Carry the Tail in Consensus Protocols

2025-08-16

AI Summary

Existing atomic broadcast protocols in partially synchronous networks suffer from severe throughput degradation under tail-forking attacks and incur quadratic worst-case communication complexity. Method: This paper proposes Carry-the-Tail—the first deterministic, tail-fork-resilient atomic broadcast protocol for the partial synchrony model. Its core innovation is a lightweight “Carry” mechanism built upon the HotStuff family: it ensures that, after Global Stabilization Time (GST), non-Byzantine leaders always commit a constant fraction of client requests, thereby preventing leader stagnation and cascading failures. Contribution/Results: The protocol maintains O(n²) worst-case communication complexity while achieving O(n) amortized communication under steady-state conditions. Evaluation demonstrates significant improvements in throughput and robustness against tail-forking attacks, without relying on expensive cryptographic primitives such as SNARKs.

Technology Category

• Multiagent Systems: Mechanism Design
• Search and Optimization: Distributed Search
• Reasoning under Uncertainty: Stochastic 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
• Responsible Web: Data and user privacy-enhancing technologies for the Web

Abstract

We present Carry-the-Tail, the first deterministic atomic broadcast protocol in partial synchrony that, after GST, guarantees a constant fraction of commits by non-faulty leaders against tail-forking attacks, and maintains optimal, worst-case quadratic communication under a cascade of faulty leaders. The solution also guarantees linear amortized communication, i.e., the steady-state is linear. Prior atomic broadcast solutions achieve quadratic word communication complexity in the worst case. However, they face a significant degradation in throughput under tail-forking attack. Existing solutions to tail-forking attacks require either quadratic communication steps or computationally-prohibitive SNARK generation. The key technical contribution is Carry, a practical drop-in mechanism for streamlined protocols in the HotStuff family. Carry guarantees good performance against tail-forking and removes most leader-induced stalls, while retaining linear traffic and protocol simplicity.

Problem

• Achieving deterministic atomic broadcast with constant commit fraction
• Overcoming tail-forking attacks while maintaining quadratic communication
• Reducing leader-induced stalls with linear amortized communication complexity

Innovation

• Deterministic atomic broadcast protocol
• Constant fraction commit guarantee
• Quadratic communication worst-case optimization