Other related network tools were available both then and now.They include the Daytime and Time protocols for recording the time of events, as well as the ICMP Timestamp and IP Timestamp option (RFC 781).NTP uses a hierarchical, semi-layered system of time sources.Each level of this hierarchy is termed a stratum and is assigned a number starting with zero for the reference clock at the top.Stratum is not always an indication of quality or reliability; it is common to find stratum 3 time sources that are higher quality than other stratum 2 time sources.These are high-precision timekeeping devices such as atomic clocks, GPS or other radio clocks.
Stratum 0 devices are also known as reference clocks.
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Network Time Protocol (NTP) is a networking protocol for clock synchronization between computer systems over packet-switched, variable-latency data networks. NTP is intended to synchronize all participating computers to within a few milliseconds of Coordinated Universal Time (UTC).
A server synchronized to a stratum n server runs at stratum n 1.
The number represents the distance from the reference clock and is used to prevent cyclical dependencies in the hierarchy.