03_6-3_Connection_Establishment

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1、ComputerNetworksConnectionEstablishment(§6.5.6,§6.5.7,§6.2.3)DavidWetherall(djw@uw.edu)ProfessorofComputerScience&EngineeringTopic•Howtosetupconnections–We’llseehowTCPdoesitSYN!ACK!SYNACK!NetworkComputerNetworks2ConnectionEstablishment•Bothsenderandreceivermust

2、bereadybeforewestartthetransferofdata–Needtoagreeonasetofparameters–e.g.,theMaximumSegmentSize(MSS)•Thisissignaling–Itsetsupstateattheendpoints–Like“dialing”foratelephonecallComputerNetworks3Three-WayHandshake•UsedinTCP;opensconnectionforActivepartyPassiveparty

3、datainbothdirections(client)(server)•EachsideprobestheotherwithafreshInitialSequenceNumber(ISN)–SendsonaSYNchronizesegment–EchoonanACKnowledgesegment•ChosentoberobustevenagainstdelayedduplicatesComputerNetworks4Three-WayHandshake(2)•Threesteps:ActivepartyPassiv

4、eparty(client)(server)–ClientsendsSYN(x)1–ServerreplieswithSYN(y)ACK(x+1)–ClientreplieswithACK(y+1)2–SYNsareretransmittediflost3•SequenceandacknumberscarriedonfurthersegmentsTimeComputerNetworks5Three-WayHandshake(3)•Supposedelayed,duplicateActivepartyPassivepa

5、rty(client)(server)copiesoftheSYNandACKarriveattheserver!–Improbable,butanyhow…ComputerNetworks6Three-WayHandshake(4)•Supposedelayed,duplicateActivepartyPassiveparty(client)(server)copiesoftheSYNandACKarriveattheserver!–Improbable,butanyhow…X•Connectionwillbecl

6、eanlyREJECTXrejectedonbothsides☺REJECTComputerNetworks7TCPConnectionStateMachine•Capturesthestates(rectangles)andtransitions(arrows)–A/BmeanseventAtriggersthetransition,withactionBBothpartiesruninstancesofthisstatemachineComputerNetworks8TCPConnections(2)•Follo

7、wthepathoftheclient:ComputerNetworks9TCPConnections(3)•Andthepathoftheserver:ComputerNetworks10TCPConnections(4)•Again,withstates…Activeparty(client)Passiveparty(server)CLOSED1CLOSEDSYN_SENTLISTEN2SYN_RCVDESTABLISHED3TimeESTABLISHEDComputerNetworks11TCPConnecti

8、ons(5)•Finitestatemachinesareausefultooltospecifyandcheckthehandlingofallcasesthatmayoccur•TCPallowsforsimultaneousopen–i.e.,bothsidesopenatonceinsteadoftheclient-serverpatt