text: /*
   *
   *   YeAH TCP
   *
   * For further details look at:
   *   https://web.archive.org/web/20080316215752/http://wil.cs.caltech.edu/pfldnet2007/paper/YeAH_TCP.pdf
   *
   */
  #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"
  
  #define TCP_YEAH_ALPHA       80 /* number of packets queued at the bottleneck */
  #define TCP_YEAH_GAMMA        1 /* fraction of queue to be removed per rtt */
  #define TCP_YEAH_DELTA        3 /* log minimum fraction of cwnd to be removed on loss */
  #define TCP_YEAH_EPSILON      1 /* log maximum fraction to be removed on early decongestion */
  #define TCP_YEAH_PHY          8 /* maximum delta from base */
  #define TCP_YEAH_RHO         16 /* minimum number of consecutive rtt to consider competition on loss */
  #define TCP_YEAH_ZETA        50 /* minimum number of state switches to reset reno_count */
  
  #define TCP_SCALABLE_AI_CNT	 100U
  
  /* YeAH variables */
  struct yeah {
  	struct vegas vegas;	/* must be first */
  
  	/* YeAH */
  	u32 lastQ;
  	u32 doing_reno_now;
  
  	u32 reno_count;
  	u32 fast_count;
  
  	u32 pkts_acked;
  };
  
  static void tcp_yeah_init(struct sock *sk)
  {
  	struct tcp_sock *tp = tcp_sk(sk);
  	struct yeah *yeah = inet_csk_ca(sk);
  
  	tcp_vegas_init(sk);
  
  	yeah->doing_reno_now = 0;
  	yeah->lastQ = 0;
  
  	yeah->reno_count = 2;
  
  	/* Ensure the MD arithmetic works.  This is somewhat pedantic,
  	 * since I don't think we will see a cwnd this large. :) */
  	tp->snd_cwnd_clamp = min_t(u32, tp->snd_cwnd_clamp, 0xffffffff/128);
  }
  
  static void tcp_yeah_pkts_acked(struct sock *sk, u32 pkts_acked, s32 rtt_us)
  {
  	const struct inet_connection_sock *icsk = inet_csk(sk);
  	struct yeah *yeah = inet_csk_ca(sk);
  
  	if (icsk->icsk_ca_state == TCP_CA_Open)
  		yeah->pkts_acked = pkts_acked;
  
  	tcp_vegas_pkts_acked(sk, pkts_acked, rtt_us);
  }
  
  static void tcp_yeah_cong_avoid(struct sock *sk, u32 ack, u32 acked)
  {
  	struct tcp_sock *tp = tcp_sk(sk);
  	struct yeah *yeah = inet_csk_ca(sk);
  
  	if (!tcp_is_cwnd_limited(sk))
  		return;
  
  	if (tp->snd_cwnd <= tp->snd_ssthresh)
  		tcp_slow_start(tp, acked);
  
  	else if (!yeah->doing_reno_now) {
  		/* Scalable */
  
  		tp->snd_cwnd_cnt += yeah->pkts_acked;
  		if (tp->snd_cwnd_cnt > min(tp->snd_cwnd, TCP_SCALABLE_AI_CNT)) {
  			if (tp->snd_cwnd < tp->snd_cwnd_clamp)
  				tp->snd_cwnd++;
  			tp->snd_cwnd_cnt = 0;
  		}
  
  		yeah->pkts_acked = 1;
  
  	} else {
  		/* Reno */
  		tcp_cong_avoid_ai(tp, tp->snd_cwnd);
  	}
  
  	/* The key players are v_vegas.beg_snd_una and v_beg_snd_nxt.
  	 *
  	 * These are so named because they represent the approximate values
  	 * of snd_una and snd_nxt at the beginning of the current RTT. More
  	 * precisely, they represent the amount of data sent during the RTT.
  	 * At the end of the RTT, when we receive an ACK for v_beg_snd_nxt,
  	 * we will calculate that (v_beg_snd_nxt - v_vegas.beg_snd_una) outstanding
  	 * bytes of data have been ACKed during the course of the RTT, giving
  	 * an "actual" rate of:
  	 *
  	 *     (v_beg_snd_nxt - v_vegas.beg_snd_una) / (rtt duration)
  	 *
  	 * Unfortunately, v_vegas.beg_snd_una is not exactly equal to snd_una,
  	 * because delayed ACKs can cover more than one segment, so they
  	 * don't line up yeahly with the boundaries of RTTs.
  	 *
  	 * Another unfortunate fact of life is that delayed ACKs delay the
  	 * advance of the left edge of our send window, so that the number
  	 * of bytes we send in an RTT is often less than our cwnd will allow.
  	 * So we keep track of our cwnd separately, in v_beg_snd_cwnd.
  	 */
  
  	if (after(ack, yeah->vegas.beg_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 (yeah->vegas.cntRTT > 2) {
  			u32 rtt, queue;
  			u64 bw;
  
