In recent years, the demand for multimedia streaming over the Internet is soaring. Due to the lack of a centralized point of administration, Peer-to-Peer (P2P) streaming systems are vulnerable to pollution attacks, in which video segments might be altered by any peer before being shared. Among existing proposals, reputation-based defense mechanisms are the most effective and practical solutions. We performed a measurement study on the effectiveness of this class of solutions. We implemented a framework that allows us to simulate different variations of the reputation rating systems, from the centralized global approaches to the decentralized local approaches, under different parameter settings and pollution models. One key finding is that a centralized reputation system is only effective in static network and in defending against light pollution attacks. In general, a fully distributed reputation system is more suitable for the “real-time” P2P streaming system, since it is better in handling network dynamics and fast in detecting the polluters. Based on this key finding, we propose DRank, a fully distributed rank-based reputation system. Experimental results show that this technique is more flexible and robust in fighting pollution attacks.