REVIEW 2 major objections 4 minor 33 references
DAG-based Consensus with Asymmetric Trust [Extended Version]
T0 review · 2 major / 4 minor · reviewed 2026-08-07 · deepseek-v4-flash
Pith's one-line read The paper presents the first randomized asynchronous DAG-based consensus protocol with asymmetric quorums, where each participant trusts its own list of peers, and claims expected constant-round progress.
desk verdict The counterexample and asymmetric gather are solid, but the headline consensus protocol has a round-transition bug that invalidates the proof as printed. read the letter →
The pith
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
The reading
What carries the argument
The load-bearing object is the asymmetric gather (Algorithm 3), which replaces threshold counting with per-process quorum conditions and inserts an amplification handshake: a process distributes its candidate set, waits for ACK messages from one of its own quorums, sends READY, waits for READY from a quorum, sends CONFIRM, and only then distributes its second-round sets. The effect is that some candidate set from a guild member is guaranteed to have reached a full quorum before second-round sets spread; quorum consistency forces every other guild member's chosen quorum to intersect that set's quorum, so one first-round candidate is contained in every delivered output. In the consensus protocol (Algorithm 4), each four-round wave is one execution of this gather structure, and the commit rule requires that a quorum of round-4 vertices have strong paths to the randomly chosen wave leader. The paper also defines the 'guild' as the set of wise processes that contains a quorum for each of its members; all guarantees are stated for this maximal guild.
What would settle it
Run Algorithm 4 on the 30-process quorum system of Figure 1 with all processes correct and check whether, along any execution, a process reaches line 107 and broadcasts a round-2 vertex while its local DAG for round 1 is missing at least one vertex from every quorum in $Q_i$. The existence of such a vertex would show that the printed code does not satisfy the quorum-membership invariant on which Lemma 4.2 and the liveness argument depend; a corrected guard at line 105 would restore it.
Extended reading notes
Core claim
The central claim is that randomized asynchronous DAG-based consensus can be realized over asymmetric quorum systems, despite the fact that simply swapping asymmetric quorums into existing common-core primitives does not work. The paper proves the failure by giving a 30-process fail-prone system satisfying the asymmetric quorum-system condition where the quorum-replacement version of the standard three-round gather leaves no common core in any output. It then introduces Algorithm 3, a constant-round asymmetric gather whose ACK/READY/CONFIRM handshake guarantees that at least one guild member's candidate first-round set is held by every guild member when the protocol delivers. Building on this, Algorithm 4 adapts DAG-Rider's wave structure to asymmetric quorums, with round changes gated by per-process quorum conditions and a commit rule requiring strong paths from a quorum of round-4 vertices to the randomly chosen wave leader. The paper argues that the expected number of waves until a leader is committed is at most $|P|/q(Q)$, where $q(Q)$ is the size of the smallest quorum, making the expected latency constant when quorums have linear size.
Load-bearing premise
The load-bearing premise is that a process advances from a round only after receiving that round's vertices from at least one of its own quorums, meaning a set of peers it trusts enough; Algorithm 4 as printed appears to violate this at lines 105-108, where a process can leave round 1 and broadcast a round-2 vertex immediately after broadcasting its own round-1 vertex, before any round-1 quorum has been received, and if that printed transition is real the quorum-membership property used by the proofs does not hold.
Editorial extensions
If this is right
- With Algorithm 4, DAG-based atomic broadcast no longer needs a single global failure threshold: each node's own quorum list determines what it waits for, and the protocol still orders messages.
- Expected progress remains constant: the number of waves until a leader is committed is at most $|P|/q(Q)$, so as long as quorum sizes grow linearly with the participant count, per-wave latency is bounded in expectation.
- The new asymmetric gather is itself a reusable constant-round common-core primitive, so other DAG protocols with common-core commit rules can be converted to the asymmetric model using this machinery rather than the failed quorum-replacement heuristic.
- The 30-process counterexample shows that existing asymmetric translations of common-core protocols cannot be used as-is, establishing that the extra control messages in Algorithm 3 are necessary rather than cosmetic.
Reading between the lines
- A testable next step is to instantiate the new asymmetric gather in a protocol with a two-round common core, mirroring the Tusk-style approach, to see whether the extra control round can be trimmed without losing the common-core guarantee.
