sys/netinet/tcp_timer.c

   1 /*
   2  * Copyright (c) 2003, 2004 Jeffrey M. Hsu.  All rights reserved.
   3  * Copyright (c) 2003, 2004 The DragonFly Project.  All rights reserved.
   4  *
   5  * This code is derived from software contributed to The DragonFly Project
   6  * by Jeffrey M. Hsu.
   7  *
   8  * Redistribution and use in source and binary forms, with or without
   9  * modification, are permitted provided that the following conditions
  10  * are met:
  11  * 1. Redistributions of source code must retain the above copyright
  12  *    notice, this list of conditions and the following disclaimer.
  13  * 2. Redistributions in binary form must reproduce the above copyright
  14  *    notice, this list of conditions and the following disclaimer in the
  15  *    documentation and/or other materials provided with the distribution.
  16  * 3. Neither the name of The DragonFly Project nor the names of its
  17  *    contributors may be used to endorse or promote products derived
  18  *    from this software without specific, prior written permission.
  19  *
  20  * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
  21  * ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
  22  * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
  23  * FOR A PARTICULAR PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE
  24  * COPYRIGHT HOLDERS OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
  25  * INCIDENTAL, SPECIAL, EXEMPLARY OR CONSEQUENTIAL DAMAGES (INCLUDING,
  26  * BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
  27  * LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED
  28  * AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
  29  * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
  30  * OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
  31  * SUCH DAMAGE.
  32  */
  33
  34 /*
  35  * Copyright (c) 1982, 1986, 1988, 1990, 1993, 1995
  36  *      The Regents of the University of California.  All rights reserved.
  37  *
  38  * Redistribution and use in source and binary forms, with or without
  39  * modification, are permitted provided that the following conditions
  40  * are met:
  41  * 1. Redistributions of source code must retain the above copyright
  42  *    notice, this list of conditions and the following disclaimer.
  43  * 2. Redistributions in binary form must reproduce the above copyright
  44  *    notice, this list of conditions and the following disclaimer in the
  45  *    documentation and/or other materials provided with the distribution.
  46  * 3. All advertising materials mentioning features or use of this software
  47  *    must display the following acknowledgement:
  48  *      This product includes software developed by the University of
  49  *      California, Berkeley and its contributors.
  50  * 4. Neither the name of the University nor the names of its contributors
  51  *    may be used to endorse or promote products derived from this software
  52  *    without specific prior written permission.
  53  *
  54  * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
  55  * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
  56  * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
  57  * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
  58  * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
  59  * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
  60  * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
  61  * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
  62  * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
  63  * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
  64  * SUCH DAMAGE.
  65  *
