[ntp:questions] Time sync on a windows domain

Jason Rabel jason at extremeoverclocking.com
Thu May 24 14:55:08 UTC 2007

Windows uses SNTP to fetch time which is more or less a one-shot deal, same
with ntpdate on *nix. SNTP is just a stripped down version of a NTP packet.
(You will notice in the internet time tab it says "next sync at...") Your
clock will be most accurate right after it syncs, but will slowly drift
without any correction. Depending on how good (or bad) the PCs crystal is,
some clocks will drift more than others. NTP is a continually running
daemon/service which will constantly be adjusting things to make sure your
time is as accurate as it can be with the sources you provide.

Depending on your needs, you might just want to stick with window's SNTP
(w32time) on the XP machines, but have them sync more often (you can change
the parameter in the registry - Google is your friend on this one). The
server machines you might want to switch over to NTP as their time accuracy
can be more critical (especially for domains - if time is too far off people
won't be able to login).

There are lots of companies out there that offer NTP based GPS devices.
Symmetricom, Zyfer, etc, etc (I'm sure there is a good list somewhere). But
you will need access to your building's roof (or at minimum a very good
window view in the right direction) for the GPS antenna. Once you hook them
up and they are locked, the device has an IP address like anything else on
your network and will respond to NTP, SNTP, and other time-related requests.
Check out some of the brochures on the Symmetricom site, they will tell you
how many thousands of requests per second each machine can handle while
still maintaining a certain level of time accuracy across the network.

Another alternative is to use a BSD or Solaris machine and hook a GPS (with
PPS signal) up to it. I say those two OSes because they already have build
in PPSAPI support. Linux will require kernel patching and you probably
wouldn't want to do it on a production machine. If you really want to be a
stickler for time on your servers you could build you own little "black box"
to distribute a PPS signal to all the Solaris machines. With a GPS based PPS
source you can get sub-millisecond accuracy on that device. However, over a
local network you can still maintain time within a couple ms.

IIRC, the 2003 time service is really just a SNTP only server (you can
actually enable the SNTP server on an XP machine for people without a MS
server product, but that's another story). If you installed a windows port
of the NTP program you would get the full blown NTP client / server
abilities, so those machines would be able to distribute more accurate time.

I think I might have rambled some, but I hope it answers your questions or
at least gets you thinking some. ;)


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