-
Notifications
You must be signed in to change notification settings - Fork 47
Commit
This commit does not belong to any branch on this repository, and may belong to a fork outside of the repository.
bLIP-0025: Allow forwarding HTLCs that underpay the onion encoded value
- Loading branch information
1 parent
4cddbed
commit cb889f3
Showing
3 changed files
with
64 additions
and
0 deletions.
There are no files selected for viewing
This file contains bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
This file contains bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
This file contains bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
Original file line number | Diff line number | Diff line change |
---|---|---|
@@ -0,0 +1,56 @@ | ||
``` | ||
bLIP: 25 | ||
Title: Allow forwarding HTLCs with less value than the onion claims to pay | ||
Status: Draft | ||
Author: Valentine Wallace <[email protected]> | ||
Created: 2023-05-22 | ||
License: CC0 | ||
``` | ||
|
||
## Abstract | ||
|
||
Penultimate hops in lightning payment paths may want to take an extra fee from the payee's final | ||
received value. To do so, they can simply forward less than the amount encoded in the onion by the | ||
sender, and specify the exact fee they took in a new `update_add_htlc` TLV for the receiver to | ||
verify. | ||
|
||
## Copyright | ||
|
||
This bLIP is licensed under the CC0 license. | ||
|
||
## Specification | ||
|
||
Penultimate hop on the path: | ||
* In their outbound `update_add_htlc` message, MUST include a TLV record keyed by type 65537 with a | ||
TLV value of `u64` containing the amount of extra fee they took from the receiver's final received | ||
value, in millisatoshis | ||
|
||
Receiver: | ||
* MUST fail the HTLC if they did not expect an extra fee to be taken or if the extra fee taken is | ||
too high | ||
* MUST either accept or reject the HTLC as if it had received `htlc_value + extra_fee` | ||
|
||
## Motivation | ||
|
||
For context, it is expected that many lightning users will be connected to the lightning network via | ||
Lightning Service Providers (LSPs), who are responsible for managing channel liquidity on end users' | ||
behalf. | ||
|
||
Often, users are onboarded to these services via a just-in-time inbound channel when they first | ||
receive a payment. However, this channel open costs money, and so liquidity providers may want to | ||
take an extra fee so that users can help bear the cost of this initial on-chain fee. | ||
|
||
## Rationale | ||
|
||
While it would be possible to avoid the extra TLV record if receivers could be trusted to calculate | ||
that the fee taken by the penultimate hop is as-expected, in practice this may be tricky to get | ||
right. If there were a bug in this logic, a sender could exploit it by forwarding less than the | ||
invoice's expected value, and receive proof-of-payment that they paid the full value. | ||
|
||
## Implementation Notes | ||
See | ||
<https://github.com/BitcoinAndLightningLayerSpecs/lsp/tree/main/LSPS2#3--invoice-generation> | ||
for invoice requirements if this feature is being used in the JIT channel context. | ||
|
||
## Reference Implementations | ||
LDK: <https://github.com/lightningdevkit/rust-lightning/pull/2319> |