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Link Layer 5-1
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
5 DataLink Layer 5-2
MAC Addresses and ARP
32-bit IP address network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address used to get datagram from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs) burned in the adapter ROM
5 DataLink Layer 5-3
LAN Addresses and ARPEach adapter on LAN has a 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)
5 DataLink Layer 5-4
LAN Address (more)
MAC address allocation administered by IEEE manufacturer buys portion of MAC address space
(to assure uniqueness) Each vendor registers one or more 3 octet OUIs
(Organizationally Unique Identifier ) httpwwwmacvendorlookupcom Many Wireless LANs use MAC address for access control
MAC flat address portability can move LAN card from one LAN to another
IP hierarchical address NOT portable depends on IP subnet to which node is attached
Link Layer 5-5
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its 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
Link Layer 5-6
ARP protocol same LAN
A wants to send datagram to B Brsquos MAC address not in Arsquos
ARP table A broadcasts ARP query
packet containing Bs IP address dest MAC address = FF-FF-
FF-FF-FF-FF all nodes on LAN receive
ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
Link Layer 5-7
walkthrough send datagram from A to B via R focus on addressing ndash at IP (datagram) and MAC layer (frame) assume A knows Brsquos IP address assume A knows IP address of first hop router R (how) assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-8
Addressing routing to another LAN
IPEthPhy
IP src 111111111111 IP dest 222222222222
A creates IP datagram with IP source A destination B A creates link-layer frame with Rs MAC address as dest frame
contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-9
Addressing routing to another LAN
IPEthPhy
frame sent from A to R
IPEthPhy
frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
IP src 111111111111 IP dest 222222222222
IP src 111111111111 IP dest 222222222222
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-10
Addressing routing to another LAN
IP src 111111111111 IP dest 222222222222
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest frame
contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-11
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest frame
contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-12
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest frame
contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
Link Layer 5-13
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
5 DataLink Layer 5-14
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for 100Mbs first widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
Old cable-line ethernet
5 DataLink Layer 5-15
Bus topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
bus coaxial cable
10BASE2 cable showing BNC Connector end
10BASE2 cable with BNC T-Connector
10BASE2 Pictures are from Wikipiedia
Star topology
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
5 DataLink Layer 5-16
switch
star
5 DataLink Layer 5-17
Images from httpatomklubchippl
5 DataLink Layer 5-18
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates
MAC addr CRC-32
5 DataLink Layer 5-19
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol
otherwise adapter discards frame
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-20
Unreliable connectionless service Connectionless No handshaking between
sending and receiving adapter Unreliable receiving adapter doesnrsquot send
acks or nacks to sending adapter Data field is 46bytes -1500 bytes
bull If data less than 46 bytes stuff to be 46bytesndash Network layer uses ldquolengthrdquo field to remove stuffing
5 DataLink Layer 5-21
Ethernet uses CSMACD
No slots (no sync clock) Preamble in Ethernet
frame is used to sync clock between sender and receiver
adapter doesnrsquot transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-22
Ethernet CSMACD algorithm
1 Adaptor receives datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal (48-bit Why)
5 After aborting adapter enters exponential backoff after the m-th collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-23
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random wait
will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Why exponentialWhy random number picking
5 DataLink Layer 5-24
CSMACD efficiency Tprop = max prop between 2 nodes in LAN
ttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0
Goes to 1 as ttrans goes to infinity Much better than ALOHA but still decentralized simple and cheap
Why
transprop tt 51
1efficiency
5 DataLink Layer 5-25
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-26
Manchester encoding
Used in 10BaseT Each bit has a transition Allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
Link Layer 5-27
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
5 DataLink Layer 5-28
10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-29
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other links at the same rate no frame buffering no CSMACD at hub adapters detect collisions provides net management functionality
twisted pair
hub
5 DataLink Layer 5-30
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-31
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-32
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-33
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
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
5 DataLink Layer 5-34
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif 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
forward on all but the interface on which the frame arrived
Link Layer 5-35
Interconnecting switches
switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-36
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-37
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-38
Summary comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
Link Layer 5-39
VLANs motivation (virtual LAN)
consider CS user moves office to EE
but wants connect to CS switch
single broadcast domain all layer-2 broadcast
traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
securityprivacy efficiency issues
Computer Science Electrical
Engineering
ComputerEngineering
Link Layer 5-40
VLANsport-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network 1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Link Layer 5-41
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined
switches plus routers
Link Layer 5-42
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be
vanilla 8021 frames (must carry VLAN ID info) 8021q protocol addsremoved additional header fields for
frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
Link Layer 5-43
type
2-byte Tag Protocol Identifier (value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
destaddress
sourceaddress data (payload) CRCpreamble
data (payload) CRC
type
Link Layer 5-44
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-45
Multiprotocol label switching (MPLS)
initial goal high-speed IP forwarding using fixed length label (instead of IP address) fast lookup using fixed length identifier (rather than
shortest prefix matching) borrowing ideas from Virtual Circuit (VC) approach but IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
Link Layer 5-46
MPLS capable routers
aka label-switched router forward packets to outgoing interface based only
on label value (donrsquot inspect IP address) MPLS forwarding table distinct from IP forwarding tables
flexibility MPLS forwarding decisions can differ from those of IP use destination and source addresses to route flows to
same destination differently (traffic engineering) re-route flows quickly if link fails pre-computed backup
paths (useful for VoIP)
Link Layer 5-47
R2
D
R3R5
A
R6
MPLS versus IP paths
IP router IP routing path to destination
determined by destination address alone
R4
Link Layer 5-48
R2
D
R3R4
R5
A
R6
MPLS versus IP paths
IP-onlyrouter
IP routing path to destination determined by destination address alone
MPLS and IP router
MPLS routing path to destination can be based on source and dest address fast reroute precompute backup routes
in case of link failure
entry router (R4) can use different MPLS routes to A based eg on source address
Link Layer 5-49
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-50
Data center networks
10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximity e-business (eg Amazon) content-servers (eg YouTube Akamai Apple Microsoft) search engines data mining (eg Google)
challenges multiple applications each
serving massive numbers of clients
managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container Chicago data center
Link Layer 5-51
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
Load balancer
Load balancer
B
1 2 3 4 5 6 7 8
A C
Border router
Access router
Internet
Data center networks
load balancer application-layer routingreceives external client requestsdirects workload within data centerreturns results to external client (hiding data center internals from client)
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
1 2 3 4 5 6 7 8
Data center networks rich interconnection among switches racks
increased throughput between racks (multiple routing paths possible) increased reliability via redundancy
Link Layer 5-53
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-54
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer5-55
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
web page
browser
router(runs DHCP)
Link Layer 5-56
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router(runs DHCP)
Link Layer 5-57
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router(runs DHCP)
Link Layer 5-58
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
router(runs DHCP)
Link Layer 5-59
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server DNS server replies to
client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router(runs DHCP)
Link Layer 5-60
A day in the lifehellipTCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router(runs DHCP)
Link Layer 5-61
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-62
Chapter 5 Summary
principles behind data link layer services error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS VLANs virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-63
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless multimedia security network management
5 DataLink Layer 5-2
MAC Addresses and ARP
32-bit IP address network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address used to get datagram from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs) burned in the adapter ROM
5 DataLink Layer 5-3
LAN Addresses and ARPEach adapter on LAN has a 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)
5 DataLink Layer 5-4
LAN Address (more)
MAC address allocation administered by IEEE manufacturer buys portion of MAC address space
(to assure uniqueness) Each vendor registers one or more 3 octet OUIs
(Organizationally Unique Identifier ) httpwwwmacvendorlookupcom Many Wireless LANs use MAC address for access control
MAC flat address portability can move LAN card from one LAN to another
IP hierarchical address NOT portable depends on IP subnet to which node is attached
Link Layer 5-5
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its 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
Link Layer 5-6
ARP protocol same LAN
A wants to send datagram to B Brsquos MAC address not in Arsquos
ARP table A broadcasts ARP query
packet containing Bs IP address dest MAC address = FF-FF-
FF-FF-FF-FF all nodes on LAN receive
ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
Link Layer 5-7
walkthrough send datagram from A to B via R focus on addressing ndash at IP (datagram) and MAC layer (frame) assume A knows Brsquos IP address assume A knows IP address of first hop router R (how) assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-8
Addressing routing to another LAN
IPEthPhy
IP src 111111111111 IP dest 222222222222
A creates IP datagram with IP source A destination B A creates link-layer frame with Rs MAC address as dest frame
contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-9
Addressing routing to another LAN
IPEthPhy
frame sent from A to R
IPEthPhy
frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
IP src 111111111111 IP dest 222222222222
IP src 111111111111 IP dest 222222222222
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-10
Addressing routing to another LAN
IP src 111111111111 IP dest 222222222222
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest frame
contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-11
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest frame
contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-12
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest frame
contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
Link Layer 5-13
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
5 DataLink Layer 5-14
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for 100Mbs first widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
Old cable-line ethernet
5 DataLink Layer 5-15
Bus topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
bus coaxial cable
10BASE2 cable showing BNC Connector end
10BASE2 cable with BNC T-Connector
10BASE2 Pictures are from Wikipiedia
Star topology
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
5 DataLink Layer 5-16
switch
star
5 DataLink Layer 5-17
Images from httpatomklubchippl
5 DataLink Layer 5-18
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates
MAC addr CRC-32
5 DataLink Layer 5-19
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol
otherwise adapter discards frame
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-20
Unreliable connectionless service Connectionless No handshaking between
sending and receiving adapter Unreliable receiving adapter doesnrsquot send
acks or nacks to sending adapter Data field is 46bytes -1500 bytes
bull If data less than 46 bytes stuff to be 46bytesndash Network layer uses ldquolengthrdquo field to remove stuffing
5 DataLink Layer 5-21
Ethernet uses CSMACD
No slots (no sync clock) Preamble in Ethernet
frame is used to sync clock between sender and receiver
adapter doesnrsquot transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-22
Ethernet CSMACD algorithm
1 Adaptor receives datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal (48-bit Why)
5 After aborting adapter enters exponential backoff after the m-th collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-23
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random wait
will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Why exponentialWhy random number picking
5 DataLink Layer 5-24
CSMACD efficiency Tprop = max prop between 2 nodes in LAN
ttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0
Goes to 1 as ttrans goes to infinity Much better than ALOHA but still decentralized simple and cheap
Why
transprop tt 51
1efficiency
5 DataLink Layer 5-25
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-26
Manchester encoding
Used in 10BaseT Each bit has a transition Allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
Link Layer 5-27
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
5 DataLink Layer 5-28
10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-29
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other links at the same rate no frame buffering no CSMACD at hub adapters detect collisions provides net management functionality
twisted pair
hub
5 DataLink Layer 5-30
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-31
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-32
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-33
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
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
5 DataLink Layer 5-34
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif 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
forward on all but the interface on which the frame arrived
Link Layer 5-35
Interconnecting switches
switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-36
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-37
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-38
Summary comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
Link Layer 5-39
VLANs motivation (virtual LAN)
consider CS user moves office to EE
but wants connect to CS switch
single broadcast domain all layer-2 broadcast
traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
securityprivacy efficiency issues
Computer Science Electrical
Engineering
ComputerEngineering
Link Layer 5-40
VLANsport-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network 1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Link Layer 5-41
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined
switches plus routers
Link Layer 5-42
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be
vanilla 8021 frames (must carry VLAN ID info) 8021q protocol addsremoved additional header fields for
frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
Link Layer 5-43
type
2-byte Tag Protocol Identifier (value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
destaddress
sourceaddress data (payload) CRCpreamble
data (payload) CRC
type
Link Layer 5-44
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-45
Multiprotocol label switching (MPLS)
initial goal high-speed IP forwarding using fixed length label (instead of IP address) fast lookup using fixed length identifier (rather than
shortest prefix matching) borrowing ideas from Virtual Circuit (VC) approach but IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
Link Layer 5-46
MPLS capable routers
aka label-switched router forward packets to outgoing interface based only
on label value (donrsquot inspect IP address) MPLS forwarding table distinct from IP forwarding tables
flexibility MPLS forwarding decisions can differ from those of IP use destination and source addresses to route flows to
same destination differently (traffic engineering) re-route flows quickly if link fails pre-computed backup
paths (useful for VoIP)
Link Layer 5-47
R2
D
R3R5
A
R6
MPLS versus IP paths
IP router IP routing path to destination
determined by destination address alone
R4
Link Layer 5-48
R2
D
R3R4
R5
A
R6
MPLS versus IP paths
IP-onlyrouter
IP routing path to destination determined by destination address alone
MPLS and IP router
MPLS routing path to destination can be based on source and dest address fast reroute precompute backup routes
in case of link failure
entry router (R4) can use different MPLS routes to A based eg on source address
Link Layer 5-49
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-50
Data center networks
10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximity e-business (eg Amazon) content-servers (eg YouTube Akamai Apple Microsoft) search engines data mining (eg Google)
challenges multiple applications each
serving massive numbers of clients
managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container Chicago data center
Link Layer 5-51
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
Load balancer
Load balancer
B
1 2 3 4 5 6 7 8
A C
Border router
Access router
Internet
Data center networks
load balancer application-layer routingreceives external client requestsdirects workload within data centerreturns results to external client (hiding data center internals from client)
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
1 2 3 4 5 6 7 8
Data center networks rich interconnection among switches racks
increased throughput between racks (multiple routing paths possible) increased reliability via redundancy
Link Layer 5-53
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-54
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer5-55
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
web page
browser
router(runs DHCP)
Link Layer 5-56
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router(runs DHCP)
Link Layer 5-57
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router(runs DHCP)
Link Layer 5-58
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
router(runs DHCP)
Link Layer 5-59
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server DNS server replies to
client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router(runs DHCP)
Link Layer 5-60
A day in the lifehellipTCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router(runs DHCP)
Link Layer 5-61
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-62
Chapter 5 Summary
principles behind data link layer services error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS VLANs virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-63
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless multimedia security network management
5 DataLink Layer 5-3
LAN Addresses and ARPEach adapter on LAN has a 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)
5 DataLink Layer 5-4
LAN Address (more)
MAC address allocation administered by IEEE manufacturer buys portion of MAC address space
(to assure uniqueness) Each vendor registers one or more 3 octet OUIs
(Organizationally Unique Identifier ) httpwwwmacvendorlookupcom Many Wireless LANs use MAC address for access control
MAC flat address portability can move LAN card from one LAN to another
IP hierarchical address NOT portable depends on IP subnet to which node is attached
Link Layer 5-5
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its 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
Link Layer 5-6
ARP protocol same LAN
A wants to send datagram to B Brsquos MAC address not in Arsquos
ARP table A broadcasts ARP query
packet containing Bs IP address dest MAC address = FF-FF-
FF-FF-FF-FF all nodes on LAN receive
ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
Link Layer 5-7
walkthrough send datagram from A to B via R focus on addressing ndash at IP (datagram) and MAC layer (frame) assume A knows Brsquos IP address assume A knows IP address of first hop router R (how) assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-8
Addressing routing to another LAN
IPEthPhy
IP src 111111111111 IP dest 222222222222
A creates IP datagram with IP source A destination B A creates link-layer frame with Rs MAC address as dest frame
contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-9
Addressing routing to another LAN
IPEthPhy
frame sent from A to R
IPEthPhy
frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
IP src 111111111111 IP dest 222222222222
IP src 111111111111 IP dest 222222222222
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-10
Addressing routing to another LAN
IP src 111111111111 IP dest 222222222222
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest frame
contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-11
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest frame
contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-12
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest frame
contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
Link Layer 5-13
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
5 DataLink Layer 5-14
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for 100Mbs first widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
Old cable-line ethernet
5 DataLink Layer 5-15
Bus topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
bus coaxial cable
10BASE2 cable showing BNC Connector end
10BASE2 cable with BNC T-Connector
10BASE2 Pictures are from Wikipiedia
Star topology
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
5 DataLink Layer 5-16
switch
star
5 DataLink Layer 5-17
Images from httpatomklubchippl
5 DataLink Layer 5-18
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates
MAC addr CRC-32
5 DataLink Layer 5-19
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol
otherwise adapter discards frame
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-20
Unreliable connectionless service Connectionless No handshaking between
sending and receiving adapter Unreliable receiving adapter doesnrsquot send
acks or nacks to sending adapter Data field is 46bytes -1500 bytes
bull If data less than 46 bytes stuff to be 46bytesndash Network layer uses ldquolengthrdquo field to remove stuffing
5 DataLink Layer 5-21
Ethernet uses CSMACD
No slots (no sync clock) Preamble in Ethernet
frame is used to sync clock between sender and receiver
adapter doesnrsquot transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-22
Ethernet CSMACD algorithm
1 Adaptor receives datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal (48-bit Why)
5 After aborting adapter enters exponential backoff after the m-th collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-23
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random wait
will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Why exponentialWhy random number picking
5 DataLink Layer 5-24
CSMACD efficiency Tprop = max prop between 2 nodes in LAN
ttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0
Goes to 1 as ttrans goes to infinity Much better than ALOHA but still decentralized simple and cheap
Why
transprop tt 51
1efficiency
5 DataLink Layer 5-25
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-26
Manchester encoding
Used in 10BaseT Each bit has a transition Allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
Link Layer 5-27
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
5 DataLink Layer 5-28
10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-29
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other links at the same rate no frame buffering no CSMACD at hub adapters detect collisions provides net management functionality
twisted pair
hub
5 DataLink Layer 5-30
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-31
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-32
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-33
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
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
5 DataLink Layer 5-34
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif 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
forward on all but the interface on which the frame arrived
Link Layer 5-35
Interconnecting switches
switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-36
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-37
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-38
Summary comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
Link Layer 5-39
VLANs motivation (virtual LAN)
consider CS user moves office to EE
but wants connect to CS switch
single broadcast domain all layer-2 broadcast
traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
securityprivacy efficiency issues
Computer Science Electrical
Engineering
ComputerEngineering
Link Layer 5-40
VLANsport-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network 1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Link Layer 5-41
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined
switches plus routers
Link Layer 5-42
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be
vanilla 8021 frames (must carry VLAN ID info) 8021q protocol addsremoved additional header fields for
frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
Link Layer 5-43
type
2-byte Tag Protocol Identifier (value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
destaddress
sourceaddress data (payload) CRCpreamble
data (payload) CRC
type
Link Layer 5-44
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-45
Multiprotocol label switching (MPLS)
initial goal high-speed IP forwarding using fixed length label (instead of IP address) fast lookup using fixed length identifier (rather than
shortest prefix matching) borrowing ideas from Virtual Circuit (VC) approach but IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
Link Layer 5-46
MPLS capable routers
aka label-switched router forward packets to outgoing interface based only
on label value (donrsquot inspect IP address) MPLS forwarding table distinct from IP forwarding tables
flexibility MPLS forwarding decisions can differ from those of IP use destination and source addresses to route flows to
same destination differently (traffic engineering) re-route flows quickly if link fails pre-computed backup
paths (useful for VoIP)
Link Layer 5-47
R2
D
R3R5
A
R6
MPLS versus IP paths
IP router IP routing path to destination
determined by destination address alone
R4
Link Layer 5-48
R2
D
R3R4
R5
A
R6
MPLS versus IP paths
IP-onlyrouter
IP routing path to destination determined by destination address alone
MPLS and IP router
MPLS routing path to destination can be based on source and dest address fast reroute precompute backup routes
in case of link failure
entry router (R4) can use different MPLS routes to A based eg on source address
Link Layer 5-49
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-50
Data center networks
10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximity e-business (eg Amazon) content-servers (eg YouTube Akamai Apple Microsoft) search engines data mining (eg Google)
challenges multiple applications each
serving massive numbers of clients
managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container Chicago data center
Link Layer 5-51
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
Load balancer
Load balancer
B
1 2 3 4 5 6 7 8
A C
Border router
Access router
Internet
Data center networks
load balancer application-layer routingreceives external client requestsdirects workload within data centerreturns results to external client (hiding data center internals from client)
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
1 2 3 4 5 6 7 8
Data center networks rich interconnection among switches racks
increased throughput between racks (multiple routing paths possible) increased reliability via redundancy
Link Layer 5-53
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-54
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer5-55
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
web page
browser
router(runs DHCP)
Link Layer 5-56
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router(runs DHCP)
Link Layer 5-57
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router(runs DHCP)
Link Layer 5-58
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
router(runs DHCP)
Link Layer 5-59
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server DNS server replies to
client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router(runs DHCP)
Link Layer 5-60
A day in the lifehellipTCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router(runs DHCP)
Link Layer 5-61
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-62
Chapter 5 Summary
principles behind data link layer services error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS VLANs virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-63
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless multimedia security network management
5 DataLink Layer 5-4
LAN Address (more)
MAC address allocation administered by IEEE manufacturer buys portion of MAC address space
(to assure uniqueness) Each vendor registers one or more 3 octet OUIs
(Organizationally Unique Identifier ) httpwwwmacvendorlookupcom Many Wireless LANs use MAC address for access control
MAC flat address portability can move LAN card from one LAN to another
IP hierarchical address NOT portable depends on IP subnet to which node is attached
Link Layer 5-5
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its 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
Link Layer 5-6
ARP protocol same LAN
A wants to send datagram to B Brsquos MAC address not in Arsquos
ARP table A broadcasts ARP query
packet containing Bs IP address dest MAC address = FF-FF-
FF-FF-FF-FF all nodes on LAN receive
ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
Link Layer 5-7
walkthrough send datagram from A to B via R focus on addressing ndash at IP (datagram) and MAC layer (frame) assume A knows Brsquos IP address assume A knows IP address of first hop router R (how) assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-8
Addressing routing to another LAN
IPEthPhy
IP src 111111111111 IP dest 222222222222
A creates IP datagram with IP source A destination B A creates link-layer frame with Rs MAC address as dest frame
contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-9
Addressing routing to another LAN
IPEthPhy
frame sent from A to R
IPEthPhy
frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
IP src 111111111111 IP dest 222222222222
IP src 111111111111 IP dest 222222222222
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-10
Addressing routing to another LAN
IP src 111111111111 IP dest 222222222222
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest frame
contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-11
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest frame
contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-12
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest frame
contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
Link Layer 5-13
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
5 DataLink Layer 5-14
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for 100Mbs first widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
Old cable-line ethernet
5 DataLink Layer 5-15
Bus topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
bus coaxial cable
10BASE2 cable showing BNC Connector end
10BASE2 cable with BNC T-Connector
10BASE2 Pictures are from Wikipiedia
Star topology
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
5 DataLink Layer 5-16
switch
star
5 DataLink Layer 5-17
Images from httpatomklubchippl
5 DataLink Layer 5-18
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates
MAC addr CRC-32
5 DataLink Layer 5-19
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol
otherwise adapter discards frame
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-20
Unreliable connectionless service Connectionless No handshaking between
sending and receiving adapter Unreliable receiving adapter doesnrsquot send
acks or nacks to sending adapter Data field is 46bytes -1500 bytes
bull If data less than 46 bytes stuff to be 46bytesndash Network layer uses ldquolengthrdquo field to remove stuffing
5 DataLink Layer 5-21
Ethernet uses CSMACD
No slots (no sync clock) Preamble in Ethernet
frame is used to sync clock between sender and receiver
adapter doesnrsquot transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-22
Ethernet CSMACD algorithm
1 Adaptor receives datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal (48-bit Why)
5 After aborting adapter enters exponential backoff after the m-th collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-23
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random wait
will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Why exponentialWhy random number picking
5 DataLink Layer 5-24
CSMACD efficiency Tprop = max prop between 2 nodes in LAN
ttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0
Goes to 1 as ttrans goes to infinity Much better than ALOHA but still decentralized simple and cheap
Why
transprop tt 51
1efficiency
5 DataLink Layer 5-25
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-26
Manchester encoding
Used in 10BaseT Each bit has a transition Allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
Link Layer 5-27
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
5 DataLink Layer 5-28
10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-29
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other links at the same rate no frame buffering no CSMACD at hub adapters detect collisions provides net management functionality
twisted pair
hub
5 DataLink Layer 5-30
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-31
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-32
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-33
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
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
5 DataLink Layer 5-34
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif 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
forward on all but the interface on which the frame arrived
Link Layer 5-35
Interconnecting switches
switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-36
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-37
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-38
Summary comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
Link Layer 5-39
VLANs motivation (virtual LAN)
consider CS user moves office to EE
but wants connect to CS switch
single broadcast domain all layer-2 broadcast
traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
securityprivacy efficiency issues
Computer Science Electrical
Engineering
ComputerEngineering
Link Layer 5-40
VLANsport-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network 1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Link Layer 5-41
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined
switches plus routers
Link Layer 5-42
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be
vanilla 8021 frames (must carry VLAN ID info) 8021q protocol addsremoved additional header fields for
frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
Link Layer 5-43
type
2-byte Tag Protocol Identifier (value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
destaddress
sourceaddress data (payload) CRCpreamble
data (payload) CRC
type
Link Layer 5-44
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-45
Multiprotocol label switching (MPLS)
initial goal high-speed IP forwarding using fixed length label (instead of IP address) fast lookup using fixed length identifier (rather than
shortest prefix matching) borrowing ideas from Virtual Circuit (VC) approach but IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
Link Layer 5-46
MPLS capable routers
aka label-switched router forward packets to outgoing interface based only
on label value (donrsquot inspect IP address) MPLS forwarding table distinct from IP forwarding tables
flexibility MPLS forwarding decisions can differ from those of IP use destination and source addresses to route flows to
same destination differently (traffic engineering) re-route flows quickly if link fails pre-computed backup
paths (useful for VoIP)
Link Layer 5-47
R2
D
R3R5
A
R6
MPLS versus IP paths
IP router IP routing path to destination
determined by destination address alone
R4
Link Layer 5-48
R2
D
R3R4
R5
A
R6
MPLS versus IP paths
IP-onlyrouter
IP routing path to destination determined by destination address alone
MPLS and IP router
MPLS routing path to destination can be based on source and dest address fast reroute precompute backup routes
in case of link failure
entry router (R4) can use different MPLS routes to A based eg on source address
Link Layer 5-49
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-50
Data center networks
10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximity e-business (eg Amazon) content-servers (eg YouTube Akamai Apple Microsoft) search engines data mining (eg Google)
challenges multiple applications each
serving massive numbers of clients
managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container Chicago data center
Link Layer 5-51
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
Load balancer
Load balancer
B
1 2 3 4 5 6 7 8
A C
Border router
Access router
Internet
Data center networks
load balancer application-layer routingreceives external client requestsdirects workload within data centerreturns results to external client (hiding data center internals from client)
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
1 2 3 4 5 6 7 8
Data center networks rich interconnection among switches racks
increased throughput between racks (multiple routing paths possible) increased reliability via redundancy
Link Layer 5-53
