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tcp_input.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 :     Fast Retransmit/Recovery.
 *                            Two receive queues.
 *                            Retransmit queue handled by TCP.
 *                            Better retransmit timer handling.
 *                            New congestion avoidance.
 *                            Header prediction.
 *                            Variable renaming.
 *
 *          Eric        :     Fast Retransmit.
 *          Randy Scott :     MSS option defines.
 *          Eric Schenk :     Fixes to slow start algorithm.
 *          Eric Schenk :     Yet another double ACK bug.
 *          Eric Schenk :     Delayed ACK bug fixes.
 *          Eric Schenk :     Floyd style fast retrans war avoidance.
 *          David S. Miller   :     Don't allow zero congestion window.
 *          Eric Schenk :     Fix retransmitter so that it sends
 *                            next packet on ack of previous packet.
 *          Andi Kleen  :     Moved open_request checking here
 *                            and process RSTs for open_requests.
 *          Andi Kleen  :     Better prune_queue, and other fixes.
 *          Andrey Savochkin: Fix RTT measurements in the presence of
 *                            timestamps.
 *          Andrey Savochkin: Check sequence numbers correctly when
 *                            removing SACKs due to in sequence incoming
 *                            data segments.
 *          Andi Kleen:       Make sure we never ack data there is not
 *                            enough room for. Also make this condition
 *                            a fatal error if it might still happen.
 *          Andi Kleen:       Add tcp_measure_rcv_mss to make
 *                            connections with MSS<min(MTU,ann. MSS)
 *                            work without delayed acks.
 *          Andi Kleen:       Process packets with PSH set in the
 *                            fast path.
 *          J Hadi Salim:           ECN support
 *          Andrei Gurtov,
 *          Pasi Sarolahti,
 *          Panu Kuhlberg:          Experimental audit of TCP (re)transmission
 *                            engine. Lots of bugs are found.
 *          Pasi Sarolahti:         F-RTO for dealing with spurious RTOs
 */

#include <linux/mm.h>
#include <linux/module.h>
#include <linux/sysctl.h>
#include <linux/kernel.h>
#include <net/dst.h>
#include <net/tcp.h>
#include <net/inet_common.h>
#include <linux/ipsec.h>
#include <asm/unaligned.h>
#include <net/netdma.h>

int sysctl_tcp_timestamps __read_mostly = 1;
int sysctl_tcp_window_scaling __read_mostly = 1;
int sysctl_tcp_sack __read_mostly = 1;
int sysctl_tcp_fack __read_mostly = 1;
int sysctl_tcp_reordering __read_mostly = TCP_FASTRETRANS_THRESH;
int sysctl_tcp_ecn __read_mostly = 2;
int sysctl_tcp_dsack __read_mostly = 1;
int sysctl_tcp_app_win __read_mostly = 31;
int sysctl_tcp_adv_win_scale __read_mostly = 2;

int sysctl_tcp_stdurg __read_mostly;
int sysctl_tcp_rfc1337 __read_mostly;
int sysctl_tcp_max_orphans __read_mostly = NR_FILE;
int sysctl_tcp_frto __read_mostly = 2;
int sysctl_tcp_frto_response __read_mostly;
int sysctl_tcp_nometrics_save __read_mostly;

int sysctl_tcp_moderate_rcvbuf __read_mostly = 1;
int sysctl_tcp_abc __read_mostly;

#define FLAG_DATA       0x01 /* Incoming frame contained data.          */
#define FLAG_WIN_UPDATE       0x02 /* Incoming ACK was a window update. */
#define FLAG_DATA_ACKED       0x04 /* This ACK acknowledged new data.         */
#define FLAG_RETRANS_DATA_ACKED     0x08 /* "" "" some of which was retransmitted.  */
#define FLAG_SYN_ACKED        0x10 /* This ACK acknowledged SYN.        */
#define FLAG_DATA_SACKED      0x20 /* New SACK.                   */
#define FLAG_ECE        0x40 /* ECE in this ACK                   */
#define FLAG_DATA_LOST        0x80 /* SACK detected data lossage.       */
#define FLAG_SLOWPATH         0x100 /* Do not skip RFC checks for window update.*/
#define FLAG_ONLY_ORIG_SACKED 0x200 /* SACKs only non-rexmit sent before RTO */
#define FLAG_SND_UNA_ADVANCED 0x400 /* Snd_una was changed (!= FLAG_DATA_ACKED) */
#define FLAG_DSACKING_ACK     0x800 /* SACK blocks contained D-SACK info */
#define FLAG_NONHEAD_RETRANS_ACKED  0x1000 /* Non-head rexmitted data was ACKed */
#define FLAG_SACK_RENEGING    0x2000 /* snd_una advanced to a sacked seq */

#define FLAG_ACKED            (FLAG_DATA_ACKED|FLAG_SYN_ACKED)
#define FLAG_NOT_DUP          (FLAG_DATA|FLAG_WIN_UPDATE|FLAG_ACKED)
#define FLAG_CA_ALERT         (FLAG_DATA_SACKED|FLAG_ECE)
#define FLAG_FORWARD_PROGRESS (FLAG_ACKED|FLAG_DATA_SACKED)
#define FLAG_ANY_PROGRESS     (FLAG_FORWARD_PROGRESS|FLAG_SND_UNA_ADVANCED)

#define TCP_REMNANT (TCP_FLAG_FIN|TCP_FLAG_URG|TCP_FLAG_SYN|TCP_FLAG_PSH)
#define TCP_HP_BITS (~(TCP_RESERVED_BITS|TCP_FLAG_PSH))

/* Adapt the MSS value used to make delayed ack decision to the
 * real world.
 */
static void tcp_measure_rcv_mss(struct sock *sk, const struct sk_buff *skb)
{
      struct inet_connection_sock *icsk = inet_csk(sk);
      const unsigned int lss = icsk->icsk_ack.last_seg_size;
      unsigned int len;

      icsk->icsk_ack.last_seg_size = 0;

      /* skb->len may jitter because of SACKs, even if peer
       * sends good full-sized frames.
       */
      len = skb_shinfo(skb)->gso_size ? : skb->len;
      if (len >= icsk->icsk_ack.rcv_mss) {
            icsk->icsk_ack.rcv_mss = len;
      } else {
            /* Otherwise, we make more careful check taking into account,
             * that SACKs block is variable.
             *
             * "len" is invariant segment length, including TCP header.
             */
            len += skb->data - skb_transport_header(skb);
            if (len >= TCP_MIN_RCVMSS + sizeof(struct tcphdr) ||
                /* If PSH is not set, packet should be
                 * full sized, provided peer TCP is not badly broken.
                 * This observation (if it is correct 8)) allows
                 * to handle super-low mtu links fairly.
                 */
                (len >= TCP_MIN_MSS + sizeof(struct tcphdr) &&
                 !(tcp_flag_word(tcp_hdr(skb)) & TCP_REMNANT))) {
                  /* Subtract also invariant (if peer is RFC compliant),
                   * tcp header plus fixed timestamp option length.
                   * Resulting "len" is MSS free of SACK jitter.
                   */
                  len -= tcp_sk(sk)->tcp_header_len;
                  icsk->icsk_ack.last_seg_size = len;
                  if (len == lss) {
                        icsk->icsk_ack.rcv_mss = len;
                        return;
                  }
            }
            if (icsk->icsk_ack.pending & ICSK_ACK_PUSHED)
                  icsk->icsk_ack.pending |= ICSK_ACK_PUSHED2;
            icsk->icsk_ack.pending |= ICSK_ACK_PUSHED;
      }
}

static void tcp_incr_quickack(struct sock *sk)
{
      struct inet_connection_sock *icsk = inet_csk(sk);
      unsigned quickacks = tcp_sk(sk)->rcv_wnd / (2 * icsk->icsk_ack.rcv_mss);

      if (quickacks == 0)
            quickacks = 2;
      if (quickacks > icsk->icsk_ack.quick)
            icsk->icsk_ack.quick = min(quickacks, TCP_MAX_QUICKACKS);
}

void tcp_enter_quickack_mode(struct sock *sk)
{
      struct inet_connection_sock *icsk = inet_csk(sk);
      tcp_incr_quickack(sk);
      icsk->icsk_ack.pingpong = 0;
      icsk->icsk_ack.ato = TCP_ATO_MIN;
}

/* Send ACKs quickly, if "quick" count is not exhausted
 * and the session is not interactive.
 */

static inline int tcp_in_quickack_mode(const struct sock *sk)
{
      const struct inet_connection_sock *icsk = inet_csk(sk);
      return icsk->icsk_ack.quick && !icsk->icsk_ack.pingpong;
}

static inline void TCP_ECN_queue_cwr(struct tcp_sock *tp)
{
      if (tp->ecn_flags & TCP_ECN_OK)
            tp->ecn_flags |= TCP_ECN_QUEUE_CWR;
}

static inline void TCP_ECN_accept_cwr(struct tcp_sock *tp, struct sk_buff *skb)
{
      if (tcp_hdr(skb)->cwr)
            tp->ecn_flags &= ~TCP_ECN_DEMAND_CWR;
}

static inline void TCP_ECN_withdraw_cwr(struct tcp_sock *tp)
{
      tp->ecn_flags &= ~TCP_ECN_DEMAND_CWR;
}

static inline void TCP_ECN_check_ce(struct tcp_sock *tp, struct sk_buff *skb)
{
      if (tp->ecn_flags & TCP_ECN_OK) {
            if (INET_ECN_is_ce(TCP_SKB_CB(skb)->flags))
                  tp->ecn_flags |= TCP_ECN_DEMAND_CWR;
            /* Funny extension: if ECT is not set on a segment,
             * it is surely retransmit. It is not in ECN RFC,
             * but Linux follows this rule. */
            else if (INET_ECN_is_not_ect((TCP_SKB_CB(skb)->flags)))
                  tcp_enter_quickack_mode((struct sock *)tp);
      }
}

static inline void TCP_ECN_rcv_synack(struct tcp_sock *tp, struct tcphdr *th)
{
      if ((tp->ecn_flags & TCP_ECN_OK) && (!th->ece || th->cwr))
            tp->ecn_flags &= ~TCP_ECN_OK;
}

static inline void TCP_ECN_rcv_syn(struct tcp_sock *tp, struct tcphdr *th)
{
      if ((tp->ecn_flags & TCP_ECN_OK) && (!th->ece || !th->cwr))
            tp->ecn_flags &= ~TCP_ECN_OK;
}

static inline int TCP_ECN_rcv_ecn_echo(struct tcp_sock *tp, struct tcphdr *th)
{
      if (th->ece && !th->syn && (tp->ecn_flags & TCP_ECN_OK))
            return 1;
      return 0;
}

/* Buffer size and advertised window tuning.
 *
 * 1. Tuning sk->sk_sndbuf, when connection enters established state.
 */

static void tcp_fixup_sndbuf(struct sock *sk)
{
      int sndmem = tcp_sk(sk)->rx_opt.mss_clamp + MAX_TCP_HEADER + 16 +
                 sizeof(struct sk_buff);

      if (sk->sk_sndbuf < 3 * sndmem)
            sk->sk_sndbuf = min(3 * sndmem, sysctl_tcp_wmem[2]);
}

/* 2. Tuning advertised window (window_clamp, rcv_ssthresh)
 *
 * All tcp_full_space() is split to two parts: "network" buffer, allocated
 * forward and advertised in receiver window (tp->rcv_wnd) and
 * "application buffer", required to isolate scheduling/application
 * latencies from network.
 * window_clamp is maximal advertised window. It can be less than
 * tcp_full_space(), in this case tcp_full_space() - window_clamp
 * is reserved for "application" buffer. The less window_clamp is
 * the smoother our behaviour from viewpoint of network, but the lower
 * throughput and the higher sensitivity of the connection to losses. 8)
 *
 * rcv_ssthresh is more strict window_clamp used at "slow start"
 * phase to predict further behaviour of this connection.
 * It is used for two goals:
 * - to enforce header prediction at sender, even when application
 *   requires some significant "application buffer". It is check #1.
 * - to prevent pruning of receive queue because of misprediction
 *   of receiver window. Check #2.
 *
 * The scheme does not work when sender sends good segments opening
 * window and then starts to feed us spaghetti. But it should work
 * in common situations. Otherwise, we have to rely on queue collapsing.
 */

/* Slow part of check#2. */
static int __tcp_grow_window(const struct sock *sk, const struct sk_buff *skb)
{
      struct tcp_sock *tp = tcp_sk(sk);
      /* Optimize this! */
      int truesize = tcp_win_from_space(skb->truesize) >> 1;
      int window = tcp_win_from_space(sysctl_tcp_rmem[2]) >> 1;

      while (tp->rcv_ssthresh <= window) {
            if (truesize <= skb->len)
                  return 2 * inet_csk(sk)->icsk_ack.rcv_mss;

            truesize >>= 1;
            window >>= 1;
      }
      return 0;
}

static void tcp_grow_window(struct sock *sk, struct sk_buff *skb)
{
      struct tcp_sock *tp = tcp_sk(sk);

      /* Check #1 */
      if (tp->rcv_ssthresh < tp->window_clamp &&
          (int)tp->rcv_ssthresh < tcp_space(sk) &&
          !tcp_memory_pressure) {
            int incr;

            /* Check #2. Increase window, if skb with such overhead
             * will fit to rcvbuf in future.
             */
            if (tcp_win_from_space(skb->truesize) <= skb->len)
                  incr = 2 * tp->advmss;
            else
                  incr = __tcp_grow_window(sk, skb);

            if (incr) {
                  tp->rcv_ssthresh = min(tp->rcv_ssthresh + incr,
                                     tp->window_clamp);
                  inet_csk(sk)->icsk_ack.quick |= 1;
            }
      }
}

/* 3. Tuning rcvbuf, when connection enters established state. */

static void tcp_fixup_rcvbuf(struct sock *sk)
{
      struct tcp_sock *tp = tcp_sk(sk);
      int rcvmem = tp->advmss + MAX_TCP_HEADER + 16 + sizeof(struct sk_buff);

      /* Try to select rcvbuf so that 4 mss-sized segments
       * will fit to window and corresponding skbs will fit to our rcvbuf.
       * (was 3; 4 is minimum to allow fast retransmit to work.)
       */
      while (tcp_win_from_space(rcvmem) < tp->advmss)
            rcvmem += 128;
      if (sk->sk_rcvbuf < 4 * rcvmem)
            sk->sk_rcvbuf = min(4 * rcvmem, sysctl_tcp_rmem[2]);
}

/* 4. Try to fixup all. It is made immediately after connection enters
 *    established state.
 */
static void tcp_init_buffer_space(struct sock *sk)
{
      struct tcp_sock *tp = tcp_sk(sk);
      int maxwin;

      if (!(sk->sk_userlocks & SOCK_RCVBUF_LOCK))
            tcp_fixup_rcvbuf(sk);
      if (!(sk->sk_userlocks & SOCK_SNDBUF_LOCK))
            tcp_fixup_sndbuf(sk);

      tp->rcvq_space.space = tp->rcv_wnd;

      maxwin = tcp_full_space(sk);

      if (tp->window_clamp >= maxwin) {
            tp->window_clamp = maxwin;

            if (sysctl_tcp_app_win && maxwin > 4 * tp->advmss)
                  tp->window_clamp = max(maxwin -
                                     (maxwin >> sysctl_tcp_app_win),
                                     4 * tp->advmss);
      }

      /* Force reservation of one segment. */
      if (sysctl_tcp_app_win &&
          tp->window_clamp > 2 * tp->advmss &&
          tp->window_clamp + tp->advmss > maxwin)
            tp->window_clamp = max(2 * tp->advmss, maxwin - tp->advmss);

      tp->rcv_ssthresh = min(tp->rcv_ssthresh, tp->window_clamp);
      tp->snd_cwnd_stamp = tcp_time_stamp;
}

/* 5. Recalculate window clamp after socket hit its memory bounds. */
static void tcp_clamp_window(struct sock *sk)
{
      struct tcp_sock *tp = tcp_sk(sk);
      struct inet_connection_sock *icsk = inet_csk(sk);

      icsk->icsk_ack.quick = 0;

      if (sk->sk_rcvbuf < sysctl_tcp_rmem[2] &&
          !(sk->sk_userlocks & SOCK_RCVBUF_LOCK) &&
          !tcp_memory_pressure &&
          atomic_read(&tcp_memory_allocated) < sysctl_tcp_mem[0]) {
            sk->sk_rcvbuf = min(atomic_read(&sk->sk_rmem_alloc),
                            sysctl_tcp_rmem[2]);
      }
      if (atomic_read(&sk->sk_rmem_alloc) > sk->sk_rcvbuf)
            tp->rcv_ssthresh = min(tp->window_clamp, 2U * tp->advmss);
}

/* Initialize RCV_MSS value.
 * RCV_MSS is an our guess about MSS used by the peer.
 * We haven't any direct information about the MSS.
 * It's better to underestimate the RCV_MSS rather than overestimate.
 * Overestimations make us ACKing less frequently than needed.
 * Underestimations are more easy to detect and fix by tcp_measure_rcv_mss().
 */
void tcp_initialize_rcv_mss(struct sock *sk)
{
      struct tcp_sock *tp = tcp_sk(sk);
      unsigned int hint = min_t(unsigned int, tp->advmss, tp->mss_cache);

      hint = min(hint, tp->rcv_wnd / 2);
      hint = min(hint, TCP_MIN_RCVMSS);
      hint = max(hint, TCP_MIN_MSS);

      inet_csk(sk)->icsk_ack.rcv_mss = hint;
}

/* Receiver "autotuning" code.
 *
 * The algorithm for RTT estimation w/o timestamps is based on
 * Dynamic Right-Sizing (DRS) by Wu Feng and Mike Fisk of LANL.
 * <http://www.lanl.gov/radiant/website/pubs/drs/lacsi2001.ps>
 *
 * More detail on this code can be found at
 * <http://www.psc.edu/~jheffner/senior_thesis.ps>,
 * though this reference is out of date.  A new paper
 * is pending.
 */
static void tcp_rcv_rtt_update(struct tcp_sock *tp, u32 sample, int win_dep)
{
      u32 new_sample = tp->rcv_rtt_est.rtt;
      long m = sample;

      if (m == 0)
            m = 1;

      if (new_sample != 0) {
            /* If we sample in larger samples in the non-timestamp
             * case, we could grossly overestimate the RTT especially
             * with chatty applications or bulk transfer apps which
             * are stalled on filesystem I/O.
             *
             * Also, since we are only going for a minimum in the
             * non-timestamp case, we do not smooth things out
             * else with timestamps disabled convergence takes too
             * long.
             */
            if (!win_dep) {
                  m -= (new_sample >> 3);
                  new_sample += m;
            } else if (m < new_sample)
                  new_sample = m << 3;
      } else {
            /* No previous measure. */
            new_sample = m << 3;
      }

      if (tp->rcv_rtt_est.rtt != new_sample)
            tp->rcv_rtt_est.rtt = new_sample;
}

static inline void tcp_rcv_rtt_measure(struct tcp_sock *tp)
{
      if (tp->rcv_rtt_est.time == 0)
            goto new_measure;
      if (before(tp->rcv_nxt, tp->rcv_rtt_est.seq))
            return;
      tcp_rcv_rtt_update(tp, jiffies - tp->rcv_rtt_est.time, 1);

new_measure:
      tp->rcv_rtt_est.seq = tp->rcv_nxt + tp->rcv_wnd;
      tp->rcv_rtt_est.time = tcp_time_stamp;
}

static inline void tcp_rcv_rtt_measure_ts(struct sock *sk,
                                const struct sk_buff *skb)
{
      struct tcp_sock *tp = tcp_sk(sk);
      if (tp->rx_opt.rcv_tsecr &&
          (TCP_SKB_CB(skb)->end_seq -
           TCP_SKB_CB(skb)->seq >= inet_csk(sk)->icsk_ack.rcv_mss))
            tcp_rcv_rtt_update(tp, tcp_time_stamp - tp->rx_opt.rcv_tsecr, 0);
}

/*
 * This function should be called every time data is copied to user space.
 * It calculates the appropriate TCP receive buffer space.
 */
void tcp_rcv_space_adjust(struct sock *sk)
{
      struct tcp_sock *tp = tcp_sk(sk);
      int time;
      int space;

      if (tp->rcvq_space.time == 0)
            goto new_measure;

      time = tcp_time_stamp - tp->rcvq_space.time;
      if (time < (tp->rcv_rtt_est.rtt >> 3) || tp->rcv_rtt_est.rtt == 0)
            return;

      space = 2 * (tp->copied_seq - tp->rcvq_space.seq);

      space = max(tp->rcvq_space.space, space);

      if (tp->rcvq_space.space != space) {
            int rcvmem;

            tp->rcvq_space.space = space;

            if (sysctl_tcp_moderate_rcvbuf &&
                !(sk->sk_userlocks & SOCK_RCVBUF_LOCK)) {
                  int new_clamp = space;

                  /* Receive space grows, normalize in order to
                   * take into account packet headers and sk_buff
                   * structure overhead.
                   */
                  space /= tp->advmss;
                  if (!space)
                        space = 1;
                  rcvmem = (tp->advmss + MAX_TCP_HEADER +
                          16 + sizeof(struct sk_buff));
                  while (tcp_win_from_space(rcvmem) < tp->advmss)
                        rcvmem += 128;
                  space *= rcvmem;
                  space = min(space, sysctl_tcp_rmem[2]);
                  if (space > sk->sk_rcvbuf) {
                        sk->sk_rcvbuf = space;

                        /* Make the window clamp follow along.  */
                        tp->window_clamp = new_clamp;
                  }
            }
      }

new_measure:
      tp->rcvq_space.seq = tp->copied_seq;
      tp->rcvq_space.time = tcp_time_stamp;
}

/* There is something which you must keep in mind when you analyze the
 * behavior of the tp->ato delayed ack timeout interval.  When a
 * connection starts up, we want to ack as quickly as possible.  The
 * problem is that "good" TCP's do slow start at the beginning of data
 * transmission.  The means that until we send the first few ACK's the
 * sender will sit on his end and only queue most of his data, because
 * he can only send snd_cwnd unacked packets at any given time.  For
 * each ACK we send, he increments snd_cwnd and transmits more of his
 * queue.  -DaveM
 */
static void tcp_event_data_recv(struct sock *sk, struct sk_buff *skb)
{
      struct tcp_sock *tp = tcp_sk(sk);
      struct inet_connection_sock *icsk = inet_csk(sk);
      u32 now;

      inet_csk_schedule_ack(sk);

      tcp_measure_rcv_mss(sk, skb);

      tcp_rcv_rtt_measure(tp);

      now = tcp_time_stamp;

      if (!icsk->icsk_ack.ato) {
            /* The _first_ data packet received, initialize
             * delayed ACK engine.
             */
            tcp_incr_quickack(sk);
            icsk->icsk_ack.ato = TCP_ATO_MIN;
      } else {
            int m = now - icsk->icsk_ack.lrcvtime;

            if (m <= TCP_ATO_MIN / 2) {
                  /* The fastest case is the first. */
                  icsk->icsk_ack.ato = (icsk->icsk_ack.ato >> 1) + TCP_ATO_MIN / 2;
            } else if (m < icsk->icsk_ack.ato) {
                  icsk->icsk_ack.ato = (icsk->icsk_ack.ato >> 1) + m;
                  if (icsk->icsk_ack.ato > icsk->icsk_rto)
                        icsk->icsk_ack.ato = icsk->icsk_rto;
            } else if (m > icsk->icsk_rto) {
                  /* Too long gap. Apparently sender failed to
                   * restart window, so that we send ACKs quickly.
                   */
                  tcp_incr_quickack(sk);
                  sk_mem_reclaim(sk);
            }
      }
      icsk->icsk_ack.lrcvtime = now;

      TCP_ECN_check_ce(tp, skb);

      if (skb->len >= 128)
            tcp_grow_window(sk, skb);
}

/* Called to compute a smoothed rtt estimate. The data fed to this
 * routine either comes from timestamps, or from segments that were
 * known _not_ to have been retransmitted [see Karn/Partridge
 * Proceedings SIGCOMM 87]. The algorithm is from the SIGCOMM 88
 * piece by Van Jacobson.
 * NOTE: the next three routines used to be one big routine.
 * To save cycles in the RFC 1323 implementation it was better to break
 * it up into three procedures. -- erics
 */
static void tcp_rtt_estimator(struct sock *sk, const __u32 mrtt)
{
      struct tcp_sock *tp = tcp_sk(sk);
      long m = mrtt; /* RTT */

      /*    The following amusing code comes from Jacobson's
       *    article in SIGCOMM '88.  Note that rtt and mdev
       *    are scaled versions of rtt and mean deviation.
       *    This is designed to be as fast as possible
       *    m stands for "measurement".
       *
       *    On a 1990 paper the rto value is changed to:
       *    RTO = rtt + 4 * mdev
       *
       * Funny. This algorithm seems to be very broken.
       * These formulae increase RTO, when it should be decreased, increase
       * too slowly, when it should be increased quickly, decrease too quickly
       * etc. I guess in BSD RTO takes ONE value, so that it is absolutely
       * does not matter how to _calculate_ it. Seems, it was trap
       * that VJ failed to avoid. 8)
       */
      if (m == 0)
            m = 1;
      if (tp->srtt != 0) {
            m -= (tp->srtt >> 3);   /* m is now error in rtt est */
            tp->srtt += m;          /* rtt = 7/8 rtt + 1/8 new */
            if (m < 0) {
                  m = -m;           /* m is now abs(error) */
                  m -= (tp->mdev >> 2);   /* similar update on mdev */
                  /* This is similar to one of Eifel findings.
                   * Eifel blocks mdev updates when rtt decreases.
                   * This solution is a bit different: we use finer gain
                   * for mdev in this case (alpha*beta).
                   * Like Eifel it also prevents growth of rto,
                   * but also it limits too fast rto decreases,
                   * happening in pure Eifel.
                   */
                  if (m > 0)
                        m >>= 3;
            } else {
                  m -= (tp->mdev >> 2);   /* similar update on mdev */
            }
            tp->mdev += m;          /* mdev = 3/4 mdev + 1/4 new */
            if (tp->mdev > tp->mdev_max) {
                  tp->mdev_max = tp->mdev;
                  if (tp->mdev_max > tp->rttvar)
                        tp->rttvar = tp->mdev_max;
            }
            if (after(tp->snd_una, tp->rtt_seq)) {
                  if (tp->mdev_max < tp->rttvar)
                        tp->rttvar -= (tp->rttvar - tp->mdev_max) >> 2;
                  tp->rtt_seq = tp->snd_nxt;
                  tp->mdev_max = tcp_rto_min(sk);
            }
      } else {
            /* no previous measure. */
            tp->srtt = m << 3;      /* take the measured time to be rtt */
            tp->mdev = m << 1;      /* make sure rto = 3*rtt */
            tp->mdev_max = tp->rttvar = max(tp->mdev, tcp_rto_min(sk));
            tp->rtt_seq = tp->snd_nxt;
      }
}

