Re: [PATCH net-next,v3 0/9] netfilter: flowtable bridge and vlan enhancements

[Tobias Waldekranz <tobias@waldekranz.com>]

On Sat, Nov 14, 2020 at 12:59, Pablo Neira Ayuso <pablo@netfilter.org> wrote:
> If any of the flowtable device goes down / removed, the entries are
> removed from the flowtable. This means packets of existing flows are
> pushed up back to classic bridge / forwarding path to re-evaluate the
> fast path.
>
> For each new flow, the fast path that is selected freshly, so they use
> the up-to-date FDB to select a new bridge port.
>
> Existing flows still follow the old path. The same happens with FIB
> currently.
>
> It should be possible to explore purging entries in the flowtable that
> are stale due to changes in the topology (either in FDB or FIB).
>
> What scenario do you have specifically in mind? Something like VM
> migrates from one bridge port to another?

This should work in the case when the bridge ports are normal NICs or
switchdev ports, right?

In that case, relying on link state is brittle as you can easily have a
switch or a media converter between the bridge and the end-station:

        br0                  br0
        / \                  / \
    eth0   eth1          eth0   eth1
     /      \      =>     /      \
  [sw0]     [sw1]      [sw0]     [sw1]
   /          \         /          \
  A                                 A

In a scenario like this, A has clearly moved. But neither eth0 nor eth1
has seen any changes in link state.

This particular example is a bit contrived. But this is essentially what
happens in redundant topologies when reconfigurations occur (e.g. STP).

These protocols will typically signal reconfigurations to all bridges
though, so as long as the affected flows are flushed at the same time as
the FDB it should work.

Interesting stuff!