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  */
  31
  32 #include <sys/cdefs.h>
  33 __FBSDID("$FreeBSD$");
  34
  35 #include "opt_inet6.h"
  36 #include "opt_tcpdebug.h"
  37
  38 #include <sys/param.h>
  39 #include <sys/kernel.h>
  40 #include <sys/lock.h>
  41 #include <sys/mbuf.h>
  42 #include <sys/mutex.h>
  43 #include <sys/protosw.h>
  44 #include <sys/socket.h>
  45 #include <sys/socketvar.h>
  46 #include <sys/sysctl.h>
  47 #include <sys/systm.h>
  48
  49 #include <net/route.h>
  50
  51 #include <netinet/in.h>
  52 #include <netinet/in_pcb.h>
  53 #include <netinet/in_systm.h>
  54 #ifdef INET6
  55 #include <netinet6/in6_pcb.h>
  56 #endif
  57 #include <netinet/ip_var.h>
  58 #include <netinet/tcp.h>
  59 #include <netinet/tcp_fsm.h>
  60 #include <netinet/tcp_timer.h>
  61 #include <netinet/tcp_var.h>
  62 #include <netinet/tcpip.h>
  63 #ifdef TCPDEBUG
  64 #include <netinet/tcp_debug.h>
  65 #endif
  66
  67 int     tcp_keepinit;
  68 SYSCTL_PROC(_net_inet_tcp, TCPCTL_KEEPINIT, keepinit, CTLTYPE_INT|CTLFLAG_RW,
  69     &tcp_keepinit, 0, sysctl_msec_to_ticks, "I", "");
  70
  71 int     tcp_keepidle;
  72 SYSCTL_PROC(_net_inet_tcp, TCPCTL_KEEPIDLE, keepidle, CTLTYPE_INT|CTLFLAG_RW,
  73     &tcp_keepidle, 0, sysctl_msec_to_ticks, "I", "");
  74
  75 int     tcp_keepintvl;
  76 SYSCTL_PROC(_net_inet_tcp, TCPCTL_KEEPINTVL, keepintvl, CTLTYPE_INT|CTLFLAG_RW,
  77     &tcp_keepintvl, 0, sysctl_msec_to_ticks, "I", "");
  78
  79 int     tcp_delacktime;
  80 SYSCTL_PROC(_net_inet_tcp, TCPCTL_DELACKTIME, delacktime, CTLTYPE_INT|CTLFLAG_RW,
  81     &tcp_delacktime, 0, sysctl_msec_to_ticks, "I",
  82     "Time before a delayed ACK is sent");
  83
  84 int     tcp_msl;
  85 SYSCTL_PROC(_net_inet_tcp, OID_AUTO, msl, CTLTYPE_INT|CTLFLAG_RW,
  86     &tcp_msl, 0, sysctl_msec_to_ticks, "I", "Maximum segment lifetime");
  87
  88 int     tcp_rexmit_min;
  89 SYSCTL_PROC(_net_inet_tcp, OID_AUTO, rexmit_min, CTLTYPE_INT|CTLFLAG_RW,
  90     &tcp_rexmit_min, 0, sysctl_msec_to_ticks, "I",
  91     "Minimum Retransmission Timeout");
  92
  93 int     tcp_rexmit_slop;
  94 SYSCTL_PROC(_net_inet_tcp, OID_AUTO, rexmit_slop, CTLTYPE_INT|CTLFLAG_RW,
  95     &tcp_rexmit_slop, 0, sysctl_msec_to_ticks, "I",
  96     "Retransmission Timer Slop");
  97
  98 static int      always_keepalive = 1;
  99 SYSCTL_INT(_net_inet_tcp, OID_AUTO, always_keepalive, CTLFLAG_RW,
 100     &always_keepalive , 0, "Assume SO_KEEPALIVE on all TCP connections");
 101
 102 int    tcp_fast_finwait2_recycle = 0;
 103 SYSCTL_INT(_net_inet_tcp, OID_AUTO, fast_finwait2_recycle, CTLFLAG_RW,
 104     &tcp_fast_finwait2_recycle, 0,
 105     "Recycle closed FIN_WAIT_2 connections faster");
 106
 107 int    tcp_finwait2_timeout;
 108 SYSCTL_PROC(_net_inet_tcp, OID_AUTO, finwait2_timeout, CTLTYPE_INT|CTLFLAG_RW,
 109     &tcp_finwait2_timeout, 0, sysctl_msec_to_ticks, "I", "FIN-WAIT2 timeout");
 110
 111
 112 static int      tcp_keepcnt = TCPTV_KEEPCNT;
 113         /* max idle probes */
 114 int     tcp_maxpersistidle;
 115         /* max idle time in persist */
 116 int     tcp_maxidle;
 117
 118 /*
 119  * Tcp protocol timeout routine called every 500 ms.
