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Subject: Re: [bitcoin-dev] [PROPOSAL] Emergency RBF (BIP 125)
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I think this needs significantly improved motivation/description. A few area=
s I'd like to see calculated out:
1) wrt rule 3, for this to be obviously-incentive-compatible-for-the-next-mi=
ner, I'd think no evicted transactions would be allowed to be in the next bl=
ock range. This would probably require some significant additional tracking i=
n today's mempool logic.
2) wrt rule 4, I'd like to see a calculation of worst-case free relay. I thi=
nk we're already not in a great place, but maybe it's worth it or maybe ther=
e is some other way to reduce this cost (intuitively it looks like this prop=
osal could make things very, very, very bad).
3) wrt rule 5, I'd like to see benchmarks, it's probably a pretty nasty DoS a=
ttack, but it may also be the case that is (a) not worse than other fundamen=
tal issues or (b) sufficiently expensive.
4) As I've indicated before, I'm generaly not a fan of such vague protection=
s for time-critical transactions such as payment channel punishment transact=
ions. At a high-level, in this context your counterparty's transactions (not=
to mention every other transaction in everyone's mempool) are still involve=
d in the decision about whether to accept an RBF, in contrast to previous pr=
oposals, which makes it much harder to reason about. As a specific example, i=
f an attacker exploits mempool policy differences they may cause your concep=
t of "top 4M weight" to be bogus for a subeset of nodes, causing propogation=
to be limited.
Obviously there is also a ton more client-side knowledge required and comple=
xity to RBF decisions here than other previous, more narrowly-targeted propo=
sals.
(I don't think this one use-case being not optimal should prevent such a pro=
posal, i agree it's quite nice for some other cases).
Matt
> On Jun 2, 2019, at 06:41, Rusty Russell <rusty@rustcorp.com.au> wrote:
>=20
> Hi all,
>=20
> I want to propose a modification to rules 3, 4 and 5 of BIP 125:
>=20
> To remind you of BIP 125:
> 3. The replacement transaction pays an absolute fee of at least the sum
> paid by the original transactions.
>=20
> 4. The replacement transaction must also pay for its own bandwidth at
> or above the rate set by the node's minimum relay fee setting.
>=20
> 5. The number of original transactions to be replaced and their
> descendant transactions which will be evicted from the mempool must not
> exceed a total of 100 transactions.
>=20
> The new "emergency RBF" rule:
>=20
> 6. If the original transaction was not in the first 4,000,000 weight
> units of the fee-ordered mempool and the replacement transaction is,
> rules 3, 4 and 5 do not apply.
>=20
> This means:
>=20
> 1. RBF can be used in adversarial conditions, such as lightning
> unilateral closes where the adversary has another valid transaction
> and can use it to block yours. This is a problem when we allow
> differential fees between the two current lightning transactions
> (aka "Bring Your Own Fees").
>=20
> 2. RBF can be used without knowing about miner's mempools, or that the
> above problem is occurring. One simply gets close to the required
> maximum height for lightning timeout, and bids to get into the next
> block.
>=20
> 3. This proposal does not open any significant new ability to RBF spam,
> since it can (usually) only be used once. IIUC bitcoind won't
> accept more that 100 descendents of an unconfirmed tx anyway.
>=20
> 4. This proposal makes RBF miner-incentive compatible. Currently the
> protocol tells miners they shouldn't accept the highest bidding tx
> for the good of the network. This conflict is particularly sharp
> in the case where the replacement tx would be immediately minable,
> which this proposal addresses.
>=20
> Unfortunately I haven't found time to code this up in bitcoin, but if
> there's positive response I can try.
>=20
> Thanks for reading!
> Rusty.
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