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 #include <sys/vimage.h>
  49
  50 #include <net/if.h>
  51 #include <net/route.h>
  52
  53 #include <netinet/in.h>
  54 #include <netinet/in_pcb.h>
  55 #include <netinet/in_systm.h>
  56 #ifdef INET6
  57 #include <netinet6/in6_pcb.h>
  58 #endif
  59 #include <netinet/ip_var.h>
  60 #include <netinet/tcp.h>
  61 #include <netinet/tcp_fsm.h>
  62 #include <netinet/tcp_timer.h>
  63 #include <netinet/tcp_var.h>
  64 #include <netinet/tcpip.h>
  65 #ifdef TCPDEBUG
  66 #include <netinet/tcp_debug.h>
  67 #endif
  68 #include <netinet/vinet.h>
  69
  70 int     tcp_keepinit;
  71 SYSCTL_PROC(_net_inet_tcp, TCPCTL_KEEPINIT, keepinit, CTLTYPE_INT|CTLFLAG_RW,
  72     &tcp_keepinit, 0, sysctl_msec_to_ticks, "I", "time to establish connection");
  73
  74 int     tcp_keepidle;
  75 SYSCTL_PROC(_net_inet_tcp, TCPCTL_KEEPIDLE, keepidle, CTLTYPE_INT|CTLFLAG_RW,
  76     &tcp_keepidle, 0, sysctl_msec_to_ticks, "I", "time before keepalive probes begin");
  77
  78 int     tcp_keepintvl;
  79 SYSCTL_PROC(_net_inet_tcp, TCPCTL_KEEPINTVL, keepintvl, CTLTYPE_INT|CTLFLAG_RW,
  80     &tcp_keepintvl, 0, sysctl_msec_to_ticks, "I", "time between keepalive probes");
  81
  82 int     tcp_delacktime;
  83 SYSCTL_PROC(_net_inet_tcp, TCPCTL_DELACKTIME, delacktime, CTLTYPE_INT|CTLFLAG_RW,
  84     &tcp_delacktime, 0, sysctl_msec_to_ticks, "I",
  85     "Time before a delayed ACK is sent");
  86
  87 int     tcp_msl;
  88 SYSCTL_PROC(_net_inet_tcp, OID_AUTO, msl, CTLTYPE_INT|CTLFLAG_RW,
  89     &tcp_msl, 0, sysctl_msec_to_ticks, "I", "Maximum segment lifetime");
  90
  91 int     tcp_rexmit_min;
  92 SYSCTL_PROC(_net_inet_tcp, OID_AUTO, rexmit_min, CTLTYPE_INT|CTLFLAG_RW,
  93     &tcp_rexmit_min, 0, sysctl_msec_to_ticks, "I",
  94     "Minimum Retransmission Timeout");
  95
  96 int     tcp_rexmit_slop;
  97 SYSCTL_PROC(_net_inet_tcp, OID_AUTO, rexmit_slop, CTLTYPE_INT|CTLFLAG_RW,
  98     &tcp_rexmit_slop, 0, sysctl_msec_to_ticks, "I",
  99     "Retransmission Timer Slop");
 100
 101 static int      always_keepalive = 1;
 102 SYSCTL_INT(_net_inet_tcp, OID_AUTO, always_keepalive, CTLFLAG_RW,
 103     &always_keepalive , 0, "Assume SO_KEEPALIVE on all TCP connections");
 104
 105 int    tcp_fast_finwait2_recycle = 0;
 106 SYSCTL_INT(_net_inet_tcp, OID_AUTO, fast_finwait2_recycle, CTLFLAG_RW,
 107     &tcp_fast_finwait2_recycle, 0,
 108     "Recycle closed FIN_WAIT_2 connections faster");
 109
 110 int    tcp_finwait2_timeout;
 111 SYSCTL_PROC(_net_inet_tcp, OID_AUTO, finwait2_timeout, CTLTYPE_INT|CTLFLAG_RW,
 112     &tcp_finwait2_timeout, 0, sysctl_msec_to_ticks, "I", "FIN-WAIT2 timeout");
 113
 114
 115 static int      tcp_keepcnt = TCPTV_KEEPCNT;
 116         /* max idle probes */
 117 int     tcp_maxpersistidle;
 118         /* max idle time in persist */
 119 int     tcp_maxidle;
 120
 121 /*
 122  * Tcp protocol timeout routine called every 500 ms.
