1. Lecture 11:
Link Layer
Slides derived from those available on the site of the book
“Computer Networking”, by Kurose and Ross, PEARSON

2. A Layered Architecture
application: network application protocol (messages)
transport: process to process data transfer (segments)
network: host to host data transfer (datagrams)
link: across individual links data transfer (frames)
physical: over the “wire” data transfer (bits)
application
transport
network
link
physical

3. Terminology nodes: hosts, switches, routers
global ISP
nodes: hosts, switches, routers
link: communication channel that connects adjacent nodes
wired links
wireless links
frame: link-layer “packet”

4. Link-layer Overview
global ISP
Data transfer from one node to a physically adjacent node
Different protocols over different links
e.g., Ethernet, WiFi, frame relay
Different services for each protocol
e.g., over-the-link reliable data delivery

5. Transportation Analogy
Trip from Princeton to Lausanne
taxi: Princeton to JFK
plane: JFK to Geneva
train: Geneva to Lausanne
Tourist = datagram
Transport segment = link
Transportation mode = link layer protocol
Travel agent = routing algorithm

6. Hardware & Software Implementation
Hardware: network adaptor (NIC)
encapsulation, decapsulation
link access, error detection
Software: CPU
addressing
NIC interaction
application
transport
network
link
physical
cpu
memory
host bus
(e.g., PCI)
controller
physical
transmission
network adapter

7. Outline Link-layer services Link-layer forwarding++
A to Z: retrieving a web page

8. Outline Link-layer services Link-layer forwarding++
A to Z: retrieving a web page

9. Link Layer Services Error detection Reliable data delivery Link access
detect bit errors caused by signal attenuation, noise
drop frames with bit errors
Reliable data delivery
error detection + correction/retransmission
used for links with high-error rate (wireless)
Link access
how to share the same link with other nodes
Flow control
pacing between adjacent sending and receiving nodes

10. Error Detection & Correction
otherwise
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

11. Error Detection & Correction (2)
Single bit parity
detect single bit errors
Two-dimensional bit parity
detect and correct single bit errors
Cyclic Redundancy Check (CRC)
more powerful error-detection than parity checking
widely used in practice: Ethernet, WiFi

12. (shared air, acoustical)
Media Access Control (MAC)
Point-to-point links
state-of-the-art Ethernet
Broadcast links (shared medium)
old-fashioned Ethernet, WiFi
shared wire (e.g.,
cabled Ethernet)
shared RF
(e.g., WiFi)
shared RF
(satellite)
humans at a
cocktail party
(shared air, acoustical)

13. MAC Protocols Given The problem An ideal multiple access protocol
single shared broadcast link of rate R
The problem
interference = two or more simultaneous transmissions
collision = node receives two or more signals at the same time
An ideal multiple access protocol
when only one node wants to transmit, it can send at rate R
when N nodes want to transmit, each can send at average rate R/N
simple, fully decentralized

14. MAC Protocols Classification
Channel partitioning
channel divided into smaller “pieces” (time slots, frequency bands)
allocate one piece to each node for exclusive use
Random access
channel not divided, allow collisions
detect and recover from collisions
Taking turns
nodes take turns

15. Efficient at high load. Inefficient at low load.
Channel Partitioning
TDMA: time divided in equal frames and slots 1 3 4
6-slot
frame
FDMA: frequency spectrum divided into frequency bands
frequency bands
time
FDM cable
Efficient at high load. Inefficient at low load.

16. Efficient at low load. Inefficient at high load.
Random Access 1 2 3
node 1
node 2
node 3 C S E
if single node - transmit at full rate R bps
if N nodes - transmit at well bellow ideal R/N bps each (collisions, empty slots)
Efficient at low load. Inefficient at high load.

17. CSMA - Carrier Sense Multiple Access
Sense the channel before transmitting
if channel sensed idle, transmit
if channel sensed busy, defer transmission

18. spatial layout of nodes
CSMA Collisions
spatial layout of nodes
Collisions can still occur
due to propagation delay
Collision probability
depends on propagation delay

19. CSMA/CD - Collision Detection
Sense the channel before transmitting
if channel sensed idle, transmit
if channel sensed busy, defer transmission
If a collision is detected
abort transmission
defer transmission

20. spatial layout of nodes
CSMA/CD - Collision Detection
spatial layout of nodes

21. 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:
more collisions -> longer wait interval

22. Taking Turns (1) Polling: master “invites” slave to transmit
Main concerns
polling overhead
single point of failure (master)
data
poll
master
data
slaves

23. Taking Turns (2)
Token passing: control token passed from one node to the next sequentially
Main concerns
token overhead
single point of failure (token) T
(nothing
to send) T
data

