Topology Restrictions for Pinning.

Would set out opt-in, stricter relay rules for unconfirmed payments, making it harder for others to block a fee bump.

Someone can sometimes 'pin' a payment, using relay limits to stop it being fee-bumped, which matters for protocols with deadlines such as Lightning. Payments marked version 3 would be kept small, with at most one unconfirmed parent or child. The BIP links the code that adds these rules to Bitcoin Core.