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1// SPDX-License-Identifier: GPL-2.0
2#include <linux/tcp.h>
3#include <net/tcp.h>
4
5static 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
10static 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
36s32 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 */
62static 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
99bool 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 false;
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 return !!timeout;
116}
117
118/* Record the most recently (re)sent time among the (s)acked packets
119 * This is "Step 3: Advance RACK.xmit_time and update RACK.RTT" from
120 * draft-cheng-tcpm-rack-00.txt
121 */
122void tcp_rack_advance(struct tcp_sock *tp, u8 sacked, u32 end_seq,
123 u64 xmit_time)
124{
125 u32 rtt_us;
126
127 rtt_us = tcp_stamp_us_delta(tp->tcp_mstamp, xmit_time);
128 if (rtt_us < tcp_min_rtt(tp) && (sacked & TCPCB_RETRANS)) {
129 /* If the sacked packet was retransmitted, it's ambiguous
130 * whether the retransmission or the original (or the prior
131 * retransmission) was sacked.
132 *
133 * If the original is lost, there is no ambiguity. Otherwise
134 * we assume the original can be delayed up to aRTT + min_rtt.
135 * the aRTT term is bounded by the fast recovery or timeout,
136 * so it's at least one RTT (i.e., retransmission is at least
137 * an RTT later).
138 */
139 return;
140 }
141 tp->rack.advanced = 1;
142 tp->rack.rtt_us = rtt_us;
143 if (tcp_rack_sent_after(xmit_time, tp->rack.mstamp,
144 end_seq, tp->rack.end_seq)) {
145 tp->rack.mstamp = xmit_time;
146 tp->rack.end_seq = end_seq;
147 }
148}
149
150/* We have waited long enough to accommodate reordering. Mark the expired
151 * packets lost and retransmit them.
152 */
153void tcp_rack_reo_timeout(struct sock *sk)
154{
155 struct tcp_sock *tp = tcp_sk(sk);
156 u32 timeout, prior_inflight;
157 u32 lost = tp->lost;
158
159 prior_inflight = tcp_packets_in_flight(tp);
160 tcp_rack_detect_loss(sk, &timeout);
161 if (prior_inflight != tcp_packets_in_flight(tp)) {
162 if (inet_csk(sk)->icsk_ca_state != TCP_CA_Recovery) {
163 tcp_enter_recovery(sk, false);
164 if (!inet_csk(sk)->icsk_ca_ops->cong_control)
165 tcp_cwnd_reduction(sk, 1, tp->lost - lost, 0);
166 }
167 tcp_xmit_retransmit_queue(sk);
168 }
169 if (inet_csk(sk)->icsk_pending != ICSK_TIME_RETRANS)
170 tcp_rearm_rto(sk);
171}
172
173/* Updates the RACK's reo_wnd based on DSACK and no. of recoveries.
174 *
175 * If DSACK is received, increment reo_wnd by min_rtt/4 (upper bounded
176 * by srtt), since there is possibility that spurious retransmission was
177 * due to reordering delay longer than reo_wnd.
178 *
179 * Persist the current reo_wnd value for TCP_RACK_RECOVERY_THRESH (16)
180 * no. of successful recoveries (accounts for full DSACK-based loss
181 * recovery undo). After that, reset it to default (min_rtt/4).
182 *
183 * At max, reo_wnd is incremented only once per rtt. So that the new
184 * DSACK on which we are reacting, is due to the spurious retx (approx)
185 * after the reo_wnd has been updated last time.
186 *
187 * reo_wnd is tracked in terms of steps (of min_rtt/4), rather than
188 * absolute value to account for change in rtt.
189 */
190void tcp_rack_update_reo_wnd(struct sock *sk, struct rate_sample *rs)
191{
192 struct tcp_sock *tp = tcp_sk(sk);
193
194 if (sock_net(sk)->ipv4.sysctl_tcp_recovery & TCP_RACK_STATIC_REO_WND ||
195 !rs->prior_delivered)
196 return;
197
198 /* Disregard DSACK if a rtt has not passed since we adjusted reo_wnd */
199 if (before(rs->prior_delivered, tp->rack.last_delivered))
200 tp->rack.dsack_seen = 0;
201
202 /* Adjust the reo_wnd if update is pending */
203 if (tp->rack.dsack_seen) {
204 tp->rack.reo_wnd_steps = min_t(u32, 0xFF,
205 tp->rack.reo_wnd_steps + 1);
206 tp->rack.dsack_seen = 0;
207 tp->rack.last_delivered = tp->delivered;
208 tp->rack.reo_wnd_persist = TCP_RACK_RECOVERY_THRESH;
209 } else if (!tp->rack.reo_wnd_persist) {
210 tp->rack.reo_wnd_steps = 1;
211 }
212}
213
214/* RFC6582 NewReno recovery for non-SACK connection. It simply retransmits
215 * the next unacked packet upon receiving
216 * a) three or more DUPACKs to start the fast recovery
217 * b) an ACK acknowledging new data during the fast recovery.
