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 1.71.2.1 2008/11/18 22:57:09 snj 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  * 3. All advertising materials mentioning features or use of this software
   51  *    must display the following acknowledgement:
   52  *      This product includes software developed by the NetBSD
   53  *      Foundation, Inc. and its contributors.
   54  * 4. Neither the name of The NetBSD Foundation nor the names of its
   55  *    contributors may be used to endorse or promote products derived
   56  *    from this software without specific prior written permission.
   57  *
   58  * THIS SOFTWARE IS PROVIDED BY THE NETBSD FOUNDATION, INC. AND CONTRIBUTORS
   59  * ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
   60  * TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
   61  * PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE FOUNDATION OR CONTRIBUTORS
   62  * BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
   63  * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
   64  * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
   65  * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
   66  * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
   67  * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
   68  * POSSIBILITY OF SUCH DAMAGE.
   69  */
   70 
   71 /*
   72  * Copyright (c) 1982, 1986, 1988, 1990, 1993, 1995
   73  *      The Regents of the University of California.  All rights reserved.
   74  *
   75  * Redistribution and use in source and binary forms, with or without
   76  * modification, are permitted provided that the following conditions
   77  * are met:
   78  * 1. Redistributions of source code must retain the above copyright
   79  *    notice, this list of conditions and the following disclaimer.
   80  * 2. Redistributions in binary form must reproduce the above copyright
   81  *    notice, this list of conditions and the following disclaimer in the
   82  *    documentation and/or other materials provided with the distribution.
   83  * 3. Neither the name of the University nor the names of its contributors
   84  *    may be used to endorse or promote products derived from this software
   85  *    without specific prior written permission.
   86  *
   87  * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
   88  * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
   89  * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
   90  * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
   91  * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
   92  * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
   93  * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
   94  * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
   95  * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
   96  * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
   97  * SUCH DAMAGE.
   98  *
   99  *      @(#)tcp_timer.c 8.2 (Berkeley) 5/24/95
  100  */
  101 
  102 #include <sys/cdefs.h>
  103 __KERNEL_RCSID(0, "$NetBSD: tcp_timer.c,v 1.71.2.1 2008/11/18 22:57:09 snj Exp $");
  104 
  105 #include "opt_inet.h"
  106 #include "opt_tcp_debug.h"
  107 
  108 #include <sys/param.h>
  109 #include <sys/systm.h>
  110 #include <sys/malloc.h>
  111 #include <sys/mbuf.h>
  112 #include <sys/socket.h>
  113 #include <sys/socketvar.h>
  114 #include <sys/protosw.h>
  115 #include <sys/errno.h>
  116 #include <sys/kernel.h>
  117 
  118 #include <net/if.h>
  119 #include <net/route.h>
  120 
  121 #include <netinet/in.h>
  122 #include <netinet/in_systm.h>
  123 #include <netinet/ip.h>
  124 #include <netinet/in_pcb.h>
  125 #include <netinet/ip_var.h>
  126 
  127 #ifdef INET6
  128 #ifndef INET
  129 #include <netinet/in.h>
  130 #endif
  131 #include <netinet/ip6.h>
  132 #include <netinet6/in6_pcb.h>
  133 #endif
  134 
  135 #include <netinet/tcp.h>
  136 #include <netinet/tcp_fsm.h>
  137 #include <netinet/tcp_seq.h>
  138 #include <netinet/tcp_timer.h>
  139 #include <netinet/tcp_var.h>
  140 #include <netinet/tcpip.h>
  141 #ifdef TCP_DEBUG
  142 #include <netinet/tcp_debug.h>
  143 #endif
  144 
  145 /*
  146  * Various tunable timer parameters.  These are initialized in tcp_init(),
  147  * unless they are patched.
  148  */
  149 int     tcp_keepidle = 0;
  150 int     tcp_keepintvl = 0;
  151 int     tcp_keepcnt = 0;                /* max idle probes */
  152 int     tcp_maxpersistidle = 0;         /* max idle time in persist */
  153 int     tcp_maxidle;                    /* computed in tcp_slowtimo() */
  154 
  155 /*
  156  * Time to delay the ACK.  This is initialized in tcp_init(), unless
  157  * its patched.
  158  */
  159 int     tcp_delack_ticks = 0;
  160 
  161 void    tcp_timer_rexmt(void *);
  162 void    tcp_timer_persist(void *);
  163 void    tcp_timer_keep(void *);
  164 void    tcp_timer_2msl(void *);
  165 
  166 const tcp_timer_func_t tcp_timer_funcs[TCPT_NTIMERS] = {
  167         tcp_timer_rexmt,
  168         tcp_timer_persist,
  169         tcp_timer_keep,
  170         tcp_timer_2msl,
  171 };
  172 
  173 /*
  174  * Timer state initialization, called from tcp_init().
