关键词: 应用层组播, 树形方案, 组播树维护策略

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