[TCP]: Add pluggable congestion control algorithm infrastructure.

[linux-2.6] / 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 1.146 2002/02/01 22:01:04 davem Exp $
 *
 * Authors:     Ross Biro
 *              Fred N. van Kempen, <waltje@uWalt.NL.Mugnet.ORG>
 *              Mark Evans, <evansmp@uhura.aston.ac.uk>
 *              Corey Minyard <wf-rch!minyard@relay.EU.net>
 *              Florian La Roche, <flla@stud.uni-sb.de>
 *              Charles Hedrick, <hedrick@klinzhai.rutgers.edu>
 *              Linus Torvalds, <torvalds@cs.helsinki.fi>
 *              Alan Cox, <gw4pts@gw4pts.ampr.org>
 *              Matthew Dillon, <dillon@apollo.west.oic.com>
 *              Arnt Gulbrandsen, <agulbra@nvg.unit.no>
 *              Jorge Cwik, <jorge@laser.satlink.net>
 */

/*
 * Changes:     Pedro Roque     :       Retransmit queue handled by TCP.
 *                              :       Fragmentation on mtu decrease
 *                              :       Segment collapse on retransmit
 *                              :       AF independence
 *
 *              Linus Torvalds  :       send_delayed_ack
 *              David S. Miller :       Charge memory using the right skb
 *                                      during syn/ack processing.
 *              David S. Miller :       Output engine completely rewritten.
 *              Andrea Arcangeli:       SYNACK carry ts_recent in tsecr.
 *              Cacophonix Gaul :       draft-minshall-nagle-01
 *              J Hadi Salim    :       ECN support
 *
 */

#include <net/tcp.h>

#include <linux/compiler.h>
#include <linux/module.h>
#include <linux/smp_lock.h>

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

/* This limits the percentage of the congestion window which we
 * will allow a single TSO frame to consume.  Building TSO frames
 * which are too large can cause TCP streams to be bursty.
 */
int sysctl_tcp_tso_win_divisor = 8;

static inline void update_send_head(struct sock *sk, struct tcp_sock *tp,
                                    struct sk_buff *skb)
{
        sk->sk_send_head = skb->next;
        if (sk->sk_send_head == (struct sk_buff *)&sk->sk_write_queue)
                sk->sk_send_head = NULL;
        tp->snd_nxt = TCP_SKB_CB(skb)->end_seq;
        tcp_packets_out_inc(sk, tp, skb);
}

/* SND.NXT, if window was not shrunk.
 * If window has been shrunk, what should we make? It is not clear at all.
 * Using SND.UNA we will fail to open window, SND.NXT is out of window. :-(
 * Anything in between SND.UNA...SND.UNA+SND.WND also can be already
 * invalid. OK, let's make this for now:
 */
static inline __u32 tcp_acceptable_seq(struct sock *sk, struct tcp_sock *tp)
{
        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 overridden via information stored in routing table.
 * 5. Value 65535 for MSS is valid in IPv6 and means "as large as possible,
 *    probably even Jumbo".
 */
static __u16 tcp_advertise_mss(struct sock *sk)
{
        struct tcp_sock *tp = tcp_sk(sk);
        struct dst_entry *dst = __sk_dst_get(sk);
        int mss = tp->advmss;

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

        return (__u16)mss;
}

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

        tcp_ca_event(tp, CA_EVENT_CWND_RESTART);

        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_sock *tp,
                                       struct sk_buff *skb, struct sock *sk)
{
        u32 now = tcp_time_stamp;

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

        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_sock *tp = tcp_sk(sk);

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

/* Determine a window scaling and initial window to offer.
 * Based on the assumption that the given amount of space
 * will be offered. Store the results in the tp structure.
 * NOTE: for smooth operation initial space offering should
 * be a multiple of mss if possible. We assume here that mss >= 1.
 * This MUST be enforced by all callers.
 */
void tcp_select_initial_window(int __space, __u32 mss,
                               __u32 *rcv_wnd, __u32 *window_clamp,
                               int wscale_ok, __u8 *rcv_wscale)
{
        unsigned int space = (__space < 0 ? 0 : __space);

        /* If no clamp set the clamp to the max possible scaled window */
        if (*window_clamp == 0)
                (*window_clamp) = (65535 << 14);
        space = min(*window_clamp, space);

        /* Quantize space offering to a multiple of mss if possible. */
        if (space > mss)
                space = (space / mss) * mss;