  175  */
  176 void
  177 tcp_timer_init(void)
  178 {
  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  * Return how many timers are currently being invoked.
  198  */
  199 int
  200 tcp_timers_invoking(struct tcpcb *tp)
  201 {
  202         int i;
  203         int count = 0;
  204 
  205         for (i = 0; i < TCPT_NTIMERS; i++)
  206                 if (callout_invoking(&tp->t_timer[i]))
  207                         count++;
  208         if (callout_invoking(&tp->t_delack_ch))
  209                 count++;
  210 
  211         return count;
  212 }
  213 
  214 /*
  215  * Callout to process delayed ACKs for a TCPCB.
  216  */
  217 void
  218 tcp_delack(void *arg)
  219 {
  220         struct tcpcb *tp = arg;
  221         int s;
  222 
  223         /*
  224          * If tcp_output() wasn't able to transmit the ACK
  225          * for whatever reason, it will restart the delayed
  226          * ACK callout.
  227          */
  228 
  229         s = splsoftnet();
  230         callout_ack(&tp->t_delack_ch);
  231         if (tcp_isdead(tp)) {
  232                 splx(s);
  233                 return;
  234         }
  235         if (!callout_expired(&tp->t_delack_ch)) {
  236                 splx(s);
  237                 return;
  238         }
  239 
  240         tp->t_flags |= TF_ACKNOW;
  241         (void) tcp_output(tp);
  242         splx(s);
  243 }
  244 
  245 /*
  246  * Tcp protocol timeout routine called every 500 ms.
  247  * Updates the timers in all active tcb's and
  248  * causes finite state machine actions if timers expire.
  249  */
  250 void
  251 tcp_slowtimo(void)
  252 {
  253         int s;
  254 
  255         s = splsoftnet();
  256         tcp_maxidle = tcp_keepcnt * tcp_keepintvl;
  257         tcp_iss_seq += TCP_ISSINCR;                     /* increment iss */
  258         tcp_now++;                                      /* for timestamps */
  259         splx(s);
  260 }
  261 
  262 /*
  263  * Cancel all timers for TCP tp.
  264  */
  265 void
  266 tcp_canceltimers(struct tcpcb *tp)
  267 {
  268         int i;
  269 
  270         for (i = 0; i < TCPT_NTIMERS; i++)
  271                 TCP_TIMER_DISARM(tp, i);
  272 }
  273 
  274 const int       tcp_backoff[TCP_MAXRXTSHIFT + 1] =
  275     { 1, 2, 4, 8, 16, 32, 64, 64, 64, 64, 64, 64, 64 };
  276 
  277 const int       tcp_totbackoff = 511;   /* sum of tcp_backoff[] */
  278 
  279 /*
  280  * TCP timer processing.
  281  */
  282 
  283 void
  284 tcp_timer_rexmt(void *arg)
  285 {
  286         struct tcpcb *tp = arg;
  287         uint32_t rto;
  288         int s;
  289 #ifdef TCP_DEBUG
  290         struct socket *so = NULL;
  291         short ostate;
  292 #endif
  293 
  294         s = splsoftnet();
  295         callout_ack(&tp->t_timer[TCPT_REXMT]);
  296         if (tcp_isdead(tp)) {
  297                 splx(s);
  298                 return;
  299         }
  300         if (!callout_expired(&tp->t_timer[TCPT_REXMT])) {
  301                 splx(s);
  302                 return;
  303         }
  304 
  305 #ifdef TCP_DEBUG
  306 #ifdef INET
  307         if (tp->t_inpcb)
  308                 so = tp->t_inpcb->inp_socket;
  309 #endif
  310 #ifdef INET6
  311         if (tp->t_in6pcb)
  312                 so = tp->t_in6pcb->in6p_socket;
  313 #endif
  314         ostate = tp->t_state;
  315 #endif /* TCP_DEBUG */
  316 
  317         /*
  318          * Clear the SACK scoreboard, reset FACK estimate.
  319          */
  320         tcp_free_sackholes(tp);
  321         tp->snd_fack = tp->snd_una;
  322 
  323         /*
  324          * Retransmission timer went off.  Message has not
  325          * been acked within retransmit interval.  Back off
  326          * to a longer retransmit interval and retransmit one segment.
