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BOLT4: don't allow a "fee" for the final node(failure messages) #718

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t-bast merged 1 commit into from
Jan 8, 2020
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BOLT4: don't allow a "fee" for the final node(failure messages) #718

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2 changes: 1 addition & 1 deletion 04-onion-routing.md
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Expand Up @@ -1076,7 +1076,7 @@ An _intermediate hop_ MUST NOT, but the _final node_:
- if the `outgoing_cltv_value` does NOT correspond with the `cltv_expiry` from
the final node's HTLC:
- MUST return `final_incorrect_cltv_expiry` error.
- if the `amt_to_forward` is greater than the `incoming_htlc_amt` from the
- if the `amt_to_forward` does NOT correspond with the `incoming_htlc_amt` from the
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@niftynei niftynei Jan 6, 2020

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what's the rationale for having these match? as long as it the amt_to_forward does not exceed the incoming_htlc_amt there's no problem.

i'm not 100% sure about this, but i believe a few use cases currently take advantage of this to allow for paying nodes via fees rather than invoice amounts.

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@nueno4 nueno4 Jan 7, 2020

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Thank you for your comment.
I read the following sentence in BOLT4:

For the final node, this value MUST be exactly equal to the incoming htlc amount, otherwise the HTLC should be rejected.

and understood as follow:

non final node:
  incoming_htlc_amt - fee >= amt_to_forward

final node:
  incoming_htlc_amt == amt_to_forward

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@t-bast t-bast Jan 7, 2020

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I understood it like you @nayuta-ueno, IIUC the change you propose should have been made at the same time as #711 but was overlooked?

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@niftynei niftynei Jan 7, 2020

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oh i see the context now. thanks.

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@niftynei niftynei Jan 7, 2020

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Oh I missed the link to #711, thanks for the clarification.

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@nueno4 nueno4 Jan 8, 2020

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@t-bast

the change you propose should have been made at the same time as #711 but was overlooked?

I think so too.

final node's HTLC:
- MUST return a `final_incorrect_htlc_amount` error.

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