/*
 * 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/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 __read_mostly = 1;
module_param(use_rtt_scaling, int, 0644);
MODULE_PARM_DESC(use_rtt_scaling, "turn on/off RTT scaling");

static int use_bandwidth_switch __read_mostly = 1;
module_param(use_bandwidth_switch, int, 0644);
MODULE_PARM_DESC(use_bandwidth_switch, "turn on/off bandwidth switcher");

struct htcp {
        u32     alpha;          /* Fixed point arith, << 7 */
        u8      beta;           /* Fixed point arith, << 7 */
        u8      modeswitch;     /* Delay modeswitch
                                   until we had at least one congestion event */
        u16     pkts_acked;
        u32     packetcount;
        u32     minRTT;
        u32     maxRTT;
        u32     last_cong;      /* Time since last congestion event end */
        u32     undo_last_cong;

        u32     undo_maxRTT;
        u32     undo_old_maxB;

        /* Bandwidth estimation */
        u32     minB;
        u32     maxB;
        u32     old_maxB;
        u32     Bi;
        u32     lasttime;
};

static inline u32 htcp_cong_time(const struct htcp *ca)
{
        return jiffies - ca->last_cong;
}

static inline u32 htcp_ccount(const struct htcp *ca)
{
        return htcp_cong_time(ca) / ca->minRTT;
}

static inline void htcp_reset(struct htcp *ca)
{
        ca->undo_last_cong = ca->last_cong;
        ca->undo_maxRTT = ca->maxRTT;
        ca->undo_old_maxB = ca->old_maxB;

        ca->last_cong = jiffies;
}

static u32 htcp_cwnd_undo(struct sock *sk)
{
        const struct tcp_sock *tp = tcp_sk(sk);
        struct htcp *ca = inet_csk_ca(sk);

        if (ca->undo_last_cong) {
                ca->last_cong = ca->undo_last_cong;
                ca->maxRTT = ca->undo_maxRTT;
                ca->old_maxB = ca->undo_old_maxB;
                ca->undo_last_cong = 0;
        }

        return max(tp->snd_cwnd, (tp->snd_ssthresh << 7) / ca->beta);
}

static inline void measure_rtt(struct sock *sk, u32 srtt)
{
        const struct inet_connection_sock *icsk = inet_csk(sk);
        struct htcp *ca = inet_csk_ca(sk);

        /* 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 (icsk->icsk_ca_state == TCP_CA_Open) {
                if (ca->maxRTT < ca->minRTT)
                        ca->maxRTT = ca->minRTT;
                if (ca->maxRTT < srtt &&
                    srtt <= ca->maxRTT + msecs_to_jiffies(20))
                        ca->maxRTT = srtt;
        }
}

static void measure_achieved_throughput(struct sock *sk, u32 pkts_acked, s32 rtt)
{
        const struct inet_connection_sock *icsk = inet_csk(sk);
        const struct tcp_sock *tp = tcp_sk(sk);
        struct htcp *ca = inet_csk_ca(sk);
        u32 now = tcp_time_stamp;

        if (icsk->icsk_ca_state == TCP_CA_Open)
                ca->pkts_acked = pkts_acked;

        if (rtt > 0)
                measure_rtt(sk, usecs_to_jiffies(rtt));

        if (!use_bandwidth_switch)
                return;

        /* achieved throughput calculations */
        if (!((1 << icsk->icsk_ca_state) & (TCPF_CA_Open | TCPF_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 (htcp_ccount(ca) <= 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 > msecs_to_jiffies(10) && 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 = htcp_cong_time(ca);

        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);

                /* clamping ratio to interval [0.5,10]<<3 */
                scale = min(max(scale, 1U << 2), 10U << 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 sock *sk)
{
        struct htcp *ca = inet_csk_ca(sk);
        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 */
        if (minRTT > 0 && maxRTT > minRTT)
                ca->maxRTT = minRTT + ((maxRTT - minRTT) * 95) / 100;
}

static u32 htcp_recalc_ssthresh(struct sock *sk)
{
        const struct tcp_sock *tp = tcp_sk(sk);
        const struct htcp *ca = inet_csk_ca(sk);

        htcp_param_update(sk);
        return max((tp->snd_cwnd * ca->beta) >> 7, 2U);
}

static void htcp_cong_avoid(struct sock *sk, u32 ack, u32 in_flight)
{
        struct tcp_sock *tp = tcp_sk(sk);
        struct htcp *ca = inet_csk_ca(sk);

        if (!tcp_is_cwnd_limited(sk, in_flight))
                return;

        if (tp->snd_cwnd <= tp->snd_ssthresh)
                tcp_slow_start(tp);
        else {
                /* 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;
                        htcp_alpha_update(ca);
                } else
                        tp->snd_cwnd_cnt += ca->pkts_acked;

                ca->pkts_acked = 1;
        }
}

static void htcp_init(struct sock *sk)
{
        struct htcp *ca = inet_csk_ca(sk);

        memset(ca, 0, sizeof(struct htcp));
        ca->alpha = ALPHA_BASE;
        ca->beta = BETA_MIN;
        ca->pkts_acked = 1;
        ca->last_cong = jiffies;
}

static void htcp_state(struct sock *sk, u8 new_state)
{
        switch (new_state) {
        case TCP_CA_Open:
                {
                        struct htcp *ca = inet_csk_ca(sk);
                        if (ca->undo_last_cong) {
                                ca->last_cong = jiffies;
                                ca->undo_last_cong = 0;
                        }
                }
                break;
        case TCP_CA_CWR:
        case TCP_CA_Recovery:
        case TCP_CA_Loss:
                htcp_reset(inet_csk_ca(sk));
                break;
        }
}

static struct tcp_congestion_ops htcp __read_mostly = {
        .init           = htcp_init,
        .ssthresh       = htcp_recalc_ssthresh,
        .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)
{
        BUILD_BUG_ON(sizeof(struct htcp) > ICSK_CA_PRIV_SIZE);
        BUILD_BUG_ON(BETA_MIN >= BETA_MAX);
        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");