The Design and Implementation of the FreeBSD Operating System, Second Edition
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FreeBSD/Linux Kernel Cross Reference
sys/netinet/tcp_timer.c

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    1 /*      $NetBSD: tcp_timer.c,v 1.85 2011/04/20 13:35:52 gdt Exp $       */
    2 
    3 /*
    4  * Copyright (C) 1995, 1996, 1997, and 1998 WIDE Project.
    5  * All rights reserved.
    6  *
    7  * Redistribution and use in source and binary forms, with or without
    8  * modification, are permitted provided that the following conditions
    9  * are met:
   10  * 1. Redistributions of source code must retain the above copyright
   11  *    notice, this list of conditions and the following disclaimer.
   12  * 2. Redistributions in binary form must reproduce the above copyright
   13  *    notice, this list of conditions and the following disclaimer in the
   14  *    documentation and/or other materials provided with the distribution.
   15  * 3. Neither the name of the project nor the names of its contributors
   16  *    may be used to endorse or promote products derived from this software
   17  *    without specific prior written permission.
   18  *
   19  * THIS SOFTWARE IS PROVIDED BY THE PROJECT AND CONTRIBUTORS ``AS IS'' AND
   20  * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
   21  * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
   22  * ARE DISCLAIMED.  IN NO EVENT SHALL THE PROJECT OR CONTRIBUTORS BE LIABLE
   23  * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
   24  * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
   25  * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
   26  * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
   27  * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
   28  * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
   29  * SUCH DAMAGE.
   30  */
   31 
   32 /*-
   33  * Copyright (c) 1997, 1998, 2001, 2005 The NetBSD Foundation, Inc.
   34  * All rights reserved.
   35  *
   36  * This code is derived from software contributed to The NetBSD Foundation
   37  * by Jason R. Thorpe and Kevin M. Lahey of the Numerical Aerospace Simulation
   38  * Facility, NASA Ames Research Center.
   39  * This code is derived from software contributed to The NetBSD Foundation
   40  * by Charles M. Hannum.
   41  *
   42  * Redistribution and use in source and binary forms, with or without
   43  * modification, are permitted provided that the following conditions
   44  * are met:
   45  * 1. Redistributions of source code must retain the above copyright
   46  *    notice, this list of conditions and the following disclaimer.
   47  * 2. Redistributions in binary form must reproduce the above copyright
   48  *    notice, this list of conditions and the following disclaimer in the
   49  *    documentation and/or other materials provided with the distribution.
   50  *
   51  * THIS SOFTWARE IS PROVIDED BY THE NETBSD FOUNDATION, INC. AND CONTRIBUTORS
   52  * ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
   53  * TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
   54  * PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE FOUNDATION OR CONTRIBUTORS
   55  * BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
   56  * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
   57  * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
   58  * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
   59  * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
   60  * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
   61  * POSSIBILITY OF SUCH DAMAGE.
   62  */
   63 
   64 /*
   65  * Copyright (c) 1982, 1986, 1988, 1990, 1993, 1995
   66  *      The Regents of the University of California.  All rights reserved.
   67  *
   68  * Redistribution and use in source and binary forms, with or without
   69  * modification, are permitted provided that the following conditions
   70  * are met:
   71  * 1. Redistributions of source code must retain the above copyright
   72  *    notice, this list of conditions and the following disclaimer.
   73  * 2. Redistributions in binary form must reproduce the above copyright
   74  *    notice, this list of conditions and the following disclaimer in the
   75  *    documentation and/or other materials provided with the distribution.
   76  * 3. Neither the name of the University nor the names of its contributors
   77  *    may be used to endorse or promote products derived from this software
   78  *    without specific prior written permission.
   79  *
   80  * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
   81  * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
   82  * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
   83  * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
   84  * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
   85  * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
   86  * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
   87  * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
   88  * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
   89  * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
   90  * SUCH DAMAGE.
