tcp: Don't prematurely drop receiving-only connections.

[dragonfly.git] / sys / netinet / tcp_output.c

/*
 * Copyright (c) 2004 Jeffrey M. Hsu.  All rights reserved.
 * Copyright (c) 2004 The DragonFly Project.  All rights reserved.
 *
 * This code is derived from software contributed to The DragonFly Project
 * by Jeffrey M. Hsu.
 *
 * 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 DragonFly Project nor the names of its
 *    contributors may be used to endorse or promote products derived
 *    from this software without specific, prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS 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
 * COPYRIGHT HOLDERS OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
 * INCIDENTAL, SPECIAL, EXEMPLARY OR CONSEQUENTIAL DAMAGES (INCLUDING,
 * BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
 * LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED
 * AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
 * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
 * OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 * SUCH DAMAGE.
 */

/*
 * Copyright (c) 1982, 1986, 1988, 1990, 1993, 1995
 *      The Regents of the University of California.  All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 * 3. Neither the name of the University nor the names of its contributors
 *    may be used to endorse or promote products derived from this software
 *    without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 * SUCH DAMAGE.
 *
 *      @(#)tcp_output.c        8.4 (Berkeley) 5/24/95
 * $FreeBSD: src/sys/netinet/tcp_output.c,v 1.39.2.20 2003/01/29 22:45:36 hsu Exp $
 */

#include "opt_inet.h"
#include "opt_inet6.h"
#include "opt_ipsec.h"
#include "opt_tcpdebug.h"

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

#include <net/if_var.h>
#include <net/route.h>
#include <net/netmsg2.h>
#include <net/netisr2.h>

#include <netinet/in.h>
#include <netinet/in_systm.h>
#include <netinet/ip.h>
#include <netinet/in_pcb.h>
#include <netinet/ip_var.h>
#include <netinet6/in6_pcb.h>
#include <netinet/ip6.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_timer2.h>
#include <netinet/tcp_var.h>
#include <netinet/tcpip.h>
#ifdef TCPDEBUG
#include <netinet/tcp_debug.h>
#endif

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

#ifdef FAST_IPSEC
#include <netproto/ipsec/ipsec.h>
#define IPSEC
#endif /*FAST_IPSEC*/

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

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

static int avoid_pure_win_update = 1;
SYSCTL_INT(_net_inet_tcp, OID_AUTO, avoid_pure_win_update, CTLFLAG_RW,
        &avoid_pure_win_update, 1, "Avoid pure window updates when possible");

/*
 * 1 - enabled for increasing and decreasing the buffer size
 * 2 - enabled only for increasing the buffer size
 */
int tcp_do_autosndbuf = 1;
SYSCTL_INT(_net_inet_tcp, OID_AUTO, sendbuf_auto, CTLFLAG_RW,
    &tcp_do_autosndbuf, 0, "Enable automatic send buffer sizing");

int tcp_autosndbuf_inc = 8*1024;
SYSCTL_INT(_net_inet_tcp, OID_AUTO, sendbuf_inc, CTLFLAG_RW,
    &tcp_autosndbuf_inc, 0, "Incrementor step size of automatic send buffer");

int tcp_autosndbuf_min = 32768;
SYSCTL_INT(_net_inet_tcp, OID_AUTO, sendbuf_min, CTLFLAG_RW,
    &tcp_autosndbuf_min, 0, "Min size of automatic send buffer");

int tcp_autosndbuf_max = 2*1024*1024;
SYSCTL_INT(_net_inet_tcp, OID_AUTO, sendbuf_max, CTLFLAG_RW,
    &tcp_autosndbuf_max, 0, "Max size of automatic send buffer");

int tcp_prio_synack = 1;
SYSCTL_INT(_net_inet_tcp, OID_AUTO, prio_synack, CTLFLAG_RW,
    &tcp_prio_synack, 0, "Prioritize SYN, SYN|ACK and pure ACK");

static int tcp_idle_cwv = 1;
SYSCTL_INT(_net_inet_tcp, OID_AUTO, idle_cwv, CTLFLAG_RW,
    &tcp_idle_cwv, 0,
    "Congestion window validation after idle period (part of RFC2861)");

static int tcp_idle_restart = 1;
SYSCTL_INT(_net_inet_tcp, OID_AUTO, idle_restart, CTLFLAG_RW,
    &tcp_idle_restart, 0, "Reset congestion window after idle period");

static int tcp_do_tso = 1;
SYSCTL_INT(_net_inet_tcp, OID_AUTO, tso, CTLFLAG_RW,
    &tcp_do_tso, 0, "Enable TCP Segmentation Offload (TSO)");

static int tcp_fairsend = 4;
SYSCTL_INT(_net_inet_tcp, OID_AUTO, fairsend, CTLFLAG_RW,
    &tcp_fairsend, 0,
    "Amount of segments sent before yield to other senders or receivers");

static void     tcp_idle_cwnd_validate(struct tcpcb *);

static int      tcp_tso_getsize(struct tcpcb *tp, u_int *segsz, u_int *hlen);
static void     tcp_output_sched(struct tcpcb *tp);

