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

unruh unruh at wormhole.physics.ubc.ca
Thu Jun 17 10:08:56 UTC 2010

On 2010-06-16, Marcelo Pimenta <marcelopimentacs at gmail.com> wrote:
> 2010/6/16 Rob <nomail at example.com>
>> Marcelo Pimenta <marcelopimentacs at gmail.com> wrote:
>> > The idea to have this is not overload the network asking time every
>> second
>> > to keep my accurancy in 1ms in 100% of time. And the second reason is
>> about
>> > how many ms your clock will be wrong in 59 sec without a frame to
>> discipline
>> > my clock again? PCs clock for example is not good, that's why I need a
>> very
>> > good crystal.
>> Why are you causing your own problems by sticking to SNTP instead of
>> using NTP which was designed to overcome the problems you fear?
> Rob, my understading about the use of SNTP and NTP is: while SNTP provides
> time synchronization within *one *network, NTP allows a global time
> 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
> 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.

No. sntp is used either for a stratum 0 time source, or a terminal
client which will never be a peer. The algorithm is suppposed to be the
same for both. There is nothing about "one network" or anything else. 
YOu seem to be describing the difference between the rdate protocol and
the ntp protocol. 

> 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,
> have back-up Ethernet Time Servers in case one is not avaiable(not two at
> the same time).

No. That is NOT sntp. It is rdate. Why anyone would use rdate nowadays I
have no idea. 

> 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

A Meingerg GPS IS a machine.

> 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.

Sure it will. ntp is designed to deliver as accurate a time as poosible
It does not care if the soruce is anohter computer, a GPS a radion
transmitter, a CDMS clock broadcast, or someone's wristwatch.

One of its key purposes is to make sure that it gets rid of time
transmission delays as much as possible, and to make sure that the
clocks it uses are sensible. 
Note that your Mienberg gps could also give bad time. Someone could
decided that they should enclose it in a coper wire mesh, making the gps
signal inaccessible. Or the computer could fail. or....

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