tcp_output.c

Go to the documentation of this file.

/*-
 * SPDX-License-Identifier: BSD-3-Clause
 *
 * 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>
__FBSDID("$FreeBSD$");
 
#include "opt_inet.h"
#include "opt_inet6.h"
#include "opt_ipsec.h"
#include "opt_kern_tls.h"
#include "opt_tcpdebug.h"
 
#include <sys/param.h>
#include <sys/systm.h>
#include <sys/arb.h>
#include <sys/domain.h>
#ifdef TCP_HHOOK
#include <sys/hhook.h>
#endif
#include <sys/kernel.h>
#ifdef KERN_TLS
#include <sys/ktls.h>
#endif
#include <sys/lock.h>
#include <sys/mbuf.h>
#include <sys/mutex.h>
#include <sys/protosw.h>
#include <sys/qmath.h>
#include <sys/sdt.h>
#include <sys/socket.h>
#include <sys/socketvar.h>
#include <sys/sysctl.h>
#include <sys/stats.h>
 
#include <net/if.h>
#include <net/route.h>
#include <net/route/nhop.h>
#include <net/vnet.h>
 
#include <netinet/in.h>
#include <netinet/in_kdtrace.h>
#include <netinet/in_systm.h>
#include <netinet/ip.h>
#include <netinet/in_pcb.h>
#include <netinet/ip_var.h>
#include <netinet/ip_options.h>
#ifdef INET6
#include <netinet6/in6_pcb.h>
#include <netinet/ip6.h>
#include <netinet6/ip6_var.h>
#endif
#include <netinet/tcp.h>
#define TCPOUTFLAGS
#include <netinet/tcp_fsm.h>
#include <netinet/tcp_log_buf.h>
#include <netinet/tcp_seq.h>
#include <netinet/tcp_var.h>
#include <netinet/tcp_syncache.h>
#include <netinet/tcp_timer.h>
#include <netinet/tcpip.h>
#include <netinet/cc/cc.h>
#include <netinet/tcp_fastopen.h>
#ifdef TCPPCAP
#include <netinet/tcp_pcap.h>
#endif
#ifdef TCPDEBUG
#include <netinet/tcp_debug.h>
#endif
#ifdef TCP_OFFLOAD
#include <netinet/tcp_offload.h>
#endif
#include <netinet/tcp_ecn.h>
 
#include <netipsec/ipsec_support.h>
 
#include <netinet/udp.h>
#include <netinet/udp_var.h>
#include <machine/in_cksum.h>
 
#include <security/mac/mac_framework.h>
 
VNET_DEFINE(int, path_mtu_discovery) = 1;
SYSCTL_INT(_net_inet_tcp, OID_AUTO, path_mtu_discovery, CTLFLAG_VNET | CTLFLAG_RW,
        &VNET_NAME(path_mtu_discovery), 1,
        "Enable Path MTU Discovery");
 
VNET_DEFINE(int, tcp_do_tso) = 1;
SYSCTL_INT(_net_inet_tcp, OID_AUTO, tso, CTLFLAG_VNET | CTLFLAG_RW,
        &VNET_NAME(tcp_do_tso), 0,
        "Enable TCP Segmentation Offload");
 
VNET_DEFINE(int, tcp_sendspace) = 1024*32;
#define V_tcp_sendspace VNET(tcp_sendspace)
SYSCTL_INT(_net_inet_tcp, TCPCTL_SENDSPACE, sendspace, CTLFLAG_VNET | CTLFLAG_RW,
        &VNET_NAME(tcp_sendspace), 0, "Initial send socket buffer size");
 
VNET_DEFINE(int, tcp_do_autosndbuf) = 1;
SYSCTL_INT(_net_inet_tcp, OID_AUTO, sendbuf_auto, CTLFLAG_VNET | CTLFLAG_RW,
        &VNET_NAME(tcp_do_autosndbuf), 0,
        "Enable automatic send buffer sizing");
 
VNET_DEFINE(int, tcp_autosndbuf_inc) = 8*1024;
SYSCTL_INT(_net_inet_tcp, OID_AUTO, sendbuf_inc, CTLFLAG_VNET | CTLFLAG_RW,
        &VNET_NAME(tcp_autosndbuf_inc), 0,
        "Incrementor step size of automatic send buffer");
 
VNET_DEFINE(int, tcp_autosndbuf_max) = 2*1024*1024;
SYSCTL_INT(_net_inet_tcp, OID_AUTO, sendbuf_max, CTLFLAG_VNET | CTLFLAG_RW,
        &VNET_NAME(tcp_autosndbuf_max), 0,
        "Max size of automatic send buffer");
 
VNET_DEFINE(int, tcp_sendbuf_auto_lowat) = 0;
#define V_tcp_sendbuf_auto_lowat        VNET(tcp_sendbuf_auto_lowat)
SYSCTL_INT(_net_inet_tcp, OID_AUTO, sendbuf_auto_lowat, CTLFLAG_VNET | CTLFLAG_RW,
        &VNET_NAME(tcp_sendbuf_auto_lowat), 0,
        "Modify threshold for auto send buffer growth to account for SO_SNDLOWAT");
 
/*
 * Make sure that either retransmit or persist timer is set for SYN, FIN and
 * non-ACK.
 */
#define TCP_XMIT_TIMER_ASSERT(tp, len, th_flags)                        \
        KASSERT(((len) == 0 && ((th_flags) & (TH_SYN | TH_FIN)) == 0) ||\
            tcp_timer_active((tp), TT_REXMT) ||                         \
            tcp_timer_active((tp), TT_PERSIST),                         \
            ("neither rexmt nor persist timer is set"))
 
static void inline      cc_after_idle(struct tcpcb *tp);
 
#ifdef TCP_HHOOK
/*
 * Wrapper for the TCP established output helper hook.
 */
void
hhook_run_tcp_est_out(struct tcpcb *tp, struct tcphdr *th,
    struct tcpopt *to, uint32_t len, int tso)
{
        struct tcp_hhook_data hhook_data;
 
        if (V_tcp_hhh[HHOOK_TCP_EST_OUT]->hhh_nhooks > 0) {
                hhook_data.tp = tp;
                hhook_data.th = th;
                hhook_data.to = to;
                hhook_data.len = len;
                hhook_data.tso = tso;
 
                hhook_run_hooks(V_tcp_hhh[HHOOK_TCP_EST_OUT], &hhook_data,
                    tp->osd);
        }
}
#endif
 
/*
 * CC wrapper hook functions
 */
static void inline
cc_after_idle(struct tcpcb *tp)
{
        INP_WLOCK_ASSERT(tp->t_inpcb);
 
        if (CC_ALGO(tp)->after_idle != NULL)
                CC_ALGO(tp)->after_idle(tp->ccv);
}
 
/*
 * Tcp output routine: figure out what should be sent and send it.
 */
int
tcp_default_output(struct tcpcb *tp)
{
        struct socket *so = tp->t_inpcb->inp_socket;
        int32_t len;
        uint32_t recwin, sendwin;
        uint16_t flags;
        int off, error = 0;     /* Keep compiler happy */
        u_int if_hw_tsomaxsegcount = 0;
        u_int if_hw_tsomaxsegsize = 0;
        struct mbuf *m;
        struct ip *ip = NULL;
#ifdef TCPDEBUG
        struct ipovly *ipov = NULL;
#endif
        struct tcphdr *th;
        u_char opt[TCP_MAXOLEN];
        unsigned ipoptlen, optlen, hdrlen, ulen;
#if defined(IPSEC) || defined(IPSEC_SUPPORT)
        unsigned ipsec_optlen = 0;
#endif
        int idle, sendalot, curticks;
        int sack_rxmit, sack_bytes_rxmt;
        struct sackhole *p;
        int tso, mtu;
        struct tcpopt to;
        struct udphdr *udp = NULL;
        unsigned int wanted_cookie = 0;
        unsigned int dont_sendalot = 0;
#if 0
        int maxburst = TCP_MAXBURST;
#endif
#ifdef INET6
        struct ip6_hdr *ip6 = NULL;
        int isipv6;
 
        isipv6 = (tp->t_inpcb->inp_vflag & INP_IPV6) != 0;
#endif
#ifdef KERN_TLS
        const bool hw_tls = (so->so_snd.sb_flags & SB_TLS_IFNET) != 0;
#else
        const bool hw_tls = false;
#endif
 
        NET_EPOCH_ASSERT();
        INP_WLOCK_ASSERT(tp->t_inpcb);
 
#ifdef TCP_OFFLOAD
        if (tp->t_flags & TF_TOE)
                return (tcp_offload_output(tp));
#endif
 
        /*
         * For TFO connections in SYN_SENT or SYN_RECEIVED,
         * only allow the initial SYN or SYN|ACK and those sent
         * by the retransmit timer.
         */
        if (IS_FASTOPEN(tp->t_flags) &&
            ((tp->t_state == TCPS_SYN_SENT) ||
             (tp->t_state == TCPS_SYN_RECEIVED)) &&
            SEQ_GT(tp->snd_max, tp->snd_una) && /* initial SYN or SYN|ACK sent */
            (tp->snd_nxt != tp->snd_una))       /* not a retransmit */
                return (0);
 
