Some level of hardware queueing is necessary to meet the response time.
It's further complicated by the need to include retries at the start of the
next aggregate, but you only learn which ones from the block-ACK received
after the last one. Given the hardware's need to have frame length well
before the data you may have only a few microseconds to make some
decisions. This is why all recent chipsets have a fairly fast CPU on board
dedicated to implementing some of these functions. Of course the quality of
the design and implementation varies hugely from vendor to vendor, and the
API in terms of queueing varies.
Simon
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On August 7, 2015 10:36:04 AM Rich Brown <[email protected]> wrote:
Ah..., the bitter bit of the reality sandwich. Yum :-(
But Dave's note about Felix's hook for per-station queueing makes it seem
feasible, if only a lot of work.
Thanks, all, for the enlightenment!
Rich
On Aug 7, 2015, at 9:28 AM, Jonathan Morton <[email protected]> wrote:
>
>> On 7 Aug, 2015, at 15:22, Rich Brown <[email protected]> wrote:
>>
>> - At that time, the wifi driver requests packets from fq_codel until a)
the the fq_codel queues are empty, or b) the wifi frame is full. In either
case, the wifi driver sends what it has.
>
> There’s one big flaw with this: if packets are available for multiple
destinations, fq_codel will generally give you a variety pack of packets
for each of them. But a wifi TXOP is for a single destination, so only
some of the packets would be eligible for the same aggregate frame.
>
> So what’s needed is a way for the wifi driver to tell the queue that it
wants packets for the *same* destination as it’s transmitting to.
>
>> - Once the transmit opportunity has come around, it's a matter of
microseconds (I assume) to pull in a wifi frame's worth of packets from
fq_codel
>
> This is hard to guarantee in software in a general-purpose OS.
>
> - Jonathan Morton
>
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