/* Calculate rto without backoff.  This is the second half of Van Jacobson's
 * routine referred to above.
 */
static inline void tcp_set_rto(struct sock *sk)
{
      const struct tcp_sock *tp = tcp_sk(sk);
      /* Old crap is replaced with new one. 8)
       *
       * More seriously:
       * 1. If rtt variance happened to be less 50msec, it is hallucination.
       *    It cannot be less due to utterly erratic ACK generation made
       *    at least by solaris and freebsd. "Erratic ACKs" has _nothing_
       *    to do with delayed acks, because at cwnd>2 true delack timeout
       *    is invisible. Actually, Linux-2.4 also generates erratic
       *    ACKs in some circumstances.
       */
      inet_csk(sk)->icsk_rto = (tp->srtt >> 3) + tp->rttvar;

      /* 2. Fixups made earlier cannot be right.
       *    If we do not estimate RTO correctly without them,
       *    all the algo is pure shit and should be replaced
       *    with correct one. It is exactly, which we pretend to do.
       */

      /* NOTE: clamping at TCP_RTO_MIN is not required, current algo
       * guarantees that rto is higher.
       */
      if (inet_csk(sk)->icsk_rto > TCP_RTO_MAX)
            inet_csk(sk)->icsk_rto = TCP_RTO_MAX;
}

/* Save metrics learned by this TCP session.
   This function is called only, when TCP finishes successfully
   i.e. when it enters TIME-WAIT or goes from LAST-ACK to CLOSE.
 */
void tcp_update_metrics(struct sock *sk)
{
      struct tcp_sock *tp = tcp_sk(sk);
      struct dst_entry *dst = __sk_dst_get(sk);

      if (sysctl_tcp_nometrics_save)
            return;

      dst_confirm(dst);

      if (dst && (dst->flags & DST_HOST)) {
            const struct inet_connection_sock *icsk = inet_csk(sk);
            int m;
            unsigned long rtt;

            if (icsk->icsk_backoff || !tp->srtt) {
                  /* This session failed to estimate rtt. Why?
                   * Probably, no packets returned in time.
                   * Reset our results.
                   */
                  if (!(dst_metric_locked(dst, RTAX_RTT)))
                        dst->metrics[RTAX_RTT - 1] = 0;
                  return;
            }

            rtt = dst_metric_rtt(dst, RTAX_RTT);
            m = rtt - tp->srtt;

            /* If newly calculated rtt larger than stored one,
             * store new one. Otherwise, use EWMA. Remember,
             * rtt overestimation is always better than underestimation.
             */
            if (!(dst_metric_locked(dst, RTAX_RTT))) {
                  if (m <= 0)
                        set_dst_metric_rtt(dst, RTAX_RTT, tp->srtt);
                  else
                        set_dst_metric_rtt(dst, RTAX_RTT, rtt - (m >> 3));
            }

            if (!(dst_metric_locked(dst, RTAX_RTTVAR))) {
                  unsigned long var;
                  if (m < 0)
                        m = -m;

                  /* Scale deviation to rttvar fixed point */
                  m >>= 1;
                  if (m < tp->mdev)
                        m = tp->mdev;

                  var = dst_metric_rtt(dst, RTAX_RTTVAR);
                  if (m >= var)
                        var = m;
                  else
                        var -= (var - m) >> 2;

                  set_dst_metric_rtt(dst, RTAX_RTTVAR, var);
            }

            if (tp->snd_ssthresh >= 0xFFFF) {
                  /* Slow start still did not finish. */
                  if (dst_metric(dst, RTAX_SSTHRESH) &&
                      !dst_metric_locked(dst, RTAX_SSTHRESH) &&
                      (tp->snd_cwnd >> 1) > dst_metric(dst, RTAX_SSTHRESH))
                        dst->metrics[RTAX_SSTHRESH-1] = tp->snd_cwnd >> 1;
                  if (!dst_metric_locked(dst, RTAX_CWND) &&
                      tp->snd_cwnd > dst_metric(dst, RTAX_CWND))
                        dst->metrics[RTAX_CWND - 1] = tp->snd_cwnd;
            } else if (tp->snd_cwnd > tp->snd_ssthresh &&
                     icsk->icsk_ca_state == TCP_CA_Open) {
                  /* Cong. avoidance phase, cwnd is reliable. */
                  if (!dst_metric_locked(dst, RTAX_SSTHRESH))
                        dst->metrics[RTAX_SSTHRESH-1] =
                              max(tp->snd_cwnd >> 1, tp->snd_ssthresh);
                  if (!dst_metric_locked(dst, RTAX_CWND))
                        dst->metrics[RTAX_CWND-1] = (dst_metric(dst, RTAX_CWND) + tp->snd_cwnd) >> 1;
            } else {
                  /* Else slow start did not finish, cwnd is non-sense,
                     ssthresh may be also invalid.
                   */
                  if (!dst_metric_locked(dst, RTAX_CWND))
                        dst->metrics[RTAX_CWND-1] = (dst_metric(dst, RTAX_CWND) + tp->snd_ssthresh) >> 1;
                  if (dst_metric(dst, RTAX_SSTHRESH) &&
                      !dst_metric_locked(dst, RTAX_SSTHRESH) &&
                      tp->snd_ssthresh > dst_metric(dst, RTAX_SSTHRESH))
                        dst->metrics[RTAX_SSTHRESH-1] = tp->snd_ssthresh;
            }

            if (!dst_metric_locked(dst, RTAX_REORDERING)) {
                  if (dst_metric(dst, RTAX_REORDERING) < tp->reordering &&
                      tp->reordering != sysctl_tcp_reordering)
                        dst->metrics[RTAX_REORDERING-1] = tp->reordering;
            }
      }
}

/* Numbers are taken from RFC3390.
 *
 * John Heffner states:
 *
 *    The RFC specifies a window of no more than 4380 bytes
 *    unless 2*MSS > 4380.  Reading the pseudocode in the RFC
 *    is a bit misleading because they use a clamp at 4380 bytes
 *    rather than use a multiplier in the relevant range.
 */
__u32 tcp_init_cwnd(struct tcp_sock *tp, struct dst_entry *dst)
{
      __u32 cwnd = (dst ? dst_metric(dst, RTAX_INITCWND) : 0);

      if (!cwnd) {
            if (tp->mss_cache > 1460)
                  cwnd = 2;
            else
                  cwnd = (tp->mss_cache > 1095) ? 3 : 4;
      }
      return min_t(__u32, cwnd, tp->snd_cwnd_clamp);
}

/* Set slow start threshold and cwnd not falling to slow start */
void tcp_enter_cwr(struct sock *sk, const int set_ssthresh)
{
      struct tcp_sock *tp = tcp_sk(sk);
      const struct inet_connection_sock *icsk = inet_csk(sk);

      tp->prior_ssthresh = 0;
      tp->bytes_acked = 0;
      if (icsk->icsk_ca_state < TCP_CA_CWR) {
            tp->undo_marker = 0;
            if (set_ssthresh)
                  tp->snd_ssthresh = icsk->icsk_ca_ops->ssthresh(sk);
            tp->snd_cwnd = min(tp->snd_cwnd,
                           tcp_packets_in_flight(tp) + 1U);
            tp->snd_cwnd_cnt = 0;
            tp->high_seq = tp->snd_nxt;
            tp->snd_cwnd_stamp = tcp_time_stamp;
            TCP_ECN_queue_cwr(tp);

            tcp_set_ca_state(sk, TCP_CA_CWR);
      }
}

/*
 * Packet counting of FACK is based on in-order assumptions, therefore TCP
 * disables it when reordering is detected
 */
static void tcp_disable_fack(struct tcp_sock *tp)
{
      /* RFC3517 uses different metric in lost marker => reset on change */
      if (tcp_is_fack(tp))
            tp->lost_skb_hint = NULL;
      tp->rx_opt.sack_ok &= ~2;
}

/* Take a notice that peer is sending D-SACKs */
static void tcp_dsack_seen(struct tcp_sock *tp)
{
      tp->rx_opt.sack_ok |= 4;
}

/* Initialize metrics on socket. */

static void tcp_init_metrics(struct sock *sk)
{
      struct tcp_sock *tp = tcp_sk(sk);
      struct dst_entry *dst = __sk_dst_get(sk);

      if (dst == NULL)
            goto reset;

      dst_confirm(dst);

      if (dst_metric_locked(dst, RTAX_CWND))
            tp->snd_cwnd_clamp = dst_metric(dst, RTAX_CWND);
      if (dst_metric(dst, RTAX_SSTHRESH)) {
            tp->snd_ssthresh = dst_metric(dst, RTAX_SSTHRESH);
            if (tp->snd_ssthresh > tp->snd_cwnd_clamp)
                  tp->snd_ssthresh = tp->snd_cwnd_clamp;
      }
      if (dst_metric(dst, RTAX_REORDERING) &&
          tp->reordering != dst_metric(dst, RTAX_REORDERING)) {
            tcp_disable_fack(tp);
            tp->reordering = dst_metric(dst, RTAX_REORDERING);
      }

      if (dst_metric(dst, RTAX_RTT) == 0)
            goto reset;

      if (!tp->srtt && dst_metric_rtt(dst, RTAX_RTT) < (TCP_TIMEOUT_INIT << 3))
            goto reset;

      /* Initial rtt is determined from SYN,SYN-ACK.
       * The segment is small and rtt may appear much
       * less than real one. Use per-dst memory
       * to make it more realistic.
       *
       * A bit of theory. RTT is time passed after "normal" sized packet
       * is sent until it is ACKed. In normal circumstances sending small
       * packets force peer to delay ACKs and calculation is correct too.
       * The algorithm is adaptive and, provided we follow specs, it
       * NEVER underestimate RTT. BUT! If peer tries to make some clever
       * tricks sort of "quick acks" for time long enough to decrease RTT
       * to low value, and then abruptly stops to do it and starts to delay
       * ACKs, wait for troubles.
       */
      if (dst_metric_rtt(dst, RTAX_RTT) > tp->srtt) {
            tp->srtt = dst_metric_rtt(dst, RTAX_RTT);
            tp->rtt_seq = tp->snd_nxt;
      }
      if (dst_metric_rtt(dst, RTAX_RTTVAR) > tp->mdev) {
            tp->mdev = dst_metric_rtt(dst, RTAX_RTTVAR);
            tp->mdev_max = tp->rttvar = max(tp->mdev, tcp_rto_min(sk));
      }
      tcp_set_rto(sk);
      if (inet_csk(sk)->icsk_rto < TCP_TIMEOUT_INIT && !tp->rx_opt.saw_tstamp)
            goto reset;

cwnd:
      tp->snd_cwnd = tcp_init_cwnd(tp, dst);
      tp->snd_cwnd_stamp = tcp_time_stamp;
      return;

reset:
      /* Play conservative. If timestamps are not
       * supported, TCP will fail to recalculate correct
       * rtt, if initial rto is too small. FORGET ALL AND RESET!
       */
      if (!tp->rx_opt.saw_tstamp && tp->srtt) {
            tp->srtt = 0;
            tp->mdev = tp->mdev_max = tp->rttvar = TCP_TIMEOUT_INIT;
            inet_csk(sk)->icsk_rto = TCP_TIMEOUT_INIT;
      }
      goto cwnd;
}

static void tcp_update_reordering(struct sock *sk, const int metric,
                          const int ts)
{
      struct tcp_sock *tp = tcp_sk(sk);
      if (metric > tp->reordering) {
            int mib_idx;

            tp->reordering = min(TCP_MAX_REORDERING, metric);

            /* This exciting event is worth to be remembered. 8) */
            if (ts)
                  mib_idx = LINUX_MIB_TCPTSREORDER;
            else if (tcp_is_reno(tp))
                  mib_idx = LINUX_MIB_TCPRENOREORDER;
            else if (tcp_is_fack(tp))
                  mib_idx = LINUX_MIB_TCPFACKREORDER;
            else
                  mib_idx = LINUX_MIB_TCPSACKREORDER;

            NET_INC_STATS_BH(sock_net(sk), mib_idx);
#if FASTRETRANS_DEBUG > 1
            printk(KERN_DEBUG "Disorder%d %d %u f%u s%u rr%d\n",
                   tp->rx_opt.sack_ok, inet_csk(sk)->icsk_ca_state,
                   tp->reordering,
                   tp->fackets_out,
                   tp->sacked_out,
                   tp->undo_marker ? tp->undo_retrans : 0);
#endif
            tcp_disable_fack(tp);
      }
}

/* This must be called before lost_out is incremented */
static void tcp_verify_retransmit_hint(struct tcp_sock *tp, struct sk_buff *skb)
{
      if ((tp->retransmit_skb_hint == NULL) ||
          before(TCP_SKB_CB(skb)->seq,
               TCP_SKB_CB(tp->retransmit_skb_hint)->seq))
            tp->retransmit_skb_hint = skb;

      if (!tp->lost_out ||
          after(TCP_SKB_CB(skb)->end_seq, tp->retransmit_high))
            tp->retransmit_high = TCP_SKB_CB(skb)->end_seq;
}

static void tcp_skb_mark_lost(struct tcp_sock *tp, struct sk_buff *skb)
{
      if (!(TCP_SKB_CB(skb)->sacked & (TCPCB_LOST|TCPCB_SACKED_ACKED))) {
            tcp_verify_retransmit_hint(tp, skb);

            tp->lost_out += tcp_skb_pcount(skb);
            TCP_SKB_CB(skb)->sacked |= TCPCB_LOST;
      }
}

static void tcp_skb_mark_lost_uncond_verify(struct tcp_sock *tp,
                                  struct sk_buff *skb)
{
      tcp_verify_retransmit_hint(tp, skb);

      if (!(TCP_SKB_CB(skb)->sacked & (TCPCB_LOST|TCPCB_SACKED_ACKED))) {
            tp->lost_out += tcp_skb_pcount(skb);
            TCP_SKB_CB(skb)->sacked |= TCPCB_LOST;
      }
}

/* This procedure tags the retransmission queue when SACKs arrive.
 *
 * We have three tag bits: SACKED(S), RETRANS(R) and LOST(L).
 * Packets in queue with these bits set are counted in variables
 * sacked_out, retrans_out and lost_out, correspondingly.
 *
 * Valid combinations are:
 * Tag  InFlight  Description
 * 0  1           - orig segment is in flight.
 * S  0           - nothing flies, orig reached receiver.
 * L  0           - nothing flies, orig lost by net.
 * R  2           - both orig and retransmit are in flight.
 * L|R      1           - orig is lost, retransmit is in flight.
 * S|R  1         - orig reached receiver, retrans is still in flight.
 * (L|S|R is logically valid, it could occur when L|R is sacked,
 *  but it is equivalent to plain S and code short-curcuits it to S.
 *  L|S is logically invalid, it would mean -1 packet in flight 8))
 *
 * These 6 states form finite state machine, controlled by the following events:
 * 1. New ACK (+SACK) arrives. (tcp_sacktag_write_queue())
 * 2. Retransmission. (tcp_retransmit_skb(), tcp_xmit_retransmit_queue())
 * 3. Loss detection event of one of three flavors:
 *    A. Scoreboard estimator decided the packet is lost.
 *       A'. Reno "three dupacks" marks head of queue lost.
 *       A''. Its FACK modfication, head until snd.fack is lost.
 *    B. SACK arrives sacking data transmitted after never retransmitted
 *       hole was sent out.
 *    C. SACK arrives sacking SND.NXT at the moment, when the
 *       segment was retransmitted.
 * 4. D-SACK added new rule: D-SACK changes any tag to S.
 *
 * It is pleasant to note, that state diagram turns out to be commutative,
 * so that we are allowed not to be bothered by order of our actions,
 * when multiple events arrive simultaneously. (see the function below).
 *
 * Reordering detection.
 * --------------------
 * Reordering metric is maximal distance, which a packet can be displaced
 * in packet stream. With SACKs we can estimate it:
 *
 * 1. SACK fills old hole and the corresponding segment was not
 *    ever retransmitted -> reordering. Alas, we cannot use it
 *    when segment was retransmitted.
 * 2. The last flaw is solved with D-SACK. D-SACK arrives
 *    for retransmitted and already SACKed segment -> reordering..
 * Both of these heuristics are not used in Loss state, when we cannot
 * account for retransmits accurately.
 *
 * SACK block validation.
 * ----------------------
 *
 * SACK block range validation checks that the received SACK block fits to
 * the expected sequence limits, i.e., it is between SND.UNA and SND.NXT.
 * Note that SND.UNA is not included to the range though being valid because
 * it means that the receiver is rather inconsistent with itself reporting
 * SACK reneging when it should advance SND.UNA. Such SACK block this is
 * perfectly valid, however, in light of RFC2018 which explicitly states
 * that "SACK block MUST reflect the newest segment.  Even if the newest
 * segment is going to be discarded ...", not that it looks very clever
 * in case of head skb. Due to potentional receiver driven attacks, we
 * choose to avoid immediate execution of a walk in write queue due to
 * reneging and defer head skb's loss recovery to standard loss recovery
 * procedure that will eventually trigger (nothing forbids us doing this).
 *
 * Implements also blockage to start_seq wrap-around. Problem lies in the
 * fact that though start_seq (s) is before end_seq (i.e., not reversed),
 * there's no guarantee that it will be before snd_nxt (n). The problem
 * happens when start_seq resides between end_seq wrap (e_w) and snd_nxt
 * wrap (s_w):
 *
 *         <- outs wnd ->                          <- wrapzone ->
 *         u     e      n                         u_w   e_w  s n_w
 *         |     |      |                          |     |   |  |
 * |<------------+------+----- TCP seqno space --------------+---------->|
 * ...-- <2^31 ->|                                           |<--------...
 * ...---- >2^31 ------>|                                    |<--------...
 *
 * Current code wouldn't be vulnerable but it's better still to discard such
 * crazy SACK blocks. Doing this check for start_seq alone closes somewhat
 * similar case (end_seq after snd_nxt wrap) as earlier reversed check in
 * snd_nxt wrap -> snd_una region will then become "well defined", i.e.,
 * equal to the ideal case (infinite seqno space without wrap caused issues).
 *
 * With D-SACK the lower bound is extended to cover sequence space below
 * SND.UNA down to undo_marker, which is the last point of interest. Yet
 * again, D-SACK block must not to go across snd_una (for the same reason as
 * for the normal SACK blocks, explained above). But there all simplicity
 * ends, TCP might receive valid D-SACKs below that. As long as they reside
 * fully below undo_marker they do not affect behavior in anyway and can
 * therefore be safely ignored. In rare cases (which are more or less
 * theoretical ones), the D-SACK will nicely cross that boundary due to skb
 * fragmentation and packet reordering past skb's retransmission. To consider
 * them correctly, the acceptable range must be extended even more though
 * the exact amount is rather hard to quantify. However, tp->max_window can
 * be used as an exaggerated estimate.
 */
static int tcp_is_sackblock_valid(struct tcp_sock *tp, int is_dsack,
                          u32 start_seq, u32 end_seq)
{
      /* Too far in future, or reversed (interpretation is ambiguous) */
      if (after(end_seq, tp->snd_nxt) || !before(start_seq, end_seq))
            return 0;

      /* Nasty start_seq wrap-around check (see comments above) */
      if (!before(start_seq, tp->snd_nxt))
            return 0;

      /* In outstanding window? ...This is valid exit for D-SACKs too.
       * start_seq == snd_una is non-sensical (see comments above)
       */
      if (after(start_seq, tp->snd_una))
            return 1;

      if (!is_dsack || !tp->undo_marker)
            return 0;

      /* ...Then it's D-SACK, and must reside below snd_una completely */
      if (!after(end_seq, tp->snd_una))
            return 0;

      if (!before(start_seq, tp->undo_marker))
            return 1;

      /* Too old */
      if (!after(end_seq, tp->undo_marker))
            return 0;

      /* Undo_marker boundary crossing (overestimates a lot). Known already:
       *   start_seq < undo_marker and end_seq >= undo_marker.
       */
      return !before(start_seq, end_seq - tp->max_window);
}

/* Check for lost retransmit. This superb idea is borrowed from "ratehalving".
 * Event "C". Later note: FACK people cheated me again 8), we have to account
 * for reordering! Ugly, but should help.
 *
 * Search retransmitted skbs from write_queue that were sent when snd_nxt was
 * less than what is now known to be received by the other end (derived from
 * highest SACK block). Also calculate the lowest snd_nxt among the remaining
 * retransmitted skbs to avoid some costly processing per ACKs.
 */
static void tcp_mark_lost_retrans(struct sock *sk)
{
      const struct inet_connection_sock *icsk = inet_csk(sk);
      struct tcp_sock *tp = tcp_sk(sk);
      struct sk_buff *skb;
      int cnt = 0;
      u32 new_low_seq = tp->snd_nxt;
      u32 received_upto = tcp_highest_sack_seq(tp);

      if (!tcp_is_fack(tp) || !tp->retrans_out ||
          !after(received_upto, tp->lost_retrans_low) ||
          icsk->icsk_ca_state != TCP_CA_Recovery)
            return;

      tcp_for_write_queue(skb, sk) {
            u32 ack_seq = TCP_SKB_CB(skb)->ack_seq;

            if (skb == tcp_send_head(sk))
                  break;
            if (cnt == tp->retrans_out)
                  break;
            if (!after(TCP_SKB_CB(skb)->end_seq, tp->snd_una))
                  continue;

            if (!(TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_RETRANS))
                  continue;

            /* TODO: We would like to get rid of tcp_is_fack(tp) only
             * constraint here (see above) but figuring out that at
             * least tp->reordering SACK blocks reside between ack_seq
             * and received_upto is not easy task to do cheaply with
             * the available datastructures.
             *
             * Whether FACK should check here for tp->reordering segs
             * in-between one could argue for either way (it would be
             * rather simple to implement as we could count fack_count
             * during the walk and do tp->fackets_out - fack_count).
             */
            if (after(received_upto, ack_seq)) {
                  TCP_SKB_CB(skb)->sacked &= ~TCPCB_SACKED_RETRANS;
                  tp->retrans_out -= tcp_skb_pcount(skb);

                  tcp_skb_mark_lost_uncond_verify(tp, skb);
                  NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPLOSTRETRANSMIT);
            } else {
                  if (before(ack_seq, new_low_seq))
                        new_low_seq = ack_seq;
                  cnt += tcp_skb_pcount(skb);
            }
      }

      if (tp->retrans_out)
            tp->lost_retrans_low = new_low_seq;
}

static int tcp_check_dsack(struct sock *sk, struct sk_buff *ack_skb,
                     struct tcp_sack_block_wire *sp, int num_sacks,
                     u32 prior_snd_una)
{
      struct tcp_sock *tp = tcp_sk(sk);
      u32 start_seq_0 = get_unaligned_be32(&sp[0].start_seq);
      u32 end_seq_0 = get_unaligned_be32(&sp[0].end_seq);
      int dup_sack = 0;

      if (before(start_seq_0, TCP_SKB_CB(ack_skb)->ack_seq)) {
            dup_sack = 1;
            tcp_dsack_seen(tp);
            NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPDSACKRECV);
      } else if (num_sacks > 1) {
            u32 end_seq_1 = get_unaligned_be32(&sp[1].end_seq);
            u32 start_seq_1 = get_unaligned_be32(&sp[1].start_seq);

            if (!after(end_seq_0, end_seq_1) &&
                !before(start_seq_0, start_seq_1)) {
                  dup_sack = 1;
                  tcp_dsack_seen(tp);
                  NET_INC_STATS_BH(sock_net(sk),
                              LINUX_MIB_TCPDSACKOFORECV);
            }
      }

      /* D-SACK for already forgotten data... Do dumb counting. */
      if (dup_sack &&
          !after(end_seq_0, prior_snd_una) &&
          after(end_seq_0, tp->undo_marker))
            tp->undo_retrans--;

      return dup_sack;
}

01238 struct tcp_sacktag_state {
      int reord;
      int fack_count;
      int flag;
};

/* Check if skb is fully within the SACK block. In presence of GSO skbs,
 * the incoming SACK may not exactly match but we can find smaller MSS
 * aligned portion of it that matches. Therefore we might need to fragment
 * which may fail and creates some hassle (caller must handle error case
 * returns).
 *
 * FIXME: this could be merged to shift decision code
 */
static int tcp_match_skb_to_sack(struct sock *sk, struct sk_buff *skb,
                         u32 start_seq, u32 end_seq)
{
      int in_sack, err;
      unsigned int pkt_len;
      unsigned int mss;

      in_sack = !after(start_seq, TCP_SKB_CB(skb)->seq) &&
              !before(end_seq, TCP_SKB_CB(skb)->end_seq);

      if (tcp_skb_pcount(skb) > 1 && !in_sack &&
          after(TCP_SKB_CB(skb)->end_seq, start_seq)) {
            mss = tcp_skb_mss(skb);
            in_sack = !after(start_seq, TCP_SKB_CB(skb)->seq);

            if (!in_sack) {
                  pkt_len = start_seq - TCP_SKB_CB(skb)->seq;
                  if (pkt_len < mss)
                        pkt_len = mss;
            } else {
                  pkt_len = end_seq - TCP_SKB_CB(skb)->seq;
                  if (pkt_len < mss)
                        return -EINVAL;
            }

            /* Round if necessary so that SACKs cover only full MSSes
             * and/or the remaining small portion (if present)
             */
            if (pkt_len > mss) {
                  unsigned int new_len = (pkt_len / mss) * mss;
                  if (!in_sack && new_len < pkt_len) {
                        new_len += mss;
                        if (new_len > skb->len)
                              return 0;
                  }
                  pkt_len = new_len;
            }
            err = tcp_fragment(sk, skb, pkt_len, mss);
            if (err < 0)
                  return err;
      }

      return in_sack;
}

static u8 tcp_sacktag_one(struct sk_buff *skb, struct sock *sk,
                    struct tcp_sacktag_state *state,
                    int dup_sack, int pcount)
{
      struct tcp_sock *tp = tcp_sk(sk);
      u8 sacked = TCP_SKB_CB(skb)->sacked;
      int fack_count = state->fack_count;

      /* Account D-SACK for retransmitted packet. */
      if (dup_sack && (sacked & TCPCB_RETRANS)) {
            if (after(TCP_SKB_CB(skb)->end_seq, tp->undo_marker))
                  tp->undo_retrans--;
            if (sacked & TCPCB_SACKED_ACKED)
                  state->reord = min(fack_count, state->reord);
      }

      /* Nothing to do; acked frame is about to be dropped (was ACKed). */
      if (!after(TCP_SKB_CB(skb)->end_seq, tp->snd_una))
            return sacked;

      if (!(sacked & TCPCB_SACKED_ACKED)) {
            if (sacked & TCPCB_SACKED_RETRANS) {
                  /* If the segment is not tagged as lost,
                   * we do not clear RETRANS, believing
                   * that retransmission is still in flight.
                   */
                  if (sacked & TCPCB_LOST) {
                        sacked &= ~(TCPCB_LOST|TCPCB_SACKED_RETRANS);
                        tp->lost_out -= pcount;
                        tp->retrans_out -= pcount;
                  }
            } else {
                  if (!(sacked & TCPCB_RETRANS)) {
                        /* New sack for not retransmitted frame,
                         * which was in hole. It is reordering.
                         */
                        if (before(TCP_SKB_CB(skb)->seq,
                                 tcp_highest_sack_seq(tp)))
                              state->reord = min(fack_count,
                                             state->reord);

