algorithmbull 1 NIC receives data-

gram from network layer creates frame

bull 2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

bull 3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

bull 4 If NIC detects an-other transmission while transmitting aborts and sends jam signal

bull 5 After aborting NIC enters exponential backoff after mth colli-sion NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

53

Ethernetrsquos CSMACD (more)bull Jam Signal make

sure all other transmitters are aware of collision 48 bits

bull Bit time 1 micro sec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Exponential Backoff bull Goal adapt retransmission

attempts to estimated cur-rent load

bull heavy load random wait will be longer

bull first collision choose K from 01 delay is K 512 bit transmission times

bull after second collision choose K from 0123hellip

bull after ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

54

CSMACD efficiencybull Tprop = max prop delay between 2 nodes in

LANbull ttrans = time to transmit max-size frame

bull efficiency goes to 1 bull as tprop goes to 0

bull as ttrans goes to infinity

bull better performance than ALOHA and simple cheap decentralized

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

transprop ttefficiency

51

1

55

8023 Ethernet Standards Link amp Physical Layersbull many different Ethernet standards

bull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps

1Gbps 10G bpsbull different physical layer media fiber cable

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

applicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twisterpair) physical layer

56

Manchester encodingbull used in 10BaseTbull each bit has a transitionbull allows clocks in sending and receiving nodes to syn-

chronize to each otherbull no need for a centralized global clock among nodes

bull Hey this is physical-layer stuff

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

57

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

58

Hubshellip physical-layer (ldquodumbrdquo) repeaters

bull bits coming in one link go out all other links at same rate

bull all nodes connected to hub can collide with one another

bull no frame bufferingbull no CSMACD at hub host NICs detect colli-

sions

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

twisted pair

hub

59

Switchbull link-layer device smarter than hubs take

active rolebull store forward Ethernet framesbull examine incoming framersquos MAC address se-

lectively forward frame to one-or-more out-going links when frame is to be forwarded on segment uses CSMACD to access segment

bull transparentbull hosts are unaware of presence of switches

bull plug-and-play self-learning (HOW)bull switches do not need to be configured

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

60

Switch allows multiple simulta-neous transmissionsbull hosts have dedicated direct

connection to switchbull switches buffer packetsbull Ethernet protocol used on

each incoming link but no collisions full duplexbull each link is its own collision

domain

bull switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions bull not possible with dumb hub

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 23

45

6

61

Switch Tablebull Q how does switch know

that Arsquo reachable via inter-face 4 Brsquo reachable via inter-face 5

bull A each switch has a switch table each entrybull (MAC address of host interface

to reach host time stamp)

bull looks like a routing tablebull Q how are entries created

maintained in switch table bull something like a routing proto-

colFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

switch with six interfaces(123456)

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

62

Switch self-learningbull switch learns which

hosts can be reached through which inter-facesbull when frame received

switch ldquolearnsrdquo location of sender incoming LAN segment

bull records senderlocation pair in switch table

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

63

Switch frame filteringforward-ingWhen frame received

1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination then if dest on segment from which frame arrived then drop the frame else

forward the frame on interface indicated else flood

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

forward on all but the interface on which the frame arrived

64

Self-learning forwarding exam-ple

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

frame destination unknownflood

Arsquo A

destination A location known

Arsquo 4 60

selective send

65

Interconnecting switchesbull switches can be connected together

bull Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3

bull A self learning (works exactly the same as in single-switch case)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

B

S1

C D

E

FS2

S4

S3

H

I

G

66

Self-learning multi-switch exam-plebull Suppose C sends frame to I I responds to

C

bull Q show switch tables and packet forward-ing in S1 S2 S3 S4

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

B

S1

CD

E F

S2

S4

S3

H

I

G

12

1

2 3

3

1 2 3 12 3

444

67

Institutional network

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

to externalnetwork

router

IP subnet

mail server

web server

68

Switches vs Routersbull both store-and-forward devices

bull routers network layer devices (examine network layer head-ers)

bull switches are link layer devices

bull routers maintain routing tables implement routing al-gorithms

bull switches maintain switch tables implement filtering learning algorithms

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

69

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

• Fall 2015 COMP 46055605 Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link layer context
• Link Layer Services
• Link Layer Services (more)
• Where is the link layer implemented
• Adaptors Communicating
• Link Layer (2)
• Error Detection
• Parity Checking
• Internet checksum (review)
• Checksumming Cyclic Redundancy Check
• Checksumming Cyclic Redundancy Check (2)
• CRC Example
• Link Layer (3)
• Multiple Access Links and Protocols
• Multiple Access protocols
• Ideal Multiple Access Protocol
• MAC Protocols a taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD collision detection
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Link Layer (4)
• MAC Addresses and ARP
• LAN Addresses and ARP
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP protocol Same LAN (network)
• Addressing routing to another LAN
• Slide 45
• Link Layer (5)
• Ethernet
• Star Topology
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester encoding
• Link Layer (6)
• Hubs
• Switch
• Switch allows multiple simultaneous transmissions
• Switch Table
• Switch self-learning
• Switch frame filteringforwarding
• Self-learning forwarding example
• Interconnecting switches
• Self-learning multi-switch example
• Institutional network
• Switches vs Routers
• Link Layer (7)

8

Where is the link layer imple-mentedbull in each and every hostbull link layer implemented

in ldquoadaptorrdquo (aka net-work interface card NIC)bull Ethernet card PCMCI

card 80211 cardbull implements link physical

layer

bull attaches into hostrsquos sys-tem buses

bull combination of hard-ware software firmware

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

controller

physicaltransmission

cpu memory

host bus (eg PCI)

network adaptercard

host schematic

applicationtransportnetwork

link

linkphysical

9

Adaptors Communicating

bull sending sidebull encapsulates datagram

in framebull adds error checking bits

rdt flow control etc

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

bull receiving sidebull looks for errors rdt flow

control etcbull extracts datagram

passes to upper layer at receiving side

controller controller

sending host receiving host

datagram datagram

datagram

frame

10

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

11

Error DetectionEDC= Error Detection and Correction bits (redundancy)D = Data protected by error checking may include header fields

bull Error detection not 100 reliablebull protocol may miss some errors but rarelybull larger EDC field yields better detection and correction

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

otherwise

12

Parity Checking

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Single Bit ParityDetect single bit errors

Two Dimensional Bit ParityDetect and correct single bit errors

0 0

13

Internet checksum (re-view)

Senderbull treat segment contents

as sequence of 16-bit integers

bull checksum addition (1rsquos complement sum) of segment contents

bull sender puts checksum value into UDP check-sum field

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Receiverbull compute checksum of

received segmentbull check if computed

checksum equals check-sum field valuebull NO - error detectedbull YES - no error detected

But maybe errors nonethe-less

Goal detect ldquoerrorsrdquo (eg flipped bits) in trans-mitted packet (note used at transport layer only

14

Checksumming Cyclic Redun-dancy Checkbull view data bits D as a binary numberbull choose r+1 bit pattern (generator) G bull goal choose r CRC bits R such that

bull ltDRgt exactly divisible by G (modulo 2) bull receiver knows G divides ltDRgt by G If non-zero remain-

der error detectedbull can detect all burst errors less than r+1 bits

bull widely used in practice (Ethernet 80211 WiFi ATM)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

15

Checksumming Cyclic Redun-dancy Checkbull 1011 XOR 0101 = 1110 bull 1001 XOR 1101 = 0100

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

16

CRC Example

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

WantD2r XOR R = nG

equivalentlyD2r = nG XOR R

equivalently if we divide D2r by

G want remainder R

R = remainder[ ]D2r

G

17

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

18

Multiple Access Links and Proto-colsTwo types of ldquolinksrdquobull point-to-point

bull PPP for dial-up accessbull point-to-point link between Ethernet switch and host

bull broadcast (shared wire or medium)bull old-fashioned Ethernetbull upstream HFC (Hybrid Fiber-Coaxial)bull 80211 wireless LAN

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

shared wire (eg cabled Ethernet)

shared RF (eg 80211 WiFi)

shared RF(satellite)

humans at acocktail party

(shared air acoustical)

19

Multiple Access protocolsbull single shared broadcast channel bull two or more simultaneous transmis-

sions by nodes interference bull collision if a node receives two or more

signals at the same time

multiple access protocolbull distributed algorithm that deter-

mines how nodes share channel ie determine when node can transmit

bull communication about channel shar-ing must use channel itself bull no out-of-band channel for coordination

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

20

Ideal Multiple Access ProtocolBroadcast channel of rate R bps1 when one node wants to transmit it

can send at rate R2 when M nodes want to transmit each

can send at average rate RM3 fully decentralized

bull no special node to coordinate transmissionsbull no synchronization of clocks slots

4 simple

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

21

MAC Protocols a taxonomyThree broad classesbull Channel Partitioning

bull divide channel into smaller ldquopiecesrdquo (time slots frequency code)

bull allocate piece to node for exclusive use

bull Random Accessbull channel not divided allow collisionsbull ldquorecoverrdquo from collisions

bull ldquoTaking turnsrdquobull nodes take turns but nodes with more to send

can take longer turnsFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

22

Channel Partitioning MAC proto-cols TDMATDMA time division multiple access bull access to channel in rounds bull each station gets fixed length slot

(length = pkt trans time) in each round bull unused slots go idle bull example 6-station LAN 134 have pkt

slots 256 idle

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

1 3 4 1 3 4

6-slotframe

23

Channel Partitioning MAC proto-cols FDMAFDMA frequency division multiple access bull channel spectrum divided into frequency bandsbull each station assigned fixed frequency bandbull unused transmission time in frequency bands go idle bull example 6-station LAN 134 have pkt frequency

bands 256 idle

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

freq

uency

bands time

FDM cable

24

Random Access Protocolsbull When node has packet to send

bull transmit at full channel data rate Rbull no a priori coordination among nodes

bull two or more transmitting nodes ldquocollisionrdquobull random access MAC protocol specifies

bull how to detect collisionsbull how to recover from collisions (eg via delayed retrans-

missions)

bull Examples of random access MAC protocolsbull slotted ALOHAbull ALOHAbull CSMA CSMACD CSMACA

bull (CS Carrier Sensing MA Multiple AccessCD Collision Detection CA Collision Avoidance)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

25

Slotted ALOHAAssumptionsbull all frames same sizebull time divided into

equal size slots (time to transmit 1 frame)

bull nodes start to transmit only slot beginning

bull nodes are synchro-nized

bull if 2 or more nodes transmit in slot all nodes detect collision

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Operationbull when node obtains

fresh frame trans-mits in next slotbull if no collision node

can send new frame in next slot

bull if collision node re-transmits frame in each subsequent slot with prob p until success

26

Slotted ALOHA

Prosbull single active node can

continuously transmit at full rate of channel

bull highly decentralized only slots in nodes need to be in sync

bull simpleFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Consbull collisions wasting slotsbull idle slotsbull nodes may be able to

detect collision in less than time to transmit packet

bull clock synchronization

27

Slotted Aloha Efficiency

bull suppose N nodes with many frames to send each transmits in slot with probability p

bull prob that given node has success in a slot = p(1-p)N-1

bull prob that any node has a success = Np(1-p)N-1

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

bull max efficiency find p that max-imizes Np(1-p)N-1

bull for many nodes take limit of Np(1- p)N-1 as N goes to infinity gives

Max efficiency = 1e = 37

Efficiency long-run fraction of successful slots (many nodes all with many frames to send)

At best channelused for useful transmissions 37of time

28

Pure (unslotted) ALOHAbull unslotted Aloha simpler no synchro-

nizationbull when frame first arrives

bull transmit immediately

bull collision probability increasesbull frame sent at t0 collides with other frames sent in

[t0-1t0+1]

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

29

Pure Aloha EfficiencyP(success by given node) = P(node transmits)

P(no other node transmits in [t0-1t0]

P(no other node transmits in [t0t0+1]

= p (1-p)N-1 (1-p)N-1

= p (1-p)2(N-1)

hellip choosing optimum p and then letting

= 1(2e) = 18

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

N

even worse than slotted Aloha

30

CSMA (Carrier Sense Multiple Access)CSMA listen before transmitIf channel sensed idle transmit entire framebull If channel sensed busy defer transmis-

sion

bull human analogy donrsquot interrupt others

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

31

CSMA Collisionscollisions can still occurbull propagation delay means

two nodes may not hear each otherrsquos transmission

collisionbull entire packet transmis-

sion time wasted

notebull role of distance amp propa-

gation delay in determin-ing collision probability

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

spatial layout of nodes

32

CSMACD (Collision Detec-tion)CSMACD carrier sensing deferral as in CSMA

bull collisions detected within short timebull colliding transmissions aborted reducing channel

wastage

bull collision detection bull easy in wired LANs measure signal strengths

compare transmitted received signalsbull difficult in wireless LANs received signal strength

overwhelmed by local transmission strength

bull human analogy the polite conversationalist

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

33

CSMACD collision detection

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

34

ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols

bull share channel efficiently and fairly at high loadbull inefficient at low load delay in channel access

1N bandwidth allocated even if only 1 active node

Random access MAC protocolsbull efficient at low load single node can fully uti-

lize channelbull high load collision overhead

ldquotaking turnsrdquo protocolsbull look for best of both worlds

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

35

ldquoTaking Turnsrdquo MAC protocolsPolling bull master node ldquoinvitesrdquo

slave nodes to transmit in turn

bull typically used with ldquodumbrdquo slave devices

bull concernsbull polling overhead bull latencybull single point of failure

(master)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

T

data

(nothingto send)

T

36

ldquoTaking Turnsrdquo MAC protocolsToken passingbull control token

passed from one node to next se-quentially

bull token messagebull concerns

bull token overhead bull latencybull single point of failure

(token)Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

master

slaves

poll

data

data

37

Summary of MAC protocolsbull channel partitioning by time frequency or

codebull Time Division Frequency Division

bull random access (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire)

hard in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211

bull taking turnsbull polling from central site token passing

bull Bluetooth FDDI IBM Token Ring Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

38

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

39

MAC Addresses and ARPbull 32-bit IP address

bull network-layer addressbull used to get datagram to destination IP subnet

bull MAC (or LAN or physical or Ethernet) ad-dress bull function get frame from one interface to an-

other physically-connected interface (same network)

bull 48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software set-

table

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

40

LAN Addresses and ARP

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Each adapter on LAN has unique LAN address

Broadcast address =FF-FF-FF-FF-FF-FF

= adapter

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired orwireless)

41

LAN Address (more)bull MAC address allocation administered by IEEEbull manufacturer buys portion of MAC address

space (to assure uniqueness)bull analogy

(a) MAC address like Social Security Num-ber

(b) IP address like postal addressbull MAC flat address portability

bull can move LAN card from one LAN to another

bull IP hierarchical address NOT portablebull address depends on IP subnet to which node is at-

tachedFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

42

ARP Address Resolution Proto-colbull Each IP node (host

router) on LAN has ARP table

bull ARP table IPMAC address mappings for some LAN nodeslt IP address MAC address

TTLgt

bull TTL (Time To Live) time after which ad-dress mapping will be forgotten (typically 20 min)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Question how to determineMAC address of Bknowing Brsquos IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN

137196723

137196778

137196714

137196788

43

ARP protocol Same LAN (net-work)bull A wants to send datagram

to B and Brsquos MAC address not in Arsquos ARP table

bull A broadcasts ARP query packet containing Bs IP address bull dest MAC address = FF-FF-FF-

FF-FF-FFbull all machines on LAN receive

ARP query

bull B receives ARP packet replies to A with its (Bs) MAC addressbull frame sent to Arsquos MAC ad-

dress (unicast)Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

bull A caches (saves) IP-to-MAC address pair in its ARP table until infor-mation becomes old (times out) bull soft state information

that times out (goes away) unless refreshed

bull ARP is ldquoplug-and-playrdquobull nodes create their ARP

tables without interven-tion from net administra-tor

44

Addressing routing to another LAN

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

R

1A-23-F9-CD-06-9B

222222222220111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

walkthrough send datagram from A to B via R assume A knows Brsquos IP address

bull two ARP tables in router R one for each IP network (LAN)

45

bull A creates IP datagram with source A destination B bull A uses ARP to get Rrsquos MAC address for 111111111110bull A creates link-layer frame with Rs MAC address as dest

frame contains A-to-B IP datagrambull Arsquos NIC sends frame bull Rrsquos NIC receives frame bull R removes IP datagram from Ethernet frame sees its des-

tined to Bbull R uses ARP to get Brsquos MAC address bull R creates frame containing A-to-B IP datagram sends to B

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

This is a really importantexample ndash make sure youunderstand

R

1A-23-F9-CD-06-9B

222222222220

111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

46

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

47

Ethernetldquodominantrdquo wired LAN technology bull cheap $20 for NICbull first widely used LAN technologybull simpler cheaper than token LANs and ATMbull kept up with speed race 10 Mbps ndash 10 Gbps

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Metcalfersquos Ethernetsketch

48

Star Topologybull bus topology popular through mid 90s

bull all nodes in same collision domain (can collide with each other)

bull today star topology prevailsbull active switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes do

not collide with each other)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

switch

bus coaxial cable star

49

Ethernet Frame Structurebull Sending adapter encapsulates IP datagram

(or other network layer protocol packet) in Ethernet frame

Preamble bull 7 bytes with pattern 10101010 followed by

one byte with pattern 10101011bull used to synchronize receiver sender clock

ratesFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

50

Ethernet Frame Structure (more)bull Addresses 6 bytes

bull if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

bull otherwise adapter discards frame

bull Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)

bull CRC checked at receiver if error is detected frame is dropped

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51

Ethernet Unreliable connection-lessbull connectionless No handshaking be-

tween sending and receiving NICs bull unreliable receiving NIC doesnrsquot send

acks or nacks to sending NICbull stream of datagrams passed to network

layer can have gaps (missing datagrams)bull gaps will be filled if app is using TCPbull otherwise app will see gaps

bull Ethernetrsquos MAC protocol unslotted CSMACD

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

52

Ethernet CSMACD algorithmbull 1 NIC receives data-

gram from network layer creates frame

bull 2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

bull 3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

bull 4 If NIC detects an-other transmission while transmitting aborts and sends jam signal

bull 5 After aborting NIC enters exponential backoff after mth colli-sion NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

53

Ethernetrsquos CSMACD (more)bull Jam Signal make

sure all other transmitters are aware of collision 48 bits

bull Bit time 1 micro sec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Exponential Backoff bull Goal adapt retransmission

attempts to estimated cur-rent load

bull heavy load random wait will be longer

bull first collision choose K from 01 delay is K 512 bit transmission times

bull after second collision choose K from 0123hellip

bull after ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

54

CSMACD efficiencybull Tprop = max prop delay between 2 nodes in

LANbull ttrans = time to transmit max-size frame

bull efficiency goes to 1 bull as tprop goes to 0

bull as ttrans goes to infinity

bull better performance than ALOHA and simple cheap decentralized

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

transprop ttefficiency

51

1

55

8023 Ethernet Standards Link amp Physical Layersbull many different Ethernet standards

bull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps

1Gbps 10G bpsbull different physical layer media fiber cable

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

applicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twisterpair) physical layer

56

Manchester encodingbull used in 10BaseTbull each bit has a transitionbull allows clocks in sending and receiving nodes to syn-

chronize to each otherbull no need for a centralized global clock among nodes

bull Hey this is physical-layer stuff

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

57

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

58

Hubshellip physical-layer (ldquodumbrdquo) repeaters

bull bits coming in one link go out all other links at same rate

bull all nodes connected to hub can collide with one another

bull no frame bufferingbull no CSMACD at hub host NICs detect colli-

sions

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

twisted pair

hub

59

Switchbull link-layer device smarter than hubs take

active rolebull store forward Ethernet framesbull examine incoming framersquos MAC address se-

lectively forward frame to one-or-more out-going links when frame is to be forwarded on segment uses CSMACD to access segment

bull transparentbull hosts are unaware of presence of switches

bull plug-and-play self-learning (HOW)bull switches do not need to be configured

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

60

Switch allows multiple simulta-neous transmissionsbull hosts have dedicated direct

connection to switchbull switches buffer packetsbull Ethernet protocol used on

each incoming link but no collisions full duplexbull each link is its own collision

domain

bull switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions bull not possible with dumb hub

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 23

45

6

61

Switch Tablebull Q how does switch know

that Arsquo reachable via inter-face 4 Brsquo reachable via inter-face 5

bull A each switch has a switch table each entrybull (MAC address of host interface

to reach host time stamp)

bull looks like a routing tablebull Q how are entries created

maintained in switch table bull something like a routing proto-

colFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

switch with six interfaces(123456)

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

62

Switch self-learningbull switch learns which

hosts can be reached through which inter-facesbull when frame received

switch ldquolearnsrdquo location of sender incoming LAN segment

bull records senderlocation pair in switch table

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

63

Switch frame filteringforward-ingWhen frame received

1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination then if dest on segment from which frame arrived then drop the frame else

forward the frame on interface indicated else flood

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

forward on all but the interface on which the frame arrived

64

Self-learning forwarding exam-ple

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

frame destination unknownflood

Arsquo A

destination A location known

Arsquo 4 60

selective send

65

Interconnecting switchesbull switches can be connected together

bull Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3

bull A self learning (works exactly the same as in single-switch case)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

B

S1

C D

E

FS2

S4

S3

H

I

G

66

Self-learning multi-switch exam-plebull Suppose C sends frame to I I responds to

C

bull Q show switch tables and packet forward-ing in S1 S2 S3 S4

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

B

S1

CD

E F

S2

S4

S3

H

I

G

12

1

2 3

3

1 2 3 12 3

444

67

Institutional network

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

to externalnetwork

router

IP subnet

mail server

web server

68

Switches vs Routersbull both store-and-forward devices

bull routers network layer devices (examine network layer head-ers)

bull switches are link layer devices

bull routers maintain routing tables implement routing al-gorithms

bull switches maintain switch tables implement filtering learning algorithms

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

69

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

• Fall 2015 COMP 46055605 Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link layer context
• Link Layer Services
• Link Layer Services (more)
• Where is the link layer implemented
• Adaptors Communicating
• Link Layer (2)
• Error Detection
• Parity Checking
• Internet checksum (review)
• Checksumming Cyclic Redundancy Check
• Checksumming Cyclic Redundancy Check (2)
• CRC Example
• Link Layer (3)
• Multiple Access Links and Protocols
• Multiple Access protocols
• Ideal Multiple Access Protocol
• MAC Protocols a taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD collision detection
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Link Layer (4)
• MAC Addresses and ARP
• LAN Addresses and ARP
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP protocol Same LAN (network)
• Addressing routing to another LAN
• Slide 45
• Link Layer (5)
• Ethernet
• Star Topology
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester encoding
• Link Layer (6)
• Hubs
• Switch
• Switch allows multiple simultaneous transmissions
• Switch Table
• Switch self-learning
• Switch frame filteringforwarding
• Self-learning forwarding example
• Interconnecting switches
• Self-learning multi-switch example
• Institutional network
• Switches vs Routers
• Link Layer (7)

9

Adaptors Communicating

bull sending sidebull encapsulates datagram

in framebull adds error checking bits

rdt flow control etc

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

bull receiving sidebull looks for errors rdt flow

control etcbull extracts datagram

passes to upper layer at receiving side

controller controller

sending host receiving host

datagram datagram

datagram

frame

10

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

11

Error DetectionEDC= Error Detection and Correction bits (redundancy)D = Data protected by error checking may include header fields

bull Error detection not 100 reliablebull protocol may miss some errors but rarelybull larger EDC field yields better detection and correction

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

otherwise

12

Parity Checking

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Single Bit ParityDetect single bit errors

Two Dimensional Bit ParityDetect and correct single bit errors

0 0

13

Internet checksum (re-view)

Senderbull treat segment contents

as sequence of 16-bit integers

bull checksum addition (1rsquos complement sum) of segment contents

bull sender puts checksum value into UDP check-sum field

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Receiverbull compute checksum of

received segmentbull check if computed

checksum equals check-sum field valuebull NO - error detectedbull YES - no error detected

But maybe errors nonethe-less

Goal detect ldquoerrorsrdquo (eg flipped bits) in trans-mitted packet (note used at transport layer only

14

Checksumming Cyclic Redun-dancy Checkbull view data bits D as a binary numberbull choose r+1 bit pattern (generator) G bull goal choose r CRC bits R such that

bull ltDRgt exactly divisible by G (modulo 2) bull receiver knows G divides ltDRgt by G If non-zero remain-

der error detectedbull can detect all burst errors less than r+1 bits

bull widely used in practice (Ethernet 80211 WiFi ATM)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

15

Checksumming Cyclic Redun-dancy Checkbull 1011 XOR 0101 = 1110 bull 1001 XOR 1101 = 0100

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

16

CRC Example

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

WantD2r XOR R = nG

equivalentlyD2r = nG XOR R

equivalently if we divide D2r by

G want remainder R

R = remainder[ ]D2r

G

17

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

18

Multiple Access Links and Proto-colsTwo types of ldquolinksrdquobull point-to-point

bull PPP for dial-up accessbull point-to-point link between Ethernet switch and host

bull broadcast (shared wire or medium)bull old-fashioned Ethernetbull upstream HFC (Hybrid Fiber-Coaxial)bull 80211 wireless LAN

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

shared wire (eg cabled Ethernet)

shared RF (eg 80211 WiFi)

shared RF(satellite)

humans at acocktail party

(shared air acoustical)

19

Multiple Access protocolsbull single shared broadcast channel bull two or more simultaneous transmis-

sions by nodes interference bull collision if a node receives two or more

signals at the same time

multiple access protocolbull distributed algorithm that deter-

mines how nodes share channel ie determine when node can transmit

bull communication about channel shar-ing must use channel itself bull no out-of-band channel for coordination

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

20

Ideal Multiple Access ProtocolBroadcast channel of rate R bps1 when one node wants to transmit it

can send at rate R2 when M nodes want to transmit each

can send at average rate RM3 fully decentralized

bull no special node to coordinate transmissionsbull no synchronization of clocks slots

4 simple

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

21

MAC Protocols a taxonomyThree broad classesbull Channel Partitioning

bull divide channel into smaller ldquopiecesrdquo (time slots frequency code)

bull allocate piece to node for exclusive use

bull Random Accessbull channel not divided allow collisionsbull ldquorecoverrdquo from collisions

bull ldquoTaking turnsrdquobull nodes take turns but nodes with more to send

can take longer turnsFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

22

Channel Partitioning MAC proto-cols TDMATDMA time division multiple access bull access to channel in rounds bull each station gets fixed length slot

(length = pkt trans time) in each round bull unused slots go idle bull example 6-station LAN 134 have pkt

slots 256 idle

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

1 3 4 1 3 4

6-slotframe

23

Channel Partitioning MAC proto-cols FDMAFDMA frequency division multiple access bull channel spectrum divided into frequency bandsbull each station assigned fixed frequency bandbull unused transmission time in frequency bands go idle bull example 6-station LAN 134 have pkt frequency

bands 256 idle

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

freq

uency

bands time

FDM cable

24

Random Access Protocolsbull When node has packet to send

bull transmit at full channel data rate Rbull no a priori coordination among nodes

bull two or more transmitting nodes ldquocollisionrdquobull random access MAC protocol specifies

bull how to detect collisionsbull how to recover from collisions (eg via delayed retrans-

missions)

bull Examples of random access MAC protocolsbull slotted ALOHAbull ALOHAbull CSMA CSMACD CSMACA

bull (CS Carrier Sensing MA Multiple AccessCD Collision Detection CA Collision Avoidance)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

25

Slotted ALOHAAssumptionsbull all frames same sizebull time divided into

equal size slots (time to transmit 1 frame)

bull nodes start to transmit only slot beginning

bull nodes are synchro-nized

bull if 2 or more nodes transmit in slot all nodes detect collision

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Operationbull when node obtains

fresh frame trans-mits in next slotbull if no collision node

can send new frame in next slot

bull if collision node re-transmits frame in each subsequent slot with prob p until success

26

Slotted ALOHA

Prosbull single active node can

continuously transmit at full rate of channel

bull highly decentralized only slots in nodes need to be in sync

bull simpleFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Consbull collisions wasting slotsbull idle slotsbull nodes may be able to

detect collision in less than time to transmit packet

bull clock synchronization

27

Slotted Aloha Efficiency

bull suppose N nodes with many frames to send each transmits in slot with probability p

bull prob that given node has success in a slot = p(1-p)N-1

bull prob that any node has a success = Np(1-p)N-1

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

bull max efficiency find p that max-imizes Np(1-p)N-1

bull for many nodes take limit of Np(1- p)N-1 as N goes to infinity gives

Max efficiency = 1e = 37

Efficiency long-run fraction of successful slots (many nodes all with many frames to send)

At best channelused for useful transmissions 37of time

28

Pure (unslotted) ALOHAbull unslotted Aloha simpler no synchro-

nizationbull when frame first arrives

bull transmit immediately

bull collision probability increasesbull frame sent at t0 collides with other frames sent in

[t0-1t0+1]

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

29

Pure Aloha EfficiencyP(success by given node) = P(node transmits)

P(no other node transmits in [t0-1t0]

P(no other node transmits in [t0t0+1]

= p (1-p)N-1 (1-p)N-1

= p (1-p)2(N-1)

hellip choosing optimum p and then letting

= 1(2e) = 18

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

N

even worse than slotted Aloha

30

CSMA (Carrier Sense Multiple Access)CSMA listen before transmitIf channel sensed idle transmit entire framebull If channel sensed busy defer transmis-

sion

bull human analogy donrsquot interrupt others

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

31

CSMA Collisionscollisions can still occurbull propagation delay means

two nodes may not hear each otherrsquos transmission

collisionbull entire packet transmis-

sion time wasted

notebull role of distance amp propa-

gation delay in determin-ing collision probability

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

spatial layout of nodes

32

CSMACD (Collision Detec-tion)CSMACD carrier sensing deferral as in CSMA

bull collisions detected within short timebull colliding transmissions aborted reducing channel

wastage

bull collision detection bull easy in wired LANs measure signal strengths

compare transmitted received signalsbull difficult in wireless LANs received signal strength

overwhelmed by local transmission strength

bull human analogy the polite conversationalist

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

33

CSMACD collision detection

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

34

ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols

bull share channel efficiently and fairly at high loadbull inefficient at low load delay in channel access

1N bandwidth allocated even if only 1 active node

Random access MAC protocolsbull efficient at low load single node can fully uti-

lize channelbull high load collision overhead

ldquotaking turnsrdquo protocolsbull look for best of both worlds

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

35

ldquoTaking Turnsrdquo MAC protocolsPolling bull master node ldquoinvitesrdquo

slave nodes to transmit in turn

bull typically used with ldquodumbrdquo slave devices

bull concernsbull polling overhead bull latencybull single point of failure

(master)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

T

data

(nothingto send)

T

36

ldquoTaking Turnsrdquo MAC protocolsToken passingbull control token

passed from one node to next se-quentially

bull token messagebull concerns

bull token overhead bull latencybull single point of failure

(token)Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

master

slaves

poll

data

data

37

Summary of MAC protocolsbull channel partitioning by time frequency or

codebull Time Division Frequency Division

bull random access (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire)

hard in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211

bull taking turnsbull polling from central site token passing

bull Bluetooth FDDI IBM Token Ring Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

38

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

39

MAC Addresses and ARPbull 32-bit IP address

bull network-layer addressbull used to get datagram to destination IP subnet

bull MAC (or LAN or physical or Ethernet) ad-dress bull function get frame from one interface to an-

other physically-connected interface (same network)

bull 48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software set-

table

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

40

LAN Addresses and ARP

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Each adapter on LAN has unique LAN address

Broadcast address =FF-FF-FF-FF-FF-FF

= adapter

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired orwireless)

41

LAN Address (more)bull MAC address allocation administered by IEEEbull manufacturer buys portion of MAC address

space (to assure uniqueness)bull analogy

(a) MAC address like Social Security Num-ber

(b) IP address like postal addressbull MAC flat address portability

bull can move LAN card from one LAN to another

bull IP hierarchical address NOT portablebull address depends on IP subnet to which node is at-

tachedFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

42

ARP Address Resolution Proto-colbull Each IP node (host

router) on LAN has ARP table

bull ARP table IPMAC address mappings for some LAN nodeslt IP address MAC address

TTLgt

bull TTL (Time To Live) time after which ad-dress mapping will be forgotten (typically 20 min)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Question how to determineMAC address of Bknowing Brsquos IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN

137196723

137196778

137196714

137196788

43

ARP protocol Same LAN (net-work)bull A wants to send datagram

to B and Brsquos MAC address not in Arsquos ARP table

bull A broadcasts ARP query packet containing Bs IP address bull dest MAC address = FF-FF-FF-

FF-FF-FFbull all machines on LAN receive

ARP query

bull B receives ARP packet replies to A with its (Bs) MAC addressbull frame sent to Arsquos MAC ad-

dress (unicast)Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

bull A caches (saves) IP-to-MAC address pair in its ARP table until infor-mation becomes old (times out) bull soft state information

that times out (goes away) unless refreshed

bull ARP is ldquoplug-and-playrdquobull nodes create their ARP

tables without interven-tion from net administra-tor

44

Addressing routing to another LAN

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

R

1A-23-F9-CD-06-9B

222222222220111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

walkthrough send datagram from A to B via R assume A knows Brsquos IP address

bull two ARP tables in router R one for each IP network (LAN)

45

bull A creates IP datagram with source A destination B bull A uses ARP to get Rrsquos MAC address for 111111111110bull A creates link-layer frame with Rs MAC address as dest

frame contains A-to-B IP datagrambull Arsquos NIC sends frame bull Rrsquos NIC receives frame bull R removes IP datagram from Ethernet frame sees its des-

tined to Bbull R uses ARP to get Brsquos MAC address bull R creates frame containing A-to-B IP datagram sends to B

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

This is a really importantexample ndash make sure youunderstand

R

1A-23-F9-CD-06-9B

222222222220

111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

46

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

47

Ethernetldquodominantrdquo wired LAN technology bull cheap $20 for NICbull first widely used LAN technologybull simpler cheaper than token LANs and ATMbull kept up with speed race 10 Mbps ndash 10 Gbps

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Metcalfersquos Ethernetsketch

48

Star Topologybull bus topology popular through mid 90s

bull all nodes in same collision domain (can collide with each other)

bull today star topology prevailsbull active switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes do

not collide with each other)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

switch

bus coaxial cable star

49

Ethernet Frame Structurebull Sending adapter encapsulates IP datagram

(or other network layer protocol packet) in Ethernet frame

Preamble bull 7 bytes with pattern 10101010 followed by

one byte with pattern 10101011bull used to synchronize receiver sender clock

ratesFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

50

Ethernet Frame Structure (more)bull Addresses 6 bytes

bull if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

bull otherwise adapter discards frame

bull Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)

bull CRC checked at receiver if error is detected frame is dropped

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51

Ethernet Unreliable connection-lessbull connectionless No handshaking be-

tween sending and receiving NICs bull unreliable receiving NIC doesnrsquot send

acks or nacks to sending NICbull stream of datagrams passed to network

layer can have gaps (missing datagrams)bull gaps will be filled if app is using TCPbull otherwise app will see gaps

bull Ethernetrsquos MAC protocol unslotted CSMACD

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

52

Ethernet CSMACD algorithmbull 1 NIC receives data-

gram from network layer creates frame

bull 2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

bull 3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

bull 4 If NIC detects an-other transmission while transmitting aborts and sends jam signal

bull 5 After aborting NIC enters exponential backoff after mth colli-sion NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

53

Ethernetrsquos CSMACD (more)bull Jam Signal make

sure all other transmitters are aware of collision 48 bits

bull Bit time 1 micro sec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Exponential Backoff bull Goal adapt retransmission

attempts to estimated cur-rent load

bull heavy load random wait will be longer

bull first collision choose K from 01 delay is K 512 bit transmission times

bull after second collision choose K from 0123hellip

bull after ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

54

CSMACD efficiencybull Tprop = max prop delay between 2 nodes in

LANbull ttrans = time to transmit max-size frame

bull efficiency goes to 1 bull as tprop goes to 0

bull as ttrans goes to infinity

bull better performance than ALOHA and simple cheap decentralized

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

transprop ttefficiency

51

1

55

8023 Ethernet Standards Link amp Physical Layersbull many different Ethernet standards

bull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps

1Gbps 10G bpsbull different physical layer media fiber cable

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

applicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twisterpair) physical layer

56

Manchester encodingbull used in 10BaseTbull each bit has a transitionbull allows clocks in sending and receiving nodes to syn-

chronize to each otherbull no need for a centralized global clock among nodes

bull Hey this is physical-layer stuff

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

57

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

58

Hubshellip physical-layer (ldquodumbrdquo) repeaters

bull bits coming in one link go out all other links at same rate

bull all nodes connected to hub can collide with one another

bull no frame bufferingbull no CSMACD at hub host NICs detect colli-

sions

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

twisted pair

hub

59

Switchbull link-layer device smarter than hubs take

active rolebull store forward Ethernet framesbull examine incoming framersquos MAC address se-

lectively forward frame to one-or-more out-going links when frame is to be forwarded on segment uses CSMACD to access segment

bull transparentbull hosts are unaware of presence of switches

bull plug-and-play self-learning (HOW)bull switches do not need to be configured

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

60

Switch allows multiple simulta-neous transmissionsbull hosts have dedicated direct

connection to switchbull switches buffer packetsbull Ethernet protocol used on

each incoming link but no collisions full duplexbull each link is its own collision

domain

bull switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions bull not possible with dumb hub

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 23

45

6

61

Switch Tablebull Q how does switch know

that Arsquo reachable via inter-face 4 Brsquo reachable via inter-face 5

bull A each switch has a switch table each entrybull (MAC address of host interface

to reach host time stamp)

bull looks like a routing tablebull Q how are entries created

maintained in switch table bull something like a routing proto-

colFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

switch with six interfaces(123456)

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

62

Switch self-learningbull switch learns which

hosts can be reached through which inter-facesbull when frame received

switch ldquolearnsrdquo location of sender incoming LAN segment

bull records senderlocation pair in switch table

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

63

Switch frame filteringforward-ingWhen frame received

1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination then if dest on segment from which frame arrived then drop the frame else

forward the frame on interface indicated else flood

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

forward on all but the interface on which the frame arrived

64

Self-learning forwarding exam-ple

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

frame destination unknownflood

Arsquo A

destination A location known

Arsquo 4 60

selective send

65

Interconnecting switchesbull switches can be connected together

bull Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3

bull A self learning (works exactly the same as in single-switch case)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

B

S1

C D

E

FS2

S4

S3

H

I

G

66

Self-learning multi-switch exam-plebull Suppose C sends frame to I I responds to

C

bull Q show switch tables and packet forward-ing in S1 S2 S3 S4

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

B

S1

CD

E F

S2

S4

S3

H

I

G

12

1

2 3

3

1 2 3 12 3

444

67

Institutional network

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

to externalnetwork

router

IP subnet

mail server

web server

68

Switches vs Routersbull both store-and-forward devices

bull routers network layer devices (examine network layer head-ers)

bull switches are link layer devices

bull routers maintain routing tables implement routing al-gorithms

bull switches maintain switch tables implement filtering learning algorithms

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

69

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

• Fall 2015 COMP 46055605 Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link layer context
• Link Layer Services
• Link Layer Services (more)
• Where is the link layer implemented
• Adaptors Communicating
• Link Layer (2)
• Error Detection
• Parity Checking
• Internet checksum (review)
• Checksumming Cyclic Redundancy Check
• Checksumming Cyclic Redundancy Check (2)
• CRC Example
• Link Layer (3)
• Multiple Access Links and Protocols
• Multiple Access protocols
• Ideal Multiple Access Protocol
• MAC Protocols a taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD collision detection
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Link Layer (4)
• MAC Addresses and ARP
• LAN Addresses and ARP
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP protocol Same LAN (network)
• Addressing routing to another LAN
• Slide 45
• Link Layer (5)
• Ethernet
• Star Topology
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester encoding
• Link Layer (6)
• Hubs
• Switch
• Switch allows multiple simultaneous transmissions
• Switch Table
• Switch self-learning
• Switch frame filteringforwarding
• Self-learning forwarding example
• Interconnecting switches
• Self-learning multi-switch example
• Institutional network
• Switches vs Routers
• Link Layer (7)

10

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

11

Error DetectionEDC= Error Detection and Correction bits (redundancy)D = Data protected by error checking may include header fields

bull Error detection not 100 reliablebull protocol may miss some errors but rarelybull larger EDC field yields better detection and correction

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

otherwise

12

Parity Checking

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Single Bit ParityDetect single bit errors

Two Dimensional Bit ParityDetect and correct single bit errors

0 0

13

Internet checksum (re-view)

Senderbull treat segment contents

as sequence of 16-bit integers

bull checksum addition (1rsquos complement sum) of segment contents

bull sender puts checksum value into UDP check-sum field

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Receiverbull compute checksum of

received segmentbull check if computed

checksum equals check-sum field valuebull NO - error detectedbull YES - no error detected

But maybe errors nonethe-less

Goal detect ldquoerrorsrdquo (eg flipped bits) in trans-mitted packet (note used at transport layer only

14

Checksumming Cyclic Redun-dancy Checkbull view data bits D as a binary numberbull choose r+1 bit pattern (generator) G bull goal choose r CRC bits R such that

bull ltDRgt exactly divisible by G (modulo 2) bull receiver knows G divides ltDRgt by G If non-zero remain-

der error detectedbull can detect all burst errors less than r+1 bits

bull widely used in practice (Ethernet 80211 WiFi ATM)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

15

Checksumming Cyclic Redun-dancy Checkbull 1011 XOR 0101 = 1110 bull 1001 XOR 1101 = 0100

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

16

CRC Example

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

WantD2r XOR R = nG

equivalentlyD2r = nG XOR R

equivalently if we divide D2r by

G want remainder R

R = remainder[ ]D2r

G

17

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

18

Multiple Access Links and Proto-colsTwo types of ldquolinksrdquobull point-to-point

bull PPP for dial-up accessbull point-to-point link between Ethernet switch and host

bull broadcast (shared wire or medium)bull old-fashioned Ethernetbull upstream HFC (Hybrid Fiber-Coaxial)bull 80211 wireless LAN

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

shared wire (eg cabled Ethernet)

shared RF (eg 80211 WiFi)

shared RF(satellite)

humans at acocktail party

(shared air acoustical)

19

Multiple Access protocolsbull single shared broadcast channel bull two or more simultaneous transmis-

sions by nodes interference bull collision if a node receives two or more

signals at the same time

multiple access protocolbull distributed algorithm that deter-

mines how nodes share channel ie determine when node can transmit

bull communication about channel shar-ing must use channel itself bull no out-of-band channel for coordination

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

20

Ideal Multiple Access ProtocolBroadcast channel of rate R bps1 when one node wants to transmit it

can send at rate R2 when M nodes want to transmit each

can send at average rate RM3 fully decentralized

bull no special node to coordinate transmissionsbull no synchronization of clocks slots

4 simple

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

21

MAC Protocols a taxonomyThree broad classesbull Channel Partitioning

bull divide channel into smaller ldquopiecesrdquo (time slots frequency code)

bull allocate piece to node for exclusive use

bull Random Accessbull channel not divided allow collisionsbull ldquorecoverrdquo from collisions

bull ldquoTaking turnsrdquobull nodes take turns but nodes with more to send

can take longer turnsFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

22

Channel Partitioning MAC proto-cols TDMATDMA time division multiple access bull access to channel in rounds bull each station gets fixed length slot

(length = pkt trans time) in each round bull unused slots go idle bull example 6-station LAN 134 have pkt

slots 256 idle

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

1 3 4 1 3 4

6-slotframe

23

Channel Partitioning MAC proto-cols FDMAFDMA frequency division multiple access bull channel spectrum divided into frequency bandsbull each station assigned fixed frequency bandbull unused transmission time in frequency bands go idle bull example 6-station LAN 134 have pkt frequency

bands 256 idle

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

freq

uency

bands time

FDM cable

24

Random Access Protocolsbull When node has packet to send

bull transmit at full channel data rate Rbull no a priori coordination among nodes

bull two or more transmitting nodes ldquocollisionrdquobull random access MAC protocol specifies

bull how to detect collisionsbull how to recover from collisions (eg via delayed retrans-

missions)

bull Examples of random access MAC protocolsbull slotted ALOHAbull ALOHAbull CSMA CSMACD CSMACA

bull (CS Carrier Sensing MA Multiple AccessCD Collision Detection CA Collision Avoidance)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

25

Slotted ALOHAAssumptionsbull all frames same sizebull time divided into

equal size slots (time to transmit 1 frame)

bull nodes start to transmit only slot beginning

bull nodes are synchro-nized

bull if 2 or more nodes transmit in slot all nodes detect collision

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Operationbull when node obtains

fresh frame trans-mits in next slotbull if no collision node

can send new frame in next slot

bull if collision node re-transmits frame in each subsequent slot with prob p until success

26

Slotted ALOHA

Prosbull single active node can

continuously transmit at full rate of channel

bull highly decentralized only slots in nodes need to be in sync

bull simpleFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Consbull collisions wasting slotsbull idle slotsbull nodes may be able to

detect collision in less than time to transmit packet

bull clock synchronization

27

Slotted Aloha Efficiency

bull suppose N nodes with many frames to send each transmits in slot with probability p

bull prob that given node has success in a slot = p(1-p)N-1

bull prob that any node has a success = Np(1-p)N-1

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

bull max efficiency find p that max-imizes Np(1-p)N-1

bull for many nodes take limit of Np(1- p)N-1 as N goes to infinity gives

Max efficiency = 1e = 37

Efficiency long-run fraction of successful slots (many nodes all with many frames to send)

At best channelused for useful transmissions 37of time

28

Pure (unslotted) ALOHAbull unslotted Aloha simpler no synchro-

nizationbull when frame first arrives

bull transmit immediately

bull collision probability increasesbull frame sent at t0 collides with other frames sent in

[t0-1t0+1]

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

29

Pure Aloha EfficiencyP(success by given node) = P(node transmits)

P(no other node transmits in [t0-1t0]

P(no other node transmits in [t0t0+1]

= p (1-p)N-1 (1-p)N-1

= p (1-p)2(N-1)

hellip choosing optimum p and then letting

= 1(2e) = 18

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

N

even worse than slotted Aloha

30

CSMA (Carrier Sense Multiple Access)CSMA listen before transmitIf channel sensed idle transmit entire framebull If channel sensed busy defer transmis-

sion

bull human analogy donrsquot interrupt others

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

31

CSMA Collisionscollisions can still occurbull propagation delay means

two nodes may not hear each otherrsquos transmission

collisionbull entire packet transmis-

sion time wasted

notebull role of distance amp propa-

gation delay in determin-ing collision probability

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

spatial layout of nodes

32

CSMACD (Collision Detec-tion)CSMACD carrier sensing deferral as in CSMA

bull collisions detected within short timebull colliding transmissions aborted reducing channel

wastage

bull collision detection bull easy in wired LANs measure signal strengths

compare transmitted received signalsbull difficult in wireless LANs received signal strength

overwhelmed by local transmission strength

bull human analogy the polite conversationalist

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

33

CSMACD collision detection

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

34

ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols

bull share channel efficiently and fairly at high loadbull inefficient at low load delay in channel access

1N bandwidth allocated even if only 1 active node

Random access MAC protocolsbull efficient at low load single node can fully uti-

lize channelbull high load collision overhead

ldquotaking turnsrdquo protocolsbull look for best of both worlds

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

35

ldquoTaking Turnsrdquo MAC protocolsPolling bull master node ldquoinvitesrdquo

slave nodes to transmit in turn

bull typically used with ldquodumbrdquo slave devices

bull concernsbull polling overhead bull latencybull single point of failure

(master)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

T

data

(nothingto send)

T

36

ldquoTaking Turnsrdquo MAC protocolsToken passingbull control token

passed from one node to next se-quentially

bull token messagebull concerns

bull token overhead bull latencybull single point of failure

(token)Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

master

slaves

poll

data

data

37

Summary of MAC protocolsbull channel partitioning by time frequency or

codebull Time Division Frequency Division

bull random access (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire)

hard in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211

bull taking turnsbull polling from central site token passing

bull Bluetooth FDDI IBM Token Ring Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

38

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

39

MAC Addresses and ARPbull 32-bit IP address

bull network-layer addressbull used to get datagram to destination IP subnet

bull MAC (or LAN or physical or Ethernet) ad-dress bull function get frame from one interface to an-

other physically-connected interface (same network)

bull 48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software set-

table

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

40

LAN Addresses and ARP

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Each adapter on LAN has unique LAN address

Broadcast address =FF-FF-FF-FF-FF-FF

= adapter

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired orwireless)

41

LAN Address (more)bull MAC address allocation administered by IEEEbull manufacturer buys portion of MAC address

space (to assure uniqueness)bull analogy

(a) MAC address like Social Security Num-ber

(b) IP address like postal addressbull MAC flat address portability

bull can move LAN card from one LAN to another

bull IP hierarchical address NOT portablebull address depends on IP subnet to which node is at-

tachedFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

42

ARP Address Resolution Proto-colbull Each IP node (host

router) on LAN has ARP table

bull ARP table IPMAC address mappings for some LAN nodeslt IP address MAC address

TTLgt

bull TTL (Time To Live) time after which ad-dress mapping will be forgotten (typically 20 min)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Question how to determineMAC address of Bknowing Brsquos IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN

137196723

137196778

137196714

137196788

43

ARP protocol Same LAN (net-work)bull A wants to send datagram

to B and Brsquos MAC address not in Arsquos ARP table

bull A broadcasts ARP query packet containing Bs IP address bull dest MAC address = FF-FF-FF-

FF-FF-FFbull all machines on LAN receive

ARP query

bull B receives ARP packet replies to A with its (Bs) MAC addressbull frame sent to Arsquos MAC ad-

dress (unicast)Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

bull A caches (saves) IP-to-MAC address pair in its ARP table until infor-mation becomes old (times out) bull soft state information

that times out (goes away) unless refreshed

bull ARP is ldquoplug-and-playrdquobull nodes create their ARP

tables without interven-tion from net administra-tor

44

Addressing routing to another LAN

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

R

1A-23-F9-CD-06-9B

222222222220111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

walkthrough send datagram from A to B via R assume A knows Brsquos IP address

bull two ARP tables in router R one for each IP network (LAN)

45

bull A creates IP datagram with source A destination B bull A uses ARP to get Rrsquos MAC address for 111111111110bull A creates link-layer frame with Rs MAC address as dest

frame contains A-to-B IP datagrambull Arsquos NIC sends frame bull Rrsquos NIC receives frame bull R removes IP datagram from Ethernet frame sees its des-

tined to Bbull R uses ARP to get Brsquos MAC address bull R creates frame containing A-to-B IP datagram sends to B

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

This is a really importantexample ndash make sure youunderstand

R

1A-23-F9-CD-06-9B

222222222220

111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

46

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

47

Ethernetldquodominantrdquo wired LAN technology bull cheap $20 for NICbull first widely used LAN technologybull simpler cheaper than token LANs and ATMbull kept up with speed race 10 Mbps ndash 10 Gbps

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Metcalfersquos Ethernetsketch

48

Star Topologybull bus topology popular through mid 90s

bull all nodes in same collision domain (can collide with each other)

bull today star topology prevailsbull active switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes do

not collide with each other)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

switch

bus coaxial cable star

49

Ethernet Frame Structurebull Sending adapter encapsulates IP datagram

(or other network layer protocol packet) in Ethernet frame

Preamble bull 7 bytes with pattern 10101010 followed by

one byte with pattern 10101011bull used to synchronize receiver sender clock

ratesFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

50

Ethernet Frame Structure (more)bull Addresses 6 bytes

bull if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

bull otherwise adapter discards frame

bull Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)

bull CRC checked at receiver if error is detected frame is dropped

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51

Ethernet Unreliable connection-lessbull connectionless No handshaking be-

tween sending and receiving NICs bull unreliable receiving NIC doesnrsquot send

acks or nacks to sending NICbull stream of datagrams passed to network

layer can have gaps (missing datagrams)bull gaps will be filled if app is using TCPbull otherwise app will see gaps

bull Ethernetrsquos MAC protocol unslotted CSMACD

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

52

Ethernet CSMACD algorithmbull 1 NIC receives data-

gram from network layer creates frame

bull 2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

bull 3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

bull 4 If NIC detects an-other transmission while transmitting aborts and sends jam signal

bull 5 After aborting NIC enters exponential backoff after mth colli-sion NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

53

Ethernetrsquos CSMACD (more)bull Jam Signal make

sure all other transmitters are aware of collision 48 bits

bull Bit time 1 micro sec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Exponential Backoff bull Goal adapt retransmission

attempts to estimated cur-rent load

bull heavy load random wait will be longer

bull first collision choose K from 01 delay is K 512 bit transmission times

bull after second collision choose K from 0123hellip

bull after ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

54

CSMACD efficiencybull Tprop = max prop delay between 2 nodes in

LANbull ttrans = time to transmit max-size frame

bull efficiency goes to 1 bull as tprop goes to 0

bull as ttrans goes to infinity

bull better performance than ALOHA and simple cheap decentralized

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

transprop ttefficiency

51

1

55

8023 Ethernet Standards Link amp Physical Layersbull many different Ethernet standards

bull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps

1Gbps 10G bpsbull different physical layer media fiber cable

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

applicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twisterpair) physical layer

56

Manchester encodingbull used in 10BaseTbull each bit has a transitionbull allows clocks in sending and receiving nodes to syn-

chronize to each otherbull no need for a centralized global clock among nodes

bull Hey this is physical-layer stuff

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

57

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

58

Hubshellip physical-layer (ldquodumbrdquo) repeaters

bull bits coming in one link go out all other links at same rate

bull all nodes connected to hub can collide with one another

bull no frame bufferingbull no CSMACD at hub host NICs detect colli-

sions

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

twisted pair

hub

59

Switchbull link-layer device smarter than hubs take

active rolebull store forward Ethernet framesbull examine incoming framersquos MAC address se-

lectively forward frame to one-or-more out-going links when frame is to be forwarded on segment uses CSMACD to access segment

bull transparentbull hosts are unaware of presence of switches

bull plug-and-play self-learning (HOW)bull switches do not need to be configured

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

60

Switch allows multiple simulta-neous transmissionsbull hosts have dedicated direct

connection to switchbull switches buffer packetsbull Ethernet protocol used on

each incoming link but no collisions full duplexbull each link is its own collision

domain

bull switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions bull not possible with dumb hub

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 23

45

6

61

Switch Tablebull Q how does switch know

that Arsquo reachable via inter-face 4 Brsquo reachable via inter-face 5

bull A each switch has a switch table each entrybull (MAC address of host interface

to reach host time stamp)

bull looks like a routing tablebull Q how are entries created

maintained in switch table bull something like a routing proto-

colFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

switch with six interfaces(123456)

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

62

Switch self-learningbull switch learns which

hosts can be reached through which inter-facesbull when frame received

switch ldquolearnsrdquo location of sender incoming LAN segment

bull records senderlocation pair in switch table

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

63

Switch frame filteringforward-ingWhen frame received

1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination then if dest on segment from which frame arrived then drop the frame else

forward the frame on interface indicated else flood

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

forward on all but the interface on which the frame arrived

64

Self-learning forwarding exam-ple

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

frame destination unknownflood

Arsquo A

destination A location known

Arsquo 4 60

selective send

65

Interconnecting switchesbull switches can be connected together

bull Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3

bull A self learning (works exactly the same as in single-switch case)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

B

S1

C D

E

FS2

S4

S3

H

I

G

66

Self-learning multi-switch exam-plebull Suppose C sends frame to I I responds to

C

bull Q show switch tables and packet forward-ing in S1 S2 S3 S4

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

B

S1

CD

E F

S2

S4

S3

H

I

G

12

1

2 3

3

1 2 3 12 3

444

67

Institutional network

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

to externalnetwork

router

IP subnet

mail server

web server

68

Switches vs Routersbull both store-and-forward devices

bull routers network layer devices (examine network layer head-ers)

bull switches are link layer devices

bull routers maintain routing tables implement routing al-gorithms

bull switches maintain switch tables implement filtering learning algorithms

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

69

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

• Fall 2015 COMP 46055605 Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link layer context
• Link Layer Services
• Link Layer Services (more)
• Where is the link layer implemented
• Adaptors Communicating
• Link Layer (2)
• Error Detection
• Parity Checking
• Internet checksum (review)
• Checksumming Cyclic Redundancy Check
• Checksumming Cyclic Redundancy Check (2)
• CRC Example
• Link Layer (3)
• Multiple Access Links and Protocols
• Multiple Access protocols
• Ideal Multiple Access Protocol
• MAC Protocols a taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD collision detection
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Link Layer (4)
• MAC Addresses and ARP
• LAN Addresses and ARP
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP protocol Same LAN (network)
• Addressing routing to another LAN
• Slide 45
• Link Layer (5)
• Ethernet
• Star Topology
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester encoding
• Link Layer (6)
• Hubs
• Switch
• Switch allows multiple simultaneous transmissions
• Switch Table
• Switch self-learning
• Switch frame filteringforwarding
• Self-learning forwarding example
• Interconnecting switches
• Self-learning multi-switch example
• Institutional network
• Switches vs Routers
• Link Layer (7)

11

Error DetectionEDC= Error Detection and Correction bits (redundancy)D = Data protected by error checking may include header fields

bull Error detection not 100 reliablebull protocol may miss some errors but rarelybull larger EDC field yields better detection and correction

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

otherwise

12

Parity Checking

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Single Bit ParityDetect single bit errors

Two Dimensional Bit ParityDetect and correct single bit errors

0 0

13

Internet checksum (re-view)

Senderbull treat segment contents

as sequence of 16-bit integers

bull checksum addition (1rsquos complement sum) of segment contents

bull sender puts checksum value into UDP check-sum field

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Receiverbull compute checksum of

received segmentbull check if computed

checksum equals check-sum field valuebull NO - error detectedbull YES - no error detected

But maybe errors nonethe-less

Goal detect ldquoerrorsrdquo (eg flipped bits) in trans-mitted packet (note used at transport layer only

14

Checksumming Cyclic Redun-dancy Checkbull view data bits D as a binary numberbull choose r+1 bit pattern (generator) G bull goal choose r CRC bits R such that

bull ltDRgt exactly divisible by G (modulo 2) bull receiver knows G divides ltDRgt by G If non-zero remain-

der error detectedbull can detect all burst errors less than r+1 bits

bull widely used in practice (Ethernet 80211 WiFi ATM)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

15

Checksumming Cyclic Redun-dancy Checkbull 1011 XOR 0101 = 1110 bull 1001 XOR 1101 = 0100

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

16

CRC Example

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

WantD2r XOR R = nG

equivalentlyD2r = nG XOR R

equivalently if we divide D2r by

G want remainder R

R = remainder[ ]D2r

G

17

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

18

Multiple Access Links and Proto-colsTwo types of ldquolinksrdquobull point-to-point

bull PPP for dial-up accessbull point-to-point link between Ethernet switch and host

bull broadcast (shared wire or medium)bull old-fashioned Ethernetbull upstream HFC (Hybrid Fiber-Coaxial)bull 80211 wireless LAN

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

shared wire (eg cabled Ethernet)

shared RF (eg 80211 WiFi)

shared RF(satellite)

humans at acocktail party

(shared air acoustical)

19

Multiple Access protocolsbull single shared broadcast channel bull two or more simultaneous transmis-

sions by nodes interference bull collision if a node receives two or more

signals at the same time

multiple access protocolbull distributed algorithm that deter-

mines how nodes share channel ie determine when node can transmit

bull communication about channel shar-ing must use channel itself bull no out-of-band channel for coordination

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

20

Ideal Multiple Access ProtocolBroadcast channel of rate R bps1 when one node wants to transmit it

can send at rate R2 when M nodes want to transmit each

can send at average rate RM3 fully decentralized

bull no special node to coordinate transmissionsbull no synchronization of clocks slots

4 simple

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

21

MAC Protocols a taxonomyThree broad classesbull Channel Partitioning

bull divide channel into smaller ldquopiecesrdquo (time slots frequency code)

bull allocate piece to node for exclusive use

bull Random Accessbull channel not divided allow collisionsbull ldquorecoverrdquo from collisions

bull ldquoTaking turnsrdquobull nodes take turns but nodes with more to send

can take longer turnsFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

22

Channel Partitioning MAC proto-cols TDMATDMA time division multiple access bull access to channel in rounds bull each station gets fixed length slot

(length = pkt trans time) in each round bull unused slots go idle bull example 6-station LAN 134 have pkt

slots 256 idle

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

1 3 4 1 3 4

6-slotframe

23

Channel Partitioning MAC proto-cols FDMAFDMA frequency division multiple access bull channel spectrum divided into frequency bandsbull each station assigned fixed frequency bandbull unused transmission time in frequency bands go idle bull example 6-station LAN 134 have pkt frequency

bands 256 idle

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

freq

uency

bands time

FDM cable

24

Random Access Protocolsbull When node has packet to send

bull transmit at full channel data rate Rbull no a priori coordination among nodes

bull two or more transmitting nodes ldquocollisionrdquobull random access MAC protocol specifies

bull how to detect collisionsbull how to recover from collisions (eg via delayed retrans-

missions)

bull Examples of random access MAC protocolsbull slotted ALOHAbull ALOHAbull CSMA CSMACD CSMACA

bull (CS Carrier Sensing MA Multiple AccessCD Collision Detection CA Collision Avoidance)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

25

Slotted ALOHAAssumptionsbull all frames same sizebull time divided into

equal size slots (time to transmit 1 frame)

bull nodes start to transmit only slot beginning

bull nodes are synchro-nized

bull if 2 or more nodes transmit in slot all nodes detect collision

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Operationbull when node obtains

fresh frame trans-mits in next slotbull if no collision node

can send new frame in next slot

bull if collision node re-transmits frame in each subsequent slot with prob p until success

26

Slotted ALOHA

Prosbull single active node can

continuously transmit at full rate of channel

bull highly decentralized only slots in nodes need to be in sync

bull simpleFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Consbull collisions wasting slotsbull idle slotsbull nodes may be able to

detect collision in less than time to transmit packet

bull clock synchronization

27

Slotted Aloha Efficiency

bull suppose N nodes with many frames to send each transmits in slot with probability p

bull prob that given node has success in a slot = p(1-p)N-1

bull prob that any node has a success = Np(1-p)N-1

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

bull max efficiency find p that max-imizes Np(1-p)N-1

bull for many nodes take limit of Np(1- p)N-1 as N goes to infinity gives

Max efficiency = 1e = 37

Efficiency long-run fraction of successful slots (many nodes all with many frames to send)

At best channelused for useful transmissions 37of time

28

Pure (unslotted) ALOHAbull unslotted Aloha simpler no synchro-

nizationbull when frame first arrives

bull transmit immediately

bull collision probability increasesbull frame sent at t0 collides with other frames sent in

[t0-1t0+1]

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

29

Pure Aloha EfficiencyP(success by given node) = P(node transmits)

P(no other node transmits in [t0-1t0]

P(no other node transmits in [t0t0+1]

= p (1-p)N-1 (1-p)N-1

= p (1-p)2(N-1)

hellip choosing optimum p and then letting

= 1(2e) = 18

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

N

even worse than slotted Aloha

30

CSMA (Carrier Sense Multiple Access)CSMA listen before transmitIf channel sensed idle transmit entire framebull If channel sensed busy defer transmis-

sion

bull human analogy donrsquot interrupt others

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

31

CSMA Collisionscollisions can still occurbull propagation delay means

two nodes may not hear each otherrsquos transmission

collisionbull entire packet transmis-

sion time wasted

notebull role of distance amp propa-

gation delay in determin-ing collision probability

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

spatial layout of nodes

32

CSMACD (Collision Detec-tion)CSMACD carrier sensing deferral as in CSMA

bull collisions detected within short timebull colliding transmissions aborted reducing channel

wastage

bull collision detection bull easy in wired LANs measure signal strengths

compare transmitted received signalsbull difficult in wireless LANs received signal strength

overwhelmed by local transmission strength

bull human analogy the polite conversationalist

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

33

CSMACD collision detection

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

34

ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols

bull share channel efficiently and fairly at high loadbull inefficient at low load delay in channel access

1N bandwidth allocated even if only 1 active node

Random access MAC protocolsbull efficient at low load single node can fully uti-

lize channelbull high load collision overhead

ldquotaking turnsrdquo protocolsbull look for best of both worlds

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

35

ldquoTaking Turnsrdquo MAC protocolsPolling bull master node ldquoinvitesrdquo

slave nodes to transmit in turn

bull typically used with ldquodumbrdquo slave devices

bull concernsbull polling overhead bull latencybull single point of failure

(master)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

T

data

(nothingto send)

T

36

ldquoTaking Turnsrdquo MAC protocolsToken passingbull control token

passed from one node to next se-quentially

bull token messagebull concerns

bull token overhead bull latencybull single point of failure

(token)Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

master

slaves

poll

data

data

37

Summary of MAC protocolsbull channel partitioning by time frequency or

codebull Time Division Frequency Division

bull random access (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire)

hard in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211

bull taking turnsbull polling from central site token passing

bull Bluetooth FDDI IBM Token Ring Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

38

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

39

MAC Addresses and ARPbull 32-bit IP address

bull network-layer addressbull used to get datagram to destination IP subnet

bull MAC (or LAN or physical or Ethernet) ad-dress bull function get frame from one interface to an-

other physically-connected interface (same network)

bull 48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software set-

table

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

40

LAN Addresses and ARP

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Each adapter on LAN has unique LAN address

Broadcast address =FF-FF-FF-FF-FF-FF

= adapter

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired orwireless)

41

LAN Address (more)bull MAC address allocation administered by IEEEbull manufacturer buys portion of MAC address

space (to assure uniqueness)bull analogy

(a) MAC address like Social Security Num-ber

(b) IP address like postal addressbull MAC flat address portability

bull can move LAN card from one LAN to another

bull IP hierarchical address NOT portablebull address depends on IP subnet to which node is at-

tachedFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

42

ARP Address Resolution Proto-colbull Each IP node (host

router) on LAN has ARP table

bull ARP table IPMAC address mappings for some LAN nodeslt IP address MAC address

TTLgt

bull TTL (Time To Live) time after which ad-dress mapping will be forgotten (typically 20 min)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Question how to determineMAC address of Bknowing Brsquos IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN

137196723

137196778

137196714

137196788

43

ARP protocol Same LAN (net-work)bull A wants to send datagram

to B and Brsquos MAC address not in Arsquos ARP table

bull A broadcasts ARP query packet containing Bs IP address bull dest MAC address = FF-FF-FF-

FF-FF-FFbull all machines on LAN receive

ARP query

bull B receives ARP packet replies to A with its (Bs) MAC addressbull frame sent to Arsquos MAC ad-

dress (unicast)Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

bull A caches (saves) IP-to-MAC address pair in its ARP table until infor-mation becomes old (times out) bull soft state information

that times out (goes away) unless refreshed

bull ARP is ldquoplug-and-playrdquobull nodes create their ARP

tables without interven-tion from net administra-tor

44

Addressing routing to another LAN

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

R

1A-23-F9-CD-06-9B

222222222220111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

walkthrough send datagram from A to B via R assume A knows Brsquos IP address

bull two ARP tables in router R one for each IP network (LAN)

45

bull A creates IP datagram with source A destination B bull A uses ARP to get Rrsquos MAC address for 111111111110bull A creates link-layer frame with Rs MAC address as dest

frame contains A-to-B IP datagrambull Arsquos NIC sends frame bull Rrsquos NIC receives frame bull R removes IP datagram from Ethernet frame sees its des-

tined to Bbull R uses ARP to get Brsquos MAC address bull R creates frame containing A-to-B IP datagram sends to B

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

This is a really importantexample ndash make sure youunderstand

R

1A-23-F9-CD-06-9B

222222222220

111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

46

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

47

Ethernetldquodominantrdquo wired LAN technology bull cheap $20 for NICbull first widely used LAN technologybull simpler cheaper than token LANs and ATMbull kept up with speed race 10 Mbps ndash 10 Gbps

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Metcalfersquos Ethernetsketch

48

Star Topologybull bus topology popular through mid 90s

bull all nodes in same collision domain (can collide with each other)

bull today star topology prevailsbull active switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes do

not collide with each other)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

switch

bus coaxial cable star

49

Ethernet Frame Structurebull Sending adapter encapsulates IP datagram

(or other network layer protocol packet) in Ethernet frame

Preamble bull 7 bytes with pattern 10101010 followed by

one byte with pattern 10101011bull used to synchronize receiver sender clock

ratesFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

50

Ethernet Frame Structure (more)bull Addresses 6 bytes

bull if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

bull otherwise adapter discards frame

bull Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)

bull CRC checked at receiver if error is detected frame is dropped

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51

Ethernet Unreliable connection-lessbull connectionless No handshaking be-

tween sending and receiving NICs bull unreliable receiving NIC doesnrsquot send

acks or nacks to sending NICbull stream of datagrams passed to network

layer can have gaps (missing datagrams)bull gaps will be filled if app is using TCPbull otherwise app will see gaps

bull Ethernetrsquos MAC protocol unslotted CSMACD

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

52

Ethernet CSMACD algorithmbull 1 NIC receives data-

gram from network layer creates frame

bull 2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

bull 3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

bull 4 If NIC detects an-other transmission while transmitting aborts and sends jam signal

bull 5 After aborting NIC enters exponential backoff after mth colli-sion NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

53

Ethernetrsquos CSMACD (more)bull Jam Signal make

sure all other transmitters are aware of collision 48 bits

bull Bit time 1 micro sec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Exponential Backoff bull Goal adapt retransmission

attempts to estimated cur-rent load

bull heavy load random wait will be longer

bull first collision choose K from 01 delay is K 512 bit transmission times

bull after second collision choose K from 0123hellip

bull after ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

54

CSMACD efficiencybull Tprop = max prop delay between 2 nodes in

LANbull ttrans = time to transmit max-size frame

bull efficiency goes to 1 bull as tprop goes to 0

bull as ttrans goes to infinity

bull better performance than ALOHA and simple cheap decentralized

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

transprop ttefficiency

51

1

55

8023 Ethernet Standards Link amp Physical Layersbull many different Ethernet standards

bull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps

1Gbps 10G bpsbull different physical layer media fiber cable

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

applicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twisterpair) physical layer

56

Manchester encodingbull used in 10BaseTbull each bit has a transitionbull allows clocks in sending and receiving nodes to syn-

chronize to each otherbull no need for a centralized global clock among nodes

bull Hey this is physical-layer stuff

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

57

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

58

Hubshellip physical-layer (ldquodumbrdquo) repeaters

bull bits coming in one link go out all other links at same rate

bull all nodes connected to hub can collide with one another

bull no frame bufferingbull no CSMACD at hub host NICs detect colli-

sions

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

twisted pair

hub

59

Switchbull link-layer device smarter than hubs take

active rolebull store forward Ethernet framesbull examine incoming framersquos MAC address se-

lectively forward frame to one-or-more out-going links when frame is to be forwarded on segment uses CSMACD to access segment

bull transparentbull hosts are unaware of presence of switches

bull plug-and-play self-learning (HOW)bull switches do not need to be configured

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

60

Switch allows multiple simulta-neous transmissionsbull hosts have dedicated direct

connection to switchbull switches buffer packetsbull Ethernet protocol used on

each incoming link but no collisions full duplexbull each link is its own collision

domain

bull switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions bull not possible with dumb hub

