Abstract

Opportunistic networks are one of the most interesting evolutions of MANETs. In opportunistic networks, mobile nodes are enabled to communicate with each other even if a route connecting them never exists. Furthermore, nodes are not supposed to possess or acquire any knowledge about the network topology, which is instead necessary in traditional MANET routing protocols. Routes are built dynamically, while messages are en route between the sender and the destination(s), and any possible node can opportunistically be used as next hop, provided it is likely to bring the message closer to the final destination. These requirements make opportunistic networks a challenging and promising research field. In this paper we survey the most interesting case studies related to opportunistic networking and discuss and organize a taxonomy for the main routing and forwarding approaches in this challenging environment. We finally envision further possible scenarios to make opportunistic networks part of the Next-Generation Internet.

Introduction

During the last few years research on multi-hop ad hoc networks has focused on a number of application environments. Originally conceived for military applications, and aimed at improving battlefield communications and survivability, multi-hop ad hoc networks have lately been proposed in many civil scenarios. As the application environments of these networks increase, their traditional communication paradigms need adequacy. Two main evolutions of multi-hop ad hoc networks are envisioned namely Mesh Networks and Opportunistic Networks. In this paper we focus on Opportunistic Networks. In opportunistic networking no assumption is made on the existence of a complete path between two nodes wishing to communicate. Source and destination nodes might never be connected to the same network, at the same time. Nevertheless, opportunistic networking techniques allow such nodes to exchange messages between them. Usually, this comes at the price of additional delay in messages delivery, since messages are often buffered in the network waiting for a path towards the destination to be available. However, there is a wide range of applications able to tolerate this. Actually, this communication paradigm is reminiscent of widespread applications such as e-mailing. Furthermore, allowing nodes to connect and disconnect at will paves the way for a number of novel application scenarios in the field of mobile ad hoc networks. So far, the main focus of research on opportunistic networks has been on routing and forwarding issues, because finding routes towards the desired destination in such disconnected environments is regarded as the most compelling issue.
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