1ntp_clock(5)                  File Formats Manual                 ntp_clock(5)
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NAME

6       ntp_clock - Reference Clock Options
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REFERENCE CLOCK SUPPORT

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
20       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.
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32       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.
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42       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.
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53       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
60       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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REFERENCE CLOCK COMMANDS

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
77               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.
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83               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.
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89               minpoll int
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91               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.
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103       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:
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111               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.
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132               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.
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138               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.
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144               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
150               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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SEE ALSO

163       ntp.conf(5)
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165       HTML documentation in ntp-doc package.
166
167       This file was automatically generated from HTML source.
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