Subgroup-based Source Recovery or Local Recovery for Reliable Multicasting

A traditional approach to design scalable reliable multicast algorithms is using proxies to recover lost packets. The proxies store the packets from the source and retransmit them to the requesting end receivers. Proxy placement in a multicast tree is a well-researched problem. However, given that the proxies require significant storage and processing resources, it is not possible to place them in arbitrary locations. Therefore, one may have to be satisfied with suboptimal proxy placement. Clearly, suboptimal proxy placement may waste significant bandwidth for recovery purposes. In addition, many proxy placement algorithms depend on the knowledge of per link packet loss probabilities, which are costly to estimate and change over time. In this paper, we propose a mechanism to alleviate the problem of suboptimal proxy placement. The usual approach to recover a packet to the receivers under a proxy is either to multicast the packet to all receivers or to unicast the packet to the receivers who lost it. We propose to partition the receivers under the proxy to several subgroups. The proxy multicasts the lost packet to the subgroup from which a NACK (negative acknowledgement) has been received. We designed two algorithms, i.e., first-fit and all-or-one to compute the subgroups. Our recovery method requires significantly less recovery bandwidth, which is a problem for sub-optimal proxy placement. In addition, our method does not depend on the knowledge of per link packet loss probabilities and needs very little network resources. Our method may also be used for source recovery (for small multicast groups or if no proxy is available).