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Release 4.14 net/ipv4/tcp_vegas.c

Directory: net/ipv4
 * TCP Vegas congestion control
 * This is based on the congestion detection/avoidance scheme described in
 *    Lawrence S. Brakmo and Larry L. Peterson.
 *    "TCP Vegas: End to end congestion avoidance on a global internet."
 *    IEEE Journal on Selected Areas in Communication, 13(8):1465--1480,
 *    October 1995. Available from:
 * See for their implementation.
 * The main aspects that distinguish this implementation from the
 * Arizona Vegas implementation are:
 *   o We do not change the loss detection or recovery mechanisms of
 *     Linux in any way. Linux already recovers from losses quite well,
 *     using fine-grained timers, NewReno, and FACK.
 *   o To avoid the performance penalty imposed by increasing cwnd
 *     only every-other RTT during slow start, we increase during
 *     every RTT during slow start, just like Reno.
 *   o Largely to allow continuous cwnd growth during slow start,
 *     we use the rate at which ACKs come back as the "actual"
 *     rate, rather than the rate at which data is sent.
 *   o To speed convergence to the right rate, we set the cwnd
 *     to achieve the right ("actual") rate when we exit slow start.
 *   o To filter out the noise caused by delayed ACKs, we use the
 *     minimum RTT sample observed during the last RTT to calculate
 *     the actual rate.
 *   o When the sender re-starts from idle, it waits until it has
 *     received ACKs for an entire flight of new data before making
 *     a cwnd adjustment decision. The original Vegas implementation
 *     assumed senders never went idle.

#include <linux/mm.h>
#include <linux/module.h>
#include <linux/skbuff.h>
#include <linux/inet_diag.h>

#include <net/tcp.h>

#include "tcp_vegas.h"

static int alpha = 2;

static int beta  = 4;

static int gamma = 1;

module_param(alpha, int, 0644);
MODULE_PARM_DESC(alpha, "lower bound of packets in network");
module_param(beta, int, 0644);
MODULE_PARM_DESC(beta, "upper bound of packets in network");
module_param(gamma, int, 0644);
MODULE_PARM_DESC(gamma, "limit on increase (scale by 2)");

/* There are several situations when we must "re-start" Vegas:
 *  o when a connection is established
 *  o after an RTO
 *  o after fast recovery
 *  o when we send a packet and there is no outstanding
 *    unacknowledged data (restarting an idle connection)
 * In these circumstances we cannot do a Vegas calculation at the
 * end of the first RTT, because any calculation we do is using
 * stale info -- both the saved cwnd and congestion feedback are
 * stale.
 * Instead we must wait until the completion of an RTT during
 * which we actually receive ACKs.

static void vegas_enable(struct sock *sk) { const struct tcp_sock *tp = tcp_sk(sk); struct vegas *vegas = inet_csk_ca(sk); /* Begin taking Vegas samples next time we send something. */ vegas->doing_vegas_now = 1; /* Set the beginning of the next send window. */ vegas->beg_snd_nxt = tp->snd_nxt; vegas->cntRTT = 0; vegas->minRTT = 0x7fffffff; }


Stephen Hemminger4575.00%150.00%
Arnaldo Carvalho de Melo1525.00%150.00%

/* Stop taking Vegas samples for now. */
static inline void vegas_disable(struct sock *sk) { struct vegas *vegas = inet_csk_ca(sk); vegas->doing_vegas_now = 0; }


Stephen Hemminger2485.71%150.00%
Arnaldo Carvalho de Melo414.29%150.00%

void tcp_vegas_init(struct sock *sk) { struct vegas *vegas = inet_csk_ca(sk); vegas->baseRTT = 0x7fffffff; vegas_enable(sk); }


Stephen Hemminger2683.87%150.00%
Arnaldo Carvalho de Melo516.13%150.00%

EXPORT_SYMBOL_GPL(tcp_vegas_init); /* Do RTT sampling needed for Vegas. * Basically we: * o min-filter RTT samples from within an RTT to get the current * propagation delay + queuing delay (we are min-filtering to try to * avoid the effects of delayed ACKs) * o min-filter RTT samples from a much longer window (forever for now) * to find the propagation delay (baseRTT) */
void tcp_vegas_pkts_acked(struct sock *sk, const struct ack_sample *sample) { struct vegas *vegas = inet_csk_ca(sk); u32 vrtt; if (sample->rtt_us < 0) return; /* Never allow zero rtt or baseRTT */ vrtt = sample->rtt_us + 1; /* Filter to find propagation delay: */ if (vrtt < vegas->baseRTT) vegas->baseRTT = vrtt; /* Find the min RTT during the last RTT to find * the current prop. delay + queuing delay: */ vegas->minRTT = min(vegas->minRTT, vrtt); vegas->cntRTT++; }


