tcp_output.c

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/*
 * INET     An implementation of the TCP/IP protocol suite for the LINUX
 *      operating system.  INET is implemented using the  BSD Socket
 *      interface as the means of communication with the user level.
 *
 *      Implementation of the Transmission Control Protocol(TCP).
 *
 * Authors: Ross Biro
 *      Fred N. van Kempen, <[email protected]>
 *      Mark Evans, <[email protected]>
 *      Corey Minyard <[email protected]>
 *      Florian La Roche, <[email protected]>
 *      Charles Hedrick, <[email protected]>
 *      Linus Torvalds, <[email protected]>
 *      Alan Cox, <[email protected]>
 *      Matthew Dillon, <[email protected]>
 *      Arnt Gulbrandsen, <[email protected]>
 *      Jorge Cwik, <[email protected]>
 */

/*
 * Changes: Pedro Roque :   Retransmit queue handled by TCP.
 *              :   Fragmentation on mtu decrease
 *              :   Segment collapse on retransmit
 *              :   AF independence
 *
 *      Linus Torvalds  :   send_delayed_ack
 *      David S. Miller :   Charge memory using the right skb
 *                  during syn/ack processing.
 *      David S. Miller :   Output engine completely rewritten.
 *      Andrea Arcangeli:   SYNACK carry ts_recent in tsecr.
 *      Cacophonix Gaul :   draft-minshall-nagle-01
 *      J Hadi Salim    :   ECN support
 *
 */

#define pr_fmt(fmt) "TCP: " fmt

#include <net/tcp.h>

#include <linux/compiler.h>
#include <linux/gfp.h>
#include <linux/module.h>

/* People can turn this off for buggy TCP's found in printers etc. */
int sysctl_tcp_retrans_collapse __read_mostly = 1;

/* People can turn this on to work with those rare, broken TCPs that
 * interpret the window field as a signed quantity.
 */
int sysctl_tcp_workaround_signed_windows __read_mostly = 0;

/* Default TSQ limit of two TSO segments */
int sysctl_tcp_limit_output_bytes __read_mostly = 131072;

/* This limits the percentage of the congestion window which we
 * will allow a single TSO frame to consume.  Building TSO frames
 * which are too large can cause TCP streams to be bursty.
 */
int sysctl_tcp_tso_win_divisor __read_mostly = 3;

int sysctl_tcp_mtu_probing __read_mostly = 0;
int sysctl_tcp_base_mss __read_mostly = TCP_BASE_MSS;

/* By default, RFC2861 behavior.  */
int sysctl_tcp_slow_start_after_idle __read_mostly = 1;

int sysctl_tcp_cookie_size __read_mostly = 0; /* TCP_COOKIE_MAX */
EXPORT_SYMBOL_GPL(sysctl_tcp_cookie_size);

static bool tcp_write_xmit(struct sock *sk, unsigned int mss_now, int nonagle,
               int push_one, gfp_t gfp);

/* Account for new data that has been sent to the network. */
static void tcp_event_new_data_sent(struct sock *sk, const struct sk_buff *skb)
{
    struct tcp_sock *tp = tcp_sk(sk);
    unsigned int prior_packets = tp->packets_out;

    tcp_advance_send_head(sk, skb);
    tp->snd_nxt = TCP_SKB_CB(skb)->end_seq;

    /* Don't override Nagle indefinitely with F-RTO */
    if (tp->frto_counter == 2)
        tp->frto_counter = 3;

    tp->packets_out += tcp_skb_pcount(skb);
    if (!prior_packets || tp->early_retrans_delayed)
        tcp_rearm_rto(sk);
}

/* SND.NXT, if window was not shrunk.
 * If window has been shrunk, what should we make? It is not clear at all.
 * Using SND.UNA we will fail to open window, SND.NXT is out of window. :-(
 * Anything in between SND.UNA...SND.UNA+SND.WND also can be already
 * invalid. OK, let's make this for now:
 */
static inline __u32 tcp_acceptable_seq(const struct sock *sk)
{
    const struct tcp_sock *tp = tcp_sk(sk);

    if (!before(tcp_wnd_end(tp), tp->snd_nxt))
        return tp->snd_nxt;
    else
        return tcp_wnd_end(tp);
}

/* Calculate mss to advertise in SYN segment.
 * RFC1122, RFC1063, draft-ietf-tcpimpl-pmtud-01 state that:
 *
 * 1. It is independent of path mtu.
 * 2. Ideally, it is maximal possible segment size i.e. 65535-40.
 * 3. For IPv4 it is reasonable to calculate it from maximal MTU of
 *    attached devices, because some buggy hosts are confused by
 *    large MSS.
 * 4. We do not make 3, we advertise MSS, calculated from first
 *    hop device mtu, but allow to raise it to ip_rt_min_advmss.
 *    This may be overridden via information stored in routing table.
 * 5. Value 65535 for MSS is valid in IPv6 and means "as large as possible,
 *    probably even Jumbo".
 */
static __u16 tcp_advertise_mss(struct sock *sk)
{
    struct tcp_sock *tp = tcp_sk(sk);
    const struct dst_entry *dst = __sk_dst_get(sk);
    int mss = tp->advmss;

    if (dst) {
        unsigned int metric = dst_metric_advmss(dst);

        if (metric < mss) {
            mss = metric;
            tp->advmss = mss;
        }
    }

    return (__u16)mss;
}

/* RFC2861. Reset CWND after idle period longer RTO to "restart window".
 * This is the first part of cwnd validation mechanism. */
static void tcp_cwnd_restart(struct sock *sk, const struct dst_entry *dst)
{
    struct tcp_sock *tp = tcp_sk(sk);
    s32 delta = tcp_time_stamp - tp->lsndtime;
    u32 restart_cwnd = tcp_init_cwnd(tp, dst);
    u32 cwnd = tp->snd_cwnd;

    tcp_ca_event(sk, CA_EVENT_CWND_RESTART);

    tp->snd_ssthresh = tcp_current_ssthresh(sk);
    restart_cwnd = min(restart_cwnd, cwnd);

    while ((delta -= inet_csk(sk)->icsk_rto) > 0 && cwnd > restart_cwnd)
        cwnd >>= 1;
    tp->snd_cwnd = max(cwnd, restart_cwnd);
    tp->snd_cwnd_stamp = tcp_time_stamp;
    tp->snd_cwnd_used = 0;
}

/* Congestion state accounting after a packet has been sent. */
static void tcp_event_data_sent(struct tcp_sock *tp,
                struct sock *sk)
{
    struct inet_connection_sock *icsk = inet_csk(sk);
    const u32 now = tcp_time_stamp;

    if (sysctl_tcp_slow_start_after_idle &&
        (!tp->packets_out && (s32)(now - tp->lsndtime) > icsk->icsk_rto))
        tcp_cwnd_restart(sk, __sk_dst_get(sk));

    tp->lsndtime = now;

    /* If it is a reply for ato after last received
     * packet, enter pingpong mode.
     */
    if ((u32)(now - icsk->icsk_ack.lrcvtime) < icsk->icsk_ack.ato)
        icsk->icsk_ack.pingpong = 1;
}

/* Account for an ACK we sent. */
static inline void tcp_event_ack_sent(struct sock *sk, unsigned int pkts)
{
    tcp_dec_quickack_mode(sk, pkts);
    inet_csk_clear_xmit_timer(sk, ICSK_TIME_DACK);
}

/* Determine a window scaling and initial window to offer.
 * Based on the assumption that the given amount of space
 * will be offered. Store the results in the tp structure.
 * NOTE: for smooth operation initial space offering should
 * be a multiple of mss if possible. We assume here that mss >= 1.
 * This MUST be enforced by all callers.
 */
void tcp_select_initial_window(int __space, __u32 mss,
                   __u32 *rcv_wnd, __u32 *window_clamp,
                   int wscale_ok, __u8 *rcv_wscale,
                   __u32 init_rcv_wnd)
{
    unsigned int space = (__space < 0 ? 0 : __space);

    /* If no clamp set the clamp to the max possible scaled window */
    if (*window_clamp == 0)
        (*window_clamp) = (65535 << 14);
    space = min(*window_clamp, space);

    /* Quantize space offering to a multiple of mss if possible. */
    if (space > mss)
        space = (space / mss) * mss;

    /* NOTE: offering an initial window larger than 32767
     * will break some buggy TCP stacks. If the admin tells us
     * it is likely we could be speaking with such a buggy stack
     * we will truncate our initial window offering to 32K-1
     * unless the remote has sent us a window scaling option,
     * which we interpret as a sign the remote TCP is not
     * misinterpreting the window field as a signed quantity.
     */
    if (sysctl_tcp_workaround_signed_windows)
        (*rcv_wnd) = min(space, MAX_TCP_WINDOW);
    else
        (*rcv_wnd) = space;

    (*rcv_wscale) = 0;
    if (wscale_ok) {
        /* Set window scaling on max possible window
         * See RFC1323 for an explanation of the limit to 14
         */
        space = max_t(u32, sysctl_tcp_rmem[2], sysctl_rmem_max);
        space = min_t(u32, space, *window_clamp);
        while (space > 65535 && (*rcv_wscale) < 14) {
            space >>= 1;
            (*rcv_wscale)++;
        }
    }

    /* Set initial window to a value enough for senders starting with
     * initial congestion window of TCP_DEFAULT_INIT_RCVWND. Place
     * a limit on the initial window when mss is larger than 1460.
     */
    if (mss > (1 << *rcv_wscale)) {
        int init_cwnd = TCP_DEFAULT_INIT_RCVWND;
        if (mss > 1460)
            init_cwnd =
            max_t(u32, (1460 * TCP_DEFAULT_INIT_RCVWND) / mss, 2);
        /* when initializing use the value from init_rcv_wnd
         * rather than the default from above
         */
        if (init_rcv_wnd)
            *rcv_wnd = min(*rcv_wnd, init_rcv_wnd * mss);
        else
            *rcv_wnd = min(*rcv_wnd, init_cwnd * mss);
    }

    /* Set the clamp no higher than max representable value */
    (*window_clamp) = min(65535U << (*rcv_wscale), *window_clamp);
}
EXPORT_SYMBOL(tcp_select_initial_window);

/* Chose a new window to advertise, update state in tcp_sock for the
 * socket, and return result with RFC1323 scaling applied.  The return
 * value can be stuffed directly into th->window for an outgoing
 * frame.
 */
static u16 tcp_select_window(struct sock *sk)
{
    struct tcp_sock *tp = tcp_sk(sk);
    u32 cur_win = tcp_receive_window(tp);
    u32 new_win = __tcp_select_window(sk);

    /* Never shrink the offered window */
    if (new_win < cur_win) {
        /* Danger Will Robinson!
         * Don't update rcv_wup/rcv_wnd here or else
         * we will not be able to advertise a zero
         * window in time.  --DaveM
         *
         * Relax Will Robinson.
         */
        new_win = ALIGN(cur_win, 1 << tp->rx_opt.rcv_wscale);
    }
    tp->rcv_wnd = new_win;
    tp->rcv_wup = tp->rcv_nxt;

    /* Make sure we do not exceed the maximum possible
     * scaled window.
     */
    if (!tp->rx_opt.rcv_wscale && sysctl_tcp_workaround_signed_windows)
        new_win = min(new_win, MAX_TCP_WINDOW);
    else
        new_win = min(new_win, (65535U << tp->rx_opt.rcv_wscale));

    /* RFC1323 scaling applied */
    new_win >>= tp->rx_opt.rcv_wscale;

    /* If we advertise zero window, disable fast path. */
    if (new_win == 0)
        tp->pred_flags = 0;

    return new_win;
}

/* Packet ECN state for a SYN-ACK */
static inline void TCP_ECN_send_synack(const struct tcp_sock *tp, struct sk_buff *skb)
{
    TCP_SKB_CB(skb)->tcp_flags &= ~TCPHDR_CWR;
    if (!(tp->ecn_flags & TCP_ECN_OK))
        TCP_SKB_CB(skb)->tcp_flags &= ~TCPHDR_ECE;
}

/* Packet ECN state for a SYN.  */
static inline void TCP_ECN_send_syn(struct sock *sk, struct sk_buff *skb)
{
    struct tcp_sock *tp = tcp_sk(sk);

    tp->ecn_flags = 0;
    if (sysctl_tcp_ecn == 1) {
        TCP_SKB_CB(skb)->tcp_flags |= TCPHDR_ECE | TCPHDR_CWR;
        tp->ecn_flags = TCP_ECN_OK;
    }
}

static __inline__ void
TCP_ECN_make_synack(const struct request_sock *req, struct tcphdr *th)
{
    if (inet_rsk(req)->ecn_ok)
        th->ece = 1;
}

/* Set up ECN state for a packet on a ESTABLISHED socket that is about to
 * be sent.
 */
static inline void TCP_ECN_send(struct sock *sk, struct sk_buff *skb,
                int tcp_header_len)
{
    struct tcp_sock *tp = tcp_sk(sk);

    if (tp->ecn_flags & TCP_ECN_OK) {
        /* Not-retransmitted data segment: set ECT and inject CWR. */
        if (skb->len != tcp_header_len &&
            !before(TCP_SKB_CB(skb)->seq, tp->snd_nxt)) {
            INET_ECN_xmit(sk);
            if (tp->ecn_flags & TCP_ECN_QUEUE_CWR) {
                tp->ecn_flags &= ~TCP_ECN_QUEUE_CWR;
                tcp_hdr(skb)->cwr = 1;
                skb_shinfo(skb)->gso_type |= SKB_GSO_TCP_ECN;
            }
        } else {
            /* ACK or retransmitted segment: clear ECT|CE */
            INET_ECN_dontxmit(sk);
        }
        if (tp->ecn_flags & TCP_ECN_DEMAND_CWR)
            tcp_hdr(skb)->ece = 1;
    }
}

/* Constructs common control bits of non-data skb. If SYN/FIN is present,
 * auto increment end seqno.
 */
static void tcp_init_nondata_skb(struct sk_buff *skb, u32 seq, u8 flags)
{
    skb->ip_summed = CHECKSUM_PARTIAL;
    skb->csum = 0;

    TCP_SKB_CB(skb)->tcp_flags = flags;
    TCP_SKB_CB(skb)->sacked = 0;

    skb_shinfo(skb)->gso_segs = 1;
    skb_shinfo(skb)->gso_size = 0;
    skb_shinfo(skb)->gso_type = 0;

    TCP_SKB_CB(skb)->seq = seq;
    if (flags & (TCPHDR_SYN | TCPHDR_FIN))
        seq++;
    TCP_SKB_CB(skb)->end_seq = seq;
}

static inline bool tcp_urg_mode(const struct tcp_sock *tp)
{
    return tp->snd_una != tp->snd_up;
}

#define OPTION_SACK_ADVERTISE   (1 << 0)
#define OPTION_TS       (1 << 1)
#define OPTION_MD5      (1 << 2)
#define OPTION_WSCALE       (1 << 3)
#define OPTION_COOKIE_EXTENSION (1 << 4)
#define OPTION_FAST_OPEN_COOKIE (1 << 8)

struct tcp_out_options {
    u16 options;        /* bit field of OPTION_* */
    u16 mss;        /* 0 to disable */
    u8 ws;          /* window scale, 0 to disable */
    u8 num_sack_blocks; /* number of SACK blocks to include */
    u8 hash_size;       /* bytes in hash_location */
    __u8 *hash_location;    /* temporary pointer, overloaded */
    __u32 tsval, tsecr; /* need to include OPTION_TS */
    struct tcp_fastopen_cookie *fastopen_cookie;    /* Fast open cookie */
};

