ccid3.c

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/*
 *  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 <[email protected]>
 *
 *  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 <[email protected]>.
 *
 *  Copyright (c) 2005 Arnaldo Carvalho de Melo <[email protected]>
 *
 *  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 bool 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
 */
#ifdef CONFIG_IP_DCCP_CCID3_DEBUG
static const char *ccid3_tx_state_name(enum ccid3_hc_tx_states state)
{
    static const char *const ccid3_state_names[] = {
    [TFRC_SSTATE_NO_SENT]  = "NO_SENT",
    [TFRC_SSTATE_NO_FBACK] = "NO_FBACK",
    [TFRC_SSTATE_FBACK]    = "FBACK",
    };

    return ccid3_state_names[state];
}
#endif

static void ccid3_hc_tx_set_state(struct sock *sk,
                  enum ccid3_hc_tx_states state)
{
    struct ccid3_hc_tx_sock *hc = ccid3_hc_tx_sk(sk);
    enum ccid3_hc_tx_states oldstate = hc->tx_state;

    ccid3_pr_debug("%s(%p) %-8.8s -> %s\n",
               dccp_role(sk), sk, ccid3_tx_state_name(oldstate),
               ccid3_tx_state_name(state));
    WARN_ON(state == oldstate);
    hc->tx_state = state;
}

/*
 * Compute the initial sending rate X_init in the manner of RFC 3390:
 *
 *  X_init  =  min(4 * s, max(2 * s, 4380 bytes)) / RTT
 *
 * Note that RFC 3390 uses MSS, RFC 4342 refers to RFC 3390, and rfc3448bis
 * (rev-02) clarifies the use of RFC 3390 with regard to the above formula.
 * 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 struct ccid3_hc_tx_sock *hc = ccid3_hc_tx_sk(sk);
    const __u32 w_init = clamp_t(__u32, 4380U, 2 * hc->tx_s, 4 * hc->tx_s);

    return scaled_div(w_init << 6, hc->tx_rtt);
}

static void ccid3_update_send_interval(struct ccid3_hc_tx_sock *hc)
{
    hc->tx_t_ipi = scaled_div32(((u64)hc->tx_s) << 6, hc->tx_x);

    DCCP_BUG_ON(hc->tx_t_ipi == 0);
    ccid3_pr_debug("t_ipi=%u, s=%u, X=%u\n", hc->tx_t_ipi,
               hc->tx_s, (unsigned int)(hc->tx_x >> 6));
}

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

    return delta / hc->tx_rtt;
}

static void ccid3_hc_tx_update_x(struct sock *sk, ktime_t *stamp)
{
    struct ccid3_hc_tx_sock *hc = ccid3_hc_tx_sk(sk);
    __u64 min_rate = 2 * hc->tx_x_recv;
    const __u64 old_x = hc->tx_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(hc, now) >= 2) {
        min_rate = rfc3390_initial_rate(sk);
        min_rate = max(min_rate, 2 * hc->tx_x_recv);
    }

    if (hc->tx_p > 0) {

        hc->tx_x = min(((__u64)hc->tx_x_calc) << 6, min_rate);
        hc->tx_x = max(hc->tx_x, (((__u64)hc->tx_s) << 6) / TFRC_T_MBI);

    } else if (ktime_us_delta(now, hc->tx_t_ld) - (s64)hc->tx_rtt >= 0) {

        hc->tx_x = min(2 * hc->tx_x, min_rate);
        hc->tx_x = max(hc->tx_x,
                   scaled_div(((__u64)hc->tx_s) << 6, hc->tx_rtt));
        hc->tx_t_ld = now;
    }

    if (hc->tx_x != old_x) {
        ccid3_pr_debug("X_prev=%u, X_now=%u, X_calc=%u, "
                   "X_recv=%u\n", (unsigned int)(old_x >> 6),
                   (unsigned int)(hc->tx_x >> 6), hc->tx_x_calc,
                   (unsigned int)(hc->tx_x_recv >> 6));

