Performance trade-offs in reliable group multicast protocols

This paper presents an extensive performance study in order to identify some tradeoffs between tree-based and ring-based reliable group multicast protocols. The motivation for such a study is the following observation. In communications network routing from one node to another over a tree embedded in the network is intuitively a good strategy, since it typically results in a route length of O(log n) links, while routing from one node to another over a ring embedded in the network would result in route length of O(n) links. However, in a group multicast (many-to-many) the overall number of links traversed by each packet for both tree and ring embedding is typically O(N), so both approaches have similar communication requirements. In reliable group multicast protocols the traffic pattern is complex, since packets are sent from a multicast source to the multiple destinations, and then some control packets are sent back to the source, and this can result in resending of some of the original packets. Consequently, determining under what condition the tree-based approach is better than ring-based approach is not obvious. The key criteria for evaluating the performance of a reliable group multicast protocol is (i) how many successful multicast were achieved per unit time, and (ii) what is the efficiency of the multicast, namely, the ratio between the number of successful transmission and the total number of packets that were transmitted. Under the above criteria it is shown that the ring-based multicast often performs better than the tree-based multicast. One of the main reasons for this result is that ring-based multicast is window-based with simple and effective management of acknowledgments and retransmissions, while the tree-base is rate-based with complex and slow management of acknowledgments and retransmissions.