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Allow nodes to overshoot the MPP total_msat when paying #1031

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Nov 8, 2022
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Allow nodes to overshoot the MPP total_msat when paying #1031

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12 changes: 10 additions & 2 deletions 04-onion-routing.md
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Expand Up @@ -292,7 +292,7 @@ The writer:
- otherwise:
- MUST set `total_msat` to the amount it wishes to pay.
- MUST ensure that the total `amount_msat` of the HTLC set which arrives at the payee
is equal to `total_msat`.
is equal to or greater than `total_msat`.
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@joostjager joostjager Nov 8, 2022 •
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Now that we have #1032, shouldn't amount_msat be changed to the final amt_to_forward? To ensure that the sender remains in control of the set total?

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We could, that's a good point!

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@joostjager joostjager Nov 8, 2022 •
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I suppose it is still at the discretion of the receiver, but a premature release of the preimage may cause the receiver to receive less (although still meeting total_msat). With a premature release, routing nodes may short-cut pending htlcs.

So incentive-compatible is to only count the minimum towards total_msat, which is amt_to_forward?

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Yes that makes sense, don't hesitate to open a follow-up PR to fix that!

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Same discussion as we had here? #1032 (comment)

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Yes, but perhaps new is that we want to at least recommend the most incentive-compatible way in the spec?

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I agree, I opened #1040 to fix that.

- MUST NOT send another HTLC if the total `amount_msat` of the HTLC set is already greater or equal to `total_msat`.
- MUST include `payment_secret`.
- otherwise:
Expand All @@ -307,7 +307,8 @@ The final node:
- MUST add it to the HTLC set corresponding to that `payment_hash`.
- SHOULD fail the entire HTLC set if `total_msat` is not the same for
all HTLCs in the set.
- if the total `amount_msat` of this HTLC set equals `total_msat`:
- if the total `amount_msat` of this HTLC set is equal to or greater than
`total_msat`:
- SHOULD fulfill all HTLCs in the HTLC set
- otherwise, if the total `amount_msat` of this HTLC set is less than
`total_msat`:
Expand All @@ -333,6 +334,13 @@ it simplifies adding noise to the amount when splitting, as well as
scenarios in which the senders are genuinely independent (friends
splitting a bill, for example).

Because a node may need to pay more than its desired amount (due to the
`htlc_minimum_msat` value of channels in the desired path), nodes are allowed
to pay more than the `total_msat` they specified. Otherwise, nodes would be
constrained in which paths they can take when retrying payments along specific
paths. However, no individual HTLC may be for less than the difference between
the total paid and `total_msat`.

The restriction on sending an HTLC once the set is over the agreed total prevents the preimage being released before all
the partial payments have arrived: that would allow any intermediate
node to immediately claim any outstanding partial payments.
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