[TCP]: Kill bogus comment above tcp_tso_acked().

[pandora-kernel.git] / net / ipv4 / 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).
 *
 * Version:     $Id: tcp_input.c,v 1.243 2002/02/01 22:01:04 davem Exp $
 *
 * 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 presnce 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/config.h>
#include <linux/mm.h>
#include <linux/module.h>
#include <linux/sysctl.h>
#include <net/tcp.h>
#include <net/inet_common.h>
#include <linux/ipsec.h>
#include <asm/unaligned.h>

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

int sysctl_tcp_stdurg;
int sysctl_tcp_rfc1337;
int sysctl_tcp_max_orphans = NR_FILE;
int sysctl_tcp_frto;
int sysctl_tcp_nometrics_save;

int sysctl_tcp_moderate_rcvbuf = 1;

#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_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 IsReno(tp) ((tp)->rx_opt.sack_ok == 0)
#define IsFack(tp) ((tp)->rx_opt.sack_ok & 2)
#define IsDSack(tp) ((tp)->rx_opt.sack_ok & 4)

#define TCP_REMNANT (TCP_FLAG_FIN|TCP_FLAG_URG|TCP_FLAG_SYN|TCP_FLAG_PSH)

/* Adapt the MSS value used to make delayed ack decision to the 
 * real world.
 */ 
static inline void tcp_measure_rcv_mss(struct tcp_sock *tp,
                                       struct sk_buff *skb)
{
        unsigned int len, lss;

        lss = tp->ack.last_seg_size; 
        tp->ack.last_seg_size = 0; 

        /* skb->len may jitter because of SACKs, even if peer
         * sends good full-sized frames.
         */
        len = skb->len;
        if (len >= tp->ack.rcv_mss) {
                tp->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->h.raw;
                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(skb->h.th)&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 -= tp->tcp_header_len;
                        tp->ack.last_seg_size = len;
                        if (len == lss) {
                                tp->ack.rcv_mss = len;
                                return;
                        }
                }
                tp->ack.pending |= TCP_ACK_PUSHED;
        }
}

static void tcp_incr_quickack(struct tcp_sock *tp)
{
        unsigned quickacks = tp->rcv_wnd/(2*tp->ack.rcv_mss);

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

void tcp_enter_quickack_mode(struct tcp_sock *tp)
{
        tcp_incr_quickack(tp);
        tp->ack.pingpong = 0;
        tp->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(struct tcp_sock *tp)
{
        return (tp->ack.quick && !tp->ack.pingpong);
}

/* 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 spagetti. 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(struct sock *sk, struct tcp_sock *tp,
                             struct sk_buff *skb)
{
        /* Optimize this! */
        int truesize = tcp_win_from_space(skb->truesize)/2;
        int window = tcp_full_space(sk)/2;

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

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

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

                if (incr) {
                        tp->rcv_ssthresh = min(tp->rcv_ssthresh + incr, tp->window_clamp);
                        tp->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 correspoding 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 iimediately 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)
{
        struct sk_buff *skb;
        unsigned int app_win = tp->rcv_nxt - tp->copied_seq;
        int ofo_win = 0;

        tp->ack.quick = 0;

        skb_queue_walk(&tp->out_of_order_queue, skb) {
                ofo_win += skb->len;
        }

        /* If overcommit is due to out of order segments,
         * do not clamp window. Try to expand rcvbuf instead.
         */
        if (ofo_win) {
                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) {
                app_win += ofo_win;
                if (atomic_read(&sk->sk_rmem_alloc) >= 2 * sk->sk_rcvbuf)
                        app_win >>= 1;
                if (app_win > tp->ack.rcv_mss)
                        app_win -= tp->ack.rcv_mss;
                app_win = max(app_win, 2U*tp->advmss);

                if (!ofo_win)
                        tp->window_clamp = min(tp->window_clamp, app_win);
                tp->rcv_ssthresh = min(tp->window_clamp, 2U*tp->advmss);
        }
}

/* 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 smoothe things out
                 * else with timestamps disabled convergance 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 mesaure. */
                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 tcp_sock *tp, struct sk_buff *skb)
{
        if (tp->rx_opt.rcv_tsecr &&
            (TCP_SKB_CB(skb)->end_seq -
             TCP_SKB_CB(skb)->seq >= tp->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) {
                        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 tcp_sock *tp, struct sk_buff *skb)
{
        u32 now;

        tcp_schedule_ack(tp);

        tcp_measure_rcv_mss(tp, skb);

        tcp_rcv_rtt_measure(tp);
        
        now = tcp_time_stamp;

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

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

        TCP_ECN_check_ce(tp, skb);

        if (skb->len >= 128)
                tcp_grow_window(sk, tp, 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 tcp_sock *tp, __u32 mrtt, u32 *usrtt)
{
        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 incresed fastly, decrease too fastly
         * 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;
                }
        } 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);
                tp->rtt_seq = tp->snd_nxt;
        }

        if (tp->ca_ops->rtt_sample)
                tp->ca_ops->rtt_sample(tp, *usrtt);
}

/* Calculate rto without backoff.  This is the second half of Van Jacobson's
 * routine referred to above.
 */
static inline void tcp_set_rto(struct tcp_sock *tp)
{
        /* 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 curcumstances.
         */
        tp->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 exaclty, which we pretend to do.
         */
}

/* NOTE: clamping at TCP_RTO_MIN is not required, current algo
 * guarantees that rto is higher.
 */
static inline void tcp_bound_rto(struct tcp_sock *tp)
{
        if (tp->rto > TCP_RTO_MAX)
                tp->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)) {
                int m;

                if (tp->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;
                }

                m = dst_metric(dst, RTAX_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)
                                dst->metrics[RTAX_RTT-1] = tp->srtt;
                        else
                                dst->metrics[RTAX_RTT-1] -= (m>>3);
                }

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

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

                        if (m >= dst_metric(dst, RTAX_RTTVAR))
                                dst->metrics[RTAX_RTTVAR-1] = m;
                        else
                                dst->metrics[RTAX_RTTVAR-1] -=
                                        (dst->metrics[RTAX_RTTVAR-1] - m)>>2;
                }

                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 &&
                           tp->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->metrics[RTAX_CWND-1] + 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->metrics[RTAX_CWND-1] + tp->snd_ssthresh) >> 1;
                        if (dst->metrics[RTAX_SSTHRESH-1] &&
                            !dst_metric_locked(dst, RTAX_SSTHRESH) &&
                            tp->snd_ssthresh > dst->metrics[RTAX_SSTHRESH-1])
                                dst->metrics[RTAX_SSTHRESH-1] = tp->snd_ssthresh;
                }

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

/* Numbers are taken from RFC2414.  */
__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_std > 1460)
                        cwnd = 2;
                else
                        cwnd = (tp->mss_cache_std > 1095) ? 3 : 4;
        }
        return min_t(__u32, cwnd, tp->snd_cwnd_clamp);
}

/* 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)) {
                tp->rx_opt.sack_ok &= ~2;
                tp->reordering = dst_metric(dst, RTAX_REORDERING);
        }

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

        if (!tp->srtt && dst_metric(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 curcumstances 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(dst, RTAX_RTT) > tp->srtt) {
                tp->srtt = dst_metric(dst, RTAX_RTT);
                tp->rtt_seq = tp->snd_nxt;
        }
        if (dst_metric(dst, RTAX_RTTVAR) > tp->mdev) {
                tp->mdev = dst_metric(dst, RTAX_RTTVAR);
                tp->mdev_max = tp->rttvar = max(tp->mdev, TCP_RTO_MIN);
        }
        tcp_set_rto(tp);
        tcp_bound_rto(tp);
        if (tp->rto < TCP_TIMEOUT_INIT && !tp->rx_opt.saw_tstamp)
                goto reset;
        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;
                tp->rto = TCP_TIMEOUT_INIT;
        }
}

static void tcp_update_reordering(struct tcp_sock *tp, int metric, int ts)
{
        if (metric > tp->reordering) {
                tp->reordering = min(TCP_MAX_REORDERING, metric);

