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 queue contains references to each of the TCP sessions
 234  * currently in the TIME_WAIT state.  The queue pointers, including the
 235  * queue pointers in each tcptw structure, are protected using the global
 236  * tcbinfo lock, which must be held over queue iteration and modification.
 237  */
 238 static TAILQ_HEAD(, tcptw)      twq_2msl;
 239
 240 void
 241 tcp_timer_init(void)
 242 {
 243
 244         TAILQ_INIT(&twq_2msl);
 245 }
 246
 247 void
 248 tcp_timer_2msl_reset(struct tcptw *tw, int rearm)
 249 {
 250
 251         INP_INFO_WLOCK_ASSERT(&tcbinfo);
 252         INP_LOCK_ASSERT(tw->tw_inpcb);
 253         if (rearm)
 254                 TAILQ_REMOVE(&twq_2msl, tw, tw_2msl);
 255         tw->tw_time = ticks + 2 * tcp_msl;
 256         TAILQ_INSERT_TAIL(&twq_2msl, tw, tw_2msl);
 257 }
 258
 259 void
 260 tcp_timer_2msl_stop(struct tcptw *tw)
 261 {
 262
 263         INP_INFO_WLOCK_ASSERT(&tcbinfo);
 264         TAILQ_REMOVE(&twq_2msl, tw, tw_2msl);
 265 }
 266
 267 struct tcptw *
 268 tcp_timer_2msl_tw(int reuse)
 269 {
 270         struct tcptw *tw;
 271
 272         INP_INFO_WLOCK_ASSERT(&tcbinfo);
 273         for (;;) {
 274                 tw = TAILQ_FIRST(&twq_2msl);
 275                 if (tw == NULL || (!reuse && tw->tw_time > ticks))
 276                         break;
 277                 INP_LOCK(tw->tw_inpcb);
 278                 tcp_twclose(tw, reuse);
 279                 if (reuse)
 280                         return (tw);
 281         }
 282         return (NULL);
 283 }
 284
 285 void
 286 tcp_timer_keep(xtp)
 287         void *xtp;
 288 {
 289         struct tcpcb *tp = xtp;
 290         struct tcptemp *t_template;
 291         struct inpcb *inp;
 292 #ifdef TCPDEBUG
 293         int ostate;
 294
 295         ostate = tp->t_state;
 296 #endif
 297         INP_INFO_WLOCK(&tcbinfo);
 298         inp = tp->t_inpcb;
 299         /*
 300          * XXXRW: While this assert is in fact correct, bugs in the tcpcb
 301          * tear-down mean we need it as a work-around for races between
 302          * timers and tcp_discardcb().
 303          *
 304          * KASSERT(inp != NULL, ("tcp_timer_keep: inp == NULL"));
 305          */
 306         if (inp == NULL) {
 307                 tcp_timer_race++;
 308                 INP_INFO_WUNLOCK(&tcbinfo);
 309                 return;
 310         }
 311         INP_LOCK(inp);
 312         if ((inp->inp_vflag & INP_DROPPED) || callout_pending(tp->tt_keep)
 313             || !callout_active(tp->tt_keep)) {
 314                 INP_UNLOCK(inp);
 315                 INP_INFO_WUNLOCK(&tcbinfo);
 316                 return;
 317         }
 318         callout_deactivate(tp->tt_keep);
 319         /*
 320          * Keep-alive timer went off; send something
 321          * or drop connection if idle for too long.
 322          */
 323         tcpstat.tcps_keeptimeo++;
 324         if (tp->t_state < TCPS_ESTABLISHED)
 325                 goto dropit;
 326         if ((always_keepalive || inp->inp_socket->so_options & SO_KEEPALIVE) &&
 327             tp->t_state <= TCPS_CLOSING) {
 328                 if ((ticks - tp->t_rcvtime) >= tcp_keepidle + tcp_maxidle)
 329                         goto dropit;
 330                 /*
 331                  * Send a packet designed to force a response
 332                  * if the peer is up and reachable:
 333                  * either an ACK if the connection is still alive,
 334                  * or an RST if the peer has closed the connection
 335                  * due to timeout or reboot.
