> For instance, the double-spend could be low-feerate and large, and effectively pin any attempt to replace it.

Yes, this is the biggest hole left. You *could* replace it with RBF when before you simply could not, so perhaps the pinning door is slightly smaller in scenarios where going feerates are significantly higher than min.

> Or it could be higher feerate and confirm and B/C have to start all over.

Coinjoins have "blame rounds" exactly for this. Ruling out the above hole where you don't want to pay the 100kvb rule#3 penalty, you can kick the griefer out. Without replacement, you likely can not.

> Or, A could stall things in the signing phase and B/C have to figure out when to give up on the channel with A.

Again, blame rounds solve this.

So to recap, it makes it *possible* to over-bid your griefer, vs simply not able to and have funds tied up for weeks(or guess you're being pinned and double-spend your input, which again looks blame-worthy).

Properly replacing rule#3 would give these protocols higher assurances, but this is where we're at now.

On Thu, Oct 27, 2022 at 1:35 PM Suhas Daftuar via bitcoin-dev <bitcoin-dev@lists.linuxfoundation.org> wrote:
I have more to say on this broader topic, but since you've brought up this particular example I think it's worth commenting: 

On Thu, Oct 27, 2022 at 1:23 PM Anthony Towns via bitcoin-dev <bitcoin-dev@lists.linuxfoundation.org> wrote:
Is that true? Antoine claims [1] that opt-in RBF isn't enough to avoid
a DoS issue when utxos are jointly funded by untrusting partners, and,
aiui, that's the main motivation for addressing this now.

[1] https://lists.linuxfoundation.org/pipermail/lightning-dev/2021-May/003033.html

The scenario he describes is: A, B, C create a tx:

  inputs: A1, B1, C1 [opts in to RBF]
  fees: normal
  outputs:
    [lightning channel, DLC, etc, who knows]

they all analyse the tx, and agree it looks great; however just before
publishing it, A spams the network with an alternative tx, double
spending her input:

  inputs: A1 [does not opt in to RBF]
  fees: low
  outputs: A

If A gets the timing right, that's bad for B and C because they've
populated their mempool with the 1st transaction, while everyone else
sees the 2nd one instead; and neither tx will replace the other. B and
C can't know that they should just cancel their transaction, eg:

  inputs: B1, C1 [opts in to RBF]
  fees: 50% above normal
  outputs:
    [smaller channel, refund, whatever]

and might instead waste time trying to fee bump the tx to get it mined,
or similar.

What should folks wanting to do coinjoins/dualfunding/dlcs/etc do to
solve that problem if they have only opt-in RBF available?

I think this is not a real example of a DoS vector that is available because we support non-rbf signaling transactions. Even in a world where all transactions are replaceable, person A could double-spend their input in a way that is annoying for B and C.  For instance, the double-spend could be low-feerate and large, and effectively pin any attempt to replace it.  Or it could be higher feerate and confirm and B/C have to start all over.  Or, A could stall things in the signing phase and B/C have to figure out when to give up on the channel with A.

So I find this example to be unconvincing.  Are there any other examples where having a non-replacement policy for some transactions causes problems for protocols people are trying to build?

Thanks,
Suhas
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