  66  *      @(#)tcp_timer.c 8.2 (Berkeley) 5/24/95
  67  * $FreeBSD: src/sys/netinet/tcp_timer.c,v 1.34.2.14 2003/02/03 02:33:41 hsu Exp $
  68  * $DragonFly: src/sys/netinet/tcp_timer.c,v 1.17 2008/03/30 20:39:01 dillon Exp $
  69  */
  70
  71 #include "opt_compat.h"
  72 #include "opt_inet6.h"
  73 #include "opt_tcpdebug.h"
  74
  75 #include <sys/param.h>
  76 #include <sys/systm.h>
  77 #include <sys/kernel.h>
  78 #include <sys/mbuf.h>
  79 #include <sys/sysctl.h>
  80 #include <sys/socket.h>
  81 #include <sys/socketvar.h>
  82 #include <sys/protosw.h>
  83 #include <sys/thread.h>
  84 #include <sys/globaldata.h>
  85 #include <sys/thread2.h>
  86
  87 #include <machine/cpu.h>        /* before tcp_seq.h, for tcp_random18() */
  88
  89 #include <net/route.h>
  90
  91 #include <netinet/in.h>
  92 #include <netinet/in_systm.h>
  93 #include <netinet/in_pcb.h>
  94 #ifdef INET6
  95 #include <netinet6/in6_pcb.h>
  96 #endif
  97 #include <netinet/ip_var.h>
  98 #include <netinet/tcp.h>
  99 #include <netinet/tcp_fsm.h>
 100 #include <netinet/tcp_seq.h>
 101 #include <netinet/tcp_timer.h>
 102 #include <netinet/tcp_var.h>
 103 #include <netinet/tcpip.h>
 104 #ifdef TCPDEBUG
 105 #include <netinet/tcp_debug.h>
 106 #endif
 107
 108 static int
 109 sysctl_msec_to_ticks(SYSCTL_HANDLER_ARGS)
 110 {
 111         int error, s, tt;
 112
 113         tt = *(int *)oidp->oid_arg1;
 114         s = (int)((int64_t)tt * 1000 / hz);
 115
 116         error = sysctl_handle_int(oidp, &s, 0, req);
 117         if (error || !req->newptr)
 118                 return (error);
 119
 120         tt = (int)((int64_t)s * hz / 1000);
 121         if (tt < 1)
 122                 return (EINVAL);
 123
 124         *(int *)oidp->oid_arg1 = tt;
 125         return (0);
 126 }
 127
 128 int     tcp_keepinit;
 129 SYSCTL_PROC(_net_inet_tcp, TCPCTL_KEEPINIT, keepinit, CTLTYPE_INT|CTLFLAG_RW,
 130     &tcp_keepinit, 0, sysctl_msec_to_ticks, "I", "");
 131
 132 int     tcp_keepidle;
 133 SYSCTL_PROC(_net_inet_tcp, TCPCTL_KEEPIDLE, keepidle, CTLTYPE_INT|CTLFLAG_RW,
 134     &tcp_keepidle, 0, sysctl_msec_to_ticks, "I", "");
 135
 136 int     tcp_keepintvl;
 137 SYSCTL_PROC(_net_inet_tcp, TCPCTL_KEEPINTVL, keepintvl, CTLTYPE_INT|CTLFLAG_RW,
 138     &tcp_keepintvl, 0, sysctl_msec_to_ticks, "I", "");
 139
 140 int     tcp_delacktime;
 141 SYSCTL_PROC(_net_inet_tcp, TCPCTL_DELACKTIME, delacktime,
 142     CTLTYPE_INT|CTLFLAG_RW, &tcp_delacktime, 0, sysctl_msec_to_ticks, "I",
 143     "Time before a delayed ACK is sent");
 144
 145 int     tcp_msl;
 146 SYSCTL_PROC(_net_inet_tcp, OID_AUTO, msl, CTLTYPE_INT|CTLFLAG_RW,
 147     &tcp_msl, 0, sysctl_msec_to_ticks, "I", "Maximum segment lifetime");
 148
 149 int     tcp_rexmit_min;
 150 SYSCTL_PROC(_net_inet_tcp, OID_AUTO, rexmit_min, CTLTYPE_INT|CTLFLAG_RW,
 151     &tcp_rexmit_min, 0, sysctl_msec_to_ticks, "I", "Minimum Retransmission Timeout");
 152
 153 int     tcp_rexmit_slop;
 154 SYSCTL_PROC(_net_inet_tcp, OID_AUTO, rexmit_slop, CTLTYPE_INT|CTLFLAG_RW,
 155     &tcp_rexmit_slop, 0, sysctl_msec_to_ticks, "I",
 156     "Retransmission Timer Slop");
 157
 158 static int      always_keepalive = 1;
 159 SYSCTL_INT(_net_inet_tcp, OID_AUTO, always_keepalive, CTLFLAG_RW,
 160     &always_keepalive , 0, "Assume SO_KEEPALIVE on all TCP connections");
 161
 162 static int      tcp_keepcnt = TCPTV_KEEPCNT;
 163         /* max idle probes */
 164 int     tcp_maxpersistidle;
 165         /* max idle time in persist */
 166 int     tcp_maxidle;
 167
 168 /*
 169  * Tcp protocol timeout routine called every 500 ms.
 170  * Updates timestamps used for TCP
 171  * causes finite state machine actions if timers expire.
