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.