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 #include "opt_tcp_sack.h" 36 37 #include <sys/param.h> 38 #include <sys/kernel.h> 39 #include <sys/lock.h> 40 #include <sys/mbuf.h> 41 #include <sys/mutex.h> 42 #include <sys/protosw.h> 43 #include <sys/socket.h> 44 #include <sys/socketvar.h> 45 #include <sys/sysctl.h> 46 #include <sys/systm.h> 47 48 #include <net/route.h> 49 50 #include <netinet/in.h> 51 #include <netinet/in_pcb.h> 52 #include <netinet/in_systm.h> 53 #ifdef INET6 54 #include <netinet6/in6_pcb.h> 55 #endif 56 #include <netinet/ip_var.h> 57 #include <netinet/tcp.h> 58 #include <netinet/tcp_fsm.h> 59 #include <netinet/tcp_timer.h> 60 #include <netinet/tcp_var.h> 61 #include <netinet/tcpip.h> 62 #ifdef TCPDEBUG 63 #include <netinet/tcp_debug.h> 64 #endif 65 66 int tcp_keepinit; 67 SYSCTL_PROC(_net_inet_tcp, TCPCTL_KEEPINIT, keepinit, CTLTYPE_INT|CTLFLAG_RW, 68 &tcp_keepinit, 0, sysctl_msec_to_ticks, "I", ""); 69 70 int tcp_keepidle; 71 SYSCTL_PROC(_net_inet_tcp, TCPCTL_KEEPIDLE, keepidle, CTLTYPE_INT|CTLFLAG_RW, 72 &tcp_keepidle, 0, sysctl_msec_to_ticks, "I", ""); 73 74 int tcp_keepintvl; 75 SYSCTL_PROC(_net_inet_tcp, TCPCTL_KEEPINTVL, keepintvl, CTLTYPE_INT|CTLFLAG_RW, 76 &tcp_keepintvl, 0, sysctl_msec_to_ticks, "I", ""); 77 78 int tcp_delacktime; 79 SYSCTL_PROC(_net_inet_tcp, TCPCTL_DELACKTIME, delacktime, 80 CTLTYPE_INT|CTLFLAG_RW, &tcp_delacktime, 0, sysctl_msec_to_ticks, "I", 81 "Time before a delayed ACK is sent"); 82 83 int tcp_msl; 84 SYSCTL_PROC(_net_inet_tcp, OID_AUTO, msl, CTLTYPE_INT|CTLFLAG_RW, 85 &tcp_msl, 0, sysctl_msec_to_ticks, "I", "Maximum segment lifetime"); 86 87 int tcp_rexmit_min; 88 SYSCTL_PROC(_net_inet_tcp, OID_AUTO, rexmit_min, CTLTYPE_INT|CTLFLAG_RW, 89 &tcp_rexmit_min, 0, sysctl_msec_to_ticks, "I", "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", "Retransmission Timer Slop"); 94 95 static int always_keepalive = 1; 96 SYSCTL_INT(_net_inet_tcp, OID_AUTO, always_keepalive, CTLFLAG_RW, 97 &always_keepalive , 0, "Assume SO_KEEPALIVE on all TCP connections"); 98 99 static int tcp_keepcnt = TCPTV_KEEPCNT; 100 /* max idle probes */ 101 int tcp_maxpersistidle; 102 /* max idle time in persist */ 103 int tcp_maxidle; 104 105 /* 106 * Tcp protocol timeout routine called every 500 ms. 107 * Updates timestamps used for TCP 108 * causes finite state machine actions if timers expire. 109 */ 110 void 111 tcp_slowtimo() 112 { 113 114 tcp_maxidle = tcp_keepcnt * tcp_keepintvl; 115 INP_INFO_WLOCK(&tcbinfo); 116 (void) tcp_timer_2msl_tw(0); 117 INP_INFO_WUNLOCK(&tcbinfo); 118 } 119 120 int tcp_syn_backoff[TCP_MAXRXTSHIFT + 1] = 121 { 1, 1, 1, 1, 1, 2, 4, 8, 16, 32, 64, 64, 64 }; 122 123 int tcp_backoff[TCP_MAXRXTSHIFT + 1] = 124 { 1, 2, 4, 8, 16, 32, 64, 128, 256, 512, 512, 512, 512 }; 125 126 static int tcp_totbackoff = 2559; /* sum of tcp_backoff[] */ 127 128 static int tcp_timer_race; 129 SYSCTL_INT(_net_inet_tcp, OID_AUTO, timer_race, CTLFLAG_RD, &tcp_timer_race, 130 0, "Count of t_inpcb races on tcp_discardcb"); 131 132 /* 133 * TCP timer processing. 