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The TCP/IP Guide

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Table Of Contents  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)

Previous Topic/Section
POP Overview, History, Versions and Standards
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POP3 Authorization State: User Authentication Process and Commands
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POP3 General Operation, Client/Server Communication and Session States
(Page 2 of 2)

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.

Note: I describe the concepts behind Finite State Machines in the topic describing the TCP FSM.

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.

Key Concept: POP3 is a client/server protocol that is described using a simple linear sequence of states. A POP3 session begins with a POP3 client making a TCP connection to a POP3 server, at which point the session is in the Authorization state. After successful authentication, the session moves to the Transaction state, where the client can perform mail access transactions. When it is done, the client ends the session and the Update state is entered automatically, where cleanup functions are performed and the POP3 session ended.

Previous Topic/Section
POP Overview, History, Versions and Standards
Previous Page
Pages in Current Topic/Section
Next Page
POP3 Authorization State: User Authentication Process and Commands
Next Topic/Section

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Version 3.0 - Version Date: September 20, 2005

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