Re: [bitcoin-dev] [PROPOSAL] Emergency RBF (BIP 125)

From: rhavar@protonmail•com
To: Rusty Russell <rusty@rustcorp•com.au>,
	Bitcoin Protocol Discussion
	<bitcoin-dev@lists•linuxfoundation.org>
Cc: Matt Corallo <matt@chaincode•com>
Subject: Re: [bitcoin-dev] [PROPOSAL] Emergency RBF (BIP 125)
Date: Mon, 03 Jun 2019 01:49:15 +0000	[thread overview]
Message-ID: <Ol3079c4eDHk0jCv6RMogcx3QPl-bZOr_AQL9siAOCtXSriqZjVhFf53iaJQbaFML0urBNmOaRVJrH848oFWs6ekCW58l4mg1hDXGjiM2WE=@protonmail.com> (raw)
In-Reply-To: <871s0c1tvg.fsf@rustcorp.com.au>

+1

From an incentive-compatible point of view, miners should be accepting transactions that increase the amount of fees that can achieved with 4M weight of transactions, so it seems like a pretty sane plan.

One common problem I've run into with RBF is since you're using RBF you probably want to low ball fees. With good coin selection (*cough* coinsayer.com *cough*), it'll use that opportunity to consolidate inputs. But now let's say fees suddenly spike (pretty common), you might want to fee bump your now stuck transaction. But now that fees are high, it doesn't make sense to be consolidating so ideally you'd just replace it with a much smaller transaction (that pays higher fee rate).

So if anything, I think your proposal doesn't go far enough. I think even in "non-emergency" cases, we could get away with removing the requirement to increase the absolute fee (as long as the fee rate is increased); which also makes it incentive compatible if you assume a reasonable fee-market.

I realize it does open potential DoS vectors, but they seem reasonably small.

-Ryan

‐‐‐‐‐‐‐ Original Message ‐‐‐‐‐‐‐
On Saturday, June 1, 2019 9:41 PM, Rusty Russell via bitcoin-dev <bitcoin-dev@lists•linuxfoundation.org> wrote:

> Hi all,
>
> I want to propose a modification to rules 3, 4 and 5 of BIP 125:
>
> To remind you of BIP 125:
> 3. The replacement transaction pays an absolute fee of at least the sum
> paid by the original transactions.
>
> 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.
>
> 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.
>
> The new "emergency RBF" rule:
>
> 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.
>
> This means:
>
> 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").
>
> 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.
>
> 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.
>
> 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.
>
>     Unfortunately I haven't found time to code this up in bitcoin, but if
>     there's positive response I can try.
>
>     Thanks for reading!
>     Rusty.
>
>
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