[rust-lightning] / lightning / src / util / config.rs

// This file is Copyright its original authors, visible in version control
// history.
//
// This file is licensed under the Apache License, Version 2.0 <LICENSE-APACHE
// or http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your option.
// You may not use this file except in accordance with one or both of these
// licenses.

//! Various user-configurable channel limits and settings which ChannelManager
//! applies for you.

use crate::ln::channel::MAX_FUNDING_SATOSHIS_NO_WUMBO;
use crate::ln::channelmanager::{BREAKDOWN_TIMEOUT, MAX_LOCAL_BREAKDOWN_TIMEOUT};

#[cfg(fuzzing)]
use crate::util::ser::Readable;

/// Configuration we set when applicable.
///
/// Default::default() provides sane defaults.
#[derive(Copy, Clone, Debug)]
pub struct ChannelHandshakeConfig {
        /// Confirmations we will wait for before considering the channel locked in.
        /// Applied only for inbound channels (see ChannelHandshakeLimits::max_minimum_depth for the
        /// equivalent limit applied to outbound channels).
        ///
        /// A lower-bound of 1 is applied, requiring all channels to have a confirmed commitment
        /// transaction before operation. If you wish to accept channels with zero confirmations, see
        /// [`UserConfig::manually_accept_inbound_channels`] and
        /// [`ChannelManager::accept_inbound_channel_from_trusted_peer_0conf`].
        ///
        /// Default value: 6.
        ///
        /// [`ChannelManager::accept_inbound_channel`]: crate::ln::channelmanager::ChannelManager::accept_inbound_channel
        /// [`ChannelManager::accept_inbound_channel_from_trusted_peer_0conf`]: crate::ln::channelmanager::ChannelManager::accept_inbound_channel_from_trusted_peer_0conf
        pub minimum_depth: u32,
        /// Set to the number of blocks we require our counterparty to wait to claim their money (ie
        /// the number of blocks we have to punish our counterparty if they broadcast a revoked
        /// transaction).
        ///
        /// This is one of the main parameters of our security model. We (or one of our watchtowers) MUST
        /// be online to check for revoked transactions on-chain at least once every our_to_self_delay
        /// blocks (minus some margin to allow us enough time to broadcast and confirm a transaction,
        /// possibly with time in between to RBF the spending transaction).
        ///
        /// Meanwhile, asking for a too high delay, we bother peer to freeze funds for nothing in
        /// case of an honest unilateral channel close, which implicitly decrease the economic value of
        /// our channel.
        ///
        /// Default value: [`BREAKDOWN_TIMEOUT`], we enforce it as a minimum at channel opening so you
        /// can tweak config to ask for more security, not less.
        pub our_to_self_delay: u16,
        /// Set to the smallest value HTLC we will accept to process.
        ///
        /// This value is sent to our counterparty on channel-open and we close the channel any time
        /// our counterparty misbehaves by sending us an HTLC with a value smaller than this.
        ///
        /// Default value: 1. If the value is less than 1, it is ignored and set to 1, as is required
        /// by the protocol.
        pub our_htlc_minimum_msat: u64,
        /// Sets the percentage of the channel value we will cap the total value of outstanding inbound
        /// HTLCs to.
        ///
        /// This can be set to a value between 1-100, where the value corresponds to the percent of the
        /// channel value in whole percentages.
        ///
        /// Note that:
        /// * If configured to another value than the default value 10, any new channels created with
        /// the non default value will cause versions of LDK prior to 0.0.104 to refuse to read the
        /// `ChannelManager`.
        ///
        /// * This caps the total value for inbound HTLCs in-flight only, and there's currently
        /// no way to configure the cap for the total value of outbound HTLCs in-flight.
        ///
        /// * The requirements for your node being online to ensure the safety of HTLC-encumbered funds
        /// are different from the non-HTLC-encumbered funds. This makes this an important knob to
        /// restrict exposure to loss due to being offline for too long.
        /// See [`ChannelHandshakeConfig::our_to_self_delay`] and [`ChannelConfig::cltv_expiry_delta`]
        /// for more information.
        ///
        /// Default value: 10.
        /// Minimum value: 1, any values less than 1 will be treated as 1 instead.
        /// Maximum value: 100, any values larger than 100 will be treated as 100 instead.
        pub max_inbound_htlc_value_in_flight_percent_of_channel: u8,
        /// If set, we attempt to negotiate the `scid_privacy` (referred to as `scid_alias` in the
        /// BOLTs) option for outbound private channels. This provides better privacy by not including
        /// our real on-chain channel UTXO in each invoice and requiring that our counterparty only
        /// relay HTLCs to us using the channel's SCID alias.
        ///
        /// If this option is set, channels may be created that will not be readable by LDK versions
        /// prior to 0.0.106, causing [`ChannelManager`]'s read method to return a
        /// [`DecodeError::InvalidValue`].
        ///
        /// Note that setting this to true does *not* prevent us from opening channels with
        /// counterparties that do not support the `scid_alias` option; we will simply fall back to a
        /// private channel without that option.
        ///
        /// Ignored if the channel is negotiated to be announced, see
        /// [`ChannelHandshakeConfig::announced_channel`] and
        /// [`ChannelHandshakeLimits::force_announced_channel_preference`] for more.
        ///
        /// Default value: false. This value is likely to change to true in the future.
        ///
        /// [`ChannelManager`]: crate::ln::channelmanager::ChannelManager
        /// [`DecodeError::InvalidValue`]: crate::ln::msgs::DecodeError::InvalidValue
        pub negotiate_scid_privacy: bool,
        /// Set to announce the channel publicly and notify all nodes that they can route via this
        /// channel.
        ///
        /// This should only be set to true for nodes which expect to be online reliably.
