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We present Hydra, the first multicast routing protocol for MANETs that establishes a multicast routing structure approximating the set of source-rooted shortest-path trees from multicast sources to receivers, without requiring the dissemination of control packets from each source of a multicast group. Hydra accomplishes this by dynamically electing a core for the mesh of a multicast group among the sources of the group, and aggregating multicast routing state in the nodes participating in multicast meshes, so that only control packets from the core are disseminated towards the receivers of a group. We prove that Hydra establishes correct routes from senders to receivers of a multicast group when multicast state information is aggregated. We also present simulations results illustrating that Hydra attains comparable or higher delivery ratios than ODMRP, but with considerably lower end-to-end delays and far less communication overhead. Results are shown for scenarios using 802.11 and TDMA as the MAC layer protocols.