 172  */
 173 void
 174 tcp_slowtimo(void)
 175 {
 176         crit_enter();
 177         tcp_maxidle = tcp_keepcnt * tcp_keepintvl;
 178         crit_exit();
 179 }
 180
 181 /*
 182  * Cancel all timers for TCP tp.
 183  */
 184 void
 185 tcp_canceltimers(struct tcpcb *tp)
 186 {
 187         callout_stop(tp->tt_2msl);
 188         callout_stop(tp->tt_persist);
 189         callout_stop(tp->tt_keep);
 190         callout_stop(tp->tt_rexmt);
 191 }
 192
 193 int     tcp_syn_backoff[TCP_MAXRXTSHIFT + 1] =
 194     { 1, 1, 1, 1, 1, 2, 4, 8, 16, 32, 64, 64, 64 };
 195
 196 int     tcp_backoff[TCP_MAXRXTSHIFT + 1] =
 197     { 1, 2, 4, 8, 16, 32, 64, 64, 64, 64, 64, 64, 64 };
 198
 199 static int tcp_totbackoff = 511;        /* sum of tcp_backoff[] */
 200
 201 /*
 202  * TCP timer processing.
 203  */
 204 void
 205 tcp_timer_delack(void *xtp)
 206 {
 207         struct tcpcb *tp = xtp;
 208
 209         crit_enter();
 210         if (callout_pending(tp->tt_delack) || !callout_active(tp->tt_delack)) {
 211                 crit_exit();
 212                 return;
 213         }
 214         callout_deactivate(tp->tt_delack);
 215
 216         tp->t_flags |= TF_ACKNOW;
 217         tcpstat.tcps_delack++;
 218         tcp_output(tp);
 219         crit_exit();
 220 }
 221
 222 void
 223 tcp_timer_2msl(void *xtp)
 224 {
 225         struct tcpcb *tp = xtp;
 226 #ifdef TCPDEBUG
 227         int ostate;
 228
 229         ostate = tp->t_state;
 230 #endif
 231         crit_enter();
 232         if (callout_pending(tp->tt_2msl) || !callout_active(tp->tt_2msl)) {
 233                 crit_exit();
 234                 return;
 235         }
 236         callout_deactivate(tp->tt_2msl);
 237         /*
 238          * 2 MSL timeout in shutdown went off.  If we're closed but
 239          * still waiting for peer to close and connection has been idle
 240          * too long, or if 2MSL time is up from TIME_WAIT, delete connection
 241          * control block.  Otherwise, check again in a bit.
 242          */
 243         if (tp->t_state != TCPS_TIME_WAIT &&
 244             (ticks - tp->t_rcvtime) <= tcp_maxidle)
 245                 callout_reset(tp->tt_2msl, tcp_keepintvl,
 246                               tcp_timer_2msl, tp);
 247         else
 248                 tp = tcp_close(tp);
 249
 250 #ifdef TCPDEBUG
 251         if (tp && (tp->t_inpcb->inp_socket->so_options & SO_DEBUG))
 252                 tcp_trace(TA_USER, ostate, tp, NULL, NULL, PRU_SLOWTIMO);
 253 #endif
 254         crit_exit();
 255 }
 256
 257 void
 258 tcp_timer_keep(void *xtp)
 259 {
 260         struct tcpcb *tp = xtp;
 261         struct tcptemp *t_template;
 262 #ifdef TCPDEBUG
 263         int ostate;
 264
 265         ostate = tp->t_state;
 266 #endif
 267         crit_enter();
 268         if (callout_pending(tp->tt_keep) || !callout_active(tp->tt_keep)) {
 269                 crit_exit();
 270                 return;
 271         }
 272         callout_deactivate(tp->tt_keep);
 273         /*
 274          * Keep-alive timer went off; send something
 275          * or drop connection if idle for too long.
 276          */
 277         tcpstat.tcps_keeptimeo++;
 278         if (tp->t_state < TCPS_ESTABLISHED)
 279                 goto dropit;
 280         if ((always_keepalive ||
 281              tp->t_inpcb->inp_socket->so_options & SO_KEEPALIVE) &&
 282             tp->t_state <= TCPS_CLOSING) {
 283                 if ((ticks - tp->t_rcvtime) >= tcp_keepidle + tcp_maxidle)
 284                         goto dropit;
 285                 /*
 286                  * Send a packet designed to force a response
 287                  * if the peer is up and reachable:
 288                  * either an ACK if the connection is still alive,
 289                  * or an RST if the peer has closed the connection
 290                  * due to timeout or reboot.
