Computer Networks Chapter 2 Data Link Layer Issues CEN 5501C - - - PowerPoint PPT Presentation

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Computer Networks Chapter 2 Data Link Layer Issues CEN 5501C - - - PowerPoint PPT Presentation

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Computer Networks Chapter 2 Data Link Layer Issues CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman 1 LAN Properties Shared medium High data rate Low delay Low error rate Native broadcast support

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CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman 1

Computer Networks

Chapter 2 – Data Link Layer Issues

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CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman 2

LAN Properties

  • Shared medium
  • High data rate
  • Low delay
  • Low error rate
  • Native broadcast support
  • Limited physical extent (a few kms)
  • Limited number of stations (100’s)
  • STAs are peers
  • Local management (not under PTT regulation)
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CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman 3

Medium Access

  • Access allocation so that

– One STA at a time accesses medium – Each STA gets a fair share – Delays are reasonable – Overhead and waste are minimized

  • Approaches

– Tokens – Contention

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CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman 4

Token-based Approaches

  • Token Ring

– STAs linked (simplex) to two neighbors – Token circulates physical ring – Add STA by insertion into ring

  • Token Bus

– STAs attached to bus – Token circulates logical ring – Add STA to bus and insert into logical ring

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CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman 5

Contention Approaches

  • ALOHA

– Can’t sense medium, so just talk!

  • CSMA

– Listen before talk

  • CSMA/CD

– Add collision detection (need sensitive PCS)

  • CSMA/CA

– Use collision avoidance (when VCS used)

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CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman 6

IEEE 802

  • 802.1 – common issues

– Addressing, management, bridging, security

  • 802.2 – LLC

– Type 1 – best effort / Type 2 - reliable

  • 802.3 – CSMA/CD LAN

– From Xerox Ethernet

  • 802.4 – Token Bus
  • 802.5 – Token Ring
  • 802.11 – Wireless LAN
  • 802.16 – Wimax
  • Note – FDDI standardized by ANSI
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CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman 7

Names, Addresses, Routes

  • Name – what

– Location-independent identifier – May be human-friendly or not

  • Address – where

– Independent of source location, but will change if destination moves

  • Route – how to get there

– Depends on both source and destination

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CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman 8

LAN Addresses

  • LANs are broadcast medium – need both

– Source address (for return messages) – Destination address (to filter)

  • IEEE 802 addresses

– 16 and 48 bits (also 60 for 802.6 DQDB)

  • 48 bit addresses managed by IEEE

– Pay to get 224 address block, Vendor Code (OUI) – G/L bit is 0 if global, 1 if locally managed

2nd Octet 4th Octet G/I bit (group/individual) G/L bit (global/local) OUI 3rd Octet 5th Octet 6th Octet

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CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman 9

Multicast Addresses

  • Service Discovery

– Solicitation (client mcast to Z-Servers address) – Advertisement (Z-Servers mcast to Z-Client addr)

  • Why Multicast (group) addresses?

– Reduce interrupt handling by hosts – Hardware filter

  • Why G/I bit?

– Allow filtering by hash buckets in HW – SW filters all hits in relevant hash buckets

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Broadcast

  • The all 1’s address (0xffffffffffff) is bcast
  • Means that all STAs must receive
  • Really, though, only those implementing

the protocol used in the broadcast packet have to…

  • Broadcast address interrupts everyone

anyway

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CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman 11

Multiplexing Field

  • Multiple higher layer protocols
  • Format and data alone cannot distinguish
  • Multiplexing field selects which one

– Protocol type in Ethernet – DSAP and SSAP in IEEE 802

  • Service Access Point (SAP) Structure

– G/L and G/I bits also, hence 6 bits – All 1’s = all SAPs (!!!) – Others assigned by IEEE – too few!

  • SNAP (Subnetwork Access Protocol)

– When DSAP = SSAP = 0xaa – extra protocol type field (5 octets) – 3 OUI octets, 3 vendor-assigned octets

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CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman 12

Bit Order

  • Bit order is order in which bits are put onto

the medium

  • Not standard ()

– LSB first canonical and for most LANs – MSB first for 802.5, FDDI

  • Bridges must convert

– Shuffle bit order within octets

  • Impact on ARP and higher layer protocols
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CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman 13

LLC

  • 802.2 LLC has two significant types

– Type 1 – datagrams (best effort) – Type 2 – reliable (connection oriented)

  • Type 1 Control – 1 octet

– UI – unnumbered info (datagram) – XID – Exchange ID (command/response)

  • ID of transmitter
  • LLC types supported

– Test – (Cmd/Rsp) – Rsp echo data in Cmd

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CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman 14

LLC Type 2

  • Control is 1 or 2 octets, per type
  • 2-octet control fields contain 7-bit seq #s