        ccid3_update_send_interval(hc);
    }
}

static inline void ccid3_hc_tx_update_s(struct ccid3_hc_tx_sock *hc, int len)
{
    const u16 old_s = hc->tx_s;

    hc->tx_s = tfrc_ewma(hc->tx_s, len, 9);

    if (hc->tx_s != old_s)
        ccid3_update_send_interval(hc);
}

/*
 *  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 *hc,
                        ktime_t now)
{
    u32 delta = ktime_us_delta(now, hc->tx_t_last_win_count),
        quarter_rtts = (4 * delta) / hc->tx_rtt;

    if (quarter_rtts > 0) {
        hc->tx_t_last_win_count = now;
        hc->tx_last_win_count  += min(quarter_rtts, 5U);
        hc->tx_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 *hc = 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, state=%s) - entry\n", dccp_role(sk), sk,
               ccid3_tx_state_name(hc->tx_state));

    /* 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" */
    if (hc->tx_state == TFRC_SSTATE_FBACK)
        ccid3_hc_tx_set_state(sk, TFRC_SSTATE_NO_FBACK);

    /*
     * 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 (hc->tx_t_rto == 0 || hc->tx_p == 0) {

        /* halve send rate directly */
        hc->tx_x = max(hc->tx_x / 2,
                   (((__u64)hc->tx_s) << 6) / TFRC_T_MBI);
        ccid3_update_send_interval(hc);
    } 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
         */
        if (hc->tx_x_calc > (hc->tx_x_recv >> 5))
            hc->tx_x_recv =
                max(hc->tx_x_recv / 2,
                    (((__u64)hc->tx_s) << 6) / (2*TFRC_T_MBI));
        else {
            hc->tx_x_recv = hc->tx_x_calc;
            hc->tx_x_recv <<= 4;
        }
        ccid3_hc_tx_update_x(sk, NULL);
    }
    ccid3_pr_debug("Reduced X to %llu/64 bytes/sec\n",
            (unsigned long long)hc->tx_x);

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

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

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 *hc = 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 (hc->tx_state == TFRC_SSTATE_NO_SENT) {
        sk_reset_timer(sk, &hc->tx_no_feedback_timer, (jiffies +
                   usecs_to_jiffies(TFRC_INITIAL_TIMEOUT)));
        hc->tx_last_win_count   = 0;
        hc->tx_t_last_win_count = now;

        /* Set t_0 for initial packet */
        hc->tx_t_nom = now;

        hc->tx_s = skb->len;

        /*
         * 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);
            hc->tx_rtt  = dp->dccps_syn_rtt;
            hc->tx_x    = rfc3390_initial_rate(sk);
            hc->tx_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.
             */
            hc->tx_rtt = DCCP_FALLBACK_RTT;
            hc->tx_x   = hc->tx_s;
            hc->tx_x <<= 6;
        }
        ccid3_update_send_interval(hc);

        ccid3_hc_tx_set_state(sk, TFRC_SSTATE_NO_FBACK);

    } else {
        delay = ktime_us_delta(hc->tx_t_nom, now);
        ccid3_pr_debug("delay=%ld\n", (long)delay);
        /*
         *  Scheduling of packet transmissions (RFC 5348, 8.3)
         *
         * 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(hc, now);
    }

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

    /* set the nominal send time for the next following packet */
    hc->tx_t_nom = ktime_add_us(hc->tx_t_nom, hc->tx_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 *hc = ccid3_hc_tx_sk(sk);

    ccid3_hc_tx_update_s(hc, len);

    if (tfrc_tx_hist_add(&hc->tx_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 *hc = 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(hc->tx_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));
    hc->tx_rtt = tfrc_ewma(hc->tx_rtt, r_sample, 9);

