[ntp:questions] Re: The asymmetry strikes again!

David J Taylor david-taylor at blueyonder.co.not-this-bit.nor-this-part.uk.invalid
Wed Jul 20 16:06:16 UTC 2005


Matt Kinard wrote:
> I'm still researching NTP issues over an asymmetric network, and I was
> wondering if I could get some suggestions as to what sort of accuracy
> I'm looking at sacrificing.
>
> Are we talking 20 ms or are we talking about 1 or 2 seconds? Is it
> contingent upon how asymmetric my network is? What is the threshold?
>
> For the particular application I'm studying, there is some acceptable
> risk in terms of lost accuracy, but I need some more concrete ideas.

How asymmetrical is the network you are considering?  I think you will 
need to know that to answer the question.

FWIW: I saw one NTP server in Europe which had an asymmetrical path to its 
reference source in the US.  The error was (I can't remember) either the 
full difference on the send and receive times or half the difference.  One 
path was via satellite and one via landline, I believe, so the offset was 
less than 100ms.

(You might ask, as I did: why did NTP prefer the server with the greater 
network distance, and why didn't the operators of the server use European 
sources).

Cheers,
David 





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