[ntp:questions] Re: NTP broadcastclient update times?
NewsGroups at US-Webmasters.com
Sat Oct 16 05:36:33 UTC 2004
Richard B. Gilbert wrote:
> 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
> >-now.cis.okstate .PSC. 1 u 307 1024 377 226.292 -46.779
> >+navobs1.wustl.e .USNO. 1 u 993 1024 377 196.782 -29.412
> >*clock.xmission. .GPS. 1 u 914 1024 377 198.391 -31.001
> >+bonehed.lcs.mit .CDMA. 1 u 853 1024 377 194.764 -29.863
> >-clock.via.net .GPS 1 u 303 1024 377 227.888 -47.328
> >-otc1.psu.edu .WWV. 1 u 874 1024 377 199.845 -48.614
> >+timekeeper.isi. .GPS. 1 u 38 1024 377 231.352 -30.935
> >-ntp-cup.externa .GPS. 1 u 318 1024 377 233.925 -35.138
> >-time-B.timefreq .ACTS. 1 u 41 1024 377 231.446 -53.467
> >-clock.isc.org clepsydra.dec.c 2 u 307 1024 377 227.118 -34.082
> > 192.168.2.255 .BCST. 16 u - 64 0 0.000 0.000
> 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.
It is a very long path to (and through) my ISP. Ping
times average 140 ms at the 'front door' and 160 ms
out the 'back door' to the rest of the Internet.
By the way, I am so out in the boonies that I am
using dial-up. They keep promising broadband, but
aren't delivering. That's probably the main reason for
the long latency. Also, these ping times are when
I am not doing anything else on the 'Net. They
are going to be much more extreme when I'm using
most of my 40 kbps for other tasks.
>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
Hmmm. 186,000 miles / second @ 3000 miles = 16.1 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.
Yep. I averaged 10 pings in choosing these servers.
>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.
But isn't NTP 'guessing' no matter what the delay is? Isn't that the
beauty of NTP? No matter what the delay is, it will intelligently
'guess' what the time *really* is, by factoring in that delay, right?
> 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.
I'll research to see if I can cut down the delay times by
using these stratum 2 servers:
> 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.
Well, I was trying to keep this as simple as possible. I called
Synergy Systems (http://www.Synergy-GPS.com/) today and they
said that I would need to spend at least $400.00 for a GPS
setup. I'd rather not spend that much money.
If I don't go for the GPS option, I guess I'll just have to
live with these plus or minus 1 second adjustments then,
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