This standard defines a protocol enabling precise synchronization of clocks in measurement and control systems implemented with technologies such as network communication@ local computing@ and distributed objects. The protocol is applicable to systems communicating by local area networks supporting multicast messaging including@ but not limited to@ 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 system-wide synchronization accuracy in the sub-microsecond range with minimal network and local clock computing resources. The default behavior of the protocol allows simple systems to be installed and operated without requiring the administrative attention of users. The standard includes mappings to User Datagram Protocol (UDP)/Internet Protocol (IP)@ DeviceNet@ and a layer-2 Ethernet implementation. It includes formal mechanisms for message extensions@ higher sampling rates@ correction for asymmetry@ a clock type to reduce error accumulation in large topologies@ and specifications on how to incorporate the resulting additional data into the synchronization protocol. The standard permits synchronization accuracies better than 1 ns. The protocol has features to address applications where redundancy and security are a requirement. The standard defines conformance and management capability. There is provision to support unicast as well as multicast messaging. The standard includes an annex on recommended practices. Annexes defining communication-medium-specific implementation details for additional network implementations are expected to be provided in future versions of this standard. Purpose Measurement and control applications are increasingly employing distributed system technologies such as network communication@ local computing@ and distributed objects. Many of these applications will be enhanced by having an accurate system-wide sense of time achieved by having local clocks in each sensor@ actuator@ or other system device. Without a standardized protocol for synchronizing these clocks@ it is unlikely that the benefits will be realized in the multivendor system component market. Existing protocols for clock synchronization are not optimum for these applications. For example@ the Network Time Protocol (NTP) targets large distributed computing systems with millisecond synchronization requirements. The protocol in this standard specifically addresses the needs of measurement and control and operational systems in the fields of test and measurement@ industrial automation@ military systems@ manufacturing systems@ power utility systems@ and certain telecommunications applications. These applications need: ?? Spatially localized systems with options for larger systems ??Microsecond to sub-microsecond accuracy ??Administration-free operation ??Applicability for both high-end devices and low-cost@ low-end devices ??Provisions for the management of redundant and fault-tolerant systems Several different application areas such as industrial automation@ telecommunication@ semiconductor manufacturing@ military systems@ and utility power generation have emerged that require the standard to be revised.