Building multicast services from unicast forwarding and ephemeral state

We present an approach to building multicast services at the network layer using unicast forwarding and two additional building blocks: (i) ephemeral state probes, i.e. extremely lightweight distributed computations based on a time-bounded associative memory; and (ii) the ability to inject or enable packet processing functions that modify router behavior in a very limited way. In our approach, senders and receivers use ephemeral state probes to determine where to inject functionality. A special function that duplicates packets matching a particular pattern and forwards them to a specific destination is then instantiated at the desired network location. Our approach eliminates the need for sophisticated multicast routing protocols and gives the end-systems control over the multicast service, allowing the application to tailor the service to its needs. At the same time, our approach creates efficient forwarding paths by using ephemeral state probes to determine (only) the relevant aspects of the network and group topology. We present two multicast implementations: one builds a multicast tree with centralized control, another provides the traditional IP multicast abstraction. Both implementations can be done in a simple and scalable manner with minimal added functionality in the routers beyond unicast forwarding.