xref: /openbmc/linux/net/ipv4/tcp_recovery.c (revision 1c2709cf)
1 // SPDX-License-Identifier: GPL-2.0
2 #include <linux/tcp.h>
3 #include <net/tcp.h>
4 
tcp_rack_reo_wnd(const struct sock * sk)5 static u32 tcp_rack_reo_wnd(const struct sock *sk)
6 {
7 	const 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 
tcp_rack_skb_timeout(struct tcp_sock * tp,struct sk_buff * skb,u32 reo_wnd)32 s32 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  */
tcp_rack_detect_loss(struct sock * sk,u32 * reo_timeout)58 static 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 
tcp_rack_mark_lost(struct sock * sk)95 bool 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_US);
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  */
tcp_rack_advance(struct tcp_sock * tp,u8 sacked,u32 end_seq,u64 xmit_time)118 void 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  */
tcp_rack_reo_timeout(struct sock * sk)149 void 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  */
tcp_rack_update_reo_wnd(struct sock * sk,struct rate_sample * rs)187 void 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  */
tcp_newreno_mark_lost(struct sock * sk,bool snd_una_advanced)217 void 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 }
238