/*
   * TCP Illinois congestion control.
   * Home page:
   *	http://www.ews.uiuc.edu/~shaoliu/tcpillinois/index.html
   *
   * The algorithm is described in:
   * "TCP-Illinois: A Loss and Delay-Based Congestion Control Algorithm
   *  for High-Speed Networks"
   * http://www.ifp.illinois.edu/~srikant/Papers/liubassri06perf.pdf
   *
   * Implemented from description in paper and ns-2 simulation.
   * Copyright (C) 2007 Stephen Hemminger <shemminger@linux-foundation.org>
   */
  
  #include <linux/module.h>
  #include <linux/skbuff.h>
  #include <linux/inet_diag.h>
  #include <asm/div64.h>
  #include <net/tcp.h>
  
  #define ALPHA_SHIFT	7
  #define ALPHA_SCALE	(1u<<ALPHA_SHIFT)
  #define ALPHA_MIN	((3*ALPHA_SCALE)/10)	/* ~0.3 */
  #define ALPHA_MAX	(10*ALPHA_SCALE)	/* 10.0 */
  #define ALPHA_BASE	ALPHA_SCALE		/* 1.0 */
  #define RTT_MAX		(U32_MAX / ALPHA_MAX)	/* 3.3 secs */
  
  #define BETA_SHIFT	6
  #define BETA_SCALE	(1u<<BETA_SHIFT)
  #define BETA_MIN	(BETA_SCALE/8)		/* 0.125 */
  #define BETA_MAX	(BETA_SCALE/2)		/* 0.5 */
  #define BETA_BASE	BETA_MAX
  
  static int win_thresh __read_mostly = 15;
  module_param(win_thresh, int, 0);
  MODULE_PARM_DESC(win_thresh, "Window threshold for starting adaptive sizing");
  
  static int theta __read_mostly = 5;
  module_param(theta, int, 0);
  MODULE_PARM_DESC(theta, "# of fast RTT's before full growth");
  
  /* TCP Illinois Parameters */
  struct illinois {
  	u64	sum_rtt;	/* sum of rtt's measured within last rtt */
  	u16	cnt_rtt;	/* # of rtts measured within last rtt */
  	u32	base_rtt;	/* min of all rtt in usec */
  	u32	max_rtt;	/* max of all rtt in usec */
  	u32	end_seq;	/* right edge of current RTT */
  	u32	alpha;		/* Additive increase */
  	u32	beta;		/* Muliplicative decrease */
  	u16	acked;		/* # packets acked by current ACK */
  	u8	rtt_above;	/* average rtt has gone above threshold */
  	u8	rtt_low;	/* # of rtts measurements below threshold */
  };
  
  static void rtt_reset(struct sock *sk)
  {
  	struct tcp_sock *tp = tcp_sk(sk);
  	struct illinois *ca = inet_csk_ca(sk);
  
  	ca->end_seq = tp->snd_nxt;
  	ca->cnt_rtt = 0;
  	ca->sum_rtt = 0;
  
  	/* TODO: age max_rtt? */
  }
  
  static void tcp_illinois_init(struct sock *sk)
  {
  	struct illinois *ca = inet_csk_ca(sk);
  
  	ca->alpha = ALPHA_MAX;
  	ca->beta = BETA_BASE;
  	ca->base_rtt = 0x7fffffff;
  	ca->max_rtt = 0;
  
  	ca->acked = 0;
  	ca->rtt_low = 0;
  	ca->rtt_above = 0;
  
  	rtt_reset(sk);
  }
  
  /* Measure RTT for each ack. */
  static void tcp_illinois_acked(struct sock *sk, u32 pkts_acked, s32 rtt)
  {
  	struct illinois *ca = inet_csk_ca(sk);
  
  	ca->acked = pkts_acked;
  
  	/* dup ack, no rtt sample */
  	if (rtt < 0)
  		return;
  
  	/* ignore bogus values, this prevents wraparound in alpha math */
  	if (rtt > RTT_MAX)
  		rtt = RTT_MAX;
  
  	/* keep track of minimum RTT seen so far */
  	if (ca->base_rtt > rtt)
  		ca->base_rtt = rtt;
  
  	/* and max */
  	if (ca->max_rtt < rtt)
  		ca->max_rtt = rtt;
  
