tcp_cubic.c

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/*
 * TCP CUBIC: Binary Increase Congestion control for TCP v2.3
 * Home page:
 *      http://netsrv.csc.ncsu.edu/twiki/bin/view/Main/BIC
 * This is from the implementation of CUBIC TCP in
 * Sangtae Ha, Injong Rhee and Lisong Xu,
 *  "CUBIC: A New TCP-Friendly High-Speed TCP Variant"
 *  in ACM SIGOPS Operating System Review, July 2008.
 * Available from:
 *  http://netsrv.csc.ncsu.edu/export/cubic_a_new_tcp_2008.pdf
 *
 * CUBIC integrates a new slow start algorithm, called HyStart.
 * The details of HyStart are presented in
 *  Sangtae Ha and Injong Rhee,
 *  "Taming the Elephants: New TCP Slow Start", NCSU TechReport 2008.
 * Available from:
 *  http://netsrv.csc.ncsu.edu/export/hystart_techreport_2008.pdf
 *
 * All testing results are available from:
 * http://netsrv.csc.ncsu.edu/wiki/index.php/TCP_Testing
 *
 * Unless CUBIC is enabled and congestion window is large
 * this behaves the same as the original Reno.
 */

#include <linux/mm.h>
#include <linux/module.h>
#include <linux/math64.h>
#include <net/tcp.h>

#define BICTCP_BETA_SCALE    1024   /* Scale factor beta calculation
                     * max_cwnd = snd_cwnd * beta
                     */
#define BICTCP_HZ       10  /* BIC HZ 2^10 = 1024 */

/* Two methods of hybrid slow start */
#define HYSTART_ACK_TRAIN   0x1
#define HYSTART_DELAY       0x2

/* Number of delay samples for detecting the increase of delay */
#define HYSTART_MIN_SAMPLES 8
#define HYSTART_DELAY_MIN   (4U<<3)
#define HYSTART_DELAY_MAX   (16U<<3)
#define HYSTART_DELAY_THRESH(x) clamp(x, HYSTART_DELAY_MIN, HYSTART_DELAY_MAX)

static int fast_convergence __read_mostly = 1;
static int beta __read_mostly = 717;    /* = 717/1024 (BICTCP_BETA_SCALE) */
static int initial_ssthresh __read_mostly;
static int bic_scale __read_mostly = 41;
static int tcp_friendliness __read_mostly = 1;

static int hystart __read_mostly = 1;
static int hystart_detect __read_mostly = HYSTART_ACK_TRAIN | HYSTART_DELAY;
static int hystart_low_window __read_mostly = 16;
static int hystart_ack_delta __read_mostly = 2;

static u32 cube_rtt_scale __read_mostly;
static u32 beta_scale __read_mostly;
static u64 cube_factor __read_mostly;

/* Note parameters that are used for precomputing scale factors are read-only */
module_param(fast_convergence, int, 0644);
MODULE_PARM_DESC(fast_convergence, "turn on/off fast convergence");
module_param(beta, int, 0644);
MODULE_PARM_DESC(beta, "beta for multiplicative increase");
module_param(initial_ssthresh, int, 0644);
MODULE_PARM_DESC(initial_ssthresh, "initial value of slow start threshold");
module_param(bic_scale, int, 0444);
MODULE_PARM_DESC(bic_scale, "scale (scaled by 1024) value for bic function (bic_scale/1024)");
module_param(tcp_friendliness, int, 0644);
MODULE_PARM_DESC(tcp_friendliness, "turn on/off tcp friendliness");
module_param(hystart, int, 0644);
MODULE_PARM_DESC(hystart, "turn on/off hybrid slow start algorithm");
module_param(hystart_detect, int, 0644);
MODULE_PARM_DESC(hystart_detect, "hyrbrid slow start detection mechanisms"
         " 1: packet-train 2: delay 3: both packet-train and delay");
module_param(hystart_low_window, int, 0644);
MODULE_PARM_DESC(hystart_low_window, "lower bound cwnd for hybrid slow start");
module_param(hystart_ack_delta, int, 0644);
MODULE_PARM_DESC(hystart_ack_delta, "spacing between ack's indicating train (msecs)");

