computer networksnemo/cen5501/slides/chapter2.pdf · 2007-01-23 · cen 5501c - computer networks -...

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

Computer Networks

Chapter 2 – Data Link Layer Issues

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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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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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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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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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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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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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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 OctetG/I bit (group/individual)G/L bit

(global/local)

OUI3rd Octet 5th Octet 6th Octet0

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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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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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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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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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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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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

DetectCollisionEnd Tx

End Tx

Start Rx

End Rx

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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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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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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 flagchecksum

HDLC format

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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 flagchecksumprotocol

PPP format

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Link Reliability IssuesEnd-to-end Success Rate

0

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

(Suc

cess

) P=0.001P=0.005P=0.01P=0.05P=0.1P=0.5

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Link Reliability IssuesExpected Cost per Attempt

0

5

10

15

20

25

1 3 5 7 9 11 13 15 17 19

Path Length (hops)

E(C

ost/a

ttem

pt) P=0.001

P=0.005P=0.01P=0.05P=0.1P=0.5

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Link Reliability IssuesExpected Number of Attempts/Success

0123456789

1 3 5 7 9 11 13 15 17 19

Path Length (hops)

E(A

ttem

pts/

succ

ess)

P=0.001P=0.005P=0.01P=0.05P=0.1

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Link Reliability IssuesExpected Cost per Success

01020304050607080

1 3 5 7 9 11 13 15 17 19

Path Length (hops)

E(C

ost/s

ucce

ss) P=0.001

P=0.005P=0.01P=0.05P=0.01

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Per Hop vs. End-to-End BECPkt Sent Pkt Sent

Pkt Recv

Pkt Recv

ACK Recv

ACK Recv

Per HopEnd-to-End