/*
 * TCP Illinois congestion control.
 * Home page:
 *      http://www.ews.uiuc.edu/~shaoliu/tcpillinois/index.html
 *
 * The algorithm is described in:
 * "TCP-Illinois: A Loss and Delay-Based Congestion Control Algorithm
 *  for High-Speed Networks"
 * http://www.ews.uiuc.edu/~shaoliu/papersandslides/liubassri06perf.pdf
 *
 * Implemented from description in paper and ns-2 simulation.
 * Copyright (C) 2007 Stephen Hemminger <shemminger@linux-foundation.org>
 */

#include <linux/module.h>
#include <linux/skbuff.h>
#include <linux/inet_diag.h>
#include <asm/div64.h>
#include <net/tcp.h>

#define ALPHA_SHIFT     7
#define ALPHA_SCALE     (1u<<ALPHA_SHIFT)
#define ALPHA_MIN       ((3*ALPHA_SCALE)/10)    /* ~0.3 */
#define ALPHA_MAX       (10*ALPHA_SCALE)        /* 10.0 */
#define ALPHA_BASE      ALPHA_SCALE             /* 1.0 */
#define U32_MAX         ((u32)~0U)
#define RTT_MAX         (U32_MAX / ALPHA_MAX)   /* 3.3 secs */

#define BETA_SHIFT      6
#define BETA_SCALE      (1u<<BETA_SHIFT)
#define BETA_MIN        (BETA_SCALE/8)          /* 0.125 */
#define BETA_MAX        (BETA_SCALE/2)          /* 0.5 */
#define BETA_BASE       BETA_MAX

static int win_thresh __read_mostly = 15;
module_param(win_thresh, int, 0);
MODULE_PARM_DESC(win_thresh, "Window threshold for starting adaptive sizing");

static int theta __read_mostly = 5;
module_param(theta, int, 0);
MODULE_PARM_DESC(theta, "# of fast RTT's before full growth");

/* TCP Illinois Parameters */
struct illinois {
        u64     sum_rtt;        /* sum of rtt's measured within last rtt */
        u16     cnt_rtt;        /* # of rtts measured within last rtt */
        u32     base_rtt;       /* min of all rtt in usec */
        u32     max_rtt;        /* max of all rtt in usec */
        u32     end_seq;        /* right edge of current RTT */
        u32     alpha;          /* Additive increase */
        u32     beta;           /* Muliplicative decrease */
        u16     acked;          /* # packets acked by current ACK */
        u8      rtt_above;      /* average rtt has gone above threshold */
        u8      rtt_low;        /* # of rtts measurements below threshold */
};

static void rtt_reset(struct sock *sk)
{
        struct tcp_sock *tp = tcp_sk(sk);
        struct illinois *ca = inet_csk_ca(sk);

        ca->end_seq = tp->snd_nxt;
        ca->cnt_rtt = 0;
        ca->sum_rtt = 0;

        /* TODO: age max_rtt? */
}

static void tcp_illinois_init(struct sock *sk)
{
        struct illinois *ca = inet_csk_ca(sk);

        ca->alpha = ALPHA_MAX;
        ca->beta = BETA_BASE;
        ca->base_rtt = 0x7fffffff;
        ca->max_rtt = 0;

        ca->acked = 0;
        ca->rtt_low = 0;
        ca->rtt_above = 0;

        rtt_reset(sk);
}

/* Measure RTT for each ack. */
static void tcp_illinois_acked(struct sock *sk, u32 pkts_acked, s32 rtt)
{
        struct illinois *ca = inet_csk_ca(sk);

        ca->acked = pkts_acked;

        /* dup ack, no rtt sample */
        if (rtt < 0)
                return;

        /* ignore bogus values, this prevents wraparound in alpha math */
        if (rtt > RTT_MAX)
                rtt = RTT_MAX;

        /* keep track of minimum RTT seen so far */
        if (ca->base_rtt > rtt)
                ca->base_rtt = rtt;