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 23

45

6

61

Switch Tablebull Q how does switch know

that Arsquo reachable via inter-face 4 Brsquo reachable via inter-face 5

bull A each switch has a switch table each entrybull (MAC address of host interface

to reach host time stamp)

bull looks like a routing tablebull Q how are entries created

maintained in switch table bull something like a routing proto-

colFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

switch with six interfaces(123456)

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

62

Switch self-learningbull switch learns which

hosts can be reached through which inter-facesbull when frame received

switch ldquolearnsrdquo location of sender incoming LAN segment

bull records senderlocation pair in switch table

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

63

Switch frame filteringforward-ingWhen frame received

1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination then if dest on segment from which frame arrived then drop the frame else

forward the frame on interface indicated else flood

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

forward on all but the interface on which the frame arrived

64

Self-learning forwarding exam-ple

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

frame destination unknownflood

Arsquo A

destination A location known

Arsquo 4 60

selective send

65

Interconnecting switchesbull switches can be connected together

bull Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3

bull A self learning (works exactly the same as in single-switch case)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

B

S1

C D

E

FS2

S4

S3

H

I

G

66

Self-learning multi-switch exam-plebull Suppose C sends frame to I I responds to

C

bull Q show switch tables and packet forward-ing in S1 S2 S3 S4

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

B

S1

CD

E F

S2

S4

S3

H

I

G

12

1

2 3

3

1 2 3 12 3

444

67

Institutional network

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

to externalnetwork

router

IP subnet

mail server

web server

68

Switches vs Routersbull both store-and-forward devices

bull routers network layer devices (examine network layer head-ers)

bull switches are link layer devices

bull routers maintain routing tables implement routing al-gorithms

bull switches maintain switch tables implement filtering learning algorithms

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

69

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

• Fall 2015 COMP 46055605 Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link layer context
• Link Layer Services
• Link Layer Services (more)
• Where is the link layer implemented
• Adaptors Communicating
• Link Layer (2)
• Error Detection
• Parity Checking
• Internet checksum (review)
• Checksumming Cyclic Redundancy Check
• Checksumming Cyclic Redundancy Check (2)
• CRC Example
• Link Layer (3)
• Multiple Access Links and Protocols
• Multiple Access protocols
• Ideal Multiple Access Protocol
• MAC Protocols a taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD collision detection
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Link Layer (4)
• MAC Addresses and ARP
• LAN Addresses and ARP
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP protocol Same LAN (network)
• Addressing routing to another LAN
• Slide 45
• Link Layer (5)
• Ethernet
• Star Topology
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester encoding
• Link Layer (6)
• Hubs
• Switch
• Switch allows multiple simultaneous transmissions
• Switch Table
• Switch self-learning
• Switch frame filteringforwarding
• Self-learning forwarding example
• Interconnecting switches
• Self-learning multi-switch example
• Institutional network
• Switches vs Routers
• Link Layer (7)

12

Parity Checking

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Single Bit ParityDetect single bit errors

Two Dimensional Bit ParityDetect and correct single bit errors

0 0

13

Internet checksum (re-view)

Senderbull treat segment contents

as sequence of 16-bit integers

bull checksum addition (1rsquos complement sum) of segment contents

bull sender puts checksum value into UDP check-sum field

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Receiverbull compute checksum of

received segmentbull check if computed

checksum equals check-sum field valuebull NO - error detectedbull YES - no error detected

But maybe errors nonethe-less

Goal detect ldquoerrorsrdquo (eg flipped bits) in trans-mitted packet (note used at transport layer only

14

Checksumming Cyclic Redun-dancy Checkbull view data bits D as a binary numberbull choose r+1 bit pattern (generator) G bull goal choose r CRC bits R such that

bull ltDRgt exactly divisible by G (modulo 2) bull receiver knows G divides ltDRgt by G If non-zero remain-

der error detectedbull can detect all burst errors less than r+1 bits

bull widely used in practice (Ethernet 80211 WiFi ATM)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

15

Checksumming Cyclic Redun-dancy Checkbull 1011 XOR 0101 = 1110 bull 1001 XOR 1101 = 0100

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

16

CRC Example

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

WantD2r XOR R = nG

equivalentlyD2r = nG XOR R

equivalently if we divide D2r by

G want remainder R

R = remainder[ ]D2r

G

17

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

18

Multiple Access Links and Proto-colsTwo types of ldquolinksrdquobull point-to-point

bull PPP for dial-up accessbull point-to-point link between Ethernet switch and host

bull broadcast (shared wire or medium)bull old-fashioned Ethernetbull upstream HFC (Hybrid Fiber-Coaxial)bull 80211 wireless LAN

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

shared wire (eg cabled Ethernet)

shared RF (eg 80211 WiFi)

shared RF(satellite)

humans at acocktail party

(shared air acoustical)

19

Multiple Access protocolsbull single shared broadcast channel bull two or more simultaneous transmis-

sions by nodes interference bull collision if a node receives two or more

signals at the same time

multiple access protocolbull distributed algorithm that deter-

mines how nodes share channel ie determine when node can transmit

bull communication about channel shar-ing must use channel itself bull no out-of-band channel for coordination

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

20

Ideal Multiple Access ProtocolBroadcast channel of rate R bps1 when one node wants to transmit it

can send at rate R2 when M nodes want to transmit each

can send at average rate RM3 fully decentralized

bull no special node to coordinate transmissionsbull no synchronization of clocks slots

4 simple

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

21

MAC Protocols a taxonomyThree broad classesbull Channel Partitioning

bull divide channel into smaller ldquopiecesrdquo (time slots frequency code)

bull allocate piece to node for exclusive use

bull Random Accessbull channel not divided allow collisionsbull ldquorecoverrdquo from collisions

bull ldquoTaking turnsrdquobull nodes take turns but nodes with more to send

can take longer turnsFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

22

Channel Partitioning MAC proto-cols TDMATDMA time division multiple access bull access to channel in rounds bull each station gets fixed length slot

(length = pkt trans time) in each round bull unused slots go idle bull example 6-station LAN 134 have pkt

slots 256 idle

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

1 3 4 1 3 4

6-slotframe

23

Channel Partitioning MAC proto-cols FDMAFDMA frequency division multiple access bull channel spectrum divided into frequency bandsbull each station assigned fixed frequency bandbull unused transmission time in frequency bands go idle bull example 6-station LAN 134 have pkt frequency

bands 256 idle

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

freq

uency

bands time

FDM cable

24

Random Access Protocolsbull When node has packet to send

bull transmit at full channel data rate Rbull no a priori coordination among nodes

bull two or more transmitting nodes ldquocollisionrdquobull random access MAC protocol specifies

bull how to detect collisionsbull how to recover from collisions (eg via delayed retrans-

missions)

bull Examples of random access MAC protocolsbull slotted ALOHAbull ALOHAbull CSMA CSMACD CSMACA

bull (CS Carrier Sensing MA Multiple AccessCD Collision Detection CA Collision Avoidance)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

25

Slotted ALOHAAssumptionsbull all frames same sizebull time divided into

equal size slots (time to transmit 1 frame)

bull nodes start to transmit only slot beginning

bull nodes are synchro-nized

bull if 2 or more nodes transmit in slot all nodes detect collision

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Operationbull when node obtains

fresh frame trans-mits in next slotbull if no collision node

can send new frame in next slot

bull if collision node re-transmits frame in each subsequent slot with prob p until success

26

Slotted ALOHA

Prosbull single active node can

continuously transmit at full rate of channel

bull highly decentralized only slots in nodes need to be in sync

bull simpleFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Consbull collisions wasting slotsbull idle slotsbull nodes may be able to

detect collision in less than time to transmit packet

bull clock synchronization

27

Slotted Aloha Efficiency

bull suppose N nodes with many frames to send each transmits in slot with probability p

bull prob that given node has success in a slot = p(1-p)N-1

bull prob that any node has a success = Np(1-p)N-1

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

bull max efficiency find p that max-imizes Np(1-p)N-1

bull for many nodes take limit of Np(1- p)N-1 as N goes to infinity gives

Max efficiency = 1e = 37

Efficiency long-run fraction of successful slots (many nodes all with many frames to send)

At best channelused for useful transmissions 37of time

28

Pure (unslotted) ALOHAbull unslotted Aloha simpler no synchro-

nizationbull when frame first arrives

bull transmit immediately

bull collision probability increasesbull frame sent at t0 collides with other frames sent in

[t0-1t0+1]

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

29

Pure Aloha EfficiencyP(success by given node) = P(node transmits)

P(no other node transmits in [t0-1t0]

P(no other node transmits in [t0t0+1]

= p (1-p)N-1 (1-p)N-1

= p (1-p)2(N-1)

hellip choosing optimum p and then letting

= 1(2e) = 18

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

N

even worse than slotted Aloha

30

CSMA (Carrier Sense Multiple Access)CSMA listen before transmitIf channel sensed idle transmit entire framebull If channel sensed busy defer transmis-

sion

bull human analogy donrsquot interrupt others

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

31

CSMA Collisionscollisions can still occurbull propagation delay means

two nodes may not hear each otherrsquos transmission

collisionbull entire packet transmis-

sion time wasted

notebull role of distance amp propa-

gation delay in determin-ing collision probability

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

spatial layout of nodes

32

CSMACD (Collision Detec-tion)CSMACD carrier sensing deferral as in CSMA

bull collisions detected within short timebull colliding transmissions aborted reducing channel

wastage

bull collision detection bull easy in wired LANs measure signal strengths

compare transmitted received signalsbull difficult in wireless LANs received signal strength

overwhelmed by local transmission strength

bull human analogy the polite conversationalist

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

33

CSMACD collision detection

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

34

ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols

bull share channel efficiently and fairly at high loadbull inefficient at low load delay in channel access

1N bandwidth allocated even if only 1 active node

Random access MAC protocolsbull efficient at low load single node can fully uti-

lize channelbull high load collision overhead

ldquotaking turnsrdquo protocolsbull look for best of both worlds

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

35

ldquoTaking Turnsrdquo MAC protocolsPolling bull master node ldquoinvitesrdquo

slave nodes to transmit in turn

bull typically used with ldquodumbrdquo slave devices

bull concernsbull polling overhead bull latencybull single point of failure

(master)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

T

data

(nothingto send)

T

36

ldquoTaking Turnsrdquo MAC protocolsToken passingbull control token

passed from one node to next se-quentially

bull token messagebull concerns

bull token overhead bull latencybull single point of failure

(token)Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

master

slaves

poll

data

data

37

Summary of MAC protocolsbull channel partitioning by time frequency or

codebull Time Division Frequency Division

bull random access (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire)

hard in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211

bull taking turnsbull polling from central site token passing

bull Bluetooth FDDI IBM Token Ring Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

38

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

39

MAC Addresses and ARPbull 32-bit IP address

bull network-layer addressbull used to get datagram to destination IP subnet

bull MAC (or LAN or physical or Ethernet) ad-dress bull function get frame from one interface to an-

other physically-connected interface (same network)

bull 48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software set-

table

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

40

LAN Addresses and ARP

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Each adapter on LAN has unique LAN address

Broadcast address =FF-FF-FF-FF-FF-FF

= adapter

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired orwireless)

41

LAN Address (more)bull MAC address allocation administered by IEEEbull manufacturer buys portion of MAC address

space (to assure uniqueness)bull analogy

(a) MAC address like Social Security Num-ber

(b) IP address like postal addressbull MAC flat address portability

bull can move LAN card from one LAN to another

bull IP hierarchical address NOT portablebull address depends on IP subnet to which node is at-

tachedFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

42

ARP Address Resolution Proto-colbull Each IP node (host

router) on LAN has ARP table

bull ARP table IPMAC address mappings for some LAN nodeslt IP address MAC address

TTLgt

bull TTL (Time To Live) time after which ad-dress mapping will be forgotten (typically 20 min)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Question how to determineMAC address of Bknowing Brsquos IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN

137196723

137196778

137196714

137196788

43

ARP protocol Same LAN (net-work)bull A wants to send datagram

to B and Brsquos MAC address not in Arsquos ARP table

bull A broadcasts ARP query packet containing Bs IP address bull dest MAC address = FF-FF-FF-

FF-FF-FFbull all machines on LAN receive

ARP query

bull B receives ARP packet replies to A with its (Bs) MAC addressbull frame sent to Arsquos MAC ad-

dress (unicast)Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

bull A caches (saves) IP-to-MAC address pair in its ARP table until infor-mation becomes old (times out) bull soft state information

that times out (goes away) unless refreshed

bull ARP is ldquoplug-and-playrdquobull nodes create their ARP

tables without interven-tion from net administra-tor

44

Addressing routing to another LAN

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

R

1A-23-F9-CD-06-9B

222222222220111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

walkthrough send datagram from A to B via R assume A knows Brsquos IP address

bull two ARP tables in router R one for each IP network (LAN)

45

bull A creates IP datagram with source A destination B bull A uses ARP to get Rrsquos MAC address for 111111111110bull A creates link-layer frame with Rs MAC address as dest

frame contains A-to-B IP datagrambull Arsquos NIC sends frame bull Rrsquos NIC receives frame bull R removes IP datagram from Ethernet frame sees its des-

tined to Bbull R uses ARP to get Brsquos MAC address bull R creates frame containing A-to-B IP datagram sends to B

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

This is a really importantexample ndash make sure youunderstand

R

1A-23-F9-CD-06-9B

222222222220

111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

46

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

47

Ethernetldquodominantrdquo wired LAN technology bull cheap $20 for NICbull first widely used LAN technologybull simpler cheaper than token LANs and ATMbull kept up with speed race 10 Mbps ndash 10 Gbps

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Metcalfersquos Ethernetsketch

48

Star Topologybull bus topology popular through mid 90s

bull all nodes in same collision domain (can collide with each other)

bull today star topology prevailsbull active switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes do

not collide with each other)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

switch

bus coaxial cable star

49

Ethernet Frame Structurebull Sending adapter encapsulates IP datagram

(or other network layer protocol packet) in Ethernet frame

Preamble bull 7 bytes with pattern 10101010 followed by

one byte with pattern 10101011bull used to synchronize receiver sender clock

ratesFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

50

Ethernet Frame Structure (more)bull Addresses 6 bytes

bull if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

bull otherwise adapter discards frame

bull Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)

bull CRC checked at receiver if error is detected frame is dropped

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51

Ethernet Unreliable connection-lessbull connectionless No handshaking be-

tween sending and receiving NICs bull unreliable receiving NIC doesnrsquot send

acks or nacks to sending NICbull stream of datagrams passed to network

layer can have gaps (missing datagrams)bull gaps will be filled if app is using TCPbull otherwise app will see gaps

bull Ethernetrsquos MAC protocol unslotted CSMACD

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

52

Ethernet CSMACD algorithmbull 1 NIC receives data-

gram from network layer creates frame

bull 2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

bull 3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

bull 4 If NIC detects an-other transmission while transmitting aborts and sends jam signal

bull 5 After aborting NIC enters exponential backoff after mth colli-sion NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

53

Ethernetrsquos CSMACD (more)bull Jam Signal make

sure all other transmitters are aware of collision 48 bits

bull Bit time 1 micro sec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Exponential Backoff bull Goal adapt retransmission

attempts to estimated cur-rent load

bull heavy load random wait will be longer

bull first collision choose K from 01 delay is K 512 bit transmission times

bull after second collision choose K from 0123hellip

bull after ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

54

CSMACD efficiencybull Tprop = max prop delay between 2 nodes in

LANbull ttrans = time to transmit max-size frame

bull efficiency goes to 1 bull as tprop goes to 0

bull as ttrans goes to infinity

bull better performance than ALOHA and simple cheap decentralized

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

transprop ttefficiency

51

1

55

8023 Ethernet Standards Link amp Physical Layersbull many different Ethernet standards

bull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps

1Gbps 10G bpsbull different physical layer media fiber cable

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

applicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twisterpair) physical layer

56

Manchester encodingbull used in 10BaseTbull each bit has a transitionbull allows clocks in sending and receiving nodes to syn-

chronize to each otherbull no need for a centralized global clock among nodes

bull Hey this is physical-layer stuff

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

57

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

58

Hubshellip physical-layer (ldquodumbrdquo) repeaters

bull bits coming in one link go out all other links at same rate

bull all nodes connected to hub can collide with one another

bull no frame bufferingbull no CSMACD at hub host NICs detect colli-

sions

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

twisted pair

hub

59

Switchbull link-layer device smarter than hubs take

active rolebull store forward Ethernet framesbull examine incoming framersquos MAC address se-

lectively forward frame to one-or-more out-going links when frame is to be forwarded on segment uses CSMACD to access segment

bull transparentbull hosts are unaware of presence of switches

bull plug-and-play self-learning (HOW)bull switches do not need to be configured

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

60

Switch allows multiple simulta-neous transmissionsbull hosts have dedicated direct

connection to switchbull switches buffer packetsbull Ethernet protocol used on

each incoming link but no collisions full duplexbull each link is its own collision

domain

bull switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions bull not possible with dumb hub

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 23

45

6

61

Switch Tablebull Q how does switch know

that Arsquo reachable via inter-face 4 Brsquo reachable via inter-face 5

bull A each switch has a switch table each entrybull (MAC address of host interface

to reach host time stamp)

bull looks like a routing tablebull Q how are entries created

maintained in switch table bull something like a routing proto-

colFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

switch with six interfaces(123456)

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

62

Switch self-learningbull switch learns which

hosts can be reached through which inter-facesbull when frame received

switch ldquolearnsrdquo location of sender incoming LAN segment

bull records senderlocation pair in switch table

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

63

Switch frame filteringforward-ingWhen frame received

1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination then if dest on segment from which frame arrived then drop the frame else

forward the frame on interface indicated else flood

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

forward on all but the interface on which the frame arrived

64

Self-learning forwarding exam-ple

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

frame destination unknownflood

Arsquo A

destination A location known

Arsquo 4 60

selective send

65

Interconnecting switchesbull switches can be connected together

bull Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3

bull A self learning (works exactly the same as in single-switch case)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

B

S1

C D

E

FS2

S4

S3

H

I

G

66

Self-learning multi-switch exam-plebull Suppose C sends frame to I I responds to

C

bull Q show switch tables and packet forward-ing in S1 S2 S3 S4

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

B

S1

CD

E F

S2

S4

S3

H

I

G

12

1

2 3

3

1 2 3 12 3

444

67

Institutional network

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

to externalnetwork

router

IP subnet

mail server

web server

68

Switches vs Routersbull both store-and-forward devices

bull routers network layer devices (examine network layer head-ers)

bull switches are link layer devices

bull routers maintain routing tables implement routing al-gorithms

bull switches maintain switch tables implement filtering learning algorithms

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

69

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

• Fall 2015 COMP 46055605 Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link layer context
• Link Layer Services
• Link Layer Services (more)
• Where is the link layer implemented
• Adaptors Communicating
• Link Layer (2)
• Error Detection
• Parity Checking
• Internet checksum (review)
• Checksumming Cyclic Redundancy Check
• Checksumming Cyclic Redundancy Check (2)
• CRC Example
• Link Layer (3)
• Multiple Access Links and Protocols
• Multiple Access protocols
• Ideal Multiple Access Protocol
• MAC Protocols a taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD collision detection
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Link Layer (4)
• MAC Addresses and ARP
• LAN Addresses and ARP
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP protocol Same LAN (network)
• Addressing routing to another LAN
• Slide 45
• Link Layer (5)
• Ethernet
• Star Topology
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester encoding
• Link Layer (6)
• Hubs
• Switch
• Switch allows multiple simultaneous transmissions
• Switch Table
• Switch self-learning
• Switch frame filteringforwarding
• Self-learning forwarding example
• Interconnecting switches
• Self-learning multi-switch example
• Institutional network
• Switches vs Routers
• Link Layer (7)

13

Internet checksum (re-view)

Senderbull treat segment contents

as sequence of 16-bit integers

bull checksum addition (1rsquos complement sum) of segment contents

bull sender puts checksum value into UDP check-sum field

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Receiverbull compute checksum of

received segmentbull check if computed

checksum equals check-sum field valuebull NO - error detectedbull YES - no error detected

But maybe errors nonethe-less

Goal detect ldquoerrorsrdquo (eg flipped bits) in trans-mitted packet (note used at transport layer only

14

Checksumming Cyclic Redun-dancy Checkbull view data bits D as a binary numberbull choose r+1 bit pattern (generator) G bull goal choose r CRC bits R such that

bull ltDRgt exactly divisible by G (modulo 2) bull receiver knows G divides ltDRgt by G If non-zero remain-

der error detectedbull can detect all burst errors less than r+1 bits

bull widely used in practice (Ethernet 80211 WiFi ATM)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

15

Checksumming Cyclic Redun-dancy Checkbull 1011 XOR 0101 = 1110 bull 1001 XOR 1101 = 0100

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

16

CRC Example

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

WantD2r XOR R = nG

equivalentlyD2r = nG XOR R

equivalently if we divide D2r by

G want remainder R

R = remainder[ ]D2r

G

17

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

18

Multiple Access Links and Proto-colsTwo types of ldquolinksrdquobull point-to-point

bull PPP for dial-up accessbull point-to-point link between Ethernet switch and host

bull broadcast (shared wire or medium)bull old-fashioned Ethernetbull upstream HFC (Hybrid Fiber-Coaxial)bull 80211 wireless LAN

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

shared wire (eg cabled Ethernet)

shared RF (eg 80211 WiFi)

shared RF(satellite)

humans at acocktail party

(shared air acoustical)

19

Multiple Access protocolsbull single shared broadcast channel bull two or more simultaneous transmis-

sions by nodes interference bull collision if a node receives two or more

signals at the same time

multiple access protocolbull distributed algorithm that deter-

mines how nodes share channel ie determine when node can transmit

bull communication about channel shar-ing must use channel itself bull no out-of-band channel for coordination

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

20

Ideal Multiple Access ProtocolBroadcast channel of rate R bps1 when one node wants to transmit it

can send at rate R2 when M nodes want to transmit each

can send at average rate RM3 fully decentralized

bull no special node to coordinate transmissionsbull no synchronization of clocks slots

4 simple

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

21

MAC Protocols a taxonomyThree broad classesbull Channel Partitioning

bull divide channel into smaller ldquopiecesrdquo (time slots frequency code)

bull allocate piece to node for exclusive use

bull Random Accessbull channel not divided allow collisionsbull ldquorecoverrdquo from collisions

bull ldquoTaking turnsrdquobull nodes take turns but nodes with more to send

can take longer turnsFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

22

Channel Partitioning MAC proto-cols TDMATDMA time division multiple access bull access to channel in rounds bull each station gets fixed length slot

(length = pkt trans time) in each round bull unused slots go idle bull example 6-station LAN 134 have pkt

slots 256 idle

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

1 3 4 1 3 4

6-slotframe

23

Channel Partitioning MAC proto-cols FDMAFDMA frequency division multiple access bull channel spectrum divided into frequency bandsbull each station assigned fixed frequency bandbull unused transmission time in frequency bands go idle bull example 6-station LAN 134 have pkt frequency

bands 256 idle

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

freq

uency

bands time

FDM cable

24

Random Access Protocolsbull When node has packet to send

bull transmit at full channel data rate Rbull no a priori coordination among nodes

bull two or more transmitting nodes ldquocollisionrdquobull random access MAC protocol specifies

bull how to detect collisionsbull how to recover from collisions (eg via delayed retrans-

missions)

bull Examples of random access MAC protocolsbull slotted ALOHAbull ALOHAbull CSMA CSMACD CSMACA

bull (CS Carrier Sensing MA Multiple AccessCD Collision Detection CA Collision Avoidance)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

25

Slotted ALOHAAssumptionsbull all frames same sizebull time divided into

equal size slots (time to transmit 1 frame)

bull nodes start to transmit only slot beginning

bull nodes are synchro-nized

bull if 2 or more nodes transmit in slot all nodes detect collision

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Operationbull when node obtains

fresh frame trans-mits in next slotbull if no collision node

can send new frame in next slot

bull if collision node re-transmits frame in each subsequent slot with prob p until success

26

Slotted ALOHA

Prosbull single active node can

continuously transmit at full rate of channel

bull highly decentralized only slots in nodes need to be in sync

bull simpleFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Consbull collisions wasting slotsbull idle slotsbull nodes may be able to

detect collision in less than time to transmit packet

bull clock synchronization

27

Slotted Aloha Efficiency

bull suppose N nodes with many frames to send each transmits in slot with probability p

bull prob that given node has success in a slot = p(1-p)N-1

bull prob that any node has a success = Np(1-p)N-1

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

bull max efficiency find p that max-imizes Np(1-p)N-1

bull for many nodes take limit of Np(1- p)N-1 as N goes to infinity gives

Max efficiency = 1e = 37

Efficiency long-run fraction of successful slots (many nodes all with many frames to send)

At best channelused for useful transmissions 37of time

28

Pure (unslotted) ALOHAbull unslotted Aloha simpler no synchro-

nizationbull when frame first arrives

bull transmit immediately

bull collision probability increasesbull frame sent at t0 collides with other frames sent in

[t0-1t0+1]

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

29

Pure Aloha EfficiencyP(success by given node) = P(node transmits)

P(no other node transmits in [t0-1t0]

P(no other node transmits in [t0t0+1]

= p (1-p)N-1 (1-p)N-1

= p (1-p)2(N-1)

hellip choosing optimum p and then letting

= 1(2e) = 18

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

N

even worse than slotted Aloha

30

CSMA (Carrier Sense Multiple Access)CSMA listen before transmitIf channel sensed idle transmit entire framebull If channel sensed busy defer transmis-

sion

bull human analogy donrsquot interrupt others

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

31

CSMA Collisionscollisions can still occurbull propagation delay means

two nodes may not hear each otherrsquos transmission

collisionbull entire packet transmis-

sion time wasted

notebull role of distance amp propa-

gation delay in determin-ing collision probability

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

spatial layout of nodes

32

CSMACD (Collision Detec-tion)CSMACD carrier sensing deferral as in CSMA

bull collisions detected within short timebull colliding transmissions aborted reducing channel

wastage

bull collision detection bull easy in wired LANs measure signal strengths

compare transmitted received signalsbull difficult in wireless LANs received signal strength

overwhelmed by local transmission strength

bull human analogy the polite conversationalist

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

33

CSMACD collision detection

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

34

ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols

bull share channel efficiently and fairly at high loadbull inefficient at low load delay in channel access

1N bandwidth allocated even if only 1 active node

Random access MAC protocolsbull efficient at low load single node can fully uti-

lize channelbull high load collision overhead

ldquotaking turnsrdquo protocolsbull look for best of both worlds

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

35

ldquoTaking Turnsrdquo MAC protocolsPolling bull master node ldquoinvitesrdquo

slave nodes to transmit in turn

bull typically used with ldquodumbrdquo slave devices

bull concernsbull polling overhead bull latencybull single point of failure

(master)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

T

data

(nothingto send)

T

36

ldquoTaking Turnsrdquo MAC protocolsToken passingbull control token

passed from one node to next se-quentially

bull token messagebull concerns

bull token overhead bull latencybull single point of failure

(token)Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

master

slaves

poll

data

data

37

Summary of MAC protocolsbull channel partitioning by time frequency or

codebull Time Division Frequency Division

bull random access (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire)

hard in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211

bull taking turnsbull polling from central site token passing

bull Bluetooth FDDI IBM Token Ring Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

38

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

39

MAC Addresses and ARPbull 32-bit IP address

bull network-layer addressbull used to get datagram to destination IP subnet

bull MAC (or LAN or physical or Ethernet) ad-dress bull function get frame from one interface to an-

other physically-connected interface (same network)

bull 48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software set-

table

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

40

LAN Addresses and ARP

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Each adapter on LAN has unique LAN address

Broadcast address =FF-FF-FF-FF-FF-FF

= adapter

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired orwireless)

41

LAN Address (more)bull MAC address allocation administered by IEEEbull manufacturer buys portion of MAC address

space (to assure uniqueness)bull analogy

(a) MAC address like Social Security Num-ber

(b) IP address like postal addressbull MAC flat address portability

bull can move LAN card from one LAN to another

bull IP hierarchical address NOT portablebull address depends on IP subnet to which node is at-

tachedFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

42

ARP Address Resolution Proto-colbull Each IP node (host

router) on LAN has ARP table

bull ARP table IPMAC address mappings for some LAN nodeslt IP address MAC address

TTLgt

bull TTL (Time To Live) time after which ad-dress mapping will be forgotten (typically 20 min)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Question how to determineMAC address of Bknowing Brsquos IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN

137196723

137196778

137196714

137196788

43

ARP protocol Same LAN (net-work)bull A wants to send datagram

to B and Brsquos MAC address not in Arsquos ARP table

bull A broadcasts ARP query packet containing Bs IP address bull dest MAC address = FF-FF-FF-

FF-FF-FFbull all machines on LAN receive

ARP query

bull B receives ARP packet replies to A with its (Bs) MAC addressbull frame sent to Arsquos MAC ad-

dress (unicast)Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

bull A caches (saves) IP-to-MAC address pair in its ARP table until infor-mation becomes old (times out) bull soft state information

that times out (goes away) unless refreshed

bull ARP is ldquoplug-and-playrdquobull nodes create their ARP

tables without interven-tion from net administra-tor

44

Addressing routing to another LAN

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

R

1A-23-F9-CD-06-9B

222222222220111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

walkthrough send datagram from A to B via R assume A knows Brsquos IP address

bull two ARP tables in router R one for each IP network (LAN)

45

bull A creates IP datagram with source A destination B bull A uses ARP to get Rrsquos MAC address for 111111111110bull A creates link-layer frame with Rs MAC address as dest

frame contains A-to-B IP datagrambull Arsquos NIC sends frame bull Rrsquos NIC receives frame bull R removes IP datagram from Ethernet frame sees its des-

tined to Bbull R uses ARP to get Brsquos MAC address bull R creates frame containing A-to-B IP datagram sends to B

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

This is a really importantexample ndash make sure youunderstand

R

1A-23-F9-CD-06-9B

222222222220

111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

46

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

47

Ethernetldquodominantrdquo wired LAN technology bull cheap $20 for NICbull first widely used LAN technologybull simpler cheaper than token LANs and ATMbull kept up with speed race 10 Mbps ndash 10 Gbps

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Metcalfersquos Ethernetsketch

48

Star Topologybull bus topology popular through mid 90s

bull all nodes in same collision domain (can collide with each other)

bull today star topology prevailsbull active switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes do

not collide with each other)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

switch

bus coaxial cable star

49

Ethernet Frame Structurebull Sending adapter encapsulates IP datagram

(or other network layer protocol packet) in Ethernet frame

Preamble bull 7 bytes with pattern 10101010 followed by

one byte with pattern 10101011bull used to synchronize receiver sender clock

ratesFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

50

Ethernet Frame Structure (more)bull Addresses 6 bytes

bull if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

bull otherwise adapter discards frame

bull Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)

bull CRC checked at receiver if error is detected frame is dropped

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51

Ethernet Unreliable connection-lessbull connectionless No handshaking be-

tween sending and receiving NICs bull unreliable receiving NIC doesnrsquot send

acks or nacks to sending NICbull stream of datagrams passed to network

layer can have gaps (missing datagrams)bull gaps will be filled if app is using TCPbull otherwise app will see gaps

bull Ethernetrsquos MAC protocol unslotted CSMACD

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

52

Ethernet CSMACD algorithmbull 1 NIC receives data-

gram from network layer creates frame

bull 2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

bull 3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

bull 4 If NIC detects an-other transmission while transmitting aborts and sends jam signal

bull 5 After aborting NIC enters exponential backoff after mth colli-sion NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

53

Ethernetrsquos CSMACD (more)bull Jam Signal make

sure all other transmitters are aware of collision 48 bits

bull Bit time 1 micro sec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Exponential Backoff bull Goal adapt retransmission

attempts to estimated cur-rent load

bull heavy load random wait will be longer

bull first collision choose K from 01 delay is K 512 bit transmission times

bull after second collision choose K from 0123hellip

bull after ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

54

CSMACD efficiencybull Tprop = max prop delay between 2 nodes in

LANbull ttrans = time to transmit max-size frame

bull efficiency goes to 1 bull as tprop goes to 0

bull as ttrans goes to infinity

bull better performance than ALOHA and simple cheap decentralized

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

transprop ttefficiency

51

1

55

8023 Ethernet Standards Link amp Physical Layersbull many different Ethernet standards

bull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps

1Gbps 10G bpsbull different physical layer media fiber cable

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

applicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twisterpair) physical layer

56

Manchester encodingbull used in 10BaseTbull each bit has a transitionbull allows clocks in sending and receiving nodes to syn-

chronize to each otherbull no need for a centralized global clock among nodes

bull Hey this is physical-layer stuff

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

57

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

58

Hubshellip physical-layer (ldquodumbrdquo) repeaters

bull bits coming in one link go out all other links at same rate

bull all nodes connected to hub can collide with one another

bull no frame bufferingbull no CSMACD at hub host NICs detect colli-

sions

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

twisted pair

hub

59

Switchbull link-layer device smarter than hubs take

active rolebull store forward Ethernet framesbull examine incoming framersquos MAC address se-

lectively forward frame to one-or-more out-going links when frame is to be forwarded on segment uses CSMACD to access segment

bull transparentbull hosts are unaware of presence of switches

bull plug-and-play self-learning (HOW)bull switches do not need to be configured

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

60

Switch allows multiple simulta-neous transmissionsbull hosts have dedicated direct

connection to switchbull switches buffer packetsbull Ethernet protocol used on

each incoming link but no collisions full duplexbull each link is its own collision

domain

bull switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions bull not possible with dumb hub

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 23

45

6

61

Switch Tablebull Q how does switch know

that Arsquo reachable via inter-face 4 Brsquo reachable via inter-face 5

bull A each switch has a switch table each entrybull (MAC address of host interface

to reach host time stamp)

bull looks like a routing tablebull Q how are entries created

maintained in switch table bull something like a routing proto-

colFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

switch with six interfaces(123456)

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

62

Switch self-learningbull switch learns which

hosts can be reached through which inter-facesbull when frame received

switch ldquolearnsrdquo location of sender incoming LAN segment

bull records senderlocation pair in switch table

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

63

Switch frame filteringforward-ingWhen frame received

1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination then if dest on segment from which frame arrived then drop the frame else

forward the frame on interface indicated else flood

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

forward on all but the interface on which the frame arrived

64

Self-learning forwarding exam-ple

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

frame destination unknownflood

Arsquo A

destination A location known

Arsquo 4 60

selective send

65

Interconnecting switchesbull switches can be connected together

bull Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3

bull A self learning (works exactly the same as in single-switch case)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

B

S1

C D

E

FS2

S4

S3

H

I

G

66

Self-learning multi-switch exam-plebull Suppose C sends frame to I I responds to

C

bull Q show switch tables and packet forward-ing in S1 S2 S3 S4

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

B

S1

CD

E F

S2

S4

S3

H

I

G

12

1

2 3

3

1 2 3 12 3

444

67

Institutional network

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

to externalnetwork

router

IP subnet

mail server

web server

68

Switches vs Routersbull both store-and-forward devices

bull routers network layer devices (examine network layer head-ers)

bull switches are link layer devices

bull routers maintain routing tables implement routing al-gorithms

bull switches maintain switch tables implement filtering learning algorithms

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

69

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

• Fall 2015 COMP 46055605 Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link layer context
• Link Layer Services
• Link Layer Services (more)
• Where is the link layer implemented
• Adaptors Communicating
• Link Layer (2)
• Error Detection
• Parity Checking
• Internet checksum (review)
• Checksumming Cyclic Redundancy Check
• Checksumming Cyclic Redundancy Check (2)
• CRC Example
• Link Layer (3)
• Multiple Access Links and Protocols
• Multiple Access protocols
• Ideal Multiple Access Protocol
• MAC Protocols a taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD collision detection
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Link Layer (4)
• MAC Addresses and ARP
• LAN Addresses and ARP
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP protocol Same LAN (network)
• Addressing routing to another LAN
• Slide 45
• Link Layer (5)
• Ethernet
• Star Topology
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester encoding
• Link Layer (6)
• Hubs
• Switch
• Switch allows multiple simultaneous transmissions
• Switch Table
• Switch self-learning
• Switch frame filteringforwarding
• Self-learning forwarding example
• Interconnecting switches
• Self-learning multi-switch example
• Institutional network
• Switches vs Routers
• Link Layer (7)

14

Checksumming Cyclic Redun-dancy Checkbull view data bits D as a binary numberbull choose r+1 bit pattern (generator) G bull goal choose r CRC bits R such that

bull ltDRgt exactly divisible by G (modulo 2) bull receiver knows G divides ltDRgt by G If non-zero remain-

der error detectedbull can detect all burst errors less than r+1 bits

bull widely used in practice (Ethernet 80211 WiFi ATM)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

15

Checksumming Cyclic Redun-dancy Checkbull 1011 XOR 0101 = 1110 bull 1001 XOR 1101 = 0100

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

16

CRC Example

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

WantD2r XOR R = nG

equivalentlyD2r = nG XOR R

equivalently if we divide D2r by

G want remainder R

R = remainder[ ]D2r

G

17

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

18

Multiple Access Links and Proto-colsTwo types of ldquolinksrdquobull point-to-point

bull PPP for dial-up accessbull point-to-point link between Ethernet switch and host

bull broadcast (shared wire or medium)bull old-fashioned Ethernetbull upstream HFC (Hybrid Fiber-Coaxial)bull 80211 wireless LAN

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

shared wire (eg cabled Ethernet)

shared RF (eg 80211 WiFi)

shared RF(satellite)

humans at acocktail party

(shared air acoustical)

19

Multiple Access protocolsbull single shared broadcast channel bull two or more simultaneous transmis-

sions by nodes interference bull collision if a node receives two or more

signals at the same time

multiple access protocolbull distributed algorithm that deter-

mines how nodes share channel ie determine when node can transmit

bull communication about channel shar-ing must use channel itself bull no out-of-band channel for coordination

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

20

Ideal Multiple Access ProtocolBroadcast channel of rate R bps1 when one node wants to transmit it

can send at rate R2 when M nodes want to transmit each

can send at average rate RM3 fully decentralized

bull no special node to coordinate transmissionsbull no synchronization of clocks slots

4 simple

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

21

MAC Protocols a taxonomyThree broad classesbull Channel Partitioning

bull divide channel into smaller ldquopiecesrdquo (time slots frequency code)

bull allocate piece to node for exclusive use

bull Random Accessbull channel not divided allow collisionsbull ldquorecoverrdquo from collisions

bull ldquoTaking turnsrdquobull nodes take turns but nodes with more to send

can take longer turnsFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

22

Channel Partitioning MAC proto-cols TDMATDMA time division multiple access bull access to channel in rounds bull each station gets fixed length slot

(length = pkt trans time) in each round bull unused slots go idle bull example 6-station LAN 134 have pkt

slots 256 idle

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

1 3 4 1 3 4

6-slotframe

23

Channel Partitioning MAC proto-cols FDMAFDMA frequency division multiple access bull channel spectrum divided into frequency bandsbull each station assigned fixed frequency bandbull unused transmission time in frequency bands go idle bull example 6-station LAN 134 have pkt frequency

bands 256 idle

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

freq

uency

bands time

FDM cable

24

Random Access Protocolsbull When node has packet to send

bull transmit at full channel data rate Rbull no a priori coordination among nodes

bull two or more transmitting nodes ldquocollisionrdquobull random access MAC protocol specifies

bull how to detect collisionsbull how to recover from collisions (eg via delayed retrans-

missions)

bull Examples of random access MAC protocolsbull slotted ALOHAbull ALOHAbull CSMA CSMACD CSMACA

bull (CS Carrier Sensing MA Multiple AccessCD Collision Detection CA Collision Avoidance)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

25

Slotted ALOHAAssumptionsbull all frames same sizebull time divided into

equal size slots (time to transmit 1 frame)

bull nodes start to transmit only slot beginning

bull nodes are synchro-nized

bull if 2 or more nodes transmit in slot all nodes detect collision

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Operationbull when node obtains

fresh frame trans-mits in next slotbull if no collision node

can send new frame in next slot

bull if collision node re-transmits frame in each subsequent slot with prob p until success

26

Slotted ALOHA

Prosbull single active node can

continuously transmit at full rate of channel

bull highly decentralized only slots in nodes need to be in sync

bull simpleFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Consbull collisions wasting slotsbull idle slotsbull nodes may be able to

detect collision in less than time to transmit packet

bull clock synchronization

27

Slotted Aloha Efficiency

bull suppose N nodes with many frames to send each transmits in slot with probability p

bull prob that given node has success in a slot = p(1-p)N-1

bull prob that any node has a success = Np(1-p)N-1

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

bull max efficiency find p that max-imizes Np(1-p)N-1

bull for many nodes take limit of Np(1- p)N-1 as N goes to infinity gives

Max efficiency = 1e = 37

Efficiency long-run fraction of successful slots (many nodes all with many frames to send)

At best channelused for useful transmissions 37of time

28

Pure (unslotted) ALOHAbull unslotted Aloha simpler no synchro-

nizationbull when frame first arrives

bull transmit immediately

bull collision probability increasesbull frame sent at t0 collides with other frames sent in

[t0-1t0+1]

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

29

Pure Aloha EfficiencyP(success by given node) = P(node transmits)

P(no other node transmits in [t0-1t0]

P(no other node transmits in [t0t0+1]

= p (1-p)N-1 (1-p)N-1

= p (1-p)2(N-1)

hellip choosing optimum p and then letting

= 1(2e) = 18

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

N

even worse than slotted Aloha

30

CSMA (Carrier Sense Multiple Access)CSMA listen before transmitIf channel sensed idle transmit entire framebull If channel sensed busy defer transmis-

sion

bull human analogy donrsquot interrupt others

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

31

CSMA Collisionscollisions can still occurbull propagation delay means

two nodes may not hear each otherrsquos transmission

collisionbull entire packet transmis-

sion time wasted

notebull role of distance amp propa-

gation delay in determin-ing collision probability

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

spatial layout of nodes

32

CSMACD (Collision Detec-tion)CSMACD carrier sensing deferral as in CSMA

bull collisions detected within short timebull colliding transmissions aborted reducing channel

wastage

bull collision detection bull easy in wired LANs measure signal strengths

compare transmitted received signalsbull difficult in wireless LANs received signal strength

overwhelmed by local transmission strength

bull human analogy the polite conversationalist

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

33

CSMACD collision detection

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

34

ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols

bull share channel efficiently and fairly at high loadbull inefficient at low load delay in channel access

1N bandwidth allocated even if only 1 active node

Random access MAC protocolsbull efficient at low load single node can fully uti-

lize channelbull high load collision overhead

ldquotaking turnsrdquo protocolsbull look for best of both worlds

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

35

ldquoTaking Turnsrdquo MAC protocolsPolling bull master node ldquoinvitesrdquo

slave nodes to transmit in turn

bull typically used with ldquodumbrdquo slave devices

bull concernsbull polling overhead bull latencybull single point of failure

(master)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

T

data

(nothingto send)

T

36

ldquoTaking Turnsrdquo MAC protocolsToken passingbull control token

passed from one node to next se-quentially

bull token messagebull concerns

bull token overhead bull latencybull single point of failure

(token)Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

master

slaves

poll

data

data

37

Summary of MAC protocolsbull channel partitioning by time frequency or

codebull Time Division Frequency Division

bull random access (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire)

hard in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211

bull taking turnsbull polling from central site token passing

bull Bluetooth FDDI IBM Token Ring Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

38

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

39

MAC Addresses and ARPbull 32-bit IP address

bull network-layer addressbull used to get datagram to destination IP subnet

bull MAC (or LAN or physical or Ethernet) ad-dress bull function get frame from one interface to an-

other physically-connected interface (same network)

bull 48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software set-

table

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

40

LAN Addresses and ARP

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Each adapter on LAN has unique LAN address

Broadcast address =FF-FF-FF-FF-FF-FF

= adapter

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired orwireless)

41

LAN Address (more)bull MAC address allocation administered by IEEEbull manufacturer buys portion of MAC address

space (to assure uniqueness)bull analogy

(a) MAC address like Social Security Num-ber

(b) IP address like postal addressbull MAC flat address portability

bull can move LAN card from one LAN to another

bull IP hierarchical address NOT portablebull address depends on IP subnet to which node is at-

tachedFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

42

ARP Address Resolution Proto-colbull Each IP node (host

router) on LAN has ARP table

bull ARP table IPMAC address mappings for some LAN nodeslt IP address MAC address

TTLgt

bull TTL (Time To Live) time after which ad-dress mapping will be forgotten (typically 20 min)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Question how to determineMAC address of Bknowing Brsquos IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN

137196723

137196778

137196714

137196788

43

ARP protocol Same LAN (net-work)bull A wants to send datagram

to B and Brsquos MAC address not in Arsquos ARP table

bull A broadcasts ARP query packet containing Bs IP address bull dest MAC address = FF-FF-FF-

FF-FF-FFbull all machines on LAN receive

ARP query

bull B receives ARP packet replies to A with its (Bs) MAC addressbull frame sent to Arsquos MAC ad-

dress (unicast)Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

bull A caches (saves) IP-to-MAC address pair in its ARP table until infor-mation becomes old (times out) bull soft state information

that times out (goes away) unless refreshed

bull ARP is ldquoplug-and-playrdquobull nodes create their ARP

tables without interven-tion from net administra-tor

44

Addressing routing to another LAN

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

R

1A-23-F9-CD-06-9B

222222222220111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

walkthrough send datagram from A to B via R assume A knows Brsquos IP address

bull two ARP tables in router R one for each IP network (LAN)

45

bull A creates IP datagram with source A destination B bull A uses ARP to get Rrsquos MAC address for 111111111110bull A creates link-layer frame with Rs MAC address as dest

frame contains A-to-B IP datagrambull Arsquos NIC sends frame bull Rrsquos NIC receives frame bull R removes IP datagram from Ethernet frame sees its des-

tined to Bbull R uses ARP to get Brsquos MAC address bull R creates frame containing A-to-B IP datagram sends to B

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

This is a really importantexample ndash make sure youunderstand

R

1A-23-F9-CD-06-9B

222222222220

111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

46

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

47

Ethernetldquodominantrdquo wired LAN technology bull cheap $20 for NICbull first widely used LAN technologybull simpler cheaper than token LANs and ATMbull kept up with speed race 10 Mbps ndash 10 Gbps

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Metcalfersquos Ethernetsketch

48

Star Topologybull bus topology popular through mid 90s

bull all nodes in same collision domain (can collide with each other)

bull today star topology prevailsbull active switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes do

not collide with each other)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

switch

bus coaxial cable star

49

Ethernet Frame Structurebull Sending adapter encapsulates IP datagram

(or other network layer protocol packet) in Ethernet frame

Preamble bull 7 bytes with pattern 10101010 followed by

one byte with pattern 10101011bull used to synchronize receiver sender clock

ratesFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

50

Ethernet Frame Structure (more)bull Addresses 6 bytes

bull if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

bull otherwise adapter discards frame

bull Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)

bull CRC checked at receiver if error is detected frame is dropped

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51

Ethernet Unreliable connection-lessbull connectionless No handshaking be-

tween sending and receiving NICs bull unreliable receiving NIC doesnrsquot send

acks or nacks to sending NICbull stream of datagrams passed to network

layer can have gaps (missing datagrams)bull gaps will be filled if app is using TCPbull otherwise app will see gaps

bull Ethernetrsquos MAC protocol unslotted CSMACD

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

52

Ethernet CSMACD algorithmbull 1 NIC receives data-

gram from network layer creates frame

bull 2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

bull 3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

bull 4 If NIC detects an-other transmission while transmitting aborts and sends jam signal

bull 5 After aborting NIC enters exponential backoff after mth colli-sion NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

53

Ethernetrsquos CSMACD (more)bull Jam Signal make

sure all other transmitters are aware of collision 48 bits

bull Bit time 1 micro sec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Exponential Backoff bull Goal adapt retransmission

attempts to estimated cur-rent load

bull heavy load random wait will be longer

bull first collision choose K from 01 delay is K 512 bit transmission times

bull after second collision choose K from 0123hellip

bull after ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

54

CSMACD efficiencybull Tprop = max prop delay between 2 nodes in

LANbull ttrans = time to transmit max-size frame

bull efficiency goes to 1 bull as tprop goes to 0

bull as ttrans goes to infinity

bull better performance than ALOHA and simple cheap decentralized

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

transprop ttefficiency

51

1

55

8023 Ethernet Standards Link amp Physical Layersbull many different Ethernet standards

bull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps

1Gbps 10G bpsbull different physical layer media fiber cable

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

applicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twisterpair) physical layer

56

Manchester encodingbull used in 10BaseTbull each bit has a transitionbull allows clocks in sending and receiving nodes to syn-

chronize to each otherbull no need for a centralized global clock among nodes

bull Hey this is physical-layer stuff

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

57

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

58

Hubshellip physical-layer (ldquodumbrdquo) repeaters

bull bits coming in one link go out all other links at same rate

bull all nodes connected to hub can collide with one another

bull no frame bufferingbull no CSMACD at hub host NICs detect colli-

sions

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

twisted pair

hub

59

Switchbull link-layer device smarter than hubs take

active rolebull store forward Ethernet framesbull examine incoming framersquos MAC address se-

lectively forward frame to one-or-more out-going links when frame is to be forwarded on segment uses CSMACD to access segment

bull transparentbull hosts are unaware of presence of switches

bull plug-and-play self-learning (HOW)bull switches do not need to be configured

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

60

Switch allows multiple simulta-neous transmissionsbull hosts have dedicated direct

connection to switchbull switches buffer packetsbull Ethernet protocol used on

each incoming link but no collisions full duplexbull each link is its own collision

domain

bull switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions bull not possible with dumb hub

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 23

45

6

61

Switch Tablebull Q how does switch know

that Arsquo reachable via inter-face 4 Brsquo reachable via inter-face 5

bull A each switch has a switch table each entrybull (MAC address of host interface

to reach host time stamp)

bull looks like a routing tablebull Q how are entries created

maintained in switch table bull something like a routing proto-

colFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

switch with six interfaces(123456)

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

62

Switch self-learningbull switch learns which

hosts can be reached through which inter-facesbull when frame received

switch ldquolearnsrdquo location of sender incoming LAN segment

bull records senderlocation pair in switch table

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

63

Switch frame filteringforward-ingWhen frame received

1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination then if dest on segment from which frame arrived then drop the frame else

forward the frame on interface indicated else flood

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

forward on all but the interface on which the frame arrived

64

Self-learning forwarding exam-ple

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

frame destination unknownflood

Arsquo A

destination A location known

Arsquo 4 60

selective send

65

Interconnecting switchesbull switches can be connected together

bull Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3

bull A self learning (works exactly the same as in single-switch case)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

B

S1

C D

E

FS2

S4

S3

H

I

G

66

Self-learning multi-switch exam-plebull Suppose C sends frame to I I responds to

C

bull Q show switch tables and packet forward-ing in S1 S2 S3 S4

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

B

S1

CD

E F

S2

S4

S3

H

I

G

12

1

2 3

3

1 2 3 12 3

444

67

Institutional network

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

to externalnetwork

router

IP subnet

mail server

web server

68

Switches vs Routersbull both store-and-forward devices

bull routers network layer devices (examine network layer head-ers)

bull switches are link layer devices

bull routers maintain routing tables implement routing al-gorithms

bull switches maintain switch tables implement filtering learning algorithms

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

69

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

• Fall 2015 COMP 46055605 Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link layer context
• Link Layer Services
• Link Layer Services (more)
• Where is the link layer implemented
• Adaptors Communicating
• Link Layer (2)
• Error Detection
• Parity Checking
• Internet checksum (review)
• Checksumming Cyclic Redundancy Check
• Checksumming Cyclic Redundancy Check (2)
• CRC Example
• Link Layer (3)
• Multiple Access Links and Protocols
• Multiple Access protocols
• Ideal Multiple Access Protocol
• MAC Protocols a taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD collision detection
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Link Layer (4)
• MAC Addresses and ARP
• LAN Addresses and ARP
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP protocol Same LAN (network)
• Addressing routing to another LAN
• Slide 45
• Link Layer (5)
• Ethernet
• Star Topology
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester encoding
• Link Layer (6)
• Hubs
• Switch
• Switch allows multiple simultaneous transmissions
• Switch Table
• Switch self-learning
• Switch frame filteringforwarding
• Self-learning forwarding example
• Interconnecting switches
• Self-learning multi-switch example
• Institutional network
• Switches vs Routers
• Link Layer (7)

15

Checksumming Cyclic Redun-dancy Checkbull 1011 XOR 0101 = 1110 bull 1001 XOR 1101 = 0100

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

16

CRC Example

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

WantD2r XOR R = nG

equivalentlyD2r = nG XOR R

equivalently if we divide D2r by

G want remainder R

R = remainder[ ]D2r

G

17

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

18

Multiple Access Links and Proto-colsTwo types of ldquolinksrdquobull point-to-point

bull PPP for dial-up accessbull point-to-point link between Ethernet switch and host

bull broadcast (shared wire or medium)bull old-fashioned Ethernetbull upstream HFC (Hybrid Fiber-Coaxial)bull 80211 wireless LAN

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

shared wire (eg cabled Ethernet)

shared RF (eg 80211 WiFi)

shared RF(satellite)

humans at acocktail party

(shared air acoustical)

19

Multiple Access protocolsbull single shared broadcast channel bull two or more simultaneous transmis-

sions by nodes interference bull collision if a node receives two or more

signals at the same time

multiple access protocolbull distributed algorithm that deter-

mines how nodes share channel ie determine when node can transmit

bull communication about channel shar-ing must use channel itself bull no out-of-band channel for coordination

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

20

Ideal Multiple Access ProtocolBroadcast channel of rate R bps1 when one node wants to transmit it

can send at rate R2 when M nodes want to transmit each

can send at average rate RM3 fully decentralized

bull no special node to coordinate transmissionsbull no synchronization of clocks slots

4 simple

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

21

MAC Protocols a taxonomyThree broad classesbull Channel Partitioning

bull divide channel into smaller ldquopiecesrdquo (time slots frequency code)

bull allocate piece to node for exclusive use

bull Random Accessbull channel not divided allow collisionsbull ldquorecoverrdquo from collisions

bull ldquoTaking turnsrdquobull nodes take turns but nodes with more to send

can take longer turnsFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

22

Channel Partitioning MAC proto-cols TDMATDMA time division multiple access bull access to channel in rounds bull each station gets fixed length slot

(length = pkt trans time) in each round bull unused slots go idle bull example 6-station LAN 134 have pkt

slots 256 idle

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

1 3 4 1 3 4

6-slotframe

23

Channel Partitioning MAC proto-cols FDMAFDMA frequency division multiple access bull channel spectrum divided into frequency bandsbull each station assigned fixed frequency bandbull unused transmission time in frequency bands go idle bull example 6-station LAN 134 have pkt frequency

bands 256 idle

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

freq

uency

bands time

FDM cable

24

Random Access Protocolsbull When node has packet to send

bull transmit at full channel data rate Rbull no a priori coordination among nodes

bull two or more transmitting nodes ldquocollisionrdquobull random access MAC protocol specifies

bull how to detect collisionsbull how to recover from collisions (eg via delayed retrans-

missions)

bull Examples of random access MAC protocolsbull slotted ALOHAbull ALOHAbull CSMA CSMACD CSMACA

bull (CS Carrier Sensing MA Multiple AccessCD Collision Detection CA Collision Avoidance)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

25

Slotted ALOHAAssumptionsbull all frames same sizebull time divided into

equal size slots (time to transmit 1 frame)

bull nodes start to transmit only slot beginning

bull nodes are synchro-nized

bull if 2 or more nodes transmit in slot all nodes detect collision

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Operationbull when node obtains

fresh frame trans-mits in next slotbull if no collision node

can send new frame in next slot

bull if collision node re-transmits frame in each subsequent slot with prob p until success

26

Slotted ALOHA

Prosbull single active node can

continuously transmit at full rate of channel

bull highly decentralized only slots in nodes need to be in sync

bull simpleFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Consbull collisions wasting slotsbull idle slotsbull nodes may be able to

detect collision in less than time to transmit packet

bull clock synchronization

27

Slotted Aloha Efficiency

bull suppose N nodes with many frames to send each transmits in slot with probability p

bull prob that given node has success in a slot = p(1-p)N-1

bull prob that any node has a success = Np(1-p)N-1

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

bull max efficiency find p that max-imizes Np(1-p)N-1

bull for many nodes take limit of Np(1- p)N-1 as N goes to infinity gives

Max efficiency = 1e = 37

Efficiency long-run fraction of successful slots (many nodes all with many frames to send)

At best channelused for useful transmissions 37of time

28

Pure (unslotted) ALOHAbull unslotted Aloha simpler no synchro-

nizationbull when frame first arrives

bull transmit immediately

bull collision probability increasesbull frame sent at t0 collides with other frames sent in

[t0-1t0+1]

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

29

Pure Aloha EfficiencyP(success by given node) = P(node transmits)

P(no other node transmits in [t0-1t0]

P(no other node transmits in [t0t0+1]

= p (1-p)N-1 (1-p)N-1

= p (1-p)2(N-1)

hellip choosing optimum p and then letting

= 1(2e) = 18

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

N

even worse than slotted Aloha

30

CSMA (Carrier Sense Multiple Access)CSMA listen before transmitIf channel sensed idle transmit entire framebull If channel sensed busy defer transmis-

sion

bull human analogy donrsquot interrupt others

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

31

CSMA Collisionscollisions can still occurbull propagation delay means

two nodes may not hear each otherrsquos transmission

collisionbull entire packet transmis-

sion time wasted

notebull role of distance amp propa-

gation delay in determin-ing collision probability

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

spatial layout of nodes

32

CSMACD (Collision Detec-tion)CSMACD carrier sensing deferral as in CSMA

bull collisions detected within short timebull colliding transmissions aborted reducing channel

wastage

bull collision detection bull easy in wired LANs measure signal strengths

compare transmitted received signalsbull difficult in wireless LANs received signal strength

overwhelmed by local transmission strength

bull human analogy the polite conversationalist

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

33

CSMACD collision detection

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

34

ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols

bull share channel efficiently and fairly at high loadbull inefficient at low load delay in channel access

1N bandwidth allocated even if only 1 active node

Random access MAC protocolsbull efficient at low load single node can fully uti-

lize channelbull high load collision overhead

ldquotaking turnsrdquo protocolsbull look for best of both worlds

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

35

ldquoTaking Turnsrdquo MAC protocolsPolling bull master node ldquoinvitesrdquo

slave nodes to transmit in turn

bull typically used with ldquodumbrdquo slave devices

bull concernsbull polling overhead bull latencybull single point of failure

(master)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

T

data

(nothingto send)

T

36

ldquoTaking Turnsrdquo MAC protocolsToken passingbull control token

passed from one node to next se-quentially

bull token messagebull concerns

bull token overhead bull latencybull single point of failure

(token)Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

master

slaves

poll

data

data

37

Summary of MAC protocolsbull channel partitioning by time frequency or

codebull Time Division Frequency Division

bull random access (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire)

hard in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211

bull taking turnsbull polling from central site token passing

bull Bluetooth FDDI IBM Token Ring Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

38

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

39

MAC Addresses and ARPbull 32-bit IP address

bull network-layer addressbull used to get datagram to destination IP subnet

bull MAC (or LAN or physical or Ethernet) ad-dress bull function get frame from one interface to an-

other physically-connected interface (same network)

bull 48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software set-

table

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

40

LAN Addresses and ARP

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Each adapter on LAN has unique LAN address

Broadcast address =FF-FF-FF-FF-FF-FF

= adapter

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired orwireless)

41

LAN Address (more)bull MAC address allocation administered by IEEEbull manufacturer buys portion of MAC address

space (to assure uniqueness)bull analogy

(a) MAC address like Social Security Num-ber

(b) IP address like postal addressbull MAC flat address portability

bull can move LAN card from one LAN to another

bull IP hierarchical address NOT portablebull address depends on IP subnet to which node is at-

tachedFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

42

ARP Address Resolution Proto-colbull Each IP node (host

router) on LAN has ARP table

bull ARP table IPMAC address mappings for some LAN nodeslt IP address MAC address

TTLgt

bull TTL (Time To Live) time after which ad-dress mapping will be forgotten (typically 20 min)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Question how to determineMAC address of Bknowing Brsquos IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN

137196723

137196778

137196714

137196788

43

ARP protocol Same LAN (net-work)bull A wants to send datagram

to B and Brsquos MAC address not in Arsquos ARP table

bull A broadcasts ARP query packet containing Bs IP address bull dest MAC address = FF-FF-FF-

FF-FF-FFbull all machines on LAN receive

ARP query

bull B receives ARP packet replies to A with its (Bs) MAC addressbull frame sent to Arsquos MAC ad-

dress (unicast)Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

bull A caches (saves) IP-to-MAC address pair in its ARP table until infor-mation becomes old (times out) bull soft state information

that times out (goes away) unless refreshed

bull ARP is ldquoplug-and-playrdquobull nodes create their ARP

tables without interven-tion from net administra-tor

44

Addressing routing to another LAN

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

R

1A-23-F9-CD-06-9B

222222222220111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

walkthrough send datagram from A to B via R assume A knows Brsquos IP address

bull two ARP tables in router R one for each IP network (LAN)

45

bull A creates IP datagram with source A destination B bull A uses ARP to get Rrsquos MAC address for 111111111110bull A creates link-layer frame with Rs MAC address as dest

frame contains A-to-B IP datagrambull Arsquos NIC sends frame bull Rrsquos NIC receives frame bull R removes IP datagram from Ethernet frame sees its des-

tined to Bbull R uses ARP to get Brsquos MAC address bull R creates frame containing A-to-B IP datagram sends to B

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

This is a really importantexample ndash make sure youunderstand

R

1A-23-F9-CD-06-9B

222222222220

111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

46

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

47

Ethernetldquodominantrdquo wired LAN technology bull cheap $20 for NICbull first widely used LAN technologybull simpler cheaper than token LANs and ATMbull kept up with speed race 10 Mbps ndash 10 Gbps

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Metcalfersquos Ethernetsketch

48

Star Topologybull bus topology popular through mid 90s

bull all nodes in same collision domain (can collide with each other)

bull today star topology prevailsbull active switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes do

not collide with each other)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

switch

bus coaxial cable star

49

Ethernet Frame Structurebull Sending adapter encapsulates IP datagram

(or other network layer protocol packet) in Ethernet frame

Preamble bull 7 bytes with pattern 10101010 followed by

one byte with pattern 10101011bull used to synchronize receiver sender clock

ratesFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

50

Ethernet Frame Structure (more)bull Addresses 6 bytes

bull if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

bull otherwise adapter discards frame

bull Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)

bull CRC checked at receiver if error is detected frame is dropped

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51

Ethernet Unreliable connection-lessbull connectionless No handshaking be-

tween sending and receiving NICs bull unreliable receiving NIC doesnrsquot send

acks or nacks to sending NICbull stream of datagrams passed to network

layer can have gaps (missing datagrams)bull gaps will be filled if app is using TCPbull otherwise app will see gaps

bull Ethernetrsquos MAC protocol unslotted CSMACD

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

52

Ethernet CSMACD algorithmbull 1 NIC receives data-

gram from network layer creates frame

bull 2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

bull 3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

bull 4 If NIC detects an-other transmission while transmitting aborts and sends jam signal

bull 5 After aborting NIC enters exponential backoff after mth colli-sion NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

53

Ethernetrsquos CSMACD (more)bull Jam Signal make

sure all other transmitters are aware of collision 48 bits

bull Bit time 1 micro sec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Exponential Backoff bull Goal adapt retransmission

attempts to estimated cur-rent load

bull heavy load random wait will be longer

bull first collision choose K from 01 delay is K 512 bit transmission times

bull after second collision choose K from 0123hellip

bull after ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

54

CSMACD efficiencybull Tprop = max prop delay between 2 nodes in

LANbull ttrans = time to transmit max-size frame

bull efficiency goes to 1 bull as tprop goes to 0

bull as ttrans goes to infinity

bull better performance than ALOHA and simple cheap decentralized

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

transprop ttefficiency

51

1

55

8023 Ethernet Standards Link amp Physical Layersbull many different Ethernet standards

bull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps

1Gbps 10G bpsbull different physical layer media fiber cable

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

applicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twisterpair) physical layer

56

Manchester encodingbull used in 10BaseTbull each bit has a transitionbull allows clocks in sending and receiving nodes to syn-

chronize to each otherbull no need for a centralized global clock among nodes

bull Hey this is physical-layer stuff

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

57

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

58

Hubshellip physical-layer (ldquodumbrdquo) repeaters

bull bits coming in one link go out all other links at same rate

bull all nodes connected to hub can collide with one another

bull no frame bufferingbull no CSMACD at hub host NICs detect colli-

sions

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

twisted pair

hub

59

Switchbull link-layer device smarter than hubs take

active rolebull store forward Ethernet framesbull examine incoming framersquos MAC address se-

lectively forward frame to one-or-more out-going links when frame is to be forwarded on segment uses CSMACD to access segment

bull transparentbull hosts are unaware of presence of switches

bull plug-and-play self-learning (HOW)bull switches do not need to be configured

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

60

Switch allows multiple simulta-neous transmissionsbull hosts have dedicated direct

connection to switchbull switches buffer packetsbull Ethernet protocol used on

each incoming link but no collisions full duplexbull each link is its own collision

domain

bull switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions bull not possible with dumb hub

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 23

45

6

61

Switch Tablebull Q how does switch know

that Arsquo reachable via inter-face 4 Brsquo reachable via inter-face 5

bull A each switch has a switch table each entrybull (MAC address of host interface

to reach host time stamp)

bull looks like a routing tablebull Q how are entries created

maintained in switch table bull something like a routing proto-

colFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

switch with six interfaces(123456)

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

62

Switch self-learningbull switch learns which

hosts can be reached through which inter-facesbull when frame received

switch ldquolearnsrdquo location of sender incoming LAN segment

bull records senderlocation pair in switch table

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

63

Switch frame filteringforward-ingWhen frame received

1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination then if dest on segment from which frame arrived then drop the frame else

forward the frame on interface indicated else flood

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

forward on all but the interface on which the frame arrived

64

Self-learning forwarding exam-ple

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

frame destination unknownflood

Arsquo A

destination A location known

Arsquo 4 60

selective send

65

Interconnecting switchesbull switches can be connected together

bull Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3

bull A self learning (works exactly the same as in single-switch case)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

B

S1

C D

E

FS2

S4

S3

H

I

G

66

Self-learning multi-switch exam-plebull Suppose C sends frame to I I responds to

C

bull Q show switch tables and packet forward-ing in S1 S2 S3 S4

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

B

S1

CD

E F

S2

S4

S3

H

I

G

12

1

2 3

3

1 2 3 12 3

444

67

Institutional network

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

to externalnetwork

router

IP subnet

mail server

web server

68

Switches vs Routersbull both store-and-forward devices

bull routers network layer devices (examine network layer head-ers)

bull switches are link layer devices

bull routers maintain routing tables implement routing al-gorithms

bull switches maintain switch tables implement filtering learning algorithms

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

69

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

• Fall 2015 COMP 46055605 Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link layer context
• Link Layer Services
• Link Layer Services (more)
• Where is the link layer implemented
• Adaptors Communicating
• Link Layer (2)
• Error Detection
• Parity Checking
• Internet checksum (review)
• Checksumming Cyclic Redundancy Check
• Checksumming Cyclic Redundancy Check (2)
• CRC Example
• Link Layer (3)
• Multiple Access Links and Protocols
• Multiple Access protocols
• Ideal Multiple Access Protocol
• MAC Protocols a taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD collision detection
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Link Layer (4)
• MAC Addresses and ARP
• LAN Addresses and ARP
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP protocol Same LAN (network)
• Addressing routing to another LAN
• Slide 45
• Link Layer (5)
• Ethernet
• Star Topology
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester encoding
• Link Layer (6)
• Hubs
• Switch
• Switch allows multiple simultaneous transmissions
• Switch Table
• Switch self-learning
• Switch frame filteringforwarding
• Self-learning forwarding example
• Interconnecting switches
• Self-learning multi-switch example
• Institutional network
• Switches vs Routers
• Link Layer (7)

16

CRC Example

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

WantD2r XOR R = nG

equivalentlyD2r = nG XOR R

equivalently if we divide D2r by

G want remainder R

R = remainder[ ]D2r

G

17

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

18

Multiple Access Links and Proto-colsTwo types of ldquolinksrdquobull point-to-point

bull PPP for dial-up accessbull point-to-point link between Ethernet switch and host

bull broadcast (shared wire or medium)bull old-fashioned Ethernetbull upstream HFC (Hybrid Fiber-Coaxial)bull 80211 wireless LAN

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

shared wire (eg cabled Ethernet)

shared RF (eg 80211 WiFi)

shared RF(satellite)

humans at acocktail party

(shared air acoustical)

19

Multiple Access protocolsbull single shared broadcast channel bull two or more simultaneous transmis-

sions by nodes interference bull collision if a node receives two or more

signals at the same time

multiple access protocolbull distributed algorithm that deter-

mines how nodes share channel ie determine when node can transmit

bull communication about channel shar-ing must use channel itself bull no out-of-band channel for coordination

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

20

Ideal Multiple Access ProtocolBroadcast channel of rate R bps1 when one node wants to transmit it

can send at rate R2 when M nodes want to transmit each

can send at average rate RM3 fully decentralized

bull no special node to coordinate transmissionsbull no synchronization of clocks slots

4 simple

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

21

MAC Protocols a taxonomyThree broad classesbull Channel Partitioning

bull divide channel into smaller ldquopiecesrdquo (time slots frequency code)

bull allocate piece to node for exclusive use

bull Random Accessbull channel not divided allow collisionsbull ldquorecoverrdquo from collisions

bull ldquoTaking turnsrdquobull nodes take turns but nodes with more to send

can take longer turnsFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

22

Channel Partitioning MAC proto-cols TDMATDMA time division multiple access bull access to channel in rounds bull each station gets fixed length slot

