/*
   * H-TCP congestion control. The algorithm is detailed in:
   * R.N.Shorten, D.J.Leith:
   *   "H-TCP: TCP for high-speed and long-distance networks"
   *   Proc. PFLDnet, Argonne, 2004.
   * http://www.hamilton.ie/net/htcp3.pdf
   */
  
  #include <linux/mm.h>
  #include <linux/module.h>
  #include <net/tcp.h>
  
  #define ALPHA_BASE	(1<<7)	/* 1.0 with shift << 7 */
  #define BETA_MIN	(1<<6)	/* 0.5 with shift << 7 */
  #define BETA_MAX	102	/* 0.8 with shift << 7 */
  
  static int use_rtt_scaling __read_mostly = 1;
  module_param(use_rtt_scaling, int, 0644);
  MODULE_PARM_DESC(use_rtt_scaling, "turn on/off RTT scaling");
  
  static int use_bandwidth_switch __read_mostly = 1;
  module_param(use_bandwidth_switch, int, 0644);
  MODULE_PARM_DESC(use_bandwidth_switch, "turn on/off bandwidth switcher");
  
  struct htcp {
  	u32	alpha;		/* Fixed point arith, << 7 */
  	u8	beta;           /* Fixed point arith, << 7 */
  	u8	modeswitch;	/* Delay modeswitch
  				   until we had at least one congestion event */
  	u16	pkts_acked;
  	u32	packetcount;
  	u32	minRTT;
  	u32	maxRTT;
  	u32	last_cong;	/* Time since last congestion event end */
  	u32	undo_last_cong;
  
  	u32	undo_maxRTT;
  	u32	undo_old_maxB;
  
  	/* Bandwidth estimation */
  	u32	minB;
  	u32	maxB;
  	u32	old_maxB;
  	u32	Bi;
  	u32	lasttime;
  };
  
  static inline u32 htcp_cong_time(const struct htcp *ca)
  {
  	return jiffies - ca->last_cong;
  }
  
  static inline u32 htcp_ccount(const struct htcp *ca)
  {
  	return htcp_cong_time(ca) / ca->minRTT;
  }
  
  static inline void htcp_reset(struct htcp *ca)
  {
  	ca->undo_last_cong = ca->last_cong;
  	ca->undo_maxRTT = ca->maxRTT;
  	ca->undo_old_maxB = ca->old_maxB;
  
  	ca->last_cong = jiffies;
  }
  
  static u32 htcp_cwnd_undo(struct sock *sk)
  {
  	const struct tcp_sock *tp = tcp_sk(sk);
  	struct htcp *ca = inet_csk_ca(sk);
  
  	if (ca->undo_last_cong) {
  		ca->last_cong = ca->undo_last_cong;
  		ca->maxRTT = ca->undo_maxRTT;
  		ca->old_maxB = ca->undo_old_maxB;
  		ca->undo_last_cong = 0;
  	}
  
  	return max(tp->snd_cwnd, (tp->snd_ssthresh << 7) / ca->beta);
  }
  
  static inline void measure_rtt(struct sock *sk, u32 srtt)
  {
  	const struct inet_connection_sock *icsk = inet_csk(sk);
  	struct htcp *ca = inet_csk_ca(sk);
  
  	/* keep track of minimum RTT seen so far, minRTT is zero at first */
  	if (ca->minRTT > srtt || !ca->minRTT)
  		ca->minRTT = srtt;
  
  	/* max RTT */
  	if (icsk->icsk_ca_state == TCP_CA_Open) {
  		if (ca->maxRTT < ca->minRTT)
  			ca->maxRTT = ca->minRTT;
  		if (ca->maxRTT < srtt &&
  		    srtt <= ca->maxRTT + msecs_to_jiffies(20))
  			ca->maxRTT = srtt;
  	}
  }
  
  static void measure_achieved_throughput(struct sock *sk, u32 pkts_acked, s32 rtt)
  {
  	const struct inet_connection_sock *icsk = inet_csk(sk);
  	const struct tcp_sock *tp = tcp_sk(sk);
  	struct htcp *ca = inet_csk_ca(sk);
  	u32 now = tcp_time_stamp;
  
  	if (icsk->icsk_ca_state == TCP_CA_Open)
  		ca->pkts_acked = pkts_acked;
  
  	if (rtt > 0)
  		measure_rtt(sk, usecs_to_jiffies(rtt));
  
  	if (!use_bandwidth_switch)
  		return;
  
