I just recently got a stratum 1 server set up on a FreeBSD box using
a Garmin 18lvc refclock (thanks Harlan!). Now that the system seems
to be working, my next step is to learn the monitoring tools. The
problem seems to be that different tools provide parameters with the
same labels, but different values making me thing I don't have a
complete understanding of the tools. I therefore want to ask this
group some basic questions to round out my understanding. I have read
the online documentation several time, but still feel I need some
holes filled. Hopefully this info will be good to have in the
archives to help future newcomers to ntp. Let me know if my thought
processes are good here and straighten me out where I need it.
I want to start with probably the most basic of monitoring tools: ntpq -p.
Here's the output from my system right now:
/home/brandt 53> ntpq -p
remote refid st t when poll reach delay offset jitter
==============================================================================
*GPS_NMEA(0) .GPS. 0 l 2 64 377 0.000 0.005 0.017
-ntp.your.org 130.126.24.44 3 u 47 64 377 62.065 4.289 2.750
+boudicca.tux.or 65.212.71.102 2 u 12 64 377 63.289 -5.525 2.273
-dake.desynched. 132.163.4.103 2 u 18 64 377 55.516 4.039 2.726
-veritas.imagepi 67.128.71.76 2 u 60 64 377 51.816 -8.385 2.495
-hudson.yutanigl 128.252.19.1 2 u 20 64 377 50.768 -5.847 14.944
+ferocious.dairi 140.142.16.34 2 u 18 64 377 119.579 -0.465 11.587
The first several columns are pretty straight forward. 'Remote' -
what peer ntp is getting info from; 'refid' - what source the peer is
using for its reference; 'st' - the stratum of that peer; and 't'the
type of peer - l being a local reference and the u by the remote
peers meaning unicast.
'When' is how long ago that a packet was received from that peer.
'Poll' is the polling interval in seconds, which seems to vary quite
a bit on my system, so here's the first real question - how does ntp
pick the polling interval (a pointer to the docs is fine, I just
haven't been able to uncover it yet).
'Reach' is explained very nicely in the ntp faq at
http://www.ntp.org/ntpfaq/NTP-s-trouble.htm#Q-MON-REACH.
'Delay' - the ntp docs say this is the 'current estimated delay' in
milliseconds. Is this based on the last query or is it an average
over several packets or otherwise cooked value? I'm assuming it
assumes the upstream and downstream transfer speeds are symmetrical.
My local clock always shows zero delay - is this an assumption that
the refclock always has zero delay or is it measured and just too
small to display?
'Offset' - this one I really don't think I understand adequately.
Docs say 'offset of the peer' again in milliseconds. What is offset
from what? Is it the current clock setting on my computer is 5 ms
faster than the refclock? Is this an instantaneous value or averaged
over some time interval? I know ntp slowly slews the clock to 'real'
time, so it's not just going to step the clock by 'offset' ms, but
how much does this value contribute to the steering algorithm?
And finally, 'jitter' - again not an easy concept. I've found these
definitions:
Short-term variations in Frequency with components greater than
10 Hz. The estimated time error of the system clock measured as
an exponential average of RMS time differences.
and
The estimated time error of the system clock measured as an
exponential average of RMS time differences.
as well as just a measure of the random noise component. Are the
units here ms also? A high jitter value would indicate a lot of
random noise in the signal, but what can I do if jitter is high? This
reading is very low, but there are other times when I'm showing
several hundred us (assuming that the readout is in ms). How do you
track down and kill jitter or do you worry about it (and at what
level do you worry about it)?
So, putting this all together, to find your best approximation of
UTC, do you take the clock time +delay +offset + jitter, or is that
all handled internally by ntp? ie., how far off is my reference
system from utc?
I know these are rather basic questions and I thank you in advance
for helping me round out my knowledge of how this all works.
Keith
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
LtCol Keith E. Brandt, MD, MPH
USAF-NASA Aerospace Medicine Liaison Officer
Johnson Space Center, Houston, Texas
[EMAIL PROTECTED]
Goodbye cruel world that was my home-
there's cleaner space out here to roam
Put my feet up on the moons of Mars-
sit back, relax, and count the stars
*This message transmitted with 100% recycled electrons
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