netdev - Re: [PATCH] net: bridge: Allow bridge to joing multicast groups

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Date:   Mon, 29 Jul 2019 15:52:06 +0200
From:   "Allan W. Nielsen" <allan.nielsen@...rochip.com>
To:     Nikolay Aleksandrov <nikolay@...ulusnetworks.com>
CC:     Horatiu Vultur <horatiu.vultur@...rochip.com>,
        <roopa@...ulusnetworks.com>, <davem@...emloft.net>,
        <bridge@...ts.linux-foundation.org>, <netdev@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] net: bridge: Allow bridge to joing multicast groups

The 07/29/2019 15:50, Nikolay Aleksandrov wrote:
> On 29/07/2019 15:22, Nikolay Aleksandrov wrote:
> > Hi Allan,
> > On 29/07/2019 15:14, Allan W. Nielsen wrote:
> >> First of all, as mentioned further down in this thread, I realized that our
> >> implementation of the multicast floodmasks does not align with the existing SW
> >> implementation. We will change this, such that all multicast packets goes to the
> >> SW bridge.
> >>
> >> This changes things a bit, not that much.
> >>
> >> I actually think you summarized the issue we have (after changing to multicast
> >> flood-masks) right here:
> >>
> >> The 07/26/2019 12:26, Nikolay Aleksandrov wrote:
> >>>>> Actually you mentioned non-IP traffic, so the querier stuff is not a problem. This
> >>>>> traffic will always be flooded by the bridge (and also a copy will be locally sent up).
> >>>>> Thus only the flooding may need to be controlled.
> >>
> >> This seems to be exactly what we need.
> >>
> >> Assuming we have a SW bridge (br0) with 4 slave interfaces (eth0-3). We use this
> >> on a network where we want to limit the flooding of frames with dmac
> >> 01:21:6C:00:00:01 (which is non IP traffic) to eth0 and eth1.
> >>
> >> One way of doing this could potentially be to support the following command:
> >>
> >> bridge fdb add    01:21:6C:00:00:01 port eth0
> >> bridge fdb append 01:21:6C:00:00:01 port eth1
> >>
> 
> And the fdbs become linked lists?
Yes, it will most likely become a linked list

> So we'll increase the complexity for something that is already supported by
> ACLs (e.g. tc) and also bridge per-port multicast flood flag ?
I do not think it can be supported with the facilities we have today in tc.

We can do half of it (copy more fraems to the CPU) with tc, but we can not limit
the floodmask of a frame with tc (say we want it to flood to 2 out of 4 slave
ports).

> I'm sorry but that doesn't sound good to me for a case which is very rare and
> there are existing ways to solve without incurring performance hits or increasing
> code complexity.
I do not consider it rarely, controling the forwarding of L2 multicast frames is
quite common in the applications we are doing.

> If you find a way to achieve this without incurring a performance hit or significant
> code complexity increase, and without breaking current use-cases (e.g. unexpected default
> forwarding behaviour changes) then please send a patch and we can discuss it further with
> all details present. People have provided enough alternatives which avoid all of the
> problems.
Will do, thanks for the guidance.

/Allan