Linux 2.4.0-test7pre1

[davej-history.git] / net / ipv4 / tcp_output.c

/*
 * INET         An implementation of the TCP/IP protocol suite for the LINUX
 *              operating system.  INET is implemented using the  BSD Socket
 *              interface as the means of communication with the user level.
 *
 *              Implementation of the Transmission Control Protocol(TCP).
 *
 * Version:     $Id: tcp_output.c,v 1.125 2000/08/09 11:59:04 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     :       Retransmit queue handled by TCP.
 *                              :       Fragmentation on mtu decrease
 *                              :       Segment collapse on retransmit
 *                              :       AF independence
 *
 *              Linus Torvalds  :       send_delayed_ack
 *              David S. Miller :       Charge memory using the right skb
 *                                      during syn/ack processing.
 *              David S. Miller :       Output engine completely rewritten.
 *              Andrea Arcangeli:       SYNACK carry ts_recent in tsecr.
 *              Cacophonix Gaul :       draft-minshall-nagle-01
 *              J Hadi Salim    :       ECN support
 *
 */

#include <net/tcp.h>

#include <linux/smp_lock.h>

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

static __inline__
void update_send_head(struct sock *sk, struct tcp_opt *tp, struct sk_buff *skb)
{
        tp->send_head = skb->next;
        if (tp->send_head == (struct sk_buff *) &sk->write_queue)
                tp->send_head = NULL;
        tp->snd_nxt = TCP_SKB_CB(skb)->end_seq;
        if (tp->packets_out++ == 0)
                tcp_reset_xmit_timer(sk, TCP_TIME_RETRANS, tp->rto);
}

/* SND.NXT, if window was not shrunk.
 * If window has been shrunk, what should we make? It is not clear at all.
 * Using SND.UNA we will fail to open window, SND.NXT is out of window. :-(
 * Anything in between SND.UNA...SND.UNA+SND.WND also can be already
 * invalid. OK, let's make this for now:
 */
static __inline__ __u32 tcp_acceptable_seq(struct sock *sk, struct tcp_opt *tp)
{
        if (!before(tp->snd_una+tp->snd_wnd, tp->snd_nxt))
                return tp->snd_nxt;
        else
                return tp->snd_una+tp->snd_wnd;
}

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

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

        return (__u16)mss;
}

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

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

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

static __inline__ void tcp_event_data_sent(struct tcp_opt *tp, struct sk_buff *skb)
{
        u32 now = tcp_time_stamp;

        if (!tp->packets_out && (s32)(now - tp->lsndtime) > tp->rto)
                tcp_cwnd_restart(tp);

        tp->lsndtime = now;

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

static __inline__ void tcp_event_ack_sent(struct sock *sk)
{
        struct tcp_opt *tp = &(sk->tp_pinfo.af_tcp);

        tcp_dec_quickack_mode(tp);
        tcp_clear_xmit_timer(sk, TCP_TIME_DACK);

        /* If we ever saw N>1 small segments from peer, it has
         * enough of send buffer to send N packets and does not nagle.
         * Hence, we may delay acks more aggresively.
         */
        if (tp->ack.rcv_small > tp->ack.rcv_thresh+1)
                tp->ack.rcv_thresh = tp->ack.rcv_small-1;
        tp->ack.rcv_small = 0;
}

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

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

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

#ifdef TCP_FORMAL_WINDOW
        if (new_win == 0) {
                /* If we advertise zero window, disable fast path. */
                tp->pred_flags = 0;
        } else if (cur_win == 0 && tp->pred_flags == 0 &&
                   skb_queue_len(&tp->out_of_order_queue) == 0 &&
                   !tp->urg_data) {
                /* If we open zero window, enable fast path.
                   Without this it will be open by the first data packet,
                   it is too late to merge checksumming to copy.
                 */
                tcp_fast_path_on(tp);
        }
#endif

        return new_win;
}


/* This routine actually transmits TCP packets queued in by
 * tcp_do_sendmsg().  This is used by both the initial
 * transmission and possible later retransmissions.
 * All SKB's seen here are completely headerless.  It is our
 * job to build the TCP header, and pass the packet down to
 * IP so it can do the same plus pass the packet off to the
 * device.
 *
 * We are working here with either a clone of the original
 * SKB, or a fresh unique copy made by the retransmit engine.
 */
int tcp_transmit_skb(struct sock *sk, struct sk_buff *skb)
{
        if(skb != NULL) {
                struct tcp_opt *tp = &(sk->tp_pinfo.af_tcp);
                struct tcp_skb_cb *tcb = TCP_SKB_CB(skb);
                int tcp_header_size = tp->tcp_header_len;
                struct tcphdr *th;
                int sysctl_flags;
                int err;

#define SYSCTL_FLAG_TSTAMPS     0x1
#define SYSCTL_FLAG_WSCALE      0x2
#define SYSCTL_FLAG_SACK        0x4

                sysctl_flags = 0;
                if (tcb->flags & TCPCB_FLAG_SYN) {
                        tcp_header_size = sizeof(struct tcphdr) + TCPOLEN_MSS;
                        if(sysctl_tcp_timestamps) {
                                tcp_header_size += TCPOLEN_TSTAMP_ALIGNED;
                                sysctl_flags |= SYSCTL_FLAG_TSTAMPS;
                        }
                        if(sysctl_tcp_window_scaling) {
                                tcp_header_size += TCPOLEN_WSCALE_ALIGNED;
                                sysctl_flags |= SYSCTL_FLAG_WSCALE;
                        }
                        if(sysctl_tcp_sack) {
                                sysctl_flags |= SYSCTL_FLAG_SACK;
                                if(!(sysctl_flags & SYSCTL_FLAG_TSTAMPS))
                                        tcp_header_size += TCPOLEN_SACKPERM_ALIGNED;
                        }
                } else if (tp->eff_sacks) {
                        /* A SACK is 2 pad bytes, a 2 byte header, plus
                         * 2 32-bit sequence numbers for each SACK block.
                         */
                        tcp_header_size += (TCPOLEN_SACK_BASE_ALIGNED +
                                            (tp->eff_sacks * TCPOLEN_SACK_PERBLOCK));
                }
                th = (struct tcphdr *) skb_push(skb, tcp_header_size);
                skb->h.th = th;
                skb_set_owner_w(skb, sk);

