dccp ccid-3: Preventing Oscillations

[linux-beck.git] / net / dccp / ccids / ccid3.c

/*
 *  net/dccp/ccids/ccid3.c
 *
 *  Copyright (c) 2007   The University of Aberdeen, Scotland, UK
 *  Copyright (c) 2005-7 The University of Waikato, Hamilton, New Zealand.
 *  Copyright (c) 2005-7 Ian McDonald <ian.mcdonald@jandi.co.nz>
 *
 *  An implementation of the DCCP protocol
 *
 *  This code has been developed by the University of Waikato WAND
 *  research group. For further information please see http://www.wand.net.nz/
 *
 *  This code also uses code from Lulea University, rereleased as GPL by its
 *  authors:
 *  Copyright (c) 2003 Nils-Erik Mattsson, Joacim Haggmark, Magnus Erixzon
 *
 *  Changes to meet Linux coding standards, to make it meet latest ccid3 draft
 *  and to make it work as a loadable module in the DCCP stack written by
 *  Arnaldo Carvalho de Melo <acme@conectiva.com.br>.
 *
 *  Copyright (c) 2005 Arnaldo Carvalho de Melo <acme@conectiva.com.br>
 *
 *  This program is free software; you can redistribute it and/or modify
 *  it under the terms of the GNU General Public License as published by
 *  the Free Software Foundation; either version 2 of the License, or
 *  (at your option) any later version.
 *
 *  This program is distributed in the hope that it will be useful,
 *  but WITHOUT ANY WARRANTY; without even the implied warranty of
 *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 *  GNU General Public License for more details.
 *
 *  You should have received a copy of the GNU General Public License
 *  along with this program; if not, write to the Free Software
 *  Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
 */
#include "../dccp.h"
#include "ccid3.h"

#include <asm/unaligned.h>

#ifdef CONFIG_IP_DCCP_CCID3_DEBUG
static int ccid3_debug;
#define ccid3_pr_debug(format, a...)    DCCP_PR_DEBUG(ccid3_debug, format, ##a)
#else
#define ccid3_pr_debug(format, a...)
#endif

/*
 *      Transmitter Half-Connection Routines
 */
/* Oscillation Prevention/Reduction: recommended by rfc3448bis, on by default */
static int do_osc_prev = true;

/*
 * Compute the initial sending rate X_init in the manner of RFC 3390:
 *
 *      X_init  =  min(4 * MPS, max(2 * MPS, 4380 bytes)) / RTT
 *
 * For consistency with other parts of the code, X_init is scaled by 2^6.
 */
static inline u64 rfc3390_initial_rate(struct sock *sk)
{
        const u32 mps = dccp_sk(sk)->dccps_mss_cache,
               w_init = clamp(4380U, 2 * mps, 4 * mps);

        return scaled_div(w_init << 6, ccid3_hc_tx_sk(sk)->rtt);
}

/**
 * ccid3_update_send_interval  -  Calculate new t_ipi = s / X
 * This respects the granularity of X (64 * bytes/second) and enforces the
 * scaled minimum of s * 64 / t_mbi = `s' bytes/second as per RFC 3448/4342.
 */
static void ccid3_update_send_interval(struct ccid3_hc_tx_sock *hctx)
{
        if (unlikely(hctx->x <= hctx->s))
                hctx->x = hctx->s;
        hctx->t_ipi = scaled_div32(((u64)hctx->s) << 6, hctx->x);
}

static u32 ccid3_hc_tx_idle_rtt(struct ccid3_hc_tx_sock *hctx, ktime_t now)
{
        u32 delta = ktime_us_delta(now, hctx->t_last_win_count);

        return delta / hctx->rtt;
}

/**
 * ccid3_hc_tx_update_x  -  Update allowed sending rate X
 * @stamp: most recent time if available - can be left NULL.
 * This function tracks draft rfc3448bis, check there for latest details.
 *
 * Note: X and X_recv are both stored in units of 64 * bytes/second, to support
 *       fine-grained resolution of sending rates. This requires scaling by 2^6
 *       throughout the code. Only X_calc is unscaled (in bytes/second).
 *
 */
static void ccid3_hc_tx_update_x(struct sock *sk, ktime_t *stamp)
{
        struct ccid3_hc_tx_sock *hctx = ccid3_hc_tx_sk(sk);
        u64 min_rate = 2 * hctx->x_recv;
        const u64 old_x = hctx->x;
        ktime_t now = stamp ? *stamp : ktime_get_real();