   91  *
   92  *      @(#)tcp_timer.c 8.2 (Berkeley) 5/24/95
   93  */
   94 
   95 #include <sys/cdefs.h>
   96 __KERNEL_RCSID(0, "$NetBSD: tcp_timer.c,v 1.85 2011/04/20 13:35:52 gdt Exp $");
   97 
   98 #include "opt_inet.h"
   99 #include "opt_tcp_debug.h"
  100 
  101 #include <sys/param.h>
  102 #include <sys/systm.h>
  103 #include <sys/malloc.h>
  104 #include <sys/mbuf.h>
  105 #include <sys/socket.h>
  106 #include <sys/socketvar.h>
  107 #include <sys/protosw.h>
  108 #include <sys/errno.h>
  109 #include <sys/kernel.h>
  110 
  111 #include <net/if.h>
  112 #include <net/route.h>
  113 
  114 #include <netinet/in.h>
  115 #include <netinet/in_systm.h>
  116 #include <netinet/ip.h>
  117 #include <netinet/in_pcb.h>
  118 #include <netinet/ip_var.h>
  119 #include <netinet/ip_icmp.h>
  120 
  121 #ifdef INET6
  122 #ifndef INET
  123 #include <netinet/in.h>
  124 #endif
  125 #include <netinet/ip6.h>
  126 #include <netinet6/in6_pcb.h>
  127 #endif
  128 
  129 #include <netinet/tcp.h>
  130 #include <netinet/tcp_fsm.h>
  131 #include <netinet/tcp_seq.h>
  132 #include <netinet/tcp_timer.h>
  133 #include <netinet/tcp_var.h>
  134 #include <netinet/tcp_private.h>
  135 #include <netinet/tcp_congctl.h>
  136 #include <netinet/tcpip.h>
  137 #ifdef TCP_DEBUG
  138 #include <netinet/tcp_debug.h>
  139 #endif
  140 
  141 /*
  142  * Various tunable timer parameters.  These are initialized in tcp_init(),
  143  * unless they are patched.
  144  */
  145 u_int   tcp_keepinit = 0;
  146 u_int   tcp_keepidle = 0;
  147 u_int   tcp_keepintvl = 0;
  148 u_int   tcp_keepcnt = 0;                /* max idle probes */
  149 
  150 int     tcp_maxpersistidle = 0;         /* max idle time in persist */
  151 
  152 /*
  153  * Time to delay the ACK.  This is initialized in tcp_init(), unless
  154  * its patched.
  155  */
  156 int     tcp_delack_ticks = 0;
  157 
  158 void    tcp_timer_rexmt(void *);
  159 void    tcp_timer_persist(void *);
  160 void    tcp_timer_keep(void *);
  161 void    tcp_timer_2msl(void *);
  162 
  163 const tcp_timer_func_t tcp_timer_funcs[TCPT_NTIMERS] = {
  164         tcp_timer_rexmt,
  165         tcp_timer_persist,
  166         tcp_timer_keep,
  167         tcp_timer_2msl,
  168 };
  169 
  170 /*
  171  * Timer state initialization, called from tcp_init().
  172  */
  173 void
  174 tcp_timer_init(void)
  175 {
  176 
  177         if (tcp_keepinit == 0)
  178                 tcp_keepinit = TCPTV_KEEP_INIT;
  179 
  180         if (tcp_keepidle == 0)
  181                 tcp_keepidle = TCPTV_KEEP_IDLE;
  182 
  183         if (tcp_keepintvl == 0)
  184                 tcp_keepintvl = TCPTV_KEEPINTVL;
  185 
  186         if (tcp_keepcnt == 0)
  187                 tcp_keepcnt = TCPTV_KEEPCNT;
  188 
  189         if (tcp_maxpersistidle == 0)
  190                 tcp_maxpersistidle = TCPTV_KEEP_IDLE;
  191 
  192         if (tcp_delack_ticks == 0)
  193                 tcp_delack_ticks = TCP_DELACK_TICKS;
  194 }
  195 
  196 /*
  197  * Callout to process delayed ACKs for a TCPCB.
  198  */
  199 void
  200 tcp_delack(void *arg)
  201 {
  202         struct tcpcb *tp = arg;
  203 
  204         /*
  205          * If tcp_output() wasn't able to transmit the ACK
  206          * for whatever reason, it will restart the delayed
  207          * ACK callout.
  208          */
  209 
  210         mutex_enter(softnet_lock);
  211         if ((tp->t_flags & (TF_DEAD | TF_DELACK)) != TF_DELACK) {
  212                 mutex_exit(softnet_lock);
  213                 return;
  214         }
  215         if (!callout_expired(&tp->t_delack_ch)) {
  216                 mutex_exit(softnet_lock);
  217                 return;
  218         }
  219 
  220         tp->t_flags |= TF_ACKNOW;
  221         KERNEL_LOCK(1, NULL);
  222         (void) tcp_output(tp);
  223         KERNEL_UNLOCK_ONE(NULL);
  224         mutex_exit(softnet_lock);
  225 }
  226 
  227 /*
  228  * Tcp protocol timeout routine called every 500 ms.