    /*
     * Update allowed sending rate X as per draft rfc3448bis-00, 4.2/3
     */
    if (hc->tx_state == TFRC_SSTATE_NO_FBACK) {
        ccid3_hc_tx_set_state(sk, TFRC_SSTATE_FBACK);

        if (hc->tx_t_rto == 0) {
            /*
             * Initial feedback packet: Larger Initial Windows (4.2)
             */
            hc->tx_x    = rfc3390_initial_rate(sk);
            hc->tx_t_ld = now;

            ccid3_update_send_interval(hc);

            goto done_computing_x;
        } else if (hc->tx_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 (hc->tx_p > 0)
        hc->tx_x_calc = tfrc_calc_x(hc->tx_s, hc->tx_rtt, hc->tx_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, hc->tx_rtt, r_sample,
                   hc->tx_s, hc->tx_p, hc->tx_x_calc,
                   (unsigned int)(hc->tx_x_recv >> 6),
                   (unsigned int)(hc->tx_x >> 6));

    /* unschedule no feedback timer */
    sk_stop_timer(sk, &hc->tx_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. In order to control
     * rate halving on networks with very low RTTs (<= 1 ms), use per-route
     * tunable RTAX_RTO_MIN value as the lower bound.
     */
    hc->tx_t_rto = max_t(u32, 4 * hc->tx_rtt,
                  USEC_PER_SEC/HZ * tcp_rto_min(sk));
    /*
     * Schedule no feedback timer to expire in
     * max(t_RTO, 2 * s/X)  =  max(t_RTO, 2 * t_ipi)
     */
    t_nfb = max(hc->tx_t_rto, 2 * hc->tx_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, &hc->tx_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 *hc = 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 */
            hc->tx_x_recv = opt_val;
            hc->tx_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 */
            hc->tx_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 *hc = ccid_priv(ccid);

    hc->tx_state = TFRC_SSTATE_NO_SENT;
    hc->tx_hist  = NULL;
    setup_timer(&hc->tx_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 *hc = ccid3_hc_tx_sk(sk);

    sk_stop_timer(sk, &hc->tx_no_feedback_timer);
    tfrc_tx_hist_purge(&hc->tx_hist);
}

static void ccid3_hc_tx_get_info(struct sock *sk, struct tcp_info *info)
{
    info->tcpi_rto = ccid3_hc_tx_sk(sk)->tx_t_rto;
    info->tcpi_rtt = ccid3_hc_tx_sk(sk)->tx_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 *hc = 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;
        memset(&tfrc, 0, sizeof(tfrc));
        tfrc.tfrctx_x      = hc->tx_x;
        tfrc.tfrctx_x_recv = hc->tx_x_recv;
        tfrc.tfrctx_x_calc = hc->tx_x_calc;
        tfrc.tfrctx_rtt    = hc->tx_rtt;
        tfrc.tfrctx_p      = hc->tx_p;
        tfrc.tfrctx_rto    = hc->tx_t_rto;
        tfrc.tfrctx_ipi    = hc->tx_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
 */

/* CCID3 feedback types */
enum ccid3_fback_type {
    CCID3_FBACK_NONE = 0,
    CCID3_FBACK_INITIAL,
    CCID3_FBACK_PERIODIC,
    CCID3_FBACK_PARAM_CHANGE
};

#ifdef CONFIG_IP_DCCP_CCID3_DEBUG
static const char *ccid3_rx_state_name(enum ccid3_hc_rx_states state)
{
    static const char *const ccid3_rx_state_names[] = {
    [TFRC_RSTATE_NO_DATA] = "NO_DATA",
    [TFRC_RSTATE_DATA]    = "DATA",
    };

    return ccid3_rx_state_names[state];
}
#endif

static void ccid3_hc_rx_set_state(struct sock *sk,
                  enum ccid3_hc_rx_states state)
{
    struct ccid3_hc_rx_sock *hc = ccid3_hc_rx_sk(sk);
    enum ccid3_hc_rx_states oldstate = hc->rx_state;

    ccid3_pr_debug("%s(%p) %-8.8s -> %s\n",
               dccp_role(sk), sk, ccid3_rx_state_name(oldstate),
               ccid3_rx_state_name(state));
    WARN_ON(state == oldstate);
    hc->rx_state = state;
}

