  It is shown that, in a precise sense, if there is no bound on the number of
faulty processes in a system with unreliable but fair communication, Uniform
Distributed Coordination (UDC) can be attained if and only if a system has
perfect failure detectors. This result is generalized to the case where there
is a bound t on the number of faulty processes. It is shown that a certain type
of generalized failure detector is necessary and sufficient for achieving UDC
in a context with at most t faulty processes. Reasoning about processes'
knowledge as to which other processes are faulty plays a key role in the
analysis.
