arq protocol

8/7/2019 arq protocol

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Flow control in the transport layerFlow control in the transport layer

refers to a set of procedures used torefers to a set of procedures used to

restrict the amount of data that therestrict the amount of data that thesender can send before waiting forsender can send before waiting for

acknowledgment.acknowledgment.

Error control in the data link layer isError control in the data link layer is

based on automatic repeat request,based on automatic repeat request,

which is the retransmission of data.which is the retransmission of data.

Flow Control & Error ControlFlow Control & Error Control


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Automatic RepeatAutomatic Repeat reQuestreQuest (ARQ) Protocols(ARQ) Protocols

Purpose: to ensure a sequence of

information packets is delivered in order

and without errors or duplicationsdespite transmission errors & losses

We will look at:

Stop-and-Wait ARQ

Go-Back N ARQ

Selective Repeat ARQ


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StopStop--andand--Wait ARQWait ARQ

Stop after Transmitting a Frame

Wait for an Acknowledgement

Transmitter Receiver

Packet

H

CRCH

Error FreePacket

ACK

Information Frame

CRC

Transmitter Receiver

Packet

H

CRCH

Error FreePacket

ACK

Information Frame

CRC

H : HeaderCRC : Cyclic Redundancy Check

(Error Detection)


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Importance ofSequence NumbersImportance ofSequence Numbers

y In Scenario B, the receiver accepts Frame 1 Twice

y How could the Receiver Know whether or Not it has Received Frame 1Correctly?

BY INCLUDING THE FRAME SEQUENCE NUMBER IN THE

HEADER

Sender Receiver

ACK

Timeout

Frame 0

Frame 1

Frame 1

ACK

Frame 2

Sender Receiver

ACK

Timeout

Frame 0

Frame 1

Frame 1

ACK

Frame 2

ACK

A B


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Sequence Numbers in ACK FramesSequence Numbers in ACK Frames

The sender may mistakenly

interpret ACK as it

assumes that the second

ACK was for Frame 1

How Could the Sender

Know which Frame does

an ACK correspond to?

BY INCLUDING THE

SEQUENCE NUMBER IN

THE ACK

Sender Receiver

ACK

Frame 0

Frame 0

Frame 1

ACK

Frame 2

ACK

C

Timeout


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StopStop--andand--Wait ARQ PerformanceWait ARQ Performance

y The Channel Idle Time (Wait for ACK) is anInefficiency because the channel is not used

Machine A Machine B

PhysicalChannel

First Frame-Bitenters Channels

Last Frame-Bitenters Channels

First Frame-Bitarrives at B

Last Frame-Bitarrives at B

LastACK

-BitArrives atA

ProcessingTime

Channelis Idle


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StopStop--andand--Wait ARQ EfficiencyWait ARQ EfficiencyMachine A Machine B

PhysicalChannel

t0=TotalTime toTransmit1 Frame

tprop

tf=Lf/R

tproc

ta=La/R

tprop

tproc

t0=2tprop+2tproc+tf+tat0=2t

prop+2t

proc+L

f/R+L

a/R

tprop Propagation Timetproc Processing Timetf Frame TransmissionTimeta ACKTransmissionTimeLf No. ofBits/Frame

La No. ofBits/ACK


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0

No. of Inormation Bits per Frame

Time Required to Deliver One Frame

f h

eff

L LR

t

! !

ErrorError--Free ChannelFree Channel

N o E r ro r

0

1

2

1

! ! !

h

e ff f h f

p r o c p r o pa

f f

t t t

Effective Transmission RateHeaders and

CRC

Transmission EfficiencyEffect ofHeader

Effect ofACK

Effect of DelayBandwidth Product

fficiency Assuming RROR FR CHANNN LNE

N


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GoGo--BackBack--N BasicsN Basics

Transmit without waiting for ACK

Each frame received correctly is

acknowledged with a sequence numbercorresponding to the NEXT frameexpected

The receiver discards all out of sequence

packetsIf number of outstanding frames isW

S:

Go-Back-N


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What doesWWhat doesWSS

Represent?Represent?

yWS

is called the sending window size

y Each frame must be buffered (stored)

until a valid ACK arrives (in caseretransmission is needed)

yWS

represents the maximum number

of frames that may be outstanding (i.e.,stored) simultaneously


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GoGo--BackBack--N ARQN ARQ

ProtocolProtocol

Anoutstandingframe:

It is a frame that hasbeen

transmitted and is yet tobe acknowledged

Sender Receiver

Fr_0

Fr_1

Fr_2

Fr_3

Fr_4

Fr_5

Fr_6

ACK_1

ACK_2

ACK_3

Fr_3

Fr_4

Fr_5

Fr_6

Fr_7

Fr_8

ACK_4

ACK_5

ACK_6

ACK_7

ACK_8

At this Pointthere areWS=4

OutstandingFrames. So,GO-BACK-4

Out of

SequenceFrames. So,do not accept

them


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TimeTime--out Driven Goout Driven Go--BackBack--NN

y For Go-Back-N withWs= 4

What happens if thesender only has 2frames to send

x No retransmission istriggered because thewindow is not full

y Time-out Driven Go-Back N

Timer per

outstand

ing

frame

If the window is full nofurther transmissionsare allowed

Sender Receiver

Fr_0

Fr_1

ACK_1

Fr_0

Fr_1

ACK_2

TimeoutOut of

sequence,ignored


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SlidingWindow ConceptSlidingWindow Concepty Go Back N is an example of what is called a sliding window protocol

y An ACK received means that all previous frames have been receivedcorrectly (previous ACKs were lost or receiver chose not to sendthem)

Sender Receiver

Fr_0

Fr_1

Fr_2

Fr_3

Fr_4

Fr_5

Fr_6

ACK_1

ACK_2

ACK_3

Sending Window

Frame Sequences thatare allowed to be sent

Outstanding FrameSequences

Fr_0

Fr_1

Fr_2

Fr_3


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Sender Receiver

Fr_0

Fr_1

Fr_2

Fr_3

Fr_4

Fr_5

Fr_6

ACK_1

ACK_2

ACK_3

Sending Window



Fr_0

Fr_1

Fr_2

Fr_3


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Sender Receiver

Fr_0

Fr_1

Fr_2

Fr_3

Fr_4

Fr_5

Fr_6

ACK_1

ACK_2

ACK_3

Sending Window



Fr_0

Fr_1

Fr_2

Fr_3


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Sender Receiver

Fr_0

Fr_1

Fr_2

Fr_3

Fr_4

Fr_5

Fr_6

ACK_1

ACK_2

ACK_3

Sending Window



Fr_0

Fr_1

Fr_2

Fr_3


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Sender Receiver

Fr_0

Fr_1

Fr_2

Fr_3

Fr_4

Fr_5

Fr_6

ACK_1

ACK_2

ACK_3

Sending Window



Fr_1

Fr_2

Fr_3

Fr_4


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Sender Receiver

Fr_0

Fr_1

Fr_2

Fr_3

Fr_4

Fr_5

Fr_6

ACK_1

ACK_2

ACK_3

Sending Window



Fr_2

Fr_3

Fr_4

Fr_5


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Sender Receiver

Fr_0

Fr_1

Fr_2

Fr_3

Fr_4

Fr_5

Fr_6

ACK_1

ACK_2

ACK_3

Sending Window



Fr_3

Fr_4

Fr_5

Fr_6


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Sender Receiver

Fr_0

Fr_1

Fr_2

Fr_3

Fr_4

Fr_5

Fr_6

ACK_1

ACK_2

ACK_3

Sending Window



Fr_4

Fr_5

Fr_6

Fr_7ACK_4

Transmitter has

nothing to send


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Header Bits for SequencingHeader Bits for Sequencing

y The frame sequence numbers cannot be allowedto keep on increasing to infinity. WHY?

