Coordinated traversal: (t+1)-round Byzantine agreement in polynomial time

The problem of efficiently performing Byzantine agreement in t +1 rounds in the face of arbitrarily malicious failures is treated. A communication-efficient polynomial-time protocol is presented for n>8t. The protocol is an early stopping protocol, halting in min{t+1, f+2} rounds in the worst case, where f is the number of processors that fail during the run. This is provably optimal. The protocol is based on a careful combination of early stopping, fault masking, and a technique called coordinated traversal. The combination of the three provides a powerful method for restricting the damage that a faulty processor, however malicious, can do. One of the byproducts of this protocol is a polynomial-time (t +1)-round protocol for the Byzantine firing squad problem