/*
   * TCP Veno congestion control
   *
   * This is based on the congestion detection/avoidance scheme described in
   *    C. P. Fu, S. C. Liew.
   *    "TCP Veno: TCP Enhancement for Transmission over Wireless Access Networks."
   *    IEEE Journal on Selected Areas in Communication,
   *    Feb. 2003.
   * 	See http://www.ie.cuhk.edu.hk/fileadmin/staff_upload/soung/Journal/J3.pdf
   */
  
  #include <linux/mm.h>
  #include <linux/module.h>
  #include <linux/skbuff.h>
  #include <linux/inet_diag.h>
  
  #include <net/tcp.h>
  
  /* Default values of the Veno variables, in fixed-point representation
   * with V_PARAM_SHIFT bits to the right of the binary point.
   */
  #define V_PARAM_SHIFT 1
  static const int beta = 3 << V_PARAM_SHIFT;
  
  /* Veno variables */
  struct veno {
  	u8 doing_veno_now;	/* if true, do veno for this rtt */
  	u16 cntrtt;		/* # of rtts measured within last rtt */
  	u32 minrtt;		/* min of rtts measured within last rtt (in usec) */
  	u32 basertt;		/* the min of all Veno rtt measurements seen (in usec) */
  	u32 inc;		/* decide whether to increase cwnd */
  	u32 diff;		/* calculate the diff rate */
  };
  
  /* There are several situations when we must "re-start" Veno:
   *
   *  o when a connection is established
   *  o after an RTO
   *  o after fast recovery
   *  o when we send a packet and there is no outstanding
   *    unacknowledged data (restarting an idle connection)
   *
   */
  static inline void veno_enable(struct sock *sk)
  {
  	struct veno *veno = inet_csk_ca(sk);
  
  	/* turn on Veno */
  	veno->doing_veno_now = 1;
  
  	veno->minrtt = 0x7fffffff;
  }
  
  static inline void veno_disable(struct sock *sk)
  {
  	struct veno *veno = inet_csk_ca(sk);
  
  	/* turn off Veno */
  	veno->doing_veno_now = 0;
  }
  
  static void tcp_veno_init(struct sock *sk)
  {
  	struct veno *veno = inet_csk_ca(sk);
  
  	veno->basertt = 0x7fffffff;
  	veno->inc = 1;
  	veno_enable(sk);
  }
  
  /* Do rtt sampling needed for Veno. */
  static void tcp_veno_pkts_acked(struct sock *sk, u32 cnt, s32 rtt_us)
  {
  	struct veno *veno = inet_csk_ca(sk);
  	u32 vrtt;
  
  	if (rtt_us < 0)
  		return;
  
  	/* Never allow zero rtt or baseRTT */
  	vrtt = rtt_us + 1;
  
  	/* Filter to find propagation delay: */
  	if (vrtt < veno->basertt)
  		veno->basertt = vrtt;
  
  	/* Find the min rtt during the last rtt to find
  	 * the current prop. delay + queuing delay:
  	 */
  	veno->minrtt = min(veno->minrtt, vrtt);
  	veno->cntrtt++;
  }
  
  static void tcp_veno_state(struct sock *sk, u8 ca_state)
  {
  	if (ca_state == TCP_CA_Open)
  		veno_enable(sk);
  	else
  		veno_disable(sk);
  }
  
  /*
   * If the connection is idle and we are restarting,
   * then we don't want to do any Veno calculations
   * until we get fresh rtt samples.  So when we
   * restart, we reset our Veno state to a clean
   * state. After we get acks for this flight of
   * packets, _then_ we can make Veno calculations
   * again.
   */
  static void tcp_veno_cwnd_event(struct sock *sk, enum tcp_ca_event event)
  {
  	if (event == CA_EVENT_CWND_RESTART || event == CA_EVENT_TX_START)
  		tcp_veno_init(sk);
  }
  
  static void tcp_veno_cong_avoid(struct sock *sk, u32 ack, u32 acked,
  				u32 in_flight)
  {
  	struct tcp_sock *tp = tcp_sk(sk);
  	struct veno *veno = inet_csk_ca(sk);
  
  	if (!veno->doing_veno_now) {
  		tcp_reno_cong_avoid(sk, ack, acked, in_flight);
  		return;
  	}
  
