[ntp:questions] Asymmetric Delay and NTP
terje.mathisen at tmsw.no
Mon Mar 17 06:23:14 UTC 2014
Joe Gwinn wrote:
> I recall seeing something from Dr. Mills saying that a formal proof had
> been found showing that no packet-exchange protocol (like NTP) could
> tell delay asymmetry from clock offset. Can anyone provide a reference
> to this proof?
This is similar to Einstein's relativity example/proof:
He claimed that there was no possible method to differentiate between
gravity and the force you would experience within an elevator box (i.e.
rocket) with constant acceleration.
Likewise, there is absolutely no way to differentiate between an actual
clock offset and a _constant_ delay asymmetry: Both cause a constant
difference in the measured (by ntpd) clock offset.
When things are not constant, including spurious packets that suffer
little or no extra delay, then you _might_ be able to see a difference
between the two possible causes.
- <Terje.Mathisen at tmsw.no>
"almost all programming can be viewed as an exercise in caching"
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