  229  * Updates the timers in all active tcb's and
  230  * causes finite state machine actions if timers expire.
  231  */
  232 void
  233 tcp_slowtimo(void)
  234 {
  235 
  236         mutex_enter(softnet_lock);
  237         tcp_iss_seq += TCP_ISSINCR;                     /* increment iss */
  238         tcp_now++;                                      /* for timestamps */
  239         mutex_exit(softnet_lock);
  240 }
  241 
  242 /*
  243  * Cancel all timers for TCP tp.
  244  */
  245 void
  246 tcp_canceltimers(struct tcpcb *tp)
  247 {
  248         int i;
  249 
  250         for (i = 0; i < TCPT_NTIMERS; i++)
  251                 TCP_TIMER_DISARM(tp, i);
  252 }
  253 
  254 const int       tcp_backoff[TCP_MAXRXTSHIFT + 1] =
  255     { 1, 2, 4, 8, 16, 32, 64, 64, 64, 64, 64, 64, 64 };
  256 
  257 const int       tcp_totbackoff = 511;   /* sum of tcp_backoff[] */
  258 
  259 /*
  260  * TCP timer processing.
  261  */
  262 
  263 void
  264 tcp_timer_rexmt(void *arg)
  265 {
  266         struct tcpcb *tp = arg;
  267         uint32_t rto;
  268 #ifdef TCP_DEBUG
  269         struct socket *so = NULL;
  270         short ostate;
  271 #endif
  272 
  273         mutex_enter(softnet_lock);
  274         if ((tp->t_flags & TF_DEAD) != 0) {
  275                 mutex_exit(softnet_lock);
  276                 return;
  277         }
  278         if (!callout_expired(&tp->t_timer[TCPT_REXMT])) {
  279                 mutex_exit(softnet_lock);
  280                 return;
  281         }
  282 
  283         KERNEL_LOCK(1, NULL);
  284         if ((tp->t_flags & TF_PMTUD_PEND) && tp->t_inpcb &&
  285             SEQ_GEQ(tp->t_pmtud_th_seq, tp->snd_una) &&
  286             SEQ_LT(tp->t_pmtud_th_seq, (int)(tp->snd_una + tp->t_ourmss))) {
  287                 extern struct sockaddr_in icmpsrc;
  288                 struct icmp icmp;
  289 
  290                 tp->t_flags &= ~TF_PMTUD_PEND;
  291 
  292                 /* XXX create fake icmp message with relevant entries */
  293                 icmp.icmp_nextmtu = tp->t_pmtud_nextmtu;
  294                 icmp.icmp_ip.ip_len = tp->t_pmtud_ip_len;
  295                 icmp.icmp_ip.ip_hl = tp->t_pmtud_ip_hl;
  296                 icmpsrc.sin_addr = tp->t_inpcb->inp_faddr;
  297                 icmp_mtudisc(&icmp, icmpsrc.sin_addr);
  298 
  299                 /*
  300                  * Notify all connections to the same peer about
  301                  * new mss and trigger retransmit.
  302                  */
  303                 in_pcbnotifyall(&tcbtable, icmpsrc.sin_addr, EMSGSIZE,
  304                     tcp_mtudisc);
  305                 KERNEL_UNLOCK_ONE(NULL);
  306                 mutex_exit(softnet_lock);
  307                 return;
  308         }
  309 #ifdef TCP_DEBUG
  310 #ifdef INET
  311         if (tp->t_inpcb)
  312                 so = tp->t_inpcb->inp_socket;
  313 #endif
  314 #ifdef INET6
  315         if (tp->t_in6pcb)
  316                 so = tp->t_in6pcb->in6p_socket;
  317 #endif
  318         ostate = tp->t_state;
  319 #endif /* TCP_DEBUG */
  320 
  321         /*
  322          * Clear the SACK scoreboard, reset FACK estimate.
  323          */
  324         tcp_free_sackholes(tp);
  325         tp->snd_fack = tp->snd_una;
  326 
  327         /*
  328          * Retransmission timer went off.  Message has not
  329          * been acked within retransmit interval.  Back off
  330          * to a longer retransmit interval and retransmit one segment.
  331          */
  332 
  333         if (++tp->t_rxtshift > TCP_MAXRXTSHIFT) {
  334                 tp->t_rxtshift = TCP_MAXRXTSHIFT;
  335                 TCP_STATINC(TCP_STAT_TIMEOUTDROP);
  336                 tp = tcp_drop(tp, tp->t_softerror ?
