我对TCP CDG拥塞控制算法的改进和优化_拥塞控制算法原理算法改进

其实这不是我的优化，我是借用了BBR之力。

        借了什么力呢？这是我一再强调的，BBR最大的共享不是为Linux贡献了一个TCP拥塞控制算法(它同时在也BSD上被实现...)，而是它重构了Linux TCP的实现！借助BBR对Linux TCP实现的重构，很多之前做不到的事情，现在可以做到了。

        简而言之，BBR算法对Linux TCP实现的重构中，将以下三件事完全分离：

1.重传哪些包；
2.传输多少包；
3.实际传输。

拥塞控制算法侧重解决上述第2点问题。

-----------------------------------

CDG必须要拥塞窗口的背后默默维护一个”自己的窗口“，称为shadow_wnd，该窗口只受”实际拥塞情况“的影响，而不受”Linux TCP拥塞状态机“的影响。所以说，即便在丢包重传的Recovery时期，也必须动态维护这个shadow_wnd，使其按照Reno方式增长(或者按照CUBIC方式，随便什么方式都可以)。

        然则这在BBR之前的Linux 4.8版本之前的内核中是无法做到的。因为tcp_congestion_ops机构体中没有一个回调函数是在Recovery阶段可以被调用的到的，而你所能控制的拥塞算法只能通过tcp_congestion_ops结构体的回调来实现。

BBR将以下的逻辑引入到了Linux：

static void tcp_cong_control(struct sock *sk, u32 ack, u32 acked_sacked,
                 int flag, const struct rate_sample *rs)
{
    const struct inet_connection_sock *icsk = inet_csk(sk);

    if (icsk->icsk_ca_ops->cong_control) {
        icsk->icsk_ca_ops->cong_control(sk, rs);
        return;
    }

    if (tcp_in_cwnd_reduction(sk)) {
        /* Reduce cwnd if state mandates */
        tcp_cwnd_reduction(sk, acked_sacked, flag);
    } else if (tcp_may_raise_cwnd(sk, flag)) {
        /* Advance cwnd if state allows */
        tcp_cong_avoid(sk, ack, acked_sacked);
    }
    tcp_update_pacing_rate(sk);
}

只要实现了cong_control回调，那就就不会再调用标准的PRR算法和拥塞避免tcp_cong_avoid函数，无论在任何阶段，均调用cong_control回调。因此，我的方法是，在Recovery或者Loss状态调用cong_control回调即可！在该回调中维护CDG的shadow窗口。

        这谈何容易！BBR引入的逻辑非常粗糙，只要实现了cong_control，该函数就无条件返回。事实上正确的做法是cong_control回调有个返回值，当满足一定条件时返回，否则继续下面的逻辑。但是BBR并没有引入这些。

-----------------------------------

但是，我将其引入了。

        请看，我将tcp_input.c中的tcp_cong_control改成了下面的样子：

static void tcp_cong_control(struct sock *sk, u32 ack, u32 prior_in_flight, u32 acked_sacked,
                                             int flag, const struct rate_sample *rs)
{
        const struct inet_connection_sock *icsk = inet_csk(sk);
#ifdef BBR
        if (icsk->icsk_ca_ops->cong_control) {
                icsk->icsk_ca_ops->cong_control(sk, rs);
#ifdef CDG
                // 以下是我添加的判断，新增了rs的flag字段，一旦置位就继续而不返回。
                if (!(rs->flag & CDG_CONT))
                        return;
#endif
        }
#endif
        if (tcp_in_cwnd_reduction(sk)) {
        /* Reduce cwnd if state mandates */
                tcp_cwnd_reduction(sk, acked_sacked, 1);
        } else if (tcp_may_raise_cwnd(sk, flag)) {
                /* Advance cwnd if state allows */
                tcp_cong_avoid(sk, ack, prior_in_flight);
        }
        tcp_update_pacing_rate(sk);
}

我添加了个判断。其实我的目的很简单，就是在Recovery状态下也能调用到CDG的逻辑，就这么简单个逻辑在不懂的人眼里显得如此高大上，在懂的人眼里显得如此傻逼...不管怎样，我做了。

