#include <net/tcp.h> /* The bandwidth estimator estimates the rate at which the network * can currently deliver outbound data packets for this flow. At a high * level, it operates by taking a delivery rate sample for each ACK. * * A rate sample records the rate at which the network delivered packets * for this flow, calculated over the time interval between the transmission * of a data packet and the acknowledgment of that packet. * * Specifically, over the interval between each transmit and corresponding ACK, * the estimator generates a delivery rate sample. Typically it uses the rate * at which packets were acknowledged. However, the approach of using only the * acknowledgment rate faces a challenge under the prevalent ACK decimation or * compression: packets can temporarily appear to be delivered much quicker * than the bottleneck rate. Since it is physically impossible to do that in a * sustained fashion, when the estimator notices that the ACK rate is faster * than the transmit rate, it uses the latter: * * send_rate = #pkts_delivered/(last_snd_time - first_snd_time) * ack_rate = #pkts_delivered/(last_ack_time - first_ack_time) * bw = min(send_rate, ack_rate) * * Notice the estimator essentially estimates the goodput, not always the * network bottleneck link rate when the sending or receiving is limited by * other factors like applications or receiver window limits. The estimator * deliberately avoids using the inter-packet spacing approach because that * approach requires a large number of samples and sophisticated filtering. * * TCP flows can often be application-limited in request/response workloads. * The estimator marks a bandwidth sample as application-limited if there * was some moment during the sampled window of packets when there was no data * ready to send in the write queue. */ /* Snapshot the current delivery information in the skb, to generate * a rate sample later when the skb is (s)acked in tcp_rate_skb_delivered(). */ void tcp_rate_skb_sent(struct sock *sk, struct sk_buff *skb) { struct tcp_sock *tp = tcp_sk(sk); /* In general we need to start delivery rate samples from the * time we received the most recent ACK, to ensure we include * the full time the network needs to deliver all in-flight * packets. If there are no packets in flight yet, then we * know that any ACKs after now indicate that the network was * able to deliver those packets completely in the sampling * interval between now and the next ACK. * * Note that we use packets_out instead of tcp_packets_in_flight(tp) * because the latter is a guess based on RTO and loss-marking * heuristics. We don't want spurious RTOs or loss markings to cause * a spuriously small time interval, causing a spuriously high * bandwidth estimate. */ if (!tp->packets_out) { tp->first_tx_mstamp = skb->skb_mstamp; tp->delivered_mstamp = skb->skb_mstamp; } TCP_SKB_CB(skb)->tx.first_tx_mstamp = tp->first_tx_mstamp; TCP_SKB_CB(skb)->tx.delivered_mstamp = tp->delivered_mstamp; TCP_SKB_CB(skb)->tx.delivered = tp->delivered; TCP_SKB_CB(skb)->tx.is_app_limited = tp->app_limited ? 1 : 0; } /* When an skb is sacked or acked, we fill in the rate sample with the (prior) * delivery information when the skb was last transmitted. * * If an ACK (s)acks multiple skbs (e.g., stretched-acks), this function is * called multiple times. We favor the information from the most recently * sent skb, i.e., the skb with the highest prior_delivered count. */ void tcp_rate_skb_delivered(struct sock *sk, struct sk_buff *skb, struct rate_sample *rs) { struct tcp_sock *tp = tcp_sk(sk); struct tcp_skb_cb *scb = TCP_SKB_CB(skb); if (!scb->tx.delivered_mstamp.v64) return; if (!rs->prior_delivered || after(scb->tx.delivered, rs->prior_delivered)) { rs->prior_delivered = scb->tx.delivered; rs->prior_mstamp = scb->tx.delivered_mstamp; rs->is_app_limited = scb->tx.is_app_limited; rs->is_retrans = scb->sacked & TCPCB_RETRANS; /* Find the duration of the "send phase" of this window: */ rs->interval_us = skb_mstamp_us_delta( &skb->skb_mstamp, &scb->tx.first_tx_mstamp); /* Record send time of most recently ACKed packet: */ tp->first_tx_mstamp = skb->skb_mstamp; } /* Mark off the skb delivered once it's sacked to avoid being * used again when it's cumulatively acked. For acked packets * we don't need to reset since it'll be freed soon. */ if (scb->sacked & TCPCB_SACKED_ACKED) scb->tx.delivered_mstamp.v64 = 0; } /* Update the connection delivery information and generate a rate sample. */ void tcp_rate_gen(struct sock *sk, u32 delivered, u32 lost, struct skb_mstamp *now, struct rate_sample *rs) { struct tcp_sock *tp = tcp_sk(sk); u32 snd_us, ack_us; /* Clear app limited if bubble is acked and gone. */ if (tp->app_limited && after(tp->delivered, tp->app_limited)) tp->app_limited = 0; /* TODO: there are multiple places throughout tcp_ack() to get * current time. Refactor the code using a new "tcp_acktag_state" * to carry current time, flags, stats like "tcp_sacktag_state". */ if (delivered) tp->delivered_mstamp = *now; rs->acked_sacked = delivered; /* freshly ACKed or SACKed */ rs->losses = lost; /* freshly marked lost */ /* Return an invalid sample if no timing information is available. */ if (!rs->prior_mstamp.v64) { rs->delivered = -1; rs->interval_us = -1; return; } rs->delivered = tp->delivered - rs->prior_delivered; /* Model sending data and receiving ACKs as separate pipeline phases * for a window. Usually the ACK phase is longer, but with ACK * compression the send phase can be longer. To be safe we use the * longer phase. */ snd_us = rs->interval_us; /* send phase */ ack_us = skb_mstamp_us_delta(now, &rs->prior_mstamp); /* ack phase */ rs->interval_us = max(snd_us, ack_us); /* Normally we expect interval_us >= min-rtt. * Note that rate may still be over-estimated when a spuriously * retransmistted skb was first (s)acked because "interval_us" * is under-estimated (up to an RTT). However continuously * measuring the delivery rate during loss recovery is crucial * for connections suffer heavy or prolonged losses. */ if (unlikely(rs->interval_us < tcp_min_rtt(tp))) { if (!rs->is_retrans) pr_debug("tcp rate: %ld %d %u %u %u\n", rs->interval_us, rs->delivered, inet_csk(sk)->icsk_ca_state, tp->rx_opt.sack_ok, tcp_min_rtt(tp)); rs->interval_us = -1; return; } /* Record the last non-app-limited or the highest app-limited bw */ if (!rs->is_app_limited || ((u64)rs->delivered * tp->rate_interval_us >= (u64)tp->rate_delivered * rs->interval_us)) { tp->rate_delivered = rs->delivered; tp->rate_interval_us = rs->interval_us; tp->rate_app_limited = rs->is_app_limited; } } /* If a gap is detected between sends, mark the socket application-limited. */ void tcp_rate_check_app_limited(struct sock *sk) { struct tcp_sock *tp = tcp_sk(sk); if (/* We have less than one packet to send. */ tp->write_seq - tp->snd_nxt < tp->mss_cache && /* Nothing in sending host's qdisc queues or NIC tx queue. */ sk_wmem_alloc_get(sk) < SKB_TRUESIZE(1) && /* We are not limited by CWND. */ tcp_packets_in_flight(tp) < tp->snd_cwnd && /* All lost packets have been retransmitted. */ tp->lost_out <= tp->retrans_out) tp->app_limited = (tp->delivered + tcp_packets_in_flight(tp)) ? : 1; }