                /* Build TCP header and checksum it. */
                th->source              = sk->sport;
                th->dest                = sk->dport;
                th->seq                 = htonl(tcb->seq);
                th->ack_seq             = htonl(tp->rcv_nxt);
                *(((__u16 *)th) + 6)    = htons(((tcp_header_size >> 2) << 12) | tcb->flags);
                if (tcb->flags & TCPCB_FLAG_SYN) {
                        /* RFC1323: The window in SYN & SYN/ACK segments
                         * is never scaled.
                         */
                        th->window      = htons(tp->rcv_wnd);
                } else {
                        th->window      = htons(tcp_select_window(sk));
                }
                th->check               = 0;
                th->urg_ptr             = ntohs(tcb->urg_ptr);

                if (tcb->flags & TCPCB_FLAG_SYN) {
                        tcp_syn_build_options((__u32 *)(th + 1),
                                              tcp_advertise_mss(sk),
                                              (sysctl_flags & SYSCTL_FLAG_TSTAMPS),
                                              (sysctl_flags & SYSCTL_FLAG_SACK),
                                              (sysctl_flags & SYSCTL_FLAG_WSCALE),
                                              tp->rcv_wscale,
                                              tcb->when,
                                              tp->ts_recent);
                } else {
                        tcp_build_and_update_options((__u32 *)(th + 1),
                                                     tp, tcb->when);

                        TCP_ECN_send(sk, tp, skb, tcp_header_size);
                }
                tp->af_specific->send_check(sk, th, skb->len, skb);

                if (tcb->flags & TCPCB_FLAG_ACK)
                        tcp_event_ack_sent(sk);

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

                TCP_INC_STATS(TcpOutSegs);

                err = tp->af_specific->queue_xmit(skb);
                if (err <= 0)
                        return err;

                tcp_enter_cwr(tp);

                /* NET_XMIT_CN is special. It does not guarantee,
                 * that this packet is lost. It tells that device
                 * is about to start to drop packets or already
                 * drops some packets of the same priority and
                 * invokes us to send less aggressively.
                 */
                return err == NET_XMIT_CN ? 0 : err;
        }
        return -ENOBUFS;
#undef SYSCTL_FLAG_TSTAMPS
#undef SYSCTL_FLAG_WSCALE
#undef SYSCTL_FLAG_SACK
}


/* This is the main buffer sending routine. We queue the buffer
 * and decide whether to queue or transmit now.
 *
 * NOTE: probe0 timer is not checked, do not forget tcp_push_pending_frames,
 * otherwise socket can stall.
 */
void tcp_send_skb(struct sock *sk, struct sk_buff *skb, int force_queue, unsigned cur_mss)
{
        struct tcp_opt *tp = &(sk->tp_pinfo.af_tcp);

        /* Advance write_seq and place onto the write_queue. */
        tp->write_seq = TCP_SKB_CB(skb)->end_seq;
        __skb_queue_tail(&sk->write_queue, skb);
        tcp_charge_skb(sk, skb);

        if (!force_queue && tp->send_head == NULL && tcp_snd_test(tp, skb, cur_mss, 1)) {
                /* Send it out now. */
                TCP_SKB_CB(skb)->when = tcp_time_stamp;
                if (tcp_transmit_skb(sk, skb_clone(skb, GFP_KERNEL)) == 0) {
                        tp->snd_nxt = TCP_SKB_CB(skb)->end_seq;
                        tcp_minshall_update(tp, cur_mss, skb);
                        if (tp->packets_out++ == 0)
                                tcp_reset_xmit_timer(sk, TCP_TIME_RETRANS, tp->rto);
                        return;
                }
        }
        /* Queue it, remembering where we must start sending. */
        if (tp->send_head == NULL)
                tp->send_head = skb;
}

/* Function to create two new TCP segments.  Shrinks the given segment
 * to the specified size and appends a new segment with the rest of the
 * packet to the list.  This won't be called frequently, I hope. 
 * Remember, these are still headerless SKBs at this point.
 */
static int tcp_fragment(struct sock *sk, struct sk_buff *skb, u32 len)
{
        struct tcp_opt *tp = &sk->tp_pinfo.af_tcp;
        struct sk_buff *buff;
        int nsize = skb->len - len;
        u16 flags;

        /* Get a new skb... force flag on. */
        buff = tcp_alloc_skb(sk, nsize + MAX_TCP_HEADER + 15, GFP_ATOMIC);
        if (buff == NULL)
                return -ENOMEM; /* We'll just try again later. */
        tcp_charge_skb(sk, buff);

        /* Reserve space for headers. */
        skb_reserve(buff, MAX_TCP_HEADER);
                
        /* Correct the sequence numbers. */
        TCP_SKB_CB(buff)->seq = TCP_SKB_CB(skb)->seq + len;
        TCP_SKB_CB(buff)->end_seq = TCP_SKB_CB(skb)->end_seq;
        
