2
Link Layer
 5.1 Introduction and
services
 5.2 Error detection
and correction
 5.3Multiple access
protocols
 5.4 Link-layer
Addressing
 5.5 Ethernet
 5.6 Link-layer switches
 5.7 PPP
 5.8 Link virtualization:
MPLS
 5.9 A day in the life of a
web request
Link Layer: Introduction
Some terminology:
 hosts and routers are nodes
 communication channels that
connect adjacent nodes along
communication path are links
 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

3
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
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 (chapter 3)!
 seldom used on low bit-error link (fiber, some twisted
pair)
 wireless links: high error rates
• Q: why both link-level and end-end reliability?

4
Link Layer Services (more)
 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
Where is the link layer implemented?
 in each and every host
 link layer implemented in
“adaptor” (aka network
interface card NIC)
 Ethernet card, PCMCI
card, 802.11 card
 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
host schematic
application
transport
network
link
link
physical

5
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
Link Layer
services
and correction
protocols
 5.4 Link-layer
Addressing
 5.5 Ethernet
 5.7 PPP
MPLS
web request

6
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
otherwise
Parity Checking
Single Bit Parity:
Detect single bit errors
Two Dimensional Bit Parity:
Detect and correct single bit errors
0 0

7
Internet checksum (review)
Sender:
 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:
 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
packet (note: used at transport layer only)
Checksumming: Cyclic Redundancy Check
 view data bits, D, as a binary number
 choose r+1 bit pattern (generator), G
 goal: choose r CRC bits, R, such that
 <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 (Ethernet, 802.11 WiFi, ATM)

8
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
Link Layer
services
and correction
protocols
 5.4 Link-layer
Addressing
 5.5 Ethernet
 5.7 PPP
MPLS
web request

9
Multiple Access Links and Protocols
Two types of “links”:
 point-to-point
 PPP for dial-up access
 point-to-point link between Ethernet switch and 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)
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

10
Ideal Multiple 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:
 no special node to coordinate transmissions
 no synchronization of clocks, slots
4. simple
MAC Protocols: a 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

11
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
Channel Partitioning MAC protocols: FDMA
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
frequencybands
FDM cable

12
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
Slotted ALOHA
Assumptions:
 all frames same size
 time divided into equal
size slots (time to
transmit 1 frame)
 nodes start to transmit
only slot beginning
 nodes are synchronized
 if 2 or more nodes
transmit in slot, all
nodes detect collision
Operation:
 when node obtains fresh
frame, transmits in next
slot
 if no collision: node can
send new frame in next
slot
 if collision: node
retransmits frame in
each subsequent slot
with prob. p until
success

13
Slotted ALOHA
Pros
 single active node can
continuously transmit
at full rate of channel
 highly decentralized:
only slots in nodes
need to be in sync
 simple
Cons
 collisions, wasting slots
 idle slots
 nodes may be able to
detect collision in less
than time to transmit
packet
 clock synchronization
Slotted Aloha efficiency
 suppose: N nodes with
many frames to send,
each transmits in slot
with probability p
 prob that given node
has success in a slot =
p(1-p)N-1
 prob that any node has
a success = Np(1-p)N-1
 max efficiency: find
p* that maximizes
Np(1-p)N-1
 for many nodes, take
limit of Np*(1-p*)N-1
as N goes to infinity,
gives:
Max efficiency = 1/e = .37
Efficiency : long-run
fraction of successful slots
(many nodes, all with many
frames to send)
At best: channel
used for useful
transmissions 37%
of time!
!

14
Pure (unslotted) ALOHA
 unslotted Aloha: simpler, no synchronization
 when frame first arrives
 transmit immediately
 collision probability increases:
 frame sent at t0 collides with other frames sent in [t0-1,t0+1]
Pure Aloha efficiency
P(success by given node) = P(node transmits) .
P(no other node transmits in [t0-1,t0] .
P(no other node transmits in [t0,t0 +1]
= p . (1-p)N-1 . (1-p)N-1
= p . (1-p)2(N-1)
… choosing optimum p and then letting n -> infty ...
= 1/(2e) = .18
even worse than slotted Aloha!

