[e2e] Accuracy achievable with NTP

[Joe Touch]

David P. Reed wrote:

> Saad's point is well worth thinking about.   The question is ill
> formed.  The accuracy of a clock stabilized and synchronized by NTP is
> in principle relatively independent of the range of delays in the network.

FWIW,

	NTP's accuracy:
		depends on the variability in the delay,
		and asymmetries in the forward vs. reverse delay

	NTP time-to-synch:
		depends on the delay itself

I.e., large RTTs will make it take longer to update the offset, and
longer to establish the correct rate, but don't per-se have any effect
on how closely the local clock can be synch'd to the remote clock (how
correct the rate is).

As David points out, larger RTTs are correllated to higher delay
variability and asymmetries are, but this needs to be weighed against
the accuracy of nearby clocks vs. remote (e.g., if nearby are stratum 4,
and remote is stratum 1...)

Joe

> At 11:53 AM 4/10/2003 -0500, Saad Biaz wrote:
> 
>> Woojune:
>>
>> It is not the delay RANGE that hinders precise clock synchronization,
>> it is the UNCERTAINTY on the delay that matters.
>>
>> Given a network of hosts, you must MEASURE the UNCERTAINTY on the delay
>> between all hosts. Then find the diameter D of your network with edges
>> weighted with the UNCERTAINTY (highest delay - lowest delay).
>>
>>
>> Your worst case imprecision on the clocks will be  1/2 D (Diameter).
>> Your average imprecision depends on the distribution of the uncertainty.
>>
>> For a formal proof, check:
>>
>> S. Biaz and Jennifer L. Welch, "Closed form Bounds for Clock
>> Synchronization under Simple Uncertainty Assumptions".
>>
>>
>>
>>
>> On Thu, 10 Apr 2003, Woojune Kim wrote:
>>
>> > Hi,
>> >
>> > I've been looking for info on how accurate NTP can be. I've looked
>> in various NTP related sites and still not quite sure.
>> >
>> > Say I have a LAN like environment with delays in the range of 10s of
>> ms. How accurate a time can I get from NTP, assuming I have the
>> appropriate HW clocks on my host ?
>> >
>> > Say I have a corporate network like environment, relatively
>> segregated from the Internet at large, with delays in the range of
>> 100ms. How accurate a time can I get ?
>> >
>> > Say the delay range goes upto 500ms, how accurate can the time be ?
>> >
>> > Any pointers to specific experiments or data would be appreciated.
>> >
>> > thanks
>> >
>> > Woojune Kim
>> >