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

[Rusty Russell <rusty@rustcorp.com.au>]

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.