School of Computing Science

1

• School of Computing Science
• Simon Fraser University
• CMPT 880 Internet Architectures and Protocols
• Introduction IV
• Instructor Dr. Mohamed Hefeeda

2
Review of Basic Networking Concepts

• Internet structure
• Protocol layering and encapsulation
• Internet services and socket programming
• Network Layer
• Network types Circuit switching, Packet
  switching
• Addressing, Forwarding, Routing
• Transport layer
• Reliability, congestion and flow control
• TCP, UDP
• Link Layer
• Multiple Access Protocols
• Ethernet, MAC addressing

3
Link Layer

• 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
4
Link layer context

• transportation analogy
• trip from Burnaby to Lausanne, Switzerland
• limo Burnaby to YVR
• plane YVR to Geneva
• train Geneva to Lausanne
• tourist datagram
• transport segment communication link
• transportation mode link layer protocol
• travel agent routing algorithm

• 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

5
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?
• LL local correction (bet adjacent nodes) ?
  faster
• e-2-e is still needed because not all LL
  protocols provide reliability

6
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

7
Adaptors Communicating
datagram
rcving node
link layer protocol
sending node
adapter
adapter

• receiving side
• looks for errors, rdt, flow control, etc
• extracts datagram, passes to rcving node
• adapter is semi-autonomous
• link physical layers

• link layer implemented in adaptor (aka NIC)
• Ethernet card, PCMCI card, 802.11 card
• sending side
• encapsulates datagram in a frame
• adds error checking bits, rdt, flow control, etc.

8
Multiple Access Links and Protocols

• Two types of links
• point-to-point
• Single sender and single receiver
• E.g., dial-up links ? point-to-point protocol
  (PPP)
• broadcast (shared wire or medium)
• Multiple senders and multiple receivers
• E.g., traditional Ethernet, 802.11 wireless LAN
• ? need Multiple Access protocol (MAC)

9
Multiple Access protocols

• Two or more simultaneous transmissions on a
  shared channel ? interference (collision)
• Collision node receives two or more signals at
  the same time
• Multiple Access (MAC) 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
MAC Protocols a taxonomy

• Three broad classes
• Channel Partitioning
• Channel Partitioning, by time, frequency or code
• TDMA, FDMA, CDMA
• 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
• E.g., Token bus and token ring

11
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

12
CSMA (Carrier Sense Multiple Access)

• CSMA listen before transmit
• If channel sensed idle transmit entire frame
• If channel sensed busy, defer transmission
• Can collisions still occur?
• Yes, because of propagation delay
• two nodes may not hear each others transmission
• During collision, entire packet transmission time
  is wasted ? detect collision and abort
  immediately (CSMA/CD)

13
Ethernet

• dominant wired LAN technology
• cheap 20 for 100Mbs!
• first widely used LAN technology
• Simpler, cheaper than token LANs and ATM
• Kept up with speed race 10 Mbps 10 Gbps

Metcalfes Ethernet sketch
14
Star topology

• Bus topology popular through mid 90s
• Now star topology prevails
• Connection choices hub or switch (more later)

hub or switch
15
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

16
Ethernet Frame Structure (more)

• Addresses 6 bytes
• if adapter receives frame with matching
  destination address, or with broadcast address
  (e.g., ARP packet), it passes data in frame to
  net-layer protocol
• otherwise, adapter discards frame
• Type indicates the higher layer protocol (mostly
  IP but others may be supported such as Novell IPX
  and AppleTalk)
• CRC checked at receiver, if error is detected,
  the frame is simply dropped

17
Unreliable, connectionless service

• Connectionless No handshaking between sending
  and receiving adapter.
• Unreliable receiving adapter doesnt send acks
  or nacks to sending adapter
• 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

18
Ethernet CSMA/CD algorithm

• 1. Adaptor receives datagram from net layer
  creates frame
• 2. If adapter senses channel idle, it starts to
  transmit frame. If it senses channel busy, waits
  until channel idle and then transmits
• 3. If adapter transmits entire frame without
  detecting another transmission, the adapter is
  done with frame!

• 4. If adapter detects another transmission while
  transmitting, aborts and sends jam signal
• 5. After aborting, adapter enters exponential
  backoff after the mth collision, adapter chooses
  K at random from 0,1,2,,2m-1. Adapter waits
  K?512 bit times and returns to Step 2

19
Ethernets CSMA/CD (more)

• Jam Signal make sure all other transmitters are
  aware of collision 48 bits
• Bit time 0.1 microsec for 10 Mbps Ethernet for
  K1023, 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 !
20
CSMA/CD efficiency

• Tprop max prop between 2 nodes in LAN
• ttrans time to transmit max-size frame
• Efficiency goes to 1 as tprop goes to 0
• Goes to 1 as ttrans goes to infinity
• Much better than ALOHA, but still decentralized,
  simple, and cheap

21
Hubs

• Hubs are essentially physical-layer repeaters
• bits coming from one link go out all other links
• at the same rate
• no frame buffering
• no CSMA/CD at hub adapters detect collisions
• provides net management functionality

22
Interconnecting with hubs

• Backbone hub interconnects LAN segments
• Extends max distance between nodes
• But individual segment collision domains become
  one large collision domain
• Cant interconnect 10BaseT 100BaseT

hub
hub
hub
hub
23
Switch

• Link layer device
• stores and forwards Ethernet frames
• examines frame header and selectively forwards
  frame based on MAC dest address
• 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

24
Forwarding
1
3
2

• How to determine onto which LAN segment to
  forward frame?
• Looks like a routing problem...

25
Self learning

• A switch has a switch table
• entry in switch table
• (MAC Address, Interface, Time Stamp)
• stale entries in table dropped (TTL can be 60
  min)
• 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

26
Switch example

• Suppose C sends frame to D

address
interface
switch
1
A B E G
1 1 2 3
3
2
hub
hub
hub
A
I
F
D
G
B
C
H
E

• Switch receives frame from C destined to D
• notes in switch table that C is on interface 1
• because D is not in table, switch forwards frame
  into interfaces 2 and 3
• frame received by D

27
Switch traffic isolation

• switch installation breaks subnet into LAN
  segments
• switch filters packets
• same-LAN-segment frames not usually forwarded
  onto other LAN segments
• segments become separate collision domains

collision domain
collision domain
collision domain
28
Switches dedicated access

• Switch with many interfaces
• Hosts have direct connection to switch
• No collisions full duplex
• Switching A-to-A and B-to-B simultaneously, no
  collisions

A
C
B
switch
C
B
A
29
Institutional network
mail server
to external network
web server
router
switch
IP subnet
hub
hub
hub
30
Switches vs. Routers

• both store-and-forward devices
• Routers network layer devices
• Switches link layer devices ? faster processing
• Routers maintain routing tables, implement
  routing algorithms
• handle complex topologies, find efficient paths
• Switches maintain switch tables, implement
  learning algorithms
• handle simpler (spanning tree) topologies, paths
  may not be optimal

31
MAC Addresses

• 32-bit IP address
• network-layer address
• used to get datagram to destination IP subnet
• MAC (or LAN or physical or Ethernet) address
• used to get frame from one interface to another
  physically-connected interface (same network)
• 48 bit MAC address (for most LANs) burned in the
  adapter ROM

32
MAC Address
Each adapter on LAN has unique LAN address
Broadcast address FF-FF-FF-FF-FF-FF
adapter
33
MAC Address (more)

• MAC address allocation administered by IEEE
• manufacturer buys portion of MAC address space
  (to assure uniqueness)
• Analogy
• (a) MAC address like Social Insurance
  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
• depends on IP subnet to which node is attached

34
MAC and IP addresses

• Why do we have TWO addresses (IP,MAC)? Do we have
  to have MAC addresses?
• Yes, we must have both
• To allow different network-layer protocols over
  same card (e.g., IP, Novell IPX, DECnet)
• Enable flexibility, mobility of cards
• Efficiency imagine that nodes have only IP
  addresses ? ALL packets sent over LAN will be
  forwarded by NIC to the IP layer ? too many
  useless interrupts

35
ARP Address Resolution Protocol

• Each IP node (Host, Router) on LAN has ARP table
• ARP Table IP/MAC address mappings for some LAN
  nodes
• lt IP address MAC address TTLgt
• TTL (Time To Live) time after which address
  mapping will be forgotten (typically 20 min)

237.196.7.78
1A-2F-BB-76-09-AD
237.196.7.23
237.196.7.14
LAN
71-65-F7-2B-08-53
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
237.196.7.88
36
ARP protocol Same LAN (network)

• 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

• A wants to send datagram to B, and Bs MAC
  address not in As 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 As MAC address (unicast)

37
Routing to another LAN

• walkthrough send datagram from A to B via R
• assume A knows Bs IP
  address
• Two ARP tables in router R, one for each IP
  network (LAN)

A
R
B
38
Routing to another LAN (contd)

• Detailed steps
• A creates datagram with source A, destination B
• A uses ARP to get Rs 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
• As adapter sends frame
• Rs adapter receives frame
• R removes IP datagram from Ethernet frame, sees
  its destined to B
• R uses ARP to get Bs MAC address
• R creates frame containing A-to-B IP datagram
  sends to B