Data Dissemination in Opportunistic Networks

Among the alternatives to pure general-purpose MANETs, one of the most promising approach is that of opportunistic networks. Differently from MANETs, opportunistic networks are designed to work properly even when the nodes of the network move. More specifically, opportunistic networks reverse the approach of MANETs, and what was before an accident to avoid (the mobility of nodes) now becomes an opportunity for communications. In fact, in an opportunistic network messages are exchanged between nodes when they come into contact, creating a multi-hop path from the source to the destination of the message. One of the most appealing applications to build upon an opportunistic network is data dissemination. Conceptually, data dissemination systems can be seen as variations of the publish/subscribe paradigm: publisher nodes generate content items and inject them into the network, subscriber nodes declare their interest in receiving certain types of content (e.g., sport news, radio podcast, blog entries, etc.) and strive to get it in some ways. Nodes can usually be publishers and subscribers at the same time. The main difference between message forwarding and content dissemination is that the source and destination of a message are typically well known when routing a message (and clearly listed in the header of the message itself), while, in content dissemination, content generators and content consumers might well be unaware of each other. Publish/subscribe systems have gained new momentum thanks to the Web 2.0 User Generated Content (UCG) paradigm, with users generating their own content and uploading it on popular platforms like Blogger, Youtube, or Flickr. The application of the UGC paradigm to opportunistic networks is particularly appealing. A future of users generating content items on the fly while moving, and distributing this content to the users in their proximity, can be realistically envisioned for the next years. In order to make this future a reality, new strategies for disseminating content items must be designed, while at the same time accounting for a wise usage of network resources, which can be easily saturated in this scenario. In this chapter we discuss the challenges connected with content dissemination in an opportunistic network and the solutions proposed in the literature. We classify current proposals that address the problem of content dissemination into six main categories, based on the specific problem targeted and the type of solution proposed. Then, we present and discuss the work that we believe best summarizes the main features of each category.