/*      $NetBSD: tcp_output.c,v 1.222 2024/09/08 09:36:52 rillig Exp $  */

/*
 * Copyright (C) 1995, 1996, 1997, and 1998 WIDE Project.
 * 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. Neither the name of the project 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 PROJECT 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 PROJECT 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.
 */

/*
 *      @(#)COPYRIGHT   1.1 (NRL) 17 January 1995
 *
 * NRL grants permission for redistribution and use in source and binary
 * forms, with or without modification, of the software and documentation
 * created at NRL 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 acknowledgements:
 *      This product includes software developed by the University of
 *      California, Berkeley and its contributors.
 *      This product includes software developed at the Information
 *      Technology Division, US Naval Research Laboratory.
 * 4. Neither the name of the NRL nor the names of its contributors
 *    may be used to endorse or promote products derived from this software
 *    without specific prior written permission.
 *
 * THE SOFTWARE PROVIDED BY NRL IS PROVIDED BY NRL 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 NRL 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.
 *
 * The views and conclusions contained in the software and documentation
 * are those of the authors and should not be interpreted as representing
 * official policies, either expressed or implied, of the US Naval
 * Research Laboratory (NRL).
 */

/*-
 * Copyright (c) 1997, 1998, 2001, 2005, 2006 The NetBSD Foundation, Inc.
 * All rights reserved.
 *
 * This code is derived from software contributed to The NetBSD Foundation
 * by Jason R. Thorpe and Kevin M. Lahey of the Numerical Aerospace Simulation
 * Facility, NASA Ames Research Center.
 * This code is derived from software contributed to The NetBSD Foundation
 * by Charles M. Hannum.
 * This code is derived from software contributed to The NetBSD Foundation
 * by Rui Paulo.
 *
 * 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.
 *
 * THIS SOFTWARE IS PROVIDED BY THE NETBSD FOUNDATION, INC. 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 FOUNDATION 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.
 */

/*
 * 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. 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
 */

#include <sys/cdefs.h>
__KERNEL_RCSID(0, "$NetBSD: tcp_output.c,v 1.222 2024/09/08 09:36:52 rillig Exp $");

#ifdef _KERNEL_OPT
#include "opt_inet.h"
#include "opt_ipsec.h"
#include "opt_tcp_debug.h"
#endif

#include <sys/param.h>
#include <sys/systm.h>
#include <sys/mbuf.h>
#include <sys/protosw.h>
#include <sys/socket.h>
#include <sys/socketvar.h>
#include <sys/errno.h>
#include <sys/domain.h>
#include <sys/kernel.h>
#ifdef TCP_SIGNATURE
#include <sys/md5.h>
#endif

#include <net/if.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>

#ifdef INET6
#include <netinet/ip6.h>
#include <netinet6/in6_var.h>
#include <netinet6/ip6_var.h>
#include <netinet6/in6_pcb.h>
#include <netinet6/nd6.h>
#endif

#ifdef IPSEC
#include <netipsec/ipsec.h>
#include <netipsec/key.h>
#ifdef INET6
#include <netipsec/ipsec6.h>
#endif
#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/tcp_private.h>
#include <netinet/tcp_congctl.h>
#include <netinet/tcp_debug.h>
#include <netinet/in_offload.h>
#include <netinet6/in6_offload.h>

/*
 * Knob to enable Congestion Window Monitoring, and control
 * the burst size it allows.  Default burst is 4 packets, per
 * the Internet draft.
 */
int     tcp_cwm = 0;
int     tcp_cwm_burstsize = 4;

int     tcp_do_autosndbuf = 1;
int     tcp_autosndbuf_inc = 8 * 1024;
int     tcp_autosndbuf_max = 256 * 1024;

#ifdef TCP_OUTPUT_COUNTERS
#include <sys/device.h>

extern struct evcnt tcp_output_bigheader;
extern struct evcnt tcp_output_predict_hit;
extern struct evcnt tcp_output_predict_miss;
extern struct evcnt tcp_output_copysmall;
extern struct evcnt tcp_output_copybig;
extern struct evcnt tcp_output_refbig;

#define TCP_OUTPUT_COUNTER_INCR(ev)     (ev)->ev_count++
#else

#define TCP_OUTPUT_COUNTER_INCR(ev)     /* nothing */

#endif /* TCP_OUTPUT_COUNTERS */

static int
tcp_segsize(struct tcpcb *tp, int *txsegsizep, int *rxsegsizep,
    bool *alwaysfragp)
{
        struct inpcb *inp = tp->t_inpcb;
        struct socket *so = NULL;
        struct rtentry *rt;
        struct ifnet *ifp;
        int size;
        int hdrlen;
        int optlen;

        *alwaysfragp = false;
        size = tcp_mssdflt;

        switch (tp->t_family) {
        case AF_INET:
                hdrlen = sizeof(struct ip) + sizeof(struct tcphdr);
                break;
#ifdef INET6
        case AF_INET6:
                hdrlen = sizeof(struct ip6_hdr) + sizeof(struct tcphdr);
                break;
#endif
        default:
                hdrlen = 1; /* prevent zero sized segments */
                goto out;
        }

        rt = inpcb_rtentry(inp);
        so = inp->inp_socket;
        if (rt == NULL) {
                goto out;
        }

        ifp = rt->rt_ifp;

