Transcript Link Layer

Chapter 5
Link Layer and LANs
A note on the use of these ppt slides:
We’re making these slides freely available to all (faculty, students, readers).
They’re in PowerPoint form so you can add, modify, and delete slides
(including this one) and slide content to suit your needs. They obviously
represent a lot of work on our part. In return for use, we only ask the
following:
 If you use these slides (e.g., in a class) in substantially unaltered form,
that you mention their source (after all, we’d like people to use our book!)
 If you post any slides in substantially unaltered form on a www site, that
you note that they are adapted from (or perhaps identical to) our slides, and
note our copyright of this material.
Computer Networking:
A Top Down Approach
Featuring the Internet,
3rd edition.
Jim Kurose, Keith Ross
Addison-Wesley, July
2004.
Thanks and enjoy! JFK/KWR
All material copyright 1996-2004
J.F Kurose and K.W. Ross, All Rights Reserved
5: DataLink Layer
5-1
Chapter 5: The Data Link Layer
Our goals:
r understand principles behind data link layer
services:
m
m
m
m
error detection, correction
sharing a broadcast channel: multiple access
link layer addressing
reliable data transfer, flow control: done!
r instantiation and implementation of various link
layer technologies
5: DataLink Layer
5-2
Link Layer
r 5.1 Introduction and
r
r
r
r
services
5.2 Error detection
and correction
5.3Multiple access
protocols
5.4 Link-Layer
Addressing
5.5 Ethernet
r 5.6 Hubs and switches
r 5.7 PPP
r 5.8 Link Virtualization:
ATM and MPLS
5: DataLink Layer
5-3
Link Layer: Introduction
Some terminology:
r
r
hosts and routers are nodes
communication channels that
connect adjacent nodes along
communication path are links
m
m
m
r
“link”
wired links
wireless links
LANs
layer-2 packet is a frame,
encapsulates datagram
data-link layer has responsibility of
transferring datagram from one node
to adjacent node over a link
5: DataLink Layer
5-4
Link layer: context
r Datagram transferred by
different link protocols
over different links:
m
e.g., Ethernet on first link,
frame relay on
intermediate links, 802.11
on last link
r Each link protocol
provides different
services
m
e.g., may or may not
provide rdt over link
transportation analogy
r
trip from Princeton to
Lausanne
m limo: Princeton to JFK
m plane: JFK to Geneva
m train: Geneva to Lausanne
r tourist = datagram
r transport segment =
communication link
r transportation mode =
link layer protocol
r travel agent = routing
algorithm
5: DataLink Layer
5-5
Link Layer Services
r Framing, link access:
m
m
m
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!
r Reliable delivery between adjacent nodes
m we learned how to do this already (chapter 3)!
m seldom used on low bit error link (fiber, some twisted
pair)
m wireless links: high error rates
• Q: why both link-level and end-end reliability?
5: DataLink Layer
5-6
Link Layer Services (more)
r Flow Control:
m
pacing between adjacent sending and receiving nodes
r Error Detection:
m
m
errors caused by signal attenuation, noise.
receiver detects presence of errors:
• signals sender for retransmission or drops frame
r Error Correction:
m
receiver identifies and corrects bit error(s) without
resorting to retransmission
r Half-duplex and full-duplex
m with half duplex, nodes at both ends of link can transmit,
but not at same time
5: DataLink Layer
5-7
Adaptors Communicating
datagram
sending
node
frame
adapter
rcving
node
link layer protocol
frame
adapter
r link layer implemented in r receiving side
“adaptor” (aka NIC)
m looks for errors, rdt, flow
control, etc
m Ethernet card, PCMCI
m extracts datagram, passes
card, 802.11 card
to rcving node
r sending side:
r adapter is semim encapsulates datagram in
autonomous
a frame
m adds error checking bits,
r link & physical layers
rdt, flow control, etc.
5: DataLink Layer
5-8
Link Layer
r 5.1 Introduction and
r
r
r
r
services
5.2 Error detection
and correction
5.3Multiple access
protocols
5.4 Link-Layer
Addressing
5.5 Ethernet
r 5.6 Hubs and switches
r 5.7 PPP
r 5.8 Link Virtualization:
ATM
5: DataLink Layer
5-9
Error Detection
EDC= 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
5: DataLink Layer
5-10
Parity Checking
Single Bit Parity:
Detect single bit errors
Two Dimensional Bit Parity:
Detect and correct single bit errors
0
0
5: DataLink Layer
5-11
Internet checksum
Goal: detect “errors” (e.g., flipped bits) in transmitted
segment (note: used at transport layer only)
Sender:
r
r
r
treat segment contents
as sequence of 16-bit
integers
checksum: addition (1’s
complement sum) of
segment contents
sender puts checksum
value into UDP checksum
field
Receiver:
r
r
compute checksum of received
segment
check if computed checksum
equals checksum field value:
m NO - error detected
m YES - no error detected. But
maybe errors nonetheless?
More later ….
5: DataLink Layer
5-12
Checksumming: Cyclic Redundancy Check
r
r
r
view data bits, D, as a binary number
choose r+1 bit pattern (generator), G
goal: choose r CRC bits, R, such that
m
m
m
r
<D,R> exactly divisible by G (modulo 2)
receiver knows G, divides <D,R> by G. If non-zero remainder:
error detected!
can detect all burst errors less than r+1 bits
widely used in practice (ATM, HDCL)
5: DataLink Layer
5-13
CRC Example
Want:
D.2r XOR R = nG
equivalently:
D.2r = nG XOR R
equivalently:
if we divide D.2r by
G, want remainder R
R = remainder[
D.2r
G
]
5: DataLink Layer
5-14
Link Layer
r 5.1 Introduction and
r
r
r
r
services
5.2 Error detection
and correction
5.3Multiple access
protocols
5.4 Link-Layer
Addressing
5.5 Ethernet
r 5.6 Hubs and switches
r 5.7 PPP
r 5.8 Link Virtualization:
ATM
5: DataLink Layer
5-15
Multiple Access Links and Protocols
Two types of “links”:
r point-to-point
m PPP for dial-up access
m point-to-point link between Ethernet switch and host
r broadcast (shared wire or medium)
m traditional Ethernet
m upstream HFC
m 802.11 wireless LAN
5: DataLink Layer
5-16
Multiple Access protocols
r single shared broadcast channel
r two or more simultaneous transmissions by nodes:
interference
m
collision if node receives two or more signals at the same time
multiple access protocol
r distributed algorithm that determines how nodes
share channel, i.e., determine when node can transmit
r communication about channel sharing must use channel
itself!