218 */
219void tcp_newreno_mark_lost(struct sock *sk, bool snd_una_advanced)
220{
221 const u8 state = inet_csk(sk)->icsk_ca_state;
222 struct tcp_sock *tp = tcp_sk(sk);
223
224 if ((state < TCP_CA_Recovery && tp->sacked_out >= tp->reordering) ||
225 (state == TCP_CA_Recovery && snd_una_advanced)) {
226 struct sk_buff *skb = tcp_rtx_queue_head(sk);
227 u32 mss;
228
229 if (TCP_SKB_CB(skb)->sacked & TCPCB_LOST)
230 return;
231
232 mss = tcp_skb_mss(skb);
233 if (tcp_skb_pcount(skb) > 1 && skb->len > mss)
234 tcp_fragment(sk, TCP_FRAG_IN_RTX_QUEUE, skb,
235 mss, mss, GFP_ATOMIC);
236
237 tcp_mark_skb_lost(sk, skb);
238 }
239}
1// SPDX-License-Identifier: GPL-2.0
2#include <linux/tcp.h>
3#include <net/tcp.h>
4
5static void tcp_rack_mark_skb_lost(struct sock *sk, struct sk_buff *skb)
6{
7 struct tcp_sock *tp = tcp_sk(sk);
8
9 tcp_skb_mark_lost_uncond_verify(tp, skb);
10 if (TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_RETRANS) {
11 /* Account for retransmits that are lost again */
12 TCP_SKB_CB(skb)->sacked &= ~TCPCB_SACKED_RETRANS;
13 tp->retrans_out -= tcp_skb_pcount(skb);
14 NET_ADD_STATS(sock_net(sk), LINUX_MIB_TCPLOSTRETRANSMIT,
15 tcp_skb_pcount(skb));
16 }
17}
18
19static bool tcp_rack_sent_after(u64 t1, u64 t2, u32 seq1, u32 seq2)
20{
21 return t1 > t2 || (t1 == t2 && after(seq1, seq2));
22}
23
24/* RACK loss detection (IETF draft draft-ietf-tcpm-rack-01):
25 *
26 * Marks a packet lost, if some packet sent later has been (s)acked.
27 * The underlying idea is similar to the traditional dupthresh and FACK
28 * but they look at different metrics:
29 *
30 * dupthresh: 3 OOO packets delivered (packet count)
31 * FACK: sequence delta to highest sacked sequence (sequence space)
32 * RACK: sent time delta to the latest delivered packet (time domain)
33 *
34 * The advantage of RACK is it applies to both original and retransmitted
35 * packet and therefore is robust against tail losses. Another advantage
36 * is being more resilient to reordering by simply allowing some
37 * "settling delay", instead of tweaking the dupthresh.
38 *
39 * When tcp_rack_detect_loss() detects some packets are lost and we
40 * are not already in the CA_Recovery state, either tcp_rack_reo_timeout()
41 * or tcp_time_to_recover()'s "Trick#1: the loss is proven" code path will
42 * make us enter the CA_Recovery state.
43 */
44static void tcp_rack_detect_loss(struct sock *sk, u32 *reo_timeout)
45{
46 struct tcp_sock *tp = tcp_sk(sk);
47 u32 min_rtt = tcp_min_rtt(tp);
48 struct sk_buff *skb, *n;
49 u32 reo_wnd;
50
51 *reo_timeout = 0;
52 /* To be more reordering resilient, allow min_rtt/4 settling delay
53 * (lower-bounded to 1000uS). We use min_rtt instead of the smoothed
54 * RTT because reordering is often a path property and less related
55 * to queuing or delayed ACKs.
56 */
57 reo_wnd = 1000;
58 if ((tp->rack.reord || inet_csk(sk)->icsk_ca_state < TCP_CA_Recovery) &&
59 min_rtt != ~0U) {
60 reo_wnd = max((min_rtt >> 2) * tp->rack.reo_wnd_steps, reo_wnd);
61 reo_wnd = min(reo_wnd, tp->srtt_us >> 3);
62 }
63
64 list_for_each_entry_safe(skb, n, &tp->tsorted_sent_queue,
65 tcp_tsorted_anchor) {
66 struct tcp_skb_cb *scb = TCP_SKB_CB(skb);
67 s32 remaining;
68
69 /* Skip ones marked lost but not yet retransmitted */
70 if ((scb->sacked & TCPCB_LOST) &&
71 !(scb->sacked & TCPCB_SACKED_RETRANS))
72 continue;
73
74 if (!tcp_rack_sent_after(tp->rack.mstamp, skb->skb_mstamp,
75 tp->rack.end_seq, scb->end_seq))
76 break;
77
78 /* A packet is lost if it has not been s/acked beyond
79 * the recent RTT plus the reordering window.
