netdev - Re: [PATCH 1/2] tcp: syncookies: reduce cookie lifetime to 128 seconds

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Date:	Wed, 28 Aug 2013 18:20:48 -0400 (EDT)
From:	David Miller <davem@...emloft.net>
To:	fw@...len.de
Cc:	netdev@...r.kernel.org, jbohac@...e.cz
Subject: Re: [PATCH 1/2] tcp: syncookies: reduce cookie lifetime to 128
 seconds

From: Florian Westphal <fw@...len.de>
Date: Sun, 25 Aug 2013 19:54:01 +0200

> We currently accept cookies that were created less than 4 minutes ago
> (ie, cookies with counter delta 0-3).  Combined with the 8 mss table
> values, this yields 32 possible values (out of 2**32) that will be valid.
> 
> Reducing the lifetime to < 2 minutes halves the guessing chance while
> still providing a large enough period (possible cookies are
> only validated if last synqueue overflow was less than 3 seconds ago).
> 
> While at it, get rid of jiffies value -- they overflow too quickly on
> 32 bit platforms.
> 
> getnstimeofday is used to create a counter that increments every 64s.
> 
> Reported-by: Jakob Lell <jakob@...oblell.com>
> Signed-off-by: Florian Westphal <fw@...len.de>

Moving from jiffies as a time seed to getnstimeofday() obviously will
have some performance impact, can you talk a little bit about that?


> +#define MAX_SYNCOOKIE_AGE 2 /* 128 seconds */

I'm sure you understand how 2 translates into 128 seconds in this syncookie
code, but I sure don't and the next person who reads this patch or this
code after I apply it may not either.

Could you in some way expose the calculation?  Thanks.
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