m
no out-of-band channel for coordination
5: DataLink Layer
5-17
Ideal Mulitple Access Protocol
Broadcast channel of rate R bps
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:
m
m
no special node to coordinate transmissions
no synchronization of clocks, slots
4. Simple
5: DataLink Layer
5-18
MAC Protocols: a taxonomy
Three broad classes:
r Channel Partitioning
m
m
divide channel into smaller “pieces” (time slots,
frequency, code)
allocate piece to node for exclusive use
r Random Access
m channel not divided, allow collisions
m “recover” from collisions
r “Taking turns”
m Nodes take turns, but nodes with more to send can take
longer turns
5: DataLink Layer
5-19
Channel Partitioning MAC protocols: TDMA
TDMA: time division multiple access
r access to channel in "rounds"
r each station gets fixed length slot (length = pkt
trans time) in each round
r unused slots go idle
r example: 6-station LAN, 1,3,4 have pkt, slots 2,5,6
idle
5: DataLink Layer
5-20
Channel Partitioning MAC protocols: FDMA
FDMA: frequency division multiple access
r channel spectrum divided into frequency bands
r each station assigned fixed frequency band
r unused transmission time in frequency bands go idle
r example: 6-station LAN, 1,3,4 have pkt, frequency
frequency bands
bands 2,5,6 idle
5: DataLink Layer
5-21
Random Access Protocols
r When node has packet to send
m transmit at full channel data rate R.
m no a priori coordination among nodes
r two or more transmitting nodes ➜ “collision”,
r random access MAC protocol specifies:
m how to detect collisions
m how to recover from collisions (e.g., via delayed
retransmissions)
r Examples of random access MAC protocols:
m slotted ALOHA
m ALOHA
m CSMA, CSMA/CD, CSMA/CA
5: DataLink Layer
5-22
Slotted ALOHA
Assumptions
r all frames same size
r time is divided into
equal size slots, time to
transmit 1 frame
r nodes start to transmit
frames only at
beginning of slots
r nodes are synchronized
r if 2 or more nodes
transmit in slot, all
nodes detect collision
Operation
r when node obtains fresh
frame, it transmits in next
slot
r no collision, node can send
new frame in next slot
r if collision, node
retransmits frame in each
subsequent slot with prob.
p until success
5: DataLink Layer
5-23
Slotted ALOHA
Pros
r single active node can
continuously transmit
at full rate of channel
r highly decentralized:
only slots in nodes
need to be in sync
r simple
Cons
r collisions, wasting slots
r idle slots
r nodes may be able to
detect collision in less
than time to transmit
packet
r clock synchronization
5: DataLink Layer
5-24
Slotted Aloha efficiency
Efficiency is the long-run
fraction of successful slots
when there are many nodes,
each with many frames to send
r Suppose N nodes with
many frames to send,
each transmits in slot
with probability p
r prob that node 1 has
success in a slot
= p(1-p)N-1
r prob that any node has
a success = Np(1-p)N-1
r For max efficiency
with N nodes, find p*
that maximizes
Np(1-p)N-1
r For many nodes, take
limit of Np*(1-p*)N-1
as N goes to infinity,
gives 1/e = .37
At best: channel
used for useful
transmissions 37%
of time!
5: DataLink Layer
5-25
Pure (unslotted) ALOHA
r unslotted Aloha: simpler, no synchronization
r when frame first arrives
m transmit immediately
r collision probability increases:
m frame sent at t0 collides with other frames sent in [t0-1,t0+1]
5: DataLink Layer
5-26
Pure Aloha efficiency
P(success by given node) = P(node transmits) .
P(no other node transmits in [p0-1,p0] .
P(no other node transmits in [p0-1,p0]
= p . (1-p)N-1 . (1-p)N-1
= p . (1-p)2(N-1)
… choosing optimum p and then letting n -> infty ...
Even worse !
= 1/(2e) = .18
5: DataLink Layer
5-27
Aloha efficiency (Slotted Vs
Pure)
5: DataLink Layer
5-28
CSMA (Carrier Sense Multiple Access)
CSMA: listen before transmit:
If channel sensed idle: transmit entire frame
r If channel sensed busy, defer transmission
r Human analogy: don’t interrupt others!
5: DataLink Layer
5-29
CSMA collisions
spatial layout of nodes
collisions can still occur:
propagation delay means
two nodes may not hear
each other’s transmission
collision:
entire packet transmission
time wasted
note:
role of distance & propagation
delay in determining collision
probability
5: DataLink Layer
5-30
CSMA/CD (Collision Detection)
CSMA/CD: carrier sensing, deferral as in CSMA
m
m
collisions detected within short time
colliding transmissions aborted, reducing channel
wastage
r collision detection:
m easy in wired LANs: measure signal strengths,
compare transmitted, received signals
m difficult in wireless LANs: receiver shut off while
transmitting
r human analogy: the polite conversationalist
5: DataLink Layer
5-31
3 States of CSMA/CD
(Contention, Idle, Transmission)
5: DataLink Layer
5-32
CSMA/CD collision detection
5: DataLink Layer
5-33
“Taking Turns” MAC protocols
channel partitioning MAC protocols:
m share channel efficiently and fairly at high load
m inefficient at low load: delay in channel access,
1/N bandwidth allocated even if only 1 active
node!
Random access MAC protocols
m efficient at low load: single node can fully
utilize channel
m high load: collision overhead
“taking turns” protocols
look for best of both worlds!
5: DataLink Layer
5-34
“Taking Turns” MAC protocols
Token passing:
Polling:
r control token passed from
r master node
one node to next
“invites” slave nodes
sequentially.
to transmit in turn
r token message
r concerns:
r concerns:
m polling overhead
m
m
latency
single point of
failure (master)
m
m
m
token overhead
latency
single point of failure (token)
5: DataLink Layer
5-35
Summary of MAC protocols
r What do you do with a shared media?
