[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>

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 &&