net/ipv4/tcp_recovery.c

// 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);
}