In a delay tolerant network (DTN), nodes are connected intermittently and the future node connections are mostly unknown. Since in these networks, a fully connected path from source to destination is unlikely to exist, message delivery relies on opportunistic routing. However, effective forwarding based on a limited knowledge of contact behavior of nodes is challenging. Most of the previous studies looked at only the pairwise node relations to decide routing. In contrast, in this paper, we analyze the correlation between the meetings of each node with other nodes and focus on the utilization of this correlation for efficient routing of messages. We introduce a new metric called conditional intermeeting time, which computes the average intermeeting time between two nodes relative to a meeting with a third node using only the local knowledge of the past contacts. Then, we show how we can utilize the proposed metric on the existing DTN routing protocols to improve their performance. For shortest-path based routing protocols in DTNs, we propose to route messages over conditional shortest paths in which the link cost between nodes are defined by conditional intermeeting times. Moreover, for metric-based forwarding protocols, we propose to use conditional intermeeting time as an additional delivery metric while making forwarding decisions of messages. Our trace-driven simulations on three different datasets show that the modified algorithms perform better than the original ones.
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