        /*
         * Handle IDLE periods: do not reduce below RFC3390 initial sending rate
         * when idling [RFC 4342, 5.1]. Definition of idling is from rfc3448bis:
         * a sender is idle if it has not sent anything over a 2-RTT-period.
         * For consistency with X and X_recv, min_rate is also scaled by 2^6.
         */
        if (ccid3_hc_tx_idle_rtt(hctx, now) >= 2) {
                min_rate = rfc3390_initial_rate(sk);
                min_rate = max(min_rate, 2 * hctx->x_recv);
        }

        if (hctx->p > 0) {

                hctx->x = min(((u64)hctx->x_calc) << 6, min_rate);

        } else if (ktime_us_delta(now, hctx->t_ld) - (s64)hctx->rtt >= 0) {

                hctx->x = min(2 * hctx->x, min_rate);
                hctx->x = max(hctx->x,
                              scaled_div(((u64)hctx->s) << 6, hctx->rtt));
                hctx->t_ld = now;
        }

        if (hctx->x != old_x) {
                ccid3_pr_debug("X_prev=%u, X_now=%u, X_calc=%u, "
                               "X_recv=%u\n", (unsigned)(old_x >> 6),
                               (unsigned)(hctx->x >> 6), hctx->x_calc,
                               (unsigned)(hctx->x_recv >> 6));

                ccid3_update_send_interval(hctx);
        }
}

/*
 * ccid3_hc_tx_measure_packet_size  -  Measuring the packet size `s' (sec 4.1)
 * @new_len: DCCP payload size in bytes (not used by all methods)
 */
static u32 ccid3_hc_tx_measure_packet_size(struct sock *sk, const u16 new_len)
{
#if   defined(CONFIG_IP_DCCP_CCID3_MEASURE_S_AS_AVG)
        return tfrc_ewma(ccid3_hc_tx_sk(sk)->s, new_len, 9);
#elif defined(CONFIG_IP_DCCP_CCID3_MEASURE_S_AS_MAX)
        return max(ccid3_hc_tx_sk(sk)->s, new_len);
#else /* CONFIG_IP_DCCP_CCID3_MEASURE_S_AS_MPS  */
        return dccp_sk(sk)->dccps_mss_cache;
#endif
}

/*
 *      Update Window Counter using the algorithm from [RFC 4342, 8.1].
 *      As elsewhere, RTT > 0 is assumed by using dccp_sample_rtt().
 */
static inline void ccid3_hc_tx_update_win_count(struct ccid3_hc_tx_sock *hctx,
                                                ktime_t now)
{
        u32 delta = ktime_us_delta(now, hctx->t_last_win_count),
            quarter_rtts = (4 * delta) / hctx->rtt;

        if (quarter_rtts > 0) {
                hctx->t_last_win_count = now;
                hctx->last_win_count  += min(quarter_rtts, 5U);
                hctx->last_win_count  &= 0xF;           /* mod 16 */
        }
}

static void ccid3_hc_tx_no_feedback_timer(unsigned long data)
{
        struct sock *sk = (struct sock *)data;
        struct ccid3_hc_tx_sock *hctx = ccid3_hc_tx_sk(sk);
        unsigned long t_nfb = USEC_PER_SEC / 5;

        bh_lock_sock(sk);
        if (sock_owned_by_user(sk)) {
                /* Try again later. */
                /* XXX: set some sensible MIB */
                goto restart_timer;
        }

        ccid3_pr_debug("%s(%p) entry with%s feedback\n", dccp_role(sk), sk,
                       hctx->feedback ? "" : "out");

        /* Ignore and do not restart after leaving the established state */
        if ((1 << sk->sk_state) & ~(DCCPF_OPEN | DCCPF_PARTOPEN))
                goto out;

        /* Reset feedback state to "no feedback received" */
        hctx->feedback = false;

        /*
         * Determine new allowed sending rate X as per draft rfc3448bis-00, 4.4
         * RTO is 0 if and only if no feedback has been received yet.
         */
        if (hctx->t_rto == 0 || hctx->p == 0) {

                /* halve send rate directly */
                hctx->x /= 2;
                ccid3_update_send_interval(hctx);

