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Date:   Tue, 19 Feb 2019 16:18:08 +0000
From:   Russell King - ARM Linux admin <linux@...linux.org.uk>
To:     Vivien Didelot <vivien.didelot@...il.com>
Cc:     Andrew Lunn <andrew@...n.ch>,
        Florian Fainelli <f.fainelli@...il.com>,
        Heiner Kallweit <hkallweit1@...il.com>,
        "David S. Miller" <davem@...emloft.net>, netdev@...r.kernel.org
Subject: Re: [PATCH net-next v2 3/3] net: dsa: mv88e6xxx: default to
 multicast and unicast flooding

On Tue, Feb 19, 2019 at 11:05:37AM -0500, Vivien Didelot wrote:
> Hi Russell,
> 
> On Mon, 18 Feb 2019 12:53:45 +0000, Russell King - ARM Linux admin <linux@...linux.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.

If you're asking whether the bridge code sends a
SWITCHDEV_ATTR_ID_PORT_BRIDGE_FLAGS message on leave/join, it seems
that it does not.

There is only one place in the bridge code that this message is
generated, that is in net/bridge/br_switchdev.c
br_switchdev_set_port_flag().  That is called from one place in the
bridge code, which is br_set_port_flag() in net/bridge/br_netlink.c,
which is in response to a RTM_SETLINK netlink message where the bridge
code processes all the various bridge link options.

There appears to be no call when adding or removing an interface to/
from a bridge.

-- 
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
FTTC broadband for 0.8mile line in suburbia: sync at 12.1Mbps down 622kbps up
According to speedtest.net: 11.9Mbps down 500kbps up

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