Introduce const MEDIAN_HOP_CLTV_EXPIRY_DELTA.

Author: tnull

lightning/src/routing/router.rs

@@ -177,6 +177,9 @@ impl_writeable_tlv_based!(RouteParameters, {
 /// Maximum total CTLV difference we allow for a full payment path.
 pub const DEFAULT_MAX_TOTAL_CLTV_EXPIRY_DELTA: u32 = 1008;
 
+/// The median hop CLTV expiry delta currently seen in the network.
+const MEDIAN_HOP_CLTV_EXPIRY_DELTA: u32 = 40;
+
 /// The recipient of a payment.
 #[derive(Clone, Debug, Hash, PartialEq, Eq)]
 pub struct PaymentParameters {
@@ -259,7 +262,6 @@ impl PaymentParameters {
 #[derive(Clone, Debug, Hash, Eq, PartialEq)]
 pub struct RouteHint(pub Vec<RouteHintHop>);
 
-
 impl Writeable for RouteHint {
 	fn write<W: ::util::ser::Writer>(&self, writer: &mut W) -> Result<(), io::Error> {
 		(self.0.len() as u64).write(writer)?;
@@ -861,10 +863,12 @@ where L::Target: Logger {
 					let contributes_sufficient_value = available_value_contribution_msat >= minimal_value_contribution_msat;
 
 					// Do not consider candidates that exceed the maximum total cltv expiry limit.
-					// We subtract 2*40 here in order to account for some of the privacy-enhancing
-					// random CLTV expiry delta offset we add on top later.
+					// In order to already account for some of the privacy enhancing random CLTV
+					// expiry delta offset we add on top later, we subtract a rough estimate
+					// (2*MEDIAN_HOP_CLTV_EXPIRY_DELTA) here.
 					let max_total_cltv_expiry_delta = payment_params.max_total_cltv_expiry_delta
-						.checked_sub(2*40).unwrap_or(payment_params.max_total_cltv_expiry_delta);
+						.checked_sub(2*MEDIAN_HOP_CLTV_EXPIRY_DELTA)
+						.unwrap_or(payment_params.max_total_cltv_expiry_delta);
 					let hop_total_cltv_delta = ($next_hops_cltv_delta as u32)
 						.checked_add($candidate.cltv_expiry_delta())
 						.unwrap_or(u32::max_value());
@@ -1592,26 +1596,27 @@ fn add_random_cltv_offset(route: &mut Route, payment_params: &PaymentParameters,
 				}
 			}
 		} else {
-			// If the entire path is private, choose a random offset from multiples of 40,
-			// which is the most prevalent cltv_expiry_delta currently used in the network
+			// If the entire path is private, choose a random offset from multiples of
+			// MEDIAN_HOP_CLTV_EXPIRY_DELTA
 			let mut prng = ChaCha20::new(random_seed_bytes, &[0u8; 8]);
 			let mut random_bytes = [0u8; 4];
 			prng.process_in_place(&mut random_bytes);
 			let random_walk_length = u32::from_be_bytes(random_bytes).wrapping_rem(3).wrapping_add(1);
-			shadow_ctlv_expiry_delta_offset = random_walk_length.wrapping_mul(40);
+			shadow_ctlv_expiry_delta_offset = random_walk_length.wrapping_mul(MEDIAN_HOP_CLTV_EXPIRY_DELTA);
 		}
 
 		// Limit the total offset to reduce the worst-case locked liquidity timevalue
 		const MAX_SHADOW_CLTV_EXPIRY_DELTA_OFFSET: u32 = 3*144;
 		shadow_ctlv_expiry_delta_offset = cmp::min(shadow_ctlv_expiry_delta_offset, MAX_SHADOW_CLTV_EXPIRY_DELTA_OFFSET);
 
 		// Limit the offset so we never exceed the max_total_cltv_expiry_delta. To improve plausibility,
-		// we choose the limit to be the largest possible multiple of 40.
+		// we choose the limit to be the largest possible multiple of MEDIAN_HOP_CLTV_EXPIRY_DELTA.
 		let mut max_path_offset = payment_params.max_total_cltv_expiry_delta
 			.checked_sub(path.iter().map(|h| h.cltv_expiry_delta).sum())
 			.unwrap_or(0);
-		max_path_offset = max_path_offset.wrapping_sub(max_path_offset.wrapping_rem(40))
-			.max(max_path_offset.wrapping_rem(40));
+		max_path_offset = max_path_offset
+			.wrapping_sub(max_path_offset.wrapping_rem(MEDIAN_HOP_CLTV_EXPIRY_DELTA))
+			.max(max_path_offset.wrapping_rem(MEDIAN_HOP_CLTV_EXPIRY_DELTA));
 		shadow_ctlv_expiry_delta_offset = cmp::min(shadow_ctlv_expiry_delta_offset, max_path_offset);
 
 		// Add 'shadow' CLTV offset to the final hop