bsd/netinet/tcp_timer.c

   1 /*
   2  * Copyright (c) 2000 Apple Computer, Inc. All rights reserved.
   3  *
   4  * @APPLE_LICENSE_HEADER_START@
   5  *
   6  * The contents of this file constitute Original Code as defined in and
   7  * are subject to the Apple Public Source License Version 1.1 (the
   8  * "License").  You may not use this file except in compliance with the
   9  * License.  Please obtain a copy of the License at
  10  * http://www.apple.com/publicsource and read it before using this file.
  11  *
  12  * This Original Code and all software distributed under the License are
  13  * distributed on an "AS IS" basis, WITHOUT WARRANTY OF ANY KIND, EITHER
  14  * EXPRESS OR IMPLIED, AND APPLE HEREBY DISCLAIMS ALL SUCH WARRANTIES,
  15  * INCLUDING WITHOUT LIMITATION, ANY WARRANTIES OF MERCHANTABILITY,
  16  * FITNESS FOR A PARTICULAR PURPOSE OR NON-INFRINGEMENT.  Please see the
  17  * License for the specific language governing rights and limitations
  18  * under the License.
  19  *
  20  * @APPLE_LICENSE_HEADER_END@
  21  */
  22 /*
  23  * Copyright (c) 1982, 1986, 1988, 1990, 1993, 1995
  24  *      The Regents of the University of California.  All rights reserved.
  25  *
  26  * Redistribution and use in source and binary forms, with or without
  27  * modification, are permitted provided that the following conditions
  28  * are met:
  29  * 1. Redistributions of source code must retain the above copyright
  30  *    notice, this list of conditions and the following disclaimer.
  31  * 2. Redistributions in binary form must reproduce the above copyright
  32  *    notice, this list of conditions and the following disclaimer in the
  33  *    documentation and/or other materials provided with the distribution.
  34  * 3. All advertising materials mentioning features or use of this software
  35  *    must display the following acknowledgement:
  36  *      This product includes software developed by the University of
  37  *      California, Berkeley and its contributors.
  38  * 4. Neither the name of the University nor the names of its contributors
  39  *    may be used to endorse or promote products derived from this software
  40  *    without specific prior written permission.
  41  *
  42  * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
  43  * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
  44  * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
  45  * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
  46  * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
  47  * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
  48  * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
  49  * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
  50  * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
  51  * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
  52  * SUCH DAMAGE.
  53  *
  54  *      @(#)tcp_timer.c 8.2 (Berkeley) 5/24/95
  55  * $FreeBSD: src/sys/netinet/tcp_timer.c,v 1.34.2.11 2001/08/22 00:59:12 silby Exp $
  56  */
  57
  58
  59 #include <sys/param.h>
  60 #include <sys/systm.h>
  61 #include <sys/kernel.h>
  62 #include <sys/mbuf.h>
  63 #include <sys/sysctl.h>
  64 #include <sys/socket.h>
  65 #include <sys/socketvar.h>
  66 #include <sys/protosw.h>
  67
  68 #include <kern/cpu_number.h>    /* before tcp_seq.h, for tcp_random18() */
  69
  70 #include <net/route.h>
  71
  72 #include <netinet/in.h>
  73 #include <netinet/in_systm.h>
  74 #include <netinet/in_pcb.h>
  75 #if INET6
  76 #include <netinet6/in6_pcb.h>
  77 #endif
  78 #include <netinet/ip_var.h>
  79 #include <netinet/tcp.h>
  80 #include <netinet/tcp_fsm.h>
  81 #include <netinet/tcp_seq.h>
  82 #include <netinet/tcp_timer.h>