 123  * Updates timestamps used for TCP
 124  * causes finite state machine actions if timers expire.
 125  */
 126 void
 127 tcp_slowtimo(void)
 128 {
 129         VNET_ITERATOR_DECL(vnet_iter);
 130
 131         VNET_LIST_RLOCK();
 132         VNET_FOREACH(vnet_iter) {
 133                 CURVNET_SET(vnet_iter);
 134                 INIT_VNET_INET(vnet_iter);
 135                 tcp_maxidle = tcp_keepcnt * tcp_keepintvl;
 136                 INP_INFO_WLOCK(&V_tcbinfo);
 137                 (void) tcp_tw_2msl_scan(0);
 138                 INP_INFO_WUNLOCK(&V_tcbinfo);
 139                 CURVNET_RESTORE();
 140         }
 141         VNET_LIST_RUNLOCK();
 142 }
 143
 144 int     tcp_syn_backoff[TCP_MAXRXTSHIFT + 1] =
 145     { 1, 1, 1, 1, 1, 2, 4, 8, 16, 32, 64, 64, 64 };
 146
 147 int     tcp_backoff[TCP_MAXRXTSHIFT + 1] =
 148     { 1, 2, 4, 8, 16, 32, 64, 128, 256, 512, 512, 512, 512 };
 149
 150 static int tcp_totbackoff = 2559;       /* sum of tcp_backoff[] */
 151
 152 static int tcp_timer_race;
 153 SYSCTL_INT(_net_inet_tcp, OID_AUTO, timer_race, CTLFLAG_RD, &tcp_timer_race,
 154     0, "Count of t_inpcb races on tcp_discardcb");
 155
 156 /*
 157  * TCP timer processing.
 158  */
 159
 160 void
 161 tcp_timer_delack(void *xtp)
 162 {
 163         struct tcpcb *tp = xtp;
 164         struct inpcb *inp;
 165         CURVNET_SET(tp->t_vnet);
 166         INIT_VNET_INET(tp->t_vnet);
 167
 168         INP_INFO_RLOCK(&V_tcbinfo);
 169         inp = tp->t_inpcb;
 170         /*
 171          * XXXRW: While this assert is in fact correct, bugs in the tcpcb
 172          * tear-down mean we need it as a work-around for races between
 173          * timers and tcp_discardcb().
 174          *
 175          * KASSERT(inp != NULL, ("tcp_timer_delack: inp == NULL"));
 176          */
 177         if (inp == NULL) {
 178                 tcp_timer_race++;
 179                 INP_INFO_RUNLOCK(&V_tcbinfo);
 180                 CURVNET_RESTORE();
 181                 return;
 182         }
 183         INP_WLOCK(inp);
 184         INP_INFO_RUNLOCK(&V_tcbinfo);
 185         if ((inp->inp_flags & INP_DROPPED) || callout_pending(&tp->t_timers->tt_delack)
 186             || !callout_active(&tp->t_timers->tt_delack)) {
 187                 INP_WUNLOCK(inp);
 188                 CURVNET_RESTORE();
 189                 return;
 190         }
 191         callout_deactivate(&tp->t_timers->tt_delack);
 192
 193         tp->t_flags |= TF_ACKNOW;
 194         V_tcpstat.tcps_delack++;
 195         (void) tcp_output(tp);
 196         INP_WUNLOCK(inp);
 197         CURVNET_RESTORE();
 198 }
 199
 200 void
 201 tcp_timer_2msl(void *xtp)
 202 {
 203         struct tcpcb *tp = xtp;
 204         struct inpcb *inp;
 205         CURVNET_SET(tp->t_vnet);
 206         INIT_VNET_INET(tp->t_vnet);
 207 #ifdef TCPDEBUG
 208         int ostate;
 209
 210         ostate = tp->t_state;
 211 #endif
 212         /*
 213          * XXXRW: Does this actually happen?