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 *hc = ccid3_hc_rx_sk(sk);
    struct dccp_sock *dp = dccp_sk(sk);
    ktime_t now = ktime_get_real();
    s64 delta = 0;

    switch (fbtype) {
    case CCID3_FBACK_INITIAL:
        hc->rx_x_recv = 0;
        hc->rx_pinv   = ~0U;   /* see RFC 4342, 8.5 */
        break;
    case CCID3_FBACK_PARAM_CHANGE:
        /*
         * 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
         * need to  reuse the previous value of X_recv. However, when
         * X_recv was 0 (due to early loss), this would kill X down to
         * s/t_mbi (i.e. one packet in 64 seconds).
         * To avoid such drastic reduction, we approximate X_recv as
         * the number of bytes since last feedback.
         * This is a safe fallback, since X is bounded above by X_calc.
         */
        if (hc->rx_x_recv > 0)
            break;
        /* fall through */
    case CCID3_FBACK_PERIODIC:
        delta = ktime_us_delta(now, hc->rx_tstamp_last_feedback);
        if (delta <= 0)
            DCCP_BUG("delta (%ld) <= 0", (long)delta);
        else
            hc->rx_x_recv = scaled_div32(hc->rx_bytes_recv, delta);
        break;
    default:
        return;
    }

    ccid3_pr_debug("Interval %ldusec, X_recv=%u, 1/p=%u\n", (long)delta,
               hc->rx_x_recv, hc->rx_pinv);

    hc->rx_tstamp_last_feedback = now;
    hc->rx_last_counter     = dccp_hdr(skb)->dccph_ccval;
    hc->rx_bytes_recv       = 0;

    dp->dccps_hc_rx_insert_options = 1;
    dccp_send_ack(sk);
}

static int ccid3_hc_rx_insert_options(struct sock *sk, struct sk_buff *skb)
{
    const struct ccid3_hc_rx_sock *hc = 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(hc->rx_x_recv);
    pinv   = htonl(hc->rx_pinv);

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

    return 0;
}

static u32 ccid3_first_li(struct sock *sk)
{
    struct ccid3_hc_rx_sock *hc = ccid3_hc_rx_sk(sk);
    u32 x_recv, p, delta;
    u64 fval;

    if (hc->rx_rtt == 0) {
        DCCP_WARN("No RTT estimate available, using fallback RTT\n");
        hc->rx_rtt = DCCP_FALLBACK_RTT;
    }

    delta  = ktime_to_us(net_timedelta(hc->rx_tstamp_last_feedback));
    x_recv = scaled_div32(hc->rx_bytes_recv, delta);
    if (x_recv == 0) {      /* would also trigger divide-by-zero */
        DCCP_WARN("X_recv==0\n");
        if (hc->rx_x_recv == 0) {
            DCCP_BUG("stored value of X_recv is zero");
            return ~0U;
        }
        x_recv = hc->rx_x_recv;
    }

    fval = scaled_div(hc->rx_s, hc->rx_rtt);
    fval = scaled_div32(fval, 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);

    return p == 0 ? ~0U : scaled_div(1, p);
}

static void ccid3_hc_rx_packet_recv(struct sock *sk, struct sk_buff *skb)
{
    struct ccid3_hc_rx_sock *hc = ccid3_hc_rx_sk(sk);
    enum ccid3_fback_type do_feedback = CCID3_FBACK_NONE;
    const u64 ndp = dccp_sk(sk)->dccps_options_received.dccpor_ndp;
    const bool is_data_packet = dccp_data_packet(skb);

    if (unlikely(hc->rx_state == TFRC_RSTATE_NO_DATA)) {
        if (is_data_packet) {
            const u32 payload = skb->len - dccp_hdr(skb)->dccph_doff * 4;
            do_feedback = CCID3_FBACK_INITIAL;
            ccid3_hc_rx_set_state(sk, TFRC_RSTATE_DATA);
            hc->rx_s = payload;
            /*
             * Not necessary to update rx_bytes_recv here,
             * since X_recv = 0 for the first feedback packet (cf.
             * RFC 3448, 6.3) -- gerrit
             */
        }
        goto update_records;
    }

