Merge tag 'for_linus' of git://git.kernel.org/pub/scm/linux/kernel/git/mst/vhost

[cris-mirror.git] / net / dccp / ccids / ccid2.c

/*
 *  Copyright (c) 2005, 2006 Andrea Bittau <a.bittau@cs.ucl.ac.uk>
 *
 *  Changes to meet Linux coding standards, and DCCP infrastructure fixes.
 *
 *  Copyright (c) 2006 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.
 */

/*
 * This implementation should follow RFC 4341
 */
#include <linux/slab.h>
#include "../feat.h"
#include "ccid2.h"


#ifdef CONFIG_IP_DCCP_CCID2_DEBUG
static bool ccid2_debug;
#define ccid2_pr_debug(format, a...)    DCCP_PR_DEBUG(ccid2_debug, format, ##a)
#else
#define ccid2_pr_debug(format, a...)
#endif

static int ccid2_hc_tx_alloc_seq(struct ccid2_hc_tx_sock *hc)
{
        struct ccid2_seq *seqp;
        int i;

        /* check if we have space to preserve the pointer to the buffer */
        if (hc->tx_seqbufc >= (sizeof(hc->tx_seqbuf) /
                               sizeof(struct ccid2_seq *)))
                return -ENOMEM;

        /* allocate buffer and initialize linked list */
        seqp = kmalloc(CCID2_SEQBUF_LEN * sizeof(struct ccid2_seq), gfp_any());
        if (seqp == NULL)
                return -ENOMEM;

        for (i = 0; i < (CCID2_SEQBUF_LEN - 1); i++) {
                seqp[i].ccid2s_next = &seqp[i + 1];
                seqp[i + 1].ccid2s_prev = &seqp[i];
        }
        seqp[CCID2_SEQBUF_LEN - 1].ccid2s_next = seqp;
        seqp->ccid2s_prev = &seqp[CCID2_SEQBUF_LEN - 1];

        /* This is the first allocation.  Initiate the head and tail.  */
        if (hc->tx_seqbufc == 0)
                hc->tx_seqh = hc->tx_seqt = seqp;
        else {
                /* link the existing list with the one we just created */
                hc->tx_seqh->ccid2s_next = seqp;
                seqp->ccid2s_prev = hc->tx_seqh;

                hc->tx_seqt->ccid2s_prev = &seqp[CCID2_SEQBUF_LEN - 1];
                seqp[CCID2_SEQBUF_LEN - 1].ccid2s_next = hc->tx_seqt;
        }

        /* store the original pointer to the buffer so we can free it */
        hc->tx_seqbuf[hc->tx_seqbufc] = seqp;
        hc->tx_seqbufc++;

        return 0;
}

static int ccid2_hc_tx_send_packet(struct sock *sk, struct sk_buff *skb)
{
        if (ccid2_cwnd_network_limited(ccid2_hc_tx_sk(sk)))
                return CCID_PACKET_WILL_DEQUEUE_LATER;
        return CCID_PACKET_SEND_AT_ONCE;
}

static void ccid2_change_l_ack_ratio(struct sock *sk, u32 val)
{
        u32 max_ratio = DIV_ROUND_UP(ccid2_hc_tx_sk(sk)->tx_cwnd, 2);

        /*
         * Ensure that Ack Ratio does not exceed ceil(cwnd/2), which is (2) from
         * RFC 4341, 6.1.2. We ignore the statement that Ack Ratio 2 is always
         * acceptable since this causes starvation/deadlock whenever cwnd < 2.
         * The same problem arises when Ack Ratio is 0 (ie. Ack Ratio disabled).
         */
        if (val == 0 || val > max_ratio) {
                DCCP_WARN("Limiting Ack Ratio (%u) to %u\n", val, max_ratio);
                val = max_ratio;
        }
        dccp_feat_signal_nn_change(sk, DCCPF_ACK_RATIO,
                                   min_t(u32, val, DCCPF_ACK_RATIO_MAX));
}

static void ccid2_check_l_ack_ratio(struct sock *sk)
{
        struct ccid2_hc_tx_sock *hc = ccid2_hc_tx_sk(sk);

