[ntp:questions] Query about NTP accuracy

Richard B. Gilbert rgilbert88 at comcast.net
Sat May 23 00:25:30 UTC 2009

Andy Yates wrote:
> Unruh wrote:
>> Andy Yates <andyy1234 at gmail.com> writes:
>>> Hal Murray wrote:
>>>> In article <4a15e001$0$18238$da0feed9 at news.zen.co.uk>,
>>>>  Andy Yates <andyy1234 at gmail.com> writes:
>>>>> Does anybody have any figures that shows the effect on accuracy of an
>>>>> NTP v3 client using a stratum 1 server rather than a stratum 2 or 3
>>>>> server? It's all in a GE LAN based scenario, commercial stratum 1
>>>>> servers connected to GPS and stratum 2 and 3 servers are typically
>>>>> dedicated Linux boxes.
>>>>> However I'm been pressed to supply an SLA for accuracy. My argument is
>>>>> that although you can get your stratum one server to synchronize to
>>>>> microseconds of UTP, as soon as the client uses NTP v3 over the LAN,
>>>>> even a GE LAN, then the accuracy degrades and putting well designed well
>>>>> specified stratum between the boxes is not going to decrease accuracy
>>>>> sufficiently to warrant purchasing many stratum one appliances.
>>>> What sort of accuracy are you interested in?  1 ms?  10 ms?  100 ms?
>>> Hi Hal
>>> Its up to us to specify what we think the SLA should be - the guide is
>>> "as accurate as possible"!
>> That is of course completely idiotic. They do NOT want it as accurate as
>> possible. That would cost them millions of dollars and is not in fact
>> needed. What are they using the time for? what kind of machines are they
>> ( Windows, linux, BSD, special home grown OS?)
>>>> How stable is your temperature?  (Both the room and the CPU load.)
>> ntp is terrible if anything varies ( absurdly long settling times).
>>> Temperature will be very stable, the DC is the very well specified and
>>> scrupulously engineered - no cables blocking air flow etc. Generally
>>> speaking the CPU is over specified.
>>>> What is the load on the LAN between the clients and servers?
>>>> (Delay is not a problem.  Variation in delay is a problem.)
>>> The NTP will be on a separate management LAN to the production traffic
>>> so not subject to the variances that application load has on the network.
>>>> I suggest you measure it.  Start with your current system.
>>>> Setup a box as a ntpd system and tell it to use several target boxes
>>>> as servers and turn on logging.  peerstats will tell you the difference
>>>> between your local clock and the target system.
>>> I'll look at the current NTP infrastructure however its completely
>>> different to the new. The old has 3 geographically diverse GPS receivers
>>> plus a GPS and radio source on the roof of the data centre and we use
>>> network components to provide the intermediate strata between stratum
>>> one and client - and have not really had many issues after almost 10
>>> years of use. However, requirements are changing and we will probably be
>>> using dedicated stratum 2/3 servers as required.
>> Does teh GPS have PPS ( puse per second) or are they just using the nmea
>> time signal? Radio about as bad as GPS with just NMEA for timing. 
> Hi Unruh
> it uses IRIG-B but can use PPS amongst others - However I should have
> said "as accurate as possible using a well designed NTP system". Our
> stratum zero sources and stratum 1 sources will be v.accurate - its the
> distribution via NTP that were trying to get our heads around.

How are you "distributing" NTP?  On a LAN?  On WAN?  What are you using 
the time for?  Do you have legal requirements for the accuracy of your 

It's possible to get all the systems on a LAN to more or less agree on 
what time it is.  +/- 20 milliseconds should be easy.  +/- 10 
milliseconds is achievable.  The tighter the agreement, the more it's 
going to cost you!  Add a WAN and it gets more difficult and more expensive.

One of the keys to getting tight synchronization is a stable and 
accurate source of time.  A GPS timing receiver can supply time of the 
required quality.  Note that GPS receivers designed for navigation 
service are generally pooly suited to timing service, and vice versa.

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