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An Active Service Framework and its
Application to Real-time Multimedia Transcoding
Elan Amir, Steven McCanne, and Randy H. Katz (UC Berkeley)
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.
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