text: /*
   * TCP CUBIC: Binary Increase Congestion control for TCP v2.3
   * Home page:
   *      http://netsrv.csc.ncsu.edu/twiki/bin/view/Main/BIC
   * This is from the implementation of CUBIC TCP in
   * Sangtae Ha, Injong Rhee and Lisong Xu,
   *  "CUBIC: A New TCP-Friendly High-Speed TCP Variant"
   *  in ACM SIGOPS Operating System Review, July 2008.
   * Available from:
   *  http://netsrv.csc.ncsu.edu/export/cubic_a_new_tcp_2008.pdf
   *
   * CUBIC integrates a new slow start algorithm, called HyStart.
   * The details of HyStart are presented in
   *  Sangtae Ha and Injong Rhee,
   *  "Taming the Elephants: New TCP Slow Start", NCSU TechReport 2008.
   * Available from:
   *  http://netsrv.csc.ncsu.edu/export/hystart_techreport_2008.pdf
   *
   * All testing results are available from:
   * http://netsrv.csc.ncsu.edu/wiki/index.php/TCP_Testing
   *
   * Unless CUBIC is enabled and congestion window is large
   * this behaves the same as the original Reno.
   */
  /*
  #include <linux/mm.h>
  #include <linux/module.h>
  #include <linux/math64.h>
  #include <net/tcp.h>
  */
  #include "tcp_cubic.h"
  #include <stdio.h>
  
  #ifdef __cplusplus
  extern "C" {
  #endif
  
  static int fast_convergence __read_mostly = 1;
  static int beta __read_mostly = 717;	/* = 717/1024 (BICTCP_BETA_SCALE) */
  static int initial_ssthresh __read_mostly;
  static int bic_scale __read_mostly = 41;
  static int tcp_friendliness __read_mostly = 1;/* 友好性 */
  
  static int hystart __read_mostly = 1;
  static int hystart_detect __read_mostly = HYSTART_ACK_TRAIN | HYSTART_DELAY;
  static int hystart_low_window __read_mostly = 16;
  
  static u32 cube_rtt_scale __read_mostly;
  static u32 beta_scale __read_mostly;
  static u64 cube_factor __read_mostly;
  
  /* Note parameters that are used for precomputing scale factors are read-only */
  module_param(fast_convergence, int, 0644);
  MODULE_PARM_DESC(fast_convergence, "turn on/off fast convergence");
  module_param(beta, int, 0644);
  MODULE_PARM_DESC(beta, "beta for multiplicative increase");
  module_param(initial_ssthresh, int, 0644);
  MODULE_PARM_DESC(initial_ssthresh, "initial value of slow start threshold");
  module_param(bic_scale, int, 0444);
  MODULE_PARM_DESC(bic_scale, "scale (scaled by 1024) value for bic function (bic_scale/1024)");
  module_param(tcp_friendliness, int, 0644);
  MODULE_PARM_DESC(tcp_friendliness, "turn on/off tcp friendliness");
  module_param(hystart, int, 0644);
  MODULE_PARM_DESC(hystart, "turn on/off hybrid slow start algorithm");
  module_param(hystart_detect, int, 0644);
  MODULE_PARM_DESC(hystart_detect, "hyrbrid slow start detection mechanisms"
  		 " 1: packet-train 2: delay 3: both packet-train and delay");
  module_param(hystart_low_window, int, 0644);
  MODULE_PARM_DESC(hystart_low_window, "lower bound cwnd for hybrid slow start");
  
