Linux 2.1.127

[davej-history.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 1.136 1998/11/07 14:36:18 davem Exp $
 *
 * Authors:     Ross Biro, <bir7@leland.Stanford.Edu>
 *              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.
 */

#include <linux/config.h>
#include <linux/mm.h>
#include <linux/sysctl.h>
#include <net/tcp.h>
#include <linux/ipsec.h>

#ifdef CONFIG_SYSCTL
#define SYNC_INIT 0 /* let the user enable it */
#else
#define SYNC_INIT 1
#endif

extern int sysctl_tcp_fin_timeout;

/* These are on by default so the code paths get tested.
 * For the final 2.2 this may be undone at our discretion. -DaveM
 */
int sysctl_tcp_timestamps = 1;
int sysctl_tcp_window_scaling = 1;
int sysctl_tcp_sack = 1;

int sysctl_tcp_syncookies = SYNC_INIT; 
int sysctl_tcp_stdurg;
int sysctl_tcp_rfc1337;

static int prune_queue(struct sock *sk);

/* 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_delack_estimator(struct tcp_opt *tp)
{
        if(tp->ato == 0) {
                tp->lrcvtime = jiffies;

                /* Help sender leave slow start quickly,
                 * this sets our initial ato value.
                 */
                tcp_enter_quickack_mode(tp);
        } else {
                int m = jiffies - tp->lrcvtime;

                tp->lrcvtime = jiffies;
                if(m <= 0)
                        m = 1;
                if(m > tp->rto)
                        tp->ato = tp->rto;
                else
                        tp->ato = (tp->ato >> 1) + m;

                /* We are not in "quick ack" mode. */
                if(tp->ato <= (HZ/100))
                        tp->ato = ((HZ/100)*2);
        }
}

/* 
 * Remember to send an ACK later.
 */
static __inline__ void tcp_remember_ack(struct tcp_opt *tp, struct tcphdr *th, 
                                        struct sk_buff *skb)
{
        tp->delayed_acks++; 
        /* Tiny-grams with PSH set make us ACK quickly. */
        if(th->psh && (skb->len < (tp->mss_cache >> 1)))
                tp->ato = HZ/50;
} 

/* 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 __inline__ void tcp_rtt_estimator(struct tcp_opt *tp, __u32 mrtt)
{
        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
         */
        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 */
                tp->mdev += m;          /* mdev = 3/4 mdev + 1/4 new */
        } else {
                /* no previous measure. */
                tp->srtt = m<<3;        /* take the measured time to be rtt */
                tp->mdev = m<<2;        /* make sure rto = 3*rtt */
        }
}

/* 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_opt *tp)
{
        tp->rto = (tp->srtt >> 3) + tp->mdev;
        tp->rto += (tp->rto >> 2) + (tp->rto >> (tp->snd_cwnd-1));
}
 

/* Keep the rto between HZ/5 and 120*HZ. 120*HZ is the upper bound
 * on packet lifetime in the internet. We need the HZ/5 lower
 * bound to behave correctly against BSD stacks with a fixed
 * delayed ack.
 * FIXME: It's not entirely clear this lower bound is the best
 * way to avoid the problem. Is it possible to drop the lower
 * bound and still avoid trouble with BSD stacks? Perhaps
 * some modification to the RTO calculation that takes delayed
 * ack bias into account? This needs serious thought. -- erics
 */
static __inline__ void tcp_bound_rto(struct tcp_opt *tp)
{
        if (tp->rto > 120*HZ)
                tp->rto = 120*HZ;
        if (tp->rto < HZ/5)
                tp->rto = HZ/5;
}

/* WARNING: this must not be called if tp->saw_timestamp was false. */
extern __inline__ void tcp_replace_ts_recent(struct sock *sk, struct tcp_opt *tp,
                                             __u32 start_seq, __u32 end_seq)
{
        /* From draft-ietf-tcplw-high-performance: the correct
         * test is last_ack_sent <= end_seq.
         * (RFC1323 stated last_ack_sent < end_seq.)
         *
         * HOWEVER: The current check contradicts the draft statements.
         * It has been done for good reasons.
         * The implemented check improves security and eliminates
         * unnecessary RTT overestimation.
         *              1998/06/27  Andrey V. Savochkin <saw@msu.ru>
         */
        if (!before(end_seq, tp->last_ack_sent - sk->rcvbuf) &&
            !after(start_seq, tp->rcv_wup + tp->rcv_wnd)) {
                /* PAWS bug workaround wrt. ACK frames, the PAWS discard
                 * extra check below makes sure this can only happen
                 * for pure ACK frames.  -DaveM
                 */
                if((s32)(tp->rcv_tsval - tp->ts_recent) >= 0) {
                        tp->ts_recent = tp->rcv_tsval;
                        tp->ts_recent_stamp = jiffies;
                }
        }
}

#define PAWS_24DAYS     (HZ * 60 * 60 * 24 * 24)

extern __inline__ int tcp_paws_discard(struct tcp_opt *tp, struct tcphdr *th, unsigned len)
{
        /* ts_recent must be younger than 24 days */
        return (((jiffies - tp->ts_recent_stamp) >= PAWS_24DAYS) ||
                (((s32)(tp->rcv_tsval-tp->ts_recent) < 0) &&
                 /* Sorry, PAWS as specified is broken wrt. pure-ACKs -DaveM */
                 (len != (th->doff * 4))));
}


static int __tcp_sequence(struct tcp_opt *tp, u32 seq, u32 end_seq)
{
        u32 end_window = tp->rcv_wup + tp->rcv_wnd;

        if (tp->rcv_wnd &&
            after(end_seq, tp->rcv_nxt) &&
            before(seq, end_window))
                return 1;
        if (seq != end_window)
                return 0;
        return (seq == end_seq);
}

/* This functions checks to see if the tcp header is actually acceptable. */
extern __inline__ int tcp_sequence(struct tcp_opt *tp, u32 seq, u32 end_seq)
{
        if (seq == tp->rcv_nxt)
                return (tp->rcv_wnd || (end_seq == seq));

        return __tcp_sequence(tp, seq, end_seq);
}

/* When we get a reset we do this. */
static void tcp_reset(struct sock *sk, struct sk_buff *skb)
{
        sk->zapped = 1;

        /* We want the right error as BSD sees it (and indeed as we do). */
        switch (sk->state) {
                case TCP_SYN_SENT:
                        sk->err = ECONNREFUSED;
                        break;
                case TCP_CLOSE_WAIT:
                        sk->err = EPIPE;
                        break;
                default:
                        sk->err = ECONNRESET;
        };
        tcp_set_state(sk,TCP_CLOSE);
        sk->shutdown = SHUTDOWN_MASK;
        if (!sk->dead) 
                sk->state_change(sk);
}

/* This tags the retransmission queue when SACKs arrive. */
static void tcp_sacktag_write_queue(struct sock *sk, struct tcp_sack_block *sp, int nsacks)
{
        struct tcp_opt *tp = &(sk->tp_pinfo.af_tcp);
        int i = nsacks;

        while(i--) {
                struct sk_buff *skb = skb_peek(&sk->write_queue);
                __u32 start_seq = ntohl(sp->start_seq);
                __u32 end_seq = ntohl(sp->end_seq);
                int fack_count = 0;

                while((skb != NULL) &&
                      (skb != tp->send_head) &&
                      (skb != (struct sk_buff *)&sk->write_queue)) {
                        /* The retransmission queue is always in order, so
                         * we can short-circuit the walk early.
                         */
                        if(!before(start_seq, TCP_SKB_CB(skb)->end_seq))
                                break;

                        /* We play conservative, we don't allow SACKS to partially
                         * tag a sequence space.
                         */
                        fack_count++;
                        if(!after(start_seq, TCP_SKB_CB(skb)->seq) &&
                           !before(end_seq, TCP_SKB_CB(skb)->end_seq)) {
                                /* If this was a retransmitted frame, account for it. */
                                if(TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_RETRANS)
                                        tp->retrans_out--;
                                TCP_SKB_CB(skb)->sacked |= TCPCB_SACKED_ACKED;

                                /* RULE: All new SACKs will either decrease retrans_out
                                 *       or advance fackets_out.
                                 */
                                if(fack_count > tp->fackets_out)
                                        tp->fackets_out = fack_count;
                        }
                        skb = skb->next;
                }
                sp++; /* Move on to the next SACK block. */
        }
}