Stephen Hemminger6479.01%350.00%
Lawrence Brakmo911.11%116.67%
Arnaldo Carvalho de Melo44.94%116.67%
Ilpo Järvinen44.94%116.67%

void tcp_vegas_state(struct sock *sk, u8 ca_state) { if (ca_state == TCP_CA_Open) vegas_enable(sk); else vegas_disable(sk); }


Stephen Hemminger2686.67%150.00%
Arnaldo Carvalho de Melo413.33%150.00%

EXPORT_SYMBOL_GPL(tcp_vegas_state); /* * If the connection is idle and we are restarting, * then we don't want to do any Vegas calculations * until we get fresh RTT samples. So when we * restart, we reset our Vegas state to a clean * slate. After we get acks for this flight of * packets, _then_ we can make Vegas calculations * again. */
void tcp_vegas_cwnd_event(struct sock *sk, enum tcp_ca_event event) { if (event == CA_EVENT_CWND_RESTART || event == CA_EVENT_TX_START) tcp_vegas_init(sk); }


Stephen Hemminger2689.66%150.00%
Arnaldo Carvalho de Melo310.34%150.00%

static inline u32 tcp_vegas_ssthresh(struct tcp_sock *tp) { return min(tp->snd_ssthresh, tp->snd_cwnd-1); }


Doug Leith26100.00%1100.00%

static void tcp_vegas_cong_avoid(struct sock *sk, u32 ack, u32 acked) { struct tcp_sock *tp = tcp_sk(sk); struct vegas *vegas = inet_csk_ca(sk); if (!vegas->doing_vegas_now) { tcp_reno_cong_avoid(sk, ack, acked); return; } if (after(ack, vegas->beg_snd_nxt)) { /* Do the Vegas once-per-RTT cwnd adjustment. */ /* Save the extent of the current window so we can use this * at the end of the next RTT. */ vegas->beg_snd_nxt = tp->snd_nxt; /* We do the Vegas calculations only if we got enough RTT * samples that we can be reasonably sure that we got * at least one RTT sample that wasn't from a delayed ACK. * If we only had 2 samples total, * then that means we're getting only 1 ACK per RTT, which * means they're almost certainly delayed ACKs. * If we have 3 samples, we should be OK. */ if (vegas->cntRTT <= 2) { /* We don't have enough RTT samples to do the Vegas * calculation, so we'll behave like Reno. */ tcp_reno_cong_avoid(sk, ack, acked); } else { u32 rtt, diff; u64 target_cwnd; /* We have enough RTT samples, so, using the Vegas * algorithm, we determine if we should increase or * decrease cwnd, and by how much. */ /* Pluck out the RTT we are using for the Vegas * calculations. This is the min RTT seen during the * last RTT. Taking the min filters out the effects * of delayed ACKs, at the cost of noticing congestion * a bit later. */ rtt = vegas->minRTT; /* Calculate the cwnd we should have, if we weren't * going too fast. * * This is: * (actual rate in segments) * baseRTT */ target_cwnd = (u64)tp->snd_cwnd * vegas->baseRTT; do_div(target_cwnd, rtt); /* Calculate the difference between the window we had, * and the window we would like to have. This quantity * is the "Diff" from the Arizona Vegas papers. */ diff = tp->snd_cwnd * (rtt-vegas->baseRTT) / vegas->baseRTT; if (diff > gamma && tcp_in_slow_start(tp)) { /* Going too fast. Time to slow down * and switch to congestion avoidance. */ /* Set cwnd to match the actual rate * exactly: * cwnd = (actual rate) * baseRTT * Then we add 1 because the integer * truncation robs us of full link * utilization. */ tp->snd_cwnd = min(tp->snd_cwnd, (u32)target_cwnd+1); tp->snd_ssthresh = tcp_vegas_ssthresh(tp); } else if (tcp_in_slow_start(tp)) { /* Slow start. */ tcp_slow_start(tp, acked); } else { /* Congestion avoidance. */ /* Figure out where we would like cwnd * to be. */ if (diff > beta) { /* The old window was too fast, so * we slow down. */ tp->snd_cwnd--; tp->snd_ssthresh = tcp_vegas_ssthresh(tp); } else if (diff < alpha) { /* We don't have enough extra packets * in the network, so speed up. */ tp->snd_cwnd++; } else { /* Sending just as fast as we * should be. */ } } if (tp->snd_cwnd < 2) tp->snd_cwnd = 2; else if (tp->snd_cwnd > tp->snd_cwnd_clamp) tp->snd_cwnd = tp->snd_cwnd_clamp; tp->snd_ssthresh = tcp_current_ssthresh(sk); } /* Wipe the slate clean for the next RTT. */ vegas->cntRTT = 0; vegas->minRTT = 0x7fffffff; } /* Use normal slow start */ else if (tcp_in_slow_start(tp)) tcp_slow_start(tp, acked); }