/* The sysctl int routines are generic, so check consistency here.
 */
static u8 tcp_cookie_size_check(u8 desired)
{
    int cookie_size;

    if (desired > 0)
        /* previously specified */
        return desired;

    cookie_size = ACCESS_ONCE(sysctl_tcp_cookie_size);
    if (cookie_size <= 0)
        /* no default specified */
        return 0;

    if (cookie_size <= TCP_COOKIE_MIN)
        /* value too small, specify minimum */
        return TCP_COOKIE_MIN;

    if (cookie_size >= TCP_COOKIE_MAX)
        /* value too large, specify maximum */
        return TCP_COOKIE_MAX;

    if (cookie_size & 1)
        /* 8-bit multiple, illegal, fix it */
        cookie_size++;

    return (u8)cookie_size;
}

/* Write previously computed TCP options to the packet.
 *
 * Beware: Something in the Internet is very sensitive to the ordering of
 * TCP options, we learned this through the hard way, so be careful here.
 * Luckily we can at least blame others for their non-compliance but from
 * inter-operatibility perspective it seems that we're somewhat stuck with
 * the ordering which we have been using if we want to keep working with
 * those broken things (not that it currently hurts anybody as there isn't
 * particular reason why the ordering would need to be changed).
 *
 * At least SACK_PERM as the first option is known to lead to a disaster
 * (but it may well be that other scenarios fail similarly).
 */
static void tcp_options_write(__be32 *ptr, struct tcp_sock *tp,
                  struct tcp_out_options *opts)
{
    u16 options = opts->options;    /* mungable copy */

    /* Having both authentication and cookies for security is redundant,
     * and there's certainly not enough room.  Instead, the cookie-less
     * extension variant is proposed.
     *
     * Consider the pessimal case with authentication.  The options
     * could look like:
     *   COOKIE|MD5(20) + MSS(4) + SACK|TS(12) + WSCALE(4) == 40
     */
    if (unlikely(OPTION_MD5 & options)) {
        if (unlikely(OPTION_COOKIE_EXTENSION & options)) {
            *ptr++ = htonl((TCPOPT_COOKIE << 24) |
                       (TCPOLEN_COOKIE_BASE << 16) |
                       (TCPOPT_MD5SIG << 8) |
                       TCPOLEN_MD5SIG);
        } else {
            *ptr++ = htonl((TCPOPT_NOP << 24) |
                       (TCPOPT_NOP << 16) |
                       (TCPOPT_MD5SIG << 8) |
                       TCPOLEN_MD5SIG);
        }
        options &= ~OPTION_COOKIE_EXTENSION;
        /* overload cookie hash location */
        opts->hash_location = (__u8 *)ptr;
        ptr += 4;
    }

    if (unlikely(opts->mss)) {
        *ptr++ = htonl((TCPOPT_MSS << 24) |
                   (TCPOLEN_MSS << 16) |
                   opts->mss);
    }

    if (likely(OPTION_TS & options)) {
        if (unlikely(OPTION_SACK_ADVERTISE & options)) {
            *ptr++ = htonl((TCPOPT_SACK_PERM << 24) |
                       (TCPOLEN_SACK_PERM << 16) |
                       (TCPOPT_TIMESTAMP << 8) |
                       TCPOLEN_TIMESTAMP);
            options &= ~OPTION_SACK_ADVERTISE;
        } else {
            *ptr++ = htonl((TCPOPT_NOP << 24) |
                       (TCPOPT_NOP << 16) |
                       (TCPOPT_TIMESTAMP << 8) |
                       TCPOLEN_TIMESTAMP);
        }
        *ptr++ = htonl(opts->tsval);
        *ptr++ = htonl(opts->tsecr);
    }

    /* Specification requires after timestamp, so do it now.
     *
     * Consider the pessimal case without authentication.  The options
     * could look like:
     *   MSS(4) + SACK|TS(12) + COOKIE(20) + WSCALE(4) == 40
     */
    if (unlikely(OPTION_COOKIE_EXTENSION & options)) {
        __u8 *cookie_copy = opts->hash_location;
        u8 cookie_size = opts->hash_size;

        /* 8-bit multiple handled in tcp_cookie_size_check() above,
         * and elsewhere.
         */
        if (0x2 & cookie_size) {
            __u8 *p = (__u8 *)ptr;

            /* 16-bit multiple */
            *p++ = TCPOPT_COOKIE;
            *p++ = TCPOLEN_COOKIE_BASE + cookie_size;
            *p++ = *cookie_copy++;
            *p++ = *cookie_copy++;
            ptr++;
            cookie_size -= 2;
        } else {
            /* 32-bit multiple */
            *ptr++ = htonl(((TCPOPT_NOP << 24) |
                    (TCPOPT_NOP << 16) |
                    (TCPOPT_COOKIE << 8) |
                    TCPOLEN_COOKIE_BASE) +
                       cookie_size);
        }

        if (cookie_size > 0) {
            memcpy(ptr, cookie_copy, cookie_size);
            ptr += (cookie_size / 4);
        }
    }

    if (unlikely(OPTION_SACK_ADVERTISE & options)) {
        *ptr++ = htonl((TCPOPT_NOP << 24) |
                   (TCPOPT_NOP << 16) |
                   (TCPOPT_SACK_PERM << 8) |
                   TCPOLEN_SACK_PERM);
    }

    if (unlikely(OPTION_WSCALE & options)) {
        *ptr++ = htonl((TCPOPT_NOP << 24) |
                   (TCPOPT_WINDOW << 16) |
                   (TCPOLEN_WINDOW << 8) |
                   opts->ws);
    }

    if (unlikely(opts->num_sack_blocks)) {
        struct tcp_sack_block *sp = tp->rx_opt.dsack ?
            tp->duplicate_sack : tp->selective_acks;
        int this_sack;

        *ptr++ = htonl((TCPOPT_NOP  << 24) |
                   (TCPOPT_NOP  << 16) |
                   (TCPOPT_SACK <<  8) |
                   (TCPOLEN_SACK_BASE + (opts->num_sack_blocks *
                             TCPOLEN_SACK_PERBLOCK)));

        for (this_sack = 0; this_sack < opts->num_sack_blocks;
             ++this_sack) {
            *ptr++ = htonl(sp[this_sack].start_seq);
            *ptr++ = htonl(sp[this_sack].end_seq);
        }

        tp->rx_opt.dsack = 0;
    }

    if (unlikely(OPTION_FAST_OPEN_COOKIE & options)) {
        struct tcp_fastopen_cookie *foc = opts->fastopen_cookie;

        *ptr++ = htonl((TCPOPT_EXP << 24) |
                   ((TCPOLEN_EXP_FASTOPEN_BASE + foc->len) << 16) |
                   TCPOPT_FASTOPEN_MAGIC);

        memcpy(ptr, foc->val, foc->len);
        if ((foc->len & 3) == 2) {
            u8 *align = ((u8 *)ptr) + foc->len;
            align[0] = align[1] = TCPOPT_NOP;
        }
        ptr += (foc->len + 3) >> 2;
    }
}

/* Compute TCP options for SYN packets. This is not the final
 * network wire format yet.
 */
static unsigned int tcp_syn_options(struct sock *sk, struct sk_buff *skb,
                struct tcp_out_options *opts,
                struct tcp_md5sig_key **md5)
{
    struct tcp_sock *tp = tcp_sk(sk);
    struct tcp_cookie_values *cvp = tp->cookie_values;
    unsigned int remaining = MAX_TCP_OPTION_SPACE;
    u8 cookie_size = (!tp->rx_opt.cookie_out_never && cvp != NULL) ?
             tcp_cookie_size_check(cvp->cookie_desired) :
             0;
    struct tcp_fastopen_request *fastopen = tp->fastopen_req;

#ifdef CONFIG_TCP_MD5SIG
    *md5 = tp->af_specific->md5_lookup(sk, sk);
    if (*md5) {
        opts->options |= OPTION_MD5;
        remaining -= TCPOLEN_MD5SIG_ALIGNED;
    }
#else
    *md5 = NULL;
#endif

    /* We always get an MSS option.  The option bytes which will be seen in
     * normal data packets should timestamps be used, must be in the MSS
     * advertised.  But we subtract them from tp->mss_cache so that
     * calculations in tcp_sendmsg are simpler etc.  So account for this
     * fact here if necessary.  If we don't do this correctly, as a
     * receiver we won't recognize data packets as being full sized when we
     * should, and thus we won't abide by the delayed ACK rules correctly.
     * SACKs don't matter, we never delay an ACK when we have any of those
     * going out.  */
    opts->mss = tcp_advertise_mss(sk);
    remaining -= TCPOLEN_MSS_ALIGNED;

    if (likely(sysctl_tcp_timestamps && *md5 == NULL)) {
        opts->options |= OPTION_TS;
        opts->tsval = TCP_SKB_CB(skb)->when;
        opts->tsecr = tp->rx_opt.ts_recent;
        remaining -= TCPOLEN_TSTAMP_ALIGNED;
    }
    if (likely(sysctl_tcp_window_scaling)) {
        opts->ws = tp->rx_opt.rcv_wscale;
        opts->options |= OPTION_WSCALE;
        remaining -= TCPOLEN_WSCALE_ALIGNED;
    }
    if (likely(sysctl_tcp_sack)) {
        opts->options |= OPTION_SACK_ADVERTISE;
        if (unlikely(!(OPTION_TS & opts->options)))
            remaining -= TCPOLEN_SACKPERM_ALIGNED;
    }

    if (fastopen && fastopen->cookie.len >= 0) {
        u32 need = TCPOLEN_EXP_FASTOPEN_BASE + fastopen->cookie.len;
        need = (need + 3) & ~3U;  /* Align to 32 bits */
        if (remaining >= need) {
            opts->options |= OPTION_FAST_OPEN_COOKIE;
            opts->fastopen_cookie = &fastopen->cookie;
            remaining -= need;
            tp->syn_fastopen = 1;
        }
    }
    /* Note that timestamps are required by the specification.
     *
     * Odd numbers of bytes are prohibited by the specification, ensuring
     * that the cookie is 16-bit aligned, and the resulting cookie pair is
     * 32-bit aligned.
     */
    if (*md5 == NULL &&
        (OPTION_TS & opts->options) &&
        cookie_size > 0) {
        int need = TCPOLEN_COOKIE_BASE + cookie_size;

        if (0x2 & need) {
            /* 32-bit multiple */
            need += 2; /* NOPs */

            if (need > remaining) {
                /* try shrinking cookie to fit */
                cookie_size -= 2;
                need -= 4;
            }
        }
        while (need > remaining && TCP_COOKIE_MIN <= cookie_size) {
            cookie_size -= 4;
            need -= 4;
        }
        if (TCP_COOKIE_MIN <= cookie_size) {
            opts->options |= OPTION_COOKIE_EXTENSION;
            opts->hash_location = (__u8 *)&cvp->cookie_pair[0];
            opts->hash_size = cookie_size;

            /* Remember for future incarnations. */
            cvp->cookie_desired = cookie_size;

            if (cvp->cookie_desired != cvp->cookie_pair_size) {
                /* Currently use random bytes as a nonce,
                 * assuming these are completely unpredictable
                 * by hostile users of the same system.
                 */
                get_random_bytes(&cvp->cookie_pair[0],
                         cookie_size);
                cvp->cookie_pair_size = cookie_size;
            }

            remaining -= need;
        }
    }
    return MAX_TCP_OPTION_SPACE - remaining;
}

/* Set up TCP options for SYN-ACKs. */
static unsigned int tcp_synack_options(struct sock *sk,
                   struct request_sock *req,
                   unsigned int mss, struct sk_buff *skb,
                   struct tcp_out_options *opts,
                   struct tcp_md5sig_key **md5,
                   struct tcp_extend_values *xvp,
                   struct tcp_fastopen_cookie *foc)
{
    struct inet_request_sock *ireq = inet_rsk(req);
    unsigned int remaining = MAX_TCP_OPTION_SPACE;
    u8 cookie_plus = (xvp != NULL && !xvp->cookie_out_never) ?
             xvp->cookie_plus :
             0;

#ifdef CONFIG_TCP_MD5SIG
    *md5 = tcp_rsk(req)->af_specific->md5_lookup(sk, req);
    if (*md5) {
        opts->options |= OPTION_MD5;
        remaining -= TCPOLEN_MD5SIG_ALIGNED;

        /* We can't fit any SACK blocks in a packet with MD5 + TS
         * options. There was discussion about disabling SACK
         * rather than TS in order to fit in better with old,
         * buggy kernels, but that was deemed to be unnecessary.
         */
        ireq->tstamp_ok &= !ireq->sack_ok;
    }
#else
    *md5 = NULL;
#endif

    /* We always send an MSS option. */
    opts->mss = mss;
    remaining -= TCPOLEN_MSS_ALIGNED;

    if (likely(ireq->wscale_ok)) {
        opts->ws = ireq->rcv_wscale;
        opts->options |= OPTION_WSCALE;
        remaining -= TCPOLEN_WSCALE_ALIGNED;
    }
    if (likely(ireq->tstamp_ok)) {
        opts->options |= OPTION_TS;
        opts->tsval = TCP_SKB_CB(skb)->when;
        opts->tsecr = req->ts_recent;
        remaining -= TCPOLEN_TSTAMP_ALIGNED;
    }
    if (likely(ireq->sack_ok)) {
        opts->options |= OPTION_SACK_ADVERTISE;
        if (unlikely(!ireq->tstamp_ok))
            remaining -= TCPOLEN_SACKPERM_ALIGNED;
    }
    if (foc != NULL) {
        u32 need = TCPOLEN_EXP_FASTOPEN_BASE + foc->len;
        need = (need + 3) & ~3U;  /* Align to 32 bits */
        if (remaining >= need) {
            opts->options |= OPTION_FAST_OPEN_COOKIE;
            opts->fastopen_cookie = foc;
            remaining -= need;
        }
    }
    /* Similar rationale to tcp_syn_options() applies here, too.
     * If the <SYN> options fit, the same options should fit now!
     */
    if (*md5 == NULL &&
        ireq->tstamp_ok &&
        cookie_plus > TCPOLEN_COOKIE_BASE) {
        int need = cookie_plus; /* has TCPOLEN_COOKIE_BASE */

        if (0x2 & need) {
            /* 32-bit multiple */
            need += 2; /* NOPs */
        }
        if (need <= remaining) {
            opts->options |= OPTION_COOKIE_EXTENSION;
            opts->hash_size = cookie_plus - TCPOLEN_COOKIE_BASE;
            remaining -= need;
        } else {
            /* There's no error return, so flag it. */
            xvp->cookie_out_never = 1; /* true */
            opts->hash_size = 0;
        }
    }
    return MAX_TCP_OPTION_SPACE - remaining;
}