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-54
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer5-55
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
web page
browser
router(runs DHCP)
Link Layer 5-56
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router(runs DHCP)
Link Layer 5-57
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router(runs DHCP)
Link Layer 5-58
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
router(runs DHCP)
Link Layer 5-59
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server DNS server replies to
client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router(runs DHCP)
Link Layer 5-60
A day in the lifehellipTCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router(runs DHCP)
Link Layer 5-61
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-62
Chapter 5 Summary
principles behind data link layer services error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS VLANs virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-63
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless multimedia security network management
Link Layer 5-5
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its 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
Link Layer 5-6
ARP protocol same LAN
A wants to send datagram to B Brsquos MAC address not in Arsquos
ARP table A broadcasts ARP query
packet containing Bs IP address dest MAC address = FF-FF-
FF-FF-FF-FF all nodes on LAN receive
ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
Link Layer 5-7
walkthrough send datagram from A to B via R focus on addressing ndash at IP (datagram) and MAC layer (frame) assume A knows Brsquos IP address assume A knows IP address of first hop router R (how) assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-8
Addressing routing to another LAN
IPEthPhy
IP src 111111111111 IP dest 222222222222
A creates IP datagram with IP source A destination B A creates link-layer frame with Rs MAC address as dest frame
contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-9
Addressing routing to another LAN
IPEthPhy
frame sent from A to R
IPEthPhy
frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
IP src 111111111111 IP dest 222222222222
IP src 111111111111 IP dest 222222222222
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-10
Addressing routing to another LAN
IP src 111111111111 IP dest 222222222222
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest frame
contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-11
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest frame
contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-12
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest frame
contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
Link Layer 5-13
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
5 DataLink Layer 5-14
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for 100Mbs first widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
Old cable-line ethernet
5 DataLink Layer 5-15
Bus topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
bus coaxial cable
10BASE2 cable showing BNC Connector end
10BASE2 cable with BNC T-Connector
10BASE2 Pictures are from Wikipiedia
Star topology
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
5 DataLink Layer 5-16
switch
star
5 DataLink Layer 5-17
Images from httpatomklubchippl
5 DataLink Layer 5-18
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates
MAC addr CRC-32
5 DataLink Layer 5-19
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol
otherwise adapter discards frame
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-20
Unreliable connectionless service Connectionless No handshaking between
sending and receiving adapter Unreliable receiving adapter doesnrsquot send
acks or nacks to sending adapter Data field is 46bytes -1500 bytes
bull If data less than 46 bytes stuff to be 46bytesndash Network layer uses ldquolengthrdquo field to remove stuffing
5 DataLink Layer 5-21
Ethernet uses CSMACD
No slots (no sync clock) Preamble in Ethernet
frame is used to sync clock between sender and receiver
adapter doesnrsquot transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-22
Ethernet CSMACD algorithm
1 Adaptor receives datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal (48-bit Why)
5 After aborting adapter enters exponential backoff after the m-th collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-23
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random wait
will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Why exponentialWhy random number picking
5 DataLink Layer 5-24
CSMACD efficiency Tprop = max prop between 2 nodes in LAN
ttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0
Goes to 1 as ttrans goes to infinity Much better than ALOHA but still decentralized simple and cheap
Why
transprop tt 51
1efficiency
5 DataLink Layer 5-25
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-26
Manchester encoding
Used in 10BaseT Each bit has a transition Allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
Link Layer 5-27
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
5 DataLink Layer 5-28
10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-29
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other links at the same rate no frame buffering no CSMACD at hub adapters detect collisions provides net management functionality
twisted pair
hub
5 DataLink Layer 5-30
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-31
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-32
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-33
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
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
5 DataLink Layer 5-34
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif 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
forward on all but the interface on which the frame arrived
Link Layer 5-35
Interconnecting switches
switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-36
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-37
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-38
Summary comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
Link Layer 5-39
VLANs motivation (virtual LAN)
consider CS user moves office to EE
but wants connect to CS switch
single broadcast domain all layer-2 broadcast
traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
securityprivacy efficiency issues
Computer Science Electrical
Engineering
ComputerEngineering
Link Layer 5-40
VLANsport-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network 1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Link Layer 5-41
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined
switches plus routers
Link Layer 5-42
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be
vanilla 8021 frames (must carry VLAN ID info) 8021q protocol addsremoved additional header fields for
frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
Link Layer 5-43
type
2-byte Tag Protocol Identifier (value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
destaddress
sourceaddress data (payload) CRCpreamble
data (payload) CRC
type
Link Layer 5-44
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-45
Multiprotocol label switching (MPLS)
initial goal high-speed IP forwarding using fixed length label (instead of IP address) fast lookup using fixed length identifier (rather than
shortest prefix matching) borrowing ideas from Virtual Circuit (VC) approach but IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
Link Layer 5-46
MPLS capable routers
aka label-switched router forward packets to outgoing interface based only
on label value (donrsquot inspect IP address) MPLS forwarding table distinct from IP forwarding tables
flexibility MPLS forwarding decisions can differ from those of IP use destination and source addresses to route flows to
same destination differently (traffic engineering) re-route flows quickly if link fails pre-computed backup
paths (useful for VoIP)
Link Layer 5-47
R2
D
R3R5
A
R6
MPLS versus IP paths
IP router IP routing path to destination
determined by destination address alone
R4
Link Layer 5-48
R2
D
R3R4
R5
A
R6
MPLS versus IP paths
IP-onlyrouter
IP routing path to destination determined by destination address alone
MPLS and IP router
MPLS routing path to destination can be based on source and dest address fast reroute precompute backup routes
in case of link failure
entry router (R4) can use different MPLS routes to A based eg on source address
Link Layer 5-49
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-50
Data center networks
10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximity e-business (eg Amazon) content-servers (eg YouTube Akamai Apple Microsoft) search engines data mining (eg Google)
challenges multiple applications each
serving massive numbers of clients
managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container Chicago data center
Link Layer 5-51
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
Load balancer
Load balancer
B
1 2 3 4 5 6 7 8
A C
Border router
Access router
Internet
Data center networks
load balancer application-layer routingreceives external client requestsdirects workload within data centerreturns results to external client (hiding data center internals from client)
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
1 2 3 4 5 6 7 8
Data center networks rich interconnection among switches racks
increased throughput between racks (multiple routing paths possible) increased reliability via redundancy
Link Layer 5-53
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-54
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer5-55
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
web page
browser
router(runs DHCP)
Link Layer 5-56
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router(runs DHCP)
Link Layer 5-57
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router(runs DHCP)
Link Layer 5-58
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
router(runs DHCP)
Link Layer 5-59
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server DNS server replies to
client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router(runs DHCP)
Link Layer 5-60
A day in the lifehellipTCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router(runs DHCP)
Link Layer 5-61
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-62
Chapter 5 Summary
principles behind data link layer services error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS VLANs virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-63
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless multimedia security network management
Link Layer 5-6
ARP protocol same LAN
A wants to send datagram to B Brsquos MAC address not in Arsquos
ARP table A broadcasts ARP query
packet containing Bs IP address dest MAC address = FF-FF-
FF-FF-FF-FF all nodes on LAN receive
ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
Link Layer 5-7
walkthrough send datagram from A to B via R focus on addressing ndash at IP (datagram) and MAC layer (frame) assume A knows Brsquos IP address assume A knows IP address of first hop router R (how) assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-8
Addressing routing to another LAN
IPEthPhy
IP src 111111111111 IP dest 222222222222
A creates IP datagram with IP source A destination B A creates link-layer frame with Rs MAC address as dest frame
contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-9
Addressing routing to another LAN
IPEthPhy
frame sent from A to R
IPEthPhy
frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
IP src 111111111111 IP dest 222222222222
IP src 111111111111 IP dest 222222222222
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-10
Addressing routing to another LAN
IP src 111111111111 IP dest 222222222222
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest frame
contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-11
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest frame
contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-12
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest frame
contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
Link Layer 5-13
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
5 DataLink Layer 5-14
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for 100Mbs first widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
Old cable-line ethernet
5 DataLink Layer 5-15
Bus topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
bus coaxial cable
10BASE2 cable showing BNC Connector end
10BASE2 cable with BNC T-Connector
10BASE2 Pictures are from Wikipiedia
Star topology
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
5 DataLink Layer 5-16
switch
star
5 DataLink Layer 5-17
Images from httpatomklubchippl
5 DataLink Layer 5-18
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates
MAC addr CRC-32
5 DataLink Layer 5-19
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol
otherwise adapter discards frame
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-20
Unreliable connectionless service Connectionless No handshaking between
sending and receiving adapter Unreliable receiving adapter doesnrsquot send
acks or nacks to sending adapter Data field is 46bytes -1500 bytes
bull If data less than 46 bytes stuff to be 46bytesndash Network layer uses ldquolengthrdquo field to remove stuffing
5 DataLink Layer 5-21
Ethernet uses CSMACD
No slots (no sync clock) Preamble in Ethernet
frame is used to sync clock between sender and receiver
adapter doesnrsquot transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-22
Ethernet CSMACD algorithm
1 Adaptor receives datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal (48-bit Why)
5 After aborting adapter enters exponential backoff after the m-th collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-23
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random wait
will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Why exponentialWhy random number picking
5 DataLink Layer 5-24
CSMACD efficiency Tprop = max prop between 2 nodes in LAN
ttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0
Goes to 1 as ttrans goes to infinity Much better than ALOHA but still decentralized simple and cheap
Why
transprop tt 51
1efficiency
5 DataLink Layer 5-25
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-26
Manchester encoding
Used in 10BaseT Each bit has a transition Allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
Link Layer 5-27
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
5 DataLink Layer 5-28
10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-29
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other links at the same rate no frame buffering no CSMACD at hub adapters detect collisions provides net management functionality
twisted pair
hub
5 DataLink Layer 5-30
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-31
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-32
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-33
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
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
5 DataLink Layer 5-34
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif 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
forward on all but the interface on which the frame arrived
Link Layer 5-35
Interconnecting switches
switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-36
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-37
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-38
Summary comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
Link Layer 5-39
VLANs motivation (virtual LAN)
consider CS user moves office to EE
but wants connect to CS switch
single broadcast domain all layer-2 broadcast
traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
securityprivacy efficiency issues
Computer Science Electrical
Engineering
ComputerEngineering
Link Layer 5-40
VLANsport-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network 1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Link Layer 5-41
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined
switches plus routers
Link Layer 5-42
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be
vanilla 8021 frames (must carry VLAN ID info) 8021q protocol addsremoved additional header fields for
frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
Link Layer 5-43
type
2-byte Tag Protocol Identifier (value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
destaddress
sourceaddress data (payload) CRCpreamble
data (payload) CRC
type
Link Layer 5-44
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-45
Multiprotocol label switching (MPLS)
initial goal high-speed IP forwarding using fixed length label (instead of IP address) fast lookup using fixed length identifier (rather than
shortest prefix matching) borrowing ideas from Virtual Circuit (VC) approach but IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
Link Layer 5-46
MPLS capable routers
aka label-switched router forward packets to outgoing interface based only
on label value (donrsquot inspect IP address) MPLS forwarding table distinct from IP forwarding tables
flexibility MPLS forwarding decisions can differ from those of IP use destination and source addresses to route flows to
same destination differently (traffic engineering) re-route flows quickly if link fails pre-computed backup
paths (useful for VoIP)
Link Layer 5-47
R2
D
R3R5
A
R6
MPLS versus IP paths
IP router IP routing path to destination
determined by destination address alone
R4
Link Layer 5-48
R2
D
R3R4
R5
A
R6
MPLS versus IP paths
IP-onlyrouter
IP routing path to destination determined by destination address alone
MPLS and IP router
MPLS routing path to destination can be based on source and dest address fast reroute precompute backup routes
in case of link failure
entry router (R4) can use different MPLS routes to A based eg on source address
Link Layer 5-49
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-50
Data center networks
10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximity e-business (eg Amazon) content-servers (eg YouTube Akamai Apple Microsoft) search engines data mining (eg Google)
challenges multiple applications each
serving massive numbers of clients
managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container Chicago data center
Link Layer 5-51
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
Load balancer
Load balancer
B
1 2 3 4 5 6 7 8
A C
Border router
Access router
Internet
Data center networks
load balancer application-layer routingreceives external client requestsdirects workload within data centerreturns results to external client (hiding data center internals from client)
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
1 2 3 4 5 6 7 8
Data center networks rich interconnection among switches racks
increased throughput between racks (multiple routing paths possible) increased reliability via redundancy
Link Layer 5-53
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-54
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer5-55
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
web page
browser
router(runs DHCP)
Link Layer 5-56
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router(runs DHCP)
Link Layer 5-57
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router(runs DHCP)
Link Layer 5-58
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
router(runs DHCP)
Link Layer 5-59
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server DNS server replies to
client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router(runs DHCP)
Link Layer 5-60
A day in the lifehellipTCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router(runs DHCP)
Link Layer 5-61
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-62
Chapter 5 Summary
principles behind data link layer services error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS VLANs virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-63
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless multimedia security network management
Link Layer 5-7
walkthrough send datagram from A to B via R focus on addressing ndash at IP (datagram) and MAC layer (frame) assume A knows Brsquos IP address assume A knows IP address of first hop router R (how) assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-8
Addressing routing to another LAN
IPEthPhy
IP src 111111111111 IP dest 222222222222
A creates IP datagram with IP source A destination B A creates link-layer frame with Rs MAC address as dest frame
contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-9
Addressing routing to another LAN
IPEthPhy
frame sent from A to R
IPEthPhy
frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
IP src 111111111111 IP dest 222222222222
IP src 111111111111 IP dest 222222222222
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-10
Addressing routing to another LAN
IP src 111111111111 IP dest 222222222222
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest frame
contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-11
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest frame
contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-12
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest frame
contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
Link Layer 5-13
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
5 DataLink Layer 5-14
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for 100Mbs first widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
Old cable-line ethernet
5 DataLink Layer 5-15
Bus topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
bus coaxial cable
10BASE2 cable showing BNC Connector end
10BASE2 cable with BNC T-Connector
10BASE2 Pictures are from Wikipiedia
Star topology
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
5 DataLink Layer 5-16
switch
star
5 DataLink Layer 5-17
Images from httpatomklubchippl
5 DataLink Layer 5-18
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates
MAC addr CRC-32
5 DataLink Layer 5-19
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol
otherwise adapter discards frame
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-20
Unreliable connectionless service Connectionless No handshaking between
sending and receiving adapter Unreliable receiving adapter doesnrsquot send
acks or nacks to sending adapter Data field is 46bytes -1500 bytes
bull If data less than 46 bytes stuff to be 46bytesndash Network layer uses ldquolengthrdquo field to remove stuffing
5 DataLink Layer 5-21
Ethernet uses CSMACD
No slots (no sync clock) Preamble in Ethernet
frame is used to sync clock between sender and receiver
adapter doesnrsquot transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-22
Ethernet CSMACD algorithm
1 Adaptor receives datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal (48-bit Why)
5 After aborting adapter enters exponential backoff after the m-th collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-23
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random wait
will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Why exponentialWhy random number picking
5 DataLink Layer 5-24
CSMACD efficiency Tprop = max prop between 2 nodes in LAN
ttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0
Goes to 1 as ttrans goes to infinity Much better than ALOHA but still decentralized simple and cheap
Why
transprop tt 51
1efficiency
5 DataLink Layer 5-25
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-26
Manchester encoding
Used in 10BaseT Each bit has a transition Allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
Link Layer 5-27
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
5 DataLink Layer 5-28
10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-29
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other links at the same rate no frame buffering no CSMACD at hub adapters detect collisions provides net management functionality
twisted pair
hub
5 DataLink Layer 5-30
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-31
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-32
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-33
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
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
5 DataLink Layer 5-34
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif 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
forward on all but the interface on which the frame arrived
Link Layer 5-35
Interconnecting switches
switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-36
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-37
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-38
Summary comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
Link Layer 5-39
VLANs motivation (virtual LAN)
consider CS user moves office to EE
but wants connect to CS switch
single broadcast domain all layer-2 broadcast
traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
securityprivacy efficiency issues
Computer Science Electrical
Engineering
ComputerEngineering
Link Layer 5-40
VLANsport-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network 1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Link Layer 5-41
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined
switches plus routers
Link Layer 5-42
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be
vanilla 8021 frames (must carry VLAN ID info) 8021q protocol addsremoved additional header fields for
frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
Link Layer 5-43
type
2-byte Tag Protocol Identifier (value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
destaddress
sourceaddress data (payload) CRCpreamble
data (payload) CRC
type
Link Layer 5-44
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-45
Multiprotocol label switching (MPLS)
initial goal high-speed IP forwarding using fixed length label (instead of IP address) fast lookup using fixed length identifier (rather than
shortest prefix matching) borrowing ideas from Virtual Circuit (VC) approach but IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
Link Layer 5-46
MPLS capable routers
aka label-switched router forward packets to outgoing interface based only
on label value (donrsquot inspect IP address) MPLS forwarding table distinct from IP forwarding tables
flexibility MPLS forwarding decisions can differ from those of IP use destination and source addresses to route flows to
same destination differently (traffic engineering) re-route flows quickly if link fails pre-computed backup
paths (useful for VoIP)
Link Layer 5-47
R2
D
R3R5
A
R6
MPLS versus IP paths
IP router IP routing path to destination
determined by destination address alone
R4
Link Layer 5-48
R2
D
R3R4
R5
A
R6
MPLS versus IP paths
IP-onlyrouter
IP routing path to destination determined by destination address alone
MPLS and IP router
MPLS routing path to destination can be based on source and dest address fast reroute precompute backup routes
in case of link failure
entry router (R4) can use different MPLS routes to A based eg on source address
Link Layer 5-49
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-50
Data center networks
10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximity e-business (eg Amazon) content-servers (eg YouTube Akamai Apple Microsoft) search engines data mining (eg Google)
challenges multiple applications each
serving massive numbers of clients
managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container Chicago data center
Link Layer 5-51
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
Load balancer
Load balancer
B
1 2 3 4 5 6 7 8
A C
Border router
Access router
Internet
Data center networks
load balancer application-layer routingreceives external client requestsdirects workload within data centerreturns results to external client (hiding data center internals from client)
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
1 2 3 4 5 6 7 8
Data center networks rich interconnection among switches racks
increased throughput between racks (multiple routing paths possible) increased reliability via redundancy
Link Layer 5-53
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-54
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer5-55
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
web page
browser
router(runs DHCP)
Link Layer 5-56
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router(runs DHCP)
Link Layer 5-57
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router(runs DHCP)
Link Layer 5-58
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
router(runs DHCP)
Link Layer 5-59
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server DNS server replies to
client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router(runs DHCP)
Link Layer 5-60
A day in the lifehellipTCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router(runs DHCP)
Link Layer 5-61
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-62
Chapter 5 Summary
principles behind data link layer services error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS VLANs virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-63
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless multimedia security network management
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-8
Addressing routing to another LAN
IPEthPhy
IP src 111111111111 IP dest 222222222222
A creates IP datagram with IP source A destination B A creates link-layer frame with Rs MAC address as dest frame
contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-9
Addressing routing to another LAN
IPEthPhy
frame sent from A to R
IPEthPhy
frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
IP src 111111111111 IP dest 222222222222
IP src 111111111111 IP dest 222222222222
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-10
Addressing routing to another LAN
IP src 111111111111 IP dest 222222222222
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest frame
contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-11
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest frame
contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-12
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest frame
contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
Link Layer 5-13
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
5 DataLink Layer 5-14
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for 100Mbs first widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
Old cable-line ethernet
5 DataLink Layer 5-15
Bus topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
bus coaxial cable
10BASE2 cable showing BNC Connector end
10BASE2 cable with BNC T-Connector
10BASE2 Pictures are from Wikipiedia
Star topology
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
5 DataLink Layer 5-16
switch
star
5 DataLink Layer 5-17
Images from httpatomklubchippl
5 DataLink Layer 5-18
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates
MAC addr CRC-32
5 DataLink Layer 5-19
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol
otherwise adapter discards frame
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-20
Unreliable connectionless service Connectionless No handshaking between
sending and receiving adapter Unreliable receiving adapter doesnrsquot send
acks or nacks to sending adapter Data field is 46bytes -1500 bytes
bull If data less than 46 bytes stuff to be 46bytesndash Network layer uses ldquolengthrdquo field to remove stuffing
5 DataLink Layer 5-21
Ethernet uses CSMACD
No slots (no sync clock) Preamble in Ethernet
frame is used to sync clock between sender and receiver
adapter doesnrsquot transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-22
Ethernet CSMACD algorithm
1 Adaptor receives datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal (48-bit Why)
5 After aborting adapter enters exponential backoff after the m-th collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-23
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random wait
will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Why exponentialWhy random number picking
5 DataLink Layer 5-24
CSMACD efficiency Tprop = max prop between 2 nodes in LAN
ttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0
Goes to 1 as ttrans goes to infinity Much better than ALOHA but still decentralized simple and cheap
Why
transprop tt 51
1efficiency
5 DataLink Layer 5-25
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-26
Manchester encoding
Used in 10BaseT Each bit has a transition Allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
Link Layer 5-27
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
5 DataLink Layer 5-28
10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-29
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other links at the same rate no frame buffering no CSMACD at hub adapters detect collisions provides net management functionality
twisted pair
hub
5 DataLink Layer 5-30
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-31
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-32
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-33
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
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
5 DataLink Layer 5-34
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif 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
forward on all but the interface on which the frame arrived
Link Layer 5-35
Interconnecting switches
switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-36
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-37
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-38
Summary comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
Link Layer 5-39
VLANs motivation (virtual LAN)
consider CS user moves office to EE
but wants connect to CS switch
single broadcast domain all layer-2 broadcast
traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
securityprivacy efficiency issues
Computer Science Electrical
Engineering
ComputerEngineering
Link Layer 5-40
VLANsport-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network 1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Link Layer 5-41
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined
switches plus routers
Link Layer 5-42
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be
vanilla 8021 frames (must carry VLAN ID info) 8021q protocol addsremoved additional header fields for
frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
Link Layer 5-43
type
2-byte Tag Protocol Identifier (value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
destaddress
sourceaddress data (payload) CRCpreamble
data (payload) CRC
type
Link Layer 5-44
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-45
Multiprotocol label switching (MPLS)
initial goal high-speed IP forwarding using fixed length label (instead of IP address) fast lookup using fixed length identifier (rather than
shortest prefix matching) borrowing ideas from Virtual Circuit (VC) approach but IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
Link Layer 5-46
MPLS capable routers
aka label-switched router forward packets to outgoing interface based only
on label value (donrsquot inspect IP address) MPLS forwarding table distinct from IP forwarding tables
flexibility MPLS forwarding decisions can differ from those of IP use destination and source addresses to route flows to
same destination differently (traffic engineering) re-route flows quickly if link fails pre-computed backup
paths (useful for VoIP)
Link Layer 5-47
R2
D
R3R5
A
R6
MPLS versus IP paths
IP router IP routing path to destination
determined by destination address alone
R4
Link Layer 5-48
R2
D
R3R4
R5
A
R6
MPLS versus IP paths
IP-onlyrouter
IP routing path to destination determined by destination address alone
MPLS and IP router
MPLS routing path to destination can be based on source and dest address fast reroute precompute backup routes
in case of link failure
entry router (R4) can use different MPLS routes to A based eg on source address
Link Layer 5-49
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-50
Data center networks
10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximity e-business (eg Amazon) content-servers (eg YouTube Akamai Apple Microsoft) search engines data mining (eg Google)
challenges multiple applications each
serving massive numbers of clients
managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container Chicago data center
Link Layer 5-51
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
Load balancer
Load balancer
B
1 2 3 4 5 6 7 8
A C
Border router
Access router
Internet
Data center networks
load balancer application-layer routingreceives external client requestsdirects workload within data centerreturns results to external client (hiding data center internals from client)
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
1 2 3 4 5 6 7 8
Data center networks rich interconnection among switches racks
increased throughput between racks (multiple routing paths possible) increased reliability via redundancy
Link Layer 5-53
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-54
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer5-55
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
web page
browser
router(runs DHCP)
Link Layer 5-56
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router(runs DHCP)
Link Layer 5-57
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router(runs DHCP)
Link Layer 5-58
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
router(runs DHCP)
Link Layer 5-59
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server DNS server replies to
client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router(runs DHCP)
Link Layer 5-60
A day in the lifehellipTCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router(runs DHCP)
Link Layer 5-61
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-62
Chapter 5 Summary
principles behind data link layer services error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS VLANs virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-63
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless multimedia security network management
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-9
Addressing routing to another LAN
IPEthPhy
frame sent from A to R
IPEthPhy
frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
IP src 111111111111 IP dest 222222222222
IP src 111111111111 IP dest 222222222222
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-10
Addressing routing to another LAN
IP src 111111111111 IP dest 222222222222
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest frame
contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-11
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest frame
contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-12
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest frame
contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
Link Layer 5-13
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
5 DataLink Layer 5-14
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for 100Mbs first widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
Old cable-line ethernet
5 DataLink Layer 5-15
Bus topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
bus coaxial cable
10BASE2 cable showing BNC Connector end
10BASE2 cable with BNC T-Connector
10BASE2 Pictures are from Wikipiedia
Star topology
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
5 DataLink Layer 5-16
switch
star
5 DataLink Layer 5-17
Images from httpatomklubchippl
5 DataLink Layer 5-18
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates
MAC addr CRC-32
5 DataLink Layer 5-19
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol
otherwise adapter discards frame
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-20
Unreliable connectionless service Connectionless No handshaking between
sending and receiving adapter Unreliable receiving adapter doesnrsquot send
