Hey, I've used the i3 cluster, cool!
The idea here is that the original point to point design of the Internet is not conducive to certain applications such as multicast, anycast, and mobile hosts. The solution is a generic overlay network on which these networking models can easily be built. This avoids the problem that application level solutions encounter which is construction of redundant underlying mechanisms between multiple applications.
i3 works by presenting a service which allows sources to send to a sort of virtual host (i.e. a logical identifier), and then receivers can pull from that identifier.
This paper provides a very thorough look at the idea they are presenting. I really liked that the paper was so broad in its analysis of the idea, implementing several non-trivial applications (such as a mobile host solution and scalable multicast), discussing scalability, security, efficiency, deployment, etc. I also like that it built on another interesting Berkeley research project (Chord).