15
CSMA (Carrier Sense Multiple Access)
CSMA: listen before transmit:
If channel sensed idle: transmit entire frame
 If channel sensed busy, defer transmission
 human analogy: don’t interrupt others!
CSMA collisions
collisions can still occur:
propagation delay means
two nodes may not hear
each other’s transmission
collision:
entire packet transmission
time wasted
spatial layout of nodes
note:
role of distance & propagation
delay in determining collision
probability

16
CSMA/CD (Collision Detection)
CSMA/CD: carrier sensing, deferral as in CSMA
 collisions detected within short time
 colliding transmissions aborted, reducing channel
wastage
 collision detection:
 easy in wired LANs: measure signal strengths,
compare transmitted, received signals
 difficult in wireless LANs: received signal strength
overwhelmed by local transmission strength
 human analogy: the polite conversationalist
CSMA/CD collision detection

17
“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” protocols
look for best of both worlds!
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

18
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
Summary of MAC protocols
 channel partitioning, by time, frequency or code
 Time Division, Frequency Division
 random access (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
 taking turns
 polling from central site, token passing
 Bluetooth, FDDI, IBM Token Ring

19
Link Layer
services
and correction
protocols
 5.4 Link-Layer
Addressing
 5.5 Ethernet
 5.7 PPP
MPLS
web request
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:
 function: get frame from one interface to another
physically-connected interface (same network)
 48 bit MAC address (for most LANs)
• burned in NIC ROM, also sometimes software settable

20
LAN Addresses and ARP
Each adapter on LAN has unique LAN address
Broadcast address =
FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
(wired or
wireless)
LAN Address (more)
 MAC address allocation administered by IEEE
 manufacturer buys portion of MAC address space
(to assure uniqueness)
 analogy:
(a) MAC address: like Social Security Number
(b) IP address: like postal address
 MAC flat address ➜ portability
 can move LAN card from one LAN to another
 IP hierarchical address NOT portable
 address depends on IP subnet to which node is attached

21
ARP: Address Resolution Protocol
 Each IP node (host,
router) on LAN has
ARP table
 ARP table: IP/MAC
address mappings for
some LAN nodes
< IP address; MAC address; TTL>
 TTL (Time To Live): time
after which address
mapping will be forgotten
(typically 20 min)
Question: how to determine
MAC address of B
knowing B’s 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
137.196.7.23
137.196.7.78
137.196.7.14
137.196.7.88
ARP protocol: Same LAN (network)
 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
 dest MAC address = FF-
FF-FF-FF-FF-FF
 all machines on LAN
receive ARP query
 B receives ARP packet,
replies to A with its (B's)
MAC address
 frame sent to A’s 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 “plug-and-play”:
 nodes create their ARP
tables without
intervention from net
administrator

22
Addressing: routing to another LAN
R
1A-23-F9-CD-06-9B
222.222.222.220
111.111.111.110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111.111.111.112
111.111.111.111
A
74-29-9C-E8-FF-55
222.222.222.221
88-B2-2F-54-1A-0F
B
222.222.222.222
49-BD-D2-C7-56-2A
walkthrough: send datagram from A to B via R
assume A knows B’s IP address
 two ARP tables in router R, one for each IP
network (LAN)
 A creates IP 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 NIC sends frame
 R’s NIC 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
R
1A-23-F9-CD-06-9B
222.222.222.220
111.111.111.110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111.111.111.112
111.111.111.111
A
74-29-9C-E8-FF-55
222.222.222.221
88-B2-2F-54-1A-0F
B
222.222.222.222
49-BD-D2-C7-56-2A
This is a really important
example – make sure you
understand!

23
Link Layer
services
and correction
protocols
 5.4 Link-Layer
Addressing
 5.5 Ethernet
 5.7 PPP
MPLS
web request
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

24
Star topology
 bus topology popular through mid 90s
 all nodes in same collision domain (can collide with each
other)
 today: star topology prevails
 active switch in center
 each “spoke” runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus: coaxial cable
star
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

25
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 network layer protocol
 otherwise, adapter discards frame
 Type: indicates higher layer protocol (mostly IP
but others possible, e.g., Novell IPX, AppleTalk)
 CRC: checked at receiver, if error is detected,
frame is dropped
Ethernet: Unreliable, connectionless
 connectionless: No handshaking between sending and
receiving NICs
 unreliable: receiving NIC doesn’t send acks or nacks
to sending NIC
 stream of datagrams passed to network layer can have gaps
(missing datagrams)
 gaps will be filled if app is using TCP
 otherwise, app will see gaps
 Ethernet’s MAC protocol: unslotted CSMA/CD

