[ntp:questions] Re: ntp & adsl

Folkert van Heusden folkert at vanheusden.com
Tue Jan 31 16:41:26 UTC 2006

> > I've heard that NTP acts funny when it tries to sync over an ADSL line.
> > That seems to be the case indeed:
> > 0 root at muur:/home/folkert/www$ ntpq -c pe
> >      remote           refid      st t when poll reach   delay   offset  jitter
> > ==============================================================================
> >  GENERIC(0)      .DCFa.           0 l   23   64    3    0.000  -17.386  26.356  <- dcf77 receiver
> > -keetweej.intran     2 u  810 1024  376    0.735    2.724   0.084  <- other server on my lan

on the lan but it also syncs against an internet-host, that explains the
offset being different from my the first entry in this list

> >  thegateap.intra .STEP.          16 u    - 1024    0    0.000    0.000 4000.00  <- other server on my lan

this one also syncs against the internet when it is (it died for some
unknown reason):
 thegateap.intra     2 u   22   64    2    5.008   18.163   0.001

> > And there it is: 20ms offset. From what I've heard this is caused by
> > ADSL being asymetric.
> > Now I was wondering, can't this be fixed by telling the NTP daemon how
> > much asynchronous the line is? I mean: it can be easily measured what
> > the down- and upload bandwidth is.
> My experience is that ADSL causes much smaller than 20ms delays.   If you do the math for  NTP
> packets which take two ATM frames you'll find it comes out to between 1 and 2 ms depending on how
> fast and how asymmetrical your line is (mine is 6000/608 yielding 1.281 ms offset).    Unless your
> line is very odd the 20ms your are seeing is far more likely to be from having the wrong propagation
> delay in the DFC77 driver.

Is there any way you know how I can fix this?

This clock is configured this way:

# mode 14, RAW DCF at 50baud
server mode 14

> After a lot of experimenting on my part I decided to give up on factoring out the DSL offset and
> live with the slight perceived error.

Hmmm ok. So far it doesn't give any problems as things will sync always
against internet NTP-servers, making the local DCF77 receiver rather
I'm a little afraid, though, that when all systems in my lan have a
DCF77-ish (well, an MFS and HFS receiver) receiver, things will become
very jumpy by switching back and forth between internet and local

Folkert van Heusden

iPod winnen? --> http://keetweej.vanheusden.com/redir.php?id=62
Phone: +31-6-41278122, PGP-key: 1F28D8AE, www.vanheusden.com

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