/*-
 * SPDX-License-Identifier: BSD-2-Clause
 *
 * Copyright (c) 2008-2010 Lawrence Stewart <lstewart@freebsd.org>
 * Copyright (c) 2010 The FreeBSD Foundation
 * All rights reserved.
 *
 * This software was developed by Lawrence Stewart while studying at the Centre
 * for Advanced Internet Architectures, Swinburne University of Technology, made
 * possible in part by a grant from the Cisco University Research Program Fund
 * at Community Foundation Silicon Valley.
 *
 * Portions of this software were developed at the Centre for Advanced
 * Internet Architectures, Swinburne University of Technology, Melbourne,
 * Australia by David Hayes under sponsorship from the FreeBSD Foundation.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 *
 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 * SUCH DAMAGE.
 */

/*
 * An implementation of the CUBIC congestion control algorithm for FreeBSD,
 * based on the Internet RFC9438 by Xu, Ha, Rhee, Goel, and Eggert.
 * Originally released as part of the NewTCP research project at Swinburne
 * University of Technology's Centre for Advanced Internet Architectures,
 * Melbourne, Australia, which was made possible in part by a grant from the
 * Cisco University Research Program Fund at Community Foundation Silicon
 * Valley. More details are available at:
 *   http://caia.swin.edu.au/urp/newtcp/
 */

#include <sys/param.h>
#include <sys/kernel.h>
#include <sys/limits.h>
#include <sys/malloc.h>
#include <sys/module.h>
#include <sys/socket.h>
#include <sys/socketvar.h>
#include <sys/sysctl.h>
#include <sys/systm.h>

#include <net/vnet.h>

#include <net/route.h>
#include <net/route/nhop.h>

#include <netinet/in_pcb.h>
#include <netinet/tcp.h>
#include <netinet/tcp_seq.h>
#include <netinet/tcp_timer.h>
#include <netinet/tcp_var.h>
#include <netinet/tcp_log_buf.h>
#include <netinet/tcp_hpts.h>
#include <netinet/cc/cc.h>
#include <netinet/cc/cc_cubic.h>
#include <netinet/cc/cc_module.h>

static void     cubic_ack_received(struct cc_var *ccv, ccsignal_t type);
static void     cubic_cb_destroy(struct cc_var *ccv);
static int      cubic_cb_init(struct cc_var *ccv, void *ptr);
static void     cubic_cong_signal(struct cc_var *ccv, ccsignal_t type);
static void     cubic_conn_init(struct cc_var *ccv);
static int      cubic_mod_init(void);
static void     cubic_post_recovery(struct cc_var *ccv);
static void     cubic_record_rtt(struct cc_var *ccv);
static uint32_t cubic_get_ssthresh(struct cc_var *ccv, uint32_t maxseg);
static void     cubic_after_idle(struct cc_var *ccv);
static size_t   cubic_data_sz(void);
static void     cubic_newround(struct cc_var *ccv, uint32_t round_cnt);
static void     cubic_rttsample(struct cc_var *ccv, uint32_t usec_rtt,
       uint32_t rxtcnt, uint32_t fas);

struct cc_algo cubic_cc_algo = {
        .name = "cubic",
        .ack_received = cubic_ack_received,
        .cb_destroy = cubic_cb_destroy,
        .cb_init = cubic_cb_init,
        .cong_signal = cubic_cong_signal,
        .conn_init = cubic_conn_init,
        .mod_init = cubic_mod_init,
        .post_recovery = cubic_post_recovery,
        .after_idle = cubic_after_idle,
        .cc_data_sz = cubic_data_sz,
        .rttsample = cubic_rttsample,
        .newround = cubic_newround
};

static void
cubic_log_hystart_event(struct cc_var *ccv, struct cubic *cubicd, uint8_t mod, uint32_t flex1)
{
        /*
         * Types of logs (mod value)
         * 1 - rtt_thresh in flex1, checking to see if RTT is to great.
         * 2 - rtt is too great, rtt_thresh in flex1.
         * 3 - CSS is active incr in flex1
         * 4 - A new round is beginning flex1 is round count
         * 5 - A new RTT measurement flex1 is the new measurement.
         * 6 - We enter CA ssthresh is also in flex1.
         * 7 - Socket option to change hystart executed opt.val in flex1.
         * 8 - Back out of CSS into SS, flex1 is the css_baseline_minrtt
         * 9 - We enter CA, via an ECN mark.
         * 10 - We enter CA, via a loss.
         * 11 - We have slipped out of SS into CA via cwnd growth.
         * 12 - After idle has re-enabled hystart++
         */
        struct tcpcb *tp;