m
Channel Partitioning, by time, frequency or code
• Time Division, Frequency Division
m
Random partitioning (dynamic),
• ALOHA, S-ALOHA, CSMA, CSMA/CD
• carrier sensing: easy in some technologies (wire), hard
in others (wireless)
• CSMA/CD used in Ethernet
• CSMA/CA used in 802.11
m
Taking Turns
• polling from a central site, token passing
5: DataLink Layer
5-36
LAN technologies
Data link layer so far:
m
services, error detection/correction, multiple
access
Next: LAN technologies
m
m
m
m
addressing
Ethernet
hubs, switches
PPP
5: DataLink Layer
5-37
Link Layer
r 5.1 Introduction and
r
r
r
r
services
5.2 Error detection
and correction
5.3Multiple access
protocols
5.4 Link-Layer
Addressing
5.5 Ethernet
r 5.6 Hubs and switches
r 5.7 PPP
r 5.8 Link Virtualization:
ATM
5: DataLink Layer
5-38
MAC Addresses and ARP
r 32-bit IP address:
m
m
network-layer address
used to get datagram to destination IP subnet
r MAC (or LAN or physical or Ethernet)
address:
m
m
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
5-39
LAN Addresses and ARP
Each adapter on LAN has unique LAN address
1A-2F-BB-76-09-AD
71-65-F7-2B-08-53
LAN
(wired or
wireless)
Broadcast address =
FF-FF-FF-FF-FF-FF
= adapter
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
5: DataLink Layer
5-40
LAN Address (more)
r MAC address allocation administered by IEEE
r manufacturer buys portion of MAC address space
(to assure uniqueness)
r Analogy:
(a) MAC address: like Social Security Number
(b) IP address: like postal address
r MAC flat address ➜ portability
m
can move LAN card from one LAN to another
r IP hierarchical address NOT portable
m depends on IP subnet to which node is attached
5: DataLink Layer
5-41
ARP: Address Resolution Protocol
Question: how to determine
MAC address of B
knowing B’s IP address?
237.196.7.78
1A-2F-BB-76-09-AD
237.196.7.23
r Each IP node (Host,
Router) on LAN has
ARP table
r ARP Table: IP/MAC
address mappings for
some LAN nodes
237.196.7.14
m
LAN
71-65-F7-2B-08-53
237.196.7.88
< IP address; MAC address; TTL>
58-23-D7-FA-20-B0
TTL (Time To Live): time
after which address
mapping will be forgotten
(typically 20 min)
0C-C4-11-6F-E3-98
5: DataLink Layer
5-42
ARP protocol: Same LAN (network)
r
r
r
A wants to send datagram
to B, and B’s MAC address
not in A’s ARP table.
A broadcasts ARP query
packet, containing B's IP
address
m Dest MAC address =
FF-FF-FF-FF-FF-FF
m all machines on LAN
receive ARP query
B receives ARP packet,
replies to A with its (B's)
MAC address
m
frame sent to A’s MAC
address (unicast)
r
A caches (saves) IP-toMAC address pair in its
ARP table until information
becomes old (times out)
m soft state: information
that times out (goes
away) unless refreshed
r ARP is “plug-and-play”:
m nodes create their ARP
tables without
intervention from net
administrator
5: DataLink Layer
5-43
Routing to another LAN
walkthrough: send datagram from A to B via R
assume A know’s B IP address
A
R
B
r Two ARP tables in router R, one for each IP
network (LAN)
5: DataLink Layer
5-44
r
r
r
r
r
r
r
r
A creates datagram with source A, destination B
A uses ARP to get R’s MAC address for 111.111.111.110
A creates link-layer frame with R's MAC address as dest,
frame contains A-to-B IP datagram
A’s adapter sends frame
R’s adapter receives frame
R removes IP datagram from Ethernet frame, sees its
destined to B
R uses ARP to get B’s MAC address
R creates frame containing A-to-B IP datagram sends to B
A
R
B
5: DataLink Layer
5-45
Unicast to server
DHCP
relay
Other netw orks
DHCP
server
Broadcast
Host
5: DataLink Layer
5-46
Operation
HType
HLen
Hops
Xid
Secs
Flags
ciaddr
yiaddr
siaddr
giaddr
chaddr (16 bytes)
sname (64 bytes)
file (128 bytes)
options
5: DataLink Layer
5-47
Link Layer
r 5.1 Introduction and
r
r
r
r
services
5.2 Error detection
and correction
5.3Multiple access
protocols
5.4 Link-Layer
Addressing
5.5 Ethernet
r 5.6 Hubs and switches
r 5.7 PPP
r 5.8 Link Virtualization:
ATM
5: DataLink Layer
5-48
Ethernet
“dominant” wired LAN technology:
r cheap $20 for 100Mbs!
r first widely used LAN technology
r Simpler, cheaper than token LANs and ATM
r Kept up with speed race: 10 Mbps – 10 Gbps
Metcalfe’s Ethernet
sketch
5: DataLink Layer
5-49
IEEE 802.2 Logical Link Control
(LLC)
r a)Position of LLC
r b) Protocol Format
5: DataLink Layer
5-50
Encoding
Bits
0 0 1 0 1 1 1 1 0 1 0 0 0 0 1 0
NRZ
Clock
Manchester
NRZI
5: DataLink Layer
5-51
Manchester encoding
r Used in 10BaseT
r Each bit has a transition
r Allows clocks in sending and receiving nodes to
synchronize to each other
m
no need for a centralized, global clock among nodes!
r Hey, this is physical-layer stuff!
5: DataLink Layer
5-52
Encoding (more)
5: DataLink Layer
5-53
Encodings (cont)
• 4B/5B
-- every 4 bits of data encoded in a 5bit code
-- 5bit codes selected to have no more than one
leading 0 and no more than two trailing 0s
-- thus, never get more than three consecutive
0s
-- resulting 5bit codes are transmitted using
NRZI
-- achieves 80% efficiency
5: DataLink Layer
5-54
Ethernet Cabling
5: DataLink Layer
5-55
Coaxial Cable
5: DataLink Layer
5-56
Coaxial Cable
5: DataLink Layer
5-57
(UTP )Unshielded Twisted Pair
Cat5 Cable
5: DataLink Layer
5-58
Cat5 Crimping tool
5: DataLink Layer
5-59
Cat 5 Crimp RJ45
5: DataLink Layer
5-60
Cat5 Tools
5: DataLink Layer
5-61
How to make a Cable
r Cutting the wire (Straightly)
5: DataLink Layer
5-62
Crimping
5: DataLink Layer
5-63
Different Connections
Crossover
Cable
Straight
Through
Cable
RJ-45
PIN
RJ-45
PIN
RJ-45
PIN
RJ-45
PIN
1 Rx+
3 Tx+
1 Tx+
1 Rc+
2 Rc-
6 Tx-
2 Tx-
2 Rc-
3 Tx+
1 Rc+
3 Rc+
3 Tx+
6 Tx-
2 Rc-
6 Rc-
6 Tx-
Note: The standard connector view shown is
color-coded for a straight thru cable
5: DataLink Layer
5-64
X-Connect
5: DataLink Layer
5-65
X-connect (PC to PC) or (Hub to
Hub)
5: DataLink Layer
5-66
X-connect (PC to PC) or (Hub to
Hub)
5: DataLink Layer
5-67
Ethernet Cabling
5: DataLink Layer
5-68
Network Interface Card (NIC)
(Adaptor)
5: DataLink Layer
5-69
Hub
5: DataLink Layer
5-70
Original Fast Ethernet Cabling
5: DataLink Layer
5-71
Gigabit Ethernet Cabling
5: DataLink Layer
5-72
Cable Topologies
r a)Linear
r b)Spine
r c) Tree
r d) Segmented
5: DataLink Layer
5-73
Bus Topology
Transceiver
Ethernet cable
Adaptor
Host
5: DataLink Layer
5-74
Bus Topology
5: DataLink Layer
5-75
Star topology
r Bus topology popular through mid 90s
r Now star topology prevails
r Connection choices: hub or switch (more later)
hub or
switch
5: DataLink Layer
5-76
Ethernet Overview
• History
-- developed by Xerox PARC in mid1970s
-- roots in Aloha packetradio network
-- standardized by Xerox, DEC, and Intel in 1978
-- similar to IEEE 802.3 standard
• CSMA/CD
-- carrier sense
-- multiple access
-- collision detection
• Frame Format
64
48
48
16
Preamble
Dest
addr
Src
addr
Type
32
Body
CRC
5: DataLink Layer
5-77
Ethernet (cont)
• Addresses
-- unique, 48bit unicast address assigned to
each adapter
-- example: 8:0:e4:b1:2
-- broadcast: all 1 s
-- multicast: first bit is 1
• Bandwidth: 10Mbps, 100Mbps, 1Gbps
• Length: 2500m (500m segments with 4
repeaters)
• Problem: Distributed algorithm to provide fair
access
5: DataLink Layer
5-78
Transmit Algorithm
• If line is idle...