        } else {
                /*
                 *  Modify the cached value of X_recv
                 *
                 *  If (X_calc > 2 * X_recv)
                 *    X_recv = max(X_recv / 2, s / (2 * t_mbi));
                 *  Else
                 *    X_recv = X_calc / 4;
                 *
                 *  Note that X_recv is scaled by 2^6 while X_calc is not
                 */
                BUG_ON(hctx->p && !hctx->x_calc);

                if (hctx->x_calc > (hctx->x_recv >> 5))
                        hctx->x_recv /= 2;
                else {
                        hctx->x_recv = hctx->x_calc;
                        hctx->x_recv <<= 4;
                }
                ccid3_hc_tx_update_x(sk, NULL);
        }
        ccid3_pr_debug("Reduced X to %llu/64 bytes/sec\n",
                        (unsigned long long)hctx->x);

        /*
         * Set new timeout for the nofeedback timer.
         * See comments in packet_recv() regarding the value of t_RTO.
         */
        if (unlikely(hctx->t_rto == 0))         /* no feedback received yet */
                t_nfb = TFRC_INITIAL_TIMEOUT;
        else
                t_nfb = max(hctx->t_rto, 2 * hctx->t_ipi);

restart_timer:
        sk_reset_timer(sk, &hctx->no_feedback_timer,
                           jiffies + usecs_to_jiffies(t_nfb));
out:
        bh_unlock_sock(sk);
        sock_put(sk);
}

/**
 * ccid3_hc_tx_send_packet  -  Delay-based dequeueing of TX packets
 * @skb: next packet candidate to send on @sk
 * This function uses the convention of ccid_packet_dequeue_eval() and
 * returns a millisecond-delay value between 0 and t_mbi = 64000 msec.
 */
static int ccid3_hc_tx_send_packet(struct sock *sk, struct sk_buff *skb)
{
        struct dccp_sock *dp = dccp_sk(sk);
        struct ccid3_hc_tx_sock *hctx = ccid3_hc_tx_sk(sk);
        ktime_t now = ktime_get_real();
        s64 delay;

        /*
         * This function is called only for Data and DataAck packets. Sending
         * zero-sized Data(Ack)s is theoretically possible, but for congestion
         * control this case is pathological - ignore it.
         */
        if (unlikely(skb->len == 0))
                return -EBADMSG;

        if (hctx->s == 0) {
                sk_reset_timer(sk, &hctx->no_feedback_timer, (jiffies +
                                usecs_to_jiffies(TFRC_INITIAL_TIMEOUT)));
                hctx->last_win_count   = 0;
                hctx->t_last_win_count = now;

                /* Set t_0 for initial packet */
                hctx->t_nom = now;

                /*
                 * Use initial RTT sample when available: recommended by erratum
                 * to RFC 4342. This implements the initialisation procedure of
                 * draft rfc3448bis, section 4.2. Remember, X is scaled by 2^6.
                 */
                if (dp->dccps_syn_rtt) {
                        ccid3_pr_debug("SYN RTT = %uus\n", dp->dccps_syn_rtt);
                        hctx->rtt  = dp->dccps_syn_rtt;
                        hctx->x    = rfc3390_initial_rate(sk);
                        hctx->t_ld = now;
                } else {
                        /*
                         * Sender does not have RTT sample:
                         * - set fallback RTT (RFC 4340, 3.4) since a RTT value
                         *   is needed in several parts (e.g.  window counter);
                         * - set sending rate X_pps = 1pps as per RFC 3448, 4.2.
                         */
                        hctx->rtt = DCCP_FALLBACK_RTT;
                        hctx->x   = dp->dccps_mss_cache;
                        hctx->x <<= 6;
                }

                /* Compute t_ipi = s / X */
                hctx->s = ccid3_hc_tx_measure_packet_size(sk, skb->len);
                ccid3_update_send_interval(hctx);

                /* Seed value for Oscillation Prevention (sec. 4.5) */
                hctx->r_sqmean = tfrc_scaled_sqrt(hctx->rtt);

        } else {
                delay = ktime_us_delta(hctx->t_nom, now);
                ccid3_pr_debug("delay=%ld\n", (long)delay);
                /*
                 *      Scheduling of packet transmissions [RFC 3448, 4.6]
                 *
                 * if (t_now > t_nom - delta)
                 *       // send the packet now
                 * else
                 *       // send the packet in (t_nom - t_now) milliseconds.
                 */
                if (delay >= TFRC_T_DELTA)
                        return (u32)delay / USEC_PER_MSEC;

                ccid3_hc_tx_update_win_count(hctx, now);
        }

        /* prepare to send now (add options etc.) */
        dp->dccps_hc_tx_insert_options = 1;
        DCCP_SKB_CB(skb)->dccpd_ccval  = hctx->last_win_count;

