As the mobility patterns of nodes are sophisticated, most of models with high accuracy to capture the mobility patterns are not applicable because of the computational complexity. At present, some simple mobility models are still used in some works, such as [17, 27]. These simple models typically have an exponentially distributed inter-meet time, which has been demonstrated with real trace by some studies [37,38]. Therefore, in this chapter, it is assumed that the meeting time epochs of each node conforms to a Poisson distribution with parameter X, giving rise to exponentially distributed inter-meet time between two nodes.
The concept of selfishness denotes that mobile nodes are not willing to help their friends to forward information due to the limited resources. Generally, mobile nodes have two patterns to exhibit selfishness. The first one is that mobile nodes occasionally contribute information to others. The second one is that some nodes do not accept or contribute information at all. Therefore, the selfishness can be divided into extreme selfishness and weak selfishness. Specially, it is assumed that (1) a mobile node does not forward information to one of its friends with a probability pnf, which denotes the weak selfishness level; (2) a mobile node does not accept the information from others absolutely, where the corresponding parameter is pi with an exponential distribution; (3) a mobile node refuses to forward the information to any nodes, where the corresponding parameter is p2 with an exponential distribution. Here, the first one refers to the weak selfishness and the next two assumptions are related to the extreme selfishness.