计算机网络：应用层.ppt

*计算机网络讲义*内容传送编码

Content-Transfer-Encoding最简单的编码就是7位ASCII码，而每行不能超过1000个字符。MIME对这种由ASCII码构成的邮件主体不进行任何转换。另一种编码称为quoted-printable，这种编码方法适用于当所传送的数据中只有少量的非ASCII码。对于任意的二进制文件，可用base64编码。*计算机网络讲义*内容类型MIME着标准规定Content-Type说明必须含有两个标识符，即内容类型(type)和子类型(subtype)，中间用“/”分开。MIME标准定义了7个基本内容类型和15种子类型。*计算机网络讲义*From:alice@crepes.frTo:bob@Subject:Pictureofyummycrepe.MIME-Version:1.0Content-Transfer-Encoding:base64Content-Type:image/jpegbase64encodeddatabase64encodeddatamultimediadatatype,subtype,parameterdeclarationmethodusedtoencodedataMIMEversionencodeddata*计算机网络讲义*MIMEtypes

Content-Type:type/subtype;parametersTextexamplesubtypes:plain,htmlImageexamplesubtypes:jpeg,gifAudioexampesubtypes:basic(8-bitmu-lawencoded),32kadpcm(32kbpscoding)Videoexamplesubtypes:mpeg,quicktimeApplicationotherdatathatmustbeprocessedbyreaderbefore“viewable”examplesubtypes:msword,octet-stream*计算机网络讲义*MultipartTypeFrom:alice@crepes.frTo:bob@Subject:Pictureofyummycrepe.MIME-Version:1.0Content-Type:multipart/mixed;boundary-Content-Transfer-Encoding:quoted-printableContent-Type:text/plainDearBob,Pleasefindapictureofacrepe.-Content-Transfer-Encoding:base64Content-Type:image/jpegbase64encodeddatabase64encodeddata--*计算机网络讲义*POP3protocolauthorizationphaseclientcommands:user:declareusernamepass:passwordserverresponses+OK-ERRtransactionphase,client:list:listmessagenumbersretr:retrievemessagebynumberdele:deletequitC:listS:1498S:2912S:.C:retr1S:message1contentsS:.C:dele1C:retr2S:message1contentsS:.C:dele2C:quitS:+OKPOP3serversigningoffS:+OKPOP3serverreadyC:useraliceS:+OKC:passhungryS:+OKusersuccessfullylogg