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Computer Networks

Lecture26:FlowControlandARQ

DataLinkLayer

TheDataLinklayercanbefurthersubdividedinto:1.  LogicalLinkControl(LLC):provides

flowanderrorcontrol•  differentlinkprotocolsmayprovidedifferentservices,e.g.,Ethernetdoesn’tprovidereliabledelivery(errorrecovery)

2.  MediaAccessControl(MAC):framingandmediaaccess

application

transport

network

LLC MAC

physical

LinkLayerServicesFlowControl:• pacingbetweenadjacentsendingandreceivingnodes

ErrorControl:• errorscausedbysignalattenuation,noise• ARQ:receiverdetectspresenceoferrorsandaskssenderforretransmission

•  FEC:receiveridentifiesandcorrectsbiterror(s),withoutresortingtoretransmission

Reliabledeliverybetweenadjacentnodes•  seldomusedonlowbiterrorlinks(fiber,sometwistedpair)• playsanimportantroleinwirelesslinkswithhigherrorrates• Q:whydoweneedbothlink-levelandend-endreliability?

FlowControl

Whatisflowcontrol?• receivertellingsendertoslowdown

Whydoyouneedflowcontrol?

Flowcontrolprotocolsatdatalinklayer(singlehop):• XON/XOFF• Stop&WaitProtocol• SlidingWindowProtocol

Similarissuesandmechanismsapplyatthetransportlayer(end-to-end)

XON/XOFF

Algorithm:• Ssendsstreamofdata• RsendsXOFF,Sstopstransmission• RsendsXON,SresumestransmissionWorksOKifτissmall,otherwisesendercanoverrunreceiver(Why?)

sender receiver

S R

! : propagationdelay

StopandWait(S&W)ProtocolAftersendingapacket,sendermustwaitforacknowledgment(ACK)beforesendingthenextpacket

Time

SenderS

ReceiverR

t

t+ 2!

t+ !

! : propagationdelay

2!

round-triptime (rtt)

ACK

pkt

Stop&WaitPerformanceDisadvantages:• slow• mustwaitforACKevenifnooverrun• maxtransmissionbandwidth1packet/round-triptime(rtt)Performanceisokifτissmall,otherwiseinefficientExample1:•  linkbandwith(µ) =1Mbps,withpacketsize(L)=1Kbits,transmissiontimeisL/µ =1ms•  ifrtt(2τ )=9ms,wecansend100pkts/sec• thethroughput(Tg)is100Kbps(10%ofcapacity)

Example2:•  linkbandwidth(µ )=1Gbps,propagationdelay(τ ) = 15ms, packetsize(L)=8Kbits,transmissiontimeisL/µ =8µs•  senderutilization(Us),fractionoftimesenderissending:

firstpacketbittransmitted, t = 0 sender receiver

RTT (2τ )

lastpacketbittransmitted, t = L/µ

firstpacketbitarriveslastpacketbitarrives,sendACK

ACKarrives,sendnextpacket,t = RTT + L/µ

Us =L / µ

2τ + L / µ=8·103 /109

30 + 8·10−6 = 0.00027

Stop&WaitPerformance

SlidingWindow:PipelinedFlowControl

Pipelining:senderallowsmultiple“in-flight,”yet-to-be-acknowledged,packets

SlidingWindowSendwnumberofpacketsbeforewaitingforanACK(canhavewoutstanding,unACKed,packets)ForeveryreceivedACK,slidewindow(overdata)by1packet(S&Wisslidingwindowwithw = 1)

Throughputoftheslidingwindowprotocol(Tw):Tw = Tg*w

sendwindowsizewlimitedbybuffersizeatreceiver(wR):Tw = Tg*MIN(w,wR)

[Stevens]

Example3:•  linkbandwidth(μ)=1Gbps,propagationdelay(τ ) = 15 ms, packetsize(L) = 8Kbits,transmissiontimeisL/µ =8µs,windowsize(w) = 3•  senderutilization(Us),fractionoftimesenderissending:

Pipelining:IncreasedUtilizationfirstpacketbittransmitted,t = 0

sender receiver

RTT (2τ )

lastbittransmitted,t = L/µ

firstpacketbitarriveslastbitof1stpktarrives,sendACK

ACKarrives,sendnextpacket,t = RTT + L/µ

lastbitof2ndpktarrives,sendACKlastbitof3rdpktarrives,sendACK

increasedutilizationbyafactorof3!Us =

w * L / µ2τ + L / µ

=3*8·103 /109

30 + 8·10−6 = 0.0008

SlidingWindow:MaxWindowSize

Whatistheoptimalwindowsize?i.e.,what’sthemaximumnumberofpacketsoncanhaveoutstanding(to“fillthepipe”)?Letμbethelinkbandwidth,pipesize=RTT*μ=2τ *μ(commonlycalledthebandwidth-delayproduct)Normallyyoudon’twantto,andcan’t,fillthepipecompletely(morewhenwediscussreliabletransportprotocol)

ErrorControlErrorsareunavoidable,causedbynoiseonchannel:• electricalinterference,thermalnoise,cosmicrays,etc.

