1// SPDX-License-Identifier: GPL-2.0
  2#include <linux/tcp.h>
  3#include <net/tcp.h>
  4
  5static u32 tcp_rack_reo_wnd(const struct sock *sk)
  6{
  7	struct tcp_sock *tp = tcp_sk(sk);
  8
  9	if (!tp->reord_seen) {
 10		/* If reordering has not been observed, be aggressive during
 11		 * the recovery or starting the recovery by DUPACK threshold.
 12		 */
 13		if (inet_csk(sk)->icsk_ca_state >= TCP_CA_Recovery)
 14			return 0;
 15
 16		if (tp->sacked_out >= tp->reordering &&
 17		    !(READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_recovery) &
 18		      TCP_RACK_NO_DUPTHRESH))
 19			return 0;
 20	}
 21
 22	/* To be more reordering resilient, allow min_rtt/4 settling delay.
 23	 * Use min_rtt instead of the smoothed RTT because reordering is
 24	 * often a path property and less related to queuing or delayed ACKs.
 25	 * Upon receiving DSACKs, linearly increase the window up to the
 26	 * smoothed RTT.
 27	 */
 28	return min((tcp_min_rtt(tp) >> 2) * tp->rack.reo_wnd_steps,
 29		   tp->srtt_us >> 3);
 30}
 31
 32s32 tcp_rack_skb_timeout(struct tcp_sock *tp, struct sk_buff *skb, u32 reo_wnd)
 33{
 34	return tp->rack.rtt_us + reo_wnd -
 35	       tcp_stamp_us_delta(tp->tcp_mstamp, tcp_skb_timestamp_us(skb));
 36}
 37
 38/* RACK loss detection (IETF draft draft-ietf-tcpm-rack-01):
 39 *
 40 * Marks a packet lost, if some packet sent later has been (s)acked.
 41 * The underlying idea is similar to the traditional dupthresh and FACK
 42 * but they look at different metrics:
 43 *
 44 * dupthresh: 3 OOO packets delivered (packet count)
 45 * FACK: sequence delta to highest sacked sequence (sequence space)
 46 * RACK: sent time delta to the latest delivered packet (time domain)
 47 *
 48 * The advantage of RACK is it applies to both original and retransmitted
 49 * packet and therefore is robust against tail losses. Another advantage
 50 * is being more resilient to reordering by simply allowing some
 51 * "settling delay", instead of tweaking the dupthresh.
 52 *
 53 * When tcp_rack_detect_loss() detects some packets are lost and we
 54 * are not already in the CA_Recovery state, either tcp_rack_reo_timeout()
 55 * or tcp_time_to_recover()'s "Trick#1: the loss is proven" code path will
 56 * make us enter the CA_Recovery state.
 57 */
 58static void tcp_rack_detect_loss(struct sock *sk, u32 *reo_timeout)
 59{
 60	struct tcp_sock *tp = tcp_sk(sk);
 61	struct sk_buff *skb, *n;
 62	u32 reo_wnd;
 63
 64	*reo_timeout = 0;
 65	reo_wnd = tcp_rack_reo_wnd(sk);
 66	list_for_each_entry_safe(skb, n, &tp->tsorted_sent_queue,
 67				 tcp_tsorted_anchor) {
 68		struct tcp_skb_cb *scb = TCP_SKB_CB(skb);
 69		s32 remaining;
 70
 71		/* Skip ones marked lost but not yet retransmitted */
 72		if ((scb->sacked & TCPCB_LOST) &&
 73		    !(scb->sacked & TCPCB_SACKED_RETRANS))
 74			continue;
 75
 76		if (!tcp_skb_sent_after(tp->rack.mstamp,
 77					tcp_skb_timestamp_us(skb),
 78					tp->rack.end_seq, scb->end_seq))
 79			break;
 80
 81		/* A packet is lost if it has not been s/acked beyond
 82		 * the recent RTT plus the reordering window.
 83		 */
 84		remaining = tcp_rack_skb_timeout(tp, skb, reo_wnd);
 85		if (remaining <= 0) {
 86			tcp_mark_skb_lost(sk, skb);
 87			list_del_init(&skb->tcp_tsorted_anchor);
 88		} else {
 89			/* Record maximum wait time */
 90			*reo_timeout = max_t(u32, *reo_timeout, remaining);
 91		}
 92	}
 93}
 94
 95bool tcp_rack_mark_lost(struct sock *sk)
 96{
 97	struct tcp_sock *tp = tcp_sk(sk);
 98	u32 timeout;
 99
100	if (!tp->rack.advanced)
101		return false;
102
103	/* Reset the advanced flag to avoid unnecessary queue scanning */
104	tp->rack.advanced = 0;
105	tcp_rack_detect_loss(sk, &timeout);
106	if (timeout) {
107		timeout = usecs_to_jiffies(timeout) + TCP_TIMEOUT_MIN;
108		inet_csk_reset_xmit_timer(sk, ICSK_TIME_REO_TIMEOUT,
109					  timeout, inet_csk(sk)->icsk_rto);
110	}
111	return !!timeout;
112}
113
114/* Record the most recently (re)sent time among the (s)acked packets
115 * This is "Step 3: Advance RACK.xmit_time and update RACK.RTT" from
116 * draft-cheng-tcpm-rack-00.txt
117 */
118void tcp_rack_advance(struct tcp_sock *tp, u8 sacked, u32 end_seq,
119		      u64 xmit_time)
120{
121	u32 rtt_us;
122
123	rtt_us = tcp_stamp_us_delta(tp->tcp_mstamp, xmit_time);
124	if (rtt_us < tcp_min_rtt(tp) && (sacked & TCPCB_RETRANS)) {
125		/* If the sacked packet was retransmitted, it's ambiguous
126		 * whether the retransmission or the original (or the prior
127		 * retransmission) was sacked.