 291                  * Using sequence number tp->snd_una-1
 292                  * causes the transmitted zero-length segment
 293                  * to lie outside the receive window;
 294                  * by the protocol spec, this requires the
 295                  * correspondent TCP to respond.
 296                  */
 297                 tcpstat.tcps_keepprobe++;
 298                 t_template = tcp_maketemplate(tp);
 299                 if (t_template) {
 300                         tcp_respond(tp, t_template->tt_ipgen,
 301                                     &t_template->tt_t, (struct mbuf *)NULL,
 302                                     tp->rcv_nxt, tp->snd_una - 1, 0);
 303                         tcp_freetemplate(t_template);
 304                 }
 305                 callout_reset(tp->tt_keep, tcp_keepintvl, tcp_timer_keep, tp);
 306         } else
 307                 callout_reset(tp->tt_keep, tcp_keepidle, tcp_timer_keep, tp);
 308
 309 #ifdef TCPDEBUG
 310         if (tp->t_inpcb->inp_socket->so_options & SO_DEBUG)
 311                 tcp_trace(TA_USER, ostate, tp, NULL, NULL, PRU_SLOWTIMO);
 312 #endif
 313         crit_exit();
 314         return;
 315
 316 dropit:
 317         tcpstat.tcps_keepdrops++;
 318         tp = tcp_drop(tp, ETIMEDOUT);
 319
 320 #ifdef TCPDEBUG
 321         if (tp && (tp->t_inpcb->inp_socket->so_options & SO_DEBUG))
 322                 tcp_trace(TA_USER, ostate, tp, NULL, NULL, PRU_SLOWTIMO);
 323 #endif
 324         crit_exit();
 325 }
 326
 327 void
 328 tcp_timer_persist(void *xtp)
 329 {
 330         struct tcpcb *tp = xtp;
 331 #ifdef TCPDEBUG
 332         int ostate;
 333
 334         ostate = tp->t_state;
 335 #endif
 336         crit_enter();
 337         if (callout_pending(tp->tt_persist) || !callout_active(tp->tt_persist)){
 338                 crit_exit();
 339                 return;
 340         }
 341         callout_deactivate(tp->tt_persist);
 342         /*
 343          * Persistance timer into zero window.
 344          * Force a byte to be output, if possible.
 345          */
 346         tcpstat.tcps_persisttimeo++;
 347         /*
 348          * Hack: if the peer is dead/unreachable, we do not
 349          * time out if the window is closed.  After a full
 350          * backoff, drop the connection if the idle time
 351          * (no responses to probes) reaches the maximum
 352          * backoff that we would use if retransmitting.
