rfc1939.txt

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      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

      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.









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RFC 1939                          POP3                          May 1996


10. Example POP3 Session

      S: <wait for connection on TCP port 110>
      C: <open connection>
      S:    +OK POP3 server ready <[e-mail]>
      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>

11. 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 (and must not) be counted twice, since the POP3 client will
   remove all byte-stuffed termination characters when it receives a
   multi-line response.



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RFC 1939                          POP3                          May 1996


12. 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.

   [RFC1730] Crispin, M., "Internet Message Access Protocol - Version
       4", RFC 1730, University of Washington, December 1994.

   [RFC1734] Myers, J., "POP3 AUTHentication command", RFC 1734,
       Carnegie Mellon, December 1994.

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 should not allow both methods of access for a given user;
   that is, for a given mailbox name, either the USER/PASS command
   sequence or the 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.

14. 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.



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RFC 1939                          POP3                          May 1996


15. Authors' Addresses

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

   EMail: [e-mail]


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

   EMail: [e-mail]



































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RFC 1939                          POP3                          May 1996


Appendix A. Differences from RFC 1725

   This memo is a revision to RFC 1725, a Draft Standard.  It makes the
   following changes from that document:

      - clarifies that command keywords are case insensitive.

      - specifies that servers must send "+OK" and "-ERR" in
        upper case.

      - specifies that the initial greeting is a positive response,
        instead of any string which should be a positive response.

      - clarifies behavior for unimplemented commands.

      - makes the USER and PASS commands optional.

      - clarified the set of possible responses to the USER command.

      - reverses the order of the examples in the USER and PASS
        commands, to reduce confusion.

      - clarifies that the PASS command may only be given immediately
        after a successful USER command.

      - clarified the persistence requirements of UIDs and added some
        implementation notes.

      - specifies a UID length limitation of one to 70 octets.

      - specifies a status indicator length limitation
        of 512 octets, including the CRLF.

      - clarifies that LIST with no arguments on an empty mailbox
        returns success.

      - adds a reference from the LIST command to the Message Format
        section

      - clarifies the behavior of QUIT upon failure

      - clarifies the security section to not imply the use of the
        USER command with the APOP command.

      - adds references to RFCs 1730 and 1734

      - clarifies the method by which a UA may enter mail into the
        transport system.



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RFC 1939                          POP3                          May 1996


      - clarifies that the second argument to the TOP command is a
        number of lines.

      - changes the suggestion in the Security Considerations section
        for a server to not accept both PASS and APOP for a given user
        from a "must" to a "should".

      - adds a section on scaling and operational considerations

Appendix B. Command Index

       APOP .......................................................   15
       DELE .......................................................    8
       LIST .......................................................    6
       NOOP .......................................................    9
       PASS .......................................................   14
       QUIT .......................................................    5
       QUIT .......................................................   10
       RETR .......................................................    8
       RSET .......................................................    9
       STAT .......................................................    6
       TOP ........................................................   11
       UIDL .......................................................   12
       USER .......................................................   13



























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