        if (hystart_bblogs == 0)
                return;
        tp = ccv->tp;
        if (tcp_bblogging_on(tp)) {
                union tcp_log_stackspecific log;
                struct timeval tv;

                memset(&log, 0, sizeof(log));
                log.u_bbr.flex1 = flex1;
                log.u_bbr.flex2 = cubicd->css_current_round_minrtt;
                log.u_bbr.flex3 = cubicd->css_lastround_minrtt;
                log.u_bbr.flex4 = cubicd->css_rttsample_count;
                log.u_bbr.flex5 = cubicd->css_entered_at_round;
                log.u_bbr.flex6 = cubicd->css_baseline_minrtt;
                /* We only need bottom 16 bits of flags */
                log.u_bbr.flex7 = cubicd->flags & 0x0000ffff;
                log.u_bbr.flex8 = mod;
                log.u_bbr.epoch = cubicd->css_current_round;
                log.u_bbr.timeStamp = tcp_get_usecs(&tv);
                log.u_bbr.lt_epoch = cubicd->css_fas_at_css_entry;
                log.u_bbr.pkts_out = cubicd->css_last_fas;
                log.u_bbr.delivered = cubicd->css_lowrtt_fas;
                log.u_bbr.pkt_epoch = ccv->flags;
                TCP_LOG_EVENTP(tp, NULL,
                    &tptosocket(tp)->so_rcv,
                    &tptosocket(tp)->so_snd,
                    TCP_HYSTART, 0,
                    0, &log, false, &tv);
        }
}

static void
cubic_does_slow_start(struct cc_var *ccv, struct cubic *cubicd)
{
        /*
         * In slow-start with ABC enabled and no RTO in sight?
         * (Must not use abc_l_var > 1 if slow starting after
         * an RTO. On RTO, snd_nxt = snd_una, so the
         * snd_nxt == snd_max check is sufficient to
         * handle this).
         *
         * XXXLAS: Find a way to signal SS after RTO that
         * doesn't rely on tcpcb vars.
         */
        u_int cw = CCV(ccv, snd_cwnd);
        uint32_t mss = tcp_fixed_maxseg(ccv->tp);
        u_int incr = mss;
        uint16_t abc_val;

        cubicd->flags |= CUBICFLAG_IN_SLOWSTART;
        if (ccv->flags & CCF_USE_LOCAL_ABC)
                abc_val = ccv->labc;
        else
                abc_val = V_tcp_abc_l_var;
        if ((ccv->flags & CCF_HYSTART_ALLOWED) &&
            (cubicd->flags & CUBICFLAG_HYSTART_ENABLED) &&
            ((cubicd->flags & CUBICFLAG_HYSTART_IN_CSS) == 0)) {
                /*
                 * Hystart is allowed and still enabled and we are not yet
                 * in CSS. Lets check to see if we can make a decision on
                 * if we need to go into CSS.
                 */
                if ((cubicd->css_rttsample_count >= hystart_n_rttsamples) &&
                    (cubicd->css_current_round_minrtt != 0xffffffff) &&
                    (cubicd->css_lastround_minrtt != 0xffffffff)) {
                        uint32_t rtt_thresh;

                        /* Clamp (minrtt_thresh, lastround/8, maxrtt_thresh) */
                        rtt_thresh = (cubicd->css_lastround_minrtt >> 3);
                        if (rtt_thresh < hystart_minrtt_thresh)
                                rtt_thresh = hystart_minrtt_thresh;
                        if (rtt_thresh > hystart_maxrtt_thresh)
                                rtt_thresh = hystart_maxrtt_thresh;
                        cubic_log_hystart_event(ccv, cubicd, 1, rtt_thresh);