        /*
         * After a loss, idle period, application limited period, or RTO we
         * need to check that the ack ratio is still less than the congestion
         * window. Otherwise, we will send an entire congestion window of
         * packets and got no response because we haven't sent ack ratio
         * packets yet.
         * If the ack ratio does need to be reduced, we reduce it to half of
         * the congestion window (or 1 if that's zero) instead of to the
         * congestion window. This prevents problems if one ack is lost.
         */
        if (dccp_feat_nn_get(sk, DCCPF_ACK_RATIO) > hc->tx_cwnd)
                ccid2_change_l_ack_ratio(sk, hc->tx_cwnd/2 ? : 1U);
}

static void ccid2_change_l_seq_window(struct sock *sk, u64 val)
{
        dccp_feat_signal_nn_change(sk, DCCPF_SEQUENCE_WINDOW,
                                   clamp_val(val, DCCPF_SEQ_WMIN,
                                                  DCCPF_SEQ_WMAX));
}

static void ccid2_hc_tx_rto_expire(struct timer_list *t)
{
        struct ccid2_hc_tx_sock *hc = from_timer(hc, t, tx_rtotimer);
        struct sock *sk = hc->sk;
        const bool sender_was_blocked = ccid2_cwnd_network_limited(hc);

        bh_lock_sock(sk);
        if (sock_owned_by_user(sk)) {
                sk_reset_timer(sk, &hc->tx_rtotimer, jiffies + HZ / 5);
                goto out;
        }

        ccid2_pr_debug("RTO_EXPIRE\n");

        if (sk->sk_state == DCCP_CLOSED)
                goto out;

        /* back-off timer */
        hc->tx_rto <<= 1;
        if (hc->tx_rto > DCCP_RTO_MAX)
                hc->tx_rto = DCCP_RTO_MAX;

        /* adjust pipe, cwnd etc */
        hc->tx_ssthresh = hc->tx_cwnd / 2;
        if (hc->tx_ssthresh < 2)
                hc->tx_ssthresh = 2;
        hc->tx_cwnd     = 1;
        hc->tx_pipe     = 0;

        /* clear state about stuff we sent */
        hc->tx_seqt = hc->tx_seqh;
        hc->tx_packets_acked = 0;

        /* clear ack ratio state. */
        hc->tx_rpseq    = 0;
        hc->tx_rpdupack = -1;
        ccid2_change_l_ack_ratio(sk, 1);

        /* if we were blocked before, we may now send cwnd=1 packet */
        if (sender_was_blocked)
                tasklet_schedule(&dccp_sk(sk)->dccps_xmitlet);
        /* restart backed-off timer */
        sk_reset_timer(sk, &hc->tx_rtotimer, jiffies + hc->tx_rto);
out:
        bh_unlock_sock(sk);
        sock_put(sk);
}

/*
 *      Congestion window validation (RFC 2861).
 */
static bool ccid2_do_cwv = true;
module_param(ccid2_do_cwv, bool, 0644);
MODULE_PARM_DESC(ccid2_do_cwv, "Perform RFC2861 Congestion Window Validation");

/**
 * ccid2_update_used_window  -  Track how much of cwnd is actually used
 * This is done in addition to CWV. The sender needs to have an idea of how many
 * packets may be in flight, to set the local Sequence Window value accordingly
 * (RFC 4340, 7.5.2). The CWV mechanism is exploited to keep track of the
 * maximum-used window. We use an EWMA low-pass filter to filter out noise.
 */
static void ccid2_update_used_window(struct ccid2_hc_tx_sock *hc, u32 new_wnd)
{
        hc->tx_expected_wnd = (3 * hc->tx_expected_wnd + new_wnd) / 4;
}