  
  
  
  static inline void bictcp_reset(struct bictcp *ca)
  {
  	ca->cnt = 0;
  	ca->last_max_cwnd = 0;
  	ca->loss_cwnd = 0;
  	ca->last_cwnd = 0;
  	ca->last_time = 0;
  	ca->bic_origin_point = 0;
  	ca->bic_K = 0;
  	ca->delay_min = 0;
  	ca->epoch_start = 0;
  	ca->delayed_ack = 2 << ACK_RATIO_SHIFT;
  	ca->ack_cnt = 0;
  	ca->tcp_cwnd = 0;
  	ca->found = 0;
  }
  
  extern unsigned long volatile __jiffy_data jiffies;
  
  static inline void bictcp_hystart_reset(struct sock *sk)
  {
  	struct tcp_sock *tp = tcp_sk(sk);
  	struct bictcp *ca = inet_csk_ca_cubic(sk);
  
  	ca->round_start = ca->last_jiffies = jiffies;
  	ca->end_seq = tp->snd_nxt;
  	ca->curr_rtt = 0;
  	ca->sample_cnt = 0;
  }
  
  static void bictcp_init(struct sock *sk)
  {
      printf("调用cubic算法\n");
  	bictcp_reset(inet_csk_ca_cubic(sk));
  
  	if (hystart)
  		bictcp_hystart_reset(sk);
  
  	if (!hystart && initial_ssthresh)
  		tcp_sk(sk)->snd_ssthresh = initial_ssthresh;
  	//printf("initial_ssthresh = %d\n", initial_ssthresh);
  }
  
  /* calculate the cubic root of x using a table lookup followed by one
   * Newton-Raphson iteration.
   * Avg err ~= 0.195%
   */
  static u32 cubic_root(u64 a)
  {
  	u32 x, b, shift;
  	/*
  	 * cbrt(x) MSB values for x MSB values in [0..63].
  	 * Precomputed then refined by hand - Willy Tarreau
  	 *
  	 * For x in [0..63],
  	 *   v = cbrt(x << 18) - 1
  	 *   cbrt(x) = (v[x] + 10) >> 6
  	 */
  	static const u8 v[] = {
  		/* 0x00 */    0,   54,   54,   54,  118,  118,  118,  118,
  		/* 0x08 */  123,  129,  134,  138,  143,  147,  151,  156,
  		/* 0x10 */  157,  161,  164,  168,  170,  173,  176,  179,
  		/* 0x18 */  181,  185,  187,  190,  192,  194,  197,  199,
  		/* 0x20 */  200,  202,  204,  206,  209,  211,  213,  215,
  		/* 0x28 */  217,  219,  221,  222,  224,  225,  227,  229,
  		/* 0x30 */  231,  232,  234,  236,  237,  239,  240,  242,
  		/* 0x38 */  244,  245,  246,  248,  250,  251,  252,  254,
  	};
  
  	b = fls64(a);
  	if (b < 7) {
  		/* a in [0..63] */
  		return ((u32)v[(u32)a] + 35) >> 6;
  	}
  
  	b = ((b * 84) >> 8) - 1;
  	shift = (a >> (b * 3));
  
  	x = ((u32)(((u32)v[shift] + 10) << b)) >> 6;
  
  	/*
  	 * Newton-Raphson iteration
  	 *                         2
  	 * x    = ( 2 * x  +  a / x  ) / 3
  	 *  k+1          k         k
  	 */
  	x = (2 * x + (u32)div64_u64(a, (u64)x * (u64)(x - 1)));
  	x = ((x * 341) >> 10);
  	return x;
  }
  
  /*
  函数关键点
  
  1.  我们最终要得到的是ca->cnt，用来控制snd_cwnd的增长。
  
  2.  ca->cnt的值，是根据cwnd和w( t + after ) 的大小来判断的。
  	w( t + after )即bic_target，它表示我们预期的
  
  在经过after时间后的snd_cwnd。如果此时cwnd < w( t + after )，那么我们就快速增加窗口，达到预期目标。
  
  如果cwnd > w( t + after )，那说明我们已经增加过快了，需要降速了，这样才能达到预期目标。
  
                cwnd / (bic_target - cwnd )   // bic_target > cwnd 
  
  cnt =   100 * cwnd   // bic_target < cwnd 
  
  3.  cwnd是传入的参数，已知。现在我们只需要计算bic_target。
  
  而根据Cubic的窗口增长函数：W(t) = C(t - K)^3 + Wmax，
  
  我们要计算时间( 当前 + after )，以及时间K。时间K即bic_K，表示函数值为Wmax所对应的时间。
  
  通过代码可以发现，after为min RTT，即连接的传播时延。
  
  4.  然后就是bic_K和t的计算了，详细可看代码。
   */
  /*
   * 计算拥塞窗口,Compute congestion window to use.
   */
  static inline void bictcp_update(struct bictcp *ca, u32 cwnd)
  {
  	u64 offs;/* 时间差，| t - K | */  
  
