sys/netinet/tcp_timer.c

   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_timer.c 8.2 (Berkeley) 5/24/95
  30  * $FreeBSD$
  31  */
  32
  33 #include "opt_inet6.h"
  34 #include "opt_tcpdebug.h"
  35 #include "opt_tcp_sack.h"
  36
  37 #include <sys/param.h>
  38 #include <sys/kernel.h>
  39 #include <sys/lock.h>
  40 #include <sys/mbuf.h>
  41 #include <sys/mutex.h>
  42 #include <sys/protosw.h>
  43 #include <sys/socket.h>
  44 #include <sys/socketvar.h>
  45 #include <sys/sysctl.h>
  46 #include <sys/systm.h>
  47
  48 #include <net/route.h>
  49
  50 #include <netinet/in.h>
  51 #include <netinet/in_pcb.h>
  52 #include <netinet/in_systm.h>
  53 #ifdef INET6
  54 #include <netinet6/in6_pcb.h>
  55 #endif
  56 #include <netinet/ip_var.h>
  57 #include <netinet/tcp.h>
  58 #include <netinet/tcp_fsm.h>
  59 #include <netinet/tcp_timer.h>
  60 #include <netinet/tcp_var.h>
  61 #include <netinet/tcpip.h>
  62 #ifdef TCPDEBUG
  63 #include <netinet/tcp_debug.h>
  64 #endif
  65
  66 int     tcp_keepinit;
  67 SYSCTL_PROC(_net_inet_tcp, TCPCTL_KEEPINIT, keepinit, CTLTYPE_INT|CTLFLAG_RW,
  68     &tcp_keepinit, 0, sysctl_msec_to_ticks, "I", "");
  69
  70 int     tcp_keepidle;
  71 SYSCTL_PROC(_net_inet_tcp, TCPCTL_KEEPIDLE, keepidle, CTLTYPE_INT|CTLFLAG_RW,
  72     &tcp_keepidle, 0, sysctl_msec_to_ticks, "I", "");
  73
  74 int     tcp_keepintvl;
  75 SYSCTL_PROC(_net_inet_tcp, TCPCTL_KEEPINTVL, keepintvl, CTLTYPE_INT|CTLFLAG_RW,
  76     &tcp_keepintvl, 0, sysctl_msec_to_ticks, "I", "");
  77
  78 int     tcp_delacktime;
  79 SYSCTL_PROC(_net_inet_tcp, TCPCTL_DELACKTIME, delacktime,
  80     CTLTYPE_INT|CTLFLAG_RW, &tcp_delacktime, 0, sysctl_msec_to_ticks, "I",
  81     "Time before a delayed ACK is sent");
  82
  83 int     tcp_msl;
  84 SYSCTL_PROC(_net_inet_tcp, OID_AUTO, msl, CTLTYPE_INT|CTLFLAG_RW,
  85     &tcp_msl, 0, sysctl_msec_to_ticks, "I", "Maximum segment lifetime");
  86
  87 int     tcp_rexmit_min;
  88 SYSCTL_PROC(_net_inet_tcp, OID_AUTO, rexmit_min, CTLTYPE_INT|CTLFLAG_RW,
  89     &tcp_rexmit_min, 0, sysctl_msec_to_ticks, "I", "Minimum Retransmission Timeout");
  90
  91 int     tcp_rexmit_slop;
  92 SYSCTL_PROC(_net_inet_tcp, OID_AUTO, rexmit_slop, CTLTYPE_INT|CTLFLAG_RW,
  93     &tcp_rexmit_slop, 0, sysctl_msec_to_ticks, "I", "Retransmission Timer Slop");
  94
  95 static int      always_keepalive = 1;
  96 SYSCTL_INT(_net_inet_tcp, OID_AUTO, always_keepalive, CTLFLAG_RW,
  97     &always_keepalive , 0, "Assume SO_KEEPALIVE on all TCP connections");
  98
  99 static int      tcp_keepcnt = TCPTV_KEEPCNT;
 100         /* max idle probes */
 101 int     tcp_maxpersistidle;
 102         /* max idle time in persist */
 103 int     tcp_maxidle;
 104
 105 /*
 106  * Tcp protocol timeout routine called every 500 ms.
 107  * Updates timestamps used for TCP
 108  * causes finite state machine actions if timers expire.