        /* PSH and FIN should only be set in the second packet. */
        flags = TCP_SKB_CB(skb)->flags;
        TCP_SKB_CB(skb)->flags = flags & ~(TCPCB_FLAG_FIN | TCPCB_FLAG_PSH);
        if(flags & TCPCB_FLAG_URG) {
                u16 old_urg_ptr = TCP_SKB_CB(skb)->urg_ptr;

                /* Urgent data is always a pain in the ass. */
                if(old_urg_ptr > len) {
                        TCP_SKB_CB(skb)->flags &= ~(TCPCB_FLAG_URG);
                        TCP_SKB_CB(skb)->urg_ptr = 0;
                        TCP_SKB_CB(buff)->urg_ptr = old_urg_ptr - len;
                } else {
                        flags &= ~(TCPCB_FLAG_URG);
                }
        }
        if(!(flags & TCPCB_FLAG_URG))
                TCP_SKB_CB(buff)->urg_ptr = 0;
        TCP_SKB_CB(buff)->flags = flags;
        TCP_SKB_CB(buff)->sacked = TCP_SKB_CB(skb)->sacked&(TCPCB_LOST|TCPCB_EVER_RETRANS);
        if (TCP_SKB_CB(buff)->sacked&TCPCB_LOST) {
                tp->lost_out++;
                tp->left_out++;
        }

        /* Copy and checksum data tail into the new buffer. */
        buff->csum = csum_partial_copy_nocheck(skb->data + len, skb_put(buff, nsize),
                                               nsize, 0);

        /* This takes care of the FIN sequence number too. */
        TCP_SKB_CB(skb)->end_seq = TCP_SKB_CB(buff)->seq;
        skb_trim(skb, len);

        /* Rechecksum original buffer. */
        skb->csum = csum_partial(skb->data, skb->len, 0);

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

        /* Link BUFF into the send queue. */
        __skb_append(skb, buff);

        return 0;
}

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

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

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

   tp->pmtu_cookie is last pmtu, seen by this function.

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

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

   NOTE2. tp->pmtu_cookie and tp->mss_cache are READ ONLY outside
   this function.                       --ANK (980731)
 */

int tcp_sync_mss(struct sock *sk, u32 pmtu)
{
        struct tcp_opt *tp = &sk->tp_pinfo.af_tcp;
        int mss_now;

        /* Calculate base mss without TCP options:
           It is MMS_S - sizeof(tcphdr) of rfc1122
         */

        mss_now = pmtu - tp->af_specific->net_header_len - sizeof(struct tcphdr);

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

        /* Now subtract optional transport overhead */
        mss_now -= tp->ext_header_len;

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

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

        /* Bound mss with half of window */
        if (tp->max_window && mss_now > (tp->max_window>>1))
                mss_now = max((tp->max_window>>1), 68 - tp->tcp_header_len);

        /* And store cached results */
        tp->pmtu_cookie = pmtu;
        tp->mss_cache = mss_now;
        return mss_now;
}


/* This routine writes packets to the network.  It advances the
 * send_head.  This happens as incoming acks open up the remote
 * window for us.
 *
 * Returns 1, if no segments are in flight and we have queued segments, but
 * cannot send anything now because of SWS or another problem.
 */
int tcp_write_xmit(struct sock *sk)
{
        struct tcp_opt *tp = &(sk->tp_pinfo.af_tcp);
        unsigned int mss_now;

        /* If we are closed, the bytes will have to remain here.
         * In time closedown will finish, we empty the write queue and all
         * will be happy.
         */
        if(sk->state != TCP_CLOSE) {
                struct sk_buff *skb;
                int sent_pkts = 0;

                /* Account for SACKS, we may need to fragment due to this.
                 * It is just like the real MSS changing on us midstream.
                 * We also handle things correctly when the user adds some
                 * IP options mid-stream.  Silly to do, but cover it.
                 */
                mss_now = tcp_current_mss(sk); 

                while((skb = tp->send_head) &&
                      tcp_snd_test(tp, skb, mss_now, tcp_skb_is_last(sk, skb))) {
                        if (skb->len > mss_now) {
                                if (tcp_fragment(sk, skb, mss_now))
                                        break;
                        }

                        TCP_SKB_CB(skb)->when = tcp_time_stamp;
                        if (tcp_transmit_skb(sk, skb_clone(skb, GFP_ATOMIC)))
                                break;
                        /* Advance the send_head.  This one is sent out. */
                        update_send_head(sk, tp, skb);
                        tcp_minshall_update(tp, mss_now, skb);
                        sent_pkts = 1;
                }

                if (sent_pkts) {
                        tcp_cwnd_validate(sk, tp);
                        return 0;
                }

                return !tp->packets_out && tp->send_head;
        }
        return 0;
}

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

        /* Sometimes free_space can be < 0. */
        free_space = tcp_space(sk); 
        if (tp->window_clamp < mss)
                mss = tp->window_clamp; 

        if (free_space < (int)min(tp->window_clamp, tcp_full_space(sk)) / 2) {
                tp->ack.quick = 0;

                if (tcp_memory_pressure)
                        tp->rcv_ssthresh = min(tp->rcv_ssthresh, 4*tp->advmss);

                if (free_space < ((int) (mss/2)))
                        return 0;
        }

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

        /* Get the largest window that is a nice multiple of mss.
         * Window clamp already applied above.
         * If our current window offering is within 1 mss of the
         * free space we just keep it. This prevents the divide
         * and multiply from happening most of the time.
         * We also don't do any window rounding when the free space
         * is too small.
         */
        window = tp->rcv_wnd;
        if ((((int) window) <= (free_space - ((int) mss))) ||
            (((int) window) > free_space))
                window = (((unsigned int) free_space)/mss)*mss;

        return window;
}

/* Attempt to collapse two adjacent SKB's during retransmission. */
static void tcp_retrans_try_collapse(struct sock *sk, struct sk_buff *skb, int mss_now)
{
        struct tcp_opt *tp = &sk->tp_pinfo.af_tcp;
        struct sk_buff *next_skb = skb->next;