/* This borrows the code of tcp_cwnd_application_limited() */
static void ccid2_cwnd_application_limited(struct sock *sk, const u32 now)
{
        struct ccid2_hc_tx_sock *hc = ccid2_hc_tx_sk(sk);
        /* don't reduce cwnd below the initial window (IW) */
        u32 init_win = rfc3390_bytes_to_packets(dccp_sk(sk)->dccps_mss_cache),
            win_used = max(hc->tx_cwnd_used, init_win);

        if (win_used < hc->tx_cwnd) {
                hc->tx_ssthresh = max(hc->tx_ssthresh,
                                     (hc->tx_cwnd >> 1) + (hc->tx_cwnd >> 2));
                hc->tx_cwnd = (hc->tx_cwnd + win_used) >> 1;
        }
        hc->tx_cwnd_used  = 0;
        hc->tx_cwnd_stamp = now;

        ccid2_check_l_ack_ratio(sk);
}

/* This borrows the code of tcp_cwnd_restart() */
static void ccid2_cwnd_restart(struct sock *sk, const u32 now)
{
        struct ccid2_hc_tx_sock *hc = ccid2_hc_tx_sk(sk);
        u32 cwnd = hc->tx_cwnd, restart_cwnd,
            iwnd = rfc3390_bytes_to_packets(dccp_sk(sk)->dccps_mss_cache);

        hc->tx_ssthresh = max(hc->tx_ssthresh, (cwnd >> 1) + (cwnd >> 2));

        /* don't reduce cwnd below the initial window (IW) */
        restart_cwnd = min(cwnd, iwnd);
        cwnd >>= (now - hc->tx_lsndtime) / hc->tx_rto;
        hc->tx_cwnd = max(cwnd, restart_cwnd);

        hc->tx_cwnd_stamp = now;
        hc->tx_cwnd_used  = 0;

        ccid2_check_l_ack_ratio(sk);
}

static void ccid2_hc_tx_packet_sent(struct sock *sk, unsigned int len)
{
        struct dccp_sock *dp = dccp_sk(sk);
        struct ccid2_hc_tx_sock *hc = ccid2_hc_tx_sk(sk);
        const u32 now = ccid2_jiffies32;
        struct ccid2_seq *next;

        /* slow-start after idle periods (RFC 2581, RFC 2861) */
        if (ccid2_do_cwv && !hc->tx_pipe &&
            (s32)(now - hc->tx_lsndtime) >= hc->tx_rto)
                ccid2_cwnd_restart(sk, now);

        hc->tx_lsndtime = now;
        hc->tx_pipe    += 1;

        /* see whether cwnd was fully used (RFC 2861), update expected window */
        if (ccid2_cwnd_network_limited(hc)) {
                ccid2_update_used_window(hc, hc->tx_cwnd);
                hc->tx_cwnd_used  = 0;
                hc->tx_cwnd_stamp = now;
        } else {
                if (hc->tx_pipe > hc->tx_cwnd_used)
                        hc->tx_cwnd_used = hc->tx_pipe;

                ccid2_update_used_window(hc, hc->tx_cwnd_used);

                if (ccid2_do_cwv && (s32)(now - hc->tx_cwnd_stamp) >= hc->tx_rto)
                        ccid2_cwnd_application_limited(sk, now);
        }

        hc->tx_seqh->ccid2s_seq   = dp->dccps_gss;
        hc->tx_seqh->ccid2s_acked = 0;
        hc->tx_seqh->ccid2s_sent  = now;

        next = hc->tx_seqh->ccid2s_next;
        /* check if we need to alloc more space */
        if (next == hc->tx_seqt) {
                if (ccid2_hc_tx_alloc_seq(hc)) {
                        DCCP_CRIT("packet history - out of memory!");
                        /* FIXME: find a more graceful way to bail out */
                        return;
                }
                next = hc->tx_seqh->ccid2s_next;
                BUG_ON(next == hc->tx_seqt);
        }
        hc->tx_seqh = next;

        ccid2_pr_debug("cwnd=%d pipe=%d\n", hc->tx_cwnd, hc->tx_pipe);

