[ntp:questions] Asymmetric Delay and NTP

William Unruh unruh at invalid.ca
Mon Mar 17 05:33:52 UTC 2014


On 2014-03-16, Joe Gwinn <joegwinn at comcast.net> wrote:
> In article <lg5bug$682$1 at dont-email.me>, David Woolley
><david at ex.djwhome.demon.invalid> wrote:
>
>> On 16/03/14 22:16, 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?
>> 
>> I would have thought it was obvious.
>
> It was generally believed that this was true, but what was lacking was
> the mathematical proof that no message-exchange protocol, however
> clever, could overcome delay asymmetry.  I recall that someone finally
> found a proof.

A single equation with two variables. The proof IS obvious.
Of course if the delay assymetry changes from one packet to the next,
then there is a possibility. 

>
> Joe Gwinn



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