FreeBSD/Linux Kernel Cross Reference
sys/netinet/tcp_output.c

     /*-
      * 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.
     * 4. Neither the name of the University nor the names of its contributors
     *    may be used to endorse or promote products derived from this software
     *    without specific prior written permission.
     *
     * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
     * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
     * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
     * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
     * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
     * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
     * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
     * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
     * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
     * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
     * SUCH DAMAGE.
     *
     *      @(#)tcp_output.c        8.4 (Berkeley) 5/24/95
     */
    
    #include <sys/cdefs.h>
    __FBSDID("$FreeBSD: releng/10.3/sys/netinet/tcp_output.c 295015 2016-01-28 21:30:49Z hiren $");
    
    #include "opt_inet.h"
    #include "opt_inet6.h"
    #include "opt_ipsec.h"
    #include "opt_kdtrace.h"
    #include "opt_tcpdebug.h"
    
    #include <sys/param.h>
    #include <sys/systm.h>
    #include <sys/domain.h>
    #include <sys/hhook.h>
    #include <sys/kernel.h>
    #include <sys/lock.h>
    #include <sys/mbuf.h>
    #include <sys/mutex.h>
    #include <sys/protosw.h>
    #include <sys/sdt.h>
    #include <sys/socket.h>
    #include <sys/socketvar.h>
    #include <sys/sysctl.h>
    
    #include <net/if.h>
    #include <net/route.h>
    #include <net/vnet.h>
    
    #include <netinet/cc.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
    #ifdef TCP_RFC7413
    #include <netinet/tcp_fastopen.h>
    #endif
    #define TCPOUTFLAGS
    #include <netinet/tcp_fsm.h>
    #include <netinet/tcp_seq.h>
    #include <netinet/tcp_timer.h>
    #include <netinet/tcp_var.h>
    #include <netinet/tcpip.h>
    #ifdef TCPDEBUG
    #include <netinet/tcp_debug.h>
    #endif
    #ifdef TCP_OFFLOAD
    #include <netinet/tcp_offload.h>
    #endif
    
    #ifdef IPSEC
    #include <netipsec/ipsec.h>
    #endif /*IPSEC*/
    
    #include <machine/in_cksum.h>
    
    #include <security/mac/mac_framework.h>
    
    VNET_DEFINE(int, path_mtu_discovery) = 1;
    SYSCTL_VNET_INT(_net_inet_tcp, OID_AUTO, path_mtu_discovery, CTLFLAG_RW,
            &VNET_NAME(path_mtu_discovery), 1,
            "Enable Path MTU Discovery");
   
   VNET_DEFINE(int, tcp_do_tso) = 1;
   #define V_tcp_do_tso            VNET(tcp_do_tso)
   SYSCTL_VNET_INT(_net_inet_tcp, OID_AUTO, tso, 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_VNET_INT(_net_inet_tcp, TCPCTL_SENDSPACE, sendspace, CTLFLAG_RW,
           &VNET_NAME(tcp_sendspace), 0, "Initial send socket buffer size");
   
   VNET_DEFINE(int, tcp_do_autosndbuf) = 1;
   #define V_tcp_do_autosndbuf     VNET(tcp_do_autosndbuf)
   SYSCTL_VNET_INT(_net_inet_tcp, OID_AUTO, sendbuf_auto, CTLFLAG_RW,
           &VNET_NAME(tcp_do_autosndbuf), 0,
           "Enable automatic send buffer sizing");
   
   VNET_DEFINE(int, tcp_autosndbuf_inc) = 8*1024;
   #define V_tcp_autosndbuf_inc    VNET(tcp_autosndbuf_inc)
   SYSCTL_VNET_INT(_net_inet_tcp, OID_AUTO, sendbuf_inc, CTLFLAG_RW,
           &VNET_NAME(tcp_autosndbuf_inc), 0,
           "Incrementor step size of automatic send buffer");
   