-----------------------------------

以下的代码只是我对标准Linux 4.3内核CDG算法的differ，想理解代码细节的，请先阅读标准CDG代码，我虽然是个传说中有求必应的人，但那只是传说...请注意，我的目标内核是3.10内核，在我移植CDG之前，我已经移植了BBR，所以说，你最好以4.9内核为准，然而这样一来，又会对3.10内核的一些接口表示费解..这里不就不多解释了，我要说的是，想彻底逃离学院派，就必须把所有这些代码都搞清楚！不然的话，首先，你根本什么都看不懂，其次，即便你有想法，你也做不来。完整的代码我会附在本文最后。

以下是patch中几个重要函数的说明：

1.CDG的cong_control回调函数cdg_main：

static void cdg_main(struct sock *sk, struct rate_sample *rs)
{
        struct inet_connection_sock *icsk = inet_csk(sk);
        struct tcp_sock *tp = tcp_sk(sk);
        struct cdg *ca = inet_csk_ca(sk);

        if (!shadow_grow) {
                rs->flag |= CDG_CONT;
                return;
        }

        if (icsk->icsk_ca_state != TCP_CA_Open) {
                // 在重传阶段，依然要采集rtt，因为链路不问包类型，重传包也会影响网络可用容量。
                if (rs->rtt_us) {
                        // 感谢BBR增加了rs结构体，从中可以取rtt_us
                        ca->rtt.min = min_not_zero(ca->rtt.min, (s32)rs->rtt_us);
                        ca->rtt.max = max(ca->rtt.max, (s32)rs->rtt_us);
                }

                if (ca->state == CDG_NONFULL && use_tolerance) {
                        if (!shadow_fast && (ca->ack_sack_cnt < 0 || ca->ack_sack_cnt == 0) && ca->rtt.v64) {
                                s32 grad = 0;

                                if (ca->rtt_prev.v64)
                                        grad = tcp_cdg_grad(ca);
                                ca->rtt_prev = ca->rtt;
                                ca->ack_sack_cnt = tcp_packets_in_flight(tp);
                                ca->rtt.v64 = 0;
                        }
                        ca->ack_sack_cnt -= rs->acked_sacked;
                        if (ca->state == CDG_NONFULL || shadow_fast) {
                                // 如果链路未完全拥塞，那么shadow窗口便默默地帮助实际窗口占据空间，等到快速恢复结束，便可以由实际窗口可用。
                                tcp_cong_avoid_ai_shadow(sk, ca->shadow_wnd, rs->acked_sacked);
                                tp->snd_cwnd = ca->shadow_wnd;
                        }

                        rs->flag |= CDG_CONT;
                }
        } else {
                // 为了让执行流继续，增加CDG_CONT标志。
                rs->flag |= CDG_CONT;
        }
}

2.状态设置回调函数cdg_state：

static void cdg_state(struct sock *sk, u8 new_state)
{
        struct cdg *ca = inet_csk_ca(sk);
        struct tcp_sock *tp = tcp_sk(sk);

        if (!recovery_restore)
                return;
        if (new_state == TCP_CA_Open)
                // 进入Open状态时，直接接管shadow窗口，这里可能会有突发问题。
                tp->snd_cwnd = max(max(tp->snd_cwnd, ca->shadow_wnd), 2U);
        if (new_state == TCP_CA_Loss) {
                // 进入Loss状态，判断是否是噪声丢包
                if (ca->state == CDG_NONFULL && use_tolerance) {
                        // 如果是噪声丢包，那么便恢复窗口。
                        tp->snd_cwnd = ca->shadow_wnd;
                        printk("#### cwnd:%u \n", tp->snd_cwnd);
                        if (loss_push)
                                // 如果是噪声丢包，那么在窗口内继续发送数据。
                                tcp_push_pending_frames(sk);
                }
                // 如果是拥塞丢包，那么执行原有流程。
        }
}

3.UNDO函数tcp_cdg_undo_cwnd：

static u32 tcp_cdg_undo_cwnd(struct sock *sk)
{
        struct cdg *ca = inet_csk_ca(sk);
        struct tcp_sock *tp = tcp_sk(sk);
        // undo到shadow窗口
        return max3(2U, ca->shadow_wnd, max(tp->snd_cwnd, ca->undo_cwnd));
}

