1. A packet transmission network comprising a plurality of nodes, a plurality of transmission links for interconnecting said nodes to each other, and multicast transmission means for transmitting the same multicast message to a plurality of said nodes, the interconnection between said nodes being represented by a spanning tree, means at each said node for transmitting an acknowledgment of the receipt of said multicast message at said each node interconnected by said spanning tree to all nodes immediately adjacent to said each node on said spanning tree, and means at each said node for retransmitting said multicast message when acknowledgments of the reception of said multicast message are not received from all of said immediately adjacent nodes on said spanning tree.