tcp: be more strict before accepting ECN negociation
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. Signed-off-by: Eric Dumazet <edumazet@google.com> Cc: Perry Lorier <perryl@google.com> Cc: Matt Mathis <mattmathis@google.com> Cc: Yuchung Cheng <ycheng@google.com> Cc: Neal Cardwell <ncardwell@google.com> Cc: Wilmer van der Gaast <wilmer@google.com> Cc: Ankur Jain <jankur@google.com> Cc: Tom Herbert <therbert@google.com> Cc: Dave Täht <dave.taht@bufferbloat.net> Acked-by: Neal Cardwell <ncardwell@google.com> Signed-off-by: David S. Miller <davem@davemloft.net>
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