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

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Date:   Mon, 16 Nov 2020 14:28:44 -0800
From:   Jakub Kicinski <kuba@...nel.org>
To:     Pablo Neira Ayuso <pablo@...filter.org>
Cc:     Tobias Waldekranz <tobias@...dekranz.com>,
        netfilter-devel@...r.kernel.org, davem@...emloft.net,
        netdev@...r.kernel.org, razor@...ckwall.org, jeremy@...zel.net
Subject: Re: [PATCH net-next,v3 0/9] netfilter: flowtable bridge and vlan
 enhancements

On Mon, 16 Nov 2020 23:18:15 +0100 Pablo Neira Ayuso wrote:
> On Sat, Nov 14, 2020 at 09:03:47AM -0800, Jakub Kicinski wrote:
> > On Sat, 14 Nov 2020 15:00:03 +0100 Tobias Waldekranz wrote:  
> > > On Sat, Nov 14, 2020 at 12:59, Pablo Neira Ayuso <pablo@...filter.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?    
> > 
> > Indeed, 2 VMs A and B, talking to each other, A is _outside_ the
> > system (reachable via eth0), B is inside (veth1). When A moves inside
> > and gets its veth. Neither B's veth1 not eth0 will change state, so
> > cache wouldn't get flushed, right?  
> 
> The flow tuple includes the input interface as part of the hash key,
> so packets will not match the existing entries in the flowtable after
> the topology update. 

To be clear - the input interface for B -> A traffic remains B.
So if B was talking to A before A moved it will keep hitting 
the cached entry.

Are you saying A -> B traffic won't match so it will update the cache,
since conntrack flows are bi-directional?

> The stale flow entries are removed after 30 seconds
> if no matching packets are seen. New flow entries will be created for
> the new topology, a few packets have to go through the classic
> forwarding path so the new flow entries that represent the flow in the
> new topology are created.