  	/* delta是cwnd差，bic_target是预测值，t为预测时间 */ 
  	u32 delta, t, bic_target, max_cnt;
  
  	ca->ack_cnt++;	/* count the number of ACKs */
  
  	 /* 31.25ms以内不更新ca！！！*/  
  	if (ca->last_cwnd == cwnd &&
  	    (s32)(tcp_time_stamp - ca->last_time) <= HZ / 32)
  		return;
  
  	ca->last_cwnd = cwnd;
  	ca->last_time = tcp_time_stamp;
  
  	/*丢包后 一个新的时段 */  
  	if (ca->epoch_start == 0) {
  		ca->epoch_start = tcp_time_stamp;	/* record the beginning of an epoch */
  		ca->ack_cnt = 1;			/* start counting */
  		ca->tcp_cwnd = cwnd;			/* syn with cubic */
  
  		/* 取max(last_max_cwnd , cwnd)作为当前Wmax */  
  		if (ca->last_max_cwnd <= cwnd) {
  			ca->bic_K = 0;
  			ca->bic_origin_point = cwnd;
  		} else {
  			/* Compute new K based on
  			 * (wmax-cwnd) * (srtt>>3 / HZ) / c * 2^(3*bictcp_HZ)
  			 * bic_K本来单位为秒，转成单位为 1 / 1024秒
  			 */
  			ca->bic_K = cubic_root(cube_factor
  					       * (ca->last_max_cwnd - cwnd));
  			ca->bic_origin_point = ca->last_max_cwnd;
  		}
  	}
  
  
  	/* cubic function - calc*/
  	/* calculate c * time^3 / rtt,
  	 *  while considering overflow in calculation of time^3
  	 * (so time^3 is done by using 64 bit)
  	 * and without the support of division of 64bit numbers
  	 * (so all divisions are done by using 32 bit)
  	 *  also NOTE the unit of those veriables
  	 *	  time  = (t - K) / 2^bictcp_HZ
  	 *	  c = bic_scale >> 10
  	 * Constant = c / srtt = 0.4, 实际参数为0.4 
  	 * rtt  = (srtt >> 3) / HZ
  	 * !!! The following code does not have overflow problems,
  	 * if the cwnd < 1 million packets !!!
  	 * 预测时间为：ca->delay_min >> 3后
  	 */
  
  	/* change the unit from HZ to bictcp_HZ */
  	t = ((tcp_time_stamp + (ca->delay_min>>3) - ca->epoch_start)
  	     << BICTCP_HZ) / HZ;
  
  	 /* 求| t - bic_K | */  
  	if (t < ca->bic_K)		/* t - K */
  		offs = ca->bic_K - t;
  	else
  		offs = t - ca->bic_K;/* 此时已经超过Wmax */
  
  	/* c/rtt * (t-K)^3 */
  	/* 计算delta =| W(t) - W(bic_K) |  
       * cube_rtt_scale = (bic_scale * 10) = c / srtt * 2^10，c/srtt = 0.4 
       */ 
  	delta = (cube_rtt_scale * offs * offs * offs) >> (10+3*BICTCP_HZ);
  	if (t < ca->bic_K)                                	/* below origin*/
  		bic_target = ca->bic_origin_point - delta;
  	else                                                	/* above origin*/
  		bic_target = ca->bic_origin_point + delta;
  
