1 // SPDX-License-Identifier: GPL-2.0 2 #include <linux/tcp.h> 3 #include <net/tcp.h> 4 5 static 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 19 static 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 */ 44 static 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 93 void 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 */ 115 void 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 */ 146 void 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 */ 182 void 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 } 205