                        if (cubicd->css_current_round_minrtt >= (cubicd->css_lastround_minrtt + rtt_thresh)) {
                                /* Enter CSS */
                                cubicd->flags |= CUBICFLAG_HYSTART_IN_CSS;
                                cubicd->css_fas_at_css_entry = cubicd->css_lowrtt_fas;
                                /* 
                                 * The draft (v4) calls for us to set baseline to css_current_round_min
                                 * but that can cause an oscillation. We probably shoudl be using
                                 * css_lastround_minrtt, but the authors insist that will cause
                                 * issues on exiting early. We will leave the draft version for now
                                 * but I suspect this is incorrect.
                                 */
                                cubicd->css_baseline_minrtt = cubicd->css_current_round_minrtt;
                                cubicd->css_entered_at_round = cubicd->css_current_round;
                                cubic_log_hystart_event(ccv, cubicd, 2, rtt_thresh);
                        }
                }
        }
        if (CCV(ccv, snd_nxt) == CCV(ccv, snd_max))
                incr = min(ccv->bytes_this_ack,
                           ccv->nsegs * abc_val * mss);
        else
                incr = min(ccv->bytes_this_ack, mss);

        /* Only if Hystart is enabled will the flag get set */
        if (cubicd->flags & CUBICFLAG_HYSTART_IN_CSS) {
                incr /= hystart_css_growth_div;
                cubic_log_hystart_event(ccv, cubicd, 3, incr);
        }
        /* ABC is on by default, so incr equals 0 frequently. */
        if (incr > 0)
                CCV(ccv, snd_cwnd) = min((cw + incr),
                                         TCP_MAXWIN << CCV(ccv, snd_scale));
}

static void
cubic_ack_received(struct cc_var *ccv, ccsignal_t type)
{
        struct cubic *cubic_data;
        uint32_t W_est, W_cubic, cwin, target, incr;
        int usecs_since_epoch;
        uint32_t mss = tcp_fixed_maxseg(ccv->tp);

        cwin = CCV(ccv, snd_cwnd);
        cubic_data = ccv->cc_data;
        cubic_record_rtt(ccv);

        /*
         * For a regular ACK and we're not in cong/fast recovery and
         * we're cwnd limited, always recalculate cwnd.
         */
        if (type == CC_ACK && !IN_RECOVERY(CCV(ccv, t_flags)) &&
            (ccv->flags & CCF_CWND_LIMITED)) {
                 /* Use the logic in NewReno ack_received() for slow start. */
                if (cwin <= CCV(ccv, snd_ssthresh) ||
                    cubic_data->min_rtt_usecs == TCPTV_SRTTBASE) {
                        cubic_does_slow_start(ccv, cubic_data);
                } else {
                        if (cubic_data->flags & CUBICFLAG_HYSTART_IN_CSS) {
                                /*
                                 * We have slipped into CA with
                                 * CSS active. Deactivate all.
                                 */
                                /* Turn off the CSS flag */
                                cubic_data->flags &= ~CUBICFLAG_HYSTART_IN_CSS;
                                /* Disable use of CSS in the future except long idle  */
                                cubic_data->flags &= ~CUBICFLAG_HYSTART_ENABLED;
                                cubic_log_hystart_event(ccv, cubic_data, 11, CCV(ccv, snd_ssthresh));
                        }
                        if (cubic_data->flags & (CUBICFLAG_IN_SLOWSTART |
                                                 CUBICFLAG_CONG_EVENT   |
                                                 CUBICFLAG_IN_APPLIMIT)) {
                                /*
                                 * At the beginning of the current congestion
                                 * avoidance stage, The epoch variables
                                 * (t_epoch, cwnd_epoch, K) are updated in the
                                 * following three cases:
                                 * 1) just exited the slow start
                                 * 2) after a congestion event
                                 * 3) application-limited
                                 */
                                cubic_data->t_epoch = ticks;
                                cubic_data->cwnd_epoch = cwin;
                                cubic_data->K = cubic_k(cubic_data->W_max / mss,
                                                        cubic_data->cwnd_epoch / mss);
                                cubic_data->flags &= ~(CUBICFLAG_IN_SLOWSTART |
                                                       CUBICFLAG_CONG_EVENT   |
                                                       CUBICFLAG_IN_APPLIMIT);

                                if (cubic_data->flags & CUBICFLAG_RTO_EVENT) {
                                        /* RFC9438 Section 4.8: Timeout */
                                        cubic_data->flags &= ~CUBICFLAG_RTO_EVENT;
                                        cubic_data->W_max = cwin;
                                        cubic_data->K = 0;
                                }
                        }
                        usecs_since_epoch = (ticks - cubic_data->t_epoch) * tick;
                        if (usecs_since_epoch < 0) {
                                /*
                                 * dragging t_epoch along
                                 */
                                usecs_since_epoch = INT_MAX;
                                cubic_data->t_epoch = ticks - INT_MAX;
                        }
                        W_est = tf_cwnd(ccv);
                        /*
                         * The mean RTT is used to best reflect the equations.
                         */
                        W_cubic = cubic_cwnd(usecs_since_epoch +
                                             cubic_data->mean_rtt_usecs,
                                             cubic_data->W_max,
                                             mss,
                                             cubic_data->K);

