Network Working Group                                            M. Rose
Request for Comments: 1225             Performance Systems International
Obsoletes: RFC 1081                                             May 1991


                    Post Office Protocol - Version 3

Status of this Memo

   This memo suggests a simple method for workstations to dynamically
   access mail from a mailbox server.  This RFC specifies an IAB
   standards track protocol for the Internet community, and requests
   discussion and suggestions for improvements.  Please refer to the
   current edition of the "IAB Official Protocol Standards" for the
   standardization state and status of this protocol.  Distribution of
   this memo is unlimited.

Overview

   This memo is a republication of RFC 1081 which was based on RFC 918
   (since revised as RFC 937).  Although similar in form to the original
   Post Office Protocol (POP) proposed for the Internet community, the
   protocol discussed in this memo is similar in spirit to the ideas
   investigated by the MZnet project at the University of California,
   Irvine.

   Further, substantial work was done on examining POP in a PC-based
   environment.  This work, which resulted in additional functionality
   in this protocol, was performed by the ACIS Networking Systems Group
   at Stanford University.  The author gratefully acknowledges their
   interest.

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 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



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RFC 1225                          POP3                          May 1991


   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.

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).

   If this method is followed, then the client host appears to the MTS
   as a user agent, and should NOT be regarded as a "trusted" MTS entity
   in any sense whatsoever.  This concept, along with the role of the
   POP3 as a part of a split-UA model is discussed later in this memo.

   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 keyword possibly followed by an
   argument.  All commands are terminated by a CRLF pair.

   Responses in the POP3 consist of a success indicator and a keyword
   possibly followed by additional information.  All responses are
   terminated by a CRLF pair.  There are currently two success
   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.



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RFC 1225                          POP3                          May 1991


   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
   finished its transactions, 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.

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 dewey POP3 server ready (Comments to: PostMaster@UDEL.EDU)

   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 issue the USER command.  If the POP3 server responds with a
   positive success indicator ("+OK"), then the client may issue either
   the PASS command to complete the authorization, or the QUIT command
   to terminate the POP3 session.  If the POP3 server responds with a
   negative success indicator ("-ERR") to the USER command, then the
   client may either issue a new USER 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.  If so,
   the POP3 server then acquires an exclusive-access lock on the
   maildrop.  If the lock is successfully acquired, the POP3 server
   parses the maildrop into individual messages (read note below),



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RFC 1225                          POP3                          May 1991


   determines the last message (if any) present in the maildrop that was
   referenced by the RETR command, and responds with a positive success
   indicator.  The POP3 session now enters the TRANSACTION state.  If
   the lock can not be acquired or the client should is denied access to
   the appropriate maildrop or the maildrop can't be parsed for some
   reason, the POP3 server responds with a negative success indicator.
   (If a lock was acquired but the POP3 server intends to respond with a
   negative success indicator, the POP3 server must release the lock
   prior to rejecting the command.)  At this point, the client may
   either issue a new USER command and start again, or the client may
   issue the QUIT command.

                 NOTE: Minimal implementations of the POP3 need only be
                 able to break a maildrop into its component messages;
                 they need NOT be able to parse individual messages.
                 More advanced implementations may wish to have this
                 capability, for reasons discussed later.

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

   It sets the "highest number accessed" to be that of the last message
   referenced by the RETR command.

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

           USER name
               Arguments: a server specific user-id (required)
               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 welcome here
                   -ERR never heard of name
               Examples:
                   C:    USER mrose
                   S:    +OK mrose is a real hoopy frood
                     ...
                   C:    USER frated
                   S:    -ERR sorry, frated doesn't get his mail here

           PASS string
               Arguments: a server/user-id specific password (required)



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RFC 1225                          POP3                          May 1991


               Restrictions: may only be given in the AUTHORIZATION
                   state after a successful USER command
               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 unable to lock mrose's maildrop, file
                         already locked

           QUIT
               Arguments: none
               Restrictions: none
               Possible Responses:
                   +OK
               Examples:
                   C:    QUIT
                   S:    +OK dewey POP3 server signing off


The TRANSACTION State

   Once the client has successfully identified itself to the POP3 server
   and the POP3 server has locked and burst 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.
               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.



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RFC 1225                          POP3                          May 1991


                 In order to simplify parsing, all POP3 servers are
                 required to use a certain format for drop listings.
                 The first octets present must indicate the number of
                 messages in the maildrop.  Following this is 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