Existing committee-based Byzantine state machine replication (SMR) protocols, typically deployed in production blockchains, face a clear trade-off: (1) they either achieve linear communication cost in the happy path, but sacrifice liveness during periods of asynchrony, or (2) they are robust (progress with probability one) but pay quadratic communication cost. We believe this trade-off is unwarranted since existing linear protocols still have asymptotic quadratic cost in the worst case. We design Ditto, a Byzantine SMR protocol that enjoys the best of both worlds: optimal communication on and off the happy path (linear and quadratic, respectively) and progress guarantee under asynchrony and DDoS attacks. We achieve this by replacing the view-synchronization of partially synchronous protocols with an asynchronous fallback mechanism at no extra asymptotic cost. Specifically, we start from HotStuff, a state-of-the-art linear protocol, and gradually build Ditto. As a separate contribution and an intermediate step, we design a 2-chain version of HotStuff, Jolteon, which leverages a quadratic view-change mechanism to reduce the latency of the standard 3-chain HotStuff. We implement and experimentally evaluate all our systems. Notably, Jolteon's commit latency outperforms HotStuff by 200-300ms with varying system size. Additionally, Ditto adapts to the network and provides better performance than Jolteon under faulty conditions and better performance than VABA (a state-of-the-art asynchronous protocol) under faultless conditions. This proves our case that breaking the robustness-efficiency trade-off is in the realm of practicality.

翻译：现有基于委员会的拜占庭状态机复制协议（通常部署于生产级区块链中）面临明确的权衡困境：（1）要么在正常路径下实现线性通信开销，但在异步期间牺牲活性；（2）要么具备强鲁棒性（以概率一持续进展）但需支付二次通信开销。我们认为这种权衡并不合理，因为现有线性协议在最坏情况下仍具有渐近二次开销。本文设计了Ditto——一种拜占庭状态机复制协议，它融合了两方面优势：在正常与非正常路径下均实现最优通信开销（分别为线性与二次），并在异步和DDoS攻击下保证进展。我们通过以零额外渐近开销，用异步回退机制取代部分同步协议的视图同步来实现这一目标。具体而言，我们从最先进的线性协议HotStuff出发，逐步构建Ditto协议。作为独立贡献与中间步骤，我们设计了HotStuff的2链变体Jolteon，该协议利用二次视图变更机制降低标准3链HotStuff的延迟。我们实现了所有系统并进行实验评估。值得注意的是，在不同系统规模下，Jolteon的提交延迟比HotStuff优化200-300毫秒。此外，Ditto能自适应网络状态：在故障条件下性能优于Jolteon，在无故障条件下性能优于最先进的异步协议VABA。这证明了打破鲁棒性与效率权衡的实践可行性。