Paper 2024/653

Aether: Approaching the Holy Grail in Asynchronous BFT

Abstract

State-of-the-art asynchronous Byzantine Fault Tolerance (BFT) protocols integrate a partially-synchronous optimistic path. The holy grail in this paradigm is to match the performance of a partially-synchronous protocol in favorable situations and match the performance of a purely asynchronous protocol in unfavorable situations. Several prior works have made progress toward this goal by matching the efficiency of a partially-synchronous protocol in favorable conditions. However, their performance compared to purely asynchronous protocols is reduced when network conditions are unfavorable. To address these shortcomings, a recent work, Abraxas (CCS'23), presents the first optimistic asynchronous BFT protocol that retains stable throughput in all situations. However, Abraxas still incurs very high worst-case latency in unfavorable situations because it is slow at detecting the failure of its optimistic path. Another recent work, ParBFT (CCS'23) guarantees good latency in all situations, but suffers from reduced throughput in unfavorable situations due to its use of extra Asynchronous Binary Agreement (ABA) instances. To approach our holy grail, we propose Aether, which delivers performance comparable to partially-synchronous protocols in favorable situations, and attains performance on par with purely asynchronous protocols in unfavorable situations—in both throughput and latency. Aether also runs the two paths simultaneously. It adopts two-chain HotStuff as the optimistic path, thus achieving high performance in favorable situations. As for the pessimistic path, we introduce a new primitive Dual-functional Byzantine Agreement (DBA), which packs the functionalities of biased ABA and Validated Asynchronous Byzantine Agreement (VABA). Aether runs DBA instances continuously as the pessimistic path. DBA’s ABA functionality quickly detects the optimistic path’s failure, ensuring Aether’s low latency in unfavorable situations. Meanwhile, the VABA functionality continuously produces blocks, maintaining Aether’s high throughput. Additionally, the biased property ensures that blocks committed via the optimistic path are respected by DBA instances, guaranteeing consistency across two paths. We conduct extensive experiments to demonstrate that Aether achieves high throughput and low latency in all situations.