  327          */
  328 
  329         if (++tp->t_rxtshift > TCP_MAXRXTSHIFT) {
  330                 tp->t_rxtshift = TCP_MAXRXTSHIFT;
  331                 tcpstat.tcps_timeoutdrop++;
  332                 tp = tcp_drop(tp, tp->t_softerror ?
  333                     tp->t_softerror : ETIMEDOUT);
  334                 goto out;
  335         }
  336         tcpstat.tcps_rexmttimeo++;
  337         rto = TCP_REXMTVAL(tp);
  338         if (rto < tp->t_rttmin)
  339                 rto = tp->t_rttmin;
  340         TCPT_RANGESET(tp->t_rxtcur, rto * tcp_backoff[tp->t_rxtshift],
  341             tp->t_rttmin, TCPTV_REXMTMAX);
  342         TCP_TIMER_ARM(tp, TCPT_REXMT, tp->t_rxtcur);
  343 
  344         /*
  345          * If we are losing and we are trying path MTU discovery,
  346          * try turning it off.  This will avoid black holes in
  347          * the network which suppress or fail to send "packet
  348          * too big" ICMP messages.  We should ideally do
  349          * lots more sophisticated searching to find the right
  350          * value here...
  351          */
  352         if (tp->t_mtudisc && tp->t_rxtshift > TCP_MAXRXTSHIFT / 6) {
  353                 tcpstat.tcps_pmtublackhole++;
  354 
  355 #ifdef INET
  356                 /* try turning PMTUD off */
  357                 if (tp->t_inpcb)
  358                         tp->t_mtudisc = 0;
  359 #endif
  360 #ifdef INET6
  361                 /* try using IPv6 minimum MTU */
  362                 if (tp->t_in6pcb)
  363                         tp->t_mtudisc = 0;
  364 #endif
  365 
  366                 /* XXX: more sophisticated Black hole recovery code? */
  367         }
  368 
  369         /*
  370          * If losing, let the lower level know and try for
  371          * a better route.  Also, if we backed off this far,
  372          * our srtt estimate is probably bogus.  Clobber it
  373          * so we'll take the next rtt measurement as our srtt;
  374          * move the current srtt into rttvar to keep the current
  375          * retransmit times until then.
  376          */
  377         if (tp->t_rxtshift > TCP_MAXRXTSHIFT / 4) {
  378 #ifdef INET
  379                 if (tp->t_inpcb)
  380                         in_losing(tp->t_inpcb);
  381 #endif
  382 #ifdef INET6
  383                 if (tp->t_in6pcb)
  384                         in6_losing(tp->t_in6pcb);
  385 #endif
  386                 tp->t_rttvar += (tp->t_srtt >> TCP_RTT_SHIFT);
  387                 tp->t_srtt = 0;
  388         }
  389         tp->snd_nxt = tp->snd_una;
  390         tp->snd_high = tp->snd_max;
  391         /*
  392          * If timing a segment in this window, stop the timer.
  393          */
  394         tp->t_rtttime = 0;
  395         /*
  396          * Remember if we are retransmitting a SYN, because if
  397          * we do, set the initial congestion window must be set
  398          * to 1 segment.
  399          */
  400         if (tp->t_state == TCPS_SYN_SENT)
  401                 tp->t_flags |= TF_SYN_REXMT;
  402         /*
  403          * Close the congestion window down to one segment
  404          * (we'll open it by one segment for each ack we get).
  405          * Since we probably have a window's worth of unacked
  406          * data accumulated, this "slow start" keeps us from
  407          * dumping all that data as back-to-back packets (which
  408          * might overwhelm an intermediate gateway).
  409          *
  410          * There are two phases to the opening: Initially we
  411          * open by one mss on each ack.  This makes the window
  412          * size increase exponentially with time.  If the
  413          * window is larger than the path can handle, this
  414          * exponential growth results in dropped packet(s)
  415          * almost immediately.  To get more time between
  416          * drops but still "push" the network to take advantage
  417          * of improving conditions, we switch from exponential
  418          * to linear window opening at some threshhold size.
  419          * For a threshhold, we use half the current window
  420          * size, truncated to a multiple of the mss.
  421          *
  422          * (the minimum cwnd that will give us exponential
  423          * growth is 2 mss.  We don't allow the threshhold
  424          * to go below this.)
