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) 2003, 2004 Jeffrey M. Hsu.  All rights reserved.
  36  *
  37  * License terms: all terms for the DragonFly license above plus the following:
  38  *
  39  * 4. All advertising materials mentioning features or use of this software
  40  *    must display the following acknowledgement:
  41  *
  42  *      This product includes software developed by Jeffrey M. Hsu
  43  *      for the DragonFly Project.
  44  *
  45  *    This requirement may be waived with permission from Jeffrey Hsu.
  46  *    This requirement will sunset and may be removed on July 8 2005,
  47  *    after which the standard DragonFly license (as shown above) will
  48  *    apply.
  49  */
  50
  51 /*
  52  * Copyright (c) 1982, 1986, 1988, 1990, 1993, 1995
  53  *      The Regents of the University of California.  All rights reserved.
  54  *
  55  * Redistribution and use in source and binary forms, with or without
  56  * modification, are permitted provided that the following conditions
  57  * are met:
  58  * 1. Redistributions of source code must retain the above copyright
  59  *    notice, this list of conditions and the following disclaimer.
  60  * 2. Redistributions in binary form must reproduce the above copyright
  61  *    notice, this list of conditions and the following disclaimer in the
  62  *    documentation and/or other materials provided with the distribution.
  63  * 3. All advertising materials mentioning features or use of this software
  64  *    must display the following acknowledgement:
  65  *      This product includes software developed by the University of
  66  *      California, Berkeley and its contributors.
  67  * 4. Neither the name of the University nor the names of its contributors
  68  *    may be used to endorse or promote products derived from this software
  69  *    without specific prior written permission.
  70  *
  71  * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
  72  * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
  73  * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
  74  * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
  75  * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
  76  * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
  77  * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
  78  * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
  79  * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
  80  * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
  81  * SUCH DAMAGE.
  82  *
  83  *      @(#)tcp_timer.c 8.2 (Berkeley) 5/24/95
  84  * $FreeBSD: src/sys/netinet/tcp_timer.c,v 1.34.2.14 2003/02/03 02:33:41 hsu Exp $
  85  * $DragonFly: src/sys/netinet/tcp_timer.c,v 1.13 2004/12/21 02:54:15 hsu Exp $
  86  */
  87
  88 #include "opt_compat.h"
  89 #include "opt_inet6.h"
  90 #include "opt_tcpdebug.h"
  91
  92 #include <sys/param.h>
  93 #include <sys/systm.h>
  94 #include <sys/kernel.h>
  95 #include <sys/mbuf.h>
  96 #include <sys/sysctl.h>
  97 #include <sys/socket.h>
  98 #include <sys/socketvar.h>
  99 #include <sys/protosw.h>
 100 #include <sys/thread.h>
 101 #include <sys/globaldata.h>
 102
