[Dovecot] May 05 07:20:21 imap: Warning: Time jumped forwards 16 seconds

[Lorens Kockum]

I did leave a little almost on-purpose dangling hint that got me
an off-list query... Since I'm replying, I might as well reply
on-list for the record, even if this is getting off-topic.  I've
just about exhausted my knowledge on the subject, so further
questions will probably find a more attentive audience on some
NTP list :-)

A reader wrote:
> You wrote "probably" but isn't it common to sync against a stratum 1 server
> having some GPS clock attached? I assume that most computer centers operate
> a stratum 1 time server today and stratum of clients should be between
> 2 (!) and 5.

Most scientific computing centers maybe, but business people
probably don't.

In my experience most people who run stratum-1 servers impose
limitations on their clients, like asking for permission,
registering, signing up for their mailing list, running public
stratum-2 servers... or being in the same organization as
them, which certainly seems to be your case :-)

I suppose the most common reason for restrictions like these
would be that with more than n clients, the server will start
having problems, bandwidth, latency, whatever. Maybe n is a
large number, but then again maybe not, after all we're talking
about milliseconds. I'm not sure what it would take to run a
stratum-1 service with under one hundredth of second of jitter
if that service gets used as the default for new Debian or
RedHat installs, but I'm quite certain I don't want to pay for
the hardware or the bandwidth! Registering for a mailing list is
also important when the service is really really important.

You don't need to take my word for it:

	http://support.ntp.org/bin/view/Servers/RulesOfEngagement

As for stratum-2 servers, there are lots of public ones with no
restrictions other than running a reasonably well-behaved ntp
implementation.

The difference being synced to a stratum-1 or a stratum-2 is
negligeable; and most people who have reasons for milli-second
accuracy want it between their own servers. They will run a set
of NTP servers, stratum 1, 2 or 3, and sync all of their other
servers to them. For many or even most uses it won't matter if
they are a several milli-seconds off with respect to some atomic
clock as long as they are internally consistent.

It is my opinion that someone like the OP, who wants his servers
to be on time but who does not seem professionally interested in
running an NTP server or in having milli-second accuracy, should
not be peering with a stratum-1 server. That is the reason I
wrote stratum-3 and not stratum-2 :-)

Just to be complete, on the other side of the spectrum, I think
we agree that with such a lot of stratum-2 servers to choose
from, it seems unnecessary to have a stratum above 5. You'd be
at 5 if you sync to your organization's stratum-4 syncing to
your ISP's stratum-3 syncing to public stratum-2s... maybe a
multi-site organization would run an NTP service for every site,
but then they'd probably sync their main servers directly to
stratum-2 servers instead of to their ISP, so that'd cancel out.

HTH