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A unified optimization framework for rate allocation among multiple video multicast sessions sharing a wireless network is presented. Our framework applies to delivery of both scalable and non-scalable video streams. In both cases, the optimization objective is to minimize the total video distortion of all peers without incurring excessive network utilization. Our system model explicitly accounts for heterogeneity in wireless link capacities, traffic contention among neighboring links, as well as different video rate-distortion (RD) characteristics. The proposed distributed rate allocation scheme leverages cross-layer information exchange between the media access control and application layers to achieve fast convergence at the optimal, media-aware allocation. Performance of the proposed media-aware rate allocation protocol is compared against a heuristic scheme based on TCP-friendly rate control (TFRC). In network simulations of standard-definition video streaming over single or multiple multicast trees, the proposed scheme consistently achieves higher overall video quality than the TFRC-based heuristics. When delivering scalable streams, the flexibility of per-peer rate adaptation inside each multicast tree yields a further slight improvement in overall video quality over multicast of non-scalable streams.