tcp_vegas.c

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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/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, u32 cnt, s32 rtt_us)
{
    struct vegas *vegas = inet_csk_ca(sk);
    u32 vrtt;

    if (rtt_us < 0)
        return;

    /* Never allow zero rtt or baseRTT */
    vrtt = 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, tp->snd_cwnd-1);
}

static void tcp_vegas_cong_avoid(struct sock *sk, u32 ack, u32 in_flight)
{
    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, in_flight);
        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, in_flight);
        } 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 = tp->snd_cwnd * vegas->baseRTT / 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 = tp->snd_cwnd * (rtt-vegas->baseRTT) / vegas->baseRTT;

            if (diff > gamma && tp->snd_cwnd <= tp->snd_ssthresh) {
                /* 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.
                 */
                tp->snd_cwnd = min(tp->snd_cwnd, (u32)target_cwnd+1);
                tp->snd_ssthresh = tcp_vegas_ssthresh(tp);

            } else if (tp->snd_cwnd <= tp->snd_ssthresh) {
                /* Slow start.  */
                tcp_slow_start(tp);
            } 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.
                     */
                    tp->snd_cwnd--;
                    tp->snd_ssthresh
                        = tcp_vegas_ssthresh(tp);
                } else if (diff < alpha) {
                    /* We don't have enough extra packets
                     * in the network, so speed up.
                     */
                    tp->snd_cwnd++;
                } else {
                    /* Sending just as fast as we
                     * should be.
                     */
                }
            }

            if (tp->snd_cwnd < 2)
                tp->snd_cwnd = 2;
            else if (tp->snd_cwnd > tp->snd_cwnd_clamp)
                tp->snd_cwnd = 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 (tp->snd_cwnd <= tp->snd_ssthresh)
        tcp_slow_start(tp);

}

/* Extract info for Tcp socket info provided via netlink. */
void tcp_vegas_get_info(struct sock *sk, u32 ext, struct sk_buff *skb)
{
    const struct vegas *ca = inet_csk_ca(sk);
    if (ext & (1 << (INET_DIAG_VEGASINFO - 1))) {
        struct tcpvegas_info info = {
            .tcpv_enabled = ca->doing_vegas_now,
            .tcpv_rttcnt = ca->cntRTT,
            .tcpv_rtt = ca->baseRTT,
            .tcpv_minrtt = ca->minRTT,
        };

        nla_put(skb, INET_DIAG_VEGASINFO, sizeof(info), &info);
    }
}
EXPORT_SYMBOL_GPL(tcp_vegas_get_info);

static struct tcp_congestion_ops tcp_vegas __read_mostly = {
    .flags      = TCP_CONG_RTT_STAMP,
    .init       = tcp_vegas_init,
    .ssthresh   = tcp_reno_ssthresh,
    .cong_avoid = tcp_vegas_cong_avoid,
    .min_cwnd   = tcp_reno_min_cwnd,
    .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");