Peer-to-peer architectures can be used to build a robust, fault
tolerant infrastructure for important services. One example is a
peer-to-peer data replication system, in which digital collections are
protected from failure by being replicated at multiple peers. We argue
that such community-based redundancy, in which multiple sites
contribute resources to build a fault-tolerant system, is an important
application of peer-to-peer networking. In such a system, there must
be flexible, effective techniques for managing resource allocation. We
propose data trading, a mechanism where a site acquires remote
resources in the community by trading away its own local resources. We
discuss the application of data trading to the data replication
problem, and examine other applications of trading. A general trading
infrastructure is a valuable part of a peer-to-peer, community-based
redundancy system.