"Dave Hart" <> wrote in message news:CAMbSiYD0cY27Ft9cadBzV4ravKcz-
[]
Retransmission is the killer issue for NTP performance over 802.11.
For practical interop with software developed on wired networks, WiFi
equipment detects packet loss and triggers retransmission invisibly to
higher layers. I suspect NTP would do better if the 802.11 layer
differentiated its handling of UDP 53 and 123 :) Where dropping DNS
queries has an awful impact on user experience, it would be preferable
for NTP compared to introducing the extra delay and thereby jitter.
I'd love to see more DNS over TCP, so that perhaps one day layer 2
wireless networks will do better letting UDP drop rather than
retransmit at layer 2. NTP is like VoIP in this regard, dropping the
traffic is likely better than unbounded delay for retransmission.
I wonder if the 90 minute periodicity to the -0.4 PPM shifts aligns
with some WiFi security renegotiation.
Cheers,
Dave Hart
Thanks for that, Dave. Initial results with no min/maxpoll=5 are showing
an offset value which initially oscillated a lot, but is now steadier at
10-14 ms, the frequency has steadied after an initial period at a rising
0.85 to 0.95 ppm (whereas the LAN-sync value was ~1.7 ppm), and the jitter
is now slowly dropping (currently 7 ms) from a peak of about 27 ms.
It seems that with min/maxpoll=5, 32 seconds, it was much more likely that
NTP would be triggered into "poor" behaviour (stepping the frequency) than
with the poll at 1024 seconds which it has now reached. Of course,
setting such short poll times over a "noisy" link is not a good idea,
although why NTP seems to settle with a higher offset and different
frequency isn't currently clear to me!
Cheers,
David
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