I have a few notes, I'll be not too rigorous in terminology anyway:

• client/server and peer-to-peer means nothing at the transport layer (broadly speaking: the layer in the "IP stack" where TCP and UDP are): it's a concept related to the application layer;
• there are client/server application protocols that are UDP based, for example the DNS protocol works over UDP (and is able to switch to TCP when query responses don't fit into an UDP packet).

Maybe you are trying to say is that the concept of connection is meaningless in UDP: packets that flow from a client to a server and those that flow in the opposite direction are, in some way, unrelated. I mean: in a TCP connection a channel is established and a client and a server communicate over that channel; in UDP a client sends a "message" to a server and closes, and the server sends a "message" to a client and closes: no complicated checks, no connections.

Ciao!
--bronto

The very nature of Perl to be like natural language--inconsistant and full of dwim and special cases--makes it impossible to know it all without simply memorizing the documentation (which is not complete or totally correct anyway).
--John M. Dlugosz