3 files changed, 12 insertions, 3 deletions

diff --git a/Documentation/networking/ip-sysctl.txt b/Documentation/networking/ip-sysctl.txt
index ec5de02f543f..7f98aa3962c6 100644
--- a/Documentation/networking/ip-sysctl.txt
+++ b/Documentation/networking/ip-sysctl.txt
@@ -168,7 +168,16 @@ tcp_dsack - BOOLEAN
         Allows TCP to send "duplicate" SACKs.
 
 tcp_ecn - BOOLEAN
-        Enable Explicit Congestion Notification in TCP.
+        Enable Explicit Congestion Notification (ECN) in TCP. ECN is only
+        used when both ends of the TCP flow support it. It is useful to
+        avoid losses due to congestion (when the bottleneck router supports
+        ECN).
+        Possible values are:
+                0 disable ECN
+                1 ECN enabled
+                2 Only server-side ECN enabled. If the other end does
+                  not support ECN, behavior is like with ECN disabled.
+        Default: 2
 
 tcp_fack - BOOLEAN
         Enable FACK congestion avoidance and fast retransmission.
diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index c96a6bb25430..56dcf97a97fb 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -77,7 +77,7 @@ int sysctl_tcp_window_scaling __read_mostly = 1;
 int sysctl_tcp_sack __read_mostly = 1;
 int sysctl_tcp_fack __read_mostly = 1;
 int sysctl_tcp_reordering __read_mostly = TCP_FASTRETRANS_THRESH;
-int sysctl_tcp_ecn __read_mostly;
+int sysctl_tcp_ecn __read_mostly = 2;
 int sysctl_tcp_dsack __read_mostly = 1;
 int sysctl_tcp_app_win __read_mostly = 31;
 int sysctl_tcp_adv_win_scale __read_mostly = 2;
diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index 59aec609cec6..79c39dc9b01c 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -288,7 +288,7 @@ static inline void TCP_ECN_send_syn(struct sock *sk, struct sk_buff *skb)
         struct tcp_sock *tp = tcp_sk(sk);
 
         tp->ecn_flags = 0;
-        if (sysctl_tcp_ecn) {
+        if (sysctl_tcp_ecn == 1) {
                 TCP_SKB_CB(skb)->flags |= TCPCB_FLAG_ECE | TCPCB_FLAG_CWR;
                 tp->ecn_flags = TCP_ECN_OK;
         }