ntp_clock(5) - phpMan

ntp_clock(5) File Formats Manual 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.

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 com-
mon 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. Refer-
ence 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, version and ttl options are not used for ref-
erence 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 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 synchronization 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 refer-
ence 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 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 cali-
bration 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 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 cali-
bration 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 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 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 interpre-
tation 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.