4.RTT梯度计算函数tcp_cdg_grad：

static s32 tcp_cdg_grad(struct cdg *ca)
{
        // rtt在pkts_acked回调和cong_control中被采样值更新
        s32 gmin = ca->rtt.min - ca->rtt_prev.min;
        s32 gmax = ca->rtt.max - ca->rtt_prev.max;
        s32 grad;

        if (ca->gradients) {
                ca->gsum.min += gmin - ca->gradients[ca->tail].min;
                ca->gsum.max += gmax - ca->gradients[ca->tail].max;
                ca->gradients[ca->tail].min = gmin;
                ca->gradients[ca->tail].max = gmax;
                ca->tail = (ca->tail + 1) & (window - 1);
                gmin = ca->gsum.min;
                gmax = ca->gsum.max;
        }
        ......
        /* Backoff was effectual: */
        if (gmin <= -32 || gmax <= -32)
                ca->backoff_cnt = 0;

        if (use_tolerance) {
                /* Reduce small variations to zero: */
                gmin = DIV_ROUND_CLOSEST(gmin, 64);
                gmax = DIV_ROUND_CLOSEST(gmax, 64);
                // 注意看上一篇文章CDG模型图示的边沿触发条件。
                if (gmin > 0 && gmax <= 0)
                        ca->state = CDG_FULL;
                else if ((gmin > 0 && gmax > 0) || gmax < 0)
                        ca->state = CDG_NONFULL;
        }
        return grad;
}

我首先盲测了一下原生的CDG，Oh NO！太垃圾，比CUBIC好，高丢包率下竟然与Westwood相当，在所有这一切中，BBR始终是另类，遥不可及，在我看了Paper之后，迅速自己实现了一版，感谢BBR对Linux TCP的重构！我承认我自己只懂Reno，BIC，CUBIC，Vegas，BBR这几种算法，其它HTCP，Westwood这些我并没有详细分析过，但是无论我怎么测，我发现我的CDG(应该是我改过的CDG)，一直跟BBR接近。

        CDG是什么？CDG实际上就是传统基于丢包的算法加上了一个抗噪声机制，本来基于丢包的算法就是以不断填充缓存为手段，直到填满缓存发生丢包进行减窗，然而有的时候并非拥塞的原因也会发生丢包，此时按照算法来看依然会减窗，这就大大降低了带宽的利用率。加上了这个CDG的RTT梯度抗噪声机制后，网络带宽的利用率大大提高了。然而可能会加重拥塞，所以CDG内置了backoff算法，这里就不赘述了。

-----------------------------------

tcp_cdg.c代码：

#include <linux/kernel.h>
#include <linux/random.h>
#include <linux/module.h>
#include <net/tcp.h>

#define HYSTART_ACK_TRAIN   1
#define HYSTART_DELAY       2

static int window __read_mostly = 8;
static unsigned int backoff_beta __read_mostly = 0.7071 * 1024; /* sqrt 0.5 */
static unsigned int backoff_factor __read_mostly = 42;
static unsigned int hystart_detect __read_mostly = 3;
static unsigned int use_ineff __read_mostly = 5;
static unsigned int use_shadow __read_mostly = 1;
static unsigned int backoff __read_mostly = 0;
static unsigned int use_tolerance __read_mostly = 1;
static unsigned int shadow_fast __read_mostly = 1;
static unsigned int shadow_grow __read_mostly = 1;
static unsigned int recovery_restore __read_mostly = 1;
static unsigned int loss_push __read_mostly = 1;

module_param(window, int, 0444);
MODULE_PARM_DESC(window, "gradient window size (power of two <= 256)");
module_param(backoff_beta, uint, 0644);
MODULE_PARM_DESC(backoff_beta, "backoff beta (0-1024)");
module_param(backoff_factor, uint, 0644);
MODULE_PARM_DESC(backoff_factor, "backoff probability scale factor");
module_param(hystart_detect, uint, 0644);
MODULE_PARM_DESC(hystart_detect, "use Hybrid Slow start "
         "(0: disabled, 1: ACK train, 2: delay threshold, 3: both)");
module_param(use_ineff, uint, 0644);
MODULE_PARM_DESC(use_ineff, "use ineffectual backoff detection (threshold)");
module_param(use_shadow, uint, 0644);
MODULE_PARM_DESC(use_shadow, "use shadow window heuristic");
module_param(backoff, uint, 0644);
MODULE_PARM_DESC(backoff, "back");
module_param(use_tolerance, uint, 0644);
MODULE_PARM_DESC(use_tolerance, "use loss tolerance heuristic");
module_param(shadow_fast, uint, 0644);
MODULE_PARM_DESC(shadow_fast, "back");
module_param(shadow_grow, uint, 0644);
MODULE_PARM_DESC(shadow_grow, "back");
module_param(recovery_restore, uint, 0644);
MODULE_PARM_DESC(recovery_restore, "back");
module_param(loss_push, uint, 0644);
MODULE_PARM_DESC(loss_push, "back");

