path: root/usr.sbin/ntp/doc/ntp_clock.8

.\"
.\" $FreeBSD$
.\"
.Dd January 12, 2000
.Dt NTP_CLOCK 8
.Os
.Sh NAME
.Nm ntp_clock
.Nd NTP daemon clock options
.Sh SYNOPSIS
.Pa /etc/ntp.conf
.Sh DESCRIPTION
The NTP Version 4 daemon supports many 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
.Qo
Reference Clock Drivers
.Qc
page
(available as part of the HTML documentation
provided in
.Pa /usr/share/doc/ntp ) .
Additional information can be found in the pages referenced there,
including the
.Qo
Debugging Hints for Reference Clock Drivers
.Qc
and
.Qo
How To Write a Reference Clock Driver
.Qc
pages.
In many drivers,
support for a PPS signal is available as described in the
.Qo
Pulse-per-second (PPS) Signal Interfacing
.Qc
page.
Many drivers support special line discipline/streams modules
which can significantly improve the accuracy using the driver.
These are described in the
.Qo
Line Disciplines and Streams Drivers
.Qc
page.
.Pp
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 United States.
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 included
or the hardware port has not been appropriately configured
results in a scalding remark to the system log file,
but is not otherwise hazardous.
.Pp
For the purposes of configuration,
.Xr ntpd 8
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
.Ar t
is an integer denoting the clock type and
.Ar u
indicates the unit number.
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.
.Pp
The
.Ic 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 reference clock
with somewhat more enthusiasm than other reference clocks or peers.
Further information on this option can be found in the
.Qo
Mitigation Rules and the prefer Keyword
.Qc
page.
The minpoll and maxpoll options have meaning
only for selected clock drivers.
See the individual clock driver document pages
for additional information.
.Pp
The stratum number of a reference clock is by default zero.
Since the
.Xr ntpd 8
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 driver.
The refid option is used for this purpose.
Except where noted,
these options apply to all clock drivers.
.Ss Reference Clock Commands
.Bl -tag -width indent
.It Xo Ic server No 127.127. Ns Xo
.Ar t Ns No . Ns Xo
.Ar u
.Op prefer
.Op mode Ar int
.Op minpoll Ar int
.Op maxpoll Ar int
.Xc
.Xc
.Xc
This command can be used to configure reference clocks
in special ways.
The options are interpreted as follows:
.Bl -tag -width indent
.It 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
.Qo
Mitigation Rules and the prefer Keyword
.Qc
page
for further information.
.It mode Ar 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.
.It minpoll Ar int
.It maxpoll Ar 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 allowable range is 4 (16 s) to 17 (36.4 h) inclusive.
.El
.It Xo Ic fudge No 127.127. Ns Xo
.Ar t Ns No . Ns Xo
.Ar u
.Op time1 Ar sec
.Op time2 Ar sec
.Op stratum Ar int
.Op refid Ar string
.Op mode Ar int
.Op flag1 Ar 0 Ns | Ns Ar 1
.Op flag2 Ar 0 Ns | Ns Ar 1
.Op flag3 Ar 0 Ns | Ns Ar 1
.Op flag4 Ar 0 Ns | Ns Ar 1
.Xc
.Xc
.Xc
This command can be used to configure reference clocks
in special ways.
It must immediately follow the
.Ic server
command which configures the driver.
Note that the same capability is possible at run time
using the
.Xr ntpdc 8
program.
The options are interpreted as follows:
.Bl -tag -width indent
.It time1 Ar 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 systematic error
or bias due to serial port 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.
.It time2 Ar 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
.Qo
Reference Clock Drivers
.Qc
page.
.It stratum Ar 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.
.It refid Ar string
Specifies an ASCII string 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.
.It mode Ar 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.
.It 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
.Ic filegen
command.
When a PPS signal is available,
a special automatic calibration facility is provided.
If the flag1 switch is set
and the PPS signal is actively disciplining the system time,
the calibration value is automatically adjusted
to maintain a residual offset of zero.
Further information on the
.Ic filegen
command can be found in the
.Xr ntp_mon 8
page.
.El
.It Ic pps device [assert|clear] [hardpps]
Specifies the name and options for the serial port device
to which the PPS signal is connected.
Note, this command replaces use of fudge flag3,
which was used for the same purpose in NTPv3.
Note that this command should preceed the
.Ic server
and
.Ic fudge
commands for the same device.
Note also that the assert,
clear and hardpps options are only available
if the ppsapi standard PPS interface is available.
.Bl -tag -width indent
.It device
Specify the device name associated with the PPS signal.
The name must match exactly the link name specified
in the driver documentation page.
.Ic assert
.Ic clear
Using assert or clear specifies
if the high going or low going edge
of the signal must be used.
The default is assert.
.Ic hardpps
This flag is used to tell the kernel that the signal
from this device must be used to drive hardpps().
The assert, clear and hardpps options are only available
if the PPSAPI is used.
.El
.El
.Sh SEE ALSO
.Xr ntp_conf 8 ,
.Xr ntpd 8 ,
.Xr ntpdc 8 ,
.Xr ntpq 8
.Pp
In addition to the manual pages provided,
comprehensive documentation is available on the world wide web
at
.Li http://www.ntp.org/ .
A snapshot of this documentation is available in HTML format in
.Pa /usr/share/doc/ntp .
.Sh HISTORY
Written by
.An Dennis Ferguson
at the University of Toronto.
Text amended by
.An David Mills
at the University of Delaware.