134 */ 135 136 void 137 tcp_timer_delack(xtp) 138 void *xtp; 139 { 140 struct tcpcb *tp = xtp; 141 struct inpcb *inp; 142 143 INP_INFO_RLOCK(&tcbinfo); 144 inp = tp->t_inpcb; 145 /* 146 * XXXRW: While this assert is in fact correct, bugs in the tcpcb 147 * tear-down mean we need it as a work-around for races between 148 * timers and tcp_discardcb(). 149 * 150 * KASSERT(inp != NULL, ("tcp_timer_delack: inp == NULL")); 151 */ 152 if (inp == NULL) { 153 tcp_timer_race++; 154 INP_INFO_RUNLOCK(&tcbinfo); 155 return; 156 } 157 INP_LOCK(inp); 158 INP_INFO_RUNLOCK(&tcbinfo); 159 if ((inp->inp_vflag & INP_DROPPED) || callout_pending(tp->tt_delack) 160 || !callout_active(tp->tt_delack)) { 161 INP_UNLOCK(inp); 162 return; 163 } 164 callout_deactivate(tp->tt_delack); 165 166 tp->t_flags |= TF_ACKNOW; 167 tcpstat.tcps_delack++; 168 (void) tcp_output(tp); 169 INP_UNLOCK(inp); 170 } 171 172 void 173 tcp_timer_2msl(xtp) 174 void *xtp; 175 { 176 struct tcpcb *tp = xtp; 177 struct inpcb *inp; 178 #ifdef TCPDEBUG 179 int ostate; 180 181 ostate = tp->t_state; 182 #endif 183 /* 184 * XXXRW: Does this actually happen? 185 */ 186 INP_INFO_WLOCK(&tcbinfo); 187 inp = tp->t_inpcb; 188 /* 189 * XXXRW: While this assert is in fact correct, bugs in the tcpcb 190 * tear-down mean we need it as a work-around for races between 191 * timers and tcp_discardcb(). 192 * 193 * KASSERT(inp != NULL, ("tcp_timer_2msl: inp == NULL")); 194 */ 195 if (inp == NULL) { 196 tcp_timer_race++; 197 INP_INFO_WUNLOCK(&tcbinfo); 198 return; 199 } 200 INP_LOCK(inp); 201 tcp_free_sackholes(tp); 202 if ((inp->inp_vflag & INP_DROPPED) || callout_pending(tp->tt_2msl) || 203 !callout_active(tp->tt_2msl)) { 204 INP_UNLOCK(tp->t_inpcb); 205 INP_INFO_WUNLOCK(&tcbinfo); 206 return; 207 } 208 callout_deactivate(tp->tt_2msl); 209 /* 210 * 2 MSL timeout in shutdown went off. If we're closed but 211 * still waiting for peer to close and connection has been idle 212 * too long, or if 2MSL time is up from TIME_WAIT, delete connection 213 * control block. Otherwise, check again in a bit. 214 */ 215 if (tp->t_state != TCPS_TIME_WAIT && 216 (ticks - tp->t_rcvtime) <= tcp_maxidle) 217 callout_reset(tp->tt_2msl, tcp_keepintvl, 218 tcp_timer_2msl, tp); 219 else 220 tp = tcp_close(tp); 221 222 #ifdef TCPDEBUG 223 if (tp != NULL && (tp->t_inpcb->inp_socket->so_options & SO_DEBUG)) 224 tcp_trace(TA_USER, ostate, tp, (void *)0, (struct tcphdr *)0, 225 PRU_SLOWTIMO); 226 #endif 227 if (tp != NULL) 228 INP_UNLOCK(inp); 229 INP_INFO_WUNLOCK(&tcbinfo); 230 } 231 232 /* 233 * The timed wait lists contain references to each of the TCP sessions 234 * currently TIME_WAIT state. The list pointers, including the list pointers 235 * in each tcptw structure, are protected using the global tcbinfo lock, 236 * which must be held over list iteration and modification. 237 */ 238 struct twlist { 239 LIST_HEAD(, tcptw) tw_list; 240 struct tcptw tw_tail; 241 }; 242 #define TWLIST_NLISTS 2 243 static struct twlist twl_2msl[TWLIST_NLISTS]; 244 static struct twlist *tw_2msl_list[] = { &twl_2msl[0], &twl_2msl[1], NULL }; 245 246 void 247 tcp_timer_init(void) 248 { 249 int i; 250 struct twlist *twl; 251 252 for (i = 0; i < TWLIST_NLISTS; i++) { 253 twl = &twl_2msl[i]; 254 LIST_INIT(&twl->tw_list); 255 LIST_INSERT_HEAD(&twl->tw_list, &twl->tw_tail, tw_2msl); 256 } 257 } 258 259 void 260 tcp_timer_2msl_reset(struct tcptw *tw, int timeo, int rearm) 261 { 262 int i; 263 struct tcptw *tw_tail; 264 265 INP_INFO_WLOCK_ASSERT(&tcbinfo); 266 INP_LOCK_ASSERT(tw->tw_inpcb); 267 if (rearm) 268 LIST_REMOVE(tw, tw_2msl); 269 tw->tw_time = timeo + ticks; 270 i = timeo > tcp_msl ? 1 : 0; 271 tw_tail = &twl_2msl[i].tw_tail; 272 LIST_INSERT_BEFORE(tw_tail, tw, tw_2msl); 273 } 274 275 void 276 tcp_timer_2msl_stop(struct tcptw *tw) 277 { 278 279 INP_INFO_WLOCK_ASSERT(&tcbinfo); 280 LIST_REMOVE(tw, tw_2msl); 281 } 282 283 struct tcptw * 284 tcp_timer_2msl_tw(int reuse) 285 { 286 struct tcptw *tw, *tw_tail; 287 struct twlist *twl; 288 int i; 289 290 INP_INFO_WLOCK_ASSERT(&tcbinfo); 291 for (i = 0; i < 2; i++) { 292 twl = tw_2msl_list[i]; 293 tw_tail = &twl->tw_tail; 294 tw = LIST_FIRST(&twl->tw_list); 295 if (tw == tw_tail || (!reuse && tw->tw_time > ticks)) 296 continue; 297 INP_LOCK(tw->tw_inpcb); 298 tcp_twclose(tw, reuse); 299 return (reuse ? tw : NULL); 300 } 301 return (NULL); 302 } 303 304 void 305 tcp_timer_keep(xtp) 306 void *xtp; 307 { 308 struct tcpcb *tp = xtp; 309 struct tcptemp *t_template; 310 struct inpcb *inp; 311 #ifdef TCPDEBUG 312 int ostate; 313 314 ostate = tp->t_state; 315 #endif 316 INP_INFO_WLOCK(&tcbinfo); 317 inp = tp->t_inpcb; 318 /* 319 * XXXRW: While this assert is in fact correct, bugs in the tcpcb 320 * tear-down mean we need it as a work-around for races between 321 * timers and tcp_discardcb(). 322 * 323 * KASSERT(inp != NULL, ("tcp_timer_keep: inp == NULL")); 324 */ 325 if (inp == NULL) { 326 tcp_timer_race++; 327 INP_INFO_WUNLOCK(&tcbinfo); 328 return; 329 } 330 INP_LOCK(inp); 331 if ((inp->inp_vflag & INP_DROPPED) || callout_pending(tp->tt_keep) 332 || !callout_active(tp->tt_keep)) { 333 INP_UNLOCK(inp); 334 INP_INFO_WUNLOCK(&tcbinfo); 335 return; 336 } 337 callout_deactivate(tp->tt_keep); 338 /* 339 * Keep-alive timer went off; send something 340 * or drop connection if idle for too long. 341 */ 342 tcpstat.tcps_keeptimeo++; 343 if (tp->t_state < TCPS_ESTABLISHED) 344 goto dropit; 345 if ((always_keepalive || inp->inp_socket->so_options & SO_KEEPALIVE) && 346 tp->t_state <= TCPS_CLOSING) { 347 if ((ticks - tp->t_rcvtime) >= tcp_keepidle + tcp_maxidle) 348 goto dropit; 349 /* 350 * Send a packet designed to force a response 351 * if the peer is up and reachable: 352 * either an ACK if the connection is still alive, 353 * or an RST if the peer has closed the connection 354 * due to timeout or reboot. 