        /*
         * 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 && (((ticks - tp->t_rcvtime) >= tp->t_rxtcur) ||
            (tp->t_sndtime && ((ticks - tp->t_sndtime) >= tp->t_rxtcur))))
                cc_after_idle(tp);
        tp->t_flags &= ~TF_LASTIDLE;
        if (idle) {
                if (tp->t_flags & TF_MORETOCOME) {
                        tp->t_flags |= TF_LASTIDLE;
                        idle = 0;
                }
        }
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->t_flags & TF_SACK_PERMIT) &&
            SEQ_LT(tp->snd_nxt, tp->snd_max))
                tcp_sack_adjust(tp);
        sendalot = 0;
        tso = 0;
        mtu = 0;
        off = tp->snd_nxt - tp->snd_una;
        sendwin = min(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;
        if ((tp->t_flags & TF_SACK_PERMIT) && IN_FASTRECOVERY(tp->t_flags) &&
            (p = tcp_sack_output(tp, &sack_bytes_rxmt))) {
                uint32_t cwin;
 
                cwin =
                    imax(min(tp->snd_wnd, tp->snd_cwnd) - sack_bytes_rxmt, 0);
                /* Do not retransmit SACK segments beyond snd_recover */
                if (SEQ_GT(p->end, tp->snd_recover)) {
                        /*
                         * (At least) part of sack hole extends beyond
                         * snd_recover. Check to see if we can rexmit data
                         * for this hole.
                         */
                        if (SEQ_GEQ(p->rxmit, tp->snd_recover)) {
                                /*
                                 * Can't rexmit any more data for this hole.
                                 * That data will be rexmitted in the next
                                 * sack recovery episode, when snd_recover
                                 * moves past p->rxmit.
                                 */
                                p = NULL;
                                goto after_sack_rexmit;
                        } else
                                /* Can rexmit part of the current hole */
                                len = ((int32_t)ulmin(cwin,
                                                   tp->snd_recover - p->rxmit));
                } else
                        len = ((int32_t)ulmin(cwin, p->end - p->rxmit));
                off = p->rxmit - tp->snd_una;
                KASSERT(off >= 0,("%s: sack block to the left of una : %d",
                    __func__, off));
                if (len > 0) {
                        sack_rxmit = 1;
                        sendalot = 1;
                        TCPSTAT_INC(tcps_sack_rexmits);
                        TCPSTAT_ADD(tcps_sack_rexmit_bytes,
                            min(len, tcp_maxseg(tp)));
                }
        }
after_sack_rexmit:
        /*
         * Get standard flags, and add SYN or FIN if requested by 'hidden'
         * state flags.
         */
        if (tp->t_flags & TF_NEEDFIN)
                flags |= TH_FIN;
        if (tp->t_flags & TF_NEEDSYN)
                flags |= TH_SYN;
 
        SOCKBUF_LOCK(&so->so_snd);
        /*
         * 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_flags & TF_FORCEDATA) {
                if (sendwin == 0) {
                        /*
                         * If we still have some data to send, then
                         * clear the FIN bit.  Usually this would
                         * happen below when it realizes that we
                         * aren't sending all the data.  However,
                         * if we have exactly 1 byte of unsent data,
                         * then it won't clear the FIN bit below,
                         * and if we are in persist state, we wind
                         * up sending the packet without recording
                         * that we sent the FIN bit.
                         *
                         * We can't just blindly clear the FIN bit,
                         * because if we don't have any more data
                         * to send then the probe will be the FIN
                         * itself.
                         */
                        if (off < sbused(&so->so_snd))
                                flags &= ~TH_FIN;
                        sendwin = 1;
                } else {
                        tcp_timer_activate(tp, TT_PERSIST, 0);
                        tp->t_rxtshift = 0;
                }
        }
 
        /*
         * If snd_nxt == snd_max and we have transmitted a FIN, the
         * offset will be > 0 even if so_snd.sb_cc is 0, resulting in
         * a negative length.  This can also occur when TCP opens up
         * its congestion window while receiving additional duplicate
         * acks after fast-retransmit because TCP will reset snd_nxt
         * to snd_max after the fast-retransmit.
         *
         * In the normal retransmit-FIN-only case, however, snd_nxt will
         * be set to snd_una, the offset will be 0, and the length may
         * wind up 0.
         *
         * If sack_rxmit is true we are retransmitting from the scoreboard
         * in which case len is already set.
         */
        if (sack_rxmit == 0) {
                if (sack_bytes_rxmt == 0)
                        len = ((int32_t)min(sbavail(&so->so_snd), sendwin) -
                            off);
                else {
                        int32_t cwin;
 
                        /*
                         * We are inside of a SACK recovery episode and are
                         * sending new data, having retransmitted all the
                         * data possible in the scoreboard.
                         */
                        len = ((int32_t)min(sbavail(&so->so_snd), tp->snd_wnd) -
                            off);
                        /*
                         * Don't remove this (len > 0) check !
                         * We explicitly check for len > 0 here (although it
                         * isn't really necessary), to work around a gcc
                         * optimization issue - to force gcc to compute
                         * len above. Without this check, the computation
                         * of len is bungled by the optimizer.
                         */
                        if (len > 0) {
                                cwin = tp->snd_cwnd -
                                        (tp->snd_nxt - tp->snd_recover) -
                                        sack_bytes_rxmt;
                                if (cwin < 0)
                                        cwin = 0;
                                len = imin(len, cwin);
                        }
                }
        }
 
        /*
         * Lop off SYN bit if it has already been sent.  However, if this
         * is SYN-SENT state and if segment contains data and if we don't
         * know that foreign host supports TAO, suppress sending segment.
         */
        if ((flags & TH_SYN) && SEQ_GT(tp->snd_nxt, tp->snd_una)) {
                if (tp->t_state != TCPS_SYN_RECEIVED)
                        flags &= ~TH_SYN;
                /*
                 * When sending additional segments following a TFO SYN|ACK,
                 * do not include the SYN bit.
                 */
                if (IS_FASTOPEN(tp->t_flags) &&
                    (tp->t_state == TCPS_SYN_RECEIVED))
                        flags &= ~TH_SYN;
                off--, len++;
        }
 
        /*
         * Be careful not to send data and/or FIN on SYN segments.
         * This measure is needed to prevent interoperability problems
         * with not fully conformant TCP implementations.
         */
        if ((flags & TH_SYN) && (tp->t_flags & TF_NOOPT)) {
                len = 0;
                flags &= ~TH_FIN;
        }
 
        /*
         * On TFO sockets, ensure no data is sent in the following cases:
         *
         *  - When retransmitting SYN|ACK on a passively-created socket
         *
         *  - When retransmitting SYN on an actively created socket
         *
         *  - When sending a zero-length cookie (cookie request) on an
         *    actively created socket
         *
         *  - When the socket is in the CLOSED state (RST is being sent)
         */
        if (IS_FASTOPEN(tp->t_flags) &&
            (((flags & TH_SYN) && (tp->t_rxtshift > 0)) ||
             ((tp->t_state == TCPS_SYN_SENT) &&
              (tp->t_tfo_client_cookie_len == 0)) ||
             (flags & TH_RST)))
                len = 0;
        if (len <= 0) {
                /*
                 * If FIN has been sent but not acked,
                 * but we haven't been called to retransmit,
                 * len will be < 0.  Otherwise, window shrank
                 * after we sent into it.  If window shrank to 0,
                 * cancel pending retransmit, pull snd_nxt back
                 * to (closed) window, and set the persist timer
                 * if it isn't already going.  If the window didn't
                 * close completely, just wait for an ACK.
                 *
                 * We also do a general check here to ensure that
                 * we will set the persist timer when we have data
                 * to send, but a 0-byte window. This makes sure
                 * the persist timer is set even if the packet
                 * hits one of the "goto send" lines below.
                 */
                len = 0;
                if ((sendwin == 0) && (TCPS_HAVEESTABLISHED(tp->t_state)) &&
                        (off < (int) sbavail(&so->so_snd))) {
                        tcp_timer_activate(tp, TT_REXMT, 0);
                        tp->t_rxtshift = 0;
                        tp->snd_nxt = tp->snd_una;
                        if (!tcp_timer_active(tp, TT_PERSIST))
                                tcp_setpersist(tp);
                }
        }
 
        /* len will be >= 0 after this point. */
        KASSERT(len >= 0, ("[%s:%d]: len < 0", __func__, __LINE__));
 
        tcp_sndbuf_autoscale(tp, so, sendwin);
 
        /*
         * Decide if we can use TCP Segmentation Offloading (if supported by
         * hardware).
         *
         * TSO may only be used if we are in a pure bulk sending state.  The
         * presence of TCP-MD5, SACK retransmits, SACK advertizements and
         * IP options prevent using TSO.  With TSO the TCP header is the same
         * (except for the sequence number) for all generated packets.  This
         * makes it impossible to transmit any options which vary per generated
         * segment or packet.
         *
         * IPv4 handling has a clear separation of ip options and ip header
         * flags while IPv6 combines both in in6p_outputopts. ip6_optlen() does
         * the right thing below to provide length of just ip options and thus
         * checking for ipoptlen is enough to decide if ip options are present.
         */
#if defined(IPSEC) || defined(IPSEC_SUPPORT)
        /*
         * Pre-calculate here as we save another lookup into the darknesses
         * of IPsec that way and can actually decide if TSO is ok.
         */
#ifdef INET6
        if (isipv6 && IPSEC_ENABLED(ipv6))
                ipsec_optlen = IPSEC_HDRSIZE(ipv6, tp->t_inpcb);
#ifdef INET
        else
#endif
#endif /* INET6 */
#ifdef INET
        if (IPSEC_ENABLED(ipv4))
                ipsec_optlen = IPSEC_HDRSIZE(ipv4, tp->t_inpcb);
#endif /* INET */
#endif /* IPSEC */
#ifdef INET6
        if (isipv6)
                ipoptlen = ip6_optlen(tp->t_inpcb);
        else
#endif
        if (tp->t_inpcb->inp_options)
                ipoptlen = tp->t_inpcb->inp_options->m_len -
                                offsetof(struct ipoption, ipopt_list);
        else
                ipoptlen = 0;
#if defined(IPSEC) || defined(IPSEC_SUPPORT)
        ipoptlen += ipsec_optlen;
#endif
 
        if ((tp->t_flags & TF_TSO) && V_tcp_do_tso && len > tp->t_maxseg &&
            (tp->t_port == 0) &&
            ((tp->t_flags & TF_SIGNATURE) == 0) &&
            tp->rcv_numsacks == 0 && sack_rxmit == 0 &&
            ipoptlen == 0 && !(flags & TH_SYN))
                tso = 1;
 
        if (sack_rxmit) {
                if (SEQ_LT(p->rxmit + len, tp->snd_una + sbused(&so->so_snd)))
                        flags &= ~TH_FIN;
        } else {
                if (SEQ_LT(tp->snd_nxt + len, tp->snd_una +
                    sbused(&so->so_snd)))
                        flags &= ~TH_FIN;
        }
 
        recwin = lmin(lmax(sbspace(&so->so_rcv), 0),
            (long)TCP_MAXWIN << tp->rcv_scale);
 