/* Compute TCP options for ESTABLISHED sockets. This is not the
 * final wire format yet.
 */
static unsigned int tcp_established_options(struct sock *sk, struct sk_buff *skb,
                    struct tcp_out_options *opts,
                    struct tcp_md5sig_key **md5)
{
    struct tcp_skb_cb *tcb = skb ? TCP_SKB_CB(skb) : NULL;
    struct tcp_sock *tp = tcp_sk(sk);
    unsigned int size = 0;
    unsigned int eff_sacks;

#ifdef CONFIG_TCP_MD5SIG
    *md5 = tp->af_specific->md5_lookup(sk, sk);
    if (unlikely(*md5)) {
        opts->options |= OPTION_MD5;
        size += TCPOLEN_MD5SIG_ALIGNED;
    }
#else
    *md5 = NULL;
#endif

    if (likely(tp->rx_opt.tstamp_ok)) {
        opts->options |= OPTION_TS;
        opts->tsval = tcb ? tcb->when : 0;
        opts->tsecr = tp->rx_opt.ts_recent;
        size += TCPOLEN_TSTAMP_ALIGNED;
    }

    eff_sacks = tp->rx_opt.num_sacks + tp->rx_opt.dsack;
    if (unlikely(eff_sacks)) {
        const unsigned int remaining = MAX_TCP_OPTION_SPACE - size;
        opts->num_sack_blocks =
            min_t(unsigned int, eff_sacks,
                  (remaining - TCPOLEN_SACK_BASE_ALIGNED) /
                  TCPOLEN_SACK_PERBLOCK);
        size += TCPOLEN_SACK_BASE_ALIGNED +
            opts->num_sack_blocks * TCPOLEN_SACK_PERBLOCK;
    }

    return size;
}


/* TCP SMALL QUEUES (TSQ)
 *
 * TSQ goal is to keep small amount of skbs per tcp flow in tx queues (qdisc+dev)
 * to reduce RTT and bufferbloat.
 * We do this using a special skb destructor (tcp_wfree).
 *
 * Its important tcp_wfree() can be replaced by sock_wfree() in the event skb
 * needs to be reallocated in a driver.
 * The invariant being skb->truesize substracted from sk->sk_wmem_alloc
 *
 * Since transmit from skb destructor is forbidden, we use a tasklet
 * to process all sockets that eventually need to send more skbs.
 * We use one tasklet per cpu, with its own queue of sockets.
 */
struct tsq_tasklet {
    struct tasklet_struct   tasklet;
    struct list_head    head; /* queue of tcp sockets */
};
static DEFINE_PER_CPU(struct tsq_tasklet, tsq_tasklet);

static void tcp_tsq_handler(struct sock *sk)
{
    if ((1 << sk->sk_state) &
        (TCPF_ESTABLISHED | TCPF_FIN_WAIT1 | TCPF_CLOSING |
         TCPF_CLOSE_WAIT  | TCPF_LAST_ACK))
        tcp_write_xmit(sk, tcp_current_mss(sk), 0, 0, GFP_ATOMIC);
}
/*
 * One tasklest per cpu tries to send more skbs.
 * We run in tasklet context but need to disable irqs when
 * transfering tsq->head because tcp_wfree() might
 * interrupt us (non NAPI drivers)
 */
static void tcp_tasklet_func(unsigned long data)
{
    struct tsq_tasklet *tsq = (struct tsq_tasklet *)data;
    LIST_HEAD(list);
    unsigned long flags;
    struct list_head *q, *n;
    struct tcp_sock *tp;
    struct sock *sk;

    local_irq_save(flags);
    list_splice_init(&tsq->head, &list);
    local_irq_restore(flags);

    list_for_each_safe(q, n, &list) {
        tp = list_entry(q, struct tcp_sock, tsq_node);
        list_del(&tp->tsq_node);

        sk = (struct sock *)tp;
        bh_lock_sock(sk);

        if (!sock_owned_by_user(sk)) {
            tcp_tsq_handler(sk);
        } else {
            /* defer the work to tcp_release_cb() */
            set_bit(TCP_TSQ_DEFERRED, &tp->tsq_flags);
        }
        bh_unlock_sock(sk);

        clear_bit(TSQ_QUEUED, &tp->tsq_flags);
        sk_free(sk);
    }
}

#define TCP_DEFERRED_ALL ((1UL << TCP_TSQ_DEFERRED) |       \
              (1UL << TCP_WRITE_TIMER_DEFERRED) |   \
              (1UL << TCP_DELACK_TIMER_DEFERRED) |  \
              (1UL << TCP_MTU_REDUCED_DEFERRED))

void tcp_release_cb(struct sock *sk)
{
    struct tcp_sock *tp = tcp_sk(sk);
    unsigned long flags, nflags;

    /* perform an atomic operation only if at least one flag is set */
    do {
        flags = tp->tsq_flags;
        if (!(flags & TCP_DEFERRED_ALL))
            return;
        nflags = flags & ~TCP_DEFERRED_ALL;
    } while (cmpxchg(&tp->tsq_flags, flags, nflags) != flags);

    if (flags & (1UL << TCP_TSQ_DEFERRED))
        tcp_tsq_handler(sk);

    if (flags & (1UL << TCP_WRITE_TIMER_DEFERRED)) {
        tcp_write_timer_handler(sk);
        __sock_put(sk);
    }
    if (flags & (1UL << TCP_DELACK_TIMER_DEFERRED)) {
        tcp_delack_timer_handler(sk);
        __sock_put(sk);
    }
    if (flags & (1UL << TCP_MTU_REDUCED_DEFERRED)) {
        sk->sk_prot->mtu_reduced(sk);
        __sock_put(sk);
    }
}
EXPORT_SYMBOL(tcp_release_cb);

void __init tcp_tasklet_init(void)
{
    int i;

    for_each_possible_cpu(i) {
        struct tsq_tasklet *tsq = &per_cpu(tsq_tasklet, i);

        INIT_LIST_HEAD(&tsq->head);
        tasklet_init(&tsq->tasklet,
                 tcp_tasklet_func,
                 (unsigned long)tsq);
    }
}

/*
 * Write buffer destructor automatically called from kfree_skb.
 * We cant xmit new skbs from this context, as we might already
 * hold qdisc lock.
 */
static void tcp_wfree(struct sk_buff *skb)
{
    struct sock *sk = skb->sk;
    struct tcp_sock *tp = tcp_sk(sk);

    if (test_and_clear_bit(TSQ_THROTTLED, &tp->tsq_flags) &&
        !test_and_set_bit(TSQ_QUEUED, &tp->tsq_flags)) {
        unsigned long flags;
        struct tsq_tasklet *tsq;

        /* Keep a ref on socket.
         * This last ref will be released in tcp_tasklet_func()
         */
        atomic_sub(skb->truesize - 1, &sk->sk_wmem_alloc);

        /* queue this socket to tasklet queue */
        local_irq_save(flags);
        tsq = &__get_cpu_var(tsq_tasklet);
        list_add(&tp->tsq_node, &tsq->head);
        tasklet_schedule(&tsq->tasklet);
        local_irq_restore(flags);
    } else {
        sock_wfree(skb);
    }
}

/* This routine actually transmits TCP packets queued in by
 * tcp_do_sendmsg().  This is used by both the initial
 * transmission and possible later retransmissions.
 * All SKB's seen here are completely headerless.  It is our
 * job to build the TCP header, and pass the packet down to
 * IP so it can do the same plus pass the packet off to the
 * device.
 *
 * We are working here with either a clone of the original
 * SKB, or a fresh unique copy made by the retransmit engine.
 */
static int tcp_transmit_skb(struct sock *sk, struct sk_buff *skb, int clone_it,
                gfp_t gfp_mask)
{
    const struct inet_connection_sock *icsk = inet_csk(sk);
    struct inet_sock *inet;
    struct tcp_sock *tp;
    struct tcp_skb_cb *tcb;
    struct tcp_out_options opts;
    unsigned int tcp_options_size, tcp_header_size;
    struct tcp_md5sig_key *md5;
    struct tcphdr *th;
    int err;

    BUG_ON(!skb || !tcp_skb_pcount(skb));

    /* If congestion control is doing timestamping, we must
     * take such a timestamp before we potentially clone/copy.
     */
    if (icsk->icsk_ca_ops->flags & TCP_CONG_RTT_STAMP)
        __net_timestamp(skb);

    if (likely(clone_it)) {
        if (unlikely(skb_cloned(skb)))
            skb = pskb_copy(skb, gfp_mask);
        else
            skb = skb_clone(skb, gfp_mask);
        if (unlikely(!skb))
            return -ENOBUFS;
    }

    inet = inet_sk(sk);
    tp = tcp_sk(sk);
    tcb = TCP_SKB_CB(skb);
    memset(&opts, 0, sizeof(opts));

    if (unlikely(tcb->tcp_flags & TCPHDR_SYN))
        tcp_options_size = tcp_syn_options(sk, skb, &opts, &md5);
    else
        tcp_options_size = tcp_established_options(sk, skb, &opts,
                               &md5);
    tcp_header_size = tcp_options_size + sizeof(struct tcphdr);

    if (tcp_packets_in_flight(tp) == 0) {
        tcp_ca_event(sk, CA_EVENT_TX_START);
        skb->ooo_okay = 1;
    } else
        skb->ooo_okay = 0;

    skb_push(skb, tcp_header_size);
    skb_reset_transport_header(skb);

    skb_orphan(skb);
    skb->sk = sk;
    skb->destructor = (sysctl_tcp_limit_output_bytes > 0) ?
              tcp_wfree : sock_wfree;
    atomic_add(skb->truesize, &sk->sk_wmem_alloc);

    /* Build TCP header and checksum it. */
    th = tcp_hdr(skb);
    th->source      = inet->inet_sport;
    th->dest        = inet->inet_dport;
    th->seq         = htonl(tcb->seq);
    th->ack_seq     = htonl(tp->rcv_nxt);
    *(((__be16 *)th) + 6)   = htons(((tcp_header_size >> 2) << 12) |
                    tcb->tcp_flags);

    if (unlikely(tcb->tcp_flags & TCPHDR_SYN)) {
        /* RFC1323: The window in SYN & SYN/ACK segments
         * is never scaled.
         */
        th->window  = htons(min(tp->rcv_wnd, 65535U));
    } else {
        th->window  = htons(tcp_select_window(sk));
    }
    th->check       = 0;
    th->urg_ptr     = 0;

    /* The urg_mode check is necessary during a below snd_una win probe */
    if (unlikely(tcp_urg_mode(tp) && before(tcb->seq, tp->snd_up))) {
        if (before(tp->snd_up, tcb->seq + 0x10000)) {
            th->urg_ptr = htons(tp->snd_up - tcb->seq);
            th->urg = 1;
        } else if (after(tcb->seq + 0xFFFF, tp->snd_nxt)) {
            th->urg_ptr = htons(0xFFFF);
            th->urg = 1;
        }
    }

    tcp_options_write((__be32 *)(th + 1), tp, &opts);
    if (likely((tcb->tcp_flags & TCPHDR_SYN) == 0))
        TCP_ECN_send(sk, skb, tcp_header_size);

#ifdef CONFIG_TCP_MD5SIG
    /* Calculate the MD5 hash, as we have all we need now */
    if (md5) {
        sk_nocaps_add(sk, NETIF_F_GSO_MASK);
        tp->af_specific->calc_md5_hash(opts.hash_location,
                           md5, sk, NULL, skb);
    }
#endif

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

    if (likely(tcb->tcp_flags & TCPHDR_ACK))
        tcp_event_ack_sent(sk, tcp_skb_pcount(skb));

    if (skb->len != tcp_header_size)
        tcp_event_data_sent(tp, sk);

    if (after(tcb->end_seq, tp->snd_nxt) || tcb->seq == tcb->end_seq)
        TCP_ADD_STATS(sock_net(sk), TCP_MIB_OUTSEGS,
                  tcp_skb_pcount(skb));

    err = icsk->icsk_af_ops->queue_xmit(skb, &inet->cork.fl);
    if (likely(err <= 0))
        return err;

    tcp_enter_cwr(sk, 1);

    return net_xmit_eval(err);
}

/* This routine just queues the buffer for sending.
 *
 * NOTE: probe0 timer is not checked, do not forget tcp_push_pending_frames,
 * otherwise socket can stall.
 */
static void tcp_queue_skb(struct sock *sk, struct sk_buff *skb)
{
    struct tcp_sock *tp = tcp_sk(sk);

    /* Advance write_seq and place onto the write_queue. */
    tp->write_seq = TCP_SKB_CB(skb)->end_seq;
    skb_header_release(skb);
    tcp_add_write_queue_tail(sk, skb);
    sk->sk_wmem_queued += skb->truesize;
    sk_mem_charge(sk, skb->truesize);
}

/* Initialize TSO segments for a packet. */
static void tcp_set_skb_tso_segs(const struct sock *sk, struct sk_buff *skb,
                 unsigned int mss_now)
{
    if (skb->len <= mss_now || !sk_can_gso(sk) ||
        skb->ip_summed == CHECKSUM_NONE) {
        /* Avoid the costly divide in the normal
         * non-TSO case.
         */
        skb_shinfo(skb)->gso_segs = 1;
        skb_shinfo(skb)->gso_size = 0;
        skb_shinfo(skb)->gso_type = 0;
    } else {
        skb_shinfo(skb)->gso_segs = DIV_ROUND_UP(skb->len, mss_now);
        skb_shinfo(skb)->gso_size = mss_now;
        skb_shinfo(skb)->gso_type = sk->sk_gso_type;
    }
}

/* When a modification to fackets out becomes necessary, we need to check
 * skb is counted to fackets_out or not.
 */
static void tcp_adjust_fackets_out(struct sock *sk, const struct sk_buff *skb,
                   int decr)
{
    struct tcp_sock *tp = tcp_sk(sk);

    if (!tp->sacked_out || tcp_is_reno(tp))
        return;

    if (after(tcp_highest_sack_seq(tp), TCP_SKB_CB(skb)->seq))
        tp->fackets_out -= decr;
}

/* Pcount in the middle of the write queue got changed, we need to do various
 * tweaks to fix counters
 */
static void tcp_adjust_pcount(struct sock *sk, const struct sk_buff *skb, int decr)
{
    struct tcp_sock *tp = tcp_sk(sk);

    tp->packets_out -= decr;

    if (TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_ACKED)
        tp->sacked_out -= decr;
    if (TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_RETRANS)
        tp->retrans_out -= decr;
    if (TCP_SKB_CB(skb)->sacked & TCPCB_LOST)
        tp->lost_out -= decr;

    /* Reno case is special. Sigh... */
    if (tcp_is_reno(tp) && decr > 0)
        tp->sacked_out -= min_t(u32, tp->sacked_out, decr);

    tcp_adjust_fackets_out(sk, skb, decr);

    if (tp->lost_skb_hint &&
        before(TCP_SKB_CB(skb)->seq, TCP_SKB_CB(tp->lost_skb_hint)->seq) &&
        (tcp_is_fack(tp) || (TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_ACKED)))
        tp->lost_cnt_hint -= decr;

    tcp_verify_left_out(tp);
}

/* Function to create two new TCP segments.  Shrinks the given segment
 * to the specified size and appends a new segment with the rest of the
 * packet to the list.  This won't be called frequently, I hope.
 * Remember, these are still headerless SKBs at this point.
 */
int tcp_fragment(struct sock *sk, struct sk_buff *skb, u32 len,
         unsigned int mss_now)
{
    struct tcp_sock *tp = tcp_sk(sk);
    struct sk_buff *buff;
    int nsize, old_factor;
    int nlen;
    u8 flags;

    if (WARN_ON(len > skb->len))
        return -EINVAL;

    nsize = skb_headlen(skb) - len;
    if (nsize < 0)
        nsize = 0;

    if (skb_cloned(skb) &&
        skb_is_nonlinear(skb) &&
        pskb_expand_head(skb, 0, 0, GFP_ATOMIC))
        return -ENOMEM;

