[ntp:questions] determining time difference between two not-directly-connected computers

Danny Mayer mayer at ntp.isc.org
Mon Jan 8 03:17:13 UTC 2007

matthew.garman at gmail.com wrote:
> On Sun, Jan 07, 2007 at 12:30:59AM -0500, Danny Mayer wrote:
>> matthew.garman at gmail.com wrote:
>>> I've got two (x86_64 Linux) machines between which I need to
>>> determine the relative time difference.  The problem is, the
>>> machines are not directly connected and only one is connected to
>>> the Internet.
>>> ...
>> What do you think that NTP does? and why don't you think that after 20+
>> years of engineering that it doesn't do a much better job?
> I think NTP syncrhonizes one computer's time to another.
> However, the two computers in question don't have a direct
> connection.  And syncing the one machine to the common server via
> NTP through VPN is not what I want to do.

You don't need to. You can just set up a machine that *can* connect to
both and use ntpq -p to look at the offsets in the billboard.

> In our application, we need to know if there's even a few
> milliseconds of time difference between the two machines... and I'm
> just looking for some feedback on how I can accurately measure that
> difference.

Then they really *should* be connected and synchronized to each other.
What happens if they do disagree by a few milliseconds? If that's a
business/scientific requirement then you need to buy and install
refclocks for each of them and configure NTP to use them.

> And I'm even curious about what kind of time difference we should
> expect when both machines are NTP time sync'ed, albeit to different
> NTP servers.

What is your *requirement*? An NTP network can be designed to meet
whatever your needs are. The needs will dictate how much money you need
to spend to meet those requirements.

> Thanks,
> Matt
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