Re: IPv6 support by wifi systems

[Owen DeLong <owen () delong com>]

On Feb 12, 2013, at 7:32 PM, Karl Auer <kauer () biplane com au> wrote:

> On Tue, 2013-02-12 at 16:29 -0500, Brandon Ross wrote:
> > It seems that, then, 
> > MLD snooping is valuable as it will prevent DAD and other ND traffic from 
> > using bandwidth towards hosts not in that group.
>
> It will prevent *all* multicast traffic from using bandwidth towards
> hosts not in the multicast groups involved. ND, DAD etc are just
> specific cases.
>
> > Other than solicited node multicast, is MLD used anywhere else in a 
> > network that does not have layer 3 multicast enabled on a router?
>
> MLD is used for all multicast - so a DHCPv6 packet, for example, will
> only go to any relays and servers in the subnet. *Any* multicast will be
> limited to its listeners. The only multicast that will go to all nodes
> will be multicast sent to the "all link-local nodes" address - and even
> that will not go to non-IPv6 nodes.
>
> MLD snooping happens on switches - you will get the benefit even if in
> an isolated network (no router at all).

In a wifi environment, however, this has additional complexity.

A multicast packet originating within the WAP or from the wired
side of the WAP and destined for more than one wireless host should
be sent to be heard by all hosts so it is only transmitted once.
Otherwise it ties up excessive air time. In this regard, a WAP
is more like a hub than a switch.

A multicast packet originating from a wifi host, OTOH, must be
repeated by the WAP so that all subscribed hosts can hear it.

Owen