Threekindsoftransmissionerrors:1.  sentsignaldestroyed(datanotreceived)2.  sentsignalchanged(receivedwrongdata)3.  spurioussignalcreated(receivedrandomdata)

AutomaticRepeatreQuest(ARQ):senderretransmitslostorcorruptedpackets

AutomaticRepeatreQuest(ARQ)Howdoessenderknowwhenandwhichpktstoretransmit?• bytheuseofACKsandtimeout

Generalalgorithm:• receiveracknowledges(ACKs)receiptofpkts• senderretransmitspacketsnotACKedbytimeout• a.k.a.PAR:PositiveAcknowledgementwithRetransmissionReliabilityprotocols:• AlternatingBitProtocol(ABP)• Go-Back-N(GBN,withorwithoutNAK)• SelectiveRepeatProtocol(SRP)

Time

SenderS

ReceiverR

t

t+ 2τ

t+ τ

τ : propagationdelay

2τ

round-triptime (rtt)

pktX

pkt lost

pktrexmitted

AlternatingBitProtocol(ABP)S&Wwithun-numberedpacketsandACKscausesconfusiononretransmission:• howtodifferentiatearetransmittedframefromthenextframe?

ABP:uses1bittonumberpacketsandACKs

Time

SenderS

ReceiverR

t

t+ 2τ

t+ τ

τ : propagationdelay

2τ

round-triptime (rtt)

ACK

pkt

XACK lost

how to detectduplicate?

ABPinAction

ABPinActionABPworks,butperformanceisbadExample:•  linkbandwidth(µ )=1Gbps•  propagationdelay(τ ) = 15 ms•  packetsize(L)=8Kbits,transmissiontimeisL/µ =8µs•  senderutilization(Us)isthesameasS&W:

•  about1packetevery30ms•  33KBpsor264Kbpsthroughputovera1Gbpslink!

PerformanceofABP

Us =L / µ

2τ + L / µ=8·103 /109

30 + 8·10−6 = 0.00027

Go-Back-N(Link-layer)Sender:•  putsk-bitsequencenumber(seq#)inpacketheader•  sendsa“window”ofuptoNpackets•  consecutiveunACKed(una)packetsallowed

• notation:ACK(n)meansACKspacketwithseq#n

snd_una snd_next

Go-Back-N(Link-layer)Sender:• associatesatimerwitheachin-flightpacket•  timeout(n):retransmitspacketwithseq#nandallhigherseq#inwindow•  resetssnd_nexttosnd_una (n)

•  resetstimerforallretransmittedpackets

snd_una snd_next

Go-Back-N(Link-layer)Receiver:• onlyneedstoremembernextexpectedseq#(next_seqn)• ACKsanddeliverstoapppacketsthatarrivedinorder• discardsout-of-orderpackets�nobuffering!• ACKsout-of-orderpacketsifseq#issmallerthannext_seqn(why?)

2τ

round-triptime (rtt)

rexmissiontimeout (rto)

SenderS

ReceiverR

ACK(1)

Pkts

X

1234

1234

5

2345

2345

discardall these

5

ACK(3)ACK(4)

ACK(5)

ACK(3)ACK(4)

ACK(2)

ACK(5)

SenderS

ReceiverR

PktsX

1234

134

5

2345

2345

discardthese

5discard

ACK(1)

ACK(3)ACK(4)

ACK(2)

ACK(5)

next_seqn= 5

next_seqn= 6

next_seqn= 2

next_seqn= 2

snd_una snd_next

Go-Back-N[K&R]Sender:• ACK(n)iscumulative:ACKsallpacketsuptoandincludingseq#n• maintainsonlyoneactivetimer,forsnd_una•  timeout(snd_una):retransmitssnd_unaandallhigherseq#sinwindow•  resetssnd_nexttosnd_una

•  resetstimerforsnd_una

Go-Back-N[K&R]Receiver:• onlyneedstoremembernextexpectedseq#(next_seqn)• cumulativeACKacknowledgesallpacketsreceivedin-order• out-of-orderpacketsrepeatthelastACK

