/* * 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/config.h> #include <linux/mm.h> #include <linux/module.h> #include <linux/skbuff.h> #include <linux/tcp_diag.h> #include <net/tcp.h> /* Default values of the Vegas variables, in fixed-point representation * with V_PARAM_SHIFT bits to the right of the binary point. */ #define V_PARAM_SHIFT 1 static int alpha = 1<<V_PARAM_SHIFT; static int beta = 3<<V_PARAM_SHIFT; static int gamma = 1<<V_PARAM_SHIFT; module_param(alpha, int, 0644); MODULE_PARM_DESC(alpha, "lower bound of packets in network (scale by 2)"); module_param(beta, int, 0644); MODULE_PARM_DESC(beta, "upper bound of packets in network (scale by 2)"); module_param(gamma, int, 0644); MODULE_PARM_DESC(gamma, "limit on increase (scale by 2)"); /* Vegas variables */ struct vegas { u32 beg_snd_nxt; /* right edge during last RTT */ u32 beg_snd_una; /* left edge during last RTT */ u32 beg_snd_cwnd; /* saves the size of the cwnd */ u8 doing_vegas_now;/* if true, do vegas for this RTT */ u16 cntRTT; /* # of RTTs measured within last RTT */ u32 minRTT; /* min of RTTs measured within last RTT (in usec) */ u32 baseRTT; /* the min of all Vegas RTT measurements seen (in usec) */ }; /* 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 inline void vegas_enable(struct tcp_sock *tp) { struct vegas *vegas = tcp_ca(tp); /* 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 tcp_sock *tp) { struct vegas *vegas = tcp_ca(tp); vegas->doing_vegas_now = 0; } static void tcp_vegas_init(struct tcp_sock *tp) { struct vegas *vegas = tcp_ca(tp); vegas->baseRTT = 0x7fffffff; vegas_enable(tp); } /* 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) */ static void tcp_vegas_rtt_calc(struct tcp_sock *tp, u32 usrtt) { struct vegas *vegas = tcp_ca(tp); u32 vrtt = usrtt + 1; /* Never allow zero rtt or baseRTT */ /* 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++; } static void tcp_vegas_state(struct tcp_sock *tp, u8 ca_state) { if (ca_state == TCP_CA_Open) vegas_enable(tp); else vegas_disable(tp); } /* * 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. */ static void tcp_vegas_cwnd_event(struct tcp_sock *tp, enum tcp_ca_event event) { if (event == CA_EVENT_CWND_RESTART || event == CA_EVENT_TX_START) tcp_vegas_init(tp); } static void tcp_vegas_cong_avoid(struct tcp_sock *tp, u32 ack, u32 seq_rtt, u32 in_flight, int flag) { struct vegas *vegas = tcp_ca(tp); if (!vegas->doing_vegas_now) return tcp_reno_cong_avoid(tp, ack, seq_rtt, in_flight, flag); /* The key players are v_beg_snd_una and v_beg_snd_nxt. * * These are so named because they represent the approximate values * of snd_una and snd_nxt at the beginning of the current RTT. More * precisely, they represent the amount of data sent during the RTT. * At the end of the RTT, when we receive an ACK for v_beg_snd_nxt, * we will calculate that (v_beg_snd_nxt - v_beg_snd_una) outstanding * bytes of data have been ACKed during the course of the RTT, giving * an "actual" rate of: * * (v_beg_snd_nxt - v_beg_snd_una) / (rtt duration) * * Unfortunately, v_beg_snd_una is not exactly equal to snd_una, * because delayed ACKs can cover more than one segment, so they * don't line up nicely with the boundaries of RTTs. * * Another unfortunate fact of life is that delayed ACKs delay the * advance of the left edge of our send window, so that the number * of bytes we send in an RTT is often less than our cwnd will allow. * So we keep track of our cwnd separately, in v_beg_snd_cwnd. */ if (after(ack, vegas->beg_snd_nxt)) { /* Do the Vegas once-per-RTT cwnd adjustment. */ u32 old_wnd, old_snd_cwnd; /* Here old_wnd is essentially the window of data that was * sent during the previous RTT, and has all * been acknowledged in the course of the RTT that ended * with the ACK we just received. Likewise, old_snd_cwnd * is the cwnd