[ntp:questions] strange behaviour of ntp peerstats entries.

Unruh unruh-spam at physics.ubc.ca
Sun Feb 3 05:32:36 UTC 2008


David Woolley <david at ex.djwhome.demon.co.uk.invalid> writes:

>Unruh wrote:

>> 
>> This is in the clock_filter algorithm. It selects the sample of the last 8
>> which has the lowest delay (suitably aged) If that sample is the most

>Yes.  That's what I am talking about.  Specifically clock_filter in
>ntp_proto.c.

>> recent, then it is actually used. Otherwise nothing is used that that
>> clokck measuremnt is tossed. Once it has a measurement sucnh that the
>> latest sample aslo has the least delay then that sample is passed to the
>> clock selection process.

>This is not what happens in RFC 1305 or ntpd 4.2.0.

>> 
>> NO I am not talking about the clock selection process. It is teh

>Nor am I.

>> clock_filter which occurs before the clock selection algorithm comes into
>> play.

>/*
>  * clock_filter - add incoming clock sample to filter register and run
>  *                the filter procedure to find the best sample.
>  */
>void
>clock_filter(
>....
>         /*
>          * Sort the samples in both lists by distance.
>          */
>         for (i = 1; i < NTP_SHIFT; i++) {
>....

Under ... is the line
dst[i]=peer->filter_delay[j]

which is the typical assignment.

Ie, the peer->filterdelay is the distance

>        /*
>          * If no acceptable samples remain in the shift register,
>          * quietly tiptoe home leaving only the dispersion. Otherwise,
>          * save the offset, delay and jitter average. Note the jitter
>          * must not be less than the system precision.
>          */
>         if (m == 0)
>                 return;
>         etemp = fabs(peer->offset - peer->filter_offset[k]);
>         dtemp = sqrt(peer->jitter);
>         peer->offset = peer->filter_offset[k];
>         peer->delay = peer->filter_delay[k];
>         if (m > 1)
>                 jit /= m - 1;
>         peer->jitter = max(jit, SQUARE(LOGTOD(sys_precision)));

>         /*


>Guess I'd better check that this hasn't been broken in 4.2.4.....

>There are quite a lot of changes there, but the basic structure seems to 
>be the same, i.e. it is based on distance, not delay, and based on 
>sorting the full list.






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