We have seen many different uses of composition in the previous chapters. In this chapter we are going to take a closer look at the use of composition as a general modeling technique to break down larger models into smaller models.

We are going to look at an example that illustrates how this technique can be applied to real-life complex application-level network protocols. Such protocols are abundant. Modern software architectures rely heavily on the fact that two parties that need to communicate, for example a client and a server, or two servers, do so by using a well-defined (application-level network) protocol.

We then discuss some of the main properties of model program composition and provide a summary of the various uses of model program composition.

Modeling protocol features

A real-life protocol can be intrinsically complex. There are several reasons for this. A protocol typically has multiple layers and depends on or uses other protocols. In a good protocol design, internal details of the underlying protocols should not leak out and the layering principle should be maintained. Another reason is that a protocol typically includes many different features within a single layer. Intuitively, a feature is a part or an aspect of the overall functionality of the protocol. Features interact and together define the protocol as a whole.

An important property of protocols is that enough information is present in messages, usually in message headers, so that it is possible to maintain a consistent view of the state of the protocol on both sides of the protocol users.