1#include <net/tcp.h>
2
3/* The bandwidth estimator estimates the rate at which the network
4 * can currently deliver outbound data packets for this flow. At a high
5 * level, it operates by taking a delivery rate sample for each ACK.
6 *
7 * A rate sample records the rate at which the network delivered packets
8 * for this flow, calculated over the time interval between the transmission
9 * of a data packet and the acknowledgment of that packet.
10 *
11 * Specifically, over the interval between each transmit and corresponding ACK,
12 * the estimator generates a delivery rate sample. Typically it uses the rate
13 * at which packets were acknowledged. However, the approach of using only the
14 * acknowledgment rate faces a challenge under the prevalent ACK decimation or
15 * compression: packets can temporarily appear to be delivered much quicker
16 * than the bottleneck rate. Since it is physically impossible to do that in a
17 * sustained fashion, when the estimator notices that the ACK rate is faster
18 * than the transmit rate, it uses the latter:
19 *
20 * send_rate = #pkts_delivered/(last_snd_time - first_snd_time)
21 * ack_rate = #pkts_delivered/(last_ack_time - first_ack_time)
22 * bw = min(send_rate, ack_rate)
23 *
24 * Notice the estimator essentially estimates the goodput, not always the
25 * network bottleneck link rate when the sending or receiving is limited by
26 * other factors like applications or receiver window limits. The estimator
27 * deliberately avoids using the inter-packet spacing approach because that
28 * approach requires a large number of samples and sophisticated filtering.
29 *
30 * TCP flows can often be application-limited in request/response workloads.
31 * The estimator marks a bandwidth sample as application-limited if there
32 * was some moment during the sampled window of packets when there was no data
33 * ready to send in the write queue.
34 */
35
36/* Snapshot the current delivery information in the skb, to generate
37 * a rate sample later when the skb is (s)acked in tcp_rate_skb_delivered().
38 */
39void tcp_rate_skb_sent(struct sock *sk, struct sk_buff *skb)
40{
41 struct tcp_sock *tp = tcp_sk(sk);
42
43 /* In general we need to start delivery rate samples from the
44 * time we received the most recent ACK, to ensure we include
45 * the full time the network needs to deliver all in-flight
46 * packets. If there are no packets in flight yet, then we
47 * know that any ACKs after now indicate that the network was
48 * able to deliver those packets completely in the sampling
49 * interval between now and the next ACK.
50 *
51 * Note that we use packets_out instead of tcp_packets_in_flight(tp)
52 * because the latter is a guess based on RTO and loss-marking
53 * heuristics. We don't want spurious RTOs or loss markings to cause
54 * a spuriously small time interval, causing a spuriously high
55 * bandwidth estimate.
56 */
57 if (!tp->packets_out) {
58 u64 tstamp_us = tcp_skb_timestamp_us(skb);
59
60 tp->first_tx_mstamp = tstamp_us;
61 tp->delivered_mstamp = tstamp_us;
62 }
63
64 TCP_SKB_CB(skb)->tx.first_tx_mstamp = tp->first_tx_mstamp;
65 TCP_SKB_CB(skb)->tx.delivered_mstamp = tp->delivered_mstamp;
66 TCP_SKB_CB(skb)->tx.delivered = tp->delivered;
67 TCP_SKB_CB(skb)->tx.is_app_limited = tp->app_limited ? 1 : 0;
68}
69
70/* When an skb is sacked or acked, we fill in the rate sample with the (prior)
71 * delivery information when the skb was last transmitted.
72 *
73 * If an ACK (s)acks multiple skbs (e.g., stretched-acks), this function is
74 * called multiple times. We favor the information from the most recently
75 * sent skb, i.e., the skb with the highest prior_delivered count.
76 */
77void tcp_rate_skb_delivered(struct sock *sk, struct sk_buff *skb,
78 struct rate_sample *rs)
79{
80 struct tcp_sock *tp = tcp_sk(sk);
81 struct tcp_skb_cb *scb = TCP_SKB_CB(skb);
82
83 if (!scb->tx.delivered_mstamp)
84 return;
85
86 if (!rs->prior_delivered ||
87 after(scb->tx.delivered, rs->prior_delivered)) {
88 rs->prior_delivered = scb->tx.delivered;
89 rs->prior_mstamp = scb->tx.delivered_mstamp;
90 rs->is_app_limited = scb->tx.is_app_limited;
91 rs->is_retrans = scb->sacked & TCPCB_RETRANS;
92
93 /* Record send time of most recently ACKed packet: */
94 tp->first_tx_mstamp = tcp_skb_timestamp_us(skb);
95 /* Find the duration of the "send phase" of this window: */
96 rs->interval_us = tcp_stamp_us_delta(tp->first_tx_mstamp,
97 scb->tx.first_tx_mstamp);
98
99 }
100 /* Mark off the skb delivered once it's sacked to avoid being
101 * used again when it's cumulatively acked. For acked packets
102 * we don't need to reset since it'll be freed soon.
