Display fee estimate for on-chain transaction

[tanx]

This would require an additional grpc call estimateFee.

[halseth]

Added issue for the rpc call: lightningnetwork/lnd#1212

[tanx]

Ok thanks. The design displays the fees in satoshis. Should the app calculate that from sat/vbyte or is that something best done on the lnd side?

[tanx]

I mean the design currenty supports only sending to a single address. But how is the app supposed to know how many inputs are required for the amount sent? Seems like something the lnd wallet will need to calculate anyway. So why duplicate that logic in JS?

[halseth]

I see, it totally makes sense not duplicating this in JS. So what should such an RPC interface return, and what information about the transaction is it given?

[tanx]

I would recommend the following:

estimateFee(amount: int64, num_outputs: int32) ->

{
  low_prio: { satoshis: int64, sat_per_byte: int64 },
  medium_prio:  { satoshis: int64, sat_per_byte: int64 },
  high_prio:  { satoshis: int64, sat_per_byte: int64 },
}

If I understand correctly all transactions in lnd are segwit, so the amount (in satoshis) and the number of outputs (in this case 1) should be enough to estimate the fee in satoshis. lnd should be able to implicitly calculate if an additional change output is required if the input amounts don't match the exact output amount?

The return value would be an object that returns both the satoshis (which can be displayed in the UI) as well as the sat_per_byte (which can be set in the SendCoins api https://api.lightning.community/#sendcoins) for low/medium/high priority transactions.

The first design iteration will likely display only the medium priority fees. But this way the rpc api is future proof and allow the user to select.