  337                     tp->t_softerror : ETIMEDOUT);
  338                 goto out;
  339         }
  340         TCP_STATINC(TCP_STAT_REXMTTIMEO);
  341         rto = TCP_REXMTVAL(tp);
  342         if (rto < tp->t_rttmin)
  343                 rto = tp->t_rttmin;
  344         TCPT_RANGESET(tp->t_rxtcur, rto * tcp_backoff[tp->t_rxtshift],
  345             tp->t_rttmin, TCPTV_REXMTMAX);
  346         TCP_TIMER_ARM(tp, TCPT_REXMT, tp->t_rxtcur);
  347 
  348         /*
  349          * If we are losing and we are trying path MTU discovery,
  350          * try turning it off.  This will avoid black holes in
  351          * the network which suppress or fail to send "packet
  352          * too big" ICMP messages.  We should ideally do
  353          * lots more sophisticated searching to find the right
  354          * value here...
  355          */
  356         if (tp->t_mtudisc && tp->t_rxtshift > TCP_MAXRXTSHIFT / 6) {
  357                 TCP_STATINC(TCP_STAT_PMTUBLACKHOLE);
  358 
  359 #ifdef INET
  360                 /* try turning PMTUD off */
  361                 if (tp->t_inpcb)
  362                         tp->t_mtudisc = 0;
  363 #endif
  364 #ifdef INET6
  365                 /* try using IPv6 minimum MTU */
  366                 if (tp->t_in6pcb)
  367                         tp->t_mtudisc = 0;
  368 #endif
  369 
  370                 /* XXX: more sophisticated Black hole recovery code? */
  371         }
  372 
  373         /*
  374          * If losing, let the lower level know and try for
  375          * a better route.  Also, if we backed off this far,
  376          * our srtt estimate is probably bogus.  Clobber it
  377          * so we'll take the next rtt measurement as our srtt;
  378          * move the current srtt into rttvar to keep the current
  379          * retransmit times until then.
  380          */
  381         if (tp->t_rxtshift > TCP_MAXRXTSHIFT / 4) {
  382 #ifdef INET
  383                 if (tp->t_inpcb)
  384                         in_losing(tp->t_inpcb);
  385 #endif
  386 #ifdef INET6
  387                 if (tp->t_in6pcb)
  388                         in6_losing(tp->t_in6pcb);
  389 #endif
  390                 /*
  391                  * This operation is not described in RFC2988.  The
  392                  * point is to keep srtt+4*rttvar constant, so we
  393                  * should shift right 2 bits to divide by 4, and then
  394                  * shift right one bit because the storage
  395                  * representation of rttvar is 1/16s vs 1/32s for
  396                  * srtt.
  397                  */
  398                 tp->t_rttvar += (tp->t_srtt >> TCP_RTT_SHIFT);
  399                 tp->t_srtt = 0;
  400         }
  401         tp->snd_nxt = tp->snd_una;
  402         tp->snd_high = tp->snd_max;
  403         /*
  404          * If timing a segment in this window, stop the timer.
  405          */
  406         tp->t_rtttime = 0;
  407         /*
  408          * Remember if we are retransmitting a SYN, because if
  409          * we do, set the initial congestion window must be set
  410          * to 1 segment.
  411          */
  412         if (tp->t_state == TCPS_SYN_SENT)
  413                 tp->t_flags |= TF_SYN_REXMT;
  414 
  415         /*
  416          * Adjust congestion control parameters.
  417          */
  418         tp->t_congctl->slow_retransmit(tp);
  419 
  420         (void) tcp_output(tp);
  421 
  422  out:
  423 #ifdef TCP_DEBUG
  424         if (tp && so->so_options & SO_DEBUG)
  425                 tcp_trace(TA_USER, ostate, tp, NULL,
  426                     PRU_SLOWTIMO | (TCPT_REXMT << 8));
  427 #endif
  428         KERNEL_UNLOCK_ONE(NULL);
  429         mutex_exit(softnet_lock);
  430 }
  431 
  432 void
  433 tcp_timer_persist(void *arg)
  434 {
  435         struct tcpcb *tp = arg;
  436         uint32_t rto;
  437 #ifdef TCP_DEBUG
  438         struct socket *so = NULL;
  439         short ostate;
  440 #endif
  441 
  442         mutex_enter(softnet_lock);
  443         if ((tp->t_flags & TF_DEAD) != 0) {
  444                 mutex_exit(softnet_lock);
  445                 return;
  446         }
  447         if (!callout_expired(&tp->t_timer[TCPT_PERSIST])) {
  448                 mutex_exit(softnet_lock);
  449                 return;
  450         }
  451 
  452         KERNEL_LOCK(1, NULL);
  453 #ifdef TCP_DEBUG
  454 #ifdef INET
  455         if (tp->t_inpcb)
  456                 so = tp->t_inpcb->inp_socket;
  457 #endif
  458 #ifdef INET6
  459         if (tp->t_in6pcb)
  460                 so = tp->t_in6pcb->in6p_socket;
  461 #endif
  462 
  463         ostate = tp->t_state;
  464 #endif /* TCP_DEBUG */
  465 
  466         /*
  467          * Persistance timer into zero window.