        /* NOTE: offering an initial window larger than 32767
         * will break some buggy TCP stacks. We try to be nice.
         * If we are not window scaling, then this truncates
         * our initial window offering to 32k. There should also
         * be a sysctl option to stop being nice.
         */
        (*rcv_wnd) = min(space, MAX_TCP_WINDOW);
        (*rcv_wscale) = 0;
        if (wscale_ok) {
                /* Set window scaling on max possible window
                 * See RFC1323 for an explanation of the limit to 14 
                 */
                space = max_t(u32, sysctl_tcp_rmem[2], sysctl_rmem_max);
                while (space > 65535 && (*rcv_wscale) < 14) {
                        space >>= 1;
                        (*rcv_wscale)++;
                }
        }

        /* Set initial window to value enough for senders,
         * following RFC1414. Senders, not following this RFC,
         * will be satisfied with 2.
         */
        if (mss > (1<<*rcv_wscale)) {
                int init_cwnd = 4;
                if (mss > 1460*3)
                        init_cwnd = 2;
                else if (mss > 1460)
                        init_cwnd = 3;
                if (*rcv_wnd > init_cwnd*mss)
                        *rcv_wnd = init_cwnd*mss;
        }

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

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

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

        /* Make sure we do not exceed the maximum possible
         * scaled window.
         */
        if (!tp->rx_opt.rcv_wscale)
                new_win = min(new_win, MAX_TCP_WINDOW);
        else
                new_win = min(new_win, (65535U << tp->rx_opt.rcv_wscale));

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

        /* If we advertise zero window, disable fast path. */
        if (new_win == 0)
                tp->pred_flags = 0;

        return new_win;
}


/* This routine actually transmits TCP packets queued in by
 * tcp_do_sendmsg().  This is used by both the initial
 * transmission and possible later retransmissions.
 * All SKB's seen here are completely headerless.  It is our
 * job to build the TCP header, and pass the packet down to
 * IP so it can do the same plus pass the packet off to the
 * device.
 *
 * We are working here with either a clone of the original
 * SKB, or a fresh unique copy made by the retransmit engine.
 */
static int tcp_transmit_skb(struct sock *sk, struct sk_buff *skb)
{
        if (skb != NULL) {
                struct inet_sock *inet = inet_sk(sk);
                struct tcp_sock *tp = tcp_sk(sk);
                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;

                BUG_ON(!tcp_skb_pcount(skb));

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

                /* If congestion control is doing timestamping */
                if (tp->ca_ops->rtt_sample)
                        do_gettimeofday(&skb->stamp);

                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->rx_opt.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->rx_opt.eff_sacks * TCPOLEN_SACK_PERBLOCK));
                }
                
                if (tcp_packets_in_flight(tp) == 0)
                        tcp_ca_event(tp, CA_EVENT_TX_START);

                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              = inet->sport;
                th->dest                = inet->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             = 0;

                if (tp->urg_mode &&
                    between(tp->snd_up, tcb->seq+1, tcb->seq+0xFFFF)) {
                        th->urg_ptr             = htons(tp->snd_up-tcb->seq);
                        th->urg                 = 1;
                }

                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->rx_opt.rcv_wscale,
                                              tcb->when,
                                              tp->rx_opt.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, sk);

                TCP_INC_STATS(TCP_MIB_OUTSEGS);

                err = tp->af_specific->queue_xmit(skb, 0);
                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 routine just queue's the buffer 
 *
 * NOTE: probe0 timer is not checked, do not forget tcp_push_pending_frames,
 * otherwise socket can stall.
 */
static void tcp_queue_skb(struct sock *sk, struct sk_buff *skb)
{
        struct tcp_sock *tp = tcp_sk(sk);

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

        /* Queue it, remembering where we must start sending. */
        if (sk->sk_send_head == NULL)
                sk->sk_send_head = skb;
}

static inline void tcp_tso_set_push(struct sk_buff *skb)
{
        /* Force push to be on for any TSO frames to workaround
         * problems with busted implementations like Mac OS-X that
         * hold off socket receive wakeups until push is seen.
         */
        if (tcp_skb_pcount(skb) > 1)
                TCP_SKB_CB(skb)->flags |= TCPCB_FLAG_PSH;
}

