[ntp:questions] Is this normal?

Russell, David David.Russell at dowjones.com
Tue May 11 23:50:03 UTC 2010


The device is a piece of networking equipment and so I doubt that the crystal is temperature controlled but since it is in a data center the temperature and power demand is steady.

If you could see the graph you would see that it seems to oscillate around -3.05 us/s over a 24 hour timeframe. The graph has a pretty nice looking sine wave with a 12 hour period.  I was expecting that if the "true" drift rate were -3.05 then NTP would converge upon that value.  Are you saying that this oscillation is typical for non-temperature (cheap) crystals or should the drift look more random?  

One new piece of information is that the reported delay is not even close to the actual delay on the network.  For example, a sniffer trace shows that the actual round trip time is 1.8ms but their NTP process is reporting the delay as 3.5ms.

I am not convinced that there isn't a coding error in the implementation and am trying to differentiate inherent limitations in the device such as normal clock drift variance, the way that the clock is adjusted etc from a coding error.  There was a coding error in the previous version so it is not too unlikely that there is another one.

Any thoughts on validating a black box NTP process?


I had not realized until I sent out the email that no attachments are allowed.  Is there a way to share a graph?


Regards,

David

-----Original Message-----
From: questions-bounces+david.russell=dowjones.com at lists.ntp.org [mailto:questions-bounces+david.russell=dowjones.com at lists.ntp.org] On Behalf Of David Woolley
Sent: Tuesday, May 11, 2010 5:08 PM
To: questions at lists.ntp.org
Subject: Re: Is this normal?

Russell, David wrote:
> I am trying to verify the correct implementation of NTP on a device
> that is configured to poll a GPS based dedicated time server on the
> local area network. The round trip times to this stratum 1 vary between
> 3.3 and 3.4ms.
> 
> I have plotted the offset vs drift in the included jpg.  The drift
> rate varies between -3 and -3.2 us/s and the offset is +/- 200us.
> Note that the first part of the graph is when NTP started.

It is not permissible to post JPGs to non-binaries Usenet groups (I 
guess you used the email gateway - the binary was stripped.)  Also, 
given the likely content of the image (i.e. line art) JPG is a very poor 
choice of image format.

You haven't told us how well controlled the temperature of the crystal 
is.  200  ppb sounds good unless the crystal is ovened or the machine is 
in an air conditioned room and has constant power demand.

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