Error control using retransmission schemes in multicast transport protocols for real-time media

We analyze different retransmission (ARQ) schemes for error control in multicast protocols geared toward real-time, multimedia applications. We discuss why retransmission schemes are not inappropriate for such applications, but in fact can be quite effective. We present a quantitative analysis of such schemes, as well as simulation results, taking into account four different parameters (and not just the source throughput): (1) the probability of dropping a packet due to limited time for retransmissions; (2) the average time required to deliver a packet correctly to end receivers; (3) the number of times a packet will be retransmitted; and (4) the cost to the network, in terms of packet duplications, of retransmitting a packet. We reach the counter-intuitive conclusion that the optimum scheme, in terms of all four of the above parameters, in the most general scenarios (where several hosts with widely varying propagation delays and 'quality of connections' are participating in the session) is to immediately retransmit packets-preferably multicast-upon reception of a NACK from any receiver. We also demonstrate, again through quantitative analysis, the circumstances under which it would be beneficial (as well as those under which it would be counter-productive) to multicast control messages in the hope of suppressing duplicates and preventing the source from being overwhelmed by control messages.