[ntp:questions] Client doesn't drop failed source

David Woolley david at ex.djwhome.demon.invalid
Mon Jan 11 23:25:39 UTC 2010

Michael Moroney wrote:
> Can anyone point me to anything similar to "How to explain NTP to Project
> Managers" esp. explaining how a preferred clock server is included even

Unfortunately its primary author fancies himself as a mathematician, so 
the main documentation is a mathematical treatise.  He has produced some 
powerpoint sort of material, but probably more for academics than 
financially oriented managers.

> though the LAN cable is known to be dangling from the rack.  Most of which

For a start, if it dropped a source on the first missing reply it would 
result in clock hopping, which is undesirable.  UDP is unrliable, and 
you cannot rely on getting every query returned.

Looking at the, slightly out of date, 4.2.4p4, a reachability of 1/8 is 
still acceptable.  Rejection during startup will be based on a high 
jitter, causing a high root distance, rather than on the number of 
replies in the last 8 polls.

Basically sources are dropped when they can no longer provide valid 
input to the time estimation process.  The quality of their input 
reduces with age, but it doesn't suddenly drop to zero after one poll 
interval; in fact a source may still have lower error bounds than the 
other options even when it has been rejected on reachability.

Using prefer will probably result in using sources whose error bounds 
are rather high.

Incidentally, another thing that managers like doing is analysing the 
dynamics of the system when they change the client clock manually.  ntpd 
can help cope with poor quality hardware, but it is working completely 
out of specification when asked to deal with a simulation of such a 
gross hardware fault.

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