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Ability to add custom TLV to update_add_htlc message #6663
Description
The LSPS2 specification requires the LSP to add a custom extra_fee TLV to the update_add_htlc message. An LSP run on top of CLN will intercept htlcs using the htlc_accepted hook. For some htlcs, LSP fees will be deducted from the outgoing amount, by modifying the onion payload.
- It will modify the next hop id record (6)
- It will modify the amount to forward record (2)
- It has to add an
extra_feerecord. (65537)
I tried adding the extra_fee record to the returned payload in the htlc_accepted hook, but I get an error: Failing HTLC because of an invalid payload. I assume this is because the TLV is not recognized. The only option currently available appears to be to enable all unknown TLVs on the node, but that is not what I want.
Relevant section in LSPS2: https://github.com/BitcoinAndLightningLayerSpecs/lsp/blob/a1d8ca8439e730d6a097585f7e1d091f12f9afb0/LSPS2/README.md?plain=1#L950-L966
So: Add support for custom TLVs for the update_add_htlc message.
Activity
cdecker commentedon Sep 11, 2023
That is expected, since the
payloadis the TLV stream for the local node. As suchlightningdwill fail the HTLC if thepayloadcontains any field that is not understood bylightningd. The onion would contain thepayloadfor the next hop (but is unmodifiable due to the encrypted and HMACd nature of the onion). There is currently no way return value that allows adding fields to the TLVs in theupdate_add_htlcbecause it was not used by anyone so far.I will add this new return field to the
htlc_acceptedhook call, and wire it through to the appropriate place.JssDWt commentedon Apr 17, 2025
I'm revisiting this subject with the
extra-feeTLV. It's still useful.The
extra_feeTLV would actually come from the local node's payload. Just like the amount to forward and next hop id.It's set as a TLV on the
update_add_htlcmessage, so there's no need to hack the onion.If there would be a setting in CLN to allow adding extra TLVs to the htlc_accepted hook payload, and CLN would pass these TLVs around up to sending the
update_add_htlcmessage this can work. The problem is right now CLN picks the fields it knows and forwards. Let's walk through it:accept-htlc-tlv-typeconfig.update_add_htlc.andrei-21 commentedon Apr 28, 2025
Hey team.
Just wanted to bump this issue as it's pretty important for enabling LSPS2 server implementations on CLN. The
extra_feeTLV field is a key piece needed to make this work.This also ties into the new bLIP-36 protocol, which will rely on similar TLV fields for on-the-fly funding. Having this in place would open up more flexibility for node operators and service providers.
Would love to see this prioritized if possible! Thanks for all the work you’re already putting in.
cdecker commentedon May 8, 2025
@nepet and I had a good discussion on how this could be implemented, since he's also working on an LSPS implementation. So this should happen soon☺️
andrei-21 commentedon Jun 25, 2025
Hi @nepet, I just wanted to kindly check in and see if there are any updates regarding the issue. Thanks.
nepet commentedon Jun 25, 2025