[ntp:questions] Re: Time mysteriously advancing with FreeBSD 5.3 and ntpd 4.2.0-a

Mxsmanic mxsmanic at hotmail.com
Sat Jan 1 11:49:24 UTC 2005


Richard B. Gilbert writes:

> Depennding on the quality of your servers and your network connections 
> to those servers, ntpd may need from twelve to thirty-six hours to 
> achieve tight synchronization.

Is this after every boot, or only after ntpd is first installed?

The time does seem to have stabilized.  For a while it was as much as
10-15 seconds off, but for the past few days (almost 72 hours now), it
has been right on the mark.  I suppose ntpd has to train itself to get
used to new servers and a new local hardware clock, no?

> If the quality of the servers etc, is 
> poor enough you may never achive tight syncronization.

The daemon seems to change its mind a lot about which server to use for
its reference, but it does seem to pick the same two or three servers
regularly.

> I normally see 
> offsets in the 100us to 1ms range from my GPS reference clock and 
> offsets in the 5 to to 20ms range for the best of the network servers I 
> use.

This is something I also don't understand very well: If you have a very
accurate local reference clock, what's the utility of querying distant
NTP servers?  Is it just a sanity check, or what?  Won't the local clock
_always_ be the final reference used by NTP?

I've been thinking of getting a DCF77 clock for my machine, if budget
ever permits.  I suppose that would be at least as accurate as network
synchronization (?).  A GPS clock isn't an option because I have no easy
way to give it a clear view of the sky in my building.

> These network servers are all within 300 miles of my site and 
> most are within 150 miles.

My daemon seems to show a preference for relatively local servers
(Germany and Switzerland), but it also considers NIST servers as
candidates and occasionally picks one of them as a reference.

I think that delays to distant servers are indeed longer, but they are
more constant, since the changes in routing tend to cancel out over long
distances; whereas nearby services may abruptly show different delays if
the routing changes locally, and frequent changes may cause delays to go
up and down a lot unpredictably.

-- 
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