acks or nacks to sending adapter Data field is 46bytes -1500 bytes
bull If data less than 46 bytes stuff to be 46bytesndash Network layer uses ldquolengthrdquo field to remove stuffing
5 DataLink Layer 5-21
Ethernet uses CSMACD
No slots (no sync clock) Preamble in Ethernet
frame is used to sync clock between sender and receiver
adapter doesnrsquot transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-22
Ethernet CSMACD algorithm
1 Adaptor receives datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal (48-bit Why)
5 After aborting adapter enters exponential backoff after the m-th collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-23
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random wait
will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Why exponentialWhy random number picking
5 DataLink Layer 5-24
CSMACD efficiency Tprop = max prop between 2 nodes in LAN
ttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0
Goes to 1 as ttrans goes to infinity Much better than ALOHA but still decentralized simple and cheap
Why
transprop tt 51
1efficiency
5 DataLink Layer 5-25
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-26
Manchester encoding
Used in 10BaseT Each bit has a transition Allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
Link Layer 5-27
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
5 DataLink Layer 5-28
10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-29
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other links at the same rate no frame buffering no CSMACD at hub adapters detect collisions provides net management functionality
twisted pair
hub
5 DataLink Layer 5-30
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-31
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-32
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-33
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
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
5 DataLink Layer 5-34
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif 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
forward on all but the interface on which the frame arrived
Link Layer 5-35
Interconnecting switches
switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-36
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-37
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-38
Summary comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
Link Layer 5-39
VLANs motivation (virtual LAN)
consider CS user moves office to EE
but wants connect to CS switch
single broadcast domain all layer-2 broadcast
traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
securityprivacy efficiency issues
Computer Science Electrical
Engineering
ComputerEngineering
Link Layer 5-40
VLANsport-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network 1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Link Layer 5-41
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined
switches plus routers
Link Layer 5-42
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be
vanilla 8021 frames (must carry VLAN ID info) 8021q protocol addsremoved additional header fields for
frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
Link Layer 5-43
type
2-byte Tag Protocol Identifier (value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
destaddress
sourceaddress data (payload) CRCpreamble
data (payload) CRC
type
Link Layer 5-44
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-45
Multiprotocol label switching (MPLS)
initial goal high-speed IP forwarding using fixed length label (instead of IP address) fast lookup using fixed length identifier (rather than
shortest prefix matching) borrowing ideas from Virtual Circuit (VC) approach but IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
Link Layer 5-46
MPLS capable routers
aka label-switched router forward packets to outgoing interface based only
on label value (donrsquot inspect IP address) MPLS forwarding table distinct from IP forwarding tables
flexibility MPLS forwarding decisions can differ from those of IP use destination and source addresses to route flows to
same destination differently (traffic engineering) re-route flows quickly if link fails pre-computed backup
paths (useful for VoIP)
Link Layer 5-47
R2
D
R3R5
A
R6
MPLS versus IP paths
IP router IP routing path to destination
determined by destination address alone
R4
Link Layer 5-48
R2
D
R3R4
R5
A
R6
MPLS versus IP paths
IP-onlyrouter
IP routing path to destination determined by destination address alone
MPLS and IP router
MPLS routing path to destination can be based on source and dest address fast reroute precompute backup routes
in case of link failure
entry router (R4) can use different MPLS routes to A based eg on source address
Link Layer 5-49
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-50
Data center networks
10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximity e-business (eg Amazon) content-servers (eg YouTube Akamai Apple Microsoft) search engines data mining (eg Google)
challenges multiple applications each
serving massive numbers of clients
managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container Chicago data center
Link Layer 5-51
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
Load balancer
Load balancer
B
1 2 3 4 5 6 7 8
A C
Border router
Access router
Internet
Data center networks
load balancer application-layer routingreceives external client requestsdirects workload within data centerreturns results to external client (hiding data center internals from client)
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
1 2 3 4 5 6 7 8
Data center networks rich interconnection among switches racks
increased throughput between racks (multiple routing paths possible) increased reliability via redundancy
Link Layer 5-53
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-54
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer5-55
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
web page
browser
router(runs DHCP)
Link Layer 5-56
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router(runs DHCP)
Link Layer 5-57
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router(runs DHCP)
Link Layer 5-58
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
router(runs DHCP)
Link Layer 5-59
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server DNS server replies to
client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router(runs DHCP)
Link Layer 5-60
A day in the lifehellipTCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router(runs DHCP)
Link Layer 5-61
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-62
Chapter 5 Summary
principles behind data link layer services error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS VLANs virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-63
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless multimedia security network management
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-10
Addressing routing to another LAN
IP src 111111111111 IP dest 222222222222
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest frame
contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-11
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest frame
contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-12
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest frame
contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
Link Layer 5-13
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
5 DataLink Layer 5-14
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for 100Mbs first widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
Old cable-line ethernet
5 DataLink Layer 5-15
Bus topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
bus coaxial cable
10BASE2 cable showing BNC Connector end
10BASE2 cable with BNC T-Connector
10BASE2 Pictures are from Wikipiedia
Star topology
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
5 DataLink Layer 5-16
switch
star
5 DataLink Layer 5-17
Images from httpatomklubchippl
5 DataLink Layer 5-18
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates
MAC addr CRC-32
5 DataLink Layer 5-19
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol
otherwise adapter discards frame
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-20
Unreliable connectionless service Connectionless No handshaking between
sending and receiving adapter Unreliable receiving adapter doesnrsquot send
acks or nacks to sending adapter Data field is 46bytes -1500 bytes
bull If data less than 46 bytes stuff to be 46bytesndash Network layer uses ldquolengthrdquo field to remove stuffing
5 DataLink Layer 5-21
Ethernet uses CSMACD
No slots (no sync clock) Preamble in Ethernet
frame is used to sync clock between sender and receiver
adapter doesnrsquot transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-22
Ethernet CSMACD algorithm
1 Adaptor receives datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal (48-bit Why)
5 After aborting adapter enters exponential backoff after the m-th collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-23
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random wait
will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Why exponentialWhy random number picking
5 DataLink Layer 5-24
CSMACD efficiency Tprop = max prop between 2 nodes in LAN
ttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0
Goes to 1 as ttrans goes to infinity Much better than ALOHA but still decentralized simple and cheap
Why
transprop tt 51
1efficiency
5 DataLink Layer 5-25
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-26
Manchester encoding
Used in 10BaseT Each bit has a transition Allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
Link Layer 5-27
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
5 DataLink Layer 5-28
10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-29
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other links at the same rate no frame buffering no CSMACD at hub adapters detect collisions provides net management functionality
twisted pair
hub
5 DataLink Layer 5-30
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-31
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-32
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-33
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
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
5 DataLink Layer 5-34
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif 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
forward on all but the interface on which the frame arrived
Link Layer 5-35
Interconnecting switches
switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-36
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-37
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-38
Summary comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
Link Layer 5-39
VLANs motivation (virtual LAN)
consider CS user moves office to EE
but wants connect to CS switch
single broadcast domain all layer-2 broadcast
traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
securityprivacy efficiency issues
Computer Science Electrical
Engineering
ComputerEngineering
Link Layer 5-40
VLANsport-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network 1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Link Layer 5-41
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined
switches plus routers
Link Layer 5-42
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be
vanilla 8021 frames (must carry VLAN ID info) 8021q protocol addsremoved additional header fields for
frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
Link Layer 5-43
type
2-byte Tag Protocol Identifier (value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
destaddress
sourceaddress data (payload) CRCpreamble
data (payload) CRC
type
Link Layer 5-44
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-45
Multiprotocol label switching (MPLS)
initial goal high-speed IP forwarding using fixed length label (instead of IP address) fast lookup using fixed length identifier (rather than
shortest prefix matching) borrowing ideas from Virtual Circuit (VC) approach but IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
Link Layer 5-46
MPLS capable routers
aka label-switched router forward packets to outgoing interface based only
on label value (donrsquot inspect IP address) MPLS forwarding table distinct from IP forwarding tables
flexibility MPLS forwarding decisions can differ from those of IP use destination and source addresses to route flows to
same destination differently (traffic engineering) re-route flows quickly if link fails pre-computed backup
paths (useful for VoIP)
Link Layer 5-47
R2
D
R3R5
A
R6
MPLS versus IP paths
IP router IP routing path to destination
determined by destination address alone
R4
Link Layer 5-48
R2
D
R3R4
R5
A
R6
MPLS versus IP paths
IP-onlyrouter
IP routing path to destination determined by destination address alone
MPLS and IP router
MPLS routing path to destination can be based on source and dest address fast reroute precompute backup routes
in case of link failure
entry router (R4) can use different MPLS routes to A based eg on source address
Link Layer 5-49
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-50
Data center networks
10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximity e-business (eg Amazon) content-servers (eg YouTube Akamai Apple Microsoft) search engines data mining (eg Google)
challenges multiple applications each
serving massive numbers of clients
managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container Chicago data center
Link Layer 5-51
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
Load balancer
Load balancer
B
1 2 3 4 5 6 7 8
A C
Border router
Access router
Internet
Data center networks
load balancer application-layer routingreceives external client requestsdirects workload within data centerreturns results to external client (hiding data center internals from client)
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
1 2 3 4 5 6 7 8
Data center networks rich interconnection among switches racks
increased throughput between racks (multiple routing paths possible) increased reliability via redundancy
Link Layer 5-53
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-54
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer5-55
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
web page
browser
router(runs DHCP)
Link Layer 5-56
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router(runs DHCP)
Link Layer 5-57
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router(runs DHCP)
Link Layer 5-58
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
router(runs DHCP)
Link Layer 5-59
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server DNS server replies to
client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router(runs DHCP)
Link Layer 5-60
A day in the lifehellipTCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router(runs DHCP)
Link Layer 5-61
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-62
Chapter 5 Summary
principles behind data link layer services error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS VLANs virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-63
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless multimedia security network management
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-11
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest frame
contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-12
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest frame
contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
Link Layer 5-13
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
5 DataLink Layer 5-14
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for 100Mbs first widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
Old cable-line ethernet
5 DataLink Layer 5-15
Bus topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
bus coaxial cable
10BASE2 cable showing BNC Connector end
10BASE2 cable with BNC T-Connector
10BASE2 Pictures are from Wikipiedia
Star topology
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
5 DataLink Layer 5-16
switch
star
5 DataLink Layer 5-17
Images from httpatomklubchippl
5 DataLink Layer 5-18
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates
MAC addr CRC-32
5 DataLink Layer 5-19
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol
otherwise adapter discards frame
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-20
Unreliable connectionless service Connectionless No handshaking between
sending and receiving adapter Unreliable receiving adapter doesnrsquot send
acks or nacks to sending adapter Data field is 46bytes -1500 bytes
bull If data less than 46 bytes stuff to be 46bytesndash Network layer uses ldquolengthrdquo field to remove stuffing
5 DataLink Layer 5-21
Ethernet uses CSMACD
No slots (no sync clock) Preamble in Ethernet
frame is used to sync clock between sender and receiver
adapter doesnrsquot transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-22
Ethernet CSMACD algorithm
1 Adaptor receives datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal (48-bit Why)
5 After aborting adapter enters exponential backoff after the m-th collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-23
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random wait
will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Why exponentialWhy random number picking
5 DataLink Layer 5-24
CSMACD efficiency Tprop = max prop between 2 nodes in LAN
ttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0
Goes to 1 as ttrans goes to infinity Much better than ALOHA but still decentralized simple and cheap
Why
transprop tt 51
1efficiency
5 DataLink Layer 5-25
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-26
Manchester encoding
Used in 10BaseT Each bit has a transition Allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
Link Layer 5-27
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
5 DataLink Layer 5-28
10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-29
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other links at the same rate no frame buffering no CSMACD at hub adapters detect collisions provides net management functionality
twisted pair
hub
5 DataLink Layer 5-30
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-31
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-32
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-33
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
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
5 DataLink Layer 5-34
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif 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
forward on all but the interface on which the frame arrived
Link Layer 5-35
Interconnecting switches
switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-36
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-37
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-38
Summary comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
Link Layer 5-39
VLANs motivation (virtual LAN)
consider CS user moves office to EE
but wants connect to CS switch
single broadcast domain all layer-2 broadcast
traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
securityprivacy efficiency issues
Computer Science Electrical
Engineering
ComputerEngineering
Link Layer 5-40
VLANsport-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network 1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Link Layer 5-41
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined
switches plus routers
Link Layer 5-42
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be
vanilla 8021 frames (must carry VLAN ID info) 8021q protocol addsremoved additional header fields for
frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
Link Layer 5-43
type
2-byte Tag Protocol Identifier (value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
destaddress
sourceaddress data (payload) CRCpreamble
data (payload) CRC
type
Link Layer 5-44
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-45
Multiprotocol label switching (MPLS)
initial goal high-speed IP forwarding using fixed length label (instead of IP address) fast lookup using fixed length identifier (rather than
shortest prefix matching) borrowing ideas from Virtual Circuit (VC) approach but IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
Link Layer 5-46
MPLS capable routers
aka label-switched router forward packets to outgoing interface based only
on label value (donrsquot inspect IP address) MPLS forwarding table distinct from IP forwarding tables
flexibility MPLS forwarding decisions can differ from those of IP use destination and source addresses to route flows to
same destination differently (traffic engineering) re-route flows quickly if link fails pre-computed backup
paths (useful for VoIP)
Link Layer 5-47
R2
D
R3R5
A
R6
MPLS versus IP paths
IP router IP routing path to destination
determined by destination address alone
R4
Link Layer 5-48
R2
D
R3R4
R5
A
R6
MPLS versus IP paths
IP-onlyrouter
IP routing path to destination determined by destination address alone
MPLS and IP router
MPLS routing path to destination can be based on source and dest address fast reroute precompute backup routes
in case of link failure
entry router (R4) can use different MPLS routes to A based eg on source address
Link Layer 5-49
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-50
Data center networks
10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximity e-business (eg Amazon) content-servers (eg YouTube Akamai Apple Microsoft) search engines data mining (eg Google)
challenges multiple applications each
serving massive numbers of clients
managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container Chicago data center
Link Layer 5-51
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
Load balancer
Load balancer
B
1 2 3 4 5 6 7 8
A C
Border router
Access router
Internet
Data center networks
load balancer application-layer routingreceives external client requestsdirects workload within data centerreturns results to external client (hiding data center internals from client)
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
1 2 3 4 5 6 7 8
Data center networks rich interconnection among switches racks
increased throughput between racks (multiple routing paths possible) increased reliability via redundancy
Link Layer 5-53
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-54
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer5-55
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
web page
browser
router(runs DHCP)
Link Layer 5-56
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router(runs DHCP)
Link Layer 5-57
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router(runs DHCP)
Link Layer 5-58
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
router(runs DHCP)
Link Layer 5-59
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server DNS server replies to
client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router(runs DHCP)
Link Layer 5-60
A day in the lifehellipTCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router(runs DHCP)
Link Layer 5-61
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-62
Chapter 5 Summary
principles behind data link layer services error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS VLANs virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-63
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless multimedia security network management
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-12
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest frame
contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
Link Layer 5-13
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
5 DataLink Layer 5-14
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for 100Mbs first widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
Old cable-line ethernet
5 DataLink Layer 5-15
Bus topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
bus coaxial cable
10BASE2 cable showing BNC Connector end
10BASE2 cable with BNC T-Connector
10BASE2 Pictures are from Wikipiedia
Star topology
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
5 DataLink Layer 5-16
switch
star
5 DataLink Layer 5-17
Images from httpatomklubchippl
5 DataLink Layer 5-18
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates
MAC addr CRC-32
5 DataLink Layer 5-19
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol
otherwise adapter discards frame
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-20
Unreliable connectionless service Connectionless No handshaking between
sending and receiving adapter Unreliable receiving adapter doesnrsquot send
acks or nacks to sending adapter Data field is 46bytes -1500 bytes
bull If data less than 46 bytes stuff to be 46bytesndash Network layer uses ldquolengthrdquo field to remove stuffing
5 DataLink Layer 5-21
Ethernet uses CSMACD
No slots (no sync clock) Preamble in Ethernet
frame is used to sync clock between sender and receiver
adapter doesnrsquot transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-22
Ethernet CSMACD algorithm
1 Adaptor receives datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal (48-bit Why)
5 After aborting adapter enters exponential backoff after the m-th collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-23
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random wait
will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Why exponentialWhy random number picking
5 DataLink Layer 5-24
CSMACD efficiency Tprop = max prop between 2 nodes in LAN
ttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0
Goes to 1 as ttrans goes to infinity Much better than ALOHA but still decentralized simple and cheap
Why
transprop tt 51
1efficiency
5 DataLink Layer 5-25
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-26
Manchester encoding
Used in 10BaseT Each bit has a transition Allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
Link Layer 5-27
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
5 DataLink Layer 5-28
10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-29
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other links at the same rate no frame buffering no CSMACD at hub adapters detect collisions provides net management functionality
twisted pair
hub
5 DataLink Layer 5-30
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-31
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-32
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-33
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
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
5 DataLink Layer 5-34
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif 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
forward on all but the interface on which the frame arrived
Link Layer 5-35
Interconnecting switches
switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-36
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-37
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-38
Summary comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
Link Layer 5-39
VLANs motivation (virtual LAN)
consider CS user moves office to EE
but wants connect to CS switch
single broadcast domain all layer-2 broadcast
traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
securityprivacy efficiency issues
Computer Science Electrical
Engineering
ComputerEngineering
Link Layer 5-40
VLANsport-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network 1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Link Layer 5-41
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined
switches plus routers
Link Layer 5-42
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be
vanilla 8021 frames (must carry VLAN ID info) 8021q protocol addsremoved additional header fields for
frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
Link Layer 5-43
type
2-byte Tag Protocol Identifier (value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
destaddress
sourceaddress data (payload) CRCpreamble
data (payload) CRC
type
Link Layer 5-44
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-45
Multiprotocol label switching (MPLS)
initial goal high-speed IP forwarding using fixed length label (instead of IP address) fast lookup using fixed length identifier (rather than
shortest prefix matching) borrowing ideas from Virtual Circuit (VC) approach but IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
Link Layer 5-46
MPLS capable routers
aka label-switched router forward packets to outgoing interface based only
on label value (donrsquot inspect IP address) MPLS forwarding table distinct from IP forwarding tables
flexibility MPLS forwarding decisions can differ from those of IP use destination and source addresses to route flows to
same destination differently (traffic engineering) re-route flows quickly if link fails pre-computed backup
paths (useful for VoIP)
Link Layer 5-47
R2
D
R3R5
A
R6
MPLS versus IP paths
IP router IP routing path to destination
determined by destination address alone
R4
Link Layer 5-48
R2
D
R3R4
R5
A
R6
MPLS versus IP paths
IP-onlyrouter
IP routing path to destination determined by destination address alone
MPLS and IP router
MPLS routing path to destination can be based on source and dest address fast reroute precompute backup routes
in case of link failure
entry router (R4) can use different MPLS routes to A based eg on source address
Link Layer 5-49
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-50
Data center networks
10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximity e-business (eg Amazon) content-servers (eg YouTube Akamai Apple Microsoft) search engines data mining (eg Google)
challenges multiple applications each
serving massive numbers of clients
managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container Chicago data center
Link Layer 5-51
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
Load balancer
Load balancer
B
1 2 3 4 5 6 7 8
A C
Border router
Access router
Internet
Data center networks
load balancer application-layer routingreceives external client requestsdirects workload within data centerreturns results to external client (hiding data center internals from client)
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
1 2 3 4 5 6 7 8
Data center networks rich interconnection among switches racks
increased throughput between racks (multiple routing paths possible) increased reliability via redundancy
Link Layer 5-53
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-54
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer5-55
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
web page
browser
router(runs DHCP)
Link Layer 5-56
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router(runs DHCP)
Link Layer 5-57
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router(runs DHCP)
Link Layer 5-58
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
router(runs DHCP)
Link Layer 5-59
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server DNS server replies to
client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router(runs DHCP)
Link Layer 5-60
A day in the lifehellipTCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router(runs DHCP)
Link Layer 5-61
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-62
Chapter 5 Summary
principles behind data link layer services error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS VLANs virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-63
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless multimedia security network management
Link Layer 5-13
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
5 DataLink Layer 5-14
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for 100Mbs first widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
Old cable-line ethernet
5 DataLink Layer 5-15
Bus topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
bus coaxial cable
10BASE2 cable showing BNC Connector end
10BASE2 cable with BNC T-Connector
10BASE2 Pictures are from Wikipiedia
Star topology
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
5 DataLink Layer 5-16
switch
star
5 DataLink Layer 5-17
Images from httpatomklubchippl
5 DataLink Layer 5-18
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates
MAC addr CRC-32
5 DataLink Layer 5-19
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol
otherwise adapter discards frame
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-20
Unreliable connectionless service Connectionless No handshaking between
sending and receiving adapter Unreliable receiving adapter doesnrsquot send
acks or nacks to sending adapter Data field is 46bytes -1500 bytes
bull If data less than 46 bytes stuff to be 46bytesndash Network layer uses ldquolengthrdquo field to remove stuffing
5 DataLink Layer 5-21
Ethernet uses CSMACD
No slots (no sync clock) Preamble in Ethernet
frame is used to sync clock between sender and receiver
adapter doesnrsquot transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-22
Ethernet CSMACD algorithm
1 Adaptor receives datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal (48-bit Why)
5 After aborting adapter enters exponential backoff after the m-th collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-23
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random wait
will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Why exponentialWhy random number picking
5 DataLink Layer 5-24
CSMACD efficiency Tprop = max prop between 2 nodes in LAN
ttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0
Goes to 1 as ttrans goes to infinity Much better than ALOHA but still decentralized simple and cheap
Why
transprop tt 51
1efficiency
5 DataLink Layer 5-25
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-26
Manchester encoding
Used in 10BaseT Each bit has a transition Allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
Link Layer 5-27
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
5 DataLink Layer 5-28
10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-29
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other links at the same rate no frame buffering no CSMACD at hub adapters detect collisions provides net management functionality
twisted pair
hub
5 DataLink Layer 5-30
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-31
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-32
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-33
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
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
5 DataLink Layer 5-34
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif 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
forward on all but the interface on which the frame arrived
Link Layer 5-35
Interconnecting switches
switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-36
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-37
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-38
Summary comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
Link Layer 5-39
VLANs motivation (virtual LAN)
consider CS user moves office to EE
but wants connect to CS switch
single broadcast domain all layer-2 broadcast
traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
securityprivacy efficiency issues
Computer Science Electrical
Engineering
ComputerEngineering
Link Layer 5-40
VLANsport-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network 1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Link Layer 5-41
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined
switches plus routers
Link Layer 5-42
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be
vanilla 8021 frames (must carry VLAN ID info) 8021q protocol addsremoved additional header fields for
frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
Link Layer 5-43
type
2-byte Tag Protocol Identifier (value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
destaddress
sourceaddress data (payload) CRCpreamble
data (payload) CRC
type
Link Layer 5-44
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-45
Multiprotocol label switching (MPLS)
initial goal high-speed IP forwarding using fixed length label (instead of IP address) fast lookup using fixed length identifier (rather than
shortest prefix matching) borrowing ideas from Virtual Circuit (VC) approach but IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
Link Layer 5-46
MPLS capable routers
aka label-switched router forward packets to outgoing interface based only
on label value (donrsquot inspect IP address) MPLS forwarding table distinct from IP forwarding tables
flexibility MPLS forwarding decisions can differ from those of IP use destination and source addresses to route flows to
same destination differently (traffic engineering) re-route flows quickly if link fails pre-computed backup
paths (useful for VoIP)
Link Layer 5-47
R2
D
R3R5
A
R6
MPLS versus IP paths
IP router IP routing path to destination
determined by destination address alone
R4
Link Layer 5-48
R2
D
R3R4
R5
A
R6
MPLS versus IP paths
IP-onlyrouter
IP routing path to destination determined by destination address alone
MPLS and IP router
MPLS routing path to destination can be based on source and dest address fast reroute precompute backup routes
in case of link failure
entry router (R4) can use different MPLS routes to A based eg on source address
Link Layer 5-49
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-50
Data center networks
10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximity e-business (eg Amazon) content-servers (eg YouTube Akamai Apple Microsoft) search engines data mining (eg Google)
challenges multiple applications each
serving massive numbers of clients
managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container Chicago data center
Link Layer 5-51
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
Load balancer
Load balancer
B
1 2 3 4 5 6 7 8
A C
Border router
Access router
Internet
Data center networks
load balancer application-layer routingreceives external client requestsdirects workload within data centerreturns results to external client (hiding data center internals from client)
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
1 2 3 4 5 6 7 8
Data center networks rich interconnection among switches racks
increased throughput between racks (multiple routing paths possible) increased reliability via redundancy
Link Layer 5-53
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-54
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer5-55
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
web page
browser
router(runs DHCP)
Link Layer 5-56
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router(runs DHCP)
Link Layer 5-57
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router(runs DHCP)
Link Layer 5-58
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
router(runs DHCP)
Link Layer 5-59
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server DNS server replies to
client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router(runs DHCP)
Link Layer 5-60
A day in the lifehellipTCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router(runs DHCP)
Link Layer 5-61
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-62
Chapter 5 Summary
principles behind data link layer services error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS VLANs virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-63
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless multimedia security network management
5 DataLink Layer 5-14
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for 100Mbs first widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
Old cable-line ethernet
5 DataLink Layer 5-15
Bus topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
bus coaxial cable
10BASE2 cable showing BNC Connector end
10BASE2 cable with BNC T-Connector
10BASE2 Pictures are from Wikipiedia
Star topology
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
5 DataLink Layer 5-16
switch
star
5 DataLink Layer 5-17