    /* Get a new skb... force flag on. */
    buff = sk_stream_alloc_skb(sk, nsize, GFP_ATOMIC);
    if (buff == NULL)
        return -ENOMEM; /* We'll just try again later. */

    sk->sk_wmem_queued += buff->truesize;
    sk_mem_charge(sk, buff->truesize);
    nlen = skb->len - len - nsize;
    buff->truesize += nlen;
    skb->truesize -= nlen;

    /* Correct the sequence numbers. */
    TCP_SKB_CB(buff)->seq = TCP_SKB_CB(skb)->seq + len;
    TCP_SKB_CB(buff)->end_seq = TCP_SKB_CB(skb)->end_seq;
    TCP_SKB_CB(skb)->end_seq = TCP_SKB_CB(buff)->seq;

    /* PSH and FIN should only be set in the second packet. */
    flags = TCP_SKB_CB(skb)->tcp_flags;
    TCP_SKB_CB(skb)->tcp_flags = flags & ~(TCPHDR_FIN | TCPHDR_PSH);
    TCP_SKB_CB(buff)->tcp_flags = flags;
    TCP_SKB_CB(buff)->sacked = TCP_SKB_CB(skb)->sacked;

    if (!skb_shinfo(skb)->nr_frags && skb->ip_summed != CHECKSUM_PARTIAL) {
        /* Copy and checksum data tail into the new buffer. */
        buff->csum = csum_partial_copy_nocheck(skb->data + len,
                               skb_put(buff, nsize),
                               nsize, 0);

        skb_trim(skb, len);

        skb->csum = csum_block_sub(skb->csum, buff->csum, len);
    } else {
        skb->ip_summed = CHECKSUM_PARTIAL;
        skb_split(skb, buff, len);
    }

    buff->ip_summed = skb->ip_summed;

    /* Looks stupid, but our code really uses when of
     * skbs, which it never sent before. --ANK
     */
    TCP_SKB_CB(buff)->when = TCP_SKB_CB(skb)->when;
    buff->tstamp = skb->tstamp;

    old_factor = tcp_skb_pcount(skb);

    /* Fix up tso_factor for both original and new SKB.  */
    tcp_set_skb_tso_segs(sk, skb, mss_now);
    tcp_set_skb_tso_segs(sk, buff, mss_now);

    /* If this packet has been sent out already, we must
     * adjust the various packet counters.
     */
    if (!before(tp->snd_nxt, TCP_SKB_CB(buff)->end_seq)) {
        int diff = old_factor - tcp_skb_pcount(skb) -
            tcp_skb_pcount(buff);

        if (diff)
            tcp_adjust_pcount(sk, skb, diff);
    }

    /* Link BUFF into the send queue. */
    skb_header_release(buff);
    tcp_insert_write_queue_after(skb, buff, sk);

    return 0;
}

/* This is similar to __pskb_pull_head() (it will go to core/skbuff.c
 * eventually). The difference is that pulled data not copied, but
 * immediately discarded.
 */
static void __pskb_trim_head(struct sk_buff *skb, int len)
{
    int i, k, eat;

    eat = min_t(int, len, skb_headlen(skb));
    if (eat) {
        __skb_pull(skb, eat);
        skb->avail_size -= eat;
        len -= eat;
        if (!len)
            return;
    }
    eat = len;
    k = 0;
    for (i = 0; i < skb_shinfo(skb)->nr_frags; i++) {
        int size = skb_frag_size(&skb_shinfo(skb)->frags[i]);

        if (size <= eat) {
            skb_frag_unref(skb, i);
            eat -= size;
        } else {
            skb_shinfo(skb)->frags[k] = skb_shinfo(skb)->frags[i];
            if (eat) {
                skb_shinfo(skb)->frags[k].page_offset += eat;
                skb_frag_size_sub(&skb_shinfo(skb)->frags[k], eat);
                eat = 0;
            }
            k++;
        }
    }
    skb_shinfo(skb)->nr_frags = k;

    skb_reset_tail_pointer(skb);
    skb->data_len -= len;
    skb->len = skb->data_len;
}

/* Remove acked data from a packet in the transmit queue. */
int tcp_trim_head(struct sock *sk, struct sk_buff *skb, u32 len)
{
    if (skb_cloned(skb) && pskb_expand_head(skb, 0, 0, GFP_ATOMIC))
        return -ENOMEM;

    __pskb_trim_head(skb, len);

    TCP_SKB_CB(skb)->seq += len;
    skb->ip_summed = CHECKSUM_PARTIAL;

    skb->truesize        -= len;
    sk->sk_wmem_queued   -= len;
    sk_mem_uncharge(sk, len);
    sock_set_flag(sk, SOCK_QUEUE_SHRUNK);

    /* Any change of skb->len requires recalculation of tso factor. */
    if (tcp_skb_pcount(skb) > 1)
        tcp_set_skb_tso_segs(sk, skb, tcp_skb_mss(skb));

    return 0;
}

/* Calculate MSS. Not accounting for SACKs here.  */
int tcp_mtu_to_mss(struct sock *sk, int pmtu)
{
    const struct tcp_sock *tp = tcp_sk(sk);
    const struct inet_connection_sock *icsk = inet_csk(sk);
    int mss_now;

    /* Calculate base mss without TCP options:
       It is MMS_S - sizeof(tcphdr) of rfc1122
     */
    mss_now = pmtu - icsk->icsk_af_ops->net_header_len - sizeof(struct tcphdr);

    /* IPv6 adds a frag_hdr in case RTAX_FEATURE_ALLFRAG is set */
    if (icsk->icsk_af_ops->net_frag_header_len) {
        const struct dst_entry *dst = __sk_dst_get(sk);

        if (dst && dst_allfrag(dst))
            mss_now -= icsk->icsk_af_ops->net_frag_header_len;
    }

    /* Clamp it (mss_clamp does not include tcp options) */
    if (mss_now > tp->rx_opt.mss_clamp)
        mss_now = tp->rx_opt.mss_clamp;

    /* Now subtract optional transport overhead */
    mss_now -= icsk->icsk_ext_hdr_len;

    /* Then reserve room for full set of TCP options and 8 bytes of data */
    if (mss_now < 48)
        mss_now = 48;

    /* Now subtract TCP options size, not including SACKs */
    mss_now -= tp->tcp_header_len - sizeof(struct tcphdr);

    return mss_now;
}

/* Inverse of above */
int tcp_mss_to_mtu(struct sock *sk, int mss)
{
    const struct tcp_sock *tp = tcp_sk(sk);
    const struct inet_connection_sock *icsk = inet_csk(sk);
    int mtu;

    mtu = mss +
          tp->tcp_header_len +
          icsk->icsk_ext_hdr_len +
          icsk->icsk_af_ops->net_header_len;

    /* IPv6 adds a frag_hdr in case RTAX_FEATURE_ALLFRAG is set */
    if (icsk->icsk_af_ops->net_frag_header_len) {
        const struct dst_entry *dst = __sk_dst_get(sk);

        if (dst && dst_allfrag(dst))
            mtu += icsk->icsk_af_ops->net_frag_header_len;
    }
    return mtu;
}

/* MTU probing init per socket */
void tcp_mtup_init(struct sock *sk)
{
    struct tcp_sock *tp = tcp_sk(sk);
    struct inet_connection_sock *icsk = inet_csk(sk);

    icsk->icsk_mtup.enabled = sysctl_tcp_mtu_probing > 1;
    icsk->icsk_mtup.search_high = tp->rx_opt.mss_clamp + sizeof(struct tcphdr) +
                   icsk->icsk_af_ops->net_header_len;
    icsk->icsk_mtup.search_low = tcp_mss_to_mtu(sk, sysctl_tcp_base_mss);
    icsk->icsk_mtup.probe_size = 0;
}
EXPORT_SYMBOL(tcp_mtup_init);

/* This function synchronize snd mss to current pmtu/exthdr set.

   tp->rx_opt.user_mss is mss set by user by TCP_MAXSEG. It does NOT counts
   for TCP options, but includes only bare TCP header.

   tp->rx_opt.mss_clamp is mss negotiated at connection setup.
   It is minimum of user_mss and mss received with SYN.
   It also does not include TCP options.

   inet_csk(sk)->icsk_pmtu_cookie is last pmtu, seen by this function.

   tp->mss_cache is current effective sending mss, including
   all tcp options except for SACKs. It is evaluated,
   taking into account current pmtu, but never exceeds
   tp->rx_opt.mss_clamp.

   NOTE1. rfc1122 clearly states that advertised MSS
   DOES NOT include either tcp or ip options.

   NOTE2. inet_csk(sk)->icsk_pmtu_cookie and tp->mss_cache
   are READ ONLY outside this function.     --ANK (980731)
 */
unsigned int tcp_sync_mss(struct sock *sk, u32 pmtu)
{
    struct tcp_sock *tp = tcp_sk(sk);
    struct inet_connection_sock *icsk = inet_csk(sk);
    int mss_now;

    if (icsk->icsk_mtup.search_high > pmtu)
        icsk->icsk_mtup.search_high = pmtu;

    mss_now = tcp_mtu_to_mss(sk, pmtu);
    mss_now = tcp_bound_to_half_wnd(tp, mss_now);

    /* And store cached results */
    icsk->icsk_pmtu_cookie = pmtu;
    if (icsk->icsk_mtup.enabled)
        mss_now = min(mss_now, tcp_mtu_to_mss(sk, icsk->icsk_mtup.search_low));
    tp->mss_cache = mss_now;

    return mss_now;
}
EXPORT_SYMBOL(tcp_sync_mss);

/* Compute the current effective MSS, taking SACKs and IP options,
 * and even PMTU discovery events into account.
 */
unsigned int tcp_current_mss(struct sock *sk)
{
    const struct tcp_sock *tp = tcp_sk(sk);
    const struct dst_entry *dst = __sk_dst_get(sk);
    u32 mss_now;
    unsigned int header_len;
    struct tcp_out_options opts;
    struct tcp_md5sig_key *md5;

    mss_now = tp->mss_cache;

    if (dst) {
        u32 mtu = dst_mtu(dst);
        if (mtu != inet_csk(sk)->icsk_pmtu_cookie)
            mss_now = tcp_sync_mss(sk, mtu);
    }

    header_len = tcp_established_options(sk, NULL, &opts, &md5) +
             sizeof(struct tcphdr);
    /* The mss_cache is sized based on tp->tcp_header_len, which assumes
     * some common options. If this is an odd packet (because we have SACK
     * blocks etc) then our calculated header_len will be different, and
     * we have to adjust mss_now correspondingly */
    if (header_len != tp->tcp_header_len) {
        int delta = (int) header_len - tp->tcp_header_len;
        mss_now -= delta;
    }

    return mss_now;
}

/* Congestion window validation. (RFC2861) */
static void tcp_cwnd_validate(struct sock *sk)
{
    struct tcp_sock *tp = tcp_sk(sk);

    if (tp->packets_out >= tp->snd_cwnd) {
        /* Network is feed fully. */
        tp->snd_cwnd_used = 0;
        tp->snd_cwnd_stamp = tcp_time_stamp;
    } else {
        /* Network starves. */
        if (tp->packets_out > tp->snd_cwnd_used)
            tp->snd_cwnd_used = tp->packets_out;

        if (sysctl_tcp_slow_start_after_idle &&
            (s32)(tcp_time_stamp - tp->snd_cwnd_stamp) >= inet_csk(sk)->icsk_rto)
            tcp_cwnd_application_limited(sk);
    }
}

/* Returns the portion of skb which can be sent right away without
 * introducing MSS oddities to segment boundaries. In rare cases where
 * mss_now != mss_cache, we will request caller to create a small skb
 * per input skb which could be mostly avoided here (if desired).
 *
 * We explicitly want to create a request for splitting write queue tail
 * to a small skb for Nagle purposes while avoiding unnecessary modulos,
 * thus all the complexity (cwnd_len is always MSS multiple which we
 * return whenever allowed by the other factors). Basically we need the
 * modulo only when the receiver window alone is the limiting factor or
 * when we would be allowed to send the split-due-to-Nagle skb fully.
 */
static unsigned int tcp_mss_split_point(const struct sock *sk, const struct sk_buff *skb,
                    unsigned int mss_now, unsigned int max_segs)
{
    const struct tcp_sock *tp = tcp_sk(sk);
    u32 needed, window, max_len;

    window = tcp_wnd_end(tp) - TCP_SKB_CB(skb)->seq;
    max_len = mss_now * max_segs;

    if (likely(max_len <= window && skb != tcp_write_queue_tail(sk)))
        return max_len;

    needed = min(skb->len, window);

    if (max_len <= needed)
        return max_len;

    return needed - needed % mss_now;
}

/* Can at least one segment of SKB be sent right now, according to the
 * congestion window rules?  If so, return how many segments are allowed.
 */
static inline unsigned int tcp_cwnd_test(const struct tcp_sock *tp,
                     const struct sk_buff *skb)
{
    u32 in_flight, cwnd;

    /* Don't be strict about the congestion window for the final FIN.  */
    if ((TCP_SKB_CB(skb)->tcp_flags & TCPHDR_FIN) &&
        tcp_skb_pcount(skb) == 1)
        return 1;

    in_flight = tcp_packets_in_flight(tp);
    cwnd = tp->snd_cwnd;
    if (in_flight < cwnd)
        return (cwnd - in_flight);

    return 0;
}

/* Initialize TSO state of a skb.
 * This must be invoked the first time we consider transmitting
 * SKB onto the wire.
 */
static int tcp_init_tso_segs(const struct sock *sk, struct sk_buff *skb,
                 unsigned int mss_now)
{
    int tso_segs = tcp_skb_pcount(skb);

    if (!tso_segs || (tso_segs > 1 && tcp_skb_mss(skb) != mss_now)) {
        tcp_set_skb_tso_segs(sk, skb, mss_now);
        tso_segs = tcp_skb_pcount(skb);
    }
    return tso_segs;
}

/* Minshall's variant of the Nagle send check. */
static inline bool tcp_minshall_check(const struct tcp_sock *tp)
{
    return after(tp->snd_sml, tp->snd_una) &&
        !after(tp->snd_sml, tp->snd_nxt);
}

/* Return false, if packet can be sent now without violation Nagle's rules:
 * 1. It is full sized.
 * 2. Or it contains FIN. (already checked by caller)
 * 3. Or TCP_CORK is not set, and TCP_NODELAY is set.
 * 4. Or TCP_CORK is not set, and all sent packets are ACKed.
 *    With Minshall's modification: all sent small packets are ACKed.
 */
static inline bool tcp_nagle_check(const struct tcp_sock *tp,
                  const struct sk_buff *skb,
                  unsigned int mss_now, int nonagle)
{
    return skb->len < mss_now &&
        ((nonagle & TCP_NAGLE_CORK) ||
         (!nonagle && tp->packets_out && tcp_minshall_check(tp)));
}