  425          */
  426         {
  427         u_int win = min(tp->snd_wnd, tp->snd_cwnd) / 2 / tp->t_segsz;
  428         if (win < 2)
  429                 win = 2;
  430         /* Loss Window MUST be one segment. */
  431         tp->snd_cwnd = tp->t_segsz;
  432         tp->snd_ssthresh = win * tp->t_segsz;
  433         tp->t_partialacks = -1;
  434         tp->t_dupacks = 0;
  435         }
  436         (void) tcp_output(tp);
  437 
  438  out:
  439 #ifdef TCP_DEBUG
  440         if (tp && so->so_options & SO_DEBUG)
  441                 tcp_trace(TA_USER, ostate, tp, NULL,
  442                     PRU_SLOWTIMO | (TCPT_REXMT << 8));
  443 #endif
  444         splx(s);
  445 }
  446 
  447 void
  448 tcp_timer_persist(void *arg)
  449 {
  450         struct tcpcb *tp = arg;
  451         uint32_t rto;
  452         int s;
  453 #ifdef TCP_DEBUG
  454         struct socket *so = NULL;
  455         short ostate;
  456 #endif
  457 
  458         s = splsoftnet();
  459         callout_ack(&tp->t_timer[TCPT_PERSIST]);
  460         if (tcp_isdead(tp)) {
  461                 splx(s);
  462                 return;
  463         }
  464         if (!callout_expired(&tp->t_timer[TCPT_PERSIST])) {
  465                 splx(s);
  466                 return;
  467         }
  468 
  469 #ifdef TCP_DEBUG
  470 #ifdef INET
  471         if (tp->t_inpcb)
  472                 so = tp->t_inpcb->inp_socket;
  473 #endif
  474 #ifdef INET6
  475         if (tp->t_in6pcb)
  476                 so = tp->t_in6pcb->in6p_socket;
  477 #endif
  478 
  479         ostate = tp->t_state;
  480 #endif /* TCP_DEBUG */
  481 
  482         /*
  483          * Persistance timer into zero window.
  484          * Force a byte to be output, if possible.
  485          */
  486 
  487         /*
  488          * Hack: if the peer is dead/unreachable, we do not
  489          * time out if the window is closed.  After a full
  490          * backoff, drop the connection if the idle time
  491          * (no responses to probes) reaches the maximum
  492          * backoff that we would use if retransmitting.
  493          */
  494         rto = TCP_REXMTVAL(tp);
  495         if (rto < tp->t_rttmin)
  496                 rto = tp->t_rttmin;
  497         if (tp->t_rxtshift == TCP_MAXRXTSHIFT &&
  498             ((tcp_now - tp->t_rcvtime) >= tcp_maxpersistidle ||
  499             (tcp_now - tp->t_rcvtime) >= rto * tcp_totbackoff)) {
  500                 tcpstat.tcps_persistdrops++;
  501                 tp = tcp_drop(tp, ETIMEDOUT);
  502                 goto out;
  503         }
  504         tcpstat.tcps_persisttimeo++;
  505         tcp_setpersist(tp);
  506         tp->t_force = 1;
  507         (void) tcp_output(tp);
  508         tp->t_force = 0;
  509 
  510  out:
  511 #ifdef TCP_DEBUG
  512         if (tp && so->so_options & SO_DEBUG)
  513                 tcp_trace(TA_USER, ostate, tp, NULL,
  514                     PRU_SLOWTIMO | (TCPT_PERSIST << 8));
  515 #endif
  516         splx(s);
  517 }
  518 
  519 void
  520 tcp_timer_keep(void *arg)
  521 {
  522         struct tcpcb *tp = arg;
  523         struct socket *so = NULL;       /* Quell compiler warning */
  524         int s;
  525 #ifdef TCP_DEBUG
  526         short ostate;
  527 #endif
  528 
  529         s = splsoftnet();
  530         callout_ack(&tp->t_timer[TCPT_KEEP]);
  531         if (tcp_isdead(tp)) {
  532                 splx(s);
  533                 return;
  534         }
  535         if (!callout_expired(&tp->t_timer[TCPT_KEEP])) {
  536                 splx(s);
  537                 return;
  538         }
  539 
  540 #ifdef TCP_DEBUG
  541         ostate = tp->t_state;
  542 #endif /* TCP_DEBUG */
  543 
  544         /*
  545          * Keep-alive timer went off; send something
  546          * or drop connection if idle for too long.
