[ntp:questions] Re: What accuracy should I expect on a LAN?
Richard B. Gilbert
rgilbert88 at comcast.net
Wed Feb 8 22:47:34 UTC 2006
skip at pobox.com wrote:
> Our LAN has a mixture of Solaris and Linux machines. Most are Intel
> boxes, but a few are SPARCs. We use xntpd across the board.
> Connection speeds are 100Mbit or Gigabit ethernet. We have three
> internal NTP servers that sync with the outside world.
> How close should we expect NTP to keep the clocks on the computers
> Maybe it's that I'm having trouble interpreting the last three columns
> of ntpq's output. The Solaris 10 man page says:
> The current estimated delay offset and dispersion of the peer, all
> in milliseconds.
> but doesn't define "delay", "offset" or "dispersion". I can guess at
> the first two, but the third means absolutely nothing to me.
> Can someone provide or point me to some more readable/complete
> description of what this all means?
Solaris 9 should be able to synchronize to within 200 microseconds of a
stable source over a 100 megabit LAN using ntpd (note no "x") V4.2.0. I
haven't installed Solaris 10 yet but I would expect it to do as well or
better with the current production release of ntpd (no "x"). The xntpd
shipped with Solaris 8 & 9 is version 3-5.93e and almost certainly will
not synchronize as well as 4.2.0; there have been many fixes and
enhancements since V3-5.93e.
Note that the "stable reference" is required! I use an Ultra 10 440MHz
machine running Solaris 8, ntpd 4.2.0 and a Motorola M12+T GPS timing
receiver. Another Ultra 10 running Solaris 9 and separated from the
first by about twelve feet of cable and a Linksys 100MB switch,
synchronizes to within 200 microseconds. It frequently does better but,
at the moment its "estimated error" as given by ntptime is 156
microseconds; I've seen as low as 41 microseconds.
A client needs many hours to pull in to really tight synchronization.
If the server is subject to the vagaries of internet transmission it
will almost certainly not be sufficiently stable. With four or five
internet servers configured as the sole source of time, I expect a
Solaris server to synch up within five or ten milliseconds. It has been
my experience that five or six good internet servers may be spread over
an interval of five or six milliseconds.
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