 214          */
 215         INP_INFO_WLOCK(&V_tcbinfo);
 216         inp = tp->t_inpcb;
 217         /*
 218          * XXXRW: While this assert is in fact correct, bugs in the tcpcb
 219          * tear-down mean we need it as a work-around for races between
 220          * timers and tcp_discardcb().
 221          *
 222          * KASSERT(inp != NULL, ("tcp_timer_2msl: inp == NULL"));
 223          */
 224         if (inp == NULL) {
 225                 tcp_timer_race++;
 226                 INP_INFO_WUNLOCK(&V_tcbinfo);
 227                 CURVNET_RESTORE();
 228                 return;
 229         }
 230         INP_WLOCK(inp);
 231         tcp_free_sackholes(tp);
 232         if ((inp->inp_flags & INP_DROPPED) || callout_pending(&tp->t_timers->tt_2msl) ||
 233             !callout_active(&tp->t_timers->tt_2msl)) {
 234                 INP_WUNLOCK(tp->t_inpcb);
 235                 INP_INFO_WUNLOCK(&V_tcbinfo);
 236                 CURVNET_RESTORE();
 237                 return;
 238         }
 239         callout_deactivate(&tp->t_timers->tt_2msl);
 240         /*
 241          * 2 MSL timeout in shutdown went off.  If we're closed but
 242          * still waiting for peer to close and connection has been idle
 243          * too long, or if 2MSL time is up from TIME_WAIT, delete connection
 244          * control block.  Otherwise, check again in a bit.
 245          *
 246          * If fastrecycle of FIN_WAIT_2, in FIN_WAIT_2 and receiver has closed,
 247          * there's no point in hanging onto FIN_WAIT_2 socket. Just close it.
 248          * Ignore fact that there were recent incoming segments.
 249          */
 250         if (tcp_fast_finwait2_recycle && tp->t_state == TCPS_FIN_WAIT_2 &&
 251             tp->t_inpcb && tp->t_inpcb->inp_socket &&
 252             (tp->t_inpcb->inp_socket->so_rcv.sb_state & SBS_CANTRCVMORE)) {
 253                 V_tcpstat.tcps_finwait2_drops++;
 254                 tp = tcp_close(tp);
 255         } else {
 256                 if (tp->t_state != TCPS_TIME_WAIT &&
 257                    (ticks - tp->t_rcvtime) <= tcp_maxidle)
 258                        callout_reset(&tp->t_timers->tt_2msl, tcp_keepintvl,
 259                                      tcp_timer_2msl, tp);
 260                else
 261                        tp = tcp_close(tp);
 262        }
 263
 264 #ifdef TCPDEBUG
 265         if (tp != NULL && (tp->t_inpcb->inp_socket->so_options & SO_DEBUG))
 266                 tcp_trace(TA_USER, ostate, tp, (void *)0, (struct tcphdr *)0,
 267                           PRU_SLOWTIMO);
 268 #endif
 269         if (tp != NULL)
 270                 INP_WUNLOCK(inp);
 271         INP_INFO_WUNLOCK(&V_tcbinfo);
 272         CURVNET_RESTORE();
 273 }
 274
 275 void
 276 tcp_timer_keep(void *xtp)
 277 {
 278         struct tcpcb *tp = xtp;
 279         struct tcptemp *t_template;
 280         struct inpcb *inp;
 281         CURVNET_SET(tp->t_vnet);
 282         INIT_VNET_INET(tp->t_vnet);
 283 #ifdef TCPDEBUG
 284         int ostate;
 285
 286         ostate = tp->t_state;
 287 #endif
 288         INP_INFO_WLOCK(&V_tcbinfo);
 289         inp = tp->t_inpcb;
 290         /*
 291          * XXXRW: While this assert is in fact correct, bugs in the tcpcb
 292          * tear-down mean we need it as a work-around for races between
 293          * timers and tcp_discardcb().