26
Ethernet CSMA/CD algorithm
1. NIC receives datagram
from network layer,
creates frame
2. If NIC senses channel idle,
starts frame transmission
If NIC senses channel
busy, waits until channel
idle, then transmits
3. If NIC transmits entire
frame without detecting
another transmission, NIC
is done with frame !
4. If NIC detects another
transmission while
transmitting, aborts and
sends jam signal
5. After aborting, NIC
enters exponential
backoff: after mth
collision, NIC chooses K at
random from
{0,1,2,…,2m-1}. NIC waits
K·512 bit times, returns to
Step 2
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
Exponential Backoff:
 Goal: adapt retransmission
attempts to estimated
current load
 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}
See/interact with Java
applet on AWL Web site:
highly recommended !

27
CSMA/CD efficiency
 Tprop = max prop delay between 2 nodes in LAN
 ttrans = time to transmit max-size frame
 efficiency goes to 1
 as tprop goes to 0
 as ttrans goes to infinity
 better performance than ALOHA: and simple,
cheap, decentralized!
transprop /tt
efficiency
51
1


802.3 Ethernet Standards: Link & 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
application
transport
network
link
physical
MAC protocol
and frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twister
pair) physical layer

28
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
services
and correction
 5.3 Multiple access
protocols
 5.4 Link-layer
Addressing
 5.5 Ethernet
 5.6 Link-layer switches,
LANs, VLANs
 5.7 PPP
MPLS
web request

29
Hubs
… physical-layer (“dumb”) repeaters:
 bits coming in one link go out all other links at
same rate
 all nodes connected to hub can collide with one
another
 no frame buffering
 no CSMA/CD at hub: host NICs detect
collisions
twisted pair
hub
Switch
 link-layer device: smarter than hubs, take
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

30
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-A’ and B-
to-B’ simultaneously,
without collisions
 not possible with dumb hub
A
A’
B
B’
C
C’
switch with six interfaces
(1,2,3,4,5,6)
1 2
3
45
6
Switch Table
 Q: how does switch know that
A’ reachable via interface 4,
B’ 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
A’
B
B’
C
C’
switch with six interfaces
(1,2,3,4,5,6)
1 2
3
45
6

31
Switch: self-learning
 switch learns which hosts
can be reached through
which interfaces
 when frame received,
switch “learns” location of
sender: incoming LAN
segment
 records sender/location
pair in switch table
A
A’
B
B’
C
C’
1 2
3
45
6
A A’
Source: A
Dest: A’
MAC addr interface TTL
Switch table
(initially empty)
A 1 60
Switch: frame filtering/forwarding
When frame received:
1. record link associated with sending host
2. index switch table using MAC dest address
3. 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

32
Self-learning,
forwarding:
example
A
A’
B
B’
C
C’
1 2
3
45
6
A A’
Source: A
Dest: A’
MAC addr interface TTL
Switch table
(initially empty)
A 1 60
A A’A A’A A’A A’A A’
 frame destination
unknown: flood
A’ A
 destination A
location known:
A’ 4 60
selective send
Interconnecting switches
 switches can be connected together
A
B
 Q: sending from A to G - how does S1 know to
forward frame destined to F via S4 and S3?
 A: self learning! (works exactly the same as in
single-switch case!)
S1
C D
E
F
S2
S4
S3
H
I
G

33
Self-learning multi-switch example
Suppose C sends frame to I, I responds to C
 Q: show switch tables and packet forwarding in S1,
S2, S3, S4
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
1
2
Institutional network
to external
network
router
IP subnet
mail server
web server

34
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
VLANs: motivation
What happens if:
 CS user moves office to EE,
but wants connect to CS
switch?
 single broadcast domain:
 all layer-2 broadcast
traffic (ARP, DHCP)
crosses entire LAN
(security/privacy,
efficiency issues)
 each lowest level switch has
only few ports in use
Computer
Science Electrical
Engineering
Computer
Engineering
What’s wrong with this picture?

35
VLANs Port-based VLAN: switch ports grouped
(by switch management software) so
that single physical switch ……
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
…
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
…
(VLAN ports 1-8)
…
1
82
7 9
1610
15
…
Computer Science
(VLAN ports 9-16)
… operates as multiple virtual switches
Port-based VLAN
1
8
9
16102
7
…
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
…
 traffic isolation: frames
to/from 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

36
VLANS spanning multiple switches
 trunk port: carries frames between VLANS defined
over multiple physical switches
 frames forwarded within VLAN between switches can’t be
vanilla 802.1 frames (must carry VLAN ID info)
 802.1q protocol adds/removed additional header fields for
frames forwarded between trunk ports
1
8
9
102
7
…
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
…
2
73
Ports 2,3,5 belong to EE VLAN
Ports 4,6,7,8 belong to CS VLAN
5
4 6 816
1
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
802.1Q VLAN frame format
802.1 frame
802.1Q frame

37
Link Layer
services
and correction
protocols
 5.4 Link-Layer
Addressing
 5.5 Ethernet
 5.7 PPP
MPLS
web request
Point to Point Data Link Control
 one sender, one receiver, one link: easier than
broadcast link:
 no Media Access Control
 no need for explicit MAC addressing
 e.g., dialup link, ISDN line
 popular point-to-point DLC protocols:
 PPP (point-to-point protocol)
 HDLC: High level data link control (Data link
used to be considered “high layer” in protocol
stack!

38
PPP Design Requirements [RFC 1557]
 packet framing: encapsulation of network-layer
datagram in data link frame
 carry network layer data of any network layer
protocol (not just IP) at same time
 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
PPP non-requirements
 no error correction/recovery
 no flow control
 out of order delivery OK
 no need to support multipoint links (e.g., polling)
Error recovery, flow control, data re-ordering
all relegated to higher layers!

39
PPP Data Frame
 Flag: delimiter (framing)
 Address: does nothing (only one option)
 Control: does nothing; in the future possible
multiple control fields
 Protocol: upper layer protocol to which frame
delivered (eg, PPP-LCP, IP, IPCP, etc)
PPP Data Frame
 info: upper layer data being carried
 check: cyclic redundancy check for error
detection

40
Byte Stuffing
 “data transparency” requirement: data field must
be allowed to include flag pattern <01111110>
 Q: is received <01111110> data or flag?
 Sender: adds (“stuffs”) extra < 01111110> byte
after each < 01111110> data byte
 Receiver:
 two 01111110 bytes in a row: discard first byte,
continue data reception
 single 01111110: flag byte
Byte Stuffing
flag byte
pattern
in data
to send
flag byte pattern plus
stuffed byte in
transmitted data

41
PPP Data Control Protocol
Before exchanging network-
layer data, data link peers
must
 configure PPP link (max.
frame length,
authentication)
 learn/configure network
layer information
 for IP: carry IP Control
Protocol (IPCP) msgs
(protocol field: 8021) to
configure/learn IP
address
Link Layer
services
and correction
protocols
 5.4 Link-Layer
Addressing
 5.5 Ethernet
 5.7 PPP
MPLS
web request

42
Virtualization of networks
Virtualization of resources: powerful abstraction in
systems engineering:
 computing examples: virtual memory, virtual
devices
 Virtual machines: e.g., java
 IBM VM os from 1960’s/70’s
 layering of abstractions: don’t sweat the details of
the lower layer, only deal with lower layers
abstractly
The Internet: virtualizing networks
1974: multiple unconnected
nets
 ARPAnet
 data-over-cable networks
 packet satellite network (Aloha)
 packet radio network
… differing in:
 addressing conventions
 packet formats
 error recovery
 routing
ARPAnet satellite net
"A Protocol for Packet Network Intercommunication",
V. Cerf, R. Kahn, IEEE Transactions on Communications,
May, 1974, pp. 637-648.

43
The Internet: virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP):
 addressing: internetwork
appears as single, uniform
entity, despite underlying local
network heterogeneity
 network of networks
Gateway:
 “embed internetwork packets in
local packet format or extract
them”
 route (at internetwork level) to
next gateway
Cerf & Kahn’s Internetwork Architecture
What is virtualized?
 two layers of addressing: internetwork and local
network
 new layer (IP) makes everything homogeneous at
internetwork layer
 underlying local network technology
 cable
 satellite
 56K telephone modem
 today: ATM, MPLS
… “invisible” at internetwork layer. Looks like a link
layer technology to IP!