        /*
         * FIXME: The code below is broken and the variables have been removed
         * from the socket struct. The `ackloss' variable was always set to 0,
         * and with arsent there are several problems:
         *  (i) it doesn't just count the number of Acks, but all sent packets;
         *  (ii) it is expressed in # of packets, not # of windows, so the
         *  comparison below uses the wrong formula: Appendix A of RFC 4341
         *  comes up with the number K = cwnd / (R^2 - R) of consecutive windows
         *  of data with no lost or marked Ack packets. If arsent were the # of
         *  consecutive Acks received without loss, then Ack Ratio needs to be
         *  decreased by 1 when
         *            arsent >=  K * cwnd / R  =  cwnd^2 / (R^3 - R^2)
         *  where cwnd / R is the number of Acks received per window of data
         *  (cf. RFC 4341, App. A). The problems are that
         *  - arsent counts other packets as well;
         *  - the comparison uses a formula different from RFC 4341;
         *  - computing a cubic/quadratic equation each time is too complicated.
         *  Hence a different algorithm is needed.
         */
#if 0
        /* Ack Ratio.  Need to maintain a concept of how many windows we sent */
        hc->tx_arsent++;
        /* We had an ack loss in this window... */
        if (hc->tx_ackloss) {
                if (hc->tx_arsent >= hc->tx_cwnd) {
                        hc->tx_arsent  = 0;
                        hc->tx_ackloss = 0;
                }
        } else {
                /* No acks lost up to now... */
                /* decrease ack ratio if enough packets were sent */
                if (dp->dccps_l_ack_ratio > 1) {
                        /* XXX don't calculate denominator each time */
                        int denom = dp->dccps_l_ack_ratio * dp->dccps_l_ack_ratio -
                                    dp->dccps_l_ack_ratio;

                        denom = hc->tx_cwnd * hc->tx_cwnd / denom;

                        if (hc->tx_arsent >= denom) {
                                ccid2_change_l_ack_ratio(sk, dp->dccps_l_ack_ratio - 1);
                                hc->tx_arsent = 0;
                        }
                } else {
                        /* we can't increase ack ratio further [1] */
                        hc->tx_arsent = 0; /* or maybe set it to cwnd*/
                }
        }
#endif

        sk_reset_timer(sk, &hc->tx_rtotimer, jiffies + hc->tx_rto);

#ifdef CONFIG_IP_DCCP_CCID2_DEBUG
        do {
                struct ccid2_seq *seqp = hc->tx_seqt;

                while (seqp != hc->tx_seqh) {
                        ccid2_pr_debug("out seq=%llu acked=%d time=%u\n",
                                       (unsigned long long)seqp->ccid2s_seq,
                                       seqp->ccid2s_acked, seqp->ccid2s_sent);
                        seqp = seqp->ccid2s_next;
                }
        } while (0);
        ccid2_pr_debug("=========\n");
#endif
}

/**
 * ccid2_rtt_estimator - Sample RTT and compute RTO using RFC2988 algorithm
 * This code is almost identical with TCP's tcp_rtt_estimator(), since
 * - it has a higher sampling frequency (recommended by RFC 1323),
 * - the RTO does not collapse into RTT due to RTTVAR going towards zero,
 * - it is simple (cf. more complex proposals such as Eifel timer or research
 *   which suggests that the gain should be set according to window size),
 * - in tests it was found to work well with CCID2 [gerrit].
 */
static void ccid2_rtt_estimator(struct sock *sk, const long mrtt)
{
        struct ccid2_hc_tx_sock *hc = ccid2_hc_tx_sk(sk);
        long m = mrtt ? : 1;

        if (hc->tx_srtt == 0) {
                /* First measurement m */
                hc->tx_srtt = m << 3;
                hc->tx_mdev = m << 1;

                hc->tx_mdev_max = max(hc->tx_mdev, tcp_rto_min(sk));
                hc->tx_rttvar   = hc->tx_mdev_max;

                hc->tx_rtt_seq  = dccp_sk(sk)->dccps_gss;
        } else {
                /* Update scaled SRTT as SRTT += 1/8 * (m - SRTT) */
                m -= (hc->tx_srtt >> 3);
                hc->tx_srtt += m;

                /* Similarly, update scaled mdev with regard to |m| */
                if (m < 0) {
                        m = -m;
                        m -= (hc->tx_mdev >> 2);
                        /*
                         * This neutralises RTO increase when RTT < SRTT - mdev
                         * (see P. Sarolahti, A. Kuznetsov,"Congestion Control
                         * in Linux TCP", USENIX 2002, pp. 49-62).
                         */
                        if (m > 0)
                                m >>= 3;
                } else {
                        m -= (hc->tx_mdev >> 2);
                }
                hc->tx_mdev += m;

                if (hc->tx_mdev > hc->tx_mdev_max) {
                        hc->tx_mdev_max = hc->tx_mdev;
                        if (hc->tx_mdev_max > hc->tx_rttvar)
                                hc->tx_rttvar = hc->tx_mdev_max;
                }