    if (tfrc_rx_hist_duplicate(&hc->rx_hist, skb))
        return; /* done receiving */

    if (is_data_packet) {
        const u32 payload = skb->len - dccp_hdr(skb)->dccph_doff * 4;
        /*
         * Update moving-average of s and the sum of received payload bytes
         */
        hc->rx_s = tfrc_ewma(hc->rx_s, payload, 9);
        hc->rx_bytes_recv += payload;
    }

    /*
     * Perform loss detection and handle pending losses
     */
    if (tfrc_rx_handle_loss(&hc->rx_hist, &hc->rx_li_hist,
                skb, ndp, ccid3_first_li, sk)) {
        do_feedback = CCID3_FBACK_PARAM_CHANGE;
        goto done_receiving;
    }

    if (tfrc_rx_hist_loss_pending(&hc->rx_hist))
        return; /* done receiving */

    /*
     * Handle data packets: RTT sampling and monitoring p
     */
    if (unlikely(!is_data_packet))
        goto update_records;

    if (!tfrc_lh_is_initialised(&hc->rx_li_hist)) {
        const u32 sample = tfrc_rx_hist_sample_rtt(&hc->rx_hist, skb);
        /*
         * Empty loss history: no loss so far, hence p stays 0.
         * Sample RTT values, since an RTT estimate is required for the
         * computation of p when the first loss occurs; RFC 3448, 6.3.1.
         */
        if (sample != 0)
            hc->rx_rtt = tfrc_ewma(hc->rx_rtt, sample, 9);

    } else if (tfrc_lh_update_i_mean(&hc->rx_li_hist, skb)) {
        /*
         * Step (3) of [RFC 3448, 6.1]: Recompute I_mean and, if I_mean
         * has decreased (resp. p has increased), send feedback now.
         */
        do_feedback = CCID3_FBACK_PARAM_CHANGE;
    }

    /*
     * Check if the periodic once-per-RTT feedback is due; RFC 4342, 10.3
     */
    if (SUB16(dccp_hdr(skb)->dccph_ccval, hc->rx_last_counter) > 3)
        do_feedback = CCID3_FBACK_PERIODIC;

update_records:
    tfrc_rx_hist_add_packet(&hc->rx_hist, skb, ndp);

done_receiving:
    if (do_feedback)
        ccid3_hc_rx_send_feedback(sk, skb, do_feedback);
}

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

    hc->rx_state = TFRC_RSTATE_NO_DATA;
    tfrc_lh_init(&hc->rx_li_hist);
    return tfrc_rx_hist_alloc(&hc->rx_hist);
}

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

    tfrc_rx_hist_purge(&hc->rx_hist);
    tfrc_lh_cleanup(&hc->rx_li_hist);
}

static void ccid3_hc_rx_get_info(struct sock *sk, struct tcp_info *info)
{
    info->tcpi_ca_state = ccid3_hc_rx_sk(sk)->rx_state;
    info->tcpi_options  |= TCPI_OPT_TIMESTAMPS;
    info->tcpi_rcv_rtt  = ccid3_hc_rx_sk(sk)->rx_rtt;
}

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 *hc = 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 = hc->rx_x_recv;
        rx_info.tfrcrx_rtt    = hc->rx_rtt;
        rx_info.tfrcrx_p      = tfrc_invert_loss_event_rate(hc->rx_pinv);
        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;
}

struct ccid_operations ccid3_ops = {
    .ccid_id           = DCCPC_CCID3,
    .ccid_name         = "TCP-Friendly Rate Control",
    .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,
};

#ifdef CONFIG_IP_DCCP_CCID3_DEBUG
module_param(ccid3_debug, bool, 0644);
MODULE_PARM_DESC(ccid3_debug, "Enable CCID-3 debug messages");
#endif