Queuing and Queue Management - Computer Science and Queue Management ... Detecon (of congeson) ... Fabric Processor Line card Line card Line card Line card data plane control plane

QueuingandQueueManagementReading:Sec1ons6.2,6.4,6.5

COS461:ComputerNetworksSpring2011

MikeFreedmanhEp://www.cs.princeton.edu/courses/archive/spring11/cos461/

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GoalsofTodaysLecture

RouterQueuingModelsLimita1onsofFIFOandDropTail

SchedulingPoliciesFairQueuing

DroppoliciesRandomEarlyDetec1on(ofconges1on)ExplicitConges1onNo1fica1on(fromrouters)

Someaddi1onalTCPmechanisms

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RouterDataandControlPlanes3

Switching Fabric

Processor

Line card

Line card

Line card

Line card

Line card

Line card

data plane

control plane

LineCards(InterfaceCards,Adaptors) Interfacing

Physicallink Switchingfabric

Packethandling PacketforwardingDecrement1metolive Buffermanagement Linkscheduling Packetfiltering Ratelimi1ng PacketmarkingMeasurement

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to/from link

to/from switch

lookup

Rec

eive

Transmit

PacketSwitchingandForwarding5

R1 Link 1

Link 2

Link 3

Link 4

Link 1, ingress Link 1, egress

Link 2, ingress Link 2, egress

Link 3, ingress Link 3, egress

Link 4, ingress Link 4, egress

Choose Egress

Choose Egress

Choose Egress

Choose Egress

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RouterDesignIssues

Schedulingdiscipline Whichpackettosend?Someno1onoffairness?Priority?

DroppolicyWhenshouldyoudiscardapacket?Whichpackettodiscard?

NeedtobalancethroughputanddelayHugebuffersminimizedrops,butaddtoqueuingdelay(thushigherRTT,longerslowstart,)

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FIFOSchedulingandDropTail Accesstothebandwidth:firstinfirstoutqueue

Packetsonlydifferen1atedwhentheyarrive

Accesstothebufferspace:droptailqueuing Ifthequeueisfull,droptheincomingpacket

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Problemswithtaildrop

Understablecondi1ons,queuealmostalwaysfull Leadstohighlatencyforalltraffic

Possiblyunfairforflowswithsmallwindows Largerflowsmayfastretransmit(detec1nglossthroughTripDupACKs),smallflowsmayhavetowaitfor1meout

Windowsynchroniza1onMoreonthislater

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SchedulingPolicies

(Weighted)FairQueuing(andClassbasedQualityofService)

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FairQueuing(FQ) Maintainsseparatequeueperflow Ensuresnoflowconsumesmorethanits1/nshare

Varia1on:weightedfairqueuing(WFQ) Ifallpacketsweresamelength,wouldbeeasy Ifnonworkconserving(resourcescangoidle),alsowouldbeeasy,yetloweru1liza1on

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RoundRobinService

Egress Link

Flow 1

Flow 2

Flow 3

Flow 4

FairQueuingBasics

Trackhowmuch1meeachflowhasusedlinkCompute1meusedifittransmitsnextpacket

SendpacketfromflowthatwillhavelowestuseifittransmitsWhynotflowwithsmallestusesofar?Becausenextpacketmaybehuge!

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FQAlgorithm

Imagineclock1ckperbit,thentx1me~length

Finish1meFi=max(Fi1,Arrive1meAi)+LengthPi

Calculatees1matedFiforallqueuedpackets TransmitpacketwithlowestFinext

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FQAlgorithm(2)

Problem:Cantpreemptcurrenttxpacket Result:Inac1veflows(Ai>Fi1)arepenalized

StandardalgorithmconsidersnohistoryEachflowgetsfairshareonlywhenpacketsqueued

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FQAlgorithm(3)

Approach:givemorepromptnesstoflowsu1lizinglessbandwidthhistorically

BidBi=max(Fi1,Ai)+Pi Intui1on:withlarger,schedulingdecisionscalculatedbylasttx1meFi1morefrequently,thuspreferringslowerflows

FQachievesmaxminfairness Firstpriority:maximizetheminimumrateofanyac1veflows Secondpriority:maximizethesecondminrate,etc.

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Usesof(W)FQ

Scalability#queuesmustbeequalto#flowsBut,canbeusedonedgerouters,lowspeedlinks,orsharedendhosts

(W)FQcanbeforclassesoftraffic,notjustflowsUseIPTOSbitstomarkimportancePartofDifferen1atedServicesarchitectureforQualityofService(QoS)

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DropPolicy

DropTailRandomEarlyDetec1on(RED)

ExplicitConges1onNo1fica1on(ECN)

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BurstyLossFromDropTailQueuing

TCPdependsonpacketloss Packetlossisindica1onofconges1onAndTCPdrivesnetworkintolossbyaddi1verateincrease

Droptailqueuingleadstoburstyloss Iflinkiscongested,manypacketsencounterfullqueue Thus,losssynchroniza1on:

Manyflowsloseoneormorepackets Inresponse,manyflowsdividesendingrateinhalf

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SlowFeedbackfromDropTail Feedbackcomeswhenbufferiscompletelyfull

eventhoughthebufferhasbeenfillingforawhile

Plus,thefillingbufferisincreasingRTTmakingdetec1onevenslower

MightbebeEertogiveearlyfeedbackAndget12connec1onstoslowdownbeforeitstoolate

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RandomEarlyDetec1on(RED) BasicideaofRED

