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+/*
+ * 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");
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