Dave Hart <[email protected]> writes:
>I recognize I'm suggesting a layer violation in wishing 802.11 devices
>treated UDP differently from TCP, or even worse in terms of layer
>violation, UDP 123 differently from UDP 53. It's not pretty, but it
>would make a positive difference. The ideal number of retries for NTP
>may be zero, assuming the radio layer loss rates are less than 87.5%
>in practice.
All QoS choices that are made at a low layer involve some amount
of layering violation, so I don't actually find this objectionable.
If the PHY rate selection algorithm is any good, then the loss rate
will be less that 87% unless the network is really clogged up with
a lot of busy stations and there are a lot of collisions.
>> An alternative would
>> be to use NTP's broadcast mode on a LAN, which would eliminate
>> retries.
>Right.
There is one other cool thing you can do with timing and 802.11
broadcasts. Broadcasts really are broadcasts - you're not seeing
copies of a packet as you would on a switch. If you can timestamp
the arrival of a broadcast, it should provide you with a common
reference point in time between a bunch of devices. It can be
a fun way to watch the drift on the clocks on a bunch of devices.
(The 802.11 cards often have relatively good time counters on them
too, for backoff and "TSF" calculations. I've a version of the
Atheros driver for FreeBSD that can work as the system timecounter.
I've still to check how it compares to other hardware.)
David.
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