ComputerNetworking

Chapter#3

Dr.AbdulrhamanAlameer

TransportLayer

Transportlayerprovideslogicalcommunicationbetweenapplicationprocessrunningondifferenthost.Applicationprocessesusethelogicalcommunicationprovidedbythetransportlayertosendmessagestoeachother.

• Whataretransport-layerservices

– MultiplexingandDemultiplexing

– Principlesofreliabledatatransfer:Making sure that the packets sent by the sender are correctly and reliably received by the receiver.

– Connectionlesstransport:User Datagram Protocol (UDP).Point-to-multiple:onesender,manyreceivers.Itisunreliabledatatransfer.NohandshakingbetweenUDPsender,receiverandnocongestioncontrol:UDPcanblastawayasfastasdesired.

– EachUDPsegmenthandledindependentlyofothers.– UDPsegmentsmaybe:lostordeliveredout-of-ordertoapp.

– Connection-orientedtransport:Transmission Control Protocol(TCP).Point-to-point:onesender,onereceiver.Itis:

§ Reliabledatatransfer§ Flowcontrol:Senderwillnotoverwhelmreceiver§ Connectionmanagement§ Fullduplexdata:Bi-directionaldataflowinsameconnection.

§ Connection-oriented:Handshaking(exchangeofcontrolmsgs)initssender,receiverstatebeforedataexchange.

§ Pipelined:TCPcongestionandflowcontrolsetwindowsize

• Reliability Technique

- Stop-and-waitprotocol:Itis a method in telecommunications to send information between two connected devices.

- Pipelinedprotocols:It is a technique in which multiple requests are written out to a single socket without waiting for the corresponding responses.

v Therearetwogenericformsofpipelinedprotocols:Go-Back-N,selectiverepeat

Go-Back-N:

v SendercanhaveuptoNunackedpacketsinpipelinev Receiveronlysendscumulativeack,doesn’tackpacketifthere’sagapv Senderhastimerforoldestunackedpacketwhentimerexpires,retransmit

allunackedpackets.

v k-bitseq#inpktheaderv “window”ofuptoN,consecutiveunack’edpktsallowedv ACK(n):ACKsallpktsupto,includingseq#n-“cumulativeACK”v mayreceiveduplicateACKs(seereceiver)v timerforoldestin-flightpktv timeout(n):retransmitpacketnandallhigherseq#pktsinwindowv ACK-only:alwayssendACKforcorrectly-receivedpktwithhighestin-order

seq#v maygenerateduplicateACKsv needonlyrememberexpectedseqnumv out-of-orderpkt:discard(don’tbuffer):noreceiverbuffering!Andre-ACKpkt

withhighestin-orderseq#

Selectiverepeat:

v receiverindividuallyacknowledgesallcorrectlyreceivedpkts§ bufferspkts,asneeded,foreventualin-orderdeliverytoupperlayer

v senderonlyresendspktsforwhichACKnotreceived§ sendertimerforeachunACKedpkt

v senderwindow§ Nconsecutiveseq#’s§ limitsseq#sofsent,unACKedpkts

§ ConnectionManagement

- Beforeexchangingdata,sender/receiver“handshake”:• Agreetoestablishconnection(eachknowingtheotherwillingtoestablish

connection)• Agreeonconnectionparameters

§ TCP:retransmissionscenarios2-wayhandshake:

2-way handshake failure scenarios:

TCP3-wayhandshake