  468          * Force a byte to be output, if possible.
  469          */
  470 
  471         /*
  472          * Hack: if the peer is dead/unreachable, we do not
  473          * time out if the window is closed.  After a full
  474          * backoff, drop the connection if the idle time
  475          * (no responses to probes) reaches the maximum
  476          * backoff that we would use if retransmitting.
  477          */
  478         rto = TCP_REXMTVAL(tp);
  479         if (rto < tp->t_rttmin)
  480                 rto = tp->t_rttmin;
  481         if (tp->t_rxtshift == TCP_MAXRXTSHIFT &&
  482             ((tcp_now - tp->t_rcvtime) >= tcp_maxpersistidle ||
  483             (tcp_now - tp->t_rcvtime) >= rto * tcp_totbackoff)) {
  484                 TCP_STATINC(TCP_STAT_PERSISTDROPS);
  485                 tp = tcp_drop(tp, ETIMEDOUT);
  486                 goto out;
  487         }
  488         TCP_STATINC(TCP_STAT_PERSISTTIMEO);
  489         tcp_setpersist(tp);
  490         tp->t_force = 1;
  491         (void) tcp_output(tp);
  492         tp->t_force = 0;
  493 
  494  out:
  495 #ifdef TCP_DEBUG
  496         if (tp && so->so_options & SO_DEBUG)
  497                 tcp_trace(TA_USER, ostate, tp, NULL,
  498                     PRU_SLOWTIMO | (TCPT_PERSIST << 8));
  499 #endif
  500         KERNEL_UNLOCK_ONE(NULL);
  501         mutex_exit(softnet_lock);
  502 }
  503 
  504 void
  505 tcp_timer_keep(void *arg)
  506 {
  507         struct tcpcb *tp = arg;
  508         struct socket *so = NULL;       /* Quell compiler warning */
  509 #ifdef TCP_DEBUG
  510         short ostate;
  511 #endif
  512 
  513         mutex_enter(softnet_lock);
  514         if ((tp->t_flags & TF_DEAD) != 0) {
  515                 mutex_exit(softnet_lock);
  516                 return;
  517         }
  518         if (!callout_expired(&tp->t_timer[TCPT_KEEP])) {
  519                 mutex_exit(softnet_lock);
  520                 return;
  521         }
  522 
  523         KERNEL_LOCK(1, NULL);
  524 
  525 #ifdef TCP_DEBUG
  526         ostate = tp->t_state;
  527 #endif /* TCP_DEBUG */
  528 
  529         /*
  530          * Keep-alive timer went off; send something
  531          * or drop connection if idle for too long.
  532          */
  533 
  534         TCP_STATINC(TCP_STAT_KEEPTIMEO);
  535         if (TCPS_HAVEESTABLISHED(tp->t_state) == 0)
  536                 goto dropit;
  537 #ifdef INET
  538         if (tp->t_inpcb)
  539                 so = tp->t_inpcb->inp_socket;
  540 #endif
  541 #ifdef INET6
  542         if (tp->t_in6pcb)
  543                 so = tp->t_in6pcb->in6p_socket;
  544 #endif
  545         KASSERT(so != NULL);
  546         if (so->so_options & SO_KEEPALIVE &&
  547             tp->t_state <= TCPS_CLOSE_WAIT) {
  548                 if ((tp->t_maxidle > 0) &&
  549                     ((tcp_now - tp->t_rcvtime) >=
  550                      tp->t_keepidle + tp->t_maxidle))
  551                         goto dropit;
  552                 /*
  553                  * Send a packet designed to force a response
  554                  * if the peer is up and reachable:
  555                  * either an ACK if the connection is still alive,
  556                  * or an RST if the peer has closed the connection
  557                  * due to timeout or reboot.
