外文翻译TCP友好的多播拥塞控制协议.docx
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外文翻译TCP友好的多播拥塞控制协议.docx
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外文翻译TCP友好的多播拥塞控制协议
外文翻译
毕业设计题目:
IP网络中单速率多播拥塞控制
算法研究
原文1:
TCP-FriendlyMulticastCongestion
Control(TFMCC):
ProtocolSpecification
译文1:
TCP友好的多播拥塞控制协议:
协议规范
原文2:
ASimpleAndEffectiveSingle-RateMulticastCongestionControl
译文2:
一种简单而有效的单速率多播拥塞控制方案
TCP-FriendlyMulticastCongestionControl(TFMCC):
ProtocolSpecification
1.Introduction
ThisdocumentspecifiesTCP-FriendlyMulticastCongestionControl(TFMCC).TFMCCisasource-based,single-ratecongestioncontrolschemethatbuildsupontheunicastTCP-FriendlyRateControlmechanism(TFRC).TFMCCisstableandresponsiveunderawiderangeofnetworkconditionsandscalestoreceiversetsontheorderofseveralthousandreceivers.Tosupportscalability,asmuchcongestioncontrolfunctionalityaspossibleislocatedatthereceivers.EachreceivercontinuouslydeterminesadesiredreceiveratethatisTCP-friendlyforthepathfromthesendertothisreceiver.Selectedreceiversthenreporttheratetothesenderinfeedbackpackets.
TFMCCisabuildingblockasdefinedinRFC3048.Insteadofspecifyingacompleteprotocol,thisdocumentsimplyspecifiesacongestioncontrolmechanismthatcouldbeusedinatransportprotocolsuchasRTP,inanapplicationincorporatingend-to-endcongestioncontrolattheapplicationlevel.Thisdocumentdoesnotdiscusspacketformats,reliability,orimplementation-relatedissues.
TFMCCisdesignedtobereasonablyfairwhencompetingforbandwidthwithTCPflows.Amulticastflowis‘‘reasonablyfair’’ifitssendingrateisgenerallywithinafactoroftwoofthesendingrateofaTCPflowfromthesendertotheslowestreceiverofthemulticastgroupunderthesamenetworkconditions.
Ingeneral,TFMCChasalowvariationofthroughput,whichmakesitsuitableforapplicationssuchasstreamingmediawherearelativelysmoothsendingrateisofimportance.Thepenaltyofhavingsmooththroughputwhilecompetingfairlyforbandwidthisareducedresponsivenesstochangesinavailablebandwidth.ThusTFMCCshouldbeusedwhentheapplicationhasarequirementforsmooththroughput,inparticular,avoidinghalvingofthesendingrateinresponse
toasinglepacketdrop.Forapplicationsthatsimplyneedtomulticastasmuchdataaspossiblein
asshortatimeaspossible,PGMCCmaybemoresuitable
2.ProtocolOverview
TFMCCextendsthebasicmechanismsofTFRCintothemulticastdomain.InordertocompetefairlywithTCP,TFMCCreceiversindividuallymeasuretheprevalentnetworkconditionsandcalculatearatethatisTCP-friendlyonthepathfromthesendertothemselves.TherateisdeterminedusinganequationforTCPthroughput,whichroughlydescribesTCP’ssendingrateasafunctionofthelosseventrate,round-triptime(RTT),andpacketsize.WedefinealosseventasoneormorelostormarkedpacketsfromthepacketsreceivedduringoneRTT,whereamarkedpacketreferstoacongestionindicationfromExplicitCongestionNotification(ECN).Thesendingrateofthemulticasttransmissionisadaptedtothereceiverexperiencingtheworstnetworkconditions.
Basically,TFMCC’scongestioncontrolmechanismworksasfollows:
•EachreceivermeasuresthelosseventrateanditsRTTtothesender.
•Eachreceiverthenusesthisinformation,togetherwithanequationforTCPthroughput,toderiveaTCP-friendlysendingrate.
•Throughadistributedfeedbacksuppressionmechanism,onlyasubsetofthereceiversareallowedtogivefeedbacktopreventafeedbackimplosionatthesender.Thefeedbackmechanismensuresthatreceiversreportingalowdesiredtransmissionratehaveahighprobabilityofsendingfeedback.
•Receiverswhosefeedbackisnotsuppressedreportthecalculatedtransmissionratebacktothesenderinso-calledreceiverreports.Thereceiverreportsservetwopurposes:
theyinformthesenderabouttheappropriatetransmitrate,andtheyallowthereceiverstomeasuretheirRTT.
•Thesenderselectsthereceiverthatreportsthelowestrateascurrentlimitingreceiver(CLR).
Wheneverfeedbackwithanevenlowerratereachesthesender,thecorrespondingreceiver
becomesCLRandthesendingrateisreducedtomatchthatreceiver’scalculatedrate.ThesendingrateincreaseswhentheCLRreportsacalculatedratehigherthanthecurrentsendingrate.
ThedynamicsofTFMCCaresensitivetohowthemeasurementsareperformedandappliedandwhatfeedbacksuppressionmechanismischosen.Werecommendspecificmechanismsbelowtoperformandapplythesemeasurements.Othermechanismsarepossible,butitisimportanttounderstandhowtheinteractionsbetweenmechanismsaffectthedynamicsofTFMCC.
3.DataSenderProtocol
Thedatasendermulticastsastreamofdatapacketstothedatareceiversatacontrolledrate.Wheneverfeedbackisreceived,thesenderchecksifitisnecessarytoswitchCLRsandtoreadjustthesendingrate.
