// SPDX-License-Identifier: GPL-2.0-only /* * 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: * ftp://ftp.cs.arizona.edu/xkernel/Papers/jsac.ps * * See http://www.cs.arizona.edu/xkernel/ 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; } /* 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; } void tcp_vegas_init(struct sock *sk) { struct vegas *vegas = inet_csk_ca(sk); vegas->baseRTT = 0x7fffffff; vegas_enable(sk); } 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++; } EXPORT_SYMBOL_GPL(tcp_vegas_pkts_acked); void tcp_vegas_state(struct sock *sk, u8 ca_state) { if (ca_state == TCP_CA_Open) vegas_enable(sk); else vegas_disable(sk); } 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); } EXPORT_SYMBOL_GPL(tcp_vegas_cwnd_event); static inline u32 tcp_vegas_ssthresh(struct tcp_sock *tp) { return min(tp->snd_ssthresh, tcp_snd_cwnd(tp)); } 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)tcp_snd_cwnd(tp) * 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 = tcp_snd_cwnd(tp) * (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. */ tcp_snd_cwnd_set(tp, min(tcp_snd_cwnd(tp), (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. */ tcp_snd_cwnd_set(tp, tcp_snd_cwnd(tp) - 1); tp->snd_ssthresh = tcp_vegas_ssthresh(tp); } else if (diff < alpha) { /* We don't have enough extra packets * in the network, so speed up. */ tcp_snd_cwnd_set(tp, tcp_snd_cwnd(tp) + 1); } else { /* Sending just as fast as we * should be. */ } } if (tcp_snd_cwnd(tp) < 2) tcp_snd_cwnd_set(tp, 2); else if (tcp_snd_cwnd(tp) > tp->snd_cwnd_clamp) tcp_snd_cwnd_set(tp, 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); } /* 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; } 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; } static void __exit tcp_vegas_unregister(void) { tcp_unregister_congestion_control(&tcp_vegas); } module_init(tcp_vegas_register); module_exit(tcp_vegas_unregister); MODULE_AUTHOR("Stephen Hemminger"); MODULE_LICENSE("GPL"); MODULE_DESCRIPTION("TCP Vegas"