        ///
        /// As the node which funds a channel picks this value this will only apply for new outbound
        /// channels unless [`ChannelHandshakeLimits::force_announced_channel_preference`] is set.
        ///
        /// Default value: false.
        pub announced_channel: bool,
        /// When set, we commit to an upfront shutdown_pubkey at channel open. If our counterparty
        /// supports it, they will then enforce the mutual-close output to us matches what we provided
        /// at intialization, preventing us from closing to an alternate pubkey.
        ///
        /// This is set to true by default to provide a slight increase in security, though ultimately
        /// any attacker who is able to take control of a channel can just as easily send the funds via
        /// lightning payments, so we never require that our counterparties support this option.
        ///
        /// The upfront key committed is provided from [`SignerProvider::get_shutdown_scriptpubkey`].
        ///
        /// Default value: true.
        ///
        /// [`SignerProvider::get_shutdown_scriptpubkey`]: crate::sign::SignerProvider::get_shutdown_scriptpubkey
        pub commit_upfront_shutdown_pubkey: bool,
        /// The Proportion of the channel value to configure as counterparty's channel reserve,
        /// i.e., `their_channel_reserve_satoshis` for both outbound and inbound channels.
        ///
        /// `their_channel_reserve_satoshis` is the minimum balance that the other node has to maintain
        /// on their side, at all times.
        /// This ensures that if our counterparty broadcasts a revoked state, we can punish them by
        /// claiming at least this value on chain.
        ///
        /// Channel reserve values greater than 30% could be considered highly unreasonable, since that
        /// amount can never be used for payments.
        /// Also, if our selected channel reserve for counterparty and counterparty's selected
        /// channel reserve for us sum up to equal or greater than channel value, channel negotiations
        /// will fail.
        ///
        /// Note: Versions of LDK earlier than v0.0.104 will fail to read channels with any channel reserve
        /// other than the default value.
        ///
        /// Default value: 1% of channel value, i.e., configured as 10,000 millionths.
        /// Minimum value: If the calculated proportional value is less than 1000 sats, it will be treated
        ///                as 1000 sats instead, which is a safe implementation-specific lower bound.
        /// Maximum value: 1,000,000, any values larger than 1 Million will be treated as 1 Million (or 100%)
        ///                instead, although channel negotiations will fail in that case.
        pub their_channel_reserve_proportional_millionths: u32,
        /// If set, we attempt to negotiate the `anchors_zero_fee_htlc_tx`option for all future
        /// channels. This feature requires having a reserve of onchain funds readily available to bump
        /// transactions in the event of a channel force close to avoid the possibility of losing funds.
        ///
        /// Note that if you wish accept inbound channels with anchor outputs, you must enable
        /// [`UserConfig::manually_accept_inbound_channels`] and manually accept them with
        /// [`ChannelManager::accept_inbound_channel`]. This is done to give you the chance to check
        /// whether your reserve of onchain funds is enough to cover the fees for all existing and new
        /// channels featuring anchor outputs in the event of a force close.
        ///
        /// If this option is set, channels may be created that will not be readable by LDK versions
        /// prior to 0.0.116, causing [`ChannelManager`]'s read method to return a
        /// [`DecodeError::InvalidValue`].
        ///
        /// Note that setting this to true does *not* prevent us from opening channels with
        /// counterparties that do not support the `anchors_zero_fee_htlc_tx` option; we will simply
        /// fall back to a `static_remote_key` channel.
        ///
        /// LDK will not support the legacy `option_anchors` commitment version due to a discovered
        /// vulnerability after its deployment. For more context, see the [`SIGHASH_SINGLE + update_fee
        /// Considered Harmful`] mailing list post.
        ///
        /// Default value: false. This value is likely to change to true in the future.
        ///
        /// [`ChannelManager`]: crate::ln::channelmanager::ChannelManager
        /// [`ChannelManager::accept_inbound_channel`]: crate::ln::channelmanager::ChannelManager::accept_inbound_channel
        /// [`DecodeError::InvalidValue`]: crate::ln::msgs::DecodeError::InvalidValue
        /// [`SIGHASH_SINGLE + update_fee Considered Harmful`]: https://lists.linuxfoundation.org/pipermail/lightning-dev/2020-September/002796.html
        pub negotiate_anchors_zero_fee_htlc_tx: bool,

        /// The maximum number of HTLCs in-flight from our counterparty towards us at the same time.
        ///
        /// Increasing the value can help improve liquidity and stability in
        /// routing at the cost of higher long term disk / DB usage.
        ///
        /// Note: Versions of LDK earlier than v0.0.115 will fail to read channels with a configuration
        /// other than the default value.
        ///
        /// Default value: 50
        /// Maximum value: 483, any values larger will be treated as 483.
        ///                     This is the BOLT #2 spec limit on `max_accepted_htlcs`.
        pub our_max_accepted_htlcs: u16,
}

impl Default for ChannelHandshakeConfig {
        fn default() -> ChannelHandshakeConfig {
                ChannelHandshakeConfig {
                        minimum_depth: 6,
                        our_to_self_delay: BREAKDOWN_TIMEOUT,
                        our_htlc_minimum_msat: 1,
                        max_inbound_htlc_value_in_flight_percent_of_channel: 10,
                        negotiate_scid_privacy: false,
                        announced_channel: false,
                        commit_upfront_shutdown_pubkey: true,
                        their_channel_reserve_proportional_millionths: 10_000,
                        negotiate_anchors_zero_fee_htlc_tx: false,
                        our_max_accepted_htlcs: 50,
                }
        }
}

