Good morning list,

While implementing support for `r` field in invoices, I stumbled upon some 
issues regarding *creating* invoices with `r` fields.

In order to receive via an unpublished channel, we need to know what 
onLightning fees the other side of that channel wants to charge.  We cannot use 
our own onLightning fees because our fees apply if we were forwarding to the 
other side.

However, in case of an unpublished channel, we do not send 
channel_announcement, and in that case we do not send channel_update.  So the 
other side of the channel never informs us of the onLightning fees they want to 
charge if we would receive funds by this channel.

An idea we want to consider is to simply send `channel_update` as soon as we 
lock in the channel: 
https://github.com/ElementsProject/lightning/pull/1330#issuecomment-383931817

I want to ask the other LN implementations (lnd, eclair, ucoin, lit) if we 
should consider standardizing this behavior (i.e. send `channel_update` after 
lockin  regardless of published/unpublished state).  It seems back-compatible: 
software which does not expect this behavior will simply drop the 
`channel_update` (as they do not follow a `channel_announcement`).

In any case, what was the intended way to get the onLightning fee rates to put 
into invoice `r` fields for private routes?

Regards,
ZmnSCPxj
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