44
ATM and MPLS
 ATM, MPLS separate networks in their own
right
 different service models, addressing, routing
from Internet
 viewed by Internet as logical link connecting
IP routers
 just like dialup link is really part of separate
network (telephone network)
 ATM, MPLS: of technical interest in their
own right
Asynchronous Transfer Mode: ATM
 1990’s/00 standard for high-speed (155Mbps to
622 Mbps and higher) Broadband Integrated
Service Digital Network architecture
 Goal: integrated, end-end transport of carry voice,
video, data
 meeting timing/QoS requirements of voice, video
(versus Internet best-effort model)
 “next generation” telephony: technical roots in
telephone world
 packet-switching (fixed length packets, called
“cells”) using virtual circuits

45
Multiprotocol label switching (MPLS)
 initial goal: speed up IP forwarding by using fixed
length label (instead of IP address) to do
forwarding
 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
MPLS capable routers
 a.k.a. label-switched router
 forwards packets to outgoing interface based
only on label value (don’t inspect IP address)
 MPLS forwarding table distinct from IP forwarding
tables
 signaling protocol needed to set up forwarding
 RSVP-TE
 forwarding possible along paths that IP alone would
not allow (e.g., source-specific routing) !!
 use MPLS for traffic engineering
 must co-exist with IP-only routers

46
R1R2
D
R3R4
R5
0
1
00
A
R6
in out out
label label dest interface
6 - A 0
in out out
10 6 A 1
12 9 D 0
in out out
10 A 0
12 D 0
1
in out out
8 6 A 0
0
8 A 1
MPLS forwarding tables
Link Layer
services
and correction
protocols
 5.4 Link-Layer
Addressing
 5.5 Ethernet
 5.7 PPP
MPLS
web request

47
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, requests/receives www.google.com
A day in the life: scenario
Comcast network
68.80.0.0/13
Google’s network
64.233.160.0/1964.233.169.105
web server
DNS server
school network
68.80.2.0/24
browser
web page

48
A day in the life… connecting to the Internet
 connecting laptop needs to
get its own IP address,
addr of first-hop router,
addr of DNS server: use
DHCP
router
(runs DHCP)
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCPDHCP
 DHCP request encapsulated
in UDP, encapsulated in IP,
encapsulated in 802.1
Ethernet
 Ethernet frame broadcast
(dest: FFFFFFFFFFFF) on LAN,
received at router running
DHCP server
 Ethernet demux’ed to IP
demux’ed, UDP demux’ed to
DHCP
A day in the life… connecting to the Internet
 DHCP server formulates
DHCP ACK containing
client’s IP address, IP
address of first-hop
router for client, name &
IP address of DNS server
router
(runs DHCP)
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
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 & addr of DNS
server, IP address of its first-hop router
 DHCP client receives DHCP
ACK reply

49
A day in the life… ARP (before DNS, before HTTP)
 before sending HTTP request,
need IP address of www.google.com:
DNS
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
 DNS query created, encapsulated
in UDP, encapsulated in IP,
encasulated in Eth. In order 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
Eth
Phy
ARP
ARP
ARP reply
A day in the life… using DNS
DNS
UDP
IP
Eth
Phy
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 and/or
BGP routing protocols) to DNS
server
 demux’ed to DNS server
 DNS server replies to
client with IP address of
www.google.com
Comcast network
68.80.0.0/13
DNS server
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS

50
A day in the life… TCP connection carrying HTTP
HTTP
TCP
IP
Eth
Phy
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 established!64.233.169.105
web server
SYN
SYN
SYN
SYN
TCP
IP
Eth
Phy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
 web server responds with
TCP SYNACK (step 2 in 3-
way handshake)
A day in the life… HTTP request/reply
HTTP
TCP
IP
Eth
Phy
HTTP
 HTTP request sent into
TCP socket
 IP datagram containing
HTTP request routed to
www.google.com
 IP datgram containing
HTTP reply routed back to
client
64.233.169.105
web server
HTTP
TCP
IP
Eth
Phy
 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

51
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
 PPP
 virtualized networks as a link layer: MPLS
 synthesis: a day in the life of a web request
Chapter 5: let’s take a breath
 journey down protocol stack complete
(except PHY)
 solid understanding of networking principles,
practice
 ….. could stop here …. but lots of interesting
topics!
 wireless
 multimedia
 security
 network management

Chapter5 link layer and la ns

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