 103 #include <machine/cpu.h>        /* before tcp_seq.h, for tcp_random18() */
 104
 105 #include <net/route.h>
 106
 107 #include <netinet/in.h>
 108 #include <netinet/in_systm.h>
 109 #include <netinet/in_pcb.h>
 110 #ifdef INET6
 111 #include <netinet6/in6_pcb.h>
 112 #endif
 113 #include <netinet/ip_var.h>
 114 #include <netinet/tcp.h>
 115 #include <netinet/tcp_fsm.h>
 116 #include <netinet/tcp_seq.h>
 117 #include <netinet/tcp_timer.h>
 118 #include <netinet/tcp_var.h>
 119 #include <netinet/tcpip.h>
 120 #ifdef TCPDEBUG
 121 #include <netinet/tcp_debug.h>
 122 #endif
 123
 124 static int
 125 sysctl_msec_to_ticks(SYSCTL_HANDLER_ARGS)
 126 {
 127         int error, s, tt;
 128
 129         tt = *(int *)oidp->oid_arg1;
 130         s = (int)((int64_t)tt * 1000 / hz);
 131
 132         error = sysctl_handle_int(oidp, &s, 0, req);
 133         if (error || !req->newptr)
 134                 return (error);
 135
 136         tt = (int)((int64_t)s * hz / 1000);
 137         if (tt < 1)
 138                 return (EINVAL);
 139
 140         *(int *)oidp->oid_arg1 = tt;
 141         return (0);
 142 }
 143
 144 int     tcp_keepinit;
 145 SYSCTL_PROC(_net_inet_tcp, TCPCTL_KEEPINIT, keepinit, CTLTYPE_INT|CTLFLAG_RW,
 146     &tcp_keepinit, 0, sysctl_msec_to_ticks, "I", "");
 147
 148 int     tcp_keepidle;
 149 SYSCTL_PROC(_net_inet_tcp, TCPCTL_KEEPIDLE, keepidle, CTLTYPE_INT|CTLFLAG_RW,
 150     &tcp_keepidle, 0, sysctl_msec_to_ticks, "I", "");
 151
 152 int     tcp_keepintvl;
 153 SYSCTL_PROC(_net_inet_tcp, TCPCTL_KEEPINTVL, keepintvl, CTLTYPE_INT|CTLFLAG_RW,
 154     &tcp_keepintvl, 0, sysctl_msec_to_ticks, "I", "");
 155
 156 int     tcp_delacktime;
 157 SYSCTL_PROC(_net_inet_tcp, TCPCTL_DELACKTIME, delacktime,
 158     CTLTYPE_INT|CTLFLAG_RW, &tcp_delacktime, 0, sysctl_msec_to_ticks, "I",
 159     "Time before a delayed ACK is sent");
 160
 161 int     tcp_msl;
 162 SYSCTL_PROC(_net_inet_tcp, OID_AUTO, msl, CTLTYPE_INT|CTLFLAG_RW,
 163     &tcp_msl, 0, sysctl_msec_to_ticks, "I", "Maximum segment lifetime");
 164
 165 int     tcp_rexmit_min;
 166 SYSCTL_PROC(_net_inet_tcp, OID_AUTO, rexmit_min, CTLTYPE_INT|CTLFLAG_RW,
 167     &tcp_rexmit_min, 0, sysctl_msec_to_ticks, "I", "Minimum Retransmission Timeout");
 168
 169 int     tcp_rexmit_slop;
 170 SYSCTL_PROC(_net_inet_tcp, OID_AUTO, rexmit_slop, CTLTYPE_INT|CTLFLAG_RW,
 171     &tcp_rexmit_slop, 0, sysctl_msec_to_ticks, "I", "Retransmission Timer Slop");
 172
 173 static int      always_keepalive = 0;
 174 SYSCTL_INT(_net_inet_tcp, OID_AUTO, always_keepalive, CTLFLAG_RW,
 175     &always_keepalive , 0, "Assume SO_KEEPALIVE on all TCP connections");
 176
 177 static int      tcp_keepcnt = TCPTV_KEEPCNT;
 178         /* max idle probes */
 179 int     tcp_maxpersistidle;
 180         /* max idle time in persist */
 181 int     tcp_maxidle;
 182
 183 /*
 184  * Tcp protocol timeout routine called every 500 ms.
 185  * Updates timestamps used for TCP
 186  * causes finite state machine actions if timers expire.
 187  */
 188 void
 189 tcp_slowtimo(void)
 190 {
 191         int s;
 192
 193         s = splnet();
 194
 195         tcp_maxidle = tcp_keepcnt * tcp_keepintvl;
 196
 197         splx(s);
 198 }
 199
 200 /*
 201  * Cancel all timers for TCP tp.
