// SPDX-License-Identifier: GPL-2.0-only
/*
 *
 *   YeAH TCP
 *
 * For further details look at:
 *   https://web.archive.org/web/20080316215752/http://wil.cs.caltech.edu/pfldnet2007/paper/YeAH_TCP.pdf
 *
 */
#include <linux/mm.h>
#include <linux/module.h>
#include <linux/skbuff.h>
#include <linux/inet_diag.h>

#include <net/tcp.h>

#include "tcp_vegas.h"

#define TCP_YEAH_ALPHA       80 /* number of packets queued at the bottleneck */
#define TCP_YEAH_GAMMA        1 /* fraction of queue to be removed per rtt */
#define TCP_YEAH_DELTA        3 /* log minimum fraction of cwnd to be removed on loss */
#define TCP_YEAH_EPSILON      1 /* log maximum fraction to be removed on early decongestion */
#define TCP_YEAH_PHY          8 /* maximum delta from base */
#define TCP_YEAH_RHO         16 /* minimum number of consecutive rtt to consider competition on loss */
#define TCP_YEAH_ZETA        50 /* minimum number of state switches to reset reno_count */

#define TCP_SCALABLE_AI_CNT      100U

/* YeAH variables */
struct yeah {
        struct vegas vegas;     /* must be first */

        /* YeAH */
        u32 lastQ;
        u32 doing_reno_now;

        u32 reno_count;
        u32 fast_count;
};

static void tcp_yeah_init(struct sock *sk)
{
        struct tcp_sock *tp = tcp_sk(sk);
        struct yeah *yeah = inet_csk_ca(sk);

        tcp_vegas_init(sk);

        yeah->doing_reno_now = 0;
        yeah->lastQ = 0;

        yeah->reno_count = 2;

        /* Ensure the MD arithmetic works.  This is somewhat pedantic,
         * since I don't think we will see a cwnd this large. :) */
        tp->snd_cwnd_clamp = min_t(u32, tp->snd_cwnd_clamp, 0xffffffff/128);
}

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

        if (!tcp_is_cwnd_limited(sk))
                return;

        if (tcp_in_slow_start(tp)) {
                acked = tcp_slow_start(tp, acked);
                if (!acked)
                        goto do_vegas;
        }

        if (!yeah->doing_reno_now) {
                /* Scalable */
                tcp_cong_avoid_ai(tp, min(tcp_snd_cwnd(tp), TCP_SCALABLE_AI_CNT),
                                  acked);
        } else {
                /* Reno */
                tcp_cong_avoid_ai(tp, tcp_snd_cwnd(tp), acked);
        }

        /* The key players are v_vegas.beg_snd_una and v_beg_snd_nxt.
         *
         * These are so named because they represent the approximate values
         * of snd_una and snd_nxt at the beginning of the current RTT. More
         * precisely, they represent the amount of data sent during the RTT.
         * At the end of the RTT, when we receive an ACK for v_beg_snd_nxt,
         * we will calculate that (v_beg_snd_nxt - v_vegas.beg_snd_una) outstanding
         * bytes of data have been ACKed during the course of the RTT, giving
         * an "actual" rate of:
         *
         *     (v_beg_snd_nxt - v_vegas.beg_snd_una) / (rtt duration)
         *
         * Unfortunately, v_vegas.beg_snd_una is not exactly equal to snd_una,
         * because delayed ACKs can cover more than one segment, so they
         * don't line up yeahly with the boundaries of RTTs.
         *
         * Another unfortunate fact of life is that delayed ACKs delay the
         * advance of the left edge of our send window, so that the number
         * of bytes we send in an RTT is often less than our cwnd will allow.
         * So we keep track of our cwnd separately, in v_beg_snd_cwnd.
         */
do_vegas:
        if (after(ack, yeah->vegas.beg_snd_nxt)) {
                /* We do the Vegas calculations only if we got enough RTT
                 * samples that we can be reasonably sure that we got
                 * at least one RTT sample that wasn't from a delayed ACK.
                 * If we only had 2 samples total,
                 * then that means we're getting only 1 ACK per RTT, which
                 * means they're almost certainly delayed ACKs.
                 * If  we have 3 samples, we should be OK.
                 */

                if (yeah->vegas.cntRTT > 2) {
                        u32 rtt, queue;
                        u64 bw;

                        /* We have enough RTT samples, so, using the Vegas
                         * algorithm, we determine if we should increase or
                         * decrease cwnd, and by how much.
                         */