/* BIC TCP Parameters */
struct bictcp {
    u32 cnt;        /* increase cwnd by 1 after ACKs */
    u32     last_max_cwnd;  /* last maximum snd_cwnd */
    u32 loss_cwnd;  /* congestion window at last loss */
    u32 last_cwnd;  /* the last snd_cwnd */
    u32 last_time;  /* time when updated last_cwnd */
    u32 bic_origin_point;/* origin point of bic function */
    u32 bic_K;      /* time to origin point from the beginning of the current epoch */
    u32 delay_min;  /* min delay (msec << 3) */
    u32 epoch_start;    /* beginning of an epoch */
    u32 ack_cnt;    /* number of acks */
    u32 tcp_cwnd;   /* estimated tcp cwnd */
#define ACK_RATIO_SHIFT 4
#define ACK_RATIO_LIMIT (32u << ACK_RATIO_SHIFT)
    u16 delayed_ack;    /* estimate the ratio of Packets/ACKs << 4 */
    u8  sample_cnt; /* number of samples to decide curr_rtt */
    u8  found;      /* the exit point is found? */
    u32 round_start;    /* beginning of each round */
    u32 end_seq;    /* end_seq of the round */
    u32 last_ack;   /* last time when the ACK spacing is close */
    u32 curr_rtt;   /* the minimum rtt of current round */
};

static inline void bictcp_reset(struct bictcp *ca)
{
    ca->cnt = 0;
    ca->last_max_cwnd = 0;
    ca->last_cwnd = 0;
    ca->last_time = 0;
    ca->bic_origin_point = 0;
    ca->bic_K = 0;
    ca->delay_min = 0;
    ca->epoch_start = 0;
    ca->delayed_ack = 2 << ACK_RATIO_SHIFT;
    ca->ack_cnt = 0;
    ca->tcp_cwnd = 0;
    ca->found = 0;
}

static inline u32 bictcp_clock(void)
{
#if HZ < 1000
    return ktime_to_ms(ktime_get_real());
#else
    return jiffies_to_msecs(jiffies);
#endif
}

static inline void bictcp_hystart_reset(struct sock *sk)
{
    struct tcp_sock *tp = tcp_sk(sk);
    struct bictcp *ca = inet_csk_ca(sk);

    ca->round_start = ca->last_ack = bictcp_clock();
    ca->end_seq = tp->snd_nxt;
    ca->curr_rtt = 0;
    ca->sample_cnt = 0;
}

static void bictcp_init(struct sock *sk)
{
    struct bictcp *ca = inet_csk_ca(sk);

    bictcp_reset(ca);
    ca->loss_cwnd = 0;

    if (hystart)
        bictcp_hystart_reset(sk);

    if (!hystart && initial_ssthresh)
        tcp_sk(sk)->snd_ssthresh = initial_ssthresh;
}

/* calculate the cubic root of x using a table lookup followed by one
 * Newton-Raphson iteration.
 * Avg err ~= 0.195%
 */
static u32 cubic_root(u64 a)
{
    u32 x, b, shift;
    /*
     * cbrt(x) MSB values for x MSB values in [0..63].
     * Precomputed then refined by hand - Willy Tarreau
     *
     * For x in [0..63],
     *   v = cbrt(x << 18) - 1
     *   cbrt(x) = (v[x] + 10) >> 6
     */
    static const u8 v[] = {
        /* 0x00 */    0,   54,   54,   54,  118,  118,  118,  118,
        /* 0x08 */  123,  129,  134,  138,  143,  147,  151,  156,
        /* 0x10 */  157,  161,  164,  168,  170,  173,  176,  179,
        /* 0x18 */  181,  185,  187,  190,  192,  194,  197,  199,
        /* 0x20 */  200,  202,  204,  206,  209,  211,  213,  215,
        /* 0x28 */  217,  219,  221,  222,  224,  225,  227,  229,
        /* 0x30 */  231,  232,  234,  236,  237,  239,  240,  242,
        /* 0x38 */  244,  245,  246,  248,  250,  251,  252,  254,
    };

    b = fls64(a);
    if (b < 7) {
        /* a in [0..63] */
        return ((u32)v[(u32)a] + 35) >> 6;
    }

    b = ((b * 84) >> 8) - 1;
    shift = (a >> (b * 3));

    x = ((u32)(((u32)v[shift] + 10) << b)) >> 6;

    /*
     * Newton-Raphson iteration
     *                         2
     * x    = ( 2 * x  +  a / x  ) / 3
     *  k+1          k         k
     */
    x = (2 * x + (u32)div64_u64(a, (u64)x * (u64)(x - 1)));
    x = ((x * 341) >> 10);
    return x;
}

/*
 * Compute congestion window to use.
 */
static inline void bictcp_update(struct bictcp *ca, u32 cwnd)
{
    u64 offs;
    u32 delta, t, bic_target, max_cnt;

    ca->ack_cnt++;  /* count the number of ACKs */

    if (ca->last_cwnd == cwnd &&
        (s32)(tcp_time_stamp - ca->last_time) <= HZ / 32)
        return;

    ca->last_cwnd = cwnd;
    ca->last_time = tcp_time_stamp;

    if (ca->epoch_start == 0) {
        ca->epoch_start = tcp_time_stamp;   /* record the beginning of an epoch */
        ca->ack_cnt = 1;            /* start counting */
        ca->tcp_cwnd = cwnd;            /* syn with cubic */

        if (ca->last_max_cwnd <= cwnd) {
            ca->bic_K = 0;
            ca->bic_origin_point = cwnd;
        } else {
            /* Compute new K based on
             * (wmax-cwnd) * (srtt>>3 / HZ) / c * 2^(3*bictcp_HZ)
             */
            ca->bic_K = cubic_root(cube_factor
                           * (ca->last_max_cwnd - cwnd));
            ca->bic_origin_point = ca->last_max_cwnd;
        }
    }

