[ntp:questions] Re: isolated network + accuracy

David J Taylor david-taylor at invalid.com
Tue Dec 14 18:14:27 UTC 2004

Bjorn Gabrielsson wrote:
> "David J Taylor" <david-taylor at invalid.com> writes:
>> Bjorn Gabrielsson wrote:
>>> "Richard B. Gilbert" <rgilbert88 at comcast.net> writes:
>>>> Using an internet server as a time source will almost certainly not
>>>> allow you to achieve 1ms accuracy!  10ms accuracy is possible over
>>>> the internet but cannot be guaranteed.
>>> I get sub 1ms time from internet servers.
>> What precise type of connection do you have to the Internet?
> Slow assymetric ADSL. (0.5Mbit downstream , 0.768Mbit/s upstream)
> The ntp servers are (today) 10 to 13 router hops away, 10 to 20 ms
> "ping"-distance and 200 to 600km physical distance.
> /Björn

The number of hops surprises me.  Ping distance sounds not bad.  I'd 
welcome comments from others on the accuracy achievable with such a 
connection, but I would have thought that better than 10ms should be 
achievable (providing your server running NTP isn't heavily loaded), but 
it might be limited if you are saturating either the uplink or the 
downlink to the Internet - with a sustained upload or download, for 
example.  Perhaps you can give NTP packets higher priority?

I would investigate whether your first or second hop (i.e. the first piece 
of the ISP's kit offers NTP.  You may find it is only 10ms away.


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