                /* This exciting event is worth to be remembered. 8) */
                if (ts)
                        NET_INC_STATS_BH(LINUX_MIB_TCPTSREORDER);
                else if (IsReno(tp))
                        NET_INC_STATS_BH(LINUX_MIB_TCPRENOREORDER);
                else if (IsFack(tp))
                        NET_INC_STATS_BH(LINUX_MIB_TCPFACKREORDER);
                else
                        NET_INC_STATS_BH(LINUX_MIB_TCPSACKREORDER);
#if FASTRETRANS_DEBUG > 1
                printk(KERN_DEBUG "Disorder%d %d %u f%u s%u rr%d\n",
                       tp->rx_opt.sack_ok, tp->ca_state,
                       tp->reordering,
                       tp->fackets_out,
                       tp->sacked_out,
                       tp->undo_marker ? tp->undo_retrans : 0);
#endif
                /* Disable FACK yet. */
                tp->rx_opt.sack_ok &= ~2;
        }
}

/* 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.
 */
static int
tcp_sacktag_write_queue(struct sock *sk, struct sk_buff *ack_skb, u32 prior_snd_una)
{
        struct tcp_sock *tp = tcp_sk(sk);
        unsigned char *ptr = ack_skb->h.raw + TCP_SKB_CB(ack_skb)->sacked;
        struct tcp_sack_block *sp = (struct tcp_sack_block *)(ptr+2);
        int num_sacks = (ptr[1] - TCPOLEN_SACK_BASE)>>3;
        int reord = tp->packets_out;
        int prior_fackets;
        u32 lost_retrans = 0;
        int flag = 0;
        int i;

        /* So, SACKs for already sent large segments will be lost.
         * Not good, but alternative is to resegment the queue. */
        if (sk->sk_route_caps & NETIF_F_TSO) {
                sk->sk_route_caps &= ~NETIF_F_TSO;
                sock_set_flag(sk, SOCK_NO_LARGESEND);
                tp->mss_cache = tp->mss_cache_std;
        }

        if (!tp->sacked_out)
                tp->fackets_out = 0;
        prior_fackets = tp->fackets_out;

        for (i=0; i<num_sacks; i++, sp++) {
                struct sk_buff *skb;
                __u32 start_seq = ntohl(sp->start_seq);
                __u32 end_seq = ntohl(sp->end_seq);
                int fack_count = 0;
                int dup_sack = 0;

                /* Check for D-SACK. */
                if (i == 0) {
                        u32 ack = TCP_SKB_CB(ack_skb)->ack_seq;

                        if (before(start_seq, ack)) {
                                dup_sack = 1;
                                tp->rx_opt.sack_ok |= 4;
                                NET_INC_STATS_BH(LINUX_MIB_TCPDSACKRECV);
                        } else if (num_sacks > 1 &&
                                   !after(end_seq, ntohl(sp[1].end_seq)) &&
                                   !before(start_seq, ntohl(sp[1].start_seq))) {
                                dup_sack = 1;
                                tp->rx_opt.sack_ok |= 4;
                                NET_INC_STATS_BH(LINUX_MIB_TCPDSACKOFORECV);
                        }

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

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

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

                sk_stream_for_retrans_queue(skb, sk) {
                        u8 sacked = TCP_SKB_CB(skb)->sacked;
                        int in_sack;

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

                        fack_count += tcp_skb_pcount(skb);

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

                        /* Account D-SACK for retransmitted packet. */
                        if ((dup_sack && in_sack) &&
                            (sacked & TCPCB_RETRANS) &&
                            after(TCP_SKB_CB(skb)->end_seq, tp->undo_marker))
                                tp->undo_retrans--;

                        /* The frame is ACKed. */
                        if (!after(TCP_SKB_CB(skb)->end_seq, tp->snd_una)) {
                                if (sacked&TCPCB_RETRANS) {
                                        if ((dup_sack && in_sack) &&
                                            (sacked&TCPCB_SACKED_ACKED))
                                                reord = min(fack_count, reord);
                                } else {
                                        /* If it was in a hole, we detected reordering. */
                                        if (fack_count < prior_fackets &&
                                            !(sacked&TCPCB_SACKED_ACKED))
                                                reord = min(fack_count, reord);
                                }

                                /* Nothing to do; acked frame is about to be dropped. */
                                continue;
                        }

                        if ((sacked&TCPCB_SACKED_RETRANS) &&
                            after(end_seq, TCP_SKB_CB(skb)->ack_seq) &&
                            (!lost_retrans || after(end_seq, lost_retrans)))
                                lost_retrans = end_seq;

                        if (!in_sack)
                                continue;

                        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) {
                                                TCP_SKB_CB(skb)->sacked &= ~(TCPCB_LOST|TCPCB_SACKED_RETRANS);
                                                tp->lost_out -= tcp_skb_pcount(skb);
                                                tp->retrans_out -= tcp_skb_pcount(skb);
                                        }
                                } else {
                                        /* New sack for not retransmitted frame,
                                         * which was in hole. It is reordering.
                                         */
                                        if (!(sacked & TCPCB_RETRANS) &&
                                            fack_count < prior_fackets)
                                                reord = min(fack_count, reord);

                                        if (sacked & TCPCB_LOST) {
                                                TCP_SKB_CB(skb)->sacked &= ~TCPCB_LOST;
                                                tp->lost_out -= tcp_skb_pcount(skb);
                                        }
                                }

                                TCP_SKB_CB(skb)->sacked |= TCPCB_SACKED_ACKED;
                                flag |= FLAG_DATA_SACKED;
                                tp->sacked_out += tcp_skb_pcount(skb);

                                if (fack_count > tp->fackets_out)
                                        tp->fackets_out = fack_count;
                        } else {
                                if (dup_sack && (sacked&TCPCB_RETRANS))
                                        reord = min(fack_count, reord);
                        }

                        /* 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 &&
                            (TCP_SKB_CB(skb)->sacked&TCPCB_SACKED_RETRANS)) {
                                TCP_SKB_CB(skb)->sacked &= ~TCPCB_SACKED_RETRANS;
                                tp->retrans_out -= tcp_skb_pcount(skb);
                        }
                }
        }

        /* 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.
         */
        if (lost_retrans && tp->ca_state == TCP_CA_Recovery) {
                struct sk_buff *skb;

                sk_stream_for_retrans_queue(skb, sk) {
                        if (after(TCP_SKB_CB(skb)->seq, lost_retrans))
                                break;
                        if (!after(TCP_SKB_CB(skb)->end_seq, tp->snd_una))
                                continue;
                        if ((TCP_SKB_CB(skb)->sacked&TCPCB_SACKED_RETRANS) &&
                            after(lost_retrans, TCP_SKB_CB(skb)->ack_seq) &&
                            (IsFack(tp) ||
                             !before(lost_retrans,
                                     TCP_SKB_CB(skb)->ack_seq + tp->reordering *
                                     tp->mss_cache_std))) {
                                TCP_SKB_CB(skb)->sacked &= ~TCPCB_SACKED_RETRANS;
                                tp->retrans_out -= tcp_skb_pcount(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;
                                        flag |= FLAG_DATA_SACKED;
                                        NET_INC_STATS_BH(LINUX_MIB_TCPLOSTRETRANSMIT);
                                }
                        }
                }
        }

        tp->left_out = tp->sacked_out + tp->lost_out;

        if ((reord < tp->fackets_out) && tp->ca_state != TCP_CA_Loss)
                tcp_update_reordering(tp, ((tp->fackets_out + 1) - reord), 0);

