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, const struct skb_mstamp *now,
  49                                  u32 *reo_timeout)
  50 {
  51         struct tcp_sock *tp = tcp_sk(sk);
  52         struct sk_buff *skb;
  53         u32 reo_wnd;
  54
  55         *reo_timeout = 0;
  56         /* To be more reordering resilient, allow min_rtt/4 settling delay
  57          * (lower-bounded to 1000uS). We use min_rtt instead of the smoothed
  58          * RTT because reordering is often a path property and less related
  59          * to queuing or delayed ACKs.
  60          */
  61         reo_wnd = 1000;
  62         if ((tp->rack.reord || !tp->lost_out) && tcp_min_rtt(tp) != ~0U)
  63                 reo_wnd = max(tcp_min_rtt(tp) >> 2, reo_wnd);
  64
  65         tcp_for_write_queue(skb, sk) {
  66                 struct tcp_skb_cb *scb = TCP_SKB_CB(skb);
  67
  68                 if (skb == tcp_send_head(sk))
  69                         break;
  70
  71                 /* Skip ones already (s)acked */
  72                 if (!after(scb->end_seq, tp->snd_una) ||
  73                     scb->sacked & TCPCB_SACKED_ACKED)
  74                         continue;
  75
  76                 if (tcp_rack_sent_after(&tp->rack.mstamp, &skb->skb_mstamp,
  77                                         tp->rack.end_seq, scb->end_seq)) {
  78                         /* Step 3 in draft-cheng-tcpm-rack-00.txt:
  79                          * A packet is lost if its elapsed time is beyond
  80                          * the recent RTT plus the reordering window.
  81                          */
  82                         u32 elapsed = skb_mstamp_us_delta(now,
  83                                                           &skb->skb_mstamp);
  84                         s32 remaining = tp->rack.rtt_us + reo_wnd - elapsed;
  85
  86                         if (remaining < 0) {
  87                                 tcp_rack_mark_skb_lost(sk, skb);
  88                                 continue;
  89                         }
  90
  91                         /* Skip ones marked lost but not yet retransmitted */
  92                         if ((scb->sacked & TCPCB_LOST) &&
  93                             !(scb->sacked & TCPCB_SACKED_RETRANS))
  94                                 continue;
  95
  96                         /* Record maximum wait time (+1 to avoid 0) */
  97                         *reo_timeout = max_t(u32, *reo_timeout, 1 + remaining);
  98
  99                 } else if (!(scb->sacked & TCPCB_RETRANS)) {
 100                         /* Original data are sent sequentially so stop early
 101                          * b/c the rest are all sent after rack_sent
 102                          */
 103                         break;
 104                 }
 105         }
 106 }
 107
 108 void tcp_rack_mark_lost(struct sock *sk, const struct skb_mstamp *now)
 109 {
 110         struct tcp_sock *tp = tcp_sk(sk);
 111         u32 timeout;
 112
 113         if (!tp->rack.advanced)
 114                 return;
 115
 116         /* Reset the advanced flag to avoid unnecessary queue scanning */
 117         tp->rack.advanced = 0;
 118         tcp_rack_detect_loss(sk, now, &timeout);
 119         if (timeout) {
 120                 timeout = usecs_to_jiffies(timeout + TCP_REO_TIMEOUT_MIN);
 121                 inet_csk_reset_xmit_timer(sk, ICSK_TIME_REO_TIMEOUT,
 122                                           timeout, inet_csk(sk)->icsk_rto);
 123         }
 124 }
 125
 126 /* Record the most recently (re)sent time among the (s)acked packets
 127  * This is "Step 3: Advance RACK.xmit_time and update RACK.RTT" from
 128  * draft-cheng-tcpm-rack-00.txt
 129  */
 130 void tcp_rack_advance(struct tcp_sock *tp, u8 sacked, u32 end_seq,
 131                       const struct skb_mstamp *xmit_time,
 132                       const struct skb_mstamp *ack_time)
 133 {
 134         u32 rtt_us;
 135
 136         if (tp->rack.mstamp.v64 &&
 137             !tcp_rack_sent_after(xmit_time, &tp->rack.mstamp,
 138                                  end_seq, tp->rack.end_seq))
 139                 return;
 140
 141         rtt_us = skb_mstamp_us_delta(ack_time, xmit_time);
 142         if (sacked & TCPCB_RETRANS) {
 143                 /* If the sacked packet was retransmitted, it's ambiguous
 144                  * whether the retransmission or the original (or the prior
 145                  * retransmission) was sacked.
 146                  *
 147                  * If the original is lost, there is no ambiguity. Otherwise
 148                  * we assume the original can be delayed up to aRTT + min_rtt.
 149                  * the aRTT term is bounded by the fast recovery or timeout,
 150                  * so it's at least one RTT (i.e., retransmission is at least
 151                  * an RTT later).
 152                  */
 153                 if (rtt_us < tcp_min_rtt(tp))
 154                         return;
 155         }
 156         tp->rack.rtt_us = rtt_us;
 157         tp->rack.mstamp = *xmit_time;
 158         tp->rack.end_seq = end_seq;
 159         tp->rack.advanced = 1;
 160 }
 161
 162 /* We have waited long enough to accommodate reordering. Mark the expired
 163  * packets lost and retransmit them.
 164  */
 165 void tcp_rack_reo_timeout(struct sock *sk)
 166 {
 167         struct tcp_sock *tp = tcp_sk(sk);
 168         struct skb_mstamp now;
 169         u32 timeout, prior_inflight;
 170
 171         skb_mstamp_get(&now);
 172         prior_inflight = tcp_packets_in_flight(tp);
 173         tcp_rack_detect_loss(sk, &now, &timeout);
 174         if (prior_inflight != tcp_packets_in_flight(tp)) {
 175                 if (inet_csk(sk)->icsk_ca_state != TCP_CA_Recovery) {
 176                         tcp_enter_recovery(sk, false);
 177                         if (!inet_csk(sk)->icsk_ca_ops->cong_control)
 178                                 tcp_cwnd_reduction(sk, 1, 0);
 179                 }
 180                 tcp_xmit_retransmit_queue(sk);
 181         }
 182         if (inet_csk(sk)->icsk_pending != ICSK_TIME_RETRANS)
 183                 tcp_rearm_rto(sk);
 184 }