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This paper proposes a new protocol, called family ACK tree (FAT), to support reliable multicast service in ad hoc networks. FAT employs a tree-based recovery mechanism that localizes ACKs and retransmissions to avoid feedback implosion. To cope with node movements, FAT constructs an ACK tree in which each node maintains reachability information to three generations of family in the tree. When a tree is fragmented due to a departed node, the fragments will be glued back to the tree using the underlying multicast routing protocol. FAT then adopts an adaptive mechanism to recover missed packets that have been multicast to the group during fragmentation. We have conducted simulations to evaluate the performance of FAT and to compare FAT with the existing solution. The results show that FAT achieves better performance in providing reliable service in ad hoc networks, in terms of reliability, scalability, and delivery efficiency.