[TCP]: Add RFC3742 Limited Slow-Start, controlled by variable sysctl_tcp_max_ssthresh.

Signed-off-by: John Heffner <jheffner@psc.edu>
Signed-off-by: David S. Miller <davem@davemloft.net>

4 files changed, 32 insertions, 9 deletions

diff --git a/include/linux/sysctl.h b/include/linux/sysctl.h
index 9a8970bf99a6..98e0fd241a25 100644
--- a/include/linux/sysctl.h
+++ b/include/linux/sysctl.h
@@ -438,6 +438,7 @@ enum
         NET_CIPSOV4_RBM_STRICTVALID=121,
         NET_TCP_AVAIL_CONG_CONTROL=122,
         NET_TCP_ALLOWED_CONG_CONTROL=123,
+        NET_TCP_MAX_SSTHRESH=124,
 };
 
 enum {
diff --git a/include/net/tcp.h b/include/net/tcp.h
index 7fd6b77519c3..6d09f5085f6a 100644
--- a/include/net/tcp.h
+++ b/include/net/tcp.h
@@ -230,6 +230,7 @@ extern int sysctl_tcp_mtu_probing;
 extern int sysctl_tcp_base_mss;
 extern int sysctl_tcp_workaround_signed_windows;
 extern int sysctl_tcp_slow_start_after_idle;
+extern int sysctl_tcp_max_ssthresh;
 
 extern atomic_t tcp_memory_allocated;
 extern atomic_t tcp_sockets_allocated;
diff --git a/net/ipv4/sysctl_net_ipv4.c b/net/ipv4/sysctl_net_ipv4.c
index 0aa304711a96..d68effe98e8d 100644
--- a/net/ipv4/sysctl_net_ipv4.c
+++ b/net/ipv4/sysctl_net_ipv4.c
@@ -803,6 +803,14 @@ ctl_table ipv4_table[] = {
                 .proc_handler   = &proc_allowed_congestion_control,
                 .strategy       = &strategy_allowed_congestion_control,
         },
+        {
+                .ctl_name       = NET_TCP_MAX_SSTHRESH,
+                .procname       = "tcp_max_ssthresh",
+                .data           = &sysctl_tcp_max_ssthresh,
+                .maxlen         = sizeof(int),
+                .mode           = 0644,
+                .proc_handler   = &proc_dointvec,
+        },
         { .ctl_name = 0 }
 };
 
diff --git a/net/ipv4/tcp_cong.c b/net/ipv4/tcp_cong.c
index 34ae3f13483a..ccd88407e0cd 100644
--- a/net/ipv4/tcp_cong.c
+++ b/net/ipv4/tcp_cong.c
@@ -12,6 +12,8 @@
 #include <linux/list.h>
 #include <net/tcp.h>
 
+int sysctl_tcp_max_ssthresh = 0;
+
 static DEFINE_SPINLOCK(tcp_cong_list_lock);
 static LIST_HEAD(tcp_cong_list);
 
@@ -274,10 +276,13 @@ int tcp_set_congestion_control(struct sock *sk, const char *name)
 
 
 /*
- * Linear increase during slow start
+ * Slow start (exponential increase) with
+ * RFC3742 Limited Slow Start (fast linear increase) support.
  */
 void tcp_slow_start(struct tcp_sock *tp)
 {
+        int cnt = 0;
+
         if (sysctl_tcp_abc) {
                 /* RFC3465: Slow Start
                  * TCP sender SHOULD increase cwnd by the number of
@@ -286,17 +291,25 @@ void tcp_slow_start(struct tcp_sock *tp)
                  */
                 if (tp->bytes_acked < tp->mss_cache)
                         return;
-
-                /* We MAY increase by 2 if discovered delayed ack */
-                if (sysctl_tcp_abc > 1 && tp->bytes_acked >= 2*tp->mss_cache) {
-                        if (tp->snd_cwnd < tp->snd_cwnd_clamp)
-                                tp->snd_cwnd++;
-                }
         }
+
+        if (sysctl_tcp_max_ssthresh > 0 &&
+            tp->snd_cwnd > sysctl_tcp_max_ssthresh)
+                cnt += sysctl_tcp_max_ssthresh>>1;
+        else
+                cnt += tp->snd_cwnd;
+
+        /* RFC3465: We MAY increase by 2 if discovered delayed ack */
+        if (sysctl_tcp_abc > 1 && tp->bytes_acked >= 2*tp->mss_cache)
+                cnt <<= 1;
         tp->bytes_acked = 0;
 
-        if (tp->snd_cwnd < tp->snd_cwnd_clamp)
-                tp->snd_cwnd++;
+        tp->snd_cwnd_cnt += cnt;
+        while (tp->snd_cwnd_cnt >= tp->snd_cwnd) {
+                tp->snd_cwnd_cnt -= tp->snd_cwnd;
+                if (tp->snd_cwnd < tp->snd_cwnd_clamp)
+                        tp->snd_cwnd++;
+        }
 }
 EXPORT_SYMBOL_GPL(tcp_slow_start);