[ntp:questions] Re: NTP broadcastclient update times?
Richard B. Gilbert
rgilbert88 at comcast.net
Fri Oct 15 21:22:59 UTC 2004
W. D. wrote:
>Harlan Stenn wrote:
>
>
>>What is ...2.122?
>>
>>
>
>FreeBSD / NTP time server
>
>
>
>
>>How many machines is it sync'd to?
>>
>>
>
>See below.
>
>
>
>
>>What OS?
>>
>>
>
>FreeBSD 4.9
>
>
>
>
>>How about an ntpq -p against 2.122?
>>
>>
>
>ntpq -p
>
> remote refid st t when poll reach delay offset
>jitter
>==============================================================================
>-now.cis.okstate .PSC. 1 u 307 1024 377 226.292 -46.779
>5.793
>+navobs1.wustl.e .USNO. 1 u 993 1024 377 196.782 -29.412
>0.498
>*clock.xmission. .GPS. 1 u 914 1024 377 198.391 -31.001
>2.665
>+bonehed.lcs.mit .CDMA. 1 u 853 1024 377 194.764 -29.863
>0.985
>-clock.via.net .GPS 1 u 303 1024 377 227.888 -47.328
>7.590
>-otc1.psu.edu .WWV. 1 u 874 1024 377 199.845 -48.614
>3.173
>+timekeeper.isi. .GPS. 1 u 38 1024 377 231.352 -30.935
>528.528
>-ntp-cup.externa .GPS. 1 u 318 1024 377 233.925 -35.138
>7.098
>-time-B.timefreq .ACTS. 1 u 41 1024 377 231.446 -53.467
>280.040
>-clock.isc.org clepsydra.dec.c 2 u 307 1024 377 227.118 -34.082
>7.550
> 192.168.2.255 .BCST. 16 u - 64 0 0.000 0.000
>4000.00
>
>
There are two very notable things about this output.
First, you have extremely high round trip delays to all the servers you
are using. This can be caused by network congestion or by very long
network paths between your site and the servers. In addition to
physical distance you must also consider the number of routers the
packets pass through; each one adds some delay in each direction.
Given a private wire from the east coast of the US to the west cost, the
speed of light delay should be less than thirty milliseconds so you are
seeing something more than simple distance.
Finding servers closer to you in net space should substantially reduce
the delay and also the offsets. Assuming that the servers have the
correct time in first place, the error/uncertainty is bounded by the delay.
With delays over 200ms, ntp is essentially guessing what time it is.
The second notable thing is that you are using numerous stratum 1
servers and, unless you are serving several hundred clients, you should
not be. A few stratum two servers much closer (lower delay) should give
you much better performance and accuracy.
If your network situation is such that there are no available servers
with delays of less than 25 milliseconds, you should probably consider a
hardware reference clock such as a GPS receiver.
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