[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 188.8.131.52 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
thegateap.intra 184.108.40.206 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 127.127.8.0 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
More information about the questions