queuing and queue management - princeton

QueuingandQueueManagementReading:Sec1ons6.2,6.4,6.5

COS461:ComputerNetworksSpring2011

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

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GoalsofToday’sLecture

•  RouterQueuingModels– Limita1onsofFIFOandDropTail

•  SchedulingPolicies– FairQueuing

•  Droppolicies– RandomEarlyDetec1on(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–  Packetforwarding– Decrement1me‐to‐live–  Buffermanagement–  Linkscheduling–  Packetfiltering–  Ratelimi1ng–  Packetmarking– Measurement

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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

“4”

“4”

RouterDesignIssues

•  Schedulingdiscipline – Whichpackettosend?

– Someno1onoffairness?Priority?

•  Droppolicy– Whenshouldyoudiscardapacket?– Whichpackettodiscard?

•  Needtobalancethroughputanddelay– Hugebuffersminimizedrops,butaddtoqueuingdelay(thushigherRTT,longerslowstart,…)

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FIFOSchedulingandDrop‐Tail•  Accesstothebandwidth:first‐infirst‐outqueue

– Packetsonlydifferen1atedwhentheyarrive

•  Accesstothebufferspace:drop‐tailqueuing–  Ifthequeueisfull,droptheincomingpacket

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✗

Problemswithtaildrop

•  Understablecondi1ons,queuealmostalwaysfull–  Leadstohighlatencyforalltraffic

•  Possiblyunfairforflowswithsmallwindows–  Largerflowsmayfastretransmit(detec1nglossthroughTripDupACKs),smallflowsmayhavetowaitfor1meout

•  Windowsynchroniza1on– Moreonthislater…

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✗

SchedulingPolicies

(Weighted)FairQueuing(andClass‐basedQualityofService)

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FairQueuing(FQ)•  Maintainsseparatequeueperflow

•  Ensuresnoflowconsumesmorethanits1/nshare–  Varia1on:weightedfairqueuing(WFQ)

•  Ifallpacketsweresamelength,wouldbeeasy

•  Ifnon‐work‐conserving(resourcescangoidle),alsowouldbeeasy,yetloweru1liza1on

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RoundRobinService

Egress Link

Flow 1

Flow 2

Flow 3

Flow 4

FairQueuingBasics

•  Trackhowmuch1meeachflowhasusedlink

– Compute1meusedifittransmitsnextpacket

•  Sendpacketfromflowthatwillhavelowestuseifittransmits– Whynotflowwithsmallestusesofar?

– Becausenextpacketmaybehuge!

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FQAlgorithm

•  Imagineclock1ckperbit,thentx1me~length

Finish1meFi=max(Fi‐1,Arrive1meAi)+LengthPi

•  Calculatees1matedFiforallqueuedpackets

•  TransmitpacketwithlowestFinext

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FQAlgorithm(2)

•  Problem:Can’tpreemptcurrenttxpacket

•  Result:Inac1veflows(Ai>Fi‐1)arepenalized– Standardalgorithmconsidersnohistory

– Eachflowgetsfairshareonlywhenpacketsqueued

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FQAlgorithm(3)

•  Approach:givemorepromptnesstoflowsu1lizinglessbandwidthhistorically

•  BidBi=max(Fi‐1,Ai–δ)+Pi–  Intui1on:withlargerδ,schedulingdecisionscalculatedbylasttx1meFi‐1morefrequently,thuspreferringslowerflows

•  FQachievesmax‐minfairness–  Firstpriority:maximizetheminimumrateofanyac1veflows

–  Secondpriority:maximizethesecondminrate,etc.

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Usesof(W)FQ

•  Scalability– #queuesmustbeequalto#flows

– But,canbeusedonedgerouters,lowspeedlinks,orsharedendhosts

•  (W)FQcanbeforclassesoftraffic,notjustflows– UseIPTOSbitstomark“importance”

– Partof“Differen1atedServices”architecturefor“Quality‐of‐Service”(QoS)

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DropPolicy

DropTailRandomEarlyDetec1on(RED)

ExplicitConges1onNo1fica1on(ECN)

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BurstyLossFromDrop‐TailQueuing

•  TCPdependsonpacketloss–  Packetlossisindica1onofconges1on– AndTCPdrivesnetworkintolossbyaddi1verateincrease

•  Drop‐tailqueuingleadstoburstyloss–  Iflinkiscongested,manypacketsencounterfullqueue–  Thus,losssynchroniza1on:

• Manyflowsloseoneormorepackets•  Inresponse,manyflowsdividesendingrateinhalf

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SlowFeedbackfromDropTail•  Feedbackcomeswhenbufferiscompletelyfull

– …eventhoughthebufferhasbeenfillingforawhile

•  Plus,thefillingbufferisincreasingRTT– …makingdetec1onevenslower

•  MightbebeEertogiveearlyfeedback– Andget1‐2connec1onstoslowdownbeforeit’stoolate

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RandomEarlyDetec1on(RED)•  BasicideaofRED

–  Routerno1cesthatqueueisgesngbacklogged– …andrandomlydropspacketstosignalconges1on

•  Packetdropprobability– Dropprobabilityincreasesasqueuelengthincreases–  Else,setdropprobabilityasfunc1onofavgqueuelengthand1mesincelastdrop

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Average Queue Length

Dro

p Pr

obab

ility

0

1

Proper1esofRED•  Dropspacketsbeforequeueisfull

–  Inthehopeofreducingtheratesofsomeflows

•  Dropspacketinpropor1ontoeachflow’srate– High‐rateflowshavemorepackets– …and,hence,ahigherchanceofbeingselected

•  Dropsarespacedoutin1me– WhichshouldhelpdesynchronizetheTCPsenders

•  Tolerantofburs1nessinthetraffic– Bybasingthedecisionsonaveragequeuelength

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ProblemsWithRED•  Hardtogettunableparametersjustright

– Howearlytostartdroppingpackets?– Whatslopeforincreaseindropprobability?– What1mescaleforaveragingqueuelength?