        /* and max */
        if (ca->max_rtt < rtt)
                ca->max_rtt = rtt;

        ++ca->cnt_rtt;
        ca->sum_rtt += rtt;
}

/* Maximum queuing delay */
static inline u32 max_delay(const struct illinois *ca)
{
        return ca->max_rtt - ca->base_rtt;
}

/* Average queuing delay */
static inline u32 avg_delay(const struct illinois *ca)
{
        u64 t = ca->sum_rtt;

        do_div(t, ca->cnt_rtt);
        return t - ca->base_rtt;
}

/*
 * Compute value of alpha used for additive increase.
 * If small window then use 1.0, equivalent to Reno.
 *
 * For larger windows, adjust based on average delay.
 * A. If average delay is at minimum (we are uncongested),
 *    then use large alpha (10.0) to increase faster.
 * B. If average delay is at maximum (getting congested)
 *    then use small alpha (0.3)
 *
 * The result is a convex window growth curve.
 */
static u32 alpha(struct illinois *ca, u32 da, u32 dm)
{
        u32 d1 = dm / 100;      /* Low threshold */

        if (da <= d1) {
                /* If never got out of low delay zone, then use max */
                if (!ca->rtt_above)
                        return ALPHA_MAX;

                /* Wait for 5 good RTT's before allowing alpha to go alpha max.
                 * This prevents one good RTT from causing sudden window increase.
                 */
                if (++ca->rtt_low < theta)
                        return ca->alpha;

                ca->rtt_low = 0;
                ca->rtt_above = 0;
                return ALPHA_MAX;
        }

        ca->rtt_above = 1;

        /*
         * Based on:
         *
         *      (dm - d1) amin amax
         * k1 = -------------------
         *         amax - amin
         *
         *       (dm - d1) amin
         * k2 = ----------------  - d1
         *        amax - amin
         *
         *             k1
         * alpha = ----------
         *          k2 + da
         */

        dm -= d1;
        da -= d1;
        return (dm * ALPHA_MAX) /
                (dm + (da  * (ALPHA_MAX - ALPHA_MIN)) / ALPHA_MIN);
}

/*
 * Beta used for multiplicative decrease.
 * For small window sizes returns same value as Reno (0.5)
 *
 * If delay is small (10% of max) then beta = 1/8
 * If delay is up to 80% of max then beta = 1/2
 * In between is a linear function
 */
static u32 beta(u32 da, u32 dm)
{
        u32 d2, d3;

        d2 = dm / 10;
        if (da <= d2)
                return BETA_MIN;

        d3 = (8 * dm) / 10;
        if (da >= d3 || d3 <= d2)
                return BETA_MAX;

        /*
         * Based on:
         *
         *       bmin d3 - bmax d2
         * k3 = -------------------
         *         d3 - d2
         *
         *       bmax - bmin
         * k4 = -------------
         *         d3 - d2
         *
         * b = k3 + k4 da
         */
        return (BETA_MIN * d3 - BETA_MAX * d2 + (BETA_MAX - BETA_MIN) * da)
                / (d3 - d2);
}

/* Update alpha and beta values once per RTT */
static void update_params(struct sock *sk)
{
        struct tcp_sock *tp = tcp_sk(sk);
        struct illinois *ca = inet_csk_ca(sk);

        if (tp->snd_cwnd < win_thresh) {
                ca->alpha = ALPHA_BASE;
                ca->beta = BETA_BASE;
        } else if (ca->cnt_rtt > 0) {
                u32 dm = max_delay(ca);
                u32 da = avg_delay(ca);

                ca->alpha = alpha(ca, da, dm);
                ca->beta = beta(da, dm);
        }

        rtt_reset(sk);
}

/*
 * In case of loss, reset to default values
 */
static void tcp_illinois_state(struct sock *sk, u8 new_state)
{
        struct illinois *ca = inet_csk_ca(sk);

        if (new_state == TCP_CA_Loss) {
                ca->alpha = ALPHA_BASE;
                ca->beta = BETA_BASE;
                ca->rtt_low = 0;
                ca->rtt_above = 0;
                rtt_reset(sk);
        }
}