 202  */
 203 void
 204 tcp_canceltimers(struct tcpcb *tp)
 205 {
 206         callout_stop(tp->tt_2msl);
 207         callout_stop(tp->tt_persist);
 208         callout_stop(tp->tt_keep);
 209         callout_stop(tp->tt_rexmt);
 210 }
 211
 212 int     tcp_syn_backoff[TCP_MAXRXTSHIFT + 1] =
 213     { 1, 1, 1, 1, 1, 2, 4, 8, 16, 32, 64, 64, 64 };
 214
 215 int     tcp_backoff[TCP_MAXRXTSHIFT + 1] =
 216     { 1, 2, 4, 8, 16, 32, 64, 64, 64, 64, 64, 64, 64 };
 217
 218 static int tcp_totbackoff = 511;        /* sum of tcp_backoff[] */
 219
 220 /*
 221  * TCP timer processing.
 222  */
 223 void
 224 tcp_timer_delack(void *xtp)
 225 {
 226         struct tcpcb *tp = xtp;
 227         int s;
 228
 229         s = splnet();
 230         if (callout_pending(tp->tt_delack) || !callout_active(tp->tt_delack)) {
 231                 splx(s);
 232                 return;
 233         }
 234         callout_deactivate(tp->tt_delack);
 235
 236         tp->t_flags |= TF_ACKNOW;
 237         tcpstat.tcps_delack++;
 238         tcp_output(tp);
 239         splx(s);
 240 }
 241
 242 void
 243 tcp_timer_2msl(void *xtp)
 244 {
 245         struct tcpcb *tp = xtp;
 246         int s;
 247 #ifdef TCPDEBUG
 248         int ostate;
 249
 250         ostate = tp->t_state;
 251 #endif
 252         s = splnet();
 253         if (callout_pending(tp->tt_2msl) || !callout_active(tp->tt_2msl)) {
 254                 splx(s);
 255                 return;
 256         }
 257         callout_deactivate(tp->tt_2msl);
 258         /*
 259          * 2 MSL timeout in shutdown went off.  If we're closed but
 260          * still waiting for peer to close and connection has been idle
 261          * too long, or if 2MSL time is up from TIME_WAIT, delete connection
 262          * control block.  Otherwise, check again in a bit.
 263          */
 264         if (tp->t_state != TCPS_TIME_WAIT &&
 265             (ticks - tp->t_rcvtime) <= tcp_maxidle)
 266                 callout_reset(tp->tt_2msl, tcp_keepintvl,
 267                               tcp_timer_2msl, tp);
 268         else
 269                 tp = tcp_close(tp);
 270
 271 #ifdef TCPDEBUG
 272         if (tp && (tp->t_inpcb->inp_socket->so_options & SO_DEBUG))
 273                 tcp_trace(TA_USER, ostate, tp, NULL, NULL, PRU_SLOWTIMO);
 274 #endif
 275         splx(s);
 276 }
 277
 278 void
 279 tcp_timer_keep(void *xtp)
 280 {
 281         struct tcpcb *tp = xtp;
 282         struct tcptemp *t_template;
 283         int s;
 284 #ifdef TCPDEBUG
 285         int ostate;
 286
 287         ostate = tp->t_state;
 288 #endif
 289         s = splnet();
 290         if (callout_pending(tp->tt_keep) || !callout_active(tp->tt_keep)) {
 291                 splx(s);
 292                 return;
 293         }
 294         callout_deactivate(tp->tt_keep);
 295         /*
 296          * Keep-alive timer went off; send something
 297          * or drop connection if idle for too long.
 298          */
 299         tcpstat.tcps_keeptimeo++;
 300         if (tp->t_state < TCPS_ESTABLISHED)
 301                 goto dropit;
 302         if ((always_keepalive ||
 303              tp->t_inpcb->inp_socket->so_options & SO_KEEPALIVE) &&
 304             tp->t_state <= TCPS_CLOSING) {
 305                 if ((ticks - tp->t_rcvtime) >= tcp_keepidle + tcp_maxidle)
 306                         goto dropit;
 307                 /*
 308                  * Send a packet designed to force a response
 309                  * if the peer is up and reachable:
 310                  * either an ACK if the connection is still alive,
 311                  * or an RST if the peer has closed the connection
 312                  * due to timeout or reboot.
