As per Relevance of the word connection, we have this rfc below:
Network Working Group R.
Request for Comments: 1090 Prime Computer, Inc
February 1989
SMTP on X.25
1. Status of this
This memo proposes a standard for SMTP on the virtual
facility provided by the X.25 standard of the CCITT
Distribution of this memo is unlimited
2.
The possibility of using the X.25 virtual circuit (ISO level 3)
directly for SMTP is mentioned in RFC 821 ("SIMPLE MAIL
PROTOCOL"), in appendix D. It suggests that "a reliable end-to-
protocol such as TCP be used on top of X.25 connections". This
undoubtedly true considering the general reliability of the PSDNs
the time (1981). The service is now (in 1989) reliable enough
allow practical direct use of the virtual circuit service
The procedures given here have proven to be successful in
production use, involving 24 PSDNs in 22 different countries.
resulting service is economical even using some of the more
PSDNs. Operation over private X.25 connections and X.25 LANs
also proven successful
An X.25 virtual circuit (VC) is opened for each SMTP session.
full duplex channel provided by the VC is used for the session.
VC is then closed, normally by the calling side
3. Protocol ID and Call User
The first four octets (bytes) of the Call User Data Field, which
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
call user data on a per-address basis, including the protocol
field
4. Data
The SMTP data is divided into (streamed into) packets in any way
sending side prefers. Sequences with the M bit (more data) set
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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
packets, or single more-data sequences, if only to
debugging the protocol. This is not a requirement
5. Qualified
Packets with the Q bit set and interrupt packets are not used,
should be ignored if received
6. Circuit
If a level 3 circuit reset is received, the VC should be cleared,
the SMTP connection attempted again. The retry may be after
delay, and may be with different call facilities
7. Call
Any negotiable features selected by the X.25 call request
field may be used. Implementations should provide the ability
specify facilities for each called address
8. Character
The character code used on X.25 is the full ASCII-8 code, with
escapes or modifications. Lines are terminated by CRLF (13 10
decimal). Implementations should, if possible, recognize
terminated only by LF (10 decimal).
9. Closing the
Unlike TCP, X.25 does not provide for synchronous delivery of data
transit when a clear request is in progress; any packets in
are discarded when the VC is cleared. Therefore, on X.25, the
session layer is closed by the calling side when the Service
message is received, either in response to a QUIT command, or
the service must shut down
10.
SMTP does not normally provide for timing out a session. On X.25,
the following has proven to be effective
10.1. call
If a call accept is not received within 100 seconds, or
Service Ready message is not received within (another) 120
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RFC 1090 SMTP on X.25 February 1989
seconds, the call should be cleared and retried later
10.2.
After the protocol session is established, the circuit
be cleared if no response is received for 10 minutes
10.3.
After the QUIT command is issued, the timeout should
shortened to 20 seconds. This will sometimes cause
ungraceful exit, but this will not affect the SMTP
already completed
10.4.
When the X.25 Clear Request packet has been sent, the VC
be timed out in accordance with the X.25
specification
11. Other
Other features of X.25, such as permanent virtual circuits
D bit selection, are not used
[1] Postel, J., "Simple Mail Transfer Protocol", RFC 821,
Information Sciences Institute, August 1982.
[2] CCITT Recommendation X.25, "Interface Between
Terminal Equipment (DTE) and Data Circuit-
Equipment (DCE) for Terminals Operating in the
Mode and Connected to Public Data Networks by
Circuit", International Telegraph and Telephone
Committee, Fascicle VIII.3, Geneva, 1976; amended
Geneva, 1980 and Malaga-Torremolinos, 1984. ("Red Book")
Author's
Robert Ullmann 23A-32
Prime Computer, Inc
Technology
Milford, MA 01757
Phone: +1 508 478 8600 x1736
Email: Ariel@Relay.Prime.
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RFC 1090 SMTP on X.25 February 1989
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