Network of proxies

Suppose that you have N web clients connected on a network. The network is represented with a graph G = (N, E). A certain number of clients can cache a small amount of the content they download. In this case, they can act as proxies for other clients.

1. How do you choose the clients which are promoted to proxies?
2. How do they index their content?
3. What content do you prefer to cache?

Can you think about metrics to evaluate your proposed methodologies for (1, 2, 3)?