 294          *
 295          * KASSERT(inp != NULL, ("tcp_timer_keep: inp == NULL"));
 296          */
 297         if (inp == NULL) {
 298                 tcp_timer_race++;
 299                 INP_INFO_WUNLOCK(&V_tcbinfo);
 300                 CURVNET_RESTORE();
 301                 return;
 302         }
 303         INP_WLOCK(inp);
 304         if ((inp->inp_flags & INP_DROPPED) || callout_pending(&tp->t_timers->tt_keep)
 305             || !callout_active(&tp->t_timers->tt_keep)) {
 306                 INP_WUNLOCK(inp);
 307                 INP_INFO_WUNLOCK(&V_tcbinfo);
 308                 CURVNET_RESTORE();
 309                 return;
 310         }
 311         callout_deactivate(&tp->t_timers->tt_keep);
 312         /*
 313          * Keep-alive timer went off; send something
 314          * or drop connection if idle for too long.
 315          */
 316         V_tcpstat.tcps_keeptimeo++;
 317         if (tp->t_state < TCPS_ESTABLISHED)
 318                 goto dropit;
 319         if ((always_keepalive || inp->inp_socket->so_options & SO_KEEPALIVE) &&
 320             tp->t_state <= TCPS_CLOSING) {
 321                 if ((ticks - tp->t_rcvtime) >= tcp_keepidle + tcp_maxidle)
 322                         goto dropit;
 323                 /*
 324                  * Send a packet designed to force a response
 325                  * if the peer is up and reachable:
 326                  * either an ACK if the connection is still alive,
 327                  * or an RST if the peer has closed the connection
 328                  * due to timeout or reboot.
 329                  * Using sequence number tp->snd_una-1
 330                  * causes the transmitted zero-length segment
 331                  * to lie outside the receive window;
 332                  * by the protocol spec, this requires the
 333                  * correspondent TCP to respond.
 334                  */
 335                 V_tcpstat.tcps_keepprobe++;
 336                 t_template = tcpip_maketemplate(inp);
 337                 if (t_template) {
 338                         tcp_respond(tp, t_template->tt_ipgen,
 339                                     &t_template->tt_t, (struct mbuf *)NULL,
 340                                     tp->rcv_nxt, tp->snd_una - 1, 0);
 341                         free(t_template, M_TEMP);
 342                 }
 343                 callout_reset(&tp->t_timers->tt_keep, tcp_keepintvl, tcp_timer_keep, tp);
 344         } else
 345                 callout_reset(&tp->t_timers->tt_keep, tcp_keepidle, tcp_timer_keep, tp);
 346
 347 #ifdef TCPDEBUG
 348         if (inp->inp_socket->so_options & SO_DEBUG)
 349                 tcp_trace(TA_USER, ostate, tp, (void *)0, (struct tcphdr *)0,
 350                           PRU_SLOWTIMO);
 351 #endif
 352         INP_WUNLOCK(inp);
 353         INP_INFO_WUNLOCK(&V_tcbinfo);
 354         CURVNET_RESTORE();
 355         return;
 356
 357 dropit:
 358         V_tcpstat.tcps_keepdrops++;
 359         tp = tcp_drop(tp, ETIMEDOUT);
 360
 361 #ifdef TCPDEBUG
 362         if (tp != NULL && (tp->t_inpcb->inp_socket->so_options & SO_DEBUG))
 363                 tcp_trace(TA_USER, ostate, tp, (void *)0, (struct tcphdr *)0,
 364                           PRU_SLOWTIMO);
 365 #endif
 366         if (tp != NULL)
 367                 INP_WUNLOCK(tp->t_inpcb);
 368         INP_INFO_WUNLOCK(&V_tcbinfo);
 369         CURVNET_RESTORE();
 370 }
 371
 372 void
 373 tcp_timer_persist(void *xtp)
 374 {
 375         struct tcpcb *tp = xtp;
 376         struct inpcb *inp;
 377         CURVNET_SET(tp->t_vnet);
 378         INIT_VNET_INET(tp->t_vnet);
 379 #ifdef TCPDEBUG
 380         int ostate;
 381
 382         ostate = tp->t_state;
 383 #endif
 384         INP_INFO_WLOCK(&V_tcbinfo);
 385         inp = tp->t_inpcb;
 386         /*
 387          * XXXRW: While this assert is in fact correct, bugs in the tcpcb
 388          * tear-down mean we need it as a work-around for races between
 389          * timers and tcp_discardcb().
