topics in networking1 data link layer services and protocols arzad a. kherani (alam@cse.iitd.ac.in)...

Topics in Networking 1

Data Link Layer Services and Data Link Layer Services and ProtocolsProtocols

Arzad A. Kherani(alam@cse.iitd.ac.in)

Dept. of Computer Sc. And Engg.

Indian Institute of Technology Delhi

OutlineOutline

Frame encoding Error detection and recovery Data Link Protocols Protocol analysis

– Performance– Verification for correctness

Data link servicesData link services Operates between two neighboring devices Provides a capability for higher-layer entities to send “packets”

– A packet is a sequence of bits, with well-identified “start” and “end” The packet is itself encapsulated into a “frame”, adding to it

“header” and “trailer”

Data link services (2)Data link services (2)

Network layer

Data link layer

Physical layer Data link protocol

Service boundary

Data link services (3)Data link services (3)

There is one data link for each physical link In a router, for instance:

Data link servicesData link services

Connection-less service– Un-acknowledged service

Useful in case of low errors, and for real-time applications

– Acknowledged service Used in wire-less networks Frames that are not acked may be re-sent

Connection-oriented service– acknowledged

Ensures delivery of frames

FramingFraming

Link layer packet = frame Problem: how to recognize beginning

and end of frame? Three methods

– byte counting (DDCMP)– bit stuffing (X.25 Level 2, 802.x)– byte stuffing (BISYNCH, IMP-IMP)

FramingFraming

Solution based on counting bits/characters

FramingFraming

Solution based on flags (01111110) and bit stuffing

Original bit string

Bit string, suitably bit-stuffed

Received, interpreted bit string

Bit Stuffing (2)Bit Stuffing (2)

Frame beginning and end marked by special

bit string ( 01111110) If 5 1's in data to be sent, sender inserts 0 If receiver sees 5 1's check next bit(s)

– if 0, remove it (stuffed bit)– if 10, end of frame marker (01111110)– if 11, error (7 1's cannot be in data)

FramingFraming Solution based on flag characters, byte stuffing

Special characters used for control

Byte Stuffing ProblemsByte Stuffing Problems

Dependence on fixed character set

Must examine every byte of data on sending

and receiving (insert / remove DLE)

Was used widely in IBM bisynch (at 9600bps)

Error detection and recoveryError detection and recovery

Two approaches:– error correction codes

quick, but ineffective in several cases:– temporary dislocation

– frame size is large, and error rate is high

– burst errors expensive

– error detection and recovery using re-transmission the preferred solution today

– efficient

Error detectionError detection

Hamming distance between pair of codeslet message of length m 2m distinct messages

with r redundant bits, codeword is of length n = m + r

hamming distance between codes x, y

= no. of bits in which x and y differ

Hamming distance for a codeconsider n dimension space, with 2m codewords

hamming distance for code (or coding scheme)

= minx, y (no. of bits in which x and y differ)

Hamming codesHamming codes

Hamming distance for code based on parity bit is 2 resulting capability: detect 1 error, correct 0 errors Result:

– to detect d errors, the Hamming distance must be d+1– to correct d errors, the Hamming distance must be at least

Hamming codes (2)Hamming codes (2) Consider 7 bit data, 4 redundancy bits, codeword is 11 bits message bits are numbered 3, 5, 6, 7, 9, 10, 11 redundancy bits are numbered 1, 2, 4, 8

check bit 1 checks error in bits 1, 3, 5, 7, 9, 11

check bit 2 checks error in bits 2, 3, 6, 7, 10, 11

check bit 4 checks error in bits 4, 5, 6, 7

check bit 8 checks error in bits 8, 9, 10, 11

or (it should be)