Routerno1cesthatqueueisgesngbackloggedandrandomlydropspacketstosignalconges1on

PacketdropprobabilityDropprobabilityincreasesasqueuelengthincreases Else,setdropprobabilityasfunc1onofavgqueuelengthand1mesincelastdrop

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Average Queue Length

Dro

p Pr

obab

ility

0

1

Proper1esofRED Dropspacketsbeforequeueisfull

Inthehopeofreducingtheratesofsomeflows

Dropspacketinpropor1ontoeachflowsrateHighrateflowshavemorepacketsand,hence,ahigherchanceofbeingselected

Dropsarespacedoutin1meWhichshouldhelpdesynchronizetheTCPsenders

Tolerantofburs1nessinthetrafficBybasingthedecisionsonaveragequeuelength

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ProblemsWithRED Hardtogettunableparametersjustright

Howearlytostartdroppingpackets?Whatslopeforincreaseindropprobability?What1mescaleforaveragingqueuelength?

REDhasmixedadop1oninprac1ce Ifparametersarentsetright,REDdoesnthelpHardtoknowhowtosettheparameters

Manyothervaria1onsinresearchcommunityNameslikeBlue(selftuning),FRED

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Feedback:Fromlosstono1fica1on

EarlydroppingofpacketsGood:givesearlyfeedbackBad:hastodropthepackettogivethefeedback

ExplicitConges1onNo1fica1onRoutermarksthepacketwithanECNbitSendinghostinterpretsasasignofconges1on

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ExplicitConges1onNo1fica1on Mustbesupportedbyrouter,sender,ANDreceiver

EndhostsdetermineifECNcapableduringTCPhandshake ECNinvolvesallthreepar1es(and4headerbits)

1. SendermarksECNcapablewhensending2. IfrouterseesECNcapableandexperiencingconges1on,

routermarkspacketasECNconges1onexperienced

3. Ifreceiverseesconges1onexperienced,marksECNechoflaginresponsesun1lconges1onACKd

4. IfsenderseesECNecho,reducescwndandmarksconges1onwindowreducedflaginnextTCPpacket

WhyextraECNflag?Conges1oncouldhappenineitherdirec1on,wantsendertoreacttoforwarddirec1on

WhyCRWACK?ECNechocouldbelost,butweideallyonlyrespondtoconges1oninforwarddirec1on

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OtherTCPMechanisms

NaglesAlgorithmandDelayedACK

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NaglesAlgorithm Waitiftheamountofdataissmall

SmallerthanMaximumSegmentSize(MSS) Andsomeotherpacketisalreadyinflight

I.e.,s1llawai1ngtheACKsforpreviouspackets Thatis,sendatmostonesmallpacketperRTT

bywai1ngun1lalloutstandingACKshavearrived

Influenceonperformance Interac1veapplica1ons:enablesbatchingofbytesBulktransfer:transmitsinMSSsizedpacketsanyway

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vs.

ACK

NaglesAlgorithm Waitiftheamountofdataissmall

SmallerthanMaximumSegmentSize(MSS) Andsomeotherpacketisalreadyinflight

I.e.,s1llawai1ngtheACKsforpreviouspackets Thatis,sendatmostonesmallpacketperRTT

bywai1ngun1lalloutstandingACKshavearrived

Influenceonperformance Interac1veapplica1ons:enablesbatchingofbytesBulktransfer:transmitsinMSSsizedpacketsanyway

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vs.

ACK

TurningNagleOffvoid tcp_nodelay (int s) {

int n = 1;

if (setsockopt (s, IPPROTO_TCP, TCP_NODELAY,

(char *) &n, sizeof (n)) < 0) warn ("TCP_NODELAY: %m\n"); }

Mo1va1onforDelayedACK

TCPtrafficisotenbidirec1onalDatatravelinginbothdirec1onsACKstravelinginbothdirec1ons

ACKpacketshavehighoverhead40bytesfortheIPheaderandTCPheaderandzerodatatraffic

PiggybackingisappealingHostBcansendanACKtohostAaspartofadatapacketfromBtoA

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TCPHeaderAllowsPiggybacking

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Source port Destination port

Sequence number

Acknowledgment

Advertised window HdrLen Flags 0

Checksum Urgent pointer

Options (variable)

Data

Flags: SYN FIN RST PSH URG ACK

ExampleofPiggybacking29

Data

Data+ACK

Data

A B

ACK

Data

Data + ACK

B has data to send

A has data to send

B doesnt have data to send

IncreasingLikelihoodofPiggybacking

Example:sshorevenHTTPHostAtypescommandHostBreceivesandexecutesthecommand

andthendataaregeneratedWouldbeniceifBcouldsendtheACKwiththenewdata

Increasepiggybacking TCPallowsthereceivertowaittosendtheACK

inthehopethatthehostwillhavedatatosend

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Data

Data+ACK

Data

A B

ACK

Data

Data + ACK

DelayedACK

DelaysendinganACKUponreceivingapacket,thehostBsetsa1mer

Typically,200msecor500msec IfBsapplica1ongeneratesdata,goaheadandsend

AndpiggybacktheACKbit Ifthe1merexpires,senda(nonpiggybacked)ACK

Limi1ngthewaitTimerof200msecor500msecACKeveryotherfullsizedpacket

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Conclusions

Conges1onisinevitable Internetdoesnotreserveresourcesinadvance TCPac1velytriestopushtheenvelope TCPcanreac