[ntp:questions] Re: status of multicast

David Woolley david at djwhome.demon.co.uk
Mon Nov 29 20:37:37 UTC 2004

In article <cofbpj$3li$1 at dewey.udel.edu>, David L. Mills <mills at udel.edu> wrote:

> The initial clock offset is computed with unicast and should not involve 
> a step adjustment, unless the initial system clock is off by over 128 

There are two possible interpretations of the system described here.

Let's assume that terrestial propagation delay is 40ms and satellite
delay is 300ms.  In the more likely scenario, the unicast measurement
will measure a round trip time delay of 340ms and estimate a propagation
delay of 170ms, which will be 130ms too low, so the time will be wrong
by more than the step limit, although things may not step unless there
was another source of time that was actually correct.  In this, case,
the time estimate from the unicast measurement will have the same error
as from multicast measurements.

The other interpretation, which better fits the wording of the enquiry,
but would be unusual for such a system, is that the unicast is purely
terrestial, in which case the estimated delay will be 40ms and the 
time measurement accurate on the unicast measurement, but the multicast
will be undercompensated, so will be slow by 260ms.  If the system were
to make use of the unicast time estimate, it would get a conflicting
estimate from the multicast and likely go into step error recovery.
If it ignores the unicast offset, it will simply have a large static
error, as in the first case.

(Normal satellites assume a consumer type user that makes small requests,
over a terrestial back channel, and receives large responses, over the

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