ThemaintasksthathavetobeprovidedbyaTFMCCsenderare:
•adjustingthesendingrate,
•controllingreceiverfeedback,and
•assistingreceiver-sideRTTmeasurements.
3.1.SenderInitialization
Atinitializationofthesender,themaximumRTTissettoavaluethatshouldbelargerthanthehighestRTTtoanyofthereceivers.Itshouldnotbesmallerthan500millisecondsforoperationinthepublicInternet.ThesendingrateXisinitializedto1packetpermaximumRTT
3.2.DeterminingtheMaximumRTT
Foreachfeedbackpacketthatarrivesatthesender,thesendercomputestheinstantaneousRTTtothereceiverasR_r=ts_now-ts_i’
Wherets_nowisthetimethefeedbackpacketarrived.Receiverswillhaveadjustedts_i’forthetimeintervalbetweenreceivingthelastdatapacketandsendingthecorrespondingreportsothatthisintervalwillnotbeincludedinR_r.IftheactualRTTissmallerthantheresolutionofthetimestampsandts_nowequalsts_i’,thenR_rissettothesmallestpositiveRTTvaluelargerthan0(i.e.,1millisecondinourcase).IftheinstantaneousRTTislargerthanthecurrent
maximumRTT,themaximumRTTisincreasedtothatvalueR_max=R_r,Otherwise,ifnofeedbackwithahigherinstantaneousRTTthanthemaximumRTTisreceivedduringafeedbackround,themaximumRTTisreducedtoR_max=MAX(R_max*0.9,R_peak)whereR_peakisthepeakreceiverRTTmeasuredduringthefeedbackround.
ThemaximumRTTismainlyusedforfeedbacksuppressionamongreceiverswithheterogeneousRTTs.Feedbacksuppressioniscloselycoupledtothesendingofdatapacketsandforthisreason,themaximumRTTmustnotdecreasebelowthemaximumtimeintervalbetweenconsecutivedatapackets:
R_max=max(R_max,8s/X+ts_gran)wherets_granisthegranularityofthesender’ssystemclock
3.3.AdjustingtheSendingRate
Whenafeedbackpacketfromreceiverrarrivesatthesender,thesenderhastocheckwhetheritisnecessarytoadjustthetransmissionrateandtoswitchtoanewCLR.HowtherateisadjusteddependsonthedesiredrateX_rofthereceiverreport.Wedistinguishfourcases:
1.IfnoCLRispresent,receiverrbecomesthecurrentlimitingreceiver.ThesendingrateXisdirectlysettoX_r,solongasthiswouldresultinarateincreaseoflessthan8s/R_maxbits/s(i.e.,1packetperR_max).OtherwiseXisgraduallyincreasedtoX_ratanincreaserateofnomorethan8s/R_maxbits/severyR_maxseconds.
2.IfreceiverrisnottheCLRbutaCLRispresent,thenreceiverrbecomesthecurrentlimitingreceiverifX_rislessthanthecurrentsendingrateXandthereceiver_leaveflagofthatreceiver’sreportisnotset.Furthermore,thesendingrateisreducedtoX_r.
3.IfreceiverrisnottheCLRbutaCLRispresentandthereceiver_leaveflagoftheCLR’slastreportwasset,thenreceiverrbecomesthecurrentlimitingreceiver.However,ifX_r>X,thesendingrateisnotincreasedtoX_rforthedurationofafeedbackroundtoallowother(lowerrate)receiverstogivefeedbackandbeselectedasCLR.
4.IfreceiverristheCLR,thesendingrateissettotheminimumofX_randX+
8s/R_maxbits/s.IfthereceiverhasnotyetmeasureditsRTTbutalreadyexperiencedpacketloss(indicatedbythecorrespondingflagsinthereceiverreport),thereceiverreportwillincludeadesiredratethatisbasedonthemaximumRTTratherthantheactualRTTtothatreceiver.Inthiscase,thesenderadjuststhedesiredrateusingitsmeasurementoftheinstantaneousRTTR_rtothatreceiver:
X_r’=X_r*R_max/R_r
X_r’isthenusedinsteadofX_rtodetectwhethertoswitchtoanewCLR.
IftheTFMCCsenderreceivesnoreportsfromtheCLRfor4RTTs,thesendingrateiscutinhalfunlesstheCLRwasselectedlessthan10RTTsago.Inaddition,ifthesenderreceivesnoreportsfromtheCLRforatleast10RTTs,itassumesthattheCLRcrashedorleftthegroup.AnewCLRisselectedfromthefeedbackthatsubsequentlyarrivesatthesender,andweincreaseasincase3above.
IfnonewCLRcanbeselected(i.e.,intheabsenceofanyfeedbackfromanyofthereceivers)itisnecessarytofurtherreducethesendingrate.Forevery10consecutiveRTTswithoutfeedback,thesendingrateiscutinhalf.Therateisatmostreducedtoonepacketevery8seconds.Notethatwhenreceiversstopreceivingdatapackets,theywillstopsendingfeedback.Thiseventuallycausesthesendingratetobereducedinthecaseofnetworkfailure.Ifthenetworksubsequentlyrecovers,alinearincreasetothecalculatedrateoftheCLRwilloccurat8s/R_maxbits/severyR_max.
3.4.AssistingReceiver-SideRTTMeasurements
ReceiversmeasuretheirRTTbysendingatimestampwithareceiverreport,whichisechoedbythesender.Ifcongestioncontrolinformationispiggybackedontodatapackets,usuallyonlyonereceiverIDandtimestampcanbeincluded.Ifmultiplefeedbackm
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