Images from httpatomklubchippl
5 DataLink Layer 5-18
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates
MAC addr CRC-32
5 DataLink Layer 5-19
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol
otherwise adapter discards frame
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-20
Unreliable connectionless service Connectionless No handshaking between
sending and receiving adapter Unreliable receiving adapter doesnrsquot send
acks or nacks to sending adapter Data field is 46bytes -1500 bytes
bull If data less than 46 bytes stuff to be 46bytesndash Network layer uses ldquolengthrdquo field to remove stuffing
5 DataLink Layer 5-21
Ethernet uses CSMACD
No slots (no sync clock) Preamble in Ethernet
frame is used to sync clock between sender and receiver
adapter doesnrsquot transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-22
Ethernet CSMACD algorithm
1 Adaptor receives datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal (48-bit Why)
5 After aborting adapter enters exponential backoff after the m-th collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-23
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random wait
will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Why exponentialWhy random number picking
5 DataLink Layer 5-24
CSMACD efficiency Tprop = max prop between 2 nodes in LAN
ttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0
Goes to 1 as ttrans goes to infinity Much better than ALOHA but still decentralized simple and cheap
Why
transprop tt 51
1efficiency
5 DataLink Layer 5-25
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-26
Manchester encoding
Used in 10BaseT Each bit has a transition Allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
Link Layer 5-27
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
5 DataLink Layer 5-28
10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-29
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other links at the same rate no frame buffering no CSMACD at hub adapters detect collisions provides net management functionality
twisted pair
hub
5 DataLink Layer 5-30
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-31
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-32
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-33
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
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
5 DataLink Layer 5-34
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif 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
forward on all but the interface on which the frame arrived
Link Layer 5-35
Interconnecting switches
switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-36
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-37
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-38
Summary comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
Link Layer 5-39
VLANs motivation (virtual LAN)
consider CS user moves office to EE
but wants connect to CS switch
single broadcast domain all layer-2 broadcast
traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
securityprivacy efficiency issues
Computer Science Electrical
Engineering
ComputerEngineering
Link Layer 5-40
VLANsport-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network 1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Link Layer 5-41
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined
switches plus routers
Link Layer 5-42
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be
vanilla 8021 frames (must carry VLAN ID info) 8021q protocol addsremoved additional header fields for
frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
Link Layer 5-43
type
2-byte Tag Protocol Identifier (value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
destaddress
sourceaddress data (payload) CRCpreamble
data (payload) CRC
type
Link Layer 5-44
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-45
Multiprotocol label switching (MPLS)
initial goal high-speed IP forwarding using fixed length label (instead of IP address) fast lookup using fixed length identifier (rather than
shortest prefix matching) borrowing ideas from Virtual Circuit (VC) approach but IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
Link Layer 5-46
MPLS capable routers
aka label-switched router forward packets to outgoing interface based only
on label value (donrsquot inspect IP address) MPLS forwarding table distinct from IP forwarding tables
flexibility MPLS forwarding decisions can differ from those of IP use destination and source addresses to route flows to
same destination differently (traffic engineering) re-route flows quickly if link fails pre-computed backup
paths (useful for VoIP)
Link Layer 5-47
R2
D
R3R5
A
R6
MPLS versus IP paths
IP router IP routing path to destination
determined by destination address alone
R4
Link Layer 5-48
R2
D
R3R4
R5
A
R6
MPLS versus IP paths
IP-onlyrouter
IP routing path to destination determined by destination address alone
MPLS and IP router
MPLS routing path to destination can be based on source and dest address fast reroute precompute backup routes
in case of link failure
entry router (R4) can use different MPLS routes to A based eg on source address
Link Layer 5-49
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-50
Data center networks
10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximity e-business (eg Amazon) content-servers (eg YouTube Akamai Apple Microsoft) search engines data mining (eg Google)
challenges multiple applications each
serving massive numbers of clients
managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container Chicago data center
Link Layer 5-51
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
Load balancer
Load balancer
B
1 2 3 4 5 6 7 8
A C
Border router
Access router
Internet
Data center networks
load balancer application-layer routingreceives external client requestsdirects workload within data centerreturns results to external client (hiding data center internals from client)
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
1 2 3 4 5 6 7 8
Data center networks rich interconnection among switches racks
increased throughput between racks (multiple routing paths possible) increased reliability via redundancy
Link Layer 5-53
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-54
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer5-55
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
web page
browser
router(runs DHCP)
Link Layer 5-56
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router(runs DHCP)
Link Layer 5-57
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router(runs DHCP)
Link Layer 5-58
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
router(runs DHCP)
Link Layer 5-59
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server DNS server replies to
client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router(runs DHCP)
Link Layer 5-60
A day in the lifehellipTCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router(runs DHCP)
Link Layer 5-61
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-62
Chapter 5 Summary
principles behind data link layer services error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS VLANs virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-63
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless multimedia security network management
5 DataLink Layer 5-15
Bus topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
bus coaxial cable
10BASE2 cable showing BNC Connector end
10BASE2 cable with BNC T-Connector
10BASE2 Pictures are from Wikipiedia
Star topology
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
5 DataLink Layer 5-16
switch
star
5 DataLink Layer 5-17
Images from httpatomklubchippl
5 DataLink Layer 5-18
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates
MAC addr CRC-32
5 DataLink Layer 5-19
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol
otherwise adapter discards frame
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-20
Unreliable connectionless service Connectionless No handshaking between
sending and receiving adapter Unreliable receiving adapter doesnrsquot send
acks or nacks to sending adapter Data field is 46bytes -1500 bytes
bull If data less than 46 bytes stuff to be 46bytesndash Network layer uses ldquolengthrdquo field to remove stuffing
5 DataLink Layer 5-21
Ethernet uses CSMACD
No slots (no sync clock) Preamble in Ethernet
frame is used to sync clock between sender and receiver
adapter doesnrsquot transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-22
Ethernet CSMACD algorithm
1 Adaptor receives datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal (48-bit Why)
5 After aborting adapter enters exponential backoff after the m-th collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-23
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random wait
will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Why exponentialWhy random number picking
5 DataLink Layer 5-24
CSMACD efficiency Tprop = max prop between 2 nodes in LAN
ttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0
Goes to 1 as ttrans goes to infinity Much better than ALOHA but still decentralized simple and cheap
Why
transprop tt 51
1efficiency
5 DataLink Layer 5-25
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-26
Manchester encoding
Used in 10BaseT Each bit has a transition Allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
Link Layer 5-27
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
5 DataLink Layer 5-28
10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-29
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other links at the same rate no frame buffering no CSMACD at hub adapters detect collisions provides net management functionality
twisted pair
hub
5 DataLink Layer 5-30
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-31
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-32
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-33
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
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
5 DataLink Layer 5-34
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif 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
forward on all but the interface on which the frame arrived
Link Layer 5-35
Interconnecting switches
switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-36
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-37
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-38
Summary comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
Link Layer 5-39
VLANs motivation (virtual LAN)
consider CS user moves office to EE
but wants connect to CS switch
single broadcast domain all layer-2 broadcast
traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
securityprivacy efficiency issues
Computer Science Electrical
Engineering
ComputerEngineering
Link Layer 5-40
VLANsport-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network 1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Link Layer 5-41
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined
switches plus routers
Link Layer 5-42
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be
vanilla 8021 frames (must carry VLAN ID info) 8021q protocol addsremoved additional header fields for
frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
Link Layer 5-43
type
2-byte Tag Protocol Identifier (value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
destaddress
sourceaddress data (payload) CRCpreamble
data (payload) CRC
type
Link Layer 5-44
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-45
Multiprotocol label switching (MPLS)
initial goal high-speed IP forwarding using fixed length label (instead of IP address) fast lookup using fixed length identifier (rather than
shortest prefix matching) borrowing ideas from Virtual Circuit (VC) approach but IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
Link Layer 5-46
MPLS capable routers
aka label-switched router forward packets to outgoing interface based only
on label value (donrsquot inspect IP address) MPLS forwarding table distinct from IP forwarding tables
flexibility MPLS forwarding decisions can differ from those of IP use destination and source addresses to route flows to
same destination differently (traffic engineering) re-route flows quickly if link fails pre-computed backup
paths (useful for VoIP)
Link Layer 5-47
R2
D
R3R5
A
R6
MPLS versus IP paths
IP router IP routing path to destination
determined by destination address alone
R4
Link Layer 5-48
R2
D
R3R4
R5
A
R6
MPLS versus IP paths
IP-onlyrouter
IP routing path to destination determined by destination address alone
MPLS and IP router
MPLS routing path to destination can be based on source and dest address fast reroute precompute backup routes
in case of link failure
entry router (R4) can use different MPLS routes to A based eg on source address
Link Layer 5-49
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-50
Data center networks
10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximity e-business (eg Amazon) content-servers (eg YouTube Akamai Apple Microsoft) search engines data mining (eg Google)
challenges multiple applications each
serving massive numbers of clients
managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container Chicago data center
Link Layer 5-51
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
Load balancer
Load balancer
B
1 2 3 4 5 6 7 8
A C
Border router
Access router
Internet
Data center networks
load balancer application-layer routingreceives external client requestsdirects workload within data centerreturns results to external client (hiding data center internals from client)
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
1 2 3 4 5 6 7 8
Data center networks rich interconnection among switches racks
increased throughput between racks (multiple routing paths possible) increased reliability via redundancy
Link Layer 5-53
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-54
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer5-55
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
web page
browser
router(runs DHCP)
Link Layer 5-56
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router(runs DHCP)
Link Layer 5-57
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router(runs DHCP)
Link Layer 5-58
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
router(runs DHCP)
Link Layer 5-59
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server DNS server replies to
client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router(runs DHCP)
Link Layer 5-60
A day in the lifehellipTCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router(runs DHCP)
Link Layer 5-61
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-62
Chapter 5 Summary
principles behind data link layer services error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS VLANs virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-63
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless multimedia security network management
Star topology
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
5 DataLink Layer 5-16
switch
star
5 DataLink Layer 5-17
Images from httpatomklubchippl
5 DataLink Layer 5-18
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates
MAC addr CRC-32
5 DataLink Layer 5-19
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol
otherwise adapter discards frame
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-20
Unreliable connectionless service Connectionless No handshaking between
sending and receiving adapter Unreliable receiving adapter doesnrsquot send
acks or nacks to sending adapter Data field is 46bytes -1500 bytes
bull If data less than 46 bytes stuff to be 46bytesndash Network layer uses ldquolengthrdquo field to remove stuffing
5 DataLink Layer 5-21
Ethernet uses CSMACD
No slots (no sync clock) Preamble in Ethernet
frame is used to sync clock between sender and receiver
adapter doesnrsquot transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-22
Ethernet CSMACD algorithm
1 Adaptor receives datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal (48-bit Why)
5 After aborting adapter enters exponential backoff after the m-th collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-23
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random wait
will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Why exponentialWhy random number picking
5 DataLink Layer 5-24
CSMACD efficiency Tprop = max prop between 2 nodes in LAN
ttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0
Goes to 1 as ttrans goes to infinity Much better than ALOHA but still decentralized simple and cheap
Why
transprop tt 51
1efficiency
5 DataLink Layer 5-25
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-26
Manchester encoding
Used in 10BaseT Each bit has a transition Allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
Link Layer 5-27
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
5 DataLink Layer 5-28
10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-29
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other links at the same rate no frame buffering no CSMACD at hub adapters detect collisions provides net management functionality
twisted pair
hub
5 DataLink Layer 5-30
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-31
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-32
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-33
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
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
5 DataLink Layer 5-34
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif 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
forward on all but the interface on which the frame arrived
Link Layer 5-35
Interconnecting switches
switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-36
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-37
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-38
Summary comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
Link Layer 5-39
VLANs motivation (virtual LAN)
consider CS user moves office to EE
but wants connect to CS switch
single broadcast domain all layer-2 broadcast
traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
securityprivacy efficiency issues
Computer Science Electrical
Engineering
ComputerEngineering
Link Layer 5-40
VLANsport-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network 1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Link Layer 5-41
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined
switches plus routers
Link Layer 5-42
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be
vanilla 8021 frames (must carry VLAN ID info) 8021q protocol addsremoved additional header fields for
frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
Link Layer 5-43
type
2-byte Tag Protocol Identifier (value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
destaddress
sourceaddress data (payload) CRCpreamble
data (payload) CRC
type
Link Layer 5-44
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-45
Multiprotocol label switching (MPLS)
initial goal high-speed IP forwarding using fixed length label (instead of IP address) fast lookup using fixed length identifier (rather than
shortest prefix matching) borrowing ideas from Virtual Circuit (VC) approach but IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
Link Layer 5-46
MPLS capable routers
aka label-switched router forward packets to outgoing interface based only
on label value (donrsquot inspect IP address) MPLS forwarding table distinct from IP forwarding tables
flexibility MPLS forwarding decisions can differ from those of IP use destination and source addresses to route flows to
same destination differently (traffic engineering) re-route flows quickly if link fails pre-computed backup
paths (useful for VoIP)
Link Layer 5-47
R2
D
R3R5
A
R6
MPLS versus IP paths
IP router IP routing path to destination
determined by destination address alone
R4
Link Layer 5-48
R2
D
R3R4
R5
A
R6
MPLS versus IP paths
IP-onlyrouter
IP routing path to destination determined by destination address alone
MPLS and IP router
MPLS routing path to destination can be based on source and dest address fast reroute precompute backup routes
in case of link failure
entry router (R4) can use different MPLS routes to A based eg on source address
Link Layer 5-49
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-50
Data center networks
10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximity e-business (eg Amazon) content-servers (eg YouTube Akamai Apple Microsoft) search engines data mining (eg Google)
challenges multiple applications each
serving massive numbers of clients
managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container Chicago data center
Link Layer 5-51
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
Load balancer
Load balancer
B
1 2 3 4 5 6 7 8
A C
Border router
Access router
Internet
Data center networks
load balancer application-layer routingreceives external client requestsdirects workload within data centerreturns results to external client (hiding data center internals from client)
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
1 2 3 4 5 6 7 8
Data center networks rich interconnection among switches racks
increased throughput between racks (multiple routing paths possible) increased reliability via redundancy
Link Layer 5-53
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-54
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer5-55
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
web page
browser
router(runs DHCP)
Link Layer 5-56
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router(runs DHCP)
Link Layer 5-57
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router(runs DHCP)
Link Layer 5-58
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
router(runs DHCP)
Link Layer 5-59
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server DNS server replies to
client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router(runs DHCP)
Link Layer 5-60
A day in the lifehellipTCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router(runs DHCP)
Link Layer 5-61
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-62
Chapter 5 Summary
principles behind data link layer services error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS VLANs virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-63
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless multimedia security network management
5 DataLink Layer 5-17
Images from httpatomklubchippl
5 DataLink Layer 5-18
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates
MAC addr CRC-32
5 DataLink Layer 5-19
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol
otherwise adapter discards frame
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-20
Unreliable connectionless service Connectionless No handshaking between
sending and receiving adapter Unreliable receiving adapter doesnrsquot send
acks or nacks to sending adapter Data field is 46bytes -1500 bytes
bull If data less than 46 bytes stuff to be 46bytesndash Network layer uses ldquolengthrdquo field to remove stuffing
5 DataLink Layer 5-21
Ethernet uses CSMACD
No slots (no sync clock) Preamble in Ethernet
frame is used to sync clock between sender and receiver
adapter doesnrsquot transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-22
Ethernet CSMACD algorithm
1 Adaptor receives datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal (48-bit Why)
5 After aborting adapter enters exponential backoff after the m-th collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-23
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random wait
will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Why exponentialWhy random number picking
5 DataLink Layer 5-24
CSMACD efficiency Tprop = max prop between 2 nodes in LAN
ttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0
Goes to 1 as ttrans goes to infinity Much better than ALOHA but still decentralized simple and cheap
Why
transprop tt 51
1efficiency
5 DataLink Layer 5-25
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-26
Manchester encoding
Used in 10BaseT Each bit has a transition Allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
Link Layer 5-27
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
5 DataLink Layer 5-28
10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-29
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other links at the same rate no frame buffering no CSMACD at hub adapters detect collisions provides net management functionality
twisted pair
hub
5 DataLink Layer 5-30
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-31
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-32
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-33
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
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
5 DataLink Layer 5-34
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif 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
forward on all but the interface on which the frame arrived
Link Layer 5-35
Interconnecting switches
switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-36
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-37
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-38
Summary comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
Link Layer 5-39
VLANs motivation (virtual LAN)
consider CS user moves office to EE
but wants connect to CS switch
single broadcast domain all layer-2 broadcast
traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
securityprivacy efficiency issues
Computer Science Electrical
Engineering
ComputerEngineering
Link Layer 5-40
VLANsport-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network 1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Link Layer 5-41
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined
switches plus routers
Link Layer 5-42
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be
vanilla 8021 frames (must carry VLAN ID info) 8021q protocol addsremoved additional header fields for
frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
Link Layer 5-43
type
2-byte Tag Protocol Identifier (value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
destaddress
sourceaddress data (payload) CRCpreamble
data (payload) CRC
type
Link Layer 5-44
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-45
Multiprotocol label switching (MPLS)
initial goal high-speed IP forwarding using fixed length label (instead of IP address) fast lookup using fixed length identifier (rather than
shortest prefix matching) borrowing ideas from Virtual Circuit (VC) approach but IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
Link Layer 5-46
MPLS capable routers
aka label-switched router forward packets to outgoing interface based only
on label value (donrsquot inspect IP address) MPLS forwarding table distinct from IP forwarding tables
flexibility MPLS forwarding decisions can differ from those of IP use destination and source addresses to route flows to
same destination differently (traffic engineering) re-route flows quickly if link fails pre-computed backup
paths (useful for VoIP)
Link Layer 5-47
R2
D
R3R5
A
R6
MPLS versus IP paths
IP router IP routing path to destination
determined by destination address alone
R4
Link Layer 5-48
R2
D
R3R4
R5
A
R6
MPLS versus IP paths
IP-onlyrouter
IP routing path to destination determined by destination address alone
MPLS and IP router
MPLS routing path to destination can be based on source and dest address fast reroute precompute backup routes
in case of link failure
entry router (R4) can use different MPLS routes to A based eg on source address
Link Layer 5-49
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-50
Data center networks
10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximity e-business (eg Amazon) content-servers (eg YouTube Akamai Apple Microsoft) search engines data mining (eg Google)
challenges multiple applications each
serving massive numbers of clients
managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container Chicago data center
Link Layer 5-51
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
Load balancer
Load balancer
B
1 2 3 4 5 6 7 8
A C
Border router
Access router
Internet
Data center networks
load balancer application-layer routingreceives external client requestsdirects workload within data centerreturns results to external client (hiding data center internals from client)
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
1 2 3 4 5 6 7 8
Data center networks rich interconnection among switches racks
increased throughput between racks (multiple routing paths possible) increased reliability via redundancy
Link Layer 5-53
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-54
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer5-55
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
web page
browser
router(runs DHCP)
Link Layer 5-56
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router(runs DHCP)
Link Layer 5-57
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router(runs DHCP)
Link Layer 5-58
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
router(runs DHCP)
Link Layer 5-59
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server DNS server replies to
client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router(runs DHCP)
Link Layer 5-60
A day in the lifehellipTCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router(runs DHCP)
Link Layer 5-61
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-62
Chapter 5 Summary
principles behind data link layer services error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS VLANs virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-63
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless multimedia security network management
5 DataLink Layer 5-18
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates
MAC addr CRC-32
5 DataLink Layer 5-19
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol
otherwise adapter discards frame
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-20
Unreliable connectionless service Connectionless No handshaking between
sending and receiving adapter Unreliable receiving adapter doesnrsquot send
acks or nacks to sending adapter Data field is 46bytes -1500 bytes
bull If data less than 46 bytes stuff to be 46bytesndash Network layer uses ldquolengthrdquo field to remove stuffing
5 DataLink Layer 5-21
Ethernet uses CSMACD
No slots (no sync clock) Preamble in Ethernet
frame is used to sync clock between sender and receiver
adapter doesnrsquot transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-22
Ethernet CSMACD algorithm
1 Adaptor receives datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal (48-bit Why)
5 After aborting adapter enters exponential backoff after the m-th collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-23
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random wait
will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Why exponentialWhy random number picking
5 DataLink Layer 5-24
CSMACD efficiency Tprop = max prop between 2 nodes in LAN
ttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0
Goes to 1 as ttrans goes to infinity Much better than ALOHA but still decentralized simple and cheap
Why
transprop tt 51
1efficiency
5 DataLink Layer 5-25
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-26
Manchester encoding
Used in 10BaseT Each bit has a transition Allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
Link Layer 5-27
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
5 DataLink Layer 5-28
10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-29
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other links at the same rate no frame buffering no CSMACD at hub adapters detect collisions provides net management functionality
twisted pair
hub
5 DataLink Layer 5-30
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-31
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-32
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-33
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
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
5 DataLink Layer 5-34
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif 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
forward on all but the interface on which the frame arrived
Link Layer 5-35
Interconnecting switches
switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-36
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-37
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-38
Summary comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
Link Layer 5-39
VLANs motivation (virtual LAN)
consider CS user moves office to EE
but wants connect to CS switch
single broadcast domain all layer-2 broadcast
traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
securityprivacy efficiency issues
Computer Science Electrical
Engineering
ComputerEngineering
Link Layer 5-40
VLANsport-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network 1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Link Layer 5-41
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined
switches plus routers
Link Layer 5-42
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be
vanilla 8021 frames (must carry VLAN ID info) 8021q protocol addsremoved additional header fields for
frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
Link Layer 5-43
type
2-byte Tag Protocol Identifier (value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
destaddress
sourceaddress data (payload) CRCpreamble
data (payload) CRC
type
Link Layer 5-44
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-45
Multiprotocol label switching (MPLS)
initial goal high-speed IP forwarding using fixed length label (instead of IP address) fast lookup using fixed length identifier (rather than
shortest prefix matching) borrowing ideas from Virtual Circuit (VC) approach but IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
Link Layer 5-46
MPLS capable routers
aka label-switched router forward packets to outgoing interface based only
on label value (donrsquot inspect IP address) MPLS forwarding table distinct from IP forwarding tables
flexibility MPLS forwarding decisions can differ from those of IP use destination and source addresses to route flows to
same destination differently (traffic engineering) re-route flows quickly if link fails pre-computed backup
paths (useful for VoIP)
Link Layer 5-47
R2
D
R3R5
A
R6
MPLS versus IP paths
IP router IP routing path to destination
determined by destination address alone
R4
Link Layer 5-48
R2
D
R3R4
R5
A
R6
MPLS versus IP paths
IP-onlyrouter
IP routing path to destination determined by destination address alone
MPLS and IP router
MPLS routing path to destination can be based on source and dest address fast reroute precompute backup routes
in case of link failure
entry router (R4) can use different MPLS routes to A based eg on source address
Link Layer 5-49
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-50
Data center networks
10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximity e-business (eg Amazon) content-servers (eg YouTube Akamai Apple Microsoft) search engines data mining (eg Google)
challenges multiple applications each
serving massive numbers of clients
managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container Chicago data center
Link Layer 5-51
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
Load balancer
Load balancer
B
1 2 3 4 5 6 7 8
A C
Border router
Access router
Internet
Data center networks
load balancer application-layer routingreceives external client requestsdirects workload within data centerreturns results to external client (hiding data center internals from client)
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
1 2 3 4 5 6 7 8
Data center networks rich interconnection among switches racks
increased throughput between racks (multiple routing paths possible) increased reliability via redundancy
Link Layer 5-53
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-54
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer5-55
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
web page
browser
router(runs DHCP)
Link Layer 5-56
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router(runs DHCP)
Link Layer 5-57
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router(runs DHCP)
Link Layer 5-58
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
router(runs DHCP)
Link Layer 5-59
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server DNS server replies to
client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router(runs DHCP)
Link Layer 5-60
A day in the lifehellipTCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router(runs DHCP)
Link Layer 5-61
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-62
Chapter 5 Summary
principles behind data link layer services error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS VLANs virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-63
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless multimedia security network management
5 DataLink Layer 5-19
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol
otherwise adapter discards frame
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-20
Unreliable connectionless service Connectionless No handshaking between
sending and receiving adapter Unreliable receiving adapter doesnrsquot send
acks or nacks to sending adapter Data field is 46bytes -1500 bytes
bull If data less than 46 bytes stuff to be 46bytesndash Network layer uses ldquolengthrdquo field to remove stuffing
5 DataLink Layer 5-21
Ethernet uses CSMACD
No slots (no sync clock) Preamble in Ethernet
frame is used to sync clock between sender and receiver
adapter doesnrsquot transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-22
Ethernet CSMACD algorithm
1 Adaptor receives datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal (48-bit Why)
5 After aborting adapter enters exponential backoff after the m-th collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-23
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random wait
will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Why exponentialWhy random number picking
5 DataLink Layer 5-24
CSMACD efficiency Tprop = max prop between 2 nodes in LAN
ttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0
Goes to 1 as ttrans goes to infinity Much better than ALOHA but still decentralized simple and cheap
Why
transprop tt 51
1efficiency
5 DataLink Layer 5-25
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-26
Manchester encoding
Used in 10BaseT Each bit has a transition Allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
Link Layer 5-27
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
5 DataLink Layer 5-28
10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-29
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other links at the same rate no frame buffering no CSMACD at hub adapters detect collisions provides net management functionality
twisted pair
hub
5 DataLink Layer 5-30
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-31
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-32
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-33
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
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
5 DataLink Layer 5-34
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif 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
forward on all but the interface on which the frame arrived
Link Layer 5-35
Interconnecting switches
switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-36
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-37
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-38
Summary comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
Link Layer 5-39
VLANs motivation (virtual LAN)
consider CS user moves office to EE
but wants connect to CS switch
single broadcast domain all layer-2 broadcast
traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
securityprivacy efficiency issues
Computer Science Electrical
Engineering
ComputerEngineering
Link Layer 5-40
VLANsport-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network 1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Link Layer 5-41
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined
switches plus routers
Link Layer 5-42
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be
vanilla 8021 frames (must carry VLAN ID info) 8021q protocol addsremoved additional header fields for
frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
Link Layer 5-43
type
2-byte Tag Protocol Identifier (value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
destaddress
sourceaddress data (payload) CRCpreamble
data (payload) CRC
type
Link Layer 5-44
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-45
Multiprotocol label switching (MPLS)
initial goal high-speed IP forwarding using fixed length label (instead of IP address) fast lookup using fixed length identifier (rather than
shortest prefix matching) borrowing ideas from Virtual Circuit (VC) approach but IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
Link Layer 5-46
MPLS capable routers
aka label-switched router forward packets to outgoing interface based only
on label value (donrsquot inspect IP address) MPLS forwarding table distinct from IP forwarding tables
flexibility MPLS forwarding decisions can differ from those of IP use destination and source addresses to route flows to
same destination differently (traffic engineering) re-route flows quickly if link fails pre-computed backup
paths (useful for VoIP)
Link Layer 5-47
R2
D
R3R5
A
R6
MPLS versus IP paths
IP router IP routing path to destination
determined by destination address alone
R4
Link Layer 5-48
R2
D
R3R4
R5
A
R6
MPLS versus IP paths
IP-onlyrouter
IP routing path to destination determined by destination address alone
MPLS and IP router
MPLS routing path to destination can be based on source and dest address fast reroute precompute backup routes
in case of link failure
entry router (R4) can use different MPLS routes to A based eg on source address
Link Layer 5-49
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-50
Data center networks
10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximity e-business (eg Amazon) content-servers (eg YouTube Akamai Apple Microsoft) search engines data mining (eg Google)
challenges multiple applications each
serving massive numbers of clients
managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container Chicago data center
Link Layer 5-51
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
Load balancer
Load balancer
B
1 2 3 4 5 6 7 8
A C
Border router
Access router
Internet
Data center networks
load balancer application-layer routingreceives external client requestsdirects workload within data centerreturns results to external client (hiding data center internals from client)
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
1 2 3 4 5 6 7 8
Data center networks rich interconnection among switches racks
increased throughput between racks (multiple routing paths possible) increased reliability via redundancy
Link Layer 5-53
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-54
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer5-55
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
web page
browser
router(runs DHCP)
Link Layer 5-56
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router(runs DHCP)
Link Layer 5-57
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router(runs DHCP)
Link Layer 5-58
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
router(runs DHCP)
Link Layer 5-59
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server DNS server replies to
client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router(runs DHCP)
Link Layer 5-60
A day in the lifehellipTCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router(runs DHCP)
Link Layer 5-61
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-62
Chapter 5 Summary
principles behind data link layer services error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS VLANs virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-63
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless multimedia security network management
5 DataLink Layer 5-20
Unreliable connectionless service Connectionless No handshaking between
sending and receiving adapter Unreliable receiving adapter doesnrsquot send
acks or nacks to sending adapter Data field is 46bytes -1500 bytes
bull If data less than 46 bytes stuff to be 46bytesndash Network layer uses ldquolengthrdquo field to remove stuffing
5 DataLink Layer 5-21
Ethernet uses CSMACD
No slots (no sync clock) Preamble in Ethernet
frame is used to sync clock between sender and receiver
adapter doesnrsquot transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-22
Ethernet CSMACD algorithm
1 Adaptor receives datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal (48-bit Why)
5 After aborting adapter enters exponential backoff after the m-th collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-23
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random wait
will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Why exponentialWhy random number picking
5 DataLink Layer 5-24
CSMACD efficiency Tprop = max prop between 2 nodes in LAN
ttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0
Goes to 1 as ttrans goes to infinity Much better than ALOHA but still decentralized simple and cheap
Why
transprop tt 51
1efficiency
5 DataLink Layer 5-25
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-26
Manchester encoding
Used in 10BaseT Each bit has a transition Allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
Link Layer 5-27
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
5 DataLink Layer 5-28
10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-29
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other links at the same rate no frame buffering no CSMACD at hub adapters detect collisions provides net management functionality
twisted pair
hub
5 DataLink Layer 5-30
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-31
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-32
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-33
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
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
5 DataLink Layer 5-34
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif 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
forward on all but the interface on which the frame arrived
Link Layer 5-35
Interconnecting switches
switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-36
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-37
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-38
Summary comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
Link Layer 5-39
VLANs motivation (virtual LAN)
consider CS user moves office to EE
but wants connect to CS switch
single broadcast domain all layer-2 broadcast
traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
securityprivacy efficiency issues
Computer Science Electrical
Engineering
ComputerEngineering
Link Layer 5-40
VLANsport-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network 1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Link Layer 5-41
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined
switches plus routers
Link Layer 5-42
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be
vanilla 8021 frames (must carry VLAN ID info) 8021q protocol addsremoved additional header fields for
frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
Link Layer 5-43
type
2-byte Tag Protocol Identifier (value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
destaddress
sourceaddress data (payload) CRCpreamble
data (payload) CRC
type
Link Layer 5-44
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-45
Multiprotocol label switching (MPLS)
initial goal high-speed IP forwarding using fixed length label (instead of IP address) fast lookup using fixed length identifier (rather than
shortest prefix matching) borrowing ideas from Virtual Circuit (VC) approach but IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
Link Layer 5-46
MPLS capable routers
aka label-switched router forward packets to outgoing interface based only
on label value (donrsquot inspect IP address) MPLS forwarding table distinct from IP forwarding tables
flexibility MPLS forwarding decisions can differ from those of IP use destination and source addresses to route flows to
same destination differently (traffic engineering) re-route flows quickly if link fails pre-computed backup
paths (useful for VoIP)
Link Layer 5-47
R2
D
R3R5
A
R6
MPLS versus IP paths
IP router IP routing path to destination
determined by destination address alone
R4
Link Layer 5-48
R2
D
R3R4
R5
A
R6
MPLS versus IP paths
IP-onlyrouter
IP routing path to destination determined by destination address alone
MPLS and IP router
MPLS routing path to destination can be based on source and dest address fast reroute precompute backup routes
in case of link failure
entry router (R4) can use different MPLS routes to A based eg on source address
Link Layer 5-49
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-50
Data center networks
10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximity e-business (eg Amazon) content-servers (eg YouTube Akamai Apple Microsoft) search engines data mining (eg Google)
challenges multiple applications each
serving massive numbers of clients
managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container Chicago data center
Link Layer 5-51
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
Load balancer
Load balancer
B
1 2 3 4 5 6 7 8
A C
Border router
Access router
Internet
Data center networks
load balancer application-layer routingreceives external client requestsdirects workload within data centerreturns results to external client (hiding data center internals from client)
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
1 2 3 4 5 6 7 8
Data center networks rich interconnection among switches racks
increased throughput between racks (multiple routing paths possible) increased reliability via redundancy
Link Layer 5-53
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-54
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer5-55
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
web page
browser
router(runs DHCP)
Link Layer 5-56
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router(runs DHCP)
Link Layer 5-57
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router(runs DHCP)
Link Layer 5-58
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
router(runs DHCP)
Link Layer 5-59
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server DNS server replies to
client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router(runs DHCP)
Link Layer 5-60
A day in the lifehellipTCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router(runs DHCP)
Link Layer 5-61
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-62
Chapter 5 Summary
principles behind data link layer services error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS VLANs virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-63
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless multimedia security network management
5 DataLink Layer 5-21
Ethernet uses CSMACD
No slots (no sync clock) Preamble in Ethernet
frame is used to sync clock between sender and receiver
adapter doesnrsquot transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-22
Ethernet CSMACD algorithm
1 Adaptor receives datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal (48-bit Why)
5 After aborting adapter enters exponential backoff after the m-th collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-23
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random wait
will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Why exponentialWhy random number picking
5 DataLink Layer 5-24
CSMACD efficiency Tprop = max prop between 2 nodes in LAN
ttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0
Goes to 1 as ttrans goes to infinity Much better than ALOHA but still decentralized simple and cheap
Why
transprop tt 51
1efficiency
5 DataLink Layer 5-25
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-26
Manchester encoding
Used in 10BaseT Each bit has a transition Allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
Link Layer 5-27
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
5 DataLink Layer 5-28
10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-29
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other links at the same rate no frame buffering no CSMACD at hub adapters detect collisions provides net management functionality
twisted pair
hub
5 DataLink Layer 5-30
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-31
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-32
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-33
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
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
5 DataLink Layer 5-34
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif 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
forward on all but the interface on which the frame arrived
Link Layer 5-35
Interconnecting switches
switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-36
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-37
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-38
Summary comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
Link Layer 5-39
VLANs motivation (virtual LAN)
consider CS user moves office to EE
but wants connect to CS switch
single broadcast domain all layer-2 broadcast
traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
securityprivacy efficiency issues
Computer Science Electrical
Engineering
ComputerEngineering
Link Layer 5-40
VLANsport-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network 1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Link Layer 5-41
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined
switches plus routers
Link Layer 5-42
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be
vanilla 8021 frames (must carry VLAN ID info) 8021q protocol addsremoved additional header fields for
frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
Link Layer 5-43
type
2-byte Tag Protocol Identifier (value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
destaddress
sourceaddress data (payload) CRCpreamble
data (payload) CRC
type
Link Layer 5-44
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-45
Multiprotocol label switching (MPLS)
initial goal high-speed IP forwarding using fixed length label (instead of IP address) fast lookup using fixed length identifier (rather than
shortest prefix matching) borrowing ideas from Virtual Circuit (VC) approach but IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
Link Layer 5-46
MPLS capable routers
aka label-switched router forward packets to outgoing interface based only
on label value (donrsquot inspect IP address) MPLS forwarding table distinct from IP forwarding tables
flexibility MPLS forwarding decisions can differ from those of IP use destination and source addresses to route flows to
same destination differently (traffic engineering) re-route flows quickly if link fails pre-computed backup
paths (useful for VoIP)
Link Layer 5-47
R2
D
R3R5
A
R6
MPLS versus IP paths
IP router IP routing path to destination
determined by destination address alone
R4
Link Layer 5-48
R2
D
R3R4
R5
A
R6
MPLS versus IP paths
IP-onlyrouter
IP routing path to destination determined by destination address alone
MPLS and IP router
MPLS routing path to destination can be based on source and dest address fast reroute precompute backup routes
in case of link failure
entry router (R4) can use different MPLS routes to A based eg on source address
Link Layer 5-49
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-50
Data center networks
10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximity e-business (eg Amazon) content-servers (eg YouTube Akamai Apple Microsoft) search engines data mining (eg Google)
challenges multiple applications each
serving massive numbers of clients
managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container Chicago data center
Link Layer 5-51
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
Load balancer
Load balancer
B
1 2 3 4 5 6 7 8
A C
Border router
Access router
Internet
Data center networks
load balancer application-layer routingreceives external client requestsdirects workload within data centerreturns results to external client (hiding data center internals from client)
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
1 2 3 4 5 6 7 8
Data center networks rich interconnection among switches racks
increased throughput between racks (multiple routing paths possible) increased reliability via redundancy
Link Layer 5-53
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-54
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer5-55
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
web page
browser
router(runs DHCP)
Link Layer 5-56
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router(runs DHCP)
Link Layer 5-57
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router(runs DHCP)
Link Layer 5-58
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
router(runs DHCP)
Link Layer 5-59
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server DNS server replies to
client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router(runs DHCP)
Link Layer 5-60
A day in the lifehellipTCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router(runs DHCP)
Link Layer 5-61
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-62
Chapter 5 Summary
principles behind data link layer services error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS VLANs virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-63
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless multimedia security network management
5 DataLink Layer 5-22
Ethernet CSMACD algorithm
1 Adaptor receives datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal (48-bit Why)
5 After aborting adapter enters exponential backoff after the m-th collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-23
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random wait
will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Why exponentialWhy random number picking
5 DataLink Layer 5-24
CSMACD efficiency Tprop = max prop between 2 nodes in LAN
ttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0
Goes to 1 as ttrans goes to infinity Much better than ALOHA but still decentralized simple and cheap
Why
transprop tt 51
1efficiency
5 DataLink Layer 5-25
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-26
Manchester encoding
Used in 10BaseT Each bit has a transition Allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
Link Layer 5-27
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
5 DataLink Layer 5-28
10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-29
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other links at the same rate no frame buffering no CSMACD at hub adapters detect collisions provides net management functionality
twisted pair
hub
5 DataLink Layer 5-30
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-31
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-32
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-33
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
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
5 DataLink Layer 5-34
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif 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
forward on all but the interface on which the frame arrived
Link Layer 5-35
Interconnecting switches
switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-36
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-37
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-38
Summary comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
Link Layer 5-39
VLANs motivation (virtual LAN)
consider CS user moves office to EE
but wants connect to CS switch
single broadcast domain all layer-2 broadcast
traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
securityprivacy efficiency issues
Computer Science Electrical
Engineering
ComputerEngineering
Link Layer 5-40
VLANsport-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network 1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Link Layer 5-41
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined
switches plus routers
Link Layer 5-42
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be
vanilla 8021 frames (must carry VLAN ID info) 8021q protocol addsremoved additional header fields for
frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
Link Layer 5-43
type
2-byte Tag Protocol Identifier (value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
destaddress
sourceaddress data (payload) CRCpreamble
data (payload) CRC
type
Link Layer 5-44
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-45
Multiprotocol label switching (MPLS)
initial goal high-speed IP forwarding using fixed length label (instead of IP address) fast lookup using fixed length identifier (rather than
shortest prefix matching) borrowing ideas from Virtual Circuit (VC) approach but IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
Link Layer 5-46
MPLS capable routers
aka label-switched router forward packets to outgoing interface based only
on label value (donrsquot inspect IP address) MPLS forwarding table distinct from IP forwarding tables
flexibility MPLS forwarding decisions can differ from those of IP use destination and source addresses to route flows to
same destination differently (traffic engineering) re-route flows quickly if link fails pre-computed backup
paths (useful for VoIP)
Link Layer 5-47
R2
D
R3R5
A
R6
MPLS versus IP paths
IP router IP routing path to destination
determined by destination address alone
R4
Link Layer 5-48
R2
D
R3R4
R5
A
R6
MPLS versus IP paths
IP-onlyrouter
IP routing path to destination determined by destination address alone
MPLS and IP router
MPLS routing path to destination can be based on source and dest address fast reroute precompute backup routes
in case of link failure
entry router (R4) can use different MPLS routes to A based eg on source address
Link Layer 5-49
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-50
Data center networks
10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximity e-business (eg Amazon) content-servers (eg YouTube Akamai Apple Microsoft) search engines data mining (eg Google)
challenges multiple applications each
serving massive numbers of clients
managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container Chicago data center
Link Layer 5-51
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
Load balancer
Load balancer
B
1 2 3 4 5 6 7 8
A C
Border router
Access router
Internet
Data center networks
load balancer application-layer routingreceives external client requestsdirects workload within data centerreturns results to external client (hiding data center internals from client)
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
1 2 3 4 5 6 7 8
Data center networks rich interconnection among switches racks
increased throughput between racks (multiple routing paths possible) increased reliability via redundancy
Link Layer 5-53
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-54
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer5-55
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
web page
browser
router(runs DHCP)
Link Layer 5-56
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router(runs DHCP)
Link Layer 5-57
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router(runs DHCP)
Link Layer 5-58
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
router(runs DHCP)
Link Layer 5-59
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server DNS server replies to
client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router(runs DHCP)
Link Layer 5-60
A day in the lifehellipTCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router(runs DHCP)
Link Layer 5-61
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-62
Chapter 5 Summary
principles behind data link layer services error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS VLANs virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-63
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless multimedia security network management
5 DataLink Layer 5-23
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random wait
will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Why exponentialWhy random number picking
5 DataLink Layer 5-24
CSMACD efficiency Tprop = max prop between 2 nodes in LAN
ttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0
Goes to 1 as ttrans goes to infinity Much better than ALOHA but still decentralized simple and cheap
Why
transprop tt 51
1efficiency
5 DataLink Layer 5-25
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-26
Manchester encoding
Used in 10BaseT Each bit has a transition Allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
Link Layer 5-27
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
5 DataLink Layer 5-28
10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-29
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other links at the same rate no frame buffering no CSMACD at hub adapters detect collisions provides net management functionality
twisted pair
hub
5 DataLink Layer 5-30
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-31
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-32
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-33
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
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
5 DataLink Layer 5-34
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif 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
forward on all but the interface on which the frame arrived
Link Layer 5-35
Interconnecting switches
switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-36
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-37
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-38
Summary comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
Link Layer 5-39
VLANs motivation (virtual LAN)
consider CS user moves office to EE
but wants connect to CS switch
single broadcast domain all layer-2 broadcast
traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
securityprivacy efficiency issues
Computer Science Electrical
Engineering
ComputerEngineering
Link Layer 5-40
VLANsport-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network 1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Link Layer 5-41
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined
switches plus routers
Link Layer 5-42
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be
vanilla 8021 frames (must carry VLAN ID info) 8021q protocol addsremoved additional header fields for
frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
Link Layer 5-43
type
2-byte Tag Protocol Identifier (value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
destaddress
sourceaddress data (payload) CRCpreamble
data (payload) CRC
type
Link Layer 5-44
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-45
Multiprotocol label switching (MPLS)
initial goal high-speed IP forwarding using fixed length label (instead of IP address) fast lookup using fixed length identifier (rather than
shortest prefix matching) borrowing ideas from Virtual Circuit (VC) approach but IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
Link Layer 5-46
MPLS capable routers
aka label-switched router forward packets to outgoing interface based only
on label value (donrsquot inspect IP address) MPLS forwarding table distinct from IP forwarding tables
flexibility MPLS forwarding decisions can differ from those of IP use destination and source addresses to route flows to
same destination differently (traffic engineering) re-route flows quickly if link fails pre-computed backup
paths (useful for VoIP)
Link Layer 5-47
R2
D
R3R5
A
R6
MPLS versus IP paths
IP router IP routing path to destination
determined by destination address alone
R4
Link Layer 5-48
R2
D
R3R4
R5
A
R6
MPLS versus IP paths
IP-onlyrouter
IP routing path to destination determined by destination address alone
MPLS and IP router
MPLS routing path to destination can be based on source and dest address fast reroute precompute backup routes
in case of link failure
entry router (R4) can use different MPLS routes to A based eg on source address
Link Layer 5-49
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-50
Data center networks
10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximity e-business (eg Amazon) content-servers (eg YouTube Akamai Apple Microsoft) search engines data mining (eg Google)
challenges multiple applications each
serving massive numbers of clients
managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container Chicago data center
Link Layer 5-51
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
Load balancer
Load balancer
B
1 2 3 4 5 6 7 8
A C
Border router
Access router
Internet
Data center networks
load balancer application-layer routingreceives external client requestsdirects workload within data centerreturns results to external client (hiding data center internals from client)
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
1 2 3 4 5 6 7 8
Data center networks rich interconnection among switches racks
increased throughput between racks (multiple routing paths possible) increased reliability via redundancy
Link Layer 5-53
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-54
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer5-55
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
web page
browser
router(runs DHCP)
Link Layer 5-56
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router(runs DHCP)
Link Layer 5-57
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router(runs DHCP)
Link Layer 5-58
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
router(runs DHCP)
Link Layer 5-59
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server DNS server replies to
client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router(runs DHCP)
Link Layer 5-60
A day in the lifehellipTCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router(runs DHCP)
Link Layer 5-61
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-62
Chapter 5 Summary
principles behind data link layer services error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS VLANs virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-63
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless multimedia security network management
5 DataLink Layer 5-24
CSMACD efficiency Tprop = max prop between 2 nodes in LAN
ttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0
Goes to 1 as ttrans goes to infinity Much better than ALOHA but still decentralized simple and cheap
Why
transprop tt 51
1efficiency
5 DataLink Layer 5-25
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-26
Manchester encoding
Used in 10BaseT Each bit has a transition Allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
Link Layer 5-27
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
5 DataLink Layer 5-28
10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-29
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other links at the same rate no frame buffering no CSMACD at hub adapters detect collisions provides net management functionality
twisted pair
hub
5 DataLink Layer 5-30
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-31
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-32
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-33
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
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
5 DataLink Layer 5-34
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif 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
forward on all but the interface on which the frame arrived
Link Layer 5-35
Interconnecting switches
switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-36
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-37
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-38
Summary comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
Link Layer 5-39
VLANs motivation (virtual LAN)
consider CS user moves office to EE
but wants connect to CS switch
single broadcast domain all layer-2 broadcast
traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
securityprivacy efficiency issues
Computer Science Electrical
Engineering
ComputerEngineering
Link Layer 5-40
VLANsport-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network 1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Link Layer 5-41
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined
switches plus routers
Link Layer 5-42
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be
vanilla 8021 frames (must carry VLAN ID info) 8021q protocol addsremoved additional header fields for
frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
Link Layer 5-43
type
2-byte Tag Protocol Identifier (value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
destaddress
sourceaddress data (payload) CRCpreamble
data (payload) CRC
type
Link Layer 5-44
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-45
Multiprotocol label switching (MPLS)
initial goal high-speed IP forwarding using fixed length label (instead of IP address) fast lookup using fixed length identifier (rather than
shortest prefix matching) borrowing ideas from Virtual Circuit (VC) approach but IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
Link Layer 5-46
MPLS capable routers
aka label-switched router forward packets to outgoing interface based only
on label value (donrsquot inspect IP address) MPLS forwarding table distinct from IP forwarding tables
flexibility MPLS forwarding decisions can differ from those of IP use destination and source addresses to route flows to
same destination differently (traffic engineering) re-route flows quickly if link fails pre-computed backup
paths (useful for VoIP)
Link Layer 5-47
R2
D
R3R5
A
R6
MPLS versus IP paths
IP router IP routing path to destination
determined by destination address alone
R4
Link Layer 5-48
R2
D
R3R4
R5
A
R6
MPLS versus IP paths
IP-onlyrouter
IP routing path to destination determined by destination address alone
MPLS and IP router
MPLS routing path to destination can be based on source and dest address fast reroute precompute backup routes
in case of link failure
entry router (R4) can use different MPLS routes to A based eg on source address
Link Layer 5-49
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-50
Data center networks
10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximity e-business (eg Amazon) content-servers (eg YouTube Akamai Apple Microsoft) search engines data mining (eg Google)
challenges multiple applications each
serving massive numbers of clients
managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container Chicago data center
Link Layer 5-51
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
Load balancer
Load balancer
B
1 2 3 4 5 6 7 8
A C
Border router
Access router
Internet
Data center networks
load balancer application-layer routingreceives external client requestsdirects workload within data centerreturns results to external client (hiding data center internals from client)
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
1 2 3 4 5 6 7 8
Data center networks rich interconnection among switches racks
increased throughput between racks (multiple routing paths possible) increased reliability via redundancy
Link Layer 5-53
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-54
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer5-55
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
web page
browser
router(runs DHCP)
Link Layer 5-56
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router(runs DHCP)
Link Layer 5-57
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router(runs DHCP)
Link Layer 5-58
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
router(runs DHCP)
Link Layer 5-59
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server DNS server replies to
client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router(runs DHCP)
Link Layer 5-60
A day in the lifehellipTCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router(runs DHCP)
Link Layer 5-61
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-62
Chapter 5 Summary
principles behind data link layer services error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS VLANs virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-63
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless multimedia security network management
5 DataLink Layer 5-25
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-26
Manchester encoding
Used in 10BaseT Each bit has a transition Allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
Link Layer 5-27
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
5 DataLink Layer 5-28
10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-29
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other links at the same rate no frame buffering no CSMACD at hub adapters detect collisions provides net management functionality
twisted pair
hub
5 DataLink Layer 5-30
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-31
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-32
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-33
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
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
5 DataLink Layer 5-34
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif 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
forward on all but the interface on which the frame arrived
Link Layer 5-35
Interconnecting switches
switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-36
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-37
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-38
Summary comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
Link Layer 5-39
VLANs motivation (virtual LAN)
consider CS user moves office to EE
but wants connect to CS switch
single broadcast domain all layer-2 broadcast
traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
securityprivacy efficiency issues
Computer Science Electrical
Engineering
ComputerEngineering
Link Layer 5-40
VLANsport-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network 1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Link Layer 5-41
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined
switches plus routers
Link Layer 5-42
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be
vanilla 8021 frames (must carry VLAN ID info) 8021q protocol addsremoved additional header fields for
frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
Link Layer 5-43
type
2-byte Tag Protocol Identifier (value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
destaddress
sourceaddress data (payload) CRCpreamble
data (payload) CRC
type
Link Layer 5-44
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-45
Multiprotocol label switching (MPLS)
initial goal high-speed IP forwarding using fixed length label (instead of IP address) fast lookup using fixed length identifier (rather than
shortest prefix matching) borrowing ideas from Virtual Circuit (VC) approach but IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
Link Layer 5-46
MPLS capable routers
aka label-switched router forward packets to outgoing interface based only
on label value (donrsquot inspect IP address) MPLS forwarding table distinct from IP forwarding tables
flexibility MPLS forwarding decisions can differ from those of IP use destination and source addresses to route flows to
same destination differently (traffic engineering) re-route flows quickly if link fails pre-computed backup
