[ntp:questions] Re: Does NTP model clock skew?

Affan affanahmed at gmail.com
Sun Feb 13 20:14:18 UTC 2005


>
> Just to be clear, let me define "clock skew":
> 	clock skew is the difference in clock signal arrival times caused
> 	by different transmission paths.
>
> NTP doesn't "cater to" clock skew, it tries to eliminate it!
I actually consider clock skew to be the frequency difference of the
oscillator in any node from what it is supposed to be. So even if it is
synchronized at one instance to the UTC, it would later go off target
because its local oscialltor does not tick at the frequency at which it
should.

>
> This is because it takes round-trip time measurements and tries to
correct
> for the propagation delay.  (By assuming it's equal in both
directions.)
>
> It's like the story of the old fellow who winds up the clock at his
farm
> and then walks to town for some shopping.  Assuming that:
> - His clock read 9:00 when he left
> - It was 12:00 (according to the clock in the town square) when he
>   arrived at town.
> - It was 2:00 when he left town
> - His clock showed 1:00 when he arrived home.
>
> What time should he set his clock to?
>
> The answer is 3:00.  Even though the clock at home doesn't know what
> time it is, it could still note that he spent 4 hours away from home.
> If he spent 2 hours in town, that's 2 hours travelling.  Assuming
equal
> speed in each direction, he arrived home 1 hour after leaving town.
>
> Of course, if there's a hill, or he was carrying a lot of stuff in
one
> direction, he may be off a bit.  If you don't know anything about how
> long it took to travel each direction, all you can say is that the
> time he arrived home is somewhere between 2:00 and 4:00.

Thank you for the very nice explanation, but i knew this before :).

>
> As for comparison with RBS... what it RBS?  Google gives me:
> - Royal Bank of Scotland
> - Research for Better Schools
> - Royal Blind Society
> - Rede Brasil Sul de Comunicação
> - Rural Business-cooperative Service
> - Royal British Society of Sculptors
> - Regionalverkehr Bern-Solothurn
> - Rutherford Backscattering Spectrometry
> - Regional Bus Stuttgart
> - Rug Book Shop

Sorry, should have been more explicit!
>
> ah... found it.  Reference Broadcast Synchronization.  A system
> wherein multiple clients receive and timestamp a common LAN
broadcast,
> and then compare their timestamps to find their clock offsets.
>
> A nice trick if you have such a network, but you need at least three
> nodes on the network (one to send the broadcast so two can receive it
> for comparison), and you need the network to support broadcasts.
>
> Further, you need a network without unbalanced transmission times.
> Suppose I have three nodes on a 100baseFX ethernet, two next to each
> other and the third connected via 40 km of single-mode fiber.
>
> Any brodcast will reach the third node 133 us (times the refractive
> index of the fiber, whatever that is) later than it reaches the first
> two.  You may also insert varying numbers of of bridges and
> store-and-forward switches.
>
> NTP is designed to work over any channel that will carry UDP packets.
Yes RBS is for only braodcast enabled networks with near simulataeous
reception times. However what they do is intersting in that they actual
fit a line to the phase offset value b/w them and another (for
simplicity consider that a aerver) node. The slope of this line (that
is the amount by which the differnce increases) gives an indication of
the skew of our clock wrt the server clock. 


Thanks.
Affan




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