If multiple multicast groups share a common tree, the inner routers of the tree need to keep forwarding state per aggragted tree instead of per group. Therefore, the number of forwarding states needed is reduced.
This paper has conducted some simulations to test PGMCC. The following problems are reported for PGMCC:
1. Adaptive timeout mechanism is required, otherwise its fairness to TCP is affected.
2. High loss rate cause PGMCC to be unfair to TCP, because it does not have cumulative ACK and thus can increase rate more than TCP.
3. NACK suppression can also be a source of unfairness. In some configurations, the worst receiver behind a long RTT path may never be chosen as acker because NACKs from closer receivers suppress its NACKs.
4. When receivers seeing similar losses have large differences in delay, acker changes can cause severe performance degradation. That is because just after acker change, its window is no longer a correct estimation of packets on flight between the sender and the acker. If the acker is changed from a long delay receiver to a short delay receiver, old ACKs from the long delay receiver won't open the window; if from short to long, the sender needs to wait a long time before ACKs can open the window.