        /* The first test we must make is that neither of these two
         * SKB's are still referenced by someone else.
         */
        if(!skb_cloned(skb) && !skb_cloned(next_skb)) {
                int skb_size = skb->len, next_skb_size = next_skb->len;
                u16 flags = TCP_SKB_CB(skb)->flags;

                /* Punt if the first SKB has URG set. */
                if(flags & TCPCB_FLAG_URG)
                        return;
        
                /* Also punt if next skb has been SACK'd. */
                if(TCP_SKB_CB(next_skb)->sacked & TCPCB_SACKED_ACKED)
                        return;

                /* Next skb is out of window. */
                if (after(TCP_SKB_CB(next_skb)->end_seq, tp->snd_una+tp->snd_wnd))
                        return;

                /* Punt if not enough space exists in the first SKB for
                 * the data in the second, or the total combined payload
                 * would exceed the MSS.
                 */
                if ((next_skb_size > skb_tailroom(skb)) ||
                    ((skb_size + next_skb_size) > mss_now))
                        return;

                /* Ok.  We will be able to collapse the packet. */
                __skb_unlink(next_skb, next_skb->list);

                if(skb->len % 4) {
                        /* Must copy and rechecksum all data. */
                        memcpy(skb_put(skb, next_skb_size), next_skb->data, next_skb_size);
                        skb->csum = csum_partial(skb->data, skb->len, 0);
                } else {
                        /* Optimize, actually we could also combine next_skb->csum
                         * to skb->csum using a single add w/carry operation too.
                         */
                        skb->csum = csum_partial_copy_nocheck(next_skb->data,
                                                              skb_put(skb, next_skb_size),
                                                              next_skb_size, skb->csum);
                }
        
                /* Update sequence range on original skb. */
                TCP_SKB_CB(skb)->end_seq = TCP_SKB_CB(next_skb)->end_seq;

                /* Merge over control information. */
                flags |= TCP_SKB_CB(next_skb)->flags; /* This moves PSH/FIN etc. over */
                if(flags & TCPCB_FLAG_URG) {
                        u16 urgptr = TCP_SKB_CB(next_skb)->urg_ptr;
                        TCP_SKB_CB(skb)->urg_ptr = urgptr + skb_size;
                }
                TCP_SKB_CB(skb)->flags = flags;

                /* All done, get rid of second SKB and account for it so
                 * packet counting does not break.
                 */
                TCP_SKB_CB(skb)->sacked |= TCP_SKB_CB(next_skb)->sacked&TCPCB_EVER_RETRANS;
                if (TCP_SKB_CB(next_skb)->sacked&TCPCB_SACKED_RETRANS)
                        tp->retrans_out--;
                if (TCP_SKB_CB(next_skb)->sacked&TCPCB_LOST) {
                        tp->lost_out--;
                        tp->left_out--;
                }
                if (!tp->sack_ok && tp->sacked_out) {
                        /* Reno case is special. Sigh... */
                        tp->sacked_out--;
                        tp->left_out--;
                }
                tcp_free_skb(sk, next_skb);
                tp->packets_out--;
        }
}

/* Do a simple retransmit without using the backoff mechanisms in
 * tcp_timer. This is used for path mtu discovery. 
 * The socket is already locked here.
 */ 
void tcp_simple_retransmit(struct sock *sk)
{
        struct tcp_opt *tp = &(sk->tp_pinfo.af_tcp);
        struct sk_buff *skb;
        unsigned int mss = tcp_current_mss(sk);
        int lost = 0;

        for_retrans_queue(skb, sk, tp) {
                if (skb->len > mss && 
                    !(TCP_SKB_CB(skb)->sacked&TCPCB_SACKED_ACKED)) {
                        if (TCP_SKB_CB(skb)->sacked&TCPCB_SACKED_RETRANS) {
                                TCP_SKB_CB(skb)->sacked &= ~TCPCB_SACKED_RETRANS;
                                tp->retrans_out--;
                        }
                        if (!(TCP_SKB_CB(skb)->sacked&TCPCB_LOST)) {
                                TCP_SKB_CB(skb)->sacked |= TCPCB_LOST;
                                tp->lost_out++;
                                lost = 1;
                        }
                }
        }

        if (!lost)
                return;

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

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

/* This retransmits one SKB.  Policy decisions and retransmit queue
 * state updates are done by the caller.  Returns non-zero if an
 * error occurred which prevented the send.
 */
int tcp_retransmit_skb(struct sock *sk, struct sk_buff *skb)
{
        struct tcp_opt *tp = &(sk->tp_pinfo.af_tcp);
        unsigned int cur_mss = tcp_current_mss(sk);
        int err;

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

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

                /* New SKB created, account for it. */
                tp->packets_out++;
        }