103 */
104 if (scb->sacked & TCPCB_SACKED_ACKED)
105 scb->tx.delivered_mstamp = 0;
106}
107
108/* Update the connection delivery information and generate a rate sample. */
109void tcp_rate_gen(struct sock *sk, u32 delivered, u32 lost,
110 bool is_sack_reneg, struct rate_sample *rs)
111{
112 struct tcp_sock *tp = tcp_sk(sk);
113 u32 snd_us, ack_us;
114
115 /* Clear app limited if bubble is acked and gone. */
116 if (tp->app_limited && after(tp->delivered, tp->app_limited))
117 tp->app_limited = 0;
118
119 /* TODO: there are multiple places throughout tcp_ack() to get
120 * current time. Refactor the code using a new "tcp_acktag_state"
121 * to carry current time, flags, stats like "tcp_sacktag_state".
122 */
123 if (delivered)
124 tp->delivered_mstamp = tp->tcp_mstamp;
125
126 rs->acked_sacked = delivered; /* freshly ACKed or SACKed */
127 rs->losses = lost; /* freshly marked lost */
128 /* Return an invalid sample if no timing information is available or
129 * in recovery from loss with SACK reneging. Rate samples taken during
130 * a SACK reneging event may overestimate bw by including packets that
131 * were SACKed before the reneg.
132 */
133 if (!rs->prior_mstamp || is_sack_reneg) {
134 rs->delivered = -1;
135 rs->interval_us = -1;
136 return;
137 }
138 rs->delivered = tp->delivered - rs->prior_delivered;
139
140 /* Model sending data and receiving ACKs as separate pipeline phases
141 * for a window. Usually the ACK phase is longer, but with ACK
142 * compression the send phase can be longer. To be safe we use the
143 * longer phase.
144 */
145 snd_us = rs->interval_us; /* send phase */
146 ack_us = tcp_stamp_us_delta(tp->tcp_mstamp,
147 rs->prior_mstamp); /* ack phase */
148 rs->interval_us = max(snd_us, ack_us);
149
150 /* Record both segment send and ack receive intervals */
151 rs->snd_interval_us = snd_us;
152 rs->rcv_interval_us = ack_us;
153
154 /* Normally we expect interval_us >= min-rtt.
155 * Note that rate may still be over-estimated when a spuriously
156 * retransmistted skb was first (s)acked because "interval_us"
157 * is under-estimated (up to an RTT). However continuously
158 * measuring the delivery rate during loss recovery is crucial
159 * for connections suffer heavy or prolonged losses.
160 */
161 if (unlikely(rs->interval_us < tcp_min_rtt(tp))) {
162 if (!rs->is_retrans)
163 pr_debug("tcp rate: %ld %d %u %u %u\n",
164 rs->interval_us, rs->delivered,
165 inet_csk(sk)->icsk_ca_state,
166 tp->rx_opt.sack_ok, tcp_min_rtt(tp));
167 rs->interval_us = -1;
168 return;
169 }
170
171 /* Record the last non-app-limited or the highest app-limited bw */
172 if (!rs->is_app_limited ||
173 ((u64)rs->delivered * tp->rate_interval_us >=
174 (u64)tp->rate_delivered * rs->interval_us)) {
175 tp->rate_delivered = rs->delivered;
176 tp->rate_interval_us = rs->interval_us;
177 tp->rate_app_limited = rs->is_app_limited;
178 }
179}
180
181/* If a gap is detected between sends, mark the socket application-limited. */
182void tcp_rate_check_app_limited(struct sock *sk)
183{
184 struct tcp_sock *tp = tcp_sk(sk);
185
186 if (/* We have less than one packet to send. */
187 tp->write_seq - tp->snd_nxt < tp->mss_cache &&
188 /* Nothing in sending host's qdisc queues or NIC tx queue. */
189 sk_wmem_alloc_get(sk) < SKB_TRUESIZE(1) &&
190 /* We are not limited by CWND. */
191 tcp_packets_in_flight(tp) < tp->snd_cwnd &&
192 /* All lost packets have been retransmitted. */
193 tp->lost_out <= tp->retrans_out)
194 tp->app_limited =
195 (tp->delivered + tcp_packets_in_flight(tp)) ? : 1;
196}
197EXPORT_SYMBOL_GPL(tcp_rate_check_app_limited);
198