  83 #include <netinet/tcp_var.h>
  84 #include <netinet/tcpip.h>
  85 #if TCPDEBUG
  86 #include <netinet/tcp_debug.h>
  87 #endif
  88 #include <sys/kdebug.h>
  89
  90 #define DBG_FNC_TCP_FAST        NETDBG_CODE(DBG_NETTCP, (5 << 8))
  91 #define DBG_FNC_TCP_SLOW        NETDBG_CODE(DBG_NETTCP, (5 << 8) | 1)
  92
  93 /*
  94  * NOTE - WARNING
  95  *
  96  *
  97  *
  98  *
  99  */
 100 static int
 101 sysctl_msec_to_ticks SYSCTL_HANDLER_ARGS
 102 {
 103         int error, s, tt;
 104
 105         tt = *(int *)oidp->oid_arg1;
 106         s = tt * 1000 / hz;
 107
 108         error = sysctl_handle_int(oidp, &s, 0, req);
 109         if (error || !req->newptr)
 110                 return (error);
 111
 112         tt = s * hz / 1000;
 113         if (tt < 1)
 114                 return (EINVAL);
 115
 116         *(int *)oidp->oid_arg1 = tt;
 117         return (0);
 118 }
 119
 120 int     tcp_keepinit;
 121 SYSCTL_PROC(_net_inet_tcp, TCPCTL_KEEPINIT, keepinit, CTLTYPE_INT|CTLFLAG_RW,
 122     &tcp_keepinit, 0, sysctl_msec_to_ticks, "I", "");
 123
 124 int     tcp_keepidle;
 125 SYSCTL_PROC(_net_inet_tcp, TCPCTL_KEEPIDLE, keepidle, CTLTYPE_INT|CTLFLAG_RW,
 126     &tcp_keepidle, 0, sysctl_msec_to_ticks, "I", "");
 127
 128 int     tcp_keepintvl;
 129 SYSCTL_PROC(_net_inet_tcp, TCPCTL_KEEPINTVL, keepintvl, CTLTYPE_INT|CTLFLAG_RW,
 130     &tcp_keepintvl, 0, sysctl_msec_to_ticks, "I", "");
 131
 132 int     tcp_delacktime;
 133 SYSCTL_PROC(_net_inet_tcp, TCPCTL_DELACKTIME, delacktime,
 134     CTLTYPE_INT|CTLFLAG_RW, &tcp_delacktime, 0, sysctl_msec_to_ticks, "I",
 135     "Time before a delayed ACK is sent");
 136
 137 int     tcp_msl;
 138 SYSCTL_PROC(_net_inet_tcp, OID_AUTO, msl, CTLTYPE_INT|CTLFLAG_RW,
 139     &tcp_msl, 0, sysctl_msec_to_ticks, "I", "Maximum segment lifetime");
 140
 141 static int      always_keepalive = 0;
 142 SYSCTL_INT(_net_inet_tcp, OID_AUTO, always_keepalive, CTLFLAG_RW,
 143     &always_keepalive , 0, "Assume SO_KEEPALIVE on all TCP connections");
 144
 145 static int      tcp_keepcnt = TCPTV_KEEPCNT;
 146         /* max idle probes */
 147 int     tcp_maxpersistidle;
 148         /* max idle time in persist */
 149 int     tcp_maxidle;
 150
 151 struct  inpcbhead       time_wait_slots[N_TIME_WAIT_SLOTS];
 152 int             cur_tw_slot = 0;
 153
 154 u_long          *delack_bitmask;
 155
 156
 157 void    add_to_time_wait(tp)
 158         struct tcpcb    *tp;
 159 {
 160         int             tw_slot;
 161
 162         LIST_REMOVE(tp->t_inpcb, inp_list);
 163
 164         if (tp->t_timer[TCPT_2MSL] == 0)
 165             tp->t_timer[TCPT_2MSL] = 1;
 166
 167         tp->t_rcvtime += tp->t_timer[TCPT_2MSL] & (N_TIME_WAIT_SLOTS - 1);
 168         tw_slot = (tp->t_timer[TCPT_2MSL] & (N_TIME_WAIT_SLOTS - 1)) + cur_tw_slot;
 169         if (tw_slot >= N_TIME_WAIT_SLOTS)
 170             tw_slot -= N_TIME_WAIT_SLOTS;
 171
 172         LIST_INSERT_HEAD(&time_wait_slots[tw_slot], tp->t_inpcb, inp_list);
 173 }
 174
 175
 176
 177
 178
 179 /*
 180  * Fast timeout routine for processing delayed acks
 181  */
 182 void
 183 tcp_fasttimo()
 184 {
 185     register struct inpcb *inp;
 186     register struct tcpcb *tp;
 187
 188
 189     register u_long                     i,j;
 190     register u_long                     temp_mask;
 191     register u_long                     elem_base = 0;
 192     struct inpcbhead                    *head;
 193     int s = splnet();
 194
 195     static
 196     int delack_checked = 0;
 197
 198     KERNEL_DEBUG(DBG_FNC_TCP_FAST | DBG_FUNC_START, 0,0,0,0,0);
 199
 200     if (!tcp_delack_enabled)
 201         return;
 202
 203     for (i=0; i < (tcbinfo.hashsize / 32); i++) {
 204             if (delack_bitmask[i]) {