/* 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 sock *sk, struct tcphdr *th, struct tcp_opt *tp, int no_fancy)
{
        unsigned char *ptr;
        int length=(th->doff*4)-sizeof(struct tcphdr);

        ptr = (unsigned char *)(th + 1);
        tp->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)
                                        break;  /* don't parse partial options */
                                switch(opcode) {
                                case TCPOPT_MSS:
                                        if(opsize==TCPOLEN_MSS && th->syn) {
                                                u16 in_mss = ntohs(*(__u16 *)ptr);
                                                if (in_mss == 0)
                                                        in_mss = 536;
                                                if (tp->mss_clamp > in_mss)
                                                        tp->mss_clamp = in_mss;
                                        }
                                        break;
                                case TCPOPT_WINDOW:
                                        if(opsize==TCPOLEN_WINDOW && th->syn)
                                                if (!no_fancy && sysctl_tcp_window_scaling) {
                                                        tp->wscale_ok = 1;
                                                        tp->snd_wscale = *(__u8 *)ptr;
                                                        if(tp->snd_wscale > 14) {
                                                                if(net_ratelimit())
                                                                        printk("tcp_parse_options: Illegal window "
                                                                               "scaling value %d >14 received.",
                                                                               tp->snd_wscale);
                                                                tp->snd_wscale = 14;
                                                        }
                                                }
                                        break;
                                case TCPOPT_TIMESTAMP:
                                        if(opsize==TCPOLEN_TIMESTAMP) {
                                                if (sysctl_tcp_timestamps && !no_fancy) {
                                                        tp->tstamp_ok = 1;
                                                        tp->saw_tstamp = 1;
                                                        tp->rcv_tsval = ntohl(*(__u32 *)ptr);
                                                        tp->rcv_tsecr = ntohl(*(__u32 *)(ptr+4));
                                                }
                                        }
                                        break;
                                case TCPOPT_SACK_PERM:
                                        if(opsize==TCPOLEN_SACK_PERM && th->syn) {
                                                if (sysctl_tcp_sack && !no_fancy) {
                                                        tp->sack_ok = 1;
                                                        tp->num_sacks = 0;
                                                }
                                        }
                                        break;

                                case TCPOPT_SACK:
                                        if((opsize >= (TCPOLEN_SACK_BASE + TCPOLEN_SACK_PERBLOCK)) &&
                                           sysctl_tcp_sack && (sk != NULL) && !th->syn) {
                                                int sack_bytes = opsize - TCPOLEN_SACK_BASE;

                                                if(!(sack_bytes % TCPOLEN_SACK_PERBLOCK)) {
                                                        int num_sacks = sack_bytes >> 3;
                                                        struct tcp_sack_block *sackp;

                                                        sackp = (struct tcp_sack_block *)ptr;
                                                        tcp_sacktag_write_queue(sk, sackp, num_sacks);
                                                }
                                        }
                                };
                                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 sock *sk, struct tcphdr *th, struct tcp_opt *tp)
{
        /* If we didn't send out any options ignore them all. */
        if (tp->tcp_header_len == sizeof(struct tcphdr))
                return 0;
        if (th->doff == sizeof(struct tcphdr)>>2) {
                tp->saw_tstamp = 0;
                return 0;
        } else if (th->doff == (sizeof(struct tcphdr)>>2)+(TCPOLEN_TSTAMP_ALIGNED>>2)) {
                __u32 *ptr = (__u32 *)(th + 1);
                if (*ptr == __constant_ntohl((TCPOPT_NOP << 24) | (TCPOPT_NOP << 16)
                                             | (TCPOPT_TIMESTAMP << 8) | TCPOLEN_TIMESTAMP)) {
                        tp->saw_tstamp = 1;
                        tp->rcv_tsval = ntohl(*++ptr);
                        tp->rcv_tsecr = ntohl(*++ptr);
                        return 1;
                }
        }
        tcp_parse_options(sk, th, tp, 0);
        return 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.  */

static __inline__ void clear_fast_retransmit(struct tcp_opt *tp)
{
        if (tp->dup_acks > 3)
                tp->snd_cwnd = (tp->snd_ssthresh);

        tp->dup_acks = 0;
}

/* NOTE: This code assumes that tp->dup_acks gets cleared when a
 * retransmit timer fires.
 */
static void tcp_fast_retrans(struct sock *sk, u32 ack, int not_dup)
{
        struct tcp_opt *tp = &(sk->tp_pinfo.af_tcp);

        /* Note: If not_dup is set this implies we got a
         * data carrying packet or a window update.
         * This carries no new information about possible
         * lost packets, so we have to ignore it for the purposes
         * of counting duplicate acks. Ideally this does not imply we
         * should stop our fast retransmit phase, more acks may come
         * later without data to help us. Unfortunately this would make
         * the code below much more complex. For now if I see such
         * a packet I clear the fast retransmit phase.
         */
        if (ack == tp->snd_una && tp->packets_out && (not_dup == 0)) {
                /* This is the standard reno style fast retransmit branch. */

                /* 1. When the third duplicate ack is received, set ssthresh 
                 * to one half the current congestion window, but no less 
                 * than two segments. Retransmit the missing segment.
                 */
                if (tp->high_seq == 0 || after(ack, tp->high_seq)) {
                        tp->dup_acks++;
                        if ((tp->fackets_out > 3) || (tp->dup_acks == 3)) {
                                tp->snd_ssthresh = max(tp->snd_cwnd >> 1, 2);
                                tp->snd_cwnd = (tp->snd_ssthresh + 3);
                                tp->high_seq = tp->snd_nxt;
                                if(!tp->fackets_out)
                                        tcp_retransmit_skb(sk, skb_peek(&sk->write_queue));
                                else
                                        tcp_fack_retransmit(sk);
                                tcp_reset_xmit_timer(sk, TIME_RETRANS, tp->rto);
                        }
                }

                /* 2. Each time another duplicate ACK arrives, increment 
                 * cwnd by the segment size. [...] Transmit a packet...
                 *
                 * Packet transmission will be done on normal flow processing
                 * since we're not in "retransmit mode".  We do not use duplicate
                 * ACKs to artificially inflate the congestion window when
                 * doing FACK.
                 */
                if (tp->dup_acks > 3) {
                        if(!tp->fackets_out) {
                                tp->snd_cwnd++;
                        } else {
                                /* Fill any further holes which may have appeared.
                                 * We may want to change this to run every further
                                 * multiple-of-3 dup ack increments, to be more robust
                                 * against out-of-order packet delivery.  -DaveM
                                 */
                                tcp_fack_retransmit(sk);
                        }
                }
        } else if (tp->high_seq != 0) {
                /* In this branch we deal with clearing the Floyd style
                 * block on duplicate fast retransmits, and if requested
                 * we do Hoe style secondary fast retransmits.
                 */
                if (!before(ack, tp->high_seq) || (not_dup & FLAG_DATA) != 0) {
                        /* Once we have acked all the packets up to high_seq
                         * we are done this fast retransmit phase.
                         * Alternatively data arrived. In this case we
                         * Have to abort the fast retransmit attempt.
                         * Note that we do want to accept a window
                         * update since this is expected with Hoe's algorithm.
                         */
                        clear_fast_retransmit(tp);

                        /* After we have cleared up to high_seq we can
                         * clear the Floyd style block.
                         */
                        if (!before(ack, tp->high_seq)) {
                                tp->high_seq = 0;
                                tp->fackets_out = 0;
                        }
                } else if (tp->dup_acks >= 3) {
                        if (!tp->fackets_out) {
                                /* Hoe Style. We didn't ack the whole
                                 * window. Take this as a cue that
                                 * another packet was lost and retransmit it.
                                 * Don't muck with the congestion window here.
                                 * Note that we have to be careful not to
                                 * act if this was a window update and it
                                 * didn't ack new data, since this does
                                 * not indicate a packet left the system.
                                 * We can test this by just checking
                                 * if ack changed from snd_una, since
                                 * the only way to get here without advancing
                                 * from snd_una is if this was a window update.
                                 */
                                if (ack != tp->snd_una && before(ack, tp->high_seq)) {
                                        tcp_retransmit_skb(sk, skb_peek(&sk->write_queue));
                                        tcp_reset_xmit_timer(sk, TIME_RETRANS, tp->rto);
                                }
                        } else {
                                /* FACK style, fill any remaining holes in
                                 * receiver's queue.
                                 */
                                tcp_fack_retransmit(sk);
                        }
                }
        }
}

