The Design and Implementation of the FreeBSD Operating System, Second Edition
Now available: The Design and Implementation of the FreeBSD Operating System (Second Edition)


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sys/netinet/tcp_output.c

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    1 /*-
    2  * Copyright (c) 1982, 1986, 1988, 1990, 1993, 1995
    3  *      The Regents of the University of California.  All rights reserved.
    4  *
    5  * Redistribution and use in source and binary forms, with or without
    6  * modification, are permitted provided that the following conditions
    7  * are met:
    8  * 1. Redistributions of source code must retain the above copyright
    9  *    notice, this list of conditions and the following disclaimer.
   10  * 2. Redistributions in binary form must reproduce the above copyright
   11  *    notice, this list of conditions and the following disclaimer in the
   12  *    documentation and/or other materials provided with the distribution.
   13  * 4. Neither the name of the University nor the names of its contributors
   14  *    may be used to endorse or promote products derived from this software
   15  *    without specific prior written permission.
   16  *
   17  * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
   18  * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
   19  * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
   20  * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
   21  * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
   22  * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
   23  * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
   24  * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
   25  * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
   26  * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
   27  * SUCH DAMAGE.
   28  *
   29  *      @(#)tcp_output.c        8.4 (Berkeley) 5/24/95
   30  */
   31 
   32 #include <sys/cdefs.h>
   33 __FBSDID("$FreeBSD: releng/7.4/sys/netinet/tcp_output.c 211871 2010-08-27 10:16:32Z andre $");
   34 
   35 #include "opt_inet.h"
   36 #include "opt_inet6.h"
   37 #include "opt_ipsec.h"
   38 #include "opt_mac.h"
   39 #include "opt_tcpdebug.h"
   40 
   41 #include <sys/param.h>
   42 #include <sys/systm.h>
   43 #include <sys/domain.h>
   44 #include <sys/kernel.h>
   45 #include <sys/lock.h>
   46 #include <sys/mbuf.h>
   47 #include <sys/mutex.h>
   48 #include <sys/protosw.h>
   49 #include <sys/socket.h>
   50 #include <sys/socketvar.h>
   51 #include <sys/sysctl.h>
   52 
   53 #include <net/route.h>
   54 
   55 #include <netinet/in.h>
   56 #include <netinet/in_systm.h>
   57 #include <netinet/ip.h>
   58 #include <netinet/in_pcb.h>
   59 #include <netinet/ip_var.h>
   60 #include <netinet/ip_options.h>
   61 #ifdef INET6
   62 #include <netinet6/in6_pcb.h>
   63 #include <netinet/ip6.h>
   64 #include <netinet6/ip6_var.h>
   65 #endif
   66 #include <netinet/tcp.h>
   67 #define TCPOUTFLAGS
   68 #include <netinet/tcp_fsm.h>
   69 #include <netinet/tcp_seq.h>
   70 #include <netinet/tcp_timer.h>
   71 #include <netinet/tcp_var.h>
   72 #include <netinet/tcpip.h>
   73 #ifdef TCPDEBUG
   74 #include <netinet/tcp_debug.h>
   75 #endif
   76 
   77 #ifdef IPSEC
   78 #include <netipsec/ipsec.h>
   79 #endif /*IPSEC*/
   80 
   81 #include <machine/in_cksum.h>
   82 
   83 #include <security/mac/mac_framework.h>
   84 
   85 #ifdef notyet
   86 extern struct mbuf *m_copypack();
   87 #endif
   88 
   89 int path_mtu_discovery = 1;
   90 SYSCTL_INT(_net_inet_tcp, OID_AUTO, path_mtu_discovery, CTLFLAG_RW,
   91         &path_mtu_discovery, 1, "Enable Path MTU Discovery");
   92 
   93 int ss_fltsz = 1;
   94 SYSCTL_INT(_net_inet_tcp, OID_AUTO, slowstart_flightsize, CTLFLAG_RW,
   95         &ss_fltsz, 1, "Slow start flight size");
   96 
   97 int ss_fltsz_local = 4;
   98 SYSCTL_INT(_net_inet_tcp, OID_AUTO, local_slowstart_flightsize, CTLFLAG_RW,
   99         &ss_fltsz_local, 1, "Slow start flight size for local networks");
  100 
  101 int     tcp_do_newreno = 1;
  102 SYSCTL_INT(_net_inet_tcp, OID_AUTO, newreno, CTLFLAG_RW,
  103         &tcp_do_newreno, 0, "Enable NewReno Algorithms");
  104 
  105 int     tcp_do_tso = 1;
  106 SYSCTL_INT(_net_inet_tcp, OID_AUTO, tso, CTLFLAG_RW,
  107         &tcp_do_tso, 0, "Enable TCP Segmentation Offload");
  108 
  109 int     tcp_do_autosndbuf = 1;
  110 SYSCTL_INT(_net_inet_tcp, OID_AUTO, sendbuf_auto, CTLFLAG_RW,
  111         &tcp_do_autosndbuf, 0, "Enable automatic send buffer sizing");
  112 
  113 int     tcp_autosndbuf_inc = 8*1024;
  114 SYSCTL_INT(_net_inet_tcp, OID_AUTO, sendbuf_inc, CTLFLAG_RW,
  115         &tcp_autosndbuf_inc, 0, "Incrementor step size of automatic send buffer");
  116 
  117 int     tcp_autosndbuf_max = 256*1024;
  118 SYSCTL_INT(_net_inet_tcp, OID_AUTO, sendbuf_max, CTLFLAG_RW,
  119         &tcp_autosndbuf_max, 0, "Max size of automatic send buffer");
  120 
  121 
  122 /*
  123  * Tcp output routine: figure out what should be sent and send it.
  124  */
  125 int
  126 tcp_output(struct tcpcb *tp)
  127 {
  128         struct socket *so = tp->t_inpcb->inp_socket;
  129         long len, recwin, sendwin;
  130         int off, flags, error, rw;
  131         struct mbuf *m;
  132         struct ip *ip = NULL;
  133         struct ipovly *ipov = NULL;
  134         struct tcphdr *th;
  135         u_char opt[TCP_MAXOLEN];
  136         unsigned ipoptlen, optlen, hdrlen;
  137 #ifdef IPSEC
  138         unsigned ipsec_optlen = 0;
  139 #endif
  140         int idle, sendalot;
  141         int sack_rxmit, sack_bytes_rxmt;
  142         struct sackhole *p;
  143         int tso;
  144         struct tcpopt to;
  145 #if 0
  146         int maxburst = TCP_MAXBURST;
  147 #endif
  148 #ifdef INET6
  149         struct ip6_hdr *ip6 = NULL;
  150         int isipv6;
  151 
  152         isipv6 = (tp->t_inpcb->inp_vflag & INP_IPV6) != 0;
  153 #endif
  154 
  155         INP_WLOCK_ASSERT(tp->t_inpcb);
  156 
  157         /*
  158          * Determine length of data that should be transmitted,
  159          * and flags that will be used.
  160          * If there is some data or critical controls (SYN, RST)
  161          * to send, then transmit; otherwise, investigate further.
  162          */
  163         idle = (tp->t_flags & TF_LASTIDLE) || (tp->snd_max == tp->snd_una);
  164         if (idle && (ticks - (int)tp->t_rcvtime) >= tp->t_rxtcur) {
  165                 /*
  166                  * If we've been idle for more than one retransmit
  167                  * timeout the old congestion window is no longer
  168                  * current and we have to reduce it to the restart
  169                  * window before we can transmit again.
  170                  *
  171                  * The restart window is the initial window or the last
  172                  * CWND, whichever is smaller.
  173                  * 
  174                  * This is done to prevent us from flooding the path with
  175                  * a full CWND at wirespeed, overloading router and switch
  176                  * buffers along the way.
  177                  *
  178                  * See RFC5681 Section 4.1. "Restarting Idle Connections".
