ntp_clock
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 drivers 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. 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 time-
code 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 spe-
cific 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 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 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
configure 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, ver-
sion and ttl options are not used for reference clock support. The
mode option is added for reference 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 max-
poll 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 indi-
vidual 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 pur-
pose. 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 spe-
cial 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 sub-
type in the parse drivers.
minpoll int
maxpoll int
These options specify the minimum and maximum polling
interval for reference clock messages, in seconds to
the power of two. For most directly connected refer-
ence clocks, both minpoll and maxpoll default to 6 (64
s). For modem reference clocks, minpoll defaults to 10
(17.1 m) and maxpoll defaults to 14 (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]
[mode int] [flag1 0|1] [flag2 0|1] [flag3 0|1] [flag4 0|1]
This command can be used to configure reference clocks in spe-
cial ways. It must immediately follow the server command which
configures the driver. Note that the same capability is possi-
ble at run time using the ntpdc program. 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 preci-
sion PPS signal. It also provides a way to correct a
systematic error or bias due to serial port or operat-
ing 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 calibra-
tion for an individual system and driver is available,
an approximate correction is noted in the driver docu-
mentation pages. Note: in order to facilitate cali-
bration when more than one radio clock or PPS signal
is supported, a special calibration feature is avail-
able. It takes the form of an argument to the enable
command described in the Miscellaneous 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 charac-
ters which defines the reference identifier used by
the driver. This string overrides the default identi-
fier ordinarily assigned by the driver itself.
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 sub-
type in the parse drivers.
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)
Primary source of documentation: /usr/share/doc/ntp-*
This file was automatically generated from HTML source.
ntp_clock(5)
