netdev - Re: [PATCH net-next] tcp: avoid tx starvation by SYNACK packets

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Date:	Sat, 02 Jun 2012 07:46:32 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Dave Taht <dave.taht@...il.com>
Cc:	Hans Schillstrom <hans.schillstrom@...csson.com>,
	netdev <netdev@...r.kernel.org>,
	Neal Cardwell <ncardwell@...gle.com>,
	Tom Herbert <therbert@...gle.com>,
	Jesper Dangaard Brouer <brouer@...hat.com>
Subject: Re: [PATCH net-next] tcp: avoid tx starvation by SYNACK packets

On Fri, 2012-06-01 at 18:28 -0700, Dave Taht wrote:
> On Thu, May 31, 2012 at 2:56 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> > From: Eric Dumazet <edumazet@...gle.com>
> >
> > pfifo_fast being the default Qdisc, its pretty easy to fill it with
> > SYNACK (small) packets while host is under SYNFLOOD attack.
> >
> > Packets of established TCP sessions are dropped and host appears almost
> > dead.
> >
> > Avoid this problem assigning TC_PRIO_FILLER priority to SYNACK
> > generated in SYNCOOKIE mode, so that these packets are enqueued into
> > pfifo_fast band 2.
> >
> > Other packets, queued to band 0 or 1 are dequeued before any SYNACK
> > packets waiting in band 2.
> 
> I am curious as to how well fq_codel survives an attack like this, without aid.

codel or fq_codel are not doing priority classification.

SYNACK will spread in all hash buckets and global queue limit can be
hit.

fq_codel wont protect you by itself, unless you use a hierarchy with one
"prio" and two or three "fq_codel".




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