    /* cubic function - calc*/
    /* calculate c * time^3 / rtt,
     *  while considering overflow in calculation of time^3
     * (so time^3 is done by using 64 bit)
     * and without the support of division of 64bit numbers
     * (so all divisions are done by using 32 bit)
     *  also NOTE the unit of those veriables
     *    time  = (t - K) / 2^bictcp_HZ
     *    c = bic_scale >> 10
     * rtt  = (srtt >> 3) / HZ
     * !!! The following code does not have overflow problems,
     * if the cwnd < 1 million packets !!!
     */

    /* change the unit from HZ to bictcp_HZ */
    t = ((tcp_time_stamp + msecs_to_jiffies(ca->delay_min>>3)
          - ca->epoch_start) << BICTCP_HZ) / HZ;

    if (t < ca->bic_K)      /* t - K */
        offs = ca->bic_K - t;
    else
        offs = t - ca->bic_K;

    /* c/rtt * (t-K)^3 */
    delta = (cube_rtt_scale * offs * offs * offs) >> (10+3*BICTCP_HZ);
    if (t < ca->bic_K)                                  /* below origin*/
        bic_target = ca->bic_origin_point - delta;
    else                                                    /* above origin*/
        bic_target = ca->bic_origin_point + delta;

    /* cubic function - calc bictcp_cnt*/
    if (bic_target > cwnd) {
        ca->cnt = cwnd / (bic_target - cwnd);
    } else {
        ca->cnt = 100 * cwnd;              /* very small increment*/
    }

    /*
     * The initial growth of cubic function may be too conservative
     * when the available bandwidth is still unknown.
     */
    if (ca->last_max_cwnd == 0 && ca->cnt > 20)
        ca->cnt = 20;   /* increase cwnd 5% per RTT */

    /* TCP Friendly */
    if (tcp_friendliness) {
        u32 scale = beta_scale;
        delta = (cwnd * scale) >> 3;
        while (ca->ack_cnt > delta) {       /* update tcp cwnd */
            ca->ack_cnt -= delta;
            ca->tcp_cwnd++;
        }

        if (ca->tcp_cwnd > cwnd){   /* if bic is slower than tcp */
            delta = ca->tcp_cwnd - cwnd;
            max_cnt = cwnd / delta;
            if (ca->cnt > max_cnt)
                ca->cnt = max_cnt;
        }
    }

    ca->cnt = (ca->cnt << ACK_RATIO_SHIFT) / ca->delayed_ack;
    if (ca->cnt == 0)           /* cannot be zero */
        ca->cnt = 1;
}

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

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

    if (tp->snd_cwnd <= tp->snd_ssthresh) {
        if (hystart && after(ack, ca->end_seq))
            bictcp_hystart_reset(sk);
        tcp_slow_start(tp);
    } else {
        bictcp_update(ca, tp->snd_cwnd);
        tcp_cong_avoid_ai(tp, ca->cnt);
    }

}

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

    ca->epoch_start = 0;    /* end of epoch */

    /* Wmax and fast convergence */
    if (tp->snd_cwnd < ca->last_max_cwnd && fast_convergence)
        ca->last_max_cwnd = (tp->snd_cwnd * (BICTCP_BETA_SCALE + beta))
            / (2 * BICTCP_BETA_SCALE);
    else
        ca->last_max_cwnd = tp->snd_cwnd;

    ca->loss_cwnd = tp->snd_cwnd;

    return max((tp->snd_cwnd * beta) / BICTCP_BETA_SCALE, 2U);
}

static u32 bictcp_undo_cwnd(struct sock *sk)
{
    struct bictcp *ca = inet_csk_ca(sk);

    return max(tcp_sk(sk)->snd_cwnd, ca->loss_cwnd);
}

static void bictcp_state(struct sock *sk, u8 new_state)
{
    if (new_state == TCP_CA_Loss) {
        bictcp_reset(inet_csk_ca(sk));
        bictcp_hystart_reset(sk);
    }
}

static void hystart_update(struct sock *sk, u32 delay)
{
    struct tcp_sock *tp = tcp_sk(sk);
    struct bictcp *ca = inet_csk_ca(sk);

    if (!(ca->found & hystart_detect)) {
        u32 now = bictcp_clock();

