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>
  
  #define BICTCP_BETA_SCALE    1024	/* Scale factor beta calculation
  					 * max_cwnd = snd_cwnd * beta
  					 */
  #define	BICTCP_HZ		10	/* BIC HZ 2^10 = 1024 */
  
  /* Two methods of hybrid slow start */
  #define HYSTART_ACK_TRAIN	0x1
  #define HYSTART_DELAY		0x2
  
  /* Number of delay samples for detecting the increase of delay */
  #define HYSTART_MIN_SAMPLES	8
  #define HYSTART_DELAY_MIN	(4U<<3)
  #define HYSTART_DELAY_MAX	(16U<<3)
  #define HYSTART_DELAY_THRESH(x)	clamp(x, HYSTART_DELAY_MIN, HYSTART_DELAY_MAX)
  
  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;
  
  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 int hystart_ack_delta __read_mostly = 2;
  
  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");
  module_param(hystart_ack_delta, int, 0644);
  MODULE_PARM_DESC(hystart_ack_delta, "spacing between ack's indicating train (msecs)");
  
  /* BIC TCP Parameters */
  struct bictcp {
  	u32	cnt;		/* increase cwnd by 1 after ACKs */
  	u32 	last_max_cwnd;	/* last maximum snd_cwnd */
  	u32	loss_cwnd;	/* congestion window at last loss */
  	u32	last_cwnd;	/* the last snd_cwnd */
  	u32	last_time;	/* time when updated last_cwnd */
  	u32	bic_origin_point;/* origin point of bic function */
  	u32	bic_K;		/* time to origin point from the beginning of the current epoch */
  	u32	delay_min;	/* min delay (msec << 3) */
  	u32	epoch_start;	/* beginning of an epoch */
  	u32	ack_cnt;	/* number of acks */
  	u32	tcp_cwnd;	/* estimated tcp cwnd */
  #define ACK_RATIO_SHIFT	4
  #define ACK_RATIO_LIMIT (32u << ACK_RATIO_SHIFT)
  	u16	delayed_ack;	/* estimate the ratio of Packets/ACKs << 4 */
  	u8	sample_cnt;	/* number of samples to decide curr_rtt */
  	u8	found;		/* the exit point is found? */
  	u32	round_start;	/* beginning of each round */
  	u32	end_seq;	/* end_seq of the round */
  	u32	last_ack;	/* last time when the ACK spacing is close */
  	u32	curr_rtt;	/* the minimum rtt of current round */
  };
  
  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;
  }
  
  static inline u32 bictcp_clock(void)
  {
  #if HZ < 1000
  	return ktime_to_ms(ktime_get_real());
  #else
  	return jiffies_to_msecs(jiffies);
  #endif
  }
  
  static inline void bictcp_hystart_reset(struct sock *sk)
  {
  	struct tcp_sock *tp = tcp_sk(sk);
  	struct bictcp *ca = inet_csk_ca(sk);
  
  	ca->round_start = ca->last_ack = bictcp_clock();
  	ca->end_seq = tp->snd_nxt;
  	ca->curr_rtt = 0;
  	ca->sample_cnt = 0;
  }
  
  static void bictcp_init(struct sock *sk)
  {
  	bictcp_reset(inet_csk_ca(sk));
  
  	if (hystart)
  		bictcp_hystart_reset(sk);
  
  	if (!hystart && initial_ssthresh)
  		tcp_sk(sk)->snd_ssthresh = 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;
  }
  
  /*
   * Compute congestion window to use.
   */
  static inline void bictcp_update(struct bictcp *ca, u32 cwnd)
  {
  	u32 delta, bic_target, max_cnt;
  	u64 offs, t;
  
  	ca->ack_cnt++;	/* count the number of ACKs */
  
  	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;
  
  	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 */
  
  		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)
  			 */
  			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
  	 * rtt  = (srtt >> 3) / HZ
  	 * !!! The following code does not have overflow problems,
  	 * if the cwnd < 1 million packets !!!
  	 */
  
  	t = (s32)(tcp_time_stamp - ca->epoch_start);
  	t += msecs_to_jiffies(ca->delay_min >> 3);
  	/* change the unit from HZ to bictcp_HZ */
  	t <<= BICTCP_HZ;
  	do_div(t, HZ);
  
  	if (t < ca->bic_K)		/* t - K */
  		offs = ca->bic_K - t;
  	else
  		offs = t - ca->bic_K;
  
