bLIP-0025: Allow forwarding HTLCs that underpay the onion encoded value

Author: valentinewallace

blip-0025.md

@@ -0,0 +1,50 @@
+```
+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 XXX with a
+  TLV value containing the amount of extra fee they took from the receiver's final received value,
+  in millisatoshis
+
+Receiver:
+* MUST check TLV XXX from the inbound `update_add_htlc` to ensure the fee taken by the previous hop
+  makes sense
+* MUST fail back the HTLC if TLV XXX is missing or its value is unexpected
+
+## Motivation
+
+For context, it is expected that many lightning users will be connected to the lightning network via
+LSPs (Lightning Service Providers), who will be 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 than the
+invoice's expected value, and receive proof-of-payment that they paid the full value.
+
+## Reference Implementations