  	/* cubic function - calc bictcp_cnt*/
  	/* 计算bic_target，即预测cwnd */ 
  	if (bic_target > cwnd) {
  		/* 相差越多，增长越快，这就是函数形状由来 */  
  		ca->cnt = cwnd / (bic_target - cwnd);
  	} else {
  		/* very small increment，目前cwnd已经超出预期了，应该降速 */
  		ca->cnt = 100 * cwnd;              /* very small increment*/
  	}
  	/* TCP Friendly —如果bic比RENO慢，则提升cwnd增长速度，即减小cnt 
       * 以上次丢包以后的时间t算起，每次RTT增长 3B / ( 2 - B)，那么可以得到 
       * 采用RENO算法的cwnd。 
       * cwnd (RENO) = cwnd + 3B / (2 - B) * ack_cnt / cwnd 
       * B为乘性减少因子，在此算法中为0.3 
       */ 
  	if (tcp_friendliness) {
  		u32 scale = 15;//通过计算获得
  		delta = (cwnd * scale) >> 3;/* delta代表多少ACK可使tcp_cwnd++ */ 
  		//printf("cwnd = %d, scale = %d\n", cwnd, scale);
  		//printf("入bictcp_update\n");
  		while (ca->ack_cnt > delta) {		/* update tcp cwnd */
  		//printf("ca->ack_cnt = %d, delta=%d\n", ca->ack_cnt, delta);
  			ca->ack_cnt -= delta;
  			ca->tcp_cwnd++;
  		}
  		//printf("出bictcp_update\n");
  
  		if (ca->tcp_cwnd > cwnd){	/* if bic is slower than tcp */
  			delta = ca->tcp_cwnd - cwnd;
  			max_cnt = cwnd / delta;
  			if (ca->cnt > max_cnt)
  				ca->cnt = max_cnt;
  		}
  	}
  
  	ca->cnt = (ca->cnt << ACK_RATIO_SHIFT) / ca->delayed_ack;
  	if (ca->cnt == 0)			/* cannot be zero */
  		ca->cnt = 1;/* 此时代表cwnd远小于bic_target，增长速度最大 */
  }
  
  /*
   * 拥塞避免
   */
  static void bictcp_cong_avoid(struct sock *sk, u32 ack, u32 in_flight)
  {
  	//printf("bictcp_cong_avoid\n");
  	struct tcp_sock *tp = tcp_sk(sk);
  	struct bictcp *ca = inet_csk_ca_cubic(sk);
  
  	 // 如果传输中的报文数量 <= 窗口数量，那么没有必要执行拥塞避免
  	// 如果发送拥塞窗口不被限制，不能再增加，则返回 
  	if (!tcp_is_cwnd_limited(sk, in_flight))
  		return;
  
  	/*  希望收到的窗口大小比门限小,则进入慢启动.否则计算拥塞窗口,并进入拥塞避免阶段. */
  	/*  after(ack, ca->end_seq): 表示end_seq-ack < 0 返回1, end_seq-ack >= 0 返回0*/
  	if (tp->snd_cwnd <= tp->snd_ssthresh) {
  		/*如果ack 包的序号大于,前面ACK 过数据*/
  		if (hystart && after(ack, ca->end_seq))
  			bictcp_hystart_reset(sk);
  		
  		//printf("进入慢启动,snd_cwnd=%d, snd_ssthresh=%d\n",tp->snd_cwnd,tp->snd_ssthresh);
  		tcp_slow_start(tp);
  	} else {
  		;//printf("进入拥塞避免,snd_cwnd=%d, snd_ssthresh=%d\n",tp->snd_cwnd,tp->snd_ssthresh);
  		bictcp_update(ca, tp->snd_cwnd);
  		tcp_cong_avoid_ai(tp, ca->cnt);
  	}
  