paths (useful for VoIP)
Link Layer 5-47
R2
D
R3R5
A
R6
MPLS versus IP paths
IP router IP routing path to destination
determined by destination address alone
R4
Link Layer 5-48
R2
D
R3R4
R5
A
R6
MPLS versus IP paths
IP-onlyrouter
IP routing path to destination determined by destination address alone
MPLS and IP router
MPLS routing path to destination can be based on source and dest address fast reroute precompute backup routes
in case of link failure
entry router (R4) can use different MPLS routes to A based eg on source address
Link Layer 5-49
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-50
Data center networks
10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximity e-business (eg Amazon) content-servers (eg YouTube Akamai Apple Microsoft) search engines data mining (eg Google)
challenges multiple applications each
serving massive numbers of clients
managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container Chicago data center
Link Layer 5-51
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
Load balancer
Load balancer
B
1 2 3 4 5 6 7 8
A C
Border router
Access router
Internet
Data center networks
load balancer application-layer routingreceives external client requestsdirects workload within data centerreturns results to external client (hiding data center internals from client)
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
1 2 3 4 5 6 7 8
Data center networks rich interconnection among switches racks
increased throughput between racks (multiple routing paths possible) increased reliability via redundancy
Link Layer 5-53
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-54
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer5-55
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
web page
browser
router(runs DHCP)
Link Layer 5-56
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router(runs DHCP)
Link Layer 5-57
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router(runs DHCP)
Link Layer 5-58
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
router(runs DHCP)
Link Layer 5-59
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server DNS server replies to
client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router(runs DHCP)
Link Layer 5-60
A day in the lifehellipTCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router(runs DHCP)
Link Layer 5-61
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-62
Chapter 5 Summary
principles behind data link layer services error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS VLANs virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-63
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless multimedia security network management
5 DataLink Layer 5-26
Manchester encoding
Used in 10BaseT Each bit has a transition Allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
Link Layer 5-27
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
5 DataLink Layer 5-28
10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-29
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other links at the same rate no frame buffering no CSMACD at hub adapters detect collisions provides net management functionality
twisted pair
hub
5 DataLink Layer 5-30
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-31
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-32
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-33
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
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
5 DataLink Layer 5-34
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif 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
forward on all but the interface on which the frame arrived
Link Layer 5-35
Interconnecting switches
switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-36
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-37
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-38
Summary comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
Link Layer 5-39
VLANs motivation (virtual LAN)
consider CS user moves office to EE
but wants connect to CS switch
single broadcast domain all layer-2 broadcast
traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
securityprivacy efficiency issues
Computer Science Electrical
Engineering
ComputerEngineering
Link Layer 5-40
VLANsport-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network 1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Link Layer 5-41
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined
switches plus routers
Link Layer 5-42
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be
vanilla 8021 frames (must carry VLAN ID info) 8021q protocol addsremoved additional header fields for
frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
Link Layer 5-43
type
2-byte Tag Protocol Identifier (value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
destaddress
sourceaddress data (payload) CRCpreamble
data (payload) CRC
type
Link Layer 5-44
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-45
Multiprotocol label switching (MPLS)
initial goal high-speed IP forwarding using fixed length label (instead of IP address) fast lookup using fixed length identifier (rather than
shortest prefix matching) borrowing ideas from Virtual Circuit (VC) approach but IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
Link Layer 5-46
MPLS capable routers
aka label-switched router forward packets to outgoing interface based only
on label value (donrsquot inspect IP address) MPLS forwarding table distinct from IP forwarding tables
flexibility MPLS forwarding decisions can differ from those of IP use destination and source addresses to route flows to
same destination differently (traffic engineering) re-route flows quickly if link fails pre-computed backup
paths (useful for VoIP)
Link Layer 5-47
R2
D
R3R5
A
R6
MPLS versus IP paths
IP router IP routing path to destination
determined by destination address alone
R4
Link Layer 5-48
R2
D
R3R4
R5
A
R6
MPLS versus IP paths
IP-onlyrouter
IP routing path to destination determined by destination address alone
MPLS and IP router
MPLS routing path to destination can be based on source and dest address fast reroute precompute backup routes
in case of link failure
entry router (R4) can use different MPLS routes to A based eg on source address
Link Layer 5-49
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-50
Data center networks
10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximity e-business (eg Amazon) content-servers (eg YouTube Akamai Apple Microsoft) search engines data mining (eg Google)
challenges multiple applications each
serving massive numbers of clients
managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container Chicago data center
Link Layer 5-51
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
Load balancer
Load balancer
B
1 2 3 4 5 6 7 8
A C
Border router
Access router
Internet
Data center networks
load balancer application-layer routingreceives external client requestsdirects workload within data centerreturns results to external client (hiding data center internals from client)
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
1 2 3 4 5 6 7 8
Data center networks rich interconnection among switches racks
increased throughput between racks (multiple routing paths possible) increased reliability via redundancy
Link Layer 5-53
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-54
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer5-55
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
web page
browser
router(runs DHCP)
Link Layer 5-56
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router(runs DHCP)
Link Layer 5-57
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router(runs DHCP)
Link Layer 5-58
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
router(runs DHCP)
Link Layer 5-59
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server DNS server replies to
client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router(runs DHCP)
Link Layer 5-60
A day in the lifehellipTCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router(runs DHCP)
Link Layer 5-61
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-62
Chapter 5 Summary
principles behind data link layer services error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS VLANs virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-63
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless multimedia security network management
Link Layer 5-27
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
5 DataLink Layer 5-28
10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-29
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other links at the same rate no frame buffering no CSMACD at hub adapters detect collisions provides net management functionality
twisted pair
hub
5 DataLink Layer 5-30
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-31
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-32
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-33
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
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
5 DataLink Layer 5-34
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif 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
forward on all but the interface on which the frame arrived
Link Layer 5-35
Interconnecting switches
switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-36
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-37
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-38
Summary comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
Link Layer 5-39
VLANs motivation (virtual LAN)
consider CS user moves office to EE
but wants connect to CS switch
single broadcast domain all layer-2 broadcast
traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
securityprivacy efficiency issues
Computer Science Electrical
Engineering
ComputerEngineering
Link Layer 5-40
VLANsport-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network 1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Link Layer 5-41
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined
switches plus routers
Link Layer 5-42
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be
vanilla 8021 frames (must carry VLAN ID info) 8021q protocol addsremoved additional header fields for
frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
Link Layer 5-43
type
2-byte Tag Protocol Identifier (value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
destaddress
sourceaddress data (payload) CRCpreamble
data (payload) CRC
type
Link Layer 5-44
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-45
Multiprotocol label switching (MPLS)
initial goal high-speed IP forwarding using fixed length label (instead of IP address) fast lookup using fixed length identifier (rather than
shortest prefix matching) borrowing ideas from Virtual Circuit (VC) approach but IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
Link Layer 5-46
MPLS capable routers
aka label-switched router forward packets to outgoing interface based only
on label value (donrsquot inspect IP address) MPLS forwarding table distinct from IP forwarding tables
flexibility MPLS forwarding decisions can differ from those of IP use destination and source addresses to route flows to
same destination differently (traffic engineering) re-route flows quickly if link fails pre-computed backup
paths (useful for VoIP)
Link Layer 5-47
R2
D
R3R5
A
R6
MPLS versus IP paths
IP router IP routing path to destination
determined by destination address alone
R4
Link Layer 5-48
R2
D
R3R4
R5
A
R6
MPLS versus IP paths
IP-onlyrouter
IP routing path to destination determined by destination address alone
MPLS and IP router
MPLS routing path to destination can be based on source and dest address fast reroute precompute backup routes
in case of link failure
entry router (R4) can use different MPLS routes to A based eg on source address
Link Layer 5-49
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-50
Data center networks
10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximity e-business (eg Amazon) content-servers (eg YouTube Akamai Apple Microsoft) search engines data mining (eg Google)
challenges multiple applications each
serving massive numbers of clients
managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container Chicago data center
Link Layer 5-51
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
Load balancer
Load balancer
B
1 2 3 4 5 6 7 8
A C
Border router
Access router
Internet
Data center networks
load balancer application-layer routingreceives external client requestsdirects workload within data centerreturns results to external client (hiding data center internals from client)
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
1 2 3 4 5 6 7 8
Data center networks rich interconnection among switches racks
increased throughput between racks (multiple routing paths possible) increased reliability via redundancy
Link Layer 5-53
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-54
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer5-55
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
web page
browser
router(runs DHCP)
Link Layer 5-56
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router(runs DHCP)
Link Layer 5-57
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router(runs DHCP)
Link Layer 5-58
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
router(runs DHCP)
Link Layer 5-59
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server DNS server replies to
client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router(runs DHCP)
Link Layer 5-60
A day in the lifehellipTCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router(runs DHCP)
Link Layer 5-61
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-62
Chapter 5 Summary
principles behind data link layer services error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS VLANs virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-63
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless multimedia security network management
5 DataLink Layer 5-28
10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-29
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other links at the same rate no frame buffering no CSMACD at hub adapters detect collisions provides net management functionality
twisted pair
hub
5 DataLink Layer 5-30
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-31
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-32
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-33
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
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
5 DataLink Layer 5-34
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif 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
forward on all but the interface on which the frame arrived
Link Layer 5-35
Interconnecting switches
switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-36
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-37
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-38
Summary comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
Link Layer 5-39
VLANs motivation (virtual LAN)
consider CS user moves office to EE
but wants connect to CS switch
single broadcast domain all layer-2 broadcast
traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
securityprivacy efficiency issues
Computer Science Electrical
Engineering
ComputerEngineering
Link Layer 5-40
VLANsport-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network 1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Link Layer 5-41
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined
switches plus routers
Link Layer 5-42
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be
vanilla 8021 frames (must carry VLAN ID info) 8021q protocol addsremoved additional header fields for
frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
Link Layer 5-43
type
2-byte Tag Protocol Identifier (value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
destaddress
sourceaddress data (payload) CRCpreamble
data (payload) CRC
type
Link Layer 5-44
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-45
Multiprotocol label switching (MPLS)
initial goal high-speed IP forwarding using fixed length label (instead of IP address) fast lookup using fixed length identifier (rather than
shortest prefix matching) borrowing ideas from Virtual Circuit (VC) approach but IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
Link Layer 5-46
MPLS capable routers
aka label-switched router forward packets to outgoing interface based only
on label value (donrsquot inspect IP address) MPLS forwarding table distinct from IP forwarding tables
flexibility MPLS forwarding decisions can differ from those of IP use destination and source addresses to route flows to
same destination differently (traffic engineering) re-route flows quickly if link fails pre-computed backup
paths (useful for VoIP)
Link Layer 5-47
R2
D
R3R5
A
R6
MPLS versus IP paths
IP router IP routing path to destination
determined by destination address alone
R4
Link Layer 5-48
R2
D
R3R4
R5
A
R6
MPLS versus IP paths
IP-onlyrouter
IP routing path to destination determined by destination address alone
MPLS and IP router
MPLS routing path to destination can be based on source and dest address fast reroute precompute backup routes
in case of link failure
entry router (R4) can use different MPLS routes to A based eg on source address
Link Layer 5-49
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-50
Data center networks
10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximity e-business (eg Amazon) content-servers (eg YouTube Akamai Apple Microsoft) search engines data mining (eg Google)
challenges multiple applications each
serving massive numbers of clients
managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container Chicago data center
Link Layer 5-51
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
Load balancer
Load balancer
B
1 2 3 4 5 6 7 8
A C
Border router
Access router
Internet
Data center networks
load balancer application-layer routingreceives external client requestsdirects workload within data centerreturns results to external client (hiding data center internals from client)
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
1 2 3 4 5 6 7 8
Data center networks rich interconnection among switches racks
increased throughput between racks (multiple routing paths possible) increased reliability via redundancy
Link Layer 5-53
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-54
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer5-55
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
web page
browser
router(runs DHCP)
Link Layer 5-56
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router(runs DHCP)
Link Layer 5-57
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router(runs DHCP)
Link Layer 5-58
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
router(runs DHCP)
Link Layer 5-59
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server DNS server replies to
client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router(runs DHCP)
Link Layer 5-60
A day in the lifehellipTCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router(runs DHCP)
Link Layer 5-61
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-62
Chapter 5 Summary
principles behind data link layer services error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS VLANs virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-63
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless multimedia security network management
5 DataLink Layer 5-29
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other links at the same rate no frame buffering no CSMACD at hub adapters detect collisions provides net management functionality
twisted pair
hub
5 DataLink Layer 5-30
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-31
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-32
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-33
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
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
5 DataLink Layer 5-34
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif 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
forward on all but the interface on which the frame arrived
Link Layer 5-35
Interconnecting switches
switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-36
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-37
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-38
Summary comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
Link Layer 5-39
VLANs motivation (virtual LAN)
consider CS user moves office to EE
but wants connect to CS switch
single broadcast domain all layer-2 broadcast
traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
securityprivacy efficiency issues
Computer Science Electrical
Engineering
ComputerEngineering
Link Layer 5-40
VLANsport-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network 1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Link Layer 5-41
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined
switches plus routers
Link Layer 5-42
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be
vanilla 8021 frames (must carry VLAN ID info) 8021q protocol addsremoved additional header fields for
frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
Link Layer 5-43
type
2-byte Tag Protocol Identifier (value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
destaddress
sourceaddress data (payload) CRCpreamble
data (payload) CRC
type
Link Layer 5-44
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-45
Multiprotocol label switching (MPLS)
initial goal high-speed IP forwarding using fixed length label (instead of IP address) fast lookup using fixed length identifier (rather than
shortest prefix matching) borrowing ideas from Virtual Circuit (VC) approach but IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
Link Layer 5-46
MPLS capable routers
aka label-switched router forward packets to outgoing interface based only
on label value (donrsquot inspect IP address) MPLS forwarding table distinct from IP forwarding tables
flexibility MPLS forwarding decisions can differ from those of IP use destination and source addresses to route flows to
same destination differently (traffic engineering) re-route flows quickly if link fails pre-computed backup
paths (useful for VoIP)
Link Layer 5-47
R2
D
R3R5
A
R6
MPLS versus IP paths
IP router IP routing path to destination
determined by destination address alone
R4
Link Layer 5-48
R2
D
R3R4
R5
A
R6
MPLS versus IP paths
IP-onlyrouter
IP routing path to destination determined by destination address alone
MPLS and IP router
MPLS routing path to destination can be based on source and dest address fast reroute precompute backup routes
in case of link failure
entry router (R4) can use different MPLS routes to A based eg on source address
Link Layer 5-49
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-50
Data center networks
10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximity e-business (eg Amazon) content-servers (eg YouTube Akamai Apple Microsoft) search engines data mining (eg Google)
challenges multiple applications each
serving massive numbers of clients
managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container Chicago data center
Link Layer 5-51
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
Load balancer
Load balancer
B
1 2 3 4 5 6 7 8
A C
Border router
Access router
Internet
Data center networks
load balancer application-layer routingreceives external client requestsdirects workload within data centerreturns results to external client (hiding data center internals from client)
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
1 2 3 4 5 6 7 8
Data center networks rich interconnection among switches racks
increased throughput between racks (multiple routing paths possible) increased reliability via redundancy
Link Layer 5-53
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-54
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer5-55
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
web page
browser
router(runs DHCP)
Link Layer 5-56
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router(runs DHCP)
Link Layer 5-57
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router(runs DHCP)
Link Layer 5-58
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
router(runs DHCP)
Link Layer 5-59
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server DNS server replies to
client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router(runs DHCP)
Link Layer 5-60
A day in the lifehellipTCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router(runs DHCP)
Link Layer 5-61
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-62
Chapter 5 Summary
principles behind data link layer services error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS VLANs virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-63
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless multimedia security network management
5 DataLink Layer 5-30
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-31
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-32
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-33
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
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
5 DataLink Layer 5-34
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif 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
forward on all but the interface on which the frame arrived
Link Layer 5-35
Interconnecting switches
switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-36
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-37
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-38
Summary comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
Link Layer 5-39
VLANs motivation (virtual LAN)
consider CS user moves office to EE
but wants connect to CS switch
single broadcast domain all layer-2 broadcast
traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
securityprivacy efficiency issues
Computer Science Electrical
Engineering
ComputerEngineering
Link Layer 5-40
VLANsport-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network 1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Link Layer 5-41
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined
switches plus routers
Link Layer 5-42
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be
vanilla 8021 frames (must carry VLAN ID info) 8021q protocol addsremoved additional header fields for
frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
Link Layer 5-43
type
2-byte Tag Protocol Identifier (value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
destaddress
sourceaddress data (payload) CRCpreamble
data (payload) CRC
type
Link Layer 5-44
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-45
Multiprotocol label switching (MPLS)
initial goal high-speed IP forwarding using fixed length label (instead of IP address) fast lookup using fixed length identifier (rather than
shortest prefix matching) borrowing ideas from Virtual Circuit (VC) approach but IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
Link Layer 5-46
MPLS capable routers
aka label-switched router forward packets to outgoing interface based only
on label value (donrsquot inspect IP address) MPLS forwarding table distinct from IP forwarding tables
flexibility MPLS forwarding decisions can differ from those of IP use destination and source addresses to route flows to
same destination differently (traffic engineering) re-route flows quickly if link fails pre-computed backup
paths (useful for VoIP)
Link Layer 5-47
R2
D
R3R5
A
R6
MPLS versus IP paths
IP router IP routing path to destination
determined by destination address alone
R4
Link Layer 5-48
R2
D
R3R4
R5
A
R6
MPLS versus IP paths
IP-onlyrouter
IP routing path to destination determined by destination address alone
MPLS and IP router
MPLS routing path to destination can be based on source and dest address fast reroute precompute backup routes
in case of link failure
entry router (R4) can use different MPLS routes to A based eg on source address
Link Layer 5-49
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-50
Data center networks
10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximity e-business (eg Amazon) content-servers (eg YouTube Akamai Apple Microsoft) search engines data mining (eg Google)
challenges multiple applications each
serving massive numbers of clients
managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container Chicago data center
Link Layer 5-51
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
Load balancer
Load balancer
B
1 2 3 4 5 6 7 8
A C
Border router
Access router
Internet
Data center networks
load balancer application-layer routingreceives external client requestsdirects workload within data centerreturns results to external client (hiding data center internals from client)
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
1 2 3 4 5 6 7 8
Data center networks rich interconnection among switches racks
increased throughput between racks (multiple routing paths possible) increased reliability via redundancy
Link Layer 5-53
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-54
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer5-55
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
web page
browser
router(runs DHCP)
Link Layer 5-56
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router(runs DHCP)
Link Layer 5-57
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router(runs DHCP)
Link Layer 5-58
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
router(runs DHCP)
Link Layer 5-59
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server DNS server replies to
client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router(runs DHCP)
Link Layer 5-60
A day in the lifehellipTCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router(runs DHCP)
Link Layer 5-61
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-62
Chapter 5 Summary
principles behind data link layer services error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS VLANs virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-63
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless multimedia security network management
5 DataLink Layer 5-31
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-32
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-33
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
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
5 DataLink Layer 5-34
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif 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
forward on all but the interface on which the frame arrived
Link Layer 5-35
Interconnecting switches
switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-36
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-37
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-38
Summary comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
Link Layer 5-39
VLANs motivation (virtual LAN)
consider CS user moves office to EE
but wants connect to CS switch
single broadcast domain all layer-2 broadcast
traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
securityprivacy efficiency issues
Computer Science Electrical
Engineering
ComputerEngineering
Link Layer 5-40
VLANsport-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network 1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Link Layer 5-41
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined
switches plus routers
Link Layer 5-42
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be
vanilla 8021 frames (must carry VLAN ID info) 8021q protocol addsremoved additional header fields for
frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
Link Layer 5-43
type
2-byte Tag Protocol Identifier (value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
destaddress
sourceaddress data (payload) CRCpreamble
data (payload) CRC
type
Link Layer 5-44
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-45
Multiprotocol label switching (MPLS)
initial goal high-speed IP forwarding using fixed length label (instead of IP address) fast lookup using fixed length identifier (rather than
shortest prefix matching) borrowing ideas from Virtual Circuit (VC) approach but IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
Link Layer 5-46
MPLS capable routers
aka label-switched router forward packets to outgoing interface based only
on label value (donrsquot inspect IP address) MPLS forwarding table distinct from IP forwarding tables
flexibility MPLS forwarding decisions can differ from those of IP use destination and source addresses to route flows to
same destination differently (traffic engineering) re-route flows quickly if link fails pre-computed backup
paths (useful for VoIP)
Link Layer 5-47
R2
D
R3R5
A
R6
MPLS versus IP paths
IP router IP routing path to destination
determined by destination address alone
R4
Link Layer 5-48
R2
D
R3R4
R5
A
R6
MPLS versus IP paths
IP-onlyrouter
IP routing path to destination determined by destination address alone
MPLS and IP router
MPLS routing path to destination can be based on source and dest address fast reroute precompute backup routes
in case of link failure
entry router (R4) can use different MPLS routes to A based eg on source address
Link Layer 5-49
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-50
Data center networks
10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximity e-business (eg Amazon) content-servers (eg YouTube Akamai Apple Microsoft) search engines data mining (eg Google)
challenges multiple applications each
serving massive numbers of clients
managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container Chicago data center
Link Layer 5-51
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
Load balancer
Load balancer
B
1 2 3 4 5 6 7 8
A C
Border router
Access router
Internet
Data center networks
load balancer application-layer routingreceives external client requestsdirects workload within data centerreturns results to external client (hiding data center internals from client)
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
1 2 3 4 5 6 7 8
Data center networks rich interconnection among switches racks
increased throughput between racks (multiple routing paths possible) increased reliability via redundancy
Link Layer 5-53
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-54
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer5-55
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
web page
browser
router(runs DHCP)
Link Layer 5-56
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router(runs DHCP)
Link Layer 5-57
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router(runs DHCP)
Link Layer 5-58
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
router(runs DHCP)
Link Layer 5-59
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server DNS server replies to
client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router(runs DHCP)
Link Layer 5-60
A day in the lifehellipTCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router(runs DHCP)
Link Layer 5-61
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-62
Chapter 5 Summary
principles behind data link layer services error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS VLANs virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-63
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless multimedia security network management
5 DataLink Layer 5-32
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-33
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
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
5 DataLink Layer 5-34
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif 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
forward on all but the interface on which the frame arrived
Link Layer 5-35
Interconnecting switches
switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-36
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-37
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-38
Summary comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
Link Layer 5-39
VLANs motivation (virtual LAN)
consider CS user moves office to EE
but wants connect to CS switch
single broadcast domain all layer-2 broadcast
traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
securityprivacy efficiency issues
Computer Science Electrical
Engineering
ComputerEngineering
Link Layer 5-40
VLANsport-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network 1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Link Layer 5-41
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined
switches plus routers
Link Layer 5-42
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be
vanilla 8021 frames (must carry VLAN ID info) 8021q protocol addsremoved additional header fields for
frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
Link Layer 5-43
type
2-byte Tag Protocol Identifier (value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
destaddress
sourceaddress data (payload) CRCpreamble
data (payload) CRC
type
Link Layer 5-44
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-45
Multiprotocol label switching (MPLS)
initial goal high-speed IP forwarding using fixed length label (instead of IP address) fast lookup using fixed length identifier (rather than
shortest prefix matching) borrowing ideas from Virtual Circuit (VC) approach but IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
Link Layer 5-46
MPLS capable routers
aka label-switched router forward packets to outgoing interface based only
on label value (donrsquot inspect IP address) MPLS forwarding table distinct from IP forwarding tables
flexibility MPLS forwarding decisions can differ from those of IP use destination and source addresses to route flows to
same destination differently (traffic engineering) re-route flows quickly if link fails pre-computed backup
paths (useful for VoIP)
Link Layer 5-47
R2
D
R3R5
A
R6
MPLS versus IP paths
IP router IP routing path to destination
determined by destination address alone
R4
Link Layer 5-48
R2
D
R3R4
R5
A
R6
MPLS versus IP paths
IP-onlyrouter
IP routing path to destination determined by destination address alone
MPLS and IP router
MPLS routing path to destination can be based on source and dest address fast reroute precompute backup routes
in case of link failure
entry router (R4) can use different MPLS routes to A based eg on source address
Link Layer 5-49
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-50
Data center networks
10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximity e-business (eg Amazon) content-servers (eg YouTube Akamai Apple Microsoft) search engines data mining (eg Google)
challenges multiple applications each
serving massive numbers of clients
managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container Chicago data center
Link Layer 5-51
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
Load balancer
Load balancer
B
1 2 3 4 5 6 7 8
A C
Border router
Access router
Internet
Data center networks
load balancer application-layer routingreceives external client requestsdirects workload within data centerreturns results to external client (hiding data center internals from client)
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
1 2 3 4 5 6 7 8
Data center networks rich interconnection among switches racks
increased throughput between racks (multiple routing paths possible) increased reliability via redundancy
Link Layer 5-53
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-54
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer5-55
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
web page
browser
router(runs DHCP)
Link Layer 5-56
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router(runs DHCP)
Link Layer 5-57
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router(runs DHCP)
Link Layer 5-58