        /* first detection parameter - ack-train detection */
        if ((s32)(now - ca->last_ack) <= hystart_ack_delta) {
            ca->last_ack = now;
            if ((s32)(now - ca->round_start) > ca->delay_min >> 4)
                ca->found |= HYSTART_ACK_TRAIN;
        }

        /* obtain the minimum delay of more than sampling packets */
        if (ca->sample_cnt < HYSTART_MIN_SAMPLES) {
            if (ca->curr_rtt == 0 || ca->curr_rtt > delay)
                ca->curr_rtt = delay;

            ca->sample_cnt++;
        } else {
            if (ca->curr_rtt > ca->delay_min +
                HYSTART_DELAY_THRESH(ca->delay_min>>4))
                ca->found |= HYSTART_DELAY;
        }
        /*
         * Either one of two conditions are met,
         * we exit from slow start immediately.
         */
        if (ca->found & hystart_detect)
            tp->snd_ssthresh = tp->snd_cwnd;
    }
}

/* Track delayed acknowledgment ratio using sliding window
 * ratio = (15*ratio + sample) / 16
 */
static void bictcp_acked(struct sock *sk, u32 cnt, s32 rtt_us)
{
    const struct inet_connection_sock *icsk = inet_csk(sk);
    const struct tcp_sock *tp = tcp_sk(sk);
    struct bictcp *ca = inet_csk_ca(sk);
    u32 delay;

    if (icsk->icsk_ca_state == TCP_CA_Open) {
        u32 ratio = ca->delayed_ack;

        ratio -= ca->delayed_ack >> ACK_RATIO_SHIFT;
        ratio += cnt;

        ca->delayed_ack = min(ratio, ACK_RATIO_LIMIT);
    }

    /* Some calls are for duplicates without timetamps */
    if (rtt_us < 0)
        return;

    /* Discard delay samples right after fast recovery */
    if ((s32)(tcp_time_stamp - ca->epoch_start) < HZ)
        return;

    delay = (rtt_us << 3) / USEC_PER_MSEC;
    if (delay == 0)
        delay = 1;

    /* first time call or link delay decreases */
    if (ca->delay_min == 0 || ca->delay_min > delay)
        ca->delay_min = delay;

    /* hystart triggers when cwnd is larger than some threshold */
    if (hystart && tp->snd_cwnd <= tp->snd_ssthresh &&
        tp->snd_cwnd >= hystart_low_window)
        hystart_update(sk, delay);
}

static struct tcp_congestion_ops cubictcp __read_mostly = {
    .init       = bictcp_init,
    .ssthresh   = bictcp_recalc_ssthresh,
    .cong_avoid = bictcp_cong_avoid,
    .set_state  = bictcp_state,
    .undo_cwnd  = bictcp_undo_cwnd,
    .pkts_acked     = bictcp_acked,
    .owner      = THIS_MODULE,
    .name       = "cubic",
};

static int __init cubictcp_register(void)
{
    BUILD_BUG_ON(sizeof(struct bictcp) > ICSK_CA_PRIV_SIZE);

    /* Precompute a bunch of the scaling factors that are used per-packet
     * based on SRTT of 100ms
     */

    beta_scale = 8*(BICTCP_BETA_SCALE+beta)/ 3 / (BICTCP_BETA_SCALE - beta);

    cube_rtt_scale = (bic_scale * 10);  /* 1024*c/rtt */

    /* calculate the "K" for (wmax-cwnd) = c/rtt * K^3
     *  so K = cubic_root( (wmax-cwnd)*rtt/c )
     * the unit of K is bictcp_HZ=2^10, not HZ
     *
     *  c = bic_scale >> 10
     *  rtt = 100ms
     *
     * the following code has been designed and tested for
     * cwnd < 1 million packets
     * RTT < 100 seconds
     * HZ < 1,000,00  (corresponding to 10 nano-second)
     */

    /* 1/c * 2^2*bictcp_HZ * srtt */
    cube_factor = 1ull << (10+3*BICTCP_HZ); /* 2^40 */

    /* divide by bic_scale and by constant Srtt (100ms) */
    do_div(cube_factor, bic_scale * 10);

    /* hystart needs ms clock resolution */
    if (hystart && HZ < 1000)
        cubictcp.flags |= TCP_CONG_RTT_STAMP;

    return tcp_register_congestion_control(&cubictcp);
}

static void __exit cubictcp_unregister(void)
{
    tcp_unregister_congestion_control(&cubictcp);
}

module_init(cubictcp_register);
module_exit(cubictcp_unregister);

MODULE_AUTHOR("Sangtae Ha, Stephen Hemminger");
MODULE_LICENSE("GPL");
MODULE_DESCRIPTION("CUBIC TCP");
MODULE_VERSION("2.3");