[ntp:questions] Confused about pool rotation

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Thu Feb 7 02:33:52 UTC 2013

Robert Scott wrote:
> As I explained in another thread last week,
>  my app is not a time app.
> It is a precision audio frequency measuring app.

Oh, makes more sense now, how was I to know who you were?
 You message was not associated with the other thread;
 You provided no references to the previous thread,
  no the application in the previous thread;
 You used an anonymous from, messages is missing posting host;
  and didn't have a sig (unlike at least once in the last thread),
 As far as I knew, you were just the next random person
  popping into the newsgroup, asking obscure questions,

 I still can't see how SNTP with random asymmetrical latency,
   and ntp not disciplining a local clock to the ns level,
   could beat the monotonic nanosecond timer available in
   the Android SDK.

  It would seem like what you could do in a short amount
   of time, would instead take several hours.

  ... Unless Android lies, and and the timer isn't monotonic,
   or doesn't have nanosecond resolution.

  You do know that NTP server randomize the least significant
   bits in the timestamps
  (for the bits below the precision of the platform).

  The 64bit fraction of seconds may resolve to .05 attosecond
   (0.5e-18), however on a ntp server with precision of -21
    anything below microseconds is randomized,
    and precision of -15 below 100 microseconds is randomized;
    {I guess I might be off by one digit}.

  What precisions are you seeing for the servers you are querying?
   Precision is in the ntp packet between poll and root delay.
   {I'd expect -20 to -22 for most in the pool,
    and I occasionally see a -24 or -29 in the pool,
    but those are fairly rare.}
    The ATT iPhone NTP servers have a precision of -19.

  Also, do you query the same server(s) at both the start,
   and end?  Otherwise the sum of offset errors between the
   two might result in more jitter than you expect.

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