128		 *
129		 * If the original is lost, there is no ambiguity. Otherwise
130		 * we assume the original can be delayed up to aRTT + min_rtt.
131		 * the aRTT term is bounded by the fast recovery or timeout,
132		 * so it's at least one RTT (i.e., retransmission is at least
133		 * an RTT later).
134		 */
135		return;
136	}
137	tp->rack.advanced = 1;
138	tp->rack.rtt_us = rtt_us;
139	if (tcp_skb_sent_after(xmit_time, tp->rack.mstamp,
140			       end_seq, tp->rack.end_seq)) {
141		tp->rack.mstamp = xmit_time;
142		tp->rack.end_seq = end_seq;
143	}
144}
145
146/* We have waited long enough to accommodate reordering. Mark the expired
147 * packets lost and retransmit them.
148 */
149void tcp_rack_reo_timeout(struct sock *sk)
150{
151	struct tcp_sock *tp = tcp_sk(sk);
152	u32 timeout, prior_inflight;
153	u32 lost = tp->lost;
154
155	prior_inflight = tcp_packets_in_flight(tp);
156	tcp_rack_detect_loss(sk, &timeout);
157	if (prior_inflight != tcp_packets_in_flight(tp)) {
158		if (inet_csk(sk)->icsk_ca_state != TCP_CA_Recovery) {
159			tcp_enter_recovery(sk, false);
160			if (!inet_csk(sk)->icsk_ca_ops->cong_control)
161				tcp_cwnd_reduction(sk, 1, tp->lost - lost, 0);
162		}
163		tcp_xmit_retransmit_queue(sk);
164	}
165	if (inet_csk(sk)->icsk_pending != ICSK_TIME_RETRANS)
166		tcp_rearm_rto(sk);
167}
168
169/* Updates the RACK's reo_wnd based on DSACK and no. of recoveries.
170 *
171 * If a DSACK is received that seems like it may have been due to reordering
172 * triggering fast recovery, increment reo_wnd by min_rtt/4 (upper bounded
173 * by srtt), since there is possibility that spurious retransmission was
174 * due to reordering delay longer than reo_wnd.
175 *
176 * Persist the current reo_wnd value for TCP_RACK_RECOVERY_THRESH (16)
177 * no. of successful recoveries (accounts for full DSACK-based loss
178 * recovery undo). After that, reset it to default (min_rtt/4).
179 *
180 * At max, reo_wnd is incremented only once per rtt. So that the new
181 * DSACK on which we are reacting, is due to the spurious retx (approx)
182 * after the reo_wnd has been updated last time.
183 *
184 * reo_wnd is tracked in terms of steps (of min_rtt/4), rather than
185 * absolute value to account for change in rtt.
186 */
187void tcp_rack_update_reo_wnd(struct sock *sk, struct rate_sample *rs)
188{
189	struct tcp_sock *tp = tcp_sk(sk);
190
191	if ((READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_recovery) &
192	     TCP_RACK_STATIC_REO_WND) ||
193	    !rs->prior_delivered)
194		return;
195
196	/* Disregard DSACK if a rtt has not passed since we adjusted reo_wnd */
197	if (before(rs->prior_delivered, tp->rack.last_delivered))
198		tp->rack.dsack_seen = 0;
199
200	/* Adjust the reo_wnd if update is pending */
201	if (tp->rack.dsack_seen) {
202		tp->rack.reo_wnd_steps = min_t(u32, 0xFF,
203					       tp->rack.reo_wnd_steps + 1);
204		tp->rack.dsack_seen = 0;
205		tp->rack.last_delivered = tp->delivered;
206		tp->rack.reo_wnd_persist = TCP_RACK_RECOVERY_THRESH;
207	} else if (!tp->rack.reo_wnd_persist) {
208		tp->rack.reo_wnd_steps = 1;
209	}
210}
211
212/* RFC6582 NewReno recovery for non-SACK connection. It simply retransmits
213 * the next unacked packet upon receiving
214 * a) three or more DUPACKs to start the fast recovery
215 * b) an ACK acknowledging new data during the fast recovery.
216 */
217void tcp_newreno_mark_lost(struct sock *sk, bool snd_una_advanced)
218{
219	const u8 state = inet_csk(sk)->icsk_ca_state;
220	struct tcp_sock *tp = tcp_sk(sk);
221
222	if ((state < TCP_CA_Recovery && tp->sacked_out >= tp->reordering) ||
223	    (state == TCP_CA_Recovery && snd_una_advanced)) {
224		struct sk_buff *skb = tcp_rtx_queue_head(sk);
225		u32 mss;
226
227		if (TCP_SKB_CB(skb)->sacked & TCPCB_LOST)
228			return;
229
230		mss = tcp_skb_mss(skb);
231		if (tcp_skb_pcount(skb) > 1 && skb->len > mss)
232			tcp_fragment(sk, TCP_FRAG_IN_RTX_QUEUE, skb,
233				     mss, mss, GFP_ATOMIC);
234
235		tcp_mark_skb_lost(sk, skb);
236	}
237}