计算机网络英文版1

Chapter1IntroductionLi-pingShen申丽萍Email:[e-mail]《ComputerNetworkv4》cs.sjtu2004-11-3TopicszWhatisComputerNetworkszUsesofComputerNetworkszComponentsofComputerNetworkszReferenceModelofComputerNetworkszBasicConceptsofDataTransmission《ComputerNetworkv4》cs.sjtu2004-11-3WhatisComputerNetworkzCollectionofautonomouscomputersinterconnectedbyasingletechnology.zTheoldmodelofalargecomputerlinkingmanyterminals,servingalloftheorganization’scomputationalneedsisnowtotallyobsolete.zDistributedSystemisasoftwaresystembuiltontopofanetwork,e.g.《ComputerNetworkv4》cs.sjtu2004-11-3UsesofComputerNetworkszBusinessApplicationszResourceSharingzE-mailzVideoconferencingzE-commercezHomeApplicationszAccesstoremoteinformationzPerson-to-personcommunicationzInteractiveentertainmentzElectroniccommercezMobileUsers:notebook,PDA《ComputerNetworkv4》cs.sjtu2004-11-3client-servermodel《ComputerNetworkv4》cs.sjtu2004-11-3peer-to-peerModelzInpeer-to-peersystemtherearenofixedclientsandservers.《ComputerNetworkv4》cs.sjtu2004-11-3ComponentsofComputerNetworkszHardware:howyoucanconfigureabunchofcomputersintoanetwork:zLocalAreaNetworks(LAN)zMetropolitanAreaNetworks(MAN)zWideAreaNetworks(WAN)zInternetworkszSoftware:Thisiswhatactuallymakescomputernetworks–notthehardware!zProtocols:describehowtwocommunicatingpartiesexchangeinformation.zServices:describewhatanetworkofferstopartiesthatwanttocommunicate.zInterfaces:describehowaclientcanmakeuseofnetworkservices,i.e.howtheservicescanbeaccessed.《ComputerNetworkv4》cs.sjtu2004-11-3NetworkHardwarezTypesoftransmissiontechnologyzBroadcastnetwork:asinglecommunicationchannelissharedbyallcomputers=sendingapacketimpliesthatallothersreceiveit.zPoint-to-pointnetwork:Computersareconnectedinpairs=sendingapacketgoesstrictlyfromthesendertothereceiver,possiblyhavingtovisitintermediatemachines(routing).《ComputerNetworkv4》cs.sjtu2004-11-3Classificationofnetworkbyscale《ComputerNetworkv4》cs.sjtu2004-11-3LocalAreaNetworkszApartfromscale,LANsdistinguishthemselvesfromothernetworksby(generally)usingbroadcasttechnology,zandhavingsimpletopologies:《ComputerNetworkv4》cs.sjtu2004-11-3MetropolitanAreaNetworksAmetropolitanareanetworkbasedoncableTV.《ComputerNetworkv4》cs.sjtu2004-11-3WideAreaNetworkszAstreamofpacketsfromsendertoreceiver.《ComputerNetworkv4》cs.sjtu2004-11-3InternetworkszTheassumptionsofaristhatanetworkishomogeneous:thereishardlyanyvariationinhardwareandsoftware.Inpractice,largenetworkscanonlybeconstructedbyinterconnectingdifferentkindsofnetworks=internet(work).zConnectingLANstoeachotherthroughaWAN(thinkofenterprisenetworksformultinationals).zConnectingWANstoeachother(theInternet).《ComputerNetworkv4》cs.sjtu2004-11-3NetworkSoftwarezProtocolszLayeringzConnection-OrientedandConnectionlessServiceszTheRelationshipofServicestoProtocols《ComputerNetworkv4》cs.sjtu2004-11-3TheNeedforProtocolszBasiccommunicationhardwareconsistsofmechanismsthatcantransferbitsfromonepointtoanother.(cumbersomeandinconvenient)zApplicationprogramsthatuseanetworkdon’tinteractdirectlywithnetworkhardware.Instead,theyinteractwithprotocolsoftwarethatfollowstherulesofagivenprotocol.zAnagreementthatspecifiestheformatandmeaningofmessagescomputersexchangeisknownasacommunicationprotocol,whichhandlesmostlow-levelcommunicationdetails《ComputerNetworkv4》cs.sjtu2004-11-3WhyLayeringzmostnetworksoftwareareorganizedasastackoflayersorlevels,eachonebuiltupontheonebelowit.zToreducedesigncomplexity,dividethecommunicationproblemintosubpiecesandtodesignaseparateprotocolforeachsubpiece,makingeachprotocoleasiertodesign,analyzeandimplement.zIndependence.Eachlayercouldbedesignedindependently,aslongaskeepinmindtheservicesthelowerlayerprovidesforitandtheservicesitshouldprovidesfortheupperlayer.zFlexibility.Allowsubsetsofprotocolsbeusedasneededandallowanyoneoftheprotocolsbereplacedorupdated.《ComputerNetworkv4》cs.sjtu2004-11-3LayeringModelStackLayer3SAPzLayers,protocols,services,interfacesandstack.《ComputerNetworkv4》cs.sjtu2004-11-3ConceptsofLayeringzTwopartiesatdifferentsites,butatthesamelevel,alwaysagreeonhowtheywillexchangeinformation=specifiedinaprotocol.Example:weallagreetospeakEnglish.zInorderforonepartytosendandreceiveinformation,itcanonlymakeuseofthecommunicationservicesofferedbythelayerdirectlyunderneathit.Example:Theuseofinterpretersinnegotiationsbetweencountries.zServicesofferedbyalayerarealwaysfullyspecifiedintermsofaninterfacethatmakesthoseservicesaccessible.Example:phoneshavebuttonsthatallowyouto“dial”anumber.《ComputerNetworkv4》cs.sjtu2004-11-3HowLayeredSoftwareWorkszMultiple,nestedHeaderszProtocolsoftwareinagivenlayeronthesendingcomputeraddsinformation(header)totheoutgoingdata,andsoftwareinthesamelayeronthereceivingcomputerusestheheadertoprocessincomingdata.zOutgoingdatapassesdownthrougheachlayer,withheadersadded,andincomingdatapassesupthrougheachlayer,withheadersverifiedandremoved.zLayeringPrinciple:LayerNsoftwareonthedestinationcomputermustreceiveexactlythemessagesentbylayerNsoftwareonthesendingcomputer.Demo《ComputerNetworkv4》cs.sjtu2004-11-3ServicestoProtocolsRela