        if (tp->t_mtudisc && rt->rt_rmx.rmx_mtu != 0) {
#ifdef INET6
                if (inp->inp_af == AF_INET6 && rt->rt_rmx.rmx_mtu < IPV6_MMTU) {
                        /*
                         * RFC2460 section 5, last paragraph: if path MTU is
                         * smaller than 1280, use 1280 as packet size and
                         * attach fragment header.
                         */
                        size = IPV6_MMTU - hdrlen - sizeof(struct ip6_frag);
                        *alwaysfragp = true;
                } else
                        size = rt->rt_rmx.rmx_mtu - hdrlen;
#else
                size = rt->rt_rmx.rmx_mtu - hdrlen;
#endif
        } else if (ifp->if_flags & IFF_LOOPBACK)
                size = ifp->if_mtu - hdrlen;
        else if (inp->inp_af == AF_INET && tp->t_mtudisc)
                size = ifp->if_mtu - hdrlen;
        else if (inp->inp_af == AF_INET && in_localaddr(in4p_faddr(inp)))
                size = ifp->if_mtu - hdrlen;
#ifdef INET6
        else if (inp->inp_af == AF_INET6) {
                if (IN6_IS_ADDR_V4MAPPED(&in6p_faddr(inp))) {
                        /* mapped addr case */
                        struct in_addr d;
                        memcpy(&d, &in6p_faddr(inp).s6_addr32[3], sizeof(d));
                        if (tp->t_mtudisc || in_localaddr(d))
                                size = ifp->if_mtu - hdrlen;
                } else {
                        /*
                         * for IPv6, path MTU discovery is always turned on,
                         * or the node must use packet size <= 1280.
                         */
                        size = tp->t_mtudisc ? ifp->if_mtu : IPV6_MMTU;
                        size -= hdrlen;
                }
        }
#endif
        inpcb_rtentry_unref(rt, inp);
 out:
        /*
         * Now we must make room for whatever extra TCP/IP options are in
         * the packet.
         */
        optlen = tcp_optlen(tp);

        /*
         * XXX tp->t_ourmss should have the right size, but without this code
         * fragmentation will occur... need more investigation
         */

        if (inp->inp_af == AF_INET) {
#if defined(IPSEC)
                if (ipsec_used &&
                    !ipsec_pcb_skip_ipsec(inp->inp_sp, IPSEC_DIR_OUTBOUND))
                        optlen += ipsec4_hdrsiz_tcp(tp);
#endif
                optlen += ip_optlen(inp);
        }

#ifdef INET6
        if (inp->inp_af == AF_INET6 && tp->t_family == AF_INET) {
#if defined(IPSEC)
                if (ipsec_used &&
                    !ipsec_pcb_skip_ipsec(inp->inp_sp, IPSEC_DIR_OUTBOUND))
                        optlen += ipsec4_hdrsiz_tcp(tp);
#endif
                /* XXX size -= ip_optlen(in6p); */
        } else if (inp->inp_af == AF_INET6) {
#if defined(IPSEC)
                if (ipsec_used &&
                    !ipsec_pcb_skip_ipsec(inp->inp_sp, IPSEC_DIR_OUTBOUND))
                        optlen += ipsec6_hdrsiz_tcp(tp);
#endif
                optlen += ip6_optlen(inp);
        }
#endif
        size -= optlen;

        /*
         * There may not be any room for data if mtu is too small. This
         * includes zero-sized.
         */
        if (size <= 0) {
                return EMSGSIZE;
        }

        /*
         * *rxsegsizep holds *estimated* inbound segment size (estimation
         * assumes that path MTU is the same for both ways).  this is only
         * for silly window avoidance, do not use the value for other purposes.
         *
         * ipseclen is subtracted from both sides, this may not be right.
         * I'm not quite sure about this (could someone comment).
         */
        *txsegsizep = uimin(tp->t_peermss - optlen, size);
        *rxsegsizep = uimin(tp->t_ourmss - optlen, size);

        /*
         * Never send more than half a buffer full.  This insures that we can
         * always keep 2 packets on the wire, no matter what SO_SNDBUF is, and
         * therefore acks will never be delayed unless we run out of data to
         * transmit.
         */
        if (so) {
                *txsegsizep = uimin(so->so_snd.sb_hiwat >> 1, *txsegsizep);
        }

        /*
         * A segment must at least store header + options
         */
        if (*txsegsizep < hdrlen + optlen) {
                return EMSGSIZE;
        }

        if (*txsegsizep != tp->t_segsz) {
                /*
                 * If the new segment size is larger, we don't want to
                 * mess up the congestion window, but if it is smaller
                 * we'll have to reduce the congestion window to ensure
                 * that we don't get into trouble with initial windows
                 * and the rest.  In any case, if the segment size
                 * has changed, chances are the path has, too, and
                 * our congestion window will be different.
                 */
                if (*txsegsizep < tp->t_segsz) {
                        tp->snd_cwnd = uimax((tp->snd_cwnd / tp->t_segsz)
                            * *txsegsizep, *txsegsizep);
                        tp->snd_ssthresh = uimax((tp->snd_ssthresh / tp->t_segsz)
                            * *txsegsizep, *txsegsizep);
                }
                tp->t_segsz = *txsegsizep;
        }

        return 0;
}

static int
tcp_build_datapkt(struct tcpcb *tp, struct socket *so, int off,
    long len, int hdrlen, struct mbuf **mp)
{
        struct mbuf *m, *m0;
        net_stat_ref_t tcps;

        tcps = TCP_STAT_GETREF();
        if (tp->t_force && len == 1)
                _NET_STATINC_REF(tcps, TCP_STAT_SNDPROBE);
        else if (SEQ_LT(tp->snd_nxt, tp->snd_max)) {
                tp->t_sndrexmitpack++;
                _NET_STATINC_REF(tcps, TCP_STAT_SNDREXMITPACK);
                _NET_STATADD_REF(tcps, TCP_STAT_SNDREXMITBYTE, len);
        } else {
                _NET_STATINC_REF(tcps, TCP_STAT_SNDPACK);
                _NET_STATADD_REF(tcps, TCP_STAT_SNDBYTE, len);
        }
        TCP_STAT_PUTREF();

        MGETHDR(m, M_DONTWAIT, MT_HEADER);
        if (__predict_false(m == NULL))
                return ENOBUFS;
        MCLAIM(m, &tcp_tx_mowner);

        /*
         * XXX Because other code assumes headers will fit in
         * XXX one header mbuf.
         *
         * (This code should almost *never* be run.)
         */
        if (__predict_false((max_linkhdr + hdrlen) > MHLEN)) {
                TCP_OUTPUT_COUNTER_INCR(&tcp_output_bigheader);
                MCLGET(m, M_DONTWAIT);
                if ((m->m_flags & M_EXT) == 0) {
                        m_freem(m);
                        return ENOBUFS;
                }
        }

        m->m_data += max_linkhdr;
        m->m_len = hdrlen;

