}
}
-/// Options for how to set the max dust HTLC exposure allowed on a channel. See
+/// Options for how to set the max dust exposure allowed on a channel. See
/// [`ChannelConfig::max_dust_htlc_exposure`] for details.
#[derive(Copy, Clone, Debug, PartialEq, Eq)]
pub enum MaxDustHTLCExposure {
/// to this maximum the channel may be unable to send/receive HTLCs between the maximum dust
/// exposure and the new minimum value for HTLCs to be economically viable to claim.
FixedLimitMsat(u64),
- /// This sets a multiplier on the estimated high priority feerate (sats/KW, as obtained from
- /// [`FeeEstimator`]) to determine the maximum allowed dust exposure. If this variant is used
- /// then the maximum dust exposure in millisatoshis is calculated as:
- /// `high_priority_feerate_per_kw * value`. For example, with our default value
- /// `FeeRateMultiplier(5000)`:
+ /// This sets a multiplier on the [`ConfirmationTarget::OnChainSweep`] feerate (in sats/KW) to
+ /// determine the maximum allowed dust exposure. If this variant is used then the maximum dust
+ /// exposure in millisatoshis is calculated as:
+ /// `feerate_per_kw * value`. For example, with our default value
+ /// `FeeRateMultiplier(10_000)`:
///
/// - For the minimum fee rate of 1 sat/vByte (250 sat/KW, although the minimum
/// defaults to 253 sats/KW for rounding, see [`FeeEstimator`]), the max dust exposure would
- /// be 253 * 5000 = 1,265,000 msats.
+ /// be 253 * 10_000 = 2,530,000 msats.
/// - For a fee rate of 30 sat/vByte (7500 sat/KW), the max dust exposure would be
- /// 7500 * 5000 = 37,500,000 msats.
- ///
- /// This allows the maximum dust exposure to automatically scale with fee rate changes.
+ /// 7500 * 50_000 = 75,000,000 msats (0.00075 BTC).
///
/// Note, if you're using a third-party fee estimator, this may leave you more exposed to a
/// fee griefing attack, where your fee estimator may purposely overestimate the fee rate,
/// by default this will be set to a [`Self::FixedLimitMsat`] of 5,000,000 msat.
///
/// [`FeeEstimator`]: crate::chain::chaininterface::FeeEstimator
+ /// [`ConfirmationTarget::OnChainSweep`]: crate::chain::chaininterface::ConfirmationTarget::OnChainSweep
FeeRateMultiplier(u64),
}
///
/// [`MIN_CLTV_EXPIRY_DELTA`]: crate::ln::channelmanager::MIN_CLTV_EXPIRY_DELTA
pub cltv_expiry_delta: u16,
- /// Limit our total exposure to in-flight HTLCs which are burned to fees as they are too
- /// small to claim on-chain.
+ /// Limit our total exposure to potential loss to on-chain fees on close, including in-flight
+ /// HTLCs which are burned to fees as they are too small to claim on-chain and fees on
+ /// commitment transaction(s) broadcasted by our counterparty in excess of our own fee estimate.
+ ///
+ /// # HTLC-based Dust Exposure
///
/// When an HTLC present in one of our channels is below a "dust" threshold, the HTLC will
/// not be claimable on-chain, instead being turned into additional miner fees if either
/// party force-closes the channel. Because the threshold is per-HTLC, our total exposure
- /// to such payments may be sustantial if there are many dust HTLCs present when the
+ /// to such payments may be substantial if there are many dust HTLCs present when the
/// channel is force-closed.
///
/// The dust threshold for each HTLC is based on the `dust_limit_satoshis` for each party in a
/// The selected limit is applied for sent, forwarded, and received HTLCs and limits the total
/// exposure across all three types per-channel.
///
- /// Default value: [`MaxDustHTLCExposure::FeeRateMultiplier`] with a multiplier of 5000.
+ /// # Transaction Fee Dust Exposure
+ ///
+ /// Further, counterparties broadcasting a commitment transaction in a force-close may result
+ /// in other balance being burned to fees, and thus all fees on commitment and HTLC
+ /// transactions in excess of our local fee estimates are included in the dust calculation.
+ ///
+ /// Because of this, another way to look at this limit is to divide it by 43,000 (or 218,750
+ /// for non-anchor channels) and see it as the maximum feerate disagreement (in sats/vB) per
+ /// non-dust HTLC we're allowed to have with our peers before risking a force-closure for
+ /// inbound channels.
+ // This works because, for anchor channels the on-chain cost is 172 weight (172+703 for
+ // non-anchors with an HTLC-Success transaction), i.e.
+ // dust_exposure_limit_msat / 1000 = 172 * feerate_in_sat_per_vb / 4 * HTLC count
+ // dust_exposure_limit_msat = 43,000 * feerate_in_sat_per_vb * HTLC count
+ // dust_exposure_limit_msat / HTLC count / 43,000 = feerate_in_sat_per_vb
+ ///
+ /// Thus, for the default value of 10_000 * a current feerate estimate of 10 sat/vB (or 2,500
+ /// sat/KW), we risk force-closure if we disagree with our peer by:
+ /// * `10_000 * 2_500 / 43_000 / (483*2)` = 0.6 sat/vB for anchor channels with 483 HTLCs in
+ /// both directions (the maximum),
+ /// * `10_000 * 2_500 / 43_000 / (50*2)` = 5.8 sat/vB for anchor channels with 50 HTLCs in both
+ /// directions (the LDK default max from [`ChannelHandshakeConfig::our_max_accepted_htlcs`])
+ /// * `10_000 * 2_500 / 218_750 / (483*2)` = 0.1 sat/vB for non-anchor channels with 483 HTLCs
+ /// in both directions (the maximum),
+ /// * `10_000 * 2_500 / 218_750 / (50*2)` = 1.1 sat/vB for non-anchor channels with 50 HTLCs
+ /// in both (the LDK default maximum from [`ChannelHandshakeConfig::our_max_accepted_htlcs`])
+ ///
+ /// Note that when using [`MaxDustHTLCExposure::FeeRateMultiplier`] this maximum disagreement
+ /// will scale linearly with increases (or decreases) in the our feerate estimates. Further,
+ /// for anchor channels we expect our counterparty to use a relatively low feerate estimate
+ /// while we use [`ConfirmationTarget::OnChainSweep`] (which should be relatively high) and
+ /// feerate disagreement force-closures should only occur when theirs is higher than ours.
+ ///
+ /// Default value: [`MaxDustHTLCExposure::FeeRateMultiplier`] with a multiplier of 10_000.
+ ///
+ /// [`ConfirmationTarget::OnChainSweep`]: crate::chain::chaininterface::ConfirmationTarget::OnChainSweep
pub max_dust_htlc_exposure: MaxDustHTLCExposure,
/// The additional fee we're willing to pay to avoid waiting for the counterparty's
/// `to_self_delay` to reclaim funds.
forwarding_fee_proportional_millionths: 0,
forwarding_fee_base_msat: 1000,
cltv_expiry_delta: 6 * 12, // 6 blocks/hour * 12 hours
- max_dust_htlc_exposure: MaxDustHTLCExposure::FeeRateMultiplier(5000),
+ max_dust_htlc_exposure: MaxDustHTLCExposure::FeeRateMultiplier(10000),
force_close_avoidance_max_fee_satoshis: 1000,
accept_underpaying_htlcs: false,
}
projects / rust-lightning / blobdiff