0 = b1 + b3 + b5 + b7 + b9 + b11

0 = b2 + b3 + b6 + b7 + b10 + b11

0 = b4 + b5 + b6 + b7

0 = b8 + b9 + b10 + b11

Error detection using block codesError detection using block codes Block of data is re-written as a matrix, say 4 x 8 last row is parity bits bits are transmitted row-by-row, including parity bits code is capable of detecting a burst of errors of length n

column --> 1 2 3 4 5 6 7 8

row 1 1 0 1 1 0 0 1 1

row 2 0 0 1 0 0 0 1 0

row 3 1 1 1 0 0 1 1 0

row 4 1 1 0 0 1 1 0 0

parity bits 1 0 1 1 1 0 1 1

Polynomial codesPolynomial codes Also known as Cyclic Redundancy Codes (CRC) Note, all arithmetic is modulo-2

x XrM(x) Xr M(x) /G(x) R(x)- T(x) = Xr M(x)-R(x)

R(x) = T(x) + E(x)

/G(x)=Yes no error

No error

Polynomial codes (2)Polynomial codes (2) Error detection capability depends upon G(x) 1 bit error R(x) = T(x) + xi

R(x)/G(x) = 0 iff E(x)/G(x) = 0

E(x)/G(x) 0 if G(x) has 2 or more terms

single errors can always be detected Similarly, two errors can always be detected if G(x) does not

divide xk + 1 Again, if G(x) is divisible by x+1 then an odd number of errors

can be detected Polynomial codes with r CRC bits will detect all bursts of length r

or less etc., etc.

Polynomial codes (3)Polynomial codes (3)

International, IEEE standards IEEE 802 standard

G(x) = x32 + x26 + x23 + x22 + x16 + x12 + x11 + x10 + x8 + x7 + x5 + x4 + x2 + x1 + 1

it is capable of detecting bursts of length up to 32, and all odd number of errors, and even no of error with high probability

Error recoveryError recovery

Error detection, followed by re-transmissions, etc.

Efficient Simultaneously address problem of “flow

control”

Elementary data link protocolsElementary data link protocols

Broad objective of data link protocol:– Error-free, loss-free, duplication-free and in-sequence

transfer of user data packets between network entities– flow-controlled– transfer user data packets in both directions

Network entity

Data link entity

Physical layer

Network entity

Data link entity

p1, p2, p3, .. p1, p2, p3, ..

Data link modulesData link modules

Network entity Network entity

Data link entity

sender

receiver sender

receiverData link

entity

Physical layer entity

Full-duplex channel

Data link modulesData link modules We consider several protocols. But to begin with we consider

moving data in one direction, only

Data link entity

sender receiverData link

entity

Physical layer entity

HALF-duplex channel

Data link protocolData link protocol

Assumptions:– errors during transmission– processing capacity at receiver end– buffer capacity at receiver end– whether the underlying physical channel is half- or

full-duplex

we will make nice assumptions to begin with, but move towards realistic assumptions later

Simple data link protocol: UtopiaSimple data link protocol: Utopia Assume no errors, infinite processing capacity and buffer space

at receiver end, and half-duplex channel

Note, F_i: Data link frame, containing data packet, i

Sender, A Receiver, B

Simple data link protocol: Utopia (2)Simple data link protocol: Utopia (2) The sender’s end

Simple data link protocol: Utopia (3)Simple data link protocol: Utopia (3) The receiver’s end

Definitions of data types/structuresDefinitions of data types/structures

Definition of proceduresDefinition of procedures

Simple data link protocol: Utopia (4)Simple data link protocol: Utopia (4)Network entity

Physical layer

Network entity

Flow control: problem and its solutionFlow control: problem and its solution

Flow control --> limit the rate at which a sender can send data to one which is consistent with the receiver’s ability to process incoming data

Two approaches to solving it:– rate-based: determine the minimum rate at which receiver can

process incoming data

– feedback based: send more data as when receiver can handle more data

Rate rcvr can handle

Acceptable rate

Stop and wait protocolStop and wait protocol

? Assume no errors, FINITE processing capacity, FINITE buffer space at receiver end, and half-duplex channel

Note, F_i: Data link frame, containing data packet, i

Stop-n-wait protocol (2)Stop-n-wait protocol (2)

Physical layer

PAR protocol for noisy channelsPAR protocol for noisy channels

PAR protocol addresses:– flow control– noisy channel

based on “positive acks” and re-transmissions

PAR protocolPAR protocol

Timer runs out

Data Frames are suitably numbered 0, 1, 0, 1, … Acks are not numbered

PAR protocol (sender)PAR protocol (sender)

