Leo Bicknell <bicknell@ufp.org> writes:
For instance, for a couple of thousand dollars you can get a Symmetricom appliance that will do GPS timing with analog dial backup to NIST. That gives you two non-internet sources at relatively low cost and low effort. Deploy four in different POP's and you have redundancy on your own network, and can market that you provide high quality NTP to your customers. It's nearly fire and forget, and a check for alarms from the box and make sure you watch for patches, that's about it. ... Notice in both cases I said deploy 4. If you understand the protocol, and in particular the decision process that really is the minimum number to have high quality NTP. Syncing everything to one or two NTP servers really doesn't work so well.
You can deploy four, which is the appropriate minimum number to deploy if you're doing it in-house, but four of the same brand and model does not protect you against *other* failure modes, like the problem we all experienced with TrueTime almost 9 years ago. A brief review is here: http://groups.google.com/group/comp.protocols.time.ntp/msg/5f4e774dccf34c47 Not only is it wise to have more than one chipset in play (I have Motorola and Garmin here), but it is good to have time sources from more than one place. Sure, the odds of the GPS C/A code getting it wrong on a global scale are pretty small and if it happens will create an enormous news event... Here in the future, we've taken an enormous step backwards in terms of precision time sources. Here, I only have GPS and WWVB as sources, and WWVB is not a 24-hour source (a better antenna might help this after I move, but the signal strength is not particularly good here on the east coast). Remember GOES? It's gone. LORAN? Canceled and shut down. GLONASS is fully restored to service as of last month after a bad multi-year post-Soviet hit, but good luck finding commodity-priced chipsets or reasonably priced NTP appliances that talk to it. It looks like Duke Nukem Forever may finally ship next year, but until it does I'll continue to draw unfavorable comparisons between it and Galileo. In answer to the original question, running a small constellation (four is the right number) of local stratum 2 servers in each datacenter is a no-brainer. A strong case can be made for running your own stratum 1 servers. They do not have to be on the same subnet as has been suggested (and in fact, you don't want that kind of non-redundancy as a general rule), but NTP really does want the path to the server to be symmetric, which is a big argument in favor of your own inside your network. The folks at NRC in Canada will do cryptographically authenticated NTP with you for an annual fee. I have no idea if there is something similar available from NIST in the US, but if they do I sure hope it doesn't go over the same links as time-a and time-b - from my location anyway, those two get tossed out as falsetickers on weekday afternoon due to too much jitter. -r