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Date: Fri, 5 Apr 2024 14:00:31 +0300
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Subject: Re: [PATCH RFC net-next 00/10] MC Flood disable and snooping
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To: Vladimir Oltean <olteanv@gmail.com>,
 Joseph Huang <joseph.huang.2024@gmail.com>
Cc: Joseph Huang <Joseph.Huang@garmin.com>, netdev@vger.kernel.org,
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 Jakub Kicinski <kuba@kernel.org>, Paolo Abeni <pabeni@redhat.com>,
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From: Nikolay Aleksandrov <razor@blackwall.org>
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On 4/5/24 13:20, Vladimir Oltean wrote:
> On Thu, Apr 04, 2024 at 06:16:12PM -0400, Joseph Huang wrote:
>>>> mcast_flood == off:
>>>> - mcast_ipv4_ctrl_flood: don't care (maybe can force to "off")
>>>> - mcast_ipv4_data_flood: don't care
>>>> - mcast_ipv6_ctrl_flood: don't care
>>>> - mcast_ipv6_data_flood: don't care
>>>> - mcast_l2_flood: don't care
>>>> mcast_flood == on:
>>>> - Flood 224.0.0.x according to mcast_ipv4_ctrl_flood
>>>> - Flood all other IPv4 multicast according to mcast_ipv4_data_flood
>>>> - Flood ff02::/16 according to mcast_ipv6_ctrl_flood
>>>> - Flood all other IPv6 multicast according to mcast_ipv6_data_flood
>>>> - Flood L2 according to mcast_l2_flood
>>
>> Did you mean
>>
>> if mcast_flood == on (meaning mcast_flood is ENABLED)
>> - mcast_ipv4_ctrl_flood: don't care (since 224.0.0.x will be flooded anyway)
>> ...
>>
>> if mcast_flood == off (meaning mcast_flood is DISABLED)
>> - Flood 224.0.0.x according to mcast_ipv4_ctrl_flood
>> ...
>>
>> ? Otherwise the problem is still not solved when mcast_flood is disabled.
> 
> No, I mean exactly as I said. My goal was not to "solve the problem"
> when mcast_flood is disabled, but to give you an option to configure the
> bridge to achieve what you want, in a way which I think is more acceptable.
> 
> AFAIU, there is not really any "problem" - the bridge behaves exactly as
> instructed given the limited language available to instruct it ("mcast_flood"
> covers all multicast). So the other knobs have the role of fine-tuning
> what gets flooded when mcast_flood is on. Like "yes, but..."
> 
> You can't "solve the problem" when it involves changing an established
> behavior that somebody probably depended on to be just like that.
> 
>>> Yep, sounds good to me. I was thinking about something in these lines
>>> as well if doing a kernel solution in order to make it simpler and more
>>> generic. The ctrl flood bits need to be handled more carefully to make
>>> sure they match only control traffic and not link-local data.
>>
>> Do we consider 224.0.0.251 (mDNS) to be control or data? What qualifies as
>> control I guess that's my question.
> 
> Well, as I said, I'm proposing that 224.0.0.x qualifies as control and
> the rest of IPv4 multicast as data. Which means that, applied to your
> case, "mcast_flood on mcast_ipv4_ctrl_flood on mcast_ipv4_data_flood off"
> will "force flood" mDNS just like the IGMP traffic from your patches.
> I'm not aware if this could be considered problematic (I don't think so).
> 
> The reason behind this proposal is that, AFAIU, endpoints may choose to
> join IGMP groups in the 224.0.0.x range or not, but RFC4541 says that
> switches shouldn't prune the destinations towards endpoints that don't
> join this range anyway: https://www.rfc-editor.org/rfc/rfc4541#page-6
> 
> Whereas for IP multicast traffic towards an address outside 224.0.0.x,
> pruning will happen as per the IGMP join tracking mechanism.

+1, non-IGMP traffic to 224.0.0.x must be flooded to all anyway
so this should allow for a better control over it, but perhaps
the naming should be link_local instead because control usually
means IGMP, maybe something like mcast_ip_link_local_flood