/* This is Jacobson's slow start and congestion avoidance. 
 * SIGCOMM '88, p. 328.
 */
static void tcp_cong_avoid(struct tcp_opt *tp)
{
        if (tp->snd_cwnd <= tp->snd_ssthresh) {
                /* In "safe" area, increase. */
                tp->snd_cwnd++;
        } else {
                /* In dangerous area, increase slowly.
                 * In theory this is tp->snd_cwnd += 1 / tp->snd_cwnd
                 */
                if (tp->snd_cwnd_cnt >= tp->snd_cwnd) {
                        tp->snd_cwnd++;
                        tp->snd_cwnd_cnt=0;
                } else
                        tp->snd_cwnd_cnt++;
        }       
}

/* Remove acknowledged frames from the retransmission queue. */
static int tcp_clean_rtx_queue(struct sock *sk, __u32 ack,
                               __u32 *seq, __u32 *seq_rtt)
{
        struct tcp_opt *tp = &(sk->tp_pinfo.af_tcp);
        struct sk_buff *skb;
        unsigned long now = jiffies;
        int acked = 0;

        while((skb=skb_peek(&sk->write_queue)) && (skb != tp->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, ack))
                        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((sacked & TCPCB_SACKED_RETRANS) && tp->retrans_out)
                        tp->retrans_out--;
                if(!(scb->flags & TCPCB_FLAG_SYN)) {
                        acked |= FLAG_DATA_ACKED;
                        if(sacked & TCPCB_SACKED_RETRANS)
                                acked |= FLAG_RETRANS_DATA_ACKED;
                        if(tp->fackets_out)
                                tp->fackets_out--;
                } else {
                        tp->retrans_head = NULL;
                }               
                tp->packets_out--;
                *seq = scb->seq;
                *seq_rtt = now - scb->when;
                __skb_unlink(skb, skb->list);
                kfree_skb(skb);
        }

        if (acked)
                tp->retrans_head = NULL;
        return acked;
}

static void tcp_ack_probe(struct sock *sk, __u32 ack)
{
        struct tcp_opt *tp = &(sk->tp_pinfo.af_tcp);
        
        /* Our probe was answered. */
        tp->probes_out = 0;
        
        /* Was it a usable window open? */
        
        /* should always be non-null */
        if (tp->send_head != NULL &&
            !before (ack + tp->snd_wnd, TCP_SKB_CB(tp->send_head)->end_seq)) {
                tp->backoff = 0;
                tp->pending = 0;
                tcp_clear_xmit_timer(sk, TIME_PROBE0);
        } else {
                tcp_reset_xmit_timer(sk, TIME_PROBE0,
                                     min(tp->rto << tp->backoff, 120*HZ));
        }
}
 
/* Read draft-ietf-tcplw-high-performance before mucking
 * with this code. (Superceeds RFC1323)
 */
static void tcp_ack_saw_tstamp(struct sock *sk, struct tcp_opt *tp,
                               u32 seq, u32 ack, 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>
         */
        if (!(flag & FLAG_DATA_ACKED))
                return;

        seq_rtt = jiffies-tp->rcv_tsecr;
        tcp_rtt_estimator(tp, seq_rtt);
        if (tp->retransmits) {
                if (tp->packets_out == 0) {
                        tp->retransmits = 0;
                        tp->fackets_out = 0;
                        tp->retrans_out = 0;
                        tp->backoff = 0;
                        tcp_set_rto(tp);
                } else {
                        /* Still retransmitting, use backoff */
                        tcp_set_rto(tp);
                        tp->rto = tp->rto << tp->backoff;
                }
        } else {
                tcp_set_rto(tp);
                tcp_cong_avoid(tp);
        }
        /* NOTE: safe here so long as cong_ctl doesn't use rto */
        tcp_bound_rto(tp);
}

static __inline__ void tcp_ack_packets_out(struct sock *sk, struct tcp_opt *tp)
{
        struct sk_buff *skb = skb_peek(&sk->write_queue);
        long when = tp->rto - (jiffies - TCP_SKB_CB(skb)->when);

        /* Some data was ACK'd, if still retransmitting (due to a
         * timeout), resend more of the retransmit queue.  The
         * congestion window is handled properly by that code.
         */
        if (tp->retransmits) {
                tp->retrans_head = NULL;
                tcp_xmit_retransmit_queue(sk);
                tcp_reset_xmit_timer(sk, TIME_RETRANS, tp->rto);
        } else {
                tcp_reset_xmit_timer(sk, TIME_RETRANS, when);
        }
}

/* This routine deals with incoming acks, but not outgoing ones. */
static int tcp_ack(struct sock *sk, struct tcphdr *th, 
                   u32 ack_seq, u32 ack, int len)
{
        struct tcp_opt *tp = &(sk->tp_pinfo.af_tcp);
        int flag = 0;
        u32 seq = 0;
        u32 seq_rtt = 0;

        if(sk->zapped)
                return(1);      /* Dead, can't ack any more so why bother */

        if (tp->pending == TIME_KEEPOPEN)
                tp->probes_out = 0;

        tp->rcv_tstamp = jiffies;

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

        dst_confirm(sk->dst_cache);

        /* If there is data set flag 1 */
        if (len != th->doff*4) {
                flag |= FLAG_DATA;
                tcp_delack_estimator(tp);
        }

        /* Update our send window. */

        /* This is the window update code as per RFC 793
         * snd_wl{1,2} are used to prevent unordered
         * segments from shrinking the window 
         */
        if (before(tp->snd_wl1, ack_seq) ||
            (tp->snd_wl1 == ack_seq && !after(tp->snd_wl2, ack))) {
                u32 nwin = ntohs(th->window) << tp->snd_wscale;

                if ((tp->snd_wl2 != ack) || (nwin > tp->snd_wnd)) {
                        flag |= FLAG_WIN_UPDATE;
                        tp->snd_wnd = nwin;

                        tp->snd_wl1 = ack_seq;
                        tp->snd_wl2 = ack;

                        if (nwin > tp->max_window)
                                tp->max_window = nwin;
                }
        }

        /* We passed data and got it acked, remove any soft error
         * log. Something worked...
         */
        sk->err_soft = 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 (tp->pending == TIME_PROBE0)
                tcp_ack_probe(sk, ack);

        /* See if we can take anything off of the retransmit queue. */
        flag |= tcp_clean_rtx_queue(sk, ack, &seq, &seq_rtt);

        /* If we have a timestamp, we always do rtt estimates. */
        if (tp->saw_tstamp) {
                tcp_ack_saw_tstamp(sk, tp, seq, ack, flag);
        } else {
                /* If we were retransmiting don't count rtt estimate. */
                if (tp->retransmits) {
                        if (tp->packets_out == 0) {
                                tp->retransmits = 0;
                                tp->fackets_out = 0;
                                tp->retrans_out = 0;
                        }
                } else {
                        /* 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_DATA_ACKED) {
                                if(!(flag & FLAG_RETRANS_DATA_ACKED)) {
                                        tp->backoff = 0;
                                        tcp_rtt_estimator(tp, seq_rtt);
                                        tcp_set_rto(tp);
                                        tcp_bound_rto(tp);
                                }
                                tcp_cong_avoid(tp);
                        }
                }
        }

        if (tp->packets_out) {
                if (flag & FLAG_DATA_ACKED)
                        tcp_ack_packets_out(sk, tp);
        } else {
                tcp_clear_xmit_timer(sk, TIME_RETRANS);
        }

        flag &= (FLAG_DATA | FLAG_WIN_UPDATE);
        if ((ack == tp->snd_una && tp->packets_out && flag == 0) ||
            (tp->high_seq != 0)) {
                tcp_fast_retrans(sk, ack, flag);
        } else {
                /* Clear any aborted fast retransmit starts. */
                tp->dup_acks = 0;
        }
        /* Remember the highest ack received. */
        tp->snd_una = ack;
        return 1;

