/*      $OpenBSD: tcp_output.c,v 1.158 2026/01/01 05:28:23 jsg Exp $    */
/*      $NetBSD: tcp_output.c,v 1.16 1997/06/03 16:17:09 kml Exp $      */

/*
 * Copyright (c) 1982, 1986, 1988, 1990, 1993
 *      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.
 *
 *      @(#)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).
 */

#include "pf.h"
#include "stoeplitz.h"

#include <sys/param.h>
#include <sys/systm.h>
#include <sys/mbuf.h>
#include <sys/protosw.h>
#include <sys/socket.h>

#include <net/if.h>
#include <net/if_var.h>
#if NPF > 0
#include <net/pfvar.h>
#endif

#include <netinet/in.h>
#include <netinet/ip.h>
#include <netinet/in_pcb.h>
#include <netinet/ip_var.h>
#include <netinet6/ip6_var.h>
#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_debug.h>

#ifdef TCP_SACK_DEBUG
void tcp_print_holes(struct tcpcb *tp);

void
tcp_print_holes(struct tcpcb *tp)
{
        struct sackhole *p = tp->snd_holes;
        if (p == NULL)
                return;
        printf("Hole report: start--end dups rxmit\n");
        while (p) {
                printf("%x--%x d %d r %x\n", p->start, p->end, p->dups,
                    p->rxmit);
                p = p->next;
        }
        printf("\n");
}
#endif /* TCP_SACK_DEBUG */

/*
 * Returns pointer to a sackhole if there are any pending retransmissions;
 * NULL otherwise.
 */
struct sackhole *
tcp_sack_output(struct tcpcb *tp)
{
        struct sackhole *p;

        if (!tp->sack_enable)
                return (NULL);
        p = tp->snd_holes;
        while (p) {
                if (p->dups >= tcprexmtthresh && SEQ_LT(p->rxmit, p->end)) {
                        if (SEQ_LT(p->rxmit, tp->snd_una)) {/* old SACK hole */
                                p = p->next;
                                continue;
                        }
#ifdef TCP_SACK_DEBUG
                        if (p)
                                tcp_print_holes(tp);
#endif
                        return (p);
                }
                p = p->next;
        }
        return (NULL);
}

/*
 * After a timeout, the SACK list may be rebuilt.  This SACK information
 * should be used to avoid retransmitting SACKed data.  This function
 * traverses the SACK list to see if snd_nxt should be moved forward.
 */

void
tcp_sack_adjust(struct tcpcb *tp)
{
        struct sackhole *cur = tp->snd_holes;
        if (cur == NULL)
                return; /* No holes */
        if (SEQ_GEQ(tp->snd_nxt, tp->rcv_lastsack))
                return; /* We're already beyond any SACKed blocks */
        /*
         * Two cases for which we want to advance snd_nxt:
         * i) snd_nxt lies between end of one hole and beginning of another
         * ii) snd_nxt lies between end of last hole and rcv_lastsack
         */
        while (cur->next) {
                if (SEQ_LT(tp->snd_nxt, cur->end))
                        return;
                if (SEQ_GEQ(tp->snd_nxt, cur->next->start))
                        cur = cur->next;
                else {
                        tp->snd_nxt = cur->next->start;
                        return;
                }
        }
        if (SEQ_LT(tp->snd_nxt, cur->end))
                return;
        tp->snd_nxt = tp->rcv_lastsack;
        return;
}

/*
 * 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, win, rcv_hiwat, txmaxseg;
        int off, flags, error;
        struct mbuf *m;
        struct tcphdr *th;
        u_int32_t optbuf[howmany(MAX_TCPOPTLEN, sizeof(u_int32_t))];
        u_char *opt = (u_char *)optbuf;
        unsigned int optlen, hdrlen, packetlen;
        int doing_sosend, idle, sendalot = 0;
        int i, sack_rxmit = 0;
        struct sackhole *p;
        uint64_t now;
#ifdef TCP_SIGNATURE
        unsigned int sigoff;
#endif /* TCP_SIGNATURE */
#ifdef TCP_ECN
        int do_ecn = atomic_load_int(&tcp_do_ecn);
        int needect;
#endif
        int tso;