355 * Using sequence number tp->snd_una-1 356 * causes the transmitted zero-length segment 357 * to lie outside the receive window; 358 * by the protocol spec, this requires the 359 * correspondent TCP to respond. 360 */ 361 tcpstat.tcps_keepprobe++; 362 t_template = tcpip_maketemplate(inp); 363 if (t_template) { 364 tcp_respond(tp, t_template->tt_ipgen, 365 &t_template->tt_t, (struct mbuf *)NULL, 366 tp->rcv_nxt, tp->snd_una - 1, 0); 367 (void) m_free(dtom(t_template)); 368 } 369 callout_reset(tp->tt_keep, tcp_keepintvl, tcp_timer_keep, tp); 370 } else 371 callout_reset(tp->tt_keep, tcp_keepidle, tcp_timer_keep, tp); 372 373 #ifdef TCPDEBUG 374 if (inp->inp_socket->so_options & SO_DEBUG) 375 tcp_trace(TA_USER, ostate, tp, (void *)0, (struct tcphdr *)0, 376 PRU_SLOWTIMO); 377 #endif 378 INP_UNLOCK(inp); 379 INP_INFO_WUNLOCK(&tcbinfo); 380 return; 381 382 dropit: 383 tcpstat.tcps_keepdrops++; 384 tp = tcp_drop(tp, ETIMEDOUT); 385 386 #ifdef TCPDEBUG 387 if (tp != NULL && (tp->t_inpcb->inp_socket->so_options & SO_DEBUG)) 388 tcp_trace(TA_USER, ostate, tp, (void *)0, (struct tcphdr *)0, 389 PRU_SLOWTIMO); 390 #endif 391 if (tp != NULL) 392 INP_UNLOCK(tp->t_inpcb); 393 INP_INFO_WUNLOCK(&tcbinfo); 394 } 395 396 void 397 tcp_timer_persist(xtp) 398 void *xtp; 399 { 400 struct tcpcb *tp = xtp; 401 struct inpcb *inp; 402 #ifdef TCPDEBUG 403 int ostate; 404 405 ostate = tp->t_state; 406 #endif 407 INP_INFO_WLOCK(&tcbinfo); 408 inp = tp->t_inpcb; 409 /* 410 * XXXRW: While this assert is in fact correct, bugs in the tcpcb 411 * tear-down mean we need it as a work-around for races between 412 * timers and tcp_discardcb(). 413 * 414 * KASSERT(inp != NULL, ("tcp_timer_persist: inp == NULL")); 415 */ 416 if (inp == NULL) { 417 tcp_timer_race++; 418 INP_INFO_WUNLOCK(&tcbinfo); 419 return; 420 } 421 INP_LOCK(inp); 422 if ((inp->inp_vflag & INP_DROPPED) || callout_pending(tp->tt_persist) 423 || !callout_active(tp->tt_persist)) { 424 INP_UNLOCK(inp); 425 INP_INFO_WUNLOCK(&tcbinfo); 426 return; 427 } 428 callout_deactivate(tp->tt_persist); 429 /* 430 * Persistance timer into zero window. 431 * Force a byte to be output, if possible. 432 */ 433 tcpstat.tcps_persisttimeo++; 434 /* 435 * Hack: if the peer is dead/unreachable, we do not 436 * time out if the window is closed. After a full 437 * backoff, drop the connection if the idle time 438 * (no responses to probes) reaches the maximum 439 * backoff that we would use if retransmitting. 