NTP question

Mike Hammett nanog at ics-il.net
Thu May 2 02:06:31 UTC 2019

What about GPS, GLONASS, Galileo, etc.? 

Mike Hammett 
Intelligent Computing Solutions 


----- Original Message -----

From: "James R Cutler" <james.cutler at consultant.com> 
To: "Harlan Stenn" <stenn at nwtime.org> 
Cc: nanog at nanog.org 
Sent: Wednesday, May 1, 2019 8:55:51 PM 
Subject: Re: NTP question 

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 


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, 

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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