        /*
         * Sender silly window avoidance.   We transmit under the following
         * conditions when len is non-zero:
         *
         *      - We have a full segment (or more with TSO)
         *      - This is the last buffer in a write()/send() and we are
         *        either idle or running NODELAY
         *      - we've timed out (e.g. persist timer)
         *      - we have more then 1/2 the maximum send window's worth of
         *        data (receiver may be limited the window size)
         *      - we need to retransmit
         */
        if (len) {
                if (len >= tp->t_maxseg)
                        goto send;
                /*
                 * As the TCP header options are now
                 * considered when setting up the initial
                 * window, we would not send the last segment
                 * if we skip considering the option length here.
                 * Note: this may not work when tcp headers change
                 * very dynamically in the future.
                 */
                if ((((tp->t_flags & TF_SIGNATURE) ?
                        PADTCPOLEN(TCPOLEN_SIGNATURE) : 0) +
                    ((tp->t_flags & TF_RCVD_TSTMP) ?
                        PADTCPOLEN(TCPOLEN_TIMESTAMP) : 0) +
                    len) >= tp->t_maxseg)
                        goto send;
                /*
                 * NOTE! on localhost connections an 'ack' from the remote
                 * end may occur synchronously with the output and cause
                 * us to flush a buffer queued with moretocome.  XXX
                 *
                 * note: the len + off check is almost certainly unnecessary.
                 */
                if (!(tp->t_flags & TF_MORETOCOME) &&   /* normal case */
                    (idle || (tp->t_flags & TF_NODELAY)) &&
                    (uint32_t)len + (uint32_t)off >= sbavail(&so->so_snd) &&
                    (tp->t_flags & TF_NOPUSH) == 0) {
                        goto send;
                }
                if (tp->t_flags & TF_FORCEDATA)         /* typ. timeout case */
                        goto send;
                if (len >= tp->max_sndwnd / 2 && tp->max_sndwnd > 0)
                        goto send;
                if (SEQ_LT(tp->snd_nxt, tp->snd_max))   /* retransmit case */
                        goto send;
                if (sack_rxmit)
                        goto send;
        }
 
        /*
         * Sending of standalone window updates.
         *
         * Window updates are important when we close our window due to a
         * full socket buffer and are opening it again after the application
         * reads data from it.  Once the window has opened again and the
         * remote end starts to send again the ACK clock takes over and
         * provides the most current window information.
         *
         * We must avoid the silly window syndrome whereas every read
         * from the receive buffer, no matter how small, causes a window
         * update to be sent.  We also should avoid sending a flurry of
         * window updates when the socket buffer had queued a lot of data
         * and the application is doing small reads.
         *
         * Prevent a flurry of pointless window updates by only sending
         * an update when we can increase the advertized window by more
         * than 1/4th of the socket buffer capacity.  When the buffer is
         * getting full or is very small be more aggressive and send an
         * update whenever we can increase by two mss sized segments.
         * In all other situations the ACK's to new incoming data will
         * carry further window increases.
         *
         * Don't send an independent window update if a delayed
         * ACK is pending (it will get piggy-backed on it) or the
         * remote side already has done a half-close and won't send
         * more data.  Skip this if the connection is in T/TCP
         * half-open state.
         */
        if (recwin > 0 && !(tp->t_flags & TF_NEEDSYN) &&
            !(tp->t_flags & TF_DELACK) &&
            !TCPS_HAVERCVDFIN(tp->t_state)) {
                /*
                 * "adv" is the amount we could increase the window,
                 * taking into account that we are limited by
                 * TCP_MAXWIN << tp->rcv_scale.
                 */
                int32_t adv;
                int oldwin;
 
                adv = recwin;
                if (SEQ_GT(tp->rcv_adv, tp->rcv_nxt)) {
                        oldwin = (tp->rcv_adv - tp->rcv_nxt);
                        if (adv > oldwin)
                                adv -= oldwin;
                        else
                                adv = 0;
                } 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 (oldwin >> tp->rcv_scale >= (adv + oldwin) >> tp->rcv_scale)
                        goto dontupdate;
 
                if (adv >= (int32_t)(2 * tp->t_maxseg) &&
                    (adv >= (int32_t)(so->so_rcv.sb_hiwat / 4) ||
                     recwin <= (so->so_rcv.sb_hiwat / 8) ||
                     so->so_rcv.sb_hiwat <= 8 * tp->t_maxseg ||
                     adv >= TCP_MAXWIN << tp->rcv_scale))
                        goto send;
                if (2 * adv >= (int32_t)so->so_rcv.sb_hiwat)
                        goto send;
        }
dontupdate:
 
        /*
         * Send if we owe the peer an ACK, RST, SYN, or urgent data.  ACKNOW
         * is also a catch-all for the retransmit timer timeout case.
         */
        if (tp->t_flags & TF_ACKNOW)
                goto send;
        if ((flags & TH_RST) ||
            ((flags & TH_SYN) && (tp->t_flags & TF_NEEDSYN) == 0))
                goto send;
        if (SEQ_GT(tp->snd_up, tp->snd_una))
                goto send;
        /*
         * If our state indicates that FIN should be sent
         * and we have not yet done so, then we need to send.
         */
        if (flags & TH_FIN &&
            ((tp->t_flags & TF_SENTFIN) == 0 || tp->snd_nxt == tp->snd_una))
                goto send;
        /*
         * In SACK, it is possible for tcp_output to fail to send a segment
         * after the retransmission timer has been turned off.  Make sure
         * that the retransmission timer is set.
         */
        if ((tp->t_flags & TF_SACK_PERMIT) &&
            SEQ_GT(tp->snd_max, tp->snd_una) &&
            !tcp_timer_active(tp, TT_REXMT) &&
            !tcp_timer_active(tp, TT_PERSIST)) {
                tcp_timer_activate(tp, TT_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
         *
         * tcp_timer_active(tp, TT_PERSIST)
         *      is true when we are in persist state.
         * (tp->t_flags & TF_FORCEDATA)
         *      is set when we are called to send a persist packet.
         * tcp_timer_active(tp, TT_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 (sbavail(&so->so_snd) && !tcp_timer_active(tp, TT_REXMT) &&
            !tcp_timer_active(tp, TT_PERSIST)) {
                tp->t_rxtshift = 0;
                tcp_setpersist(tp);
        }
 
        /*
         * No reason to send a segment, just return.
         */
just_return:
        SOCKBUF_UNLOCK(&so->so_snd);
        return (0);
 
send:
        SOCKBUF_LOCK_ASSERT(&so->so_snd);
        if (len > 0) {
                if (len >= tp->t_maxseg)
                        tp->t_flags2 |= TF2_PLPMTU_MAXSEGSNT;
                else
                        tp->t_flags2 &= ~TF2_PLPMTU_MAXSEGSNT;
        }
        /*
         * Before ESTABLISHED, force sending of initial options
         * unless TCP set not to do any options.
         * NOTE: we assume that the IP/TCP header plus TCP options
         * always fit in a single mbuf, leaving room for a maximum
         * link header, i.e.
         *      max_linkhdr + sizeof (struct tcpiphdr) + optlen <= MCLBYTES
         */
        optlen = 0;
#ifdef INET6
        if (isipv6)
                hdrlen = sizeof (struct ip6_hdr) + sizeof (struct tcphdr);
        else
#endif
                hdrlen = sizeof (struct tcpiphdr);
 
        if (flags & TH_SYN) {
                tp->snd_nxt = tp->iss;
        }
 
        /*
         * Compute options for segment.
         * We only have to care about SYN and established connection
         * segments.  Options for SYN-ACK segments are handled in TCP
         * syncache.
         */
        to.to_flags = 0;
        if ((tp->t_flags & TF_NOOPT) == 0) {
                /* Maximum segment size. */
                if (flags & TH_SYN) {
                        to.to_mss = tcp_mssopt(&tp->t_inpcb->inp_inc);
                        if (tp->t_port)
                                to.to_mss -= V_tcp_udp_tunneling_overhead;
                        to.to_flags |= TOF_MSS;
 
                        /*
                         * On SYN or SYN|ACK transmits on TFO connections,
                         * only include the TFO option if it is not a
                         * retransmit, as the presence of the TFO option may
                         * have caused the original SYN or SYN|ACK to have
                         * been dropped by a middlebox.
                         */
                        if (IS_FASTOPEN(tp->t_flags) &&
                            (tp->t_rxtshift == 0)) {
                                if (tp->t_state == TCPS_SYN_RECEIVED) {
                                        to.to_tfo_len = TCP_FASTOPEN_COOKIE_LEN;
                                        to.to_tfo_cookie =
                                            (u_int8_t *)&tp->t_tfo_cookie.server;
                                        to.to_flags |= TOF_FASTOPEN;
                                        wanted_cookie = 1;
                                } else if (tp->t_state == TCPS_SYN_SENT) {
                                        to.to_tfo_len =
                                            tp->t_tfo_client_cookie_len;
                                        to.to_tfo_cookie =
                                            tp->t_tfo_cookie.client;
                                        to.to_flags |= TOF_FASTOPEN;
                                        wanted_cookie = 1;
                                        /*
                                         * If we wind up having more data to
                                         * send with the SYN than can fit in
                                         * one segment, don't send any more
                                         * until the SYN|ACK comes back from
                                         * the other end.
                                         */
                                        dont_sendalot = 1;
                                }
                        }
                }
                /* Window scaling. */
                if ((flags & TH_SYN) && (tp->t_flags & TF_REQ_SCALE)) {
                        to.to_wscale = tp->request_r_scale;
                        to.to_flags |= TOF_SCALE;
                }
                /* Timestamps. */
                if ((tp->t_flags & TF_RCVD_TSTMP) ||
                    ((flags & TH_SYN) && (tp->t_flags & TF_REQ_TSTMP))) {
                        curticks = tcp_ts_getticks();
                        to.to_tsval = curticks + tp->ts_offset;
                        to.to_tsecr = tp->ts_recent;
                        to.to_flags |= TOF_TS;
                        if (tp->t_rxtshift == 1)
                                tp->t_badrxtwin = curticks;
                }
 
                /* Set receive buffer autosizing timestamp. */
                if (tp->rfbuf_ts == 0 &&
                    (so->so_rcv.sb_flags & SB_AUTOSIZE))
                        tp->rfbuf_ts = tcp_ts_getticks();
 
                /* Selective ACK's. */
                if (tp->t_flags & TF_SACK_PERMIT) {
                        if (flags & TH_SYN)
                                to.to_flags |= TOF_SACKPERM;
                        else if (TCPS_HAVEESTABLISHED(tp->t_state) &&
                            tp->rcv_numsacks > 0) {
                                to.to_flags |= TOF_SACK;
                                to.to_nsacks = tp->rcv_numsacks;
                                to.to_sacks = (u_char *)tp->sackblks;
                        }
                }
#if defined(IPSEC_SUPPORT) || defined(TCP_SIGNATURE)
                /* TCP-MD5 (RFC2385). */
                /*
                 * Check that TCP_MD5SIG is enabled in tcpcb to
                 * account the size needed to set this TCP option.
                 */
                if (tp->t_flags & TF_SIGNATURE)
                        to.to_flags |= TOF_SIGNATURE;
#endif /* TCP_SIGNATURE */
 
                /* Processing the options. */
                hdrlen += optlen = tcp_addoptions(&to, opt);
                /*
                 * If we wanted a TFO option to be added, but it was unable
                 * to fit, ensure no data is sent.
                 */
                if (IS_FASTOPEN(tp->t_flags) && wanted_cookie &&
                    !(to.to_flags & TOF_FASTOPEN))
                        len = 0;
        }
        if (tp->t_port) {
                if (V_tcp_udp_tunneling_port == 0) {
                        /* The port was removed?? */
                        SOCKBUF_UNLOCK(&so->so_snd);
                        return (EHOSTUNREACH);
                }
                hdrlen += sizeof(struct udphdr);
        }
        /*
         * Adjust data length if insertion of options will
         * bump the packet length beyond the t_maxseg length.
         * Clear the FIN bit because we cut off the tail of
         * the segment.
         */
        if (len + optlen + ipoptlen > tp->t_maxseg) {
                flags &= ~TH_FIN;
 
                if (tso) {
                        u_int if_hw_tsomax;
                        u_int moff;
                        int max_len;
 
                        /* extract TSO information */
                        if_hw_tsomax = tp->t_tsomax;
                        if_hw_tsomaxsegcount = tp->t_tsomaxsegcount;
                        if_hw_tsomaxsegsize = tp->t_tsomaxsegsize;
 
                        /*
                         * Limit a TSO burst to prevent it from
                         * overflowing or exceeding the maximum length
                         * allowed by the network interface:
                         */
                        KASSERT(ipoptlen == 0,
                            ("%s: TSO can't do IP options", __func__));
 
                        /*
                         * Check if we should limit by maximum payload
                         * length:
                         */
                        if (if_hw_tsomax != 0) {
                                /* compute maximum TSO length */
                                max_len = (if_hw_tsomax - hdrlen -
                                    max_linkhdr);
                                if (max_len <= 0) {
                                        len = 0;
                                } else if (len > max_len) {
                                        sendalot = 1;
                                        len = max_len;
                                }
                        }
 
                        /*
                         * Prevent the last segment from being
                         * fractional unless the send sockbuf can be
                         * emptied:
                         */
                        max_len = (tp->t_maxseg - optlen);
                        if (((uint32_t)off + (uint32_t)len) <
                            sbavail(&so->so_snd)) {
                                moff = len % max_len;
                                if (moff != 0) {
                                        len -= moff;
                                        sendalot = 1;
                                }
                        }
 
                        /*
                         * In case there are too many small fragments
                         * don't use TSO:
                         */
                        if (len <= max_len) {
                                len = max_len;
                                sendalot = 1;
                                tso = 0;
                        }
 
                        /*
                         * Send the FIN in a separate segment
                         * after the bulk sending is done.
                         * We don't trust the TSO implementations
                         * to clear the FIN flag on all but the
                         * last segment.
                         */
                        if (tp->t_flags & TF_NEEDFIN)
                                sendalot = 1;
                } else {
                        if (optlen + ipoptlen >= tp->t_maxseg) {
                                /*
                                 * Since we don't have enough space to put
                                 * the IP header chain and the TCP header in
                                 * one packet as required by RFC 7112, don't
                                 * send it. Also ensure that at least one
                                 * byte of the payload can be put into the
                                 * TCP segment.
                                 */
                                SOCKBUF_UNLOCK(&so->so_snd);
                                error = EMSGSIZE;
                                sack_rxmit = 0;
                                goto out;
                        }
                        len = tp->t_maxseg - optlen - ipoptlen;
                        sendalot = 1;
                        if (dont_sendalot)
                                sendalot = 0;
                }
        } else
                tso = 0;
 
        KASSERT(len + hdrlen + ipoptlen <= IP_MAXPACKET,
            ("%s: len > IP_MAXPACKET", __func__));
 
/*#ifdef DIAGNOSTIC*/
#ifdef INET6
        if (max_linkhdr + hdrlen > MCLBYTES)
#else
        if (max_linkhdr + hdrlen > MHLEN)
#endif
                panic("tcphdr too big");
/*#endif*/
 
        /*
         * This KASSERT is here to catch edge cases at a well defined place.
         * Before, those had triggered (random) panic conditions further down.
         */
        KASSERT(len >= 0, ("[%s:%d]: len < 0", __func__, __LINE__));
 
        /*
         * 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) {
                struct mbuf *mb;
                struct sockbuf *msb;
                u_int moff;
 
                if ((tp->t_flags & TF_FORCEDATA) && len == 1) {
                        TCPSTAT_INC(tcps_sndprobe);
#ifdef STATS
                        if (SEQ_LT(tp->snd_nxt, tp->snd_max))
                                stats_voi_update_abs_u32(tp->t_stats,
                                VOI_TCP_RETXPB, len);
                        else
                                stats_voi_update_abs_u64(tp->t_stats,
                                    VOI_TCP_TXPB, len);
#endif /* STATS */
                } else if (SEQ_LT(tp->snd_nxt, tp->snd_max) || sack_rxmit) {
                        tp->t_sndrexmitpack++;
                        TCPSTAT_INC(tcps_sndrexmitpack);
                        TCPSTAT_ADD(tcps_sndrexmitbyte, len);
#ifdef STATS
                        stats_voi_update_abs_u32(tp->t_stats, VOI_TCP_RETXPB,
                            len);
#endif /* STATS */
                } else {
                        TCPSTAT_INC(tcps_sndpack);
                        TCPSTAT_ADD(tcps_sndbyte, len);
#ifdef STATS
                        stats_voi_update_abs_u64(tp->t_stats, VOI_TCP_TXPB,
                            len);
#endif /* STATS */
                }
#ifdef INET6
                if (MHLEN < hdrlen + max_linkhdr)
                        m = m_getcl(M_NOWAIT, MT_DATA, M_PKTHDR);
                else
#endif
                        m = m_gethdr(M_NOWAIT, MT_DATA);
 
                if (m == NULL) {
                        SOCKBUF_UNLOCK(&so->so_snd);
                        error = ENOBUFS;
                        sack_rxmit = 0;
                        goto out;
                }
 
                m->m_data += max_linkhdr;
                m->m_len = hdrlen;
 
