Abstract: Application Level Multicast becomes an effective solution for live media streaming nowadays.Tree based ALM solution suitable for large-scale streaming is simple to construct and easy to maintain but weak in reliability.In this paper，with the study on peers stability and distance evaluation，a new tree-topology maintain policy is proposed based on peercast，a peer-peer streaming system is designed and implemented with this policy.Application shows that this algorithm works fine and can greatly deduce the influence on QoS when peers joining/leaving.

Key words: Application Level Multicast（ALM）, tree topology, maintenance policy for ALM tree

摘要： 在IP组播模型、Overlay Network和Peer-to-Peer等技术的基础上发展而来的应用层组播技术正逐步成为一个流媒体直播的有效解决方案。树形组播方案构造简单，维护开销较小，可扩展性较好，比较适合于大规模的组播应用，但是其可靠性较差。通过比较研究其他P2P算法对节点稳定性和节点距离的衡量策略，对树形拓扑维护策略PeerCast进行改进，设计并实现了一个基于该改进算法的流媒体直播系统。应用表明，该算法能动态适应节点数达数百，节点生存周期较短的组播应用，较大程度上减小节点加入退出对QoS的影响。

关键词: 应用层组播, 树形方案, 组播树维护策略