        /*
         * To avoid traversing the whole sb_mb chain for correct
         * data to send, remember last sent mbuf, its offset and
         * the sent size.  When called the next time, see if the
         * data to send is directly following the previous transfer.
         * This is important for large TCP windows.
         */
        if (off == 0 || tp->t_lastm == NULL ||
            (tp->t_lastoff + tp->t_lastlen) != off) {
                TCP_OUTPUT_COUNTER_INCR(&tcp_output_predict_miss);
                /*
                 * Either a new packet or a retransmit.
                 * Start from the beginning.
                 */
                tp->t_lastm = so->so_snd.sb_mb;
                tp->t_inoff = off;
        } else {
                TCP_OUTPUT_COUNTER_INCR(&tcp_output_predict_hit);
                tp->t_inoff += tp->t_lastlen;
        }

        /* Traverse forward to next packet */
        while (tp->t_inoff > 0) {
                if (tp->t_lastm == NULL)
                        panic("tp->t_lastm == NULL");
                if (tp->t_inoff < tp->t_lastm->m_len)
                        break;
                tp->t_inoff -= tp->t_lastm->m_len;
                tp->t_lastm = tp->t_lastm->m_next;
        }

        tp->t_lastoff = off;
        tp->t_lastlen = len;
        m0 = tp->t_lastm;
        off = tp->t_inoff;

        if (len <= M_TRAILINGSPACE(m)) {
                m_copydata(m0, off, (int)len, mtod(m, char *) + hdrlen);
                m->m_len += len;
                TCP_OUTPUT_COUNTER_INCR(&tcp_output_copysmall);
        } else {
                m->m_next = m_copym(m0, off, (int)len, M_DONTWAIT);
                if (m->m_next == NULL) {
                        m_freem(m);
                        return ENOBUFS;
                }
#ifdef TCP_OUTPUT_COUNTERS
                if (m->m_next->m_flags & M_EXT)
                        TCP_OUTPUT_COUNTER_INCR(&tcp_output_refbig);
                else
                        TCP_OUTPUT_COUNTER_INCR(&tcp_output_copybig);
#endif
        }

        *mp = m;
        return 0;
}

/*
 * Tcp output routine: figure out what should be sent and send it.
 */
int
tcp_output(struct tcpcb *tp)
{
        struct rtentry *rt = NULL;
        struct socket *so;
        struct route *ro;
        long len, win;
        int off, flags, error;
        struct mbuf *m;
        struct ip *ip;
#ifdef INET6
        struct ip6_hdr *ip6;
#endif
        struct tcphdr *th;
        u_char opt[MAX_TCPOPTLEN], *optp;
#define OPT_FITS(more)  ((optlen + (more)) <= sizeof(opt))
        unsigned optlen, hdrlen, packetlen;
        unsigned int sack_numblks;
        int idle, sendalot, txsegsize, rxsegsize;
        int txsegsize_nosack;
        int maxburst = TCP_MAXBURST;
        int af;         /* address family on the wire */
        int iphdrlen;
        int has_tso4, has_tso6;
        int has_tso, use_tso;
        bool alwaysfrag;
        int sack_rxmit;
        int sack_bytes_rxmt;
        int ecn_tos;
        struct sackhole *p;
#ifdef TCP_SIGNATURE
        int sigoff = 0;
#endif
        net_stat_ref_t tcps;

        so = tp->t_inpcb->inp_socket;
        ro = &tp->t_inpcb->inp_route;

        switch (af = tp->t_family) {
        case AF_INET:
        case AF_INET6:
                if (tp->t_inpcb)
                        break;
                return EINVAL;
        default:
                return EAFNOSUPPORT;
        }

        if (tcp_segsize(tp, &txsegsize, &rxsegsize, &alwaysfrag))
                return EMSGSIZE;

        idle = (tp->snd_max == tp->snd_una);

        /*
         * Determine if we can use TCP segmentation offload:
         * - If we're using IPv4
         * - If there is not an IPsec policy that prevents it
         * - If the interface can do it
         */
        has_tso4 = has_tso6 = false;

        has_tso4 = tp->t_inpcb->inp_af == AF_INET &&
#if defined(IPSEC)
            (!ipsec_used || ipsec_pcb_skip_ipsec(tp->t_inpcb->inp_sp,
            IPSEC_DIR_OUTBOUND)) &&
#endif
            (rt = rtcache_validate(&tp->t_inpcb->inp_route)) != NULL &&
            (rt->rt_ifp->if_capenable & IFCAP_TSOv4) != 0;
        if (rt != NULL) {
                rtcache_unref(rt, &tp->t_inpcb->inp_route);
                rt = NULL;
        }

#if defined(INET6)
        has_tso6 = tp->t_inpcb->inp_af == AF_INET6 &&
#if defined(IPSEC)
            (!ipsec_used || ipsec_pcb_skip_ipsec(tp->t_inpcb->inp_sp,
            IPSEC_DIR_OUTBOUND)) &&
#endif
            (rt = rtcache_validate(&tp->t_inpcb->inp_route)) != NULL &&
            (rt->rt_ifp->if_capenable & IFCAP_TSOv6) != 0;
        if (rt != NULL)
                rtcache_unref(rt, &tp->t_inpcb->inp_route);
#endif /* defined(INET6) */
        has_tso = (has_tso4 || has_tso6) && !alwaysfrag;

        /*
         * Restart Window computation.  From draft-floyd-incr-init-win-03:
         *
         *      Optionally, a TCP MAY set the restart window to the
         *      minimum of the value used for the initial window and
         *      the current value of cwnd (in other words, using a
         *      larger value for the restart window should never increase
         *      the size of cwnd).
         */
        if (tcp_cwm) {
                /*
                 * Hughes/Touch/Heidemann Congestion Window Monitoring.
                 * Count the number of packets currently pending
                 * acknowledgement, and limit our congestion window
                 * to a pre-determined allowed burst size plus that count.
                 * This prevents bursting once all pending packets have
                 * been acknowledged (i.e. transmission is idle).
                 *
                 * XXX Link this to Initial Window?
                 */
                tp->snd_cwnd = uimin(tp->snd_cwnd,
                    (tcp_cwm_burstsize * txsegsize) +
                    (tp->snd_nxt - tp->snd_una));
        } else {
                if (idle && (tcp_now - 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.
                         */
                        int ss = tcp_init_win;
                        if (tp->t_inpcb->inp_af == AF_INET &&
                            in_localaddr(in4p_faddr(tp->t_inpcb)))
                                ss = tcp_init_win_local;
#ifdef INET6
                        else if (tp->t_inpcb->inp_af == AF_INET6 &&
                            in6_localaddr(&in6p_faddr(tp->t_inpcb)))
                                ss = tcp_init_win_local;
#endif
                        tp->snd_cwnd = uimin(tp->snd_cwnd,
                            TCP_INITIAL_WINDOW(ss, txsegsize));
                }
        }

