第15章计算机学科基础

Chapter15,Networks网络,15-2,ChapterGoals,Describethecoreissuesrelatedtocomputernetworks描述与计算机网络相关的核心问题Listvarioustypesofnetworksandtheircharacteristics列出各种类型的网络和它们的特征Explainvarioustopologiesoflocal-areanetworks解释局域网的各种拓扑Explainwhynetworktechnologiesarebestimplementedasopensystems解释为什么用开放式系统实现网络技术,15-3,ChapterGoals,CompareandcontrastvarioustechnologiesforhomeInternetconnections比较家庭Internet连接的各种技术Explainpacketswitching解释包交换Describethebasicrolesofvariousnetworkprotocols说明各种网络协议的基本职责Explaintheroleofafirewall解释防火墙的功能CompareandcontrastnetworkhostnamesandIPaddresses比较网络的主机名和IP地址Explainthedomainnamesystem解释域名系统,15-4,Networking网络,ComputernetworkAcollectionofcomputingdevicesthatareconnectedinvariouswaysinordertocommunicateandshareresources计算机网络是为了通信和共享资源而以各种方式连在一起。Usually,theconnectionsbetweencomputersinanetworkaremadeusingphysicalwiresorcables计算机之间的连接通常是靠物理电线或电缆实现的。However,someconnectionsarewireless,usingradiowavesorinfraredsignals但是，有些连接使用无线电波或红外信号传导数据。,15-5,Networking网络,Thegenerictermnodeorhostreferstoanydeviceonanetwork用通常的术语节点或主机来指网络中的所有设备。DatatransferrateThespeedwithwhichdataismovedfromoneplaceonanetworktoanother数据传输速率：数据从网络中的一点到另一点的速率。Datatransferrateisakeyissueincomputernetworks数据传输速率是计算机网络中的一个关键问题。数据传输速率又称带宽，单位:kbps,mbps,15-6,Networking网络,Computernetworkshaveopenedupanentirefrontierintheworldofcomputingcalledtheclient/servermodel计算机网络开创了一个新的计算领域客户/服务器模式,Figure15.1Client/Serverinteraction,请求,响应,15-7,Networking网络,FileserverAcomputerthatstoresandmanagesfilesformultipleusersonanetwork文件服务器：专用于为网络用户存储和管理文件的计算机。WebserverAcomputerdedicatedtorespondingtorequests(fromthebrowserclient)forwebpagesWeb服务器：专用于响应网页请求的计算机。,15-8,TypesofNetworks网络的类型,Local-areanetwork(LAN)Anetworkthatconnectsarelativelysmallnumberofmachinesinarelativelyclosegeographicalarea局域网(LAN)：是连接在较小地理范围内的少量计算机的网络。,15-9,TypesofNetworks网络类型,Variousconfigurations,calledtopologies,havebeenusedtoadministerLANs管理LAN的各种配置叫做拓扑Ringtopology环形拓扑AconfigurationthatconnectsallnodesinaclosedlooponwhichmessagestravelinonedirectionStartopology星形拓扑AconfigurationthatcentersaroundonenodetowhichallothersareconnectedandthroughwhichallmessagesaresentBustopology总线拓扑Allnodesareconnectedtoasinglecommunicationlinethatcarriesmessagesinbothdirections,TypesofNetworks网络类型,AbustechnologycalledEthernethasbecometheindustrystandardforlocal-areanetworks被称为以太网的总线技术已经成为局域网的业界标准。,Figure15.2Variousnetworktopologies,15-10,15-11,TypesofNetworks网络类型,Wide-areanetwork(WAN)Anetworkthatconnectstwoormorelocal-areanetworksoverapotentiallylargegeographicdistance广域网(WAN)是连接两个或多个相距较远的局域网的网络。OftenoneparticularnodeonaLANissetuptoserveasagatewaytohandleallcommunicationgoingbetweenthatLANandothernetworksLAN中通常会有一个特殊节点作为网关，处理这个LAN和其他网络之间的通信。