 120  * Updates timestamps used for TCP
 121  * causes finite state machine actions if timers expire.
 122  */
 123 void
 124 tcp_slowtimo(void)
 125 {
 126
 127         tcp_maxidle = tcp_keepcnt * tcp_keepintvl;
 128         INP_INFO_WLOCK(&tcbinfo);
 129         (void) tcp_tw_2msl_scan(0);
 130         INP_INFO_WUNLOCK(&tcbinfo);
 131 }
 132
 133 int     tcp_syn_backoff[TCP_MAXRXTSHIFT + 1] =
 134     { 1, 1, 1, 1, 1, 2, 4, 8, 16, 32, 64, 64, 64 };
 135
 136 int     tcp_backoff[TCP_MAXRXTSHIFT + 1] =
 137     { 1, 2, 4, 8, 16, 32, 64, 128, 256, 512, 512, 512, 512 };
 138
 139 static int tcp_totbackoff = 2559;       /* sum of tcp_backoff[] */
 140
 141 static int tcp_timer_race;
 142 SYSCTL_INT(_net_inet_tcp, OID_AUTO, timer_race, CTLFLAG_RD, &tcp_timer_race,
 143     0, "Count of t_inpcb races on tcp_discardcb");
 144
 145 /*
 146  * TCP timer processing.
 147  */
 148
 149 void
 150 tcp_timer_delack(void *xtp)
 151 {
 152         struct tcpcb *tp = xtp;
 153         struct inpcb *inp;
 154
 155         INP_INFO_RLOCK(&tcbinfo);
 156         inp = tp->t_inpcb;
 157         /*
 158          * XXXRW: While this assert is in fact correct, bugs in the tcpcb
 159          * tear-down mean we need it as a work-around for races between
 160          * timers and tcp_discardcb().
 161          *
 162          * KASSERT(inp != NULL, ("tcp_timer_delack: inp == NULL"));
 163          */
 164         if (inp == NULL) {
 165                 tcp_timer_race++;
 166                 INP_INFO_RUNLOCK(&tcbinfo);
 167                 return;
 168         }
 169         INP_WLOCK(inp);
 170         INP_INFO_RUNLOCK(&tcbinfo);
 171         if ((inp->inp_flags & INP_DROPPED) || callout_pending(&tp->t_timers->tt_delack)
 172             || !callout_active(&tp->t_timers->tt_delack)) {
 173                 INP_WUNLOCK(inp);
 174                 return;
 175         }
 176         callout_deactivate(&tp->t_timers->tt_delack);
 177
 178         tp->t_flags |= TF_ACKNOW;
 179         tcpstat.tcps_delack++;
 180         (void) tcp_output(tp);
 181         INP_WUNLOCK(inp);
 182 }
 183
 184 void
 185 tcp_timer_2msl(void *xtp)
 186 {
 187         struct tcpcb *tp = xtp;
 188         struct inpcb *inp;
 189 #ifdef TCPDEBUG
 190         int ostate;
 191
 192         ostate = tp->t_state;
 193 #endif
 194         /*
 195          * XXXRW: Does this actually happen?
