[ntp:questions] Can NTP sync within 1ms

William Unruh unruh at invalid.ca
Fri Apr 25 15:02:34 UTC 2014

On 2014-04-25, Rob <nomail at example.com> wrote:
> Henry Hallam <henry at pericynthion.org> wrote:
>> On Thu, Apr 24, 2014 at 8:07 AM, Montgomery, Peter         BIS
>> <Peter.Montgomery at fs.utc.com> wrote:
>>> I am new to NTP. But I have a quick question that I need to answer soon.
>>> I would like to know whether NTP can sync between a client and a server within 1ms if the client and server are Linux applications on a simple local network ( less than 10 nodes).
>> Yes, absolutely and easily.  It can usually achieve that even over the Internet.
>> Henry
> Of course there is a bit of a difference between "usually this will
> work very well" and "it better works OK or we will lose millions of
> dollars".  You know, the financial trade markets where they try to
> rip eachother off by sending transactions across links 1ms shorter
> then someone else to use prior knowledge.

If you need for accurate time is that great, get a local refclock-- gps
receiver, local atomic clock, etc. attached to each of your machines. 
And even then there is no guarentee (gps goes down due to a war between
the USSR and USA, atmonic clock burns out, computer crystal cracks,...)

Also do not use Windows. 

ntpd has a number of mitigation factors against network congestion. 
The delay filter ( choosing only that offset whose delay is the smallest
of the past 8 tries for example, slowly changing the rate due to
non-zero offsets, etc) and you CANNOT go by the offsets to see if your
clock is off. You have to get in another better time source (eg gps pps)
to see how far off your clocks are, precisely because the offsets could
be off while the clocks are not "due to network congestion" which the
clock-filter may ignore. 

If you have network problems such that your network is full 24 hours a
day, then clearly a network is not the way to get the clocks

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