Attribution data (feature 36/37)

[joostjager]

Failure attribution is important to properly penalize nodes after a payment failure occurs. The goal of the penalty is to give the next attempt a better chance at succeeding. In the happy failure flow, the sender is able to determine the origin of the failure and penalizes a single node or pair of nodes.

Unfortunately it is possible for nodes on the route to hide themselves. If they return random data as the failure message, the sender won't know where the failure happened.

This PR proposes a new failure message format that lets each node commit to the failure message. If one of the nodes corrupts the failure message, the sender will be able to identify that node.

For more information, see https://lists.linuxfoundation.org/pipermail/lightning-dev/2022-October/003723.html.

Furthermore, the htlc fail and fulfill flows are extended to convey self-reported htlc hold times to the sender.

Fail flow implementations

LND implementation: lightningnetwork/lnd#9888

LDK implementation: lightningdevkit/rust-lightning#2256

Eclair implementation: ACINQ/eclair#3065

CLN implementation: ElementsProject/lightning#8291

Fulfill flow implementations

LDK implementation: lightningdevkit/rust-lightning#3801

Eclair implementation: ACINQ/eclair#3100

[thomash-acinq]

I've started implementing it in eclair, do you have some test vectors so we can check that we are compatible?
The design seems good to me, but as I've said previously, I think keeping hop payloads and hmacs for 8 nodes only (instead of 27) is enough for almost all use cases and would give us huge size savings.

[joostjager]

I don't have test vectors yet, but I can produce them. Will add them to this PR when ready.

Capping the max hops at a lower number is fine to me, but do you have a scenario in mind where this would really make the difference? Or is it to more generally that everything above 8 is wasteful?

[joostjager]

@thomash-acinq added a happy fat error test vector.

[joostjager]

I think this big gap in the bits has emerged here because of tentative spec changes that may or may not make it. Not sure why that is necessary. I thought for unofficial extensions, the custom range is supposed to be used?

I can see that with unofficial features deployed in the wild, it is easier to keep the same bit when something becomes official. But not sure if that is worth creating the gap here? An alternative is to deploy unofficial features in the custom range first, and then later recognize both the official and unofficial bit. Slightly more code, but this feature list remains clean.

[joostjager]

Added fat error signaling to the PR.

[thomash-acinq]

I've spent a lot of time trying to make the test vector pass and I've finally found what was wrong:
In the spec you write that the hmac covers

• failure_len, failuremsg, pad_len and pad.

• The first y+1 payloads in payloads. For example, hmac_0_2 would cover all three payloads.

• y downstream hmacs that correspond to downstream node positions relative to x. For example, hmac_0_2 would cover hmac_1_1 and hmac_2_0.

implying that we need to concatenate them in that order. But in your code you follow a different order:

// Include payloads including our own.
_, _ = hash.Write(payloads[:(NumMaxHops-position)*payloadLen])

// Include downstream hmacs.
var hmacsIdx = position + NumMaxHops
for j := 0; j < NumMaxHops-position-1; j++ {
	_, _ = hash.Write(
		hmacs[hmacsIdx*sha256.Size : (hmacsIdx+1)*sha256.Size],
	)

	hmacsIdx += NumMaxHops - j - 1
}

// Include message.
_, _ = hash.Write(message)

I think the order message + hop payloads + hmacs is more intuitive as it matches the order of the fields in the packet.

[joostjager]

Oh great catch! Will produce a new vector.

[joostjager]

@thomash-acinq updated vector

[joostjager]

Updated LND implementation with sender-picked fat error structure parameters: lightningnetwork/lnd#7139

[joostjager]

I still believe in a future of the lightning network where failures aren't tolerated at all, and all routing nodes deliver excellent service. Then it's just a matter of measuring latency and going for the sub-10 ms ones.

[joostjager]

Hold time filtering to avoid users doing things we think they shouldn't. OP_RETURN vibes 😂

[t-bast]

I still believe in a future of the lightning network where failures aren't tolerated at all, and all routing nodes deliver excellent service.

I agree, but that's orthogonal to payment latency? I honestly don't understand why it's important to aim for minimal latency: is it for performance's sake, to boast about numbers? A payment that takes 500ms to complete is almost indistinguishable from being instantaneous for our brains, so why does it matter that it takes 50ms instead of 500ms if you have to sacrifice privacy for it?

[joostjager]

For me it is two-fold:

• I don't want to rule out potential future uses cases that somehow do require minimal latency.

• It is unclear to me how much an arbitrary 100 ms granularity is going to help with privacy today and also in a future in which we are stuck with this hard-coded value in the protocol. Yes it probably helps, but it may not be substantial.

Furthermore I think that an instant instant payment with no observable latency at all is just cool too indeed.

[GeorgeTsagk]

Are we talking about introducing actual time delays here, or just bucketing the reported value of the hold time?

If we're talking about further delaying the actual forwarding of the HTLC then I'm very much against it. Keep in mind this is done per payment attempt so if I need to try out 20 different routes this will accumulate quite fast, significantly killing ux.

Seems like the concern is that the accurate reporting of hold times does not directly ruin the privacy for the nodes reporting the hold times, but will eventually lead to intense competition around latency, contributing to further centralization in the network?

If we're talking about hold times leaking information about the routing nodes (fingerprinting?) then this is already too noisy to produce consistent results. Every node has different hardware and internet connectivity configuration, I wouldn't expect all LNDs or LDKs to have some entropy in their reports, and there exist better ways to fingerprint anyway.

I don't think we should pollute the protocol with this kind of safeguard/filter against more precise reports. If we assume that a user is willing to get their hands dirty to somehow signal lower hold times, they'll find a way. Also let's not underestimate the power of what implementations default to.