/* Send _single_ skb sitting at the send head. This function requires
 * true push pending frames to setup probe timer etc.
 */
void tcp_push_one(struct sock *sk, unsigned cur_mss)
{
        struct tcp_sock *tp = tcp_sk(sk);
        struct sk_buff *skb = sk->sk_send_head;

        if (tcp_snd_test(sk, skb, cur_mss, TCP_NAGLE_PUSH)) {
                /* Send it out now. */
                TCP_SKB_CB(skb)->when = tcp_time_stamp;
                tcp_tso_set_push(skb);
                if (!tcp_transmit_skb(sk, skb_clone(skb, sk->sk_allocation))) {
                        sk->sk_send_head = NULL;
                        tp->snd_nxt = TCP_SKB_CB(skb)->end_seq;
                        tcp_packets_out_inc(sk, tp, skb);
                        return;
                }
        }
}

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

        if (skb->len <= tp->mss_cache_std ||
            !(sk->sk_route_caps & NETIF_F_TSO)) {
                /* Avoid the costly divide in the normal
                 * non-TSO case.
                 */
                skb_shinfo(skb)->tso_segs = 1;
                skb_shinfo(skb)->tso_size = 0;
        } else {
                unsigned int factor;

                factor = skb->len + (tp->mss_cache_std - 1);
                factor /= tp->mss_cache_std;
                skb_shinfo(skb)->tso_segs = factor;
                skb_shinfo(skb)->tso_size = tp->mss_cache_std;
        }
}

/* 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_sock *tp = tcp_sk(sk);
        struct sk_buff *buff;
        int nsize;
        u16 flags;

        nsize = skb_headlen(skb) - len;
        if (nsize < 0)
                nsize = 0;

        if (skb_cloned(skb) &&
            skb_is_nonlinear(skb) &&
            pskb_expand_head(skb, 0, 0, GFP_ATOMIC))
                return -ENOMEM;

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

        /* Correct the sequence numbers. */
        TCP_SKB_CB(buff)->seq = TCP_SKB_CB(skb)->seq + len;
        TCP_SKB_CB(buff)->end_seq = TCP_SKB_CB(skb)->end_seq;
        TCP_SKB_CB(skb)->end_seq = TCP_SKB_CB(buff)->seq;

        /* PSH and FIN should only be set in the second packet. */
        flags = TCP_SKB_CB(skb)->flags;
        TCP_SKB_CB(skb)->flags = flags & ~(TCPCB_FLAG_FIN|TCPCB_FLAG_PSH);
        TCP_SKB_CB(buff)->flags = flags;
        TCP_SKB_CB(buff)->sacked =
                (TCP_SKB_CB(skb)->sacked &
                 (TCPCB_LOST | TCPCB_EVER_RETRANS | TCPCB_AT_TAIL));
        TCP_SKB_CB(skb)->sacked &= ~TCPCB_AT_TAIL;

        if (!skb_shinfo(skb)->nr_frags && skb->ip_summed != CHECKSUM_HW) {
                /* Copy and checksum data tail into the new buffer. */
                buff->csum = csum_partial_copy_nocheck(skb->data + len, skb_put(buff, nsize),
                                                       nsize, 0);

                skb_trim(skb, len);

                skb->csum = csum_block_sub(skb->csum, buff->csum, len);
        } else {
                skb->ip_summed = CHECKSUM_HW;
                skb_split(skb, buff, len);
        }

        buff->ip_summed = skb->ip_summed;

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

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

        /* Fix up tso_factor for both original and new SKB.  */
        tcp_set_skb_tso_segs(sk, skb);
        tcp_set_skb_tso_segs(sk, buff);

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

        if (TCP_SKB_CB(buff)->sacked&TCPCB_LOST) {
                tp->lost_out += tcp_skb_pcount(buff);
                tp->left_out += tcp_skb_pcount(buff);
        }

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

        return 0;
}

/* This is similar to __pskb_pull_head() (it will go to core/skbuff.c
 * eventually). The difference is that pulled data not copied, but
 * immediately discarded.
 */
static unsigned char *__pskb_trim_head(struct sk_buff *skb, int len)
{
        int i, k, eat;

        eat = len;
        k = 0;
        for (i=0; i<skb_shinfo(skb)->nr_frags; i++) {
                if (skb_shinfo(skb)->frags[i].size <= eat) {
                        put_page(skb_shinfo(skb)->frags[i].page);
                        eat -= skb_shinfo(skb)->frags[i].size;
                } else {
                        skb_shinfo(skb)->frags[k] = skb_shinfo(skb)->frags[i];
                        if (eat) {
                                skb_shinfo(skb)->frags[k].page_offset += eat;
                                skb_shinfo(skb)->frags[k].size -= eat;
                                eat = 0;
                        }
                        k++;
                }
        }
        skb_shinfo(skb)->nr_frags = k;

        skb->tail = skb->data;
        skb->data_len -= len;
        skb->len = skb->data_len;
        return skb->tail;
}

int tcp_trim_head(struct sock *sk, struct sk_buff *skb, u32 len)
{
        if (skb_cloned(skb) &&
            pskb_expand_head(skb, 0, 0, GFP_ATOMIC))
                return -ENOMEM;

        if (len <= skb_headlen(skb)) {
                __skb_pull(skb, len);
        } else {
                if (__pskb_trim_head(skb, len-skb_headlen(skb)) == NULL)
                        return -ENOMEM;
        }

