103 lines
4.4 KiB
Markdown
103 lines
4.4 KiB
Markdown
# AIP-9: Consensus Mechanism: QBFT, not HotStuff
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## Preamble
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| Field | Value |
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| --- | --- |
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| AIP | 9 |
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| Title | Consensus Mechanism: QBFT, not HotStuff |
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| Author | Aere Network Foundation |
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| Type | Informational |
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| Category | (none) |
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| Status | Final |
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| Created | 2026-07-19 |
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| Requires | 3 |
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| Supersedes | None |
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| Superseded-By | None |
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| Ratification | Foundation-ratified (pre-decentralization) |
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## Abstract
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This AIP records the decision to run QBFT as Aere's consensus protocol rather
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than HotStuff or a pipelined HotStuff variant. It backfills a decision that is
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already live on chain 2800. It is a decision record: it explains why a shipped
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choice was made and mandates nothing new.
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## Motivation
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Aere is a settlement layer whose core promise is deterministic, single-slot
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finality at a sub-second cadence, running a small permissioned validator set:
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seven validators today, with a roadmap target in the 9 to 21 range. The consensus
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choice determines whether that promise is deliverable at all, and it was taken
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without being written down anywhere citable.
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## Specification
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Aere runs QBFT, the Byzantine-fault-tolerant IBFT-family protocol as implemented
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in Hyperledger Besu, producing deterministic single-block finality.
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- Validator count N = 7 (**measured**, all Foundation-operated).
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- Fault tolerance f = 2, commit quorum 2f + 1 = 5-of-7 (**spec**).
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- Target block period 500 ms (**spec**, see AIP-3); measured mean interval
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516.4 ms.
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- Validator seals are classical secp256k1 ECDSA. **Consensus is not
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post-quantum** and no part of this record implies otherwise.
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## Rationale
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At Aere's validator-set size, QBFT's O(N^2) message complexity is negligible in
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absolute terms: a 5-of-7 quorum verify costs on the order of a few milliseconds
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per block, a small fraction of the 500 ms slot. Against that non-cost, QBFT gives
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three concrete advantages. It provides deterministic instant single-block
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finality, which is the exact settlement guarantee the network sells. It is mature
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and already integrated in Besu, so there is no new-protocol implementation risk.
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And Aere's post-quantum consensus research, second-client work, and zk finality
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proofs are all built against QBFT, so changing it would invalidate that work.
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**Alternatives rejected.** HotStuff and its pipelined variants, whose linear
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per-round communication complexity is engineered to scale BFT to hundreds of
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validators. That scaling regime is one Aere does not operate in and does not plan
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to enter under this design, so HotStuff's headline benefit is unavailable, while
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adopting it would add the risk and integration cost of a protocol less
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battle-tested in the chosen client.
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## Backwards Compatibility
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None. This records the existing protocol.
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## Security Considerations
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At N = 7 the fault tolerance against node outages is real, but **all seven
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validators are Foundation-operated**, so the effective Nakamoto coefficient is 1.
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Decentralization, not the consensus mechanism, is the binding trust assumption on
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this chain. A reader should not read "Byzantine fault tolerant" as "trustless".
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Consensus seals are secp256k1 ECDSA and are therefore classically breakable by a
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cryptographically relevant quantum computer. Work on a hybrid post-quantum
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consensus path is recorded in AIP-15 and is **not live**.
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## Reference Implementation and On-Chain Deployment
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Chain ID 2800. Configuration lives in the Besu QBFT genesis and chain config; the
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block period transition is documented in AIP-3. There is no contract address for
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a consensus choice.
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## Errata
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**Erratum 1 (2026-09-12).** Every validator count in this document is a
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measurement of 2026-07-19, the day it was written, and none of them carries that
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date in its own sentence, so a sentence quoted on its own reads as a claim about
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today. Read them as dated: "seven validators today" in the Motivation,
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"Validator count N = 7 (**measured**)" and "commit quorum 2f + 1 = 5-of-7" in the
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Specification, and "At N = 7 ... all seven validators are Foundation-operated" in
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the Security Considerations were all true on 2026-07-19. Measured 2026-09-12 on
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the live chain: N = 10, f = 3, commit quorum 7-of-10, and one of the ten runs a
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different client implementation. The decision this AIP records, QBFT rather than
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HotStuff, is unaffected, and so is the Nakamoto argument in the Security
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Considerations: the set grew, the operator count did not. The original text is
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left exactly as published, per AIP-1 section 5.
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## Copyright
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Released to the public domain (CC0). No rights reserved.
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