                /*
                 * Start the m_copy functions from the closest mbuf
                 * to the offset in the socket buffer chain.
                 */
                mb = sbsndptr_noadv(&so->so_snd, off, &moff);
                if (len <= MHLEN - hdrlen - max_linkhdr && !hw_tls) {
                        m_copydata(mb, moff, len,
                            mtod(m, caddr_t) + hdrlen);
                        if (SEQ_LT(tp->snd_nxt, tp->snd_max))
                                sbsndptr_adv(&so->so_snd, mb, len);
                        m->m_len += len;
                } else {
                        if (SEQ_LT(tp->snd_nxt, tp->snd_max))
                                msb = NULL;
                        else
                                msb = &so->so_snd;
                        m->m_next = tcp_m_copym(mb, moff,
                            &len, if_hw_tsomaxsegcount,
                            if_hw_tsomaxsegsize, msb, hw_tls);
                        if (len <= (tp->t_maxseg - optlen)) {
                                /*
                                 * Must have ran out of mbufs for the copy
                                 * shorten it to no longer need tso. Lets
                                 * not put on sendalot since we are low on
                                 * mbufs.
                                 */
                                tso = 0;
                        }
                        if (m->m_next == NULL) {
                                SOCKBUF_UNLOCK(&so->so_snd);
                                (void) m_free(m);
                                error = ENOBUFS;
                                sack_rxmit = 0;
                                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 (((uint32_t)off + (uint32_t)len == sbused(&so->so_snd)) &&
                    !(flags & TH_SYN))
                        flags |= TH_PUSH;
                SOCKBUF_UNLOCK(&so->so_snd);
        } else {
                SOCKBUF_UNLOCK(&so->so_snd);
                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);
 
                m = m_gethdr(M_NOWAIT, MT_DATA);
                if (m == NULL) {
                        error = ENOBUFS;
                        sack_rxmit = 0;
                        goto out;
                }
#ifdef INET6
                if (isipv6 && (MHLEN < hdrlen + max_linkhdr) &&
                    MHLEN >= hdrlen) {
                        M_ALIGN(m, hdrlen);
                } else
#endif
                m->m_data += max_linkhdr;
                m->m_len = hdrlen;
        }
        SOCKBUF_UNLOCK_ASSERT(&so->so_snd);
        m->m_pkthdr.rcvif = (struct ifnet *)0;
#ifdef MAC
        mac_inpcb_create_mbuf(tp->t_inpcb, m);
#endif
#ifdef INET6
        if (isipv6) {
                ip6 = mtod(m, struct ip6_hdr *);
                if (tp->t_port) {
                        udp = (struct udphdr *)((caddr_t)ip6 + sizeof(struct ip6_hdr));
                        udp->uh_sport = htons(V_tcp_udp_tunneling_port);
                        udp->uh_dport = tp->t_port;
                        ulen = hdrlen + len - sizeof(struct ip6_hdr);
                        udp->uh_ulen = htons(ulen);
                        th = (struct tcphdr *)(udp + 1);
                } else {
                        th = (struct tcphdr *)(ip6 + 1);
                }
                tcpip_fillheaders(tp->t_inpcb, tp->t_port, ip6, th);
        } else
#endif /* INET6 */
        {
                ip = mtod(m, struct ip *);
#ifdef TCPDEBUG
                ipov = (struct ipovly *)ip;
#endif
                if (tp->t_port) {
                        udp = (struct udphdr *)((caddr_t)ip + sizeof(struct ip));
                        udp->uh_sport = htons(V_tcp_udp_tunneling_port);
                        udp->uh_dport = tp->t_port;
                        ulen = hdrlen + len - sizeof(struct ip);
                        udp->uh_ulen = htons(ulen);
                        th = (struct tcphdr *)(udp + 1);
                } else
                        th = (struct tcphdr *)(ip + 1);
                tcpip_fillheaders(tp->t_inpcb, tp->t_port, ip, th);
        }
 
        /*
         * Fill in fields, remembering maximum advertised
         * window for use in delaying messages about window sizes.
         * If resending a FIN, be sure not to use a new sequence number.
         */
        if (flags & TH_FIN && tp->t_flags & TF_SENTFIN &&
            tp->snd_nxt == tp->snd_max)
                tp->snd_nxt--;
        /*
         * If we are 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 && V_tcp_do_ecn) {
                flags |= tcp_ecn_output_syn_sent(tp);
        }
        /* Also handle parallel SYN for ECN */
        if ((TCPS_HAVERCVDSYN(tp->t_state)) &&
            (tp->t_flags2 & TF2_ECN_PERMIT)) {
                int ect = tcp_ecn_output_established(tp, &flags, len, sack_rxmit);
                if ((tp->t_state == TCPS_SYN_RECEIVED) &&
                    (tp->t_flags2 & TF2_ECN_SND_ECE))
                        tp->t_flags2 &= ~TF2_ECN_SND_ECE;
#ifdef INET6
                if (isipv6) {
                        ip6->ip6_flow &= ~htonl(IPTOS_ECN_MASK << 20);
                        ip6->ip6_flow |= htonl(ect << 20);
                }
                else
#endif
                {
                        ip->ip_tos &= ~IPTOS_ECN_MASK;
                        ip->ip_tos |= ect;
                }
        }
 
        /*
         * If we are doing retransmissions, then snd_nxt will
         * not reflect the first unsent octet.  For ACK only
         * packets, we do not want the sequence number of the
         * retransmitted packet, we want the sequence number
         * of the next unsent octet.  So, if there is no data
         * (and no SYN or FIN), use snd_max instead of snd_nxt
         * when filling in ti_seq.  But if we are in persist
         * state, snd_max might reflect one byte beyond the
         * right edge of the window, so use snd_nxt in that
         * case, since we know we aren't doing a retransmission.
         * (retransmit and persist are mutually exclusive...)
         */
        if (sack_rxmit == 0) {
                if (len || (flags & (TH_SYN|TH_FIN)) ||
                    tcp_timer_active(tp, TT_PERSIST))
                        th->th_seq = htonl(tp->snd_nxt);
                else
                        th->th_seq = htonl(tp->snd_max);
        } else {
                th->th_seq = htonl(p->rxmit);
                p->rxmit += len;
                /*
                 * Lost Retransmission Detection
                 * trigger resending of a (then
                 * still existing) hole, when
                 * fack acks recoverypoint.
                 */
                if ((tp->t_flags & TF_LRD) && SEQ_GEQ(p->rxmit, p->end))
                        p->rxmit = tp->snd_recover;
                tp->sackhint.sack_bytes_rexmit += len;
        }
        if (IN_RECOVERY(tp->t_flags)) {
                /*
                 * Account all bytes transmitted while
                 * IN_RECOVERY, simplifying PRR and
                 * Lost Retransmit Detection
                 */
                tp->sackhint.prr_out += len;
        }
        th->th_ack = htonl(tp->rcv_nxt);
        if (optlen) {
                bcopy(opt, th + 1, optlen);
                th->th_off = (sizeof (struct tcphdr) + optlen) >> 2;
        }
        tcp_set_flags(th, flags);
        /*
         * Calculate receive window.  Don't shrink window,
         * but avoid silly window syndrome.
         * If a RST segment is sent, advertise a window of zero.
         */
        if (flags & TH_RST) {
                recwin = 0;
        } else {
                if (recwin < (so->so_rcv.sb_hiwat / 4) &&
                    recwin < tp->t_maxseg)
                        recwin = 0;
                if (SEQ_GT(tp->rcv_adv, tp->rcv_nxt) &&
                    recwin < (tp->rcv_adv - tp->rcv_nxt))
                        recwin = (tp->rcv_adv - tp->rcv_nxt);
        }
        /*
         * According to RFC1323 the window field in a SYN (i.e., a <SYN>
         * or <SYN,ACK>) segment itself is never scaled.  The <SYN,ACK>
         * case is handled in syncache.
         */
        if (flags & TH_SYN)
                th->th_win = htons((u_short)
                                (min(sbspace(&so->so_rcv), TCP_MAXWIN)));
        else {
                /* Avoid shrinking window with window scaling. */
                recwin = roundup2(recwin, 1 << tp->rcv_scale);
                th->th_win = htons((u_short)(recwin >> tp->rcv_scale));
        }
 
        /*
         * Adjust the RXWIN0SENT flag - indicate that we have advertised
         * a 0 window.  This may cause the remote transmitter to stall.  This
         * flag tells soreceive() to disable delayed acknowledgements when
         * draining the buffer.  This can occur if the receiver is attempting
         * to read more data than can be buffered prior to transmitting on
         * the connection.
         */
        if (th->th_win == 0) {
                tp->t_sndzerowin++;
                tp->t_flags |= TF_RXWIN0SENT;
        } else
                tp->t_flags &= ~TF_RXWIN0SENT;
        if (SEQ_GT(tp->snd_up, tp->snd_nxt)) {
                th->th_urp = htons((u_short)(tp->snd_up - tp->snd_nxt));
                th->th_flags |= TH_URG;
        } else
                /*
                 * If no urgent pointer to send, then we pull
                 * the urgent pointer to the left edge of the send window
                 * so that it doesn't drift into the send window on sequence
                 * number wraparound.
                 */
                tp->snd_up = tp->snd_una;               /* drag it along */
 
        /*
         * Put TCP length in extended header, and then
         * checksum extended header and data.
         */
        m->m_pkthdr.len = hdrlen + len; /* in6_cksum() need this */
 