                        if (W_cubic < W_est) {
                                /* RFC9438 Section 4.3: Reno-friendly region */
                                CCV(ccv, snd_cwnd) = W_est;
                                cubic_data->flags |= CUBICFLAG_IN_TF;
                        } else {
                                /*
                                 * RFC9438 Section 4.4 or 4.5:
                                 * Concave or Convex Region
                                 */
                                if (W_cubic < cwin) {
                                        target = cwin;
                                } else if (W_cubic > ((cwin * 3) >> 1)) {
                                        target = (cwin * 3) >> 1;
                                } else {
                                        target = W_cubic;
                                }
                                incr = (((target - cwin) << CUBIC_SHIFT) /
                                        cwin * mss) >> CUBIC_SHIFT;
                                CCV(ccv, snd_cwnd) = cwin + incr;
                        }
                }
        } else if (type == CC_ACK && !IN_RECOVERY(CCV(ccv, t_flags)) &&
            !(ccv->flags & CCF_CWND_LIMITED)) {
                cubic_data->flags |= CUBICFLAG_IN_APPLIMIT;
        }
}

/*
 * This is a CUBIC specific implementation of after_idle.
 *   - Reset cwnd by calling New Reno implementation of after_idle.
 *   - Reset t_epoch.
 */
static void
cubic_after_idle(struct cc_var *ccv)
{
        struct cubic *cubic_data = ccv->cc_data;
        uint32_t mss = tcp_fixed_maxseg(ccv->tp);

        cubic_data->W_max = ulmax(cubic_data->W_max, CCV(ccv, snd_cwnd));
        cubic_data->K = cubic_k(cubic_data->W_max / mss, cubic_data->cwnd_epoch / mss);
        if ((cubic_data->flags & CUBICFLAG_HYSTART_ENABLED) == 0) {
                /*
                 * Re-enable hystart if we have been idle.
                 */
                cubic_data->flags &= ~CUBICFLAG_HYSTART_IN_CSS;
                cubic_data->flags |= CUBICFLAG_HYSTART_ENABLED;
                cubic_log_hystart_event(ccv, cubic_data, 12, CCV(ccv, snd_ssthresh));
        }
        newreno_cc_after_idle(ccv);
        cubic_data->t_epoch = ticks;
}

static void
cubic_cb_destroy(struct cc_var *ccv)
{
        free(ccv->cc_data, M_CC_MEM);
}

static size_t
cubic_data_sz(void)
{
        return (sizeof(struct cubic));
}

static int
cubic_cb_init(struct cc_var *ccv, void *ptr)
{
        struct cubic *cubic_data;

        INP_WLOCK_ASSERT(tptoinpcb(ccv->tp));
        if (ptr == NULL) {
                cubic_data = malloc(sizeof(struct cubic), M_CC_MEM, M_NOWAIT|M_ZERO);
                if (cubic_data == NULL)
                        return (ENOMEM);
        } else
                cubic_data = ptr;

        /* Init some key variables with sensible defaults. */
        cubic_data->t_epoch = 0;
        cubic_data->cwnd_epoch = 0;
        cubic_data->K = 0;
        cubic_data->min_rtt_usecs = TCPTV_SRTTBASE;
        cubic_data->mean_rtt_usecs = 1;

        ccv->cc_data = cubic_data;
        cubic_data->flags = CUBICFLAG_HYSTART_ENABLED;
        /* At init set both to infinity */
        cubic_data->css_lastround_minrtt = 0xffffffff;
        cubic_data->css_current_round_minrtt = 0xffffffff;
        cubic_data->css_current_round = 0;
        cubic_data->css_baseline_minrtt = 0xffffffff;
        cubic_data->css_rttsample_count = 0;
        cubic_data->css_entered_at_round = 0;
        cubic_data->css_fas_at_css_entry = 0;
        cubic_data->css_lowrtt_fas = 0;
        cubic_data->css_last_fas = 0;

