计算机网络英文版课件：2-2 Web与HTTP

Web和HTTPWeb/WWWFirstsomejargonWebpageconsistsofobjectsObjectcanbeHTMLfile,JPEGimage,Javaapplet,audiofile,…WebpageconsistsofbaseHTML-filewhichincludesseveralreferencedobjectsEachobjectisaddressablebyaURLExampleURL:/someDept/pic.gifhostnamepathnameHTML–HyperTextMarkupLanguage(a)Theformattedpage.(b)Theformattedpage.HTTPoverviewHTTP:hypertexttransferprotocolWeb’sapplicationlayerprotocolclient/servermodelclient:browserthatrequests,receives,“displays”Webobjectsserver:WebserversendsobjectsinresponsetorequestsPCrunningExplorerServerrunningApacheWebserverMacrunningNavigatorHTTPrequestHTTPrequestHTTPresponseHTTPresponseHTTPoverview(continued)HTTPis“stateless”servermaintainsnoinformationaboutpastclientrequestsUsesTCP:clientinitiatesTCPconnection(createssocket)toserver,port80serveracceptsTCPconnectionfromclientHTTPmessages(application-layerprotocolmessages)exchangedbetweenbrowser(HTTPclient)andWebserver(HTTPserver)TCPconnectionclosedProtocolsthatmaintain“state”arecomplex!pasthistory(state)mustbemaintainedifserver/clientcrashes,theirviewsof“state”maybeinconsistent,mustbereconciledasideHTTPconnectionsNonpersistentHTTPAtmostoneobjectissentoveraTCPconnection.PersistentHTTPMultipleobjectscanbesentoversingleTCPconnectionbetweenclientandserver.NonpersistentHTTPSupposeuserentersURLwww.someS/someDepartment/home.index(containstext,referencesto10jpegimages)time1a.HTTPclientinitiatesTCPconnectiontoHTTPserver(process)atwww.someSonport802.HTTPclientsendsHTTPrequestmessage(containingURL)intoTCPconnectionsocket.MessageindicatesthatclientwantsobjectsomeDepartment/home.index1b.HTTPserverathostwww.someSwaitingforTCPconnectionatport80.“accepts”connection,notifyingclientNon-persistentHTTP(cont.)3.HTTPserverreceivesrequestmessage,formsresponsemessagecontainingrequestedobject,andsendsmessageintoitssockettime4.HTTPserverclosesTCPconnection.5.HTTPclientreceivesresponsemessagecontaininghtmlfile,displayshtml.Parsinghtmlfile,finds10referencedjpegobjects6.

Steps1-5repeatedforeachof10jpegobjectsNon-PersistentHTTP:ResponsetimeDefinitionofRTT:timeforasmallpackettotravelfromclienttoserverandback.Responsetime:oneRTTtoinitiateTCPconnectiononeRTTforHTTPrequestandfirstfewbytesofHTTPresponsetoreturnfiletransmissiontimetotal=2RTT+transmittimetimetotransmitfileinitiateTCPconnectionRTTrequestfileRTTfilereceivedtimetimePersistentHTTPNonpersistentHTTPissues:requires2RTTsperobjectOSoverheadforeachTCPconnectionbrowsersoftenopenparallelTCPconnectionstofetchreferencedobjectsPersistentHTTPserverleavesconnectionopenaftersendingresponsesubsequentHTTPmessagesbetweensameclient/serversentoveropenconnectionclientsendsrequestsassoonasitencountersareferencedobjectaslittleasoneRTTforallthereferencedobjectsHTTPrequestmessagetwotypesofHTTPmessages:request,responseHTTPrequestmessage:ASCII(human-readableformat)GET/somedir/page.htmlHTTP/1.1Host:User-agent:Mozilla/4.0Connection:closeAccept-language:fr(extracarriagereturn,linefeed)

requestline(GET,POST,HEADcommands)headerlinesCarriagereturn,line

feedindicatesendofmessageHTTPrequestmessage:generalformatMethodtypesHTTP/1.0GETPOSTHEADasksservertoleaverequestedobjectoutofresponseHTTP/1.1GET,POST,HEADPUTuploadsfileinentitybodytopathspecifiedinURLfieldDELETEdeletesfilespecifiedintheURLfieldHTTPresponsemessageHTTP/1.1200OKConnectioncloseDate:Thu,06Aug199812:00:15GMTServer:Apache/1.3.0(Unix)Last-Modified:Mon,22Jun1998…...Content-Length:6821Content-Type:text/html

datadatadatadatadata...statusline(protocolstatuscodestatusphrase)headerlinesdata,e.g.,requestedHTMLfileUser-serverstate:cookiesManymajorWebsitesusecookiesFourcomponents:1)cookieheaderlineofHTTPresponsemessage2)cookieheaderlineinHTTPrequestmessage3)cookiefilekeptonuser’shost,managedbyuser’sbrowser4)back-enddatabaseatWebsiteExample:SusanalwaysaccessInternetalwaysfromPCvisitsspecifice-commercesiteforfirsttimewheninitialHTTPrequestsarrivesatsite,sitecreates:uniqueIDentryinbackenddatabaseforIDCookies:keeping“state”(cont.)clientserverusualhttpresponsemsgusualhttpresponsemsgcookiefileoneweeklater:usualhttprequestmsgcookie:1678cookie-specificactionaccessebay8734usualhttprequestmsgAmazonservercreatesID1678forusercreateentryusualhttpresponseSet-cookie:1678ebay8734amazon1678usualhttprequestmsgcookie:1678cookie-spectificactionaccessebay8734amazon1678backenddatabaseCookies(continued)Whatcookiescanbring:authorizationshoppingcartsrecommendationsusersessionstate(Webe-mail)Howtokeep“state”:protocolendpoints:maintainstateatsender/receiverovermultipletransactionscookies:httpmessagescarrystateCookiesandprivacy:cookiespermitsitestolearnalotaboutyouyoumaysupplynameande-mailtositesasideWebcaches(proxyserver)Goal:satisfyclientrequestwithoutinvolvingoriginserverusersetsbrowser:WebaccessesviacachebrowsersendsallHTTPrequeststocacheobjectincache:cachereturnsobjectelsecacherequestsobjectfromoriginserver,thenreturnsobjecttoclientclientProxyserverclientHTTPrequestHTTPresponseHTTPrequestHTTPrequestoriginserveroriginserverHTTPresponseHTTPresponseMoreaboutWebcachingcacheactsasbothclientandservertypicallycacheisinstalledbyISP(university,company,residentialISP)WhyWebcaching?reduceresponsetimeforclientrequestreducetrafficonanin