        txsegsize_nosack = txsegsize;
again:
        ecn_tos = 0;
        use_tso = has_tso;
        if ((tp->t_flags & (TF_ECN_SND_CWR|TF_ECN_SND_ECE)) != 0) {
                /* don't duplicate CWR/ECE. */
                use_tso = 0;
        }
        TCP_REASS_LOCK(tp);
        sack_numblks = tcp_sack_numblks(tp);
        if (sack_numblks) {
                int sackoptlen;

                sackoptlen = TCP_SACK_OPTLEN(sack_numblks);
                if (sackoptlen > txsegsize_nosack) {
                        sack_numblks = 0; /* give up SACK */
                        txsegsize = txsegsize_nosack;
                } else {
                        if ((tp->rcv_sack_flags & TCPSACK_HAVED) != 0) {
                                /* don't duplicate D-SACK. */
                                use_tso = 0;
                        }
                        txsegsize = txsegsize_nosack - sackoptlen;
                }
        } else {
                txsegsize = txsegsize_nosack;
        }

        /*
         * Determine length of data that should be transmitted, and
         * flags that should be used.  If there is some data or critical
         * controls (SYN, RST) to send, then transmit; otherwise,
         * investigate further.
         *
         * Readjust SACK information to avoid resending duplicate data.
         */
        if (TCP_SACK_ENABLED(tp) && SEQ_LT(tp->snd_nxt, tp->snd_max))
                tcp_sack_adjust(tp);
        sendalot = 0;
        off = tp->snd_nxt - tp->snd_una;
        win = uimin(tp->snd_wnd, tp->snd_cwnd);

        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;
        do {
                long cwin;
                if (!TCP_SACK_ENABLED(tp))
                        break;
                if (tp->t_partialacks < 0)
                        break;
                p = tcp_sack_output(tp, &sack_bytes_rxmt);
                if (p == NULL)
                        break;

                cwin = uimin(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;
                                break;
                        }
                        /* 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 (off + len > so->so_snd.sb_cc) {
                        /* 1 for TH_FIN */
                        KASSERT(off + len == so->so_snd.sb_cc + 1);
                        KASSERT(p->rxmit + len == tp->snd_max);
                        len = so->so_snd.sb_cc - off;
                }
                if (len > 0) {
                        sack_rxmit = 1;
                        sendalot = 1;
                }
        } while (/*CONSTCOND*/0);

        /*
         * 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 (win == 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 unset 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;
                        win = 1;
                } else {
                        TCP_TIMER_DISARM(tp, TCPT_PERSIST);
                        tp->t_rxtshift = 0;
                }
        }

        if (sack_rxmit == 0) {
                if (TCP_SACK_ENABLED(tp) && tp->t_partialacks >= 0) {
                        long cwin;

                        /*
                         * We are inside of a SACK recovery episode and are
                         * sending new data, having retransmitted all the
                         * data possible in the scoreboard.
                         */
                        if (tp->snd_wnd < so->so_snd.sb_cc) {
                                len = tp->snd_wnd - off;
                                flags &= ~TH_FIN;
                        } else {
                                len = so->so_snd.sb_cc - off;
                        }

                        /*
                         * From FreeBSD:
                         *  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;
                                if (cwin < len) {
                                        len = cwin;
                                        flags &= ~TH_FIN;
                                }
                        }
                } else if (win < so->so_snd.sb_cc) {
                        len = win - off;
                        flags &= ~TH_FIN;
                } else {
                        len = so->so_snd.sb_cc - off;
                }
        }

        if (len < 0) {
                /*
                 * If FIN has been sent but not acked,
                 * but we haven't been called to retransmit,
                 * len will be -1.  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.
                 *
                 * If we have a pending FIN, either it has already been
                 * transmitted or it is outside the window, so drop it.
                 * If the FIN has been transmitted, but this is not a
                 * retransmission, then len must be -1.  Therefore we also
                 * prevent here the sending of `gratuitous FINs'.  This
                 * eliminates the need to check for that case below (e.g.
                 * to back up snd_nxt before the FIN so that the sequence
                 * number is correct).
                 */
                len = 0;
                flags &= ~TH_FIN;
                if (win == 0) {
                        TCP_TIMER_DISARM(tp, TCPT_REXMT);
                        tp->t_rxtshift = 0;
                        tp->snd_nxt = tp->snd_una;
                        if (TCP_TIMER_ISARMED(tp, TCPT_PERSIST) == 0)
                                tcp_setpersist(tp);
                }
        }

        /*
         * Automatic sizing enables the performance of large buffers
         * and most of the efficiency of small ones by only allocating
         * space when it is needed.
         *
         * The criteria to step up the send buffer one notch are:
         *  1. receive window of remote host is larger than send buffer
         *     (with a fudge factor of 5/4th);
         *  2. send buffer is filled to 7/8th with data (so we actually
         *     have data to make use of it);
         *  3. send buffer fill has not hit maximal automatic size;
         *  4. our send window (slow start and cogestion controlled) is
         *     larger than sent but unacknowledged data in send buffer.
         *
         * The remote host receive window scaling factor may limit the
         * growing of the send buffer before it reaches its allowed
         * maximum.
         *
         * It scales directly with slow start or congestion window
         * and does at most one step per received ACK.  This fast
         * scaling has the drawback of growing the send buffer beyond
         * what is strictly necessary to make full use of a given
         * delay*bandwidth product.  However testing has shown this not
         * to be much of a problem.  At worst we are trading wasting
         * of available bandwidth (the non-use of it) for wasting some
         * socket buffer memory.
         *
         * TODO: Shrink send buffer during idle periods together
         * with congestion window.  Requires another timer.
         */
        if (tcp_do_autosndbuf && so->so_snd.sb_flags & SB_AUTOSIZE) {
                if ((tp->snd_wnd / 4 * 5) >= so->so_snd.sb_hiwat &&
                    so->so_snd.sb_cc >= (so->so_snd.sb_hiwat / 8 * 7) &&
                    so->so_snd.sb_cc < tcp_autosndbuf_max &&
                    win >= (so->so_snd.sb_cc - (tp->snd_nxt - tp->snd_una))) {
                        if (!sbreserve(&so->so_snd,
                            uimin(so->so_snd.sb_hiwat + tcp_autosndbuf_inc,
                             tcp_autosndbuf_max), so))
                                so->so_snd.sb_flags &= ~SB_AUTOSIZE;
                }
        }

