On 12/02/2014, at 1:07 pm, Nathan Ward
At least 3 - you need 3 to NTP to do its job and eliminate bad tickers. If you only have 2, you end up with an average between them, even if one is an hour out.
Last I heard, you need four servers to eliminate false tickers from which I’m guessing that eliminating false tickers is a Byzantine Generals problem. Note that this is to eliminate false tickers from within said four servers i.e. it detects which of your servers is telling fibs and not which of your upstreams might be telling fibs. I think the only way you can protect against a faulty or malicious upstream NTP server is to have more upstream NTP servers. Jay mentioned a while back that since their servers are not peers, but individually locked to GPS (a stratum 0 clock) they don’t have quite the same issues that other setups might have. Cheers, Lloyd