rfc1939.txt

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Network Working Group                                           J. Myers
Request for Comments: 1939                               Carnegie Mellon
STD: 53                                                          M. Rose
Obsoletes: 1725                             Dover Beach Consulting, Inc.
Category: Standards Track                                       May 1996


                    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.

Table of Contents

   1. Introduction ................................................    2
   2. A Short Digression ..........................................    2
   3. Basic Operation .............................................    3
   4. The AUTHORIZATION State .....................................    4
      QUIT Command ................................................    5
   5. The TRANSACTION State .......................................    5
      STAT Command ................................................    6
      LIST Command ................................................    6
      RETR Command ................................................    8
      DELE Command ................................................    8
      NOOP Command ................................................    9
      RSET Command ................................................    9
   6. The UPDATE State ............................................   10
      QUIT Command ................................................   10
   7. Optional POP3 Commands ......................................   11
      TOP Command .................................................   11
      UIDL Command ................................................   12
      USER Command ................................................   13
      PASS Command ................................................   14
      APOP Command ................................................   15
   8. Scaling and Operational Considerations ......................   16
   9. POP3 Command Summary ........................................   18
   10. Example POP3 Session .......................................   19
   11. Message Format .............................................   19
   12. References .................................................   20
   13. Security Considerations ....................................   20
   14. Acknowledgements ...........................................   20
   15. Authors' Addresses .........................................   21
   Appendix A. Differences from RFC 1725 ..........................   22



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RFC 1939                          POP3                          May 1996


   Appendix B. Command Index ......................................   23

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 protocol
   is used to allow a workstation to retrieve mail that the server is
   holding for it.

   POP3 is not intended to provide extensive manipulation operations of
   mail on the server; normally, mail is downloaded and then deleted.  A
   more advanced (and complex) protocol, IMAP4, is discussed in
   [RFC1730].

   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 and sends all mail to it.  This relay host could
      be, but need not be, the POP3 server host for the client host.  Of
      course, the relay host must accept mail for delivery to arbitrary
      recipient addresses, that functionality is not required of all
      SMTP servers.





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RFC 1939                          POP3                          May 1996


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
   (respectively) until the connection is closed or aborted.

   Commands in the POP3 consist of a case-insensitive 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.  Responses may be up to 512 characters
   long, including the terminating CRLF.  There are currently two status
   indicators: positive ("+OK") and negative ("-ERR").  Servers MUST
   send the "+OK" and "-ERR" in upper case.

   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 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



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RFC 1939                          POP3                          May 1996


   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 server MUST respond to an unrecognized, unimplemented, or
   syntactically invalid command by responding with a negative status
   indicator.  A server MUST respond to a command issued when the
   session is in an incorrect state by responding with a negative status
   indicator.  There is no general method for a client to distinguish
   between a server which does not implement an optional command and a
   server which is unwilling or unable to process the command.

   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
   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 positive
   response.  An example might be:

      S:  +OK POP3 server ready

   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.  Both
   mechanisms are described later in this document.  Additional
   authentication mechanisms are described in [RFC1734].  While there is
   no single authentication mechanism that is required of all POP3
   servers, a POP3 server must of course support at least one
   authentication mechanism.

   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
   maildrop cannot be opened for some reason (for example, a lock can
   not be acquired, the client is denied access to the appropriate



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RFC 1939                          POP3                          May 1996


   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
   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 nth message
   in a maildrop is assigned a message-number of "n".  In POP3 commands
   and responses, all message-numbers and message sizes are expressed in
   base-10 (i.e., decimal).

   Here is the summary for the QUIT command when used in the
   AUTHORIZATION state:

      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.










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RFC 1939                          POP3                          May 1996


   Here are the POP3 commands valid in the TRANSACTION state:

      STAT

         Arguments: none

         Restrictions:
             may only be given in the TRANSACTION state

         Discussion:
             The POP3 server issues a positive response with a line
             containing information for the maildrop.  This line is
             called a "drop listing" for that maildrop.

             In order to simplify parsing, all POP3 servers are
             required to use a certain format for drop listings.  The
             positive response consists of "+OK" followed by a single
             space, the number of messages in the maildrop, a single
             space, and the size of the maildrop in octets.  This memo
             makes no requirement on what follows the maildrop size.
             Minimal implementations should just end that line of the
             response with a CRLF pair.  More advanced implementations
             may include other information.

                NOTE: This memo STRONGLY discourages implementations
                from supplying additional information in the drop
                listing.  Other, optional, facilities are discussed
                later on which permit the client to parse the messages
                in the maildrop.

             Note that messages marked as deleted are not counted in
             either total.

         Possible Responses:
             +OK nn mm

         Examples: