BOLT4: unclear handling of invalid onions at final node

[f321x]

In BOLT 4 it is written that only forwarding nodes, but not final nodes, should fail an htlc e.g. for an invalid hmac or onion version. Why should the final node not fail a htlc with these errors if they receive an invalid onion? Should final nodes use some different error type or is this an issue of the spec?

bolts/04-onion-routing.md

Lines 1333 to 1339 in 14272b1

	A _forwarding node_ MAY, but a _final node_ MUST NOT:
	- if the onion `version` byte is unknown:
	- return an `invalid_onion_version` error.
	- if the onion HMAC is incorrect:
	- return an `invalid_onion_hmac` error.
	- if the ephemeral key in the onion is unparsable:
	- return an `invalid_onion_key` error.

[t-bast]

You're right, I think that those requirements shouldn't be in that section: AFAIK all implementations today do send those errors even when they're the final node.

[Roasbeef]

i think only the first 3 should be moved to another section.

The others in that section contain errors that the second-to-last node can/should send, but the final node shouldn't. Some examples:

  - if the receiving peer specified by the onion is NOT known:
    - return an `unknown_next_peer` error.
  - if the HTLC amount is less than the currently specified minimum amount:
    - report the amount of the outgoing HTLC and the current channel setting for
    the outgoing channel.
    - return an `amount_below_minimum` error.
  - if the HTLC does NOT pay a sufficient fee:
    - report the amount of the incoming HTLC and the current channel setting for
    the outgoing channel.
    - return a `fee_insufficient` error.

All of these can be sent by the penultimate node, but it doesn't make sense for the destination to send them.