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(const struct skb_mstamp *t1,
  21                                 const struct skb_mstamp *t2,
  22                                 u32 seq1, u32 seq2)
  23 {
  24         return skb_mstamp_after(t1, t2) ||
  25                (t1->v64 == t2->v64 && after(seq1, seq2));
  26 }
  27
  28 /* RACK loss detection (IETF draft draft-ietf-tcpm-rack-01):
  29  *
  30  * Marks a packet lost, if some packet sent later has been (s)acked.
  31  * The underlying idea is similar to the traditional dupthresh and FACK
  32  * but they look at different metrics:
  33  *
  34  * dupthresh: 3 OOO packets delivered (packet count)
  35  * FACK: sequence delta to highest sacked sequence (sequence space)
  36  * RACK: sent time delta to the latest delivered packet (time domain)
  37  *
  38  * The advantage of RACK is it applies to both original and retransmitted
  39  * packet and therefore is robust against tail losses. Another advantage
  40  * is being more resilient to reordering by simply allowing some
  41  * "settling delay", instead of tweaking the dupthresh.
  42  *
  43  * When tcp_rack_detect_loss() detects some packets are lost and we
  44  * are not already in the CA_Recovery state, either tcp_rack_reo_timeout()
  45  * or tcp_time_to_recover()'s "Trick#1: the loss is proven" code path will
  46  * make us enter the CA_Recovery state.
  47  */
  48 static void tcp_rack_detect_loss(struct sock *sk, 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(&tp->tcp_mstamp,
  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)
 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, &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 {
 132         u32 rtt_us;
 133
 134         if (tp->rack.mstamp.v64 &&
 135             !tcp_rack_sent_after(xmit_time, &tp->rack.mstamp,
 136                                  end_seq, tp->rack.end_seq))
 137                 return;
 138
 139         rtt_us = skb_mstamp_us_delta(&tp->tcp_mstamp, xmit_time);
 140         if (sacked & TCPCB_RETRANS) {
 141                 /* If the sacked packet was retransmitted, it's ambiguous
 142                  * whether the retransmission or the original (or the prior
 143                  * retransmission) was sacked.
 144                  *
 145                  * If the original is lost, there is no ambiguity. Otherwise
 146                  * we assume the original can be delayed up to aRTT + min_rtt.
 147                  * the aRTT term is bounded by the fast recovery or timeout,
 148                  * so it's at least one RTT (i.e., retransmission is at least
 149                  * an RTT later).
 150                  */
 151                 if (rtt_us < tcp_min_rtt(tp))
 152                         return;
 153         }
 154         tp->rack.rtt_us = rtt_us;
 155         tp->rack.mstamp = *xmit_time;
 156         tp->rack.end_seq = end_seq;
 157         tp->rack.advanced = 1;
 158 }
 159
 160 /* We have waited long enough to accommodate reordering. Mark the expired
 161  * packets lost and retransmit them.
 162  */
 163 void tcp_rack_reo_timeout(struct sock *sk)
 164 {
 165         struct tcp_sock *tp = tcp_sk(sk);
 166         u32 timeout, prior_inflight;
 167
 168         prior_inflight = tcp_packets_in_flight(tp);
 169         skb_mstamp_get(&tp->tcp_mstamp);
 170         tcp_rack_detect_loss(sk, &timeout);
 171         if (prior_inflight != tcp_packets_in_flight(tp)) {
 172                 if (inet_csk(sk)->icsk_ca_state != TCP_CA_Recovery) {
 173                         tcp_enter_recovery(sk, false);
 174                         if (!inet_csk(sk)->icsk_ca_ops->cong_control)
 175                                 tcp_cwnd_reduction(sk, 1, 0);
 176                 }
 177                 tcp_xmit_retransmit_queue(sk);
 178         }
 179         if (inet_csk(sk)->icsk_pending != ICSK_TIME_RETRANS)
 180                 tcp_rearm_rto(sk);
 181 }