                /*
                 * Decay RTTVAR at most once per flight, exploiting that
                 *  1) pipe <= cwnd <= Sequence_Window = W  (RFC 4340, 7.5.2)
                 *  2) AWL = GSS-W+1 <= GAR <= GSS          (RFC 4340, 7.5.1)
                 * GAR is a useful bound for FlightSize = pipe.
                 * AWL is probably too low here, as it over-estimates pipe.
                 */
                if (after48(dccp_sk(sk)->dccps_gar, hc->tx_rtt_seq)) {
                        if (hc->tx_mdev_max < hc->tx_rttvar)
                                hc->tx_rttvar -= (hc->tx_rttvar -
                                                  hc->tx_mdev_max) >> 2;
                        hc->tx_rtt_seq  = dccp_sk(sk)->dccps_gss;
                        hc->tx_mdev_max = tcp_rto_min(sk);
                }
        }

        /*
         * Set RTO from SRTT and RTTVAR
         * As in TCP, 4 * RTTVAR >= TCP_RTO_MIN, giving a minimum RTO of 200 ms.
         * This agrees with RFC 4341, 5:
         *      "Because DCCP does not retransmit data, DCCP does not require
         *       TCP's recommended minimum timeout of one second".
         */
        hc->tx_rto = (hc->tx_srtt >> 3) + hc->tx_rttvar;

        if (hc->tx_rto > DCCP_RTO_MAX)
                hc->tx_rto = DCCP_RTO_MAX;
}

static void ccid2_new_ack(struct sock *sk, struct ccid2_seq *seqp,
                          unsigned int *maxincr)
{
        struct ccid2_hc_tx_sock *hc = ccid2_hc_tx_sk(sk);
        struct dccp_sock *dp = dccp_sk(sk);
        int r_seq_used = hc->tx_cwnd / dp->dccps_l_ack_ratio;

        if (hc->tx_cwnd < dp->dccps_l_seq_win &&
            r_seq_used < dp->dccps_r_seq_win) {
                if (hc->tx_cwnd < hc->tx_ssthresh) {
                        if (*maxincr > 0 && ++hc->tx_packets_acked >= 2) {
                                hc->tx_cwnd += 1;
                                *maxincr    -= 1;
                                hc->tx_packets_acked = 0;
                        }
                } else if (++hc->tx_packets_acked >= hc->tx_cwnd) {
                        hc->tx_cwnd += 1;
                        hc->tx_packets_acked = 0;
                }
        }

        /*
         * Adjust the local sequence window and the ack ratio to allow about
         * 5 times the number of packets in the network (RFC 4340 7.5.2)
         */
        if (r_seq_used * CCID2_WIN_CHANGE_FACTOR >= dp->dccps_r_seq_win)
                ccid2_change_l_ack_ratio(sk, dp->dccps_l_ack_ratio * 2);
        else if (r_seq_used * CCID2_WIN_CHANGE_FACTOR < dp->dccps_r_seq_win/2)
                ccid2_change_l_ack_ratio(sk, dp->dccps_l_ack_ratio / 2 ? : 1U);

        if (hc->tx_cwnd * CCID2_WIN_CHANGE_FACTOR >= dp->dccps_l_seq_win)
                ccid2_change_l_seq_window(sk, dp->dccps_l_seq_win * 2);
        else if (hc->tx_cwnd * CCID2_WIN_CHANGE_FACTOR < dp->dccps_l_seq_win/2)
                ccid2_change_l_seq_window(sk, dp->dccps_l_seq_win / 2);