        return (0);
}

/*
 * Perform any necessary tasks before we enter congestion recovery.
 */
static void
cubic_cong_signal(struct cc_var *ccv, ccsignal_t type)
{
        struct cubic *cubic_data;
        uint32_t mss, pipe, ssthresh;

        cubic_data = ccv->cc_data;
        mss = tcp_fixed_maxseg(ccv->tp);

        switch (type) {
        case CC_NDUPACK:
                if (cubic_data->flags & CUBICFLAG_HYSTART_ENABLED) {
                        /* Make sure the flags are all off we had a loss */
                        cubic_data->flags &= ~CUBICFLAG_HYSTART_ENABLED;
                        cubic_data->flags &= ~CUBICFLAG_HYSTART_IN_CSS;
                        cubic_log_hystart_event(ccv, cubic_data, 10, CCV(ccv, snd_ssthresh));
                }
                if (!IN_FASTRECOVERY(CCV(ccv, t_flags))) {
                        if (!IN_CONGRECOVERY(CCV(ccv, t_flags))) {
                                ssthresh = cubic_get_ssthresh(ccv, mss);
                                CCV(ccv, snd_ssthresh) = max(ssthresh, 2 * mss);
                                /*
                                 * The congestion flag will recalculate K at the
                                 * beginning of the congestion avoidance stage.
                                 */
                                cubic_data->flags |= CUBICFLAG_CONG_EVENT;
                        }
                        ENTER_RECOVERY(CCV(ccv, t_flags));
                }
                break;

        case CC_ECN:
                if (cubic_data->flags & CUBICFLAG_HYSTART_ENABLED) {
                        /* Make sure the flags are all off we had a loss */
                        cubic_data->flags &= ~CUBICFLAG_HYSTART_ENABLED;
                        cubic_data->flags &= ~CUBICFLAG_HYSTART_IN_CSS;
                        cubic_log_hystart_event(ccv, cubic_data, 9, CCV(ccv, snd_ssthresh));
                }
                if (!IN_CONGRECOVERY(CCV(ccv, t_flags))) {
                        ssthresh = cubic_get_ssthresh(ccv, mss);
                        CCV(ccv, snd_ssthresh) = max(ssthresh, 2 * mss);
                        CCV(ccv, snd_cwnd) = max(ssthresh, mss);
                        /*
                         * The congestion flag will recalculate K at the
                         * beginning of the congestion avoidance stage.
                         */
                        cubic_data->flags |= CUBICFLAG_CONG_EVENT;
                        ENTER_CONGRECOVERY(CCV(ccv, t_flags));
                }
                break;

        case CC_RTO:
                /* RFC9438 Section 4.8: Timeout */
                if (CCV(ccv, t_rxtshift) == 1) {
                        /*
                         * Remember the state only for the first RTO event. This
                         * will help us restore the state to the values seen
                         * at the most recent congestion avoidance stage before
                         * the current RTO event.
                         */
                        cubic_data->undo_t_epoch = cubic_data->t_epoch;
                        cubic_data->undo_cwnd_epoch = cubic_data->cwnd_epoch;
                        cubic_data->undo_W_max = cubic_data->W_max;
                        cubic_data->undo_K = cubic_data->K;
                        pipe = tcp_compute_pipe(ccv->tp);
                        CCV(ccv, snd_ssthresh) = max(2,
                                (((uint64_t)min(CCV(ccv, snd_wnd), pipe) *
                                CUBIC_BETA) >> CUBIC_SHIFT) / mss) * mss;
                }
                /*
                 * The RTO flag will recalculate K at the
                 * beginning of the congestion avoidance stage.
                 */
                cubic_data->flags |= CUBICFLAG_RTO_EVENT;
                CCV(ccv, snd_cwnd) = mss;
                break;

        case CC_RTO_ERR:
                cubic_data->flags &= ~CUBICFLAG_RTO_EVENT;
                cubic_data->K = cubic_data->undo_K;
                cubic_data->W_max = cubic_data->undo_W_max;
                cubic_data->cwnd_epoch = cubic_data->undo_cwnd_epoch;
                cubic_data->t_epoch = cubic_data->undo_t_epoch;
                break;
        default:
                break;
        }
}

static void
cubic_conn_init(struct cc_var *ccv)
{
        struct cubic *cubic_data;

        cubic_data = ccv->cc_data;