 390          *
 391          * KASSERT(inp != NULL, ("tcp_timer_persist: inp == NULL"));
 392          */
 393         if (inp == NULL) {
 394                 tcp_timer_race++;
 395                 INP_INFO_WUNLOCK(&V_tcbinfo);
 396                 CURVNET_RESTORE();
 397                 return;
 398         }
 399         INP_WLOCK(inp);
 400         if ((inp->inp_flags & INP_DROPPED) || callout_pending(&tp->t_timers->tt_persist)
 401             || !callout_active(&tp->t_timers->tt_persist)) {
 402                 INP_WUNLOCK(inp);
 403                 INP_INFO_WUNLOCK(&V_tcbinfo);
 404                 CURVNET_RESTORE();
 405                 return;
 406         }
 407         callout_deactivate(&tp->t_timers->tt_persist);
 408         /*
 409          * Persistance timer into zero window.
 410          * Force a byte to be output, if possible.
 411          */
 412         V_tcpstat.tcps_persisttimeo++;
 413         /*
 414          * Hack: if the peer is dead/unreachable, we do not
 415          * time out if the window is closed.  After a full
 416          * backoff, drop the connection if the idle time
 417          * (no responses to probes) reaches the maximum
 418          * backoff that we would use if retransmitting.
 419          */
 420         if (tp->t_rxtshift == TCP_MAXRXTSHIFT &&
 421             ((ticks - tp->t_rcvtime) >= tcp_maxpersistidle ||
 422              (ticks - tp->t_rcvtime) >= TCP_REXMTVAL(tp) * tcp_totbackoff)) {
 423                 V_tcpstat.tcps_persistdrop++;
 424                 tp = tcp_drop(tp, ETIMEDOUT);
 425                 goto out;
 426         }
 427         tcp_setpersist(tp);
 428         tp->t_flags |= TF_FORCEDATA;
 429         (void) tcp_output(tp);
 430         tp->t_flags &= ~TF_FORCEDATA;
 431
 432 out:
 433 #ifdef TCPDEBUG
 434         if (tp != NULL && tp->t_inpcb->inp_socket->so_options & SO_DEBUG)
 435                 tcp_trace(TA_USER, ostate, tp, NULL, NULL, PRU_SLOWTIMO);
 436 #endif
 437         if (tp != NULL)
 438                 INP_WUNLOCK(inp);
 439         INP_INFO_WUNLOCK(&V_tcbinfo);
 440         CURVNET_RESTORE();
 441 }
 442
 443 void
 444 tcp_timer_rexmt(void * xtp)
 445 {
 446         struct tcpcb *tp = xtp;
 447         CURVNET_SET(tp->t_vnet);
 448         INIT_VNET_INET(tp->t_vnet);
 449         int rexmt;
 450         int headlocked;
 451         struct inpcb *inp;
 452 #ifdef TCPDEBUG
 453         int ostate;
 454
 455         ostate = tp->t_state;
 456 #endif
 457         INP_INFO_WLOCK(&V_tcbinfo);
 458         headlocked = 1;
 459         inp = tp->t_inpcb;
 460         /*
 461          * XXXRW: While this assert is in fact correct, bugs in the tcpcb
 462          * tear-down mean we need it as a work-around for races between
 463          * timers and tcp_discardcb().
 464          *
 465          * KASSERT(inp != NULL, ("tcp_timer_rexmt: inp == NULL"));
 466          */
 467         if (inp == NULL) {
 468                 tcp_timer_race++;
 469                 INP_INFO_WUNLOCK(&V_tcbinfo);
 470                 CURVNET_RESTORE();
 471                 return;
 472         }
 473         INP_WLOCK(inp);
 474         if ((inp->inp_flags & INP_DROPPED) || callout_pending(&tp->t_timers->tt_rexmt)
 475             || !callout_active(&tp->t_timers->tt_rexmt)) {
 476                 INP_WUNLOCK(inp);
 477                 INP_INFO_WUNLOCK(&V_tcbinfo);
 478                 CURVNET_RESTORE();
 479                 return;
 480         }
 481         callout_deactivate(&tp->t_timers->tt_rexmt);
 482         tcp_free_sackholes(tp);
 483         /*
 484          * Retransmission timer went off.  Message has not
 485          * been acked within retransmit interval.  Back off
 486          * to a longer retransmit interval and retransmit one segment.
