Async payments are LN payments that are made when the receiver is offline, are held by a forwarding node (ideally in a trustless manner), and are delivered when the receiver comes back online.

Traditional onchain Bitcoin payments are asynchronous (async) because the receiver can generate an output script (Bitcoin address) and give that address to the spender at any time, and then the spender can pay that address at any time—even when the receiver is offline. The process of securing that payment (receiving block confirmations) doesn’t require any action from the receiver.

For LN, the receiver needs to release a secret at the time a payment is received in order to secure that payment. This requires that both the sender and receiver of a payment both be online at the same time. In many cases, it’s not a significant problem for a spender to be online because they’ve initiated the spending process and can trigger actions to ensure the payment gets sent. But for some receivers, being online to receive a payment is more of a challenge. For example, an LN node running on a mobile phone may be entirely disconnected from the internet some of the time and may not have access to the network other times because the node’s app is running in the background.

A 2021 discussion about improving this user experience led to several ideas about allowing a forwarding node to hold a payment for a receiving node until the receiver was known to be online. The best described trustless method in that discussion required the use of PTLCs, which have not yet been added to LN as of the end of 2022. An alternative method, which could be implemented in the existing protocol, involved the use of trampoline relays.

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