// When fuzzing, we want to allow the fuzzer to pick any configuration parameters. Thus, we
// implement Readable here in a naive way (which is a bit easier for the fuzzer to handle). We
// don't really want to ever expose this to users (if we did we'd want to use TLVs).
#[cfg(fuzzing)]
impl Readable for ChannelHandshakeConfig {
        fn read<R: crate::io::Read>(reader: &mut R) -> Result<Self, crate::ln::msgs::DecodeError> {
                Ok(Self {
                        minimum_depth: Readable::read(reader)?,
                        our_to_self_delay: Readable::read(reader)?,
                        our_htlc_minimum_msat: Readable::read(reader)?,
                        max_inbound_htlc_value_in_flight_percent_of_channel: Readable::read(reader)?,
                        negotiate_scid_privacy: Readable::read(reader)?,
                        announced_channel: Readable::read(reader)?,
                        commit_upfront_shutdown_pubkey: Readable::read(reader)?,
                        their_channel_reserve_proportional_millionths: Readable::read(reader)?,
                        negotiate_anchors_zero_fee_htlc_tx: Readable::read(reader)?,
                        our_max_accepted_htlcs: Readable::read(reader)?,
                })
        }
}

/// Optional channel limits which are applied during channel creation.
///
/// These limits are only applied to our counterparty's limits, not our own.
///
/// Use 0/`<type>::max_value()` as appropriate to skip checking.
///
/// Provides sane defaults for most configurations.
///
/// Most additional limits are disabled except those with which specify a default in individual
/// field documentation. Note that this may result in barely-usable channels, but since they
/// are applied mostly only to incoming channels that's not much of a problem.
#[derive(Copy, Clone, Debug)]
pub struct ChannelHandshakeLimits {
        /// Minimum allowed satoshis when a channel is funded. This is supplied by the sender and so
        /// only applies to inbound channels.
        ///
        /// Default value: 0.
        pub min_funding_satoshis: u64,
        /// Maximum allowed satoshis when a channel is funded. This is supplied by the sender and so
        /// only applies to inbound channels.
        ///
        /// Default value: 2^24 - 1.
        pub max_funding_satoshis: u64,
        /// The remote node sets a limit on the minimum size of HTLCs we can send to them. This allows
        /// you to limit the maximum minimum-size they can require.
        ///
        /// Default value: u64::max_value.
        pub max_htlc_minimum_msat: u64,
        /// The remote node sets a limit on the maximum value of pending HTLCs to them at any given
        /// time to limit their funds exposure to HTLCs. This allows you to set a minimum such value.
        ///
        /// Default value: 0.
        pub min_max_htlc_value_in_flight_msat: u64,
        /// The remote node will require we keep a certain amount in direct payment to ourselves at all
        /// time, ensuring that we are able to be punished if we broadcast an old state. This allows to
        /// you limit the amount which we will have to keep to ourselves (and cannot use for HTLCs).
        ///
        /// Default value: u64::max_value.
        pub max_channel_reserve_satoshis: u64,
        /// The remote node sets a limit on the maximum number of pending HTLCs to them at any given
        /// time. This allows you to set a minimum such value.
        ///
        /// Default value: 0.
        pub min_max_accepted_htlcs: u16,
        /// Before a channel is usable the funding transaction will need to be confirmed by at least a
        /// certain number of blocks, specified by the node which is not the funder (as the funder can
        /// assume they aren't going to double-spend themselves).
        /// This config allows you to set a limit on the maximum amount of time to wait.
        ///
        /// Default value: 144, or roughly one day and only applies to outbound channels.
        pub max_minimum_depth: u32,
        /// Whether we implicitly trust funding transactions generated by us for our own outbound
        /// channels to not be double-spent.
        ///
        /// If this is set, we assume that our own funding transactions are *never* double-spent, and
        /// thus we can trust them without any confirmations. This is generally a reasonable
        /// assumption, given we're the only ones who could ever double-spend it (assuming we have sole
        /// control of the signing keys).
        ///
        /// You may wish to un-set this if you allow the user to (or do in an automated fashion)
        /// double-spend the funding transaction to RBF with an alternative channel open.
        ///
        /// This only applies if our counterparty set their confirmations-required value to 0, and we
        /// always trust our own funding transaction at 1 confirmation irrespective of this value.
        /// Thus, this effectively acts as a `min_minimum_depth`, with the only possible values being
        /// `true` (0) and `false` (1).
        ///
        /// Default value: true
        pub trust_own_funding_0conf: bool,
        /// Set to force an incoming channel to match our announced channel preference in
        /// [`ChannelHandshakeConfig::announced_channel`].
        ///
        /// For a node which is not online reliably, this should be set to true and
        /// [`ChannelHandshakeConfig::announced_channel`] set to false, ensuring that no announced (aka public)
        /// channels will ever be opened.
        ///
        /// Default value: true.
        pub force_announced_channel_preference: bool,
        /// Set to the amount of time we're willing to wait to claim money back to us.
        ///
        /// Not checking this value would be a security issue, as our peer would be able to set it to
        /// max relative lock-time (a year) and we would "lose" money as it would be locked for a long time.
        ///
        /// Default value: 2016, which we also enforce as a maximum value so you can tweak config to
        /// reduce the loss of having useless locked funds (if your peer accepts)
        pub their_to_self_delay: u16
}

