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339 lines
9.7 KiB
339 lines
9.7 KiB
// SPDX-License-Identifier: GPL-2.0-only |
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/* |
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* TCP Vegas congestion control |
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* |
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* This is based on the congestion detection/avoidance scheme described in |
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* Lawrence S. Brakmo and Larry L. Peterson. |
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* "TCP Vegas: End to end congestion avoidance on a global internet." |
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* IEEE Journal on Selected Areas in Communication, 13(8):1465--1480, |
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* October 1995. Available from: |
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* ftp://ftp.cs.arizona.edu/xkernel/Papers/jsac.ps |
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* |
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* See http://www.cs.arizona.edu/xkernel/ for their implementation. |
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* The main aspects that distinguish this implementation from the |
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* Arizona Vegas implementation are: |
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* o We do not change the loss detection or recovery mechanisms of |
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* Linux in any way. Linux already recovers from losses quite well, |
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* using fine-grained timers, NewReno, and FACK. |
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* o To avoid the performance penalty imposed by increasing cwnd |
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* only every-other RTT during slow start, we increase during |
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* every RTT during slow start, just like Reno. |
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* o Largely to allow continuous cwnd growth during slow start, |
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* we use the rate at which ACKs come back as the "actual" |
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* rate, rather than the rate at which data is sent. |
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* o To speed convergence to the right rate, we set the cwnd |
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* to achieve the right ("actual") rate when we exit slow start. |
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* o To filter out the noise caused by delayed ACKs, we use the |
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* minimum RTT sample observed during the last RTT to calculate |
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* the actual rate. |
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* o When the sender re-starts from idle, it waits until it has |
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* received ACKs for an entire flight of new data before making |
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* a cwnd adjustment decision. The original Vegas implementation |
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* assumed senders never went idle. |
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*/ |
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#include <linux/mm.h> |
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#include <linux/module.h> |
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#include <linux/skbuff.h> |
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#include <linux/inet_diag.h> |
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#include <net/tcp.h> |
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#include "tcp_vegas.h" |
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static int alpha = 2; |
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static int beta = 4; |
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static int gamma = 1; |
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module_param(alpha, int, 0644); |
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MODULE_PARM_DESC(alpha, "lower bound of packets in network"); |
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module_param(beta, int, 0644); |
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MODULE_PARM_DESC(beta, "upper bound of packets in network"); |
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module_param(gamma, int, 0644); |
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MODULE_PARM_DESC(gamma, "limit on increase (scale by 2)"); |
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/* There are several situations when we must "re-start" Vegas: |
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* |
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* o when a connection is established |
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* o after an RTO |
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* o after fast recovery |
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* o when we send a packet and there is no outstanding |
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* unacknowledged data (restarting an idle connection) |
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* |
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* In these circumstances we cannot do a Vegas calculation at the |
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* end of the first RTT, because any calculation we do is using |
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* stale info -- both the saved cwnd and congestion feedback are |
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* stale. |
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* |
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* Instead we must wait until the completion of an RTT during |
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* which we actually receive ACKs. |
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*/ |
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static void vegas_enable(struct sock *sk) |
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{ |
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const struct tcp_sock *tp = tcp_sk(sk); |
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struct vegas *vegas = inet_csk_ca(sk); |
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/* Begin taking Vegas samples next time we send something. */ |
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vegas->doing_vegas_now = 1; |
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/* Set the beginning of the next send window. */ |
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vegas->beg_snd_nxt = tp->snd_nxt; |
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vegas->cntRTT = 0; |
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vegas->minRTT = 0x7fffffff; |
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} |
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/* Stop taking Vegas samples for now. */ |
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static inline void vegas_disable(struct sock *sk) |
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{ |
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struct vegas *vegas = inet_csk_ca(sk); |
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vegas->doing_vegas_now = 0; |
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} |
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void tcp_vegas_init(struct sock *sk) |
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{ |
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struct vegas *vegas = inet_csk_ca(sk); |
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vegas->baseRTT = 0x7fffffff; |
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vegas_enable(sk); |
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} |
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EXPORT_SYMBOL_GPL(tcp_vegas_init); |
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/* Do RTT sampling needed for Vegas. |
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* Basically we: |
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* o min-filter RTT samples from within an RTT to get the current |
