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
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