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We present hBFT, a hybrid, Byzantine fault-tolerant, replicated state machine protocol with optimal resilience. Under normal circumstances, hBFT uses speculation, i.e., replicas directly adopt the order from the primary and send replies to the clients. As in prior work such as Zyzzyva, when replicas are out of order, clients can detect the inconsistency and help replicas converge on the total ordering. However, we take a different approach than previous work that has four distinct benefits: it requires many fewer cryptographic operations, it moves critical jobs to the clients with no additional costs, faulty clients can be detected and identified, and performance in the presence of client participation will not degrade as long as the primary is correct. The correctness is guaranteed by a three-phase checkpoint subprotocol similar to PBFT, which is tailored to our needs. The protocol is triggered by the primary when a certain number of requests are executed or by clients when they detect an inconsistency.

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Abstract-Building robust network services that can withstand a wide range of failure types is a fundamental problem in distributed systems. The most general approach, called Byzantine fault tolerance, can mask arbitrary failures. Yet it is often considered too costly to deploy in practice, and many solutions are not resilient to performance attacks. To address this concern we leverage two key technologies already widely deployed in cloud computing infrastructures: replicated state machines and intrusion detection systems.First, we have designed a general framework for constructing Byzantine failure detectors based on an intrusion detection system. Based on such a failure detector, we have designed and built a practical Byzantine fault-tolerant protocol, which has costs comparable to crash-resilient protocols like Paxos. More importantly, our protocol is particularly robust against several key attacks such as flooding attacks, timing attacks, and fairness attacks, that are typically not handled well by Byzantine fault masking procedures.

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