/* Return true if the Nagle test allows this packet to be
 * sent now.
 */
static inline bool tcp_nagle_test(const struct tcp_sock *tp, const struct sk_buff *skb,
                  unsigned int cur_mss, int nonagle)
{
    /* Nagle rule does not apply to frames, which sit in the middle of the
     * write_queue (they have no chances to get new data).
     *
     * This is implemented in the callers, where they modify the 'nonagle'
     * argument based upon the location of SKB in the send queue.
     */
    if (nonagle & TCP_NAGLE_PUSH)
        return true;

    /* Don't use the nagle rule for urgent data (or for the final FIN).
     * Nagle can be ignored during F-RTO too (see RFC4138).
     */
    if (tcp_urg_mode(tp) || (tp->frto_counter == 2) ||
        (TCP_SKB_CB(skb)->tcp_flags & TCPHDR_FIN))
        return true;

    if (!tcp_nagle_check(tp, skb, cur_mss, nonagle))
        return true;

    return false;
}

/* Does at least the first segment of SKB fit into the send window? */
static bool tcp_snd_wnd_test(const struct tcp_sock *tp,
                 const struct sk_buff *skb,
                 unsigned int cur_mss)
{
    u32 end_seq = TCP_SKB_CB(skb)->end_seq;

    if (skb->len > cur_mss)
        end_seq = TCP_SKB_CB(skb)->seq + cur_mss;

    return !after(end_seq, tcp_wnd_end(tp));
}

/* This checks if the data bearing packet SKB (usually tcp_send_head(sk))
 * should be put on the wire right now.  If so, it returns the number of
 * packets allowed by the congestion window.
 */
static unsigned int tcp_snd_test(const struct sock *sk, struct sk_buff *skb,
                 unsigned int cur_mss, int nonagle)
{
    const struct tcp_sock *tp = tcp_sk(sk);
    unsigned int cwnd_quota;

    tcp_init_tso_segs(sk, skb, cur_mss);

    if (!tcp_nagle_test(tp, skb, cur_mss, nonagle))
        return 0;

    cwnd_quota = tcp_cwnd_test(tp, skb);
    if (cwnd_quota && !tcp_snd_wnd_test(tp, skb, cur_mss))
        cwnd_quota = 0;

    return cwnd_quota;
}

/* Test if sending is allowed right now. */
bool tcp_may_send_now(struct sock *sk)
{
    const struct tcp_sock *tp = tcp_sk(sk);
    struct sk_buff *skb = tcp_send_head(sk);

    return skb &&
        tcp_snd_test(sk, skb, tcp_current_mss(sk),
                 (tcp_skb_is_last(sk, skb) ?
                  tp->nonagle : TCP_NAGLE_PUSH));
}

/* Trim TSO SKB to LEN bytes, put the remaining data into a new packet
 * which is put after SKB on the list.  It is very much like
 * tcp_fragment() except that it may make several kinds of assumptions
 * in order to speed up the splitting operation.  In particular, we
 * know that all the data is in scatter-gather pages, and that the
 * packet has never been sent out before (and thus is not cloned).
 */
static int tso_fragment(struct sock *sk, struct sk_buff *skb, unsigned int len,
            unsigned int mss_now, gfp_t gfp)
{
    struct sk_buff *buff;
    int nlen = skb->len - len;
    u8 flags;

    /* All of a TSO frame must be composed of paged data.  */
    if (skb->len != skb->data_len)
        return tcp_fragment(sk, skb, len, mss_now);

    buff = sk_stream_alloc_skb(sk, 0, gfp);
    if (unlikely(buff == NULL))
        return -ENOMEM;

    sk->sk_wmem_queued += buff->truesize;
    sk_mem_charge(sk, buff->truesize);
    buff->truesize += nlen;
    skb->truesize -= nlen;

    /* Correct the sequence numbers. */
    TCP_SKB_CB(buff)->seq = TCP_SKB_CB(skb)->seq + len;
    TCP_SKB_CB(buff)->end_seq = TCP_SKB_CB(skb)->end_seq;
    TCP_SKB_CB(skb)->end_seq = TCP_SKB_CB(buff)->seq;

    /* PSH and FIN should only be set in the second packet. */
    flags = TCP_SKB_CB(skb)->tcp_flags;
    TCP_SKB_CB(skb)->tcp_flags = flags & ~(TCPHDR_FIN | TCPHDR_PSH);
    TCP_SKB_CB(buff)->tcp_flags = flags;

    /* This packet was never sent out yet, so no SACK bits. */
    TCP_SKB_CB(buff)->sacked = 0;

    buff->ip_summed = skb->ip_summed = CHECKSUM_PARTIAL;
    skb_split(skb, buff, len);

    /* Fix up tso_factor for both original and new SKB.  */
    tcp_set_skb_tso_segs(sk, skb, mss_now);
    tcp_set_skb_tso_segs(sk, buff, mss_now);

    /* Link BUFF into the send queue. */
    skb_header_release(buff);
    tcp_insert_write_queue_after(skb, buff, sk);

    return 0;
}

/* Try to defer sending, if possible, in order to minimize the amount
 * of TSO splitting we do.  View it as a kind of TSO Nagle test.
 *
 * This algorithm is from John Heffner.
 */
static bool tcp_tso_should_defer(struct sock *sk, struct sk_buff *skb)
{
    struct tcp_sock *tp = tcp_sk(sk);
    const struct inet_connection_sock *icsk = inet_csk(sk);
    u32 send_win, cong_win, limit, in_flight;
    int win_divisor;

    if (TCP_SKB_CB(skb)->tcp_flags & TCPHDR_FIN)
        goto send_now;

    if (icsk->icsk_ca_state != TCP_CA_Open)
        goto send_now;

    /* Defer for less than two clock ticks. */
    if (tp->tso_deferred &&
        (((u32)jiffies << 1) >> 1) - (tp->tso_deferred >> 1) > 1)
        goto send_now;

    in_flight = tcp_packets_in_flight(tp);

    BUG_ON(tcp_skb_pcount(skb) <= 1 || (tp->snd_cwnd <= in_flight));

    send_win = tcp_wnd_end(tp) - TCP_SKB_CB(skb)->seq;

    /* From in_flight test above, we know that cwnd > in_flight.  */
    cong_win = (tp->snd_cwnd - in_flight) * tp->mss_cache;

    limit = min(send_win, cong_win);

    /* If a full-sized TSO skb can be sent, do it. */
    if (limit >= min_t(unsigned int, sk->sk_gso_max_size,
               sk->sk_gso_max_segs * tp->mss_cache))
        goto send_now;

    /* Middle in queue won't get any more data, full sendable already? */
    if ((skb != tcp_write_queue_tail(sk)) && (limit >= skb->len))
        goto send_now;

    win_divisor = ACCESS_ONCE(sysctl_tcp_tso_win_divisor);
    if (win_divisor) {
        u32 chunk = min(tp->snd_wnd, tp->snd_cwnd * tp->mss_cache);

        /* If at least some fraction of a window is available,
         * just use it.
         */
        chunk /= win_divisor;
        if (limit >= chunk)
            goto send_now;
    } else {
        /* Different approach, try not to defer past a single
         * ACK.  Receiver should ACK every other full sized
         * frame, so if we have space for more than 3 frames
         * then send now.
         */
        if (limit > tcp_max_tso_deferred_mss(tp) * tp->mss_cache)
            goto send_now;
    }

    /* Ok, it looks like it is advisable to defer.  */
    tp->tso_deferred = 1 | (jiffies << 1);

    return true;

send_now:
    tp->tso_deferred = 0;
    return false;
}

/* Create a new MTU probe if we are ready.
 * MTU probe is regularly attempting to increase the path MTU by
 * deliberately sending larger packets.  This discovers routing
 * changes resulting in larger path MTUs.
 *
 * Returns 0 if we should wait to probe (no cwnd available),
 *         1 if a probe was sent,
 *         -1 otherwise
 */
static int tcp_mtu_probe(struct sock *sk)
{
    struct tcp_sock *tp = tcp_sk(sk);
    struct inet_connection_sock *icsk = inet_csk(sk);
    struct sk_buff *skb, *nskb, *next;
    int len;
    int probe_size;
    int size_needed;
    int copy;
    int mss_now;

    /* Not currently probing/verifying,
     * not in recovery,
     * have enough cwnd, and
     * not SACKing (the variable headers throw things off) */
    if (!icsk->icsk_mtup.enabled ||
        icsk->icsk_mtup.probe_size ||
        inet_csk(sk)->icsk_ca_state != TCP_CA_Open ||
        tp->snd_cwnd < 11 ||
        tp->rx_opt.num_sacks || tp->rx_opt.dsack)
        return -1;

    /* Very simple search strategy: just double the MSS. */
    mss_now = tcp_current_mss(sk);
    probe_size = 2 * tp->mss_cache;
    size_needed = probe_size + (tp->reordering + 1) * tp->mss_cache;
    if (probe_size > tcp_mtu_to_mss(sk, icsk->icsk_mtup.search_high)) {
        /* TODO: set timer for probe_converge_event */
        return -1;
    }

    /* Have enough data in the send queue to probe? */
    if (tp->write_seq - tp->snd_nxt < size_needed)
        return -1;

    if (tp->snd_wnd < size_needed)
        return -1;
    if (after(tp->snd_nxt + size_needed, tcp_wnd_end(tp)))
        return 0;

    /* Do we need to wait to drain cwnd? With none in flight, don't stall */
    if (tcp_packets_in_flight(tp) + 2 > tp->snd_cwnd) {
        if (!tcp_packets_in_flight(tp))
            return -1;
        else
            return 0;
    }

    /* We're allowed to probe.  Build it now. */
    if ((nskb = sk_stream_alloc_skb(sk, probe_size, GFP_ATOMIC)) == NULL)
        return -1;
    sk->sk_wmem_queued += nskb->truesize;
    sk_mem_charge(sk, nskb->truesize);

    skb = tcp_send_head(sk);

    TCP_SKB_CB(nskb)->seq = TCP_SKB_CB(skb)->seq;
    TCP_SKB_CB(nskb)->end_seq = TCP_SKB_CB(skb)->seq + probe_size;
    TCP_SKB_CB(nskb)->tcp_flags = TCPHDR_ACK;
    TCP_SKB_CB(nskb)->sacked = 0;
    nskb->csum = 0;
    nskb->ip_summed = skb->ip_summed;

    tcp_insert_write_queue_before(nskb, skb, sk);

    len = 0;
    tcp_for_write_queue_from_safe(skb, next, sk) {
        copy = min_t(int, skb->len, probe_size - len);
        if (nskb->ip_summed)
            skb_copy_bits(skb, 0, skb_put(nskb, copy), copy);
        else
            nskb->csum = skb_copy_and_csum_bits(skb, 0,
                                skb_put(nskb, copy),
                                copy, nskb->csum);

        if (skb->len <= copy) {
            /* We've eaten all the data from this skb.
             * Throw it away. */
            TCP_SKB_CB(nskb)->tcp_flags |= TCP_SKB_CB(skb)->tcp_flags;
            tcp_unlink_write_queue(skb, sk);
            sk_wmem_free_skb(sk, skb);
        } else {
            TCP_SKB_CB(nskb)->tcp_flags |= TCP_SKB_CB(skb)->tcp_flags &
                           ~(TCPHDR_FIN|TCPHDR_PSH);
            if (!skb_shinfo(skb)->nr_frags) {
                skb_pull(skb, copy);
                if (skb->ip_summed != CHECKSUM_PARTIAL)
                    skb->csum = csum_partial(skb->data,
                                 skb->len, 0);
            } else {
                __pskb_trim_head(skb, copy);
                tcp_set_skb_tso_segs(sk, skb, mss_now);
            }
            TCP_SKB_CB(skb)->seq += copy;
        }

        len += copy;

        if (len >= probe_size)
            break;
    }
    tcp_init_tso_segs(sk, nskb, nskb->len);

    /* We're ready to send.  If this fails, the probe will
     * be resegmented into mss-sized pieces by tcp_write_xmit(). */
    TCP_SKB_CB(nskb)->when = tcp_time_stamp;
    if (!tcp_transmit_skb(sk, nskb, 1, GFP_ATOMIC)) {
        /* Decrement cwnd here because we are sending
         * effectively two packets. */
        tp->snd_cwnd--;
        tcp_event_new_data_sent(sk, nskb);

        icsk->icsk_mtup.probe_size = tcp_mss_to_mtu(sk, nskb->len);
        tp->mtu_probe.probe_seq_start = TCP_SKB_CB(nskb)->seq;
        tp->mtu_probe.probe_seq_end = TCP_SKB_CB(nskb)->end_seq;

        return 1;
    }

    return -1;
}

/* This routine writes packets to the network.  It advances the
 * send_head.  This happens as incoming acks open up the remote
 * window for us.
 *
 * LARGESEND note: !tcp_urg_mode is overkill, only frames between
 * snd_up-64k-mss .. snd_up cannot be large. However, taking into
 * account rare use of URG, this is not a big flaw.
 *
 * Returns true, if no segments are in flight and we have queued segments,
 * but cannot send anything now because of SWS or another problem.
 */
static bool tcp_write_xmit(struct sock *sk, unsigned int mss_now, int nonagle,
               int push_one, gfp_t gfp)
{
    struct tcp_sock *tp = tcp_sk(sk);
    struct sk_buff *skb;
    unsigned int tso_segs, sent_pkts;
    int cwnd_quota;
    int result;

    sent_pkts = 0;

    if (!push_one) {
        /* Do MTU probing. */
        result = tcp_mtu_probe(sk);
        if (!result) {
            return false;
        } else if (result > 0) {
            sent_pkts = 1;
        }
    }

    while ((skb = tcp_send_head(sk))) {
        unsigned int limit;


        tso_segs = tcp_init_tso_segs(sk, skb, mss_now);
        BUG_ON(!tso_segs);

        if (unlikely(tp->repair) && tp->repair_queue == TCP_SEND_QUEUE)
            goto repair; /* Skip network transmission */

        cwnd_quota = tcp_cwnd_test(tp, skb);
        if (!cwnd_quota)
            break;

        if (unlikely(!tcp_snd_wnd_test(tp, skb, mss_now)))
            break;

        if (tso_segs == 1) {
            if (unlikely(!tcp_nagle_test(tp, skb, mss_now,
                             (tcp_skb_is_last(sk, skb) ?
                              nonagle : TCP_NAGLE_PUSH))))
                break;
        } else {
            if (!push_one && tcp_tso_should_defer(sk, skb))
                break;
        }

        /* TSQ : sk_wmem_alloc accounts skb truesize,
         * including skb overhead. But thats OK.
         */
        if (atomic_read(&sk->sk_wmem_alloc) >= sysctl_tcp_limit_output_bytes) {
            set_bit(TSQ_THROTTLED, &tp->tsq_flags);
            break;
        }
        limit = mss_now;
        if (tso_segs > 1 && !tcp_urg_mode(tp))
            limit = tcp_mss_split_point(sk, skb, mss_now,
                            min_t(unsigned int,
                              cwnd_quota,
                              sk->sk_gso_max_segs));

        if (skb->len > limit &&
            unlikely(tso_fragment(sk, skb, limit, mss_now, gfp)))
            break;

