[ntp:questions] why do internet servers not poll at longer intervals
David Lord
snews at lordynet.org
Wed Feb 15 10:55:37 UTC 2012
Ron Frazier (NTP) wrote:
> Hi all,
>
> I have my USB GPS working pretty good at this point on both Windows and
> Linux. I have some more Linux specific questions that I'll post later.
> At the moment, I'm working on Windows. As has been suggested by others,
> I know I can get hugely better accuracy using a different GPS and PPS,
> and I plan to experiment with that when I get time and money. (I've
> been ignoring lots of other household things while pursuing this
> project.) For the moment, I just want to get optimum performance out of
> the equipment that I have. For my immediate needs, +/- 6-10 ms accuracy
> is perfectly adequate for what I'm doing.
>
> Here's a recent ntpq -p printout:
>
> C:\Windows\system32>ntpq -p
> remote refid st t when poll reach delay offset
> jitter
> ==============================================================================
>
> *GPS_NMEA(5) .GPS1. 0 l 1 8 377 0.000 -1.181
> 0.977
> +nist1-ny.ustimi .ACTS. 1 u 25 64 377 53.990 3.458
> 6.160
> #216.119.63.113 .ACTS. 1 u 42 64 377 59.998 22.534
> 5.595
> +india.colorado. .ACTS. 1 u 36 64 377 63.986 1.739
> 6.604
> +ping-audit-207- .ACTS. 1 u 50 64 377 83.998 0.715
> 3.206
>
> Note that my GPS is closely synchronized with 3 of the 4 NIST Stratum 1
> servers. I have intentionally set the fudge factor on the GPS so this
> is the case.
>
> Now, here are the relevant server lines in ntp.conf:
>
> server 127.127.20.5 prefer minpoll 3 maxpoll 3 mode 72
> fudge 127.127.20.5 time2 0.2950 refid GPS1
>
> server nist1-ny.ustiming.org prefer minpoll 6 maxpoll 13
> server nist1.columbiacountyga.gov minpoll 6 maxpoll 13
> server utcnist.colorado.edu minpoll 6 maxpoll 13
> server nist1.aol-ca.truetime.com minpoll 6 maxpoll 13
>
> Now, note that the NIST servers are being polled every 64 seconds. This
> thing has been running this way since yesterday evening. However, in
> the configuration file, those servers are allowed to poll up to every 2
> hours. My question is, why are the internet servers being polled so
> often continuously even though I have a stable GPS source available
> which is the currently selected active clock? If I comment out the GPS
> line, the internet servers will eventually go back to polling at longer
> intervals. Note that I don't want to get banned from using any of these
> public servers for hitting them too often.
You are best using servers from the nearest pool.ntp.org group
rather than using distant stratum1 servers.
My own internet connection is 2 Mbit/s adsl. I chose the nearest
from the list of public ntp servers at ntp.org. By nearest I mean
with the lowest delay. I then removed any that were using the
same source. The tools for doing this should be with your ntpd
package.
Note that just now the only stratum-1 source is my own ntp0.
The reason for selection for lowest delay is that internet
connections can be assymetric with route to a server being
different from the route back from the server. I convinced myself
of this by ssh into a remote on a different network and doing
traceroute in both directions. So having selected for lowest
delay that gives the maximum for offset error from that source
as 1/2 of the delay. For my ISP's timeserver the delay is
17.572 msec so maximum possible error is +/- 8.786 msec.
ntpq -c rv -p ntp1:
version="ntpd 4.2.6p5 at 1.2349-o Tue Dec 27 11:31:58 UTC 2011 (1)",
processor="i386", stratum=2, precision=-18, rootdelay=0.426,
rootdisp=36.828, refid=81.187.61.74, Wed, Feb 15 2012 9:14:14.232,
offset=0.237, frequency=0.157, sys_jitter=0.562, clk_jitter=0.101,
clk_wander=0.006, tai=34
remote refid st t poll reach delay offset jitter
==========================================================================
*ntp0.lordynet.o .MSFa. 1 u 256 377 0.426 -0.059 0.058
-xxxxxxxxxxxxxxx 195.66.241.10 2 u 256 377 17.911 2.872 0.640
+xxxxxxxxxxxxxxx 195.66.241.2 2 u 256 377 17.572 0.532 0.355
-xxxxxxxxxxxxxxx 140.203.16.5 2 u 256 377 30.898 2.129 0.332
-xxxxxxxxxxxxxxx 194.66.31.14 2 u 256 377 26.400 7.166 0.423
+xxxxxxxxxxxxxxx 193.62.22.74 2 u 256 377 32.417 0.811 0.315
ntpq -c rv -p ntp0:
version="ntpd 4.2.6p5 at 1.2349-o Tue Dec 27 09:47:55 UTC 2011 (1)",
processor="i386", stratum=1, precision=-18, rootdelay=0.000,
rootdisp=11.479, refid=MSFa, Wed, Feb 15 2012 9:14:35.778,
offset=0.400, frequency=-48.742, sys_jitter=0.341, clk_jitter=0.480,
clk_wander=0.000, tai=34
remote refid st t poll reach delay offset jitter
==========================================================================
*SHM(0) .MSFa. 0 l 512 377 0.000 0.242 0.404
-xxxxxxxxxxxxxxx 81.187.61.74 2 u 256 377 0.425 0.220 0.059
+xxxxxxxxxxxxxxx 195.66.241.10 2 u 512 377 17.630 0.941 0.495
-xxxxxxxxxxxxxxx 131.188.3.222 2 u 512 377 32.280 1.153 0.347
-xxxxxxxxxxxxxxx 193.67.79.202 2 u 512 377 20.314 1.401 1.097
-xxxxxxxxxxxxxxx 194.66.31.14 2 u 512 377 26.781 9.677 0.273
+xxxxxxxxxxxxxxx 195.66.241.2 2 u 512 377 18.267 0.978 0.391
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