[ntp:questions] Win7: ntpd adjusting time backwards

David Taylor david-taylor at blueyonder.co.uk.invalid
Wed Dec 12 08:15:12 UTC 2012


On 11/12/2012 21:46, David Woolley wrote:
> David Taylor wrote:
>> On 10/12/2012 21:18, David Woolley wrote:
>>> Jeroen Mostert wrote:
>>>
>>>>>
>>>> For what it's worth, after all of that, the offset is steadily
>>>> zigzagging between 27 and 41 ms, which I'm guessing is about the best
>>>> you can hope for on a Windows machine with Internet sync. There have
>>>> not been any major time step adjustments.
>>>
>>> Offsets should be scattered around zero.  If they are all the same sign,
>>> something is wrong.
>>
>> What happens if the link to the Internet is rather asymmetrical?  For
>> example, here I am stuck with 30 Mb/s down, but only 3 Mb/s up.
>
> If there is an actual asymmetry in the delays (the slow uplink may have
> lower delays, because it is unloaded!), the offsets will still be spread
> across zero, but zero will not correspond to the true time.

So, if the PC also has a stratum-1 clock connected (e.g. PPS), to which 
it was synced, then the offsets would be spread across the the non-zero 
difference between "computed true time from the Internet" and UTC, 
wouldn't they?  In this example:

C:\Users\David>ntpq -p narvik
      remote           refid      st t when poll reach   delay   offset 
  jitter
==============================================================================
*pixie           .PPS.            1 u   20   32  377    0.153    0.371 
  0.024
+FEENIX          .PPS.            1 u   25   32  377    0.217    0.308 
  0.018
+Stamsund        .PPS.            1 u   25   32  377    0.221    0.420 
  0.050
  uk.pool.ntp.org .POOL.          16 p    - 1024    0    0.000    0.000 
   0.001
-utserv.mcc.ac.u 194.66.31.14     2 u  800 1024  377   23.998   10.566 
  7.972
-linnaeus.inf.ed 129.215.64.32    2 u    9 1024  377   29.272    3.559 
  3.337
-ntp.uk.syrahost 192.93.2.20      2 u   36 1024  377   27.957    4.717 
  1.551
-cpc2-derb13-2-0 213.248.206.227  2 u  922 1024  377   26.324    3.124 
  3.752
-ntp1.exa-networ 33.117.170.50    2 u  750 1024  377   42.560    4.435 
  1.156
-ntp2.warwicknet 195.66.241.2     2 u  660 1024  377   22.819    4.221 
  4.440

PC Narvik is synced to a local stratum-1 server, and the WAN servers 
listed below the POOL entry, mostly have offsets in the 3-4 millisecond 
region, suggesting (if I understand this correctly) that the delay 
introduced by the asymmetry of the WAN may be in the 3-4 millisecond region.
-- 
Cheers,
David
Web: http://www.satsignal.eu



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