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* propagation delay + queuing delay (we are min-filtering to try to |
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* avoid the effects of delayed ACKs) |
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* o min-filter RTT samples from a much longer window (forever for now) |
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* to find the propagation delay (baseRTT) |
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*/ |
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void tcp_vegas_pkts_acked(struct sock *sk, const struct ack_sample *sample) |
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{ |
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struct vegas *vegas = inet_csk_ca(sk); |
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u32 vrtt; |
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if (sample->rtt_us < 0) |
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return; |
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/* Never allow zero rtt or baseRTT */ |
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vrtt = sample->rtt_us + 1; |
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/* Filter to find propagation delay: */ |
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if (vrtt < vegas->baseRTT) |
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vegas->baseRTT = vrtt; |
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/* Find the min RTT during the last RTT to find |
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* the current prop. delay + queuing delay: |
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*/ |
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vegas->minRTT = min(vegas->minRTT, vrtt); |
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vegas->cntRTT++; |
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} |
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EXPORT_SYMBOL_GPL(tcp_vegas_pkts_acked); |
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void tcp_vegas_state(struct sock *sk, u8 ca_state) |
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{ |
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if (ca_state == TCP_CA_Open) |
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vegas_enable(sk); |
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else |
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vegas_disable(sk); |
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} |
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EXPORT_SYMBOL_GPL(tcp_vegas_state); |
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/* |
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* If the connection is idle and we are restarting, |
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* then we don't want to do any Vegas calculations |
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* until we get fresh RTT samples. So when we |
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* restart, we reset our Vegas state to a clean |
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* slate. After we get acks for this flight of |
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* packets, _then_ we can make Vegas calculations |
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* again. |
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*/ |
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void tcp_vegas_cwnd_event(struct sock *sk, enum tcp_ca_event event) |
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{ |
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if (event == CA_EVENT_CWND_RESTART || |
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event == CA_EVENT_TX_START) |
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tcp_vegas_init(sk); |
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} |
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EXPORT_SYMBOL_GPL(tcp_vegas_cwnd_event); |
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static inline u32 tcp_vegas_ssthresh(struct tcp_sock *tp) |
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{ |
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return min(tp->snd_ssthresh, tp->snd_cwnd); |
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} |
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static void tcp_vegas_cong_avoid(struct sock *sk, u32 ack, u32 acked) |
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{ |
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struct tcp_sock *tp = tcp_sk(sk); |
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struct vegas *vegas = inet_csk_ca(sk); |
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if (!vegas->doing_vegas_now) { |
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tcp_reno_cong_avoid(sk, ack, acked); |
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return; |
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} |
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if (after(ack, vegas->beg_snd_nxt)) { |
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/* Do the Vegas once-per-RTT cwnd adjustment. */ |
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/* Save the extent of the current window so we can use this |
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* at the end of the next RTT. |
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*/ |
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vegas->beg_snd_nxt = tp->snd_nxt; |
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/* We do the Vegas calculations only if we got enough RTT |
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* samples that we can be reasonably sure that we got |
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* at least one RTT sample that wasn't from a delayed ACK. |
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* If we only had 2 samples total, |
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* then that means we're getting only 1 ACK per RTT, which |
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* means they're almost certainly delayed ACKs. |
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* If we have 3 samples, we should be OK. |
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*/ |
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if (vegas->cntRTT <= 2) { |
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/* We don't have enough RTT samples to do the Vegas |
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* calculation, so we'll behave like Reno. |
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*/ |
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tcp_reno_cong_avoid(sk, ack, acked); |
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} else { |
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u32 rtt, diff; |
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u64 target_cwnd; |
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/* We have enough RTT samples, so, using the Vegas |
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* algorithm, we determine if we should increase or |
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* decrease cwnd, and by how much. |
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*/ |
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/* Pluck out the RTT we are using for the Vegas |
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* calculations. This is the min RTT seen during the |
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* last RTT. Taking the min filters out the effects |
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* of delayed ACKs, at the cost of noticing congestion |
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* a bit later. |
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*/ |
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rtt = vegas->minRTT; |
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/* Calculate the cwnd we should have, if we weren't |
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* going too fast. |
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* |
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* This is: |
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* (actual rate in segments) * baseRTT |
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*/ |
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target_cwnd = (u64)tp->snd_cwnd * vegas->baseRTT; |
