Ark is a layer 2 protocol for making off-chain bitcoin transactions. Originally published as TBDXXX by Burak on the bitcoin-dev mailing list, it has since been named Ark and the protocol design has advanced significantly. The goal of Ark is to enable bitcoin transactions at very low cost without requiring extensive setup (like opening and funding channels). The Ark model closely resembles bitcoin’s UTXO model. There are currently two implementations of the Ark protocol in development: