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Work on delay tolerant networks (DTN) and peer-to-peer replication systems has made rapid advances with the similar goal of permitting reliable message delivery in challenged communication environments. Techniques developed for both types of systems also bear some similarity. Both exploit opportunistic connectivity to route messages and updates to their desired destinations while making minimal assumptions about end-to-end connectivity. However, they also have some unique characteristics. DTNs have recently incorporated forwarding algorithms that make use of historical information on past encounters between participants and predictions of future connectivity. Modern replication systems utilize protocols with low overheads that guarantee eventual consistency and at-most-once delivery while supporting content-based filters. In this paper, we show how a DTN-like messaging system can be readily built as a simple application on top of a peer-to-peer replication platform. To reduce delivery delays while retaining the desirable replication guarantees, we then extend the replication substrate to permit pluggable DTN routing protocols. We describe the implementation of four representative DTN schemes as replication policies and evaluate these extensions with emulations driven by traces of e-mail messaging and vehicular mobility. We conclude that DTNs and replication systems can benefit substantially from a cross-fertilization of ideas.