// SPDX-License-Identifier: GPL-2.0

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

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

  {

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

  }

  static u32 tcp_rack_reo_wnd(const struct sock *sk)

  {
  	struct tcp_sock *tp = tcp_sk(sk);

  	if (!tp->reord_seen) {

  		/* If reordering has not been observed, be aggressive during
  		 * the recovery or starting the recovery by DUPACK threshold.
  		 */
  		if (inet_csk(sk)->icsk_ca_state >= TCP_CA_Recovery)
  			return 0;
  
  		if (tp->sacked_out >= tp->reordering &&
  		    !(sock_net(sk)->ipv4.sysctl_tcp_recovery & TCP_RACK_NO_DUPTHRESH))
  			return 0;
  	}
  
  	/* To be more reordering resilient, allow min_rtt/4 settling delay.
  	 * Use min_rtt instead of the smoothed RTT because reordering is
  	 * often a path property and less related to queuing or delayed ACKs.
  	 * Upon receiving DSACKs, linearly increase the window up to the
  	 * smoothed RTT.
  	 */
  	return min((tcp_min_rtt(tp) >> 2) * tp->rack.reo_wnd_steps,
  		   tp->srtt_us >> 3);
  }

  s32 tcp_rack_skb_timeout(struct tcp_sock *tp, struct sk_buff *skb, u32 reo_wnd)
  {
  	return tp->rack.rtt_us + reo_wnd -

  	       tcp_stamp_us_delta(tp->tcp_mstamp, tcp_skb_timestamp_us(skb));

  }

  /* 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, *n;

  	u32 reo_wnd;

  	*reo_timeout = 0;

  	reo_wnd = tcp_rack_reo_wnd(sk);

  	list_for_each_entry_safe(skb, n, &tp->tsorted_sent_queue,
  				 tcp_tsorted_anchor) {

  		struct tcp_skb_cb *scb = TCP_SKB_CB(skb);

  		s32 remaining;

  		/* Skip ones marked lost but not yet retransmitted */
  		if ((scb->sacked & TCPCB_LOST) &&
  		    !(scb->sacked & TCPCB_SACKED_RETRANS))
  			continue;

  		if (!tcp_rack_sent_after(tp->rack.mstamp,
  					 tcp_skb_timestamp_us(skb),

  					 tp->rack.end_seq, scb->end_seq))
  			break;

  		/* A packet is lost if it has not been s/acked beyond
  		 * the recent RTT plus the reordering window.
  		 */

  		remaining = tcp_rack_skb_timeout(tp, skb, reo_wnd);

  		if (remaining <= 0) {

  			tcp_mark_skb_lost(sk, skb);

  			list_del_init(&skb->tcp_tsorted_anchor);
  		} else {

  			/* Record maximum wait time */
  			*reo_timeout = max_t(u32, *reo_timeout, remaining);

  		}
  	}

  }

  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;

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

  	if (rtt_us < tcp_min_rtt(tp) && (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).
  		 */

  		return;

  	}

  	tp->rack.advanced = 1;

  	tp->rack.rtt_us = rtt_us;
  	if (tcp_rack_sent_after(xmit_time, tp->rack.mstamp,
  				end_seq, tp->rack.end_seq)) {
  		tp->rack.mstamp = xmit_time;
  		tp->rack.end_seq = end_seq;
  	}

  }

  
  /* 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);
  }

  
  /* Updates the RACK's reo_wnd based on DSACK and no. of recoveries.
   *
   * If DSACK is received, increment reo_wnd by min_rtt/4 (upper bounded
   * by srtt), since there is possibility that spurious retransmission was
   * due to reordering delay longer than reo_wnd.
   *
   * Persist the current reo_wnd value for TCP_RACK_RECOVERY_THRESH (16)
   * no. of successful recoveries (accounts for full DSACK-based loss
   * recovery undo). After that, reset it to default (min_rtt/4).
   *
   * At max, reo_wnd is incremented only once per rtt. So that the new
   * DSACK on which we are reacting, is due to the spurious retx (approx)
   * after the reo_wnd has been updated last time.
   *
   * reo_wnd is tracked in terms of steps (of min_rtt/4), rather than
   * absolute value to account for change in rtt.
   */
  void tcp_rack_update_reo_wnd(struct sock *sk, struct rate_sample *rs)
  {
  	struct tcp_sock *tp = tcp_sk(sk);
  
  	if (sock_net(sk)->ipv4.sysctl_tcp_recovery & TCP_RACK_STATIC_REO_WND ||
  	    !rs->prior_delivered)
  		return;
  
  	/* Disregard DSACK if a rtt has not passed since we adjusted reo_wnd */
  	if (before(rs->prior_delivered, tp->rack.last_delivered))
  		tp->rack.dsack_seen = 0;
  
  	/* Adjust the reo_wnd if update is pending */
  	if (tp->rack.dsack_seen) {
  		tp->rack.reo_wnd_steps = min_t(u32, 0xFF,
  					       tp->rack.reo_wnd_steps + 1);
  		tp->rack.dsack_seen = 0;
  		tp->rack.last_delivered = tp->delivered;
  		tp->rack.reo_wnd_persist = TCP_RACK_RECOVERY_THRESH;
  	} else if (!tp->rack.reo_wnd_persist) {
  		tp->rack.reo_wnd_steps = 1;
  	}
  }

  
  /* RFC6582 NewReno recovery for non-SACK connection. It simply retransmits
   * the next unacked packet upon receiving
   * a) three or more DUPACKs to start the fast recovery
   * b) an ACK acknowledging new data during the fast recovery.
   */
  void tcp_newreno_mark_lost(struct sock *sk, bool snd_una_advanced)
  {
  	const u8 state = inet_csk(sk)->icsk_ca_state;
  	struct tcp_sock *tp = tcp_sk(sk);
  
  	if ((state < TCP_CA_Recovery && tp->sacked_out >= tp->reordering) ||
  	    (state == TCP_CA_Recovery && snd_una_advanced)) {
  		struct sk_buff *skb = tcp_rtx_queue_head(sk);
  		u32 mss;
  
  		if (TCP_SKB_CB(skb)->sacked & TCPCB_LOST)
  			return;
  
  		mss = tcp_skb_mss(skb);
  		if (tcp_skb_pcount(skb) > 1 && skb->len > mss)
  			tcp_fragment(sk, TCP_FRAG_IN_RTX_QUEUE, skb,
  				     mss, mss, GFP_ATOMIC);

  		tcp_mark_skb_lost(sk, skb);

  	}
  }