   VNET_DEFINE(int, tcp_autosndbuf_max) = 2*1024*1024;
   #define V_tcp_autosndbuf_max    VNET(tcp_autosndbuf_max)
   SYSCTL_VNET_INT(_net_inet_tcp, OID_AUTO, sendbuf_max, CTLFLAG_RW,
           &VNET_NAME(tcp_autosndbuf_max), 0,
           "Max size of automatic send buffer");
   
   static void inline      hhook_run_tcp_est_out(struct tcpcb *tp,
                               struct tcphdr *th, struct tcpopt *to,
                               long len, int tso);
   static void inline      cc_after_idle(struct tcpcb *tp);
   
   /*
    * Wrapper for the TCP established output helper hook.
    */
   static void inline
   hhook_run_tcp_est_out(struct tcpcb *tp, struct tcphdr *th,
       struct tcpopt *to, long 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);
           }
   }
   
   /*
    * 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_output(struct tcpcb *tp)
   {
           struct socket *so = tp->t_inpcb->inp_socket;
           long len, recwin, sendwin;
           int off, flags, error = 0;      /* Keep compiler happy */
           struct mbuf *m;
           struct ip *ip = NULL;
           struct ipovly *ipov = NULL;
           struct tcphdr *th;
           u_char opt[TCP_MAXOLEN];
           unsigned ipoptlen, optlen, hdrlen;
   #ifdef IPSEC
           unsigned ipsec_optlen = 0;
   #endif
           int idle, sendalot;
           int sack_rxmit, sack_bytes_rxmt;
           struct sackhole *p;
           int tso, mtu;
           struct tcpopt to;
   #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
   
           INP_WLOCK_ASSERT(tp->t_inpcb);
   
   #ifdef TCP_OFFLOAD
           if (tp->t_flags & TF_TOE)
                   return (tcp_offload_output(tp));
   #endif
   
   #ifdef TCP_RFC7413
           /*
            * For TFO connections in SYN_RECEIVED, only allow the initial
            * SYN|ACK and those sent by the retransmit timer.
            */
           if ((tp->t_flags & TF_FASTOPEN) &&
               (tp->t_state == TCPS_SYN_RECEIVED) &&
               SEQ_GT(tp->snd_max, tp->snd_una) &&    /* inital SYN|ACK sent */
               (tp->snd_nxt != tp->snd_una))          /* not a retransmit */
                   return (0);
   #endif
           /*
            * 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)
                   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))) {
                   long cwin;
                   
                   cwin = min(tp->snd_wnd, tp->snd_cwnd) - sack_bytes_rxmt;
                   if (cwin < 0)
                           cwin = 0;
                   /* Do not retransmit SACK segments beyond snd_recover */
                   if (SEQ_GT(p->end, tp->snd_recover)) {
                           /*
                            * (At least) part of sack hole extends beyond
                            * snd_recover. Check to see if we can rexmit data
                            * for this hole.
                            */
                           if (SEQ_GEQ(p->rxmit, tp->snd_recover)) {
                                   /*
                                    * Can't rexmit any more data for this hole.
                                    * That data will be rexmitted in the next
                                    * sack recovery episode, when snd_recover
                                    * moves past p->rxmit.
                                    */
                                   p = NULL;
                                   goto after_sack_rexmit;
                           } else
                                   /* Can rexmit part of the current hole */
                                   len = ((long)ulmin(cwin,
                                                      tp->snd_recover - p->rxmit));
                   } else
                           len = ((long)ulmin(cwin, p->end - p->rxmit));
                   off = p->rxmit - tp->snd_una;
                   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, tp->t_maxseg));
                   }
           }
   after_sack_rexmit:
           /*
            * Get standard flags, and add SYN or FIN if requested by 'hidden'
            * state flags.
            */
           if (tp->t_flags & TF_NEEDFIN)
                   flags |= TH_FIN;
           if (tp->t_flags & TF_NEEDSYN)
                   flags |= TH_SYN;
   