-- send immediately
-- upper bound message size of 1500 bytes
-- must wait 9.6us between backtoback frames
• If line is busy...
-- wait until idle and transmit immediately
-- called 1persistent (special case of ppersistent)
5: DataLink Layer
5-79
Transmit (cont)
• If collision...
-- jam for 32 bits, then stop transmitting frame
-- minimum frame is 64 bytes (header + 46
bytes of data)
-- delay and try again
• 1st time: 0 or 51.2us
• 2nd time: 0, 51.2, or 102.4us
• 3rd time51.2, 102.4, or 153.6us
• nth time: k x 51.2us, for randomly selected k =
0..2 n 1
• give up after several tries (usually 16)
• exponential backoff
5: DataLink Layer
5-80
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble:
r 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011
r used to synchronize receiver, sender clock rates
5: DataLink Layer
5-81
Ethernet Frame Structure
(more)
r Addresses: 6 bytes
m if adapter receives frame with matching destination
address, or with broadcast address (eg ARP packet), it
passes data in frame to net-layer protocol
m otherwise, adapter discards frame
r Type: indicates the higher layer protocol (mostly
IP but others may be supported such as Novell
IPX and AppleTalk)
r CRC: checked at receiver, if error is detected, the
frame is simply dropped
5: DataLink Layer
5-82
Unreliable, connectionless service
r Connectionless: No handshaking between sending
and receiving adapter.
r Unreliable: receiving adapter doesn’t send acks or
nacks to sending adapter
m
m
m
stream of datagrams passed to network layer can have
gaps
gaps will be filled if app is using TCP
otherwise, app will see the gaps
5: DataLink Layer
5-83
Ethernet uses CSMA/CD
r No slots
r adapter doesn’t transmit
if it senses that some
other adapter is
transmitting, that is,
carrier sense
r transmitting adapter
aborts when it senses
that another adapter is
transmitting, that is,
collision detection
r Before attempting a
retransmission,
adapter waits a
random time, that is,
random access
5: DataLink Layer
5-84
Ethernet CSMA/CD algorithm
1. Adaptor receives
4. If adapter detects
datagram from net layer &
another transmission while
creates frame
transmitting, aborts and
sends jam signal
2. If adapter senses channel
idle, it starts to transmit 5. After aborting, adapter
frame. If it senses
enters exponential
channel busy, waits until
backoff: after the mth
channel idle and then
collision, adapter chooses
transmits
a K at random from
{0,1,2,…,2m-1}. Adapter
3. If adapter transmits
waits K·512 bit times and
entire frame without
returns to Step 2
detecting another
transmission, the adapter
is done with frame !
5: DataLink Layer 5-85
Ethernet’s CSMA/CD (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
See/interact with Java
applet on AWL Web site:
highly recommended !
Exponential Backoff:
r Goal: adapt retransmission
attempts to estimated
current load
m
r
r
r
heavy load: random wait
will be longer
first collision: choose K
from {0,1}; delay is K· 512
bit transmission times
after second collision:
choose K from {0,1,2,3}…
after ten collisions, choose
K from {0,1,2,3,4,…,1023}
5: DataLink Layer
5-86
Collision Detection (2tprop)
5: DataLink Layer
5-87
Collision Detection (2tprop)
A
B
A
B
A
B
A
B
(a)
(b)
(c)
(d)
5: DataLink Layer
5-88
CSMA/CD efficiency
r Tprop = max prop between 2 nodes in LAN
r ttrans = time to transmit max-size frame
efficiency 
1
1  5t prop / ttrans
r Efficiency goes to 1 as tprop goes to 0
r Goes to 1 as ttrans goes to infinity
r Much better than ALOHA, but still decentralized,
simple, and cheap
5: DataLink Layer
5-89
Performance
5: DataLink Layer
5-90
10BaseT and 100BaseT
r 10/100 Mbps rate; latter called “fast ethernet”
r T stands for Twisted Pair
r Nodes connect to a hub: “star topology”; 100 m
max distance between nodes and hub
twisted pair
hub
5: DataLink Layer
5-91
Repeaters and Multiport
Repeaters (Hubs)
Hubs are essentially physical-layer repeaters:
m bits coming from one link go out all other links
m at the same rate
m no frame buffering
m no CSMA/CD at hub: adapters detect collisions
m provides net management functionality
twisted pair
hub
5: DataLink Layer
5-92
Gbit Ethernet
r uses standard Ethernet frame format
r allows for point-to-point links and shared
r
r
r
r
broadcast channels
in shared mode, CSMA/CD is used; short distances
between nodes required for efficiency
uses hubs, called here “Buffered Distributors”
Full-Duplex at 1 Gbps for point-to-point links
10 Gbps now !
5: DataLink Layer
5-93
Link Layer
r 5.1 Introduction and
r
r
r
r
services
5.2 Error detection
and correction
5.3Multiple access
protocols
5.4 Link-Layer
Addressing
5.5 Ethernet
r 5.6 Interconnections:
Hubs and switches
r 5.7 PPP
r 5.8 Link Virtualization:
ATM
5: DataLink Layer
5-94
Extending LANs
(Interconnecting LANs)
5: DataLink Layer
5-95
Interconnecting with hubs
Hub
Hub
Interconnecting with hubs
r Backbone hub interconnects LAN segments
r Extends max distance between nodes
r But individual segment collision domains become one
large collision domain
r Can’t interconnect 10BaseT & 100BaseT
hub
hub
hub
hub
5: DataLink Layer
5-97
Bridges and Multiport Bridges
(Switches)
r Link layer device
stores and forwards Ethernet frames
m examines frame header and selectively
forwards frame based on MAC dest address
m when frame is to be forwarded on segment,
uses CSMA/CD to access segment
r transparent
m hosts are unaware of presence of switches
r plug-and-play, self-learning
m switches do not need to be configured
m
5: DataLink Layer
5-98
Interconnecting with hubs and
Switch
5: DataLink Layer
5-99
Interconnecting with Switches
(Forwarding)
switch
1
2
hub
3
hub
hub
• How do determine onto which LAN segment to
forward frame?
