RFC 1090 (rfc1090) - Page 1 of 4
SMTP on X
Alternative Format: Original Text Document
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
Ullmann
