Re: [PATCH net] net: bridge: remove ipv6 zero address check in mcast queries

[Ying Xu <yinxu@redhat.com>]

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 I think the scenario mentioned above is abnormal.
According to rfc 4541, multicast router port means this port is attached to
a real router.

The source of query indicats that is a real router or only a
switch.(0.0.0.0 means switch,non-zero means router).
In the scenario above,the switch A was selected to be a querier that means
A performs as a router,
so switch A should config its query source address to non-zero,and then
Host A can recieve the traffic from B.

On Fri, Dec 14, 2018 at 12:10 AM Linus Lüssing <linus.luessing@c0d3.blue>
wrote:

> Even though RFC4541 recommends this, I'm not quite sure whether
> this works... even for IGMP.
>
> I think this would lead to multicast packet loss in a scenario
> like this:
>
> ----------
>
>      [Switch A] -------------- [Switch B]
>         /                          /
>        /                          /
>       /                          /
>  (Host A)                   (Host B)
>
>
> - Snooping Switches: Switch A + Switch B
> - Selected Querier: Switch A, with 0.0.0.0 query source
> - Multicast Listener: Host A
> - Multicast Data Sender: Host B
>
> 1) Host A sends IGMP report to Switch A
> 2) Switch A refrains from forwarding it to Switch B
>    (reports are only forwarded to multicast routers according to
>     RFC4541)
>    => Switch B does not learn about listeners on Host A
>
> Now, with this patch and recommendation in RFC4541 to not add queries
> with a 0.0.0.0 source address to the multicast router port list:
>
> 3) Host B sends multicast data to Switch B
>    => Switch B does not forward it to Switch A as it neither
>       detected a multicast listener nor multicast router on
>       the according port.
>    => Host A does not receive the multicast data it signed up for
>
> (Or with colors:
>
> https://metameute.de/~tux/linux/bridge/query-zero-source-no-mcrouter-port.png
> )
>
> ----------
>
> Alternatively we would need to ignore 0.0.0.0 for the querier
> election and "querier present" detection. And by that disable
> multicast snooping if there are no queries from a non-zero source
> address.
>
> But I'm a little hesitant whether ignoring is a reliable way as
> IGMPv3 (RFC3376) and IGMPv2 (RFC2236) make no such restrictions
> regarding the query source address.
>
> With no such restrictions according to RFC3376/RFC2236 a 0.0.0.0
> would always win the querier election. Meaning any potential
> querier with a non-zero source address would remain silent.
> Meaning we would always disable multicast snooping then?
>
>
> Adding queriers with a 0.0.0.0 source address to the multicast
> router list, too, seems like a less harmful way then disabling multicast
> snooping completely?
>
> ----------
>
> However, one of the two options seems to be necessary. Either
> reverting the patch for the IGMP part, too. Or Ignoring 0.0.0.0
> sources for querier eletcion and presence detection.
>
> The current state seems broken to me unless I'm missing something.
>

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