[apple/xnu.git] / bsd / netinet / tcp_output.c

/*
 * Copyright (c) 2000 Apple Computer, Inc. All rights reserved.
 *
 * @APPLE_OSREFERENCE_LICENSE_HEADER_START@
 * 
 * This file contains Original Code and/or Modifications of Original Code
 * as defined in and that are subject to the Apple Public Source License
 * Version 2.0 (the 'License'). You may not use this file except in
 * compliance with the License. The rights granted to you under the License
 * may not be used to create, or enable the creation or redistribution of,
 * unlawful or unlicensed copies of an Apple operating system, or to
 * circumvent, violate, or enable the circumvention or violation of, any
 * terms of an Apple operating system software license agreement.
 * 
 * Please obtain a copy of the License at
 * http://www.opensource.apple.com/apsl/ and read it before using this file.
 * 
 * The Original Code and all software distributed under the License are
 * distributed on an 'AS IS' basis, WITHOUT WARRANTY OF ANY KIND, EITHER
 * EXPRESS OR IMPLIED, AND APPLE HEREBY DISCLAIMS ALL SUCH WARRANTIES,
 * INCLUDING WITHOUT LIMITATION, ANY WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE, QUIET ENJOYMENT OR NON-INFRINGEMENT.
 * Please see the License for the specific language governing rights and
 * limitations under the License.
 * 
 * @APPLE_OSREFERENCE_LICENSE_HEADER_END@
 */
/*
 * Copyright (c) 1982, 1986, 1988, 1990, 1993, 1995
 *      The Regents of the University of California.  All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 * 3. All advertising materials mentioning features or use of this software
 *    must display the following acknowledgement:
 *      This product includes software developed by the University of
 *      California, Berkeley and its contributors.
 * 4. Neither the name of the University nor the names of its contributors
 *    may be used to endorse or promote products derived from this software
 *    without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 * SUCH DAMAGE.
 *
 *      @(#)tcp_output.c        8.4 (Berkeley) 5/24/95
 * $FreeBSD: src/sys/netinet/tcp_output.c,v 1.39.2.10 2001/07/07 04:30:38 silby Exp $
 */

#define _IP_VHL


#include <sys/param.h>
#include <sys/systm.h>
#include <sys/kernel.h>
#include <sys/sysctl.h>
#include <sys/mbuf.h>
#include <sys/domain.h>
#include <sys/protosw.h>
#include <sys/socket.h>
#include <sys/socketvar.h>

#include <net/route.h>

#include <netinet/in.h>
#include <netinet/in_systm.h>
#include <netinet/ip.h>
#include <netinet/in_pcb.h>
#include <netinet/ip_var.h>
#if INET6
#include <netinet6/in6_pcb.h>
#include <netinet/ip6.h>
#include <netinet6/ip6_var.h>
#endif
#include <netinet/tcp.h>
#define TCPOUTFLAGS
#include <netinet/tcp_fsm.h>
#include <netinet/tcp_seq.h>
#include <netinet/tcp_timer.h>
#include <netinet/tcp_var.h>
#include <netinet/tcpip.h>
#if TCPDEBUG
#include <netinet/tcp_debug.h>
#endif
#include <sys/kdebug.h>

#if IPSEC
#include <netinet6/ipsec.h>
#endif /*IPSEC*/

#define DBG_LAYER_BEG           NETDBG_CODE(DBG_NETTCP, 1)
#define DBG_LAYER_END           NETDBG_CODE(DBG_NETTCP, 3)
#define DBG_FNC_TCP_OUTPUT      NETDBG_CODE(DBG_NETTCP, (4 << 8) | 1)


#ifdef notyet
extern struct mbuf *m_copypack();
#endif

static int path_mtu_discovery = 1;
SYSCTL_INT(_net_inet_tcp, OID_AUTO, path_mtu_discovery, CTLFLAG_RW,
        &path_mtu_discovery, 1, "Enable Path MTU Discovery");

int ss_fltsz = 1;
SYSCTL_INT(_net_inet_tcp, OID_AUTO, slowstart_flightsize, CTLFLAG_RW,
        &ss_fltsz, 1, "Slow start flight size");

int ss_fltsz_local = 4; /* starts with four segments max */
SYSCTL_INT(_net_inet_tcp, OID_AUTO, local_slowstart_flightsize, CTLFLAG_RW,
        &ss_fltsz_local, 1, "Slow start flight size for local networks");

int     tcp_do_newreno = 0;
SYSCTL_INT(_net_inet_tcp, OID_AUTO, newreno, CTLFLAG_RW, &tcp_do_newreno,
        0, "Enable NewReno Algorithms");

int     tcp_packet_chaining = 50;
SYSCTL_INT(_net_inet_tcp, OID_AUTO, packetchain, CTLFLAG_RW, &tcp_packet_chaining,
        0, "Enable TCP output packet chaining");

struct  mbuf *m_copym_with_hdrs(struct mbuf*, int, int, int, struct mbuf**, int*);
static long packchain_newlist = 0;
static long packchain_looped = 0;
static long packchain_sent = 0;


/* temporary: for testing */
#if IPSEC
extern int ipsec_bypass;
#endif

extern int slowlink_wsize;      /* window correction for slow links */
extern u_long  route_generation;
extern int fw_enable;           /* firewall is on: disable packet chaining */
extern int ipsec_bypass;

extern vm_size_t        so_cache_zone_element_size;

static __inline__ u_int16_t
get_socket_id(struct socket * s)
{
        u_int16_t               val;

        if (so_cache_zone_element_size == 0) {
                return (0);
        }
        val = (u_int16_t)(((u_int32_t)s) / so_cache_zone_element_size);
        if (val == 0) {
                val = 0xffff;
        }
        return (val);
}

/*
 * Tcp output routine: figure out what should be sent and send it.
 */
int
tcp_output(struct tcpcb *tp)
{
        struct socket *so = tp->t_inpcb->inp_socket;
        long len, recwin, sendwin;
        int off, flags, error;
        register struct mbuf *m;
        struct ip *ip = NULL;
        register struct ipovly *ipov = NULL;
#if INET6
        struct ip6_hdr *ip6 = NULL;
#endif /* INET6 */
        register struct tcphdr *th;
        u_char opt[TCP_MAXOLEN];
        unsigned ipoptlen, optlen, hdrlen;
        int idle, sendalot, howmuchsent = 0;
        int i, sack_rxmit;
        int sack_bytes_rxmt;
        struct sackhole *p;

        int maxburst = TCP_MAXBURST;
        struct rmxp_tao *taop;
        struct rmxp_tao tao_noncached;
        int    last_off = 0;
        int    m_off;
        struct mbuf *m_last = 0;
        struct mbuf *m_head = 0;
        struct mbuf *packetlist = 0;
        struct mbuf *lastpacket = 0;
#if INET6
        int isipv6 = tp->t_inpcb->inp_vflag & INP_IPV6 ;
#endif
        short packchain_listadd = 0;
        u_int16_t       socket_id = get_socket_id(so);


