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Ensure minimum RBF feerate satisfies BIP125 #4494

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Ensure minimum RBF feerate satisfies BIP125 #4494

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9b9c114
Use +25 sat/kwu increment for our minimum RBF feerate
jkczyz Mar 17, 2026
f56d3c8
f - Use max of flat increment and spec's 25/24 rule
jkczyz Mar 30, 2026
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23 changes: 15 additions & 8 deletions lightning/src/ln/channel.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -6484,6 +6484,19 @@ fn get_v2_channel_reserve_satoshis(channel_value_satoshis: u64, dust_limit_satos
cmp::min(channel_value_satoshis, cmp::max(q, dust_limit_satoshis))
}

/// Returns the minimum feerate for our own RBF attempts given a previous feerate.
///
/// The spec (tx_init_rbf) requires the new feerate to be >= 25/24 of the previous feerate.
/// However, at low feerates that multiplier doesn't always satisfy BIP125's relay requirement of
/// an absolute fee increase, so we take the max of a flat +25 sat/kwu (0.1 sat/vB) increment
/// and the spec's multiplicative rule. We still accept the bare 25/24 rule from counterparties
/// in [`FundedChannel::validate_tx_init_rbf`].
fn min_rbf_feerate(prev_feerate: u32) -> FeeRate {
let flat_increment = (prev_feerate as u64).saturating_add(25);
let spec_increment = ((prev_feerate as u64) * 25).div_ceil(24);
FeeRate::from_sat_per_kwu(cmp::max(flat_increment, spec_increment))
}
Comment on lines +6487 to +6498
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@ldk-claude-review-bot ldk-claude-review-bot Mar 30, 2026

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Bug: For prev_feerate > 600, this returns a value below the spec's 25/24 rule that counterparties enforce in validate_tx_init_rbf (line 12719). This means our own RBF attempts will be rejected by any spec-compliant counterparty at higher feerates.

Example at prev_feerate = 1000:

  • min_rbf_feerate returns 1025
  • Counterparty checks 1025 * 24 = 24600 < 1000 * 25 = 25000 → rejected

The crossover is at exactly 600 sat/kwu (25 * 24 = 600). Above that, +25 is insufficient to satisfy >= ceil(prev * 25/24). Real-world feerates are often well above 600 sat/kwu.

Additionally, maybe_adjust_for_rbf (line 12172) adjusts contributions to this floor, so automated feerate adjustment will also produce rejected RBFs.

Consider using max(prev + 25, ceil(prev * 25/24)) to satisfy both the BIP125-motivated flat increment and the spec's multiplicative rule:

Suggested change
/// Returns the minimum feerate for our own RBF attempts given a previous feerate.
///
/// The spec (tx_init_rbf) requires the new feerate to be >= 25/24 of the previous feerate.
/// However, that multiplier doesn't always satisfy BIP125's relay requirement of an absolute fee
/// increase, so for our own RBFs we use a flat +25 sat/kwu (0.1 sat/vB) increment instead. We
/// still accept the 25/24 rule from counterparties in [`FundedChannel::validate_tx_init_rbf`].
fn min_rbf_feerate(prev_feerate: u32) -> FeeRate {
FeeRate::from_sat_per_kwu((prev_feerate as u64).saturating_add(25))
}
fn min_rbf_feerate(prev_feerate: u32) -> FeeRate {
let flat_increment = (prev_feerate as u64).saturating_add(25);
let spec_increment = ((prev_feerate as u64) * 25).div_ceil(24);
FeeRate::from_sat_per_kwu(cmp::max(flat_increment, spec_increment))
}

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@jkczyz jkczyz Mar 30, 2026

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Using max now which also matches the alternative spec proposal.


/// Context for negotiating channels (dual-funded V2 open, splicing)
#[derive(Debug)]
pub(super) struct FundingNegotiationContext {
Expand Down Expand Up @@ -12019,10 +12032,7 @@ where
prev_feerate.is_some(),
"pending_splice should have last_funding_feerate or funding_negotiation",
);
let min_rbf_feerate = prev_feerate.map(|f| {
let min_feerate_kwu = ((f as u64) * 25).div_ceil(24);
FeeRate::from_sat_per_kwu(min_feerate_kwu)
});
let min_rbf_feerate = prev_feerate.map(min_rbf_feerate);
let prior = if pending_splice.last_funding_feerate_sat_per_1000_weight.is_some() {
self.build_prior_contribution()
} else {
Expand Down Expand Up @@ -12114,10 +12124,7 @@ where
}