PAR protocol (receiver)PAR protocol (receiver)

Physical layer

PAR protocolPAR protocol If the underlying physical layer is full-duplex, then the protocol fails

Timer runs out

Alternating bit protocolAlternating bit protocol PAR protocol, with numbered acks

Timer runs out

Alternating bit protocolAlternating bit protocol

Alternating bit protocolAlternating bit protocol Two possibilities:

Alternating bit protocolAlternating bit protocol Use link A B to carry data from A to B, and acks from B to A

and use link B A to carry data from B to A, and acks from A to B

Piggyback acks onto data frames, if one has data to send Else, just an ack Redundant acks are OK An ack takes the form “I am waiting to receive data frame no. X” Introduce a field for frame type, “data frame” or “ack frame”

Performance of PAR protocolPerformance of PAR protocol Link utilization

Utilization = L / (L+b*R), where

L = size of data frame

b = data rate

R is round-trip delay For a satellite channel, let L = 10K bits, b = 100 Kbps, R=500ms

Utilization is 10K / (10K + 50K) = 1/6 For a fibre-optic channel, let L = 10K bits, b = 100 Mbps, R = 1ms

Utilization is 10K / (10K + 100K) = 1/11

Delay=BW product is the key Let sender send many data frames before it receives an ack

PipeliningPipelining

Problems arise when one or more packets are lost

Pipelining, with error recoveryPipelining, with error recovery Two approaches to recover from loss of data frame:

– go-back n– selective repeat

Pipelining, with error recoveryPipelining, with error recovery Go-back n scheme:

Pipelining, with error recoveryPipelining, with error recovery Selective repeat

Sliding window protocolsSliding window protocols

Each data frame is sequentially numbered 0 through 2n-1

Sender maintains “transmit window” indicating:– which data frames can be sent– which data frames have been sent

receiver maintains a “Receive window”– which data frames can it receive– which data frames have been received

Sliding window protocolsSliding window protocols Transmit window:

– size = 4, waiting for acks for data frames 1, 2, 3, frame 4 not sent

Receive window:– size = 3, ack for frame 1 sent, data frame 3 received, waiting for

frames 2 and 4

0 1 2 4 53 76

Sliding window protocolsSliding window protocols

Assuming sequence numbering 0 .. 7– Go back n protocol:

Transmit window is size, say 7 receive window is size 1

– selective repeat protocol: transmit window is size 4 receive window is size 4

Go back n protocol: declarationsGo back n protocol: declarations

Go back n protocol: more declrationsGo back n protocol: more declrations

Go back n protocol: initializationsGo back n protocol: initializations

Go back n protocol (loop)Go back n protocol (loop)

Go back n protocol: event processingGo back n protocol: event processing

Go back n protocolGo back n protocol

Buffer requirements– at sender’s end: N-1, where seq no is 0 .. N-1– at receiver’s end: 1

Go-back n works well when error are infrequent– Because several frames need to be re-transmitted

when an error occurs

Select Repeat ProtocolSelect Repeat Protocol

Sequence numbering: 0 through n-1 Transmit window size is n/2 Receive window size is n/2

Receiver buffers each correctly recd data frame, but does not deliver it to the layer 3

Receiver sends a NACK frame when it suspects loss of a data frame– But makes sure that a NACK is not repeated

Protocol AnalysisProtocol Analysis

Performance Verification

Protocol PerformanceProtocol Performance

Performance– Delay– Efficient use of available bandwidth

Channel utilizationChannel utilization It can be shown for “stop-and-wait” protocol:

U = F1 * F2 * F3where

F1 = D/ (H+D)F2 = (1-E)(H+D) * (1-E)D F3 = (H+D) / (H+D+CT)

Above,D, H are resp. length of data and header (or Ack)E is bit-error-rate (BER)C is channel capacityT is timer interval

Channel utilizationChannel utilization

Channel utilization in “stop-and-wait” protocol is maximum when

Doptimum = (H+CT)/E

Channel utilization in “sliding-window” protocols is complex. Consider:– No error, large Tx window– No error, small Tx window– With possibility of errors, large Tx window– With possibility of errors, small Tx window

What is a large enough window size?