 205                 temp_mask = 1;
 206                 for (j=0; j < 32; j++) {
 207                     if (temp_mask & delack_bitmask[i]) {
 208                         head = &tcbinfo.hashbase[elem_base + j];
 209                         for (inp=head->lh_first; inp != 0; inp = inp->inp_hash.le_next) {
 210                             delack_checked++;
 211                             if ((tp = (struct tcpcb *)inp->inp_ppcb) && (tp->t_flags & TF_DELACK)) {
 212                                 tp->t_flags &= ~TF_DELACK;
 213                                 tp->t_flags |= TF_ACKNOW;
 214                                 tcpstat.tcps_delack++;
 215                                 (void) tcp_output(tp);
 216                             }
 217                         }
 218                     }
 219                     temp_mask <<=  1;
 220                 }
 221                 delack_bitmask[i] = 0;
 222             }
 223             elem_base += 32;
 224     }
 225     KERNEL_DEBUG(DBG_FNC_TCP_FAST | DBG_FUNC_END, delack_checked,tcpstat.tcps_delack,0,0,0);
 226     splx(s);
 227
 228 }
 229
 230 /*
 231  * Tcp protocol timeout routine called every 500 ms.
 232  * Updates the timers in all active tcb's and
 233  * causes finite state machine actions if timers expire.
 234  */
 235 void
 236 tcp_slowtimo()
 237 {
 238         register struct inpcb *ip, *ipnxt;
 239         register struct tcpcb *tp;
 240         register int i;
 241         int s;
 242 #if TCPDEBUG
 243         int ostate;
 244 #endif
 245 #if KDEBUG
 246         static int tws_checked;
 247 #endif
 248
 249         KERNEL_DEBUG(DBG_FNC_TCP_SLOW | DBG_FUNC_START, 0,0,0,0,0);
 250         s = splnet();
 251
 252         tcp_maxidle = tcp_keepcnt * tcp_keepintvl;
 253
 254         ip = tcb.lh_first;
 255         if (ip == NULL) {
 256                 splx(s);
 257                 return;
 258         }
 259         /*
 260          * Search through tcb's and update active timers.
 261          */
 262         for (; ip != NULL; ip = ipnxt) {
 263                 ipnxt = ip->inp_list.le_next;
 264                 tp = intotcpcb(ip);
 265                 if (tp == 0 || tp->t_state == TCPS_LISTEN)
 266                         continue;
 267                 /*
 268                  * Bogus state when port owned by SharedIP with loopback as the
 269                  * only configured interface: BlueBox does not filters loopback
 270                  */
 271                 if (tp->t_state == TCP_NSTATES)
 272                         continue;
 273
 274                 for (i = 0; i < TCPT_NTIMERS; i++) {
 275                         if (tp->t_timer[i] && --tp->t_timer[i] == 0) {
 276 #if TCPDEBUG
 277                                 ostate = tp->t_state;
 278 #endif
 279                                 tp = tcp_timers(tp, i);
 280                                 if (tp == NULL)
 281                                         goto tpgone;
 282 #if TCPDEBUG
 283                                 if (tp->t_inpcb->inp_socket->so_options
 284                                     & SO_DEBUG)
 285                                         tcp_trace(TA_USER, ostate, tp,
 286                                                   (void *)0,
 287                                                   (struct tcphdr *)0,
 288                                                   PRU_SLOWTIMO);
 289 #endif
 290                         }
 291                 }
 292                 tp->t_rcvtime++;
 293                 tp->t_starttime++;
 294                 if (tp->t_rtttime)
 295                         tp->t_rtttime++;
 296 tpgone:
 297                 ;
 298         }
 299
 300 #if KDEBUG
 301         tws_checked = 0;
 302 #endif
