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

- 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

   (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

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

   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.

         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

         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 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:
             C: STAT
             S: +OK 2 320

      LIST [msg]

         Arguments:
             a message-number (optional), which, if present, may NOT
             refer to a message marked as deleted

         Restrictions:
             may only be given in the TRANSACTION state

         Discussion:
             If an argument was given and the POP3 server issues a
             positive response with a line containing information for
             that message.  This line is called a "scan listing" for
             that message.

             If no argument was given and the POP3 server issues a
             positive response, then the response given is multi-line.

             After the initial +OK, for each message in the maildrop,
             the POP3 server responds with a line containing information
             for that message.  This line is also called a "scan
             listing" for that message.

             In order to simplify parsing, all POP3 servers are required
             to use a certain format for scan listings.  A scan listing
             consists of the message-number of the message, followed by
             a single space and the exact size of the message in octets.
             This memo makes no requirement on what follows the message
             size in the scan listing.  Minimal implementations should
             just end that line of the response with a CRLF pair.  More
             advanced implementations may include other information, as
             parsed from the message.

                NOTE: This memo STRONGLY discourages implementations
                from supplying additional information in the scan
                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 listed.

         Possible Responses:
             +OK scan listing follows
             -ERR no such message

         Examples:
             C: LIST
             S: +OK 2 messages (320 octets)
             S: 1 120
             S: 2 200
             S: .
               ...
             C: LIST 2
             S: +OK 2 200
               ...
             C: LIST 3
             S: -ERR no such message, only 2 messages in maildrop

      RETR msg

         Arguments:
             a message-number (required) which may not refer to a
             message marked as deleted

         Restrictions:
             may only be given in the TRANSACTION state

         Discussion:
             If the POP3 server issues a positive response, then the
             response given is multi-line.  After the initial +OK, the
             POP3 server sends the message corresponding to the given
             message-number, being careful to byte-stuff the termination
             character (as with all multi-line responses).

         Possible Responses:
             +OK message follows
             -ERR no such message

         Examples:
             C: RETR 1
             S: +OK 120 octets
             S: <the POP3 server sends the entire message here>
             S: .

      DELE msg

         Arguments:
             a message-number (required) which may not refer to a
             message marked as deleted

         Restrictions:
             may only be given in the TRANSACTION state

         Discussion:
             The POP3 server marks the message as deleted.  Any future
             reference to the message-number associated with the message
             in a POP3 command generates an error.  The POP3 server does
             not actually delete the message until the POP3 session
             enters the UPDATE state.

         Possible Responses:
             +OK message deleted
             -ERR no such message

         Examples:
             C: DELE 1
             S: +OK message 1 deleted
                ...
             C: DELE 2
             S: -ERR message 2 already deleted

      NOOP

         Arguments: none

         Restrictions:
             may only be given in the TRANSACTION state

         Discussion:
             The POP3 server does nothing, it merely replies with a
             positive response.

         Possible Responses:
             +OK

         Examples:
             C: NOOP
             S: +OK

      RSET

         Arguments: none

         Restrictions:
             may only be given in the TRANSACTION state

         Discussion:
             If any messages have been marked as deleted by the POP3
             server, they are unmarked.  The POP3 server then replies
             with a positive response.

         Possible Responses:
             +OK

         Examples:
             C: RSET
             S: +OK maildrop has 2 messages (320 octets)

6. The UPDATE State

   When the client issues the QUIT command from the TRANSACTION state,
   the POP3 session enters the UPDATE state.  (Note that if the client
   issues the QUIT command from the AUTHORIZATION state, the POP3
   session terminates but does NOT enter the UPDATE state.)

   If a session terminates for some reason other than a client-issued
   QUIT command, the POP3 session does NOT enter the UPDATE state and
   MUST not remove any messages from the maildrop.

      QUIT

         Arguments: none

         Restrictions: none

         Discussion:
             The POP3 server removes all messages marked as deleted from
             the maildrop.  It then releases any exclusive-access lock
             on the maildrop and replies as to the status of these
             operations.  The TCP connection is then closed.

         Possible Responses:
             +OK

         Examples:
             C: QUIT
             S: +OK dewey POP3 server signing off (maildrop empty)
                ...
             C: QUIT
             S: +OK dewey POP3 server signing off (2 messages left)
                ...

7. Optional POP3 Commands

   The POP3 commands discussed above must be supported by all minimal
   implementations of POP3 servers.

   The optional POP3 commands described below permit a POP3 client
   greater freedom in message handling, while preserving a simple POP3
   server implementation.

      NOTE: This memo STRONGLY encourages implementations to support
      these commands in lieu of developing augmented drop and scan
      listings.  In short, the philosophy of this memo is to put
      intelligence in the part of the POP3 client and not the POP3
      server.