#if defined(IPSEC_SUPPORT) || defined(TCP_SIGNATURE)
        if (to.to_flags & TOF_SIGNATURE) {
                /*
                 * Calculate MD5 signature and put it into the place
                 * determined before.
                 * NOTE: since TCP options buffer doesn't point into
                 * mbuf's data, calculate offset and use it.
                 */
                if (!TCPMD5_ENABLED() || (error = TCPMD5_OUTPUT(m, th,
                    (u_char *)(th + 1) + (to.to_signature - opt))) != 0) {
                        /*
                         * Do not send segment if the calculation of MD5
                         * digest has failed.
                         */
                        m_freem(m);
                        goto out;
                }
        }
#endif
#ifdef INET6
        if (isipv6) {
                /*
                 * There is no need to fill in ip6_plen right now.
                 * It will be filled later by ip6_output.
                 */
                if (tp->t_port) {
                        m->m_pkthdr.csum_flags = CSUM_UDP_IPV6;
                        m->m_pkthdr.csum_data = offsetof(struct udphdr, uh_sum);
                        udp->uh_sum = in6_cksum_pseudo(ip6, ulen, IPPROTO_UDP, 0);
                        th->th_sum = htons(0);
                        UDPSTAT_INC(udps_opackets);
                } else {
                        m->m_pkthdr.csum_flags = CSUM_TCP_IPV6;
                        m->m_pkthdr.csum_data = offsetof(struct tcphdr, th_sum);
                        th->th_sum = in6_cksum_pseudo(ip6,
                            sizeof(struct tcphdr) + optlen + len, IPPROTO_TCP,
                            0);
                }
        }
#endif
#if defined(INET6) && defined(INET)
        else
#endif
#ifdef INET
        {
                if (tp->t_port) {
                        m->m_pkthdr.csum_flags = CSUM_UDP;
                        m->m_pkthdr.csum_data = offsetof(struct udphdr, uh_sum);
                        udp->uh_sum = in_pseudo(ip->ip_src.s_addr,
                           ip->ip_dst.s_addr, htons(ulen + IPPROTO_UDP));
                        th->th_sum = htons(0);
                        UDPSTAT_INC(udps_opackets);
                } else {
                        m->m_pkthdr.csum_flags = CSUM_TCP;
                        m->m_pkthdr.csum_data = offsetof(struct tcphdr, th_sum);
                        th->th_sum = in_pseudo(ip->ip_src.s_addr,
                            ip->ip_dst.s_addr, htons(sizeof(struct tcphdr) +
                            IPPROTO_TCP + len + optlen));
                }
 
                /* IP version must be set here for ipv4/ipv6 checking later */
                KASSERT(ip->ip_v == IPVERSION,
                    ("%s: IP version incorrect: %d", __func__, ip->ip_v));
        }
#endif
 
        /*
         * Enable TSO and specify the size of the segments.
         * The TCP pseudo header checksum is always provided.
         */
        if (tso) {
                KASSERT(len > tp->t_maxseg - optlen,
                    ("%s: len <= tso_segsz", __func__));
                m->m_pkthdr.csum_flags |= CSUM_TSO;
                m->m_pkthdr.tso_segsz = tp->t_maxseg - optlen;
        }
 
        KASSERT(len + hdrlen == m_length(m, NULL),
            ("%s: mbuf chain shorter than expected: %d + %u != %u",
            __func__, len, hdrlen, m_length(m, NULL)));
 
#ifdef TCP_HHOOK
        /* Run HHOOK_TCP_ESTABLISHED_OUT helper hooks. */
        hhook_run_tcp_est_out(tp, th, &to, len, tso);
#endif
 
#ifdef TCPDEBUG
        /*
         * Trace.
         */
        if (so->so_options & SO_DEBUG) {
                u_short save = 0;
#ifdef INET6
                if (!isipv6)
#endif
                {
                        save = ipov->ih_len;
                        ipov->ih_len = htons(m->m_pkthdr.len /* - hdrlen + (th->th_off << 2) */);
                }
                tcp_trace(TA_OUTPUT, tp->t_state, tp, mtod(m, void *), th, 0);
#ifdef INET6
                if (!isipv6)
#endif
                ipov->ih_len = save;
        }
#endif /* TCPDEBUG */
        TCP_PROBE3(debug__output, tp, th, m);
 
        /* We're getting ready to send; log now. */
        TCP_LOG_EVENT(tp, th, &so->so_rcv, &so->so_snd, TCP_LOG_OUT, ERRNO_UNK,
            len, NULL, false);
 
        /*
         * Fill in IP length and desired time to live and
         * send to IP level.  There should be a better way
         * to handle ttl and tos; we could keep them in
         * the template, but need a way to checksum without them.
         */
        /*
         * m->m_pkthdr.len should have been set before checksum calculation,
         * because in6_cksum() need it.
         */
#ifdef INET6
        if (isipv6) {
                /*
                 * we separately set hoplimit for every segment, since the
                 * user might want to change the value via setsockopt.
                 * Also, desired default hop limit might be changed via
                 * Neighbor Discovery.
                 */
                ip6->ip6_hlim = in6_selecthlim(tp->t_inpcb, NULL);
 
                /*
                 * Set the packet size here for the benefit of DTrace probes.
                 * ip6_output() will set it properly; it's supposed to include
                 * the option header lengths as well.
                 */
                ip6->ip6_plen = htons(m->m_pkthdr.len - sizeof(*ip6));
 
                if (V_path_mtu_discovery && tp->t_maxseg > V_tcp_minmss)
                        tp->t_flags2 |= TF2_PLPMTU_PMTUD;
                else
                        tp->t_flags2 &= ~TF2_PLPMTU_PMTUD;
 
                if (tp->t_state == TCPS_SYN_SENT)
                        TCP_PROBE5(connect__request, NULL, tp, ip6, tp, th);
 
                TCP_PROBE5(send, NULL, tp, ip6, tp, th);
 
#ifdef TCPPCAP
                /* Save packet, if requested. */
                tcp_pcap_add(th, m, &(tp->t_outpkts));
#endif
 
                /* TODO: IPv6 IP6TOS_ECT bit on */
                error = ip6_output(m, tp->t_inpcb->in6p_outputopts,
                    &tp->t_inpcb->inp_route6,
                    ((so->so_options & SO_DONTROUTE) ?  IP_ROUTETOIF : 0),
                    NULL, NULL, tp->t_inpcb);
 
                if (error == EMSGSIZE && tp->t_inpcb->inp_route6.ro_nh != NULL)
                        mtu = tp->t_inpcb->inp_route6.ro_nh->nh_mtu;
        }
#endif /* INET6 */
#if defined(INET) && defined(INET6)
        else
#endif
#ifdef INET
    {
        ip->ip_len = htons(m->m_pkthdr.len);
#ifdef INET6
        if (tp->t_inpcb->inp_vflag & INP_IPV6PROTO)
                ip->ip_ttl = in6_selecthlim(tp->t_inpcb, NULL);
#endif /* INET6 */
        /*
         * If we do path MTU discovery, then we set DF on every packet.
         * This might not be the best thing to do according to RFC3390
         * Section 2. However the tcp hostcache migitates the problem
         * so it affects only the first tcp connection with a host.
         *
         * NB: Don't set DF on small MTU/MSS to have a safe fallback.
         */
        if (V_path_mtu_discovery && tp->t_maxseg > V_tcp_minmss) {
                tp->t_flags2 |= TF2_PLPMTU_PMTUD;
                if (tp->t_port == 0 || len < V_tcp_minmss) {
                        ip->ip_off |= htons(IP_DF);
                }
        } else {
                tp->t_flags2 &= ~TF2_PLPMTU_PMTUD;
        }
 
        if (tp->t_state == TCPS_SYN_SENT)
                TCP_PROBE5(connect__request, NULL, tp, ip, tp, th);
 
        TCP_PROBE5(send, NULL, tp, ip, tp, th);
 
#ifdef TCPPCAP
        /* Save packet, if requested. */
        tcp_pcap_add(th, m, &(tp->t_outpkts));
#endif
 
        error = ip_output(m, tp->t_inpcb->inp_options, &tp->t_inpcb->inp_route,
            ((so->so_options & SO_DONTROUTE) ? IP_ROUTETOIF : 0), 0,
            tp->t_inpcb);
 
        if (error == EMSGSIZE && tp->t_inpcb->inp_route.ro_nh != NULL)
                mtu = tp->t_inpcb->inp_route.ro_nh->nh_mtu;
    }
#endif /* INET */
 
out:
        if (error == 0)
                tcp_account_for_send(tp, len, (tp->snd_nxt != tp->snd_max), 0, hw_tls);
        /*
         * In transmit state, time the transmission and arrange for
         * the retransmit.  In persist state, just set snd_max.
         */
        if ((tp->t_flags & TF_FORCEDATA) == 0 ||
            !tcp_timer_active(tp, TT_PERSIST)) {
                tcp_seq startseq = tp->snd_nxt;
 
                /*
                 * Advance snd_nxt over sequence space of this segment.
                 */
                if (flags & (TH_SYN|TH_FIN)) {
                        if (flags & TH_SYN)
                                tp->snd_nxt++;
                        if (flags & TH_FIN) {
                                tp->snd_nxt++;
                                tp->t_flags |= TF_SENTFIN;
                        }
                }
                if (sack_rxmit)
                        goto timer;
                tp->snd_nxt += len;
                if (SEQ_GT(tp->snd_nxt, tp->snd_max)) {
                        tp->snd_max = tp->snd_nxt;
                        /*
                         * Time this transmission if not a retransmission and
                         * not currently timing anything.
                         */
                        tp->t_sndtime = ticks;
                        if (tp->t_rtttime == 0) {
                                tp->t_rtttime = ticks;
                                tp->t_rtseq = startseq;
                                TCPSTAT_INC(tcps_segstimed);
                        }
#ifdef STATS
                        if (!(tp->t_flags & TF_GPUTINPROG) && len) {
                                tp->t_flags |= TF_GPUTINPROG;
                                tp->gput_seq = startseq;
                                tp->gput_ack = startseq +
                                    ulmin(sbavail(&so->so_snd) - off, sendwin);
                                tp->gput_ts = tcp_ts_getticks();
                        }
#endif /* STATS */
                }
 