                        /* SACK enhanced F-RTO (RFC4138; Appendix B) */
                        if (!after(TCP_SKB_CB(skb)->end_seq, tp->frto_highmark))
                              state->flag |= FLAG_ONLY_ORIG_SACKED;
                  }

                  if (sacked & TCPCB_LOST) {
                        sacked &= ~TCPCB_LOST;
                        tp->lost_out -= pcount;
                  }
            }

            sacked |= TCPCB_SACKED_ACKED;
            state->flag |= FLAG_DATA_SACKED;
            tp->sacked_out += pcount;

            fack_count += pcount;

            /* Lost marker hint past SACKed? Tweak RFC3517 cnt */
            if (!tcp_is_fack(tp) && (tp->lost_skb_hint != NULL) &&
                before(TCP_SKB_CB(skb)->seq,
                     TCP_SKB_CB(tp->lost_skb_hint)->seq))
                  tp->lost_cnt_hint += pcount;

            if (fack_count > tp->fackets_out)
                  tp->fackets_out = fack_count;
      }

      /* D-SACK. We can detect redundant retransmission in S|R and plain R
       * frames and clear it. undo_retrans is decreased above, L|R frames
       * are accounted above as well.
       */
      if (dup_sack && (sacked & TCPCB_SACKED_RETRANS)) {
            sacked &= ~TCPCB_SACKED_RETRANS;
            tp->retrans_out -= pcount;
      }

      return sacked;
}

static int tcp_shifted_skb(struct sock *sk, struct sk_buff *skb,
                     struct tcp_sacktag_state *state,
                     unsigned int pcount, int shifted, int mss,
                     int dup_sack)
{
      struct tcp_sock *tp = tcp_sk(sk);
      struct sk_buff *prev = tcp_write_queue_prev(sk, skb);

      BUG_ON(!pcount);

      /* Tweak before seqno plays */
      if (!tcp_is_fack(tp) && tcp_is_sack(tp) && tp->lost_skb_hint &&
          !before(TCP_SKB_CB(tp->lost_skb_hint)->seq, TCP_SKB_CB(skb)->seq))
            tp->lost_cnt_hint += pcount;

      TCP_SKB_CB(prev)->end_seq += shifted;
      TCP_SKB_CB(skb)->seq += shifted;

      skb_shinfo(prev)->gso_segs += pcount;
      BUG_ON(skb_shinfo(skb)->gso_segs < pcount);
      skb_shinfo(skb)->gso_segs -= pcount;

      /* When we're adding to gso_segs == 1, gso_size will be zero,
       * in theory this shouldn't be necessary but as long as DSACK
       * code can come after this skb later on it's better to keep
       * setting gso_size to something.
       */
      if (!skb_shinfo(prev)->gso_size) {
            skb_shinfo(prev)->gso_size = mss;
            skb_shinfo(prev)->gso_type = sk->sk_gso_type;
      }

      /* CHECKME: To clear or not to clear? Mimics normal skb currently */
      if (skb_shinfo(skb)->gso_segs <= 1) {
            skb_shinfo(skb)->gso_size = 0;
            skb_shinfo(skb)->gso_type = 0;
      }

      /* We discard results */
      tcp_sacktag_one(skb, sk, state, dup_sack, pcount);

      /* Difference in this won't matter, both ACKed by the same cumul. ACK */
      TCP_SKB_CB(prev)->sacked |= (TCP_SKB_CB(skb)->sacked & TCPCB_EVER_RETRANS);

      if (skb->len > 0) {
            BUG_ON(!tcp_skb_pcount(skb));
            NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_SACKSHIFTED);
            return 0;
      }

      /* Whole SKB was eaten :-) */

      if (skb == tp->retransmit_skb_hint)
            tp->retransmit_skb_hint = prev;
      if (skb == tp->scoreboard_skb_hint)
            tp->scoreboard_skb_hint = prev;
      if (skb == tp->lost_skb_hint) {
            tp->lost_skb_hint = prev;
            tp->lost_cnt_hint -= tcp_skb_pcount(prev);
      }

      TCP_SKB_CB(skb)->flags |= TCP_SKB_CB(prev)->flags;
      if (skb == tcp_highest_sack(sk))
            tcp_advance_highest_sack(sk, skb);

      tcp_unlink_write_queue(skb, sk);
      sk_wmem_free_skb(sk, skb);

      NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_SACKMERGED);

      return 1;
}

/* I wish gso_size would have a bit more sane initialization than
 * something-or-zero which complicates things
 */
static int tcp_skb_seglen(struct sk_buff *skb)
{
      return tcp_skb_pcount(skb) == 1 ? skb->len : tcp_skb_mss(skb);
}

/* Shifting pages past head area doesn't work */
static int skb_can_shift(struct sk_buff *skb)
{
      return !skb_headlen(skb) && skb_is_nonlinear(skb);
}

/* Try collapsing SACK blocks spanning across multiple skbs to a single
 * skb.
 */
static struct sk_buff *tcp_shift_skb_data(struct sock *sk, struct sk_buff *skb,
                                struct tcp_sacktag_state *state,
                                u32 start_seq, u32 end_seq,
                                int dup_sack)
{
      struct tcp_sock *tp = tcp_sk(sk);
      struct sk_buff *prev;
      int mss;
      int pcount = 0;
      int len;
      int in_sack;

      if (!sk_can_gso(sk))
            goto fallback;

      /* Normally R but no L won't result in plain S */
      if (!dup_sack &&
          (TCP_SKB_CB(skb)->sacked & (TCPCB_LOST|TCPCB_SACKED_RETRANS)) == TCPCB_SACKED_RETRANS)
            goto fallback;
      if (!skb_can_shift(skb))
            goto fallback;
      /* This frame is about to be dropped (was ACKed). */
      if (!after(TCP_SKB_CB(skb)->end_seq, tp->snd_una))
            goto fallback;

      /* Can only happen with delayed DSACK + discard craziness */
      if (unlikely(skb == tcp_write_queue_head(sk)))
            goto fallback;
      prev = tcp_write_queue_prev(sk, skb);

      if ((TCP_SKB_CB(prev)->sacked & TCPCB_TAGBITS) != TCPCB_SACKED_ACKED)
            goto fallback;

      in_sack = !after(start_seq, TCP_SKB_CB(skb)->seq) &&
              !before(end_seq, TCP_SKB_CB(skb)->end_seq);

      if (in_sack) {
            len = skb->len;
            pcount = tcp_skb_pcount(skb);
            mss = tcp_skb_seglen(skb);

            /* TODO: Fix DSACKs to not fragment already SACKed and we can
             * drop this restriction as unnecessary
             */
            if (mss != tcp_skb_seglen(prev))
                  goto fallback;
      } else {
            if (!after(TCP_SKB_CB(skb)->end_seq, start_seq))
                  goto noop;
            /* CHECKME: This is non-MSS split case only?, this will
             * cause skipped skbs due to advancing loop btw, original
             * has that feature too
             */
            if (tcp_skb_pcount(skb) <= 1)
                  goto noop;

            in_sack = !after(start_seq, TCP_SKB_CB(skb)->seq);
            if (!in_sack) {
                  /* TODO: head merge to next could be attempted here
                   * if (!after(TCP_SKB_CB(skb)->end_seq, end_seq)),
                   * though it might not be worth of the additional hassle
                   *
                   * ...we can probably just fallback to what was done
                   * previously. We could try merging non-SACKed ones
                   * as well but it probably isn't going to buy off
                   * because later SACKs might again split them, and
                   * it would make skb timestamp tracking considerably
                   * harder problem.
                   */
                  goto fallback;
            }

            len = end_seq - TCP_SKB_CB(skb)->seq;
            BUG_ON(len < 0);
            BUG_ON(len > skb->len);

            /* MSS boundaries should be honoured or else pcount will
             * severely break even though it makes things bit trickier.
             * Optimize common case to avoid most of the divides
             */
            mss = tcp_skb_mss(skb);

            /* TODO: Fix DSACKs to not fragment already SACKed and we can
             * drop this restriction as unnecessary
             */
            if (mss != tcp_skb_seglen(prev))
                  goto fallback;

            if (len == mss) {
                  pcount = 1;
            } else if (len < mss) {
                  goto noop;
            } else {
                  pcount = len / mss;
                  len = pcount * mss;
            }
      }

      if (!skb_shift(prev, skb, len))
            goto fallback;
      if (!tcp_shifted_skb(sk, skb, state, pcount, len, mss, dup_sack))
            goto out;

      /* Hole filled allows collapsing with the next as well, this is very
       * useful when hole on every nth skb pattern happens
       */
      if (prev == tcp_write_queue_tail(sk))
            goto out;
      skb = tcp_write_queue_next(sk, prev);

      if (!skb_can_shift(skb) ||
          (skb == tcp_send_head(sk)) ||
          ((TCP_SKB_CB(skb)->sacked & TCPCB_TAGBITS) != TCPCB_SACKED_ACKED) ||
          (mss != tcp_skb_seglen(skb)))
            goto out;

      len = skb->len;
      if (skb_shift(prev, skb, len)) {
            pcount += tcp_skb_pcount(skb);
            tcp_shifted_skb(sk, skb, state, tcp_skb_pcount(skb), len, mss, 0);
      }

out:
      state->fack_count += pcount;
      return prev;

noop:
      return skb;

fallback:
      NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_SACKSHIFTFALLBACK);
      return NULL;
}

static struct sk_buff *tcp_sacktag_walk(struct sk_buff *skb, struct sock *sk,
                              struct tcp_sack_block *next_dup,
                              struct tcp_sacktag_state *state,
                              u32 start_seq, u32 end_seq,
                              int dup_sack_in)
{
      struct tcp_sock *tp = tcp_sk(sk);
      struct sk_buff *tmp;

      tcp_for_write_queue_from(skb, sk) {
            int in_sack = 0;
            int dup_sack = dup_sack_in;

            if (skb == tcp_send_head(sk))
                  break;

            /* queue is in-order => we can short-circuit the walk early */
            if (!before(TCP_SKB_CB(skb)->seq, end_seq))
                  break;

            if ((next_dup != NULL) &&
                before(TCP_SKB_CB(skb)->seq, next_dup->end_seq)) {
                  in_sack = tcp_match_skb_to_sack(sk, skb,
                                          next_dup->start_seq,
                                          next_dup->end_seq);
                  if (in_sack > 0)
                        dup_sack = 1;
            }

            /* skb reference here is a bit tricky to get right, since
             * shifting can eat and free both this skb and the next,
             * so not even _safe variant of the loop is enough.
             */
            if (in_sack <= 0) {
                  tmp = tcp_shift_skb_data(sk, skb, state,
                                     start_seq, end_seq, dup_sack);
                  if (tmp != NULL) {
                        if (tmp != skb) {
                              skb = tmp;
                              continue;
                        }

                        in_sack = 0;
                  } else {
                        in_sack = tcp_match_skb_to_sack(sk, skb,
                                                start_seq,
                                                end_seq);
                  }
            }

            if (unlikely(in_sack < 0))
                  break;

            if (in_sack) {
                  TCP_SKB_CB(skb)->sacked = tcp_sacktag_one(skb, sk,
                                                  state,
                                                  dup_sack,
                                                  tcp_skb_pcount(skb));

                  if (!before(TCP_SKB_CB(skb)->seq,
                            tcp_highest_sack_seq(tp)))
                        tcp_advance_highest_sack(sk, skb);
            }

            state->fack_count += tcp_skb_pcount(skb);
      }
      return skb;
}

/* Avoid all extra work that is being done by sacktag while walking in
 * a normal way
 */
static struct sk_buff *tcp_sacktag_skip(struct sk_buff *skb, struct sock *sk,
                              struct tcp_sacktag_state *state,
                              u32 skip_to_seq)
{
      tcp_for_write_queue_from(skb, sk) {
            if (skb == tcp_send_head(sk))
                  break;

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

            state->fack_count += tcp_skb_pcount(skb);
      }
      return skb;
}

static struct sk_buff *tcp_maybe_skipping_dsack(struct sk_buff *skb,
                                    struct sock *sk,
                                    struct tcp_sack_block *next_dup,
                                    struct tcp_sacktag_state *state,
                                    u32 skip_to_seq)
{
      if (next_dup == NULL)
            return skb;

      if (before(next_dup->start_seq, skip_to_seq)) {
            skb = tcp_sacktag_skip(skb, sk, state, next_dup->start_seq);
            skb = tcp_sacktag_walk(skb, sk, NULL, state,
                               next_dup->start_seq, next_dup->end_seq,
                               1);
      }

      return skb;
}

static int tcp_sack_cache_ok(struct tcp_sock *tp, struct tcp_sack_block *cache)
{
      return cache < tp->recv_sack_cache + ARRAY_SIZE(tp->recv_sack_cache);
}

static int
tcp_sacktag_write_queue(struct sock *sk, struct sk_buff *ack_skb,
                  u32 prior_snd_una)
{
      const struct inet_connection_sock *icsk = inet_csk(sk);
      struct tcp_sock *tp = tcp_sk(sk);
      unsigned char *ptr = (skb_transport_header(ack_skb) +
                        TCP_SKB_CB(ack_skb)->sacked);
      struct tcp_sack_block_wire *sp_wire = (struct tcp_sack_block_wire *)(ptr+2);
      struct tcp_sack_block sp[TCP_NUM_SACKS];
      struct tcp_sack_block *cache;
      struct tcp_sacktag_state state;
      struct sk_buff *skb;
      int num_sacks = min(TCP_NUM_SACKS, (ptr[1] - TCPOLEN_SACK_BASE) >> 3);
      int used_sacks;
      int found_dup_sack = 0;
      int i, j;
      int first_sack_index;

      state.flag = 0;
      state.reord = tp->packets_out;

      if (!tp->sacked_out) {
            if (WARN_ON(tp->fackets_out))
                  tp->fackets_out = 0;
            tcp_highest_sack_reset(sk);
      }

      found_dup_sack = tcp_check_dsack(sk, ack_skb, sp_wire,
                               num_sacks, prior_snd_una);
      if (found_dup_sack)
            state.flag |= FLAG_DSACKING_ACK;

      /* Eliminate too old ACKs, but take into
       * account more or less fresh ones, they can
       * contain valid SACK info.
       */
      if (before(TCP_SKB_CB(ack_skb)->ack_seq, prior_snd_una - tp->max_window))
            return 0;

      if (!tp->packets_out)
            goto out;

      used_sacks = 0;
      first_sack_index = 0;
      for (i = 0; i < num_sacks; i++) {
            int dup_sack = !i && found_dup_sack;

            sp[used_sacks].start_seq = get_unaligned_be32(&sp_wire[i].start_seq);
            sp[used_sacks].end_seq = get_unaligned_be32(&sp_wire[i].end_seq);

            if (!tcp_is_sackblock_valid(tp, dup_sack,
                                  sp[used_sacks].start_seq,
                                  sp[used_sacks].end_seq)) {
                  int mib_idx;

                  if (dup_sack) {
                        if (!tp->undo_marker)
                              mib_idx = LINUX_MIB_TCPDSACKIGNOREDNOUNDO;
                        else
                              mib_idx = LINUX_MIB_TCPDSACKIGNOREDOLD;
                  } else {
                        /* Don't count olds caused by ACK reordering */
                        if ((TCP_SKB_CB(ack_skb)->ack_seq != tp->snd_una) &&
                            !after(sp[used_sacks].end_seq, tp->snd_una))
                              continue;
                        mib_idx = LINUX_MIB_TCPSACKDISCARD;
                  }

                  NET_INC_STATS_BH(sock_net(sk), mib_idx);
                  if (i == 0)
                        first_sack_index = -1;
                  continue;
            }

            /* Ignore very old stuff early */
            if (!after(sp[used_sacks].end_seq, prior_snd_una))
                  continue;

            used_sacks++;
      }

      /* order SACK blocks to allow in order walk of the retrans queue */
      for (i = used_sacks - 1; i > 0; i--) {
            for (j = 0; j < i; j++) {
                  if (after(sp[j].start_seq, sp[j + 1].start_seq)) {
                        swap(sp[j], sp[j + 1]);

                        /* Track where the first SACK block goes to */
                        if (j == first_sack_index)
                              first_sack_index = j + 1;
                  }
            }
      }

      skb = tcp_write_queue_head(sk);
      state.fack_count = 0;
      i = 0;

      if (!tp->sacked_out) {
            /* It's already past, so skip checking against it */
            cache = tp->recv_sack_cache + ARRAY_SIZE(tp->recv_sack_cache);
      } else {
            cache = tp->recv_sack_cache;
            /* Skip empty blocks in at head of the cache */
            while (tcp_sack_cache_ok(tp, cache) && !cache->start_seq &&
                   !cache->end_seq)
                  cache++;
      }

      while (i < used_sacks) {
            u32 start_seq = sp[i].start_seq;
            u32 end_seq = sp[i].end_seq;
            int dup_sack = (found_dup_sack && (i == first_sack_index));
            struct tcp_sack_block *next_dup = NULL;

            if (found_dup_sack && ((i + 1) == first_sack_index))
                  next_dup = &sp[i + 1];

            /* Event "B" in the comment above. */
            if (after(end_seq, tp->high_seq))
                  state.flag |= FLAG_DATA_LOST;

            /* Skip too early cached blocks */
            while (tcp_sack_cache_ok(tp, cache) &&
                   !before(start_seq, cache->end_seq))
                  cache++;

            /* Can skip some work by looking recv_sack_cache? */
            if (tcp_sack_cache_ok(tp, cache) && !dup_sack &&
                after(end_seq, cache->start_seq)) {

                  /* Head todo? */
                  if (before(start_seq, cache->start_seq)) {
                        skb = tcp_sacktag_skip(skb, sk, &state,
                                           start_seq);
                        skb = tcp_sacktag_walk(skb, sk, next_dup,
                                           &state,
                                           start_seq,
                                           cache->start_seq,
                                           dup_sack);
                  }

                  /* Rest of the block already fully processed? */
                  if (!after(end_seq, cache->end_seq))
                        goto advance_sp;

                  skb = tcp_maybe_skipping_dsack(skb, sk, next_dup,
                                           &state,
                                           cache->end_seq);

                  /* ...tail remains todo... */
                  if (tcp_highest_sack_seq(tp) == cache->end_seq) {
                        /* ...but better entrypoint exists! */
                        skb = tcp_highest_sack(sk);
                        if (skb == NULL)
                              break;
                        state.fack_count = tp->fackets_out;
                        cache++;
                        goto walk;
                  }

                  skb = tcp_sacktag_skip(skb, sk, &state, cache->end_seq);
                  /* Check overlap against next cached too (past this one already) */
                  cache++;
                  continue;
            }

            if (!before(start_seq, tcp_highest_sack_seq(tp))) {
                  skb = tcp_highest_sack(sk);
                  if (skb == NULL)
                        break;
                  state.fack_count = tp->fackets_out;
            }
            skb = tcp_sacktag_skip(skb, sk, &state, start_seq);

walk:
            skb = tcp_sacktag_walk(skb, sk, next_dup, &state,
                               start_seq, end_seq, dup_sack);

advance_sp:
            /* SACK enhanced FRTO (RFC4138, Appendix B): Clearing correct
             * due to in-order walk
             */
            if (after(end_seq, tp->frto_highmark))
                  state.flag &= ~FLAG_ONLY_ORIG_SACKED;

            i++;
      }

      /* Clear the head of the cache sack blocks so we can skip it next time */
      for (i = 0; i < ARRAY_SIZE(tp->recv_sack_cache) - used_sacks; i++) {
            tp->recv_sack_cache[i].start_seq = 0;
            tp->recv_sack_cache[i].end_seq = 0;
      }
      for (j = 0; j < used_sacks; j++)
            tp->recv_sack_cache[i++] = sp[j];

      tcp_mark_lost_retrans(sk);

      tcp_verify_left_out(tp);

      if ((state.reord < tp->fackets_out) &&
          ((icsk->icsk_ca_state != TCP_CA_Loss) || tp->undo_marker) &&
          (!tp->frto_highmark || after(tp->snd_una, tp->frto_highmark)))
            tcp_update_reordering(sk, tp->fackets_out - state.reord, 0);

out:

#if FASTRETRANS_DEBUG > 0
      WARN_ON((int)tp->sacked_out < 0);
      WARN_ON((int)tp->lost_out < 0);
      WARN_ON((int)tp->retrans_out < 0);
      WARN_ON((int)tcp_packets_in_flight(tp) < 0);
#endif
      return state.flag;
}

/* Limits sacked_out so that sum with lost_out isn't ever larger than
 * packets_out. Returns zero if sacked_out adjustement wasn't necessary.
 */
static int tcp_limit_reno_sacked(struct tcp_sock *tp)
{
      u32 holes;

      holes = max(tp->lost_out, 1U);
      holes = min(holes, tp->packets_out);

      if ((tp->sacked_out + holes) > tp->packets_out) {
            tp->sacked_out = tp->packets_out - holes;
            return 1;
      }
      return 0;
}

/* If we receive more dupacks than we expected counting segments
 * in assumption of absent reordering, interpret this as reordering.
 * The only another reason could be bug in receiver TCP.
 */
static void tcp_check_reno_reordering(struct sock *sk, const int addend)
{
      struct tcp_sock *tp = tcp_sk(sk);
      if (tcp_limit_reno_sacked(tp))
            tcp_update_reordering(sk, tp->packets_out + addend, 0);
}

/* Emulate SACKs for SACKless connection: account for a new dupack. */

static void tcp_add_reno_sack(struct sock *sk)
{
      struct tcp_sock *tp = tcp_sk(sk);
      tp->sacked_out++;
      tcp_check_reno_reordering(sk, 0);
      tcp_verify_left_out(tp);
}

/* Account for ACK, ACKing some data in Reno Recovery phase. */

static void tcp_remove_reno_sacks(struct sock *sk, int acked)
{
      struct tcp_sock *tp = tcp_sk(sk);

      if (acked > 0) {
            /* One ACK acked hole. The rest eat duplicate ACKs. */
            if (acked - 1 >= tp->sacked_out)
                  tp->sacked_out = 0;
            else
                  tp->sacked_out -= acked - 1;
      }
      tcp_check_reno_reordering(sk, acked);
      tcp_verify_left_out(tp);
}

static inline void tcp_reset_reno_sack(struct tcp_sock *tp)
{
      tp->sacked_out = 0;
}

static int tcp_is_sackfrto(const struct tcp_sock *tp)
{
      return (sysctl_tcp_frto == 0x2) && !tcp_is_reno(tp);
}

/* F-RTO can only be used if TCP has never retransmitted anything other than
 * head (SACK enhanced variant from Appendix B of RFC4138 is more robust here)
 */
int tcp_use_frto(struct sock *sk)
{
      const struct tcp_sock *tp = tcp_sk(sk);
      const struct inet_connection_sock *icsk = inet_csk(sk);
      struct sk_buff *skb;

      if (!sysctl_tcp_frto)
            return 0;

      /* MTU probe and F-RTO won't really play nicely along currently */
      if (icsk->icsk_mtup.probe_size)
            return 0;

      if (tcp_is_sackfrto(tp))
            return 1;

      /* Avoid expensive walking of rexmit queue if possible */
      if (tp->retrans_out > 1)
            return 0;

      skb = tcp_write_queue_head(sk);
      if (tcp_skb_is_last(sk, skb))
            return 1;
      skb = tcp_write_queue_next(sk, skb);      /* Skips head */
      tcp_for_write_queue_from(skb, sk) {
            if (skb == tcp_send_head(sk))
                  break;
            if (TCP_SKB_CB(skb)->sacked & TCPCB_RETRANS)
                  return 0;
            /* Short-circuit when first non-SACKed skb has been checked */
            if (!(TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_ACKED))
                  break;
      }
      return 1;
}

/* RTO occurred, but do not yet enter Loss state. Instead, defer RTO
 * recovery a bit and use heuristics in tcp_process_frto() to detect if
 * the RTO was spurious. Only clear SACKED_RETRANS of the head here to
 * keep retrans_out counting accurate (with SACK F-RTO, other than head
 * may still have that bit set); TCPCB_LOST and remaining SACKED_RETRANS
 * bits are handled if the Loss state is really to be entered (in
 * tcp_enter_frto_loss).
 *
 * Do like tcp_enter_loss() would; when RTO expires the second time it
 * does:
 *  "Reduce ssthresh if it has not yet been made inside this window."
 */
void tcp_enter_frto(struct sock *sk)
{
      const struct inet_connection_sock *icsk = inet_csk(sk);
      struct tcp_sock *tp = tcp_sk(sk);
      struct sk_buff *skb;

      if ((!tp->frto_counter && icsk->icsk_ca_state <= TCP_CA_Disorder) ||
          tp->snd_una == tp->high_seq ||
          ((icsk->icsk_ca_state == TCP_CA_Loss || tp->frto_counter) &&
           !icsk->icsk_retransmits)) {
            tp->prior_ssthresh = tcp_current_ssthresh(sk);
            /* Our state is too optimistic in ssthresh() call because cwnd
             * is not reduced until tcp_enter_frto_loss() when previous F-RTO
             * recovery has not yet completed. Pattern would be this: RTO,
             * Cumulative ACK, RTO (2xRTO for the same segment does not end
             * up here twice).
             * RFC4138 should be more specific on what to do, even though
             * RTO is quite unlikely to occur after the first Cumulative ACK
             * due to back-off and complexity of triggering events ...
             */
            if (tp->frto_counter) {
                  u32 stored_cwnd;
                  stored_cwnd = tp->snd_cwnd;
                  tp->snd_cwnd = 2;
                  tp->snd_ssthresh = icsk->icsk_ca_ops->ssthresh(sk);
                  tp->snd_cwnd = stored_cwnd;
            } else {
                  tp->snd_ssthresh = icsk->icsk_ca_ops->ssthresh(sk);
            }
            /* ... in theory, cong.control module could do "any tricks" in
             * ssthresh(), which means that ca_state, lost bits and lost_out
             * counter would have to be faked before the call occurs. We
             * consider that too expensive, unlikely and hacky, so modules
             * using these in ssthresh() must deal these incompatibility
             * issues if they receives CA_EVENT_FRTO and frto_counter != 0
             */
            tcp_ca_event(sk, CA_EVENT_FRTO);
      }

      tp->undo_marker = tp->snd_una;
      tp->undo_retrans = 0;

      skb = tcp_write_queue_head(sk);
      if (TCP_SKB_CB(skb)->sacked & TCPCB_RETRANS)
            tp->undo_marker = 0;
      if (TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_RETRANS) {
            TCP_SKB_CB(skb)->sacked &= ~TCPCB_SACKED_RETRANS;
            tp->retrans_out -= tcp_skb_pcount(skb);
      }
      tcp_verify_left_out(tp);

      /* Too bad if TCP was application limited */
      tp->snd_cwnd = min(tp->snd_cwnd, tcp_packets_in_flight(tp) + 1);

