Good morning Thomas,
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‐‐‐‐‐‐‐ Original Message ‐‐‐‐‐‐‐
On March 19, 2018 6:24 AM, Thomas Steenholdt
<tsteenho...@cascadetechnologypartners.com> wrote:
> Hi,
>
> I've been trying to figure out the mechanics of Lightning fees, especially in
> the case of routed payments. Unfortunately, I haven't had any success in
> finding a high level description on the topic.
>
> I'm hoping somebody is able to point me in the right direction?
BOLT spec https://github.com/lightningnetwork/lightning-rfc contains
everything, but is very detailed and contains the topic in multiple places.
> Example:
>
> A multi-hop routed payment where A needs to pay D through B and C.
> Established channels are A -> B -> C -> D.
>
> What I'm looking for is a high level explanation of how fees are established,
> announced and ultimately claimed in a payment like this. Some of the
> questions that come to mind are:
>
> - Does A know ahead of time the fees on B and C, or only when trying to set
> up the payment? And how?
Yes. Node gossip, the `channel_update` message in BOLT#7. This message,
contains `fee_base_msat` and `fee_proportional_millionths`. For each channel,
there are two `channel_update` messages, one from each direction. For example
B<->C channel, B announces its fee for B->C transfers while C announnces its
fee for C->B transfers.
The A may have obsolete information about fees (e.g. B or C change their fee
but their `channel_update` has not propagated to A yet). In this case, payment
routing will fail, but the `channel_update` will also be sent as part of the
error message returned by payment routing failure.
> - How does A know the amount of fees that need to be added to the payment to
> cover all fees?
It computes it. If D is to be given a payment with value `msatoshi` then it
computes first the C->D fee, which is the C->D `fee_base_msat` + (C->D
`fee_proportion_millionts` * `msatoshi` / 1,000,000). Add that to `msatoshi`
and that is the payment that needs to reach C, so A computes the payment from
B->C similarly, except the `msatoshi` is replaced with the payment that should
reach C. Then A knows how much it has to give to B.
> - Is D aware of the full amount including fees or is that somehow hidden?
No. D is only aware of how much C offers it.
> - How are the fees actually claimed (who ends up paying whom)?
A offer B a value that is higher than what A instruct B to forward to C. The
difference is the fee. Since the highest value is at the source A, A is the
one, who ends up paying the entire fee.
Regards,
ZmnSCPxj
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