Causal and Δ-Causal Broadcast in Opportunistic Networks

Causal broadcast is a fundamental communication abstraction for many distributed applications. Several implementations of this abstraction have been proposed over the last decades for traditional networks, that is, networks that assume the existence of a continuous bi-directional end-to-end path between any pair of nodes. Opportunistic networks constitute a kind of networks in which this assumption cannot be made, though, so the implementation of causal broadcast in such networks must be addressed differently. This paper presents two algorithms based on causal barriers that can ensure the causally-ordered delivery of broadcast messages in an opportunistic network, considering both cases where the messages propagate in the network without or with a bounded lifetime. The latter case is especially interesting in networks that must run for a long time, or with a population of nodes that changes continuously.