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.

> 3'. The replacement transaction pays a fee rate of at least the effective
> fee rate of any chain of transactions from the set of original transactions
> that begins with the root of the original transaction set.

I think what you mean here should be the effective fee rate of the maximum
feerate package that can be built from the set of transactions that begins with
the candidate replacement. But actually calculating this is I believe
non-trivial, which is why I didn't implement it this way when RBF was first
implemented.

Yes, that is what I mean. My proposal was off-the-mark.

Surely CPFP is already computing the package-fee rates of mempool transactions. That is the value we need to compute.