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1、第四章 传输层WWW clienthost CSource IP: CDest IP: BSource IP: CDest IP: B数据通信与计算机网络Lecture 4WWW clienthost AWWWserver Bsource port: xdest. port: 80source port: ydest. port: 80port use: WWW serverSource IP: ADest IP: Bsource port: xdest. port: 80西安工业大学学习要点：What You Will Learn In This Lecture The Transport
2、Layer is concerned about getting packets from source to destination in a reliable, cost-effectivemanner. 了解传输层在网络工作过程中的作用； 了解QoS的概念及传输层与网络层服务的关系； 了解传输层协议的分类； 理解网络环境中实现分布式进程通信的基本方法； 理解传输层协议设计的基本问题； 掌握TCP/UDP协议的基本内容； 掌握传输层、网络层的内部工作过程。1 TheTransportService：Overviewofthislayer2 Elements of Transport Pro
3、tocols：Jobs done in this layer3 TCP and UDP：Description of popular protocols一.传输层基本概念 传输层在网络中的作用：克服通信子网自身难以克服的问题，改善和优化通信子网的服务质量，使得通信子网对网络用户是透明的；AL ALPL PL面向SLTL TLNL NL NL NLDLL DLL DLL DLLPhL PhL PhL PhLPL信息处理面向通信传输层使通信子网对用户是透明的Transport services Providelogical communication between app processes r
4、unning on different hosts Transport protocols run in end systems sender side: breaks messages from app layer into segments, passes to network layerapplicationtransportnetworkdata linkphysicalnetworknetworkdata linkphysicalnetworkdata linkphysical receiver side: reassembles segments into messages, pa
5、sses to app layer More than one transport protocol available to applications Famous Internet Transport Protocols: TCP and UDPapplicationtransportnetworkdata linkphysicalnetworkdata linkphysicalnetworkdata linkphysicaldata linkphysical服务与服务质量QoS(Quality of Service) 连接建立延时 Connection Establishment Del
6、ay Time between request for aconnectionanditsacknowledgment. Becomingincreasinglyimportantforweb-relatedactivity. 释放延时； DisconnectDelay 连接建立失败概率 Connection Establishment Failure Probability Chancethattheconnectionwontbeestablishedwithinadefinedtime. (Acongestionissue.) 释放失败概率 Disconnect FailureProba
7、bility 传输延时 Transit Delay Time between when data is sent and when itsreceived(usertouser). Again,thisdependsonhostCPUtimeandsubnetcongestion.Quality of Service con.吞吐率Throughput: Number of bytes of data transferred pertimeunit. ThisisafactorofhostCPUtime(onbenchmarktests)andcongestion,windowsize,and
8、numberofhops,(inreallife).残留误码率Residual Error Ratio: What fraction of packetsare lost or garbled. This is from the point of view of the user -hopefully there are none, but in practice there is some none-zeroprobabilityoferror.数据安全防护Protection: The ability to provide securityagainstathirdpartyreading
9、ormodifyingthetransmitteddata.Quality of Service con.优先权Priority: A way of designating that when congestionoccurs,somemessagesshouldbegivenmorefavorabletreatmentthanothers.恢复能力Resilience: The probability that the transport layeritselfwillmessupandterminateaconnection. Italsorelatestotheabilityofatra
10、nsport layertorespond toproblems andgivegracefulrecovery.传输层协议的复杂性与网络层提供服务质量的关系传输层的服务质量与网络层协议及网络层服务质量直接相关；高 层 协 议传输层协议传输层协议高质量的网络服务低质量的网络服务传输层服务网络层服务2.通信子网与传输层 网络层服务-面向连接服务；无连接服务； 网络层服务质量分类-A类-面向连接，可靠通信；B类-面向连接，不可靠通信；C类-无连接，不可靠通信； 传输层利用网络层服务； 传输层服务质量取决于传输层协议类型；传输层协议类型取决于网络层服务质量；IP协议所提供的服务质量 Network
11、Layer (NL) provides “Best Effort” service: 尽力传送 NL will “attempt” to deliver a segment to the requested destination, but makes no guaranty 不保证 To guaranty delivery, keep sending and ask for acknowledgment (ACKs themselves are delivered via “Best Effort”)端端应答Best Effort leaves a lot to be desired: 无法
12、实现 segments can be delayed, lost, reordered (with respect to other transmitted packets) 延迟、丢失、乱序 sender and receiver flow capabilities can be mismatched (e.g., sender can send packets faster than receiver can buffer)流量 high transmission rates can induce network congestion doesnt (want to know) which
13、 transport-layer segments belong to which message拥塞Transport Layer provides services (protocols) to handle these problems传输层协议类型分类原则-QoS（OSI）0类：简单，面向A类子网1类：基本错误恢复，面向B类子网2类：多路复用，面向A类子网3类：错误恢复与多路复用，面向B类子网4类：错误检查与恢复，面向C类子网A类-面向连接，可靠通信；B类-面向连接，不可靠通信；C类-无连接，不可靠通信；3.传输层功能数据链路层 点点连接传 输 层 端端连接共性：差错控制/流量控制/报
14、文排序/连接管理传输层基本功能：*传输连接管理 建立传输连接 报文传输 释放传输连接*流量控制*多路复用The Transport Service VS Data Link LayerDifferencesbetweenthetransportandlowerlayers:1. UserOriented. 面向用户Application programmers interact directly with the transport layer; from theprogrammers perspective, the transport layer is the network. The t
15、ransport layershould be oriented more towards user services. Should not simply reflect what theunderlyinglayershappentoprovide. ProvidesanAPI.2.NegotiationofQualityandTypeofServices.服务质量和类型The user and transport protocol may need to negotiate as to the quality or type ofservice to be provided. A use
16、r may want to negotiate such options as: throughput,delay,protection,priority,reliability,etc.3.GuaranteedService.传输层服务The transport layer may have to overcome service deficiencies of the lower layers (e.g.providingreliableserviceoveranunreliablenetworklayer).The Transport ServiceVS Data Link Layer4. AddressResolution.地址解决方案Naming and
