Analysis of simple counting protocols for delay-tolerant networks

Mobile Wireless Delay-Tolerant Networks (DTNs) are wireless networks that suffer from intermittent connectivity, but enjoy the benefit of mobile nodes that can store and forward packets or messages, and can act as relays, bringing packets and messages closer to their destination through a selective forwarding policy. Many DTN protocols compensate for the unpredictability of the network by distributing multiple message copies in the hopes that at least one will eventually be delivered. As the number of message carriers becomes large these schemes experience diminishing marginal benefits from the addition of more message carriers. We describe and analyze the Simple Counting Protocol, an extremely simple and robust method for limiting the fraction of nodes that carry a copy of a message. We examine the performance of this protocol in conjunction with several abstract mobility models and show that the protocol performs reasonably well in diverse circumstances. The Simple Counting Protocol does not assume much about node mobility, and therefore should be useful for applications where little is known about node encounter patterns. The simplicity of its implementation will hopefully make it a useful substitute for epidemic routing as a naive lower bound in protocol performance comparisons. We also show how the same simple techniques and principles can be applied in conjunction with more complex heuristic DTN protocols to reduce network resource usage, a scheme we call Intermediate Immunity.