[PATCH] Scalable TCP

[John Heffner <jheffner@xxxxxxx>]

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@xxxxxxx>

Index: net/ipv4/Kconfig
===================================================================
--- ba5d2c7b7d035e88291b9692bd7006907e4c46f5/net/ipv4/Kconfig  (mode:100644)
+++ 4130df8533b79d0ccfd0195e707f4aa60a527e41/net/ipv4/Kconfig  (mode:100644)
@@ -481,6 +481,15 @@
        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 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
 
Index: net/ipv4/Makefile
===================================================================
--- ba5d2c7b7d035e88291b9692bd7006907e4c46f5/net/ipv4/Makefile  (mode:100644)
+++ 4130df8533b79d0ccfd0195e707f4aa60a527e41/net/ipv4/Makefile  (mode:100644)
@@ -35,6 +35,7 @@
 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
Index: net/ipv4/tcp_scalable.c
===================================================================
--- /dev/null  (tree:ba5d2c7b7d035e88291b9692bd7006907e4c46f5)
+++ 
4130df8533b79d0ccfd0195e707f4aa60a527e41/net/ipv4/tcp_scalable.c  (mode:100644)
@@ -0,0 +1,71 @@
+/* Tom Kelly's Scalable TCP
+ *
+ * See http://www-lce.eng.cam.ac.uk/~ctk21/scalable/
+ *
+ * John Heffner <jheffner@xxxxxxx>
+ */
+
+#include <linux/config.h>
+#include <linux/module.h>
+#include <net/tcp.h>
+
+/* These factors derived from the recommended values in the paper:
+ * .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(tp->snd_cwnd, (__u32)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 u32 tcp_scalable_min_cwnd(struct tcp_sock *tp)
+{
+       return tp->snd_ssthresh / 2;
+}
+
+static struct tcp_congestion_ops tcp_scalable = {
+       .ssthresh       = tcp_scalable_ssthresh,
+       .cong_avoid     = tcp_scalable_cong_avoid,
+       .min_cwnd       = tcp_scalable_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");