From mboxrd@z Thu Jan  1 00:00:00 1970
From: Eric Dumazet <eric.dumazet@gmail.com>
Subject: Re: [PATCH net-next] tcp: be more strict before accepting ECN
 negociation
Date: Fri, 04 May 2012 20:23:35 +0200
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Cc: David Miller <davem@davemloft.net>,
	netdev <netdev@vger.kernel.org>,
	Perry Lorier <perryl@google.com>,
	Matt Mathis <mattmathis@google.com>,
	Yuchung Cheng <ycheng@google.com>,
	Neal Cardwell <ncardwell@google.com>,
	Tom Herbert <therbert@google.com>,
	Wilmer van der Gaast <wilmer@google.com>,
	Dave =?ISO-8859-1?Q?T=E4ht?= <dave.taht@bufferbloat.net>,
	Ankur Jain <jankur@google.com>
To: Rick Jones <rick.jones2@hp.com>
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On Fri, 2012-05-04 at 11:09 -0700, Rick Jones wrote:
> On 05/04/2012 08:14 AM, Eric Dumazet wrote:
> > From: Eric Dumazet<edumazet@google.com>
> >
> > It appears some networks play bad games with the two bits reserved for
> > ECN. This can trigger false congestion notifications and very slow
> > transferts.
> >
> > Since RFC 3168 (6.1.1) forbids SYN packets to carry CT bits, we can
> > disable TCP ECN negociation if it happens we receive mangled CT bits in
> > the SYN packet.
> 
> What sort of networks were these?  Any chance it was some sort of 
> attempt to add ECN to FastOpen?

Nothing to do with fastopen.

Just take a look at a random http server and sample all SYN packets it
receives.

Some of them have TOS bits 0 or 1 set, or even both bits set.