 336                  * Using sequence number tp->snd_una-1
 337                  * causes the transmitted zero-length segment
 338                  * to lie outside the receive window;
 339                  * by the protocol spec, this requires the
 340                  * correspondent TCP to respond.
 341                  */
 342                 tcpstat.tcps_keepprobe++;
 343                 t_template = tcpip_maketemplate(inp);
 344                 if (t_template) {
 345                         tcp_respond(tp, t_template->tt_ipgen,
 346                                     &t_template->tt_t, (struct mbuf *)NULL,
 347                                     tp->rcv_nxt, tp->snd_una - 1, 0);
 348                         (void) m_free(dtom(t_template));
 349                 }
 350                 callout_reset(tp->tt_keep, tcp_keepintvl, tcp_timer_keep, tp);
 351         } else
 352                 callout_reset(tp->tt_keep, tcp_keepidle, tcp_timer_keep, tp);
 353
 354 #ifdef TCPDEBUG
 355         if (inp->inp_socket->so_options & SO_DEBUG)
 356                 tcp_trace(TA_USER, ostate, tp, (void *)0, (struct tcphdr *)0,
 357                           PRU_SLOWTIMO);
 358 #endif
 359         INP_UNLOCK(inp);
 360         INP_INFO_WUNLOCK(&tcbinfo);
 361         return;
 362
 363 dropit:
 364         tcpstat.tcps_keepdrops++;
 365         tp = tcp_drop(tp, ETIMEDOUT);
 366
 367 #ifdef TCPDEBUG
 368         if (tp != NULL && (tp->t_inpcb->inp_socket->so_options & SO_DEBUG))
 369                 tcp_trace(TA_USER, ostate, tp, (void *)0, (struct tcphdr *)0,
 370                           PRU_SLOWTIMO);
 371 #endif
 372         if (tp != NULL)
 373                 INP_UNLOCK(tp->t_inpcb);
 374         INP_INFO_WUNLOCK(&tcbinfo);
 375 }
 376
 377 void
 378 tcp_timer_persist(xtp)
 379         void *xtp;
 380 {
 381         struct tcpcb *tp = xtp;
 382         struct inpcb *inp;
 383 #ifdef TCPDEBUG
 384         int ostate;
 385
 386         ostate = tp->t_state;
 387 #endif
 388         INP_INFO_WLOCK(&tcbinfo);
 389         inp = tp->t_inpcb;
 390         /*
 391          * XXXRW: While this assert is in fact correct, bugs in the tcpcb
 392          * tear-down mean we need it as a work-around for races between
 393          * timers and tcp_discardcb().
 394          *
 395          * KASSERT(inp != NULL, ("tcp_timer_persist: inp == NULL"));
 396          */
 397         if (inp == NULL) {
 398                 tcp_timer_race++;
 399                 INP_INFO_WUNLOCK(&tcbinfo);
 400                 return;
 401         }
 402         INP_LOCK(inp);
 403         if ((inp->inp_vflag & INP_DROPPED) || callout_pending(tp->tt_persist)
 404             || !callout_active(tp->tt_persist)) {
 405                 INP_UNLOCK(inp);
 406                 INP_INFO_WUNLOCK(&tcbinfo);
 407                 return;
 408         }
 409         callout_deactivate(tp->tt_persist);
 410         /*
 411          * Persistance timer into zero window.
 412          * Force a byte to be output, if possible.
 413          */
 414         tcpstat.tcps_persisttimeo++;
 415         /*
 416          * Hack: if the peer is dead/unreachable, we do not
 417          * time out if the window is closed.  After a full
 418          * backoff, drop the connection if the idle time
 419          * (no responses to probes) reaches the maximum
 420          * backoff that we would use if retransmitting.
