[ntp:questions] strange behaviour of ntp peerstats entries.

Brian Utterback brian.utterback at sun.com
Tue Jan 29 19:19:13 UTC 2008

Unruh wrote:
> "David L. Mills" <mills at udel.edu> writes:
>> Unruh,
>> It would seem self evident from the equations that minimizing the delay 
>> variance truly does minimize the offset variance. Further evidence of 
>> that is in the raw versus filtered offset graphs in the architecture 
>> briefings. If nothing else, the filter reduces the variance by some 10 
>> dB. More to the point, emphasis added, the wedge scattergrams show just 
> I guess then I am confused because my data does not support that. While the
> delay variance IS reduced, the offset variance is not. The correleation
> between dely and offset IS reduced by a factor of 10, but the clock
> variance is reduced not at all. 
> Here are the results from one day gathered brom one clock (I had ntp not
> only print out the peer->offset peer->delay as it does in the
> record_peer_stats , but also the p_offset and p_del, the offset and delays
> calculated for each packet. I alsy throw out the outliers ( for some reason
> the system would all of a sudden have packets with were 4ms round trip,
> rather than 160usec. These "popcorn" spikes are clearly bad. The difference
> between the variance as calculated from the peer->offset values, and the
> p_offset values was

I do not know your network configuration at all, so I am just guessing,
but My guess is that you are talking about a client connected on the
same subnet with one or more servers, right? Connected by ethernet?

In that case, you are talking about a situation where the error
introduced by factors that increase in correlation with the round
trip time is minimal at best. When they do kick in, you see what
looks like huge jumps and filter them. A 4ms increase is just what
you would expect when ethernet timers kick in. Now imagine a RTT of
60-70ms. A 9ms delay from a collision introduces a 4ms change in the
delay value and a 2ms change in the offset, but with a delay might not
perturb the delay value enough to make it obviously an outlyer.

Brian Utterback

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