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() 123 { 124 125 tcp_maxidle = tcp_keepcnt * tcp_keepintvl; 126 INP_INFO_WLOCK(&tcbinfo); 127 (void) tcp_timer_2msl_tw(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->tt_delack) 170 || !callout_active(tp->tt_delack)) { 171 INP_UNLOCK(inp); 172 return; 173 } 174 callout_deactivate(tp->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->tt_2msl) || 212 !callout_active(tp->tt_2msl)) { 213 INP_UNLOCK(tp->t_inpcb); 214 INP_INFO_WUNLOCK(&tcbinfo); 215 return; 216 } 217 callout_deactivate(tp->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->tt_2msl, tcp_keepintvl, 237 tcp_timer_2msl, tp); 238 else 239 tp = tcp_close(tp); 240 } 241 242 #ifdef TCPDEBUG 243 if (tp != NULL && (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 /* 253 * The timed wait queue contains references to each of the TCP sessions 254 * currently in the TIME_WAIT state. The queue pointers, including the 255 * queue pointers in each tcptw structure, are protected using the global 256 * tcbinfo lock, which must be held over queue iteration and modification. 257 */ 258 static TAILQ_HEAD(, tcptw) twq_2msl; 259 260 void 261 tcp_timer_init(void) 262 { 263 264 TAILQ_INIT(&twq_2msl); 265 } 266 267 void 268 tcp_timer_2msl_reset(struct tcptw *tw, int rearm) 269 { 270 271 INP_INFO_WLOCK_ASSERT(&tcbinfo); 272 INP_LOCK_ASSERT(tw->tw_inpcb); 273 if (rearm) 274 TAILQ_REMOVE(&twq_2msl, tw, tw_2msl); 275 tw->tw_time = ticks + 2 * tcp_msl; 276 TAILQ_INSERT_TAIL(&twq_2msl, tw, tw_2msl); 277 } 278 279 void 280 tcp_timer_2msl_stop(struct tcptw *tw) 281 { 282 283 INP_INFO_WLOCK_ASSERT(&tcbinfo); 284 TAILQ_REMOVE(&twq_2msl, tw, tw_2msl); 285 } 286 287 struct tcptw * 288 tcp_timer_2msl_tw(int reuse) 289 { 290 struct tcptw *tw; 291 292 INP_INFO_WLOCK_ASSERT(&tcbinfo); 293 for (;;) { 294 tw = TAILQ_FIRST(&twq_2msl); 295 if (tw == NULL || (!reuse && tw->tw_time > ticks)) 296 break; 297 INP_LOCK(tw->tw_inpcb); 298 tcp_twclose(tw, reuse); 299 if (reuse) 300 return (tw); 301 } 302 return (NULL); 303 } 304 305 void 306 tcp_timer_keep(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(void *xtp) 398 { 399 struct tcpcb *tp = xtp; 400 struct inpcb *inp; 401 #ifdef TCPDEBUG 402 int ostate; 403 404 ostate = tp->t_state; 405 #endif 406 INP_INFO_WLOCK(&tcbinfo); 407 inp = tp->t_inpcb; 408 /* 409 * XXXRW: While this assert is in fact correct, bugs in the tcpcb 410 * tear-down mean we need it as a work-around for races between 411 * timers and tcp_discardcb(). 412 * 413 * KASSERT(inp != NULL, ("tcp_timer_persist: inp == NULL")); 414 */ 415 if (inp == NULL) { 416 tcp_timer_race++; 417 INP_INFO_WUNLOCK(&tcbinfo); 418 return; 419 } 420 INP_LOCK(inp); 421 if ((inp->inp_vflag & INP_DROPPED) || callout_pending(tp->tt_persist) 422 || !callout_active(tp->tt_persist)) { 423 INP_UNLOCK(inp); 424 INP_INFO_WUNLOCK(&tcbinfo); 425 return; 426 } 427 callout_deactivate(tp->tt_persist); 428 /* 429 * Persistance timer into zero window. 430 * Force a byte to be output, if possible. 