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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.
+ *
+ *
+ * TCP Compound based on TCP Vegas
+ *
+ * further details can be found here:
+ * ftp://ftp.research.microsoft.com/pub/tr/TR-2005-86.pdf
+ */
+
+#include <linux/config.h>
+#include <linux/mm.h>
+#include <linux/module.h>
+#include <linux/skbuff.h>
+#include <linux/inet_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
+
+#define TCP_COMPOUND_ALPHA 3U
+#define TCP_COMPOUND_BETA 1U
+#define TCP_COMPOUND_GAMMA 30
+#define TCP_COMPOUND_ZETA 1
+
+/* TCP compound variables */
+struct compound {
+ 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) */
+
+ u32 cwnd;
+ u32 dwnd;
+};
+
+/* 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 sock *sk)
+{
+ const struct tcp_sock *tp = tcp_sk(sk);
+ struct compound *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 compound *vegas = inet_csk_ca(sk);
+
+ vegas->doing_vegas_now = 0;
+}
+
+static void tcp_compound_init(struct sock *sk)
+{
+ struct compound *vegas = inet_csk_ca(sk);
+ const struct tcp_sock *tp = tcp_sk(sk);
+
+ vegas->baseRTT = 0x7fffffff;
+ vegas_enable(sk);
+
+ vegas->dwnd = 0;
+ vegas->cwnd = tp->snd_cwnd;
+}
+
+/* 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_compound_rtt_calc(struct sock *sk, u32 usrtt)
+{
+ struct compound *vegas = inet_csk_ca(sk);
+ 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_compound_state(struct sock *sk, u8 ca_state)
+{
+
+ if (ca_state == TCP_CA_Open)
+ vegas_enable(sk);
+ else
+ vegas_disable(sk);
+}
+
+
+/* 64bit divisor, dividend and result. dynamic precision */
+static inline u64 div64_64(u64 dividend, u64 divisor)
+{
+ u32 d = divisor;
+
+ if (divisor > 0xffffffffULL) {
+ unsigned int shift = fls(divisor >> 32);
+
+ d = divisor >> shift;
+ dividend >>= shift;
+ }
+
+ /* avoid 64 bit division if possible */
+ if (dividend >> 32)
+ do_div(dividend, d);
+ else
+ dividend = (u32) dividend / d;
+
+ return dividend;
+}
+
+/* calculate the quartic root of "a" using Newton-Raphson */
+static u32 qroot(u64 a)
+{
+ u32 x, x1;
+
+ /* Initial estimate is based on:
+ * qrt(x) = exp(log(x) / 4)
+ */
+ x = 1u << (fls64(a) >> 2);
+
+ /*
+ * Iteration based on:
+ * 3
+ * x = ( 3 * x + a / x ) / 4
+ * k+1 k k
+ */
+ do {
+ u64 x3 = x;
+
+ x1 = x;
+ x3 *= x;
+ x3 *= x;
+
+ x = (3 * x + (u32) div64_64(a, x3)) / 4;
+ } while (abs(x1 - x) > 1);
+
+ return x;
+}
+
+
+/*
+ * 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_compound_cwnd_event(struct sock *sk, enum tcp_ca_event event)
+{
+ if (event == CA_EVENT_CWND_RESTART || event == CA_EVENT_TX_START)
+ tcp_compound_init(sk);
+}
+
+static void tcp_compound_cong_avoid(struct sock *sk, u32 ack,
+ u32 seq_rtt, u32 in_flight, int flag)
+{
+ struct tcp_sock *tp = tcp_sk(sk);
+ struct compound *vegas = inet_csk_ca(sk);
+ u8 inc = 0;
+
+ if (vegas->cwnd + vegas->dwnd > tp->snd_cwnd) {
+ if (vegas->cwnd > tp->snd_cwnd || vegas->dwnd > tp->snd_cwnd) {
+ vegas->cwnd = tp->snd_cwnd;
+ vegas->dwnd = 0;
+ } else
+ vegas->cwnd = tp->snd_cwnd - vegas->dwnd;
+
+ }
+
+ if (!tcp_is_cwnd_limited(sk, in_flight))
+ return;
+
+ if (vegas->cwnd <= tp->snd_ssthresh)
+ inc = 1;
+ else if (tp->snd_cwnd_cnt < tp->snd_cwnd)
+ tp->snd_cwnd_cnt++;
+
+ if (tp->snd_cwnd_cnt >= tp->snd_cwnd) {
+ inc = 1;
+ tp->snd_cwnd_cnt = 0;
+ }
+
+ if (inc && tp->snd_cwnd < tp->snd_cwnd_clamp)
+ vegas->cwnd++;
+
+ /* 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.
