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 (◇ RC) protocol that does not store messages for long is used for attempting at consensus, and an eventually quiescent (◇ QC) one that stops forwarding messages after a while is used for concluding consensus. Use of the ◇ 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.