Strangers help friends to communicate in opportunistic networks

People's inclination to move and meet while engaging in social relationships generates a wide set of contacts ranging from a few seconds to hours. This paper focuses mainly on the role of short contacts and shows how they are essential to the efficiency, stability and simple computation of the forwarding process, thus making affordable in practice efforts to deploy opportunistic networks on top of human contacts. Starting from a rich dataset of contacts that we collected in a university workplace, the paper offers three main contributions. Firstly, it shows that brief contacts contribute significantly to reducing delivery latency and increasing delivery likelihood. Secondly, the brief contacts enable the construction of a taxonomy of social relationships which has proven to be stable over time and easily computable by each handheld device in a distributed way. Finally, the novel description of sociality can be profitably applied to achieve forwarding requirements that combine efficiency of data delivery with stability and simple computation of the adopted metrics.