      /* Earlier loss recovery underway (see RFC4138; Appendix B).
       * The last condition is necessary at least in tp->frto_counter case.
       */
      if (tcp_is_sackfrto(tp) && (tp->frto_counter ||
          ((1 << icsk->icsk_ca_state) & (TCPF_CA_Recovery|TCPF_CA_Loss))) &&
          after(tp->high_seq, tp->snd_una)) {
            tp->frto_highmark = tp->high_seq;
      } else {
            tp->frto_highmark = tp->snd_nxt;
      }
      tcp_set_ca_state(sk, TCP_CA_Disorder);
      tp->high_seq = tp->snd_nxt;
      tp->frto_counter = 1;
}

/* Enter Loss state after F-RTO was applied. Dupack arrived after RTO,
 * which indicates that we should follow the traditional RTO recovery,
 * i.e. mark everything lost and do go-back-N retransmission.
 */
static void tcp_enter_frto_loss(struct sock *sk, int allowed_segments, int flag)
{
      struct tcp_sock *tp = tcp_sk(sk);
      struct sk_buff *skb;

      tp->lost_out = 0;
      tp->retrans_out = 0;
      if (tcp_is_reno(tp))
            tcp_reset_reno_sack(tp);

      tcp_for_write_queue(skb, sk) {
            if (skb == tcp_send_head(sk))
                  break;

            TCP_SKB_CB(skb)->sacked &= ~TCPCB_LOST;
            /*
             * Count the retransmission made on RTO correctly (only when
             * waiting for the first ACK and did not get it)...
             */
            if ((tp->frto_counter == 1) && !(flag & FLAG_DATA_ACKED)) {
                  /* For some reason this R-bit might get cleared? */
                  if (TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_RETRANS)
                        tp->retrans_out += tcp_skb_pcount(skb);
                  /* ...enter this if branch just for the first segment */
                  flag |= FLAG_DATA_ACKED;
            } else {
                  if (TCP_SKB_CB(skb)->sacked & TCPCB_RETRANS)
                        tp->undo_marker = 0;
                  TCP_SKB_CB(skb)->sacked &= ~TCPCB_SACKED_RETRANS;
            }

            /* Marking forward transmissions that were made after RTO lost
             * can cause unnecessary retransmissions in some scenarios,
             * SACK blocks will mitigate that in some but not in all cases.
             * We used to not mark them but it was causing break-ups with
             * receivers that do only in-order receival.
             *
             * TODO: we could detect presence of such receiver and select
             * different behavior per flow.
             */
            if (!(TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_ACKED)) {
                  TCP_SKB_CB(skb)->sacked |= TCPCB_LOST;
                  tp->lost_out += tcp_skb_pcount(skb);
                  tp->retransmit_high = TCP_SKB_CB(skb)->end_seq;
            }
      }
      tcp_verify_left_out(tp);

      tp->snd_cwnd = tcp_packets_in_flight(tp) + allowed_segments;
      tp->snd_cwnd_cnt = 0;
      tp->snd_cwnd_stamp = tcp_time_stamp;
      tp->frto_counter = 0;
      tp->bytes_acked = 0;

      tp->reordering = min_t(unsigned int, tp->reordering,
                         sysctl_tcp_reordering);
      tcp_set_ca_state(sk, TCP_CA_Loss);
      tp->high_seq = tp->snd_nxt;
      TCP_ECN_queue_cwr(tp);

      tcp_clear_all_retrans_hints(tp);
}

static void tcp_clear_retrans_partial(struct tcp_sock *tp)
{
      tp->retrans_out = 0;
      tp->lost_out = 0;

      tp->undo_marker = 0;
      tp->undo_retrans = 0;
}

void tcp_clear_retrans(struct tcp_sock *tp)
{
      tcp_clear_retrans_partial(tp);

      tp->fackets_out = 0;
      tp->sacked_out = 0;
}

/* Enter Loss state. If "how" is not zero, forget all SACK information
 * and reset tags completely, otherwise preserve SACKs. If receiver
 * dropped its ofo queue, we will know this due to reneging detection.
 */
void tcp_enter_loss(struct sock *sk, int how)
{
      const struct inet_connection_sock *icsk = inet_csk(sk);
      struct tcp_sock *tp = tcp_sk(sk);
      struct sk_buff *skb;

      /* Reduce ssthresh if it has not yet been made inside this window. */
      if (icsk->icsk_ca_state <= TCP_CA_Disorder || tp->snd_una == tp->high_seq ||
          (icsk->icsk_ca_state == TCP_CA_Loss && !icsk->icsk_retransmits)) {
            tp->prior_ssthresh = tcp_current_ssthresh(sk);
            tp->snd_ssthresh = icsk->icsk_ca_ops->ssthresh(sk);
            tcp_ca_event(sk, CA_EVENT_LOSS);
      }
      tp->snd_cwnd         = 1;
      tp->snd_cwnd_cnt   = 0;
      tp->snd_cwnd_stamp = tcp_time_stamp;

      tp->bytes_acked = 0;
      tcp_clear_retrans_partial(tp);

      if (tcp_is_reno(tp))
            tcp_reset_reno_sack(tp);

      if (!how) {
            /* Push undo marker, if it was plain RTO and nothing
             * was retransmitted. */
            tp->undo_marker = tp->snd_una;
      } else {
            tp->sacked_out = 0;
            tp->fackets_out = 0;
      }
      tcp_clear_all_retrans_hints(tp);

      tcp_for_write_queue(skb, sk) {
            if (skb == tcp_send_head(sk))
                  break;

            if (TCP_SKB_CB(skb)->sacked & TCPCB_RETRANS)
                  tp->undo_marker = 0;
            TCP_SKB_CB(skb)->sacked &= (~TCPCB_TAGBITS)|TCPCB_SACKED_ACKED;
            if (!(TCP_SKB_CB(skb)->sacked&TCPCB_SACKED_ACKED) || how) {
                  TCP_SKB_CB(skb)->sacked &= ~TCPCB_SACKED_ACKED;
                  TCP_SKB_CB(skb)->sacked |= TCPCB_LOST;
                  tp->lost_out += tcp_skb_pcount(skb);
                  tp->retransmit_high = TCP_SKB_CB(skb)->end_seq;
            }
      }
      tcp_verify_left_out(tp);

      tp->reordering = min_t(unsigned int, tp->reordering,
                         sysctl_tcp_reordering);
      tcp_set_ca_state(sk, TCP_CA_Loss);
      tp->high_seq = tp->snd_nxt;
      TCP_ECN_queue_cwr(tp);
      /* Abort F-RTO algorithm if one is in progress */
      tp->frto_counter = 0;
}

/* If ACK arrived pointing to a remembered SACK, it means that our
 * remembered SACKs do not reflect real state of receiver i.e.
 * receiver _host_ is heavily congested (or buggy).
 *
 * Do processing similar to RTO timeout.
 */
static int tcp_check_sack_reneging(struct sock *sk, int flag)
{
      if (flag & FLAG_SACK_RENEGING) {
            struct inet_connection_sock *icsk = inet_csk(sk);
            NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPSACKRENEGING);

            tcp_enter_loss(sk, 1);
            icsk->icsk_retransmits++;
            tcp_retransmit_skb(sk, tcp_write_queue_head(sk));
            inet_csk_reset_xmit_timer(sk, ICSK_TIME_RETRANS,
                                icsk->icsk_rto, TCP_RTO_MAX);
            return 1;
      }
      return 0;
}

static inline int tcp_fackets_out(struct tcp_sock *tp)
{
      return tcp_is_reno(tp) ? tp->sacked_out + 1 : tp->fackets_out;
}

/* Heurestics to calculate number of duplicate ACKs. There's no dupACKs
 * counter when SACK is enabled (without SACK, sacked_out is used for
 * that purpose).
 *
 * Instead, with FACK TCP uses fackets_out that includes both SACKed
 * segments up to the highest received SACK block so far and holes in
 * between them.
 *
 * With reordering, holes may still be in flight, so RFC3517 recovery
 * uses pure sacked_out (total number of SACKed segments) even though
 * it violates the RFC that uses duplicate ACKs, often these are equal
 * but when e.g. out-of-window ACKs or packet duplication occurs,
 * they differ. Since neither occurs due to loss, TCP should really
 * ignore them.
 */
static inline int tcp_dupack_heurestics(struct tcp_sock *tp)
{
      return tcp_is_fack(tp) ? tp->fackets_out : tp->sacked_out + 1;
}

static inline int tcp_skb_timedout(struct sock *sk, struct sk_buff *skb)
{
      return (tcp_time_stamp - TCP_SKB_CB(skb)->when > inet_csk(sk)->icsk_rto);
}

static inline int tcp_head_timedout(struct sock *sk)
{
      struct tcp_sock *tp = tcp_sk(sk);

      return tp->packets_out &&
             tcp_skb_timedout(sk, tcp_write_queue_head(sk));
}

/* Linux NewReno/SACK/FACK/ECN state machine.
 * --------------------------------------
 *
 * "Open"   Normal state, no dubious events, fast path.
 * "Disorder"   In all the respects it is "Open",
 *          but requires a bit more attention. It is entered when
 *          we see some SACKs or dupacks. It is split of "Open"
 *          mainly to move some processing from fast path to slow one.
 * "CWR"    CWND was reduced due to some Congestion Notification event.
 *          It can be ECN, ICMP source quench, local device congestion.
 * "Recovery"     CWND was reduced, we are fast-retransmitting.
 * "Loss"   CWND was reduced due to RTO timeout or SACK reneging.
 *
 * tcp_fastretrans_alert() is entered:
 * - each incoming ACK, if state is not "Open"
 * - when arrived ACK is unusual, namely:
 *    * SACK
 *    * Duplicate ACK.
 *    * ECN ECE.
 *
 * Counting packets in flight is pretty simple.
 *
 *    in_flight = packets_out - left_out + retrans_out
 *
 *    packets_out is SND.NXT-SND.UNA counted in packets.
 *
 *    retrans_out is number of retransmitted segments.
 *
 *    left_out is number of segments left network, but not ACKed yet.
 *
 *          left_out = sacked_out + lost_out
 *
 *     sacked_out: Packets, which arrived to receiver out of order
 *             and hence not ACKed. With SACKs this number is simply
 *             amount of SACKed data. Even without SACKs
 *             it is easy to give pretty reliable estimate of this number,
 *             counting duplicate ACKs.
 *
 *       lost_out: Packets lost by network. TCP has no explicit
 *             "loss notification" feedback from network (for now).
 *             It means that this number can be only _guessed_.
 *             Actually, it is the heuristics to predict lossage that
 *             distinguishes different algorithms.
 *
 *    F.e. after RTO, when all the queue is considered as lost,
 *    lost_out = packets_out and in_flight = retrans_out.
 *
 *          Essentially, we have now two algorithms counting
 *          lost packets.
 *
 *          FACK: It is the simplest heuristics. As soon as we decided
 *          that something is lost, we decide that _all_ not SACKed
 *          packets until the most forward SACK are lost. I.e.
 *          lost_out = fackets_out - sacked_out and left_out = fackets_out.
 *          It is absolutely correct estimate, if network does not reorder
 *          packets. And it loses any connection to reality when reordering
 *          takes place. We use FACK by default until reordering
 *          is suspected on the path to this destination.
 *
 *          NewReno: when Recovery is entered, we assume that one segment
 *          is lost (classic Reno). While we are in Recovery and
 *          a partial ACK arrives, we assume that one more packet
 *          is lost (NewReno). This heuristics are the same in NewReno
 *          and SACK.
 *
 *  Imagine, that's all! Forget about all this shamanism about CWND inflation
 *  deflation etc. CWND is real congestion window, never inflated, changes
 *  only according to classic VJ rules.
 *
 * Really tricky (and requiring careful tuning) part of algorithm
 * is hidden in functions tcp_time_to_recover() and tcp_xmit_retransmit_queue().
 * The first determines the moment _when_ we should reduce CWND and,
 * hence, slow down forward transmission. In fact, it determines the moment
 * when we decide that hole is caused by loss, rather than by a reorder.
 *
 * tcp_xmit_retransmit_queue() decides, _what_ we should retransmit to fill
 * holes, caused by lost packets.
 *
 * And the most logically complicated part of algorithm is undo
 * heuristics. We detect false retransmits due to both too early
 * fast retransmit (reordering) and underestimated RTO, analyzing
 * timestamps and D-SACKs. When we detect that some segments were
 * retransmitted by mistake and CWND reduction was wrong, we undo
 * window reduction and abort recovery phase. This logic is hidden
 * inside several functions named tcp_try_undo_<something>.
 */

/* This function decides, when we should leave Disordered state
 * and enter Recovery phase, reducing congestion window.
 *
 * Main question: may we further continue forward transmission
 * with the same cwnd?
 */
static int tcp_time_to_recover(struct sock *sk)
{
      struct tcp_sock *tp = tcp_sk(sk);
      __u32 packets_out;

      /* Do not perform any recovery during F-RTO algorithm */
      if (tp->frto_counter)
            return 0;

      /* Trick#1: The loss is proven. */
      if (tp->lost_out)
            return 1;

      /* Not-A-Trick#2 : Classic rule... */
      if (tcp_dupack_heurestics(tp) > tp->reordering)
            return 1;

      /* Trick#3 : when we use RFC2988 timer restart, fast
       * retransmit can be triggered by timeout of queue head.
       */
      if (tcp_is_fack(tp) && tcp_head_timedout(sk))
            return 1;

      /* Trick#4: It is still not OK... But will it be useful to delay
       * recovery more?
       */
      packets_out = tp->packets_out;
      if (packets_out <= tp->reordering &&
          tp->sacked_out >= max_t(__u32, packets_out/2, sysctl_tcp_reordering) &&
          !tcp_may_send_now(sk)) {
            /* We have nothing to send. This connection is limited
             * either by receiver window or by application.
             */
            return 1;
      }

      return 0;
}

/* New heuristics: it is possible only after we switched to restart timer
 * each time when something is ACKed. Hence, we can detect timed out packets
 * during fast retransmit without falling to slow start.
 *
 * Usefulness of this as is very questionable, since we should know which of
 * the segments is the next to timeout which is relatively expensive to find
 * in general case unless we add some data structure just for that. The
 * current approach certainly won't find the right one too often and when it
 * finally does find _something_ it usually marks large part of the window
 * right away (because a retransmission with a larger timestamp blocks the
 * loop from advancing). -ij
 */
static void tcp_timeout_skbs(struct sock *sk)
{
      struct tcp_sock *tp = tcp_sk(sk);
      struct sk_buff *skb;

      if (!tcp_is_fack(tp) || !tcp_head_timedout(sk))
            return;

      skb = tp->scoreboard_skb_hint;
      if (tp->scoreboard_skb_hint == NULL)
            skb = tcp_write_queue_head(sk);

      tcp_for_write_queue_from(skb, sk) {
            if (skb == tcp_send_head(sk))
                  break;
            if (!tcp_skb_timedout(sk, skb))
                  break;

            tcp_skb_mark_lost(tp, skb);
      }

      tp->scoreboard_skb_hint = skb;

      tcp_verify_left_out(tp);
}

/* Mark head of queue up as lost. With RFC3517 SACK, the packets is
 * is against sacked "cnt", otherwise it's against facked "cnt"
 */
static void tcp_mark_head_lost(struct sock *sk, int packets)
{
      struct tcp_sock *tp = tcp_sk(sk);
      struct sk_buff *skb;
      int cnt, oldcnt;
      int err;
      unsigned int mss;

      WARN_ON(packets > tp->packets_out);
      if (tp->lost_skb_hint) {
            skb = tp->lost_skb_hint;
            cnt = tp->lost_cnt_hint;
      } else {
            skb = tcp_write_queue_head(sk);
            cnt = 0;
      }

      tcp_for_write_queue_from(skb, sk) {
            if (skb == tcp_send_head(sk))
                  break;
            /* TODO: do this better */
            /* this is not the most efficient way to do this... */
            tp->lost_skb_hint = skb;
            tp->lost_cnt_hint = cnt;

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

            oldcnt = cnt;
            if (tcp_is_fack(tp) || tcp_is_reno(tp) ||
                (TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_ACKED))
                  cnt += tcp_skb_pcount(skb);

            if (cnt > packets) {
                  if (tcp_is_sack(tp) || (oldcnt >= packets))
                        break;

                  mss = skb_shinfo(skb)->gso_size;
                  err = tcp_fragment(sk, skb, (packets - oldcnt) * mss, mss);
                  if (err < 0)
                        break;
                  cnt = packets;
            }

            tcp_skb_mark_lost(tp, skb);
      }
      tcp_verify_left_out(tp);
}

/* Account newly detected lost packet(s) */

static void tcp_update_scoreboard(struct sock *sk, int fast_rexmit)
{
      struct tcp_sock *tp = tcp_sk(sk);

      if (tcp_is_reno(tp)) {
            tcp_mark_head_lost(sk, 1);
      } else if (tcp_is_fack(tp)) {
            int lost = tp->fackets_out - tp->reordering;
            if (lost <= 0)
                  lost = 1;
            tcp_mark_head_lost(sk, lost);
      } else {
            int sacked_upto = tp->sacked_out - tp->reordering;
            if (sacked_upto < fast_rexmit)
                  sacked_upto = fast_rexmit;
            tcp_mark_head_lost(sk, sacked_upto);
      }

      tcp_timeout_skbs(sk);
}

/* CWND moderation, preventing bursts due to too big ACKs
 * in dubious situations.
 */
static inline void tcp_moderate_cwnd(struct tcp_sock *tp)
{
      tp->snd_cwnd = min(tp->snd_cwnd,
                     tcp_packets_in_flight(tp) + tcp_max_burst(tp));
      tp->snd_cwnd_stamp = tcp_time_stamp;
}

/* Lower bound on congestion window is slow start threshold
 * unless congestion avoidance choice decides to overide it.
 */
static inline u32 tcp_cwnd_min(const struct sock *sk)
{
      const struct tcp_congestion_ops *ca_ops = inet_csk(sk)->icsk_ca_ops;

      return ca_ops->min_cwnd ? ca_ops->min_cwnd(sk) : tcp_sk(sk)->snd_ssthresh;
}

/* Decrease cwnd each second ack. */
static void tcp_cwnd_down(struct sock *sk, int flag)
{
      struct tcp_sock *tp = tcp_sk(sk);
      int decr = tp->snd_cwnd_cnt + 1;

      if ((flag & (FLAG_ANY_PROGRESS | FLAG_DSACKING_ACK)) ||
          (tcp_is_reno(tp) && !(flag & FLAG_NOT_DUP))) {
            tp->snd_cwnd_cnt = decr & 1;
            decr >>= 1;

            if (decr && tp->snd_cwnd > tcp_cwnd_min(sk))
                  tp->snd_cwnd -= decr;

            tp->snd_cwnd = min(tp->snd_cwnd, tcp_packets_in_flight(tp) + 1);
            tp->snd_cwnd_stamp = tcp_time_stamp;
      }
}

/* Nothing was retransmitted or returned timestamp is less
 * than timestamp of the first retransmission.
 */
static inline int tcp_packet_delayed(struct tcp_sock *tp)
{
      return !tp->retrans_stamp ||
            (tp->rx_opt.saw_tstamp && tp->rx_opt.rcv_tsecr &&
             before(tp->rx_opt.rcv_tsecr, tp->retrans_stamp));
}

/* Undo procedures. */

#if FASTRETRANS_DEBUG > 1
static void DBGUNDO(struct sock *sk, const char *msg)
{
      struct tcp_sock *tp = tcp_sk(sk);
      struct inet_sock *inet = inet_sk(sk);

      if (sk->sk_family == AF_INET) {
            printk(KERN_DEBUG "Undo %s %pI4/%u c%u l%u ss%u/%u p%u\n",
                   msg,
                   &inet->daddr, ntohs(inet->dport),
                   tp->snd_cwnd, tcp_left_out(tp),
                   tp->snd_ssthresh, tp->prior_ssthresh,
                   tp->packets_out);
      }
#if defined(CONFIG_IPV6) || defined(CONFIG_IPV6_MODULE)
      else if (sk->sk_family == AF_INET6) {
            struct ipv6_pinfo *np = inet6_sk(sk);
            printk(KERN_DEBUG "Undo %s %pI6/%u c%u l%u ss%u/%u p%u\n",
                   msg,
                   &np->daddr, ntohs(inet->dport),
                   tp->snd_cwnd, tcp_left_out(tp),
                   tp->snd_ssthresh, tp->prior_ssthresh,
                   tp->packets_out);
      }
#endif
}
#else
#define DBGUNDO(x...) do { } while (0)
#endif

static void tcp_undo_cwr(struct sock *sk, const int undo)
{
      struct tcp_sock *tp = tcp_sk(sk);

      if (tp->prior_ssthresh) {
            const struct inet_connection_sock *icsk = inet_csk(sk);

            if (icsk->icsk_ca_ops->undo_cwnd)
                  tp->snd_cwnd = icsk->icsk_ca_ops->undo_cwnd(sk);
            else
                  tp->snd_cwnd = max(tp->snd_cwnd, tp->snd_ssthresh << 1);

            if (undo && tp->prior_ssthresh > tp->snd_ssthresh) {
                  tp->snd_ssthresh = tp->prior_ssthresh;
                  TCP_ECN_withdraw_cwr(tp);
            }
      } else {
            tp->snd_cwnd = max(tp->snd_cwnd, tp->snd_ssthresh);
      }
      tcp_moderate_cwnd(tp);
      tp->snd_cwnd_stamp = tcp_time_stamp;
}

static inline int tcp_may_undo(struct tcp_sock *tp)
{
      return tp->undo_marker && (!tp->undo_retrans || tcp_packet_delayed(tp));
}

/* People celebrate: "We love our President!" */
static int tcp_try_undo_recovery(struct sock *sk)
{
      struct tcp_sock *tp = tcp_sk(sk);

      if (tcp_may_undo(tp)) {
            int mib_idx;

            /* Happy end! We did not retransmit anything
             * or our original transmission succeeded.
             */
            DBGUNDO(sk, inet_csk(sk)->icsk_ca_state == TCP_CA_Loss ? "loss" : "retrans");
            tcp_undo_cwr(sk, 1);
            if (inet_csk(sk)->icsk_ca_state == TCP_CA_Loss)
                  mib_idx = LINUX_MIB_TCPLOSSUNDO;
            else
                  mib_idx = LINUX_MIB_TCPFULLUNDO;

            NET_INC_STATS_BH(sock_net(sk), mib_idx);
            tp->undo_marker = 0;
      }
      if (tp->snd_una == tp->high_seq && tcp_is_reno(tp)) {
            /* Hold old state until something *above* high_seq
             * is ACKed. For Reno it is MUST to prevent false
             * fast retransmits (RFC2582). SACK TCP is safe. */
            tcp_moderate_cwnd(tp);
            return 1;
      }
      tcp_set_ca_state(sk, TCP_CA_Open);
      return 0;
}

/* Try to undo cwnd reduction, because D-SACKs acked all retransmitted data */
static void tcp_try_undo_dsack(struct sock *sk)
{
      struct tcp_sock *tp = tcp_sk(sk);

      if (tp->undo_marker && !tp->undo_retrans) {
            DBGUNDO(sk, "D-SACK");
            tcp_undo_cwr(sk, 1);
            tp->undo_marker = 0;
            NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPDSACKUNDO);
      }
}

/* Undo during fast recovery after partial ACK. */

static int tcp_try_undo_partial(struct sock *sk, int acked)
{
      struct tcp_sock *tp = tcp_sk(sk);
      /* Partial ACK arrived. Force Hoe's retransmit. */
      int failed = tcp_is_reno(tp) || (tcp_fackets_out(tp) > tp->reordering);

      if (tcp_may_undo(tp)) {
            /* Plain luck! Hole if filled with delayed
             * packet, rather than with a retransmit.
             */
            if (tp->retrans_out == 0)
                  tp->retrans_stamp = 0;

            tcp_update_reordering(sk, tcp_fackets_out(tp) + acked, 1);

            DBGUNDO(sk, "Hoe");
            tcp_undo_cwr(sk, 0);
            NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPPARTIALUNDO);

            /* So... Do not make Hoe's retransmit yet.
             * If the first packet was delayed, the rest
             * ones are most probably delayed as well.
             */
            failed = 0;
      }
      return failed;
}

/* Undo during loss recovery after partial ACK. */
static int tcp_try_undo_loss(struct sock *sk)
{
      struct tcp_sock *tp = tcp_sk(sk);

      if (tcp_may_undo(tp)) {
            struct sk_buff *skb;
            tcp_for_write_queue(skb, sk) {
                  if (skb == tcp_send_head(sk))
                        break;
                  TCP_SKB_CB(skb)->sacked &= ~TCPCB_LOST;
            }

            tcp_clear_all_retrans_hints(tp);

            DBGUNDO(sk, "partial loss");
            tp->lost_out = 0;
            tcp_undo_cwr(sk, 1);
            NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPLOSSUNDO);
            inet_csk(sk)->icsk_retransmits = 0;
            tp->undo_marker = 0;
            if (tcp_is_sack(tp))
                  tcp_set_ca_state(sk, TCP_CA_Open);
            return 1;
      }
      return 0;
}

static inline void tcp_complete_cwr(struct sock *sk)
{
      struct tcp_sock *tp = tcp_sk(sk);
      tp->snd_cwnd = min(tp->snd_cwnd, tp->snd_ssthresh);
      tp->snd_cwnd_stamp = tcp_time_stamp;
      tcp_ca_event(sk, CA_EVENT_COMPLETE_CWR);
}

static void tcp_try_keep_open(struct sock *sk)
{
      struct tcp_sock *tp = tcp_sk(sk);
      int state = TCP_CA_Open;

      if (tcp_left_out(tp) || tp->retrans_out || tp->undo_marker)
            state = TCP_CA_Disorder;

      if (inet_csk(sk)->icsk_ca_state != state) {
            tcp_set_ca_state(sk, state);
            tp->high_seq = tp->snd_nxt;
      }
}

static void tcp_try_to_open(struct sock *sk, int flag)
{
      struct tcp_sock *tp = tcp_sk(sk);

      tcp_verify_left_out(tp);

      if (!tp->frto_counter && tp->retrans_out == 0)
            tp->retrans_stamp = 0;

      if (flag & FLAG_ECE)
            tcp_enter_cwr(sk, 1);

      if (inet_csk(sk)->icsk_ca_state != TCP_CA_CWR) {
            tcp_try_keep_open(sk);
            tcp_moderate_cwnd(tp);
      } else {
            tcp_cwnd_down(sk, flag);
      }
}

static void tcp_mtup_probe_failed(struct sock *sk)
{
      struct inet_connection_sock *icsk = inet_csk(sk);

      icsk->icsk_mtup.search_high = icsk->icsk_mtup.probe_size - 1;
      icsk->icsk_mtup.probe_size = 0;
}

static void tcp_mtup_probe_success(struct sock *sk)
{
      struct tcp_sock *tp = tcp_sk(sk);
      struct inet_connection_sock *icsk = inet_csk(sk);

