Re: [bitcoin-dev] Thoughts on fee bumping

From: Jeremy Rubin <jeremy.l.rubin@gmail•com>
To: "Russell O'Connor" <roconnor@blockstream•com>,
	 Bitcoin Protocol Discussion
	<bitcoin-dev@lists•linuxfoundation.org>
Subject: Re: [bitcoin-dev] Thoughts on fee bumping
Date: Tue, 15 Feb 2022 13:38:11 -0800	[thread overview]
Message-ID: <CAD5xwhjGqEu5z1O0ho9pHnUD+woSKeGUxh+7rdHq5fPZU+mzww@mail.gmail.com> (raw)
In-Reply-To: <CAMZUoKnhyzJ=6W-=hxpmCyjiPyYMuS=eKjLN+bu5cuLRQ42nxA@mail.gmail.com>

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The difference between sponsors and this issue is more subtle. The issue
Suhas raised was with a variant of sponsors trying to address a second
criticism, not sponsors itself, which is secure against this.

I think I can make this clear by defining a few different properties:

Strong Reorgability: The transaction graph can be arbitrarily reorged into
any series of blocks as long as dependency order/timelocks are respected.
Simple Existential Reorgability: The transaction graph can be reorged into
a different series of blocks, and it is not computationally difficult to
find such an ordering.
Epsilon-Strong Reorgability: The transaction graph can be arbitrarily
reorged into any series of blocks as long as dependency order/timelocks are
respected, up to Epsilon blocks.
Epsilon: Simple Existential Reorgability: The transaction graph can be
reorged into a different series of blocks, and it is not computationally
difficult to find such an ordering, up to epsilon blocks.
Perfect Reorgability: The transaction graph can be reorged into a different
series of blocks, but the transactions themselves are already locked in.

Perfect Reorgability doesn't exist in Bitcoin because unconfirmed
transactions can be double spent which invalidates descendants. Notably,
for a subset of the graph which is CTV Congestion control tree expansions,
perfect reorg ability would exist, so it's not just a bullshit concept to
think about :)

The sponsors proposal is a change from Epsilon-Strong Reorgability to
Epsilon-Weak Reorgability. It's not clear to me that there is any
functional reason to rely on Strongness when Bitcoin's reorgability is
already not Perfect, so a reorg generator with malicious intent can already
disturb the tx graph. Epsion-Weak Reorgability seems to be a sufficient
property.

Do you disagree with that?

Best,

Jeremy

--
@JeremyRubin <https://twitter.com/JeremyRubin>

On Tue, Feb 15, 2022 at 12:25 PM Russell O'Connor via bitcoin-dev <
bitcoin-dev@lists•linuxfoundation.org> wrote:

>
>
>> >> 2. (from Suhas) "once a valid transaction is created, it should not
>> become invalid later on unless the inputs are double-spent."
>> > This doesn't seem like a huge concern to me
>>
>> I agree that this shouldn't be a concern. In fact, I've asked numerous
>> people in numerous places what practical downside there is to transactions
>> that become invalid, and I've heard basically radio silence other than one
>> off hand remark by satoshi at the dawn of time which didn't seem to me to
>> have good reasoning. I haven't seen any downside whatsoever of transactions
>> that can become invalid for anyone waiting the standard 6 confirmations -
>> the reorg risks only exists for people not waiting for standard
>> finalization. So I don't think we should consider that aspect of a
>> sponsorship transaction that can only be mined with the transaction it
>> sponsors to be a problem unless a specific practical problem case can be
>> identified. Even if a significant such case was identified, an easy
>> solution would be to simply allow sponsorship transactions to be mined on
>> or after the sponsored transaction is mined.
>>
>
> The downside is that in a 6 block reorg any transaction that is moved past
> its expiration date becomes invalid and all its descendants become invalid
> too.
>
> The current consensus threshold for transactions to become invalid is a
> 100 block reorg, and I see no reason to change this threshold.  I promise
> to personally build a wallet that always creates transactions on the verge
> of becoming invalid should anyone ever implement a feature that violates
> this tx validity principle.
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