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：现有的基于委员会结构的拜占庭状态机复制（SMR）协议（通常部署在生产级区块链中）面临明确的两难选择：（1）要么在正常路径下实现线性通信成本，但牺牲异步期间的活性保证；（2）要么具备鲁棒性（以概率1推进），但需承担二次通信成本。我们认为这种权衡并不必要，因为现有线性协议在最坏情况下仍具有渐近二次成本。我们设计了Ditto——一种兼具两者优势的拜占庭SMR协议：在正常路径与非正常路径下均实现最优通信（分别为线性与二次），并在异步与DDoS攻击下保持进度保证。通过以无额外渐近成本的方式，用异步回退机制替代部分同步协议中的视图同步模块，我们实现了这一目标。具体而言，我们从当前最先进的线性协议HotStuff出发，逐步构建出Ditto。作为独立贡献与中间步骤，我们设计了HotStuff的2链版本Jolteon，该方案利用二次视图变更机制降低了标准3链HotStuff的延迟。我们对所有系统进行了实现与实验评估。值得注意的是，在不同系统规模下，Jolteon的提交延迟较HotStuff降低200-300毫秒。此外，Ditto能够自适应网络环境：在故障条件下性能优于Jolteon，在无故障条件下性能优于VABA（当前最先进的异步协议）。这证明了我们打破鲁棒性-效率权衡的实践可行性。