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In content networks, messages are routed on the basis of their content and the interests (subscriptions) of the message consumers. This form of routing offers an interesting alternative to unicast or multicast communication in loosely-coupled distributed systems with large number of consumers, with diverse interests, wide geographical dispersion, and heterogeneous resources (e.g., CPU, bandwidth). In this paper, we propose a novel protocol for content-based routing in overlay networks. This protocol guarantees perfect routing (i.e., a message is received by all, and only those, consumers that have registered a matching subscription) and optimizes the usage of the network bandwidth. Furthermore, our protocol takes advantage of subscription aggregation to dramatically reduce the size of the routing tables, and it fully supports dynamic subscription registrations and cancellations without impacting the routing accuracy. We have implemented this protocol in the application-level routers of an overlay network to build a scalable XML-based data dissemination system. Experimental evaluation shows that the size of the routing tables remains small, even with very large populations of consumers.