 196          */
 197         INP_INFO_WLOCK(&tcbinfo);
 198         inp = tp->t_inpcb;
 199         /*
 200          * XXXRW: While this assert is in fact correct, bugs in the tcpcb
 201          * tear-down mean we need it as a work-around for races between
 202          * timers and tcp_discardcb().
 203          *
 204          * KASSERT(inp != NULL, ("tcp_timer_2msl: inp == NULL"));
 205          */
 206         if (inp == NULL) {
 207                 tcp_timer_race++;
 208                 INP_INFO_WUNLOCK(&tcbinfo);
 209                 return;
 210         }
 211         INP_WLOCK(inp);
 212         tcp_free_sackholes(tp);
 213         if ((inp->inp_flags & INP_DROPPED) || callout_pending(&tp->t_timers->tt_2msl) ||
 214             !callout_active(&tp->t_timers->tt_2msl)) {
 215                 INP_WUNLOCK(tp->t_inpcb);
 216                 INP_INFO_WUNLOCK(&tcbinfo);
 217                 return;
 218         }
 219         callout_deactivate(&tp->t_timers->tt_2msl);
 220         /*
 221          * 2 MSL timeout in shutdown went off.  If we're closed but
 222          * still waiting for peer to close and connection has been idle
 223          * too long, or if 2MSL time is up from TIME_WAIT, delete connection
 224          * control block.  Otherwise, check again in a bit.
 225          *
 226          * If fastrecycle of FIN_WAIT_2, in FIN_WAIT_2 and receiver has closed,
 227          * there's no point in hanging onto FIN_WAIT_2 socket. Just close it.
 228          * Ignore fact that there were recent incoming segments.
 229          */
 230         if (tcp_fast_finwait2_recycle && tp->t_state == TCPS_FIN_WAIT_2 &&
 231             tp->t_inpcb && tp->t_inpcb->inp_socket &&
 232             (tp->t_inpcb->inp_socket->so_rcv.sb_state & SBS_CANTRCVMORE)) {
 233                 tcpstat.tcps_finwait2_drops++;
 234                 tp = tcp_close(tp);
 235         } else {
 236                 if (tp->t_state != TCPS_TIME_WAIT &&
 237                    (ticks - tp->t_rcvtime) <= tcp_maxidle)
 238                        callout_reset(&tp->t_timers->tt_2msl, tcp_keepintvl,
 239                                      tcp_timer_2msl, tp);
 240                else
 241                        tp = tcp_close(tp);
 242        }
 243
 244 #ifdef TCPDEBUG
 245         if (tp != NULL && (tp->t_inpcb->inp_socket->so_options & SO_DEBUG))
 246                 tcp_trace(TA_USER, ostate, tp, (void *)0, (struct tcphdr *)0,
 247                           PRU_SLOWTIMO);
 248 #endif
 249         if (tp != NULL)
 250                 INP_WUNLOCK(inp);
 251         INP_INFO_WUNLOCK(&tcbinfo);
 252 }
 253
 254 void
 255 tcp_timer_keep(void *xtp)
 256 {
 257         struct tcpcb *tp = xtp;
 258         struct tcptemp *t_template;
 259         struct inpcb *inp;
 260 #ifdef TCPDEBUG
 261         int ostate;
 262
 263         ostate = tp->t_state;
 264 #endif
 265         INP_INFO_WLOCK(&tcbinfo);
 266         inp = tp->t_inpcb;
 267         /*
 268          * XXXRW: While this assert is in fact correct, bugs in the tcpcb
 269          * tear-down mean we need it as a work-around for races between
 270          * timers and tcp_discardcb().