(length = pkt trans time) in each round bull unused slots go idle bull example 6-station LAN 134 have pkt

slots 256 idle

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

1 3 4 1 3 4

6-slotframe

23

Channel Partitioning MAC proto-cols FDMAFDMA frequency division multiple access bull channel spectrum divided into frequency bandsbull each station assigned fixed frequency bandbull unused transmission time in frequency bands go idle bull example 6-station LAN 134 have pkt frequency

bands 256 idle

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

freq

uency

bands time

FDM cable

24

Random Access Protocolsbull When node has packet to send

bull transmit at full channel data rate Rbull no a priori coordination among nodes

bull two or more transmitting nodes ldquocollisionrdquobull random access MAC protocol specifies

bull how to detect collisionsbull how to recover from collisions (eg via delayed retrans-

missions)

bull Examples of random access MAC protocolsbull slotted ALOHAbull ALOHAbull CSMA CSMACD CSMACA

bull (CS Carrier Sensing MA Multiple AccessCD Collision Detection CA Collision Avoidance)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

25

Slotted ALOHAAssumptionsbull all frames same sizebull time divided into

equal size slots (time to transmit 1 frame)

bull nodes start to transmit only slot beginning

bull nodes are synchro-nized

bull if 2 or more nodes transmit in slot all nodes detect collision

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Operationbull when node obtains

fresh frame trans-mits in next slotbull if no collision node

can send new frame in next slot

bull if collision node re-transmits frame in each subsequent slot with prob p until success

26

Slotted ALOHA

Prosbull single active node can

continuously transmit at full rate of channel

bull highly decentralized only slots in nodes need to be in sync

bull simpleFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Consbull collisions wasting slotsbull idle slotsbull nodes may be able to

detect collision in less than time to transmit packet

bull clock synchronization

27

Slotted Aloha Efficiency

bull suppose N nodes with many frames to send each transmits in slot with probability p

bull prob that given node has success in a slot = p(1-p)N-1

bull prob that any node has a success = Np(1-p)N-1

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

bull max efficiency find p that max-imizes Np(1-p)N-1

bull for many nodes take limit of Np(1- p)N-1 as N goes to infinity gives

Max efficiency = 1e = 37

Efficiency long-run fraction of successful slots (many nodes all with many frames to send)

At best channelused for useful transmissions 37of time

28

Pure (unslotted) ALOHAbull unslotted Aloha simpler no synchro-

nizationbull when frame first arrives

bull transmit immediately

bull collision probability increasesbull frame sent at t0 collides with other frames sent in

[t0-1t0+1]

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

29

Pure Aloha EfficiencyP(success by given node) = P(node transmits)

P(no other node transmits in [t0-1t0]

P(no other node transmits in [t0t0+1]

= p (1-p)N-1 (1-p)N-1

= p (1-p)2(N-1)

hellip choosing optimum p and then letting

= 1(2e) = 18

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

N

even worse than slotted Aloha

30

CSMA (Carrier Sense Multiple Access)CSMA listen before transmitIf channel sensed idle transmit entire framebull If channel sensed busy defer transmis-

sion

bull human analogy donrsquot interrupt others

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

31

CSMA Collisionscollisions can still occurbull propagation delay means

two nodes may not hear each otherrsquos transmission

collisionbull entire packet transmis-

sion time wasted

notebull role of distance amp propa-

gation delay in determin-ing collision probability

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

spatial layout of nodes

32

CSMACD (Collision Detec-tion)CSMACD carrier sensing deferral as in CSMA

bull collisions detected within short timebull colliding transmissions aborted reducing channel

wastage

bull collision detection bull easy in wired LANs measure signal strengths

compare transmitted received signalsbull difficult in wireless LANs received signal strength

overwhelmed by local transmission strength

bull human analogy the polite conversationalist

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

33

CSMACD collision detection

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

34

ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols

bull share channel efficiently and fairly at high loadbull inefficient at low load delay in channel access

1N bandwidth allocated even if only 1 active node

Random access MAC protocolsbull efficient at low load single node can fully uti-

lize channelbull high load collision overhead

ldquotaking turnsrdquo protocolsbull look for best of both worlds

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

35

ldquoTaking Turnsrdquo MAC protocolsPolling bull master node ldquoinvitesrdquo

slave nodes to transmit in turn

bull typically used with ldquodumbrdquo slave devices

bull concernsbull polling overhead bull latencybull single point of failure

(master)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

T

data

(nothingto send)

T

36

ldquoTaking Turnsrdquo MAC protocolsToken passingbull control token

passed from one node to next se-quentially

bull token messagebull concerns

bull token overhead bull latencybull single point of failure

(token)Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

master

slaves

poll

data

data

37

Summary of MAC protocolsbull channel partitioning by time frequency or

codebull Time Division Frequency Division

bull random access (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire)

hard in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211

bull taking turnsbull polling from central site token passing

bull Bluetooth FDDI IBM Token Ring Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

38

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

39

MAC Addresses and ARPbull 32-bit IP address

bull network-layer addressbull used to get datagram to destination IP subnet

bull MAC (or LAN or physical or Ethernet) ad-dress bull function get frame from one interface to an-

other physically-connected interface (same network)

bull 48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software set-

table

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

40

LAN Addresses and ARP

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Each adapter on LAN has unique LAN address

Broadcast address =FF-FF-FF-FF-FF-FF

= adapter

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired orwireless)

41

LAN Address (more)bull MAC address allocation administered by IEEEbull manufacturer buys portion of MAC address

space (to assure uniqueness)bull analogy

(a) MAC address like Social Security Num-ber

(b) IP address like postal addressbull MAC flat address portability

bull can move LAN card from one LAN to another

bull IP hierarchical address NOT portablebull address depends on IP subnet to which node is at-

tachedFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

42

ARP Address Resolution Proto-colbull Each IP node (host

router) on LAN has ARP table

bull ARP table IPMAC address mappings for some LAN nodeslt IP address MAC address

TTLgt

bull TTL (Time To Live) time after which ad-dress mapping will be forgotten (typically 20 min)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Question how to determineMAC address of Bknowing Brsquos IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN

137196723

137196778

137196714

137196788

43

ARP protocol Same LAN (net-work)bull A wants to send datagram

to B and Brsquos MAC address not in Arsquos ARP table

bull A broadcasts ARP query packet containing Bs IP address bull dest MAC address = FF-FF-FF-

FF-FF-FFbull all machines on LAN receive

ARP query

bull B receives ARP packet replies to A with its (Bs) MAC addressbull frame sent to Arsquos MAC ad-

dress (unicast)Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

bull A caches (saves) IP-to-MAC address pair in its ARP table until infor-mation becomes old (times out) bull soft state information

that times out (goes away) unless refreshed

bull ARP is ldquoplug-and-playrdquobull nodes create their ARP

tables without interven-tion from net administra-tor

44

Addressing routing to another LAN

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

R

1A-23-F9-CD-06-9B

222222222220111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

walkthrough send datagram from A to B via R assume A knows Brsquos IP address

bull two ARP tables in router R one for each IP network (LAN)

45

bull A creates IP datagram with source A destination B bull A uses ARP to get Rrsquos MAC address for 111111111110bull A creates link-layer frame with Rs MAC address as dest

frame contains A-to-B IP datagrambull Arsquos NIC sends frame bull Rrsquos NIC receives frame bull R removes IP datagram from Ethernet frame sees its des-

tined to Bbull R uses ARP to get Brsquos MAC address bull R creates frame containing A-to-B IP datagram sends to B

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

This is a really importantexample ndash make sure youunderstand

R

1A-23-F9-CD-06-9B

222222222220

111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

46

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

47

Ethernetldquodominantrdquo wired LAN technology bull cheap $20 for NICbull first widely used LAN technologybull simpler cheaper than token LANs and ATMbull kept up with speed race 10 Mbps ndash 10 Gbps

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Metcalfersquos Ethernetsketch

48

Star Topologybull bus topology popular through mid 90s

bull all nodes in same collision domain (can collide with each other)

bull today star topology prevailsbull active switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes do

not collide with each other)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

switch

bus coaxial cable star

49

Ethernet Frame Structurebull Sending adapter encapsulates IP datagram

(or other network layer protocol packet) in Ethernet frame

Preamble bull 7 bytes with pattern 10101010 followed by

one byte with pattern 10101011bull used to synchronize receiver sender clock

ratesFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

50

Ethernet Frame Structure (more)bull Addresses 6 bytes

bull if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

bull otherwise adapter discards frame

bull Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)

bull CRC checked at receiver if error is detected frame is dropped

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51

Ethernet Unreliable connection-lessbull connectionless No handshaking be-

tween sending and receiving NICs bull unreliable receiving NIC doesnrsquot send

acks or nacks to sending NICbull stream of datagrams passed to network

layer can have gaps (missing datagrams)bull gaps will be filled if app is using TCPbull otherwise app will see gaps

bull Ethernetrsquos MAC protocol unslotted CSMACD

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

52

Ethernet CSMACD algorithmbull 1 NIC receives data-

gram from network layer creates frame

bull 2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

bull 3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

bull 4 If NIC detects an-other transmission while transmitting aborts and sends jam signal

bull 5 After aborting NIC enters exponential backoff after mth colli-sion NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

53

Ethernetrsquos CSMACD (more)bull Jam Signal make

sure all other transmitters are aware of collision 48 bits

bull Bit time 1 micro sec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Exponential Backoff bull Goal adapt retransmission

attempts to estimated cur-rent load

bull heavy load random wait will be longer

bull first collision choose K from 01 delay is K 512 bit transmission times

bull after second collision choose K from 0123hellip

bull after ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

54

CSMACD efficiencybull Tprop = max prop delay between 2 nodes in

LANbull ttrans = time to transmit max-size frame

bull efficiency goes to 1 bull as tprop goes to 0

bull as ttrans goes to infinity

bull better performance than ALOHA and simple cheap decentralized

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

transprop ttefficiency

51

1

55

8023 Ethernet Standards Link amp Physical Layersbull many different Ethernet standards

bull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps

1Gbps 10G bpsbull different physical layer media fiber cable

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

applicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twisterpair) physical layer

56

Manchester encodingbull used in 10BaseTbull each bit has a transitionbull allows clocks in sending and receiving nodes to syn-

chronize to each otherbull no need for a centralized global clock among nodes

bull Hey this is physical-layer stuff

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

57

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

58

Hubshellip physical-layer (ldquodumbrdquo) repeaters

bull bits coming in one link go out all other links at same rate

bull all nodes connected to hub can collide with one another

bull no frame bufferingbull no CSMACD at hub host NICs detect colli-

sions

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

twisted pair

hub

59

Switchbull link-layer device smarter than hubs take

active rolebull store forward Ethernet framesbull examine incoming framersquos MAC address se-

lectively forward frame to one-or-more out-going links when frame is to be forwarded on segment uses CSMACD to access segment

bull transparentbull hosts are unaware of presence of switches

bull plug-and-play self-learning (HOW)bull switches do not need to be configured

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

60

Switch allows multiple simulta-neous transmissionsbull hosts have dedicated direct

connection to switchbull switches buffer packetsbull Ethernet protocol used on

each incoming link but no collisions full duplexbull each link is its own collision

domain

bull switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions bull not possible with dumb hub

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 23

45

6

61

Switch Tablebull Q how does switch know

that Arsquo reachable via inter-face 4 Brsquo reachable via inter-face 5

bull A each switch has a switch table each entrybull (MAC address of host interface

to reach host time stamp)

bull looks like a routing tablebull Q how are entries created

maintained in switch table bull something like a routing proto-

colFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

switch with six interfaces(123456)

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

62

Switch self-learningbull switch learns which

hosts can be reached through which inter-facesbull when frame received

switch ldquolearnsrdquo location of sender incoming LAN segment

bull records senderlocation pair in switch table

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

63

Switch frame filteringforward-ingWhen frame received

1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination then if dest on segment from which frame arrived then drop the frame else

forward the frame on interface indicated else flood

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

forward on all but the interface on which the frame arrived

64

Self-learning forwarding exam-ple

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

frame destination unknownflood

Arsquo A

destination A location known

Arsquo 4 60

selective send

65

Interconnecting switchesbull switches can be connected together

bull Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3

bull A self learning (works exactly the same as in single-switch case)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

B

S1

C D

E

FS2

S4

S3

H

I

G

66

Self-learning multi-switch exam-plebull Suppose C sends frame to I I responds to

C

bull Q show switch tables and packet forward-ing in S1 S2 S3 S4

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

B

S1

CD

E F

S2

S4

S3

H

I

G

12

1

2 3

3

1 2 3 12 3

444

67

Institutional network

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

to externalnetwork

router

IP subnet

mail server

web server

68

Switches vs Routersbull both store-and-forward devices

bull routers network layer devices (examine network layer head-ers)

bull switches are link layer devices

bull routers maintain routing tables implement routing al-gorithms

bull switches maintain switch tables implement filtering learning algorithms

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

69

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

• Fall 2015 COMP 46055605 Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link layer context
• Link Layer Services
• Link Layer Services (more)
• Where is the link layer implemented
• Adaptors Communicating
• Link Layer (2)
• Error Detection
• Parity Checking
• Internet checksum (review)
• Checksumming Cyclic Redundancy Check
• Checksumming Cyclic Redundancy Check (2)
• CRC Example
• Link Layer (3)
• Multiple Access Links and Protocols
• Multiple Access protocols
• Ideal Multiple Access Protocol
• MAC Protocols a taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD collision detection
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Link Layer (4)
• MAC Addresses and ARP
• LAN Addresses and ARP
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP protocol Same LAN (network)
• Addressing routing to another LAN
• Slide 45
• Link Layer (5)
• Ethernet
• Star Topology
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester encoding
• Link Layer (6)
• Hubs
• Switch
• Switch allows multiple simultaneous transmissions
• Switch Table
• Switch self-learning
• Switch frame filteringforwarding
• Self-learning forwarding example
• Interconnecting switches
• Self-learning multi-switch example
• Institutional network
• Switches vs Routers
• Link Layer (7)

17

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

18

Multiple Access Links and Proto-colsTwo types of ldquolinksrdquobull point-to-point

bull PPP for dial-up accessbull point-to-point link between Ethernet switch and host

bull broadcast (shared wire or medium)bull old-fashioned Ethernetbull upstream HFC (Hybrid Fiber-Coaxial)bull 80211 wireless LAN

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

shared wire (eg cabled Ethernet)

shared RF (eg 80211 WiFi)

shared RF(satellite)

humans at acocktail party

(shared air acoustical)

19

Multiple Access protocolsbull single shared broadcast channel bull two or more simultaneous transmis-

sions by nodes interference bull collision if a node receives two or more

signals at the same time

multiple access protocolbull distributed algorithm that deter-

mines how nodes share channel ie determine when node can transmit

bull communication about channel shar-ing must use channel itself bull no out-of-band channel for coordination

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

20

Ideal Multiple Access ProtocolBroadcast channel of rate R bps1 when one node wants to transmit it

can send at rate R2 when M nodes want to transmit each

can send at average rate RM3 fully decentralized

bull no special node to coordinate transmissionsbull no synchronization of clocks slots

4 simple

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

21

MAC Protocols a taxonomyThree broad classesbull Channel Partitioning

bull divide channel into smaller ldquopiecesrdquo (time slots frequency code)

bull allocate piece to node for exclusive use

bull Random Accessbull channel not divided allow collisionsbull ldquorecoverrdquo from collisions

bull ldquoTaking turnsrdquobull nodes take turns but nodes with more to send

can take longer turnsFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

22

Channel Partitioning MAC proto-cols TDMATDMA time division multiple access bull access to channel in rounds bull each station gets fixed length slot

(length = pkt trans time) in each round bull unused slots go idle bull example 6-station LAN 134 have pkt

slots 256 idle

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

1 3 4 1 3 4

6-slotframe

23

Channel Partitioning MAC proto-cols FDMAFDMA frequency division multiple access bull channel spectrum divided into frequency bandsbull each station assigned fixed frequency bandbull unused transmission time in frequency bands go idle bull example 6-station LAN 134 have pkt frequency

bands 256 idle

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

freq

uency

bands time

FDM cable

24

Random Access Protocolsbull When node has packet to send

bull transmit at full channel data rate Rbull no a priori coordination among nodes

bull two or more transmitting nodes ldquocollisionrdquobull random access MAC protocol specifies

bull how to detect collisionsbull how to recover from collisions (eg via delayed retrans-

missions)

bull Examples of random access MAC protocolsbull slotted ALOHAbull ALOHAbull CSMA CSMACD CSMACA

bull (CS Carrier Sensing MA Multiple AccessCD Collision Detection CA Collision Avoidance)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

25

Slotted ALOHAAssumptionsbull all frames same sizebull time divided into

equal size slots (time to transmit 1 frame)

bull nodes start to transmit only slot beginning

bull nodes are synchro-nized

bull if 2 or more nodes transmit in slot all nodes detect collision

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Operationbull when node obtains

fresh frame trans-mits in next slotbull if no collision node

can send new frame in next slot

bull if collision node re-transmits frame in each subsequent slot with prob p until success

26

Slotted ALOHA

Prosbull single active node can

continuously transmit at full rate of channel

bull highly decentralized only slots in nodes need to be in sync

bull simpleFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Consbull collisions wasting slotsbull idle slotsbull nodes may be able to

detect collision in less than time to transmit packet

bull clock synchronization

27

Slotted Aloha Efficiency

bull suppose N nodes with many frames to send each transmits in slot with probability p

bull prob that given node has success in a slot = p(1-p)N-1

bull prob that any node has a success = Np(1-p)N-1

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

bull max efficiency find p that max-imizes Np(1-p)N-1

bull for many nodes take limit of Np(1- p)N-1 as N goes to infinity gives

Max efficiency = 1e = 37

Efficiency long-run fraction of successful slots (many nodes all with many frames to send)

At best channelused for useful transmissions 37of time

28

Pure (unslotted) ALOHAbull unslotted Aloha simpler no synchro-

nizationbull when frame first arrives

bull transmit immediately

bull collision probability increasesbull frame sent at t0 collides with other frames sent in

[t0-1t0+1]

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

29

Pure Aloha EfficiencyP(success by given node) = P(node transmits)

P(no other node transmits in [t0-1t0]

P(no other node transmits in [t0t0+1]

= p (1-p)N-1 (1-p)N-1

= p (1-p)2(N-1)

hellip choosing optimum p and then letting

= 1(2e) = 18

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

N

even worse than slotted Aloha

30

CSMA (Carrier Sense Multiple Access)CSMA listen before transmitIf channel sensed idle transmit entire framebull If channel sensed busy defer transmis-

sion

bull human analogy donrsquot interrupt others

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

31

CSMA Collisionscollisions can still occurbull propagation delay means

two nodes may not hear each otherrsquos transmission

collisionbull entire packet transmis-

sion time wasted

notebull role of distance amp propa-

gation delay in determin-ing collision probability

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

spatial layout of nodes

32

CSMACD (Collision Detec-tion)CSMACD carrier sensing deferral as in CSMA

bull collisions detected within short timebull colliding transmissions aborted reducing channel

wastage

bull collision detection bull easy in wired LANs measure signal strengths

compare transmitted received signalsbull difficult in wireless LANs received signal strength

overwhelmed by local transmission strength

bull human analogy the polite conversationalist

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

33

CSMACD collision detection

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

34

ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols

bull share channel efficiently and fairly at high loadbull inefficient at low load delay in channel access

1N bandwidth allocated even if only 1 active node

Random access MAC protocolsbull efficient at low load single node can fully uti-

lize channelbull high load collision overhead

ldquotaking turnsrdquo protocolsbull look for best of both worlds

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

35

ldquoTaking Turnsrdquo MAC protocolsPolling bull master node ldquoinvitesrdquo

slave nodes to transmit in turn

bull typically used with ldquodumbrdquo slave devices

bull concernsbull polling overhead bull latencybull single point of failure

(master)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

T

data

(nothingto send)

T

36

ldquoTaking Turnsrdquo MAC protocolsToken passingbull control token

passed from one node to next se-quentially

bull token messagebull concerns

bull token overhead bull latencybull single point of failure

(token)Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

master

slaves

poll

data

data

37

Summary of MAC protocolsbull channel partitioning by time frequency or

codebull Time Division Frequency Division

bull random access (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire)

hard in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211

bull taking turnsbull polling from central site token passing

bull Bluetooth FDDI IBM Token Ring Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

38

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

39

MAC Addresses and ARPbull 32-bit IP address

bull network-layer addressbull used to get datagram to destination IP subnet

bull MAC (or LAN or physical or Ethernet) ad-dress bull function get frame from one interface to an-

other physically-connected interface (same network)

bull 48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software set-

table

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

40

LAN Addresses and ARP

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Each adapter on LAN has unique LAN address

Broadcast address =FF-FF-FF-FF-FF-FF

= adapter

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired orwireless)

41

LAN Address (more)bull MAC address allocation administered by IEEEbull manufacturer buys portion of MAC address

space (to assure uniqueness)bull analogy

(a) MAC address like Social Security Num-ber

(b) IP address like postal addressbull MAC flat address portability

bull can move LAN card from one LAN to another

bull IP hierarchical address NOT portablebull address depends on IP subnet to which node is at-

tachedFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

42

ARP Address Resolution Proto-colbull Each IP node (host

router) on LAN has ARP table

bull ARP table IPMAC address mappings for some LAN nodeslt IP address MAC address

TTLgt

bull TTL (Time To Live) time after which ad-dress mapping will be forgotten (typically 20 min)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Question how to determineMAC address of Bknowing Brsquos IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN

137196723

137196778

137196714

137196788

43

ARP protocol Same LAN (net-work)bull A wants to send datagram

to B and Brsquos MAC address not in Arsquos ARP table

bull A broadcasts ARP query packet containing Bs IP address bull dest MAC address = FF-FF-FF-

FF-FF-FFbull all machines on LAN receive

ARP query

bull B receives ARP packet replies to A with its (Bs) MAC addressbull frame sent to Arsquos MAC ad-

dress (unicast)Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

bull A caches (saves) IP-to-MAC address pair in its ARP table until infor-mation becomes old (times out) bull soft state information

that times out (goes away) unless refreshed

bull ARP is ldquoplug-and-playrdquobull nodes create their ARP

tables without interven-tion from net administra-tor

44

Addressing routing to another LAN

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

R

1A-23-F9-CD-06-9B

222222222220111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

walkthrough send datagram from A to B via R assume A knows Brsquos IP address

bull two ARP tables in router R one for each IP network (LAN)

45

bull A creates IP datagram with source A destination B bull A uses ARP to get Rrsquos MAC address for 111111111110bull A creates link-layer frame with Rs MAC address as dest

frame contains A-to-B IP datagrambull Arsquos NIC sends frame bull Rrsquos NIC receives frame bull R removes IP datagram from Ethernet frame sees its des-

tined to Bbull R uses ARP to get Brsquos MAC address bull R creates frame containing A-to-B IP datagram sends to B

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

This is a really importantexample ndash make sure youunderstand

R

1A-23-F9-CD-06-9B

222222222220

111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

46

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

47

Ethernetldquodominantrdquo wired LAN technology bull cheap $20 for NICbull first widely used LAN technologybull simpler cheaper than token LANs and ATMbull kept up with speed race 10 Mbps ndash 10 Gbps

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Metcalfersquos Ethernetsketch

48

Star Topologybull bus topology popular through mid 90s

bull all nodes in same collision domain (can collide with each other)

bull today star topology prevailsbull active switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes do

not collide with each other)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

switch

bus coaxial cable star

49

Ethernet Frame Structurebull Sending adapter encapsulates IP datagram

(or other network layer protocol packet) in Ethernet frame

Preamble bull 7 bytes with pattern 10101010 followed by

one byte with pattern 10101011bull used to synchronize receiver sender clock

ratesFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

50

Ethernet Frame Structure (more)bull Addresses 6 bytes

bull if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

bull otherwise adapter discards frame

bull Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)

bull CRC checked at receiver if error is detected frame is dropped

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51

Ethernet Unreliable connection-lessbull connectionless No handshaking be-

tween sending and receiving NICs bull unreliable receiving NIC doesnrsquot send

acks or nacks to sending NICbull stream of datagrams passed to network

layer can have gaps (missing datagrams)bull gaps will be filled if app is using TCPbull otherwise app will see gaps

bull Ethernetrsquos MAC protocol unslotted CSMACD

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

52

Ethernet CSMACD algorithmbull 1 NIC receives data-

gram from network layer creates frame

bull 2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

bull 3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

bull 4 If NIC detects an-other transmission while transmitting aborts and sends jam signal

bull 5 After aborting NIC enters exponential backoff after mth colli-sion NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

53

Ethernetrsquos CSMACD (more)bull Jam Signal make

sure all other transmitters are aware of collision 48 bits

bull Bit time 1 micro sec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Exponential Backoff bull Goal adapt retransmission

attempts to estimated cur-rent load

bull heavy load random wait will be longer

bull first collision choose K from 01 delay is K 512 bit transmission times

bull after second collision choose K from 0123hellip

bull after ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

54

CSMACD efficiencybull Tprop = max prop delay between 2 nodes in

LANbull ttrans = time to transmit max-size frame

bull efficiency goes to 1 bull as tprop goes to 0

bull as ttrans goes to infinity

bull better performance than ALOHA and simple cheap decentralized

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

transprop ttefficiency

51

1

55

8023 Ethernet Standards Link amp Physical Layersbull many different Ethernet standards

bull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps

1Gbps 10G bpsbull different physical layer media fiber cable

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

applicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twisterpair) physical layer

56

Manchester encodingbull used in 10BaseTbull each bit has a transitionbull allows clocks in sending and receiving nodes to syn-

chronize to each otherbull no need for a centralized global clock among nodes

bull Hey this is physical-layer stuff

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

57

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

58

Hubshellip physical-layer (ldquodumbrdquo) repeaters

bull bits coming in one link go out all other links at same rate

bull all nodes connected to hub can collide with one another

bull no frame bufferingbull no CSMACD at hub host NICs detect colli-

sions

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

twisted pair

hub

59

Switchbull link-layer device smarter than hubs take

active rolebull store forward Ethernet framesbull examine incoming framersquos MAC address se-

lectively forward frame to one-or-more out-going links when frame is to be forwarded on segment uses CSMACD to access segment

bull transparentbull hosts are unaware of presence of switches

bull plug-and-play self-learning (HOW)bull switches do not need to be configured

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

60

Switch allows multiple simulta-neous transmissionsbull hosts have dedicated direct

connection to switchbull switches buffer packetsbull Ethernet protocol used on

each incoming link but no collisions full duplexbull each link is its own collision

domain

bull switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions bull not possible with dumb hub

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 23

45

6

61

Switch Tablebull Q how does switch know

that Arsquo reachable via inter-face 4 Brsquo reachable via inter-face 5

bull A each switch has a switch table each entrybull (MAC address of host interface

to reach host time stamp)

bull looks like a routing tablebull Q how are entries created

maintained in switch table bull something like a routing proto-

colFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

switch with six interfaces(123456)

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

62

Switch self-learningbull switch learns which

hosts can be reached through which inter-facesbull when frame received

switch ldquolearnsrdquo location of sender incoming LAN segment

bull records senderlocation pair in switch table

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

63

Switch frame filteringforward-ingWhen frame received

1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination then if dest on segment from which frame arrived then drop the frame else

forward the frame on interface indicated else flood

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

forward on all but the interface on which the frame arrived

64

Self-learning forwarding exam-ple

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

frame destination unknownflood

Arsquo A

destination A location known

Arsquo 4 60

selective send

65

Interconnecting switchesbull switches can be connected together

bull Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3

bull A self learning (works exactly the same as in single-switch case)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

B

S1

C D

E

FS2

S4

S3

H

I

G

66

Self-learning multi-switch exam-plebull Suppose C sends frame to I I responds to

C

bull Q show switch tables and packet forward-ing in S1 S2 S3 S4

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

B

S1

CD

E F

S2

S4

S3

H

I

G

12

1

2 3

3

1 2 3 12 3

444

67

Institutional network

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

to externalnetwork

router

IP subnet

mail server

web server

68

Switches vs Routersbull both store-and-forward devices

bull routers network layer devices (examine network layer head-ers)

bull switches are link layer devices

bull routers maintain routing tables implement routing al-gorithms

bull switches maintain switch tables implement filtering learning algorithms

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

69

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

• Fall 2015 COMP 46055605 Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link layer context
• Link Layer Services
• Link Layer Services (more)
• Where is the link layer implemented
• Adaptors Communicating
• Link Layer (2)
• Error Detection
• Parity Checking
• Internet checksum (review)
• Checksumming Cyclic Redundancy Check
• Checksumming Cyclic Redundancy Check (2)
• CRC Example
• Link Layer (3)
• Multiple Access Links and Protocols
• Multiple Access protocols
• Ideal Multiple Access Protocol
• MAC Protocols a taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD collision detection
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Link Layer (4)
• MAC Addresses and ARP
• LAN Addresses and ARP
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP protocol Same LAN (network)
• Addressing routing to another LAN
• Slide 45
• Link Layer (5)
• Ethernet
• Star Topology
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester encoding
• Link Layer (6)
• Hubs
• Switch
• Switch allows multiple simultaneous transmissions
• Switch Table
• Switch self-learning
• Switch frame filteringforwarding
• Self-learning forwarding example
• Interconnecting switches
• Self-learning multi-switch example
• Institutional network
• Switches vs Routers
• Link Layer (7)

18

Multiple Access Links and Proto-colsTwo types of ldquolinksrdquobull point-to-point

bull PPP for dial-up accessbull point-to-point link between Ethernet switch and host

bull broadcast (shared wire or medium)bull old-fashioned Ethernetbull upstream HFC (Hybrid Fiber-Coaxial)bull 80211 wireless LAN

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

shared wire (eg cabled Ethernet)

shared RF (eg 80211 WiFi)

shared RF(satellite)

humans at acocktail party

(shared air acoustical)

19

Multiple Access protocolsbull single shared broadcast channel bull two or more simultaneous transmis-

sions by nodes interference bull collision if a node receives two or more

signals at the same time

multiple access protocolbull distributed algorithm that deter-

mines how nodes share channel ie determine when node can transmit

bull communication about channel shar-ing must use channel itself bull no out-of-band channel for coordination

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

20

Ideal Multiple Access ProtocolBroadcast channel of rate R bps1 when one node wants to transmit it

can send at rate R2 when M nodes want to transmit each

can send at average rate RM3 fully decentralized

bull no special node to coordinate transmissionsbull no synchronization of clocks slots

4 simple

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

21

MAC Protocols a taxonomyThree broad classesbull Channel Partitioning

bull divide channel into smaller ldquopiecesrdquo (time slots frequency code)

bull allocate piece to node for exclusive use

bull Random Accessbull channel not divided allow collisionsbull ldquorecoverrdquo from collisions

bull ldquoTaking turnsrdquobull nodes take turns but nodes with more to send

can take longer turnsFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

22

Channel Partitioning MAC proto-cols TDMATDMA time division multiple access bull access to channel in rounds bull each station gets fixed length slot

(length = pkt trans time) in each round bull unused slots go idle bull example 6-station LAN 134 have pkt

slots 256 idle

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

1 3 4 1 3 4

6-slotframe

23

Channel Partitioning MAC proto-cols FDMAFDMA frequency division multiple access bull channel spectrum divided into frequency bandsbull each station assigned fixed frequency bandbull unused transmission time in frequency bands go idle bull example 6-station LAN 134 have pkt frequency

bands 256 idle

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

freq

uency

bands time

FDM cable

24

Random Access Protocolsbull When node has packet to send

bull transmit at full channel data rate Rbull no a priori coordination among nodes

bull two or more transmitting nodes ldquocollisionrdquobull random access MAC protocol specifies

bull how to detect collisionsbull how to recover from collisions (eg via delayed retrans-

missions)

bull Examples of random access MAC protocolsbull slotted ALOHAbull ALOHAbull CSMA CSMACD CSMACA

bull (CS Carrier Sensing MA Multiple AccessCD Collision Detection CA Collision Avoidance)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

25

Slotted ALOHAAssumptionsbull all frames same sizebull time divided into

equal size slots (time to transmit 1 frame)

bull nodes start to transmit only slot beginning

bull nodes are synchro-nized

bull if 2 or more nodes transmit in slot all nodes detect collision

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Operationbull when node obtains

fresh frame trans-mits in next slotbull if no collision node

can send new frame in next slot

bull if collision node re-transmits frame in each subsequent slot with prob p until success

26

Slotted ALOHA

Prosbull single active node can

continuously transmit at full rate of channel

bull highly decentralized only slots in nodes need to be in sync

bull simpleFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Consbull collisions wasting slotsbull idle slotsbull nodes may be able to

detect collision in less than time to transmit packet

bull clock synchronization

27

Slotted Aloha Efficiency

bull suppose N nodes with many frames to send each transmits in slot with probability p

bull prob that given node has success in a slot = p(1-p)N-1

bull prob that any node has a success = Np(1-p)N-1

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

bull max efficiency find p that max-imizes Np(1-p)N-1

bull for many nodes take limit of Np(1- p)N-1 as N goes to infinity gives

Max efficiency = 1e = 37

Efficiency long-run fraction of successful slots (many nodes all with many frames to send)

At best channelused for useful transmissions 37of time

28

Pure (unslotted) ALOHAbull unslotted Aloha simpler no synchro-

nizationbull when frame first arrives

bull transmit immediately

bull collision probability increasesbull frame sent at t0 collides with other frames sent in

[t0-1t0+1]

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

29

Pure Aloha EfficiencyP(success by given node) = P(node transmits)

P(no other node transmits in [t0-1t0]

P(no other node transmits in [t0t0+1]

= p (1-p)N-1 (1-p)N-1

= p (1-p)2(N-1)

hellip choosing optimum p and then letting

= 1(2e) = 18

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

N

even worse than slotted Aloha

30

CSMA (Carrier Sense Multiple Access)CSMA listen before transmitIf channel sensed idle transmit entire framebull If channel sensed busy defer transmis-

sion

bull human analogy donrsquot interrupt others

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

31

CSMA Collisionscollisions can still occurbull propagation delay means

two nodes may not hear each otherrsquos transmission

collisionbull entire packet transmis-

sion time wasted

notebull role of distance amp propa-

gation delay in determin-ing collision probability

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

spatial layout of nodes

32

CSMACD (Collision Detec-tion)CSMACD carrier sensing deferral as in CSMA

bull collisions detected within short timebull colliding transmissions aborted reducing channel

wastage

bull collision detection bull easy in wired LANs measure signal strengths

compare transmitted received signalsbull difficult in wireless LANs received signal strength

overwhelmed by local transmission strength

bull human analogy the polite conversationalist

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

33

CSMACD collision detection

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

34

ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols

bull share channel efficiently and fairly at high loadbull inefficient at low load delay in channel access

1N bandwidth allocated even if only 1 active node

Random access MAC protocolsbull efficient at low load single node can fully uti-

lize channelbull high load collision overhead

ldquotaking turnsrdquo protocolsbull look for best of both worlds

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

35

ldquoTaking Turnsrdquo MAC protocolsPolling bull master node ldquoinvitesrdquo

slave nodes to transmit in turn

bull typically used with ldquodumbrdquo slave devices

bull concernsbull polling overhead bull latencybull single point of failure

(master)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

T

data

(nothingto send)

T

36

ldquoTaking Turnsrdquo MAC protocolsToken passingbull control token

passed from one node to next se-quentially

bull token messagebull concerns

bull token overhead bull latencybull single point of failure

(token)Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

master

slaves

poll

data

data

37

Summary of MAC protocolsbull channel partitioning by time frequency or

codebull Time Division Frequency Division

bull random access (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire)

hard in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211

bull taking turnsbull polling from central site token passing

bull Bluetooth FDDI IBM Token Ring Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

38

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

39

MAC Addresses and ARPbull 32-bit IP address

bull network-layer addressbull used to get datagram to destination IP subnet

bull MAC (or LAN or physical or Ethernet) ad-dress bull function get frame from one interface to an-

other physically-connected interface (same network)

bull 48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software set-

table

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

40

LAN Addresses and ARP

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Each adapter on LAN has unique LAN address

Broadcast address =FF-FF-FF-FF-FF-FF

= adapter

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired orwireless)

41

LAN Address (more)bull MAC address allocation administered by IEEEbull manufacturer buys portion of MAC address

space (to assure uniqueness)bull analogy

(a) MAC address like Social Security Num-ber

(b) IP address like postal addressbull MAC flat address portability

bull can move LAN card from one LAN to another

bull IP hierarchical address NOT portablebull address depends on IP subnet to which node is at-

tachedFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

42

ARP Address Resolution Proto-colbull Each IP node (host

router) on LAN has ARP table

bull ARP table IPMAC address mappings for some LAN nodeslt IP address MAC address

TTLgt

bull TTL (Time To Live) time after which ad-dress mapping will be forgotten (typically 20 min)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Question how to determineMAC address of Bknowing Brsquos IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN

137196723

137196778

137196714

137196788

43

ARP protocol Same LAN (net-work)bull A wants to send datagram

to B and Brsquos MAC address not in Arsquos ARP table

bull A broadcasts ARP query packet containing Bs IP address bull dest MAC address = FF-FF-FF-

FF-FF-FFbull all machines on LAN receive

ARP query

bull B receives ARP packet replies to A with its (Bs) MAC addressbull frame sent to Arsquos MAC ad-

dress (unicast)Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

bull A caches (saves) IP-to-MAC address pair in its ARP table until infor-mation becomes old (times out) bull soft state information

that times out (goes away) unless refreshed

bull ARP is ldquoplug-and-playrdquobull nodes create their ARP

tables without interven-tion from net administra-tor

44

Addressing routing to another LAN

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

R

1A-23-F9-CD-06-9B

222222222220111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

walkthrough send datagram from A to B via R assume A knows Brsquos IP address

bull two ARP tables in router R one for each IP network (LAN)

45

bull A creates IP datagram with source A destination B bull A uses ARP to get Rrsquos MAC address for 111111111110bull A creates link-layer frame with Rs MAC address as dest

frame contains A-to-B IP datagrambull Arsquos NIC sends frame bull Rrsquos NIC receives frame bull R removes IP datagram from Ethernet frame sees its des-

tined to Bbull R uses ARP to get Brsquos MAC address bull R creates frame containing A-to-B IP datagram sends to B

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

This is a really importantexample ndash make sure youunderstand

R

1A-23-F9-CD-06-9B

222222222220

111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

46

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

47

Ethernetldquodominantrdquo wired LAN technology bull cheap $20 for NICbull first widely used LAN technologybull simpler cheaper than token LANs and ATMbull kept up with speed race 10 Mbps ndash 10 Gbps

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Metcalfersquos Ethernetsketch

48

Star Topologybull bus topology popular through mid 90s

bull all nodes in same collision domain (can collide with each other)

bull today star topology prevailsbull active switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes do

not collide with each other)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

switch

bus coaxial cable star

49

Ethernet Frame Structurebull Sending adapter encapsulates IP datagram

(or other network layer protocol packet) in Ethernet frame

Preamble bull 7 bytes with pattern 10101010 followed by

one byte with pattern 10101011bull used to synchronize receiver sender clock

ratesFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

50

Ethernet Frame Structure (more)bull Addresses 6 bytes

bull if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

bull otherwise adapter discards frame

bull Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)

bull CRC checked at receiver if error is detected frame is dropped

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51

Ethernet Unreliable connection-lessbull connectionless No handshaking be-

tween sending and receiving NICs bull unreliable receiving NIC doesnrsquot send

acks or nacks to sending NICbull stream of datagrams passed to network

layer can have gaps (missing datagrams)bull gaps will be filled if app is using TCPbull otherwise app will see gaps

bull Ethernetrsquos MAC protocol unslotted CSMACD

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

52

Ethernet CSMACD algorithmbull 1 NIC receives data-

gram from network layer creates frame

bull 2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

bull 3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

bull 4 If NIC detects an-other transmission while transmitting aborts and sends jam signal

bull 5 After aborting NIC enters exponential backoff after mth colli-sion NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

53

Ethernetrsquos CSMACD (more)bull Jam Signal make

sure all other transmitters are aware of collision 48 bits

bull Bit time 1 micro sec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Exponential Backoff bull Goal adapt retransmission

attempts to estimated cur-rent load

bull heavy load random wait will be longer

bull first collision choose K from 01 delay is K 512 bit transmission times

bull after second collision choose K from 0123hellip

bull after ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

54

CSMACD efficiencybull Tprop = max prop delay between 2 nodes in

LANbull ttrans = time to transmit max-size frame

bull efficiency goes to 1 bull as tprop goes to 0

bull as ttrans goes to infinity

bull better performance than ALOHA and simple cheap decentralized

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

transprop ttefficiency

51

1

55

8023 Ethernet Standards Link amp Physical Layersbull many different Ethernet standards

bull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps

1Gbps 10G bpsbull different physical layer media fiber cable

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

applicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twisterpair) physical layer

56

Manchester encodingbull used in 10BaseTbull each bit has a transitionbull allows clocks in sending and receiving nodes to syn-

chronize to each otherbull no need for a centralized global clock among nodes

bull Hey this is physical-layer stuff

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

57

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

58

Hubshellip physical-layer (ldquodumbrdquo) repeaters

bull bits coming in one link go out all other links at same rate

bull all nodes connected to hub can collide with one another

bull no frame bufferingbull no CSMACD at hub host NICs detect colli-

sions

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

twisted pair

hub

59

Switchbull link-layer device smarter than hubs take

active rolebull store forward Ethernet framesbull examine incoming framersquos MAC address se-

lectively forward frame to one-or-more out-going links when frame is to be forwarded on segment uses CSMACD to access segment

bull transparentbull hosts are unaware of presence of switches

bull plug-and-play self-learning (HOW)bull switches do not need to be configured

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

60

Switch allows multiple simulta-neous transmissionsbull hosts have dedicated direct

connection to switchbull switches buffer packetsbull Ethernet protocol used on

each incoming link but no collisions full duplexbull each link is its own collision

domain

bull switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions bull not possible with dumb hub

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 23

45

6

61

Switch Tablebull Q how does switch know

that Arsquo reachable via inter-face 4 Brsquo reachable via inter-face 5

bull A each switch has a switch table each entrybull (MAC address of host interface

to reach host time stamp)

bull looks like a routing tablebull Q how are entries created

maintained in switch table bull something like a routing proto-

colFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

switch with six interfaces(123456)

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

62

Switch self-learningbull switch learns which

hosts can be reached through which inter-facesbull when frame received

switch ldquolearnsrdquo location of sender incoming LAN segment

bull records senderlocation pair in switch table

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

63

Switch frame filteringforward-ingWhen frame received

1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination then if dest on segment from which frame arrived then drop the frame else

forward the frame on interface indicated else flood

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

forward on all but the interface on which the frame arrived

64

Self-learning forwarding exam-ple

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

frame destination unknownflood

Arsquo A

destination A location known

Arsquo 4 60

selective send

65

Interconnecting switchesbull switches can be connected together

bull Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3

bull A self learning (works exactly the same as in single-switch case)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

B

S1

C D

E

FS2

S4

S3

H

I

G

66

Self-learning multi-switch exam-plebull Suppose C sends frame to I I responds to

C

bull Q show switch tables and packet forward-ing in S1 S2 S3 S4

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

B

S1

CD

E F

S2

S4

S3

H

I

G

12

1

2 3

3

1 2 3 12 3

444

67

Institutional network

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

to externalnetwork

router

IP subnet

mail server

web server

68

Switches vs Routersbull both store-and-forward devices

bull routers network layer devices (examine network layer head-ers)

bull switches are link layer devices

bull routers maintain routing tables implement routing al-gorithms

bull switches maintain switch tables implement filtering learning algorithms

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

69

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

• Fall 2015 COMP 46055605 Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link layer context
• Link Layer Services
• Link Layer Services (more)
• Where is the link layer implemented
• Adaptors Communicating
• Link Layer (2)
• Error Detection
• Parity Checking
• Internet checksum (review)
• Checksumming Cyclic Redundancy Check
• Checksumming Cyclic Redundancy Check (2)
• CRC Example
• Link Layer (3)
• Multiple Access Links and Protocols
• Multiple Access protocols
• Ideal Multiple Access Protocol
• MAC Protocols a taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD collision detection
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Link Layer (4)
• MAC Addresses and ARP
• LAN Addresses and ARP
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP protocol Same LAN (network)
• Addressing routing to another LAN
• Slide 45
• Link Layer (5)
• Ethernet
• Star Topology
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester encoding
• Link Layer (6)
• Hubs
• Switch
• Switch allows multiple simultaneous transmissions
• Switch Table
• Switch self-learning
• Switch frame filteringforwarding
• Self-learning forwarding example
• Interconnecting switches
• Self-learning multi-switch example
• Institutional network
• Switches vs Routers
• Link Layer (7)

19

Multiple Access protocolsbull single shared broadcast channel bull two or more simultaneous transmis-

sions by nodes interference bull collision if a node receives two or more

signals at the same time

multiple access protocolbull distributed algorithm that deter-

mines how nodes share channel ie determine when node can transmit

bull communication about channel shar-ing must use channel itself bull no out-of-band channel for coordination

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

20

Ideal Multiple Access ProtocolBroadcast channel of rate R bps1 when one node wants to transmit it

can send at rate R2 when M nodes want to transmit each

can send at average rate RM3 fully decentralized

bull no special node to coordinate transmissionsbull no synchronization of clocks slots

4 simple

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

21

MAC Protocols a taxonomyThree broad classesbull Channel Partitioning

bull divide channel into smaller ldquopiecesrdquo (time slots frequency code)

bull allocate piece to node for exclusive use

bull Random Accessbull channel not divided allow collisionsbull ldquorecoverrdquo from collisions

bull ldquoTaking turnsrdquobull nodes take turns but nodes with more to send

can take longer turnsFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

22

Channel Partitioning MAC proto-cols TDMATDMA time division multiple access bull access to channel in rounds bull each station gets fixed length slot

(length = pkt trans time) in each round bull unused slots go idle bull example 6-station LAN 134 have pkt

slots 256 idle

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

1 3 4 1 3 4

6-slotframe

23

Channel Partitioning MAC proto-cols FDMAFDMA frequency division multiple access bull channel spectrum divided into frequency bandsbull each station assigned fixed frequency bandbull unused transmission time in frequency bands go idle bull example 6-station LAN 134 have pkt frequency

bands 256 idle

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

freq

uency

bands time

FDM cable

24

Random Access Protocolsbull When node has packet to send

bull transmit at full channel data rate Rbull no a priori coordination among nodes

bull two or more transmitting nodes ldquocollisionrdquobull random access MAC protocol specifies

bull how to detect collisionsbull how to recover from collisions (eg via delayed retrans-

missions)

bull Examples of random access MAC protocolsbull slotted ALOHAbull ALOHAbull CSMA CSMACD CSMACA

bull (CS Carrier Sensing MA Multiple AccessCD Collision Detection CA Collision Avoidance)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

25

Slotted ALOHAAssumptionsbull all frames same sizebull time divided into

equal size slots (time to transmit 1 frame)

bull nodes start to transmit only slot beginning

bull nodes are synchro-nized

bull if 2 or more nodes transmit in slot all nodes detect collision

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Operationbull when node obtains

fresh frame trans-mits in next slotbull if no collision node

can send new frame in next slot

bull if collision node re-transmits frame in each subsequent slot with prob p until success

26

Slotted ALOHA

Prosbull single active node can

continuously transmit at full rate of channel

bull highly decentralized only slots in nodes need to be in sync

bull simpleFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Consbull collisions wasting slotsbull idle slotsbull nodes may be able to

detect collision in less than time to transmit packet

bull clock synchronization

27

Slotted Aloha Efficiency

bull suppose N nodes with many frames to send each transmits in slot with probability p

bull prob that given node has success in a slot = p(1-p)N-1

bull prob that any node has a success = Np(1-p)N-1

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

bull max efficiency find p that max-imizes Np(1-p)N-1

bull for many nodes take limit of Np(1- p)N-1 as N goes to infinity gives

Max efficiency = 1e = 37

Efficiency long-run fraction of successful slots (many nodes all with many frames to send)

At best channelused for useful transmissions 37of time

28

Pure (unslotted) ALOHAbull unslotted Aloha simpler no synchro-

nizationbull when frame first arrives

bull transmit immediately

bull collision probability increasesbull frame sent at t0 collides with other frames sent in

[t0-1t0+1]

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

29

Pure Aloha EfficiencyP(success by given node) = P(node transmits)

P(no other node transmits in [t0-1t0]

P(no other node transmits in [t0t0+1]

= p (1-p)N-1 (1-p)N-1

= p (1-p)2(N-1)

hellip choosing optimum p and then letting

= 1(2e) = 18

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

N

even worse than slotted Aloha

30

CSMA (Carrier Sense Multiple Access)CSMA listen before transmitIf channel sensed idle transmit entire framebull If channel sensed busy defer transmis-

sion

bull human analogy donrsquot interrupt others

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

31

CSMA Collisionscollisions can still occurbull propagation delay means

two nodes may not hear each otherrsquos transmission

collisionbull entire packet transmis-

sion time wasted

notebull role of distance amp propa-

gation delay in determin-ing collision probability

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

spatial layout of nodes

32

CSMACD (Collision Detec-tion)CSMACD carrier sensing deferral as in CSMA

bull collisions detected within short timebull colliding transmissions aborted reducing channel

wastage

bull collision detection bull easy in wired LANs measure signal strengths

compare transmitted received signalsbull difficult in wireless LANs received signal strength

overwhelmed by local transmission strength

bull human analogy the polite conversationalist

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

33

CSMACD collision detection

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

34

ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols

bull share channel efficiently and fairly at high loadbull inefficient at low load delay in channel access

1N bandwidth allocated even if only 1 active node

Random access MAC protocolsbull efficient at low load single node can fully uti-

lize channelbull high load collision overhead

ldquotaking turnsrdquo protocolsbull look for best of both worlds

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

35

ldquoTaking Turnsrdquo MAC protocolsPolling bull master node ldquoinvitesrdquo

slave nodes to transmit in turn

bull typically used with ldquodumbrdquo slave devices

bull concernsbull polling overhead bull latencybull single point of failure

(master)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

T

data

(nothingto send)

T

36

ldquoTaking Turnsrdquo MAC protocolsToken passingbull control token

passed from one node to next se-quentially

bull token messagebull concerns

bull token overhead bull latencybull single point of failure

(token)Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

master

slaves

poll

data

data

37

Summary of MAC protocolsbull channel partitioning by time frequency or

codebull Time Division Frequency Division

bull random access (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire)

hard in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211

bull taking turnsbull polling from central site token passing

bull Bluetooth FDDI IBM Token Ring Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

38

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

39

MAC Addresses and ARPbull 32-bit IP address

bull network-layer addressbull used to get datagram to destination IP subnet

bull MAC (or LAN or physical or Ethernet) ad-dress bull function get frame from one interface to an-

other physically-connected interface (same network)

bull 48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software set-

table

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

40

LAN Addresses and ARP

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Each adapter on LAN has unique LAN address

Broadcast address =FF-FF-FF-FF-FF-FF

= adapter

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired orwireless)

41

LAN Address (more)bull MAC address allocation administered by IEEEbull manufacturer buys portion of MAC address

space (to assure uniqueness)bull analogy

(a) MAC address like Social Security Num-ber

(b) IP address like postal addressbull MAC flat address portability

bull can move LAN card from one LAN to another

bull IP hierarchical address NOT portablebull address depends on IP subnet to which node is at-

tachedFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

42

ARP Address Resolution Proto-colbull Each IP node (host

router) on LAN has ARP table

bull ARP table IPMAC address mappings for some LAN nodeslt IP address MAC address

TTLgt

bull TTL (Time To Live) time after which ad-dress mapping will be forgotten (typically 20 min)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Question how to determineMAC address of Bknowing Brsquos IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN

137196723

137196778

137196714

137196788

43

ARP protocol Same LAN (net-work)bull A wants to send datagram

to B and Brsquos MAC address not in Arsquos ARP table

bull A broadcasts ARP query packet containing Bs IP address bull dest MAC address = FF-FF-FF-

FF-FF-FFbull all machines on LAN receive

ARP query

bull B receives ARP packet replies to A with its (Bs) MAC addressbull frame sent to Arsquos MAC ad-

dress (unicast)Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

bull A caches (saves) IP-to-MAC address pair in its ARP table until infor-mation becomes old (times out) bull soft state information

that times out (goes away) unless refreshed

bull ARP is ldquoplug-and-playrdquobull nodes create their ARP

tables without interven-tion from net administra-tor

44

Addressing routing to another LAN

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

R

1A-23-F9-CD-06-9B

222222222220111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

walkthrough send datagram from A to B via R assume A knows Brsquos IP address

bull two ARP tables in router R one for each IP network (LAN)

45

bull A creates IP datagram with source A destination B bull A uses ARP to get Rrsquos MAC address for 111111111110bull A creates link-layer frame with Rs MAC address as dest

frame contains A-to-B IP datagrambull Arsquos NIC sends frame bull Rrsquos NIC receives frame bull R removes IP datagram from Ethernet frame sees its des-

tined to Bbull R uses ARP to get Brsquos MAC address bull R creates frame containing A-to-B IP datagram sends to B

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

This is a really importantexample ndash make sure youunderstand

R

1A-23-F9-CD-06-9B

222222222220

111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

46

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

47

Ethernetldquodominantrdquo wired LAN technology bull cheap $20 for NICbull first widely used LAN technologybull simpler cheaper than token LANs and ATMbull kept up with speed race 10 Mbps ndash 10 Gbps

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Metcalfersquos Ethernetsketch

48

Star Topologybull bus topology popular through mid 90s

bull all nodes in same collision domain (can collide with each other)

bull today star topology prevailsbull active switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes do

not collide with each other)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

switch

bus coaxial cable star

49

Ethernet Frame Structurebull Sending adapter encapsulates IP datagram

(or other network layer protocol packet) in Ethernet frame

Preamble bull 7 bytes with pattern 10101010 followed by

one byte with pattern 10101011bull used to synchronize receiver sender clock

ratesFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

50

Ethernet Frame Structure (more)bull Addresses 6 bytes

bull if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

bull otherwise adapter discards frame

bull Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)

bull CRC checked at receiver if error is detected frame is dropped

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51

Ethernet Unreliable connection-lessbull connectionless No handshaking be-

tween sending and receiving NICs bull unreliable receiving NIC doesnrsquot send

acks or nacks to sending NICbull stream of datagrams passed to network

layer can have gaps (missing datagrams)bull gaps will be filled if app is using TCPbull otherwise app will see gaps

bull Ethernetrsquos MAC protocol unslotted CSMACD

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

52

Ethernet CSMACD algorithmbull 1 NIC receives data-

gram from network layer creates frame

bull 2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

bull 3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

bull 4 If NIC detects an-other transmission while transmitting aborts and sends jam signal

bull 5 After aborting NIC enters exponential backoff after mth colli-sion NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

53

Ethernetrsquos CSMACD (more)bull Jam Signal make

sure all other transmitters are aware of collision 48 bits

bull Bit time 1 micro sec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Exponential Backoff bull Goal adapt retransmission

attempts to estimated cur-rent load

bull heavy load random wait will be longer

bull first collision choose K from 01 delay is K 512 bit transmission times

bull after second collision choose K from 0123hellip

bull after ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

54

CSMACD efficiencybull Tprop = max prop delay between 2 nodes in

LANbull ttrans = time to transmit max-size frame

bull efficiency goes to 1 bull as tprop goes to 0

bull as ttrans goes to infinity

bull better performance than ALOHA and simple cheap decentralized

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

transprop ttefficiency

51

1

55

8023 Ethernet Standards Link amp Physical Layersbull many different Ethernet standards

bull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps

1Gbps 10G bpsbull different physical layer media fiber cable

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

applicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twisterpair) physical layer

56

Manchester encodingbull used in 10BaseTbull each bit has a transitionbull allows clocks in sending and receiving nodes to syn-

chronize to each otherbull no need for a centralized global clock among nodes

bull Hey this is physical-layer stuff

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

57

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

58

Hubshellip physical-layer (ldquodumbrdquo) repeaters

bull bits coming in one link go out all other links at same rate

bull all nodes connected to hub can collide with one another

bull no frame bufferingbull no CSMACD at hub host NICs detect colli-

sions

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

twisted pair

hub

59

Switchbull link-layer device smarter than hubs take

active rolebull store forward Ethernet framesbull examine incoming framersquos MAC address se-

lectively forward frame to one-or-more out-going links when frame is to be forwarded on segment uses CSMACD to access segment

bull transparentbull hosts are unaware of presence of switches

bull plug-and-play self-learning (HOW)bull switches do not need to be configured

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

60

Switch allows multiple simulta-neous transmissionsbull hosts have dedicated direct

connection to switchbull switches buffer packetsbull Ethernet protocol used on

each incoming link but no collisions full duplexbull each link is its own collision

domain

bull switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions bull not possible with dumb hub

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 23

45

6

61

Switch Tablebull Q how does switch know

that Arsquo reachable via inter-face 4 Brsquo reachable via inter-face 5

bull A each switch has a switch table each entrybull (MAC address of host interface

to reach host time stamp)

bull looks like a routing tablebull Q how are entries created

maintained in switch table bull something like a routing proto-

colFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

switch with six interfaces(123456)

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

62

Switch self-learningbull switch learns which

hosts can be reached through which inter-facesbull when frame received

switch ldquolearnsrdquo location of sender incoming LAN segment

bull records senderlocation pair in switch table

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

63

Switch frame filteringforward-ingWhen frame received

1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination then if dest on segment from which frame arrived then drop the frame else

forward the frame on interface indicated else flood

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

forward on all but the interface on which the frame arrived

64

Self-learning forwarding exam-ple

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

frame destination unknownflood

Arsquo A

destination A location known

Arsquo 4 60

selective send

65

Interconnecting switchesbull switches can be connected together

bull Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3

bull A self learning (works exactly the same as in single-switch case)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

B

S1

C D

E

FS2

S4

S3

H

I

G

66

Self-learning multi-switch exam-plebull Suppose C sends frame to I I responds to

C

bull Q show switch tables and packet forward-ing in S1 S2 S3 S4

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

B

S1

CD

E F

S2

S4

S3

H

I

G

12

1

2 3

3

1 2 3 12 3

444

67

Institutional network

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

to externalnetwork

router

IP subnet

mail server

web server

68

Switches vs Routersbull both store-and-forward devices

bull routers network layer devices (examine network layer head-ers)

bull switches are link layer devices

bull routers maintain routing tables implement routing al-gorithms

bull switches maintain switch tables implement filtering learning algorithms

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

69

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

• Fall 2015 COMP 46055605 Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link layer context
• Link Layer Services
• Link Layer Services (more)
• Where is the link layer implemented
• Adaptors Communicating
• Link Layer (2)
• Error Detection
• Parity Checking
• Internet checksum (review)
• Checksumming Cyclic Redundancy Check
• Checksumming Cyclic Redundancy Check (2)
• CRC Example
• Link Layer (3)
• Multiple Access Links and Protocols
• Multiple Access protocols
• Ideal Multiple Access Protocol
• MAC Protocols a taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD collision detection
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Link Layer (4)
• MAC Addresses and ARP
• LAN Addresses and ARP
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP protocol Same LAN (network)
• Addressing routing to another LAN
• Slide 45
• Link Layer (5)
• Ethernet
• Star Topology
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester encoding
• Link Layer (6)
• Hubs
• Switch
• Switch allows multiple simultaneous transmissions
• Switch Table
• Switch self-learning
• Switch frame filteringforwarding
• Self-learning forwarding example
• Interconnecting switches
• Self-learning multi-switch example
• Institutional network
• Switches vs Routers
• Link Layer (7)

20

Ideal Multiple Access ProtocolBroadcast channel of rate R bps1 when one node wants to transmit it

can send at rate R2 when M nodes want to transmit each

can send at average rate RM3 fully decentralized

bull no special node to coordinate transmissionsbull no synchronization of clocks slots

4 simple

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

21

MAC Protocols a taxonomyThree broad classesbull Channel Partitioning

bull divide channel into smaller ldquopiecesrdquo (time slots frequency code)

bull allocate piece to node for exclusive use

bull Random Accessbull channel not divided allow collisionsbull ldquorecoverrdquo from collisions

bull ldquoTaking turnsrdquobull nodes take turns but nodes with more to send

can take longer turnsFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

22

Channel Partitioning MAC proto-cols TDMATDMA time division multiple access bull access to channel in rounds bull each station gets fixed length slot

(length = pkt trans time) in each round bull unused slots go idle bull example 6-station LAN 134 have pkt

slots 256 idle

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

1 3 4 1 3 4

6-slotframe

23

Channel Partitioning MAC proto-cols FDMAFDMA frequency division multiple access bull channel spectrum divided into frequency bandsbull each station assigned fixed frequency bandbull unused transmission time in frequency bands go idle bull example 6-station LAN 134 have pkt frequency

bands 256 idle

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

freq

uency

bands time

FDM cable

24

Random Access Protocolsbull When node has packet to send

bull transmit at full channel data rate Rbull no a priori coordination among nodes

bull two or more transmitting nodes ldquocollisionrdquobull random access MAC protocol specifies

bull how to detect collisionsbull how to recover from collisions (eg via delayed retrans-

missions)

bull Examples of random access MAC protocolsbull slotted ALOHAbull ALOHAbull CSMA CSMACD CSMACA

bull (CS Carrier Sensing MA Multiple AccessCD Collision Detection CA Collision Avoidance)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

25

Slotted ALOHAAssumptionsbull all frames same sizebull time divided into

equal size slots (time to transmit 1 frame)

bull nodes start to transmit only slot beginning

bull nodes are synchro-nized

bull if 2 or more nodes transmit in slot all nodes detect collision

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Operationbull when node obtains

fresh frame trans-mits in next slotbull if no collision node

can send new frame in next slot

bull if collision node re-transmits frame in each subsequent slot with prob p until success

26

Slotted ALOHA

Prosbull single active node can

continuously transmit at full rate of channel

bull highly decentralized only slots in nodes need to be in sync

bull simpleFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Consbull collisions wasting slotsbull idle slotsbull nodes may be able to

detect collision in less than time to transmit packet

bull clock synchronization

27

Slotted Aloha Efficiency

bull suppose N nodes with many frames to send each transmits in slot with probability p

bull prob that given node has success in a slot = p(1-p)N-1

bull prob that any node has a success = Np(1-p)N-1

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

bull max efficiency find p that max-imizes Np(1-p)N-1

bull for many nodes take limit of Np(1- p)N-1 as N goes to infinity gives

Max efficiency = 1e = 37

Efficiency long-run fraction of successful slots (many nodes all with many frames to send)

At best channelused for useful transmissions 37of time

28

Pure (unslotted) ALOHAbull unslotted Aloha simpler no synchro-

nizationbull when frame first arrives

bull transmit immediately

bull collision probability increasesbull frame sent at t0 collides with other frames sent in

[t0-1t0+1]

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

29

Pure Aloha EfficiencyP(success by given node) = P(node transmits)

P(no other node transmits in [t0-1t0]

P(no other node transmits in [t0t0+1]

= p (1-p)N-1 (1-p)N-1

= p (1-p)2(N-1)

hellip choosing optimum p and then letting

= 1(2e) = 18

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

N

even worse than slotted Aloha

30

CSMA (Carrier Sense Multiple Access)CSMA listen before transmitIf channel sensed idle transmit entire framebull If channel sensed busy defer transmis-

sion

bull human analogy donrsquot interrupt others

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

31

CSMA Collisionscollisions can still occurbull propagation delay means

two nodes may not hear each otherrsquos transmission

collisionbull entire packet transmis-

sion time wasted

notebull role of distance amp propa-

gation delay in determin-ing collision probability

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

spatial layout of nodes

32

CSMACD (Collision Detec-tion)CSMACD carrier sensing deferral as in CSMA

bull collisions detected within short timebull colliding transmissions aborted reducing channel

wastage

bull collision detection bull easy in wired LANs measure signal strengths

compare transmitted received signalsbull difficult in wireless LANs received signal strength

overwhelmed by local transmission strength

bull human analogy the polite conversationalist

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

33

CSMACD collision detection

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

34

ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols

bull share channel efficiently and fairly at high loadbull inefficient at low load delay in channel access

1N bandwidth allocated even if only 1 active node

Random access MAC protocolsbull efficient at low load single node can fully uti-

lize channelbull high load collision overhead

ldquotaking turnsrdquo protocolsbull look for best of both worlds

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

35

ldquoTaking Turnsrdquo MAC protocolsPolling bull master node ldquoinvitesrdquo

slave nodes to transmit in turn

bull typically used with ldquodumbrdquo slave devices

bull concernsbull polling overhead bull latencybull single point of failure

(master)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

T

data

(nothingto send)

T

36

ldquoTaking Turnsrdquo MAC protocolsToken passingbull control token

passed from one node to next se-quentially

bull token messagebull concerns

bull token overhead bull latencybull single point of failure

(token)Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

master

slaves

poll

data

data

37

Summary of MAC protocolsbull channel partitioning by time frequency or

codebull Time Division Frequency Division

bull random access (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire)

hard in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211

bull taking turnsbull polling from central site token passing

bull Bluetooth FDDI IBM Token Ring Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

38

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

39

MAC Addresses and ARPbull 32-bit IP address

bull network-layer addressbull used to get datagram to destination IP subnet

bull MAC (or LAN or physical or Ethernet) ad-dress bull function get frame from one interface to an-

other physically-connected interface (same network)

bull 48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software set-

table

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

40

LAN Addresses and ARP

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Each adapter on LAN has unique LAN address

Broadcast address =FF-FF-FF-FF-FF-FF

= adapter

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired orwireless)

41

LAN Address (more)bull MAC address allocation administered by IEEEbull manufacturer buys portion of MAC address

space (to assure uniqueness)bull analogy

(a) MAC address like Social Security Num-ber

(b) IP address like postal addressbull MAC flat address portability

bull can move LAN card from one LAN to another

bull IP hierarchical address NOT portablebull address depends on IP subnet to which node is at-

tachedFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

42

ARP Address Resolution Proto-colbull Each IP node (host

router) on LAN has ARP table

bull ARP table IPMAC address mappings for some LAN nodeslt IP address MAC address

TTLgt

bull TTL (Time To Live) time after which ad-dress mapping will be forgotten (typically 20 min)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Question how to determineMAC address of Bknowing Brsquos IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN

137196723

137196778

137196714

137196788

43

ARP protocol Same LAN (net-work)bull A wants to send datagram

to B and Brsquos MAC address not in Arsquos ARP table

bull A broadcasts ARP query packet containing Bs IP address bull dest MAC address = FF-FF-FF-

FF-FF-FFbull all machines on LAN receive

ARP query

bull B receives ARP packet replies to A with its (Bs) MAC addressbull frame sent to Arsquos MAC ad-

dress (unicast)Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

bull A caches (saves) IP-to-MAC address pair in its ARP table until infor-mation becomes old (times out) bull soft state information

that times out (goes away) unless refreshed

bull ARP is ldquoplug-and-playrdquobull nodes create their ARP

tables without interven-tion from net administra-tor

44

Addressing routing to another LAN

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

R

1A-23-F9-CD-06-9B

222222222220111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

walkthrough send datagram from A to B via R assume A knows Brsquos IP address

bull two ARP tables in router R one for each IP network (LAN)

45

bull A creates IP datagram with source A destination B bull A uses ARP to get Rrsquos MAC address for 111111111110bull A creates link-layer frame with Rs MAC address as dest

frame contains A-to-B IP datagrambull Arsquos NIC sends frame bull Rrsquos NIC receives frame bull R removes IP datagram from Ethernet frame sees its des-

tined to Bbull R uses ARP to get Brsquos MAC address bull R creates frame containing A-to-B IP datagram sends to B

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

This is a really importantexample ndash make sure youunderstand

R

1A-23-F9-CD-06-9B

222222222220

111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

46

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

47

Ethernetldquodominantrdquo wired LAN technology bull cheap $20 for NICbull first widely used LAN technologybull simpler cheaper than token LANs and ATMbull kept up with speed race 10 Mbps ndash 10 Gbps

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Metcalfersquos Ethernetsketch

48

Star Topologybull bus topology popular through mid 90s

bull all nodes in same collision domain (can collide with each other)

bull today star topology prevailsbull active switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes do

not collide with each other)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

switch

bus coaxial cable star

49

Ethernet Frame Structurebull Sending adapter encapsulates IP datagram

(or other network layer protocol packet) in Ethernet frame

Preamble bull 7 bytes with pattern 10101010 followed by

one byte with pattern 10101011bull used to synchronize receiver sender clock

ratesFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

50

Ethernet Frame Structure (more)bull Addresses 6 bytes

bull if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

bull otherwise adapter discards frame

bull Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)

bull CRC checked at receiver if error is detected frame is dropped

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51

Ethernet Unreliable connection-lessbull connectionless No handshaking be-

tween sending and receiving NICs bull unreliable receiving NIC doesnrsquot send

acks or nacks to sending NICbull stream of datagrams passed to network

layer can have gaps (missing datagrams)bull gaps will be filled if app is using TCPbull otherwise app will see gaps

bull Ethernetrsquos MAC protocol unslotted CSMACD

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

52

Ethernet CSMACD algorithmbull 1 NIC receives data-

gram from network layer creates frame

bull 2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

bull 3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

bull 4 If NIC detects an-other transmission while transmitting aborts and sends jam signal

bull 5 After aborting NIC enters exponential backoff after mth colli-sion NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

53

Ethernetrsquos CSMACD (more)bull Jam Signal make

sure all other transmitters are aware of collision 48 bits

bull Bit time 1 micro sec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Exponential Backoff bull Goal adapt retransmission

attempts to estimated cur-rent load

bull heavy load random wait will be longer

bull first collision choose K from 01 delay is K 512 bit transmission times

bull after second collision choose K from 0123hellip

bull after ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

54

CSMACD efficiencybull Tprop = max prop delay between 2 nodes in

LANbull ttrans = time to transmit max-size frame

bull efficiency goes to 1 bull as tprop goes to 0

bull as ttrans goes to infinity

bull better performance than ALOHA and simple cheap decentralized

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

transprop ttefficiency

51

1

55

8023 Ethernet Standards Link amp Physical Layersbull many different Ethernet standards

bull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps

1Gbps 10G bpsbull different physical layer media fiber cable

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

applicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twisterpair) physical layer

56

Manchester encodingbull used in 10BaseTbull each bit has a transitionbull allows clocks in sending and receiving nodes to syn-

chronize to each otherbull no need for a centralized global clock among nodes

bull Hey this is physical-layer stuff

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

57

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

58

Hubshellip physical-layer (ldquodumbrdquo) repeaters

bull bits coming in one link go out all other links at same rate

bull all nodes connected to hub can collide with one another

bull no frame bufferingbull no CSMACD at hub host NICs detect colli-

sions

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

twisted pair

hub

59

Switchbull link-layer device smarter than hubs take

active rolebull store forward Ethernet framesbull examine incoming framersquos MAC address se-

lectively forward frame to one-or-more out-going links when frame is to be forwarded on segment uses CSMACD to access segment

bull transparentbull hosts are unaware of presence of switches

bull plug-and-play self-learning (HOW)bull switches do not need to be configured

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

60

Switch allows multiple simulta-neous transmissionsbull hosts have dedicated direct

connection to switchbull switches buffer packetsbull Ethernet protocol used on

each incoming link but no collisions full duplexbull each link is its own collision

domain

bull switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions bull not possible with dumb hub

