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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}
1#include <linux/tcp.h>
2#include <net/tcp.h>
3
4int sysctl_tcp_recovery __read_mostly = TCP_RACK_LOST_RETRANS;
5
6/* Marks a packet lost, if some packet sent later has been (s)acked.
7 * The underlying idea is similar to the traditional dupthresh and FACK
8 * but they look at different metrics:
9 *
10 * dupthresh: 3 OOO packets delivered (packet count)
11 * FACK: sequence delta to highest sacked sequence (sequence space)
12 * RACK: sent time delta to the latest delivered packet (time domain)
13 *
14 * The advantage of RACK is it applies to both original and retransmitted
15 * packet and therefore is robust against tail losses. Another advantage
16 * is being more resilient to reordering by simply allowing some
17 * "settling delay", instead of tweaking the dupthresh.
18 *
19 * The current version is only used after recovery starts but can be
20 * easily extended to detect the first loss.
21 */
22int tcp_rack_mark_lost(struct sock *sk)
23{
24 struct tcp_sock *tp = tcp_sk(sk);
25 struct sk_buff *skb;
26 u32 reo_wnd, prior_retrans = tp->retrans_out;
27
28 if (inet_csk(sk)->icsk_ca_state < TCP_CA_Recovery || !tp->rack.advanced)
29 return 0;
30
31 /* Reset the advanced flag to avoid unnecessary queue scanning */
32 tp->rack.advanced = 0;
33
34 /* To be more reordering resilient, allow min_rtt/4 settling delay
35 * (lower-bounded to 1000uS). We use min_rtt instead of the smoothed
36 * RTT because reordering is often a path property and less related
37 * to queuing or delayed ACKs.
38 *
39 * TODO: measure and adapt to the observed reordering delay, and
40 * use a timer to retransmit like the delayed early retransmit.
41 */
42 reo_wnd = 1000;
43 if (tp->rack.reord && tcp_min_rtt(tp) != ~0U)
44 reo_wnd = max(tcp_min_rtt(tp) >> 2, reo_wnd);
45
46 tcp_for_write_queue(skb, sk) {
47 struct tcp_skb_cb *scb = TCP_SKB_CB(skb);
48
49 if (skb == tcp_send_head(sk))
50 break;
51
52 /* Skip ones already (s)acked */
53 if (!after(scb->end_seq, tp->snd_una) ||
54 scb->sacked & TCPCB_SACKED_ACKED)
55 continue;
56
57 if (skb_mstamp_after(&tp->rack.mstamp, &skb->skb_mstamp)) {
58
59 if (skb_mstamp_us_delta(&tp->rack.mstamp,
60 &skb->skb_mstamp) <= reo_wnd)
61 continue;
62
63 /* skb is lost if packet sent later is sacked */
64 tcp_skb_mark_lost_uncond_verify(tp, skb);
65 if (scb->sacked & TCPCB_SACKED_RETRANS) {
66 scb->sacked &= ~TCPCB_SACKED_RETRANS;
67 tp->retrans_out -= tcp_skb_pcount(skb);
68 NET_INC_STATS_BH(sock_net(sk),
69 LINUX_MIB_TCPLOSTRETRANSMIT);
70 }
71 } else if (!(scb->sacked & TCPCB_RETRANS)) {
72 /* Original data are sent sequentially so stop early
73 * b/c the rest are all sent after rack_sent
74 */
75 break;
76 }
77 }
78 return prior_retrans - tp->retrans_out;
79}
80
81/* Record the most recently (re)sent time among the (s)acked packets */
82void tcp_rack_advance(struct tcp_sock *tp,
83 const struct skb_mstamp *xmit_time, u8 sacked)
84{
85 if (tp->rack.mstamp.v64 &&
86 !skb_mstamp_after(xmit_time, &tp->rack.mstamp))
87 return;
88
89 if (sacked & TCPCB_RETRANS) {
90 struct skb_mstamp now;
91
92 /* If the sacked packet was retransmitted, it's ambiguous
93 * whether the retransmission or the original (or the prior
94 * retransmission) was sacked.
95 *
96 * If the original is lost, there is no ambiguity. Otherwise
97 * we assume the original can be delayed up to aRTT + min_rtt.
98 * the aRTT term is bounded by the fast recovery or timeout,
99 * so it's at least one RTT (i.e., retransmission is at least
100 * an RTT later).
101 */
102 skb_mstamp_get(&now);
103 if (skb_mstamp_us_delta(&now, xmit_time) < tcp_min_rtt(tp))
104 return;
105 }
106
107 tp->rack.mstamp = *xmit_time;
108 tp->rack.advanced = 1;
109}