[ntp:questions] ntp server pool advice

Danny Mayer mayer at ntp.org
Tue Dec 27 22:30:57 UTC 2011


On 12/27/2011 1:16 PM, unruh wrote:
> On 2011-12-27, Danny Mayer <mayer at ntp.org> wrote:
>> On 12/26/2011 11:17 PM, ben slimup wrote:
>>> Thanks Danny for your reply,
>>>
>>> but is it a big problem, if the client round-trip packet comes from a
>>> different servers each time? why?
>>>
>>
>> Because NTP uses multiple packets to gain data on the round-trip delay,
>> jitter, etc. of each server it gets responses from. The round-trip delay
> 
> No it doesn't. It uses one outbound and one inbound packet to get the
> delay time. Ie, one packet arrives at the server, and one exits the
> server. Now if you are talking about statistics, that is different, and
> using many will increase the jitter. If the two machines are "good" then
> their times should agree within the jitter anyway.
> 
>> is different if it comes from different systems. In addition each system
>> has its own idea of what the correct time is and at the point that it
>> receives and sends out the reply packet. The resulting data points will
> 
> Not if they are all synchronised to UTC.
> 

What UTC is is not necessarily exactly identical. NIST has one idea of
it and NPL (UK) has a slightly different idea. However that is not what
I was referring to. Each server gets its information from different
sources whether it's a refclock, GPS, another server, etc.. As such
these sources differ somewhat from each other and while NTP tries to get
the best answer possible, each server will have a slight different
answer to the question. They may be only milliseconds apart but they
will be different.

Danny


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