output.c

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/*
 *  net/dccp/output.c
 *
 *  An implementation of the DCCP protocol
 *  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.
 */

#include <linux/dccp.h>
#include <linux/kernel.h>
#include <linux/skbuff.h>
#include <linux/slab.h>

#include <net/inet_sock.h>
#include <net/sock.h>

#include "ackvec.h"
#include "ccid.h"
#include "dccp.h"

static inline void dccp_event_ack_sent(struct sock *sk)
{
    inet_csk_clear_xmit_timer(sk, ICSK_TIME_DACK);
}

/* enqueue @skb on sk_send_head for retransmission, return clone to send now */
static struct sk_buff *dccp_skb_entail(struct sock *sk, struct sk_buff *skb)
{
    skb_set_owner_w(skb, sk);
    WARN_ON(sk->sk_send_head);
    sk->sk_send_head = skb;
    return skb_clone(sk->sk_send_head, gfp_any());
}

/*
 * All SKB's seen here are completely headerless. It is our
 * job to build the DCCP header, and pass the packet down to
 * IP so it can do the same plus pass the packet off to the
 * device.
 */
static int dccp_transmit_skb(struct sock *sk, struct sk_buff *skb)
{
    if (likely(skb != NULL)) {
        struct inet_sock *inet = inet_sk(sk);
        const struct inet_connection_sock *icsk = inet_csk(sk);
        struct dccp_sock *dp = dccp_sk(sk);
        struct dccp_skb_cb *dcb = DCCP_SKB_CB(skb);
        struct dccp_hdr *dh;
        /* XXX For now we're using only 48 bits sequence numbers */
        const u32 dccp_header_size = sizeof(*dh) +
                         sizeof(struct dccp_hdr_ext) +
                      dccp_packet_hdr_len(dcb->dccpd_type);
        int err, set_ack = 1;
        u64 ackno = dp->dccps_gsr;
        /*
         * Increment GSS here already in case the option code needs it.
         * Update GSS for real only if option processing below succeeds.
         */
        dcb->dccpd_seq = ADD48(dp->dccps_gss, 1);

        switch (dcb->dccpd_type) {
        case DCCP_PKT_DATA:
            set_ack = 0;
            /* fall through */
        case DCCP_PKT_DATAACK:
        case DCCP_PKT_RESET:
            break;

        case DCCP_PKT_REQUEST:
            set_ack = 0;
            /* Use ISS on the first (non-retransmitted) Request. */
            if (icsk->icsk_retransmits == 0)
                dcb->dccpd_seq = dp->dccps_iss;
            /* fall through */

        case DCCP_PKT_SYNC:
        case DCCP_PKT_SYNCACK:
            ackno = dcb->dccpd_ack_seq;
            /* fall through */
        default:
            /*
             * Set owner/destructor: some skbs are allocated via
             * alloc_skb (e.g. when retransmission may happen).
             * Only Data, DataAck, and Reset packets should come
             * through here with skb->sk set.
             */
            WARN_ON(skb->sk);
            skb_set_owner_w(skb, sk);
            break;
        }

        if (dccp_insert_options(sk, skb)) {
            kfree_skb(skb);
            return -EPROTO;
        }


        /* Build DCCP header and checksum it. */
        dh = dccp_zeroed_hdr(skb, dccp_header_size);
        dh->dccph_type  = dcb->dccpd_type;
        dh->dccph_sport = inet->inet_sport;
        dh->dccph_dport = inet->inet_dport;
        dh->dccph_doff  = (dccp_header_size + dcb->dccpd_opt_len) / 4;
        dh->dccph_ccval = dcb->dccpd_ccval;
        dh->dccph_cscov = dp->dccps_pcslen;
        /* XXX For now we're using only 48 bits sequence numbers */
        dh->dccph_x = 1;

        dccp_update_gss(sk, dcb->dccpd_seq);
        dccp_hdr_set_seq(dh, dp->dccps_gss);
        if (set_ack)
            dccp_hdr_set_ack(dccp_hdr_ack_bits(skb), ackno);

        switch (dcb->dccpd_type) {
        case DCCP_PKT_REQUEST:
            dccp_hdr_request(skb)->dccph_req_service =
                            dp->dccps_service;
            /*
             * Limit Ack window to ISS <= P.ackno <= GSS, so that
             * only Responses to Requests we sent are considered.
             */
            dp->dccps_awl = dp->dccps_iss;
            break;
        case DCCP_PKT_RESET:
            dccp_hdr_reset(skb)->dccph_reset_code =
                            dcb->dccpd_reset_code;
            break;
        }

        icsk->icsk_af_ops->send_check(sk, skb);

        if (set_ack)
            dccp_event_ack_sent(sk);