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 23

45

6

61

Switch Tablebull Q how does switch know

that Arsquo reachable via inter-face 4 Brsquo reachable via inter-face 5

bull A each switch has a switch table each entrybull (MAC address of host interface

to reach host time stamp)

bull looks like a routing tablebull Q how are entries created

maintained in switch table bull something like a routing proto-

colFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

switch with six interfaces(123456)

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

62

Switch self-learningbull switch learns which

hosts can be reached through which inter-facesbull when frame received

switch ldquolearnsrdquo location of sender incoming LAN segment

bull records senderlocation pair in switch table

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

63

Switch frame filteringforward-ingWhen frame received

1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination then if dest on segment from which frame arrived then drop the frame else

forward the frame on interface indicated else flood

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

forward on all but the interface on which the frame arrived

64

Self-learning forwarding exam-ple

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

frame destination unknownflood

Arsquo A

destination A location known

Arsquo 4 60

selective send

65

Interconnecting switchesbull switches can be connected together

bull Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3

bull A self learning (works exactly the same as in single-switch case)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

B

S1

C D

E

FS2

S4

S3

H

I

G

66

Self-learning multi-switch exam-plebull Suppose C sends frame to I I responds to

C

bull Q show switch tables and packet forward-ing in S1 S2 S3 S4

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

B

S1

CD

E F

S2

S4

S3

H

I

G

12

1

2 3

3

1 2 3 12 3

444

67

Institutional network

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

to externalnetwork

router

IP subnet

mail server

web server

68

Switches vs Routersbull both store-and-forward devices

bull routers network layer devices (examine network layer head-ers)

bull switches are link layer devices

bull routers maintain routing tables implement routing al-gorithms

bull switches maintain switch tables implement filtering learning algorithms

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

69

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

• Fall 2015 COMP 46055605 Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link layer context
• Link Layer Services
• Link Layer Services (more)
• Where is the link layer implemented
• Adaptors Communicating
• Link Layer (2)
• Error Detection
• Parity Checking
• Internet checksum (review)
• Checksumming Cyclic Redundancy Check
• Checksumming Cyclic Redundancy Check (2)
• CRC Example
• Link Layer (3)
• Multiple Access Links and Protocols
• Multiple Access protocols
• Ideal Multiple Access Protocol
• MAC Protocols a taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD collision detection
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Link Layer (4)
• MAC Addresses and ARP
• LAN Addresses and ARP
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP protocol Same LAN (network)
• Addressing routing to another LAN
• Slide 45
• Link Layer (5)
• Ethernet
• Star Topology
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester encoding
• Link Layer (6)
• Hubs
• Switch
• Switch allows multiple simultaneous transmissions
• Switch Table
• Switch self-learning
• Switch frame filteringforwarding
• Self-learning forwarding example
• Interconnecting switches
• Self-learning multi-switch example
• Institutional network
• Switches vs Routers
• Link Layer (7)

21

MAC Protocols a taxonomyThree broad classesbull Channel Partitioning

bull divide channel into smaller ldquopiecesrdquo (time slots frequency code)

bull allocate piece to node for exclusive use

bull Random Accessbull channel not divided allow collisionsbull ldquorecoverrdquo from collisions

bull ldquoTaking turnsrdquobull nodes take turns but nodes with more to send

can take longer turnsFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

22

Channel Partitioning MAC proto-cols TDMATDMA time division multiple access bull access to channel in rounds bull each station gets fixed length slot

(length = pkt trans time) in each round bull unused slots go idle bull example 6-station LAN 134 have pkt

slots 256 idle

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

1 3 4 1 3 4

6-slotframe

23

Channel Partitioning MAC proto-cols FDMAFDMA frequency division multiple access bull channel spectrum divided into frequency bandsbull each station assigned fixed frequency bandbull unused transmission time in frequency bands go idle bull example 6-station LAN 134 have pkt frequency

bands 256 idle

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

freq

uency

bands time

FDM cable

24

Random Access Protocolsbull When node has packet to send

bull transmit at full channel data rate Rbull no a priori coordination among nodes

bull two or more transmitting nodes ldquocollisionrdquobull random access MAC protocol specifies

bull how to detect collisionsbull how to recover from collisions (eg via delayed retrans-

missions)

bull Examples of random access MAC protocolsbull slotted ALOHAbull ALOHAbull CSMA CSMACD CSMACA

bull (CS Carrier Sensing MA Multiple AccessCD Collision Detection CA Collision Avoidance)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

25

Slotted ALOHAAssumptionsbull all frames same sizebull time divided into

equal size slots (time to transmit 1 frame)

bull nodes start to transmit only slot beginning

bull nodes are synchro-nized

bull if 2 or more nodes transmit in slot all nodes detect collision

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Operationbull when node obtains

fresh frame trans-mits in next slotbull if no collision node

can send new frame in next slot

bull if collision node re-transmits frame in each subsequent slot with prob p until success

26

Slotted ALOHA

Prosbull single active node can

continuously transmit at full rate of channel

bull highly decentralized only slots in nodes need to be in sync

bull simpleFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Consbull collisions wasting slotsbull idle slotsbull nodes may be able to

detect collision in less than time to transmit packet

bull clock synchronization

27

Slotted Aloha Efficiency

bull suppose N nodes with many frames to send each transmits in slot with probability p

bull prob that given node has success in a slot = p(1-p)N-1

bull prob that any node has a success = Np(1-p)N-1

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

bull max efficiency find p that max-imizes Np(1-p)N-1

bull for many nodes take limit of Np(1- p)N-1 as N goes to infinity gives

Max efficiency = 1e = 37

Efficiency long-run fraction of successful slots (many nodes all with many frames to send)

At best channelused for useful transmissions 37of time

28

Pure (unslotted) ALOHAbull unslotted Aloha simpler no synchro-

nizationbull when frame first arrives

bull transmit immediately

bull collision probability increasesbull frame sent at t0 collides with other frames sent in

[t0-1t0+1]

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

29

Pure Aloha EfficiencyP(success by given node) = P(node transmits)

P(no other node transmits in [t0-1t0]

P(no other node transmits in [t0t0+1]

= p (1-p)N-1 (1-p)N-1

= p (1-p)2(N-1)

hellip choosing optimum p and then letting

= 1(2e) = 18

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

N

even worse than slotted Aloha

30

CSMA (Carrier Sense Multiple Access)CSMA listen before transmitIf channel sensed idle transmit entire framebull If channel sensed busy defer transmis-

sion

bull human analogy donrsquot interrupt others

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

31

CSMA Collisionscollisions can still occurbull propagation delay means

two nodes may not hear each otherrsquos transmission

collisionbull entire packet transmis-

sion time wasted

notebull role of distance amp propa-

gation delay in determin-ing collision probability

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

spatial layout of nodes

32

CSMACD (Collision Detec-tion)CSMACD carrier sensing deferral as in CSMA

bull collisions detected within short timebull colliding transmissions aborted reducing channel

wastage

bull collision detection bull easy in wired LANs measure signal strengths

compare transmitted received signalsbull difficult in wireless LANs received signal strength

overwhelmed by local transmission strength

bull human analogy the polite conversationalist

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

33

CSMACD collision detection

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

34

ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols

bull share channel efficiently and fairly at high loadbull inefficient at low load delay in channel access

1N bandwidth allocated even if only 1 active node

Random access MAC protocolsbull efficient at low load single node can fully uti-

lize channelbull high load collision overhead

ldquotaking turnsrdquo protocolsbull look for best of both worlds

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

35

ldquoTaking Turnsrdquo MAC protocolsPolling bull master node ldquoinvitesrdquo

slave nodes to transmit in turn

bull typically used with ldquodumbrdquo slave devices

bull concernsbull polling overhead bull latencybull single point of failure

(master)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

T

data

(nothingto send)

T

36

ldquoTaking Turnsrdquo MAC protocolsToken passingbull control token

passed from one node to next se-quentially

bull token messagebull concerns

bull token overhead bull latencybull single point of failure

(token)Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

master

slaves

poll

data

data

37

Summary of MAC protocolsbull channel partitioning by time frequency or

codebull Time Division Frequency Division

bull random access (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire)

hard in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211

bull taking turnsbull polling from central site token passing

bull Bluetooth FDDI IBM Token Ring Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

38

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

39

MAC Addresses and ARPbull 32-bit IP address

bull network-layer addressbull used to get datagram to destination IP subnet

bull MAC (or LAN or physical or Ethernet) ad-dress bull function get frame from one interface to an-

other physically-connected interface (same network)

bull 48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software set-

table

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

40

LAN Addresses and ARP

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Each adapter on LAN has unique LAN address

Broadcast address =FF-FF-FF-FF-FF-FF

= adapter

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired orwireless)

41

LAN Address (more)bull MAC address allocation administered by IEEEbull manufacturer buys portion of MAC address

space (to assure uniqueness)bull analogy

(a) MAC address like Social Security Num-ber

(b) IP address like postal addressbull MAC flat address portability

bull can move LAN card from one LAN to another

bull IP hierarchical address NOT portablebull address depends on IP subnet to which node is at-

tachedFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

42

ARP Address Resolution Proto-colbull Each IP node (host

router) on LAN has ARP table

bull ARP table IPMAC address mappings for some LAN nodeslt IP address MAC address

TTLgt

bull TTL (Time To Live) time after which ad-dress mapping will be forgotten (typically 20 min)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Question how to determineMAC address of Bknowing Brsquos IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN

137196723

137196778

137196714

137196788

43

ARP protocol Same LAN (net-work)bull A wants to send datagram

to B and Brsquos MAC address not in Arsquos ARP table

bull A broadcasts ARP query packet containing Bs IP address bull dest MAC address = FF-FF-FF-

FF-FF-FFbull all machines on LAN receive

ARP query

bull B receives ARP packet replies to A with its (Bs) MAC addressbull frame sent to Arsquos MAC ad-

dress (unicast)Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

bull A caches (saves) IP-to-MAC address pair in its ARP table until infor-mation becomes old (times out) bull soft state information

that times out (goes away) unless refreshed

bull ARP is ldquoplug-and-playrdquobull nodes create their ARP

tables without interven-tion from net administra-tor

44

Addressing routing to another LAN

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

R

1A-23-F9-CD-06-9B

222222222220111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

walkthrough send datagram from A to B via R assume A knows Brsquos IP address

bull two ARP tables in router R one for each IP network (LAN)

45

bull A creates IP datagram with source A destination B bull A uses ARP to get Rrsquos MAC address for 111111111110bull A creates link-layer frame with Rs MAC address as dest

frame contains A-to-B IP datagrambull Arsquos NIC sends frame bull Rrsquos NIC receives frame bull R removes IP datagram from Ethernet frame sees its des-

tined to Bbull R uses ARP to get Brsquos MAC address bull R creates frame containing A-to-B IP datagram sends to B

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

This is a really importantexample ndash make sure youunderstand

R

1A-23-F9-CD-06-9B

222222222220

111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

46

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

47

Ethernetldquodominantrdquo wired LAN technology bull cheap $20 for NICbull first widely used LAN technologybull simpler cheaper than token LANs and ATMbull kept up with speed race 10 Mbps ndash 10 Gbps

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Metcalfersquos Ethernetsketch

48

Star Topologybull bus topology popular through mid 90s

bull all nodes in same collision domain (can collide with each other)

bull today star topology prevailsbull active switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes do

not collide with each other)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

switch

bus coaxial cable star

49

Ethernet Frame Structurebull Sending adapter encapsulates IP datagram

(or other network layer protocol packet) in Ethernet frame

Preamble bull 7 bytes with pattern 10101010 followed by

one byte with pattern 10101011bull used to synchronize receiver sender clock

ratesFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

50

Ethernet Frame Structure (more)bull Addresses 6 bytes

bull if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

bull otherwise adapter discards frame

bull Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)

bull CRC checked at receiver if error is detected frame is dropped

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51

Ethernet Unreliable connection-lessbull connectionless No handshaking be-

tween sending and receiving NICs bull unreliable receiving NIC doesnrsquot send

acks or nacks to sending NICbull stream of datagrams passed to network

layer can have gaps (missing datagrams)bull gaps will be filled if app is using TCPbull otherwise app will see gaps

bull Ethernetrsquos MAC protocol unslotted CSMACD

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

52

Ethernet CSMACD algorithmbull 1 NIC receives data-

gram from network layer creates frame

bull 2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

bull 3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

bull 4 If NIC detects an-other transmission while transmitting aborts and sends jam signal

bull 5 After aborting NIC enters exponential backoff after mth colli-sion NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

53

Ethernetrsquos CSMACD (more)bull Jam Signal make

sure all other transmitters are aware of collision 48 bits

bull Bit time 1 micro sec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Exponential Backoff bull Goal adapt retransmission

attempts to estimated cur-rent load

bull heavy load random wait will be longer

bull first collision choose K from 01 delay is K 512 bit transmission times

bull after second collision choose K from 0123hellip

bull after ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

54

CSMACD efficiencybull Tprop = max prop delay between 2 nodes in

LANbull ttrans = time to transmit max-size frame

bull efficiency goes to 1 bull as tprop goes to 0

bull as ttrans goes to infinity

bull better performance than ALOHA and simple cheap decentralized

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

transprop ttefficiency

51

1

55

8023 Ethernet Standards Link amp Physical Layersbull many different Ethernet standards

bull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps

1Gbps 10G bpsbull different physical layer media fiber cable

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

applicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twisterpair) physical layer

56

Manchester encodingbull used in 10BaseTbull each bit has a transitionbull allows clocks in sending and receiving nodes to syn-

chronize to each otherbull no need for a centralized global clock among nodes

bull Hey this is physical-layer stuff

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

57

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

58

Hubshellip physical-layer (ldquodumbrdquo) repeaters

bull bits coming in one link go out all other links at same rate

bull all nodes connected to hub can collide with one another

bull no frame bufferingbull no CSMACD at hub host NICs detect colli-

sions

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

twisted pair

hub

59

Switchbull link-layer device smarter than hubs take

active rolebull store forward Ethernet framesbull examine incoming framersquos MAC address se-

lectively forward frame to one-or-more out-going links when frame is to be forwarded on segment uses CSMACD to access segment

bull transparentbull hosts are unaware of presence of switches

bull plug-and-play self-learning (HOW)bull switches do not need to be configured

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

60

Switch allows multiple simulta-neous transmissionsbull hosts have dedicated direct

connection to switchbull switches buffer packetsbull Ethernet protocol used on

each incoming link but no collisions full duplexbull each link is its own collision

domain

bull switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions bull not possible with dumb hub

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 23

45

6

61

Switch Tablebull Q how does switch know

that Arsquo reachable via inter-face 4 Brsquo reachable via inter-face 5

bull A each switch has a switch table each entrybull (MAC address of host interface

to reach host time stamp)

bull looks like a routing tablebull Q how are entries created

maintained in switch table bull something like a routing proto-

colFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

switch with six interfaces(123456)

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

62

Switch self-learningbull switch learns which

hosts can be reached through which inter-facesbull when frame received

switch ldquolearnsrdquo location of sender incoming LAN segment

bull records senderlocation pair in switch table

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

63

Switch frame filteringforward-ingWhen frame received

1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination then if dest on segment from which frame arrived then drop the frame else

forward the frame on interface indicated else flood

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

forward on all but the interface on which the frame arrived

64

Self-learning forwarding exam-ple

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

frame destination unknownflood

Arsquo A

destination A location known

Arsquo 4 60

selective send

65

Interconnecting switchesbull switches can be connected together

bull Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3

bull A self learning (works exactly the same as in single-switch case)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

B

S1

C D

E

FS2

S4

S3

H

I

G

66

Self-learning multi-switch exam-plebull Suppose C sends frame to I I responds to

C

bull Q show switch tables and packet forward-ing in S1 S2 S3 S4

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

B

S1

CD

E F

S2

S4

S3

H

I

G

12

1

2 3

3

1 2 3 12 3

444

67

Institutional network

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

to externalnetwork

router

IP subnet

mail server

web server

68

Switches vs Routersbull both store-and-forward devices

bull routers network layer devices (examine network layer head-ers)

bull switches are link layer devices

bull routers maintain routing tables implement routing al-gorithms

bull switches maintain switch tables implement filtering learning algorithms

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

69

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

• Fall 2015 COMP 46055605 Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link layer context
• Link Layer Services
• Link Layer Services (more)
• Where is the link layer implemented
• Adaptors Communicating
• Link Layer (2)
• Error Detection
• Parity Checking
• Internet checksum (review)
• Checksumming Cyclic Redundancy Check
• Checksumming Cyclic Redundancy Check (2)
• CRC Example
• Link Layer (3)
• Multiple Access Links and Protocols
• Multiple Access protocols
• Ideal Multiple Access Protocol
• MAC Protocols a taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD collision detection
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Link Layer (4)
• MAC Addresses and ARP
• LAN Addresses and ARP
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP protocol Same LAN (network)
• Addressing routing to another LAN
• Slide 45
• Link Layer (5)
• Ethernet
• Star Topology
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester encoding
• Link Layer (6)
• Hubs
• Switch
• Switch allows multiple simultaneous transmissions
• Switch Table
• Switch self-learning
• Switch frame filteringforwarding
• Self-learning forwarding example
• Interconnecting switches
• Self-learning multi-switch example
• Institutional network
• Switches vs Routers
• Link Layer (7)

22

Channel Partitioning MAC proto-cols TDMATDMA time division multiple access bull access to channel in rounds bull each station gets fixed length slot

(length = pkt trans time) in each round bull unused slots go idle bull example 6-station LAN 134 have pkt

slots 256 idle

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

1 3 4 1 3 4

6-slotframe

23

Channel Partitioning MAC proto-cols FDMAFDMA frequency division multiple access bull channel spectrum divided into frequency bandsbull each station assigned fixed frequency bandbull unused transmission time in frequency bands go idle bull example 6-station LAN 134 have pkt frequency

bands 256 idle

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

freq

uency

bands time

FDM cable

24

Random Access Protocolsbull When node has packet to send

bull transmit at full channel data rate Rbull no a priori coordination among nodes

bull two or more transmitting nodes ldquocollisionrdquobull random access MAC protocol specifies

bull how to detect collisionsbull how to recover from collisions (eg via delayed retrans-

missions)

bull Examples of random access MAC protocolsbull slotted ALOHAbull ALOHAbull CSMA CSMACD CSMACA

bull (CS Carrier Sensing MA Multiple AccessCD Collision Detection CA Collision Avoidance)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

25

Slotted ALOHAAssumptionsbull all frames same sizebull time divided into

equal size slots (time to transmit 1 frame)

bull nodes start to transmit only slot beginning

bull nodes are synchro-nized

bull if 2 or more nodes transmit in slot all nodes detect collision

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Operationbull when node obtains

fresh frame trans-mits in next slotbull if no collision node

can send new frame in next slot

bull if collision node re-transmits frame in each subsequent slot with prob p until success

26

Slotted ALOHA

Prosbull single active node can

continuously transmit at full rate of channel

bull highly decentralized only slots in nodes need to be in sync

bull simpleFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Consbull collisions wasting slotsbull idle slotsbull nodes may be able to

detect collision in less than time to transmit packet

bull clock synchronization

27

Slotted Aloha Efficiency

bull suppose N nodes with many frames to send each transmits in slot with probability p

bull prob that given node has success in a slot = p(1-p)N-1

bull prob that any node has a success = Np(1-p)N-1

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

bull max efficiency find p that max-imizes Np(1-p)N-1

bull for many nodes take limit of Np(1- p)N-1 as N goes to infinity gives

Max efficiency = 1e = 37

Efficiency long-run fraction of successful slots (many nodes all with many frames to send)

At best channelused for useful transmissions 37of time

28

Pure (unslotted) ALOHAbull unslotted Aloha simpler no synchro-

nizationbull when frame first arrives

bull transmit immediately

bull collision probability increasesbull frame sent at t0 collides with other frames sent in

[t0-1t0+1]

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

29

Pure Aloha EfficiencyP(success by given node) = P(node transmits)

P(no other node transmits in [t0-1t0]

P(no other node transmits in [t0t0+1]

= p (1-p)N-1 (1-p)N-1

= p (1-p)2(N-1)

hellip choosing optimum p and then letting

= 1(2e) = 18

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

N

even worse than slotted Aloha

30

CSMA (Carrier Sense Multiple Access)CSMA listen before transmitIf channel sensed idle transmit entire framebull If channel sensed busy defer transmis-

sion

bull human analogy donrsquot interrupt others

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

31

CSMA Collisionscollisions can still occurbull propagation delay means

two nodes may not hear each otherrsquos transmission

collisionbull entire packet transmis-

sion time wasted

notebull role of distance amp propa-

gation delay in determin-ing collision probability

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

spatial layout of nodes

32

CSMACD (Collision Detec-tion)CSMACD carrier sensing deferral as in CSMA

bull collisions detected within short timebull colliding transmissions aborted reducing channel

wastage

bull collision detection bull easy in wired LANs measure signal strengths

compare transmitted received signalsbull difficult in wireless LANs received signal strength

overwhelmed by local transmission strength

bull human analogy the polite conversationalist

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

33

CSMACD collision detection

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

34

ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols

bull share channel efficiently and fairly at high loadbull inefficient at low load delay in channel access

1N bandwidth allocated even if only 1 active node

Random access MAC protocolsbull efficient at low load single node can fully uti-

lize channelbull high load collision overhead

ldquotaking turnsrdquo protocolsbull look for best of both worlds

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

35

ldquoTaking Turnsrdquo MAC protocolsPolling bull master node ldquoinvitesrdquo

slave nodes to transmit in turn

bull typically used with ldquodumbrdquo slave devices

bull concernsbull polling overhead bull latencybull single point of failure

(master)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

T

data

(nothingto send)

T

36

ldquoTaking Turnsrdquo MAC protocolsToken passingbull control token

passed from one node to next se-quentially

bull token messagebull concerns

bull token overhead bull latencybull single point of failure

(token)Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

master

slaves

poll

data

data

37

Summary of MAC protocolsbull channel partitioning by time frequency or

codebull Time Division Frequency Division

bull random access (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire)

hard in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211

bull taking turnsbull polling from central site token passing

bull Bluetooth FDDI IBM Token Ring Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

38

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

39

MAC Addresses and ARPbull 32-bit IP address

bull network-layer addressbull used to get datagram to destination IP subnet

bull MAC (or LAN or physical or Ethernet) ad-dress bull function get frame from one interface to an-

other physically-connected interface (same network)

bull 48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software set-

table

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

40

LAN Addresses and ARP

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Each adapter on LAN has unique LAN address

Broadcast address =FF-FF-FF-FF-FF-FF

= adapter

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired orwireless)

41

LAN Address (more)bull MAC address allocation administered by IEEEbull manufacturer buys portion of MAC address

space (to assure uniqueness)bull analogy

(a) MAC address like Social Security Num-ber

(b) IP address like postal addressbull MAC flat address portability

bull can move LAN card from one LAN to another

bull IP hierarchical address NOT portablebull address depends on IP subnet to which node is at-

tachedFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

42

ARP Address Resolution Proto-colbull Each IP node (host

router) on LAN has ARP table

bull ARP table IPMAC address mappings for some LAN nodeslt IP address MAC address

TTLgt

bull TTL (Time To Live) time after which ad-dress mapping will be forgotten (typically 20 min)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Question how to determineMAC address of Bknowing Brsquos IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN

137196723

137196778

137196714

137196788

43

ARP protocol Same LAN (net-work)bull A wants to send datagram

to B and Brsquos MAC address not in Arsquos ARP table

bull A broadcasts ARP query packet containing Bs IP address bull dest MAC address = FF-FF-FF-

FF-FF-FFbull all machines on LAN receive

ARP query

bull B receives ARP packet replies to A with its (Bs) MAC addressbull frame sent to Arsquos MAC ad-

dress (unicast)Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

bull A caches (saves) IP-to-MAC address pair in its ARP table until infor-mation becomes old (times out) bull soft state information

that times out (goes away) unless refreshed

bull ARP is ldquoplug-and-playrdquobull nodes create their ARP

tables without interven-tion from net administra-tor

44

Addressing routing to another LAN

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

R

1A-23-F9-CD-06-9B

222222222220111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

walkthrough send datagram from A to B via R assume A knows Brsquos IP address

bull two ARP tables in router R one for each IP network (LAN)

45

bull A creates IP datagram with source A destination B bull A uses ARP to get Rrsquos MAC address for 111111111110bull A creates link-layer frame with Rs MAC address as dest

frame contains A-to-B IP datagrambull Arsquos NIC sends frame bull Rrsquos NIC receives frame bull R removes IP datagram from Ethernet frame sees its des-

tined to Bbull R uses ARP to get Brsquos MAC address bull R creates frame containing A-to-B IP datagram sends to B

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

This is a really importantexample ndash make sure youunderstand

R

1A-23-F9-CD-06-9B

222222222220

111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

46

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

47

Ethernetldquodominantrdquo wired LAN technology bull cheap $20 for NICbull first widely used LAN technologybull simpler cheaper than token LANs and ATMbull kept up with speed race 10 Mbps ndash 10 Gbps

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Metcalfersquos Ethernetsketch

48

Star Topologybull bus topology popular through mid 90s

bull all nodes in same collision domain (can collide with each other)

bull today star topology prevailsbull active switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes do

not collide with each other)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

switch

bus coaxial cable star

49

Ethernet Frame Structurebull Sending adapter encapsulates IP datagram

(or other network layer protocol packet) in Ethernet frame

Preamble bull 7 bytes with pattern 10101010 followed by

one byte with pattern 10101011bull used to synchronize receiver sender clock

ratesFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

50

Ethernet Frame Structure (more)bull Addresses 6 bytes

bull if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

bull otherwise adapter discards frame

bull Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)

bull CRC checked at receiver if error is detected frame is dropped

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51

Ethernet Unreliable connection-lessbull connectionless No handshaking be-

tween sending and receiving NICs bull unreliable receiving NIC doesnrsquot send

acks or nacks to sending NICbull stream of datagrams passed to network

layer can have gaps (missing datagrams)bull gaps will be filled if app is using TCPbull otherwise app will see gaps

bull Ethernetrsquos MAC protocol unslotted CSMACD

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

52

Ethernet CSMACD algorithmbull 1 NIC receives data-

gram from network layer creates frame

bull 2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

bull 3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

bull 4 If NIC detects an-other transmission while transmitting aborts and sends jam signal

bull 5 After aborting NIC enters exponential backoff after mth colli-sion NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

53

Ethernetrsquos CSMACD (more)bull Jam Signal make

sure all other transmitters are aware of collision 48 bits

bull Bit time 1 micro sec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Exponential Backoff bull Goal adapt retransmission

attempts to estimated cur-rent load

bull heavy load random wait will be longer

bull first collision choose K from 01 delay is K 512 bit transmission times

bull after second collision choose K from 0123hellip

bull after ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

54

CSMACD efficiencybull Tprop = max prop delay between 2 nodes in

LANbull ttrans = time to transmit max-size frame

bull efficiency goes to 1 bull as tprop goes to 0

bull as ttrans goes to infinity

bull better performance than ALOHA and simple cheap decentralized

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

transprop ttefficiency

51

1

55

8023 Ethernet Standards Link amp Physical Layersbull many different Ethernet standards

bull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps

1Gbps 10G bpsbull different physical layer media fiber cable

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

applicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twisterpair) physical layer

56

Manchester encodingbull used in 10BaseTbull each bit has a transitionbull allows clocks in sending and receiving nodes to syn-

chronize to each otherbull no need for a centralized global clock among nodes

bull Hey this is physical-layer stuff

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

57

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

58

Hubshellip physical-layer (ldquodumbrdquo) repeaters

bull bits coming in one link go out all other links at same rate

bull all nodes connected to hub can collide with one another

bull no frame bufferingbull no CSMACD at hub host NICs detect colli-

sions

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

twisted pair

hub

59

Switchbull link-layer device smarter than hubs take

active rolebull store forward Ethernet framesbull examine incoming framersquos MAC address se-

lectively forward frame to one-or-more out-going links when frame is to be forwarded on segment uses CSMACD to access segment

bull transparentbull hosts are unaware of presence of switches

bull plug-and-play self-learning (HOW)bull switches do not need to be configured

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

60

Switch allows multiple simulta-neous transmissionsbull hosts have dedicated direct

connection to switchbull switches buffer packetsbull Ethernet protocol used on

each incoming link but no collisions full duplexbull each link is its own collision

domain

bull switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions bull not possible with dumb hub

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 23

45

6

61

Switch Tablebull Q how does switch know

that Arsquo reachable via inter-face 4 Brsquo reachable via inter-face 5

bull A each switch has a switch table each entrybull (MAC address of host interface

to reach host time stamp)

bull looks like a routing tablebull Q how are entries created

maintained in switch table bull something like a routing proto-

colFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

switch with six interfaces(123456)

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

62

Switch self-learningbull switch learns which

hosts can be reached through which inter-facesbull when frame received

switch ldquolearnsrdquo location of sender incoming LAN segment

bull records senderlocation pair in switch table

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

63

Switch frame filteringforward-ingWhen frame received

1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination then if dest on segment from which frame arrived then drop the frame else

forward the frame on interface indicated else flood

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

forward on all but the interface on which the frame arrived

64

Self-learning forwarding exam-ple

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

frame destination unknownflood

Arsquo A

destination A location known

Arsquo 4 60

selective send

65

Interconnecting switchesbull switches can be connected together

bull Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3

bull A self learning (works exactly the same as in single-switch case)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

B

S1

C D

E

FS2

S4

S3

H

I

G

66

Self-learning multi-switch exam-plebull Suppose C sends frame to I I responds to

C

bull Q show switch tables and packet forward-ing in S1 S2 S3 S4

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

B

S1

CD

E F

S2

S4

S3

H

I

G

12

1

2 3

3

1 2 3 12 3

444

67

Institutional network

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

to externalnetwork

router

IP subnet

mail server

web server

68

Switches vs Routersbull both store-and-forward devices

bull routers network layer devices (examine network layer head-ers)

bull switches are link layer devices

bull routers maintain routing tables implement routing al-gorithms

bull switches maintain switch tables implement filtering learning algorithms

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

69

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

• Fall 2015 COMP 46055605 Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link layer context
• Link Layer Services
• Link Layer Services (more)
• Where is the link layer implemented
• Adaptors Communicating
• Link Layer (2)
• Error Detection
• Parity Checking
• Internet checksum (review)
• Checksumming Cyclic Redundancy Check
• Checksumming Cyclic Redundancy Check (2)
• CRC Example
• Link Layer (3)
• Multiple Access Links and Protocols
• Multiple Access protocols
• Ideal Multiple Access Protocol
• MAC Protocols a taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD collision detection
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Link Layer (4)
• MAC Addresses and ARP
• LAN Addresses and ARP
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP protocol Same LAN (network)
• Addressing routing to another LAN
• Slide 45
• Link Layer (5)
• Ethernet
• Star Topology
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester encoding
• Link Layer (6)
• Hubs
• Switch
• Switch allows multiple simultaneous transmissions
• Switch Table
• Switch self-learning
• Switch frame filteringforwarding
• Self-learning forwarding example
• Interconnecting switches
• Self-learning multi-switch example
• Institutional network
• Switches vs Routers
• Link Layer (7)

23

Channel Partitioning MAC proto-cols FDMAFDMA frequency division multiple access bull channel spectrum divided into frequency bandsbull each station assigned fixed frequency bandbull unused transmission time in frequency bands go idle bull example 6-station LAN 134 have pkt frequency

bands 256 idle

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

freq

uency

bands time

FDM cable

24

Random Access Protocolsbull When node has packet to send

bull transmit at full channel data rate Rbull no a priori coordination among nodes

bull two or more transmitting nodes ldquocollisionrdquobull random access MAC protocol specifies

bull how to detect collisionsbull how to recover from collisions (eg via delayed retrans-

missions)

bull Examples of random access MAC protocolsbull slotted ALOHAbull ALOHAbull CSMA CSMACD CSMACA

bull (CS Carrier Sensing MA Multiple AccessCD Collision Detection CA Collision Avoidance)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

25

Slotted ALOHAAssumptionsbull all frames same sizebull time divided into

equal size slots (time to transmit 1 frame)

bull nodes start to transmit only slot beginning

bull nodes are synchro-nized

bull if 2 or more nodes transmit in slot all nodes detect collision

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Operationbull when node obtains

fresh frame trans-mits in next slotbull if no collision node

can send new frame in next slot

bull if collision node re-transmits frame in each subsequent slot with prob p until success

26

Slotted ALOHA

Prosbull single active node can

continuously transmit at full rate of channel

bull highly decentralized only slots in nodes need to be in sync

bull simpleFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Consbull collisions wasting slotsbull idle slotsbull nodes may be able to

detect collision in less than time to transmit packet

bull clock synchronization

27

Slotted Aloha Efficiency

bull suppose N nodes with many frames to send each transmits in slot with probability p

bull prob that given node has success in a slot = p(1-p)N-1

bull prob that any node has a success = Np(1-p)N-1

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

bull max efficiency find p that max-imizes Np(1-p)N-1

bull for many nodes take limit of Np(1- p)N-1 as N goes to infinity gives

Max efficiency = 1e = 37

Efficiency long-run fraction of successful slots (many nodes all with many frames to send)

At best channelused for useful transmissions 37of time

28

Pure (unslotted) ALOHAbull unslotted Aloha simpler no synchro-

nizationbull when frame first arrives

bull transmit immediately

bull collision probability increasesbull frame sent at t0 collides with other frames sent in

[t0-1t0+1]

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

29

Pure Aloha EfficiencyP(success by given node) = P(node transmits)

P(no other node transmits in [t0-1t0]

P(no other node transmits in [t0t0+1]

= p (1-p)N-1 (1-p)N-1

= p (1-p)2(N-1)

hellip choosing optimum p and then letting

= 1(2e) = 18

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

N

even worse than slotted Aloha

30

CSMA (Carrier Sense Multiple Access)CSMA listen before transmitIf channel sensed idle transmit entire framebull If channel sensed busy defer transmis-

sion

bull human analogy donrsquot interrupt others

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

31

CSMA Collisionscollisions can still occurbull propagation delay means

two nodes may not hear each otherrsquos transmission

collisionbull entire packet transmis-

sion time wasted

notebull role of distance amp propa-

gation delay in determin-ing collision probability

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

spatial layout of nodes

32

CSMACD (Collision Detec-tion)CSMACD carrier sensing deferral as in CSMA

bull collisions detected within short timebull colliding transmissions aborted reducing channel

wastage

bull collision detection bull easy in wired LANs measure signal strengths

compare transmitted received signalsbull difficult in wireless LANs received signal strength

overwhelmed by local transmission strength

bull human analogy the polite conversationalist

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

33

CSMACD collision detection

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

34

ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols

bull share channel efficiently and fairly at high loadbull inefficient at low load delay in channel access

1N bandwidth allocated even if only 1 active node

Random access MAC protocolsbull efficient at low load single node can fully uti-

lize channelbull high load collision overhead

ldquotaking turnsrdquo protocolsbull look for best of both worlds

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

35

ldquoTaking Turnsrdquo MAC protocolsPolling bull master node ldquoinvitesrdquo

slave nodes to transmit in turn

bull typically used with ldquodumbrdquo slave devices

bull concernsbull polling overhead bull latencybull single point of failure

(master)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

T

data

(nothingto send)

T

36

ldquoTaking Turnsrdquo MAC protocolsToken passingbull control token

passed from one node to next se-quentially

bull token messagebull concerns

bull token overhead bull latencybull single point of failure

(token)Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

master

slaves

poll

data

data

37

Summary of MAC protocolsbull channel partitioning by time frequency or

codebull Time Division Frequency Division

bull random access (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire)

hard in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211

bull taking turnsbull polling from central site token passing

bull Bluetooth FDDI IBM Token Ring Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

38

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

39

MAC Addresses and ARPbull 32-bit IP address

bull network-layer addressbull used to get datagram to destination IP subnet

bull MAC (or LAN or physical or Ethernet) ad-dress bull function get frame from one interface to an-

other physically-connected interface (same network)

bull 48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software set-

table

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

40

LAN Addresses and ARP

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Each adapter on LAN has unique LAN address

Broadcast address =FF-FF-FF-FF-FF-FF

= adapter

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired orwireless)

41

LAN Address (more)bull MAC address allocation administered by IEEEbull manufacturer buys portion of MAC address

space (to assure uniqueness)bull analogy

(a) MAC address like Social Security Num-ber

(b) IP address like postal addressbull MAC flat address portability

bull can move LAN card from one LAN to another

bull IP hierarchical address NOT portablebull address depends on IP subnet to which node is at-

tachedFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

42

ARP Address Resolution Proto-colbull Each IP node (host

router) on LAN has ARP table

bull ARP table IPMAC address mappings for some LAN nodeslt IP address MAC address

TTLgt

bull TTL (Time To Live) time after which ad-dress mapping will be forgotten (typically 20 min)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Question how to determineMAC address of Bknowing Brsquos IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN

137196723

137196778

137196714

137196788

43

ARP protocol Same LAN (net-work)bull A wants to send datagram

to B and Brsquos MAC address not in Arsquos ARP table

bull A broadcasts ARP query packet containing Bs IP address bull dest MAC address = FF-FF-FF-

FF-FF-FFbull all machines on LAN receive

ARP query

bull B receives ARP packet replies to A with its (Bs) MAC addressbull frame sent to Arsquos MAC ad-

dress (unicast)Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

bull A caches (saves) IP-to-MAC address pair in its ARP table until infor-mation becomes old (times out) bull soft state information

that times out (goes away) unless refreshed

bull ARP is ldquoplug-and-playrdquobull nodes create their ARP

tables without interven-tion from net administra-tor

44

Addressing routing to another LAN

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

R

1A-23-F9-CD-06-9B

222222222220111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

walkthrough send datagram from A to B via R assume A knows Brsquos IP address

bull two ARP tables in router R one for each IP network (LAN)

45

bull A creates IP datagram with source A destination B bull A uses ARP to get Rrsquos MAC address for 111111111110bull A creates link-layer frame with Rs MAC address as dest

frame contains A-to-B IP datagrambull Arsquos NIC sends frame bull Rrsquos NIC receives frame bull R removes IP datagram from Ethernet frame sees its des-

tined to Bbull R uses ARP to get Brsquos MAC address bull R creates frame containing A-to-B IP datagram sends to B

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

This is a really importantexample ndash make sure youunderstand

R

1A-23-F9-CD-06-9B

222222222220

111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

46

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

47

Ethernetldquodominantrdquo wired LAN technology bull cheap $20 for NICbull first widely used LAN technologybull simpler cheaper than token LANs and ATMbull kept up with speed race 10 Mbps ndash 10 Gbps

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Metcalfersquos Ethernetsketch

48

Star Topologybull bus topology popular through mid 90s

bull all nodes in same collision domain (can collide with each other)

bull today star topology prevailsbull active switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes do

not collide with each other)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

switch

bus coaxial cable star

49

Ethernet Frame Structurebull Sending adapter encapsulates IP datagram

(or other network layer protocol packet) in Ethernet frame

Preamble bull 7 bytes with pattern 10101010 followed by

one byte with pattern 10101011bull used to synchronize receiver sender clock

ratesFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

50

Ethernet Frame Structure (more)bull Addresses 6 bytes

bull if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

bull otherwise adapter discards frame

bull Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)

bull CRC checked at receiver if error is detected frame is dropped

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51

Ethernet Unreliable connection-lessbull connectionless No handshaking be-

tween sending and receiving NICs bull unreliable receiving NIC doesnrsquot send

acks or nacks to sending NICbull stream of datagrams passed to network

layer can have gaps (missing datagrams)bull gaps will be filled if app is using TCPbull otherwise app will see gaps

bull Ethernetrsquos MAC protocol unslotted CSMACD

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

52

Ethernet CSMACD algorithmbull 1 NIC receives data-

gram from network layer creates frame

bull 2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

bull 3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

bull 4 If NIC detects an-other transmission while transmitting aborts and sends jam signal

bull 5 After aborting NIC enters exponential backoff after mth colli-sion NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

53

Ethernetrsquos CSMACD (more)bull Jam Signal make

sure all other transmitters are aware of collision 48 bits

bull Bit time 1 micro sec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Exponential Backoff bull Goal adapt retransmission

attempts to estimated cur-rent load

bull heavy load random wait will be longer

bull first collision choose K from 01 delay is K 512 bit transmission times

bull after second collision choose K from 0123hellip

bull after ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

54

CSMACD efficiencybull Tprop = max prop delay between 2 nodes in

LANbull ttrans = time to transmit max-size frame

bull efficiency goes to 1 bull as tprop goes to 0

bull as ttrans goes to infinity

bull better performance than ALOHA and simple cheap decentralized

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

transprop ttefficiency

51

1

55

8023 Ethernet Standards Link amp Physical Layersbull many different Ethernet standards

bull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps

1Gbps 10G bpsbull different physical layer media fiber cable

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

applicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twisterpair) physical layer

56

Manchester encodingbull used in 10BaseTbull each bit has a transitionbull allows clocks in sending and receiving nodes to syn-

chronize to each otherbull no need for a centralized global clock among nodes

bull Hey this is physical-layer stuff

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

57

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

58

Hubshellip physical-layer (ldquodumbrdquo) repeaters

bull bits coming in one link go out all other links at same rate

bull all nodes connected to hub can collide with one another

bull no frame bufferingbull no CSMACD at hub host NICs detect colli-

sions

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

twisted pair

hub

59

Switchbull link-layer device smarter than hubs take

active rolebull store forward Ethernet framesbull examine incoming framersquos MAC address se-

lectively forward frame to one-or-more out-going links when frame is to be forwarded on segment uses CSMACD to access segment

bull transparentbull hosts are unaware of presence of switches

bull plug-and-play self-learning (HOW)bull switches do not need to be configured

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

60

Switch allows multiple simulta-neous transmissionsbull hosts have dedicated direct

connection to switchbull switches buffer packetsbull Ethernet protocol used on

each incoming link but no collisions full duplexbull each link is its own collision

domain

bull switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions bull not possible with dumb hub

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 23

45

6

61

Switch Tablebull Q how does switch know

that Arsquo reachable via inter-face 4 Brsquo reachable via inter-face 5

bull A each switch has a switch table each entrybull (MAC address of host interface

to reach host time stamp)

bull looks like a routing tablebull Q how are entries created

maintained in switch table bull something like a routing proto-

colFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

switch with six interfaces(123456)

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

62

Switch self-learningbull switch learns which

hosts can be reached through which inter-facesbull when frame received

switch ldquolearnsrdquo location of sender incoming LAN segment

bull records senderlocation pair in switch table

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

63

Switch frame filteringforward-ingWhen frame received

1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination then if dest on segment from which frame arrived then drop the frame else

forward the frame on interface indicated else flood

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

forward on all but the interface on which the frame arrived

64

Self-learning forwarding exam-ple

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

frame destination unknownflood

Arsquo A

destination A location known

Arsquo 4 60

selective send

65

Interconnecting switchesbull switches can be connected together

bull Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3

bull A self learning (works exactly the same as in single-switch case)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

B

S1

C D

E

FS2

S4

S3

H

I

G

66

Self-learning multi-switch exam-plebull Suppose C sends frame to I I responds to

C

bull Q show switch tables and packet forward-ing in S1 S2 S3 S4

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

B

S1

CD

E F

S2

S4

S3

H

I

G

12

1

2 3

3

1 2 3 12 3

444

67

Institutional network

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

to externalnetwork

router

IP subnet

mail server

web server

68

Switches vs Routersbull both store-and-forward devices

bull routers network layer devices (examine network layer head-ers)

bull switches are link layer devices

bull routers maintain routing tables implement routing al-gorithms

bull switches maintain switch tables implement filtering learning algorithms

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

69

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

• Fall 2015 COMP 46055605 Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link layer context
• Link Layer Services
• Link Layer Services (more)
• Where is the link layer implemented
• Adaptors Communicating
• Link Layer (2)
• Error Detection
• Parity Checking
• Internet checksum (review)
• Checksumming Cyclic Redundancy Check
• Checksumming Cyclic Redundancy Check (2)
• CRC Example
• Link Layer (3)
• Multiple Access Links and Protocols
• Multiple Access protocols
• Ideal Multiple Access Protocol
• MAC Protocols a taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD collision detection
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Link Layer (4)
• MAC Addresses and ARP
• LAN Addresses and ARP
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP protocol Same LAN (network)
• Addressing routing to another LAN
• Slide 45
• Link Layer (5)
• Ethernet
• Star Topology
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester encoding
• Link Layer (6)
• Hubs
• Switch
• Switch allows multiple simultaneous transmissions
• Switch Table
• Switch self-learning
• Switch frame filteringforwarding
• Self-learning forwarding example
• Interconnecting switches
• Self-learning multi-switch example
• Institutional network
• Switches vs Routers
• Link Layer (7)

24

Random Access Protocolsbull When node has packet to send

bull transmit at full channel data rate Rbull no a priori coordination among nodes

bull two or more transmitting nodes ldquocollisionrdquobull random access MAC protocol specifies

bull how to detect collisionsbull how to recover from collisions (eg via delayed retrans-

missions)

bull Examples of random access MAC protocolsbull slotted ALOHAbull ALOHAbull CSMA CSMACD CSMACA

bull (CS Carrier Sensing MA Multiple AccessCD Collision Detection CA Collision Avoidance)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

25

Slotted ALOHAAssumptionsbull all frames same sizebull time divided into

equal size slots (time to transmit 1 frame)

bull nodes start to transmit only slot beginning

bull nodes are synchro-nized

bull if 2 or more nodes transmit in slot all nodes detect collision

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Operationbull when node obtains

fresh frame trans-mits in next slotbull if no collision node

can send new frame in next slot

bull if collision node re-transmits frame in each subsequent slot with prob p until success

26

Slotted ALOHA

Prosbull single active node can

continuously transmit at full rate of channel

bull highly decentralized only slots in nodes need to be in sync

bull simpleFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Consbull collisions wasting slotsbull idle slotsbull nodes may be able to

detect collision in less than time to transmit packet

bull clock synchronization

27

Slotted Aloha Efficiency

bull suppose N nodes with many frames to send each transmits in slot with probability p

bull prob that given node has success in a slot = p(1-p)N-1

bull prob that any node has a success = Np(1-p)N-1

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

bull max efficiency find p that max-imizes Np(1-p)N-1

bull for many nodes take limit of Np(1- p)N-1 as N goes to infinity gives

Max efficiency = 1e = 37

Efficiency long-run fraction of successful slots (many nodes all with many frames to send)

At best channelused for useful transmissions 37of time

28

Pure (unslotted) ALOHAbull unslotted Aloha simpler no synchro-

nizationbull when frame first arrives

bull transmit immediately

bull collision probability increasesbull frame sent at t0 collides with other frames sent in

[t0-1t0+1]

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

29

Pure Aloha EfficiencyP(success by given node) = P(node transmits)

P(no other node transmits in [t0-1t0]

P(no other node transmits in [t0t0+1]

= p (1-p)N-1 (1-p)N-1

= p (1-p)2(N-1)

hellip choosing optimum p and then letting

= 1(2e) = 18

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

N

even worse than slotted Aloha

30

CSMA (Carrier Sense Multiple Access)CSMA listen before transmitIf channel sensed idle transmit entire framebull If channel sensed busy defer transmis-

sion

bull human analogy donrsquot interrupt others

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

31

CSMA Collisionscollisions can still occurbull propagation delay means

two nodes may not hear each otherrsquos transmission

collisionbull entire packet transmis-

sion time wasted

notebull role of distance amp propa-

gation delay in determin-ing collision probability

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

spatial layout of nodes

32

CSMACD (Collision Detec-tion)CSMACD carrier sensing deferral as in CSMA

bull collisions detected within short timebull colliding transmissions aborted reducing channel

wastage

bull collision detection bull easy in wired LANs measure signal strengths

compare transmitted received signalsbull difficult in wireless LANs received signal strength

overwhelmed by local transmission strength

bull human analogy the polite conversationalist

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

33

CSMACD collision detection

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

34

ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols

bull share channel efficiently and fairly at high loadbull inefficient at low load delay in channel access

1N bandwidth allocated even if only 1 active node

Random access MAC protocolsbull efficient at low load single node can fully uti-

lize channelbull high load collision overhead

ldquotaking turnsrdquo protocolsbull look for best of both worlds

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

35

ldquoTaking Turnsrdquo MAC protocolsPolling bull master node ldquoinvitesrdquo

slave nodes to transmit in turn

bull typically used with ldquodumbrdquo slave devices

bull concernsbull polling overhead bull latencybull single point of failure

(master)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

T

data

(nothingto send)

T

36

ldquoTaking Turnsrdquo MAC protocolsToken passingbull control token

passed from one node to next se-quentially

bull token messagebull concerns

bull token overhead bull latencybull single point of failure

(token)Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

master

slaves

poll

data

data

37

Summary of MAC protocolsbull channel partitioning by time frequency or

codebull Time Division Frequency Division

bull random access (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire)

hard in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211

bull taking turnsbull polling from central site token passing

bull Bluetooth FDDI IBM Token Ring Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

38

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

39

MAC Addresses and ARPbull 32-bit IP address

bull network-layer addressbull used to get datagram to destination IP subnet

bull MAC (or LAN or physical or Ethernet) ad-dress bull function get frame from one interface to an-

other physically-connected interface (same network)

bull 48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software set-

table

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

40

LAN Addresses and ARP

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Each adapter on LAN has unique LAN address

Broadcast address =FF-FF-FF-FF-FF-FF

= adapter

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired orwireless)

41

LAN Address (more)bull MAC address allocation administered by IEEEbull manufacturer buys portion of MAC address

space (to assure uniqueness)bull analogy

(a) MAC address like Social Security Num-ber

(b) IP address like postal addressbull MAC flat address portability

bull can move LAN card from one LAN to another

bull IP hierarchical address NOT portablebull address depends on IP subnet to which node is at-

tachedFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

42

ARP Address Resolution Proto-colbull Each IP node (host

router) on LAN has ARP table

bull ARP table IPMAC address mappings for some LAN nodeslt IP address MAC address

TTLgt

bull TTL (Time To Live) time after which ad-dress mapping will be forgotten (typically 20 min)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Question how to determineMAC address of Bknowing Brsquos IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN

137196723

137196778

137196714

137196788

43

ARP protocol Same LAN (net-work)bull A wants to send datagram

to B and Brsquos MAC address not in Arsquos ARP table

bull A broadcasts ARP query packet containing Bs IP address bull dest MAC address = FF-FF-FF-

FF-FF-FFbull all machines on LAN receive

ARP query

bull B receives ARP packet replies to A with its (Bs) MAC addressbull frame sent to Arsquos MAC ad-

dress (unicast)Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

bull A caches (saves) IP-to-MAC address pair in its ARP table until infor-mation becomes old (times out) bull soft state information

that times out (goes away) unless refreshed

bull ARP is ldquoplug-and-playrdquobull nodes create their ARP

tables without interven-tion from net administra-tor

44

Addressing routing to another LAN

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

R

1A-23-F9-CD-06-9B

222222222220111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

walkthrough send datagram from A to B via R assume A knows Brsquos IP address

bull two ARP tables in router R one for each IP network (LAN)

45

bull A creates IP datagram with source A destination B bull A uses ARP to get Rrsquos MAC address for 111111111110bull A creates link-layer frame with Rs MAC address as dest

frame contains A-to-B IP datagrambull Arsquos NIC sends frame bull Rrsquos NIC receives frame bull R removes IP datagram from Ethernet frame sees its des-

tined to Bbull R uses ARP to get Brsquos MAC address bull R creates frame containing A-to-B IP datagram sends to B

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

This is a really importantexample ndash make sure youunderstand

R

1A-23-F9-CD-06-9B

222222222220

111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

46

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

47

Ethernetldquodominantrdquo wired LAN technology bull cheap $20 for NICbull first widely used LAN technologybull simpler cheaper than token LANs and ATMbull kept up with speed race 10 Mbps ndash 10 Gbps

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Metcalfersquos Ethernetsketch

48

Star Topologybull bus topology popular through mid 90s

bull all nodes in same collision domain (can collide with each other)

bull today star topology prevailsbull active switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes do

not collide with each other)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

switch

bus coaxial cable star

49

Ethernet Frame Structurebull Sending adapter encapsulates IP datagram

(or other network layer protocol packet) in Ethernet frame

Preamble bull 7 bytes with pattern 10101010 followed by

one byte with pattern 10101011bull used to synchronize receiver sender clock

ratesFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

50

Ethernet Frame Structure (more)bull Addresses 6 bytes

bull if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

bull otherwise adapter discards frame

bull Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)

bull CRC checked at receiver if error is detected frame is dropped

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51

Ethernet Unreliable connection-lessbull connectionless No handshaking be-

tween sending and receiving NICs bull unreliable receiving NIC doesnrsquot send

acks or nacks to sending NICbull stream of datagrams passed to network

layer can have gaps (missing datagrams)bull gaps will be filled if app is using TCPbull otherwise app will see gaps

bull Ethernetrsquos MAC protocol unslotted CSMACD

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

52

Ethernet CSMACD algorithmbull 1 NIC receives data-

gram from network layer creates frame

bull 2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

bull 3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

bull 4 If NIC detects an-other transmission while transmitting aborts and sends jam signal

bull 5 After aborting NIC enters exponential backoff after mth colli-sion NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

53

Ethernetrsquos CSMACD (more)bull Jam Signal make

sure all other transmitters are aware of collision 48 bits

bull Bit time 1 micro sec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Exponential Backoff bull Goal adapt retransmission

attempts to estimated cur-rent load

bull heavy load random wait will be longer

bull first collision choose K from 01 delay is K 512 bit transmission times

bull after second collision choose K from 0123hellip

bull after ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

54

CSMACD efficiencybull Tprop = max prop delay between 2 nodes in

LANbull ttrans = time to transmit max-size frame

bull efficiency goes to 1 bull as tprop goes to 0

bull as ttrans goes to infinity

bull better performance than ALOHA and simple cheap decentralized

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

transprop ttefficiency

51

1

55

8023 Ethernet Standards Link amp Physical Layersbull many different Ethernet standards

bull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps

1Gbps 10G bpsbull different physical layer media fiber cable

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

applicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twisterpair) physical layer

56

Manchester encodingbull used in 10BaseTbull each bit has a transitionbull allows clocks in sending and receiving nodes to syn-

chronize to each otherbull no need for a centralized global clock among nodes

bull Hey this is physical-layer stuff

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

57

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

58

Hubshellip physical-layer (ldquodumbrdquo) repeaters

bull bits coming in one link go out all other links at same rate

bull all nodes connected to hub can collide with one another

bull no frame bufferingbull no CSMACD at hub host NICs detect colli-

sions

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

twisted pair

hub

59

Switchbull link-layer device smarter than hubs take

active rolebull store forward Ethernet framesbull examine incoming framersquos MAC address se-

lectively forward frame to one-or-more out-going links when frame is to be forwarded on segment uses CSMACD to access segment

bull transparentbull hosts are unaware of presence of switches

bull plug-and-play self-learning (HOW)bull switches do not need to be configured

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

60

Switch allows multiple simulta-neous transmissionsbull hosts have dedicated direct

connection to switchbull switches buffer packetsbull Ethernet protocol used on

each incoming link but no collisions full duplexbull each link is its own collision

domain

bull switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions bull not possible with dumb hub

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 23

45

6

61

Switch Tablebull Q how does switch know

that Arsquo reachable via inter-face 4 Brsquo reachable via inter-face 5

bull A each switch has a switch table each entrybull (MAC address of host interface

to reach host time stamp)

bull looks like a routing tablebull Q how are entries created

maintained in switch table bull something like a routing proto-

colFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

switch with six interfaces(123456)

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

62

Switch self-learningbull switch learns which

hosts can be reached through which inter-facesbull when frame received

switch ldquolearnsrdquo location of sender incoming LAN segment

bull records senderlocation pair in switch table

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

63

Switch frame filteringforward-ingWhen frame received

1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination then if dest on segment from which frame arrived then drop the frame else

forward the frame on interface indicated else flood

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

forward on all but the interface on which the frame arrived

64

Self-learning forwarding exam-ple

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

frame destination unknownflood

Arsquo A

destination A location known

Arsquo 4 60

selective send

65

Interconnecting switchesbull switches can be connected together

bull Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3

bull A self learning (works exactly the same as in single-switch case)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

B

S1

C D

E

FS2

S4

S3

H

I

G

66

Self-learning multi-switch exam-plebull Suppose C sends frame to I I responds to

C

bull Q show switch tables and packet forward-ing in S1 S2 S3 S4

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

B

S1

CD

E F

S2

S4

S3

H

I

G

12

1

2 3

3

1 2 3 12 3

444

67

Institutional network

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

to externalnetwork

router

IP subnet

mail server

web server

68

Switches vs Routersbull both store-and-forward devices

bull routers network layer devices (examine network layer head-ers)

bull switches are link layer devices

bull routers maintain routing tables implement routing al-gorithms

bull switches maintain switch tables implement filtering learning algorithms

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

69

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

• Fall 2015 COMP 46055605 Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link layer context
• Link Layer Services
• Link Layer Services (more)
• Where is the link layer implemented
• Adaptors Communicating
• Link Layer (2)
• Error Detection
• Parity Checking
• Internet checksum (review)
• Checksumming Cyclic Redundancy Check
• Checksumming Cyclic Redundancy Check (2)
• CRC Example
• Link Layer (3)
• Multiple Access Links and Protocols
• Multiple Access protocols
• Ideal Multiple Access Protocol
• MAC Protocols a taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD collision detection
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Link Layer (4)
• MAC Addresses and ARP
• LAN Addresses and ARP
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP protocol Same LAN (network)
• Addressing routing to another LAN
• Slide 45
• Link Layer (5)
• Ethernet
• Star Topology
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester encoding
• Link Layer (6)
• Hubs
• Switch
• Switch allows multiple simultaneous transmissions
• Switch Table
• Switch self-learning
• Switch frame filteringforwarding
• Self-learning forwarding example
• Interconnecting switches
• Self-learning multi-switch example
• Institutional network
• Switches vs Routers
• Link Layer (7)

25

Slotted ALOHAAssumptionsbull all frames same sizebull time divided into

equal size slots (time to transmit 1 frame)

bull nodes start to transmit only slot beginning

bull nodes are synchro-nized

bull if 2 or more nodes transmit in slot all nodes detect collision

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Operationbull when node obtains

fresh frame trans-mits in next slotbull if no collision node

can send new frame in next slot

bull if collision node re-transmits frame in each subsequent slot with prob p until success

26

Slotted ALOHA

Prosbull single active node can

continuously transmit at full rate of channel

bull highly decentralized only slots in nodes need to be in sync

bull simpleFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Consbull collisions wasting slotsbull idle slotsbull nodes may be able to

detect collision in less than time to transmit packet

bull clock synchronization

27

Slotted Aloha Efficiency

bull suppose N nodes with many frames to send each transmits in slot with probability p

bull prob that given node has success in a slot = p(1-p)N-1

bull prob that any node has a success = Np(1-p)N-1

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

bull max efficiency find p that max-imizes Np(1-p)N-1

bull for many nodes take limit of Np(1- p)N-1 as N goes to infinity gives

Max efficiency = 1e = 37

Efficiency long-run fraction of successful slots (many nodes all with many frames to send)

At best channelused for useful transmissions 37of time

28

Pure (unslotted) ALOHAbull unslotted Aloha simpler no synchro-

nizationbull when frame first arrives

bull transmit immediately

bull collision probability increasesbull frame sent at t0 collides with other frames sent in

[t0-1t0+1]

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

29

Pure Aloha EfficiencyP(success by given node) = P(node transmits)

P(no other node transmits in [t0-1t0]

P(no other node transmits in [t0t0+1]

= p (1-p)N-1 (1-p)N-1

= p (1-p)2(N-1)

hellip choosing optimum p and then letting

= 1(2e) = 18

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

N

even worse than slotted Aloha

30

CSMA (Carrier Sense Multiple Access)CSMA listen before transmitIf channel sensed idle transmit entire framebull If channel sensed busy defer transmis-

sion

bull human analogy donrsquot interrupt others

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

31

CSMA Collisionscollisions can still occurbull propagation delay means

two nodes may not hear each otherrsquos transmission

collisionbull entire packet transmis-

sion time wasted

notebull role of distance amp propa-

gation delay in determin-ing collision probability

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

spatial layout of nodes

32

CSMACD (Collision Detec-tion)CSMACD carrier sensing deferral as in CSMA

bull collisions detected within short timebull colliding transmissions aborted reducing channel

wastage

bull collision detection bull easy in wired LANs measure signal strengths

compare transmitted received signalsbull difficult in wireless LANs received signal strength

overwhelmed by local transmission strength

bull human analogy the polite conversationalist

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

33

CSMACD collision detection

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

34

ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols

bull share channel efficiently and fairly at high loadbull inefficient at low load delay in channel access

1N bandwidth allocated even if only 1 active node

Random access MAC protocolsbull efficient at low load single node can fully uti-

lize channelbull high load collision overhead

ldquotaking turnsrdquo protocolsbull look for best of both worlds

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

35

ldquoTaking Turnsrdquo MAC protocolsPolling bull master node ldquoinvitesrdquo

slave nodes to transmit in turn

bull typically used with ldquodumbrdquo slave devices

bull concernsbull polling overhead bull latencybull single point of failure

(master)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

T

data

(nothingto send)

T

36

ldquoTaking Turnsrdquo MAC protocolsToken passingbull control token

passed from one node to next se-quentially

bull token messagebull concerns

bull token overhead bull latencybull single point of failure

(token)Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

master

slaves

poll

data

data

37

Summary of MAC protocolsbull channel partitioning by time frequency or

codebull Time Division Frequency Division

bull random access (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire)

hard in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211

bull taking turnsbull polling from central site token passing

bull Bluetooth FDDI IBM Token Ring Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

38

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

39

MAC Addresses and ARPbull 32-bit IP address

bull network-layer addressbull used to get datagram to destination IP subnet

bull MAC (or LAN or physical or Ethernet) ad-dress bull function get frame from one interface to an-

other physically-connected interface (same network)

bull 48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software set-

table

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

40

LAN Addresses and ARP

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Each adapter on LAN has unique LAN address

Broadcast address =FF-FF-FF-FF-FF-FF

= adapter

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired orwireless)

41

LAN Address (more)bull MAC address allocation administered by IEEEbull manufacturer buys portion of MAC address

space (to assure uniqueness)bull analogy

(a) MAC address like Social Security Num-ber

(b) IP address like postal addressbull MAC flat address portability

bull can move LAN card from one LAN to another

bull IP hierarchical address NOT portablebull address depends on IP subnet to which node is at-

tachedFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

42

ARP Address Resolution Proto-colbull Each IP node (host

router) on LAN has ARP table

bull ARP table IPMAC address mappings for some LAN nodeslt IP address MAC address

TTLgt

bull TTL (Time To Live) time after which ad-dress mapping will be forgotten (typically 20 min)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Question how to determineMAC address of Bknowing Brsquos IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN

137196723

137196778

137196714

137196788

43

ARP protocol Same LAN (net-work)bull A wants to send datagram

to B and Brsquos MAC address not in Arsquos ARP table

bull A broadcasts ARP query packet containing Bs IP address bull dest MAC address = FF-FF-FF-

FF-FF-FFbull all machines on LAN receive

ARP query

bull B receives ARP packet replies to A with its (Bs) MAC addressbull frame sent to Arsquos MAC ad-

dress (unicast)Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

bull A caches (saves) IP-to-MAC address pair in its ARP table until infor-mation becomes old (times out) bull soft state information

that times out (goes away) unless refreshed

bull ARP is ldquoplug-and-playrdquobull nodes create their ARP

tables without interven-tion from net administra-tor

44

Addressing routing to another LAN

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

R

1A-23-F9-CD-06-9B

222222222220111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

walkthrough send datagram from A to B via R assume A knows Brsquos IP address

bull two ARP tables in router R one for each IP network (LAN)

45

bull A creates IP datagram with source A destination B bull A uses ARP to get Rrsquos MAC address for 111111111110bull A creates link-layer frame with Rs MAC address as dest

frame contains A-to-B IP datagrambull Arsquos NIC sends frame bull Rrsquos NIC receives frame bull R removes IP datagram from Ethernet frame sees its des-

tined to Bbull R uses ARP to get Brsquos MAC address bull R creates frame containing A-to-B IP datagram sends to B

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

This is a really importantexample ndash make sure youunderstand

R

1A-23-F9-CD-06-9B

222222222220

111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

46

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

47

Ethernetldquodominantrdquo wired LAN technology bull cheap $20 for NICbull first widely used LAN technologybull simpler cheaper than token LANs and ATMbull kept up with speed race 10 Mbps ndash 10 Gbps

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Metcalfersquos Ethernetsketch

48

Star Topologybull bus topology popular through mid 90s

bull all nodes in same collision domain (can collide with each other)

bull today star topology prevailsbull active switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes do

not collide with each other)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

switch

bus coaxial cable star

49

Ethernet Frame Structurebull Sending adapter encapsulates IP datagram

(or other network layer protocol packet) in Ethernet frame

Preamble bull 7 bytes with pattern 10101010 followed by

one byte with pattern 10101011bull used to synchronize receiver sender clock

ratesFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

50

Ethernet Frame Structure (more)bull Addresses 6 bytes

bull if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

bull otherwise adapter discards frame

bull Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)

bull CRC checked at receiver if error is detected frame is dropped

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51

Ethernet Unreliable connection-lessbull connectionless No handshaking be-

tween sending and receiving NICs bull unreliable receiving NIC doesnrsquot send

acks or nacks to sending NICbull stream of datagrams passed to network

layer can have gaps (missing datagrams)bull gaps will be filled if app is using TCPbull otherwise app will see gaps

bull Ethernetrsquos MAC protocol unslotted CSMACD

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

52

Ethernet CSMACD algorithmbull 1 NIC receives data-

gram from network layer creates frame

bull 2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

bull 3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

bull 4 If NIC detects an-other transmission while transmitting aborts and sends jam signal

bull 5 After aborting NIC enters exponential backoff after mth colli-sion NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

53

Ethernetrsquos CSMACD (more)bull Jam Signal make

sure all other transmitters are aware of collision 48 bits

bull Bit time 1 micro sec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Exponential Backoff bull Goal adapt retransmission

attempts to estimated cur-rent load

bull heavy load random wait will be longer

bull first collision choose K from 01 delay is K 512 bit transmission times

bull after second collision choose K from 0123hellip

bull after ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

54

CSMACD efficiencybull Tprop = max prop delay between 2 nodes in

LANbull ttrans = time to transmit max-size frame

bull efficiency goes to 1 bull as tprop goes to 0

bull as ttrans goes to infinity

bull better performance than ALOHA and simple cheap decentralized

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

transprop ttefficiency

51

1

55

8023 Ethernet Standards Link amp Physical Layersbull many different Ethernet standards

bull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps

1Gbps 10G bpsbull different physical layer media fiber cable

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

applicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twisterpair) physical layer

56

Manchester encodingbull used in 10BaseTbull each bit has a transitionbull allows clocks in sending and receiving nodes to syn-

chronize to each otherbull no need for a centralized global clock among nodes

bull Hey this is physical-layer stuff

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

57

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

58

Hubshellip physical-layer (ldquodumbrdquo) repeaters

bull bits coming in one link go out all other links at same rate

bull all nodes connected to hub can collide with one another

bull no frame bufferingbull no CSMACD at hub host NICs detect colli-

sions

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

twisted pair

hub

59

Switchbull link-layer device smarter than hubs take

active rolebull store forward Ethernet framesbull examine incoming framersquos MAC address se-

lectively forward frame to one-or-more out-going links when frame is to be forwarded on segment uses CSMACD to access segment

bull transparentbull hosts are unaware of presence of switches

bull plug-and-play self-learning (HOW)bull switches do not need to be configured

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

60

Switch allows multiple simulta-neous transmissionsbull hosts have dedicated direct

connection to switchbull switches buffer packetsbull Ethernet protocol used on

each incoming link but no collisions full duplexbull each link is its own collision

domain

bull switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions bull not possible with dumb hub

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 23

45

6

61

Switch Tablebull Q how does switch know

that Arsquo reachable via inter-face 4 Brsquo reachable via inter-face 5

bull A each switch has a switch table each entrybull (MAC address of host interface

to reach host time stamp)

bull looks like a routing tablebull Q how are entries created

maintained in switch table bull something like a routing proto-

colFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

switch with six interfaces(123456)

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

62

Switch self-learningbull switch learns which

hosts can be reached through which inter-facesbull when frame received

switch ldquolearnsrdquo location of sender incoming LAN segment

bull records senderlocation pair in switch table

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

63

Switch frame filteringforward-ingWhen frame received

1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination then if dest on segment from which frame arrived then drop the frame else

forward the frame on interface indicated else flood

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

forward on all but the interface on which the frame arrived

64

Self-learning forwarding exam-ple

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

frame destination unknownflood

Arsquo A

destination A location known

Arsquo 4 60

selective send

65

Interconnecting switchesbull switches can be connected together

bull Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3

bull A self learning (works exactly the same as in single-switch case)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

B

S1

C D

E

FS2

S4

S3

H

I

G

66

Self-learning multi-switch exam-plebull Suppose C sends frame to I I responds to

C

bull Q show switch tables and packet forward-ing in S1 S2 S3 S4

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

B

S1

CD

E F

S2

S4

S3

H

I

G

12

1

2 3

3

1 2 3 12 3

444

67

Institutional network

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

to externalnetwork

router

IP subnet

mail server

web server

68

Switches vs Routersbull both store-and-forward devices

bull routers network layer devices (examine network layer head-ers)

bull switches are link layer devices

bull routers maintain routing tables implement routing al-gorithms

bull switches maintain switch tables implement filtering learning algorithms

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

69

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

• Fall 2015 COMP 46055605 Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link layer context
• Link Layer Services
• Link Layer Services (more)
• Where is the link layer implemented
• Adaptors Communicating
• Link Layer (2)
• Error Detection
• Parity Checking
• Internet checksum (review)
• Checksumming Cyclic Redundancy Check
• Checksumming Cyclic Redundancy Check (2)
• CRC Example
• Link Layer (3)
• Multiple Access Links and Protocols
• Multiple Access protocols
• Ideal Multiple Access Protocol
• MAC Protocols a taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD collision detection
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Link Layer (4)
• MAC Addresses and ARP
• LAN Addresses and ARP
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP protocol Same LAN (network)
• Addressing routing to another LAN
• Slide 45
• Link Layer (5)
• Ethernet
• Star Topology
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester encoding
• Link Layer (6)
• Hubs
• Switch
• Switch allows multiple simultaneous transmissions
• Switch Table
• Switch self-learning
• Switch frame filteringforwarding
• Self-learning forwarding example
• Interconnecting switches
• Self-learning multi-switch example
• Institutional network
• Switches vs Routers
• Link Layer (7)

