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 addi-
tion, 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 dis-
tribution; 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 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 reference clock support, as described below. The
prefer option can be useful to persuade the server to cherish a refer-
ence clock with somewhat more enthusiasm than other reference clocks or
peers. Further information on this option can be found in the Mitiga-
tion 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 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 reference
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 allow-
able 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 inter-
preted 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 correct a system-
atic error or bias due to serial port or operating sys-
tem 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 indi-
vidual 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 Mis-
cellaneous 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 identifier
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 informa-
tion on the filegen command can be found in the Moni-
toring Options page.
SEE ALSO
ntp.conf(5)
Primary source of documentation: /usr/share/doc/ntp-*
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ntp_clock(5)
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