      TOP msg n

         Arguments:
             a message-number (required) which may NOT refer to to a
             message marked as deleted, and a non-negative number
             (required)

         Restrictions:
             may only be given in the TRANSACTION state

         Discussion:
             If the POP3 server issues a positive response, then the
             response given is multi-line.  After the initial +OK, the
             POP3 server sends the headers of the message, the blank

             line separating the headers from the body, and then the
             number of lines indicated message's body, being careful to
             byte-stuff the termination character (as with all multi-
             line responses).

             Note that if the number of lines requested by the POP3
             client is greater than than the number of lines in the
             body, then the POP3 server sends the entire message.

         Possible Responses:
             +OK top of message follows
             -ERR no such message

         Examples:
             C: TOP 1 10
             S: +OK
             S: <the POP3 server sends the headers of the
                message, a blank line, and the first 10 lines
                of the body of the message>
             S: .
                ...
             C: TOP 100 3
             S: -ERR no such message

      UIDL [msg]

      Arguments:
          a message-number (optionally)  If a message-number is given,
          it may NOT refer to a message marked as deleted.

      Restrictions:
          may only be given in the TRANSACTION state.

      Discussion:
          If an argument was given and the POP3 server issues a positive
          response with a line containing information for that message.
          This line is called a "unique-id listing" for that message.

          If no argument was given and the POP3 server issues a positive
          response, then the response given is multi-line.  After the
          initial +OK, for each message in the maildrop, the POP3 server
          responds with a line containing information for that message.
          This line is called a "unique-id listing" for that message.

          In order to simplify parsing, all POP3 servers are required to
          use a certain format for unique-id listings.  A unique-id
          listing consists of the message-number of the message,
          followed by a single space and the unique-id of the message.

          No information follows the unique-id in the unique-id listing.

          The unique-id of a message is an arbitrary server-determined
          string, consisting of characters in the range 0x21 to 0x7E,
          which uniquely identifies a message within a maildrop and
          which persists across sessions. The server should never reuse
          an unique-id in a given maildrop, for as long as the entity
          using the unique-id exists.

          Note that messages marked as deleted are not listed.

      Possible Responses:
          +OK unique-id listing follows
          -ERR no such message

      Examples:
          C: UIDL
          S: +OK
          S: 1 whqtswO00WBw418f9t5JxYwZ
          S: 2 QhdPYR:00WBw1Ph7x7
          S: .
             ...
          C: UIDL 2
          S: +OK 2 QhdPYR:00WBw1Ph7x7
             ...
          C: UIDL 3
          S: -ERR no such message, only 2 messages in maildrop

      APOP name digest

         Arguments:
             a string identifying a mailbox and a MD5 digest string
             (both required)

         Restrictions:
             may only be given in the AUTHORIZATION state after the POP3
             greeting

         Discussion:
             Normally, each POP3 session starts with a USER/PASS
             exchange.  This results in a server/user-id specific
             password being sent in the clear on the network.  For
             intermittent use of POP3, this may not introduce a sizable
             risk.  However, many POP3 client implementations connect to
             the POP3 server on a regular basis -- to check for new
             mail.  Further the interval of session initiation may be on
             the order of five minutes.  Hence, the risk of password
             capture is greatly enhanced.

             An alternate method of authentication is required which
             provides for both origin authentication and replay
             protection, but which does not involve sending a password
             in the clear over the network.  The APOP command provides
             this functionality.

             A POP3 server which implements the APOP command will
             include a timestamp in its banner greeting.  The syntax of
             the timestamp corresponds to the `msg-id' in [RFC822], and
             MUST be different each time the POP3 server issues a banner
             greeting.  For example, on a UNIX implementation in which a
             separate UNIX process is used for each instance of a POP3
             server, the syntax of the timestamp might be:

                <process-ID.clock@hostname>

             where `process-ID' is the decimal value of the process's
             PID, clock is the decimal value of the system clock, and
             hostname is the fully-qualified domain-name corresponding
             to the host where the POP3 server is running.

             The POP3 client makes note of this timestamp, and then
             issues the APOP command.  The `name' parameter has
             identical semantics to the `name' parameter of the USER
             command. The `digest' parameter is calculated by applying
             the MD5 algorithm [RFC1321] to a string consisting of the
             timestamp (including angle-brackets) followed by a shared
             secret.  This shared secret is a string known only to the
             POP3 client and server.  Great care should be taken to
             prevent unauthorized disclosure of the secret, as knowledge
             of the secret will allow any entity to successfully
             masquerade as the named user.  The `digest' parameter
             itself is a 16-octet value which is sent in hexadecimal
             format, using lower-case ASCII characters.

             When the POP3 server receives the APOP command, it verifies
             the digest provided.  If the digest is correct, the POP3
             server issues a positive response, and the POP3 session
             enters the TRANSACTION state.  Otherwise, a negative
             response is issued and the POP3 session remains in the
             AUTHORIZATION state.

             Note that as the length of the shared secret increases, so
             does the difficulty of deriving it.  As such, shared
             secrets should be long strings (considerably longer than
             the 8-character example shown below).

