diff --git a/lightning/src/routing/router.rs b/lightning/src/routing/router.rs index c52e5e14964..5363a89d161 100644 --- a/lightning/src/routing/router.rs +++ b/lightning/src/routing/router.rs @@ -746,7 +746,7 @@ where L::Target: Logger, GL::Target: Logger { pub(crate) fn get_route( our_node_pubkey: &PublicKey, payment_params: &PaymentParameters, network_graph: &ReadOnlyNetworkGraph, first_hops: Option<&[&ChannelDetails]>, final_value_msat: u64, final_cltv_expiry_delta: u32, - logger: L, scorer: &S, random_seed_bytes: &[u8; 32] + logger: L, scorer: &S, _random_seed_bytes: &[u8; 32] ) -> Result where L::Target: Logger { let payee_node_id = NodeId::from_pubkey(&payment_params.payee_pubkey); @@ -788,11 +788,11 @@ where L::Target: Logger { // 4. See if we managed to collect paths which aggregately are able to transfer target value // (not recommended value). // 5. If yes, proceed. If not, fail routing. - // 6. Randomly combine paths into routes having enough to fulfill the payment. (TODO: knapsack) - // 7. Of all the found paths, select only those with the lowest total fee. - // 8. The last path in every selected route is likely to be more than we need. - // Reduce its value-to-transfer and recompute fees. - // 9. Choose the best route by the lowest total fee. + // 6. Select the paths which have the lowest cost (fee plus scorer penalty) per amount + // transferred up to the transfer target value. + // 7. Reduce the value of the last path until we are sending only the target value. + // 8. If our maximum channel saturation limit caused us to pick two identical paths, combine + // them so that we're not sending two HTLCs along the same path. // As for the actual search algorithm, // we do a payee-to-payer pseudo-Dijkstra's sorting by each node's distance from the payee @@ -1643,96 +1643,57 @@ where L::Target: Logger { return Err(LightningError{err: "Failed to find a sufficient route to the given destination".to_owned(), action: ErrorAction::IgnoreError}); } - // Sort by total fees and take the best paths. - payment_paths.sort_unstable_by_key(|path| path.get_total_fee_paid_msat()); - if payment_paths.len() > 50 { - payment_paths.truncate(50); - } + // Step (6). + let mut selected_route = payment_paths; - // Draw multiple sufficient routes by randomly combining the selected paths. - let mut drawn_routes = Vec::new(); - let mut prng = ChaCha20::new(random_seed_bytes, &[0u8; 12]); - let mut random_index_bytes = [0u8; ::core::mem::size_of::()]; + debug_assert_eq!(selected_route.iter().map(|p| p.get_value_msat()).sum::(), already_collected_value_msat); + let mut overpaid_value_msat = already_collected_value_msat - final_value_msat; - let num_permutations = payment_paths.len(); - for _ in 0..num_permutations { - let mut cur_route = Vec::::new(); - let mut aggregate_route_value_msat = 0; + // First, sort by the cost-per-value of the path, selecting only the paths which + // contribute the most per cost. + // We sort backwards as we will remove from the front in `retain`, next. + selected_route.sort_unstable_by(|a, b| + (((b.get_cost_msat() as u128) << 64) / (b.get_value_msat() as u128)) + .cmp(&(((a.get_cost_msat() as u128) << 64) / (a.get_value_msat() as u128))) + ); - // Step (6). - // Do a Fisher-Yates shuffle to create a random permutation of the payment paths - for cur_index in (1..payment_paths.len()).rev() { - prng.process_in_place(&mut random_index_bytes); - let random_index = usize::from_be_bytes(random_index_bytes).wrapping_rem(cur_index+1); - payment_paths.swap(cur_index, random_index); + // We should make sure that at least 1 path left. + let mut paths_left = selected_route.len(); + selected_route.retain(|path| { + if paths_left == 1 { + return true + } + let mut keep = true; + let path_value_msat = path.get_value_msat(); + if path_value_msat <= overpaid_value_msat { + keep = false; + overpaid_value_msat -= path_value_msat; + paths_left -= 1; } + keep + }); + assert!