1// SPDX-License-Identifier: GPL-2.0
2#include <linux/tcp.h>
3#include <net/tcp.h>
4
5void tcp_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
24static u32 tcp_rack_reo_wnd(const struct sock *sk)
25{
26 struct tcp_sock *tp = tcp_sk(sk);
27
28 if (!tp->reord_seen) {
29 /* If reordering has not been observed, be aggressive during
30 * the recovery or starting the recovery by DUPACK threshold.
31 */
32 if (inet_csk(sk)->icsk_ca_state >= TCP_CA_Recovery)
33 return 0;
34
35 if (tp->sacked_out >= tp->reordering &&
36 !(sock_net(sk)->ipv4.sysctl_tcp_recovery & TCP_RACK_NO_DUPTHRESH))
37 return 0;
38 }
39
40 /* To be more reordering resilient, allow min_rtt/4 settling delay.
41 * Use min_rtt instead of the smoothed RTT because reordering is
42 * often a path property and less related to queuing or delayed ACKs.
43 * Upon receiving DSACKs, linearly increase the window up to the
44 * smoothed RTT.
45 */
46 return min((tcp_min_rtt(tp) >> 2) * tp->rack.reo_wnd_steps,
47 tp->srtt_us >> 3);
48}
49
50s32 tcp_rack_skb_timeout(struct tcp_sock *tp, struct sk_buff *skb, u32 reo_wnd)
51{
52 return tp->rack.rtt_us + reo_wnd -
53 tcp_stamp_us_delta(tp->tcp_mstamp, tcp_skb_timestamp_us(skb));
54}
55
56/* RACK loss detection (IETF draft draft-ietf-tcpm-rack-01):
57 *
58 * Marks a packet lost, if some packet sent later has been (s)acked.
59 * The underlying idea is similar to the traditional dupthresh and FACK
60 * but they look at different metrics:
61 *
62 * dupthresh: 3 OOO packets delivered (packet count)
63 * FACK: sequence delta to highest sacked sequence (sequence space)
64 * RACK: sent time delta to the latest delivered packet (time domain)
65 *
66 * The advantage of RACK is it applies to both original and retransmitted
67 * packet and therefore is robust against tail losses. Another advantage
68 * is being more resilient to reordering by simply allowing some
69 * "settling delay", instead of tweaking the dupthresh.
70 *
71 * When tcp_rack_detect_loss() detects some packets are lost and we
72 * are not already in the CA_Recovery state, either tcp_rack_reo_timeout()
73 * or tcp_time_to_recover()'s "Trick#1: the loss is proven" code path will
74 * make us enter the CA_Recovery state.
75 */
76static void tcp_rack_detect_loss(struct sock *sk, u32 *reo_timeout)
77{
78 struct tcp_sock *tp = tcp_sk(sk);
79 struct sk_buff *skb, *n;
80 u32 reo_wnd;
81
82 *reo_timeout = 0;
83 reo_wnd = tcp_rack_reo_wnd(sk);
84 list_for_each_entry_safe(skb, n, &tp->tsorted_sent_queue,
85 tcp_tsorted_anchor) {
86 struct tcp_skb_cb *scb = TCP_SKB_CB(skb);
87 s32 remaining;
88
89 /* Skip ones marked lost but not yet retransmitted */
90 if ((scb->sacked & TCPCB_LOST) &&
91 !(scb->sacked & TCPCB_SACKED_RETRANS))
92 continue;
93
94 if (!tcp_rack_sent_after(tp->rack.mstamp,
95 tcp_skb_timestamp_us(skb),
96 tp->rack.end_seq, scb->end_seq))
97 break;
98
99 /* A packet is lost if it has not been s/acked beyond
100 * the recent RTT plus the reordering window.
101 */
102 remaining = tcp_rack_skb_timeout(tp, skb, reo_wnd);
103 if (remaining <= 0) {
104 tcp_mark_skb_lost(sk, skb);
105 list_del_init(&skb->tcp_tsorted_anchor);
106 } else {
107 /* Record maximum wait time */
108 *reo_timeout = max_t(u32, *reo_timeout, remaining);
109 }
110 }
111}
112
113void tcp_rack_mark_lost(struct sock *sk)
114{
115 struct tcp_sock *tp = tcp_sk(sk);
116 u32 timeout;
117
118 if (!tp->rack.advanced)
119 return;
120
121 /* Reset the advanced flag to avoid unnecessary queue scanning */
122 tp->rack.advanced = 0;
123 tcp_rack_detect_loss(sk, &timeout);
124 if (timeout) {
125 timeout = usecs_to_jiffies(timeout) + TCP_TIMEOUT_MIN;
126 inet_csk_reset_xmit_timer(sk, ICSK_TIME_REO_TIMEOUT,
127 timeout, inet_csk(sk)->icsk_rto);
128 }
129}
130
131/* Record the most recently (re)sent time among the (s)acked packets
132 * This is "Step 3: Advance RACK.xmit_time and update RACK.RTT" from
133 * draft-cheng-tcpm-rack-00.txt
134 */
135void tcp_rack_advance(struct tcp_sock *tp, u8 sacked, u32 end_seq,
136 u64 xmit_time)
137{
138 u32 rtt_us;
139
140 rtt_us = tcp_stamp_us_delta(tp->tcp_mstamp, xmit_time);
141 if (rtt_us < tcp_min_rtt(tp) && (sacked & TCPCB_RETRANS)) {
142 /* If the sacked packet was retransmitted, it's ambiguous
143 * whether the retransmission or the original (or the prior
144 * retransmission) was sacked.