impl Default for ChannelHandshakeLimits {
        fn default() -> Self {
                ChannelHandshakeLimits {
                        min_funding_satoshis: 0,
                        max_funding_satoshis: MAX_FUNDING_SATOSHIS_NO_WUMBO,
                        max_htlc_minimum_msat: <u64>::max_value(),
                        min_max_htlc_value_in_flight_msat: 0,
                        max_channel_reserve_satoshis: <u64>::max_value(),
                        min_max_accepted_htlcs: 0,
                        trust_own_funding_0conf: true,
                        max_minimum_depth: 144,
                        force_announced_channel_preference: true,
                        their_to_self_delay: MAX_LOCAL_BREAKDOWN_TIMEOUT,
                }
        }
}

// When fuzzing, we want to allow the fuzzer to pick any configuration parameters. Thus, we
// implement Readable here in a naive way (which is a bit easier for the fuzzer to handle). We
// don't really want to ever expose this to users (if we did we'd want to use TLVs).
#[cfg(fuzzing)]
impl Readable for ChannelHandshakeLimits {
        fn read<R: crate::io::Read>(reader: &mut R) -> Result<Self, crate::ln::msgs::DecodeError> {
                Ok(Self {
                        min_funding_satoshis: Readable::read(reader)?,
                        max_funding_satoshis: Readable::read(reader)?,
                        max_htlc_minimum_msat: Readable::read(reader)?,
                        min_max_htlc_value_in_flight_msat: Readable::read(reader)?,
                        max_channel_reserve_satoshis: Readable::read(reader)?,
                        min_max_accepted_htlcs: Readable::read(reader)?,
                        trust_own_funding_0conf: Readable::read(reader)?,
                        max_minimum_depth: Readable::read(reader)?,
                        force_announced_channel_preference: Readable::read(reader)?,
                        their_to_self_delay: Readable::read(reader)?,
                })
        }
}

/// 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 {
        /// This sets a fixed limit on the total dust exposure in millisatoshis. Setting this too low
        /// may prevent the sending or receipt of low-value HTLCs on high-traffic nodes, however this
        /// limit is very important to prevent stealing of large amounts of dust HTLCs by miners
        /// through [fee griefing
        /// attacks](https://lists.linuxfoundation.org/pipermail/lightning-dev/2020-May/002714.html).
        ///
        /// Note that if the feerate increases significantly, without a manual increase
        /// 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 [`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 * 10_000 = 2,530,000 msats.
        /// - For a fee rate of 30 sat/vByte (7500 sat/KW), the max dust exposure would be
        /// 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,
        /// causing you to accept more dust HTLCs than you would otherwise.
        ///
        /// This variant is primarily meant to serve pre-anchor channels, as HTLC fees being included
        /// on HTLC outputs means your channel may be subject to more dust exposure in the event of
        /// increases in fee rate.
        ///
        /// # Backwards Compatibility
        /// This variant only became available in LDK 0.0.116, so if you downgrade to a prior version
        /// 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),
}

impl_writeable_tlv_based_enum!(MaxDustHTLCExposure, ;
        (1, FixedLimitMsat),
        (3, FeeRateMultiplier),
);