        /*
         * Determine length of data that should be transmitted,
         * and flags that will be used.
         * If there is some data or critical controls (SYN, RST)
         * to send, then transmit; otherwise, investigate further.
         */
        idle = (tp->t_flags & TF_LASTIDLE) || (tp->snd_max == tp->snd_una);
        if (idle && tp->t_rcvtime >= tp->t_rxtcur) {
                /*
                 * We have been idle for "a while" and no acks are
                 * expected to clock out any data we send --
                 * slow start to get ack "clock" running again.
                 *
                 * Set the slow-start flight size depending on whether
                 * this is a local network or not.
                 */
                if (
#if INET6
                    (isipv6 && in6_localaddr(&tp->t_inpcb->in6p_faddr)) ||
                    (!isipv6 &&
#endif
                     in_localaddr(tp->t_inpcb->inp_faddr)
#if INET6
                     )
#endif
                    )
                        tp->snd_cwnd = tp->t_maxseg * ss_fltsz_local;
                else     
                        tp->snd_cwnd = tp->t_maxseg * ss_fltsz;
        }
        tp->t_flags &= ~TF_LASTIDLE;
        if (idle) {
                if (tp->t_flags & TF_MORETOCOME) {
                        tp->t_flags |= TF_LASTIDLE;
                        idle = 0;
                }
        }
again:
        KERNEL_DEBUG(DBG_FNC_TCP_OUTPUT | DBG_FUNC_START, 0,0,0,0,0);

#if INET6
        if (isipv6) {
        
                KERNEL_DEBUG(DBG_LAYER_BEG,
                     ((tp->t_inpcb->inp_fport << 16) | tp->t_inpcb->inp_lport),
                     (((tp->t_inpcb->in6p_laddr.s6_addr16[0] & 0xffff) << 16) |
                      (tp->t_inpcb->in6p_faddr.s6_addr16[0] & 0xffff)),
                     sendalot,0,0);
        }
        else
#endif

        {
                KERNEL_DEBUG(DBG_LAYER_BEG,
                     ((tp->t_inpcb->inp_fport << 16) | tp->t_inpcb->inp_lport),
                     (((tp->t_inpcb->inp_laddr.s_addr & 0xffff) << 16) |
                      (tp->t_inpcb->inp_faddr.s_addr & 0xffff)),
                     sendalot,0,0);
        /*
         * If the route generation id changed, we need to check that our
         * local (source) IP address is still valid. If it isn't either
         * return error or silently do nothing (assuming the address will
         * come back before the TCP connection times out).
         */

      if ((tp->t_inpcb->inp_route.ro_rt != NULL &&
           (tp->t_inpcb->inp_route.ro_rt->generation_id != route_generation)) || (tp->t_inpcb->inp_route.ro_rt == NULL)) {
                /* check that the source address is still valid */
                if (ifa_foraddr(tp->t_inpcb->inp_laddr.s_addr) == 0) {
                        if (tp->t_state >= TCPS_CLOSE_WAIT) {
                                tcp_close(tp);
                                return(EADDRNOTAVAIL);
                        }

                        /* set Retransmit  timer if it wasn't set
                         * reset Persist timer and shift register as the
                         * adversed peer window may not be valid anymore
                         */

                        if (!tp->t_timer[TCPT_REXMT]) {
                                tp->t_timer[TCPT_REXMT] = tp->t_rxtcur;
                                if (tp->t_timer[TCPT_PERSIST]) {
                                        tp->t_timer[TCPT_PERSIST] = 0;
                                        tp->t_rxtshift = 0;
                                }
                        }

                        if (packetlist) {
                                error = ip_output_list(packetlist, packchain_listadd, tp->t_inpcb->inp_options, &tp->t_inpcb->inp_route,
                                        (so->so_options & SO_DONTROUTE), 0);
                                tp->t_lastchain = 0;
                        }
                        if (so->so_flags & SOF_NOADDRAVAIL)
                                return(EADDRNOTAVAIL);
                        else
                                return(0); /* silently ignore and keep data in socket */
                }
        }
        }

        /*
         * If we've recently taken a timeout, snd_max will be greater than
         * snd_nxt.  There may be SACK information that allows us to avoid
         * resending already delivered data.  Adjust snd_nxt accordingly.
         */
        if (tp->sack_enable && SEQ_LT(tp->snd_nxt, tp->snd_max))
                tcp_sack_adjust(tp);
        sendalot = 0;
        off = tp->snd_nxt - tp->snd_una;
        sendwin = min(tp->snd_wnd, tp->snd_cwnd);

        if (tp->t_flags & TF_SLOWLINK && slowlink_wsize > 0)
                sendwin = min(sendwin, slowlink_wsize);

        flags = tcp_outflags[tp->t_state];
        /*
         * Send any SACK-generated retransmissions.  If we're explicitly trying
         * to send out new data (when sendalot is 1), bypass this function.
         * If we retransmit in fast recovery mode, decrement snd_cwnd, since
         * we're replacing a (future) new transmission with a retransmission
         * now, and we previously incremented snd_cwnd in tcp_input().
         */
        /*
         * Still in sack recovery , reset rxmit flag to zero.
         */
        sack_rxmit = 0;
        sack_bytes_rxmt = 0;
        len = 0;
        p = NULL;
        if (tp->sack_enable && IN_FASTRECOVERY(tp) &&
            (p = tcp_sack_output(tp, &sack_bytes_rxmt))) {
                long cwin;
                
                cwin = min(tp->snd_wnd, tp->snd_cwnd) - sack_bytes_rxmt;
                if (cwin < 0)
                        cwin = 0;
                /* Do not retransmit SACK segments beyond snd_recover */
                if (SEQ_GT(p->end, tp->snd_recover)) {
                        /*
                         * (At least) part of sack hole extends beyond
                         * snd_recover. Check to see if we can rexmit data
                         * for this hole.
                         */
                        if (SEQ_GEQ(p->rxmit, tp->snd_recover)) {
                                /*
                                 * Can't rexmit any more data for this hole.
                                 * That data will be rexmitted in the next
                                 * sack recovery episode, when snd_recover
                                 * moves past p->rxmit.
                                 */
                                p = NULL;
                                goto after_sack_rexmit;
                        } else
                                /* Can rexmit part of the current hole */
                                len = ((long)ulmin(cwin,
                                                   tp->snd_recover - p->rxmit));
                } else
                        len = ((long)ulmin(cwin, p->end - p->rxmit));
                off = p->rxmit - tp->snd_una;
                if (len > 0) {
                        sack_rxmit = 1;
                        sendalot = 1;
                        tcpstat.tcps_sack_rexmits++;
                        tcpstat.tcps_sack_rexmit_bytes +=
                            min(len, tp->t_maxseg);
                }
        }
after_sack_rexmit:
        /*
         * Get standard flags, and add SYN or FIN if requested by 'hidden'
         * state flags.
         */
        if (tp->t_flags & TF_NEEDFIN)
                flags |= TH_FIN;
        if (tp->t_flags & TF_NEEDSYN)
                flags |= TH_SYN;