        /*
         * Ensure we have a sane initial value for W_max recorded. Without
         * this here bad things happen when entries from the TCP hostcache
         * get used.
         */
        cubic_data->W_max = UINT_MAX;
}

static int
cubic_mod_init(void)
{
        return (0);
}

/*
 * Perform any necessary tasks before we exit congestion recovery.
 */
static void
cubic_post_recovery(struct cc_var *ccv)
{
        struct cubic *cubic_data;
        int pipe;
        uint32_t mss = tcp_fixed_maxseg(ccv->tp);

        cubic_data = ccv->cc_data;
        pipe = 0;

        if (IN_FASTRECOVERY(CCV(ccv, t_flags))) {
                /*
                 * If inflight data is less than ssthresh, set cwnd
                 * conservatively to avoid a burst of data, as suggested in
                 * the NewReno RFC. Otherwise, use the CUBIC method.
                 */
                pipe = tcp_compute_pipe(ccv->tp);
                if (pipe < CCV(ccv, snd_ssthresh))
                        /*
                         * Ensure that cwnd does not collapse to 1 MSS under
                         * adverse conditions. Implements RFC6582
                         */
                        CCV(ccv, snd_cwnd) = max(pipe, mss) + mss;
                else
                        /* Update cwnd based on beta and adjusted W_max. */
                        CCV(ccv, snd_cwnd) = max(((uint64_t)cubic_data->W_max *
                            CUBIC_BETA) >> CUBIC_SHIFT,
                            2 * mss);
        }

        /* Calculate the average RTT between congestion epochs. */
        if (cubic_data->epoch_ack_count > 0 &&
            cubic_data->sum_rtt_usecs >= cubic_data->epoch_ack_count) {
                cubic_data->mean_rtt_usecs = (int)(cubic_data->sum_rtt_usecs /
                    cubic_data->epoch_ack_count);
        }

        cubic_data->epoch_ack_count = 0;
        cubic_data->sum_rtt_usecs = 0;
}

/*
 * Record the min RTT and sum samples for the epoch average RTT calculation.
 */
static void
cubic_record_rtt(struct cc_var *ccv)
{
        struct cubic *cubic_data;
        uint32_t t_srtt_usecs;

        /* Ignore srtt until a min number of samples have been taken. */
        if (CCV(ccv, t_rttupdated) >= CUBIC_MIN_RTT_SAMPLES) {
                cubic_data = ccv->cc_data;
                t_srtt_usecs = tcp_get_srtt(ccv->tp,
                                            TCP_TMR_GRANULARITY_USEC);
                /*
                 * Record the current SRTT as our minrtt if it's the smallest
                 * we've seen or minrtt is currently equal to its initialised
                 * value.
                 *
                 * XXXLAS: Should there be some hysteresis for minrtt?
                 */
                if ((t_srtt_usecs < cubic_data->min_rtt_usecs ||
                    cubic_data->min_rtt_usecs == TCPTV_SRTTBASE)) {
                        /* A minimal rtt is a single unshifted tick of a ticks
                         * timer. */
                        cubic_data->min_rtt_usecs = max(tick >> TCP_RTT_SHIFT,
                                                        t_srtt_usecs);

                        /*
                         * If the connection is within its first congestion
                         * epoch, ensure we prime mean_rtt_usecs with a
                         * reasonable value until the epoch average RTT is
                         * calculated in cubic_post_recovery().
                         */
                        if (cubic_data->min_rtt_usecs >
                            cubic_data->mean_rtt_usecs)
                                cubic_data->mean_rtt_usecs =
                                    cubic_data->min_rtt_usecs;
                }

                /* Sum samples for epoch average RTT calculation. */
                cubic_data->sum_rtt_usecs += t_srtt_usecs;
                cubic_data->epoch_ack_count++;
        }
}

/*
 * Return the new value for ssthresh in the event of a congestion.
 */
static uint32_t
cubic_get_ssthresh(struct cc_var *ccv, uint32_t maxseg)
{
        struct cubic *cubic_data;
        uint32_t cwnd, pipe;

        cubic_data = ccv->cc_data;
        cwnd = CCV(ccv, snd_cwnd);

        /* RFC9438 Section 4.7: Fast convergence */
        if (cwnd < cubic_data->W_max) {
                cwnd = ((uint64_t)cwnd * CUBIC_FC_FACTOR) >> CUBIC_SHIFT;
        }
        cubic_data->W_max = cwnd;

        if (cubic_data->flags & CUBICFLAG_IN_TF) {
                /* If in the TCP friendly region, follow what newreno does. */
                return (newreno_cc_cwnd_on_multiplicative_decrease(ccv, maxseg));

