[ntp:questions] ntp server specs

Richard B. Gilbert rgilbert88 at comcast.net
Thu Apr 21 00:21:10 UTC 2011

On 4/20/2011 5:49 PM, unruh wrote:
> On 2011-04-20, C BlacK<rblak at non.net>  wrote:
>> If I am setting up an ntp server for a lot machines, I have some questions.
>> If the timing on client machines needs to be tight, should the ntp server be an x86 machine with a less than accurate clock
>> or a ntp server with a very accurate clock.
> ? Clearly the server should have as good a clock as possible. And it
> should not CPU speed type power management. And it should not be on a GB
> network.
> Note that if you run the server with a GPS PPS you can get it down to
> 5usec accuracy, at which point the type of machine is largely
> irrelevant.
>> I want the clients to track closely to server.
> What does "track closely" mean? 1 hour? 1 sec? 1ms? 1us, 1ns?

It's reasonable to expect NTPD to track the correct time within 10 
milliseconds or better using a server that is close to you in *Net 
Space*.  It's conceivable that a server could be physically no more than 
five miles away and yet have a network path between the two with a
link that is 200 miles long.  Always remember that the shortest network 
path will usually give you the best results.

In principle, you could be in New York City and use a server in Tokyo. 
In practice the "phase noise" will kill you!  There are hundreds or even 
thousands of possible paths between New York and Tokyo and you could get 
a different one with each request and thus a different delay with every 

If you have a budget that will stand it, you can get a hardware 
reference clock such as a GPS Receiver, LORAN receiver, etc, etc.
A good reference clock should be able to keep you within 50 nanoseconds 
or better.  Keep in mind that, while the PPS output of a GPS or other 
timing receiver can be within 50 nanoseconds of true time, getting the 
signal into a computer can introduce delays that are extremely difficult 
to measure!

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