Probabilistic channel scoring
[rust-lightning] / lightning / src / routing / scoring.rs
index 6a0174096917e591b9d2479ba8d897ca496cdd23..643425bb214a05347a307144ae965b934fe52780 100644 (file)
@@ -9,8 +9,8 @@
 
 //! Utilities for scoring payment channels.
 //!
-//! [`Scorer`] may be given to [`find_route`] to score payment channels during path finding when a
-//! custom [`Score`] implementation is not needed.
+//! [`ProbabilisticScorer`] may be given to [`find_route`] to score payment channels during path
+//! finding when a custom [`Score`] implementation is not needed.
 //!
 //! # Example
 //!
@@ -19,7 +19,7 @@
 //! #
 //! # use lightning::routing::network_graph::NetworkGraph;
 //! # use lightning::routing::router::{RouteParameters, find_route};
-//! # use lightning::routing::scoring::{Scorer, ScoringParameters};
+//! # use lightning::routing::scoring::{ProbabilisticScorer, ProbabilisticScoringParameters, Scorer, ScoringParameters};
 //! # use lightning::util::logger::{Logger, Record};
 //! # use secp256k1::key::PublicKey;
 //! #
 //! # impl Logger for FakeLogger {
 //! #     fn log(&self, record: &Record) { unimplemented!() }
 //! # }
-//! # fn find_scored_route(payer: PublicKey, params: RouteParameters, network_graph: NetworkGraph) {
+//! # fn find_scored_route(payer: PublicKey, route_params: RouteParameters, network_graph: NetworkGraph) {
 //! # let logger = FakeLogger {};
 //! #
 //! // Use the default channel penalties.
-//! let scorer = Scorer::default();
+//! let params = ProbabilisticScoringParameters::default();
+//! let scorer = ProbabilisticScorer::new(params, &network_graph);
 //!
 //! // Or use custom channel penalties.
-//! let scorer = Scorer::new(ScoringParameters {
-//!     base_penalty_msat: 1000,
-//!     failure_penalty_msat: 2 * 1024 * 1000,
-//!     ..ScoringParameters::default()
-//! });
+//! let params = ProbabilisticScoringParameters {
+//!     liquidity_penalty_multiplier_msat: 2 * 1000,
+//!     ..ProbabilisticScoringParameters::default()
+//! };
+//! let scorer = ProbabilisticScorer::new(params, &network_graph);
 //!
-//! let route = find_route(&payer, &params, &network_graph, None, &logger, &scorer);
+//! let route = find_route(&payer, &route_params, &network_graph, None, &logger, &scorer);
 //! # }
 //! ```
 //!
 //! [`find_route`]: crate::routing::router::find_route
 
 use ln::msgs::DecodeError;
-use routing::network_graph::NodeId;
+use routing::network_graph::{NetworkGraph, NodeId};
 use routing::router::RouteHop;
-use util::ser::{Readable, Writeable, Writer};
+use util::ser::{Readable, ReadableArgs, Writeable, Writer};
 
 use prelude::*;
 use core::cell::{RefCell, RefMut};
-use core::ops::DerefMut;
+use core::ops::{Deref, DerefMut};
 use core::time::Duration;
 use io::{self, Read};
 use sync::{Mutex, MutexGuard};
@@ -452,6 +453,325 @@ impl<T: Time> Readable for ChannelFailure<T> {
        }
 }
 
+/// [`Score`] implementation using channel success probability distributions.
+///
+/// Based on *Optimally Reliable & Cheap Payment Flows on the Lightning Network* by Rene Pickhardt
+/// and Stefan Richter [[1]]. Given the uncertainty of channel liquidity balances, probability
+/// distributions are defined based on knowledge learned from successful and unsuccessful attempts.
+/// Then the negative `log10` of the success probability is used to determine the cost of routing a
+/// specific HTLC amount through a channel.
+///
+/// [1]: https://arxiv.org/abs/2107.05322
+pub struct ProbabilisticScorer<G: Deref<Target = NetworkGraph>> {
+       params: ProbabilisticScoringParameters,
+       network_graph: G,
+       // TODO: Remove entries of closed channels.
+       channel_liquidities: HashMap<u64, ChannelLiquidity>,
+}
+
+/// Parameters for configuring [`ProbabilisticScorer`].
+pub struct ProbabilisticScoringParameters {
+       /// A penalty applied after multiplying by the negative `log10` of the channel's success
+       /// probability for a payment.
+       ///
+       /// The success probability is determined by the effective channel capacity, the payment amount,
+       /// and knowledge learned from prior successful and unsuccessful payments. The lower bound of
+       /// the success probability is 0.01, effectively limiting the penalty to the range
+       /// `0..=2*liquidity_penalty_multiplier_msat`.
+       ///
+       /// Default value: 10,000 msat
+       pub liquidity_penalty_multiplier_msat: u64,
+}
+
+impl_writeable_tlv_based!(ProbabilisticScoringParameters, {
+       (0, liquidity_penalty_multiplier_msat, required),
+});
+
+/// Accounting for channel liquidity balance uncertainty.
+///
+/// Direction is defined in terms of [`NodeId`] partial ordering, where the source node is the
+/// first node in the ordering of the channel's counterparties. Thus, swapping the two liquidity
+/// offset fields gives the opposite direction.
+struct ChannelLiquidity {
+       /// Lower channel liquidity bound in terms of an offset from zero.
+       min_liquidity_offset_msat: u64,
+
+       /// Upper channel liquidity bound in terms of an offset from the effective capacity.
+       max_liquidity_offset_msat: u64,
+}
+
+/// A view of [`ChannelLiquidity`] in one direction assuming a certain channel capacity.
+struct DirectedChannelLiquidity<L: Deref<Target = u64>> {
+       min_liquidity_offset_msat: L,
+       max_liquidity_offset_msat: L,
+       capacity_msat: u64,
+}
+
+impl<G: Deref<Target = NetworkGraph>> ProbabilisticScorer<G> {
+       /// Creates a new scorer using the given scoring parameters for sending payments from a node
+       /// through a network graph.
+       pub fn new(params: ProbabilisticScoringParameters, network_graph: G) -> Self {
+               Self {
+                       params,
+                       network_graph,
+                       channel_liquidities: HashMap::new(),
+               }
+       }
+
+       #[cfg(test)]
+       fn with_channel(mut self, short_channel_id: u64, liquidity: ChannelLiquidity) -> Self {
+               assert!(self.channel_liquidities.insert(short_channel_id, liquidity).is_none());
+               self
+       }
+}
+
+impl Default for ProbabilisticScoringParameters {
+       fn default() -> Self {
+               Self {
+                       liquidity_penalty_multiplier_msat: 10_000,
+               }
+       }
+}
+
+impl ChannelLiquidity {
+       #[inline]
+       fn new() -> Self {
+               Self {
+                       min_liquidity_offset_msat: 0,
+                       max_liquidity_offset_msat: 0,
+               }
+       }
+
+       /// Returns a view of the channel liquidity directed from `source` to `target` assuming
+       /// `capacity_msat`.
+       fn as_directed(
+               &self, source: &NodeId, target: &NodeId, capacity_msat: u64
+       ) -> DirectedChannelLiquidity<&u64> {
+               let (min_liquidity_offset_msat, max_liquidity_offset_msat) = if source < target {
+                       (&self.min_liquidity_offset_msat, &self.max_liquidity_offset_msat)
+               } else {
+                       (&self.max_liquidity_offset_msat, &self.min_liquidity_offset_msat)
+               };
+
+               DirectedChannelLiquidity {
+                       min_liquidity_offset_msat,
+                       max_liquidity_offset_msat,
+                       capacity_msat,
+               }
+       }
+
+       /// Returns a mutable view of the channel liquidity directed from `source` to `target` assuming
+       /// `capacity_msat`.
+       fn as_directed_mut(
+               &mut self, source: &NodeId, target: &NodeId, capacity_msat: u64
+       ) -> DirectedChannelLiquidity<&mut u64> {
+               let (min_liquidity_offset_msat, max_liquidity_offset_msat) = if source < target {
+                       (&mut self.min_liquidity_offset_msat, &mut self.max_liquidity_offset_msat)
+               } else {
+                       (&mut self.max_liquidity_offset_msat, &mut self.min_liquidity_offset_msat)
+               };
+
+               DirectedChannelLiquidity {
+                       min_liquidity_offset_msat,
+                       max_liquidity_offset_msat,
+                       capacity_msat,
+               }
+       }
+}
+
+impl<L: Deref<Target = u64>> DirectedChannelLiquidity<L> {
+       /// Returns the success probability of routing the given HTLC `amount_msat` through the channel
+       /// in this direction.
+       fn success_probability(&self, amount_msat: u64) -> f64 {
+               let max_liquidity_msat = self.max_liquidity_msat();
+               let min_liquidity_msat = core::cmp::min(self.min_liquidity_msat(), max_liquidity_msat);
+               if amount_msat > max_liquidity_msat {
+                       0.0
+               } else if amount_msat <= min_liquidity_msat {
+                       1.0
+               } else {
+                       let numerator = max_liquidity_msat + 1 - amount_msat;
+                       let denominator = max_liquidity_msat + 1 - min_liquidity_msat;
+                       numerator as f64 / denominator as f64
+               }.max(0.01) // Lower bound the success probability to ensure some channel is selected.
+       }
+
+       /// Returns the lower bound of the channel liquidity balance in this direction.
+       fn min_liquidity_msat(&self) -> u64 {
+               *self.min_liquidity_offset_msat
+       }
+
+       /// Returns the upper bound of the channel liquidity balance in this direction.
+       fn max_liquidity_msat(&self) -> u64 {
+               self.capacity_msat.checked_sub(*self.max_liquidity_offset_msat).unwrap_or(0)
+       }
+}
+
+impl<L: DerefMut<Target = u64>> DirectedChannelLiquidity<L> {
+       /// Adjusts the channel liquidity balance bounds when failing to route `amount_msat`.
+       fn failed_at_channel(&mut self, amount_msat: u64) {
+               if amount_msat < self.max_liquidity_msat() {
+                       self.set_max_liquidity_msat(amount_msat);
+               }
+       }
+
+       /// Adjusts the channel liquidity balance bounds when failing to route `amount_msat` downstream.
+       fn failed_downstream(&mut self, amount_msat: u64) {
+               if amount_msat > self.min_liquidity_msat() {
+                       self.set_min_liquidity_msat(amount_msat);
+               }
+       }
+
+       /// Adjusts the channel liquidity balance bounds when successfully routing `amount_msat`.
+       fn successful(&mut self, amount_msat: u64) {
+               let max_liquidity_msat = self.max_liquidity_msat().checked_sub(amount_msat).unwrap_or(0);
+               self.set_max_liquidity_msat(max_liquidity_msat);
+       }
+
+       /// Adjusts the lower bound of the channel liquidity balance in this direction.
+       fn set_min_liquidity_msat(&mut self, amount_msat: u64) {
+               *self.min_liquidity_offset_msat = amount_msat;
+
+               if amount_msat > self.max_liquidity_msat() {
+                       *self.max_liquidity_offset_msat = 0;
+               }
+       }
+
+       /// Adjusts the upper bound of the channel liquidity balance in this direction.
+       fn set_max_liquidity_msat(&mut self, amount_msat: u64) {
+               *self.max_liquidity_offset_msat = self.capacity_msat.checked_sub(amount_msat).unwrap_or(0);
+
+               if amount_msat < self.min_liquidity_msat() {
+                       *self.min_liquidity_offset_msat = 0;
+               }
+       }
+}
+
+impl<G: Deref<Target = NetworkGraph>> Score for ProbabilisticScorer<G> {
+       fn channel_penalty_msat(
+               &self, short_channel_id: u64, amount_msat: u64, capacity_msat: u64, source: &NodeId,
+               target: &NodeId
+       ) -> u64 {
+               let liquidity_penalty_multiplier_msat = self.params.liquidity_penalty_multiplier_msat;
+               let success_probability = self.channel_liquidities
+                       .get(&short_channel_id)
+                       .unwrap_or(&ChannelLiquidity::new())
+                       .as_directed(source, target, capacity_msat)
+                       .success_probability(amount_msat);
+               // NOTE: If success_probability is ever changed to return 0.0, log10 is undefined so return
+               // u64::max_value instead.
+               debug_assert!(success_probability > core::f64::EPSILON);
+               (-(success_probability.log10()) * liquidity_penalty_multiplier_msat as f64) as u64
+       }
+
+       fn payment_path_failed(&mut self, path: &[&RouteHop], short_channel_id: u64) {
+               let amount_msat = path.split_last().map(|(hop, _)| hop.fee_msat).unwrap_or(0);
+               let network_graph = self.network_graph.read_only();
+               for hop in path {
+                       let target = NodeId::from_pubkey(&hop.pubkey);
+                       let channel_directed_from_source = network_graph.channels()
+                               .get(&hop.short_channel_id)
+                               .and_then(|channel| channel.as_directed_to(&target));
+
+                       // Only score announced channels.
+                       if let Some((channel, source)) = channel_directed_from_source {
+                               let capacity_msat = channel.effective_capacity().as_msat();
+                               if hop.short_channel_id == short_channel_id {
+                                       self.channel_liquidities
+                                               .entry(hop.short_channel_id)
+                                               .or_insert_with(ChannelLiquidity::new)
+                                               .as_directed_mut(source, &target, capacity_msat)
+                                               .failed_at_channel(amount_msat);
+                                       break;
+                               }
+
+                               self.channel_liquidities
+                                       .entry(hop.short_channel_id)
+                                       .or_insert_with(ChannelLiquidity::new)
+                                       .as_directed_mut(source, &target, capacity_msat)
+                                       .failed_downstream(amount_msat);
+                       }
+               }
+       }
+
+       fn payment_path_successful(&mut self, path: &[&RouteHop]) {
+               let amount_msat = path.split_last().map(|(hop, _)| hop.fee_msat).unwrap_or(0);
+               let network_graph = self.network_graph.read_only();
+               for hop in path {
+                       let target = NodeId::from_pubkey(&hop.pubkey);
+                       let channel_directed_from_source = network_graph.channels()
+                               .get(&hop.short_channel_id)
+                               .and_then(|channel| channel.as_directed_to(&target));
+
+                       // Only score announced channels.
+                       if let Some((channel, source)) = channel_directed_from_source {
+                               let capacity_msat = channel.effective_capacity().as_msat();
+                               self.channel_liquidities
+                                       .entry(hop.short_channel_id)
+                                       .or_insert_with(ChannelLiquidity::new)
+                                       .as_directed_mut(source, &target, capacity_msat)
+                                       .successful(amount_msat);
+                       }
+               }
+       }
+}
+
+impl<G: Deref<Target = NetworkGraph>> Writeable for ProbabilisticScorer<G> {
+       #[inline]
+       fn write<W: Writer>(&self, w: &mut W) -> Result<(), io::Error> {
+               write_tlv_fields!(w, {
+                       (0, self.channel_liquidities, required)
+               });
+               Ok(())
+       }
+}
+
+impl<G: Deref<Target = NetworkGraph>> ReadableArgs<(ProbabilisticScoringParameters, G)>
+for ProbabilisticScorer<G> {
+       #[inline]
+       fn read<R: Read>(
+               r: &mut R, args: (ProbabilisticScoringParameters, G)
+       ) -> Result<Self, DecodeError> {
+               let (params, network_graph) = args;
+               let mut channel_liquidities = HashMap::new();
+               read_tlv_fields!(r, {
+                       (0, channel_liquidities, required)
+               });
+               Ok(Self {
+                       params,
+                       network_graph,
+                       channel_liquidities,
+               })
+       }
+}
+
+impl Writeable for ChannelLiquidity {
+       #[inline]
+       fn write<W: Writer>(&self, w: &mut W) -> Result<(), io::Error> {
+               write_tlv_fields!(w, {
+                       (0, self.min_liquidity_offset_msat, required),
+                       (2, self.max_liquidity_offset_msat, required),
+               });
+               Ok(())
+       }
+}
+
+impl Readable for ChannelLiquidity {
+       #[inline]
+       fn read<R: Read>(r: &mut R) -> Result<Self, DecodeError> {
+               let mut min_liquidity_offset_msat = 0;
+               let mut max_liquidity_offset_msat = 0;
+               read_tlv_fields!(r, {
+                       (0, min_liquidity_offset_msat, required),
+                       (2, max_liquidity_offset_msat, required),
+               });
+               Ok(Self {
+                       min_liquidity_offset_msat,
+                       max_liquidity_offset_msat
+               })
+       }
+}
+
 pub(crate) mod time {
        use core::ops::Sub;
        use core::time::Duration;
@@ -516,21 +836,28 @@ pub(crate) use self::time::Time;
 
 #[cfg(test)]
 mod tests {
-       use super::{ScoringParameters, ScorerUsingTime, Time};
+       use super::{ChannelLiquidity, ProbabilisticScoringParameters, ProbabilisticScorer, ScoringParameters, ScorerUsingTime, Time};
        use super::time::Eternity;
 
        use ln::features::{ChannelFeatures, NodeFeatures};
+       use ln::msgs::{ChannelAnnouncement, ChannelUpdate, OptionalField, UnsignedChannelAnnouncement, UnsignedChannelUpdate};
        use routing::scoring::Score;
-       use routing::network_graph::NodeId;
+       use routing::network_graph::{NetworkGraph, NodeId};
        use routing::router::RouteHop;
        use util::ser::{Readable, Writeable};
 
-       use bitcoin::secp256k1::PublicKey;
+       use bitcoin::blockdata::constants::genesis_block;
+       use bitcoin::hashes::Hash;
+       use bitcoin::hashes::sha256d::Hash as Sha256dHash;
+       use bitcoin::network::constants::Network;
+       use bitcoin::secp256k1::{PublicKey, Secp256k1, SecretKey};
        use core::cell::Cell;
        use core::ops::Sub;
        use core::time::Duration;
        use io;
 
+       // `Time` tests
+
        /// Time that can be advanced manually in tests.
        #[derive(Debug, PartialEq, Eq)]
        struct SinceEpoch(Duration);
@@ -592,15 +919,35 @@ mod tests {
                assert_eq!(later - elapsed, now);
        }
 
+       // `Scorer` tests
+
        /// A scorer for testing with time that can be manually advanced.
        type Scorer = ScorerUsingTime::<SinceEpoch>;
 
+       fn source_privkey() -> SecretKey {
+               SecretKey::from_slice(&[42; 32]).unwrap()
+       }
+
+       fn target_privkey() -> SecretKey {
+               SecretKey::from_slice(&[43; 32]).unwrap()
+       }
+
+       fn source_pubkey() -> PublicKey {
+               let secp_ctx = Secp256k1::new();
+               PublicKey::from_secret_key(&secp_ctx, &source_privkey())
+       }
+
+       fn target_pubkey() -> PublicKey {
+               let secp_ctx = Secp256k1::new();
+               PublicKey::from_secret_key(&secp_ctx, &target_privkey())
+       }
+
        fn source_node_id() -> NodeId {
-               NodeId::from_pubkey(&PublicKey::from_slice(&hex::decode("02eec7245d6b7d2ccb30380bfbe2a3648cd7a942653f5aa340edcea1f283686619").unwrap()[..]).unwrap())
+               NodeId::from_pubkey(&source_pubkey())
        }
 
        fn target_node_id() -> NodeId {
-               NodeId::from_pubkey(&PublicKey::from_slice(&hex::decode("0324653eac434488002cc06bbfb7f10fe18991e35f9fe4302dbea6d2353dc0ab1c").unwrap()[..]).unwrap())
+               NodeId::from_pubkey(&target_pubkey())
        }
 
        #[test]
@@ -834,4 +1181,413 @@ mod tests {
                assert_eq!(scorer.channel_penalty_msat(42, 258_000, 1_024_000, &source, &target), 200);
                assert_eq!(scorer.channel_penalty_msat(42, 512_000, 1_024_000, &source, &target), 256 * 100);