 109  */
 110 void
 111 tcp_slowtimo()
 112 {
 113
 114         tcp_maxidle = tcp_keepcnt * tcp_keepintvl;
 115         INP_INFO_WLOCK(&tcbinfo);
 116         (void) tcp_timer_2msl_tw(0);
 117         INP_INFO_WUNLOCK(&tcbinfo);
 118 }
 119
 120 int     tcp_syn_backoff[TCP_MAXRXTSHIFT + 1] =
 121     { 1, 1, 1, 1, 1, 2, 4, 8, 16, 32, 64, 64, 64 };
 122
 123 int     tcp_backoff[TCP_MAXRXTSHIFT + 1] =
 124     { 1, 2, 4, 8, 16, 32, 64, 128, 256, 512, 512, 512, 512 };
 125
 126 static int tcp_totbackoff = 2559;       /* sum of tcp_backoff[] */
 127
 128 static int tcp_timer_race;
 129 SYSCTL_INT(_net_inet_tcp, OID_AUTO, timer_race, CTLFLAG_RD, &tcp_timer_race,
 130     0, "Count of t_inpcb races on tcp_discardcb");
 131
 132 /*
 133  * TCP timer processing.
 134  */
 135
 136 void
 137 tcp_timer_delack(xtp)
 138         void *xtp;
 139 {
 140         struct tcpcb *tp = xtp;
 141         struct inpcb *inp;
 142
 143         INP_INFO_RLOCK(&tcbinfo);
 144         inp = tp->t_inpcb;
 145         /*
 146          * XXXRW: While this assert is in fact correct, bugs in the tcpcb
 147          * tear-down mean we need it as a work-around for races between
 148          * timers and tcp_discardcb().
 149          *
 150          * KASSERT(inp != NULL, ("tcp_timer_delack: inp == NULL"));
 151          */
 152         if (inp == NULL) {
 153                 tcp_timer_race++;
 154                 INP_INFO_RUNLOCK(&tcbinfo);
 155                 return;
 156         }
 157         INP_LOCK(inp);
 158         INP_INFO_RUNLOCK(&tcbinfo);
 159         if ((inp->inp_vflag & INP_DROPPED) || callout_pending(tp->tt_delack)
 160             || !callout_active(tp->tt_delack)) {
 161                 INP_UNLOCK(inp);
 162                 return;
 163         }
 164         callout_deactivate(tp->tt_delack);
 165
 166         tp->t_flags |= TF_ACKNOW;
 167         tcpstat.tcps_delack++;
 168         (void) tcp_output(tp);
 169         INP_UNLOCK(inp);
 170 }
 171
 172 void
 173 tcp_timer_2msl(xtp)
 174         void *xtp;
 175 {
 176         struct tcpcb *tp = xtp;
 177         struct inpcb *inp;
 178 #ifdef TCPDEBUG
 179         int ostate;
 180
 181         ostate = tp->t_state;
 182 #endif
 183         /*
 184          * XXXRW: Does this actually happen?
 185          */
 186         INP_INFO_WLOCK(&tcbinfo);
 187         inp = tp->t_inpcb;
 188         /*
 189          * XXXRW: While this assert is in fact correct, bugs in the tcpcb
 190          * tear-down mean we need it as a work-around for races between
 191          * timers and tcp_discardcb().
 192          *
 193          * KASSERT(inp != NULL, ("tcp_timer_2msl: inp == NULL"));
 194          */
 195         if (inp == NULL) {
 196                 tcp_timer_race++;
 197                 INP_INFO_WUNLOCK(&tcbinfo);
 198                 return;
 199         }
 200         INP_LOCK(inp);
 201         tcp_free_sackholes(tp);
 202         if ((inp->inp_vflag & INP_DROPPED) || callout_pending(tp->tt_2msl) ||
 203             !callout_active(tp->tt_2msl)) {
 204                 INP_UNLOCK(tp->t_inpcb);
 205                 INP_INFO_WUNLOCK(&tcbinfo);
 206                 return;
 207         }
 208         callout_deactivate(tp->tt_2msl);
 209         /*
 210          * 2 MSL timeout in shutdown went off.  If we're closed but
 211          * still waiting for peer to close and connection has been idle
 212          * too long, or if 2MSL time is up from TIME_WAIT, delete connection
 213          * control block.  Otherwise, check again in a bit.
