Danny Mayer <[email protected]> writes:
>No you don't want to do DNS over TCP if you can avoid it. It would be a
>major hit on the resolver servers and with the kind of high volume that
>you get as mobile devices make increasing use of such networks. You want
>WiFi to drop UDP packets if they are lost rather than attempting to
>retransmit them.
\begin{ramble}
All unicast MAC layer traffic is retransmitted on WiFi if not
recieved correctly, probably for the same reason that Ethernet also
retransmits packets that collide. If NTP can handle 10base2 Ethernet,
then it should be able to handle WiFi.
The maximum number of retries depends somewhat on the hardware
(usually between 7 and 11 tries), and is subject to backoff, in a
similar way to traditional Ethernet. Determining if a packet was
successful depends on on a MAC-layer ACK, because you can't easily
do collision detection in the same way as Ethernet. The reason that
only unicast packets are retransmitted is because it's tricky to
figure out who sends the ACK if it is multicast or broadcast. I
suspect that if the 802.11 guys could figure it out, multi-/broadcast
traffic would also be retransmitted as needed.
Modern hardware that supports 802.11e (or 802.11n, which requires
much of the QoS part of 11e) can control things like the number of
retries, and you could hack the driver to inspect the packets and
if it is NTP to reduce the number of retires. An alternative would
be to use NTP's broadcast mode on a LAN, which would eliminate
retries. However, I suspect that bufferbloat and asymetric delays
on DSL is probably a much bigger problem for NTP than 802.11 retries.
\end{ramble}
David.
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