Many works have recently addressed the issue of congestion control for multicast communications and the problem is known to be highly complex. Scalability, responsiveness, stability and fairness with TCP are some of the required properties. In this report, we present a congestion avoidance scheme for bulk data distribution called amca (active-based multicast congestion avoidance algorithm) that tries to meet these properties. we use the active networking technology to perform on a per-section dialogue to probe for available bandwidth along a multicast tree. the solution uses the rtt variations experienced by every branch to estimate the congestion situation in the multicast tree. the physical multicast tree is also used to appropriate- ly aggregate the rtt variations at intermediate nodes before they reach the source. simulations show that amca converges / makes use of the available bandwidth and reacts rapidly to dynamic changes while being tcp-fair. the results also show that losses are very few demonstrating the conservative property of amca which reacts to congestion before a loss occurs