- The paper's guarantees are stated for the maximal guild; an open question is whether a weaker liveness property can be proven for correct processes that are not wise, since the paper itself notes that naive processes can affect safety and liveness.
- Another stress test is to simulate Algorithm 4 on the paper's 30-process counterexample quorum system and compare the empirical commit rate against the $|P|/q(Q)$ bound, using a system engineered to defeat quorum replacement as a clean benchmark.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper proposes adapting DAG-based consensus to the asymmetric-trust model. It first shows, via a 30-process counterexample and a verifying Python script, that the standard three-round gather protocol fails when threshold quorums are replaced by asymmetric quorums (Section 3.2, Lemma 3.2). It then introduces Algorithm 3, a constant-round asymmetric gather protocol with proofs in Lemmas 3.3-3.8, and Algorithm 4, a DAG-Rider-style asymmetric atomic broadcast protocol claimed to terminate in an expected constant number of rounds proportional to |P|/q(Q). The main contribution is stated as the first randomized asynchronous DAG-based consensus protocol with asymmetric quorums.
Significance. If the protocol were correct as intended, this would be a genuinely novel contribution: it extends the DAG-Rider paradigm to a model where each process chooses its own quorums, and it identifies a non-obvious obstacle in adapting common-core primitives to asymmetric trust. The counterexample of Section 3.2 is a useful negative result, and the provision of a runnable Python verifier is a concrete checkability asset. The intended asymmetric gather protocol (Algorithm 3) and the expected-constant-round argument (Lemma 4.4) are plausible. However, the paper currently contains load-bearing pseudocode errors: Algorithm 4 does not implement the round-change rule on which the proofs of Lemmas 4.2 and 4.3 rely, and Algorithm 3's line 60 has a type-incorrect guard. These issues must be fixed before the central claims can be accepted.
major comments (2)
- [Section 4.3, Algorithm 4, lines 105-108 and 117-120] Section 4.3 states that a round is considered complete only when a process has delivered vertices from all members of at least one of its quorums, and that this rule applies to round 1 and round 3 sets. Algorithm 4 does not implement this rule. After line 99 is satisfied for r=0, lines 100-104 create and broadcast the round-1 vertex; then lines 105-108 immediately increment r from 1 to 2 and create/broadcast a round-2 vertex, without checking Q |= DAG[1]. Likewise, after lines 109-116 move from round 2 to round 3, lines 117-120 immediately create a round-4 vertex without waiting for a quorum of round-3 vertices. Because line 81 records strong edges only to the creator's current DAG[round-1] at creation time, a round-2 or round-4 vertex created in this way may have strong edges to only a subset of the previous round, not to a quorum. The invariant used in Lemma 4.2 ("any vertex in DAG_i[r] has a strong path to the vertices produced by some quorum in DAG_i[r-1]") is therefore false for r=2 and r=4, and the claim in Lemma 4.3 that each wave executes an asymmetric gather is not supported. The proofs in Section 4.4 are proofs of a different, more constrained protocol than the one printed.
- [Algorithm 3, line 60] The guard "T_j ⊆ S_i" in Algorithm 3 is type-incorrect and appears to be a typo: S_i (lines 38/45) is the set of (process, value) pairs delivered from reliable broadcast, whereas T_j is a set of E-sets. The condition can never be satisfied for a non-empty T_j, so line 60 will never add received T_j sets to U and processes would ag-deliver an empty set even after receiving DISTRIBUTE T messages from a quorum (line 62). The surrounding text says "all correct processes wait to deliver the original input messages before accepting any E or T set that contains them (Lines 48 and 60)", which suggests the intended guard is "T_j ⊆ T_i" or an explicit check that all S-sets in T_j have been accepted. As printed, the asymmetric gather protocol of Section 3.3 does not satisfy the liveness part of Definition 3.1.
minor comments (4)
- [Algorithms 4 and 6] The procedure getWaveVertexLeader is defined twice, once in Algorithm 4 (lines 89-93) and again in Algorithm 6 (lines 158-162); one definition should be removed.
- [Section 3.3, paragraph 4] There is a typo "DISTRIBTUE T" where "DISTRIBUTE T" is meant.
