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_INC_STATS(sock_net(sk), LINUX_MIB_TCPLOSTRETRANSMIT); 16 } 17 } 18 19 static bool tcp_rack_sent_after(const struct skb_mstamp *t1, 20 const struct skb_mstamp *t2, 21 u32 seq1, u32 seq2) 22 { 23 return skb_mstamp_after(t1, t2) || 24 (t1->v64 == t2->v64 && after(seq1, seq2)); 25 } 26 27 /* RACK loss detection (IETF draft draft-ietf-tcpm-rack-01): 28 * 29 * Marks a packet lost, if some packet sent later has been (s)acked. 30 * The underlying idea is similar to the traditional dupthresh and FACK 31 * but they look at different metrics: 32 * 33 * dupthresh: 3 OOO packets delivered (packet count) 34 * FACK: sequence delta to highest sacked sequence (sequence space) 35 * RACK: sent time delta to the latest delivered packet (time domain) 36 * 37 * The advantage of RACK is it applies to both original and retransmitted 38 * packet and therefore is robust against tail losses. Another advantage 39 * is being more resilient to reordering by simply allowing some 40 * "settling delay", instead of tweaking the dupthresh. 41 * 42 * When tcp_rack_detect_loss() detects some packets are lost and we 43 * are not already in the CA_Recovery state, either tcp_rack_reo_timeout() 44 * or tcp_time_to_recover()'s "Trick#1: the loss is proven" code path will 45 * make us enter the CA_Recovery state. 46 */ 47 static void tcp_rack_detect_loss(struct sock *sk, const struct skb_mstamp *now, 48 u32 *reo_timeout) 49 { 50 struct tcp_sock *tp = tcp_sk(sk); 51 struct sk_buff *skb; 52 u32 reo_wnd; 53 54 *reo_timeout = 0; 55 /* To be more reordering resilient, allow min_rtt/4 settling delay 56 * (lower-bounded to 1000uS). We use min_rtt instead of the smoothed 57 * RTT because reordering is often a path property and less related 58 * to queuing or delayed ACKs. 59 */ 60 reo_wnd = 1000; 61 if ((tp->rack.reord || !tp->lost_out) && tcp_min_rtt(tp) != ~0U) 62 reo_wnd = max(tcp_min_rtt(tp) >> 2, reo_wnd); 63 64 tcp_for_write_queue(skb, sk) { 65 struct tcp_skb_cb *scb = TCP_SKB_CB(skb); 66 67 if (skb == tcp_send_head(sk)) 68 break; 69 70 /* Skip ones already (s)acked */ 71 if (!after(scb->end_seq, tp->snd_una) || 72 scb->sacked & TCPCB_SACKED_ACKED) 73 continue; 74 75 if (tcp_rack_sent_after(&tp->rack.mstamp, &skb->skb_mstamp, 76 tp->rack.end_seq, scb->end_seq)) { 77 /* Step 3 in draft-cheng-tcpm-rack-00.txt: 78 * A packet is lost if its elapsed time is beyond 79 * the recent RTT plus the reordering window. 80 */ 81 u32 elapsed = skb_mstamp_us_delta(now, 82 &skb->skb_mstamp); 83 s32 remaining = tp->rack.rtt_us + reo_wnd - elapsed; 84 85 if (remaining < 0) { 86 tcp_rack_mark_skb_lost(sk, skb); 87 continue; 88 } 89 90 /* Skip ones marked lost but not yet retransmitted */ 91 if ((scb->sacked & TCPCB_LOST) && 92 !(scb->sacked & TCPCB_SACKED_RETRANS)) 93 continue; 94 95 /* Record maximum wait time (+1 to avoid 0) */ 96 *reo_timeout = max_t(u32, *reo_timeout, 1 + remaining); 97 98 } else if (!(scb->sacked & TCPCB_RETRANS)) { 99 /* Original data are sent sequentially so stop early 100 * b/c the rest are all sent after rack_sent 101 */ 102 break; 103 } 104 } 105 } 106 107 void tcp_rack_mark_lost(struct sock *sk, const struct skb_mstamp *now) 108 { 109 struct tcp_sock *tp = tcp_sk(sk); 110 u32 timeout; 111 112 if (!tp->rack.advanced) 113 return; 114 115 /* Reset the advanced flag to avoid unnecessary queue scanning */ 116 tp->rack.advanced = 0; 117 tcp_rack_detect_loss(sk, now, &timeout); 118 if (timeout) { 119 timeout = usecs_to_jiffies(timeout + TCP_REO_TIMEOUT_MIN); 120 inet_csk_reset_xmit_timer(sk, ICSK_TIME_REO_TIMEOUT, 121 timeout, inet_csk(sk)->icsk_rto); 122 } 123 } 124 125 /* Record the most recently (re)sent time among the (s)acked packets 126 * This is "Step 3: Advance RACK.xmit_time and update RACK.RTT" from 127 * draft-cheng-tcpm-rack-00.txt 128 */ 129 void tcp_rack_advance(struct tcp_sock *tp, u8 sacked, u32 end_seq, 130 const struct skb_mstamp *xmit_time, 131 const struct skb_mstamp *ack_time) 132 { 133 u32 rtt_us; 134 135 if (tp->rack.mstamp.v64 && 136 !tcp_rack_sent_after(xmit_time, &tp->rack.mstamp, 137 end_seq, tp->rack.end_seq)) 138 return; 139 140 rtt_us = skb_mstamp_us_delta(ack_time, xmit_time); 141 if (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 if (rtt_us < tcp_min_rtt(tp)) 153 return; 154 } 155 tp->rack.rtt_us = rtt_us; 156 tp->rack.mstamp = *xmit_time; 157 tp->rack.end_seq = end_seq; 158 tp->rack.advanced = 1; 159 } 160 161 /* We have waited long enough to accommodate reordering. Mark the expired 162 * packets lost and retransmit them. 163 */ 164 void tcp_rack_reo_timeout(struct sock *sk) 165 { 166 struct tcp_sock *tp = tcp_sk(sk); 167 struct skb_mstamp now; 168 u32 timeout, prior_inflight; 169 170 skb_mstamp_get(&now); 171 prior_inflight = tcp_packets_in_flight(tp); 172 tcp_rack_detect_loss(sk, &now, &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