The P2P case this article discusses is "peer-to-peer lockstep", which is only a small subset of possible p2p systems.
(it's called peer-to-peer instead of client-to-client, because there is no client role in such a system).
(it's called peer-to-peer instead of client-to-client, because there is no client role in such a system).
A great series for you to read.
tl;dr
Udp is good. client to client sessions are terrible over the internet.