1. CSCD 330 Network Programming
Winter 2019
Lecture 18
Link Layer Protocols Continued
Reading: Chapter 5
Who is this?
Some slides provided courtesy of
J.F Kurose and K.W. Ross, All Rights Reserved, copyright 1 1

2. Overview Link Layer Hardware Hubs vs Switches vs Routers Ethernet
ARP and MAC Addressing 2 2

3. LAN and Network Hardware

4. Shuttling Data at Different Layers
Different devices switch different things
Physical Layer: Electrical signals
(repeaters and hubs)‏
Link layer: Frames (bridges and switches)‏
Network layer: Packets (routers)‏
Application gateway
Transport gateway
Router
Frame header
Packet header
TCP header
User
data
Bridge, switch
Repeater, hub 4 4

5. Key Distinction (Smart) Routers (Dumber) Switches/Bridges
Forward based on IP headers
(Dumber) Switches/Bridges
00:13:02:BA:43:56
Forward based on MAC addresses
(Dumbest) Repeaters/Hubs
Broadcast all bits
Also talk about devices being “smart” or “stupid”
Smart means they can detect addresses
Stupid means they can't make decisions – too dumb 5

6. Repeaters Length of cable influences quality of communication
Repeaters repeat signals
Clean and boost digital transmission
Analog networks use amplifiers to boost signal
Repeaters only work with the physical signal
Cannot reformat, resize, or manipulate the data
Physical layer, Layer 1 device

7. Repeaters (continued)‏

8. Repeaters (continued)‏

9. Hubs Generic connection device
Operates at the Physical Layer
Connect several networking cables together
Active hubs
Known as ... Multiport repeaters
Passive hubs
Something that does not boost signal, just connects the wires

10. Hubs (continued)‏

11. Switches Operate at the Data Link layer
Increases network performance
Virtual circuits between source and destination
Micro segmentation at the port level
More complicated and expensive than hubs

12. Switches Switch Filter based on MAC addresses
Build tables in memory of host locations
Creates singular collision domains
Collision Domain
Hub
Switch

13. Collision Domain What is a Collision Domain?
Group of nodes in Ethernet network that compete with each other for access
If two or more devices try to access network at exact same time, collision will occur
In switched Ethernet environment, each transmitting-receiving pair of nodes is essentially its own collision domain, except that no collisions can occur, because there is no sharing of bandwidth

14. Use of Ethernet Switches Versus Hubs in a LAN
Collisions with Switch and Hub
Hubs
Switch

15. Switch Link-layer device, “Layer 2” Switch Transparent
Store, forward Ethernet frames
Examine incoming frame’s MAC address, selectively forward frame outgoing link when frame is to be forwarded on segment
Transparent
Important !!!!! Note that
Hosts are unaware of presence of switches
Operate at lower levels of protocol stack
Plug-and-play, self-learning
Switches do not need to be manually set
5-15 15

16. Switches Advantages of Switches Increase available network bandwidth
Reduced workload, computers only receive packets intended for them specifically
Increase network performance
Each host gets full speed use of link
Smaller collision domains
Single host as opposed to everyone is competing for bandwidth

17. Switches ‏ Disadvantages of Switches
More expensive than hubs and bridges
Difficult to trace network connectivity problems through a switch
Does not filter broadcast traffic

18. Switches (continued)‏

19. Routers What does a router do?
Provides filtering and network traffic control
Used for LANs to connect segments and WANs
Connect multiple segments and networks
Multiple routers create an “internetwork”
Operates at Network layer
Layer 3 device

20. Routers Creates a table to determine how to forward packets
Filtering and traffic control base on logical addresses, IP addresses

21. Differences Logical vs. Physical
Look at the Differences Between Logical and Physical Addresses ...

22. Physical Versus Logical Addresses
MAC addresses
Data Link layer
Used by switches
Used for directly connected devices
Mostly encoded in firmware, does not change
Logical addresses, IP
Network layer
Use by routers
Changes with network segment
IP addresses are assigned manually or by software