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

/* New-style handling of TIME_WAIT sockets. */
extern void tcp_tw_schedule(struct tcp_tw_bucket *tw);
extern void tcp_tw_reschedule(struct tcp_tw_bucket *tw);
extern void tcp_tw_deschedule(struct tcp_tw_bucket *tw);

void tcp_timewait_kill(struct tcp_tw_bucket *tw)
{
        /* Unlink from various places. */
        if(tw->bind_next)
                tw->bind_next->bind_pprev = tw->bind_pprev;
        *(tw->bind_pprev) = tw->bind_next;
        if(tw->tb->owners == NULL)
                tcp_inc_slow_timer(TCP_SLT_BUCKETGC);

        if(tw->next)
                tw->next->pprev = tw->pprev;
        *tw->pprev = tw->next;

        /* We decremented the prot->inuse count when we entered TIME_WAIT
         * and the sock from which this came was destroyed.
         */
        tw->sklist_next->sklist_prev = tw->sklist_prev;
        tw->sklist_prev->sklist_next = tw->sklist_next;

        /* Ok, now free it up. */
        kmem_cache_free(tcp_timewait_cachep, tw);
}

/* We come here as a special case from the AF specific TCP input processing,
 * and the SKB has no owner.  Essentially handling this is very simple,
 * we just keep silently eating rx'd packets until none show up for the
 * entire timeout period.  The only special cases are for BSD TIME_WAIT
 * reconnects and SYN/RST bits being set in the TCP header.
 */
int tcp_timewait_state_process(struct tcp_tw_bucket *tw, struct sk_buff *skb,
                               struct tcphdr *th, unsigned len)
{
        /*      RFC 1122:
         *      "When a connection is [...] on TIME-WAIT state [...]
         *      [a TCP] MAY accept a new SYN from the remote TCP to
         *      reopen the connection directly, if it:
         *      
         *      (1)  assigns its initial sequence number for the new
         *      connection to be larger than the largest sequence
         *      number it used on the previous connection incarnation,
         *      and
         *
         *      (2)  returns to TIME-WAIT state if the SYN turns out 
         *      to be an old duplicate".
         */
        if(th->syn && !th->rst && after(TCP_SKB_CB(skb)->seq, tw->rcv_nxt)) {
                struct sock *sk;
                struct tcp_func *af_specific = tw->af_specific;
                __u32 isn;

                isn = tw->rcv_nxt + 128000;
                if(isn == 0)
                        isn++;
                tcp_tw_deschedule(tw);
                tcp_timewait_kill(tw);
                sk = af_specific->get_sock(skb, th);
                if(sk == NULL || !ipsec_sk_policy(sk,skb))
                        return 0;
                skb_set_owner_r(skb, sk);
                af_specific = sk->tp_pinfo.af_tcp.af_specific;
                if(af_specific->conn_request(sk, skb, isn) < 0)
                        return 1; /* Toss a reset back. */
                return 0; /* Discard the frame. */
        }

        /* Check RST or SYN */
        if(th->rst || th->syn) {
                /* This is TIME_WAIT assasination, in two flavors.
                 * Oh well... nobody has a sufficient solution to this
                 * protocol bug yet.
                 */
                if(sysctl_tcp_rfc1337 == 0) {
                        tcp_tw_deschedule(tw);
                        tcp_timewait_kill(tw);
                }
                if(!th->rst)
                        return 1; /* toss a reset back */
        } else {
                /* In this case we must reset the TIMEWAIT timer. */
                if(th->ack)
                        tcp_tw_reschedule(tw);
        }
        return 0; /* Discard the frame. */
}

/* Enter the time wait state.  This is always called from BH
 * context.  Essentially we whip up a timewait bucket, copy the
 * relevant info into it from the SK, and mess with hash chains
 * and list linkage.
 */
static __inline__ void tcp_tw_hashdance(struct sock *sk, struct tcp_tw_bucket *tw)
{
        struct sock **head, *sktw;

        /* Step 1: Remove SK from established hash. */
        if(sk->next)
                sk->next->pprev = sk->pprev;
        *sk->pprev = sk->next;
        sk->pprev = NULL;
        tcp_reg_zap(sk);

        /* Step 2: Put TW into bind hash where SK was. */
        tw->tb = (struct tcp_bind_bucket *)sk->prev;
        if((tw->bind_next = sk->bind_next) != NULL)
                sk->bind_next->bind_pprev = &tw->bind_next;
        tw->bind_pprev = sk->bind_pprev;
        *sk->bind_pprev = (struct sock *)tw;

        /* Step 3: Same for the protocol sklist. */
        (tw->sklist_next = sk->sklist_next)->sklist_prev = (struct sock *)tw;
        (tw->sklist_prev = sk->sklist_prev)->sklist_next = (struct sock *)tw;
        sk->sklist_next = NULL;
        sk->prot->inuse--;

        /* Step 4: Hash TW into TIMEWAIT half of established hash table. */
        head = &tcp_established_hash[sk->hashent + (TCP_HTABLE_SIZE/2)];
        sktw = (struct sock *)tw;
        if((sktw->next = *head) != NULL)
                (*head)->pprev = &sktw->next;
        *head = sktw;
        sktw->pprev = head;
}

void tcp_time_wait(struct sock *sk)
{
        struct tcp_tw_bucket *tw;

        tw = kmem_cache_alloc(tcp_timewait_cachep, SLAB_ATOMIC);
        if(tw != NULL) {
                /* Give us an identity. */
                tw->daddr       = sk->daddr;
                tw->rcv_saddr   = sk->rcv_saddr;
                tw->bound_dev_if= sk->bound_dev_if;
                tw->num         = sk->num;
                tw->state       = TCP_TIME_WAIT;
                tw->sport       = sk->sport;
                tw->dport       = sk->dport;
                tw->family      = sk->family;
                tw->reuse       = sk->reuse;
                tw->rcv_nxt     = sk->tp_pinfo.af_tcp.rcv_nxt;
                tw->af_specific = sk->tp_pinfo.af_tcp.af_specific;

#if defined(CONFIG_IPV6) || defined(CONFIG_IPV6_MODULE)
                if(tw->family == PF_INET6) {
                        memcpy(&tw->v6_daddr,
                               &sk->net_pinfo.af_inet6.daddr,
                               sizeof(struct in6_addr));
                        memcpy(&tw->v6_rcv_saddr,
                               &sk->net_pinfo.af_inet6.rcv_saddr,
                               sizeof(struct in6_addr));
                }
#endif
                /* Linkage updates. */
                tcp_tw_hashdance(sk, tw);

                /* Get the TIME_WAIT timeout firing. */
                tcp_tw_schedule(tw);

                /* CLOSE the SK. */
                if(sk->state == TCP_ESTABLISHED)
                        tcp_statistics.TcpCurrEstab--;
                sk->state = TCP_CLOSE;
                net_reset_timer(sk, TIME_DONE,
                                min(sk->tp_pinfo.af_tcp.srtt * 2, TCP_DONE_TIME));
        } else {
                /* Sorry, we're out of memory, just CLOSE this
                 * socket up.  We've got bigger problems than
                 * non-graceful socket closings.
                 */
                tcp_set_state(sk, TCP_CLOSE);
        }

        /* Prevent rcvmsg/sndmsg calls, and wake people up. */
        sk->shutdown = SHUTDOWN_MASK;
        if(!sk->dead)
                sk->state_change(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)
{
        sk->tp_pinfo.af_tcp.fin_seq = TCP_SKB_CB(skb)->end_seq;

        tcp_send_ack(sk);

        if (!sk->dead) {
                sk->state_change(sk);
                sock_wake_async(sk->socket, 1);
        }

        switch(sk->state) {
                case TCP_SYN_RECV:
                case TCP_ESTABLISHED:
                        /* Move to CLOSE_WAIT */
                        tcp_set_state(sk, TCP_CLOSE_WAIT);
                        if (th->rst)
                                sk->shutdown = SHUTDOWN_MASK;
                        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.
                         *
                         * This causes a WRITE timeout, which will either
                         * move on to TIME_WAIT when we timeout, or resend
                         * the FIN properly (maybe we get rid of that annoying
                         * FIN lost hang). The TIME_WRITE code is already 
                         * correct for handling this timeout.
                         */
                        tcp_set_state(sk, TCP_CLOSING);
                        break;
                case TCP_FIN_WAIT2:
                        /* Received a FIN -- send ACK and enter TIME_WAIT. */
                        tcp_time_wait(sk);
                        break;
                default:
                        /* Only TCP_LISTEN and TCP_CLOSE are left, in these
                         * cases we should never reach this piece of code.
                         */
                        printk("tcp_fin: Impossible, sk->state=%d\n", sk->state);
                        break;
        };
}