  179                  */
  180                 if (tcp_do_rfc3390)
  181                         rw = min(4 * tp->t_maxseg,
  182                                  max(2 * tp->t_maxseg, 4380));
  183 #ifdef INET6
  184                 else if ((isipv6 ? in6_localaddr(&tp->t_inpcb->in6p_faddr) :
  185                           in_localaddr(tp->t_inpcb->inp_faddr)))
  186 #else
  187                 else if (in_localaddr(tp->t_inpcb->inp_faddr))
  188 #endif
  189                         rw = ss_fltsz_local * tp->t_maxseg;
  190                 else
  191                         rw = ss_fltsz * tp->t_maxseg;
  192 
  193                 tp->snd_cwnd = min(rw, tp->snd_cwnd);
  194         }
  195         tp->t_flags &= ~TF_LASTIDLE;
  196         if (idle) {
  197                 if (tp->t_flags & TF_MORETOCOME) {
  198                         tp->t_flags |= TF_LASTIDLE;
  199                         idle = 0;
  200                 }
  201         }
  202 again:
  203         /*
  204          * If we've recently taken a timeout, snd_max will be greater than
  205          * snd_nxt.  There may be SACK information that allows us to avoid
  206          * resending already delivered data.  Adjust snd_nxt accordingly.
  207          */
  208         if ((tp->t_flags & TF_SACK_PERMIT) &&
  209             SEQ_LT(tp->snd_nxt, tp->snd_max))
  210                 tcp_sack_adjust(tp);
  211         sendalot = 0;
  212         tso = 0;
  213         off = tp->snd_nxt - tp->snd_una;
  214         sendwin = min(tp->snd_wnd, tp->snd_cwnd);
  215         sendwin = min(sendwin, tp->snd_bwnd);
  216 
  217         flags = tcp_outflags[tp->t_state];
  218         /*
  219          * Send any SACK-generated retransmissions.  If we're explicitly trying
  220          * to send out new data (when sendalot is 1), bypass this function.
  221          * If we retransmit in fast recovery mode, decrement snd_cwnd, since
  222          * we're replacing a (future) new transmission with a retransmission
  223          * now, and we previously incremented snd_cwnd in tcp_input().
  224          */
  225         /*
  226          * Still in sack recovery , reset rxmit flag to zero.
  227          */
  228         sack_rxmit = 0;
  229         sack_bytes_rxmt = 0;
  230         len = 0;
  231         p = NULL;
  232         if ((tp->t_flags & TF_SACK_PERMIT) && IN_FASTRECOVERY(tp) &&
  233             (p = tcp_sack_output(tp, &sack_bytes_rxmt))) {
  234                 long cwin;
  235                 
  236                 cwin = min(tp->snd_wnd, tp->snd_cwnd) - sack_bytes_rxmt;
  237                 if (cwin < 0)
  238                         cwin = 0;
  239                 /* Do not retransmit SACK segments beyond snd_recover */
  240                 if (SEQ_GT(p->end, tp->snd_recover)) {
  241                         /*
  242                          * (At least) part of sack hole extends beyond
  243                          * snd_recover. Check to see if we can rexmit data
  244                          * for this hole.
  245                          */
  246                         if (SEQ_GEQ(p->rxmit, tp->snd_recover)) {
  247                                 /*
  248                                  * Can't rexmit any more data for this hole.
  249                                  * That data will be rexmitted in the next
  250                                  * sack recovery episode, when snd_recover
  251                                  * moves past p->rxmit.
  252                                  */
  253                                 p = NULL;
  254                                 goto after_sack_rexmit;
  255                         } else
  256                                 /* Can rexmit part of the current hole */
  257                                 len = ((long)ulmin(cwin,
  258                                                    tp->snd_recover - p->rxmit));
  259                 } else
  260                         len = ((long)ulmin(cwin, p->end - p->rxmit));
  261                 off = p->rxmit - tp->snd_una;
  262                 KASSERT(off >= 0,("%s: sack block to the left of una : %d",
  263                     __func__, off));
  264                 if (len > 0) {
  265                         sack_rxmit = 1;
  266                         sendalot = 1;
  267                         tcpstat.tcps_sack_rexmits++;
  268                         tcpstat.tcps_sack_rexmit_bytes +=
  269                             min(len, tp->t_maxseg);
  270                 }
  271         }
  272 after_sack_rexmit:
  273         /*
  274          * Get standard flags, and add SYN or FIN if requested by 'hidden'
  275          * state flags.
  276          */
  277         if (tp->t_flags & TF_NEEDFIN)
  278                 flags |= TH_FIN;
  279         if (tp->t_flags & TF_NEEDSYN)
  280                 flags |= TH_SYN;
  281 
  282         SOCKBUF_LOCK(&so->so_snd);
  283         /*
  284          * If in persist timeout with window of 0, send 1 byte.
  285          * Otherwise, if window is small but nonzero
  286          * and timer expired, we will send what we can
  287          * and go to transmit state.
  288          */
  289         if (tp->t_flags & TF_FORCEDATA) {
  290                 if (sendwin == 0) {
  291                         /*
  292                          * If we still have some data to send, then
  293                          * clear the FIN bit.  Usually this would
  294                          * happen below when it realizes that we
  295                          * aren't sending all the data.  However,
  296                          * if we have exactly 1 byte of unsent data,
  297                          * then it won't clear the FIN bit below,
  298                          * and if we are in persist state, we wind
  299                          * up sending the packet without recording
  300                          * that we sent the FIN bit.
  301                          *
  302                          * We can't just blindly clear the FIN bit,
  303                          * because if we don't have any more data
  304                          * to send then the probe will be the FIN
  305                          * itself.
  306                          */
  307                         if (off < so->so_snd.sb_cc)
  308                                 flags &= ~TH_FIN;
  309                         sendwin = 1;
  310                 } else {
  311                         tcp_timer_activate(tp, TT_PERSIST, 0);
  312                         tp->t_rxtshift = 0;
  313                 }
  314         }
  315 
  316         /*
  317          * If snd_nxt == snd_max and we have transmitted a FIN, the
  318          * offset will be > 0 even if so_snd.sb_cc is 0, resulting in
  319          * a negative length.  This can also occur when TCP opens up
  320          * its congestion window while receiving additional duplicate
  321          * acks after fast-retransmit because TCP will reset snd_nxt
  322          * to snd_max after the fast-retransmit.
  323          *
  324          * In the normal retransmit-FIN-only case, however, snd_nxt will
  325          * be set to snd_una, the offset will be 0, and the length may
  326          * wind up 0.
  327          *
  328          * If sack_rxmit is true we are retransmitting from the scoreboard
  329          * in which case len is already set.
  330          */
  331         if (sack_rxmit == 0) {
  332                 if (sack_bytes_rxmt == 0)
  333                         len = ((long)ulmin(so->so_snd.sb_cc, sendwin) - off);
  334                 else {
  335                         long cwin;
  336 
  337                         /*
  338                          * We are inside of a SACK recovery episode and are
  339                          * sending new data, having retransmitted all the
  340                          * data possible in the scoreboard.
  341                          */
  342                         len = ((long)ulmin(so->so_snd.sb_cc, tp->snd_wnd) 
  343                                - off);
  344                         /*
  345                          * Don't remove this (len > 0) check !
  346                          * We explicitly check for len > 0 here (although it 
  347                          * isn't really necessary), to work around a gcc 
  348                          * optimization issue - to force gcc to compute
  349                          * len above. Without this check, the computation
  350                          * of len is bungled by the optimizer.
  351                          */
  352                         if (len > 0) {
  353                                 cwin = tp->snd_cwnd - 
  354                                         (tp->snd_nxt - tp->sack_newdata) -
  355                                         sack_bytes_rxmt;
  356                                 if (cwin < 0)
  357                                         cwin = 0;
  358                                 len = lmin(len, cwin);
  359                         }
  360                 }
  361         }
  362 
  363         /*
  364          * Lop off SYN bit if it has already been sent.  However, if this
  365          * is SYN-SENT state and if segment contains data and if we don't
  366          * know that foreign host supports TAO, suppress sending segment.
  367          */
  368         if ((flags & TH_SYN) && SEQ_GT(tp->snd_nxt, tp->snd_una)) {
  369                 if (tp->t_state != TCPS_SYN_RECEIVED)
  370                         flags &= ~TH_SYN;
  371                 off--, len++;
  372         }
  373 
  374         /*
  375          * Be careful not to send data and/or FIN on SYN segments.
