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北京工业大学毕业设计(译文)7TheDIL/NetPCsDNP/1110–UsingtheEmbeddedLinuxWebServerforEmbeddedSystemsAftertheeverybodyintheInternetwavenowobviouslyfollowstheeverythingintheInternetwave.Themostcoffee,vendingandwashingmachinesarestillnotavailableabouttheworldwidenet.HowevertheembeddedInternetintegrationforremotemaintenanceanddiagnosticaswellasthesocalledM2Mcommunicationisgrowingwithaconsiderablespeedrate.JusttheremotemaintenanceanddiagnosticofcomponentsandsystemsbyWebbrowsersviatheInternet,oralocalIntranethasaveryhighweightformanydevelopmentprojects.InnumerousdevelopmentdepartmentspeopleworkoncompletelyWebbasedconfigurationsandservicesforembeddedsystems.TheremainingdaysoftheclassicuserinterfacemadebyasmallLCdisplaywithfrontpanelandafewfunctionkeysareover.ThroughfutureevolutionsinthefieldofthemobileInternet,BluetoothbasedPANsPersonalAreaNetworksandtherapidlygrowingM2McommunicationM2MMachinetoMachineafurtherinnovatingadvanceistobeexpected.ThecentralfunctionunittogetaccessonanembeddedsystemviaWebbrowseristheWebserver.SuchWebserversbringthedesiredHTMLpagesHTMLHyperTextMarkupLanguageandpicturesovertheworldwideInternetoralocalnetworktotheWebbrowser.ThishappensHTTPbasedHyperTextTransferProtocol.ATCP/IPprotocolstack–thatmeansitisbasedonsophisticatedandestablishedstandards–managestheentirecommunication.WebserverHTTPserverandbrowserHTTPclientbuildTCP/IPapplications.HTTPachievedaphenomenaldistributioninthelastyears.MeanwhilemillionsofuseraroundtheworldsurfHTTPbasedintheWorldWideWeb.Todayalmosteverypersonalcomputeroffersthenecessaryassistanceforthisprotocol.Thisstatusisvalidmoreandmoreforembeddedsystemsalso.TheHTTPspreadsupwithafastratetoo.北京工业大学毕业设计(译文)8TheDIL/NetPCsDNP/1110–UsingtheEmbeddedLinux1.TCP/IPbasedHTTPasCommunicationPlatformHTTPisasimpleprotocolthatisbasedonaTCP/IPprotocolstackpicture1.A.HTTPusesTCPTransmissionControlProtocol.TCPisarelativecomplexandhighqualityprotocoltotransferdatabythesubordinateIPprotocol.TCPitselfalwaysguaranteesasafeguardedconnectionbetweentwocommunicationpartnersbasedonanextensivethreewayhandshakeprocedure.AsaresultthedatatransferviaHTTPisalwaysprotected.DuetotheextensiveTCPprotocolmechanismsHTTPoffersonlyalowgradeperformance.Figure1TCP/IPstackandHTTPprogrammingmodelHTTPisbasedonasimpleclient/serverconcept.HTTPserverandclientcommunicateviaaTCPconnection.AsdefaultTCPportvaluetheportnumber80willbeused.Theserverworkscompletelypassive.Hewaitsforarequestorderofaclient.ThisrequestnormallyreferstothetransmitionofspecificHTMLdocuments.ThisHTMLdocumentspossiblyhavetobegenerateddynamicallybyCGI.Asresultoftherequests,theserverwillanswerwitharesponsethatusuallycontainsthedesiredHTMLdocumentsamongotherspicture1.B.GET/test.htmHTTP/1.1Acceptimage/gif,image/jpeg,/UsersellingagentMozilla/4.0Host192.168.0.1Listing1.AHTTPGETrequestHTTP/1.1200OKDateMon,06Dec1999205512GMTServerApache/1.3.6LinuxContentlength82Contenttypetext/htmlTestSeite北京工业大学毕业设计(译文)9TheDIL/NetPCsDNP/1110–UsingtheEmbeddedLinuxListing1.BHTTPresponseasresultoftheGETrequestfromlisting1.AHTTPrequestsnormallyconsistofseveraltextlines,whicharetransmittedtotheserverbyTCP.Thelisting1.Ashowsanexample.ThefirstlinecharacterizestherequesttypeGET,therequestedobject/test1.htmandtheusedHTTPversionHTTP/1.1.Inthesecondrequestlinetheclienttellstheserver,whichkindoffilesitisabletoevaluate.Thethirdlineincludesinformationabouttheclientsoftware.Thefourthandlastlineoftherequestfromlisting1.AisusedtoinformtheserverabouttheIPaddressoftheclient.Inaccordingtothetypeofrequestandtheusedclientsoftwaretherecouldfollowsomefurtherlines.Asanendoftherequestablanklineisexpected.TheHTTPresponsesasrequestanswermostlyconsistoftwoparts.Atfirstthereisaheaderofindividuallinesoftext.Thenfollowsacontentobjectoptional.Thiscontentobjectmaybeconsistsofsometextlines–incaseofaHTMLfile–orabinaryfilewhenaGIForJPEGimageshouldbetransferred.Thefirstlineoftheheaderisespeciallyimportant.Itworksasstatusorerrormessage.Ifanerroroccurs,onlytheheaderorapartofitwillbetransmittedasanswer.2.FunctionalprincipleofaWebServerSimplifiedaWebservercanbeimaginedlikeaspecialkindofafileserver.Picture2.Ashowsanoverview.TheWebserverreceivesaHTTPGETrequestfromtheWebbrowser.Bythisrequest,aspecificfileisrequiredasanswerseestep1intopicture2.A.Afterthat,theWebservertriestogetaccessonthefilesystemoftherequestedcomputer.Thenitattemptstofindthedesiredfilestep2.AfterthesuccessfulsearchtheWebserverreadtheentirefilestep3andtransmititasananswerHTTPresponsecomprisingofheaderandcontentobjecttotheWebbrowserstep4.IftheWebservercannotfindtheappropriatefileinthefilesystem,anerrormessageHTTPresponsewhichonlycontainstheheaderissimplybesendasresponsetotheclient.Figure2FunctionalprinciplefromWebserverandbrowser北京工业大学毕业设计(译文)10TheDIL/NetPCsDNP/1110–UsingtheEmbeddedLinuxThewebcontentisbuildbyindividualfiles.ThebaseisbuildbystaticfileswithHTMLpages.WithinsuchHTMLfilestherearereferencestofurtherfilesembedded–thesefilesaretypicallypicturesinGIForJPEGformat.However,alsoreferencestootherobjects,forexampleJavaApplets,arepossible.AfteraWebbrowserhasreceivedaHTMLfileofaWebserver,thisfilewillbeevaluatedandthensearchedforexternalreferences.Nowthesteps1to4frompicture2.AwillrunagainforeveryexternalreferenceinordertorequesttherespectivefilefromthecorrespondingWebserver.Pleasenote,thatsuchareferenceconsistsofthenameorIPaddressofaWebservere.g.dilnetpc.com,aswellasthenameofthedesiredfilee.g.picture1.gif.SovirtuallyeveryreferencecanrefertoanotherWebserver.Inotherwords,aHTMLfilecouldbelocatedontheserverssvembedded.debuttherequiredpicturewhichisexternalreferencedbythisHTMLfileislocatedontheWebserverdilnetpc.com.FinallythisworldwidenetworkingofseparateobjectsisthecauseforthenameWorldWideWebWWW.Allfiles,whicharerequiredbyaWebserver,arerequestedfromabrowserliketheprocedureshownonpicture2.A.Normallythesefilesarestoredinthefilesystemoftheserver.TheWebmasterhastoupdatethesefilesfromtimetotime.AfurtherelementaryfunctionalityofaWebserveristheCommonGatewayInterfaceCGIwehavementionedbefore.Originallythistechnologyismadeonlyforsimpleforms,whichareembeddedintoHTMLpages.Thedata,resultingfromthepaddingofaform,willbetransmittedtoaWebserverviaHTTPGETorPOSTrequestseestep1intopicture2.B.InsuchaGETorPOSTrequestthenameoftheCGIprogram,whichisneededfortheevaluationofaform,isfundamentallyincluded.ThisprogramhastobeontheWebserver.Normallythedirectory/cgibinisusedasstoragelocation.AsresultoftheGETorPOSTrequesttheWebserverstartstheCGIprogramlocatedinthesubdirectory/cgibinanddeliversthereceiveddatainformofparametersstep2.TheoutputsofaCGIprogramareguidedtotheWebserverstep3.ThentheWebserversendsthemallasresponsestotheWebbrowserstep4.3.DynamicgeneratedHTMLPagesIncontradictiontoacompanyWebsiteserver,whichinformspeopleabouttheproductprogramandservicesbystaticpagesandpictures,anembeddedWebserverhastosupplydynamicallygeneratedcontents.TheembeddedWebserverwillgeneratethedynamicpagesinthemomentofthefirstaccessbyabrowser.HowelsecouldwechecktheactualtemperatureofasystemviaInternetStaticHTMLfilesarenotinterestingforanembeddedWebserver.ThemostinformationaboutthefirmwareversionandserviceinstructionsarestoredinHTMLformat.AllothertasksarenormallymadeviadynamicgeneratedHTML.TherearetwodifferenttechnologiestogenerateaspecificHTMLpageinthemomentoftherequestFirstthesocalledserversidescriptingandsecondtheCGIprogramming.Attheserversidescripting,scriptcodeisembeddedintoaHTMLpage.Ifrequired,thiscodewillbecarriedoutontheserverserversided.
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