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do_div(target_cwnd, rtt); |
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/* Calculate the difference between the window we had, |
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* and the window we would like to have. This quantity |
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* is the "Diff" from the Arizona Vegas papers. |
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*/ |
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diff = tp->snd_cwnd * (rtt-vegas->baseRTT) / vegas->baseRTT; |
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if (diff > gamma && tcp_in_slow_start(tp)) { |
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/* Going too fast. Time to slow down |
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* and switch to congestion avoidance. |
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*/ |
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/* Set cwnd to match the actual rate |
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* exactly: |
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* cwnd = (actual rate) * baseRTT |
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* Then we add 1 because the integer |
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* truncation robs us of full link |
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* utilization. |
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*/ |
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tp->snd_cwnd = min(tp->snd_cwnd, (u32)target_cwnd+1); |
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tp->snd_ssthresh = tcp_vegas_ssthresh(tp); |
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} else if (tcp_in_slow_start(tp)) { |
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/* Slow start. */ |
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tcp_slow_start(tp, acked); |
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} else { |
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/* Congestion avoidance. */ |
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/* Figure out where we would like cwnd |
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* to be. |
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*/ |
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if (diff > beta) { |
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/* The old window was too fast, so |
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* we slow down. |
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*/ |
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tp->snd_cwnd--; |
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tp->snd_ssthresh |
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= tcp_vegas_ssthresh(tp); |
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} else if (diff < alpha) { |
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/* We don't have enough extra packets |
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* in the network, so speed up. |
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*/ |
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tp->snd_cwnd++; |
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} else { |
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/* Sending just as fast as we |
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* should be. |
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*/ |
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} |
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} |
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if (tp->snd_cwnd < 2) |
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tp->snd_cwnd = 2; |
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else if (tp->snd_cwnd > tp->snd_cwnd_clamp) |
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tp->snd_cwnd = tp->snd_cwnd_clamp; |
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tp->snd_ssthresh = tcp_current_ssthresh(sk); |
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} |
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/* Wipe the slate clean for the next RTT. */ |
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vegas->cntRTT = 0; |
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vegas->minRTT = 0x7fffffff; |
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} |
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/* Use normal slow start */ |
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else if (tcp_in_slow_start(tp)) |
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tcp_slow_start(tp, acked); |
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} |
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/* Extract info for Tcp socket info provided via netlink. */ |
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size_t tcp_vegas_get_info(struct sock *sk, u32 ext, int *attr, |
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union tcp_cc_info *info) |
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{ |
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const struct vegas *ca = inet_csk_ca(sk); |
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if (ext & (1 << (INET_DIAG_VEGASINFO - 1))) { |
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info->vegas.tcpv_enabled = ca->doing_vegas_now; |
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info->vegas.tcpv_rttcnt = ca->cntRTT; |
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info->vegas.tcpv_rtt = ca->baseRTT; |
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info->vegas.tcpv_minrtt = ca->minRTT; |
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*attr = INET_DIAG_VEGASINFO; |
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return sizeof(struct tcpvegas_info); |
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} |
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return 0; |
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} |
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EXPORT_SYMBOL_GPL(tcp_vegas_get_info); |
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static struct tcp_congestion_ops tcp_vegas __read_mostly = { |
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.init = tcp_vegas_init, |
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.ssthresh = tcp_reno_ssthresh, |
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.undo_cwnd = tcp_reno_undo_cwnd, |
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.cong_avoid = tcp_vegas_cong_avoid, |
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.pkts_acked = tcp_vegas_pkts_acked, |
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.set_state = tcp_vegas_state, |
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.cwnd_event = tcp_vegas_cwnd_event, |
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.get_info = tcp_vegas_get_info, |
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.owner = THIS_MODULE, |
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.name = "vegas", |
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}; |
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static int __init tcp_vegas_register(void) |
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{ |
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BUILD_BUG_ON(sizeof(struct vegas) > ICSK_CA_PRIV_SIZE); |
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tcp_register_congestion_control(&tcp_vegas); |
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return 0; |
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} |
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static void __exit tcp_vegas_unregister(void) |
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{ |
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tcp_unregister_congestion_control(&tcp_vegas); |
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} |
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module_init(tcp_vegas_register); |
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module_exit(tcp_vegas_unregister); |
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MODULE_AUTHOR("Stephen Hemminger"); |
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MODULE_LICENSE("GPL"); |
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MODULE_DESCRIPTION("TCP Vegas");
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