《计算机网络——自顶向下方法与Internet特色》幻灯片Lecture35

Chapter3outline 3 1Transport layerservices3 2Multiplexinganddemultiplexing3 3Connectionlesstransport UDP3 4Principlesofreliabledatatransfer 3 5Connection orientedtransport TCPsegmentstructurereliabledatatransferflowcontrolconnectionmanagement3 6Principlesofcongestioncontrol3 7TCPcongestioncontrol 3 7TCPcongestioncontrol TCPusesend to endcongestioncontrolratherthennetwork assistedcongestioncontrol sincetheIPlayerprovidesnoexplicitfeedbacktotheendsystemsregardingnetworkcongestion TheapproachtakenbyTCPistohaveeachsenderlimittherateatwhichitsendstrafficintoitsconnectionasafunctionofperceivednetworkcongestion Question Howlimittherate congestionwindow CongWin Howperceiveacongestion Timeoutor3duplicateACKsHowchangethesendrate TCPRenocongestioncontrolalgorithm TCPCongestionControl details senderlimitstransmission LastByteSent LastByteAcked min CongWin RcvWindow Roughly CongWinisdynamic functionofperceivednetworkcongestion 新增加的一个变量 叫做拥塞窗口 若RcvWindow足够大 仅受拥塞窗口控制 Howdoessenderperceivecongestion lossevent timeoutor3duplicateacksTCPsenderreducesrate CongWin afterlosseventthreemechanismsofTCPcongestioncontrol AIMDslowstartconservativeaftertimeoutevents TCPcongestioncontrol additiveincrease multiplicativedecrease AIMD Approach increasetransmissionrate windowsize probingforusablebandwidth untillossoccursadditiveincrease increaseCongWinby1MSSeveryRTTuntillossdetectedmultiplicativedecrease cutCongWininhalfafterloss time congestionwindowsize Sawtoothbehavior probingforbandwidth 2 TCPSlowStart Whenconnectionbegins CongWin 1MSSExample MSS 500bytes RTT 200msecinitialrate 20kbpsavailablebandwidthmaybe MSS RTTdesirabletoquicklyrampuptorespectablerate Whenconnectionbegins increaserateexponentiallyfastuntilfirstlossevent连接的开始阶段采用指数的增长方式 即一个RTT后若没有丢失 则CongWin增加一倍 直到发生第一次丢失为止 降低一半 叫慢启动 TCPSlowStart more Whenconnectionbegins increaserateexponentiallyuntilfirstlossevent doubleCongWineveryRTTdonebyincrementingCongWinforeveryACKreceivedSummary initialrateisslowbutrampsupexponentiallyfast HostA onesegment RTT HostB twosegments foursegments 3 Refinement Q Whenshouldtheexponentialincreaseswitchtolinear A WhenCongWingetsto1 2ofitsvaluebeforetimeout Implementation VariableThreshold来管理复杂的动态过程 开始时设置一个很大的值 65KB Atlossevent Thresholdissetto1 2ofCongWinjustbeforelossevent 3 Refinement inferringloss After3dupACKs CongWiniscutinhalfwindowthengrowslinearlyButaftertimeoutevent CongWininsteadsetto1MSS windowthengrowsexponentiallytoathreshold thengrowslinearly 3dupACKsindicatesnetworkcapableofdeliveringsomesegmentstimeoutindicatesa morealarming congestionscenario Philosophy Summary TCPCongestionControl WhenCongWinisbelowThreshold senderinslow startphase windowgrowsexponentially WhenCongWinisaboveThreshold senderisincongestion avoidancephase windowgrowslinearly WhenatripleduplicateACKoccurs ThresholdsettoCongWin 2andCongWinsettoThreshold Whentimeoutoccurs ThresholdsettoCongWin 2andCongWinissetto1MSS TCPsendercongestioncontrol 4 TCPthroughput What stheaveragethroughoutofTCPasafunctionofwindowsizeandRTT