struct cdg_minmax {
    union {
        struct {
            s32 min;
            s32 max;
        };
        u64 v64;
    };
};

enum cdg_state {
    CDG_UNKNOWN = 0,
    CDG_NONFULL = 1,
    CDG_FULL    = 2,
    CDG_BACKOFF = 3,
};

struct cdg {
    struct cdg_minmax rtt;
    struct cdg_minmax rtt_prev;
    struct cdg_minmax *gradients;
    struct cdg_minmax gsum;
    bool gfilled;
    u8  tail;
    u8  state;
    u8  delack;
    u32 rtt_seq;
    u32 undo_cwnd;
    u32 shadow_wnd;
    u32 snd_cwnd_cnt;
    u16 backoff_cnt;
    u16 sample_cnt;
    s32 delay_min;
    s32 ack_sack_cnt;
    u32 last_ack;
    u32 round_start;
};

/**
 * nexp_u32 - negative base-e exponential
 * @ux: x in units of micro
 *
 * Returns exp(ux * -1e-6) * U32_MAX.
 */
static u32 __pure nexp_u32(u32 ux)
{
    static const u16 v[] = {
        /* exp(-x)*65536-1 for x = 0, 0.000256, 0.000512, ... */
        65535,
        65518, 65501, 65468, 65401, 65267, 65001, 64470, 63422,
        61378, 57484, 50423, 38795, 22965, 8047,  987,   14,
    };
    u32 msb = ux >> 8;
    u32 res;
    int i;

    /* Cut off when ux >= 2^24 (actual result is <= 222/U32_MAX). */
    if (msb > U16_MAX)
        return 0;

    /* Scale first eight bits linearly: */
    res = U32_MAX - (ux & 0xff) * (U32_MAX / 1000000);

    /* Obtain e^(x + y + ...) by computing e^x * e^y * ...: */
    for (i = 1; msb; i++, msb >>= 1) {
        u32 y = v[i & -(msb & 1)] + U32_C(1);

        res = ((u64)res * y) >> 16;
    }

    return res;
}

/* Based on the HyStart algorithm (by Ha et al.) that is implemented in
 * tcp_cubic. Differences/experimental changes:
 *   o Using Hayes' delayed ACK filter.
 *   o Using a usec clock for the ACK train.
 *   o Reset ACK train when application limited.
 *   o Invoked at any cwnd (i.e. also when cwnd < 16).
 *   o Invoked only when cwnd < ssthresh (i.e. not when cwnd == ssthresh).
 */
static void tcp_cdg_hystart_update(struct sock *sk)
{
    struct cdg *ca = inet_csk_ca(sk);
    struct tcp_sock *tp = tcp_sk(sk);

    ca->delay_min = min_not_zero(ca->delay_min, ca->rtt.min);
    if (ca->delay_min == 0)
        return;

    if (hystart_detect & HYSTART_ACK_TRAIN) {
        u32 now_us = div_u64(local_clock(), NSEC_PER_USEC);

        if (ca->last_ack == 0 || !tcp_is_cwnd_limited(sk, tcp_packets_in_flight(tp))) {
            ca->last_ack = now_us;
            ca->round_start = now_us;
        } else if (before(now_us, ca->last_ack + 3000)) {
            u32 base_owd = max(ca->delay_min / 2U, 125U);

            ca->last_ack = now_us;
            if (after(now_us, ca->round_start + base_owd)) {
                tp->snd_ssthresh = tp->snd_cwnd;
                return;
            }
        }
    }

    if (hystart_detect & HYSTART_DELAY) {
        if (ca->sample_cnt < 8) {
            ca->sample_cnt++;
        } else {
            s32 thresh = max(ca->delay_min + ca->delay_min / 8U,
                     125U);

