[ntp:questions] ARRGH!!! I woke up to a 50 SECOND clock error.

David J Taylor david-taylor at blueyonder.co.uk.invalid
Sat Mar 17 19:20:30 UTC 2012

"Ron Frazier (NTP)" <timekeepingntplist at c3energy.com> wrote in message 
news:4F64D793.9010708 at c3energy.com...
> Hi David,
> I'm not sure what will happen if you simultaneously prefer and noselect 
> the local server.  Assuming the local stratum 1 server is the most 
> stable time source, you'll get a much better picture of what the 
> internet servers are doing relative to it if you allow it to be 
> selectable as well as being preferred.  When you graph it, if the local 
> server is the active clock, all the lines for the internet servers will 
> be gathered around and relative to the local server.  When I tried to do 
> things the other way around, with an internet server preferred, the 
> graph looked awful because there was so much variation.  Also, if your 
> local server starts reporting time that looks too far from the internet 
> servers, regardless of who's fault it is, ntp will clock hop over to the 
> internet servers.
> I don't THINK your internet servers will ever poll above their default 
> minpoll value of 6, or 64 seconds.
> I realize you don't have a gps attached to this pc, but the iburst lines 
> reminded me of something.  I read somewhere that having iburst on 
> internet server lines, if a local gps is attached, could prevent the PC 
> from synchronizing to the gps before it synchronizes to the internet. 
> On my pc with the gps attached, I don't use the iburst command.
> Sincerely,
> Ron


The local stratum-1 server shows without a tally code against it in the 
ntpq -p output, so it's being recorded in the peerstats, but not used for 
syncing.  The noselect msut override the prefer.

After about three hours running, the Internet servers are all at 512 
seconds poll interval.  The averaged jitter has been below 1 millisecond 
for the last couple of hours.  The offset is reporting between -0.7 
and -1.8 milliseconds, and the frequency is stabilising very nicely 
(because of the long poll interval).  I'll leave this running overnight 
and tomorrow to see how it handles temperature changes and any Internet 
access changes, and to get a few more points on the graph.

One caveat is that I am using the most recent NTP (ntpd 4.2.7p263 from 
Dave Hart's download page), and that with Windows-8, it may be using the 
new precision time system call.  From my own tests, this is similar, on 
earlier versions of NTP, to setting the environment variable 
NTPD_USE_INTERP_DANGEROUS, thus forcing the NTP time interpolation to be 

The configuration I have is:

- cable modem (with built-in router, but working as a bridge by putting my 
own router in a device in the DMZ).

- Samknows network monitor (modified WRT54GL router)

- WRT54GL router running DD-WRT firmware

- Netgear 8-port consumer 1 Gb/s switch (G5108)

- wired connection to ~2 year old laptop PC

I only mention this to show that (a) it's not a direct connection and (b) 
there's no wireless involved.  My aim here is simply to see what 
performance may be had with just Internet servers.  The PC is only running 
NTP and monitoring software - no user programs and no interactive work, so 
it is a best-case scenario.


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