 313                  * Using sequence number tp->snd_una-1
 314                  * causes the transmitted zero-length segment
 315                  * to lie outside the receive window;
 316                  * by the protocol spec, this requires the
 317                  * correspondent TCP to respond.
 318                  */
 319                 tcpstat.tcps_keepprobe++;
 320                 t_template = tcp_maketemplate(tp);
 321                 if (t_template) {
 322                         tcp_respond(tp, t_template->tt_ipgen,
 323                                     &t_template->tt_t, (struct mbuf *)NULL,
 324                                     tp->rcv_nxt, tp->snd_una - 1, 0);
 325                         tcp_freetemplate(t_template);
 326                 }
 327                 callout_reset(tp->tt_keep, tcp_keepintvl, tcp_timer_keep, tp);
 328         } else
 329                 callout_reset(tp->tt_keep, tcp_keepidle, tcp_timer_keep, tp);
 330
 331 #ifdef TCPDEBUG
 332         if (tp->t_inpcb->inp_socket->so_options & SO_DEBUG)
 333                 tcp_trace(TA_USER, ostate, tp, NULL, NULL, PRU_SLOWTIMO);
 334 #endif
 335         splx(s);
 336         return;
 337
 338 dropit:
 339         tcpstat.tcps_keepdrops++;
 340         tp = tcp_drop(tp, ETIMEDOUT);
 341
 342 #ifdef TCPDEBUG
 343         if (tp && (tp->t_inpcb->inp_socket->so_options & SO_DEBUG))
 344                 tcp_trace(TA_USER, ostate, tp, NULL, NULL, PRU_SLOWTIMO);
 345 #endif
 346         splx(s);
 347 }
 348
 349 void
 350 tcp_timer_persist(void *xtp)
 351 {
 352         struct tcpcb *tp = xtp;
 353         int s;
 354 #ifdef TCPDEBUG
 355         int ostate;
 356
 357         ostate = tp->t_state;
 358 #endif
 359         s = splnet();
 360         if (callout_pending(tp->tt_persist) || !callout_active(tp->tt_persist)){
 361                 splx(s);
 362                 return;
 363         }
 364         callout_deactivate(tp->tt_persist);
 365         /*
 366          * Persistance timer into zero window.
 367          * Force a byte to be output, if possible.
 368          */
 369         tcpstat.tcps_persisttimeo++;
 370         /*
 371          * Hack: if the peer is dead/unreachable, we do not
 372          * time out if the window is closed.  After a full
 373          * backoff, drop the connection if the idle time
 374          * (no responses to probes) reaches the maximum
 375          * backoff that we would use if retransmitting.
 376          */
 377         if (tp->t_rxtshift == TCP_MAXRXTSHIFT &&
 378             ((ticks - tp->t_rcvtime) >= tcp_maxpersistidle ||
 379              (ticks - tp->t_rcvtime) >= TCP_REXMTVAL(tp) * tcp_totbackoff)) {
 380                 tcpstat.tcps_persistdrop++;
 381                 tp = tcp_drop(tp, ETIMEDOUT);
 382                 goto out;
 383         }
 384         tcp_setpersist(tp);
 385         tp->t_flags |= TF_FORCE;
 386         tcp_output(tp);
 387         tp->t_flags &= ~TF_FORCE;
 388
 389 out:
 390 #ifdef TCPDEBUG
 391         if (tp && tp->t_inpcb->inp_socket->so_options & SO_DEBUG)
 392                 tcp_trace(TA_USER, ostate, tp, NULL, NULL, PRU_SLOWTIMO);
 393 #endif
 394         splx(s);
 395 }
 396
 397 void
 398 tcp_save_congestion_state(struct tcpcb *tp)
 399 {
 400         tp->snd_cwnd_prev = tp->snd_cwnd;
 401         tp->snd_ssthresh_prev = tp->snd_ssthresh;
 402         tp->snd_recover_prev = tp->snd_recover;
 403         if (IN_FASTRECOVERY(tp))
 404           tp->t_flags |= TF_WASFRECOVERY;
 405         else
 406           tp->t_flags &= ~TF_WASFRECOVERY;
 407         if (tp->t_flags & TF_RCVD_TSTMP) {
 408                 tp->t_rexmtTS = ticks;