/*
 * Tcp output routine: figure out what should be sent and send it.
 */
int
tcp_output(struct tcpcb *tp)
{
        struct inpcb * const inp = tp->t_inpcb;
        struct socket *so = inp->inp_socket;
        long len, recvwin, sendwin;
        int nsacked = 0;
        int off, flags, error = 0;
#ifdef TCP_SIGNATURE
        int sigoff = 0;
#endif
        struct mbuf *m;
        struct ip *ip;
        struct tcphdr *th;
        u_char opt[TCP_MAXOLEN];
        unsigned int ipoptlen, optlen, hdrlen;
        int idle;
        boolean_t sendalot;
        struct ip6_hdr *ip6;
#ifdef INET6
        const boolean_t isipv6 = INP_ISIPV6(inp);
#else
        const boolean_t isipv6 = FALSE;
#endif
        boolean_t can_tso = FALSE, use_tso;
        boolean_t report_sack, idle_cwv = FALSE;
        u_int segsz, tso_hlen, tso_lenmax = 0;
        int segcnt = 0;
        boolean_t need_sched = FALSE;

        KKASSERT(so->so_port == &curthread->td_msgport);

        /*
         * 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.
         */

        /*
         * If we have been idle for a while, the send congestion window
         * could be no longer representative of the current state of the
         * link; need to validate congestion window.  However, we should
         * not perform congestion window validation here, since we could
         * be asked to send pure ACK.
         */
        if (tp->snd_max == tp->snd_una &&
            (ticks - tp->snd_last) >= tp->t_rxtcur && tcp_idle_restart)
                idle_cwv = TRUE;

        /*
         * Calculate whether the transmit stream was previously idle 
         * and adjust TF_LASTIDLE for the next time.
         */
        idle = (tp->t_flags & TF_LASTIDLE) || (tp->snd_max == tp->snd_una);
        if (idle && (tp->t_flags & TF_MORETOCOME))
                tp->t_flags |= TF_LASTIDLE;
        else
                tp->t_flags &= ~TF_LASTIDLE;

        if (TCP_DO_SACK(tp) && tp->snd_nxt != tp->snd_max &&
            !IN_FASTRECOVERY(tp))
                nsacked = tcp_sack_bytes_below(&tp->scb, tp->snd_nxt);

        /*
         * Find out whether TSO could be used or not
         *
         * For TSO capable devices, the following assumptions apply to
         * the processing of TCP flags:
         * - If FIN is set on the large TCP segment, the device must set
         *   FIN on the last segment that it creates from the large TCP
         *   segment.
         * - If PUSH is set on the large TCP segment, the device must set
         *   PUSH on the last segment that it creates from the large TCP
         *   segment.
         */
#if !defined(IPSEC) && !defined(FAST_IPSEC)
        if (tcp_do_tso
#ifdef TCP_SIGNATURE
            && (tp->t_flags & TF_SIGNATURE) == 0
#endif
        ) {
                if (!isipv6) {
                        struct rtentry *rt = inp->inp_route.ro_rt;

                        if (rt != NULL && (rt->rt_flags & RTF_UP) &&
                            (rt->rt_ifp->if_hwassist & CSUM_TSO)) {
                                can_tso = TRUE;
                                tso_lenmax = rt->rt_ifp->if_tsolen;
                        }
                }
        }
#endif  /* !IPSEC && !FAST_IPSEC */

again:
        m = NULL;
        ip = NULL;
        th = NULL;
        ip6 = NULL;

        if ((tp->t_flags & (TF_SACK_PERMITTED | TF_NOOPT)) ==
                TF_SACK_PERMITTED &&
            (!TAILQ_EMPTY(&tp->t_segq) ||
             tp->reportblk.rblk_start != tp->reportblk.rblk_end))
                report_sack = TRUE;
        else
                report_sack = FALSE;

        /* Make use of SACK information when slow-starting after a RTO. */
        if (TCP_DO_SACK(tp) && tp->snd_nxt != tp->snd_max &&
            !IN_FASTRECOVERY(tp)) {
                tcp_seq old_snd_nxt = tp->snd_nxt;

                tcp_sack_skip_sacked(&tp->scb, &tp->snd_nxt);
                nsacked += tp->snd_nxt - old_snd_nxt;
        }

        sendalot = FALSE;
        off = tp->snd_nxt - tp->snd_una;
        sendwin = min(tp->snd_wnd, tp->snd_cwnd + nsacked);
        sendwin = min(sendwin, tp->snd_bwnd);

        flags = tcp_outflags[tp->t_state];
        /*
         * Get standard flags, and add SYN or FIN if requested by 'hidden'
         * state flags.
         */
        if (tp->t_flags & TF_NEEDFIN)
                flags |= TH_FIN;
        if (tp->t_flags & TF_NEEDSYN)
                flags |= TH_SYN;

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

        /*
         * If snd_nxt == snd_max and we have transmitted a FIN, the
         * offset will be > 0 even if so_snd.ssb_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.
         *
         * A negative length can also occur when we are in the
         * TCPS_SYN_RECEIVED state due to a simultanious connect where
         * our SYN has not been acked yet.
         *
         * 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.
         */
        len = (long)ulmin(so->so_snd.ssb_cc, sendwin) - off;

        /*
         * Lop off SYN bit if it has already been sent.  However, if this
         * is SYN-SENT state and if segment contains data, suppress sending
         * segment (sending the segment would be an option if we still
         * did TAO and the remote host supported it).
         */
        if ((flags & TH_SYN) && SEQ_GT(tp->snd_nxt, tp->snd_una)) {
                flags &= ~TH_SYN;
                off--, len++;
                if (len > 0 && tp->t_state == TCPS_SYN_SENT) {
                        tp->t_flags &= ~(TF_ACKNOW | TF_XMITNOW);
                        return 0;
                }
        }

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

        if (len < 0) {
                /*
                 * A negative len can occur if our FIN has been sent but not
                 * acked, or if we are in a simultanious connect in the
                 * TCPS_SYN_RECEIVED state with our SYN sent but not yet
                 * acked.
                 *
                 * If our window has contracted to 0 in the FIN case
                 * (which can only occur if we have NOT been called to
                 * retransmit as per code a few paragraphs up) then we
                 * want to shift the retransmit timer over to the
                 * persist timer.
                 *
                 * However, if we are in the TCPS_SYN_RECEIVED state
                 * (the SYN case) we will be in a simultanious connect and
                 * the window may be zero degeneratively.  In this case we
                 * do not want to shift to the persist timer after the SYN
                 * or the SYN+ACK transmission.
                 */
                len = 0;
                if (sendwin == 0 && tp->t_state != TCPS_SYN_RECEIVED) {
                        tcp_callout_stop(tp, tp->tt_rexmt);
                        tp->t_rxtshift = 0;
                        tp->snd_nxt = tp->snd_una;
                        if (!tcp_callout_active(tp, tp->tt_persist))
                                tcp_setpersist(tp);
                }
        }

        KASSERT(len >= 0, ("%s: len < 0", __func__));
        /*
         * 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. hiwat has not significantly exceeded bwnd (inflight)
         *     (bwnd is a maximal value if inflight is disabled).
         *  3. send buffer is filled to 7/8th with data (so we actually
         *     have data to make use of it);
         *  4. hiwat has not hit maximal automatic size;
         *  5. 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*bandwith product.  However testing has shown this not
         * to be much of an problem.  At worst we are trading wasting
         * of available bandwith (the non-use of it) for wasting some
         * socket buffer memory.
         *
         * The criteria for shrinking the buffer is based solely on
         * the inflight code (snd_bwnd).  If inflight is disabled,
         * the buffer will not be shrinked.  Note that snd_bwnd already
         * has a fudge factor.  Our test adds a little hysteresis.
         */
        if (tcp_do_autosndbuf && (so->so_snd.ssb_flags & SSB_AUTOSIZE)) {
                const int asbinc = tcp_autosndbuf_inc;
                const int hiwat = so->so_snd.ssb_hiwat;
                const int lowat = so->so_snd.ssb_lowat;
                u_long newsize;

                if ((tp->snd_wnd / 4 * 5) >= hiwat &&
                    so->so_snd.ssb_cc >= (hiwat / 8 * 7) &&
                    hiwat < tp->snd_bwnd + hiwat / 10 &&
                    hiwat + asbinc < tcp_autosndbuf_max &&
                    hiwat < (TCP_MAXWIN << tp->snd_scale) &&
                    sendwin >= (so->so_snd.ssb_cc -
                                (tp->snd_nxt - tp->snd_una))) {
                        newsize = ulmin(hiwat + asbinc, tcp_autosndbuf_max);
                        if (!ssb_reserve(&so->so_snd, newsize, so, NULL))
                                atomic_clear_int(&so->so_snd.ssb_flags, SSB_AUTOSIZE);
#if 0
                        if (newsize >= (TCP_MAXWIN << tp->snd_scale))
                                atomic_clear_int(&so->so_snd.ssb_flags, SSB_AUTOSIZE);
#endif
                } else if ((long)tp->snd_bwnd <
                           (long)(hiwat * 3 / 4 - lowat - asbinc) &&
                           hiwat > tp->t_maxseg * 2 + asbinc &&
                           hiwat + asbinc >= tcp_autosndbuf_min &&
                           tcp_do_autosndbuf == 1) {
                        newsize = ulmax(hiwat - asbinc, tp->t_maxseg * 2);
                        ssb_reserve(&so->so_snd, newsize, so, NULL);
                }
        }

        /*
         * Don't use TSO, if:
         * - Congestion window needs validation
         * - There are SACK blocks to report
         * - RST or SYN flags is set
         * - URG will be set
         *
         * XXX
         * Checking for SYN|RST looks overkill, just to be safe than sorry
         */
        use_tso = can_tso;
        if (report_sack || idle_cwv || (flags & (TH_RST | TH_SYN)))
                use_tso = FALSE;
        if (use_tso) {
                tcp_seq ugr_nxt = tp->snd_nxt;

                if ((flags & TH_FIN) && (tp->t_flags & TF_SENTFIN) &&
                    tp->snd_nxt == tp->snd_max)
                        --ugr_nxt;

                if (SEQ_GT(tp->snd_up, ugr_nxt))
                        use_tso = FALSE;
        }

        if (use_tso) {
                /*
                 * Find out segment size and header length for TSO
                 */
                error = tcp_tso_getsize(tp, &segsz, &tso_hlen);
                if (error)
                        use_tso = FALSE;
        }
        if (!use_tso) {
                segsz = tp->t_maxseg;
                tso_hlen = 0; /* not used */
        }

