Abstract: Application layer multicast (ALM) without expanding the basic underlying network can support greater range of multicast communications, which makes it a popular mechanism for multicast applications. However, compared with the traditional IP multicast, ALM has larger network delay and the node stability is poor, which makes it become a unique and challenging problem to implement multi-source interactive multicast application. In this paper, an ALM protocol called Thunder is proposed. According to the roles of members in the group, Thunder divides the interactive group into core-network and peripheral-tree. The core-network consists of the multicast data sources. By using Mesh-Tree structure, it constructs an optimal distribution tree for each data source and pursues fast forwarding to optimize the interactive process. The peripherals-tree is a tree structure composed by the unstable members which just receive data or only produce a small amount of data. Additionally, Peripherals-tree structure allows more members to receive multicast data. Those unstable nodes will not influence the interaction process of core-network, so the response speed and the protocol scalability of the interactive process are improved. Experiments demonstrate that Thunder can decrease network delay of interactive ALM and improve the scalability, which makes the multi-source interactive multicast applications be deployed and implemented more easily.