                /*
                 * Set retransmit timer if not currently set,
                 * and not doing a pure 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_active(tp, TT_REXMT) &&
                    ((sack_rxmit && tp->snd_nxt != tp->snd_max) ||
                     (tp->snd_nxt != tp->snd_una))) {
                        if (tcp_timer_active(tp, TT_PERSIST)) {
                                tcp_timer_activate(tp, TT_PERSIST, 0);
                                tp->t_rxtshift = 0;
                        }
                        tcp_timer_activate(tp, TT_REXMT, tp->t_rxtcur);
                } else if (len == 0 && sbavail(&so->so_snd) &&
                    !tcp_timer_active(tp, TT_REXMT) &&
                    !tcp_timer_active(tp, TT_PERSIST)) {
                        /*
                         * 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 {
                /*
                 * Persist case, update snd_max but since we are in
                 * persist mode (no window) we do not update snd_nxt.
                 */
                int xlen = len;
                if (flags & TH_SYN)
                        ++xlen;
                if (flags & TH_FIN) {
                        ++xlen;
                        tp->t_flags |= TF_SENTFIN;
                }
                if (SEQ_GT(tp->snd_nxt + xlen, tp->snd_max))
                        tp->snd_max = tp->snd_nxt + xlen;
        }
        if ((error == 0) &&
            (TCPS_HAVEESTABLISHED(tp->t_state) &&
             (tp->t_flags & TF_SACK_PERMIT) &&
             tp->rcv_numsacks > 0)) {
                    /* Clean up any DSACK's sent */
                    tcp_clean_dsack_blocks(tp);
        }
        if (error) {
                /* Record the error. */
                TCP_LOG_EVENT(tp, NULL, &so->so_rcv, &so->so_snd, TCP_LOG_OUT,
                    error, 0, NULL, false);
 
                /*
                 * We know that the packet was lost, so back out the
                 * sequence number advance, if any.
                 *
                 * If the error is EPERM the packet got blocked by the
                 * local firewall.  Normally we should terminate the
                 * connection but the blocking may have been spurious
                 * due to a firewall reconfiguration cycle.  So we treat
                 * it like a packet loss and let the retransmit timer and
                 * timeouts do their work over time.
                 * XXX: It is a POLA question whether calling tcp_drop right
                 * away would be the really correct behavior instead.
                 */
                if (((tp->t_flags & TF_FORCEDATA) == 0 ||
                    !tcp_timer_active(tp, TT_PERSIST)) &&
                    ((flags & TH_SYN) == 0) &&
                    (error != EPERM)) {
                        if (sack_rxmit) {
                                p->rxmit -= len;
                                tp->sackhint.sack_bytes_rexmit -= len;
                                KASSERT(tp->sackhint.sack_bytes_rexmit >= 0,
                                    ("sackhint bytes rtx >= 0"));
                        } else
                                tp->snd_nxt -= len;
                }
                SOCKBUF_UNLOCK_ASSERT(&so->so_snd);     /* Check gotos. */
                switch (error) {
                case EACCES:
                case EPERM:
                        tp->t_softerror = error;
                        return (error);
                case ENOBUFS:
                        TCP_XMIT_TIMER_ASSERT(tp, len, flags);
                        tp->snd_cwnd = tp->t_maxseg;
                        return (0);
                case EMSGSIZE:
                        /*
                         * For some reason the interface we used initially
                         * to send segments changed to another or lowered
                         * its MTU.
                         * If TSO was active we either got an interface
                         * without TSO capabilits or TSO was turned off.
                         * If we obtained mtu from ip_output() then update
                         * it and try again.
                         */
                        if (tso)
                                tp->t_flags &= ~TF_TSO;
                        if (mtu != 0) {
                                tcp_mss_update(tp, -1, mtu, NULL, NULL);
                                goto again;
                        }
                        return (error);
                case EHOSTDOWN:
                case EHOSTUNREACH:
                case ENETDOWN:
                case ENETUNREACH:
                        if (TCPS_HAVERCVDSYN(tp->t_state)) {
                                tp->t_softerror = error;
                                return (0);
                        }
                        /* FALLTHROUGH */
                default:
                        return (error);
                }
        }
        TCPSTAT_INC(tcps_sndtotal);
 
        /*
         * Data sent (as far as we can tell).
         * If this advertises a larger window than any other segment,
         * then remember the size of the advertised window.
         * Any pending ACK has now been sent.
         */
        if (SEQ_GT(tp->rcv_nxt + recwin, tp->rcv_adv))
                tp->rcv_adv = tp->rcv_nxt + recwin;
        tp->last_ack_sent = tp->rcv_nxt;
        tp->t_flags &= ~(TF_ACKNOW | TF_DELACK);
        if (tcp_timer_active(tp, TT_DELACK))
                tcp_timer_activate(tp, TT_DELACK, 0);
#if 0
        /*
         * This completely breaks TCP if newreno is turned on.  What happens
         * is that if delayed-acks are turned on on the receiver, this code
         * on the transmitter effectively destroys the TCP window, forcing
         * it to four packets (1.5Kx4 = 6K window).
         */
        if (sendalot && --maxburst)
                goto again;
#endif
        if (sendalot)
                goto again;
        return (0);
}
 
void
tcp_setpersist(struct tcpcb *tp)
{
        int t = ((tp->t_srtt >> 2) + tp->t_rttvar) >> 1;
        int tt;
 
        tp->t_flags &= ~TF_PREVVALID;
        if (tcp_timer_active(tp, TT_REXMT))
                panic("tcp_setpersist: retransmit pending");
        /*
         * Start/restart persistence timer.
         */
        TCPT_RANGESET(tt, t * tcp_backoff[tp->t_rxtshift],
                      tcp_persmin, tcp_persmax);
        tcp_timer_activate(tp, TT_PERSIST, tt);
        if (tp->t_rxtshift < TCP_MAXRXTSHIFT)
                tp->t_rxtshift++;
}
 
/*
 * Insert TCP options according to the supplied parameters to the place
 * optp in a consistent way.  Can handle unaligned destinations.
 *
 * The order of the option processing is crucial for optimal packing and
 * alignment for the scarce option space.
 *
 * The optimal order for a SYN/SYN-ACK segment is:
 *   MSS (4) + NOP (1) + Window scale (3) + SACK permitted (2) +
 *   Timestamp (10) + Signature (18) = 38 bytes out of a maximum of 40.
 *
 * The SACK options should be last.  SACK blocks consume 8*n+2 bytes.
 * So a full size SACK blocks option is 34 bytes (with 4 SACK blocks).
 * At minimum we need 10 bytes (to generate 1 SACK block).  If both
 * TCP Timestamps (12 bytes) and TCP Signatures (18 bytes) are present,
 * we only have 10 bytes for SACK options (40 - (12 + 18)).
 */
int
tcp_addoptions(struct tcpopt *to, u_char *optp)
{
        u_int32_t mask, optlen = 0;
 
        for (mask = 1; mask < TOF_MAXOPT; mask <<= 1) {
                if ((to->to_flags & mask) != mask)
                        continue;
                if (optlen == TCP_MAXOLEN)
                        break;
                switch (to->to_flags & mask) {
                case TOF_MSS:
                        while (optlen % 4) {
                                optlen += TCPOLEN_NOP;
                                *optp++ = TCPOPT_NOP;
                        }
                        if (TCP_MAXOLEN - optlen < TCPOLEN_MAXSEG)
                                continue;
                        optlen += TCPOLEN_MAXSEG;
                        *optp++ = TCPOPT_MAXSEG;
                        *optp++ = TCPOLEN_MAXSEG;
                        to->to_mss = htons(to->to_mss);
                        bcopy((u_char *)&to->to_mss, optp, sizeof(to->to_mss));
                        optp += sizeof(to->to_mss);
                        break;
                case TOF_SCALE:
                        while (!optlen || optlen % 2 != 1) {
                                optlen += TCPOLEN_NOP;
                                *optp++ = TCPOPT_NOP;
                        }
                        if (TCP_MAXOLEN - optlen < TCPOLEN_WINDOW)
                                continue;
                        optlen += TCPOLEN_WINDOW;
                        *optp++ = TCPOPT_WINDOW;
                        *optp++ = TCPOLEN_WINDOW;
                        *optp++ = to->to_wscale;
                        break;
                case TOF_SACKPERM:
                        while (optlen % 2) {
                                optlen += TCPOLEN_NOP;
                                *optp++ = TCPOPT_NOP;
                        }
                        if (TCP_MAXOLEN - optlen < TCPOLEN_SACK_PERMITTED)
                                continue;
                        optlen += TCPOLEN_SACK_PERMITTED;
                        *optp++ = TCPOPT_SACK_PERMITTED;
                        *optp++ = TCPOLEN_SACK_PERMITTED;
                        break;
                case TOF_TS:
                        while (!optlen || optlen % 4 != 2) {
                                optlen += TCPOLEN_NOP;
                                *optp++ = TCPOPT_NOP;
                        }
                        if (TCP_MAXOLEN - optlen < TCPOLEN_TIMESTAMP)
                                continue;
                        optlen += TCPOLEN_TIMESTAMP;
                        *optp++ = TCPOPT_TIMESTAMP;
                        *optp++ = TCPOLEN_TIMESTAMP;
                        to->to_tsval = htonl(to->to_tsval);
                        to->to_tsecr = htonl(to->to_tsecr);
                        bcopy((u_char *)&to->to_tsval, optp, sizeof(to->to_tsval));
                        optp += sizeof(to->to_tsval);
                        bcopy((u_char *)&to->to_tsecr, optp, sizeof(to->to_tsecr));
                        optp += sizeof(to->to_tsecr);
                        break;
                case TOF_SIGNATURE:
                        {
                        int siglen = TCPOLEN_SIGNATURE - 2;
 
                        while (!optlen || optlen % 4 != 2) {
                                optlen += TCPOLEN_NOP;
                                *optp++ = TCPOPT_NOP;
                        }
                        if (TCP_MAXOLEN - optlen < TCPOLEN_SIGNATURE) {
                                to->to_flags &= ~TOF_SIGNATURE;
                                continue;
                        }
                        optlen += TCPOLEN_SIGNATURE;
                        *optp++ = TCPOPT_SIGNATURE;
                        *optp++ = TCPOLEN_SIGNATURE;
                        to->to_signature = optp;
                        while (siglen--)
                                 *optp++ = 0;
                        break;
                        }
                case TOF_SACK:
                        {
                        int sackblks = 0;
                        struct sackblk *sack = (struct sackblk *)to->to_sacks;
                        tcp_seq sack_seq;
 
