The NTP Version 3 daemon currently supports several different radio, satellite and modem reference clocks plus a special pseudo-clock used for backup or when no other clock source is available. Detailed descriptions of individual device drivers and options can be found in the Reference Clock Drivers page. Additional information can be found in the pages referenced there, including the Debugging Hints for Reference Clock Drivers and How To Write a Reference Clock Driver pages. In many drivers, support for a PPS signal is available as described in Pulse-per-second (PPS) Signal Interfacing page. Many drivers support special line discipline/streams modules which can significantly improve the accuracy using the driver. These are described in the Line Disciplines and Streams Drivers page.
A reference clock will generally (though not always) be a radio timecode receiver which is synchronized to a source of standard time such as the services offered by the NRC in Canada and NIST and USNO in the U.S. The interface between the computer and the timecode receiver is device dependent and will vary, but is often a serial port. A device driver specific to each clock must be selected and compiled in the distribution; however, most common radio, satellite and modem clocks are included by default. Note that an attempt to configure a reference clock when the driver has not been included or the hardware port has not been appropriately configured results in a scalding remark to the system log file, but is otherwise non hazardous.
For the purposes of configuration,
reference clocks in a manner analogous to normal NTP peers as much as
possible. Reference clocks are identified by a syntactically correct but
invalid IP address, in order to distinguish them from normal NTP peers.
Reference clock addresses are of the form
t is an
integer denoting the clock type and
u indicates the
type-specific unit number.
server command is used to configure a reference clock,
address argument in that command is the
clock address. The
ttl options are not used for reference clock support. The
prefer option can be useful to persuade the server to
cherish a reference clock with somewhat more enthusiasm than other
reference clocks or peers. Further information on this option can be
found in the Mitigation Rules and the
prefer Keyword page. Some reference clocks require
additional options to further specify the operation.
The stratum of a reference clock is by default zero. Since the
xntpd daemon adds one to the stratum of each peer, a
primary server ordinarily displays stratum one. In order to provide
engineered backups, it is often useful to specify the reference clock
stratum as greater than zero. The
stratum option is used
for this purpose. Also, in cases involving both a reference clock and a
pulse-per-second (PPS) discipline signal, it is useful to specify the
reference clock identifier as other than the default, depending on the
refid option is used for this purpose. Except
where noted, these options apply to all clock drivers.
fudge 127.127.t.u [ time1 secs ] [ time2
secs ] [ stratum int ] [ refid string ] [ mode
int ] [ flag1 0 | 1 ] [ flag2 0 | 1 ] [ flag3 0 | 1] [ flag4 0 |
servercommand which configures the driver. Note that the same capability is possible at run time using the
xntpdcprogram. The options are interpreted as follows:
flag3is used to attach the
ppsclockstreams module to the configured driver, while
flag4is used to enable recording verbose monitoring data to the
clockstatsfile configured with the
filegencommand. Further information on the
ppsclockstreams module can be found in the Pulse-per-second (PPS) Signal Interfacing page. Further information on the
filegencommand can be found in the Monitoring Options page.