        DCCP_INC_STATS(DCCP_MIB_OUTSEGS);

        err = icsk->icsk_af_ops->queue_xmit(skb, &inet->cork.fl);
        return net_xmit_eval(err);
    }
    return -ENOBUFS;
}

static u32 dccp_determine_ccmps(const struct dccp_sock *dp)
{
    const struct ccid *tx_ccid = dp->dccps_hc_tx_ccid;

    if (tx_ccid == NULL || tx_ccid->ccid_ops == NULL)
        return 0;
    return tx_ccid->ccid_ops->ccid_ccmps;
}

unsigned int dccp_sync_mss(struct sock *sk, u32 pmtu)
{
    struct inet_connection_sock *icsk = inet_csk(sk);
    struct dccp_sock *dp = dccp_sk(sk);
    u32 ccmps = dccp_determine_ccmps(dp);
    u32 cur_mps = ccmps ? min(pmtu, ccmps) : pmtu;

    /* Account for header lengths and IPv4/v6 option overhead */
    cur_mps -= (icsk->icsk_af_ops->net_header_len + icsk->icsk_ext_hdr_len +
            sizeof(struct dccp_hdr) + sizeof(struct dccp_hdr_ext));

    /*
     * Leave enough headroom for common DCCP header options.
     * This only considers options which may appear on DCCP-Data packets, as
     * per table 3 in RFC 4340, 5.8. When running out of space for other
     * options (eg. Ack Vector which can take up to 255 bytes), it is better
     * to schedule a separate Ack. Thus we leave headroom for the following:
     *  - 1 byte for Slow Receiver (11.6)
     *  - 6 bytes for Timestamp (13.1)
     *  - 10 bytes for Timestamp Echo (13.3)
     *  - 8 bytes for NDP count (7.7, when activated)
     *  - 6 bytes for Data Checksum (9.3)
     *  - %DCCPAV_MIN_OPTLEN bytes for Ack Vector size (11.4, when enabled)
     */
    cur_mps -= roundup(1 + 6 + 10 + dp->dccps_send_ndp_count * 8 + 6 +
               (dp->dccps_hc_rx_ackvec ? DCCPAV_MIN_OPTLEN : 0), 4);

    /* And store cached results */
    icsk->icsk_pmtu_cookie = pmtu;
    dp->dccps_mss_cache = cur_mps;

    return cur_mps;
}

EXPORT_SYMBOL_GPL(dccp_sync_mss);

void dccp_write_space(struct sock *sk)
{
    struct socket_wq *wq;

    rcu_read_lock();
    wq = rcu_dereference(sk->sk_wq);
    if (wq_has_sleeper(wq))
        wake_up_interruptible(&wq->wait);
    /* Should agree with poll, otherwise some programs break */
    if (sock_writeable(sk))
        sk_wake_async(sk, SOCK_WAKE_SPACE, POLL_OUT);

    rcu_read_unlock();
}

static int dccp_wait_for_ccid(struct sock *sk, unsigned long delay)
{
    DEFINE_WAIT(wait);
    long remaining;

    prepare_to_wait(sk_sleep(sk), &wait, TASK_INTERRUPTIBLE);
    sk->sk_write_pending++;
    release_sock(sk);

    remaining = schedule_timeout(delay);

    lock_sock(sk);
    sk->sk_write_pending--;
    finish_wait(sk_sleep(sk), &wait);

    if (signal_pending(current) || sk->sk_err)
        return -1;
    return remaining;
}

static void dccp_xmit_packet(struct sock *sk)
{
    int err, len;
    struct dccp_sock *dp = dccp_sk(sk);
    struct sk_buff *skb = dccp_qpolicy_pop(sk);

    if (unlikely(skb == NULL))
        return;
    len = skb->len;