#if FASTRETRANS_DEBUG > 0
        BUG_TRAP((int)tp->sacked_out >= 0);
        BUG_TRAP((int)tp->lost_out >= 0);
        BUG_TRAP((int)tp->retrans_out >= 0);
        BUG_TRAP((int)tcp_packets_in_flight(tp) >= 0);
#endif
        return flag;
}

/* RTO occurred, but do not yet enter loss state. Instead, transmit two new
 * segments to see from the next ACKs whether any data was really missing.
 * If the RTO was spurious, new ACKs should arrive.
 */
void tcp_enter_frto(struct sock *sk)
{
        struct tcp_sock *tp = tcp_sk(sk);
        struct sk_buff *skb;

        tp->frto_counter = 1;

        if (tp->ca_state <= TCP_CA_Disorder ||
            tp->snd_una == tp->high_seq ||
            (tp->ca_state == TCP_CA_Loss && !tp->retransmits)) {
                tp->prior_ssthresh = tcp_current_ssthresh(tp);
                tp->snd_ssthresh = tp->ca_ops->ssthresh(tp);
                tcp_ca_event(tp, CA_EVENT_FRTO);
        }

        /* Have to clear retransmission markers here to keep the bookkeeping
         * in shape, even though we are not yet in Loss state.
         * If something was really lost, it is eventually caught up
         * in tcp_enter_frto_loss.
         */
        tp->retrans_out = 0;
        tp->undo_marker = tp->snd_una;
        tp->undo_retrans = 0;

        sk_stream_for_retrans_queue(skb, sk) {
                TCP_SKB_CB(skb)->sacked &= ~TCPCB_RETRANS;
        }
        tcp_sync_left_out(tp);

        tcp_set_ca_state(tp, TCP_CA_Open);
        tp->frto_highmark = tp->snd_nxt;
}

/* 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)
{
        struct tcp_sock *tp = tcp_sk(sk);
        struct sk_buff *skb;
        int cnt = 0;

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

        sk_stream_for_retrans_queue(skb, sk) {
                cnt += tcp_skb_pcount(skb);
                TCP_SKB_CB(skb)->sacked &= ~TCPCB_LOST;
                if (!(TCP_SKB_CB(skb)->sacked&TCPCB_SACKED_ACKED)) {

                        /* Do not mark those segments lost that were
                         * forward transmitted after RTO
                         */
                        if (!after(TCP_SKB_CB(skb)->end_seq,
                                   tp->frto_highmark)) {
                                TCP_SKB_CB(skb)->sacked |= TCPCB_LOST;
                                tp->lost_out += tcp_skb_pcount(skb);
                        }
                } else {
                        tp->sacked_out += tcp_skb_pcount(skb);
                        tp->fackets_out = cnt;
                }
        }
        tcp_sync_left_out(tp);

        tp->snd_cwnd = tp->frto_counter + tcp_packets_in_flight(tp)+1;
        tp->snd_cwnd_cnt = 0;
        tp->snd_cwnd_stamp = tcp_time_stamp;
        tp->undo_marker = 0;
        tp->frto_counter = 0;

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

void tcp_clear_retrans(struct tcp_sock *tp)
{
        tp->left_out = 0;
        tp->retrans_out = 0;

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

        tp->undo_marker = 0;
        tp->undo_retrans = 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)
{
        struct tcp_sock *tp = tcp_sk(sk);
        struct sk_buff *skb;
        int cnt = 0;

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

        tcp_clear_retrans(tp);

        /* Push undo marker, if it was plain RTO and nothing
         * was retransmitted. */
        if (!how)
                tp->undo_marker = tp->snd_una;

        sk_stream_for_retrans_queue(skb, sk) {
                cnt += tcp_skb_pcount(skb);
                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);
                } else {
                        tp->sacked_out += tcp_skb_pcount(skb);
                        tp->fackets_out = cnt;
                }
        }
        tcp_sync_left_out(tp);

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

static int tcp_check_sack_reneging(struct sock *sk, struct tcp_sock *tp)
{
        struct sk_buff *skb;

        /* 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.
         */
        if ((skb = skb_peek(&sk->sk_write_queue)) != NULL &&
            (TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_ACKED)) {
                NET_INC_STATS_BH(LINUX_MIB_TCPSACKRENEGING);

                tcp_enter_loss(sk, 1);
                tp->retransmits++;
                tcp_retransmit_skb(sk, skb_peek(&sk->sk_write_queue));
                tcp_reset_xmit_timer(sk, TCP_TIME_RETRANS, tp->rto);
                return 1;
        }
        return 0;
}

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

static inline int tcp_skb_timedout(struct tcp_sock *tp, struct sk_buff *skb)
{
        return (tcp_time_stamp - TCP_SKB_CB(skb)->when > tp->rto);
}

static inline int tcp_head_timedout(struct sock *sk, struct tcp_sock *tp)
{
        return tp->packets_out &&
               tcp_skb_timedout(tp, skb_peek(&sk->sk_write_queue));
}

/* 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)
{
        __u32 packets_out;

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

        /* Not-A-Trick#2 : Classic rule... */
        if (tcp_fackets_out(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_head_timedout(sk, tp))
                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, tp)) {
                /* We have nothing to send. This connection is limited
                 * either by receiver window or by application.
                 */
                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 tcp_sock *tp, int addend)
{
        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;
                tcp_update_reordering(tp, tp->packets_out+addend, 0);
        }
}

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

static void tcp_add_reno_sack(struct tcp_sock *tp)
{
        tp->sacked_out++;
        tcp_check_reno_reordering(tp, 0);
        tcp_sync_left_out(tp);
}

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

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

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

/* Mark head of queue up as lost. */
static void tcp_mark_head_lost(struct sock *sk, struct tcp_sock *tp,
                               int packets, u32 high_seq)
{
        struct sk_buff *skb;
        int cnt = packets;

        BUG_TRAP(cnt <= tp->packets_out);

        sk_stream_for_retrans_queue(skb, sk) {
                cnt -= tcp_skb_pcount(skb);
                if (cnt < 0 || after(TCP_SKB_CB(skb)->end_seq, high_seq))
                        break;
                if (!(TCP_SKB_CB(skb)->sacked&TCPCB_TAGBITS)) {
                        TCP_SKB_CB(skb)->sacked |= TCPCB_LOST;
                        tp->lost_out += tcp_skb_pcount(skb);
                }
        }
        tcp_sync_left_out(tp);
}

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

static void tcp_update_scoreboard(struct sock *sk, struct tcp_sock *tp)
{
        if (IsFack(tp)) {
                int lost = tp->fackets_out - tp->reordering;
                if (lost <= 0)
                        lost = 1;
                tcp_mark_head_lost(sk, tp, lost, tp->high_seq);
        } else {
                tcp_mark_head_lost(sk, tp, 1, tp->high_seq);
        }

        /* 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.
         */
        if (tcp_head_timedout(sk, tp)) {
                struct sk_buff *skb;

                sk_stream_for_retrans_queue(skb, sk) {
                        if (tcp_skb_timedout(tp, skb) &&
                            !(TCP_SKB_CB(skb)->sacked&TCPCB_TAGBITS)) {
                                TCP_SKB_CB(skb)->sacked |= TCPCB_LOST;
                                tp->lost_out += tcp_skb_pcount(skb);
                        }
                }
                tcp_sync_left_out(tp);
        }
}

/* 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;
}

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

        tp->snd_cwnd_cnt = decr&1;
        decr >>= 1;

        if (decr && tp->snd_cwnd > tp->ca_ops->min_cwnd(tp))
                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 &&
                 (__s32)(tp->rx_opt.rcv_tsecr - tp->retrans_stamp) < 0);
}

