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Transport, Network and Link Layers 3-1
Chapter 3Transport, Network and
Link Layers
All material copyright 1996-2012J.F Kurose and K.W. Ross, All Rights Reserved
Informática y Comunicaciones
Chapter 3: outline
3.1 Transport layer
3.2 Network layer
3.3 Link layer
Transport, Network and Link Layers 3-2
Transport services and protocols
provide logical communicationbetween app processes running on different hosts
transport protocols run in end systems
send side: breaks app messages into segments, passes to network layer
rcv side: reassembles segments into messages, passes to app layer
more than one transport protocol available to apps
Internet: TCP and UDP
application
transport
network
data link
physical
application
transport
network
data link
physical
Transport, Network and Link Layers 3-3
Transport vs. network layer
household analogy: network layer:logical communication between hosts
transport layer:logical communication between processes relies on, enhances,
network layer services
12 kids in Ann’s house sending letters to 12 kids in Bill’s house:
hosts = houses processes = kids app messages = letters
in envelopes transport protocol = Ann
and Bill who demux to in-house siblings
network-layer protocol = postal service
Transport, Network and Link Layers 3-4
Multiplexing/demultiplexing
process
socket
use header info to deliver
received segments to correct
socket
demultiplexing at receiver:handle data from multiple
sockets, add transport header (later used for demultiplexing)
multiplexing at sender:
transport
application
physical
link
network
P2P1
transport
application
physical
link
network
P4
transport
application
physical
link
network
P3
Transport, Network and Link Layers 3-5
How demultiplexing works
host receives IP datagrams each datagram has source
IP address, destination IP address
each datagram carries one transport-layer segment
each segment has source, destination port number
host uses IP addresses & port numbers to direct segment to appropriate socket
source port # dest port #
32 bits
application
data
(payload)
other header fields
TCP/UDP segment format
Transport, Network and Link Layers 3-6
UDP: User Datagram Protocol
“no frills,” “bare bones”Internet transport protocol
“best effort” service, UDP segments may be:
lost
delivered out-of-order to app
connectionless:
no handshaking between UDP sender, receiver
each UDP segment handled independently of others
UDP use: streaming multimedia
apps (loss tolerant, rate sensitive)
DNS
SNMP
reliable transfer over UDP: add reliability at
application layer
application-specific error recovery!
Transport, Network and Link Layers 3-7
UDP: segment header
source port # dest port #
32 bits
application
data
(payload)
UDP segment format
length checksum
length, in bytes of UDP segment,
including header
no connection establishment (which can add delay)
simple: no connection state at sender, receiver
small header size
no congestion control: UDP can blast away as fast as desired
why is there a UDP?
Transport, Network and Link Layers 3-8
UDP checksum
sender: treat segment
contents, including header fields, as sequence of 16-bit integers
checksum: addition (one’s complement sum) of segment contents
sender puts checksum value into UDP checksum field
receiver: compute checksum of
received segment
check if computed checksum equals checksum field value:
NO - error detected
YES - no error detected. But maybe errors nonetheless?
Goal: detect “errors” (e.g., flipped bits) in transmitted segment
Transport, Network and Link Layers 3-9
TCP: Overview
full duplex data: bi-directional data flow
in same connection
MSS: maximum segment size
connection-oriented: handshaking (exchange
of control msgs) initssender, receiver state before data exchange
flow controlled: sender will not
overwhelm receiver
point-to-point: one sender, one
receiver
reliable, in-order byte stream: no “message
boundaries”
pipelined: TCP congestion and
flow control set window size
Transport, Network and Link Layers 3-10
TCP segment structure
source port # dest port #
32 bits
application
data
(variable length)
sequence number
acknowledgement number
receive window
Urg data pointerchecksum
FSRPAUhead
len
not
used
options (variable length)
URG: urgent data
(generally not used)
ACK: ACK #
valid
PSH: push data now
(generally not used)
RST, SYN, FIN:
connection estab
(setup, teardown
commands)
# bytes
rcvr willing
to accept
counting
by bytes
of data
(not segments!)
