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