Protocols and time transfer
Stratum
In NTP, the stratum number indicates the distance to the reference clock. Stratum 0 is the physical time source (for example GPS or an atomic clock), stratum 1 is a server that synchronises directly with it, stratum 2 synchronises with a stratum-1 server, and so on.
The maximum is stratum 15; stratum 16 means 'not synchronised'. The lower the stratum, the closer to the source.