24. MAC Protocols Channel partitioning Random access Taking turns
efficient at high load
inefficient at low load
Random access
efficient at low load
inefficient at high load (due to collisions)
Taking turns
look for best of both worlds
single point of failure

25. Outline Link-layer services Link-layer forwarding++
A to Z: retrieving a web page

26. MAC Addresses Used to move frames between link-layer nodes
one address per NIC
Format: 48 bit address, flat
usually shown in hexadecimal format
1A-2F-BB AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN

27. Address Resolution Protocol (ARP)
Question:
- How to determine interface’s MAC address, knowing its IP address?
1A-2F-BB AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
Answer:
- Source broadcasts request for destination’s MAC address
- Stores response in local ARP table < IP address; MAC address; TTL>

28. Ethernet The dominant wired LAN technology
First widely used LAN technology
Simpler, cheaper than token LANs and ATM
Kept up with speed race: 10 Mbps – 100 Gbps

29. Ethernet Services Connectionless Unreliable MAC protocol
no handshaking between sending and receiving NICs
Unreliable
receiving NIC does not send ACKs or NACKs to sending NIC
recover dropped frames only if sender uses higher layer “rdt” (TCP)
MAC protocol
CSMA/CD with exponential backoff

30. Ethernet Frame Structure
Sending adapter encapsulates datagram in Ethernet frame
preamble: 7 x followed by 1 x
synchronize sender, receiver clock rates
addresses: 48 bit source, source/destination MAC address
receive frame only if destination matches or broadcast
type: indicates higher layer protocol
mostly IP, others possible
CRC: cyclic redundancy check at receiver
dest.
address
source
data (payload)
CRC
preamble
type

31. Ethernet Physical Topology
bus: popular in the mid 90s
all nodes in same collision domain (frames can collide)
star: prevails today
active switch in the middle - each “spoke” in a separate collision domain (no collisions)
switch
bus
star

32. Link-layer Switch Forwards frames within LAN Self-learning
determines output port based on destination MAC address
similar to router forwarding process
Self-learning
forwarding table populated automatically
no need for manual configuration or routing protocol

33. switch forwarding table
Self-learning
Source: A
Dest: A’ A A’ B B’ C C’ 1 2 3 4 5 6
A A’
- frame destination A’, location unknown:
flood
- frame destination A, location known:
selectively send on just one link
A A’
A A’
A A’
A A’
A A’
A’ A
MAC addr interface TTL A 1 60
switch forwarding table
(initially empty) A’ 4 60

34. Self-learning (2) When a frame is received at the switch:
1. Record pair (source MAC address; incoming link)
2. Index forwarding table using destination MAC address
3. If entry found: - if destination on link from which frame arrived drop frame
- else forward frame on link indicated by entry
(else) If entry not found:
- flood: forward on all links except the arriving interface

35. Self-learning: Interconnected SwitchesB S1 C
Scenario: A wants to send frame to H
- Question: how does S1 know to forward via S4 and S3?
- Answer: self learning, exactly the same way

36. Switches vs. Routers Both are store-and-forward
application
transport
network
link
physical
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: using routing
switches: using self-learning
datagram
frame
link
physical
frame
switch
network
link
physical
datagram
frame
application
transport
network
link
physical

37. Forwarding to Another LAN
Scenario:
- A sends a datagram to B via router R - A knows IP address of B (how?)
- A knows IP address of R (how?) - A knows MAC address of R (how?) R
1A-23-F9-CD-06-9B
E6-E BB-4B
CC-49-DE-D0-AB-7D
C-E8-FF-55 A
49-BD-D2-C7-56-2A
88-B2-2F-54-1A-0F B

38. Forwarding to Another LAN
- create IP datagram with source IP-A, destination IP-B
- create link-layer frame with source MAC-A, destination MAC-R, frame contains A-to-B IP datagram
MAC src: C-E8-FF-55
MAC dest: E6-E BB-4B IP
Eth
Phy
IP src:
IP dest: R
1A-23-F9-CD-06-9B
E6-E BB-4B
CC-49-DE-D0-AB-7D
C-E8-FF-55 A
49-BD-D2-C7-56-2A
88-B2-2F-54-1A-0F B

39. Forwarding to Another LAN
- send frame from A to R
- receive frame at R, extract datagram, pass it up to IP layer
MAC src: C-E8-FF-55
MAC dest: E6-E BB-4B
IP src:
IP dest:
IP src:
IP dest: IP
Eth
Phy IP
Eth
Phy R
1A-23-F9-CD-06-9B
E6-E BB-4B
CC-49-DE-D0-AB-7D
C-E8-FF-55 A
49-BD-D2-C7-56-2A
88-B2-2F-54-1A-0F B

