嵌入式系统的网络服务器外文文献翻译.docx

嵌入式系统的网络服务器外文文献翻译.docx

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嵌入式系统的网络服务器外文文献翻译

TheDIL/NetPCsDNP/1110–UsingtheEmbeddedLinux

WebServerforEmbeddedSystems

Afterthe“everybody-in-the-Internet-wave”nowobviouslyfollowsthe

“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.

TheDIL/NetPCsDNP/1110–UsingtheEmbeddedLinux

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

Test-Seite

TheDIL/NetPCsDNP/1110–UsingtheEmbeddedLinux

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,anerrormessage（HTTPresponsewhichonlycontainstheheader）issimplybesendasresponsetotheclient.

Figure2:

FunctionalprinciplefromWebserverandbrowser

TheDIL/NetPCsDNP/1110–UsingtheEmbeddedLinux

Thewebcontentisbuildbyindividualfiles.ThebaseisbuildbystaticfileswithHTMLpages.WithinsuchHTMLfilestherearereferencestofurtherfilesembedded–thesefilesaretypicallypicturesinGIForJPEGformat.However,alsoreferencestootherobjects,forexampleJava-Applets,arepossible.AfteraWebbrowserhasreceivedaHTMLfileofaWebserver,thisfilewillbeevaluatedandthensearchedforexternalreferences.Nowthesteps1to4frompicture2.AwillrunagainforeveryexternalreferenceinordertorequesttherespectivefilefromthecorrespondingWebserver.Pleasenote,thatsuchareferenceconsistsofthenameorIPaddressofaWebserver（e.g.""）,aswellasthenameofthedesiredfile（e.g."picture1.gif"）.SovirtuallyeveryreferencecanrefertoanotherWebserver.Inotherwords,aHTMLfilecouldbelocatedontheserver"ssv-embedded.de"buttherequiredpicture-whichisexternalreferencedbythisHTMLfile-islocatedontheWebserver

"".Finallythis（worldwide）networkingofseparateobjectsisthecauseforthenameWorldWideWeb（WWW）.Allfiles,whicharerequiredbyaWebserver,arerequestedfromabrowserliketheprocedureshownonpicture

2.A.Normallythesefilesarestoredinthefilesystemoftheserver.The

Webmasterhastoupdatethesefilesfromtimetotime.

AfurtherelementaryfunctionalityofaWebserveristheCommonGatewayInterface（CGI）-wehavementionedbefore.Originallythistechnologyismadeonlyforsimpleforms,whichareembeddedintoHTMLpages.Thedata,resultingfromthepaddingofaform,willbetransmittedtoaWebserverviaHTTP-GETorPOST-request（seestep1intopicture2.B）.InsuchaGET-orPOST-requestthenameoftheCGIprogram,whichisneededfortheevaluationofaform,isfundamentallyincluded.ThisprogramhastobeontheWebserver.Normallythedirectory"/cgi-bin"isusedasstoragelocation.

AsresultoftheGET-orPOST-requesttheWebserverstartstheCGIprogramlocatedinthesubdirectory"/cgi-bin"anddeliversthereceiveddatainformofparameters（step2）.TheoutputsofaCGIprogramareguidedtotheWebserver

（step3）.ThentheWebserversendsthemallasresponsestotheWebbrowser

（step4）.

3.DynamicgeneratedHTMLPages

IncontradictiontoacompanyWebsiteserver,whichinformspeopleabouttheproductprogramandservicesbystaticpagesandpictures,anembeddedWebserverhastosupplydynamicallygeneratedcontents.TheembeddedWebserverwillgeneratethedynamicpagesinthemomentofthefirstaccessbyabrowser.HowelsecouldwechecktheactualtemperatureofasystemviaInternet?

StaticHTMLfilesarenotinterestingforanembeddedWebserver.ThemostinformationaboutthefirmwareversionandserviceinstructionsarestoredinHTMLformat.AllothertasksarenormallymadeviadynamicgeneratedHTML.

TherearetwodifferenttechnologiestogenerateaspecificHTMLpageinthemomentoftherequest:

Firsttheso-calledserver-side-scriptingandsecondtheCGIprogramming.Attheserver-side-scripting,scriptcodeisembeddedintoaHTMLpage.Ifrequired,thiscodewillbecarriedoutontheserver（server-sided）.

TheDIL/NetPCsDNP/1110–UsingtheEmbeddedLinux

Forthis,therearenumerousscriptlanguagesavailable.AlltheselanguagesareusableinsideaHTML-page.IntheLinuxcommunityPHPisusedmostly.ThefavouriteofMicrosoftisVBScript.ItisalsopossibletoinsertJavadirectlyintoHTMLpages.SunhasnamedthistechnologyJSP（JavaServerPages）.TheHTMLpagewiththescriptcodeisstaticallystoredinthefilesystemoftheWebserver.Beforethisserverfileisdeliveredtotheclient,aspecialprogramreplacestheentirescriptcodewithdynamicgeneratedstandardHTML.TheWebbrowserwillnotseeanythingfromthescriptlanguage.

Figure3:

SinglestepsoftheServer-Side-Scripting

Picture3showsthesinglestepsoftheserver-side-scripting.Instep1theWebbrowserrequestsaspecif