The effects of NAK-based loss recovery mechanism on window-based multicast congestion control

Window-based multicast congestion control protocols such as pgmcc and AER/NCA try to emulate TCP congestion control behaviors for achieving TCP-compatibility. However, they employ NAK-based loss recovery scheme instead of the ACK-based approach, which is used in TCP. The improvement introduced by NAK-based approach on loss recovery capabilities can bring higher throughput for the multicast session under the same network conditions. In this paper, we provide a quantitative evaluation on the influences of NAK-based loss recovery mechanism on the behaviors and throughput properties of window-based multicast congestion control. First, we identify the differences on congestion control behaviors caused by the NAK-based retransmission mechanism. Based on the observations, we mathematically model the throughput of window-based multicast congestion control protocols. Accordingly, the multicast throughput can be expressed as a function of the round-trip time and loss rate experienced by the "worst" receiver. Moreover, selecting the "worst" receiver as the nominee to send congestion control feedbacks are critical to ensure fairness and congestion avoidance in single-rate multicast congestion control schemes. As the TCP throughput equations are used in existing schemes for nominee selection, the obtained multicast throughput equations can be used to enhance the mechanisms since they predict the multicast throughput more accurately than TCP throughput equations