Network Working Group                                         R. Ullmann
Request for Comments: 1090                          Prime Computer, Inc.
                                                          February 1989


                             SMTP on X.25

1. Status of this Memo

  This memo proposes a standard for SMTP on the virtual circuit
  facility provided by the X.25 standard of the CCITT.

  Distribution of this memo is unlimited.

2. Introduction

  The possibility of using the X.25 virtual circuit (ISO level 3)
  directly for SMTP is mentioned in RFC 821 ("SIMPLE MAIL TRANSPORT
  PROTOCOL"), in appendix D.  It suggests that "a reliable end-to-end
  protocol such as TCP be used on top of X.25 connections".  This was
  undoubtedly true considering the general reliability of the PSDNs at
  the time (1981).  The service is now (in 1989) reliable enough to
  allow practical direct use of the virtual circuit service.

  The procedures given here have proven to be successful in extensive
  production use, involving 24 PSDNs in 22 different countries.  The
  resulting service is economical even using some of the more expensive
  PSDNs.  Operation over private X.25 connections and X.25 LANs has
  also proven successful.

  An X.25 virtual circuit (VC) is opened for each SMTP session.  The
  full duplex channel provided by the VC is used for the session.  The
  VC is then closed, normally by the calling side.

3. Protocol ID and Call User Data

  The first four octets (bytes) of the Call User Data Field, which are
  commonly used as a protocol identifier, or PRID, should be (hex)
  C0F70000.  (In decimal, 192 247 0 0.)

  Implementations should, however, provide the ability to configure the
  call user data on a per-address basis, including the protocol ID
  field.

4. Data stream

  The SMTP data is divided into (streamed into) packets in any way the
  sending side prefers.  Sequences with the M bit (more data) set are



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RFC 1090                      SMTP on X.25                 February 1989


  encouraged, and may be up to 2048 bytes in total length.

  It is recommended that SMTP commands and responses be sent as single
  packets, or single more-data sequences, if only to facilitate
  debugging the protocol.  This is not a requirement.

5. Qualified data

  Packets with the Q bit set and interrupt packets are not used, and
  should be ignored if received.

6. Circuit resets

  If a level 3 circuit reset is received, the VC should be cleared, and
  the SMTP connection attempted again.  The retry may be after some
  delay, and may be with different call facilities.

7. Call facilities

  Any negotiable features selected by the X.25 call request facilities
  field may be used.  Implementations should provide the ability to
  specify facilities for each called address.

8. Character code

  The character code used on X.25 is the full ASCII-8 code, with no
  escapes or modifications.  Lines are terminated by CRLF (13 10
  decimal).  Implementations should, if possible, recognize lines
  terminated only by LF (10 decimal).

9. Closing the connection

  Unlike TCP, X.25 does not provide for synchronous delivery of data in
  transit when a clear request is in progress; any packets in transit
  are discarded when the VC is cleared.  Therefore, on X.25, the SMTP
  session layer is closed by the calling side when the Service Closing
  message is received, either in response to a QUIT command, or because
  the service must shut down.

10. Timeouts

  SMTP does not normally provide for timing out a session.  On X.25,
  the following has proven to be effective:

     10.1. call request

        If a call accept is not received within 100 seconds, or the
        Service Ready message is not received within (another) 120



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        seconds, the call should be cleared and retried later.

     10.2. established

        After the protocol session is established, the circuit should
        be cleared if no response is received for 10 minutes.

     10.3. closing

        After the QUIT command is issued, the timeout should be
        shortened to 20 seconds.  This will sometimes cause an
        ungraceful exit, but this will not affect the SMTP transactions
        already completed.

     10.4. clearing

        When the X.25 Clear Request packet has been sent, the VC should
        be timed out in accordance with the X.25 protocol
        specification.

     11. Other features

        Other features of X.25, such as permanent virtual circuits and
        D bit selection, are not used.

References

    [1]  Postel, J., "Simple Mail Transfer Protocol", RFC 821, USC
         Information Sciences Institute, August 1982.

    [2]  CCITT Recommendation X.25, "Interface Between Data
         Terminal Equipment (DTE) and Data Circuit-Terminating
         Equipment (DCE) for Terminals Operating in the Packet
         Mode and Connected to Public Data Networks by Dedicated
         Circuit", International Telegraph and Telephone Consultative
         Committee, Fascicle VIII.3, Geneva, 1976; amended at
         Geneva, 1980 and Malaga-Torremolinos, 1984. ("Red Book")

Author's Address

  Robert Ullmann 23A-32
  Prime Computer, Inc.
  Technology Drive
  Milford, MA 01757

  Phone: +1 508 478 8600 x1736

  Email: [email protected]



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