1ntp_clock(5) File Formats Manual ntp_clock(5)
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6 ntp_clock - Reference Clock Options
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10 The NTP Version 4 daemon supports some three dozen different radio,
11 satellite and modem reference clocks plus a special pseudo-clock used
12 for backup or when no other clock source is available. Detailed
13 descriptions of individual device drivers and options can be found in
14 the Reference Clock Drivers page. Additional information can be found
15 in the pages linked there, including the Debugging Hints for Reference
16 Clock Drivers and How To Write a Reference Clock Driver pages. In addi‐
17 tion, support for a PPS signal is available as described in Pulse-per-
18 second (PPS) Signal Interfacing page.
19 A reference clock will generally (though not always) be a radio time‐
21 code receiver which is synchronized to a source of standard time such
22 as the services offered by the NRC in Canada and NIST and USNO in the
23 US. The interface between the computer and the timecode receiver is
24 device dependent, but is usually a serial port. A device driver spe‐
25 cific to each reference clock must be selected and compiled in the dis‐
26 tribution; however, most common radio, satellite and modem clocks are
27 included by default. Note that an attempt to configure a reference
28 clock when the driver has not been compiled or the hardware port has
29 not been appropriately configured results in a scalding remark to the
30 system log file, but is otherwise non hazardous.
31 For the purposes of configuration, ntpd treats reference clocks in a
33 manner analogous to normal NTP peers as much as possible. Reference
34 clocks are identified by a syntactically correct but invalid IP
35 address, in order to distinguish them from normal NTP peers. Reference
36 clock addresses are of the form 127.127.t.u, where t is an integer
37 denoting the clock type and u indicates the unit number in the range
38 0-3. While it may seem overkill, it is in fact sometimes useful to con‐
39 figure multiple reference clocks of the same type, in which case the
40 unit numbers must be unique.
41 The server command is used to configure a reference clock, where the
43 address argument in that command is the clock address. The key, version
44 and ttl options are not used for reference clock support. The mode
45 option is added for reference clock support, as described below. The
46 prefer option can be useful to persuade the server to cherish a refer‐
47 ence clock with somewhat more enthusiasm than other reference clocks or
48 peers. Further information on this option can be found in the Mitiga‐
49 tion Rules and the prefer Keyword page. The minpoll and maxpoll options
50 have meaning only for selected clock drivers. See the individual clock
51 driver document pages for additional information.
52 The fudge command is used to provide additional information for indi‐
54 vidual clock drivers and normally follows immediately after the server
55 command. The address argument specifies the clock address. The refid
56 and stratum options control can be used to override the defaults for
57 the device. There are two optional device-dependent time offsets and
58 four flags that can be included in the fudge command as well.
59 The stratum number of a reference clock is by default zero. Since the
61 ntpd daemon adds one to the stratum of each peer, a primary server
62 ordinarily displays an external stratum of one. In order to provide
63 engineered backups, it is often useful to specify the reference clock
64 stratum as greater than zero. The stratum option is used for this pur‐
65 pose. Also, in cases involving both a reference clock and a pulse-per-
66 second (PPS) discipline signal, it is useful to specify the reference
67 clock identifier as other than the default, depending on the driver.
68 The refid option is used for this purpose. Except where noted, these
69 options apply to all clock drivers.
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73 server 127.127.t.u [prefer] [mode int] [minpoll int] [maxpoll int]
74 This command can be used to configure reference clocks in spe‐
75 cial ways. The options are interpreted as follows:
76 prefer Marks the reference clock as preferred. All other
78 things being equal, this host will be chosen for syn‐
79 chronization among a set of correctly operating hosts.
80 See the Mitigation Rules and the prefer Keyword page
81 for further information.
82 mode int
84 Specifies a mode number which is interpreted in a
85 device-specific fashion. For instance, it selects a
86 dialing protocol in the ACTS driver and a device sub‐
87 type in the parse drivers.
88 minpoll int
90 maxpoll int
92 These options specify the minimum and maximum polling
93 interval for reference clock messages in seconds,
94 interpreted as dual logarithms (2 ^ x). For most
95 directly connected reference clocks, both minpoll and
96 maxpoll default to 6 (2^16 = 64 s). For modem reference
97 clocks, minpoll defaults to 10 (2^10 = 1024 s = 17.1 m)
98 and maxpoll defaults to 14 (2^14 = 16384 s = 4.5 h).
99 The allowable range is 4 (16 s) to 17 (36.4 h) inclu‐
100 sive.
101 fudge 127.127.t.u [time1 sec] [time2 sec] [stratum int] [refid string]
104 [flag1 0|1] [flag2 0|1] [flag3 0|1] [flag4 0|1]
105 This command can be used to configure reference clocks in spe‐
106 cial ways. It must immediately follow the server command which
107 configures the driver. Note that the same capability is possi‐
108 ble at run time using the ntpdc program. The options are inter‐
109 preted as follows:
110 time1 sec
112 Specifies a constant to be added to the time offset
113 produced by the driver, a fixed-point decimal number in
114 seconds. This is used as a calibration constant to
115 adjust the nominal time offset of a particular clock to
116 agree with an external standard, such as a precision
117 PPS signal. It also provides a way to correct a system‐
118 atic error or bias due to serial port or operating sys‐
119 tem latencies, different cable lengths or receiver
120 internal delay. The specified offset is in addition to
121 the propagation delay provided by other means, such as
122 internal DIPswitches. Where a calibration for an indi‐
123 vidual system and driver is available, an approximate
124 correction is noted in the driver documentation pages.
125 Note: in order to facilitate calibration when more than
126 one radio clock or PPS signal is supported, a special
127 calibration feature is available. It takes the form of
128 an argument to the enable command described in the Mis‐
129 cellaneous Options page and operates as described in
130 the Reference Clock Drivers page.
131 time2 secs
133 Specifies a fixed-point decimal number in seconds,
134 which is interpreted in a driver-dependent way. See the
135 descriptions of specific drivers in the reference clock
136 drivers page.
137 stratum int
139 Specifies the stratum number assigned to the driver, an
140 integer between 0 and 15. This number overrides the
141 default stratum number ordinarily assigned by the
142 driver itself, usually zero.
143 refid string
145 Specifies an ASCII string of from one to four charac‐
146 ters which defines the reference identifier used by the
147 driver. This string overrides the default identifier
148 ordinarily assigned by the driver itself.
149 flag1 flag2 flag3 flag4
151 These four flags are used for customizing the clock
152 driver. The interpretation of these values, and whether
153 they are used at all, is a function of the particular
154 clock driver. However, by convention flag4 is used to
155 enable recording monitoring data to the clockstats file
156 configured with the filegen command. Further informa‐
157 tion on the filegen command can be found in the Moni‐
158 toring Options page.
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163 ntp.conf(5)
164 HTML documentation in ntp-doc package.
166 This file was automatically generated from HTML source.
168 ntp_clock(5)