Chapter 3: Message Structure

Chapter 3: Message Structure

Overview

The primary function of a mail system is to deliver messages from one user to another. Before discussing how the transfer works, it is good to understand what data we are actually working with. The OSI and Internet formats are quite different, but they do offer analogous contents.

X.400 Message Structure

P2 versus P22

There are not many differences between the 1984 and 1988 X.400 standards, but there is one that is impossible to miss. The structure of the messages is different. The implication of this is that if an MTA (message transfer agent) sends a message encoded with the 1988 standard to an MTA that was implemented using the 1984 standard, the message will not be accepted.

In 1984 the name P2 was given to the protocol for user agents to speak to each other, and it defines the structure of a message. In 1988 it was extended, and the new protocol was baptized as P22. It is important to realize that 1984 MTAs do not understand P22, so it is necessary for 1988 MTAs to know if their connection is with a 1984 MTA. If so it can send the message in P2 format.

Message Types

The unit of communication between two MTAs is an MPDU (message protocol data unit). When one user sends a message to others, we call it an interpersonal message (IPM). The bulk of the MPDUs are IPMs, but there are also other kinds.

Reports

Reports are notifications...

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X.400 and SMTP: Battle of the E-Mail Protocols

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Chapter 3: Message Structure

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