[OpenWrt-Devel] Regarding bridge multicast-to-unicast patch

Linus Lüssing linus.luessing at web.de
Thu Sep 11 11:49:40 EDT 2014

On Thu, Sep 11, 2014 at 04:51:41PM +0200, Felix Fietkau wrote:
> On 2014-09-11 16:28, Linus Lüssing wrote:
> > On Wed, Sep 10, 2014 at 04:31:37PM +0200, Linus Lüssing wrote:
> >> Another approach would be
> >> to add a feature to mac80211 to disable the forwarding of
> >> IGMPv2/MLDv1 reports to other STAs, only forwarding it on the own
> >> AP interface so that the bridge would see all reports.
> > 
> > Hrm, on the other hand, if a querier is actually on one of the
> > other STAs then mac80211 would have to forward the report there.
> > Which would mean either needing query-snooping in mac80211 (or
> > netdev in general) or interaction between mac80211 and the
> > bridge-snooping code.
> > 
> > I really hate this IGMPv2/MLDv1 report suppression...
> > (that's where all the complexity in the batman-adv multicast
> > optimizations came from, too)
> How widely used is IGMPv2/MLDv1 compared to the newer version?

According to Wikipedia, MLDv2 is present in operating systems

* Windows Vista
* FreeBSD 8.0
* Linux 2.5.68

However, unfortunately, MLDv2 has a lot of MLDv1-compatibility
sections: A single MLDv1 listener/report can cause report
suppresion for MLDv2 listeners, too. An MLDv1 router/query leads
to MLDv2 listeners to switch to MLDv1 compatibility mode. The
querier code in the Linux bridge sends MLDv1 queries which is even
enabled by default in widely used kernels like 3.2 (and which you
enabled by default in OpenWRT Barrier Breaker, too).

In the Freifunk mesh networks in Hamburg and Lübeck we do have
MLDv1 reports, though it's hard to tell how many MLDv1-only
devices there are due to the suppressions and
MLDv1-compatibility-mode on MLDv2 hosts.

Cheers, Linus
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