        /*
         * FIXME: RTT is sampled several times per acknowledgment (for each
         * entry in the Ack Vector), instead of once per Ack (as in TCP SACK).
         * This causes the RTT to be over-estimated, since the older entries
         * in the Ack Vector have earlier sending times.
         * The cleanest solution is to not use the ccid2s_sent field at all
         * and instead use DCCP timestamps: requires changes in other places.
         */
        ccid2_rtt_estimator(sk, ccid2_jiffies32 - seqp->ccid2s_sent);
}

static void ccid2_congestion_event(struct sock *sk, struct ccid2_seq *seqp)
{
        struct ccid2_hc_tx_sock *hc = ccid2_hc_tx_sk(sk);

        if ((s32)(seqp->ccid2s_sent - hc->tx_last_cong) < 0) {
                ccid2_pr_debug("Multiple losses in an RTT---treating as one\n");
                return;
        }

        hc->tx_last_cong = ccid2_jiffies32;

        hc->tx_cwnd      = hc->tx_cwnd / 2 ? : 1U;
        hc->tx_ssthresh  = max(hc->tx_cwnd, 2U);

        ccid2_check_l_ack_ratio(sk);
}

static int ccid2_hc_tx_parse_options(struct sock *sk, u8 packet_type,
                                     u8 option, u8 *optval, u8 optlen)
{
        struct ccid2_hc_tx_sock *hc = ccid2_hc_tx_sk(sk);

        switch (option) {
        case DCCPO_ACK_VECTOR_0:
        case DCCPO_ACK_VECTOR_1:
                return dccp_ackvec_parsed_add(&hc->tx_av_chunks, optval, optlen,
                                              option - DCCPO_ACK_VECTOR_0);
        }
        return 0;
}

static void ccid2_hc_tx_packet_recv(struct sock *sk, struct sk_buff *skb)
{
        struct dccp_sock *dp = dccp_sk(sk);
        struct ccid2_hc_tx_sock *hc = ccid2_hc_tx_sk(sk);
        const bool sender_was_blocked = ccid2_cwnd_network_limited(hc);
        struct dccp_ackvec_parsed *avp;
        u64 ackno, seqno;
        struct ccid2_seq *seqp;
        int done = 0;
        unsigned int maxincr = 0;

        /* check reverse path congestion */
        seqno = DCCP_SKB_CB(skb)->dccpd_seq;

        /* XXX this whole "algorithm" is broken.  Need to fix it to keep track
         * of the seqnos of the dupacks so that rpseq and rpdupack are correct
         * -sorbo.
         */
        /* need to bootstrap */
        if (hc->tx_rpdupack == -1) {
                hc->tx_rpdupack = 0;
                hc->tx_rpseq    = seqno;
        } else {
                /* check if packet is consecutive */
                if (dccp_delta_seqno(hc->tx_rpseq, seqno) == 1)
                        hc->tx_rpseq = seqno;
                /* it's a later packet */
                else if (after48(seqno, hc->tx_rpseq)) {
                        hc->tx_rpdupack++;

                        /* check if we got enough dupacks */
                        if (hc->tx_rpdupack >= NUMDUPACK) {
                                hc->tx_rpdupack = -1; /* XXX lame */
                                hc->tx_rpseq    = 0;
#ifdef __CCID2_COPES_GRACEFULLY_WITH_ACK_CONGESTION_CONTROL__
                                /*
                                 * FIXME: Ack Congestion Control is broken; in
                                 * the current state instabilities occurred with
                                 * Ack Ratios greater than 1; causing hang-ups
                                 * and long RTO timeouts. This needs to be fixed
                                 * before opening up dynamic changes. -- gerrit
                                 */
                                ccid2_change_l_ack_ratio(sk, 2 * dp->dccps_l_ack_ratio);
#endif
                        }
                }
        }

        /* check forward path congestion */
        if (dccp_packet_without_ack(skb))
                return;

        /* still didn't send out new data packets */
        if (hc->tx_seqh == hc->tx_seqt)
                goto done;

        ackno = DCCP_SKB_CB(skb)->dccpd_ack_seq;
        if (after48(ackno, hc->tx_high_ack))
                hc->tx_high_ack = ackno;

        seqp = hc->tx_seqt;
        while (before48(seqp->ccid2s_seq, ackno)) {
                seqp = seqp->ccid2s_next;
                if (seqp == hc->tx_seqh) {
                        seqp = hc->tx_seqh->ccid2s_prev;
                        break;
                }
        }