/// Options which apply on a per-channel basis and may change at runtime or based on negotiation
/// with our counterparty.
#[derive(Copy, Clone, Debug, PartialEq, Eq)]
pub struct ChannelConfig {
        /// Amount (in millionths of a satoshi) charged per satoshi for payments forwarded outbound
        /// over the channel.
        /// This may be allowed to change at runtime in a later update, however doing so must result in
        /// update messages sent to notify all nodes of our updated relay fee.
        ///
        /// Default value: 0.
        pub forwarding_fee_proportional_millionths: u32,
        /// Amount (in milli-satoshi) charged for payments forwarded outbound over the channel, in
        /// excess of [`forwarding_fee_proportional_millionths`].
        /// This may be allowed to change at runtime in a later update, however doing so must result in
        /// update messages sent to notify all nodes of our updated relay fee.
        ///
        /// The default value of a single satoshi roughly matches the market rate on many routing nodes
        /// as of July 2021. Adjusting it upwards or downwards may change whether nodes route through
        /// this node.
        ///
        /// Default value: 1000.
        ///
        /// [`forwarding_fee_proportional_millionths`]: ChannelConfig::forwarding_fee_proportional_millionths
        pub forwarding_fee_base_msat: u32,
        /// The difference in the CLTV value between incoming HTLCs and an outbound HTLC forwarded over
        /// the channel this config applies to.
        ///
        /// This is analogous to [`ChannelHandshakeConfig::our_to_self_delay`] but applies to in-flight
        /// HTLC balance when a channel appears on-chain whereas
        /// [`ChannelHandshakeConfig::our_to_self_delay`] applies to the remaining
        /// (non-HTLC-encumbered) balance.
        ///
        /// Thus, for HTLC-encumbered balances to be enforced on-chain when a channel is force-closed,
        /// we (or one of our watchtowers) MUST be online to check for broadcast of the current
        /// commitment transaction at least once per this many blocks (minus some margin to allow us
        /// enough time to broadcast and confirm a transaction, possibly with time in between to RBF
        /// the spending transaction).
        ///
        /// Default value: 72 (12 hours at an average of 6 blocks/hour).
        /// Minimum value: [`MIN_CLTV_EXPIRY_DELTA`], any values less than this will be treated as
        ///                [`MIN_CLTV_EXPIRY_DELTA`] instead.
        ///
        /// [`MIN_CLTV_EXPIRY_DELTA`]: crate::ln::channelmanager::MIN_CLTV_EXPIRY_DELTA
        pub cltv_expiry_delta: u16,
        /// 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 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
        /// channel negotiated throughout the channel open process, along with the fees required to have
        /// a broadcastable HTLC spending transaction. When a channel supports anchor outputs
        /// (specifically the zero fee HTLC transaction variant), this threshold no longer takes into
        /// account the HTLC transaction fee as it is zero. Because of this, you may want to set this
        /// value to a fixed limit for channels using anchor outputs, while the fee rate multiplier
        /// variant is primarily intended for use with pre-anchor channels.
        ///
        /// The selected limit is applied for sent, forwarded, and received HTLCs and limits the total
        /// exposure across all three types per-channel.
        ///
        /// # 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.
        ///
        /// When we close a channel cooperatively with our counterparty, we negotiate a fee for the
        /// closing transaction which both sides find acceptable, ultimately paid by the channel
        /// funder/initiator.
        ///
        /// When we are the funder, because we have to pay the channel closing fee, we bound the
        /// acceptable fee by our [`ChannelCloseMinimum`] and [`NonAnchorChannelFee`] fees, with the upper bound increased by
        /// this value. Because the on-chain fee we'd pay to force-close the channel is kept near our
        /// [`NonAnchorChannelFee`] feerate during normal operation, this value represents the additional fee we're
        /// willing to pay in order to avoid waiting for our counterparty's to_self_delay to reclaim our
        /// funds.
        ///
        /// When we are not the funder, we require the closing transaction fee pay at least our
        /// [`ChannelCloseMinimum`] fee estimate, but allow our counterparty to pay as much fee as they like.
        /// Thus, this value is ignored when we are not the funder.
        ///
        /// Default value: 1000 satoshis.
        ///
        /// [`NonAnchorChannelFee`]: crate::chain::chaininterface::ConfirmationTarget::NonAnchorChannelFee
        /// [`ChannelCloseMinimum`]: crate::chain::chaininterface::ConfirmationTarget::ChannelCloseMinimum
        pub force_close_avoidance_max_fee_satoshis: u64,
        /// If set, allows this channel's counterparty to skim an additional fee off this node's inbound
        /// HTLCs. Useful for liquidity providers to offload on-chain channel costs to end users.
        ///
        /// Usage:
        /// - The payee will set this option and set its invoice route hints to use [intercept scids]
        ///   generated by this channel's counterparty.
        /// - The counterparty will get an [`HTLCIntercepted`] event upon payment forward, and call
        ///   [`forward_intercepted_htlc`] with less than the amount provided in
        ///   [`HTLCIntercepted::expected_outbound_amount_msat`]. The difference between the expected and
        ///   actual forward amounts is their fee. See
        ///   <https://github.com/BitcoinAndLightningLayerSpecs/lsp/tree/main/LSPS2#flow-lsp-trusts-client-model>
        ///   for how this feature may be used in the LSP use case.
        ///
        /// # Note
        /// It's important for payee wallet software to verify that [`PaymentClaimable::amount_msat`] is
        /// as-expected if this feature is activated, otherwise they may lose money!
        /// [`PaymentClaimable::counterparty_skimmed_fee_msat`] provides the fee taken by the
        /// counterparty.
        ///
        /// # Note
        /// Switching this config flag on may break compatibility with versions of LDK prior to 0.0.116.
        /// Unsetting this flag between restarts may lead to payment receive failures.
        ///
        /// Default value: false.
        ///
        /// [intercept scids]: crate::ln::channelmanager::ChannelManager::get_intercept_scid
        /// [`forward_intercepted_htlc`]: crate::ln::channelmanager::ChannelManager::forward_intercepted_htlc
        /// [`HTLCIntercepted`]: crate::events::Event::HTLCIntercepted
        /// [`HTLCIntercepted::expected_outbound_amount_msat`]: crate::events::Event::HTLCIntercepted::expected_outbound_amount_msat
        /// [`PaymentClaimable::amount_msat`]: crate::events::Event::PaymentClaimable::amount_msat
        /// [`PaymentClaimable::counterparty_skimmed_fee_msat`]: crate::events::Event::PaymentClaimable::counterparty_skimmed_fee_msat
        //  TODO: link to bLIP when it's merged
        pub accept_underpaying_htlcs: bool,
}

