blip18 negative inbound fee support #4285
Description
This issue is created in the hopes that LDK will add support for blip18 inbound fees.
LND introduced inbound fees in version 0.18.0 back in May 2024.
There is an open but ready to be merged PR for eclair since October 2024 that adds blip18 support.
Inbound fees have been accepted by the community and are now widely used.
You can check the status of adoption in this sheet
At the time of writing, 40% of all public channels have the blip18 TLV stream set and 14% of the channels offer negative inbound fees.
Negative inbound fees open up more route in the network and make routes cheaper.
It makes routes through nodes that are deep below a starting node economically viable.
Support comes in different levels and one does not have to implement full support in one step.
Level 0: preparation
Implement the necessary functions to parse the TLV stream from the gossip and make the information available.
Level 1: simple sending support
Include the inbound fees when building the onion packages for a route. But not yet use it when trying to find a route.
This allows to make use of negative inbound fees that happen to be present on a picked route.
Level 2: full sending support
Take negative inbound fees into consideration when searching for routes.
This allows to take full use of the saving that negative inbound fees offer.
Level 3: routing support
The ability to set negative inbound fees on own channels.