// SPDX-License-Identifier: GPL-2.0-only

  /*
   * TCP Vegas congestion control
   *
   * This is based on the congestion detection/avoidance scheme described in
   *    Lawrence S. Brakmo and Larry L. Peterson.
   *    "TCP Vegas: End to end congestion avoidance on a global internet."
   *    IEEE Journal on Selected Areas in Communication, 13(8):1465--1480,
   *    October 1995. Available from:
   *	ftp://ftp.cs.arizona.edu/xkernel/Papers/jsac.ps
   *
   * See http://www.cs.arizona.edu/xkernel/ for their implementation.
   * The main aspects that distinguish this implementation from the
   * Arizona Vegas implementation are:
   *   o We do not change the loss detection or recovery mechanisms of
   *     Linux in any way. Linux already recovers from losses quite well,
   *     using fine-grained timers, NewReno, and FACK.
   *   o To avoid the performance penalty imposed by increasing cwnd
   *     only every-other RTT during slow start, we increase during
   *     every RTT during slow start, just like Reno.
   *   o Largely to allow continuous cwnd growth during slow start,
   *     we use the rate at which ACKs come back as the "actual"
   *     rate, rather than the rate at which data is sent.
   *   o To speed convergence to the right rate, we set the cwnd
   *     to achieve the right ("actual") rate when we exit slow start.
   *   o To filter out the noise caused by delayed ACKs, we use the
   *     minimum RTT sample observed during the last RTT to calculate
   *     the actual rate.
   *   o When the sender re-starts from idle, it waits until it has
   *     received ACKs for an entire flight of new data before making
   *     a cwnd adjustment decision. The original Vegas implementation
   *     assumed senders never went idle.
   */

  #include <linux/mm.h>
  #include <linux/module.h>
  #include <linux/skbuff.h>

  #include <linux/inet_diag.h>

  
  #include <net/tcp.h>

  #include "tcp_vegas.h"

  static int alpha = 2;
  static int beta  = 4;
  static int gamma = 1;

  
  module_param(alpha, int, 0644);

  MODULE_PARM_DESC(alpha, "lower bound of packets in network");

  module_param(beta, int, 0644);

  MODULE_PARM_DESC(beta, "upper bound of packets in network");

  module_param(gamma, int, 0644);
  MODULE_PARM_DESC(gamma, "limit on increase (scale by 2)");

  /* There are several situations when we must "re-start" Vegas:
   *
   *  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)
   *
   * In these circumstances we cannot do a Vegas calculation at the
   * end of the first RTT, because any calculation we do is using
   * stale info -- both the saved cwnd and congestion feedback are
   * stale.
   *
   * Instead we must wait until the completion of an RTT during
   * which we actually receive ACKs.
   */

  static void vegas_enable(struct sock *sk)

  {

  	const struct tcp_sock *tp = tcp_sk(sk);
  	struct vegas *vegas = inet_csk_ca(sk);

  
  	/* Begin taking Vegas samples next time we send something. */
  	vegas->doing_vegas_now = 1;
  
  	/* Set the beginning of the next send window. */
  	vegas->beg_snd_nxt = tp->snd_nxt;
  
  	vegas->cntRTT = 0;
  	vegas->minRTT = 0x7fffffff;
  }
  
  /* Stop taking Vegas samples for now. */

  static inline void vegas_disable(struct sock *sk)

  {

  	struct vegas *vegas = inet_csk_ca(sk);

  
  	vegas->doing_vegas_now = 0;
  }

  void tcp_vegas_init(struct sock *sk)

  {

  	struct vegas *vegas = inet_csk_ca(sk);

  
  	vegas->baseRTT = 0x7fffffff;

  	vegas_enable(sk);

  }

  EXPORT_SYMBOL_GPL(tcp_vegas_init);

  
  /* Do RTT sampling needed for Vegas.
   * Basically we:
   *   o min-filter RTT samples from within an RTT to get the current
   *     propagation delay + queuing delay (we are min-filtering to try to
   *     avoid the effects of delayed ACKs)
   *   o min-filter RTT samples from a much longer window (forever for now)
   *     to find the propagation delay (baseRTT)
   */

  void tcp_vegas_pkts_acked(struct sock *sk, const struct ack_sample *sample)