/* Undo procedures. */

#if FASTRETRANS_DEBUG > 1
static void DBGUNDO(struct sock *sk, struct tcp_sock *tp, const char *msg)
{
        struct inet_sock *inet = inet_sk(sk);
        printk(KERN_DEBUG "Undo %s %u.%u.%u.%u/%u c%u l%u ss%u/%u p%u\n",
               msg,
               NIPQUAD(inet->daddr), ntohs(inet->dport),
               tp->snd_cwnd, tp->left_out,
               tp->snd_ssthresh, tp->prior_ssthresh,
               tp->packets_out);
}
#else
#define DBGUNDO(x...) do { } while (0)
#endif

static void tcp_undo_cwr(struct tcp_sock *tp, int undo)
{
        if (tp->prior_ssthresh) {
                if (tp->ca_ops->undo_cwnd)
                        tp->snd_cwnd = tp->ca_ops->undo_cwnd(tp);
                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)
{
        if (tcp_may_undo(tp)) {
                /* Happy end! We did not retransmit anything
                 * or our original transmission succeeded.
                 */
                DBGUNDO(sk, tp, tp->ca_state == TCP_CA_Loss ? "loss" : "retrans");
                tcp_undo_cwr(tp, 1);
                if (tp->ca_state == TCP_CA_Loss)
                        NET_INC_STATS_BH(LINUX_MIB_TCPLOSSUNDO);
                else
                        NET_INC_STATS_BH(LINUX_MIB_TCPFULLUNDO);
                tp->undo_marker = 0;
        }
        if (tp->snd_una == tp->high_seq && IsReno(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(tp, 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)
{
        if (tp->undo_marker && !tp->undo_retrans) {
                DBGUNDO(sk, tp, "D-SACK");
                tcp_undo_cwr(tp, 1);
                tp->undo_marker = 0;
                NET_INC_STATS_BH(LINUX_MIB_TCPDSACKUNDO);
        }
}

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

static int tcp_try_undo_partial(struct sock *sk, struct tcp_sock *tp,
                                int acked)
{
        /* Partial ACK arrived. Force Hoe's retransmit. */
        int failed = IsReno(tp) || tp->fackets_out>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(tp, tcp_fackets_out(tp)+acked, 1);

                DBGUNDO(sk, tp, "Hoe");
                tcp_undo_cwr(tp, 0);
                NET_INC_STATS_BH(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)
{
        if (tcp_may_undo(tp)) {
                struct sk_buff *skb;
                sk_stream_for_retrans_queue(skb, sk) {
                        TCP_SKB_CB(skb)->sacked &= ~TCPCB_LOST;
                }
                DBGUNDO(sk, tp, "partial loss");
                tp->lost_out = 0;
                tp->left_out = tp->sacked_out;
                tcp_undo_cwr(tp, 1);
                NET_INC_STATS_BH(LINUX_MIB_TCPLOSSUNDO);
                tp->retransmits = 0;
                tp->undo_marker = 0;
                if (!IsReno(tp))
                        tcp_set_ca_state(tp, TCP_CA_Open);
                return 1;
        }
        return 0;
}

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

static void tcp_try_to_open(struct sock *sk, struct tcp_sock *tp, int flag)
{
        tp->left_out = tp->sacked_out;

        if (tp->retrans_out == 0)
                tp->retrans_stamp = 0;

        if (flag&FLAG_ECE)
                tcp_enter_cwr(tp);

        if (tp->ca_state != TCP_CA_CWR) {
                int state = TCP_CA_Open;

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

                if (tp->ca_state != state) {
                        tcp_set_ca_state(tp, state);
                        tp->high_seq = tp->snd_nxt;
                }
                tcp_moderate_cwnd(tp);
        } else {
                tcp_cwnd_down(tp);
        }
}

/* 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, u32 prior_snd_una,
                      int prior_packets, int flag)
{
        struct tcp_sock *tp = tcp_sk(sk);
        int is_dupack = (tp->snd_una == prior_snd_una && !(flag&FLAG_NOT_DUP));

        /* Some technical things:
         * 1. Reno does not count dupacks (sacked_out) automatically. */
        if (!tp->packets_out)
                tp->sacked_out = 0;
        /* 2. SACK counts snd_fack in packets inaccurately. */
        if (tp->sacked_out == 0)
                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 (tp->sacked_out && tcp_check_sack_reneging(sk, tp))
                return;

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

        /* D. Synchronize left_out to current state. */
        tcp_sync_left_out(tp);

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

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

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

        /* F. Process state. */
        switch (tp->ca_state) {
        case TCP_CA_Recovery:
                if (prior_snd_una == tp->snd_una) {
                        if (IsReno(tp) && is_dupack)
                                tcp_add_reno_sack(tp);
                } else {
                        int acked = prior_packets - tp->packets_out;
                        if (IsReno(tp))
                                tcp_remove_reno_sacks(sk, tp, acked);
                        is_dupack = tcp_try_undo_partial(sk, tp, acked);
                }
                break;
        case TCP_CA_Loss:
                if (flag&FLAG_DATA_ACKED)
                        tp->retransmits = 0;
                if (!tcp_try_undo_loss(sk, tp)) {
                        tcp_moderate_cwnd(tp);
                        tcp_xmit_retransmit_queue(sk);
                        return;
                }
                if (tp->ca_state != TCP_CA_Open)
                        return;
                /* Loss is undone; fall through to processing in Open state. */
        default:
                if (IsReno(tp)) {
                        if (tp->snd_una != prior_snd_una)
                                tcp_reset_reno_sack(tp);
                        if (is_dupack)
                                tcp_add_reno_sack(tp);
                }

                if (tp->ca_state == TCP_CA_Disorder)
                        tcp_try_undo_dsack(sk, tp);

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

                /* Otherwise enter Recovery state */

                if (IsReno(tp))
                        NET_INC_STATS_BH(LINUX_MIB_TCPRENORECOVERY);
                else
                        NET_INC_STATS_BH(LINUX_MIB_TCPSACKRECOVERY);

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

                if (tp->ca_state < TCP_CA_CWR) {
                        if (!(flag&FLAG_ECE))
                                tp->prior_ssthresh = tcp_current_ssthresh(tp);
                        tp->snd_ssthresh = tp->ca_ops->ssthresh(tp);
                        TCP_ECN_queue_cwr(tp);
                }

                tp->snd_cwnd_cnt = 0;
                tcp_set_ca_state(tp, TCP_CA_Recovery);
        }

        if (is_dupack || tcp_head_timedout(sk, tp))
                tcp_update_scoreboard(sk, tp);
        tcp_cwnd_down(tp);
        tcp_xmit_retransmit_queue(sk);
}

/* Read draft-ietf-tcplw-high-performance before mucking
 * with this code. (Superceeds RFC1323)
 */
static void tcp_ack_saw_tstamp(struct tcp_sock *tp, u32 *usrtt, int flag)
{
        __u32 seq_rtt;

        /* 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 overstimated 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)
         */
        seq_rtt = tcp_time_stamp - tp->rx_opt.rcv_tsecr;
        tcp_rtt_estimator(tp, seq_rtt, usrtt);
        tcp_set_rto(tp);
        tp->backoff = 0;
        tcp_bound_rto(tp);
}

static void tcp_ack_no_tstamp(struct tcp_sock *tp, u32 seq_rtt, u32 *usrtt, 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_rtt_estimator(tp, seq_rtt, usrtt);
        tcp_set_rto(tp);
        tp->backoff = 0;
        tcp_bound_rto(tp);
}

static inline void tcp_ack_update_rtt(struct tcp_sock *tp,
                                      int flag, s32 seq_rtt, u32 *usrtt)
{
        /* 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(tp, usrtt, flag);
        else if (seq_rtt >= 0)
                tcp_ack_no_tstamp(tp, seq_rtt, usrtt, flag);
}

static inline void tcp_cong_avoid(struct tcp_sock *tp, u32 ack, u32 rtt,
                                  u32 in_flight, int good)
{
        tp->ca_ops->cong_avoid(tp, ack, rtt, in_flight, good);
        tp->snd_cwnd_stamp = tcp_time_stamp;
}

