— This paper describes the design and implementation of a novel reliable multicast protocol, totally reliable and scalable to large number of receivers. MAF relies on Active Networks technology: active routers in the multicast tree store sender’s transmissions in order to be able to later retransmit them to repair downstream losses. To address scalability, MAF organizes those active routers into a hierarchical structure obtained by dividing the multicast tree into subtrees. Since a sender initiated approach is used within each of those subtrees, MAF has the particularity of operating correctly with finite buffers. This paper also describes the implementation of MAF over the active network platform deployed by the RNRT project AMARRAGE.