        /*
         * In slow-start, cwnd can increase up to a maximum of Ack Ratio/2
         * packets per acknowledgement. Rounding up avoids that cwnd is not
         * advanced when Ack Ratio is 1 and gives a slight edge otherwise.
         */
        if (hc->tx_cwnd < hc->tx_ssthresh)
                maxincr = DIV_ROUND_UP(dp->dccps_l_ack_ratio, 2);

        /* go through all ack vectors */
        list_for_each_entry(avp, &hc->tx_av_chunks, node) {
                /* go through this ack vector */
                for (; avp->len--; avp->vec++) {
                        u64 ackno_end_rl = SUB48(ackno,
                                                 dccp_ackvec_runlen(avp->vec));

                        ccid2_pr_debug("ackvec %llu |%u,%u|\n",
                                       (unsigned long long)ackno,
                                       dccp_ackvec_state(avp->vec) >> 6,
                                       dccp_ackvec_runlen(avp->vec));
                        /* if the seqno we are analyzing is larger than the
                         * current ackno, then move towards the tail of our
                         * seqnos.
                         */
                        while (after48(seqp->ccid2s_seq, ackno)) {
                                if (seqp == hc->tx_seqt) {
                                        done = 1;
                                        break;
                                }
                                seqp = seqp->ccid2s_prev;
                        }
                        if (done)
                                break;

                        /* check all seqnos in the range of the vector
                         * run length
                         */
                        while (between48(seqp->ccid2s_seq,ackno_end_rl,ackno)) {
                                const u8 state = dccp_ackvec_state(avp->vec);

                                /* new packet received or marked */
                                if (state != DCCPAV_NOT_RECEIVED &&
                                    !seqp->ccid2s_acked) {
                                        if (state == DCCPAV_ECN_MARKED)
                                                ccid2_congestion_event(sk,
                                                                       seqp);
                                        else
                                                ccid2_new_ack(sk, seqp,
                                                              &maxincr);

                                        seqp->ccid2s_acked = 1;
                                        ccid2_pr_debug("Got ack for %llu\n",
                                                       (unsigned long long)seqp->ccid2s_seq);
                                        hc->tx_pipe--;
                                }
                                if (seqp == hc->tx_seqt) {
                                        done = 1;
                                        break;
                                }
                                seqp = seqp->ccid2s_prev;
                        }
                        if (done)
                                break;

                        ackno = SUB48(ackno_end_rl, 1);
                }
                if (done)
                        break;
        }

        /* The state about what is acked should be correct now
         * Check for NUMDUPACK
         */
        seqp = hc->tx_seqt;
        while (before48(seqp->ccid2s_seq, hc->tx_high_ack)) {
                seqp = seqp->ccid2s_next;
                if (seqp == hc->tx_seqh) {
                        seqp = hc->tx_seqh->ccid2s_prev;
                        break;
                }
        }
        done = 0;
        while (1) {
                if (seqp->ccid2s_acked) {
                        done++;
                        if (done == NUMDUPACK)
                                break;
                }
                if (seqp == hc->tx_seqt)
                        break;
                seqp = seqp->ccid2s_prev;
        }

        /* If there are at least 3 acknowledgements, anything unacknowledged
         * below the last sequence number is considered lost
         */
        if (done == NUMDUPACK) {
                struct ccid2_seq *last_acked = seqp;

                /* check for lost packets */
                while (1) {
                        if (!seqp->ccid2s_acked) {
                                ccid2_pr_debug("Packet lost: %llu\n",
                                               (unsigned long long)seqp->ccid2s_seq);
                                /* XXX need to traverse from tail -> head in
                                 * order to detect multiple congestion events in
                                 * one ack vector.
                                 */
                                ccid2_congestion_event(sk, seqp);
                                hc->tx_pipe--;
                        }
                        if (seqp == hc->tx_seqt)
                                break;
                        seqp = seqp->ccid2s_prev;
                }

                hc->tx_seqt = last_acked;
        }

        /* trim acked packets in tail */
        while (hc->tx_seqt != hc->tx_seqh) {
                if (!hc->tx_seqt->ccid2s_acked)
                        break;

