This strategy chooses fee rates based on Bitcoin Core’s (or some other) fee rate estimator. The estimator is queried with the HTLC deadline as the confirmation target, and the returned fee rate is used for commitment and HTLC transactions. Typically the estimator is requeried every block to update fee rates and RBF any unconfirmed transactions.

CLN and LND prior to v0.18.0 use this strategy exclusively. eclair uses this strategy until deadlines are within 6 blocks, after which it switches to exponential bumping. LDK uses a combined strategy that sometimes uses the fee rate from the estimator and other times uses exponential bumping.

The new sweeper uses HTLC deadlines and fee budgets to compute a fee rate curve, dynamically adjusting fees (fee bumping) to prioritize urgent transactions.

Based on the transaction budget and deadline, a fee function is computed that determines how much of the budget is spent as the deadline approaches. By default, a linear fee function is used which starts at a low fee (determined by the minimum relay fee rate or an external estimator) and ends with the total budget being allocated to fees when the deadline is one block away.