IgnoreslowstartLetWbethewindowsizewhenlossoccurs WhenwindowisW throughputisW RTTJustafterloss windowdropstoW 2 throughputtoW 2RTT Averagethroughout 75W RTT 5 TCPFutures Example 1500bytesegments 100msRTT want10GbpsthroughputRequireswindowsizeW 83 333in flightsegmentsThroughputintermsoflossrate L 2 10 10for10GbpsNewversionsofTCPforhigh speedneeded Fairnessgoal ifKTCPsessionssharesamebottlenecklinkofbandwidthR eachshouldhaveaveragerateofR K 3 7 1TCPFairness WhyisTCPfair Twocompetingsessions Additiveincreasegivesslopeof1 asthroughoutincreasesmultiplicativedecreasedecreasesthroughputproportionally R R equalbandwidthshare Connection1throughput Connection2throughput congestionavoidance additiveincrease loss decreasewindowbyfactorof2 congestionavoidance additiveincrease loss decreasewindowbyfactorof2 Fairness more FairnessandUDP UDP是不公平的 它没有限制地向网络发送流量 MultimediaappsoftendonotuseTCPdonotwantratethrottledbycongestioncontrolInsteaduseUDP pumpaudio videoatconstantrate toleratepacketlossResearcharea TCPfriendly 限制UDP流量 FairnessandparallelTCPconnections 这种方式是不公平的 nothingpreventsappfromopeningparallelconnectionsbetween2hosts WebbrowsersdothisExample linkofrateRsupporting9cnctions newappasksfor1TCP getsrateR 10newappasksfor11TCPs getsR 2 3 7 2Delaymodeling Q HowlongdoesittaketoreceiveanobjectfromaWebserveraftersendingarequest Ignoringcongestion delayisinfluencedby TCPconnectionestablishmentdatatransmissiondelayslowstart Notation assumptions AssumeonelinkbetweenclientandserverofrateRS MSS bits O objectsize bits noretransmissions noloss nocorruption Windowsize Firstassume fixedcongestionwindow WsegmentsThendynamicwindow modelingslowstart 1 Fixedcongestionwindow 1 Firstcase 完成第一窗口内的所有报文段发送前 收到了第一个报文段的应答WS R RTT S R ACKforfirstsegmentinwindowreturnsbeforewindow sworthofdatasent delay 2RTT O R 假设窗口长度为4 2 Fixedcongestionwindow 2 Secondcase 收到窗口中第一个报文段之前已经发完了第一个窗口中的所有内容WS R RTT S R waitforACKaftersendingwindow sworthofdatasent delay 2RTT O R K 1 S R RTT WS R 假设窗口长度为2K是涉及一个对象的数据窗口的数量 3 TCPDelayModeling SlowStart 1 NowsupposewindowgrowsaccordingtoslowstartWillshowthatthedelayforoneobjectis wherePisthenumberoftimesTCPidlesatserver whereQisthenumberoftimestheserveridlesiftheobjectwereofinfinitesize andKisthenumberofwindowsthatcovertheobject 4 TCPDelayModeling SlowStart 2 Example O S 15segmentsK 4windowsQ 2P min K 1 Q 2ServeridlesP 2times Delaycomponents 2RTTforconnectionestabandrequestO RtotransmitobjecttimeserveridlesduetoslowstartServeridles P min K 1 Q times 5 TCPDelayModeling 3 6 TCPDelayModeling 4 CalculationofQ numberofidlesforinfinite sizeobject issimilar RecallK numberofwindowsthatcoverobjectHowdowecalculateK 7 HTTPModeling AssumeWebpageconsistsof 1baseHTMLpage ofsizeObits Mimages eachofsizeObits Non persistentHTTP M 1TCPconnectionsinseriesResponsetime M 1 O R M 1 2RTT sumofidletimesPersistentHTTP 2RTTtorequestandreceivebaseHTMLfile1RTTtorequestandreceiveMimagesResponsetime M 1 O R 3RTT sumofidletimesNon persistentHTTPwithXparallelconnectionsSupposeM Xinteger 1TCPconnectionforbasefileM Xsetsofparallelconnectionsforimages Responsetime M 1 O R M X 1 2RTT sumofi