NTP question

James R Cutler james.cutler at consultant.com
Thu May 2 01:55:51 UTC 2019

> On May 1, 2019, at 9:45 PM, Harlan Stenn <stenn at nwtime.org> wrote:
> On 5/1/19 5:39 PM, William Herrin wrote:
>> On Wed, May 1, 2019 at 12:23 PM Mehmet Akcin <mehmet at akcin.net> wrote:
>>> I am trying to buy a GPS based NTP server like this one
>>> https://timemachinescorp.com/product/gps-time-server-tm1000a/
>>> but I will be placing this inside a data center, do these need an actual
>>> view of a sky to be able to get signal or will they work fine inside a data
>>> center building? if you have any other hardware requirements to be able to
>>> provide stable time service for hundreds of customers, please let me know.
>> You buy a powered GPS antenna for it. Which antenna depends on the cable
>> length and type. The amplifier in the antenna amplifies the signal just
>> enough to overcome the cable loss between the antenna and the receiver.
>> Nice thick cables lose less signal. Dinky thin ones are easier to work with.
>> You sure you need a GPS NTP server? You understand that if you do, you need
>> two for reliability right, and probably at geographically diverse
>> locations? If you're not on an air-gapped network, consider syncing a
>> couple head-end NTP servers against tick and tock (.usno.navy.mil, the
>> naval observatory) and not worrying about it. One less piece of equipment
>> to manage, update, secure, etc.
> Two is not a great number.  If they disagree, there is no majority
> clique to be found.
> Also, there is something to be said for using different models/vendors
> for the time sources.  If you only have the same model from one vendor
> and there is a bug, you can lose all your time sources at once.   The
> GPS week rollover happens every ~19.7 years, and when that problem hits
> is a function of the firmware and a manufacturing date put in the firmware.
> These problems can be mitigated if you have "enough" time sources for
> your internal NTP servers and you peer with enough other, possibly your,
> servers.
>> Regards,
>> Bill Herrin
> -- 
> Harlan Stenn <stenn at nwtime.org>
> http://networktimefoundation.org - be a member!

To amplify the points made by Harlan Stenn:

Four is a better number locally for ntpd instances. As for different models/vendors for the time sources, I consider the GPS constellation as one vendor so I add multiple internet-connected sources as well to my ntp.conf instances.

James R. Cutler
James.cutler at consultant.com
GPG keys: hkps://hkps.pool.sks-keyservers.net

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