        /*
         * Truncate to the maximum segment length if not TSO, and ensure that
         * FIN is removed if the length no longer contains the last data byte.
         */
        if (len > segsz) {
                if (!use_tso) {
                        len = segsz;
                        ++segcnt;
                } else {
                        int nsegs;

                        if (__predict_false(tso_lenmax < segsz))
                                tso_lenmax = segsz << 1;

                        /*
                         * Truncate TSO transfers to (IP_MAXPACKET - iphlen -
                         * thoff), and make sure that we send equal size
                         * transfers down the stack (rather than big-small-
                         * big-small-...).
                         */
                        len = min(len, tso_lenmax);
                        nsegs = min(len, (IP_MAXPACKET - tso_hlen)) / segsz;
                        KKASSERT(nsegs > 0);

                        len = nsegs * segsz;

                        if (len <= segsz) {
                                use_tso = FALSE;
                                ++segcnt;
                        } else {
                                segcnt += nsegs;
                        }
                }
                sendalot = TRUE;
        } else {
                use_tso = FALSE;
                if (len > 0)
                        ++segcnt;
        }
        if (SEQ_LT(tp->snd_nxt + len, tp->snd_una + so->so_snd.ssb_cc))
                flags &= ~TH_FIN;

        recvwin = ssb_space(&so->so_rcv);

        /*
         * Sender silly window avoidance.   We transmit under the following
         * conditions when len is non-zero:
         *
         *      - We have a full segment
         *      - This is the last buffer in a write()/send() and we are
         *        either idle or running NODELAY
         *      - we've timed out (e.g. persist timer)
         *      - we have more then 1/2 the maximum send window's worth of
         *        data (receiver may be limiting the window size)
         *      - we need to retransmit
         */
        if (len) {
                if (len >= segsz)
                        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)) &&
                    len + off >= so->so_snd.ssb_cc &&
                    !(tp->t_flags & TF_NOPUSH)) {
                        goto send;
                }
                if (tp->t_flags & TF_FORCE)             /* 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 (tp->t_flags & TF_XMITNOW)
                        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 (recvwin > 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 = min(recvwin, (long)TCP_MAXWIN << tp->rcv_scale) -
                        (tp->rcv_adv - tp->rcv_nxt);
                long hiwat;

                /*
                 * This ack case typically occurs when the user has drained
                 * the TCP socket buffer sufficiently to warrent an ack
                 * containing a 'pure window update'... that is, an ack that
                 * ONLY updates the tcp window.
                 *
                 * It is unclear why we would need to do a pure window update
                 * past 2 segments if we are going to do one at 1/2 the high
                 * water mark anyway, especially since under normal conditions
                 * the user program will drain the socket buffer quickly.
                 * The 2-segment pure window update will often add a large
                 * number of extra, unnecessary acks to the stream.
                 *
                 * avoid_pure_win_update now defaults to 1.
                 */
                if (avoid_pure_win_update == 0 ||
                    (tp->t_flags & TF_RXRESIZED)) {
                        if (adv >= (long) (2 * segsz)) {
                                goto send;
                        }
                }
                hiwat = (long)(TCP_MAXWIN << tp->rcv_scale);
                if (hiwat > (long)so->so_rcv.ssb_hiwat)
                        hiwat = (long)so->so_rcv.ssb_hiwat;
                if (adv >= hiwat / 2)
                        goto send;
        }

        /*
         * Send if we owe the peer an ACK, RST, SYN, or urgent data.  ACKNOW
         * is also a catch-all for the retransmit timer timeout case.
         */
        if (tp->t_flags & TF_ACKNOW)
                goto send;
        if ((flags & TH_RST) ||
            ((flags & TH_SYN) && !(tp->t_flags & TF_NEEDSYN)))
                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) || tp->snd_nxt == tp->snd_una))
                goto send;

        /*
         * 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_callout_active(tp, tp->tt_persist)
         *      is true when we are in persist state.
         * The TF_FORCE flag in tp->t_flags
         *      is set when we are called to send a persist packet.
         * tcp_callout_active(tp, 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.
         *
         * Don't try to set the persist state if we are in TCPS_SYN_RECEIVED
         * with data pending.  This situation can occur during a
         * simultanious connect.
         */
        if (so->so_snd.ssb_cc > 0 &&
            tp->t_state != TCPS_SYN_RECEIVED &&
            !tcp_callout_active(tp, tp->tt_rexmt) &&
            !tcp_callout_active(tp, tp->tt_persist)) {
                tp->t_rxtshift = 0;
                tcp_setpersist(tp);
        }

        /*
         * No reason to send a segment, just return.
         */
        tp->t_flags &= ~TF_XMITNOW;
        return (0);

send:
        if (need_sched && len > 0) {
                tcp_output_sched(tp);
                return 0;
        }

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

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

                        if ((tp->t_flags & TF_REQ_SCALE) &&
                            (!(flags & TH_ACK) ||
                             (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;
                        }

                        if ((tcp_do_sack && !(flags & TH_ACK)) ||
                            tp->t_flags & TF_SACK_PERMITTED) {
                                uint32_t *lp = (uint32_t *)(opt + optlen);

                                *lp = htonl(TCPOPT_SACK_PERMITTED_ALIGNED);
                                optlen += TCPOLEN_SACK_PERMITTED_ALIGNED;
                        }
                }
        }

