中科大,计算机网络课件07.ppt

TCP IPProtocolSuite 1 Chapter7 Uponcompletionyouwillbeableto ARPandRARP UnderstandtheneedforARPUnderstandthecasesinwhichARPisusedUnderstandthecomponentsandinteractionsinanARPpackageUnderstandtheneedforRARP Objectives TCP IPProtocolSuite 2 Figure7 1ARPandRARP TCP IPProtocolSuite 3 Figure7 2PositionofARPandRARPinTCP IPprotocolsuite TCP IPProtocolSuite 4 7 1ARP ARPassociatesanIPaddresswithitsphysicaladdress Onatypicalphysicalnetwork suchasaLAN eachdeviceonalinkisidentifiedbyaphysicalorstationaddressthatisusuallyimprintedontheNIC Thetopicsdiscussedinthissectioninclude PacketFormatEncapsulationOperationProxyARP TCP IPProtocolSuite 5 Figure7 3ARPoperation TCP IPProtocolSuite 6 Figure7 4ARPpacket 硬件类型表示硬件地址的类型 以太网为1 协议类型字段表示要映射的协议地址类型 IP协议对应的值为0 x0800 硬件地址长度 以太网为6 协议地址长度 以太网为4 即IP地址长度 IPv4 ipv6添16 这里单位是字节 操作字段 ARP请求 1 ARP应答 2 RARP请求 3 RARP应答 4 当系统收到一份目的端为本机的ARP请求报文时 它就把硬件地址填进去 然后用两个目的端地址分别替换两个发送端地址 源和目的地址交换 并把操作字段置为2 最后把它发送出去 TCP IPProtocolSuite 7 TCP IPProtocolSuite 8 Figure7 5EncapsulationofARPpacket 目的地址 源地址 类型 CRC 数据 46 1500字节 6 6 2 4 0800IP数据报 46 1500字节 2 0806ARP请求 应答PAD 0835RARP请求 应答PAD 2 28 18 类型 TCP IPProtocolSuite 9 TCP IPProtocolSuite 10 含有ARP消息的以太网帧 TCP IPProtocolSuite 11 含有ARP消息的以太网帧 129 84 25 02 TCP IPProtocolSuite 12 Figure7 6FourcasesusingARP TCP IPProtocolSuite 13 AnARPrequestisbroadcast anARPreplyisunicast Note TCP IPProtocolSuite 14 AhostwithIPaddress130 23 43 20andphysicaladdressB2 34 55 10 22 10hasapackettosendtoanotherhostwithIPaddress130 23 43 25andphysicaladdressA4 6E F4 59 83 AB whichisunknowntothefirsthost ThetwohostsareonthesameEthernetnetwork ShowtheARPrequestandreplypacketsencapsulatedinEthernetframes Example1 SeeNextSlide TCP IPProtocolSuite 15 SolutionFigure7 7showstheARPrequestandreplypackets NotethattheARPdatafieldinthiscaseis28bytes andthattheindividualaddressesdonotfitinthe4 byteboundary Thatiswhywedonotshowtheregular4 byteboundariesfortheseaddresses AlsonotethattheIPaddressesareshowninhexadecimal ForinformationonbinaryorhexadecimalnotationseeAppendixB Example1 Continued SeeNextSlide TCP IPProtocolSuite 16 Figure7 7Example1 TCP IPProtocolSuite 17 Figure7 8ProxyARP TCP IPProtocolSuite 18 7 2ARPPACKAGE Inthissection wegiveanexampleofasimplifiedARPsoftwarepackagetoshowthecomponentsandtherelationshipsbetweenthecomponents ThisARPpackageinvolvesfivemodules acachetable queues anoutputmodule aninputmodule andacache controlmodule Thetopicsdiscussedinthissectioninclude CacheTableQueuesOutputModuleInputModuleCache ControlModule TCP IPProtocolSuite 19 Figure7 9ARPcomponents TCP IPProtocolSuite 20 Table7 1Originalcachetableusedforexamples TCP IPProtocolSuite 21 输出模块 算法 睡眠 直到从IP软件收到IP分组 检查高速缓存表 查找对应该IP分组的目的端的项目 若找到若状态是RESOLVED从项目中提取硬件地址值 将分组连同硬件地址一起发送到数据链路层 返回 若状态是PENDINGpending把分组放入相应的队列 返回 若未找到创建一个高速缓存项目 状态置为P且尝试置1 