        /*
         * If in persist timeout with window of 0, send 1 byte.
         * Otherwise, if window is small but nonzero
         * and timer expired, we will send what we can
         * and go to transmit state.
         */
        if (tp->t_force) {
                if (sendwin == 0) {
                        /*
                         * If we still have some data to send, then
                         * clear the FIN bit.  Usually this would
                         * happen below when it realizes that we
                         * aren't sending all the data.  However,
                         * if we have exactly 1 byte of unsent data,
                         * then it won't clear the FIN bit below,
                         * and if we are in persist state, we wind
                         * up sending the packet without recording
                         * that we sent the FIN bit.
                         *
                         * We can't just blindly clear the FIN bit,
                         * because if we don't have any more data
                         * to send then the probe will be the FIN
                         * itself.
                         */
                        if (off < so->so_snd.sb_cc)
                                flags &= ~TH_FIN;
                        sendwin = 1;
                } else {
                        tp->t_timer[TCPT_PERSIST] = 0;
                        tp->t_rxtshift = 0;
                }
        }

        /*
         * If snd_nxt == snd_max and we have transmitted a FIN, the
         * offset will be > 0 even if so_snd.sb_cc is 0, resulting in
         * a negative length.  This can also occur when TCP opens up
         * its congestion window while receiving additional duplicate
         * acks after fast-retransmit because TCP will reset snd_nxt
         * to snd_max after the fast-retransmit.
         *
         * In the normal retransmit-FIN-only case, however, snd_nxt will
         * be set to snd_una, the offset will be 0, and the length may
         * wind up 0.
         *
         * If sack_rxmit is true we are retransmitting from the scoreboard
         * in which case len is already set.
         */
        if (sack_rxmit == 0) {
                if (sack_bytes_rxmt == 0)
                        len = ((long)ulmin(so->so_snd.sb_cc, sendwin) - off);
                else {
                        long cwin;

                        /*
                         * We are inside of a SACK recovery episode and are
                         * sending new data, having retransmitted all the
                         * data possible in the scoreboard.
                         */
                        len = ((long)ulmin(so->so_snd.sb_cc, tp->snd_wnd) 
                               - off);
                        /*
                         * Don't remove this (len > 0) check !
                         * We explicitly check for len > 0 here (although it 
                         * isn't really necessary), to work around a gcc 
                         * optimization issue - to force gcc to compute
                         * len above. Without this check, the computation
                         * of len is bungled by the optimizer.
                         */
                        if (len > 0) {
                                cwin = tp->snd_cwnd - 
                                        (tp->snd_nxt - tp->sack_newdata) -
                                        sack_bytes_rxmt;
                                if (cwin < 0)
                                        cwin = 0;
                                len = lmin(len, cwin);
                        }
                }
        }

        /*
         * Lop off SYN bit if it has already been sent.  However, if this
         * is SYN-SENT state and if segment contains data and if we don't
         * know that foreign host supports TAO, suppress sending segment.
         */
        if ((flags & TH_SYN) && SEQ_GT(tp->snd_nxt, tp->snd_una)) {
                flags &= ~TH_SYN;
                off--, len++;
                if (len > 0 && tp->t_state == TCPS_SYN_SENT) {
                        if (packetlist) {
                                error = ip_output_list(packetlist, packchain_listadd, tp->t_inpcb->inp_options, &tp->t_inpcb->inp_route,
                                (so->so_options & SO_DONTROUTE), 0);
                                tp->t_lastchain = 0;
                        }
                  KERNEL_DEBUG(DBG_FNC_TCP_OUTPUT | DBG_FUNC_END, 0,0,0,0,0);
                  return 0;
                }
        }

        /*
         * Be careful not to send data and/or FIN on SYN segments.
         * This measure is needed to prevent interoperability problems
         * with not fully conformant TCP implementations.
         */
        if ((flags & TH_SYN) && (tp->t_flags & TF_NOOPT)) {
                len = 0;
                flags &= ~TH_FIN;
        }

        if (len < 0) {
                /*
                 * If FIN has been sent but not acked,
                 * but we haven't been called to retransmit,
                 * len will be < 0.  Otherwise, window shrank
                 * after we sent into it.  If window shrank to 0,
                 * cancel pending retransmit, pull snd_nxt back
                 * to (closed) window, and set the persist timer
                 * if it isn't already going.  If the window didn't
                 * close completely, just wait for an ACK.
                 */
                len = 0;
                if (sendwin == 0) {
                        tp->t_timer[TCPT_REXMT] = 0;
                        tp->t_rxtshift = 0;
                        tp->snd_nxt = tp->snd_una;
                        if (tp->t_timer[TCPT_PERSIST] == 0)
                                tcp_setpersist(tp);
                }
        }

        /*
         * len will be >= 0 after this point.  Truncate to the maximum
         * segment length and ensure that FIN is removed if the length
         * no longer contains the last data byte.
         */
        if (len > tp->t_maxseg) {
                len = tp->t_maxseg;
                howmuchsent += len;
                sendalot = 1;
        }
        if (sack_rxmit) {
                if (SEQ_LT(p->rxmit + len, tp->snd_una + so->so_snd.sb_cc))
                        flags &= ~TH_FIN;
        } else {
                if (SEQ_LT(tp->snd_nxt + len, tp->snd_una + so->so_snd.sb_cc))
                        flags &= ~TH_FIN;
        }

        if (tp->t_flags & TF_SLOWLINK && slowlink_wsize > 0 )   /* Clips window size for slow links */
                recwin = min(sbspace(&so->so_rcv), slowlink_wsize);
        else
                recwin = sbspace(&so->so_rcv);

        /*
         * Sender silly window avoidance.   We transmit under the following
         * conditions when len is non-zero:
         *
         *      - We have a full segment
         *      - This is the last buffer in a write()/send() and we are
         *        either idle or running NODELAY
         *      - we've timed out (e.g. persist timer)
         *      - we have more then 1/2 the maximum send window's worth of
         *        data (receiver may be limited the window size)
         *      - we need to retransmit
         */
        if (len) {
                if (len == tp->t_maxseg)
                        goto send;
                if (!(tp->t_flags & TF_MORETOCOME) &&
                    (idle || tp->t_flags & TF_NODELAY) &&
                    (tp->t_flags & TF_NOPUSH) == 0 &&
                    len + off >= so->so_snd.sb_cc)
                        goto send;
                if (tp->t_force)
                        goto send;
                if (len >= tp->max_sndwnd / 2 && tp->max_sndwnd > 0)
                        goto send;
                if (SEQ_LT(tp->snd_nxt, tp->snd_max))   /* retransmit case */
                        goto send;
                if (sack_rxmit)
                        goto send;
        }

