[TCP]: TCP Compound congestion control

TCP Compound is a sender-side only change to TCP that uses
a mixed Reno/Vegas approach to calculate the cwnd.

For further details look here:
  ftp://ftp.research.microsoft.com/pub/tr/TR-2005-86.pdf

Signed-off-by: Angelo P. Castellani <angelo.castellani@gmail.com>
Signed-off-by: Stephen Hemminger <shemminger@osdl.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 407 insertions, 0 deletions

diff --git a/net/ipv4/tcp_compound.c b/net/ipv4/tcp_compound.c
new file mode 100644
index 000000000000..8c1ebfb7659e
--- /dev/null
+++ b/net/ipv4/tcp_compound.c
@@ -0,0 +1,407 @@
+/*
+ * 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_KAPPA_POW      3
+#define TCP_COMPOUND_KAPPA_NSQRT    2
+#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);
+}
+
+/*
+ * 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)) {
+                                u32 i, j, x, x2;
+                                u64 v;
+
+                                v = 1;
+
+                                for (i = 0; i < TCP_COMPOUND_KAPPA_POW; i++)
+                                        v *= old_wnd;
+
+                                for (i = 0; i < TCP_COMPOUND_KAPPA_NSQRT; i++) {
+                                        x = 1;
+                                        for (j = 0; j < 200; j++) {
+                                                x2 = (x + v / x) / 2;
+
+                                                if (x2 == x || !x2)
+                                                        break;
+
+                                                x = x2;
+                                        }
+                                        v = x;
+                                }
+
+                                x = (u32) 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,
+        .min_cwnd       = tcp_reno_min_cwnd,
+        .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");