AOMP (Ad-hoc Overlay Multicast Protocol) is a novel approach for application-layer multicast in ad-hoc networks. We introduce in this paper a new algorithm that exploits a few properties of IP-routing to extract underlying topology information. The basic idea is to match nodes' path to the source in order to detect near neighbors in the physical topology. Then, in a dynamic and decentralized way, we construct a minimum cost mobility-aware delivery tree, connecting nodes that are close to each other. We design a tree improvement algorithm in order to enhance the global performance of AOMP during data distribution. Our simulations results show that, compared to previously proposed application-layer multicast structures, AOMP yields trees with lower cost and traffic redundancy. In addition, it performs well in terms of packet losses, especially in case of node mobility.