  558                  * Using sequence number tp->snd_una-1
  559                  * causes the transmitted zero-length segment
  560                  * to lie outside the receive window;
  561                  * by the protocol spec, this requires the
  562                  * correspondent TCP to respond.
  563                  */
  564                 TCP_STATINC(TCP_STAT_KEEPPROBE);
  565                 if (tcp_compat_42) {
  566                         /*
  567                          * The keepalive packet must have nonzero
  568                          * length to get a 4.2 host to respond.
  569                          */
  570                         (void)tcp_respond(tp, tp->t_template,
  571                             (struct mbuf *)NULL, NULL, tp->rcv_nxt - 1,
  572                             tp->snd_una - 1, 0);
  573                 } else {
  574                         (void)tcp_respond(tp, tp->t_template,
  575                             (struct mbuf *)NULL, NULL, tp->rcv_nxt,
  576                             tp->snd_una - 1, 0);
  577                 }
  578                 TCP_TIMER_ARM(tp, TCPT_KEEP, tp->t_keepintvl);
  579         } else
  580                 TCP_TIMER_ARM(tp, TCPT_KEEP, tp->t_keepidle);
  581 
  582 #ifdef TCP_DEBUG
  583         if (tp && so->so_options & SO_DEBUG)
  584                 tcp_trace(TA_USER, ostate, tp, NULL,
  585                     PRU_SLOWTIMO | (TCPT_KEEP << 8));
  586 #endif
  587         KERNEL_UNLOCK_ONE(NULL);
  588         mutex_exit(softnet_lock);
  589         return;
  590 
  591  dropit:
  592         TCP_STATINC(TCP_STAT_KEEPDROPS);
  593         (void) tcp_drop(tp, ETIMEDOUT);
  594         KERNEL_UNLOCK_ONE(NULL);
  595         mutex_exit(softnet_lock);
  596 }
  597 
  598 void
  599 tcp_timer_2msl(void *arg)
  600 {
  601         struct tcpcb *tp = arg;
  602 #ifdef TCP_DEBUG
  603         struct socket *so = NULL;
  604         short ostate;
  605 #endif
  606 
  607         mutex_enter(softnet_lock);
  608         if ((tp->t_flags & TF_DEAD) != 0) {
  609                 mutex_exit(softnet_lock);
  610                 return;
  611         }
  612         if (!callout_expired(&tp->t_timer[TCPT_2MSL])) {
  613                 mutex_exit(softnet_lock);
  614                 return;
  615         }
  616 
  617         /*
  618          * 2 MSL timeout went off, clear the SACK scoreboard, reset
  619          * the FACK estimate.
  620          */
  621         KERNEL_LOCK(1, NULL);
  622         tcp_free_sackholes(tp);
  623         tp->snd_fack = tp->snd_una;
  624 
  625 #ifdef TCP_DEBUG
  626 #ifdef INET
  627         if (tp->t_inpcb)
  628                 so = tp->t_inpcb->inp_socket;
  629 #endif
  630 #ifdef INET6
  631         if (tp->t_in6pcb)
  632                 so = tp->t_in6pcb->in6p_socket;
  633 #endif
  634 
  635         ostate = tp->t_state;
  636 #endif /* TCP_DEBUG */
  637 
  638         /*
  639          * 2 MSL timeout in shutdown went off.  If we're closed but
  640          * still waiting for peer to close and connection has been idle
  641          * too long, or if 2MSL time is up from TIME_WAIT, delete connection
  642          * control block.  Otherwise, check again in a bit.
  643          */
  644         if (tp->t_state != TCPS_TIME_WAIT &&
  645             ((tp->t_maxidle == 0) ||
  646             ((tcp_now - tp->t_rcvtime) <= tp->t_maxidle)))
  647             TCP_TIMER_ARM(tp, TCPT_2MSL, tp->t_keepintvl);
  648         else
  649                 tp = tcp_close(tp);
  650 
  651 #ifdef TCP_DEBUG
  652         if (tp && so->so_options & SO_DEBUG)
  653                 tcp_trace(TA_USER, ostate, tp, NULL,
  654                     PRU_SLOWTIMO | (TCPT_2MSL << 8));
  655 #endif
  656         mutex_exit(softnet_lock);
  657         KERNEL_UNLOCK_ONE(NULL);
  658 }

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