1. CS440 Computer Networks 1 Email Neil Tang 12/01/2008

2. CS440 Computer Networks 2 Outline  Email Message Format  SMTP  Mail Reader

3. CS440 Computer Networks 3 Email Message Format (RFC822)  Two parts: header and body  Header: The header includes a series of - terminated lines and each header line contains a type and value separated by a colon.  Type: From, To, Subject, Date, Received, …

4. CS440 Computer Networks 4 Email Message Format (MIME)  New MIME header lines describe the data being carried in the message body. They include MIME-Version (the version of MIME being used), Content-Description: (a human-readable description of what’s in the message), Content-Type: (the type of data contained in the message), and Content-Transfer-Encoding: (how the data in the message body is encoded).  Content-Type: image/jpeg, text/plain, text/richtext, application/msword multipart/mixed.  An Email message contain only ASCII characters. The binary data sequence will be divided to 6-bit pieces, each of which maps onto one of 64 valid ASCII characters (52 upper- and lowercase letters, the 10 digits 0 through 9, and the special characters + and /).  Content-Transfer-Encoding: base64

5. CS440 Computer Networks 5 SMTP  Mail reader: the client program used by users to read and compose an email message.  Mail daemon: the process which sends or receives email messages.  Mail readers give the daemon messages they want to send to other users, the daemon uses SMTP running over TCP to transmit the message to a daemon running on another machine.

6. CS440 Computer Networks 6 SMTP  SMTP is an ASCII based protocol.  SMTP involves a sequence of exchanges between the client and the server.  In each exchange, the client posts a command (e.g., HELO, MAIL, RCPT,DATA, QUIT) and the server responds with a code (e.g., 250, 550, 354, 221). The server also returns a human-readable explanation for the code (e.g., code 550: No such user).

7. CS440 Computer Networks 7 SMTP (Example) HELLO cs.princeton.edu 250 Hello daemon@mail.cs.princeton.edu [128.12.169.24] MAIL FROM: 250 OK RCPT TO: 250 OK RCPT TO: 550 No such user here DATA 354 Start mail input; end with. Blah blah blah......etc. etc. etc.. 250 OK QUIT 221 Closing connection

8. CS440 Computer Networks 8 Mail Reader  The user uses a mail reader to actually retrieve his or her messages from the mailbox, read them, reply to them, and possibly save a copy for future reference.  Usually, the user accesses his or her mailbox from a remote machine using another protocol, such as the Post Office Protocol (POP) or the Internet Message Access Protocol (IMAP).