         Possible Responses:
             +OK maildrop locked and ready
             -ERR permission denied

         Examples:
             S: +OK POP3 server ready <1896.697170952@dbc.mtview.ca.us>
             C: APOP mrose c4c9334bac560ecc979e58001b3e22fb
             S: +OK maildrop has 1 message (369 octets)

             In this example, the shared  secret  is  the  string  `tan-
             staaf'.  Hence, the MD5 algorithm is applied to the string

                <1896.697170952@dbc.mtview.ca.us>tanstaaf

             which produces a digest value of

                c4c9334bac560ecc979e58001b3e22fb

8. POP3 Command Summary

   Minimal POP3 Commands:

      USER name               valid in the AUTHORIZATION state
      PASS string
      QUIT

      STAT                    valid in the TRANSACTION state
      LIST [msg]
      RETR msg
      DELE msg
      NOOP
      RSET

      QUIT                    valid in the UPDATE state

   Optional POP3 Commands:

      APOP name digest        valid in the AUTHORIZATION state

      TOP msg n               valid in the TRANSACTION state
      UIDL [msg]

   POP3 Replies:

      +OK
      -ERR

   Note that with the exception of the STAT, LIST, and UIDL commands,
   the reply given by the POP3 server to any command is significant only
   to "+OK" and "-ERR".  Any text occurring after this reply may be
   ignored by the client.

9. Example POP3 Session

   S: <wait for connection on TCP port 110>
   C: <open connection>
   S:    +OK POP3 server ready <1896.697170952@dbc.mtview.ca.us>
   C:    APOP mrose c4c9334bac560ecc979e58001b3e22fb
   S:    +OK mrose's maildrop has 2 messages (320 octets)
   C:    STAT
   S:    +OK 2 320
   C:    LIST
   S:    +OK 2 messages (320 octets)
   S:    1 120
   S:    2 200
   S:    .
   C:    RETR 1
   S:    +OK 120 octets
   S:    <the POP3 server sends message 1>
   S:    .
   C:    DELE 1
   S:    +OK message 1 deleted
   C:    RETR 2
   S:    +OK 200 octets
   S:    <the POP3 server sends message 2>
   S:    .
   C:    DELE 2
   S:    +OK message 2 deleted
   C:    QUIT
   S:    +OK dewey POP3 server signing off (maildrop empty)
   C:  <close connection>
   S:  <wait for next connection>

10. Message Format

   All messages transmitted during a POP3 session are assumed to conform
   to the standard for the format of Internet text messages [RFC822].

   It is important to note that the octet count for a message on the
   server host may differ from the octet count assigned to that message
   due to local conventions for designating end-of-line.  Usually,
   during the AUTHORIZATION state of the POP3 session, the POP3 server
   can calculate the size of each message in octets when it opens the
   maildrop.  For example, if the POP3 server host internally represents
   end-of-line as a single character, then the POP3 server simply counts

   each occurrence of this character in a message as two octets.  Note
   that lines in the message which start with the termination octet need
   not be counted twice, since the POP3 client will remove all byte-
   stuffed termination characters when it receives a multi-line
   response.

11. References

   [RFC821] Postel, J., "Simple Mail Transfer Protocol", STD 10, RFC
       821, USC/Information Sciences Institute, August 1982.

   [RFC822] Crocker, D., "Standard for the Format of ARPA-Internet Text
       Messages", STD 11, RFC 822, University of Delaware, August 1982.

   [RFC1321] Rivest, R. "The MD5 Message-Digest Algorithm", RFC 1321,
       MIT Laboratory for Computer Science, April, 1992.

12. Security Considerations

   It is conjectured that use of the APOP command provides origin
   identification and replay protection for a POP3 session.
   Accordingly, a POP3 server which implements both the PASS and APOP
   commands must not allow both methods of access for a given user; that
   is, for a given "USER name" either the PASS or APOP command is
   allowed, but not both.

   Further, note that as the length of the shared secret increases, so
   does the difficulty of deriving it.

   Servers that answer -ERR to the USER command are giving potential
   attackers clues about which names are valid

   Use of the PASS command sends passwords in the clear over the
   network.

   Use of the RETR and TOP commands sends mail in the clear over the
   network.

   Otherwise, security issues are not discussed in this memo.

13. Acknowledgements

   The POP family has a long and checkered history.  Although primarily
   a minor revision to RFC 1460, POP3 is based on the ideas presented in
   RFCs 918, 937, and 1081.

   In addition, Alfred Grimstad, Keith McCloghrie, and Neil Ostroff
   provided significant comments on the APOP command.

14. Authors' Addresses

   John G. Myers
   Carnegie-Mellon University
   5000 Forbes Ave
   Pittsburgh, PA 15213

   EMail: jgm+@cmu.edu

   Marshall T. Rose
   Dover Beach Consulting, Inc.
   420 Whisman Court
   Mountain View, CA  94043-2186

   EMail: mrose@dbc.mtview.ca.us