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

diff --git a/target/linux/generic/patches-3.3/060-tcp-ecn-dont-delay-ACKS-after-CE.patch b/target/linux/generic/patches-3.3/060-tcp-ecn-dont-delay-ACKS-after-CE.patch
new file mode 100644 (file)
index 0000000..547e4fa
--- /dev/null
+++ b/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 <eric.dumazet@gmail.com>
+X-Patchwork-Id: 175453
+Message-Id: <1344287083.26674.83.camel@edumazet-glaptop>
+To: David Miller <davem@davemloft.net>
+Cc: netdev <netdev@vger.kernel.org>,
+       Neal Cardwell <ncardwell@google.com>
+
+From: Eric Dumazet <edumazet@google.com>
+
+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 <edumazet@google.com>
+Cc: Neal Cardwell <ncardwell@google.com>
+Acked-by: Neal Cardwell <ncardwell@google.com>
+
+---
+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;