        TCP_SKB_CB(skb)->when = tcp_time_stamp;

        if (unlikely(tcp_transmit_skb(sk, skb, 1, gfp)))
            break;

repair:
        /* Advance the send_head.  This one is sent out.
         * This call will increment packets_out.
         */
        tcp_event_new_data_sent(sk, skb);

        tcp_minshall_update(tp, mss_now, skb);
        sent_pkts += tcp_skb_pcount(skb);

        if (push_one)
            break;
    }

    if (likely(sent_pkts)) {
        if (tcp_in_cwnd_reduction(sk))
            tp->prr_out += sent_pkts;
        tcp_cwnd_validate(sk);
        return false;
    }
    return !tp->packets_out && tcp_send_head(sk);
}

/* Push out any pending frames which were held back due to
 * TCP_CORK or attempt at coalescing tiny packets.
 * The socket must be locked by the caller.
 */
void __tcp_push_pending_frames(struct sock *sk, unsigned int cur_mss,
                   int nonagle)
{
    /* If we are closed, the bytes will have to remain here.
     * In time closedown will finish, we empty the write queue and
     * all will be happy.
     */
    if (unlikely(sk->sk_state == TCP_CLOSE))
        return;

    if (tcp_write_xmit(sk, cur_mss, nonagle, 0,
               sk_gfp_atomic(sk, GFP_ATOMIC)))
        tcp_check_probe_timer(sk);
}

/* Send _single_ skb sitting at the send head. This function requires
 * true push pending frames to setup probe timer etc.
 */
void tcp_push_one(struct sock *sk, unsigned int mss_now)
{
    struct sk_buff *skb = tcp_send_head(sk);

    BUG_ON(!skb || skb->len < mss_now);

    tcp_write_xmit(sk, mss_now, TCP_NAGLE_PUSH, 1, sk->sk_allocation);
}

/* This function returns the amount that we can raise the
 * usable window based on the following constraints
 *
 * 1. The window can never be shrunk once it is offered (RFC 793)
 * 2. We limit memory per socket
 *
 * RFC 1122:
 * "the suggested [SWS] avoidance algorithm for the receiver is to keep
 *  RECV.NEXT + RCV.WIN fixed until:
 *  RCV.BUFF - RCV.USER - RCV.WINDOW >= min(1/2 RCV.BUFF, MSS)"
 *
 * i.e. don't raise the right edge of the window until you can raise
 * it at least MSS bytes.
 *
 * Unfortunately, the recommended algorithm breaks header prediction,
 * since header prediction assumes th->window stays fixed.
 *
 * Strictly speaking, keeping th->window fixed violates the receiver
 * side SWS prevention criteria. The problem is that under this rule
 * a stream of single byte packets will cause the right side of the
 * window to always advance by a single byte.
 *
 * Of course, if the sender implements sender side SWS prevention
 * then this will not be a problem.
 *
 * BSD seems to make the following compromise:
 *
 *  If the free space is less than the 1/4 of the maximum
 *  space available and the free space is less than 1/2 mss,
 *  then set the window to 0.
 *  [ Actually, bsd uses MSS and 1/4 of maximal _window_ ]
 *  Otherwise, just prevent the window from shrinking
 *  and from being larger than the largest representable value.
 *
 * This prevents incremental opening of the window in the regime
 * where TCP is limited by the speed of the reader side taking
 * data out of the TCP receive queue. It does nothing about
 * those cases where the window is constrained on the sender side
 * because the pipeline is full.
 *
 * BSD also seems to "accidentally" limit itself to windows that are a
 * multiple of MSS, at least until the free space gets quite small.
 * This would appear to be a side effect of the mbuf implementation.
 * Combining these two algorithms results in the observed behavior
 * of having a fixed window size at almost all times.
 *
 * Below we obtain similar behavior by forcing the offered window to
 * a multiple of the mss when it is feasible to do so.
 *
 * Note, we don't "adjust" for TIMESTAMP or SACK option bytes.
 * Regular options like TIMESTAMP are taken into account.
 */
u32 __tcp_select_window(struct sock *sk)
{
    struct inet_connection_sock *icsk = inet_csk(sk);
    struct tcp_sock *tp = tcp_sk(sk);
    /* MSS for the peer's data.  Previous versions used mss_clamp
     * here.  I don't know if the value based on our guesses
     * of peer's MSS is better for the performance.  It's more correct
     * but may be worse for the performance because of rcv_mss
     * fluctuations.  --SAW  1998/11/1
     */
    int mss = icsk->icsk_ack.rcv_mss;
    int free_space = tcp_space(sk);
    int full_space = min_t(int, tp->window_clamp, tcp_full_space(sk));
    int window;

    if (mss > full_space)
        mss = full_space;

    if (free_space < (full_space >> 1)) {
        icsk->icsk_ack.quick = 0;

        if (sk_under_memory_pressure(sk))
            tp->rcv_ssthresh = min(tp->rcv_ssthresh,
                           4U * tp->advmss);

        if (free_space < mss)
            return 0;
    }

    if (free_space > tp->rcv_ssthresh)
        free_space = tp->rcv_ssthresh;

    /* Don't do rounding if we are using window scaling, since the
     * scaled window will not line up with the MSS boundary anyway.
     */
    window = tp->rcv_wnd;
    if (tp->rx_opt.rcv_wscale) {
        window = free_space;

        /* Advertise enough space so that it won't get scaled away.
         * Import case: prevent zero window announcement if
         * 1<<rcv_wscale > mss.
         */
        if (((window >> tp->rx_opt.rcv_wscale) << tp->rx_opt.rcv_wscale) != window)
            window = (((window >> tp->rx_opt.rcv_wscale) + 1)
                  << tp->rx_opt.rcv_wscale);
    } else {
        /* Get the largest window that is a nice multiple of mss.
         * Window clamp already applied above.
         * If our current window offering is within 1 mss of the
         * free space we just keep it. This prevents the divide
         * and multiply from happening most of the time.
         * We also don't do any window rounding when the free space
         * is too small.
         */
        if (window <= free_space - mss || window > free_space)
            window = (free_space / mss) * mss;
        else if (mss == full_space &&
             free_space > window + (full_space >> 1))
            window = free_space;
    }

    return window;
}

/* Collapses two adjacent SKB's during retransmission. */
static void tcp_collapse_retrans(struct sock *sk, struct sk_buff *skb)
{
    struct tcp_sock *tp = tcp_sk(sk);
    struct sk_buff *next_skb = tcp_write_queue_next(sk, skb);
    int skb_size, next_skb_size;

    skb_size = skb->len;
    next_skb_size = next_skb->len;

    BUG_ON(tcp_skb_pcount(skb) != 1 || tcp_skb_pcount(next_skb) != 1);

    tcp_highest_sack_combine(sk, next_skb, skb);

    tcp_unlink_write_queue(next_skb, sk);

    skb_copy_from_linear_data(next_skb, skb_put(skb, next_skb_size),
                  next_skb_size);

    if (next_skb->ip_summed == CHECKSUM_PARTIAL)
        skb->ip_summed = CHECKSUM_PARTIAL;

    if (skb->ip_summed != CHECKSUM_PARTIAL)
        skb->csum = csum_block_add(skb->csum, next_skb->csum, skb_size);

    /* Update sequence range on original skb. */
    TCP_SKB_CB(skb)->end_seq = TCP_SKB_CB(next_skb)->end_seq;

    /* Merge over control information. This moves PSH/FIN etc. over */
    TCP_SKB_CB(skb)->tcp_flags |= TCP_SKB_CB(next_skb)->tcp_flags;

    /* All done, get rid of second SKB and account for it so
     * packet counting does not break.
     */
    TCP_SKB_CB(skb)->sacked |= TCP_SKB_CB(next_skb)->sacked & TCPCB_EVER_RETRANS;

    /* changed transmit queue under us so clear hints */
    tcp_clear_retrans_hints_partial(tp);
    if (next_skb == tp->retransmit_skb_hint)
        tp->retransmit_skb_hint = skb;

    tcp_adjust_pcount(sk, next_skb, tcp_skb_pcount(next_skb));

    sk_wmem_free_skb(sk, next_skb);
}

/* Check if coalescing SKBs is legal. */
static bool tcp_can_collapse(const struct sock *sk, const struct sk_buff *skb)
{
    if (tcp_skb_pcount(skb) > 1)
        return false;
    /* TODO: SACK collapsing could be used to remove this condition */
    if (skb_shinfo(skb)->nr_frags != 0)
        return false;
    if (skb_cloned(skb))
        return false;
    if (skb == tcp_send_head(sk))
        return false;
    /* Some heurestics for collapsing over SACK'd could be invented */
    if (TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_ACKED)
        return false;

    return true;
}

/* Collapse packets in the retransmit queue to make to create
 * less packets on the wire. This is only done on retransmission.
 */
static void tcp_retrans_try_collapse(struct sock *sk, struct sk_buff *to,
                     int space)
{
    struct tcp_sock *tp = tcp_sk(sk);
    struct sk_buff *skb = to, *tmp;
    bool first = true;

    if (!sysctl_tcp_retrans_collapse)
        return;
    if (TCP_SKB_CB(skb)->tcp_flags & TCPHDR_SYN)
        return;

    tcp_for_write_queue_from_safe(skb, tmp, sk) {
        if (!tcp_can_collapse(sk, skb))
            break;

        space -= skb->len;

        if (first) {
            first = false;
            continue;
        }

        if (space < 0)
            break;
        /* Punt if not enough space exists in the first SKB for
         * the data in the second
         */
        if (skb->len > skb_availroom(to))
            break;

        if (after(TCP_SKB_CB(skb)->end_seq, tcp_wnd_end(tp)))
            break;

        tcp_collapse_retrans(sk, to);
    }
}

/* This retransmits one SKB.  Policy decisions and retransmit queue
 * state updates are done by the caller.  Returns non-zero if an
 * error occurred which prevented the send.
 */
int __tcp_retransmit_skb(struct sock *sk, struct sk_buff *skb)
{
    struct tcp_sock *tp = tcp_sk(sk);
    struct inet_connection_sock *icsk = inet_csk(sk);
    unsigned int cur_mss;

    /* Inconslusive MTU probe */
    if (icsk->icsk_mtup.probe_size) {
        icsk->icsk_mtup.probe_size = 0;
    }

    /* Do not sent more than we queued. 1/4 is reserved for possible
     * copying overhead: fragmentation, tunneling, mangling etc.
     */
    if (atomic_read(&sk->sk_wmem_alloc) >
        min(sk->sk_wmem_queued + (sk->sk_wmem_queued >> 2), sk->sk_sndbuf))
        return -EAGAIN;

    if (before(TCP_SKB_CB(skb)->seq, tp->snd_una)) {
        if (before(TCP_SKB_CB(skb)->end_seq, tp->snd_una))
            BUG();
        if (tcp_trim_head(sk, skb, tp->snd_una - TCP_SKB_CB(skb)->seq))
            return -ENOMEM;
    }

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

    cur_mss = tcp_current_mss(sk);

    /* If receiver has shrunk his window, and skb is out of
     * new window, do not retransmit it. The exception is the
     * case, when window is shrunk to zero. In this case
     * our retransmit serves as a zero window probe.
     */
    if (!before(TCP_SKB_CB(skb)->seq, tcp_wnd_end(tp)) &&
        TCP_SKB_CB(skb)->seq != tp->snd_una)
        return -EAGAIN;

    if (skb->len > cur_mss) {
        if (tcp_fragment(sk, skb, cur_mss, cur_mss))
            return -ENOMEM; /* We'll try again later. */
    } else {
        int oldpcount = tcp_skb_pcount(skb);

        if (unlikely(oldpcount > 1)) {
            tcp_init_tso_segs(sk, skb, cur_mss);
            tcp_adjust_pcount(sk, skb, oldpcount - tcp_skb_pcount(skb));
        }
    }

    tcp_retrans_try_collapse(sk, skb, cur_mss);

    /* Some Solaris stacks overoptimize and ignore the FIN on a
     * retransmit when old data is attached.  So strip it off
     * since it is cheap to do so and saves bytes on the network.
     */
    if (skb->len > 0 &&
        (TCP_SKB_CB(skb)->tcp_flags & TCPHDR_FIN) &&
        tp->snd_una == (TCP_SKB_CB(skb)->end_seq - 1)) {
        if (!pskb_trim(skb, 0)) {
            /* Reuse, even though it does some unnecessary work */
            tcp_init_nondata_skb(skb, TCP_SKB_CB(skb)->end_seq - 1,
                         TCP_SKB_CB(skb)->tcp_flags);
            skb->ip_summed = CHECKSUM_NONE;
        }
    }

    /* Make a copy, if the first transmission SKB clone we made
     * is still in somebody's hands, else make a clone.
     */
    TCP_SKB_CB(skb)->when = tcp_time_stamp;

    /* make sure skb->data is aligned on arches that require it */
    if (unlikely(NET_IP_ALIGN && ((unsigned long)skb->data & 3))) {
        struct sk_buff *nskb = __pskb_copy(skb, MAX_TCP_HEADER,
                           GFP_ATOMIC);
        return nskb ? tcp_transmit_skb(sk, nskb, 0, GFP_ATOMIC) :
                  -ENOBUFS;
    } else {
        return tcp_transmit_skb(sk, skb, 1, GFP_ATOMIC);
    }
}

int tcp_retransmit_skb(struct sock *sk, struct sk_buff *skb)
{
    struct tcp_sock *tp = tcp_sk(sk);
    int err = __tcp_retransmit_skb(sk, skb);

    if (err == 0) {
        /* Update global TCP statistics. */
        TCP_INC_STATS(sock_net(sk), TCP_MIB_RETRANSSEGS);

        tp->total_retrans++;

#if FASTRETRANS_DEBUG > 0
        if (TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_RETRANS) {
            net_dbg_ratelimited("retrans_out leaked\n");
        }
#endif
        if (!tp->retrans_out)
            tp->lost_retrans_low = tp->snd_nxt;
        TCP_SKB_CB(skb)->sacked |= TCPCB_RETRANS;
        tp->retrans_out += tcp_skb_pcount(skb);

        /* Save stamp of the first retransmit. */
        if (!tp->retrans_stamp)
            tp->retrans_stamp = TCP_SKB_CB(skb)->when;

        tp->undo_retrans += tcp_skb_pcount(skb);

        /* snd_nxt is stored to detect loss of retransmitted segment,
         * see tcp_input.c tcp_sacktag_write_queue().
         */
        TCP_SKB_CB(skb)->ack_seq = tp->snd_nxt;
    }
    return err;
}

/* Check if we forward retransmits are possible in the current
 * window/congestion state.
 */
static bool tcp_can_forward_retransmit(struct sock *sk)
{
    const struct inet_connection_sock *icsk = inet_csk(sk);
    const struct tcp_sock *tp = tcp_sk(sk);

    /* Forward retransmissions are possible only during Recovery. */
    if (icsk->icsk_ca_state != TCP_CA_Recovery)
        return false;

    /* No forward retransmissions in Reno are possible. */
    if (tcp_is_reno(tp))
        return false;