        }
+
+       // `ProbabilisticScorer` tests
+
+       fn sender_privkey() -> SecretKey {
+               SecretKey::from_slice(&[41; 32]).unwrap()
+       }
+
+       fn recipient_privkey() -> SecretKey {
+               SecretKey::from_slice(&[45; 32]).unwrap()
+       }
+
+       fn sender_pubkey() -> PublicKey {
+               let secp_ctx = Secp256k1::new();
+               PublicKey::from_secret_key(&secp_ctx, &sender_privkey())
+       }
+
+       fn recipient_pubkey() -> PublicKey {
+               let secp_ctx = Secp256k1::new();
+               PublicKey::from_secret_key(&secp_ctx, &recipient_privkey())
+       }
+
+       fn sender_node_id() -> NodeId {
+               NodeId::from_pubkey(&sender_pubkey())
+       }
+
+       fn recipient_node_id() -> NodeId {
+               NodeId::from_pubkey(&recipient_pubkey())
+       }
+
+       fn network_graph() -> NetworkGraph {
+               let genesis_hash = genesis_block(Network::Testnet).header.block_hash();
+               let mut network_graph = NetworkGraph::new(genesis_hash);
+               add_channel(&mut network_graph, 42, source_privkey(), target_privkey());
+               add_channel(&mut network_graph, 43, target_privkey(), recipient_privkey());
+
+               network_graph
+       }
+
+       fn add_channel(
+               network_graph: &mut NetworkGraph, short_channel_id: u64, node_1_key: SecretKey,
+               node_2_key: SecretKey
+       ) {
+               let genesis_hash = genesis_block(Network::Testnet).header.block_hash();
+               let node_1_secret = &SecretKey::from_slice(&[39; 32]).unwrap();
+               let node_2_secret = &SecretKey::from_slice(&[40; 32]).unwrap();
+               let secp_ctx = Secp256k1::new();
+               let unsigned_announcement = UnsignedChannelAnnouncement {
+                       features: ChannelFeatures::known(),
+                       chain_hash: genesis_hash,
+                       short_channel_id,
+                       node_id_1: PublicKey::from_secret_key(&secp_ctx, &node_1_key),
+                       node_id_2: PublicKey::from_secret_key(&secp_ctx, &node_2_key),
+                       bitcoin_key_1: PublicKey::from_secret_key(&secp_ctx, &node_1_secret),
+                       bitcoin_key_2: PublicKey::from_secret_key(&secp_ctx, &node_2_secret),
+                       excess_data: Vec::new(),
+               };
+               let msghash = hash_to_message!(&Sha256dHash::hash(&unsigned_announcement.encode()[..])[..]);
+               let signed_announcement = ChannelAnnouncement {
+                       node_signature_1: secp_ctx.sign(&msghash, &node_1_key),
+                       node_signature_2: secp_ctx.sign(&msghash, &node_2_key),
+                       bitcoin_signature_1: secp_ctx.sign(&msghash, &node_1_secret),
+                       bitcoin_signature_2: secp_ctx.sign(&msghash, &node_2_secret),
+                       contents: unsigned_announcement,
+               };
+               let chain_source: Option<&::util::test_utils::TestChainSource> = None;
+               network_graph.update_channel_from_announcement(
+                       &signed_announcement, &chain_source, &secp_ctx).unwrap();
+               update_channel(network_graph, short_channel_id, node_1_key, 0);
+               update_channel(network_graph, short_channel_id, node_2_key, 1);
+       }
+
+       fn update_channel(
+               network_graph: &mut NetworkGraph, short_channel_id: u64, node_key: SecretKey, flags: u8
+       ) {
+               let genesis_hash = genesis_block(Network::Testnet).header.block_hash();
+               let secp_ctx = Secp256k1::new();
+               let unsigned_update = UnsignedChannelUpdate {
+                       chain_hash: genesis_hash,
+                       short_channel_id,
+                       timestamp: 100,
+                       flags,
+                       cltv_expiry_delta: 18,
+                       htlc_minimum_msat: 0,
+                       htlc_maximum_msat: OptionalField::Present(1_000),
+                       fee_base_msat: 1,
+                       fee_proportional_millionths: 0,
+                       excess_data: Vec::new(),
+               };
+               let msghash = hash_to_message!(&Sha256dHash::hash(&unsigned_update.encode()[..])[..]);
+               let signed_update = ChannelUpdate {
+                       signature: secp_ctx.sign(&msghash, &node_key),
+                       contents: unsigned_update,
+               };
+               network_graph.update_channel(&signed_update, &secp_ctx).unwrap();
+       }
+
+       fn payment_path_for_amount(amount_msat: u64) -> Vec<RouteHop> {
+               vec![
+                       RouteHop {
+                               pubkey: source_pubkey(),
+                               node_features: NodeFeatures::known(),
+                               short_channel_id: 41,
+                               channel_features: ChannelFeatures::known(),
+                               fee_msat: 1,
+                               cltv_expiry_delta: 18,
+                       },
+                       RouteHop {
+                               pubkey: target_pubkey(),
+                               node_features: NodeFeatures::known(),
+                               short_channel_id: 42,
+                               channel_features: ChannelFeatures::known(),
+                               fee_msat: 2,
+                               cltv_expiry_delta: 18,
