Allow single-part payments to trampoline nodes

We don't actually need to use MPP between trampoline nodes: a single
part payment could work. We're thus removing the requirement to always
provide the MPP field in the outer onion.

Always using MPP (even for single-part payments) simplified our eclair
implementation, but other implementations may work differently, so it
shouldn't impact the spec.

Author: t-bast

04-onion-routing.md

@@ -513,7 +513,9 @@ A sending node:
     - MUST include `outgoing_node_id`.
 - MUST use a different `session_key` for the `trampoline_onion_packet` and the `onion_packet`.
 - MUST include the `trampoline_onion_packet` tlv in the _last_ hop's payload of the `onion_packet`.
-- MUST generate a random `payment_secret` to use in the outer onion.
+- If it sends a multi-part payment:
+  - MUST generate a random `payment_secret` to use in the outer onion.
+  - MUST NOT use the invoice's `payment_secret` in the outer onion.
 
 When processing a `trampoline_onion_packet`, a receiving node:
 

09-features.md

@@ -51,7 +51,7 @@ The Context column decodes as follows:
 | 46/47 | `option_scid_alias`               | Supply channel aliases for routing                        | IN       |                           | [BOLT #2][bolt02-channel-ready]                                       |
 | 48/49 | `option_payment_metadata`         | Payment metadata in tlv record                            | 9        |                           | [BOLT #11](11-payment-encoding.md#tagged-fields)                      |
 | 50/51 | `option_zeroconf`                 | Understands zeroconf channel types                        | IN       | `option_scid_alias`       | [BOLT #2][bolt02-channel-ready]                                       |
-| 56/57 | `trampoline_routing`              | This node supports trampoline routing                     | IN9      | `basic_mpp`               | [BOLT #4](bolt04-trampoline)                                          |
+| 56/57 | `trampoline_routing`              | This node supports trampoline routing                     | IN9      |                           | [BOLT #4](bolt04-trampoline)                                          |
 
 ## Requirements