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Date:	Fri, 04 May 2012 20:23:35 +0200
From:	Eric Dumazet <>
To:	Rick Jones <>
Cc:	David Miller <>,
	netdev <>,
	Perry Lorier <>,
	Matt Mathis <>,
	Yuchung Cheng <>,
	Neal Cardwell <>,
	Tom Herbert <>,
	Wilmer van der Gaast <>,
	Dave Täht <>,
	Ankur Jain <>
Subject: Re: [PATCH net-next] tcp: be more strict before accepting ECN

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<>
> >
> > 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

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

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