[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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