NTP distributes time through a hierarchy of primary and secondary servers, with each server adopting a stratum, see NTP system implementation model. A stratum defines how many NTP hops away a particular secondary time server is from an authoritative time source (primary time server) in the synchronization subnet. A stratum 1 time server is located at the top of the hierarchy and is directly attached to an external time source, typically a wire or radio clock; a stratum 2 time server receives its time through NTP from a stratum 1 time server; a stratum 3 time server receives its time through NTP from a stratum 2 time server, and so forth.
Each NTP client in the synchronization subnet chooses as its time source the server with the lowest stratum number with which it is configured to communicate through NTP. This strategy effectively builds a self-organizing tree of NTP speakers. The number of strata is limited to 15 to avoid long synchronization loops.
NTP avoids synchronizing to a remote time server with inaccurate time. NTP never synchronizes to a remote time server that is not itself synchronized. NTP compares the times reported by several remote time servers.