/* Restart timer after forward progress on connection.
 * RFC2988 recommends to restart timer to now+rto.
 */

static inline void tcp_ack_packets_out(struct sock *sk, struct tcp_sock *tp)
{
        if (!tp->packets_out) {
                tcp_clear_xmit_timer(sk, TCP_TIME_RETRANS);
        } else {
                tcp_reset_xmit_timer(sk, TCP_TIME_RETRANS, tp->rto);
        }
}

static int tcp_tso_acked(struct sock *sk, struct sk_buff *skb,
                         __u32 now, __s32 *seq_rtt)
{
        struct tcp_sock *tp = tcp_sk(sk);
        struct tcp_skb_cb *scb = TCP_SKB_CB(skb); 
        __u32 seq = tp->snd_una;
        __u32 packets_acked;
        int acked = 0;

        /* If we get here, the whole TSO packet has not been
         * acked.
         */
        BUG_ON(!after(scb->end_seq, seq));

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

        if (packets_acked) {
                __u8 sacked = scb->sacked;

                acked |= FLAG_DATA_ACKED;
                if (sacked) {
                        if (sacked & TCPCB_RETRANS) {
                                if (sacked & TCPCB_SACKED_RETRANS)
                                        tp->retrans_out -= packets_acked;
                                acked |= FLAG_RETRANS_DATA_ACKED;
                                *seq_rtt = -1;
                        } else if (*seq_rtt < 0)
                                *seq_rtt = now - scb->when;
                        if (sacked & TCPCB_SACKED_ACKED)
                                tp->sacked_out -= packets_acked;
                        if (sacked & TCPCB_LOST)
                                tp->lost_out -= packets_acked;
                        if (sacked & TCPCB_URG) {
                                if (tp->urg_mode &&
                                    !before(seq, tp->snd_up))
                                        tp->urg_mode = 0;
                        }
                } else if (*seq_rtt < 0)
                        *seq_rtt = now - scb->when;

                if (tp->fackets_out) {
                        __u32 dval = min(tp->fackets_out, packets_acked);
                        tp->fackets_out -= dval;
                }
                tp->packets_out -= packets_acked;

                BUG_ON(tcp_skb_pcount(skb) == 0);
                BUG_ON(!before(scb->seq, scb->end_seq));
        }

        return acked;
}


/* Remove acknowledged frames from the retransmission queue. */
static int tcp_clean_rtx_queue(struct sock *sk, __s32 *seq_rtt_p, s32 *seq_usrtt)
{
        struct tcp_sock *tp = tcp_sk(sk);
        struct sk_buff *skb;
        __u32 now = tcp_time_stamp;
        int acked = 0;
        __s32 seq_rtt = -1;
        struct timeval usnow;
        u32 pkts_acked = 0;

        if (seq_usrtt)
                do_gettimeofday(&usnow);

        while ((skb = skb_peek(&sk->sk_write_queue)) &&
               skb != sk->sk_send_head) {
                struct tcp_skb_cb *scb = TCP_SKB_CB(skb); 
                __u8 sacked = scb->sacked;

                /* If our packet is before the ack sequence we can
                 * discard it as it's confirmed to have arrived at
                 * the other end.
                 */
                if (after(scb->end_seq, tp->snd_una)) {
                        if (tcp_skb_pcount(skb) > 1)
                                acked |= tcp_tso_acked(sk, skb,
                                                       now, &seq_rtt);
                        break;
                }

                /* 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)) {
                        acked |= FLAG_DATA_ACKED;
                        ++pkts_acked;
                } else {
                        acked |= FLAG_SYN_ACKED;
                        tp->retrans_stamp = 0;
                }

                if (sacked) {
                        if (sacked & TCPCB_RETRANS) {
                                if(sacked & TCPCB_SACKED_RETRANS)
                                        tp->retrans_out -= tcp_skb_pcount(skb);
                                acked |= FLAG_RETRANS_DATA_ACKED;
                                seq_rtt = -1;
                        } else if (seq_rtt < 0)
                                seq_rtt = now - scb->when;
                        if (seq_usrtt)
                                *seq_usrtt = (usnow.tv_sec - skb->stamp.tv_sec) * 1000000
                                        + (usnow.tv_usec - skb->stamp.tv_usec);

                        if (sacked & TCPCB_SACKED_ACKED)
                                tp->sacked_out -= tcp_skb_pcount(skb);
                        if (sacked & TCPCB_LOST)
                                tp->lost_out -= tcp_skb_pcount(skb);
                        if (sacked & TCPCB_URG) {
                                if (tp->urg_mode &&
                                    !before(scb->end_seq, tp->snd_up))
                                        tp->urg_mode = 0;
                        }
                } else if (seq_rtt < 0)
                        seq_rtt = now - scb->when;
                tcp_dec_pcount_approx(&tp->fackets_out, skb);
                tcp_packets_out_dec(tp, skb);
                __skb_unlink(skb, skb->list);
                sk_stream_free_skb(sk, skb);
        }

        if (acked&FLAG_ACKED) {
                tcp_ack_update_rtt(tp, acked, seq_rtt, seq_usrtt);
                tcp_ack_packets_out(sk, tp);

                if (tp->ca_ops->pkts_acked)
                        tp->ca_ops->pkts_acked(tp, pkts_acked);
        }

#if FASTRETRANS_DEBUG > 0
        BUG_TRAP((int)tp->sacked_out >= 0);
        BUG_TRAP((int)tp->lost_out >= 0);
        BUG_TRAP((int)tp->retrans_out >= 0);
        if (!tp->packets_out && tp->rx_opt.sack_ok) {
                if (tp->lost_out) {
                        printk(KERN_DEBUG "Leak l=%u %d\n",
                               tp->lost_out, tp->ca_state);
                        tp->lost_out = 0;
                }
                if (tp->sacked_out) {
                        printk(KERN_DEBUG "Leak s=%u %d\n",
                               tp->sacked_out, tp->ca_state);
                        tp->sacked_out = 0;
                }
                if (tp->retrans_out) {
                        printk(KERN_DEBUG "Leak r=%u %d\n",
                               tp->retrans_out, tp->ca_state);
                        tp->retrans_out = 0;
                }
        }
#endif
        *seq_rtt_p = seq_rtt;
        return acked;
}

static void tcp_ack_probe(struct sock *sk)
{
        struct tcp_sock *tp = tcp_sk(sk);

        /* Was it a usable window open? */

        if (!after(TCP_SKB_CB(sk->sk_send_head)->end_seq,
                   tp->snd_una + tp->snd_wnd)) {
                tp->backoff = 0;
                tcp_clear_xmit_timer(sk, TCP_TIME_PROBE0);
                /* Socket must be waked up by subsequent tcp_data_snd_check().
                 * This function is not for random using!
                 */
        } else {
                tcp_reset_xmit_timer(sk, TCP_TIME_PROBE0,
                                     min(tp->rto << tp->backoff, TCP_RTO_MAX));
        }
}

static inline int tcp_ack_is_dubious(struct tcp_sock *tp, int flag)
{
        return (!(flag & FLAG_NOT_DUP) || (flag & FLAG_CA_ALERT) ||
                tp->ca_state != TCP_CA_Open);
}

static inline int tcp_may_raise_cwnd(struct tcp_sock *tp, int flag)
{
        return (!(flag & FLAG_ECE) || tp->snd_cwnd < tp->snd_ssthresh) &&
                !((1<<tp->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(struct tcp_sock *tp, u32 ack,
                                        u32 ack_seq, 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 tcp_sock *tp,
                                 struct sk_buff *skb, u32 ack, u32 ack_seq)
{
        int flag = 0;
        u32 nwin = ntohs(skb->h.th->window);

        if (likely(!skb->h.th->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, 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.
                         */
                        tcp_fast_path_check(sk, tp);

