/*
   * TCP Westwood+: end-to-end bandwidth estimation for TCP
   *
   *      Angelo Dell'Aera: author of the first version of TCP Westwood+ in Linux 2.4
   *
   * Support at http://c3lab.poliba.it/index.php/Westwood
   * Main references in literature:
   *
   * - Mascolo S, Casetti, M. Gerla et al.
   *   "TCP Westwood: bandwidth estimation for TCP" Proc. ACM Mobicom 2001
   *
   * - A. Grieco, s. Mascolo
   *   "Performance evaluation of New Reno, Vegas, Westwood+ TCP" ACM Computer
   *     Comm. Review, 2004
   *
   * - A. Dell'Aera, L. Grieco, S. Mascolo.
   *   "Linux 2.4 Implementation of Westwood+ TCP with Rate-Halving :
   *    A Performance Evaluation Over the Internet" (ICC 2004), Paris, June 2004
   *
   * Westwood+ employs end-to-end bandwidth measurement to set cwnd and
   * ssthresh after packet loss. The probing phase is as the original Reno.
   */
  
  #include <linux/mm.h>
  #include <linux/module.h>
  #include <linux/skbuff.h>
  #include <linux/inet_diag.h>
  #include <net/tcp.h>
  
  /* TCP Westwood structure */
  struct westwood {
  	u32    bw_ns_est;        /* first bandwidth estimation..not too smoothed 8) */
  	u32    bw_est;           /* bandwidth estimate */
  	u32    rtt_win_sx;       /* here starts a new evaluation... */
  	u32    bk;
  	u32    snd_una;          /* used for evaluating the number of acked bytes */
  	u32    cumul_ack;
  	u32    accounted;
  	u32    rtt;
  	u32    rtt_min;          /* minimum observed RTT */
  	u8     first_ack;        /* flag which infers that this is the first ack */
  	u8     reset_rtt_min;    /* Reset RTT min to next RTT sample*/
  };
  
  
  /* TCP Westwood functions and constants */
  #define TCP_WESTWOOD_RTT_MIN   (HZ/20)	/* 50ms */
  #define TCP_WESTWOOD_INIT_RTT  (20*HZ)	/* maybe too conservative?! */
  
  /*
   * @tcp_westwood_create
   * This function initializes fields used in TCP Westwood+,
   * it is called after the initial SYN, so the sequence numbers
   * are correct but new passive connections we have no
   * information about RTTmin at this time so we simply set it to
   * TCP_WESTWOOD_INIT_RTT. This value was chosen to be too conservative
   * since in this way we're sure it will be updated in a consistent
   * way as soon as possible. It will reasonably happen within the first
   * RTT period of the connection lifetime.
   */
  static void tcp_westwood_init(struct sock *sk)
  {
  	struct westwood *w = inet_csk_ca(sk);
  
  	w->bk = 0;
  	w->bw_ns_est = 0;
  	w->bw_est = 0;
  	w->accounted = 0;
  	w->cumul_ack = 0;
  	w->reset_rtt_min = 1;
  	w->rtt_min = w->rtt = TCP_WESTWOOD_INIT_RTT;
  	w->rtt_win_sx = tcp_time_stamp;
  	w->snd_una = tcp_sk(sk)->snd_una;
  	w->first_ack = 1;
  }
  
  /*
   * @westwood_do_filter
   * Low-pass filter. Implemented using constant coefficients.
   */
  static inline u32 westwood_do_filter(u32 a, u32 b)
  {
  	return ((7 * a) + b) >> 3;
  }
  
  static void westwood_filter(struct westwood *w, u32 delta)
  {
  	/* If the filter is empty fill it with the first sample of bandwidth  */
  	if (w->bw_ns_est == 0 && w->bw_est == 0) {
  		w->bw_ns_est = w->bk / delta;
  		w->bw_est = w->bw_ns_est;
  	} else {
  		w->bw_ns_est = westwood_do_filter(w->bw_ns_est, w->bk / delta);
  		w->bw_est = westwood_do_filter(w->bw_est, w->bw_ns_est);
  	}
  }
  
  /*
   * @westwood_pkts_acked
   * Called after processing group of packets.
   * but all westwood needs is the last sample of srtt.
   */
  static void tcp_westwood_pkts_acked(struct sock *sk, u32 cnt, s32 rtt)
  {
  	struct westwood *w = inet_csk_ca(sk);
  
  	if (rtt > 0)
  		w->rtt = usecs_to_jiffies(rtt);
  }
  
  /*
   * @westwood_update_window
   * It updates RTT evaluation window if it is the right moment to do
   * it. If so it calls filter for evaluating bandwidth.
   */
  static void westwood_update_window(struct sock *sk)
  {
  	struct westwood *w = inet_csk_ca(sk);
  	s32 delta = tcp_time_stamp - w->rtt_win_sx;
  
  	/* Initialize w->snd_una with the first acked sequence number in order
  	 * to fix mismatch between tp->snd_una and w->snd_una for the first
  	 * bandwidth sample
  	 */
  	if (w->first_ack) {
  		w->snd_una = tcp_sk(sk)->snd_una;
  		w->first_ack = 0;
  	}
  
  	/*
  	 * See if a RTT-window has passed.
  	 * Be careful since if RTT is less than
  	 * 50ms we don't filter but we continue 'building the sample'.
  	 * This minimum limit was chosen since an estimation on small
  	 * time intervals is better to avoid...
  	 * Obviously on a LAN we reasonably will always have
  	 * right_bound = left_bound + WESTWOOD_RTT_MIN
  	 */
  	if (w->rtt && delta > max_t(u32, w->rtt, TCP_WESTWOOD_RTT_MIN)) {
  		westwood_filter(w, delta);
  
  		w->bk = 0;
  		w->rtt_win_sx = tcp_time_stamp;
  	}
  }
  
  static inline void update_rtt_min(struct westwood *w)
  {
  	if (w->reset_rtt_min) {
  		w->rtt_min = w->rtt;
  		w->reset_rtt_min = 0;
  	} else
  		w->rtt_min = min(w->rtt, w->rtt_min);
  }
  