  	++ca->cnt_rtt;
  	ca->sum_rtt += rtt;
  }
  
  /* Maximum queuing delay */
  static inline u32 max_delay(const struct illinois *ca)
  {
  	return ca->max_rtt - ca->base_rtt;
  }
  
  /* Average queuing delay */
  static inline u32 avg_delay(const struct illinois *ca)
  {
  	u64 t = ca->sum_rtt;
  
  	do_div(t, ca->cnt_rtt);
  	return t - ca->base_rtt;
  }
  
  /*
   * Compute value of alpha used for additive increase.
   * If small window then use 1.0, equivalent to Reno.
   *
   * For larger windows, adjust based on average delay.
   * A. If average delay is at minimum (we are uncongested),
   *    then use large alpha (10.0) to increase faster.
   * B. If average delay is at maximum (getting congested)
   *    then use small alpha (0.3)
   *
   * The result is a convex window growth curve.
   */
  static u32 alpha(struct illinois *ca, u32 da, u32 dm)
  {
  	u32 d1 = dm / 100;	/* Low threshold */
  
  	if (da <= d1) {
  		/* If never got out of low delay zone, then use max */
  		if (!ca->rtt_above)
  			return ALPHA_MAX;
  
  		/* Wait for 5 good RTT's before allowing alpha to go alpha max.
  		 * This prevents one good RTT from causing sudden window increase.
  		 */
  		if (++ca->rtt_low < theta)
  			return ca->alpha;
  
  		ca->rtt_low = 0;
  		ca->rtt_above = 0;
  		return ALPHA_MAX;
  	}
  
  	ca->rtt_above = 1;
  
  	/*
  	 * Based on:
  	 *
  	 *      (dm - d1) amin amax
  	 * k1 = -------------------
  	 *         amax - amin
  	 *
  	 *       (dm - d1) amin
  	 * k2 = ----------------  - d1
  	 *        amax - amin
  	 *
  	 *             k1
  	 * alpha = ----------
  	 *          k2 + da
  	 */
  
  	dm -= d1;
  	da -= d1;
  	return (dm * ALPHA_MAX) /
  		(dm + (da  * (ALPHA_MAX - ALPHA_MIN)) / ALPHA_MIN);
  }
  
  /*
   * Beta used for multiplicative decrease.
   * For small window sizes returns same value as Reno (0.5)
   *
   * If delay is small (10% of max) then beta = 1/8
   * If delay is up to 80% of max then beta = 1/2
   * In between is a linear function
   */
  static u32 beta(u32 da, u32 dm)
  {
  	u32 d2, d3;
  
  	d2 = dm / 10;
  	if (da <= d2)
  		return BETA_MIN;
  
  	d3 = (8 * dm) / 10;
  	if (da >= d3 || d3 <= d2)
  		return BETA_MAX;
  
  	/*
  	 * Based on:
  	 *
  	 *       bmin d3 - bmax d2
  	 * k3 = -------------------
  	 *         d3 - d2
  	 *
  	 *       bmax - bmin
  	 * k4 = -------------
  	 *         d3 - d2
  	 *
  	 * b = k3 + k4 da
  	 */
  	return (BETA_MIN * d3 - BETA_MAX * d2 + (BETA_MAX - BETA_MIN) * da)
  		/ (d3 - d2);
  }
  
  /* Update alpha and beta values once per RTT */
  static void update_params(struct sock *sk)
  {
  	struct tcp_sock *tp = tcp_sk(sk);
  	struct illinois *ca = inet_csk_ca(sk);
  
  	if (tp->snd_cwnd < win_thresh) {
  		ca->alpha = ALPHA_BASE;
  		ca->beta = BETA_BASE;
  	} else if (ca->cnt_rtt > 0) {
  		u32 dm = max_delay(ca);
  		u32 da = avg_delay(ca);
  
  		ca->alpha = alpha(ca, da, dm);
  		ca->beta = beta(da, dm);
  	}
  
  	rtt_reset(sk);
  }
  
  /*
   * In case of loss, reset to default values
   */
  static void tcp_illinois_state(struct sock *sk, u8 new_state)
  {
  	struct illinois *ca = inet_csk_ca(sk);
  
  	if (new_state == TCP_CA_Loss) {
  		ca->alpha = ALPHA_BASE;
  		ca->beta = BETA_BASE;
  		ca->rtt_low = 0;
  		ca->rtt_above = 0;
  		rtt_reset(sk);
  	}
  }
  