  			/* 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 = yeah->vegas.minRTT;
  
  			/* Compute excess number of packets above bandwidth
  			 * Avoid doing full 64 bit divide.
  			 */
  			bw = tp->snd_cwnd;
  			bw *= rtt - yeah->vegas.baseRTT;
  			do_div(bw, rtt);
  			queue = bw;
  
  			if (queue > TCP_YEAH_ALPHA ||
  			    rtt - yeah->vegas.baseRTT > (yeah->vegas.baseRTT / TCP_YEAH_PHY)) {
  				if (queue > TCP_YEAH_ALPHA &&
  				    tp->snd_cwnd > yeah->reno_count) {
  					u32 reduction = min(queue / TCP_YEAH_GAMMA ,
  							    tp->snd_cwnd >> TCP_YEAH_EPSILON);
  
  					tp->snd_cwnd -= reduction;
  
  					tp->snd_cwnd = max(tp->snd_cwnd,
  							   yeah->reno_count);
  
  					tp->snd_ssthresh = tp->snd_cwnd;
  				}
  
  				if (yeah->reno_count <= 2)
  					yeah->reno_count = max(tp->snd_cwnd>>1, 2U);
  				else
  					yeah->reno_count++;
  
  				yeah->doing_reno_now = min(yeah->doing_reno_now + 1,
  							   0xffffffU);
  			} else {
  				yeah->fast_count++;
  
  				if (yeah->fast_count > TCP_YEAH_ZETA) {
  					yeah->reno_count = 2;
  					yeah->fast_count = 0;
  				}
  
  				yeah->doing_reno_now = 0;
  			}
  
  			yeah->lastQ = queue;
  		}
  
  		/* Save the extent of the current window so we can use this
  		 * at the end of the next RTT.
  		 */
  		yeah->vegas.beg_snd_una  = yeah->vegas.beg_snd_nxt;
  		yeah->vegas.beg_snd_nxt  = tp->snd_nxt;
  		yeah->vegas.beg_snd_cwnd = tp->snd_cwnd;
  
  		/* Wipe the slate clean for the next RTT. */
  		yeah->vegas.cntRTT = 0;
  		yeah->vegas.minRTT = 0x7fffffff;
  	}
  }
  
  static u32 tcp_yeah_ssthresh(struct sock *sk)
  {
  	const struct tcp_sock *tp = tcp_sk(sk);
  	struct yeah *yeah = inet_csk_ca(sk);
  	u32 reduction;
  
  	if (yeah->doing_reno_now < TCP_YEAH_RHO) {
  		reduction = yeah->lastQ;
  
  		reduction = min(reduction, max(tp->snd_cwnd>>1, 2U));
  
  		reduction = max(reduction, tp->snd_cwnd >> TCP_YEAH_DELTA);
  	} else
  		reduction = max(tp->snd_cwnd>>1, 2U);
  
  	yeah->fast_count = 0;
  	yeah->reno_count = max(yeah->reno_count>>1, 2U);
  
  	return tp->snd_cwnd - reduction;
  }
  
  static struct tcp_congestion_ops tcp_yeah __read_mostly = {
  	.init		= tcp_yeah_init,
  	.ssthresh	= tcp_yeah_ssthresh,
  	.cong_avoid	= tcp_yeah_cong_avoid,
  	.set_state	= tcp_vegas_state,
  	.cwnd_event	= tcp_vegas_cwnd_event,
  	.get_info	= tcp_vegas_get_info,
  	.pkts_acked	= tcp_yeah_pkts_acked,
  
  	.owner		= THIS_MODULE,
  	.name		= "yeah",
  };
  
  static int __init tcp_yeah_register(void)
  {
  	BUG_ON(sizeof(struct yeah) > ICSK_CA_PRIV_SIZE);
  	tcp_register_congestion_control(&tcp_yeah);
  	return 0;
  }
  
  static void __exit tcp_yeah_unregister(void)
  {
  	tcp_unregister_congestion_control(&tcp_yeah);
  }
  
  module_init(tcp_yeah_register);
  module_exit(tcp_yeah_unregister);
  
  MODULE_AUTHOR("Angelo P. Castellani");
  MODULE_LICENSE("GPL");
  MODULE_DESCRIPTION("YeAH TCP");
  