145 *
146 * If the original is lost, there is no ambiguity. Otherwise
147 * we assume the original can be delayed up to aRTT + min_rtt.
148 * the aRTT term is bounded by the fast recovery or timeout,
149 * so it's at least one RTT (i.e., retransmission is at least
150 * an RTT later).
151 */
152 return;
153 }
154 tp->rack.advanced = 1;
155 tp->rack.rtt_us = rtt_us;
156 if (tcp_rack_sent_after(xmit_time, tp->rack.mstamp,
157 end_seq, tp->rack.end_seq)) {
158 tp->rack.mstamp = xmit_time;
159 tp->rack.end_seq = end_seq;
160 }
161}
162
163/* We have waited long enough to accommodate reordering. Mark the expired
164 * packets lost and retransmit them.
165 */
166void tcp_rack_reo_timeout(struct sock *sk)
167{
168 struct tcp_sock *tp = tcp_sk(sk);
169 u32 timeout, prior_inflight;
170
171 prior_inflight = tcp_packets_in_flight(tp);
172 tcp_rack_detect_loss(sk, &timeout);
173 if (prior_inflight != tcp_packets_in_flight(tp)) {
174 if (inet_csk(sk)->icsk_ca_state != TCP_CA_Recovery) {
175 tcp_enter_recovery(sk, false);
176 if (!inet_csk(sk)->icsk_ca_ops->cong_control)
177 tcp_cwnd_reduction(sk, 1, 0);
178 }
179 tcp_xmit_retransmit_queue(sk);
180 }
181 if (inet_csk(sk)->icsk_pending != ICSK_TIME_RETRANS)
182 tcp_rearm_rto(sk);
183}
184
185/* Updates the RACK's reo_wnd based on DSACK and no. of recoveries.
186 *
187 * If DSACK is received, increment reo_wnd by min_rtt/4 (upper bounded
188 * by srtt), since there is possibility that spurious retransmission was
189 * due to reordering delay longer than reo_wnd.
190 *
191 * Persist the current reo_wnd value for TCP_RACK_RECOVERY_THRESH (16)
192 * no. of successful recoveries (accounts for full DSACK-based loss
193 * recovery undo). After that, reset it to default (min_rtt/4).
194 *
195 * At max, reo_wnd is incremented only once per rtt. So that the new
196 * DSACK on which we are reacting, is due to the spurious retx (approx)
197 * after the reo_wnd has been updated last time.
198 *
199 * reo_wnd is tracked in terms of steps (of min_rtt/4), rather than
200 * absolute value to account for change in rtt.
201 */
202void tcp_rack_update_reo_wnd(struct sock *sk, struct rate_sample *rs)
203{
204 struct tcp_sock *tp = tcp_sk(sk);
205
206 if (sock_net(sk)->ipv4.sysctl_tcp_recovery & TCP_RACK_STATIC_REO_WND ||
207 !rs->prior_delivered)
208 return;
209
210 /* Disregard DSACK if a rtt has not passed since we adjusted reo_wnd */
211 if (before(rs->prior_delivered, tp->rack.last_delivered))
212 tp->rack.dsack_seen = 0;
213
214 /* Adjust the reo_wnd if update is pending */
215 if (tp->rack.dsack_seen) {
216 tp->rack.reo_wnd_steps = min_t(u32, 0xFF,
217 tp->rack.reo_wnd_steps + 1);
218 tp->rack.dsack_seen = 0;
219 tp->rack.last_delivered = tp->delivered;
220 tp->rack.reo_wnd_persist = TCP_RACK_RECOVERY_THRESH;
221 } else if (!tp->rack.reo_wnd_persist) {
222 tp->rack.reo_wnd_steps = 1;
223 }
224}
225
226/* RFC6582 NewReno recovery for non-SACK connection. It simply retransmits
227 * the next unacked packet upon receiving
228 * a) three or more DUPACKs to start the fast recovery
229 * b) an ACK acknowledging new data during the fast recovery.
230 */
231void tcp_newreno_mark_lost(struct sock *sk, bool snd_una_advanced)
232{
233 const u8 state = inet_csk(sk)->icsk_ca_state;
234 struct tcp_sock *tp = tcp_sk(sk);
235
236 if ((state < TCP_CA_Recovery && tp->sacked_out >= tp->reordering) ||
237 (state == TCP_CA_Recovery && snd_una_advanced)) {
238 struct sk_buff *skb = tcp_rtx_queue_head(sk);
239 u32 mss;
240
241 if (TCP_SKB_CB(skb)->sacked & TCPCB_LOST)
242 return;
243
244 mss = tcp_skb_mss(skb);
245 if (tcp_skb_pcount(skb) > 1 && skb->len > mss)
246 tcp_fragment(sk, TCP_FRAG_IN_RTX_QUEUE, skb,
247 mss, mss, GFP_ATOMIC);
248
249 tcp_skb_mark_lost_uncond_verify(tp, skb);
250 }
251}
252