无线移动互联网：原理、技术与应用 教学课件 作者 崔勇 CH7 Wireless TCP.ppt

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1、WirelessTCPCS80240333Instructor:CUIYong11OutlineTraditionalTCPSolutionsSplitapproachbasedsolutionsLinklayersolutionsEndtoendsolutionsImpactofmobility22OutlineTraditionalTCPSolutionsSplitapproachbasedsolutionsLinklayersolutionsEndtoendsolutionsImpactofmobility33ReviewofTCP/IPIPPacketsmaybedel

2、iveredout-of-orderPacketsmaybelostPacketsmaybeduplicated44NitinH.Vaidya@Infocom’06ReviewofTCP/IP(cont’d)TCPconnection-orientedReliabledeliveryAchievedbymeansofretransmissionsifnecessaryCongestionavoidanceandcontrolEnd-to-endsemanticsAcknowledgementssenttoTCPsenderconfirmdeliveryofdatareceive

3、dbyTCPreceiverAckfordatasentonlyafterdatahasreachedreceiver556SlowstartCongestionavoidanceSlowstartthresholdExampleassumesthatacksarenotdelayedReviewofTCP/IP(cont’d)WindowbasedflowcontrolCongestionwindowsizeboundstheamountofdatathatcanbesentperround-triptimeThroughput<=W/RTTCongestionAvoidan

4、ceandControlHowdoesTCPdetectapacketloss?Retransmissiontimeout(RTO)TCPsendersetsretransmissiontimerforonlyonepacketIfACKforthetimedpacketisnotreceivedbeforetimergoesoff,thepacketisassumedtobelostRTOdynamicallycalculatedDuplicateacknowledgementsTCPsenderassumesthatapacketlosshasoccurredifitrec

5、eivesthreedupacksconsecutively77ReviewofTCP/IP(cont’d)8ssthresh=8ssthresh=10cwnd=20AftertimeoutCongestionControlFastRetransmitFastretransmitoccurswhenmultiple(>=3)dupackscomebackFastrecoveryfollowsfastretransmitFastRecoveryssthresh=min(cwnd,receiver’sadvertisedwindow)/2(atleast2MSS)retransmi

6、tthemissingsegment(fastretransmit)cwnd=ssthresh+numberofdupackswhenanewackcomes:cwnd=ssthreshentercongestionavoidance9CongestionControl10WindowsizeisreducedinhalfafterfastretransmitandfastrecoveryReceiver’sadvertisedwindowAfterfastrecoveryTCPproblemsinwirelessnetworkTCPassumptionPacketlossis

7、duetocongestionNeedtoreducethetransmittingrateButinwirelessnetworkPacketlossmaybecausedbyHighBER,BERofawirelesschannelis10-3-10-5,whilethatofwiredlinkis10-8-10-10orbetterUnstablechannelUsermobilityNeedfastretransmission1111Impactoftransmissionerror