Delay Tolerant Networks (DTNs) are usually intermittently connected mobile wireless networks. Routing in delay tolerant networks is challenging because many well-known assumptions of traditional networks do not hold in DTNs. Various routing schemes have been proposed in literature which exploit redundancy to improve the message delivery. However, there lacks a systematical analytic work on modeling the redundancy-based routing schemes for delay tolerant networks. This motivates us to develop a unified model to analyze the various redundancy-based routing schemes. In this paper, we model the message delivering process in DTNs as a continuous time Markov random process with absorbing state. With this model, we have derived the probability distribution functions of the message delivery delay for different routing schemes, including direct transmission routing, source forward, and binary forward schemes. We conduct simulations to validate our model and our analytic results. The simulations show that our model can precisely capture the characteristics of those redundancy-based routing schemes.
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