440 */ 441 if (tp->t_rxtshift == TCP_MAXRXTSHIFT && 442 ((ticks - tp->t_rcvtime) >= tcp_maxpersistidle || 443 (ticks - tp->t_rcvtime) >= TCP_REXMTVAL(tp) * tcp_totbackoff)) { 444 tcpstat.tcps_persistdrop++; 445 tp = tcp_drop(tp, ETIMEDOUT); 446 goto out; 447 } 448 tcp_setpersist(tp); 449 tp->t_flags |= TF_FORCEDATA; 450 (void) tcp_output(tp); 451 tp->t_flags &= ~TF_FORCEDATA; 452 453 out: 454 #ifdef TCPDEBUG 455 if (tp != NULL && tp->t_inpcb->inp_socket->so_options & SO_DEBUG) 456 tcp_trace(TA_USER, ostate, tp, NULL, NULL, PRU_SLOWTIMO); 457 #endif 458 if (tp != NULL) 459 INP_UNLOCK(inp); 460 INP_INFO_WUNLOCK(&tcbinfo); 461 } 462 463 void 464 tcp_timer_rexmt(xtp) 465 void *xtp; 466 { 467 struct tcpcb *tp = xtp; 468 int rexmt; 469 int headlocked; 470 struct inpcb *inp; 471 #ifdef TCPDEBUG 472 int ostate; 473 474 ostate = tp->t_state; 475 #endif 476 INP_INFO_WLOCK(&tcbinfo); 477 headlocked = 1; 478 inp = tp->t_inpcb; 479 /* 480 * XXXRW: While this assert is in fact correct, bugs in the tcpcb 481 * tear-down mean we need it as a work-around for races between 482 * timers and tcp_discardcb(). 483 * 484 * KASSERT(inp != NULL, ("tcp_timer_rexmt: inp == NULL")); 485 */ 486 if (inp == NULL) { 487 tcp_timer_race++; 488 INP_INFO_WUNLOCK(&tcbinfo); 489 return; 490 } 491 INP_LOCK(inp); 492 if ((inp->inp_vflag & INP_DROPPED) || callout_pending(tp->tt_rexmt) 493 || !callout_active(tp->tt_rexmt)) { 494 INP_UNLOCK(inp); 495 INP_INFO_WUNLOCK(&tcbinfo); 496 return; 497 } 498 callout_deactivate(tp->tt_rexmt); 499 tcp_free_sackholes(tp); 500 /* 501 * Retransmission timer went off. Message has not 502 * been acked within retransmit interval. Back off 503 * to a longer retransmit interval and retransmit one segment. 504 */ 505 if (++tp->t_rxtshift > TCP_MAXRXTSHIFT) { 506 tp->t_rxtshift = TCP_MAXRXTSHIFT; 507 tcpstat.tcps_timeoutdrop++; 508 tp = tcp_drop(tp, tp->t_softerror ? 509 tp->t_softerror : ETIMEDOUT); 510 goto out; 511 } 512 INP_INFO_WUNLOCK(&tcbinfo); 513 headlocked = 0; 514 if (tp->t_rxtshift == 1) { 515 /* 516 * first retransmit; record ssthresh and cwnd so they can 517 * be recovered if this turns out to be a "bad" retransmit. 518 * A retransmit is considered "bad" if an ACK for this 519 * segment is received within RTT/2 interval; the assumption 520 * here is that the ACK was already in flight. See 521 * "On Estimating End-to-End Network Path Properties" by 522 * Allman and Paxson for more details. 523 */ 524 tp->snd_cwnd_prev = tp->snd_cwnd; 525 tp->snd_ssthresh_prev = tp->snd_ssthresh; 526 tp->snd_recover_prev = tp->snd_recover; 527 if (IN_FASTRECOVERY(tp)) 528 tp->t_flags |= TF_WASFRECOVERY; 529 else 530 tp->t_flags &= ~TF_WASFRECOVERY; 531 tp->t_badrxtwin = ticks + (tp->t_srtt >> (TCP_RTT_SHIFT + 1)); 532 } 533 tcpstat.tcps_rexmttimeo++; 534 if (tp->t_state == TCPS_SYN_SENT) 535 rexmt = TCP_REXMTVAL(tp) * tcp_syn_backoff[tp->t_rxtshift]; 536 else 537 rexmt = TCP_REXMTVAL(tp) * tcp_backoff[tp->t_rxtshift]; 538 TCPT_RANGESET(tp->t_rxtcur, rexmt, 539 tp->t_rttmin, TCPTV_REXMTMAX); 540 /* 541 * Disable rfc1323 if we havn't got any response to 542 * our third SYN to work-around some broken terminal servers 543 * (most of which have hopefully been retired) that have bad VJ 544 * header compression code which trashes TCP segments containing 545 * unknown-to-them TCP options. 