        /* set the nominal send time for the next following packet */
        hctx->t_nom = ktime_add_us(hctx->t_nom, hctx->t_ipi);
        return CCID_PACKET_SEND_AT_ONCE;
}

static void ccid3_hc_tx_packet_sent(struct sock *sk, unsigned int len)
{
        struct ccid3_hc_tx_sock *hctx = ccid3_hc_tx_sk(sk);

        /* Changes to s will become effective the next time X is computed */
        hctx->s = ccid3_hc_tx_measure_packet_size(sk, len);

        if (tfrc_tx_hist_add(&hctx->hist, dccp_sk(sk)->dccps_gss))
                DCCP_CRIT("packet history - out of memory!");
}

static void ccid3_hc_tx_packet_recv(struct sock *sk, struct sk_buff *skb)
{
        struct ccid3_hc_tx_sock *hctx = ccid3_hc_tx_sk(sk);
        struct tfrc_tx_hist_entry *acked;
        ktime_t now;
        unsigned long t_nfb;
        u32 r_sample;

        /* we are only interested in ACKs */
        if (!(DCCP_SKB_CB(skb)->dccpd_type == DCCP_PKT_ACK ||
              DCCP_SKB_CB(skb)->dccpd_type == DCCP_PKT_DATAACK))
                return;
        /*
         * Locate the acknowledged packet in the TX history.
         *
         * Returning "entry not found" here can for instance happen when
         *  - the host has not sent out anything (e.g. a passive server),
         *  - the Ack is outdated (packet with higher Ack number was received),
         *  - it is a bogus Ack (for a packet not sent on this connection).
         */
        acked = tfrc_tx_hist_find_entry(hctx->hist, dccp_hdr_ack_seq(skb));
        if (acked == NULL)
                return;
        /* For the sake of RTT sampling, ignore/remove all older entries */
        tfrc_tx_hist_purge(&acked->next);

        /* Update the moving average for the RTT estimate (RFC 3448, 4.3) */
        now       = ktime_get_real();
        r_sample  = dccp_sample_rtt(sk, ktime_us_delta(now, acked->stamp));
        hctx->rtt = tfrc_ewma(hctx->rtt, r_sample, 9);

        /*
         * Update allowed sending rate X as per draft rfc3448bis-00, 4.2/3
         */
        if (!hctx->feedback) {
                hctx->feedback = true;

                if (hctx->t_rto == 0) {
                        /*
                         * Initial feedback packet: Larger Initial Windows (4.2)
                         */
                        hctx->x    = rfc3390_initial_rate(sk);
                        hctx->t_ld = now;

                        ccid3_update_send_interval(hctx);

                        goto done_computing_x;
                } else if (hctx->p == 0) {
                        /*
                         * First feedback after nofeedback timer expiry (4.3)
                         */
                        goto done_computing_x;
                }
        }

        /* Update sending rate (step 4 of [RFC 3448, 4.3]) */
        if (hctx->p > 0)
                hctx->x_calc = tfrc_calc_x(hctx->s, hctx->rtt, hctx->p);
        ccid3_hc_tx_update_x(sk, &now);

done_computing_x:
        ccid3_pr_debug("%s(%p), RTT=%uus (sample=%uus), s=%u, "
                               "p=%u, X_calc=%u, X_recv=%u, X=%u\n",
                               dccp_role(sk), sk, hctx->rtt, r_sample,
                               hctx->s, hctx->p, hctx->x_calc,
                               (unsigned)(hctx->x_recv >> 6),
                               (unsigned)(hctx->x >> 6));
        /*
         * Oscillation Reduction (RFC 3448, 4.5) - modifying t_ipi according to
         * RTT changes, multiplying by X/X_inst = sqrt(R_sample)/R_sqmean. This
         * can be useful if few connections share a link, avoiding that buffer
         * fill levels (RTT) oscillate as a result of frequent adjustments to X.
         * A useful presentation with background information is in
         *    Joerg Widmer, "Equation-Based Congestion Control",
         *    MSc Thesis, University of Mannheim, Germany, 2000
         * (sec. 3.6.4), who calls this ISM ("Inter-packet Space Modulation").
         */
        if (do_osc_prev) {
                r_sample = tfrc_scaled_sqrt(r_sample);
                /*
                 * The modulation can work in both ways: increase/decrease t_ipi
                 * according to long-term increases/decreases of the RTT. The
                 * former is a useful measure, since it works against queue
                 * build-up. The latter temporarily increases the sending rate,
                 * so that buffers fill up more quickly. This in turn causes
                 * the RTT to increase, so that either later reduction becomes
                 * necessary or the RTT stays at a very high level. Decreasing
                 * t_ipi is therefore not supported.
                 * Furthermore, during the initial slow-start phase the RTT
                 * naturally increases, where using the algorithm would cause
                 * delays. Hence it is disabled during the initial slow-start.
                 */
                if (r_sample > hctx->r_sqmean && hctx->p > 0)
                        hctx->t_ipi = div_u64((u64)hctx->t_ipi * (u64)r_sample,
                                              hctx->r_sqmean);
                hctx->t_ipi = min_t(u32, hctx->t_ipi, TFRC_T_MBI);
                /* update R_sqmean _after_ computing the modulation factor */
                hctx->r_sqmean = tfrc_ewma(hctx->r_sqmean, r_sample, 9);
        }

