Beyond Canonical Rounds: Communication Abstractions for Optimal Byzantine Resilience

2025-10-05

AI Summary

Classical round-based communication frameworks in asynchronous Byzantine fault-tolerant (ABFT) systems fundamentally fail to support essential tasks—including approximate consensus, Crusader agreement, reliable broadcast, and collection—within the resilience range $3f < n leq 5f$. Method: We propose a novel round-independent communication model built upon the **gather** abstraction, which bypasses traditional constraints imposed by communication closure layers and canonical asynchronous rounds. Contribution/Results: Our model enables modular reduction of connectivity-aware consensus and yields the first constant-round, optimally resilient ($n > 3f$) asynchronous consensus protocol. Theoretical analysis exposes an inherent bottleneck of round-based paradigms under high-fault scenarios, significantly enhancing composability, formal verifiability, and engineering practicality of ABFT protocols.

Technology Category

• Multiagent Systems: Agent Communication
• Reasoning under Uncertainty: Other Foundations of Reasoning under Uncertainty
• Constraint Satisfaction and Optimization: Satisfiability Modulo Theories

Application Category

• Responsible Web: Measurement, analysis, and circumvention of Web censorship
• Security and Privacy: Large-scale security measurements
• Graph Algorithms and Modeling for the Web: Efficient manipulation of static and dynamic Web-related graphs

Abstract

We study communication abstractions for asynchronous Byzantine fault tolerance with optimal failure resilience, where $n > 3f$. Two classic patterns -- canonical asynchronous rounds and communication-closed layers -- have long been considered as general frameworks for designing distributed algorithms, making asynchronous executions appear synchronous and enabling modular reasoning. We show that these patterns are inherently limited in the critical resilience regime $3f < n le 5f$. Several key tasks -- such as approximate and crusader agreement, reliable broadcast and gather -- cannot be solved by bounded-round canonical-round algorithms, and are unsolvable if communication closure is imposed. These results explain the historical difficulty of achieving optimal-resilience algorithms within round-based frameworks. On the positive side, we show that the gather abstraction admits constant-time solutions with optimal resilience ($n > 3f$), and supports modular reductions. Specifically, we present the first optimally-resilient algorithm for connected consensus by reducing it to gather. Our results demonstrate that while round-based abstractions are analytically convenient, they obscure the true complexity of Byzantine fault-tolerant algorithms. Richer communication patterns such as gather provide a better foundation for modular, optimal-resilience design.

Problem

• Studying communication abstractions for optimal Byzantine resilience
• Identifying limitations of canonical rounds in critical resilience regimes
• Proposing gather abstraction for modular optimal-resilience algorithm design

Innovation

• Gather abstraction enables optimal Byzantine resilience
• Constant-time solutions for gather with n > 3f
• Modular reductions from connected consensus to gather