Introduction to Networking for NonStop S-Series Servers

Open Systems Interconnection (OSI) Network
Connections
Introduction to Networking for HP NonStop S-Series Servers—520670-005
8-11
Message Handling System (OSI/MHS)
Message handling, or messaging, is the computing function that allows exchange of
electronic messages among users in a network. Several kinds of messages are
explicitly defined by the standard, and many more are conceivable. Among the most
familiar ones are interpersonal messages—equivalent to interoffice memoranda—
and electronic data interchange (EDI) messages, which convey other kinds of
business data in a prescribed format.
An MHS includes the following major components:
•
User Agents (UAs). A UA is a processing entity that acts on behalf of a person or
an application to submit and receive messages, either directly (through an MTA) or
indirectly (through a Message Store, or MS).
•
Message Transfer Agents (MTAs). MTAs are processing entities that accept
messages from UAs (or from other MTAs) and relay the messages across the
network to their destinations. At its destination, a message is either delivered
directly to a UA or is deposited in an MS for later retrieval. A message can pass
through any number of MTAs on its route through the network.
A user who sends a message is called the originator, and a user who receives a
message is called the recipient. Each user—and therefore the UA—has a unique name
called an Originator/Recipient (O/R) name. Several standard forms are possible for
an O/R name.
OSI/MHS consists of several kinds of processes and interfaces; Figure 8-4 on
page 8-12 illustrates some of the major ones. Multiple, identical processes operate in
parallel as a single logical MTA. A requester has access to MTA functions through any
one of the Message Relay (MR) processes. Each MR process uses another process
called the Reliable Transfer Service (RTS), which provides an interface to the HP
NonStop OSI stack. RTS is defined as a standard service within X.400; the HP
implementation is compatible with the 1988 and 1984 definitions of that service.
A third kind of process is a Message Store (MS) process; each MS process controls
multiple X.400 message stores: that is, a single MS process serves multiple X.400
users. There can be many MS processes, each serving a different set of users.
Another kind of process, called a Remote Operations Service (RS) process, gives a
remote UA access to an MS on the NonStop S-series server.