  }
  
  /**
   * 重新计算门限
   */
  static u32 bictcp_recalc_ssthresh(struct sock *sk)
  {
  
  	const struct tcp_sock *tp = tcp_sk(sk);
  	struct bictcp *ca = inet_csk_ca_cubic(sk);
  	printf("重新计算门限,snd_cwnd=%d, snd_ssthresh=%d\n",tp->snd_cwnd,tp->snd_ssthresh);
  
  	ca->epoch_start = 0;	/* end of epoch */
  
  	/* 当发送窗口小于上一次的最大窗口的时候,快速收敛;Wmax and fast convergence */
  	/*如果第二拥塞, 并且cwnd 值小于前面一次的cwnd, 进入该流程*/
  	if (tp->snd_cwnd < ca->last_max_cwnd && fast_convergence)
  	{
  		//last_max_cwnd = (snd_cwnd * (1024+717))/(2*1024)
  		ca->last_max_cwnd = (tp->snd_cwnd * (BICTCP_BETA_SCALE + beta))
  			/ (2 * BICTCP_BETA_SCALE);
  	}
  	else
  	{
  		ca->last_max_cwnd = tp->snd_cwnd;
  	}
  
  	/*丢包时候的窗口值*/
  	ca->loss_cwnd = tp->snd_cwnd;
  
  	return max((tp->snd_cwnd * beta) / BICTCP_BETA_SCALE, 2U);
  }
  
  static u32 bictcp_undo_cwnd(struct sock *sk)
  {
  	struct bictcp *ca = inet_csk_ca_cubic(sk);
  
  	return max(tcp_sk(sk)->snd_cwnd, ca->last_max_cwnd);
  }
  
  static void bictcp_state(struct sock *sk, u8 new_state)
  {
  	if (new_state == TCP_CA_Loss) {
  		bictcp_reset(inet_csk_ca_cubic(sk));
  		bictcp_hystart_reset(sk);
  	}
  }
  
  /*Hybrid Start算法*/
  static void hystart_update(struct sock *sk, u32 delay)
  {
  	//printf("--->进入Hybrid Start\n");
  	struct tcp_sock *tp = tcp_sk(sk);
  	struct bictcp *ca = inet_csk_ca_cubic(sk);
  
  	//如果找到了, 只能在下次出现丢包的时候才能在次进入该流程
  	if (!(ca->found & hystart_detect)) {
  		u32 curr_jiffies = jiffies;
  
  		/* first detection parameter - ack-train detection */
  		/*如果ack  过来的速度小于等于2 ms */
  		if (curr_jiffies - ca->last_jiffies <= msecs_to_jiffies(2)) {
  			ca->last_jiffies = curr_jiffies;
  			if (curr_jiffies - ca->round_start >= ca->delay_min>>4)
  				ca->found |= HYSTART_ACK_TRAIN;
  		}
  
  		/* obtain the minimum delay of more than sampling packets */
  		if (ca->sample_cnt < HYSTART_MIN_SAMPLES) {
  			if (ca->curr_rtt == 0 || ca->curr_rtt > delay)
  				ca->curr_rtt = delay;
  
  			ca->sample_cnt++;
  		} else {
  			if (ca->curr_rtt > ca->delay_min + HYSTART_DELAY_THRESH(ca->delay_min>>4))
  				ca->found |= HYSTART_DELAY;
  		}
  		/*
  		 * Either one of two conditions are met,
  		 * we exit from slow start immediately.
  		 */
  		if (ca->found & hystart_detect);
  			tp->snd_ssthresh = tp->snd_cwnd;
  	}
  }
  
  /* Track delayed acknowledgment ratio using sliding window
   * ratio = (15*ratio + sample) / 16
   */
  static void bictcp_acked(struct sock *sk, u32 cnt, s32 rtt_us)
  {
  
  	const struct inet_connection_sock *icsk = inet_csk(sk);
  	const struct tcp_sock *tp = tcp_sk(sk);
  	struct bictcp *ca = inet_csk_ca_cubic(sk);
  	u32 delay;
  
  	//printf("入bictcp_acked,snd_cwnd=%d, snd_ssthresh=%d\n",tp->snd_cwnd,tp->snd_ssthresh);
  
  	if (icsk->icsk_ca_state == TCP_CA_Open) {
  		cnt -= ca->delayed_ack >> ACK_RATIO_SHIFT;
  		ca->delayed_ack += cnt;
  	}
  
