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tcp_output.c

/*
 * 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, <waltje@uWalt.NL.Mugnet.ORG>
 *          Mark Evans, <evansmp@uhura.aston.ac.uk>
 *          Corey Minyard <wf-rch!minyard@relay.EU.net>
 *          Florian La Roche, <flla@stud.uni-sb.de>
 *          Charles Hedrick, <hedrick@klinzhai.rutgers.edu>
 *          Linus Torvalds, <torvalds@cs.helsinki.fi>
 *          Alan Cox, <gw4pts@gw4pts.ampr.org>
 *          Matthew Dillon, <dillon@apollo.west.oic.com>
 *          Arnt Gulbrandsen, <agulbra@nvg.unit.no>
 *          Jorge Cwik, <jorge@laser.satlink.net>
 */

/*
 * 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
 *
 */

#include <net/tcp.h>

#include <linux/compiler.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;

/* 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 = 512;

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

static void tcp_event_new_data_sent(struct sock *sk, 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 indefinately with F-RTO */
      if (tp->frto_counter == 2)
            tp->frto_counter = 3;

      tp->packets_out += tcp_skb_pcount(skb);
      if (!prior_packets)
            inet_csk_reset_xmit_timer(sk, ICSK_TIME_RETRANS,
                                inet_csk(sk)->icsk_rto, TCP_RTO_MAX);
}

/* 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(struct sock *sk)
{
      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);
      struct dst_entry *dst = __sk_dst_get(sk);
      int mss = tp->advmss;

      if (dst && dst_metric(dst, RTAX_ADVMSS) < mss) {
            mss = dst_metric(dst, RTAX_ADVMSS);
            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, 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;
}

static void tcp_event_data_sent(struct tcp_sock *tp,
                        struct sk_buff *skb, 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;
}

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)
{
      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 value enough for senders,
       * following RFC2414. Senders, not following this RFC,
       * will be satisfied with 2.
       */
      if (mss > (1 << *rcv_wscale)) {
            int init_cwnd = 4;
            if (mss > 1460 * 3)
                  init_cwnd = 2;
            else if (mss > 1460)
                  init_cwnd = 3;
            if (*rcv_wnd > init_cwnd * mss)
                  *rcv_wnd = init_cwnd * mss;
      }

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

/* 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;
}

static inline void TCP_ECN_send_synack(struct tcp_sock *tp, struct sk_buff *skb)
{
      TCP_SKB_CB(skb)->flags &= ~TCPCB_FLAG_CWR;
      if (!(tp->ecn_flags & TCP_ECN_OK))
            TCP_SKB_CB(skb)->flags &= ~TCPCB_FLAG_ECE;
}

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)->flags |= TCPCB_FLAG_ECE | TCPCB_FLAG_CWR;
            tp->ecn_flags = TCP_ECN_OK;
      }
}

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

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->csum = 0;

      TCP_SKB_CB(skb)->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 & (TCPCB_FLAG_SYN | TCPCB_FLAG_FIN))
            seq++;
      TCP_SKB_CB(skb)->end_seq = seq;
}

static inline int 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)

00357 struct tcp_out_options {
      u8 options;       /* bit field of OPTION_* */
      u8 ws;                  /* window scale, 0 to disable */
      u8 num_sack_blocks;     /* number of SACK blocks to include */
      u16 mss;          /* 0 to disable */
      __u32 tsval, tsecr;     /* need to include OPTION_TS */
};

/* 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,
                        const struct tcp_out_options *opts,
                        __u8 **md5_hash) {
      if (unlikely(OPTION_MD5 & opts->options)) {
            *ptr++ = htonl((TCPOPT_NOP << 24) |
                         (TCPOPT_NOP << 16) |
                         (TCPOPT_MD5SIG << 8) |
                         TCPOLEN_MD5SIG);
            *md5_hash = (__u8 *)ptr;
            ptr += 4;
      } else {
            *md5_hash = NULL;
      }

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

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

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

      if (unlikely(opts->ws)) {
            *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;
      }
}

static unsigned 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);
      unsigned size = 0;