 214          */
 215         if (tp->t_state != TCPS_TIME_WAIT &&
 216             (ticks - tp->t_rcvtime) <= tcp_maxidle)
 217                 callout_reset(tp->tt_2msl, tcp_keepintvl,
 218                               tcp_timer_2msl, tp);
 219         else
 220                 tp = tcp_close(tp);
 221
 222 #ifdef TCPDEBUG
 223         if (tp != NULL && (tp->t_inpcb->inp_socket->so_options & SO_DEBUG))
 224                 tcp_trace(TA_USER, ostate, tp, (void *)0, (struct tcphdr *)0,
 225                           PRU_SLOWTIMO);
 226 #endif
 227         if (tp != NULL)
 228                 INP_UNLOCK(inp);
 229         INP_INFO_WUNLOCK(&tcbinfo);
 230 }
 231
 232 /*
 233  * The timed wait lists contain references to each of the TCP sessions
 234  * currently TIME_WAIT state.  The list pointers, including the list pointers
 235  * in each tcptw structure, are protected using the global tcbinfo lock,
 236  * which must be held over list iteration and modification.
 237  */
 238 struct twlist {
 239         LIST_HEAD(, tcptw)      tw_list;
 240         struct tcptw    tw_tail;
 241 };
 242 #define TWLIST_NLISTS   2
 243 static struct twlist twl_2msl[TWLIST_NLISTS];
 244 static struct twlist *tw_2msl_list[] = { &twl_2msl[0], &twl_2msl[1], NULL };
 245
 246 void
 247 tcp_timer_init(void)
 248 {
 249         int i;
 250         struct twlist *twl;
 251
 252         for (i = 0; i < TWLIST_NLISTS; i++) {
 253                 twl = &twl_2msl[i];
 254                 LIST_INIT(&twl->tw_list);
 255                 LIST_INSERT_HEAD(&twl->tw_list, &twl->tw_tail, tw_2msl);
 256         }
 257 }
 258
 259 void
 260 tcp_timer_2msl_reset(struct tcptw *tw, int timeo, int rearm)
 261 {
 262         int i;
 263         struct tcptw *tw_tail;
 264
 265         INP_INFO_WLOCK_ASSERT(&tcbinfo);
 266         INP_LOCK_ASSERT(tw->tw_inpcb);
 267         if (rearm)
 268                 LIST_REMOVE(tw, tw_2msl);
 269         tw->tw_time = timeo + ticks;
 270         i = timeo > tcp_msl ? 1 : 0;
 271         tw_tail = &twl_2msl[i].tw_tail;
 272         LIST_INSERT_BEFORE(tw_tail, tw, tw_2msl);
 273 }
 274
 275 void
 276 tcp_timer_2msl_stop(struct tcptw *tw)
 277 {
 278
 279         INP_INFO_WLOCK_ASSERT(&tcbinfo);
 280         LIST_REMOVE(tw, tw_2msl);
 281 }
 282
 283 struct tcptw *
 284 tcp_timer_2msl_tw(int reuse)
 285 {
 286         struct tcptw *tw, *tw_tail;
 287         struct twlist *twl;
 288         int i;
 289
 290         INP_INFO_WLOCK_ASSERT(&tcbinfo);
 291         for (i = 0; i < 2; i++) {
 292                 twl = tw_2msl_list[i];
 293                 tw_tail = &twl->tw_tail;
 294                 tw = LIST_FIRST(&twl->tw_list);
 295                 if (tw == tw_tail || (!reuse && tw->tw_time > ticks))
 296                         continue;
 297                 INP_LOCK(tw->tw_inpcb);
 298                 tcp_twclose(tw, reuse);
 299                 return (reuse ? tw : NULL);
 300         }
 301         return (NULL);
 302 }
 303
 304 void
 305 tcp_timer_keep(xtp)
 306         void *xtp;
 307 {
 308         struct tcpcb *tp = xtp;
 309         struct tcptemp *t_template;
 310         struct inpcb *inp;
 311 #ifdef TCPDEBUG
 312         int ostate;
 313
 314         ostate = tp->t_state;
 315 #endif
 316         INP_INFO_WLOCK(&tcbinfo);
 317         inp = tp->t_inpcb;
 318         /*
 319          * XXXRW: While this assert is in fact correct, bugs in the tcpcb
 320          * tear-down mean we need it as a work-around for races between
 321          * timers and tcp_discardcb().
