TCP CDG算法与TCP Westwood算法

自从我修改了CDG算法以及Westwood算法之后，通过微信，QQ还有邮件与很多志同道合的朋友进行了交流，本文将这些交流的结论总结一下。

TCP CDG算法有多好？

很多人不屑于这个算法，包括社区的一些自诩清高的人，然而把问题细化的话，如果无法回答“TCP CDG算法有多不好？”这个问题，那就是无知了。CDG有多好取决于你的网络质量，你的网络有多大的概率是线路噪声引发的丢包决定了这个问题的答案。

1.抗噪声丢包

修改过的CDG对噪声丢包的抵抗性非常强，在它明确判断出是由于噪声而发生丢包而不是由于拥塞丢包的时候，在重传期间(TCP的快速恢复期间)，仍然可以保持几乎不变的窗口发送重传数据段以及新数据。

        可以多说一句，CDG抗噪声的能力甚至比BBR都好。首先BBR+RACK可能会在多个短连接流竞争队列时带来频繁的盲目重传，其次，BBR可能根本就不适合普通运营商网络，因此很多关于BBR的迷信都是盲目的。然而CDG由传统的拥塞控制算法衍生而来(还包括Westwood也是这么衍生过来的)，非常容易切进没有Google B4基因的国内运营商网络。

2.公平性相关

公平收敛性是所有TCP拥塞控制算法必须遵守的戒律，凡违反该戒律的，都是傻逼，数不清的越界行为就是让我越发恶心TCP的首要原因。CDG的公平性由其Backoff机制来保障，即在RTT持续增大但尚未丢包的期间，CDG预判此时开始有拥塞迹象，大家都开始抢节点队列缓存了，此时CDG会主动随机退避降低发送速率，即降窗。然而这只是CDG单方面的君子行为，在降窗期间，一个shadow窗口默默地继续增加，如果再发生丢包，CDG就不再谦让了，其不会像Reno或者CUBIC那样把当前窗口降低一定比例，而是将已经在Backoff阶段默默增长的shadow窗口降低一定比例作为新的拥塞窗口。

3.拥塞导致的丢包处理

这里你会发现，实际上CDG是在传统的NewReno/CUBIC算法上做了以上两项优化，首先它跟踪RTT变化梯度以区别拥塞与噪声，其次在噪声情况下不降窗，而在拥塞未丢包情况下随机降窗并与此同时保持shadow增长。然而在拥塞丢包时，CDG的行为会像NewReno那样乘性减半当前窗口并设置新的ssthresh值。

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上面描述的CDG使用场景看明白了吗？如果你的网络经常是拥塞发生丢包，那么CDG的表现其实几乎与NewReno无异，大量的测试数据已经表明NewReno已经是要被淘汰的算法了，CUBIC早就取代了NewReno。

        然而我接下来并无意要移植CUBIC的逻辑到CDG。

        注意分割线以上的第3点，这个第3点明确的抛出一个问题，显然在拥塞丢包发生的时候，NewReno对这个问题回答的并不好，CUBIC似乎比NewReno要好些，但是显然二者师出同门，直线变成了三次曲线而已，一个回答不好，能指望另一个回答好吗，显然这是徒劳的。

        Westwood算法完美回答了这个问题。

        接下来，让我们简单看一下Westwood。

        在CDG与Westwood的PK中，你可能会发现Westwood会持续性占据优势，然而当你用tc工具设置x%丢包率(注意这是随机的丢包)的时候，CDG的优势便表现了出来。这正是CDG发力的地方，然而一旦真正发生了拥塞而频繁丢包，特别是在浅队列环境中，Westwood算法将压倒性超越CDG，即便我对CDG进行了修改，所占优势也只是略微。现在的问题是，是什么机制让Westwood可以如此坦然面对拥塞？！

        事实上，Westwood什么新东西都没有，它的核心就是精确定位拥塞丢包重传恢复后窗口应该降低到多少(当然必须使用我修改过的版本)，除此之外，没别的什么了。它是怎么做到的呢？

