Add probing service by randomlogin · Pull Request #815 · lightningdevkit/ldk-node

randomlogin · 2026-03-01T11:20:36Z

Added a probing service which is used to send probes to estimate channels' capacities.

Related issue: #765.

Probing is intended to be used in two ways:

on a 'normal' node that allocates some liquidity to probe channels;
on a probing node which doesn't make any real payments, just observes the network.

For probing a new abstraction Prober is defined and is (optionally) created during node building.
Prober periodically sends probes to feed the data to the scorer.
Prober sends probes using a ProbingStrategy.

ProbingStrategy trait has only one method: fn next_probe(&self) -> Option<Probe>; every tick it generates a probe, where Probe represents how to send a probe.

To accommodate two different ways the probing is used, we either construct a probing route manually (Probe::PrebuiltRoute) or rely on the router/scorer (Probe::Destination).

Prober tracks how much liquidity is locked in-flight in probes, prevents the new probes from firing if the cap is reached.

There are two probing strategies implemented:

Random probing strategy, it picks a random route from the current node, the route is probed via send_probe, thus ignores scoring parameters (what hops to pick), it also ignores liquidity_limit_multiplier which prohibits taking a hop if its capacity is too small. It is a true random route.
High degree probing strategy, it examines the graph and finds the nodes with the biggest number of (public) channels and probes routes to them using send_spontaneous_preflight_probes which uses the current router/scorer.

The former is meant to be used on payment nodes, while the latter on probing nodes. For the HighDegreeStrategy to work it is recommended to set probing_diversity_penalty_msat to some nonzero value to prevent routes reuse, however it may fail to find any available routes.

There are three tests added:

check the probing locked amount increases/decreases
check that the new probes are not fired if the current locked amount cap is reached
performance testing which sets up a network of nodes and 4 observing nodes

Example output (runs for ~1 minute, needs --nocapture flag):

SCID            Direction         Probing Random               Probing HiDeg                Probing HiDeg+P              Probing nostrat              Real outbound msat
-------------------------------------------------------------------------------------------------------------------------------------------------------------------------
114349209354241 C→Random          [0, 90 100 235]              unknown                      unknown                      unknown                      0
114349209354241 Random→C          [9 899 765, 100 000 000]     unknown                      unknown                      unknown                      917 184 187
114349209419777 D→HiDeg+P         unknown                      unknown                      [0, 91 127 725]              unknown                      0
114349209419777 HiDeg+P→D         unknown                      unknown                      [8 872 275, 100 000 000]     unknown                      905 313 697
114349209485312 B→F               [0, 3 418 855]               unknown                      unknown                      unknown                      160 288 291
114349209485312 F→B               [96 581 145, 100 000 000]    unknown                      unknown                      unknown                      819 711 709
114349209550848 B→D               [0, 6 141 420]               [0, 2 988 399]               [0, 91 127 725]              unknown                      30 052 300
114349209550848 D→B               [93 858 580, 100 000 000]    [97 011 601, 100 000 000]    [8 872 275, 100 000 000]     unknown                      874 555 512
114349209616384 E→HiDeg           [0, 2 097 950]               [0, 90 455 967]              unknown                      unknown                      0
114349209616384 HiDeg→E           [97 902 050, 100 000 000]    [9 544 033, 100 000 000]     unknown                      unknown                      913 366 836
114349209681920 B→C               [98 411 698, 100 000 000]    unknown                      unknown                      unknown                      86 511 108
114349209681920 C→B               [0, 1 588 302]               unknown                      unknown                      unknown                      812 521 355
114349209747457 B→E               [0, 6 625 595]               [0, 90 455 967]              [0, 4 795 311]               unknown                      0
114349209747457 E→B               [93 374 405, 100 000 000]    [9 544 033, 100 000 000]     [95 204 689, 100 000 000]    unknown                      907 339 924
114349209812993 C→nostrat         [0, 1 113 591]               unknown                      unknown                      unknown                      0
114349209812993 nostrat→C         [98 886 409, 100 000 000]    unknown                      unknown                      unknown                      990 000 000
114349209878528 C→E               [2 097 950, 5 335 740]       [0, 90 808 929]              unknown                      unknown                      55 569 306
114349209878528 E→C               [94 664 260, 97 902 050]     [9 191 071, 100 000 000]     unknown                      unknown                      878 423 826
114349209944065 B→C               [97 243 524, 100 000 000]    unknown                      [1 698 550, 6 086 572]       unknown                      938 012 264
114349209944065 C→B               [0, 2 756 476]               unknown                      [93 913 428, 98 301 450]     unknown                      34 367 637
-------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Known directions                   18/20                        8/20                         8/20                         0/20

For performance testing I had to expose the scoring data (scorer_channel_liquidity).
Also exposed scoring_fee_params: ProbabilisticScoringFeeParameters to Config.

TODOs:

adjust default parameters
improve HighDegree strategy to take into account channel capacities
improve HighDegree strategy to cache results
improve performance test to better estimate real channel capacity

Create new background task for probing, add its initialization to the builder. Add new trait ProobingStrategy with two default implementations. Total amount of sats in current non-finished probes is tracked, it is changed on ProbeSucceded and ProbeFailed events.

There are 3 probing tests: - check that probing lock liquidity amount changes - test that new probes are not fired if the max locked liquidity is reached (for example if on of the nodes goes down) - probing perfomance test which sets up 4 probing nodes (random strategy, high-degree without penalty, high-degree with penalty, control node without probing). A network of several nodes is set up; these nodes make payments to one another and probing nodes observe their behaviour. Next the scorer estimates are printed out. Scorer channel estimates are exposed for the purposes of the test. Scoring parameters (probing penalty) is exposed to be set up during node builiding. Random probing strategy constructs the maximal possible route (up to the set limit) instead of failing when the next hop is not possible to construct.

Probing locked amount involves hops' fees, not only the last hop. Remove usage of cursor in favor of Atomicusize for pseudorandom pick of channels for random strategy. Add check that min_probe amount is indeed smaller than max_probe_amount. Add liquidity_limit_multiplier to probing config instead of hardocded None value.

Remove Prober::run method in favor of distinct function. Tidy probing tests.

ldk-reviews-bot · 2026-03-01T11:20:38Z

👋 Thanks for assigning @tnull as a reviewer!
I'll wait for their review and will help manage the review process.
Once they submit their review, I'll check if a second reviewer would be helpful.

ldk-reviews-bot · 2026-03-04T07:57:35Z

🔔 1st Reminder