Internet
checksum
(as in UDP)
Transport, Network and Link Layers 3-11
TCP reliable data transfer
TCP creates rdt service on top of IP’s unreliable service pipelined segments
cumulative acks
single retransmission timer
retransmissions triggered by: timeout events
duplicate acks
Transport, Network and Link Layers 3-12
congestion: informally: “too many sources sending too
much data too fast for network to handle”
different from flow control!
manifestations:
lost packets (buffer overflow at routers)
long delays (queueing in router buffers)
a top-10 problem!
Principles of congestion control
Transport, Network and Link Layers 3-13
Causes/costs of congestion: scenario 1
two senders, two receivers
one router, infinite buffers
output link capacity: R
no retransmission
maximum per-connection throughput: R/2
unlimited shared
output link buffers
Host A
original data: lin
Host B
throughput:
lout
R/2
R/2
lo
ut
lin R/2d
ela
ylin
large delays as arrival rate, lin, approaches capacity
Transport, Network and Link Layers 3-14
one router, finite buffers
sender retransmission of timed-out packet application-layer input = application-layer output: lin = lout
transport-layer input includes retransmissions : lin lin
finite shared
output link
buffers
Host A
lin : original
data
Host B
lout
l'in: original data,
plus retransmitted
data
‘
Causes/costs of congestion: scenario 2
Transport, Network and Link Layers 3-15
idealization: perfect knowledge
sender sends only when router buffers available
finite shared
output link
buffers
lin : original
data lout
l'in: original data,
plus retransmitted
data
copy
free buffer space!
R/2
R/2
lo
ut
lin
Causes/costs of congestion: scenario 2
Host B
Host A
Transport, Network and Link Layers 3-16
lin : original
data lout
l'in: original data,
plus retransmitted
data
copy
no buffer space!
Idealization: known loss packets can be lost, dropped at router due to full buffers
sender only resends if packet known to be lost
Causes/costs of congestion: scenario 2
A
Host
B Transport, Network and Link Layers 3-17
lin : original
data lout
l'in: original data,
plus retransmitted
datafree buffer space!
Causes/costs of congestion: scenario 2
Idealization: known loss packets can be lost, dropped at router due to full buffers
sender only resends if packet known to be lost
R/2
R/2lin
lo
ut
when sending at R/2,
some packets are
retransmissions but
asymptotic goodput
is still R/2
A
Host
B Transport, Network and Link Layers 3-18
A
linloutl'in
copy
free buffer space!
timeout
R/2
R/2lin
lo
ut
when sending at R/2,
some packets are
retransmissions
including duplicated
that are delivered!
Host
B
Realistic: duplicates packets can be lost, dropped
at router due to full buffers
sender times out prematurely, sending two copies, both of which are delivered
Causes/costs of congestion: scenario 2
Transport, Network and Link Layers 3-19
R/2
lo
ut
when sending at R/2,
some packets are
retransmissions
including duplicated
that are delivered!
“costs” of congestion: more work (retrans) for given “goodput” unneeded retransmissions: link carries multiple copies
of pkt
decreasing goodput
R/2lin
Causes/costs of congestion: scenario 2
Realistic: duplicates packets can be lost,
dropped at router due to full buffers
sender times out prematurely, sending twocopies, both of which are delivered
Transport, Network and Link Layers 3-20
four senders
multihop paths
timeout/retransmit
Q: what happens as lin and lin’
increase ?
finite shared output link
buffers
Host A lout
Causes/costs of congestion: scenario 3
Host B
Host C
Host D
lin : original data
l'in: original data, plus
retransmitted data
A: as red lin’ increases, all arriving
blue pkts at upper queue are dropped, blue throughput g 0
Transport, Network and Link Layers 3-21
another “cost” of congestion:
when packet dropped, any “upstream transmission capacity used for that packet was wasted!
