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ntp_clock(5) ntp_clock(5)
NAME
ntp_clock - Reference Clock Options
REFERENCE CLOCK SUPPORT
The NTP Version 4 daemon supports some three dozen 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 driv-
ers and options can be found in the Reference Clock Drivers page. Additional information can be found in the pages linked
there, including the Debugging Hints for Reference Clock Drivers and How To Write a Reference Clock Driver pages. In
addition, support for a PPS signal is available as described in Pulse-per-second (PPS) Signal Interfacing 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 US. The interface between the
computer and the timecode receiver is device dependent, but is usually a serial port. A device driver specific to each
reference 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 compiled 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, ntpd treats reference clocks in a manner analogous to normal NTP peers as much as pos-
sible. 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 127.127.t.u, where t is an integer denoting the clock
type and u indicates the unit number in the range 0-3. While it may seem overkill, it is in fact sometimes useful to con-
figure multiple reference clocks of the same type, in which case the unit numbers must be unique.
The server command is used to configure a reference clock, where the address argument in that command is the clock
address. The key, version and ttl options are not used for reference clock support. The mode option is added for refer-
ence clock support, as described below. 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. The minpoll and maxpoll options have meaning only for selected clock
drivers. See the individual clock driver document pages for additional information.
The fudge command is used to provide additional information for individual clock drivers and normally follows immediately
after the server command. The address argument specifies the clock address. The refid and stratum options control can be
used to override the defaults for the device. There are two optional device-dependent time offsets and four flags that
can be included in the fudge command as well.
The stratum number of a reference clock is by default zero. Since the ntpd daemon adds one to the stratum of each peer, a
primary server ordinarily displays an external stratum of 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 driver. The refid option is used for this purpose. Except
where noted, these options apply to all clock drivers.
REFERENCE CLOCK COMMANDS
server 127.127.t.u [prefer] [mode int] [minpoll int] [maxpoll int]
This command can be used to configure reference clocks in special ways. The options are interpreted as follows:
prefer Marks the reference clock as preferred. All other things being equal, this host will be chosen for syn-
chronization among a set of correctly operating hosts. See the Mitigation Rules and the prefer Keyword
page for further information.
mode int
Specifies a mode number which is interpreted in a device-specific fashion. For instance, it selects a
dialing protocol in the ACTS driver and a device subtype in the parse drivers.
minpoll int
maxpoll int
These options specify the minimum and maximum polling interval for reference clock messages in seconds,
interpreted as dual logarithms (2 ^ x). For most directly connected reference clocks, both minpoll and
maxpoll default to 6 (2^16 = 64 s). For modem reference clocks, minpoll defaults to 10 (2^10 = 1024 s =
17.1 m) and maxpoll defaults to 14 (2^14 = 16384 s = 4.5 h). The allowable range is 4 (16 s) to 17 (36.4
h) inclusive.
fudge 127.127.t.u [time1 sec] [time2 sec] [stratum int] [refid string] [flag1 0|1] [flag2 0|1] [flag3 0|1] [flag4 0|1]
This command can be used to configure reference clocks in special ways. It must immediately follow the server
command which configures the driver. Note that the same capability is possible at run time using the ntpdc pro-
gram. The options are interpreted as follows:
time1 sec
Specifies a constant to be added to the time offset produced by the driver, a fixed-point decimal number
in seconds. This is used as a calibration constant to adjust the nominal time offset of a particular
clock to agree with an external standard, such as a precision PPS signal. It also provides a way to cor-
rect a systematic error or bias due to serial port or operating system latencies, different cable lengths
or receiver internal delay. The specified offset is in addition to the propagation delay provided by
other means, such as internal DIPswitches. Where a calibration for an individual system and driver is
available, an approximate correction is noted in the driver documentation pages. Note: in order to
facilitate calibration when more than one radio clock or PPS signal is supported, a special calibration
feature is available. It takes the form of an argument to the enable command described in the Miscella-
neous Options page and operates as described in the Reference Clock Drivers page.
time2 secs
Specifies a fixed-point decimal number in seconds, which is interpreted in a driver-dependent way. See
the descriptions of specific drivers in the reference clock drivers page.
stratum int
Specifies the stratum number assigned to the driver, an integer between 0 and 15. This number overrides
the default stratum number ordinarily assigned by the driver itself, usually zero.
refid string
Specifies an ASCII string of from one to four characters which defines the reference identifier used by
the driver. This string overrides the default identifier ordinarily assigned by the driver itself.
flag1 flag2 flag3 flag4
These four flags are used for customizing the clock driver. The interpretation of these values, and
whether they are used at all, is a function of the particular clock driver. However, by convention flag4
is used to enable recording monitoring data to the clockstats file configured with the filegen command.
Further information on the filegen command can be found in the Monitoring Options page.
SEE ALSO
ntp.conf(5)
HTML documentation in ntp-doc package.
This file was automatically generated from HTML source.
ntp_clock(5)