23. Physical Versus Logical Addresses (continued)‏

24. Ethernet

25. Ethernet History
1973 Xerox Corporation’s Palo Alto Research Center developed bus topology Local Area Network (LAN)
1976, carrier sensing added,
Xerox built 2.94 Mbps network to connect over 100 personal workstations on 1 km cable
Network called Ethernet, named after ether, single coaxial cable used to connect machines
"Ethernet" refers to product which predates IEEE Standard
But, nowadays any compliant network is referred to as an Ethernet

26. Ethernet Bob Metcalf Original Paper published in 1976
Metcalfe’s Ethernet
sketch
Original Paper published in 1976 26 26

27. 802.3 Standard Project
In 1980, (IEEE) started project 802 to standardize local area networks (LAN)‏
IEEE wanted to put forward one standard
IBM Token Ring,
Token Bus and
Ethernet were all contenders
Eventually, Ethernet won and it became a standard in 1985 … 28 years ago!
First standard was for 10 Mbps
Entire list of Ethernet Standards

28. Ethernet Terminology Physical Layer
First number (10, 100, or 1000) transmission speed in megabits per second, Mbps
Second term indicates transmission type
BASE = baseband or BROAD = broadband
Baseband – entire channel is taken up by one signal
Broadband – multiple signals can occupy one channel
Last number indicates segment length
5 means 500-meter (500-m) segment length from original Thicknet
Recent versions, letters replace numbers
Ex. 10BASE-T, T means unshielded twisted-pair cables
Numbers show number of twisted pairs
Ex.100BASE-T4, T4 indicates four twisted pair

29. Classical Ethernet Broadcast
Classical Ethernet, is Broadcast Network
Hosts connected to network through single shared medium
If two nodes try to send at same time,
Called collision and prevents information passed along network
Multiple messages would collide and corrupt each other

30. Early Ethernet 10Base5 and 10Base2
The initial Ethernet implementations used coaxial cable to connect the stations to each other
Two forms of coaxial cable were used
10Base5 cable known as thick Ethernet
10Base2 also known as thin Ethernet

31. Ethernet Technology: First Attempt
10Base2: under 656 ft cable length
Thin coaxial cable in a bus topology
10Base5 uses Thick coaxial cable which allows up to 100 nodes over a maximum distance of 1640 ft
Repeaters used to connect multiple segments

32. Ethernet with Hubs: 2nd Attempt
Next form of Ethernet
10BaseT with Hubs
10BaseT, used twisted pair wiring
instead of coaxial cable
10BaseT could use Category 3 twisted pair wire, already present in modern office buildings for telephones
Contributed hugely to Ethernet's popularity

33. Ethernet Technologies: Next Attempt 10BaseT and 100BaseT
Hub(s) connected by twisted pair in
“star topology”
Distance of any node to hub < 326 ft

34. The 10Mb/s Ethernet Standard IEEE 802.3
Ethernet MAC Protocol
10Base-5
10Base-2
10Base-T
10Base-F
Different physical layer options
10Base-5: Original Ethernet: large thick coaxial cable.
10Base-2: Thin coaxial cable version.
10Base-T: Voice-grade unshielded twisted-pair
Category-3 telephone cable.
10Base-F: Two optical fibers in a single cable.

35. 802.3u Fast Ethernet In 1995, IEEE adopted
802.3u Fast Ethernet standard
Fast Ethernet is 100 Mbps Ethernet standard
With Fast Ethernet came full-duplex Ethernet
Two stations could transmit at the same time!
Previously, Ethernets worked in half-duplex mode

36. The 100Mb/s Ethernet Standard “Fast Ethernet”
Different physical layer options
Ethernet MAC Protocol
100Base-T4
100Base-TX
100Base-FX
Up to 100m of cable per segment.
100Base-T4: Uses four pairs of voice grade Category-3 cable .
100Base-TX: Uses two pairs of data grade Category-5 cable.
100Base-FX: Uses two optical fibers.

37. The 1Gb/s Ethernet Standard
June Gigabit Ethernet was first defined in 802.3z supplement to standard, published in June 1998
802.3z defines network running at 1000 Mbps in either half-duplex or full-duplex mode, over variety of different network media

38. The 1Gb/s Ethernet Standard “Gigabit Ethernet”
Ethernet MAC Protocol
1000Base-TX
1000Base-FX
1000Base-TX: Uses four pairs of data grade Category-5 cable.
1000Base-FX: Uses two optical fibers.

39. Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble:
7 bytes with pattern followed by one byte with pattern
Used to synchronize receiver, sender clock rates
5: DataLink Layer
5-39 39

40. Ethernet Frame Structure (more)‏
Addresses:
If adapter receives frame with matching destination address, or with broadcast address , it passes data in frame to network layer protocol, accepts the frame
Otherwise, adapter discards frame
What kind of addresses at this layer?
MAC - Media Access Control
Example: 00:13:02:BA:43:56
Type: Indicates higher layer protocol (mostly IP but others possible, e.g., Novell IPX)‏
CRC: checked at receiver, if error is detected, frame is dropped 40 40

41. CSMA/CD in
Ethernet

42. Carrier Sense Media Access/ Collison Detection (CSMA/CD) Protocol
All hosts transmit & receive on one channel
Packets are of variable size.
When a host has a packet to transmit:
1. Carrier Sense: Check that the line is quiet before transmitting.
2. Collision Detection: Detect collision as soon as possible. If a collision is detected, stop transmitting; wait a random time, then return to step 1.
binary exponential backoff

43. Ethernet CSMA/CD algorithm
Carrier Sense Multiple Access/
Collision Detection
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 ! 43 43

44. Ethernet CSMA/CD algorithm
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 a K, small
integer, at random from {0,1,2,…,2m-1}
NIC then waits K·512 bit time,
Returns to Step 2
More details follow … 44

45. Ethernet CSMA/CD algorithm
Features
Transmitting station intentionally transmits a "jam sequence" to ensure all stations are notified frame transmission failed due to collision
Station then remains silent for a random period of time before attempting to transmit again
Repeats: Until frame is eventually transmitted successfully 45

46. Ethernet’s CSMA/CD (more)‏
Exponential Backoff
Goal Adapt retransmission attempts to estimated current load
Heavy load -> random wait will be longer and more varied
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}
Set size grows Exponentially 46 46

47. Ethernet and Switches

48. switch with six interfaces
Switch: Allows multiple simultaneous transmissions A
Hosts have dedicated, direct connection to switch
Switches “buffer” packets
Ethernet protocol used on each incoming link, no collisions AND 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 C’ B 1 2 6 3 4 5 C B’ A’
switch with six interfaces
(1,2,3,4,5,6)
5: DataLink Layer
5-48 48

49. switch with six interfaces
Switch Table A
Q: How does switch know that A’ reachable via interface 4, B’ reachable via interface 5?
A: Each switch has 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?
Self-Learning C’ B 1 2 6 3 4 5 C B’ A’
switch with six interfaces
(1,2,3,4,5,6)
5: DataLink Layer
5-49 49

50. 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 A C’ B 1 2 6 3 4 5 C B’ A’
MAC addr interface TTL
Switch table
(initially empty)‏ A 1 60
5-50 50

51. Switch: self-learning
Source: A
Dest: A’ A
A A’
Frame with Destination A' arrives at switch from interface 1
Two possibilities:
1. No entry in table, for A',
switch forwards frame to all interfaces except 1
- Entry for A' added when
A' answers
2. Entry in table, for A'
interface of 4, frame
would get forwarded
only to A' C’ B 1 2 6 3 4 5 C B’ A’
MAC addr interface TTL A 1 60
Switch table
(initially empty)‏ A' 4
5: DataLink Layer
5-51 51

52. Switches and CSMA/CD
Do we need to use CSMA/CD on today's switched network?
Collision domain has pretty much been relegated to history
Hubs still use CSMA/CD, but if network
uses Fast Ethernet switches,
in full-duplex mode, then CSMA/CD
no longer comes into play
Full-duplex switches use separate wire pairs so switch port can send data to attached computer, while receiving data from that computer on another wire pair

53. Link Layer 5.1 Introduction and services
5.2 Error detection and correction
5.3Multiple access protocols
5.4 Link-Layer Addressing
5.6 Link-layer switches 53 53

