Chapter 5 : Message Transfer

Chapter 5 : Message Transfer

Overview

We now know what a message looks like and what path it traverses to get from its source to its destination. What is missing is a closer look at how a message is transferred from one system to the next.

In this area there are a few points to focus on:


	1.	We need to establish the protocols of mail transfer.


	2.	In the case of store-and-forward (both X.400 and SMTP), we need an interim storage.


	3.	We need to decide at each system where the next step toward the destination is.

X.400 Protocols

Figure 5-1 shows the protocols defined by the ITU?T for communication between the messaging components. P1 defines the protocol to be used between two adjacent MTAs. If an MTA does not support P1, it cannot be said to conform to X.400 and it cannot interoperate with other X.400-compliant MTAs. P2 is similarly important, as it defines what a user agent can expect from another user agent. As we saw earlier, this is called the content of a message, and it should conform to a certain structure or the recipient UA will not be able to use it.

Figure 5-1. X.400 Protocols

The relationship between P1 and P2 is quite similar to the relationship between the protocols at different layers in the OSI stack. P2 describes the content of a message. P1 will include messages in its protocol, so it essentially packages the P2 protocol with some...

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