26

Slotted ALOHA

Prosbull single active node can

continuously transmit at full rate of channel

bull highly decentralized only slots in nodes need to be in sync

bull simpleFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Consbull collisions wasting slotsbull idle slotsbull nodes may be able to

detect collision in less than time to transmit packet

bull clock synchronization

27

Slotted Aloha Efficiency

bull suppose N nodes with many frames to send each transmits in slot with probability p

bull prob that given node has success in a slot = p(1-p)N-1

bull prob that any node has a success = Np(1-p)N-1

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

bull max efficiency find p that max-imizes Np(1-p)N-1

bull for many nodes take limit of Np(1- p)N-1 as N goes to infinity gives

Max efficiency = 1e = 37

Efficiency long-run fraction of successful slots (many nodes all with many frames to send)

At best channelused for useful transmissions 37of time

28

Pure (unslotted) ALOHAbull unslotted Aloha simpler no synchro-

nizationbull when frame first arrives

bull transmit immediately

bull collision probability increasesbull frame sent at t0 collides with other frames sent in

[t0-1t0+1]

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

29

Pure Aloha EfficiencyP(success by given node) = P(node transmits)

P(no other node transmits in [t0-1t0]

P(no other node transmits in [t0t0+1]

= p (1-p)N-1 (1-p)N-1

= p (1-p)2(N-1)

hellip choosing optimum p and then letting

= 1(2e) = 18

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

N

even worse than slotted Aloha

30

CSMA (Carrier Sense Multiple Access)CSMA listen before transmitIf channel sensed idle transmit entire framebull If channel sensed busy defer transmis-

sion

bull human analogy donrsquot interrupt others

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

31

CSMA Collisionscollisions can still occurbull propagation delay means

two nodes may not hear each otherrsquos transmission

collisionbull entire packet transmis-

sion time wasted

notebull role of distance amp propa-

gation delay in determin-ing collision probability

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

spatial layout of nodes

32

CSMACD (Collision Detec-tion)CSMACD carrier sensing deferral as in CSMA

bull collisions detected within short timebull colliding transmissions aborted reducing channel

wastage

bull collision detection bull easy in wired LANs measure signal strengths

compare transmitted received signalsbull difficult in wireless LANs received signal strength

overwhelmed by local transmission strength

bull human analogy the polite conversationalist

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

33

CSMACD collision detection

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

34

ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols

bull share channel efficiently and fairly at high loadbull inefficient at low load delay in channel access

1N bandwidth allocated even if only 1 active node

Random access MAC protocolsbull efficient at low load single node can fully uti-

lize channelbull high load collision overhead

ldquotaking turnsrdquo protocolsbull look for best of both worlds

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

35

ldquoTaking Turnsrdquo MAC protocolsPolling bull master node ldquoinvitesrdquo

slave nodes to transmit in turn

bull typically used with ldquodumbrdquo slave devices

bull concernsbull polling overhead bull latencybull single point of failure

(master)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

T

data

(nothingto send)

T

36

ldquoTaking Turnsrdquo MAC protocolsToken passingbull control token

passed from one node to next se-quentially

bull token messagebull concerns

bull token overhead bull latencybull single point of failure

(token)Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

master

slaves

poll

data

data

37

Summary of MAC protocolsbull channel partitioning by time frequency or

codebull Time Division Frequency Division

bull random access (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire)

hard in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211

bull taking turnsbull polling from central site token passing

bull Bluetooth FDDI IBM Token Ring Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

38

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

39

MAC Addresses and ARPbull 32-bit IP address

bull network-layer addressbull used to get datagram to destination IP subnet

bull MAC (or LAN or physical or Ethernet) ad-dress bull function get frame from one interface to an-

other physically-connected interface (same network)

bull 48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software set-

table

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

40

LAN Addresses and ARP

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Each adapter on LAN has unique LAN address

Broadcast address =FF-FF-FF-FF-FF-FF

= adapter

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired orwireless)

41

LAN Address (more)bull MAC address allocation administered by IEEEbull manufacturer buys portion of MAC address

space (to assure uniqueness)bull analogy

(a) MAC address like Social Security Num-ber

(b) IP address like postal addressbull MAC flat address portability

bull can move LAN card from one LAN to another

bull IP hierarchical address NOT portablebull address depends on IP subnet to which node is at-

tachedFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

42

ARP Address Resolution Proto-colbull Each IP node (host

router) on LAN has ARP table

bull ARP table IPMAC address mappings for some LAN nodeslt IP address MAC address

TTLgt

bull TTL (Time To Live) time after which ad-dress mapping will be forgotten (typically 20 min)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Question how to determineMAC address of Bknowing Brsquos IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN

137196723

137196778

137196714

137196788

43

ARP protocol Same LAN (net-work)bull A wants to send datagram

to B and Brsquos MAC address not in Arsquos ARP table

bull A broadcasts ARP query packet containing Bs IP address bull dest MAC address = FF-FF-FF-

FF-FF-FFbull all machines on LAN receive

ARP query

bull B receives ARP packet replies to A with its (Bs) MAC addressbull frame sent to Arsquos MAC ad-

dress (unicast)Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

bull A caches (saves) IP-to-MAC address pair in its ARP table until infor-mation becomes old (times out) bull soft state information

that times out (goes away) unless refreshed

bull ARP is ldquoplug-and-playrdquobull nodes create their ARP

tables without interven-tion from net administra-tor

44

Addressing routing to another LAN

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

R

1A-23-F9-CD-06-9B

222222222220111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

walkthrough send datagram from A to B via R assume A knows Brsquos IP address

bull two ARP tables in router R one for each IP network (LAN)

45

bull A creates IP datagram with source A destination B bull A uses ARP to get Rrsquos MAC address for 111111111110bull A creates link-layer frame with Rs MAC address as dest

frame contains A-to-B IP datagrambull Arsquos NIC sends frame bull Rrsquos NIC receives frame bull R removes IP datagram from Ethernet frame sees its des-

tined to Bbull R uses ARP to get Brsquos MAC address bull R creates frame containing A-to-B IP datagram sends to B

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

This is a really importantexample ndash make sure youunderstand

R

1A-23-F9-CD-06-9B

222222222220

111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

46

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

47

Ethernetldquodominantrdquo wired LAN technology bull cheap $20 for NICbull first widely used LAN technologybull simpler cheaper than token LANs and ATMbull kept up with speed race 10 Mbps ndash 10 Gbps

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Metcalfersquos Ethernetsketch

48

Star Topologybull bus topology popular through mid 90s

bull all nodes in same collision domain (can collide with each other)

bull today star topology prevailsbull active switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes do

not collide with each other)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

switch

bus coaxial cable star

49

Ethernet Frame Structurebull Sending adapter encapsulates IP datagram

(or other network layer protocol packet) in Ethernet frame

Preamble bull 7 bytes with pattern 10101010 followed by

one byte with pattern 10101011bull used to synchronize receiver sender clock

ratesFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

50

Ethernet Frame Structure (more)bull Addresses 6 bytes

bull if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

bull otherwise adapter discards frame

bull Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)

bull CRC checked at receiver if error is detected frame is dropped

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51

Ethernet Unreliable connection-lessbull connectionless No handshaking be-

tween sending and receiving NICs bull unreliable receiving NIC doesnrsquot send

acks or nacks to sending NICbull stream of datagrams passed to network

layer can have gaps (missing datagrams)bull gaps will be filled if app is using TCPbull otherwise app will see gaps

bull Ethernetrsquos MAC protocol unslotted CSMACD

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

52

Ethernet CSMACD algorithmbull 1 NIC receives data-

gram from network layer creates frame

bull 2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

bull 3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

bull 4 If NIC detects an-other transmission while transmitting aborts and sends jam signal

bull 5 After aborting NIC enters exponential backoff after mth colli-sion NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

53

Ethernetrsquos CSMACD (more)bull Jam Signal make

sure all other transmitters are aware of collision 48 bits

bull Bit time 1 micro sec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Exponential Backoff bull Goal adapt retransmission

attempts to estimated cur-rent load

bull heavy load random wait will be longer

bull first collision choose K from 01 delay is K 512 bit transmission times

bull after second collision choose K from 0123hellip

bull after ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

54

CSMACD efficiencybull Tprop = max prop delay between 2 nodes in

LANbull ttrans = time to transmit max-size frame

bull efficiency goes to 1 bull as tprop goes to 0

bull as ttrans goes to infinity

bull better performance than ALOHA and simple cheap decentralized

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

transprop ttefficiency

51

1

55

8023 Ethernet Standards Link amp Physical Layersbull many different Ethernet standards

bull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps

1Gbps 10G bpsbull different physical layer media fiber cable

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

applicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twisterpair) physical layer

56

Manchester encodingbull used in 10BaseTbull each bit has a transitionbull allows clocks in sending and receiving nodes to syn-

chronize to each otherbull no need for a centralized global clock among nodes

bull Hey this is physical-layer stuff

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

57

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

58

Hubshellip physical-layer (ldquodumbrdquo) repeaters

bull bits coming in one link go out all other links at same rate

bull all nodes connected to hub can collide with one another

bull no frame bufferingbull no CSMACD at hub host NICs detect colli-

sions

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

twisted pair

hub

59

Switchbull link-layer device smarter than hubs take

active rolebull store forward Ethernet framesbull examine incoming framersquos MAC address se-

lectively forward frame to one-or-more out-going links when frame is to be forwarded on segment uses CSMACD to access segment

bull transparentbull hosts are unaware of presence of switches

bull plug-and-play self-learning (HOW)bull switches do not need to be configured

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

60

Switch allows multiple simulta-neous transmissionsbull hosts have dedicated direct

connection to switchbull switches buffer packetsbull Ethernet protocol used on

each incoming link but no collisions full duplexbull each link is its own collision

domain

bull switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions bull not possible with dumb hub

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 23

45

6

61

Switch Tablebull Q how does switch know

that Arsquo reachable via inter-face 4 Brsquo reachable via inter-face 5

bull A each switch has a switch table each entrybull (MAC address of host interface

to reach host time stamp)

bull looks like a routing tablebull Q how are entries created

maintained in switch table bull something like a routing proto-

colFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

switch with six interfaces(123456)

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

62

Switch self-learningbull switch learns which

hosts can be reached through which inter-facesbull when frame received

switch ldquolearnsrdquo location of sender incoming LAN segment

bull records senderlocation pair in switch table

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

63

Switch frame filteringforward-ingWhen frame received

1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination then if dest on segment from which frame arrived then drop the frame else

forward the frame on interface indicated else flood

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

forward on all but the interface on which the frame arrived

64

Self-learning forwarding exam-ple

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

frame destination unknownflood

Arsquo A

destination A location known

Arsquo 4 60

selective send

65

Interconnecting switchesbull switches can be connected together

bull Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3

bull A self learning (works exactly the same as in single-switch case)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

B

S1

C D

E

FS2

S4

S3

H

I

G

66

Self-learning multi-switch exam-plebull Suppose C sends frame to I I responds to

C

bull Q show switch tables and packet forward-ing in S1 S2 S3 S4

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

B

S1

CD

E F

S2

S4

S3

H

I

G

12

1

2 3

3

1 2 3 12 3

444

67

Institutional network

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

to externalnetwork

router

IP subnet

mail server

web server

68

Switches vs Routersbull both store-and-forward devices

bull routers network layer devices (examine network layer head-ers)

bull switches are link layer devices

bull routers maintain routing tables implement routing al-gorithms

bull switches maintain switch tables implement filtering learning algorithms

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

69

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

• Fall 2015 COMP 46055605 Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link layer context
• Link Layer Services
• Link Layer Services (more)
• Where is the link layer implemented
• Adaptors Communicating
• Link Layer (2)
• Error Detection
• Parity Checking
• Internet checksum (review)
• Checksumming Cyclic Redundancy Check
• Checksumming Cyclic Redundancy Check (2)
• CRC Example
• Link Layer (3)
• Multiple Access Links and Protocols
• Multiple Access protocols
• Ideal Multiple Access Protocol
• MAC Protocols a taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD collision detection
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Link Layer (4)
• MAC Addresses and ARP
• LAN Addresses and ARP
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP protocol Same LAN (network)
• Addressing routing to another LAN
• Slide 45
• Link Layer (5)
• Ethernet
• Star Topology
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester encoding
• Link Layer (6)
• Hubs
• Switch
• Switch allows multiple simultaneous transmissions
• Switch Table
• Switch self-learning
• Switch frame filteringforwarding
• Self-learning forwarding example
• Interconnecting switches
• Self-learning multi-switch example
• Institutional network
• Switches vs Routers
• Link Layer (7)

27

Slotted Aloha Efficiency

bull suppose N nodes with many frames to send each transmits in slot with probability p

bull prob that given node has success in a slot = p(1-p)N-1

bull prob that any node has a success = Np(1-p)N-1

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

bull max efficiency find p that max-imizes Np(1-p)N-1

bull for many nodes take limit of Np(1- p)N-1 as N goes to infinity gives

Max efficiency = 1e = 37

Efficiency long-run fraction of successful slots (many nodes all with many frames to send)

At best channelused for useful transmissions 37of time

28

Pure (unslotted) ALOHAbull unslotted Aloha simpler no synchro-

nizationbull when frame first arrives

bull transmit immediately

bull collision probability increasesbull frame sent at t0 collides with other frames sent in

[t0-1t0+1]

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

29

Pure Aloha EfficiencyP(success by given node) = P(node transmits)

P(no other node transmits in [t0-1t0]

P(no other node transmits in [t0t0+1]

= p (1-p)N-1 (1-p)N-1

= p (1-p)2(N-1)

hellip choosing optimum p and then letting

= 1(2e) = 18

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

N

even worse than slotted Aloha

30

CSMA (Carrier Sense Multiple Access)CSMA listen before transmitIf channel sensed idle transmit entire framebull If channel sensed busy defer transmis-

sion

bull human analogy donrsquot interrupt others

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

31

CSMA Collisionscollisions can still occurbull propagation delay means

two nodes may not hear each otherrsquos transmission

collisionbull entire packet transmis-

sion time wasted

notebull role of distance amp propa-

gation delay in determin-ing collision probability

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

spatial layout of nodes

32

CSMACD (Collision Detec-tion)CSMACD carrier sensing deferral as in CSMA

bull collisions detected within short timebull colliding transmissions aborted reducing channel

wastage

bull collision detection bull easy in wired LANs measure signal strengths

compare transmitted received signalsbull difficult in wireless LANs received signal strength

overwhelmed by local transmission strength

bull human analogy the polite conversationalist

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

33

CSMACD collision detection

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

34

ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols

bull share channel efficiently and fairly at high loadbull inefficient at low load delay in channel access

1N bandwidth allocated even if only 1 active node

Random access MAC protocolsbull efficient at low load single node can fully uti-

lize channelbull high load collision overhead

ldquotaking turnsrdquo protocolsbull look for best of both worlds

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

35

ldquoTaking Turnsrdquo MAC protocolsPolling bull master node ldquoinvitesrdquo

slave nodes to transmit in turn

bull typically used with ldquodumbrdquo slave devices

bull concernsbull polling overhead bull latencybull single point of failure

(master)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

T

data

(nothingto send)

T

36

ldquoTaking Turnsrdquo MAC protocolsToken passingbull control token

passed from one node to next se-quentially

bull token messagebull concerns

bull token overhead bull latencybull single point of failure

(token)Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

master

slaves

poll

data

data

37

Summary of MAC protocolsbull channel partitioning by time frequency or

codebull Time Division Frequency Division

bull random access (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire)

hard in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211

bull taking turnsbull polling from central site token passing

bull Bluetooth FDDI IBM Token Ring Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

38

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

39

MAC Addresses and ARPbull 32-bit IP address

bull network-layer addressbull used to get datagram to destination IP subnet

bull MAC (or LAN or physical or Ethernet) ad-dress bull function get frame from one interface to an-

other physically-connected interface (same network)

bull 48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software set-

table

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

40

LAN Addresses and ARP

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Each adapter on LAN has unique LAN address

Broadcast address =FF-FF-FF-FF-FF-FF

= adapter

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired orwireless)

41

LAN Address (more)bull MAC address allocation administered by IEEEbull manufacturer buys portion of MAC address

space (to assure uniqueness)bull analogy

(a) MAC address like Social Security Num-ber

(b) IP address like postal addressbull MAC flat address portability

bull can move LAN card from one LAN to another

bull IP hierarchical address NOT portablebull address depends on IP subnet to which node is at-

tachedFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

42

ARP Address Resolution Proto-colbull Each IP node (host

router) on LAN has ARP table

bull ARP table IPMAC address mappings for some LAN nodeslt IP address MAC address

TTLgt

bull TTL (Time To Live) time after which ad-dress mapping will be forgotten (typically 20 min)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Question how to determineMAC address of Bknowing Brsquos IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN

137196723

137196778

137196714

137196788

43

ARP protocol Same LAN (net-work)bull A wants to send datagram

to B and Brsquos MAC address not in Arsquos ARP table

bull A broadcasts ARP query packet containing Bs IP address bull dest MAC address = FF-FF-FF-

FF-FF-FFbull all machines on LAN receive

ARP query

bull B receives ARP packet replies to A with its (Bs) MAC addressbull frame sent to Arsquos MAC ad-

dress (unicast)Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

bull A caches (saves) IP-to-MAC address pair in its ARP table until infor-mation becomes old (times out) bull soft state information

that times out (goes away) unless refreshed

bull ARP is ldquoplug-and-playrdquobull nodes create their ARP

tables without interven-tion from net administra-tor

44

Addressing routing to another LAN

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

R

1A-23-F9-CD-06-9B

222222222220111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

walkthrough send datagram from A to B via R assume A knows Brsquos IP address

bull two ARP tables in router R one for each IP network (LAN)

45

bull A creates IP datagram with source A destination B bull A uses ARP to get Rrsquos MAC address for 111111111110bull A creates link-layer frame with Rs MAC address as dest

frame contains A-to-B IP datagrambull Arsquos NIC sends frame bull Rrsquos NIC receives frame bull R removes IP datagram from Ethernet frame sees its des-

tined to Bbull R uses ARP to get Brsquos MAC address bull R creates frame containing A-to-B IP datagram sends to B

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

This is a really importantexample ndash make sure youunderstand

R

1A-23-F9-CD-06-9B

222222222220

111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

46

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

47

Ethernetldquodominantrdquo wired LAN technology bull cheap $20 for NICbull first widely used LAN technologybull simpler cheaper than token LANs and ATMbull kept up with speed race 10 Mbps ndash 10 Gbps

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Metcalfersquos Ethernetsketch

48

Star Topologybull bus topology popular through mid 90s

bull all nodes in same collision domain (can collide with each other)

bull today star topology prevailsbull active switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes do

not collide with each other)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

switch

bus coaxial cable star

49

Ethernet Frame Structurebull Sending adapter encapsulates IP datagram

(or other network layer protocol packet) in Ethernet frame

Preamble bull 7 bytes with pattern 10101010 followed by

one byte with pattern 10101011bull used to synchronize receiver sender clock

ratesFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

50

Ethernet Frame Structure (more)bull Addresses 6 bytes

bull if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

bull otherwise adapter discards frame

bull Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)

bull CRC checked at receiver if error is detected frame is dropped

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51

Ethernet Unreliable connection-lessbull connectionless No handshaking be-

tween sending and receiving NICs bull unreliable receiving NIC doesnrsquot send

acks or nacks to sending NICbull stream of datagrams passed to network

layer can have gaps (missing datagrams)bull gaps will be filled if app is using TCPbull otherwise app will see gaps

bull Ethernetrsquos MAC protocol unslotted CSMACD

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

52

Ethernet CSMACD algorithmbull 1 NIC receives data-

gram from network layer creates frame

bull 2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

bull 3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

bull 4 If NIC detects an-other transmission while transmitting aborts and sends jam signal

bull 5 After aborting NIC enters exponential backoff after mth colli-sion NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

53

Ethernetrsquos CSMACD (more)bull Jam Signal make

sure all other transmitters are aware of collision 48 bits

bull Bit time 1 micro sec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Exponential Backoff bull Goal adapt retransmission

attempts to estimated cur-rent load

bull heavy load random wait will be longer

bull first collision choose K from 01 delay is K 512 bit transmission times

bull after second collision choose K from 0123hellip

bull after ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

54

CSMACD efficiencybull Tprop = max prop delay between 2 nodes in

LANbull ttrans = time to transmit max-size frame

bull efficiency goes to 1 bull as tprop goes to 0

bull as ttrans goes to infinity

bull better performance than ALOHA and simple cheap decentralized

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

transprop ttefficiency

51

1

55

8023 Ethernet Standards Link amp Physical Layersbull many different Ethernet standards

bull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps

1Gbps 10G bpsbull different physical layer media fiber cable

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

applicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twisterpair) physical layer

56

Manchester encodingbull used in 10BaseTbull each bit has a transitionbull allows clocks in sending and receiving nodes to syn-

chronize to each otherbull no need for a centralized global clock among nodes

bull Hey this is physical-layer stuff

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

57

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

58

Hubshellip physical-layer (ldquodumbrdquo) repeaters

bull bits coming in one link go out all other links at same rate

bull all nodes connected to hub can collide with one another

bull no frame bufferingbull no CSMACD at hub host NICs detect colli-

sions

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

twisted pair

hub

59

Switchbull link-layer device smarter than hubs take

active rolebull store forward Ethernet framesbull examine incoming framersquos MAC address se-

lectively forward frame to one-or-more out-going links when frame is to be forwarded on segment uses CSMACD to access segment

bull transparentbull hosts are unaware of presence of switches

bull plug-and-play self-learning (HOW)bull switches do not need to be configured

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

60

Switch allows multiple simulta-neous transmissionsbull hosts have dedicated direct

connection to switchbull switches buffer packetsbull Ethernet protocol used on

each incoming link but no collisions full duplexbull each link is its own collision

domain

bull switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions bull not possible with dumb hub

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 23

45

6

61

Switch Tablebull Q how does switch know

that Arsquo reachable via inter-face 4 Brsquo reachable via inter-face 5

bull A each switch has a switch table each entrybull (MAC address of host interface

to reach host time stamp)

bull looks like a routing tablebull Q how are entries created

maintained in switch table bull something like a routing proto-

colFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

switch with six interfaces(123456)

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

62

Switch self-learningbull switch learns which

hosts can be reached through which inter-facesbull when frame received

switch ldquolearnsrdquo location of sender incoming LAN segment

bull records senderlocation pair in switch table

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

63

Switch frame filteringforward-ingWhen frame received

1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination then if dest on segment from which frame arrived then drop the frame else

forward the frame on interface indicated else flood

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

forward on all but the interface on which the frame arrived

64

Self-learning forwarding exam-ple

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

frame destination unknownflood

Arsquo A

destination A location known

Arsquo 4 60

selective send

65

Interconnecting switchesbull switches can be connected together

bull Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3

bull A self learning (works exactly the same as in single-switch case)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

B

S1

C D

E

FS2

S4

S3

H

I

G

66

Self-learning multi-switch exam-plebull Suppose C sends frame to I I responds to

C

bull Q show switch tables and packet forward-ing in S1 S2 S3 S4

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

B

S1

CD

E F

S2

S4

S3

H

I

G

12

1

2 3

3

1 2 3 12 3

444

67

Institutional network

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

to externalnetwork

router

IP subnet

mail server

web server

68

Switches vs Routersbull both store-and-forward devices

bull routers network layer devices (examine network layer head-ers)

bull switches are link layer devices

bull routers maintain routing tables implement routing al-gorithms

bull switches maintain switch tables implement filtering learning algorithms

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

69

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

• Fall 2015 COMP 46055605 Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link layer context
• Link Layer Services
• Link Layer Services (more)
• Where is the link layer implemented
• Adaptors Communicating
• Link Layer (2)
• Error Detection
• Parity Checking
• Internet checksum (review)
• Checksumming Cyclic Redundancy Check
• Checksumming Cyclic Redundancy Check (2)
• CRC Example
• Link Layer (3)
• Multiple Access Links and Protocols
• Multiple Access protocols
• Ideal Multiple Access Protocol
• MAC Protocols a taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD collision detection
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Link Layer (4)
• MAC Addresses and ARP
• LAN Addresses and ARP
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP protocol Same LAN (network)
• Addressing routing to another LAN
• Slide 45
• Link Layer (5)
• Ethernet
• Star Topology
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester encoding
• Link Layer (6)
• Hubs
• Switch
• Switch allows multiple simultaneous transmissions
• Switch Table
• Switch self-learning
• Switch frame filteringforwarding
• Self-learning forwarding example
• Interconnecting switches
• Self-learning multi-switch example
• Institutional network
• Switches vs Routers
• Link Layer (7)

28

Pure (unslotted) ALOHAbull unslotted Aloha simpler no synchro-

nizationbull when frame first arrives

bull transmit immediately

bull collision probability increasesbull frame sent at t0 collides with other frames sent in

[t0-1t0+1]

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

29

Pure Aloha EfficiencyP(success by given node) = P(node transmits)

P(no other node transmits in [t0-1t0]

P(no other node transmits in [t0t0+1]

= p (1-p)N-1 (1-p)N-1

= p (1-p)2(N-1)

hellip choosing optimum p and then letting

= 1(2e) = 18

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

N

even worse than slotted Aloha

30

CSMA (Carrier Sense Multiple Access)CSMA listen before transmitIf channel sensed idle transmit entire framebull If channel sensed busy defer transmis-

sion

bull human analogy donrsquot interrupt others

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

31

CSMA Collisionscollisions can still occurbull propagation delay means

two nodes may not hear each otherrsquos transmission

collisionbull entire packet transmis-

sion time wasted

notebull role of distance amp propa-

gation delay in determin-ing collision probability

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

spatial layout of nodes

32

CSMACD (Collision Detec-tion)CSMACD carrier sensing deferral as in CSMA

bull collisions detected within short timebull colliding transmissions aborted reducing channel

wastage

bull collision detection bull easy in wired LANs measure signal strengths

compare transmitted received signalsbull difficult in wireless LANs received signal strength

overwhelmed by local transmission strength

bull human analogy the polite conversationalist

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

33

CSMACD collision detection

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

34

ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols

bull share channel efficiently and fairly at high loadbull inefficient at low load delay in channel access

1N bandwidth allocated even if only 1 active node

Random access MAC protocolsbull efficient at low load single node can fully uti-

lize channelbull high load collision overhead

ldquotaking turnsrdquo protocolsbull look for best of both worlds

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

35

ldquoTaking Turnsrdquo MAC protocolsPolling bull master node ldquoinvitesrdquo

slave nodes to transmit in turn

bull typically used with ldquodumbrdquo slave devices

bull concernsbull polling overhead bull latencybull single point of failure

(master)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

T

data

(nothingto send)

T

36

ldquoTaking Turnsrdquo MAC protocolsToken passingbull control token

passed from one node to next se-quentially

bull token messagebull concerns

bull token overhead bull latencybull single point of failure

(token)Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

master

slaves

poll

data

data

37

Summary of MAC protocolsbull channel partitioning by time frequency or

codebull Time Division Frequency Division

bull random access (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire)

hard in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211

bull taking turnsbull polling from central site token passing

bull Bluetooth FDDI IBM Token Ring Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

38

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

39

MAC Addresses and ARPbull 32-bit IP address

bull network-layer addressbull used to get datagram to destination IP subnet

bull MAC (or LAN or physical or Ethernet) ad-dress bull function get frame from one interface to an-

other physically-connected interface (same network)

bull 48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software set-

table

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

40

LAN Addresses and ARP

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Each adapter on LAN has unique LAN address

Broadcast address =FF-FF-FF-FF-FF-FF

= adapter

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired orwireless)

41

LAN Address (more)bull MAC address allocation administered by IEEEbull manufacturer buys portion of MAC address

space (to assure uniqueness)bull analogy

(a) MAC address like Social Security Num-ber

(b) IP address like postal addressbull MAC flat address portability

bull can move LAN card from one LAN to another

bull IP hierarchical address NOT portablebull address depends on IP subnet to which node is at-

tachedFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

42

ARP Address Resolution Proto-colbull Each IP node (host

router) on LAN has ARP table

bull ARP table IPMAC address mappings for some LAN nodeslt IP address MAC address

TTLgt

bull TTL (Time To Live) time after which ad-dress mapping will be forgotten (typically 20 min)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Question how to determineMAC address of Bknowing Brsquos IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN

137196723

137196778

137196714

137196788

43

ARP protocol Same LAN (net-work)bull A wants to send datagram

to B and Brsquos MAC address not in Arsquos ARP table

bull A broadcasts ARP query packet containing Bs IP address bull dest MAC address = FF-FF-FF-

FF-FF-FFbull all machines on LAN receive

ARP query

bull B receives ARP packet replies to A with its (Bs) MAC addressbull frame sent to Arsquos MAC ad-

dress (unicast)Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

bull A caches (saves) IP-to-MAC address pair in its ARP table until infor-mation becomes old (times out) bull soft state information

that times out (goes away) unless refreshed

bull ARP is ldquoplug-and-playrdquobull nodes create their ARP

tables without interven-tion from net administra-tor

44

Addressing routing to another LAN

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

R

1A-23-F9-CD-06-9B

222222222220111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

walkthrough send datagram from A to B via R assume A knows Brsquos IP address

bull two ARP tables in router R one for each IP network (LAN)

45

bull A creates IP datagram with source A destination B bull A uses ARP to get Rrsquos MAC address for 111111111110bull A creates link-layer frame with Rs MAC address as dest

frame contains A-to-B IP datagrambull Arsquos NIC sends frame bull Rrsquos NIC receives frame bull R removes IP datagram from Ethernet frame sees its des-

tined to Bbull R uses ARP to get Brsquos MAC address bull R creates frame containing A-to-B IP datagram sends to B

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

This is a really importantexample ndash make sure youunderstand

R

1A-23-F9-CD-06-9B

222222222220

111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

46

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

47

Ethernetldquodominantrdquo wired LAN technology bull cheap $20 for NICbull first widely used LAN technologybull simpler cheaper than token LANs and ATMbull kept up with speed race 10 Mbps ndash 10 Gbps

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Metcalfersquos Ethernetsketch

48

Star Topologybull bus topology popular through mid 90s

bull all nodes in same collision domain (can collide with each other)

bull today star topology prevailsbull active switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes do

not collide with each other)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

switch

bus coaxial cable star

49

Ethernet Frame Structurebull Sending adapter encapsulates IP datagram

(or other network layer protocol packet) in Ethernet frame

Preamble bull 7 bytes with pattern 10101010 followed by

one byte with pattern 10101011bull used to synchronize receiver sender clock

ratesFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

50

Ethernet Frame Structure (more)bull Addresses 6 bytes

bull if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

bull otherwise adapter discards frame

bull Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)

bull CRC checked at receiver if error is detected frame is dropped

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51

Ethernet Unreliable connection-lessbull connectionless No handshaking be-

tween sending and receiving NICs bull unreliable receiving NIC doesnrsquot send

acks or nacks to sending NICbull stream of datagrams passed to network

layer can have gaps (missing datagrams)bull gaps will be filled if app is using TCPbull otherwise app will see gaps

bull Ethernetrsquos MAC protocol unslotted CSMACD

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

52

Ethernet CSMACD algorithmbull 1 NIC receives data-

gram from network layer creates frame

bull 2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

bull 3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

bull 4 If NIC detects an-other transmission while transmitting aborts and sends jam signal

bull 5 After aborting NIC enters exponential backoff after mth colli-sion NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

53

Ethernetrsquos CSMACD (more)bull Jam Signal make

sure all other transmitters are aware of collision 48 bits

bull Bit time 1 micro sec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Exponential Backoff bull Goal adapt retransmission

attempts to estimated cur-rent load

bull heavy load random wait will be longer

bull first collision choose K from 01 delay is K 512 bit transmission times

bull after second collision choose K from 0123hellip

bull after ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

54

CSMACD efficiencybull Tprop = max prop delay between 2 nodes in

LANbull ttrans = time to transmit max-size frame

bull efficiency goes to 1 bull as tprop goes to 0

bull as ttrans goes to infinity

bull better performance than ALOHA and simple cheap decentralized

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

transprop ttefficiency

51

1

55

8023 Ethernet Standards Link amp Physical Layersbull many different Ethernet standards

bull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps

1Gbps 10G bpsbull different physical layer media fiber cable

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

applicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twisterpair) physical layer

56

Manchester encodingbull used in 10BaseTbull each bit has a transitionbull allows clocks in sending and receiving nodes to syn-

chronize to each otherbull no need for a centralized global clock among nodes

bull Hey this is physical-layer stuff

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

57

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

58

Hubshellip physical-layer (ldquodumbrdquo) repeaters

bull bits coming in one link go out all other links at same rate

bull all nodes connected to hub can collide with one another

bull no frame bufferingbull no CSMACD at hub host NICs detect colli-

sions

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

twisted pair

hub

59

Switchbull link-layer device smarter than hubs take

active rolebull store forward Ethernet framesbull examine incoming framersquos MAC address se-

lectively forward frame to one-or-more out-going links when frame is to be forwarded on segment uses CSMACD to access segment

bull transparentbull hosts are unaware of presence of switches

bull plug-and-play self-learning (HOW)bull switches do not need to be configured

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

60

Switch allows multiple simulta-neous transmissionsbull hosts have dedicated direct

connection to switchbull switches buffer packetsbull Ethernet protocol used on

each incoming link but no collisions full duplexbull each link is its own collision

domain

bull switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions bull not possible with dumb hub

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 23

45

6

61

Switch Tablebull Q how does switch know

that Arsquo reachable via inter-face 4 Brsquo reachable via inter-face 5

bull A each switch has a switch table each entrybull (MAC address of host interface

to reach host time stamp)

bull looks like a routing tablebull Q how are entries created

maintained in switch table bull something like a routing proto-

colFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

switch with six interfaces(123456)

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

62

Switch self-learningbull switch learns which

hosts can be reached through which inter-facesbull when frame received

switch ldquolearnsrdquo location of sender incoming LAN segment

bull records senderlocation pair in switch table

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

63

Switch frame filteringforward-ingWhen frame received

1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination then if dest on segment from which frame arrived then drop the frame else

forward the frame on interface indicated else flood

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

forward on all but the interface on which the frame arrived

64

Self-learning forwarding exam-ple

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

frame destination unknownflood

Arsquo A

destination A location known

Arsquo 4 60

selective send

65

Interconnecting switchesbull switches can be connected together

bull Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3

bull A self learning (works exactly the same as in single-switch case)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

B

S1

C D

E

FS2

S4

S3

H

I

G

66

Self-learning multi-switch exam-plebull Suppose C sends frame to I I responds to

C

bull Q show switch tables and packet forward-ing in S1 S2 S3 S4

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

B

S1

CD

E F

S2

S4

S3

H

I

G

12

1

2 3

3

1 2 3 12 3

444

67

Institutional network

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

to externalnetwork

router

IP subnet

mail server

web server

68

Switches vs Routersbull both store-and-forward devices

bull routers network layer devices (examine network layer head-ers)

bull switches are link layer devices

bull routers maintain routing tables implement routing al-gorithms

bull switches maintain switch tables implement filtering learning algorithms

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

69

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

• Fall 2015 COMP 46055605 Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link layer context
• Link Layer Services
• Link Layer Services (more)
• Where is the link layer implemented
• Adaptors Communicating
• Link Layer (2)
• Error Detection
• Parity Checking
• Internet checksum (review)
• Checksumming Cyclic Redundancy Check
• Checksumming Cyclic Redundancy Check (2)
• CRC Example
• Link Layer (3)
• Multiple Access Links and Protocols
• Multiple Access protocols
• Ideal Multiple Access Protocol
• MAC Protocols a taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD collision detection
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Link Layer (4)
• MAC Addresses and ARP
• LAN Addresses and ARP
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP protocol Same LAN (network)
• Addressing routing to another LAN
• Slide 45
• Link Layer (5)
• Ethernet
• Star Topology
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester encoding
• Link Layer (6)
• Hubs
• Switch
• Switch allows multiple simultaneous transmissions
• Switch Table
• Switch self-learning
• Switch frame filteringforwarding
• Self-learning forwarding example
• Interconnecting switches
• Self-learning multi-switch example
• Institutional network
• Switches vs Routers
• Link Layer (7)

29

Pure Aloha EfficiencyP(success by given node) = P(node transmits)

P(no other node transmits in [t0-1t0]

P(no other node transmits in [t0t0+1]

= p (1-p)N-1 (1-p)N-1

= p (1-p)2(N-1)

hellip choosing optimum p and then letting

= 1(2e) = 18

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

N

even worse than slotted Aloha

30

CSMA (Carrier Sense Multiple Access)CSMA listen before transmitIf channel sensed idle transmit entire framebull If channel sensed busy defer transmis-

sion

bull human analogy donrsquot interrupt others

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

31

CSMA Collisionscollisions can still occurbull propagation delay means

two nodes may not hear each otherrsquos transmission

collisionbull entire packet transmis-

sion time wasted

notebull role of distance amp propa-

gation delay in determin-ing collision probability

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

spatial layout of nodes

32

CSMACD (Collision Detec-tion)CSMACD carrier sensing deferral as in CSMA

bull collisions detected within short timebull colliding transmissions aborted reducing channel

wastage

bull collision detection bull easy in wired LANs measure signal strengths

compare transmitted received signalsbull difficult in wireless LANs received signal strength

overwhelmed by local transmission strength

bull human analogy the polite conversationalist

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

33

CSMACD collision detection

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

34

ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols

bull share channel efficiently and fairly at high loadbull inefficient at low load delay in channel access

1N bandwidth allocated even if only 1 active node

Random access MAC protocolsbull efficient at low load single node can fully uti-

lize channelbull high load collision overhead

ldquotaking turnsrdquo protocolsbull look for best of both worlds

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

35

ldquoTaking Turnsrdquo MAC protocolsPolling bull master node ldquoinvitesrdquo

slave nodes to transmit in turn

bull typically used with ldquodumbrdquo slave devices

bull concernsbull polling overhead bull latencybull single point of failure

(master)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

T

data

(nothingto send)

T

36

ldquoTaking Turnsrdquo MAC protocolsToken passingbull control token

passed from one node to next se-quentially

bull token messagebull concerns

bull token overhead bull latencybull single point of failure

(token)Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

master

slaves

poll

data

data

37

Summary of MAC protocolsbull channel partitioning by time frequency or

codebull Time Division Frequency Division

bull random access (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire)

hard in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211

bull taking turnsbull polling from central site token passing

bull Bluetooth FDDI IBM Token Ring Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

38

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

39

MAC Addresses and ARPbull 32-bit IP address

bull network-layer addressbull used to get datagram to destination IP subnet

bull MAC (or LAN or physical or Ethernet) ad-dress bull function get frame from one interface to an-

other physically-connected interface (same network)

bull 48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software set-

table

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

40

LAN Addresses and ARP

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Each adapter on LAN has unique LAN address

Broadcast address =FF-FF-FF-FF-FF-FF

= adapter

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired orwireless)

41

LAN Address (more)bull MAC address allocation administered by IEEEbull manufacturer buys portion of MAC address

space (to assure uniqueness)bull analogy

(a) MAC address like Social Security Num-ber

(b) IP address like postal addressbull MAC flat address portability

bull can move LAN card from one LAN to another

bull IP hierarchical address NOT portablebull address depends on IP subnet to which node is at-

tachedFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

42

ARP Address Resolution Proto-colbull Each IP node (host

router) on LAN has ARP table

bull ARP table IPMAC address mappings for some LAN nodeslt IP address MAC address

TTLgt

bull TTL (Time To Live) time after which ad-dress mapping will be forgotten (typically 20 min)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Question how to determineMAC address of Bknowing Brsquos IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN

137196723

137196778

137196714

137196788

43

ARP protocol Same LAN (net-work)bull A wants to send datagram

to B and Brsquos MAC address not in Arsquos ARP table

bull A broadcasts ARP query packet containing Bs IP address bull dest MAC address = FF-FF-FF-

FF-FF-FFbull all machines on LAN receive

ARP query

bull B receives ARP packet replies to A with its (Bs) MAC addressbull frame sent to Arsquos MAC ad-

dress (unicast)Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

bull A caches (saves) IP-to-MAC address pair in its ARP table until infor-mation becomes old (times out) bull soft state information

that times out (goes away) unless refreshed

bull ARP is ldquoplug-and-playrdquobull nodes create their ARP

tables without interven-tion from net administra-tor

44

Addressing routing to another LAN

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

R

1A-23-F9-CD-06-9B

222222222220111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

walkthrough send datagram from A to B via R assume A knows Brsquos IP address

bull two ARP tables in router R one for each IP network (LAN)

45

bull A creates IP datagram with source A destination B bull A uses ARP to get Rrsquos MAC address for 111111111110bull A creates link-layer frame with Rs MAC address as dest

frame contains A-to-B IP datagrambull Arsquos NIC sends frame bull Rrsquos NIC receives frame bull R removes IP datagram from Ethernet frame sees its des-

tined to Bbull R uses ARP to get Brsquos MAC address bull R creates frame containing A-to-B IP datagram sends to B

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

This is a really importantexample ndash make sure youunderstand

R

1A-23-F9-CD-06-9B

222222222220

111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

46

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

47

Ethernetldquodominantrdquo wired LAN technology bull cheap $20 for NICbull first widely used LAN technologybull simpler cheaper than token LANs and ATMbull kept up with speed race 10 Mbps ndash 10 Gbps

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Metcalfersquos Ethernetsketch

48

Star Topologybull bus topology popular through mid 90s

bull all nodes in same collision domain (can collide with each other)

bull today star topology prevailsbull active switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes do

not collide with each other)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

switch

bus coaxial cable star

49

Ethernet Frame Structurebull Sending adapter encapsulates IP datagram

(or other network layer protocol packet) in Ethernet frame

Preamble bull 7 bytes with pattern 10101010 followed by

one byte with pattern 10101011bull used to synchronize receiver sender clock

ratesFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

50

Ethernet Frame Structure (more)bull Addresses 6 bytes

bull if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

bull otherwise adapter discards frame

bull Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)

bull CRC checked at receiver if error is detected frame is dropped

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51

Ethernet Unreliable connection-lessbull connectionless No handshaking be-

tween sending and receiving NICs bull unreliable receiving NIC doesnrsquot send

acks or nacks to sending NICbull stream of datagrams passed to network

layer can have gaps (missing datagrams)bull gaps will be filled if app is using TCPbull otherwise app will see gaps

bull Ethernetrsquos MAC protocol unslotted CSMACD

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

52

Ethernet CSMACD algorithmbull 1 NIC receives data-

gram from network layer creates frame

bull 2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

bull 3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

bull 4 If NIC detects an-other transmission while transmitting aborts and sends jam signal

bull 5 After aborting NIC enters exponential backoff after mth colli-sion NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

53

Ethernetrsquos CSMACD (more)bull Jam Signal make

sure all other transmitters are aware of collision 48 bits

bull Bit time 1 micro sec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Exponential Backoff bull Goal adapt retransmission

attempts to estimated cur-rent load

bull heavy load random wait will be longer

bull first collision choose K from 01 delay is K 512 bit transmission times

bull after second collision choose K from 0123hellip

bull after ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

54

CSMACD efficiencybull Tprop = max prop delay between 2 nodes in

LANbull ttrans = time to transmit max-size frame

bull efficiency goes to 1 bull as tprop goes to 0

bull as ttrans goes to infinity

bull better performance than ALOHA and simple cheap decentralized

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

transprop ttefficiency

51

1

55

8023 Ethernet Standards Link amp Physical Layersbull many different Ethernet standards

bull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps

1Gbps 10G bpsbull different physical layer media fiber cable

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

applicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twisterpair) physical layer

56

Manchester encodingbull used in 10BaseTbull each bit has a transitionbull allows clocks in sending and receiving nodes to syn-

chronize to each otherbull no need for a centralized global clock among nodes

bull Hey this is physical-layer stuff

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

57

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

58

Hubshellip physical-layer (ldquodumbrdquo) repeaters

bull bits coming in one link go out all other links at same rate

bull all nodes connected to hub can collide with one another

bull no frame bufferingbull no CSMACD at hub host NICs detect colli-

sions

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

twisted pair

hub

59

Switchbull link-layer device smarter than hubs take

active rolebull store forward Ethernet framesbull examine incoming framersquos MAC address se-

lectively forward frame to one-or-more out-going links when frame is to be forwarded on segment uses CSMACD to access segment

bull transparentbull hosts are unaware of presence of switches

bull plug-and-play self-learning (HOW)bull switches do not need to be configured

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

60

Switch allows multiple simulta-neous transmissionsbull hosts have dedicated direct

connection to switchbull switches buffer packetsbull Ethernet protocol used on

each incoming link but no collisions full duplexbull each link is its own collision

domain

bull switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions bull not possible with dumb hub

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 23

45

6

61

Switch Tablebull Q how does switch know

that Arsquo reachable via inter-face 4 Brsquo reachable via inter-face 5

bull A each switch has a switch table each entrybull (MAC address of host interface

to reach host time stamp)

bull looks like a routing tablebull Q how are entries created

maintained in switch table bull something like a routing proto-

colFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

switch with six interfaces(123456)

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

62

Switch self-learningbull switch learns which

hosts can be reached through which inter-facesbull when frame received

switch ldquolearnsrdquo location of sender incoming LAN segment

bull records senderlocation pair in switch table

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

63

Switch frame filteringforward-ingWhen frame received

1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination then if dest on segment from which frame arrived then drop the frame else

forward the frame on interface indicated else flood

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

forward on all but the interface on which the frame arrived

64

Self-learning forwarding exam-ple

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

frame destination unknownflood

Arsquo A

destination A location known

Arsquo 4 60

selective send

65

Interconnecting switchesbull switches can be connected together

bull Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3

bull A self learning (works exactly the same as in single-switch case)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

B

S1

C D

E

FS2

S4

S3

H

I

G

66

Self-learning multi-switch exam-plebull Suppose C sends frame to I I responds to

C

bull Q show switch tables and packet forward-ing in S1 S2 S3 S4

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

B

S1

CD

E F

S2

S4

S3

H

I

G

12

1

2 3

3

1 2 3 12 3

444

67

Institutional network

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

to externalnetwork

router

IP subnet

mail server

web server

68

Switches vs Routersbull both store-and-forward devices

bull routers network layer devices (examine network layer head-ers)

bull switches are link layer devices

bull routers maintain routing tables implement routing al-gorithms

bull switches maintain switch tables implement filtering learning algorithms

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

69

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

• Fall 2015 COMP 46055605 Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link layer context
• Link Layer Services
• Link Layer Services (more)
• Where is the link layer implemented
• Adaptors Communicating
• Link Layer (2)
• Error Detection
• Parity Checking
• Internet checksum (review)
• Checksumming Cyclic Redundancy Check
• Checksumming Cyclic Redundancy Check (2)
• CRC Example
• Link Layer (3)
• Multiple Access Links and Protocols
• Multiple Access protocols
• Ideal Multiple Access Protocol
• MAC Protocols a taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD collision detection
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Link Layer (4)
• MAC Addresses and ARP
• LAN Addresses and ARP
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP protocol Same LAN (network)
• Addressing routing to another LAN
• Slide 45
• Link Layer (5)
• Ethernet
• Star Topology
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester encoding
• Link Layer (6)
• Hubs
• Switch
• Switch allows multiple simultaneous transmissions
• Switch Table
• Switch self-learning
• Switch frame filteringforwarding
• Self-learning forwarding example
• Interconnecting switches
• Self-learning multi-switch example
• Institutional network
• Switches vs Routers
• Link Layer (7)

30

CSMA (Carrier Sense Multiple Access)CSMA listen before transmitIf channel sensed idle transmit entire framebull If channel sensed busy defer transmis-

sion

bull human analogy donrsquot interrupt others

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

31

CSMA Collisionscollisions can still occurbull propagation delay means

two nodes may not hear each otherrsquos transmission

collisionbull entire packet transmis-

sion time wasted

notebull role of distance amp propa-

gation delay in determin-ing collision probability

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

spatial layout of nodes

32

CSMACD (Collision Detec-tion)CSMACD carrier sensing deferral as in CSMA

bull collisions detected within short timebull colliding transmissions aborted reducing channel

wastage

bull collision detection bull easy in wired LANs measure signal strengths

compare transmitted received signalsbull difficult in wireless LANs received signal strength

overwhelmed by local transmission strength

bull human analogy the polite conversationalist

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

33

CSMACD collision detection

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

34

ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols

bull share channel efficiently and fairly at high loadbull inefficient at low load delay in channel access

1N bandwidth allocated even if only 1 active node

Random access MAC protocolsbull efficient at low load single node can fully uti-

lize channelbull high load collision overhead

ldquotaking turnsrdquo protocolsbull look for best of both worlds

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

35

ldquoTaking Turnsrdquo MAC protocolsPolling bull master node ldquoinvitesrdquo

slave nodes to transmit in turn

bull typically used with ldquodumbrdquo slave devices

bull concernsbull polling overhead bull latencybull single point of failure

(master)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

T

data

(nothingto send)

T

36

ldquoTaking Turnsrdquo MAC protocolsToken passingbull control token

passed from one node to next se-quentially

bull token messagebull concerns

bull token overhead bull latencybull single point of failure

(token)Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

master

slaves

poll

data

data

37

Summary of MAC protocolsbull channel partitioning by time frequency or

codebull Time Division Frequency Division

bull random access (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire)

hard in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211

bull taking turnsbull polling from central site token passing

bull Bluetooth FDDI IBM Token Ring Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

38

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

39

MAC Addresses and ARPbull 32-bit IP address

bull network-layer addressbull used to get datagram to destination IP subnet

bull MAC (or LAN or physical or Ethernet) ad-dress bull function get frame from one interface to an-

other physically-connected interface (same network)

bull 48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software set-

table

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

40

LAN Addresses and ARP

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Each adapter on LAN has unique LAN address

Broadcast address =FF-FF-FF-FF-FF-FF

= adapter

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired orwireless)

41

LAN Address (more)bull MAC address allocation administered by IEEEbull manufacturer buys portion of MAC address

space (to assure uniqueness)bull analogy

(a) MAC address like Social Security Num-ber

(b) IP address like postal addressbull MAC flat address portability

bull can move LAN card from one LAN to another

bull IP hierarchical address NOT portablebull address depends on IP subnet to which node is at-

tachedFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

42

ARP Address Resolution Proto-colbull Each IP node (host

router) on LAN has ARP table

bull ARP table IPMAC address mappings for some LAN nodeslt IP address MAC address

TTLgt

bull TTL (Time To Live) time after which ad-dress mapping will be forgotten (typically 20 min)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Question how to determineMAC address of Bknowing Brsquos IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN

137196723

137196778

137196714

137196788

43

ARP protocol Same LAN (net-work)bull A wants to send datagram

to B and Brsquos MAC address not in Arsquos ARP table

bull A broadcasts ARP query packet containing Bs IP address bull dest MAC address = FF-FF-FF-

FF-FF-FFbull all machines on LAN receive

ARP query

bull B receives ARP packet replies to A with its (Bs) MAC addressbull frame sent to Arsquos MAC ad-

dress (unicast)Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

bull A caches (saves) IP-to-MAC address pair in its ARP table until infor-mation becomes old (times out) bull soft state information

that times out (goes away) unless refreshed

bull ARP is ldquoplug-and-playrdquobull nodes create their ARP

tables without interven-tion from net administra-tor

44

Addressing routing to another LAN

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

R

1A-23-F9-CD-06-9B

222222222220111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

walkthrough send datagram from A to B via R assume A knows Brsquos IP address

bull two ARP tables in router R one for each IP network (LAN)

45

bull A creates IP datagram with source A destination B bull A uses ARP to get Rrsquos MAC address for 111111111110bull A creates link-layer frame with Rs MAC address as dest

frame contains A-to-B IP datagrambull Arsquos NIC sends frame bull Rrsquos NIC receives frame bull R removes IP datagram from Ethernet frame sees its des-

tined to Bbull R uses ARP to get Brsquos MAC address bull R creates frame containing A-to-B IP datagram sends to B

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

This is a really importantexample ndash make sure youunderstand

R

1A-23-F9-CD-06-9B

222222222220

111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

46

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

47

Ethernetldquodominantrdquo wired LAN technology bull cheap $20 for NICbull first widely used LAN technologybull simpler cheaper than token LANs and ATMbull kept up with speed race 10 Mbps ndash 10 Gbps

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Metcalfersquos Ethernetsketch

48

Star Topologybull bus topology popular through mid 90s

bull all nodes in same collision domain (can collide with each other)

bull today star topology prevailsbull active switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes do

not collide with each other)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

switch

bus coaxial cable star

49

Ethernet Frame Structurebull Sending adapter encapsulates IP datagram

(or other network layer protocol packet) in Ethernet frame

Preamble bull 7 bytes with pattern 10101010 followed by

one byte with pattern 10101011bull used to synchronize receiver sender clock

ratesFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

50

Ethernet Frame Structure (more)bull Addresses 6 bytes

bull if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

bull otherwise adapter discards frame

bull Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)

bull CRC checked at receiver if error is detected frame is dropped

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51

Ethernet Unreliable connection-lessbull connectionless No handshaking be-

tween sending and receiving NICs bull unreliable receiving NIC doesnrsquot send

acks or nacks to sending NICbull stream of datagrams passed to network

layer can have gaps (missing datagrams)bull gaps will be filled if app is using TCPbull otherwise app will see gaps

bull Ethernetrsquos MAC protocol unslotted CSMACD

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

52

Ethernet CSMACD algorithmbull 1 NIC receives data-

gram from network layer creates frame

bull 2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

bull 3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

bull 4 If NIC detects an-other transmission while transmitting aborts and sends jam signal

bull 5 After aborting NIC enters exponential backoff after mth colli-sion NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

53

Ethernetrsquos CSMACD (more)bull Jam Signal make

sure all other transmitters are aware of collision 48 bits

bull Bit time 1 micro sec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Exponential Backoff bull Goal adapt retransmission

attempts to estimated cur-rent load

bull heavy load random wait will be longer

bull first collision choose K from 01 delay is K 512 bit transmission times

bull after second collision choose K from 0123hellip

bull after ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

54

CSMACD efficiencybull Tprop = max prop delay between 2 nodes in

LANbull ttrans = time to transmit max-size frame

bull efficiency goes to 1 bull as tprop goes to 0

bull as ttrans goes to infinity

bull better performance than ALOHA and simple cheap decentralized

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

transprop ttefficiency

51

1

55

8023 Ethernet Standards Link amp Physical Layersbull many different Ethernet standards

bull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps

1Gbps 10G bpsbull different physical layer media fiber cable

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

applicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twisterpair) physical layer

56

Manchester encodingbull used in 10BaseTbull each bit has a transitionbull allows clocks in sending and receiving nodes to syn-

chronize to each otherbull no need for a centralized global clock among nodes

bull Hey this is physical-layer stuff

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

57

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

58

Hubshellip physical-layer (ldquodumbrdquo) repeaters

bull bits coming in one link go out all other links at same rate

bull all nodes connected to hub can collide with one another

bull no frame bufferingbull no CSMACD at hub host NICs detect colli-

sions

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

twisted pair

hub

59

Switchbull link-layer device smarter than hubs take

active rolebull store forward Ethernet framesbull examine incoming framersquos MAC address se-

lectively forward frame to one-or-more out-going links when frame is to be forwarded on segment uses CSMACD to access segment

bull transparentbull hosts are unaware of presence of switches

bull plug-and-play self-learning (HOW)bull switches do not need to be configured

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

60

Switch allows multiple simulta-neous transmissionsbull hosts have dedicated direct

connection to switchbull switches buffer packetsbull Ethernet protocol used on

each incoming link but no collisions full duplexbull each link is its own collision

domain

bull switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions bull not possible with dumb hub

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 23

45

6

61

Switch Tablebull Q how does switch know

that Arsquo reachable via inter-face 4 Brsquo reachable via inter-face 5

bull A each switch has a switch table each entrybull (MAC address of host interface

to reach host time stamp)

bull looks like a routing tablebull Q how are entries created

maintained in switch table bull something like a routing proto-

colFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

switch with six interfaces(123456)

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

62

Switch self-learningbull switch learns which

hosts can be reached through which inter-facesbull when frame received

switch ldquolearnsrdquo location of sender incoming LAN segment

bull records senderlocation pair in switch table

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

63

Switch frame filteringforward-ingWhen frame received

1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination then if dest on segment from which frame arrived then drop the frame else

forward the frame on interface indicated else flood

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

forward on all but the interface on which the frame arrived

64

Self-learning forwarding exam-ple

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

frame destination unknownflood

Arsquo A

destination A location known

Arsquo 4 60

selective send

65

Interconnecting switchesbull switches can be connected together

bull Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3

bull A self learning (works exactly the same as in single-switch case)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

B

S1

C D

E

FS2

S4

S3

H

I

G

66

Self-learning multi-switch exam-plebull Suppose C sends frame to I I responds to

C

bull Q show switch tables and packet forward-ing in S1 S2 S3 S4

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

B

S1

CD

E F

S2

S4

S3

H

I

G

12

1

2 3

3

1 2 3 12 3

444

67

Institutional network

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

to externalnetwork

router

IP subnet

mail server

web server

68

Switches vs Routersbull both store-and-forward devices

bull routers network layer devices (examine network layer head-ers)

bull switches are link layer devices

bull routers maintain routing tables implement routing al-gorithms

bull switches maintain switch tables implement filtering learning algorithms

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

69

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

• Fall 2015 COMP 46055605 Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link layer context
• Link Layer Services
• Link Layer Services (more)
• Where is the link layer implemented
• Adaptors Communicating
• Link Layer (2)
• Error Detection
• Parity Checking
• Internet checksum (review)
• Checksumming Cyclic Redundancy Check
• Checksumming Cyclic Redundancy Check (2)
• CRC Example
• Link Layer (3)
• Multiple Access Links and Protocols
• Multiple Access protocols
• Ideal Multiple Access Protocol
• MAC Protocols a taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD collision detection
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Link Layer (4)
• MAC Addresses and ARP
• LAN Addresses and ARP
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP protocol Same LAN (network)
• Addressing routing to another LAN
• Slide 45
• Link Layer (5)
• Ethernet
• Star Topology
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester encoding
• Link Layer (6)
• Hubs
• Switch
• Switch allows multiple simultaneous transmissions
• Switch Table
• Switch self-learning
• Switch frame filteringforwarding
• Self-learning forwarding example
• Interconnecting switches
• Self-learning multi-switch example
• Institutional network
• Switches vs Routers
• Link Layer (7)

31

CSMA Collisionscollisions can still occurbull propagation delay means

two nodes may not hear each otherrsquos transmission

collisionbull entire packet transmis-

sion time wasted

notebull role of distance amp propa-

gation delay in determin-ing collision probability

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

spatial layout of nodes

32

CSMACD (Collision Detec-tion)CSMACD carrier sensing deferral as in CSMA

bull collisions detected within short timebull colliding transmissions aborted reducing channel

wastage

bull collision detection bull easy in wired LANs measure signal strengths

compare transmitted received signalsbull difficult in wireless LANs received signal strength

overwhelmed by local transmission strength

bull human analogy the polite conversationalist

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

33

CSMACD collision detection

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

34

ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols

bull share channel efficiently and fairly at high loadbull inefficient at low load delay in channel access

1N bandwidth allocated even if only 1 active node

Random access MAC protocolsbull efficient at low load single node can fully uti-

lize channelbull high load collision overhead

ldquotaking turnsrdquo protocolsbull look for best of both worlds

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

35

ldquoTaking Turnsrdquo MAC protocolsPolling bull master node ldquoinvitesrdquo

slave nodes to transmit in turn

bull typically used with ldquodumbrdquo slave devices

bull concernsbull polling overhead bull latencybull single point of failure

(master)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

T

data

(nothingto send)

T

36

ldquoTaking Turnsrdquo MAC protocolsToken passingbull control token

passed from one node to next se-quentially

bull token messagebull concerns

bull token overhead bull latencybull single point of failure

(token)Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

master

slaves

poll

data

data

37

Summary of MAC protocolsbull channel partitioning by time frequency or

codebull Time Division Frequency Division

bull random access (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire)

hard in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211

bull taking turnsbull polling from central site token passing

bull Bluetooth FDDI IBM Token Ring Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

38

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

39

MAC Addresses and ARPbull 32-bit IP address

bull network-layer addressbull used to get datagram to destination IP subnet

bull MAC (or LAN or physical or Ethernet) ad-dress bull function get frame from one interface to an-

other physically-connected interface (same network)

bull 48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software set-

table

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

40

LAN Addresses and ARP

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Each adapter on LAN has unique LAN address

Broadcast address =FF-FF-FF-FF-FF-FF

= adapter

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired orwireless)

41

LAN Address (more)bull MAC address allocation administered by IEEEbull manufacturer buys portion of MAC address

space (to assure uniqueness)bull analogy

(a) MAC address like Social Security Num-ber

(b) IP address like postal addressbull MAC flat address portability

bull can move LAN card from one LAN to another

bull IP hierarchical address NOT portablebull address depends on IP subnet to which node is at-

tachedFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

42

ARP Address Resolution Proto-colbull Each IP node (host

router) on LAN has ARP table

bull ARP table IPMAC address mappings for some LAN nodeslt IP address MAC address

TTLgt

bull TTL (Time To Live) time after which ad-dress mapping will be forgotten (typically 20 min)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Question how to determineMAC address of Bknowing Brsquos IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN

137196723

137196778

137196714

137196788

43

ARP protocol Same LAN (net-work)bull A wants to send datagram

to B and Brsquos MAC address not in Arsquos ARP table

bull A broadcasts ARP query packet containing Bs IP address bull dest MAC address = FF-FF-FF-

FF-FF-FFbull all machines on LAN receive

ARP query

bull B receives ARP packet replies to A with its (Bs) MAC addressbull frame sent to Arsquos MAC ad-

dress (unicast)Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

bull A caches (saves) IP-to-MAC address pair in its ARP table until infor-mation becomes old (times out) bull soft state information

that times out (goes away) unless refreshed

bull ARP is ldquoplug-and-playrdquobull nodes create their ARP

tables without interven-tion from net administra-tor

44

Addressing routing to another LAN

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

R

1A-23-F9-CD-06-9B

222222222220111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

walkthrough send datagram from A to B via R assume A knows Brsquos IP address

bull two ARP tables in router R one for each IP network (LAN)

45

bull A creates IP datagram with source A destination B bull A uses ARP to get Rrsquos MAC address for 111111111110bull A creates link-layer frame with Rs MAC address as dest

frame contains A-to-B IP datagrambull Arsquos NIC sends frame bull Rrsquos NIC receives frame bull R removes IP datagram from Ethernet frame sees its des-

tined to Bbull R uses ARP to get Brsquos MAC address bull R creates frame containing A-to-B IP datagram sends to B

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

This is a really importantexample ndash make sure youunderstand

R

1A-23-F9-CD-06-9B

222222222220

111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

46

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

47

Ethernetldquodominantrdquo wired LAN technology bull cheap $20 for NICbull first widely used LAN technologybull simpler cheaper than token LANs and ATMbull kept up with speed race 10 Mbps ndash 10 Gbps

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Metcalfersquos Ethernetsketch

48

Star Topologybull bus topology popular through mid 90s

bull all nodes in same collision domain (can collide with each other)

bull today star topology prevailsbull active switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes do

not collide with each other)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

switch

bus coaxial cable star

49

Ethernet Frame Structurebull Sending adapter encapsulates IP datagram

(or other network layer protocol packet) in Ethernet frame

Preamble bull 7 bytes with pattern 10101010 followed by

one byte with pattern 10101011bull used to synchronize receiver sender clock

ratesFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

50

Ethernet Frame Structure (more)bull Addresses 6 bytes

bull if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

bull otherwise adapter discards frame

bull Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)

bull CRC checked at receiver if error is detected frame is dropped

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51

Ethernet Unreliable connection-lessbull connectionless No handshaking be-

tween sending and receiving NICs bull unreliable receiving NIC doesnrsquot send

acks or nacks to sending NICbull stream of datagrams passed to network

layer can have gaps (missing datagrams)bull gaps will be filled if app is using TCPbull otherwise app will see gaps

bull Ethernetrsquos MAC protocol unslotted CSMACD

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

52

Ethernet CSMACD algorithmbull 1 NIC receives data-

gram from network layer creates frame

bull 2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

bull 3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

bull 4 If NIC detects an-other transmission while transmitting aborts and sends jam signal

bull 5 After aborting NIC enters exponential backoff after mth colli-sion NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

53

Ethernetrsquos CSMACD (more)bull Jam Signal make

sure all other transmitters are aware of collision 48 bits

bull Bit time 1 micro sec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Exponential Backoff bull Goal adapt retransmission

attempts to estimated cur-rent load

bull heavy load random wait will be longer

bull first collision choose K from 01 delay is K 512 bit transmission times

bull after second collision choose K from 0123hellip

bull after ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

54

CSMACD efficiencybull Tprop = max prop delay between 2 nodes in

LANbull ttrans = time to transmit max-size frame

bull efficiency goes to 1 bull as tprop goes to 0

bull as ttrans goes to infinity

bull better performance than ALOHA and simple cheap decentralized

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

transprop ttefficiency

51

1

55

8023 Ethernet Standards Link amp Physical Layersbull many different Ethernet standards

bull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps

1Gbps 10G bpsbull different physical layer media fiber cable

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

applicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twisterpair) physical layer

56

Manchester encodingbull used in 10BaseTbull each bit has a transitionbull allows clocks in sending and receiving nodes to syn-

chronize to each otherbull no need for a centralized global clock among nodes

bull Hey this is physical-layer stuff

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

57

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

58

Hubshellip physical-layer (ldquodumbrdquo) repeaters

bull bits coming in one link go out all other links at same rate

bull all nodes connected to hub can collide with one another

bull no frame bufferingbull no CSMACD at hub host NICs detect colli-

sions

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

twisted pair

hub

59

Switchbull link-layer device smarter than hubs take

active rolebull store forward Ethernet framesbull examine incoming framersquos MAC address se-

lectively forward frame to one-or-more out-going links when frame is to be forwarded on segment uses CSMACD to access segment

bull transparentbull hosts are unaware of presence of switches

bull plug-and-play self-learning (HOW)bull switches do not need to be configured

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

60

Switch allows multiple simulta-neous transmissionsbull hosts have dedicated direct

connection to switchbull switches buffer packetsbull Ethernet protocol used on

each incoming link but no collisions full duplexbull each link is its own collision

domain

bull switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions bull not possible with dumb hub

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 23

45

6

61

Switch Tablebull Q how does switch know

that Arsquo reachable via inter-face 4 Brsquo reachable via inter-face 5

bull A each switch has a switch table each entrybull (MAC address of host interface

to reach host time stamp)

bull looks like a routing tablebull Q how are entries created

maintained in switch table bull something like a routing proto-

colFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

switch with six interfaces(123456)

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

62

Switch self-learningbull switch learns which

hosts can be reached through which inter-facesbull when frame received

switch ldquolearnsrdquo location of sender incoming LAN segment

bull records senderlocation pair in switch table

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

63

Switch frame filteringforward-ingWhen frame received

1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination then if dest on segment from which frame arrived then drop the frame else

forward the frame on interface indicated else flood

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

forward on all but the interface on which the frame arrived

64

Self-learning forwarding exam-ple

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

frame destination unknownflood

Arsquo A

destination A location known

Arsquo 4 60

selective send

65

Interconnecting switchesbull switches can be connected together

bull Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3

bull A self learning (works exactly the same as in single-switch case)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

B

S1

C D

E

FS2

S4

S3

H

I

G

66

Self-learning multi-switch exam-plebull Suppose C sends frame to I I responds to

C

bull Q show switch tables and packet forward-ing in S1 S2 S3 S4

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

B

S1

CD

E F

S2

S4

S3

H

I

G

12

1

2 3

3

1 2 3 12 3

444

67

Institutional network

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

to externalnetwork

router

IP subnet

mail server

web server

68

Switches vs Routersbull both store-and-forward devices

bull routers network layer devices (examine network layer head-ers)

bull switches are link layer devices

bull routers maintain routing tables implement routing al-gorithms

bull switches maintain switch tables implement filtering learning algorithms

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

69

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

• Fall 2015 COMP 46055605 Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link layer context
• Link Layer Services
• Link Layer Services (more)
• Where is the link layer implemented
• Adaptors Communicating
• Link Layer (2)
• Error Detection
• Parity Checking
• Internet checksum (review)
• Checksumming Cyclic Redundancy Check
• Checksumming Cyclic Redundancy Check (2)
• CRC Example
• Link Layer (3)
• Multiple Access Links and Protocols
• Multiple Access protocols
• Ideal Multiple Access Protocol
• MAC Protocols a taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD collision detection
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Link Layer (4)
• MAC Addresses and ARP
• LAN Addresses and ARP
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP protocol Same LAN (network)
• Addressing routing to another LAN
• Slide 45
• Link Layer (5)
• Ethernet
• Star Topology
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester encoding
• Link Layer (6)
• Hubs
• Switch
• Switch allows multiple simultaneous transmissions
• Switch Table
• Switch self-learning
• Switch frame filteringforwarding
• Self-learning forwarding example
• Interconnecting switches
• Self-learning multi-switch example
• Institutional network
• Switches vs Routers
• Link Layer (7)

32

CSMACD (Collision Detec-tion)CSMACD carrier sensing deferral as in CSMA

bull collisions detected within short timebull colliding transmissions aborted reducing channel

wastage

bull collision detection bull easy in wired LANs measure signal strengths

compare transmitted received signalsbull difficult in wireless LANs received signal strength

overwhelmed by local transmission strength

bull human analogy the polite conversationalist

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

33

CSMACD collision detection

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

34

ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols

bull share channel efficiently and fairly at high loadbull inefficient at low load delay in channel access

1N bandwidth allocated even if only 1 active node

Random access MAC protocolsbull efficient at low load single node can fully uti-

lize channelbull high load collision overhead

ldquotaking turnsrdquo protocolsbull look for best of both worlds

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

35

ldquoTaking Turnsrdquo MAC protocolsPolling bull master node ldquoinvitesrdquo

slave nodes to transmit in turn

bull typically used with ldquodumbrdquo slave devices

bull concernsbull polling overhead bull latencybull single point of failure

(master)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

T

data

(nothingto send)

T

36

ldquoTaking Turnsrdquo MAC protocolsToken passingbull control token

passed from one node to next se-quentially

bull token messagebull concerns

bull token overhead bull latencybull single point of failure

(token)Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

master

slaves

poll

data

data

37

Summary of MAC protocolsbull channel partitioning by time frequency or

codebull Time Division Frequency Division

bull random access (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire)

hard in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211

bull taking turnsbull polling from central site token passing

bull Bluetooth FDDI IBM Token Ring Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

38

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

39

MAC Addresses and ARPbull 32-bit IP address

bull network-layer addressbull used to get datagram to destination IP subnet

bull MAC (or LAN or physical or Ethernet) ad-dress bull function get frame from one interface to an-

other physically-connected interface (same network)

bull 48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software set-

table

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

40

LAN Addresses and ARP

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Each adapter on LAN has unique LAN address