•  REDhasmixedadop1oninprac1ce–  Ifparametersaren’tsetright,REDdoesn’thelp– Hardtoknowhowtosettheparameters

•  Manyothervaria1onsinresearchcommunity– Nameslike“Blue”(self‐tuning),“FRED”…

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Feedback:Fromlosstono1fica1on

•  Earlydroppingofpackets– Good:givesearlyfeedback– Bad:hastodropthepackettogivethefeedback

•  ExplicitConges1onNo1fica1on– RoutermarksthepacketwithanECNbit– Sendinghostinterpretsasasignofconges1on

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ExplicitConges1onNo1fica1on•  Mustbesupportedbyrouter,sender,ANDreceiver

–  End‐hostsdetermineifECN‐capableduringTCPhandshake

•  ECNinvolvesallthreepar1es(and4headerbits)1.  Sendermarks“ECN‐capable”whensending

2.  Ifroutersees“ECN‐capable”andexperiencingconges1on,routermarkspacketas“ECNconges1onexperienced”

3.  Ifreceiversees“conges1onexperienced”,marks“ECNecho”flaginresponsesun1lconges1onACK’d

4.  Ifsendersees“ECNecho”,reducescwndandmarks“conges1onwindowreduced”flaginnextTCPpacket

•  WhyextraECNflag?Conges1oncouldhappenineitherdirec1on,wantsendertoreacttoforwarddirec1on

•  WhyCRWACK?ECN‐echocouldbelost,butweideallyonlyrespondtoconges1oninforwarddirec1on

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OtherTCPMechanisms

Nagle’sAlgorithmandDelayedACK

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Nagle’sAlgorithm•  Waitiftheamountofdataissmall

– SmallerthanMaximumSegmentSize(MSS)

•  Andsomeotherpacketisalreadyinflight–  I.e.,s1llawai1ngtheACKsforpreviouspackets

•  Thatis,sendatmostonesmallpacketperRTT– …bywai1ngun1lalloutstandingACKshavearrived

•  Influenceonperformance–  Interac1veapplica1ons:enablesbatchingofbytes– Bulktransfer:transmitsinMSS‐sizedpacketsanyway

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vs.

ACK

Nagle’sAlgorithm•  Waitiftheamountofdataissmall

– SmallerthanMaximumSegmentSize(MSS)

•  Andsomeotherpacketisalreadyinflight–  I.e.,s1llawai1ngtheACKsforpreviouspackets

•  Thatis,sendatmostonesmallpacketperRTT– …bywai1ngun1lalloutstandingACKshavearrived

•  Influenceonperformance–  Interac1veapplica1ons:enablesbatchingofbytes– Bulktransfer:transmitsinMSS‐sizedpacketsanyway

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vs.

ACK

TurningNagleOff void 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

•  TCPtrafficisotenbidirec1onal– Datatravelinginbothdirec1ons– ACKstravelinginbothdirec1ons

•  ACKpacketshavehighoverhead– 40bytesfortheIPheaderandTCPheader– …andzerodatatraffic

•  Piggybackingisappealing– HostBcansendanACKtohostA– …aspartofadatapacketfromBtoA

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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 doesn’t have data to send

IncreasingLikelihoodofPiggybacking

•  Example:sshorevenHTTP– HostAtypescommand– HostBreceivesandexecutesthecommand

– …andthendataaregenerated– WouldbeniceifBcouldsendtheACKwiththenewdata

•  Increasepiggybacking–  TCPallowsthereceivertowaittosendtheACK

– …inthehopethatthehostwillhavedatatosend

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Data

Data+ACK

Data

A B

ACK

Data

Data + ACK

DelayedACK

•  DelaysendinganACK– Uponreceivingapacket,thehostBsetsa1mer

•  Typically,200msecor500msec

–  IfB’sapplica1ongeneratesdata,goaheadandsend•  AndpiggybacktheACKbit

–  Ifthe1merexpires,senda(non‐piggybacked)ACK

•  Limi1ngthewait– Timerof200msecor500msec– ACKeveryotherfull‐sizedpacket

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Conclusions

•  Conges1onisinevitable–  Internetdoesnotreserveresourcesinadvance–  TCPac1velytriestopushtheenvelope–  TCPcanreacttoconges1on(mul1plica1vedecrease)

•  Ac1veQueueManagementcanfurtherhelp–  RandomEarlyDetec1on(RED)–  ExplicitConges1onNo1fica1on(ECN)

•  Fundamentaltensions–  Feedbackfromthenetwork?–  Enforcementof“TCPfriendly”behavior?Otherschedulingpolicies(FQ)cangivenstrongerguarantees

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queuing and queue management - princeton

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