 271          *
 272          * KASSERT(inp != NULL, ("tcp_timer_keep: inp == NULL"));
 273          */
 274         if (inp == NULL) {
 275                 tcp_timer_race++;
 276                 INP_INFO_WUNLOCK(&tcbinfo);
 277                 return;
 278         }
 279         INP_WLOCK(inp);
 280         if ((inp->inp_flags & INP_DROPPED) || callout_pending(&tp->t_timers->tt_keep)
 281             || !callout_active(&tp->t_timers->tt_keep)) {
 282                 INP_WUNLOCK(inp);
 283                 INP_INFO_WUNLOCK(&tcbinfo);
 284                 return;
 285         }
 286         callout_deactivate(&tp->t_timers->tt_keep);
 287         /*
 288          * Keep-alive timer went off; send something
 289          * or drop connection if idle for too long.
 290          */
 291         tcpstat.tcps_keeptimeo++;
 292         if (tp->t_state < TCPS_ESTABLISHED)
 293                 goto dropit;
 294         if ((always_keepalive || inp->inp_socket->so_options & SO_KEEPALIVE) &&
 295             tp->t_state <= TCPS_CLOSING) {
 296                 if ((ticks - tp->t_rcvtime) >= tcp_keepidle + tcp_maxidle)
 297                         goto dropit;
 298                 /*
 299                  * Send a packet designed to force a response
 300                  * if the peer is up and reachable:
 301                  * either an ACK if the connection is still alive,
 302                  * or an RST if the peer has closed the connection
 303                  * due to timeout or reboot.
 304                  * Using sequence number tp->snd_una-1
 305                  * causes the transmitted zero-length segment
 306                  * to lie outside the receive window;
 307                  * by the protocol spec, this requires the
 308                  * correspondent TCP to respond.
 309                  */
 310                 tcpstat.tcps_keepprobe++;
 311                 t_template = tcpip_maketemplate(inp);
 312                 if (t_template) {
 313                         tcp_respond(tp, t_template->tt_ipgen,
 314                                     &t_template->tt_t, (struct mbuf *)NULL,
 315                                     tp->rcv_nxt, tp->snd_una - 1, 0);
 316                         free(t_template, M_TEMP);
 317                 }
 318                 callout_reset(&tp->t_timers->tt_keep, tcp_keepintvl, tcp_timer_keep, tp);
 319         } else
 320                 callout_reset(&tp->t_timers->tt_keep, tcp_keepidle, tcp_timer_keep, tp);
 321
 322 #ifdef TCPDEBUG
 323         if (inp->inp_socket->so_options & SO_DEBUG)
 324                 tcp_trace(TA_USER, ostate, tp, (void *)0, (struct tcphdr *)0,
 325                           PRU_SLOWTIMO);
 326 #endif
 327         INP_WUNLOCK(inp);
 328         INP_INFO_WUNLOCK(&tcbinfo);
 329         return;
 330
 331 dropit:
 332         tcpstat.tcps_keepdrops++;
 333         tp = tcp_drop(tp, ETIMEDOUT);
 334
 335 #ifdef TCPDEBUG
 336         if (tp != NULL && (tp->t_inpcb->inp_socket->so_options & SO_DEBUG))
 337                 tcp_trace(TA_USER, ostate, tp, (void *)0, (struct tcphdr *)0,
 338                           PRU_SLOWTIMO);
 339 #endif
 340         if (tp != NULL)
 341                 INP_WUNLOCK(tp->t_inpcb);
 342         INP_INFO_WUNLOCK(&tcbinfo);
 343 }
 344
 345 void
 346 tcp_timer_persist(void *xtp)
 347 {
 348         struct tcpcb *tp = xtp;
 349         struct inpcb *inp;
 350 #ifdef TCPDEBUG
 351         int ostate;
 352
 353         ostate = tp->t_state;
 354 #endif
 355         INP_INFO_WLOCK(&tcbinfo);
 356         inp = tp->t_inpcb;
 357         /*
 358          * XXXRW: While this assert is in fact correct, bugs in the tcpcb
 359          * tear-down mean we need it as a work-around for races between
 360          * timers and tcp_discardcb().
