Carry the Tail in Consensus Protocols

TL;DR

Carry-the-Tail introduces a deterministic atomic broadcast protocol in partial synchrony that ensures a constant fraction of commits by non-faulty leaders post-GST, effectively countering tail-forking attacks with optimal communication complexity.

Contribution

It presents Carry, a practical mechanism that enhances HotStuff-like protocols to resist tail-forking attacks while maintaining linear communication and simplicity.

Findings

Guarantees a constant fraction of commits post-GST.

Maintains linear amortized communication in steady state.

Resists tail-forking attacks without quadratic communication overhead.

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.