1 #include <linux/tcp.h> 2 #include <net/tcp.h> 3 4 int sysctl_tcp_recovery __read_mostly = TCP_RACK_LOSS_DETECTION; 5 6 static void tcp_rack_mark_skb_lost(struct sock *sk, struct sk_buff *skb) 7 { 8 struct tcp_sock *tp = tcp_sk(sk); 9 10 tcp_skb_mark_lost_uncond_verify(tp, skb); 11 if (TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_RETRANS) { 12 /* Account for retransmits that are lost again */ 13 TCP_SKB_CB(skb)->sacked &= ~TCPCB_SACKED_RETRANS; 14 tp->retrans_out -= tcp_skb_pcount(skb); 15 NET_ADD_STATS(sock_net(sk), LINUX_MIB_TCPLOSTRETRANSMIT, 16 tcp_skb_pcount(skb)); 17 } 18 } 19 20 static bool tcp_rack_sent_after(u64 t1, u64 t2, u32 seq1, u32 seq2) 21 { 22 return t1 > t2 || (t1 == t2 && after(seq1, seq2)); 23 } 24 25 /* RACK loss detection (IETF draft draft-ietf-tcpm-rack-01): 26 * 27 * Marks a packet lost, if some packet sent later has been (s)acked. 28 * The underlying idea is similar to the traditional dupthresh and FACK 29 * but they look at different metrics: 30 * 31 * dupthresh: 3 OOO packets delivered (packet count) 32 * FACK: sequence delta to highest sacked sequence (sequence space) 33 * RACK: sent time delta to the latest delivered packet (time domain) 34 * 35 * The advantage of RACK is it applies to both original and retransmitted 36 * packet and therefore is robust against tail losses. Another advantage 37 * is being more resilient to reordering by simply allowing some 38 * "settling delay", instead of tweaking the dupthresh. 39 * 40 * When tcp_rack_detect_loss() detects some packets are lost and we 41 * are not already in the CA_Recovery state, either tcp_rack_reo_timeout() 42 * or tcp_time_to_recover()'s "Trick#1: the loss is proven" code path will 43 * make us enter the CA_Recovery state. 44 */ 45 static void tcp_rack_detect_loss(struct sock *sk, u32 *reo_timeout) 46 { 47 struct tcp_sock *tp = tcp_sk(sk); 48 struct sk_buff *skb; 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) && tcp_min_rtt(tp) != ~0U) 59 reo_wnd = max(tcp_min_rtt(tp) >> 2, reo_wnd); 60 61 tcp_for_write_queue(skb, sk) { 62 struct tcp_skb_cb *scb = TCP_SKB_CB(skb); 63 64 if (skb == tcp_send_head(sk)) 65 break; 66 67 /* Skip ones already (s)acked */ 68 if (!after(scb->end_seq, tp->snd_una) || 69 scb->sacked & TCPCB_SACKED_ACKED) 70 continue; 71 72 if (tcp_rack_sent_after(tp->rack.mstamp, skb->skb_mstamp, 73 tp->rack.end_seq, scb->end_seq)) { 74 /* Step 3 in draft-cheng-tcpm-rack-00.txt: 75 * A packet is lost if its elapsed time is beyond 76 * the recent RTT plus the reordering window. 77 */ 78 u32 elapsed = tcp_stamp_us_delta(tp->tcp_mstamp, 79 skb->skb_mstamp); 80 s32 remaining = tp->rack.rtt_us + reo_wnd - elapsed; 81 82 if (remaining < 0) { 83 tcp_rack_mark_skb_lost(sk, skb); 84 continue; 85 } 86 87 /* Skip ones marked lost but not yet retransmitted */ 88 if ((scb->sacked & TCPCB_LOST) && 89 !(scb->sacked & TCPCB_SACKED_RETRANS)) 90 continue; 91 92 /* Record maximum wait time (+1 to avoid 0) */ 93 *reo_timeout = max_t(u32, *reo_timeout, 1 + remaining); 94 95 } else if (!(scb->sacked & TCPCB_RETRANS)) { 96 /* Original data are sent sequentially so stop early 97 * b/c the rest are all sent after rack_sent 98 */ 99 break; 100 } 101 } 102 } 103 104 void tcp_rack_mark_lost(struct sock *sk) 105 { 106 struct tcp_sock *tp = tcp_sk(sk); 107 u32 timeout; 108 109 if (!tp->rack.advanced) 110 return; 111 112 /* Reset the advanced flag to avoid unnecessary queue scanning */ 113 tp->rack.advanced = 0; 114 tcp_rack_detect_loss(sk, &timeout); 115 if (timeout) { 116 timeout = usecs_to_jiffies(timeout) + TCP_TIMEOUT_MIN; 117 inet_csk_reset_xmit_timer(sk, ICSK_TIME_REO_TIMEOUT, 118 timeout, inet_csk(sk)->icsk_rto); 119 } 120 } 121 122 /* Record the most recently (re)sent time among the (s)acked packets 123 * This is "Step 3: Advance RACK.xmit_time and update RACK.RTT" from 124 * draft-cheng-tcpm-rack-00.txt 125 */ 126 void tcp_rack_advance(struct tcp_sock *tp, u8 sacked, u32 end_seq, 127 u64 xmit_time) 128 { 129 u32 rtt_us; 130 131 if (tp->rack.mstamp && 132 !tcp_rack_sent_after(xmit_time, tp->rack.mstamp, 133 end_seq, tp->rack.end_seq)) 134 return; 135 136 rtt_us = tcp_stamp_us_delta(tp->tcp_mstamp, xmit_time); 137 if (sacked & TCPCB_RETRANS) { 138 /* If the sacked packet was retransmitted, it's ambiguous 139 * whether the retransmission or the original (or the prior 140 * retransmission) was sacked. 141 * 142 * If the original is lost, there is no ambiguity. Otherwise 143 * we assume the original can be delayed up to aRTT + min_rtt. 144 * the aRTT term is bounded by the fast recovery or timeout, 145 * so it's at least one RTT (i.e., retransmission is at least 146 * an RTT later). 147 */ 148 if (rtt_us < tcp_min_rtt(tp)) 149 return; 150 } 151 tp->rack.rtt_us = rtt_us; 152 tp->rack.mstamp = xmit_time; 153 tp->rack.end_seq = end_seq; 154 tp->rack.advanced = 1; 155 } 156 157 /* We have waited long enough to accommodate reordering. Mark the expired 158 * packets lost and retransmit them. 159 */ 160 void tcp_rack_reo_timeout(struct sock *sk) 161 { 162 struct tcp_sock *tp = tcp_sk(sk); 163 u32 timeout, prior_inflight; 164 165 prior_inflight = tcp_packets_in_flight(tp); 166 tcp_rack_detect_loss(sk, &timeout); 167 if (prior_inflight != tcp_packets_in_flight(tp)) { 168 if (inet_csk(sk)->icsk_ca_state != TCP_CA_Recovery) { 169 tcp_enter_recovery(sk, false); 170 if (!inet_csk(sk)->icsk_ca_ops->cong_control) 171 tcp_cwnd_reduction(sk, 1, 0); 172 } 173 tcp_xmit_retransmit_queue(sk); 174 } 175 if (inet_csk(sk)->icsk_pending != ICSK_TIME_RETRANS) 176 tcp_rearm_rto(sk); 177 } 178