[TCP]: Add H-TCP congestion control module.

H-TCP is a congestion control algorithm developed at the Hamilton Institute, by
Douglas Leith and Robert Shorten. It is extending the standard Reno algorithm
with mode switching is thus a relatively simple modification.

H-TCP is defined in a layered manner as it is still a research platform. The
basic form includes the modification of beta according to the ratio of maxRTT
to min RTT and the alpha=2*factor*(1-beta) relation, where factor is dependant
on the time since last congestion.

The other layers improve convergence by adding appropriate factors to alpha.

The following patch implements the H-TCP algorithm in it's basic form.

Signed-Off-By: Baruch Even <baruch@ev-en.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 289 insertions, 0 deletions

diff --git a/net/ipv4/tcp_htcp.c b/net/ipv4/tcp_htcp.c
new file mode 100644
index 000000000000..40168275acf9
--- /dev/null
+++ b/net/ipv4/tcp_htcp.c
@@ -0,0 +1,289 @@
+/*
+ * H-TCP congestion control. The algorithm is detailed in:
+ * R.N.Shorten, D.J.Leith:
+ *   "H-TCP: TCP for high-speed and long-distance networks"
+ *   Proc. PFLDnet, Argonne, 2004.
+ * http://www.hamilton.ie/net/htcp3.pdf
+ */
+
+#include <linux/config.h>
+#include <linux/mm.h>
+#include <linux/module.h>
+#include <net/tcp.h>
+
+#define ALPHA_BASE      (1<<7)  /* 1.0 with shift << 7 */
+#define BETA_MIN        (1<<6)  /* 0.5 with shift << 7 */
+#define BETA_MAX        102     /* 0.8 with shift << 7 */
+
+static int use_rtt_scaling = 1;
+module_param(use_rtt_scaling, int, 0644);
+MODULE_PARM_DESC(use_rtt_scaling, "turn on/off RTT scaling");
+
+static int use_bandwidth_switch = 1;
+module_param(use_bandwidth_switch, int, 0644);
+MODULE_PARM_DESC(use_bandwidth_switch, "turn on/off bandwidth switcher");
+
+struct htcp {
+        u16     alpha;          /* Fixed point arith, << 7 */
+        u8      beta;           /* Fixed point arith, << 7 */
+        u8      modeswitch;     /* Delay modeswitch until we had at least one congestion event */
+        u8      ccount;         /* Number of RTTs since last congestion event */
+        u8      undo_ccount;
+        u16     packetcount;
+        u32     minRTT;
+        u32     maxRTT;
+        u32     snd_cwnd_cnt2;
+
+        u32     undo_maxRTT;
+        u32     undo_old_maxB;
+
+        /* Bandwidth estimation */
+        u32     minB;
+        u32     maxB;
+        u32     old_maxB;
+        u32     Bi;
+        u32     lasttime;
+};
+
+static inline void htcp_reset(struct htcp *ca)
+{
+        ca->undo_ccount = ca->ccount;
+        ca->undo_maxRTT = ca->maxRTT;
+        ca->undo_old_maxB = ca->old_maxB;
+
+        ca->ccount = 0;
+        ca->snd_cwnd_cnt2 = 0;
+}
+
+static u32 htcp_cwnd_undo(struct tcp_sock *tp)
+{
+        struct htcp *ca = tcp_ca(tp);
+        ca->ccount = ca->undo_ccount;
+        ca->maxRTT = ca->undo_maxRTT;
+        ca->old_maxB = ca->undo_old_maxB;
+        return max(tp->snd_cwnd, (tp->snd_ssthresh<<7)/ca->beta);
+}
+
+static inline void measure_rtt(struct tcp_sock *tp)
+{
+        struct htcp *ca = tcp_ca(tp);
+        u32 srtt = tp->srtt>>3;
+
+        /* keep track of minimum RTT seen so far, minRTT is zero at first */
+        if (ca->minRTT > srtt || !ca->minRTT)
+                ca->minRTT = srtt;
+
+        /* max RTT */
+        if (tp->ca_state == TCP_CA_Open && tp->snd_ssthresh < 0xFFFF && ca->ccount > 3) {
+                if (ca->maxRTT < ca->minRTT)