4.[这里前面的标号4没有任何问题]Westwood在TCP连接采样了最小的RTT，同时周期性采集带宽，然后Westwood会认为二者的乘积就是不排队时的BDP，即拥塞丢包发生后比较合理的拥塞窗口值，其实更加合理的是应该在一个时间窗口内采样最小RTT和最大带宽，以应对波动，然而在CDG和BBR之前，几乎所有算法都是采用“移动指数平均”的方式来进行低通滤波，包括Fast TCP。现在，我们知道了，Westwood的核心在于“丢包并处理完重传后cwnd以及ssthresh的设置”：

case CA_EVENT_COMPLETE_CWR:
    tp->snd_cwnd = tp->snd_ssthresh = tcp_westwood_bw_rttmin(sk);
    break;

遗憾的是，Westwood并无法区分丢包是由于噪声引起的，还是由于拥塞引起的，所以即便是“加性减”，也扛不住频繁加性减。幸运的是，CDG可以来补充！

        现在你应该已经知道了Westwood的应用场景，总结起来就是Westwood试图用加性增，加性减来应对拥塞避免和快速恢复的过程。因此在常规的拥塞避免阶段，Westwood同样采用NewReno算法，这点和CDG是完全一样的。

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你还在想用CUBIC替换Westwood或者CDG里默认的NewReno算法吗？嗯，这样想是对的，但是收益不大。

        更加合理的做法难道不是将上面的1，2，3，4结合起来吗？！如果抛开BBR的BW & RTT模型不论，其基本思路实际上就是CDG+Westwood的公平收敛版：

比BBR稍微有点过头，且好像丢失了乘性减的公平性，但我依然称其为公平收敛版本。不管怎么说，谁在乎呢？只要保证自己不吃亏即可吧。

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经常有人觉得BBR非常复杂，事实上其特别简单，同样简单的还有CDG和Westwood。

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温州皮鞋厂老板的测试结论非常好，我把初版代码贴在本文的最后，特意修改了名字，该算法改名为bugs，非常凶险的一个名字。该代码也放在
github上：

#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
#define TCP_EASTWOOD_RTT_MIN   (HZ/20)  /* 50ms */
#define TCP_EASTWOOD_INIT_RTT  (20*HZ)  /* maybe too conservative?! */

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 = 0;
static unsigned int loss_push __read_mostly = 1;
static unsigned int use_sack __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");
module_param(use_sack, uint, 0644);
MODULE_PARM_DESC(use_sack, "back");

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

enum bugs_state {
    BU_GS_UNKNOWN = 0,
    BU_GS_NONFULL = 1,
    BU_GS_FULL    = 2,
    BU_GS_BACKOFF = 3,
};

struct bugs {
    struct bugs_minmax rtt;
    struct bugs_minmax rtt_prev;
    struct bugs_minmax *gradients;
    struct bugs_minmax gsum;
    bool gfilled;
    u8  tail;
    u8  state;
    u8  delack;
    u8     first_ack;        /* flag which infers that this is the first ack */
    u8     reset_rtt_min;    /* Reset RTT min to next RTT sample*/
    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;

    u32    bw_ns_est;        /* first bandwidth estimation..not too smoothed 8) */
    u32    bw_est;           /* bandwidth estimate */
    u32    rtt_win_sx;       /* here starts a new evaluation... */
    u32    bk;
    u32    snd_una;          /* used for evaluating the number of acked bytes */
    u32    cumul_ack;
    u32    accounted;
    u32    east_rtt;
    u32    rtt_min;          /* minimum observed RTT */
};

/**
 * 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_bugs_hystart_update(struct sock *sk)
{
    struct bugs *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_bugs_grad(struct bugs *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 = BU_GS_FULL;
    else if ((gmin > 0 && gmax > 0) || gmax < 0)
        ca->state = BU_GS_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_bugs_backoff(struct sock *sk, u32 grad)
{
    struct bugs *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 = BU_GS_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 bugs *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_bugs_cong_avoid(struct sock *sk, u32 ack, u32 acked)
{
    struct bugs *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_bugs_hystart_update(sk);