      /* FIXME: breaks with very large cwnd */
      tp->prior_ssthresh = tcp_current_ssthresh(sk);
      tp->snd_cwnd = tp->snd_cwnd *
                   tcp_mss_to_mtu(sk, tp->mss_cache) /
                   icsk->icsk_mtup.probe_size;
      tp->snd_cwnd_cnt = 0;
      tp->snd_cwnd_stamp = tcp_time_stamp;
      tp->rcv_ssthresh = tcp_current_ssthresh(sk);

      icsk->icsk_mtup.search_low = icsk->icsk_mtup.probe_size;
      icsk->icsk_mtup.probe_size = 0;
      tcp_sync_mss(sk, icsk->icsk_pmtu_cookie);
}

/* Do a simple retransmit without using the backoff mechanisms in
 * tcp_timer. This is used for path mtu discovery.
 * The socket is already locked here.
 */
void tcp_simple_retransmit(struct sock *sk)
{
      const struct inet_connection_sock *icsk = inet_csk(sk);
      struct tcp_sock *tp = tcp_sk(sk);
      struct sk_buff *skb;
      unsigned int mss = tcp_current_mss(sk);
      u32 prior_lost = tp->lost_out;

      tcp_for_write_queue(skb, sk) {
            if (skb == tcp_send_head(sk))
                  break;
            if (tcp_skb_seglen(skb) > mss &&
                !(TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_ACKED)) {
                  if (TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_RETRANS) {
                        TCP_SKB_CB(skb)->sacked &= ~TCPCB_SACKED_RETRANS;
                        tp->retrans_out -= tcp_skb_pcount(skb);
                  }
                  tcp_skb_mark_lost_uncond_verify(tp, skb);
            }
      }

      tcp_clear_retrans_hints_partial(tp);

      if (prior_lost == tp->lost_out)
            return;

      if (tcp_is_reno(tp))
            tcp_limit_reno_sacked(tp);

      tcp_verify_left_out(tp);

      /* Don't muck with the congestion window here.
       * Reason is that we do not increase amount of _data_
       * in network, but units changed and effective
       * cwnd/ssthresh really reduced now.
       */
      if (icsk->icsk_ca_state != TCP_CA_Loss) {
            tp->high_seq = tp->snd_nxt;
            tp->snd_ssthresh = tcp_current_ssthresh(sk);
            tp->prior_ssthresh = 0;
            tp->undo_marker = 0;
            tcp_set_ca_state(sk, TCP_CA_Loss);
      }
      tcp_xmit_retransmit_queue(sk);
}

/* Process an event, which can update packets-in-flight not trivially.
 * Main goal of this function is to calculate new estimate for left_out,
 * taking into account both packets sitting in receiver's buffer and
 * packets lost by network.
 *
 * Besides that it does CWND reduction, when packet loss is detected
 * and changes state of machine.
 *
 * It does _not_ decide what to send, it is made in function
 * tcp_xmit_retransmit_queue().
 */
static void tcp_fastretrans_alert(struct sock *sk, int pkts_acked, int flag)
{
      struct inet_connection_sock *icsk = inet_csk(sk);
      struct tcp_sock *tp = tcp_sk(sk);
      int is_dupack = !(flag & (FLAG_SND_UNA_ADVANCED | FLAG_NOT_DUP));
      int do_lost = is_dupack || ((flag & FLAG_DATA_SACKED) &&
                            (tcp_fackets_out(tp) > tp->reordering));
      int fast_rexmit = 0, mib_idx;

      if (WARN_ON(!tp->packets_out && tp->sacked_out))
            tp->sacked_out = 0;
      if (WARN_ON(!tp->sacked_out && tp->fackets_out))
            tp->fackets_out = 0;

      /* Now state machine starts.
       * A. ECE, hence prohibit cwnd undoing, the reduction is required. */
      if (flag & FLAG_ECE)
            tp->prior_ssthresh = 0;

      /* B. In all the states check for reneging SACKs. */
      if (tcp_check_sack_reneging(sk, flag))
            return;

      /* C. Process data loss notification, provided it is valid. */
      if (tcp_is_fack(tp) && (flag & FLAG_DATA_LOST) &&
          before(tp->snd_una, tp->high_seq) &&
          icsk->icsk_ca_state != TCP_CA_Open &&
          tp->fackets_out > tp->reordering) {
            tcp_mark_head_lost(sk, tp->fackets_out - tp->reordering);
            NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPLOSS);
      }

      /* D. Check consistency of the current state. */
      tcp_verify_left_out(tp);

      /* E. Check state exit conditions. State can be terminated
       *    when high_seq is ACKed. */
      if (icsk->icsk_ca_state == TCP_CA_Open) {
            WARN_ON(tp->retrans_out != 0);
            tp->retrans_stamp = 0;
      } else if (!before(tp->snd_una, tp->high_seq)) {
            switch (icsk->icsk_ca_state) {
            case TCP_CA_Loss:
                  icsk->icsk_retransmits = 0;
                  if (tcp_try_undo_recovery(sk))
                        return;
                  break;

            case TCP_CA_CWR:
                  /* CWR is to be held something *above* high_seq
                   * is ACKed for CWR bit to reach receiver. */
                  if (tp->snd_una != tp->high_seq) {
                        tcp_complete_cwr(sk);
                        tcp_set_ca_state(sk, TCP_CA_Open);
                  }
                  break;

            case TCP_CA_Disorder:
                  tcp_try_undo_dsack(sk);
                  if (!tp->undo_marker ||
                      /* For SACK case do not Open to allow to undo
                       * catching for all duplicate ACKs. */
                      tcp_is_reno(tp) || tp->snd_una != tp->high_seq) {
                        tp->undo_marker = 0;
                        tcp_set_ca_state(sk, TCP_CA_Open);
                  }
                  break;

            case TCP_CA_Recovery:
                  if (tcp_is_reno(tp))
                        tcp_reset_reno_sack(tp);
                  if (tcp_try_undo_recovery(sk))
                        return;
                  tcp_complete_cwr(sk);
                  break;
            }
      }

      /* F. Process state. */
      switch (icsk->icsk_ca_state) {
      case TCP_CA_Recovery:
            if (!(flag & FLAG_SND_UNA_ADVANCED)) {
                  if (tcp_is_reno(tp) && is_dupack)
                        tcp_add_reno_sack(sk);
            } else
                  do_lost = tcp_try_undo_partial(sk, pkts_acked);
            break;
      case TCP_CA_Loss:
            if (flag & FLAG_DATA_ACKED)
                  icsk->icsk_retransmits = 0;
            if (tcp_is_reno(tp) && flag & FLAG_SND_UNA_ADVANCED)
                  tcp_reset_reno_sack(tp);
            if (!tcp_try_undo_loss(sk)) {
                  tcp_moderate_cwnd(tp);
                  tcp_xmit_retransmit_queue(sk);
                  return;
            }
            if (icsk->icsk_ca_state != TCP_CA_Open)
                  return;
            /* Loss is undone; fall through to processing in Open state. */
      default:
            if (tcp_is_reno(tp)) {
                  if (flag & FLAG_SND_UNA_ADVANCED)
                        tcp_reset_reno_sack(tp);
                  if (is_dupack)
                        tcp_add_reno_sack(sk);
            }

            if (icsk->icsk_ca_state == TCP_CA_Disorder)
                  tcp_try_undo_dsack(sk);

            if (!tcp_time_to_recover(sk)) {
                  tcp_try_to_open(sk, flag);
                  return;
            }

            /* MTU probe failure: don't reduce cwnd */
            if (icsk->icsk_ca_state < TCP_CA_CWR &&
                icsk->icsk_mtup.probe_size &&
                tp->snd_una == tp->mtu_probe.probe_seq_start) {
                  tcp_mtup_probe_failed(sk);
                  /* Restores the reduction we did in tcp_mtup_probe() */
                  tp->snd_cwnd++;
                  tcp_simple_retransmit(sk);
                  return;
            }

            /* Otherwise enter Recovery state */

            if (tcp_is_reno(tp))
                  mib_idx = LINUX_MIB_TCPRENORECOVERY;
            else
                  mib_idx = LINUX_MIB_TCPSACKRECOVERY;

            NET_INC_STATS_BH(sock_net(sk), mib_idx);

            tp->high_seq = tp->snd_nxt;
            tp->prior_ssthresh = 0;
            tp->undo_marker = tp->snd_una;
            tp->undo_retrans = tp->retrans_out;

            if (icsk->icsk_ca_state < TCP_CA_CWR) {
                  if (!(flag & FLAG_ECE))
                        tp->prior_ssthresh = tcp_current_ssthresh(sk);
                  tp->snd_ssthresh = icsk->icsk_ca_ops->ssthresh(sk);
                  TCP_ECN_queue_cwr(tp);
            }

            tp->bytes_acked = 0;
            tp->snd_cwnd_cnt = 0;
            tcp_set_ca_state(sk, TCP_CA_Recovery);
            fast_rexmit = 1;
      }

      if (do_lost || (tcp_is_fack(tp) && tcp_head_timedout(sk)))
            tcp_update_scoreboard(sk, fast_rexmit);
      tcp_cwnd_down(sk, flag);
      tcp_xmit_retransmit_queue(sk);
}

static void tcp_valid_rtt_meas(struct sock *sk, u32 seq_rtt)
{
      tcp_rtt_estimator(sk, seq_rtt);
      tcp_set_rto(sk);
      inet_csk(sk)->icsk_backoff = 0;
}

/* Read draft-ietf-tcplw-high-performance before mucking
 * with this code. (Supersedes RFC1323)
 */
static void tcp_ack_saw_tstamp(struct sock *sk, int flag)
{
      /* RTTM Rule: A TSecr value received in a segment is used to
       * update the averaged RTT measurement only if the segment
       * acknowledges some new data, i.e., only if it advances the
       * left edge of the send window.
       *
       * See draft-ietf-tcplw-high-performance-00, section 3.3.
       * 1998/04/10 Andrey V. Savochkin <saw@msu.ru>
       *
       * Changed: reset backoff as soon as we see the first valid sample.
       * If we do not, we get strongly overestimated rto. With timestamps
       * samples are accepted even from very old segments: f.e., when rtt=1
       * increases to 8, we retransmit 5 times and after 8 seconds delayed
       * answer arrives rto becomes 120 seconds! If at least one of segments
       * in window is lost... Voila.                        --ANK (010210)
       */
      struct tcp_sock *tp = tcp_sk(sk);

      tcp_valid_rtt_meas(sk, tcp_time_stamp - tp->rx_opt.rcv_tsecr);
}

static void tcp_ack_no_tstamp(struct sock *sk, u32 seq_rtt, int flag)
{
      /* We don't have a timestamp. Can only use
       * packets that are not retransmitted to determine
       * rtt estimates. Also, we must not reset the
       * backoff for rto until we get a non-retransmitted
       * packet. This allows us to deal with a situation
       * where the network delay has increased suddenly.
       * I.e. Karn's algorithm. (SIGCOMM '87, p5.)
       */

      if (flag & FLAG_RETRANS_DATA_ACKED)
            return;

      tcp_valid_rtt_meas(sk, seq_rtt);
}

static inline void tcp_ack_update_rtt(struct sock *sk, const int flag,
                              const s32 seq_rtt)
{
      const struct tcp_sock *tp = tcp_sk(sk);
      /* Note that peer MAY send zero echo. In this case it is ignored. (rfc1323) */
      if (tp->rx_opt.saw_tstamp && tp->rx_opt.rcv_tsecr)
            tcp_ack_saw_tstamp(sk, flag);
      else if (seq_rtt >= 0)
            tcp_ack_no_tstamp(sk, seq_rtt, flag);
}

static void tcp_cong_avoid(struct sock *sk, u32 ack, u32 in_flight)
{
      const struct inet_connection_sock *icsk = inet_csk(sk);
      icsk->icsk_ca_ops->cong_avoid(sk, ack, in_flight);
      tcp_sk(sk)->snd_cwnd_stamp = tcp_time_stamp;
}

/* Restart timer after forward progress on connection.
 * RFC2988 recommends to restart timer to now+rto.
 */
static void tcp_rearm_rto(struct sock *sk)
{
      struct tcp_sock *tp = tcp_sk(sk);

      if (!tp->packets_out) {
            inet_csk_clear_xmit_timer(sk, ICSK_TIME_RETRANS);
      } else {
            inet_csk_reset_xmit_timer(sk, ICSK_TIME_RETRANS,
                                inet_csk(sk)->icsk_rto, TCP_RTO_MAX);
      }
}

/* If we get here, the whole TSO packet has not been acked. */
static u32 tcp_tso_acked(struct sock *sk, struct sk_buff *skb)
{
      struct tcp_sock *tp = tcp_sk(sk);
      u32 packets_acked;

      BUG_ON(!after(TCP_SKB_CB(skb)->end_seq, tp->snd_una));

      packets_acked = tcp_skb_pcount(skb);
      if (tcp_trim_head(sk, skb, tp->snd_una - TCP_SKB_CB(skb)->seq))
            return 0;
      packets_acked -= tcp_skb_pcount(skb);

      if (packets_acked) {
            BUG_ON(tcp_skb_pcount(skb) == 0);
            BUG_ON(!before(TCP_SKB_CB(skb)->seq, TCP_SKB_CB(skb)->end_seq));
      }

      return packets_acked;
}

/* Remove acknowledged frames from the retransmission queue. If our packet
 * is before the ack sequence we can discard it as it's confirmed to have
 * arrived at the other end.
 */
static int tcp_clean_rtx_queue(struct sock *sk, int prior_fackets,
                         u32 prior_snd_una)
{
      struct tcp_sock *tp = tcp_sk(sk);
      const struct inet_connection_sock *icsk = inet_csk(sk);
      struct sk_buff *skb;
      u32 now = tcp_time_stamp;
      int fully_acked = 1;
      int flag = 0;
      u32 pkts_acked = 0;
      u32 reord = tp->packets_out;
      u32 prior_sacked = tp->sacked_out;
      s32 seq_rtt = -1;
      s32 ca_seq_rtt = -1;
      ktime_t last_ackt = net_invalid_timestamp();

      while ((skb = tcp_write_queue_head(sk)) && skb != tcp_send_head(sk)) {
            struct tcp_skb_cb *scb = TCP_SKB_CB(skb);
            u32 acked_pcount;
            u8 sacked = scb->sacked;

            /* Determine how many packets and what bytes were acked, tso and else */
            if (after(scb->end_seq, tp->snd_una)) {
                  if (tcp_skb_pcount(skb) == 1 ||
                      !after(tp->snd_una, scb->seq))
                        break;

                  acked_pcount = tcp_tso_acked(sk, skb);
                  if (!acked_pcount)
                        break;

                  fully_acked = 0;
            } else {
                  acked_pcount = tcp_skb_pcount(skb);
            }

            if (sacked & TCPCB_RETRANS) {
                  if (sacked & TCPCB_SACKED_RETRANS)
                        tp->retrans_out -= acked_pcount;
                  flag |= FLAG_RETRANS_DATA_ACKED;
                  ca_seq_rtt = -1;
                  seq_rtt = -1;
                  if ((flag & FLAG_DATA_ACKED) || (acked_pcount > 1))
                        flag |= FLAG_NONHEAD_RETRANS_ACKED;
            } else {
                  ca_seq_rtt = now - scb->when;
                  last_ackt = skb->tstamp;
                  if (seq_rtt < 0) {
                        seq_rtt = ca_seq_rtt;
                  }
                  if (!(sacked & TCPCB_SACKED_ACKED))
                        reord = min(pkts_acked, reord);
            }

            if (sacked & TCPCB_SACKED_ACKED)
                  tp->sacked_out -= acked_pcount;
            if (sacked & TCPCB_LOST)
                  tp->lost_out -= acked_pcount;

            tp->packets_out -= acked_pcount;
            pkts_acked += acked_pcount;

            /* Initial outgoing SYN's get put onto the write_queue
             * just like anything else we transmit.  It is not
             * true data, and if we misinform our callers that
             * this ACK acks real data, we will erroneously exit
             * connection startup slow start one packet too
             * quickly.  This is severely frowned upon behavior.
             */
            if (!(scb->flags & TCPCB_FLAG_SYN)) {
                  flag |= FLAG_DATA_ACKED;
            } else {
                  flag |= FLAG_SYN_ACKED;
                  tp->retrans_stamp = 0;
            }

            if (!fully_acked)
                  break;

            tcp_unlink_write_queue(skb, sk);
            sk_wmem_free_skb(sk, skb);
            tp->scoreboard_skb_hint = NULL;
            if (skb == tp->retransmit_skb_hint)
                  tp->retransmit_skb_hint = NULL;
            if (skb == tp->lost_skb_hint)
                  tp->lost_skb_hint = NULL;
      }

      if (likely(between(tp->snd_up, prior_snd_una, tp->snd_una)))
            tp->snd_up = tp->snd_una;

      if (skb && (TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_ACKED))
            flag |= FLAG_SACK_RENEGING;

      if (flag & FLAG_ACKED) {
            const struct tcp_congestion_ops *ca_ops
                  = inet_csk(sk)->icsk_ca_ops;

            if (unlikely(icsk->icsk_mtup.probe_size &&
                       !after(tp->mtu_probe.probe_seq_end, tp->snd_una))) {
                  tcp_mtup_probe_success(sk);
            }

            tcp_ack_update_rtt(sk, flag, seq_rtt);
            tcp_rearm_rto(sk);

            if (tcp_is_reno(tp)) {
                  tcp_remove_reno_sacks(sk, pkts_acked);
            } else {
                  int delta;

                  /* Non-retransmitted hole got filled? That's reordering */
                  if (reord < prior_fackets)
                        tcp_update_reordering(sk, tp->fackets_out - reord, 0);

                  delta = tcp_is_fack(tp) ? pkts_acked :
                                      prior_sacked - tp->sacked_out;
                  tp->lost_cnt_hint -= min(tp->lost_cnt_hint, delta);
            }

            tp->fackets_out -= min(pkts_acked, tp->fackets_out);

            if (ca_ops->pkts_acked) {
                  s32 rtt_us = -1;

                  /* Is the ACK triggering packet unambiguous? */
                  if (!(flag & FLAG_RETRANS_DATA_ACKED)) {
                        /* High resolution needed and available? */
                        if (ca_ops->flags & TCP_CONG_RTT_STAMP &&
                            !ktime_equal(last_ackt,
                                     net_invalid_timestamp()))
                              rtt_us = ktime_us_delta(ktime_get_real(),
                                                last_ackt);
                        else if (ca_seq_rtt > 0)
                              rtt_us = jiffies_to_usecs(ca_seq_rtt);
                  }

                  ca_ops->pkts_acked(sk, pkts_acked, rtt_us);
            }
      }

#if FASTRETRANS_DEBUG > 0
      WARN_ON((int)tp->sacked_out < 0);
      WARN_ON((int)tp->lost_out < 0);
      WARN_ON((int)tp->retrans_out < 0);
      if (!tp->packets_out && tcp_is_sack(tp)) {
            icsk = inet_csk(sk);
            if (tp->lost_out) {
                  printk(KERN_DEBUG "Leak l=%u %d\n",
                         tp->lost_out, icsk->icsk_ca_state);
                  tp->lost_out = 0;
            }
            if (tp->sacked_out) {
                  printk(KERN_DEBUG "Leak s=%u %d\n",
                         tp->sacked_out, icsk->icsk_ca_state);
                  tp->sacked_out = 0;
            }
            if (tp->retrans_out) {
                  printk(KERN_DEBUG "Leak r=%u %d\n",
                         tp->retrans_out, icsk->icsk_ca_state);
                  tp->retrans_out = 0;
            }
      }
#endif
      return flag;
}

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

      /* Was it a usable window open? */

      if (!after(TCP_SKB_CB(tcp_send_head(sk))->end_seq, tcp_wnd_end(tp))) {
            icsk->icsk_backoff = 0;
            inet_csk_clear_xmit_timer(sk, ICSK_TIME_PROBE0);
            /* Socket must be waked up by subsequent tcp_data_snd_check().
             * This function is not for random using!
             */
      } else {
            inet_csk_reset_xmit_timer(sk, ICSK_TIME_PROBE0,
                                min(icsk->icsk_rto << icsk->icsk_backoff, TCP_RTO_MAX),
                                TCP_RTO_MAX);
      }
}

static inline int tcp_ack_is_dubious(const struct sock *sk, const int flag)
{
      return (!(flag & FLAG_NOT_DUP) || (flag & FLAG_CA_ALERT) ||
            inet_csk(sk)->icsk_ca_state != TCP_CA_Open);
}

static inline int tcp_may_raise_cwnd(const struct sock *sk, const int flag)
{
      const struct tcp_sock *tp = tcp_sk(sk);
      return (!(flag & FLAG_ECE) || tp->snd_cwnd < tp->snd_ssthresh) &&
            !((1 << inet_csk(sk)->icsk_ca_state) & (TCPF_CA_Recovery | TCPF_CA_CWR));
}

/* Check that window update is acceptable.
 * The function assumes that snd_una<=ack<=snd_next.
 */
static inline int tcp_may_update_window(const struct tcp_sock *tp,
                              const u32 ack, const u32 ack_seq,
                              const u32 nwin)
{
      return (after(ack, tp->snd_una) ||
            after(ack_seq, tp->snd_wl1) ||
            (ack_seq == tp->snd_wl1 && nwin > tp->snd_wnd));
}

/* Update our send window.
 *
 * Window update algorithm, described in RFC793/RFC1122 (used in linux-2.2
 * and in FreeBSD. NetBSD's one is even worse.) is wrong.
 */
static int tcp_ack_update_window(struct sock *sk, struct sk_buff *skb, u32 ack,
                         u32 ack_seq)
{
      struct tcp_sock *tp = tcp_sk(sk);
      int flag = 0;
      u32 nwin = ntohs(tcp_hdr(skb)->window);

      if (likely(!tcp_hdr(skb)->syn))
            nwin <<= tp->rx_opt.snd_wscale;

      if (tcp_may_update_window(tp, ack, ack_seq, nwin)) {
            flag |= FLAG_WIN_UPDATE;
            tcp_update_wl(tp, ack_seq);

            if (tp->snd_wnd != nwin) {
                  tp->snd_wnd = nwin;

                  /* Note, it is the only place, where
                   * fast path is recovered for sending TCP.
                   */
                  tp->pred_flags = 0;
                  tcp_fast_path_check(sk);

                  if (nwin > tp->max_window) {
                        tp->max_window = nwin;
                        tcp_sync_mss(sk, inet_csk(sk)->icsk_pmtu_cookie);
                  }
            }
      }

      tp->snd_una = ack;

      return flag;
}

/* A very conservative spurious RTO response algorithm: reduce cwnd and
 * continue in congestion avoidance.
 */
static void tcp_conservative_spur_to_response(struct tcp_sock *tp)
{
      tp->snd_cwnd = min(tp->snd_cwnd, tp->snd_ssthresh);
      tp->snd_cwnd_cnt = 0;
      tp->bytes_acked = 0;
      TCP_ECN_queue_cwr(tp);
      tcp_moderate_cwnd(tp);
}

/* A conservative spurious RTO response algorithm: reduce cwnd using
 * rate halving and continue in congestion avoidance.
 */
static void tcp_ratehalving_spur_to_response(struct sock *sk)
{
      tcp_enter_cwr(sk, 0);
}

static void tcp_undo_spur_to_response(struct sock *sk, int flag)
{
      if (flag & FLAG_ECE)
            tcp_ratehalving_spur_to_response(sk);
      else
            tcp_undo_cwr(sk, 1);
}

/* F-RTO spurious RTO detection algorithm (RFC4138)
 *
 * F-RTO affects during two new ACKs following RTO (well, almost, see inline
 * comments). State (ACK number) is kept in frto_counter. When ACK advances
 * window (but not to or beyond highest sequence sent before RTO):
 *   On First ACK,  send two new segments out.
 *   On Second ACK, RTO was likely spurious. Do spurious response (response
 *                  algorithm is not part of the F-RTO detection algorithm
 *                  given in RFC4138 but can be selected separately).
 * Otherwise (basically on duplicate ACK), RTO was (likely) caused by a loss
 * and TCP falls back to conventional RTO recovery. F-RTO allows overriding
 * of Nagle, this is done using frto_counter states 2 and 3, when a new data
 * segment of any size sent during F-RTO, state 2 is upgraded to 3.
 *
 * Rationale: if the RTO was spurious, new ACKs should arrive from the
 * original window even after we transmit two new data segments.
 *
 * SACK version:
 *   on first step, wait until first cumulative ACK arrives, then move to
 *   the second step. In second step, the next ACK decides.
 *
 * F-RTO is implemented (mainly) in four functions:
 *   - tcp_use_frto() is used to determine if TCP is can use F-RTO
 *   - tcp_enter_frto() prepares TCP state on RTO if F-RTO is used, it is
 *     called when tcp_use_frto() showed green light
 *   - tcp_process_frto() handles incoming ACKs during F-RTO algorithm
 *   - tcp_enter_frto_loss() is called if there is not enough evidence
 *     to prove that the RTO is indeed spurious. It transfers the control
 *     from F-RTO to the conventional RTO recovery
 */
static int tcp_process_frto(struct sock *sk, int flag)
{
      struct tcp_sock *tp = tcp_sk(sk);

      tcp_verify_left_out(tp);

      /* Duplicate the behavior from Loss state (fastretrans_alert) */
      if (flag & FLAG_DATA_ACKED)
            inet_csk(sk)->icsk_retransmits = 0;

      if ((flag & FLAG_NONHEAD_RETRANS_ACKED) ||
          ((tp->frto_counter >= 2) && (flag & FLAG_RETRANS_DATA_ACKED)))
            tp->undo_marker = 0;

      if (!before(tp->snd_una, tp->frto_highmark)) {
            tcp_enter_frto_loss(sk, (tp->frto_counter == 1 ? 2 : 3), flag);
            return 1;
      }

      if (!tcp_is_sackfrto(tp)) {
            /* RFC4138 shortcoming in step 2; should also have case c):
             * ACK isn't duplicate nor advances window, e.g., opposite dir
             * data, winupdate
             */
            if (!(flag & FLAG_ANY_PROGRESS) && (flag & FLAG_NOT_DUP))
                  return 1;

            if (!(flag & FLAG_DATA_ACKED)) {
                  tcp_enter_frto_loss(sk, (tp->frto_counter == 1 ? 0 : 3),
                                  flag);
                  return 1;
            }
      } else {
            if (!(flag & FLAG_DATA_ACKED) && (tp->frto_counter == 1)) {
                  /* Prevent sending of new data. */
                  tp->snd_cwnd = min(tp->snd_cwnd,
                                 tcp_packets_in_flight(tp));
                  return 1;
            }

            if ((tp->frto_counter >= 2) &&
                (!(flag & FLAG_FORWARD_PROGRESS) ||
                 ((flag & FLAG_DATA_SACKED) &&
                  !(flag & FLAG_ONLY_ORIG_SACKED)))) {
                  /* RFC4138 shortcoming (see comment above) */
                  if (!(flag & FLAG_FORWARD_PROGRESS) &&
                      (flag & FLAG_NOT_DUP))
                        return 1;

                  tcp_enter_frto_loss(sk, 3, flag);
                  return 1;
            }
      }

      if (tp->frto_counter == 1) {
            /* tcp_may_send_now needs to see updated state */
            tp->snd_cwnd = tcp_packets_in_flight(tp) + 2;
            tp->frto_counter = 2;

            if (!tcp_may_send_now(sk))
                  tcp_enter_frto_loss(sk, 2, flag);

            return 1;
      } else {
            switch (sysctl_tcp_frto_response) {
            case 2:
                  tcp_undo_spur_to_response(sk, flag);
                  break;
            case 1:
                  tcp_conservative_spur_to_response(tp);
                  break;
            default:
                  tcp_ratehalving_spur_to_response(sk);
                  break;
            }
            tp->frto_counter = 0;
            tp->undo_marker = 0;
            NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPSPURIOUSRTOS);
      }
      return 0;
}

