Expiring HTLCs without free relay

[josh]

@instagibbs

Took me a while to get the claim, probably should have started with this. Many implementations do no mempool monitoring at all, so I was confused what this was accomplishing. What the claim is is that with proper expiry, replacement cycling is a just a more time insensitive grief vector, which makes the mempool scanning some may do as a partial mitigation, like LND , completely unnecessary. Wtxid grinding and rebroacasting also become unnecessary at least in theory (I think there are valid reasons to do it anyway)

Thanks for clarifying the idea! I was under the impression that most implementations did mempool monitoring, so to hear otherwise is a surprise.

As for grinding and rebroadcasting, I had hoped that this proposal would make those unnecessary, but my primary motivation was relay-safe expiry and the elimination of the time-sensitive replacement cycling attack.

Lastly, I would add that per @ajtowns, it may be reasonable to eliminate the 100 block delay entirely. I included it here to be conservative and match the 100 block delay in OP_EXPIRE, but consensus and relay may be unaffected by a substantially lower number. I plan to explore this in a future post.