TRANSFER Programming Manual

6 X400 Services
069138, Update 1 to 040970 Tandem Computers Incorporated 6–1
The X400 Gateway allows TRANSFER correspondents to exchange messages with
users of different message systems that are based on international X.400 standards.
The gateway translates the format of a TRANSFER message into a format defined by
the International Telegraph and Telephone Consultative Committee (CCITT)
recommendations for X.400 message handling systems.
X.400 has its own architecture and protocols, but you need not understand these as
long as you know how to address X400 correspondents. There are a few X.400-specific
units of work (UOWs) and several X.400 delivery features, called service elements, but
you create, submit, and receive X.400 packages exactly as you do standard TRANSFER
packages. When you specify a valid X400 recipient for a package, the TRANSFER
delivery system automatically forwards the package to the X400 Gateway.
This section introduces a few X.400 concepts that may help you understand X.400
features. Topics include:
A brief overview of X.400 functionality
How to send and receive X.400 messages
How to use X.400 service elements
How the TRANSFER delivery system maps message elements from TRANSFER
format to X.400 format
For more information about the X400 gateway, refer to the TRANSFER Installation and
Management Guide , the Tandem OSI/MHS Configuration and Management Guide, the
Tandem OSI/MHS Management Programming Manual, and the SCF Reference Manual for
Tandem OSI/MHS.
An Overview
of X.400
X.400 is a set of recommendations that define and standardize electronic message
handling. The recommendations are similar in concept to those of the TRANSFER
delivery system; an X.400 correspondent can request the following services:
Submit messages on behalf of an application or a human user
Send mail messages or application-defined messages, or request services from
asynchronously connected processes
Include data and/or text in a message
X.400 is based on the Open Systems Interconnect (OSI) communication model. The
OSI model uses discrete layers of functionality to provide communication and routing
support for messages. Each layer in the OSI model has its own protocol for
communication with peer processes at other nodes and provides services for the next
higher layer.
X.400 defines two OSI sublayers—the User Agent Layer (UAL) and the Message
Transfer Layer (MTL). Both reside in the OSI application layer. The UAL provides
support for mail features; the MTL is responsible for message delivery.