/* This routine deals with incoming acks, but not outgoing ones. */
static int tcp_ack(struct sock *sk, struct sk_buff *skb, int flag)
{
      struct inet_connection_sock *icsk = inet_csk(sk);
      struct tcp_sock *tp = tcp_sk(sk);
      u32 prior_snd_una = tp->snd_una;
      u32 ack_seq = TCP_SKB_CB(skb)->seq;
      u32 ack = TCP_SKB_CB(skb)->ack_seq;
      u32 prior_in_flight;
      u32 prior_fackets;
      int prior_packets;
      int frto_cwnd = 0;

      /* If the ack is older than previous acks
       * then we can probably ignore it.
       */
      if (before(ack, prior_snd_una))
            goto old_ack;

      /* If the ack includes data we haven't sent yet, discard
       * this segment (RFC793 Section 3.9).
       */
      if (after(ack, tp->snd_nxt))
            goto invalid_ack;

      if (after(ack, prior_snd_una))
            flag |= FLAG_SND_UNA_ADVANCED;

      if (sysctl_tcp_abc) {
            if (icsk->icsk_ca_state < TCP_CA_CWR)
                  tp->bytes_acked += ack - prior_snd_una;
            else if (icsk->icsk_ca_state == TCP_CA_Loss)
                  /* we assume just one segment left network */
                  tp->bytes_acked += min(ack - prior_snd_una,
                                     tp->mss_cache);
      }

      prior_fackets = tp->fackets_out;
      prior_in_flight = tcp_packets_in_flight(tp);

      if (!(flag & FLAG_SLOWPATH) && after(ack, prior_snd_una)) {
            /* Window is constant, pure forward advance.
             * No more checks are required.
             * Note, we use the fact that SND.UNA>=SND.WL2.
             */
            tcp_update_wl(tp, ack_seq);
            tp->snd_una = ack;
            flag |= FLAG_WIN_UPDATE;

            tcp_ca_event(sk, CA_EVENT_FAST_ACK);

            NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPHPACKS);
      } else {
            if (ack_seq != TCP_SKB_CB(skb)->end_seq)
                  flag |= FLAG_DATA;
            else
                  NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPPUREACKS);

            flag |= tcp_ack_update_window(sk, skb, ack, ack_seq);

            if (TCP_SKB_CB(skb)->sacked)
                  flag |= tcp_sacktag_write_queue(sk, skb, prior_snd_una);

            if (TCP_ECN_rcv_ecn_echo(tp, tcp_hdr(skb)))
                  flag |= FLAG_ECE;

            tcp_ca_event(sk, CA_EVENT_SLOW_ACK);
      }

      /* We passed data and got it acked, remove any soft error
       * log. Something worked...
       */
      sk->sk_err_soft = 0;
      icsk->icsk_probes_out = 0;
      tp->rcv_tstamp = tcp_time_stamp;
      prior_packets = tp->packets_out;
      if (!prior_packets)
            goto no_queue;

      /* See if we can take anything off of the retransmit queue. */
      flag |= tcp_clean_rtx_queue(sk, prior_fackets, prior_snd_una);

      if (tp->frto_counter)
            frto_cwnd = tcp_process_frto(sk, flag);
      /* Guarantee sacktag reordering detection against wrap-arounds */
      if (before(tp->frto_highmark, tp->snd_una))
            tp->frto_highmark = 0;

      if (tcp_ack_is_dubious(sk, flag)) {
            /* Advance CWND, if state allows this. */
            if ((flag & FLAG_DATA_ACKED) && !frto_cwnd &&
                tcp_may_raise_cwnd(sk, flag))
                  tcp_cong_avoid(sk, ack, prior_in_flight);
            tcp_fastretrans_alert(sk, prior_packets - tp->packets_out,
                              flag);
      } else {
            if ((flag & FLAG_DATA_ACKED) && !frto_cwnd)
                  tcp_cong_avoid(sk, ack, prior_in_flight);
      }

      if ((flag & FLAG_FORWARD_PROGRESS) || !(flag & FLAG_NOT_DUP))
            dst_confirm(sk->sk_dst_cache);

      return 1;

no_queue:
      /* If this ack opens up a zero window, clear backoff.  It was
       * being used to time the probes, and is probably far higher than
       * it needs to be for normal retransmission.
       */
      if (tcp_send_head(sk))
            tcp_ack_probe(sk);
      return 1;

invalid_ack:
      SOCK_DEBUG(sk, "Ack %u after %u:%u\n", ack, tp->snd_una, tp->snd_nxt);
      return -1;

old_ack:
      if (TCP_SKB_CB(skb)->sacked) {
            tcp_sacktag_write_queue(sk, skb, prior_snd_una);
            if (icsk->icsk_ca_state == TCP_CA_Open)
                  tcp_try_keep_open(sk);
      }

      SOCK_DEBUG(sk, "Ack %u before %u:%u\n", ack, tp->snd_una, tp->snd_nxt);
      return 0;
}

/* Look for tcp options. Normally only called on SYN and SYNACK packets.
 * But, this can also be called on packets in the established flow when
 * the fast version below fails.
 */
void tcp_parse_options(struct sk_buff *skb, struct tcp_options_received *opt_rx,
                   int estab)
{
      unsigned char *ptr;
      struct tcphdr *th = tcp_hdr(skb);
      int length = (th->doff * 4) - sizeof(struct tcphdr);

      ptr = (unsigned char *)(th + 1);
      opt_rx->saw_tstamp = 0;

      while (length > 0) {
            int opcode = *ptr++;
            int opsize;

            switch (opcode) {
            case TCPOPT_EOL:
                  return;
            case TCPOPT_NOP:  /* Ref: RFC 793 section 3.1 */
                  length--;
                  continue;
            default:
                  opsize = *ptr++;
                  if (opsize < 2) /* "silly options" */
                        return;
                  if (opsize > length)
                        return;     /* don't parse partial options */
                  switch (opcode) {
                  case TCPOPT_MSS:
                        if (opsize == TCPOLEN_MSS && th->syn && !estab) {
                              u16 in_mss = get_unaligned_be16(ptr);
                              if (in_mss) {
                                    if (opt_rx->user_mss &&
                                        opt_rx->user_mss < in_mss)
                                          in_mss = opt_rx->user_mss;
                                    opt_rx->mss_clamp = in_mss;
                              }
                        }
                        break;
                  case TCPOPT_WINDOW:
                        if (opsize == TCPOLEN_WINDOW && th->syn &&
                            !estab && sysctl_tcp_window_scaling) {
                              __u8 snd_wscale = *(__u8 *)ptr;
                              opt_rx->wscale_ok = 1;
                              if (snd_wscale > 14) {
                                    if (net_ratelimit())
                                          printk(KERN_INFO "tcp_parse_options: Illegal window "
                                                 "scaling value %d >14 received.\n",
                                                 snd_wscale);
                                    snd_wscale = 14;
                              }
                              opt_rx->snd_wscale = snd_wscale;
                        }
                        break;
                  case TCPOPT_TIMESTAMP:
                        if ((opsize == TCPOLEN_TIMESTAMP) &&
                            ((estab && opt_rx->tstamp_ok) ||
                             (!estab && sysctl_tcp_timestamps))) {
                              opt_rx->saw_tstamp = 1;
                              opt_rx->rcv_tsval = get_unaligned_be32(ptr);
                              opt_rx->rcv_tsecr = get_unaligned_be32(ptr + 4);
                        }
                        break;
                  case TCPOPT_SACK_PERM:
                        if (opsize == TCPOLEN_SACK_PERM && th->syn &&
                            !estab && sysctl_tcp_sack) {
                              opt_rx->sack_ok = 1;
                              tcp_sack_reset(opt_rx);
                        }
                        break;

                  case TCPOPT_SACK:
                        if ((opsize >= (TCPOLEN_SACK_BASE + TCPOLEN_SACK_PERBLOCK)) &&
                           !((opsize - TCPOLEN_SACK_BASE) % TCPOLEN_SACK_PERBLOCK) &&
                           opt_rx->sack_ok) {
                              TCP_SKB_CB(skb)->sacked = (ptr - 2) - (unsigned char *)th;
                        }
                        break;
#ifdef CONFIG_TCP_MD5SIG
                  case TCPOPT_MD5SIG:
                        /*
                         * The MD5 Hash has already been
                         * checked (see tcp_v{4,6}_do_rcv()).
                         */
                        break;
#endif
                  }

                  ptr += opsize-2;
                  length -= opsize;
            }
      }
}

static int tcp_parse_aligned_timestamp(struct tcp_sock *tp, struct tcphdr *th)
{
      __be32 *ptr = (__be32 *)(th + 1);

      if (*ptr == htonl((TCPOPT_NOP << 24) | (TCPOPT_NOP << 16)
                    | (TCPOPT_TIMESTAMP << 8) | TCPOLEN_TIMESTAMP)) {
            tp->rx_opt.saw_tstamp = 1;
            ++ptr;
            tp->rx_opt.rcv_tsval = ntohl(*ptr);
            ++ptr;
            tp->rx_opt.rcv_tsecr = ntohl(*ptr);
            return 1;
      }
      return 0;
}

/* Fast parse options. This hopes to only see timestamps.
 * If it is wrong it falls back on tcp_parse_options().
 */
static int tcp_fast_parse_options(struct sk_buff *skb, struct tcphdr *th,
                          struct tcp_sock *tp)
{
      if (th->doff == sizeof(struct tcphdr) >> 2) {
            tp->rx_opt.saw_tstamp = 0;
            return 0;
      } else if (tp->rx_opt.tstamp_ok &&
               th->doff == (sizeof(struct tcphdr)>>2)+(TCPOLEN_TSTAMP_ALIGNED>>2)) {
            if (tcp_parse_aligned_timestamp(tp, th))
                  return 1;
      }
      tcp_parse_options(skb, &tp->rx_opt, 1);
      return 1;
}

#ifdef CONFIG_TCP_MD5SIG
/*
 * Parse MD5 Signature option
 */
u8 *tcp_parse_md5sig_option(struct tcphdr *th)
{
      int length = (th->doff << 2) - sizeof (*th);
      u8 *ptr = (u8*)(th + 1);

      /* If the TCP option is too short, we can short cut */
      if (length < TCPOLEN_MD5SIG)
            return NULL;

      while (length > 0) {
            int opcode = *ptr++;
            int opsize;

            switch(opcode) {
            case TCPOPT_EOL:
                  return NULL;
            case TCPOPT_NOP:
                  length--;
                  continue;
            default:
                  opsize = *ptr++;
                  if (opsize < 2 || opsize > length)
                        return NULL;
                  if (opcode == TCPOPT_MD5SIG)
                        return ptr;
            }
            ptr += opsize - 2;
            length -= opsize;
      }
      return NULL;
}
#endif

static inline void tcp_store_ts_recent(struct tcp_sock *tp)
{
      tp->rx_opt.ts_recent = tp->rx_opt.rcv_tsval;
      tp->rx_opt.ts_recent_stamp = get_seconds();
}

static inline void tcp_replace_ts_recent(struct tcp_sock *tp, u32 seq)
{
      if (tp->rx_opt.saw_tstamp && !after(seq, tp->rcv_wup)) {
            /* PAWS bug workaround wrt. ACK frames, the PAWS discard
             * extra check below makes sure this can only happen
             * for pure ACK frames.  -DaveM
             *
             * Not only, also it occurs for expired timestamps.
             */

            if (tcp_paws_check(&tp->rx_opt, 0))
                  tcp_store_ts_recent(tp);
      }
}

/* Sorry, PAWS as specified is broken wrt. pure-ACKs -DaveM
 *
 * It is not fatal. If this ACK does _not_ change critical state (seqs, window)
 * it can pass through stack. So, the following predicate verifies that
 * this segment is not used for anything but congestion avoidance or
 * fast retransmit. Moreover, we even are able to eliminate most of such
 * second order effects, if we apply some small "replay" window (~RTO)
 * to timestamp space.
 *
 * All these measures still do not guarantee that we reject wrapped ACKs
 * on networks with high bandwidth, when sequence space is recycled fastly,
 * but it guarantees that such events will be very rare and do not affect
 * connection seriously. This doesn't look nice, but alas, PAWS is really
 * buggy extension.
 *
 * [ Later note. Even worse! It is buggy for segments _with_ data. RFC
 * states that events when retransmit arrives after original data are rare.
 * It is a blatant lie. VJ forgot about fast retransmit! 8)8) It is
 * the biggest problem on large power networks even with minor reordering.
 * OK, let's give it small replay window. If peer clock is even 1hz, it is safe
 * up to bandwidth of 18Gigabit/sec. 8) ]
 */

static int tcp_disordered_ack(const struct sock *sk, const struct sk_buff *skb)
{
      struct tcp_sock *tp = tcp_sk(sk);
      struct tcphdr *th = tcp_hdr(skb);
      u32 seq = TCP_SKB_CB(skb)->seq;
      u32 ack = TCP_SKB_CB(skb)->ack_seq;

      return (/* 1. Pure ACK with correct sequence number. */
            (th->ack && seq == TCP_SKB_CB(skb)->end_seq && seq == tp->rcv_nxt) &&

            /* 2. ... and duplicate ACK. */
            ack == tp->snd_una &&

            /* 3. ... and does not update window. */
            !tcp_may_update_window(tp, ack, seq, ntohs(th->window) << tp->rx_opt.snd_wscale) &&

            /* 4. ... and sits in replay window. */
            (s32)(tp->rx_opt.ts_recent - tp->rx_opt.rcv_tsval) <= (inet_csk(sk)->icsk_rto * 1024) / HZ);
}

static inline int tcp_paws_discard(const struct sock *sk,
                           const struct sk_buff *skb)
{
      const struct tcp_sock *tp = tcp_sk(sk);

      return !tcp_paws_check(&tp->rx_opt, TCP_PAWS_WINDOW) &&
             !tcp_disordered_ack(sk, skb);
}

/* Check segment sequence number for validity.
 *
 * Segment controls are considered valid, if the segment
 * fits to the window after truncation to the window. Acceptability
 * of data (and SYN, FIN, of course) is checked separately.
 * See tcp_data_queue(), for example.
 *
 * Also, controls (RST is main one) are accepted using RCV.WUP instead
 * of RCV.NXT. Peer still did not advance his SND.UNA when we
 * delayed ACK, so that hisSND.UNA<=ourRCV.WUP.
 * (borrowed from freebsd)
 */

static inline int tcp_sequence(struct tcp_sock *tp, u32 seq, u32 end_seq)
{
      return      !before(end_seq, tp->rcv_wup) &&
            !after(seq, tp->rcv_nxt + tcp_receive_window(tp));
}

/* When we get a reset we do this. */
static void tcp_reset(struct sock *sk)
{
      /* We want the right error as BSD sees it (and indeed as we do). */
      switch (sk->sk_state) {
      case TCP_SYN_SENT:
            sk->sk_err = ECONNREFUSED;
            break;
      case TCP_CLOSE_WAIT:
            sk->sk_err = EPIPE;
            break;
      case TCP_CLOSE:
            return;
      default:
            sk->sk_err = ECONNRESET;
      }

      if (!sock_flag(sk, SOCK_DEAD))
            sk->sk_error_report(sk);

      tcp_done(sk);
}

/*
 *    Process the FIN bit. This now behaves as it is supposed to work
 *    and the FIN takes effect when it is validly part of sequence
 *    space. Not before when we get holes.
 *
 *    If we are ESTABLISHED, a received fin moves us to CLOSE-WAIT
 *    (and thence onto LAST-ACK and finally, CLOSE, we never enter
 *    TIME-WAIT)
 *
 *    If we are in FINWAIT-1, a received FIN indicates simultaneous
 *    close and we go into CLOSING (and later onto TIME-WAIT)
 *
 *    If we are in FINWAIT-2, a received FIN moves us to TIME-WAIT.
 */
static void tcp_fin(struct sk_buff *skb, struct sock *sk, struct tcphdr *th)
{
      struct tcp_sock *tp = tcp_sk(sk);

      inet_csk_schedule_ack(sk);

      sk->sk_shutdown |= RCV_SHUTDOWN;
      sock_set_flag(sk, SOCK_DONE);

      switch (sk->sk_state) {
      case TCP_SYN_RECV:
      case TCP_ESTABLISHED:
            /* Move to CLOSE_WAIT */
            tcp_set_state(sk, TCP_CLOSE_WAIT);
            inet_csk(sk)->icsk_ack.pingpong = 1;
            break;

      case TCP_CLOSE_WAIT:
      case TCP_CLOSING:
            /* Received a retransmission of the FIN, do
             * nothing.
             */
            break;
      case TCP_LAST_ACK:
            /* RFC793: Remain in the LAST-ACK state. */
            break;

      case TCP_FIN_WAIT1:
            /* This case occurs when a simultaneous close
             * happens, we must ack the received FIN and
             * enter the CLOSING state.
             */
            tcp_send_ack(sk);
            tcp_set_state(sk, TCP_CLOSING);
            break;
      case TCP_FIN_WAIT2:
            /* Received a FIN -- send ACK and enter TIME_WAIT. */
            tcp_send_ack(sk);
            tcp_time_wait(sk, TCP_TIME_WAIT, 0);
            break;
      default:
            /* Only TCP_LISTEN and TCP_CLOSE are left, in these
             * cases we should never reach this piece of code.
             */
            printk(KERN_ERR "%s: Impossible, sk->sk_state=%d\n",
                   __func__, sk->sk_state);
            break;
      }

      /* It _is_ possible, that we have something out-of-order _after_ FIN.
       * Probably, we should reset in this case. For now drop them.
       */
      __skb_queue_purge(&tp->out_of_order_queue);
      if (tcp_is_sack(tp))
            tcp_sack_reset(&tp->rx_opt);
      sk_mem_reclaim(sk);

      if (!sock_flag(sk, SOCK_DEAD)) {
            sk->sk_state_change(sk);

            /* Do not send POLL_HUP for half duplex close. */
            if (sk->sk_shutdown == SHUTDOWN_MASK ||
                sk->sk_state == TCP_CLOSE)
                  sk_wake_async(sk, SOCK_WAKE_WAITD, POLL_HUP);
            else
                  sk_wake_async(sk, SOCK_WAKE_WAITD, POLL_IN);
      }
}

static inline int tcp_sack_extend(struct tcp_sack_block *sp, u32 seq,
                          u32 end_seq)
{
      if (!after(seq, sp->end_seq) && !after(sp->start_seq, end_seq)) {
            if (before(seq, sp->start_seq))
                  sp->start_seq = seq;
            if (after(end_seq, sp->end_seq))
                  sp->end_seq = end_seq;
            return 1;
      }
      return 0;
}

static void tcp_dsack_set(struct sock *sk, u32 seq, u32 end_seq)
{
      struct tcp_sock *tp = tcp_sk(sk);

      if (tcp_is_sack(tp) && sysctl_tcp_dsack) {
            int mib_idx;

            if (before(seq, tp->rcv_nxt))
                  mib_idx = LINUX_MIB_TCPDSACKOLDSENT;
            else
                  mib_idx = LINUX_MIB_TCPDSACKOFOSENT;

            NET_INC_STATS_BH(sock_net(sk), mib_idx);

            tp->rx_opt.dsack = 1;
            tp->duplicate_sack[0].start_seq = seq;
            tp->duplicate_sack[0].end_seq = end_seq;
      }
}

static void tcp_dsack_extend(struct sock *sk, u32 seq, u32 end_seq)
{
      struct tcp_sock *tp = tcp_sk(sk);

      if (!tp->rx_opt.dsack)
            tcp_dsack_set(sk, seq, end_seq);
      else
            tcp_sack_extend(tp->duplicate_sack, seq, end_seq);
}

static void tcp_send_dupack(struct sock *sk, struct sk_buff *skb)
{
      struct tcp_sock *tp = tcp_sk(sk);

      if (TCP_SKB_CB(skb)->end_seq != TCP_SKB_CB(skb)->seq &&
          before(TCP_SKB_CB(skb)->seq, tp->rcv_nxt)) {
            NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_DELAYEDACKLOST);
            tcp_enter_quickack_mode(sk);

            if (tcp_is_sack(tp) && sysctl_tcp_dsack) {
                  u32 end_seq = TCP_SKB_CB(skb)->end_seq;

                  if (after(TCP_SKB_CB(skb)->end_seq, tp->rcv_nxt))
                        end_seq = tp->rcv_nxt;
                  tcp_dsack_set(sk, TCP_SKB_CB(skb)->seq, end_seq);
            }
      }

      tcp_send_ack(sk);
}

/* These routines update the SACK block as out-of-order packets arrive or
 * in-order packets close up the sequence space.
 */
static void tcp_sack_maybe_coalesce(struct tcp_sock *tp)
{
      int this_sack;
      struct tcp_sack_block *sp = &tp->selective_acks[0];
      struct tcp_sack_block *swalk = sp + 1;

      /* See if the recent change to the first SACK eats into
       * or hits the sequence space of other SACK blocks, if so coalesce.
       */
      for (this_sack = 1; this_sack < tp->rx_opt.num_sacks;) {
            if (tcp_sack_extend(sp, swalk->start_seq, swalk->end_seq)) {
                  int i;

                  /* Zap SWALK, by moving every further SACK up by one slot.
                   * Decrease num_sacks.
                   */
                  tp->rx_opt.num_sacks--;
                  for (i = this_sack; i < tp->rx_opt.num_sacks; i++)
                        sp[i] = sp[i + 1];
                  continue;
            }
            this_sack++, swalk++;
      }
}

static void tcp_sack_new_ofo_skb(struct sock *sk, u32 seq, u32 end_seq)
{
      struct tcp_sock *tp = tcp_sk(sk);
      struct tcp_sack_block *sp = &tp->selective_acks[0];
      int cur_sacks = tp->rx_opt.num_sacks;
      int this_sack;

      if (!cur_sacks)
            goto new_sack;

      for (this_sack = 0; this_sack < cur_sacks; this_sack++, sp++) {
            if (tcp_sack_extend(sp, seq, end_seq)) {
                  /* Rotate this_sack to the first one. */
                  for (; this_sack > 0; this_sack--, sp--)
                        swap(*sp, *(sp - 1));
                  if (cur_sacks > 1)
                        tcp_sack_maybe_coalesce(tp);
                  return;
            }
      }

      /* Could not find an adjacent existing SACK, build a new one,
       * put it at the front, and shift everyone else down.  We
       * always know there is at least one SACK present already here.
       *
       * If the sack array is full, forget about the last one.
       */
      if (this_sack >= TCP_NUM_SACKS) {
            this_sack--;
            tp->rx_opt.num_sacks--;
            sp--;
      }
      for (; this_sack > 0; this_sack--, sp--)
            *sp = *(sp - 1);

new_sack:
      /* Build the new head SACK, and we're done. */
      sp->start_seq = seq;
      sp->end_seq = end_seq;
      tp->rx_opt.num_sacks++;
}

/* RCV.NXT advances, some SACKs should be eaten. */

static void tcp_sack_remove(struct tcp_sock *tp)
{
      struct tcp_sack_block *sp = &tp->selective_acks[0];
      int num_sacks = tp->rx_opt.num_sacks;
      int this_sack;

      /* Empty ofo queue, hence, all the SACKs are eaten. Clear. */
      if (skb_queue_empty(&tp->out_of_order_queue)) {
            tp->rx_opt.num_sacks = 0;
            return;
      }

      for (this_sack = 0; this_sack < num_sacks;) {
            /* Check if the start of the sack is covered by RCV.NXT. */
            if (!before(tp->rcv_nxt, sp->start_seq)) {
                  int i;

                  /* RCV.NXT must cover all the block! */
                  WARN_ON(before(tp->rcv_nxt, sp->end_seq));

                  /* Zap this SACK, by moving forward any other SACKS. */
                  for (i=this_sack+1; i < num_sacks; i++)
                        tp->selective_acks[i-1] = tp->selective_acks[i];
                  num_sacks--;
                  continue;
            }
            this_sack++;
            sp++;
      }
      tp->rx_opt.num_sacks = num_sacks;
}

/* This one checks to see if we can put data from the
 * out_of_order queue into the receive_queue.
 */
static void tcp_ofo_queue(struct sock *sk)
{
      struct tcp_sock *tp = tcp_sk(sk);
      __u32 dsack_high = tp->rcv_nxt;
      struct sk_buff *skb;

      while ((skb = skb_peek(&tp->out_of_order_queue)) != NULL) {
            if (after(TCP_SKB_CB(skb)->seq, tp->rcv_nxt))
                  break;

            if (before(TCP_SKB_CB(skb)->seq, dsack_high)) {
                  __u32 dsack = dsack_high;
                  if (before(TCP_SKB_CB(skb)->end_seq, dsack_high))
                        dsack_high = TCP_SKB_CB(skb)->end_seq;
                  tcp_dsack_extend(sk, TCP_SKB_CB(skb)->seq, dsack);
            }

            if (!after(TCP_SKB_CB(skb)->end_seq, tp->rcv_nxt)) {
                  SOCK_DEBUG(sk, "ofo packet was already received \n");
                  __skb_unlink(skb, &tp->out_of_order_queue);
                  __kfree_skb(skb);
                  continue;
            }
            SOCK_DEBUG(sk, "ofo requeuing : rcv_next %X seq %X - %X\n",
                     tp->rcv_nxt, TCP_SKB_CB(skb)->seq,
                     TCP_SKB_CB(skb)->end_seq);

            __skb_unlink(skb, &tp->out_of_order_queue);
            __skb_queue_tail(&sk->sk_receive_queue, skb);
            tp->rcv_nxt = TCP_SKB_CB(skb)->end_seq;
            if (tcp_hdr(skb)->fin)
                  tcp_fin(skb, sk, tcp_hdr(skb));
      }
}

static int tcp_prune_ofo_queue(struct sock *sk);
static int tcp_prune_queue(struct sock *sk);

static inline int tcp_try_rmem_schedule(struct sock *sk, unsigned int size)
{
      if (atomic_read(&sk->sk_rmem_alloc) > sk->sk_rcvbuf ||
          !sk_rmem_schedule(sk, size)) {

            if (tcp_prune_queue(sk) < 0)
                  return -1;

            if (!sk_rmem_schedule(sk, size)) {
                  if (!tcp_prune_ofo_queue(sk))
                        return -1;

                  if (!sk_rmem_schedule(sk, size))
                        return -1;
            }
      }
      return 0;
}

static void tcp_data_queue(struct sock *sk, struct sk_buff *skb)
{
      struct tcphdr *th = tcp_hdr(skb);
      struct tcp_sock *tp = tcp_sk(sk);
      int eaten = -1;

      if (TCP_SKB_CB(skb)->seq == TCP_SKB_CB(skb)->end_seq)
            goto drop;

