Abstract.  Many peer-to-peer (p2p) system designs assume cooperative environments, with all clients correctly running the same software. Any client who modifies its software may be able to unfairly benefit. This paper considers such fairness issues in the context of p2p multicast streaming services. We present mechanisms that can distinguish nodes with selfish behavior and reduce the quality of service experienced by these selfish nodes from their peers. The peers make their judgments strictly by observing the behavior of their upstream peers. We only require that the multicast trees be periodically rebuilt, increasing the likelihood that a freeloading node's downstream peers will later be upstream of the freeloader and can retaliate by refusing to serve the offender.