  376          * This measure is needed to prevent interoperability problems
  377          * with not fully conformant TCP implementations.
  378          */
  379         if ((flags & TH_SYN) && (tp->t_flags & TF_NOOPT)) {
  380                 len = 0;
  381                 flags &= ~TH_FIN;
  382         }
  383 
  384         if (len < 0) {
  385                 /*
  386                  * If FIN has been sent but not acked,
  387                  * but we haven't been called to retransmit,
  388                  * len will be < 0.  Otherwise, window shrank
  389                  * after we sent into it.  If window shrank to 0,
  390                  * cancel pending retransmit, pull snd_nxt back
  391                  * to (closed) window, and set the persist timer
  392                  * if it isn't already going.  If the window didn't
  393                  * close completely, just wait for an ACK.
  394                  */
  395                 len = 0;
  396                 if (sendwin == 0) {
  397                         tcp_timer_activate(tp, TT_REXMT, 0);
  398                         tp->t_rxtshift = 0;
  399                         tp->snd_nxt = tp->snd_una;
  400                         if (!tcp_timer_active(tp, TT_PERSIST))
  401                                 tcp_setpersist(tp);
  402                 }
  403         }
  404 
  405         /* len will be >= 0 after this point. */
  406         KASSERT(len >= 0, ("[%s:%d]: len < 0", __func__, __LINE__));
  407 
  408         /*
  409          * Automatic sizing of send socket buffer.  Often the send buffer
  410          * size is not optimally adjusted to the actual network conditions
  411          * at hand (delay bandwidth product).  Setting the buffer size too
  412          * small limits throughput on links with high bandwidth and high
  413          * delay (eg. trans-continental/oceanic links).  Setting the
  414          * buffer size too big consumes too much real kernel memory,
  415          * especially with many connections on busy servers.
  416          *
  417          * The criteria to step up the send buffer one notch are:
  418          *  1. receive window of remote host is larger than send buffer
  419          *     (with a fudge factor of 5/4th);
  420          *  2. send buffer is filled to 7/8th with data (so we actually
  421          *     have data to make use of it);
  422          *  3. send buffer fill has not hit maximal automatic size;
  423          *  4. our send window (slow start and cogestion controlled) is
  424          *     larger than sent but unacknowledged data in send buffer.
  425          *
  426          * The remote host receive window scaling factor may limit the
  427          * growing of the send buffer before it reaches its allowed
  428          * maximum.
  429          *
  430          * It scales directly with slow start or congestion window
  431          * and does at most one step per received ACK.  This fast
  432          * scaling has the drawback of growing the send buffer beyond
  433          * what is strictly necessary to make full use of a given
  434          * delay*bandwith product.  However testing has shown this not
  435          * to be much of an problem.  At worst we are trading wasting
  436          * of available bandwith (the non-use of it) for wasting some
  437          * socket buffer memory.
  438          *
  439          * TODO: Shrink send buffer during idle periods together
  440          * with congestion window.  Requires another timer.  Has to
  441          * wait for upcoming tcp timer rewrite.
  442          */
  443         if (tcp_do_autosndbuf && so->so_snd.sb_flags & SB_AUTOSIZE) {
  444                 if ((tp->snd_wnd / 4 * 5) >= so->so_snd.sb_hiwat &&
  445                     so->so_snd.sb_cc >= (so->so_snd.sb_hiwat / 8 * 7) &&
  446                     so->so_snd.sb_cc < tcp_autosndbuf_max &&
  447                     sendwin >= (so->so_snd.sb_cc - (tp->snd_nxt - tp->snd_una))) {
  448                         if (!sbreserve_locked(&so->so_snd,
  449                             min(so->so_snd.sb_hiwat + tcp_autosndbuf_inc,
  450                              tcp_autosndbuf_max), so, curthread))
  451                                 so->so_snd.sb_flags &= ~SB_AUTOSIZE;
  452                 }
  453         }
  454 
  455         /*
  456          * Truncate to the maximum segment length or enable TCP Segmentation
  457          * Offloading (if supported by hardware) and ensure that FIN is removed
  458          * if the length no longer contains the last data byte.
  459          *
  460          * TSO may only be used if we are in a pure bulk sending state.  The
  461          * presence of TCP-MD5, SACK retransmits, SACK advertizements and
  462          * IP options prevent using TSO.  With TSO the TCP header is the same
  463          * (except for the sequence number) for all generated packets.  This
  464          * makes it impossible to transmit any options which vary per generated
  465          * segment or packet.
  466          *
  467          * The length of TSO bursts is limited to TCP_MAXWIN.  That limit and
  468          * removal of FIN (if not already catched here) are handled later after
  469          * the exact length of the TCP options are known.
  470          */
  471 #ifdef IPSEC
  472         /*
  473          * Pre-calculate here as we save another lookup into the darknesses
  474          * of IPsec that way and can actually decide if TSO is ok.
  475          */
  476         ipsec_optlen = ipsec_hdrsiz_tcp(tp);
  477 #endif
  478         if (len > tp->t_maxseg) {
  479                 if ((tp->t_flags & TF_TSO) && tcp_do_tso &&
  480                     ((tp->t_flags & TF_SIGNATURE) == 0) &&
  481                     tp->rcv_numsacks == 0 && sack_rxmit == 0 &&
  482                     tp->t_inpcb->inp_options == NULL &&
  483                     tp->t_inpcb->in6p_options == NULL
  484 #ifdef IPSEC
  485                     && ipsec_optlen == 0
  486 #endif
  487                     ) {
  488                         tso = 1;
  489                 } else {
  490                         len = tp->t_maxseg;
  491                         sendalot = 1;
  492                 }
  493         }
  494 
  495         if (sack_rxmit) {
  496                 if (SEQ_LT(p->rxmit + len, tp->snd_una + so->so_snd.sb_cc))
  497                         flags &= ~TH_FIN;
  498         } else {
  499                 if (SEQ_LT(tp->snd_nxt + len, tp->snd_una + so->so_snd.sb_cc))
  500                         flags &= ~TH_FIN;
  501         }
  502 
  503         recwin = sbspace(&so->so_rcv);
  504 
  505         /*
  506          * Sender silly window avoidance.   We transmit under the following
  507          * conditions when len is non-zero:
  508          *
  509          *      - We have a full segment (or more with TSO)
  510          *      - This is the last buffer in a write()/send() and we are
  511          *        either idle or running NODELAY
  512          *      - we've timed out (e.g. persist timer)
  513          *      - we have more then 1/2 the maximum send window's worth of
  514          *        data (receiver may be limited the window size)
  515          *      - we need to retransmit
  516          */
  517         if (len) {
  518                 if (len >= tp->t_maxseg)
  519                         goto send;
  520                 /*
  521                  * NOTE! on localhost connections an 'ack' from the remote
  522                  * end may occur synchronously with the output and cause
  523                  * us to flush a buffer queued with moretocome.  XXX
  524                  *
  525                  * note: the len + off check is almost certainly unnecessary.
  526                  */
  527                 if (!(tp->t_flags & TF_MORETOCOME) &&   /* normal case */
  528                     (idle || (tp->t_flags & TF_NODELAY)) &&
  529                     len + off >= so->so_snd.sb_cc &&
  530                     (tp->t_flags & TF_NOPUSH) == 0) {
  531                         goto send;
  532                 }
  533                 if (tp->t_flags & TF_FORCEDATA)         /* typ. timeout case */
  534                         goto send;
  535                 if (len >= tp->max_sndwnd / 2 && tp->max_sndwnd > 0)
  536                         goto send;
  537                 if (SEQ_LT(tp->snd_nxt, tp->snd_max))   /* retransmit case */
  538                         goto send;
  539                 if (sack_rxmit)
  540                         goto send;
  541         }
  542 
  543         /*
  544          * Compare available window to amount of window
  545          * known to peer (as advertised window less
  546          * next expected input).  If the difference is at least two
  547          * max size segments, or at least 50% of the maximum possible
  548          * window, then want to send a window update to peer.