创建一个队列将分组放入队列 发送ARP请求 返回 TCP IPProtocolSuite 22 输入模块 算法 睡眠 直到ARP分组 请求或回答 到达 检查高速缓存表 查询对应该ARP分组的项目 若找到更新这个项目 若状态是PENDING当队列非空把一个分组从队列中取出 将分组连同硬件地址一起发送给数据链路层 若找不到创建一个项目 在高速缓存表中添加该项目 若分组是请求发送ARP回答 返回 TCP IPProtocolSuite 23 高速缓存控制模块 算法 睡眠 直到周期性计时器到时间 对高速缓存表中的每一个项目若状态为FREE继续 若状态为PENDING把尝试值加1 若尝试值大于最大数把状态改为FREE 撤销相应的队列 否则发送ARP请求 继续若状态为RESOLVED把超时字段的值减去已经过的时间 若超时字段的值小于或等于零把状态改为FREE 撤销相应的队列 返回 TCP IPProtocolSuite 24 TheARPoutputmodulereceivesanIPdatagram fromtheIPlayer withthedestinationaddress114 5 7 89 ItchecksthecachetableandfindsthatanentryexistsforthisdestinationwiththeRESOLVEDstate Rinthetable Itextractsthehardwareaddress whichis457342ACAE32 andsendsthepacketandtheaddresstothedatalinklayerfortransmission Thecachetableremainsthesame Example2 TCP IPProtocolSuite 25 Twentysecondslater theARPoutputmodulereceivesanIPdatagram fromtheIPlayer withthedestinationaddress116 1 7 22 Itchecksthecachetableanddoesnotfindthisdestinationinthetable ThemoduleaddsanentrytothetablewiththestatePENDINGandtheAttemptvalue1 Itcreatesanewqueueforthisdestinationandenqueuesthepacket ItthensendsanARPrequesttothedatalinklayerforthisdestination ThenewcachetableisshowninTable7 2 Example3 SeeNextSlide TCP IPProtocolSuite 26 Table7 2UpdatedcachetableforExample3 TCP IPProtocolSuite 27 Fifteensecondslater theARPinputmodulereceivesanARPpacketwithtargetprotocol IP address188 11 8 71 Themodulechecksthetableandfindsthisaddress ItchangesthestateoftheentrytoRESOLVEDandsetsthetime outvalueto900 Themodulethenaddsthetargethardwareaddress E34573242ACA totheentry Nowitaccessesqueue18andsendsallthepacketsinthisqueue onebyone tothedatalinklayer ThenewcachetableisshowninTable7 3 Example4 SeeNextSlide TCP IPProtocolSuite 28 Table7 3UpdatedcachetableforExample4 TCP IPProtocolSuite 29 Twenty fivesecondslater thecache controlmoduleupdateseveryentry Thetime outvaluesforthefirstthreeresolvedentriesaredecrementedby60 Thetime outvalueforthelastresolvedentryisdecrementedby25 Thestateofthenext to thelastentryischangedtoFREEbecausethetime outiszero Foreachofthethreependingentries thevalueoftheattempts Example5 SeeNextSlide TCP IPProtocolSuite 30 Table7 4UpdatedcachetableforExample5 TCP IPProtocolSuite 31 7 3RARP RARPfindsthelogicaladdressforamachinethatonlyknowsitsphysicaladdress Thetopicsdiscussedinthissectioninclude PacketFormatEncapsulationRARPServerAlternativeSolutionstoRARP TCP IPProtocolSuite 32 TheRARPrequestpacketsarebroadcast theRARPreplypacketsareunicast Note TCP IPProtocolSuite 33 Figure7 10RARPoperation TCP IPProtocolSuite 34 Figure7 11RARPpacket TCP IPProtocolSuite 35 Figure7 12EncapsulationofRARPpacket TCP IPProtocolSuite 36 AlternativeSolutionstoRARPWhenadisklesscomputerisbooted itneedsmoreinformationinadditiontoitsIPaddress Itneedstoknowitssubnetmask theIPaddressofarouter andtheIPaddressofanam