[ntp:hackers] First sample after f15 minutes used. Really?

Brian Utterback Brian.Utterback at Sun.COM
Sun Sep 9 21:33:09 UTC 2007


Yes, that is what I was saying, that the standard mitigation algorithms 
are still in effect. I
thought that the wording in the docs made it sound like this was not the 
case. I read the doc as
saying that the first sample after 15 minutes (i.e. from the packet 
currently being processed) would
be accepted, not the offset calculated after the mitigation and 
selection algorithms.

Another thing, in the html man page for ntpd, it says that if the TOY 
chip is not working,
then ntpd exits. Here is the text:

"In case there is no TOY chip or for some reason its time is more than 
1000 s"..."exit
with a panic message to the system log"

Now, of course we are all aware of the panic threshold for the 
calculation of the offset, but
I am not sure what is meant here about the TOY chip.  If there is no TOY 
chip ntpd exits?
That doesn't seem right.

David L. Mills wrote:
> Brian,
>
> You might misunderstand the purpose of the scheme. First, the mitigation 
> is done before the threshold is checked, so presumably we have the best 
> candidates to wiggle the clock. Although a rogue sample or two might 
> exceed the step threshold, it's highly likely a good sample will come 
> along and reset the stepout counter.
[snip]
>
> Dave
>
> Brian Utterback wrote:
>
>   
>> To answer my own question, the wording isn't exactly right. What 
>> happens is that any offset greater
>> than 128ms is ignored for 15 minutes. If it has been over 15 minutes 
>> since the last clock update, then
>> the offset (Still calculated by the normal method) is allowed. So, the 
>> standard clustering and combining
>> is still in effect.
>>
>>
>> Brian Utterback wrote:
>>
>>     
>>> (Resending, including Dave this time.)
>>>
>>>
>>> In the documentation for ntpd, it says:
>>>
>>>  
>>>
>>>       
>>>> The ntpd  algorithms  discard  sample
>>>>      offsets  exceeding  128 ms, unless there is no sample offset
>>>>      of less than 128 ms for 15 minutes. The first  sample  after
>>>>      that,  no  matter  what  the  offset, steps the clock to the
>>>>      indicatedA time.
>>>>     
>>>>         
>>> Is this really true? The first sample after 15 minutes is used, even if
>>> it is a lone wolf? What if you have 5 servers and 4 of them give an
>>> offset of 2 seconds and one has an offset of 10 seconds. If the 10
>>> second guy happens to be the first one after 15 minutes of such
>>> samples, he gets used? This seems wrong to me.
>>>
>>> Brian Utterback
>>>
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>>>   
>>>       
>>     
>
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