On tx-monotonicity, Andrew Poelstra remarked:
"As you say, the benefits to monotonicity include
- You can’t get out of paying fees by broadcasting near-expiring transactions (assume additional network rules like RBF prevent you doing the same thing by double-spending)
- Harms to aggregation, both noninteractive and coinjoin-style
- Reorg safety
- Harms to second-layer protocols (though I assume these would work if you just required participants to disable non-monotonic features).
The only thing I can think of that you didn’t mention is:
- Caching: if transactions expire at a fixed time this is not too bad, but
if the expiry conditions are more complicated then you have to constantly re-validate transactions in your mempool to check if they’ve expired
"
On your reply, he remarked:
"But he seems not to understand the concern about fees (he says “miners will include higher-fee transactions instead” as though that were a solution, rather than the problem).
He suggests solving problems with aggregation by adding epicycles, which I don’t care to analyze in detail, but he says “this is just a special case of users aggregating low-fee transactions” which is definitely untrue, both because the aggregation function is different (weighted average for feerate, minimum for expiry times), and more importantly, because reasoning about fees is a 1D problem (efficiently optimizable) while fees + expiry is a 2D problem (knapsack problem, NP complete).
Then he suggests that the reorg safety issues around transactions which automatically become invalid regardless of user/miner choice are commensurate with reorg safety issues in the presence of active double-spend attackers.
It is not. In the case of expiring transactions the result is an unreliable
network. In the case of a double-spend attacker the result is unreliable
transactions when dealing with unreliable people."
He clarified that “It is a term from history of science to refer to addition of orbits upon orbits upon orbits, to try to make a broken solar system model match observation. I’m using it by analogy, see
https://en.wikipedia.org/wiki/Deferent_and_epicycle#Bad_science”