Fundamental observations on multicast congestion control in the Internet

We study congestion control for one-to-many multicast applications in the Internet and establish a three-way relationship between the choice of regulation parameter (i.e., rate or window size), the requirement to estimate receiver round trip times, and the type of fairness that may be accomplished. In particular, we show that in order to provide TCP-compatible fairness in rate-based regulation, receiver round trip times must be known. However, such a requirement does not exist in window-based regulation. We further show that measurement of receiver round-trip times in multicast communication, is fundamentally different and more complex than unicast communication, in order to avoid implosion of acknowledgments at the source. A major part of the paper deals with extending window-based regulation to multicast communications. We show that window-based regulation using a common window size for the whole session leads to unnecessary restrictions on the throughput. To alleviate this problem, we propose a multicast window scheme using a distinct window size for each receiver, and enforcing it as the limit on the number of outstanding packets to that receiver. The complexity of window-based regulation can be defused by a receiver-driven implementation and by consolidation of receiver feedback in successive stages, e.g., using a hierarchical architecture. This hierarchical approach is also useful for scalable consolidation of receiver feedback in the case of rate-based regulation, and for distributed estimation of receiver round trip times, when such estimation is necessary.