+                        ca->maxRTT = ca->minRTT;
+                if (ca->maxRTT < srtt && srtt <= ca->maxRTT+HZ/50)
+                        ca->maxRTT = srtt;
+        }
+}
+
+static void measure_achieved_throughput(struct tcp_sock *tp, u32 pkts_acked)
+{
+        struct htcp *ca = tcp_ca(tp);
+        u32 now = tcp_time_stamp;
+
+        /* achieved throughput calculations */
+        if (tp->ca_state != TCP_CA_Open && tp->ca_state != TCP_CA_Disorder) {
+                ca->packetcount = 0;
+                ca->lasttime = now;
+                return;
+        }
+
+        ca->packetcount += pkts_acked;
+
+        if (ca->packetcount >= tp->snd_cwnd - (ca->alpha>>7? : 1)
+                        && now - ca->lasttime >= ca->minRTT
+                        && ca->minRTT > 0) {
+                __u32 cur_Bi = ca->packetcount*HZ/(now - ca->lasttime);
+                if (ca->ccount <= 3) {
+                        /* just after backoff */
+                        ca->minB = ca->maxB = ca->Bi = cur_Bi;
+                } else {
+                        ca->Bi = (3*ca->Bi + cur_Bi)/4;
+                        if (ca->Bi > ca->maxB)
+                                ca->maxB = ca->Bi;
+                        if (ca->minB > ca->maxB)
+                                ca->minB = ca->maxB;
+                }
+                ca->packetcount = 0;
+                ca->lasttime = now;
+        }
+}
+
+static inline void htcp_beta_update(struct htcp *ca, u32 minRTT, u32 maxRTT)
+{
+        if (use_bandwidth_switch) {
+                u32 maxB = ca->maxB;
+                u32 old_maxB = ca->old_maxB;
+                ca->old_maxB = ca->maxB;
+
+                if (!between(5*maxB, 4*old_maxB, 6*old_maxB)) {
+                        ca->beta = BETA_MIN;
+                        ca->modeswitch = 0;
+                        return;
+                }
+        }
+
+        if (ca->modeswitch && minRTT > max(HZ/100, 1) && maxRTT) {
+                ca->beta = (minRTT<<7)/maxRTT;
+                if (ca->beta < BETA_MIN)
+                        ca->beta = BETA_MIN;
+                else if (ca->beta > BETA_MAX)
+                        ca->beta = BETA_MAX;
+        } else {
+                ca->beta = BETA_MIN;
+                ca->modeswitch = 1;
+        }
+}
+
+static inline void htcp_alpha_update(struct htcp *ca)
+{
+        u32 minRTT = ca->minRTT;
+        u32 factor = 1;
+        u32 diff = ca->ccount * minRTT; /* time since last backoff */
+
+        if (diff > HZ) {
+                diff -= HZ;
+                factor = 1+ ( 10*diff + ((diff/2)*(diff/2)/HZ) )/HZ;
+        }
+
+        if (use_rtt_scaling && minRTT) {
+                u32 scale = (HZ<<3)/(10*minRTT);
+                scale = min(max(scale, 1U<<2), 10U<<3); /* clamping ratio to interval [0.5,10]<<3 */
+                factor = (factor<<3)/scale;
+                if (!factor)
+                        factor = 1;
+        }
+
+        ca->alpha = 2*factor*((1<<7)-ca->beta);
+        if (!ca->alpha)
+                ca->alpha = ALPHA_BASE;
+}
+
+/* After we have the rtt data to calculate beta, we'd still prefer to wait one
+ * rtt before we adjust our beta to ensure we are working from a consistent
+ * data.
+ *
+ * This function should be called when we hit a congestion event since only at
+ * that point do we really have a real sense of maxRTT (the queues en route
+ * were getting just too full now).
+ */
