Abstract : Opportunistic networks constitute an appealing solution to complement fixed network infrastructures – or make up for the lack thereof – in challenged areas. In this type of mobile networks, contacts between devices are intermittent and are hardly predictable. Additionally, because of the sparse and irregular distribution of mobile devices, neither end-to-end connectivity nor transmission delays can be guaranteed. The many forwarding protocols designed for Mobile Ad hoc Networks (MANETs) over the last two decades are usually ineffective in such conditions. Unplanned radio contacts between neighbor devices must be exploited opportunistically instead, and network-wide information dissemination can only rely on the store-carry-and-forward principle: any mobile device that gets an opportunity to obtain a message from a neighbor can serve as a mobile carrier (a.k.a. data mule) for this message for a while, and forward the message later to one or several other devices .
A typical opportunistic network is shown in Fig. 1. In this example the mobile devices are smartphones, laptops or tablets carried by users. The network appears as a collection of distinct, continuously changing “islands” rather than as a single, connected network. Although no end-to-end path exists for example between islands 1 and 2, a user moving – deliberately or by chance – from island 1 to island 2 can serve as a carrier for messages addressed to devices located in island 2. The lack of end-to-end connectivity in the network is thus tolerated thanks to the mobility of devices. Yet there is no guarantee that a message ever gets delivered to its destination(s): message dissemination in an opportunistic network is a best-effort activity, which is directly constrained by how devices move and get in radio contact.