431 */ 432 tcpstat.tcps_persisttimeo++; 433 /* 434 * Hack: if the peer is dead/unreachable, we do not 435 * time out if the window is closed. After a full 436 * backoff, drop the connection if the idle time 437 * (no responses to probes) reaches the maximum 438 * backoff that we would use if retransmitting. 439 */ 440 if (tp->t_rxtshift == TCP_MAXRXTSHIFT && 441 ((ticks - tp->t_rcvtime) >= tcp_maxpersistidle || 442 (ticks - tp->t_rcvtime) >= TCP_REXMTVAL(tp) * tcp_totbackoff)) { 443 tcpstat.tcps_persistdrop++; 444 tp = tcp_drop(tp, ETIMEDOUT); 445 goto out; 446 } 447 tcp_setpersist(tp); 448 tp->t_flags |= TF_FORCEDATA; 449 (void) tcp_output(tp); 450 tp->t_flags &= ~TF_FORCEDATA; 451 452 out: 453 #ifdef TCPDEBUG 454 if (tp != NULL && tp->t_inpcb->inp_socket->so_options & SO_DEBUG) 455 tcp_trace(TA_USER, ostate, tp, NULL, NULL, PRU_SLOWTIMO); 456 #endif 457 if (tp != NULL) 458 INP_UNLOCK(inp); 459 INP_INFO_WUNLOCK(&tcbinfo); 460 } 461 462 void 463 tcp_timer_rexmt(void * xtp) 464 { 465 struct tcpcb *tp = xtp; 466 int rexmt; 467 int headlocked; 468 struct inpcb *inp; 469 #ifdef TCPDEBUG 470 int ostate; 471 472 ostate = tp->t_state; 473 #endif 474 INP_INFO_WLOCK(&tcbinfo); 475 headlocked = 1; 476 inp = tp->t_inpcb; 477 /* 478 * XXXRW: While this assert is in fact correct, bugs in the tcpcb 479 * tear-down mean we need it as a work-around for races between 480 * timers and tcp_discardcb(). 481 * 482 * KASSERT(inp != NULL, ("tcp_timer_rexmt: inp == NULL")); 483 */ 484 if (inp == NULL) { 485 tcp_timer_race++; 486 INP_INFO_WUNLOCK(&tcbinfo); 487 return; 488 } 489 INP_LOCK(inp); 490 if ((inp->inp_vflag & INP_DROPPED) || callout_pending(tp->tt_rexmt) 491 || !callout_active(tp->tt_rexmt)) { 492 INP_UNLOCK(inp); 493 INP_INFO_WUNLOCK(&tcbinfo); 494 return; 495 } 496 callout_deactivate(tp->tt_rexmt); 497 tcp_free_sackholes(tp); 498 /* 499 * Retransmission timer went off. Message has not 500 * been acked within retransmit interval. Back off 501 * to a longer retransmit interval and retransmit one segment. 502 */ 503 if (++tp->t_rxtshift > TCP_MAXRXTSHIFT) { 504 tp->t_rxtshift = TCP_MAXRXTSHIFT; 505 tcpstat.tcps_timeoutdrop++; 506 tp = tcp_drop(tp, tp->t_softerror ? 507 tp->t_softerror : ETIMEDOUT); 508 goto out; 509 } 510 INP_INFO_WUNLOCK(&tcbinfo); 511 headlocked = 0; 512 if (tp->t_rxtshift == 1) { 513 /* 514 * first retransmit; record ssthresh and cwnd so they can 515 * be recovered if this turns out to be a "bad" retransmit. 516 * A retransmit is considered "bad" if an ACK for this 517 * segment is received within RTT/2 interval; the assumption 518 * here is that the ACK was already in flight. See 519 * "On Estimating End-to-End Network Path Properties" by 520 * Allman and Paxson for more details. 