What is NTP Server Stratum?
Large computer networks can have many devices acting as NTP servers. Where multiple NTP sources are available, NTP clients need some way of (a) judging which time sources are likely to be the most accurate, and (b) preventing timing loops*. The NTP protocol achieves these aims by including a simple measure of the synchronisation distance from the primary time source. This is known as the Stratum level.
Stratum 0 devices (including atomic and gps clocks) are the most accurate, but cannot be connected to via a network connection. A Stratum 0 device is usually used as a reference clock (or synchronisation source) for a Stratum 1 time server.
Stratum 1 devices are the most accurate ntp time sources accessible via a network connection. A Stratum 1 device would normally be synchronised via a Stratum 0 reference clock.
Stratum 2 devices are synchronised from a Stratum 1 device across a network connection. Due to network jitter and delays, Stratum 2 servers are not as accurate as Stratum 1 time servers. An NTP client synchronised from a Stratum 2 source would be a Stratum 3 device etc...