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EEC484584 Computer Networks

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(Part of the s are based on materials supplied by. Dr. Louise Moser at UCSB and ... Frames received with non-matching destination address is discarded ... – PowerPoint PPT presentation

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Title: EEC484584 Computer Networks


1
EEC-484/584Computer Networks
  • Lecture 10
  • Wenbing Zhao
  • wenbing_at_ieee.org
  • (Part of the slides are based on materials
    supplied by Dr. Louise Moser at UCSB and
    Prentice-Hall)

2
Outline
  • Manchester Encoding
  • The Ethernet MAC Sublayer Protocol
  • The Binary Exponential Backoff Algorithm
  • Switched Ethernet
  • ARP and DHCP
  • Reminder Quiz 2
  • Oct 19 Thursday 4-6pm (TTh Session)
  • Oct 23 Monday 2-4pm (MW Session)

3
Manchester Encoding
  • Binary encoding
  • Hard to distinguish 0 bit (0-volt) from idle
    (0-volt)
  • Requires clocks of all stations synchronized
  • Manchester encoding and differential Manchester
    encoding

4
Ethernet Frame Structure
  • Preamble for clock synchronization
  • First 7 bytes with pattern 10101010, last byte
    with pattern 10101011
  • The two consecutive 1s indicate the start of a
    frame
  • How can the receiver tell the end of the frame?
  • No current on the wire

gt 64 bytes
5
Ethernet Frame Structure
  • Destination address 6 bytes (48 bits)
  • Highest order bit 0 individual, 1 multicast
    all 1s broadcast
  • Frames received with non-matching destination
    address is discarded
  • Type type of network layer protocol
  • Pad used to produce valid frame gt 64 bytes
  • Checksum 32-bit cyclic redundancy check

6
Minimum Frame Length
  • To ensure the sender can detect collision
  • All frames must take more than 2t to send so that
    transmission is still taking place when the noise
    burst gets back to the sender

7
Ethernet MAC Sublayer Protocol
  • Uses 1-persistent CSMA/CD
  • Binary exponential backoff
  • Provides unreliable connectionless service

8
Randomization and Binary Exponential Backoff
  • Time divided into slots
  • Length of slot 2t worst-case round-trip
    propagation time
  • To accommodate longest path, slot time 512 bit
    times 51.2 msec (10Mbps Ethernet)
  • Binary exponential backoff

9
Randomization and Binary Exponential Backoff
  • After 1st collision, station picks 0 or 1 at
    random, waits that number of slots and tries
    again
  • After 2nd collision, station picks 0,1,2,3 at
    random, waits that number of slots and tries
    again
  • .
  • After i-th collision, station picks 0,1,,2i-1 at
    random,
  • If 10 lt i lt 16, station picks 0,1,,210-1 at
    random
  • If i16, controller reports failure to computer

10
Ethernet Performance
  • Binary exponential backoff results in
  • Low delay when few stations collide
  • Reasonable delay for collision resolution when
    many stations collide
  • When other factors are fixed, channel efficiency
    decreases when
  • Network bandwidth increases
  • Cable length increases
  • Number of stations increases
  • Frame length decreases

11
Ethernet Performance
  • Efficiency of Ethernet at 10 Mbps with 512-bit
    slot times

12
Switched Ethernet
  • Switch contains a high-speed backplane and room
    for typically 4 to 32 plug-in line cards, each
    containing 1-8 connectors
  • Possibly each card forms its own collision
    domain, or
  • Full-duplex operation if each input port is
    buffered

13
ARP Address Resolution Protocol
  • How do IP addresses get mapped onto data link
    layer addresses, such as Ethernet?

14
ARP Optimization
  • ARP result is cached (step 5 in figure)
  • When A wants to communicate with B, A includes
    its IP-to-Ethernet mapping in the ARP packet so
    that B knows the mapping right away (step 3 in
    figure)
  • Have every machine broadcast its mapping when it
    boots, so that everyone else knows the mapping
  • To accommodate changes, entries in the ARP cache
    time out after a few minutes

15
ARP How to Handle Remote Traffic
  • Proxy ARP A router is configured to answer ARP
    requests on one of its networks for a host on
    another network

16
ARP Exercise
  • Node 1 wants to send a packet to node 3, what
    will be returned by ARP?
  • Node 1 wants to send a packet to node 2, what
    will be returned by ARP?

