[ntp:questions] Is this normal?

Chuck Swiger cswiger at mac.com
Wed May 12 00:09:26 UTC 2010


Hi, David--

On May 11, 2010, at 4:50 PM, Russell, David wrote:
> 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.

It's pretty common for system loads to be different during the day and night, and even if this specific machine doesn't have such a change, other machines in the rack, network switches, and so forth produce different levels of heat which mildly change ambient temps, even with good air-conditioning in the datacenter.

> 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?  

A 24-hour period to the drift is very typical, yes.  12-hour period is more interesting....

> 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.

Are you sure something isn't measuring latency one-way, and the other isn't measuring the round-trip time?

 1.8 ms * 2 ~= 3.5 ms

> 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.

You're welcome to request a refund of whatever you've paid for ntpd.  :-)

> Any thoughts on validating a black box NTP process?

Sure: find a more reliable timing source-- cesium or rubidium atomic clock, GPS receiver, WWVB, etc-- and compare that to your ntpd timestamps.

However, typical network latency and jitter (which should be less than a microsecond for local gigabit switched traffic, but your latency is somewhat higher) means that it's tough to get time significantly more accurate than around a microsecond, unless you are measuring locally.

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

Please put it on a website somewhere, and mail a link to it.

Regards,
-- 
-Chuck




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