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When you set a routing fee on your Lightning node there are two fees you can set: a fixed base fee (non-proportional to the amount being transferred) and a variable proportional fee.

What is the purpose of the base fee? Should a routing node operator set it to zero? Should it be set to zero by default in implementations and/or removed from the spec entirely?

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This was discussed on a c-lightning developer call on January 10th 2022. AJ Towns also discussed it on the lightning-dev mailing list.

The base fee was introduced back in 2016 after a very short whiteboard discussion between protocol developers. lnd currently sets a default base fee of 1 satoshi and c-lightning currently sets a default base fee of 1 millisatoshi.

The base fee adds a non-linear factor which is complicating ongoing research by Rene Pickhardt and others on optimal path finding.

According to Rene’s research it would be much nicer to have just the proportional size. That brings us to a cost function when we compute flows and paths where everything is proportional to the size of the payment that massively simplifies the computations.

during the reformulations for the calculation of the min cost flow these these fall out as remainders and accumulate, basically just mess everything up

To utilize Rene's research routing node operators could set the base fee to zero, it could be set by default to zero in individual Lightning implementations or theoretically it could be taken out of the spec entirely.

There are some arguments against the latter. Some consider the base fee as commensurate to the non-proportional risk of routing payments. Even the smallest HTLC above the dust threshold can end up causing a channel closure.

Another concern is that if implementations set a default base fee of zero then routing algorithms will assume zero and start filtering out routing nodes that set a non-zero base fee.

However, if the base fee set across the network is negligible but still non-zero researchers may still choose to ignore it in their search for optimal path finding algorithms.

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