The purpose of the example developed in this chapter is to present an interesting routing algorithm for sending messages to a mobile phone. In this example, we shall again encounter a tree structure as in the previous chapter, but this time the tree structure will be modified dynamically. We shall also encounter another example (besides the bounded re-transmission protocol in Chapter 6) where the usage of clocks will play a fundamental role. This example is taken from [1].

Informal description of the problem

A, so-called, mobile agent ℳ is supposed to travel between various sites. Fixed agents situated in the sites in question want to establish some communications with it. To simplify matters, such communications are supposed to be unidirectional: they take the practical form of messages sent from the fixed agents to ℳ.

Abstract informal specification

In an ideal abstract world, the moves of the mobile agent ℳ from one site to another are instantaneous. Likewise, the knowledge by the fixed agents of the exact position of ℳ is also supposed to be instantaneous. In that case, the fixed agents follow the mobile agent ℳ by sending messages where it currently is. Notice that such messages are (for the moment) received immediately by ℳ. This is illustrated in Fig. 12.1 where the mobile agent ℳ (represented by a black square) originally situated at site c, moves then successively to sites d, a, c, and b.