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 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
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(t1: tp->tcp_mstamp, t0: tcp_skb_timestamp_us(skb));
36}
37
38/* RACK loss detection (IETF RFC8985):
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(t1: tp->rack.mstamp,
77 t2: tcp_skb_timestamp_us(skb),
78 seq1: tp->rack.end_seq, seq2: 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(entry: &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, reo_timeout: &timeout);
106 if (timeout) {
107 timeout = usecs_to_jiffies(u: timeout + TCP_TIMEOUT_MIN_US);
108 inet_csk_reset_xmit_timer(sk, ICSK_TIME_REO_TIMEOUT,
109 when: 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(t1: tp->tcp_mstamp, t0: 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(t1: xmit_time, t2: tp->rack.mstamp,
140 seq1: end_seq, seq2: 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, reo_timeout: &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, ece_ack: false);
160 if (!inet_csk(sk)->icsk_ca_ops->cong_control)
161 tcp_cwnd_reduction(sk, newly_acked_sacked: 1, newly_lost: tp->lost - lost, flag: 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(seq1: rs->prior_delivered, seq2: 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_queue: TCP_FRAG_IN_RTX_QUEUE, skb,
233 len: mss, mss_now: mss, GFP_ATOMIC);
234
235 tcp_mark_skb_lost(sk, skb);
236 }
237}
238