  	/* achieved throughput calculations */
  	if (!((1 << icsk->icsk_ca_state) & (TCPF_CA_Open | TCPF_CA_Disorder))) {
  		ca->packetcount = 0;
  		ca->lasttime = now;
  		return;
  	}
  
  	ca->packetcount += pkts_acked;
  
  	if (ca->packetcount >= tp->snd_cwnd - (ca->alpha >> 7 ? : 1) &&
  	    now - ca->lasttime >= ca->minRTT &&
  	    ca->minRTT > 0) {
  		__u32 cur_Bi = ca->packetcount * HZ / (now - ca->lasttime);
  
  		if (htcp_ccount(ca) <= 3) {
  			/* just after backoff */
  			ca->minB = ca->maxB = ca->Bi = cur_Bi;
  		} else {
  			ca->Bi = (3 * ca->Bi + cur_Bi) / 4;
  			if (ca->Bi > ca->maxB)
  				ca->maxB = ca->Bi;
  			if (ca->minB > ca->maxB)
  				ca->minB = ca->maxB;
  		}
  		ca->packetcount = 0;
  		ca->lasttime = now;
  	}
  }
  
  static inline void htcp_beta_update(struct htcp *ca, u32 minRTT, u32 maxRTT)
  {
  	if (use_bandwidth_switch) {
  		u32 maxB = ca->maxB;
  		u32 old_maxB = ca->old_maxB;
  		ca->old_maxB = ca->maxB;
  
  		if (!between(5 * maxB, 4 * old_maxB, 6 * old_maxB)) {
  			ca->beta = BETA_MIN;
  			ca->modeswitch = 0;
  			return;
  		}
  	}
  
  	if (ca->modeswitch && minRTT > msecs_to_jiffies(10) && maxRTT) {
  		ca->beta = (minRTT << 7) / maxRTT;
  		if (ca->beta < BETA_MIN)
  			ca->beta = BETA_MIN;
  		else if (ca->beta > BETA_MAX)
  			ca->beta = BETA_MAX;
  	} else {
  		ca->beta = BETA_MIN;
  		ca->modeswitch = 1;
  	}
  }
  
  static inline void htcp_alpha_update(struct htcp *ca)
  {
  	u32 minRTT = ca->minRTT;
  	u32 factor = 1;
  	u32 diff = htcp_cong_time(ca);
  
  	if (diff > HZ) {
  		diff -= HZ;
  		factor = 1 + (10 * diff + ((diff / 2) * (diff / 2) / HZ)) / HZ;
  	}
  
  	if (use_rtt_scaling && minRTT) {
  		u32 scale = (HZ << 3) / (10 * minRTT);
  
  		/* clamping ratio to interval [0.5,10]<<3 */
  		scale = min(max(scale, 1U << 2), 10U << 3);
  		factor = (factor << 3) / scale;
  		if (!factor)
  			factor = 1;
  	}
  
  	ca->alpha = 2 * factor * ((1 << 7) - ca->beta);
  	if (!ca->alpha)
  		ca->alpha = ALPHA_BASE;
  }
  
  /*
   * After we have the rtt data to calculate beta, we'd still prefer to wait one
   * rtt before we adjust our beta to ensure we are working from a consistent
   * data.
   *
   * This function should be called when we hit a congestion event since only at
   * that point do we really have a real sense of maxRTT (the queues en route
   * were getting just too full now).
   */
  static void htcp_param_update(struct sock *sk)
  {
  	struct htcp *ca = inet_csk_ca(sk);
  	u32 minRTT = ca->minRTT;
  	u32 maxRTT = ca->maxRTT;
  
  	htcp_beta_update(ca, minRTT, maxRTT);
  	htcp_alpha_update(ca);
  
