[bug]: The fee in the asset channel was not successfully collected.

[shuishihang]

I created three nodes (A, B, C). Node A and B established an asset channel, while node B and C have a BTC channel. I set the asset price at 1 asset = 100 sats. When I created a 10,000 sats invoice using node C, node A paid it with assets but only transferred 354 sats + 100 assets.

However, when I set the price to 1 asset = 1 sat, an extra 1 asset was charged as a fee. At the 1 asset = 100 sats rate with a routing fee of 10 sats, after dozens of transactions, no asset-based fee was deducted. Shouldn’t the system charge 1 asset as a fee every 10 payments (since 10 × 10 sats = 100 sats = 1 asset)? Is there a way to collect fees in sats instead of assets in the asset channel?

[guggero]

Fees are always collected in milli-satoshis. But because assets aren't divisible, a round-down mechanism is implemented.
Which means, the fees can only be paid if the assets have a high enough "resolution", meaning that one asset unit can represent one or more milli-satoshis.
If that isn't the case, then things are simply rounded down. And there's no over time accounting, which means "a fee every 10 payments" isn't what is happening. A fee can either be expressed in assets and be paid or it won't.

See https://github.com/lightninglabs/taproot-assets/blob/main/docs/rfq-and-decimal-display.md for more explanation.

If you want to test that this works, I suggest changing the following parameters:

• Issue an asset with the recommended decimal display of 6
• Set the rate as 1 asset = 1 milli-satoshi

But bear in mind that the 354 sats that are required to send a non-dust HTLC will always be paid, which might be larger than any possible fee that's accrued. But that's a current protocol limitation that we're going to remove in future versions.

[shuishihang]

Thank you for your response. I have another question: Suppose there are three nodes (A-B-C), where A and B are connected via a BTC Lightning channel, and B and C are also connected via a BTC Lightning channel. If node B sets its base_fee_msat to 2000 using lncli updatechanpolicy, then when node C generates an invoice and node A makes a payment, node B will earn a 2-sat routing fee. However, if these two BTC channels are instead asset channels (e.g., for a different asset), node B’s routing fee is determined by node C’s fee policy. Why does this happen?

[guggero]

node B’s routing fee is determined by node C’s fee policy. Why does this happen?

Hmm, I think this is a bug here:

taproot-assets/rpcserver.go

Line 8557 in 33b94a9

if edge.Node2Pub == remotePubStr {

Let me create a fix for that.