[ntp:questions] how to have offset < 1ms

nemo_outis abc at xyz.com
Wed Apr 14 20:19:16 UTC 2010

Rob <nomail at example.com> wrote in
news:slrnhsc00l.l0m.nomail at xs8.xs4all.nl: 

> nemo_outis <abc at xyz.com> wrote:
>> True, one can chase any number of will-o-the-wisps such as the causes
>> of the network latency and asymmetry, but that is not the central
>> problem posed by the OP.
> But it is!
>> In short, Unruh may be boring in his reiteration of his simple GPS 
>> solution, but that is because it works for a very wide range of
>> problems - including this one!
> No, not at all.  The poster wants to be within 1ms of his chosen
> reference clock, and syncing to GPS is bringing nothing towards that.

First, the OP isn't going to synchronize anything with anything to 1 ms 
given the high and irregular latencies on his network. In short, he doesn't 
have a time problem, he has a network problem.

Second, it is true that the OP presented his problem as merely keeping 
Lamport time linked to an arbitrary shipboard server (ntpgmtaceb) of 
unknown stability and accuracy - a task which is presently unachievable and 
may ultimately be of limited utility.  But, of course, it's up to the OP to 
decide if such a makeshift approach encapsulating what may be charitably 
called ntp "worst practices" is "good enough."  

However, it would be irresponsible for Unruh and others here not to point 
out that a much better timekeeping solution is readily available - a 
solution which is technically easy to implement.  

Unruh has rightly pointed out that installing a GPS sensor would be clearly 
better than the current ludicrous 18th century policy of setting the master 
clock at voyage outset and then earnestly hoping that subsequent drift (or 
other timekeeping irregularities) is not "too bad."   That the OP may not 
be able or willing to implement such a straightforward solution (whether 
from his lack of stroke or his superiors' indifference) does not invalidate 
the suggestion.

Moreover, that you find Unruh's pointing out the elephant in the room 
tiresome only indicts your judgment, not his.


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