A fulfillment payload is an encrypted message the recipient of a message can send back to the sender. Like failure messages, the fulfillment_payload message is relayed back to the sender through the payment’s route. The sender of the payment, who recevied the message, can check it originated from the recipient, and that it was not tampered with by intermediate nodes.

One of the primary use cases for fulfillment payloads is to provide proof of payment for spontaneous (e.g. keysend) payments, where the preimage cannot serve as proof since it’s sender-generated. Another potential use case could be Lightning-native content delivery, conditioned by payment, but the 32 KiB maximum size of such messages constrains this use case.

Ressources

• Bolt 4 “Successful Payments” section
• Bolts Pull Request 1344