        /* Collapse two adjacent packets if worthwhile and we can. */
        if(!(TCP_SKB_CB(skb)->flags & TCPCB_FLAG_SYN) &&
           (skb->len < (cur_mss >> 1)) &&
           (skb->next != tp->send_head) &&
           (skb->next != (struct sk_buff *)&sk->write_queue) &&
           (sysctl_tcp_retrans_collapse != 0))
                tcp_retrans_try_collapse(sk, skb, cur_mss);

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

        /* Some Solaris stacks overoptimize and ignore the FIN on a
         * retransmit when old data is attached.  So strip it off
         * since it is cheap to do so and saves bytes on the network.
         */
        if(skb->len > 0 &&
           (TCP_SKB_CB(skb)->flags & TCPCB_FLAG_FIN) &&
           tp->snd_una == (TCP_SKB_CB(skb)->end_seq - 1)) {
                TCP_SKB_CB(skb)->seq = TCP_SKB_CB(skb)->end_seq - 1;
                skb_trim(skb, 0);
                skb->csum = 0;
        }

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

        err = tcp_transmit_skb(sk, (skb_cloned(skb) ?
                                    skb_copy(skb, GFP_ATOMIC):
                                    skb_clone(skb, GFP_ATOMIC)));

        if (err == 0) {
                /* Update global TCP statistics. */
                TCP_INC_STATS(TcpRetransSegs);

#if FASTRETRANS_DEBUG > 0
                if (TCP_SKB_CB(skb)->sacked&TCPCB_SACKED_RETRANS) {
                        if (net_ratelimit())
                                printk(KERN_DEBUG "retrans_out leaked.\n");
                }
#endif
                TCP_SKB_CB(skb)->sacked |= TCPCB_RETRANS;
                tp->retrans_out++;

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

                tp->undo_retrans++;

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

/* This gets called after a retransmit timeout, and the initially
 * retransmitted data is acknowledged.  It tries to continue
 * resending the rest of the retransmit queue, until either
 * we've sent it all or the congestion window limit is reached.
 * If doing SACK, the first ACK which comes back for a timeout
 * based retransmit packet might feed us FACK information again.
 * If so, we use it to avoid unnecessarily retransmissions.
 */
void tcp_xmit_retransmit_queue(struct sock *sk)
{
        struct tcp_opt *tp = &(sk->tp_pinfo.af_tcp);
        struct sk_buff *skb;
        int packet_cnt = tp->lost_out;

        /* First pass: retransmit lost packets. */
        if (packet_cnt) {
                for_retrans_queue(skb, sk, tp) {
                        __u8 sacked = TCP_SKB_CB(skb)->sacked;

                        if (tcp_packets_in_flight(tp) >= tp->snd_cwnd)
                                return;

                        if (sacked&TCPCB_LOST) {
                                if (!(sacked&(TCPCB_SACKED_ACKED|TCPCB_SACKED_RETRANS))) {
                                        if (tcp_retransmit_skb(sk, skb))
                                                return;
                                        if (tp->ca_state != TCP_CA_Loss)
                                                NET_INC_STATS_BH(TCPFastRetrans);
                                        else
                                                NET_INC_STATS_BH(TCPSlowStartRetrans);

                                        if (skb == skb_peek(&sk->write_queue))
                                                tcp_reset_xmit_timer(sk, TCP_TIME_RETRANS, tp->rto);
                                }

                                if (--packet_cnt <= 0)
                                        break;
                        }
                }
        }

        /* OK, demanded retransmission is finished. */

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

        /* No forward retransmissions in Reno are possible. */
        if (!tp->sack_ok)
                return;

        /* Yeah, we have to make difficult choice between forward transmission
         * and retransmission... Both ways have their merits...
         *
         * For now we do not retrnamsit anything, while we have some new
         * segments to send.
         */

        if (tcp_may_send_now(sk, tp))
                return;

        packet_cnt = 0;

        for_retrans_queue(skb, sk, tp) {
                if(++packet_cnt > tp->fackets_out)
                        break;

                if (tcp_packets_in_flight(tp) >= tp->snd_cwnd)
                        break;

                if(TCP_SKB_CB(skb)->sacked & TCPCB_TAGBITS)
                        continue;

                /* Ok, retransmit it. */
                if(tcp_retransmit_skb(sk, skb))
                        break;

                if (skb == skb_peek(&sk->write_queue))
                        tcp_reset_xmit_timer(sk, TCP_TIME_RETRANS, tp->rto);

                NET_INC_STATS_BH(TCPForwardRetrans);
        }
}


/* Send a fin.  The caller locks the socket for us.  This cannot be
 * allowed to fail queueing a FIN frame under any circumstances.
 */
void tcp_send_fin(struct sock *sk)
{
        struct tcp_opt *tp = &(sk->tp_pinfo.af_tcp);    
        struct sk_buff *skb = skb_peek_tail(&sk->write_queue);
        unsigned int mss_now;
        
        /* Optimization, tack on the FIN if we have a queue of
         * unsent frames.  But be careful about outgoing SACKS
         * and IP options.
         */
        mss_now = tcp_current_mss(sk); 

        /* Please, find seven differences of 2.3.33 and loook
         * what I broke here. 8) --ANK
         */

        if(tp->send_head != NULL) {
                /* tcp_write_xmit() takes care of the rest. */
                TCP_SKB_CB(skb)->flags |= TCPCB_FLAG_FIN;
                TCP_SKB_CB(skb)->end_seq++;
                tp->write_seq++;

