netdev - Re: [PATCH] net: bridge: add max_fdb

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Date:   Fri, 17 Nov 2017 19:44:31 +0100
From:   Willy Tarreau <w@....eu>
To:     Andrew Lunn <andrew@...n.ch>
Cc:     Stephen Hemminger <stephen@...workplumber.org>,
        Vincent Bernat <bernat@...fy.cx>, Sarah Newman <srn@...mr.com>,
        Nikolay Aleksandrov <nikolay@...ulusnetworks.com>,
        netdev@...r.kernel.org, roopa <roopa@...ulusnetworks.com>
Subject: Re: [PATCH] net: bridge: add max_fdb_count

Hi Andrew,

On Fri, Nov 17, 2017 at 03:06:23PM +0100, Andrew Lunn wrote:
> > Usually it's better to apply LRU or random here in my opinion, as the
> > new entry is much more likely to be needed than older ones by definition.
> 
> Hi Willy
> 
> I think this depends on why you need to discard. If it is normal
> operation and the limits are simply too low, i would agree.
> 
> If however it is a DoS, throwing away the new entries makes sense,
> leaving the old ones which are more likely to be useful.
> 
> Most of the talk in this thread has been about limits for DoS
> prevention...

Sure but my point is that it can kick in on regular traffic and in
this case it can be catastrophic. That's only what bothers me. If
we have an unlimited default value with this algorithm I'm fine
because nobody will get caught by accident with a bridge suddenly
replicating high traffic on all ports because an unknown limit was
reached. That's the principle of least surprise.

I know that when fighting DoSes there's never any universally good
solutions and one has to make tradeoffs. I'm perfectly fine with this.

Cheers,
Willy