计算机网络和协议分析3

AdvancedComputerNetworks计算机网络WeihongChenDepartmentofComputerScienceHunanCityUniversitySep,2010ReviewNetworkHardware•Thetransmissiontechniques:Broadcastinglinks,Point-to-pointlinks•Thetypesofthenetworksbyscale:LAN,MAN,WANNetworksoftware•ProtocolHierarchies•DesignIssuesfortheLayers•Connection-OrientedandConnectionlessServices•ServicePrimitives•TheRelationshipofServicestoProtocolsConnection-OrientedandConnectionlessServicesconnection-orientedconnectionlessConnection-orientedserviceismodeledafterthetelephonesystem.Connectionlessserviceismodeledafterthepostalsystem.Itjustsendsthedata.Threephases:setupaconnection,datatransmission,releasetheconnection.TheRelationshipofServicestoProtocolsServicesandprotocolsaredistinctconcepts.Fig.6TherelationshipbetweenaserviceandaprotocolServiceprovidedbylayerkProtocolAserviceisasetofprimitives(operations)thatalayerprovidestothelayeraboveit.Aservicerelatestoaninterfacebetweentwolayers,withthelowerlayerbeingtheserviceproviderandtheupperlayerbeingtheserviceuser.Aprotocol,incontrast,isasetofrulesgoverningtheformatandmeaningofthepackets,ormessagesthatareexchangedbythepeerentitieswithinalayer.Entitiesuseprotocolstoimplementtheirservicedefinitions.Back第一章引言Chapter1IntroductionWeihongChenSpring’10CN4.NetworkSoftware–ProtocolHierarchies–DesignIssuesfortheLayers–Connection-OrientedandConnectionlessServices–ServicePrimitives–TheRelationshipofServicestoProtocolsWeihongChenSpring’10CN–协议层次–典型的设计问题–面向连接和无连接服务–服务原语–服务和协议的关系WeihongChenSpring’10CN4.1ProtocolHierarchiesToreducetheirdesigncomplexity,mostnetworksareorganizedasastackoflayersorlevels,eachonebuiltupontheonebelowit.Thenumberoflayers,thenameofeachlayer,thecontentsofeachlayer,andthefunctionofeachlayerdifferfromnetworktonetwork.Thepurposeofeachlayeristooffercertainservicestothehigherlayers,shieldingthoselayersfromthedetailsofhowtheofferedservicesareactuallyimplemented.Inasense,eachlayerisakindofvirtualmachine,offeringcertainservicestothelayeraboveit.WeihongChenSpring’10CN分层的好处•各层之间是独立的;•灵活性好;•结构上可分割开;•易于实现和维护;•能促进标准化工作;WeihongChenSpring’10CNFig.9ProtocolHierarchiesAprotocolisanagreementbetweenthecommunicatingpartiesonhowcommunicationistoproceed.Itcontainsthreeelements:syntax,meaningandsynchronization.Theentitiescomprisingthecorrespondinglayersondifferentmachinesarecalledpeers.Betweeneachpairofadjacentlayersisaninterface.Asetoflayersandprotocolsiscalledanetworkarchitecture.Inreality,nodataaredirectlytransferredfromlayernononemachinetolayernonanothermachine.Instead,eachlayerpassesdataandcontrolinformationtothelayerimmediatelybelowit,untilthelowestlayerisreached.Belowlayer1isthephysicalmediumthroughwhichactualcommunicationoccurs.InFig.9,virtualcommunicationisshownbydottedlinesandphysicalcommunicationbysolidlines.1010100001110110100110100101Communicationondifferentmachines5432154321Computer1AP2AP1Computer2H5H4H5H4H3H5321H2H4H3H5H5H4H5H4H3H5H2H4H3H5RouterWeihongChenSpring’10CNFig.10Exampleinformationflowsupportingvirtualcommunicationinlayer5WeihongChenSpring’10CN一些概念•对等实体：在不同的系统中位于同一层次上的实体。（不同系统之间的通信实际上就是各对等实体之间的通信。）•协议：对等实体间通信所需遵循的规则的总和。•接口：定义了上层调用下层服务的原语操作。•网络体系结构：层次与协议的集合。•协议栈：将系统使用的协议按层次顺序（每层一个协议）列出而得到的协议列表。•实通信和虚通信。•分层网络中实际的通信过程。WeihongChenSpring’10CN4.2DesignIssuesfortheLayersErrorcontrolisanimportantissuebecausephysicalcommunicationcircuitsarenotperfect.error-detecting,error-correctingFlowcontrol:limitthesendertoanagreed-ontransmissionrate.Multiplexing:usethesameconnectionformultiple,unrelatedconversations.Routing:Whentherearemultiplepathsbetweensourceanddestination,aroutemustbechosen.WeihongChenSpring’10CN4.3Connection-OrientedandConnectionlessServicesconnection-orientedconnectionlessConnection-orientedserviceismodeledafterthetelephonesystem.Connectionlessserviceismodeledafterthepostalsystem.Itjustsendsthedata.Threephases:setupaconnection,datatransmission,releasetheconnection.WeihongChenSpring’10CNEachservicecanbecharacterizedbyaqualityofservice.Connection-orientedserviceisreliable,butConnectionlessserviceisunreliable.Connection-orientedcommunicationhasthreephases.Intheestablishmentphasearequestismadetosetupaconnection.Onlyafterthisphasehasbeensuccessfullycompletedcanthedatatransferphasebestartedanddatatransported.Thencomesthereleasephase.Connectionlesscommunicationdoesnothavethesephases.Itjustsendsthedata.WeihongChenSpring’10CN•面向连接服务–建立连接、传输数据、拆除连接三步曲。类似电话系统服务，保证数据传输顺序。•无连接服务–数据不需要沿着相同的路径传输。类似邮政系统服务，不保证数据传输顺序。•可靠的服务–通过接收方的应答来实现，会引入开销和延迟。WeihongChenSpring’10CN4.5TheRelationshipofServicestoProtocolsServicesandprotocolsaredistinctconcepts.Fig.6TherelationshipbetweenaserviceandaprotocolServiceprovidedbylayerkProtocolAserviceisasetofprimitives(operations)thatalayerprovidestothelayeraboveit.Aservicerelatestoaninterfacebetweentwolayers,withthelowerlayerbeingtheserviceproviderandtheupperlayerbeingtheserviceuser.Aprotocol,incontrast,isasetofrulesgoverningtheformatandmeaningofthepackets,ormessagesthatareexchangedbythepeerentitieswithinalayer.Entitiesuseprotocolstoimplementtheirservicedefinitions.BackWeihongChenSpring’10CNInotherwords,servicesr