    if (sk->sk_state == DCCP_PARTOPEN) {
        const u32 cur_mps = dp->dccps_mss_cache - DCCP_FEATNEG_OVERHEAD;
        /*
         * See 8.1.5 - Handshake Completion.
         *
         * For robustness we resend Confirm options until the client has
         * entered OPEN. During the initial feature negotiation, the MPS
         * is smaller than usual, reduced by the Change/Confirm options.
         */
        if (!list_empty(&dp->dccps_featneg) && len > cur_mps) {
            DCCP_WARN("Payload too large (%d) for featneg.\n", len);
            dccp_send_ack(sk);
            dccp_feat_list_purge(&dp->dccps_featneg);
        }

        inet_csk_schedule_ack(sk);
        inet_csk_reset_xmit_timer(sk, ICSK_TIME_DACK,
                          inet_csk(sk)->icsk_rto,
                          DCCP_RTO_MAX);
        DCCP_SKB_CB(skb)->dccpd_type = DCCP_PKT_DATAACK;
    } else if (dccp_ack_pending(sk)) {
        DCCP_SKB_CB(skb)->dccpd_type = DCCP_PKT_DATAACK;
    } else {
        DCCP_SKB_CB(skb)->dccpd_type = DCCP_PKT_DATA;
    }

    err = dccp_transmit_skb(sk, skb);
    if (err)
        dccp_pr_debug("transmit_skb() returned err=%d\n", err);
    /*
     * Register this one as sent even if an error occurred. To the remote
     * end a local packet drop is indistinguishable from network loss, i.e.
     * any local drop will eventually be reported via receiver feedback.
     */
    ccid_hc_tx_packet_sent(dp->dccps_hc_tx_ccid, sk, len);

    /*
     * If the CCID needs to transfer additional header options out-of-band
     * (e.g. Ack Vectors or feature-negotiation options), it activates this
     * flag to schedule a Sync. The Sync will automatically incorporate all
     * currently pending header options, thus clearing the backlog.
     */
    if (dp->dccps_sync_scheduled)
        dccp_send_sync(sk, dp->dccps_gsr, DCCP_PKT_SYNC);
}

void dccp_flush_write_queue(struct sock *sk, long *time_budget)
{
    struct dccp_sock *dp = dccp_sk(sk);
    struct sk_buff *skb;
    long delay, rc;

    while (*time_budget > 0 && (skb = skb_peek(&sk->sk_write_queue))) {
        rc = ccid_hc_tx_send_packet(dp->dccps_hc_tx_ccid, sk, skb);

        switch (ccid_packet_dequeue_eval(rc)) {
        case CCID_PACKET_WILL_DEQUEUE_LATER:
            /*
             * If the CCID determines when to send, the next sending
             * time is unknown or the CCID may not even send again
             * (e.g. remote host crashes or lost Ack packets).
             */
            DCCP_WARN("CCID did not manage to send all packets\n");
            return;
        case CCID_PACKET_DELAY:
            delay = msecs_to_jiffies(rc);
            if (delay > *time_budget)
                return;
            rc = dccp_wait_for_ccid(sk, delay);
            if (rc < 0)
                return;
            *time_budget -= (delay - rc);
            /* check again if we can send now */
            break;
        case CCID_PACKET_SEND_AT_ONCE:
            dccp_xmit_packet(sk);
            break;
        case CCID_PACKET_ERR:
            skb_dequeue(&sk->sk_write_queue);
            kfree_skb(skb);
            dccp_pr_debug("packet discarded due to err=%ld\n", rc);
        }
    }
}

void dccp_write_xmit(struct sock *sk)
{
    struct dccp_sock *dp = dccp_sk(sk);
    struct sk_buff *skb;

    while ((skb = dccp_qpolicy_top(sk))) {
        int rc = ccid_hc_tx_send_packet(dp->dccps_hc_tx_ccid, sk, skb);

        switch (ccid_packet_dequeue_eval(rc)) {
        case CCID_PACKET_WILL_DEQUEUE_LATER:
            return;
        case CCID_PACKET_DELAY:
            sk_reset_timer(sk, &dp->dccps_xmit_timer,
                       jiffies + msecs_to_jiffies(rc));
            return;
        case CCID_PACKET_SEND_AT_ONCE:
            dccp_xmit_packet(sk);
            break;
        case CCID_PACKET_ERR:
            dccp_qpolicy_drop(sk, skb);
            dccp_pr_debug("packet discarded due to err=%d\n", rc);
        }
    }
}

int dccp_retransmit_skb(struct sock *sk)
{
    WARN_ON(sk->sk_send_head == NULL);

    if (inet_csk(sk)->icsk_af_ops->rebuild_header(sk) != 0)
        return -EHOSTUNREACH; /* Routing failure or similar. */

