[ntp:questions] Order of servers in ntp.conf

Steve Kostecke kostecke at ntp.org
Thu Aug 15 14:30:34 UTC 2013


On 2013-08-15, unruh <unruh at invalid.ca> wrote:

> On 2013-08-14, Steve Kostecke <kostecke at ntp.org> wrote:
>
>> The time server specified in each of those lines is the one which is
>> currently selected as the "sys_peer".
>
> But as I understand it, it is simply one of the systems which is
> not a false ticker, and has not real significance other than that.
> Ie, its time is not treated any differently than any of the other
> systems regarded as true chimers by the selection algorithm. Or do I
> misunderstand something?

The sys_peer is chosen after the select[ion] alrorithm scans the
associations for selectable candiates _and_ after the cluster
(combine?) algorithm casts out the outliers.

In the last paragraph at
http://www.eecis.udel.edu/~mills/ntp/html/warp.html we see:

"The algorithms described on the Mitigation Rules and the prefer Keyword
page combine the survivor offsets, designate one of them as the system
peer and produces the final offset used by the algorithm described on
the Clock Discipline Algorithm page to adjust the system clock time and
frequency."

Section 6 of the Mitigation Rules page
(http://www.eecis.udel.edu/~mills/ntp/html/prefer.html) 
clarifies how the process works:

"As previously noted, the cluster algorithm casts out outliers, leaving
the survivor list for later processing. The survivor list is then sorted
by increasing root distance and the first entry temporarily designated
the system peer. At this point the following contributors to the system
clock discipline may be available:

* (potential) system peer, if there are survivors;
* orphan parent, if present;
* local driver, if present;
* modem driver, if present;
* prefer peer, if present;
* PPS driver, if present.

The mitigation algorithm proceeds in three steps in turn.

1. If there are no survivors, the modem driver becomes the only survivor
if there is one. If not, the local driver becomes the only survivor if
there is one. If not, the orphan parent becomes the only survivor if
there is one. If the number of survivors at this point is less than the
minsane option of the tos command, the algorithm is terminated and the
system variables remain unchanged. Note that minsane is by default 1,
but can be set at any value including 0.

2. If the prefer peer is among the survivors, it becomes the system peer
and its offset and jitter are inherited by the corresponding system
variables. Otherwise, the combine algorithm computes these variables
from the survivor population.

3. If there is a PPS driver and the system clock offset at this point is
less than 0.4 s, and if there is a prefer peer among the survivors or
if the PPS peer is designated as a prefer peer, the PPS driver becomes
the system peer and its offset and jitter are inherited by the system
variables, thus overriding any variables already computed. Note that a
PPS driver is present only if PPS signals are actually being received
and enabled by the associated driver.

If none of the above is the case, the data are disregarded and the
system variables remain as they are."

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



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