By having implementations default to scoring all hold times <=100ms the same, or reporting buckets of 100ms by default, most of the concerns seem to be eliminated. Of course, if we assume the majority of the network to be running custom software then what's even the point of this discussion?

[DerEwige]

I’m very much against the intentional slowing down of HTLC resolves.

Every milli second a HTLC is unresolved it poses a risk to the node runner.
One of the common causes of force closures are stuck HTLCs.

A HTLC gets stuck when a node goes offline after the HTLC passed that node.
So the failure or success can not propagate backwards through it.
The chances of a HTLC getting stuck is proportional to the time the HTLC needs to resolve.

So if you intentionally slow down the HTLC resolve speed, you also increase the number of force closures.

I would consider the intentional hold of a HTLC of more than 10ms as an jamming attack.

[t-bast]

A HTLC gets stuck when a node goes offline after the HTLC passed that node.
So the failure or success can not propagate backwards through it.
The chances of a HTLC getting stuck is proportional to the time the HTLC needs to resolve.
So if you intentionally slow down the HTLC resolve speed, you also increase the number of force closures.

That's not at all a valid conclusion. HTLCs get stuck and channels force-close because of bugs, not because nodes go temporarily offline or introduce batching/delays. That's completely unrelated.

I 100% agree with you that all bugs that lead to stuck HTLCs or force-closed channels must be fixed and should be the highest priority for all implementations. But that has absolutely nothing to do with whether or not we should introduce delays or measure relay latency in buckets of 100ms.

[DerEwige]

That's not at all a valid conclusion. HTLCs get stuck and channels force-close because of bugs, not because nodes go temporarily offline or introduce batching/delays. That's completely unrelated.

I 100% agree with you that all bugs that lead to stuck HTLCs or force-closed channels must be fixed and should be the highest priority for all implementations. But that has absolutely nothing to do with whether or not we should introduce delays or measure relay latency in buckets of 100ms.

I do agree, that it is usually a bug or sometimes negligence, that is the root cause of most force-closes.

I also totally agree that there should be an easy way to measure relay latency.
Currently the only way to measure relay latency is by aggregating a lot of data (100’000+ payments) and calculate the time for each channel from that data.

In fact I do exactly this to generate the list you can find here on the “node speeds” tab
https://docs.google.com/spreadsheets/d/1gn2TY3zDoPdk46yYQfRhGb2_YDepQEn7l3qtHWCxov4
And I would love to have simpler and more accurate way to do this. And 100ms accuracy in the reported time would also be good enough.

But saying that adding a relay delay would not have an impact of the number of force-closes is - in my opinion - wrong.

If a channel with active HTLC goes offline and does not come online again before the HTLC reaches its timeout, this will lead to a force-close.
The chance of a channel going offline with an active HTLC on it increases with the time a HTLC lingers on the channel.

I send out more than 100’000 HTLCs each day.
0.1s extra per hop would add hours of exposure each day.

[carlaKC]

Wrote up some thoughts on delving that I think are worth considering when we think about latency measurements and privacy: https://delvingbitcoin.org/t/latency-and-privacy-in-lightning/1723.

[joostjager]

I did a rough implementation in LDK of hold times for the success case. It turned out to be straight-forward, mostly reusing code from the failure case. Test vectors in #1261

[carlaKC]

Logic all looks good, but would like to restructure the Returning Errors section to follow the requirements / rationale structure that we have elsewhere in the spec - as is the two are interspersed.

Also needs to refer to option_attributable_failures rather than "attributable failures" in a few place.

[carlaKC]

Doesn't need to be in invoices (9)?

[joostjager]

It is indeed the question whether there are scenarios where senders would refuse a payment that pays to a node that doesn't support attribution data? If they'd return random data, and their predecessor adds attribution data, blame still lands on the final node pair.

Maybe attribution data is unnecessary for the final hop? 🤔 Of course they still want to pass back something to not stand out as a recipient.

For hold times, they will probably always report zero anyway.

[carlaKC]

We don't have any instructions in this PR for how to react to the presence / absence of this feature bit in an invoice, so I think we can take it out?

Seems easy enough to come back and add signaling in invoices (+ handling instructions) if we want it in the future?

[carlaKC]

nit: note that nodes on -> note that nodes in?

[joostjager]

Fixed. Language skills insufficient for me to flag 'on the route' as incorrect.

[carlaKC]

I think that we can more formally specify this:

• if option_attributable_failure is advertised:
  • if path_key is not set in the incoming update_add_htlc:
    • MUST include htlc_hold_times in payload.

[joostjager]

Added

[carlaKC]

Duplicated here and in BOLT02 - perhaps just link to the BOLT02 description?

[carlaKC]

General comment on this section: I'm missing the structure of requirements/rationale that we have elsewhere in the specification.

For example, the section for the erring node notes "The sender can use this information to score nodes on latency". To me this should be in the origin node section or all contained in a single rationale at the end. Ditto with the game theory of guessing HMACs.

[joostjager]

I see what you mean. I've added a commit trying to address this and improve it a little, but it remains difficult to capture this in requirements and still have explanation too I found.

[brh28]

I didn't see the start time and end time of the htlc_hold_time defined. I assume these are upon receiving the update_add_htlc and before sending the update_fulfill_htlc/update_fail_htlc, respectively?

Overall, I like the idea of htlc_hold_time but suspect it'll be gamed by routing nodes.

[Roasbeef]

Overall, I like the idea of htlc_hold_time but suspect it'll be gamed by routing nodes.

A motivated sender can always point point what the actual forwarding delays are by bisecting the route. They also always know how long the entire route took as well. You are correct though that it's intended mainly on a best effort basis, the overall assumption is that this can be used to allow a "fast" node to distinguish themselves, and be rewarded for that.