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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)
POP3 General Operation, Client/Server Communication and Session States
(Page 1 of 2)
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
server software that receives mail for those boxesa 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.
Commands and Response Codes
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
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.
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The TCP/IP Guide (http://www.TCPIPGuide.com)
Version 3.0 - Version Date: September 20, 2005
© Copyright 2001-2005 Charles M. Kozierok. All Rights Reserved.
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