Re: [RFC PATCH 2/2] tcp: Early SYN limit and SYN cookie handling to mitigate SYN floods

Eric Dumazet
2012-05-30T08:15:08Z

On Wed, 2012-05-30 at 09:45 +0200, Jesper Dangaard Brouer wrote:

> Sounds interesting, but TCP Fast Open is primarily concerned with
> enabling data exchange during SYN establishment. I don't see any
> indication that they have implemented parallel SYN handling.
>

Not at all, TCP fast open main goal is to allow connection establishment
with a single packet (thus removing one RTT). This also removes the
whole idea of having half-sockets (in SYN_RCV state)

Then, allowing DATA in the SYN packet is an extra bonus, only if the
whole request can fit in the packet (it is unlikely for typical http
requests)

> Implementing parallel SYN handling, should also benefit their work.

Why do you think I am working on this ? Hint : I am a Google coworker.

> After studying this code path, I also see great performance benefit in
> also optimizing the normal 3WHS on sock's in sk_state == LISTEN.
> Perhaps we should split up the code path for LISTEN vs. ESTABLISHED, as
> they are very entangled at the moment AFAIKS.
>
> > Yuchung Cheng and Jerry Chu should upstream this code in a very near
> > future.
>
> Looking forward to see the code, and the fallout discussions, on
> transferring data on SYN packets.
>

Problem is this code will be delayed if we change net-next code in this
area, because we'll have to rebase and retest everything.

>
> > Another way to mitigate SYN scalability issues before the full RCU
> > solution I was cooking is to either :
> >
> > 1) Use a hardware filter (like on Intel NICS) to force all SYN packets
> > going to one queue (so that they are all serviced on one CPU)
> >
> > 2) Tweak RPS (__skb_get_rxhash()) so that SYN packets rxhash is not
> > dependent on src port/address, to get same effect (All SYN packets
> > processed by one cpu). Note this only address the SYN flood problem, not
> > the general 3WHS scalability one, since if real connection is
> > established, the third packet (ACK from client) will have the 'real'
> > rxhash and will be processed by another cpu.
>
> I don't like the idea of overloading one CPU with SYN packets. As the
> attacker can still cause a DoS on new connections.
>

One CPU can handle more than one million SYN per second, while 32 cpus
fighting on socket lock can not handle 1 % of this load.

If Intel chose to implement this hardware filter in their NIC, its for a
good reason.

> My "unlocked" parallel SYN cookie approach, should favor established
> connections, as they are allowed to run under a BH lock, and thus don't
> let new SYN packets in (on this CPU), until the establish conn packet is
> finished. Unless I have misunderstood something... I think I have,
> established connections have their own/seperate struck sock, and thus
> this is another slock spinlock, right?. (Well let Eric bash me for
> this ;-))

It seems you forgot I have patches to have full parallelism, not only
the SYNCOOKIE hack.

I am still polishing them, its a _long_ process, especially if network
tree changes a lot.

If you believe you can beat me on this, please let me know so that I can
switch to other tasks.