From Few to Many Faults: Adaptive Byzantine Agreement with Optimal Communication

2025-05-26

AI Summary

Byzantine fault-tolerant (BFT) consensus suffers from communication bottlenecks, particularly under strong consistency guarantees in partial-synchrony and asynchrony. Method: This work establishes tight communication complexity bounds for strongly consistent protocols in both models and introduces adaptive optimizations for actual fault counts $f leq t$. In partial synchrony, it proposes a new protocol achieving $O(n + tf)$ word communication—optimal and preserving $t < n/3$ fault tolerance. In asynchrony, it designs an information dissemination mechanism based on bipartite expander graphs, integrated with randomized aggregation and adaptive failure detection, attaining $O((n + t^2)log n)$ word communication. Contribution/Results: The protocol asymptotically approaches the $Omega(n + t^2)$ lower bound, substantially improving over conventional $Omega(nt)$ schemes. Collectively, these results unify and enhance the communication scalability and practicality of BFT consensus across both network models.

Technology Category

• Multiagent Systems: Agent Communication
• Search and Optimization: Distributed Search
• Constraint Satisfaction and Optimization: Distributed CSP/Optimization

Application Category

• Security and Privacy: Large-scale security measurements
• Graph Algorithms and Modeling for the Web: Efficient manipulation of static and dynamic Web-related graphs
• Systems and Infrastructure for Web, Mobile and WoT: Web performance, measurement, and characterization

Abstract

Achieving agreement among distributed parties is a fundamental task in modern systems, underpinning applications such as consensus in blockchains, coordination in cloud infrastructure, and fault tolerance in critical services. However, this task can be communication-intensive, often requiring a large number of messages to be exchanged, especially in the presence of Byzantine faults, making efficiency a central challenge in the design of practical agreement protocols. In this paper, we study the problem of Strong Byzantine Agreement and establish tight upper and lower bounds on communication complexity, parameterized by the actual number of Byzantine faults. Specifically, for a system of $n$ parties tolerating up to $t$ Byzantine faults, out of which only $f leq t$ are actually faulty, we obtain the following results: In the partially synchronous setting, we present the first Byzantine Agreement protocol that achieves adaptive communication complexity of $mathcal{O}(n + t cdot f)$ words, which is asymptotically optimal. Our protocol has an optimal resilience of $t<n/3$. In the asynchronous setting, we prove a lower bound of $Omega(n + t^2)$ on the expected number of messages, and design an almost matching protocol with an optimal resilience that solves agreement with $mathcal{O}((n + t^2)cdot log n)$ words. Our main technical contribution in the asynchronous setting is the utilization of a bipartite expander graph that allows for low-cost information dissemination.

Problem

• Achieving efficient Byzantine Agreement with optimal communication.
• Establishing tight bounds on communication complexity for faults.
• Designing adaptive protocols for synchronous and asynchronous settings.

Innovation

• Adaptive Byzantine Agreement protocol
• Optimal communication complexity O(n + t*f)
• Bipartite expander graph utilization