 409                 tp->t_flags |= TF_FIRSTACCACK;
 410         }
 411 #ifdef later
 412         tcp_sack_save_scoreboard(&tp->scb);
 413 #endif
 414 }
 415
 416 void
 417 tcp_revert_congestion_state(struct tcpcb *tp)
 418 {
 419         tp->snd_cwnd = tp->snd_cwnd_prev;
 420         tp->snd_ssthresh = tp->snd_ssthresh_prev;
 421         tp->snd_recover = tp->snd_recover_prev;
 422         if (tp->t_flags & TF_WASFRECOVERY)
 423             ENTER_FASTRECOVERY(tp);
 424         if (tp->t_flags & TF_FASTREXMT) {
 425                 ++tcpstat.tcps_sndfastrexmitbad;
 426                 if (tp->t_flags & TF_EARLYREXMT)
 427                         ++tcpstat.tcps_sndearlyrexmitbad;
 428         } else
 429                 ++tcpstat.tcps_sndrtobad;
 430         tp->t_badrxtwin = 0;
 431         tp->t_rxtshift = 0;
 432         tp->snd_nxt = tp->snd_max;
 433 #ifdef later
 434         tcp_sack_revert_scoreboard(&tp->scb, tp->snd_una);
 435 #endif
 436 }
 437
 438 void
 439 tcp_timer_rexmt(void *xtp)
 440 {
 441         struct tcpcb *tp = xtp;
 442         int s;
 443         int rexmt;
 444 #ifdef TCPDEBUG
 445         int ostate;
 446
 447         ostate = tp->t_state;
 448 #endif
 449         s = splnet();
 450         if (callout_pending(tp->tt_rexmt) || !callout_active(tp->tt_rexmt)) {
 451                 splx(s);
 452                 return;
 453         }
 454         callout_deactivate(tp->tt_rexmt);
 455         /*
 456          * Retransmission timer went off.  Message has not
 457          * been acked within retransmit interval.  Back off
 458          * to a longer retransmit interval and retransmit one segment.
 459          */
 460         if (++tp->t_rxtshift > TCP_MAXRXTSHIFT) {
 461                 tp->t_rxtshift = TCP_MAXRXTSHIFT;
 462                 tcpstat.tcps_timeoutdrop++;
 463                 tp = tcp_drop(tp, tp->t_softerror ?
 464                               tp->t_softerror : ETIMEDOUT);
 465                 goto out;
 466         }
 467         if (tp->t_rxtshift == 1) {
 468                 /*
 469                  * first retransmit; record ssthresh and cwnd so they can
 470                  * be recovered if this turns out to be a "bad" retransmit.
 471                  * A retransmit is considered "bad" if an ACK for this
 472                  * segment is received within RTT/2 interval; the assumption
 473                  * here is that the ACK was already in flight.  See
 474                  * "On Estimating End-to-End Network Path Properties" by
 475                  * Allman and Paxson for more details.
 476                  */
 477                 tp->t_badrxtwin = ticks + (tp->t_srtt >> (TCP_RTT_SHIFT + 1));
 478                 tcp_save_congestion_state(tp);
 479                 tp->t_flags &= ~(TF_FASTREXMT | TF_EARLYREXMT);
 480         }
 481         /* Throw away SACK blocks on a RTO, as specified by RFC2018. */
 482         tcp_sack_cleanup(&tp->scb);
 483         tcpstat.tcps_rexmttimeo++;
 484         if (tp->t_state == TCPS_SYN_SENT)
 485                 rexmt = TCP_REXMTVAL(tp) * tcp_syn_backoff[tp->t_rxtshift];
 486         else
 487                 rexmt = TCP_REXMTVAL(tp) * tcp_backoff[tp->t_rxtshift];
 488         TCPT_RANGESET(tp->t_rxtcur, rexmt,
 489                       tp->t_rttmin, TCPTV_REXMTMAX);
 490         /*
 491          * Disable rfc1323 and rfc1644 if we havn't got any response to
 492          * our third SYN to work-around some broken terminal servers
 493          * (most of which have hopefully been retired) that have bad VJ
 494          * header compression code which trashes TCP segments containing
 495          * unknown-to-them TCP options.
