net/ipv4/tcp_recovery.c

   1 // SPDX-License-Identifier: GPL-2.0
   2 #include <linux/tcp.h>
   3 #include <net/tcp.h>
   4
   5 void tcp_mark_skb_lost(struct sock *sk, struct sk_buff *skb)
   6 {
   7         struct tcp_sock *tp = tcp_sk(sk);
   8
   9         tcp_skb_mark_lost_uncond_verify(tp, skb);
  10         if (TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_RETRANS) {
  11                 /* Account for retransmits that are lost again */
  12                 TCP_SKB_CB(skb)->sacked &= ~TCPCB_SACKED_RETRANS;
  13                 tp->retrans_out -= tcp_skb_pcount(skb);
  14                 NET_ADD_STATS(sock_net(sk), LINUX_MIB_TCPLOSTRETRANSMIT,
  15                               tcp_skb_pcount(skb));
  16         }
  17 }
  18
  19 static bool tcp_rack_sent_after(u64 t1, u64 t2, u32 seq1, u32 seq2)
  20 {
  21         return t1 > t2 || (t1 == t2 && after(seq1, seq2));
  22 }
  23
  24 static u32 tcp_rack_reo_wnd(const struct sock *sk)
  25 {
  26         struct tcp_sock *tp = tcp_sk(sk);
  27
  28         if (!tp->reord_seen) {
  29                 /* If reordering has not been observed, be aggressive during
  30                  * the recovery or starting the recovery by DUPACK threshold.
  31                  */
  32                 if (inet_csk(sk)->icsk_ca_state >= TCP_CA_Recovery)
  33                         return 0;
  34
  35                 if (tp->sacked_out >= tp->reordering &&
  36                     !(sock_net(sk)->ipv4.sysctl_tcp_recovery & TCP_RACK_NO_DUPTHRESH))
  37                         return 0;
  38         }
  39
  40         /* To be more reordering resilient, allow min_rtt/4 settling delay.
  41          * Use min_rtt instead of the smoothed RTT because reordering is
  42          * often a path property and less related to queuing or delayed ACKs.
  43          * Upon receiving DSACKs, linearly increase the window up to the
  44          * smoothed RTT.
  45          */
  46         return min((tcp_min_rtt(tp) >> 2) * tp->rack.reo_wnd_steps,
  47                    tp->srtt_us >> 3);
  48 }
  49
  50 s32 tcp_rack_skb_timeout(struct tcp_sock *tp, struct sk_buff *skb, u32 reo_wnd)
  51 {
  52         return tp->rack.rtt_us + reo_wnd -
  53                tcp_stamp_us_delta(tp->tcp_mstamp, skb->skb_mstamp);
  54 }
  55
  56 /* RACK loss detection (IETF draft draft-ietf-tcpm-rack-01):
  57  *
  58  * Marks a packet lost, if some packet sent later has been (s)acked.
  59  * The underlying idea is similar to the traditional dupthresh and FACK
  60  * but they look at different metrics:
  61  *
  62  * dupthresh: 3 OOO packets delivered (packet count)
  63  * FACK: sequence delta to highest sacked sequence (sequence space)
  64  * RACK: sent time delta to the latest delivered packet (time domain)
  65  *
  66  * The advantage of RACK is it applies to both original and retransmitted
  67  * packet and therefore is robust against tail losses. Another advantage
  68  * is being more resilient to reordering by simply allowing some
  69  * "settling delay", instead of tweaking the dupthresh.
  70  *
  71  * When tcp_rack_detect_loss() detects some packets are lost and we
  72  * are not already in the CA_Recovery state, either tcp_rack_reo_timeout()
  73  * or tcp_time_to_recover()'s "Trick#1: the loss is proven" code path will
  74  * make us enter the CA_Recovery state.
  75  */
  76 static void tcp_rack_detect_loss(struct sock *sk, u32 *reo_timeout)
  77 {
  78         struct tcp_sock *tp = tcp_sk(sk);
  79         struct sk_buff *skb, *n;
  80         u32 reo_wnd;
  81
  82         *reo_timeout = 0;
  83         reo_wnd = tcp_rack_reo_wnd(sk);
  84         list_for_each_entry_safe(skb, n, &tp->tsorted_sent_queue,
  85                                  tcp_tsorted_anchor) {
  86                 struct tcp_skb_cb *scb = TCP_SKB_CB(skb);
  87                 s32 remaining;
  88
  89                 /* Skip ones marked lost but not yet retransmitted */
  90                 if ((scb->sacked & TCPCB_LOST) &&
  91                     !(scb->sacked & TCPCB_SACKED_RETRANS))
  92                         continue;
  93
  94                 if (!tcp_rack_sent_after(tp->rack.mstamp, skb->skb_mstamp,
  95                                          tp->rack.end_seq, scb->end_seq))
  96                         break;
  97
  98                 /* A packet is lost if it has not been s/acked beyond
  99                  * the recent RTT plus the reordering window.
