The loss path multiplicity problem in multicast congestion control

An important concern for source-based multicast congestion control algorithms is the loss path multiplicity (LPM) problem that arises because a transmitted packet can be lost on one or more of the many end-to-end paths in a multicast tree. Consequently, if a multicast source's transmission rate is regulated according to loss indications from receivers, the rate may be completely throttled as the number of loss paths increases. In this paper, we analyze a family of additive increase multiplicative decrease congestion control algorithms and show that, unless careful attention is paid to the LPM problem, the average session bandwidth of a multicast session may be reduced drastically as the size of the multicast group increases. This makes it impossible to share bandwidth in a max-min fair manner among unicast and multicast sessions. We show that max-min fairness can be achieved however if every multicast session regulates its rate according to the most congested end-to-end path in its multicast tree. We present an idealized protocol for tracking the most congested path under changing network conditions, and use simulations to illustrate that tracking the most congested path is indeed a promising approach.