                        while (!optlen || optlen % 4 != 2) {
                                optlen += TCPOLEN_NOP;
                                *optp++ = TCPOPT_NOP;
                        }
                        if (TCP_MAXOLEN - optlen < TCPOLEN_SACKHDR + TCPOLEN_SACK)
                                continue;
                        optlen += TCPOLEN_SACKHDR;
                        *optp++ = TCPOPT_SACK;
                        sackblks = min(to->to_nsacks,
                                        (TCP_MAXOLEN - optlen) / TCPOLEN_SACK);
                        *optp++ = TCPOLEN_SACKHDR + sackblks * TCPOLEN_SACK;
                        while (sackblks--) {
                                sack_seq = htonl(sack->start);
                                bcopy((u_char *)&sack_seq, optp, sizeof(sack_seq));
                                optp += sizeof(sack_seq);
                                sack_seq = htonl(sack->end);
                                bcopy((u_char *)&sack_seq, optp, sizeof(sack_seq));
                                optp += sizeof(sack_seq);
                                optlen += TCPOLEN_SACK;
                                sack++;
                        }
                        TCPSTAT_INC(tcps_sack_send_blocks);
                        break;
                        }
                case TOF_FASTOPEN:
                        {
                        int total_len;
 
                        /* XXX is there any point to aligning this option? */
                        total_len = TCPOLEN_FAST_OPEN_EMPTY + to->to_tfo_len;
                        if (TCP_MAXOLEN - optlen < total_len) {
                                to->to_flags &= ~TOF_FASTOPEN;
                                continue;
                        }
                        *optp++ = TCPOPT_FAST_OPEN;
                        *optp++ = total_len;
                        if (to->to_tfo_len > 0) {
                                bcopy(to->to_tfo_cookie, optp, to->to_tfo_len);
                                optp += to->to_tfo_len;
                        }
                        optlen += total_len;
                        break;
                        }
                default:
                        panic("%s: unknown TCP option type", __func__);
                        break;
                }
        }
 
        /* Terminate and pad TCP options to a 4 byte boundary. */
        if (optlen % 4) {
                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) {
                optlen += TCPOLEN_PAD;
                *optp++ = TCPOPT_PAD;
        }
 
        KASSERT(optlen <= TCP_MAXOLEN, ("%s: TCP options too long", __func__));
        return (optlen);
}
 
/*
 * This is a copy of m_copym(), taking the TSO segment size/limit
 * constraints into account, and advancing the sndptr as it goes.
 */
struct mbuf *
tcp_m_copym(struct mbuf *m, int32_t off0, int32_t *plen,
    int32_t seglimit, int32_t segsize, struct sockbuf *sb, bool hw_tls)
{
#ifdef KERN_TLS
        struct ktls_session *tls, *ntls;
        struct mbuf *start;
#endif
        struct mbuf *n, **np;
        struct mbuf *top;
        int32_t off = off0;
        int32_t len = *plen;
        int32_t fragsize;
        int32_t len_cp = 0;
        int32_t *pkthdrlen;
        uint32_t mlen, frags;
        bool copyhdr;
 
        KASSERT(off >= 0, ("tcp_m_copym, negative off %d", off));
        KASSERT(len >= 0, ("tcp_m_copym, negative len %d", len));
        if (off == 0 && m->m_flags & M_PKTHDR)
                copyhdr = true;
        else
                copyhdr = false;
        while (off > 0) {
                KASSERT(m != NULL, ("tcp_m_copym, offset > size of mbuf chain"));
                if (off < m->m_len)
                        break;
                off -= m->m_len;
                if ((sb) && (m == sb->sb_sndptr)) {
                        sb->sb_sndptroff += m->m_len;
                        sb->sb_sndptr = m->m_next;
                }
                m = m->m_next;
        }
        np = &top;
        top = NULL;
        pkthdrlen = NULL;
#ifdef KERN_TLS
        if (hw_tls && (m->m_flags & M_EXTPG))
                tls = m->m_epg_tls;
        else
                tls = NULL;
        start = m;
#endif
        while (len > 0) {
                if (m == NULL) {
                        KASSERT(len == M_COPYALL,
                            ("tcp_m_copym, length > size of mbuf chain"));
                        *plen = len_cp;
                        if (pkthdrlen != NULL)
                                *pkthdrlen = len_cp;
                        break;
                }
#ifdef KERN_TLS
                if (hw_tls) {
                        if (m->m_flags & M_EXTPG)
                                ntls = m->m_epg_tls;
                        else
                                ntls = NULL;
 
                        /*
                         * Avoid mixing TLS records with handshake
                         * data or TLS records from different
                         * sessions.
                         */
                        if (tls != ntls) {
                                MPASS(m != start);
                                *plen = len_cp;
                                if (pkthdrlen != NULL)
                                        *pkthdrlen = len_cp;
                                break;
                        }
                }
#endif
                mlen = min(len, m->m_len - off);
                if (seglimit) {
                        /*
                         * For M_EXTPG mbufs, add 3 segments
                         * + 1 in case we are crossing page boundaries
                         * + 2 in case the TLS hdr/trailer are used
                         * It is cheaper to just add the segments
                         * than it is to take the cache miss to look
                         * at the mbuf ext_pgs state in detail.
                         */
                        if (m->m_flags & M_EXTPG) {
                                fragsize = min(segsize, PAGE_SIZE);
                                frags = 3;
                        } else {
                                fragsize = segsize;
                                frags = 0;
                        }
 
                        /* Break if we really can't fit anymore. */
                        if ((frags + 1) >= seglimit) {
                                *plen = len_cp;
                                if (pkthdrlen != NULL)
                                        *pkthdrlen = len_cp;
                                break;
                        }
 
                        /*
                         * Reduce size if you can't copy the whole
                         * mbuf. If we can't copy the whole mbuf, also
                         * adjust len so the loop will end after this
                         * mbuf.
                         */
                        if ((frags + howmany(mlen, fragsize)) >= seglimit) {
                                mlen = (seglimit - frags - 1) * fragsize;
                                len = mlen;
                                *plen = len_cp + len;
                                if (pkthdrlen != NULL)
                                        *pkthdrlen = *plen;
                        }
                        frags += howmany(mlen, fragsize);
                        if (frags == 0)
                                frags++;
                        seglimit -= frags;
                        KASSERT(seglimit > 0,
                            ("%s: seglimit went too low", __func__));
                }
                if (copyhdr)
                        n = m_gethdr(M_NOWAIT, m->m_type);
                else
                        n = m_get(M_NOWAIT, m->m_type);
                *np = n;
                if (n == NULL)
                        goto nospace;
                if (copyhdr) {
                        if (!m_dup_pkthdr(n, m, M_NOWAIT))
                                goto nospace;
                        if (len == M_COPYALL)
                                n->m_pkthdr.len -= off0;
                        else
                                n->m_pkthdr.len = len;
                        pkthdrlen = &n->m_pkthdr.len;
                        copyhdr = false;
                }
                n->m_len = mlen;
                len_cp += n->m_len;
                if (m->m_flags & (M_EXT|M_EXTPG)) {
                        n->m_data = m->m_data + off;
                        mb_dupcl(n, m);
                } else
                        bcopy(mtod(m, caddr_t)+off, mtod(n, caddr_t),
                            (u_int)n->m_len);
 
                if (sb && (sb->sb_sndptr == m) &&
                    ((n->m_len + off) >= m->m_len) && m->m_next) {
                        sb->sb_sndptroff += m->m_len;
                        sb->sb_sndptr = m->m_next;
                }
                off = 0;
                if (len != M_COPYALL) {
                        len -= n->m_len;
                }
                m = m->m_next;
                np = &n->m_next;
        }
        return (top);
nospace:
        m_freem(top);
        return (NULL);
}
 
void
tcp_sndbuf_autoscale(struct tcpcb *tp, struct socket *so, uint32_t sendwin)
{
 
        /*
         * Automatic sizing of send socket buffer.  Often the send buffer
         * size is not optimally adjusted to the actual network conditions
         * at hand (delay bandwidth product).  Setting the buffer size too
         * small limits throughput on links with high bandwidth and high
         * delay (eg. trans-continental/oceanic links).  Setting the
         * buffer size too big consumes too much real kernel memory,
         * especially with many connections on busy servers.
         *
         * 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 an 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.  Has to
         * wait for upcoming tcp timer rewrite.
         *
         * XXXGL: should there be used sbused() or sbavail()?
         */
        if (V_tcp_do_autosndbuf && so->so_snd.sb_flags & SB_AUTOSIZE) {
                int lowat;
 
                lowat = V_tcp_sendbuf_auto_lowat ? so->so_snd.sb_lowat : 0;
                if ((tp->snd_wnd / 4 * 5) >= so->so_snd.sb_hiwat - lowat &&
                    sbused(&so->so_snd) >=
                    (so->so_snd.sb_hiwat / 8 * 7) - lowat &&
                    sbused(&so->so_snd) < V_tcp_autosndbuf_max &&
                    sendwin >= (sbused(&so->so_snd) -
                    (tp->snd_nxt - tp->snd_una))) {
                        if (!sbreserve_locked(&so->so_snd,
                            min(so->so_snd.sb_hiwat + V_tcp_autosndbuf_inc,
                             V_tcp_autosndbuf_max), so, curthread))
                                so->so_snd.sb_flags &= ~SB_AUTOSIZE;
                }
        }
}