net/ipv4/tcp_recovery.c

   1 // SPDX-License-Identifier: GPL-2.0
   2 #include <linux/tcp.h>
   3 #include <net/tcp.h>
   4
   5 static u32 tcp_rack_reo_wnd(const struct sock *sk)
   6 {
   7         const struct tcp_sock *tp = tcp_sk(sk);
   8
   9         if (!tp->reord_seen) {
  10                 /* If reordering has not been observed, be aggressive during
  11                  * the recovery or starting the recovery by DUPACK threshold.
  12                  */
  13                 if (inet_csk(sk)->icsk_ca_state >= TCP_CA_Recovery)
  14                         return 0;
  15
  16                 if (tp->sacked_out >= tp->reordering &&
  17                     !(READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_recovery) &
  18                       TCP_RACK_NO_DUPTHRESH))
  19                         return 0;
  20         }
  21
  22         /* To be more reordering resilient, allow min_rtt/4 settling delay.
  23          * Use min_rtt instead of the smoothed RTT because reordering is
  24          * often a path property and less related to queuing or delayed ACKs.
  25          * Upon receiving DSACKs, linearly increase the window up to the
  26          * smoothed RTT.
  27          */
  28         return min((tcp_min_rtt(tp) >> 2) * tp->rack.reo_wnd_steps,
  29                    tp->srtt_us >> 3);
  30 }
  31
  32 s32 tcp_rack_skb_timeout(struct tcp_sock *tp, struct sk_buff *skb, u32 reo_wnd)
  33 {
  34         return tp->rack.rtt_us + reo_wnd -
  35                tcp_stamp_us_delta(tp->tcp_mstamp, tcp_skb_timestamp_us(skb));
  36 }
  37
  38 /* RACK loss detection (IETF draft draft-ietf-tcpm-rack-01):
  39  *
  40  * Marks a packet lost, if some packet sent later has been (s)acked.
  41  * The underlying idea is similar to the traditional dupthresh and FACK
  42  * but they look at different metrics:
  43  *
  44  * dupthresh: 3 OOO packets delivered (packet count)
  45  * FACK: sequence delta to highest sacked sequence (sequence space)
  46  * RACK: sent time delta to the latest delivered packet (time domain)
  47  *
  48  * The advantage of RACK is it applies to both original and retransmitted
  49  * packet and therefore is robust against tail losses. Another advantage
  50  * is being more resilient to reordering by simply allowing some
  51  * "settling delay", instead of tweaking the dupthresh.
  52  *
  53  * When tcp_rack_detect_loss() detects some packets are lost and we
  54  * are not already in the CA_Recovery state, either tcp_rack_reo_timeout()
  55  * or tcp_time_to_recover()'s "Trick#1: the loss is proven" code path will
  56  * make us enter the CA_Recovery state.
  57  */
  58 static void tcp_rack_detect_loss(struct sock *sk, u32 *reo_timeout)
  59 {
  60         struct tcp_sock *tp = tcp_sk(sk);
  61         struct sk_buff *skb, *n;
  62         u32 reo_wnd;
  63
  64         *reo_timeout = 0;
  65         reo_wnd = tcp_rack_reo_wnd(sk);
  66         list_for_each_entry_safe(skb, n, &tp->tsorted_sent_queue,
  67                                  tcp_tsorted_anchor) {
  68                 struct tcp_skb_cb *scb = TCP_SKB_CB(skb);
  69                 s32 remaining;
  70
  71                 /* Skip ones marked lost but not yet retransmitted */
  72                 if ((scb->sacked & TCPCB_LOST) &&
  73                     !(scb->sacked & TCPCB_SACKED_RETRANS))
  74                         continue;
  75
  76                 if (!tcp_skb_sent_after(tp->rack.mstamp,
  77                                         tcp_skb_timestamp_us(skb),
  78                                         tp->rack.end_seq, scb->end_seq))
  79                         break;
  80
  81                 /* A packet is lost if it has not been s/acked beyond
  82                  * the recent RTT plus the reordering window.
  83                  */
  84                 remaining = tcp_rack_skb_timeout(tp, skb, reo_wnd);
  85                 if (remaining <= 0) {
  86                         tcp_mark_skb_lost(sk, skb);
  87                         list_del_init(&skb->tcp_tsorted_anchor);
  88                 } else {
  89                         /* Record maximum wait time */
  90                         *reo_timeout = max_t(u32, *reo_timeout, remaining);
  91                 }
  92         }
  93 }
  94
  95 bool tcp_rack_mark_lost(struct sock *sk)
  96 {
  97         struct tcp_sock *tp = tcp_sk(sk);
  98         u32 timeout;
  99
 100         if (!tp->rack.advanced)
 101                 return false;
 102
 103         /* Reset the advanced flag to avoid unnecessary queue scanning */
 104         tp->rack.advanced = 0;
 105         tcp_rack_detect_loss(sk, &timeout);
 106         if (timeout) {
 107                 timeout = usecs_to_jiffies(timeout + TCP_TIMEOUT_MIN_US);
 108                 inet_csk_reset_xmit_timer(sk, ICSK_TIME_REO_TIMEOUT,
 109                                           timeout, inet_csk(sk)->icsk_rto);
 110         }
 111         return !!timeout;
 112 }
 113
 114 /* Record the most recently (re)sent time among the (s)acked packets
 115  * This is "Step 3: Advance RACK.xmit_time and update RACK.RTT" from
 116  * draft-cheng-tcpm-rack-00.txt
 117  */
 118 void tcp_rack_advance(struct tcp_sock *tp, u8 sacked, u32 end_seq,
 119                       u64 xmit_time)
 120 {
 121         u32 rtt_us;
 122
 123         rtt_us = tcp_stamp_us_delta(tp->tcp_mstamp, xmit_time);
 124         if (rtt_us < tcp_min_rtt(tp) && (sacked & TCPCB_RETRANS)) {
 125                 /* If the sacked packet was retransmitted, it's ambiguous
 126                  * whether the retransmission or the original (or the prior
 127                  * retransmission) was sacked.