 303         KERNEL_DEBUG(DBG_FNC_TCP_SLOW | DBG_FUNC_NONE, tws_checked,0,0,0,0);
 304
 305         /*
 306          * Process the items in the current time-wait slot
 307          */
 308
 309         for (ip = time_wait_slots[cur_tw_slot].lh_first; ip; ip = ipnxt)
 310         {
 311 #if KDEBUG
 312                 tws_checked++;
 313 #endif
 314                 ipnxt = ip->inp_list.le_next;
 315                 tp = intotcpcb(ip);
 316                 if (tp == NULL) { /* tp already closed, remove from list */
 317                         LIST_REMOVE(ip, inp_list);
 318                         continue;
 319                 }
 320                 if (tp->t_timer[TCPT_2MSL] >= N_TIME_WAIT_SLOTS) {
 321                     tp->t_timer[TCPT_2MSL] -= N_TIME_WAIT_SLOTS;
 322                     tp->t_rcvtime += N_TIME_WAIT_SLOTS;
 323                 }
 324                 else
 325                     tp->t_timer[TCPT_2MSL] = 0;
 326
 327                 if (tp->t_timer[TCPT_2MSL] == 0)
 328                     tp = tcp_timers(tp, TCPT_2MSL);
 329         }
 330
 331         if (++cur_tw_slot >= N_TIME_WAIT_SLOTS)
 332                 cur_tw_slot = 0;
 333         tcp_now++;                                      /* for timestamps */
 334         splx(s);
 335         KERNEL_DEBUG(DBG_FNC_TCP_SLOW | DBG_FUNC_END, tws_checked, cur_tw_slot,0,0,0);
 336 }
 337
 338 /*
 339  * Cancel all timers for TCP tp.
 340  */
 341 void
 342 tcp_canceltimers(tp)
 343         struct tcpcb *tp;
 344 {
 345         register int i;
 346
 347         for (i = 0; i < TCPT_NTIMERS; i++)
 348                 tp->t_timer[i] = 0;
 349 }
 350
 351 int     tcp_syn_backoff[TCP_MAXRXTSHIFT + 1] =
 352     { 1, 1, 1, 1, 1, 2, 4, 8, 16, 32, 64, 64, 64 };
 353
 354 int     tcp_backoff[TCP_MAXRXTSHIFT + 1] =
 355     { 1, 2, 4, 8, 16, 32, 64, 64, 64, 64, 64, 64, 64 };
 356
 357 static int tcp_totbackoff = 511;        /* sum of tcp_backoff[] */
 358
 359 /*
 360  * TCP timer processing.
 361  */
 362 struct tcpcb *
 363 tcp_timers(tp, timer)
 364         register struct tcpcb *tp;
 365         int timer;
 366 {
 367         register int rexmt;
 368         struct socket *so_tmp;
 369         struct tcptemp *t_template;
 370
 371 #if TCPDEBUG
 372         int ostate;
 373 #endif
 374
 375 #if INET6
 376         int isipv6 = (tp->t_inpcb->inp_vflag & INP_IPV4) == 0;
 377 #endif /* INET6 */
 378
 379
 380         switch (timer) {
 381
 382         /*
 383          * 2 MSL timeout in shutdown went off.  If we're closed but
 384          * still waiting for peer to close and connection has been idle
 385          * too long, or if 2MSL time is up from TIME_WAIT, delete connection
 386          * control block.  Otherwise, check again in a bit.
 387          */
 388         case TCPT_2MSL:
 389                 if (tp->t_state != TCPS_TIME_WAIT &&
 390                     tp->t_rcvtime <= tcp_maxidle) {
 391                         tp->t_timer[TCPT_2MSL] = tcp_keepintvl;
 392                         add_to_time_wait(tp);
 393                 }
 394                 else
 395                         tp = tcp_close(tp);
 396                 break;
 397
 398         /*
 399          * Retransmission timer went off.  Message has not
 400          * been acked within retransmit interval.  Back off
 401          * to a longer retransmit interval and retransmit one segment.
 402          */
 403         case TCPT_REXMT:
 404                 if (++tp->t_rxtshift > TCP_MAXRXTSHIFT) {
 405                         tp->t_rxtshift = TCP_MAXRXTSHIFT;
 406                         tcpstat.tcps_timeoutdrop++;
 407                         so_tmp = tp->t_inpcb->inp_socket;
 408                         tp = tcp_drop(tp, tp->t_softerror ?
