[ntp:hackers] Fw: Internet delays affecting NTP timing - feature or bug?

Miroslav Lichvar mlichvar at redhat.com
Tue May 28 15:32:26 UTC 2019


On Tue, May 28, 2019 at 10:57:21AM -0400, Danny Mayer wrote:
> Can you send details of this to at least Miroslav and myself so we can
> look at what you are seeing? It's quite possible that the networks are
> not giving the NTP packet priority as it isn't coming from the NTP port.
> Pure speculation on my part right now without knowing any details.

The question was asked on a different list and there was a pointer to
a paper which explains that routers may have several routes with the
same weight and the hashing may include the source port, so NTP
exchanges between the same addresses, but different ports may be
identified as different flows and routed differently.

http://leapsecond.com/ntp/NTP_Paper_Sommars_PTTI2017.pdf

On page 7 there is a nice example.

This should be possible to detect with UDP traceroute. I don't think
it's a big problem for NTP, because the clock filter should prefer
measurements that took the shortest path.

-- 
Miroslav Lichvar


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