/*
 * Increase window in response to successful acknowledgment.
 */
static void tcp_illinois_cong_avoid(struct sock *sk, u32 ack, u32 in_flight)
{
        struct tcp_sock *tp = tcp_sk(sk);
        struct illinois *ca = inet_csk_ca(sk);

        if (after(ack, ca->end_seq))
                update_params(sk);

        /* RFC2861 only increase cwnd if fully utilized */
        if (!tcp_is_cwnd_limited(sk, in_flight))
                return;

        /* In slow start */
        if (tp->snd_cwnd <= tp->snd_ssthresh)
                tcp_slow_start(tp);

        else {
                u32 delta;

                /* snd_cwnd_cnt is # of packets since last cwnd increment */
                tp->snd_cwnd_cnt += ca->acked;
                ca->acked = 1;

                /* This is close approximation of:
                 * tp->snd_cwnd += alpha/tp->snd_cwnd
                */
                delta = (tp->snd_cwnd_cnt * ca->alpha) >> ALPHA_SHIFT;
                if (delta >= tp->snd_cwnd) {
                        tp->snd_cwnd = min(tp->snd_cwnd + delta / tp->snd_cwnd,
                                           (u32) tp->snd_cwnd_clamp);
                        tp->snd_cwnd_cnt = 0;
                }
        }
}

static u32 tcp_illinois_ssthresh(struct sock *sk)
{
        struct tcp_sock *tp = tcp_sk(sk);
        struct illinois *ca = inet_csk_ca(sk);

        /* Multiplicative decrease */
        return max(tp->snd_cwnd - ((tp->snd_cwnd * ca->beta) >> BETA_SHIFT), 2U);
}


/* Extract info for Tcp socket info provided via netlink. */
static void tcp_illinois_info(struct sock *sk, u32 ext,
                              struct sk_buff *skb)
{
        const struct illinois *ca = inet_csk_ca(sk);

        if (ext & (1 << (INET_DIAG_VEGASINFO - 1))) {
                struct tcpvegas_info info = {
                        .tcpv_enabled = 1,
                        .tcpv_rttcnt = ca->cnt_rtt,
                        .tcpv_minrtt = ca->base_rtt,
                };
                u64 t = ca->sum_rtt;

                do_div(t, ca->cnt_rtt);
                info.tcpv_rtt = t;

                nla_put(skb, INET_DIAG_VEGASINFO, sizeof(info), &info);
        }
}

static struct tcp_congestion_ops tcp_illinois = {
        .flags          = TCP_CONG_RTT_STAMP,
        .init           = tcp_illinois_init,
        .ssthresh       = tcp_illinois_ssthresh,
        .min_cwnd       = tcp_reno_min_cwnd,
        .cong_avoid     = tcp_illinois_cong_avoid,
        .set_state      = tcp_illinois_state,
        .get_info       = tcp_illinois_info,
        .pkts_acked     = tcp_illinois_acked,

        .owner          = THIS_MODULE,
        .name           = "illinois",
};

static int __init tcp_illinois_register(void)
{
        BUILD_BUG_ON(sizeof(struct illinois) > ICSK_CA_PRIV_SIZE);
        return tcp_register_congestion_control(&tcp_illinois);
}

static void __exit tcp_illinois_unregister(void)
{
        tcp_unregister_congestion_control(&tcp_illinois);
}

module_init(tcp_illinois_register);
module_exit(tcp_illinois_unregister);

MODULE_AUTHOR("Stephen Hemminger, Shao Liu");
MODULE_LICENSE("GPL");
MODULE_DESCRIPTION("TCP Illinois");
MODULE_VERSION("1.0");