Broadcast address =FF-FF-FF-FF-FF-FF

= adapter

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired orwireless)

41

LAN Address (more)bull MAC address allocation administered by IEEEbull manufacturer buys portion of MAC address

space (to assure uniqueness)bull analogy

(a) MAC address like Social Security Num-ber

(b) IP address like postal addressbull MAC flat address portability

bull can move LAN card from one LAN to another

bull IP hierarchical address NOT portablebull address depends on IP subnet to which node is at-

tachedFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

42

ARP Address Resolution Proto-colbull Each IP node (host

router) on LAN has ARP table

bull ARP table IPMAC address mappings for some LAN nodeslt IP address MAC address

TTLgt

bull TTL (Time To Live) time after which ad-dress mapping will be forgotten (typically 20 min)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Question how to determineMAC address of Bknowing Brsquos IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN

137196723

137196778

137196714

137196788

43

ARP protocol Same LAN (net-work)bull A wants to send datagram

to B and Brsquos MAC address not in Arsquos ARP table

bull A broadcasts ARP query packet containing Bs IP address bull dest MAC address = FF-FF-FF-

FF-FF-FFbull all machines on LAN receive

ARP query

bull B receives ARP packet replies to A with its (Bs) MAC addressbull frame sent to Arsquos MAC ad-

dress (unicast)Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

bull A caches (saves) IP-to-MAC address pair in its ARP table until infor-mation becomes old (times out) bull soft state information

that times out (goes away) unless refreshed

bull ARP is ldquoplug-and-playrdquobull nodes create their ARP

tables without interven-tion from net administra-tor

44

Addressing routing to another LAN

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

R

1A-23-F9-CD-06-9B

222222222220111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

walkthrough send datagram from A to B via R assume A knows Brsquos IP address

bull two ARP tables in router R one for each IP network (LAN)

45

bull A creates IP datagram with source A destination B bull A uses ARP to get Rrsquos MAC address for 111111111110bull A creates link-layer frame with Rs MAC address as dest

frame contains A-to-B IP datagrambull Arsquos NIC sends frame bull Rrsquos NIC receives frame bull R removes IP datagram from Ethernet frame sees its des-

tined to Bbull R uses ARP to get Brsquos MAC address bull R creates frame containing A-to-B IP datagram sends to B

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

This is a really importantexample ndash make sure youunderstand

R

1A-23-F9-CD-06-9B

222222222220

111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

46

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

47

Ethernetldquodominantrdquo wired LAN technology bull cheap $20 for NICbull first widely used LAN technologybull simpler cheaper than token LANs and ATMbull kept up with speed race 10 Mbps ndash 10 Gbps

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Metcalfersquos Ethernetsketch

48

Star Topologybull bus topology popular through mid 90s

bull all nodes in same collision domain (can collide with each other)

bull today star topology prevailsbull active switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes do

not collide with each other)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

switch

bus coaxial cable star

49

Ethernet Frame Structurebull Sending adapter encapsulates IP datagram

(or other network layer protocol packet) in Ethernet frame

Preamble bull 7 bytes with pattern 10101010 followed by

one byte with pattern 10101011bull used to synchronize receiver sender clock

ratesFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

50

Ethernet Frame Structure (more)bull Addresses 6 bytes

bull if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

bull otherwise adapter discards frame

bull Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)

bull CRC checked at receiver if error is detected frame is dropped

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51

Ethernet Unreliable connection-lessbull connectionless No handshaking be-

tween sending and receiving NICs bull unreliable receiving NIC doesnrsquot send

acks or nacks to sending NICbull stream of datagrams passed to network

layer can have gaps (missing datagrams)bull gaps will be filled if app is using TCPbull otherwise app will see gaps

bull Ethernetrsquos MAC protocol unslotted CSMACD

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

52

Ethernet CSMACD algorithmbull 1 NIC receives data-

gram from network layer creates frame

bull 2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

bull 3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

bull 4 If NIC detects an-other transmission while transmitting aborts and sends jam signal

bull 5 After aborting NIC enters exponential backoff after mth colli-sion NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

53

Ethernetrsquos CSMACD (more)bull Jam Signal make

sure all other transmitters are aware of collision 48 bits

bull Bit time 1 micro sec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Exponential Backoff bull Goal adapt retransmission

attempts to estimated cur-rent load

bull heavy load random wait will be longer

bull first collision choose K from 01 delay is K 512 bit transmission times

bull after second collision choose K from 0123hellip

bull after ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

54

CSMACD efficiencybull Tprop = max prop delay between 2 nodes in

LANbull ttrans = time to transmit max-size frame

bull efficiency goes to 1 bull as tprop goes to 0

bull as ttrans goes to infinity

bull better performance than ALOHA and simple cheap decentralized

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

transprop ttefficiency

51

1

55

8023 Ethernet Standards Link amp Physical Layersbull many different Ethernet standards

bull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps

1Gbps 10G bpsbull different physical layer media fiber cable

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

applicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twisterpair) physical layer

56

Manchester encodingbull used in 10BaseTbull each bit has a transitionbull allows clocks in sending and receiving nodes to syn-

chronize to each otherbull no need for a centralized global clock among nodes

bull Hey this is physical-layer stuff

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

57

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

58

Hubshellip physical-layer (ldquodumbrdquo) repeaters

bull bits coming in one link go out all other links at same rate

bull all nodes connected to hub can collide with one another

bull no frame bufferingbull no CSMACD at hub host NICs detect colli-

sions

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

twisted pair

hub

59

Switchbull link-layer device smarter than hubs take

active rolebull store forward Ethernet framesbull examine incoming framersquos MAC address se-

lectively forward frame to one-or-more out-going links when frame is to be forwarded on segment uses CSMACD to access segment

bull transparentbull hosts are unaware of presence of switches

bull plug-and-play self-learning (HOW)bull switches do not need to be configured

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

60

Switch allows multiple simulta-neous transmissionsbull hosts have dedicated direct

connection to switchbull switches buffer packetsbull Ethernet protocol used on

each incoming link but no collisions full duplexbull each link is its own collision

domain

bull switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions bull not possible with dumb hub

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 23

45

6

61

Switch Tablebull Q how does switch know

that Arsquo reachable via inter-face 4 Brsquo reachable via inter-face 5

bull A each switch has a switch table each entrybull (MAC address of host interface

to reach host time stamp)

bull looks like a routing tablebull Q how are entries created

maintained in switch table bull something like a routing proto-

colFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

switch with six interfaces(123456)

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

62

Switch self-learningbull switch learns which

hosts can be reached through which inter-facesbull when frame received

switch ldquolearnsrdquo location of sender incoming LAN segment

bull records senderlocation pair in switch table

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

63

Switch frame filteringforward-ingWhen frame received

1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination then if dest on segment from which frame arrived then drop the frame else

forward the frame on interface indicated else flood

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

forward on all but the interface on which the frame arrived

64

Self-learning forwarding exam-ple

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

frame destination unknownflood

Arsquo A

destination A location known

Arsquo 4 60

selective send

65

Interconnecting switchesbull switches can be connected together

bull Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3

bull A self learning (works exactly the same as in single-switch case)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

B

S1

C D

E

FS2

S4

S3

H

I

G

66

Self-learning multi-switch exam-plebull Suppose C sends frame to I I responds to

C

bull Q show switch tables and packet forward-ing in S1 S2 S3 S4

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

B

S1

CD

E F

S2

S4

S3

H

I

G

12

1

2 3

3

1 2 3 12 3

444

67

Institutional network

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

to externalnetwork

router

IP subnet

mail server

web server

68

Switches vs Routersbull both store-and-forward devices

bull routers network layer devices (examine network layer head-ers)

bull switches are link layer devices

bull routers maintain routing tables implement routing al-gorithms

bull switches maintain switch tables implement filtering learning algorithms

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

69

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

• Fall 2015 COMP 46055605 Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link layer context
• Link Layer Services
• Link Layer Services (more)
• Where is the link layer implemented
• Adaptors Communicating
• Link Layer (2)
• Error Detection
• Parity Checking
• Internet checksum (review)
• Checksumming Cyclic Redundancy Check
• Checksumming Cyclic Redundancy Check (2)
• CRC Example
• Link Layer (3)
• Multiple Access Links and Protocols
• Multiple Access protocols
• Ideal Multiple Access Protocol
• MAC Protocols a taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD collision detection
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Link Layer (4)
• MAC Addresses and ARP
• LAN Addresses and ARP
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP protocol Same LAN (network)
• Addressing routing to another LAN
• Slide 45
• Link Layer (5)
• Ethernet
• Star Topology
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester encoding
• Link Layer (6)
• Hubs
• Switch
• Switch allows multiple simultaneous transmissions
• Switch Table
• Switch self-learning
• Switch frame filteringforwarding
• Self-learning forwarding example
• Interconnecting switches
• Self-learning multi-switch example
• Institutional network
• Switches vs Routers
• Link Layer (7)

33

CSMACD collision detection

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

34

ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols

bull share channel efficiently and fairly at high loadbull inefficient at low load delay in channel access

1N bandwidth allocated even if only 1 active node

Random access MAC protocolsbull efficient at low load single node can fully uti-

lize channelbull high load collision overhead

ldquotaking turnsrdquo protocolsbull look for best of both worlds

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

35

ldquoTaking Turnsrdquo MAC protocolsPolling bull master node ldquoinvitesrdquo

slave nodes to transmit in turn

bull typically used with ldquodumbrdquo slave devices

bull concernsbull polling overhead bull latencybull single point of failure

(master)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

T

data

(nothingto send)

T

36

ldquoTaking Turnsrdquo MAC protocolsToken passingbull control token

passed from one node to next se-quentially

bull token messagebull concerns

bull token overhead bull latencybull single point of failure

(token)Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

master

slaves

poll

data

data

37

Summary of MAC protocolsbull channel partitioning by time frequency or

codebull Time Division Frequency Division

bull random access (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire)

hard in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211

bull taking turnsbull polling from central site token passing

bull Bluetooth FDDI IBM Token Ring Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

38

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

39

MAC Addresses and ARPbull 32-bit IP address

bull network-layer addressbull used to get datagram to destination IP subnet

bull MAC (or LAN or physical or Ethernet) ad-dress bull function get frame from one interface to an-

other physically-connected interface (same network)

bull 48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software set-

table

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

40

LAN Addresses and ARP

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Each adapter on LAN has unique LAN address

Broadcast address =FF-FF-FF-FF-FF-FF

= adapter

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired orwireless)

41

LAN Address (more)bull MAC address allocation administered by IEEEbull manufacturer buys portion of MAC address

space (to assure uniqueness)bull analogy

(a) MAC address like Social Security Num-ber

(b) IP address like postal addressbull MAC flat address portability

bull can move LAN card from one LAN to another

bull IP hierarchical address NOT portablebull address depends on IP subnet to which node is at-

tachedFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

42

ARP Address Resolution Proto-colbull Each IP node (host

router) on LAN has ARP table

bull ARP table IPMAC address mappings for some LAN nodeslt IP address MAC address

TTLgt

bull TTL (Time To Live) time after which ad-dress mapping will be forgotten (typically 20 min)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Question how to determineMAC address of Bknowing Brsquos IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN

137196723

137196778

137196714

137196788

43

ARP protocol Same LAN (net-work)bull A wants to send datagram

to B and Brsquos MAC address not in Arsquos ARP table

bull A broadcasts ARP query packet containing Bs IP address bull dest MAC address = FF-FF-FF-

FF-FF-FFbull all machines on LAN receive

ARP query

bull B receives ARP packet replies to A with its (Bs) MAC addressbull frame sent to Arsquos MAC ad-

dress (unicast)Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

bull A caches (saves) IP-to-MAC address pair in its ARP table until infor-mation becomes old (times out) bull soft state information

that times out (goes away) unless refreshed

bull ARP is ldquoplug-and-playrdquobull nodes create their ARP

tables without interven-tion from net administra-tor

44

Addressing routing to another LAN

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

R

1A-23-F9-CD-06-9B

222222222220111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

walkthrough send datagram from A to B via R assume A knows Brsquos IP address

bull two ARP tables in router R one for each IP network (LAN)

45

bull A creates IP datagram with source A destination B bull A uses ARP to get Rrsquos MAC address for 111111111110bull A creates link-layer frame with Rs MAC address as dest

frame contains A-to-B IP datagrambull Arsquos NIC sends frame bull Rrsquos NIC receives frame bull R removes IP datagram from Ethernet frame sees its des-

tined to Bbull R uses ARP to get Brsquos MAC address bull R creates frame containing A-to-B IP datagram sends to B

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

This is a really importantexample ndash make sure youunderstand

R

1A-23-F9-CD-06-9B

222222222220

111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

46

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

47

Ethernetldquodominantrdquo wired LAN technology bull cheap $20 for NICbull first widely used LAN technologybull simpler cheaper than token LANs and ATMbull kept up with speed race 10 Mbps ndash 10 Gbps

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Metcalfersquos Ethernetsketch

48

Star Topologybull bus topology popular through mid 90s

bull all nodes in same collision domain (can collide with each other)

bull today star topology prevailsbull active switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes do

not collide with each other)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

switch

bus coaxial cable star

49

Ethernet Frame Structurebull Sending adapter encapsulates IP datagram

(or other network layer protocol packet) in Ethernet frame

Preamble bull 7 bytes with pattern 10101010 followed by

one byte with pattern 10101011bull used to synchronize receiver sender clock

ratesFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

50

Ethernet Frame Structure (more)bull Addresses 6 bytes

bull if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

bull otherwise adapter discards frame

bull Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)

bull CRC checked at receiver if error is detected frame is dropped

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51

Ethernet Unreliable connection-lessbull connectionless No handshaking be-

tween sending and receiving NICs bull unreliable receiving NIC doesnrsquot send

acks or nacks to sending NICbull stream of datagrams passed to network

layer can have gaps (missing datagrams)bull gaps will be filled if app is using TCPbull otherwise app will see gaps

bull Ethernetrsquos MAC protocol unslotted CSMACD

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

52

Ethernet CSMACD algorithmbull 1 NIC receives data-

gram from network layer creates frame

bull 2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

bull 3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

bull 4 If NIC detects an-other transmission while transmitting aborts and sends jam signal

bull 5 After aborting NIC enters exponential backoff after mth colli-sion NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

53

Ethernetrsquos CSMACD (more)bull Jam Signal make

sure all other transmitters are aware of collision 48 bits

bull Bit time 1 micro sec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Exponential Backoff bull Goal adapt retransmission

attempts to estimated cur-rent load

bull heavy load random wait will be longer

bull first collision choose K from 01 delay is K 512 bit transmission times

bull after second collision choose K from 0123hellip

bull after ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

54

CSMACD efficiencybull Tprop = max prop delay between 2 nodes in

LANbull ttrans = time to transmit max-size frame

bull efficiency goes to 1 bull as tprop goes to 0

bull as ttrans goes to infinity

bull better performance than ALOHA and simple cheap decentralized

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

transprop ttefficiency

51

1

55

8023 Ethernet Standards Link amp Physical Layersbull many different Ethernet standards

bull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps

1Gbps 10G bpsbull different physical layer media fiber cable

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

applicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twisterpair) physical layer

56

Manchester encodingbull used in 10BaseTbull each bit has a transitionbull allows clocks in sending and receiving nodes to syn-

chronize to each otherbull no need for a centralized global clock among nodes

bull Hey this is physical-layer stuff

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

57

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

58

Hubshellip physical-layer (ldquodumbrdquo) repeaters

bull bits coming in one link go out all other links at same rate

bull all nodes connected to hub can collide with one another

bull no frame bufferingbull no CSMACD at hub host NICs detect colli-

sions

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

twisted pair

hub

59

Switchbull link-layer device smarter than hubs take

active rolebull store forward Ethernet framesbull examine incoming framersquos MAC address se-

lectively forward frame to one-or-more out-going links when frame is to be forwarded on segment uses CSMACD to access segment

bull transparentbull hosts are unaware of presence of switches

bull plug-and-play self-learning (HOW)bull switches do not need to be configured

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

60

Switch allows multiple simulta-neous transmissionsbull hosts have dedicated direct

connection to switchbull switches buffer packetsbull Ethernet protocol used on

each incoming link but no collisions full duplexbull each link is its own collision

domain

bull switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions bull not possible with dumb hub

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 23

45

6

61

Switch Tablebull Q how does switch know

that Arsquo reachable via inter-face 4 Brsquo reachable via inter-face 5

bull A each switch has a switch table each entrybull (MAC address of host interface

to reach host time stamp)

bull looks like a routing tablebull Q how are entries created

maintained in switch table bull something like a routing proto-

colFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

switch with six interfaces(123456)

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

62

Switch self-learningbull switch learns which

hosts can be reached through which inter-facesbull when frame received

switch ldquolearnsrdquo location of sender incoming LAN segment

bull records senderlocation pair in switch table

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

63

Switch frame filteringforward-ingWhen frame received

1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination then if dest on segment from which frame arrived then drop the frame else

forward the frame on interface indicated else flood

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

forward on all but the interface on which the frame arrived

64

Self-learning forwarding exam-ple

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

frame destination unknownflood

Arsquo A

destination A location known

Arsquo 4 60

selective send

65

Interconnecting switchesbull switches can be connected together

bull Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3

bull A self learning (works exactly the same as in single-switch case)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

B

S1

C D

E

FS2

S4

S3

H

I

G

66

Self-learning multi-switch exam-plebull Suppose C sends frame to I I responds to

C

bull Q show switch tables and packet forward-ing in S1 S2 S3 S4

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

B

S1

CD

E F

S2

S4

S3

H

I

G

12

1

2 3

3

1 2 3 12 3

444

67

Institutional network

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

to externalnetwork

router

IP subnet

mail server

web server

68

Switches vs Routersbull both store-and-forward devices

bull routers network layer devices (examine network layer head-ers)

bull switches are link layer devices

bull routers maintain routing tables implement routing al-gorithms

bull switches maintain switch tables implement filtering learning algorithms

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

69

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

• Fall 2015 COMP 46055605 Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link layer context
• Link Layer Services
• Link Layer Services (more)
• Where is the link layer implemented
• Adaptors Communicating
• Link Layer (2)
• Error Detection
• Parity Checking
• Internet checksum (review)
• Checksumming Cyclic Redundancy Check
• Checksumming Cyclic Redundancy Check (2)
• CRC Example
• Link Layer (3)
• Multiple Access Links and Protocols
• Multiple Access protocols
• Ideal Multiple Access Protocol
• MAC Protocols a taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD collision detection
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Link Layer (4)
• MAC Addresses and ARP
• LAN Addresses and ARP
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP protocol Same LAN (network)
• Addressing routing to another LAN
• Slide 45
• Link Layer (5)
• Ethernet
• Star Topology
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester encoding
• Link Layer (6)
• Hubs
• Switch
• Switch allows multiple simultaneous transmissions
• Switch Table
• Switch self-learning
• Switch frame filteringforwarding
• Self-learning forwarding example
• Interconnecting switches
• Self-learning multi-switch example
• Institutional network
• Switches vs Routers
• Link Layer (7)

34

ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols

bull share channel efficiently and fairly at high loadbull inefficient at low load delay in channel access

1N bandwidth allocated even if only 1 active node

Random access MAC protocolsbull efficient at low load single node can fully uti-

lize channelbull high load collision overhead

ldquotaking turnsrdquo protocolsbull look for best of both worlds

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

35

ldquoTaking Turnsrdquo MAC protocolsPolling bull master node ldquoinvitesrdquo

slave nodes to transmit in turn

bull typically used with ldquodumbrdquo slave devices

bull concernsbull polling overhead bull latencybull single point of failure

(master)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

T

data

(nothingto send)

T

36

ldquoTaking Turnsrdquo MAC protocolsToken passingbull control token

passed from one node to next se-quentially

bull token messagebull concerns

bull token overhead bull latencybull single point of failure

(token)Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

master

slaves

poll

data

data

37

Summary of MAC protocolsbull channel partitioning by time frequency or

codebull Time Division Frequency Division

bull random access (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire)

hard in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211

bull taking turnsbull polling from central site token passing

bull Bluetooth FDDI IBM Token Ring Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

38

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

39

MAC Addresses and ARPbull 32-bit IP address

bull network-layer addressbull used to get datagram to destination IP subnet

bull MAC (or LAN or physical or Ethernet) ad-dress bull function get frame from one interface to an-

other physically-connected interface (same network)

bull 48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software set-

table

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

40

LAN Addresses and ARP

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Each adapter on LAN has unique LAN address

Broadcast address =FF-FF-FF-FF-FF-FF

= adapter

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired orwireless)

41

LAN Address (more)bull MAC address allocation administered by IEEEbull manufacturer buys portion of MAC address

space (to assure uniqueness)bull analogy

(a) MAC address like Social Security Num-ber

(b) IP address like postal addressbull MAC flat address portability

bull can move LAN card from one LAN to another

bull IP hierarchical address NOT portablebull address depends on IP subnet to which node is at-

tachedFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

42

ARP Address Resolution Proto-colbull Each IP node (host

router) on LAN has ARP table

bull ARP table IPMAC address mappings for some LAN nodeslt IP address MAC address

TTLgt

bull TTL (Time To Live) time after which ad-dress mapping will be forgotten (typically 20 min)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Question how to determineMAC address of Bknowing Brsquos IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN

137196723

137196778

137196714

137196788

43

ARP protocol Same LAN (net-work)bull A wants to send datagram

to B and Brsquos MAC address not in Arsquos ARP table

bull A broadcasts ARP query packet containing Bs IP address bull dest MAC address = FF-FF-FF-

FF-FF-FFbull all machines on LAN receive

ARP query

bull B receives ARP packet replies to A with its (Bs) MAC addressbull frame sent to Arsquos MAC ad-

dress (unicast)Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

bull A caches (saves) IP-to-MAC address pair in its ARP table until infor-mation becomes old (times out) bull soft state information

that times out (goes away) unless refreshed

bull ARP is ldquoplug-and-playrdquobull nodes create their ARP

tables without interven-tion from net administra-tor

44

Addressing routing to another LAN

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

R

1A-23-F9-CD-06-9B

222222222220111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

walkthrough send datagram from A to B via R assume A knows Brsquos IP address

bull two ARP tables in router R one for each IP network (LAN)

45

bull A creates IP datagram with source A destination B bull A uses ARP to get Rrsquos MAC address for 111111111110bull A creates link-layer frame with Rs MAC address as dest

frame contains A-to-B IP datagrambull Arsquos NIC sends frame bull Rrsquos NIC receives frame bull R removes IP datagram from Ethernet frame sees its des-

tined to Bbull R uses ARP to get Brsquos MAC address bull R creates frame containing A-to-B IP datagram sends to B

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

This is a really importantexample ndash make sure youunderstand

R

1A-23-F9-CD-06-9B

222222222220

111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

46

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

47

Ethernetldquodominantrdquo wired LAN technology bull cheap $20 for NICbull first widely used LAN technologybull simpler cheaper than token LANs and ATMbull kept up with speed race 10 Mbps ndash 10 Gbps

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Metcalfersquos Ethernetsketch

48

Star Topologybull bus topology popular through mid 90s

bull all nodes in same collision domain (can collide with each other)

bull today star topology prevailsbull active switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes do

not collide with each other)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

switch

bus coaxial cable star

49

Ethernet Frame Structurebull Sending adapter encapsulates IP datagram

(or other network layer protocol packet) in Ethernet frame

Preamble bull 7 bytes with pattern 10101010 followed by

one byte with pattern 10101011bull used to synchronize receiver sender clock

ratesFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

50

Ethernet Frame Structure (more)bull Addresses 6 bytes

bull if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

bull otherwise adapter discards frame

bull Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)

bull CRC checked at receiver if error is detected frame is dropped

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51

Ethernet Unreliable connection-lessbull connectionless No handshaking be-

tween sending and receiving NICs bull unreliable receiving NIC doesnrsquot send

acks or nacks to sending NICbull stream of datagrams passed to network

layer can have gaps (missing datagrams)bull gaps will be filled if app is using TCPbull otherwise app will see gaps

bull Ethernetrsquos MAC protocol unslotted CSMACD

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

52

Ethernet CSMACD algorithmbull 1 NIC receives data-

gram from network layer creates frame

bull 2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

bull 3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

bull 4 If NIC detects an-other transmission while transmitting aborts and sends jam signal

bull 5 After aborting NIC enters exponential backoff after mth colli-sion NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

53

Ethernetrsquos CSMACD (more)bull Jam Signal make

sure all other transmitters are aware of collision 48 bits

bull Bit time 1 micro sec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Exponential Backoff bull Goal adapt retransmission

attempts to estimated cur-rent load

bull heavy load random wait will be longer

bull first collision choose K from 01 delay is K 512 bit transmission times

bull after second collision choose K from 0123hellip

bull after ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

54

CSMACD efficiencybull Tprop = max prop delay between 2 nodes in

LANbull ttrans = time to transmit max-size frame

bull efficiency goes to 1 bull as tprop goes to 0

bull as ttrans goes to infinity

bull better performance than ALOHA and simple cheap decentralized

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

transprop ttefficiency

51

1

55

8023 Ethernet Standards Link amp Physical Layersbull many different Ethernet standards

bull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps

1Gbps 10G bpsbull different physical layer media fiber cable

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

applicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twisterpair) physical layer

56

Manchester encodingbull used in 10BaseTbull each bit has a transitionbull allows clocks in sending and receiving nodes to syn-

chronize to each otherbull no need for a centralized global clock among nodes

bull Hey this is physical-layer stuff

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

57

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

58

Hubshellip physical-layer (ldquodumbrdquo) repeaters

bull bits coming in one link go out all other links at same rate

bull all nodes connected to hub can collide with one another

bull no frame bufferingbull no CSMACD at hub host NICs detect colli-

sions

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

twisted pair

hub

59

Switchbull link-layer device smarter than hubs take

active rolebull store forward Ethernet framesbull examine incoming framersquos MAC address se-

lectively forward frame to one-or-more out-going links when frame is to be forwarded on segment uses CSMACD to access segment

bull transparentbull hosts are unaware of presence of switches

bull plug-and-play self-learning (HOW)bull switches do not need to be configured

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

60

Switch allows multiple simulta-neous transmissionsbull hosts have dedicated direct

connection to switchbull switches buffer packetsbull Ethernet protocol used on

each incoming link but no collisions full duplexbull each link is its own collision

domain

bull switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions bull not possible with dumb hub

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 23

45

6

61

Switch Tablebull Q how does switch know

that Arsquo reachable via inter-face 4 Brsquo reachable via inter-face 5

bull A each switch has a switch table each entrybull (MAC address of host interface

to reach host time stamp)

bull looks like a routing tablebull Q how are entries created

maintained in switch table bull something like a routing proto-

colFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

switch with six interfaces(123456)

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

62

Switch self-learningbull switch learns which

hosts can be reached through which inter-facesbull when frame received

switch ldquolearnsrdquo location of sender incoming LAN segment

bull records senderlocation pair in switch table

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

63

Switch frame filteringforward-ingWhen frame received

1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination then if dest on segment from which frame arrived then drop the frame else

forward the frame on interface indicated else flood

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

forward on all but the interface on which the frame arrived

64

Self-learning forwarding exam-ple

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

frame destination unknownflood

Arsquo A

destination A location known

Arsquo 4 60

selective send

65

Interconnecting switchesbull switches can be connected together

bull Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3

bull A self learning (works exactly the same as in single-switch case)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

B

S1

C D

E

FS2

S4

S3

H

I

G

66

Self-learning multi-switch exam-plebull Suppose C sends frame to I I responds to

C

bull Q show switch tables and packet forward-ing in S1 S2 S3 S4

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

B

S1

CD

E F

S2

S4

S3

H

I

G

12

1

2 3

3

1 2 3 12 3

444

67

Institutional network

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

to externalnetwork

router

IP subnet

mail server

web server

68

Switches vs Routersbull both store-and-forward devices

bull routers network layer devices (examine network layer head-ers)

bull switches are link layer devices

bull routers maintain routing tables implement routing al-gorithms

bull switches maintain switch tables implement filtering learning algorithms

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

69

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

• Fall 2015 COMP 46055605 Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link layer context
• Link Layer Services
• Link Layer Services (more)
• Where is the link layer implemented
• Adaptors Communicating
• Link Layer (2)
• Error Detection
• Parity Checking
• Internet checksum (review)
• Checksumming Cyclic Redundancy Check
• Checksumming Cyclic Redundancy Check (2)
• CRC Example
• Link Layer (3)
• Multiple Access Links and Protocols
• Multiple Access protocols
• Ideal Multiple Access Protocol
• MAC Protocols a taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD collision detection
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Link Layer (4)
• MAC Addresses and ARP
• LAN Addresses and ARP
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP protocol Same LAN (network)
• Addressing routing to another LAN
• Slide 45
• Link Layer (5)
• Ethernet
• Star Topology
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester encoding
• Link Layer (6)
• Hubs
• Switch
• Switch allows multiple simultaneous transmissions
• Switch Table
• Switch self-learning
• Switch frame filteringforwarding
• Self-learning forwarding example
• Interconnecting switches
• Self-learning multi-switch example
• Institutional network
• Switches vs Routers
• Link Layer (7)

35

ldquoTaking Turnsrdquo MAC protocolsPolling bull master node ldquoinvitesrdquo

slave nodes to transmit in turn

bull typically used with ldquodumbrdquo slave devices

bull concernsbull polling overhead bull latencybull single point of failure

(master)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

T

data

(nothingto send)

T

36

ldquoTaking Turnsrdquo MAC protocolsToken passingbull control token

passed from one node to next se-quentially

bull token messagebull concerns

bull token overhead bull latencybull single point of failure

(token)Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

master

slaves

poll

data

data

37

Summary of MAC protocolsbull channel partitioning by time frequency or

codebull Time Division Frequency Division

bull random access (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire)

hard in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211

bull taking turnsbull polling from central site token passing

bull Bluetooth FDDI IBM Token Ring Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

38

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

39

MAC Addresses and ARPbull 32-bit IP address

bull network-layer addressbull used to get datagram to destination IP subnet

bull MAC (or LAN or physical or Ethernet) ad-dress bull function get frame from one interface to an-

other physically-connected interface (same network)

bull 48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software set-

table

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

40

LAN Addresses and ARP

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Each adapter on LAN has unique LAN address

Broadcast address =FF-FF-FF-FF-FF-FF

= adapter

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired orwireless)

41

LAN Address (more)bull MAC address allocation administered by IEEEbull manufacturer buys portion of MAC address

space (to assure uniqueness)bull analogy

(a) MAC address like Social Security Num-ber

(b) IP address like postal addressbull MAC flat address portability

bull can move LAN card from one LAN to another

bull IP hierarchical address NOT portablebull address depends on IP subnet to which node is at-

tachedFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

42

ARP Address Resolution Proto-colbull Each IP node (host

router) on LAN has ARP table

bull ARP table IPMAC address mappings for some LAN nodeslt IP address MAC address

TTLgt

bull TTL (Time To Live) time after which ad-dress mapping will be forgotten (typically 20 min)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Question how to determineMAC address of Bknowing Brsquos IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN

137196723

137196778

137196714

137196788

43

ARP protocol Same LAN (net-work)bull A wants to send datagram

to B and Brsquos MAC address not in Arsquos ARP table

bull A broadcasts ARP query packet containing Bs IP address bull dest MAC address = FF-FF-FF-

FF-FF-FFbull all machines on LAN receive

ARP query

bull B receives ARP packet replies to A with its (Bs) MAC addressbull frame sent to Arsquos MAC ad-

dress (unicast)Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

bull A caches (saves) IP-to-MAC address pair in its ARP table until infor-mation becomes old (times out) bull soft state information

that times out (goes away) unless refreshed

bull ARP is ldquoplug-and-playrdquobull nodes create their ARP

tables without interven-tion from net administra-tor

44

Addressing routing to another LAN

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

R

1A-23-F9-CD-06-9B

222222222220111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

walkthrough send datagram from A to B via R assume A knows Brsquos IP address

bull two ARP tables in router R one for each IP network (LAN)

45

bull A creates IP datagram with source A destination B bull A uses ARP to get Rrsquos MAC address for 111111111110bull A creates link-layer frame with Rs MAC address as dest

frame contains A-to-B IP datagrambull Arsquos NIC sends frame bull Rrsquos NIC receives frame bull R removes IP datagram from Ethernet frame sees its des-

tined to Bbull R uses ARP to get Brsquos MAC address bull R creates frame containing A-to-B IP datagram sends to B

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

This is a really importantexample ndash make sure youunderstand

R

1A-23-F9-CD-06-9B

222222222220

111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

46

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

47

Ethernetldquodominantrdquo wired LAN technology bull cheap $20 for NICbull first widely used LAN technologybull simpler cheaper than token LANs and ATMbull kept up with speed race 10 Mbps ndash 10 Gbps

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Metcalfersquos Ethernetsketch

48

Star Topologybull bus topology popular through mid 90s

bull all nodes in same collision domain (can collide with each other)

bull today star topology prevailsbull active switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes do

not collide with each other)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

switch

bus coaxial cable star

49

Ethernet Frame Structurebull Sending adapter encapsulates IP datagram

(or other network layer protocol packet) in Ethernet frame

Preamble bull 7 bytes with pattern 10101010 followed by

one byte with pattern 10101011bull used to synchronize receiver sender clock

ratesFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

50

Ethernet Frame Structure (more)bull Addresses 6 bytes

bull if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

bull otherwise adapter discards frame

bull Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)

bull CRC checked at receiver if error is detected frame is dropped

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51

Ethernet Unreliable connection-lessbull connectionless No handshaking be-

tween sending and receiving NICs bull unreliable receiving NIC doesnrsquot send

acks or nacks to sending NICbull stream of datagrams passed to network

layer can have gaps (missing datagrams)bull gaps will be filled if app is using TCPbull otherwise app will see gaps

bull Ethernetrsquos MAC protocol unslotted CSMACD

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

52

Ethernet CSMACD algorithmbull 1 NIC receives data-

gram from network layer creates frame

bull 2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

bull 3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

bull 4 If NIC detects an-other transmission while transmitting aborts and sends jam signal

bull 5 After aborting NIC enters exponential backoff after mth colli-sion NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

53

Ethernetrsquos CSMACD (more)bull Jam Signal make

sure all other transmitters are aware of collision 48 bits

bull Bit time 1 micro sec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Exponential Backoff bull Goal adapt retransmission

attempts to estimated cur-rent load

bull heavy load random wait will be longer

bull first collision choose K from 01 delay is K 512 bit transmission times

bull after second collision choose K from 0123hellip

bull after ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

54

CSMACD efficiencybull Tprop = max prop delay between 2 nodes in

LANbull ttrans = time to transmit max-size frame

bull efficiency goes to 1 bull as tprop goes to 0

bull as ttrans goes to infinity

bull better performance than ALOHA and simple cheap decentralized

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

transprop ttefficiency

51

1

55

8023 Ethernet Standards Link amp Physical Layersbull many different Ethernet standards

bull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps

1Gbps 10G bpsbull different physical layer media fiber cable

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

applicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twisterpair) physical layer

56

Manchester encodingbull used in 10BaseTbull each bit has a transitionbull allows clocks in sending and receiving nodes to syn-

chronize to each otherbull no need for a centralized global clock among nodes

bull Hey this is physical-layer stuff

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

57

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

58

Hubshellip physical-layer (ldquodumbrdquo) repeaters

bull bits coming in one link go out all other links at same rate

bull all nodes connected to hub can collide with one another

bull no frame bufferingbull no CSMACD at hub host NICs detect colli-

sions

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

twisted pair

hub

59

Switchbull link-layer device smarter than hubs take

active rolebull store forward Ethernet framesbull examine incoming framersquos MAC address se-

lectively forward frame to one-or-more out-going links when frame is to be forwarded on segment uses CSMACD to access segment

bull transparentbull hosts are unaware of presence of switches

bull plug-and-play self-learning (HOW)bull switches do not need to be configured

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

60

Switch allows multiple simulta-neous transmissionsbull hosts have dedicated direct

connection to switchbull switches buffer packetsbull Ethernet protocol used on

each incoming link but no collisions full duplexbull each link is its own collision

domain

bull switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions bull not possible with dumb hub

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 23

45

6

61

Switch Tablebull Q how does switch know

that Arsquo reachable via inter-face 4 Brsquo reachable via inter-face 5

bull A each switch has a switch table each entrybull (MAC address of host interface

to reach host time stamp)

bull looks like a routing tablebull Q how are entries created

maintained in switch table bull something like a routing proto-

colFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

switch with six interfaces(123456)

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

62

Switch self-learningbull switch learns which

hosts can be reached through which inter-facesbull when frame received

switch ldquolearnsrdquo location of sender incoming LAN segment

bull records senderlocation pair in switch table

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

63

Switch frame filteringforward-ingWhen frame received

1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination then if dest on segment from which frame arrived then drop the frame else

forward the frame on interface indicated else flood

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

forward on all but the interface on which the frame arrived

64

Self-learning forwarding exam-ple

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

frame destination unknownflood

Arsquo A

destination A location known

Arsquo 4 60

selective send

65

Interconnecting switchesbull switches can be connected together

bull Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3

bull A self learning (works exactly the same as in single-switch case)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

B

S1

C D

E

FS2

S4

S3

H

I

G

66

Self-learning multi-switch exam-plebull Suppose C sends frame to I I responds to

C

bull Q show switch tables and packet forward-ing in S1 S2 S3 S4

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

B

S1

CD

E F

S2

S4

S3

H

I

G

12

1

2 3

3

1 2 3 12 3

444

67

Institutional network

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

to externalnetwork

router

IP subnet

mail server

web server

68

Switches vs Routersbull both store-and-forward devices

bull routers network layer devices (examine network layer head-ers)

bull switches are link layer devices

bull routers maintain routing tables implement routing al-gorithms

bull switches maintain switch tables implement filtering learning algorithms

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

69

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

• Fall 2015 COMP 46055605 Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link layer context
• Link Layer Services
• Link Layer Services (more)
• Where is the link layer implemented
• Adaptors Communicating
• Link Layer (2)
• Error Detection
• Parity Checking
• Internet checksum (review)
• Checksumming Cyclic Redundancy Check
• Checksumming Cyclic Redundancy Check (2)
• CRC Example
• Link Layer (3)
• Multiple Access Links and Protocols
• Multiple Access protocols
• Ideal Multiple Access Protocol
• MAC Protocols a taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD collision detection
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Link Layer (4)
• MAC Addresses and ARP
• LAN Addresses and ARP
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP protocol Same LAN (network)
• Addressing routing to another LAN
• Slide 45
• Link Layer (5)
• Ethernet
• Star Topology
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester encoding
• Link Layer (6)
• Hubs
• Switch
• Switch allows multiple simultaneous transmissions
• Switch Table
• Switch self-learning
• Switch frame filteringforwarding
• Self-learning forwarding example
• Interconnecting switches
• Self-learning multi-switch example
• Institutional network
• Switches vs Routers
• Link Layer (7)

36

ldquoTaking Turnsrdquo MAC protocolsToken passingbull control token

passed from one node to next se-quentially

bull token messagebull concerns

bull token overhead bull latencybull single point of failure

(token)Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

master

slaves

poll

data

data

37

Summary of MAC protocolsbull channel partitioning by time frequency or

codebull Time Division Frequency Division

bull random access (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire)

hard in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211

bull taking turnsbull polling from central site token passing

bull Bluetooth FDDI IBM Token Ring Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

38

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

39

MAC Addresses and ARPbull 32-bit IP address

bull network-layer addressbull used to get datagram to destination IP subnet

bull MAC (or LAN or physical or Ethernet) ad-dress bull function get frame from one interface to an-

other physically-connected interface (same network)

bull 48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software set-

table

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

40

LAN Addresses and ARP

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Each adapter on LAN has unique LAN address

Broadcast address =FF-FF-FF-FF-FF-FF

= adapter

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired orwireless)

41

LAN Address (more)bull MAC address allocation administered by IEEEbull manufacturer buys portion of MAC address

space (to assure uniqueness)bull analogy

(a) MAC address like Social Security Num-ber

(b) IP address like postal addressbull MAC flat address portability

bull can move LAN card from one LAN to another

bull IP hierarchical address NOT portablebull address depends on IP subnet to which node is at-

tachedFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

42

ARP Address Resolution Proto-colbull Each IP node (host

router) on LAN has ARP table

bull ARP table IPMAC address mappings for some LAN nodeslt IP address MAC address

TTLgt

bull TTL (Time To Live) time after which ad-dress mapping will be forgotten (typically 20 min)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Question how to determineMAC address of Bknowing Brsquos IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN

137196723

137196778

137196714

137196788

43

ARP protocol Same LAN (net-work)bull A wants to send datagram

to B and Brsquos MAC address not in Arsquos ARP table

bull A broadcasts ARP query packet containing Bs IP address bull dest MAC address = FF-FF-FF-

FF-FF-FFbull all machines on LAN receive

ARP query

bull B receives ARP packet replies to A with its (Bs) MAC addressbull frame sent to Arsquos MAC ad-

dress (unicast)Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

bull A caches (saves) IP-to-MAC address pair in its ARP table until infor-mation becomes old (times out) bull soft state information

that times out (goes away) unless refreshed

bull ARP is ldquoplug-and-playrdquobull nodes create their ARP

tables without interven-tion from net administra-tor

44

Addressing routing to another LAN

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

R

1A-23-F9-CD-06-9B

222222222220111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

walkthrough send datagram from A to B via R assume A knows Brsquos IP address

bull two ARP tables in router R one for each IP network (LAN)

45

bull A creates IP datagram with source A destination B bull A uses ARP to get Rrsquos MAC address for 111111111110bull A creates link-layer frame with Rs MAC address as dest

frame contains A-to-B IP datagrambull Arsquos NIC sends frame bull Rrsquos NIC receives frame bull R removes IP datagram from Ethernet frame sees its des-

tined to Bbull R uses ARP to get Brsquos MAC address bull R creates frame containing A-to-B IP datagram sends to B

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

This is a really importantexample ndash make sure youunderstand

R

1A-23-F9-CD-06-9B

222222222220

111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

46

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

47

Ethernetldquodominantrdquo wired LAN technology bull cheap $20 for NICbull first widely used LAN technologybull simpler cheaper than token LANs and ATMbull kept up with speed race 10 Mbps ndash 10 Gbps

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Metcalfersquos Ethernetsketch

48

Star Topologybull bus topology popular through mid 90s

bull all nodes in same collision domain (can collide with each other)

bull today star topology prevailsbull active switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes do

not collide with each other)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

switch

bus coaxial cable star

49

Ethernet Frame Structurebull Sending adapter encapsulates IP datagram

(or other network layer protocol packet) in Ethernet frame

Preamble bull 7 bytes with pattern 10101010 followed by

one byte with pattern 10101011bull used to synchronize receiver sender clock

ratesFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

50

Ethernet Frame Structure (more)bull Addresses 6 bytes

bull if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

bull otherwise adapter discards frame

bull Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)

bull CRC checked at receiver if error is detected frame is dropped

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51

Ethernet Unreliable connection-lessbull connectionless No handshaking be-

tween sending and receiving NICs bull unreliable receiving NIC doesnrsquot send

acks or nacks to sending NICbull stream of datagrams passed to network

layer can have gaps (missing datagrams)bull gaps will be filled if app is using TCPbull otherwise app will see gaps

bull Ethernetrsquos MAC protocol unslotted CSMACD

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

52

Ethernet CSMACD algorithmbull 1 NIC receives data-

gram from network layer creates frame

bull 2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

bull 3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

bull 4 If NIC detects an-other transmission while transmitting aborts and sends jam signal

bull 5 After aborting NIC enters exponential backoff after mth colli-sion NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

53

Ethernetrsquos CSMACD (more)bull Jam Signal make

sure all other transmitters are aware of collision 48 bits

bull Bit time 1 micro sec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Exponential Backoff bull Goal adapt retransmission

attempts to estimated cur-rent load

bull heavy load random wait will be longer

bull first collision choose K from 01 delay is K 512 bit transmission times

bull after second collision choose K from 0123hellip

bull after ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

54

CSMACD efficiencybull Tprop = max prop delay between 2 nodes in

LANbull ttrans = time to transmit max-size frame

bull efficiency goes to 1 bull as tprop goes to 0

bull as ttrans goes to infinity

bull better performance than ALOHA and simple cheap decentralized

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

transprop ttefficiency

51

1

55

8023 Ethernet Standards Link amp Physical Layersbull many different Ethernet standards

bull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps

1Gbps 10G bpsbull different physical layer media fiber cable

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

applicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twisterpair) physical layer

56

Manchester encodingbull used in 10BaseTbull each bit has a transitionbull allows clocks in sending and receiving nodes to syn-

chronize to each otherbull no need for a centralized global clock among nodes

bull Hey this is physical-layer stuff

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

57

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

58

Hubshellip physical-layer (ldquodumbrdquo) repeaters

bull bits coming in one link go out all other links at same rate

bull all nodes connected to hub can collide with one another

bull no frame bufferingbull no CSMACD at hub host NICs detect colli-

sions

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

twisted pair

hub

59

Switchbull link-layer device smarter than hubs take

active rolebull store forward Ethernet framesbull examine incoming framersquos MAC address se-

lectively forward frame to one-or-more out-going links when frame is to be forwarded on segment uses CSMACD to access segment

bull transparentbull hosts are unaware of presence of switches

bull plug-and-play self-learning (HOW)bull switches do not need to be configured

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

60

Switch allows multiple simulta-neous transmissionsbull hosts have dedicated direct

connection to switchbull switches buffer packetsbull Ethernet protocol used on

each incoming link but no collisions full duplexbull each link is its own collision

domain

bull switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions bull not possible with dumb hub

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 23

45

6

61

Switch Tablebull Q how does switch know

that Arsquo reachable via inter-face 4 Brsquo reachable via inter-face 5

bull A each switch has a switch table each entrybull (MAC address of host interface

to reach host time stamp)

bull looks like a routing tablebull Q how are entries created

maintained in switch table bull something like a routing proto-

colFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

switch with six interfaces(123456)

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

62

Switch self-learningbull switch learns which

hosts can be reached through which inter-facesbull when frame received

switch ldquolearnsrdquo location of sender incoming LAN segment

bull records senderlocation pair in switch table

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

63

Switch frame filteringforward-ingWhen frame received

1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination then if dest on segment from which frame arrived then drop the frame else

forward the frame on interface indicated else flood

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

forward on all but the interface on which the frame arrived

64

Self-learning forwarding exam-ple

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

frame destination unknownflood

Arsquo A

destination A location known

Arsquo 4 60

selective send

65

Interconnecting switchesbull switches can be connected together

bull Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3

bull A self learning (works exactly the same as in single-switch case)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

B

S1

C D

E

FS2

S4

S3

H

I

G

66

Self-learning multi-switch exam-plebull Suppose C sends frame to I I responds to

C

bull Q show switch tables and packet forward-ing in S1 S2 S3 S4

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

B

S1

CD

E F

S2

S4

S3

H

I

G

12

1

2 3

3

1 2 3 12 3

444

67

Institutional network

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

to externalnetwork

router

IP subnet

mail server

web server

68

Switches vs Routersbull both store-and-forward devices

bull routers network layer devices (examine network layer head-ers)

bull switches are link layer devices

bull routers maintain routing tables implement routing al-gorithms

bull switches maintain switch tables implement filtering learning algorithms

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

69

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

• Fall 2015 COMP 46055605 Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link layer context
• Link Layer Services
• Link Layer Services (more)
• Where is the link layer implemented
• Adaptors Communicating
• Link Layer (2)
• Error Detection
• Parity Checking
• Internet checksum (review)
• Checksumming Cyclic Redundancy Check
• Checksumming Cyclic Redundancy Check (2)
• CRC Example
• Link Layer (3)
• Multiple Access Links and Protocols
• Multiple Access protocols
• Ideal Multiple Access Protocol
• MAC Protocols a taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD collision detection
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Link Layer (4)
• MAC Addresses and ARP
• LAN Addresses and ARP
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP protocol Same LAN (network)
• Addressing routing to another LAN
• Slide 45
• Link Layer (5)
• Ethernet
• Star Topology
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester encoding
• Link Layer (6)
• Hubs
• Switch
• Switch allows multiple simultaneous transmissions
• Switch Table
• Switch self-learning
• Switch frame filteringforwarding
• Self-learning forwarding example
• Interconnecting switches
• Self-learning multi-switch example
• Institutional network
• Switches vs Routers
• Link Layer (7)

37

Summary of MAC protocolsbull channel partitioning by time frequency or

codebull Time Division Frequency Division

bull random access (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire)

hard in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211

bull taking turnsbull polling from central site token passing

bull Bluetooth FDDI IBM Token Ring Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

38

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

39

MAC Addresses and ARPbull 32-bit IP address

bull network-layer addressbull used to get datagram to destination IP subnet

bull MAC (or LAN or physical or Ethernet) ad-dress bull function get frame from one interface to an-

other physically-connected interface (same network)

bull 48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software set-

table

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

40

LAN Addresses and ARP

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Each adapter on LAN has unique LAN address

Broadcast address =FF-FF-FF-FF-FF-FF

= adapter

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired orwireless)

41

LAN Address (more)bull MAC address allocation administered by IEEEbull manufacturer buys portion of MAC address

space (to assure uniqueness)bull analogy

(a) MAC address like Social Security Num-ber

(b) IP address like postal addressbull MAC flat address portability

bull can move LAN card from one LAN to another

bull IP hierarchical address NOT portablebull address depends on IP subnet to which node is at-

tachedFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

42

ARP Address Resolution Proto-colbull Each IP node (host

router) on LAN has ARP table

bull ARP table IPMAC address mappings for some LAN nodeslt IP address MAC address

TTLgt

bull TTL (Time To Live) time after which ad-dress mapping will be forgotten (typically 20 min)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Question how to determineMAC address of Bknowing Brsquos IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN

137196723

137196778

137196714

137196788

43

ARP protocol Same LAN (net-work)bull A wants to send datagram

to B and Brsquos MAC address not in Arsquos ARP table

bull A broadcasts ARP query packet containing Bs IP address bull dest MAC address = FF-FF-FF-

FF-FF-FFbull all machines on LAN receive

ARP query

bull B receives ARP packet replies to A with its (Bs) MAC addressbull frame sent to Arsquos MAC ad-

dress (unicast)Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

bull A caches (saves) IP-to-MAC address pair in its ARP table until infor-mation becomes old (times out) bull soft state information

that times out (goes away) unless refreshed

bull ARP is ldquoplug-and-playrdquobull nodes create their ARP

tables without interven-tion from net administra-tor

44

Addressing routing to another LAN

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

R

1A-23-F9-CD-06-9B

222222222220111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

walkthrough send datagram from A to B via R assume A knows Brsquos IP address

bull two ARP tables in router R one for each IP network (LAN)

45

bull A creates IP datagram with source A destination B bull A uses ARP to get Rrsquos MAC address for 111111111110bull A creates link-layer frame with Rs MAC address as dest

frame contains A-to-B IP datagrambull Arsquos NIC sends frame bull Rrsquos NIC receives frame bull R removes IP datagram from Ethernet frame sees its des-

tined to Bbull R uses ARP to get Brsquos MAC address bull R creates frame containing A-to-B IP datagram sends to B

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

This is a really importantexample ndash make sure youunderstand

R

1A-23-F9-CD-06-9B

222222222220

111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

46

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

47

Ethernetldquodominantrdquo wired LAN technology bull cheap $20 for NICbull first widely used LAN technologybull simpler cheaper than token LANs and ATMbull kept up with speed race 10 Mbps ndash 10 Gbps

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Metcalfersquos Ethernetsketch

48

Star Topologybull bus topology popular through mid 90s

bull all nodes in same collision domain (can collide with each other)

bull today star topology prevailsbull active switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes do

not collide with each other)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

switch

bus coaxial cable star

49

Ethernet Frame Structurebull Sending adapter encapsulates IP datagram

(or other network layer protocol packet) in Ethernet frame

Preamble bull 7 bytes with pattern 10101010 followed by

one byte with pattern 10101011bull used to synchronize receiver sender clock

ratesFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

50

Ethernet Frame Structure (more)bull Addresses 6 bytes

bull if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

bull otherwise adapter discards frame

bull Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)

bull CRC checked at receiver if error is detected frame is dropped

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51

Ethernet Unreliable connection-lessbull connectionless No handshaking be-

tween sending and receiving NICs bull unreliable receiving NIC doesnrsquot send

acks or nacks to sending NICbull stream of datagrams passed to network

layer can have gaps (missing datagrams)bull gaps will be filled if app is using TCPbull otherwise app will see gaps

bull Ethernetrsquos MAC protocol unslotted CSMACD

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

52

Ethernet CSMACD algorithmbull 1 NIC receives data-

gram from network layer creates frame

bull 2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

bull 3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

bull 4 If NIC detects an-other transmission while transmitting aborts and sends jam signal

bull 5 After aborting NIC enters exponential backoff after mth colli-sion NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

53

Ethernetrsquos CSMACD (more)bull Jam Signal make

sure all other transmitters are aware of collision 48 bits

bull Bit time 1 micro sec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Exponential Backoff bull Goal adapt retransmission

attempts to estimated cur-rent load

bull heavy load random wait will be longer

bull first collision choose K from 01 delay is K 512 bit transmission times

bull after second collision choose K from 0123hellip

bull after ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

54

CSMACD efficiencybull Tprop = max prop delay between 2 nodes in

LANbull ttrans = time to transmit max-size frame

bull efficiency goes to 1 bull as tprop goes to 0

bull as ttrans goes to infinity

bull better performance than ALOHA and simple cheap decentralized

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

transprop ttefficiency

51

1

55

8023 Ethernet Standards Link amp Physical Layersbull many different Ethernet standards

bull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps

1Gbps 10G bpsbull different physical layer media fiber cable

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

applicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twisterpair) physical layer

56

Manchester encodingbull used in 10BaseTbull each bit has a transitionbull allows clocks in sending and receiving nodes to syn-

chronize to each otherbull no need for a centralized global clock among nodes

bull Hey this is physical-layer stuff

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

57

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

58

Hubshellip physical-layer (ldquodumbrdquo) repeaters

bull bits coming in one link go out all other links at same rate

bull all nodes connected to hub can collide with one another

bull no frame bufferingbull no CSMACD at hub host NICs detect colli-

sions

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

twisted pair

hub

59

Switchbull link-layer device smarter than hubs take

active rolebull store forward Ethernet framesbull examine incoming framersquos MAC address se-

lectively forward frame to one-or-more out-going links when frame is to be forwarded on segment uses CSMACD to access segment

bull transparentbull hosts are unaware of presence of switches

bull plug-and-play self-learning (HOW)bull switches do not need to be configured

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

60

Switch allows multiple simulta-neous transmissionsbull hosts have dedicated direct

connection to switchbull switches buffer packetsbull Ethernet protocol used on

each incoming link but no collisions full duplexbull each link is its own collision

domain

bull switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions bull not possible with dumb hub

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 23

45

6

61

Switch Tablebull Q how does switch know

that Arsquo reachable via inter-face 4 Brsquo reachable via inter-face 5

bull A each switch has a switch table each entrybull (MAC address of host interface

to reach host time stamp)

bull looks like a routing tablebull Q how are entries created

maintained in switch table bull something like a routing proto-

colFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

switch with six interfaces(123456)

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

62

Switch self-learningbull switch learns which

hosts can be reached through which inter-facesbull when frame received

switch ldquolearnsrdquo location of sender incoming LAN segment

bull records senderlocation pair in switch table

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

63

Switch frame filteringforward-ingWhen frame received

1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination then if dest on segment from which frame arrived then drop the frame else

forward the frame on interface indicated else flood

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

forward on all but the interface on which the frame arrived

64

Self-learning forwarding exam-ple

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

frame destination unknownflood

Arsquo A

destination A location known

Arsquo 4 60

selective send

65

Interconnecting switchesbull switches can be connected together

bull Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3

bull A self learning (works exactly the same as in single-switch case)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

B

S1

C D

E

FS2

S4

S3

H

I

G

66

Self-learning multi-switch exam-plebull Suppose C sends frame to I I responds to

C

bull Q show switch tables and packet forward-ing in S1 S2 S3 S4

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

B

S1

CD

E F

S2

S4

S3

H

I

G

12

1

2 3

3

1 2 3 12 3

444

67

Institutional network

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

to externalnetwork

router

IP subnet

mail server

web server

68

Switches vs Routersbull both store-and-forward devices

bull routers network layer devices (examine network layer head-ers)

bull switches are link layer devices

bull routers maintain routing tables implement routing al-gorithms

bull switches maintain switch tables implement filtering learning algorithms

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

69

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

• Fall 2015 COMP 46055605 Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link layer context
• Link Layer Services
• Link Layer Services (more)
• Where is the link layer implemented
• Adaptors Communicating
• Link Layer (2)
• Error Detection
• Parity Checking
• Internet checksum (review)
• Checksumming Cyclic Redundancy Check
• Checksumming Cyclic Redundancy Check (2)
• CRC Example
• Link Layer (3)
• Multiple Access Links and Protocols
• Multiple Access protocols
• Ideal Multiple Access Protocol
• MAC Protocols a taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD collision detection
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Link Layer (4)
• MAC Addresses and ARP
• LAN Addresses and ARP
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP protocol Same LAN (network)
• Addressing routing to another LAN
• Slide 45
• Link Layer (5)
• Ethernet
• Star Topology
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester encoding
• Link Layer (6)
• Hubs
• Switch
• Switch allows multiple simultaneous transmissions
• Switch Table
• Switch self-learning
• Switch frame filteringforwarding
• Self-learning forwarding example
• Interconnecting switches
• Self-learning multi-switch example
• Institutional network
• Switches vs Routers
• Link Layer (7)

38

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

39

MAC Addresses and ARPbull 32-bit IP address

bull network-layer addressbull used to get datagram to destination IP subnet

bull MAC (or LAN or physical or Ethernet) ad-dress bull function get frame from one interface to an-

other physically-connected interface (same network)

bull 48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software set-

table

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

40

LAN Addresses and ARP

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Each adapter on LAN has unique LAN address

Broadcast address =FF-FF-FF-FF-FF-FF

= adapter

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired orwireless)

41

LAN Address (more)bull MAC address allocation administered by IEEEbull manufacturer buys portion of MAC address

space (to assure uniqueness)bull analogy

(a) MAC address like Social Security Num-ber

(b) IP address like postal addressbull MAC flat address portability

bull can move LAN card from one LAN to another

bull IP hierarchical address NOT portablebull address depends on IP subnet to which node is at-

tachedFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

42

ARP Address Resolution Proto-colbull Each IP node (host

router) on LAN has ARP table

bull ARP table IPMAC address mappings for some LAN nodeslt IP address MAC address

TTLgt

bull TTL (Time To Live) time after which ad-dress mapping will be forgotten (typically 20 min)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Question how to determineMAC address of Bknowing Brsquos IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN

137196723

137196778

137196714

137196788

43

ARP protocol Same LAN (net-work)bull A wants to send datagram

to B and Brsquos MAC address not in Arsquos ARP table

bull A broadcasts ARP query packet containing Bs IP address bull dest MAC address = FF-FF-FF-

FF-FF-FFbull all machines on LAN receive

ARP query

bull B receives ARP packet replies to A with its (Bs) MAC addressbull frame sent to Arsquos MAC ad-

dress (unicast)Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

bull A caches (saves) IP-to-MAC address pair in its ARP table until infor-mation becomes old (times out) bull soft state information

that times out (goes away) unless refreshed

bull ARP is ldquoplug-and-playrdquobull nodes create their ARP

tables without interven-tion from net administra-tor

44

Addressing routing to another LAN

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

R

1A-23-F9-CD-06-9B

222222222220111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

walkthrough send datagram from A to B via R assume A knows Brsquos IP address

bull two ARP tables in router R one for each IP network (LAN)

45

bull A creates IP datagram with source A destination B bull A uses ARP to get Rrsquos MAC address for 111111111110bull A creates link-layer frame with Rs MAC address as dest

frame contains A-to-B IP datagrambull Arsquos NIC sends frame bull Rrsquos NIC receives frame bull R removes IP datagram from Ethernet frame sees its des-

tined to Bbull R uses ARP to get Brsquos MAC address bull R creates frame containing A-to-B IP datagram sends to B

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

This is a really importantexample ndash make sure youunderstand

R

1A-23-F9-CD-06-9B

222222222220

111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

46

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

47

Ethernetldquodominantrdquo wired LAN technology bull cheap $20 for NICbull first widely used LAN technologybull simpler cheaper than token LANs and ATMbull kept up with speed race 10 Mbps ndash 10 Gbps

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Metcalfersquos Ethernetsketch

48

Star Topologybull bus topology popular through mid 90s

bull all nodes in same collision domain (can collide with each other)

bull today star topology prevailsbull active switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes do

not collide with each other)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

switch

bus coaxial cable star

49

Ethernet Frame Structurebull Sending adapter encapsulates IP datagram

(or other network layer protocol packet) in Ethernet frame

Preamble bull 7 bytes with pattern 10101010 followed by

one byte with pattern 10101011bull used to synchronize receiver sender clock

ratesFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

50

Ethernet Frame Structure (more)bull Addresses 6 bytes

bull if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

bull otherwise adapter discards frame

bull Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)

bull CRC checked at receiver if error is detected frame is dropped

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51

Ethernet Unreliable connection-lessbull connectionless No handshaking be-

tween sending and receiving NICs bull unreliable receiving NIC doesnrsquot send

acks or nacks to sending NICbull stream of datagrams passed to network

layer can have gaps (missing datagrams)bull gaps will be filled if app is using TCPbull otherwise app will see gaps

bull Ethernetrsquos MAC protocol unslotted CSMACD

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

52

Ethernet CSMACD algorithmbull 1 NIC receives data-

gram from network layer creates frame

bull 2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

bull 3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

bull 4 If NIC detects an-other transmission while transmitting aborts and sends jam signal

bull 5 After aborting NIC enters exponential backoff after mth colli-sion NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

53

Ethernetrsquos CSMACD (more)bull Jam Signal make

sure all other transmitters are aware of collision 48 bits

bull Bit time 1 micro sec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Exponential Backoff bull Goal adapt retransmission

attempts to estimated cur-rent load

bull heavy load random wait will be longer

bull first collision choose K from 01 delay is K 512 bit transmission times

bull after second collision choose K from 0123hellip

bull after ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

54

CSMACD efficiencybull Tprop = max prop delay between 2 nodes in

LANbull ttrans = time to transmit max-size frame

bull efficiency goes to 1 bull as tprop goes to 0

bull as ttrans goes to infinity

bull better performance than ALOHA and simple cheap decentralized

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

transprop ttefficiency

51

1

55

8023 Ethernet Standards Link amp Physical Layersbull many different Ethernet standards

bull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps

1Gbps 10G bpsbull different physical layer media fiber cable

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

applicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twisterpair) physical layer

56

Manchester encodingbull used in 10BaseTbull each bit has a transitionbull allows clocks in sending and receiving nodes to syn-

chronize to each otherbull no need for a centralized global clock among nodes

bull Hey this is physical-layer stuff

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

57

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

58

Hubshellip physical-layer (ldquodumbrdquo) repeaters

bull bits coming in one link go out all other links at same rate

bull all nodes connected to hub can collide with one another

bull no frame bufferingbull no CSMACD at hub host NICs detect colli-

sions

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

twisted pair

hub

59

Switchbull link-layer device smarter than hubs take

active rolebull store forward Ethernet framesbull examine incoming framersquos MAC address se-

lectively forward frame to one-or-more out-going links when frame is to be forwarded on segment uses CSMACD to access segment

bull transparentbull hosts are unaware of presence of switches

bull plug-and-play self-learning (HOW)bull switches do not need to be configured

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

60

Switch allows multiple simulta-neous transmissionsbull hosts have dedicated direct

connection to switchbull switches buffer packetsbull Ethernet protocol used on

each incoming link but no collisions full duplexbull each link is its own collision

domain

bull switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions bull not possible with dumb hub

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 23

45

6

61

Switch Tablebull Q how does switch know

that Arsquo reachable via inter-face 4 Brsquo reachable via inter-face 5

bull A each switch has a switch table each entrybull (MAC address of host interface

to reach host time stamp)

bull looks like a routing tablebull Q how are entries created

maintained in switch table bull something like a routing proto-

colFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

switch with six interfaces(123456)

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

62

Switch self-learningbull switch learns which

hosts can be reached through which inter-facesbull when frame received

switch ldquolearnsrdquo location of sender incoming LAN segment

bull records senderlocation pair in switch table

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

63

Switch frame filteringforward-ingWhen frame received

1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination then if dest on segment from which frame arrived then drop the frame else

forward the frame on interface indicated else flood

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

forward on all but the interface on which the frame arrived

64

Self-learning forwarding exam-ple

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

frame destination unknownflood

Arsquo A

destination A location known

Arsquo 4 60

selective send

65

Interconnecting switchesbull switches can be connected together

bull Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3

bull A self learning (works exactly the same as in single-switch case)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

B

S1

C D

E

FS2

S4

S3

H

I

G

66

Self-learning multi-switch exam-plebull Suppose C sends frame to I I responds to

C

bull Q show switch tables and packet forward-ing in S1 S2 S3 S4

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

B

S1

CD

E F

S2

S4

S3

H

I

G

12

1

2 3

3

1 2 3 12 3

444

67

Institutional network

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

to externalnetwork

router

IP subnet

mail server

web server

68

Switches vs Routersbull both store-and-forward devices

bull routers network layer devices (examine network layer head-ers)

bull switches are link layer devices

bull routers maintain routing tables implement routing al-gorithms

bull switches maintain switch tables implement filtering learning algorithms

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

69

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

• Fall 2015 COMP 46055605 Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link layer context
• Link Layer Services
• Link Layer Services (more)
• Where is the link layer implemented
• Adaptors Communicating
• Link Layer (2)
• Error Detection
• Parity Checking
• Internet checksum (review)
• Checksumming Cyclic Redundancy Check
• Checksumming Cyclic Redundancy Check (2)
• CRC Example
• Link Layer (3)
• Multiple Access Links and Protocols
• Multiple Access protocols
• Ideal Multiple Access Protocol
• MAC Protocols a taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD collision detection
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Link Layer (4)
• MAC Addresses and ARP
• LAN Addresses and ARP
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP protocol Same LAN (network)
• Addressing routing to another LAN
• Slide 45
• Link Layer (5)
• Ethernet
• Star Topology
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester encoding
• Link Layer (6)
• Hubs
• Switch
• Switch allows multiple simultaneous transmissions
• Switch Table
• Switch self-learning
• Switch frame filteringforwarding
• Self-learning forwarding example
• Interconnecting switches
• Self-learning multi-switch example
• Institutional network
• Switches vs Routers
• Link Layer (7)

39

MAC Addresses and ARPbull 32-bit IP address

bull network-layer addressbull used to get datagram to destination IP subnet

bull MAC (or LAN or physical or Ethernet) ad-dress bull function get frame from one interface to an-

other physically-connected interface (same network)

bull 48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software set-

table

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

40

LAN Addresses and ARP

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Each adapter on LAN has unique LAN address

Broadcast address =FF-FF-FF-FF-FF-FF

= adapter

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired orwireless)

41

LAN Address (more)bull MAC address allocation administered by IEEEbull manufacturer buys portion of MAC address

space (to assure uniqueness)bull analogy

(a) MAC address like Social Security Num-ber

(b) IP address like postal addressbull MAC flat address portability

bull can move LAN card from one LAN to another

bull IP hierarchical address NOT portablebull address depends on IP subnet to which node is at-

tachedFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

42

ARP Address Resolution Proto-colbull Each IP node (host

router) on LAN has ARP table

bull ARP table IPMAC address mappings for some LAN nodeslt IP address MAC address

TTLgt

bull TTL (Time To Live) time after which ad-dress mapping will be forgotten (typically 20 min)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Question how to determineMAC address of Bknowing Brsquos IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN

137196723

137196778

137196714

137196788

43

ARP protocol Same LAN (net-work)bull A wants to send datagram

to B and Brsquos MAC address not in Arsquos ARP table

bull A broadcasts ARP query packet containing Bs IP address bull dest MAC address = FF-FF-FF-

FF-FF-FFbull all machines on LAN receive

ARP query

bull B receives ARP packet replies to A with its (Bs) MAC addressbull frame sent to Arsquos MAC ad-

dress (unicast)Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

bull A caches (saves) IP-to-MAC address pair in its ARP table until infor-mation becomes old (times out) bull soft state information

that times out (goes away) unless refreshed

bull ARP is ldquoplug-and-playrdquobull nodes create their ARP

tables without interven-tion from net administra-tor

44

Addressing routing to another LAN

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

R

1A-23-F9-CD-06-9B

222222222220111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

walkthrough send datagram from A to B via R assume A knows Brsquos IP address

bull two ARP tables in router R one for each IP network (LAN)

45

bull A creates IP datagram with source A destination B bull A uses ARP to get Rrsquos MAC address for 111111111110bull A creates link-layer frame with Rs MAC address as dest

frame contains A-to-B IP datagrambull Arsquos NIC sends frame bull Rrsquos NIC receives frame bull R removes IP datagram from Ethernet frame sees its des-

tined to Bbull R uses ARP to get Brsquos MAC address bull R creates frame containing A-to-B IP datagram sends to B

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

This is a really importantexample ndash make sure youunderstand

R

1A-23-F9-CD-06-9B

222222222220

111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

46

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

47

Ethernetldquodominantrdquo wired LAN technology bull cheap $20 for NICbull first widely used LAN technologybull simpler cheaper than token LANs and ATMbull kept up with speed race 10 Mbps ndash 10 Gbps

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Metcalfersquos Ethernetsketch

48

Star Topologybull bus topology popular through mid 90s

bull all nodes in same collision domain (can collide with each other)

bull today star topology prevailsbull active switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes do

not collide with each other)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

switch

bus coaxial cable star

49

Ethernet Frame Structurebull Sending adapter encapsulates IP datagram

(or other network layer protocol packet) in Ethernet frame

Preamble bull 7 bytes with pattern 10101010 followed by

one byte with pattern 10101011bull used to synchronize receiver sender clock

ratesFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

50

Ethernet Frame Structure (more)bull Addresses 6 bytes

bull if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

bull otherwise adapter discards frame

bull Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)

bull CRC checked at receiver if error is detected frame is dropped

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51

Ethernet Unreliable connection-lessbull connectionless No handshaking be-

tween sending and receiving NICs bull unreliable receiving NIC doesnrsquot send

acks or nacks to sending NICbull stream of datagrams passed to network

layer can have gaps (missing datagrams)bull gaps will be filled if app is using TCPbull otherwise app will see gaps

bull Ethernetrsquos MAC protocol unslotted CSMACD

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

52

Ethernet CSMACD algorithmbull 1 NIC receives data-

gram from network layer creates frame

bull 2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

bull 3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

bull 4 If NIC detects an-other transmission while transmitting aborts and sends jam signal

bull 5 After aborting NIC enters exponential backoff after mth colli-sion NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