 496          */
 497         if ((tp->t_state == TCPS_SYN_SENT) && (tp->t_rxtshift == 3))
 498                 tp->t_flags &= ~(TF_REQ_SCALE|TF_REQ_TSTMP|TF_REQ_CC);
 499         /*
 500          * If losing, let the lower level know and try for
 501          * a better route.  Also, if we backed off this far,
 502          * our srtt estimate is probably bogus.  Clobber it
 503          * so we'll take the next rtt measurement as our srtt;
 504          * move the current srtt into rttvar to keep the current
 505          * retransmit times until then.
 506          */
 507         if (tp->t_rxtshift > TCP_MAXRXTSHIFT / 4) {
 508 #ifdef INET6
 509                 if ((tp->t_inpcb->inp_vflag & INP_IPV6) != 0)
 510                         in6_losing(tp->t_inpcb);
 511                 else
 512 #endif
 513                 in_losing(tp->t_inpcb);
 514                 tp->t_rttvar += (tp->t_srtt >> TCP_RTT_SHIFT);
 515                 tp->t_srtt = 0;
 516         }
 517         tp->snd_nxt = tp->snd_una;
 518         tp->rexmt_high = tp->snd_una;
 519         tp->snd_recover = tp->snd_max;
 520         /*
 521          * Force a segment to be sent.
 522          */
 523         tp->t_flags |= TF_ACKNOW;
 524         /*
 525          * If timing a segment in this window, stop the timer.
 526          */
 527         tp->t_rtttime = 0;
 528         /*
 529          * Close the congestion window down to one segment
 530          * (we'll open it by one segment for each ack we get).
 531          * Since we probably have a window's worth of unacked
 532          * data accumulated, this "slow start" keeps us from
 533          * dumping all that data as back-to-back packets (which
 534          * might overwhelm an intermediate gateway).
 535          *
 536          * There are two phases to the opening: Initially we
 537          * open by one mss on each ack.  This makes the window
 538          * size increase exponentially with time.  If the
 539          * window is larger than the path can handle, this
 540          * exponential growth results in dropped packet(s)
 541          * almost immediately.  To get more time between
 542          * drops but still "push" the network to take advantage
 543          * of improving conditions, we switch from exponential
 544          * to linear window opening at some threshhold size.
 545          * For a threshhold, we use half the current window
 546          * size, truncated to a multiple of the mss.
 547          *
 548          * (the minimum cwnd that will give us exponential
 549          * growth is 2 mss.  We don't allow the threshhold
 550          * to go below this.)
 551          */
 552         {
 553                 u_int win = min(tp->snd_wnd, tp->snd_cwnd) / 2 / tp->t_maxseg;
 554                 if (win < 2)
 555                         win = 2;
 556                 tp->snd_cwnd = tp->t_maxseg;
 557                 tp->snd_ssthresh = win * tp->t_maxseg;
 558                 tp->t_dupacks = 0;
 559         }
 560         EXIT_FASTRECOVERY(tp);
 561         tcp_output(tp);
 562
 563 out:
 564 #ifdef TCPDEBUG
 565         if (tp && (tp->t_inpcb->inp_socket->so_options & SO_DEBUG))
 566                 tcp_trace(TA_USER, ostate, tp, NULL, NULL, PRU_SLOWTIMO);
 567 #endif
 568         splx(s);
 569 }