 353          */
 354         if (tp->t_rxtshift == TCP_MAXRXTSHIFT &&
 355             ((ticks - tp->t_rcvtime) >= tcp_maxpersistidle ||
 356              (ticks - tp->t_rcvtime) >= TCP_REXMTVAL(tp) * tcp_totbackoff)) {
 357                 tcpstat.tcps_persistdrop++;
 358                 tp = tcp_drop(tp, ETIMEDOUT);
 359                 goto out;
 360         }
 361         tcp_setpersist(tp);
 362         tp->t_flags |= TF_FORCE;
 363         tcp_output(tp);
 364         tp->t_flags &= ~TF_FORCE;
 365
 366 out:
 367 #ifdef TCPDEBUG
 368         if (tp && tp->t_inpcb->inp_socket->so_options & SO_DEBUG)
 369                 tcp_trace(TA_USER, ostate, tp, NULL, NULL, PRU_SLOWTIMO);
 370 #endif
 371         crit_exit();
 372 }
 373
 374 void
 375 tcp_save_congestion_state(struct tcpcb *tp)
 376 {
 377         tp->snd_cwnd_prev = tp->snd_cwnd;
 378         tp->snd_wacked_prev = tp->snd_wacked;
 379         tp->snd_ssthresh_prev = tp->snd_ssthresh;
 380         tp->snd_recover_prev = tp->snd_recover;
 381         if (IN_FASTRECOVERY(tp))
 382                 tp->t_flags |= TF_WASFRECOVERY;
 383         else
 384                 tp->t_flags &= ~TF_WASFRECOVERY;
 385         if (tp->t_flags & TF_RCVD_TSTMP) {
 386                 tp->t_rexmtTS = ticks;
 387                 tp->t_flags |= TF_FIRSTACCACK;
 388         }
 389 #ifdef later
 390         tcp_sack_save_scoreboard(&tp->scb);
 391 #endif
 392 }
 393
 394 void
 395 tcp_revert_congestion_state(struct tcpcb *tp)
 396 {
 397         tp->snd_cwnd = tp->snd_cwnd_prev;
 398         tp->snd_wacked = tp->snd_wacked_prev;
 399         tp->snd_ssthresh = tp->snd_ssthresh_prev;
 400         tp->snd_recover = tp->snd_recover_prev;
 401         if (tp->t_flags & TF_WASFRECOVERY)
 402                 ENTER_FASTRECOVERY(tp);
 403         if (tp->t_flags & TF_FASTREXMT) {
 404                 ++tcpstat.tcps_sndfastrexmitbad;
 405                 if (tp->t_flags & TF_EARLYREXMT)
 406                         ++tcpstat.tcps_sndearlyrexmitbad;
 407         } else
 408                 ++tcpstat.tcps_sndrtobad;
 409         tp->t_badrxtwin = 0;
 410         tp->t_rxtshift = 0;
 411         tp->snd_nxt = tp->snd_max;
 412 #ifdef later
 413         tcp_sack_revert_scoreboard(&tp->scb, tp->snd_una);
 414 #endif
 415 }
 416
 417 void
 418 tcp_timer_rexmt(void *xtp)
 419 {
 420         struct tcpcb *tp = xtp;
 421         int rexmt;
 422 #ifdef TCPDEBUG
 423         int ostate;
 424
 425         ostate = tp->t_state;
 426 #endif
 427         crit_enter();
 428         if (callout_pending(tp->tt_rexmt) || !callout_active(tp->tt_rexmt)) {
 429                 crit_exit();
 430                 return;
 431         }
 432         callout_deactivate(tp->tt_rexmt);
 433         /*
 434          * Retransmission timer went off.  Message has not
 435          * been acked within retransmit interval.  Back off
 436          * to a longer retransmit interval and retransmit one segment.
 437          */
 438         if (++tp->t_rxtshift > TCP_MAXRXTSHIFT) {
 439                 tp->t_rxtshift = TCP_MAXRXTSHIFT;
 440                 tcpstat.tcps_timeoutdrop++;
 441                 tp = tcp_drop(tp, tp->t_softerror ?
 442                               tp->t_softerror : ETIMEDOUT);
 443                 goto out;
 444         }
 445         if (tp->t_rxtshift == 1) {
 446                 /*
 447                  * first retransmit; record ssthresh and cwnd so they can
 448                  * be recovered if this turns out to be a "bad" retransmit.
 449                  * A retransmit is considered "bad" if an ACK for this
 450                  * segment is received within RTT/2 interval; the assumption
 451                  * here is that the ACK was already in flight.  See
 452                  * "On Estimating End-to-End Network Path Properties" by
 453                  * Allman and Paxson for more details.