40. Forwarding to Another LAN
- forward datagram with source IP-A, destination IP-B
- create link-layer frame with source MAC-R, destination MAC-B, frame contains A-to-B IP datagram
MAC src: 1A-23-F9-CD-06-9B
MAC dest: 49-BD-D2-C7-56-2A
IP src:
IP dest: IP
Eth
Phy R
1A-23-F9-CD-06-9B
E6-E BB-4B
CC-49-DE-D0-AB-7D
C-E8-FF-55 A
49-BD-D2-C7-56-2A
88-B2-2F-54-1A-0F B

41. Forwarding to Another LAN
- send frame from R to B
IP src:
IP dest:
MAC src: 1A-23-F9-CD-06-9B
MAC dest: 49-BD-D2-C7-56-2A IP
Eth
Phy IP
Eth
Phy R
1A-23-F9-CD-06-9B
E6-E BB-4B
CC-49-DE-D0-AB-7D
C-E8-FF-55 A
49-BD-D2-C7-56-2A
88-B2-2F-54-1A-0F B

42. Forwarding to Another LAN
- send frame from R to B
- receive frame at B, extract datagram, pass it up to IP layer
MAC src: 1A-23-F9-CD-06-9B
MAC dest: 49-BD-D2-C7-56-2A
IP src:
IP dest: IP
Eth
Phy B A R
49-BD-D2-C7-56-2A
C-E8-FF-55
1A-23-F9-CD-06-9B
E6-E BB-4B
CC-49-DE-D0-AB-7D
88-B2-2F-54-1A-0F

43. Outline Link-layer services Link-layer forwarding++
A to Z: retrieving a web page

44. Synthesis - A day in the life of a web request
application
transport
network
link
physical
scenario:
- attach laptop to campus network
- request/receive page
goal:
- review the protocols involved at all layers

45. A day in the life… scenario
browser
DNS server
Comcast network
/13
school network
/24
web page
web server
Google’s network
/19

46. … connecting to the Internet (1)
DHCP
UDP IP
Eth
Phy
DHCP
DHCP
Laptop uses DHCP to get: - IP address, subnet mask - IP address of first-hop router - IP address of DNS server
router
(runs DHCP)
DHCP
DHCP discovery message
- encapsulated in UDP
- encapsulated in IP
- encapsulated in Ethernet
DHCP
DHCP
UDP IP
Eth
Phy
DHCP
Ethernet frame broadcast on LAN
- destination MAC: 0xffffffffffff
- received by DHCP server (running on the router)

47. … connecting to the Internet (2)
DHCP
DHCP
UDP IP
Eth
Phy
router
(runs DHCP)
- DHCP server and laptop exchange
-offer
-request
-acknowledgement
DHCP
UDP IP
Eth
Phy
DHCP
DHCP
DHCP
Laptop has IP address, knows IP address of the first-hop router and a DNS server

48. Laptop knows MAC address of the first-hop router
… L2 learning
DNS
UDP IP
Eth
Phy
DNS
- Laptop uses DNS to get the IP address for
router
(runs DHCP)
ARP
ARP query
- DNS query
-encapsulated in UDP
-encapsulated in IP
-encapsulated in.. ooops!
- Laptop needs to get the MAC address of the first-hop router
Eth
Phy
ARP
ARP reply
- ARP request broadcasted on LAN, received at first-hop router
- Router sends ARP response back
Laptop knows MAC address of the first-hop router

49. Laptop knows IP address of www.google.com
… L3 learning
DNS
UDP IP
Eth
Phy
DNS
DNS server
DNS
UDP IP
Eth
Phy
DNS
router
(runs DHCP)
DNS
DNS
DNS
DNS
Comcast network
/13
- DNS query (IP datagram) forwarded to first-hop router via switch
- DNS query (IP datagram) forwarded to Comcast network, DNS server
- DNS server sends DNS response back to laptop
Laptop knows IP address of

50. TCP connection established
… TCP connection setup
HTTP
HTTP
TCP IP
Eth
Phy
router
(runs DHCP)
SYN
SYNACK
SYN
- Web browser process opens TCP socket
TCP IP
Eth
Phy
- Sends SYN segment
SYN
SYNACK
- Web server responds with SYNACK segment
SYNACK
web server
TCP connection established

51. Web page is finally displayed!
… HTTP request/reply
HTTP
HTTP
HTTP
TCP IP
Eth
Phy
router
(runs DHCP)
HTTP
HTTP
- Web browser sends HTTP request to web server
HTTP
TCP IP
Eth
Phy
HTTP
- Web server sends HTTP reply to web client (the web page)
HTTP
web server
Web page is finally displayed!