RFC1090 - SMTP on X.25

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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 RFC821 ("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

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

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", RFC821, 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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