80 */
81 remaining = tp->rack.rtt_us + reo_wnd -
82 tcp_stamp_us_delta(tp->tcp_mstamp, skb->skb_mstamp);
83 if (remaining <= 0) {
84 tcp_rack_mark_skb_lost(sk, skb);
85 list_del_init(&skb->tcp_tsorted_anchor);
86 } else {
87 /* Record maximum wait time */
88 *reo_timeout = max_t(u32, *reo_timeout, remaining);
89 }
90 }
91}
92
93void tcp_rack_mark_lost(struct sock *sk)
94{
95 struct tcp_sock *tp = tcp_sk(sk);
96 u32 timeout;
97
98 if (!tp->rack.advanced)
99 return;
100
101 /* Reset the advanced flag to avoid unnecessary queue scanning */
102 tp->rack.advanced = 0;
103 tcp_rack_detect_loss(sk, &timeout);
104 if (timeout) {
105 timeout = usecs_to_jiffies(timeout) + TCP_TIMEOUT_MIN;
106 inet_csk_reset_xmit_timer(sk, ICSK_TIME_REO_TIMEOUT,
107 timeout, inet_csk(sk)->icsk_rto);
108 }
109}
110
111/* Record the most recently (re)sent time among the (s)acked packets
112 * This is "Step 3: Advance RACK.xmit_time and update RACK.RTT" from
113 * draft-cheng-tcpm-rack-00.txt
114 */
115void tcp_rack_advance(struct tcp_sock *tp, u8 sacked, u32 end_seq,
116 u64 xmit_time)
117{
118 u32 rtt_us;
119
120 rtt_us = tcp_stamp_us_delta(tp->tcp_mstamp, xmit_time);
121 if (rtt_us < tcp_min_rtt(tp) && (sacked & TCPCB_RETRANS)) {
122 /* If the sacked packet was retransmitted, it's ambiguous
123 * whether the retransmission or the original (or the prior
124 * retransmission) was sacked.
125 *
126 * If the original is lost, there is no ambiguity. Otherwise
127 * we assume the original can be delayed up to aRTT + min_rtt.
128 * the aRTT term is bounded by the fast recovery or timeout,
129 * so it's at least one RTT (i.e., retransmission is at least
130 * an RTT later).
131 */
132 return;
133 }
134 tp->rack.advanced = 1;
135 tp->rack.rtt_us = rtt_us;
136 if (tcp_rack_sent_after(xmit_time, tp->rack.mstamp,
137 end_seq, tp->rack.end_seq)) {
138 tp->rack.mstamp = xmit_time;
139 tp->rack.end_seq = end_seq;
140 }
141}
142
143/* We have waited long enough to accommodate reordering. Mark the expired
144 * packets lost and retransmit them.
145 */
146void tcp_rack_reo_timeout(struct sock *sk)
147{
148 struct tcp_sock *tp = tcp_sk(sk);
149 u32 timeout, prior_inflight;
150
151 prior_inflight = tcp_packets_in_flight(tp);
152 tcp_rack_detect_loss(sk, &timeout);
153 if (prior_inflight != tcp_packets_in_flight(tp)) {
154 if (inet_csk(sk)->icsk_ca_state != TCP_CA_Recovery) {
155 tcp_enter_recovery(sk, false);
156 if (!inet_csk(sk)->icsk_ca_ops->cong_control)
157 tcp_cwnd_reduction(sk, 1, 0);
158 }
159 tcp_xmit_retransmit_queue(sk);
160 }
161 if (inet_csk(sk)->icsk_pending != ICSK_TIME_RETRANS)
162 tcp_rearm_rto(sk);
163}
164
165/* Updates the RACK's reo_wnd based on DSACK and no. of recoveries.
166 *
167 * If DSACK is received, increment reo_wnd by min_rtt/4 (upper bounded
168 * by srtt), since there is possibility that spurious retransmission was
169 * due to reordering delay longer than reo_wnd.
170 *
171 * Persist the current reo_wnd value for TCP_RACK_RECOVERY_THRESH (16)
172 * no. of successful recoveries (accounts for full DSACK-based loss
173 * recovery undo). After that, reset it to default (min_rtt/4).
174 *
175 * At max, reo_wnd is incremented only once per rtt. So that the new
176 * DSACK on which we are reacting, is due to the spurious retx (approx)
177 * after the reo_wnd has been updated last time.
178 *
179 * reo_wnd is tracked in terms of steps (of min_rtt/4), rather than
180 * absolute value to account for change in rtt.
181 */
182void tcp_rack_update_reo_wnd(struct sock *sk, struct rate_sample *rs)
183{
184 struct tcp_sock *tp = tcp_sk(sk);
185
186 if (sock_net(sk)->ipv4.sysctl_tcp_recovery & TCP_RACK_STATIC_REO_WND ||
187 !rs->prior_delivered)
188 return;
189
190 /* Disregard DSACK if a rtt has not passed since we adjusted reo_wnd */
191 if (before(rs->prior_delivered, tp->rack.last_delivered))
192 tp->rack.dsack_seen = 0;
193
194 /* Adjust the reo_wnd if update is pending */
195 if (tp->rack.dsack_seen) {
196 tp->rack.reo_wnd_steps = min_t(u32, 0xFF,
197 tp->rack.reo_wnd_steps + 1);
198 tp->rack.dsack_seen = 0;
199 tp->rack.last_delivered = tp->delivered;
200 tp->rack.reo_wnd_persist = TCP_RACK_RECOVERY_THRESH;
201 } else if (!tp->rack.reo_wnd_persist) {
202 tp->rack.reo_wnd_steps = 1;
203 }
204}