blip-0041: channel funding fee credit

Onboarding users to the lightning network is hard, because opening a
channel requires paying on-chain fees and allocating capital, hoping
that it will be used to earn routing fees. It is not economical nor
sustainable to open channels for users that receive a tiny amount and
may not use the network frequently enough.

Wallet providers usually work around this issue by requiring users to
receive a large enough first payment to pay on-chain fees for their
channel.

However, a collection of unrelated tiny payments received over a period
of time may be enough to justify creating a channel. We thus introduce
a funding fee credit, that lets users prepay for future channel(s) by
proactively revealing htlc preimages for small payments. When that fee
credit reaches a sufficient amount, it is used by the remote node to
cover the on-chain costs of opening a channel or splicing additional
liquidity.

Author: t-bast

README.md

@@ -27,3 +27,4 @@ For more detail on the process, please read [bLIP-0001](./blip-0001.md) and
 | [17](./blip-0017.md)     | Hosted Channels                | Anton Kumaigorodskiy        | Active |
 | [25](./blip-0025.md)     | Forward less than onion value  | Valentine Wallace           | Active |
 | [32](./blip-0032.md)     | Onion Message DNS Resolution   | Matt Corallo                | Active |
+| [41](./blip-0041.md)     | Channel Funding Fee Credit     | Bastien Teinturier          | Active |

blip-0002.md

@@ -50,6 +50,7 @@ bLIPs may reserve feature bits by adding them to the following table:
 | 54/55       | `keysend`              | A form of spontaneous payment                     | N                | `var_onion_optin`                | [bLIP 3](./blip-0003.md)         |
 | 256/257     | `hosted_channels`      | This node accepts requests for hosted channels    | IN               |                                  | [bLIP 17](./blip-0017.md)        |
 | 258/259     | `dns_resolver`         | This node accepts DNSSEC proof requests           | N                |                                  | [bLIP 32](./blip-0032.md)        |
+| 562/563     | `funding_fee_credit`   | This node provides fee credit for channel funding | IN               | `on_the_fly_funding`             | [bLIP 41](./blip-0041.md)        |
 
 ### Messages
 
@@ -61,6 +62,8 @@ bLIPs may create new messages and reserve their type in the following table:
 
 | Type    | Name                            | Link                       |
 | ------- | ------------------------------- | -------------------------- |
+| 41045   | `add_fee_credit`                | [bLIP 41](./blip-0041.md)  |
+| 41046   | `current_fee_credit`            | [bLIP 41](./blip-0041.md)  |
 | 65535   | `invoke_hosted_channel`         | [bLIP 17](./blip-0017.md)  |
 | 65533   | `init_hosted_channel`           | [bLIP 17](./blip-0017.md)  |
 | 65531   | `last_cross_signed_state`       | [bLIP 17](./blip-0017.md)  |