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

        tp->snd_una = ack;

        return flag;
}

static void tcp_process_frto(struct sock *sk, u32 prior_snd_una)
{
        struct tcp_sock *tp = tcp_sk(sk);
        
        tcp_sync_left_out(tp);
        
        if (tp->snd_una == prior_snd_una ||
            !before(tp->snd_una, tp->frto_highmark)) {
                /* RTO was caused by loss, start retransmitting in
                 * go-back-N slow start
                 */
                tcp_enter_frto_loss(sk);
                return;
        }

        if (tp->frto_counter == 1) {
                /* First ACK after RTO advances the window: allow two new
                 * segments out.
                 */
                tp->snd_cwnd = tcp_packets_in_flight(tp) + 2;
        } else {
                /* Also the second ACK after RTO advances the window.
                 * The RTO was likely spurious. Reduce cwnd and continue
                 * in congestion avoidance
                 */
                tp->snd_cwnd = min(tp->snd_cwnd, tp->snd_ssthresh);
                tcp_moderate_cwnd(tp);
        }

        /* F-RTO affects on two new ACKs following RTO.
         * At latest on third ACK the TCP behavor is back to normal.
         */
        tp->frto_counter = (tp->frto_counter + 1) % 3;
}

/* This routine deals with incoming acks, but not outgoing ones. */
static int tcp_ack(struct sock *sk, struct sk_buff *skb, int flag)
{
        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;
        s32 seq_rtt;
        s32 seq_usrtt = 0;
        int prior_packets;

        /* If the ack is newer than sent or older than previous acks
         * then we can probably ignore it.
         */
        if (after(ack, tp->snd_nxt))
                goto uninteresting_ack;

        if (before(ack, prior_snd_una))
                goto old_ack;

        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, ack_seq);
                tp->snd_una = ack;
                flag |= FLAG_WIN_UPDATE;

                tcp_ca_event(tp, CA_EVENT_FAST_ACK);

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

                flag |= tcp_ack_update_window(sk, tp, 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, skb->h.th))
                        flag |= FLAG_ECE;

                tcp_ca_event(tp, CA_EVENT_SLOW_ACK);
        }

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

        prior_in_flight = tcp_packets_in_flight(tp);

        /* See if we can take anything off of the retransmit queue. */
        flag |= tcp_clean_rtx_queue(sk, &seq_rtt,
                                    tp->ca_ops->rtt_sample ? &seq_usrtt : NULL);

        if (tp->frto_counter)
                tcp_process_frto(sk, prior_snd_una);

        if (tcp_ack_is_dubious(tp, flag)) {
                /* Advanve CWND, if state allows this. */
                if ((flag & FLAG_DATA_ACKED) && tcp_may_raise_cwnd(tp, flag))
                        tcp_cong_avoid(tp, ack,  seq_rtt, prior_in_flight, 0);
                tcp_fastretrans_alert(sk, prior_snd_una, prior_packets, flag);
        } else {
                if ((flag & FLAG_DATA_ACKED))
                        tcp_cong_avoid(tp, ack, seq_rtt, prior_in_flight, 1);
        }

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

        return 1;

no_queue:
        tp->probes_out = 0;

        /* 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 (sk->sk_send_head)
                tcp_ack_probe(sk);
        return 1;

old_ack:
        if (TCP_SKB_CB(skb)->sacked)
                tcp_sacktag_write_queue(sk, skb, prior_snd_una);

uninteresting_ack:
        SOCK_DEBUG(sk, "Ack %u out of %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 = skb->h.th;
        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 = ntohs(get_unaligned((__u16 *)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)
                                                if (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) {
                                                if ((estab && opt_rx->tstamp_ok) ||
                                                    (!estab && sysctl_tcp_timestamps)) {
                                                        opt_rx->saw_tstamp = 1;
                                                        opt_rx->rcv_tsval = ntohl(get_unaligned((__u32 *)ptr));
                                                        opt_rx->rcv_tsecr = ntohl(get_unaligned((__u32 *)(ptr+4)));
                                                }
                                        }
                                        break;
                                case TCPOPT_SACK_PERM:
                                        if(opsize==TCPOLEN_SACK_PERM && th->syn && !estab) {
                                                if (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;
                                        }
                                };
                                ptr+=opsize-2;
                                length-=opsize;
                };
        }
}

/* Fast parse options. This hopes to only see timestamps.
 * If it is wrong it falls back on tcp_parse_options().
 */
static inline 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)) {
                __u32 *ptr = (__u32 *)(th + 1);
                if (*ptr == ntohl((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;
                }
        }
        tcp_parse_options(skb, &tp->rx_opt, 1);
        return 1;
}

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 = xtime.tv_sec;
}

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((s32)(tp->rx_opt.rcv_tsval - tp->rx_opt.ts_recent) >= 0 ||
                   xtime.tv_sec >= tp->rx_opt.ts_recent_stamp + TCP_PAWS_24DAYS)
                        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(struct tcp_sock *tp, struct sk_buff *skb)
{
        struct tcphdr *th = skb->h.th;
        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) <= (tp->rto*1024)/HZ);
}

static inline int tcp_paws_discard(struct tcp_sock *tp, struct sk_buff *skb)
{
        return ((s32)(tp->rx_opt.ts_recent - tp->rx_opt.rcv_tsval) > TCP_PAWS_WINDOW &&
                xtime.tv_sec < tp->rx_opt.ts_recent_stamp + TCP_PAWS_24DAYS &&
                !tcp_disordered_ack(tp, 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);

        tcp_schedule_ack(tp);

        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);
                        tp->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",
                               __FUNCTION__, 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 (tp->rx_opt.sack_ok)
                tcp_sack_reset(&tp->rx_opt);
        sk_stream_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, 1, POLL_HUP);
                else
                        sk_wake_async(sk, 1, 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 inline void tcp_dsack_set(struct tcp_sock *tp, u32 seq, u32 end_seq)
{
        if (tp->rx_opt.sack_ok && sysctl_tcp_dsack) {
                if (before(seq, tp->rcv_nxt))
                        NET_INC_STATS_BH(LINUX_MIB_TCPDSACKOLDSENT);
                else
                        NET_INC_STATS_BH(LINUX_MIB_TCPDSACKOFOSENT);

                tp->rx_opt.dsack = 1;
                tp->duplicate_sack[0].start_seq = seq;
                tp->duplicate_sack[0].end_seq = end_seq;
                tp->rx_opt.eff_sacks = min(tp->rx_opt.num_sacks + 1, 4 - tp->rx_opt.tstamp_ok);
        }
}

static inline void tcp_dsack_extend(struct tcp_sock *tp, u32 seq, u32 end_seq)
{
        if (!tp->rx_opt.dsack)
                tcp_dsack_set(tp, 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(LINUX_MIB_DELAYEDACKLOST);
                tcp_enter_quickack_mode(tp);

                if (tp->rx_opt.sack_ok && 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(tp, 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--;
                        tp->rx_opt.eff_sacks = min(tp->rx_opt.num_sacks + tp->rx_opt.dsack, 4 - tp->rx_opt.tstamp_ok);
                        for(i=this_sack; i < tp->rx_opt.num_sacks; i++)
                                sp[i] = sp[i+1];
                        continue;
                }
                this_sack++, swalk++;
        }
}

static __inline__ void tcp_sack_swap(struct tcp_sack_block *sack1, struct tcp_sack_block *sack2)
{
        __u32 tmp;

        tmp = sack1->start_seq;
        sack1->start_seq = sack2->start_seq;
        sack2->start_seq = tmp;

        tmp = sack1->end_seq;
        sack1->end_seq = sack2->end_seq;
        sack2->end_seq = tmp;
}

