// SPDX-License-Identifier: GPL-2.0-only #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) { … } /* 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 most recently sent time and the highest * sequence. */ void tcp_rate_skb_delivered(struct sock *sk, struct sk_buff *skb, struct rate_sample *rs) { … } /* Update the connection delivery information and generate a rate sample. */ void tcp_rate_gen(struct sock *sk, u32 delivered, u32 lost, bool is_sack_reneg, struct rate_sample *rs) { … } /* If a gap is detected between sends, mark the socket application-limited. */ void tcp_rate_check_app_limited(struct sock *sk) { … } EXPORT_SYMBOL_GPL(…);
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