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.
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
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
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.
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
seconds, the call should be cleared and retried later.
After the protocol session is established, the circuit should
be cleared if no response is received for 10 minutes.
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
When the X.25 Clear Request packet has been sent, the VC should
be timed out in accordance with the X.25 protocol
11. Other features
Other features of X.25, such as permanent virtual circuits and
D bit selection, are not used.
 Postel, J., "Simple Mail Transfer Protocol", RFC 821, USC
Information Sciences Institute, August 1982.
 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")
Robert Ullmann 23A-32
Prime Computer, Inc.
Milford, MA 01757
Phone: +1 508 478 8600 x1736