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PTP
Sub-microsecond time sync on a LAN, timestamping packets as they cross the wire rather than when software reads them.
The Precision Time Protocol (IEEE 1588) syncs clocks on a LAN far more tightly than NTP. A grandmaster (chosen by the best-master-clock algorithm) sends a Sync message; its exact send time follows in a Follow_Up. The slave replies with a Delay_Req, and the master answers its arrival time in a Delay_Resp. From those four stamps the slave finds both its offset and the path delay, and corrects its clock. Precision comes from stamping each message in the NIC hardware as it leaves and arrives (nanoseconds); software timestamping, in a container without a PTP NIC, is real PTP but only microseconds. NTP, stamping in software over the open internet, is milliseconds.