Practical Byzantine Fault Tolerance Using Fewer than 3f+1 Active Replicas

Byzantine fault tolerant state machine replication (BFT-SMR) is a foundation for implementations of highly reliable services. Existing algorithms for BFT-SMR require at least 3f +1 active replicas to tolerate f faulty replicas. We show that BFT-SMR can be achieved with fewer than 3f +1 active replicas, as long as standby spare replicas are available, such that the number of active replicas plus the number of spares is at least 3f +1. In particular, a single Byzantine fault can be tolerated with only three active replicas and a single standby spare. The spares are rarely used — only for reconfigurations triggered by faults or suspected faults. Each reconfiguration leads to a different subset of nodes running the active replicas. By reducing the number of active replicas, our algorithm reduces power consumption for processing and communication. It also has the potential to provide better performance. We present a 1-resilient algorithm that achieves BFT-SMR with only three active replicas and one standby spare, and show the results of preliminary performance evaluation.