impl ChannelConfig {
        /// Applies the given [`ChannelConfigUpdate`] as a partial update to the [`ChannelConfig`].
        pub fn apply(&mut self, update: &ChannelConfigUpdate) {
                if let Some(forwarding_fee_proportional_millionths) = update.forwarding_fee_proportional_millionths {
                        self.forwarding_fee_proportional_millionths = forwarding_fee_proportional_millionths;
                }
                if let Some(forwarding_fee_base_msat) = update.forwarding_fee_base_msat {
                        self.forwarding_fee_base_msat = forwarding_fee_base_msat;
                }
                if let Some(cltv_expiry_delta) = update.cltv_expiry_delta {
                        self.cltv_expiry_delta = cltv_expiry_delta;
                }
                if let Some(max_dust_htlc_exposure_msat) = update.max_dust_htlc_exposure_msat {
                        self.max_dust_htlc_exposure = max_dust_htlc_exposure_msat;
                }
                if let Some(force_close_avoidance_max_fee_satoshis) = update.force_close_avoidance_max_fee_satoshis {
                        self.force_close_avoidance_max_fee_satoshis = force_close_avoidance_max_fee_satoshis;
                }
        }
}

impl Default for ChannelConfig {
        /// Provides sane defaults for most configurations (but with zero relay fees!).
        fn default() -> Self {
                ChannelConfig {
                        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(10000),
                        force_close_avoidance_max_fee_satoshis: 1000,
                        accept_underpaying_htlcs: false,
                }
        }
}

impl crate::util::ser::Writeable for ChannelConfig {
        fn write<W: crate::util::ser::Writer>(&self, writer: &mut W) -> Result<(), crate::io::Error> {
                let max_dust_htlc_exposure_msat_fixed_limit = match self.max_dust_htlc_exposure {
                        MaxDustHTLCExposure::FixedLimitMsat(limit) => limit,
                        MaxDustHTLCExposure::FeeRateMultiplier(_) => 5_000_000,
                };
                write_tlv_fields!(writer, {
                        (0, self.forwarding_fee_proportional_millionths, required),
                        (1, self.accept_underpaying_htlcs, (default_value, false)),
                        (2, self.forwarding_fee_base_msat, required),
                        (3, self.max_dust_htlc_exposure, required),
                        (4, self.cltv_expiry_delta, required),
                        (6, max_dust_htlc_exposure_msat_fixed_limit, required),
                        // ChannelConfig serialized this field with a required type of 8 prior to the introduction of
                        // LegacyChannelConfig. To make sure that serialization is not compatible with this one, we use
                        // the next required type of 10, which if seen by the old serialization will always fail.
                        (10, self.force_close_avoidance_max_fee_satoshis, required),
                });
                Ok(())
        }
}

impl crate::util::ser::Readable for ChannelConfig {
        fn read<R: crate::io::Read>(reader: &mut R) -> Result<Self, crate::ln::msgs::DecodeError> {
                let mut forwarding_fee_proportional_millionths = 0;
                let mut accept_underpaying_htlcs = false;
                let mut forwarding_fee_base_msat = 1000;
                let mut cltv_expiry_delta = 6 * 12;
                let mut max_dust_htlc_exposure_msat = None;
                let mut max_dust_htlc_exposure_enum = None;
                let mut force_close_avoidance_max_fee_satoshis = 1000;
                read_tlv_fields!(reader, {
                        (0, forwarding_fee_proportional_millionths, required),
                        (1, accept_underpaying_htlcs, (default_value, false)),
                        (2, forwarding_fee_base_msat, required),
                        (3, max_dust_htlc_exposure_enum, option),
                        (4, cltv_expiry_delta, required),
                        // Has always been written, but became optionally read in 0.0.116
                        (6, max_dust_htlc_exposure_msat, option),
                        (10, force_close_avoidance_max_fee_satoshis, required),
                });
                let max_dust_htlc_fixed_limit = max_dust_htlc_exposure_msat.unwrap_or(5_000_000);
                let max_dust_htlc_exposure_msat = max_dust_htlc_exposure_enum
                        .unwrap_or(MaxDustHTLCExposure::FixedLimitMsat(max_dust_htlc_fixed_limit));
                Ok(Self {
                        forwarding_fee_proportional_millionths,
                        accept_underpaying_htlcs,
                        forwarding_fee_base_msat,
                        cltv_expiry_delta,
                        max_dust_htlc_exposure: max_dust_htlc_exposure_msat,
                        force_close_avoidance_max_fee_satoshis,
                })
        }
}

/// A parallel struct to [`ChannelConfig`] to define partial updates.
#[allow(missing_docs)]
pub struct ChannelConfigUpdate {
        pub forwarding_fee_proportional_millionths: Option<u32>,
        pub forwarding_fee_base_msat: Option<u32>,
        pub cltv_expiry_delta: Option<u16>,
        pub max_dust_htlc_exposure_msat: Option<MaxDustHTLCExposure>,
        pub force_close_avoidance_max_fee_satoshis: Option<u64>,
}

impl Default for ChannelConfigUpdate {
        fn default() -> ChannelConfigUpdate {
                ChannelConfigUpdate {
                        forwarding_fee_proportional_millionths: None,
                        forwarding_fee_base_msat: None,
                        cltv_expiry_delta: None,
                        max_dust_htlc_exposure_msat: None,
                        force_close_avoidance_max_fee_satoshis: None,
                }
        }
}

impl From<ChannelConfig> for ChannelConfigUpdate {
        fn from(config: ChannelConfig) -> ChannelConfigUpdate {
                ChannelConfigUpdate {
                        forwarding_fee_proportional_millionths: Some(config.forwarding_fee_proportional_millionths),
                        forwarding_fee_base_msat: Some(config.forwarding_fee_base_msat),
                        cltv_expiry_delta: Some(config.cltv_expiry_delta),
                        max_dust_htlc_exposure_msat: Some(config.max_dust_htlc_exposure),
                        force_close_avoidance_max_fee_satoshis: Some(config.force_close_avoidance_max_fee_satoshis),
                }
        }
}