        TCP_SKB_CB(skb)->seq += len;
        skb->ip_summed = CHECKSUM_HW;

        skb->truesize        -= len;
        sk->sk_wmem_queued   -= len;
        sk->sk_forward_alloc += len;
        sock_set_flag(sk, SOCK_QUEUE_SHRUNK);

        /* Any change of skb->len requires recalculation of tso
         * factor and mss.
         */
        if (tcp_skb_pcount(skb) > 1)
                tcp_set_skb_tso_segs(sk, skb);

        return 0;
}

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

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

   tp->rx_opt.mss_clamp is mss negotiated at connection setup.
   It is 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->rx_opt.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)
 */

unsigned int tcp_sync_mss(struct sock *sk, u32 pmtu)
{
        struct tcp_sock *tp = tcp_sk(sk);
        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->rx_opt.mss_clamp)
                mss_now = tp->rx_opt.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), 68U - tp->tcp_header_len);

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

        return mss_now;
}

/* Compute the current effective MSS, taking SACKs and IP options,
 * and even PMTU discovery events into account.
 *
 * LARGESEND note: !urg_mode is overkill, only frames up to snd_up
 * cannot be large. However, taking into account rare use of URG, this
 * is not a big flaw.
 */

unsigned int tcp_current_mss(struct sock *sk, int large)
{
        struct tcp_sock *tp = tcp_sk(sk);
        struct dst_entry *dst = __sk_dst_get(sk);
        unsigned int do_large, mss_now;

        mss_now = tp->mss_cache_std;
        if (dst) {
                u32 mtu = dst_mtu(dst);
                if (mtu != tp->pmtu_cookie)
                        mss_now = tcp_sync_mss(sk, mtu);
        }

        do_large = (large &&
                    (sk->sk_route_caps & NETIF_F_TSO) &&
                    !tp->urg_mode);

        if (do_large) {
                unsigned int large_mss, factor, limit;

                large_mss = 65535 - tp->af_specific->net_header_len -
                        tp->ext_header_len - tp->tcp_header_len;

                if (tp->max_window && large_mss > (tp->max_window>>1))
                        large_mss = max((tp->max_window>>1),
                                        68U - tp->tcp_header_len);

                factor = large_mss / mss_now;

                /* Always keep large mss multiple of real mss, but
                 * do not exceed 1/tso_win_divisor of the congestion window
                 * so we can keep the ACK clock ticking and minimize
                 * bursting.
                 */
                limit = tp->snd_cwnd;
                if (sysctl_tcp_tso_win_divisor)
                        limit /= sysctl_tcp_tso_win_divisor;
                limit = max(1U, limit);
                if (factor > limit)
                        factor = limit;

                tp->mss_cache = mss_now * factor;

                mss_now = tp->mss_cache;
        }

        if (tp->rx_opt.eff_sacks)
                mss_now -= (TCPOLEN_SACK_BASE_ALIGNED +
                            (tp->rx_opt.eff_sacks * TCPOLEN_SACK_PERBLOCK));
        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, int nonagle)
{
        struct tcp_sock *tp = tcp_sk(sk);
        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->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, 1);

                while ((skb = sk->sk_send_head) &&
                       tcp_snd_test(sk, skb, mss_now,
                                    tcp_skb_is_last(sk, skb) ? nonagle :
                                                               TCP_NAGLE_PUSH)) {
                        if (skb->len > mss_now) {
                                if (tcp_fragment(sk, skb, mss_now))
                                        break;
                        }

                        TCP_SKB_CB(skb)->when = tcp_time_stamp;
                        tcp_tso_set_push(skb);
                        if (tcp_transmit_skb(sk, skb_clone(skb, GFP_ATOMIC)))
                                break;

