[ntp:questions] Re: Time mysteriously advancing with FreeBSD 5.3 and ntpd 4.2.0-a

Mxsmanic mxsmanic at hotmail.com
Tue Dec 28 08:25:59 UTC 2004


Richard B. Gilbert writes:

> You may have multiple problems but I think you have at least a problem 
> with your network connection and/or choice of servers.  Delay figures 
> of  over 100 milliseconds seem unreasonably high and leave a lot of 
> uncertainty as to the correct time!

I'm in Europe, so delays of 100+ ms to the US are unfortunately typical.

> Ideally, the round trip delays would be symmetrical but the world
> is seldom ideal.

Oddly enough, my impression is that delays to US servers are longer, but
more consistent over time, at least over intermediate periods (hours).

> The servers with the lowest delays appear to be in Denmark which is a 
> little strange if you are in the USA.

They're in Germany.  I figured German servers would be trustworthy.

> If you are located in or near Denmark, why are you using all
> those servers located in the USA?

Because I've had a hard time finding trustworthy, public servers in
Europe.

> The best servers will be those located closest to you in terms of 
> delay.  The delay has many components: the physical distance the signal 
> must travel, the number of hubs, switches, and routers the signal must 
> pass through, how heavily the server is loaded, and how busy the network 
> is.  Low numbers are good; high numbers are bad.

I've tried servers right here in my metropolitan area a few times, but
they seemed to be more random in their response times and delays than
faraway servers.

> Next, your offsets are rather large.  This may be due to a number of 
> things but the most likely two are that your ntp daemon has not been 
> running for very long and that you did not set the clock before starting 
> the daemon!   Use the -g option to start ntpd.  That will set the clock 
> to the correct time.

I set the -g option but the daemon had not been running long when I ran
those numbers.  This is what they look like now, after many hours:

 # ntpq -p
      remote           refid      st t when poll reach   delay   offset  jitter
 ==============================================================================
  tick.usno.navy. .USNO.           1 u    9 1024   17  147.285  -87.954  23.298
 +time-a.nist.gov .ACTS.           1 u    4   64  377  109.996  -103.77   3.370
 *time-b.nist.gov .ACTS.           1 u   12   64  377  110.707  -108.44   1.700
 -tock.usno.navy. .USNO.           1 u   49   64  277  147.667  -112.26 157.350
 -time-nw.nist.go .ACTS.           1 u   27   64  377  193.522  -110.77   3.410
  time-A.timefreq .STEP.          16 u    - 1024    0    0.000    0.000 4000.00
  time-A.timefreq .STEP.          16 u    - 1024    0    0.000    0.000 4000.00
 -time-A.timefreq .ACTS.           1 u    5   64  377  177.909  -122.46   3.375
 +ntp1.ptb.de     .PTB.            1 u    2   64  377   54.645  -103.43   2.509
 +ntp2.ptb.de     .PTB.            1 u   59   64  377   53.357  -105.80  13.954
  pix.mxsmanic.co 10.0.0.80        3 u   43  256  376    0.002  -39.003   1.586
 # 

Looks like ntpd is watching time-b.nist.gov now--I notice that it's the
server with the lowest jitter.  Is that a good thing?  What do each of
these numbers represent?  I can't seem to find clear documentation on
understanding which figures are key to accuracy.

> Consider using stratum 2 servers.   Servers closer to you will provide 
> much better synchronization.  The public stratum 1 servers are all 
> extremely heavily loaded and cannot respond quickly which means that 
> they cannot provide really good synchronization.

OK, but where can I look to discover the names of public servers I can
use?

The weird thing is that I had very good sync with the same configuration
previously, in FreeBSD 4.2.  What has changed?  Or is it just a matter
of building up drift information again (the motherboard and hence the
system clock hardware has changed, and I erased the old ntp.drift file
since I figured ntpd would need to collect fresh data).

Right now the clock is perceptibly fast (about .5 second).  Would it
still be that far off normally after eight hours of operation?

-- 
Transpose hotmail and mxsmanic in my e-mail address to reach me directly.



More information about the questions mailing list