/// Legacy version of [`ChannelConfig`] that stored the static
/// [`ChannelHandshakeConfig::announced_channel`] and
/// [`ChannelHandshakeConfig::commit_upfront_shutdown_pubkey`] fields.
#[derive(Copy, Clone, Debug)]
pub(crate) struct LegacyChannelConfig {
        pub(crate) options: ChannelConfig,
        /// Deprecated but may still be read from. See [`ChannelHandshakeConfig::announced_channel`] to
        /// set this when opening/accepting a channel.
        pub(crate) announced_channel: bool,
        /// Deprecated but may still be read from. See
        /// [`ChannelHandshakeConfig::commit_upfront_shutdown_pubkey`] to set this when
        /// opening/accepting a channel.
        pub(crate) commit_upfront_shutdown_pubkey: bool,
}

impl Default for LegacyChannelConfig {
        fn default() -> Self {
                Self {
                        options: ChannelConfig::default(),
                        announced_channel: false,
                        commit_upfront_shutdown_pubkey: true,
                }
        }
}

impl crate::util::ser::Writeable for LegacyChannelConfig {
        fn write<W: crate::util::ser::Writer>(&self, writer: &mut W) -> Result<(), crate::io::Error> {
                let max_dust_htlc_exposure_msat_fixed_limit = match self.options.max_dust_htlc_exposure {
                        MaxDustHTLCExposure::FixedLimitMsat(limit) => limit,
                        MaxDustHTLCExposure::FeeRateMultiplier(_) => 5_000_000,
                };
                write_tlv_fields!(writer, {
                        (0, self.options.forwarding_fee_proportional_millionths, required),
                        (1, max_dust_htlc_exposure_msat_fixed_limit, required),
                        (2, self.options.cltv_expiry_delta, required),
                        (3, self.options.force_close_avoidance_max_fee_satoshis, (default_value, 1000)),
                        (4, self.announced_channel, required),
                        (5, self.options.max_dust_htlc_exposure, required),
                        (6, self.commit_upfront_shutdown_pubkey, required),
                        (8, self.options.forwarding_fee_base_msat, required),
                });
                Ok(())
        }
}

impl crate::util::ser::Readable for LegacyChannelConfig {
        fn read<R: crate::io::Read>(reader: &mut R) -> Result<Self, crate::ln::msgs::DecodeError> {
                let mut forwarding_fee_proportional_millionths = 0;
                let mut max_dust_htlc_exposure_msat_fixed_limit = None;
                let mut cltv_expiry_delta = 0;
                let mut force_close_avoidance_max_fee_satoshis = 1000;
                let mut announced_channel = false;
                let mut commit_upfront_shutdown_pubkey = false;
                let mut forwarding_fee_base_msat = 0;
                let mut max_dust_htlc_exposure_enum = None;
                read_tlv_fields!(reader, {
                        (0, forwarding_fee_proportional_millionths, required),
                        // Has always been written, but became optionally read in 0.0.116
                        (1, max_dust_htlc_exposure_msat_fixed_limit, option),
                        (2, cltv_expiry_delta, required),
                        (3, force_close_avoidance_max_fee_satoshis, (default_value, 1000u64)),
                        (4, announced_channel, required),
                        (5, max_dust_htlc_exposure_enum, option),
                        (6, commit_upfront_shutdown_pubkey, required),
                        (8, forwarding_fee_base_msat, required),
                });
                let max_dust_htlc_exposure_msat_fixed_limit =
                        max_dust_htlc_exposure_msat_fixed_limit.unwrap_or(5_000_000);
                let max_dust_htlc_exposure_msat = max_dust_htlc_exposure_enum
                        .unwrap_or(MaxDustHTLCExposure::FixedLimitMsat(max_dust_htlc_exposure_msat_fixed_limit));
                Ok(Self {
                        options: ChannelConfig {
                                forwarding_fee_proportional_millionths,
                                max_dust_htlc_exposure: max_dust_htlc_exposure_msat,
                                cltv_expiry_delta,
                                force_close_avoidance_max_fee_satoshis,
                                forwarding_fee_base_msat,
                                accept_underpaying_htlcs: false,
                        },
                        announced_channel,
                        commit_upfront_shutdown_pubkey,
                })
        }
}