           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 < so->so_snd.sb_cc)
                                   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 = ((long)ulmin(so->so_snd.sb_cc, sendwin) - off);
                   else {
                           long cwin;
   
                           /*
                            * We are inside of a SACK recovery episode and are
                            * sending new data, having retransmitted all the
                            * data possible in the scoreboard.
                            */
                           len = ((long)ulmin(so->so_snd.sb_cc, tp->snd_wnd) 
                                  - off);
                           /*
                            * Don't remove this (len > 0) check !
                            * We explicitly check for len > 0 here (although it 
                            * isn't really necessary), to work around a gcc 
                            * optimization issue - to force gcc to compute
                            * len above. Without this check, the computation
                            * of len is bungled by the optimizer.
                            */
                           if (len > 0) {
                                   cwin = tp->snd_cwnd - 
                                           (tp->snd_nxt - tp->sack_newdata) -
                                           sack_bytes_rxmt;
                                   if (cwin < 0)
                                           cwin = 0;
                                   len = lmin(len, cwin);
                           }
                   }
           }
   
           /*
            * Lop off SYN bit if it has already been sent.  However, if this
            * is SYN-SENT state and if segment contains data and if we don't
            * know that foreign host supports TAO, suppress sending segment.
            */
           if ((flags & TH_SYN) && SEQ_GT(tp->snd_nxt, tp->snd_una)) {
                   if (tp->t_state != TCPS_SYN_RECEIVED)
                           flags &= ~TH_SYN;
   #ifdef TCP_RFC7413
                   /*
                    * When sending additional segments following a TFO SYN|ACK,
                    * do not include the SYN bit.
                    */
                   if ((tp->t_flags & TF_FASTOPEN) &&
                       (tp->t_state == TCPS_SYN_RECEIVED))
                           flags &= ~TH_SYN;
   #endif
                   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;
           }
   
   #ifdef TCP_RFC7413
           /*
            * When retransmitting SYN|ACK on a passively-created TFO socket,
            * don't include data, as the presence of data may have caused the
            * original SYN|ACK to have been dropped by a middlebox.
            */
           if ((tp->t_flags & TF_FASTOPEN) &&
               (((tp->t_state == TCPS_SYN_RECEIVED) && (tp->t_rxtshift > 0)) ||
                (flags & TH_RST)))
                   len = 0;
   #endif
           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) so->so_snd.sb_cc)) {
                           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__));
   
           /*
            * 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*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.
            *
            * TODO: Shrink send buffer during idle periods together
            * with congestion window.  Requires another timer.  Has to
            * wait for upcoming tcp timer rewrite.
            */
           if (V_tcp_do_autosndbuf && so->so_snd.sb_flags & SB_AUTOSIZE) {
                   if ((tp->snd_wnd / 4 * 5) >= so->so_snd.sb_hiwat &&
                       so->so_snd.sb_cc >= (so->so_snd.sb_hiwat / 8 * 7) &&
                       so->so_snd.sb_cc < V_tcp_autosndbuf_max &&
                       sendwin >= (so->so_snd.sb_cc - (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;
                   }
           }
   
           /*
            * 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.
            */
   #ifdef IPSEC
           /*
            * Pre-calculate here as we save another lookup into the darknesses
            * of IPsec that way and can actually decide if TSO is ok.
            */
           ipsec_optlen = ipsec_hdrsiz_tcp(tp);
   #endif
           if ((tp->t_flags & TF_TSO) && V_tcp_do_tso && len > tp->t_maxseg &&
               ((tp->t_flags & TF_SIGNATURE) == 0) &&
               tp->rcv_numsacks == 0 && sack_rxmit == 0 &&
   #ifdef IPSEC
               ipsec_optlen == 0 &&
   #endif
               tp->t_inpcb->inp_options == NULL &&
               tp->t_inpcb->in6p_options == NULL)
                   tso = 1;
   
           if (sack_rxmit) {
                   if (SEQ_LT(p->rxmit + len, tp->snd_una + so->so_snd.sb_cc))
                           flags &= ~TH_FIN;
           } else {
                   if (SEQ_LT(tp->snd_nxt + len, tp->snd_una + so->so_snd.sb_cc))
                           flags &= ~TH_FIN;
           }
   
           recwin = sbspace(&so->so_rcv);
   