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
router(runs DHCP)
Link Layer 5-59
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server DNS server replies to
client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router(runs DHCP)
Link Layer 5-60
A day in the lifehellipTCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router(runs DHCP)
Link Layer 5-61
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-62
Chapter 5 Summary
principles behind data link layer services error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS VLANs virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-63
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless multimedia security network management
5 DataLink Layer 5-33
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
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
5 DataLink Layer 5-34
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif 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
forward on all but the interface on which the frame arrived
Link Layer 5-35
Interconnecting switches
switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-36
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-37
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-38
Summary comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
Link Layer 5-39
VLANs motivation (virtual LAN)
consider CS user moves office to EE
but wants connect to CS switch
single broadcast domain all layer-2 broadcast
traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
securityprivacy efficiency issues
Computer Science Electrical
Engineering
ComputerEngineering
Link Layer 5-40
VLANsport-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network 1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Link Layer 5-41
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined
switches plus routers
Link Layer 5-42
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be
vanilla 8021 frames (must carry VLAN ID info) 8021q protocol addsremoved additional header fields for
frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
Link Layer 5-43
type
2-byte Tag Protocol Identifier (value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
destaddress
sourceaddress data (payload) CRCpreamble
data (payload) CRC
type
Link Layer 5-44
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-45
Multiprotocol label switching (MPLS)
initial goal high-speed IP forwarding using fixed length label (instead of IP address) fast lookup using fixed length identifier (rather than
shortest prefix matching) borrowing ideas from Virtual Circuit (VC) approach but IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
Link Layer 5-46
MPLS capable routers
aka label-switched router forward packets to outgoing interface based only
on label value (donrsquot inspect IP address) MPLS forwarding table distinct from IP forwarding tables
flexibility MPLS forwarding decisions can differ from those of IP use destination and source addresses to route flows to
same destination differently (traffic engineering) re-route flows quickly if link fails pre-computed backup
paths (useful for VoIP)
Link Layer 5-47
R2
D
R3R5
A
R6
MPLS versus IP paths
IP router IP routing path to destination
determined by destination address alone
R4
Link Layer 5-48
R2
D
R3R4
R5
A
R6
MPLS versus IP paths
IP-onlyrouter
IP routing path to destination determined by destination address alone
MPLS and IP router
MPLS routing path to destination can be based on source and dest address fast reroute precompute backup routes
in case of link failure
entry router (R4) can use different MPLS routes to A based eg on source address
Link Layer 5-49
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-50
Data center networks
10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximity e-business (eg Amazon) content-servers (eg YouTube Akamai Apple Microsoft) search engines data mining (eg Google)
challenges multiple applications each
serving massive numbers of clients
managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container Chicago data center
Link Layer 5-51
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
Load balancer
Load balancer
B
1 2 3 4 5 6 7 8
A C
Border router
Access router
Internet
Data center networks
load balancer application-layer routingreceives external client requestsdirects workload within data centerreturns results to external client (hiding data center internals from client)
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
1 2 3 4 5 6 7 8
Data center networks rich interconnection among switches racks
increased throughput between racks (multiple routing paths possible) increased reliability via redundancy
Link Layer 5-53
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-54
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer5-55
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
web page
browser
router(runs DHCP)
Link Layer 5-56
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router(runs DHCP)
Link Layer 5-57
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router(runs DHCP)
Link Layer 5-58
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
router(runs DHCP)
Link Layer 5-59
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server DNS server replies to
client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router(runs DHCP)
Link Layer 5-60
A day in the lifehellipTCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router(runs DHCP)
Link Layer 5-61
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-62
Chapter 5 Summary
principles behind data link layer services error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS VLANs virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-63
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless multimedia security network management
5 DataLink Layer 5-34
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif 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
forward on all but the interface on which the frame arrived
Link Layer 5-35
Interconnecting switches
switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-36
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-37
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-38
Summary comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
Link Layer 5-39
VLANs motivation (virtual LAN)
consider CS user moves office to EE
but wants connect to CS switch
single broadcast domain all layer-2 broadcast
traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
securityprivacy efficiency issues
Computer Science Electrical
Engineering
ComputerEngineering
Link Layer 5-40
VLANsport-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network 1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Link Layer 5-41
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined
switches plus routers
Link Layer 5-42
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be
vanilla 8021 frames (must carry VLAN ID info) 8021q protocol addsremoved additional header fields for
frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
Link Layer 5-43
type
2-byte Tag Protocol Identifier (value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
destaddress
sourceaddress data (payload) CRCpreamble
data (payload) CRC
type
Link Layer 5-44
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-45
Multiprotocol label switching (MPLS)
initial goal high-speed IP forwarding using fixed length label (instead of IP address) fast lookup using fixed length identifier (rather than
shortest prefix matching) borrowing ideas from Virtual Circuit (VC) approach but IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
Link Layer 5-46
MPLS capable routers
aka label-switched router forward packets to outgoing interface based only
on label value (donrsquot inspect IP address) MPLS forwarding table distinct from IP forwarding tables
flexibility MPLS forwarding decisions can differ from those of IP use destination and source addresses to route flows to
same destination differently (traffic engineering) re-route flows quickly if link fails pre-computed backup
paths (useful for VoIP)
Link Layer 5-47
R2
D
R3R5
A
R6
MPLS versus IP paths
IP router IP routing path to destination
determined by destination address alone
R4
Link Layer 5-48
R2
D
R3R4
R5
A
R6
MPLS versus IP paths
IP-onlyrouter
IP routing path to destination determined by destination address alone
MPLS and IP router
MPLS routing path to destination can be based on source and dest address fast reroute precompute backup routes
in case of link failure
entry router (R4) can use different MPLS routes to A based eg on source address
Link Layer 5-49
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-50
Data center networks
10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximity e-business (eg Amazon) content-servers (eg YouTube Akamai Apple Microsoft) search engines data mining (eg Google)
challenges multiple applications each
serving massive numbers of clients
managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container Chicago data center
Link Layer 5-51
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
Load balancer
Load balancer
B
1 2 3 4 5 6 7 8
A C
Border router
Access router
Internet
Data center networks
load balancer application-layer routingreceives external client requestsdirects workload within data centerreturns results to external client (hiding data center internals from client)
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
1 2 3 4 5 6 7 8
Data center networks rich interconnection among switches racks
increased throughput between racks (multiple routing paths possible) increased reliability via redundancy
Link Layer 5-53
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-54
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer5-55
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
web page
browser
router(runs DHCP)
Link Layer 5-56
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router(runs DHCP)
Link Layer 5-57
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router(runs DHCP)
Link Layer 5-58
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
router(runs DHCP)
Link Layer 5-59
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server DNS server replies to
client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router(runs DHCP)
Link Layer 5-60
A day in the lifehellipTCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router(runs DHCP)
Link Layer 5-61
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-62
Chapter 5 Summary
principles behind data link layer services error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS VLANs virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-63
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless multimedia security network management
Link Layer 5-35
Interconnecting switches
switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-36
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-37
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-38
Summary comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
Link Layer 5-39
VLANs motivation (virtual LAN)
consider CS user moves office to EE
but wants connect to CS switch
single broadcast domain all layer-2 broadcast
traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
securityprivacy efficiency issues
Computer Science Electrical
Engineering
ComputerEngineering
Link Layer 5-40
VLANsport-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network 1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Link Layer 5-41
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined
switches plus routers
Link Layer 5-42
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be
vanilla 8021 frames (must carry VLAN ID info) 8021q protocol addsremoved additional header fields for
frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
Link Layer 5-43
type
2-byte Tag Protocol Identifier (value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
destaddress
sourceaddress data (payload) CRCpreamble
data (payload) CRC
type
Link Layer 5-44
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-45
Multiprotocol label switching (MPLS)
initial goal high-speed IP forwarding using fixed length label (instead of IP address) fast lookup using fixed length identifier (rather than
shortest prefix matching) borrowing ideas from Virtual Circuit (VC) approach but IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
Link Layer 5-46
MPLS capable routers
aka label-switched router forward packets to outgoing interface based only
on label value (donrsquot inspect IP address) MPLS forwarding table distinct from IP forwarding tables
flexibility MPLS forwarding decisions can differ from those of IP use destination and source addresses to route flows to
same destination differently (traffic engineering) re-route flows quickly if link fails pre-computed backup
paths (useful for VoIP)
Link Layer 5-47
R2
D
R3R5
A
R6
MPLS versus IP paths
IP router IP routing path to destination
determined by destination address alone
R4
Link Layer 5-48
R2
D
R3R4
R5
A
R6
MPLS versus IP paths
IP-onlyrouter
IP routing path to destination determined by destination address alone
MPLS and IP router
MPLS routing path to destination can be based on source and dest address fast reroute precompute backup routes
in case of link failure
entry router (R4) can use different MPLS routes to A based eg on source address
Link Layer 5-49
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-50
Data center networks
10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximity e-business (eg Amazon) content-servers (eg YouTube Akamai Apple Microsoft) search engines data mining (eg Google)
challenges multiple applications each
serving massive numbers of clients
managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container Chicago data center
Link Layer 5-51
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
Load balancer
Load balancer
B
1 2 3 4 5 6 7 8
A C
Border router
Access router
Internet
Data center networks
load balancer application-layer routingreceives external client requestsdirects workload within data centerreturns results to external client (hiding data center internals from client)
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
1 2 3 4 5 6 7 8
Data center networks rich interconnection among switches racks
increased throughput between racks (multiple routing paths possible) increased reliability via redundancy
Link Layer 5-53
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-54
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer5-55
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
web page
browser
router(runs DHCP)
Link Layer 5-56
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router(runs DHCP)
Link Layer 5-57
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router(runs DHCP)
Link Layer 5-58
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
router(runs DHCP)
Link Layer 5-59
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server DNS server replies to
client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router(runs DHCP)
Link Layer 5-60
A day in the lifehellipTCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router(runs DHCP)
Link Layer 5-61
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-62
Chapter 5 Summary
principles behind data link layer services error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS VLANs virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-63
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless multimedia security network management
Link Layer 5-36
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-37
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-38
Summary comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
Link Layer 5-39
VLANs motivation (virtual LAN)
consider CS user moves office to EE
but wants connect to CS switch
single broadcast domain all layer-2 broadcast
traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
securityprivacy efficiency issues
Computer Science Electrical
Engineering
ComputerEngineering
Link Layer 5-40
VLANsport-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network 1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Link Layer 5-41
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined
switches plus routers
Link Layer 5-42
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be
vanilla 8021 frames (must carry VLAN ID info) 8021q protocol addsremoved additional header fields for
frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
Link Layer 5-43
type
2-byte Tag Protocol Identifier (value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
destaddress
sourceaddress data (payload) CRCpreamble
data (payload) CRC
type
Link Layer 5-44
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-45
Multiprotocol label switching (MPLS)
initial goal high-speed IP forwarding using fixed length label (instead of IP address) fast lookup using fixed length identifier (rather than
shortest prefix matching) borrowing ideas from Virtual Circuit (VC) approach but IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
Link Layer 5-46
MPLS capable routers
aka label-switched router forward packets to outgoing interface based only
on label value (donrsquot inspect IP address) MPLS forwarding table distinct from IP forwarding tables
flexibility MPLS forwarding decisions can differ from those of IP use destination and source addresses to route flows to
same destination differently (traffic engineering) re-route flows quickly if link fails pre-computed backup
paths (useful for VoIP)
Link Layer 5-47
R2
D
R3R5
A
R6
MPLS versus IP paths
IP router IP routing path to destination
determined by destination address alone
R4
Link Layer 5-48
R2
D
R3R4
R5
A
R6
MPLS versus IP paths
IP-onlyrouter
IP routing path to destination determined by destination address alone
MPLS and IP router
MPLS routing path to destination can be based on source and dest address fast reroute precompute backup routes
in case of link failure
entry router (R4) can use different MPLS routes to A based eg on source address
Link Layer 5-49
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-50
Data center networks
10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximity e-business (eg Amazon) content-servers (eg YouTube Akamai Apple Microsoft) search engines data mining (eg Google)
challenges multiple applications each
serving massive numbers of clients
managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container Chicago data center
Link Layer 5-51
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
Load balancer
Load balancer
B
1 2 3 4 5 6 7 8
A C
Border router
Access router
Internet
Data center networks
load balancer application-layer routingreceives external client requestsdirects workload within data centerreturns results to external client (hiding data center internals from client)
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
1 2 3 4 5 6 7 8
Data center networks rich interconnection among switches racks
increased throughput between racks (multiple routing paths possible) increased reliability via redundancy
Link Layer 5-53
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-54
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer5-55
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
web page
browser
router(runs DHCP)
Link Layer 5-56
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router(runs DHCP)
Link Layer 5-57
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router(runs DHCP)
Link Layer 5-58
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
router(runs DHCP)
Link Layer 5-59
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server DNS server replies to
client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router(runs DHCP)
Link Layer 5-60
A day in the lifehellipTCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router(runs DHCP)
Link Layer 5-61
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-62
Chapter 5 Summary
principles behind data link layer services error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS VLANs virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-63
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless multimedia security network management
5 DataLink Layer 5-37
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-38
Summary comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
Link Layer 5-39
VLANs motivation (virtual LAN)
consider CS user moves office to EE
but wants connect to CS switch
single broadcast domain all layer-2 broadcast
traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
securityprivacy efficiency issues
Computer Science Electrical
Engineering
ComputerEngineering
Link Layer 5-40
VLANsport-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network 1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Link Layer 5-41
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined
switches plus routers
Link Layer 5-42
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be
vanilla 8021 frames (must carry VLAN ID info) 8021q protocol addsremoved additional header fields for
frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
Link Layer 5-43
type
2-byte Tag Protocol Identifier (value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
destaddress
sourceaddress data (payload) CRCpreamble
data (payload) CRC
type
Link Layer 5-44
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-45
Multiprotocol label switching (MPLS)
initial goal high-speed IP forwarding using fixed length label (instead of IP address) fast lookup using fixed length identifier (rather than
shortest prefix matching) borrowing ideas from Virtual Circuit (VC) approach but IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
Link Layer 5-46
MPLS capable routers
aka label-switched router forward packets to outgoing interface based only
on label value (donrsquot inspect IP address) MPLS forwarding table distinct from IP forwarding tables
flexibility MPLS forwarding decisions can differ from those of IP use destination and source addresses to route flows to
same destination differently (traffic engineering) re-route flows quickly if link fails pre-computed backup
paths (useful for VoIP)
Link Layer 5-47
R2
D
R3R5
A
R6
MPLS versus IP paths
IP router IP routing path to destination
determined by destination address alone
R4
Link Layer 5-48
R2
D
R3R4
R5
A
R6
MPLS versus IP paths
IP-onlyrouter
IP routing path to destination determined by destination address alone
MPLS and IP router
MPLS routing path to destination can be based on source and dest address fast reroute precompute backup routes
in case of link failure
entry router (R4) can use different MPLS routes to A based eg on source address
Link Layer 5-49
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-50
Data center networks
10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximity e-business (eg Amazon) content-servers (eg YouTube Akamai Apple Microsoft) search engines data mining (eg Google)
challenges multiple applications each
serving massive numbers of clients
managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container Chicago data center
Link Layer 5-51
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
Load balancer
Load balancer
B
1 2 3 4 5 6 7 8
A C
Border router
Access router
Internet
Data center networks
load balancer application-layer routingreceives external client requestsdirects workload within data centerreturns results to external client (hiding data center internals from client)
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
1 2 3 4 5 6 7 8
Data center networks rich interconnection among switches racks
increased throughput between racks (multiple routing paths possible) increased reliability via redundancy
Link Layer 5-53
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-54
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer5-55
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
web page
browser
router(runs DHCP)
Link Layer 5-56
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router(runs DHCP)
Link Layer 5-57
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router(runs DHCP)
Link Layer 5-58
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
router(runs DHCP)
Link Layer 5-59
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server DNS server replies to
client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router(runs DHCP)
Link Layer 5-60
A day in the lifehellipTCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router(runs DHCP)
Link Layer 5-61
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-62
Chapter 5 Summary
principles behind data link layer services error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS VLANs virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-63
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless multimedia security network management
5 DataLink Layer 5-38
Summary comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
Link Layer 5-39
VLANs motivation (virtual LAN)
consider CS user moves office to EE
but wants connect to CS switch
single broadcast domain all layer-2 broadcast
traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
securityprivacy efficiency issues
Computer Science Electrical
Engineering
ComputerEngineering
Link Layer 5-40
VLANsport-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network 1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Link Layer 5-41
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined
switches plus routers
Link Layer 5-42
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be
vanilla 8021 frames (must carry VLAN ID info) 8021q protocol addsremoved additional header fields for
frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
Link Layer 5-43
type
2-byte Tag Protocol Identifier (value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
destaddress
sourceaddress data (payload) CRCpreamble
data (payload) CRC
type
Link Layer 5-44
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-45
Multiprotocol label switching (MPLS)
initial goal high-speed IP forwarding using fixed length label (instead of IP address) fast lookup using fixed length identifier (rather than
shortest prefix matching) borrowing ideas from Virtual Circuit (VC) approach but IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
Link Layer 5-46
MPLS capable routers
aka label-switched router forward packets to outgoing interface based only
on label value (donrsquot inspect IP address) MPLS forwarding table distinct from IP forwarding tables
flexibility MPLS forwarding decisions can differ from those of IP use destination and source addresses to route flows to
same destination differently (traffic engineering) re-route flows quickly if link fails pre-computed backup
paths (useful for VoIP)
Link Layer 5-47
R2
D
R3R5
A
R6
MPLS versus IP paths
IP router IP routing path to destination
determined by destination address alone
R4
Link Layer 5-48
R2
D
R3R4
R5
A
R6
MPLS versus IP paths
IP-onlyrouter
IP routing path to destination determined by destination address alone
MPLS and IP router
MPLS routing path to destination can be based on source and dest address fast reroute precompute backup routes
in case of link failure
entry router (R4) can use different MPLS routes to A based eg on source address
Link Layer 5-49
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-50
Data center networks
10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximity e-business (eg Amazon) content-servers (eg YouTube Akamai Apple Microsoft) search engines data mining (eg Google)
challenges multiple applications each
serving massive numbers of clients
managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container Chicago data center
Link Layer 5-51
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
Load balancer
Load balancer
B
1 2 3 4 5 6 7 8
A C
Border router
Access router
Internet
Data center networks
load balancer application-layer routingreceives external client requestsdirects workload within data centerreturns results to external client (hiding data center internals from client)
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
1 2 3 4 5 6 7 8
Data center networks rich interconnection among switches racks
increased throughput between racks (multiple routing paths possible) increased reliability via redundancy
Link Layer 5-53
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-54
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer5-55
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
web page
browser
router(runs DHCP)
Link Layer 5-56
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router(runs DHCP)
Link Layer 5-57
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router(runs DHCP)
Link Layer 5-58
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
router(runs DHCP)
Link Layer 5-59
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server DNS server replies to
client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router(runs DHCP)
Link Layer 5-60
A day in the lifehellipTCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router(runs DHCP)
Link Layer 5-61
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-62
Chapter 5 Summary
principles behind data link layer services error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS VLANs virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-63
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless multimedia security network management
Link Layer 5-39
VLANs motivation (virtual LAN)
consider CS user moves office to EE
but wants connect to CS switch
single broadcast domain all layer-2 broadcast
traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
securityprivacy efficiency issues
Computer Science Electrical
Engineering
ComputerEngineering
Link Layer 5-40
VLANsport-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network 1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Link Layer 5-41
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined
switches plus routers
Link Layer 5-42
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be
vanilla 8021 frames (must carry VLAN ID info) 8021q protocol addsremoved additional header fields for
frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
Link Layer 5-43
type
2-byte Tag Protocol Identifier (value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
destaddress
sourceaddress data (payload) CRCpreamble
data (payload) CRC
type
Link Layer 5-44
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-45
Multiprotocol label switching (MPLS)
initial goal high-speed IP forwarding using fixed length label (instead of IP address) fast lookup using fixed length identifier (rather than
shortest prefix matching) borrowing ideas from Virtual Circuit (VC) approach but IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
Link Layer 5-46
MPLS capable routers
aka label-switched router forward packets to outgoing interface based only
on label value (donrsquot inspect IP address) MPLS forwarding table distinct from IP forwarding tables
flexibility MPLS forwarding decisions can differ from those of IP use destination and source addresses to route flows to
same destination differently (traffic engineering) re-route flows quickly if link fails pre-computed backup
paths (useful for VoIP)
Link Layer 5-47
R2
D
R3R5
A
R6
MPLS versus IP paths
IP router IP routing path to destination
determined by destination address alone
R4
Link Layer 5-48
R2
D
R3R4
R5
A
R6
MPLS versus IP paths
IP-onlyrouter
IP routing path to destination determined by destination address alone
MPLS and IP router
MPLS routing path to destination can be based on source and dest address fast reroute precompute backup routes
in case of link failure
entry router (R4) can use different MPLS routes to A based eg on source address
Link Layer 5-49
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-50
Data center networks
10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximity e-business (eg Amazon) content-servers (eg YouTube Akamai Apple Microsoft) search engines data mining (eg Google)
challenges multiple applications each
serving massive numbers of clients
managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container Chicago data center
Link Layer 5-51
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
Load balancer
Load balancer
B
1 2 3 4 5 6 7 8
A C
Border router
Access router
Internet
Data center networks
load balancer application-layer routingreceives external client requestsdirects workload within data centerreturns results to external client (hiding data center internals from client)
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
1 2 3 4 5 6 7 8
Data center networks rich interconnection among switches racks
increased throughput between racks (multiple routing paths possible) increased reliability via redundancy
Link Layer 5-53
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-54
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer5-55
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
web page
browser
router(runs DHCP)
Link Layer 5-56
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router(runs DHCP)
Link Layer 5-57
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router(runs DHCP)
Link Layer 5-58
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
router(runs DHCP)
Link Layer 5-59
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server DNS server replies to
client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router(runs DHCP)
Link Layer 5-60
A day in the lifehellipTCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router(runs DHCP)
Link Layer 5-61
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-62
Chapter 5 Summary
principles behind data link layer services error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS VLANs virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-63
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless multimedia security network management
Link Layer 5-40
VLANsport-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network 1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Link Layer 5-41
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined
switches plus routers
Link Layer 5-42
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be
vanilla 8021 frames (must carry VLAN ID info) 8021q protocol addsremoved additional header fields for
frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
Link Layer 5-43
type
2-byte Tag Protocol Identifier (value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
destaddress
sourceaddress data (payload) CRCpreamble
data (payload) CRC
type
Link Layer 5-44
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-45
Multiprotocol label switching (MPLS)
initial goal high-speed IP forwarding using fixed length label (instead of IP address) fast lookup using fixed length identifier (rather than
shortest prefix matching) borrowing ideas from Virtual Circuit (VC) approach but IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
Link Layer 5-46
MPLS capable routers
aka label-switched router forward packets to outgoing interface based only
on label value (donrsquot inspect IP address) MPLS forwarding table distinct from IP forwarding tables
flexibility MPLS forwarding decisions can differ from those of IP use destination and source addresses to route flows to
same destination differently (traffic engineering) re-route flows quickly if link fails pre-computed backup
paths (useful for VoIP)
Link Layer 5-47
R2
D
R3R5
A
R6
MPLS versus IP paths
IP router IP routing path to destination
determined by destination address alone
R4
Link Layer 5-48
R2
D
R3R4
R5
A
R6
MPLS versus IP paths
IP-onlyrouter
IP routing path to destination determined by destination address alone
MPLS and IP router
MPLS routing path to destination can be based on source and dest address fast reroute precompute backup routes
in case of link failure
entry router (R4) can use different MPLS routes to A based eg on source address
Link Layer 5-49
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-50
Data center networks
10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximity e-business (eg Amazon) content-servers (eg YouTube Akamai Apple Microsoft) search engines data mining (eg Google)
challenges multiple applications each
serving massive numbers of clients
managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container Chicago data center
Link Layer 5-51
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
Load balancer
Load balancer
B
1 2 3 4 5 6 7 8
A C
Border router
Access router
Internet
Data center networks
load balancer application-layer routingreceives external client requestsdirects workload within data centerreturns results to external client (hiding data center internals from client)
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
1 2 3 4 5 6 7 8
Data center networks rich interconnection among switches racks
increased throughput between racks (multiple routing paths possible) increased reliability via redundancy
Link Layer 5-53
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-54
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer5-55
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
web page
browser
router(runs DHCP)
Link Layer 5-56
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router(runs DHCP)
Link Layer 5-57
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router(runs DHCP)
Link Layer 5-58
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
router(runs DHCP)
Link Layer 5-59
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server DNS server replies to
client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router(runs DHCP)
Link Layer 5-60
A day in the lifehellipTCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router(runs DHCP)
Link Layer 5-61
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-62
Chapter 5 Summary
principles behind data link layer services error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS VLANs virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-63
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless multimedia security network management
Link Layer 5-41
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined
switches plus routers
Link Layer 5-42
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be
vanilla 8021 frames (must carry VLAN ID info) 8021q protocol addsremoved additional header fields for
frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
Link Layer 5-43
type
2-byte Tag Protocol Identifier (value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
destaddress
sourceaddress data (payload) CRCpreamble
data (payload) CRC
type
Link Layer 5-44
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-45
Multiprotocol label switching (MPLS)
initial goal high-speed IP forwarding using fixed length label (instead of IP address) fast lookup using fixed length identifier (rather than
shortest prefix matching) borrowing ideas from Virtual Circuit (VC) approach but IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
Link Layer 5-46
MPLS capable routers
aka label-switched router forward packets to outgoing interface based only
on label value (donrsquot inspect IP address) MPLS forwarding table distinct from IP forwarding tables
flexibility MPLS forwarding decisions can differ from those of IP use destination and source addresses to route flows to
same destination differently (traffic engineering) re-route flows quickly if link fails pre-computed backup
paths (useful for VoIP)
Link Layer 5-47
R2
D
R3R5
A
R6
MPLS versus IP paths
IP router IP routing path to destination
determined by destination address alone
R4
Link Layer 5-48
R2
D
R3R4
R5
A
R6
MPLS versus IP paths
IP-onlyrouter
IP routing path to destination determined by destination address alone
MPLS and IP router
MPLS routing path to destination can be based on source and dest address fast reroute precompute backup routes
in case of link failure
entry router (R4) can use different MPLS routes to A based eg on source address
Link Layer 5-49
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-50
Data center networks