/// Top-level config which holds ChannelHandshakeLimits and ChannelConfig.
///
/// Default::default() provides sane defaults for most configurations
/// (but currently with 0 relay fees!)
#[derive(Copy, Clone, Debug)]
pub struct UserConfig {
        /// Channel handshake config that we propose to our counterparty.
        pub channel_handshake_config: ChannelHandshakeConfig,
        /// Limits applied to our counterparty's proposed channel handshake config settings.
        pub channel_handshake_limits: ChannelHandshakeLimits,
        /// Channel config which affects behavior during channel lifetime.
        pub channel_config: ChannelConfig,
        /// If this is set to false, we will reject any HTLCs which were to be forwarded over private
        /// channels. This prevents us from taking on HTLC-forwarding risk when we intend to run as a
        /// node which is not online reliably.
        ///
        /// For nodes which are not online reliably, you should set all channels to *not* be announced
        /// (using [`ChannelHandshakeConfig::announced_channel`] and
        /// [`ChannelHandshakeLimits::force_announced_channel_preference`]) and set this to false to
        /// ensure you are not exposed to any forwarding risk.
        ///
        /// Note that because you cannot change a channel's announced state after creation, there is no
        /// way to disable forwarding on public channels retroactively. Thus, in order to change a node
        /// from a publicly-announced forwarding node to a private non-forwarding node you must close
        /// all your channels and open new ones. For privacy, you should also change your node_id
        /// (swapping all private and public key material for new ones) at that time.
        ///
        /// Default value: false.
        pub accept_forwards_to_priv_channels: bool,
        /// If this is set to false, we do not accept inbound requests to open a new channel.
        /// Default value: true.
        pub accept_inbound_channels: bool,
        /// If this is set to true, the user needs to manually accept inbound requests to open a new
        /// channel.
        ///
        /// When set to true, [`Event::OpenChannelRequest`] will be triggered once a request to open a
        /// new inbound channel is received through a [`msgs::OpenChannel`] message. In that case, a
        /// [`msgs::AcceptChannel`] message will not be sent back to the counterparty node unless the
        /// user explicitly chooses to accept the request.
        ///
        /// Default value: false.
        ///
        /// [`Event::OpenChannelRequest`]: crate::events::Event::OpenChannelRequest
        /// [`msgs::OpenChannel`]: crate::ln::msgs::OpenChannel
        /// [`msgs::AcceptChannel`]: crate::ln::msgs::AcceptChannel
        pub manually_accept_inbound_channels: bool,
        ///  If this is set to true, LDK will intercept HTLCs that are attempting to be forwarded over
        ///  fake short channel ids generated via [`ChannelManager::get_intercept_scid`]. Upon HTLC
        ///  intercept, LDK will generate an [`Event::HTLCIntercepted`] which MUST be handled by the user.
        ///
        ///  Setting this to true may break backwards compatibility with LDK versions < 0.0.113.
        ///
        ///  Default value: false.
        ///
        /// [`ChannelManager::get_intercept_scid`]: crate::ln::channelmanager::ChannelManager::get_intercept_scid
        /// [`Event::HTLCIntercepted`]: crate::events::Event::HTLCIntercepted
        pub accept_intercept_htlcs: bool,
        /// If this is set to false, when receiving a keysend payment we'll fail it if it has multiple
        /// parts. If this is set to true, we'll accept the payment.
        ///
        /// Setting this to true will break backwards compatibility upon downgrading to an LDK
        /// version < 0.0.116 while receiving an MPP keysend. If we have already received an MPP
        /// keysend, downgrading will cause us to fail to deserialize [`ChannelManager`].
        ///
        /// Default value: false.
        ///
        /// [`ChannelManager`]: crate::ln::channelmanager::ChannelManager
        pub accept_mpp_keysend: bool,
        /// If this is set to true, the user needs to manually pay [`Bolt12Invoice`]s when received.
        ///
        /// When set to true, [`Event::InvoiceReceived`] will be generated for each received
        /// [`Bolt12Invoice`] instead of being automatically paid after verification.
        ///
        /// Default value: false.
        ///
        /// [`Bolt12Invoice`]: crate::offers::invoice::Bolt12Invoice
        /// [`Event::InvoiceReceived`]: crate::events::Event::InvoiceReceived
        pub manually_handle_bolt12_invoices: bool,
}

impl Default for UserConfig {
        fn default() -> Self {
                UserConfig {
                        channel_handshake_config: ChannelHandshakeConfig::default(),
                        channel_handshake_limits: ChannelHandshakeLimits::default(),
                        channel_config: ChannelConfig::default(),
                        accept_forwards_to_priv_channels: false,
                        accept_inbound_channels: true,
                        manually_accept_inbound_channels: false,
                        accept_intercept_htlcs: false,
                        accept_mpp_keysend: false,
                        manually_handle_bolt12_invoices: false,
                }
        }
}

// When fuzzing, we want to allow the fuzzer to pick any configuration parameters. Thus, we
// implement Readable here in a naive way (which is a bit easier for the fuzzer to handle). We
// don't really want to ever expose this to users (if we did we'd want to use TLVs).
#[cfg(fuzzing)]
impl Readable for UserConfig {
        fn read<R: crate::io::Read>(reader: &mut R) -> Result<Self, crate::ln::msgs::DecodeError> {
                Ok(Self {
                        channel_handshake_config: Readable::read(reader)?,
                        channel_handshake_limits: Readable::read(reader)?,
                        channel_config: Readable::read(reader)?,
                        accept_forwards_to_priv_channels: Readable::read(reader)?,
                        accept_inbound_channels: Readable::read(reader)?,
                        manually_accept_inbound_channels: Readable::read(reader)?,
                        accept_intercept_htlcs: Readable::read(reader)?,
                        accept_mpp_keysend: Readable::read(reader)?,
                        manually_handle_bolt12_invoices: Readable::read(reader)?,
                })
        }
}