Allow setting a negative fee rate.

[legendarymartini]

Background

In section "8.5 Fees" (page 49) of the lightning network white paper, a single sentence at the end of the section suggests using negative fees to encourage routing on certain channels. When trying to set a negative fee rate, though, I get an error that it is below the minimum. I tracked down where the error is generated in the code, and where the referenced minFeeRate constant is set , and I understand the rational outlined in the comments (a lower fee rate would result in a ppm that is less than 1 but greater than 0).

However, I believe that changing case req.FeeRate != 0 && req.FeeRate < minFeeRate: to case req.FeeRate != 0 && Abs(req.FeeRate) < minFeeRate: on line 6304 would provide the same protection, while allowing nodes to set negative fees, to essentially pay senders to help push liquidity out on a channel with excess local liquidity.

Your environment

• version of lnd
  0.13.3-beta

• which operating system (uname -a on *Nix)
  Linux lightning1 5.11.0-1019-raspi 20-Ubuntu SMP PREEMPT Tue Sep 21 15:23:42 UTC 2021 aarch64 aarch64 aarch64 GNU/Linux

• version of btcd, bitcoind, or other backend
  Bitcoin Core version v0.21.1

• any other relevant environment details

Steps to reproduce

See above

Expected behaviour

Negative Fees FTW

Actual behaviour

Error Message
fee rate of -1e-06 is too small, min fee rate is 1e-06

[alexbosworth]

Fees are not signed in the current gossip protocol, maybe you would want to discuss this idea on the protocol level: https://github.com/lightningnetwork/lightning-rfc/blob/master/07-routing-gossip.md#the-channel_update-message

[legendarymartini]

Ok, good to know. Thank you.