           /*
            * Sender silly window avoidance.   We transmit under the following
            * conditions when len is non-zero:
            *
            *      - We have a full segment (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;
                   /*
                    * 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.sb_cc &&
                       (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.
                    */
                   long adv;
                   int oldwin;
   
                   adv = min(recwin, (long)TCP_MAXWIN << tp->rcv_scale);
                   if (SEQ_GT(tp->rcv_adv, tp->rcv_nxt)) {
                           oldwin = (tp->rcv_adv - tp->rcv_nxt);
                           adv -= oldwin;
                   } else
                           oldwin = 0;
   
                   /* 
                    * If the new window size ends up being the same as 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 >= (long)(2 * tp->t_maxseg) &&
                       (adv >= (long)(so->so_rcv.sb_hiwat / 4) ||
                        recwin <= (long)(so->so_rcv.sb_hiwat / 8) ||
                        so->so_rcv.sb_hiwat <= 8 * tp->t_maxseg))
                           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 (so->so_snd.sb_cc && !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);
   
           /*
            * 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) {
                           tp->snd_nxt = tp->iss;
                           to.to_mss = tcp_mssopt(&tp->t_inpcb->inp_inc);
                           to.to_flags |= TOF_MSS;
   #ifdef TCP_RFC7413
                           /*
                            * Only include the TFO option on the first
                            * transmission of the SYN|ACK on a
                            * passively-created TFO socket, as the presence of
                            * the TFO option may have caused the original
                            * SYN|ACK to have been dropped by a middlebox.
                            */
                           if ((tp->t_flags & TF_FASTOPEN) &&
                               (tp->t_state == TCPS_SYN_RECEIVED) &&
                               (tp->t_rxtshift == 0)) {
                                   to.to_tfo_len = TCP_FASTOPEN_COOKIE_LEN;
                                   to.to_tfo_cookie = (u_char *)&tp->t_tfo_cookie;
                                   to.to_flags |= TOF_FASTOPEN;
                           }
   #endif
                   }
                   /* 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))) {
                           to.to_tsval = tcp_ts_getticks() + tp->ts_offset;
                           to.to_tsecr = tp->ts_recent;
                           to.to_flags |= TOF_TS;
                           /* 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->t_flags & TF_SACK_PERMIT) &&
                               tp->rcv_numsacks > 0) {
                                   to.to_flags |= TOF_SACK;
                                   to.to_nsacks = tp->rcv_numsacks;
                                   to.to_sacks = (u_char *)tp->sackblks;
                           }
                   }
   #ifdef TCP_SIGNATURE
                   /* TCP-MD5 (RFC2385). */
                   if (tp->t_flags & TF_SIGNATURE)
                           to.to_flags |= TOF_SIGNATURE;
   #endif /* TCP_SIGNATURE */
   
                   /* Processing the options. */
                   hdrlen += optlen = tcp_addoptions(&to, opt);
           }
   
   #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;
   #ifdef IPSEC
           ipoptlen += ipsec_optlen;
   #endif
   
           /*
            * Adjust data length if insertion of options will
            * bump the packet length beyond the t_maxopd length.
            * Clear the FIN bit because we cut off the tail of
            * the segment.
            */
           if (len + optlen + ipoptlen > tp->t_maxopd) {
                   flags &= ~TH_FIN;
   
                   if (tso) {
                           u_int if_hw_tsomax;
                           u_int if_hw_tsomaxsegcount;
                           u_int if_hw_tsomaxsegsize;
                           struct mbuf *mb;
                           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;
                                   }
                           }
   
