[Lightning-dev] Link-level payment splitting via intermediary rendezvous nodes

[ZmnSCPxj]

Good morning list,

As was discussed directly in summit, we accept link-lvel payment splitting (scid is not binding), and provisionally accept rendez-vous routing.

It strikes me, that even if your node has only a single channel to the next node (c-lightning), it is possible, to still perform link-level payment splitting/re-routing.

For instance, consider this below graph:

      E<---D--->C<---B
           ^  /
           | /
           |L
           A

In the above, B requests a route from B->C->D->E.

However, C cannot send to D, since the channel direction is saturated in favor of D.

Alternately, C can route to D via A instead.  It holds the (encrypted) route from D to E.  It can take that sub-route and treat it as a partial route-to-payee under rendez-vous routing, as long as node A supports rendez-vous routing.

This can allow re-routing or payment splitting over multiple hops.

Even though C does not know the number of remaining hops between D and the destination, its alternative is to earn nothing anyway as its only alternative is to fail the routing.  At least with this, there is a chance it can succeed to send the payment to the final destination.

Regards,
ZmnSCPxj
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