  /*
   * Increase window in response to successful acknowledgment.
   */
  static void tcp_illinois_cong_avoid(struct sock *sk, u32 ack, u32 acked,
  				    u32 in_flight)
  {
  	struct tcp_sock *tp = tcp_sk(sk);
  	struct illinois *ca = inet_csk_ca(sk);
  
  	if (after(ack, ca->end_seq))
  		update_params(sk);
  
  	/* RFC2861 only increase cwnd if fully utilized */
  	if (!tcp_is_cwnd_limited(sk, in_flight))
  		return;
  
  	/* In slow start */
  	if (tp->snd_cwnd <= tp->snd_ssthresh)
  		tcp_slow_start(tp, acked);
  
  	else {
  		u32 delta;
  
  		/* snd_cwnd_cnt is # of packets since last cwnd increment */
  		tp->snd_cwnd_cnt += ca->acked;
  		ca->acked = 1;
  
  		/* This is close approximation of:
  		 * tp->snd_cwnd += alpha/tp->snd_cwnd
  		*/
  		delta = (tp->snd_cwnd_cnt * ca->alpha) >> ALPHA_SHIFT;
  		if (delta >= tp->snd_cwnd) {
  			tp->snd_cwnd = min(tp->snd_cwnd + delta / tp->snd_cwnd,
  					   (u32) tp->snd_cwnd_clamp);
  			tp->snd_cwnd_cnt = 0;
  		}
  	}
  }
  
  static u32 tcp_illinois_ssthresh(struct sock *sk)
  {
  	struct tcp_sock *tp = tcp_sk(sk);
  	struct illinois *ca = inet_csk_ca(sk);
  
  	/* Multiplicative decrease */
  	return max(tp->snd_cwnd - ((tp->snd_cwnd * ca->beta) >> BETA_SHIFT), 2U);
  }
  
  
  /* Extract info for Tcp socket info provided via netlink. */
  static void tcp_illinois_info(struct sock *sk, u32 ext,
  			      struct sk_buff *skb)
  {
  	const struct illinois *ca = inet_csk_ca(sk);
  
  	if (ext & (1 << (INET_DIAG_VEGASINFO - 1))) {
  		struct tcpvegas_info info = {
  			.tcpv_enabled = 1,
  			.tcpv_rttcnt = ca->cnt_rtt,
  			.tcpv_minrtt = ca->base_rtt,
  		};
  
  		if (info.tcpv_rttcnt > 0) {
  			u64 t = ca->sum_rtt;
  
  			do_div(t, info.tcpv_rttcnt);
  			info.tcpv_rtt = t;
  		}
  		nla_put(skb, INET_DIAG_VEGASINFO, sizeof(info), &info);
  	}
  }
  
  static struct tcp_congestion_ops tcp_illinois __read_mostly = {
  	.flags		= TCP_CONG_RTT_STAMP,
  	.init		= tcp_illinois_init,
  	.ssthresh	= tcp_illinois_ssthresh,
  	.min_cwnd	= tcp_reno_min_cwnd,
  	.cong_avoid	= tcp_illinois_cong_avoid,
  	.set_state	= tcp_illinois_state,
  	.get_info	= tcp_illinois_info,
  	.pkts_acked	= tcp_illinois_acked,
  
  	.owner		= THIS_MODULE,
  	.name		= "illinois",
  };
  
  static int __init tcp_illinois_register(void)
  {
  	BUILD_BUG_ON(sizeof(struct illinois) > ICSK_CA_PRIV_SIZE);
  	return tcp_register_congestion_control(&tcp_illinois);
  }
  
  static void __exit tcp_illinois_unregister(void)
  {
  	tcp_unregister_congestion_control(&tcp_illinois);
  }
  
  module_init(tcp_illinois_register);
  module_exit(tcp_illinois_unregister);
  
  MODULE_AUTHOR("Stephen Hemminger, Shao Liu");
  MODULE_LICENSE("GPL");
  MODULE_DESCRIPTION("TCP Illinois");
  MODULE_VERSION("1.0");