        } else {
                /*
                 * RFC9438 Section 4.6: Multiplicative Decrease
                 * Outside the TCP friendly region, set ssthresh to the size of
                 * inflight_size * beta.
                 */
                pipe = tcp_compute_pipe(ccv->tp);
                return ((pipe * CUBIC_BETA) >> CUBIC_SHIFT);
        }
}

static void
cubic_rttsample(struct cc_var *ccv, uint32_t usec_rtt, uint32_t rxtcnt, uint32_t fas)
{
        struct cubic *cubicd;

        cubicd = ccv->cc_data;
        if (rxtcnt > 1) {
                /*
                 * Only look at RTT's that are non-ambiguous.
                 */
                return;
        }
        cubicd->css_rttsample_count++;
        cubicd->css_last_fas = fas;
        if (cubicd->css_current_round_minrtt > usec_rtt) {
                cubicd->css_current_round_minrtt = usec_rtt;
                cubicd->css_lowrtt_fas = cubicd->css_last_fas;
        }
        if ((cubicd->css_rttsample_count >= hystart_n_rttsamples) &&
            (cubicd->css_current_round_minrtt != 0xffffffff) &&
            (cubicd->css_current_round_minrtt < cubicd->css_baseline_minrtt) &&
            (cubicd->css_lastround_minrtt != 0xffffffff)) {
                /*
                 * We were in CSS and the RTT is now less, we
                 * entered CSS erroneously.
                 */
                cubicd->flags &= ~CUBICFLAG_HYSTART_IN_CSS;
                cubic_log_hystart_event(ccv, cubicd, 8, cubicd->css_baseline_minrtt);
                cubicd->css_baseline_minrtt = 0xffffffff;
        }
        if (cubicd->flags & CUBICFLAG_HYSTART_ENABLED)
                cubic_log_hystart_event(ccv, cubicd, 5, usec_rtt);
}

static void
cubic_newround(struct cc_var *ccv, uint32_t round_cnt)
{
        struct cubic *cubicd;

        cubicd = ccv->cc_data;
        /* We have entered a new round */
        cubicd->css_lastround_minrtt = cubicd->css_current_round_minrtt;
        cubicd->css_current_round_minrtt = 0xffffffff;
        cubicd->css_rttsample_count = 0;
        cubicd->css_current_round = round_cnt;
        if ((cubicd->flags & CUBICFLAG_HYSTART_IN_CSS) &&
            ((round_cnt - cubicd->css_entered_at_round) >= hystart_css_rounds)) {
                /* Enter CA */
                if (ccv->flags & CCF_HYSTART_CAN_SH_CWND) {
                        /*
                         * We engage more than snd_ssthresh, engage
                         * the brakes!! Though we will stay in SS to
                         * creep back up again, so lets leave CSS active
                         * and give us hystart_css_rounds more rounds.
                         */
                        if (ccv->flags & CCF_HYSTART_CONS_SSTH) {
                                CCV(ccv, snd_ssthresh) = ((cubicd->css_lowrtt_fas + cubicd->css_fas_at_css_entry) / 2);
                        } else {
                                CCV(ccv, snd_ssthresh) = cubicd->css_lowrtt_fas;
                        }
                        CCV(ccv, snd_cwnd) = cubicd->css_fas_at_css_entry;
                        cubicd->css_entered_at_round = round_cnt;
                } else {
                        CCV(ccv, snd_ssthresh) = CCV(ccv, snd_cwnd);
                        /* Turn off the CSS flag */
                        cubicd->flags &= ~CUBICFLAG_HYSTART_IN_CSS;
                        /* Disable use of CSS in the future except long idle  */
                        cubicd->flags &= ~CUBICFLAG_HYSTART_ENABLED;
                }
                cubic_log_hystart_event(ccv, cubicd, 6, CCV(ccv, snd_ssthresh));
        }
        if (cubicd->flags & CUBICFLAG_HYSTART_ENABLED)
                cubic_log_hystart_event(ccv, cubicd, 4, round_cnt);
}

DECLARE_CC_MODULE(cubic, &cubic_cc_algo);
MODULE_VERSION(cubic, 2);