 322          *
 323          * KASSERT(inp != NULL, ("tcp_timer_keep: inp == NULL"));
 324          */
 325         if (inp == NULL) {
 326                 tcp_timer_race++;
 327                 INP_INFO_WUNLOCK(&tcbinfo);
 328                 return;
 329         }
 330         INP_LOCK(inp);
 331         if ((inp->inp_vflag & INP_DROPPED) || callout_pending(tp->tt_keep)
 332             || !callout_active(tp->tt_keep)) {
 333                 INP_UNLOCK(inp);
 334                 INP_INFO_WUNLOCK(&tcbinfo);
 335                 return;
 336         }
 337         callout_deactivate(tp->tt_keep);
 338         /*
 339          * Keep-alive timer went off; send something
 340          * or drop connection if idle for too long.
 341          */
 342         tcpstat.tcps_keeptimeo++;
 343         if (tp->t_state < TCPS_ESTABLISHED)
 344                 goto dropit;
 345         if ((always_keepalive || inp->inp_socket->so_options & SO_KEEPALIVE) &&
 346             tp->t_state <= TCPS_CLOSING) {
 347                 if ((ticks - tp->t_rcvtime) >= tcp_keepidle + tcp_maxidle)
 348                         goto dropit;
 349                 /*
 350                  * Send a packet designed to force a response
 351                  * if the peer is up and reachable:
 352                  * either an ACK if the connection is still alive,
 353                  * or an RST if the peer has closed the connection
 354                  * due to timeout or reboot.
 355                  * Using sequence number tp->snd_una-1
 356                  * causes the transmitted zero-length segment
 357                  * to lie outside the receive window;
 358                  * by the protocol spec, this requires the
 359                  * correspondent TCP to respond.
 360                  */
 361                 tcpstat.tcps_keepprobe++;
 362                 t_template = tcpip_maketemplate(inp);
 363                 if (t_template) {
 364                         tcp_respond(tp, t_template->tt_ipgen,
 365                                     &t_template->tt_t, (struct mbuf *)NULL,
 366                                     tp->rcv_nxt, tp->snd_una - 1, 0);
 367                         (void) m_free(dtom(t_template));
 368                 }
 369                 callout_reset(tp->tt_keep, tcp_keepintvl, tcp_timer_keep, tp);
 370         } else
 371                 callout_reset(tp->tt_keep, tcp_keepidle, tcp_timer_keep, tp);
 372
 373 #ifdef TCPDEBUG
 374         if (inp->inp_socket->so_options & SO_DEBUG)
 375                 tcp_trace(TA_USER, ostate, tp, (void *)0, (struct tcphdr *)0,
 376                           PRU_SLOWTIMO);
 377 #endif
 378         INP_UNLOCK(inp);
 379         INP_INFO_WUNLOCK(&tcbinfo);
 380         return;
 381
 382 dropit:
 383         tcpstat.tcps_keepdrops++;
 384         tp = tcp_drop(tp, ETIMEDOUT);
 385
 386 #ifdef TCPDEBUG
 387         if (tp != NULL && (tp->t_inpcb->inp_socket->so_options & SO_DEBUG))
 388                 tcp_trace(TA_USER, ostate, tp, (void *)0, (struct tcphdr *)0,
 389                           PRU_SLOWTIMO);
 390 #endif
 391         if (tp != NULL)
 392                 INP_UNLOCK(tp->t_inpcb);
 393         INP_INFO_WUNLOCK(&tcbinfo);
 394 }
 395
 396 void
 397 tcp_timer_persist(xtp)
 398         void *xtp;
 399 {
 400         struct tcpcb *tp = xtp;
 401         struct inpcb *inp;
 402 #ifdef TCPDEBUG
 403         int ostate;
 404
 405         ostate = tp->t_state;
 406 #endif
 407         INP_INFO_WLOCK(&tcbinfo);
 408         inp = tp->t_inpcb;
 409         /*
 410          * XXXRW: While this assert is in fact correct, bugs in the tcpcb
 411          * tear-down mean we need it as a work-around for races between
 412          * timers and tcp_discardcb().
 413          *
 414          * KASSERT(inp != NULL, ("tcp_timer_persist: inp == NULL"));
 415          */
 416         if (inp == NULL) {
 417                 tcp_timer_race++;
 418                 INP_INFO_WUNLOCK(&tcbinfo);
 419                 return;
 420         }
 421         INP_LOCK(inp);
 422         if ((inp->inp_vflag & INP_DROPPED) || callout_pending(tp->tt_persist)
 423             || !callout_active(tp->tt_persist)) {
 424                 INP_UNLOCK(inp);
 425                 INP_INFO_WUNLOCK(&tcbinfo);
 426                 return;
 427         }
 428         callout_deactivate(tp->tt_persist);
 429         /*
 430          * Persistance timer into zero window.