                        /* Advance the send_head.  This one is sent out.
                         * This call will increment packets_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 && sk->sk_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.
 *      [ Actually, bsd uses MSS and 1/4 of maximal _window_ ]
 *      Otherwise, just prevent the window from shrinking
 *      and from being larger than the largest representable value.
 *
 * This prevents incremental opening of the window in the regime
 * where TCP is limited by the speed of the reader side taking
 * data out of the TCP receive queue. It does nothing about
 * those cases where the window is constrained on the sender side
 * because the pipeline is full.
 *
 * BSD also seems to "accidentally" limit itself to windows that are a
 * multiple of MSS, at least until the free space gets quite small.
 * This would appear to be a side effect of the mbuf implementation.
 * Combining these two algorithms results in the observed behavior
 * of having a fixed window size at almost all times.
 *
 * Below we obtain similar behavior by forcing the offered window to
 * a multiple of the mss when it is feasible to do so.
 *
 * Note, we don't "adjust" for TIMESTAMP or SACK option bytes.
 * Regular options like TIMESTAMP are taken into account.
 */
u32 __tcp_select_window(struct sock *sk)
{
        struct tcp_sock *tp = tcp_sk(sk);
        /* 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
         */
        int mss = tp->ack.rcv_mss;
        int free_space = tcp_space(sk);
        int full_space = min_t(int, tp->window_clamp, tcp_full_space(sk));
        int window;

        if (mss > full_space)
                mss = full_space; 

        if (free_space < full_space/2) {
                tp->ack.quick = 0;

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

                if (free_space < mss)
                        return 0;
        }

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

        /* Don't do rounding if we are using window scaling, since the
         * scaled window will not line up with the MSS boundary anyway.
         */
        window = tp->rcv_wnd;
        if (tp->rx_opt.rcv_wscale) {
                window = free_space;

                /* Advertise enough space so that it won't get scaled away.
                 * Import case: prevent zero window announcement if
                 * 1<<rcv_wscale > mss.
                 */
                if (((window >> tp->rx_opt.rcv_wscale) << tp->rx_opt.rcv_wscale) != window)
                        window = (((window >> tp->rx_opt.rcv_wscale) + 1)
                                  << tp->rx_opt.rcv_wscale);
        } else {
                /* Get the largest window that is a nice multiple of mss.
                 * Window clamp already applied above.
                 * If our current window offering is within 1 mss of the
                 * free space we just keep it. This prevents the divide
                 * and multiply from happening most of the time.
                 * We also don't do any window rounding when the free space
                 * is too small.
                 */
                if (window <= free_space - mss || window > free_space)
                        window = (free_space/mss)*mss;
        }

        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_sock *tp = tcp_sk(sk);
        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;

                /* 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;

                BUG_ON(tcp_skb_pcount(skb) != 1 ||
                       tcp_skb_pcount(next_skb) != 1);

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

                memcpy(skb_put(skb, next_skb_size), next_skb->data, next_skb_size);

                if (next_skb->ip_summed == CHECKSUM_HW)
                        skb->ip_summed = CHECKSUM_HW;

                if (skb->ip_summed != CHECKSUM_HW)
                        skb->csum = csum_block_add(skb->csum, next_skb->csum, skb_size);

                /* Update sequence range on original skb. */
                TCP_SKB_CB(skb)->end_seq = TCP_SKB_CB(next_skb)->end_seq;

                /* Merge over control information. */
                flags |= TCP_SKB_CB(next_skb)->flags; /* This moves PSH/FIN etc. over */
                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|TCPCB_AT_TAIL);
                if (TCP_SKB_CB(next_skb)->sacked&TCPCB_SACKED_RETRANS)
                        tp->retrans_out -= tcp_skb_pcount(next_skb);
                if (TCP_SKB_CB(next_skb)->sacked&TCPCB_LOST) {
                        tp->lost_out -= tcp_skb_pcount(next_skb);
                        tp->left_out -= tcp_skb_pcount(next_skb);
                }
                /* Reno case is special. Sigh... */
                if (!tp->rx_opt.sack_ok && tp->sacked_out) {
                        tcp_dec_pcount_approx(&tp->sacked_out, next_skb);
                        tp->left_out -= tcp_skb_pcount(next_skb);
                }

                /* Not quite right: it can be > snd.fack, but
                 * it is better to underestimate fackets.
                 */
                tcp_dec_pcount_approx(&tp->fackets_out, next_skb);
                tcp_packets_out_dec(tp, next_skb);
                sk_stream_free_skb(sk, next_skb);
        }
}

