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

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Date:   Tue, 10 Jan 2017 09:17:14 -0800
From:   Dave Taht <dave.taht@...il.com>
To:     Johannes Berg <johannes@...solutions.net>
Cc:     Linus Lüssing <linus.luessing@...3.blue>,
        Stephen Hemminger <stephen@...workplumber.org>,
        "M. Braun" <michael-dev@...i-braun.de>,
        Felix Fietkau <nbd@....name>,
        "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

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....

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.

Has anyone actually looked at the effects of this with, say, 5-10
stations at middlin to poor quality (longer distance)? using something
to measure the real effect of the multicast conversion? (uftp, mdns?)