– I = Information (data) – SN plus ACK SN – RR = Receive Ready (ACK) – ACK SN – RNR = Rcv Not Ready (Busy) – ACK SN – REJ = Reject – ACK SN

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CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman 15

LLC Type 2

  • Control is 1 or 2 octets, per type
  • 1-octet control field types

– SABME = Set Asynchronous Balanced Mode Extended (start connection) – DISC = Disconnect (end connection) – DM = Disconnected Mode (confirm DISC) – FRMR = Frame Reject (receipt of invalid pkt) – UA = Unnumbered ACK (for DISC/SABME)

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802.3 Issues

  • CSMA/CD

– Listen before talk – Detect collisions – Binary exponential backoff

  • Minimum frame length

– So all STAs detect collision – Slot time = 2τ (512 bits at 2.5km and 10Mbps)

Start Tx Start Tx Detect Collision End Tx End Tx Start Rx End Rx

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CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman 17

802.5 Issues

  • Each packet traverses every station in

physical ring

  • Each STA has transceiver buffer with

special HW to recognize token, addresses

  • Two bits at end of each frame for ACK:

– A bit (address recognized) – C bit (frame copied)

  • Each STA may modify bits
  • Sender sees A/C bits when frame returns
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CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman 18

802.5 Issues

  • A/C Bits on return:

– A=0: Address not recognized (give up) – A=1, C=0: Address recognized, but STA busy (try again) – A=1, C=1: Address recognized and frame copied (success!)

  • What does a bridge do with these?

– Clear both? – Leave unmodified – Set A and C if bridge forwards – Clear A and set C if bridge forwards

  • A/C used for other purposes:

– Ring order (bcast frame with A bit clear indicates predecessor)

  • Only 31 functional addresses for multicast

– Mapping – Oversubscription

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Packet Bursts

  • Station processing rate may be slower

than the LAN data rate

  • While OK on average, packets may be

sent in a burst

  • Early packets received, later ones lost
  • Problem if naïve protocol retransmits

whole burst every time

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Why Bridges?

  • Limit number of stations in LAN

– Packet lengths – Delay

  • Size limitations

– 802.3 collision detection

  • Traffic

– Capacity is shared

  • Simple, high performance, allow limited

location transparency (keep IP address)

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CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman 21

Point-to-Point Links

  • Flag

– Special pattern to delimit frames

  • In HDLC, 01111110
  • In DDCMP, DLE-SOF & DLE-EOF

– Bit-stuffing/character-stuffing for data transparency

  • In HDLC, 011111… -> 0111110… on Transmit
  • In DDCMP, … DLE … -> … DLE DLE … on Transmit
  • Addressing

– Needed if multiple stations on medium – Traditionally assume master/slave

  • Control – Like LLC Type 2
  • Checksum – 16 bit CRC

flag address control data flag checksum HDLC format

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CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman 22

Point-to-Point Links

  • Multiplexing

– Protocol field in PPP (16 bits – see RFC 1700)

  • Service

– If links not reliable, need reliable transfer per hop (HDLC, DDCMP, LLC Type 1)

  • What is probability of success for k hops with FER P?
  • What is cost per attempt for k hops with FER P?
  • What is overall cost for success for k hops with FER P?

– If links reliable, then datagram service OK (PPP, LLC Type 2)

flag Addr=0xff Ctl=0x03 data flag checksum protocol PPP format

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CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman 23

Link Reliability Issues

End-to-end Success Rate

0.2 0.4 0.6 0.8 1 1.2 1 3 5 7 9 11 13 15 17 19 Path Length (Hops) Prob(Success) P=0.001 P=0.005 P=0.01 P=0.05 P=0.1 P=0.5

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Link Reliability Issues

Expected Cost per Attempt

5 10 15 20 25 1 3 5 7 9 11 13 15 17 19 Path Length (hops) E(Cost/attempt) P=0.001 P=0.005 P=0.01 P=0.05 P=0.1 P=0.5

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Link Reliability Issues

Expected Number of Attempts/Success

1 2 3 4 5 6 7 8 9 1 3 5 7 9 11 13 15 17 19 Path Length (hops) E(Attempts/success) P=0.001 P=0.005 P=0.01 P=0.05 P=0.1

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Link Reliability Issues

Expected Cost per Success

10 20 30 40 50 60 70 80 1 3 5 7 9 11 13 15 17 19 Path Length (hops) E(Cost/success) P=0.001 P=0.005 P=0.01 P=0.05 P=0.01

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Per Hop vs. End-to-End BEC

Pkt Sent Pkt Sent Pkt Recv Pkt Recv ACK Recv ACK Recv Per Hop End-to-End