Re: MDB offloading of local ipv4 multicast groups

[Ido Schimmel <idosch@idosch.org>]

On Thu, Sep 15, 2016 at 08:58:50PM +0200, John Crispin wrote:
> Hi,
> 
> While adding MDB support to the qca8k dsa driver I found that ipv4 mcast
> groups don't always get propagated to the dsa driver. In my setup there
> are 2 clients connected to the switch, both running a mdns client. The
> .port_mdb_add() callback is properly called for 33:33:00:00:00:FB but
> 01:00:5E:00:00:FB never got propagated to the dsa driver.
> 
> The reason is that the call to ipv4_is_local_multicast() here [1] will
> return true and the notifier is never called. Is this intentional or is
> there something missing in the code ?

I believe this is based on RFC 4541:

"Packets with a destination IP (DIP) address in the 224.0.0.X range
which are not IGMP must be forwarded on all ports."
https://tools.ietf.org/html/rfc4541

But, we are missing the offloading of router ports, which is needed for
the device to correctly flood unregistered multicast packets. That's
also according to the mentioned RFC:

"If a switch receives an unregistered packet, it must forward that
packet on all ports to which an IGMP router is attached."

Implemented at br_flood_multicast()

However, the marking is done per-port and not per-{port, VID}. We need
that in case vlan filtering is enabled. I think Nik is working on that,
but he can correct me if I'm wrong :). The switchdev bits can be added
soon after.

> 
> 	John
> 
> [1]
> https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/tree/net/bridge/br_multicast.c?id=refs/tags/v4.8-rc6#n737