• Looks like a routing problem...
5: DataLink Layer 5-100
Bridge (Switch) Self learning
. LANs have physical limitations (e.g.,
2500m)
. Connect two or more LANs with a bridge
-- accept and forward strategy
-- level 2 connection (does not add packet
header)
A
B
C
Port 1
Bridge
Port 2
X
Y
Z
. Ethernet Switch = Bridge on Steroids
5: DataLink Layer 5-101
Bridge (Switch) Self learning
r A switch has a switch table
r entry in switch table:
(MAC Address, Interface, Time Stamp)
m stale entries in table dropped (TTL can be 60 min)
r switch learns which hosts can be reached through
which interfaces
m when frame received, switch “learns” location of
sender: incoming LAN segment
m records sender/location pair in switch table
m
5: DataLink Layer 5-102
Bridge (Switch)
Filtering/Forwarding
When switch receives a frame:
index switch table using MAC dest address
if entry found for destination
then{
if dest on segment from which frame arrived
then drop the frame
else forward the frame on interface indicated
}
else flood
forward on all but the interface
on which the frame arrived
5: DataLink Layer 5-103
Bridges and Extended LANs
(Example1)
. Do not forward when unnecessary
. Maintain forwarding table
A
B
C
Host
Port
A
1
Port 1
B
1
Port 2
C
1
X
2
Y
2
Z
2
Bridge
X
Y
Z
. Learn table entries based on source
address
. Table is an optimization; need not be
complete
. Always forward broadcast frames
5: DataLink Layer 5-104
Switch example2
Suppose C sends frame to D
1
B
C
A
B
E
G
3
2
hub
hub
hub
A
address interface
switch
1
1
2
3
I
D
E
F
G
H
r Switch receives frame from from C
m notes in bridge table that C is on interface 1
m because D is not in table, switch forwards frame into
interfaces 2 and 3
r frame received by D
5: DataLink Layer 5-105
Switch example2 (cntd)
Suppose D replies back with frame to C.
address interface
switch
B
C
hub
hub
hub
A
I
D
E
F
G
A
B
E
G
C
1
1
2
3
1
H
r Switch receives frame from from D
m notes in bridge table that D is on interface 2
m because C is in table, switch forwards frame only to
interface 1
r frame received by C
5: DataLink Layer 5-106
Switch: traffic isolation
r switch installation breaks subnet into LAN
segments
r switch filters packets:
m same-LAN-segment frames not usually
forwarded onto other LAN segments
m segments become separate collision domains
switch
collision
domain
hub
collision domain
hub
collision domain
hub
5: DataLink Layer 5-107
Switches: dedicated access
r Switch with many
interfaces
r Hosts have direct
connection to switch
r No collisions; full duplex
Switching: A-to-A’ and B-to-B’
simultaneously, no collisions
A
C’
B
switch
C
B’
A’
5: DataLink Layer 5-108
More on Switches
r cut-through switching: frame forwarded
from input to output port without first
collecting entire frame
m slight reduction in latency
r combinations of shared/dedicated,
10/100/1000 Mbps interfaces
5: DataLink Layer 5-109
Institutional network
to external
network
mail server
web server
router
switch
IP subnet
hub
hub
hub
5: DataLink Layer
5-110
Institutional network
5: DataLink Layer
5-111
Switches vs. Routers
r both store-and-forward devices
m routers: network layer devices (examine network layer
headers)
m switches are link layer devices
r routers maintain routing tables, implement routing
algorithms
r switches maintain switch tables, implement
filtering, learning algorithms
5: DataLink Layer
5-112
Summary comparison
hubs
routers
switches
traffic
isolation
no
yes
yes
plug & play
yes
no
yes
optimal
routing
cut
through
no
yes
no
yes
no
yes
5: DataLink Layer
5-113
More on Bridge-Spanning Tree
(SPT) Algorithm
• Problem: loops
• Bridges run a distributed spanning
tree algorithm
-- select which bridges actively
forward
-- developed by Radia Perlman
-- now IEEE 802.1 specification
(a)
(b)
5: DataLink Layer
5-114
SPT Algorithm Overview
•Each bridge has unique id (e.g., B1, B2,
B3)
•Select bridge with smallest id as root
•Select bridge on each LAN closest to
root as
•designated bridge (use id to break ties)
•Each bridge forwards frames over each
LAN for which it is the designated
bridge
A
B
B3
C
B5
D
B2
B7
E
K
F
B1
G
H
B6
B4
I
J
5: DataLink Layer
5-115
SPT Algorithm Details
• Bridges exchange configuration messages
-- id for bridge sending the message
-- id for what the sending bridge believes to be
root bridge
-- distance (hops) from sending bridge to root
bridge
• Each bridge records current best configuration
•message for each port
• Initially, each bridge believes it is the root
5: DataLink Layer
5-116
SPT Algorithm Detail (cont)
. When learn not root, stop generating config messages
-- in steady state, only root generates configuration
messages
. When learn not designated bridge, stop forwarding
config
messages
-- in steady state, only designated bridges forward config
messages
. Root continues to periodically send config messages
. If any bridge does not receive config message after a
period
of time, it starts generating config messages claiming to
be
the root
5: DataLink Layer
5-117
More on Bridge- Broadcast and
Multicast
• Forward all broadcast/multicastframes
-- current practice
• Learn when no group members
downstream
• Accomplished by having each member
of group G send a frame to bridge
multicast address with G in source field
5: DataLink Layer
5-118
More on Bridge- Limitations of
Bridges
• Do not scale
-- spanning tree algorithm does not
scale
-- broadcast does not scale
• Do not accommodate heterogeneity
• Caution: beware of transparency
5: DataLink Layer
5-119
More on Bridge- Virtual LAN
(VLAN)
W
X
VLAN 100
VLAN 100
B1
B2
VLAN 200
VLAN 200
Y
Z
5: DataLink Layer 5-120
Link Layer
r 5.1 Introduction and
r
r
r
r
services
5.2 Error detection
and correction
5.3Multiple access
protocols
5.4 Link-Layer
Addressing
5.5 Ethernet
r 5.6 Hubs and switches
r 5.7 Point to Point Links
r 5.8 Link Virtualization:
ATM
5: DataLink Layer 5-121
Point to Point Links - Outline
Encoding (Done)
Framing
Error Detection (Done)
Sliding Window Algorithm(Done)
Internet PointtoPoint Links
5: DataLink Layer 5-122
Point to Point Data Link Control
r one sender, one receiver, one link: easier than
broadcast link:
m no Media Access Control
m no need for explicit MAC addressing
m e.g., dialup link, ISDN line
r popular point-to-point DLC protocols:
m PPP (point-to-point protocol)
m HDLC: High level data link control (Data link
used to be considered “high layer” in protocol
stack!