  {

  	struct vegas *vegas = inet_csk_ca(sk);

  	u32 vrtt;

  	if (sample->rtt_us < 0)

  		return;

  	/* Never allow zero rtt or baseRTT */

  	vrtt = sample->rtt_us + 1;

  
  	/* Filter to find propagation delay: */
  	if (vrtt < vegas->baseRTT)
  		vegas->baseRTT = vrtt;
  
  	/* Find the min RTT during the last RTT to find
  	 * the current prop. delay + queuing delay:
  	 */
  	vegas->minRTT = min(vegas->minRTT, vrtt);
  	vegas->cntRTT++;
  }

  EXPORT_SYMBOL_GPL(tcp_vegas_pkts_acked);

  void tcp_vegas_state(struct sock *sk, u8 ca_state)

  {

  	if (ca_state == TCP_CA_Open)

  		vegas_enable(sk);

  	else

  		vegas_disable(sk);

  }

  EXPORT_SYMBOL_GPL(tcp_vegas_state);

  
  /*
   * If the connection is idle and we are restarting,
   * then we don't want to do any Vegas calculations
   * until we get fresh RTT samples.  So when we
   * restart, we reset our Vegas state to a clean
   * slate. After we get acks for this flight of
   * packets, _then_ we can make Vegas calculations
   * again.
   */

  void tcp_vegas_cwnd_event(struct sock *sk, enum tcp_ca_event event)

  {
  	if (event == CA_EVENT_CWND_RESTART ||
  	    event == CA_EVENT_TX_START)

  		tcp_vegas_init(sk);

  }

  EXPORT_SYMBOL_GPL(tcp_vegas_cwnd_event);

  static inline u32 tcp_vegas_ssthresh(struct tcp_sock *tp)
  {

  	return  min(tp->snd_ssthresh, tp->snd_cwnd);

  }

  static void tcp_vegas_cong_avoid(struct sock *sk, u32 ack, u32 acked)

  {

  	struct tcp_sock *tp = tcp_sk(sk);
  	struct vegas *vegas = inet_csk_ca(sk);

  	if (!vegas->doing_vegas_now) {

  		tcp_reno_cong_avoid(sk, ack, acked);

  		return;
  	}

  	if (after(ack, vegas->beg_snd_nxt)) {
  		/* Do the Vegas once-per-RTT cwnd adjustment. */

  
  		/* Save the extent of the current window so we can use this
  		 * at the end of the next RTT.
  		 */

  		vegas->beg_snd_nxt  = tp->snd_nxt;

  		/* We do the Vegas calculations only if we got enough RTT
  		 * samples that we can be reasonably sure that we got
  		 * at least one RTT sample that wasn't from a delayed ACK.
  		 * If we only had 2 samples total,
  		 * then that means we're getting only 1 ACK per RTT, which
  		 * means they're almost certainly delayed ACKs.
  		 * If  we have 3 samples, we should be OK.
  		 */
  
  		if (vegas->cntRTT <= 2) {
  			/* We don't have enough RTT samples to do the Vegas
  			 * calculation, so we'll behave like Reno.
  			 */

  			tcp_reno_cong_avoid(sk, ack, acked);

  		} else {

  			u32 rtt, diff;
  			u64 target_cwnd;

  
  			/* We have enough RTT samples, so, using the Vegas
  			 * algorithm, we determine if we should increase or
  			 * decrease cwnd, and by how much.
  			 */
  
  			/* Pluck out the RTT we are using for the Vegas
  			 * calculations. This is the min RTT seen during the
  			 * last RTT. Taking the min filters out the effects
  			 * of delayed ACKs, at the cost of noticing congestion
  			 * a bit later.
  			 */
  			rtt = vegas->minRTT;
  
  			/* Calculate the cwnd we should have, if we weren't
  			 * going too fast.
  			 *
  			 * This is:
  			 *     (actual rate in segments) * baseRTT

  			 */

  			target_cwnd = (u64)tp->snd_cwnd * vegas->baseRTT;
  			do_div(target_cwnd, rtt);

  
  			/* Calculate the difference between the window we had,
  			 * and the window we would like to have. This quantity
  			 * is the "Diff" from the Arizona Vegas papers.

