// SPDX-License-Identifier: GPL-2.0-only
/*
 * TCP Veno congestion control
 *
 * This is based on the congestion detection/avoidance scheme described in
 *    C. P. Fu, S. C. Liew.
 *    "TCP Veno: TCP Enhancement for Transmission over Wireless Access Networks."
 *    IEEE Journal on Selected Areas in Communication,
 *    Feb. 2003.
 *      See https://www.ie.cuhk.edu.hk/fileadmin/staff_upload/soung/Journal/J3.pdf
 */

#include <linux/mm.h>
#include <linux/module.h>
#include <linux/skbuff.h>
#include <linux/inet_diag.h>

#include <net/tcp.h>

/* Default values of the Veno variables, in fixed-point representation
 * with V_PARAM_SHIFT bits to the right of the binary point.
 */
#define V_PARAM_SHIFT 1
static const int beta = 3 << V_PARAM_SHIFT;

/* Veno variables */
struct veno {
        u8 doing_veno_now;      /* if true, do veno for this rtt */
        u16 cntrtt;             /* # of rtts measured within last rtt */
        u32 minrtt;             /* min of rtts measured within last rtt (in usec) */
        u32 basertt;            /* the min of all Veno rtt measurements seen (in usec) */
        u32 inc;                /* decide whether to increase cwnd */
        u32 diff;               /* calculate the diff rate */
};

/* There are several situations when we must "re-start" Veno:
 *
 *  o when a connection is established
 *  o after an RTO
 *  o after fast recovery
 *  o when we send a packet and there is no outstanding
 *    unacknowledged data (restarting an idle connection)
 *
 */
static inline void veno_enable(struct sock *sk)
{
        struct veno *veno = inet_csk_ca(sk);

        /* turn on Veno */
        veno->doing_veno_now = 1;

        veno->minrtt = 0x7fffffff;
}

static inline void veno_disable(struct sock *sk)
{
        struct veno *veno = inet_csk_ca(sk);

        /* turn off Veno */
        veno->doing_veno_now = 0;
}

static void tcp_veno_init(struct sock *sk)
{
        struct veno *veno = inet_csk_ca(sk);

        veno->basertt = 0x7fffffff;
        veno->inc = 1;
        veno_enable(sk);
}

/* Do rtt sampling needed for Veno. */
static void tcp_veno_pkts_acked(struct sock *sk,
                                const struct ack_sample *sample)
{
        struct veno *veno = inet_csk_ca(sk);
        u32 vrtt;

        if (sample->rtt_us < 0)
                return;

        /* Never allow zero rtt or baseRTT */
        vrtt = sample->rtt_us + 1;

        /* Filter to find propagation delay: */
        if (vrtt < veno->basertt)
                veno->basertt = vrtt;

        /* Find the min rtt during the last rtt to find
         * the current prop. delay + queuing delay:
         */
        veno->minrtt = min(veno->minrtt, vrtt);
        veno->cntrtt++;
}

static void tcp_veno_state(struct sock *sk, u8 ca_state)
{
        if (ca_state == TCP_CA_Open)
                veno_enable(sk);
        else
                veno_disable(sk);
}

/*
 * If the connection is idle and we are restarting,
 * then we don't want to do any Veno calculations
 * until we get fresh rtt samples.  So when we
 * restart, we reset our Veno state to a clean
 * state. After we get acks for this flight of
 * packets, _then_ we can make Veno calculations
 * again.
 */
static void tcp_veno_cwnd_event(struct sock *sk, enum tcp_ca_event event)
{
        if (event == CA_EVENT_CWND_RESTART || event == CA_EVENT_TX_START)
                tcp_veno_init(sk);
}

static void tcp_veno_cong_avoid(struct sock *sk, u32 ack, u32 acked)
{
        struct tcp_sock *tp = tcp_sk(sk);
        struct veno *veno = inet_csk_ca(sk);

        if (!veno->doing_veno_now) {
                tcp_reno_cong_avoid(sk, ack, acked);
                return;
        }

