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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 Authorization State: User Authentication Process and Commands
(Page 2 of 3)
Standard POP3 Authentication
The normal method of authorization
in POP3 is a standard username / password login. This is
pretty much identical to how
a login is performed in FTP; even the
commands are the same. First the client issues a USER command
along with the user's mailbox name (his or her user name or e-mail address).
The server responds with an intermediate acknowledgment. The client
then uses the PASS command to send the user's password. Assuming
the login is valid, the server responds to the client with an acknowledgment
that indicates successful authentication. The response will also typically
specify the number of messages waiting for the user in the mailbox.
This process is illustrated in Figure 307.
Figure 307: Post Office Protocol (POP3) User Authentication Process
Once the TCP connection is established from the client to the server, the server responds with a greeting message, and the simple POP3 authentication process begins. The client sends a user name and password to the server using the USER and PASS commands, and the server evaluates the information to determine whether or not it will allow the client access.
shows an example POP3 authorization, with the client's commands highlighted
and the server's responses in italics.
Table 256: Example POP3 Authorization
+OK email@example.com has 3 messages
Note: Some servers may require only the name of the user (jane) while others require the full e-mail address as shown in the example. Also, I have not shown Jane's password in Table 256; that wouldn't be very nice, would it? J
If authorization is successful, the
POP3 session transitions to the Transaction state where mail
access commands can be performed. If the
user name or password are incorrect, an error response is given and
the session cannot proceed. The authorization may also fail due to technical
problems, such as an inability by the server to lock the mailbox (perhaps
due to new mail arriving via SMTP).
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The TCP/IP Guide (http://www.TCPIPGuide.com)
Version 3.0 - Version Date: September 20, 2005
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