Causes/costs of congestion: scenario 3
C/2
C/2
lo
ut
lin’
Transport, Network and Link Layers 3-22
Approaches towards congestion control
two broad approaches towards congestion control:
end-end congestion control:
no explicit feedback from network
congestion inferred from end-system observed loss, delay
approach taken by TCP
network-assisted congestion control:
routers provide feedback to end systems
single bit indicating congestion (SNA, DECbit, TCP/IP ECN, ATM)
explicit rate for sender to send at
Transport, Network and Link Layers 3-23
fairness goal: if K TCP sessions share same bottleneck link of bandwidth R, each should have average rate of R/K
TCP connection 1
bottleneck
router
capacity R
TCP Fairness
TCP connection 2
Transport, Network and Link Layers 3-24
Chapter 3: outline
3.1 Transport layer
3.2 Network layer
3.3 Link layer
Transport, Network and Link Layers 3-25
Network layer
transport segment from sending to receiving host
on sending side encapsulates segments into datagrams
on receiving side, delivers segments to transport layer
network layer protocols in every host, router
router examines header fields in all IP datagrams passing through it
application
transport
network
data link
physical
application
transport
network
data link
physical
network
data link
physical network
data link
physical
network
data link
physical
network
data link
physical
network
data link
physical
network
data link
physical
network
data link
physical
network
data link
physical
network
data link
physical
network
data link
physicalnetwork
data link
physical
Transport, Network and Link Layers 3-26
Two key network-layer functions
forwarding: move packets from router’s input to appropriate router output
routing: determine route taken by packets from source to dest.
routing algorithms
analogy:
routing: process of planning trip from source to dest
forwarding: process of getting through single interchange
Transport, Network and Link Layers 3-27
1
23
0111
value in arriving
packet’s header
routing algorithm
local forwarding table
header value output link
0100
0101
0111
1001
3
2
2
1
Interplay between routing and forwarding
routing algorithm determines
end-end-path through network
forwarding table determines
local forwarding at this router
Transport, Network and Link Layers 3-28
The Internet network layer
forwarding
table
host, router network layer functions:
routing
protocols
• path selection
• RIP, OSPF, BGP
IP protocol
• addressing conventions
• datagram format
• packet handling conventions
ICMP protocol
• error reporting
• router “signaling”
transport layer: TCP, UDP
link layer
physical layer
network
layer
Transport, Network and Link Layers 3-29
ver length
32 bits
data
(variable length,
typically a TCP
or UDP segment)
16-bit identifier
header
checksum
time to
live
32 bit source IP address
head.
len
type of
serviceflgs
fragment
offsetupper
layer
32 bit destination IP address
options (if any)
IP datagram formatIP protocol version
numberheader length
(bytes)
upper layer protocol
to deliver payload to
total datagram
length (bytes)
“type” of data for
fragmentation/
reassemblymax number
remaining hops
(decremented at
each router)
e.g. timestamp,
record route
taken, specify
list of routers
to visit.
how much overhead?
20 bytes of TCP
20 bytes of IP
= 40 bytes + app layer overhead
Transport, Network and Link Layers 3-30
IP addressing: introduction
IP address: 32-bit identifier for host, router interface
interface: connection between host/router and physical link router’s typically have
multiple interfaces
host typically has one or two interfaces (e.g., wired Ethernet, wireless 802.11)
IP addresses associated with each interface
223.1.1.1
223.1.1.2
223.1.1.3
223.1.1.4 223.1.2.9
223.1.2.2
223.1.2.1
223.1.3.2223.1.3.1
223.1.3.27
223.1.1.1 = 11011111 00000001 00000001 00000001
223 1 11
Transport, Network and Link Layers 3-31
IP addressing: introduction223.1.1.1
223.1.1.2
223.1.1.3
223.1.1.4 223.1.2.9
223.1.2.2
223.1.2.1
223.1.3.2223.1.3.1
223.1.3.27
wired Ethernet interfaces connected
by Ethernet switches
wireless WiFi interfaces connected
by WiFi base station
Transport, Network and Link Layers 3-32
Subnets
IP address:subnet part - high order bits
host part - low order bits
what’s a subnet ?device interfaces with same subnet part of IP address
can physically reach each other without intervening router network consisting of 3 subnets
223.1.1.1
223.1.1.3
223.1.1.4 223.1.2.9
223.1.3.2223.1.3.1
subnet
223.1.1.2
223.1.3.27223.1.2.2
223.1.2.1
Transport, Network and Link Layers 3-33
recipe
to determine the subnets, detach each interface from its host or router, creating islands of isolated networks
each isolated network is called a subnet
subnet mask: /24
Subnets223.1.1.0/24
223.1.2.0/24
223.1.3.0/24
223.1.1.1
223.1.1.3
223.1.1.4 223.1.2.9
223.1.3.2223.1.3.1
subnet
223.1.1.2
223.1.3.27223.1.2.2
223.1.2.1
Transport, Network and Link Layers 3-34
how many? 223.1.1.1
223.1.1.3
223.1.1.4
223.1.2.2223.1.2.1
223.1.2.6
223.1.3.2223.1.3.1
223.1.3.27
223.1.1.2
223.1.7.0
223.1.7.1
223.1.8.0223.1.8.1
223.1.9.1
223.1.9.2
Subnets
Transport, Network and Link Layers 3-35
IP addressing: CIDR
CIDR: Classless InterDomain Routing subnet portion of address of arbitrary length
address format: a.b.c.d/x, where x is # bits in subnet portion of address
11001000 00010111 00010000 00000000
subnet
part
host
part
200.23.16.0/23
Transport, Network and Link Layers 3-36
IP addresses: how to get one?
