X.400

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X.400 is a suite of ITU-T Recommendations that define standards for Data Communication Networks for Message Handling Systems (MHS).

1 History
2 Addressing
3 See also
4 Footnotes
5 Reference
6 External links
7 Internal links
8 Further reading

[edit] History

The first X.400 Recommendations were 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

Developed in cooperation with the ISO, the X.400 Recommendations specify OSI standard protocols for exchanging and addressing electronic messages. However, the F.400-series of Recommendations define Message Handling Services built on Message Handling Systems (MHS), as well as access to and from the MHS for public services. Therefore, in the late 1990s the ITU-T consolidated the F.400- and X.400-series of Recommendations and published the ITU-T F.400/X.400 (06/1999) Message handling system and service overview Recommendations.

The X.400 Recommendations define the technical aspects of the MHS whereas ITU-T Rec. X.402 | (ISO/IEC 10021-2) defines the overall system architecture of a MHS. All ITU-T Recommendations provide specific terms for descriptions of system entities and procedures and as such, must be strictly adhered to in literature or any form of personal communications. For example, messages (email) exchanged among people is referred to as Interpersonal Messaging (IPM); electronically structured business documents (e.g., invoices, P.O.s, dispatch advice, etc.) exchanged among trading partners’ computers fall under the EDI protocols.

As with most ISO standards dealing with application-level networking, X.400 failed to compete successfully with SMTP, the Internet-based equivalent in North America. However, in Europe, South America, and Asia, X.400 is quite widely implemented, especially for EDI services. In North America X.400 is still used in some limited applications, such as the military, intelligence services 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.

Message handling is a distributed information processing task that integrates two related subtasks: message transfer and message storage. The ITU-T Recommendations define specific protocols for a wide-range of communication tasks. For example, the P1 protocol is used explicitly for communication among MTAs, P3 between the user agent and a 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 been a factor why it never became popular.

X.400 has been extended for use in military applications (see MMHS) and aviation (see AMHS.)

[edit] Addressing

An X.400 address consists of several elements, including:

C (Country name)
ADMD (Administration Management Domain), usually a public mail service provider
PRMD (Private Management Domain)
O (Organization name)
OU (Organizational Unit Names)
G (Given name)
I (Initials)
S (Surname)

The standards themselves originally did not specify how these email addresses should be written (for instance on a business card); RFC 1685 specified one encoding, based on a 1993 draft of ITU-T Recommendation F.401 which looked like "G=Harald;S=Alvestrand;O=Uninett;P=Uninett;A=;C=no" - the unwieldiness of this addressing format is believed by many to be one factor in the lack of success of X.400. ^[1]