  			 */

  			diff = tp->snd_cwnd * (rtt-vegas->baseRTT) / vegas->baseRTT;

  			if (diff > gamma && tcp_in_slow_start(tp)) {

  				/* Going too fast. Time to slow down
  				 * and switch to congestion avoidance.
  				 */

  
  				/* Set cwnd to match the actual rate
  				 * exactly:
  				 *   cwnd = (actual rate) * baseRTT
  				 * Then we add 1 because the integer
  				 * truncation robs us of full link
  				 * utilization.
  				 */

  				tp->snd_cwnd = min(tp->snd_cwnd, (u32)target_cwnd+1);

  				tp->snd_ssthresh = tcp_vegas_ssthresh(tp);

  			} else if (tcp_in_slow_start(tp)) {

  				/* Slow start.  */

  				tcp_slow_start(tp, acked);

  			} else {
  				/* Congestion avoidance. */

  
  				/* Figure out where we would like cwnd
  				 * to be.
  				 */
  				if (diff > beta) {
  					/* The old window was too fast, so
  					 * we slow down.
  					 */

  					tp->snd_cwnd--;

  					tp->snd_ssthresh
  						= tcp_vegas_ssthresh(tp);

  				} else if (diff < alpha) {
  					/* We don't have enough extra packets
  					 * in the network, so speed up.
  					 */

  					tp->snd_cwnd++;

  				} else {
  					/* Sending just as fast as we
  					 * should be.
  					 */

  				}

  			}

  			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;

  
  			tp->snd_ssthresh = tcp_current_ssthresh(sk);

  		}

  		/* Wipe the slate clean for the next RTT. */
  		vegas->cntRTT = 0;
  		vegas->minRTT = 0x7fffffff;
  	}

  	/* Use normal slow start */

  	else if (tcp_in_slow_start(tp))

  		tcp_slow_start(tp, acked);

  }
  
  /* Extract info for Tcp socket info provided via netlink. */

  size_t tcp_vegas_get_info(struct sock *sk, u32 ext, int *attr,
  			  union tcp_cc_info *info)

  {

  	const struct vegas *ca = inet_csk_ca(sk);

  	if (ext & (1 << (INET_DIAG_VEGASINFO - 1))) {

  		info->vegas.tcpv_enabled = ca->doing_vegas_now;
  		info->vegas.tcpv_rttcnt = ca->cntRTT;
  		info->vegas.tcpv_rtt = ca->baseRTT;
  		info->vegas.tcpv_minrtt = ca->minRTT;

  
  		*attr = INET_DIAG_VEGASINFO;
  		return sizeof(struct tcpvegas_info);

  	}

  	return 0;

  }

  EXPORT_SYMBOL_GPL(tcp_vegas_get_info);

  static struct tcp_congestion_ops tcp_vegas __read_mostly = {

  	.init		= tcp_vegas_init,
  	.ssthresh	= tcp_reno_ssthresh,

  	.undo_cwnd	= tcp_reno_undo_cwnd,

  	.cong_avoid	= tcp_vegas_cong_avoid,

  	.pkts_acked	= tcp_vegas_pkts_acked,

  	.set_state	= tcp_vegas_state,
  	.cwnd_event	= tcp_vegas_cwnd_event,
  	.get_info	= tcp_vegas_get_info,
  
  	.owner		= THIS_MODULE,
  	.name		= "vegas",
  };
  
  static int __init tcp_vegas_register(void)
  {

  	BUILD_BUG_ON(sizeof(struct vegas) > ICSK_CA_PRIV_SIZE);

  	tcp_register_congestion_control(&tcp_vegas);
  	return 0;
  }
  
  static void __exit tcp_vegas_unregister(void)
  {
  	tcp_unregister_congestion_control(&tcp_vegas);
  }
  
  module_init(tcp_vegas_register);
  module_exit(tcp_vegas_unregister);
  
  MODULE_AUTHOR("Stephen Hemminger");
  MODULE_LICENSE("GPL");
  MODULE_DESCRIPTION("TCP Vegas");