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Date:	Sun, 1 Feb 2015 23:04:41 -0500
From:	Avery Pennarun <apenwarr@...gle.com>
To:	Andrew McGregor <andrewmcgr@...il.com>
Cc:	David Reed <dpreed@...d.com>,
	Jonathan Morton <chromatix99@...il.com>,
	Dave Taht <dave.taht@...il.com>,
	Matt Mathis <mattmathis@...gle.com>,
	Tim Shepard <shep@...m.mit.edu>, dstanley@...banetworks.com,
	Kathy Giori <kgiori@....qualcomm.com>,
	Stig Thormodsrud <stig@...t.com>,
	Derrick Pallas <pallas@...aki.com>,
	"cerowrt-devel@...ts.bufferbloat.net" 
	<cerowrt-devel@...ts.bufferbloat.net>,
	Mahesh Paolini-Subramanya <mahesh@...swaytoofast.com>,
	Jim Gettys <jg@...edesktop.org>,
	Jesper Dangaard Brouer <jbrouer@...hat.com>,
	linux-wireless <linux-wireless@...r.kernel.org>,
	netdev@...r.kernel.org
Subject: Re: [Cerowrt-devel] Fwd: Throughput regression with `tcp: refine TSO autosizing`

On Sun, Feb 1, 2015 at 6:34 PM, Andrew McGregor <andrewmcgr@...il.com> wrote:
> I missed one item in my list of potential improvements: the most braindead
> thing 802.11 has to say about rates is that broadcast and multicast packets
> should be sent at 'the lowest basic rate in the current supported rate set',
> which is really wasteful.  There are a couple of ways of dealing with this:
> one, ignore the standard and pick the rate that is most likely to get the
> frame to as many neighbours as possible (by a scan of the Minstrel tables).
> Or two, fan it out as unicast, which might well take less airtime (due to
> aggregation) as well as being much more likely to be delivered, since you
> get ACKs and retries by doing that.

As far as I can see, the only sensible thing to do with
multicast/broadcast is some variation of the unicast fanout, unless
you've got a truly huge number of nodes.  I don't know of any
protocols (certainly not video streams) that actually work well with
the kind of packet loss you see at medium/long range with wifi if
retransmits aren't used.  I've heard that openwrt already has a patch
included that does this kind of fanout at the bridge layer.

I've also heard of a new "reliable multicast" in some newer 802.11
variant, which essentially sends out a single multicast packet and
expects an ACK from each intended recipient.  Other than adding
complexity, it seems like the best of both worlds.
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