 409                             tp->t_softerror : ETIMEDOUT);
 410                         postevent(so_tmp, 0, EV_TIMEOUT);
 411                         break;
 412                 }
 413
 414                 if (tp->t_rxtshift == 1) {
 415                         /*
 416                          * first retransmit; record ssthresh and cwnd so they can
 417                          * be recovered if this turns out to be a "bad" retransmit.
 418                          * A retransmit is considered "bad" if an ACK for this
 419                          * segment is received within RTT/2 interval; the assumption
 420                          * here is that the ACK was already in flight.  See
 421                          * "On Estimating End-to-End Network Path Properties" by
 422                          * Allman and Paxson for more details.
 423                          */
 424                         tp->snd_cwnd_prev = tp->snd_cwnd;
 425                         tp->snd_ssthresh_prev = tp->snd_ssthresh;
 426                         tp->t_badrxtwin = tcp_now + (tp->t_srtt >> (TCP_RTT_SHIFT + 1));
 427                 }
 428                 tcpstat.tcps_rexmttimeo++;
 429                 if (tp->t_state == TCPS_SYN_SENT)
 430                         rexmt = TCP_REXMTVAL(tp) * tcp_syn_backoff[tp->t_rxtshift];
 431                 else
 432                         rexmt = TCP_REXMTVAL(tp) * tcp_backoff[tp->t_rxtshift];
 433                 TCPT_RANGESET(tp->t_rxtcur, rexmt,
 434                         tp->t_rttmin, TCPTV_REXMTMAX);
 435                 tp->t_timer[TCPT_REXMT] = tp->t_rxtcur;
 436
 437                 /*
 438                  * Disable rfc1323 and rfc1644 if we havn't got any response to
 439                  * our third SYN to work-around some broken terminal servers
 440                  * (most of which have hopefully been retired) that have bad VJ
 441                  * header compression code which trashes TCP segments containing
 442                  * unknown-to-them TCP options.
 443                  */
 444                 if ((tp->t_state == TCPS_SYN_SENT) && (tp->t_rxtshift == 3))
 445                                 tp->t_flags &= ~(TF_REQ_SCALE|TF_REQ_TSTMP|TF_REQ_CC);
 446                 /*
 447                  * If losing, let the lower level know and try for
 448                  * a better route.  Also, if we backed off this far,
 449                  * our srtt estimate is probably bogus.  Clobber it
 450                  * so we'll take the next rtt measurement as our srtt;
 451                  * move the current srtt into rttvar to keep the current
 452                  * retransmit times until then.
 453                  */
 454                 if (tp->t_rxtshift > TCP_MAXRXTSHIFT / 4) {
 455 #if INET6
 456                         if (isipv6)
 457                                 in6_losing(tp->t_inpcb);
 458                         else
 459 #endif /* INET6 */
 460                         in_losing(tp->t_inpcb);
 461                         tp->t_rttvar += (tp->t_srtt >> TCP_RTT_SHIFT);
 462                         tp->t_srtt = 0;
 463                 }
 464                 tp->snd_nxt = tp->snd_una;
 465                 /*
 466                  * Note:  We overload snd_recover to function also as the
 467                  * snd_last variable described in RFC 2582
 468                  */
 469                 tp->snd_recover = tp->snd_max;
 470                 /*
 471                  * Force a segment to be sent.
 472                  */
 473                 tp->t_flags |= TF_ACKNOW;
 474                 /*
 475                  * If timing a segment in this window, stop the timer.
 476                  */
 477                 tp->t_rtttime = 0;
 478                 /*
 479                  * Close the congestion window down to one segment
 480                  * (we'll open it by one segment for each ack we get).
 481                  * Since we probably have a window's worth of unacked
 482                  * data accumulated, this "slow start" keeps us from
 483                  * dumping all that data as back-to-back packets (which
 484                  * might overwhelm an intermediate gateway).
 485                  *
 486                  * There are two phases to the opening: Initially we
 487                  * open by one mss on each ack.  This makes the window
 488                  * size increase exponentially with time.  If the
 489                  * window is larger than the path can handle, this
 490                  * exponential growth results in dropped packet(s)
 491                  * almost immediately.  To get more time between
 492                  * drops but still "push" the network to take advantage
 493                  * of improving conditions, we switch from exponential
 494                  * to linear window opening at some threshhold size.
 495                  * For a threshhold, we use half the current window
 496                  * size, truncated to a multiple of the mss.
