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From: "Russell O'Connor" <roconnor@blockstream.io>
Date: Mon, 12 Feb 2018 18:46:43 -0500
Message-ID: <CAMZUoK=BSOpUsJ=3n1jgHEAEq2M-4rigGML3Z7WN0eTgXsPp7g@mail.gmail.com>
To: Peter Todd <pete@petertodd.org>
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Cc: Bitcoin Protocol Discussion <bitcoin-dev@lists.linuxfoundation.org>
Subject: Re: [bitcoin-dev] Revisiting BIP 125 RBF policy.
Precedence: list

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On Mon, Feb 12, 2018 at 6:42 PM, Peter Todd <pete@petertodd.org> wrote:

> On Mon, Feb 12, 2018 at 06:19:40PM -0500, Russell O'Connor wrote:
> > On Mon, Feb 12, 2018 at 5:58 PM, Peter Todd <pete@petertodd.org> wrote:
> >
> > >
> > > I don't actually see where the problem is here. First of all, suppose
> we
> > > have a
> > > transaction T_a that already pays Alice with a feerate sufficiently
> high
> > > that
> > > we expect it to get mined in the near future. If we want to pay Bob, we
> > > can do
> > > that by simply creating a double-spend of T_a that pays both Bob and
> Alice,
> > > T_{ab}. BIP125 only requires that double-spend to have an absolute fee
> > > higher
> > > than the minimum relay feerate * size of the transaction.
> > >
> >
> > The problem is that rule 3 of BIP 125 requires you pay a fee that is
> higher
> > than the the fee of T_a *plus* the fee of the sweep-transaction that the
> > Alice has added as a unconfirmed child transaction to T_a because
> > double-spending to pay Alice and Bob invalidates Alice's
> > sweep-transaction.  Alice's sweep-transaction is very large, and hence
> pays
> > a large absolute fee even though her fee-rate is very low.  We do not
> have
> > any control over its value, hence Alice has "pinned" our RBF transaction.
>
> Ah ok, I misunderstood and didn't realise you were talking about the case
> where
> Alice re-spends her unconfirmed payment. Unfortunately I don't think that
> case
> is possible to solve without putting some kind of restriction on spending
> unconfirmed outputs; with a restriction it's fairly simple to solve.
>

Adding such a restriction was Rhavar's original suggestion in
https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2017-July/014688.html,
but it seems the proposal wasn't well received because it kinda destroys
CPFP.

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<div dir=3D"ltr"><br><div class=3D"gmail_extra"><br><div class=3D"gmail_quo=
te">On Mon, Feb 12, 2018 at 6:42 PM, Peter Todd <span dir=3D"ltr">&lt;<a hr=
ef=3D"mailto:pete@petertodd.org" target=3D"_blank">pete@petertodd.org</a>&g=
t;</span> wrote:<br><blockquote class=3D"gmail_quote" style=3D"margin:0px 0=
px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><span=
 class=3D"gmail-">On Mon, Feb 12, 2018 at 06:19:40PM -0500, Russell O&#39;C=
onnor wrote:<br>
&gt; On Mon, Feb 12, 2018 at 5:58 PM, Peter Todd &lt;<a href=3D"mailto:pete=
@petertodd.org">pete@petertodd.org</a>&gt; wrote:<br>
&gt;<br>
&gt; &gt;<br>
&gt; &gt; I don&#39;t actually see where the problem is here. First of all,=
 suppose we<br>
&gt; &gt; have a<br>
&gt; &gt; transaction T_a that already pays Alice with a feerate sufficient=
ly high<br>
&gt; &gt; that<br>
&gt; &gt; we expect it to get mined in the near future. If we want to pay B=
ob, we<br>
&gt; &gt; can do<br>
&gt; &gt; that by simply creating a double-spend of T_a that pays both Bob =
and Alice,<br>
&gt; &gt; T_{ab}. BIP125 only requires that double-spend to have an absolut=
e fee<br>
&gt; &gt; higher<br>
&gt; &gt; than the minimum relay feerate * size of the transaction.<br>
&gt; &gt;<br>
&gt;<br>
&gt; The problem is that rule 3 of BIP 125 requires you pay a fee that is h=
igher<br>
&gt; than the the fee of T_a *plus* the fee of the sweep-transaction that t=
he<br>
&gt; Alice has added as a unconfirmed child transaction to T_a because<br>
&gt; double-spending to pay Alice and Bob invalidates Alice&#39;s<br>
&gt; sweep-transaction.=C2=A0 Alice&#39;s sweep-transaction is very large, =
and hence pays<br>
&gt; a large absolute fee even though her fee-rate is very low.=C2=A0 We do=
 not have<br>
&gt; any control over its value, hence Alice has &quot;pinned&quot; our RBF=
 transaction.<br>
<br>
</span>Ah ok, I misunderstood and didn&#39;t realise you were talking about=
 the case where<br>
Alice re-spends her unconfirmed payment. Unfortunately I don&#39;t think th=
at case<br>
is possible to solve without putting some kind of restriction on spending<b=
r>
unconfirmed outputs; with a restriction it&#39;s fairly simple to solve.<br=
>
<span class=3D"gmail-"></span></blockquote><div><br></div><div>Adding such =
a restriction was Rhavar&#39;s original suggestion in <a href=3D"https://li=
sts.linuxfoundation.org/pipermail/bitcoin-dev/2017-July/014688.html">https:=
//lists.linuxfoundation.org/pipermail/bitcoin-dev/2017-July/014688.html</a>=
, but it seems the proposal wasn&#39;t well received because it kinda destr=
oys CPFP.<br></div></div><br></div></div>

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