  	/* limited by applications */
  	if (!tcp_is_cwnd_limited(sk, in_flight))
  		return;
  
  	/* We do the Veno calculations only if we got enough rtt samples */
  	if (veno->cntrtt <= 2) {
  		/* We don't have enough rtt samples to do the Veno
  		 * calculation, so we'll behave like Reno.
  		 */
  		tcp_reno_cong_avoid(sk, ack, acked, in_flight);
  	} else {
  		u64 target_cwnd;
  		u32 rtt;
  
  		/* We have enough rtt samples, so, using the Veno
  		 * algorithm, we determine the state of the network.
  		 */
  
  		rtt = veno->minrtt;
  
  		target_cwnd = (u64)tp->snd_cwnd * veno->basertt;
  		target_cwnd <<= V_PARAM_SHIFT;
  		do_div(target_cwnd, rtt);
  
  		veno->diff = (tp->snd_cwnd << V_PARAM_SHIFT) - target_cwnd;
  
  		if (tp->snd_cwnd <= tp->snd_ssthresh) {
  			/* Slow start.  */
  			tcp_slow_start(tp, acked);
  		} else {
  			/* Congestion avoidance. */
  			if (veno->diff < beta) {
  				/* In the "non-congestive state", increase cwnd
  				 *  every rtt.
  				 */
  				tcp_cong_avoid_ai(tp, tp->snd_cwnd);
  			} else {
  				/* In the "congestive state", increase cwnd
  				 * every other rtt.
  				 */
  				if (tp->snd_cwnd_cnt >= tp->snd_cwnd) {
  					if (veno->inc &&
  					    tp->snd_cwnd < tp->snd_cwnd_clamp) {
  						tp->snd_cwnd++;
  						veno->inc = 0;
  					} else
  						veno->inc = 1;
  					tp->snd_cwnd_cnt = 0;
  				} else
  					tp->snd_cwnd_cnt++;
  			}
  
  		}
  		if (tp->snd_cwnd < 2)
  			tp->snd_cwnd = 2;
  		else if (tp->snd_cwnd > tp->snd_cwnd_clamp)
  			tp->snd_cwnd = tp->snd_cwnd_clamp;
  	}
  	/* Wipe the slate clean for the next rtt. */
  	/* veno->cntrtt = 0; */
  	veno->minrtt = 0x7fffffff;
  }
  
  /* Veno MD phase */
  static u32 tcp_veno_ssthresh(struct sock *sk)
  {
  	const struct tcp_sock *tp = tcp_sk(sk);
  	struct veno *veno = inet_csk_ca(sk);
  
  	if (veno->diff < beta)
  		/* in "non-congestive state", cut cwnd by 1/5 */
  		return max(tp->snd_cwnd * 4 / 5, 2U);
  	else
  		/* in "congestive state", cut cwnd by 1/2 */
  		return max(tp->snd_cwnd >> 1U, 2U);
  }
  
  static struct tcp_congestion_ops tcp_veno __read_mostly = {
  	.flags		= TCP_CONG_RTT_STAMP,
  	.init		= tcp_veno_init,
  	.ssthresh	= tcp_veno_ssthresh,
  	.cong_avoid	= tcp_veno_cong_avoid,
  	.pkts_acked	= tcp_veno_pkts_acked,
  	.set_state	= tcp_veno_state,
  	.cwnd_event	= tcp_veno_cwnd_event,
  
  	.owner		= THIS_MODULE,
  	.name		= "veno",
  };
  
  static int __init tcp_veno_register(void)
  {
  	BUILD_BUG_ON(sizeof(struct veno) > ICSK_CA_PRIV_SIZE);
  	tcp_register_congestion_control(&tcp_veno);
  	return 0;
  }
  
  static void __exit tcp_veno_unregister(void)
  {
  	tcp_unregister_congestion_control(&tcp_veno);
  }
  
  module_init(tcp_veno_register);
  module_exit(tcp_veno_unregister);
  
  MODULE_AUTHOR("Bin Zhou, Cheng Peng Fu");
  MODULE_LICENSE("GPL");
  MODULE_DESCRIPTION("TCP Veno");