[ntp:questions] NIST vs. pool.ntp.org ?

Robert Scott no-one at notreal.invalid
Thu Mar 28 13:08:23 UTC 2013


On 28 Mar 2013 09:57:37 GMT, Rob <nomail at example.com> wrote:


>To achieve an uncertainty below 100ms over connections like that you
>will need to do some clever programming where you get multiple time
>stamps, measure the roundtriptime on each of them, and discard
>outliers before you calculate an average.  The kind of thing that ntpd
>already does.

If I could use a packaged implementation of NTP I would.  But I don't
have that option.  If I understand correctly, packaged implementations
of NTP are designed specifically for setting the system time.  I
cannot impose that effect on users of my app.  My app must be
stand-alone and not attempt to affect the system time.  So I will have
to take the algorithms and basic ideas of NTP and re-implement them in
a stand-along fashion.  In any case the language of implementation
will have to depend on the target: Java for Android devices,
Objective-C for iPhones.


>Radio protocols often use polling to grant the permission to the mobiles
>to send, and adaptive polling intervals depending on the recent traffic.
>(or they drop clients from the polling list when there is no traffic,
>offering some method for them to join again)
>
>The result is that when you start traffic on an otherwise idle client,
>you will observe roundtrip intervals like this:
>
>time=531 ms
>time=500 ms
>time=540 ms
>time=874 ms
>time=99.7 ms
>time=101 ms
>time=96.3 ms
>time=97.3 ms
>time=101 ms
>time=96.9 ms
>time=102 ms
>time=102 ms
>time=97.4 ms
>time=102 ms
>time=103 ms

Are these exchanges all with the same Time Server?  I thought good
citizen use of these Time Servers required at least 4 seconds
inbetween queries.  Or does the start-up polling time penalty only
once even if every query is to a different Time Server?


>in this example, the first four exchanges were with the device still
>in "idle mode" and only then the cell network (UMTS in this case)
>decided to step down the polling interval on that client, and from
>the fifth one the roundtriptime is pretty consistent.  you will need
>to discard those first 4 samples and then calculate an average over
>a couple more.
>however, that "4" is not a constant!  it depends on parameters and
>circumstances beyond your control.  so you will need to write an adaptive
>algorithm that recognizes what is happening here, and send the queries
>quickly enough (I would say at least two per second, maybe 4) to force
>the "active user" algorithm to kick in.
>
>even then, I have seen links (e.g. over WiFi) where a stable state
>is never reached and varying roundtriptimes between nearly zero and
>about 200ms are seen no matter how often and how frequently you ping
>over them.

In those cases how asymmetric is the polling delay likely to be if I
just take the midpoint of the polling interval, just like NTP?  I
realize that it is "possible" to be extremely assymetric, but in
practice?

Robert Scott
Hopkins, MN



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