This paper targets the problem of optimal resource allocation in overlay multicast, which poses both theoretical and practical challenges. Theoretically, resource allocation among overlay flows is not only subject to the network capacity constraint, but also the data constraint, mainly due to the dual role of end hosts as both receivers and senders. Practically, existing distributed resource allocation schemes assume the network links to be capable of measuring flow rates, calculating and communicating price signals, none of which actually exists in the Internet today. We address these challenges as follows. First, we formalize the problem using non-linear optimization theory, which incorporates both network constraint and data constraint. Based on our theoretical framework, we propose a distributed algorithm, which is proved to converge to the optimal point, where the aggregate utility of all receivers is maximized. Second, we propose an end-host-based solution, which relies on the coordination of end hosts to accomplish tasks originally assigned to network links. Our solution can be directly deployed without any changes to the existing network infrastructure.