 421          */
 422         if (tp->t_rxtshift == TCP_MAXRXTSHIFT &&
 423             ((ticks - tp->t_rcvtime) >= tcp_maxpersistidle ||
 424              (ticks - tp->t_rcvtime) >= TCP_REXMTVAL(tp) * tcp_totbackoff)) {
 425                 tcpstat.tcps_persistdrop++;
 426                 tp = tcp_drop(tp, ETIMEDOUT);
 427                 goto out;
 428         }
 429         tcp_setpersist(tp);
 430         tp->t_flags |= TF_FORCEDATA;
 431         (void) tcp_output(tp);
 432         tp->t_flags &= ~TF_FORCEDATA;
 433
 434 out:
 435 #ifdef TCPDEBUG
 436         if (tp != NULL && tp->t_inpcb->inp_socket->so_options & SO_DEBUG)
 437                 tcp_trace(TA_USER, ostate, tp, NULL, NULL, PRU_SLOWTIMO);
 438 #endif
 439         if (tp != NULL)
 440                 INP_UNLOCK(inp);
 441         INP_INFO_WUNLOCK(&tcbinfo);
 442 }
 443
 444 void
 445 tcp_timer_rexmt(xtp)
 446         void *xtp;
 447 {
 448         struct tcpcb *tp = xtp;
 449         int rexmt;
 450         int headlocked;
 451         struct inpcb *inp;
 452 #ifdef TCPDEBUG
 453         int ostate;
 454
 455         ostate = tp->t_state;
 456 #endif
 457         INP_INFO_WLOCK(&tcbinfo);
 458         headlocked = 1;
 459         inp = tp->t_inpcb;
 460         /*
 461          * XXXRW: While this assert is in fact correct, bugs in the tcpcb
 462          * tear-down mean we need it as a work-around for races between
 463          * timers and tcp_discardcb().
 464          *
 465          * KASSERT(inp != NULL, ("tcp_timer_rexmt: inp == NULL"));
 466          */
 467         if (inp == NULL) {
 468                 tcp_timer_race++;
 469                 INP_INFO_WUNLOCK(&tcbinfo);
 470                 return;
 471         }
 472         INP_LOCK(inp);
 473         if ((inp->inp_vflag & INP_DROPPED) || callout_pending(tp->tt_rexmt)
 474             || !callout_active(tp->tt_rexmt)) {
 475                 INP_UNLOCK(inp);
 476                 INP_INFO_WUNLOCK(&tcbinfo);
 477                 return;
 478         }
 479         callout_deactivate(tp->tt_rexmt);
 480         tcp_free_sackholes(tp);
 481         /*
 482          * Retransmission timer went off.  Message has not
 483          * been acked within retransmit interval.  Back off
 484          * to a longer retransmit interval and retransmit one segment.
 485          */
 486         if (++tp->t_rxtshift > TCP_MAXRXTSHIFT) {
 487                 tp->t_rxtshift = TCP_MAXRXTSHIFT;
 488                 tcpstat.tcps_timeoutdrop++;
 489                 tp = tcp_drop(tp, tp->t_softerror ?
 490                               tp->t_softerror : ETIMEDOUT);
 491                 goto out;
 492         }
 493         INP_INFO_WUNLOCK(&tcbinfo);
 494         headlocked = 0;
 495         if (tp->t_rxtshift == 1) {
 496                 /*
 497                  * first retransmit; record ssthresh and cwnd so they can
 498                  * be recovered if this turns out to be a "bad" retransmit.
 499                  * A retransmit is considered "bad" if an ACK for this
 500                  * segment is received within RTT/2 interval; the assumption
 501                  * here is that the ACK was already in flight.  See
 502                  * "On Estimating End-to-End Network Path Properties" by
 503                  * Allman and Paxson for more details.