+ */
+ if (!tp->mss_cache)
+ return;
+
+ 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;
+
+ /* 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) {
+ u32 rtt, target_cwnd, diff;
+ u32 brtt, dwnd;
+
+ /* 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.
+ */
+ if (!rtt)
+ return;
+
+ brtt = vegas->baseRTT;
+ target_cwnd = ((old_wnd * brtt)
+ << 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;
+
+ dwnd = vegas->dwnd;
+
+ if (diff < (TCP_COMPOUND_GAMMA << V_PARAM_SHIFT)) {
+ u64 v;
+ u32 x;
+
+ /*
+ * The TCP Compound paper describes the choice
+ * of "k" determines the agressiveness,
+ * ie. slope of the response function.
+ *
+ * For same value as HSTCP would be 0.8
+ * but for computaional reasons, both the
+ * original authors and this implementation
+ * use 0.75.
+ */
+ v = old_wnd;
+ x = qroot(v * v * v) >> TCP_COMPOUND_ALPHA;
+ if (x > 1)
+ dwnd = x - 1;
+ else
+ dwnd = 0;
+
+ dwnd += vegas->dwnd;
+
+ } else if ((dwnd << V_PARAM_SHIFT) <
+ (diff * TCP_COMPOUND_BETA))
+ dwnd = 0;
+ else
+ dwnd =
+ ((dwnd << V_PARAM_SHIFT) -
+ (diff *
+ TCP_COMPOUND_BETA)) >> V_PARAM_SHIFT;
+
+ vegas->dwnd = dwnd;
+
+ }
+
+ /* Wipe the slate clean for the next RTT. */
+ vegas->cntRTT = 0;
+ vegas->minRTT = 0x7fffffff;
+ }
+
+ tp->snd_cwnd = vegas->cwnd + vegas->dwnd;
+}
+
+/* Extract info for Tcp socket info provided via netlink. */
+static void tcp_compound_get_info(struct sock *sk, u32 ext, struct sk_buff *skb)
+{
+ const struct compound *ca = inet_csk_ca(sk);
+ if (ext & (1 << (INET_DIAG_VEGASINFO - 1))) {
+ struct tcpvegas_info *info;
+
+ info = RTA_DATA(__RTA_PUT(skb, INET_DIAG_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_compound = {
+ .init = tcp_compound_init,
+ .ssthresh = tcp_reno_ssthresh,
+ .cong_avoid = tcp_compound_cong_avoid,
+ .rtt_sample = tcp_compound_rtt_calc,
+ .set_state = tcp_compound_state,
+ .cwnd_event = tcp_compound_cwnd_event,
+ .get_info = tcp_compound_get_info,
+
+ .owner = THIS_MODULE,
+ .name = "compound",
+};
+
+static int __init tcp_compound_register(void)
+{
+ BUG_ON(sizeof(struct compound) > ICSK_CA_PRIV_SIZE);
+ tcp_register_congestion_control(&tcp_compound);
+ return 0;
+}
+
+static void __exit tcp_compound_unregister(void)
+{
+ tcp_unregister_congestion_control(&tcp_compound);
+}
+
+module_init(tcp_compound_register);
+module_exit(tcp_compound_unregister);
+
+MODULE_AUTHOR("Angelo P. Castellani, Stephen Hemminger");
+MODULE_LICENSE("GPL");
+MODULE_DESCRIPTION("TCP Compound");
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