  547          */
  548 
  549         tcpstat.tcps_keeptimeo++;
  550         if (TCPS_HAVEESTABLISHED(tp->t_state) == 0)
  551                 goto dropit;
  552 #ifdef INET
  553         if (tp->t_inpcb)
  554                 so = tp->t_inpcb->inp_socket;
  555 #endif
  556 #ifdef INET6
  557         if (tp->t_in6pcb)
  558                 so = tp->t_in6pcb->in6p_socket;
  559 #endif
  560         if (so->so_options & SO_KEEPALIVE &&
  561             tp->t_state <= TCPS_CLOSE_WAIT) {
  562                 if ((tcp_maxidle > 0) &&
  563                     ((tcp_now - tp->t_rcvtime) >=
  564                      tcp_keepidle + tcp_maxidle))
  565                         goto dropit;
  566                 /*
  567                  * Send a packet designed to force a response
  568                  * if the peer is up and reachable:
  569                  * either an ACK if the connection is still alive,
  570                  * or an RST if the peer has closed the connection
  571                  * due to timeout or reboot.
  572                  * Using sequence number tp->snd_una-1
  573                  * causes the transmitted zero-length segment
  574                  * to lie outside the receive window;
  575                  * by the protocol spec, this requires the
  576                  * correspondent TCP to respond.
  577                  */
  578                 tcpstat.tcps_keepprobe++;
  579                 if (tcp_compat_42) {
  580                         /*
  581                          * The keepalive packet must have nonzero
  582                          * length to get a 4.2 host to respond.
  583                          */
  584                         (void)tcp_respond(tp, tp->t_template,
  585                             (struct mbuf *)NULL, NULL, tp->rcv_nxt - 1,
  586                             tp->snd_una - 1, 0);
  587                 } else {
  588                         (void)tcp_respond(tp, tp->t_template,
  589                             (struct mbuf *)NULL, NULL, tp->rcv_nxt,
  590                             tp->snd_una - 1, 0);
  591                 }
  592                 TCP_TIMER_ARM(tp, TCPT_KEEP, tcp_keepintvl);
  593         } else
  594                 TCP_TIMER_ARM(tp, TCPT_KEEP, tcp_keepidle);
  595 
  596 #ifdef TCP_DEBUG
  597         if (tp && so->so_options & SO_DEBUG)
  598                 tcp_trace(TA_USER, ostate, tp, NULL,
  599                     PRU_SLOWTIMO | (TCPT_KEEP << 8));
  600 #endif
  601         splx(s);
  602         return;
  603 
  604  dropit:
  605         tcpstat.tcps_keepdrops++;
  606         (void) tcp_drop(tp, ETIMEDOUT);
  607         splx(s);
  608 }
  609 
  610 void
  611 tcp_timer_2msl(void *arg)
  612 {
  613         struct tcpcb *tp = arg;
  614         int s;
  615 #ifdef TCP_DEBUG
  616         struct socket *so = NULL;
  617         short ostate;
  618 #endif
  619 
  620         s = splsoftnet();
  621         callout_ack(&tp->t_timer[TCPT_2MSL]);
  622         if (tcp_isdead(tp)) {
  623                 splx(s);
  624                 return;
  625         }
  626         if (!callout_expired(&tp->t_timer[TCPT_2MSL])) {
  627                 splx(s);
  628                 return;
  629         }
  630 
  631         /*
  632          * 2 MSL timeout went off, clear the SACK scoreboard, reset
  633          * the FACK estimate.
  634          */
  635         tcp_free_sackholes(tp);
  636         tp->snd_fack = tp->snd_una;
  637 
  638 #ifdef TCP_DEBUG
  639 #ifdef INET
  640         if (tp->t_inpcb)
  641                 so = tp->t_inpcb->inp_socket;
  642 #endif
  643 #ifdef INET6
  644         if (tp->t_in6pcb)
  645                 so = tp->t_in6pcb->in6p_socket;
  646 #endif
  647 
  648         ostate = tp->t_state;
  649 #endif /* TCP_DEBUG */
  650 
  651         /*
  652          * 2 MSL timeout in shutdown went off.  If we're closed but
  653          * still waiting for peer to close and connection has been idle
  654          * too long, or if 2MSL time is up from TIME_WAIT, delete connection
  655          * control block.  Otherwise, check again in a bit.
  656          */
  657         if (tp->t_state != TCPS_TIME_WAIT &&
  658             ((tcp_maxidle == 0) || ((tcp_now - tp->t_rcvtime) <= tcp_maxidle)))
  659                 TCP_TIMER_ARM(tp, TCPT_2MSL, tcp_keepintvl);
  660         else
  661                 tp = tcp_close(tp);
  662 
  663 #ifdef TCP_DEBUG
  664         if (tp && so->so_options & SO_DEBUG)
  665                 tcp_trace(TA_USER, ostate, tp, NULL,
  666                     PRU_SLOWTIMO | (TCPT_2MSL << 8));
  667 #endif
  668         splx(s);
  669 }

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