Re: [bitcoin-dev] Congestion Control via OP_CHECKOUTPUTSHASHVERIFY proposal

[Johnson Lau <jl2012@xbt.hk>]

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Functionally, COHV is a proper subset of ANYPREVOUT (NOINPUT). The only justification to do both is better space efficiency when making covenant.

With eltoo as a clear usecase of ANYPREVOUT, I’m not sure if we really want a very restricted opcode like COHV. But these are my comments, anyway:

1. The “one input” rule could be relaxed to “first input” rule. This allows adding more inputs as fees, as an alternative to CPFP. In case the value is insufficient to pay the required outputs, it is also possible to rescue the UTXO by adding more inputs.

2. While there is no reason to use non-minimal push, there is neither a reason to require minimal push. Since minimal push is never a consensus rule, COHV shouldn’t be a special case.

3. As I suggested in a different post (https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2019-May/016963.html <https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2019-May/016963.html>), the argument for requiring a prevout binding signature may also be applicable to COHV

> On 21 May 2019, at 4:58 AM, Jeremy via bitcoin-dev <bitcoin-dev@lists.linuxfoundation.org> wrote:
> 
> Hello bitcoin-devs,
> 
> Below is a link to a BIP Draft for a new opcode, OP_CHECKOUTPUTSHASHVERIFY. This opcode enables an easy-to-use trustless congestion control techniques via a rudimentary, limited form of covenant which does not bear the same technical and social risks of prior covenant designs.
> 
> Congestion control allows Bitcoin users to confirm payments to many users in a single transaction without creating the UTXO on-chain until a later time. This therefore improves the throughput of confirmed payments, at the expense of latency on spendability and increased average block space utilization. The BIP covers this use case in detail, and a few other use cases lightly.
> 
> The BIP draft is here:
> https://github.com/JeremyRubin/bips/blob/op-checkoutputshashverify/bip-coshv.mediawiki <https://github.com/JeremyRubin/bips/blob/op-checkoutputshashverify/bip-coshv.mediawiki>
> 
> The BIP proposes to deploy the change simultaneously with Taproot as an OPSUCCESS, but it could be deployed separately if needed.
> 
> An initial reference implementation of the consensus changes and  tests which demonstrate how to use it for basic congestion control is available at https://github.com/JeremyRubin/bitcoin/tree/congestion-control <https://github.com/JeremyRubin/bitcoin/tree/congestion-control>.  The changes are about 74 lines of code on top of sipa's Taproot reference implementation.
> 
> Best regards,
> 
> Jeremy Rubin
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