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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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 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: Ariel@Relay.Prime.COM

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