#if defined(TCP_SIGNATURE) && defined(DIAGNOSTIC)
        if (tp->sack_enable && (tp->t_flags & TF_SIGNATURE))
                return (EINVAL);
#endif /* defined(TCP_SIGNATURE) && defined(DIAGNOSTIC) */

        now = tcp_now();

        mtx_enter(&so->so_snd.sb_mtx);
        doing_sosend = soissending(so);
        mtx_leave(&so->so_snd.sb_mtx);

        /*
         * 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 && (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.
                 */
                tp->snd_cwnd = 2 * tp->t_maxseg;

        /* remember 'idle' for next invocation of tcp_output */
        if (idle && doing_sosend) {
                tp->t_flags |= TF_LASTIDLE;
                idle = 0;
        } else
                tp->t_flags &= ~TF_LASTIDLE;

again:
        /*
         * 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);
        off = tp->snd_nxt - tp->snd_una;
        win = ulmin(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().
         */
        if (tp->sack_enable && !sendalot) {
                if (tp->t_dupacks >= tcprexmtthresh &&
                    (p = tcp_sack_output(tp))) {
                        off = p->rxmit - tp->snd_una;
                        sack_rxmit = 1;
                        /* Coalesce holes into a single retransmission */
                        len = min(tp->t_maxseg, p->end - p->rxmit);
                        if (SEQ_LT(tp->snd_una, tp->snd_last))
                                tp->snd_cwnd -= tp->t_maxseg;
                }
        }

        sendalot = 0;
        tso = 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) {
                len = ulmin(so->so_snd.sb_cc, win) - 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.
                 */
                len = 0;
                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);
                }
        }

        /*
         * 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.
         */
        txmaxseg = ulmin(so->so_snd.sb_hiwat / 2, tp->t_maxseg);

        if (len > txmaxseg) {
                if (atomic_load_int(&tcp_do_tso) &&
                    tp->t_inpcb->inp_options == NULL &&
                    tp->t_inpcb->inp_outputopts6 == NULL &&
#ifdef TCP_SIGNATURE
                    ((tp->t_flags & TF_SIGNATURE) == 0) &&
#endif
                    len >= 2 * tp->t_maxseg &&
                    tp->rcv_numsacks == 0 && sack_rxmit == 0 &&
                    !(flags & (TH_SYN|TH_RST|TH_FIN))) {
                        tso = 1;
                        /* avoid small chopped packets */
                        if (len > (len / tp->t_maxseg) * tp->t_maxseg) {
                                len = (len / tp->t_maxseg) * tp->t_maxseg;
                                sendalot = 1;
                        }
                } else {
                        len = txmaxseg;
                        sendalot = 1;
                }
        }
        if (off + len < so->so_snd.sb_cc)
                flags &= ~TH_FIN;

        mtx_enter(&so->so_rcv.sb_mtx);
        win = sbspace_locked(&so->so_rcv);
        rcv_hiwat = (long) so->so_rcv.sb_hiwat;
        mtx_leave(&so->so_rcv.sb_mtx);

        /*
         * 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 >= txmaxseg)
                        goto send;
                if ((idle || (tp->t_flags & TF_NODELAY)) &&
                    len + off >= so->so_snd.sb_cc && !doing_sosend &&
                    (tp->t_flags & TF_NOPUSH) == 0)
                        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))
                        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.
         */
        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 adv = lmin(win, (long)TCP_MAXWIN << tp->rcv_scale) -
                        (tp->rcv_adv - tp->rcv_nxt);

                if (adv >= (long) (2 * tp->t_maxseg))
                        goto send;
                if (2 * adv >= rcv_hiwat)
                        goto send;
        }

        /*
         * Send if we owe peer an ACK.
         */
        if (tp->t_flags & TF_ACKNOW)
                goto send;
        if (flags & (TH_SYN|TH_RST))
                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, or we're retransmitting the FIN,
         * 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 (SEQ_GT(tp->snd_max, tp->snd_una) &&
            TCP_TIMER_ISARMED(tp, TCPT_REXMT) == 0 &&
            TCP_TIMER_ISARMED(tp, TCPT_PERSIST) == 0) {
                TCP_TIMER_ARM(tp, TCPT_REXMT, tp->t_rxtcur);
                return (0);
        }

        /*
         * 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.
         */
        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(network header) + sizeof(struct tcphdr +
         *              optlen <= MHLEN
         */
        optlen = 0;

        switch (tp->pf) {
        case 0: /*default to PF_INET*/
        case PF_INET:
                hdrlen = sizeof(struct ip) + sizeof(struct tcphdr);
                break;
#ifdef INET6
        case PF_INET6:
                hdrlen = sizeof(struct ip6_hdr) + sizeof(struct tcphdr);
                break;
#endif /* INET6 */
        default:
                return (EPFNOSUPPORT);
        }

        if (flags & TH_SYN) {
                tp->snd_nxt = tp->iss;
                if ((tp->t_flags & TF_NOOPT) == 0) {
                        u_int16_t mss;

                        opt[0] = TCPOPT_MAXSEG;
                        opt[1] = 4;
                        mss = htons((u_int16_t) tcp_mss(tp, 0));
                        memcpy(opt + 2, &mss, sizeof(mss));
                        optlen = 4;

                        if (flags & TH_ACK)
                                tcp_mss_update(tp);
                        /*
                         * If this is the first SYN of connection (not a SYN
                         * ACK), include SACK_PERMIT_HDR option.  If this is a
                         * SYN ACK, include SACK_PERMIT_HDR option if peer has
                         * already done so.
                         */
                        if (tp->sack_enable && ((flags & TH_ACK) == 0 ||
                            (tp->t_flags & TF_SACK_PERMIT))) {
                                *((u_int32_t *) (opt + optlen)) =
                                    htonl(TCPOPT_SACK_PERMIT_HDR);
                                optlen += 4;
                        }
                        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_SYN|TH_ACK)) == TH_SYN ||
             (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(now + tp->ts_modulate);
                *lp   = htonl(tp->ts_recent);
                optlen += TCPOLEN_TSTAMP_APPA;
        }
        /* Set receive buffer autosizing timestamp. */
        if (tp->rfbuf_ts == 0) {
                tp->rfbuf_ts = now;
                tp->rfbuf_cnt = 0;
        }

#ifdef TCP_SIGNATURE
        if (tp->t_flags & TF_SIGNATURE) {
                u_int8_t *bp = (u_int8_t *)(opt + optlen);

                /* Send signature option */
                *(bp++) = TCPOPT_SIGNATURE;
                *(bp++) = TCPOLEN_SIGNATURE;
                sigoff = optlen + 2;

                {
                        unsigned int i;

                        for (i = 0; i < 16; i++)
                                *(bp++) = 0;
                }


                /* Pad options list to the next 32 bit boundary and
                 * terminate it.
                 */
                *bp++ = TCPOPT_NOP;
                *bp++ = TCPOPT_NOP;

                optlen += TCPOLEN_SIGLEN;
        }
#endif /* TCP_SIGNATURE */

        /*
         * 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.  No more than three SACKs are sent.
         */
        if (tp->sack_enable && tp->t_state == TCPS_ESTABLISHED &&
            (tp->t_flags & (TF_SACK_PERMIT|TF_NOOPT)) == TF_SACK_PERMIT &&
            tp->rcv_numsacks) {
                u_int32_t *lp = (u_int32_t *)(opt + optlen);
                u_int32_t *olp = lp++;
                int count = 0;  /* actual number of SACKs inserted */
                int maxsack = (MAX_TCPOPTLEN - (optlen + 4))/TCPOLEN_SACK;

                tcpstat_inc(tcps_sack_snd_opts);
                maxsack = min(maxsack, TCP_MAX_SACK);
                for (i = 0; (i < tp->rcv_numsacks && count < maxsack); i++) {
                        struct sackblk sack = tp->sackblks[i];
                        if (sack.start == 0 && sack.end == 0)
                                continue;
                        *lp++ = htonl(sack.start);
                        *lp++ = htonl(sack.end);
                        count++;
                }
                *olp = htonl(TCPOPT_SACK_HDR|(TCPOLEN_SACK*count+2));
                optlen += TCPOLEN_SACK*count + 4; /* including leading NOPs */
        }

#ifdef DIAGNOSTIC
        if (optlen > MAX_TCPOPTLEN)
                panic("tcp_output: options too long");
#endif /* DIAGNOSTIC */

        hdrlen += optlen;