        /*
         * Compare available window to amount of window
         * known to peer (as advertised window less
         * next expected input).  If the difference is at least two
         * max size segments, or at least 50% of the maximum possible
         * window, then want to send a window update to peer.
         * Skip this if the connection is in T/TCP half-open state.
         */
        if (recwin > 0 && !(tp->t_flags & TF_NEEDSYN)) {
                /*
                 * "adv" is the amount we can increase the window,
                 * taking into account that we are limited by
                 * TCP_MAXWIN << tp->rcv_scale.
                 */
                long adv = min(recwin, (long)TCP_MAXWIN << tp->rcv_scale) -
                        (tp->rcv_adv - tp->rcv_nxt);

                if (adv >= (long) (2 * tp->t_maxseg))
                        goto send;
                if (2 * adv >= (long) so->so_rcv.sb_hiwat)
                        goto send;
        }

        /*
         * Send if we owe the peer an ACK, RST, SYN, or urgent data.  ACKNOW
         * is also a catch-all for the retransmit timer timeout case.
         */
        if (tp->t_flags & TF_ACKNOW)
                goto send;
        if ((flags & TH_RST) ||
            ((flags & TH_SYN) && (tp->t_flags & TF_NEEDSYN) == 0))
                goto send;
        if (SEQ_GT(tp->snd_up, tp->snd_una))
                goto send;
        /*
         * If our state indicates that FIN should be sent
         * and we have not yet done so, then we need to send.
         */
        if (flags & TH_FIN &&
            ((tp->t_flags & TF_SENTFIN) == 0 || tp->snd_nxt == tp->snd_una))
                goto send;
        /*
         * In SACK, it is possible for tcp_output to fail to send a segment
         * after the retransmission timer has been turned off.  Make sure
         * that the retransmission timer is set.
         */
        if (tp->sack_enable && SEQ_GT(tp->snd_max, tp->snd_una) &&
                tp->t_timer[TCPT_REXMT] == 0 &&
            tp->t_timer[TCPT_PERSIST] == 0) {
                        tp->t_timer[TCPT_REXMT] = tp->t_rxtcur;
                        goto just_return;
        } 
        /*
         * TCP window updates are not reliable, rather a polling protocol
         * using ``persist'' packets is used to insure receipt of window
         * updates.  The three ``states'' for the output side are:
         *      idle                    not doing retransmits or persists
         *      persisting              to move a small or zero window
         *      (re)transmitting        and thereby not persisting
         *
         * tp->t_timer[TCPT_PERSIST]
         *      is set when we are in persist state.
         * tp->t_force
         *      is set when we are called to send a persist packet.
         * tp->t_timer[TCPT_REXMT]
         *      is set when we are retransmitting
         * The output side is idle when both timers are zero.
         *
         * If send window is too small, there is data to transmit, and no
         * retransmit or persist is pending, then go to persist state.
         * If nothing happens soon, send when timer expires:
         * if window is nonzero, transmit what we can,
         * otherwise force out a byte.
         */
        if (so->so_snd.sb_cc && tp->t_timer[TCPT_REXMT] == 0 &&
            tp->t_timer[TCPT_PERSIST] == 0) {
                tp->t_rxtshift = 0;
                tcp_setpersist(tp);
        }
just_return:
        /*
         * If there is no reason to send a segment, just return.
         * but if there is some packets left in the packet list, send them now.
         */
        if (packetlist) {
                error = ip_output_list(packetlist, packchain_listadd, tp->t_inpcb->inp_options, &tp->t_inpcb->inp_route,
                        (so->so_options & SO_DONTROUTE), 0);
        }
        KERNEL_DEBUG(DBG_FNC_TCP_OUTPUT | DBG_FUNC_END, 0,0,0,0,0);
        return (0);

send:
        /*
         * Before ESTABLISHED, force sending of initial options
         * unless TCP set not to do any options.
         * NOTE: we assume that the IP/TCP header plus TCP options
         * always fit in a single mbuf, leaving room for a maximum
         * link header, i.e.
         *      max_linkhdr + sizeof (struct tcpiphdr) + optlen <= MCLBYTES
         */
        optlen = 0;
#if INET6
        if (isipv6)
                hdrlen = sizeof (struct ip6_hdr) + sizeof (struct tcphdr);
        else
#endif
        hdrlen = sizeof (struct tcpiphdr);
        if (flags & TH_SYN) {
                tp->snd_nxt = tp->iss;
                if ((tp->t_flags & TF_NOOPT) == 0) {
                        u_short mss;

                        opt[0] = TCPOPT_MAXSEG;
                        opt[1] = TCPOLEN_MAXSEG;
                        mss = htons((u_short) tcp_mssopt(tp));
                        (void)memcpy(opt + 2, &mss, sizeof(mss));
                        optlen = TCPOLEN_MAXSEG;

                        if ((tp->t_flags & TF_REQ_SCALE) &&
                            ((flags & TH_ACK) == 0 ||
                            (tp->t_flags & TF_RCVD_SCALE))) {
                                *((u_int32_t *)(opt + optlen)) = htonl(
                                        TCPOPT_NOP << 24 |
                                        TCPOPT_WINDOW << 16 |
                                        TCPOLEN_WINDOW << 8 |
                                        tp->request_r_scale);
                                optlen += 4;
                        }
                }
        }

        /*
         * Send a timestamp and echo-reply if this is a SYN and our side
         * wants to use timestamps (TF_REQ_TSTMP is set) or both our side
         * and our peer have sent timestamps in our SYN's.
         */
        if ((tp->t_flags & (TF_REQ_TSTMP|TF_NOOPT)) == TF_REQ_TSTMP &&
            (flags & TH_RST) == 0 &&
            ((flags & TH_ACK) == 0 ||
             (tp->t_flags & TF_RCVD_TSTMP))) {
                u_int32_t *lp = (u_int32_t *)(opt + optlen);