 361          *
 362          * KASSERT(inp != NULL, ("tcp_timer_persist: inp == NULL"));
 363          */
 364         if (inp == NULL) {
 365                 tcp_timer_race++;
 366                 INP_INFO_WUNLOCK(&tcbinfo);
 367                 return;
 368         }
 369         INP_WLOCK(inp);
 370         if ((inp->inp_flags & INP_DROPPED) || callout_pending(&tp->t_timers->tt_persist)
 371             || !callout_active(&tp->t_timers->tt_persist)) {
 372                 INP_WUNLOCK(inp);
 373                 INP_INFO_WUNLOCK(&tcbinfo);
 374                 return;
 375         }
 376         callout_deactivate(&tp->t_timers->tt_persist);
 377         /*
 378          * Persistance timer into zero window.
 379          * Force a byte to be output, if possible.
 380          */
 381         tcpstat.tcps_persisttimeo++;
 382         /*
 383          * Hack: if the peer is dead/unreachable, we do not
 384          * time out if the window is closed.  After a full
 385          * backoff, drop the connection if the idle time
 386          * (no responses to probes) reaches the maximum
 387          * backoff that we would use if retransmitting.
 388          */
 389         if (tp->t_rxtshift == TCP_MAXRXTSHIFT &&
 390             ((ticks - tp->t_rcvtime) >= tcp_maxpersistidle ||
 391              (ticks - tp->t_rcvtime) >= TCP_REXMTVAL(tp) * tcp_totbackoff)) {
 392                 tcpstat.tcps_persistdrop++;
 393                 tp = tcp_drop(tp, ETIMEDOUT);
 394                 goto out;
 395         }
 396         tcp_setpersist(tp);
 397         tp->t_flags |= TF_FORCEDATA;
 398         (void) tcp_output(tp);
 399         tp->t_flags &= ~TF_FORCEDATA;
 400
 401 out:
 402 #ifdef TCPDEBUG
 403         if (tp != NULL && tp->t_inpcb->inp_socket->so_options & SO_DEBUG)
 404                 tcp_trace(TA_USER, ostate, tp, NULL, NULL, PRU_SLOWTIMO);
 405 #endif
 406         if (tp != NULL)
 407                 INP_WUNLOCK(inp);
 408         INP_INFO_WUNLOCK(&tcbinfo);
 409 }
 410
 411 void
 412 tcp_timer_rexmt(void * xtp)
 413 {
 414         struct tcpcb *tp = xtp;
 415         int rexmt;
 416         int headlocked;
 417         struct inpcb *inp;
 418 #ifdef TCPDEBUG
 419         int ostate;
 420
 421         ostate = tp->t_state;
 422 #endif
 423         INP_INFO_WLOCK(&tcbinfo);
 424         headlocked = 1;
 425         inp = tp->t_inpcb;
 426         /*
 427          * XXXRW: While this assert is in fact correct, bugs in the tcpcb
 428          * tear-down mean we need it as a work-around for races between
 429          * timers and tcp_discardcb().
 430          *
 431          * KASSERT(inp != NULL, ("tcp_timer_rexmt: inp == NULL"));
 432          */
 433         if (inp == NULL) {
 434                 tcp_timer_race++;
 435                 INP_INFO_WUNLOCK(&tcbinfo);
 436                 return;
 437         }
 438         INP_WLOCK(inp);
 439         if ((inp->inp_flags & INP_DROPPED) || callout_pending(&tp->t_timers->tt_rexmt)
 440             || !callout_active(&tp->t_timers->tt_rexmt)) {
 441                 INP_WUNLOCK(inp);
 442                 INP_INFO_WUNLOCK(&tcbinfo);
 443                 return;
 444         }
 445         callout_deactivate(&tp->t_timers->tt_rexmt);
 446         tcp_free_sackholes(tp);
 447         /*
 448          * Retransmission timer went off.  Message has not
 449          * been acked within retransmit interval.  Back off
 450          * to a longer retransmit interval and retransmit one segment.