Communicationbetweennetworksiscalledinternetworking网络之间的通信叫做网际互通。TheInternet,asweknowittoday,isessentiallytheultimatewide-areanetwork,spanningtheentireglobe我们现在熟知的Internet本质上就是一个最大的广域网，遍布整个地球。,15-12,TypesofNetworks网络类型,Metropolitan-areanetwork(MAN)Thecommunicationinfrastructuresthathavebeendevelopedinandaroundlargecities城域网(MAN)是指在大城市内或周边开发的基础通讯设施。,15-13,TypesofNetworks网络类型,Figure15.1Local-areanetworksconnectedacrossadistancetocreateawide-areanetwork,15-14,InternetConnectionsInternet连接,InternetbackboneAsetofhigh-speednetworksthatcarryInternettrafficInternet骨干网：承载Internet通信的一组高速网络。ThesenetworksareprovidedbycompaniessuchasAT&T,GTE,andIBMInternetserviceprovider(ISP)AcompanythatprovidesothercompaniesorindividualswithaccesstotheInternetISP：提供Internet访问的公司。,15-15,InternetConnectionsInternet连接,TherearevarioustechnologiesavailablethatyoucanusetoconnectahomecomputertotheInternet把家里的计算机连接到Internet上的方法有很多。Aphonemodemconvertscomputerdataintoananalogaudiosignalfortransferoveratelephoneline,andthenamodematthedestinationconvertsitbackagainintodata电话调制解调器(Modem)Adigitalsubscriberline(DSL)usesregularcopperphonelinestotransferdigitaldatatoandfromthephonecompanyscentraloffice数字用户线路，如ADSLAcablemodemusesthesamelinethatyourcableTVsignalscomeinontotransferthedatabackandforth线缆调制解调器，有线电视上网,15-16,InternetConnectionsInternet连接,BroadbandAconnectioninwhichtransferspeedsarefasterthan128Kbitspersecond宽带：提供的数据传输速率大于128Kbps的连网技术。DSLconnectionsandcablemodemsarebroadbandconnectionsDSL和Cable上网属于宽带连接Thespeedfordownloads(gettingdatafromtheInternettoyourhomecomputer)maynotbethesameasuploads(sendingdatafromyourhomecomputertotheInternet)下载（从Internet上把数据传到家用计算机上）速度可以和上传（把家用计算机上的数据发送到Internet上）速度不同。,PacketSwitching包交换,Toimprovetheefficiencyoftransferringinformationoverasharedcommunicationline,messagesaredividedintofixed-sized,numberedpackets为了提高在共享线路上传输信息的有效性，消息被分割为大小固定、有编号的包。每个包将独立在网上传输，直到到达目的地，它们将在此被重新组合为原始的消息。这种方法叫作包交换。Networkdevicescalledroutersareusedtodirectpacketsbetweennetworks用于指导包在网络之间传输的设备叫做路由器。,15-18,15-18,PacketSwitching包交换,Figure15.4Messagessentbypacketswitching,15-19,OpenSystems开放系统,Proprietarysystem专有系统Asystemthatusestechnologieskeptprivatebyaparticularcommercialvendor使用特定销售商的私有技术的系统OnesystemcouldntcommunicatewithanotherInteroperability互通性Theabilityofsoftwareandhardwareonmultiplemachinesandfrommultiplecommercialvendorstocommunicate多台机器上的来自多个销售商的软件和硬件互相通信的能力。,15-20,OpenSystems开放系统,Opensystems开放系统Systemsbasedonacommonmodelofnetworkarchitectureandasuiteofprotocolsusedinitsimplementation以网络体系结构的通用模型为基础并且伴有一组协议的系统。