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）面临明显的权衡：（1）要么在"快乐路径"实现线性通信成本，但牺牲异步期间的活性；要么（2）具备鲁棒性（以概率1推进），但需承担二次通信成本。我们认为这种权衡缺乏依据，因为现有线性协议在最坏情况下仍存在渐近二次成本。我们设计了Ditto——一种兼得两者优势的拜占庭SMR协议：在快乐路径内外均实现最优通信（分别为线性和二次），并在异步及DDoS攻击下保证协议推进。我们通过用异步回退机制替代部分同步协议的视图同步，且不增加额外渐近成本来实现这一目标。具体而言，我们从当前最优的线性协议HotStuff出发，逐步构建Ditto。作为独立贡献和中间步骤，我们设计了2链版HotStuff——Jolteon，其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，Jolteon的提交延迟在不同系统规模下均优于HotStuff 200-300毫秒。此外，Ditto能自适应网络环境：在故障条件下性能优于Jolteon，在无故障条件下性能优于VABA（当前最优的异步协议）。这证明打破鲁棒性-效率权衡确实具备实践可行性。