 128                  *
 129                  * If the original is lost, there is no ambiguity. Otherwise
 130                  * we assume the original can be delayed up to aRTT + min_rtt.
 131                  * the aRTT term is bounded by the fast recovery or timeout,
 132                  * so it's at least one RTT (i.e., retransmission is at least
 133                  * an RTT later).
 134                  */
 135                 return;
 136         }
 137         tp->rack.advanced = 1;
 138         tp->rack.rtt_us = rtt_us;
 139         if (tcp_skb_sent_after(xmit_time, tp->rack.mstamp,
 140                                end_seq, tp->rack.end_seq)) {
 141                 tp->rack.mstamp = xmit_time;
 142                 tp->rack.end_seq = end_seq;
 143         }
 144 }
 145
 146 /* We have waited long enough to accommodate reordering. Mark the expired
 147  * packets lost and retransmit them.
 148  */
 149 void tcp_rack_reo_timeout(struct sock *sk)
 150 {
 151         struct tcp_sock *tp = tcp_sk(sk);
 152         u32 timeout, prior_inflight;
 153         u32 lost = tp->lost;
 154
 155         prior_inflight = tcp_packets_in_flight(tp);
 156         tcp_rack_detect_loss(sk, &timeout);
 157         if (prior_inflight != tcp_packets_in_flight(tp)) {
 158                 if (inet_csk(sk)->icsk_ca_state != TCP_CA_Recovery) {
 159                         tcp_enter_recovery(sk, false);
 160                         if (!inet_csk(sk)->icsk_ca_ops->cong_control)
 161                                 tcp_cwnd_reduction(sk, 1, tp->lost - lost, 0);
 162                 }
 163                 tcp_xmit_retransmit_queue(sk);
 164         }
 165         if (inet_csk(sk)->icsk_pending != ICSK_TIME_RETRANS)
 166                 tcp_rearm_rto(sk);
 167 }
 168
 169 /* Updates the RACK's reo_wnd based on DSACK and no. of recoveries.
 170  *
 171  * If a DSACK is received that seems like it may have been due to reordering
 172  * triggering fast recovery, increment reo_wnd by min_rtt/4 (upper bounded
 173  * by srtt), since there is possibility that spurious retransmission was
 174  * due to reordering delay longer than reo_wnd.
 175  *
 176  * Persist the current reo_wnd value for TCP_RACK_RECOVERY_THRESH (16)
 177  * no. of successful recoveries (accounts for full DSACK-based loss
 178  * recovery undo). After that, reset it to default (min_rtt/4).
 179  *
 180  * At max, reo_wnd is incremented only once per rtt. So that the new
 181  * DSACK on which we are reacting, is due to the spurious retx (approx)
 182  * after the reo_wnd has been updated last time.
 183  *
 184  * reo_wnd is tracked in terms of steps (of min_rtt/4), rather than
 185  * absolute value to account for change in rtt.
 186  */
 187 void tcp_rack_update_reo_wnd(struct sock *sk, struct rate_sample *rs)
 188 {
 189         struct tcp_sock *tp = tcp_sk(sk);
 190
 191         if ((READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_recovery) &
 192              TCP_RACK_STATIC_REO_WND) ||
 193             !rs->prior_delivered)
 194                 return;
 195
 196         /* Disregard DSACK if a rtt has not passed since we adjusted reo_wnd */
 197         if (before(rs->prior_delivered, tp->rack.last_delivered))
 198                 tp->rack.dsack_seen = 0;
 199
 200         /* Adjust the reo_wnd if update is pending */
 201         if (tp->rack.dsack_seen) {
 202                 tp->rack.reo_wnd_steps = min_t(u32, 0xFF,
 203                                                tp->rack.reo_wnd_steps + 1);
 204                 tp->rack.dsack_seen = 0;
 205                 tp->rack.last_delivered = tp->delivered;
 206                 tp->rack.reo_wnd_persist = TCP_RACK_RECOVERY_THRESH;
 207         } else if (!tp->rack.reo_wnd_persist) {
 208                 tp->rack.reo_wnd_steps = 1;
 209         }
 210 }
 211
 212 /* RFC6582 NewReno recovery for non-SACK connection. It simply retransmits
 213  * the next unacked packet upon receiving
 214  * a) three or more DUPACKs to start the fast recovery
 215  * b) an ACK acknowledging new data during the fast recovery.
 216  */
 217 void tcp_newreno_mark_lost(struct sock *sk, bool snd_una_advanced)
 218 {
 219         const u8 state = inet_csk(sk)->icsk_ca_state;
 220         struct tcp_sock *tp = tcp_sk(sk);
 221
 222         if ((state < TCP_CA_Recovery && tp->sacked_out >= tp->reordering) ||
 223             (state == TCP_CA_Recovery && snd_una_advanced)) {
 224                 struct sk_buff *skb = tcp_rtx_queue_head(sk);
 225                 u32 mss;
 226
 227                 if (TCP_SKB_CB(skb)->sacked & TCPCB_LOST)
 228                         return;
 229
 230                 mss = tcp_skb_mss(skb);
 231                 if (tcp_skb_pcount(skb) > 1 && skb->len > mss)
 232                         tcp_fragment(sk, TCP_FRAG_IN_RTX_QUEUE, skb,
 233                                      mss, mss, GFP_ATOMIC);
 234
 235                 tcp_mark_skb_lost(sk, skb);
 236         }
 237 }