Q: How does a host get IP address?
hard-coded by system admin in a file Windows: control-panel->network->configuration-
>tcp/ip->properties
UNIX: /etc/rc.config
DHCP: Dynamic Host Configuration Protocol: dynamically get address from as server
“plug-and-play”
Transport, Network and Link Layers 3-37
DHCP: Dynamic Host Configuration Protocol
goal: allow host to dynamically obtain its IP address from network server when it joins network
can renew its lease on address in use
allows reuse of addresses (only hold address while connected/“on”)
support for mobile users who want to join network (more shortly)
DHCP overview: host broadcasts “DHCP discover” msg [optional]
DHCP server responds with “DHCP offer” msg [optional]
host requests IP address: “DHCP request” msg
DHCP server sends address: “DHCP ack” msg
Transport, Network and Link Layers 3-38
DHCP client-server scenario
223.1.1.0/24
223.1.2.0/24
223.1.3.0/24
223.1.1.1
223.1.1.3
223.1.1.4 223.1.2.9
223.1.3.2223.1.3.1
223.1.1.2
223.1.3.27223.1.2.2
223.1.2.1
DHCP
server
arriving DHCP
client needs
address in this
network
Transport, Network and Link Layers 3-39
DHCP: more than IP addresses
DHCP can return more than just allocated IP address on subnet: address of first-hop router for client
name and IP address of DNS sever
network mask (indicating network versus host portion of address)
Transport, Network and Link Layers 3-40
IP addresses: how to get one?
Q: how does network get subnet part of IP addr?
A: gets allocated portion of its provider ISP’s address space
ISP's block 11001000 00010111 00010000 00000000
200.23.16.0/20
Organization 0 11001000 00010111 00010000 00000000
200.23.16.0/23
Organization 1 11001000 00010111 00010010 00000000
200.23.18.0/23
Organization 2 11001000 00010111 00010100 00000000
200.23.20.0/23
... ….. …. ….
Organization 7 11001000 00010111 00011110 00000000
200.23.30.0/23 Transport, Network and Link Layers 3-41
IP addressing: the last word...
Q: how does an ISP get block of addresses?