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

    if (ca->rtt_prev.v64)
        grad = tcp_bugs_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_bugs_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_bugs_acked(struct sock *sk, u32 num_acked, s32 rtt_us)
{
    struct bugs *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);

    if (rtt_us > 0)
    ca->east_rtt = usecs_to_jiffies(rtt_us);
}

static u32 tcp_bugs_ssthresh(struct sock *sk)
{
    struct bugs *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 == BU_GS_BACKOFF)
    return max(2U, (tp->snd_cwnd * min(1024U, backoff_beta)) >> 10);

    if (ca->state == BU_GS_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_bugs_undo_cwnd(struct sock *sk)
{
    struct bugs *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_bugs_init(struct sock *sk)
{
    struct bugs *ca = inet_csk_ca(sk);
    struct tcp_sock *tp = tcp_sk(sk);

    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;
    // eastwood
    ca->bk = 0;
    ca->bw_ns_est = 0;
    ca->bw_est = 0;
    ca->accounted = 0;
    ca->cumul_ack = 0;
    ca->reset_rtt_min = 1;
    ca->rtt_min = ca->east_rtt = TCP_EASTWOOD_INIT_RTT;
    ca->rtt_win_sx = tcp_time_stamp;
    ca->snd_una = tcp_sk(sk)->snd_una;
    ca->first_ack = 1;
}

static void tcp_bugs_release(struct sock *sk)
{
    struct bugs *ca = inet_csk_ca(sk);

    kfree(ca->gradients);
}

static inline u32 eastwood_do_filter(u32 a, u32 b)
{
    return ((7 * a) + b) >> 3;
}

static void eastwood_filter(struct bugs *ca, u32 delta)
{
    if (ca->bw_ns_est == 0 && ca->bw_est == 0) {
    ca->bw_ns_est = ca->bk / delta;
    ca->bw_est = ca->bw_ns_est;
    } else {
    ca->bw_ns_est = eastwood_do_filter(ca->bw_ns_est, ca->bk / delta);
    ca->bw_est = eastwood_do_filter(ca->bw_est, ca->bw_ns_est);
    }
}

static void eastwood_update_window(struct sock *sk)
{
    struct bugs *ca = inet_csk_ca(sk);
    s32 delta = tcp_time_stamp - ca->rtt_win_sx;

    if (ca->first_ack) {
    ca->snd_una = tcp_sk(sk)->snd_una;
    ca->first_ack = 0;
    }

    if (ca->east_rtt && delta > max_t(u32, ca->east_rtt, TCP_EASTWOOD_RTT_MIN)) {
    eastwood_filter(ca, delta);
    ca->bk = 0;
    ca->rtt_win_sx = tcp_time_stamp;
    }
}

static inline void update_rtt_min(struct bugs *ca)
{
    if (ca->reset_rtt_min) {
    ca->rtt_min = ca->east_rtt;
    ca->reset_rtt_min = 0;
    } else
    ca->rtt_min = min(ca->east_rtt, ca->rtt_min);
}

static inline void eastwood_fast_bw(struct sock *sk)
{
    const struct tcp_sock *tp = tcp_sk(sk);
    struct bugs *ca = inet_csk_ca(sk);

    eastwood_update_window(sk);

    ca->bk += tp->snd_una - ca->snd_una;
    ca->snd_una = tp->snd_una;
    update_rtt_min(ca);
}

static inline u32 eastwood_acked_count(struct sock *sk)
{
    const struct tcp_sock *tp = tcp_sk(sk);
    struct bugs *ca = inet_csk_ca(sk);

    ca->cumul_ack = tp->snd_una - ca->snd_una;

    if (!ca->cumul_ack) {
    ca->accounted += tp->mss_cache;
    ca->cumul_ack = tp->mss_cache;
    }

    if (ca->cumul_ack > tp->mss_cache) {
    /* Partial or delayed ack */
    if (ca->accounted >= ca->cumul_ack) {
        ca->accounted -= ca->cumul_ack;
        ca->cumul_ack = tp->mss_cache;
    } else {
        ca->cumul_ack -= ca->accounted;
        ca->accounted = 0;
    }
    }
    ca->snd_una = tp->snd_una;
    return ca->cumul_ack;
}