        /*
         * 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) &&
            (!(flags & TH_ACK) || (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(ticks);
                *lp   = htonl(tp->ts_recent);
                optlen += TCPOLEN_TSTAMP_APPA;
        }

        /* Set receive buffer autosizing timestamp. */
        if (tp->rfbuf_ts == 0 && (so->so_rcv.ssb_flags & SSB_AUTOSIZE))
                tp->rfbuf_ts = ticks;

        /*
         * If this is a SACK connection and we have a block to report,
         * fill in the SACK blocks in the TCP options.
         */
        if (report_sack)
                tcp_sack_fill_report(tp, opt, &optlen);

#ifdef TCP_SIGNATURE
        if (tp->t_flags & TF_SIGNATURE) {
                int i;
                u_char *bp;
                /*
                 * Initialize TCP-MD5 option (RFC2385)
                 */
                bp = (u_char *)opt + optlen;
                *bp++ = TCPOPT_SIGNATURE;
                *bp++ = TCPOLEN_SIGNATURE;
                sigoff = optlen + 2;
                for (i = 0; i < TCP_SIGLEN; i++)
                        *bp++ = 0;
                optlen += TCPOLEN_SIGNATURE;
                /*
                 * Terminate options list and maintain 32-bit alignment.
                 */
                *bp++ = TCPOPT_NOP;
                *bp++ = TCPOPT_EOL;
                optlen += 2;
        }
#endif /* TCP_SIGNATURE */
        KASSERT(optlen <= TCP_MAXOLEN, ("too many TCP options"));
        hdrlen += optlen;

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

        if (use_tso) {
                /* TSO segment length must be multiple of segment size */
                KASSERT(len >= (2 * segsz) && (len % segsz == 0),
                    ("invalid TSO len %ld, segsz %u", len, segsz));
        } else {
                KASSERT(len <= segsz,
                    ("invalid len %ld, segsz %u", len, segsz));

                /*
                 * Adjust data length if insertion of options will bump
                 * the packet length beyond the t_maxopd length.  Clear
                 * FIN to prevent premature closure since there is still
                 * more data to send after this (now truncated) packet.
                 *
                 * If just the options do not fit we are in a no-win
                 * situation and we treat it as an unreachable host.
                 */
                if (len + optlen + ipoptlen > tp->t_maxopd) {
                        if (tp->t_maxopd <= optlen + ipoptlen) {
                                static time_t last_optlen_report;

                                if (last_optlen_report != time_uptime) {
                                        last_optlen_report = time_uptime;
                                        kprintf("tcpcb %p: MSS (%d) too "
                                            "small to hold options!\n",
                                            tp, tp->t_maxopd);
                                }
                                error = EHOSTUNREACH;
                                goto out;
                        } else {
                                flags &= ~TH_FIN;
                                len = tp->t_maxopd - optlen - ipoptlen;
                                sendalot = TRUE;
                        }
                }
        }

#ifdef INET6
        KASSERT(max_linkhdr + hdrlen <= MCLBYTES, ("tcphdr too big"));
#else
        KASSERT(max_linkhdr + hdrlen <= MHLEN, ("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_flags & TF_FORCE) && len == 1)
                        tcpstat.tcps_sndprobe++;
                else if (SEQ_LT(tp->snd_nxt, tp->snd_max)) {
                        if (tp->snd_nxt == tp->snd_una)
                                tp->snd_max_rexmt = tp->snd_max;
                        if (nsacked) {
                                tcpstat.tcps_sndsackrtopack++;
                                tcpstat.tcps_sndsackrtobyte += len;
                        }
                        tcpstat.tcps_sndrexmitpack++;
                        tcpstat.tcps_sndrexmitbyte += len;
                } else {
                        tcpstat.tcps_sndpack++;
                        tcpstat.tcps_sndbyte += len;
                }
                if (idle_cwv) {
                        idle_cwv = FALSE;
                        tcp_idle_cwnd_validate(tp);
                }
                /* Update last send time after CWV */
                tp->snd_last = ticks;
#ifdef notyet
                if ((m = m_copypack(so->so_snd.ssb_mb, off, (int)len,
                    max_linkhdr + hdrlen)) == NULL) {
                        error = ENOBUFS;
                        goto after_th;
                }
                /*
                 * m_copypack left space for our hdr; use it.
                 */
                m->m_len += hdrlen;
                m->m_data -= hdrlen;
#else
#ifndef INET6
                m = m_gethdr(M_NOWAIT, MT_HEADER);
#else
                m = m_getl(hdrlen + max_linkhdr, M_NOWAIT, MT_HEADER,
                           M_PKTHDR, NULL);
#endif
                if (m == NULL) {
                        error = ENOBUFS;
                        goto after_th;
                }
                m->m_data += max_linkhdr;
                m->m_len = hdrlen;
                if (len <= MHLEN - hdrlen - max_linkhdr) {
                        m_copydata(so->so_snd.ssb_mb, off, (int) len,
                            mtod(m, caddr_t) + hdrlen);
                        m->m_len += len;
                } else {
                        m->m_next = m_copy(so->so_snd.ssb_mb, off, (int) len);
                        if (m->m_next == NULL) {
                                m_free(m);
                                m = NULL;
                                error = ENOBUFS;
                                goto after_th;
                        }
                }
#endif
                /*
                 * If we're sending everything we've got, set PUSH.
                 * (This will keep happy those implementations which only
                 * give data to the user when a buffer fills or
                 * a PUSH comes in.)
                 */
                if (off + len == so->so_snd.ssb_cc)
                        flags |= TH_PUSH;
        } else {
                if (tp->t_flags & TF_ACKNOW)
                        tcpstat.tcps_sndacks++;
                else if (flags & (TH_SYN | TH_FIN | TH_RST))
                        tcpstat.tcps_sndctrl++;
                else if (SEQ_GT(tp->snd_up, tp->snd_una))
                        tcpstat.tcps_sndurg++;
                else
                        tcpstat.tcps_sndwinup++;

