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Traffic shaping



DefinitionofTrafficShaping

TrafficShapingisameasuretoactivelyadjusttherateoftrafficoutput.

AtypicalapplicationistocontroltheoutputoflocaltrafficbasedontheTPindexofthedownstreamnetworknode.Themaindifferencebetweentrafficshapingandtrafficpolicingisthattrafficshapingbuffersthepacketsthatneedtobediscardedintrafficpolicing—usuallyputtingtheminabufferorqueue,alsoknownasTrafficShaping(TS).Whenthereareenoughtokensinthetokenbucket,thesecachedmessagesaresentoutevenly.Anotherdifferencebetweentrafficshapingandtrafficpolicingisthatshapingmayincreasedelay,whilepolicinghardlyintroducesadditionaldelay.

Asshowninthefigureabove,whenamessagearrivesattheinterface,themessageisfirstclassifiedandprocessedaccordingtothefollowingsituations.

  • Forpacketsthatdonotparticipateintrafficshaping,continuetosenddirectly.

  • Forpacketsparticipatingintrafficshaping,ifthereisashapingqueue,thepacketsenterthequeuedirectly,andthewaitingqueuewillscheduleandforwardthepacketsaccordingtotheshapingalgorithm.

  • Forpacketsparticipatingintrafficshaping,ifthereisnoshapingqueue,theshapingalgorithmdetermineswhetherthepacketissent.

Whenamessagearrivesattheinterfaceandtheshapingqueueisfull,themessageisdiscarded.

Thecorealgorithmoftrafficshaping

Therearetwocorealgorithmsoftrafficshaping.ThespecifictechnologyusedisGTS(GenericTrafficShaping),generaltrafficshaping:

LeakyBucketAlgorithm

LeakyBucket

LeakyBucketalgorithmisoftenusedinthenetworkworldintrafficshaping(TrafficShaping)orratelimiting(RateLimiting)Analgorithmwhosemainpurposeistocontroltherateofdatainjectionintothenetworkandsmooththebursttrafficonthenetwork.Theleakybucketalgorithmprovidesamechanismthroughwhichbursttrafficcanbeshapedtoprovideastableflowforthenetwork.

TokenBucketAlgorithm

TokenBucket

Sometimespeoplemistakenlyconfusetheleakybucketalgorithmwiththetokenbucketalgorithm.Infact,thesetwoalgorithmshavecompletelydifferentcharacteristicsandareusedforcompletelydifferentpurposes.Themaindifferencebetweenthemisthattheleakybucketalgorithmcanforciblylimitthedatatransmissionrate,whilethetokenbucketalgorithmcanlimittheaveragedatatransmissionratewhilealsoallowingacertaindegreeofbursttransmission.

Insomecases,theleakybucketalgorithmcannoteffectivelyusenetworkresources.Becausetheleakagerateoftheleakybucketisafixedparameter,evenifthereisnoresourceconflictinthenetwork(nocongestionoccurs),theleakybucketalgorithmcannotmakeasingleflowbursttotheportrate.Therefore,theleakybucketalgorithmisinefficientfortrafficwithburstcharacteristics.Thetokenbucketalgorithmcansatisfytheseburstytraffic.Generally,leakybucketalgorithmandtokenbucketalgorithmcanbecombinedtoprovidegreatercontrolovernetworktraffic.

Commontrafficshapingclassification

GTSgeneraltrafficshaping

Trafficshapingforcesthetraffictofollowacertainbandwidthallocationlimitbyreducingtherateofoutgoingtraffic.Unliketrafficpolicing,whichdiscardsthetrafficthatexceedstheburstsize,itputstheburstoftrafficintothetrafficshapingbuffer,andthensendsitoutwhenthebandwidthisavailable,orthenumberofbuffereddatapackets.Whenitislowerthantheconfiguredlimit,itwillbesentoutagain.

Trafficshapingusesatokenbucketsystemtodeterminewhethertotransmit,delayordiscardnewpackets.Usingthistokenbucketsystem,eachinterfacehasacommittedinformationrate(CIR),whichistherateatwhichtheinterfacecantransmitdatapacketsinaperiodoftime.Thecontinuousburstrate(Bc)definesthemaximumnumberoftokensthatthetokenbucketcancontaininanothertimeinterval.Whenadatapacketarrivesataninterface,itwilltakeoutatokenfromthetokenbucket.Whenthedatapacketissent,thetokenwillbereleased.Whenthetimeinterval(Tc)haspassed,thetokenwillbereturnedtothetokenbucket.Ifthetokenbucketisempty,anynewpacketsarrivingatthatinterfacewillbeputinthequeueuntilthetimeintervalhaspassedandthetokenwillberefilled.IftheCIRcontinuestoexceed,tokenswillberemovedfromthetokenbucketatarategreaterthantheywereaddedtofillthequeueandcausepacketstobediscarded.

TrafficshapingcanbeappliedtoanumberofdifferentLayer2technologies,suchasEthernet,ATM,HDLC,PPP(ISDNanddial-upinterfacesarenotsupported),andframerelay.Withtheexceptionofframerelay,allofthesetechnologiessupportGeneralTrafficShaping(GTS).GTScanalsoshapeacertaintypeoftrafficdefinedintheaccesscontrollistbyspecifyingagroupinthetrafficshaping.