            if (ca->rtt.min > thresh) {
                tp->snd_ssthresh = tp->snd_cwnd;
            }
        }
    }
}

static s32 tcp_cdg_grad(struct cdg *ca)
{
    s32 gmin = ca->rtt.min - ca->rtt_prev.min;
    s32 gmax = ca->rtt.max - ca->rtt_prev.max;
    s32 grad;

    if (ca->gradients) {
        ca->gsum.min += gmin - ca->gradients[ca->tail].min;
        ca->gsum.max += gmax - ca->gradients[ca->tail].max;
        ca->gradients[ca->tail].min = gmin;
        ca->gradients[ca->tail].max = gmax;
        ca->tail = (ca->tail + 1) & (window - 1);
        gmin = ca->gsum.min;
        gmax = ca->gsum.max;
    }

    /* We keep sums to ignore gradients during cwnd reductions;
     * the paper's smoothed gradients otherwise simplify to:
     * (rtt_latest - rtt_oldest) / window.
     *
     * We also drop division by window here.
     */
    grad = gmin > 0 ? gmin : gmax;

    /* Extrapolate missing values in gradient window: */
    if (!ca->gfilled) {
        if (!ca->gradients && window > 1)
            grad *= window; /* Memory allocation failed. */
        else if (ca->tail == 0)
            ca->gfilled = true;
        else
            grad = (grad * window) / (int)ca->tail;
    }

    /* Backoff was effectual: */
    if (gmin <= -32 || gmax <= -32)
        ca->backoff_cnt = 0;

    if (use_tolerance) {
        /* Reduce small variations to zero: */
        gmin = DIV_ROUND_CLOSEST(gmin, 64);
        gmax = DIV_ROUND_CLOSEST(gmax, 64);
        if (gmin > 0 && gmax <= 0)
            ca->state = CDG_FULL;
        else if ((gmin > 0 && gmax > 0) || gmax < 0)
            ca->state = CDG_NONFULL;
    }
    return grad;
}

void tcp_enter_cwr_1(struct sock *sk)
{
    struct tcp_sock *tp = tcp_sk(sk);

    tp->prior_ssthresh = 0;
    if (inet_csk(sk)->icsk_ca_state < TCP_CA_CWR) {
        tp->undo_marker = 0;
        tp->high_seq = tp->snd_nxt;
        tp->tlp_high_seq = 0;
        tp->snd_cwnd_cnt = 0;
        tp->prior_cwnd = tp->snd_cwnd;
        tp->prr_delivered = 0;
        tp->prr_out = 0;
        tp->snd_ssthresh = inet_csk(sk)->icsk_ca_ops->ssthresh(sk);
        if (tp->ecn_flags & TCP_ECN_OK)
            tp->ecn_flags |= TCP_ECN_QUEUE_CWR;
        tcp_set_ca_state(sk, TCP_CA_CWR);
    }
}

static bool tcp_cdg_backoff(struct sock *sk, u32 grad)
{
    struct cdg *ca = inet_csk_ca(sk);
    struct tcp_sock *tp = tcp_sk(sk);

    if (prandom_u32() <= nexp_u32(grad * backoff_factor))
        return false;

    if (use_ineff) {
        ca->backoff_cnt++;
        if (ca->backoff_cnt > use_ineff)
            return false;
    }

    ca->shadow_wnd = max(ca->shadow_wnd, tp->snd_cwnd);
    ca->state = CDG_BACKOFF;
    tcp_enter_cwr_1(sk);
    return true;
}

void tcp_cong_avoid_ai_shadow(struct sock *sk, u32 w, u32 acked)
{
    struct tcp_sock *tp = tcp_sk(sk);
    struct cdg *ca = inet_csk_ca(sk);
    if (ca->snd_cwnd_cnt >= w) {
        ca->snd_cwnd_cnt = 0;
        ca->shadow_wnd ++;
    }

    ca->snd_cwnd_cnt += acked;
    if (ca->snd_cwnd_cnt >= w) {
        u32 delta = ca->snd_cwnd_cnt / w;

        ca->snd_cwnd_cnt -= delta * w;
        ca->shadow_wnd += delta;
    }
    ca->shadow_wnd = min(ca->shadow_wnd, tp->snd_cwnd_clamp);
}

