xref: /openbmc/linux/net/ipv4/tcp_recovery.c (revision 8b8f095b)
1 // SPDX-License-Identifier: GPL-2.0
2 #include <linux/tcp.h>
3 #include <net/tcp.h>
4 
5 static bool tcp_rack_sent_after(u64 t1, u64 t2, u32 seq1, u32 seq2)
6 {
7 	return t1 > t2 || (t1 == t2 && after(seq1, seq2));
8 }
9 
10 static u32 tcp_rack_reo_wnd(const struct sock *sk)
11 {
12 	struct tcp_sock *tp = tcp_sk(sk);
13 
14 	if (!tp->reord_seen) {
15 		/* If reordering has not been observed, be aggressive during
16 		 * the recovery or starting the recovery by DUPACK threshold.
17 		 */
18 		if (inet_csk(sk)->icsk_ca_state >= TCP_CA_Recovery)
19 			return 0;
20 
21 		if (tp->sacked_out >= tp->reordering &&
22 		    !(sock_net(sk)->ipv4.sysctl_tcp_recovery & TCP_RACK_NO_DUPTHRESH))
23 			return 0;
24 	}
25 
26 	/* To be more reordering resilient, allow min_rtt/4 settling delay.
27 	 * Use min_rtt instead of the smoothed RTT because reordering is
28 	 * often a path property and less related to queuing or delayed ACKs.
29 	 * Upon receiving DSACKs, linearly increase the window up to the
30 	 * smoothed RTT.
31 	 */
32 	return min((tcp_min_rtt(tp) >> 2) * tp->rack.reo_wnd_steps,
33 		   tp->srtt_us >> 3);
34 }
35 
36 s32 tcp_rack_skb_timeout(struct tcp_sock *tp, struct sk_buff *skb, u32 reo_wnd)
37 {
38 	return tp->rack.rtt_us + reo_wnd -
39 	       tcp_stamp_us_delta(tp->tcp_mstamp, tcp_skb_timestamp_us(skb));
40 }
41 
42 /* RACK loss detection (IETF draft draft-ietf-tcpm-rack-01):
43  *
44  * Marks a packet lost, if some packet sent later has been (s)acked.
45  * The underlying idea is similar to the traditional dupthresh and FACK
46  * but they look at different metrics:
47  *
48  * dupthresh: 3 OOO packets delivered (packet count)
49  * FACK: sequence delta to highest sacked sequence (sequence space)
50  * RACK: sent time delta to the latest delivered packet (time domain)
51  *
52  * The advantage of RACK is it applies to both original and retransmitted
53  * packet and therefore is robust against tail losses. Another advantage
54  * is being more resilient to reordering by simply allowing some
55  * "settling delay", instead of tweaking the dupthresh.
56  *
57  * When tcp_rack_detect_loss() detects some packets are lost and we
58  * are not already in the CA_Recovery state, either tcp_rack_reo_timeout()
59  * or tcp_time_to_recover()'s "Trick#1: the loss is proven" code path will
60  * make us enter the CA_Recovery state.
61  */
62 static void tcp_rack_detect_loss(struct sock *sk, u32 *reo_timeout)
63 {
64 	struct tcp_sock *tp = tcp_sk(sk);
65 	struct sk_buff *skb, *n;
66 	u32 reo_wnd;
67 
68 	*reo_timeout = 0;
69 	reo_wnd = tcp_rack_reo_wnd(sk);
70 	list_for_each_entry_safe(skb, n, &tp->tsorted_sent_queue,
71 				 tcp_tsorted_anchor) {
72 		struct tcp_skb_cb *scb = TCP_SKB_CB(skb);
73 		s32 remaining;
74 
75 		/* Skip ones marked lost but not yet retransmitted */
76 		if ((scb->sacked & TCPCB_LOST) &&
77 		    !(scb->sacked & TCPCB_SACKED_RETRANS))
78 			continue;
79 
80 		if (!tcp_rack_sent_after(tp->rack.mstamp,
81 					 tcp_skb_timestamp_us(skb),
82 					 tp->rack.end_seq, scb->end_seq))
83 			break;
84 
85 		/* A packet is lost if it has not been s/acked beyond
86 		 * the recent RTT plus the reordering window.
87 		 */
88 		remaining = tcp_rack_skb_timeout(tp, skb, reo_wnd);
89 		if (remaining <= 0) {
90 			tcp_mark_skb_lost(sk, skb);
91 			list_del_init(&skb->tcp_tsorted_anchor);
92 		} else {
93 			/* Record maximum wait time */
94 			*reo_timeout = max_t(u32, *reo_timeout, remaining);
95 		}
96 	}
97 }
98 
99 void tcp_rack_mark_lost(struct sock *sk)
100 {
101 	struct tcp_sock *tp = tcp_sk(sk);
102 	u32 timeout;
103 
104 	if (!tp->rack.advanced)
105 		return;
106 
107 	/* Reset the advanced flag to avoid unnecessary queue scanning */
108 	tp->rack.advanced = 0;
109 	tcp_rack_detect_loss(sk, &timeout);
110 	if (timeout) {
111 		timeout = usecs_to_jiffies(timeout) + TCP_TIMEOUT_MIN;
112 		inet_csk_reset_xmit_timer(sk, ICSK_TIME_REO_TIMEOUT,
113 					  timeout, inet_csk(sk)->icsk_rto);
114 	}
115 }
116 
117 /* Record the most recently (re)sent time among the (s)acked packets
118  * This is "Step 3: Advance RACK.xmit_time and update RACK.RTT" from
119  * draft-cheng-tcpm-rack-00.txt
120  */
121 void tcp_rack_advance(struct tcp_sock *tp, u8 sacked, u32 end_seq,
122 		      u64 xmit_time)
123 {
124 	u32 rtt_us;
125 
126 	rtt_us = tcp_stamp_us_delta(tp->tcp_mstamp, xmit_time);
127 	if (rtt_us < tcp_min_rtt(tp) && (sacked & TCPCB_RETRANS)) {
128 		/* If the sacked packet was retransmitted, it's ambiguous
129 		 * whether the retransmission or the original (or the prior
130 		 * retransmission) was sacked.
