Please Whitelist This Site? I know everyone hates ads. But please understand that I am providing premium content for free that takes hundreds of hours of time to research and write. I don't want to go to a pay-only model like some sites, but when more and more people block ads, I end up working for free. And I have a family to support, just like you. :) If you like The TCP/IP Guide, please consider the download version. It's priced very economically and you can read all of it in a convenient format without ads. If you want to use this site for free, I'd be grateful if you could add the site to the whitelist for Adblock. To do so, just open the Adblock menu and select "Disable on tcpipguide.com". Or go to the Tools menu and select "Adblock Plus Preferences...". Then click "Add Filter..." at the bottom, and add this string: "@@||tcpipguide.com^$document". Then just click OK. Thanks for your understanding! Sincerely, Charles Kozierok Author and Publisher, The TCP/IP Guide

NOTE: Using software to mass-download the site degrades the server and is prohibited. If you want to read The TCP/IP Guide offline, please consider licensing it. Thank you.

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

Session States

POP3 is described in terms of a finite state machine (FSM), with a session transitioning through three states during the course of its “lifetime”, as shown in Figure 306.

Before you get too intimidated, realize that unlike the FSMs of protocols like TCP, this one really is simple, because it is linear. The session goes through each state once and only once, in the following sequence:

1. Authorization State: The server provides a greeting to the client to indicate that it is ready for commands. The client then provides authentication information to allow access to the user's mailbox.
   
   
2. Transaction State: The client is allowed to perform various operations on the mailbox. These include listing and retrieving messages, and marking retrieved messages for deletion.
   
   
3. Update State: When the client is done with all of its tasks and issues the QUIT command, the session enters this state automatically, where the server actually deletes the messages marked for deletion in the Transaction state. The session is then concluded and the TCP connection between the two terminated.
   

   Figure 306: Post Office Protocol (POP3) Finite State Machine

   POP uses a finite state machine to describe its operation, but it is very simple because it is linear. Once a TCP connection is established between a POP3 client and POP3 server, the session proceeds through three states in sequence, after which the connection is terminated.

   
   

POP3 is designed so that only certain commands may be sent in each of these states. The next three topics describe the activities that take place in these three states, including the commands that are issued by the client in each.

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