        /* unschedule no feedback timer */
        sk_stop_timer(sk, &hctx->no_feedback_timer);

        /*
         * As we have calculated new ipi, delta, t_nom it is possible
         * that we now can send a packet, so wake up dccp_wait_for_ccid
         */
        sk->sk_write_space(sk);

        /*
         * Update timeout interval for the nofeedback timer.
         * We use a configuration option to increase the lower bound.
         * This can help avoid triggering the nofeedback timer too
         * often ('spinning') on LANs with small RTTs.
         */
        hctx->t_rto = max_t(u32, 4 * hctx->rtt, (CONFIG_IP_DCCP_CCID3_RTO *
                                                 (USEC_PER_SEC / 1000)));
        /*
         * Schedule no feedback timer to expire in
         * max(t_RTO, 2 * s/X)  =  max(t_RTO, 2 * t_ipi)
         */
        t_nfb = max(hctx->t_rto, 2 * hctx->t_ipi);

        ccid3_pr_debug("%s(%p), Scheduled no feedback timer to "
                       "expire in %lu jiffies (%luus)\n",
                       dccp_role(sk), sk, usecs_to_jiffies(t_nfb), t_nfb);

        sk_reset_timer(sk, &hctx->no_feedback_timer,
                           jiffies + usecs_to_jiffies(t_nfb));
}

static int ccid3_hc_tx_parse_options(struct sock *sk, u8 packet_type,
                                     u8 option, u8 *optval, u8 optlen)
{
        struct ccid3_hc_tx_sock *hctx = ccid3_hc_tx_sk(sk);
        __be32 opt_val;

        switch (option) {
        case TFRC_OPT_RECEIVE_RATE:
        case TFRC_OPT_LOSS_EVENT_RATE:
                /* Must be ignored on Data packets, cf. RFC 4342 8.3 and 8.5 */
                if (packet_type == DCCP_PKT_DATA)
                        break;
                if (unlikely(optlen != 4)) {
                        DCCP_WARN("%s(%p), invalid len %d for %u\n",
                                  dccp_role(sk), sk, optlen, option);
                        return -EINVAL;
                }
                opt_val = ntohl(get_unaligned((__be32 *)optval));

                if (option == TFRC_OPT_RECEIVE_RATE) {
                        /* Receive Rate is kept in units of 64 bytes/second */
                        hctx->x_recv = opt_val;
                        hctx->x_recv <<= 6;

                        ccid3_pr_debug("%s(%p), RECEIVE_RATE=%u\n",
                                       dccp_role(sk), sk, opt_val);
                } else {
                        /* Update the fixpoint Loss Event Rate fraction */
                        hctx->p = tfrc_invert_loss_event_rate(opt_val);

                        ccid3_pr_debug("%s(%p), LOSS_EVENT_RATE=%u\n",
                                       dccp_role(sk), sk, opt_val);
                }
        }
        return 0;
}

static int ccid3_hc_tx_init(struct ccid *ccid, struct sock *sk)
{
        struct ccid3_hc_tx_sock *hctx = ccid_priv(ccid);

        hctx->hist  = NULL;
        setup_timer(&hctx->no_feedback_timer,
                    ccid3_hc_tx_no_feedback_timer, (unsigned long)sk);
        return 0;
}

static void ccid3_hc_tx_exit(struct sock *sk)
{
        struct ccid3_hc_tx_sock *hctx = ccid3_hc_tx_sk(sk);

        sk_stop_timer(sk, &hctx->no_feedback_timer);
        tfrc_tx_hist_purge(&hctx->hist);
}