InordertoconfiguretheGTS,youmustknowthetargetbitratefortheinterface,thatis,thecommittedinformationrate(CIR),whichreferstotherateatwhichtrafficissentundernormalconditions.Continuousburstrate(Bc)referstotherateatwhichtrafficisallowedtoburstbeyondthenormaltrafficrateineachtimeinterval,expressedinbits.Excessiveburstrate(Be)referstotherateatwhichtrafficisallowedtoburstbeyondthesustainedburstrateinthefirsttimeinterval.Everytimeinterval(Tc),trafficwillbefilledintothetokenbucketoftrafficshaping.

Tc=Bc/CIR

Thetimeintervaloftrafficshapingcannotbelessthan10msorgreaterthan125ms.TherouterfindsthebesttimeintervalbasedontheformulaTc=Bc/CIR.Thedefaulttimeintervalis125ms.ThistimeintervalistheresultofCIRandBcconfigurationandisusedfornon-configurability.CiscorecommendsthatBcshouldbe1/8ofCIR.Itwillgenerate8intervalsof125mspersecond.

ConfigureGTSforallinterfacetraffic,andusethetraffic-shapingratecommandontheinterfacethatneedstrafficshaping.Forspecifictraffic,usethetraffic-shapinggroupcommandandanaccesscontrollist.

traffic-shaping{rate|groupaccess-list-number}target-bit-rate[sustained][excess][buffer-limit]

groupaccess-list-numberspecifiedAlltrafficmatchingtheaccesscontrollist(1-2699)isshaped.

ratespecifiesthatalltrafficonthisinterfacewillbeshaped.

target-bit-rateThenormalrate(CIR)atwhichthetrafficwillbetransmitted,rangingfrom8000tothecompleteCIRexpressedinbitspersecondoftheinterface.

sustainedsustainedbitrate(Bc)referstothevalueoftrafficallowedtoburst,expressedinbitsineachtimeinterval.

ExcessExcessivebitrate(Be)referstotheburstoftrafficexceedingthecontinuousbitrateinthefirsttimeinterval,expressedinbitsineachtimeinterval.Itisanoptionalparameter,itwillassumethatthetokenbucketiscompletelyfull:Be=Bc*2.

bufferisusedtospecifyabufferlimit,rangingfrom1to4096.

GTSconfigurationrequirestwosteps:discoverthevalueoftrafficshaping,andconfiguretrafficshapingontheinterface.

1.Findthecorrectvaluefortrafficshaping.CIR,Bc,Be

Bcspecifiesthenumberofbitsthattheinterfacecantransmitinacertaintimeinterval.Bc=CIR*Tc

Bespecifiesthebursttrafficthatcanbesupportedwhentheinterfaceisfilledwithenoughtokens,usuallyinthefirsttimeinterval.Be=Bc*2

Iftheinterfacedoesnotsupportburst,youcanusethefollowingformula:Be=Bc

2,intheinterfacemode,usethetraffic-shapingcommandtoenabletrafficshaping.

ToviewtheGTSconfiguration,usetheshowtraffic-shaping[statistics]command

FrameRelayTrafficShaping(FRTS)

Underthefollowing4situationsWhenusingFRTS:1centerhighspeedandlowbranchspeed.2AsinglephysicallinecarriesmanyVCstodifferentdestinations.3IfcongestionoccursinFR,Iwanttheroutertoblockthedataflow(Throttle).4Itisnecessarytotransmitdatastreamsofmultipleprotocols(IP,SNA)onthesameFRVC,andhopethateachdatastreamcanaccountforacertainamountofBW.

TheFECNandBECNinFRareusedtoimplythatcongestionoccursonthenetwork.WhenadatapacketwithaBECNmarkisreceived,FRTrafficShaping(FRTS)willdynamicallyshapethetraffic.Note:FRTScanonlybeusedonFRPVCandSVC.AmongthemisanadaptiveFRTS.IneachTcinterval,theprocesswillcheckwhetherBECNisreceivedfromtheframerelaynetwork.IfBECNisreceivedinaTcinterval,thetransmissionratewillbereducedby25%untilitdropstohalfoftheCIR.until.IfandonlyifBECNisnotreceivedwithin16Tc,thecommunicationrateisrestoredtoCIR.

FRTSconfigurationsteps

1)CreateaMAP-CLASS,thenameiscasesensitive.

2)Definethemethodoftrafficshaping,suchassettingtheaveragerateandthemaximumrate.

3)EncapsulateFRAME-RELAYontheinterface.

4)ApplyMAP-CLASS5totheportandenabletrafficshaping,whichisgenerallyusedforthesourceinterface.

Trafficsupervision

RouterAsendspacketstoRouterB.Inordertoreduceunnecessarylossofpackets,thepacketscanbetraffic-shapedattheexitofRouterA.CachethepacketsthatexceedthetrafficshapingfeatureinRouterA.Whentheshapingalgorithmallowsthenextbatchofmessagestocontinuetobesent,themessagesaretakenoutofthebufferqueuefortransmission.Inthisway,thepacketssenttoRouterBwillallcomplywithRouterB'strafficregulations.

Themaindifferencebetweentrafficshapingandtrafficpolicingis:

  • Trafficshapingcachespacketsthatexceedthetrafficspecificationsintrafficpolicing.Whentheshapingalgorithmallowsmessagestobesent,thesebufferedmessagesaresentoutevenly.

  • Trafficshapingmayincreasedelay,andtrafficpolicinghardlyintroducesadditionaldelay.

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