      __skb_pull(skb, th->doff * 4);

      TCP_ECN_accept_cwr(tp, skb);

      tp->rx_opt.dsack = 0;

      /*  Queue data for delivery to the user.
       *  Packets in sequence go to the receive queue.
       *  Out of sequence packets to the out_of_order_queue.
       */
      if (TCP_SKB_CB(skb)->seq == tp->rcv_nxt) {
            if (tcp_receive_window(tp) == 0)
                  goto out_of_window;

            /* Ok. In sequence. In window. */
            if (tp->ucopy.task == current &&
                tp->copied_seq == tp->rcv_nxt && tp->ucopy.len &&
                sock_owned_by_user(sk) && !tp->urg_data) {
                  int chunk = min_t(unsigned int, skb->len,
                                tp->ucopy.len);

                  __set_current_state(TASK_RUNNING);

                  local_bh_enable();
                  if (!skb_copy_datagram_iovec(skb, 0, tp->ucopy.iov, chunk)) {
                        tp->ucopy.len -= chunk;
                        tp->copied_seq += chunk;
                        eaten = (chunk == skb->len && !th->fin);
                        tcp_rcv_space_adjust(sk);
                  }
                  local_bh_disable();
            }

            if (eaten <= 0) {
queue_and_out:
                  if (eaten < 0 &&
                      tcp_try_rmem_schedule(sk, skb->truesize))
                        goto drop;

                  skb_set_owner_r(skb, sk);
                  __skb_queue_tail(&sk->sk_receive_queue, skb);
            }
            tp->rcv_nxt = TCP_SKB_CB(skb)->end_seq;
            if (skb->len)
                  tcp_event_data_recv(sk, skb);
            if (th->fin)
                  tcp_fin(skb, sk, th);

            if (!skb_queue_empty(&tp->out_of_order_queue)) {
                  tcp_ofo_queue(sk);

                  /* RFC2581. 4.2. SHOULD send immediate ACK, when
                   * gap in queue is filled.
                   */
                  if (skb_queue_empty(&tp->out_of_order_queue))
                        inet_csk(sk)->icsk_ack.pingpong = 0;
            }

            if (tp->rx_opt.num_sacks)
                  tcp_sack_remove(tp);

            tcp_fast_path_check(sk);

            if (eaten > 0)
                  __kfree_skb(skb);
            else if (!sock_flag(sk, SOCK_DEAD))
                  sk->sk_data_ready(sk, 0);
            return;
      }

      if (!after(TCP_SKB_CB(skb)->end_seq, tp->rcv_nxt)) {
            /* A retransmit, 2nd most common case.  Force an immediate ack. */
            NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_DELAYEDACKLOST);
            tcp_dsack_set(sk, TCP_SKB_CB(skb)->seq, TCP_SKB_CB(skb)->end_seq);

out_of_window:
            tcp_enter_quickack_mode(sk);
            inet_csk_schedule_ack(sk);
drop:
            __kfree_skb(skb);
            return;
      }

      /* Out of window. F.e. zero window probe. */
      if (!before(TCP_SKB_CB(skb)->seq, tp->rcv_nxt + tcp_receive_window(tp)))
            goto out_of_window;

      tcp_enter_quickack_mode(sk);

      if (before(TCP_SKB_CB(skb)->seq, tp->rcv_nxt)) {
            /* Partial packet, seq < rcv_next < end_seq */
            SOCK_DEBUG(sk, "partial packet: rcv_next %X seq %X - %X\n",
                     tp->rcv_nxt, TCP_SKB_CB(skb)->seq,
                     TCP_SKB_CB(skb)->end_seq);

            tcp_dsack_set(sk, TCP_SKB_CB(skb)->seq, tp->rcv_nxt);

            /* If window is closed, drop tail of packet. But after
             * remembering D-SACK for its head made in previous line.
             */
            if (!tcp_receive_window(tp))
                  goto out_of_window;
            goto queue_and_out;
      }

      TCP_ECN_check_ce(tp, skb);

      if (tcp_try_rmem_schedule(sk, skb->truesize))
            goto drop;

      /* Disable header prediction. */
      tp->pred_flags = 0;
      inet_csk_schedule_ack(sk);

      SOCK_DEBUG(sk, "out of order segment: rcv_next %X seq %X - %X\n",
               tp->rcv_nxt, TCP_SKB_CB(skb)->seq, TCP_SKB_CB(skb)->end_seq);

      skb_set_owner_r(skb, sk);

      if (!skb_peek(&tp->out_of_order_queue)) {
            /* Initial out of order segment, build 1 SACK. */
            if (tcp_is_sack(tp)) {
                  tp->rx_opt.num_sacks = 1;
                  tp->selective_acks[0].start_seq = TCP_SKB_CB(skb)->seq;
                  tp->selective_acks[0].end_seq =
                                    TCP_SKB_CB(skb)->end_seq;
            }
            __skb_queue_head(&tp->out_of_order_queue, skb);
      } else {
            struct sk_buff *skb1 = skb_peek_tail(&tp->out_of_order_queue);
            u32 seq = TCP_SKB_CB(skb)->seq;
            u32 end_seq = TCP_SKB_CB(skb)->end_seq;

            if (seq == TCP_SKB_CB(skb1)->end_seq) {
                  __skb_queue_after(&tp->out_of_order_queue, skb1, skb);

                  if (!tp->rx_opt.num_sacks ||
                      tp->selective_acks[0].end_seq != seq)
                        goto add_sack;

                  /* Common case: data arrive in order after hole. */
                  tp->selective_acks[0].end_seq = end_seq;
                  return;
            }

            /* Find place to insert this segment. */
            while (1) {
                  if (!after(TCP_SKB_CB(skb1)->seq, seq))
                        break;
                  if (skb_queue_is_first(&tp->out_of_order_queue, skb1)) {
                        skb1 = NULL;
                        break;
                  }
                  skb1 = skb_queue_prev(&tp->out_of_order_queue, skb1);
            }

            /* Do skb overlap to previous one? */
            if (skb1 && before(seq, TCP_SKB_CB(skb1)->end_seq)) {
                  if (!after(end_seq, TCP_SKB_CB(skb1)->end_seq)) {
                        /* All the bits are present. Drop. */
                        __kfree_skb(skb);
                        tcp_dsack_set(sk, seq, end_seq);
                        goto add_sack;
                  }
                  if (after(seq, TCP_SKB_CB(skb1)->seq)) {
                        /* Partial overlap. */
                        tcp_dsack_set(sk, seq,
                                    TCP_SKB_CB(skb1)->end_seq);
                  } else {
                        if (skb_queue_is_first(&tp->out_of_order_queue,
                                           skb1))
                              skb1 = NULL;
                        else
                              skb1 = skb_queue_prev(
                                    &tp->out_of_order_queue,
                                    skb1);
                  }
            }
            if (!skb1)
                  __skb_queue_head(&tp->out_of_order_queue, skb);
            else
                  __skb_queue_after(&tp->out_of_order_queue, skb1, skb);

            /* And clean segments covered by new one as whole. */
            while (!skb_queue_is_last(&tp->out_of_order_queue, skb)) {
                  skb1 = skb_queue_next(&tp->out_of_order_queue, skb);

                  if (!after(end_seq, TCP_SKB_CB(skb1)->seq))
                        break;
                  if (before(end_seq, TCP_SKB_CB(skb1)->end_seq)) {
                        tcp_dsack_extend(sk, TCP_SKB_CB(skb1)->seq,
                                     end_seq);
                        break;
                  }
                  __skb_unlink(skb1, &tp->out_of_order_queue);
                  tcp_dsack_extend(sk, TCP_SKB_CB(skb1)->seq,
                               TCP_SKB_CB(skb1)->end_seq);
                  __kfree_skb(skb1);
            }

add_sack:
            if (tcp_is_sack(tp))
                  tcp_sack_new_ofo_skb(sk, seq, end_seq);
      }
}

static struct sk_buff *tcp_collapse_one(struct sock *sk, struct sk_buff *skb,
                              struct sk_buff_head *list)
{
      struct sk_buff *next = NULL;

      if (!skb_queue_is_last(list, skb))
            next = skb_queue_next(list, skb);

      __skb_unlink(skb, list);
      __kfree_skb(skb);
      NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPRCVCOLLAPSED);

      return next;
}

/* Collapse contiguous sequence of skbs head..tail with
 * sequence numbers start..end.
 *
 * If tail is NULL, this means until the end of the list.
 *
 * Segments with FIN/SYN are not collapsed (only because this
 * simplifies code)
 */
static void
tcp_collapse(struct sock *sk, struct sk_buff_head *list,
           struct sk_buff *head, struct sk_buff *tail,
           u32 start, u32 end)
{
      struct sk_buff *skb, *n;
      bool end_of_skbs;

      /* First, check that queue is collapsible and find
       * the point where collapsing can be useful. */
      skb = head;
restart:
      end_of_skbs = true;
      skb_queue_walk_from_safe(list, skb, n) {
            if (skb == tail)
                  break;
            /* No new bits? It is possible on ofo queue. */
            if (!before(start, TCP_SKB_CB(skb)->end_seq)) {
                  skb = tcp_collapse_one(sk, skb, list);
                  if (!skb)
                        break;
                  goto restart;
            }

            /* The first skb to collapse is:
             * - not SYN/FIN and
             * - bloated or contains data before "start" or
             *   overlaps to the next one.
             */
            if (!tcp_hdr(skb)->syn && !tcp_hdr(skb)->fin &&
                (tcp_win_from_space(skb->truesize) > skb->len ||
                 before(TCP_SKB_CB(skb)->seq, start))) {
                  end_of_skbs = false;
                  break;
            }

            if (!skb_queue_is_last(list, skb)) {
                  struct sk_buff *next = skb_queue_next(list, skb);
                  if (next != tail &&
                      TCP_SKB_CB(skb)->end_seq != TCP_SKB_CB(next)->seq) {
                        end_of_skbs = false;
                        break;
                  }
            }

            /* Decided to skip this, advance start seq. */
            start = TCP_SKB_CB(skb)->end_seq;
      }
      if (end_of_skbs || tcp_hdr(skb)->syn || tcp_hdr(skb)->fin)
            return;

      while (before(start, end)) {
            struct sk_buff *nskb;
            unsigned int header = skb_headroom(skb);
            int copy = SKB_MAX_ORDER(header, 0);

            /* Too big header? This can happen with IPv6. */
            if (copy < 0)
                  return;
            if (end - start < copy)
                  copy = end - start;
            nskb = alloc_skb(copy + header, GFP_ATOMIC);
            if (!nskb)
                  return;

            skb_set_mac_header(nskb, skb_mac_header(skb) - skb->head);
            skb_set_network_header(nskb, (skb_network_header(skb) -
                                    skb->head));
            skb_set_transport_header(nskb, (skb_transport_header(skb) -
                                    skb->head));
            skb_reserve(nskb, header);
            memcpy(nskb->head, skb->head, header);
            memcpy(nskb->cb, skb->cb, sizeof(skb->cb));
            TCP_SKB_CB(nskb)->seq = TCP_SKB_CB(nskb)->end_seq = start;
            __skb_queue_before(list, skb, nskb);
            skb_set_owner_r(nskb, sk);

            /* Copy data, releasing collapsed skbs. */
            while (copy > 0) {
                  int offset = start - TCP_SKB_CB(skb)->seq;
                  int size = TCP_SKB_CB(skb)->end_seq - start;

                  BUG_ON(offset < 0);
                  if (size > 0) {
                        size = min(copy, size);
                        if (skb_copy_bits(skb, offset, skb_put(nskb, size), size))
                              BUG();
                        TCP_SKB_CB(nskb)->end_seq += size;
                        copy -= size;
                        start += size;
                  }
                  if (!before(start, TCP_SKB_CB(skb)->end_seq)) {
                        skb = tcp_collapse_one(sk, skb, list);
                        if (!skb ||
                            skb == tail ||
                            tcp_hdr(skb)->syn ||
                            tcp_hdr(skb)->fin)
                              return;
                  }
            }
      }
}

/* Collapse ofo queue. Algorithm: select contiguous sequence of skbs
 * and tcp_collapse() them until all the queue is collapsed.
 */
static void tcp_collapse_ofo_queue(struct sock *sk)
{
      struct tcp_sock *tp = tcp_sk(sk);
      struct sk_buff *skb = skb_peek(&tp->out_of_order_queue);
      struct sk_buff *head;
      u32 start, end;

      if (skb == NULL)
            return;

      start = TCP_SKB_CB(skb)->seq;
      end = TCP_SKB_CB(skb)->end_seq;
      head = skb;

      for (;;) {
            struct sk_buff *next = NULL;

            if (!skb_queue_is_last(&tp->out_of_order_queue, skb))
                  next = skb_queue_next(&tp->out_of_order_queue, skb);
            skb = next;

            /* Segment is terminated when we see gap or when
             * we are at the end of all the queue. */
            if (!skb ||
                after(TCP_SKB_CB(skb)->seq, end) ||
                before(TCP_SKB_CB(skb)->end_seq, start)) {
                  tcp_collapse(sk, &tp->out_of_order_queue,
                             head, skb, start, end);
                  head = skb;
                  if (!skb)
                        break;
                  /* Start new segment */
                  start = TCP_SKB_CB(skb)->seq;
                  end = TCP_SKB_CB(skb)->end_seq;
            } else {
                  if (before(TCP_SKB_CB(skb)->seq, start))
                        start = TCP_SKB_CB(skb)->seq;
                  if (after(TCP_SKB_CB(skb)->end_seq, end))
                        end = TCP_SKB_CB(skb)->end_seq;
            }
      }
}

/*
 * Purge the out-of-order queue.
 * Return true if queue was pruned.
 */
static int tcp_prune_ofo_queue(struct sock *sk)
{
      struct tcp_sock *tp = tcp_sk(sk);
      int res = 0;

      if (!skb_queue_empty(&tp->out_of_order_queue)) {
            NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_OFOPRUNED);
            __skb_queue_purge(&tp->out_of_order_queue);

            /* Reset SACK state.  A conforming SACK implementation will
             * do the same at a timeout based retransmit.  When a connection
             * is in a sad state like this, we care only about integrity
             * of the connection not performance.
             */
            if (tp->rx_opt.sack_ok)
                  tcp_sack_reset(&tp->rx_opt);
            sk_mem_reclaim(sk);
            res = 1;
      }
      return res;
}

/* Reduce allocated memory if we can, trying to get
 * the socket within its memory limits again.
 *
 * Return less than zero if we should start dropping frames
 * until the socket owning process reads some of the data
 * to stabilize the situation.
 */
static int tcp_prune_queue(struct sock *sk)
{
      struct tcp_sock *tp = tcp_sk(sk);

      SOCK_DEBUG(sk, "prune_queue: c=%x\n", tp->copied_seq);

      NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_PRUNECALLED);

      if (atomic_read(&sk->sk_rmem_alloc) >= sk->sk_rcvbuf)
            tcp_clamp_window(sk);
      else if (tcp_memory_pressure)
            tp->rcv_ssthresh = min(tp->rcv_ssthresh, 4U * tp->advmss);

      tcp_collapse_ofo_queue(sk);
      if (!skb_queue_empty(&sk->sk_receive_queue))
            tcp_collapse(sk, &sk->sk_receive_queue,
                       skb_peek(&sk->sk_receive_queue),
                       NULL,
                       tp->copied_seq, tp->rcv_nxt);
      sk_mem_reclaim(sk);

      if (atomic_read(&sk->sk_rmem_alloc) <= sk->sk_rcvbuf)
            return 0;

      /* Collapsing did not help, destructive actions follow.
       * This must not ever occur. */

      tcp_prune_ofo_queue(sk);

      if (atomic_read(&sk->sk_rmem_alloc) <= sk->sk_rcvbuf)
            return 0;

      /* If we are really being abused, tell the caller to silently
       * drop receive data on the floor.  It will get retransmitted
       * and hopefully then we'll have sufficient space.
       */
      NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_RCVPRUNED);

      /* Massive buffer overcommit. */
      tp->pred_flags = 0;
      return -1;
}

/* RFC2861, slow part. Adjust cwnd, after it was not full during one rto.
 * As additional protections, we do not touch cwnd in retransmission phases,
 * and if application hit its sndbuf limit recently.
 */
void tcp_cwnd_application_limited(struct sock *sk)
{
      struct tcp_sock *tp = tcp_sk(sk);

      if (inet_csk(sk)->icsk_ca_state == TCP_CA_Open &&
          sk->sk_socket && !test_bit(SOCK_NOSPACE, &sk->sk_socket->flags)) {
            /* Limited by application or receiver window. */
            u32 init_win = tcp_init_cwnd(tp, __sk_dst_get(sk));
            u32 win_used = max(tp->snd_cwnd_used, init_win);
            if (win_used < tp->snd_cwnd) {
                  tp->snd_ssthresh = tcp_current_ssthresh(sk);
                  tp->snd_cwnd = (tp->snd_cwnd + win_used) >> 1;
            }
            tp->snd_cwnd_used = 0;
      }
      tp->snd_cwnd_stamp = tcp_time_stamp;
}

static int tcp_should_expand_sndbuf(struct sock *sk)
{
      struct tcp_sock *tp = tcp_sk(sk);

      /* If the user specified a specific send buffer setting, do
       * not modify it.
       */
      if (sk->sk_userlocks & SOCK_SNDBUF_LOCK)
            return 0;

      /* If we are under global TCP memory pressure, do not expand.  */
      if (tcp_memory_pressure)
            return 0;

      /* If we are under soft global TCP memory pressure, do not expand.  */
      if (atomic_read(&tcp_memory_allocated) >= sysctl_tcp_mem[0])
            return 0;

      /* If we filled the congestion window, do not expand.  */
      if (tp->packets_out >= tp->snd_cwnd)
            return 0;

      return 1;
}

/* When incoming ACK allowed to free some skb from write_queue,
 * we remember this event in flag SOCK_QUEUE_SHRUNK and wake up socket
 * on the exit from tcp input handler.
 *
 * PROBLEM: sndbuf expansion does not work well with largesend.
 */
static void tcp_new_space(struct sock *sk)
{
      struct tcp_sock *tp = tcp_sk(sk);

      if (tcp_should_expand_sndbuf(sk)) {
            int sndmem = max_t(u32, tp->rx_opt.mss_clamp, tp->mss_cache) +
                  MAX_TCP_HEADER + 16 + sizeof(struct sk_buff);
            int demanded = max_t(unsigned int, tp->snd_cwnd,
                             tp->reordering + 1);
            sndmem *= 2 * demanded;
            if (sndmem > sk->sk_sndbuf)
                  sk->sk_sndbuf = min(sndmem, sysctl_tcp_wmem[2]);
            tp->snd_cwnd_stamp = tcp_time_stamp;
      }

      sk->sk_write_space(sk);
}

static void tcp_check_space(struct sock *sk)
{
      if (sock_flag(sk, SOCK_QUEUE_SHRUNK)) {
            sock_reset_flag(sk, SOCK_QUEUE_SHRUNK);
            if (sk->sk_socket &&
                test_bit(SOCK_NOSPACE, &sk->sk_socket->flags))
                  tcp_new_space(sk);
      }
}

static inline void tcp_data_snd_check(struct sock *sk)
{
      tcp_push_pending_frames(sk);
      tcp_check_space(sk);
}

/*
 * Check if sending an ack is needed.
 */
static void __tcp_ack_snd_check(struct sock *sk, int ofo_possible)
{
      struct tcp_sock *tp = tcp_sk(sk);

          /* More than one full frame received... */
      if (((tp->rcv_nxt - tp->rcv_wup) > inet_csk(sk)->icsk_ack.rcv_mss
           /* ... and right edge of window advances far enough.
            * (tcp_recvmsg() will send ACK otherwise). Or...
            */
           && __tcp_select_window(sk) >= tp->rcv_wnd) ||
          /* We ACK each frame or... */
          tcp_in_quickack_mode(sk) ||
          /* We have out of order data. */
          (ofo_possible && skb_peek(&tp->out_of_order_queue))) {
            /* Then ack it now */
            tcp_send_ack(sk);
      } else {
            /* Else, send delayed ack. */
            tcp_send_delayed_ack(sk);
      }
}

static inline void tcp_ack_snd_check(struct sock *sk)
{
      if (!inet_csk_ack_scheduled(sk)) {
            /* We sent a data segment already. */
            return;
      }
      __tcp_ack_snd_check(sk, 1);
}

/*
 *    This routine is only called when we have urgent data
 *    signaled. Its the 'slow' part of tcp_urg. It could be
 *    moved inline now as tcp_urg is only called from one
 *    place. We handle URGent data wrong. We have to - as
 *    BSD still doesn't use the correction from RFC961.
 *    For 1003.1g we should support a new option TCP_STDURG to permit
 *    either form (or just set the sysctl tcp_stdurg).
 */

static void tcp_check_urg(struct sock *sk, struct tcphdr *th)
{
      struct tcp_sock *tp = tcp_sk(sk);
      u32 ptr = ntohs(th->urg_ptr);

      if (ptr && !sysctl_tcp_stdurg)
            ptr--;
      ptr += ntohl(th->seq);

      /* Ignore urgent data that we've already seen and read. */
      if (after(tp->copied_seq, ptr))
            return;

      /* Do not replay urg ptr.
       *
       * NOTE: interesting situation not covered by specs.
       * Misbehaving sender may send urg ptr, pointing to segment,
       * which we already have in ofo queue. We are not able to fetch
       * such data and will stay in TCP_URG_NOTYET until will be eaten
       * by recvmsg(). Seems, we are not obliged to handle such wicked
       * situations. But it is worth to think about possibility of some
       * DoSes using some hypothetical application level deadlock.
       */
      if (before(ptr, tp->rcv_nxt))
            return;

      /* Do we already have a newer (or duplicate) urgent pointer? */
      if (tp->urg_data && !after(ptr, tp->urg_seq))
            return;

      /* Tell the world about our new urgent pointer. */
      sk_send_sigurg(sk);

      /* We may be adding urgent data when the last byte read was
       * urgent. To do this requires some care. We cannot just ignore
       * tp->copied_seq since we would read the last urgent byte again
       * as data, nor can we alter copied_seq until this data arrives
       * or we break the semantics of SIOCATMARK (and thus sockatmark())
       *
       * NOTE. Double Dutch. Rendering to plain English: author of comment
       * above did something sort of      send("A", MSG_OOB); send("B", MSG_OOB);
       * and expect that both A and B disappear from stream. This is _wrong_.
       * Though this happens in BSD with high probability, this is occasional.
       * Any application relying on this is buggy. Note also, that fix "works"
       * only in this artificial test. Insert some normal data between A and B and we will
       * decline of BSD again. Verdict: it is better to remove to trap
       * buggy users.
       */
      if (tp->urg_seq == tp->copied_seq && tp->urg_data &&
          !sock_flag(sk, SOCK_URGINLINE) && tp->copied_seq != tp->rcv_nxt) {
            struct sk_buff *skb = skb_peek(&sk->sk_receive_queue);
            tp->copied_seq++;
            if (skb && !before(tp->copied_seq, TCP_SKB_CB(skb)->end_seq)) {
                  __skb_unlink(skb, &sk->sk_receive_queue);
                  __kfree_skb(skb);
            }
      }

      tp->urg_data = TCP_URG_NOTYET;
      tp->urg_seq = ptr;

      /* Disable header prediction. */
      tp->pred_flags = 0;
}

/* This is the 'fast' part of urgent handling. */
static void tcp_urg(struct sock *sk, struct sk_buff *skb, struct tcphdr *th)
{
      struct tcp_sock *tp = tcp_sk(sk);

      /* Check if we get a new urgent pointer - normally not. */
      if (th->urg)
            tcp_check_urg(sk, th);

      /* Do we wait for any urgent data? - normally not... */
      if (tp->urg_data == TCP_URG_NOTYET) {
            u32 ptr = tp->urg_seq - ntohl(th->seq) + (th->doff * 4) -
                    th->syn;

            /* Is the urgent pointer pointing into this packet? */
            if (ptr < skb->len) {
                  u8 tmp;
                  if (skb_copy_bits(skb, ptr, &tmp, 1))
                        BUG();
                  tp->urg_data = TCP_URG_VALID | tmp;
                  if (!sock_flag(sk, SOCK_DEAD))
                        sk->sk_data_ready(sk, 0);
            }
      }
}

static int tcp_copy_to_iovec(struct sock *sk, struct sk_buff *skb, int hlen)
{
      struct tcp_sock *tp = tcp_sk(sk);
      int chunk = skb->len - hlen;
      int err;

      local_bh_enable();
      if (skb_csum_unnecessary(skb))
            err = skb_copy_datagram_iovec(skb, hlen, tp->ucopy.iov, chunk);
      else
            err = skb_copy_and_csum_datagram_iovec(skb, hlen,
                                           tp->ucopy.iov);

      if (!err) {
            tp->ucopy.len -= chunk;
            tp->copied_seq += chunk;
            tcp_rcv_space_adjust(sk);
      }

      local_bh_disable();
      return err;
}

static __sum16 __tcp_checksum_complete_user(struct sock *sk,
                                  struct sk_buff *skb)
{
      __sum16 result;

      if (sock_owned_by_user(sk)) {
            local_bh_enable();
            result = __tcp_checksum_complete(skb);
            local_bh_disable();
      } else {
            result = __tcp_checksum_complete(skb);
      }
      return result;
}

static inline int tcp_checksum_complete_user(struct sock *sk,
                                   struct sk_buff *skb)
{
      return !skb_csum_unnecessary(skb) &&
             __tcp_checksum_complete_user(sk, skb);
}

#ifdef CONFIG_NET_DMA
static int tcp_dma_try_early_copy(struct sock *sk, struct sk_buff *skb,
                          int hlen)
{
      struct tcp_sock *tp = tcp_sk(sk);
      int chunk = skb->len - hlen;
      int dma_cookie;
      int copied_early = 0;

      if (tp->ucopy.wakeup)
            return 0;

      if (!tp->ucopy.dma_chan && tp->ucopy.pinned_list)
            tp->ucopy.dma_chan = dma_find_channel(DMA_MEMCPY);

      if (tp->ucopy.dma_chan && skb_csum_unnecessary(skb)) {

            dma_cookie = dma_skb_copy_datagram_iovec(tp->ucopy.dma_chan,
                                           skb, hlen,
                                           tp->ucopy.iov, chunk,
                                           tp->ucopy.pinned_list);

            if (dma_cookie < 0)
                  goto out;

            tp->ucopy.dma_cookie = dma_cookie;
            copied_early = 1;

            tp->ucopy.len -= chunk;
            tp->copied_seq += chunk;
            tcp_rcv_space_adjust(sk);

            if ((tp->ucopy.len == 0) ||
                (tcp_flag_word(tcp_hdr(skb)) & TCP_FLAG_PSH) ||
                (atomic_read(&sk->sk_rmem_alloc) > (sk->sk_rcvbuf >> 1))) {
                  tp->ucopy.wakeup = 1;
                  sk->sk_data_ready(sk, 0);
            }
      } else if (chunk > 0) {
            tp->ucopy.wakeup = 1;
            sk->sk_data_ready(sk, 0);
      }
out:
      return copied_early;
}
#endif /* CONFIG_NET_DMA */