#ifdef CONFIG_TCP_MD5SIG
      *md5 = tp->af_specific->md5_lookup(sk, sk);
      if (*md5) {
            opts->options |= OPTION_MD5;
            size += 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);
      size += 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;
            size += TCPOLEN_TSTAMP_ALIGNED;
      }
      if (likely(sysctl_tcp_window_scaling)) {
            opts->ws = tp->rx_opt.rcv_wscale;
            if (likely(opts->ws))
                  size += TCPOLEN_WSCALE_ALIGNED;
      }
      if (likely(sysctl_tcp_sack)) {
            opts->options |= OPTION_SACK_ADVERTISE;
            if (unlikely(!(OPTION_TS & opts->options)))
                  size += TCPOLEN_SACKPERM_ALIGNED;
      }

      return size;
}

static unsigned tcp_synack_options(struct sock *sk,
                           struct request_sock *req,
                           unsigned mss, struct sk_buff *skb,
                           struct tcp_out_options *opts,
                           struct tcp_md5sig_key **md5) {
      unsigned size = 0;
      struct inet_request_sock *ireq = inet_rsk(req);
      char doing_ts;

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

      /* 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. */
      doing_ts = ireq->tstamp_ok && !(*md5 && ireq->sack_ok);

      opts->mss = mss;
      size += TCPOLEN_MSS_ALIGNED;

      if (likely(ireq->wscale_ok)) {
            opts->ws = ireq->rcv_wscale;
            if (likely(opts->ws))
                  size += TCPOLEN_WSCALE_ALIGNED;
      }
      if (likely(doing_ts)) {
            opts->options |= OPTION_TS;
            opts->tsval = TCP_SKB_CB(skb)->when;
            opts->tsecr = req->ts_recent;
            size += TCPOLEN_TSTAMP_ALIGNED;
      }
      if (likely(ireq->sack_ok)) {
            opts->options |= OPTION_SACK_ADVERTISE;
            if (unlikely(!doing_ts))
                  size += TCPOLEN_SACKPERM_ALIGNED;
      }

      return size;
}

static unsigned 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 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 remaining = MAX_TCP_OPTION_SPACE - size;
            opts->num_sack_blocks =
                  min_t(unsigned, eff_sacks,
                        (remaining - TCPOLEN_SACK_BASE_ALIGNED) /
                        TCPOLEN_SACK_PERBLOCK);
            size += TCPOLEN_SACK_BASE_ALIGNED +
                  opts->num_sack_blocks * TCPOLEN_SACK_PERBLOCK;
      }

      return size;
}

/* 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 tcp_options_size, tcp_header_size;
      struct tcp_md5sig_key *md5;
      __u8 *md5_hash_location;
      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->flags & TCPCB_FLAG_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_push(skb, tcp_header_size);
      skb_reset_transport_header(skb);
      skb_set_owner_w(skb, sk);

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

      if (unlikely(tcb->flags & TCPCB_FLAG_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 = 0xFFFF;
                  th->urg = 1;
            }
      }

      tcp_options_write((__be32 *)(th + 1), tp, &opts, &md5_hash_location);
      if (likely((tcb->flags & TCPCB_FLAG_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->sk_route_caps &= ~NETIF_F_GSO_MASK;
            tp->af_specific->calc_md5_hash(md5_hash_location,
                                     md5, sk, NULL, skb);
      }
#endif

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

      if (likely(tcb->flags & TCPCB_FLAG_ACK))
            tcp_event_ack_sent(sk, tcp_skb_pcount(skb));

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

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

      err = icsk->icsk_af_ops->queue_xmit(skb, 0);
      if (likely(err <= 0))
            return err;

      tcp_enter_cwr(sk, 1);

      return net_xmit_eval(err);
}

/* This routine just queue's the buffer
 *
 * 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);
}

static void tcp_set_skb_tso_segs(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, 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, 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;

      BUG_ON(len > skb->len);

      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)->flags;
      TCP_SKB_CB(skb)->flags = flags & ~(TCPCB_FLAG_FIN | TCPCB_FLAG_PSH);
      TCP_SKB_CB(buff)->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 = len;
      k = 0;
      for (i = 0; i < skb_shinfo(skb)->nr_frags; i++) {
            if (skb_shinfo(skb)->frags[i].size <= eat) {
                  put_page(skb_shinfo(skb)->frags[i].page);
                  eat -= skb_shinfo(skb)->frags[i].size;
            } else {
                  skb_shinfo(skb)->frags[k] = skb_shinfo(skb)->frags[i];
                  if (eat) {
                        skb_shinfo(skb)->frags[k].page_offset += eat;
                        skb_shinfo(skb)->frags[k].size -= eat;
                        eat = 0;
                  }
                  k++;
            }
      }
      skb_shinfo(skb)->nr_frags = k;

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

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;

      /* If len == headlen, we avoid __skb_pull to preserve alignment. */
      if (unlikely(len < skb_headlen(skb)))
            __skb_pull(skb, len);
      else
            __pskb_trim_head(skb, len - skb_headlen(skb));

