[ntp:questions] Compensating for asymmetric delay on a per-peer/server basis?

Harlan Stenn stenn at ntp.org
Thu Sep 11 22:46:18 UTC 2014


There are a bunch of issues here, and I don't think there is a simple
answer.

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".
-- 
Harlan Stenn <stenn at ntp.org>
http://networktimefoundation.org - be a member!


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