 504                  */
 505                 tp->snd_cwnd_prev = tp->snd_cwnd;
 506                 tp->snd_ssthresh_prev = tp->snd_ssthresh;
 507                 tp->snd_recover_prev = tp->snd_recover;
 508                 if (IN_FASTRECOVERY(tp))
 509                   tp->t_flags |= TF_WASFRECOVERY;
 510                 else
 511                   tp->t_flags &= ~TF_WASFRECOVERY;
 512                 tp->t_badrxtwin = ticks + (tp->t_srtt >> (TCP_RTT_SHIFT + 1));
 513         }
 514         tcpstat.tcps_rexmttimeo++;
 515         if (tp->t_state == TCPS_SYN_SENT)
 516                 rexmt = TCP_REXMTVAL(tp) * tcp_syn_backoff[tp->t_rxtshift];
 517         else
 518                 rexmt = TCP_REXMTVAL(tp) * tcp_backoff[tp->t_rxtshift];
 519         TCPT_RANGESET(tp->t_rxtcur, rexmt,
 520                       tp->t_rttmin, TCPTV_REXMTMAX);
 521         /*
 522          * Disable rfc1323 if we havn't got any response to
 523          * our third SYN to work-around some broken terminal servers
 524          * (most of which have hopefully been retired) that have bad VJ
 525          * header compression code which trashes TCP segments containing
 526          * unknown-to-them TCP options.
 527          */
 528         if ((tp->t_state == TCPS_SYN_SENT) && (tp->t_rxtshift == 3))
 529                 tp->t_flags &= ~(TF_REQ_SCALE|TF_REQ_TSTMP);
 530         /*
 531          * If we backed off this far, our srtt estimate is probably bogus.
 532          * Clobber it so we'll take the next rtt measurement as our srtt;
 533          * move the current srtt into rttvar to keep the current
 534          * retransmit times until then.
 535          */
 536         if (tp->t_rxtshift > TCP_MAXRXTSHIFT / 4) {
 537 #ifdef INET6
 538                 if ((tp->t_inpcb->inp_vflag & INP_IPV6) != 0)
 539                         in6_losing(tp->t_inpcb);
 540                 else
 541 #endif
 542                 tp->t_rttvar += (tp->t_srtt >> TCP_RTT_SHIFT);
 543                 tp->t_srtt = 0;
 544         }
 545         tp->snd_nxt = tp->snd_una;
 546         tp->snd_recover = tp->snd_max;
 547         /*
 548          * Force a segment to be sent.
 549          */
 550         tp->t_flags |= TF_ACKNOW;
 551         /*
 552          * If timing a segment in this window, stop the timer.
 553          */
 554         tp->t_rtttime = 0;
 555         /*
 556          * Close the congestion window down to one segment
 557          * (we'll open it by one segment for each ack we get).
 558          * Since we probably have a window's worth of unacked
 559          * data accumulated, this "slow start" keeps us from
 560          * dumping all that data as back-to-back packets (which
 561          * might overwhelm an intermediate gateway).
 562          *
 563          * There are two phases to the opening: Initially we
 564          * open by one mss on each ack.  This makes the window
 565          * size increase exponentially with time.  If the
 566          * window is larger than the path can handle, this
 567          * exponential growth results in dropped packet(s)
 568          * almost immediately.  To get more time between
 569          * drops but still "push" the network to take advantage
 570          * of improving conditions, we switch from exponential
 571          * to linear window opening at some threshhold size.
 572          * For a threshhold, we use half the current window
 573          * size, truncated to a multiple of the mss.
 574          *
 575          * (the minimum cwnd that will give us exponential
 576          * growth is 2 mss.  We don't allow the threshhold
 577          * to go below this.)
 578          */
 579         {
 580                 u_int win = min(tp->snd_wnd, tp->snd_cwnd) / 2 / tp->t_maxseg;
 581                 if (win < 2)
 582                         win = 2;
 583                 tp->snd_cwnd = tp->t_maxseg;
 584                 tp->snd_ssthresh = win * tp->t_maxseg;
 585                 tp->t_dupacks = 0;
 586         }
 587         EXIT_FASTRECOVERY(tp);
 588         (void) tcp_output(tp);
 589
 590 out:
 591 #ifdef TCPDEBUG
 592         if (tp != NULL && (tp->t_inpcb->inp_socket->so_options & SO_DEBUG))
 593                 tcp_trace(TA_USER, ostate, tp, (void *)0, (struct tcphdr *)0,
 594                           PRU_SLOWTIMO);
 595 #endif
 596         if (tp != NULL)
 597                 INP_UNLOCK(inp);
 598         if (headlocked)
 599                 INP_INFO_WUNLOCK(&tcbinfo);
 600 }