 487          */
 488         if (++tp->t_rxtshift > TCP_MAXRXTSHIFT) {
 489                 tp->t_rxtshift = TCP_MAXRXTSHIFT;
 490                 V_tcpstat.tcps_timeoutdrop++;
 491                 tp = tcp_drop(tp, tp->t_softerror ?
 492                               tp->t_softerror : ETIMEDOUT);
 493                 goto out;
 494         }
 495         INP_INFO_WUNLOCK(&V_tcbinfo);
 496         headlocked = 0;
 497         if (tp->t_rxtshift == 1) {
 498                 /*
 499                  * first retransmit; record ssthresh and cwnd so they can
 500                  * be recovered if this turns out to be a "bad" retransmit.
 501                  * A retransmit is considered "bad" if an ACK for this
 502                  * segment is received within RTT/2 interval; the assumption
 503                  * here is that the ACK was already in flight.  See
 504                  * "On Estimating End-to-End Network Path Properties" by
 505                  * Allman and Paxson for more details.
 506                  */
 507                 tp->snd_cwnd_prev = tp->snd_cwnd;
 508                 tp->snd_ssthresh_prev = tp->snd_ssthresh;
 509                 tp->snd_recover_prev = tp->snd_recover;
 510                 if (IN_FASTRECOVERY(tp))
 511                   tp->t_flags |= TF_WASFRECOVERY;
 512                 else
 513                   tp->t_flags &= ~TF_WASFRECOVERY;
 514                 tp->t_badrxtwin = ticks + (tp->t_srtt >> (TCP_RTT_SHIFT + 1));
 515         }
 516         V_tcpstat.tcps_rexmttimeo++;
 517         if (tp->t_state == TCPS_SYN_SENT)
 518                 rexmt = TCP_REXMTVAL(tp) * tcp_syn_backoff[tp->t_rxtshift];
 519         else
 520                 rexmt = TCP_REXMTVAL(tp) * tcp_backoff[tp->t_rxtshift];
 521         TCPT_RANGESET(tp->t_rxtcur, rexmt,
 522                       tp->t_rttmin, TCPTV_REXMTMAX);
 523         /*
 524          * Disable rfc1323 if we havn't got any response to
 525          * our third SYN to work-around some broken terminal servers
 526          * (most of which have hopefully been retired) that have bad VJ
 527          * header compression code which trashes TCP segments containing
 528          * unknown-to-them TCP options.
 529          */
 530         if ((tp->t_state == TCPS_SYN_SENT) && (tp->t_rxtshift == 3))
 531                 tp->t_flags &= ~(TF_REQ_SCALE|TF_REQ_TSTMP);
 532         /*
 533          * If we backed off this far, our srtt estimate is probably bogus.
 534          * Clobber it so we'll take the next rtt measurement as our srtt;
 535          * move the current srtt into rttvar to keep the current
 536          * retransmit times until then.
 537          */
 538         if (tp->t_rxtshift > TCP_MAXRXTSHIFT / 4) {
 539 #ifdef INET6
 540                 if ((tp->t_inpcb->inp_vflag & INP_IPV6) != 0)
 541                         in6_losing(tp->t_inpcb);
 542                 else
 543 #endif
 544                 tp->t_rttvar += (tp->t_srtt >> TCP_RTT_SHIFT);
 545                 tp->t_srtt = 0;
 546         }
 547         tp->snd_nxt = tp->snd_una;
 548         tp->snd_recover = tp->snd_max;
 549         /*
 550          * Force a segment to be sent.
 551          */
 552         tp->t_flags |= TF_ACKNOW;
 553         /*
 554          * If timing a segment in this window, stop the timer.
 555          */
 556         tp->t_rtttime = 0;
 557         /*
 558          * Close the congestion window down to one segment
 559          * (we'll open it by one segment for each ack we get).