                           /*
                            * Check if we should limit by maximum segment
                            * size and count:
                            */
                           if (if_hw_tsomaxsegcount != 0 &&
                               if_hw_tsomaxsegsize != 0) {
                                   /*
                                    * Subtract one segment for the LINK
                                    * and TCP/IP headers mbuf that will
                                    * be prepended to this mbuf chain
                                    * after the code in this section
                                    * limits the number of mbufs in the
                                    * chain to if_hw_tsomaxsegcount.
                                    */
                                   if_hw_tsomaxsegcount -= 1;
                                   max_len = 0;
                                   mb = sbsndmbuf(&so->so_snd, off, &moff);
   
                                   while (mb != NULL && max_len < len) {
                                           u_int mlen;
                                           u_int frags;
   
                                           /*
                                            * Get length of mbuf fragment
                                            * and how many hardware frags,
                                            * rounded up, it would use:
                                            */
                                           mlen = (mb->m_len - moff);
                                           frags = howmany(mlen,
                                               if_hw_tsomaxsegsize);
   
                                           /* Handle special case: Zero Length Mbuf */
                                           if (frags == 0)
                                                   frags = 1;
   
                                           /*
                                            * Check if the fragment limit
                                            * will be reached or exceeded:
                                            */
                                           if (frags >= if_hw_tsomaxsegcount) {
                                                   max_len += min(mlen,
                                                       if_hw_tsomaxsegcount *
                                                       if_hw_tsomaxsegsize);
                                                   break;
                                           }
                                           max_len += mlen;
                                           if_hw_tsomaxsegcount -= frags;
                                           moff = 0;
                                           mb = mb->m_next;
                                   }
                                   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_maxopd - optlen);
                           if ((off + len) < so->so_snd.sb_cc) {
                                   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 {
                           len = tp->t_maxopd - optlen - ipoptlen;
                           sendalot = 1;
                   }
           } 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;
                   u_int moff;
   
                   if ((tp->t_flags & TF_FORCEDATA) && len == 1)
                           TCPSTAT_INC(tcps_sndprobe);
                   else if (SEQ_LT(tp->snd_nxt, tp->snd_max) || sack_rxmit) {
                           tp->t_sndrexmitpack++;
                          TCPSTAT_INC(tcps_sndrexmitpack);
                          TCPSTAT_ADD(tcps_sndrexmitbyte, len);
                  } else {
                          TCPSTAT_INC(tcps_sndpack);
                          TCPSTAT_ADD(tcps_sndbyte, len);
                  }
  #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(&so->so_snd, off, len, &moff);
  
                  if (len <= MHLEN - hdrlen - max_linkhdr) {
                          m_copydata(mb, moff, (int)len,
                              mtod(m, caddr_t) + hdrlen);
                          m->m_len += len;
                  } else {
                          m->m_next = m_copy(mb, moff, (int)len);
                          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 ((off + len == so->so_snd.sb_cc) && !(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) {
                          MH_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 *);
                  th = (struct tcphdr *)(ip6 + 1);
                  tcpip_fillheaders(tp->t_inpcb, ip6, th);
          } else
  #endif /* INET6 */
          {
                  ip = mtod(m, struct ip *);
                  ipov = (struct ipovly *)ip;
                  th = (struct tcphdr *)(ip + 1);
                  tcpip_fillheaders(tp->t_inpcb, 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) {
                  if (tp->t_rxtshift >= 1) {
                          if (tp->t_rxtshift <= V_tcp_ecn_maxretries)
                                  flags |= TH_ECE|TH_CWR;
                  } else
                          flags |= TH_ECE|TH_CWR;
          }
          
          if (tp->t_state == TCPS_ESTABLISHED &&
              (tp->t_flags & TF_ECN_PERMIT)) {
                  /*
                   * If the peer has ECN, mark data packets with
                   * ECN capable transmission (ECT).
                   * Ignore pure ack packets, retransmissions and window probes.
                   */
                  if (len > 0 && SEQ_GEQ(tp->snd_nxt, tp->snd_max) &&
                      !((tp->t_flags & TF_FORCEDATA) && len == 1)) {
  #ifdef INET6
                          if (isipv6)
                                  ip6->ip6_flow |= htonl(IPTOS_ECN_ECT0 << 20);
                          else
  #endif
                                  ip->ip_tos |= IPTOS_ECN_ECT0;
                          TCPSTAT_INC(tcps_ecn_ect0);
                  }
                  