  549          * Skip this if the connection is in T/TCP half-open state.
  550          * Don't send pure window updates when the peer has closed
  551          * the connection and won't ever send more data.
  552          */
  553         if (recwin > 0 && !(tp->t_flags & TF_NEEDSYN) &&
  554             !TCPS_HAVERCVDFIN(tp->t_state)) {
  555                 /*
  556                  * "adv" is the amount we can increase the window,
  557                  * taking into account that we are limited by
  558                  * TCP_MAXWIN << tp->rcv_scale.
  559                  */
  560                 long adv = min(recwin, (long)TCP_MAXWIN << tp->rcv_scale) -
  561                         (tp->rcv_adv - tp->rcv_nxt);
  562 
  563                 if (adv >= (long) (2 * tp->t_maxseg))
  564                         goto send;
  565                 if (2 * adv >= (long) so->so_rcv.sb_hiwat)
  566                         goto send;
  567         }
  568 
  569         /*
  570          * Send if we owe the peer an ACK, RST, SYN, or urgent data.  ACKNOW
  571          * is also a catch-all for the retransmit timer timeout case.
  572          */
  573         if (tp->t_flags & TF_ACKNOW)
  574                 goto send;
  575         if ((flags & TH_RST) ||
  576             ((flags & TH_SYN) && (tp->t_flags & TF_NEEDSYN) == 0))
  577                 goto send;
  578         if (SEQ_GT(tp->snd_up, tp->snd_una))
  579                 goto send;
  580         /*
  581          * If our state indicates that FIN should be sent
  582          * and we have not yet done so, then we need to send.
  583          */
  584         if (flags & TH_FIN &&
  585             ((tp->t_flags & TF_SENTFIN) == 0 || tp->snd_nxt == tp->snd_una))
  586                 goto send;
  587         /*
  588          * In SACK, it is possible for tcp_output to fail to send a segment
  589          * after the retransmission timer has been turned off.  Make sure
  590          * that the retransmission timer is set.
  591          */
  592         if ((tp->t_flags & TF_SACK_PERMIT) &&
  593             SEQ_GT(tp->snd_max, tp->snd_una) &&
  594             !tcp_timer_active(tp, TT_REXMT) &&
  595             !tcp_timer_active(tp, TT_PERSIST)) {
  596                 tcp_timer_activate(tp, TT_REXMT, tp->t_rxtcur);
  597                 goto just_return;
  598         } 
  599         /*
  600          * TCP window updates are not reliable, rather a polling protocol
  601          * using ``persist'' packets is used to insure receipt of window
  602          * updates.  The three ``states'' for the output side are:
  603          *      idle                    not doing retransmits or persists
  604          *      persisting              to move a small or zero window
  605          *      (re)transmitting        and thereby not persisting
  606          *
  607          * tcp_timer_active(tp, TT_PERSIST)
  608          *      is true when we are in persist state.
  609          * (tp->t_flags & TF_FORCEDATA)
  610          *      is set when we are called to send a persist packet.
  611          * tcp_timer_active(tp, TT_REXMT)
  612          *      is set when we are retransmitting
  613          * The output side is idle when both timers are zero.
  614          *
  615          * If send window is too small, there is data to transmit, and no
  616          * retransmit or persist is pending, then go to persist state.
  617          * If nothing happens soon, send when timer expires:
  618          * if window is nonzero, transmit what we can,
  619          * otherwise force out a byte.
  620          */
  621         if (so->so_snd.sb_cc && !tcp_timer_active(tp, TT_REXMT) &&
  622             !tcp_timer_active(tp, TT_PERSIST)) {
  623                 tp->t_rxtshift = 0;
  624                 tcp_setpersist(tp);
  625         }
  626 
  627         /*
  628          * No reason to send a segment, just return.
  629          */
  630 just_return:
  631         SOCKBUF_UNLOCK(&so->so_snd);
  632         return (0);
  633 
  634 send:
  635         SOCKBUF_LOCK_ASSERT(&so->so_snd);
  636         /*
  637          * Before ESTABLISHED, force sending of initial options
  638          * unless TCP set not to do any options.
  639          * NOTE: we assume that the IP/TCP header plus TCP options
  640          * always fit in a single mbuf, leaving room for a maximum
  641          * link header, i.e.
  642          *      max_linkhdr + sizeof (struct tcpiphdr) + optlen <= MCLBYTES
  643          */
  644         optlen = 0;
  645 #ifdef INET6
  646         if (isipv6)
  647                 hdrlen = sizeof (struct ip6_hdr) + sizeof (struct tcphdr);
  648         else
  649 #endif
  650         hdrlen = sizeof (struct tcpiphdr);
  651 
  652         /*
  653          * Compute options for segment.
  654          * We only have to care about SYN and established connection
  655          * segments.  Options for SYN-ACK segments are handled in TCP
  656          * syncache.
  657          */
  658         if ((tp->t_flags & TF_NOOPT) == 0) {
  659                 to.to_flags = 0;
  660                 /* Maximum segment size. */
  661                 if (flags & TH_SYN) {
  662                         tp->snd_nxt = tp->iss;
  663                         to.to_mss = tcp_mssopt(&tp->t_inpcb->inp_inc);
  664                         to.to_flags |= TOF_MSS;
  665                 }
  666                 /* Window scaling. */
  667                 if ((flags & TH_SYN) && (tp->t_flags & TF_REQ_SCALE)) {
  668                         to.to_wscale = tp->request_r_scale;
  669                         to.to_flags |= TOF_SCALE;
  670                 }
  671                 /* Timestamps. */
  672                 if ((tp->t_flags & TF_RCVD_TSTMP) ||
  673                     ((flags & TH_SYN) && (tp->t_flags & TF_REQ_TSTMP))) {
  674                         to.to_tsval = ticks + tp->ts_offset;
  675                         to.to_tsecr = tp->ts_recent;
  676                         to.to_flags |= TOF_TS;
  677                         /* Set receive buffer autosizing timestamp. */
  678                         if (tp->rfbuf_ts == 0 &&
  679                             (so->so_rcv.sb_flags & SB_AUTOSIZE))
  680                                 tp->rfbuf_ts = ticks;
  681                 }
  682                 /* Selective ACK's. */
  683                 if (tp->t_flags & TF_SACK_PERMIT) {
  684                         if (flags & TH_SYN)
  685                                 to.to_flags |= TOF_SACKPERM;
  686                         else if (TCPS_HAVEESTABLISHED(tp->t_state) &&
  687                             (tp->t_flags & TF_SACK_PERMIT) &&
  688                             tp->rcv_numsacks > 0) {
  689                                 to.to_flags |= TOF_SACK;
  690                                 to.to_nsacks = tp->rcv_numsacks;
  691                                 to.to_sacks = (u_char *)tp->sackblks;
  692                         }
  693                 }
  694 #ifdef TCP_SIGNATURE
  695                 /* TCP-MD5 (RFC2385). */
  696                 if (tp->t_flags & TF_SIGNATURE)
  697                         to.to_flags |= TOF_SIGNATURE;
  698 #endif /* TCP_SIGNATURE */
  699 
  700                 /* Processing the options. */
  701                 hdrlen += optlen = tcp_addoptions(&to, opt);
  702         }
  703 
  704 #ifdef INET6
  705         if (isipv6)
  706                 ipoptlen = ip6_optlen(tp->t_inpcb);
  707         else
  708 #endif
  709         if (tp->t_inpcb->inp_options)
  710                 ipoptlen = tp->t_inpcb->inp_options->m_len -
  711                                 offsetof(struct ipoption, ipopt_list);
  712         else
  713                 ipoptlen = 0;
  714 #ifdef IPSEC
  715         ipoptlen += ipsec_optlen;
  716 #endif
  717 
  718         /*
  719          * Adjust data length if insertion of options will
  720          * bump the packet length beyond the t_maxopd length.
  721          * Clear the FIN bit because we cut off the tail of
  722          * the segment.
