netdev - Re: [PATCH RFC net-next 08/13] net: bridge: avoid classifying unknown multicast as mrouters

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Date:   Wed, 13 Apr 2022 12:00:40 +0300
From:   Nikolay Aleksandrov <razor@...ckwall.org>
To:     Joachim Wiberg <troglobit@...il.com>,
        Roopa Prabhu <roopa@...dia.com>
Cc:     netdev@...r.kernel.org, bridge@...ts.linux-foundation.org,
        "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Tobias Waldekranz <tobias@...dekranz.com>,
        Vladimir Oltean <vladimir.oltean@....com>
Subject: Re: [PATCH RFC net-next 08/13] net: bridge: avoid classifying unknown
 multicast as mrouters_only

On 13/04/2022 11:55, Nikolay Aleksandrov wrote:
> On 13/04/2022 11:51, Joachim Wiberg wrote:
>> On Tue, Apr 12, 2022 at 20:37, Nikolay Aleksandrov <razor@...ckwall.org> wrote:
>>> On 12/04/2022 20:27, Joachim Wiberg wrote:
>>>> [snip]
>>>> From this I'd like to argue that our current behavior in the bridge is
>>>> wrong.  To me it's clear that, since we have a confiugration option, we
>>>> should forward unknown IP multicast to all MCAST_FLOOD ports (as well as
>>>> the router ports).
>>> Definitely not wrong. In fact:
>>> "Switches that do not forward unregistered packets to all ports must
>>>  include a configuration option to force the flooding of unregistered
>>>  packets on specified ports. [..]"
>>> is already implemented because the admin can mark any port as a router and
>>> enable flooding to it.
>>
>> Hmm, I understand your point (here and below), and won't drive this
>> point further.  Instead I'll pick up on what you said in your first
>> reply ... (below, last)
>>
>> Btw, thank you for taking the time to reply and explain your standpoint,
>> really helps my understanding of how we can develop the bridge further,
>> without breaking userspace! :)
>>
>>>> [1]: https://www.rfc-editor.org/rfc/rfc4541.html#section-2.1.2
>>> RFC4541 is only recommending, it's not a mandatory behaviour. This
>>> default has been placed for a very long time and a lot of users and
>>> tests take it into consideration.
>>
>> Noted.
>>
>>> We cannot break such assumptions and start suddenly flooding packets,
>>> but we can leave it up to the admin or distribution/network software
>>> to configure it as default.
>>
>> So, if I add a bridge flag, default off as you mentioned out earlier,
>> which changes the default behavior of MCAST_FLOOD, then you'd be OK with
>> that?  Something cheeky like this perhaps:
>>
>>     if (!ipv4_is_local_multicast(ip_hdr(skb)->daddr))
>>        	BR_INPUT_SKB_CB(skb)->mrouters_only = !br_opt_get(br, BROPT_MCAST_FLOOD_RFC4541);
> 
> Exactly! And that is exactly what I had in mind when I wrote it. :)
> 

Just please use a different option name that better suggests what it does.

> Thanks,
>  Nik
> 
>>
>>
>> Best regards
>>  /Joachim
>