    /* this count is used to distinguish original and retransmitted skb */
    inet_csk(sk)->icsk_retransmits++;

    return dccp_transmit_skb(sk, skb_clone(sk->sk_send_head, GFP_ATOMIC));
}

struct sk_buff *dccp_make_response(struct sock *sk, struct dst_entry *dst,
                   struct request_sock *req)
{
    struct dccp_hdr *dh;
    struct dccp_request_sock *dreq;
    const u32 dccp_header_size = sizeof(struct dccp_hdr) +
                     sizeof(struct dccp_hdr_ext) +
                     sizeof(struct dccp_hdr_response);
    struct sk_buff *skb = sock_wmalloc(sk, sk->sk_prot->max_header, 1,
                       GFP_ATOMIC);
    if (skb == NULL)
        return NULL;

    /* Reserve space for headers. */
    skb_reserve(skb, sk->sk_prot->max_header);

    skb_dst_set(skb, dst_clone(dst));

    dreq = dccp_rsk(req);
    if (inet_rsk(req)->acked)   /* increase GSS upon retransmission */
        dccp_inc_seqno(&dreq->dreq_gss);
    DCCP_SKB_CB(skb)->dccpd_type = DCCP_PKT_RESPONSE;
    DCCP_SKB_CB(skb)->dccpd_seq  = dreq->dreq_gss;

    /* Resolve feature dependencies resulting from choice of CCID */
    if (dccp_feat_server_ccid_dependencies(dreq))
        goto response_failed;

    if (dccp_insert_options_rsk(dreq, skb))
        goto response_failed;

    /* Build and checksum header */
    dh = dccp_zeroed_hdr(skb, dccp_header_size);

    dh->dccph_sport = inet_rsk(req)->loc_port;
    dh->dccph_dport = inet_rsk(req)->rmt_port;
    dh->dccph_doff  = (dccp_header_size +
               DCCP_SKB_CB(skb)->dccpd_opt_len) / 4;
    dh->dccph_type  = DCCP_PKT_RESPONSE;
    dh->dccph_x = 1;
    dccp_hdr_set_seq(dh, dreq->dreq_gss);
    dccp_hdr_set_ack(dccp_hdr_ack_bits(skb), dreq->dreq_gsr);
    dccp_hdr_response(skb)->dccph_resp_service = dreq->dreq_service;

    dccp_csum_outgoing(skb);

    /* We use `acked' to remember that a Response was already sent. */
    inet_rsk(req)->acked = 1;
    DCCP_INC_STATS(DCCP_MIB_OUTSEGS);
    return skb;
response_failed:
    kfree_skb(skb);
    return NULL;
}

EXPORT_SYMBOL_GPL(dccp_make_response);

/* answer offending packet in @rcv_skb with Reset from control socket @ctl */
struct sk_buff *dccp_ctl_make_reset(struct sock *sk, struct sk_buff *rcv_skb)
{
    struct dccp_hdr *rxdh = dccp_hdr(rcv_skb), *dh;
    struct dccp_skb_cb *dcb = DCCP_SKB_CB(rcv_skb);
    const u32 dccp_hdr_reset_len = sizeof(struct dccp_hdr) +
                       sizeof(struct dccp_hdr_ext) +
                       sizeof(struct dccp_hdr_reset);
    struct dccp_hdr_reset *dhr;
    struct sk_buff *skb;

    skb = alloc_skb(sk->sk_prot->max_header, GFP_ATOMIC);
    if (skb == NULL)
        return NULL;

    skb_reserve(skb, sk->sk_prot->max_header);

    /* Swap the send and the receive. */
    dh = dccp_zeroed_hdr(skb, dccp_hdr_reset_len);
    dh->dccph_type  = DCCP_PKT_RESET;
    dh->dccph_sport = rxdh->dccph_dport;
    dh->dccph_dport = rxdh->dccph_sport;
    dh->dccph_doff  = dccp_hdr_reset_len / 4;
    dh->dccph_x = 1;

    dhr = dccp_hdr_reset(skb);
    dhr->dccph_reset_code = dcb->dccpd_reset_code;