 451          */
 452         if (++tp->t_rxtshift > TCP_MAXRXTSHIFT) {
 453                 tp->t_rxtshift = TCP_MAXRXTSHIFT;
 454                 tcpstat.tcps_timeoutdrop++;
 455                 tp = tcp_drop(tp, tp->t_softerror ?
 456                               tp->t_softerror : ETIMEDOUT);
 457                 goto out;
 458         }
 459         INP_INFO_WUNLOCK(&tcbinfo);
 460         headlocked = 0;
 461         if (tp->t_rxtshift == 1) {
 462                 /*
 463                  * first retransmit; record ssthresh and cwnd so they can
 464                  * be recovered if this turns out to be a "bad" retransmit.
 465                  * A retransmit is considered "bad" if an ACK for this
 466                  * segment is received within RTT/2 interval; the assumption
 467                  * here is that the ACK was already in flight.  See
 468                  * "On Estimating End-to-End Network Path Properties" by
 469                  * Allman and Paxson for more details.
 470                  */
 471                 tp->snd_cwnd_prev = tp->snd_cwnd;
 472                 tp->snd_ssthresh_prev = tp->snd_ssthresh;
 473                 tp->snd_recover_prev = tp->snd_recover;
 474                 if (IN_FASTRECOVERY(tp))
 475                   tp->t_flags |= TF_WASFRECOVERY;
 476                 else
 477                   tp->t_flags &= ~TF_WASFRECOVERY;
 478                 tp->t_badrxtwin = ticks + (tp->t_srtt >> (TCP_RTT_SHIFT + 1));
 479         }
 480         tcpstat.tcps_rexmttimeo++;
 481         if (tp->t_state == TCPS_SYN_SENT)
 482                 rexmt = TCP_REXMTVAL(tp) * tcp_syn_backoff[tp->t_rxtshift];
 483         else
 484                 rexmt = TCP_REXMTVAL(tp) * tcp_backoff[tp->t_rxtshift];
 485         TCPT_RANGESET(tp->t_rxtcur, rexmt,
 486                       tp->t_rttmin, TCPTV_REXMTMAX);
 487         /*
 488          * Disable rfc1323 if we havn't got any response to
 489          * our third SYN to work-around some broken terminal servers
 490          * (most of which have hopefully been retired) that have bad VJ
 491          * header compression code which trashes TCP segments containing
 492          * unknown-to-them TCP options.
 493          */
 494         if ((tp->t_state == TCPS_SYN_SENT) && (tp->t_rxtshift == 3))
 495                 tp->t_flags &= ~(TF_REQ_SCALE|TF_REQ_TSTMP);
 496         /*
 497          * If we backed off this far, our srtt estimate is probably bogus.
 498          * Clobber it so we'll take the next rtt measurement as our srtt;
 499          * move the current srtt into rttvar to keep the current
 500          * retransmit times until then.
 501          */
 502         if (tp->t_rxtshift > TCP_MAXRXTSHIFT / 4) {
 503 #ifdef INET6
 504                 if ((tp->t_inpcb->inp_vflag & INP_IPV6) != 0)
 505                         in6_losing(tp->t_inpcb);
 506                 else
 507 #endif
 508                 tp->t_rttvar += (tp->t_srtt >> TCP_RTT_SHIFT);
 509                 tp->t_srtt = 0;
 510         }
 511         tp->snd_nxt = tp->snd_una;
 512         tp->snd_recover = tp->snd_max;
 513         /*
 514          * Force a segment to be sent.
 515          */
 516         tp->t_flags |= TF_ACKNOW;
 517         /*
 518          * If timing a segment in this window, stop the timer.
 519          */
 520         tp->t_rtttime = 0;
 521         /*
 522          * Close the congestion window down to one segment
 523          * (we'll open it by one segment for each ack we get).
 524          * Since we probably have a window's worth of unacked
 525          * data accumulated, this "slow start" keeps us from
 526          * dumping all that data as back-to-back packets (which
 527          * might overwhelm an intermediate gateway).