53

Ethernetrsquos CSMACD (more)bull Jam Signal make

sure all other transmitters are aware of collision 48 bits

bull Bit time 1 micro sec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Exponential Backoff bull Goal adapt retransmission

attempts to estimated cur-rent load

bull heavy load random wait will be longer

bull first collision choose K from 01 delay is K 512 bit transmission times

bull after second collision choose K from 0123hellip

bull after ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

54

CSMACD efficiencybull Tprop = max prop delay between 2 nodes in

LANbull ttrans = time to transmit max-size frame

bull efficiency goes to 1 bull as tprop goes to 0

bull as ttrans goes to infinity

bull better performance than ALOHA and simple cheap decentralized

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

transprop ttefficiency

51

1

55

8023 Ethernet Standards Link amp Physical Layersbull many different Ethernet standards

bull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps

1Gbps 10G bpsbull different physical layer media fiber cable

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

applicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twisterpair) physical layer

56

Manchester encodingbull used in 10BaseTbull each bit has a transitionbull allows clocks in sending and receiving nodes to syn-

chronize to each otherbull no need for a centralized global clock among nodes

bull Hey this is physical-layer stuff

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

57

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

58

Hubshellip physical-layer (ldquodumbrdquo) repeaters

bull bits coming in one link go out all other links at same rate

bull all nodes connected to hub can collide with one another

bull no frame bufferingbull no CSMACD at hub host NICs detect colli-

sions

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

twisted pair

hub

59

Switchbull link-layer device smarter than hubs take

active rolebull store forward Ethernet framesbull examine incoming framersquos MAC address se-

lectively forward frame to one-or-more out-going links when frame is to be forwarded on segment uses CSMACD to access segment

bull transparentbull hosts are unaware of presence of switches

bull plug-and-play self-learning (HOW)bull switches do not need to be configured

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

60

Switch allows multiple simulta-neous transmissionsbull hosts have dedicated direct

connection to switchbull switches buffer packetsbull Ethernet protocol used on

each incoming link but no collisions full duplexbull each link is its own collision

domain

bull switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions bull not possible with dumb hub

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 23

45

6

61

Switch Tablebull Q how does switch know

that Arsquo reachable via inter-face 4 Brsquo reachable via inter-face 5

bull A each switch has a switch table each entrybull (MAC address of host interface

to reach host time stamp)

bull looks like a routing tablebull Q how are entries created

maintained in switch table bull something like a routing proto-

colFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

switch with six interfaces(123456)

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

62

Switch self-learningbull switch learns which

hosts can be reached through which inter-facesbull when frame received

switch ldquolearnsrdquo location of sender incoming LAN segment

bull records senderlocation pair in switch table

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

63

Switch frame filteringforward-ingWhen frame received

1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination then if dest on segment from which frame arrived then drop the frame else

forward the frame on interface indicated else flood

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

forward on all but the interface on which the frame arrived

64

Self-learning forwarding exam-ple

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

frame destination unknownflood

Arsquo A

destination A location known

Arsquo 4 60

selective send

65

Interconnecting switchesbull switches can be connected together

bull Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3

bull A self learning (works exactly the same as in single-switch case)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

B

S1

C D

E

FS2

S4

S3

H

I

G

66

Self-learning multi-switch exam-plebull Suppose C sends frame to I I responds to

C

bull Q show switch tables and packet forward-ing in S1 S2 S3 S4

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

B

S1

CD

E F

S2

S4

S3

H

I

G

12

1

2 3

3

1 2 3 12 3

444

67

Institutional network

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

to externalnetwork

router

IP subnet

mail server

web server

68

Switches vs Routersbull both store-and-forward devices

bull routers network layer devices (examine network layer head-ers)

bull switches are link layer devices

bull routers maintain routing tables implement routing al-gorithms

bull switches maintain switch tables implement filtering learning algorithms

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

69

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

• Fall 2015 COMP 46055605 Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link layer context
• Link Layer Services
• Link Layer Services (more)
• Where is the link layer implemented
• Adaptors Communicating
• Link Layer (2)
• Error Detection
• Parity Checking
• Internet checksum (review)
• Checksumming Cyclic Redundancy Check
• Checksumming Cyclic Redundancy Check (2)
• CRC Example
• Link Layer (3)
• Multiple Access Links and Protocols
• Multiple Access protocols
• Ideal Multiple Access Protocol
• MAC Protocols a taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD collision detection
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Link Layer (4)
• MAC Addresses and ARP
• LAN Addresses and ARP
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP protocol Same LAN (network)
• Addressing routing to another LAN
• Slide 45
• Link Layer (5)
• Ethernet
• Star Topology
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester encoding
• Link Layer (6)
• Hubs
• Switch
• Switch allows multiple simultaneous transmissions
• Switch Table
• Switch self-learning
• Switch frame filteringforwarding
• Self-learning forwarding example
• Interconnecting switches
• Self-learning multi-switch example
• Institutional network
• Switches vs Routers
• Link Layer (7)

40

LAN Addresses and ARP

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Each adapter on LAN has unique LAN address

Broadcast address =FF-FF-FF-FF-FF-FF

= adapter

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired orwireless)

41

LAN Address (more)bull MAC address allocation administered by IEEEbull manufacturer buys portion of MAC address

space (to assure uniqueness)bull analogy

(a) MAC address like Social Security Num-ber

(b) IP address like postal addressbull MAC flat address portability

bull can move LAN card from one LAN to another

bull IP hierarchical address NOT portablebull address depends on IP subnet to which node is at-

tachedFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

42

ARP Address Resolution Proto-colbull Each IP node (host

router) on LAN has ARP table

bull ARP table IPMAC address mappings for some LAN nodeslt IP address MAC address

TTLgt

bull TTL (Time To Live) time after which ad-dress mapping will be forgotten (typically 20 min)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Question how to determineMAC address of Bknowing Brsquos IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN

137196723

137196778

137196714

137196788

43

ARP protocol Same LAN (net-work)bull A wants to send datagram

to B and Brsquos MAC address not in Arsquos ARP table

bull A broadcasts ARP query packet containing Bs IP address bull dest MAC address = FF-FF-FF-

FF-FF-FFbull all machines on LAN receive

ARP query

bull B receives ARP packet replies to A with its (Bs) MAC addressbull frame sent to Arsquos MAC ad-

dress (unicast)Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

bull A caches (saves) IP-to-MAC address pair in its ARP table until infor-mation becomes old (times out) bull soft state information

that times out (goes away) unless refreshed

bull ARP is ldquoplug-and-playrdquobull nodes create their ARP

tables without interven-tion from net administra-tor

44

Addressing routing to another LAN

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

R

1A-23-F9-CD-06-9B

222222222220111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

walkthrough send datagram from A to B via R assume A knows Brsquos IP address

bull two ARP tables in router R one for each IP network (LAN)

45

bull A creates IP datagram with source A destination B bull A uses ARP to get Rrsquos MAC address for 111111111110bull A creates link-layer frame with Rs MAC address as dest

frame contains A-to-B IP datagrambull Arsquos NIC sends frame bull Rrsquos NIC receives frame bull R removes IP datagram from Ethernet frame sees its des-

tined to Bbull R uses ARP to get Brsquos MAC address bull R creates frame containing A-to-B IP datagram sends to B

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

This is a really importantexample ndash make sure youunderstand

R

1A-23-F9-CD-06-9B

222222222220

111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

46

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

47

Ethernetldquodominantrdquo wired LAN technology bull cheap $20 for NICbull first widely used LAN technologybull simpler cheaper than token LANs and ATMbull kept up with speed race 10 Mbps ndash 10 Gbps

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Metcalfersquos Ethernetsketch

48

Star Topologybull bus topology popular through mid 90s

bull all nodes in same collision domain (can collide with each other)

bull today star topology prevailsbull active switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes do

not collide with each other)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

switch

bus coaxial cable star

49

Ethernet Frame Structurebull Sending adapter encapsulates IP datagram

(or other network layer protocol packet) in Ethernet frame

Preamble bull 7 bytes with pattern 10101010 followed by

one byte with pattern 10101011bull used to synchronize receiver sender clock

ratesFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

50

Ethernet Frame Structure (more)bull Addresses 6 bytes

bull if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

bull otherwise adapter discards frame

bull Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)

bull CRC checked at receiver if error is detected frame is dropped

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51

Ethernet Unreliable connection-lessbull connectionless No handshaking be-

tween sending and receiving NICs bull unreliable receiving NIC doesnrsquot send

acks or nacks to sending NICbull stream of datagrams passed to network

layer can have gaps (missing datagrams)bull gaps will be filled if app is using TCPbull otherwise app will see gaps

bull Ethernetrsquos MAC protocol unslotted CSMACD

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

52

Ethernet CSMACD algorithmbull 1 NIC receives data-

gram from network layer creates frame

bull 2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

bull 3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

bull 4 If NIC detects an-other transmission while transmitting aborts and sends jam signal

bull 5 After aborting NIC enters exponential backoff after mth colli-sion NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

53

Ethernetrsquos CSMACD (more)bull Jam Signal make

sure all other transmitters are aware of collision 48 bits

bull Bit time 1 micro sec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Exponential Backoff bull Goal adapt retransmission

attempts to estimated cur-rent load

bull heavy load random wait will be longer

bull first collision choose K from 01 delay is K 512 bit transmission times

bull after second collision choose K from 0123hellip

bull after ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

54

CSMACD efficiencybull Tprop = max prop delay between 2 nodes in

LANbull ttrans = time to transmit max-size frame

bull efficiency goes to 1 bull as tprop goes to 0

bull as ttrans goes to infinity

bull better performance than ALOHA and simple cheap decentralized

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

transprop ttefficiency

51

1

55

8023 Ethernet Standards Link amp Physical Layersbull many different Ethernet standards

bull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps

1Gbps 10G bpsbull different physical layer media fiber cable

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

applicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twisterpair) physical layer

56

Manchester encodingbull used in 10BaseTbull each bit has a transitionbull allows clocks in sending and receiving nodes to syn-

chronize to each otherbull no need for a centralized global clock among nodes

bull Hey this is physical-layer stuff

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

57

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

58

Hubshellip physical-layer (ldquodumbrdquo) repeaters

bull bits coming in one link go out all other links at same rate

bull all nodes connected to hub can collide with one another

bull no frame bufferingbull no CSMACD at hub host NICs detect colli-

sions

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

twisted pair

hub

59

Switchbull link-layer device smarter than hubs take

active rolebull store forward Ethernet framesbull examine incoming framersquos MAC address se-

lectively forward frame to one-or-more out-going links when frame is to be forwarded on segment uses CSMACD to access segment

bull transparentbull hosts are unaware of presence of switches

bull plug-and-play self-learning (HOW)bull switches do not need to be configured

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

60

Switch allows multiple simulta-neous transmissionsbull hosts have dedicated direct

connection to switchbull switches buffer packetsbull Ethernet protocol used on

each incoming link but no collisions full duplexbull each link is its own collision

domain

bull switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions bull not possible with dumb hub

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 23

45

6

61

Switch Tablebull Q how does switch know

that Arsquo reachable via inter-face 4 Brsquo reachable via inter-face 5

bull A each switch has a switch table each entrybull (MAC address of host interface

to reach host time stamp)

bull looks like a routing tablebull Q how are entries created

maintained in switch table bull something like a routing proto-

colFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

switch with six interfaces(123456)

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

62

Switch self-learningbull switch learns which

hosts can be reached through which inter-facesbull when frame received

switch ldquolearnsrdquo location of sender incoming LAN segment

bull records senderlocation pair in switch table

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

63

Switch frame filteringforward-ingWhen frame received

1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination then if dest on segment from which frame arrived then drop the frame else

forward the frame on interface indicated else flood

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

forward on all but the interface on which the frame arrived

64

Self-learning forwarding exam-ple

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

frame destination unknownflood

Arsquo A

destination A location known

Arsquo 4 60

selective send

65

Interconnecting switchesbull switches can be connected together

bull Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3

bull A self learning (works exactly the same as in single-switch case)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

B

S1

C D

E

FS2

S4

S3

H

I

G

66

Self-learning multi-switch exam-plebull Suppose C sends frame to I I responds to

C

bull Q show switch tables and packet forward-ing in S1 S2 S3 S4

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

B

S1

CD

E F

S2

S4

S3

H

I

G

12

1

2 3

3

1 2 3 12 3

444

67

Institutional network

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

to externalnetwork

router

IP subnet

mail server

web server

68

Switches vs Routersbull both store-and-forward devices

bull routers network layer devices (examine network layer head-ers)

bull switches are link layer devices

bull routers maintain routing tables implement routing al-gorithms

bull switches maintain switch tables implement filtering learning algorithms

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

69

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

• Fall 2015 COMP 46055605 Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link layer context
• Link Layer Services
• Link Layer Services (more)
• Where is the link layer implemented
• Adaptors Communicating
• Link Layer (2)
• Error Detection
• Parity Checking
• Internet checksum (review)
• Checksumming Cyclic Redundancy Check
• Checksumming Cyclic Redundancy Check (2)
• CRC Example
• Link Layer (3)
• Multiple Access Links and Protocols
• Multiple Access protocols
• Ideal Multiple Access Protocol
• MAC Protocols a taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD collision detection
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Link Layer (4)
• MAC Addresses and ARP
• LAN Addresses and ARP
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP protocol Same LAN (network)
• Addressing routing to another LAN
• Slide 45
• Link Layer (5)
• Ethernet
• Star Topology
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester encoding
• Link Layer (6)
• Hubs
• Switch
• Switch allows multiple simultaneous transmissions
• Switch Table
• Switch self-learning
• Switch frame filteringforwarding
• Self-learning forwarding example
• Interconnecting switches
• Self-learning multi-switch example
• Institutional network
• Switches vs Routers
• Link Layer (7)

41

LAN Address (more)bull MAC address allocation administered by IEEEbull manufacturer buys portion of MAC address

space (to assure uniqueness)bull analogy

(a) MAC address like Social Security Num-ber

(b) IP address like postal addressbull MAC flat address portability

bull can move LAN card from one LAN to another

bull IP hierarchical address NOT portablebull address depends on IP subnet to which node is at-

tachedFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

42

ARP Address Resolution Proto-colbull Each IP node (host

router) on LAN has ARP table

bull ARP table IPMAC address mappings for some LAN nodeslt IP address MAC address

TTLgt

bull TTL (Time To Live) time after which ad-dress mapping will be forgotten (typically 20 min)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Question how to determineMAC address of Bknowing Brsquos IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN

137196723

137196778

137196714

137196788

43

ARP protocol Same LAN (net-work)bull A wants to send datagram

to B and Brsquos MAC address not in Arsquos ARP table

bull A broadcasts ARP query packet containing Bs IP address bull dest MAC address = FF-FF-FF-

FF-FF-FFbull all machines on LAN receive

ARP query

bull B receives ARP packet replies to A with its (Bs) MAC addressbull frame sent to Arsquos MAC ad-

dress (unicast)Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

bull A caches (saves) IP-to-MAC address pair in its ARP table until infor-mation becomes old (times out) bull soft state information

that times out (goes away) unless refreshed

bull ARP is ldquoplug-and-playrdquobull nodes create their ARP

tables without interven-tion from net administra-tor

44

Addressing routing to another LAN

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

R

1A-23-F9-CD-06-9B

222222222220111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

walkthrough send datagram from A to B via R assume A knows Brsquos IP address

bull two ARP tables in router R one for each IP network (LAN)

45

bull A creates IP datagram with source A destination B bull A uses ARP to get Rrsquos MAC address for 111111111110bull A creates link-layer frame with Rs MAC address as dest

frame contains A-to-B IP datagrambull Arsquos NIC sends frame bull Rrsquos NIC receives frame bull R removes IP datagram from Ethernet frame sees its des-

tined to Bbull R uses ARP to get Brsquos MAC address bull R creates frame containing A-to-B IP datagram sends to B

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

This is a really importantexample ndash make sure youunderstand

R

1A-23-F9-CD-06-9B

222222222220

111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

46

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

47

Ethernetldquodominantrdquo wired LAN technology bull cheap $20 for NICbull first widely used LAN technologybull simpler cheaper than token LANs and ATMbull kept up with speed race 10 Mbps ndash 10 Gbps

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Metcalfersquos Ethernetsketch

48

Star Topologybull bus topology popular through mid 90s

bull all nodes in same collision domain (can collide with each other)

bull today star topology prevailsbull active switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes do

not collide with each other)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

switch

bus coaxial cable star

49

Ethernet Frame Structurebull Sending adapter encapsulates IP datagram

(or other network layer protocol packet) in Ethernet frame

Preamble bull 7 bytes with pattern 10101010 followed by

one byte with pattern 10101011bull used to synchronize receiver sender clock

ratesFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

50

Ethernet Frame Structure (more)bull Addresses 6 bytes

bull if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

bull otherwise adapter discards frame

bull Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)

bull CRC checked at receiver if error is detected frame is dropped

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51

Ethernet Unreliable connection-lessbull connectionless No handshaking be-

tween sending and receiving NICs bull unreliable receiving NIC doesnrsquot send

acks or nacks to sending NICbull stream of datagrams passed to network

layer can have gaps (missing datagrams)bull gaps will be filled if app is using TCPbull otherwise app will see gaps

bull Ethernetrsquos MAC protocol unslotted CSMACD

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

52

Ethernet CSMACD algorithmbull 1 NIC receives data-

gram from network layer creates frame

bull 2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

bull 3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

bull 4 If NIC detects an-other transmission while transmitting aborts and sends jam signal

bull 5 After aborting NIC enters exponential backoff after mth colli-sion NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

53

Ethernetrsquos CSMACD (more)bull Jam Signal make

sure all other transmitters are aware of collision 48 bits

bull Bit time 1 micro sec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Exponential Backoff bull Goal adapt retransmission

attempts to estimated cur-rent load

bull heavy load random wait will be longer

bull first collision choose K from 01 delay is K 512 bit transmission times

bull after second collision choose K from 0123hellip

bull after ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

54

CSMACD efficiencybull Tprop = max prop delay between 2 nodes in

LANbull ttrans = time to transmit max-size frame

bull efficiency goes to 1 bull as tprop goes to 0

bull as ttrans goes to infinity

bull better performance than ALOHA and simple cheap decentralized

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

transprop ttefficiency

51

1

55

8023 Ethernet Standards Link amp Physical Layersbull many different Ethernet standards

bull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps

1Gbps 10G bpsbull different physical layer media fiber cable

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

applicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twisterpair) physical layer

56

Manchester encodingbull used in 10BaseTbull each bit has a transitionbull allows clocks in sending and receiving nodes to syn-

chronize to each otherbull no need for a centralized global clock among nodes

bull Hey this is physical-layer stuff

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

57

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

58

Hubshellip physical-layer (ldquodumbrdquo) repeaters

bull bits coming in one link go out all other links at same rate

bull all nodes connected to hub can collide with one another

bull no frame bufferingbull no CSMACD at hub host NICs detect colli-

sions

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

twisted pair

hub

59

Switchbull link-layer device smarter than hubs take

active rolebull store forward Ethernet framesbull examine incoming framersquos MAC address se-

lectively forward frame to one-or-more out-going links when frame is to be forwarded on segment uses CSMACD to access segment

bull transparentbull hosts are unaware of presence of switches

bull plug-and-play self-learning (HOW)bull switches do not need to be configured

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

60

Switch allows multiple simulta-neous transmissionsbull hosts have dedicated direct

connection to switchbull switches buffer packetsbull Ethernet protocol used on

each incoming link but no collisions full duplexbull each link is its own collision

domain

bull switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions bull not possible with dumb hub

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 23

45

6

61

Switch Tablebull Q how does switch know

that Arsquo reachable via inter-face 4 Brsquo reachable via inter-face 5

bull A each switch has a switch table each entrybull (MAC address of host interface

to reach host time stamp)

bull looks like a routing tablebull Q how are entries created

maintained in switch table bull something like a routing proto-

colFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

switch with six interfaces(123456)

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

62

Switch self-learningbull switch learns which

hosts can be reached through which inter-facesbull when frame received

switch ldquolearnsrdquo location of sender incoming LAN segment

bull records senderlocation pair in switch table

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

63

Switch frame filteringforward-ingWhen frame received

1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination then if dest on segment from which frame arrived then drop the frame else

forward the frame on interface indicated else flood

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

forward on all but the interface on which the frame arrived

64

Self-learning forwarding exam-ple

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

frame destination unknownflood

Arsquo A

destination A location known

Arsquo 4 60

selective send

65

Interconnecting switchesbull switches can be connected together

bull Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3

bull A self learning (works exactly the same as in single-switch case)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

B

S1

C D

E

FS2

S4

S3

H

I

G

66

Self-learning multi-switch exam-plebull Suppose C sends frame to I I responds to

C

bull Q show switch tables and packet forward-ing in S1 S2 S3 S4

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

B

S1

CD

E F

S2

S4

S3

H

I

G

12

1

2 3

3

1 2 3 12 3

444

67

Institutional network

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

to externalnetwork

router

IP subnet

mail server

web server

68

Switches vs Routersbull both store-and-forward devices

bull routers network layer devices (examine network layer head-ers)

bull switches are link layer devices

bull routers maintain routing tables implement routing al-gorithms

bull switches maintain switch tables implement filtering learning algorithms

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

69

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

• Fall 2015 COMP 46055605 Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link layer context
• Link Layer Services
• Link Layer Services (more)
• Where is the link layer implemented
• Adaptors Communicating
• Link Layer (2)
• Error Detection
• Parity Checking
• Internet checksum (review)
• Checksumming Cyclic Redundancy Check
• Checksumming Cyclic Redundancy Check (2)
• CRC Example
• Link Layer (3)
• Multiple Access Links and Protocols
• Multiple Access protocols
• Ideal Multiple Access Protocol
• MAC Protocols a taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD collision detection
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Link Layer (4)
• MAC Addresses and ARP
• LAN Addresses and ARP
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP protocol Same LAN (network)
• Addressing routing to another LAN
• Slide 45
• Link Layer (5)
• Ethernet
• Star Topology
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester encoding
• Link Layer (6)
• Hubs
• Switch
• Switch allows multiple simultaneous transmissions
• Switch Table
• Switch self-learning
• Switch frame filteringforwarding
• Self-learning forwarding example
• Interconnecting switches
• Self-learning multi-switch example
• Institutional network
• Switches vs Routers
• Link Layer (7)

42

ARP Address Resolution Proto-colbull Each IP node (host

router) on LAN has ARP table

bull ARP table IPMAC address mappings for some LAN nodeslt IP address MAC address

TTLgt

bull TTL (Time To Live) time after which ad-dress mapping will be forgotten (typically 20 min)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Question how to determineMAC address of Bknowing Brsquos IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN

137196723

137196778

137196714

137196788

43

ARP protocol Same LAN (net-work)bull A wants to send datagram

to B and Brsquos MAC address not in Arsquos ARP table

bull A broadcasts ARP query packet containing Bs IP address bull dest MAC address = FF-FF-FF-

FF-FF-FFbull all machines on LAN receive

ARP query

bull B receives ARP packet replies to A with its (Bs) MAC addressbull frame sent to Arsquos MAC ad-

dress (unicast)Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

bull A caches (saves) IP-to-MAC address pair in its ARP table until infor-mation becomes old (times out) bull soft state information

that times out (goes away) unless refreshed

bull ARP is ldquoplug-and-playrdquobull nodes create their ARP

tables without interven-tion from net administra-tor

44

Addressing routing to another LAN

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

R

1A-23-F9-CD-06-9B

222222222220111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

walkthrough send datagram from A to B via R assume A knows Brsquos IP address

bull two ARP tables in router R one for each IP network (LAN)

45

bull A creates IP datagram with source A destination B bull A uses ARP to get Rrsquos MAC address for 111111111110bull A creates link-layer frame with Rs MAC address as dest

frame contains A-to-B IP datagrambull Arsquos NIC sends frame bull Rrsquos NIC receives frame bull R removes IP datagram from Ethernet frame sees its des-

tined to Bbull R uses ARP to get Brsquos MAC address bull R creates frame containing A-to-B IP datagram sends to B

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

This is a really importantexample ndash make sure youunderstand

R

1A-23-F9-CD-06-9B

222222222220

111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

46

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

47

Ethernetldquodominantrdquo wired LAN technology bull cheap $20 for NICbull first widely used LAN technologybull simpler cheaper than token LANs and ATMbull kept up with speed race 10 Mbps ndash 10 Gbps

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Metcalfersquos Ethernetsketch

48

Star Topologybull bus topology popular through mid 90s

bull all nodes in same collision domain (can collide with each other)

bull today star topology prevailsbull active switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes do

not collide with each other)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

switch

bus coaxial cable star

49

Ethernet Frame Structurebull Sending adapter encapsulates IP datagram

(or other network layer protocol packet) in Ethernet frame

Preamble bull 7 bytes with pattern 10101010 followed by

one byte with pattern 10101011bull used to synchronize receiver sender clock

ratesFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

50

Ethernet Frame Structure (more)bull Addresses 6 bytes

bull if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

bull otherwise adapter discards frame

bull Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)

bull CRC checked at receiver if error is detected frame is dropped

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51

Ethernet Unreliable connection-lessbull connectionless No handshaking be-

tween sending and receiving NICs bull unreliable receiving NIC doesnrsquot send

acks or nacks to sending NICbull stream of datagrams passed to network

layer can have gaps (missing datagrams)bull gaps will be filled if app is using TCPbull otherwise app will see gaps

bull Ethernetrsquos MAC protocol unslotted CSMACD

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

52

Ethernet CSMACD algorithmbull 1 NIC receives data-

gram from network layer creates frame

bull 2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

bull 3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

bull 4 If NIC detects an-other transmission while transmitting aborts and sends jam signal

bull 5 After aborting NIC enters exponential backoff after mth colli-sion NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

53

Ethernetrsquos CSMACD (more)bull Jam Signal make

sure all other transmitters are aware of collision 48 bits

bull Bit time 1 micro sec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Exponential Backoff bull Goal adapt retransmission

attempts to estimated cur-rent load

bull heavy load random wait will be longer

bull first collision choose K from 01 delay is K 512 bit transmission times

bull after second collision choose K from 0123hellip

bull after ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

54

CSMACD efficiencybull Tprop = max prop delay between 2 nodes in

LANbull ttrans = time to transmit max-size frame

bull efficiency goes to 1 bull as tprop goes to 0

bull as ttrans goes to infinity

bull better performance than ALOHA and simple cheap decentralized

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

transprop ttefficiency

51

1

55

8023 Ethernet Standards Link amp Physical Layersbull many different Ethernet standards

bull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps

1Gbps 10G bpsbull different physical layer media fiber cable

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

applicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twisterpair) physical layer

56

Manchester encodingbull used in 10BaseTbull each bit has a transitionbull allows clocks in sending and receiving nodes to syn-

chronize to each otherbull no need for a centralized global clock among nodes

bull Hey this is physical-layer stuff

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

57

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

58

Hubshellip physical-layer (ldquodumbrdquo) repeaters

bull bits coming in one link go out all other links at same rate

bull all nodes connected to hub can collide with one another

bull no frame bufferingbull no CSMACD at hub host NICs detect colli-

sions

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

twisted pair

hub

59

Switchbull link-layer device smarter than hubs take

active rolebull store forward Ethernet framesbull examine incoming framersquos MAC address se-

lectively forward frame to one-or-more out-going links when frame is to be forwarded on segment uses CSMACD to access segment

bull transparentbull hosts are unaware of presence of switches

bull plug-and-play self-learning (HOW)bull switches do not need to be configured

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

60

Switch allows multiple simulta-neous transmissionsbull hosts have dedicated direct

connection to switchbull switches buffer packetsbull Ethernet protocol used on

each incoming link but no collisions full duplexbull each link is its own collision

domain

bull switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions bull not possible with dumb hub

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 23

45

6

61

Switch Tablebull Q how does switch know

that Arsquo reachable via inter-face 4 Brsquo reachable via inter-face 5

bull A each switch has a switch table each entrybull (MAC address of host interface

to reach host time stamp)

bull looks like a routing tablebull Q how are entries created

maintained in switch table bull something like a routing proto-

colFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

switch with six interfaces(123456)

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

62

Switch self-learningbull switch learns which

hosts can be reached through which inter-facesbull when frame received

switch ldquolearnsrdquo location of sender incoming LAN segment

bull records senderlocation pair in switch table

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

63

Switch frame filteringforward-ingWhen frame received

1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination then if dest on segment from which frame arrived then drop the frame else

forward the frame on interface indicated else flood

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

forward on all but the interface on which the frame arrived

64

Self-learning forwarding exam-ple

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

frame destination unknownflood

Arsquo A

destination A location known

Arsquo 4 60

selective send

65

Interconnecting switchesbull switches can be connected together

bull Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3

bull A self learning (works exactly the same as in single-switch case)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

B

S1

C D

E

FS2

S4

S3

H

I

G

66

Self-learning multi-switch exam-plebull Suppose C sends frame to I I responds to

C

bull Q show switch tables and packet forward-ing in S1 S2 S3 S4

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

B

S1

CD

E F

S2

S4

S3

H

I

G

12

1

2 3

3

1 2 3 12 3

444

67

Institutional network

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

to externalnetwork

router

IP subnet

mail server

web server

68

Switches vs Routersbull both store-and-forward devices

bull routers network layer devices (examine network layer head-ers)

bull switches are link layer devices

bull routers maintain routing tables implement routing al-gorithms

bull switches maintain switch tables implement filtering learning algorithms

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

69

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

• Fall 2015 COMP 46055605 Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link layer context
• Link Layer Services
• Link Layer Services (more)
• Where is the link layer implemented
• Adaptors Communicating
• Link Layer (2)
• Error Detection
• Parity Checking
• Internet checksum (review)
• Checksumming Cyclic Redundancy Check
• Checksumming Cyclic Redundancy Check (2)
• CRC Example
• Link Layer (3)
• Multiple Access Links and Protocols
• Multiple Access protocols
• Ideal Multiple Access Protocol
• MAC Protocols a taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD collision detection
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Link Layer (4)
• MAC Addresses and ARP
• LAN Addresses and ARP
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP protocol Same LAN (network)
• Addressing routing to another LAN
• Slide 45
• Link Layer (5)
• Ethernet
• Star Topology
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester encoding
• Link Layer (6)
• Hubs
• Switch
• Switch allows multiple simultaneous transmissions
• Switch Table
• Switch self-learning
• Switch frame filteringforwarding
• Self-learning forwarding example
• Interconnecting switches
• Self-learning multi-switch example
• Institutional network
• Switches vs Routers
• Link Layer (7)

43

ARP protocol Same LAN (net-work)bull A wants to send datagram

to B and Brsquos MAC address not in Arsquos ARP table

bull A broadcasts ARP query packet containing Bs IP address bull dest MAC address = FF-FF-FF-

FF-FF-FFbull all machines on LAN receive

ARP query

bull B receives ARP packet replies to A with its (Bs) MAC addressbull frame sent to Arsquos MAC ad-

dress (unicast)Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

bull A caches (saves) IP-to-MAC address pair in its ARP table until infor-mation becomes old (times out) bull soft state information

that times out (goes away) unless refreshed

bull ARP is ldquoplug-and-playrdquobull nodes create their ARP

tables without interven-tion from net administra-tor

44

Addressing routing to another LAN

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

R

1A-23-F9-CD-06-9B

222222222220111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

walkthrough send datagram from A to B via R assume A knows Brsquos IP address

bull two ARP tables in router R one for each IP network (LAN)

45

bull A creates IP datagram with source A destination B bull A uses ARP to get Rrsquos MAC address for 111111111110bull A creates link-layer frame with Rs MAC address as dest

frame contains A-to-B IP datagrambull Arsquos NIC sends frame bull Rrsquos NIC receives frame bull R removes IP datagram from Ethernet frame sees its des-

tined to Bbull R uses ARP to get Brsquos MAC address bull R creates frame containing A-to-B IP datagram sends to B

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

This is a really importantexample ndash make sure youunderstand

R

1A-23-F9-CD-06-9B

222222222220

111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

46

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

47

Ethernetldquodominantrdquo wired LAN technology bull cheap $20 for NICbull first widely used LAN technologybull simpler cheaper than token LANs and ATMbull kept up with speed race 10 Mbps ndash 10 Gbps

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Metcalfersquos Ethernetsketch

48

Star Topologybull bus topology popular through mid 90s

bull all nodes in same collision domain (can collide with each other)

bull today star topology prevailsbull active switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes do

not collide with each other)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

switch

bus coaxial cable star

49

Ethernet Frame Structurebull Sending adapter encapsulates IP datagram

(or other network layer protocol packet) in Ethernet frame

Preamble bull 7 bytes with pattern 10101010 followed by

one byte with pattern 10101011bull used to synchronize receiver sender clock

ratesFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

50

Ethernet Frame Structure (more)bull Addresses 6 bytes

bull if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

bull otherwise adapter discards frame

bull Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)

bull CRC checked at receiver if error is detected frame is dropped

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51

Ethernet Unreliable connection-lessbull connectionless No handshaking be-

tween sending and receiving NICs bull unreliable receiving NIC doesnrsquot send

acks or nacks to sending NICbull stream of datagrams passed to network

layer can have gaps (missing datagrams)bull gaps will be filled if app is using TCPbull otherwise app will see gaps

bull Ethernetrsquos MAC protocol unslotted CSMACD

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

52

Ethernet CSMACD algorithmbull 1 NIC receives data-

gram from network layer creates frame

bull 2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

bull 3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

bull 4 If NIC detects an-other transmission while transmitting aborts and sends jam signal

bull 5 After aborting NIC enters exponential backoff after mth colli-sion NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

53

Ethernetrsquos CSMACD (more)bull Jam Signal make

sure all other transmitters are aware of collision 48 bits

bull Bit time 1 micro sec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Exponential Backoff bull Goal adapt retransmission

attempts to estimated cur-rent load

bull heavy load random wait will be longer

bull first collision choose K from 01 delay is K 512 bit transmission times

bull after second collision choose K from 0123hellip

bull after ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

54

CSMACD efficiencybull Tprop = max prop delay between 2 nodes in

LANbull ttrans = time to transmit max-size frame

bull efficiency goes to 1 bull as tprop goes to 0

bull as ttrans goes to infinity

bull better performance than ALOHA and simple cheap decentralized

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

transprop ttefficiency

51

1

55

8023 Ethernet Standards Link amp Physical Layersbull many different Ethernet standards

bull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps

1Gbps 10G bpsbull different physical layer media fiber cable

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

applicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twisterpair) physical layer

56

Manchester encodingbull used in 10BaseTbull each bit has a transitionbull allows clocks in sending and receiving nodes to syn-

chronize to each otherbull no need for a centralized global clock among nodes

bull Hey this is physical-layer stuff

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

57

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

58

Hubshellip physical-layer (ldquodumbrdquo) repeaters

bull bits coming in one link go out all other links at same rate

bull all nodes connected to hub can collide with one another

bull no frame bufferingbull no CSMACD at hub host NICs detect colli-

sions

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

twisted pair

hub

59

Switchbull link-layer device smarter than hubs take

active rolebull store forward Ethernet framesbull examine incoming framersquos MAC address se-

lectively forward frame to one-or-more out-going links when frame is to be forwarded on segment uses CSMACD to access segment

bull transparentbull hosts are unaware of presence of switches

bull plug-and-play self-learning (HOW)bull switches do not need to be configured

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

60

Switch allows multiple simulta-neous transmissionsbull hosts have dedicated direct

connection to switchbull switches buffer packetsbull Ethernet protocol used on

each incoming link but no collisions full duplexbull each link is its own collision

domain

bull switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions bull not possible with dumb hub

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 23

45

6

61

Switch Tablebull Q how does switch know

that Arsquo reachable via inter-face 4 Brsquo reachable via inter-face 5

bull A each switch has a switch table each entrybull (MAC address of host interface

to reach host time stamp)

bull looks like a routing tablebull Q how are entries created

maintained in switch table bull something like a routing proto-

colFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

switch with six interfaces(123456)

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

62

Switch self-learningbull switch learns which

hosts can be reached through which inter-facesbull when frame received

switch ldquolearnsrdquo location of sender incoming LAN segment

bull records senderlocation pair in switch table

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

63

Switch frame filteringforward-ingWhen frame received

1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination then if dest on segment from which frame arrived then drop the frame else

forward the frame on interface indicated else flood

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

forward on all but the interface on which the frame arrived

64

Self-learning forwarding exam-ple

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

frame destination unknownflood

Arsquo A

destination A location known

Arsquo 4 60

selective send

65

Interconnecting switchesbull switches can be connected together

bull Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3

bull A self learning (works exactly the same as in single-switch case)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

B

S1

C D

E

FS2

S4

S3

H

I

G

66

Self-learning multi-switch exam-plebull Suppose C sends frame to I I responds to

C

bull Q show switch tables and packet forward-ing in S1 S2 S3 S4

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

B

S1

CD

E F

S2

S4

S3

H

I

G

12

1

2 3

3

1 2 3 12 3

444

67

Institutional network

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

to externalnetwork

router

IP subnet

mail server

web server

68

Switches vs Routersbull both store-and-forward devices

bull routers network layer devices (examine network layer head-ers)

bull switches are link layer devices

bull routers maintain routing tables implement routing al-gorithms

bull switches maintain switch tables implement filtering learning algorithms

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

69

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

• Fall 2015 COMP 46055605 Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link layer context
• Link Layer Services
• Link Layer Services (more)
• Where is the link layer implemented
• Adaptors Communicating
• Link Layer (2)
• Error Detection
• Parity Checking
• Internet checksum (review)
• Checksumming Cyclic Redundancy Check
• Checksumming Cyclic Redundancy Check (2)
• CRC Example
• Link Layer (3)
• Multiple Access Links and Protocols
• Multiple Access protocols
• Ideal Multiple Access Protocol
• MAC Protocols a taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD collision detection
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Link Layer (4)
• MAC Addresses and ARP
• LAN Addresses and ARP
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP protocol Same LAN (network)
• Addressing routing to another LAN
• Slide 45
• Link Layer (5)
• Ethernet
• Star Topology
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester encoding
• Link Layer (6)
• Hubs
• Switch
• Switch allows multiple simultaneous transmissions
• Switch Table
• Switch self-learning
• Switch frame filteringforwarding
• Self-learning forwarding example
• Interconnecting switches
• Self-learning multi-switch example
• Institutional network
• Switches vs Routers
• Link Layer (7)

44

Addressing routing to another LAN

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

R

1A-23-F9-CD-06-9B

222222222220111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

walkthrough send datagram from A to B via R assume A knows Brsquos IP address

bull two ARP tables in router R one for each IP network (LAN)

45

bull A creates IP datagram with source A destination B bull A uses ARP to get Rrsquos MAC address for 111111111110bull A creates link-layer frame with Rs MAC address as dest

frame contains A-to-B IP datagrambull Arsquos NIC sends frame bull Rrsquos NIC receives frame bull R removes IP datagram from Ethernet frame sees its des-

tined to Bbull R uses ARP to get Brsquos MAC address bull R creates frame containing A-to-B IP datagram sends to B

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

This is a really importantexample ndash make sure youunderstand

R

1A-23-F9-CD-06-9B

222222222220

111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

46

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

47

Ethernetldquodominantrdquo wired LAN technology bull cheap $20 for NICbull first widely used LAN technologybull simpler cheaper than token LANs and ATMbull kept up with speed race 10 Mbps ndash 10 Gbps

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Metcalfersquos Ethernetsketch

48

Star Topologybull bus topology popular through mid 90s

bull all nodes in same collision domain (can collide with each other)

bull today star topology prevailsbull active switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes do

not collide with each other)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

switch

bus coaxial cable star

49

Ethernet Frame Structurebull Sending adapter encapsulates IP datagram

(or other network layer protocol packet) in Ethernet frame

Preamble bull 7 bytes with pattern 10101010 followed by

one byte with pattern 10101011bull used to synchronize receiver sender clock

ratesFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

50

Ethernet Frame Structure (more)bull Addresses 6 bytes

bull if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

bull otherwise adapter discards frame

bull Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)

bull CRC checked at receiver if error is detected frame is dropped

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51

Ethernet Unreliable connection-lessbull connectionless No handshaking be-

tween sending and receiving NICs bull unreliable receiving NIC doesnrsquot send

acks or nacks to sending NICbull stream of datagrams passed to network

layer can have gaps (missing datagrams)bull gaps will be filled if app is using TCPbull otherwise app will see gaps

bull Ethernetrsquos MAC protocol unslotted CSMACD

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

52

Ethernet CSMACD algorithmbull 1 NIC receives data-

gram from network layer creates frame

bull 2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

bull 3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

bull 4 If NIC detects an-other transmission while transmitting aborts and sends jam signal

bull 5 After aborting NIC enters exponential backoff after mth colli-sion NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

53

Ethernetrsquos CSMACD (more)bull Jam Signal make

sure all other transmitters are aware of collision 48 bits

bull Bit time 1 micro sec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Exponential Backoff bull Goal adapt retransmission

attempts to estimated cur-rent load

bull heavy load random wait will be longer

bull first collision choose K from 01 delay is K 512 bit transmission times

bull after second collision choose K from 0123hellip

bull after ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

54

CSMACD efficiencybull Tprop = max prop delay between 2 nodes in

LANbull ttrans = time to transmit max-size frame

bull efficiency goes to 1 bull as tprop goes to 0

bull as ttrans goes to infinity

bull better performance than ALOHA and simple cheap decentralized

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

transprop ttefficiency

51

1

55

8023 Ethernet Standards Link amp Physical Layersbull many different Ethernet standards

bull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps

1Gbps 10G bpsbull different physical layer media fiber cable

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

applicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twisterpair) physical layer

56

Manchester encodingbull used in 10BaseTbull each bit has a transitionbull allows clocks in sending and receiving nodes to syn-

chronize to each otherbull no need for a centralized global clock among nodes

bull Hey this is physical-layer stuff

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

57

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

58

Hubshellip physical-layer (ldquodumbrdquo) repeaters

bull bits coming in one link go out all other links at same rate

bull all nodes connected to hub can collide with one another

bull no frame bufferingbull no CSMACD at hub host NICs detect colli-

sions

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

twisted pair

hub

59

Switchbull link-layer device smarter than hubs take

active rolebull store forward Ethernet framesbull examine incoming framersquos MAC address se-

lectively forward frame to one-or-more out-going links when frame is to be forwarded on segment uses CSMACD to access segment

bull transparentbull hosts are unaware of presence of switches

bull plug-and-play self-learning (HOW)bull switches do not need to be configured

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

60

Switch allows multiple simulta-neous transmissionsbull hosts have dedicated direct

connection to switchbull switches buffer packetsbull Ethernet protocol used on

each incoming link but no collisions full duplexbull each link is its own collision

domain

bull switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions bull not possible with dumb hub

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 23

45

6

61

Switch Tablebull Q how does switch know

that Arsquo reachable via inter-face 4 Brsquo reachable via inter-face 5

bull A each switch has a switch table each entrybull (MAC address of host interface

to reach host time stamp)

bull looks like a routing tablebull Q how are entries created

maintained in switch table bull something like a routing proto-

colFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

switch with six interfaces(123456)

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

62

Switch self-learningbull switch learns which

hosts can be reached through which inter-facesbull when frame received

switch ldquolearnsrdquo location of sender incoming LAN segment

bull records senderlocation pair in switch table

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

63

Switch frame filteringforward-ingWhen frame received

1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination then if dest on segment from which frame arrived then drop the frame else

forward the frame on interface indicated else flood

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

forward on all but the interface on which the frame arrived

64

Self-learning forwarding exam-ple

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

frame destination unknownflood

Arsquo A

destination A location known

Arsquo 4 60

selective send

65

Interconnecting switchesbull switches can be connected together

bull Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3

bull A self learning (works exactly the same as in single-switch case)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

B

S1

C D

E

FS2

S4

S3

H

I

G

66

Self-learning multi-switch exam-plebull Suppose C sends frame to I I responds to

C

bull Q show switch tables and packet forward-ing in S1 S2 S3 S4

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

B

S1

CD

E F

S2

S4

S3

H

I

G

12

1

2 3

3

1 2 3 12 3

444

67

Institutional network

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

to externalnetwork

router

IP subnet

mail server

web server

68

Switches vs Routersbull both store-and-forward devices

bull routers network layer devices (examine network layer head-ers)

bull switches are link layer devices

bull routers maintain routing tables implement routing al-gorithms

bull switches maintain switch tables implement filtering learning algorithms

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

69

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

• Fall 2015 COMP 46055605 Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link layer context
• Link Layer Services
• Link Layer Services (more)
• Where is the link layer implemented
• Adaptors Communicating
• Link Layer (2)
• Error Detection
• Parity Checking
• Internet checksum (review)
• Checksumming Cyclic Redundancy Check
• Checksumming Cyclic Redundancy Check (2)
• CRC Example
• Link Layer (3)
• Multiple Access Links and Protocols
• Multiple Access protocols
• Ideal Multiple Access Protocol
• MAC Protocols a taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD collision detection
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Link Layer (4)
• MAC Addresses and ARP
• LAN Addresses and ARP
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP protocol Same LAN (network)
• Addressing routing to another LAN
• Slide 45
• Link Layer (5)
• Ethernet
• Star Topology
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester encoding
• Link Layer (6)
• Hubs
• Switch
• Switch allows multiple simultaneous transmissions
• Switch Table
• Switch self-learning
• Switch frame filteringforwarding
• Self-learning forwarding example
• Interconnecting switches
• Self-learning multi-switch example
• Institutional network
• Switches vs Routers
• Link Layer (7)

45

bull A creates IP datagram with source A destination B bull A uses ARP to get Rrsquos MAC address for 111111111110bull A creates link-layer frame with Rs MAC address as dest

frame contains A-to-B IP datagrambull Arsquos NIC sends frame bull Rrsquos NIC receives frame bull R removes IP datagram from Ethernet frame sees its des-

tined to Bbull R uses ARP to get Brsquos MAC address bull R creates frame containing A-to-B IP datagram sends to B

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

This is a really importantexample ndash make sure youunderstand

R

1A-23-F9-CD-06-9B

222222222220

111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

46

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

47

Ethernetldquodominantrdquo wired LAN technology bull cheap $20 for NICbull first widely used LAN technologybull simpler cheaper than token LANs and ATMbull kept up with speed race 10 Mbps ndash 10 Gbps

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Metcalfersquos Ethernetsketch

48

Star Topologybull bus topology popular through mid 90s

bull all nodes in same collision domain (can collide with each other)

bull today star topology prevailsbull active switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes do

not collide with each other)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

switch

bus coaxial cable star

49

Ethernet Frame Structurebull Sending adapter encapsulates IP datagram

(or other network layer protocol packet) in Ethernet frame

Preamble bull 7 bytes with pattern 10101010 followed by

one byte with pattern 10101011bull used to synchronize receiver sender clock

ratesFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

50

Ethernet Frame Structure (more)bull Addresses 6 bytes

bull if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

bull otherwise adapter discards frame

bull Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)

bull CRC checked at receiver if error is detected frame is dropped

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51

Ethernet Unreliable connection-lessbull connectionless No handshaking be-

tween sending and receiving NICs bull unreliable receiving NIC doesnrsquot send

acks or nacks to sending NICbull stream of datagrams passed to network

layer can have gaps (missing datagrams)bull gaps will be filled if app is using TCPbull otherwise app will see gaps

bull Ethernetrsquos MAC protocol unslotted CSMACD

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

52

Ethernet CSMACD algorithmbull 1 NIC receives data-

gram from network layer creates frame

bull 2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

bull 3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

bull 4 If NIC detects an-other transmission while transmitting aborts and sends jam signal

bull 5 After aborting NIC enters exponential backoff after mth colli-sion NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

53

Ethernetrsquos CSMACD (more)bull Jam Signal make

sure all other transmitters are aware of collision 48 bits

bull Bit time 1 micro sec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Exponential Backoff bull Goal adapt retransmission

attempts to estimated cur-rent load

bull heavy load random wait will be longer

bull first collision choose K from 01 delay is K 512 bit transmission times

bull after second collision choose K from 0123hellip

bull after ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

54

CSMACD efficiencybull Tprop = max prop delay between 2 nodes in

LANbull ttrans = time to transmit max-size frame

bull efficiency goes to 1 bull as tprop goes to 0

bull as ttrans goes to infinity

bull better performance than ALOHA and simple cheap decentralized

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

transprop ttefficiency

51

1

55

8023 Ethernet Standards Link amp Physical Layersbull many different Ethernet standards

bull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps

1Gbps 10G bpsbull different physical layer media fiber cable

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

applicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twisterpair) physical layer

56

Manchester encodingbull used in 10BaseTbull each bit has a transitionbull allows clocks in sending and receiving nodes to syn-

chronize to each otherbull no need for a centralized global clock among nodes

bull Hey this is physical-layer stuff

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

57

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

58

Hubshellip physical-layer (ldquodumbrdquo) repeaters

bull bits coming in one link go out all other links at same rate

bull all nodes connected to hub can collide with one another

bull no frame bufferingbull no CSMACD at hub host NICs detect colli-

sions

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

twisted pair

hub

59

Switchbull link-layer device smarter than hubs take

active rolebull store forward Ethernet framesbull examine incoming framersquos MAC address se-

lectively forward frame to one-or-more out-going links when frame is to be forwarded on segment uses CSMACD to access segment

bull transparentbull hosts are unaware of presence of switches

bull plug-and-play self-learning (HOW)bull switches do not need to be configured

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

60

Switch allows multiple simulta-neous transmissionsbull hosts have dedicated direct

connection to switchbull switches buffer packetsbull Ethernet protocol used on

each incoming link but no collisions full duplexbull each link is its own collision

domain

bull switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions bull not possible with dumb hub

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 23

45

6

61

Switch Tablebull Q how does switch know

that Arsquo reachable via inter-face 4 Brsquo reachable via inter-face 5

bull A each switch has a switch table each entrybull (MAC address of host interface

to reach host time stamp)

bull looks like a routing tablebull Q how are entries created

maintained in switch table bull something like a routing proto-

colFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

switch with six interfaces(123456)

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

62

Switch self-learningbull switch learns which

hosts can be reached through which inter-facesbull when frame received

switch ldquolearnsrdquo location of sender incoming LAN segment

bull records senderlocation pair in switch table

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

63

Switch frame filteringforward-ingWhen frame received

1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination then if dest on segment from which frame arrived then drop the frame else

forward the frame on interface indicated else flood

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

forward on all but the interface on which the frame arrived

64

Self-learning forwarding exam-ple

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

frame destination unknownflood

Arsquo A

destination A location known

Arsquo 4 60

selective send

65

Interconnecting switchesbull switches can be connected together

bull Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3

bull A self learning (works exactly the same as in single-switch case)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

B

S1

C D

E

FS2

S4

S3

H

I

G

66

Self-learning multi-switch exam-plebull Suppose C sends frame to I I responds to

C

bull Q show switch tables and packet forward-ing in S1 S2 S3 S4

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

B

S1

CD

E F

S2

S4

S3

H

I

G

12

1

2 3

3

1 2 3 12 3

444

67

Institutional network

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

to externalnetwork

router

IP subnet

mail server

web server

68

Switches vs Routersbull both store-and-forward devices

bull routers network layer devices (examine network layer head-ers)

bull switches are link layer devices

bull routers maintain routing tables implement routing al-gorithms

bull switches maintain switch tables implement filtering learning algorithms

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

69

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

• Fall 2015 COMP 46055605 Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link layer context
• Link Layer Services
• Link Layer Services (more)
• Where is the link layer implemented
• Adaptors Communicating
• Link Layer (2)
• Error Detection
• Parity Checking
• Internet checksum (review)
• Checksumming Cyclic Redundancy Check
• Checksumming Cyclic Redundancy Check (2)
• CRC Example
• Link Layer (3)
• Multiple Access Links and Protocols
• Multiple Access protocols
• Ideal Multiple Access Protocol
• MAC Protocols a taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD collision detection
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Link Layer (4)
• MAC Addresses and ARP
• LAN Addresses and ARP
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP protocol Same LAN (network)
• Addressing routing to another LAN
• Slide 45
• Link Layer (5)
• Ethernet
• Star Topology
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester encoding
• Link Layer (6)
• Hubs
• Switch
• Switch allows multiple simultaneous transmissions
• Switch Table
• Switch self-learning
• Switch frame filteringforwarding
• Self-learning forwarding example
• Interconnecting switches
• Self-learning multi-switch example
• Institutional network
• Switches vs Routers
• Link Layer (7)

46

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

47

Ethernetldquodominantrdquo wired LAN technology bull cheap $20 for NICbull first widely used LAN technologybull simpler cheaper than token LANs and ATMbull kept up with speed race 10 Mbps ndash 10 Gbps

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Metcalfersquos Ethernetsketch

48

Star Topologybull bus topology popular through mid 90s

bull all nodes in same collision domain (can collide with each other)

bull today star topology prevailsbull active switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes do

not collide with each other)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

switch

bus coaxial cable star

49

Ethernet Frame Structurebull Sending adapter encapsulates IP datagram

(or other network layer protocol packet) in Ethernet frame

Preamble bull 7 bytes with pattern 10101010 followed by

one byte with pattern 10101011bull used to synchronize receiver sender clock

ratesFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

50

Ethernet Frame Structure (more)bull Addresses 6 bytes

bull if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

bull otherwise adapter discards frame

bull Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)

bull CRC checked at receiver if error is detected frame is dropped

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51

Ethernet Unreliable connection-lessbull connectionless No handshaking be-

tween sending and receiving NICs bull unreliable receiving NIC doesnrsquot send

acks or nacks to sending NICbull stream of datagrams passed to network

layer can have gaps (missing datagrams)bull gaps will be filled if app is using TCPbull otherwise app will see gaps

bull Ethernetrsquos MAC protocol unslotted CSMACD

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

52

Ethernet CSMACD algorithmbull 1 NIC receives data-

gram from network layer creates frame

bull 2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

bull 3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

bull 4 If NIC detects an-other transmission while transmitting aborts and sends jam signal

bull 5 After aborting NIC enters exponential backoff after mth colli-sion NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

53

Ethernetrsquos CSMACD (more)bull Jam Signal make

sure all other transmitters are aware of collision 48 bits

bull Bit time 1 micro sec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Exponential Backoff bull Goal adapt retransmission

attempts to estimated cur-rent load

bull heavy load random wait will be longer

bull first collision choose K from 01 delay is K 512 bit transmission times

bull after second collision choose K from 0123hellip

bull after ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

54

CSMACD efficiencybull Tprop = max prop delay between 2 nodes in

LANbull ttrans = time to transmit max-size frame

bull efficiency goes to 1 bull as tprop goes to 0

bull as ttrans goes to infinity

bull better performance than ALOHA and simple cheap decentralized

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

transprop ttefficiency

51

1

55

8023 Ethernet Standards Link amp Physical Layersbull many different Ethernet standards

bull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps

1Gbps 10G bpsbull different physical layer media fiber cable

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

applicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twisterpair) physical layer

56

Manchester encodingbull used in 10BaseTbull each bit has a transitionbull allows clocks in sending and receiving nodes to syn-

chronize to each otherbull no need for a centralized global clock among nodes

bull Hey this is physical-layer stuff

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

57

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

58

Hubshellip physical-layer (ldquodumbrdquo) repeaters

bull bits coming in one link go out all other links at same rate

bull all nodes connected to hub can collide with one another

bull no frame bufferingbull no CSMACD at hub host NICs detect colli-

sions

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

twisted pair

hub

59

Switchbull link-layer device smarter than hubs take

active rolebull store forward Ethernet framesbull examine incoming framersquos MAC address se-

lectively forward frame to one-or-more out-going links when frame is to be forwarded on segment uses CSMACD to access segment

bull transparentbull hosts are unaware of presence of switches

bull plug-and-play self-learning (HOW)bull switches do not need to be configured

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

60

Switch allows multiple simulta-neous transmissionsbull hosts have dedicated direct

connection to switchbull switches buffer packetsbull Ethernet protocol used on

each incoming link but no collisions full duplexbull each link is its own collision

domain

bull switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions bull not possible with dumb hub

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 23

45

6

61

Switch Tablebull Q how does switch know

that Arsquo reachable via inter-face 4 Brsquo reachable via inter-face 5

bull A each switch has a switch table each entrybull (MAC address of host interface

to reach host time stamp)

bull looks like a routing tablebull Q how are entries created

maintained in switch table bull something like a routing proto-

colFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

switch with six interfaces(123456)

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

62

Switch self-learningbull switch learns which

hosts can be reached through which inter-facesbull when frame received

switch ldquolearnsrdquo location of sender incoming LAN segment

bull records senderlocation pair in switch table

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

63

Switch frame filteringforward-ingWhen frame received

1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination then if dest on segment from which frame arrived then drop the frame else

forward the frame on interface indicated else flood

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

forward on all but the interface on which the frame arrived

64

Self-learning forwarding exam-ple

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

frame destination unknownflood

Arsquo A

destination A location known

Arsquo 4 60

selective send

65

Interconnecting switchesbull switches can be connected together

bull Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3

bull A self learning (works exactly the same as in single-switch case)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

B

S1

C D

E

FS2

S4

S3

H

I

G

66

Self-learning multi-switch exam-plebull Suppose C sends frame to I I responds to

C

bull Q show switch tables and packet forward-ing in S1 S2 S3 S4

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

B

S1

CD

E F

S2

S4

S3

H

I

G

12

1

2 3

3

1 2 3 12 3

444

67

Institutional network

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

to externalnetwork

router

IP subnet

mail server

web server

68

Switches vs Routersbull both store-and-forward devices

bull routers network layer devices (examine network layer head-ers)

bull switches are link layer devices

bull routers maintain routing tables implement routing al-gorithms

bull switches maintain switch tables implement filtering learning algorithms

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

69

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

• Fall 2015 COMP 46055605 Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link layer context
• Link Layer Services
• Link Layer Services (more)
• Where is the link layer implemented
• Adaptors Communicating
• Link Layer (2)
• Error Detection
• Parity Checking
• Internet checksum (review)
• Checksumming Cyclic Redundancy Check
• Checksumming Cyclic Redundancy Check (2)
• CRC Example
• Link Layer (3)
• Multiple Access Links and Protocols
• Multiple Access protocols
• Ideal Multiple Access Protocol
• MAC Protocols a taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD collision detection
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Link Layer (4)
• MAC Addresses and ARP
• LAN Addresses and ARP
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP protocol Same LAN (network)
• Addressing routing to another LAN
• Slide 45
• Link Layer (5)
• Ethernet
• Star Topology
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester encoding
• Link Layer (6)
• Hubs
• Switch
• Switch allows multiple simultaneous transmissions
• Switch Table
• Switch self-learning
• Switch frame filteringforwarding
• Self-learning forwarding example
• Interconnecting switches
• Self-learning multi-switch example
• Institutional network
• Switches vs Routers
• Link Layer (7)

47

Ethernetldquodominantrdquo wired LAN technology bull cheap $20 for NICbull first widely used LAN technologybull simpler cheaper than token LANs and ATMbull kept up with speed race 10 Mbps ndash 10 Gbps

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Metcalfersquos Ethernetsketch

48

Star Topologybull bus topology popular through mid 90s

bull all nodes in same collision domain (can collide with each other)

bull today star topology prevailsbull active switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes do

not collide with each other)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

switch

bus coaxial cable star

49

Ethernet Frame Structurebull Sending adapter encapsulates IP datagram

(or other network layer protocol packet) in Ethernet frame

Preamble bull 7 bytes with pattern 10101010 followed by

one byte with pattern 10101011bull used to synchronize receiver sender clock

ratesFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

50

Ethernet Frame Structure (more)bull Addresses 6 bytes

bull if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

bull otherwise adapter discards frame

bull Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)

bull CRC checked at receiver if error is detected frame is dropped

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51

Ethernet Unreliable connection-lessbull connectionless No handshaking be-

tween sending and receiving NICs bull unreliable receiving NIC doesnrsquot send

acks or nacks to sending NICbull stream of datagrams passed to network

layer can have gaps (missing datagrams)bull gaps will be filled if app is using TCPbull otherwise app will see gaps

bull Ethernetrsquos MAC protocol unslotted CSMACD

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

52

Ethernet CSMACD algorithmbull 1 NIC receives data-

gram from network layer creates frame

bull 2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

bull 3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

bull 4 If NIC detects an-other transmission while transmitting aborts and sends jam signal

bull 5 After aborting NIC enters exponential backoff after mth colli-sion NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

53

Ethernetrsquos CSMACD (more)bull Jam Signal make

sure all other transmitters are aware of collision 48 bits

bull Bit time 1 micro sec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Exponential Backoff bull Goal adapt retransmission

attempts to estimated cur-rent load

bull heavy load random wait will be longer

bull first collision choose K from 01 delay is K 512 bit transmission times

bull after second collision choose K from 0123hellip

bull after ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

54

CSMACD efficiencybull Tprop = max prop delay between 2 nodes in

LANbull ttrans = time to transmit max-size frame

bull efficiency goes to 1 bull as tprop goes to 0

bull as ttrans goes to infinity

bull better performance than ALOHA and simple cheap decentralized

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

transprop ttefficiency

51

1

55

8023 Ethernet Standards Link amp Physical Layersbull many different Ethernet standards

bull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps

1Gbps 10G bpsbull different physical layer media fiber cable

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

applicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twisterpair) physical layer

56

Manchester encodingbull used in 10BaseTbull each bit has a transitionbull allows clocks in sending and receiving nodes to syn-

chronize to each otherbull no need for a centralized global clock among nodes

bull Hey this is physical-layer stuff

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

57

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

58

Hubshellip physical-layer (ldquodumbrdquo) repeaters

bull bits coming in one link go out all other links at same rate

bull all nodes connected to hub can collide with one another

bull no frame bufferingbull no CSMACD at hub host NICs detect colli-

sions

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

twisted pair

hub

59

Switchbull link-layer device smarter than hubs take

active rolebull store forward Ethernet framesbull examine incoming framersquos MAC address se-

lectively forward frame to one-or-more out-going links when frame is to be forwarded on segment uses CSMACD to access segment

bull transparentbull hosts are unaware of presence of switches

bull plug-and-play self-learning (HOW)bull switches do not need to be configured

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

60

Switch allows multiple simulta-neous transmissionsbull hosts have dedicated direct

connection to switchbull switches buffer packetsbull Ethernet protocol used on

each incoming link but no collisions full duplexbull each link is its own collision

domain

bull switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions bull not possible with dumb hub

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 23

45

6

61

Switch Tablebull Q how does switch know

that Arsquo reachable via inter-face 4 Brsquo reachable via inter-face 5

bull A each switch has a switch table each entrybull (MAC address of host interface

to reach host time stamp)

bull looks like a routing tablebull Q how are entries created

maintained in switch table bull something like a routing proto-

colFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

switch with six interfaces(123456)

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

62

Switch self-learningbull switch learns which

hosts can be reached through which inter-facesbull when frame received

switch ldquolearnsrdquo location of sender incoming LAN segment

bull records senderlocation pair in switch table

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

63

Switch frame filteringforward-ingWhen frame received

1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination then if dest on segment from which frame arrived then drop the frame else

forward the frame on interface indicated else flood

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

forward on all but the interface on which the frame arrived

64

Self-learning forwarding exam-ple

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

frame destination unknownflood

Arsquo A

destination A location known

Arsquo 4 60

selective send

65

Interconnecting switchesbull switches can be connected together

bull Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3

bull A self learning (works exactly the same as in single-switch case)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

B

S1

C D

E

FS2

S4

S3

H

I

G

66

Self-learning multi-switch exam-plebull Suppose C sends frame to I I responds to

C

bull Q show switch tables and packet forward-ing in S1 S2 S3 S4

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

B

S1

CD

E F

S2

S4

S3

H

I

G

12

1

2 3

3

1 2 3 12 3

444

67

Institutional network

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

to externalnetwork

router

IP subnet

mail server

web server

68

Switches vs Routersbull both store-and-forward devices

bull routers network layer devices (examine network layer head-ers)

bull switches are link layer devices

bull routers maintain routing tables implement routing al-gorithms

bull switches maintain switch tables implement filtering learning algorithms

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

69

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

• Fall 2015 COMP 46055605 Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link layer context
• Link Layer Services
• Link Layer Services (more)
• Where is the link layer implemented
• Adaptors Communicating
• Link Layer (2)
• Error Detection
• Parity Checking
• Internet checksum (review)
• Checksumming Cyclic Redundancy Check
• Checksumming Cyclic Redundancy Check (2)
• CRC Example
• Link Layer (3)
• Multiple Access Links and Protocols
• Multiple Access protocols
• Ideal Multiple Access Protocol
• MAC Protocols a taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD collision detection
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Link Layer (4)
• MAC Addresses and ARP
• LAN Addresses and ARP
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP protocol Same LAN (network)
• Addressing routing to another LAN
• Slide 45
• Link Layer (5)
• Ethernet
• Star Topology
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester encoding
• Link Layer (6)
• Hubs
• Switch
• Switch allows multiple simultaneous transmissions
• Switch Table
• Switch self-learning
• Switch frame filteringforwarding
• Self-learning forwarding example
• Interconnecting switches
• Self-learning multi-switch example
• Institutional network
• Switches vs Routers
• Link Layer (7)

48

Star Topologybull bus topology popular through mid 90s

bull all nodes in same collision domain (can collide with each other)

bull today star topology prevailsbull active switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes do

not collide with each other)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

switch

bus coaxial cable star

49

Ethernet Frame Structurebull Sending adapter encapsulates IP datagram

(or other network layer protocol packet) in Ethernet frame

Preamble bull 7 bytes with pattern 10101010 followed by

one byte with pattern 10101011bull used to synchronize receiver sender clock

ratesFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

50

Ethernet Frame Structure (more)bull Addresses 6 bytes

bull if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

bull otherwise adapter discards frame

bull Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)

bull CRC checked at receiver if error is detected frame is dropped

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51

Ethernet Unreliable connection-lessbull connectionless No handshaking be-

tween sending and receiving NICs bull unreliable receiving NIC doesnrsquot send

acks or nacks to sending NICbull stream of datagrams passed to network

layer can have gaps (missing datagrams)bull gaps will be filled if app is using TCPbull otherwise app will see gaps

bull Ethernetrsquos MAC protocol unslotted CSMACD

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

52

Ethernet CSMACD algorithmbull 1 NIC receives data-

gram from network layer creates frame

bull 2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

bull 3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

bull 4 If NIC detects an-other transmission while transmitting aborts and sends jam signal

bull 5 After aborting NIC enters exponential backoff after mth colli-sion NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

53

Ethernetrsquos CSMACD (more)bull Jam Signal make

sure all other transmitters are aware of collision 48 bits

bull Bit time 1 micro sec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Exponential Backoff bull Goal adapt retransmission

attempts to estimated cur-rent load

bull heavy load random wait will be longer

bull first collision choose K from 01 delay is K 512 bit transmission times

bull after second collision choose K from 0123hellip

bull after ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

54

CSMACD efficiencybull Tprop = max prop delay between 2 nodes in

LANbull ttrans = time to transmit max-size frame

bull efficiency goes to 1 bull as tprop goes to 0

bull as ttrans goes to infinity

bull better performance than ALOHA and simple cheap decentralized

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

transprop ttefficiency

51

1

55

8023 Ethernet Standards Link amp Physical Layersbull many different Ethernet standards

bull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps

1Gbps 10G bpsbull different physical layer media fiber cable

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

applicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twisterpair) physical layer

56

Manchester encodingbull used in 10BaseTbull each bit has a transitionbull allows clocks in sending and receiving nodes to syn-

chronize to each otherbull no need for a centralized global clock among nodes

bull Hey this is physical-layer stuff

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

57

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

58

Hubshellip physical-layer (ldquodumbrdquo) repeaters

bull bits coming in one link go out all other links at same rate

bull all nodes connected to hub can collide with one another

bull no frame bufferingbull no CSMACD at hub host NICs detect colli-

sions

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

twisted pair

hub

59

Switchbull link-layer device smarter than hubs take

active rolebull store forward Ethernet framesbull examine incoming framersquos MAC address se-

lectively forward frame to one-or-more out-going links when frame is to be forwarded on segment uses CSMACD to access segment

bull transparentbull hosts are unaware of presence of switches

bull plug-and-play self-learning (HOW)bull switches do not need to be configured

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

60

Switch allows multiple simulta-neous transmissionsbull hosts have dedicated direct

connection to switchbull switches buffer packetsbull Ethernet protocol used on

each incoming link but no collisions full duplexbull each link is its own collision

domain

bull switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions bull not possible with dumb hub

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 23

45

6

61

Switch Tablebull Q how does switch know

that Arsquo reachable via inter-face 4 Brsquo reachable via inter-face 5

bull A each switch has a switch table each entrybull (MAC address of host interface

to reach host time stamp)

bull looks like a routing tablebull Q how are entries created

maintained in switch table bull something like a routing proto-

colFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

switch with six interfaces(123456)

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

62

Switch self-learningbull switch learns which

hosts can be reached through which inter-facesbull when frame received

switch ldquolearnsrdquo location of sender incoming LAN segment

bull records senderlocation pair in switch table

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

63

Switch frame filteringforward-ingWhen frame received

1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination then if dest on segment from which frame arrived then drop the frame else

forward the frame on interface indicated else flood

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

forward on all but the interface on which the frame arrived

64

Self-learning forwarding exam-ple

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

frame destination unknownflood

Arsquo A

destination A location known

Arsquo 4 60

selective send

65

Interconnecting switchesbull switches can be connected together

bull Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3

bull A self learning (works exactly the same as in single-switch case)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

B

S1

C D

E

FS2

S4

S3

H

I

G

66

Self-learning multi-switch exam-plebull Suppose C sends frame to I I responds to

C

bull Q show switch tables and packet forward-ing in S1 S2 S3 S4

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

B

S1

CD

E F

S2

S4

S3

H

I

G

12

1

2 3

3

1 2 3 12 3

444

67

Institutional network

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

to externalnetwork

router

IP subnet

mail server

web server

68

Switches vs Routersbull both store-and-forward devices

bull routers network layer devices (examine network layer head-ers)

bull switches are link layer devices

bull routers maintain routing tables implement routing al-gorithms

bull switches maintain switch tables implement filtering learning algorithms

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

69

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

• Fall 2015 COMP 46055605 Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link layer context
• Link Layer Services
• Link Layer Services (more)
• Where is the link layer implemented
• Adaptors Communicating
• Link Layer (2)
• Error Detection
• Parity Checking
• Internet checksum (review)
• Checksumming Cyclic Redundancy Check
• Checksumming Cyclic Redundancy Check (2)
• CRC Example
• Link Layer (3)
• Multiple Access Links and Protocols
• Multiple Access protocols
• Ideal Multiple Access Protocol
• MAC Protocols a taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD collision detection
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Link Layer (4)
• MAC Addresses and ARP
• LAN Addresses and ARP
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP protocol Same LAN (network)
• Addressing routing to another LAN
• Slide 45
• Link Layer (5)
• Ethernet
• Star Topology
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester encoding
• Link Layer (6)
• Hubs
• Switch
• Switch allows multiple simultaneous transmissions
• Switch Table
• Switch self-learning
• Switch frame filteringforwarding
• Self-learning forwarding example
• Interconnecting switches
• Self-learning multi-switch example
• Institutional network
• Switches vs Routers
• Link Layer (7)