/* 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_opt *tp, struct tcp_sack_block *sp)
{
        int this_sack, num_sacks = tp->num_sacks;
        struct tcp_sack_block *swalk = &tp->selective_acks[0];

        /* If more than one SACK block, see if the recent change to SP eats into
         * or hits the sequence space of other SACK blocks, if so coalesce.
         */
        if(num_sacks != 1) {
                for(this_sack = 0; this_sack < num_sacks; this_sack++, swalk++) {
                        if(swalk == sp)
                                continue;

                        /* First case, bottom of SP moves into top of the
                         * sequence space of SWALK.
                         */
                        if(between(sp->start_seq, swalk->start_seq, swalk->end_seq)) {
                                sp->start_seq = swalk->start_seq;
                                goto coalesce;
                        }
                        /* Second case, top of SP moves into bottom of the
                         * sequence space of SWALK.
                         */
                        if(between(sp->end_seq, swalk->start_seq, swalk->end_seq)) {
                                sp->end_seq = swalk->end_seq;
                                goto coalesce;
                        }
                }
        }
        /* SP is the only SACK, or no coalescing cases found. */
        return;

coalesce:
        /* Zap SWALK, by moving every further SACK up by one slot.
         * Decrease num_sacks.
         */
        for(this_sack += 1; this_sack < num_sacks-1; this_sack++, swalk++) {
                struct tcp_sack_block *next = (swalk + 1);
                swalk->start_seq = next->start_seq;
                swalk->end_seq = next->end_seq;
        }
        tp->num_sacks--;
}

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, struct sk_buff *skb)
{
        struct tcp_opt *tp = &(sk->tp_pinfo.af_tcp);
        struct tcp_sack_block *sp = &tp->selective_acks[0];
        int cur_sacks = tp->num_sacks;

        if (!cur_sacks)
                goto new_sack;

        /* Optimize for the common case, new ofo frames arrive
         * "in order". ;-)  This also satisfies the requirements
         * of RFC2018 about ordering of SACKs.
         */
        if(sp->end_seq == TCP_SKB_CB(skb)->seq) {
                sp->end_seq = TCP_SKB_CB(skb)->end_seq;
                tcp_sack_maybe_coalesce(tp, sp);
        } else if(sp->start_seq == TCP_SKB_CB(skb)->end_seq) {
                /* Re-ordered arrival, in this case, can be optimized
                 * as well.
                 */
                sp->start_seq = TCP_SKB_CB(skb)->seq;
                tcp_sack_maybe_coalesce(tp, sp);
        } else {
                struct tcp_sack_block *swap = sp + 1;
                int this_sack, max_sacks = (tp->tstamp_ok ? 3 : 4);

                /* Oh well, we have to move things around.
                 * Try to find a SACK we can tack this onto.
                 */

                for(this_sack = 1; this_sack < cur_sacks; this_sack++, swap++) {
                        if((swap->end_seq == TCP_SKB_CB(skb)->seq) ||
                           (swap->start_seq == TCP_SKB_CB(skb)->end_seq)) {
                                if(swap->end_seq == TCP_SKB_CB(skb)->seq)
                                        swap->end_seq = TCP_SKB_CB(skb)->end_seq;
                                else
                                        swap->start_seq = TCP_SKB_CB(skb)->seq;
                                tcp_sack_swap(sp, swap);
                                tcp_sack_maybe_coalesce(tp, sp);
                                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 (cur_sacks >= max_sacks) {
                        cur_sacks--;
                        tp->num_sacks--;
                }
                while(cur_sacks >= 1) {
                        struct tcp_sack_block *this = &tp->selective_acks[cur_sacks];
                        struct tcp_sack_block *prev = (this - 1);
                        this->start_seq = prev->start_seq;
                        this->end_seq = prev->end_seq;
                        cur_sacks--;
                }

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

static void tcp_sack_remove_skb(struct tcp_opt *tp, struct sk_buff *skb)
{
        struct tcp_sack_block *sp = &tp->selective_acks[0];
        int num_sacks = tp->num_sacks;
        int this_sack;

        /* This is an in order data segment _or_ an out-of-order SKB being
         * moved to the receive queue, so we know this removed SKB will eat
         * from the front of a SACK.
         */
        for(this_sack = 0; this_sack < num_sacks; this_sack++, sp++) {
                /* Check if the start of the sack is covered by skb. */
                if(!before(sp->start_seq, TCP_SKB_CB(skb)->seq) &&
                   before(sp->start_seq, TCP_SKB_CB(skb)->end_seq))
                        break;
        }

        /* This should only happen if so many SACKs get built that some get
         * pushed out before we get here, or we eat some in sequence packets
         * which are before the first SACK block.
         */
        if(this_sack >= num_sacks)
                return;

        sp->start_seq = TCP_SKB_CB(skb)->end_seq;
        if(!before(sp->start_seq, sp->end_seq)) {
                /* Zap this SACK, by moving forward any other SACKS. */
                for(this_sack += 1; this_sack < num_sacks; this_sack++, sp++) {
                        struct tcp_sack_block *next = (sp + 1);
                        sp->start_seq = next->start_seq;
                        sp->end_seq = next->end_seq;
                }
                tp->num_sacks--;
        }
}

static void tcp_sack_extend(struct tcp_opt *tp, struct sk_buff *old_skb, struct sk_buff *new_skb)
{
        struct tcp_sack_block *sp = &tp->selective_acks[0];
        int num_sacks = tp->num_sacks;
        int this_sack;

        for(this_sack = 0; this_sack < num_sacks; this_sack++, tp++) {
                if(sp->end_seq == TCP_SKB_CB(old_skb)->end_seq)
                        break;
        }
        if(this_sack >= num_sacks)
                return;
        sp->end_seq = TCP_SKB_CB(new_skb)->end_seq;
}

/* 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 sk_buff *skb;
        struct tcp_opt *tp = &(sk->tp_pinfo.af_tcp);

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

                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);

                if(tp->sack_ok)
                        tcp_sack_remove_skb(tp, skb);
                __skb_unlink(skb, skb->list);
                __skb_queue_tail(&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 void tcp_data_queue(struct sock *sk, struct sk_buff *skb)
{
        struct sk_buff *skb1;
        struct tcp_opt *tp = &(sk->tp_pinfo.af_tcp);

        /*  Queue data for delivery to the user.
         *  Packets in sequence go to the receive queue.
         *  Out of sequence packets to out_of_order_queue.
         */
        if (TCP_SKB_CB(skb)->seq == tp->rcv_nxt) {
                /* Ok. In sequence. */
        queue_and_out:
                dst_confirm(sk->dst_cache);
                __skb_queue_tail(&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);
                } else {
                        tcp_remember_ack(tp, skb->h.th, skb); 
                }
                /* This may have eaten into a SACK block. */
                if(tp->sack_ok && tp->num_sacks)
                        tcp_sack_remove_skb(tp, skb);
                tcp_ofo_queue(sk);

                /* Turn on fast path. */ 
                if (skb_queue_len(&tp->out_of_order_queue) == 0)
                        tp->pred_flags = htonl(((tp->tcp_header_len >> 2) << 28) |
                                               (0x10 << 16) |
                                               tp->snd_wnd);
                return;
        }
        
        /* An old packet, either a retransmit or some packet got lost. */
        if (!after(TCP_SKB_CB(skb)->end_seq, tp->rcv_nxt)) {
                /* A retransmit, 2nd most common case.  Force an imediate ack. */
                SOCK_DEBUG(sk, "retransmit received: seq %X\n", TCP_SKB_CB(skb)->seq);
                tcp_enter_quickack_mode(tp);
                kfree_skb(skb);
                return;
        }

        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);

                goto queue_and_out;
        }

        /* Ok. This is an out_of_order segment, force an ack. */
        tp->delayed_acks++;
        tcp_enter_quickack_mode(tp);