 431          * Force a byte to be output, if possible.
 432          */
 433         tcpstat.tcps_persisttimeo++;
 434         /*
 435          * Hack: if the peer is dead/unreachable, we do not
 436          * time out if the window is closed.  After a full
 437          * backoff, drop the connection if the idle time
 438          * (no responses to probes) reaches the maximum
 439          * backoff that we would use if retransmitting.
 440          */
 441         if (tp->t_rxtshift == TCP_MAXRXTSHIFT &&
 442             ((ticks - tp->t_rcvtime) >= tcp_maxpersistidle ||
 443              (ticks - tp->t_rcvtime) >= TCP_REXMTVAL(tp) * tcp_totbackoff)) {
 444                 tcpstat.tcps_persistdrop++;
 445                 tp = tcp_drop(tp, ETIMEDOUT);
 446                 goto out;
 447         }
 448         tcp_setpersist(tp);
 449         tp->t_flags |= TF_FORCEDATA;
 450         (void) tcp_output(tp);
 451         tp->t_flags &= ~TF_FORCEDATA;
 452
 453 out:
 454 #ifdef TCPDEBUG
 455         if (tp != NULL && tp->t_inpcb->inp_socket->so_options & SO_DEBUG)
 456                 tcp_trace(TA_USER, ostate, tp, NULL, NULL, PRU_SLOWTIMO);
 457 #endif
 458         if (tp != NULL)
 459                 INP_UNLOCK(inp);
 460         INP_INFO_WUNLOCK(&tcbinfo);
 461 }
 462
 463 void
 464 tcp_timer_rexmt(xtp)
 465         void *xtp;
 466 {
 467         struct tcpcb *tp = xtp;
 468         int rexmt;
 469         int headlocked;
 470         struct inpcb *inp;
 471 #ifdef TCPDEBUG
 472         int ostate;
 473
 474         ostate = tp->t_state;
 475 #endif
 476         INP_INFO_WLOCK(&tcbinfo);
 477         headlocked = 1;
 478         inp = tp->t_inpcb;
 479         /*
 480          * XXXRW: While this assert is in fact correct, bugs in the tcpcb
 481          * tear-down mean we need it as a work-around for races between
 482          * timers and tcp_discardcb().
 483          *
 484          * KASSERT(inp != NULL, ("tcp_timer_rexmt: inp == NULL"));
 485          */
 486         if (inp == NULL) {
 487                 tcp_timer_race++;
 488                 INP_INFO_WUNLOCK(&tcbinfo);
 489                 return;
 490         }
 491         INP_LOCK(inp);
 492         if ((inp->inp_vflag & INP_DROPPED) || callout_pending(tp->tt_rexmt)
 493             || !callout_active(tp->tt_rexmt)) {
 494                 INP_UNLOCK(inp);
 495                 INP_INFO_WUNLOCK(&tcbinfo);
 496                 return;
 497         }
 498         callout_deactivate(tp->tt_rexmt);
 499         tcp_free_sackholes(tp);
 500         /*
 501          * Retransmission timer went off.  Message has not
 502          * been acked within retransmit interval.  Back off
 503          * to a longer retransmit interval and retransmit one segment.
 504          */
 505         if (++tp->t_rxtshift > TCP_MAXRXTSHIFT) {
 506                 tp->t_rxtshift = TCP_MAXRXTSHIFT;
 507                 tcpstat.tcps_timeoutdrop++;
 508                 tp = tcp_drop(tp, tp->t_softerror ?
 509                               tp->t_softerror : ETIMEDOUT);
 510                 goto out;
 511         }
 512         INP_INFO_WUNLOCK(&tcbinfo);
 513         headlocked = 0;
 514         if (tp->t_rxtshift == 1) {
 515                 /*
 516                  * first retransmit; record ssthresh and cwnd so they can
 517                  * be recovered if this turns out to be a "bad" retransmit.
 518                  * A retransmit is considered "bad" if an ACK for this
 519                  * segment is received within RTT/2 interval; the assumption
 520                  * here is that the ACK was already in flight.  See
 521                  * "On Estimating End-to-End Network Path Properties" by
 522                  * Allman and Paxson for more details.
