计算机网络：ICMP协议

ICMP协议IP协议的缺陷ErrorReporting/ErrorcorrectingmechanismAssistancemechanism–queryInformationfromanotherhostorrouter.ICMP协议的层次ICMP消息差错报告消息查询消息ICMP消息格式差错报告消息IP不关心差错检查和差错控制ICMP只报告差错，不纠错，纠错的处理？差错消息总是被发送给源端。（？）差错报告消息类型差错报告消息对差错报告消息，不会生成差错报告消息去响应。即一个差错报告消息独立完成一个任务。对分片报文，只有第一个分片引发差错报告消息对多播报文，不会产生差错报文对环回地址不会产生差错报文差错消息内容为什么要加IP报文数据部分的8个字节UDP和TCP报文头格式目的不可达消息报文Code0.Thenetworkisunreachable,possiblyduetohardwarefailure.Code1.Thehostisunreachable.Thiscanalsobeduetohardwarefailure.Code2.Theprotocolisunreachable.AnIPdatagramcancarrydatabelongingto

higher-levelprotocolssuchasUDP,TCP,andOSPF.Ifthedestinationhost

receives

adatagramthatmustbedelivered,forexample,totheTCPprotocol,but

theTCPprotocolisnotrunningatthemoment,acode2messageissent.Code3.Theportisunreachable.Theapplicationprogram(process)thatthedatagramisdestinedforisnotrunningatthemoment.Code4.Fragmentationisrequired,buttheDF(donotfragment)fieldofthedatagram

hasbeenset.Inotherwords,thesenderofthedatagramhasspecifiedthatthe

datagramnotbefragmented,butroutingisimpossiblewithoutfragmentation.Code5.Sourceroutingcannotbeaccomplished.Inotherwords,oneormorerouters

definedinthesourceroutingoptioncannotbevisited.Code6.Thedestinationnetworkisunknown.Thisisdifferentfromcode0.In

code0,therouterknowsthatthedestinationnetworkexists,butitis

unreachable

atthemoment.Forcode6,therouterhasnoinformationaboutthedestination

network.Code7.Thedestinationhostisunknown.Thisisdifferentfromcode1.Incode1,

therouterknowsthatthedestinationhostexists,butitisunreachableatthemoment.Forcode7,therouterisunawareoftheexistenceofthedestinationhost.Code8.Thesourcehostisisolated.Code9.Communicationwiththedestinationnetworkisadministratively

prohibited.Code10.Communicationwiththedestinationhostisadministrativelyprohibited.Code11.Thenetworkisunreachableforthespecifiedtypeofservice.Thisisdifferent

fromcode0.Heretheroutercanroutethedatagramifthesourcehad

requestedanavailabletypeofservice.Code12.Thehostisunreachableforthespecifiedtypeofservice.Thisisdifferent

fromcode1.Heretheroutercanroutethedatagramifthesourcehadrequestedan

availabletypeofservice.Code13.Thehostisunreachablebecausetheadministratorhasputafilteronit.Code14.Thehostisunreachablebecausethehostprecedenceisviolated.The

messageissentbyaroutertoindicatethattherequestedprecedenceisnotpermitted

forthedestination.Code15.Thehostisunreachablebecauseitsprecedencewascutoff.Thismessage

isgeneratedwhenthenetworkoperatorshaveimposedaminimumlevelofprecedencefortheoperationofthenetwork,butthedatagramwassentwithaprecedence

belowthislevel.SourceQuench消息流控制和拥赛控制报文格式TimeExceeded消息ParameterProblem消息IP头部歧义会引起严重问题。路由器或主机发现任何头部歧义问题时，会丢弃此数据报，并向源端报告。0头部字段有歧义1头部要求的选项丢失Redirection消息报文格式Code0.Redirectionforanetwork-specificroute.Code1.Redirectionforahost-specificroute.Code2.Redirectionforanetwork-specificroutebasedonaspecifiedtypeof

service.Code3.Redirectionforahost-specificroutebasedonaspecifiedtypeofservice.查询消息用于诊断网络问题现在只使用两组：echorequestechoreply

timestamprequesttimestampreply.echo消息Timestamp消息两个测试工具PINGTRACEROUTE作业111、主机A发送一个时间戳请求消息给主机B，但是没有收到响应，给出三个原因及主机或路由器对其采取的处理。2、主机A发送一个IP数据报给主机B。主机B没接收到任何数据报，主机A也没接收到任何失败的