    /* Yeah, we have to make difficult choice between forward transmission
     * and retransmission... Both ways have their merits...
     *
     * For now we do not retransmit anything, while we have some new
     * segments to send. In the other cases, follow rule 3 for
     * NextSeg() specified in RFC3517.
     */

    if (tcp_may_send_now(sk))
        return false;

    return true;
}

/* This gets called after a retransmit timeout, and the initially
 * retransmitted data is acknowledged.  It tries to continue
 * resending the rest of the retransmit queue, until either
 * we've sent it all or the congestion window limit is reached.
 * If doing SACK, the first ACK which comes back for a timeout
 * based retransmit packet might feed us FACK information again.
 * If so, we use it to avoid unnecessarily retransmissions.
 */
void tcp_xmit_retransmit_queue(struct sock *sk)
{
    const struct inet_connection_sock *icsk = inet_csk(sk);
    struct tcp_sock *tp = tcp_sk(sk);
    struct sk_buff *skb;
    struct sk_buff *hole = NULL;
    u32 last_lost;
    int mib_idx;
    int fwd_rexmitting = 0;

    if (!tp->packets_out)
        return;

    if (!tp->lost_out)
        tp->retransmit_high = tp->snd_una;

    if (tp->retransmit_skb_hint) {
        skb = tp->retransmit_skb_hint;
        last_lost = TCP_SKB_CB(skb)->end_seq;
        if (after(last_lost, tp->retransmit_high))
            last_lost = tp->retransmit_high;
    } else {
        skb = tcp_write_queue_head(sk);
        last_lost = tp->snd_una;
    }

    tcp_for_write_queue_from(skb, sk) {
        __u8 sacked = TCP_SKB_CB(skb)->sacked;

        if (skb == tcp_send_head(sk))
            break;
        /* we could do better than to assign each time */
        if (hole == NULL)
            tp->retransmit_skb_hint = skb;

        /* Assume this retransmit will generate
         * only one packet for congestion window
         * calculation purposes.  This works because
         * tcp_retransmit_skb() will chop up the
         * packet to be MSS sized and all the
         * packet counting works out.
         */
        if (tcp_packets_in_flight(tp) >= tp->snd_cwnd)
            return;

        if (fwd_rexmitting) {
begin_fwd:
            if (!before(TCP_SKB_CB(skb)->seq, tcp_highest_sack_seq(tp)))
                break;
            mib_idx = LINUX_MIB_TCPFORWARDRETRANS;

        } else if (!before(TCP_SKB_CB(skb)->seq, tp->retransmit_high)) {
            tp->retransmit_high = last_lost;
            if (!tcp_can_forward_retransmit(sk))
                break;
            /* Backtrack if necessary to non-L'ed skb */
            if (hole != NULL) {
                skb = hole;
                hole = NULL;
            }
            fwd_rexmitting = 1;
            goto begin_fwd;

        } else if (!(sacked & TCPCB_LOST)) {
            if (hole == NULL && !(sacked & (TCPCB_SACKED_RETRANS|TCPCB_SACKED_ACKED)))
                hole = skb;
            continue;

        } else {
            last_lost = TCP_SKB_CB(skb)->end_seq;
            if (icsk->icsk_ca_state != TCP_CA_Loss)
                mib_idx = LINUX_MIB_TCPFASTRETRANS;
            else
                mib_idx = LINUX_MIB_TCPSLOWSTARTRETRANS;
        }

        if (sacked & (TCPCB_SACKED_ACKED|TCPCB_SACKED_RETRANS))
            continue;

        if (tcp_retransmit_skb(sk, skb)) {
            NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPRETRANSFAIL);
            return;
        }
        NET_INC_STATS_BH(sock_net(sk), mib_idx);

        if (tcp_in_cwnd_reduction(sk))
            tp->prr_out += tcp_skb_pcount(skb);

        if (skb == tcp_write_queue_head(sk))
            inet_csk_reset_xmit_timer(sk, ICSK_TIME_RETRANS,
                          inet_csk(sk)->icsk_rto,
                          TCP_RTO_MAX);
    }
}

/* Send a fin.  The caller locks the socket for us.  This cannot be
 * allowed to fail queueing a FIN frame under any circumstances.
 */
void tcp_send_fin(struct sock *sk)
{
    struct tcp_sock *tp = tcp_sk(sk);
    struct sk_buff *skb = tcp_write_queue_tail(sk);
    int mss_now;

    /* Optimization, tack on the FIN if we have a queue of
     * unsent frames.  But be careful about outgoing SACKS
     * and IP options.
     */
    mss_now = tcp_current_mss(sk);

    if (tcp_send_head(sk) != NULL) {
        TCP_SKB_CB(skb)->tcp_flags |= TCPHDR_FIN;
        TCP_SKB_CB(skb)->end_seq++;
        tp->write_seq++;
    } else {
        /* Socket is locked, keep trying until memory is available. */
        for (;;) {
            skb = alloc_skb_fclone(MAX_TCP_HEADER,
                           sk->sk_allocation);
            if (skb)
                break;
            yield();
        }

        /* Reserve space for headers and prepare control bits. */
        skb_reserve(skb, MAX_TCP_HEADER);
        /* FIN eats a sequence byte, write_seq advanced by tcp_queue_skb(). */
        tcp_init_nondata_skb(skb, tp->write_seq,
                     TCPHDR_ACK | TCPHDR_FIN);
        tcp_queue_skb(sk, skb);
    }
    __tcp_push_pending_frames(sk, mss_now, TCP_NAGLE_OFF);
}

/* We get here when a process closes a file descriptor (either due to
 * an explicit close() or as a byproduct of exit()'ing) and there
 * was unread data in the receive queue.  This behavior is recommended
 * by RFC 2525, section 2.17.  -DaveM
 */
void tcp_send_active_reset(struct sock *sk, gfp_t priority)
{
    struct sk_buff *skb;

    /* NOTE: No TCP options attached and we never retransmit this. */
    skb = alloc_skb(MAX_TCP_HEADER, priority);
    if (!skb) {
        NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPABORTFAILED);
        return;
    }

    /* Reserve space for headers and prepare control bits. */
    skb_reserve(skb, MAX_TCP_HEADER);
    tcp_init_nondata_skb(skb, tcp_acceptable_seq(sk),
                 TCPHDR_ACK | TCPHDR_RST);
    /* Send it off. */
    TCP_SKB_CB(skb)->when = tcp_time_stamp;
    if (tcp_transmit_skb(sk, skb, 0, priority))
        NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPABORTFAILED);

    TCP_INC_STATS(sock_net(sk), TCP_MIB_OUTRSTS);
}

/* Send a crossed SYN-ACK during socket establishment.
 * WARNING: This routine must only be called when we have already sent
 * a SYN packet that crossed the incoming SYN that caused this routine
 * to get called. If this assumption fails then the initial rcv_wnd
 * and rcv_wscale values will not be correct.
 */
int tcp_send_synack(struct sock *sk)
{
    struct sk_buff *skb;

    skb = tcp_write_queue_head(sk);
    if (skb == NULL || !(TCP_SKB_CB(skb)->tcp_flags & TCPHDR_SYN)) {
        pr_debug("%s: wrong queue state\n", __func__);
        return -EFAULT;
    }
    if (!(TCP_SKB_CB(skb)->tcp_flags & TCPHDR_ACK)) {
        if (skb_cloned(skb)) {
            struct sk_buff *nskb = skb_copy(skb, GFP_ATOMIC);
            if (nskb == NULL)
                return -ENOMEM;
            tcp_unlink_write_queue(skb, sk);
            skb_header_release(nskb);
            __tcp_add_write_queue_head(sk, nskb);
            sk_wmem_free_skb(sk, skb);
            sk->sk_wmem_queued += nskb->truesize;
            sk_mem_charge(sk, nskb->truesize);
            skb = nskb;
        }

        TCP_SKB_CB(skb)->tcp_flags |= TCPHDR_ACK;
        TCP_ECN_send_synack(tcp_sk(sk), skb);
    }
    TCP_SKB_CB(skb)->when = tcp_time_stamp;
    return tcp_transmit_skb(sk, skb, 1, GFP_ATOMIC);
}

struct sk_buff *tcp_make_synack(struct sock *sk, struct dst_entry *dst,
                struct request_sock *req,
                struct request_values *rvp,
                struct tcp_fastopen_cookie *foc)
{
    struct tcp_out_options opts;
    struct tcp_extend_values *xvp = tcp_xv(rvp);
    struct inet_request_sock *ireq = inet_rsk(req);
    struct tcp_sock *tp = tcp_sk(sk);
    const struct tcp_cookie_values *cvp = tp->cookie_values;
    struct tcphdr *th;
    struct sk_buff *skb;
    struct tcp_md5sig_key *md5;
    int tcp_header_size;
    int mss;
    int s_data_desired = 0;

    if (cvp != NULL && cvp->s_data_constant && cvp->s_data_desired)
        s_data_desired = cvp->s_data_desired;
    skb = alloc_skb(MAX_TCP_HEADER + 15 + s_data_desired,
            sk_gfp_atomic(sk, GFP_ATOMIC));
    if (unlikely(!skb)) {
        dst_release(dst);
        return NULL;
    }
    /* Reserve space for headers. */
    skb_reserve(skb, MAX_TCP_HEADER);

    skb_dst_set(skb, dst);

    mss = dst_metric_advmss(dst);
    if (tp->rx_opt.user_mss && tp->rx_opt.user_mss < mss)
        mss = tp->rx_opt.user_mss;

    if (req->rcv_wnd == 0) { /* ignored for retransmitted syns */
        __u8 rcv_wscale;
        /* Set this up on the first call only */
        req->window_clamp = tp->window_clamp ? : dst_metric(dst, RTAX_WINDOW);

        /* limit the window selection if the user enforce a smaller rx buffer */
        if (sk->sk_userlocks & SOCK_RCVBUF_LOCK &&
            (req->window_clamp > tcp_full_space(sk) || req->window_clamp == 0))
            req->window_clamp = tcp_full_space(sk);

        /* tcp_full_space because it is guaranteed to be the first packet */
        tcp_select_initial_window(tcp_full_space(sk),
            mss - (ireq->tstamp_ok ? TCPOLEN_TSTAMP_ALIGNED : 0),
            &req->rcv_wnd,
            &req->window_clamp,
            ireq->wscale_ok,
            &rcv_wscale,
            dst_metric(dst, RTAX_INITRWND));
        ireq->rcv_wscale = rcv_wscale;
    }

    memset(&opts, 0, sizeof(opts));
#ifdef CONFIG_SYN_COOKIES
    if (unlikely(req->cookie_ts))
        TCP_SKB_CB(skb)->when = cookie_init_timestamp(req);
    else
#endif
    TCP_SKB_CB(skb)->when = tcp_time_stamp;
    tcp_header_size = tcp_synack_options(sk, req, mss,
                         skb, &opts, &md5, xvp, foc)
            + sizeof(*th);

    skb_push(skb, tcp_header_size);
    skb_reset_transport_header(skb);

    th = tcp_hdr(skb);
    memset(th, 0, sizeof(struct tcphdr));
    th->syn = 1;
    th->ack = 1;
    TCP_ECN_make_synack(req, th);
    th->source = ireq->loc_port;
    th->dest = ireq->rmt_port;
    /* Setting of flags are superfluous here for callers (and ECE is
     * not even correctly set)
     */
    tcp_init_nondata_skb(skb, tcp_rsk(req)->snt_isn,
                 TCPHDR_SYN | TCPHDR_ACK);

    if (OPTION_COOKIE_EXTENSION & opts.options) {
        if (s_data_desired) {
            u8 *buf = skb_put(skb, s_data_desired);

            /* copy data directly from the listening socket. */
            memcpy(buf, cvp->s_data_payload, s_data_desired);
            TCP_SKB_CB(skb)->end_seq += s_data_desired;
        }

        if (opts.hash_size > 0) {
            __u32 workspace[SHA_WORKSPACE_WORDS];
            u32 *mess = &xvp->cookie_bakery[COOKIE_DIGEST_WORDS];
            u32 *tail = &mess[COOKIE_MESSAGE_WORDS-1];

            /* Secret recipe depends on the Timestamp, (future)
             * Sequence and Acknowledgment Numbers, Initiator
             * Cookie, and others handled by IP variant caller.
             */
            *tail-- ^= opts.tsval;
            *tail-- ^= tcp_rsk(req)->rcv_isn + 1;
            *tail-- ^= TCP_SKB_CB(skb)->seq + 1;

            /* recommended */
            *tail-- ^= (((__force u32)th->dest << 16) | (__force u32)th->source);
            *tail-- ^= (u32)(unsigned long)cvp; /* per sockopt */

            sha_transform((__u32 *)&xvp->cookie_bakery[0],
                      (char *)mess,
                      &workspace[0]);
            opts.hash_location =
                (__u8 *)&xvp->cookie_bakery[0];
        }
    }

    th->seq = htonl(TCP_SKB_CB(skb)->seq);
    /* XXX data is queued and acked as is. No buffer/window check */
    th->ack_seq = htonl(tcp_rsk(req)->rcv_nxt);

    /* RFC1323: The window in SYN & SYN/ACK segments is never scaled. */
    th->window = htons(min(req->rcv_wnd, 65535U));
    tcp_options_write((__be32 *)(th + 1), tp, &opts);
    th->doff = (tcp_header_size >> 2);
    TCP_ADD_STATS(sock_net(sk), TCP_MIB_OUTSEGS, tcp_skb_pcount(skb));

#ifdef CONFIG_TCP_MD5SIG
    /* Okay, we have all we need - do the md5 hash if needed */
    if (md5) {
        tcp_rsk(req)->af_specific->calc_md5_hash(opts.hash_location,
                           md5, NULL, req, skb);
    }
#endif

    return skb;
}
EXPORT_SYMBOL(tcp_make_synack);

/* Do all connect socket setups that can be done AF independent. */
void tcp_connect_init(struct sock *sk)
{
    const struct dst_entry *dst = __sk_dst_get(sk);
    struct tcp_sock *tp = tcp_sk(sk);
    __u8 rcv_wscale;

    /* We'll fix this up when we get a response from the other end.
     * See tcp_input.c:tcp_rcv_state_process case TCP_SYN_SENT.
     */
    tp->tcp_header_len = sizeof(struct tcphdr) +
        (sysctl_tcp_timestamps ? TCPOLEN_TSTAMP_ALIGNED : 0);

#ifdef CONFIG_TCP_MD5SIG
    if (tp->af_specific->md5_lookup(sk, sk) != NULL)
        tp->tcp_header_len += TCPOLEN_MD5SIG_ALIGNED;
#endif

    /* If user gave his TCP_MAXSEG, record it to clamp */
    if (tp->rx_opt.user_mss)
        tp->rx_opt.mss_clamp = tp->rx_opt.user_mss;
    tp->max_window = 0;
    tcp_mtup_init(sk);
    tcp_sync_mss(sk, dst_mtu(dst));

    if (!tp->window_clamp)
        tp->window_clamp = dst_metric(dst, RTAX_WINDOW);
    tp->advmss = dst_metric_advmss(dst);
    if (tp->rx_opt.user_mss && tp->rx_opt.user_mss < tp->advmss)
        tp->advmss = tp->rx_opt.user_mss;

    tcp_initialize_rcv_mss(sk);