static u32 tcp_eastwood_bw_rttmin(const struct sock *sk)
{
    const struct tcp_sock *tp = tcp_sk(sk);
    const struct bugs *ca = inet_csk_ca(sk);
    return max_t(u32, (ca->bw_est * ca->rtt_min) / tp->mss_cache, 2);
}

static int bugs_main(struct sock *sk, struct rate_sample *rs)
{
    struct inet_connection_sock *icsk = inet_csk(sk);
    struct tcp_sock *tp = tcp_sk(sk);
    struct bugs *ca = inet_csk_ca(sk);

    if (!shadow_grow) {
    goto eastwood;
    }

    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 == BU_GS_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_bugs_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 == BU_GS_NONFULL || shadow_fast) {
        tcp_cong_avoid_ai_shadow(sk, ca->shadow_wnd, rs->acked_sacked);   
        tp->snd_cwnd = ca->shadow_wnd;
        }
    }
    } 

eastwood:
    if (use_sack) {
    eastwood_update_window(sk); 
    ca->bk += rs->acked_sacked * tp->mss_cache;
    update_rtt_min(ca);
    }
    return CONG_CONT;
}

static void bugs_state(struct sock *sk, u8 new_state)
{
    struct inet_connection_sock *icsk = inet_csk(sk);
    struct bugs *ca = inet_csk_ca(sk);
    struct tcp_sock *tp = tcp_sk(sk);

    if (!recovery_restore) {
    return;
    }

    if (icsk->icsk_ca_state && new_state == TCP_CA_Open) {
    ca->rtt_seq = tp->snd_nxt;
    ca->rtt_prev = ca->rtt;
    ca->rtt.v64 = 0;
    ca->state = BU_GS_UNKNOWN;
    }
    if (new_state == TCP_CA_Open && ca->state == BU_GS_NONFULL) {
    tp->snd_cwnd = max(max(tp->snd_cwnd, ca->shadow_wnd), 2U);
    }
    if (new_state == TCP_CA_Loss) {
    if (ca->state == BU_GS_NONFULL && use_tolerance) {
        tp->snd_cwnd = ca->shadow_wnd;
        if (loss_push)
        tcp_push_pending_frames(sk);
    } 
    }
}

static void tcp_bugs_cwnd_event(struct sock *sk, const enum tcp_ca_event ev)
{
    struct bugs *ca = inet_csk_ca(sk);
    struct tcp_sock *tp = tcp_sk(sk);
    struct bugs_minmax *gradients;

    switch (ev) {
    case CA_EVENT_FAST_ACK:
        if (!use_sack)
        eastwood_fast_bw(sk);
        break;
    case CA_EVENT_SLOW_ACK:
        if (!use_sack) {
        eastwood_update_window(sk);
        ca->bk += eastwood_acked_count(sk);
        update_rtt_min(ca);
        }
        break;
    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:
        tp->snd_cwnd = tp->snd_ssthresh = tcp_eastwood_bw_rttmin(sk);
        break;
    case CA_EVENT_LOSS:
        tp->snd_ssthresh = tcp_eastwood_bw_rttmin(sk);
        ca->reset_rtt_min = 1;
        break;
    default:
        break;
    }
}


struct tcp_congestion_ops tcp_bugs __read_mostly = {
    .cong_avoid = tcp_bugs_cong_avoid,
    .cong_collect   = bugs_main,
    .set_state = bugs_state,
    .cwnd_event = tcp_bugs_cwnd_event,
    .pkts_acked = tcp_bugs_acked,
    .undo_cwnd = tcp_bugs_undo_cwnd,
    .ssthresh = tcp_bugs_ssthresh,
    .release = tcp_bugs_release,
    .init = tcp_bugs_init,
    .owner = THIS_MODULE,
    .name = "bugs",
};

static int __init tcp_bugs_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 bugs) > ICSK_CA_PRIV_SIZE);
    tcp_register_congestion_control(&tcp_bugs);
    return 0;
}

static void __exit tcp_bugs_unregister(void)
{
    tcp_unregister_congestion_control(&tcp_bugs);
}

module_init(tcp_bugs_register);
module_exit(tcp_bugs_unregister);
MODULE_AUTHOR("CAO NI MA");
MODULE_LICENSE("GPL");
MODULE_DESCRIPTION("TCP BU_GS");