X.400: Стандарт обмена сообщениями OSI

OSI standard for email
X.400 is a suite of ITU T recommendations that define the ITU T Message Handling System (MHS). At one time, the designers of X.400 were expecting it to be the predominant form of email, but this role has been taken by the SMTP based Internet e mail. Despite this, it has been widely used within organizations and was a core part of Microsoft Exchange Server until 2006; variants continue to be important in military and aviation contexts.

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 ISO/IEC, the X.400 series recommendations specify OSI standard protocols for exchanging and addressing electronic messages. The companion 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. In the late 1990s, the ITU T consolidated recommendations F.400 and X.400 and published the ITU T F.400/X.400 (06/1999) recommendation "Message handling system and service overview". The X.400 series recommendations define the technical aspects of the MHS: ITU T Rec. X.402 | (ISO/IEC 10021 2) defines the overall system architecture of an MHS, ITU T Rec. X.411 | (ISO/IEC 10021 4) defines the Message Transfer Service (MTS) and its functional component the Message Transfer Agent (MTA), and ITU T Rec. X.413 | (ISO/IEC 10021 5) defines the Message Store. All ITU T recommendations provide specific terms for descriptions of system entities and procedures. For example, messages (email) exchanged among people is referred to as Interpersonal Messaging (IPM); electronically structured business documents (e. g., invoices, purchase orders, dispatch advice, etc.) exchanged among trading partners’ computers fall under the EDI protocols. 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 an MTA, and P7 between the user agent and message store. In the 1994 version, P7 was enhanced to provide folders in the message store, allow storage of submitted messages, and provide many automatic actions such as auto foldering and correlation of replies, delivery reports and receipt notifications with submitted messages. X.400 message content standards are defined for communication between user agents. These are modelled as conceptual protocols that treat P1 and P3/P7 as providing an underlying reliable transport of message contents. The message content standard for interpersonal messaging, IPM, defined in ITU T Rec. X.420 | ISO/IEC 10021 7 was named P2 in the Red Book. The extended version of IPM in the Blue Book was given content type 22 (for P2 version 2) and is often referred to informally as P22, although that term is not used in the standards. The message content standard for EDI is defined in ITU T Rec. F.435 | ISO/IEC 10021 8 and ITU T Rec. X.435 | ISO/IEC 10021 9, and informally referred to as P35. A voice messaging content type is defined in ITU T Recs. F.440 and X.440. Exchange Server 2007 does not use the MTA object, and the X.400 connector (which must use the MTA) is gone in Exchange Server 2007. There are no longer any X.400 default proxy e mail addresses in Exchange Server 2007. Important features of X.400 include structured addressing, ASN.1 binary code enabling multimedia content (predating and more efficient than MIME), and integrated security capabilities. As X.400 inter domain relay services were assumed by ITU to be run by PTTs, X.400 incorporated fields for the automated transfer of messages between X.400 and other PTT services, such as Telex, facsimile and physical postal mail. ISO later added open routing standards (ITU T Rec. X.412 | ISO/IEC 10021 10 and ITU T Rec. X.404 | ISO/IEC 10021 11), but the initial misconception that X.400 required PTT relay services, coupled with PTT volume based charges for these, were factors that inhibited the widespread uptake of X.400.

From the late 1980s, many major countries committed to the OSI stack, via GOSIP Government Open Systems Interconnection Profiles. In the United States this was in the form of the 1990 NIST "Federal Information Processing Standard" (FIPS #146). In turn, major computer vendors committed to producing OSI compliant products, including X.400. Microsoft's Exchange Server was developed in this time period, and internally based on X.400/X.500 with the initial release "equally happy to dispatch messages via Messaging API (MAPI), X.400, or Simple Mail Transfer Protocol (SMTP)". In practice however, most of these were poorly produced, and seldom put into operation. In North America, many large defense contractors and universities had also already committed to the Internet and TCP/IP standards, including SMTP for email. There, X.400 is still used in some applications, such as the military, intelligence services and aviation, mainly because the X.400 protocol supports encryption, and its 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. In Europe, South America, and Asia, X.400 is quite widely implemented, especially for EDI services. X.400 has been extended for use in military applications see Military Message Handling System and aviation see Aeronautical Message Handling System. Furthermore, NATO uses STANAG 4406 as a standard for Military Messaging based on X.400.

The confusion caused by the X.400 addressing format led to the creation of the X.500 standard for directory services. The idea was to create a hierarchical and standardized email address directory with replication and distribution functionality that allowed multiple organizations to produce a single public directory. For instance, each Administration Management Domain (service provider) could optionally upload their directory to a shared X.500 server, and then allow this database to be searched by the X.400 User Agents during email creation, thereby avoiding needing to know anything about the address other than the recipient's name and some sort of organizational name like a company. Unfortunately, the X.500 protocol proved to be every bit as complex and unwieldy as X.400, and this led to the creation of the Lightweight Directory Access Protocol, or LDAP, that standardized a simple subset of the X.500 protocols suitable for use by end user software searching for addresses. LDAP is widely used in directory services like Microsoft's Active Directory.

RFC 1615 Migrating from X.400(84) to X.400(88)
RFC 1649 Operational Requirements for X.400 Management Domains in the GO MHS Community

ITU T Rec. F.400/X.400 |ISO/IEC 10021 1 Message handling system and service overview
ITU T Rec. X.402 |ISO/IEC 10021 2 Message Handling Systems (MHS): Overall architecture
ITU T Rec. X.411 |ISO/IEC 10021 4 Message Handling Systems (MHS): Message Transfer System: Abstract Service Definition and Procedures
ITU T Rec. X.413 |ISO/IEC 10021 5 Message Handling Systems (MHS): Message store Abstract service definition
ITU T Rec. X.419 |ISO/IEC 10021 6 Message Handling Systems (MHS): Protocol specifications
ITU T Rec. X.420 |ISO/IEC 10021 7 Message Handling Systems (MHS): Interpersonal Messaging System
ITU T Rec. X.435 |ISO/IEC 10021 9 Message Handling Systems (MHS): Electronic data interchange messaging system
ITU T Rec. X.412 |ISO/IEC 10021 10 Message Handling Systems (MHS): MHS routing
ITU T Rec. X.404 |ISO/IEC 10021 11 Message Handling Systems (MHS): MHS routing Guide for messaging systems managers