  	/* c/rtt * (t-K)^3 */
  	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*/
  	if (bic_target > cwnd) {
  		ca->cnt = cwnd / (bic_target - cwnd);
  	} else {
  		ca->cnt = 100 * cwnd;              /* very small increment*/
  	}
  
  	/*
  	 * The initial growth of cubic function may be too conservative
  	 * when the available bandwidth is still unknown.
  	 */
  	if (ca->loss_cwnd == 0 && ca->cnt > 20)
  		ca->cnt = 20;	/* increase cwnd 5% per RTT */
  
  	/* TCP Friendly */
  	if (tcp_friendliness) {
  		u32 scale = beta_scale;
  		delta = (cwnd * scale) >> 3;
  		while (ca->ack_cnt > delta) {		/* update tcp cwnd */
  			ca->ack_cnt -= delta;
  			ca->tcp_cwnd++;
  		}
  
  		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;
  }
  
  static void bictcp_cong_avoid(struct sock *sk, u32 ack, u32 in_flight)
  {
  	struct tcp_sock *tp = tcp_sk(sk);
  	struct bictcp *ca = inet_csk_ca(sk);
  
  	if (!tcp_is_cwnd_limited(sk, in_flight))
  		return;
  
  	if (tp->snd_cwnd <= tp->snd_ssthresh) {
  		if (hystart && after(ack, ca->end_seq))
  			bictcp_hystart_reset(sk);
  		tcp_slow_start(tp);
  	} else {
  		bictcp_update(ca, tp->snd_cwnd);
  		tcp_cong_avoid_ai(tp, ca->cnt);
  	}
  
  }
  
  static u32 bictcp_recalc_ssthresh(struct sock *sk)
  {
  	const struct tcp_sock *tp = tcp_sk(sk);
  	struct bictcp *ca = inet_csk_ca(sk);
  
  	ca->epoch_start = 0;	/* end of epoch */
  
  	/* Wmax and fast convergence */
  	if (tp->snd_cwnd < ca->last_max_cwnd && fast_convergence)
  		ca->last_max_cwnd = (tp->snd_cwnd * (BICTCP_BETA_SCALE + beta))
  			/ (2 * BICTCP_BETA_SCALE);
  	else
  		ca->last_max_cwnd = tp->snd_cwnd;
  
  	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(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(sk));
  		bictcp_hystart_reset(sk);
  	}
  }
  
  static void hystart_update(struct sock *sk, u32 delay)
  {
  	struct tcp_sock *tp = tcp_sk(sk);
  	struct bictcp *ca = inet_csk_ca(sk);
  
  	if (!(ca->found & hystart_detect)) {
  		u32 now = bictcp_clock();
  
  		/* first detection parameter - ack-train detection */
  		if ((s32)(now - ca->last_ack) <= hystart_ack_delta) {
  			ca->last_ack = now;
  			if ((s32)(now - 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(sk);
  	u32 delay;
  
  	if (icsk->icsk_ca_state == TCP_CA_Open) {
  		u32 ratio = ca->delayed_ack;
  
  		ratio -= ca->delayed_ack >> ACK_RATIO_SHIFT;
  		ratio += cnt;
  
  		ca->delayed_ack = min(ratio, ACK_RATIO_LIMIT);
  	}
  
  	/* Some calls are for duplicates without timetamps */
  	if (rtt_us < 0)
  		return;
  
  	/* Discard delay samples right after fast recovery */
  	if (ca->epoch_start && (s32)(tcp_time_stamp - ca->epoch_start) < HZ)
  		return;
  
  	delay = (rtt_us << 3) / USEC_PER_MSEC;
  	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);
  }
  
  static struct tcp_congestion_ops cubictcp __read_mostly = {
  	.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",
  };
  
  static int __init cubictcp_register(void)
  {
  	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*(BICTCP_BETA_SCALE+beta)/ 3 / (BICTCP_BETA_SCALE - beta);
  
  	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);
  
  	/* hystart needs ms clock resolution */
  	if (hystart && HZ < 1000)
  		cubictcp.flags |= TCP_CONG_RTT_STAMP;
  
  	return tcp_register_congestion_control(&cubictcp);
  }
  
  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");
  