5: DataLink Layer 5-123
Framing
• Break sequence of bits into a
frame
• Typically implemented by network
adaptor
Bits
Node A
Adaptor
Adaptor
Node B
Frames
5: DataLink Layer 5-124
Approaches
• Sentinelbased
-- delineate frame with special pattern:
01111110
-- e.g., HDLC, SDLC, PPP
8
16
Beginning
sequence
Header
16
Body
8
Ending
CRC sequence
-- problem: special pattern appears in the
payload
-- solution: Byte Stuffing (CharacterOriented)
and Bit stuffing
5: DataLink Layer 5-125
Character-Oriented Framing
Byte Stuffing
Data to be sent
A
DLE
B
ETX DLE
STX E
After stuffing and framing
DLE
r
STX A
r
DLE
B
ETX DLE
DLE
STX E
DLE
Frames consist of integer number of bytes
ETX
m
Asynchronous transmission systems using ASCII to transmit printable
characters
Octets with HEX value <20 are nonprintable
m
STX (start of text) = 0x02; ETX (end of text) = 0x03;
m
r
DLE
Special 8-bit patterns used as control characters
Byte used to carry non-printable characters in frame
m
m
m
m
DLE (data link escape) = 0x10
DLE STX (DLE ETX) used to indicate beginning (end) of frame
Insert extra DLE in front of occurrence of DLE STX (DLE ETX) in
frame
All DLEs occur in pairs except at frame boundaries
Byte Stuffing (Examples)
5: DataLink Layer 5-127
Bit Oriented (Bit Stuffing)
HDLC frame
Flag
Address
Control
Information
FCS
Flag
any number of bits
r Frame delineated by flag character
r HDLC uses bit stuffing to prevent occurrence of
flag 01111110 inside the frame
r Transmitter inserts extra 0 after each
consecutive five 1s inside the frame
r Receiver checks for five consecutive 1s
m
m
m
if next bit = 0, it is removed
if next two bits are 10, then flag is detected
If next two bits are 11, then frame has errors
Example: Bit stuffing & de-stuffing
(a)
Data to be sent
0110111111111100
After stuffing and framing
0111111001101111101111100001111110
(b)
Data received
01111110000111011111011111011001111110
After destuffing and deframing
*000111011111-11111-110*
Approaches (cont)- Counterbased
• Counterbased
-- include payload length in header
-- e.g., DDCMP
8
8
8
14
42
Count
Header
16
Body
CRC
-- problem: count field corrupted
-- solution: catch when CRC fails
5: DataLink Layer 5-130
Counter-based
5: DataLink Layer 5-131
Approaches (cont)- Clockbased
• Clockbased
-- each frame is 125us long
-- e.g., SONET: Synchronous
Optical Network
-- STSn (STS1 = 51.84 Mbps)
Overhead
Payload
STS-1
STS-1
STS-1
9 row s
Hdr
STS-3c
90 columns
5: DataLink Layer 5-132
Internet PointtoPoint Links
PPP Design Requirements [RFC 1557]
r packet framing: encapsulation of network-layer
r
r
r
r
datagram in data link frame
m carry network layer data of any network layer
protocol (not just IP) at same time
m ability to demultiplex upwards
bit transparency: must carry any bit pattern in the
data field
error detection (no correction)
connection liveness: detect, signal link failure to
network layer
network layer address negotiation: endpoint can
learn/configure each other’s network address
5: DataLink Layer 5-133
PPP non-requirements
r no error correction/recovery
r no flow control
r out of order delivery OK
r no need to support multipoint links (e.g., polling)
Error recovery, flow control, data re-ordering
all relegated to higher layers!
5: DataLink Layer 5-134
PPP Data Frame
r Flag: delimiter (framing)
r Address: does nothing (only one option)
r Control: does nothing; in the future possible
multiple control fields
r Protocol: upper layer protocol to which frame
delivered (eg, PPP-LCP, IP, IPCP, etc)
5: DataLink Layer 5-135
PPP Data Frame
r info: upper layer data being carried
r check: cyclic redundancy check for error
detection
5: DataLink Layer 5-136
PPP Frame
Flag
01111110
Address
1111111
Control
00000011
Protocol
Information
CRC
Flag
01111110
integer # of bytes
All stations are to
accept the frame
r
Unnumbered
frame
Specifies what kind of packet is contained in
the payload, e.g., LCP, NCP, IP, OSI CLNP,
IPX
PPP uses similar frame structure as HDLC, except
m
m
Protocol type field
Payload contains an integer number of bytes
PPP uses the same flag, but uses byte stuffing
r Problems with PPP byte stuffing
r
m
m
Size of frame varies unpredictably due to byte insertion
Malicious users can inflate bandwidth by inserting 7D & 7E
Byte-Stuffing in PPP
PPP is character-oriented version of HDLC
r Flag is 0x7E (01111110)
r Control escape 0x7D (01111101)
r Any occurrence of flag or control escape inside of frame
is replaced with 0x7D followed by
original octet XORed with 0x20 (00100000)
r
Data to be sent
7E
41
41
7D
42
7E
50
70
46
7D
5D
42
7D
5E
50
70
After stuffing and framing
46
7E
PPP Data Control Protocol
Before exchanging networklayer data, data link peers
must
r configure PPP link (max.
frame length,
authentication)
r learn/configure network
layer information
m for IP: carry IP Control
Protocol (IPCP) msgs
(protocol field: 8021) to
configure/learn IP
address
5: DataLink Layer 5-139
PPP Phases
Dead
7. Carrier
dropped
Terminate
6. Done
5. Open
Failed
Failed
4. NCP
configuration Network
Home PC to Internet Service
Provider
1. PC calls router via modem
2. PC and router exchange LCP
packets to negotiate PPP
parameters
Establish
3. Check on identities
4. NCP packets exchanged to
2. Options
configure the network layer, e.g.
