[ntp:questions] Compensating for asymmetric delay on a per-peer/server basis?
martin.burnicki at meinberg.de
Fri Sep 12 07:48:19 UTC 2014
Harlan Stenn wrote:
> There are a bunch of issues here, and I don't think there is a simple
> For starters, there is static asymmetry and dynamic asymmetry.
> One of the core issues is that NTP is frequently multihop, and the
> routing for at least some of these connections can spontaneously change.
> Declaring an asymmetry correction for an interface will affect all
> connections over that interface. Sometimes that's OK, sometimes not.
> Declaring an asymmetry correction for a given remote server hardcodes
> assumptions that almost certainly will change over time.
> The trick is that we don't know how many hops there are between "here"
> and "there", and the number and location of these hops can change.
> Precision Time Protocol looks closer at these issues, and PTP is
> designed to work on point-to-point links.
> So if one can use a local good reference time and use those timestamps
> to compare with remote good reference times, one can have a better
> chance to identify some of the static asymmetry issues.
> Dealing with the dynamic ones is harder...
> The soution seems to depend on having multiple sources of good time, and
> having access to these good time sources via different "paths".
I think an IP based rules could do the job. See my other reply.
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