Network Engineering English - Assignment 1-Layers in the TCP_IP Model.doc

《Network Engineering English - Assignment 1-Layers in the TCP_IP Model.doc》由会员上传分享，免费在线阅读，更多相关内容在行业资料-天天文库。

1、LayersintheTCP/IPModelTCP/IP模型中的分层Thelayersnearthetoparelogicallyclosertotheuserapplication,whilethosenearthebottomarelogicallyclosertothephysicaltransmissionofthedata.Viewinglayersasprovidingorconsumingaserviceisamethodofabstractiontoisolateupperlayerprotocolsfromthedetailoft

2、ransmittingbitsover,forexample,Ethernetandcollisiondetection,whilethelowerlayersavoidhavingtoknowthedetailsofeachandeveryapplicationanditsprotocol.在逻辑上，接近于顶部的层次对用户更加亲和，而接近于底部的层次对于数据的物理传输更加亲和。有一种方法是将层次视为可以提供并使用服务的，这样就可以抽象地隔离传输比特细节的上层协议，例如，以太网与碰撞检测，此时低分层不必须了解任何设备及其协议。Thisabstrac

3、tionalsoallowsupperlayerstoprovideservicesthatthelowerlayerscannot,orchoosenotto,provide.Again,theoriginalOSIReferenceModelwasextendedtoincludeconnectionlessservices.Forexample,IPisnotdesignedtobereliableandisabesteffortdeliveryprotocol.Thismeansthatalltransportlayerimplementa

4、tionsmustchoosewhetherornottoprovidereliabilityandtowhatdegree.UDPprovidesdataintegrity(viaachecksum)butdoesnotguaranteedelivery;TCPprovidesbothdataintegrityanddeliveryguarantee(byretransmittinguntilthereceiveracknowledgesthereceptionofthepacket).此抽象概念也允许上层提供服务，然而低层不可提供服务，或选择型

5、不可提供服务。此外，原始的OSI参照模型可被扩展用于包含非面向连接的服务。例如，IP协议并非设计为可以靠的，但却是一种最有效的传输协议。那意味着所有的传输层的实现必须选择是否拥有可依靠性及可依靠性的级别。UDP协议提供数据的保存（通过校检和），但并不保证传输；TCP供数据的保存，也保证数据的传输（通过中继站的转发，直至接收方承认包的获取时中继站进行转发）。LinkLayer链路层TheLinkLayeristhenetworkingscopeofthelocalnetworkconnectiontowhichahostisattached.This

6、regimeiscalledthelinkinInternetliterature.ThisisthelowestcomponentlayeroftheInternetprotocols,asTCP/IPisdesignedtobehardwareindependent.AsaresultTCP/IPhasbeenimplementedontopofvirtuallyanyhardwarenetworkingtechnologyinexistence.链路层是指某个主机所依附的局域网的网络范围。这个制度被称为是网络的连接。这是互联网协议的最低组成部

7、分，例如TCP/IP协议是独立于硬件的。结果就是TCP/IP协议可以在现今几乎所有的硬件网络技术上实现。TheLinkLayerisusedtomovepacketsbetweentheInternetLayerinterfacesoftwodifferenthostsonthesamelink.Theprocessesoftransmittingandreceivingpacketsonagivenlinkcanbecontrolledinthesoftwaredevicedriverforthenetworkcard,aswellasonfir

8、mwareorspecializedchipsets.Thesewillperformdatalinkfunctionss