      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 and mss.
       */
      if (tcp_skb_pcount(skb) > 1)
            tcp_set_skb_tso_segs(sk, skb, tcp_current_mss(sk));

      return 0;
}

/* Not accounting for SACKs here. */
int tcp_mtu_to_mss(struct sock *sk, int pmtu)
{
      struct tcp_sock *tp = tcp_sk(sk);
      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);

      /* 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)
{
      struct tcp_sock *tp = tcp_sk(sk);
      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;

      return mtu;
}

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;
}

/* 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;
}

/* 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)
{
      struct tcp_sock *tp = tcp_sk(sk);
      struct dst_entry *dst = __sk_dst_get(sk);
      u32 mss_now;
      unsigned 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(struct sock *sk, struct sk_buff *skb,
                              unsigned int mss_now, unsigned int cwnd)
{
      struct tcp_sock *tp = tcp_sk(sk);
      u32 needed, window, cwnd_len;

      window = tcp_wnd_end(tp) - TCP_SKB_CB(skb)->seq;
      cwnd_len = mss_now * cwnd;

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

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

      if (cwnd_len <= needed)
            return cwnd_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(struct tcp_sock *tp,
                               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)->flags & TCPCB_FLAG_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;
}

/* This must be invoked the first time we consider transmitting
 * SKB onto the wire.
 */
static int tcp_init_tso_segs(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;
}

static inline int tcp_minshall_check(const struct tcp_sock *tp)
{
      return after(tp->snd_sml, tp->snd_una) &&
            !after(tp->snd_sml, tp->snd_nxt);
}

/* Return 0, 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_NODELAY was 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 int tcp_nagle_check(const struct tcp_sock *tp,
                          const struct sk_buff *skb,
                          unsigned mss_now, int nonagle)
{
      return (skb->len < mss_now &&
            ((nonagle & TCP_NAGLE_CORK) ||
             (!nonagle && tp->packets_out && tcp_minshall_check(tp))));
}

/* Return non-zero if the Nagle test allows this packet to be
 * sent now.
 */
static inline int tcp_nagle_test(struct tcp_sock *tp, 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 1;

      /* 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)->flags & TCPCB_FLAG_FIN))
            return 1;

      if (!tcp_nagle_check(tp, skb, cur_mss, nonagle))
            return 1;

      return 0;
}

/* Does at least the first segment of SKB fit into the send window? */
static inline int tcp_snd_wnd_test(struct tcp_sock *tp, 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(struct sock *sk, struct sk_buff *skb,
                         unsigned int cur_mss, int nonagle)
{
      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;
}

int tcp_may_send_now(struct sock *sk)
{
      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)
{
      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_ATOMIC);
      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)->flags;
      TCP_SKB_CB(skb)->flags = flags & ~(TCPCB_FLAG_FIN | TCPCB_FLAG_PSH);
      TCP_SKB_CB(buff)->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 int 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;

      if (TCP_SKB_CB(skb)->flags & TCPCB_FLAG_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 >= sk->sk_gso_max_size)
            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;

      if (sysctl_tcp_tso_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 /= sysctl_tcp_tso_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_burst(tp) * tp->mss_cache)
                  goto send_now;
      }

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

      return 1;

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

/* Create a new MTU probe if we are ready.
 * 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)->flags = TCPCB_FLAG_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)->flags |= TCP_SKB_CB(skb)->flags;
                  tcp_unlink_write_queue(skb, sk);
                  sk_wmem_free_skb(sk, skb);
            } else {
                  TCP_SKB_CB(nskb)->flags |= TCP_SKB_CB(skb)->flags &
                                       ~(TCPCB_FLAG_FIN|TCPCB_FLAG_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 1, if no segments are in flight and we have queued segments, but
 * cannot send anything now because of SWS or another problem.
 */
static int 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 0;
            } 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);

            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;
            }

            limit = mss_now;
            if (tso_segs > 1 && !tcp_urg_mode(tp))
                  limit = tcp_mss_split_point(sk, skb, mss_now,
                                        cwnd_quota);

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

            TCP_SKB_CB(skb)->when = tcp_time_stamp;

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

            /* 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++;

            if (push_one)
                  break;
      }

      if (likely(sent_pkts)) {
            tcp_cwnd_validate(sk);
            return 0;
      }
      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)
{
      struct sk_buff *skb = tcp_send_head(sk);

      if (!skb)
            return;

      /* 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, 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 (tcp_memory_pressure)
                  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)->flags |= TCP_SKB_CB(next_skb)->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);
}

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

      return 1;
}

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;
      int first = 1;

      if (!sysctl_tcp_retrans_collapse)
            return;
      if (TCP_SKB_CB(skb)->flags & TCPCB_FLAG_SYN)
            return;