    switch (dcb->dccpd_reset_code) {
    case DCCP_RESET_CODE_PACKET_ERROR:
        dhr->dccph_reset_data[0] = rxdh->dccph_type;
        break;
    case DCCP_RESET_CODE_OPTION_ERROR:  /* fall through */
    case DCCP_RESET_CODE_MANDATORY_ERROR:
        memcpy(dhr->dccph_reset_data, dcb->dccpd_reset_data, 3);
        break;
    }
    /*
     * From RFC 4340, 8.3.1:
     *   If P.ackno exists, set R.seqno := P.ackno + 1.
     *   Else set R.seqno := 0.
     */
    if (dcb->dccpd_ack_seq != DCCP_PKT_WITHOUT_ACK_SEQ)
        dccp_hdr_set_seq(dh, ADD48(dcb->dccpd_ack_seq, 1));
    dccp_hdr_set_ack(dccp_hdr_ack_bits(skb), dcb->dccpd_seq);

    dccp_csum_outgoing(skb);
    return skb;
}

EXPORT_SYMBOL_GPL(dccp_ctl_make_reset);

/* send Reset on established socket, to close or abort the connection */
int dccp_send_reset(struct sock *sk, enum dccp_reset_codes code)
{
    struct sk_buff *skb;
    /*
     * FIXME: what if rebuild_header fails?
     * Should we be doing a rebuild_header here?
     */
    int err = inet_csk(sk)->icsk_af_ops->rebuild_header(sk);

    if (err != 0)
        return err;

    skb = sock_wmalloc(sk, sk->sk_prot->max_header, 1, GFP_ATOMIC);
    if (skb == NULL)
        return -ENOBUFS;

    /* Reserve space for headers and prepare control bits. */
    skb_reserve(skb, sk->sk_prot->max_header);
    DCCP_SKB_CB(skb)->dccpd_type       = DCCP_PKT_RESET;
    DCCP_SKB_CB(skb)->dccpd_reset_code = code;

    return dccp_transmit_skb(sk, skb);
}

/*
 * Do all connect socket setups that can be done AF independent.
 */
int dccp_connect(struct sock *sk)
{
    struct sk_buff *skb;
    struct dccp_sock *dp = dccp_sk(sk);
    struct dst_entry *dst = __sk_dst_get(sk);
    struct inet_connection_sock *icsk = inet_csk(sk);

    sk->sk_err = 0;
    sock_reset_flag(sk, SOCK_DONE);

    dccp_sync_mss(sk, dst_mtu(dst));

    /* do not connect if feature negotiation setup fails */
    if (dccp_feat_finalise_settings(dccp_sk(sk)))
        return -EPROTO;

    /* Initialise GAR as per 8.5; AWL/AWH are set in dccp_transmit_skb() */
    dp->dccps_gar = dp->dccps_iss;

    skb = alloc_skb(sk->sk_prot->max_header, sk->sk_allocation);
    if (unlikely(skb == NULL))
        return -ENOBUFS;

    /* Reserve space for headers. */
    skb_reserve(skb, sk->sk_prot->max_header);

    DCCP_SKB_CB(skb)->dccpd_type = DCCP_PKT_REQUEST;

    dccp_transmit_skb(sk, dccp_skb_entail(sk, skb));
    DCCP_INC_STATS(DCCP_MIB_ACTIVEOPENS);

    /* Timer for repeating the REQUEST until an answer. */
    icsk->icsk_retransmits = 0;
    inet_csk_reset_xmit_timer(sk, ICSK_TIME_RETRANS,
                  icsk->icsk_rto, DCCP_RTO_MAX);
    return 0;
}

EXPORT_SYMBOL_GPL(dccp_connect);

void dccp_send_ack(struct sock *sk)
{
    /* If we have been reset, we may not send again. */
    if (sk->sk_state != DCCP_CLOSED) {
        struct sk_buff *skb = alloc_skb(sk->sk_prot->max_header,
                        GFP_ATOMIC);

        if (skb == NULL) {
            inet_csk_schedule_ack(sk);
            inet_csk(sk)->icsk_ack.ato = TCP_ATO_MIN;
            inet_csk_reset_xmit_timer(sk, ICSK_TIME_DACK,
                          TCP_DELACK_MAX,
                          DCCP_RTO_MAX);
            return;
        }

        /* Reserve space for headers */
        skb_reserve(skb, sk->sk_prot->max_header);
        DCCP_SKB_CB(skb)->dccpd_type = DCCP_PKT_ACK;
        dccp_transmit_skb(sk, skb);
    }
}