/* 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_sock *tp = tcp_sk(sk);
        struct sk_buff *skb;
        unsigned int mss = tcp_current_mss(sk, 0);
        int lost = 0;

        sk_stream_for_retrans_queue(skb, sk) {
                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 -= tcp_skb_pcount(skb);
                        }
                        if (!(TCP_SKB_CB(skb)->sacked&TCPCB_LOST)) {
                                TCP_SKB_CB(skb)->sacked |= TCPCB_LOST;
                                tp->lost_out += tcp_skb_pcount(skb);
                                lost = 1;
                        }
                }
        }

        if (!lost)
                return;

        tcp_sync_left_out(tp);

        /* Don't muck with the congestion window here.
         * Reason is that we do not increase amount of _data_
         * in network, but units changed and effective
         * cwnd/ssthresh really reduced now.
         */
        if (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;
                tcp_set_ca_state(tp, 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_sock *tp = tcp_sk(sk);
        unsigned int cur_mss = tcp_current_mss(sk, 0);
        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->sk_wmem_alloc) >
            min(sk->sk_wmem_queued + (sk->sk_wmem_queued >> 2), sk->sk_sndbuf))
                return -EAGAIN;

        if (before(TCP_SKB_CB(skb)->seq, tp->snd_una)) {
                if (before(TCP_SKB_CB(skb)->end_seq, tp->snd_una))
                        BUG();

                if (sk->sk_route_caps & NETIF_F_TSO) {
                        sk->sk_route_caps &= ~NETIF_F_TSO;
                        sock_set_flag(sk, SOCK_NO_LARGESEND);
                        tp->mss_cache = tp->mss_cache_std;
                }

                if (tcp_trim_head(sk, skb, tp->snd_una - TCP_SKB_CB(skb)->seq))
                        return -ENOMEM;
        }

        /* If receiver has shrunk his window, and skb is out of
         * new window, do not retransmit it. The exception is the
         * case, when window is shrunk to zero. In this case
         * our retransmit serves as a zero window probe.
         */
        if (!before(TCP_SKB_CB(skb)->seq, tp->snd_una+tp->snd_wnd)
            && TCP_SKB_CB(skb)->seq != tp->snd_una)
                return -EAGAIN;

        if (skb->len > cur_mss) {
                int old_factor = tcp_skb_pcount(skb);
                int new_factor;

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

                /* New SKB created, account for it. */
                new_factor = tcp_skb_pcount(skb);
                tp->packets_out -= old_factor - new_factor;
                tp->packets_out += tcp_skb_pcount(skb->next);
        }

        /* 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 != sk->sk_send_head) &&
           (skb->next != (struct sk_buff *)&sk->sk_write_queue) &&
           (skb_shinfo(skb)->nr_frags == 0 && skb_shinfo(skb->next)->nr_frags == 0) &&
           (tcp_skb_pcount(skb) == 1 && tcp_skb_pcount(skb->next) == 1) &&
           (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)) {
                if (!pskb_trim(skb, 0)) {
                        TCP_SKB_CB(skb)->seq = TCP_SKB_CB(skb)->end_seq - 1;
                        skb_shinfo(skb)->tso_segs = 1;
                        skb_shinfo(skb)->tso_size = 0;
                        skb->ip_summed = CHECKSUM_NONE;
                        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;
        tcp_tso_set_push(skb);

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

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

                tp->total_retrans++;

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

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

                tp->undo_retrans++;

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

/* 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_sock *tp = tcp_sk(sk);
        struct sk_buff *skb;
        int packet_cnt = tp->lost_out;

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

                        /* Assume this retransmit will generate
                         * only one packet for congestion window
                         * calculation purposes.  This works because
                         * tcp_retransmit_skb() will chop up the
                         * packet to be MSS sized and all the
                         * packet counting works out.
                         */
                        if (tcp_packets_in_flight(tp) >= tp->snd_cwnd)
                                return;

                        if (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(LINUX_MIB_TCPFASTRETRANS);
                                        else
                                                NET_INC_STATS_BH(LINUX_MIB_TCPSLOWSTARTRETRANS);

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

                                packet_cnt -= tcp_skb_pcount(skb);
                                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->rx_opt.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 retransmit anything, while we have some new
         * segments to send.
         */

        if (tcp_may_send_now(sk, tp))
                return;

        packet_cnt = 0;

        sk_stream_for_retrans_queue(skb, sk) {
                /* Similar to the retransmit loop above we
                 * can pretend that the retransmitted SKB
                 * we send out here will be composed of one
                 * real MSS sized packet because tcp_retransmit_skb()
                 * will fragment it if necessary.
                 */
                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->sk_write_queue))
                        tcp_reset_xmit_timer(sk, TCP_TIME_RETRANS, tp->rto);

