kernel: dont delay acks after ECN CE

Bugfix from linux head - don't delay acks from ECN congestion
experienced in some situations.

SVN-Revision: 33686

target/linux/generic/patches-3.3/060-tcp-ecn-dont-delay-ACKS-after-CE.patch

@@ -0,0 +1,63 @@
+From patchwork Mon Aug  6 21:04:43 2012
+Content-Type: text/plain; charset="utf-8"
+MIME-Version: 1.0
+Content-Transfer-Encoding: 7bit
+Subject: [net-next] tcp: ecn: dont delay ACKS after CE
+Date: Mon, 06 Aug 2012 11:04:43 -0000
+From: Eric Dumazet <[email protected]>
+X-Patchwork-Id: 175453
+Message-Id: <1344287083.26674.83.camel@edumazet-glaptop>
+To: David Miller <[email protected]>
+Cc: netdev <[email protected]>,
+	Neal Cardwell <[email protected]>
+
+From: Eric Dumazet <[email protected]>
+
+While playing with CoDel and ECN marking, I discovered a
+non optimal behavior of receiver of CE (Congestion Encountered)
+segments.
+
+In pathological cases, sender has reduced its cwnd to low values,
+and receiver delays its ACK (by 40 ms).
+
+While RFC 3168 6.1.3 (The TCP Receiver) doesn't explicitly recommend
+to send immediate ACKS, we believe its better to not delay ACKS, because
+a CE segment should give same signal than a dropped segment, and its
+quite important to reduce RTT to give ECE/CWR signals as fast as
+possible.
+
+Note we already call tcp_enter_quickack_mode() from TCP_ECN_check_ce()
+if we receive a retransmit, for the same reason.
+
+Signed-off-by: Eric Dumazet <[email protected]>
+Cc: Neal Cardwell <[email protected]>
+Acked-by: Neal Cardwell <[email protected]>
+
+---
+net/ipv4/tcp_input.c |    6 +++++-
+ 1 file changed, 5 insertions(+), 1 deletion(-)
+
+
+
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+
+diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
+index 2fd2bc9..fa2c2c2 100644
+--- a/net/ipv4/tcp_input.c
++++ b/net/ipv4/tcp_input.c
+@@ -237,7 +237,11 @@ static inline void TCP_ECN_check_ce(struct tcp_sock *tp, const struct sk_buff *s
+ 			tcp_enter_quickack_mode((struct sock *)tp);
+ 		break;
+ 	case INET_ECN_CE:
+-		tp->ecn_flags |= TCP_ECN_DEMAND_CWR;
++		if (!(tp->ecn_flags & TCP_ECN_DEMAND_CWR)) {
++			/* Better not delay acks, sender can have a very low cwnd */
++			tcp_enter_quickack_mode((struct sock *)tp);
++			tp->ecn_flags |= TCP_ECN_DEMAND_CWR;
++		}
+ 		/* fallinto */
+ 	default:
+ 		tp->ecn_flags |= TCP_ECN_SEEN;