Stephen Hemminger21062.31%214.29%
Doug Leith5315.73%321.43%
Yuchung Cheng164.75%214.29%
Arnaldo Carvalho de Melo154.45%17.14%
Xiaoliang (David) Wei133.86%17.14%
Thomas Young123.56%214.29%
Christoph Paasch92.67%17.14%
Lachlan Andrew51.48%17.14%
Harvey Harrison41.19%17.14%

/* Extract info for Tcp socket info provided via netlink. */
size_t tcp_vegas_get_info(struct sock *sk, u32 ext, int *attr, union tcp_cc_info *info) { const struct vegas *ca = inet_csk_ca(sk); if (ext & (1 << (INET_DIAG_VEGASINFO - 1))) { info->vegas.tcpv_enabled = ca->doing_vegas_now, info->vegas.tcpv_rttcnt = ca->cntRTT, info->vegas.tcpv_rtt = ca->baseRTT, info->vegas.tcpv_minrtt = ca->minRTT, *attr = INET_DIAG_VEGASINFO; return sizeof(struct tcpvegas_info); } return 0; }


Stephen Hemminger5754.81%116.67%
Eric Dumazet3432.69%233.33%
Thomas Graf87.69%116.67%
Arnaldo Carvalho de Melo54.81%233.33%

EXPORT_SYMBOL_GPL(tcp_vegas_get_info); static struct tcp_congestion_ops tcp_vegas __read_mostly = { .init = tcp_vegas_init, .ssthresh = tcp_reno_ssthresh, .undo_cwnd = tcp_reno_undo_cwnd, .cong_avoid = tcp_vegas_cong_avoid, .pkts_acked = tcp_vegas_pkts_acked, .set_state = tcp_vegas_state, .cwnd_event = tcp_vegas_cwnd_event, .get_info = tcp_vegas_get_info, .owner = THIS_MODULE, .name = "vegas", };
static int __init tcp_vegas_register(void) { BUILD_BUG_ON(sizeof(struct vegas) > ICSK_CA_PRIV_SIZE); tcp_register_congestion_control(&tcp_vegas); return 0; }


Stephen Hemminger2793.10%133.33%
Arnaldo Carvalho de Melo13.45%133.33%
Alexey Dobriyan13.45%133.33%

static void __exit tcp_vegas_unregister(void) { tcp_unregister_congestion_control(&tcp_vegas); }


Stephen Hemminger15100.00%1100.00%

module_init(tcp_vegas_register); module_exit(tcp_vegas_unregister); MODULE_AUTHOR("Stephen Hemminger"); MODULE_LICENSE("GPL"); MODULE_DESCRIPTION("TCP Vegas");

Overall Contributors

Stephen Hemminger70873.14%621.43%
Doug Leith818.37%310.71%
Arnaldo Carvalho de Melo575.89%310.71%
Eric Dumazet343.51%27.14%
Yuchung Cheng161.65%27.14%
Xiaoliang (David) Wei131.34%13.57%
Thomas Young121.24%27.14%
Lawrence Brakmo90.93%13.57%
Christoph Paasch90.93%13.57%
Thomas Graf80.83%13.57%
Florian Westphal50.52%13.57%
Lachlan Andrew50.52%13.57%
Ilpo Järvinen40.41%13.57%
Harvey Harrison40.41%13.57%
David S. Miller20.21%13.57%
Alexey Dobriyan10.10%13.57%
Directory: net/ipv4
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