/* Not called in CWR or Recovery state. */
static void tcp_cdg_cong_avoid(struct sock *sk, u32 ack, u32 acked)
{
    struct cdg *ca = inet_csk_ca(sk);
    struct tcp_sock *tp = tcp_sk(sk);
    u32 prior_snd_cwnd;
    u32 incr;

    if (tp->snd_cwnd <= tp->snd_ssthresh && hystart_detect)
        tcp_cdg_hystart_update(sk);

    if (after(ack, ca->rtt_seq) && ca->rtt.v64) {
        s32 grad = 0;

        if (ca->rtt_prev.v64)
            grad = tcp_cdg_grad(ca);
        ca->rtt_seq = tp->snd_nxt;
        ca->rtt_prev = ca->rtt;
        ca->rtt.v64 = 0;
        ca->last_ack = 0;
        ca->sample_cnt = 0;

        if (backoff && grad > 0 && tcp_cdg_backoff(sk, grad))
            return;
    }

    if (!tcp_is_cwnd_limited(sk, tcp_packets_in_flight(tp))) {
        ca->shadow_wnd = min(ca->shadow_wnd, tp->snd_cwnd);
        return;
    }

    prior_snd_cwnd = tp->snd_cwnd;
    tcp_reno_cong_avoid(sk, ack, acked);

    incr = tp->snd_cwnd - prior_snd_cwnd;
    ca->shadow_wnd = max(ca->shadow_wnd, ca->shadow_wnd + incr);
}

static void tcp_cdg_acked(struct sock *sk, u32 num_acked, s32 rtt_us)
{
    struct cdg *ca = inet_csk_ca(sk);
    struct tcp_sock *tp = tcp_sk(sk);

    if (rtt_us <= 0)
        return;

    /* A heuristic for filtering delayed ACKs, adapted from:
     * D.A. Hayes. "Timing enhancements to the FreeBSD kernel to support
     * delay and rate based TCP mechanisms." TR 100219A. CAIA, 2010.
     */
    if (tp->sacked_out == 0) {
        if (num_acked == 1 && ca->delack) {
            /* A delayed ACK is only used for the minimum if it is
             * provenly lower than an existing non-zero minimum.
             */
            ca->rtt.min = min(ca->rtt.min, rtt_us);
            ca->delack--;
            return;
        } else if (num_acked > 1 && ca->delack < 5) {
            ca->delack++;
        }
    }

    ca->rtt.min = min_not_zero(ca->rtt.min, rtt_us);
    ca->rtt.max = max(ca->rtt.max, rtt_us);
}

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

    ca->undo_cwnd = tp->snd_cwnd;
    ca->snd_cwnd_cnt = 0;
    ca->ack_sack_cnt = tcp_packets_in_flight(tp);

    if (ca->state == CDG_BACKOFF)
        return max(2U, (tp->snd_cwnd * min(1024U, backoff_beta)) >> 10);

    if (ca->state == CDG_NONFULL && use_tolerance)
        return tp->snd_cwnd;

    ca->shadow_wnd = max(min(ca->shadow_wnd >> 1, tp->snd_cwnd), 2U);
    if (use_shadow)
        return max3(2U, ca->shadow_wnd, tp->snd_cwnd >> 1);
    return max(2U, tp->snd_cwnd >> 1);
}

static u32 tcp_cdg_undo_cwnd(struct sock *sk)
{
    struct cdg *ca = inet_csk_ca(sk);
    struct tcp_sock *tp = tcp_sk(sk);
    return max3(2U, ca->shadow_wnd, max(tp->snd_cwnd, ca->undo_cwnd));
}

static void tcp_cdg_cwnd_event(struct sock *sk, const enum tcp_ca_event ev)
{
    struct cdg *ca = inet_csk_ca(sk);
    struct tcp_sock *tp = tcp_sk(sk);
    struct cdg_minmax *gradients;

    switch (ev) {
    case CA_EVENT_CWND_RESTART:
        gradients = ca->gradients;
        if (gradients)
            memset(gradients, 0, window * sizeof(gradients[0]));
        memset(ca, 0, sizeof(*ca));

        ca->gradients = gradients;
        ca->rtt_seq = tp->snd_nxt;
        ca->shadow_wnd = tp->snd_cwnd;
        break;
    case CA_EVENT_COMPLETE_CWR:
        ca->state = CDG_UNKNOWN;
        ca->rtt_seq = tp->snd_nxt;
        ca->rtt_prev = ca->rtt;
        ca->rtt.v64 = 0;
        break;
    default:
        break;
    }
}

