  Delay Tolerant Networks (DTNs) rely on the cooperation of nodes in a network
to forward a message from its source to its destination. Most of previous
studies on DTNs have focused on the design of routing schemes under the
hypothesis that each relay node is willing to participate in the forwarding
process. However, the delivery of a message incurs energy and memory costs. In
this paper we handle the problem of how to incentivize mobile nodes to
participate in relaying messages using a reward mechanism. We consider
heterogeneous relay nodes where the cost for taking part in the forwarding
process varies as a function of the mobility patterns of the relays. We show
that under fairly weak assumptions on the mobility pattern, the expected reward
the source pays remains the same irrespective of the information it conveys to
the relays, provided that the type of information does not vary dynamically
over time. We also characterize the effect of time to live (TTL) counters on
the delivery probability and memory usage. Using simulations based on a
synthetic mobility model and real mobility traces, we perform a few tests that
show a good accordance among theoretical and simulation results.
