Layered multicast recovery

We study the problem of localizing repair packets when packets are lost during multicasts. When repair packets are multicasted, a highly lossy receiver may swamp the entire multicast `group' with duplicate repair packets thereby wasting bandwidth; thus, the protocols need repair locality. We present a multicast layering scheme where the sender proactively distributes FEC repair packets among multiple multicast groups. Each receiver can selectively tune in to a subset of these multicast groups to obtain packets close in number to what it needs. We develop an efficient algorithm that dynamically determines the optimal distribution of FEC repair packets to a given (small constant) number of multicast groups. The running time of this algorithm is independent of the number of receivers in the multicast session, and is hence highly scalable. However, the optimal algorithm requires the knowledge of the FEC repair requirements of all the receivers in the multicast group, and hence is subject to the implosion problem. To handle the implosion problem, we develop a heuristic algorithm that achieves repair locality very similar to that of the optimal algorithm, but does not require as much global knowledge. Our multicast layering scheme can be integrated into known reliable multicast protocols to make them more scalable. For concreteness, we focus on (i) singly scoped SRM, (ii) hierarchically scoped SRM, and (iii) a tree-based reliable multicasting protocol RMTP, and present combined protocols incorporating our solutions into each of them. Simulation experiments show that our solutions can substantially enhance the scalability of these reliable multicast protocols.