 528          *
 529          * There are two phases to the opening: Initially we
 530          * open by one mss on each ack.  This makes the window
 531          * size increase exponentially with time.  If the
 532          * window is larger than the path can handle, this
 533          * exponential growth results in dropped packet(s)
 534          * almost immediately.  To get more time between
 535          * drops but still "push" the network to take advantage
 536          * of improving conditions, we switch from exponential
 537          * to linear window opening at some threshhold size.
 538          * For a threshhold, we use half the current window
 539          * size, truncated to a multiple of the mss.
 540          *
 541          * (the minimum cwnd that will give us exponential
 542          * growth is 2 mss.  We don't allow the threshhold
 543          * to go below this.)
 544          */
 545         {
 546                 u_int win = min(tp->snd_wnd, tp->snd_cwnd) / 2 / tp->t_maxseg;
 547                 if (win < 2)
 548                         win = 2;
 549                 tp->snd_cwnd = tp->t_maxseg;
 550                 tp->snd_ssthresh = win * tp->t_maxseg;
 551                 tp->t_dupacks = 0;
 552         }
 553         EXIT_FASTRECOVERY(tp);
 554         (void) tcp_output(tp);
 555
 556 out:
 557 #ifdef TCPDEBUG
 558         if (tp != NULL && (tp->t_inpcb->inp_socket->so_options & SO_DEBUG))
 559                 tcp_trace(TA_USER, ostate, tp, (void *)0, (struct tcphdr *)0,
 560                           PRU_SLOWTIMO);
 561 #endif
 562         if (tp != NULL)
 563                 INP_WUNLOCK(inp);
 564         if (headlocked)
 565                 INP_INFO_WUNLOCK(&tcbinfo);
 566 }
 567
 568 void
 569 tcp_timer_activate(struct tcpcb *tp, int timer_type, u_int delta)
 570 {
 571         struct callout *t_callout;
 572         void *f_callout;
 573
 574         switch (timer_type) {
 575                 case TT_DELACK:
 576                         t_callout = &tp->t_timers->tt_delack;
 577                         f_callout = tcp_timer_delack;
 578                         break;
 579                 case TT_REXMT:
 580                         t_callout = &tp->t_timers->tt_rexmt;
 581                         f_callout = tcp_timer_rexmt;
 582                         break;
 583                 case TT_PERSIST:
 584                         t_callout = &tp->t_timers->tt_persist;
 585                         f_callout = tcp_timer_persist;
 586                         break;
 587                 case TT_KEEP:
 588                         t_callout = &tp->t_timers->tt_keep;
 589                         f_callout = tcp_timer_keep;
 590                         break;
 591                 case TT_2MSL:
 592                         t_callout = &tp->t_timers->tt_2msl;
 593                         f_callout = tcp_timer_2msl;
 594                         break;
 595                 default:
 596                         panic("bad timer_type");
 597                 }
 598         if (delta == 0) {
 599                 callout_stop(t_callout);
 600         } else {
 601                 callout_reset(t_callout, delta, f_callout, tp);
 602         }
 603 }
 604
 605 int
 606 tcp_timer_active(struct tcpcb *tp, int timer_type)
 607 {
 608         struct callout *t_callout;
 609
 610         switch (timer_type) {
 611                 case TT_DELACK:
 612                         t_callout = &tp->t_timers->tt_delack;
 613                         break;
 614                 case TT_REXMT:
 615                         t_callout = &tp->t_timers->tt_rexmt;
 616                         break;
 617                 case TT_PERSIST:
 618                         t_callout = &tp->t_timers->tt_persist;
 619                         break;
 620                 case TT_KEEP:
 621                         t_callout = &tp->t_timers->tt_keep;
 622                         break;
 623                 case TT_2MSL:
 624                         t_callout = &tp->t_timers->tt_2msl;
 625                         break;
 626                 default:
 627                         panic("bad timer_type");
 628                 }
 629         return callout_active(t_callout);
 630 }