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

    /* We silently fall back to window = 1 if allocation fails. */
    if (window > 1)
        ca->gradients = kcalloc(window, sizeof(ca->gradients[0]),
                    GFP_NOWAIT | __GFP_NOWARN);
    ca->rtt_seq = tp->snd_nxt;
    ca->shadow_wnd = tp->snd_cwnd;
    ca->ack_sack_cnt = 0;
}

static void tcp_cdg_release(struct sock *sk)
{
    struct cdg *ca = inet_csk_ca(sk);

    kfree(ca->gradients);
}

static void cdg_main(struct sock *sk, struct rate_sample *rs)
{
    struct inet_connection_sock *icsk = inet_csk(sk);
    struct tcp_sock *tp = tcp_sk(sk);
    struct cdg *ca = inet_csk_ca(sk);

    if (!shadow_grow) {
        rs->flag |= CDG_CONT;
        return;
    }

    if (icsk->icsk_ca_state != TCP_CA_Open) {
        if (rs->rtt_us) {
            ca->rtt.min = min_not_zero(ca->rtt.min, (s32)rs->rtt_us);
            ca->rtt.max = max(ca->rtt.max, (s32)rs->rtt_us);
        }

        if (ca->state == CDG_NONFULL && use_tolerance) { 
            if (!shadow_fast && (ca->ack_sack_cnt < 0 || ca->ack_sack_cnt == 0) && ca->rtt.v64) {
                s32 grad = 0;

                if (ca->rtt_prev.v64)
                    grad = tcp_cdg_grad(ca);
                ca->rtt_prev = ca->rtt;
                ca->ack_sack_cnt = tcp_packets_in_flight(tp);
                ca->rtt.v64 = 0;
            }
            ca->ack_sack_cnt -= rs->acked_sacked;
            if (ca->state == CDG_NONFULL || shadow_fast) {
                tcp_cong_avoid_ai_shadow(sk, ca->shadow_wnd, rs->acked_sacked);   
                tp->snd_cwnd = ca->shadow_wnd;
            }

            rs->flag |= CDG_CONT;
        }
    } else {
        rs->flag |= CDG_CONT;
    }
}

static void cdg_state(struct sock *sk, u8 new_state)
{
    struct cdg *ca = inet_csk_ca(sk);
    struct tcp_sock *tp = tcp_sk(sk);

    if (!recovery_restore)
        return;
    if (new_state == TCP_CA_Open)
        tp->snd_cwnd = max(max(tp->snd_cwnd, ca->shadow_wnd), 2U);
    if (new_state == TCP_CA_Loss) {
        if (ca->state == CDG_NONFULL && use_tolerance) {
            tp->snd_cwnd = ca->shadow_wnd;
            if (loss_push)
                tcp_push_pending_frames(sk);
        } 
    }
}

struct tcp_congestion_ops tcp_cdg __read_mostly = {
    .cong_avoid = tcp_cdg_cong_avoid,
    .cong_control   = cdg_main,
    .set_state = cdg_state,
    .cwnd_event = tcp_cdg_cwnd_event,
    .pkts_acked = tcp_cdg_acked,
    .undo_cwnd = tcp_cdg_undo_cwnd,
    .ssthresh = tcp_cdg_ssthresh,
    .release = tcp_cdg_release,
    .init = tcp_cdg_init,
    .owner = THIS_MODULE,
    .name = "cdg",
};

static int __init tcp_cdg_register(void)
{
    if (backoff_beta > 1024 || window < 1 || window > 256)
        return -ERANGE;
    if (!is_power_of_2(window))
        return -EINVAL;

    BUILD_BUG_ON(sizeof(struct cdg) > ICSK_CA_PRIV_SIZE);
    tcp_register_congestion_control(&tcp_cdg);
    return 0;
}

static void __exit tcp_cdg_unregister(void)
{
    tcp_unregister_congestion_control(&tcp_cdg);
}

module_init(tcp_cdg_register);
module_exit(tcp_cdg_unregister);
MODULE_AUTHOR("...");
MODULE_LICENSE("GPL");
MODULE_DESCRIPTION("TCP CDG");