                /* Special case to avoid Nagle bogosity.  If this
                 * segment is the last segment, and it was queued
                 * due to Nagle/SWS-avoidance, send it out now.
                 */
                if(tp->send_head == skb &&
                   !after(tp->write_seq, tp->snd_una + tp->snd_wnd)) {
                        TCP_SKB_CB(skb)->when = tcp_time_stamp;
                        if (!tcp_transmit_skb(sk, skb_clone(skb, GFP_KERNEL)))
                                update_send_head(sk, tp, skb);
                        else
                                tcp_check_probe_timer(sk, tp);
                }
        } else {
                /* Socket is locked, keep trying until memory is available. */
                for (;;) {
                        skb = alloc_skb(MAX_TCP_HEADER + 15, GFP_KERNEL);
                        if (skb)
                                break;
                        current->policy |= SCHED_YIELD;
                        schedule();
                }

                /* Reserve space for headers and prepare control bits. */
                skb_reserve(skb, MAX_TCP_HEADER);
                skb->csum = 0;
                TCP_SKB_CB(skb)->flags = (TCPCB_FLAG_ACK | TCPCB_FLAG_FIN);
                TCP_SKB_CB(skb)->sacked = 0;
                TCP_SKB_CB(skb)->urg_ptr = 0;

                /* FIN eats a sequence byte, write_seq advanced by tcp_send_skb(). */
                TCP_SKB_CB(skb)->seq = tp->write_seq;
                TCP_SKB_CB(skb)->end_seq = TCP_SKB_CB(skb)->seq + 1;
                tcp_send_skb(sk, skb, 0, mss_now);
                __tcp_push_pending_frames(sk, tp, mss_now);
        }
}

/* We get here when a process closes a file descriptor (either due to
 * an explicit close() or as a byproduct of exit()'ing) and there
 * was unread data in the receive queue.  This behavior is recommended
 * by draft-ietf-tcpimpl-prob-03.txt section 3.10.  -DaveM
 */
void tcp_send_active_reset(struct sock *sk, int priority)
{
        struct tcp_opt *tp = &(sk->tp_pinfo.af_tcp);
        struct sk_buff *skb;

        /* NOTE: No TCP options attached and we never retransmit this. */
        skb = alloc_skb(MAX_TCP_HEADER + 15, priority);
        if (!skb) {
                NET_INC_STATS(TCPAbortFailed);
                return;
        }

        /* Reserve space for headers and prepare control bits. */
        skb_reserve(skb, MAX_TCP_HEADER);
        skb->csum = 0;
        TCP_SKB_CB(skb)->flags = (TCPCB_FLAG_ACK | TCPCB_FLAG_RST);
        TCP_SKB_CB(skb)->sacked = 0;
        TCP_SKB_CB(skb)->urg_ptr = 0;

        /* Send it off. */
        TCP_SKB_CB(skb)->seq = tcp_acceptable_seq(sk, tp);
        TCP_SKB_CB(skb)->end_seq = TCP_SKB_CB(skb)->seq;
        TCP_SKB_CB(skb)->when = tcp_time_stamp;
        if (tcp_transmit_skb(sk, skb))
                NET_INC_STATS(TCPAbortFailed);
}

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

        skb = skb_peek(&sk->write_queue);
        if (skb == NULL || !(TCP_SKB_CB(skb)->flags&TCPCB_FLAG_SYN)) {
                printk(KERN_DEBUG "tcp_send_synack: wrong queue state\n");
                return -EFAULT;
        }
        if (!(TCP_SKB_CB(skb)->flags&TCPCB_FLAG_ACK)) {
                if (skb_cloned(skb)) {
                        struct sk_buff *nskb = skb_copy(skb, GFP_ATOMIC);
                        if (nskb == NULL)
                                return -ENOMEM;
                        __skb_unlink(skb, &sk->write_queue);
                        __skb_queue_head(&sk->write_queue, nskb);
                        tcp_free_skb(sk, skb);
                        tcp_charge_skb(sk, nskb);
                        skb = nskb;
                }

                TCP_SKB_CB(skb)->flags |= TCPCB_FLAG_ACK;
                TCP_ECN_send_synack(&sk->tp_pinfo.af_tcp, skb);
        }
        TCP_SKB_CB(skb)->when = tcp_time_stamp;
        return tcp_transmit_skb(sk, skb_clone(skb, GFP_ATOMIC));
}

/*
 * Prepare a SYN-ACK.
 */
struct sk_buff * tcp_make_synack(struct sock *sk, struct dst_entry *dst,
                                 struct open_request *req)
{
        struct tcp_opt *tp = &(sk->tp_pinfo.af_tcp);
        struct tcphdr *th;
        int tcp_header_size;
        struct sk_buff *skb;

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

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

        skb->dst = dst_clone(dst);

        tcp_header_size = (sizeof(struct tcphdr) + TCPOLEN_MSS +
                           (req->tstamp_ok ? TCPOLEN_TSTAMP_ALIGNED : 0) +
                           (req->wscale_ok ? TCPOLEN_WSCALE_ALIGNED : 0) +
                           /* SACK_PERM is in the place of NOP NOP of TS */
                           ((req->sack_ok && !req->tstamp_ok) ? TCPOLEN_SACKPERM_ALIGNED : 0));
        skb->h.th = th = (struct tcphdr *) skb_push(skb, tcp_header_size);

        memset(th, 0, sizeof(struct tcphdr));
        th->syn = 1;
        th->ack = 1;
        TCP_ECN_make_synack(req, th);
        th->source = sk->sport;
        th->dest = req->rmt_port;
        TCP_SKB_CB(skb)->seq = req->snt_isn;
        TCP_SKB_CB(skb)->end_seq = TCP_SKB_CB(skb)->seq + 1;
        th->seq = htonl(TCP_SKB_CB(skb)->seq);
        th->ack_seq = htonl(req->rcv_isn + 1);
        if (req->rcv_wnd == 0) { /* ignored for retransmitted syns */
                __u8 rcv_wscale; 
                /* Set this up on the first call only */
                req->window_clamp = tp->window_clamp ? : dst->window;
                /* tcp_full_space because it is guaranteed to be the first packet */
                tcp_select_initial_window(tcp_full_space(sk), 
                        dst->advmss - (req->tstamp_ok ? TCPOLEN_TSTAMP_ALIGNED : 0),
                        &req->rcv_wnd,
                        &req->window_clamp,
                        req->wscale_ok,
                        &rcv_wscale);
                req->rcv_wscale = rcv_wscale; 
        }

