[ntp:questions] why does asterisk still show after the ntp server is shutdown?

Rob nomail at example.com
Sat Nov 17 08:49:57 UTC 2012

Brian Utterback <brian.utterback at oracle.com> wrote:
> On 11/16/2012 6:03 PM, Rob wrote:
>> Harlan Stenn <stenn at ntp.org> wrote:
>>> I don't understand the problem.
>>> It doesn't matter why the destination machine is unreachable, or when it
>>> becomes unreachable.
>>> Time from that source is valid for "a while", and after a known number
>>> of unsuccessful attempts to reach that server it will be unselected.
>>> So what's the issue?
>> It is just another incarnation of a FAQ in this group.
>> People apparently get tasked with evaluating and testing the performance
>> of NTP in the case of errors and failures.  When it does not perform
>> as well as they (or their bosses) hope, they come and ask here.
>> They like instant detection of server outage, predictable action in
>> case of failure of primary or secondary servers, and usable interfaces
>> for error reporting and handling.
>> It is like the other FAQ: how can I achieve perfect time synchronization
>> on my lousy hardware.  Just when you think 10ms is pretty good, they
>> ask for 1ms.  And when 1ms seems reachable, they want 1us.  And not just
>> a best effort to be in that ballpark, but a "guarantee".
>> It is not like most people really need that accuracy, but they get
>> specifications that are written up by people who do not understand how
>> hard it is to achieve them on standard computer and network hardware.
>> The problem is not that ntpd would not perform well in cases occuring
>> during day-to-day use, they want to write documents that tell the
>> client or user what will happen when there is a problem and how this
>> will be signaled and handled.
> Harlan's point is a good one. Whether or not it would be difficult to 
> receive ICMP's and match them up, it is not worth the trouble since 
> there is no action to take from the information. The mechanism for error 
> detection and recovery is the same regardless of whether you got 
> notification actively through ICMP or passively through a missing packet.
> I might see some value of reacting to a port unreachable ICMP, but not 
> to a host unreachable one.

In the above I was not specificially referring to possible handling
of ICMP, but to two frequently asked questions on this group.

They usually are asked by people apparently new to NTP and obviously
tasked with deploying NTP in a setup where there are specifications
and there are tests to be done to see if those specifications are met.

They deploy a test setup, do harsh things like shutting down servers
or jerking system clocks (that probably are written up in a test
document), and observe behavior that is different from what they hoped
would happen.

Sometimes it can be explained by use of old versions that had bugs,
sometimes it is the intended behavior that is simply different from
what the user expected, and sometimes they have a reasonable point
and it could be discussed.

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