        /*
         * 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 > tp->t_maxopd - optlen) {
                if (tso) {
                        if (len + hdrlen + max_linkhdr > MAXMCLBYTES) {
                                len = MAXMCLBYTES - hdrlen - max_linkhdr;
                                sendalot = 1;
                        }
                } else {
                        len = tp->t_maxopd - optlen;
                        sendalot = 1;
                }
                flags &= ~TH_FIN;
        }

#ifdef DIAGNOSTIC
        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) {
                if (tp->t_force && len == 1)
                        tcpstat_inc(tcps_sndprobe);
                else if (SEQ_LT(tp->snd_nxt, tp->snd_max)) {
                        tcpstat_pkt(tcps_sndrexmitpack, tcps_sndrexmitbyte,
                            len);
                        tp->t_sndrexmitpack++;
                } else {
                        tcpstat_pkt(tcps_sndpack, tcps_sndbyte, len);
                }
                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;
                }
                m->m_data += max_linkhdr;
                m->m_len = hdrlen;
                if (len <= m_trailingspace(m)) {
                        m_copydata(so->so_snd.sb_mb, off, (int) len,
                            mtod(m, caddr_t) + hdrlen);
                        m->m_len += len;
                } else {
                        m->m_next = m_copym(so->so_snd.sb_mb, off, (int) len,
                            M_NOWAIT);
                        if (m->m_next == 0) {
                                (void) m_free(m);
                                error = ENOBUFS;
                                goto out;
                        }
                }
                if (so->so_snd.sb_mb->m_flags & M_PKTHDR)
                        m->m_pkthdr.ph_loopcnt =
                            so->so_snd.sb_mb->m_pkthdr.ph_loopcnt;
                /*
                 * 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 && !doing_sosend)
                        flags |= TH_PUSH;
                tp->t_sndtime = now;
        } else {
                if (tp->t_flags & TF_ACKNOW)
                        tcpstat_inc(tcps_sndacks);
                else if (flags & (TH_SYN|TH_FIN|TH_RST))
                        tcpstat_inc(tcps_sndctrl);
                else if (SEQ_GT(tp->snd_up, tp->snd_una))
                        tcpstat_inc(tcps_sndurg);
                else
                        tcpstat_inc(tcps_sndwinup);

                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;
                }
                m->m_data += max_linkhdr;
                m->m_len = hdrlen;
        }
        m->m_pkthdr.ph_ifidx = 0;
        m->m_pkthdr.len = hdrlen + len;

        /* Enable TSO and specify the size of the resulting segments. */
        if (tso) {
                SET(m->m_pkthdr.csum_flags, M_TCP_TSO);
                m->m_pkthdr.ph_mss = tp->t_maxseg;
        }

        if (!tp->t_template)
                panic("tcp_output");
#ifdef DIAGNOSTIC
        if (tp->t_template->m_len != hdrlen - optlen)
                panic("tcp_output: template len != hdrlen - optlen");
#endif /* DIAGNOSTIC */
        memcpy(mtod(m, caddr_t), mtod(tp->t_template, caddr_t),
            tp->t_template->m_len);
        th = (struct tcphdr *)(mtod(m, caddr_t) + tp->t_template->m_len -
                sizeof(struct tcphdr));

        /*
         * 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 (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);

        if (sack_rxmit) {
                /*
                 * If sendalot was turned on (due to option stuffing), turn it
                 * off. Properly set th_seq field.  Advance the ret'x pointer
                 * by len.
                 */
                if (sendalot)
                        sendalot = 0;
                th->th_seq = htonl(p->rxmit);
                p->rxmit += len;
                tcpstat_pkt(tcps_sack_rexmits, tcps_sack_rexmit_bytes, len);
        }

