X.400
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In computing, X.400 is an OSI standard developed by the ITU-T (at the time the CCITT) in cooperation with ISO for the exchange of messages, both between people and between software applications. Messages between people (that is, e-mail) are known as IPMs — Inter Personal Messages. Like most ISO standards dealing with application-level networking, it failed to compete successfully with Internet-based equivalents, in this case SMTP. X.400 was at one time quite widely implemented, especially in Europe, but its use for e-mail was almost entirely confined to in-house applications. One reason was that the pricing structures proposed by public telecomms operators were prohibitive, especially in comparison with the Internet; another was that the e-mail addressing mechanisms (based on X.500) were too complex for everyday use. X.400 is still used in some limited applications, such as military/intelligence use and aviation, mainly because the X.400 functions for integrity and security were developed and deployed much earlier than their SMTP counterparts (S/MIME, PGP and SMTP-TLS). For similar reasons it is sometimes used for transmission of EDI messages between applications.
X.400 was first published in 1984 (Red Book), and a substantially revised version was published in 1988 (Blue Book). New features were added in 1992 (White Book) and subsequent updates. Although X.400 was originally designed to run over the OSI Transport service, an adaptation to allow operation over TCP/IP, RFC 1006, has become the most popular way to run X.400
The main protocols of X.400 are: P1 for communication between MTAs, P3 between the user agent and MTA, and P7 between the user agent and message store.
Conceptual protocols are defined for the communication between user agents, even if this would not occur directly, treating P1 and P3 as providing an underlying reliable transport of message contents. The inter-user agent protocol (the message content standard) was named P2 in the Red Book, and P22 in the Blue Book.
Important features of X.400 include structured addressing, the possibility of multimedia content (predating MIME), and integrated security capabilities. As X.400 MTAs were assumed to be run by PTTs, X.400 incorporated fields for the automated transfer of messages between X.400 and other 'protocols', such as Telex, facsimile and physical postal mail. Its need for a central control may have something to do with why it never became popular.
X.400 has been extended for use in military applications (see MMHS) and aviation (see AMHS.) See also: X.500
[edit] External links
- Microsoft Exchange Server: Using Industry Standards for Greater Compatibility - The Use of x.400 in Microsoft Exchange Server 5.5 (Published August 1st 1995)
- Harald T. Alvestrand's x.400 FAQ - A comprehensive list of resources on x.400 series of standards
- X400.org
[edit] Further reading
- Betanov, Cemil (1993). Introduction to X.400. Boston: Artech House. ISBN 0-89006-597-7.
