Network Working Group                                           J. Myers
Request for Comments: 1725                               Carnegie Mellon
Obsoletes: 1460                                                  M. Rose
Category: Standards Track                   Dover Beach Consulting, Inc.
                                                           November 1994


                    Post Office Protocol - Version 3

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Overview

   This memo is a revision to RFC 1460, a Draft Standard.  It makes the
   following changes from that document:

      - removed text regarding "split-UA model", which didn't add
        anything to the understanding of POP

      - clarified syntax of commands, keywords, and arguments

      - clarified behavior on broken connection

      - explicitly permitted an inactivity autologout timer

      - clarified the requirements of the "exclusive-access lock"

      - removed implementation-specific wording regarding the parsing of
        the maildrop

      - allowed servers to close the connection after a failed
        authentication command

      - removed the LAST command

      - fixed typo in example of TOP command

      - clarified that the second argument to the TOP command is non-
        negative

      - added the optional UIDL command




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RFC 1725                          POP3                     November 1994


      - added warning regarding length of shared secrets with APOP

      - added additional warnings to the security considerations section

1. Introduction

   On certain types of smaller nodes in the Internet it is often
   impractical to maintain a message transport system (MTS).  For
   example, a workstation may not have sufficient resources (cycles,
   disk space) in order to permit a SMTP server [RFC821] and associated
   local mail delivery system to be kept resident and continuously
   running.  Similarly, it may be expensive (or impossible) to keep a
   personal computer interconnected to an IP-style network for long
   amounts of time (the node is lacking the resource known as
   "connectivity").

   Despite this, it is often very useful to be able to manage mail on
   these smaller nodes, and they often support a user agent (UA) to aid
   the tasks of mail handling.  To solve this problem, a node which can
   support an MTS entity offers a maildrop service to these less endowed
   nodes.  The Post Office Protocol - Version 3 (POP3) is intended to
   permit a workstation to dynamically access a maildrop on a server
   host in a useful fashion.  Usually, this means that the POP3 is used
   to allow a workstation to retrieve mail that the server is holding
   for it.

   For the remainder of this memo, the term "client host" refers to a
   host making use of the POP3 service, while the term "server host"
   refers to a host which offers the POP3 service.

2. A Short Digression

   This memo does not specify how a client host enters mail into the
   transport system, although a method consistent with the philosophy of
   this memo is presented here:

      When the user agent on a client host wishes to enter a message
      into the transport system, it establishes an SMTP connection to
      its relay host (this relay host could be, but need not be, the
      POP3 server host for the client host).

3. Basic Operation

   Initially, the server host starts the POP3 service by listening on
   TCP port 110.  When a client host wishes to make use of the service,
   it establishes a TCP connection with the server host.  When the
   connection is established, the POP3 server sends a greeting.  The
   client and POP3 server then exchange commands and responses



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   (respectively) until the connection is closed or aborted.

   Commands in the POP3 consist of a keyword, possibly followed by one
   or more arguments.  All commands are terminated by a CRLF pair.
   Keywords and arguments consist of printable ASCII characters.
   Keywords and arguments are each separated by a single SPACE
   character.  Keywords are three or four characters long. Each argument
   may be up to 40 characters long.

   Responses in the POP3 consist of a status indicator and a keyword
   possibly followed by additional information.  All responses are
   terminated by a CRLF pair.  There are currently two status
   indicators: positive ("+OK") and negative ("-ERR").

   Responses to certain commands are multi-line.  In these cases, which
   are clearly indicated below, after sending the first line of the
   response and a CRLF, any additional lines are sent, each terminated
   by a CRLF pair.  When all lines of the response have been sent, a
   final line is sent, consisting of a termination octet (decimal code
   046, ".") and a CRLF pair.  If any line of the multi-line response
   begins with the termination octet, the line is "byte-stuffed" by
   pre-pending the termination octet to that line of the response.
   Hence a multi-line response is terminated with the five octets
   "CRLF.CRLF".  When examining a multi-line response, the client checks
   to see if the line begins with the termination octet.  If so and if
   octets other than CRLF follow, the the first octet of the line (the
   termination octet) is stripped away.  If so and if CRLF immediately
   follows the termination character, then the response from the POP
   server is ended and the line containing ".CRLF" is not considered
   part of the multi-line response.

   A POP3 session progresses through a number of states during its
   lifetime.  Once the TCP connection has been opened and the POP3
   server has sent the greeting, the session enters the AUTHORIZATION
   state.  In this state, the client must identify itself to the POP3
   server.  Once the client has successfully done this, the server
   acquires resources associated with the client's maildrop, and the
   session enters the TRANSACTION state.  In this state, the client
   requests actions on the part of the POP3 server.  When the client has
   issued the QUIT command, the session enters the UPDATE state.  In
   this state, the POP3 server releases any resources acquired during
   the TRANSACTION state and says goodbye.  The TCP connection is then
   closed.