negotiated
TCP/IP ( requires IP address
assignment)
Authenticate
5. Data transport, e.g.
send/receive IP packets
6. NCP used to tear down the
3. Authentication network layer connection (free
up IP address); LCP used to shut
completed
down data link layer connection
1. Carrier
detected
7. Modem hangs up
PPP Authentication
r Password Authentication Protocol
m Initiator must send ID & password
m Authenticator replies with authentication
success/fail
m After several attempts, LCP closes link
m Transmitted unencrypted, susceptible to
eavesdropping
r Challenge-Handshake Authentication Protocol
(CHAP)
m
m
m
m
m
Initiator & authenticator share a secret key
Authenticator sends a challenge (random # & ID)
Initiator computes cryptographic checksum of
random # & ID using the shared secret key
Authenticator also calculates cryptocgraphic
checksum & compares to response
Authenticator can reissue challenge during session
Link Layer
r 5.1 Introduction and
r
r
r
r
services
5.2 Error detection
and correction
5.3Multiple access
protocols
5.4 Link-Layer
Addressing
5.5 Ethernet
r 5.6 Hubs and switches
r 5.7 PPP
r 5.8 Link Virtualization:
ATM and MPLS
5: DataLink Layer 5-142
Virtualization of networks
Virtualization of resources: a powerful abstraction in
systems engineering:
r computing examples: virtual memory, virtual
devices
m Virtual machines: e.g., java
m IBM VM os from 1960’s/70’s
r layering of abstractions: don’t sweat the details of
the lower layer, only deal with lower layers
abstractly
5: DataLink Layer 5-143
The Internet: virtualizing networks
1974: multiple unconnected
nets
ARPAnet
m data-over-cable networks
m packet satellite network (Aloha)
m packet radio network
m
ARPAnet
"A Protocol for Packet Network Intercommunication",
V. Cerf, R. Kahn, IEEE Transactions on Communications,
May, 1974, pp. 637-648.
… differing in:
addressing conventions
m packet formats
m error recovery
m routing
m
satellite net
5: DataLink Layer 5-144
The Internet: virtualizing networks
Internetwork layer (IP):
r addressing: internetwork
appears as a single, uniform
entity, despite underlying local
network heterogeneity
r network of networks
Gateway:
r “embed internetwork packets in
local packet format or extract
them”
r route (at internetwork level) to
next gateway
gateway
ARPAnet
satellite net
5: DataLink Layer 5-145
Cerf & Kahn’s Internetwork Architecture
What is virtualized?
r two layers of addressing: internetwork and local
network
r new layer (IP) makes everything homogeneous at
internetwork layer
r underlying local network technology
m cable
m satellite
m 56K telephone modem
m today: ATM, MPLS
… “invisible” at internetwork layer. Looks like a link
layer technology to IP!
5: DataLink Layer 5-146
ATM and MPLS
r ATM, MPLS separate networks in their own
right
m
different service models, addressing, routing
from Internet
r viewed by Internet as logical link connecting
IP routers
m
just like dialup link is really part of separate
network (telephone network)
r ATM, MPSL: of technical interest in their
own right
5: DataLink Layer 5-147
Asynchronous Transfer Mode: ATM
r 1990’s/00 standard for high-speed (155Mbps to
622 Mbps and higher) Broadband Integrated
Service Digital Network architecture
r Goal: integrated, end-end transport of carry voice,
video, data
m meeting timing/QoS requirements of voice, video
(versus Internet best-effort model)
m “next generation” telephony: technical roots in
telephone world
m packet-switching (fixed length packets, called
“cells”) using virtual circuits
5: DataLink Layer 5-148
ATM architecture
r adaptation layer: only at edge of ATM network
data segmentation/reassembly
m roughly analagous to Internet transport layer
r ATM layer: “network” layer
m cell switching, routing
r physical layer
m
5: DataLink Layer 5-149
ATM: network or link layer?
Vision: end-to-end
transport: “ATM from
desktop to desktop”
m ATM is a network
technology
Reality: used to connect
IP backbone routers
m “IP over ATM”
m ATM as switched
link layer,
connecting IP
routers
IP
network
ATM
network
5: DataLink Layer 5-150
ATM Adaptation Layer (AAL)
r ATM Adaptation Layer (AAL): “adapts” upper
layers (IP or native ATM applications) to ATM
layer below
r AAL present only in end systems, not in switches
r AAL layer segment (header/trailer fields, data)
fragmented across multiple ATM cells
m analogy: TCP segment in many IP packets
5: DataLink Layer 5-151
ATM Adaptation Layer (AAL) [more]
Different versions of AAL layers, depending on ATM
service class:
r
r
r
AAL1: for CBR (Constant Bit Rate) services, e.g. circuit emulation
AAL2: for VBR (Variable Bit Rate) services, e.g., MPEG video
AAL5: for data (eg, IP datagrams)
User data
AAL PDU
ATM cell
5: DataLink Layer 5-152
ATM Layer
Service: transport cells across ATM network
r analogous to IP network layer
r very different services than IP network layer
Network
Architecture
Internet
Service
Model
Guarantees ?
Congestion
Bandwidth Loss Order Timing feedback
best effort none
ATM
CBR
ATM
VBR
ATM
ABR
ATM
UBR
constant
rate
guaranteed
rate
guaranteed
minimum
none
no
no
no
yes
yes
yes
yes
yes
yes
no
yes
no
no (inferred
via loss)
no
congestion
no
congestion
yes
no
yes
no
no
5: DataLink Layer 5-153
ATM Layer: Virtual Circuits
r VC transport: cells carried on VC from source to dest
m call setup, teardown for each call before data can flow
m each packet carries VC identifier (not destination ID)
m every switch on source-dest path maintain “state” for each
passing connection
m link,switch resources (bandwidth, buffers) may be allocated to
VC: to get circuit-like perf.
r Permanent VCs (PVCs)
long lasting connections
m typically: “permanent” route between to IP routers
r Switched VCs (SVC):
m dynamically set up on per-call basis
m
5: DataLink Layer 5-154
ATM VCs
r Advantages of ATM VC approach:
QoS performance guarantee for connection
mapped to VC (bandwidth, delay, delay jitter)
r Drawbacks of ATM VC approach:
m Inefficient support of datagram traffic
m one PVC between each source/dest pair) does
not scale (N*2 connections needed)
m SVC introduces call setup latency, processing
overhead for short lived connections
m
5: DataLink Layer 5-155
ATM Layer: ATM cell
r 5-byte ATM cell header
r 48-byte payload
m
m
Why?: small payload -> short cell-creation delay
for digitized voice
halfway between 32 and 64 (compromise!)
