// SPDX-License-Identifier: GPL-2.0

  #include <linux/tcp.h>
  #include <net/tcp.h>

  int sysctl_tcp_recovery __read_mostly = TCP_RACK_LOSS_DETECTION;

  static void tcp_rack_mark_skb_lost(struct sock *sk, struct sk_buff *skb)
  {
  	struct tcp_sock *tp = tcp_sk(sk);
  
  	tcp_skb_mark_lost_uncond_verify(tp, skb);
  	if (TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_RETRANS) {
  		/* Account for retransmits that are lost again */
  		TCP_SKB_CB(skb)->sacked &= ~TCPCB_SACKED_RETRANS;
  		tp->retrans_out -= tcp_skb_pcount(skb);

  		NET_ADD_STATS(sock_net(sk), LINUX_MIB_TCPLOSTRETRANSMIT,
  			      tcp_skb_pcount(skb));

  	}
  }

  static bool tcp_rack_sent_after(u64 t1, u64 t2, u32 seq1, u32 seq2)

  {

  	return t1 > t2 || (t1 == t2 && after(seq1, seq2));

  }

  /* RACK loss detection (IETF draft draft-ietf-tcpm-rack-01):
   *
   * Marks a packet lost, if some packet sent later has been (s)acked.

   * The underlying idea is similar to the traditional dupthresh and FACK
   * but they look at different metrics:
   *
   * dupthresh: 3 OOO packets delivered (packet count)
   * FACK: sequence delta to highest sacked sequence (sequence space)
   * RACK: sent time delta to the latest delivered packet (time domain)
   *
   * The advantage of RACK is it applies to both original and retransmitted
   * packet and therefore is robust against tail losses. Another advantage
   * is being more resilient to reordering by simply allowing some
   * "settling delay", instead of tweaking the dupthresh.
   *

   * When tcp_rack_detect_loss() detects some packets are lost and we
   * are not already in the CA_Recovery state, either tcp_rack_reo_timeout()
   * or tcp_time_to_recover()'s "Trick#1: the loss is proven" code path will
   * make us enter the CA_Recovery state.

   */

  static void tcp_rack_detect_loss(struct sock *sk, u32 *reo_timeout)

  {
  	struct tcp_sock *tp = tcp_sk(sk);
  	struct sk_buff *skb;

  	u32 reo_wnd;

  	*reo_timeout = 0;

  	/* To be more reordering resilient, allow min_rtt/4 settling delay
  	 * (lower-bounded to 1000uS). We use min_rtt instead of the smoothed
  	 * RTT because reordering is often a path property and less related
  	 * to queuing or delayed ACKs.

  	 */
  	reo_wnd = 1000;

  	if ((tp->rack.reord || !tp->lost_out) && tcp_min_rtt(tp) != ~0U)

  		reo_wnd = max(tcp_min_rtt(tp) >> 2, reo_wnd);
  
  	tcp_for_write_queue(skb, sk) {
  		struct tcp_skb_cb *scb = TCP_SKB_CB(skb);
  
  		if (skb == tcp_send_head(sk))
  			break;
  
  		/* Skip ones already (s)acked */
  		if (!after(scb->end_seq, tp->snd_una) ||
  		    scb->sacked & TCPCB_SACKED_ACKED)
  			continue;

  		if (tcp_rack_sent_after(tp->rack.mstamp, skb->skb_mstamp,

  					tp->rack.end_seq, scb->end_seq)) {

  			/* Step 3 in draft-cheng-tcpm-rack-00.txt:
  			 * A packet is lost if its elapsed time is beyond
  			 * the recent RTT plus the reordering window.
  			 */

  			u32 elapsed = tcp_stamp_us_delta(tp->tcp_mstamp,
  							 skb->skb_mstamp);

  			s32 remaining = tp->rack.rtt_us + reo_wnd - elapsed;
  
  			if (remaining < 0) {

  				tcp_rack_mark_skb_lost(sk, skb);

  				continue;

  			}

  
  			/* Skip ones marked lost but not yet retransmitted */
  			if ((scb->sacked & TCPCB_LOST) &&
  			    !(scb->sacked & TCPCB_SACKED_RETRANS))
  				continue;
  
  			/* Record maximum wait time (+1 to avoid 0) */
  			*reo_timeout = max_t(u32, *reo_timeout, 1 + remaining);

  		} else if (!(scb->sacked & TCPCB_RETRANS)) {
  			/* Original data are sent sequentially so stop early
  			 * b/c the rest are all sent after rack_sent
  			 */
  			break;
  		}
  	}

  }

  void tcp_rack_mark_lost(struct sock *sk)

  {
  	struct tcp_sock *tp = tcp_sk(sk);

  	u32 timeout;

  	if (!tp->rack.advanced)

  		return;

  	/* Reset the advanced flag to avoid unnecessary queue scanning */
  	tp->rack.advanced = 0;

  	tcp_rack_detect_loss(sk, &timeout);

  	if (timeout) {

  		timeout = usecs_to_jiffies(timeout) + TCP_TIMEOUT_MIN;

  		inet_csk_reset_xmit_timer(sk, ICSK_TIME_REO_TIMEOUT,
  					  timeout, inet_csk(sk)->icsk_rto);
  	}

  }

  /* Record the most recently (re)sent time among the (s)acked packets
   * This is "Step 3: Advance RACK.xmit_time and update RACK.RTT" from
   * draft-cheng-tcpm-rack-00.txt
   */

  void tcp_rack_advance(struct tcp_sock *tp, u8 sacked, u32 end_seq,

  		      u64 xmit_time)

  {

  	u32 rtt_us;

  	if (tp->rack.mstamp &&
  	    !tcp_rack_sent_after(xmit_time, tp->rack.mstamp,

  				 end_seq, tp->rack.end_seq))

  		return;

  	rtt_us = tcp_stamp_us_delta(tp->tcp_mstamp, xmit_time);

  	if (sacked & TCPCB_RETRANS) {

  		/* If the sacked packet was retransmitted, it's ambiguous
  		 * whether the retransmission or the original (or the prior
  		 * retransmission) was sacked.
  		 *
  		 * If the original is lost, there is no ambiguity. Otherwise
  		 * we assume the original can be delayed up to aRTT + min_rtt.
  		 * the aRTT term is bounded by the fast recovery or timeout,
  		 * so it's at least one RTT (i.e., retransmission is at least
  		 * an RTT later).
  		 */

  		if (rtt_us < tcp_min_rtt(tp))

  			return;
  	}

  	tp->rack.rtt_us = rtt_us;

  	tp->rack.mstamp = xmit_time;

  	tp->rack.end_seq = end_seq;

  	tp->rack.advanced = 1;
  }

  
  /* We have waited long enough to accommodate reordering. Mark the expired
   * packets lost and retransmit them.
   */
  void tcp_rack_reo_timeout(struct sock *sk)
  {
  	struct tcp_sock *tp = tcp_sk(sk);

  	u32 timeout, prior_inflight;

  	prior_inflight = tcp_packets_in_flight(tp);

  	tcp_rack_detect_loss(sk, &timeout);

  	if (prior_inflight != tcp_packets_in_flight(tp)) {
  		if (inet_csk(sk)->icsk_ca_state != TCP_CA_Recovery) {
  			tcp_enter_recovery(sk, false);
  			if (!inet_csk(sk)->icsk_ca_ops->cong_control)
  				tcp_cwnd_reduction(sk, 1, 0);
  		}
  		tcp_xmit_retransmit_queue(sk);
  	}
  	if (inet_csk(sk)->icsk_pending != ICSK_TIME_RETRANS)
  		tcp_rearm_rto(sk);
  }