 100                  */
 101                 remaining = tcp_rack_skb_timeout(tp, skb, reo_wnd);
 102                 if (remaining <= 0) {
 103                         tcp_mark_skb_lost(sk, skb);
 104                         list_del_init(&skb->tcp_tsorted_anchor);
 105                 } else {
 106                         /* Record maximum wait time */
 107                         *reo_timeout = max_t(u32, *reo_timeout, remaining);
 108                 }
 109         }
 110 }
 111
 112 void tcp_rack_mark_lost(struct sock *sk)
 113 {
 114         struct tcp_sock *tp = tcp_sk(sk);
 115         u32 timeout;
 116
 117         if (!tp->rack.advanced)
 118                 return;
 119
 120         /* Reset the advanced flag to avoid unnecessary queue scanning */
 121         tp->rack.advanced = 0;
 122         tcp_rack_detect_loss(sk, &timeout);
 123         if (timeout) {
 124                 timeout = usecs_to_jiffies(timeout) + TCP_TIMEOUT_MIN;
 125                 inet_csk_reset_xmit_timer(sk, ICSK_TIME_REO_TIMEOUT,
 126                                           timeout, inet_csk(sk)->icsk_rto);
 127         }
 128 }
 129
 130 /* Record the most recently (re)sent time among the (s)acked packets
 131  * This is "Step 3: Advance RACK.xmit_time and update RACK.RTT" from
 132  * draft-cheng-tcpm-rack-00.txt
 133  */
 134 void tcp_rack_advance(struct tcp_sock *tp, u8 sacked, u32 end_seq,
 135                       u64 xmit_time)
 136 {
 137         u32 rtt_us;
 138
 139         rtt_us = tcp_stamp_us_delta(tp->tcp_mstamp, xmit_time);
 140         if (rtt_us < tcp_min_rtt(tp) && (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                 return;
 152         }
 153         tp->rack.advanced = 1;
 154         tp->rack.rtt_us = rtt_us;
 155         if (tcp_rack_sent_after(xmit_time, tp->rack.mstamp,
 156                                 end_seq, tp->rack.end_seq)) {
 157                 tp->rack.mstamp = xmit_time;
 158                 tp->rack.end_seq = end_seq;
 159         }
 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         u32 timeout, prior_inflight;
 169
 170         prior_inflight = tcp_packets_in_flight(tp);
 171         tcp_rack_detect_loss(sk, &timeout);
 172         if (prior_inflight != tcp_packets_in_flight(tp)) {
 173                 if (inet_csk(sk)->icsk_ca_state != TCP_CA_Recovery) {
 174                         tcp_enter_recovery(sk, false);
 175                         if (!inet_csk(sk)->icsk_ca_ops->cong_control)
 176                                 tcp_cwnd_reduction(sk, 1, 0);
 177                 }
 178                 tcp_xmit_retransmit_queue(sk);
 179         }
 180         if (inet_csk(sk)->icsk_pending != ICSK_TIME_RETRANS)
 181                 tcp_rearm_rto(sk);
 182 }
 183
 184 /* Updates the RACK's reo_wnd based on DSACK and no. of recoveries.
 185  *
 186  * If DSACK is received, increment reo_wnd by min_rtt/4 (upper bounded
 187  * by srtt), since there is possibility that spurious retransmission was
 188  * due to reordering delay longer than reo_wnd.
 189  *
 190  * Persist the current reo_wnd value for TCP_RACK_RECOVERY_THRESH (16)
 191  * no. of successful recoveries (accounts for full DSACK-based loss
 192  * recovery undo). After that, reset it to default (min_rtt/4).
 193  *
 194  * At max, reo_wnd is incremented only once per rtt. So that the new
 195  * DSACK on which we are reacting, is due to the spurious retx (approx)
 196  * after the reo_wnd has been updated last time.
 197  *
 198  * reo_wnd is tracked in terms of steps (of min_rtt/4), rather than
 199  * absolute value to account for change in rtt.
 200  */
 201 void tcp_rack_update_reo_wnd(struct sock *sk, struct rate_sample *rs)
 202 {
 203         struct tcp_sock *tp = tcp_sk(sk);
 204
 205         if (sock_net(sk)->ipv4.sysctl_tcp_recovery & TCP_RACK_STATIC_REO_WND ||
 206             !rs->prior_delivered)
 207                 return;
 208
 209         /* Disregard DSACK if a rtt has not passed since we adjusted reo_wnd */
 210         if (before(rs->prior_delivered, tp->rack.last_delivered))
 211                 tp->rack.dsack_seen = 0;
 212
 213         /* Adjust the reo_wnd if update is pending */
 214         if (tp->rack.dsack_seen) {
 215                 tp->rack.reo_wnd_steps = min_t(u32, 0xFF,
 216                                                tp->rack.reo_wnd_steps + 1);
 217                 tp->rack.dsack_seen = 0;
 218                 tp->rack.last_delivered = tp->delivered;
 219                 tp->rack.reo_wnd_persist = TCP_RACK_RECOVERY_THRESH;
 220         } else if (!tp->rack.reo_wnd_persist) {
 221                 tp->rack.reo_wnd_steps = 1;
 222         }
 223 }
 224
 225 /* RFC6582 NewReno recovery for non-SACK connection. It simply retransmits
 226  * the next unacked packet upon receiving
 227  * a) three or more DUPACKs to start the fast recovery
 228  * b) an ACK acknowledging new data during the fast recovery.
 229  */
 230 void tcp_newreno_mark_lost(struct sock *sk, bool snd_una_advanced)
 231 {
 232         const u8 state = inet_csk(sk)->icsk_ca_state;
 233         struct tcp_sock *tp = tcp_sk(sk);
 234
 235         if ((state < TCP_CA_Recovery && tp->sacked_out >= tp->reordering) ||
 236             (state == TCP_CA_Recovery && snd_una_advanced)) {
 237                 struct sk_buff *skb = tcp_rtx_queue_head(sk);
 238                 u32 mss;
 239
 240                 if (TCP_SKB_CB(skb)->sacked & TCPCB_LOST)
 241                         return;
 242
 243                 mss = tcp_skb_mss(skb);
 244                 if (tcp_skb_pcount(skb) > 1 && skb->len > mss)
 245                         tcp_fragment(sk, TCP_FRAG_IN_RTX_QUEUE, skb,
 246                                      mss, mss, GFP_ATOMIC);
 247
 248                 tcp_skb_mark_lost_uncond_verify(tp, skb);
 249         }
 250 }