/* Does PAWS and seqno based validation of an incoming segment, flags will
 * play significant role here.
 */
static int tcp_validate_incoming(struct sock *sk, struct sk_buff *skb,
                        struct tcphdr *th, int syn_inerr)
{
      struct tcp_sock *tp = tcp_sk(sk);

      /* RFC1323: H1. Apply PAWS check first. */
      if (tcp_fast_parse_options(skb, th, tp) && tp->rx_opt.saw_tstamp &&
          tcp_paws_discard(sk, skb)) {
            if (!th->rst) {
                  NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_PAWSESTABREJECTED);
                  tcp_send_dupack(sk, skb);
                  goto discard;
            }
            /* Reset is accepted even if it did not pass PAWS. */
      }

      /* Step 1: check sequence number */
      if (!tcp_sequence(tp, TCP_SKB_CB(skb)->seq, TCP_SKB_CB(skb)->end_seq)) {
            /* RFC793, page 37: "In all states except SYN-SENT, all reset
             * (RST) segments are validated by checking their SEQ-fields."
             * And page 69: "If an incoming segment is not acceptable,
             * an acknowledgment should be sent in reply (unless the RST
             * bit is set, if so drop the segment and return)".
             */
            if (!th->rst)
                  tcp_send_dupack(sk, skb);
            goto discard;
      }

      /* Step 2: check RST bit */
      if (th->rst) {
            tcp_reset(sk);
            goto discard;
      }

      /* ts_recent update must be made after we are sure that the packet
       * is in window.
       */
      tcp_replace_ts_recent(tp, TCP_SKB_CB(skb)->seq);

      /* step 3: check security and precedence [ignored] */

      /* step 4: Check for a SYN in window. */
      if (th->syn && !before(TCP_SKB_CB(skb)->seq, tp->rcv_nxt)) {
            if (syn_inerr)
                  TCP_INC_STATS_BH(sock_net(sk), TCP_MIB_INERRS);
            NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPABORTONSYN);
            tcp_reset(sk);
            return -1;
      }

      return 1;

discard:
      __kfree_skb(skb);
      return 0;
}

/*
 *    TCP receive function for the ESTABLISHED state.
 *
 *    It is split into a fast path and a slow path. The fast path is
 *    disabled when:
 *    - A zero window was announced from us - zero window probing
 *        is only handled properly in the slow path.
 *    - Out of order segments arrived.
 *    - Urgent data is expected.
 *    - There is no buffer space left
 *    - Unexpected TCP flags/window values/header lengths are received
 *      (detected by checking the TCP header against pred_flags)
 *    - Data is sent in both directions. Fast path only supports pure senders
 *      or pure receivers (this means either the sequence number or the ack
 *      value must stay constant)
 *    - Unexpected TCP option.
 *
 *    When these conditions are not satisfied it drops into a standard
 *    receive procedure patterned after RFC793 to handle all cases.
 *    The first three cases are guaranteed by proper pred_flags setting,
 *    the rest is checked inline. Fast processing is turned on in
 *    tcp_data_queue when everything is OK.
 */
int tcp_rcv_established(struct sock *sk, struct sk_buff *skb,
                  struct tcphdr *th, unsigned len)
{
      struct tcp_sock *tp = tcp_sk(sk);
      int res;

      /*
       *    Header prediction.
       *    The code loosely follows the one in the famous
       *    "30 instruction TCP receive" Van Jacobson mail.
       *
       *    Van's trick is to deposit buffers into socket queue
       *    on a device interrupt, to call tcp_recv function
       *    on the receive process context and checksum and copy
       *    the buffer to user space. smart...
       *
       *    Our current scheme is not silly either but we take the
       *    extra cost of the net_bh soft interrupt processing...
       *    We do checksum and copy also but from device to kernel.
       */

      tp->rx_opt.saw_tstamp = 0;

      /*    pred_flags is 0xS?10 << 16 + snd_wnd
       *    if header_prediction is to be made
       *    'S' will always be tp->tcp_header_len >> 2
       *    '?' will be 0 for the fast path, otherwise pred_flags is 0 to
       *  turn it off   (when there are holes in the receive
       *     space for instance)
       *    PSH flag is ignored.
       */

      if ((tcp_flag_word(th) & TCP_HP_BITS) == tp->pred_flags &&
          TCP_SKB_CB(skb)->seq == tp->rcv_nxt &&
          !after(TCP_SKB_CB(skb)->ack_seq, tp->snd_nxt)) {
            int tcp_header_len = tp->tcp_header_len;

            /* Timestamp header prediction: tcp_header_len
             * is automatically equal to th->doff*4 due to pred_flags
             * match.
             */

            /* Check timestamp */
            if (tcp_header_len == sizeof(struct tcphdr) + TCPOLEN_TSTAMP_ALIGNED) {
                  /* No? Slow path! */
                  if (!tcp_parse_aligned_timestamp(tp, th))
                        goto slow_path;

                  /* If PAWS failed, check it more carefully in slow path */
                  if ((s32)(tp->rx_opt.rcv_tsval - tp->rx_opt.ts_recent) < 0)
                        goto slow_path;

                  /* DO NOT update ts_recent here, if checksum fails
                   * and timestamp was corrupted part, it will result
                   * in a hung connection since we will drop all
                   * future packets due to the PAWS test.
                   */
            }

            if (len <= tcp_header_len) {
                  /* Bulk data transfer: sender */
                  if (len == tcp_header_len) {
                        /* Predicted packet is in window by definition.
                         * seq == rcv_nxt and rcv_wup <= rcv_nxt.
                         * Hence, check seq<=rcv_wup reduces to:
                         */
                        if (tcp_header_len ==
                            (sizeof(struct tcphdr) + TCPOLEN_TSTAMP_ALIGNED) &&
                            tp->rcv_nxt == tp->rcv_wup)
                              tcp_store_ts_recent(tp);

                        /* We know that such packets are checksummed
                         * on entry.
                         */
                        tcp_ack(sk, skb, 0);
                        __kfree_skb(skb);
                        tcp_data_snd_check(sk);
                        return 0;
                  } else { /* Header too small */
                        TCP_INC_STATS_BH(sock_net(sk), TCP_MIB_INERRS);
                        goto discard;
                  }
            } else {
                  int eaten = 0;
                  int copied_early = 0;

                  if (tp->copied_seq == tp->rcv_nxt &&
                      len - tcp_header_len <= tp->ucopy.len) {
#ifdef CONFIG_NET_DMA
                        if (tcp_dma_try_early_copy(sk, skb, tcp_header_len)) {
                              copied_early = 1;
                              eaten = 1;
                        }
#endif
                        if (tp->ucopy.task == current &&
                            sock_owned_by_user(sk) && !copied_early) {
                              __set_current_state(TASK_RUNNING);

                              if (!tcp_copy_to_iovec(sk, skb, tcp_header_len))
                                    eaten = 1;
                        }
                        if (eaten) {
                              /* Predicted packet is in window by definition.
                               * seq == rcv_nxt and rcv_wup <= rcv_nxt.
                               * Hence, check seq<=rcv_wup reduces to:
                               */
                              if (tcp_header_len ==
                                  (sizeof(struct tcphdr) +
                                   TCPOLEN_TSTAMP_ALIGNED) &&
                                  tp->rcv_nxt == tp->rcv_wup)
                                    tcp_store_ts_recent(tp);

                              tcp_rcv_rtt_measure_ts(sk, skb);

                              __skb_pull(skb, tcp_header_len);
                              tp->rcv_nxt = TCP_SKB_CB(skb)->end_seq;
                              NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPHPHITSTOUSER);
                        }
                        if (copied_early)
                              tcp_cleanup_rbuf(sk, skb->len);
                  }
                  if (!eaten) {
                        if (tcp_checksum_complete_user(sk, skb))
                              goto csum_error;

                        /* Predicted packet is in window by definition.
                         * seq == rcv_nxt and rcv_wup <= rcv_nxt.
                         * Hence, check seq<=rcv_wup reduces to:
                         */
                        if (tcp_header_len ==
                            (sizeof(struct tcphdr) + TCPOLEN_TSTAMP_ALIGNED) &&
                            tp->rcv_nxt == tp->rcv_wup)
                              tcp_store_ts_recent(tp);

                        tcp_rcv_rtt_measure_ts(sk, skb);

                        if ((int)skb->truesize > sk->sk_forward_alloc)
                              goto step5;

                        NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPHPHITS);

                        /* Bulk data transfer: receiver */
                        __skb_pull(skb, tcp_header_len);
                        __skb_queue_tail(&sk->sk_receive_queue, skb);
                        skb_set_owner_r(skb, sk);
                        tp->rcv_nxt = TCP_SKB_CB(skb)->end_seq;
                  }

                  tcp_event_data_recv(sk, skb);

                  if (TCP_SKB_CB(skb)->ack_seq != tp->snd_una) {
                        /* Well, only one small jumplet in fast path... */
                        tcp_ack(sk, skb, FLAG_DATA);
                        tcp_data_snd_check(sk);
                        if (!inet_csk_ack_scheduled(sk))
                              goto no_ack;
                  }

                  if (!copied_early || tp->rcv_nxt != tp->rcv_wup)
                        __tcp_ack_snd_check(sk, 0);
no_ack:
#ifdef CONFIG_NET_DMA
                  if (copied_early)
                        __skb_queue_tail(&sk->sk_async_wait_queue, skb);
                  else
#endif
                  if (eaten)
                        __kfree_skb(skb);
                  else
                        sk->sk_data_ready(sk, 0);
                  return 0;
            }
      }

slow_path:
      if (len < (th->doff << 2) || tcp_checksum_complete_user(sk, skb))
            goto csum_error;

      /*
       *    Standard slow path.
       */

      res = tcp_validate_incoming(sk, skb, th, 1);
      if (res <= 0)
            return -res;

step5:
      if (th->ack && tcp_ack(sk, skb, FLAG_SLOWPATH) < 0)
            goto discard;

      tcp_rcv_rtt_measure_ts(sk, skb);

      /* Process urgent data. */
      tcp_urg(sk, skb, th);

      /* step 7: process the segment text */
      tcp_data_queue(sk, skb);

      tcp_data_snd_check(sk);
      tcp_ack_snd_check(sk);
      return 0;

csum_error:
      TCP_INC_STATS_BH(sock_net(sk), TCP_MIB_INERRS);

discard:
      __kfree_skb(skb);
      return 0;
}

static int tcp_rcv_synsent_state_process(struct sock *sk, struct sk_buff *skb,
                               struct tcphdr *th, unsigned len)
{
      struct tcp_sock *tp = tcp_sk(sk);
      struct inet_connection_sock *icsk = inet_csk(sk);
      int saved_clamp = tp->rx_opt.mss_clamp;

      tcp_parse_options(skb, &tp->rx_opt, 0);

      if (th->ack) {
            /* rfc793:
             * "If the state is SYN-SENT then
             *    first check the ACK bit
             *      If the ACK bit is set
             *      If SEG.ACK =< ISS, or SEG.ACK > SND.NXT, send
             *        a reset (unless the RST bit is set, if so drop
             *        the segment and return)"
             *
             *  We do not send data with SYN, so that RFC-correct
             *  test reduces to:
             */
            if (TCP_SKB_CB(skb)->ack_seq != tp->snd_nxt)
                  goto reset_and_undo;

            if (tp->rx_opt.saw_tstamp && tp->rx_opt.rcv_tsecr &&
                !between(tp->rx_opt.rcv_tsecr, tp->retrans_stamp,
                       tcp_time_stamp)) {
                  NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_PAWSACTIVEREJECTED);
                  goto reset_and_undo;
            }

            /* Now ACK is acceptable.
             *
             * "If the RST bit is set
             *    If the ACK was acceptable then signal the user "error:
             *    connection reset", drop the segment, enter CLOSED state,
             *    delete TCB, and return."
             */

            if (th->rst) {
                  tcp_reset(sk);
                  goto discard;
            }

            /* rfc793:
             *   "fifth, if neither of the SYN or RST bits is set then
             *    drop the segment and return."
             *
             *    See note below!
             *                                        --ANK(990513)
             */
            if (!th->syn)
                  goto discard_and_undo;

            /* rfc793:
             *   "If the SYN bit is on ...
             *    are acceptable then ...
             *    (our SYN has been ACKed), change the connection
             *    state to ESTABLISHED..."
             */

            TCP_ECN_rcv_synack(tp, th);

            tp->snd_wl1 = TCP_SKB_CB(skb)->seq;
            tcp_ack(sk, skb, FLAG_SLOWPATH);

            /* Ok.. it's good. Set up sequence numbers and
             * move to established.
             */
            tp->rcv_nxt = TCP_SKB_CB(skb)->seq + 1;
            tp->rcv_wup = TCP_SKB_CB(skb)->seq + 1;

            /* RFC1323: The window in SYN & SYN/ACK segments is
             * never scaled.
             */
            tp->snd_wnd = ntohs(th->window);
            tcp_init_wl(tp, TCP_SKB_CB(skb)->seq);

            if (!tp->rx_opt.wscale_ok) {
                  tp->rx_opt.snd_wscale = tp->rx_opt.rcv_wscale = 0;
                  tp->window_clamp = min(tp->window_clamp, 65535U);
            }

            if (tp->rx_opt.saw_tstamp) {
                  tp->rx_opt.tstamp_ok       = 1;
                  tp->tcp_header_len =
                        sizeof(struct tcphdr) + TCPOLEN_TSTAMP_ALIGNED;
                  tp->advmss      -= TCPOLEN_TSTAMP_ALIGNED;
                  tcp_store_ts_recent(tp);
            } else {
                  tp->tcp_header_len = sizeof(struct tcphdr);
            }

            if (tcp_is_sack(tp) && sysctl_tcp_fack)
                  tcp_enable_fack(tp);

            tcp_mtup_init(sk);
            tcp_sync_mss(sk, icsk->icsk_pmtu_cookie);
            tcp_initialize_rcv_mss(sk);

            /* Remember, tcp_poll() does not lock socket!
             * Change state from SYN-SENT only after copied_seq
             * is initialized. */
            tp->copied_seq = tp->rcv_nxt;
            smp_mb();
            tcp_set_state(sk, TCP_ESTABLISHED);

            security_inet_conn_established(sk, skb);

            /* Make sure socket is routed, for correct metrics.  */
            icsk->icsk_af_ops->rebuild_header(sk);

            tcp_init_metrics(sk);

            tcp_init_congestion_control(sk);

            /* Prevent spurious tcp_cwnd_restart() on first data
             * packet.
             */
            tp->lsndtime = tcp_time_stamp;

            tcp_init_buffer_space(sk);

            if (sock_flag(sk, SOCK_KEEPOPEN))
                  inet_csk_reset_keepalive_timer(sk, keepalive_time_when(tp));

            if (!tp->rx_opt.snd_wscale)
                  __tcp_fast_path_on(tp, tp->snd_wnd);
            else
                  tp->pred_flags = 0;

            if (!sock_flag(sk, SOCK_DEAD)) {
                  sk->sk_state_change(sk);
                  sk_wake_async(sk, SOCK_WAKE_IO, POLL_OUT);
            }

            if (sk->sk_write_pending ||
                icsk->icsk_accept_queue.rskq_defer_accept ||
                icsk->icsk_ack.pingpong) {
                  /* Save one ACK. Data will be ready after
                   * several ticks, if write_pending is set.
                   *
                   * It may be deleted, but with this feature tcpdumps
                   * look so _wonderfully_ clever, that I was not able
                   * to stand against the temptation 8)     --ANK
                   */
                  inet_csk_schedule_ack(sk);
                  icsk->icsk_ack.lrcvtime = tcp_time_stamp;
                  icsk->icsk_ack.ato       = TCP_ATO_MIN;
                  tcp_incr_quickack(sk);
                  tcp_enter_quickack_mode(sk);
                  inet_csk_reset_xmit_timer(sk, ICSK_TIME_DACK,
                                      TCP_DELACK_MAX, TCP_RTO_MAX);

discard:
                  __kfree_skb(skb);
                  return 0;
            } else {
                  tcp_send_ack(sk);
            }
            return -1;
      }

      /* No ACK in the segment */

      if (th->rst) {
            /* rfc793:
             * "If the RST bit is set
             *
             *      Otherwise (no ACK) drop the segment and return."
             */

            goto discard_and_undo;
      }

      /* PAWS check. */
      if (tp->rx_opt.ts_recent_stamp && tp->rx_opt.saw_tstamp &&
          tcp_paws_reject(&tp->rx_opt, 0))
            goto discard_and_undo;

      if (th->syn) {
            /* We see SYN without ACK. It is attempt of
             * simultaneous connect with crossed SYNs.
             * Particularly, it can be connect to self.
             */
            tcp_set_state(sk, TCP_SYN_RECV);

            if (tp->rx_opt.saw_tstamp) {
                  tp->rx_opt.tstamp_ok = 1;
                  tcp_store_ts_recent(tp);
                  tp->tcp_header_len =
                        sizeof(struct tcphdr) + TCPOLEN_TSTAMP_ALIGNED;
            } else {
                  tp->tcp_header_len = sizeof(struct tcphdr);
            }

            tp->rcv_nxt = TCP_SKB_CB(skb)->seq + 1;
            tp->rcv_wup = TCP_SKB_CB(skb)->seq + 1;

            /* RFC1323: The window in SYN & SYN/ACK segments is
             * never scaled.
             */
            tp->snd_wnd    = ntohs(th->window);
            tp->snd_wl1    = TCP_SKB_CB(skb)->seq;
            tp->max_window = tp->snd_wnd;

            TCP_ECN_rcv_syn(tp, th);

            tcp_mtup_init(sk);
            tcp_sync_mss(sk, icsk->icsk_pmtu_cookie);
            tcp_initialize_rcv_mss(sk);

            tcp_send_synack(sk);
#if 0
            /* Note, we could accept data and URG from this segment.
             * There are no obstacles to make this.
             *
             * However, if we ignore data in ACKless segments sometimes,
             * we have no reasons to accept it sometimes.
             * Also, seems the code doing it in step6 of tcp_rcv_state_process
             * is not flawless. So, discard packet for sanity.
             * Uncomment this return to process the data.
             */
            return -1;
#else
            goto discard;
#endif
      }
      /* "fifth, if neither of the SYN or RST bits is set then
       * drop the segment and return."
       */

discard_and_undo:
      tcp_clear_options(&tp->rx_opt);
      tp->rx_opt.mss_clamp = saved_clamp;
      goto discard;

reset_and_undo:
      tcp_clear_options(&tp->rx_opt);
      tp->rx_opt.mss_clamp = saved_clamp;
      return 1;
}

/*
 *    This function implements the receiving procedure of RFC 793 for
 *    all states except ESTABLISHED and TIME_WAIT.
 *    It's called from both tcp_v4_rcv and tcp_v6_rcv and should be
 *    address independent.
 */

int tcp_rcv_state_process(struct sock *sk, struct sk_buff *skb,
                    struct tcphdr *th, unsigned len)
{
      struct tcp_sock *tp = tcp_sk(sk);
      struct inet_connection_sock *icsk = inet_csk(sk);
      int queued = 0;
      int res;

      tp->rx_opt.saw_tstamp = 0;

      switch (sk->sk_state) {
      case TCP_CLOSE:
            goto discard;

      case TCP_LISTEN:
            if (th->ack)
                  return 1;

            if (th->rst)
                  goto discard;

            if (th->syn) {
                  if (icsk->icsk_af_ops->conn_request(sk, skb) < 0)
                        return 1;

                  /* Now we have several options: In theory there is
                   * nothing else in the frame. KA9Q has an option to
                   * send data with the syn, BSD accepts data with the
                   * syn up to the [to be] advertised window and
                   * Solaris 2.1 gives you a protocol error. For now
                   * we just ignore it, that fits the spec precisely
                   * and avoids incompatibilities. It would be nice in
                   * future to drop through and process the data.
                   *
                   * Now that TTCP is starting to be used we ought to
                   * queue this data.
                   * But, this leaves one open to an easy denial of
                   * service attack, and SYN cookies can't defend
                   * against this problem. So, we drop the data
                   * in the interest of security over speed unless
                   * it's still in use.
                   */
                  kfree_skb(skb);
                  return 0;
            }
            goto discard;

      case TCP_SYN_SENT:
            queued = tcp_rcv_synsent_state_process(sk, skb, th, len);
            if (queued >= 0)
                  return queued;

            /* Do step6 onward by hand. */
            tcp_urg(sk, skb, th);
            __kfree_skb(skb);
            tcp_data_snd_check(sk);
            return 0;
      }

      res = tcp_validate_incoming(sk, skb, th, 0);
      if (res <= 0)
            return -res;

      /* step 5: check the ACK field */
      if (th->ack) {
            int acceptable = tcp_ack(sk, skb, FLAG_SLOWPATH) > 0;

            switch (sk->sk_state) {
            case TCP_SYN_RECV:
                  if (acceptable) {
                        tp->copied_seq = tp->rcv_nxt;
                        smp_mb();
                        tcp_set_state(sk, TCP_ESTABLISHED);
                        sk->sk_state_change(sk);

                        /* Note, that this wakeup is only for marginal
                         * crossed SYN case. Passively open sockets
                         * are not waked up, because sk->sk_sleep ==
                         * NULL and sk->sk_socket == NULL.
                         */
                        if (sk->sk_socket)
                              sk_wake_async(sk,
                                          SOCK_WAKE_IO, POLL_OUT);

                        tp->snd_una = TCP_SKB_CB(skb)->ack_seq;
                        tp->snd_wnd = ntohs(th->window) <<
                                    tp->rx_opt.snd_wscale;
                        tcp_init_wl(tp, TCP_SKB_CB(skb)->seq);

                        /* tcp_ack considers this ACK as duplicate
                         * and does not calculate rtt.
                         * Fix it at least with timestamps.
                         */
                        if (tp->rx_opt.saw_tstamp &&
                            tp->rx_opt.rcv_tsecr && !tp->srtt)
                              tcp_ack_saw_tstamp(sk, 0);

                        if (tp->rx_opt.tstamp_ok)
                              tp->advmss -= TCPOLEN_TSTAMP_ALIGNED;

                        /* Make sure socket is routed, for
                         * correct metrics.
                         */
                        icsk->icsk_af_ops->rebuild_header(sk);

                        tcp_init_metrics(sk);

                        tcp_init_congestion_control(sk);

                        /* Prevent spurious tcp_cwnd_restart() on
                         * first data packet.
                         */
                        tp->lsndtime = tcp_time_stamp;

                        tcp_mtup_init(sk);
                        tcp_initialize_rcv_mss(sk);
                        tcp_init_buffer_space(sk);
                        tcp_fast_path_on(tp);
                  } else {
                        return 1;
                  }
                  break;

            case TCP_FIN_WAIT1:
                  if (tp->snd_una == tp->write_seq) {
                        tcp_set_state(sk, TCP_FIN_WAIT2);
                        sk->sk_shutdown |= SEND_SHUTDOWN;
                        dst_confirm(sk->sk_dst_cache);

                        if (!sock_flag(sk, SOCK_DEAD))
                              /* Wake up lingering close() */
                              sk->sk_state_change(sk);
                        else {
                              int tmo;

                              if (tp->linger2 < 0 ||
                                  (TCP_SKB_CB(skb)->end_seq != TCP_SKB_CB(skb)->seq &&
                                   after(TCP_SKB_CB(skb)->end_seq - th->fin, tp->rcv_nxt))) {
                                    tcp_done(sk);
                                    NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPABORTONDATA);
                                    return 1;
                              }

                              tmo = tcp_fin_time(sk);
                              if (tmo > TCP_TIMEWAIT_LEN) {
                                    inet_csk_reset_keepalive_timer(sk, tmo - TCP_TIMEWAIT_LEN);
                              } else if (th->fin || sock_owned_by_user(sk)) {
                                    /* Bad case. We could lose such FIN otherwise.
                                     * It is not a big problem, but it looks confusing
                                     * and not so rare event. We still can lose it now,
                                     * if it spins in bh_lock_sock(), but it is really
                                     * marginal case.
                                     */
                                    inet_csk_reset_keepalive_timer(sk, tmo);
                              } else {
                                    tcp_time_wait(sk, TCP_FIN_WAIT2, tmo);
                                    goto discard;
                              }
                        }
                  }
                  break;

            case TCP_CLOSING:
                  if (tp->snd_una == tp->write_seq) {
                        tcp_time_wait(sk, TCP_TIME_WAIT, 0);
                        goto discard;
                  }
                  break;

            case TCP_LAST_ACK:
                  if (tp->snd_una == tp->write_seq) {
                        tcp_update_metrics(sk);
                        tcp_done(sk);
                        goto discard;
                  }
                  break;
            }
      } else
            goto discard;

      /* step 6: check the URG bit */
      tcp_urg(sk, skb, th);

      /* step 7: process the segment text */
      switch (sk->sk_state) {
      case TCP_CLOSE_WAIT:
      case TCP_CLOSING:
      case TCP_LAST_ACK:
            if (!before(TCP_SKB_CB(skb)->seq, tp->rcv_nxt))
                  break;
      case TCP_FIN_WAIT1:
      case TCP_FIN_WAIT2:
            /* RFC 793 says to queue data in these states,
             * RFC 1122 says we MUST send a reset.
             * BSD 4.4 also does reset.
             */
            if (sk->sk_shutdown & RCV_SHUTDOWN) {
                  if (TCP_SKB_CB(skb)->end_seq != TCP_SKB_CB(skb)->seq &&
                      after(TCP_SKB_CB(skb)->end_seq - th->fin, tp->rcv_nxt)) {
                        NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPABORTONDATA);
                        tcp_reset(sk);
                        return 1;
                  }
            }
            /* Fall through */
      case TCP_ESTABLISHED:
            tcp_data_queue(sk, skb);
            queued = 1;
            break;
      }

      /* tcp_data could move socket to TIME-WAIT */
      if (sk->sk_state != TCP_CLOSE) {
            tcp_data_snd_check(sk);
            tcp_ack_snd_check(sk);
      }

      if (!queued) {
discard:
            __kfree_skb(skb);
      }
      return 0;
}

EXPORT_SYMBOL(sysctl_tcp_ecn);
EXPORT_SYMBOL(sysctl_tcp_reordering);
EXPORT_SYMBOL(sysctl_tcp_adv_win_scale);
EXPORT_SYMBOL(tcp_parse_options);
#ifdef CONFIG_TCP_MD5SIG
EXPORT_SYMBOL(tcp_parse_md5sig_option);
#endif
EXPORT_SYMBOL(tcp_rcv_established);
EXPORT_SYMBOL(tcp_rcv_state_process);
EXPORT_SYMBOL(tcp_initialize_rcv_mss);

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