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, tcp_skb_timestamp_us(skb));
  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,
  95                                          tcp_skb_timestamp_us(skb),
  96                                          tp->rack.end_seq, scb->end_seq))
  97                         break;
  98
  99                 /* A packet is lost if it has not been s/acked beyond
 100                  * the recent RTT plus the reordering window.
 101                  */
 102                 remaining = tcp_rack_skb_timeout(tp, skb, reo_wnd);
 103                 if (remaining <= 0) {
 104                         tcp_mark_skb_lost(sk, skb);
 105                         list_del_init(&skb->tcp_tsorted_anchor);
 106                 } else {
 107                         /* Record maximum wait time */
 108                         *reo_timeout = max_t(u32, *reo_timeout, remaining);
 109                 }
 110         }
 111 }
 112
 113 void tcp_rack_mark_lost(struct sock *sk)
 114 {
 115         struct tcp_sock *tp = tcp_sk(sk);
 116         u32 timeout;
 117
 118         if (!tp->rack.advanced)
 119                 return;
 120
 121         /* Reset the advanced flag to avoid unnecessary queue scanning */
 122         tp->rack.advanced = 0;
 123         tcp_rack_detect_loss(sk, &timeout);
 124         if (timeout) {
 125                 timeout = usecs_to_jiffies(timeout) + TCP_TIMEOUT_MIN;
 126                 inet_csk_reset_xmit_timer(sk, ICSK_TIME_REO_TIMEOUT,
 127                                           timeout, inet_csk(sk)->icsk_rto);
 128         }
 129 }
 130
 131 /* Record the most recently (re)sent time among the (s)acked packets
 132  * This is "Step 3: Advance RACK.xmit_time and update RACK.RTT" from
 133  * draft-cheng-tcpm-rack-00.txt
 134  */
 135 void tcp_rack_advance(struct tcp_sock *tp, u8 sacked, u32 end_seq,
 136                       u64 xmit_time)
 137 {
 138         u32 rtt_us;
 139
 140         rtt_us = tcp_stamp_us_delta(tp->tcp_mstamp, xmit_time);
 141         if (rtt_us < tcp_min_rtt(tp) && (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                 return;
 153         }
 154         tp->rack.advanced = 1;
 155         tp->rack.rtt_us = rtt_us;
 156         if (tcp_rack_sent_after(xmit_time, tp->rack.mstamp,
 157                                 end_seq, tp->rack.end_seq)) {
 158                 tp->rack.mstamp = xmit_time;
 159                 tp->rack.end_seq = end_seq;
 160         }
 161 }
 162
 163 /* We have waited long enough to accommodate reordering. Mark the expired
 164  * packets lost and retransmit them.
 165  */
 166 void tcp_rack_reo_timeout(struct sock *sk)
 167 {
 168         struct tcp_sock *tp = tcp_sk(sk);
 169         u32 timeout, prior_inflight;
 170
 171         prior_inflight = tcp_packets_in_flight(tp);
 172         tcp_rack_detect_loss(sk, &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
 185 /* Updates the RACK's reo_wnd based on DSACK and no. of recoveries.
 186  *
 187  * If DSACK is received, increment reo_wnd by min_rtt/4 (upper bounded
 188  * by srtt), since there is possibility that spurious retransmission was
 189  * due to reordering delay longer than reo_wnd.
 190  *
 191  * Persist the current reo_wnd value for TCP_RACK_RECOVERY_THRESH (16)
 192  * no. of successful recoveries (accounts for full DSACK-based loss
 193  * recovery undo). After that, reset it to default (min_rtt/4).
 194  *
 195  * At max, reo_wnd is incremented only once per rtt. So that the new
 196  * DSACK on which we are reacting, is due to the spurious retx (approx)
 197  * after the reo_wnd has been updated last time.
 198  *
 199  * reo_wnd is tracked in terms of steps (of min_rtt/4), rather than
 200  * absolute value to account for change in rtt.
 201  */
 202 void tcp_rack_update_reo_wnd(struct sock *sk, struct rate_sample *rs)
 203 {
 204         struct tcp_sock *tp = tcp_sk(sk);
 205
 206         if (sock_net(sk)->ipv4.sysctl_tcp_recovery & TCP_RACK_STATIC_REO_WND ||
 207             !rs->prior_delivered)
 208                 return;
 209
 210         /* Disregard DSACK if a rtt has not passed since we adjusted reo_wnd */
 211         if (before(rs->prior_delivered, tp->rack.last_delivered))
 212                 tp->rack.dsack_seen = 0;
 213
 214         /* Adjust the reo_wnd if update is pending */
 215         if (tp->rack.dsack_seen) {
 216                 tp->rack.reo_wnd_steps = min_t(u32, 0xFF,
 217                                                tp->rack.reo_wnd_steps + 1);
 218                 tp->rack.dsack_seen = 0;
 219                 tp->rack.last_delivered = tp->delivered;
 220                 tp->rack.reo_wnd_persist = TCP_RACK_RECOVERY_THRESH;
 221         } else if (!tp->rack.reo_wnd_persist) {
 222                 tp->rack.reo_wnd_steps = 1;
 223         }
 224 }
 225
 226 /* RFC6582 NewReno recovery for non-SACK connection. It simply retransmits
 227  * the next unacked packet upon receiving
 228  * a) three or more DUPACKs to start the fast recovery
 229  * b) an ACK acknowledging new data during the fast recovery.
 230  */
 231 void tcp_newreno_mark_lost(struct sock *sk, bool snd_una_advanced)
 232 {
 233         const u8 state = inet_csk(sk)->icsk_ca_state;
 234         struct tcp_sock *tp = tcp_sk(sk);
 235
 236         if ((state < TCP_CA_Recovery && tp->sacked_out >= tp->reordering) ||
 237             (state == TCP_CA_Recovery && snd_una_advanced)) {
 238                 struct sk_buff *skb = tcp_rtx_queue_head(sk);
 239                 u32 mss;
 240
 241                 if (TCP_SKB_CB(skb)->sacked & TCPCB_LOST)
 242                         return;
 243
 244                 mss = tcp_skb_mss(skb);
 245                 if (tcp_skb_pcount(skb) > 1 && skb->len > mss)
 246                         tcp_fragment(sk, TCP_FRAG_IN_RTX_QUEUE, skb,
 247                                      mss, mss, GFP_ATOMIC);
 248
 249                 tcp_skb_mark_lost_uncond_verify(tp, skb);
 250         }
 251 }