                MGETHDR(m, M_NOWAIT, MT_HEADER);
                if (m == NULL) {
                        error = ENOBUFS;
                        goto after_th;
                }
                if (isipv6 &&
                    (hdrlen + max_linkhdr > MHLEN) && hdrlen <= MHLEN)
                        MH_ALIGN(m, hdrlen);
                else
                        m->m_data += max_linkhdr;
                m->m_len = hdrlen;

                /*
                 * Prioritize SYN, SYN|ACK and pure ACK.
                 * Leave FIN and RST as they are.
                 */
                if (tcp_prio_synack && (flags & (TH_FIN | TH_RST)) == 0)
                        m->m_flags |= M_PRIO;
        }
        m->m_pkthdr.rcvif = NULL;
        if (isipv6) {
                ip6 = mtod(m, struct ip6_hdr *);
                th = (struct tcphdr *)(ip6 + 1);
                tcp_fillheaders(tp, ip6, th, use_tso);
        } else {
                ip = mtod(m, struct ip *);
                th = (struct tcphdr *)(ip + 1);
                /* this picks up the pseudo header (w/o the length) */
                tcp_fillheaders(tp, ip, th, use_tso);
        }
after_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 (th != NULL) {
                /*
                 * 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_callout_active(tp, tp->tt_persist))
                        th->th_seq = htonl(tp->snd_nxt);
                else
                        th->th_seq = htonl(tp->snd_max);
                th->th_ack = htonl(tp->rcv_nxt);
                if (optlen) {
                        bcopy(opt, th + 1, optlen);
                        th->th_off = (sizeof(struct tcphdr) + optlen) >> 2;
                }
                th->th_flags = flags;
        }

        /*
         * Calculate receive window.  Don't shrink window, but avoid
         * silly window syndrome by sending a 0 window if the actual
         * window is less then one segment.
         */
        if (recvwin < (long)(so->so_rcv.ssb_hiwat / 4) &&
            recvwin < (long)segsz)
                recvwin = 0;
        if (recvwin < (tcp_seq_diff_t)(tp->rcv_adv - tp->rcv_nxt))
                recvwin = (tcp_seq_diff_t)(tp->rcv_adv - tp->rcv_nxt);
        if (recvwin > (long)TCP_MAXWIN << tp->rcv_scale)
                recvwin = (long)TCP_MAXWIN << tp->rcv_scale;

        /*
         * Adjust the RXWIN0SENT flag - indicate that we have advertised
         * a 0 window.  This may cause the remote transmitter to stall.  This
         * flag tells soreceive() to disable delayed acknowledgements when
         * draining the buffer.  This can occur if the receiver is attempting
         * to read more data then can be buffered prior to transmitting on
         * the connection.
         */
        if (recvwin == 0)
                tp->t_flags |= TF_RXWIN0SENT;
        else
                tp->t_flags &= ~TF_RXWIN0SENT;

        if (th != NULL)
                th->th_win = htons((u_short) (recvwin>>tp->rcv_scale));

        if (SEQ_GT(tp->snd_up, tp->snd_nxt)) {
                KASSERT(!use_tso, ("URG with TSO"));
                if (th != NULL) {
                        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 */
        }

        if (th != NULL) {
#ifdef TCP_SIGNATURE
                if (tp->t_flags & TF_SIGNATURE) {
                        tcpsignature_compute(m, len, optlen,
                            (u_char *)(th + 1) + sigoff, IPSEC_DIR_OUTBOUND);
                }
#endif /* TCP_SIGNATURE */

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

                        /*
                         * 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));
                }
        }

        /*
         * In transmit state, time the transmission and arrange for
         * the retransmit.  In persist state, just set snd_max.
         */
        if (!(tp->t_flags & TF_FORCE) ||
            !tcp_callout_active(tp, 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;
                        }
                }
                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 = ticks;
                                tp->t_rtseq = startseq;
                                tcpstat.tcps_segstimed++;
                        }
                }

                /*
                 * 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.
                 */
                if (!tcp_callout_active(tp, tp->tt_rexmt) &&
                    tp->snd_nxt != tp->snd_una) {
                        if (tcp_callout_active(tp, tp->tt_persist)) {
                                tcp_callout_stop(tp, tp->tt_persist);
                                tp->t_rxtshift = 0;
                        }
                        tcp_callout_reset(tp, tp->tt_rexmt, tp->t_rxtcur,
                            tcp_timer_rexmt);
                } else if (len == 0 && so->so_snd.ssb_cc &&
                           tp->t_state > TCPS_SYN_RECEIVED &&
                           !tcp_callout_active(tp, tp->tt_rexmt) &&
                           !tcp_callout_active(tp, 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)
                        panic("tcp_output: persist timer to send SYN");
                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 (th != NULL) {
#ifdef TCPDEBUG
                /* Trace. */
                if (so->so_options & SO_DEBUG) {
                        tcp_trace(TA_OUTPUT, tp->t_state, tp,
                            mtod(m, void *), th, 0);
                }
#endif

