[ntp:questions] ntp server pool advice

Danny Mayer mayer at ntp.org
Tue Dec 27 05:09:23 UTC 2011


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
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
be so uneven if it's coming from the different systems that it will
never select that system as accurate and will almost certainly remove it
from the list of possible candidates. The data for each specific IP
address is collected this way and compared to data from other IP
addresses. Load-balancing breaks that since it's really a different
system that it can get data from. Instead of load balancing you give
each system its own IP address and it can synchronize against those systems.

Danny
> thank you again
> 
>> Date: Mon, 26 Dec 2011 22:39:22 -0500
>> From: mayer at ntp.org
>> To: "terje.mathisen at tmsw.no"@ntp.org
>> CC: questions at lists.ntp.org
>> Subject: Re: [ntp:questions] ntp server pool advice
>>
>> On 12/22/2011 6:40 AM, Terje Mathisen wrote:
>> > ben slimup wrote:
>> >>
>> >> Dear all,
>> >>
>> >> Thank you very much for support,
>> >>
>> >> i do not have 1000,000 client, i need those ntp servers to serve a
>> >> load between 100000 to 1000000 clients
>> >> over a public network with an accuracy of 100ms
>> >>
>> >> those clients will use dns round robin to resolve 4 external ip, 2 IPs
>> >> on each site.
>> >
>> > DO NOT USE ROUND ROBIN DNS for NTP!
>>
>> You can use round robin dns for NTP. There's nothing wrong with that.
>> It's load balancing that must NOT be used as you would get different
>> answers from different systems each time.
>>
>> Danny
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