  723          *
  724          * When doing TSO limit a burst to TCP_MAXWIN minus the
  725          * IP, TCP and Options length to keep ip->ip_len from
  726          * overflowing.  Prevent the last segment from being
  727          * fractional thus making them all equal sized and set
  728          * the flag to continue sending.  TSO is disabled when
  729          * IP options or IPSEC are present.
  730          */
  731         if (len + optlen + ipoptlen > tp->t_maxopd) {
  732                 flags &= ~TH_FIN;
  733                 if (tso) {
  734                         if (len > TCP_MAXWIN - hdrlen - optlen) {
  735                                 len = TCP_MAXWIN - hdrlen - optlen;
  736                                 len = len - (len % (tp->t_maxopd - optlen));
  737                                 sendalot = 1;
  738                         } else if (tp->t_flags & TF_NEEDFIN)
  739                                 sendalot = 1;
  740                 } else {
  741                         len = tp->t_maxopd - optlen - ipoptlen;
  742                         sendalot = 1;
  743                 }
  744         }
  745 
  746 /*#ifdef DIAGNOSTIC*/
  747 #ifdef INET6
  748         if (max_linkhdr + hdrlen > MCLBYTES)
  749 #else
  750         if (max_linkhdr + hdrlen > MHLEN)
  751 #endif
  752                 panic("tcphdr too big");
  753 /*#endif*/
  754 
  755         /*
  756          * This KASSERT is here to catch edge cases at a well defined place.
  757          * Before, those had triggered (random) panic conditions further down.
  758          */
  759         KASSERT(len >= 0, ("[%s:%d]: len < 0", __func__, __LINE__));
  760 
  761         /*
  762          * Grab a header mbuf, attaching a copy of data to
  763          * be transmitted, and initialize the header from
  764          * the template for sends on this connection.
  765          */
  766         if (len) {
  767                 struct mbuf *mb;
  768                 u_int moff;
  769 
  770                 if ((tp->t_flags & TF_FORCEDATA) && len == 1)
  771                         tcpstat.tcps_sndprobe++;
  772                 else if (SEQ_LT(tp->snd_nxt, tp->snd_max) || sack_rxmit) {
  773                         tcpstat.tcps_sndrexmitpack++;
  774                         tcpstat.tcps_sndrexmitbyte += len;
  775                 } else {
  776                         tcpstat.tcps_sndpack++;
  777                         tcpstat.tcps_sndbyte += len;
  778                 }
  779 #ifdef notyet
  780                 if ((m = m_copypack(so->so_snd.sb_mb, off,
  781                     (int)len, max_linkhdr + hdrlen)) == 0) {
  782                         SOCKBUF_UNLOCK(&so->so_snd);
  783                         error = ENOBUFS;
  784                         goto out;
  785                 }
  786                 /*
  787                  * m_copypack left space for our hdr; use it.
  788                  */
  789                 m->m_len += hdrlen;
  790                 m->m_data -= hdrlen;
  791 #else
  792                 MGETHDR(m, M_DONTWAIT, MT_DATA);
  793                 if (m == NULL) {
  794                         SOCKBUF_UNLOCK(&so->so_snd);
  795                         error = ENOBUFS;
  796                         goto out;
  797                 }
  798 #ifdef INET6
  799                 if (MHLEN < hdrlen + max_linkhdr) {
  800                         MCLGET(m, M_DONTWAIT);
  801                         if ((m->m_flags & M_EXT) == 0) {
  802                                 SOCKBUF_UNLOCK(&so->so_snd);
  803                                 m_freem(m);
  804                                 error = ENOBUFS;
  805                                 goto out;
  806                         }
  807                 }
  808 #endif
  809                 m->m_data += max_linkhdr;
  810                 m->m_len = hdrlen;
  811 
  812                 /*
  813                  * Start the m_copy functions from the closest mbuf
  814                  * to the offset in the socket buffer chain.
  815                  */
  816                 mb = sbsndptr(&so->so_snd, off, len, &moff);
  817 
  818                 if (len <= MHLEN - hdrlen - max_linkhdr) {
  819                         m_copydata(mb, moff, (int)len,
  820                             mtod(m, caddr_t) + hdrlen);
  821                         m->m_len += len;
  822                 } else {
  823                         m->m_next = m_copy(mb, moff, (int)len);
  824                         if (m->m_next == NULL) {
  825                                 SOCKBUF_UNLOCK(&so->so_snd);
  826                                 (void) m_free(m);
  827                                 error = ENOBUFS;
  828                                 goto out;
  829                         }
  830                 }
  831 #endif
  832                 /*
  833                  * If we're sending everything we've got, set PUSH.
  834                  * (This will keep happy those implementations which only
  835                  * give data to the user when a buffer fills or
  836                  * a PUSH comes in.)
  837                  */
  838                 if (off + len == so->so_snd.sb_cc)
  839                         flags |= TH_PUSH;
  840                 SOCKBUF_UNLOCK(&so->so_snd);
  841         } else {
  842                 SOCKBUF_UNLOCK(&so->so_snd);
  843                 if (tp->t_flags & TF_ACKNOW)
  844                         tcpstat.tcps_sndacks++;
  845                 else if (flags & (TH_SYN|TH_FIN|TH_RST))
  846                         tcpstat.tcps_sndctrl++;
  847                 else if (SEQ_GT(tp->snd_up, tp->snd_una))
  848                         tcpstat.tcps_sndurg++;
  849                 else
  850                         tcpstat.tcps_sndwinup++;
  851 
  852                 MGETHDR(m, M_DONTWAIT, MT_DATA);
  853                 if (m == NULL) {
  854                         error = ENOBUFS;
  855                         goto out;
  856                 }
  857 #ifdef INET6
  858                 if (isipv6 && (MHLEN < hdrlen + max_linkhdr) &&
  859                     MHLEN >= hdrlen) {
  860                         MH_ALIGN(m, hdrlen);
  861                 } else
  862 #endif
  863                 m->m_data += max_linkhdr;
  864                 m->m_len = hdrlen;
  865         }
  866         SOCKBUF_UNLOCK_ASSERT(&so->so_snd);
  867         m->m_pkthdr.rcvif = (struct ifnet *)0;
  868 #ifdef MAC
  869         mac_create_mbuf_from_inpcb(tp->t_inpcb, m);
  870 #endif
  871 #ifdef INET6
  872         if (isipv6) {
  873                 ip6 = mtod(m, struct ip6_hdr *);
  874                 th = (struct tcphdr *)(ip6 + 1);
  875                 tcpip_fillheaders(tp->t_inpcb, ip6, th);
  876         } else
  877 #endif /* INET6 */
  878         {
  879                 ip = mtod(m, struct ip *);
  880                 ipov = (struct ipovly *)ip;
  881                 th = (struct tcphdr *)(ip + 1);
  882                 tcpip_fillheaders(tp->t_inpcb, ip, th);
  883         }
  884 
  885         /*
  886          * Fill in fields, remembering maximum advertised
  887          * window for use in delaying messages about window sizes.
  888          * If resending a FIN, be sure not to use a new sequence number.
  889          */
  890         if (flags & TH_FIN && tp->t_flags & TF_SENTFIN &&
  891             tp->snd_nxt == tp->snd_max)
  892                 tp->snd_nxt--;
  893         /*
  894          * If we are doing retransmissions, then snd_nxt will
  895          * not reflect the first unsent octet.  For ACK only
  896          * packets, we do not want the sequence number of the
  897          * retransmitted packet, we want the sequence number
  898          * of the next unsent octet.  So, if there is no data
  899          * (and no SYN or FIN), use snd_max instead of snd_nxt
  900          * when filling in ti_seq.  But if we are in persist
  901          * state, snd_max might reflect one byte beyond the
  902          * right edge of the window, so use snd_nxt in that
  903          * case, since we know we aren't doing a retransmission.
  904          * (retransmit and persist are mutually exclusive...)
