[ntp:questions] 500ppm - is it too small?

nemo_outis abc at xyz.com
Mon Aug 17 19:15:22 UTC 2009


"Richard B. Gilbert" <rgilbert88 at comcast.net> wrote in
news:RfWdneXv8elEAhTXnZ2dnUVZ_sKdnZ2d at giganews.com: 

> nemo_outis wrote:
>> "Richard B. Gilbert" <rgilbert88 at comcast.net> wrote in
>> news:poydnd1spod4pBTXnZ2dnUVZ_vudnZ2d at giganews.com: 
>> 
>>> I still haven't seen any argument that supporting a value greater
>>> than 500 PPM is worth the trouble!
>> 
>> I still haven't seen any evidence that there is much trouble - only 
>> anticipatory speculative whinging. 
>> 
>> Nor, for that matter, has there been much other than speculation and
>> vague anecdotal recollections that the 500 ppm limit *may* have had
>> some justification and not been entirely arbitrary.
>> 
>> This doesn't inspire much confdence about the documentation of the 
>> architecture and design decisions regarding ntp.
>> 
>> Regards,
>> 
>> 
> 
> I think "Das Buch" a/k/a  "Computer Network Time Synchronization: The 
> Network Time Protocol" by David L. Mills might help you understand the
> design and how it was derived.
> 
> See:
> http://www.amazon.com/s/ref=nb_ss?url=search-alias%3Daps&field-keywords
> =NTP&x=0&y=0 
> 
> I think you will need some background in "control systems", "phase 
> locked loops" and other such esoterica.  I don't pretend to understand
> the math.  I simply use the software derived from it to synchronize my
> clocks.
> 
> NTPD does that nicely!
> 

I have the book and I've read it.  And although I am a mechanical 
engineer, not electrical, I have a solid background in control theory and 
I do understand the math.  

The justification for 500 ppm in "Das Buch" is very thin gruel and 
amounts to litle more than "argument by repeated assertion."  In fact, 
the book itself discusses a 1997 survey (section 6.6) that shows time 
offsets of hundreds of milliseconds, so far from being unusual, are 
common. This (inter alia) makes the book's light regard for large offsets 
seem more than a little cavalier.

Regards,

PS  I too only use the software to synchronize my clocks.  However, while 
I am grateful for the years of effort that ntp reflects, I am not blind 
to its warts.  And while the opinions of a strong personality may have 
driven the creation of ntp, it seems those same rigidly-held opinions may 
now be restricting its further growth and development.




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