+                       },
+                       RouteHop {
+                               pubkey: recipient_pubkey(),
+                               node_features: NodeFeatures::known(),
+                               short_channel_id: 43,
+                               channel_features: ChannelFeatures::known(),
+                               fee_msat: amount_msat,
+                               cltv_expiry_delta: 18,
+                       },
+               ]
+       }
+
+       #[test]
+       fn liquidity_bounds_directed_from_lowest_node_id() {
+               let network_graph = network_graph();
+               let params = ProbabilisticScoringParameters::default();
+               let mut scorer = ProbabilisticScorer::new(params, &network_graph)
+                       .with_channel(42,
+                               ChannelLiquidity {
+                                       min_liquidity_offset_msat: 700, max_liquidity_offset_msat: 100
+                               })
+                       .with_channel(43,
+                               ChannelLiquidity {
+                                       min_liquidity_offset_msat: 700, max_liquidity_offset_msat: 100
+                               });
+               let source = source_node_id();
+               let target = target_node_id();
+               let recipient = recipient_node_id();
+
+               let liquidity = scorer.channel_liquidities.get_mut(&42).unwrap();
+               assert!(source > target);
+               assert_eq!(liquidity.as_directed(&source, &target, 1_000).min_liquidity_msat(), 100);
+               assert_eq!(liquidity.as_directed(&source, &target, 1_000).max_liquidity_msat(), 300);
+               assert_eq!(liquidity.as_directed(&target, &source, 1_000).min_liquidity_msat(), 700);
+               assert_eq!(liquidity.as_directed(&target, &source, 1_000).max_liquidity_msat(), 900);
+
+               liquidity.as_directed_mut(&source, &target, 1_000).set_min_liquidity_msat(200);
+               assert_eq!(liquidity.as_directed(&source, &target, 1_000).min_liquidity_msat(), 200);
+               assert_eq!(liquidity.as_directed(&source, &target, 1_000).max_liquidity_msat(), 300);
+               assert_eq!(liquidity.as_directed(&target, &source, 1_000).min_liquidity_msat(), 700);
+               assert_eq!(liquidity.as_directed(&target, &source, 1_000).max_liquidity_msat(), 800);
+
+               let liquidity = scorer.channel_liquidities.get_mut(&43).unwrap();
+               assert!(target < recipient);
+               assert_eq!(liquidity.as_directed(&target, &recipient, 1_000).min_liquidity_msat(), 700);
+               assert_eq!(liquidity.as_directed(&target, &recipient, 1_000).max_liquidity_msat(), 900);
+               assert_eq!(liquidity.as_directed(&recipient, &target, 1_000).min_liquidity_msat(), 100);
+               assert_eq!(liquidity.as_directed(&recipient, &target, 1_000).max_liquidity_msat(), 300);
+
+               liquidity.as_directed_mut(&target, &recipient, 1_000).set_max_liquidity_msat(200);
+               assert_eq!(liquidity.as_directed(&target, &recipient, 1_000).min_liquidity_msat(), 0);
+               assert_eq!(liquidity.as_directed(&target, &recipient, 1_000).max_liquidity_msat(), 200);
+               assert_eq!(liquidity.as_directed(&recipient, &target, 1_000).min_liquidity_msat(), 800);
+               assert_eq!(liquidity.as_directed(&recipient, &target, 1_000).max_liquidity_msat(), 1000);
+       }
+
+       #[test]
+       fn resets_liquidity_upper_bound_when_crossed_by_lower_bound() {
+               let network_graph = network_graph();
+               let params = ProbabilisticScoringParameters::default();
+               let mut scorer = ProbabilisticScorer::new(params, &network_graph)
+                       .with_channel(42,
+                               ChannelLiquidity {
+                                       min_liquidity_offset_msat: 200, max_liquidity_offset_msat: 400
+                               });
+               let source = source_node_id();
+               let target = target_node_id();
+               assert!(source > target);
+
+               // Check initial bounds.
+               let liquidity = scorer.channel_liquidities.get(&42).unwrap()
+                       .as_directed(&source, &target, 1_000);
+               assert_eq!(liquidity.min_liquidity_msat(), 400);
+               assert_eq!(liquidity.max_liquidity_msat(), 800);
+
+               let liquidity = scorer.channel_liquidities.get(&42).unwrap()
+                       .as_directed(&target, &source, 1_000);
+               assert_eq!(liquidity.min_liquidity_msat(), 200);
+               assert_eq!(liquidity.max_liquidity_msat(), 600);
+
+               // Reset from source to target.
+               scorer.channel_liquidities.get_mut(&42).unwrap()
+                       .as_directed_mut(&source, &target, 1_000)
+                       .set_min_liquidity_msat(900);
+
+               let liquidity = scorer.channel_liquidities.get(&42).unwrap()
+                       .as_directed(&source, &target, 1_000);
+               assert_eq!(liquidity.min_liquidity_msat(), 900);
+               assert_eq!(liquidity.max_liquidity_msat(), 1_000);
+
+               let liquidity = scorer.channel_liquidities.get(&42).unwrap()
+                       .as_directed(&target, &source, 1_000);
+               assert_eq!(liquidity.min_liquidity_msat(), 0);
+               assert_eq!(liquidity.max_liquidity_msat(), 100);
+
+               // Reset from target to source.