        /* limited by applications */
        if (!tcp_is_cwnd_limited(sk))
                return;

        /* We do the Veno calculations only if we got enough rtt samples */
        if (veno->cntrtt <= 2) {
                /* We don't have enough rtt samples to do the Veno
                 * calculation, so we'll behave like Reno.
                 */
                tcp_reno_cong_avoid(sk, ack, acked);
        } else {
                u64 target_cwnd;
                u32 rtt;

                /* We have enough rtt samples, so, using the Veno
                 * algorithm, we determine the state of the network.
                 */

                rtt = veno->minrtt;

                target_cwnd = (u64)tp->snd_cwnd * veno->basertt;
                target_cwnd <<= V_PARAM_SHIFT;
                do_div(target_cwnd, rtt);

                veno->diff = (tp->snd_cwnd << V_PARAM_SHIFT) - target_cwnd;

                if (tcp_in_slow_start(tp)) {
                        /* Slow start. */
                        acked = tcp_slow_start(tp, acked);
                        if (!acked)
                                goto done;
                }

                /* Congestion avoidance. */
                if (veno->diff < beta) {
                        /* In the "non-congestive state", increase cwnd
                         * every rtt.
                         */
                        tcp_cong_avoid_ai(tp, tp->snd_cwnd, acked);
                } else {
                        /* In the "congestive state", increase cwnd
                         * every other rtt.
                         */
                        if (tp->snd_cwnd_cnt >= tp->snd_cwnd) {
                                if (veno->inc &&
                                    tp->snd_cwnd < tp->snd_cwnd_clamp) {
                                        tp->snd_cwnd++;
                                        veno->inc = 0;
                                } else
                                        veno->inc = 1;
                                tp->snd_cwnd_cnt = 0;
                        } else
                                tp->snd_cwnd_cnt += acked;
                }
done:
                if (tp->snd_cwnd < 2)
                        tp->snd_cwnd = 2;
                else if (tp->snd_cwnd > tp->snd_cwnd_clamp)
                        tp->snd_cwnd = tp->snd_cwnd_clamp;
        }
        /* Wipe the slate clean for the next rtt. */
        /* veno->cntrtt = 0; */
        veno->minrtt = 0x7fffffff;
}

/* Veno MD phase */
static u32 tcp_veno_ssthresh(struct sock *sk)
{
        const struct tcp_sock *tp = tcp_sk(sk);
        struct veno *veno = inet_csk_ca(sk);

        if (veno->diff < beta)
                /* in "non-congestive state", cut cwnd by 1/5 */
                return max(tp->snd_cwnd * 4 / 5, 2U);
        else
                /* in "congestive state", cut cwnd by 1/2 */
                return max(tp->snd_cwnd >> 1U, 2U);
}

static struct tcp_congestion_ops tcp_veno __read_mostly = {
        .init           = tcp_veno_init,
        .ssthresh       = tcp_veno_ssthresh,
        .undo_cwnd      = tcp_reno_undo_cwnd,
        .cong_avoid     = tcp_veno_cong_avoid,
        .pkts_acked     = tcp_veno_pkts_acked,
        .set_state      = tcp_veno_state,
        .cwnd_event     = tcp_veno_cwnd_event,

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

static int __init tcp_veno_register(void)
{
        BUILD_BUG_ON(sizeof(struct veno) > ICSK_CA_PRIV_SIZE);
        tcp_register_congestion_control(&tcp_veno);
        return 0;
}

static void __exit tcp_veno_unregister(void)
{
        tcp_unregister_congestion_control(&tcp_veno);
}

module_init(tcp_veno_register);
module_exit(tcp_veno_unregister);

MODULE_AUTHOR("Bin Zhou, Cheng Peng Fu");
MODULE_LICENSE("GPL");
MODULE_DESCRIPTION("TCP Veno");