        /* RFC1323: The window in SYN & SYN/ACK segments is never scaled. */
        th->window = htons(req->rcv_wnd);

        TCP_SKB_CB(skb)->when = tcp_time_stamp;
        tcp_syn_build_options((__u32 *)(th + 1), dst->advmss, req->tstamp_ok,
                              req->sack_ok, req->wscale_ok, req->rcv_wscale,
                              TCP_SKB_CB(skb)->when,
                              req->ts_recent);

        skb->csum = 0;
        th->doff = (tcp_header_size >> 2);
        TCP_INC_STATS(TcpOutSegs);
        return skb;
}

int tcp_connect(struct sock *sk, struct sk_buff *buff)
{
        struct dst_entry *dst = __sk_dst_get(sk);
        struct tcp_opt *tp = &(sk->tp_pinfo.af_tcp);

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

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

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

        if (!tp->window_clamp)
                tp->window_clamp = dst->window;
        tp->advmss = dst->advmss;
        tcp_initialize_rcv_mss(sk);

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

        tp->rcv_ssthresh = tp->rcv_wnd;

        /* Socket identity change complete, no longer
         * in TCP_CLOSE, so enter ourselves into the
         * hash tables.
         */
        tcp_set_state(sk,TCP_SYN_SENT);
        if (tp->af_specific->hash_connecting(sk))
                goto err_out;

        sk->err = 0;
        sk->done = 0;
        tp->snd_wnd = 0;
        tcp_init_wl(tp, tp->write_seq, 0);
        tp->snd_una = tp->write_seq;
        tp->snd_sml = tp->write_seq;
        tp->rcv_nxt = 0;
        tp->rcv_wup = 0;
        tp->copied_seq = 0;

        tp->rto = TCP_TIMEOUT_INIT;
        tcp_init_xmit_timers(sk);
        tp->retransmits = 0;
        tcp_clear_retrans(tp);

        TCP_SKB_CB(buff)->flags = TCPCB_FLAG_SYN;
        TCP_ECN_send_syn(tp, buff);
        TCP_SKB_CB(buff)->sacked = 0;
        TCP_SKB_CB(buff)->urg_ptr = 0;
        buff->csum = 0;
        TCP_SKB_CB(buff)->seq = tp->write_seq++;
        TCP_SKB_CB(buff)->end_seq = tp->write_seq;
        tp->snd_nxt = tp->write_seq;
        tp->pushed_seq = tp->write_seq;

        /* Send it off. */
        TCP_SKB_CB(buff)->when = tcp_time_stamp;
        tp->retrans_stamp = TCP_SKB_CB(buff)->when;
        __skb_queue_tail(&sk->write_queue, buff);
        tcp_charge_skb(sk, buff);
        tp->packets_out++;
        tcp_transmit_skb(sk, skb_clone(buff, GFP_KERNEL));
        TCP_INC_STATS(TcpActiveOpens);

        /* Timer for repeating the SYN until an answer. */
        tcp_reset_xmit_timer(sk, TCP_TIME_RETRANS, tp->rto);
        return 0;

err_out:
        tcp_set_state(sk,TCP_CLOSE);
        kfree_skb(buff);
        return -EADDRNOTAVAIL;
}

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

        if (ato > TCP_DELACK_MIN) {
                int max_ato = (tp->ack.pingpong || tp->ack.rcv_small) ?
                        TCP_DELACK_MAX : (HZ/2);

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

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

                        if (rtt < max_ato)
                                max_ato = rtt;
                }

                ato = min(ato, max_ato);
        }

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

        /* Use new timeout only if there wasn't a older one earlier. */
        if (tp->ack.pending&2) {
                /* If delack timer was blocked or is about to expire,
                 * send ACK now.
                 */
                if (tp->ack.blocked || time_before_eq(tp->ack.timeout, jiffies+(ato>>2))) {
                        tcp_send_ack(sk);
                        return;
                }

                if (!time_before(timeout, tp->ack.timeout))
                        timeout = tp->ack.timeout;
        }
        tp->ack.pending = 3;
        tp->ack.timeout = timeout;
        if (!mod_timer(&tp->delack_timer, timeout))
                sock_hold(sk);

#ifdef TCP_FORMAL_WINDOW
        /* Explanation. Header prediction path does not handle
         * case of zero window. If we send ACK immediately, pred_flags
         * are reset when sending ACK. If rcv_nxt is advanced and
         * ack is not sent, than delayed ack is scheduled.
         * Hence, it is the best place to check for zero window.
         */
        if (tp->pred_flags) {
                if (tcp_receive_window(tp) == 0)
                        tp->pred_flags = 0;
        } else {
                if (skb_queue_len(&tp->out_of_order_queue) == 0 &&
                    !tp->urg_data)
                        tcp_fast_path_on(tp);
        }
#endif
}

