net/ipv4/tcp_cubic.c - third_party/linux - Git at Google

 /*
  * TCP CUBIC: Binary Increase Congestion control for TCP v2.0
  *
  * This is from the implementation of CUBIC TCP in
  * Injong Rhee, Lisong Xu.
  *  "CUBIC: A New TCP-Friendly High-Speed TCP Variant
  *  in PFLDnet 2005
  * Available from:
  *  http://www.csc.ncsu.edu/faculty/rhee/export/bitcp/cubic-paper.pdf
  *
  * 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 <net/tcp.h>
 #include <asm/div64.h>

 #define BICTCP_BETA_SCALE    1024	/* Scale factor beta calculation
 					 * max_cwnd = snd_cwnd * beta
 					 */
 #define BICTCP_B		4	 /*
 					  * In binary search,
 					  * go to point (max+min)/N
 					  */
 #define	BICTCP_HZ		10	/* BIC HZ 2^10 = 1024 */

 static int fast_convergence = 1;
 static int max_increment = 16;
 static int beta = 819;		/* = 819/1024 (BICTCP_BETA_SCALE) */
 static int initial_ssthresh = 100;
 static int bic_scale = 41;
 static int tcp_friendliness = 1;

 static u32 cube_rtt_scale;
 static u32 beta_scale;
 static u64 cube_factor;

 /* 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(max_increment, int, 0644);
 MODULE_PARM_DESC(max_increment, "Limit on increment allowed during binary search");
 module_param(beta, int, 0444);
 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");

 #include <asm/div64.h>

 /* 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 */
 	u32	epoch_start;	/* beginning of an epoch */
 	u32	ack_cnt;	/* number of acks */
 	u32	tcp_cwnd;	/* estimated tcp cwnd */
 #define ACK_RATIO_SHIFT	4
 	u32	delayed_ack;	/* estimate the ratio of Packets/ACKs << 4 */
 };

 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;
 }

 static void bictcp_init(struct sock *sk)
 {
 	bictcp_reset(inet_csk_ca(sk));
 	if (initial_ssthresh)
 		tcp_sk(sk)->snd_ssthresh = initial_ssthresh;
 }

 /* 64bit divisor, dividend and result. dynamic precision */
 static inline u_int64_t div64_64(u_int64_t dividend, u_int64_t divisor)
 {
 	u_int32_t d = divisor;

 	if (divisor > 0xffffffffULL) {
 		unsigned int shift = fls(divisor >> 32);

 		d = divisor >> shift;
 		dividend >>= shift;
 	}

 	/* avoid 64 bit division if possible */
 	if (dividend >> 32)
 		do_div(dividend, d);
 	else
 		dividend = (uint32_t) dividend / d;

 	return dividend;
 }

 /*
  * calculate the cubic root of x using Newton-Raphson
  */
 static u32 cubic_root(u64 a)
 {
 	u32 x, x1;

 	/* Initial estimate is based on:
 	 * cbrt(x) = exp(log(x) / 3)
 	 */
 	x = 1u << (fls64(a)/3);

 	/*
 	 * Iteration based on:
 	 *                         2
 	 * x    = ( 2 * x  +  a / x  ) / 3
 	 *  k+1          k         k
 	 */
 	do {
 		x1 = x;
 		x = (2 * x + (uint32_t) div64_64(a, x*x)) / 3;
 	} while (abs(x1 - x) > 1);

 	return x;
 }

 /*
  * Compute congestion window to use.
  */
 static inline void bictcp_update(struct bictcp *ca, u32 cwnd)
 {
 	u64 offs;
 	u32 delta, t, bic_target, min_cnt, max_cnt;

 	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 !!!
          */

 	/* change the unit from HZ to bictcp_HZ */
         t = ((tcp_time_stamp + ca->delay_min - ca->epoch_start)
 	     << BICTCP_HZ) / 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*/
         }

 	if (ca->delay_min > 0) {
 		/* max increment = Smax * rtt / 0.1  */
 		min_cnt = (cwnd * HZ * 8)/(10 * max_increment * ca->delay_min);
 		if (ca->cnt < min_cnt)
 			ca->cnt = min_cnt;
 	}

         /* slow start and low utilization  */
 	if (ca->loss_cwnd == 0)		/* could be aggressive in slow start */
 		ca->cnt = 50;

