•         As client it queries the reference time from one        or more servers.
•         As server it makes its own time available as        reference time for other clients.
•         As peer it compares its system time to other peers        until all the peers finally agree about the “true” time        to synchchronize to.

These features can be used to set up a hierarchical time        synchronization structure.        The hierarchical levels of the time synchronization structure        are called stratum levels. A smaller stratum        number means a higher level in the hierarchy structure.        On top of the hierarchy there is the daemon which has the most        accurate time and therefore the smallest stratumnumber.

By default, a daemon’s stratum level is always one level        below the level of its reference time source.        The top level daemon often uses a radio clock as reference        time source. By default, radio clocks have a stratum number        of 0, so a daemon who uses that radio clock as reference time will        be a stratum 1 time server, which has the highest        priority level in the NTP hierarchy.        In large networks it is a good practice to install one ore more        stratum 1 time servers which make a reference time available        to several server computers in each department.        Thus the servers in the departments become stratum 2 time servers        which can be used as reference time source for workstations and        other network devices of the department.

Unlike in telecom applications where the word stratum is used        e.g. to classify oscillators according to their absolute accuracy,        the term stratum in the NTP context does not indicate a certain        class of accuracy, it’s just an indicator of the hierarchy level.