/* This routine sends an ack and also updates the window. */
void tcp_send_ack(struct sock *sk)
{
        /* If we have been reset, we may not send again. */
        if(sk->state != TCP_CLOSE) {
                struct tcp_opt *tp = &(sk->tp_pinfo.af_tcp);
                struct sk_buff *buff;

                /* We are not putting this on the write queue, so
                 * tcp_transmit_skb() will set the ownership to this
                 * sock.
                 */
                buff = alloc_skb(MAX_TCP_HEADER + 15, GFP_ATOMIC);
                if (buff == NULL) {
                        tcp_schedule_ack(tp);
                        tp->ack.ato = TCP_ATO_MIN;
                        tcp_reset_xmit_timer(sk, TCP_TIME_DACK, TCP_DELACK_MAX);
                        return;
                }

                /* Reserve space for headers and prepare control bits. */
                skb_reserve(buff, MAX_TCP_HEADER);
                buff->csum = 0;
                TCP_SKB_CB(buff)->flags = TCPCB_FLAG_ACK;
                TCP_SKB_CB(buff)->sacked = 0;
                TCP_SKB_CB(buff)->urg_ptr = 0;

                /* Send it off, this clears delayed acks for us. */
                TCP_SKB_CB(buff)->seq = TCP_SKB_CB(buff)->end_seq = tcp_acceptable_seq(sk, tp);
                TCP_SKB_CB(buff)->when = tcp_time_stamp;
                tcp_transmit_skb(sk, buff);
        }
}

/* This routine sends a packet with an out of date sequence
 * number. It assumes the other end will try to ack it.
 */
static int tcp_xmit_probe_skb(struct sock *sk)
{
        struct tcp_opt *tp = &(sk->tp_pinfo.af_tcp);
        struct sk_buff *skb;

        /* We don't queue it, tcp_transmit_skb() sets ownership. */
        skb = alloc_skb(MAX_TCP_HEADER + 15, GFP_ATOMIC);
        if (skb == NULL) 
                return -1;

        /* Reserve space for headers and set control bits. */
        skb_reserve(skb, MAX_TCP_HEADER);
        skb->csum = 0;
        TCP_SKB_CB(skb)->flags = TCPCB_FLAG_ACK;
        TCP_SKB_CB(skb)->sacked = 0;
        TCP_SKB_CB(skb)->urg_ptr = 0;

        /* Use a previous sequence.  This should cause the other
         * end to send an ack.  Don't queue or clone SKB, just
         * send it.
         */
        TCP_SKB_CB(skb)->seq = tp->snd_una - 1;
        TCP_SKB_CB(skb)->end_seq = TCP_SKB_CB(skb)->seq;
        TCP_SKB_CB(skb)->when = tcp_time_stamp;
        return tcp_transmit_skb(sk, skb);
}

int tcp_write_wakeup(struct sock *sk)
{
        if (sk->state != TCP_CLOSE) {
                struct tcp_opt *tp = &(sk->tp_pinfo.af_tcp);
                struct sk_buff *skb;

                if ((skb = tp->send_head) != NULL &&
                    before(TCP_SKB_CB(skb)->seq, tp->snd_una+tp->snd_wnd)) {
                        int err;
                        int mss = tcp_current_mss(sk);
                        int seg_size = tp->snd_una+tp->snd_wnd-TCP_SKB_CB(skb)->seq;

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

                        /* We are probing the opening of a window
                         * but the window size is != 0
                         * must have been a result SWS avoidance ( sender )
                         */
                        if (seg_size < TCP_SKB_CB(skb)->end_seq - TCP_SKB_CB(skb)->seq ||
                            skb->len > mss) {
                                seg_size = min(seg_size, mss);
                                TCP_SKB_CB(skb)->flags |= TCPCB_FLAG_PSH;
                                if (tcp_fragment(sk, skb, seg_size))
                                        return -1;
                        }
                        TCP_SKB_CB(skb)->flags |= TCPCB_FLAG_PSH;
                        TCP_SKB_CB(skb)->when = tcp_time_stamp;
                        err = tcp_transmit_skb(sk, skb_clone(skb, GFP_ATOMIC));
                        if (!err) {
                                update_send_head(sk, tp, skb);
                        }
                        return err;
                } else {
                        return tcp_xmit_probe_skb(sk);
                }
        }
        return -1;
}

/* A window probe timeout has occurred.  If window is not closed send
 * a partial packet else a zero probe.
 */
void tcp_send_probe0(struct sock *sk)
{
        struct tcp_opt *tp = &(sk->tp_pinfo.af_tcp);
        int err;

        err = tcp_write_wakeup(sk);

        if (tp->packets_out || !tp->send_head) {
                /* Cancel probe timer, if it is not required. */
                tp->probes_out = 0;
                tp->backoff = 0;
                return;
        }

        if (err <= 0) {
                tp->backoff++;
                tp->probes_out++;
                tcp_reset_xmit_timer (sk, TCP_TIME_PROBE0, 
                                      min(tp->rto << tp->backoff, TCP_RTO_MAX));
        } else {
                /* If packet was not sent due to local congestion,
                 * do not backoff and do not remember probes_out.
                 * Let local senders to fight for local resources.
                 *
                 * Use accumulated backoff yet.
                 */
                if (!tp->probes_out)
                        tp->probes_out=1;
                tcp_reset_xmit_timer (sk, TCP_TIME_PROBE0, 
                                      min(tp->rto << tp->backoff, TCP_RESOURCE_PROBE_INTERVAL));
        }
}