[ntp:questions] SNTP with 1ms of precision?

David Woolley david at ex.djwhome.demon.invalid
Wed Jun 16 20:42:52 UTC 2010

Marcelo Pimenta wrote:

> Rob, my understading about the use of SNTP and NTP is: while SNTP provides
> time synchronization within *one *network, NTP allows a global time

You are confusing it with timed.  SNTP also expects to use global time, 
and NTP can be used with an arbitrary timebase, providing all stratum 0s 
have the same arbitrary time (i.e., typically there is only one).

> synchronization on the internet. SNTP provides the current time, the current
> number of leap seconds and the warning flags marking the intriduction of a
> leap second correction.
> The NTP algorithm is much more complicated than the SNTP algorithm. NTP
> normally uses multiple time servers to verify the time and then controls the

There is no SNTP algorithm, beyond basic validity checking.  SNTP covers 
any use of NTP wire formats that falls short of being a compliant NTP 
implementation.  SNTP implementations could still use PLL clock 
discipline code, or could use linear regression solutions for offset and 

> slew rate of the device. The algorithm determines if the values are accurate
> using several methods including fudge factors and identifying time servers
> that don't agree with the other time servers. It then speeds up or slows
> down the device drift rate so device time is always correct and there won't
> be any subsequent time jumps after the initial correction.
> SNTP usually uses just one Ethernet Time Server to calculate the time and
> then it "jumps" the system time to the calculated time. It can, however,

One issue that has maybe not been stressed is that this strategy makes 
you extremely vulnerable to scheduling delays, e.g. you would likely 
never get a 1ms error bound on Windows if you used this strategy.

Early W32Time implementations, and current ones in out of the box 
configurations are SNTP implementations, but often use internet sources. 
  They do violate the specification by allowing multiple SNTP strata.

> have back-up Ethernet Time Servers in case one is not avaiable(not two at
> the same time).
> In my case, I have* only one* network. My Time Server is not a machine, is a
> meinberg GPS. In my point of view, if my source time were machines, maybe
> NTP could be better to find a middle line between all these machines used as
> time servers. But if I am using a very good and reliable GPS(Meinberg) with
> a lot of satellites giving it the correct time, and it's pluged directly in
> my switch, I think that in this case, NTP will not make any difference.

I think you are being asked why you don't just use a standard ntpd for 
the platform, and also how you know that your total system meets the 
scheduling constraints for 1ms accuracy with SNTP.

More information about the questions mailing list