 497                  *
 498                  * (the minimum cwnd that will give us exponential
 499                  * growth is 2 mss.  We don't allow the threshhold
 500                  * to go below this.)
 501                  */
 502                 {
 503                 u_int win = min(tp->snd_wnd, tp->snd_cwnd) / 2 / tp->t_maxseg;
 504                 if (win < 2)
 505                         win = 2;
 506                 tp->snd_cwnd = tp->t_maxseg;
 507                 tp->snd_ssthresh = win * tp->t_maxseg;
 508                 tp->t_dupacks = 0;
 509                 }
 510                 (void) tcp_output(tp);
 511                 break;
 512
 513         /*
 514          * Persistance timer into zero window.
 515          * Force a byte to be output, if possible.
 516          */
 517         case TCPT_PERSIST:
 518                 tcpstat.tcps_persisttimeo++;
 519                 /*
 520                  * Hack: if the peer is dead/unreachable, we do not
 521                  * time out if the window is closed.  After a full
 522                  * backoff, drop the connection if the idle time
 523                  * (no responses to probes) reaches the maximum
 524                  * backoff that we would use if retransmitting.
 525                  */
 526                 if (tp->t_rxtshift == TCP_MAXRXTSHIFT &&
 527                     (tp->t_rcvtime >= tcp_maxpersistidle ||
 528                     tp->t_rcvtime >= TCP_REXMTVAL(tp) * tcp_totbackoff)) {
 529                         tcpstat.tcps_persistdrop++;
 530                         so_tmp = tp->t_inpcb->inp_socket;
 531                         tp = tcp_drop(tp, ETIMEDOUT);
 532                         postevent(so_tmp, 0, EV_TIMEOUT);
 533                         break;
 534                 }
 535                 tcp_setpersist(tp);
 536                 tp->t_force = 1;
 537                 (void) tcp_output(tp);
 538                 tp->t_force = 0;
 539                 break;
 540
 541         /*
 542          * Keep-alive timer went off; send something
 543          * or drop connection if idle for too long.
 544          */
 545         case TCPT_KEEP:
 546                 tcpstat.tcps_keeptimeo++;
 547                 if (tp->t_state < TCPS_ESTABLISHED)
 548                         goto dropit;
 549                 if ((always_keepalive ||
 550                     tp->t_inpcb->inp_socket->so_options & SO_KEEPALIVE) &&
 551                     tp->t_state <= TCPS_CLOSING) {
 552                         if (tp->t_rcvtime >= TCP_KEEPIDLE(tp) + tcp_maxidle)
 553                                 goto dropit;
 554                         /*
 555                          * Send a packet designed to force a response
 556                          * if the peer is up and reachable:
 557                          * either an ACK if the connection is still alive,
 558                          * or an RST if the peer has closed the connection
 559                          * due to timeout or reboot.
 560                          * Using sequence number tp->snd_una-1
 561                          * causes the transmitted zero-length segment
 562                          * to lie outside the receive window;
 563                          * by the protocol spec, this requires the
 564                          * correspondent TCP to respond.
 565                          */
 566                         tcpstat.tcps_keepprobe++;
 567                         t_template = tcp_maketemplate(tp);
 568                         if (t_template) {
 569                                 tcp_respond(tp, t_template->tt_ipgen,
 570                                     &t_template->tt_t, (struct mbuf *)NULL,
 571                                     tp->rcv_nxt, tp->snd_una - 1, 0);
 572                                 (void) m_free(dtom(t_template));
 573                         }
 574                         tp->t_timer[TCPT_KEEP] = tcp_keepintvl;
 575                 } else
 576                         tp->t_timer[TCPT_KEEP] = TCP_KEEPIDLE(tp);
 577                 break;
 578
 579 #if TCPDEBUG
 580         if (tp->t_inpcb->inp_socket->so_options & SO_DEBUG)
 581                 tcp_trace(TA_USER, ostate, tp, (void *)0, (struct tcphdr *)0,
 582                           PRU_SLOWTIMO);
 583 #endif
 584         dropit:
 585                 tcpstat.tcps_keepdrops++;
 586                 so_tmp = tp->t_inpcb->inp_socket;
 587                 tp = tcp_drop(tp, ETIMEDOUT);
 588                 postevent(so_tmp, 0, EV_TIMEOUT);
 589                 break;
 590         }
 591         return (tp);
 592 }