 454                  */
 455                 tp->t_badrxtwin = ticks + (tp->t_srtt >> (TCP_RTT_SHIFT + 1));
 456                 tcp_save_congestion_state(tp);
 457                 tp->t_flags &= ~(TF_FASTREXMT | TF_EARLYREXMT);
 458         }
 459         /* Throw away SACK blocks on a RTO, as specified by RFC2018. */
 460         tcp_sack_cleanup(&tp->scb);
 461         tcpstat.tcps_rexmttimeo++;
 462         if (tp->t_state == TCPS_SYN_SENT)
 463                 rexmt = TCP_REXMTVAL(tp) * tcp_syn_backoff[tp->t_rxtshift];
 464         else
 465                 rexmt = TCP_REXMTVAL(tp) * tcp_backoff[tp->t_rxtshift];
 466         TCPT_RANGESET(tp->t_rxtcur, rexmt,
 467                       tp->t_rttmin, TCPTV_REXMTMAX);
 468         /*
 469          * Disable rfc1323 and rfc1644 if we havn't got any response to
 470          * our third SYN to work-around some broken terminal servers
 471          * (most of which have hopefully been retired) that have bad VJ
 472          * header compression code which trashes TCP segments containing
 473          * unknown-to-them TCP options.
 474          */
 475         if ((tp->t_state == TCPS_SYN_SENT) && (tp->t_rxtshift == 3))
 476                 tp->t_flags &= ~(TF_REQ_SCALE|TF_REQ_TSTMP|TF_REQ_CC);
 477         /*
 478          * If losing, let the lower level know and try for
 479          * a better route.  Also, if we backed off this far,
 480          * our srtt estimate is probably bogus.  Clobber it
 481          * so we'll take the next rtt measurement as our srtt;
 482          * move the current srtt into rttvar to keep the current
 483          * retransmit times until then.
 484          */
 485         if (tp->t_rxtshift > TCP_MAXRXTSHIFT / 4) {
 486 #ifdef INET6
 487                 if ((tp->t_inpcb->inp_vflag & INP_IPV6) != 0)
 488                         in6_losing(tp->t_inpcb);
 489                 else
 490 #endif
 491                 in_losing(tp->t_inpcb);
 492                 tp->t_rttvar += (tp->t_srtt >> TCP_RTT_SHIFT);
 493                 tp->t_srtt = 0;
 494         }
 495         tp->snd_nxt = tp->snd_una;
 496         tp->rexmt_high = tp->snd_una;
 497         tp->snd_recover = tp->snd_max;
 498         /*
 499          * Force a segment to be sent.
 500          */
 501         tp->t_flags |= TF_ACKNOW;
 502         /*
 503          * If timing a segment in this window, stop the timer.
 504          */
 505         tp->t_rtttime = 0;
 506         /*
 507          * Close the congestion window down to one segment
 508          * (we'll open it by one segment for each ack we get).
 509          * Since we probably have a window's worth of unacked
 510          * data accumulated, this "slow start" keeps us from
 511          * dumping all that data as back-to-back packets (which
 512          * might overwhelm an intermediate gateway).
 513          *
 514          * There are two phases to the opening: Initially we
 515          * open by one mss on each ack.  This makes the window
 516          * size increase exponentially with time.  If the
 517          * window is larger than the path can handle, this
 518          * exponential growth results in dropped packet(s)
 519          * almost immediately.  To get more time between
 520          * drops but still "push" the network to take advantage
 521          * of improving conditions, we switch from exponential
 522          * to linear window opening at some threshhold size.
 523          * For a threshhold, we use half the current window
 524          * size, truncated to a multiple of the mss.
 525          *
 526          * (the minimum cwnd that will give us exponential
 527          * growth is 2 mss.  We don't allow the threshhold
 528          * to go below this.)
 529          */
 530         {
 531                 u_int win = min(tp->snd_wnd, tp->snd_cwnd) / 2 / tp->t_maxseg;
 532
 533                 if (win < 2)
 534                         win = 2;
 535                 tp->snd_cwnd = tp->t_maxseg;
 536                 tp->snd_wacked = 0;
 537                 tp->snd_ssthresh = win * tp->t_maxseg;
 538                 tp->t_dupacks = 0;
 539         }
 540         EXIT_FASTRECOVERY(tp);
 541         tcp_output(tp);
 542
 543 out:
 544 #ifdef TCPDEBUG
 545         if (tp && (tp->t_inpcb->inp_socket->so_options & SO_DEBUG))
 546                 tcp_trace(TA_USER, ostate, tp, NULL, NULL, PRU_SLOWTIMO);
 547 #endif
 548         crit_exit();
 549 }