[Bloat] tiny monsters: multicast packets

[Dave Taht]

On Sun, May 29, 2011 at 8:10 AM, Jonathan Morton <chromatix99 at gmail.com>wrote:

>
> On 29 May, 2011, at 4:23 pm, Dave Taht wrote:
>
> > In my last 2 months of travel, I have seen multicast packets, such as
> ARP, DHCP, MDNS, and now babel, all failing far, far, far more often than is
> desirable. I have seen DHCP fail completely for hours at a time, I've seen
> ARP take dozens of queries to resolve.
>
> 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.
>  Broadcast packets are not supposed to be large ones, but wireless framing
> must add a lot of fixed overhead.
>
> Given that the AP surely knows which hosts are attached to it at any given
> time, and what link rate they are currently sustaining, surely a saner
> design would have been either:
>
> 1) Broadcast the packet at the lowest link rate for all known attached
> hosts.
>
> 2) Unicast the packet to each attached host in turn, at that host's current
> link rate.
>
> The latter sounds wasteful, but would still be a win on 802.11g in
> compatibility mode.  It also turns the AP into a star-topology hub, so hosts
> would send their broadcast packets by unicast to the AP, which would repeat
> them.
>
> But presumably the brokenness is now baked firmly into the standard, and is
> therefore inescapable.  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.
>
> I should check whether my Airport Base Station already supports that.
>

I just managed (after fighting with the switch in the wndr3700, and having
to disable vlan support) to treat the wireless and wired lans separately
[1].

Result - 130+Mbit performance on iperf on the lan (up from 60Mbit), which is
still pretty low - [2]

AND multicast stopped failing, in the limited testing I'd done so far.
Yea... [3]

Yes, I think home routers should route, not bridge between widely disparate
network types.

Bridging between a gige interface, a wireless N 5.x gigE interface, and a
2.4 ghz interface in compatability mode is just begging for trouble.
Especially with a switch on the gigE interface that is also doing buffering
itself.

Switching to routing throughout does induce more complexity in the network
(3 or more subnets, rather than 1), and for the end-user...

... and all the multicast-isms that have arisen in the last decade such as
mdns and daap would need to be looked at, to see if they can actually be
made to work... (IGMPv2 ?)

but I can no longer think of a better solution to the nearly intolerable
performance of today's wireless networks than starting to route them, and
dealing with the consequences above.



>  - Jonathan
>
>

1: http://www.bufferbloat.net/issues/186
2: I'm still seeing enormous delays in the switch, however
3: http://www.bufferbloat.net/projects/bismark-testbed/wiki/Experiment_-_QoS
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