match pending_splice.last_funding_feerate_sat_per_1000_weight {
Some(prev_feerate) => {
let min_feerate_kwu = ((prev_feerate as u64) * 25).div_ceil(24);
Ok(FeeRate::from_sat_per_kwu(min_feerate_kwu))
},
Some(prev_feerate) => Ok(min_rbf_feerate(prev_feerate)),
None => Err(format!(
"Channel {} has no prior feerate to compute RBF minimum",
self.context.channel_id(),
Expand Down
26 changes: 14 additions & 12 deletions lightning/src/ln/funding.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -218,8 +218,9 @@ impl PriorContribution {
/// prior contribution logic internally — reusing an adjusted prior when possible, re-running
/// coin selection when needed, or creating a fee-bump-only contribution.
///
/// Check [`FundingTemplate::min_rbf_feerate`] for the minimum feerate required (25/24 of
/// the previous feerate). Use [`FundingTemplate::prior_contribution`] to inspect the prior
/// Check [`FundingTemplate::min_rbf_feerate`] for the minimum feerate required (the greater of
/// the previous feerate + 25 sat/kwu and the spec's 25/24 rule). Use
/// [`FundingTemplate::prior_contribution`] to inspect the prior
/// contribution's parameters (e.g., [`FundingContribution::value_added`],
/// [`FundingContribution::outputs`]) before deciding whether to reuse it via the RBF methods
/// or build a fresh contribution with different parameters using the splice methods above.
Expand All @@ -232,8 +233,9 @@ pub struct FundingTemplate {
/// transaction.
shared_input: Option<Input>,

/// The minimum RBF feerate (25/24 of the previous feerate), if this template is for an
/// RBF attempt. `None` for fresh splices with no pending splice candidates.
/// The minimum RBF feerate (the greater of previous feerate + 25 sat/kwu and the spec's
/// 25/24 rule), if this template is for an RBF attempt. `None` for fresh splices with no
/// pending splice candidates.
min_rbf_feerate: Option<FeeRate>,

/// The user's prior contribution from a previous splice negotiation, if available.
Expand Down Expand Up @@ -2262,8 +2264,8 @@ mod tests {
// When the caller's max_feerate is below the minimum RBF feerate, rbf_sync should
// return Err(()).
let prior_feerate = FeeRate::from_sat_per_kwu(2000);
let min_rbf_feerate = FeeRate::from_sat_per_kwu(5000);
let max_feerate = FeeRate::from_sat_per_kwu(3000);
let min_rbf_feerate = FeeRate::from_sat_per_kwu(2025);
let max_feerate = FeeRate::from_sat_per_kwu(2020);

let prior = FundingContribution {
value_added: Amount::from_sat(50_000),
Expand All @@ -2276,7 +2278,7 @@ mod tests {
is_splice: true,
};

// max_feerate (3000) < min_rbf_feerate (5000).
// max_feerate (2020) < min_rbf_feerate (2025).
let template = FundingTemplate::new(
None,
Some(min_rbf_feerate),
Expand Down Expand Up @@ -2359,8 +2361,8 @@ mod tests {
// When the prior contribution's feerate is below the minimum RBF feerate and no
// holder balance is available, rbf_sync should run coin selection to add inputs that
// cover the higher RBF fee.
let min_rbf_feerate = FeeRate::from_sat_per_kwu(5000);
let prior_feerate = FeeRate::from_sat_per_kwu(2000);
let min_rbf_feerate = FeeRate::from_sat_per_kwu(2025);
let withdrawal = funding_output_sats(20_000);

let prior = FundingContribution {
Expand Down Expand Up @@ -2397,7 +2399,7 @@ mod tests {
fn test_rbf_sync_no_prior_fee_bump_only_runs_coin_selection() {
// When there is no prior contribution (e.g., acceptor), rbf_sync should run coin
// selection to add inputs for a fee-bump-only contribution.
let min_rbf_feerate = FeeRate::from_sat_per_kwu(5000);
let min_rbf_feerate = FeeRate::from_sat_per_kwu(2025);

let template =
FundingTemplate::new(Some(shared_input(100_000)), Some(min_rbf_feerate), None);
Expand All @@ -2419,7 +2421,7 @@ mod tests {
// When the prior contribution's feerate is below the minimum RBF feerate and no
// holder balance is available, rbf_sync should use the caller's max_feerate (not the
// prior's) for the resulting contribution.
let min_rbf_feerate = FeeRate::from_sat_per_kwu(5000);
let min_rbf_feerate = FeeRate::from_sat_per_kwu(2025);
let prior_max_feerate = FeeRate::from_sat_per_kwu(50_000);
let callers_max_feerate = FeeRate::from_sat_per_kwu(10_000);
let withdrawal = funding_output_sats(20_000);
Expand Down Expand Up @@ -2458,8 +2460,8 @@ mod tests {
// When splice_out_sync is called on a template with min_rbf_feerate set (user
// choosing a fresh splice-out instead of rbf_sync), coin selection should NOT run.
// Fees come from the channel balance.
let min_rbf_feerate = FeeRate::from_sat_per_kwu(5000);
let feerate = FeeRate::from_sat_per_kwu(5000);
let min_rbf_feerate = FeeRate::from_sat_per_kwu(2025);
let feerate = FeeRate::from_sat_per_kwu(2025);
let withdrawal = funding_output_sats(20_000);

let template =
Expand Down
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