netdev - Re: [PATCH net-next 2/5] tcp: TFO: search for correct cookie and accept data

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Date:   Mon, 17 Dec 2018 14:59:42 -0800
From:   Christoph Paasch <cpaasch@...le.com>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     netdev@...r.kernel.org, Eric Dumazet <edumazet@...gle.com>,
        Yuchung Cheng <ycheng@...gle.com>,
        David Miller <davem@...emloft.net>
Subject: Re: [PATCH net-next 2/5] tcp: TFO: search for correct cookie and
 accept data

On 16/12/18 - 22:30:51, Eric Dumazet wrote:
> 
> 
> On 12/14/2018 02:40 PM, Christoph Paasch wrote:
> > This change allows to search for the right cookie and accepts old ones
> > (announcing a new one if it has changed).
> > 
> > __tcp_fastopen_cookie_gen_with_ctx() allows to generate a cookie based
> > on a given TFO-context. A later patch will cleanup the duplicate code.
> 
> How long is kept the secondary (old) context ?

There is no time-limit on keeping the older context.

In an older version of this series I had the pool-size as a sysctl so one
could try out different configurations. For us, a size of 2 was good enough.

I could bring that back if you think it's useful.

> I do not know exact crypto_cipher_encrypt_one() cost, but it looks like
> your patch could double the cost of some TFO based attacks ?

True, we are doing more crypto if we are getting a lot of invalid or old cookies.
I don't have a good answer to that besides that one should probably disable
TFO at that point ;-)

On the other hand, AFAICS tcp_conn_request will end up setting want_cookie
to true under SYN-flooding so we won't even enter tcp_try_fastopen if it's
really an attack.

Christoph