                  /*
                   * Reply with proper ECN notifications.
                   */
                  if (tp->t_flags & TF_ECN_SND_CWR) {
                          flags |= TH_CWR;
                          tp->t_flags &= ~TF_ECN_SND_CWR;
                  } 
                  if (tp->t_flags & TF_ECN_SND_ECE)
                          flags |= TH_ECE;
          }
          
          /*
           * If we are doing retransmissions, then snd_nxt will
           * not reflect the first unsent octet.  For ACK only
           * packets, we do not want the sequence number of the
           * retransmitted packet, we want the sequence number
           * of the next unsent octet.  So, if there is no data
           * (and no SYN or FIN), use snd_max instead of snd_nxt
           * when filling in ti_seq.  But if we are in persist
           * state, snd_max might reflect one byte beyond the
           * right edge of the window, so use snd_nxt in that
           * case, since we know we aren't doing a retransmission.
           * (retransmit and persist are mutually exclusive...)
           */
          if (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;
                  tp->sackhint.sack_bytes_rexmit += len;
          }
          th->th_ack = htonl(tp->rcv_nxt);
          if (optlen) {
                  bcopy(opt, th + 1, optlen);
                  th->th_off = (sizeof (struct tcphdr) + optlen) >> 2;
          }
          th->th_flags = flags;
          /*
           * Calculate receive window.  Don't shrink window,
           * but avoid silly window syndrome.
           */
          if (recwin < (long)(so->so_rcv.sb_hiwat / 4) &&
              recwin < (long)tp->t_maxseg)
                  recwin = 0;
          if (SEQ_GT(tp->rcv_adv, tp->rcv_nxt) &&
              recwin < (long)(tp->rcv_adv - tp->rcv_nxt))
                  recwin = (long)(tp->rcv_adv - tp->rcv_nxt);
          if (recwin > (long)TCP_MAXWIN << tp->rcv_scale)
                  recwin = (long)TCP_MAXWIN << tp->rcv_scale;
  
          /*
           * 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
                  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 */
  
  #ifdef TCP_SIGNATURE
          if (to.to_flags & TOF_SIGNATURE) {
                  int sigoff = to.to_signature - opt;
                  tcp_signature_compute(m, 0, len, optlen,
                      (u_char *)(th + 1) + sigoff, IPSEC_DIR_OUTBOUND);
          }
  #endif
  
          /*
           * Put TCP length in extended header, and then
           * checksum extended header and data.
           */
          m->m_pkthdr.len = hdrlen + len; /* in6_cksum() need this */
          m->m_pkthdr.csum_data = offsetof(struct tcphdr, th_sum);
  #ifdef INET6
          if (isipv6) {
                  /*
                   * ip6_plen is not need to be filled now, and will be filled
                   * in ip6_output.
                   */
                  m->m_pkthdr.csum_flags = CSUM_TCP_IPV6;
                  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
          {
                  m->m_pkthdr.csum_flags = CSUM_TCP;
                  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.
           * XXX: Fixme: This is currently not the case for IPv6.
           */
          if (tso) {
                  KASSERT(len > tp->t_maxopd - optlen,
                      ("%s: len <= tso_segsz", __func__));
                  m->m_pkthdr.csum_flags |= CSUM_TSO;
                  m->m_pkthdr.tso_segsz = tp->t_maxopd - optlen;
          }
  
