Dynamic Byzantine Quorum Systems

Byzantine quorum systems [13] enhance the availability and efficiency of fault-tolerant replicated services when servers may suffer Byzantine failures. An important limitation, however, is their dependence on a static threshold limit on the number of server faults. The correctness of the system is only guaranteed if at all times the threshold is higher than the actual number of faults, yet a conservatively chosen (high) threshold wastes expensive replication in the common situation where the number of faults averages well below the worst case. In this paper we present protocols for dynamically raising and lowering the resilience threshold of a quorumbased Byzantine fault-tolerant data service in response to current information on the number of server failures. Using such protocols, a system can operate in an efficient lowthreshold mode with relatively small quorums in the absence of faults, increasing and decreasing the quorum size (and thus the tolerance) as faults appear and are dealt with,