/*
   * TCP HYBLA
   *
   * TCP-HYBLA Congestion control algorithm, based on:
   *   C.Caini, R.Firrincieli, "TCP-Hybla: A TCP Enhancement
   *   for Heterogeneous Networks",
   *   International Journal on satellite Communications,
   *				       September 2004
   *    Daniele Lacamera
   *    root at danielinux.net
   */
  
  #include <linux/module.h>
  #include <net/tcp.h>
  
  /* Tcp Hybla structure. */
  struct hybla {
  	bool  hybla_en;
  	u32   snd_cwnd_cents; /* Keeps increment values when it is <1, <<7 */
  	u32   rho;	      /* Rho parameter, integer part  */
  	u32   rho2;	      /* Rho * Rho, integer part */
  	u32   rho_3ls;	      /* Rho parameter, <<3 */
  	u32   rho2_7ls;	      /* Rho^2, <<7	*/
  	u32   minrtt;	      /* Minimum smoothed round trip time value seen */
  };
  
  /* Hybla reference round trip time (default= 1/40 sec = 25 ms), in ms */
  static int rtt0 = 25;
  module_param(rtt0, int, 0644);
  MODULE_PARM_DESC(rtt0, "reference rout trip time (ms)");
  
  
  /* This is called to refresh values for hybla parameters */
  static inline void hybla_recalc_param (struct sock *sk)
  {
  	struct hybla *ca = inet_csk_ca(sk);
  
  	ca->rho_3ls = max_t(u32, tcp_sk(sk)->srtt / msecs_to_jiffies(rtt0), 8);
  	ca->rho = ca->rho_3ls >> 3;
  	ca->rho2_7ls = (ca->rho_3ls * ca->rho_3ls) << 1;
  	ca->rho2 = ca->rho2_7ls >> 7;
  }
  
  static void hybla_init(struct sock *sk)
  {
  	struct tcp_sock *tp = tcp_sk(sk);
  	struct hybla *ca = inet_csk_ca(sk);
  
  	ca->rho = 0;
  	ca->rho2 = 0;
  	ca->rho_3ls = 0;
  	ca->rho2_7ls = 0;
  	ca->snd_cwnd_cents = 0;
  	ca->hybla_en = true;
  	tp->snd_cwnd = 2;
  	tp->snd_cwnd_clamp = 65535;
  
  	/* 1st Rho measurement based on initial srtt */
  	hybla_recalc_param(sk);
  
  	/* set minimum rtt as this is the 1st ever seen */
  	ca->minrtt = tp->srtt;
  	tp->snd_cwnd = ca->rho;
  }
  
  static void hybla_state(struct sock *sk, u8 ca_state)
  {
  	struct hybla *ca = inet_csk_ca(sk);
  
  	ca->hybla_en = (ca_state == TCP_CA_Open);
  }
  
  static inline u32 hybla_fraction(u32 odds)
  {
  	static const u32 fractions[] = {
  		128, 139, 152, 165, 181, 197, 215, 234,
  	};
  
  	return (odds < ARRAY_SIZE(fractions)) ? fractions[odds] : 128;
  }
  
  /* TCP Hybla main routine.
   * This is the algorithm behavior:
   *     o Recalc Hybla parameters if min_rtt has changed
   *     o Give cwnd a new value based on the model proposed
   *     o remember increments <1
   */
  static void hybla_cong_avoid(struct sock *sk, u32 ack, u32 acked,
  			     u32 in_flight)
  {
  	struct tcp_sock *tp = tcp_sk(sk);
  	struct hybla *ca = inet_csk_ca(sk);
  	u32 increment, odd, rho_fractions;
  	int is_slowstart = 0;
  
  	/*  Recalculate rho only if this srtt is the lowest */
  	if (tp->srtt < ca->minrtt){
  		hybla_recalc_param(sk);
  		ca->minrtt = tp->srtt;
  	}
  
  	if (!tcp_is_cwnd_limited(sk, in_flight))
  		return;
  
  	if (!ca->hybla_en) {
  		tcp_reno_cong_avoid(sk, ack, acked, in_flight);
  		return;
  	}
  
  	if (ca->rho == 0)
  		hybla_recalc_param(sk);
  
  	rho_fractions = ca->rho_3ls - (ca->rho << 3);
  
  	if (tp->snd_cwnd < tp->snd_ssthresh) {
  		/*
  		 * slow start
  		 *      INC = 2^RHO - 1
  		 * This is done by splitting the rho parameter
  		 * into 2 parts: an integer part and a fraction part.
  		 * Inrement<<7 is estimated by doing:
  		 *	       [2^(int+fract)]<<7
  		 * that is equal to:
  		 *	       (2^int)	*  [(2^fract) <<7]
  		 * 2^int is straightly computed as 1<<int,
  		 * while we will use hybla_slowstart_fraction_increment() to
  		 * calculate 2^fract in a <<7 value.
  		 */
  		is_slowstart = 1;
  		increment = ((1 << min(ca->rho, 16U)) *
  			hybla_fraction(rho_fractions)) - 128;
  	} else {
  		/*
  		 * congestion avoidance
  		 * INC = RHO^2 / W
  		 * as long as increment is estimated as (rho<<7)/window
  		 * it already is <<7 and we can easily count its fractions.
  		 */
  		increment = ca->rho2_7ls / tp->snd_cwnd;
  		if (increment < 128)
  			tp->snd_cwnd_cnt++;
  	}
  
  	odd = increment % 128;
  	tp->snd_cwnd += increment >> 7;
  	ca->snd_cwnd_cents += odd;
  
  	/* check when fractions goes >=128 and increase cwnd by 1. */
  	while (ca->snd_cwnd_cents >= 128) {
  		tp->snd_cwnd++;
  		ca->snd_cwnd_cents -= 128;
  		tp->snd_cwnd_cnt = 0;
  	}
  	/* check when cwnd has not been incremented for a while */
  	if (increment == 0 && odd == 0 && tp->snd_cwnd_cnt >= tp->snd_cwnd) {
  		tp->snd_cwnd++;
  		tp->snd_cwnd_cnt = 0;
  	}
  	/* clamp down slowstart cwnd to ssthresh value. */
  	if (is_slowstart)
  		tp->snd_cwnd = min(tp->snd_cwnd, tp->snd_ssthresh);
  
  	tp->snd_cwnd = min_t(u32, tp->snd_cwnd, tp->snd_cwnd_clamp);
  }
  
  static struct tcp_congestion_ops tcp_hybla __read_mostly = {
  	.init		= hybla_init,
  	.ssthresh	= tcp_reno_ssthresh,
  	.min_cwnd	= tcp_reno_min_cwnd,
  	.cong_avoid	= hybla_cong_avoid,
  	.set_state	= hybla_state,
  
  	.owner		= THIS_MODULE,
  	.name		= "hybla"
  };
  
  static int __init hybla_register(void)
  {
  	BUILD_BUG_ON(sizeof(struct hybla) > ICSK_CA_PRIV_SIZE);
  	return tcp_register_congestion_control(&tcp_hybla);
  }
  
  static void __exit hybla_unregister(void)
  {
  	tcp_unregister_congestion_control(&tcp_hybla);
  }
  
  module_init(hybla_register);
  module_exit(hybla_unregister);
  
  MODULE_AUTHOR("Daniele Lacamera");
  MODULE_LICENSE("GPL");
  MODULE_DESCRIPTION("TCP Hybla");