[ntp:questions] GPS driver failure mode question

Steve Kostecke kostecke at ntp.org
Tue Jan 13 02:13:16 UTC 2009


On 2009-01-12, Unruh <unruh-spam at physics.ubc.ca> wrote:

> Steve Kostecke <kostecke at ntp.org> writes:
>
>>On 2009-01-11, Terje Mathisen <terje.mathisen at hda.hydro.com> wrote:
>
>>> Steve Kostecke wrote:
>>>
>>>> ntpd _never_ serves time "from" the Undisciplined Local Clock.
>>>
>>> Never?
>
>>Never.
>
>>The Undisciplined Clock Driver does poll the system clock (often
>>misleadingly referred to as the "local clock") _but_ the poll
>>results are discarded. So ntpd never actually serves time _from_ the
>>Undisciplined Local Clock.
>
> Sorry there is some sematic distinction going on here that I do not
> follow.

The only thing that the Undisciplined Local Clock driver does is allow
an ntpd to operate at something other that Stratum 16 regardless of
whether or not the ntpd is synced to a real time source.

> ntp always "serves time" from the system clock.

The problem here is that people conflate the Undisciplined Local Clock
driver with the system clock through the misuse of the term "local
clock."

> The question is how and whether ntp thinks that system clock itself is
> being disciplined.

ntpd knows if it has enough time sources to discipline the clock.

ntpd can be tricked with the Undisciplined Local Clock driver.

ntpd has no way of knowing if some outside force is disciplining the
system clock in the "background".

> If it is disciplined by nothing it does not act as a server, and says
> so. If it is "disciplined " by local clock, it is still disciplined by
> nothing, but can act as an ntp server,

The Undisciplined Local Clock driver is not used by ntpd to discipline
the system clock.

> and tells outside clients that it is being disciplined by the local
> clock.

Use of the Undisciplined Local Clock driver allows an ntpd to claim that
it is operating a some arbitrary stratum level with an acceptable root
dispersion. The reported Ref-ID is irrelevant.

-- 
Steve Kostecke <kostecke at ntp.org>
NTP Public Services Project - http://support.ntp.org/




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