        if (len > txsegsize) {
                if (use_tso) {
                        /*
                         * Truncate TSO transfers to IP_MAXPACKET, and make
                         * sure that we send equal size transfers down the
                         * stack (rather than big-small-big-small-...).
                         */
#ifdef INET6
                        CTASSERT(IPV6_MAXPACKET == IP_MAXPACKET);
#endif
                        len = (uimin(len, IP_MAXPACKET) / txsegsize) * txsegsize;
                        if (len <= txsegsize) {
                                use_tso = 0;
                        }
                } else
                        len = txsegsize;
                flags &= ~TH_FIN;
                sendalot = 1;
        } else
                use_tso = 0;
        if (sack_rxmit) {
                if (SEQ_LT(p->rxmit + len, tp->snd_una + so->so_snd.sb_cc))
                        flags &= ~TH_FIN;
        }

        win = sbspace(&so->so_rcv);

        /*
         * Sender silly window avoidance.  If connection is idle
         * and can send all data, a maximum segment,
         * at least a maximum default-size segment do it,
         * or are forced, do it; otherwise don't bother.
         * If peer's buffer is tiny, then send
         * when window is at least half open.
         * If retransmitting (possibly after persist timer forced us
         * to send into a small window), then must resend.
         */
        if (len) {
                if (len >= txsegsize)
                        goto send;
                if ((so->so_state & SS_MORETOCOME) == 0 &&
                    ((idle || tp->t_flags & TF_NODELAY) &&
                     len + off >= so->so_snd.sb_cc))
                        goto send;
                if (tp->t_force)
                        goto send;
                if (len >= tp->max_sndwnd / 2)
                        goto send;
                if (SEQ_LT(tp->snd_nxt, tp->snd_max))
                        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 twice the size of the largest segment
         * we expect to receive (i.e. two segments) or at least 50% of
         * the maximum possible window, then want to send a window update
         * to peer.
         */
        if (win > 0) {
                /*
                 * "adv" is the amount we can increase the window,
                 * taking into account that we are limited by
                 * TCP_MAXWIN << tp->rcv_scale.
                 */
                long recwin = uimin(win, (long)TCP_MAXWIN << tp->rcv_scale);
                long oldwin, adv;

                /*
                 * rcv_nxt may overtake rcv_adv when we accept a
                 * zero-window probe.
                 */
                if (SEQ_GT(tp->rcv_adv, tp->rcv_nxt))
                        oldwin = tp->rcv_adv - tp->rcv_nxt;
                else
                        oldwin = 0;

                /*
                 * If the new window size ends up being the same as or
                 * less than the old size when it is scaled, then
                 * don't force a window update.
                 */
                if (recwin >> tp->rcv_scale <= oldwin >> tp->rcv_scale)
                        goto dontupdate;

                adv = recwin - oldwin;
                if (adv >= (long) (2 * rxsegsize))
                        goto send;
                if (2 * adv >= (long) so->so_rcv.sb_hiwat)
                        goto send;
        }
dontupdate:

        /*
         * Send if we owe peer an ACK.
         */
        if (tp->t_flags & TF_ACKNOW)
                goto send;
        if (flags & (TH_SYN|TH_FIN|TH_RST))
                goto send;
        if (SEQ_GT(tp->snd_up, 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 (TCP_SACK_ENABLED(tp) && SEQ_GT(tp->snd_max, tp->snd_una) &&
            !TCP_TIMER_ISARMED(tp, TCPT_REXMT) &&
            !TCP_TIMER_ISARMED(tp, TCPT_PERSIST)) {
                TCP_TIMER_ARM(tp, 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 && TCP_TIMER_ISARMED(tp, TCPT_REXMT) == 0 &&
            TCP_TIMER_ISARMED(tp, TCPT_PERSIST) == 0) {
                tp->t_rxtshift = 0;
                tcp_setpersist(tp);
        }

        /*
         * No reason to send a segment, just return.
         */
just_return:
        TCP_REASS_UNLOCK(tp);
        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 + IP_header + TCP_header + optlen <= MCLBYTES
         */
        optlen = 0;
        optp = opt;
        switch (af) {
        case AF_INET:
                iphdrlen = sizeof(struct ip) + sizeof(struct tcphdr);
                break;
#ifdef INET6
        case AF_INET6:
                iphdrlen = sizeof(struct ip6_hdr) + sizeof(struct tcphdr);
                break;
#endif
        default:        /*pacify gcc*/
                iphdrlen = 0;
                break;
        }
        hdrlen = iphdrlen;
        if (flags & TH_SYN) {
                struct rtentry *synrt;

                synrt = inpcb_rtentry(tp->t_inpcb);
                tp->snd_nxt = tp->iss;
                tp->t_ourmss = tcp_mss_to_advertise(synrt != NULL ?
                                                    synrt->rt_ifp : NULL, af);
                inpcb_rtentry_unref(synrt, tp->t_inpcb);
                if ((tp->t_flags & TF_NOOPT) == 0 && OPT_FITS(TCPOLEN_MAXSEG)) {
                        *optp++ = TCPOPT_MAXSEG;
                        *optp++ = TCPOLEN_MAXSEG;
                        *optp++ = (tp->t_ourmss >> 8) & 0xff;
                        *optp++ = tp->t_ourmss & 0xff;
                        optlen += TCPOLEN_MAXSEG;

                        if ((tp->t_flags & TF_REQ_SCALE) &&
                            ((flags & TH_ACK) == 0 ||
                            (tp->t_flags & TF_RCVD_SCALE)) &&
                            OPT_FITS(TCPOLEN_WINDOW + TCPOLEN_NOP)) {
                                *((uint32_t *)optp) = htonl(
                                        TCPOPT_NOP << 24 |
                                        TCPOPT_WINDOW << 16 |
                                        TCPOLEN_WINDOW << 8 |
                                        tp->request_r_scale);
                                optp += TCPOLEN_WINDOW + TCPOLEN_NOP;
                                optlen += TCPOLEN_WINDOW + TCPOLEN_NOP;
                        }
                        if (tcp_do_sack && OPT_FITS(TCPOLEN_SACK_PERMITTED)) {
                                *optp++ = TCPOPT_SACK_PERMITTED;
                                *optp++ = TCPOLEN_SACK_PERMITTED;
                                optlen += TCPOLEN_SACK_PERMITTED;
                        }
                }
        }