- [Lemma 4.4 and preceding paragraph] The notation q(Q) is used in the abstract and in Lemma 4.4 but is defined only informally in the sentence before Lemma 4.4; please introduce it as a formal definition for precision.
- [Algorithm 4, line 140] The condition "∃Q ∈ Q_j for some Q_j ∈ Q : Q |= v.strongEdges" is ambiguous: it should state clearly whether the quorum must belong to the sender's quorum system Q_j or may be drawn from any process's quorum system, since this affects the soundness of vertex validation.
Circularity Check
No substantive circularity; the gather counterexample and Algorithms 3/4 proofs are self-contained, and the only overlapping-author citations are to prior published primitives that are not derived in this paper.
full rationale
The paper's main derivation chain is: (i) show that the quorum-replacement gather fails via an explicit 30-process counterexample with a Python verifier (Section 3.2 and Appendix A); (ii) propose Algorithm 3 and prove common core, validity, and agreement from quorum consistency and the asymmetric reliable-broadcast interface (Lemmas 3.3-3.8); and (iii) build Algorithm 4 as a DAG-embedding of Algorithm 3 and prove safety and liveness via the common-core property and common-coin matching (Lemmas 4.2-4.10). None of these steps defines a claimed result in terms of itself, fits a parameter and then presents it as a prediction, or imports a uniqueness theorem from the authors' prior work to force a choice. The only overlapping-author citations are to Alpos et al. [3] for asymmetric reliable broadcast and common coin and to Amores-Sesar and Cachin [4] in related work; these are prior published building blocks with independent content and are not re-derived or re-defined here, so they do not make the central claim circular. The reader-identified issue in Algorithm 4 lines 105-108, where a process advances from round 1 to round 2 before receiving a quorum of round-1 vertices, is a correctness gap between the printed code and the invariant assumed in Lemma 4.2; it concerns soundness, not circularity, since it does not reduce a prediction to an input by construction. Accordingly, no specific circular step meets the evidentiary bar, and the score reflects only the minor, non-load-bearing self-citation.
Assumptions & free parameters
assumptions (4)
- domain assumption Asymmetric Byzantine quorum systems satisfy consistency and availability as defined in Definition 2.1, taken from Alpos et al. [3].
- domain assumption The maximal guild contains a quorum for each of its members, as stated in Definition 2.2.
- domain assumption There exists an asymmetric reliable broadcast and an asymmetric common coin with the properties required by Algorithm 4, attributed to Alpos et al. [3].
- ad hoc to paper The asymmetric common coin outputs a uniformly random process as the wave leader.
Cite this review
Pith. "Pith review of DAG-based Consensus with Asymmetric Trust [Extended Version]." pith.science (2026). https://pith.science/paper/3AHR6DJS
@misc{pith2026250517891,
author = {Pith},
title = {Pith review of: DAG-based Consensus with Asymmetric Trust [Extended Version]},
year = {2026},
howpublished = {\url{https://pith.science/paper/3AHR6DJS}},
note = {Machine review of arXiv:2505.17891}
}
read the original abstract
In protocols with asymmetric trust, each participant is free to make its own individual trust assumptions about others, captured by an asymmetric quorum system. This contrasts with ordinary, symmetric quorum systems and with threshold models, where all participants share the same trust assumption. It is already known how to realize reliable broadcasts, shared-memory emulations, and binary consensus with asymmetric quorums. In this work, we introduce Directed Acyclic Graph (DAG)-based consensus protocols with asymmetric trust. To achieve this, we extend the key building-blocks of the well-known DAG-Rider protocol to the asymmetric model. Counter to expectation, we find that replacing threshold quorums with their asymmetric counterparts in the existing constant-round gather protocol does not result in a sound asymmetric gather primitive. This implies that asymmetric DAG-based consensus protocols, specifically those based on the existence of common-core primitives, need new ideas in an asymmetric-trust model. Consequently, we introduce the first asymmetric protocol for computing a common core, equivalent to that in the threshold model. This leads to the first randomized asynchronous DAG-based consensus protocol with asymmetric quorums. It decides within an expected constant number of rounds after an input has been submitted, where the constant depends on the quorum system.
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Reviewed August 7, 2026 · model on record in the stance chip above.
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