 	/* 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;
 }


 /* Keep track of minimum rtt */
 static inline void measure_delay(struct sock *sk)
 {
 	const struct tcp_sock *tp = tcp_sk(sk);
 	struct bictcp *ca = inet_csk_ca(sk);
 	u32 delay;

 	/* No time stamp */
 	if (!(tp->rx_opt.saw_tstamp && tp->rx_opt.rcv_tsecr) ||
 	     /* Discard delay samples right after fast recovery */
 	    (s32)(tcp_time_stamp - ca->epoch_start) < HZ)
 		return;

 	delay = tcp_time_stamp - tp->rx_opt.rcv_tsecr;
 	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;
 }

 static void bictcp_cong_avoid(struct sock *sk, u32 ack,
 			      u32 seq_rtt, u32 in_flight, int data_acked)
 {
 	struct tcp_sock *tp = tcp_sk(sk);
 	struct bictcp *ca = inet_csk_ca(sk);

 	if (data_acked)
 		measure_delay(sk);

 	if (!tcp_is_cwnd_limited(sk, in_flight))
 		return;

 	if (tp->snd_cwnd <= tp->snd_ssthresh)
 		tcp_slow_start(tp);
 	else {
 		bictcp_update(ca, tp->snd_cwnd);

 		/* In dangerous area, increase slowly.
 		 * In theory this is tp->snd_cwnd += 1 / tp->snd_cwnd
 		 */
 		if (tp->snd_cwnd_cnt >= ca->cnt) {
 			if (tp->snd_cwnd < tp->snd_cwnd_clamp)
 				tp->snd_cwnd++;
 			tp->snd_cwnd_cnt = 0;
 		} else
 			tp->snd_cwnd_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));
 }

 /* Track delayed acknowledgment ratio using sliding window
  * ratio = (15*ratio + sample) / 16
  */
 static void bictcp_acked(struct sock *sk, u32 cnt)
 {
 	const struct inet_connection_sock *icsk = inet_csk(sk);

 	if (cnt > 0 && icsk->icsk_ca_state == TCP_CA_Open) {
 		struct bictcp *ca = inet_csk_ca(sk);
 		cnt -= ca->delayed_ack >> ACK_RATIO_SHIFT;
 		ca->delayed_ack += cnt;
 	}
 }


 static struct tcp_congestion_ops cubictcp = {
 	.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 << 3) / 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);

 	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("2.0");
	/*
	* TCP CUBIC: Binary Increase Congestion control for TCP v2.0
	*
	* This is from the implementation of CUBIC TCP in
	* Injong Rhee, Lisong Xu.
	* "CUBIC: A New TCP-Friendly High-Speed TCP Variant
	* in PFLDnet 2005
	* Available from:
	* http://www.csc.ncsu.edu/faculty/rhee/export/bitcp/cubic-paper.pdf
	*
	* 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 <net/tcp.h>
	#include <asm/div64.h>

	#define BICTCP_BETA_SCALE 1024 /* Scale factor beta calculation
	* max_cwnd = snd_cwnd * beta
	*/
	#define BICTCP_B 4 /*
	* In binary search,
	* go to point (max+min)/N
	*/
	#define BICTCP_HZ 10 /* BIC HZ 2^10 = 1024 */

	static int fast_convergence = 1;
	static int max_increment = 16;
	static int beta = 819; /* = 819/1024 (BICTCP_BETA_SCALE) */
	static int initial_ssthresh = 100;
	static int bic_scale = 41;
	static int tcp_friendliness = 1;

	static u32 cube_rtt_scale;
	static u32 beta_scale;
	static u64 cube_factor;

	/* 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(max_increment, int, 0644);
	MODULE_PARM_DESC(max_increment, "Limit on increment allowed during binary search");
	module_param(beta, int, 0444);
	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");

	#include <asm/div64.h>

	/* 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 */
	u32 epoch_start; /* beginning of an epoch */
	u32 ack_cnt; /* number of acks */
	u32 tcp_cwnd; /* estimated tcp cwnd */
	#define ACK_RATIO_SHIFT 4
	u32 delayed_ack; /* estimate the ratio of Packets/ACKs << 4 */
	};

	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;
	}

	static void bictcp_init(struct sock *sk)
	{
	bictcp_reset(inet_csk_ca(sk));
	if (initial_ssthresh)
	tcp_sk(sk)->snd_ssthresh = initial_ssthresh;
	}