static void ccid3_hc_tx_get_info(struct sock *sk, struct tcp_info *info)
{
        info->tcpi_rto = ccid3_hc_tx_sk(sk)->t_rto;
        info->tcpi_rtt = ccid3_hc_tx_sk(sk)->rtt;
}

static int ccid3_hc_tx_getsockopt(struct sock *sk, const int optname, int len,
                                  u32 __user *optval, int __user *optlen)
{
        const struct ccid3_hc_tx_sock *hctx = ccid3_hc_tx_sk(sk);
        struct tfrc_tx_info tfrc;
        const void *val;

        switch (optname) {
        case DCCP_SOCKOPT_CCID_TX_INFO:
                if (len < sizeof(tfrc))
                        return -EINVAL;
                tfrc.tfrctx_x      = hctx->x;
                tfrc.tfrctx_x_recv = hctx->x_recv;
                tfrc.tfrctx_x_calc = hctx->x_calc;
                tfrc.tfrctx_rtt    = hctx->rtt;
                tfrc.tfrctx_p      = hctx->p;
                tfrc.tfrctx_rto    = hctx->t_rto;
                tfrc.tfrctx_ipi    = hctx->t_ipi;
                len = sizeof(tfrc);
                val = &tfrc;
                break;
        default:
                return -ENOPROTOOPT;
        }

        if (put_user(len, optlen) || copy_to_user(optval, val, len))
                return -EFAULT;

        return 0;
}

/*
 *      Receiver Half-Connection Routines
 */
static void ccid3_hc_rx_send_feedback(struct sock *sk,
                                      const struct sk_buff *skb,
                                      enum ccid3_fback_type fbtype)
{
        struct ccid3_hc_rx_sock *hcrx = ccid3_hc_rx_sk(sk);

        switch (fbtype) {
        case CCID3_FBACK_INITIAL:
                hcrx->x_recv = 0;
                hcrx->p_inverse = ~0U;   /* see RFC 4342, 8.5 */
                break;
        case CCID3_FBACK_PARAM_CHANGE:
                if (unlikely(hcrx->feedback == CCID3_FBACK_NONE)) {
                        /*
                         * rfc3448bis-06, 6.3.1: First packet(s) lost or marked
                         * FIXME: in rfc3448bis the receiver returns X_recv=0
                         * here as it normally would in the first feedback packet.
                         * However this is not possible yet, since the code still
                         * uses RFC 3448, i.e.
                         *    If (p > 0)
                         *      Calculate X_calc using the TCP throughput equation.
                         *      X = max(min(X_calc, 2*X_recv), s/t_mbi);
                         * would bring X down to s/t_mbi. That is why we return
                         * X_recv according to rfc3448bis-06 for the moment.
                         */
                        u32 s = tfrc_rx_hist_packet_size(&hcrx->hist),
                            rtt = tfrc_rx_hist_rtt(&hcrx->hist);

                        hcrx->x_recv = scaled_div32(s, 2 * rtt);
                        break;
                }
                /*
                 * When parameters change (new loss or p > p_prev), we do not
                 * have a reliable estimate for R_m of [RFC 3448, 6.2] and so
                 * always check whether at least RTT time units were covered.
                 */
                hcrx->x_recv = tfrc_rx_hist_x_recv(&hcrx->hist, hcrx->x_recv);
                break;
        case CCID3_FBACK_PERIODIC:
                /*
                 * Step (2) of rfc3448bis-06, 6.2:
                 * - if no data packets have been received, just restart timer
                 * - if data packets have been received, re-compute X_recv
                 */
                if (hcrx->hist.bytes_recvd == 0)
                        goto prepare_for_next_time;
                hcrx->x_recv = tfrc_rx_hist_x_recv(&hcrx->hist, hcrx->x_recv);
                break;
        default:
                return;
        }

        ccid3_pr_debug("X_recv=%u, 1/p=%u\n", hcrx->x_recv, hcrx->p_inverse);

        dccp_sk(sk)->dccps_hc_rx_insert_options = 1;
        dccp_send_ack(sk);

prepare_for_next_time:
        tfrc_rx_hist_restart_byte_counter(&hcrx->hist);
        hcrx->last_counter = dccp_hdr(skb)->dccph_ccval;
        hcrx->feedback     = fbtype;
}

static int ccid3_hc_rx_insert_options(struct sock *sk, struct sk_buff *skb)
{
        const struct ccid3_hc_rx_sock *hcrx = ccid3_hc_rx_sk(sk);
        __be32 x_recv, pinv;