  #ifdef IPSEC
          KASSERT(len + hdrlen + ipoptlen - ipsec_optlen == m_length(m, NULL),
              ("%s: mbuf chain shorter than expected: %ld + %u + %u - %u != %u",
              __func__, len, hdrlen, ipoptlen, ipsec_optlen, m_length(m, NULL)));
  #else
          KASSERT(len + hdrlen + ipoptlen == m_length(m, NULL),
              ("%s: mbuf chain shorter than expected: %ld + %u + %u != %u",
              __func__, len, hdrlen, ipoptlen, m_length(m, NULL)));
  #endif
  
          /* Run HHOOK_TCP_ESTABLISHED_OUT helper hooks. */
          hhook_run_tcp_est_out(tp, th, &to, len, tso);
  
  #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 */
  
          /*
           * 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.
           */
  #ifdef INET6
          if (isipv6) {
                  struct route_in6 ro;
  
                  bzero(&ro, sizeof(ro));
                  /*
                   * 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_maxopd > 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);
  
                  /* TODO: IPv6 IP6TOS_ECT bit on */
                  error = ip6_output(m, tp->t_inpcb->in6p_outputopts, &ro,
                      ((so->so_options & SO_DONTROUTE) ?  IP_ROUTETOIF : 0),
                      NULL, NULL, tp->t_inpcb);
  
                  if (error == EMSGSIZE && ro.ro_rt != NULL)
                          mtu = ro.ro_rt->rt_mtu;
                  RO_RTFREE(&ro);
          }
  #endif /* INET6 */
  #if defined(INET) && defined(INET6)
          else
  #endif
  #ifdef INET
      {
          struct route ro;
  
          bzero(&ro, sizeof(ro));
          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_maxopd > V_tcp_minmss) {
                  ip->ip_off |= htons(IP_DF);
                  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, ip, tp, th);
  
          TCP_PROBE5(send, NULL, tp, ip, tp, th);
  
          error = ip_output(m, tp->t_inpcb->inp_options, &ro,
              ((so->so_options & SO_DONTROUTE) ? IP_ROUTETOIF : 0), 0,
              tp->t_inpcb);
  
          if (error == EMSGSIZE && ro.ro_rt != NULL)
                  mtu = ro.ro_rt->rt_mtu;
          RO_RTFREE(&ro);
      }
  #endif /* INET */
  
  out:
          /*
           * 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.
                           */
                          if (tp->t_rtttime == 0) {
                                  tp->t_rtttime = ticks;
                                  tp->t_rtseq = startseq;
                                  TCPSTAT_INC(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.
                   */
  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 && so->so_snd.sb_cc &&
                      !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 + len;
          }
  
          if (error) {
  
                  /*
                   * 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 EPERM:
                          tp->t_softerror = error;
                          return (error);
                  case ENOBUFS:
                          if (!tcp_timer_active(tp, TT_REXMT) &&
                              !tcp_timer_active(tp, TT_PERSIST))
                                  tcp_timer_activate(tp, TT_REXMT, tp->t_rxtcur);
                          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 (recwin >= 0 && SEQ_GT(tp->rcv_nxt + recwin, tp->rcv_adv))
                  tp->rcv_adv = tp->rcv_nxt + recwin;
          tp->last_ack_sent = tp->rcv_nxt;
          tp->t_flags &= ~(TF_ACKNOW | TF_DELACK);
          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 persistance 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_int 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;
  #ifdef TCP_SIGNATURE
                  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)
                                  continue;
                          optlen += TCPOLEN_SIGNATURE;
                          *optp++ = TCPOPT_SIGNATURE;
                          *optp++ = TCPOLEN_SIGNATURE;
                          to->to_signature = optp;
                          while (siglen--)
                                   *optp++ = 0;
                          break;
                          }
  #endif
                  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;
                          }
  #ifdef TCP_RFC7413
                  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)
                                  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;
                          }
  #endif
                  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);
  }

Cache object: 5973565890398e1558dff8721e9a5fd7