    /* limit the window selection if the user enforce a smaller rx buffer */
    if (sk->sk_userlocks & SOCK_RCVBUF_LOCK &&
        (tp->window_clamp > tcp_full_space(sk) || tp->window_clamp == 0))
        tp->window_clamp = tcp_full_space(sk);

    tcp_select_initial_window(tcp_full_space(sk),
                  tp->advmss - (tp->rx_opt.ts_recent_stamp ? tp->tcp_header_len - sizeof(struct tcphdr) : 0),
                  &tp->rcv_wnd,
                  &tp->window_clamp,
                  sysctl_tcp_window_scaling,
                  &rcv_wscale,
                  dst_metric(dst, RTAX_INITRWND));

    tp->rx_opt.rcv_wscale = rcv_wscale;
    tp->rcv_ssthresh = tp->rcv_wnd;

    sk->sk_err = 0;
    sock_reset_flag(sk, SOCK_DONE);
    tp->snd_wnd = 0;
    tcp_init_wl(tp, 0);
    tp->snd_una = tp->write_seq;
    tp->snd_sml = tp->write_seq;
    tp->snd_up = tp->write_seq;
    tp->snd_nxt = tp->write_seq;

    if (likely(!tp->repair))
        tp->rcv_nxt = 0;
    tp->rcv_wup = tp->rcv_nxt;
    tp->copied_seq = tp->rcv_nxt;

    inet_csk(sk)->icsk_rto = TCP_TIMEOUT_INIT;
    inet_csk(sk)->icsk_retransmits = 0;
    tcp_clear_retrans(tp);
}

static void tcp_connect_queue_skb(struct sock *sk, struct sk_buff *skb)
{
    struct tcp_sock *tp = tcp_sk(sk);
    struct tcp_skb_cb *tcb = TCP_SKB_CB(skb);

    tcb->end_seq += skb->len;
    skb_header_release(skb);
    __tcp_add_write_queue_tail(sk, skb);
    sk->sk_wmem_queued += skb->truesize;
    sk_mem_charge(sk, skb->truesize);
    tp->write_seq = tcb->end_seq;
    tp->packets_out += tcp_skb_pcount(skb);
}

/* Build and send a SYN with data and (cached) Fast Open cookie. However,
 * queue a data-only packet after the regular SYN, such that regular SYNs
 * are retransmitted on timeouts. Also if the remote SYN-ACK acknowledges
 * only the SYN sequence, the data are retransmitted in the first ACK.
 * If cookie is not cached or other error occurs, falls back to send a
 * regular SYN with Fast Open cookie request option.
 */
static int tcp_send_syn_data(struct sock *sk, struct sk_buff *syn)
{
    struct tcp_sock *tp = tcp_sk(sk);
    struct tcp_fastopen_request *fo = tp->fastopen_req;
    int syn_loss = 0, space, i, err = 0, iovlen = fo->data->msg_iovlen;
    struct sk_buff *syn_data = NULL, *data;
    unsigned long last_syn_loss = 0;

    tp->rx_opt.mss_clamp = tp->advmss;  /* If MSS is not cached */
    tcp_fastopen_cache_get(sk, &tp->rx_opt.mss_clamp, &fo->cookie,
                   &syn_loss, &last_syn_loss);
    /* Recurring FO SYN losses: revert to regular handshake temporarily */
    if (syn_loss > 1 &&
        time_before(jiffies, last_syn_loss + (60*HZ << syn_loss))) {
        fo->cookie.len = -1;
        goto fallback;
    }

    if (sysctl_tcp_fastopen & TFO_CLIENT_NO_COOKIE)
        fo->cookie.len = -1;
    else if (fo->cookie.len <= 0)
        goto fallback;

    /* MSS for SYN-data is based on cached MSS and bounded by PMTU and
     * user-MSS. Reserve maximum option space for middleboxes that add
     * private TCP options. The cost is reduced data space in SYN :(
     */
    if (tp->rx_opt.user_mss && tp->rx_opt.user_mss < tp->rx_opt.mss_clamp)
        tp->rx_opt.mss_clamp = tp->rx_opt.user_mss;
    space = tcp_mtu_to_mss(sk, inet_csk(sk)->icsk_pmtu_cookie) -
        MAX_TCP_OPTION_SPACE;

    syn_data = skb_copy_expand(syn, skb_headroom(syn), space,
                   sk->sk_allocation);
    if (syn_data == NULL)
        goto fallback;

    for (i = 0; i < iovlen && syn_data->len < space; ++i) {
        struct iovec *iov = &fo->data->msg_iov[i];
        unsigned char __user *from = iov->iov_base;
        int len = iov->iov_len;

        if (syn_data->len + len > space)
            len = space - syn_data->len;
        else if (i + 1 == iovlen)
            /* No more data pending in inet_wait_for_connect() */
            fo->data = NULL;

        if (skb_add_data(syn_data, from, len))
            goto fallback;
    }

    /* Queue a data-only packet after the regular SYN for retransmission */
    data = pskb_copy(syn_data, sk->sk_allocation);
    if (data == NULL)
        goto fallback;
    TCP_SKB_CB(data)->seq++;
    TCP_SKB_CB(data)->tcp_flags &= ~TCPHDR_SYN;
    TCP_SKB_CB(data)->tcp_flags = (TCPHDR_ACK|TCPHDR_PSH);
    tcp_connect_queue_skb(sk, data);
    fo->copied = data->len;

    if (tcp_transmit_skb(sk, syn_data, 0, sk->sk_allocation) == 0) {
        tp->syn_data = (fo->copied > 0);
        NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPFASTOPENACTIVE);
        goto done;
    }
    syn_data = NULL;

fallback:
    /* Send a regular SYN with Fast Open cookie request option */
    if (fo->cookie.len > 0)
        fo->cookie.len = 0;
    err = tcp_transmit_skb(sk, syn, 1, sk->sk_allocation);
    if (err)
        tp->syn_fastopen = 0;
    kfree_skb(syn_data);
done:
    fo->cookie.len = -1;  /* Exclude Fast Open option for SYN retries */
    return err;
}

/* Build a SYN and send it off. */
int tcp_connect(struct sock *sk)
{
    struct tcp_sock *tp = tcp_sk(sk);
    struct sk_buff *buff;
    int err;

    tcp_connect_init(sk);

    buff = alloc_skb_fclone(MAX_TCP_HEADER + 15, sk->sk_allocation);
    if (unlikely(buff == NULL))
        return -ENOBUFS;

    /* Reserve space for headers. */
    skb_reserve(buff, MAX_TCP_HEADER);

    tcp_init_nondata_skb(buff, tp->write_seq++, TCPHDR_SYN);
    tp->retrans_stamp = TCP_SKB_CB(buff)->when = tcp_time_stamp;
    tcp_connect_queue_skb(sk, buff);
    TCP_ECN_send_syn(sk, buff);

    /* Send off SYN; include data in Fast Open. */
    err = tp->fastopen_req ? tcp_send_syn_data(sk, buff) :
          tcp_transmit_skb(sk, buff, 1, sk->sk_allocation);
    if (err == -ECONNREFUSED)
        return err;

    /* We change tp->snd_nxt after the tcp_transmit_skb() call
     * in order to make this packet get counted in tcpOutSegs.
     */
    tp->snd_nxt = tp->write_seq;
    tp->pushed_seq = tp->write_seq;
    TCP_INC_STATS(sock_net(sk), TCP_MIB_ACTIVEOPENS);

    /* Timer for repeating the SYN until an answer. */
    inet_csk_reset_xmit_timer(sk, ICSK_TIME_RETRANS,
                  inet_csk(sk)->icsk_rto, TCP_RTO_MAX);
    return 0;
}
EXPORT_SYMBOL(tcp_connect);

/* Send out a delayed ack, the caller does the policy checking
 * to see if we should even be here.  See tcp_input.c:tcp_ack_snd_check()
 * for details.
 */
void tcp_send_delayed_ack(struct sock *sk)
{
    struct inet_connection_sock *icsk = inet_csk(sk);
    int ato = icsk->icsk_ack.ato;
    unsigned long timeout;

    if (ato > TCP_DELACK_MIN) {
        const struct tcp_sock *tp = tcp_sk(sk);
        int max_ato = HZ / 2;

        if (icsk->icsk_ack.pingpong ||
            (icsk->icsk_ack.pending & ICSK_ACK_PUSHED))
            max_ato = TCP_DELACK_MAX;

        /* Slow path, intersegment interval is "high". */

        /* If some rtt estimate is known, use it to bound delayed ack.
         * Do not use inet_csk(sk)->icsk_rto here, use results of rtt measurements
         * directly.
         */
        if (tp->srtt) {
            int rtt = max(tp->srtt >> 3, TCP_DELACK_MIN);

            if (rtt < max_ato)
                max_ato = rtt;
        }

        ato = min(ato, max_ato);
    }

    /* Stay within the limit we were given */
    timeout = jiffies + ato;

    /* 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.
         */
        if (icsk->icsk_ack.blocked ||
            time_before_eq(icsk->icsk_ack.timeout, jiffies + (ato >> 2))) {
            tcp_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);
}

/* This routine sends an ack and also updates the window. */
void tcp_send_ack(struct sock *sk)
{
    struct sk_buff *buff;

    /* If we have been reset, we may not send again. */
    if (sk->sk_state == TCP_CLOSE)
        return;

    /* We are not putting this on the write queue, so
     * tcp_transmit_skb() will set the ownership to this
     * sock.
     */
    buff = alloc_skb(MAX_TCP_HEADER, sk_gfp_atomic(sk, GFP_ATOMIC));
    if (buff == 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, TCP_RTO_MAX);
        return;
    }

    /* Reserve space for headers and prepare control bits. */
    skb_reserve(buff, MAX_TCP_HEADER);
    tcp_init_nondata_skb(buff, tcp_acceptable_seq(sk), TCPHDR_ACK);

    /* Send it off, this clears delayed acks for us. */
    TCP_SKB_CB(buff)->when = tcp_time_stamp;
    tcp_transmit_skb(sk, buff, 0, sk_gfp_atomic(sk, GFP_ATOMIC));
}

/* This routine sends a packet with an out of date sequence
 * number. It assumes the other end will try to ack it.
 *
 * Question: what should we make while urgent mode?
 * 4.4BSD forces sending single byte of data. We cannot send
 * out of window data, because we have SND.NXT==SND.MAX...
 *
 * Current solution: to send TWO zero-length segments in urgent mode:
 * one is with SEG.SEQ=SND.UNA to deliver urgent pointer, another is
 * out-of-date with SND.UNA-1 to probe window.
 */
static int tcp_xmit_probe_skb(struct sock *sk, int urgent)
{
    struct tcp_sock *tp = tcp_sk(sk);
    struct sk_buff *skb;

    /* We don't queue it, tcp_transmit_skb() sets ownership. */
    skb = alloc_skb(MAX_TCP_HEADER, sk_gfp_atomic(sk, GFP_ATOMIC));
    if (skb == NULL)
        return -1;

    /* Reserve space for headers and set control bits. */
    skb_reserve(skb, MAX_TCP_HEADER);
    /* Use a previous sequence.  This should cause the other
     * end to send an ack.  Don't queue or clone SKB, just
     * send it.
     */
    tcp_init_nondata_skb(skb, tp->snd_una - !urgent, TCPHDR_ACK);
    TCP_SKB_CB(skb)->when = tcp_time_stamp;
    return tcp_transmit_skb(sk, skb, 0, GFP_ATOMIC);
}

void tcp_send_window_probe(struct sock *sk)
{
    if (sk->sk_state == TCP_ESTABLISHED) {
        tcp_sk(sk)->snd_wl1 = tcp_sk(sk)->rcv_nxt - 1;
        tcp_sk(sk)->snd_nxt = tcp_sk(sk)->write_seq;
        tcp_xmit_probe_skb(sk, 0);
    }
}

/* Initiate keepalive or window probe from timer. */
int tcp_write_wakeup(struct sock *sk)
{
    struct tcp_sock *tp = tcp_sk(sk);
    struct sk_buff *skb;

    if (sk->sk_state == TCP_CLOSE)
        return -1;

    if ((skb = tcp_send_head(sk)) != NULL &&
        before(TCP_SKB_CB(skb)->seq, tcp_wnd_end(tp))) {
        int err;
        unsigned int mss = tcp_current_mss(sk);
        unsigned int seg_size = tcp_wnd_end(tp) - TCP_SKB_CB(skb)->seq;

        if (before(tp->pushed_seq, TCP_SKB_CB(skb)->end_seq))
            tp->pushed_seq = TCP_SKB_CB(skb)->end_seq;

        /* We are probing the opening of a window
         * but the window size is != 0
         * must have been a result SWS avoidance ( sender )
         */
        if (seg_size < TCP_SKB_CB(skb)->end_seq - TCP_SKB_CB(skb)->seq ||
            skb->len > mss) {
            seg_size = min(seg_size, mss);
            TCP_SKB_CB(skb)->tcp_flags |= TCPHDR_PSH;
            if (tcp_fragment(sk, skb, seg_size, mss))
                return -1;
        } else if (!tcp_skb_pcount(skb))
            tcp_set_skb_tso_segs(sk, skb, mss);

        TCP_SKB_CB(skb)->tcp_flags |= TCPHDR_PSH;
        TCP_SKB_CB(skb)->when = tcp_time_stamp;
        err = tcp_transmit_skb(sk, skb, 1, GFP_ATOMIC);
        if (!err)
            tcp_event_new_data_sent(sk, skb);
        return err;
    } else {
        if (between(tp->snd_up, tp->snd_una + 1, tp->snd_una + 0xFFFF))
            tcp_xmit_probe_skb(sk, 1);
        return tcp_xmit_probe_skb(sk, 0);
    }
}

/* A window probe timeout has occurred.  If window is not closed send
 * a partial packet else a zero probe.
 */
void tcp_send_probe0(struct sock *sk)
{
    struct inet_connection_sock *icsk = inet_csk(sk);
    struct tcp_sock *tp = tcp_sk(sk);
    int err;

    err = tcp_write_wakeup(sk);

    if (tp->packets_out || !tcp_send_head(sk)) {
        /* Cancel probe timer, if it is not required. */
        icsk->icsk_probes_out = 0;
        icsk->icsk_backoff = 0;
        return;
    }

    if (err <= 0) {
        if (icsk->icsk_backoff < sysctl_tcp_retries2)
            icsk->icsk_backoff++;
        icsk->icsk_probes_out++;
        inet_csk_reset_xmit_timer(sk, ICSK_TIME_PROBE0,
                      min(icsk->icsk_rto << icsk->icsk_backoff, TCP_RTO_MAX),
                      TCP_RTO_MAX);
    } else {
        /* If packet was not sent due to local congestion,
         * do not backoff and do not remember icsk_probes_out.
         * Let local senders to fight for local resources.
         *
         * Use accumulated backoff yet.
         */
        if (!icsk->icsk_probes_out)
            icsk->icsk_probes_out = 1;
        inet_csk_reset_xmit_timer(sk, ICSK_TIME_PROBE0,
                      min(icsk->icsk_rto << icsk->icsk_backoff,
                          TCP_RESOURCE_PROBE_INTERVAL),
                      TCP_RTO_MAX);
    }
}