                /*
                 * Fill in IP length and desired time to live and
                 * send to IP level.  There should be a better way
                 * to handle ttl and tos; we could keep them in
                 * the template, but need a way to checksum without them.
                 */
                /*
                 * m->m_pkthdr.len should have been set before cksum
                 * calcuration, because in6_cksum() need it.
                 */
                if (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(inp,
                            (inp->in6p_route.ro_rt ?
                             inp->in6p_route.ro_rt->rt_ifp : NULL));

                        /* TODO: IPv6 IP6TOS_ECT bit on */
                        error = ip6_output(m, inp->in6p_outputopts,
                            &inp->in6p_route, (so->so_options & SO_DONTROUTE),
                            NULL, NULL, inp);
                } else {
                        struct rtentry *rt;

                        KASSERT(!INP_CHECK_SOCKAF(so, AF_INET6), ("inet6 pcb"));

                        ip->ip_len = m->m_pkthdr.len;
                        ip->ip_ttl = inp->inp_ip_ttl;   /* XXX */
                        ip->ip_tos = inp->inp_ip_tos;   /* XXX */
                        /*
                         * See if we should do MTU discovery.
                         * We do it only if the following are true:
                         *      1) we have a valid route to the destination
                         *      2) the MTU is not locked (if it is,
                         *         then discovery has been disabled)
                         */
                        if (path_mtu_discovery &&
                            (rt = inp->inp_route.ro_rt) &&
                            (rt->rt_flags & RTF_UP) &&
                            !(rt->rt_rmx.rmx_locks & RTV_MTU))
                                ip->ip_off |= IP_DF;

                        error = ip_output(m, inp->inp_options, &inp->inp_route,
                                          (so->so_options & SO_DONTROUTE) |
                                          IP_DEBUGROUTE, NULL, inp);
                }
        } else {
                KASSERT(error != 0, ("no error, but th not set"));
        }
        if (error) {
                tp->t_flags &= ~(TF_ACKNOW | TF_XMITNOW);

                /*
                 * We know that the packet was lost, so back out the
                 * sequence number advance, if any.
                 */
                if (!(tp->t_flags & TF_FORCE) ||
                    !tcp_callout_active(tp, tp->tt_persist)) {
                        /*
                         * No need to check for TH_FIN here because
                         * the TF_SENTFIN flag handles that case.
                         */
                        if (!(flags & TH_SYN))
                                tp->snd_nxt -= len;
                }

out:
                if (error == ENOBUFS) {
                        KASSERT((len == 0 && (flags & (TH_SYN | TH_FIN)) == 0) ||
                            tcp_callout_active(tp, tp->tt_rexmt) ||
                            tcp_callout_active(tp, tp->tt_persist),
                            ("neither rexmt nor persist timer is set"));
                        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(inp, 0);
                        return 0;
                }
                if ((error == EHOSTUNREACH || error == ENETDOWN) &&
                    TCPS_HAVERCVDSYN(tp->t_state)) {
                        tp->t_softerror = error;
                        return (0);
                }
                return (error);
        }
        tcpstat.tcps_sndtotal++;

        /*
         * Data sent (as far as we can tell).
         *
         * If this advertises a larger window than any other segment,
         * then remember the size of the advertised window.
         *
         * Any pending ACK has now been sent.
         */
        if (recvwin > 0 && SEQ_GT(tp->rcv_nxt + recvwin, tp->rcv_adv)) {
                tp->rcv_adv = tp->rcv_nxt + recvwin;
                tp->t_flags &= ~TF_RXRESIZED;
        }
        tp->last_ack_sent = tp->rcv_nxt;
        tp->t_flags &= ~(TF_ACKNOW | TF_XMITNOW);
        if (tcp_delack_enabled)
                tcp_callout_stop(tp, tp->tt_delack);
        if (sendalot) {
                if (tcp_fairsend > 0 && (tp->t_flags & TF_FAIRSEND) &&
                    segcnt >= tcp_fairsend)
                        need_sched = TRUE;
                goto again;
        }
        return (0);
}

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

        if (tp->t_state == TCPS_SYN_SENT ||
            tp->t_state == TCPS_SYN_RECEIVED) {
                panic("tcp_setpersist: not established yet, current %s",
                      tp->t_state == TCPS_SYN_SENT ?
                      "SYN_SENT" : "SYN_RECEIVED");
        }

        if (tcp_callout_active(tp, tp->tt_rexmt))
                panic("tcp_setpersist: retransmit pending");
        /*
         * Start/restart persistance timer.
         */
        TCPT_RANGESET(tt, t * tcp_backoff[tp->t_rxtshift], TCPTV_PERSMIN,
                      TCPTV_PERSMAX);
        tcp_callout_reset(tp, tp->tt_persist, tt, tcp_timer_persist);
        if (tp->t_rxtshift < TCP_MAXRXTSHIFT)
                tp->t_rxtshift++;
}

static void
tcp_idle_cwnd_validate(struct tcpcb *tp)
{
        u_long initial_cwnd = tcp_initial_window(tp);
        u_long min_cwnd;

        tcpstat.tcps_sndidle++;