  
  /*
   * @westwood_fast_bw
   * It is called when we are in fast path. In particular it is called when
   * header prediction is successful. In such case in fact update is
   * straight forward and doesn't need any particular care.
   */
  static inline void westwood_fast_bw(struct sock *sk)
  {
  	const struct tcp_sock *tp = tcp_sk(sk);
  	struct westwood *w = inet_csk_ca(sk);
  
  	westwood_update_window(sk);
  
  	w->bk += tp->snd_una - w->snd_una;
  	w->snd_una = tp->snd_una;
  	update_rtt_min(w);
  }
  
  /*
   * @westwood_acked_count
   * This function evaluates cumul_ack for evaluating bk in case of
   * delayed or partial acks.
   */
  static inline u32 westwood_acked_count(struct sock *sk)
  {
  	const struct tcp_sock *tp = tcp_sk(sk);
  	struct westwood *w = inet_csk_ca(sk);
  
  	w->cumul_ack = tp->snd_una - w->snd_una;
  
  	/* If cumul_ack is 0 this is a dupack since it's not moving
  	 * tp->snd_una.
  	 */
  	if (!w->cumul_ack) {
  		w->accounted += tp->mss_cache;
  		w->cumul_ack = tp->mss_cache;
  	}
  
  	if (w->cumul_ack > tp->mss_cache) {
  		/* Partial or delayed ack */
  		if (w->accounted >= w->cumul_ack) {
  			w->accounted -= w->cumul_ack;
  			w->cumul_ack = tp->mss_cache;
  		} else {
  			w->cumul_ack -= w->accounted;
  			w->accounted = 0;
  		}
  	}
  
  	w->snd_una = tp->snd_una;
  
  	return w->cumul_ack;
  }
  
  
  /*
   * TCP Westwood
   * Here limit is evaluated as Bw estimation*RTTmin (for obtaining it
   * in packets we use mss_cache). Rttmin is guaranteed to be >= 2
   * so avoids ever returning 0.
   */
  static u32 tcp_westwood_bw_rttmin(const struct sock *sk)
  {
  	const struct tcp_sock *tp = tcp_sk(sk);
  	const struct westwood *w = inet_csk_ca(sk);
  	return max_t(u32, (w->bw_est * w->rtt_min) / tp->mss_cache, 2);
  }
  
  static void tcp_westwood_event(struct sock *sk, enum tcp_ca_event event)
  {
  	struct tcp_sock *tp = tcp_sk(sk);
  	struct westwood *w = inet_csk_ca(sk);
  
  	switch (event) {
  	case CA_EVENT_FAST_ACK:
  		westwood_fast_bw(sk);
  		break;
  
  	case CA_EVENT_COMPLETE_CWR:
  		tp->snd_cwnd = tp->snd_ssthresh = tcp_westwood_bw_rttmin(sk);
  		break;
  
  	case CA_EVENT_LOSS:
  		tp->snd_ssthresh = tcp_westwood_bw_rttmin(sk);
  		/* Update RTT_min when next ack arrives */
  		w->reset_rtt_min = 1;
  		break;
  
  	case CA_EVENT_SLOW_ACK:
  		westwood_update_window(sk);
  		w->bk += westwood_acked_count(sk);
  		update_rtt_min(w);
  		break;
  
  	default:
  		/* don't care */
  		break;
  	}
  }
  
  
  /* Extract info for Tcp socket info provided via netlink. */
  static void tcp_westwood_info(struct sock *sk, u32 ext,
  			      struct sk_buff *skb)
  {
  	const struct westwood *ca = inet_csk_ca(sk);
  	if (ext & (1 << (INET_DIAG_VEGASINFO - 1))) {
  		struct tcpvegas_info info = {
  			.tcpv_enabled = 1,
  			.tcpv_rtt = jiffies_to_usecs(ca->rtt),
  			.tcpv_minrtt = jiffies_to_usecs(ca->rtt_min),
  		};
  
  		nla_put(skb, INET_DIAG_VEGASINFO, sizeof(info), &info);
  	}
  }
  
  
  static struct tcp_congestion_ops tcp_westwood __read_mostly = {
  	.init		= tcp_westwood_init,
  	.ssthresh	= tcp_reno_ssthresh,
  	.cong_avoid	= tcp_reno_cong_avoid,
  	.min_cwnd	= tcp_westwood_bw_rttmin,
  	.cwnd_event	= tcp_westwood_event,
  	.get_info	= tcp_westwood_info,
  	.pkts_acked	= tcp_westwood_pkts_acked,
  
  	.owner		= THIS_MODULE,
  	.name		= "westwood"
  };
  
  static int __init tcp_westwood_register(void)
  {
  	BUILD_BUG_ON(sizeof(struct westwood) > ICSK_CA_PRIV_SIZE);
  	return tcp_register_congestion_control(&tcp_westwood);
  }
  
  static void __exit tcp_westwood_unregister(void)
  {
  	tcp_unregister_congestion_control(&tcp_westwood);
  }
  
  module_init(tcp_westwood_register);
  module_exit(tcp_westwood_unregister);
  
  MODULE_AUTHOR("Stephen Hemminger, Angelo Dell'Aera");
  MODULE_LICENSE("GPL");
  MODULE_DESCRIPTION("TCP Westwood+");