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

        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);

        if (skb_peek(&tp->out_of_order_queue) == NULL) {
                /* Initial out of order segment, build 1 SACK. */
                if(tp->sack_ok) {
                        tp->num_sacks = 1;
                        tp->selective_acks[0].start_seq = TCP_SKB_CB(skb)->seq;
                        tp->selective_acks[0].end_seq = TCP_SKB_CB(skb)->end_seq;
                }
                __skb_queue_head(&tp->out_of_order_queue,skb);
        } else {
                for(skb1=tp->out_of_order_queue.prev; ; skb1 = skb1->prev) {
                        /* Already there. */
                        if (TCP_SKB_CB(skb)->seq == TCP_SKB_CB(skb1)->seq) {
                                if (skb->len >= skb1->len) {
                                        if(tp->sack_ok)
                                                tcp_sack_extend(tp, skb1, skb);
                                        __skb_append(skb1, skb);
                                        __skb_unlink(skb1, skb1->list);
                                        kfree_skb(skb1);
                                } else {
                                        /* A duplicate, smaller than what is in the
                                         * out-of-order queue right now, toss it.
                                         */
                                        kfree_skb(skb);
                                }
                                break;
                        }
                        
                        if (after(TCP_SKB_CB(skb)->seq, TCP_SKB_CB(skb1)->seq)) {
                                __skb_append(skb1, skb);
                                if(tp->sack_ok)
                                        tcp_sack_new_ofo_skb(sk, skb);
                                break;
                        }

                        /* See if we've hit the start. If so insert. */
                        if (skb1 == skb_peek(&tp->out_of_order_queue)) {
                                __skb_queue_head(&tp->out_of_order_queue,skb);
                                if(tp->sack_ok)
                                        tcp_sack_new_ofo_skb(sk, skb);
                                break;
                        }
                }
        }
}


/*
 *      This routine handles the data.  If there is room in the buffer,
 *      it will be have already been moved into it.  If there is no
 *      room, then we will just have to discard the packet.
 */

static int tcp_data(struct sk_buff *skb, struct sock *sk, unsigned int len)
{
        struct tcphdr *th;
        struct tcp_opt *tp = &(sk->tp_pinfo.af_tcp);

        th = skb->h.th;
        skb_pull(skb, th->doff*4);
        skb_trim(skb, len - (th->doff*4));

        if (skb->len == 0 && !th->fin)
                return(0);

        /* 
         *      If our receive queue has grown past its limits shrink it.
         *      Make sure to do this before moving snd_nxt, otherwise
         *      data might be acked for that we don't have enough room.
         */
        if (atomic_read(&sk->rmem_alloc) > sk->rcvbuf) { 
                if (prune_queue(sk) < 0) { 
                        /* Still not enough room. That can happen when
                         * skb->true_size differs significantly from skb->len.
                         */
                        return 0;
                }
        }

        tcp_data_queue(sk, skb);

        if (before(tp->rcv_nxt, tp->copied_seq)) {
                printk(KERN_DEBUG "*** tcp.c:tcp_data bug acked < copied\n");
                tp->rcv_nxt = tp->copied_seq;
        }

        /* Above, tcp_data_queue() increments delayed_acks appropriately.
         * Now tell the user we may have some data.
         */
        if (!sk->dead) {
                SOCK_DEBUG(sk, "Data wakeup.\n");
                sk->data_ready(sk,0);
        }
        return(1);
}

static void __tcp_data_snd_check(struct sock *sk, struct sk_buff *skb)
{
        struct tcp_opt *tp = &(sk->tp_pinfo.af_tcp);

        if (!after(TCP_SKB_CB(skb)->end_seq, tp->snd_una + tp->snd_wnd) &&
            tcp_packets_in_flight(tp) < tp->snd_cwnd) {
                /* Put more data onto the wire. */
                tcp_write_xmit(sk);
        } else if (tp->packets_out == 0 && !tp->pending) {
                /* Start probing the receivers window. */
                tcp_reset_xmit_timer(sk, TIME_PROBE0, tp->rto);
        }
}

static __inline__ void tcp_data_snd_check(struct sock *sk)
{
        struct sk_buff *skb = sk->tp_pinfo.af_tcp.send_head;

        if (skb != NULL)
                __tcp_data_snd_check(sk, skb); 
}

/* 
 * Adapt the MSS value used to make delayed ack decision to the 
 * real world. 
 */ 
static __inline__ void tcp_measure_rcv_mss(struct sock *sk, struct sk_buff *skb)
{
        struct tcp_opt *tp = &(sk->tp_pinfo.af_tcp);
        unsigned int len = skb->len, lss; 

        if (len > tp->rcv_mss) 
                tp->rcv_mss = len; 
        lss = tp->last_seg_size; 
        tp->last_seg_size = 0; 
        if (len >= 536) {
                if (len == lss) 
                        tp->rcv_mss = len; 
                tp->last_seg_size = len; 
        }
}

/*
 * Check if sending an ack is needed.
 */
static __inline__ void __tcp_ack_snd_check(struct sock *sk)
{
        struct tcp_opt *tp = &(sk->tp_pinfo.af_tcp);

        /* This also takes care of updating the window.
         * This if statement needs to be simplified.
         *
         * Rules for delaying an ack:
         *      - delay time <= 0.5 HZ
         *      - we don't have a window update to send
         *      - must send at least every 2 full sized packets
         *      - must send an ACK if we have any out of order data
         *
         * With an extra heuristic to handle loss of packet
         * situations and also helping the sender leave slow
         * start in an expediant manner.
         */

            /* Two full frames received or... */
        if (((tp->rcv_nxt - tp->rcv_wup) >= tp->rcv_mss * MAX_DELAY_ACK) ||
            /* We will update the window "significantly" or... */
            tcp_raise_window(sk) ||
            /* We entered "quick ACK" mode or... */
            tcp_in_quickack_mode(tp) ||
            /* We have out of order data */
            (skb_peek(&tp->out_of_order_queue) != NULL)) {
                /* Then ack it now */
                tcp_send_ack(sk);
        } else {
                /* Else, send delayed ack. */
                tcp_send_delayed_ack(tp, HZ/2);
        }
}

static __inline__ void tcp_ack_snd_check(struct sock *sk)
{
        struct tcp_opt *tp = &(sk->tp_pinfo.af_tcp);
        if (tp->delayed_acks == 0) {
                /* We sent a data segment already. */
                return;
        }
        __tcp_ack_snd_check(sk);
}


/*
 *      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_opt *tp = &(sk->tp_pinfo.af_tcp);
        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 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. */
        if (sk->proc != 0) {
                if (sk->proc > 0)
                        kill_proc(sk->proc, SIGURG, 1);
                else
                        kill_pg(-sk->proc, SIGURG, 1);
        }

        /* 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())
         */
        if (tp->urg_seq == tp->copied_seq)
                tp->copied_seq++;       /* Move the copied sequence on correctly */
        tp->urg_data = URG_NOTYET;
        tp->urg_seq = ptr;

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

/* This is the 'fast' part of urgent handling. */
static inline void tcp_urg(struct sock *sk, struct tcphdr *th, unsigned long len)
{
        struct tcp_opt *tp = &(sk->tp_pinfo.af_tcp);

        /* 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 == URG_NOTYET) {
                u32 ptr = tp->urg_seq - ntohl(th->seq) + (th->doff*4);

                /* Is the urgent pointer pointing into this packet? */   
                if (ptr < len) {
                        tp->urg_data = URG_VALID | *(ptr + (unsigned char *) th);
                        if (!sk->dead)
                                sk->data_ready(sk,0);
                }
        }
}

/*
 * Clean first the out_of_order queue, then the receive queue until
 * the socket is in its memory limits again.
 */
static int prune_queue(struct sock *sk)
{
        struct tcp_opt *tp = &sk->tp_pinfo.af_tcp; 
        struct sk_buff * skb;

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

        net_statistics.PruneCalled++; 

        /* First Clean the out_of_order queue. */
        /* Start with the end because there are probably the least
         * useful packets (crossing fingers).
         */
        while ((skb = __skb_dequeue_tail(&tp->out_of_order_queue))) { 
                net_statistics.OfoPruned += skb->len; 
                kfree_skb(skb);
                if (atomic_read(&sk->rmem_alloc) <= sk->rcvbuf)
                        return 0;
        }
        
