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The TCP/IP Guide  9  TCP/IP Application Layer Protocols, Services and Applications (OSI Layers 5, 6 and 7)       9  TCP/IP Key Applications and Application Protocols            9  TCP/IP File and Message Transfer Applications and Protocols (FTP, TFTP, Electronic Mail, USENET, HTTP/WWW, Gopher)                 9  TCP/IP Electronic Mail System: Concepts and Protocols (RFC 822, MIME, SMTP, POP3, IMAP)                      9  TCP/IP Electronic Mail Access and Retrieval Protocols and Methods                           9  TCP/IP Post Office Protocol (POP/POP3)

POP Overview, History, Versions and Standards 1 2 POP3 Authorization State: User Authentication Process and Commands

As I mentioned in the previous topic, the Post Office Protocol was designed to perform only one main function, and to do it quickly and simply: allow a client computer to access and retrieve e-mail from a server. The operation of the protocol is simple and straight-forward, which is good news for you, the person trying to understand how it works.

POP3 is a regular TCP/IP client/server protocol. To provide access to mailboxes, POP3 server software must be installed and continuously running on the server where the mailboxes are located. This does not necessarily have to be the same physical hardware device that runs the SMTP server software that receives mail for those boxes—a mechanism such as NFS may be used to allow the POP3 and SMTP servers to both “see” mailboxes locally. POP3 clients are regular end-user e-mail programs that make connections to POP3 servers to get mail; examples include Microsoft Outlook, Eudora Email and so forth.

POP3 uses the Transmission Control Protocol (TCP) for communication, to ensure the reliable transfer of commands, responses and message data. POP3 servers “listen” on well-known port number 110 for incoming connection requests from POP3 clients. After a TCP connection is established, the POP3 session is activated. The client sends commands to the server, which replies with responses and/or e-mail message contents.

POP3 commands are three or four letters long and are case-insensitive. They are all sent in plain ASCII text and terminated with a “CRLF” sequence, just as with FTP and SMTP commands. POP3 replies are also textual, but the protocol does not use the complex three-digit reply code mechanism of FTP (and SMTP). In fact, it defines only two basic responses:

• +OK: A positive response, sent when a command or action is successful; and
  
  
• -ERR: A negative response, sent to indicate that an error has occurred.

These messages may be accompanied by explanatory text, especially in the case of an -ERR response, to provide more information about the nature of the error.

POP Overview, History, Versions and Standards 1 2 POP3 Authorization State: User Authentication Process and Commands