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

            space -= skb->len;

            if (first) {
                  first = 0;
                  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_tailroom(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;
      int err;

      /* 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)->flags & TCPCB_FLAG_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)->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;

      err = tcp_transmit_skb(sk, skb, 1, GFP_ATOMIC);

      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) {
                  if (net_ratelimit())
                        printk(KERN_DEBUG "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++;

            /* 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;
}

static int tcp_can_forward_retransmit(struct sock *sk)
{
      const struct inet_connection_sock *icsk = inet_csk(sk);
      struct tcp_sock *tp = tcp_sk(sk);

      /* Forward retransmissions are possible only during Recovery. */
      if (icsk->icsk_ca_state != TCP_CA_Recovery)
            return 0;

      /* No forward retransmissions in Reno are possible. */
      if (tcp_is_reno(tp))
            return 0;

      /* 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 0;

      return 1;
}

/* 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->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))
                  return;
            NET_INC_STATS_BH(sock_net(sk), mib_idx);

            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)->flags |= TCPCB_FLAG_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, GFP_KERNEL);
                  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,
                             TCPCB_FLAG_ACK | TCPCB_FLAG_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),
                       TCPCB_FLAG_ACK | TCPCB_FLAG_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);
}

/* 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)->flags & TCPCB_FLAG_SYN)) {
            printk(KERN_DEBUG "tcp_send_synack: wrong queue state\n");
            return -EFAULT;
      }
      if (!(TCP_SKB_CB(skb)->flags & TCPCB_FLAG_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)->flags |= TCPCB_FLAG_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);
}

/*
 * Prepare a SYN-ACK.
 */
struct sk_buff *tcp_make_synack(struct sock *sk, struct dst_entry *dst,
                        struct request_sock *req)
{
      struct inet_request_sock *ireq = inet_rsk(req);
      struct tcp_sock *tp = tcp_sk(sk);
      struct tcphdr *th;
      int tcp_header_size;
      struct tcp_out_options opts;
      struct sk_buff *skb;
      struct tcp_md5sig_key *md5;
      __u8 *md5_hash_location;
      int mss;

      skb = sock_wmalloc(sk, MAX_TCP_HEADER + 15, 1, GFP_ATOMIC);
      if (skb == NULL)
            return NULL;

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

      skb_dst_set(skb, dst_clone(dst));

      mss = dst_metric(dst, RTAX_ADVMSS);
      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);
            /* 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);
            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) +
                    sizeof(struct tcphdr);

      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,
                       TCPCB_FLAG_SYN | TCPCB_FLAG_ACK);
      th->seq = htonl(TCP_SKB_CB(skb)->seq);
      th->ack_seq = htonl(tcp_rsk(req)->rcv_isn + 1);

      /* 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, &md5_hash_location);
      th->doff = (tcp_header_size >> 2);
      TCP_INC_STATS(sock_net(sk), TCP_MIB_OUTSEGS);

#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(md5_hash_location,
                                     md5, NULL, req, skb);
      }
#endif

      return skb;
}

/*
 * Do all connect socket setups that can be done AF independent.
 */
static void tcp_connect_init(struct sock *sk)
{
      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(dst, RTAX_ADVMSS);
      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);

      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);

      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->rcv_nxt = 0;
      tp->rcv_wup = 0;
      tp->copied_seq = 0;

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

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

      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);

      tp->snd_nxt = tp->write_seq;
      tcp_init_nondata_skb(buff, tp->write_seq++, TCPCB_FLAG_SYN);
      TCP_ECN_send_syn(sk, buff);

      /* Send it off. */
      TCP_SKB_CB(buff)->when = tcp_time_stamp;
      tp->retrans_stamp = TCP_SKB_CB(buff)->when;
      skb_header_release(buff);
      __tcp_add_write_queue_tail(sk, buff);
      sk->sk_wmem_queued += buff->truesize;
      sk_mem_charge(sk, buff->truesize);
      tp->packets_out += tcp_skb_pcount(buff);
      tcp_transmit_skb(sk, buff, 1, GFP_KERNEL);

      /* 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;
}

/* 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, 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), TCPCB_FLAG_ACK);

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

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)->flags |= TCPCB_FLAG_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)->flags |= TCPCB_FLAG_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);
      }
}

EXPORT_SYMBOL(tcp_select_initial_window);
EXPORT_SYMBOL(tcp_connect);
EXPORT_SYMBOL(tcp_make_synack);
EXPORT_SYMBOL(tcp_simple_retransmit);
EXPORT_SYMBOL(tcp_sync_mss);
EXPORT_SYMBOL(tcp_mtup_init);

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