+               scorer.channel_liquidities.get_mut(&42).unwrap()
+                       .as_directed_mut(&target, &source, 1_000)
+                       .set_min_liquidity_msat(400);
+
+               let liquidity = scorer.channel_liquidities.get(&42).unwrap()
+                       .as_directed(&source, &target, 1_000);
+               assert_eq!(liquidity.min_liquidity_msat(), 0);
+               assert_eq!(liquidity.max_liquidity_msat(), 600);
+
+               let liquidity = scorer.channel_liquidities.get(&42).unwrap()
+                       .as_directed(&target, &source, 1_000);
+               assert_eq!(liquidity.min_liquidity_msat(), 400);
+               assert_eq!(liquidity.max_liquidity_msat(), 1_000);
+       }
+
+       #[test]
+       fn resets_liquidity_lower_bound_when_crossed_by_upper_bound() {
+               let network_graph = network_graph();
+               let params = ProbabilisticScoringParameters::default();
+               let mut scorer = ProbabilisticScorer::new(params, &network_graph)
+                       .with_channel(42,
+                               ChannelLiquidity {
+                                       min_liquidity_offset_msat: 200, max_liquidity_offset_msat: 400
+                               });
+               let source = source_node_id();
+               let target = target_node_id();
+               assert!(source > target);
+
+               // Check initial bounds.
+               let liquidity = scorer.channel_liquidities.get(&42).unwrap()
+                       .as_directed(&source, &target, 1_000);
+               assert_eq!(liquidity.min_liquidity_msat(), 400);
+               assert_eq!(liquidity.max_liquidity_msat(), 800);
+
+               let liquidity = scorer.channel_liquidities.get(&42).unwrap()
+                       .as_directed(&target, &source, 1_000);
+               assert_eq!(liquidity.min_liquidity_msat(), 200);
+               assert_eq!(liquidity.max_liquidity_msat(), 600);
+
+               // Reset from source to target.
+               scorer.channel_liquidities.get_mut(&42).unwrap()
+                       .as_directed_mut(&source, &target, 1_000)
+                       .set_max_liquidity_msat(300);
+
+               let liquidity = scorer.channel_liquidities.get(&42).unwrap()
+                       .as_directed(&source, &target, 1_000);
+               assert_eq!(liquidity.min_liquidity_msat(), 0);
+               assert_eq!(liquidity.max_liquidity_msat(), 300);
+
+               let liquidity = scorer.channel_liquidities.get(&42).unwrap()
+                       .as_directed(&target, &source, 1_000);
+               assert_eq!(liquidity.min_liquidity_msat(), 700);
+               assert_eq!(liquidity.max_liquidity_msat(), 1_000);
+
+               // Reset from target to source.
+               scorer.channel_liquidities.get_mut(&42).unwrap()
+                       .as_directed_mut(&target, &source, 1_000)
+                       .set_max_liquidity_msat(600);
+
+               let liquidity = scorer.channel_liquidities.get(&42).unwrap()
+                       .as_directed(&source, &target, 1_000);
+               assert_eq!(liquidity.min_liquidity_msat(), 400);
+               assert_eq!(liquidity.max_liquidity_msat(), 1_000);
+
+               let liquidity = scorer.channel_liquidities.get(&42).unwrap()
+                       .as_directed(&target, &source, 1_000);
+               assert_eq!(liquidity.min_liquidity_msat(), 0);
+               assert_eq!(liquidity.max_liquidity_msat(), 600);
+       }
+
+       #[test]
+       fn increased_penalty_nearing_liquidity_upper_bound() {
+               let network_graph = network_graph();
+               let params = ProbabilisticScoringParameters {
+                       liquidity_penalty_multiplier_msat: 1_000, ..Default::default()
+               };
+               let scorer = ProbabilisticScorer::new(params, &network_graph);
+               let source = source_node_id();
+               let target = target_node_id();
+
+               assert_eq!(scorer.channel_penalty_msat(42, 100, 100_000, &source, &target), 0);
+               assert_eq!(scorer.channel_penalty_msat(42, 1_000, 100_000, &source, &target), 4);
+               assert_eq!(scorer.channel_penalty_msat(42, 10_000, 100_000, &source, &target), 45);
+               assert_eq!(scorer.channel_penalty_msat(42, 100_000, 100_000, &source, &target), 2_000);
+
+               assert_eq!(scorer.channel_penalty_msat(42, 125, 1_000, &source, &target), 57);
+               assert_eq!(scorer.channel_penalty_msat(42, 250, 1_000, &source, &target), 124);
+               assert_eq!(scorer.channel_penalty_msat(42, 375, 1_000, &source, &target), 203);
+               assert_eq!(scorer.channel_penalty_msat(42, 500, 1_000, &source, &target), 300);
+               assert_eq!(scorer.channel_penalty_msat(42, 625, 1_000, &source, &target), 425);
+               assert_eq!(scorer.channel_penalty_msat(42, 750, 1_000, &source, &target), 600);
+               assert_eq!(scorer.channel_penalty_msat(42, 875, 1_000, &source, &target), 900);
+       }
+
+       #[test]
+       fn constant_penalty_outside_liquidity_bounds() {
+               let network_graph = network_graph();
+               let params = ProbabilisticScoringParameters {
+                       liquidity_penalty_multiplier_msat: 1_000, ..Default::default()
+               };
+               let scorer = ProbabilisticScorer::new(params, &network_graph)
