[TCP]: Add Scalable TCP congestion control module.

This patch implements Tom Kelly's Scalable TCP congestion control algorithm 
for the modular framework.

The algorithm has some nice scaling properties, and has been used a fair bit 
in research, though is known to have significant fairness issues, so it's not 
really suitable for general purpose use.

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

3 files changed, 78 insertions, 0 deletions

diff --git a/net/ipv4/Kconfig b/net/ipv4/Kconfig
index 73a25b52bf7d..690e88ba2484 100644
--- a/net/ipv4/Kconfig
+++ b/net/ipv4/Kconfig
@@ -511,6 +511,15 @@ config TCP_CONG_VEGAS
         window. TCP Vegas should provide less packet loss, but it is
         not as aggressive as TCP Reno.
 
+config TCP_CONG_SCALABLE
+        tristate "Scalable TCP"
+        depends on INET && EXPERIMENTAL
+        default n
+        ---help---
+        Scalable TCP is a sender-side only change to TCP which uses a
+        MIMD congestion control algorithm which has some nice scaling
+        properties, though is known to have fairness issues.
+        See http://www-lce.eng.cam.ac.uk/~ctk21/scalable/
 
 endmenu
 
diff --git a/net/ipv4/Makefile b/net/ipv4/Makefile
index e96ed17deb96..5718cdb3a61e 100644
--- a/net/ipv4/Makefile
+++ b/net/ipv4/Makefile
@@ -37,6 +37,7 @@ obj-$(CONFIG_TCP_CONG_HSTCP) += tcp_highspeed.o
 obj-$(CONFIG_TCP_CONG_HYBLA) += tcp_hybla.o
 obj-$(CONFIG_TCP_CONG_HTCP) += tcp_htcp.o
 obj-$(CONFIG_TCP_CONG_VEGAS) += tcp_vegas.o
+obj-$(CONFIG_TCP_CONG_SCALABLE) += tcp_scalable.o
 
 obj-$(CONFIG_XFRM) += xfrm4_policy.o xfrm4_state.o xfrm4_input.o \
                       xfrm4_output.o
diff --git a/net/ipv4/tcp_scalable.c b/net/ipv4/tcp_scalable.c
new file mode 100644
index 000000000000..70e108e15c71
--- /dev/null
+++ b/net/ipv4/tcp_scalable.c
@@ -0,0 +1,68 @@
+/* Tom Kelly's Scalable TCP
+ *
+ * See htt://www-lce.eng.cam.ac.uk/~ctk21/scalable/
+ *
+ * John Heffner <jheffner@sc.edu>
+ */
+
+#include <linux/config.h>
+#include <linux/module.h>
+#include <net/tcp.h>
+
+/* These factors derived from the recommended values in the aer:
+ * .01 and and 7/8. We use 50 instead of 100 to account for
+ * delayed ack.
+ */
+#define TCP_SCALABLE_AI_CNT     50U
+#define TCP_SCALABLE_MD_SCALE   3
+
+static void tcp_scalable_cong_avoid(struct tcp_sock *tp, u32 ack, u32 rtt,
+                                    u32 in_flight, int flag)
+{
+        if (in_flight < tp->snd_cwnd)
+                return;
+
+        if (tp->snd_cwnd <= tp->snd_ssthresh) {
+                tp->snd_cwnd++;
+        } else {
+                tp->snd_cwnd_cnt++;
+                if (tp->snd_cwnd_cnt > min(tp->snd_cwnd, TCP_SCALABLE_AI_CNT)){
+                        tp->snd_cwnd++;
+                        tp->snd_cwnd_cnt = 0;
+                }
+        }
+        tp->snd_cwnd = min_t(u32, tp->snd_cwnd, tp->snd_cwnd_clamp);
+        tp->snd_cwnd_stamp = tcp_time_stamp;
+}
+
+static u32 tcp_scalable_ssthresh(struct tcp_sock *tp)
+{
+        return max(tp->snd_cwnd - (tp->snd_cwnd>>TCP_SCALABLE_MD_SCALE), 2U);
+}
+
+
+static struct tcp_congestion_ops tcp_scalable = {
+        .ssthresh       = tcp_scalable_ssthresh,
+        .cong_avoid     = tcp_scalable_cong_avoid,
+        .min_cwnd       = tcp_reno_min_cwnd,
+
+        .owner          = THIS_MODULE,
+        .name           = "scalable",
+};
+
+static int __init tcp_scalable_register(void)
+{
+        return tcp_register_congestion_control(&tcp_scalable);
+}
+
+static void __exit tcp_scalable_unregister(void)
+{
+        tcp_unregister_congestion_control(&tcp_scalable);
+}
+
+module_init(tcp_scalable_register);
+module_exit(tcp_scalable_unregister);
+
+MODULE_AUTHOR("John Heffner");
+MODULE_LICENSE("GPL");
+MODULE_DESCRIPTION("Scalable TCP");