netfront and promiscuous/multicast packets

netfront and promiscuous/multicast packets

[James Harper]

Am I right in saying that there is no way for netfront to convey to
netback its desire to switch to promiscuous mode? Not that that is
really a bit deal in a switched environment.

What about multicast? How is that handled? Are multicast packets just
treated like broadcasts and sent to all interfaces?

The Windows NDIS api asks for various packet filters (DIRECTED,
MULTICAST, BROADCAST, PROMISCUOUS, ALL_MULTICAST, etc) and I'm not quite
sure what the gplpv drivers should be telling it...

Thanks

James

Re: netfront and promiscuous/multicast packets

[Samuel Thibault]

AIUI netfront sees network from an always promiscuous possibly switched
point of view, i.e. it should already receive all ethernet broadcast
and multicast packets.  Whether it is actually switched depends on the
network configuration in dom0 of course.

Samuel

RE: netfront and promiscuous/multicast packets

[James Harper]

> 
> Am I right in saying that there is no way for netfront to convey to
> netback its desire to switch to promiscuous mode? Not that that is
> really a bit deal in a switched environment.
> 
> What about multicast? How is that handled? Are multicast packets just
> treated like broadcasts and sent to all interfaces?
> 
> The Windows NDIS api asks for various packet filters (DIRECTED,
> MULTICAST, BROADCAST, PROMISCUOUS, ALL_MULTICAST, etc) and I'm not
quite
> sure what the gplpv drivers should be telling it...

Okay google tells me that Keir submitted a patch late last year to
enable netfront to send multicast filter lists to netback via extra_info
structures on the ring.

What is the best way to tell if the particular netback I am connected to
actually supports this? Send a dummy EXTRA_TYPE_MCAST_ADD request and
see if it succeeds?

I'm pretty certain that my Dom0's (Debian based) won't support this so
the point is moot for now anyway as I can't test it.

I guess I'll just advertise to Windows that I support DIRECTED,
BROADCAST, and ALL_MULTICAST, and maybe PROMISCUOUS if things don't work
without it.

James

Re: netfront and promiscuous/multicast packets

[David Edmondson]

On Thu, Aug 14, 2008 at 09:49:08AM +1000, James Harper wrote:
> Okay google tells me that Keir submitted a patch late last year to
> enable netfront to send multicast filter lists to netback via extra_info
> structures on the ring.
> 
> What is the best way to tell if the particular netback I am connected to
> actually supports this? Send a dummy EXTRA_TYPE_MCAST_ADD request and
> see if it succeeds?

Look for the 'feature-multicast-control' flag on the backend node in
xenstore. If it's present and set to '1', you can enable the behaviour
(see below) by setting 'request-multicast-control' to '1' on the
frontend node.

> I'm pretty certain that my Dom0's (Debian based) won't support this so
> the point is moot for now anyway as I can't test it.

This was added to support Solaris, where we have a different model for
dom0 networking.

The behaviour without any flag set is that the guest should get all of
the packets it might rightfully be interested in. That would be:
    - all packets to its' unicast address,
    - all broadcast packets,
    - all multicast packets.
It may of course receive more packets than this set.

If the flag is set, the frontend won't receive all multicast packets,
only those for which is has issued an MCAST_ADD request. It still gets
the unitcast and broadcast packets.

This feature means that we avoid pushing the dom0 NIC into promiscuous
mode on NICs that support multiple unicast addresses (and our drivers
know how to use that support, obviously).