Linux Ethernet Bridging - Apr 2022 - [Bridge] [PATCH RFC net-next 08/13] net: bridge: avoid classifying unknown multicast as mrouters_only

On 13/04/2022 11:51, Joachim Wiberg wrote:
> On Tue, Apr 12, 2022 at 20:37, Nikolay Aleksandrov <razor at
blackwall.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. :)

Thanks,
 Nik
> 
> 
> Best regards
>  /Joachim

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 at
blackwall.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
>