An active service framework and its application to real-time multimedia transcoding

Several recent proposals for an "active networks" architecture advocate the placement of user-defined computation within the network as a key mechanism to enable a wide range of new applications and protocols, including reliable multicast transports, mechanisms to foil denial of service attacks, intra-network real-time signal transcoding, and so forth. This laudable goal, however, creates a number of very difficult research problems, and although a number of pioneering research efforts in active networks have solved some of the preliminary small-scale problems, a large number of wide open problems remain. In this paper, we propose an alternative to active networks that addresses a restricted and more tractable subset of the active-networks design space. Our approach, which we (and others) call "active services", advocates the placement of user-defined computation within the network as with active networks, but unlike active networks preserves all of the routing and forwarding semantics of current Internet architecture by restricting the computation environment to the application layer. Because active services do not require changes to the Internet architecture, they can be deployed incrementally in today's Internet.We believe that many of the applications and protocols targeted by the active networks initiative can be solved with active services and, toward this end, we propose herein a specific architecture for an active service and develop one such service in detail --- the Media Gateway (MeGa) service --- that exploits this architecture. In defining our active service, we encountered six key problems --- service location, service control, service management, service attachment, service composition, and the definition of the service environment --- and have crafted solutions for these problems in the context of the MeGa service. To verify our design, we implemented and fielded MeGa on the UC Berkeley campus, where it has been used regularly for several months by real users who connect via ISDN to an "on-line classroom". Our initial experience indicates that our active services prototype provides a very flexible and programmable platform for intra-network computation that strikes a good balance between the flexibility of the active networks architecture and the practical constraints of incremental deployment in the current Internet.