[ntp:questions] SNTP with 1ms of precision?
unruh at wormhole.physics.ubc.ca
Fri Jun 18 11:05:48 UTC 2010
On 2010-06-17, Marcelo Pimenta <marcelopimentacs at gmail.com> wrote:
> 2010/6/16 David Woolley <david at ex.djwhome.demon.invalid>
>> 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).
> I thought that SNTP was used in a kind of "Intranet" with no routers and no
> internet, so the low latency there is no need of so much calculations to
> adjust the clock.
No. SNTP is a system that uses the ntp protocol to exchange time (
timestamp the output packet, the other side timestamps the receive time
and emit time, and the client then uses that to estimate the "true time
SNTP thenb uses this to discipline the local clock by some method
>> synchronization on the internet. SNTP provides the current time, the
>>> 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
>> 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 rate.
> Yeh!! You are right!! RFC is the same. But, the server can respond SNTP e
> NTP request? Or the request is the same? If not the same, who decide it? The
> client? If client ask SNTP to the server, it will respond SNTP?
NO idea what this means. SNTP uses the ntp ehternet protocol Ie, the
packets interchanged are ntp packets.
There is no "asks SNTP" There is the ntp packets and that is what sntp
>> slew rate of the device. The algorithm determines if the values are
>>> 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
>>> 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.
> Yep, Windows have a lot of "things" to do. I am using a private SO for
> specific use.
>> 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.
> So, Windows XP use SNTP when I put an IP server to sincronize through
No idea what Windows uses. It is not necessary that it uses sntp. It
could use anything, including guessing the time.
>> 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
>>> 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
>>> time servers. But if I am using a very good and reliable GPS(Meinberg)
>>> a lot of satellites giving it the correct time, and it's pluged directly
>>> 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.
> I can't use ntpd, I have an equipment for specific use with RFC2030
> implemented because of a standard that say it is mandatory SNTP - RFC2030.
> My equipment ask for time every minute and frequence tolerance of the
> crystal is less then 1ms/min. They are in a intranet with 2 switches
> isolated so... my question was about the SNTP because some people said to me
> that SNTP cannot garantee 1ms accuracy. Is it possible, in that described
> cenario to no have 1ms of accuracy?
Your specifications are silly. Yes, it is possible on a local internet
to have accuracy ( at least most of the time) be less than 1ms. I have
done it. If some kid cuts the intern et wre of switches off hte server,
there is no guarentee of any accuracy at all.
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