net/ipv4/tcp_recovery.c

   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_REO_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 }