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-toend 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 acheived however, if every multicast session regulates its rate according to the most congested end-to-end path in its mu..