UDP as a substrate
UDP has shown up in talk about peer to peer far more often than in talk about the web. The reason is that peer to peer often requires larger numbers of connections than the web. Server hubs as used on the web consolidate connections, so when you move the action away from hubs you lose the consolidation. In an mesh topography there are too many interconnections for TCP/IP to support gracefully. Since many of the connections are unreliable and short lived anyway, whether or not TCP/IP is used, UDP's weaknesses are not such a serious problem.