during the previous RTT. */ old_wnd = (vegas->beg_snd_nxt - vegas->beg_snd_una) / tp->mss_cache; old_snd_cwnd = vegas->beg_snd_cwnd; /* Save the extent of the current window so we can use this * at the end of the next RTT. */ vegas->beg_snd_una = vegas->beg_snd_nxt; vegas->beg_snd_nxt = tp->snd_nxt; vegas->beg_snd_cwnd = tp->snd_cwnd; /* Take into account the current RTT sample too, to * decrease the impact of delayed acks. This double counts * this sample since we count it for the next window as well, * but that's not too awful, since we're taking the min, * rather than averaging. */ tcp_vegas_rtt_calc(tp, seq_rtt*1000); /* 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. */ if (tp->snd_cwnd > tp->snd_ssthresh) tp->snd_cwnd++; } else { u32 rtt, target_cwnd, diff; /* 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 * We keep it as a fixed point number with * V_PARAM_SHIFT bits to the right of the binary point. */ target_cwnd = ((old_wnd * vegas->baseRTT) << V_PARAM_SHIFT) / 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. * * Again, this is a fixed point number with * V_PARAM_SHIFT bits to the right of the binary * point. */ diff = (old_wnd << V_PARAM_SHIFT) - target_cwnd; if (tp->snd_cwnd < tp->snd_ssthresh) { /* Slow start. */ if (diff > gamma) { /* Going too fast. Time to slow down * and switch to congestion avoidance. */ tp->snd_ssthresh = 2; /* 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, (target_cwnd >> V_PARAM_SHIFT)+1); } } else { /* Congestion avoidance. */ u32 next_snd_cwnd; /* Figure out where we would like cwnd * to be. */ if (diff > beta) { /* The old window was too fast, so * we slow down. */ next_snd_cwnd = old_snd_cwnd - 1; } else if (diff < alpha) { /* We don't have enough extra packets * in the network, so speed up. */ next_snd_cwnd = old_snd_cwnd + 1; } else { /* Sending just as fast as we * should be. */ next_snd_cwnd = old_snd_cwnd; } /* Adjust cwnd upward or downward, toward the * desired value. */ if (next_snd_cwnd > tp->snd_cwnd) tp->snd_cwnd++; else if (next_snd_cwnd < tp->snd_cwnd) tp->snd_cwnd--; } } /* Wipe the slate clean for the next RTT. */ vegas->cntRTT = 0; vegas->minRTT = 0x7fffffff; } /* The following code is executed for every ack we receive, * except for conditions checked in should_advance_cwnd() * before the call to tcp_cong_avoid(). Mainly this means that * we only execute this code if the ack actually acked some * data. */ /* If we are in slow start, increase our cwnd in response to this ACK. * (If we are not in slow start then we are in congestion avoidance, * and adjust our congestion window only once per RTT. See the code * above.) */ if (tp->snd_cwnd <= tp->snd_ssthresh) tp->snd_cwnd++; /* to keep cwnd from growing without bound */ tp->snd_cwnd = min_t(u32, tp->snd_cwnd, tp->snd_cwnd_clamp); /* Make sure that we are never so timid as to reduce our cwnd below * 2 MSS. * * Going below 2 MSS would risk huge delayed ACKs from our receiver. */ tp->snd_cwnd = max(tp->snd_cwnd, 2U); } /* Extract info for Tcp socket info provided via netlink. */ static void tcp_vegas_get_info(struct tcp_sock *tp, u32 ext, struct sk_buff *skb) { const struct vegas *ca = tcp_ca(tp); if (ext & (1<<(TCPDIAG_VEGASINFO-1))) { struct tcpvegas_info *info; info = RTA_DATA(__RTA_PUT(skb, TCPDIAG_VEGASINFO, sizeof(*info))); info->tcpv_enabled = ca->doing_vegas_now; info->tcpv_rttcnt = ca->cntRTT; info->tcpv_rtt = ca->baseRTT; info->tcpv_minrtt = ca->minRTT; rtattr_failure: ; } } static struct tcp_congestion_ops tcp_vegas = { .init = tcp_vegas_init, .ssthresh = tcp_reno_ssthresh, .cong_avoid = tcp_vegas_cong_avoid, .min_cwnd = tcp_reno_min_cwnd, .rtt_sample = tcp_vegas_rtt_calc, .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) { BUG_ON(sizeof(struct vegas) > TCP_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");