





Network Working Group                                            M. Rose
Request for Comments: 1460                  Dover Beach Consulting, Inc.
Obsoletes: 1225                                                June 1993


                    Post Office Protocol - Version 3


Status of this Memo

   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 revision to RFC 1225, a Draft Standard.  It makes the
   following changes from that document:

        - the RPOP facility is removed;

        - the optional APOP facility is added (which is in interoperable,
          operational use in at least three implementations);

        - a typo was corrected with respect to the interaction of LAST
          and RSET;

        - section numbers were added; and,

        - an acknowledgements section was added.

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



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   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).

   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.

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 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



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   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
   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.

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 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.




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   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),
   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



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                     ...
                   C:    USER frated
                   S:    -ERR sorry, frated doesn't get his mail here

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

5. 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.



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               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 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
                      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-id (optionally)  If a message-id 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 "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



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                 containing information for that message.  This line
                 is 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.
                 The first octets present must be the message-id of
                 the message.  Following the message-id is the 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-id (required)  This message-id 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



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                 given message-id, being careful to byte-stuff the
                 termination character (as with all multi-line
                 responses).

                 If the number associated with this message is higher
                 than the "highest number accessed" in the maildrop, the
                 POP3 server updates the "highest number accessed" to
                 the number associated with this message.

               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-id (required)  This message-id
                   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-id 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.

                 If the number associated with this message is higher
                 than the "highest number accessed" in the maildrop,
                 the POP3 server updates the "highest number accessed"
                 to the number associated with this message.

               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.



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               Discussion:

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

               Possible Responses:
                   +OK
               Examples:
                   C:    NOOP
                   S:    +OK

           LAST
               Arguments: none
               Restrictions: may only be issued in the TRANSACTION state.
               Discussion:

                 The POP3 server issues a positive response with a line
                 containing the highest message number which accessed.
                 Zero is returned in case no message in the maildrop has
                 been accessed during previous transactions.  A client
                 may thereafter infer that messages, if any, numbered
                 greater than the response to the LAST command are
                 messages not yet accessed by the client.

             Possible Response:
                   +OK nn

             Examples:
                   C:      STAT
                   S:      +OK 4 320
                   C:      LAST
                   S:      +OK 1
                   C:      RETR 3
                   S:      +OK 120 octets
                   S:      <the POP3 server sends the entire message
                           here>
                   S:      .
                   C:      LAST
                   S:      +OK 3
                   C:      DELE 2
                   S:      +OK message 2 deleted
                   C:      LAST
                   S:      +OK 3
                   C:      RSET
                   S:      +OK
                   C:      LAST
                   S:      +OK 0




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           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.  In addition, the
                 "highest number accessed" is also reset to zero.

               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.)

           QUIT
               Arguments: none
               Restrictions: none
               Discussion:

                 The POP3 server removes all messages marked as deleted
                 from the maildrop.  It then releases the
                 exclusive-access lock on the maildrop and replies as
                 to the success 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)
                     ...





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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-id (required) and a number.  This
                   message-id 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 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 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
                   S:    -ERR no such message




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           APOP name digest
               Arguments: a server specific user-id and a 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



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                 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.

               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 "tanstaaf".
                 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
           LAST
           RSET




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           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

       POP3 Replies:
           +OK
           -ERR

       Note that with the exception of the STAT command, 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>




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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 byte 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 client
   can calculate the size of each message in octets when it parses the
   maildrop into messages.  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. The POP and the Split-UA model

   The underlying paradigm in which the POP3 functions is that of a
   split-UA model.  The POP3 client host, being a remote PC based
   workstation, acts solely as a client to the message transport system.
   It does not provide delivery/authentication services to others.
   Hence, it is acting as a UA, on behalf of the person using the
   workstation.  Furthermore, the workstation uses SMTP to enter mail
   into the MTS.

   In this sense, we have two UA functions which interface to the
   message transport system: Posting (SMTP) and Retrieval (POP3).  The
   entity which supports this type of environment is called a split-UA
   (since the user agent is split between two hosts which must
   interoperate to provide these functions).

                 ASIDE:  Others might term this a remote-UA instead.
                 There are arguments supporting the use of both terms.

   This memo has explicitly referenced TCP as the underlying transport
   agent for the POP3.  This need not be the case.  In the MZnet split-
   UA, for example, personal micro-computer systems are used which do
   not have IP-style networking capability [MZnet].  To connect to the
   POP3 server host, a PC establishes a terminal connection using some
   simple protocol (PhoneNet).  A program on the PC drives the
   connection, first establishing a login session as a normal user.  The
   login shell for this pseudo-user is a program which drives the other
   half of the terminal protocol and communicates with one of two
   servers.  Although MZnet can support several PCs, a single pseudo-
   user login is present on the server host.  The user-id and password



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   for this pseudo-user login is known to all members of MZnet.  Hence,
   the first action of the login shell, after starting the terminal
   protocol, is to demand a USER/PASS authorization pair from the PC.
   This second level of authorization is used to ascertain who is
   interacting with the MTS.  Although the server host is deemed to
   support a "trusted" MTS entity, PCs in MZnet are not.  Naturally, the
   USER/PASS authorization pair for a PC is known only to the owner of
   the PC (in theory, at least).

   After successfully verifying the identity of the client, a modified
   SMTP server is started, and the PC posts mail with the server host.
   After the QUIT command is given to the SMTP server and it terminates,
   a modified POP3 server is started, and the PC retrieves mail from the
   server host.  After the QUIT command is given to the POP3 server and
   it terminates, the login shell for the pseudo-user terminates the
   terminal protocol and logs the job out.  The PC then closes the
   terminal connection to the server host.

   The SMTP server used by MZnet is modified in the sense that it knows
   that it's talking to a user agent and not a "trusted" entity in the
   message transport system.  Hence, it does performs the validation
   activities normally performed by an entity in the MTS when it accepts
   a message from a UA.

   The POP3 server used by MZnet is modified in the sense that it does
   not require a USER/PASS combination before entering the TRANSACTION
   state.  The reason for this (of course) is that the PC has already
   identified itself during the second-level authorization step
   described above.

                 NOTE: Truth in advertising laws require that the author
                 of this memo state that MZnet has not actually been
                 fully implemented.  The concepts presented and proven
                 by the project led to the notion of the MZnet
                 split-slot model.  This notion has inspired the
                 split-UA concept described in this memo, led to the
                 author's interest in the POP, and heavily influenced
                 the the description of the POP3 herein.

   In fact, some UAs present in the Internet already support the notion
   of posting directly to an SMTP server and retrieving mail directly
   from a POP3 server, even if the POP3 server and client resided on the
   same host!

                 ASIDE: this discussion raises an issue which this memo
                 purposedly avoids: how does SMTP know that it's talking
                 to a "trusted" MTS entity?




Rose                                                           [Page 16]

RFC 1460                          POP3                         June 1993


12. References

   [MZnet]   Stefferud, E., Sweet, J., and T. Domae, "MZnet: Mail
             Service for Personal Micro-Computer Systems,:
             Proceedings, IFIP 6.5 International Conference on
             Computer Message Systems, Nottingham, U.K., May 1984.

   [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", MIT
             Laboratory for Computer Science, April 1992.

13. 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.

   Otherwise, security issues are not discussed in this memo.

14. Acknowledgements

   The POP family has a long and checkered history.  Although primarily
   a minor revision to [RFC1225], 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.

15. Author's Address

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

   Phone: +1 415 968 1052
   Fax:   +1 415 968 2510

   EMail: mrose@dbc.mtview.ca.us
   X.500: rose, dbc, us



Rose                                                           [Page 17]