  905          */
  906         if (sack_rxmit == 0) {
  907                 if (len || (flags & (TH_SYN|TH_FIN)) ||
  908                     tcp_timer_active(tp, TT_PERSIST))
  909                         th->th_seq = htonl(tp->snd_nxt);
  910                 else
  911                         th->th_seq = htonl(tp->snd_max);
  912         } else {
  913                 th->th_seq = htonl(p->rxmit);
  914                 p->rxmit += len;
  915                 tp->sackhint.sack_bytes_rexmit += len;
  916         }
  917         th->th_ack = htonl(tp->rcv_nxt);
  918         if (optlen) {
  919                 bcopy(opt, th + 1, optlen);
  920                 th->th_off = (sizeof (struct tcphdr) + optlen) >> 2;
  921         }
  922         th->th_flags = flags;
  923         /*
  924          * Calculate receive window.  Don't shrink window,
  925          * but avoid silly window syndrome.
  926          */
  927         if (recwin < (long)(so->so_rcv.sb_hiwat / 4) &&
  928             recwin < (long)tp->t_maxseg)
  929                 recwin = 0;
  930         if (recwin < (long)(tp->rcv_adv - tp->rcv_nxt))
  931                 recwin = (long)(tp->rcv_adv - tp->rcv_nxt);
  932         if (recwin > (long)TCP_MAXWIN << tp->rcv_scale)
  933                 recwin = (long)TCP_MAXWIN << tp->rcv_scale;
  934 
  935         /*
  936          * According to RFC1323 the window field in a SYN (i.e., a <SYN>
  937          * or <SYN,ACK>) segment itself is never scaled.  The <SYN,ACK>
  938          * case is handled in syncache.
  939          */
  940         if (flags & TH_SYN)
  941                 th->th_win = htons((u_short)
  942                                 (min(sbspace(&so->so_rcv), TCP_MAXWIN)));
  943         else
  944                 th->th_win = htons((u_short)(recwin >> tp->rcv_scale));
  945 
  946         /*
  947          * Adjust the RXWIN0SENT flag - indicate that we have advertised
  948          * a 0 window.  This may cause the remote transmitter to stall.  This
  949          * flag tells soreceive() to disable delayed acknowledgements when
  950          * draining the buffer.  This can occur if the receiver is attempting
  951          * to read more data then can be buffered prior to transmitting on
  952          * the connection.
  953          */
  954         if (th->th_win == 0)
  955                 tp->t_flags |= TF_RXWIN0SENT;
  956         else
  957                 tp->t_flags &= ~TF_RXWIN0SENT;
  958         if (SEQ_GT(tp->snd_up, tp->snd_nxt)) {
  959                 th->th_urp = htons((u_short)(tp->snd_up - tp->snd_nxt));
  960                 th->th_flags |= TH_URG;
  961         } else
  962                 /*
  963                  * If no urgent pointer to send, then we pull
  964                  * the urgent pointer to the left edge of the send window
  965                  * so that it doesn't drift into the send window on sequence
  966                  * number wraparound.
  967                  */
  968                 tp->snd_up = tp->snd_una;               /* drag it along */
  969 
  970 #ifdef TCP_SIGNATURE
  971         if (tp->t_flags & TF_SIGNATURE) {
  972                 int sigoff = to.to_signature - opt;
  973                 tcp_signature_compute(m, 0, len, optlen,
  974                     (u_char *)(th + 1) + sigoff, IPSEC_DIR_OUTBOUND);
  975         }
  976 #endif
  977 
  978         /*
  979          * Put TCP length in extended header, and then
  980          * checksum extended header and data.
  981          */
  982         m->m_pkthdr.len = hdrlen + len; /* in6_cksum() need this */
  983 #ifdef INET6
  984         if (isipv6)
  985                 /*
  986                  * ip6_plen is not need to be filled now, and will be filled
  987                  * in ip6_output.
  988                  */
  989                 th->th_sum = in6_cksum(m, IPPROTO_TCP, sizeof(struct ip6_hdr),
  990                                        sizeof(struct tcphdr) + optlen + len);
  991         else
  992 #endif /* INET6 */
  993         {
  994                 m->m_pkthdr.csum_flags = CSUM_TCP;
  995                 m->m_pkthdr.csum_data = offsetof(struct tcphdr, th_sum);
  996                 th->th_sum = in_pseudo(ip->ip_src.s_addr, ip->ip_dst.s_addr,
  997                     htons(sizeof(struct tcphdr) + IPPROTO_TCP + len + optlen));
  998 
  999                 /* IP version must be set here for ipv4/ipv6 checking later */
 1000                 KASSERT(ip->ip_v == IPVERSION,
 1001                     ("%s: IP version incorrect: %d", __func__, ip->ip_v));
 1002         }
 1003 
 1004         /*
 1005          * Enable TSO and specify the size of the segments.
 1006          * The TCP pseudo header checksum is always provided.
 1007          * XXX: Fixme: This is currently not the case for IPv6.
 1008          */
 1009         if (tso) {
 1010                 KASSERT(len > tp->t_maxopd - optlen,
 1011                     ("%s: len <= tso_segsz", __func__));
 1012                 m->m_pkthdr.csum_flags |= CSUM_TSO;
 1013                 m->m_pkthdr.tso_segsz = tp->t_maxopd - optlen;
 1014         }
 1015 
 1016         /*
 1017          * In transmit state, time the transmission and arrange for
 1018          * the retransmit.  In persist state, just set snd_max.
 1019          */
 1020         if ((tp->t_flags & TF_FORCEDATA) == 0 || 
 1021             !tcp_timer_active(tp, TT_PERSIST)) {
 1022                 tcp_seq startseq = tp->snd_nxt;
 1023 
 1024                 /*
 1025                  * Advance snd_nxt over sequence space of this segment.
 1026                  */
 1027                 if (flags & (TH_SYN|TH_FIN)) {
 1028                         if (flags & TH_SYN)
 1029                                 tp->snd_nxt++;
 1030                         if (flags & TH_FIN) {
 1031                                 tp->snd_nxt++;
 1032                                 tp->t_flags |= TF_SENTFIN;
 1033                         }
 1034                 }
 1035                 if (sack_rxmit)
 1036                         goto timer;
 1037                 tp->snd_nxt += len;
 1038                 if (SEQ_GT(tp->snd_nxt, tp->snd_max)) {
 1039                         tp->snd_max = tp->snd_nxt;
 1040                         /*
 1041                          * Time this transmission if not a retransmission and
 1042                          * not currently timing anything.
 1043                          */
 1044                         if (tp->t_rtttime == 0) {
 1045                                 tp->t_rtttime = ticks;
 1046                                 tp->t_rtseq = startseq;
 1047                                 tcpstat.tcps_segstimed++;
 1048                         }
 1049                 }
 1050 
 1051                 /*
 1052                  * Set retransmit timer if not currently set,
 1053                  * and not doing a pure ack or a keep-alive probe.
 1054                  * Initial value for retransmit timer is smoothed
 1055                  * round-trip time + 2 * round-trip time variance.
 1056                  * Initialize shift counter which is used for backoff
 1057                  * of retransmit time.
 1058                  */
 1059 timer:
 1060                 if (!tcp_timer_active(tp, TT_REXMT) &&
 1061                     ((sack_rxmit && tp->snd_nxt != tp->snd_max) ||
 1062                      (tp->snd_nxt != tp->snd_una))) {
 1063                         if (tcp_timer_active(tp, TT_PERSIST)) {
 1064                                 tcp_timer_activate(tp, TT_PERSIST, 0);
 1065                                 tp->t_rxtshift = 0;
 1066                         }
 1067                         tcp_timer_activate(tp, TT_REXMT, tp->t_rxtcur);
 1068                 }
 1069         } else {
 1070                 /*
 1071                  * Persist case, update snd_max but since we are in
 1072                  * persist mode (no window) we do not update snd_nxt.
 1073                  */
 1074                 int xlen = len;
 1075                 if (flags & TH_SYN)
 1076                         ++xlen;
 1077                 if (flags & TH_FIN) {
 1078                         ++xlen;
 1079                         tp->t_flags |= TF_SENTFIN;
 1080                 }
 1081                 if (SEQ_GT(tp->snd_nxt + xlen, tp->snd_max))
 1082                         tp->snd_max = tp->snd_nxt + len;
 1083         }
 1084 
 1085 #ifdef TCPDEBUG
 1086         /*
 1087          * Trace.
