  Motivated by distributed implementations of game-theoretical algorithms, we
study symmetric process systems and the problem of attaining common knowledge
between processes. We formalize our setting by defining a notion of
peer-to-peer networks(*) and appropriate symmetry concepts in the context of
Communicating Sequential Processes (CSP), due to the common knowledge creating
effects of its synchronous communication primitives. We then prove that CSP
with input and output guards makes common knowledge in symmetric peer-to-peer
networks possible, but not the restricted version which disallows output
statements in guards and is commonly implemented.
  (*) Please note that we are not dealing with fashionable incarnations such as
file-sharing networks, but merely use this name for a mathematical notion of a
network consisting of directly connected peers "treated on an equal footing",
i.e. not having a client-server structure or otherwise pre-determined roles.)
