[Bloat] tiny monsters: multicast packets

[Dave Taht]

On Sun, May 29, 2011 at 9:21 AM, Juliusz Chroboczek <jch at pps.jussieu.fr>wrote:

> > And the irony is that the lower speed is specifically chosen for
> > multicast in order to make sure all clients in range can hear them
> > reliably.
>
> It was my understanding that it was done for compatibility with older
> devices, since 2 Mbit/s is the fastest rate supported by pre-B
> spread-spectrum hardware.
>
>
And thus, everybody loses. I doubt there is much 802.11b gear still active
in the field.


> > 2) Unicast the packet to each attached host in turn,
>
> Both DVMR and the multicast part of BATMAN-adv do that for router-router
> links.
>
> A better link-layer solution, IMHO, would be to multicast (with ARQ) the
> packet at a reasonably high rate (say, the median of the STAs subscribed
> to the multicast group),


I'm not deeply familiar with many of the  multicast protocols in use today,
such as MDNS. I AM under the impression that IGMP has not been well used or
tested recently.

I did start building uftp which I think I understand, and lets me have very
tunable rates, and definately uses igmp.


> and then unicast it to all STAs that failed to
> return an ACK.  Interestingly, if the new multicast frame format is
> incompatible with the normal 802.11 format, then this scheme is
> compatible with legacy devices, which won't ever see the multicast frame
> and hence won't return an ACK.
>
>
While I can see this helping for many protocols ARP seems a problem, in that
you'd like to stop broadcasting after you get a good response, and that's at
a higher layer.

> So the workaround is to isolate the broadcast domains of wired
> > networks and wireless networks by making the home router into...
> > a router.  Wireless on one subnet, wired on another, and so ARP
> > between the two turns into ARP to the router alone - much more
> > scalable.
>
> OpenWRT is your friend.
>
>
It certainly is! I would never have got this much insight into these
problems, so fast, had I not poked into the depths of the linux kernel,
patched it up (with the help of many here on this list) and tried to build a
version of openwrt based on everything we've learned about bufferbloat so
far, and setup a lab to stress it out.

Future directions are becoming apparent, rapidly.


> -- Juliusz
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> Bloat at lists.bufferbloat.net
> https://lists.bufferbloat.net/listinfo/bloat
>



-- 
Dave Täht
SKYPE: davetaht
US Tel: 1-239-829-5608
http://the-edge.blogspot.com
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