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--)
                                tcp_sack_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 >= 4) {
                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++;
        tp->rx_opt.eff_sacks = min(tp->rx_opt.num_sacks + tp->rx_opt.dsack, 4 - tp->rx_opt.tstamp_ok);
}

/* 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_len(&tp->out_of_order_queue) == 0) {
                tp->rx_opt.num_sacks = 0;
                tp->rx_opt.eff_sacks = tp->rx_opt.dsack;
                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! */
                        BUG_TRAP(!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++;
        }
        if (num_sacks != tp->rx_opt.num_sacks) {
                tp->rx_opt.num_sacks = num_sacks;
                tp->rx_opt.eff_sacks = min(tp->rx_opt.num_sacks + tp->rx_opt.dsack, 4 - tp->rx_opt.tstamp_ok);
        }
}

/* 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(tp, 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, skb->list);
                        __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, skb->list);
                __skb_queue_tail(&sk->sk_receive_queue, skb);
                tp->rcv_nxt = TCP_SKB_CB(skb)->end_seq;
                if(skb->h.th->fin)
                        tcp_fin(skb, sk, skb->h.th);
        }
}

static int tcp_prune_queue(struct sock *sk);

static void tcp_data_queue(struct sock *sk, struct sk_buff *skb)
{
        struct tcphdr *th = skb->h.th;
        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);

        if (tp->rx_opt.dsack) {
                tp->rx_opt.dsack = 0;
                tp->rx_opt.eff_sacks = min_t(unsigned int, tp->rx_opt.num_sacks,
                                                    4 - tp->rx_opt.tstamp_ok);
        }

        /*  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 &&
                            (atomic_read(&sk->sk_rmem_alloc) > sk->sk_rcvbuf ||
                             !sk_stream_rmem_schedule(sk, skb))) {
                                if (tcp_prune_queue(sk) < 0 ||
                                    !sk_stream_rmem_schedule(sk, skb))
                                        goto drop;
                        }
                        sk_stream_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, tp, skb);
                if(th->fin)
                        tcp_fin(skb, sk, th);

                if (skb_queue_len(&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_len(&tp->out_of_order_queue))
                                tp->ack.pingpong = 0;
                }

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

                tcp_fast_path_check(sk, tp);

                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(LINUX_MIB_DELAYEDACKLOST);
                tcp_dsack_set(tp, TCP_SKB_CB(skb)->seq, TCP_SKB_CB(skb)->end_seq);

out_of_window:
                tcp_enter_quickack_mode(tp);
                tcp_schedule_ack(tp);
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(tp);

        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(tp, 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 (atomic_read(&sk->sk_rmem_alloc) > sk->sk_rcvbuf ||
            !sk_stream_rmem_schedule(sk, skb)) {
                if (tcp_prune_queue(sk) < 0 ||
                    !sk_stream_rmem_schedule(sk, skb))
                        goto drop;
        }

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

        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);

        sk_stream_set_owner_r(skb, sk);

        if (!skb_peek(&tp->out_of_order_queue)) {
                /* Initial out of order segment, build 1 SACK. */
                if (tp->rx_opt.sack_ok) {
                        tp->rx_opt.num_sacks = 1;
                        tp->rx_opt.dsack     = 0;
                        tp->rx_opt.eff_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 = tp->out_of_order_queue.prev;
                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_append(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. */
                do {
                        if (!after(TCP_SKB_CB(skb1)->seq, seq))
                                break;
                } while ((skb1 = skb1->prev) !=
                         (struct sk_buff*)&tp->out_of_order_queue);

                /* Do skb overlap to previous one? */
                if (skb1 != (struct sk_buff*)&tp->out_of_order_queue &&
                    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(tp, seq, end_seq);
                                goto add_sack;
                        }
                        if (after(seq, TCP_SKB_CB(skb1)->seq)) {
                                /* Partial overlap. */
                                tcp_dsack_set(tp, seq, TCP_SKB_CB(skb1)->end_seq);
                        } else {
                                skb1 = skb1->prev;
                        }
                }
                __skb_insert(skb, skb1, skb1->next, &tp->out_of_order_queue);
                
                /* And clean segments covered by new one as whole. */
                while ((skb1 = skb->next) !=
                       (struct sk_buff*)&tp->out_of_order_queue &&
                       after(end_seq, TCP_SKB_CB(skb1)->seq)) {
                       if (before(end_seq, TCP_SKB_CB(skb1)->end_seq)) {
                               tcp_dsack_extend(tp, TCP_SKB_CB(skb1)->seq, end_seq);
                               break;
                       }
                       __skb_unlink(skb1, skb1->list);
                       tcp_dsack_extend(tp, TCP_SKB_CB(skb1)->seq, TCP_SKB_CB(skb1)->end_seq);
                       __kfree_skb(skb1);
                }

add_sack:
                if (tp->rx_opt.sack_ok)
                        tcp_sack_new_ofo_skb(sk, seq, end_seq);
        }
}

/* Collapse contiguous sequence of skbs head..tail with
 * sequence numbers start..end.
 * Segments with FIN/SYN are not collapsed (only because this
 * simplifies code)
 */
static void
tcp_collapse(struct sock *sk, struct sk_buff *head,
             struct sk_buff *tail, u32 start, u32 end)
{
        struct sk_buff *skb;

        /* First, check that queue is collapsable and find
         * the point where collapsing can be useful. */
        for (skb = head; skb != tail; ) {
                /* No new bits? It is possible on ofo queue. */
                if (!before(start, TCP_SKB_CB(skb)->end_seq)) {
                        struct sk_buff *next = skb->next;
                        __skb_unlink(skb, skb->list);
                        __kfree_skb(skb);
                        NET_INC_STATS_BH(LINUX_MIB_TCPRCVCOLLAPSED);
                        skb = next;
                        continue;
                }

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

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

        while (before(start, end)) {
                struct sk_buff *nskb;
                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_reserve(nskb, header);
                memcpy(nskb->head, skb->head, header);
                nskb->nh.raw = nskb->head + (skb->nh.raw-skb->head);
                nskb->h.raw = nskb->head + (skb->h.raw-skb->head);
                nskb->mac.raw = nskb->head + (skb->mac.raw-skb->head);
                memcpy(nskb->cb, skb->cb, sizeof(skb->cb));
                TCP_SKB_CB(nskb)->seq = TCP_SKB_CB(nskb)->end_seq = start;
                __skb_insert(nskb, skb->prev, skb, skb->list);
                sk_stream_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;

                        if (offset < 0) BUG();
                        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)) {
                                struct sk_buff *next = skb->next;
                                __skb_unlink(skb, skb->list);
                                __kfree_skb(skb);
                                NET_INC_STATS_BH(LINUX_MIB_TCPRCVCOLLAPSED);
                                skb = next;
                                if (skb == tail || skb->h.th->syn || skb->h.th->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 (;;) {
                skb = skb->next;

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

/* 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(LINUX_MIB_PRUNECALLED);

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

        tcp_collapse_ofo_queue(sk);
        tcp_collapse(sk, sk->sk_receive_queue.next,
                     (struct sk_buff*)&sk->sk_receive_queue,
                     tp->copied_seq, tp->rcv_nxt);
        sk_stream_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. */

