netdev - Re: [PATCH net-next] bridge: multicast to unicast

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Date:   Tue, 10 Jan 2017 18:23:29 +0100
From:   Felix Fietkau <nbd@....name>
To:     Dave Taht <dave.taht@...il.com>,
        Johannes Berg <johannes@...solutions.net>
Cc:     Linus Lüssing <linus.luessing@...3.blue>,
        Stephen Hemminger <stephen@...workplumber.org>,
        "M. Braun" <michael-dev@...i-braun.de>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "David S . Miller" <davem@...emloft.net>,
        bridge@...ts.linux-foundation.org,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        linux-wireless <linux-wireless@...r.kernel.org>
Subject: Re: [PATCH net-next] bridge: multicast to unicast

On 2017-01-10 18:17, Dave Taht wrote:
> In the case of wifi I have 3 issues with this line of thought.
> 
> multicast in wifi has generally supposed to be unreliable. This makes
> it reliable. reliability comes at a cost -
> 
> multicast is typically set at a fixed low rate today. unicast is
> retried at different rates until it succeeds - for every station
> listening. If one station is already at the lowest rate, the total
> cost of the transmit increases, rather than decreases.
> 
> unicast gets block acks until it succeeds. Again, more delay.
> 
> I think there is something like 31 soft-retries in the ath9k driver....
If I remember correctly, hardware retries are counted here as well.

> what happens to diffserv markings here? for unicast CS1 goes into the
> BE queue, CS6, the VO queue. Do we go from one flat queue for all of
> multicast to punching it through one of the hardware queues based on
> the diffserv mark now with this patch?
> 
> I would like it if there was a way to preserve the unreliability
> (which multiple mesh protocols depend on), send stuff with QoSNoack,
> etc - or dynamically choose (based on the rates of the stations)
> between conventional multicast and unicast.
> 
> Or - better, IMHO, keep sending multicast as is but pick the best of
> the rates available to all the listening stations for it.
The advantage of the multicast-to-unicast conversion goes beyond simply
selecting a better rate - aggregation matters a lot as well, and that is
simply incompatible with normal multicast.

Some multicast streams use lots of small-ish packets, the airtime impact
of those is vastly reduced, even if the transmission has to be
duplicated for a few stations.

- Felix