Making Speculative BFT Resilient with Trusted Monotonic Counters
This addresses efficiency and scalability problems in distributed ledgers for systems requiring Byzantine fault tolerance, though it is incremental as it builds on existing speculative BFT protocols.
The paper tackles the trade-offs in speculative BFT protocols like Zyzzyva and Zyzzyva5, which are efficient and scalable but suffer from issues like fallback due to slow replicas or high replica requirements, by introducing SACZyzzyva, which achieves resilience to slow replicas with only 3f+1 replicas and one active monotonic counter, demonstrating low latency and high scalability in experiments.
Consensus mechanisms used by popular distributed ledgers are highly scalable but notoriously inefficient. Byzantine fault tolerance (BFT) protocols are efficient but far less scalable. Speculative BFT protocols such as Zyzzyva and Zyzzyva5 are efficient and scalable but require a trade-off: Zyzzyva requires only $3f + 1$ replicas to tolerate $f$ faults, but even a single slow replica will make Zyzzyva fall back to more expensive non-speculative operation. Zyzzyva5 does not require a non-speculative fallback, but requires $5f + 1$ replicas in order to tolerate $f$ faults. BFT variants using hardware-assisted trusted components can tolerate a greater proportion of faults, but require that every replica have this hardware. We present SACZyzzyva, addressing these concerns: resilience to slow replicas and requiring only $3f + 1$ replicas, with only one replica needing an active monotonic counter at any given time. We experimentally evaluate our protocols, demonstrating low latency and high scalability. We prove that SACZyzzyva is optimally robust and that trusted components cannot increase fault tolerance unless they are present in greater than two-thirds of replicas.