 1088          */
 1089         if (so->so_options & SO_DEBUG) {
 1090                 u_short save = 0;
 1091 #ifdef INET6
 1092                 if (!isipv6)
 1093 #endif
 1094                 {
 1095                         save = ipov->ih_len;
 1096                         ipov->ih_len = htons(m->m_pkthdr.len /* - hdrlen + (th->th_off << 2) */);
 1097                 }
 1098                 tcp_trace(TA_OUTPUT, tp->t_state, tp, mtod(m, void *), th, 0);
 1099 #ifdef INET6
 1100                 if (!isipv6)
 1101 #endif
 1102                 ipov->ih_len = save;
 1103         }
 1104 #endif
 1105 
 1106         /*
 1107          * Fill in IP length and desired time to live and
 1108          * send to IP level.  There should be a better way
 1109          * to handle ttl and tos; we could keep them in
 1110          * the template, but need a way to checksum without them.
 1111          */
 1112         /*
 1113          * m->m_pkthdr.len should have been set before cksum calcuration,
 1114          * because in6_cksum() need it.
 1115          */
 1116 #ifdef INET6
 1117         if (isipv6) {
 1118                 /*
 1119                  * we separately set hoplimit for every segment, since the
 1120                  * user might want to change the value via setsockopt.
 1121                  * Also, desired default hop limit might be changed via
 1122                  * Neighbor Discovery.
 1123                  */
 1124                 ip6->ip6_hlim = in6_selecthlim(tp->t_inpcb, NULL);
 1125 
 1126                 /* TODO: IPv6 IP6TOS_ECT bit on */
 1127                 error = ip6_output(m,
 1128                             tp->t_inpcb->in6p_outputopts, NULL,
 1129                             ((so->so_options & SO_DONTROUTE) ?
 1130                             IP_ROUTETOIF : 0), NULL, NULL, tp->t_inpcb);
 1131         } else
 1132 #endif /* INET6 */
 1133     {
 1134         ip->ip_len = m->m_pkthdr.len;
 1135 #ifdef INET6
 1136         if (tp->t_inpcb->inp_vflag & INP_IPV6PROTO)
 1137                 ip->ip_ttl = in6_selecthlim(tp->t_inpcb, NULL);
 1138 #endif /* INET6 */
 1139         /*
 1140          * If we do path MTU discovery, then we set DF on every packet.
 1141          * This might not be the best thing to do according to RFC3390
 1142          * Section 2. However the tcp hostcache migitates the problem
 1143          * so it affects only the first tcp connection with a host.
 1144          *
 1145          * NB: Don't set DF on small MTU/MSS to have a safe fallback.
 1146          */
 1147         if (path_mtu_discovery && tp->t_maxopd > tcp_minmss)
 1148                 ip->ip_off |= IP_DF;
 1149 
 1150         error = ip_output(m, tp->t_inpcb->inp_options, NULL,
 1151             ((so->so_options & SO_DONTROUTE) ? IP_ROUTETOIF : 0), 0,
 1152             tp->t_inpcb);
 1153     }
 1154         if (error) {
 1155 
 1156                 /*
 1157                  * We know that the packet was lost, so back out the
 1158                  * sequence number advance, if any.
 1159                  *
 1160                  * If the error is EPERM the packet got blocked by the
 1161                  * local firewall.  Normally we should terminate the
 1162                  * connection but the blocking may have been spurious
 1163                  * due to a firewall reconfiguration cycle.  So we treat
 1164                  * it like a packet loss and let the retransmit timer and
 1165                  * timeouts do their work over time.
 1166                  * XXX: It is a POLA question whether calling tcp_drop right
 1167                  * away would be the really correct behavior instead.
 1168                  */
 1169                 if (((tp->t_flags & TF_FORCEDATA) == 0 ||
 1170                     !tcp_timer_active(tp, TT_PERSIST)) &&
 1171                     ((flags & TH_SYN) == 0) &&
 1172                     (error != EPERM)) {
 1173                         if (sack_rxmit) {
 1174                                 p->rxmit -= len;
 1175                                 tp->sackhint.sack_bytes_rexmit -= len;
 1176                                 KASSERT(tp->sackhint.sack_bytes_rexmit >= 0,
 1177                                     ("sackhint bytes rtx >= 0"));
 1178                         } else
 1179                                 tp->snd_nxt -= len;
 1180                 }
 1181 out:
 1182                 SOCKBUF_UNLOCK_ASSERT(&so->so_snd);     /* Check gotos. */
 1183                 switch (error) {
 1184                 case EPERM:
 1185                         tp->t_softerror = error;
 1186                         return (error);
 1187                 case ENOBUFS:
 1188                         if (!tcp_timer_active(tp, TT_REXMT) &&
 1189                             !tcp_timer_active(tp, TT_PERSIST))
 1190                                 tcp_timer_activate(tp, TT_REXMT, tp->t_rxtcur);
 1191                         tp->snd_cwnd = tp->t_maxseg;
 1192                         return (0);
 1193                 case EMSGSIZE:
 1194                         /*
 1195                          * For some reason the interface we used initially
 1196                          * to send segments changed to another or lowered
 1197                          * its MTU.
 1198                          *
 1199                          * tcp_mtudisc() will find out the new MTU and as
 1200                          * its last action, initiate retransmission, so it
 1201                          * is important to not do so here.
 1202                          *
 1203                          * If TSO was active we either got an interface
 1204                          * without TSO capabilits or TSO was turned off.
 1205                          * Disable it for this connection as too and
 1206                          * immediatly retry with MSS sized segments generated
 1207                          * by this function.
 1208                          */
 1209                         if (tso)
 1210                                 tp->t_flags &= ~TF_TSO;
 1211                         tcp_mtudisc(tp->t_inpcb, 0);
 1212                         return (0);
 1213                 case EHOSTDOWN:
 1214                 case EHOSTUNREACH:
 1215                 case ENETDOWN:
 1216                 case ENETUNREACH:
 1217                         if (TCPS_HAVERCVDSYN(tp->t_state)) {
 1218                                 tp->t_softerror = error;
 1219                                 return (0);
 1220                         }
 1221                         /* FALLTHROUGH */
 1222                 default:
 1223                         return (error);
 1224                 }
 1225         }
 1226         tcpstat.tcps_sndtotal++;
 1227 
 1228         /*
 1229          * Data sent (as far as we can tell).
 1230          * If this advertises a larger window than any other segment,
 1231          * then remember the size of the advertised window.
 1232          * Any pending ACK has now been sent.
 1233          */
 1234         if (recwin > 0 && SEQ_GT(tp->rcv_nxt + recwin, tp->rcv_adv))
 1235                 tp->rcv_adv = tp->rcv_nxt + recwin;
 1236         tp->last_ack_sent = tp->rcv_nxt;
 1237         tp->t_flags &= ~(TF_ACKNOW | TF_DELACK);
 1238         if (tcp_timer_active(tp, TT_DELACK))
 1239                 tcp_timer_activate(tp, TT_DELACK, 0);
 1240 #if 0
 1241         /*
 1242          * This completely breaks TCP if newreno is turned on.  What happens
 1243          * is that if delayed-acks are turned on on the receiver, this code
 1244          * on the transmitter effectively destroys the TCP window, forcing
 1245          * it to four packets (1.5Kx4 = 6K window).
 1246          */
 1247         if (sendalot && (!tcp_do_newreno || --maxburst))
 1248                 goto again;
 1249 #endif
 1250         if (sendalot)
 1251                 goto again;
 1252         return (0);
 1253 }
 1254 
 1255 void
 1256 tcp_setpersist(struct tcpcb *tp)
 1257 {
 1258         int t = ((tp->t_srtt >> 2) + tp->t_rttvar) >> 1;
 1259         int tt;
 1260 
 1261         if (tcp_timer_active(tp, TT_REXMT))
 1262                 panic("tcp_setpersist: retransmit pending");
 1263         /*
 1264          * Start/restart persistance timer.