        th->th_ack = htonl(tp->rcv_nxt);
        if (optlen) {
                memcpy(th + 1, opt, optlen);
                th->th_off = (sizeof (struct tcphdr) + optlen) >> 2;
        }
#ifdef TCP_ECN
        if (do_ecn) {
                /*
                 * if we have received congestion experienced segs,
                 * set ECE bit.
                 */
                if (tp->t_flags & TF_RCVD_CE) {
                        flags |= TH_ECE;
                        tcpstat_inc(tcps_ecn_sndece);
                }
                if (!(tp->t_flags & TF_DISABLE_ECN)) {
                        /*
                         * if this is a SYN seg, set ECE and CWR.
                         * set only ECE for SYN-ACK if peer supports ECN.
                         */
                        if ((flags & (TH_SYN|TH_ACK)) == TH_SYN)
                                flags |= (TH_ECE|TH_CWR);
                        else if ((tp->t_flags & TF_ECN_PERMIT) &&
                            (flags & (TH_SYN|TH_ACK)) == (TH_SYN|TH_ACK))
                                flags |= TH_ECE;
                }
                /*
                 * if we have reduced the congestion window, notify
                 * the peer by setting CWR bit.
                 */
                if ((tp->t_flags & TF_ECN_PERMIT) &&
                    (tp->t_flags & TF_SEND_CWR)) {
                        flags |= TH_CWR;
                        tp->t_flags &= ~TF_SEND_CWR;
                        tcpstat_inc(tcps_ecn_sndcwr);
                }
        }
#endif
        th->th_flags = flags;

        /*
         * Calculate receive window.  Don't shrink window,
         * but avoid silly window syndrome.
         */
        if (win < (rcv_hiwat / 4) && win < (long)tp->t_maxseg)
                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);
        if (flags & TH_RST)
                win = 0;
        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 (tp->t_flags & TF_SIGNATURE) {
                int iphlen;
                union sockaddr_union src, dst;
                struct tdb *tdb;

                bzero(&src, sizeof(union sockaddr_union));
                bzero(&dst, sizeof(union sockaddr_union));

                switch (tp->pf) {
                case 0: /*default to PF_INET*/
                case AF_INET:
                        iphlen = sizeof(struct ip);
                        src.sa.sa_len = sizeof(struct sockaddr_in);
                        src.sa.sa_family = AF_INET;
                        src.sin.sin_addr = mtod(m, struct ip *)->ip_src;
                        dst.sa.sa_len = sizeof(struct sockaddr_in);
                        dst.sa.sa_family = AF_INET;
                        dst.sin.sin_addr = mtod(m, struct ip *)->ip_dst;
                        break;
#ifdef INET6
                case AF_INET6:
                        iphlen = sizeof(struct ip6_hdr);
                        src.sa.sa_len = sizeof(struct sockaddr_in6);
                        src.sa.sa_family = AF_INET6;
                        src.sin6.sin6_addr = mtod(m, struct ip6_hdr *)->ip6_src;
                        dst.sa.sa_len = sizeof(struct sockaddr_in6);
                        dst.sa.sa_family = AF_INET6;
                        dst.sin6.sin6_addr = mtod(m, struct ip6_hdr *)->ip6_dst;
                        break;
#endif /* INET6 */
                }

                tdb = gettdbbysrcdst(rtable_l2(tp->t_inpcb->inp_rtableid),
                    0, &src, &dst, IPPROTO_TCP);
                if (tdb == NULL) {
                        m_freem(m);
                        return (EPERM);
                }

                if (tcp_signature(tdb, tp->pf, m, th, iphlen, 0,
                    mtod(m, caddr_t) + hdrlen - optlen + sigoff) < 0) {
                        m_freem(m);
                        tdb_unref(tdb);
                        return (EINVAL);
                }
                tdb_unref(tdb);
        }
#endif /* TCP_SIGNATURE */

        /* Defer checksumming until later (ip_output() or hardware) */
        m->m_pkthdr.csum_flags |= M_TCP_CSUM_OUT;

        /*
         * 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.
                 */
                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 (tp->sack_enable) {
                        if (sack_rxmit && (p->rxmit != tp->snd_nxt)) {
                                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 = now;
                                tp->t_rtseq = startseq;
                                tcpstat_inc(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->sack_enable && sack_rxmit &&
                    TCP_TIMER_ISARMED(tp, TCPT_REXMT) == 0 &&
                    tp->snd_nxt != tp->snd_max) {
                        TCP_TIMER_ARM(tp, TCPT_REXMT, tp->t_rxtcur);
                        if (TCP_TIMER_ISARMED(tp, TCPT_PERSIST)) {
                                TCP_TIMER_DISARM(tp, TCPT_PERSIST);
                                tp->t_rxtshift = 0;
                        }
                }