                /* Form timestamp option as shown in appendix A of RFC 1323. */
                *lp++ = htonl(TCPOPT_TSTAMP_HDR);
                *lp++ = htonl(tcp_now);
                *lp   = htonl(tp->ts_recent);
                optlen += TCPOLEN_TSTAMP_APPA;
        }

        if (tp->sack_enable && ((tp->t_flags & TF_NOOPT) == 0)) {
                /* 
                 * Tack on the SACK permitted option *last*.
                 * And do padding of options after tacking this on.
                 * This is because of MSS, TS, WinScale and Signatures are
                 * all present, we have just 2 bytes left for the SACK
                 * permitted option, which is just enough.
                 */
                /*
                 * If this is the first SYN of connection (not a SYN
                 * ACK), include SACK permitted option.  If this is a
                 * SYN ACK, include SACK permitted option if peer has
                 * already done so. This is only for active connect,
                 * since the syncache takes care of the passive connect.
                 */
                if ((flags & TH_SYN) &&
                    (!(flags & TH_ACK) || (tp->t_flags & TF_SACK_PERMIT))) {
                        u_char *bp;
                        bp = (u_char *)opt + optlen;

                        *bp++ = TCPOPT_SACK_PERMITTED;
                        *bp++ = TCPOLEN_SACK_PERMITTED;
                        optlen += TCPOLEN_SACK_PERMITTED;
                }

                /*
                 * Send SACKs if necessary.  This should be the last
                 * option processed.  Only as many SACKs are sent as
                 * are permitted by the maximum options size.
                 *
                 * In general, SACK blocks consume 8*n+2 bytes.
                 * So a full size SACK blocks option is 34 bytes
                 * (to generate 4 SACK blocks).  At a minimum,
                 * we need 10 bytes (to generate 1 SACK block).
                 * If TCP Timestamps (12 bytes) and TCP Signatures
                 * (18 bytes) are both present, we'll just have
                 * 10 bytes for SACK options 40 - (12 + 18).
                 */
                if (TCPS_HAVEESTABLISHED(tp->t_state) &&
                    (tp->t_flags & TF_SACK_PERMIT) && tp->rcv_numsacks > 0 &&
                    MAX_TCPOPTLEN - optlen - 2 >= TCPOLEN_SACK) {
                        int nsack, sackoptlen, padlen;
                        u_char *bp = (u_char *)opt + optlen;
                        u_int32_t *lp;

                        nsack = (MAX_TCPOPTLEN - optlen - 2) / TCPOLEN_SACK;
                        nsack = min(nsack, tp->rcv_numsacks);
                        sackoptlen = (2 + nsack * TCPOLEN_SACK);

                        /*
                         * First we need to pad options so that the
                         * SACK blocks can start at a 4-byte boundary
                         * (sack option and length are at a 2 byte offset).
                         */
                        padlen = (MAX_TCPOPTLEN - optlen - sackoptlen) % 4;
                        optlen += padlen;
                        while (padlen-- > 0)
                                *bp++ = TCPOPT_NOP;

                        tcpstat.tcps_sack_send_blocks++;
                        *bp++ = TCPOPT_SACK;
                        *bp++ = sackoptlen;
                        lp = (u_int32_t *)bp;
                        for (i = 0; i < nsack; i++) {
                                struct sackblk sack = tp->sackblks[i];
                                *lp++ = htonl(sack.start);
                                *lp++ = htonl(sack.end);
                        }
                        optlen += sackoptlen;
                }
        }

        /* Pad TCP options to a 4 byte boundary */
        if (optlen < MAX_TCPOPTLEN && (optlen % sizeof(u_int32_t))) {
                int pad = sizeof(u_int32_t) - (optlen % sizeof(u_int32_t));
                u_char *bp = (u_char *)opt + optlen;

                optlen += pad;
                while (pad) {
                        *bp++ = TCPOPT_EOL;
                        pad--;
                }
        }

        hdrlen += optlen;

#if INET6
        if (isipv6)
                ipoptlen = ip6_optlen(tp->t_inpcb);
        else
#endif
        {
                if (tp->t_inpcb->inp_options) {
                        ipoptlen = tp->t_inpcb->inp_options->m_len -
                                offsetof(struct ipoption, ipopt_list);
                } else
                        ipoptlen = 0;
        }
#if IPSEC
        if (ipsec_bypass == 0)
                ipoptlen += ipsec_hdrsiz_tcp(tp);
#endif

        /*
         * Adjust data length if insertion of options will
         * bump the packet length beyond the t_maxopd length.
         * Clear the FIN bit because we cut off the tail of
         * the segment.
         */
        if (len + optlen + ipoptlen > tp->t_maxopd) {
                /*
                 * If there is still more to send, don't close the connection.
                 */
                flags &= ~TH_FIN;
                len = tp->t_maxopd - optlen - ipoptlen;
                howmuchsent += len;
                sendalot = 1;
        }

/*#ifdef DIAGNOSTIC*/
#if INET6
        if (max_linkhdr + hdrlen > MCLBYTES)
                panic("tcphdr too big");
#else
        if (max_linkhdr + hdrlen > MHLEN)
                panic("tcphdr too big");
#endif
/*#endif*/

        /*
         * Grab a header mbuf, attaching a copy of data to
         * be transmitted, and initialize the header from
         * the template for sends on this connection.
         */
        if (len) {
                if (tp->t_force && len == 1)
                        tcpstat.tcps_sndprobe++;
                else if (SEQ_LT(tp->snd_nxt, tp->snd_max)) {
                        tcpstat.tcps_sndrexmitpack++;
                        tcpstat.tcps_sndrexmitbyte += len;
                } else {
                        tcpstat.tcps_sndpack++;
                        tcpstat.tcps_sndbyte += len;
                }
#ifdef notyet
                if ((m = m_copypack(so->so_snd.sb_mb, off,
                    (int)len, max_linkhdr + hdrlen)) == 0) {
                        error = ENOBUFS;
                        goto out;
                }
                /*
                 * m_copypack left space for our hdr; use it.
                 */
                m->m_len += hdrlen;
                m->m_data -= hdrlen;
#else
                /*
                 * try to use the new interface that allocates all 
                 * the necessary mbuf hdrs under 1 mbuf lock and 
                 * avoids rescanning the socket mbuf list if 
                 * certain conditions are met.  This routine can't
                 * be used in the following cases...
                 * 1) the protocol headers exceed the capacity of
                 * of a single mbuf header's data area (no cluster attached)
                 * 2) the length of the data being transmitted plus
                 * the protocol headers fits into a single mbuf header's
                 * data area (no cluster attached)
                 */
                m = NULL;
#if INET6
                if (MHLEN < hdrlen + max_linkhdr) {
                        MGETHDR(m, M_DONTWAIT, MT_HEADER);
                        if (m == NULL) {
                                error = ENOBUFS;
                                goto out;
                        }
                        MCLGET(m, M_DONTWAIT);
                        if ((m->m_flags & M_EXT) == 0) {
                                m_freem(m);
                                error = ENOBUFS;
                                goto out;
                        }
                        m->m_data += max_linkhdr;
                        m->m_len = hdrlen;
                }
#endif
                if (len <= MHLEN - hdrlen - max_linkhdr) {
                        if (m == NULL) {
                                MGETHDR(m, M_DONTWAIT, MT_HEADER);
                                if (m == NULL) {
                                        error = ENOBUFS;
                                        goto out;
                                }
                                m->m_data += max_linkhdr;
                                m->m_len = hdrlen;
                        }
                        /* makes sure we still have data left to be sent at this point */
                        if (so->so_snd.sb_mb == NULL || off == -1) {
                                if (m != NULL)  m_freem(m);
                                error = 0; /* should we return an error? */
                                goto out;
                        }
                        m_copydata(so->so_snd.sb_mb, off, (int) len,
                            mtod(m, caddr_t) + hdrlen);
                        m->m_len += len;
                } else {
                        if (m != NULL) {
                                m->m_next = m_copy(so->so_snd.sb_mb, off, (int) len);
                                if (m->m_next == 0) {
                                        (void) m_free(m);
                                        error = ENOBUFS;
                                        goto out;
                                }
                        } else {
                                /*
                                 * determine whether the mbuf pointer and offset passed back by the 'last' call
                                 * to m_copym_with_hdrs are still valid... if the head of the socket chain has
                                 * changed (due to an incoming ACK for instance), or the offset into the chain we
                                 * just computed is different from the one last returned by m_copym_with_hdrs (perhaps
                                 * we're re-transmitting a packet sent earlier), than we can't pass the mbuf pointer and
                                 * offset into it as valid hints for m_copym_with_hdrs to use (if valid, these hints allow
                                 * m_copym_with_hdrs to avoid rescanning from the beginning of the socket buffer mbuf list.
                                 * setting the mbuf pointer to NULL is sufficient to disable the hint mechanism.
                                 */
                                if (m_head != so->so_snd.sb_mb || last_off != off)
                                        m_last = NULL;
                                last_off = off + len;
                                m_head = so->so_snd.sb_mb;
        