+static void htcp_param_update(struct tcp_sock *tp)
+{
+        struct htcp *ca = tcp_ca(tp);
+        u32 minRTT = ca->minRTT;
+        u32 maxRTT = ca->maxRTT;
+
+        htcp_beta_update(ca, minRTT, maxRTT);
+        htcp_alpha_update(ca);
+
+        /* add slowly fading memory for maxRTT to accommodate routing changes etc */
+        if (minRTT > 0 && maxRTT > minRTT)
+                ca->maxRTT = minRTT + ((maxRTT-minRTT)*95)/100;
+}
+
+static u32 htcp_recalc_ssthresh(struct tcp_sock *tp)
+{
+        struct htcp *ca = tcp_ca(tp);
+        htcp_param_update(tp);
+        return max((tp->snd_cwnd * ca->beta) >> 7, 2U);
+}
+
+static void htcp_cong_avoid(struct tcp_sock *tp, u32 ack, u32 rtt,
+                            u32 in_flight, int data_acked)
+{
+        struct htcp *ca = tcp_ca(tp);
+
+        if (in_flight < tp->snd_cwnd)
+                return;
+
+        if (tp->snd_cwnd <= tp->snd_ssthresh) {
+                /* In "safe" area, increase. */
+                if (tp->snd_cwnd < tp->snd_cwnd_clamp)
+                        tp->snd_cwnd++;
+        } else {
+                measure_rtt(tp);
+
+                /* keep track of number of round-trip times since last backoff event */
+                if (ca->snd_cwnd_cnt2++ > tp->snd_cwnd) {
+                        ca->ccount++;
+                        ca->snd_cwnd_cnt2 = 0;
+                        htcp_alpha_update(ca);
+                }
+
+                /* In dangerous area, increase slowly.
+                 * In theory this is tp->snd_cwnd += alpha / tp->snd_cwnd
+                 */
+                if ((tp->snd_cwnd_cnt++ * ca->alpha)>>7 >= tp->snd_cwnd) {
+                        if (tp->snd_cwnd < tp->snd_cwnd_clamp)
+                                tp->snd_cwnd++;
+                        tp->snd_cwnd_cnt = 0;
+                        ca->ccount++;
+                }
+        }
+}
+
+/* Lower bound on congestion window. */
+static u32 htcp_min_cwnd(struct tcp_sock *tp)
+{
+        return tp->snd_ssthresh;
+}
+
+
+static void htcp_init(struct tcp_sock *tp)
+{
+        struct htcp *ca = tcp_ca(tp);
+
+        memset(ca, 0, sizeof(struct htcp));
+        ca->alpha = ALPHA_BASE;
+        ca->beta = BETA_MIN;
+}
+
+static void htcp_state(struct tcp_sock *tp, u8 new_state)
+{
+        switch (new_state) {
+        case TCP_CA_CWR:
+        case TCP_CA_Recovery:
+        case TCP_CA_Loss:
+                htcp_reset(tcp_ca(tp));
+                break;
+        }
+}
+
+static struct tcp_congestion_ops htcp = {
+        .init           = htcp_init,
+        .ssthresh       = htcp_recalc_ssthresh,
+        .min_cwnd       = htcp_min_cwnd,
+        .cong_avoid     = htcp_cong_avoid,
+        .set_state      = htcp_state,
+        .undo_cwnd      = htcp_cwnd_undo,
+        .pkts_acked     = measure_achieved_throughput,
+        .owner          = THIS_MODULE,
+        .name           = "htcp",
+};
+
+static int __init htcp_register(void)
+{
+        BUG_ON(sizeof(struct htcp) > TCP_CA_PRIV_SIZE);
+        BUILD_BUG_ON(BETA_MIN >= BETA_MAX);
+        if (!use_bandwidth_switch)
+                htcp.pkts_acked = NULL;
+        return tcp_register_congestion_control(&htcp);
+}
+
+static void __exit htcp_unregister(void)
+{
+        tcp_unregister_congestion_control(&htcp);
+}
+
+module_init(htcp_register);
+module_exit(htcp_unregister);
+
+MODULE_AUTHOR("Baruch Even");
+MODULE_LICENSE("GPL");
+MODULE_DESCRIPTION("H-TCP");