(selected_route.len() > 0); + if overpaid_value_msat != 0 { // Step (7). - for payment_path in &payment_paths { - cur_route.push(payment_path.clone()); - aggregate_route_value_msat += payment_path.get_value_msat(); - if aggregate_route_value_msat > final_value_msat { - // Last path likely overpaid. Substract it from the most expensive - // (in terms of proportional fee) path in this route and recompute fees. - // This might be not the most economically efficient way, but fewer paths - // also makes routing more reliable. - let mut overpaid_value_msat = aggregate_route_value_msat - final_value_msat; - - // First, we drop some expensive low-value paths entirely if possible, since fewer - // paths is better: the payment is less likely to fail. In order to do so, we sort - // by value and fall back to total fees paid, i.e., in case of equal values we - // prefer lower cost paths. - cur_route.sort_unstable_by(|a, b| { - a.get_value_msat().cmp(&b.get_value_msat()) - // Reverse ordering for cost, so we drop higher-cost paths first - .then_with(|| b.get_cost_msat().cmp(&a.get_cost_msat())) - }); - - // We should make sure that at least 1 path left. - let mut paths_left = cur_route.len(); - cur_route.retain(|path| { - if paths_left == 1 { - return true - } - let mut keep = true; - let path_value_msat = path.get_value_msat(); - if path_value_msat <= overpaid_value_msat { - keep = false; - overpaid_value_msat -= path_value_msat; - paths_left -= 1; - } - keep - }); - - if overpaid_value_msat == 0 { - break; - } + // Now, subtract the overpaid value from the most-expensive path. + // TODO: this could also be optimized by also sorting by feerate_per_sat_routed, + // so that the sender pays less fees overall. And also htlc_minimum_msat. + selected_route.sort_unstable_by(|a, b| { + let a_f = a.hops.iter().map(|hop| hop.0.candidate.fees().proportional_millionths as u64).sum::(); + let b_f = b.hops.iter().map(|hop| hop.0.candidate.fees().proportional_millionths as u64).sum::(); + a_f.cmp(&b_f).then_with(|| b.get_cost_msat().cmp(&a.get_cost_msat())) + }); + let expensive_payment_path = selected_route.first_mut().unwrap(); - assert!(cur_route.len() > 0); - - // Step (8). - // Now, subtract the overpaid value from the most-expensive path. - // TODO: this could also be optimized by also sorting by feerate_per_sat_routed, - // so that the sender pays less fees overall. And also htlc_minimum_msat. - cur_route.sort_unstable_by_key(|path| { path.hops.iter().map(|hop| hop.0.candidate.fees().proportional_millionths as u64).sum::() }); - let expensive_payment_path = cur_route.first_mut().unwrap(); - - // We already dropped all the small value paths above, meaning all the - // remaining paths are larger than remaining overpaid_value_msat. - // Thus, this can't be negative. - let expensive_path_new_value_msat = expensive_payment_path.get_value_msat() - overpaid_value_msat; - expensive_payment_path.update_value_and_recompute_fees(expensive_path_new_value_msat); - break; - } - } - drawn_routes.push(cur_route); + // We already dropped all the small value paths above, meaning all the + // remaining paths are larger than remaining overpaid_value_msat. + // Thus, this can't be negative. + let expensive_path_new_value_msat = expensive_payment_path.get_value_msat() - overpaid_value_msat; + expensive_payment_path.update_value_and_recompute_fees(expensive_path_new_value_msat); } - // Step (9). - // Select the best route by lowest total cost. - drawn_routes.sort_unstable_by_key(|paths| paths.iter().map(|path| path.get_cost_msat()).sum::()); - let selected_route = drawn_routes.first_mut().unwrap(); - + // Step (8). // Sort by the path itself and combine redundant paths. // Note that we sort by SCIDs alone as its simpler but when combining we have to ensure we // compare both SCIDs and NodeIds as individual nodes may use random aliases causing collisions