        if (!(sk->sk_state == DCCP_OPEN || sk->sk_state == DCCP_PARTOPEN))
                return 0;

        if (dccp_packet_without_ack(skb))
                return 0;

        x_recv = htonl(hcrx->x_recv);
        pinv   = htonl(hcrx->p_inverse);

        if (dccp_insert_option(sk, skb, TFRC_OPT_LOSS_EVENT_RATE,
                               &pinv, sizeof(pinv)) ||
            dccp_insert_option(sk, skb, TFRC_OPT_RECEIVE_RATE,
                               &x_recv, sizeof(x_recv)))
                return -1;

        return 0;
}

/** ccid3_first_li  -  Implements [RFC 3448, 6.3.1]
 *
 * Determine the length of the first loss interval via inverse lookup.
 * Assume that X_recv can be computed by the throughput equation
 *                  s
 *      X_recv = --------
 *               R * fval
 * Find some p such that f(p) = fval; return 1/p (scaled).
 */
static u32 ccid3_first_li(struct sock *sk)
{
        struct ccid3_hc_rx_sock *hcrx = ccid3_hc_rx_sk(sk);
        u32 s = tfrc_rx_hist_packet_size(&hcrx->hist),
            rtt = tfrc_rx_hist_rtt(&hcrx->hist), x_recv, p;
        u64 fval;

        /*
         * rfc3448bis-06, 6.3.1: First data packet(s) are marked or lost. Set p
         * to give the equivalent of X_target = s/(2*R). Thus fval = 2 and so p
         * is about 20.64%. This yields an interval length of 4.84 (rounded up).
         */
        if (unlikely(hcrx->feedback == CCID3_FBACK_NONE))
                return 5;

        x_recv = tfrc_rx_hist_x_recv(&hcrx->hist, hcrx->x_recv);
        if (x_recv == 0)
                goto failed;

        fval = scaled_div32(scaled_div(s, rtt), x_recv);
        p = tfrc_calc_x_reverse_lookup(fval);

        ccid3_pr_debug("%s(%p), receive rate=%u bytes/s, implied "
                       "loss rate=%u\n", dccp_role(sk), sk, x_recv, p);

        if (p > 0)
                return scaled_div(1, p);
failed:
        return UINT_MAX;
}

static void ccid3_hc_rx_packet_recv(struct sock *sk, struct sk_buff *skb)
{
        struct ccid3_hc_rx_sock *hcrx = ccid3_hc_rx_sk(sk);
        const u64 ndp = dccp_sk(sk)->dccps_options_received.dccpor_ndp;
        const bool is_data_packet = dccp_data_packet(skb);

        /*
         * Perform loss detection and handle pending losses
         */
        if (tfrc_rx_congestion_event(&hcrx->hist, &hcrx->li_hist,
                                     skb, ndp, ccid3_first_li, sk))
                ccid3_hc_rx_send_feedback(sk, skb, CCID3_FBACK_PARAM_CHANGE);
        /*
         * Feedback for first non-empty data packet (RFC 3448, 6.3)
         */
        else if (unlikely(hcrx->feedback == CCID3_FBACK_NONE && is_data_packet))
                ccid3_hc_rx_send_feedback(sk, skb, CCID3_FBACK_INITIAL);
        /*
         * Check if the periodic once-per-RTT feedback is due; RFC 4342, 10.3
         */
        else if (!tfrc_rx_hist_loss_pending(&hcrx->hist) && is_data_packet &&
                 SUB16(dccp_hdr(skb)->dccph_ccval, hcrx->last_counter) > 3)
                ccid3_hc_rx_send_feedback(sk, skb, CCID3_FBACK_PERIODIC);
}

static int ccid3_hc_rx_init(struct ccid *ccid, struct sock *sk)
{
        struct ccid3_hc_rx_sock *hcrx = ccid_priv(ccid);

        tfrc_lh_init(&hcrx->li_hist);
        return tfrc_rx_hist_init(&hcrx->hist, sk);
}

static void ccid3_hc_rx_exit(struct sock *sk)
{
        struct ccid3_hc_rx_sock *hcrx = ccid3_hc_rx_sk(sk);

        tfrc_rx_hist_purge(&hcrx->hist);
        tfrc_lh_cleanup(&hcrx->li_hist);
}

static void ccid3_hc_rx_get_info(struct sock *sk, struct tcp_info *info)
{
        info->tcpi_options  |= TCPI_OPT_TIMESTAMPS;
        info->tcpi_rcv_rtt  = tfrc_rx_hist_rtt(&ccid3_hc_rx_sk(sk)->hist);
}