49

Ethernet Frame Structurebull Sending adapter encapsulates IP datagram

(or other network layer protocol packet) in Ethernet frame

Preamble bull 7 bytes with pattern 10101010 followed by

one byte with pattern 10101011bull used to synchronize receiver sender clock

ratesFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

50

Ethernet Frame Structure (more)bull Addresses 6 bytes

bull if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

bull otherwise adapter discards frame

bull Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)

bull CRC checked at receiver if error is detected frame is dropped

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51

Ethernet Unreliable connection-lessbull connectionless No handshaking be-

tween sending and receiving NICs bull unreliable receiving NIC doesnrsquot send

acks or nacks to sending NICbull stream of datagrams passed to network

layer can have gaps (missing datagrams)bull gaps will be filled if app is using TCPbull otherwise app will see gaps

bull Ethernetrsquos MAC protocol unslotted CSMACD

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

52

Ethernet CSMACD algorithmbull 1 NIC receives data-

gram from network layer creates frame

bull 2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

bull 3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

bull 4 If NIC detects an-other transmission while transmitting aborts and sends jam signal

bull 5 After aborting NIC enters exponential backoff after mth colli-sion NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

53

Ethernetrsquos CSMACD (more)bull Jam Signal make

sure all other transmitters are aware of collision 48 bits

bull Bit time 1 micro sec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Exponential Backoff bull Goal adapt retransmission

attempts to estimated cur-rent load

bull heavy load random wait will be longer

bull first collision choose K from 01 delay is K 512 bit transmission times

bull after second collision choose K from 0123hellip

bull after ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

54

CSMACD efficiencybull Tprop = max prop delay between 2 nodes in

LANbull ttrans = time to transmit max-size frame

bull efficiency goes to 1 bull as tprop goes to 0

bull as ttrans goes to infinity

bull better performance than ALOHA and simple cheap decentralized

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

transprop ttefficiency

51

1

55

8023 Ethernet Standards Link amp Physical Layersbull many different Ethernet standards

bull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps

1Gbps 10G bpsbull different physical layer media fiber cable

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

applicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twisterpair) physical layer

56

Manchester encodingbull used in 10BaseTbull each bit has a transitionbull allows clocks in sending and receiving nodes to syn-

chronize to each otherbull no need for a centralized global clock among nodes

bull Hey this is physical-layer stuff

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

57

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

58

Hubshellip physical-layer (ldquodumbrdquo) repeaters

bull bits coming in one link go out all other links at same rate

bull all nodes connected to hub can collide with one another

bull no frame bufferingbull no CSMACD at hub host NICs detect colli-

sions

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

twisted pair

hub

59

Switchbull link-layer device smarter than hubs take

active rolebull store forward Ethernet framesbull examine incoming framersquos MAC address se-

lectively forward frame to one-or-more out-going links when frame is to be forwarded on segment uses CSMACD to access segment

bull transparentbull hosts are unaware of presence of switches

bull plug-and-play self-learning (HOW)bull switches do not need to be configured

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

60

Switch allows multiple simulta-neous transmissionsbull hosts have dedicated direct

connection to switchbull switches buffer packetsbull Ethernet protocol used on

each incoming link but no collisions full duplexbull each link is its own collision

domain

bull switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions bull not possible with dumb hub

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 23

45

6

61

Switch Tablebull Q how does switch know

that Arsquo reachable via inter-face 4 Brsquo reachable via inter-face 5

bull A each switch has a switch table each entrybull (MAC address of host interface

to reach host time stamp)

bull looks like a routing tablebull Q how are entries created

maintained in switch table bull something like a routing proto-

colFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

switch with six interfaces(123456)

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

62

Switch self-learningbull switch learns which

hosts can be reached through which inter-facesbull when frame received

switch ldquolearnsrdquo location of sender incoming LAN segment

bull records senderlocation pair in switch table

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

63

Switch frame filteringforward-ingWhen frame received

1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination then if dest on segment from which frame arrived then drop the frame else

forward the frame on interface indicated else flood

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

forward on all but the interface on which the frame arrived

64

Self-learning forwarding exam-ple

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

frame destination unknownflood

Arsquo A

destination A location known

Arsquo 4 60

selective send

65

Interconnecting switchesbull switches can be connected together

bull Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3

bull A self learning (works exactly the same as in single-switch case)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

B

S1

C D

E

FS2

S4

S3

H

I

G

66

Self-learning multi-switch exam-plebull Suppose C sends frame to I I responds to

C

bull Q show switch tables and packet forward-ing in S1 S2 S3 S4

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

B

S1

CD

E F

S2

S4

S3

H

I

G

12

1

2 3

3

1 2 3 12 3

444

67

Institutional network

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

to externalnetwork

router

IP subnet

mail server

web server

68

Switches vs Routersbull both store-and-forward devices

bull routers network layer devices (examine network layer head-ers)

bull switches are link layer devices

bull routers maintain routing tables implement routing al-gorithms

bull switches maintain switch tables implement filtering learning algorithms

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

69

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

• Fall 2015 COMP 46055605 Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link layer context
• Link Layer Services
• Link Layer Services (more)
• Where is the link layer implemented
• Adaptors Communicating
• Link Layer (2)
• Error Detection
• Parity Checking
• Internet checksum (review)
• Checksumming Cyclic Redundancy Check
• Checksumming Cyclic Redundancy Check (2)
• CRC Example
• Link Layer (3)
• Multiple Access Links and Protocols
• Multiple Access protocols
• Ideal Multiple Access Protocol
• MAC Protocols a taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD collision detection
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Link Layer (4)
• MAC Addresses and ARP
• LAN Addresses and ARP
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP protocol Same LAN (network)
• Addressing routing to another LAN
• Slide 45
• Link Layer (5)
• Ethernet
• Star Topology
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester encoding
• Link Layer (6)
• Hubs
• Switch
• Switch allows multiple simultaneous transmissions
• Switch Table
• Switch self-learning
• Switch frame filteringforwarding
• Self-learning forwarding example
• Interconnecting switches
• Self-learning multi-switch example
• Institutional network
• Switches vs Routers
• Link Layer (7)

50

Ethernet Frame Structure (more)bull Addresses 6 bytes

bull if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

bull otherwise adapter discards frame

bull Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)

bull CRC checked at receiver if error is detected frame is dropped

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51

Ethernet Unreliable connection-lessbull connectionless No handshaking be-

tween sending and receiving NICs bull unreliable receiving NIC doesnrsquot send

acks or nacks to sending NICbull stream of datagrams passed to network

layer can have gaps (missing datagrams)bull gaps will be filled if app is using TCPbull otherwise app will see gaps

bull Ethernetrsquos MAC protocol unslotted CSMACD

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

52

Ethernet CSMACD algorithmbull 1 NIC receives data-

gram from network layer creates frame

bull 2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

bull 3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

bull 4 If NIC detects an-other transmission while transmitting aborts and sends jam signal

bull 5 After aborting NIC enters exponential backoff after mth colli-sion NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

53

Ethernetrsquos CSMACD (more)bull Jam Signal make

sure all other transmitters are aware of collision 48 bits

bull Bit time 1 micro sec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Exponential Backoff bull Goal adapt retransmission

attempts to estimated cur-rent load

bull heavy load random wait will be longer

bull first collision choose K from 01 delay is K 512 bit transmission times

bull after second collision choose K from 0123hellip

bull after ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

54

CSMACD efficiencybull Tprop = max prop delay between 2 nodes in

LANbull ttrans = time to transmit max-size frame

bull efficiency goes to 1 bull as tprop goes to 0

bull as ttrans goes to infinity

bull better performance than ALOHA and simple cheap decentralized

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

transprop ttefficiency

51

1

55

8023 Ethernet Standards Link amp Physical Layersbull many different Ethernet standards

bull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps

1Gbps 10G bpsbull different physical layer media fiber cable

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

applicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twisterpair) physical layer

56

Manchester encodingbull used in 10BaseTbull each bit has a transitionbull allows clocks in sending and receiving nodes to syn-

chronize to each otherbull no need for a centralized global clock among nodes

bull Hey this is physical-layer stuff

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

57

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

58

Hubshellip physical-layer (ldquodumbrdquo) repeaters

bull bits coming in one link go out all other links at same rate

bull all nodes connected to hub can collide with one another

bull no frame bufferingbull no CSMACD at hub host NICs detect colli-

sions

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

twisted pair

hub

59

Switchbull link-layer device smarter than hubs take

active rolebull store forward Ethernet framesbull examine incoming framersquos MAC address se-

lectively forward frame to one-or-more out-going links when frame is to be forwarded on segment uses CSMACD to access segment

bull transparentbull hosts are unaware of presence of switches

bull plug-and-play self-learning (HOW)bull switches do not need to be configured

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

60

Switch allows multiple simulta-neous transmissionsbull hosts have dedicated direct

connection to switchbull switches buffer packetsbull Ethernet protocol used on

each incoming link but no collisions full duplexbull each link is its own collision

domain

bull switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions bull not possible with dumb hub

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 23

45

6

61

Switch Tablebull Q how does switch know

that Arsquo reachable via inter-face 4 Brsquo reachable via inter-face 5

bull A each switch has a switch table each entrybull (MAC address of host interface

to reach host time stamp)

bull looks like a routing tablebull Q how are entries created

maintained in switch table bull something like a routing proto-

colFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

switch with six interfaces(123456)

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

62

Switch self-learningbull switch learns which

hosts can be reached through which inter-facesbull when frame received

switch ldquolearnsrdquo location of sender incoming LAN segment

bull records senderlocation pair in switch table

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

63

Switch frame filteringforward-ingWhen frame received

1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination then if dest on segment from which frame arrived then drop the frame else

forward the frame on interface indicated else flood

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

forward on all but the interface on which the frame arrived

64

Self-learning forwarding exam-ple

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

frame destination unknownflood

Arsquo A

destination A location known

Arsquo 4 60

selective send

65

Interconnecting switchesbull switches can be connected together

bull Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3

bull A self learning (works exactly the same as in single-switch case)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

B

S1

C D

E

FS2

S4

S3

H

I

G

66

Self-learning multi-switch exam-plebull Suppose C sends frame to I I responds to

C

bull Q show switch tables and packet forward-ing in S1 S2 S3 S4

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

B

S1

CD

E F

S2

S4

S3

H

I

G

12

1

2 3

3

1 2 3 12 3

444

67

Institutional network

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

to externalnetwork

router

IP subnet

mail server

web server

68

Switches vs Routersbull both store-and-forward devices

bull routers network layer devices (examine network layer head-ers)

bull switches are link layer devices

bull routers maintain routing tables implement routing al-gorithms

bull switches maintain switch tables implement filtering learning algorithms

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

69

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

• Fall 2015 COMP 46055605 Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link layer context
• Link Layer Services
• Link Layer Services (more)
• Where is the link layer implemented
• Adaptors Communicating
• Link Layer (2)
• Error Detection
• Parity Checking
• Internet checksum (review)
• Checksumming Cyclic Redundancy Check
• Checksumming Cyclic Redundancy Check (2)
• CRC Example
• Link Layer (3)
• Multiple Access Links and Protocols
• Multiple Access protocols
• Ideal Multiple Access Protocol
• MAC Protocols a taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD collision detection
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Link Layer (4)
• MAC Addresses and ARP
• LAN Addresses and ARP
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP protocol Same LAN (network)
• Addressing routing to another LAN
• Slide 45
• Link Layer (5)
• Ethernet
• Star Topology
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester encoding
• Link Layer (6)
• Hubs
• Switch
• Switch allows multiple simultaneous transmissions
• Switch Table
• Switch self-learning
• Switch frame filteringforwarding
• Self-learning forwarding example
• Interconnecting switches
• Self-learning multi-switch example
• Institutional network
• Switches vs Routers
• Link Layer (7)

51

Ethernet Unreliable connection-lessbull connectionless No handshaking be-

tween sending and receiving NICs bull unreliable receiving NIC doesnrsquot send

acks or nacks to sending NICbull stream of datagrams passed to network

layer can have gaps (missing datagrams)bull gaps will be filled if app is using TCPbull otherwise app will see gaps

bull Ethernetrsquos MAC protocol unslotted CSMACD

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

52

Ethernet CSMACD algorithmbull 1 NIC receives data-

gram from network layer creates frame

bull 2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

bull 3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

bull 4 If NIC detects an-other transmission while transmitting aborts and sends jam signal

bull 5 After aborting NIC enters exponential backoff after mth colli-sion NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

53

Ethernetrsquos CSMACD (more)bull Jam Signal make

sure all other transmitters are aware of collision 48 bits

bull Bit time 1 micro sec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Exponential Backoff bull Goal adapt retransmission

attempts to estimated cur-rent load

bull heavy load random wait will be longer

bull first collision choose K from 01 delay is K 512 bit transmission times

bull after second collision choose K from 0123hellip

bull after ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

54

CSMACD efficiencybull Tprop = max prop delay between 2 nodes in

LANbull ttrans = time to transmit max-size frame

bull efficiency goes to 1 bull as tprop goes to 0

bull as ttrans goes to infinity

bull better performance than ALOHA and simple cheap decentralized

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

transprop ttefficiency

51

1

55

8023 Ethernet Standards Link amp Physical Layersbull many different Ethernet standards

bull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps

1Gbps 10G bpsbull different physical layer media fiber cable

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

applicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twisterpair) physical layer

56

Manchester encodingbull used in 10BaseTbull each bit has a transitionbull allows clocks in sending and receiving nodes to syn-

chronize to each otherbull no need for a centralized global clock among nodes

bull Hey this is physical-layer stuff

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

57

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

58

Hubshellip physical-layer (ldquodumbrdquo) repeaters

bull bits coming in one link go out all other links at same rate

bull all nodes connected to hub can collide with one another

bull no frame bufferingbull no CSMACD at hub host NICs detect colli-

sions

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

twisted pair

hub

59

Switchbull link-layer device smarter than hubs take

active rolebull store forward Ethernet framesbull examine incoming framersquos MAC address se-

lectively forward frame to one-or-more out-going links when frame is to be forwarded on segment uses CSMACD to access segment

bull transparentbull hosts are unaware of presence of switches

bull plug-and-play self-learning (HOW)bull switches do not need to be configured

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

60

Switch allows multiple simulta-neous transmissionsbull hosts have dedicated direct

connection to switchbull switches buffer packetsbull Ethernet protocol used on

each incoming link but no collisions full duplexbull each link is its own collision

domain

bull switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions bull not possible with dumb hub

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 23

45

6

61

Switch Tablebull Q how does switch know

that Arsquo reachable via inter-face 4 Brsquo reachable via inter-face 5

bull A each switch has a switch table each entrybull (MAC address of host interface

to reach host time stamp)

bull looks like a routing tablebull Q how are entries created

maintained in switch table bull something like a routing proto-

colFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

switch with six interfaces(123456)

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

62

Switch self-learningbull switch learns which

hosts can be reached through which inter-facesbull when frame received

switch ldquolearnsrdquo location of sender incoming LAN segment

bull records senderlocation pair in switch table

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

63

Switch frame filteringforward-ingWhen frame received

1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination then if dest on segment from which frame arrived then drop the frame else

forward the frame on interface indicated else flood

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

forward on all but the interface on which the frame arrived

64

Self-learning forwarding exam-ple

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

frame destination unknownflood

Arsquo A

destination A location known

Arsquo 4 60

selective send

65

Interconnecting switchesbull switches can be connected together

bull Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3

bull A self learning (works exactly the same as in single-switch case)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

B

S1

C D

E

FS2

S4

S3

H

I

G

66

Self-learning multi-switch exam-plebull Suppose C sends frame to I I responds to

C

bull Q show switch tables and packet forward-ing in S1 S2 S3 S4

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

B

S1

CD

E F

S2

S4

S3

H

I

G

12

1

2 3

3

1 2 3 12 3

444

67

Institutional network

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

to externalnetwork

router

IP subnet

mail server

web server

68

Switches vs Routersbull both store-and-forward devices

bull routers network layer devices (examine network layer head-ers)

bull switches are link layer devices

bull routers maintain routing tables implement routing al-gorithms

bull switches maintain switch tables implement filtering learning algorithms

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

69

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

• Fall 2015 COMP 46055605 Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link layer context
• Link Layer Services
• Link Layer Services (more)
• Where is the link layer implemented
• Adaptors Communicating
• Link Layer (2)
• Error Detection
• Parity Checking
• Internet checksum (review)
• Checksumming Cyclic Redundancy Check
• Checksumming Cyclic Redundancy Check (2)
• CRC Example
• Link Layer (3)
• Multiple Access Links and Protocols
• Multiple Access protocols
• Ideal Multiple Access Protocol
• MAC Protocols a taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD collision detection
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Link Layer (4)
• MAC Addresses and ARP
• LAN Addresses and ARP
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP protocol Same LAN (network)
• Addressing routing to another LAN
• Slide 45
• Link Layer (5)
• Ethernet
• Star Topology
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester encoding
• Link Layer (6)
• Hubs
• Switch
• Switch allows multiple simultaneous transmissions
• Switch Table
• Switch self-learning
• Switch frame filteringforwarding
• Self-learning forwarding example
• Interconnecting switches
• Self-learning multi-switch example
• Institutional network
• Switches vs Routers
• Link Layer (7)

52

Ethernet CSMACD algorithmbull 1 NIC receives data-

gram from network layer creates frame

bull 2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

bull 3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

bull 4 If NIC detects an-other transmission while transmitting aborts and sends jam signal

bull 5 After aborting NIC enters exponential backoff after mth colli-sion NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

53

Ethernetrsquos CSMACD (more)bull Jam Signal make

sure all other transmitters are aware of collision 48 bits

bull Bit time 1 micro sec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Exponential Backoff bull Goal adapt retransmission

attempts to estimated cur-rent load

bull heavy load random wait will be longer

bull first collision choose K from 01 delay is K 512 bit transmission times

bull after second collision choose K from 0123hellip

bull after ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

54

CSMACD efficiencybull Tprop = max prop delay between 2 nodes in

LANbull ttrans = time to transmit max-size frame

bull efficiency goes to 1 bull as tprop goes to 0

bull as ttrans goes to infinity

bull better performance than ALOHA and simple cheap decentralized

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

transprop ttefficiency

51

1

55

8023 Ethernet Standards Link amp Physical Layersbull many different Ethernet standards

bull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps

1Gbps 10G bpsbull different physical layer media fiber cable

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

applicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twisterpair) physical layer

56

Manchester encodingbull used in 10BaseTbull each bit has a transitionbull allows clocks in sending and receiving nodes to syn-

chronize to each otherbull no need for a centralized global clock among nodes

bull Hey this is physical-layer stuff

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

57

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

58

Hubshellip physical-layer (ldquodumbrdquo) repeaters

bull bits coming in one link go out all other links at same rate

bull all nodes connected to hub can collide with one another

bull no frame bufferingbull no CSMACD at hub host NICs detect colli-

sions

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

twisted pair

hub

59

Switchbull link-layer device smarter than hubs take

active rolebull store forward Ethernet framesbull examine incoming framersquos MAC address se-

lectively forward frame to one-or-more out-going links when frame is to be forwarded on segment uses CSMACD to access segment

bull transparentbull hosts are unaware of presence of switches

bull plug-and-play self-learning (HOW)bull switches do not need to be configured

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

60

Switch allows multiple simulta-neous transmissionsbull hosts have dedicated direct

connection to switchbull switches buffer packetsbull Ethernet protocol used on

each incoming link but no collisions full duplexbull each link is its own collision

domain

bull switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions bull not possible with dumb hub

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 23

45

6

61

Switch Tablebull Q how does switch know

that Arsquo reachable via inter-face 4 Brsquo reachable via inter-face 5

bull A each switch has a switch table each entrybull (MAC address of host interface

to reach host time stamp)

bull looks like a routing tablebull Q how are entries created

maintained in switch table bull something like a routing proto-

colFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

switch with six interfaces(123456)

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

62

Switch self-learningbull switch learns which

hosts can be reached through which inter-facesbull when frame received

switch ldquolearnsrdquo location of sender incoming LAN segment

bull records senderlocation pair in switch table

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

63

Switch frame filteringforward-ingWhen frame received

1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination then if dest on segment from which frame arrived then drop the frame else

forward the frame on interface indicated else flood

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

forward on all but the interface on which the frame arrived

64

Self-learning forwarding exam-ple

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

frame destination unknownflood

Arsquo A

destination A location known

Arsquo 4 60

selective send

65

Interconnecting switchesbull switches can be connected together

bull Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3

bull A self learning (works exactly the same as in single-switch case)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

B

S1

C D

E

FS2

S4

S3

H

I

G

66

Self-learning multi-switch exam-plebull Suppose C sends frame to I I responds to

C

bull Q show switch tables and packet forward-ing in S1 S2 S3 S4

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

B

S1

CD

E F

S2

S4

S3

H

I

G

12

1

2 3

3

1 2 3 12 3

444

67

Institutional network

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

to externalnetwork

router

IP subnet

mail server

web server

68

Switches vs Routersbull both store-and-forward devices

bull routers network layer devices (examine network layer head-ers)

bull switches are link layer devices

bull routers maintain routing tables implement routing al-gorithms

bull switches maintain switch tables implement filtering learning algorithms

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

69

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

• Fall 2015 COMP 46055605 Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link layer context
• Link Layer Services
• Link Layer Services (more)
• Where is the link layer implemented
• Adaptors Communicating
• Link Layer (2)
• Error Detection
• Parity Checking
• Internet checksum (review)
• Checksumming Cyclic Redundancy Check
• Checksumming Cyclic Redundancy Check (2)
• CRC Example
• Link Layer (3)
• Multiple Access Links and Protocols
• Multiple Access protocols
• Ideal Multiple Access Protocol
• MAC Protocols a taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD collision detection
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Link Layer (4)
• MAC Addresses and ARP
• LAN Addresses and ARP
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP protocol Same LAN (network)
• Addressing routing to another LAN
• Slide 45
• Link Layer (5)
• Ethernet
• Star Topology
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester encoding
• Link Layer (6)
• Hubs
• Switch
• Switch allows multiple simultaneous transmissions
• Switch Table
• Switch self-learning
• Switch frame filteringforwarding
• Self-learning forwarding example
• Interconnecting switches
• Self-learning multi-switch example
• Institutional network
• Switches vs Routers
• Link Layer (7)

53

Ethernetrsquos CSMACD (more)bull Jam Signal make

sure all other transmitters are aware of collision 48 bits

bull Bit time 1 micro sec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

Exponential Backoff bull Goal adapt retransmission

attempts to estimated cur-rent load

bull heavy load random wait will be longer

bull first collision choose K from 01 delay is K 512 bit transmission times

bull after second collision choose K from 0123hellip

bull after ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

54

CSMACD efficiencybull Tprop = max prop delay between 2 nodes in

LANbull ttrans = time to transmit max-size frame

bull efficiency goes to 1 bull as tprop goes to 0

bull as ttrans goes to infinity

bull better performance than ALOHA and simple cheap decentralized

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

transprop ttefficiency

51

1

55

8023 Ethernet Standards Link amp Physical Layersbull many different Ethernet standards

bull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps

1Gbps 10G bpsbull different physical layer media fiber cable

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

applicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twisterpair) physical layer

56

Manchester encodingbull used in 10BaseTbull each bit has a transitionbull allows clocks in sending and receiving nodes to syn-

chronize to each otherbull no need for a centralized global clock among nodes

bull Hey this is physical-layer stuff

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

57

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

58

Hubshellip physical-layer (ldquodumbrdquo) repeaters

bull bits coming in one link go out all other links at same rate

bull all nodes connected to hub can collide with one another

bull no frame bufferingbull no CSMACD at hub host NICs detect colli-

sions

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

twisted pair

hub

59

Switchbull link-layer device smarter than hubs take

active rolebull store forward Ethernet framesbull examine incoming framersquos MAC address se-

lectively forward frame to one-or-more out-going links when frame is to be forwarded on segment uses CSMACD to access segment

bull transparentbull hosts are unaware of presence of switches

bull plug-and-play self-learning (HOW)bull switches do not need to be configured

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

60

Switch allows multiple simulta-neous transmissionsbull hosts have dedicated direct

connection to switchbull switches buffer packetsbull Ethernet protocol used on

each incoming link but no collisions full duplexbull each link is its own collision

domain

bull switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions bull not possible with dumb hub

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 23

45

6

61

Switch Tablebull Q how does switch know

that Arsquo reachable via inter-face 4 Brsquo reachable via inter-face 5

bull A each switch has a switch table each entrybull (MAC address of host interface

to reach host time stamp)

bull looks like a routing tablebull Q how are entries created

maintained in switch table bull something like a routing proto-

colFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

switch with six interfaces(123456)

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

62

Switch self-learningbull switch learns which

hosts can be reached through which inter-facesbull when frame received

switch ldquolearnsrdquo location of sender incoming LAN segment

bull records senderlocation pair in switch table

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

63

Switch frame filteringforward-ingWhen frame received

1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination then if dest on segment from which frame arrived then drop the frame else

forward the frame on interface indicated else flood

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

forward on all but the interface on which the frame arrived

64

Self-learning forwarding exam-ple

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

frame destination unknownflood

Arsquo A

destination A location known

Arsquo 4 60

selective send

65

Interconnecting switchesbull switches can be connected together

bull Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3

bull A self learning (works exactly the same as in single-switch case)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

B

S1

C D

E

FS2

S4

S3

H

I

G

66

Self-learning multi-switch exam-plebull Suppose C sends frame to I I responds to

C

bull Q show switch tables and packet forward-ing in S1 S2 S3 S4

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

B

S1

CD

E F

S2

S4

S3

H

I

G

12

1

2 3

3

1 2 3 12 3

444

67

Institutional network

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

to externalnetwork

router

IP subnet

mail server

web server

68

Switches vs Routersbull both store-and-forward devices

bull routers network layer devices (examine network layer head-ers)

bull switches are link layer devices

bull routers maintain routing tables implement routing al-gorithms

bull switches maintain switch tables implement filtering learning algorithms

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

69

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

• Fall 2015 COMP 46055605 Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link layer context
• Link Layer Services
• Link Layer Services (more)
• Where is the link layer implemented
• Adaptors Communicating
• Link Layer (2)
• Error Detection
• Parity Checking
• Internet checksum (review)
• Checksumming Cyclic Redundancy Check
• Checksumming Cyclic Redundancy Check (2)
• CRC Example
• Link Layer (3)
• Multiple Access Links and Protocols
• Multiple Access protocols
• Ideal Multiple Access Protocol
• MAC Protocols a taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD collision detection
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Link Layer (4)
• MAC Addresses and ARP
• LAN Addresses and ARP
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP protocol Same LAN (network)
• Addressing routing to another LAN
• Slide 45
• Link Layer (5)
• Ethernet
• Star Topology
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester encoding
• Link Layer (6)
• Hubs
• Switch
• Switch allows multiple simultaneous transmissions
• Switch Table
• Switch self-learning
• Switch frame filteringforwarding
• Self-learning forwarding example
• Interconnecting switches
• Self-learning multi-switch example
• Institutional network
• Switches vs Routers
• Link Layer (7)

54

CSMACD efficiencybull Tprop = max prop delay between 2 nodes in

LANbull ttrans = time to transmit max-size frame

bull efficiency goes to 1 bull as tprop goes to 0

bull as ttrans goes to infinity

bull better performance than ALOHA and simple cheap decentralized

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

transprop ttefficiency

51

1

55

8023 Ethernet Standards Link amp Physical Layersbull many different Ethernet standards

bull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps

1Gbps 10G bpsbull different physical layer media fiber cable

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

applicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twisterpair) physical layer

56

Manchester encodingbull used in 10BaseTbull each bit has a transitionbull allows clocks in sending and receiving nodes to syn-

chronize to each otherbull no need for a centralized global clock among nodes

bull Hey this is physical-layer stuff

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

57

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

58

Hubshellip physical-layer (ldquodumbrdquo) repeaters

bull bits coming in one link go out all other links at same rate

bull all nodes connected to hub can collide with one another

bull no frame bufferingbull no CSMACD at hub host NICs detect colli-

sions

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

twisted pair

hub

59

Switchbull link-layer device smarter than hubs take

active rolebull store forward Ethernet framesbull examine incoming framersquos MAC address se-

lectively forward frame to one-or-more out-going links when frame is to be forwarded on segment uses CSMACD to access segment

bull transparentbull hosts are unaware of presence of switches

bull plug-and-play self-learning (HOW)bull switches do not need to be configured

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

60

Switch allows multiple simulta-neous transmissionsbull hosts have dedicated direct

connection to switchbull switches buffer packetsbull Ethernet protocol used on

each incoming link but no collisions full duplexbull each link is its own collision

domain

bull switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions bull not possible with dumb hub

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 23

45

6

61

Switch Tablebull Q how does switch know

that Arsquo reachable via inter-face 4 Brsquo reachable via inter-face 5

bull A each switch has a switch table each entrybull (MAC address of host interface

to reach host time stamp)

bull looks like a routing tablebull Q how are entries created

maintained in switch table bull something like a routing proto-

colFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

switch with six interfaces(123456)

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

62

Switch self-learningbull switch learns which

hosts can be reached through which inter-facesbull when frame received

switch ldquolearnsrdquo location of sender incoming LAN segment

bull records senderlocation pair in switch table

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

63

Switch frame filteringforward-ingWhen frame received

1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination then if dest on segment from which frame arrived then drop the frame else

forward the frame on interface indicated else flood

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

forward on all but the interface on which the frame arrived

64

Self-learning forwarding exam-ple

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

frame destination unknownflood

Arsquo A

destination A location known

Arsquo 4 60

selective send

65

Interconnecting switchesbull switches can be connected together

bull Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3

bull A self learning (works exactly the same as in single-switch case)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

B

S1

C D

E

FS2

S4

S3

H

I

G

66

Self-learning multi-switch exam-plebull Suppose C sends frame to I I responds to

C

bull Q show switch tables and packet forward-ing in S1 S2 S3 S4

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

B

S1

CD

E F

S2

S4

S3

H

I

G

12

1

2 3

3

1 2 3 12 3

444

67

Institutional network

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

to externalnetwork

router

IP subnet

mail server

web server

68

Switches vs Routersbull both store-and-forward devices

bull routers network layer devices (examine network layer head-ers)

bull switches are link layer devices

bull routers maintain routing tables implement routing al-gorithms

bull switches maintain switch tables implement filtering learning algorithms

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

69

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

• Fall 2015 COMP 46055605 Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link layer context
• Link Layer Services
• Link Layer Services (more)
• Where is the link layer implemented
• Adaptors Communicating
• Link Layer (2)
• Error Detection
• Parity Checking
• Internet checksum (review)
• Checksumming Cyclic Redundancy Check
• Checksumming Cyclic Redundancy Check (2)
• CRC Example
• Link Layer (3)
• Multiple Access Links and Protocols
• Multiple Access protocols
• Ideal Multiple Access Protocol
• MAC Protocols a taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD collision detection
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Link Layer (4)
• MAC Addresses and ARP
• LAN Addresses and ARP
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP protocol Same LAN (network)
• Addressing routing to another LAN
• Slide 45
• Link Layer (5)
• Ethernet
• Star Topology
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester encoding
• Link Layer (6)
• Hubs
• Switch
• Switch allows multiple simultaneous transmissions
• Switch Table
• Switch self-learning
• Switch frame filteringforwarding
• Self-learning forwarding example
• Interconnecting switches
• Self-learning multi-switch example
• Institutional network
• Switches vs Routers
• Link Layer (7)

55

8023 Ethernet Standards Link amp Physical Layersbull many different Ethernet standards

bull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps

1Gbps 10G bpsbull different physical layer media fiber cable

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

applicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twisterpair) physical layer

56

Manchester encodingbull used in 10BaseTbull each bit has a transitionbull allows clocks in sending and receiving nodes to syn-

chronize to each otherbull no need for a centralized global clock among nodes

bull Hey this is physical-layer stuff

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

57

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

58

Hubshellip physical-layer (ldquodumbrdquo) repeaters

bull bits coming in one link go out all other links at same rate

bull all nodes connected to hub can collide with one another

bull no frame bufferingbull no CSMACD at hub host NICs detect colli-

sions

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

twisted pair

hub

59

Switchbull link-layer device smarter than hubs take

active rolebull store forward Ethernet framesbull examine incoming framersquos MAC address se-

lectively forward frame to one-or-more out-going links when frame is to be forwarded on segment uses CSMACD to access segment

bull transparentbull hosts are unaware of presence of switches

bull plug-and-play self-learning (HOW)bull switches do not need to be configured

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

60

Switch allows multiple simulta-neous transmissionsbull hosts have dedicated direct

connection to switchbull switches buffer packetsbull Ethernet protocol used on

each incoming link but no collisions full duplexbull each link is its own collision

domain

bull switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions bull not possible with dumb hub

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 23

45

6

61

Switch Tablebull Q how does switch know

that Arsquo reachable via inter-face 4 Brsquo reachable via inter-face 5

bull A each switch has a switch table each entrybull (MAC address of host interface

to reach host time stamp)

bull looks like a routing tablebull Q how are entries created

maintained in switch table bull something like a routing proto-

colFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

switch with six interfaces(123456)

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

62

Switch self-learningbull switch learns which

hosts can be reached through which inter-facesbull when frame received

switch ldquolearnsrdquo location of sender incoming LAN segment

bull records senderlocation pair in switch table

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

63

Switch frame filteringforward-ingWhen frame received

1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination then if dest on segment from which frame arrived then drop the frame else

forward the frame on interface indicated else flood

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

forward on all but the interface on which the frame arrived

64

Self-learning forwarding exam-ple

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

frame destination unknownflood

Arsquo A

destination A location known

Arsquo 4 60

selective send

65

Interconnecting switchesbull switches can be connected together

bull Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3

bull A self learning (works exactly the same as in single-switch case)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

B

S1

C D

E

FS2

S4

S3

H

I

G

66

Self-learning multi-switch exam-plebull Suppose C sends frame to I I responds to

C

bull Q show switch tables and packet forward-ing in S1 S2 S3 S4

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

B

S1

CD

E F

S2

S4

S3

H

I

G

12

1

2 3

3

1 2 3 12 3

444

67

Institutional network

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

to externalnetwork

router

IP subnet

mail server

web server

68

Switches vs Routersbull both store-and-forward devices

bull routers network layer devices (examine network layer head-ers)

bull switches are link layer devices

bull routers maintain routing tables implement routing al-gorithms

bull switches maintain switch tables implement filtering learning algorithms

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

69

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

• Fall 2015 COMP 46055605 Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link layer context
• Link Layer Services
• Link Layer Services (more)
• Where is the link layer implemented
• Adaptors Communicating
• Link Layer (2)
• Error Detection
• Parity Checking
• Internet checksum (review)
• Checksumming Cyclic Redundancy Check
• Checksumming Cyclic Redundancy Check (2)
• CRC Example
• Link Layer (3)
• Multiple Access Links and Protocols
• Multiple Access protocols
• Ideal Multiple Access Protocol
• MAC Protocols a taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD collision detection
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Link Layer (4)
• MAC Addresses and ARP
• LAN Addresses and ARP
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP protocol Same LAN (network)
• Addressing routing to another LAN
• Slide 45
• Link Layer (5)
• Ethernet
• Star Topology
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester encoding
• Link Layer (6)
• Hubs
• Switch
• Switch allows multiple simultaneous transmissions
• Switch Table
• Switch self-learning
• Switch frame filteringforwarding
• Self-learning forwarding example
• Interconnecting switches
• Self-learning multi-switch example
• Institutional network
• Switches vs Routers
• Link Layer (7)

56

Manchester encodingbull used in 10BaseTbull each bit has a transitionbull allows clocks in sending and receiving nodes to syn-

chronize to each otherbull no need for a centralized global clock among nodes

bull Hey this is physical-layer stuff

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

57

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

58

Hubshellip physical-layer (ldquodumbrdquo) repeaters

bull bits coming in one link go out all other links at same rate

bull all nodes connected to hub can collide with one another

bull no frame bufferingbull no CSMACD at hub host NICs detect colli-

sions

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

twisted pair

hub

59

Switchbull link-layer device smarter than hubs take

active rolebull store forward Ethernet framesbull examine incoming framersquos MAC address se-

lectively forward frame to one-or-more out-going links when frame is to be forwarded on segment uses CSMACD to access segment

bull transparentbull hosts are unaware of presence of switches

bull plug-and-play self-learning (HOW)bull switches do not need to be configured

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

60

Switch allows multiple simulta-neous transmissionsbull hosts have dedicated direct

connection to switchbull switches buffer packetsbull Ethernet protocol used on

each incoming link but no collisions full duplexbull each link is its own collision

domain

bull switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions bull not possible with dumb hub

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 23

45

6

61

Switch Tablebull Q how does switch know

that Arsquo reachable via inter-face 4 Brsquo reachable via inter-face 5

bull A each switch has a switch table each entrybull (MAC address of host interface

to reach host time stamp)

bull looks like a routing tablebull Q how are entries created

maintained in switch table bull something like a routing proto-

colFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

switch with six interfaces(123456)

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

62

Switch self-learningbull switch learns which

hosts can be reached through which inter-facesbull when frame received

switch ldquolearnsrdquo location of sender incoming LAN segment

bull records senderlocation pair in switch table

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

63

Switch frame filteringforward-ingWhen frame received

1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination then if dest on segment from which frame arrived then drop the frame else

forward the frame on interface indicated else flood

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

forward on all but the interface on which the frame arrived

64

Self-learning forwarding exam-ple

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

frame destination unknownflood

Arsquo A

destination A location known

Arsquo 4 60

selective send

65

Interconnecting switchesbull switches can be connected together

bull Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3

bull A self learning (works exactly the same as in single-switch case)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

B

S1

C D

E

FS2

S4

S3

H

I

G

66

Self-learning multi-switch exam-plebull Suppose C sends frame to I I responds to

C

bull Q show switch tables and packet forward-ing in S1 S2 S3 S4

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

B

S1

CD

E F

S2

S4

S3

H

I

G

12

1

2 3

3

1 2 3 12 3

444

67

Institutional network

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

to externalnetwork

router

IP subnet

mail server

web server

68

Switches vs Routersbull both store-and-forward devices

bull routers network layer devices (examine network layer head-ers)

bull switches are link layer devices

bull routers maintain routing tables implement routing al-gorithms

bull switches maintain switch tables implement filtering learning algorithms

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

69

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

• Fall 2015 COMP 46055605 Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link layer context
• Link Layer Services
• Link Layer Services (more)
• Where is the link layer implemented
• Adaptors Communicating
• Link Layer (2)
• Error Detection
• Parity Checking
• Internet checksum (review)
• Checksumming Cyclic Redundancy Check
• Checksumming Cyclic Redundancy Check (2)
• CRC Example
• Link Layer (3)
• Multiple Access Links and Protocols
• Multiple Access protocols
• Ideal Multiple Access Protocol
• MAC Protocols a taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD collision detection
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Link Layer (4)
• MAC Addresses and ARP
• LAN Addresses and ARP
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP protocol Same LAN (network)
• Addressing routing to another LAN
• Slide 45
• Link Layer (5)
• Ethernet
• Star Topology
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester encoding
• Link Layer (6)
• Hubs
• Switch
• Switch allows multiple simultaneous transmissions
• Switch Table
• Switch self-learning
• Switch frame filteringforwarding
• Self-learning forwarding example
• Interconnecting switches
• Self-learning multi-switch example
• Institutional network
• Switches vs Routers
• Link Layer (7)

57

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53 Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

58

Hubshellip physical-layer (ldquodumbrdquo) repeaters

bull bits coming in one link go out all other links at same rate

bull all nodes connected to hub can collide with one another

bull no frame bufferingbull no CSMACD at hub host NICs detect colli-

sions

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

twisted pair

hub

59

Switchbull link-layer device smarter than hubs take

active rolebull store forward Ethernet framesbull examine incoming framersquos MAC address se-

lectively forward frame to one-or-more out-going links when frame is to be forwarded on segment uses CSMACD to access segment

bull transparentbull hosts are unaware of presence of switches

bull plug-and-play self-learning (HOW)bull switches do not need to be configured

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

60

Switch allows multiple simulta-neous transmissionsbull hosts have dedicated direct

connection to switchbull switches buffer packetsbull Ethernet protocol used on

each incoming link but no collisions full duplexbull each link is its own collision

domain

bull switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions bull not possible with dumb hub

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 23

45

6

61

Switch Tablebull Q how does switch know

that Arsquo reachable via inter-face 4 Brsquo reachable via inter-face 5

bull A each switch has a switch table each entrybull (MAC address of host interface

to reach host time stamp)

bull looks like a routing tablebull Q how are entries created

maintained in switch table bull something like a routing proto-

colFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

switch with six interfaces(123456)

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

62

Switch self-learningbull switch learns which

hosts can be reached through which inter-facesbull when frame received

switch ldquolearnsrdquo location of sender incoming LAN segment

bull records senderlocation pair in switch table

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

63

Switch frame filteringforward-ingWhen frame received

1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination then if dest on segment from which frame arrived then drop the frame else

forward the frame on interface indicated else flood

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

forward on all but the interface on which the frame arrived

64

Self-learning forwarding exam-ple

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

frame destination unknownflood

Arsquo A

destination A location known

Arsquo 4 60

selective send

65

Interconnecting switchesbull switches can be connected together

bull Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3

bull A self learning (works exactly the same as in single-switch case)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

B

S1

C D

E

FS2

S4

S3

H

I

G

66

Self-learning multi-switch exam-plebull Suppose C sends frame to I I responds to

C

bull Q show switch tables and packet forward-ing in S1 S2 S3 S4

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

B

S1

CD

E F

S2

S4

S3

H

I

G

12

1

2 3

3

1 2 3 12 3

444

67

Institutional network

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

to externalnetwork

router

IP subnet

mail server

web server

68

Switches vs Routersbull both store-and-forward devices

bull routers network layer devices (examine network layer head-ers)

bull switches are link layer devices

bull routers maintain routing tables implement routing al-gorithms

bull switches maintain switch tables implement filtering learning algorithms

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

69

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

• Fall 2015 COMP 46055605 Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link layer context
• Link Layer Services
• Link Layer Services (more)
• Where is the link layer implemented
• Adaptors Communicating
• Link Layer (2)
• Error Detection
• Parity Checking
• Internet checksum (review)
• Checksumming Cyclic Redundancy Check
• Checksumming Cyclic Redundancy Check (2)
• CRC Example
• Link Layer (3)
• Multiple Access Links and Protocols
• Multiple Access protocols
• Ideal Multiple Access Protocol
• MAC Protocols a taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD collision detection
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Link Layer (4)
• MAC Addresses and ARP
• LAN Addresses and ARP
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP protocol Same LAN (network)
• Addressing routing to another LAN
• Slide 45
• Link Layer (5)
• Ethernet
• Star Topology
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester encoding
• Link Layer (6)
• Hubs
• Switch
• Switch allows multiple simultaneous transmissions
• Switch Table
• Switch self-learning
• Switch frame filteringforwarding
• Self-learning forwarding example
• Interconnecting switches
• Self-learning multi-switch example
• Institutional network
• Switches vs Routers
• Link Layer (7)

58

Hubshellip physical-layer (ldquodumbrdquo) repeaters

bull bits coming in one link go out all other links at same rate

bull all nodes connected to hub can collide with one another

bull no frame bufferingbull no CSMACD at hub host NICs detect colli-

sions

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

twisted pair

hub

59

Switchbull link-layer device smarter than hubs take

active rolebull store forward Ethernet framesbull examine incoming framersquos MAC address se-

lectively forward frame to one-or-more out-going links when frame is to be forwarded on segment uses CSMACD to access segment

bull transparentbull hosts are unaware of presence of switches

bull plug-and-play self-learning (HOW)bull switches do not need to be configured

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

60

Switch allows multiple simulta-neous transmissionsbull hosts have dedicated direct

connection to switchbull switches buffer packetsbull Ethernet protocol used on

each incoming link but no collisions full duplexbull each link is its own collision

domain

bull switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions bull not possible with dumb hub

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 23

45

6

61

Switch Tablebull Q how does switch know

that Arsquo reachable via inter-face 4 Brsquo reachable via inter-face 5

bull A each switch has a switch table each entrybull (MAC address of host interface

to reach host time stamp)

bull looks like a routing tablebull Q how are entries created

maintained in switch table bull something like a routing proto-

colFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

switch with six interfaces(123456)

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

62

Switch self-learningbull switch learns which

hosts can be reached through which inter-facesbull when frame received

switch ldquolearnsrdquo location of sender incoming LAN segment

bull records senderlocation pair in switch table

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

63

Switch frame filteringforward-ingWhen frame received

1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination then if dest on segment from which frame arrived then drop the frame else

forward the frame on interface indicated else flood

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

forward on all but the interface on which the frame arrived

64

Self-learning forwarding exam-ple

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

frame destination unknownflood

Arsquo A

destination A location known

Arsquo 4 60

selective send

65

Interconnecting switchesbull switches can be connected together

bull Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3

bull A self learning (works exactly the same as in single-switch case)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

B

S1

C D

E

FS2

S4

S3

H

I

G

66

Self-learning multi-switch exam-plebull Suppose C sends frame to I I responds to

C

bull Q show switch tables and packet forward-ing in S1 S2 S3 S4

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

B

S1

CD

E F

S2

S4

S3

H

I

G

12

1

2 3

3

1 2 3 12 3

444

67

Institutional network

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

to externalnetwork

router

IP subnet

mail server

web server

68

Switches vs Routersbull both store-and-forward devices

bull routers network layer devices (examine network layer head-ers)

bull switches are link layer devices

bull routers maintain routing tables implement routing al-gorithms

bull switches maintain switch tables implement filtering learning algorithms

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

69

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

• Fall 2015 COMP 46055605 Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link layer context
• Link Layer Services
• Link Layer Services (more)
• Where is the link layer implemented
• Adaptors Communicating
• Link Layer (2)
• Error Detection
• Parity Checking
• Internet checksum (review)
• Checksumming Cyclic Redundancy Check
• Checksumming Cyclic Redundancy Check (2)
• CRC Example
• Link Layer (3)
• Multiple Access Links and Protocols
• Multiple Access protocols
• Ideal Multiple Access Protocol
• MAC Protocols a taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD collision detection
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Link Layer (4)
• MAC Addresses and ARP
• LAN Addresses and ARP
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP protocol Same LAN (network)
• Addressing routing to another LAN
• Slide 45
• Link Layer (5)
• Ethernet
• Star Topology
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester encoding
• Link Layer (6)
• Hubs
• Switch
• Switch allows multiple simultaneous transmissions
• Switch Table
• Switch self-learning
• Switch frame filteringforwarding
• Self-learning forwarding example
• Interconnecting switches
• Self-learning multi-switch example
• Institutional network
• Switches vs Routers
• Link Layer (7)

59

Switchbull link-layer device smarter than hubs take

active rolebull store forward Ethernet framesbull examine incoming framersquos MAC address se-

lectively forward frame to one-or-more out-going links when frame is to be forwarded on segment uses CSMACD to access segment

bull transparentbull hosts are unaware of presence of switches

bull plug-and-play self-learning (HOW)bull switches do not need to be configured

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

60

Switch allows multiple simulta-neous transmissionsbull hosts have dedicated direct

connection to switchbull switches buffer packetsbull Ethernet protocol used on

each incoming link but no collisions full duplexbull each link is its own collision

domain

bull switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions bull not possible with dumb hub

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 23

45

6

61

Switch Tablebull Q how does switch know

that Arsquo reachable via inter-face 4 Brsquo reachable via inter-face 5

bull A each switch has a switch table each entrybull (MAC address of host interface

to reach host time stamp)

bull looks like a routing tablebull Q how are entries created

maintained in switch table bull something like a routing proto-

colFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

switch with six interfaces(123456)

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

62

Switch self-learningbull switch learns which

hosts can be reached through which inter-facesbull when frame received

switch ldquolearnsrdquo location of sender incoming LAN segment

bull records senderlocation pair in switch table

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

63

Switch frame filteringforward-ingWhen frame received

1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination then if dest on segment from which frame arrived then drop the frame else

forward the frame on interface indicated else flood

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

forward on all but the interface on which the frame arrived

64

Self-learning forwarding exam-ple

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

frame destination unknownflood

Arsquo A

destination A location known

Arsquo 4 60

selective send

65

Interconnecting switchesbull switches can be connected together

bull Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3

bull A self learning (works exactly the same as in single-switch case)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

B

S1

C D

E

FS2

S4

S3

H

I

G

66

Self-learning multi-switch exam-plebull Suppose C sends frame to I I responds to

C

bull Q show switch tables and packet forward-ing in S1 S2 S3 S4

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

B

S1

CD

E F

S2

S4

S3

H

I

G

12

1

2 3

3

1 2 3 12 3

444

67

Institutional network

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

to externalnetwork

router

IP subnet

mail server

web server

68

Switches vs Routersbull both store-and-forward devices

bull routers network layer devices (examine network layer head-ers)

bull switches are link layer devices

bull routers maintain routing tables implement routing al-gorithms

bull switches maintain switch tables implement filtering learning algorithms

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

69

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

• Fall 2015 COMP 46055605 Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link layer context
• Link Layer Services
• Link Layer Services (more)
• Where is the link layer implemented
• Adaptors Communicating
• Link Layer (2)
• Error Detection
• Parity Checking
• Internet checksum (review)
• Checksumming Cyclic Redundancy Check
• Checksumming Cyclic Redundancy Check (2)
• CRC Example
• Link Layer (3)
• Multiple Access Links and Protocols
• Multiple Access protocols
• Ideal Multiple Access Protocol
• MAC Protocols a taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD collision detection
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Link Layer (4)
• MAC Addresses and ARP
• LAN Addresses and ARP
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP protocol Same LAN (network)
• Addressing routing to another LAN
• Slide 45
• Link Layer (5)
• Ethernet
• Star Topology
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester encoding
• Link Layer (6)
• Hubs
• Switch
• Switch allows multiple simultaneous transmissions
• Switch Table
• Switch self-learning
• Switch frame filteringforwarding
• Self-learning forwarding example
• Interconnecting switches
• Self-learning multi-switch example
• Institutional network
• Switches vs Routers
• Link Layer (7)

60

Switch allows multiple simulta-neous transmissionsbull hosts have dedicated direct

connection to switchbull switches buffer packetsbull Ethernet protocol used on

each incoming link but no collisions full duplexbull each link is its own collision

domain

bull switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions bull not possible with dumb hub

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 23

45

6

61

Switch Tablebull Q how does switch know

that Arsquo reachable via inter-face 4 Brsquo reachable via inter-face 5

bull A each switch has a switch table each entrybull (MAC address of host interface

to reach host time stamp)

bull looks like a routing tablebull Q how are entries created

maintained in switch table bull something like a routing proto-

colFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

switch with six interfaces(123456)

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

62

Switch self-learningbull switch learns which

hosts can be reached through which inter-facesbull when frame received

switch ldquolearnsrdquo location of sender incoming LAN segment

bull records senderlocation pair in switch table

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

63

Switch frame filteringforward-ingWhen frame received

1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination then if dest on segment from which frame arrived then drop the frame else

forward the frame on interface indicated else flood

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

forward on all but the interface on which the frame arrived

64

Self-learning forwarding exam-ple

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

frame destination unknownflood

Arsquo A

destination A location known

Arsquo 4 60

selective send

65

Interconnecting switchesbull switches can be connected together

bull Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3

bull A self learning (works exactly the same as in single-switch case)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

B

S1

C D

E

FS2

S4

S3

H

I

G

66

Self-learning multi-switch exam-plebull Suppose C sends frame to I I responds to

C

bull Q show switch tables and packet forward-ing in S1 S2 S3 S4

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

B

S1

CD

E F

S2

S4

S3

H

I

G

12

1

2 3

3

1 2 3 12 3

444

67

Institutional network

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

to externalnetwork

router

IP subnet

mail server

web server

68

Switches vs Routersbull both store-and-forward devices

bull routers network layer devices (examine network layer head-ers)

bull switches are link layer devices

bull routers maintain routing tables implement routing al-gorithms

bull switches maintain switch tables implement filtering learning algorithms

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

69

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

• Fall 2015 COMP 46055605 Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link layer context
• Link Layer Services
• Link Layer Services (more)
• Where is the link layer implemented
• Adaptors Communicating
• Link Layer (2)
• Error Detection
• Parity Checking
• Internet checksum (review)
• Checksumming Cyclic Redundancy Check
• Checksumming Cyclic Redundancy Check (2)
• CRC Example
• Link Layer (3)
• Multiple Access Links and Protocols
• Multiple Access protocols
• Ideal Multiple Access Protocol
• MAC Protocols a taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD collision detection
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Link Layer (4)
• MAC Addresses and ARP
• LAN Addresses and ARP
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP protocol Same LAN (network)
• Addressing routing to another LAN
• Slide 45
• Link Layer (5)
• Ethernet
• Star Topology
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester encoding
• Link Layer (6)
• Hubs
• Switch
• Switch allows multiple simultaneous transmissions
• Switch Table
• Switch self-learning
• Switch frame filteringforwarding
• Self-learning forwarding example
• Interconnecting switches
• Self-learning multi-switch example
• Institutional network
• Switches vs Routers
• Link Layer (7)

61

Switch Tablebull Q how does switch know

that Arsquo reachable via inter-face 4 Brsquo reachable via inter-face 5

bull A each switch has a switch table each entrybull (MAC address of host interface

to reach host time stamp)

bull looks like a routing tablebull Q how are entries created

maintained in switch table bull something like a routing proto-

colFall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

switch with six interfaces(123456)

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

62

Switch self-learningbull switch learns which

hosts can be reached through which inter-facesbull when frame received

switch ldquolearnsrdquo location of sender incoming LAN segment

bull records senderlocation pair in switch table

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

63

Switch frame filteringforward-ingWhen frame received

1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination then if dest on segment from which frame arrived then drop the frame else

forward the frame on interface indicated else flood

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

forward on all but the interface on which the frame arrived

64

Self-learning forwarding exam-ple

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

frame destination unknownflood

Arsquo A

destination A location known

Arsquo 4 60

selective send

65

Interconnecting switchesbull switches can be connected together

bull Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3

bull A self learning (works exactly the same as in single-switch case)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

B

S1

C D

E

FS2

S4

S3

H

I

G

66

Self-learning multi-switch exam-plebull Suppose C sends frame to I I responds to

C

bull Q show switch tables and packet forward-ing in S1 S2 S3 S4

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

B

S1

CD

E F

S2

S4

S3

H

I

G

12

1

2 3

3

1 2 3 12 3

444

67

Institutional network

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

to externalnetwork

router

IP subnet

mail server

web server

68

Switches vs Routersbull both store-and-forward devices

bull routers network layer devices (examine network layer head-ers)

bull switches are link layer devices

bull routers maintain routing tables implement routing al-gorithms

bull switches maintain switch tables implement filtering learning algorithms

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

69

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

• Fall 2015 COMP 46055605 Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link layer context
• Link Layer Services
• Link Layer Services (more)
• Where is the link layer implemented
• Adaptors Communicating
• Link Layer (2)
• Error Detection
• Parity Checking
• Internet checksum (review)
• Checksumming Cyclic Redundancy Check
• Checksumming Cyclic Redundancy Check (2)
• CRC Example
• Link Layer (3)
• Multiple Access Links and Protocols
• Multiple Access protocols
• Ideal Multiple Access Protocol
• MAC Protocols a taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD collision detection
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Link Layer (4)
• MAC Addresses and ARP
• LAN Addresses and ARP
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP protocol Same LAN (network)
• Addressing routing to another LAN
• Slide 45
• Link Layer (5)
• Ethernet
• Star Topology
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester encoding
• Link Layer (6)
• Hubs
• Switch
• Switch allows multiple simultaneous transmissions
• Switch Table
• Switch self-learning
• Switch frame filteringforwarding
• Self-learning forwarding example
• Interconnecting switches
• Self-learning multi-switch example
• Institutional network
• Switches vs Routers
• Link Layer (7)

62

Switch self-learningbull switch learns which

hosts can be reached through which inter-facesbull when frame received

switch ldquolearnsrdquo location of sender incoming LAN segment

bull records senderlocation pair in switch table

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

63

Switch frame filteringforward-ingWhen frame received

1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination then if dest on segment from which frame arrived then drop the frame else

forward the frame on interface indicated else flood

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

forward on all but the interface on which the frame arrived

64

Self-learning forwarding exam-ple

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

frame destination unknownflood

Arsquo A

destination A location known

Arsquo 4 60

selective send

65

Interconnecting switchesbull switches can be connected together

bull Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3

bull A self learning (works exactly the same as in single-switch case)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

B

S1

C D

E

FS2

S4

S3

H

I

G

66

Self-learning multi-switch exam-plebull Suppose C sends frame to I I responds to

C

bull Q show switch tables and packet forward-ing in S1 S2 S3 S4

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

B

S1

CD

E F

S2

S4

S3

H

I

G

12

1

2 3

3

1 2 3 12 3

444

67

Institutional network

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

to externalnetwork

router

IP subnet

mail server

web server

68

Switches vs Routersbull both store-and-forward devices

bull routers network layer devices (examine network layer head-ers)

bull switches are link layer devices

bull routers maintain routing tables implement routing al-gorithms

bull switches maintain switch tables implement filtering learning algorithms

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

69

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

• Fall 2015 COMP 46055605 Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link layer context
• Link Layer Services
• Link Layer Services (more)
• Where is the link layer implemented
• Adaptors Communicating
• Link Layer (2)
• Error Detection
• Parity Checking
• Internet checksum (review)
• Checksumming Cyclic Redundancy Check
• Checksumming Cyclic Redundancy Check (2)
• CRC Example
• Link Layer (3)
• Multiple Access Links and Protocols
• Multiple Access protocols
• Ideal Multiple Access Protocol
• MAC Protocols a taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD collision detection
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Link Layer (4)
• MAC Addresses and ARP
• LAN Addresses and ARP
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP protocol Same LAN (network)
• Addressing routing to another LAN
• Slide 45
• Link Layer (5)
• Ethernet
• Star Topology
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester encoding
• Link Layer (6)
• Hubs
• Switch
• Switch allows multiple simultaneous transmissions
• Switch Table
• Switch self-learning
• Switch frame filteringforwarding
• Self-learning forwarding example
• Interconnecting switches
• Self-learning multi-switch example
• Institutional network
• Switches vs Routers
• Link Layer (7)

63

Switch frame filteringforward-ingWhen frame received

1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination then if dest on segment from which frame arrived then drop the frame else

forward the frame on interface indicated else flood

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

forward on all but the interface on which the frame arrived

64

Self-learning forwarding exam-ple

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

frame destination unknownflood

Arsquo A

destination A location known

Arsquo 4 60

selective send

65

Interconnecting switchesbull switches can be connected together

bull Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3

bull A self learning (works exactly the same as in single-switch case)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

B

S1

C D

E

FS2

S4

S3

H

I

G

66

Self-learning multi-switch exam-plebull Suppose C sends frame to I I responds to

C

bull Q show switch tables and packet forward-ing in S1 S2 S3 S4

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

B

S1

CD

E F

S2

S4

S3

H

I

G

12

1

2 3

3

1 2 3 12 3

444

67

Institutional network

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

to externalnetwork

router

IP subnet

mail server

web server

68

Switches vs Routersbull both store-and-forward devices

bull routers network layer devices (examine network layer head-ers)

bull switches are link layer devices

bull routers maintain routing tables implement routing al-gorithms

bull switches maintain switch tables implement filtering learning algorithms

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

69

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

• Fall 2015 COMP 46055605 Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link layer context
• Link Layer Services
• Link Layer Services (more)
• Where is the link layer implemented
• Adaptors Communicating
• Link Layer (2)
• Error Detection
• Parity Checking
• Internet checksum (review)
• Checksumming Cyclic Redundancy Check
• Checksumming Cyclic Redundancy Check (2)
• CRC Example
• Link Layer (3)
• Multiple Access Links and Protocols
• Multiple Access protocols
• Ideal Multiple Access Protocol
• MAC Protocols a taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD collision detection
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Link Layer (4)
• MAC Addresses and ARP
• LAN Addresses and ARP
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP protocol Same LAN (network)
• Addressing routing to another LAN
• Slide 45
• Link Layer (5)
• Ethernet
• Star Topology
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester encoding
• Link Layer (6)
• Hubs
• Switch
• Switch allows multiple simultaneous transmissions
• Switch Table
• Switch self-learning
• Switch frame filteringforwarding
• Self-learning forwarding example
• Interconnecting switches
• Self-learning multi-switch example
• Institutional network
• Switches vs Routers
• Link Layer (7)

64

Self-learning forwarding exam-ple

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

frame destination unknownflood

Arsquo A

destination A location known

Arsquo 4 60

selective send

65

Interconnecting switchesbull switches can be connected together

bull Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3

bull A self learning (works exactly the same as in single-switch case)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

B

S1

C D

E

FS2

S4

S3

H

I

G

66

Self-learning multi-switch exam-plebull Suppose C sends frame to I I responds to

C

bull Q show switch tables and packet forward-ing in S1 S2 S3 S4

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

B

S1

CD

E F

S2

S4

S3

H

I

G

12

1

2 3

3

1 2 3 12 3

444

67

Institutional network

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

to externalnetwork

router

IP subnet

mail server

web server

68

Switches vs Routersbull both store-and-forward devices

bull routers network layer devices (examine network layer head-ers)

bull switches are link layer devices

bull routers maintain routing tables implement routing al-gorithms

bull switches maintain switch tables implement filtering learning algorithms

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

69

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

• Fall 2015 COMP 46055605 Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link layer context
• Link Layer Services
• Link Layer Services (more)
• Where is the link layer implemented
• Adaptors Communicating
• Link Layer (2)
• Error Detection
• Parity Checking
• Internet checksum (review)
• Checksumming Cyclic Redundancy Check
• Checksumming Cyclic Redundancy Check (2)
• CRC Example
• Link Layer (3)
• Multiple Access Links and Protocols
• Multiple Access protocols
• Ideal Multiple Access Protocol
• MAC Protocols a taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD collision detection
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Link Layer (4)
• MAC Addresses and ARP
• LAN Addresses and ARP
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP protocol Same LAN (network)
• Addressing routing to another LAN
• Slide 45
• Link Layer (5)
• Ethernet
• Star Topology
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester encoding
• Link Layer (6)
• Hubs
• Switch
• Switch allows multiple simultaneous transmissions
• Switch Table
• Switch self-learning
• Switch frame filteringforwarding
• Self-learning forwarding example
• Interconnecting switches
• Self-learning multi-switch example
• Institutional network
• Switches vs Routers
• Link Layer (7)

65

Interconnecting switchesbull switches can be connected together

bull Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3

bull A self learning (works exactly the same as in single-switch case)

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

B

S1

C D

E

FS2

S4

S3

H

I

G

66

Self-learning multi-switch exam-plebull Suppose C sends frame to I I responds to

C

bull Q show switch tables and packet forward-ing in S1 S2 S3 S4

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

B

S1

CD

E F

S2

S4

S3

H

I

G

12

1

2 3

3

1 2 3 12 3

444

67

Institutional network

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

to externalnetwork

router

IP subnet

mail server

web server

68

Switches vs Routersbull both store-and-forward devices

bull routers network layer devices (examine network layer head-ers)

bull switches are link layer devices

bull routers maintain routing tables implement routing al-gorithms

bull switches maintain switch tables implement filtering learning algorithms

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

69

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

• Fall 2015 COMP 46055605 Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link layer context
• Link Layer Services
• Link Layer Services (more)
• Where is the link layer implemented
• Adaptors Communicating
• Link Layer (2)
• Error Detection
• Parity Checking
• Internet checksum (review)
• Checksumming Cyclic Redundancy Check
• Checksumming Cyclic Redundancy Check (2)
• CRC Example
• Link Layer (3)
• Multiple Access Links and Protocols
• Multiple Access protocols
• Ideal Multiple Access Protocol
• MAC Protocols a taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD collision detection
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Link Layer (4)
• MAC Addresses and ARP
• LAN Addresses and ARP
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP protocol Same LAN (network)
• Addressing routing to another LAN
• Slide 45
• Link Layer (5)
• Ethernet
• Star Topology
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester encoding
• Link Layer (6)
• Hubs
• Switch
• Switch allows multiple simultaneous transmissions
• Switch Table
• Switch self-learning
• Switch frame filteringforwarding
• Self-learning forwarding example
• Interconnecting switches
• Self-learning multi-switch example
• Institutional network
• Switches vs Routers
• Link Layer (7)

66

Self-learning multi-switch exam-plebull Suppose C sends frame to I I responds to

C

bull Q show switch tables and packet forward-ing in S1 S2 S3 S4

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

A

B

S1

CD

E F

S2

S4

S3

H

I

G

12

1

2 3

3

1 2 3 12 3

444

67

Institutional network

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

to externalnetwork

router

IP subnet

mail server

web server

68

Switches vs Routersbull both store-and-forward devices

bull routers network layer devices (examine network layer head-ers)

bull switches are link layer devices

bull routers maintain routing tables implement routing al-gorithms

bull switches maintain switch tables implement filtering learning algorithms

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

69

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

• Fall 2015 COMP 46055605 Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link layer context
• Link Layer Services
• Link Layer Services (more)
• Where is the link layer implemented
• Adaptors Communicating
• Link Layer (2)
• Error Detection
• Parity Checking
• Internet checksum (review)
• Checksumming Cyclic Redundancy Check
• Checksumming Cyclic Redundancy Check (2)
• CRC Example
• Link Layer (3)
• Multiple Access Links and Protocols
• Multiple Access protocols
• Ideal Multiple Access Protocol
• MAC Protocols a taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD collision detection
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Link Layer (4)
• MAC Addresses and ARP
• LAN Addresses and ARP
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP protocol Same LAN (network)
• Addressing routing to another LAN
• Slide 45
• Link Layer (5)
• Ethernet
• Star Topology
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester encoding
• Link Layer (6)
• Hubs
• Switch
• Switch allows multiple simultaneous transmissions
• Switch Table
• Switch self-learning
• Switch frame filteringforwarding
• Self-learning forwarding example
• Interconnecting switches
• Self-learning multi-switch example
• Institutional network
• Switches vs Routers
• Link Layer (7)

67

Institutional network

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

to externalnetwork

router

IP subnet

mail server

web server

68

Switches vs Routersbull both store-and-forward devices

bull routers network layer devices (examine network layer head-ers)

bull switches are link layer devices

bull routers maintain routing tables implement routing al-gorithms

bull switches maintain switch tables implement filtering learning algorithms

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

69

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

• Fall 2015 COMP 46055605 Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link layer context
• Link Layer Services
• Link Layer Services (more)
• Where is the link layer implemented
• Adaptors Communicating
• Link Layer (2)
• Error Detection
• Parity Checking
• Internet checksum (review)
• Checksumming Cyclic Redundancy Check
• Checksumming Cyclic Redundancy Check (2)
• CRC Example
• Link Layer (3)
• Multiple Access Links and Protocols
• Multiple Access protocols
• Ideal Multiple Access Protocol
• MAC Protocols a taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD collision detection
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Link Layer (4)
• MAC Addresses and ARP
• LAN Addresses and ARP
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP protocol Same LAN (network)
• Addressing routing to another LAN
• Slide 45
• Link Layer (5)
• Ethernet
• Star Topology
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester encoding
• Link Layer (6)
• Hubs
• Switch
• Switch allows multiple simultaneous transmissions
• Switch Table
• Switch self-learning
• Switch frame filteringforwarding
• Self-learning forwarding example
• Interconnecting switches
• Self-learning multi-switch example
• Institutional network
• Switches vs Routers
• Link Layer (7)

68

Switches vs Routersbull both store-and-forward devices

bull routers network layer devices (examine network layer head-ers)

bull switches are link layer devices

bull routers maintain routing tables implement routing al-gorithms

bull switches maintain switch tables implement filtering learning algorithms

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

69

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

• Fall 2015 COMP 46055605 Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link layer context
• Link Layer Services
• Link Layer Services (more)
• Where is the link layer implemented
• Adaptors Communicating
• Link Layer (2)
• Error Detection
• Parity Checking
• Internet checksum (review)
• Checksumming Cyclic Redundancy Check
• Checksumming Cyclic Redundancy Check (2)
• CRC Example
• Link Layer (3)
• Multiple Access Links and Protocols
• Multiple Access protocols
• Ideal Multiple Access Protocol
• MAC Protocols a taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD collision detection
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Link Layer (4)
• MAC Addresses and ARP
• LAN Addresses and ARP
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP protocol Same LAN (network)
• Addressing routing to another LAN
• Slide 45
• Link Layer (5)
• Ethernet
• Star Topology
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester encoding
• Link Layer (6)
• Hubs
• Switch
• Switch allows multiple simultaneous transmissions
• Switch Table
• Switch self-learning
• Switch frame filteringforwarding
• Self-learning forwarding example
• Interconnecting switches
• Self-learning multi-switch example
• Institutional network
• Switches vs Routers
• Link Layer (7)

69

Link Layer

Fall 2015 COMP 46055605 Department of Mathematics and Physics Donghyun (David) Kim North Carolina Central University

51 Introduction and services52 Error detection and correction 53Multiple access protocols54 Link-layer Addressing55 Ethernet56 Link-layer switches57 PPP58 Link virtualization MPLS59 A day in the life of a web request

• Fall 2015 COMP 46055605 Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link layer context
• Link Layer Services
• Link Layer Services (more)
• Where is the link layer implemented
• Adaptors Communicating
• Link Layer (2)
• Error Detection
• Parity Checking
• Internet checksum (review)
• Checksumming Cyclic Redundancy Check
• Checksumming Cyclic Redundancy Check (2)
• CRC Example
• Link Layer (3)
• Multiple Access Links and Protocols
• Multiple Access protocols
• Ideal Multiple Access Protocol
• MAC Protocols a taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD collision detection
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Link Layer (4)
• MAC Addresses and ARP
• LAN Addresses and ARP
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP protocol Same LAN (network)
• Addressing routing to another LAN
• Slide 45
• Link Layer (5)
• Ethernet
• Star Topology
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester encoding
• Link Layer (6)
• Hubs
• Switch
• Switch allows multiple simultaneous transmissions
• Switch Table
• Switch self-learning
• Switch frame filteringforwarding
• Self-learning forwarding example
• Interconnecting switches
• Self-learning multi-switch example
• Institutional network
• Switches vs Routers
• Link Layer (7)