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 || !tp->lost_out) && min_rtt != ~0U) { 59 reo_wnd = max((min_rtt >> 2) * tp->rack.reo_wnd_steps, reo_wnd); 60 reo_wnd = min(reo_wnd, tp->srtt_us >> 3); 61 } 62 63 list_for_each_entry_safe(skb, n, &tp->tsorted_sent_queue, 64 tcp_tsorted_anchor) { 65 struct tcp_skb_cb *scb = TCP_SKB_CB(skb); 66 s32 remaining; 67 68 /* Skip ones marked lost but not yet retransmitted */ 69 if ((scb->sacked & TCPCB_LOST) && 70 !(scb->sacked & TCPCB_SACKED_RETRANS)) 71 continue; 72 73 if (!tcp_rack_sent_after(tp->rack.mstamp, skb->skb_mstamp, 74 tp->rack.end_seq, scb->end_seq)) 75 break; 76 77 /* A packet is lost if it has not been s/acked beyond 78 * the recent RTT plus the reordering window. 79 */ 80 remaining = tp->rack.rtt_us + reo_wnd - 81 tcp_stamp_us_delta(tp->tcp_mstamp, skb->skb_mstamp); 82 if (remaining < 0) { 83 tcp_rack_mark_skb_lost(sk, skb); 84 list_del_init(&skb->tcp_tsorted_anchor); 85 } else { 86 /* Record maximum wait time (+1 to avoid 0) */ 87 *reo_timeout = max_t(u32, *reo_timeout, 1 + remaining); 88 } 89 } 90 } 91 92 void tcp_rack_mark_lost(struct sock *sk) 93 { 94 struct tcp_sock *tp = tcp_sk(sk); 95 u32 timeout; 96 97 if (!tp->rack.advanced) 98 return; 99 100 /* Reset the advanced flag to avoid unnecessary queue scanning */ 101 tp->rack.advanced = 0; 102 tcp_rack_detect_loss(sk, &timeout); 103 if (timeout) { 104 timeout = usecs_to_jiffies(timeout) + TCP_TIMEOUT_MIN; 105 inet_csk_reset_xmit_timer(sk, ICSK_TIME_REO_TIMEOUT, 106 timeout, inet_csk(sk)->icsk_rto); 107 } 108 } 109 110 /* Record the most recently (re)sent time among the (s)acked packets 111 * This is "Step 3: Advance RACK.xmit_time and update RACK.RTT" from 112 * draft-cheng-tcpm-rack-00.txt 113 */ 114 void tcp_rack_advance(struct tcp_sock *tp, u8 sacked, u32 end_seq, 115 u64 xmit_time) 116 { 117 u32 rtt_us; 118 119 if (tp->rack.mstamp && 120 !tcp_rack_sent_after(xmit_time, tp->rack.mstamp, 121 end_seq, tp->rack.end_seq)) 122 return; 123 124 rtt_us = tcp_stamp_us_delta(tp->tcp_mstamp, xmit_time); 125 if (sacked & TCPCB_RETRANS) { 126 /* If the sacked packet was retransmitted, it's ambiguous 127 * whether the retransmission or the original (or the prior 128 * retransmission) was sacked. 129 * 130 * If the original is lost, there is no ambiguity. Otherwise 131 * we assume the original can be delayed up to aRTT + min_rtt. 132 * the aRTT term is bounded by the fast recovery or timeout, 133 * so it's at least one RTT (i.e., retransmission is at least 134 * an RTT later). 135 */ 136 if (rtt_us < tcp_min_rtt(tp)) 137 return; 138 } 139 tp->rack.rtt_us = rtt_us; 140 tp->rack.mstamp = xmit_time; 141 tp->rack.end_seq = end_seq; 142 tp->rack.advanced = 1; 143 } 144 145 /* We have waited long enough to accommodate reordering. Mark the expired 146 * packets lost and retransmit them. 147 */ 148 void tcp_rack_reo_timeout(struct sock *sk) 149 { 150 struct tcp_sock *tp = tcp_sk(sk); 151 u32 timeout, prior_inflight; 152 153 prior_inflight = tcp_packets_in_flight(tp); 154 tcp_rack_detect_loss(sk, &timeout); 155 if (prior_inflight != tcp_packets_in_flight(tp)) { 156 if (inet_csk(sk)->icsk_ca_state != TCP_CA_Recovery) { 157 tcp_enter_recovery(sk, false); 158 if (!inet_csk(sk)->icsk_ca_ops->cong_control) 159 tcp_cwnd_reduction(sk, 1, 0); 160 } 161 tcp_xmit_retransmit_queue(sk); 162 } 163 if (inet_csk(sk)->icsk_pending != ICSK_TIME_RETRANS) 164 tcp_rearm_rto(sk); 165 } 166 167 /* Updates the RACK's reo_wnd based on DSACK and no. of recoveries. 168 * 169 * If DSACK is received, increment reo_wnd by min_rtt/4 (upper bounded 170 * by srtt), since there is possibility that spurious retransmission was 171 * due to reordering delay longer than reo_wnd. 172 * 173 * Persist the current reo_wnd value for TCP_RACK_RECOVERY_THRESH (16) 174 * no. of successful recoveries (accounts for full DSACK-based loss 175 * recovery undo). After that, reset it to default (min_rtt/4). 176 * 177 * At max, reo_wnd is incremented only once per rtt. So that the new 178 * DSACK on which we are reacting, is due to the spurious retx (approx) 179 * after the reo_wnd has been updated last time. 180 * 181 * reo_wnd is tracked in terms of steps (of min_rtt/4), rather than 182 * absolute value to account for change in rtt. 183 */ 184 void tcp_rack_update_reo_wnd(struct sock *sk, struct rate_sample *rs) 185 { 186 struct tcp_sock *tp = tcp_sk(sk); 187 188 if (sock_net(sk)->ipv4.sysctl_tcp_recovery & TCP_RACK_STATIC_REO_WND || 189 !rs->prior_delivered) 190 return; 191 192 /* Disregard DSACK if a rtt has not passed since we adjusted reo_wnd */ 193 if (before(rs->prior_delivered, tp->rack.last_delivered)) 194 tp->rack.dsack_seen = 0; 195 196 /* Adjust the reo_wnd if update is pending */ 197 if (tp->rack.dsack_seen) { 198 tp->rack.reo_wnd_steps = min_t(u32, 0xFF, 199 tp->rack.reo_wnd_steps + 1); 200 tp->rack.dsack_seen = 0; 201 tp->rack.last_delivered = tp->delivered; 202 tp->rack.reo_wnd_persist = TCP_RACK_RECOVERY_THRESH; 203 } else if (!tp->rack.reo_wnd_persist) { 204 tp->rack.reo_wnd_steps = 1; 205 } 206 } 207