I found this article . It deals with various P2P networks, including kademlia, which is the basis of the bitoral DHT. It defines a probabilistic metric called routing, not to mention connectivity, but I think these two things are connected. (The graph is probably related to high routing.) From the article:
... consider a routing measure, which is defined as the expected number of routed node pairs divided by the number of possible node pairs among the surviving nodes ....
- http://j.imagehost.org/0431/p2pgraph.png