[Ntp] Re: NTP speed of synchronization question

[Martin Burnicki <[e-mail]>]

Toerless Eckert 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:

All of the answers to the questions below depend completely on the NTP 
implementation running on the *client*.

> 1. Is there any reasonw why we should NOT be able to achieve synchronization of less than 5 msec ?

Assuming that the time provided the NTP server(s) is sufficiently 
accurate and precise, this should not be a problem if the client runs a 
good NTP implementation. It's the client who has to evaluate the time 
stamps involved in the packet exchange, calculate time offset vs. packet 
delay, identify and discard outliers, etc. See also:
https://kb.meinbergglobal.com/kb/time_sync/time_synchronization_accuracy_with_ntp

A systematic residual time offset may affect the final accuracy if there 
is some asymmetric delay on the network path between the client and the 
server(s), maybe due to different upload and download speeds. See:
https://kb.meinbergglobal.com/kb/time_sync/time_synchronization_errors_caused_by_network_asymmetries

> 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 ?

It depends on the initial range of the time offset on the client, and 
the way the client compensates the time offset: The time on the client 
can be stepped to the time received from the server(s), or it can be 
slewed fast or slow.

> 3. Would synchronization to a local peer be faster ? If so, how fast could it be ?

Basically, I don't think so, provided that the server responds in time 
and the network connection is stable enough.

The first KB article I mentioned shows that even on a WAN connection 
with a huge network delay the resulting accuracy achieved with ntpd can 
be in the range of 1 ms.

> 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 depends on whether the NTP program actually supports this. Again, if 
there is a systematic residual time offset, the client can't report it. 
If the client could determine the offset, it could also compensate it. 
So to detect such systematic offset, you need another reference time 
that is suitable for time comparisons.

Martin
-- 
Martin Burnicki

Senior Software Engineer

MEINBERG Funkuhren GmbH & Co. KG
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