                                /* makes sure we still have data left to be sent at this point */
                                if (m_head == NULL) {
                                        error = 0; /* should we return an error? */
                                        goto out;
                                }
                                
                                /*
                                 * m_copym_with_hdrs will always return the last mbuf pointer and the offset into it that
                                 * it acted on to fullfill the current request, whether a valid 'hint' was passed in or not
                                 */
                                if ((m = m_copym_with_hdrs(so->so_snd.sb_mb, off, (int) len, M_DONTWAIT, &m_last, &m_off)) == NULL) {
                                        error = ENOBUFS;
                                        goto out;
                                }
                                m->m_data += max_linkhdr;
                                m->m_len = hdrlen;
                        }
                }
#endif
                /*
                 * If we're sending everything we've got, set PUSH.
                 * (This will keep happy those implementations which only
                 * give data to the user when a buffer fills or
                 * a PUSH comes in.)
                 */
                if (off + len == so->so_snd.sb_cc)
                        flags |= TH_PUSH;
        } else {
                if (tp->t_flags & TF_ACKNOW)
                        tcpstat.tcps_sndacks++;
                else if (flags & (TH_SYN|TH_FIN|TH_RST))
                        tcpstat.tcps_sndctrl++;
                else if (SEQ_GT(tp->snd_up, tp->snd_una))
                        tcpstat.tcps_sndurg++;
                else
                        tcpstat.tcps_sndwinup++;

                MGETHDR(m, M_DONTWAIT, MT_HEADER);
                if (m == NULL) {
                        error = ENOBUFS;
                        goto out;
                }
#if INET6
                if (isipv6 && (MHLEN < hdrlen + max_linkhdr) &&
                    MHLEN >= hdrlen) {
                        MH_ALIGN(m, hdrlen);
                } else
#endif
                m->m_data += max_linkhdr;
                m->m_len = hdrlen;
        }
        m->m_pkthdr.rcvif = 0;
#if INET6
        if (isipv6) {
                ip6 = mtod(m, struct ip6_hdr *);
                th = (struct tcphdr *)(ip6 + 1);
                tcp_fillheaders(tp, ip6, th);
        } else
#endif /* INET6 */
        {
                ip = mtod(m, struct ip *);
                ipov = (struct ipovly *)ip;
                th = (struct tcphdr *)(ip + 1);
                /* this picks up the pseudo header (w/o the length) */
                tcp_fillheaders(tp, ip, th);
        }

        /*
         * Fill in fields, remembering maximum advertised
         * window for use in delaying messages about window sizes.
         * If resending a FIN, be sure not to use a new sequence number.
         */
        if (flags & TH_FIN && tp->t_flags & TF_SENTFIN &&
            tp->snd_nxt == tp->snd_max)
                tp->snd_nxt--;
        /*
         * If we are doing retransmissions, then snd_nxt will
         * not reflect the first unsent octet.  For ACK only
         * packets, we do not want the sequence number of the
         * retransmitted packet, we want the sequence number
         * of the next unsent octet.  So, if there is no data
         * (and no SYN or FIN), use snd_max instead of snd_nxt
         * when filling in ti_seq.  But if we are in persist
         * state, snd_max might reflect one byte beyond the
         * right edge of the window, so use snd_nxt in that
         * case, since we know we aren't doing a retransmission.
         * (retransmit and persist are mutually exclusive...)
         */
        if (sack_rxmit == 0) {
                if (len || (flags & (TH_SYN|TH_FIN)) || tp->t_timer[TCPT_PERSIST])
                        th->th_seq = htonl(tp->snd_nxt);
                else
                        th->th_seq = htonl(tp->snd_max);
        } else {
                th->th_seq = htonl(p->rxmit);
                p->rxmit += len;
                tp->sackhint.sack_bytes_rexmit += len;
        }
        th->th_ack = htonl(tp->rcv_nxt);
        if (optlen) {
                bcopy(opt, th + 1, optlen);
                th->th_off = (sizeof (struct tcphdr) + optlen) >> 2;
        }
        th->th_flags = flags;
        /*
         * Calculate receive window.  Don't shrink window,
         * but avoid silly window syndrome.
         */
        if (recwin < (long)(so->so_rcv.sb_hiwat / 4) && recwin < (long)tp->t_maxseg)
                recwin = 0;
        if (recwin < (long)(tp->rcv_adv - tp->rcv_nxt))
                recwin = (long)(tp->rcv_adv - tp->rcv_nxt);
        if (tp->t_flags & TF_SLOWLINK && slowlink_wsize > 0) {
                if (recwin > (long)slowlink_wsize) 
                        recwin = slowlink_wsize;
                        th->th_win = htons((u_short) (recwin>>tp->rcv_scale));
        }
        else {
                if (recwin > (long)TCP_MAXWIN << tp->rcv_scale)
                        recwin = (long)TCP_MAXWIN << tp->rcv_scale;
                th->th_win = htons((u_short) (recwin>>tp->rcv_scale));
        }

