netdev - Re: [PATCH -next v2 1/2] syncookies: remove ecn_ok validation when decoding option timestamp

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Date:	Fri, 31 Oct 2014 08:47:40 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Florian Westphal <fw@...len.de>
Cc:	netdev@...r.kernel.org
Subject: Re: [PATCH -next v2 1/2] syncookies: remove ecn_ok validation when
 decoding option timestamp

On Fri, 2014-10-31 at 15:15 +0100, Florian Westphal wrote:

> So if you have a per route ecn setting, and syncookies are used,
> and tcp_ecn sysctl is 0:

This part I do not understand.

Why should tcp_ecn be 0 here, and not 2 (default value) ?

> 
> 1. we receive syn with ecn on and timestamps
> 2. we send cookie synack, with timestamp and ecn (route allowed it),
> the lower bits of the timestamp have a "magic" bit set that allows
> us to infer that ecn was negotiated successfully.
> 3. we drop the ack from the client, since timestamp decoding sees
> "ecn is on according to timestamp, but the tcp_ecn sysctl is off".
> 
> So to fix this, step 3 either has to check the dst setting
> in addition to the global sysctl, or to rely on the timestamp alone
> that ecn was requested by the original client and allowed by our host
> at the time synack timestamp was generated/sent.
> 
> I hope that explains the reason behind patch #1 up.



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