The purpose of ISO 21324:2016 is to specify the Space Data Link Security Protocol (hereafter referred as the Security Protocol) for CCSDS data links. This protocol provides a security header and trailer along with associated procedures that may be used with the CCSDS Telemetry, Telecommand, and Advanced Orbiting Systems Space Data Link Protocols (references [1]-[3]) to provide a structured method for applying data authentication and/or data confidentiality at the Data Link Layer.