[Cerowrt-devel] [Make-wifi-fast] [tsvwg] Comments on draft-szigeti-tsvwg-ieee-802-11e

Simon Barber simon at superduper.net
Sat Aug 8 10:25:04 EDT 2015

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.


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On August 7, 2015 10:36:04 AM Rich Brown <richb.hanover at gmail.com> 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 <chromatix99 at gmail.com> wrote:
> >
> >> On 7 Aug, 2015, at 15:22, Rich Brown <richb.hanover at gmail.com> 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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