  	/* Some calls are for duplicates without timetamps */
  	if (rtt_us < 0)
  		return;
  
  	/* Discard delay samples right after fast recovery */
  	if ((s32)(tcp_time_stamp - ca->epoch_start) < HZ)
  		return;
  
  	delay = usecs_to_jiffies(rtt_us) << 3;
  	if (delay == 0)
  		delay = 1;
  
  	/* first time call or link delay decreases */
  	if (ca->delay_min == 0 || ca->delay_min > delay)
  		ca->delay_min = delay;
  
  	/* hystart triggers when cwnd is larger than some threshold */
  	if (hystart && tp->snd_cwnd <= tp->snd_ssthresh &&
  	    tp->snd_cwnd >= hystart_low_window)
  		hystart_update(sk, delay);
  	
  }
  
  struct tcp_congestion_ops tcp_cubic = {
  	.init		= bictcp_init,
  	.ssthresh	= bictcp_recalc_ssthresh,
  	.cong_avoid	= bictcp_cong_avoid,
  	.set_state	= bictcp_state,
  	.undo_cwnd	= bictcp_undo_cwnd,
  	.pkts_acked = bictcp_acked,
  	.owner		= THIS_MODULE,
  	.name		= "cubic",
  };
  
  /*注册tcp_cubic算法*/
  static int __init cubictcp_register(void)
  {
  	/* 始终会成立,因为bictcp结构的大小< 16*4字节*/
  	BUILD_BUG_ON(sizeof(struct bictcp) > ICSK_CA_PRIV_SIZE);
  
  	/* Precompute a bunch of the scaling factors that are used per-packet
  	 * based on SRTT of 100ms
  	 * beta_scale = 8*(1024 + 717) / 3 / (1024 -717 )，大约为15 
  	 */
  
  	beta_scale = 8 * (BICTCP_BETA_SCALE + beta) / 3 / (BICTCP_BETA_SCALE - beta);
  
  	/* 1024 * c / rtt ，值为410 
  	 * c = bic_scale >> 10 = 41 / 2^10 = 0.04 
       *  rtt = 100ms = 0.1s 
       * 如此算来，cube_rtt_scale = 1024 * c / rtt 
       * c / rtt = 0.4 
       */ 
  	cube_rtt_scale = (bic_scale * 10);	/* 1024*c/rtt */
  
  
  	/* calculate the "K" for (wmax-cwnd) = c/rtt * K^3
  	 *  so K = cubic_root( (wmax-cwnd)*rtt/c )
  	 * the unit of K is bictcp_HZ=2^10, not HZ
  	 *
  	 *  c = bic_scale >> 10
  	 *  rtt = 100ms
  	 *
  	 * the following code has been designed and tested for
  	 * cwnd < 1 million packets
  	 * RTT < 100 seconds
  	 * HZ < 1,000,00  (corresponding to 10 nano-second)
  	 */
  
  	/* 1/c * 2^2*bictcp_HZ * srtt */
  	cube_factor = 1ull << (10+3*BICTCP_HZ); /* 2^40 */
  
  	/* divide by bic_scale and by constant Srtt (100ms) */
  	do_div(cube_factor, bic_scale * 10);
  
  	// 注册拥塞控制算法
  	tcp_register_congestion_control(&cubictcp);
  	return 0;
  }
  
  static void __exit cubictcp_unregister(void)
  {
  	tcp_unregister_congestion_control(&cubictcp);
  }
  
  module_init(cubictcp_register);
  module_exit(cubictcp_unregister);
  
  MODULE_AUTHOR("Sangtae Ha, Stephen Hemminger");
  MODULE_LICENSE("GPL");
  MODULE_DESCRIPTION("CUBIC TCP");
  MODULE_VERSION("2.3");
  
  #ifdef __cplusplus
  }
  #endif
  