built timeout trees 

 Out of round payments 

 OOR Payments changes the trust model, as they rely on at least one paritcipant being honest and deleting the key (used to hold the pre-signed exit tree).

https://arkdev.info/docs/learn/payments-oor

That being said, normal Ark payment (using the so called Ruber shortcut) are already async.

https://arkdev.info/docs/learn/payments 

 Payment in Ark are very similar to ecash, in the sense the Sender and ASP co-signing and delegating the recipient to claim the payment once it comes online, basically a self-transfer. If all the senders are also recipient, there is no online requirement.

https://arkdev.info/docs/learn/payments