static int ccid3_hc_rx_getsockopt(struct sock *sk, const int optname, int len,
                                  u32 __user *optval, int __user *optlen)
{
        const struct ccid3_hc_rx_sock *hcrx = ccid3_hc_rx_sk(sk);
        struct tfrc_rx_info rx_info;
        const void *val;

        switch (optname) {
        case DCCP_SOCKOPT_CCID_RX_INFO:
                if (len < sizeof(rx_info))
                        return -EINVAL;
                rx_info.tfrcrx_x_recv = hcrx->x_recv;
                rx_info.tfrcrx_rtt    = tfrc_rx_hist_rtt(&hcrx->hist);
                rx_info.tfrcrx_p      = tfrc_invert_loss_event_rate(hcrx->p_inverse);
                len = sizeof(rx_info);
                val = &rx_info;
                break;
        default:
                return -ENOPROTOOPT;
        }

        if (put_user(len, optlen) || copy_to_user(optval, val, len))
                return -EFAULT;

        return 0;
}

static struct ccid_operations ccid3 = {
        .ccid_id                   = DCCPC_CCID3,
        .ccid_name                 = "TCP-Friendly Rate Control",
        .ccid_owner                = THIS_MODULE,
        .ccid_hc_tx_obj_size       = sizeof(struct ccid3_hc_tx_sock),
        .ccid_hc_tx_init           = ccid3_hc_tx_init,
        .ccid_hc_tx_exit           = ccid3_hc_tx_exit,
        .ccid_hc_tx_send_packet    = ccid3_hc_tx_send_packet,
        .ccid_hc_tx_packet_sent    = ccid3_hc_tx_packet_sent,
        .ccid_hc_tx_packet_recv    = ccid3_hc_tx_packet_recv,
        .ccid_hc_tx_parse_options  = ccid3_hc_tx_parse_options,
        .ccid_hc_rx_obj_size       = sizeof(struct ccid3_hc_rx_sock),
        .ccid_hc_rx_init           = ccid3_hc_rx_init,
        .ccid_hc_rx_exit           = ccid3_hc_rx_exit,
        .ccid_hc_rx_insert_options = ccid3_hc_rx_insert_options,
        .ccid_hc_rx_packet_recv    = ccid3_hc_rx_packet_recv,
        .ccid_hc_rx_get_info       = ccid3_hc_rx_get_info,
        .ccid_hc_tx_get_info       = ccid3_hc_tx_get_info,
        .ccid_hc_rx_getsockopt     = ccid3_hc_rx_getsockopt,
        .ccid_hc_tx_getsockopt     = ccid3_hc_tx_getsockopt,
};

module_param(do_osc_prev, bool, 0644);
MODULE_PARM_DESC(do_osc_prev, "Use Oscillation Prevention (RFC 3448, 4.5)");

#ifdef CONFIG_IP_DCCP_CCID3_DEBUG
module_param(ccid3_debug, bool, 0644);
MODULE_PARM_DESC(ccid3_debug, "Enable debug messages");
#endif

static __init int ccid3_module_init(void)
{
        struct timespec tp;

        /*
         * Without a fine-grained clock resolution, RTTs/X_recv are not sampled
         * correctly and feedback is sent either too early or too late.
         */
        hrtimer_get_res(CLOCK_MONOTONIC, &tp);
        if (tp.tv_sec || tp.tv_nsec > DCCP_TIME_RESOLUTION * NSEC_PER_USEC) {
                printk(KERN_ERR "%s: Timer too coarse (%ld usec), need %u-usec"
                       " resolution - check your clocksource.\n", __func__,
                       tp.tv_nsec/NSEC_PER_USEC, DCCP_TIME_RESOLUTION);
                return -ESOCKTNOSUPPORT;
        }
        return ccid_register(&ccid3);
}
module_init(ccid3_module_init);

static __exit void ccid3_module_exit(void)
{
        ccid_unregister(&ccid3);
}
module_exit(ccid3_module_exit);

MODULE_AUTHOR("Ian McDonald <ian.mcdonald@jandi.co.nz>, "
              "Arnaldo Carvalho de Melo <acme@ghostprotocols.net>");
MODULE_DESCRIPTION("DCCP TFRC CCID3 CCID");
MODULE_LICENSE("GPL");
MODULE_ALIAS("net-dccp-ccid-3");