Controller Area Network (CAN) has been the de facto standard in the automotive industry for the past two decades, and recently CAN with flexible data-rate has been standardized, which achieves higher speed. Further improvements are possible by exploiting the peculiar physical layer of CAN to perform distributed operations among network nodes. In this paper a proposal is made for an extension to the CAN protocol, denoted CAN with eXtensible in-frame Reply (CAN XR), which enables upper protocol layers to define new custom services devoted to, e.g, network management, applicationspecific functions, and high-efficiency data transfer. Since CAN XR retains full backward compatibility with CAN FD, there is no need to change the protocol specification.