Asynchronous Byzantine consensus

Reaching agreement in an asynchronous environment is essential to guarantee consistency in distributed data processing. All previous asynchronous protocols were either probabilistic or they assumed a fail-stop mode of failure. The deterministic protocol presented in this paper reaches a Strong Byzantine Agreement in a system of asynchronous processors; and therefore can sustain arbitrary faults. In our model, processors can be completely asynchronous, though the communication network has the property that a message being sent by a correctly operating processor to a set of processors will reach its destinations within a predetermined period Δ. Additional results presented in the paper prove that in the above model one cannot reach a consensus within a bounded time. A correctly operating processor should wait to receive messages from other processors before making a decision. This result holds also for Weak Byzantine Agreement, but not for nontrivial consensus. We present a trivial protocol to reach a nontrivial consensus in bounded time.