2τ

round-triptime (rtt)

SenderS

ReceiverR

ACK(1)

Pkts

X

1234

1234

5678

678

5

ACK(3)ACK(4)

ACK(5)

ACK(6)ACK(7)ACK(8)

SenderS

ReceiverR

PktsX

1234

134

5

2345

2345

discardthese

5discard

ACK(1)

ACK(3)ACK(4)

ACK(2)

ACK(5)

ACK(1)

ACK(1)

rexmissiontimeout (rto)

next_seqn= 5

next_seqn= 9

next_seqn= 2

next_seqn= 2

Go-Back-NwithNegativeACK(NAK)

sender

receiver

Walrand

Receiver:• ACKsanddeliversin-orderpackets• sendsNAKforfirstoutoforderpacketanddiscardspacket• ACKsanddiscardssubsequentoutoforderpacketsSender:retransmitsonreceivingNAKorifRTOexpires

SelectiveRepeatProtocol(SRP)

wR

wR

snd_una snd_next

next_seqn

Receiver:• buffersout-of-orderpackets(uptowR),forin-orderdelivery• ACKsallcorrectly(noerror,butmaybeout-of-order)receivedpacketsindividually,notcumulatively

Sender:• keepstrackofwRandensuresthatwR>(snd_next−snd_una)• keepsaretransmittimerforeachpacket• retransmitsonlyunACKedpackets

SelectiveRepeatProtocol(SRP)Receiver:• buffersout-of-orderpackets(uptowR),forin-orderdelivery• ACKsallcorrectlyreceivedpacketsindividually

Sender:• keepstrackofwRandensuresthatwR>(snd_next−snd_una),intheexample,wR=4

• keepsaretransmittimerforeachpacket• retransmitsonlyunACKedpackets

SelectiveAcknowledgement:Piggy-backNAKwithACK.,e.g.[ACK2,NAK1],[ACK4,NAK3]

cannotsendpkt5(why?)

oktosendpkt6(why?)

1 2 3 4 5 6 71 2 3 4 5 6 7

Walrand

SelectiveRepeatinAction

non-cumulativeACK

SelectiveRepeatSummary

• datafromupperlayer:•  ifnextavailableseq#inwindow,sendpacket

• timeout(n):•  resendpacketn,restarttimer

• receivedACK(n)in[snd_una,snd_next]:• markpktnasreceived•  ifnissmallestunACKedpacket,advancesnd_unatonextunACKedseq#

sender

• packetnin[next_seqn,next_seqn+wR�1]•  sendACK(n)•  out-of-order:buffer•  in-order:deliver(alsodeliverbuffered,in-orderpackets),advancewindowtonextnot-yet-receivedpacket

• packetn<next_seqn•  sendACK(n)

• otherwise:•  ignore

receiver

ARQProtocolsSummaryGo-Back-N[K&R]:• sender:allowsuptoNunACKedpacketsinpipeline• receiver:sendscumulativeACKs•  repeatlastACKifthere’sagap• sender:keepstimeronlyforoldestunACKedpkt•  iftimerexpires:retransmitsallunACKedpackets

SelectiveRepeat• sender:allowsuptoNunACKedpacketsinpipeline• receiver:ACKsindividualpackets• sender:maintainstimerforeachunACKedpkt•  iftimerexpires:retransmitsonlyunACKedpacket

SimplifyingAssumptionInfinitesequence#space

Supposeyouhaveonlya2-bitsequencespace:Q:what’stherelationshipbetweenseq#spaceandwindowsize?

SenderS

ReceiverR

ACKs

Pkts

rexmissiontimeout (rto)

1234

1234

1

X

1st or5th pkt?

Ethernet:ConnectionlessServiceNohandshakingbetweensendingandreceivingadaptor

Receivingadaptordoesn’tsendACKsorNACKstosendingadaptor•  streamofdatagramspasseduptonetworklayercanhavegaps•  gapswillbefilledifapplicationusesreliabletransportlayer•  otherwise,applicationwillseethegaps

Otherdatalinkprotocolsmayprovideerrorcorrectionandflowcontrol

OtherIssuesatTransportLayerConnectionlessnetworklayermeanseachpacketcan:• takeadifferentpath• experiencedifferentcongestion

Implications:• non-deterministicround-triptime• out-of-orderpacketsmustbebufferedforGo-Back-N(soasnottomistakelatepacketsaslostpackets)• complicatescomputationofreceiver’swindow(w)