[ntp:questions] Tighter regulation?

Brian Utterback brian.utterback at oracle.com
Sat May 25 17:14:38 UTC 2013

On 5/25/2013 5:55 AM, Mischanko, Edward T wrote:
> [Mischanko, Edward T]
> I would modify the current algorithm with an exception that if offsets
>   exceed 1 millisecond for more than one polling cycle, then, polling will be
>   reduced by one interval, else, continue normal operation.

What if 1 millisecond doesn't happen to by the tolerance that some 
particular system needs? Ten years ago, I was thrilled if my customers 
reported off sets in the 10-20ms range and sub-10ms was rare. Not to 
mention that what should be expected in the way of maximum offset is a 
function to the polling period and the frequency measurement accuracy, 
the latter of which is probably large and unknown when the clock system 

The current algorithm is supposed to use the amount of jitter and the 
increase in the offset between polls to determine when to increase the 
polling period. If the noise in the samples is greater than the amount 
of offset between polls, then there is no way to increase the accuracy 
of the clock at the current polling rate. So, if you are seeing large 
offsets at a particular poll rate, then that would indicate that your 
jitter is too high to prevent it. However, conditions change, and I 
believe that the algorithm for reducing the poll rate again is known to 
be too "stiff".

So, are you seeing offsets that are large immediately after the poll 
rate is increased, or does it cruise along fine for awhile and then you 
see increasing offsets? The former would indicate too much jitter, the 
latter woudl indicate an actual failure of NTP to respond quickly to 
change in clock frequency.

Brian Utterback

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