                NET_INC_STATS_BH(LINUX_MIB_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_sock *tp = tcp_sk(sk);       
        struct sk_buff *skb = skb_peek_tail(&sk->sk_write_queue);
        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, 1);

        if (sk->sk_send_head != NULL) {
                TCP_SKB_CB(skb)->flags |= TCPCB_FLAG_FIN;
                TCP_SKB_CB(skb)->end_seq++;
                tp->write_seq++;
        } else {
                /* Socket is locked, keep trying until memory is available. */
                for (;;) {
                        skb = alloc_skb(MAX_TCP_HEADER, GFP_KERNEL);
                        if (skb)
                                break;
                        yield();
                }

                /* 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;
                skb_shinfo(skb)->tso_segs = 1;
                skb_shinfo(skb)->tso_size = 0;

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

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

        /* NOTE: No TCP options attached and we never retransmit this. */
        skb = alloc_skb(MAX_TCP_HEADER, priority);
        if (!skb) {
                NET_INC_STATS(LINUX_MIB_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;
        skb_shinfo(skb)->tso_segs = 1;
        skb_shinfo(skb)->tso_size = 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(LINUX_MIB_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->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->sk_write_queue);
                        skb_header_release(nskb);
                        __skb_queue_head(&sk->sk_write_queue, nskb);
                        sk_stream_free_skb(sk, skb);
                        sk_charge_skb(sk, nskb);
                        skb = nskb;
                }

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

/*
 * Prepare a SYN-ACK.
 */
struct sk_buff * tcp_make_synack(struct sock *sk, struct dst_entry *dst,
                                 struct request_sock *req)
{
        struct inet_request_sock *ireq = inet_rsk(req);
        struct tcp_sock *tp = tcp_sk(sk);
        struct tcphdr *th;
        int tcp_header_size;
        struct 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 +
                           (ireq->tstamp_ok ? TCPOLEN_TSTAMP_ALIGNED : 0) +
                           (ireq->wscale_ok ? TCPOLEN_WSCALE_ALIGNED : 0) +
                           /* SACK_PERM is in the place of NOP NOP of TS */
                           ((ireq->sack_ok && !ireq->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;
        if (dst->dev->features&NETIF_F_TSO)
                ireq->ecn_ok = 0;
        TCP_ECN_make_synack(req, th);
        th->source = inet_sk(sk)->sport;
        th->dest = ireq->rmt_port;
        TCP_SKB_CB(skb)->seq = tcp_rsk(req)->snt_isn;
        TCP_SKB_CB(skb)->end_seq = TCP_SKB_CB(skb)->seq + 1;
        TCP_SKB_CB(skb)->sacked = 0;
        skb_shinfo(skb)->tso_segs = 1;
        skb_shinfo(skb)->tso_size = 0;
        th->seq = htonl(TCP_SKB_CB(skb)->seq);
        th->ack_seq = htonl(tcp_rsk(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_metric(dst, RTAX_WINDOW);
                /* tcp_full_space because it is guaranteed to be the first packet */
                tcp_select_initial_window(tcp_full_space(sk), 
                        dst_metric(dst, RTAX_ADVMSS) - (ireq->tstamp_ok ? TCPOLEN_TSTAMP_ALIGNED : 0),
                        &req->rcv_wnd,
                        &req->window_clamp,
                        ireq->wscale_ok,
                        &rcv_wscale);
                ireq->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_metric(dst, RTAX_ADVMSS), ireq->tstamp_ok,
                              ireq->sack_ok, ireq->wscale_ok, ireq->rcv_wscale,
                              TCP_SKB_CB(skb)->when,
                              req->ts_recent);

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

/* 
 * Do all connect socket setups that can be done AF independent.
 */ 
static inline void tcp_connect_init(struct sock *sk)
{
        struct dst_entry *dst = __sk_dst_get(sk);
        struct tcp_sock *tp = tcp_sk(sk);
        __u8 rcv_wscale;

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

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

        if (!tp->window_clamp)
                tp->window_clamp = dst_metric(dst, RTAX_WINDOW);
        tp->advmss = dst_metric(dst, RTAX_ADVMSS);
        tcp_initialize_rcv_mss(sk);

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

        tp->rx_opt.rcv_wscale = rcv_wscale;
        tp->rcv_ssthresh = tp->rcv_wnd;

        sk->sk_err = 0;
        sock_reset_flag(sk, SOCK_DONE);
        tp->snd_wnd = 0;
        tcp_init_wl(tp, 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;
        tp->retransmits = 0;
        tcp_clear_retrans(tp);
}