        /*
         * 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_SYN|TH_ACK)) == TH_SYN ||
             (tp->t_flags & TF_RCVD_TSTMP))) {
                int alen = 0;
                while (optlen % 4 != 2) {
                        optlen += TCPOLEN_NOP;
                        *optp++ = TCPOPT_NOP;
                        alen++;
                }
                if (OPT_FITS(TCPOLEN_TIMESTAMP)) {
                        *optp++ = TCPOPT_TIMESTAMP;
                        *optp++ = TCPOLEN_TIMESTAMP;
                        uint32_t *lp = (uint32_t *)optp;
                        /* Form timestamp option (appendix A of RFC 1323) */
                        *lp++ = htonl(TCP_TIMESTAMP(tp));
                        *lp   = htonl(tp->ts_recent);
                        optp += TCPOLEN_TIMESTAMP - 2;
                        optlen += TCPOLEN_TIMESTAMP;

                        /* Set receive buffer autosizing timestamp. */
                        if (tp->rfbuf_ts == 0 &&
                            (so->so_rcv.sb_flags & SB_AUTOSIZE))
                                tp->rfbuf_ts = TCP_TIMESTAMP(tp);
                } else {
                        optp -= alen;
                        optlen -= alen;
                }
        }

#ifdef TCP_SIGNATURE
        if (tp->t_flags & TF_SIGNATURE) {
                /*
                 * Initialize TCP-MD5 option (RFC2385)
                 */
                if (!OPT_FITS(TCPOLEN_SIGNATURE))
                        goto reset;

                *optp++ = TCPOPT_SIGNATURE;
                *optp++ = TCPOLEN_SIGNATURE;
                sigoff = optlen + 2;
                memset(optp, 0, TCP_SIGLEN);
                optlen += TCPOLEN_SIGNATURE;
                optp += TCP_SIGLEN;
        }
#endif

        /*
         * Tack on the SACK block if it is necessary.
         */
        if (sack_numblks) {
                int alen = 0;
                int sack_len = sack_numblks * 8;
                while (optlen % 4 != 2) {
                        optlen += TCPOLEN_NOP;
                        *optp++ = TCPOPT_NOP;
                        alen++;
                }
                if (OPT_FITS(sack_len + 2)) {
                        struct ipqent *tiqe;
                        *optp++ = TCPOPT_SACK;
                        *optp++ = sack_len + 2;
                        uint32_t *lp = (uint32_t *)optp;
                        if ((tp->rcv_sack_flags & TCPSACK_HAVED) != 0) {
                                sack_numblks--;
                                *lp++ = htonl(tp->rcv_dsack_block.left);
                                *lp++ = htonl(tp->rcv_dsack_block.right);
                                tp->rcv_sack_flags &= ~TCPSACK_HAVED;
                        }
                        for (tiqe = TAILQ_FIRST(&tp->timeq);
                            sack_numblks > 0;
                            tiqe = TAILQ_NEXT(tiqe, ipqe_timeq)) {
                                KASSERT(tiqe != NULL);
                                sack_numblks--;
                                *lp++ = htonl(tiqe->ipqe_seq);
                                *lp++ = htonl(tiqe->ipqe_seq + tiqe->ipqe_len +
                                    ((tiqe->ipqe_flags & TH_FIN) != 0 ? 1 : 0));
                        }
                        optlen += sack_len + 2;
                        optp += sack_len;
                } else {
                        optp -= alen;
                        optlen -= alen;
                }
        }

        /* Terminate and pad TCP options to a 4 byte boundary. */
        if (optlen % 4) {
                if (!OPT_FITS(TCPOLEN_EOL)) {
reset:                  TCP_REASS_UNLOCK(tp);
                        error = ECONNABORTED;
                        goto out;
                }
                optlen += TCPOLEN_EOL;
                *optp++ = TCPOPT_EOL;
        }
        /*
         * According to RFC 793 (STD0007):
         *   "The content of the header beyond the End-of-Option option
         *    must be header padding (i.e., zero)."
         *   and later: "The padding is composed of zeros."
         */
        while (optlen % 4) {
                if (!OPT_FITS(TCPOLEN_PAD))
                        goto reset;
                optlen += TCPOLEN_PAD;
                *optp++ = TCPOPT_PAD;
        }

        TCP_REASS_UNLOCK(tp);

        hdrlen += optlen;

#ifdef DIAGNOSTIC
        if (!use_tso && len > txsegsize)
                panic("tcp data to be sent is larger than segment");
        else if (use_tso && len > IP_MAXPACKET)
                panic("tcp data to be sent is larger than max TSO size");
        if (max_linkhdr + hdrlen > MCLBYTES)
                panic("tcphdr too big");
#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) {
                error = tcp_build_datapkt(tp, so, off, len, hdrlen, &m);
                if (error)
                        goto out;
                /*
                 * 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 {
                tcps = TCP_STAT_GETREF();
                if (tp->t_flags & TF_ACKNOW)
                        _NET_STATINC_REF(tcps, TCP_STAT_SNDACKS);
                else if (flags & (TH_SYN|TH_FIN|TH_RST))
                        _NET_STATINC_REF(tcps, TCP_STAT_SNDCTRL);
                else if (SEQ_GT(tp->snd_up, tp->snd_una))
                        _NET_STATINC_REF(tcps, TCP_STAT_SNDURG);
                else
                        _NET_STATINC_REF(tcps, TCP_STAT_SNDWINUP);
                TCP_STAT_PUTREF();