A: ICANN: Internet Corporation for Assigned
Names and Numbers http://www.icann.org/
allocates addresses
manages DNS
assigns domain names, resolves disputes
Transport, Network and Link Layers 3-42
NAT: network address translation
10.0.0.1
10.0.0.2
10.0.0.3
10.0.0.4
138.76.29.7
local network
(e.g., home network)
10.0.0/24
rest of
Internet
datagrams with source or
destination in this network
have 10.0.0/24 address for
source, destination (as usual)
all datagrams leaving local
network have same single source NAT IP address:
138.76.29.7,different source port numbers
Transport, Network and Link Layers 3-43
motivation: local network uses just one IP address as far as outside world is concerned:
range of addresses not needed from ISP: just one IP address for all devices
can change addresses of devices in local network without notifying outside world
can change ISP without changing addresses of devices in local network
devices inside local net not explicitly addressable, visible by outside world (a security plus)
NAT: network address translation
Transport, Network and Link Layers 3-44
implementation: NAT router must:
outgoing datagrams: replace (source IP address, port #) of every outgoing datagram to (NAT IP address, new port #)
. . . remote clients/servers will respond using (NAT IP address, new port #) as destination addr
remember (in NAT translation table) every (source IP address, port #) to (NAT IP address, new port #) translation pair
incoming datagrams: replace (NAT IP address, new port #) in dest fields of every incoming datagram with corresponding (source IP address, port #) stored in NAT table
NAT: network address translation
Transport, Network and Link Layers 3-45
10.0.0.1
10.0.0.2
10.0.0.3
S: 10.0.0.1, 3345
D: 128.119.40.186, 80
1
10.0.0.4
138.76.29.7
1: host 10.0.0.1
sends datagram to
128.119.40.186, 80
NAT translation table
WAN side addr LAN side addr
138.76.29.7, 5001 10.0.0.1, 3345
…… ……
S: 128.119.40.186, 80
D: 10.0.0.1, 33454
S: 138.76.29.7, 5001
D: 128.119.40.186, 802
2: NAT router
changes datagram
source addr from
10.0.0.1, 3345 to
138.76.29.7, 5001,
updates table
S: 128.119.40.186, 80
D: 138.76.29.7, 50013
3: reply arrives
dest. address:
138.76.29.7, 5001
4: NAT router
changes datagram
dest addr from
138.76.29.7, 5001 to 10.0.0.1, 3345
NAT: network address translation
Transport, Network and Link Layers 3-46
16-bit port-number field:
60,000 simultaneous connections with a single LAN-side address!
NAT is controversial:
routers should only process up to layer 3
violates end-to-end argument
• NAT possibility must be taken into account by
app designers, e.g., P2P applications
address shortage should instead be solved by IPv6
NAT: network address translation
Transport, Network and Link Layers 3-47
IPv6: motivation
initial motivation: 32-bit address space soon to be completely allocated.
additional motivation: header format helps speed processing/forwarding
header changes to facilitate QoS
IPv6 datagram format: fixed-length 40 byte header
no fragmentation allowed
Transport, Network and Link Layers 3-48
IPv6 datagram format
priority: identify priority among datagrams in flow
flow Label: identify datagrams in same “flow.”
(concept of“flow” not well defined).
next header: identify upper layer protocol for data
data
destination address
(128 bits)
source address
(128 bits)
payload len next hdr hop limitflow labelpriver
32 bitsTransport, Network and Link Layers 3-49
Transition from IPv4 to IPv6
not all routers can be upgraded simultaneously no “flag days” how will network operate with mixed IPv4 and
IPv6 routers?
tunneling: IPv6 datagram carried as payload in IPv4 datagram among IPv4 routers
IPv4 source, dest addr
IPv4 header fields
IPv4 datagram
IPv6 datagram
IPv4 payload
UDP/TCP payload
IPv6 source dest addr
IPv6 header fields
Transport, Network and Link Layers 3-50
Chapter 3: outline
3.1 Transport layer
3.2 Network layer
3.3 Link layer
Transport, Network and Link Layers 3-51
Link layer: introduction
terminology: hosts and routers: nodes
communication channels that connect adjacent nodes along communication path: links
wired links
wireless links
LANs
layer-2 packet: frame,encapsulates datagram
data-link layer has responsibility of
transferring datagram from one node
to physically adjacent node over a link
global ISP
Transport, Network and Link Layers 3-52
Link layer: context
datagram transferred by different link protocols over different links:
e.g., Ethernet on first link, frame relay on intermediate links, 802.11 on last link
each link protocol provides different services
e.g., may or may not provide rdt over link
transportation analogy: trip from Princeton to Lausanne
limo: Princeton to JFK
plane: JFK to Geneva
train: Geneva to Lausanne
tourist = datagram
transport segment = communication link
transportation mode = link layer protocol
travel agent = routing algorithm
Transport, Network and Link Layers 3-53
Link layer services
framing, link access: encapsulate datagram into frame, adding header,
trailer channel access if shared medium “MAC” addresses used in frame headers to identify
source, dest
• different from IP address! reliable delivery between adjacent nodes
we learned how to do this already! seldom used on low bit-error link (fiber, some twisted
pair) wireless links: high error rates
Transport, Network and Link Layers 3-54
flow control: pacing between adjacent sending and receiving nodes
error detection: errors caused by signal attenuation, noise.