Headers must be limited in size

An increase in header size means inefficiency

y Ifm bits are devoted for carrying the sequencenumbers

Frame sequence numbers may go from 0up to 2m-1

y IMPORTANT REQUIREMENT: The receivermust have the ability to unambiguously identify the

received frame

Window Size MUST BE smaller than 2m


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Go-Back-N ARQ: sender window size


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Selective Repeat ARQ ProtocolSelective Repeat ARQ Protocol

What is the problem of Go-Back N?Go Back-N retransmits the erroneous

frame and all subsequent frames

This reduces efficiency especially forHigh Error Channels

Selective Repeat ARQIntroduce a receive window to store

CORRECT out of sequence framesRetransmit individual frames

TCP uses a form of selective repeat


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Error Recovery in SelectiveError Recovery in Selective

Repeat ARQRepeat ARQ

Sender Receiver

Fr_0

Fr_1

Fr_2

Fr_3

Fr_4

Fr_5

ACK_1

ACK_2

NAK_2

Fr_2

ACK_2

ACK_2

Fr_6 ACK_6

ACK_7


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Operation ofSelective RepeatOperation ofSelective Repeat

Sending Window



Fr_0

Fr_1

Fr_0

Fr_1

Frame Sequences that areallowed to be received

Frame Sequences thatare received correctlyand buffered

Receiving Window

Sender Receiver

Fr_0

Fr_1

Fr_0

NAK_0

WS=WR=2 , m=2

ACK_2

Fr_2

ACK_3Fr_3

ACK_0


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WS=WR=2 , m=2

Sending Window



Fr_0

Fr_1

Fr_0

Fr_1



Receiving Window

Sender Receiver

Fr_0

Fr_1

Fr_0

NAK_0

ACK_2

Fr_2

ACK_3Fr_3

ACK_0


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Sending Window



Fr_0

Fr_1

Fr_0

Fr_1



Receiving Window

Sender Receiver

Fr_0

Fr_1

Fr_0

NAK_0

ACK_2

Fr_2

ACK_3Fr_3

ACK_0

WS=WR=2 , m=2


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Sending Window



Fr_0

Fr_1

Fr_0

Fr_1



Receiving Window

Sender Receiver

Fr_0

Fr_1

Fr_0

NAK_0

ACK_2

Fr_2

ACK_3Fr_3

ACK_0

WS=WR=2 , m=2


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Sending Window



Fr_0

Fr_1

Fr_0

Fr_1



Receiving Window

Sender Receiver

Fr_0

Fr_1

Fr_0

NAK_0

ACK_2

Fr_2

ACK_3Fr_3

ACK_0

WS=WR=2 , m=2


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Sending Window



Fr_0

Fr_1

Fr_2

Fr_3



Receiving Window

Sender Receiver

Fr_0

Fr_1

Fr_0

NAK_0

ACK_2

Fr_2

ACK_3Fr_3

ACK_0

WS=WR=2 , m=2


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Sending Window



Fr_2

Fr_3

Fr_2

Fr_3



Receiving Window

Sender Receiver

Fr_0

Fr_1

Fr_0

NAK_0

ACK_2

Fr_2

ACK_3Fr_3

ACK_0

WS=WR=2 , m=2


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Sending Window



Fr_2

Fr_3

Fr_2

Fr_3



Receiving Window

Sender Receiver

Fr_0

Fr_1

Fr_0

NAK_0

ACK_2

Fr_2

ACK_3Fr_3

ACK_0

WS=WR=2 , m=2


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Sending Window



Fr_2

Fr_3

Fr_3

Fr_0



Receiving Window

Sender Receiver

Fr_0

Fr_1

Fr_0

NAK_0

ACK_2

Fr_2

ACK_3Fr_3

ACK_0

WS=WR=2 , m=2


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Sending Window



Fr_2

Fr_3

Fr_3

Fr_0



Receiving Window

Sender Receiver

Fr_0

Fr_1

Fr_0

NAK_0

ACK_2

Fr_2

ACK_3Fr_3

ACK_0

WS=WR=2 , m=2


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Selective Repeat ARQ, sender window size

arq protocol

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