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Distributing video over the Internet is an increasingly important application. Nevertheless, the real-time and high bandwidth requirements of video make video distribution over today's Internet a challenge. Adaptive approaches can be used to respond to changes in bandwidth availability while limiting the effect of such changes on perceptual quality and resource consumption. Nevertheless, most existing adaptation mechanisms have limited scalability and do not effectively exploit the heterogeneity of the Internet. In this paper, we describe the design and implementation of a MPEG video broadcasting service based on active networks. In an active network, routers can be programmed to make routing decisions based on local conditions. Because decisions are made locally, adaptation reacts rapidly to changing conditions and is unaffected by conditions elsewhere in the network. Programmability allows the adaptation policy to be tuned to the structure of the transmitted data, and to the properties of local clients. We use the PLAN-P domain-specific language for programming active routers; this language provides high-level abstractions and safety guarantees that allow complex protocols to be developed rapidly and reliably. Our experiments show that our approach to video distribution permits the decoding of up to 9 times as many frames in a heavily loaded network as distribution using standard routers.