 560          * Since we probably have a window's worth of unacked
 561          * data accumulated, this "slow start" keeps us from
 562          * dumping all that data as back-to-back packets (which
 563          * might overwhelm an intermediate gateway).
 564          *
 565          * There are two phases to the opening: Initially we
 566          * open by one mss on each ack.  This makes the window
 567          * size increase exponentially with time.  If the
 568          * window is larger than the path can handle, this
 569          * exponential growth results in dropped packet(s)
 570          * almost immediately.  To get more time between
 571          * drops but still "push" the network to take advantage
 572          * of improving conditions, we switch from exponential
 573          * to linear window opening at some threshhold size.
 574          * For a threshhold, we use half the current window
 575          * size, truncated to a multiple of the mss.
 576          *
 577          * (the minimum cwnd that will give us exponential
 578          * growth is 2 mss.  We don't allow the threshhold
 579          * to go below this.)
 580          */
 581         {
 582                 u_int win = min(tp->snd_wnd, tp->snd_cwnd) / 2 / tp->t_maxseg;
 583                 if (win < 2)
 584                         win = 2;
 585                 tp->snd_cwnd = tp->t_maxseg;
 586                 tp->snd_ssthresh = win * tp->t_maxseg;
 587                 tp->t_dupacks = 0;
 588         }
 589         EXIT_FASTRECOVERY(tp);
 590         tp->t_bytes_acked = 0;
 591         (void) tcp_output(tp);
 592
 593 out:
 594 #ifdef TCPDEBUG
 595         if (tp != NULL && (tp->t_inpcb->inp_socket->so_options & SO_DEBUG))
 596                 tcp_trace(TA_USER, ostate, tp, (void *)0, (struct tcphdr *)0,
 597                           PRU_SLOWTIMO);
 598 #endif
 599         if (tp != NULL)
 600                 INP_WUNLOCK(inp);
 601         if (headlocked)
 602                 INP_INFO_WUNLOCK(&V_tcbinfo);
 603         CURVNET_RESTORE();
 604 }
 605
 606 void
 607 tcp_timer_activate(struct tcpcb *tp, int timer_type, u_int delta)
 608 {
 609         struct callout *t_callout;
 610         void *f_callout;
 611
 612         switch (timer_type) {
 613                 case TT_DELACK:
 614                         t_callout = &tp->t_timers->tt_delack;
 615                         f_callout = tcp_timer_delack;
 616                         break;
 617                 case TT_REXMT:
 618                         t_callout = &tp->t_timers->tt_rexmt;
 619                         f_callout = tcp_timer_rexmt;
 620                         break;
 621                 case TT_PERSIST:
 622                         t_callout = &tp->t_timers->tt_persist;
 623                         f_callout = tcp_timer_persist;
 624                         break;
 625                 case TT_KEEP:
 626                         t_callout = &tp->t_timers->tt_keep;
 627                         f_callout = tcp_timer_keep;
 628                         break;
 629                 case TT_2MSL:
 630                         t_callout = &tp->t_timers->tt_2msl;
 631                         f_callout = tcp_timer_2msl;
 632                         break;
 633                 default:
 634                         panic("bad timer_type");
 635                 }
 636         if (delta == 0) {
 637                 callout_stop(t_callout);
 638         } else {
 639                 callout_reset(t_callout, delta, f_callout, tp);
 640         }
 641 }
 642
 643 int
 644 tcp_timer_active(struct tcpcb *tp, int timer_type)
 645 {
 646         struct callout *t_callout;
 647
 648         switch (timer_type) {
 649                 case TT_DELACK:
 650                         t_callout = &tp->t_timers->tt_delack;
 651                         break;
 652                 case TT_REXMT:
 653                         t_callout = &tp->t_timers->tt_rexmt;
 654                         break;
 655                 case TT_PERSIST:
 656                         t_callout = &tp->t_timers->tt_persist;
 657                         break;
 658                 case TT_KEEP:
 659                         t_callout = &tp->t_timers->tt_keep;
 660                         break;
 661                 case TT_2MSL:
 662                         t_callout = &tp->t_timers->tt_2msl;
 663                         break;
 664                 default:
 665                         panic("bad timer_type");
 666                 }
 667         return callout_active(t_callout);
 668 }