Hydrozoan: Latency-Adaptive DAG Consensus under Mixed Byzantine and Crash Faults

DAG-based consensus protocols have become a leading design for high-throughput Byzantine fault-tolerant (BFT) replication. They achieve great throughput and reach the optimal three-message-delay limit for consensus among $n=3f+1$ validators. However, two-delay protocols come at a cost: they either reduce resilience by requiring larger committees (e.g., $5f+1$-style quorums) or rely on fallback mechanisms that sacrifice the DAG's high throughput.

This paper introduces **Hydrozoan**, the first DAG protocol with a *dual commit path* under a hybrid fault model. The model tolerates $f$ Byzantine validators and $c$ crashed validators on $n=3f+c+2p+1$ validators, where $p$ is a tunable parameter that sizes the fast quorum. The central idea is that leaders commit in **two message delays** whenever at most $p$ validators are faulty, and in **three message delays** otherwise, without extra messages, without view changes, and while allowing multiple leaders per round. Both paths are evaluated on the same DAG, and a novel *graded indirect rule* reconciles them so that every honest validator reaches the same decision.

The authors also present **Optimal-Hydrozoan**, a variant that

The geo-distributed evaluation demonstrates that Hydrozoan matches Mysticeti's throughput while committing approximately **25% faster** when the fast quorum fits fast regions. When the fast quorum does not fit, or after more than $p$ faults, Hydrozoan falls back to three message delays, whereas existing two-delay protocols stall.

Specifically, the paper reports that under geo-distributed conditions, the latency advantage of the fast path depends on geography: rounds that reach a remote region cost far more than those that do not. This means that the decision of which path is faster is not a property of the protocol but of the deployment's network topology. The $(f,c,p)$ knobs allow operators to configure the fast quorum to match the regions where low latency is required.

Optimal-Hydrozoan, the variant that tolerates one more fault on the fast path, is the first construction to match the known lower bound. The authors do not report a specific throughput number beyond matching Mysticeti, but they emphasize that the fallback to three message delays occurs without extra messages or view changes, preserving liveness and safety.

The machine-checked proofs in Lean 4 cover both Hydroz