        /* Now continue with the receive queue if it wasn't enough.
         * But only do this if we are really being abused.
         */
        while ((atomic_read(&sk->rmem_alloc) >= (sk->rcvbuf * 2)) &&
               (skb = skb_peek_tail(&sk->receive_queue))) {
                /* Never toss anything when we've seen the FIN.
                 * It's just too complex to recover from it.
                 */
                if(skb->h.th->fin)
                        break;

                /* Never remove packets that have been already acked */
                if (before(TCP_SKB_CB(skb)->end_seq, tp->last_ack_sent+1)) {
                        SOCK_DEBUG(sk, "prune_queue: hit acked data c=%x,%x,%x\n",
                                   tp->copied_seq, TCP_SKB_CB(skb)->end_seq,
                                   tp->last_ack_sent);
                        return -1;
                }

                net_statistics.RcvPruned += skb->len; 

                __skb_unlink(skb, skb->list);
                tp->rcv_nxt = TCP_SKB_CB(skb)->seq;
                SOCK_DEBUG(sk, "prune_queue: removing %x-%x (c=%x)\n",
                           TCP_SKB_CB(skb)->seq, TCP_SKB_CB(skb)->end_seq,
                           tp->copied_seq); 
                kfree_skb(skb);
        }
        return 0;
}

/*
 *      TCP receive function for the ESTABLISHED state. 
 *
 *      It is split into a fast path and a slow path. The fast path is 
 *      disabled when:
 *      - A zero window was announced from us - zero window probing
 *        is only handled properly in the slow path. 
 *      - Out of order segments arrived.
 *      - Urgent data is expected.
 *      - There is no buffer space left
 *      - Unexpected TCP flags/window values/header lengths are received
 *        (detected by checking the TCP header against pred_flags) 
 *      - Data is sent in both directions. Fast path only supports pure senders
 *        or pure receivers (this means either the sequence number or the ack
 *        value must stay constant)
 *
 *      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_opt *tp = &(sk->tp_pinfo.af_tcp);
        int queued;
        u32 flg;

        /*
         *      Header prediction.
         *      The code 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.
         */

        /*
         * RFC1323: H1. Apply PAWS check first.
         */
        if (tcp_fast_parse_options(sk, th, tp)) {
                if (tp->saw_tstamp) {
                        if (tcp_paws_discard(tp, th, len)) {
                                if (!th->rst) {
                                        tcp_send_ack(sk);
                                        goto discard;
                                }
                        }
                        tcp_replace_ts_recent(sk, tp,
                                              TCP_SKB_CB(skb)->seq,
                                              TCP_SKB_CB(skb)->end_seq);
                }
        }

        flg = *(((u32 *)th) + 3) & ~htonl(0x8 << 16);

        /*      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 else it will be !0
         *      (when there are holes in the receive 
         *       space for instance)
         *      PSH flag is ignored.
         */

        if (flg == tp->pred_flags && TCP_SKB_CB(skb)->seq == tp->rcv_nxt) {
                if (len <= th->doff*4) {
                        /* Bulk data transfer: sender */
                        if (len == th->doff*4) {
                                tcp_ack(sk, th, TCP_SKB_CB(skb)->seq,
                                        TCP_SKB_CB(skb)->ack_seq, len); 
                                kfree_skb(skb); 
                                tcp_data_snd_check(sk);
                                return 0;
                        } else { /* Header too small */
                                tcp_statistics.TcpInErrs++;
                                goto discard;
                        }
                } else if (TCP_SKB_CB(skb)->ack_seq == tp->snd_una &&
                           atomic_read(&sk->rmem_alloc) <= sk->rcvbuf) {
                        /* Bulk data transfer: receiver */
                        __skb_pull(skb,th->doff*4);

                        tcp_measure_rcv_mss(sk, skb); 

                        /* DO NOT notify forward progress here.
                         * It saves dozen of CPU instructions in fast path. --ANK
                         */
                        __skb_queue_tail(&sk->receive_queue, skb);
                        tp->rcv_nxt = TCP_SKB_CB(skb)->end_seq;

                        /* FIN bit check is not done since if FIN is set in
                         * this frame, the pred_flags won't match up. -DaveM
                         */
                        sk->data_ready(sk, 0);
                        tcp_delack_estimator(tp);

                        tcp_remember_ack(tp, th, skb); 

                        __tcp_ack_snd_check(sk);
                        return 0;
                }
        }

        /*
         *      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)
                        goto discard;
                if (after(TCP_SKB_CB(skb)->seq, tp->rcv_nxt)) {
                        SOCK_DEBUG(sk, "seq:%d end:%d wup:%d wnd:%d\n",
                                   TCP_SKB_CB(skb)->seq, TCP_SKB_CB(skb)->end_seq,
                                   tp->rcv_wup, tp->rcv_wnd);
                }
                tcp_send_ack(sk);
                goto discard;
        }

        if(th->syn && TCP_SKB_CB(skb)->seq != tp->syn_seq) {
                SOCK_DEBUG(sk, "syn in established state\n");
                tcp_statistics.TcpInErrs++;
                tcp_reset(sk, skb);
                return 1;
        }
        
        if(th->rst) {
                tcp_reset(sk,skb);
                goto discard;
        }

        if(th->ack)
                tcp_ack(sk, th, TCP_SKB_CB(skb)->seq, TCP_SKB_CB(skb)->ack_seq, len);
        
        /* Process urgent data. */
        tcp_urg(sk, th, len);

        /* step 7: process the segment text */
        queued = tcp_data(skb, sk, len);

        /* This must be after tcp_data() does the skb_pull() to
         * remove the header size from skb->len.
         *
         * Dave!!! Phrase above (and all about rcv_mss) has 
         * nothing to do with reality. rcv_mss must measure TOTAL
         * size, including sacks, IP options etc. Hence, measure_rcv_mss
         * must occure before pulling etc, otherwise it will flap
         * like hell. Even putting it before tcp_data is wrong,
         * it should use skb->tail - skb->nh.raw instead.
         *                                      --ANK (980805)
         * 
         * BTW I broke it. Now all TCP options are handled equally
         * in mss_clamp calculations (i.e. ignored, rfc1122),
         * and mss_cache does include all of them (i.e. tstamps)
         * except for sacks, to calulate effective mss faster.
         *                                      --ANK (980805)
         */
        tcp_measure_rcv_mss(sk, skb); 

        /* Be careful, tcp_data() may have put this into TIME_WAIT. */
        if(sk->state != TCP_CLOSE) {
                tcp_data_snd_check(sk);
                tcp_ack_snd_check(sk);
        }

        if (!queued) {
        discard:
                kfree_skb(skb);
        }

        return 0;
}

/* 
 *      Process an incoming SYN or SYN-ACK for SYN_RECV sockets represented
 *      as an open_request. 
 */

struct sock *tcp_check_req(struct sock *sk, struct sk_buff *skb, 
                           struct open_request *req)
{
        struct tcp_opt *tp = &(sk->tp_pinfo.af_tcp);
        u32 flg;

        /*      assumption: the socket is not in use.
         *      as we checked the user count on tcp_rcv and we're
         *      running from a soft interrupt.
         */

        /* Check for syn retransmission */
        flg = *(((u32 *)skb->h.th) + 3);
        
        flg &= __constant_htonl(0x00170000);
        /* Only SYN set? */
        if (flg == __constant_htonl(0x00020000)) {
                if (!after(TCP_SKB_CB(skb)->seq, req->rcv_isn)) {
                        /*      retransmited syn.
                         */
                        req->class->rtx_syn_ack(sk, req); 
                        return NULL;
                } else {
                        return sk; /* Pass new SYN to the listen socket. */
                }
        }

        /* We know it's an ACK here */  
        if (req->sk) {
                /*      socket already created but not
                 *      yet accepted()...
                 */
                sk = req->sk;
        } else {
                /* In theory the packet could be for a cookie, but
                 * TIME_WAIT should guard us against this. 
                 * XXX: Nevertheless check for cookies?
                 * This sequence number check is done again later,
                 * but we do it here to prevent syn flood attackers
                 * from creating big SYN_RECV sockets.
                 */ 
                if (!between(TCP_SKB_CB(skb)->ack_seq, req->snt_isn, req->snt_isn+1) ||
                    !between(TCP_SKB_CB(skb)->seq, req->rcv_isn, 
                             req->rcv_isn+1+req->rcv_wnd)) {
                        req->class->send_reset(skb);
                        return NULL;
                }
        
                sk = tp->af_specific->syn_recv_sock(sk, skb, req, NULL);
                tcp_dec_slow_timer(TCP_SLT_SYNACK);
                if (sk == NULL)
                        return NULL;
                
                req->expires = 0UL;
                req->sk = sk;
        }
        skb_orphan(skb); 
        skb_set_owner_r(skb, sk);
        return sk; 
}

