Abstract: Protocol-independent multicast sparse mode (PIM-SM) is one of the most widely used multicast routing protocols. According to its mechanism, multicast applications establish shared tree that based on the RP to transport data. Then, the receivers start to establish the shortest path tree to the source and at last transport multicast data through it. In the course of the switch from shared tree to the shortest path tree, packet loss often happens during the switch from shared tree to the shortest path tree. The paper gives a particular analysis of the reasons and other factors for packet loss. It also provides the resolving and the results of pertinent assaying.

Key words: Multicast, Protocol independent multicast-sparse mode, Rendezvous point tree, Shortest path tree

摘要： 协议无关组播-稀疏模式是目前应用最广泛的组播路由协议之一。它以汇聚点为基础建立共享树进行数据传输，由接收者发起到源的最短路径树的建立，通过最短路径树进行组播数据的分发。在共享树向最短路径树切换时会发生部分数据包丢失现象。该文分析了RPT向SPT切换时数据包丢失的原因及相关因素，提出了改进和解决方案。

关键词: 组播, 协议无关组播-稀疏模式, 共享树, 最短路径树