From: Matt Corallo Date: Tue, 2 Mar 2021 01:20:16 +0000 (-0500) Subject: Add comment in get_route describing our dijkstra's mods X-Git-Url: http://git.bitcoin.ninja/?a=commitdiff_plain;h=562de9b4df06b7c4255d26e0ed6e0d844ec9c4b7;p=rust-lightning Add comment in get_route describing our dijkstra's mods --- diff --git a/lightning/src/routing/router.rs b/lightning/src/routing/router.rs index c9d7cc253..0b3232697 100644 --- a/lightning/src/routing/router.rs +++ b/lightning/src/routing/router.rs @@ -372,8 +372,42 @@ pub fn get_route(our_node_id: &PublicKey, network: &NetworkGraph, paye // 8. Choose the best route by the lowest total fee. // As for the actual search algorithm, - // we do a payee-to-payer Dijkstra's sorting by each node's distance from the payee - // plus the minimum per-HTLC fee to get from it to another node (aka "shitty A*"). + // we do a payee-to-payer pseudo-Dijkstra's sorting by each node's distance from the payee + // plus the minimum per-HTLC fee to get from it to another node (aka "shitty pseudo-A*"). + // + // We are not a faithful Dijkstra's implementation because we can change values which impact + // earlier nodes while processing later nodes. Specifically, if we reach a channel with a lower + // liquidity limit (via htlc_maximum_msat, on-chain capacity or assumed liquidity limits) then + // the value we are currently attempting to send over a path, we simply reduce the value being + // sent along the path for any hops after that channel. This may imply that later fees (which + // we've already tabulated) are lower because a smaller value is passing through the channels + // (and the proportional fee is thus lower). There isn't a trivial way to recalculate the + // channels which were selected earlier (and which may still be used for other paths without a + // lower liquidity limit), so we simply accept that some liquidity-limited paths may be + // de-preferenced. + // + // One potentially problematic case for this algorithm would be if there are many + // liquidity-limited paths which are liquidity-limited near the destination (ie early in our + // graph walking), we may never find a liquidity-unlimited path which has lower proportional + // fee (and only lower absolute fee when considering the ultimate value sent). Because we only + // consider paths with at least 5% of the total value being sent, the damage from such a case + // should be limited, however this could be further reduced in the future by calculating fees + // on the amount we wish to route over a path, not the amount we are routing over a path. + // + // Alternatively, we could store more detailed path information in the heap (targets, below) + // and index the best-path map (dist, below) by node *and* HTLC limits, however that would blow + // up the runtime significantly both algorithmically (as we'd traverse nodes multiple times) + // and practically (as we would need to store dynamically-allocated path information in heap + // objects, increasing malloc traffic and indirect memory access significantly). Further, the + // results of such an algorithm would likely be biased towards lower-value paths. + // + // Further, we could return to a faithful Dijkstra's algorithm by rejecting paths with limits + // outside of our current search value, running a path search more times to gather candidate + // paths at different values. While this may be acceptable, further path searches may increase + // runtime for little gain. Specifically, the current algorithm rather efficiently explores the + // graph for candidate paths, calculating the maximum value which can realistically be sent at + // the same time, remaining generic across different payment values. + // // TODO: There are a few tweaks we could do, including possibly pre-calculating more stuff // to use as the A* heuristic beyond just the cost to get one node further than the current // one.