        /*
         * Adjust the RXWIN0SENT flag - indicate that we have advertised
         * a 0 window.  This may cause the remote transmitter to stall.  This
         * flag tells soreceive() to disable delayed acknowledgements when
         * draining the buffer.  This can occur if the receiver is attempting
         * to read more data then can be buffered prior to transmitting on
         * the connection.
         */
        if (recwin == 0)
                tp->t_flags |= TF_RXWIN0SENT;
        else
                tp->t_flags &= ~TF_RXWIN0SENT;
        if (SEQ_GT(tp->snd_up, tp->snd_nxt)) {
                th->th_urp = htons((u_short)(tp->snd_up - tp->snd_nxt));
                th->th_flags |= TH_URG;
        } else
                /*
                 * If no urgent pointer to send, then we pull
                 * the urgent pointer to the left edge of the send window
                 * so that it doesn't drift into the send window on sequence
                 * number wraparound.
                 */
                tp->snd_up = tp->snd_una;               /* drag it along */

        /*
         * Put TCP length in extended header, and then
         * checksum extended header and data.
         */
        m->m_pkthdr.len = hdrlen + len; /* in6_cksum() need this */
#if INET6
        if (isipv6)
                /*
                 * ip6_plen is not need to be filled now, and will be filled
                 * in ip6_output.
                 */
                th->th_sum = in6_cksum(m, IPPROTO_TCP, sizeof(struct ip6_hdr),
                                       sizeof(struct tcphdr) + optlen + len);
        else
#endif /* INET6 */
        {
                m->m_pkthdr.csum_flags = CSUM_TCP;
                m->m_pkthdr.csum_data = offsetof(struct tcphdr, th_sum);
                if (len + optlen)
                        th->th_sum = in_addword(th->th_sum, 
                                htons((u_short)(optlen + len)));
      }

        /*
         * In transmit state, time the transmission and arrange for
         * the retransmit.  In persist state, just set snd_max.
         */
        if (tp->t_force == 0 || tp->t_timer[TCPT_PERSIST] == 0) {
                tcp_seq startseq = tp->snd_nxt;

                /*
                 * Advance snd_nxt over sequence space of this segment.
                 */
                if (flags & (TH_SYN|TH_FIN)) {
                        if (flags & TH_SYN)
                                tp->snd_nxt++;
                        if (flags & TH_FIN) {
                                tp->snd_nxt++;
                                tp->t_flags |= TF_SENTFIN;
                        }
                }
                if (sack_rxmit)
                        goto timer;
                tp->snd_nxt += len;
                if (SEQ_GT(tp->snd_nxt, tp->snd_max)) {
                        tp->snd_max = tp->snd_nxt;
                        /*
                         * Time this transmission if not a retransmission and
                         * not currently timing anything.
                         */
                        if (tp->t_rtttime == 0) {
                                tp->t_rtttime = 1;
                                tp->t_rtseq = startseq;
                                tcpstat.tcps_segstimed++;
                        }
                }

                /*
                 * Set retransmit timer if not currently set,
                 * and not doing an ack or a keep-alive probe.
                 * Initial value for retransmit timer is smoothed
                 * round-trip time + 2 * round-trip time variance.
                 * Initialize shift counter which is used for backoff
                 * of retransmit time.
                 */
timer:
                if (tp->t_timer[TCPT_REXMT] == 0 &&
                    ((sack_rxmit && tp->snd_nxt != tp->snd_max) ||
                        tp->snd_nxt != tp->snd_una)) {
                        if (tp->t_timer[TCPT_PERSIST]) {
                                tp->t_timer[TCPT_PERSIST] = 0;
                                tp->t_rxtshift = 0;
                        }
                        tp->t_timer[TCPT_REXMT] = tp->t_rxtcur;
                }
        } else {
                /*
                 * Persist case, update snd_max but since we are in
                 * persist mode (no window) we do not update snd_nxt.
                 */
                int xlen = len;
                if (flags & TH_SYN)
                        ++xlen;
                if (flags & TH_FIN) {
                        ++xlen;
                        tp->t_flags |= TF_SENTFIN;
                }
                if (SEQ_GT(tp->snd_nxt + xlen, tp->snd_max))
                        tp->snd_max = tp->snd_nxt + len;
        }

#if TCPDEBUG
        /*
         * Trace.
         */
        if (so->so_options & SO_DEBUG)
                tcp_trace(TA_OUTPUT, tp->t_state, tp, mtod(m, void *), th, 0);
#endif

        /*
         * Fill in IP length and desired time to live and
         * send to IP level.  There should be a better way
         * to handle ttl and tos; we could keep them in
         * the template, but need a way to checksum without them.
         */
        /*
         * m->m_pkthdr.len should have been set before cksum calcuration,
         * because in6_cksum() need it.
         */
#if INET6
        if (isipv6) {
                /*
                 * we separately set hoplimit for every segment, since the
                 * user might want to change the value via setsockopt.
                 * Also, desired default hop limit might be changed via
                 * Neighbor Discovery.
                 */
                ip6->ip6_hlim = in6_selecthlim(tp->t_inpcb,
                                               tp->t_inpcb->in6p_route.ro_rt ?
                                               tp->t_inpcb->in6p_route.ro_rt->rt_ifp
                                               : NULL);

                /* TODO: IPv6 IP6TOS_ECT bit on */
#if IPSEC
                if (ipsec_bypass == 0 && ipsec_setsocket(m, so) != 0) {
                        m_freem(m);
                        error = ENOBUFS;
                        goto out;
                }
#endif /*IPSEC*/
                m->m_pkthdr.socket_id = socket_id;
                error = ip6_output(m,
                            tp->t_inpcb->in6p_outputopts,
                            &tp->t_inpcb->in6p_route,
                            (so->so_options & SO_DONTROUTE), NULL, NULL, 0);
        } else
#endif /* INET6 */
    {
        struct rtentry *rt;
        ip->ip_len = m->m_pkthdr.len;
#if INET6
        if (isipv6)
                ip->ip_ttl = in6_selecthlim(tp->t_inpcb,
                                            tp->t_inpcb->in6p_route.ro_rt ?
                                            tp->t_inpcb->in6p_route.ro_rt->rt_ifp
                                            : NULL);
        else
#endif /* INET6 */
        ip->ip_ttl = tp->t_inpcb->inp_ip_ttl;   /* XXX */
        ip->ip_tos = tp->t_inpcb->inp_ip_tos;   /* XXX */