521 */ 522 tp->snd_cwnd_prev = tp->snd_cwnd; 523 tp->snd_ssthresh_prev = tp->snd_ssthresh; 524 tp->snd_recover_prev = tp->snd_recover; 525 if (IN_FASTRECOVERY(tp)) 526 tp->t_flags |= TF_WASFRECOVERY; 527 else 528 tp->t_flags &= ~TF_WASFRECOVERY; 529 tp->t_badrxtwin = ticks + (tp->t_srtt >> (TCP_RTT_SHIFT + 1)); 530 } 531 tcpstat.tcps_rexmttimeo++; 532 if (tp->t_state == TCPS_SYN_SENT) 533 rexmt = TCP_REXMTVAL(tp) * tcp_syn_backoff[tp->t_rxtshift]; 534 else 535 rexmt = TCP_REXMTVAL(tp) * tcp_backoff[tp->t_rxtshift]; 536 TCPT_RANGESET(tp->t_rxtcur, rexmt, 537 tp->t_rttmin, TCPTV_REXMTMAX); 538 /* 539 * Disable rfc1323 if we havn't got any response to 540 * our third SYN to work-around some broken terminal servers 541 * (most of which have hopefully been retired) that have bad VJ 542 * header compression code which trashes TCP segments containing 543 * unknown-to-them TCP options. 544 */ 545 if ((tp->t_state == TCPS_SYN_SENT) && (tp->t_rxtshift == 3)) 546 tp->t_flags &= ~(TF_REQ_SCALE|TF_REQ_TSTMP); 547 /* 548 * If we backed off this far, our srtt estimate is probably bogus. 549 * Clobber it so we'll take the next rtt measurement as our srtt; 550 * move the current srtt into rttvar to keep the current 551 * retransmit times until then. 552 */ 553 if (tp->t_rxtshift > TCP_MAXRXTSHIFT / 4) { 554 #ifdef INET6 555 if ((tp->t_inpcb->inp_vflag & INP_IPV6) != 0) 556 in6_losing(tp->t_inpcb); 557 else 558 #endif 559 tp->t_rttvar += (tp->t_srtt >> TCP_RTT_SHIFT); 560 tp->t_srtt = 0; 561 } 562 tp->snd_nxt = tp->snd_una; 563 tp->snd_recover = tp->snd_max; 564 /* 565 * Force a segment to be sent. 566 */ 567 tp->t_flags |= TF_ACKNOW; 568 /* 569 * If timing a segment in this window, stop the timer. 570 */ 571 tp->t_rtttime = 0; 572 /* 573 * Close the congestion window down to one segment 574 * (we'll open it by one segment for each ack we get). 575 * Since we probably have a window's worth of unacked 576 * data accumulated, this "slow start" keeps us from 577 * dumping all that data as back-to-back packets (which 578 * might overwhelm an intermediate gateway). 579 * 580 * There are two phases to the opening: Initially we 581 * open by one mss on each ack. This makes the window 582 * size increase exponentially with time. If the 583 * window is larger than the path can handle, this 584 * exponential growth results in dropped packet(s) 585 * almost immediately. To get more time between 586 * drops but still "push" the network to take advantage 587 * of improving conditions, we switch from exponential 588 * to linear window opening at some threshhold size. 589 * For a threshhold, we use half the current window 590 * size, truncated to a multiple of the mss. 591 * 592 * (the minimum cwnd that will give us exponential 593 * growth is 2 mss. We don't allow the threshhold 594 * to go below this.) 595 */ 596 { 597 u_int win = min(tp->snd_wnd, tp->snd_cwnd) / 2 / tp->t_maxseg; 598 if (win < 2) 599 win = 2; 600 tp->snd_cwnd = tp->t_maxseg; 601 tp->snd_ssthresh = win * tp->t_maxseg; 602 tp->t_dupacks = 0; 603 } 604 EXIT_FASTRECOVERY(tp); 605 (void) tcp_output(tp); 606 607 out: 608 #ifdef TCPDEBUG 609 if (tp != NULL && (tp->t_inpcb->inp_socket->so_options & SO_DEBUG)) 610 tcp_trace(TA_USER, ostate, tp, (void *)0, (struct tcphdr *)0, 611 PRU_SLOWTIMO); 612 #endif 613 if (tp != NULL) 614 INP_UNLOCK(inp); 615 if (headlocked) 616 INP_INFO_WUNLOCK(&tcbinfo); 617 } 618