                        /* Pluck out the RTT we are using for the Vegas
                         * calculations. This is the min RTT seen during the
                         * last RTT. Taking the min filters out the effects
                         * of delayed ACKs, at the cost of noticing congestion
                         * a bit later.
                         */
                        rtt = yeah->vegas.minRTT;

                        /* Compute excess number of packets above bandwidth
                         * Avoid doing full 64 bit divide.
                         */
                        bw = tcp_snd_cwnd(tp);
                        bw *= rtt - yeah->vegas.baseRTT;
                        do_div(bw, rtt);
                        queue = bw;

                        if (queue > TCP_YEAH_ALPHA ||
                            rtt - yeah->vegas.baseRTT > (yeah->vegas.baseRTT / TCP_YEAH_PHY)) {
                                if (queue > TCP_YEAH_ALPHA &&
                                    tcp_snd_cwnd(tp) > yeah->reno_count) {
                                        u32 reduction = min(queue / TCP_YEAH_GAMMA ,
                                                            tcp_snd_cwnd(tp) >> TCP_YEAH_EPSILON);

                                        tcp_snd_cwnd_set(tp, tcp_snd_cwnd(tp) - reduction);

                                        tcp_snd_cwnd_set(tp, max(tcp_snd_cwnd(tp),
                                                                 yeah->reno_count));

                                        tp->snd_ssthresh = tcp_snd_cwnd(tp);
                                }

                                if (yeah->reno_count <= 2)
                                        yeah->reno_count = max(tcp_snd_cwnd(tp)>>1, 2U);
                                else
                                        yeah->reno_count++;

                                yeah->doing_reno_now = min(yeah->doing_reno_now + 1,
                                                           0xffffffU);
                        } else {
                                yeah->fast_count++;

                                if (yeah->fast_count > TCP_YEAH_ZETA) {
                                        yeah->reno_count = 2;
                                        yeah->fast_count = 0;
                                }

                                yeah->doing_reno_now = 0;
                        }

                        yeah->lastQ = queue;
                }

                /* Save the extent of the current window so we can use this
                 * at the end of the next RTT.
                 */
                yeah->vegas.beg_snd_una  = yeah->vegas.beg_snd_nxt;
                yeah->vegas.beg_snd_nxt  = tp->snd_nxt;
                yeah->vegas.beg_snd_cwnd = tcp_snd_cwnd(tp);

                /* Wipe the slate clean for the next RTT. */
                yeah->vegas.cntRTT = 0;
                yeah->vegas.minRTT = 0x7fffffff;
        }
}

static u32 tcp_yeah_ssthresh(struct sock *sk)
{
        const struct tcp_sock *tp = tcp_sk(sk);
        struct yeah *yeah = inet_csk_ca(sk);
        u32 reduction;

        if (yeah->doing_reno_now < TCP_YEAH_RHO) {
                reduction = yeah->lastQ;

                reduction = min(reduction, max(tcp_snd_cwnd(tp)>>1, 2U));

                reduction = max(reduction, tcp_snd_cwnd(tp) >> TCP_YEAH_DELTA);
        } else
                reduction = max(tcp_snd_cwnd(tp)>>1, 2U);

        yeah->fast_count = 0;
        yeah->reno_count = max(yeah->reno_count>>1, 2U);

        return max_t(int, tcp_snd_cwnd(tp) - reduction, 2);
}

static struct tcp_congestion_ops tcp_yeah __read_mostly = {
        .init           = tcp_yeah_init,
        .ssthresh       = tcp_yeah_ssthresh,
        .undo_cwnd      = tcp_reno_undo_cwnd,
        .cong_avoid     = tcp_yeah_cong_avoid,
        .set_state      = tcp_vegas_state,
        .cwnd_event     = tcp_vegas_cwnd_event,
        .get_info       = tcp_vegas_get_info,
        .pkts_acked     = tcp_vegas_pkts_acked,

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

static int __init tcp_yeah_register(void)
{
        BUILD_BUG_ON(sizeof(struct yeah) > ICSK_CA_PRIV_SIZE);
        tcp_register_congestion_control(&tcp_yeah);
        return 0;
}

static void __exit tcp_yeah_unregister(void)
{
        tcp_unregister_congestion_control(&tcp_yeah);
}

module_init(tcp_yeah_register);
module_exit(tcp_yeah_unregister);

MODULE_AUTHOR("Angelo P. Castellani");
MODULE_LICENSE("GPL");
MODULE_DESCRIPTION("YeAH TCP");