Compared with file-sharing and distributed hash table (DHT) network, P2P video streaming is more vulnerable to denial of service (DoS) attacks because of its high bandwidth demand and stringent time requirement. This paper studies the design of DoS resilient streaming networks using credit systems. We propose a novel framework-ripple-stream-to improve DoS resilience of P2P streaming. Ripple-stream leverages existing credit systems to introduce credit constraints in overlay construction such that malicious nodes are pushed to the fringe of overlays. Combining credit constraints with overlay optimization techniques, ripple-stream can achieve both DoS resilience and overlay efficiency.