嵌入式系统的网络服务器外文文献翻译Word格式.docx
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嵌入式系统的网络服务器外文文献翻译Word格式.docx
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“everything-in-the-Internet-wave”.Themostcoffee,vendingandwashingmachinesarestillnotavailableabouttheworldwidenet.HowevertheembeddedInternetintegrationforremotemaintenanceanddiagnosticaswellastheso-calledM2Mcommunicationisgrowingwithaconsiderablespeedrate.
JusttheremotemaintenanceanddiagnosticofcomponentsandsystemsbyWebbrowsersviatheInternet,oralocalIntranethasaveryhighweightformanydevelopmentprojects.InnumerousdevelopmentdepartmentspeopleworkoncompletelyWebbasedconfigurationsandservicesforembeddedsystems.TheremainingdaysoftheclassicuserinterfacemadebyasmallLC-displaywithfrontpanelandafewfunctionkeysareover.ThroughfutureevolutionsinthefieldofthemobileInternet,Bluetooth-basedPANs(PersonalAreaNetwork'
s)andtherapidlygrowingM2Mcommunication(M2M=Machine-to-Machine)afurtherinnovatingadvanceistobeexpected.
ThecentralfunctionunittogetaccessonanembeddedsystemviaWebbrowseristheWebserver.SuchWebserversbringthedesiredHTMLpages(HTML=HyperTextMarkupLanguage)andpicturesovertheworldwideInternetoralocalnetworktotheWebbrowser.ThishappensHTTP-based(HyperTextTransferProtocol).ATCP/IPprotocolstack–thatmeansitisbasedonsophisticatedandestablishedstandards–managestheentirecommunication.Webserver(HTTPserver)andbrowser(HTTPclient)buildTCP/IP-applications.HTTPachievedaphenomenaldistributioninthelastyears.MeanwhilemillionsofuseraroundtheworldsurfHTTP-basedintheWorldWideWeb.Todayalmosteverypersonalcomputeroffersthenecessaryassistanceforthisprotocol.Thisstatusisvalidmoreandmoreforembeddedsystemsalso.TheHTTPspreadsupwithafastratetoo.
1.TCP/IP-basedHTTPasCommunicationPlatform
HTTPisasimpleprotocolthatisbasedonaTCP/IPprotocolstack(picture1.A).HTTPusesTCP(TransmissionControlProtocol).TCPisarelativecomplexandhigh-qualityprotocoltotransferdatabythesubordinateIPprotocol.TCPitselfalwaysguaranteesasafeguardedconnectionbetweentwocommunicationpartnersbasedonanextensivethree-way-handshakeprocedure.AsaresultthedatatransferviaHTTPisalwaysprotected.DuetotheextensiveTCPprotocolmechanismsHTTPoffersonlyalow-gradeperformance.
Figure1:
TCP/IPstackandHTTPprogrammingmodel
HTTPisbasedonasimpleclient/server-concept.HTTPserverandclientcommunicateviaaTCPconnection.AsdefaultTCPportvaluetheportnumber
80willbeused.Theserverworkscompletelypassive.Hewaitsforarequest
(order)ofaclient.ThisrequestnormallyreferstothetransmitionofspecificHTMLdocuments.ThisHTMLdocumentspossiblyhavetobegenerateddynamicallybyCGI.Asresultoftherequests,theserverwillanswerwitharesponsethatusuallycontainsthedesiredHTMLdocumentsamongothers
(picture1.B).
GET/test.htmHTTP/1.1
Accept]:
image/gif,image/jpeg,*/*Usersellingagent:
Mozilla/4.0
Host:
192.168.0.1
Listing1.A:
HTTPGET-request
HTTP/1.1200OK
Date:
Mon,06Dec199920:
55:
12GMTServer:
Apache/1.3.6(Linux)
Content-length:
82
Content-type:
text/html
<
html>
head>
title>
Test-Seite<
/title>
/head>
body>
Test-Seite
/body>
/html>
Listing1.B:
HTTPresponseasresultoftheGET-requestfromlisting1.A
HTTPrequestsnormallyconsistofseveraltextlines,whicharetransmittedtotheserverbyTCP.Thelisting1.Ashowsanexample.Thefirstlinecharacterizestherequesttype(GET),therequestedobject(/test1.htm)andtheusedHTTPversion
(HTTP/1.1).Inthesecondrequestlinetheclienttellstheserver,whichkindoffilesitisabletoevaluate.Thethirdlineincludesinformationabouttheclient-software.Thefourthandlastlineoftherequestfromlisting1.AisusedtoinformtheserverabouttheIPaddressoftheclient.Inaccordingtothetypeofrequestandtheusedclientsoftwaretherecouldfollowsomefurtherlines.Asanendoftherequestablanklineisexpected.
TheHTTPresponsesasrequestanswermostlyconsistoftwoparts.Atfirstthereisaheaderofindividuallinesoftext.Thenfollowsacontentobject(optional).Thiscontentobjectmaybeconsistsofsometextlines–incaseofaHTMLfile–orabinaryfilewhenaGIForJPEGimageshouldbetransferred.Thefirstlineoftheheaderisespeciallyimportant.Itworksasstatusorerrormessage.Ifanerroroccurs,onlytheheaderorapartofitwillbetransmittedasanswer.
2.FunctionalprincipleofaWebServer
SimplifiedaWebservercanbeimaginedlikeaspecialkindofafileserver.Picture2.Ashowsanoverview.TheWebserverreceivesaHTTPGET-requestfromtheWebbrowser.Bythisrequest,aspecificfileisrequiredasanswer(seestep1intopicture2.A).Afterthat,theWebservertriestogetaccessonthefilesystemoftherequestedcomputer.Thenitattemptstofindthedesiredfile(step2).AfterthesuccessfulsearchtheWebserverreadtheentirefile(step3)andtransmititasananswer(HTTPresponsecomprisingofheaderandcontentobject)totheWebbrowser(step4).IftheWebservercannotfindtheappropriatefileinthefilesystem,ane
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