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 21:05:59 +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 11:48 -0700, Rick Jones wrote:
> On 05/04/2012 11:23 AM, Eric Dumazet wrote:
> > On Fri, 2012-05-04 at 11:09 -0700, Rick Jones wrote:
> >> 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.
> 
> I'll fire-up tcpdump on netperf.org:
> 
> tcpdump -i eth0 -vvv '(tcp[tcpflags] & tcp-syn != 0) && (ip[1] != 0x0)'
> 
> and see what appears.
> 
> rick

of (ip[1] & 3 != 0)


Note that you could catch SYNACK with this filter (if your machine
initiates some active TCP sessions), since SYNACK might have ECT bits,
if some stacks implemented :

http://tools.ietf.org/html/draft-kuzmanovic-ecn-syn-00  ( Adding
Explicit Congestion Notification (ECN) Capability to TCP's SYN/ACK
Packets )

http://tools.ietf.org/id/draft-ietf-tcpm-ecnsyn-04.txt