	/* 64bit divisor, dividend and result. dynamic precision */
	static inline u_int64_t div64_64(u_int64_t dividend, u_int64_t divisor)
	{
	u_int32_t d = divisor;

	if (divisor > 0xffffffffULL) {
	unsigned int shift = fls(divisor >> 32);

	d = divisor >> shift;
	dividend >>= shift;
	}

	/* avoid 64 bit division if possible */
	if (dividend >> 32)
	do_div(dividend, d);
	else
	dividend = (uint32_t) dividend / d;

	return dividend;
	}

	/*
	* calculate the cubic root of x using Newton-Raphson
	*/
	static u32 cubic_root(u64 a)
	{
	u32 x, x1;

	/* Initial estimate is based on:
	* cbrt(x) = exp(log(x) / 3)
	*/
	x = 1u << (fls64(a)/3);

	/*
	* Iteration based on:
	* 2
	* x = ( 2 * x + a / x ) / 3
	* k+1 k k
	*/
	do {
	x1 = x;
	x = (2 * x + (uint32_t) div64_64(a, x*x)) / 3;
	} while (abs(x1 - x) > 1);

	return x;
	}

	/*
	* Compute congestion window to use.
	*/
	static inline void bictcp_update(struct bictcp *ca, u32 cwnd)
	{
	u64 offs;
	u32 delta, t, bic_target, min_cnt, max_cnt;

	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 !!!
	*/

	/* change the unit from HZ to bictcp_HZ */
	t = ((tcp_time_stamp + ca->delay_min - ca->epoch_start)
	<< BICTCP_HZ) / 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*/
	}

	if (ca->delay_min > 0) {
	/* max increment = Smax * rtt / 0.1 */
	min_cnt = (cwnd * HZ * 8)/(10 * max_increment * ca->delay_min);
	if (ca->cnt < min_cnt)
	ca->cnt = min_cnt;
	}

	/* slow start and low utilization */
	if (ca->loss_cwnd == 0) /* could be aggressive in slow start */
	ca->cnt = 50;

	/* 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;
	}


	/* Keep track of minimum rtt */
	static inline void measure_delay(struct sock *sk)
	{
	const struct tcp_sock *tp = tcp_sk(sk);
	struct bictcp *ca = inet_csk_ca(sk);
	u32 delay;

	/* No time stamp */
	if (!(tp->rx_opt.saw_tstamp && tp->rx_opt.rcv_tsecr) \|\|
	/* Discard delay samples right after fast recovery */
	(s32)(tcp_time_stamp - ca->epoch_start) < HZ)
	return;

	delay = tcp_time_stamp - tp->rx_opt.rcv_tsecr;
	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;
	}

	static void bictcp_cong_avoid(struct sock *sk, u32 ack,
	u32 seq_rtt, u32 in_flight, int data_acked)
	{
	struct tcp_sock *tp = tcp_sk(sk);
	struct bictcp *ca = inet_csk_ca(sk);

	if (data_acked)
	measure_delay(sk);

	if (!tcp_is_cwnd_limited(sk, in_flight))
	return;

	if (tp->snd_cwnd <= tp->snd_ssthresh)
	tcp_slow_start(tp);
	else {
	bictcp_update(ca, tp->snd_cwnd);

	/* In dangerous area, increase slowly.
	* In theory this is tp->snd_cwnd += 1 / tp->snd_cwnd
	*/
	if (tp->snd_cwnd_cnt >= ca->cnt) {
	if (tp->snd_cwnd < tp->snd_cwnd_clamp)
	tp->snd_cwnd++;
	tp->snd_cwnd_cnt = 0;
	} else
	tp->snd_cwnd_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));
	}

	/* Track delayed acknowledgment ratio using sliding window
	* ratio = (15*ratio + sample) / 16
	*/
	static void bictcp_acked(struct sock *sk, u32 cnt)
	{
	const struct inet_connection_sock *icsk = inet_csk(sk);

	if (cnt > 0 && icsk->icsk_ca_state == TCP_CA_Open) {
	struct bictcp *ca = inet_csk_ca(sk);
	cnt -= ca->delayed_ack >> ACK_RATIO_SHIFT;
	ca->delayed_ack += cnt;
	}
	}


	static struct tcp_congestion_ops cubictcp = {
	.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 << 3) / 10; /* 1024c/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 * 2^2bictcp_HZ srtt */
	cube_factor = 1ull << (10+3BICTCP_HZ); / 2^40 */

	/* divide by bic_scale and by constant Srtt (100ms) */
	do_div(cube_factor, bic_scale * 10);

	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("2.0");