[Cerowrt-devel] Fwd: Throughput regression with `tcp: refine TSO autosizing`

Avery Pennarun apenwarr at google.com
Sun Feb 1 23:04:41 EST 2015

On Sun, Feb 1, 2015 at 6:34 PM, Andrew McGregor <andrewmcgr at gmail.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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