e-计算机网络基础4-UDP+TCP课件.ppt

e-计算机网络基础4-UDP+TCP课件.ppt

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时间:2020-07-30

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1、TransportLayer:UDP&TCP (Commonalities)End-to-EndCommunicationHowaprocessisidentifiedonInternet:EachprocesscommunicatingviaTCPorUDPisassignedanhost-unique16-bitbinarycalled“portnumber”;AportnumberiseitherTCP-orUDP-type;AprocessisuniquelyidentifiedbyanIPaddressanditsportnumber.Portnumber

2、sinrangeof1-1024areassignedbyIANA。TransportLayer:UDP(1)UDPisend-to-endandconnectionless.UDPisNOTreliableperse;UDPdoesNOTdoflow-adjustment;UDPCAN(!)goinbroadcastormulticastway(incontrastTCPCANNOTdo).Transportlayer:UDP(2)Transportlayer:UDP(3)Transportlayer:UDP(4)TransportLayer:TCP(1)TCPi

3、send-to-endandconnection-oriented;TCPisbi-directional;TCPisreliable:receiverisguaranteedtogetexactlywhateverisoutgoingfromthesender;TCPPdoesflow-adjustment;TCPcansupportneitherbroadcastnormulticast(!).TransportLayer:TCP(2)WhatsortofreliabilityissuesonIPpacketstransmittingoverInternet?P

4、acketLoss(becauseof,e.g.,link/gatewaysaturation、mediainterference、gatewayoverloading、interfacefault,etc);PacketDisorder(packetsinthesamestreamarriveatdestinationhostindifferentorderfromthatofsender);Flow-mismatch(Lower-speedreceivercannotcatchupwithahigher-speedsender,asaresult,somepac

5、ketsaredropped).TransportLayer:TCP(3-1)HowTCPresolvethosereliabilityissues?Rule#1:sequencenumberTCPregardswhateverisintransmission(Email/filedata,Web-page,voice,video,etc.)astheorderedbytesequence(calledstream).Eachbyteisassignedanuniquesequencenumber(a32-bitbinary)whichiscontiguouswit

6、hthebyte’spositioninthestream.TransportLayer:TCP(3-2)HowTCPresolvethosereliabilityissues?Rule#2:Acknowledgement/resending1)EverytimeTCPsendingablockofbytes(asegment),thesendermustwaituntilexplicitlyacknowledgedbyitsreceiver.2)EverytimeTCPreceivesablockofbytes,thereceivermustexplicitlya

7、cknowledgethesender(byacknowledgenumberwhichvalueissequencenumberofthefirstbytereceived+thenumberofreceivedbytesorequivalently1+sequencenumberofthelastreceivedbyte);3)IfthesenderisNOTacknowledgedafteraperiod(1-2ms),itmustresendthesegmentagain,repeattheprocedureuntilexplicitlyacknowle

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