Congestion control and traffic management in ATM n

CongestionControlandTracManagementinATMNetworks:RecentAdvancesandASurvey1RajJainDepartmentofComputerandInformationScienceTheOhioStateUniversityColumbus,OH43210Email:[e-mail]:January26,1995AbstractCongestioncontrolmechanismsforATMnetworksasselectedbytheATMForumtracmanagementgrouparedescribed.Reasonsbehindtheseselectionsareexplained.Inpartic-ular,selectioncriterionforselectionbetweenrate-basedandcredit-basedapproachandthekeypointsofthedebatebetweenthesetwoapproachesarepresented.Theapprachthatwasnallyselectedandseveralotherschemesthatwereconsideredaredescribed.1IntroductionFuturehighspeednetworksareexpectedtousetheAsynchronousTransferMode(ATM)inwhichtheinformationistransmittedusingshortxed-sizecellsconsistingof48bytesofpayloadand5bytesofheader.Thexedsizeofthecellsreducesthevarianceofdelaymakingthenetworkssuitableforintegratedtracconsistingofvoice,video,anddata.Providingthedesiredqualityofserviceforthesevarioustractypesismuchmorecomplexthanthedatanetworksoftoday.Propertracmanagementhelpsensureecientandfairoperationofnetworksinspiteofconstantlyvaryingdemand.Thisisparticularlyimportantforthedatatracwhichhasverylittlepredictabilityand,therefore,cannotreserveresourcesinadvanceasinthecaseofvoicetelecommunicationsnetworks.Tracmanagementisconcernedwithensuringthatusersgettheirdesiredqualityofservice.Theproblemisespeciallydicultduringperiodsofheavyloadparticularlyifthetracdemandscannotbepredictedinadvance.Thisiswhycongestioncontrol,althoughonlyapartofthetracmanagementissues,isthemostessentialaspectoftracmanagement.CongestioncontroliscriticalinbothATMandnon-ATMnetworks.Whentwoburstsarrivesimulataneouslyatanode,thequeuelengthsmaybecomelargeveryfastresultinginbueroverow.Also,therangeoflinkspeedsisgrowingfastashigherspeedlinksarebeingintroducedinslowernetworksofthepast.Atthepoints,wherethetotalinputrateislargerthantheoutputlinkcapacity,congestionbecomesaproblem.1InvitedsubmissiontoComputerNetworksandISDNSystems.1TheprotocolsforATMnetworksbeganbeingdesignedin1984whentheConsultativeCom-mitteeonInternationalTelecommunicationandTelegraph(CCITT)-aUnitedNationsOrganizationresponsiblefortelecommunicationsstandards-selectedATMastheparadigmforitsbroadbandintegratedservicedigitalnetworks(B-ISDN).Likemostothertelecommu-nicationsstandards,theATMstandardsspecifytheinterfacebetweenvariousnetworkingcomponents.Aprincipalfocusofthesestandardsistheuser-networkinterface(UNI),whichspecieshowacomputersystem(whichisownedbyauser)shouldcommunicatewithaswitch(whichisownedbythenetworkserviceprovider).ATMnetworksareconnection-orientedinthesensethatbeforetwosystemsonthenetworkcancommunicate,theyshouldinformallintermediateswitchesabouttheirservicerequire-mentsandtracparameters.Thisissimilartothetelephonenetworkswhereacircuitissetupfromthecallingpartytothecalledparty.InATMnetworks,suchcircuitsarecalledvirtualcircuits(VCs).TheconnectionsallowthenetworktoguaranteethequalityofservicebylimitingthenumberofVCs.Typically,auserdeclareskeyservicerequirementsatthetimeofconnectionsetup,declaresthetracparametersandmayagreetocontroltheseparametersdynamicallyasdemandedbythenetwork.Thispaperisorganizedasfollows.Section2denesvariousqualityofserviceattributes.TheseattributeshelpdenevariousclassesofserviceinSection3.WethenprovideageneraloverviewofcongestioncontrolmechanisimsinSection4anddescribethegeneralizedcellalgorithminSection4.1.Section5describesthecriteriathatweresetupforselectingthenalapproach.AnumberofcongestionschemesaredescribedbrieyinSection6withthecredit-basedandrate-basedapproachesdescribedinmoredetailsinSections7and8.Thedebatethatleadtotheeventualselectionoftherate-basedapproachispresentedinSection9.Thedescriptionpresentedhereisnotintendedtobeaprecisedescriptionofthestandard.Inordertomaketheconcepteasytounderstand,wehaveattimessimpliedthedescription.Thoseinterestedinprecisedetailsshouldconsultthestandardsdocuments,whicharestillbeingdevelopedasofthiswritinginJanuary1995.2QualityofService(QoS)AttributesWhilesettingupaconnectiononATMnetworks,userscanspecifythefollowingparametersrelatedtothedesiredqualityofservice.1.PeakCellRate(PCR):Themaximuminstantaneousrateatwhichtheuserwilltransmit.Forburstytrac,theinter-cellintervalandthecellratevariesconsiderably.AsshowninFigure1,PCRistheinverseoftheminimuminter-cellinterval.2.SustainedCellRate(SCR):Thisistheaveragerateasmeasuredoveralonginterval.2PCR=1/MinimumInter-cellTimeCellarrivalsTimeFigure1:Peakcellrateandinter-celltime3.CellLossRatio(CLR):Thepercentageofcellsthatarelostinthenetworkduetoerrorandcongestionandarenotdeliveredtothedestination.CellLossRatio=LostCellsTransmittedCellsEachATMcellhasa\CellLossPriority(CLP)bitintheheader.Duringcongestion,thenetworkrstdropscellsthathaveCLPbitset.SincethelossofCLP=0cellismoreharmfultotheoperationoftheapplication,CLRcanbespeciedseparatelyforcellswithCLP=1andforthosewithCLP=0.4.CellTransferDelay(CTD):Thedelayexperiencedbyacellbetweennetworkentryandexitpointsiscalledthecelltransferdelay.Itincludespropagationdelays,queueingdelaysatvari