                hc->tx_seqt = hc->tx_seqt->ccid2s_next;
        }

        /* restart RTO timer if not all outstanding data has been acked */
        if (hc->tx_pipe == 0)
                sk_stop_timer(sk, &hc->tx_rtotimer);
        else
                sk_reset_timer(sk, &hc->tx_rtotimer, jiffies + hc->tx_rto);
done:
        /* check if incoming Acks allow pending packets to be sent */
        if (sender_was_blocked && !ccid2_cwnd_network_limited(hc))
                tasklet_schedule(&dccp_sk(sk)->dccps_xmitlet);
        dccp_ackvec_parsed_cleanup(&hc->tx_av_chunks);
}

static int ccid2_hc_tx_init(struct ccid *ccid, struct sock *sk)
{
        struct ccid2_hc_tx_sock *hc = ccid_priv(ccid);
        struct dccp_sock *dp = dccp_sk(sk);
        u32 max_ratio;

        /* RFC 4341, 5: initialise ssthresh to arbitrarily high (max) value */
        hc->tx_ssthresh = ~0U;

        /* Use larger initial windows (RFC 4341, section 5). */
        hc->tx_cwnd = rfc3390_bytes_to_packets(dp->dccps_mss_cache);
        hc->tx_expected_wnd = hc->tx_cwnd;

        /* Make sure that Ack Ratio is enabled and within bounds. */
        max_ratio = DIV_ROUND_UP(hc->tx_cwnd, 2);
        if (dp->dccps_l_ack_ratio == 0 || dp->dccps_l_ack_ratio > max_ratio)
                dp->dccps_l_ack_ratio = max_ratio;

        /* XXX init ~ to window size... */
        if (ccid2_hc_tx_alloc_seq(hc))
                return -ENOMEM;

        hc->tx_rto       = DCCP_TIMEOUT_INIT;
        hc->tx_rpdupack  = -1;
        hc->tx_last_cong = hc->tx_lsndtime = hc->tx_cwnd_stamp = ccid2_jiffies32;
        hc->tx_cwnd_used = 0;
        hc->sk           = sk;
        timer_setup(&hc->tx_rtotimer, ccid2_hc_tx_rto_expire, 0);
        INIT_LIST_HEAD(&hc->tx_av_chunks);
        return 0;
}

static void ccid2_hc_tx_exit(struct sock *sk)
{
        struct ccid2_hc_tx_sock *hc = ccid2_hc_tx_sk(sk);
        int i;

        sk_stop_timer(sk, &hc->tx_rtotimer);

        for (i = 0; i < hc->tx_seqbufc; i++)
                kfree(hc->tx_seqbuf[i]);
        hc->tx_seqbufc = 0;
        dccp_ackvec_parsed_cleanup(&hc->tx_av_chunks);
}

static void ccid2_hc_rx_packet_recv(struct sock *sk, struct sk_buff *skb)
{
        struct ccid2_hc_rx_sock *hc = ccid2_hc_rx_sk(sk);

        if (!dccp_data_packet(skb))
                return;

        if (++hc->rx_num_data_pkts >= dccp_sk(sk)->dccps_r_ack_ratio) {
                dccp_send_ack(sk);
                hc->rx_num_data_pkts = 0;
        }
}

struct ccid_operations ccid2_ops = {
        .ccid_id                  = DCCPC_CCID2,
        .ccid_name                = "TCP-like",
        .ccid_hc_tx_obj_size      = sizeof(struct ccid2_hc_tx_sock),
        .ccid_hc_tx_init          = ccid2_hc_tx_init,
        .ccid_hc_tx_exit          = ccid2_hc_tx_exit,
        .ccid_hc_tx_send_packet   = ccid2_hc_tx_send_packet,
        .ccid_hc_tx_packet_sent   = ccid2_hc_tx_packet_sent,
        .ccid_hc_tx_parse_options = ccid2_hc_tx_parse_options,
        .ccid_hc_tx_packet_recv   = ccid2_hc_tx_packet_recv,
        .ccid_hc_rx_obj_size      = sizeof(struct ccid2_hc_rx_sock),
        .ccid_hc_rx_packet_recv   = ccid2_hc_rx_packet_recv,
};

#ifdef CONFIG_IP_DCCP_CCID2_DEBUG
module_param(ccid2_debug, bool, 0644);
MODULE_PARM_DESC(ccid2_debug, "Enable CCID-2 debug messages");
#endif