[TCP]: Limited slow start for Highspeed TCP

Implementation of RFC3742 limited slow start. Added as part
of the TCP highspeed congestion control module.

Signed-off-by: Stephen Hemminger <shemminger@osdl.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 21 insertions, 3 deletions

diff --git a/net/ipv4/tcp_highspeed.c b/net/ipv4/tcp_highspeed.c
index ba7c63ca5bb1..1120245b2373 100644
--- a/net/ipv4/tcp_highspeed.c
+++ b/net/ipv4/tcp_highspeed.c
@@ -98,6 +98,10 @@ struct hstcp {
         u32     ai;
 };
 
+static int max_ssthresh = 100;
+module_param(max_ssthresh, int, 0644);
+MODULE_PARM_DESC(max_ssthresh, "limited slow start threshold (RFC3742)");
+
 static void hstcp_init(struct sock *sk)
 {
         struct tcp_sock *tp = tcp_sk(sk);
@@ -119,9 +123,23 @@ static void hstcp_cong_avoid(struct sock *sk, u32 adk, u32 rtt,
         if (!tcp_is_cwnd_limited(sk, in_flight))
                 return;
 
-        if (tp->snd_cwnd <= tp->snd_ssthresh)
-                tcp_slow_start(tp);
-        else {
+        if (tp->snd_cwnd <= tp->snd_ssthresh) {
+                /* RFC3742: limited slow start
+                 * the window is increased by 1/K MSS for each arriving ACK,
+                 * for K = int(cwnd/(0.5 max_ssthresh))
+                 */
+                if (max_ssthresh > 0 && tp->snd_cwnd > max_ssthresh) {
+                        u32 k = max(tp->snd_cwnd / (max_ssthresh >> 1), 1U);
+                        if (++tp->snd_cwnd_cnt >= k) {
+                                if (tp->snd_cwnd < tp->snd_cwnd_clamp)
+                                        tp->snd_cwnd++;
+                                tp->snd_cwnd_cnt = 0;
+                        }
+                } else {
+                        if (tp->snd_cwnd < tp->snd_cwnd_clamp)
+                                tp->snd_cwnd++;
+                }
+        } else {
                 /* Update AIMD parameters */
                 if (tp->snd_cwnd > hstcp_aimd_vals[ca->ai].cwnd) {
                         while (tp->snd_cwnd > hstcp_aimd_vals[ca->ai].cwnd &&