546 */ 547 if ((tp->t_state == TCPS_SYN_SENT) && (tp->t_rxtshift == 3)) 548 tp->t_flags &= ~(TF_REQ_SCALE|TF_REQ_TSTMP); 549 /* 550 * If we backed off this far, our srtt estimate is probably bogus. 551 * Clobber it so we'll take the next rtt measurement as our srtt; 552 * move the current srtt into rttvar to keep the current 553 * retransmit times until then. 554 */ 555 if (tp->t_rxtshift > TCP_MAXRXTSHIFT / 4) { 556 #ifdef INET6 557 if ((tp->t_inpcb->inp_vflag & INP_IPV6) != 0) 558 in6_losing(tp->t_inpcb); 559 else 560 #endif 561 tp->t_rttvar += (tp->t_srtt >> TCP_RTT_SHIFT); 562 tp->t_srtt = 0; 563 } 564 tp->snd_nxt = tp->snd_una; 565 tp->snd_recover = tp->snd_max; 566 /* 567 * Force a segment to be sent. 568 */ 569 tp->t_flags |= TF_ACKNOW; 570 /* 571 * If timing a segment in this window, stop the timer. 572 */ 573 tp->t_rtttime = 0; 574 /* 575 * Close the congestion window down to one segment 576 * (we'll open it by one segment for each ack we get). 577 * Since we probably have a window's worth of unacked 578 * data accumulated, this "slow start" keeps us from 579 * dumping all that data as back-to-back packets (which 580 * might overwhelm an intermediate gateway). 581 * 582 * There are two phases to the opening: Initially we 583 * open by one mss on each ack. This makes the window 584 * size increase exponentially with time. If the 585 * window is larger than the path can handle, this 586 * exponential growth results in dropped packet(s) 587 * almost immediately. To get more time between 588 * drops but still "push" the network to take advantage 589 * of improving conditions, we switch from exponential 590 * to linear window opening at some threshhold size. 591 * For a threshhold, we use half the current window 592 * size, truncated to a multiple of the mss. 593 * 594 * (the minimum cwnd that will give us exponential 595 * growth is 2 mss. We don't allow the threshhold 596 * to go below this.) 597 */ 598 { 599 u_int win = min(tp->snd_wnd, tp->snd_cwnd) / 2 / tp->t_maxseg; 600 if (win < 2) 601 win = 2; 602 tp->snd_cwnd = tp->t_maxseg; 603 tp->snd_ssthresh = win * tp->t_maxseg; 604 tp->t_dupacks = 0; 605 } 606 EXIT_FASTRECOVERY(tp); 607 (void) tcp_output(tp); 608 609 out: 610 #ifdef TCPDEBUG 611 if (tp != NULL && (tp->t_inpcb->inp_socket->so_options & SO_DEBUG)) 612 tcp_trace(TA_USER, ostate, tp, (void *)0, (struct tcphdr *)0, 613 PRU_SLOWTIMO); 614 #endif 615 if (tp != NULL) 616 INP_UNLOCK(inp); 617 if (headlocked) 618 INP_INFO_WUNLOCK(&tcbinfo); 619 } 620