《计算机网络和 与因特网》课件第四章电子邮件表示和传输.pptVIP

Computer Networks and Internets 《计算机网络与因特网》课件 林坤辉 PART IV Network Applications Chapter 4 Electronic Mail Representation And Transfer 电子邮件表示和传输 Introduction Electronic mail is a client-server model . The chapter describe: Mail transport service. Mail forwarding. Mailbox access. The TCP/IP protocol that supports electronic mail on the Internet is called simple Mail Transfer Protocol (SMTP). SMTP provides for mail exchange between users on the same or different computers. Format of an email Email address: a local part and a domain name, separated by an @ sign The Local part defines the name of a special file,called the user mailbox. An organization usually selects one or more hosts to receive and send email. After the user agent creates a message, it is delivered to spool, which is a storage structure. The mail transfer system periodically checks the mail stored in the spool. If The IP address of the server has been obtained ? If the receiver is ready? After the message has been received, it does not have to be read by the recipient immediately. The mail can be stored in the mailbox of the receiver. Alias expansion. The entire email system SMTP uses commands and responses to transfer messages between an MTA client and an MTA server. Each command or reply is terminated by a two-character(carriage return and line feed) end-of-line token. Commands are sent from the client to the server. The format of Command: Keyword:argument(s) Example: HELO: MAIL FROM:abc@ RCPT TO: def@ Responses are sent from the server to the client. A response is a three-digit code that may be followed by additional textual information. Three phases: connection establishment, mail transfer, and connection termination. Connection establishment: After a client has made a TCP connection to the well-known port 25, the SMTP server starts the connection phase. Mail Transfer Phases Message transfer 上图的前一部分 上图的后一部分 Connection termination: After the