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

David Woolley david at ex.djwhome.demon.invalid
Sat Jan 9 09:55:47 UTC 2010

Michael Moroney wrote:
> David Woolley <david at ex.djwhome.demon.invalid> writes:

>> If it remains the best choice it could take up to 8192 seconds before 
>> all the samples are flushed from the FIFO and it becomes completely 
>> ineligible.  There is logic to discourage early switching, as clock 
>> hopping is, itself, considered a bad thing.
> OK, is 8192 the default time before a nonresponsive host gets dropped?

8192 was an oversimplification, as:

1) I think that the reachability has to be more than 1/8 before the 
source is considered valid - however a reachability of 7/8 is definitely 
still considered valid;

2) In a failing situation, the poll rate may start to reduce;

3) If the peer isn't the preferred peer, you might trip the rules for 
switching peers simply because another one is better and enough time has 
passed (I'd have to check whether the anti-clock hopping is based on 
sample counts or elapsed time).

8 * maxpoll guarantees that all samples for a source have been flushed 
and it cannot possibly be used, but the cutoff may be more like 4 * 
maxpoll (I need to check the code - this apparent limit may actually be 
more the result of jitter calculations during startup and might not 
apply to the loss of a source).

NTP tries to set the poll rate so that the error due to using a sample 
that is 8 poll intervals old is negligible when taking into account the 
PLL bandwidth.  It is oversampling the source by a factor of more than 
8.  If a source continues to respond to polls it can and often does use 
a sample that is 8192 seconds old for a poll interval of 1024.

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