netdev - Re: [PATCH 1/2] syncookies: do not store rcv

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Message-ID: <20100623202759.GA3581@nuttenaction>
Date:	Wed, 23 Jun 2010 22:28:00 +0200
From:	Hagen Paul Pfeifer <hagen@...u.net>
To:	Florian Westphal <fw@...len.de>
Cc:	netdev@...r.kernel.org,
	Ilpo Järvinen <ilpo.jarvinen@...sinki.fi>
Subject: Re: [PATCH 1/2] syncookies: do not store rcv_wscale in tcp
 timestamp

* Florian Westphal | 2010-06-21 23:48:44 [+0200]:

>As pointed out by Fernando Gont there is no need to encode rcv_wscale
>into the cookie.
>
>We did not use the restored rcv_wscale anyway; it is recomputed
>via tcp_select_initial_window().

I speculate that this behavior was and is not correct. I suppose that their is
a race between the SYN/ACK where we initial force a particular window scale
and the next time where we recalculate the window via tcp_select_initial_window().

If the user change net.core.rmem_max or net.ipv4.tcp_rmem in between this
time, the recalculated window scale (rcv_wscale) can be smaller. But the
receiver still operates with the initial window scale and can overshot the
granted window - and bang.

There are several solutions: encode rcv_wscale into the syn cookie and don't
recalculate or disable window scaling and don't transmit any scaling option
when SYN cookies are active.

HGN
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