Active Directory Domain - time critical application

Hi guys.

we use a typical NTP configuration in our AD domain (NTP Server --> Root Domain PDC Emulator --> other PDC Emulators --> member servers.

We now need to install a particularly time critical application (say +/- 1 sec tolerance). Do you think the standard NTP configuration can guarantee such tolerance ? Or should I connect this server to a dedicated stratum one device ? Or other setup ?

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Cliff GaliherCommented:
That depends *entirely* on the hardware that the application will be running on.  *MOST* server class hardware has a predictable clock skew, and so the windows NTP algorithms for tracking and adjusting skew would fall well below the 1 second variance.  However I've seen hardware where the timing mechanism is very erratic, and honestly even a dedicated device wouldn't help. The device would make sure that the adjustment gets the device back on track, but the skew between adjustment intervals could still be an issue.

Ultimately windows is not an RTOS and was never architected to be. Everything is processed "best effort" and if RTOS is an absolute requirement, windows is not a good fit.

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ferraristaAuthor Commented:
Hi Cliff.

this would most probably be a vmware virtual machine, hosted on enterprise class hardware, if that makes a difference.

Cliff GaliherCommented:
VMs are more susceptible to clock skew, regardless of the host technology.  Same issue as already described, NTP can usually handle it, but when it fails, so would having a higher stratum dedicated device. The OS itself would be preempted by another host task and when it "comes back" its clock is off until the designated task to adjust it fires off.  With VMs, this usually is resynced with the host itself, but it basically means you don't get a guaranteed +/- 1 second and NTP vs dedicated device doesn't really change that.

In general, where that is important, I would not use a VM and I would dedicate a device. Not because NTP is unreliable, but because NTP still has a dependency on a network connection to a machine not in your control.  That's where a good dedicated machine and a dedicated time source are still the only sure-fire solution.  But given the cost, I'd usually only do this for mission critical needs, such as 911 dispatch centers where an accurate accounting of time is important even well after an initial incident has ended.  Not quite "real time" (again, windows isn't an RTOS), but reasonably accurate per human standards.
I see two scenarios I'd recommend:

For a server with dedicated hardware I'd think about a built in RTC card, like i.e. this one or one of its relatives for other time sources.

For any VM, I'd think about one (or two) dedicated time server appliances, like this one or one of its relative for other time sources or form factors. To minimize network disturbance, I'd connect it/'em to the VM host on speparate network port(s), dedicated only to that VM with crossover cable(s) (no switch).

With both solutions I'd expect a stable time in the range of 10...20 ms compared to the source servers, if the server (THE server if dedicated, VM-host and VM if virtual) is not stressed too much.

BTW: I'd recommend a Windows port of the classic *ux NTP daemon in favor of W32time ... see my article on NTP basics for details.
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