  	/* add slowly fading memory for maxRTT to accommodate routing changes */
  	if (minRTT > 0 && maxRTT > minRTT)
  		ca->maxRTT = minRTT + ((maxRTT - minRTT) * 95) / 100;
  }
  
  static u32 htcp_recalc_ssthresh(struct sock *sk)
  {
  	const struct tcp_sock *tp = tcp_sk(sk);
  	const struct htcp *ca = inet_csk_ca(sk);
  
  	htcp_param_update(sk);
  	return max((tp->snd_cwnd * ca->beta) >> 7, 2U);
  }
  
  static void htcp_cong_avoid(struct sock *sk, u32 ack, u32 acked, u32 in_flight)
  {
  	struct tcp_sock *tp = tcp_sk(sk);
  	struct htcp *ca = inet_csk_ca(sk);
  
  	if (!tcp_is_cwnd_limited(sk, in_flight))
  		return;
  
  	if (tp->snd_cwnd <= tp->snd_ssthresh)
  		tcp_slow_start(tp, acked);
  	else {
  		/* In dangerous area, increase slowly.
  		 * In theory this is tp->snd_cwnd += alpha / tp->snd_cwnd
  		 */
  		if ((tp->snd_cwnd_cnt * ca->alpha)>>7 >= tp->snd_cwnd) {
  			if (tp->snd_cwnd < tp->snd_cwnd_clamp)
  				tp->snd_cwnd++;
  			tp->snd_cwnd_cnt = 0;
  			htcp_alpha_update(ca);
  		} else
  			tp->snd_cwnd_cnt += ca->pkts_acked;
  
  		ca->pkts_acked = 1;
  	}
  }
  
  static void htcp_init(struct sock *sk)
  {
  	struct htcp *ca = inet_csk_ca(sk);
  
  	memset(ca, 0, sizeof(struct htcp));
  	ca->alpha = ALPHA_BASE;
  	ca->beta = BETA_MIN;
  	ca->pkts_acked = 1;
  	ca->last_cong = jiffies;
  }
  
  static void htcp_state(struct sock *sk, u8 new_state)
  {
  	switch (new_state) {
  	case TCP_CA_Open:
  		{
  			struct htcp *ca = inet_csk_ca(sk);
  			if (ca->undo_last_cong) {
  				ca->last_cong = jiffies;
  				ca->undo_last_cong = 0;
  			}
  		}
  		break;
  	case TCP_CA_CWR:
  	case TCP_CA_Recovery:
  	case TCP_CA_Loss:
  		htcp_reset(inet_csk_ca(sk));
  		break;
  	}
  }
  
  static struct tcp_congestion_ops htcp __read_mostly = {
  	.init		= htcp_init,
  	.ssthresh	= htcp_recalc_ssthresh,
  	.cong_avoid	= htcp_cong_avoid,
  	.set_state	= htcp_state,
  	.undo_cwnd	= htcp_cwnd_undo,
  	.pkts_acked	= measure_achieved_throughput,
  	.owner		= THIS_MODULE,
  	.name		= "htcp",
  };
  
  static int __init htcp_register(void)
  {
  	BUILD_BUG_ON(sizeof(struct htcp) > ICSK_CA_PRIV_SIZE);
  	BUILD_BUG_ON(BETA_MIN >= BETA_MAX);
  	return tcp_register_congestion_control(&htcp);
  }
  
  static void __exit htcp_unregister(void)
  {
  	tcp_unregister_congestion_control(&htcp);
  }
  
  module_init(htcp_register);
  module_exit(htcp_unregister);
  
  MODULE_AUTHOR("Baruch Even");
  MODULE_LICENSE("GPL");
  MODULE_DESCRIPTION("H-TCP");