Consensus in Sparse, Mobile Ad Hoc Networks

Consensus is central to several applications including collaborative ones which a wireless ad hoc network can facilitate for mobile users in terrains with no infrastructure support for communication. We solve the consensus problem in a sparse network in which a node can at times have no other node in its wireless range and useful end-to-end connectivity between nodes can just be a temporary feature that emerges at arbitrary intervals of time for any given node pair. Efficient one-to-many dissemination, essential for consensus, now becomes a challenge; enough number of destinations cannot deliver a multicast unless nodes retain the multicast message for exercising opportunistic forwarding. Seeking to keep storage and bandwidth costs low, we propose two protocols. An eventually relinquishing (lozenge RC) protocol that does not store messages for long is used for attempting at consensus, and an eventually quiescent (lozenge QC) one that stops forwarding messages after a while is used for concluding consensus. Use of the lozenge RC protocol poses additional challenges for consensus, when the fraction, f/n, of nodes that can crash is 1/4 <= f/n < 1/2. Consensus latency and packet overhead are measured through simulations and both decrease considerably even for a modest increase in network density.