   A POP3 server MAY have an inactivity autologout timer.  Such a timer
   MUST be of at least 10 minutes' duration.  The receipt of any command
   from the client during that interval should suffice to reset the
   autologout timer.  When the timer expires, the session does NOT enter



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   the UPDATE state--the server should close the TCP connection without
   removing any messages or sending any response to the client.

4. The AUTHORIZATION State

   Once the TCP connection has been opened by a POP3 client, the POP3
   server issues a one line greeting.  This can be any string terminated
   by CRLF.  An example might be:

      S:  +OK POP3 server ready

   Note that this greeting is a POP3 reply.  The POP3 server should
   always give a positive response as the greeting.

   The POP3 session is now in the AUTHORIZATION state.  The client must
   now identify and authenticate itself to the POP3 server.  Two
   possible mechanisms for doing this are described in this document,
   the USER and PASS command combination and the APOP command.  The APOP
   command is described later in this document.

   To authenticate using the USER and PASS command combination, the
   client must first issue the USER command.  If the POP3 server
   responds with a positive status indicator ("+OK"), then the client
   may issue either the PASS command to complete the authentication, or
   the QUIT command to terminate the POP3 session.  If the POP3 server
   responds with a negative status indicator ("-ERR") to the USER
   command, then the client may either issue a new authentication
   command or may issue the QUIT command.

   When the client issues the PASS command, the POP3 server uses the
   argument pair from the USER and PASS commands to determine if the
   client should be given access to the appropriate maildrop.

   Once the POP3 server has determined through the use of any
   authentication command that the client should be given access to the
   appropriate maildrop, the POP3 server then acquires an exclusive-
   access lock on the maildrop, as necessary to prevent messages from
   being modified or removed before the session enters the UPDATE state.
   If the lock is successfully acquired, the POP3 server responds with a
   positive status indicator.  The POP3 session now enters the
   TRANSACTION state, with no messages marked as deleted.  If the the
   maildrop cannot be opened for some reason (for example, a lock can
   not be acquired, the client is denied access to the appropriate
   maildrop, or the maildrop cannot be parsed), the POP3 server responds
   with a negative status indicator.  (If a lock was acquired but the
   POP3 server intends to respond with a negative status indicator, the
   POP3 server must release the lock prior to rejecting the command.)
   After returning a negative status indicator, the server may close the



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   connection.  If the server does not close the connection, the client
   may either issue a new authentication command and start again, or the
   client may issue the QUIT command.

   After the POP3 server has opened the maildrop, it assigns a message-
   number to each message, and notes the size of each message in octets.
   The first message in the maildrop is assigned a message-number of
   "1", the second is assigned "2", and so on, so that the n'th message
   in a maildrop is assigned a message-number of "n".  In POP3 commands
   and responses, all message-number's and message sizes are expressed
   in base-10 (i.e., decimal).

   Here are summaries for the three POP3 commands discussed thus far:

      USER name

         Arguments:
             a string identifying a mailbox (required), which is of
             significance ONLY to the server

         Restrictions:
             may only be given in the AUTHORIZATION state after the POP3
             greeting or after an unsuccessful USER or PASS command

         Possible Responses:
             +OK name is a valid mailbox
             -ERR never heard of mailbox name

         Examples:
             C: USER mrose
             S: +OK mrose is a real hoopy frood
                ...
             C: USER frated
             S: -ERR sorry, no mailbox for frated here

      PASS string

         Arguments:
             a server/mailbox-specific password (required)

         Restrictions:
             may only be given in the AUTHORIZATION state after a
             successful USER command

         Discussion:
             Since the PASS command has exactly one argument, a POP3
             server may treat spaces in the argument as part of the
             password, instead of as argument separators.



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RFC 1725                          POP3                     November 1994


         Possible Responses:
             +OK maildrop locked and ready
             -ERR invalid password
             -ERR unable to lock maildrop

         Examples:
             C: USER mrose
             S: +OK mrose is a real hoopy frood
             C: PASS secret
             S: +OK mrose's maildrop has 2 messages (320 octets)
               ...
             C: USER mrose
             S: +OK mrose is a real hoopy frood
             C: PASS secret
             S: -ERR maildrop already locked

      QUIT

         Arguments: none

         Restrictions: none

         Possible Responses:
             +OK

         Examples:
             C: QUIT
             S: +OK dewey POP3 server signing off

5. The TRANSACTION State

   Once the client has successfully identified itself to the POP3 server
   and the POP3 server has locked and opened the appropriate maildrop,
   the POP3 session is now in the TRANSACTION state.  The client may now
   issue any of the following POP3 commands repeatedly.  After each
   command, the POP3 server issues a response.  Eventually, the client
   issues the QUIT command and the POP3 session enters the UPDATE state.

   Here are the POP3 commands valid in the TRANSACTION state:

      STAT

         Arguments: none

         Restrictions:
             may only be given in the TRANSACTION state





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