                MGETHDR(m, M_DONTWAIT, MT_HEADER);
                if (m != NULL && max_linkhdr + hdrlen > MHLEN) {
                        MCLGET(m, M_DONTWAIT);
                        if ((m->m_flags & M_EXT) == 0) {
                                m_freem(m);
                                m = NULL;
                        }
                }
                if (m == NULL) {
                        error = ENOBUFS;
                        goto out;
                }
                MCLAIM(m, &tcp_tx_mowner);
                m->m_data += max_linkhdr;
                m->m_len = hdrlen;
        }
        m_reset_rcvif(m);
        switch (af) {
        case AF_INET:
                ip = mtod(m, struct ip *);
#ifdef INET6
                ip6 = NULL;
#endif
                th = (struct tcphdr *)(ip + 1);
                break;
#ifdef INET6
        case AF_INET6:
                ip = NULL;
                ip6 = mtod(m, struct ip6_hdr *);
                th = (struct tcphdr *)(ip6 + 1);
                break;
#endif
        default:        /*pacify gcc*/
                ip = NULL;
#ifdef INET6
                ip6 = NULL;
#endif
                th = NULL;
                break;
        }
        if (tp->t_template == NULL)
                panic("%s: no template", __func__);
        if (tp->t_template->m_len < iphdrlen)
                panic("%s: %d < %d", __func__, tp->t_template->m_len, iphdrlen);
        bcopy(mtod(tp->t_template, void *), mtod(m, void *), iphdrlen);

        /*
         * If we are starting a connection, send ECN setup
         * SYN packet. If we are on a retransmit, we may
         * resend those bits a number of times as per
         * RFC 3168.
         */
        if (tp->t_state == TCPS_SYN_SENT && tcp_do_ecn) {
                if (tp->t_flags & TF_SYN_REXMT) {
                        if (tp->t_ecn_retries--)
                                flags |= TH_ECE|TH_CWR;
                } else {
                        flags |= TH_ECE|TH_CWR;
                        tp->t_ecn_retries = tcp_ecn_maxretries;
                }
        }

        if (TCP_ECN_ALLOWED(tp)) {
                /*
                 * If the peer has ECN, mark data packets
                 * ECN capable. Ignore pure ack packets, retransmissions
                 * and window probes.
                 */
                if (len > 0 && SEQ_GEQ(tp->snd_nxt, tp->snd_max) &&
                    !(tp->t_force && len == 1)) {
                        ecn_tos = IPTOS_ECN_ECT0;
                        TCP_STATINC(TCP_STAT_ECN_ECT);
                }

                /*
                 * Reply with proper ECN notifications.
                 */
                if (tp->t_flags & TF_ECN_SND_CWR) {
                        flags |= TH_CWR;
                        tp->t_flags &= ~TF_ECN_SND_CWR;
                }
                if (tp->t_flags & TF_ECN_SND_ECE) {
                        flags |= TH_ECE;
                }
        }

        /*
         * 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 (TCP_SACK_ENABLED(tp) && sack_rxmit) {
                th->th_seq = htonl(p->rxmit);
                p->rxmit += len;
        } else {
                if (len || (flags & (TH_SYN|TH_FIN)) ||
                    TCP_TIMER_ISARMED(tp, TCPT_PERSIST))
                        th->th_seq = htonl(tp->snd_nxt);
                else
                        th->th_seq = htonl(tp->snd_max);
        }
        th->th_ack = htonl(tp->rcv_nxt);
        if (optlen) {
                memcpy(th + 1, opt, 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 (win < (long)(so->so_rcv.sb_hiwat / 4) && win < (long)rxsegsize)
                win = 0;
        if (win > (long)TCP_MAXWIN << tp->rcv_scale)
                win = (long)TCP_MAXWIN << tp->rcv_scale;
        if (win < (long)(int32_t)(tp->rcv_adv - tp->rcv_nxt))
                win = (long)(int32_t)(tp->rcv_adv - tp->rcv_nxt);
        th->th_win = htons((u_int16_t) (win>>tp->rcv_scale));
        if (th->th_win == 0) {
                tp->t_sndzerowin++;
        }
        if (SEQ_GT(tp->snd_up, tp->snd_nxt)) {
                u_int32_t urp = tp->snd_up - tp->snd_nxt;
                if (urp > IP_MAXPACKET)
                        urp = IP_MAXPACKET;
                th->th_urp = htons((u_int16_t)urp);
                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 */

#ifdef TCP_SIGNATURE
        if (sigoff && (tp->t_flags & TF_SIGNATURE)) {
                struct secasvar *sav;
                u_int8_t *sigp;

                sav = tcp_signature_getsav(m);
                if (sav == NULL) {
                        m_freem(m);
                        return EPERM;
                }

                m->m_pkthdr.len = hdrlen + len;
                sigp = (char *)th + sizeof(*th) + sigoff;
                tcp_signature(m, th, (char *)th - mtod(m, char *), sav, sigp);

                key_sa_recordxfer(sav, m);
                KEY_SA_UNREF(&sav);
        }
#endif

        /*
         * Set ourselves up to be checksummed just before the packet
         * hits the wire.
         */
        switch (af) {
        case AF_INET:
                m->m_pkthdr.csum_data = offsetof(struct tcphdr, th_sum);
                if (use_tso) {
                        m->m_pkthdr.segsz = txsegsize;
                        m->m_pkthdr.csum_flags = M_CSUM_TSOv4;
                } else {
                        m->m_pkthdr.csum_flags = M_CSUM_TCPv4;
                        if (len + optlen) {
                                /* Fixup the pseudo-header checksum. */
                                /* XXXJRT Not IP Jumbogram safe. */
                                th->th_sum = in_cksum_addword(th->th_sum,
                                    htons((u_int16_t) (len + optlen)));
                        }
                }
                break;
#ifdef INET6
        case AF_INET6:
                m->m_pkthdr.csum_data = offsetof(struct tcphdr, th_sum);
                if (use_tso) {
                        m->m_pkthdr.segsz = txsegsize;
                        m->m_pkthdr.csum_flags = M_CSUM_TSOv6;
                } else {
                        m->m_pkthdr.csum_flags = M_CSUM_TCPv6;
                        if (len + optlen) {
                                /* Fixup the pseudo-header checksum. */
                                /* XXXJRT: Not IPv6 Jumbogram safe. */
                                th->th_sum = in_cksum_addword(th->th_sum,
                                    htons((u_int16_t) (len + optlen)));
                        }
                }
                break;
#endif
        }

