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FinWhale: An Optimally Resilient 2 Rounds Terminating DAG Protocol

Authors Razya Ladelsky , Roy Friedman

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LIPIcs.DISC.2026.2.pdf
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ACM Subject Classification
  • Computer systems organization → Dependable and fault-tolerant systems and networks
Keywords
  • DAG protocols
  • Byzantine Atomic Broadcast
  • Consensus
  • Fast Termination

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Abstract

DAG-based Byzantine Fault Tolerant (BFT) protocols provide high-throughput consensus under partial synchrony, but existing DAG protocols still require at least three message delays to commit decisions. In contrast, fast-path BFT protocols can achieve optimal two-message-delay termination under favorable conditions, though they do not naturally extend to DAGs.
We present FinWhale, the first DAG-based BFT protocol with a two-message-delay fast path. FinWhale extends Mysticeti with a novel fast-path commit mechanism that safely coexists with the protocol’s original slow-path rules. To preserve safety across different local DAG views, we introduce new commit structures based on FP-evidence blocks, enabling validators to combine fast-path and slow-path reasoning consistently.
FinWhale operates in the partially synchronous model with n = 3f+2p-1 validators, matching the known lower bound for fast Byzantine consensus. The protocol tolerates up to f Byzantine faults and achieves fast termination whenever at most 1 ≤ ~p ≤ ~f validators fail during the fast path. Our results show that optimal-latency fast paths can be integrated into uncertified DAG consensus protocols.

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Razya Ladelsky and Roy Friedman. FinWhale: An Optimally Resilient 2 Rounds Terminating DAG Protocol. In 40th International Symposium on Distributed Computing (DISC 2026). Leibniz International Proceedings in Informatics (LIPIcs), Volume 397, pp. 2:1-2:22, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2026) https://doi.org/10.4230/LIPIcs.DISC.2026.2

Author Details

Razya Ladelsky
  • Technion, Haifa, Israel
Roy Friedman
  • Technion, Haifa, Israel

Acknowledgements

We thank Alberto Sonnino and Sebastian Müller for insightful discussions that helped us better understand Mysticeti and Starfish. We also thank the anonymous reviewers for their valuable comments, which helped us improve the presentation of the paper.

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