 1265          */
 1266         TCPT_RANGESET(tt, t * tcp_backoff[tp->t_rxtshift],
 1267                       TCPTV_PERSMIN, TCPTV_PERSMAX);
 1268         tcp_timer_activate(tp, TT_PERSIST, tt);
 1269         if (tp->t_rxtshift < TCP_MAXRXTSHIFT)
 1270                 tp->t_rxtshift++;
 1271 }
 1272 
 1273 /*
 1274  * Insert TCP options according to the supplied parameters to the place
 1275  * optp in a consistent way.  Can handle unaligned destinations.
 1276  *
 1277  * The order of the option processing is crucial for optimal packing and
 1278  * alignment for the scarce option space.
 1279  *
 1280  * The optimal order for a SYN/SYN-ACK segment is:
 1281  *   MSS (4) + NOP (1) + Window scale (3) + SACK permitted (2) +
 1282  *   Timestamp (10) + Signature (18) = 38 bytes out of a maximum of 40.
 1283  *
 1284  * The SACK options should be last.  SACK blocks consume 8*n+2 bytes.
 1285  * So a full size SACK blocks option is 34 bytes (with 4 SACK blocks).
 1286  * At minimum we need 10 bytes (to generate 1 SACK block).  If both
 1287  * TCP Timestamps (12 bytes) and TCP Signatures (18 bytes) are present,
 1288  * we only have 10 bytes for SACK options (40 - (12 + 18)).
 1289  */
 1290 int
 1291 tcp_addoptions(struct tcpopt *to, u_char *optp)
 1292 {
 1293         u_int mask, optlen = 0;
 1294 
 1295         for (mask = 1; mask < TOF_MAXOPT; mask <<= 1) {
 1296                 if ((to->to_flags & mask) != mask)
 1297                         continue;
 1298                 if (optlen == TCP_MAXOLEN)
 1299                         break;
 1300                 switch (to->to_flags & mask) {
 1301                 case TOF_MSS:
 1302                         while (optlen % 4) {
 1303                                 optlen += TCPOLEN_NOP;
 1304                                 *optp++ = TCPOPT_NOP;
 1305                         }
 1306                         if (TCP_MAXOLEN - optlen < TCPOLEN_MAXSEG)
 1307                                 continue;
 1308                         optlen += TCPOLEN_MAXSEG;
 1309                         *optp++ = TCPOPT_MAXSEG;
 1310                         *optp++ = TCPOLEN_MAXSEG;
 1311                         to->to_mss = htons(to->to_mss);
 1312                         bcopy((u_char *)&to->to_mss, optp, sizeof(to->to_mss));
 1313                         optp += sizeof(to->to_mss);
 1314                         break;
 1315                 case TOF_SCALE:
 1316                         while (!optlen || optlen % 2 != 1) {
 1317                                 optlen += TCPOLEN_NOP;
 1318                                 *optp++ = TCPOPT_NOP;
 1319                         }
 1320                         if (TCP_MAXOLEN - optlen < TCPOLEN_WINDOW)
 1321                                 continue;
 1322                         optlen += TCPOLEN_WINDOW;
 1323                         *optp++ = TCPOPT_WINDOW;
 1324                         *optp++ = TCPOLEN_WINDOW;
 1325                         *optp++ = to->to_wscale;
 1326                         break;
 1327                 case TOF_SACKPERM:
 1328                         while (optlen % 2) {
 1329                                 optlen += TCPOLEN_NOP;
 1330                                 *optp++ = TCPOPT_NOP;
 1331                         }
 1332                         if (TCP_MAXOLEN - optlen < TCPOLEN_SACK_PERMITTED)
 1333                                 continue;
 1334                         optlen += TCPOLEN_SACK_PERMITTED;
 1335                         *optp++ = TCPOPT_SACK_PERMITTED;
 1336                         *optp++ = TCPOLEN_SACK_PERMITTED;
 1337                         break;
 1338                 case TOF_TS:
 1339                         while (!optlen || optlen % 4 != 2) {
 1340                                 optlen += TCPOLEN_NOP;
 1341                                 *optp++ = TCPOPT_NOP;
 1342                         }
 1343                         if (TCP_MAXOLEN - optlen < TCPOLEN_TIMESTAMP)
 1344                                 continue;
 1345                         optlen += TCPOLEN_TIMESTAMP;
 1346                         *optp++ = TCPOPT_TIMESTAMP;
 1347                         *optp++ = TCPOLEN_TIMESTAMP;
 1348                         to->to_tsval = htonl(to->to_tsval);
 1349                         to->to_tsecr = htonl(to->to_tsecr);
 1350                         bcopy((u_char *)&to->to_tsval, optp, sizeof(to->to_tsval));
 1351                         optp += sizeof(to->to_tsval);
 1352                         bcopy((u_char *)&to->to_tsecr, optp, sizeof(to->to_tsecr));
 1353                         optp += sizeof(to->to_tsecr);
 1354                         break;
 1355                 case TOF_SIGNATURE:
 1356                         {
 1357                         int siglen = TCPOLEN_SIGNATURE - 2;
 1358 
 1359                         while (!optlen || optlen % 4 != 2) {
 1360                                 optlen += TCPOLEN_NOP;
 1361                                 *optp++ = TCPOPT_NOP;
 1362                         }
 1363                         if (TCP_MAXOLEN - optlen < TCPOLEN_SIGNATURE)
 1364                                 continue;
 1365                         optlen += TCPOLEN_SIGNATURE;
 1366                         *optp++ = TCPOPT_SIGNATURE;
 1367                         *optp++ = TCPOLEN_SIGNATURE;
 1368                         to->to_signature = optp;
 1369                         while (siglen--)
 1370                                  *optp++ = 0;
 1371                         break;
 1372                         }
 1373                 case TOF_SACK:
 1374                         {
 1375                         int sackblks = 0;
 1376                         struct sackblk *sack = (struct sackblk *)to->to_sacks;
 1377                         tcp_seq sack_seq;
 1378 
 1379                         while (!optlen || optlen % 4 != 2) {
 1380                                 optlen += TCPOLEN_NOP;
 1381                                 *optp++ = TCPOPT_NOP;
 1382                         }
 1383                         if (TCP_MAXOLEN - optlen < TCPOLEN_SACKHDR + TCPOLEN_SACK)
 1384                                 continue;
 1385                         optlen += TCPOLEN_SACKHDR;
 1386                         *optp++ = TCPOPT_SACK;
 1387                         sackblks = min(to->to_nsacks,
 1388                                         (TCP_MAXOLEN - optlen) / TCPOLEN_SACK);
 1389                         *optp++ = TCPOLEN_SACKHDR + sackblks * TCPOLEN_SACK;
 1390                         while (sackblks--) {
 1391                                 sack_seq = htonl(sack->start);
 1392                                 bcopy((u_char *)&sack_seq, optp, sizeof(sack_seq));
 1393                                 optp += sizeof(sack_seq);
 1394                                 sack_seq = htonl(sack->end);
 1395                                 bcopy((u_char *)&sack_seq, optp, sizeof(sack_seq));
 1396                                 optp += sizeof(sack_seq);
 1397                                 optlen += TCPOLEN_SACK;
 1398                                 sack++;
 1399                         }
 1400                         tcpstat.tcps_sack_send_blocks++;
 1401                         break;
 1402                         }
 1403                 default:
 1404                         panic("%s: unknown TCP option type", __func__);
 1405                         break;
 1406                 }
 1407         }
 1408 
 1409         /* Terminate and pad TCP options to a 4 byte boundary. */
 1410         if (optlen % 4) {
 1411                 optlen += TCPOLEN_EOL;
 1412                 *optp++ = TCPOPT_EOL;
 1413         }
 1414         /*
 1415          * According to RFC 793 (STD0007):
 1416          *   "The content of the header beyond the End-of-Option option
 1417          *    must be header padding (i.e., zero)."
 1418          *   and later: "The padding is composed of zeros."
 1419          */
 1420         while (optlen % 4) {
 1421                 optlen += TCPOLEN_PAD;
 1422                 *optp++ = TCPOPT_PAD;
 1423         }
 1424 
 1425         KASSERT(optlen <= TCP_MAXOLEN, ("%s: TCP options too long", __func__));
 1426         return (optlen);
 1427 }

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