+                       .with_channel(42,
+                               ChannelLiquidity { min_liquidity_offset_msat: 40, max_liquidity_offset_msat: 40 });
+               let source = source_node_id();
+               let target = target_node_id();
+
+               assert_eq!(scorer.channel_penalty_msat(42, 39, 100, &source, &target), 0);
+               assert_ne!(scorer.channel_penalty_msat(42, 50, 100, &source, &target), 0);
+               assert_ne!(scorer.channel_penalty_msat(42, 50, 100, &source, &target), 2_000);
+               assert_eq!(scorer.channel_penalty_msat(42, 61, 100, &source, &target), 2_000);
+       }
+
+       #[test]
+       fn does_not_further_penalize_own_channel() {
+               let network_graph = network_graph();
+               let params = ProbabilisticScoringParameters {
+                       liquidity_penalty_multiplier_msat: 1_000, ..Default::default()
+               };
+               let mut scorer = ProbabilisticScorer::new(params, &network_graph);
+               let sender = sender_node_id();
+               let source = source_node_id();
+               let failed_path = payment_path_for_amount(500);
+               let successful_path = payment_path_for_amount(200);
+
+               assert_eq!(scorer.channel_penalty_msat(41, 500, 1_000, &sender, &source), 300);
+
+               scorer.payment_path_failed(&failed_path.iter().collect::<Vec<_>>(), 41);
+               assert_eq!(scorer.channel_penalty_msat(41, 500, 1_000, &sender, &source), 300);
+
+               scorer.payment_path_successful(&successful_path.iter().collect::<Vec<_>>());
+               assert_eq!(scorer.channel_penalty_msat(41, 500, 1_000, &sender, &source), 300);
+       }
+
+       #[test]
+       fn sets_liquidity_lower_bound_on_downstream_failure() {
+               let network_graph = network_graph();
+               let params = ProbabilisticScoringParameters {
+                       liquidity_penalty_multiplier_msat: 1_000, ..Default::default()
+               };
+               let mut scorer = ProbabilisticScorer::new(params, &network_graph);
+               let source = source_node_id();
+               let target = target_node_id();
+               let path = payment_path_for_amount(500);
+
+               assert_eq!(scorer.channel_penalty_msat(42, 250, 1_000, &source, &target), 124);
+               assert_eq!(scorer.channel_penalty_msat(42, 500, 1_000, &source, &target), 300);
+               assert_eq!(scorer.channel_penalty_msat(42, 750, 1_000, &source, &target), 600);
+
+               scorer.payment_path_failed(&path.iter().collect::<Vec<_>>(), 43);
+
+               assert_eq!(scorer.channel_penalty_msat(42, 250, 1_000, &source, &target), 0);
+               assert_eq!(scorer.channel_penalty_msat(42, 500, 1_000, &source, &target), 0);
+               assert_eq!(scorer.channel_penalty_msat(42, 750, 1_000, &source, &target), 300);
+       }
+
+       #[test]
+       fn sets_liquidity_upper_bound_on_failure() {
+               let network_graph = network_graph();
+               let params = ProbabilisticScoringParameters {
+                       liquidity_penalty_multiplier_msat: 1_000, ..Default::default()
+               };
+               let mut scorer = ProbabilisticScorer::new(params, &network_graph);
+               let source = source_node_id();
+               let target = target_node_id();
+               let path = payment_path_for_amount(500);
+
+               assert_eq!(scorer.channel_penalty_msat(42, 250, 1_000, &source, &target), 124);
+               assert_eq!(scorer.channel_penalty_msat(42, 500, 1_000, &source, &target), 300);
+               assert_eq!(scorer.channel_penalty_msat(42, 750, 1_000, &source, &target), 600);
+
+               scorer.payment_path_failed(&path.iter().collect::<Vec<_>>(), 42);
+
+               assert_eq!(scorer.channel_penalty_msat(42, 250, 1_000, &source, &target), 300);
+               assert_eq!(scorer.channel_penalty_msat(42, 500, 1_000, &source, &target), 2_000);
+               assert_eq!(scorer.channel_penalty_msat(42, 750, 1_000, &source, &target), 2_000);
+       }
+
+       #[test]
+       fn reduces_liquidity_upper_bound_along_path_on_success() {
+               let network_graph = network_graph();
+               let params = ProbabilisticScoringParameters {
+                       liquidity_penalty_multiplier_msat: 1_000, ..Default::default()
+               };
+               let mut scorer = ProbabilisticScorer::new(params, &network_graph);
+               let sender = sender_node_id();
+               let source = source_node_id();
+               let target = target_node_id();
+               let recipient = recipient_node_id();
+               let path = payment_path_for_amount(500);
+
+               assert_eq!(scorer.channel_penalty_msat(41, 250, 1_000, &sender, &source), 124);
+               assert_eq!(scorer.channel_penalty_msat(42, 250, 1_000, &source, &target), 124);
+               assert_eq!(scorer.channel_penalty_msat(43, 250, 1_000, &target, &recipient), 124);
+
+               scorer.payment_path_successful(&path.iter().collect::<Vec<_>>());
+
+               assert_eq!(scorer.channel_penalty_msat(41, 250, 1_000, &sender, &source), 124);
+               assert_eq!(scorer.channel_penalty_msat(42, 250, 1_000, &source, &target), 300);
+               assert_eq!(scorer.channel_penalty_msat(43, 250, 1_000, &target, &recipient), 300);
+       }
 }