On Tue, Feb 27, 2018 at 11:25:59AM -0500, Russell O'Connor wrote:
> 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?

I mean, I think in general solving this problem is probably not possible.
Basically, the fundamental problem is someone else has consumed network
bandwidth that should be paid for with fees. What you're trying to do is
replace a transaction without paying those fees, which is identical to what an
attacker is trying to do, and thus any such scheme will be as vulnerable to
attack as not having that protection in the first place.

...which does give you an out: maybe the attack isn't important enough to
matter. :)