#if INET6
        if (isipv6) {
                KERNEL_DEBUG(DBG_LAYER_BEG,
                     ((tp->t_inpcb->inp_fport << 16) | tp->t_inpcb->inp_lport),
                     (((tp->t_inpcb->in6p_laddr.s6_addr16[0] & 0xffff) << 16) |
                      (tp->t_inpcb->in6p_faddr.s6_addr16[0] & 0xffff)),
                     0,0,0);
        }
        else 
#endif
        {
                KERNEL_DEBUG(DBG_LAYER_BEG,
                     ((tp->t_inpcb->inp_fport << 16) | tp->t_inpcb->inp_lport),
                     (((tp->t_inpcb->inp_laddr.s_addr & 0xffff) << 16) |
                      (tp->t_inpcb->inp_faddr.s_addr & 0xffff)),
                     0,0,0);
        }

        /*
         * See if we should do MTU discovery.  We do it only if the following
         * are true:
         *      1) we have a valid route to the destination
         *      2) the MTU is not locked (if it is, then discovery has been
         *         disabled)
         */
        if (path_mtu_discovery
            && (rt = tp->t_inpcb->inp_route.ro_rt)
            && rt->rt_flags & RTF_UP
            && !(rt->rt_rmx.rmx_locks & RTV_MTU)) {
                ip->ip_off |= IP_DF;
        }
#if IPSEC
        if (ipsec_bypass == 0)
                ipsec_setsocket(m, so);
#endif /*IPSEC*/

        /*
         * The socket is kept locked while sending out packets in ip_output, even if packet chaining is not active.
         */

        m->m_pkthdr.socket_id = socket_id;
        if (packetlist) {
                m->m_nextpkt = NULL;
                lastpacket->m_nextpkt = m;
                lastpacket = m;
                packchain_listadd++;
        }
        else {
                m->m_nextpkt = NULL;
                packchain_newlist++;
                packetlist = lastpacket = m;
                packchain_listadd=0;
        }

       if ((ipsec_bypass == 0) || fw_enable || sendalot == 0 || (tp->t_state != TCPS_ESTABLISHED) || 
                      (tp->snd_cwnd <= (tp->snd_wnd / 4)) || 
                      (tp->t_flags & (TH_PUSH | TF_ACKNOW)) || tp->t_force != 0 ||
                      packchain_listadd >= tcp_packet_chaining) {
                lastpacket->m_nextpkt = 0;
                error = ip_output_list(packetlist, packchain_listadd, tp->t_inpcb->inp_options, &tp->t_inpcb->inp_route,
                        (so->so_options & SO_DONTROUTE), 0);
                tp->t_lastchain = packchain_listadd;
                packchain_sent++;
                packetlist = NULL;
                if (error == 0)
                        howmuchsent = 0;
        }
        else {
                error = 0;
                packchain_looped++;
                tcpstat.tcps_sndtotal++;
                if (recwin > 0 && SEQ_GT(tp->rcv_nxt+recwin, tp->rcv_adv))
                        tp->rcv_adv = tp->rcv_nxt + recwin;
                tp->last_ack_sent = tp->rcv_nxt;
                tp->t_flags &= ~(TF_ACKNOW|TF_DELACK);
                goto again;
        }
   }
        if (error) {

                /*
                 * We know that the packet was lost, so back out the
                 * sequence number advance, if any.
                 */
                if (tp->t_force == 0 || tp->t_timer[TCPT_PERSIST] == 0) {
                        /*
                         * No need to check for TH_FIN here because
                         * the TF_SENTFIN flag handles that case.
                         */
                        if ((flags & TH_SYN) == 0) {
                                if (sack_rxmit) {
                                        p->rxmit -= howmuchsent;
                                        tp->sackhint.sack_bytes_rexmit -= howmuchsent;
                                } else
                                        tp->snd_nxt -= howmuchsent;
                        }
                }
                howmuchsent = 0;
out:
                if (error == ENOBUFS) {
                        if (!tp->t_timer[TCPT_REXMT] &&
                             !tp->t_timer[TCPT_PERSIST])
                                tp->t_timer[TCPT_REXMT] = tp->t_rxtcur;
                        tcp_quench(tp->t_inpcb, 0);
                        if (packetlist)
                                m_freem_list(packetlist);
                        tp->t_lastchain = 0;
                        KERNEL_DEBUG(DBG_FNC_TCP_OUTPUT | DBG_FUNC_END, 0,0,0,0,0);
                        return (0);
                }
                if (error == EMSGSIZE) {
                        /*
                         * ip_output() will have already fixed the route
                         * for us.  tcp_mtudisc() will, as its last action,
                         * initiate retransmission, so it is important to
                         * not do so here.
                         */
                        tcp_mtudisc(tp->t_inpcb, 0);
                        if (packetlist)
                                m_freem_list(packetlist);
                        tp->t_lastchain = 0;
                        KERNEL_DEBUG(DBG_FNC_TCP_OUTPUT | DBG_FUNC_END, 0,0,0,0,0);
                        return 0;
                }
                if ((error == EHOSTUNREACH || error == ENETDOWN)
                    && TCPS_HAVERCVDSYN(tp->t_state)) {
                        tp->t_softerror = error;
                        if (packetlist)
                                m_freem_list(packetlist);
                        tp->t_lastchain = 0;
                        KERNEL_DEBUG(DBG_FNC_TCP_OUTPUT | DBG_FUNC_END, 0,0,0,0,0);
                        return (0);
                }
                if (packetlist)
                        m_freem_list(packetlist);
                tp->t_lastchain = 0;
                KERNEL_DEBUG(DBG_FNC_TCP_OUTPUT | DBG_FUNC_END, 0,0,0,0,0);
                return (error);
        }
sentit:
        tcpstat.tcps_sndtotal++;

        /*
         * Data sent (as far as we can tell).
         * If this advertises a larger window than any other segment,
         * then remember the size of the advertised window.
         * Any pending ACK has now been sent.
         */
        if (recwin > 0 && SEQ_GT(tp->rcv_nxt+recwin, tp->rcv_adv))
                tp->rcv_adv = tp->rcv_nxt + recwin;
        tp->last_ack_sent = tp->rcv_nxt;
        tp->t_flags &= ~(TF_ACKNOW|TF_DELACK);

        KERNEL_DEBUG(DBG_FNC_TCP_OUTPUT | DBG_FUNC_END,0,0,0,0,0);
        if (sendalot && (!tcp_do_newreno || --maxburst))
                goto again;
        return (0);
}

void
tcp_setpersist(tp)
        register struct tcpcb *tp;
{
        int t = ((tp->t_srtt >> 2) + tp->t_rttvar) >> 1;

        if (tp->t_timer[TCPT_REXMT])
                panic("tcp_setpersist: retransmit pending");
        /*
         * Start/restart persistance timer.
         */
        TCPT_RANGESET(tp->t_timer[TCPT_PERSIST],
            t * tcp_backoff[tp->t_rxtshift],
            TCPTV_PERSMIN, TCPTV_PERSMAX);
        if (tp->t_rxtshift < TCP_MAXRXTSHIFT)
                tp->t_rxtshift++;
}