                if (TCP_TIMER_ISARMED(tp, TCPT_REXMT) == 0 &&
                    tp->snd_nxt != tp->snd_una) {
                        TCP_TIMER_ARM(tp, TCPT_REXMT, tp->t_rxtcur);
                        if (TCP_TIMER_ISARMED(tp, TCPT_PERSIST)) {
                                TCP_TIMER_DISARM(tp, TCPT_PERSIST);
                                tp->t_rxtshift = 0;
                        }
                }

                if (len == 0 && so->so_snd.sb_cc &&
                    TCP_TIMER_ISARMED(tp, TCPT_REXMT) == 0 &&
                    TCP_TIMER_ISARMED(tp, TCPT_PERSIST) == 0) {
                        /*
                         * Avoid a situation where we do not set persist timer
                         * after a zero window condition. For example:
                         * 1) A -> B: packet with enough data to fill the window
                         * 2) B -> A: ACK for #1 + new data (0 window
                         *    advertisement)
                         * 3) A -> B: ACK for #2, 0 len packet
                         *
                         * In this case, A will not activate the persist timer,
                         * because it chose to send a packet. Unless tcp_output
                         * is called for some other reason (delayed ack timer,
                         * another input packet from B, socket syscall), A will
                         * not send zero window probes.
                         *
                         * So, if you send a 0-length packet, but there is data
                         * in the socket buffer, and neither the rexmt or
                         * persist timer is already set, then activate the
                         * persist timer.
                         */
                        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;

        tcp_update_sndspace(tp);

        /*
         * Trace.
         */
        if (so->so_options & SO_DEBUG)
                tcp_trace(TA_OUTPUT, tp->t_state, tp, tp, mtod(m, caddr_t), 0,
                        len);

        /*
         * 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.
         */

#ifdef TCP_ECN
        /*
         * if peer is ECN capable, set the ECT bit in the IP header.
         * but don't set ECT for a pure ack, a retransmit or a window probe.
         */
        needect = 0;
        if (do_ecn && (tp->t_flags & TF_ECN_PERMIT)) {
                if (len == 0 || SEQ_LT(tp->snd_nxt, tp->snd_max) ||
                    (tp->t_force && len == 1)) {
                        /* don't set ECT */
                } else {
                        needect = 1;
                        tcpstat_inc(tcps_ecn_sndect);
                }
        }
#endif

        /* force routing table */
        m->m_pkthdr.ph_rtableid = tp->t_inpcb->inp_rtableid;

#if NPF > 0
        pf_mbuf_link_inpcb(m, tp->t_inpcb);
#endif
#if NSTOEPLITZ > 0
        m->m_pkthdr.ph_flowid = tp->t_inpcb->inp_flowid;
        SET(m->m_pkthdr.csum_flags, M_FLOWID);
#endif

        switch (tp->pf) {
        case 0: /*default to PF_INET*/
        case AF_INET:
                {
                        struct ip *ip;

                        ip = mtod(m, struct ip *);
                        ip->ip_len = htons(m->m_pkthdr.len);
                        packetlen = m->m_pkthdr.len;
                        ip->ip_ttl = tp->t_inpcb->inp_ip.ip_ttl;
                        ip->ip_tos = tp->t_inpcb->inp_ip.ip_tos;
#ifdef TCP_ECN
                        if (needect)
                                ip->ip_tos |= IPTOS_ECN_ECT0;
#endif
                }
                error = ip_output(m, tp->t_inpcb->inp_options,
                    &tp->t_inpcb->inp_route,
                    (atomic_load_int(&ip_mtudisc) ? IP_MTUDISC : 0), NULL,
                    &tp->t_inpcb->inp_seclevel, 0);
                break;
#ifdef INET6
        case AF_INET6:
                {
                        struct ip6_hdr *ip6;

