On Mon, Feb 12, 2018 at 6:42 PM, Peter Todd <pete@petertodd.org> wrote:

Ah ok, I misunderstood and didn't realise you were talking about the case where
Alice re-spends her unconfirmed payment. Unfortunately I don't think that case
is possible to solve without putting some kind of restriction on spending
unconfirmed outputs; with a restriction it's fairly simple to solve.

When you say that you don't think it is possible to solve, do you mean that there is a specific problem with this proposal of replacing transactions when offered a new transaction whose fee rate exceeds the package fee rate of the original transaction (and ensuring that the fee increase covers the size of the transactions being ejected)? Is your concern only about the ability to computing and track the package fee rate for transactions within the mempool or is there some other issue you foresee?