Re: [PATCH net-next v2 3/3] net: dsa: mv88e6xxx: default to multicast and unicast flooding

[Vivien Didelot <vivien.didelot@gmail.com>]

Hi Russell,

On Mon, 18 Feb 2019 12:53:45 +0000, Russell King - ARM Linux admin <linux@armlinux.org.uk> wrote:
> On Sun, Feb 17, 2019 at 04:32:40PM +0000, Russell King wrote:
> > Switches work by learning the MAC address for each attached station by
> > monitoring traffic from each station.  When a station sends a packet,
> > the switch records which port the MAC address is connected to.
> > 
> > With IPv4 networking, before communication commences with a neighbour,
> > an ARP packet is broadcasted to all stations asking for the MAC address
> > corresponding with the IPv4.  The desired station responds with an ARP
> > reply, and the ARP reply causes the switch to learn which port the
> > station is connected to.
> > 
> > With IPv6 networking, the situation is rather different.  Rather than
> > broadcasting ARP packets, a "neighbour solicitation" is multicasted
> > rather than broadcasted.  This multicast needs to reach the intended
> > station in order for the neighbour to be discovered.
> > 
> > Once a neighbour has been discovered, and entered into the sending
> > stations neighbour cache, communication can restart at a point later
> > without sending a new neighbour solicitation, even if the entry in
> > the neighbour cache is marked as stale.  This can be after the MAC
> > address has expired from the forwarding cache of the DSA switch -
> > when that occurs, there is a long pause in communication.

Thank you for the very informative message above.

> > Our DSA implementation for mv88e6xxx switches has defaulted to having
> > multicast and unicast flooding disabled.  As per the above description,
> > this is fine for IPv4 networking, since the broadcasted ARP queries
> > will be sent to and received by all stations on the same network.
> > However, this breaks IPv6 very badly - blocking neighbour solicitations
> > and later causing connections to stall.
> > 
> > The defaults that the Linux bridge code expect from bridges are that
> > unknown unicast frames and unknown multicast frames are flooded to
> > all stations, which is at odds to the defaults adopted by our DSA
> > implementation for mv88e6xxx switches.
> > 
> > This commit enables by default flooding of both unknown unicast and
> > unknown multicast frames.  This means that mv88e6xxx DSA switches now
> > behave as per the bridge(8) man page, and IPv6 works flawlessly through
> > such a switch.
> 
> Thinking about this a bit more, this approach probably isn't the best.
> If we have a port that goes through this life-cycle:
> 
> 1. assigned to a bridge
> 2. configured not to flood
> 3. reassigned to a new bridge
> 
> the port will retain its settings from the first bridge, which will be
> at odds with the settings that the Linux bridge code expects and the
> settings visible to the user.
> 
> So, how about this, which basically reverts this patch and applies the
> flood settings each time a port joins a bridge, and clears them when
> the port leaves a bridge.

Isn't the bridge code programming flooding on the port correctly on leave/join,
because the BR_*FLOOD flags have been learned? I would expect that.


Thanks,

	Vivien