 523                  */
 524                 tp->snd_cwnd_prev = tp->snd_cwnd;
 525                 tp->snd_ssthresh_prev = tp->snd_ssthresh;
 526                 tp->snd_recover_prev = tp->snd_recover;
 527                 if (IN_FASTRECOVERY(tp))
 528                   tp->t_flags |= TF_WASFRECOVERY;
 529                 else
 530                   tp->t_flags &= ~TF_WASFRECOVERY;
 531                 tp->t_badrxtwin = ticks + (tp->t_srtt >> (TCP_RTT_SHIFT + 1));
 532         }
 533         tcpstat.tcps_rexmttimeo++;
 534         if (tp->t_state == TCPS_SYN_SENT)
 535                 rexmt = TCP_REXMTVAL(tp) * tcp_syn_backoff[tp->t_rxtshift];
 536         else
 537                 rexmt = TCP_REXMTVAL(tp) * tcp_backoff[tp->t_rxtshift];
 538         TCPT_RANGESET(tp->t_rxtcur, rexmt,
 539                       tp->t_rttmin, TCPTV_REXMTMAX);
 540         /*
 541          * Disable rfc1323 if we havn't got any response to
 542          * our third SYN to work-around some broken terminal servers
 543          * (most of which have hopefully been retired) that have bad VJ
 544          * header compression code which trashes TCP segments containing
 545          * unknown-to-them TCP options.
 546          */
 547         if ((tp->t_state == TCPS_SYN_SENT) && (tp->t_rxtshift == 3))
 548                 tp->t_flags &= ~(TF_REQ_SCALE|TF_REQ_TSTMP);
 549         /*
 550          * If we backed off this far, our srtt estimate is probably bogus.
 551          * Clobber it so we'll take the next rtt measurement as our srtt;
 552          * move the current srtt into rttvar to keep the current
 553          * retransmit times until then.
 554          */
 555         if (tp->t_rxtshift > TCP_MAXRXTSHIFT / 4) {
 556 #ifdef INET6
 557                 if ((tp->t_inpcb->inp_vflag & INP_IPV6) != 0)
 558                         in6_losing(tp->t_inpcb);
 559                 else
 560 #endif
 561                 tp->t_rttvar += (tp->t_srtt >> TCP_RTT_SHIFT);
 562                 tp->t_srtt = 0;
 563         }
 564         tp->snd_nxt = tp->snd_una;
 565         tp->snd_recover = tp->snd_max;
 566         /*
 567          * Force a segment to be sent.
 568          */
 569         tp->t_flags |= TF_ACKNOW;
 570         /*
 571          * If timing a segment in this window, stop the timer.
 572          */
 573         tp->t_rtttime = 0;
 574         /*
 575          * Close the congestion window down to one segment
 576          * (we'll open it by one segment for each ack we get).
 577          * Since we probably have a window's worth of unacked
 578          * data accumulated, this "slow start" keeps us from
 579          * dumping all that data as back-to-back packets (which
 580          * might overwhelm an intermediate gateway).
 581          *
 582          * There are two phases to the opening: Initially we
 583          * open by one mss on each ack.  This makes the window
 584          * size increase exponentially with time.  If the
 585          * window is larger than the path can handle, this
 586          * exponential growth results in dropped packet(s)
 587          * almost immediately.  To get more time between
 588          * drops but still "push" the network to take advantage
 589          * of improving conditions, we switch from exponential
 590          * to linear window opening at some threshhold size.
 591          * For a threshhold, we use half the current window
 592          * size, truncated to a multiple of the mss.
 593          *
 594          * (the minimum cwnd that will give us exponential
 595          * growth is 2 mss.  We don't allow the threshhold
 596          * to go below this.)
 597          */
 598         {
 599                 u_int win = min(tp->snd_wnd, tp->snd_cwnd) / 2 / tp->t_maxseg;
 600                 if (win < 2)
 601                         win = 2;
 602                 tp->snd_cwnd = tp->t_maxseg;
 603                 tp->snd_ssthresh = win * tp->t_maxseg;
 604                 tp->t_dupacks = 0;
 605         }
 606         EXIT_FASTRECOVERY(tp);
 607         (void) tcp_output(tp);
 608
 609 out:
 610 #ifdef TCPDEBUG
 611         if (tp != NULL && (tp->t_inpcb->inp_socket->so_options & SO_DEBUG))
 612                 tcp_trace(TA_USER, ostate, tp, (void *)0, (struct tcphdr *)0,
 613                           PRU_SLOWTIMO);
 614 #endif
 615         if (tp != NULL)
 616                 INP_UNLOCK(inp);
 617         if (headlocked)
 618                 INP_INFO_WUNLOCK(&tcbinfo);
 619 }