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

           RPOP user
               Arguments: a client specific user-id (required)
               Restrictions: may only be given in the AUTHORIZATION



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                   state after a successful USER command; in addition,
                   may only be given if the client used a reserved
                   (privileged) TCP port to connect to the server.
               Discussion:

                 The RPOP command may be used instead of the PASS
                 command to authenticate access to the maildrop.  In
                 order for this command to be successful, the POP3
                 client must use a reserved TCP port (port < 1024) to
                 connect tothe server.  The POP3 server uses the
                 argument pair from the USER and RPOP commands to
                 determine if the client should be given access to
                 the appropriate maildrop.  Unlike the PASS command
                 however, the POP3 server considers if the remote user
                 specified by the RPOP command who resides on the POP3
                 client host is allowed to access the maildrop for the
                 user specified by the USER command (e.g., on Berkeley
                 UNIX, the .rhosts mechanism is used).  With the
                 exception of this differing in authentication, this
                 command is identical to the PASS command.

                 Note that the use of this feature has allowed much wider
                 penetration into numerous hosts on local networks (and
                 sometimes remote networks) by those who gain illegal
                 access to computers by guessing passwords or otherwise
                 breaking into the system.

               Possible Responses:
                   +OK maildrop locked and ready
                   -ERR permission denied
               Examples:
                   C:    USER mrose
                   S:    +OK mrose is a real hoopy frood
                   C:    RPOP mrose
                   S:    +OK mrose's maildrop has 2 messages (320
                         octets)

       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



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           RSET

           QUIT                    valid in the UPDATE state

       Optional POP3 Commands:
           RPOP user               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.

Example POP3 Session

    S: <wait for connection on TCP port 110>
        ...
    C: <open connection>
    S:    +OK dewey POP3 server ready (Comments to: PostMaster@UDEL.EDU)
    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:    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



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    S:    +OK dewey POP3 server signing off (maildrop empty)
    C:  <close connection>
    S:  <wait for next connection>

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.

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.  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



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   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 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 POP server, even if the POP server and client resided on the
   same host!

                 ASIDE: this discussion raises an issue which this memo



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                 purposedly avoids: how does SMTP know that it's talking
                 to a "trusted" MTS entity?

References

     [MZnet]   Stefferud, E., J. Sweet, 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",
               USC/Information Sciences Institute, August 1982.

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

     [RFC937]  Butler, M., J. Postel, D. Chase, J. Goldberger, and J.
               Reynolds, "Post Office Protocol - Version 2", RFC 937,
               USC/Information Sciences Institute, February 1985.

     [RFC1060] Reynolds, J., and J. Postel, "Assigned Numbers", RFC
               1060, USC/Information Sciences Institute, March 1990.

Security Considerations

   Security issues are not discussed in this memo.

Author's Address:

   Marshall T. Rose
   Performance Systems International
   5201 Great America Parkway
   Suite 3106
   Santa Clara, CA  95054

   Phone: +1 408 562 6222

   EMail: mrose@psi.com
   X.500:  rose, psi, us












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