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

        tcp_connect_init(sk);

        buff = alloc_skb(MAX_TCP_HEADER + 15, sk->sk_allocation);
        if (unlikely(buff == NULL))
                return -ENOBUFS;

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

        TCP_SKB_CB(buff)->flags = TCPCB_FLAG_SYN;
        TCP_ECN_send_syn(sk, tp, buff);
        TCP_SKB_CB(buff)->sacked = 0;
        skb_shinfo(buff)->tso_segs = 1;
        skb_shinfo(buff)->tso_size = 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_header_release(buff);
        __skb_queue_tail(&sk->sk_write_queue, buff);
        sk_charge_skb(sk, buff);
        tp->packets_out += tcp_skb_pcount(buff);
        tcp_transmit_skb(sk, skb_clone(buff, GFP_KERNEL));
        TCP_INC_STATS(TCP_MIB_ACTIVEOPENS);

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

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

        if (ato > TCP_DELACK_MIN) {
                int max_ato = HZ/2;

                if (tp->ack.pingpong || (tp->ack.pending&TCP_ACK_PUSHED))
                        max_ato = TCP_DELACK_MAX;

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

                /* If some rtt estimate is known, use it to bound delayed ack.
                 * Do not use 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&TCP_ACK_TIMER) {
                /* 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 |= TCP_ACK_SCHED|TCP_ACK_TIMER;
        tp->ack.timeout = timeout;
        sk_reset_timer(sk, &tp->delack_timer, timeout);
}

/* 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->sk_state != TCP_CLOSE) {
                struct tcp_sock *tp = tcp_sk(sk);
                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, 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;
                skb_shinfo(buff)->tso_segs = 1;
                skb_shinfo(buff)->tso_size = 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.
 *
 * Question: what should we make while urgent mode?
 * 4.4BSD forces sending single byte of data. We cannot send
 * out of window data, because we have SND.NXT==SND.MAX...
 *
 * Current solution: to send TWO zero-length segments in urgent mode:
 * one is with SEG.SEQ=SND.UNA to deliver urgent pointer, another is
 * out-of-date with SND.UNA-1 to probe window.
 */
static int tcp_xmit_probe_skb(struct sock *sk, int urgent)
{
        struct tcp_sock *tp = tcp_sk(sk);
        struct sk_buff *skb;

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

        /* Reserve space for headers and set control bits. */
        skb_reserve(skb, MAX_TCP_HEADER);
        skb->csum = 0;
        TCP_SKB_CB(skb)->flags = TCPCB_FLAG_ACK;
        TCP_SKB_CB(skb)->sacked = urgent;
        skb_shinfo(skb)->tso_segs = 1;
        skb_shinfo(skb)->tso_size = 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 = urgent ? tp->snd_una : 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->sk_state != TCP_CLOSE) {
                struct tcp_sock *tp = tcp_sk(sk);
                struct sk_buff *skb;

                if ((skb = sk->sk_send_head) != NULL &&
                    before(TCP_SKB_CB(skb)->seq, tp->snd_una+tp->snd_wnd)) {
                        int err;
                        unsigned int mss = tcp_current_mss(sk, 0);
                        unsigned 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;
                                /* SWS override triggered forced fragmentation.
                                 * Disable TSO, the connection is too sick. */
                                if (sk->sk_route_caps & NETIF_F_TSO) {
                                        sock_set_flag(sk, SOCK_NO_LARGESEND);
                                        sk->sk_route_caps &= ~NETIF_F_TSO;
                                        tp->mss_cache = tp->mss_cache_std;
                                }
                        } else if (!tcp_skb_pcount(skb))
                                tcp_set_skb_tso_segs(sk, skb);

                        TCP_SKB_CB(skb)->flags |= TCPCB_FLAG_PSH;
                        TCP_SKB_CB(skb)->when = tcp_time_stamp;
                        tcp_tso_set_push(skb);
                        err = tcp_transmit_skb(sk, skb_clone(skb, GFP_ATOMIC));
                        if (!err) {
                                update_send_head(sk, tp, skb);
                        }
                        return err;
                } else {
                        if (tp->urg_mode &&
                            between(tp->snd_up, tp->snd_una+1, tp->snd_una+0xFFFF))
                                tcp_xmit_probe_skb(sk, TCPCB_URG);
                        return tcp_xmit_probe_skb(sk, 0);
                }
        }
        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_sock *tp = tcp_sk(sk);
        int err;

        err = tcp_write_wakeup(sk);

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

        if (err <= 0) {
                if (tp->backoff < sysctl_tcp_retries2)
                        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));
        }
}

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