Cell header
Cell format
5: DataLink Layer 5-156
ATM cell header
r VCI: virtual channel ID
will change from link to link thru net
r PT: Payload type (e.g. RM cell versus data cell)
r CLP: Cell Loss Priority bit
m CLP = 1 implies low priority cell, can be
discarded if congestion
r HEC: Header Error Checksum
m cyclic redundancy check
m
5: DataLink Layer 5-157
ATM Physical Layer (more)
Two pieces (sublayers) of physical layer:
r Transmission Convergence Sublayer (TCS): adapts
ATM layer above to PMD sublayer below
r Physical Medium Dependent: depends on physical
medium being used
TCS Functions:
m Header checksum generation: 8 bits CRC
m Cell delineation
m With “unstructured” PMD sublayer, transmission
of idle cells when no data cells to send
5: DataLink Layer 5-158
ATM Physical Layer
Physical Medium Dependent (PMD) sublayer
r SONET/SDH: transmission frame structure (like a
container carrying bits);
m bit synchronization;
m bandwidth partitions (TDM);
m several speeds: OC3 = 155.52 Mbps; OC12 = 622.08
Mbps; OC48 = 2.45 Gbps, OC192 = 9.6 Gbps
r TI/T3: transmission frame structure (old
telephone hierarchy): 1.5 Mbps/ 45 Mbps
r unstructured: just cells (busy/idle)
5: DataLink Layer 5-159
IP-Over-ATM
Classic IP only
r 3 “networks” (e.g.,
LAN segments)
r MAC (802.3) and IP
addresses
IP over ATM
r replace “network”
(e.g., LAN segment)
with ATM network
r ATM addresses, IP
addresses
ATM
network
Ethernet
LANs
Ethernet
LANs
5: DataLink Layer 5-160
IP-Over-ATM
app
transport
IP
Eth
phy
IP
AAL
Eth
ATM
phy phy
ATM
phy
ATM
phy
app
transport
IP
AAL
ATM
phy
5: DataLink Layer 5-161
Datagram Journey in IP-over-ATM Network
r at Source Host:
m IP layer maps between IP, ATM dest address (using ARP)
m passes datagram to AAL5
m AAL5 encapsulates data, segments cells, passes to ATM layer
r ATM network: moves cell along VC to destination
r at Destination Host:
m
m
AAL5 reassembles cells into original datagram
if CRC OK, datagram is passed to IP
5: DataLink Layer 5-162
IP-Over-ATM
Issues:
r IP datagrams into
ATM AAL5 PDUs
r from IP addresses
to ATM addresses
m just like IP
addresses to
802.3 MAC
addresses!
ATM
network
Ethernet
LANs
5: DataLink Layer 5-163
Multiprotocol label switching (MPLS)
r initial goal: speed up IP forwarding by using fixed
length label (instead of IP address) to do
forwarding
m
m
borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address!
PPP or Ethernet
header
MPLS header
label
20
IP header
remainder of link-layer frame
Exp S TTL
3
1
5
5: DataLink Layer 5-164
MPLS capable routers
r a.k.a. label-switched router
r forwards packets to outgoing interface based
only on label value (don’t inspect IP address)
m
MPLS forwarding table distinct from IP forwarding
tables
r signaling protocol needed to set up forwarding
m RSVP-TE
m forwarding possible along paths that IP alone would
not allow (e.g., source-specific routing) !!
m use MPLS for traffic engineering
r must co-exist with IP-only routers
5: DataLink Layer 5-165
MPLS forwarding tables
in
label
out
label dest
10
12
8
out
interface
A
D
A
0
0
1
in
label
out
label dest
out
interface
10
6
A
1
12
9
D
0
R6
0
0
D
1
1
R3
R4
R5
0
0
R2
in
label
8
out
label dest
6
A
out
interface
in
label
6
outR1
label dest
-
A
A
out
interface
0
0
5: DataLink Layer 5-166
10.1.1/24
R3
1
0
R1
0
R2
Prefix
Interface
Prefix
Interface
10.1.1
0
10.1.1
1
10.3.3
0
10.3.3
0
■■■
10.3.3/24
R4
■■■
5: DataLink Layer 5-167
10.1.1/24
Label = 15, Prefix = 10.1.1
R3
1
0
R1
Prefix
0
R2
Interface
10.3.3/24
Label
Prefix
10.1.1
0
15
10.1.1
1
10.3.3
0
16
10.3.3
0
■■■
R4
Interface
■■■
(a)
10.1.1/24
R3
1
R1
Prefix
10.1.1
10.3.3
R2
0
Remote
Interface Label
0
15
0
16
0
10.3.3/24
R4
Label
Prefix
15
10.1.1
Interface
1
16
10.3.3
0
■■■
■■■
(b)
5: DataLink Layer 5-168
Label = 24, Prefix = 10.1.1
10.1.1/24
R3
1
0
R1
0
R2
10.3.3/24
R4
Prefix
10. 1. 1
Interface
0
Remote
Label
15
10. 3. 3
0
16
■■■
Label Prefix
15 10.1.1
16
10.3.3
Interface
1
Remote
Label
24
0
■■■
5: DataLink Layer 5-169
(a)
ATM cell
header
GFC
VPI
VCI
PTI
CLP
HEC
DATA
Label
(b)
“ Shim “ header
(for PPP, Ethernet,
etc.)
PPP header
Label header
Layer 3 header
5: DataLink Layer 5-170
R6
R1
R5
R2
R3
R4
(a)
R6
R1
LSR1
LSR3
R5
R2
LSR2
R4
R3
(b)
5: DataLink Layer 5-171
R1
R6
R7
R3
R2
R4
R5
5: DataLink Layer 5-172
ATM cells arrive
ATM cells sent
Tail
Head
R2
Cells sent into
tunnel at head
R3
Tunneled data
arrives at tail
5: DataLink Layer 5-173
1. ATM cells arrive
101
6. ATM cells sent
202
Tail
Head
R2
2. Demux label added
DL 101
3. Tunnel label added
TL DL 101
R3
DL 101
TL DL 101
5. Demux label examined
4. Packet is forw arded to tail
5: DataLink Layer 5-174
VPN A / Site 2
VPN B / Site 2
VPN B / Site 1
Provider
netw ork
VPN A / Site 3
VPN A / Site 1
VPN B / Site 3
5: DataLink Layer 5-175
Chapter 5: Summary
r
principles behind data link layer services:
m
m
m
error detection, correction
sharing a broadcast channel: multiple access
link layer addressing
r instantiation and implementation of various link
layer technologies
m Ethernet
m switched LANS
m PPP
m virtualized networks as a link layer: ATM, MPLS
5: DataLink Layer 5-176