EXPORT_SYMBOL_GPL(dccp_send_ack);

#if 0
/* FIXME: Is this still necessary (11.3) - currently nowhere used by DCCP. */
void dccp_send_delayed_ack(struct sock *sk)
{
    struct inet_connection_sock *icsk = inet_csk(sk);
    /*
     * FIXME: tune this timer. elapsed time fixes the skew, so no problem
     * with using 2s, and active senders also piggyback the ACK into a
     * DATAACK packet, so this is really for quiescent senders.
     */
    unsigned long timeout = jiffies + 2 * HZ;

    /* Use new timeout only if there wasn't a older one earlier. */
    if (icsk->icsk_ack.pending & ICSK_ACK_TIMER) {
        /* If delack timer was blocked or is about to expire,
         * send ACK now.
         *
         * FIXME: check the "about to expire" part
         */
        if (icsk->icsk_ack.blocked) {
            dccp_send_ack(sk);
            return;
        }

        if (!time_before(timeout, icsk->icsk_ack.timeout))
            timeout = icsk->icsk_ack.timeout;
    }
    icsk->icsk_ack.pending |= ICSK_ACK_SCHED | ICSK_ACK_TIMER;
    icsk->icsk_ack.timeout = timeout;
    sk_reset_timer(sk, &icsk->icsk_delack_timer, timeout);
}
#endif

void dccp_send_sync(struct sock *sk, const u64 ackno,
            const enum dccp_pkt_type pkt_type)
{
    /*
     * We are not putting this on the write queue, so
     * dccp_transmit_skb() will set the ownership to this
     * sock.
     */
    struct sk_buff *skb = alloc_skb(sk->sk_prot->max_header, GFP_ATOMIC);

    if (skb == NULL) {
        /* FIXME: how to make sure the sync is sent? */
        DCCP_CRIT("could not send %s", dccp_packet_name(pkt_type));
        return;
    }

    /* Reserve space for headers and prepare control bits. */
    skb_reserve(skb, sk->sk_prot->max_header);
    DCCP_SKB_CB(skb)->dccpd_type = pkt_type;
    DCCP_SKB_CB(skb)->dccpd_ack_seq = ackno;

    /*
     * Clear the flag in case the Sync was scheduled for out-of-band data,
     * such as carrying a long Ack Vector.
     */
    dccp_sk(sk)->dccps_sync_scheduled = 0;

    dccp_transmit_skb(sk, skb);
}

EXPORT_SYMBOL_GPL(dccp_send_sync);

/*
 * Send a DCCP_PKT_CLOSE/CLOSEREQ. The caller locks the socket for us. This
 * cannot be allowed to fail queueing a DCCP_PKT_CLOSE/CLOSEREQ frame under
 * any circumstances.
 */
void dccp_send_close(struct sock *sk, const int active)
{
    struct dccp_sock *dp = dccp_sk(sk);
    struct sk_buff *skb;
    const gfp_t prio = active ? GFP_KERNEL : GFP_ATOMIC;

    skb = alloc_skb(sk->sk_prot->max_header, prio);
    if (skb == NULL)
        return;

    /* Reserve space for headers and prepare control bits. */
    skb_reserve(skb, sk->sk_prot->max_header);
    if (dp->dccps_role == DCCP_ROLE_SERVER && !dp->dccps_server_timewait)
        DCCP_SKB_CB(skb)->dccpd_type = DCCP_PKT_CLOSEREQ;
    else
        DCCP_SKB_CB(skb)->dccpd_type = DCCP_PKT_CLOSE;

    if (active) {
        skb = dccp_skb_entail(sk, skb);
        /*
         * Retransmission timer for active-close: RFC 4340, 8.3 requires
         * to retransmit the Close/CloseReq until the CLOSING/CLOSEREQ
         * state can be left. The initial timeout is 2 RTTs.
         * Since RTT measurement is done by the CCIDs, there is no easy
         * way to get an RTT sample. The fallback RTT from RFC 4340, 3.4
         * is too low (200ms); we use a high value to avoid unnecessary
         * retransmissions when the link RTT is > 0.2 seconds.
         * FIXME: Let main module sample RTTs and use that instead.
         */
        inet_csk_reset_xmit_timer(sk, ICSK_TIME_RETRANS,
                      DCCP_TIMEOUT_INIT, DCCP_RTO_MAX);
    }
    dccp_transmit_skb(sk, skb);
}