What is J1939/21 Transport Protocol? Learn How It Supports More Than 8 Bytes Data Over CAN.J1939 stack, for automotive ECU and tooling applications in commercial vehicles, is an embedded software code with layered architecture and is compliant with J1939 standard. CAN based J1939/21 layer defines the format, sending and reassembling of the packets. It also defines the specifications related to handshake.