receiver detects presence of errors:
• signals sender for retransmission or drops frame
error correction: receiver identifies and corrects bit error(s) without resorting
to retransmission
half-duplex and full-duplex with half duplex, nodes at both ends of link can transmit, but
not at same time
Link layer services (more)
Transport, Network and Link Layers 3-55
Where is the link layer implemented?
in each and every host
link layer implemented in “adaptor” (aka network interface card NIC) or on a chip
Ethernet card, 802.11 card; Ethernet chipset
implements link, physical layer
attaches into host’s system buses
combination of hardware, software, firmware
controller
physical
transmission
cpu memory
host
bus
(e.g., PCI)
network adapter
card
application
transport
network
link
link
physical
Transport, Network and Link Layers 3-56
Adaptors communicating
sending side:
encapsulates datagram in frame
adds error checking bits, rdt, flow control, etc.
receiving side
looks for errors, rdt, flow control, etc
extracts datagram, passes to upper layer at receiving side
controller controller
sending host receiving host
datagram datagram
datagram
frame
Transport, Network and Link Layers 3-57
Error detectionEDC= Error Detection and Correction bits (redundancy)
D = Data protected by error checking, may include header fields
• Error detection not 100% reliable!
• protocol may miss some errors, but rarely
• larger EDC field yields better detection and correction
otherwise
Transport, Network and Link Layers 3-58
Multiple access links, protocols
two types of “links”:
point-to-point PPP for dial-up access
point-to-point link between Ethernet switch, host
broadcast (shared wire or medium) old-fashioned Ethernet
upstream HFC
802.11 wireless LAN
shared wire (e.g.,
cabled Ethernet)
shared RF
(e.g., 802.11 WiFi)
shared RF
(satellite)
humans at a
cocktail party
(shared air, acoustical)
Transport, Network and Link Layers 3-59
Multiple access protocols
single shared broadcast channel
two or more simultaneous transmissions by nodes: interference
collision if node receives two or more signals at the same time
multiple access protocol distributed algorithm that determines how nodes share
channel, i.e., determine when node can transmit
communication about channel sharing must use channel itself! no out-of-band channel for coordination
Transport, Network and Link Layers 3-60
An ideal multiple access protocol
given: broadcast channel of rate R bps
desiderata:1. when one node wants to transmit, it can send at rate
R.
2. when M nodes want to transmit, each can send at average rate R/M
3. fully decentralized:
• no special node to coordinate transmissions
• no synchronization of clocks, slots4. simple
Transport, Network and Link Layers 3-61
MAC protocols: taxonomy
three broad classes:
channel partitioning divide channel into smaller “pieces” (time slots, frequency,
code)
allocate piece to node for exclusive use
random access channel not divided, allow collisions
“recover” from collisions
“taking turns” nodes take turns, but nodes with more to send can take
longer turns
Transport, Network and Link Layers 3-62
Channel partitioning MAC protocols: TDMA
TDMA: time division multiple access access to channel in "rounds" each station gets fixed length slot (length =
pkt trans time) in each round unused slots go idle example: 6-station LAN, 1,3,4 have pkt, slots
2,5,6 idle
1 3 4 1 3 4
6-slot
frame
6-slot
frame
Transport, Network and Link Layers 3-63
FDMA: frequency division multiple access channel spectrum divided into frequency bands
each station assigned fixed frequency band
unused transmission time in frequency bands go idle
example: 6-station LAN, 1,3,4 have pkt, frequency bands 2,5,6 idle
fre
qu
en
cy b
an
ds
FDM cable
Channel partitioning MAC protocols: FDMA
Transport, Network and Link Layers 3-64
Random access protocols
when node has packet to send transmit at full channel data rate R. no a priori coordination among nodes
two or more transmitting nodes ➜ “collision”, random access MAC protocol specifies:
how to detect collisions how to recover from collisions (e.g., via delayed
retransmissions)
examples of random access MAC protocols: slotted ALOHA ALOHA CSMA, CSMA/CD, CSMA/CA
Transport, Network and Link Layers 3-65
“Taking turns” MAC protocols
channel partitioning MAC protocols: share channel efficiently and fairly at high load
inefficient at low load: delay in channel access, 1/N bandwidth allocated even if only 1 active node!
random access MAC protocols efficient at low load: single node can fully utilize
channel
high load: collision overhead
“taking turns” protocolslook for best of both worlds!
Transport, Network and Link Layers 3-66
polling: master node “invites” slave nodes to transmit in turn
typically used with “dumb” slave devices
concerns:
polling overhead
latency
single point of failure (master)
master
slaves
poll
data
data
“Taking turns” MAC protocols
Transport, Network and Link Layers 3-67
token passing: control token passed
from one node to next sequentially.
token message
concerns:
token overhead
latency
single point of failure (token)
T
data
(nothing
to send)
T
“Taking turns” MAC protocols
Transport, Network and Link Layers 3-68
MAC addresses and ARP
32-bit IP address: network-layer address for interface
used for layer 3 (network layer) forwarding
MAC (or LAN or physical or Ethernet) address: function: used ‘locally” to get frame from one interface
to another physically-connected interface (same network, in IP-addressing sense)
48 bit MAC address (for most LANs) burned in NIC ROM, also sometimes software settable
e.g.: 1A-2F-BB-76-09-AD
hexadecimal (base 16) notation
(each “number” represents 4 bits)
Transport, Network and Link Layers 3-69
LAN addresses and ARP
each adapter on LAN has unique LAN address
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 or
wireless)
Transport, Network and Link Layers 3-70
Ethernet
“dominant” wired LAN technology:
cheap $20 for NIC
first widely used LAN technology
simpler, cheaper than token LANs and ATM
kept up with speed race: 10 Mbps – 10 Gbps
Metcalfe’s Ethernet
sketch Transport, Network and Link Layers 3-71
Ethernet: physical topology
bus: popular through mid 90s all nodes in same collision domain (can collide with
each other)
star: prevails today active switch in center
each “spoke” runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus: coaxial cablestar
Transport, Network and Link Layers 3-72
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
dest.address
sourceaddress
data (payload) CRCpreamble
type
Transport, Network and Link Layers 3-73
Ethernet frame structure (more)
addresses: 6 byte source, destination MAC addresses if adapter receives frame with matching destination
address, or with broadcast address (e.g. ARP packet), it passes data in frame to network layer protocol
otherwise, adapter discards frame
type: indicates higher layer protocol (mostly IP but others possible, e.g., Novell IPX, AppleTalk)
CRC: cyclic redundancy check at receiver error detected: frame is dropped
dest.address
sourceaddress
data (payload) CRCpreamble
type
Transport, Network and Link Layers 3-74
Ethernet switch link-layer device: takes an active role
store, forward Ethernet frames
examine incoming frame’s MAC address, selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment, uses CSMA/CD to access segment
transparent
hosts are unaware of presence of switches
plug-and-play, self-learning
switches do not need to be configured
Transport, Network and Link Layers 3-75
Switch: 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-A’ and B-to-B’ can transmit simultaneously, without collisions
switch with six interfaces
(1,2,3,4,5,6)
A
A’
B
B’ C
C’
1 2
345
6
Transport, Network and Link Layers 3-76
Interconnecting switches
switches can be connected together
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Transport, Network and Link Layers 3-77
Institutional network
to external
networkrouter
IP subnet
mail server
web server
Transport, Network and Link Layers 3-78
Switches vs. routers
both are store-and-forward:
routers: network-layer devices (examine network-layer headers)
switches: link-layer devices (examine link-layer headers)
both have forwarding tables:
routers: compute tables using routing algorithms, IP addresses
switches: learn forwarding table using flooding, learning, MAC addresses
application
transport
network
link
physical
network
link
physical
link
physical
switch
datagram
application
transport
network
link
physical
frame
frame
frame
datagram
Transport, Network and Link Layers 3-79
Chapter 3: summary
transport layer
UDP
TCP
Congestion
network layer
Routing
IP
DHCP
NAT
link layer
Error detection
MAC
Ethernet
our study of layers now complete!
Transport, Network and Link Layers 3-80
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