/*
 *      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_opt *tp = &(sk->tp_pinfo.af_tcp);
        int queued = 0;

        /* state == CLOSED, hash lookup always fails, so no worries. -DaveM */
        switch (sk->state) {
        case TCP_LISTEN:
                /* These use the socket TOS.. 
                 * might want to be the received TOS 
                 */
                if(th->ack)
                        return 1;
                
                if(th->syn) {
                        if(tp->af_specific->conn_request(sk, skb, 0) < 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;
                break;

        case TCP_SYN_SENT:
                /* SYN sent means we have to look for a suitable ack and 
                 * either reset for bad matches or go to connected. 
                 * The SYN_SENT case is unusual and should
                 * not be in line code. [AC]
                 */
                if(th->ack) {
                        tp->snd_wl1 = TCP_SKB_CB(skb)->seq;

                        /* We got an ack, but it's not a good ack. */
                        if(!tcp_ack(sk,th, TCP_SKB_CB(skb)->seq,
                                    TCP_SKB_CB(skb)->ack_seq, len)) {
                                sk->err = ECONNRESET;
                                sk->state_change(sk);
                                tcp_statistics.TcpAttemptFails++;
                                return 1;
                        }

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

                        if(!th->syn) {
                                /* A valid ack from a different connection
                                 * start.  Shouldn't happen but cover it.
                                 */
                                sk->err = ECONNRESET;
                                sk->state_change(sk);
                                tcp_statistics.TcpAttemptFails++;
                                return 1;
                        }

                        /* 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 = htons(th->window);
                        tp->snd_wl1 = TCP_SKB_CB(skb)->seq;
                        tp->snd_wl2 = TCP_SKB_CB(skb)->ack_seq;
                        tp->fin_seq = TCP_SKB_CB(skb)->seq;

                        tcp_set_state(sk, TCP_ESTABLISHED);
                        tcp_parse_options(sk, th, tp, 0);

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

                        if (tp->tstamp_ok) {
                                tp->tcp_header_len =
                                        sizeof(struct tcphdr) + TCPOLEN_TSTAMP_ALIGNED;
                        } else
                                tp->tcp_header_len = sizeof(struct tcphdr);
                        if (tp->saw_tstamp) {
                                tp->ts_recent = tp->rcv_tsval;
                                tp->ts_recent_stamp = jiffies;
                        }

                        /* Can't be earlier, doff would be wrong. */
                        tcp_send_ack(sk);

                        sk->dport = th->source;
                        tp->copied_seq = tp->rcv_nxt;

                        if(!sk->dead) {
                                sk->state_change(sk);
                                sock_wake_async(sk->socket, 0);
                        }
                } else {
                        if(th->syn && !th->rst) {
                                /* The previous version of the code
                                 * checked for "connecting to self"
                                 * here. that check is done now in
                                 * tcp_connect.
                                 */
                                tcp_set_state(sk, TCP_SYN_RECV);
                                tcp_parse_options(sk, th, tp, 0);
                                if (tp->saw_tstamp) {
                                        tp->ts_recent = tp->rcv_tsval;
                                        tp->ts_recent_stamp = jiffies;
                                }
                                
                                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 = htons(th->window);
                                tp->snd_wl1 = TCP_SKB_CB(skb)->seq;
                                
                                tcp_send_synack(sk);
                        } else
                                break; 
                }

                /* tp->tcp_header_len and tp->mss_clamp
                   probably changed, synchronize mss.
                   */
                tcp_sync_mss(sk, tp->pmtu_cookie);
                tp->rcv_mss = tp->mss_cache;

                if (sk->state == TCP_SYN_RECV)
                        goto discard;
                
                goto step6; 
        }

        /*   Parse the tcp_options present on this header.
         *   By this point we really only expect timestamps.
         *   Note that this really has to be here and not later for PAWS
         *   (RFC1323) to work.
         */
        if (tcp_fast_parse_options(sk, th, tp)) {
                /* NOTE: assumes saw_tstamp is never set if we didn't
                 * negotiate the option. tcp_fast_parse_options() must
                 * guarantee this.
                 */
                if (tp->saw_tstamp) {
                        if (tcp_paws_discard(tp, th, len)) {
                                if (!th->rst) {
                                        tcp_send_ack(sk);
                                        goto discard;
                                }
                        }
                        tcp_replace_ts_recent(sk, tp,
                                              TCP_SKB_CB(skb)->seq,
                                              TCP_SKB_CB(skb)->end_seq);
                }
        }

        /* 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_ack(sk);
                        goto discard;
                }
        }

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

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

        /*      step 4:
         *
         *      Check for a SYN, and ensure it matches the SYN we were
         *      first sent. We have to handle the rather unusual (but valid)
         *      sequence that KA9Q derived products may generate of
         *
         *      SYN
         *                              SYN|ACK Data
         *      ACK     (lost)
         *                              SYN|ACK Data + More Data
         *      .. we must ACK not RST...
         *
         *      We keep syn_seq as the sequence space occupied by the 
         *      original syn. 
         */

        if (th->syn && TCP_SKB_CB(skb)->seq != tp->syn_seq) {
                tcp_reset(sk, skb);
                return 1;
        }

        /* step 5: check the ACK field */
        if (th->ack) {
                int acceptable = tcp_ack(sk, th, TCP_SKB_CB(skb)->seq,
                                         TCP_SKB_CB(skb)->ack_seq, len);
                
                switch(sk->state) {
                case TCP_SYN_RECV:
                        if (acceptable) {
                                tcp_set_state(sk, TCP_ESTABLISHED);
                                sk->dport = th->source;
                                tp->copied_seq = tp->rcv_nxt;

                                if(!sk->dead)
                                        sk->state_change(sk);           

                                tp->snd_una = TCP_SKB_CB(skb)->ack_seq;
                                tp->snd_wnd = htons(th->window) << tp->snd_wscale;
                                tp->snd_wl1 = TCP_SKB_CB(skb)->seq;
                                tp->snd_wl2 = TCP_SKB_CB(skb)->ack_seq;

                        } else {
                                SOCK_DEBUG(sk, "bad ack\n");
                                return 1;
                        }
                        break;

                case TCP_FIN_WAIT1:
                        if (tp->snd_una == tp->write_seq) {
                                sk->shutdown |= SEND_SHUTDOWN;
                                tcp_set_state(sk, TCP_FIN_WAIT2);
                                if (!sk->dead)
                                        sk->state_change(sk);
                                else
                                        tcp_reset_msl_timer(sk, TIME_CLOSE, sysctl_tcp_fin_timeout);
                        }
                        break;

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

                case TCP_LAST_ACK:
                        if (tp->snd_una == tp->write_seq) {
                                sk->shutdown = SHUTDOWN_MASK;
                                tcp_set_state(sk,TCP_CLOSE);
                                if (!sk->dead)
                                        sk->state_change(sk);
                                goto discard;
                        }
                        break;
                }
        } else
                goto discard;

step6:
        /* step 6: check the URG bit */
        tcp_urg(sk, th, len);

        /* step 7: process the segment text */
        switch (sk->state) {
        case TCP_CLOSE_WAIT:
        case TCP_CLOSING:
                if (!before(TCP_SKB_CB(skb)->seq, tp->fin_seq))
                        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->shutdown & RCV_SHUTDOWN) && sk->dead) {
                        if (after(TCP_SKB_CB(skb)->end_seq - th->fin, tp->rcv_nxt)) {
                                tcp_reset(sk, skb);
                                return 1;
                        }
                }
                
        case TCP_ESTABLISHED: 
                queued = tcp_data(skb, sk, len);

                /* This must be after tcp_data() does the skb_pull() to
                 * remove the header size from skb->len.
                 */
                tcp_measure_rcv_mss(sk, skb); 
                break;
        }

        tcp_data_snd_check(sk);
        tcp_ack_snd_check(sk);

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