This standard defines a network protocol@ the Precision Time Protocol (PTP)@ enabling accurate and precise synchronization of the real-time clocks of devices in networked distributed systems. The protocol is applicable to systems where devices communicate via networks@ including Ethernet. The standard allows multicast communication@ unicast communication or both. The standard specifies requirements for mapping the protocol to specific network implementations and defines such mappings@ including User Datagram Protocol (UDP)/Internet Protocol (IP versions 4 and 6)@ and layer-2 IEEE 802.3 Ethernet. The protocol enables heterogeneous systems that include clocks of various inherent precision@ resolution@ and stability to synchronize to a grandmaster clock. The protocol supports synchronization in the sub-microsecond range with minimal network bandwidth and local clock computing resources. The protocol enhances support for synchronization to better than 1 nanosecond. The protocol specifies how corrections for path asymmetry are made@ if the asymmetry values are known. The grandmaster can be synchronized to a source of time external to the system@ if time traceable to international standards or other source of time is required. The protocol provides information for devices to compute Coordinated Universal Time (UTC) from the protocol distributed time@ if the grandmaster is traceable to international standards and is able to access pending leap second changes. Options are also provided to allow end devices to compute other time scales from the protocol distributed time scale. The protocol defines timing domains in which system timing is consistent. The protocol establishes the timing topology. The default behavior of the protocol allows simple systems to be installed and operated without requiring the administrative attention of users to determine the system timing topology. The standard defines all needed data types@ message formats@ required computations@ internal states@ the behavior of devices with respect to transmitting@ receiving@ and processing protocol communications. The standard provides for the management of protocol artifacts in devices. The standard defines formal mechanisms for message extensions and the requirements for profiles that allow customization for specific application domains. The standard defines conformance requirements. Optional specifications are provided for protocol security. This standard documents conditions under which this standard is backward compatible with IEEE 1588-2008.