W > 1 + 2 C I / (D+H)where I = propagation delay + interrupt handling time

Channel utilization in “sliding-window” when there is no error, large Tx window

U = D / (D+H)

Channel utilization in “sliding-window” when there is no error, but small Tx window

U = D/(D+H) * W/(1 + 2 C I / (D+H))

Channel utilization in “sliding-window” when there may be errors, but Tx window is small U = D/(D+H) * (1-L)

where L is the probability that a frame or an ack to it is lost

Channel utilization in “sliding-window” when there may be errors, but Tx window is small U = D/(D+H) * (1-L) * W/(1 + 2 C I / (D+H))

C*I is also the “delay-BW product”, And CI/F is the delay-BW product in terms of

no of frames

Window size

Channel util. Increasing delay-BW

Protocol VerificationProtocol Verification

Verification– Formal specification– Verification of protocol design– Conformance of an implementation to a given

design

Two parts:– protocol modeling– verification

Look upon the communicating entities as ONE finite state machine

Use two different, although equivalent, ways to model the machines:– State transition diagrams– Petrinet based model

Protocol verification: model using state Protocol verification: model using state transitionstransitions

Consider an example: stop-n-wait protocol– Consider the sender-receiver pair, together with

channels as ONE single system– Focus on significant states, not intermediate states

encountered while executing commands or program instructions

Protocol Verification: model using Protocol Verification: model using state transitionsstate transitions

Stop-n-wait protocol components, and their states– Sender: “sending” 0 or 1– Receiver: receiving 0 or 1– Half-duplex channel: carrying frame 0 or 1, or Ack, or “-”– Acks are not numbered

Total no of states is 16, of which 6 are unreachable– These are states that correspond to sender and receiver waiting for

next event to occur Further,

– Consider initial state– Any enabled transition may take place next– Transitions take place at any time

Protocol Verification: model using Protocol Verification: model using state transitionsstate transitions

Can a protocol drop a user packet?– Protocol never delivers packets in two data

frames, numbered 0, without delivering packet contained in a data frame numbered 1

– Or, machine does not make two transitions numbered “1”, without an intervening transition “3”

– There does not a path with two edges corresponding to “transition 1” without an intervening edge “transition 3”

Can a protocol drop a sequence of two user packets?– Or, sender should not state 1 twice while receiver

does not change its state in between

Protocol modeling using PetrinetsProtocol modeling using Petrinets

A Petrinet, with– places: set of possible states– tokens: define the current state– transitions, input and output arcs– firing of transitions:

conditions, and resulting re-distribution of tokens

eating

sleeping

wake-upburp

eating

sleeping

wake-upburp

Producer/consumer modelProducer/consumer model

producing

waiting1

consuming

waiting2

Model of a producer- consumer system with one buffer, using Petrinet

Stop-n-wait Stop-n-wait protocol modelprotocol model

Petrinet is formally described by its transitions:1: BD --> AC

2: A --> A

3: AD --> BE

4: B --> B

5: C -->

6: D -->

7: E -->

8: CF --> DF

9: EG --> DG

10: CG --> DF

11: EF --> DG

Using the formal specs one can formally argue about properties that are (are not) satisfied:

– e.g. consider all possible sequences of transitions. Then are there two “10” transitions without an intervening transition “11”?

– How would ensure that two consecutive packets are not lost

Example Data Link ProtocolsExample Data Link Protocols

HDLC (bit-oriented, high-level data link control)– Connection establishment, release, data transfer, and even connection reset

Example Data Link ProtocolsExample Data Link Protocols PPP (point-to-point protocol)

– Mainly used router-to-router and dial-up connections– Connection establishment and release a data link, data transfer, and even connection reset– Help establish network layer (IP) addresses

Over LANs, the protocol is typically Ethernet

ThanksThanks

topics in networking1 data link layer services and protocols arzad a. kherani (alam@cse.iitd.ac.in)...

data topics

redundancy bits

sequence of bits

bitsmessage bits

data link layer services

byte of data

codewordshamming distance

y differhamming distance

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