54. Topics
LAN Addressing
Arp Protocol

55. MAC Addresses Network Layer 32-bit IP address
Network-layer address, dotted decimal
Ex.:
To route datagram to destination machine
MAC (or LAN or physical or Ethernet) Address
MAC stands for Media Access Control
48 bit MAC address (for most LANs)‏
Burned in NIC ROM, also sometimes software settable
24 bits set for manufacturer, bits for NIC adapter
Ex.: 00:E0:B8:9C:A6:60 55 55

56. MAC Address Why would you want to change your MAC address?
Many reasons ...
1. To get around MAC address filtering of wireless routers. You sniff for Mac address of someone already on network, then change your Mac address to one that's acceptable
Why filtering by Mac address is not very secure!!
2. To keep a burned-in MAC address out of IDS and security logs, keeps deviant behavior from being connected to hardware
3.To pull off a denial of service attack, assume MAC of gateway to a subnet lots of WiFi routers will lock up if a client tries to connect with the same MAC as router's BSSID

57. Change Your MAC Address
How to change your MAC address
Windows XP/2000/Vista
Use regedit to edit registry or use a utility
Mac Makeup,
MadMACs
spoofer
Smac
Etherchange -
Linux
$ ifconfig eth0 down hw ether 00:00:00:00:00:01
$ ifconfig eth0 up

58. MAC Addresses Each adapter on LAN has unique MAC address except for
Broadcast address which is FF-FF-FF-FF-FF-FF
1A-2F-BB AD
LAN
(wired or
wireless)‏
= adapter
71-65-F7-2B-08-53
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98 58 58

59. MAC Address Analogy: MAC address allocation administered by IEEE
Manufacturer buys portion of MAC address space
(to assure uniqueness)‏
Table:
Analogy:
(a) MAC address … like Social Security Number
Take it with you
(b) IP address … like postal address,
Changes when you move
Flat MAC address increases 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
Must change IP address if move to a different subnet 59 59

60. ARP: Address Resolution Protocol
Question: How to determine
MAC address of B
knowing B’s IP address?
Each IP node 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 discarded
Varies 1 to 20 minutes on average A
1A-2F-BB AD
LAN
71-65-F7-2B-08-53
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98 B 60 60

61. ARP Cache For every outgoing packet sending ARP request
and wait for response is inefficient
Requires more bandwidth
Consumes Time
So, ARP cache maintained at each node
Size limit = 512 entries

62. ARP Protocol: Same LAN
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
Shouts to everyone on LAN!!!
Destination 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)‏ 62 62

63. Types of ARP Messages ARP request ARP reply
Who is IP addr X.X.X.X tell IP addr Y.Y.Y.Y
ARP reply
IP addr X.X.X.X is Ethernet Address hh:hh:hh:hh:hh:hh

64. ARP Protocol: Same LAN A caches (saves) IP-to-MAC address pair
Called: ARP table until information becomes old
Eventually
Times out
ARP table keeps Soft state information that times out unless refreshed
ARP is “plug-and-play”
Nodes create their ARP tables without intervention from, you, the network administrator
Look at example within one LAN … in class

65. ARP Protocol: Routing to another LAN
Walkthrough: Send datagram from A to B via router, R
Assume A knows B’s IP address
Two ARP tables in router R, one for each IP network (LAN)‏ R
1A-23-F9-CD-06-9B
E6-E BB-4B
CC-49-DE-D0-AB-7D A
C-E8-FF-55
88-B2-2F-54-1A-0F B
49-BD-D2-C7-56-2A 65 65

66. R ARP Protocol example continued
A creates IP datagram with source A, destination B
A uses ARP to get R’s MAC address for
A creates link-layer frame with R's MAC address as destination, 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
C-E8-FF-55
88-B2-2F-54-1A-0F A
E6-E BB-4B
1A-23-F9-CD-06-9B B R
49-BD-D2-C7-56-2A
CC-49-DE-D0-AB-7D 66 66

67. Summary Ethernet highly successful LAN technology
Simple, cheap and adaptable
Can adapt to new faster underlying medium
Hubs, Switches and Routers
Good to know what each does
Hubs and switches at Link Layer
Router at higher layer 67

68. End 68