                        ip6 = mtod(m, struct ip6_hdr *);
                        ip6->ip6_plen = m->m_pkthdr.len -
                                sizeof(struct ip6_hdr);
                        packetlen = m->m_pkthdr.len;
                        ip6->ip6_nxt = IPPROTO_TCP;
                        ip6->ip6_hlim = in6_selecthlim(tp->t_inpcb);
#ifdef TCP_ECN
                        if (needect)
                                ip6->ip6_flow |= htonl(IPTOS_ECN_ECT0 << 20);
#endif
                }
                error = ip6_output(m, tp->t_inpcb->inp_outputopts6,
                    &tp->t_inpcb->inp_route, 0, NULL,
                    &tp->t_inpcb->inp_seclevel);
                break;
#endif /* INET6 */
        }

        if (error) {
out:
                if (error == ENOBUFS) {
                        /*
                         * If the interface queue is full, or IP cannot
                         * get an mbuf, trigger TCP slow start.
                         */
                        tp->snd_cwnd = tp->t_maxseg;
                        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, -1);
                        return (0);
                }
                if ((error == EHOSTUNREACH || error == ENETDOWN) &&
                    TCPS_HAVERCVDSYN(tp->t_state)) {
                        tp->t_softerror = error;
                        return (0);
                }

                /* Restart the delayed ACK timer, if necessary. */
                if (TCP_TIMER_ISARMED(tp, TCPT_DELACK))
                        TCP_TIMER_ARM(tp, TCPT_DELACK, tcp_delack_msecs);

                return (error);
        }

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

        tcpstat_inc(tcps_sndtotal);
        if (TCP_TIMER_ISARMED(tp, TCPT_DELACK))
                tcpstat_inc(tcps_delack);

        /*
         * 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_sndacktime = now;
        tp->t_flags &= ~TF_ACKNOW;
        TCP_TIMER_DISARM(tp, TCPT_DELACK);
        if (sendalot)
                goto again;
        return (0);
}

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

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

int
tcp_softtso_chop(struct mbuf_list *ml, struct mbuf *m0, struct ifnet *ifp,
    u_int mss)
{
        struct ip *ip = NULL;
#ifdef INET6
        struct ip6_hdr *ip6 = NULL;
#endif
        struct tcphdr *th;
        int firstlen, iphlen, hlen, tlen, off;
        int error;

        ml_init(ml);
        ml_enqueue(ml, m0);

        if (mss == 0) {
                error = EINVAL;
                goto bad;
        }

        ip = mtod(m0, struct ip *);
        switch (ip->ip_v) {
        case 4:
                iphlen = ip->ip_hl << 2;
                if (ISSET(ip->ip_off, htons(IP_OFFMASK | IP_MF)) ||
                    iphlen != sizeof(struct ip) || ip->ip_p != IPPROTO_TCP) {
                        /* only TCP without fragment or IP option supported */
                        error = EPROTOTYPE;
                        goto bad;
                }
                break;
#ifdef INET6
        case 6:
                ip = NULL;
                ip6 = mtod(m0, struct ip6_hdr *);
                iphlen = sizeof(struct ip6_hdr);
                if (ip6->ip6_nxt != IPPROTO_TCP) {
                        /* only TCP without IPv6 header chain supported */
                        error = EPROTOTYPE;
                        goto bad;
                }
                break;
#endif
        default:
                panic("%s: unknown ip version %d", __func__, ip->ip_v);
        }

        tlen = m0->m_pkthdr.len;
        if (tlen < iphlen + sizeof(struct tcphdr)) {
                error = ENOPROTOOPT;
                goto bad;
        }
        /* IP and TCP header should be contiguous, this check is paranoia */
        if (m0->m_len < iphlen + sizeof(*th)) {
                ml_dequeue(ml);
                if ((m0 = m_pullup(m0, iphlen + sizeof(*th))) == NULL) {
                        error = ENOBUFS;
                        goto bad;
                }
                ml_enqueue(ml, m0);
        }
        th = (struct tcphdr *)(mtod(m0, caddr_t) + iphlen);
        hlen = iphlen + (th->th_off << 2);
        if (tlen < hlen) {
                error = ENOPROTOOPT;
                goto bad;
        }
        firstlen = MIN(tlen - hlen, mss);

        CLR(m0->m_pkthdr.csum_flags, M_TCP_TSO);

        /*
         * Loop through length of payload after first segment,
         * make new header and copy data of each part and link onto chain.
         */
        for (off = hlen + firstlen; off < tlen; off += mss) {
                struct mbuf *m;
                struct tcphdr *mhth;
                int len;

                len = MIN(tlen - off, mss);

                MGETHDR(m, M_DONTWAIT, MT_HEADER);
                if (m == NULL) {
                        error = ENOBUFS;
                        goto bad;
                }
                ml_enqueue(ml, m);
                if ((error = m_dup_pkthdr(m, m0, M_DONTWAIT)) != 0)
                        goto bad;