10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximity e-business (eg Amazon) content-servers (eg YouTube Akamai Apple Microsoft) search engines data mining (eg Google)
challenges multiple applications each
serving massive numbers of clients
managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container Chicago data center
Link Layer 5-51
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
Load balancer
Load balancer
B
1 2 3 4 5 6 7 8
A C
Border router
Access router
Internet
Data center networks
load balancer application-layer routingreceives external client requestsdirects workload within data centerreturns results to external client (hiding data center internals from client)
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
1 2 3 4 5 6 7 8
Data center networks rich interconnection among switches racks
increased throughput between racks (multiple routing paths possible) increased reliability via redundancy
Link Layer 5-53
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-54
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer5-55
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
web page
browser
router(runs DHCP)
Link Layer 5-56
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router(runs DHCP)
Link Layer 5-57
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router(runs DHCP)
Link Layer 5-58
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
router(runs DHCP)
Link Layer 5-59
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server DNS server replies to
client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router(runs DHCP)
Link Layer 5-60
A day in the lifehellipTCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router(runs DHCP)
Link Layer 5-61
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-62
Chapter 5 Summary
principles behind data link layer services error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS VLANs virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-63
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless multimedia security network management
Link Layer 5-42
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be
vanilla 8021 frames (must carry VLAN ID info) 8021q protocol addsremoved additional header fields for
frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
Link Layer 5-43
type
2-byte Tag Protocol Identifier (value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
destaddress
sourceaddress data (payload) CRCpreamble
data (payload) CRC
type
Link Layer 5-44
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-45
Multiprotocol label switching (MPLS)
initial goal high-speed IP forwarding using fixed length label (instead of IP address) fast lookup using fixed length identifier (rather than
shortest prefix matching) borrowing ideas from Virtual Circuit (VC) approach but IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
Link Layer 5-46
MPLS capable routers
aka label-switched router forward packets to outgoing interface based only
on label value (donrsquot inspect IP address) MPLS forwarding table distinct from IP forwarding tables
flexibility MPLS forwarding decisions can differ from those of IP use destination and source addresses to route flows to
same destination differently (traffic engineering) re-route flows quickly if link fails pre-computed backup
paths (useful for VoIP)
Link Layer 5-47
R2
D
R3R5
A
R6
MPLS versus IP paths
IP router IP routing path to destination
determined by destination address alone
R4
Link Layer 5-48
R2
D
R3R4
R5
A
R6
MPLS versus IP paths
IP-onlyrouter
IP routing path to destination determined by destination address alone
MPLS and IP router
MPLS routing path to destination can be based on source and dest address fast reroute precompute backup routes
in case of link failure
entry router (R4) can use different MPLS routes to A based eg on source address
Link Layer 5-49
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-50
Data center networks
10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximity e-business (eg Amazon) content-servers (eg YouTube Akamai Apple Microsoft) search engines data mining (eg Google)
challenges multiple applications each
serving massive numbers of clients
managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container Chicago data center
Link Layer 5-51
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
Load balancer
Load balancer
B
1 2 3 4 5 6 7 8
A C
Border router
Access router
Internet
Data center networks
load balancer application-layer routingreceives external client requestsdirects workload within data centerreturns results to external client (hiding data center internals from client)
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
1 2 3 4 5 6 7 8
Data center networks rich interconnection among switches racks
increased throughput between racks (multiple routing paths possible) increased reliability via redundancy
Link Layer 5-53
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-54
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer5-55
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
web page
browser
router(runs DHCP)
Link Layer 5-56
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router(runs DHCP)
Link Layer 5-57
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router(runs DHCP)
Link Layer 5-58
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
router(runs DHCP)
Link Layer 5-59
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server DNS server replies to
client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router(runs DHCP)
Link Layer 5-60
A day in the lifehellipTCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router(runs DHCP)
Link Layer 5-61
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-62
Chapter 5 Summary
principles behind data link layer services error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS VLANs virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-63
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless multimedia security network management
Link Layer 5-43
type
2-byte Tag Protocol Identifier (value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
destaddress
sourceaddress data (payload) CRCpreamble
data (payload) CRC
type
Link Layer 5-44
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-45
Multiprotocol label switching (MPLS)
initial goal high-speed IP forwarding using fixed length label (instead of IP address) fast lookup using fixed length identifier (rather than
shortest prefix matching) borrowing ideas from Virtual Circuit (VC) approach but IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
Link Layer 5-46
MPLS capable routers
aka label-switched router forward packets to outgoing interface based only
on label value (donrsquot inspect IP address) MPLS forwarding table distinct from IP forwarding tables
flexibility MPLS forwarding decisions can differ from those of IP use destination and source addresses to route flows to
same destination differently (traffic engineering) re-route flows quickly if link fails pre-computed backup
paths (useful for VoIP)
Link Layer 5-47
R2
D
R3R5
A
R6
MPLS versus IP paths
IP router IP routing path to destination
determined by destination address alone
R4
Link Layer 5-48
R2
D
R3R4
R5
A
R6
MPLS versus IP paths
IP-onlyrouter
IP routing path to destination determined by destination address alone
MPLS and IP router
MPLS routing path to destination can be based on source and dest address fast reroute precompute backup routes
in case of link failure
entry router (R4) can use different MPLS routes to A based eg on source address
Link Layer 5-49
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-50
Data center networks
10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximity e-business (eg Amazon) content-servers (eg YouTube Akamai Apple Microsoft) search engines data mining (eg Google)
challenges multiple applications each
serving massive numbers of clients
managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container Chicago data center
Link Layer 5-51
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
Load balancer
Load balancer
B
1 2 3 4 5 6 7 8
A C
Border router
Access router
Internet
Data center networks
load balancer application-layer routingreceives external client requestsdirects workload within data centerreturns results to external client (hiding data center internals from client)
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
1 2 3 4 5 6 7 8
Data center networks rich interconnection among switches racks
increased throughput between racks (multiple routing paths possible) increased reliability via redundancy
Link Layer 5-53
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-54
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer5-55
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
web page
browser
router(runs DHCP)
Link Layer 5-56
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router(runs DHCP)
Link Layer 5-57
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router(runs DHCP)
Link Layer 5-58
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
router(runs DHCP)
Link Layer 5-59
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server DNS server replies to
client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router(runs DHCP)
Link Layer 5-60
A day in the lifehellipTCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router(runs DHCP)
Link Layer 5-61
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-62
Chapter 5 Summary
principles behind data link layer services error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS VLANs virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-63
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless multimedia security network management
Link Layer 5-44
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-45
Multiprotocol label switching (MPLS)
initial goal high-speed IP forwarding using fixed length label (instead of IP address) fast lookup using fixed length identifier (rather than
shortest prefix matching) borrowing ideas from Virtual Circuit (VC) approach but IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
Link Layer 5-46
MPLS capable routers
aka label-switched router forward packets to outgoing interface based only
on label value (donrsquot inspect IP address) MPLS forwarding table distinct from IP forwarding tables
flexibility MPLS forwarding decisions can differ from those of IP use destination and source addresses to route flows to
same destination differently (traffic engineering) re-route flows quickly if link fails pre-computed backup
paths (useful for VoIP)
Link Layer 5-47
R2
D
R3R5
A
R6
MPLS versus IP paths
IP router IP routing path to destination
determined by destination address alone
R4
Link Layer 5-48
R2
D
R3R4
R5
A
R6
MPLS versus IP paths
IP-onlyrouter
IP routing path to destination determined by destination address alone
MPLS and IP router
MPLS routing path to destination can be based on source and dest address fast reroute precompute backup routes
in case of link failure
entry router (R4) can use different MPLS routes to A based eg on source address
Link Layer 5-49
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-50
Data center networks
10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximity e-business (eg Amazon) content-servers (eg YouTube Akamai Apple Microsoft) search engines data mining (eg Google)
challenges multiple applications each
serving massive numbers of clients
managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container Chicago data center
Link Layer 5-51
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
Load balancer
Load balancer
B
1 2 3 4 5 6 7 8
A C
Border router
Access router
Internet
Data center networks
load balancer application-layer routingreceives external client requestsdirects workload within data centerreturns results to external client (hiding data center internals from client)
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
1 2 3 4 5 6 7 8
Data center networks rich interconnection among switches racks
increased throughput between racks (multiple routing paths possible) increased reliability via redundancy
Link Layer 5-53
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-54
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer5-55
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
web page
browser
router(runs DHCP)
Link Layer 5-56
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router(runs DHCP)
Link Layer 5-57
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router(runs DHCP)
Link Layer 5-58
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
router(runs DHCP)
Link Layer 5-59
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server DNS server replies to
client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router(runs DHCP)
Link Layer 5-60
A day in the lifehellipTCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router(runs DHCP)
Link Layer 5-61
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-62
Chapter 5 Summary
principles behind data link layer services error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS VLANs virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-63
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless multimedia security network management
Link Layer 5-45
Multiprotocol label switching (MPLS)
initial goal high-speed IP forwarding using fixed length label (instead of IP address) fast lookup using fixed length identifier (rather than
shortest prefix matching) borrowing ideas from Virtual Circuit (VC) approach but IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
Link Layer 5-46
MPLS capable routers
aka label-switched router forward packets to outgoing interface based only
on label value (donrsquot inspect IP address) MPLS forwarding table distinct from IP forwarding tables
flexibility MPLS forwarding decisions can differ from those of IP use destination and source addresses to route flows to
same destination differently (traffic engineering) re-route flows quickly if link fails pre-computed backup
paths (useful for VoIP)
Link Layer 5-47
R2
D
R3R5
A
R6
MPLS versus IP paths
IP router IP routing path to destination
determined by destination address alone
R4
Link Layer 5-48
R2
D
R3R4
R5
A
R6
MPLS versus IP paths
IP-onlyrouter
IP routing path to destination determined by destination address alone
MPLS and IP router
MPLS routing path to destination can be based on source and dest address fast reroute precompute backup routes
in case of link failure
entry router (R4) can use different MPLS routes to A based eg on source address
Link Layer 5-49
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-50
Data center networks
10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximity e-business (eg Amazon) content-servers (eg YouTube Akamai Apple Microsoft) search engines data mining (eg Google)
challenges multiple applications each
serving massive numbers of clients
managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container Chicago data center
Link Layer 5-51
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
Load balancer
Load balancer
B
1 2 3 4 5 6 7 8
A C
Border router
Access router
Internet
Data center networks
load balancer application-layer routingreceives external client requestsdirects workload within data centerreturns results to external client (hiding data center internals from client)
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
1 2 3 4 5 6 7 8
Data center networks rich interconnection among switches racks
increased throughput between racks (multiple routing paths possible) increased reliability via redundancy
Link Layer 5-53
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-54
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer5-55
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
web page
browser
router(runs DHCP)
Link Layer 5-56
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router(runs DHCP)
Link Layer 5-57
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router(runs DHCP)
Link Layer 5-58
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
router(runs DHCP)
Link Layer 5-59
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server DNS server replies to
client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router(runs DHCP)
Link Layer 5-60
A day in the lifehellipTCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router(runs DHCP)
Link Layer 5-61
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-62
Chapter 5 Summary
principles behind data link layer services error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS VLANs virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-63
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless multimedia security network management
Link Layer 5-46
MPLS capable routers
aka label-switched router forward packets to outgoing interface based only
on label value (donrsquot inspect IP address) MPLS forwarding table distinct from IP forwarding tables
flexibility MPLS forwarding decisions can differ from those of IP use destination and source addresses to route flows to
same destination differently (traffic engineering) re-route flows quickly if link fails pre-computed backup
paths (useful for VoIP)
Link Layer 5-47
R2
D
R3R5
A
R6
MPLS versus IP paths
IP router IP routing path to destination
determined by destination address alone
R4
Link Layer 5-48
R2
D
R3R4
R5
A
R6
MPLS versus IP paths
IP-onlyrouter
IP routing path to destination determined by destination address alone
MPLS and IP router
MPLS routing path to destination can be based on source and dest address fast reroute precompute backup routes
in case of link failure
entry router (R4) can use different MPLS routes to A based eg on source address
Link Layer 5-49
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-50
Data center networks
10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximity e-business (eg Amazon) content-servers (eg YouTube Akamai Apple Microsoft) search engines data mining (eg Google)
challenges multiple applications each
serving massive numbers of clients
managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container Chicago data center
Link Layer 5-51
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
Load balancer
Load balancer
B
1 2 3 4 5 6 7 8
A C
Border router
Access router
Internet
Data center networks
load balancer application-layer routingreceives external client requestsdirects workload within data centerreturns results to external client (hiding data center internals from client)
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
1 2 3 4 5 6 7 8
Data center networks rich interconnection among switches racks
increased throughput between racks (multiple routing paths possible) increased reliability via redundancy
Link Layer 5-53
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-54
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer5-55
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
web page
browser
router(runs DHCP)
Link Layer 5-56
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router(runs DHCP)
Link Layer 5-57
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router(runs DHCP)
Link Layer 5-58
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
router(runs DHCP)
Link Layer 5-59
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server DNS server replies to
client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router(runs DHCP)
Link Layer 5-60
A day in the lifehellipTCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router(runs DHCP)
Link Layer 5-61
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-62
Chapter 5 Summary
principles behind data link layer services error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS VLANs virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-63
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless multimedia security network management
Link Layer 5-47
R2
D
R3R5
A
R6
MPLS versus IP paths
IP router IP routing path to destination
determined by destination address alone
R4
Link Layer 5-48
R2
D
R3R4
R5
A
R6
MPLS versus IP paths
IP-onlyrouter
IP routing path to destination determined by destination address alone
MPLS and IP router
MPLS routing path to destination can be based on source and dest address fast reroute precompute backup routes
in case of link failure
entry router (R4) can use different MPLS routes to A based eg on source address
Link Layer 5-49
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-50
Data center networks
10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximity e-business (eg Amazon) content-servers (eg YouTube Akamai Apple Microsoft) search engines data mining (eg Google)
challenges multiple applications each
serving massive numbers of clients
managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container Chicago data center
Link Layer 5-51
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
Load balancer
Load balancer
B
1 2 3 4 5 6 7 8
A C
Border router
Access router
Internet
Data center networks
load balancer application-layer routingreceives external client requestsdirects workload within data centerreturns results to external client (hiding data center internals from client)
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
1 2 3 4 5 6 7 8
Data center networks rich interconnection among switches racks
increased throughput between racks (multiple routing paths possible) increased reliability via redundancy
Link Layer 5-53
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-54
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer5-55
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
web page
browser
router(runs DHCP)
Link Layer 5-56
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router(runs DHCP)
Link Layer 5-57
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router(runs DHCP)
Link Layer 5-58
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
router(runs DHCP)
Link Layer 5-59
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server DNS server replies to
client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router(runs DHCP)
Link Layer 5-60
A day in the lifehellipTCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router(runs DHCP)
Link Layer 5-61
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-62
Chapter 5 Summary
principles behind data link layer services error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS VLANs virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-63
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless multimedia security network management
Link Layer 5-48
R2
D
R3R4
R5
A
R6
MPLS versus IP paths
IP-onlyrouter
IP routing path to destination determined by destination address alone
MPLS and IP router
MPLS routing path to destination can be based on source and dest address fast reroute precompute backup routes
in case of link failure
entry router (R4) can use different MPLS routes to A based eg on source address
Link Layer 5-49
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-50
Data center networks
10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximity e-business (eg Amazon) content-servers (eg YouTube Akamai Apple Microsoft) search engines data mining (eg Google)
challenges multiple applications each
serving massive numbers of clients
managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container Chicago data center
Link Layer 5-51
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
Load balancer
Load balancer
B
1 2 3 4 5 6 7 8
A C
Border router
Access router
Internet
Data center networks
load balancer application-layer routingreceives external client requestsdirects workload within data centerreturns results to external client (hiding data center internals from client)
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
1 2 3 4 5 6 7 8
Data center networks rich interconnection among switches racks
increased throughput between racks (multiple routing paths possible) increased reliability via redundancy
Link Layer 5-53
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-54
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer5-55
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
web page
browser
router(runs DHCP)
Link Layer 5-56
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router(runs DHCP)
Link Layer 5-57
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router(runs DHCP)
Link Layer 5-58
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
router(runs DHCP)
Link Layer 5-59
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server DNS server replies to
client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router(runs DHCP)
Link Layer 5-60
A day in the lifehellipTCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router(runs DHCP)
Link Layer 5-61
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-62
Chapter 5 Summary
principles behind data link layer services error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS VLANs virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-63
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless multimedia security network management
Link Layer 5-49
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-50
Data center networks
10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximity e-business (eg Amazon) content-servers (eg YouTube Akamai Apple Microsoft) search engines data mining (eg Google)
challenges multiple applications each
serving massive numbers of clients
managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container Chicago data center
Link Layer 5-51
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
Load balancer
Load balancer
B
1 2 3 4 5 6 7 8
A C
Border router
Access router
Internet
Data center networks
load balancer application-layer routingreceives external client requestsdirects workload within data centerreturns results to external client (hiding data center internals from client)
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
1 2 3 4 5 6 7 8
Data center networks rich interconnection among switches racks
increased throughput between racks (multiple routing paths possible) increased reliability via redundancy
Link Layer 5-53
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-54
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer5-55
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
web page
browser
router(runs DHCP)
Link Layer 5-56
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router(runs DHCP)
Link Layer 5-57
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router(runs DHCP)
Link Layer 5-58
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
router(runs DHCP)
Link Layer 5-59
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server DNS server replies to
client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router(runs DHCP)
Link Layer 5-60
A day in the lifehellipTCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router(runs DHCP)
Link Layer 5-61
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-62
Chapter 5 Summary
principles behind data link layer services error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS VLANs virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-63
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless multimedia security network management
Link Layer 5-50
Data center networks
10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximity e-business (eg Amazon) content-servers (eg YouTube Akamai Apple Microsoft) search engines data mining (eg Google)
challenges multiple applications each
serving massive numbers of clients
managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container Chicago data center
Link Layer 5-51
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
Load balancer
Load balancer
B
1 2 3 4 5 6 7 8
A C
Border router
Access router
Internet
Data center networks
load balancer application-layer routingreceives external client requestsdirects workload within data centerreturns results to external client (hiding data center internals from client)
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
1 2 3 4 5 6 7 8
Data center networks rich interconnection among switches racks
increased throughput between racks (multiple routing paths possible) increased reliability via redundancy
Link Layer 5-53
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-54
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer5-55
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
web page
browser
router(runs DHCP)
Link Layer 5-56
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router(runs DHCP)
Link Layer 5-57
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router(runs DHCP)
Link Layer 5-58
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
router(runs DHCP)
Link Layer 5-59
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server DNS server replies to
client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router(runs DHCP)
Link Layer 5-60
A day in the lifehellipTCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router(runs DHCP)
Link Layer 5-61
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-62
Chapter 5 Summary
principles behind data link layer services error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS VLANs virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-63
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless multimedia security network management
Link Layer 5-51
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
Load balancer
Load balancer
B
1 2 3 4 5 6 7 8
A C
Border router
Access router
Internet
Data center networks
load balancer application-layer routingreceives external client requestsdirects workload within data centerreturns results to external client (hiding data center internals from client)
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
1 2 3 4 5 6 7 8
Data center networks rich interconnection among switches racks
increased throughput between racks (multiple routing paths possible) increased reliability via redundancy
Link Layer 5-53
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-54
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer5-55
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
web page
browser
router(runs DHCP)
Link Layer 5-56
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router(runs DHCP)
Link Layer 5-57
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router(runs DHCP)
Link Layer 5-58
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
router(runs DHCP)
Link Layer 5-59
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server DNS server replies to
client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router(runs DHCP)
Link Layer 5-60
A day in the lifehellipTCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router(runs DHCP)
Link Layer 5-61
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-62
Chapter 5 Summary
principles behind data link layer services error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS VLANs virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-63
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless multimedia security network management
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
1 2 3 4 5 6 7 8
Data center networks rich interconnection among switches racks
increased throughput between racks (multiple routing paths possible) increased reliability via redundancy
Link Layer 5-53
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-54
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer5-55
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
web page
browser
router(runs DHCP)
Link Layer 5-56
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router(runs DHCP)
Link Layer 5-57
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router(runs DHCP)
Link Layer 5-58
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
router(runs DHCP)
Link Layer 5-59
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server DNS server replies to
client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router(runs DHCP)
Link Layer 5-60
A day in the lifehellipTCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router(runs DHCP)
Link Layer 5-61
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-62
Chapter 5 Summary
principles behind data link layer services error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS VLANs virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-63
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless multimedia security network management
Link Layer 5-53
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-54
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer5-55
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
web page
browser
router(runs DHCP)
Link Layer 5-56
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router(runs DHCP)
Link Layer 5-57
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router(runs DHCP)
Link Layer 5-58
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
router(runs DHCP)
Link Layer 5-59
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server DNS server replies to
client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router(runs DHCP)
Link Layer 5-60
A day in the lifehellipTCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router(runs DHCP)
Link Layer 5-61
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-62
Chapter 5 Summary
principles behind data link layer services error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS VLANs virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-63
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless multimedia security network management
Link Layer 5-54
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer5-55
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
web page
browser
router(runs DHCP)
Link Layer 5-56
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router(runs DHCP)
Link Layer 5-57
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router(runs DHCP)
Link Layer 5-58
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
router(runs DHCP)
Link Layer 5-59
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server DNS server replies to
client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router(runs DHCP)
Link Layer 5-60
A day in the lifehellipTCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router(runs DHCP)
Link Layer 5-61
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-62
Chapter 5 Summary
principles behind data link layer services error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS VLANs virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-63
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless multimedia security network management
Link Layer5-55
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
web page
browser
router(runs DHCP)
Link Layer 5-56
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router(runs DHCP)
Link Layer 5-57
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router(runs DHCP)
Link Layer 5-58
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
router(runs DHCP)
Link Layer 5-59
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server DNS server replies to
client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router(runs DHCP)
Link Layer 5-60
A day in the lifehellipTCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router(runs DHCP)
Link Layer 5-61
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-62
Chapter 5 Summary
principles behind data link layer services error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS VLANs virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-63
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless multimedia security network management
router(runs DHCP)
Link Layer 5-56
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router(runs DHCP)
Link Layer 5-57
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router(runs DHCP)
Link Layer 5-58
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
router(runs DHCP)
Link Layer 5-59
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server DNS server replies to
client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router(runs DHCP)
Link Layer 5-60
A day in the lifehellipTCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router(runs DHCP)
Link Layer 5-61
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-62
Chapter 5 Summary
principles behind data link layer services error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS VLANs virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-63
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless multimedia security network management
router(runs DHCP)
Link Layer 5-57
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router(runs DHCP)
Link Layer 5-58
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
router(runs DHCP)
Link Layer 5-59
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server DNS server replies to
client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router(runs DHCP)
Link Layer 5-60
A day in the lifehellipTCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router(runs DHCP)
Link Layer 5-61
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-62
Chapter 5 Summary
principles behind data link layer services error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS VLANs virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-63
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless multimedia security network management
router(runs DHCP)
Link Layer 5-58
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
router(runs DHCP)
Link Layer 5-59
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server DNS server replies to
client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router(runs DHCP)
Link Layer 5-60
A day in the lifehellipTCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router(runs DHCP)
Link Layer 5-61
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-62
Chapter 5 Summary
principles behind data link layer services error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS VLANs virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-63
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless multimedia security network management
router(runs DHCP)
Link Layer 5-59
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server DNS server replies to
client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router(runs DHCP)
Link Layer 5-60
A day in the lifehellipTCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router(runs DHCP)
Link Layer 5-61
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-62
Chapter 5 Summary
principles behind data link layer services error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS VLANs virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-63
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless multimedia security network management
router(runs DHCP)
Link Layer 5-60
A day in the lifehellipTCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router(runs DHCP)
Link Layer 5-61
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-62
Chapter 5 Summary
principles behind data link layer services error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS VLANs virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-63
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless multimedia security network management
router(runs DHCP)
Link Layer 5-61
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-62
Chapter 5 Summary
principles behind data link layer services error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS VLANs virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-63
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless multimedia security network management
Link Layer 5-62
Chapter 5 Summary
principles behind data link layer services error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS VLANs virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-63
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless multimedia security network management
Link Layer 5-63
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless multimedia security network management