        /* According to RFC5681: RW=min(IW,cwnd) */
        min_cwnd = min(tp->snd_cwnd, initial_cwnd);

        if (tcp_idle_cwv) {
                u_long idle_time, decay_cwnd;

                /*
                 * RFC2861, but only after idle period.
                 */

                /*
                 * Before the congestion window is reduced, ssthresh
                 * is set to the maximum of its current value and 3/4
                 * cwnd.  If the sender then has more data to send
                 * than the decayed cwnd allows, the TCP will slow-
                 * start (perform exponential increase) at least
                 * half-way back up to the old value of cwnd.
                 */
                tp->snd_ssthresh = max(tp->snd_ssthresh,
                    (3 * tp->snd_cwnd) / 4);

                /*
                 * Decay the congestion window by half for every RTT
                 * that the flow remains inactive.
                 *
                 * The difference between our implementation and
                 * RFC2861 is that we don't allow cwnd to go below
                 * the value allowed by RFC5681 (min_cwnd).
                 */
                idle_time = ticks - tp->snd_last;
                decay_cwnd = tp->snd_cwnd;
                while (idle_time >= tp->t_rxtcur &&
                    decay_cwnd > min_cwnd) {
                        decay_cwnd >>= 1;
                        idle_time -= tp->t_rxtcur;
                }
                tp->snd_cwnd = max(decay_cwnd, min_cwnd);
        } else {
                /*
                 * Slow-start from scratch to re-determine the send
                 * congestion window.
                 */
                tp->snd_cwnd = min_cwnd;
        }

        /* Restart ABC counting during congestion avoidance */
        tp->snd_wacked = 0;
}

static int
tcp_tso_getsize(struct tcpcb *tp, u_int *segsz, u_int *hlen0)
{
        struct inpcb * const inp = tp->t_inpcb;
#ifdef INET6
        const boolean_t isipv6 = INP_ISIPV6(inp);
#else
        const boolean_t isipv6 = FALSE;
#endif
        unsigned int ipoptlen, optlen;
        u_int hlen;

        hlen = sizeof(struct ip) + sizeof(struct tcphdr);

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

        optlen = 0;
        if ((tp->t_flags & (TF_REQ_TSTMP | TF_NOOPT)) == TF_REQ_TSTMP &&
            (tp->t_flags & TF_RCVD_TSTMP))
                optlen += TCPOLEN_TSTAMP_APPA;
        hlen += optlen;

        if (tp->t_maxopd <= optlen + ipoptlen)
                return EHOSTUNREACH;

        *segsz = tp->t_maxopd - optlen - ipoptlen;
        *hlen0 = hlen;
        return 0;
}

static void
tcp_output_sched_handler(netmsg_t nmsg)
{
        struct tcpcb *tp = nmsg->lmsg.u.ms_resultp;

        /* Reply ASAP */
        crit_enter();
        lwkt_replymsg(&nmsg->lmsg, 0);
        crit_exit();

        tcp_output_fair(tp);
}

void
tcp_output_init(struct tcpcb *tp)
{
        netmsg_init(tp->tt_sndmore, NULL, &netisr_adone_rport, MSGF_DROPABLE,
            tcp_output_sched_handler);
        tp->tt_sndmore->lmsg.u.ms_resultp = tp;
}

void
tcp_output_cancel(struct tcpcb *tp)
{
        /*
         * This message is still pending to be processed;
         * drop it.  Optimized.
         */
        crit_enter();
        if ((tp->tt_sndmore->lmsg.ms_flags & MSGF_DONE) == 0) {
                lwkt_dropmsg(&tp->tt_sndmore->lmsg);
        }
        crit_exit();
}

boolean_t
tcp_output_pending(struct tcpcb *tp)
{
        if ((tp->tt_sndmore->lmsg.ms_flags & MSGF_DONE) == 0)
                return TRUE;
        else
                return FALSE;
}

static void
tcp_output_sched(struct tcpcb *tp)
{
        crit_enter();
        if (tp->tt_sndmore->lmsg.ms_flags & MSGF_DONE)
                lwkt_sendmsg(netisr_cpuport(mycpuid), &tp->tt_sndmore->lmsg);
        crit_exit();
}

/*
 * Fairsend
 *
 * Yield to other senders or receivers on the same netisr if the current
 * TCP stream has sent tcp_fairsend segments and is going to burst more
 * segments.  Bursting large amount of segements in a single TCP stream
 * could delay other senders' segments and receivers' ACKs quite a lot,
 * if others segments and ACKs are queued on to the same hardware transmit
 * queue; thus cause unfairness between senders and suppress receiving
 * performance.
 * 
 * Fairsend should be performed at the places that do not affect segment
 * sending during congestion control, e.g.
 * - User requested output
 * - ACK input triggered output
 *
 * NOTE:
 * For devices that are TSO capable, their TSO aggregation size limit could
 * affect fairsend.
 */
int
tcp_output_fair(struct tcpcb *tp)
{
        int ret;

        tp->t_flags |= TF_FAIRSEND;
        ret = tcp_output(tp);
        tp->t_flags &= ~TF_FAIRSEND;

        return ret;
}