[Ntp] Re: NTP speed of synchronization question

[Watson Ladd <[e-mail]>]

On Wed, Aug 14, 2024 at 9:09 AM Toerless Eckert <[e-mail]> wrote:
>
> Dear NTP WG
>
> I have a few quick ops question against NTP which would take me a lot longer to investigate
> on actual devices. This is for reviewing draft-ietf-bess-evpn-fast-df-recovery which wants
> to utilize clock synchronization between routers.
>
> Assuming typical high-end routers and their implementation of NTP:
>
> 1. Is there any reasonw why we should NOT be able to achieve synchronization of less than 5 msec

What's the performance characteristic you want, and how much do you
want to do to get to it? You can get to single digit milliseconds with
right design, and start pushing beyond that with PTP easily. Some
exotic designs go down to nanoseconds, but it's not that exotic, just
a matter of making the pieces go together. How bad can it get when
people screw up? Pretty bad.

>
> 2. Assuming synchronization only across some WAN connection, how long would it typically
>    take for a restarting router to synchronize against a peer to that level of accuracy or better ?
>
> 3. Would synchronization to a local peer be faster ? If so, how fast could it be ?

The synchronization speed is entirely dependent on the PLL
characteristics. Implementations and hardware vary in how good they
are at this.

If the router comes up with no idea of the current time, and limits on
the maximum frequency shift, it could take a while to stabilize,
especially if steps aren't allowed. If the clock can be stepped to the
right time, it would be a lot faster. If the RTC is good, again a lot
faster. Putting a clock on each physical hop and arranging the
inheritance accordingly can do good things or bad things depending on
network conditions and the implementation.

>
> 4. Am i correct to remember that the accuracy of synchronization should be accessible to
>    user of NTP (such as the aforementioned drafts mechanism) through some diagnostics interface ?
>    ( think to remmeber that to be the case from typical NTP implementation CLI).

It is in most implementations but they vary in how they do that.

>
> Thanks so much!
>     Toerless
>
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