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

[Jim Gettys <jg@freedesktop.org>]

On Sun, Feb 1, 2015 at 11:04 PM, Avery Pennarun <apenwarr@google.com> wrote:
> On Sun, Feb 1, 2015 at 6:34 PM, Andrew McGregor <andrewmcgr@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 gather some Windows drivers from some vendors do this unicast fanout
(claim made by one of their engineers in an early homenet meeting).

>
> 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.

So long as it times out in some very small, finite time.  We don't
want a repeat of the infinite retry bugs Dave found in drivers a few
years back...

"Reliable multicast" ultimately is an oxymoron, particularly on a
medium with hundreds/one bandwidth variation.  One remote low
bandwidth station cannot be allowed to drag the entire network to the
basement.
                                     - Jim