                /* IP and TCP header to the end, space for link layer header */
                m->m_len = hlen;
                m_align(m, hlen);

                /* copy and adjust TCP header */
                mhth = (struct tcphdr *)(mtod(m, caddr_t) + iphlen);
                memcpy(mhth, th, hlen - iphlen);
                mhth->th_seq = htonl(ntohl(th->th_seq) + (off - hlen));
                if (off + len < tlen)
                        CLR(mhth->th_flags, TH_PUSH|TH_FIN);

                /* add mbuf chain with payload */
                m->m_pkthdr.len = hlen + len;
                if ((m->m_next = m_copym(m0, off, len, M_DONTWAIT)) == NULL) {
                        error = ENOBUFS;
                        goto bad;
                }

                /* copy and adjust IP header, calculate checksum */
                SET(m->m_pkthdr.csum_flags, M_TCP_CSUM_OUT);
                if (ip) {
                        struct ip *mhip;

                        mhip = mtod(m, struct ip *);
                        *mhip = *ip;
                        mhip->ip_len = htons(hlen + len);
                        mhip->ip_id = htons(ip_randomid());
                        in_hdr_cksum_out(m, ifp);
                        in_proto_cksum_out(m, ifp);
                }
#ifdef INET6
                if (ip6) {
                        struct ip6_hdr *mhip6;

                        mhip6 = mtod(m, struct ip6_hdr *);
                        *mhip6 = *ip6;
                        mhip6->ip6_plen = htons(hlen - iphlen + len);
                        in6_proto_cksum_out(m, ifp);
                }
#endif
        }

        /*
         * Update first segment by trimming what's been copied out
         * and updating header, then send each segment (in order).
         */
        if (hlen + firstlen < tlen) {
                m_adj(m0, hlen + firstlen - tlen);
                CLR(th->th_flags, TH_PUSH|TH_FIN);
        }
        /* adjust IP header, calculate checksum */
        SET(m0->m_pkthdr.csum_flags, M_TCP_CSUM_OUT);
        if (ip) {
                ip->ip_len = htons(m0->m_pkthdr.len);
                in_hdr_cksum_out(m0, ifp);
                in_proto_cksum_out(m0, ifp);
        }
#ifdef INET6
        if (ip6) {
                ip6->ip6_plen = htons(m0->m_pkthdr.len - iphlen);
                in6_proto_cksum_out(m0, ifp);
        }
#endif

        tcpstat_add(tcps_outpkttso, ml_len(ml));
        return 0;

 bad:
        tcpstat_inc(tcps_outbadtso);
        ml_purge(ml);
        return error;
}

int
tcp_if_output_tso(struct ifnet *ifp, struct mbuf **mp, struct sockaddr *dst,
    struct rtentry *rt, uint32_t ifcap, u_int mtu)
{
        struct mbuf_list ml;
        int error;

        /* caller must fail later or fragment */
        if (!ISSET((*mp)->m_pkthdr.csum_flags, M_TCP_TSO))
                return 0;
        if ((*mp)->m_pkthdr.ph_mss > mtu) {
                CLR((*mp)->m_pkthdr.csum_flags, M_TCP_TSO);
                return 0;
        }

        /* network interface hardware will do TSO */
        if (in_ifcap_cksum(*mp, ifp, ifcap)) {
                if (ISSET(ifcap, IFCAP_TSOv4)) {
                        in_hdr_cksum_out(*mp, ifp);
                        in_proto_cksum_out(*mp, ifp);
                }
#ifdef INET6
                if (ISSET(ifcap, IFCAP_TSOv6))
                        in6_proto_cksum_out(*mp, ifp);
#endif
                error = ifp->if_output(ifp, *mp, dst, rt);
                if (!error)
                        tcpstat_inc(tcps_outhwtso);
                goto done;
        }

        /* as fallback do TSO in software */
        if ((error = tcp_softtso_chop(&ml, *mp, ifp, (*mp)->m_pkthdr.ph_mss)) ||
            (error = if_output_ml(ifp, &ml, dst, rt)))
                goto done;
        tcpstat_inc(tcps_outswtso);

 done:
        *mp = NULL;
        return error;
}