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NTP Issues Today

On Wed, Nov 21, 2012 at 10:41:01AM -0500, Jay Ashworth wrote:
> "... against multiple [Stratum 1] sources..."
> Baby, if you've ever wondered... whether it matters whether your sources
> are strat 1 or not, now you know -- since there's no real way to get 
> provenance on down-strat time sources that I'm aware of.
> Does the NTP code, people who know, give any extra credence to strat-1
> sources in it's byzantine code?

	Not in a way that matters if one of them suddenly becomes a 
falseticker.  If a reference clock goes insane, it's pretty easily 
detected provided you have at least two more servers (or even
peers configured.)

	Stratum 1 just means it thinks it has a reference clock
attached, but those clocks fail, go into holdover, what have you
all the time.

	NTP will happily select a stratum 2 or lower clock instead
provided it appears stable (low jitter, responded to our last 255
queries, and is an eligible candidate.)

	To get an idea what your NTP server will do, try ntpq -p:

msa at paladin:/home/msa (582)$ ntpq -p
     remote           refid      st t when poll reach   delay   offset
-nist1.symmetric .ACTS.           1 u  304 1024  377    5.140    3.271
+nist1-sj.ustimi .ACTS.           1 u  307 1024  377    7.843    5.227
+  .ACTS.           1 u  414 1024  377    9.406    5.742
*usno.pa-x.dec.c .USNO.           1 u  540 1024  377    1.373    4.242
-pegasus.latt.ne   2 u  304 1024  377   61.383    5.920
-pyramid.latt.ne   2 u  361 1024  377    1.076    4.181

	This is a stratum 2 server in the public pool.  It's peering
with two other stratum 2 servers that I run.  Those two are deselected
(-).  The server marked with a * is selected, and those with a + are
included in a weighted averdage used to maintain the system clock.
If the primary selected server does something wonky, it's going to 
select one of the candidates marked with a +.

	In this case it has enough stratum 1 servers that it's not
likely to fall back to its peers, but it can do so if those servers
suddenly give it a set of unexpected replies.