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Change RBF feerate bump rule to ensure BIP125 relay conformance #1327

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Change RBF feerate bump rule to ensure BIP125 relay conformance #1327

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29 changes: 20 additions & 9 deletions 02-peer-protocol.md
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Original file line number Diff line number Diff line change
Expand Up @@ -458,8 +458,11 @@ completed.
#### Requirements

The sender:
- MUST set `feerate` greater than or equal to 25/24 times the `feerate`
of the previously constructed transaction, rounded down.
- MUST set `feerate` greater than or equal to the maximum of:
- 25/24 times the `feerate` of the previously constructed transaction,
rounded down.
- 25 sat per kw greater than the `feerate` of the previously
constructed transaction.
- If it contributes to the transaction's funding output:
- MUST set `funding_output_contribution`
- If it requires the receiving node to only use confirmed inputs:
Expand All @@ -472,20 +475,28 @@ The sender:
The recipient:
- MUST respond either with `tx_abort` or with `tx_ack_rbf`
- MUST respond with `tx_abort` if:
- the `feerate` is not greater than or equal to 25/24 times `feerate`
of the last successfully constructed transaction
- the `feerate` is not greater than or equal to the maximum of:
- 25/24 times the `feerate` of the last successfully constructed
transaction, rounded down.
- 25 sat per kw greater than the `feerate` of the last successfully
constructed transaction
- MAY send `tx_abort` for any reason
- MUST fail the negotiation if:
- `require_confirmed_inputs` is set but it cannot provide confirmed inputs

#### Rationale

`feerate` is the feerate this transaction will pay. It must be at least
1/24 greater than the last used `feerate`, rounded down to the nearest
satoshi to ensure there is progress.

E.g. if the last `feerate` was 520, the next sent `feerate` must be 541
(520 * 25 / 24 = 541.667, rounded down to 541).
the maximum of 25/24 times the previous `feerate` (rounded down) and 25
sat per kw above the previous `feerate`. The multiplicative rule ensures
progress at higher feerates, while the additive floor of 25 sat per kw
(0.1 sat/vB) ensures that the replacement transaction meets BIP125's
minimum relay fee requirement (default `incrementalRelayFee` of 0.1
sat/vB since Bitcoin Core v30.0), which the multiplicative increase alone
could fail to satisfy at low feerates.

E.g. if the last `feerate` was 520, the next sent `feerate` must be at
least 545 (max(520 * 25 / 24, 520 + 25) = max(541, 545) = 545).

If the previous transaction confirms in the middle of an RBF attempt,
the attempt MUST be abandoned.
Expand Down