,15-21,OpenSystems开放系统,TheInternationalOrganizationforStandardization(ISO)establishedtheOpenSystemsInterconnection(OSI)ReferenceModel开放系统互联参考模型Eachlayerdealswithaparticularaspectofnetworkcommunication每一层处理网络通信的一个特定方面,Figure15.5ThelayersoftheOSIReferenceModel,应用层表示层会话层传输层网络层数据链路层物理层,15-22,NetworkProtocols网络协议,Networkprotocolsarelayeredsuchthateachonereliesontheprotocolsthatunderlieit网络协议被分层，以便每一层都能依靠自己的基础协议。Sometimesreferredtoasaprotocolstack这种分层有时叫做协议栈。,Figure15.6Layeringofkeynetworkprotocols,15-23,TCP/IP,TCPstandsforTransmissionControlProtocolTCP是传输控制协议的缩写。TCPsoftwarebreaksmessagesintopackets,handsthemofftotheIPsoftwarefordelivery,andthenordersandreassemblesthepacketsattheirdestinationTCP负责把消息分割成包，交给IP软件传递，目的地机器上的TCP负责把包排序，重新组合为消息。IPstandsforInternetProtocolIPsoftwaredealswiththeroutingofpacketsthroughthemazeofinterconnectednetworkstotheirfinaldestinationIP软件处理的是包通过相互连接的网络传递到最终目的地的路由选择。,15-24,TCP/IP(cont.),UDPstandsforUserDatagramProtocolUDP是用户数据报协议的所写。ItisanalternativetoTCP它是TCP的替代品ThemaindifferenceisthatTCPishighlyreliable,atthecostofdecreasedperformance,whileUDPislessreliable,butgenerallyfaster主要的不同之处在于TCP牺牲了一定的性能，提供了高度可靠性，而UDP更快，但不那么可靠。,15-25,High-LevelProtocols高层协议,OtherprotocolsbuildonthefoundationestablishedbytheTCP/IPprotocolsuite其他协议都是在TCP/IP协议组建立的基础上构建的SimpleMailTransferProtocol(SMTP)简单邮件传输协议FileTransferProtocol(FTP)文件传输协议Telnet远程登录协议HyperTextTransferProtocol(http)超文本传输协议,15-26,协议和它们的端口,Figure15.7Someprotocolsandtheportstheyuse,15-27,Firewalls防火墙,FirewallAmachineanditssoftwarethatserveasaspecialgatewaytoanetwork,protectingitfrominappropriateaccess防火墙是一台机器，它的软件将作为网络的特殊网关，保护它免受不正当的访问。Filtersthenetworktrafficthatcomesin,checkingthevalidityofthemessagesasmuchaspossibleandperhapsdenyingsomemessagesaltogether过滤到来的网络通信，尽可能地检查消息的有效性，可能会拒绝某些消息。Enforcesanorganizationsaccesscontrolpolicy会强制执行一个组织的访问控制策略,15-28,Firewalls防火墙,Figure15.8AfirewallprotectingaLAN,15-29,NetworkAddresses网络地址,HostnameAuniqueidentificationthatspecifiesaparticularcomputerontheInternet主机名是Internet上的计算机的唯一标识。Forexample,15-30,NetworkAddresses网络地址,NetworksoftwaretranslatesahostnameintoitscorrespondingIPaddress网络软件把主机名翻译成它对应的IP地址Forexample54,15-31,NetworkAddresses网络地址,AnIPaddresscanbesplitintoIP地址分割成：networkaddress,whichspecifiesaspecificnetwork网络地址，指定特定网络hostnumber,whichspecifiesaparticularmachineinthatnetwork主机地址，指定特定网络主机,Figure15.9AnIPaddressisstoredinfourbytes,15-32,DomainNameSystem域名系统,Ahostnameconsistsofthecomputernamefollowedbythedomainname主机名由计算机名加域名构成isthedomainnameAdomainnameisseparatedintotwoormoresectionsthatspecifytheorganization,andpossiblyasubsetofanorganization,ofwhichthecomputerisapartTwoorganizationscanhaveacomputernamedthesamethingbecausethedomainnamemakesitclearwhichoneisbeingreferredto,15-33,DomainNameSystem,Theverylastsectionofthedomainiscalleditstop-leveldomain(TLD)name,Figure15.10Top-leveldomains,includ