Re: [bitcoin-dev] Transaction Merging (bip125 relaxation)

From: Alan Evans <thealanevans@gmail•com>
To: Rhavar <rhavar@protonmail•com>,
	 Bitcoin Protocol Discussion
	<bitcoin-dev@lists•linuxfoundation.org>
Subject: Re: [bitcoin-dev] Transaction Merging (bip125 relaxation)
Date: Mon, 22 Jan 2018 14:16:13 -0400	[thread overview]
Message-ID: <CALPhJazXpUAsd9qeHPZdwK6eeoEfsM7i3FouMifkKNv9YFzQ9w@mail.gmail.com> (raw)
In-Reply-To: <M8yPGuNmrXfNNwrYDDLpTVb__BhGysVW060Cq_tMc-AC6F7pKd1Vvb4wWbpmhhEvfoQ7fn-EcgfxRwJSVkFAZ5x57hg9XxpdZlDPi2IBJZg=@protonmail.com>

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> So now I still owe John 1 BTC, however it's not immediately clear if it's
safe to send to him

If you spent your change from transaction A, that would be safe. There'd be
no way you John could end up with 2 BTC from you then.

On Mon, Jan 22, 2018 at 1:40 PM, Rhavar via bitcoin-dev <
bitcoin-dev@lists•linuxfoundation.org> wrote:

> So my half-baked idea is very simple:
>
> Allow users to merge multiple unconfirmed transactions, stripping
> extraneous inputs and change as they go.
>
> This is currently not possible because of the bip125 rule:
> "The replacement transaction pays an absolute fee of at least the sum paid
> by the original transactions."
>
> Because the size of the merged transaction is smaller than the original
> transactions, unless there is a considerable feerate bump, this rule isn't
> possible to observe.
>
>
> I my question is: is it possible or reasonable to relax this rule? If this
> rule was removed in its entirety, does it introduce any DoS vectors? Or can
> it be changed to allow my use-case?
>
>
> ---
> Full backstory: I have been trying to use bip125 (Opt-in Full
> Replace-by-Fee) to do "transaction merging" on the fly. Let's say that I
> owe John 1 bitcoin, and have promised to pay him immediately: Instead of
> creating a whole new transaction if I have an in-flight (unconfirmed)
> transaction, I can follow the rules of bip125 to create a replacement that
> accomplishes this goal.
>
> From a "coin selection" point of view, this was significantly easier than
> I had anticipated. I was able to encode the rules in my linear model and
> feed in all my unspent and in-flight transactions and it can solve it
> without difficulty.
>
> However, the real problem is tracking the mess. Consider this sequence of
> events:
> 1) I have unconfirmed transaction A
> 2) I replace it with B, which pays John 1 BTC
> 3) Transaction A gets confirmed
>
> So now I still owe John 1 BTC, however it's not immediately clear if
> it's safe to send to him without waiting $n transactions. However even
> for a small $n, this breaks my promise to pay him immediately.
>
> One possible solution is to only consider a transaction "replaceable" if
> it has change, so if the original transaction confirms -- payments can
> immediately be made that source the change, and provide safety in a reorg.
>
> However, this will only work <50% of the time for me (most transactions
> don't have change) and opens a pandora's box of complexity.
>
> There's a few other hacks you can do to make it work in a few more cases,
> but nothing that is realistic to expect anyone to implement any time soon.
>
> However, if there was a straight foward way to merge N unconfirmed
> transactions, it would be easy get into production, and potentially offer
> some pretty nice savings for everyone.
>
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>

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