131 		 *
132 		 * If the original is lost, there is no ambiguity. Otherwise
133 		 * we assume the original can be delayed up to aRTT + min_rtt.
134 		 * the aRTT term is bounded by the fast recovery or timeout,
135 		 * so it's at least one RTT (i.e., retransmission is at least
136 		 * an RTT later).
137 		 */
138 		return;
139 	}
140 	tp->rack.advanced = 1;
141 	tp->rack.rtt_us = rtt_us;
142 	if (tcp_rack_sent_after(xmit_time, tp->rack.mstamp,
143 				end_seq, tp->rack.end_seq)) {
144 		tp->rack.mstamp = xmit_time;
145 		tp->rack.end_seq = end_seq;
146 	}
147 }
148 
149 /* We have waited long enough to accommodate reordering. Mark the expired
150  * packets lost and retransmit them.
151  */
152 void tcp_rack_reo_timeout(struct sock *sk)
153 {
154 	struct tcp_sock *tp = tcp_sk(sk);
155 	u32 timeout, prior_inflight;
156 	u32 lost = tp->lost;
157 
158 	prior_inflight = tcp_packets_in_flight(tp);
159 	tcp_rack_detect_loss(sk, &timeout);
160 	if (prior_inflight != tcp_packets_in_flight(tp)) {
161 		if (inet_csk(sk)->icsk_ca_state != TCP_CA_Recovery) {
162 			tcp_enter_recovery(sk, false);
163 			if (!inet_csk(sk)->icsk_ca_ops->cong_control)
164 				tcp_cwnd_reduction(sk, 1, tp->lost - lost, 0);
165 		}
166 		tcp_xmit_retransmit_queue(sk);
167 	}
168 	if (inet_csk(sk)->icsk_pending != ICSK_TIME_RETRANS)
169 		tcp_rearm_rto(sk);
170 }
171 
172 /* Updates the RACK's reo_wnd based on DSACK and no. of recoveries.
173  *
174  * If DSACK is received, increment reo_wnd by min_rtt/4 (upper bounded
175  * by srtt), since there is possibility that spurious retransmission was
176  * due to reordering delay longer than reo_wnd.
177  *
178  * Persist the current reo_wnd value for TCP_RACK_RECOVERY_THRESH (16)
179  * no. of successful recoveries (accounts for full DSACK-based loss
180  * recovery undo). After that, reset it to default (min_rtt/4).
181  *
182  * At max, reo_wnd is incremented only once per rtt. So that the new
183  * DSACK on which we are reacting, is due to the spurious retx (approx)
184  * after the reo_wnd has been updated last time.
185  *
186  * reo_wnd is tracked in terms of steps (of min_rtt/4), rather than
187  * absolute value to account for change in rtt.
188  */
189 void tcp_rack_update_reo_wnd(struct sock *sk, struct rate_sample *rs)
190 {
191 	struct tcp_sock *tp = tcp_sk(sk);
192 
193 	if (sock_net(sk)->ipv4.sysctl_tcp_recovery & TCP_RACK_STATIC_REO_WND ||
194 	    !rs->prior_delivered)
195 		return;
196 
197 	/* Disregard DSACK if a rtt has not passed since we adjusted reo_wnd */
198 	if (before(rs->prior_delivered, tp->rack.last_delivered))
199 		tp->rack.dsack_seen = 0;
200 
201 	/* Adjust the reo_wnd if update is pending */
202 	if (tp->rack.dsack_seen) {
203 		tp->rack.reo_wnd_steps = min_t(u32, 0xFF,
204 					       tp->rack.reo_wnd_steps + 1);
205 		tp->rack.dsack_seen = 0;
206 		tp->rack.last_delivered = tp->delivered;
207 		tp->rack.reo_wnd_persist = TCP_RACK_RECOVERY_THRESH;
208 	} else if (!tp->rack.reo_wnd_persist) {
209 		tp->rack.reo_wnd_steps = 1;
210 	}
211 }
212 
213 /* RFC6582 NewReno recovery for non-SACK connection. It simply retransmits
214  * the next unacked packet upon receiving
215  * a) three or more DUPACKs to start the fast recovery
216  * b) an ACK acknowledging new data during the fast recovery.
217  */
218 void tcp_newreno_mark_lost(struct sock *sk, bool snd_una_advanced)
219 {
220 	const u8 state = inet_csk(sk)->icsk_ca_state;
221 	struct tcp_sock *tp = tcp_sk(sk);
222 
223 	if ((state < TCP_CA_Recovery && tp->sacked_out >= tp->reordering) ||
224 	    (state == TCP_CA_Recovery && snd_una_advanced)) {
225 		struct sk_buff *skb = tcp_rtx_queue_head(sk);
226 		u32 mss;
227 
228 		if (TCP_SKB_CB(skb)->sacked & TCPCB_LOST)
229 			return;
230 
231 		mss = tcp_skb_mss(skb);
232 		if (tcp_skb_pcount(skb) > 1 && skb->len > mss)
233 			tcp_fragment(sk, TCP_FRAG_IN_RTX_QUEUE, skb,
234 				     mss, mss, GFP_ATOMIC);
235 
236 		tcp_mark_skb_lost(sk, skb);
237 	}
238 }
239