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

                /*
                 * Advance snd_nxt over sequence space of this segment.
                 * There are no states in which we send both a SYN and a FIN,
                 * so we collapse the tests for these flags.
                 */
                if (flags & (TH_SYN|TH_FIN))
                        tp->snd_nxt++;
                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 = tcp_now;
                                tp->t_rtseq = startseq;
                                TCP_STATINC(TCP_STAT_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 (TCP_TIMER_ISARMED(tp, TCPT_REXMT) == 0) {
                        if ((sack_rxmit && tp->snd_nxt != tp->snd_max)
                            || tp->snd_nxt != tp->snd_una) {
                                if (TCP_TIMER_ISARMED(tp, TCPT_PERSIST)) {
                                        TCP_TIMER_DISARM(tp, TCPT_PERSIST);
                                        tp->t_rxtshift = 0;
                                }
                                TCP_TIMER_ARM(tp, TCPT_REXMT, tp->t_rxtcur);
                        } else if (len == 0 && so->so_snd.sb_cc > 0
                            && TCP_TIMER_ISARMED(tp, TCPT_PERSIST) == 0) {
                                /*
                                 * If we are sending a window probe and there's
                                 * unacked data in the socket, make sure at
                                 * least the persist timer is running.
                                 */
                                tp->t_rxtshift = 0;
                                tcp_setpersist(tp);
                        }
                }
        } else
                if (SEQ_GT(tp->snd_nxt + len, tp->snd_max))
                        tp->snd_max = tp->snd_nxt + len;

#ifdef TCP_DEBUG
        /*
         * Trace.
         */
        if (so->so_options & SO_DEBUG)
                tcp_trace(TA_OUTPUT, tp->t_state, tp, m, 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 = hdrlen + len;

        switch (af) {
        case AF_INET:
                ip->ip_len = htons(m->m_pkthdr.len);
                packetlen = m->m_pkthdr.len;
                if (tp->t_inpcb->inp_af == AF_INET) {
                        ip->ip_ttl = in4p_ip(tp->t_inpcb).ip_ttl;
                        ip->ip_tos = in4p_ip(tp->t_inpcb).ip_tos | ecn_tos;
                }
#ifdef INET6
                else if (tp->t_inpcb->inp_af == AF_INET6) {
                        ip->ip_ttl = in6pcb_selecthlim(tp->t_inpcb, NULL); /*XXX*/
                        ip->ip_tos = ecn_tos;   /*XXX*/
                }
#endif
                break;
#ifdef INET6
        case AF_INET6:
                packetlen = m->m_pkthdr.len;
                ip6->ip6_nxt = IPPROTO_TCP;
                if (tp->t_family == AF_INET6) {
                        /*
                         * 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 = in6pcb_selecthlim_rt(tp->t_inpcb);
                }
                ip6->ip6_flow |= htonl(ecn_tos << 20);
                /* ip6->ip6_flow = ??? (from template) */
                /* ip6_plen will be filled in ip6_output(). */
                break;
#endif
        default:        /*pacify gcc*/
                packetlen = 0;
                break;
        }

        switch (af) {
        case AF_INET:
            {
                struct mbuf *opts;

                if (tp->t_inpcb->inp_af == AF_INET)
                        opts = tp->t_inpcb->inp_options;
                else
                        opts = NULL;
                error = ip_output(m, opts, ro,
                        (tp->t_mtudisc ? IP_MTUDISC : 0) |
                        (so->so_options & SO_DONTROUTE), NULL, tp->t_inpcb);
                break;
            }
#ifdef INET6
        case AF_INET6:
            {
                struct ip6_pktopts *opts;

                if (tp->t_inpcb->inp_af == AF_INET6)
                        opts = in6p_outputopts(tp->t_inpcb);
                else
                        opts = NULL;
                error = ip6_output(m, opts, ro, so->so_options & SO_DONTROUTE,
                        NULL, tp->t_inpcb, NULL);
                break;
            }
#endif
        default:
                error = EAFNOSUPPORT;
                break;
        }
        if (error) {
out:
                if (error == ENOBUFS) {
                        TCP_STATINC(TCP_STAT_SELFQUENCH);
                        tcp_quench(tp->t_inpcb);
                        error = 0;
                } else if ((error == EHOSTUNREACH || error == ENETDOWN ||
                    error == EHOSTDOWN) && TCPS_HAVERCVDSYN(tp->t_state)) {
                        tp->t_softerror = error;
                        error = 0;
                }

                /* Back out the sequence number advance. */
                if (sack_rxmit)
                        p->rxmit -= len;

                /* Restart the delayed ACK timer, if necessary. */
                if (tp->t_flags & TF_DELACK)
                        TCP_RESTART_DELACK(tp);

                return error;
        }

        if (packetlen > tp->t_pmtud_mtu_sent)
                tp->t_pmtud_mtu_sent = packetlen;

        tcps = TCP_STAT_GETREF();
        _NET_STATINC_REF(tcps, TCP_STAT_SNDTOTAL);
        if (tp->t_flags & TF_DELACK)
                _NET_STATINC_REF(tcps, TCP_STAT_DELACK);
        TCP_STAT_PUTREF();

        /*
         * 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 (win > 0 && SEQ_GT(tp->rcv_nxt+win, tp->rcv_adv))
                tp->rcv_adv = tp->rcv_nxt + win;
        tp->last_ack_sent = tp->rcv_nxt;
        tp->t_flags &= ~TF_ACKNOW;
        TCP_CLEAR_DELACK(tp);
#ifdef DIAGNOSTIC
        if (maxburst < 0)
                printf("tcp_output: maxburst exceeded by %d\n", -maxburst);
#endif
        if (sendalot && (tp->t_congctl == &tcp_reno_ctl || --maxburst))
                goto again;
        return 0;
}

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

        if (TCP_TIMER_ISARMED(tp, TCPT_REXMT))
                panic("tcp_output REXMT");
        /*
         * Start/restart persistance timer.
         */
        if (t < tp->t_rttmin)
                t = tp->t_rttmin;
        TCPT_RANGESET(nticks, t * tcp_backoff[tp->t_rxtshift],
            TCPTV_PERSMIN, TCPTV_PERSMAX);
        TCP_TIMER_ARM(tp, TCPT_PERSIST, nticks);
        if (tp->t_rxtshift < TCP_MAXRXTSHIFT)
                tp->t_rxtshift++;
}