        /* First, purge the out_of_order queue. */
        if (skb_queue_len(&tp->out_of_order_queue)) {
                NET_ADD_STATS_BH(LINUX_MIB_OFOPRUNED, 
                                 skb_queue_len(&tp->out_of_order_queue));
                __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_stream_mem_reclaim(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(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 (tp->ca_state == TCP_CA_Open &&
            sk->sk_socket && !test_bit(SOCK_NOSPACE, &sk->sk_socket->flags)) {
                /* Limited by application or receiver window. */
                u32 win_used = max(tp->snd_cwnd_used, 2U);
                if (win_used < tp->snd_cwnd) {
                        tp->snd_ssthresh = tcp_current_ssthresh(tp);
                        tp->snd_cwnd = (tp->snd_cwnd + win_used) >> 1;
                }
                tp->snd_cwnd_used = 0;
        }
        tp->snd_cwnd_stamp = tcp_time_stamp;
}


/* 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 (tp->packets_out < tp->snd_cwnd &&
            !(sk->sk_userlocks & SOCK_SNDBUF_LOCK) &&
            !tcp_memory_pressure &&
            atomic_read(&tcp_memory_allocated) < sysctl_tcp_mem[0]) {
                int sndmem = max_t(u32, tp->rx_opt.mss_clamp, tp->mss_cache_std) +
                        MAX_TCP_HEADER + 16 + sizeof(struct sk_buff),
                    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 inline 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, struct tcp_sock *tp)
{
        tcp_push_pending_frames(sk, tp);
        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) > tp->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(tp) ||
            /* 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)
{
        struct tcp_sock *tp = tcp_sk(sk);
        if (!tcp_ack_scheduled(tp)) {
                /* We sent a data segment already. */
                return;
        }
        __tcp_ack_snd_check(sk, 1);
}

/*
 *      This routine is only called when we have urgent data
 *      signalled. 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 sematics 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, skb->list);
                        __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->ip_summed==CHECKSUM_UNNECESSARY)
                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 int __tcp_checksum_complete_user(struct sock *sk, struct sk_buff *skb)
{
        int 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->ip_summed != CHECKSUM_UNNECESSARY &&
                __tcp_checksum_complete_user(sk, skb);
}

/*
 *      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);

        /*
         *      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_predition 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) {
                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) {
                        __u32 *ptr = (__u32 *)(th + 1);

                        /* No? Slow path! */
                        if (*ptr != ntohl((TCPOPT_NOP << 24) | (TCPOPT_NOP << 16)
                                          | (TCPOPT_TIMESTAMP << 8) | TCPOLEN_TIMESTAMP))
                                goto slow_path;

                        tp->rx_opt.saw_tstamp = 1;
                        ++ptr; 
                        tp->rx_opt.rcv_tsval = ntohl(*ptr);
                        ++ptr;
                        tp->rx_opt.rcv_tsecr = ntohl(*ptr);

                        /* 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);

                                tcp_rcv_rtt_measure_ts(tp, skb);

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

                        if (tp->ucopy.task == current &&
                            tp->copied_seq == tp->rcv_nxt &&
                            len - tcp_header_len <= tp->ucopy.len &&
                            sock_owned_by_user(sk)) {
                                __set_current_state(TASK_RUNNING);

                                if (!tcp_copy_to_iovec(sk, skb, 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);

                                        tcp_rcv_rtt_measure_ts(tp, skb);

                                        __skb_pull(skb, tcp_header_len);
                                        tp->rcv_nxt = TCP_SKB_CB(skb)->end_seq;
                                        NET_INC_STATS_BH(LINUX_MIB_TCPHPHITSTOUSER);
                                        eaten = 1;
                                }
                        }
                        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(tp, skb);

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

                                NET_INC_STATS_BH(LINUX_MIB_TCPHPHITS);

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

                        tcp_event_data_recv(sk, tp, 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, tp);
                                if (!tcp_ack_scheduled(tp))
                                        goto no_ack;
                        }

                        __tcp_ack_snd_check(sk, 0);
no_ack:
                        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;

        /*
         * RFC1323: H1. Apply PAWS check first.
         */
        if (tcp_fast_parse_options(skb, th, tp) && tp->rx_opt.saw_tstamp &&
            tcp_paws_discard(tp, skb)) {
                if (!th->rst) {
                        NET_INC_STATS_BH(LINUX_MIB_PAWSESTABREJECTED);
                        tcp_send_dupack(sk, skb);
                        goto discard;
                }
                /* Resets are accepted even if PAWS failed.

                   ts_recent update must be made after we are sure
                   that the packet is in window.
                 */
        }

        /*
         *      Standard slow path.
         */

        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;
        }

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

        tcp_replace_ts_recent(tp, TCP_SKB_CB(skb)->seq);

        if (th->syn && !before(TCP_SKB_CB(skb)->seq, tp->rcv_nxt)) {
                TCP_INC_STATS_BH(TCP_MIB_INERRS);
                NET_INC_STATS_BH(LINUX_MIB_TCPABORTONSYN);
                tcp_reset(sk);
                return 1;
        }

step5:
        if(th->ack)
                tcp_ack(sk, skb, FLAG_SLOWPATH);

        tcp_rcv_rtt_measure_ts(tp, 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, tp);
        tcp_ack_snd_check(sk);
        return 0;

csum_error:
        TCP_INC_STATS_BH(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);
        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(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);
                if (tp->ecn_flags&TCP_ECN_OK)
                        sock_set_flag(sk, SOCK_NO_LARGESEND);

                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)->ack_seq, 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 (tp->rx_opt.sack_ok && sysctl_tcp_fack)
                        tp->rx_opt.sack_ok |= 2;

                tcp_sync_mss(sk, tp->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;
                mb();
                tcp_set_state(sk, TCP_ESTABLISHED);

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

                tcp_init_metrics(sk);

                tcp_init_congestion_control(tp);

                /* 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))
                        tcp_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, 0, POLL_OUT);
                }

                if (sk->sk_write_pending || tp->defer_accept || tp->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
                         */
                        tcp_schedule_ack(tp);
                        tp->ack.lrcvtime = tcp_time_stamp;
                        tp->ack.ato      = TCP_ATO_MIN;
                        tcp_incr_quickack(tp);
                        tcp_enter_quickack_mode(tp);
                        tcp_reset_xmit_timer(sk, TCP_TIME_DACK, TCP_DELACK_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_check(&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);
                if (tp->ecn_flags&TCP_ECN_OK)
                        sock_set_flag(sk, SOCK_NO_LARGESEND);

                tcp_sync_mss(sk, tp->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);
        int queued = 0;

        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(tp->af_specific->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.
                         */
                        goto discard;
                }
                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, tp);
                return 0;
        }

        if (tcp_fast_parse_options(skb, th, tp) && tp->rx_opt.saw_tstamp &&
            tcp_paws_discard(tp, skb)) {
                if (!th->rst) {
                        NET_INC_STATS_BH(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)) {
                if (!th->rst)
                        tcp_send_dupack(sk, skb);
                goto discard;
        }

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

        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)) {
                NET_INC_STATS_BH(LINUX_MIB_TCPABORTONSYN);
                tcp_reset(sk);
                return 1;
        }

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

                switch(sk->sk_state) {
                case TCP_SYN_RECV:
                        if (acceptable) {
                                tp->copied_seq = tp->rcv_nxt;
                                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,0,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)->ack_seq,
                                            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(tp, 0, 0);

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

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

                                tcp_init_metrics(sk);

                                tcp_init_congestion_control(tp);

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

                                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(LINUX_MIB_TCPABORTONDATA);
                                                return 1;
                                        }

                                        tmo = tcp_fin_time(tp);
                                        if (tmo > TCP_TIMEWAIT_LEN) {
                                                tcp_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.
                                                 */
                                                tcp_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(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, tp);
                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(tcp_parse_options);
EXPORT_SYMBOL(tcp_rcv_established);
EXPORT_SYMBOL(tcp_rcv_state_process);