Inbound routing fee discount

Author: joostjager

blip-0002.md

@@ -104,6 +104,14 @@ The following table contains extension tlv fields for the `update_add_htlc` mess
 |------------|-----------------------------|--------------------------------|
 | 5482373484 | `keysend_preimage`          | [bLIP 3](./blip-0003.md)       |
 
+#### `channel_update`
+
+The following table contains extension tlv fields for the `channel_update` message:
+
+| Type       | Name                        | Link                           |
+|------------|-----------------------------|--------------------------------|
+| 32769      | `inbound_discount`          | [bLIP 18](./blip-0018.md)      |
+
 ## Copyright
 
 This bLIP is licensed under the CC0 license.

blip-0018.md

@@ -0,0 +1,61 @@
+```
+bLIP: 14
+Title: Inbound routing fee discount
+Status: Draft
+Author: Joost Jager <[email protected]>
+Created: 2022-08-29
+License: CC0
+```
+
+## Abstract
+
+The Lightning BOLTs define a routing fee that is based on the policy of the
+_outgoing_ channel. This makes it impossible for routing node operators to
+differentiate in fees between incoming channels.
+
+Incoming traffic from some peers may be more desired than from others, for
+example because it balances out traffic in the opposite direction.
+
+This bLIP defines an optional `channel_update` message field to express an
+inbound fee discount. Senders who are able to read this field can benefit from
+lower routing fees using those channels.
+
+## Copyright
+
+This bLIP is licensed under the CC0 license.
+
+## Specification
+
+Origin node:
+
+* MAY include a `channel_update` optional TLV record keyed by type `32769` with
+  the following value:
+
+  * [i32:inbound_fee_base_msat]
+  * [i32:inbound_fee_proportional_millionths]
+
+  These fields SHOULD be negative numbers that define the inbound discount for
+  that channel.
+  
+Receiving node:
+
+* MAY pay each routing node the sum of its advertised inbound and outbound fee
+for the incoming and outgoing channel that is used. This should amount to a
+lower value than just the outbound fee.
+
+  `outbound_fee(amt_to_fwd) + inbound_fee(amt_to_fwd + outbound_fee(amt_to_fwd))`
+
+Note that the inbound fee is calculated over the total balance shift of the
+incoming channel. This includes the outbound fee.
+
+* For the final node, the inbound discount does not apply.
+
+## Universality
+
+This feature is defined as a BLIP because it's optional.
+
+## Backwards Compatibility 
+
+By defining the inbound fee as a negative number, senders who don't understand
+the new field will still be able to make payments. Routing nodes will accept
+overpayment on fees.