The LINX protocol (previously called RLNH protocol) is designed to be light-weight and efficient. The RLNH-to-RLNH control PDUs are described in detail below. The required overhead associated with user signal transmission is not carried in any dedicated RLNH PDU. Instead it is passed as arguments along with the signal data to the Connection Manager layer, where an optimized transmission scheme and message layout can be implemented based on knowledge of the underlying media and/or protocols. In particular the source and destination link addresses are sent this way, the addresses identifies the sending and receiving endpoints respectively. LINX protocol messages are sent with source and destination link addresses set to zero (0).