17
RARP Reverse Address Resolution Protocol
  • RARP - Allows a newly-booted diskless-workstation
    (e.g., X terminal) to broadcast its Ethernet
    address and ask for its IP address
  • RARP server responds to a RARP request with the
    assigned IP address

18
Limitations of RARP
  • RARP uses a link-layer broadcast, RARP requests
    are not forwarded by routers, therefore, an RARP
    server must be present on every network
  • The only thing returned by the RARP server is the
    IP address

19
BOOTP Bootstrap Protocol
  • BOOTP uses UDP
  • A client broadcasts to 255.255.255.255
  • The source IP address is set to 0.0.0.0 if client
    does not know its own IP address yet
  • Port number 67 for server, 68 for client
  • BOOTP drawbacks
  • Requires manual configuration of tables mapping
    IP address to Ethernet address at the BOOTP
    server
  • Replaced by DHCP

20
Dynamic Host Configuration Protocol
  • Allow host to dynamically obtain its IP address
    from network server when it joins network
  • IP address assignment is lease-based (to cope
    with client failure, also enables reuse of
    addresses)
  • Can renew its lease on address in use
  • DHCP overview
  • Host broadcasts DHCP discover msg
  • DHCP server responds with DHCP offer msg
  • Host requests IP address DHCP request msg
  • DHCP server sends address DHCP ack msg

21
DHCP Client-Server Scenario
223.1.2.1
DHCP

223.1.1.1
server

223.1.1.2
223.1.2.9
223.1.1.4
223.1.2.2
arriving DHCP client needs address in
this network
223.1.1.3
223.1.3.27

223.1.3.2
223.1.3.1

22
DHCP Client-Server Scenario
arriving client
DHCP server 223.1.2.5
DHCP offer
src 223.1.2.5, 67 dest 255.255.255.255,
68 yiaddr 223.1.2.4 transaction ID
654 Lifetime 3600 secs
DHCP request
src 0.0.0.0, 68 dest 255.255.255.255,
67 yiaddr 223.1.2.4 transaction ID
655 Lifetime 3600 secs
time
DHCP ACK
src 223.1.2.5, 67 dest 255.255.255.255,
68 yiaddr 223.1.2.4 transaction ID
655 Lifetime 3600 secs
23
DHCP Replay
  • A DHCP relay agent is can be configured on each
    LAN
  • The agent stores the IP address of the DHCP
    server and forward the request to the server

24
DHCP with Replay Agent
  • To find its IP address, a newly-booted machine
    broadcasts a DHCP Discover packet
  • The DHCP relay agent on its LAN receives all DHCP
    broadcasts
  • On receiving a DHCP Discover packet, the agent
    sends the packet as a unicast packet to the DHCP
    server, possibly on a distant network

25
Exercise
  • An IP packet to be transmitted by Ethernet is 60
    bytes long, including all its headers. Is padding
    needed in the Ethernet frame, and if so, how many
    bytes?

26
Exercise
  • Consider building a CSMA/CD network running at 1
    Gbps over a 1-km cable. The signal speed in the
    cable is 200,000 km/sec. What is the minimum
    frame size?

27
Exercise
  • A switch designed for use with fast Ethernet has
    a backplane that can move 10 Gbps. How many
    frames/sec can it handle? Assume there is an
    endless stream of 64-byte (512-bit) frames.

28
Exercise
  • How long does a station, s, have to wait in the
    worst case before it can start transmitting its
    frame over a LAN that uses the basic bit-map
    protocol? (Assume N stations, each frame is d
    bits)

29
Exercise
  • Six stations, A through F, communicate using the
    MACA protocol. Is it possible that two
    transmissions take place simultaneously?
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