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From: Hagen Paul Pfeifer <hagen@jauu.net>
To: Florian Westphal <fw@strlen.de>
Cc: netdev@vger.kernel.org, "Ilpo Järvinen" <ilpo.jarvinen@helsinki.fi>
Subject: Re: [PATCH 1/2] syncookies: do not store rcv_wscale in tcp timestamp
Date: Wed, 23 Jun 2010 22:28:00 +0200	[thread overview]
Message-ID: <20100623202759.GA3581@nuttenaction> (raw)
In-Reply-To: <1277156925-7295-1-git-send-email-fw@strlen.de>

* 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

  parent reply	other threads:[~2010-06-23 20:28 UTC|newest]

Thread overview: 8+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2010-06-21 21:48 [PATCH 1/2] syncookies: do not store rcv_wscale in tcp timestamp Florian Westphal
2010-06-21 21:48 ` [PATCH 2/2] syncookies: add support for ECN Florian Westphal
2010-06-27  5:00   ` David Miller
2010-06-23 20:28 ` Hagen Paul Pfeifer [this message]
2010-06-24  7:53   ` [PATCH 1/2] syncookies: do not store rcv_wscale in tcp timestamp Florian Westphal
2010-06-24 22:14     ` Hagen Paul Pfeifer
2010-06-27  4:27       ` David Miller
2010-06-27  5:00 ` David Miller

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