The Design and Implementation of the FreeBSD Operating System, Second Edition
Now available: The Design and Implementation of the FreeBSD Operating System (Second Edition)


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FreeBSD/Linux Kernel Cross Reference
sys/netinet/tcp_timer.c

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    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 static int
   67 sysctl_msec_to_ticks(SYSCTL_HANDLER_ARGS)
   68 {
   69         int error, s, tt;
   70 
   71         tt = *(int *)oidp->oid_arg1;
   72         s = (int)((int64_t)tt * 1000 / hz);
   73 
   74         error = sysctl_handle_int(oidp, &s, 0, req);
   75         if (error || !req->newptr)
   76                 return (error);
   77 
   78         tt = (int)((int64_t)s * hz / 1000);
   79         if (tt < 1)
   80                 return (EINVAL);
   81 
   82         *(int *)oidp->oid_arg1 = tt;
   83         return (0);
   84 }
   85 
   86 int     tcp_keepinit;
   87 SYSCTL_PROC(_net_inet_tcp, TCPCTL_KEEPINIT, keepinit, CTLTYPE_INT|CTLFLAG_RW,
   88     &tcp_keepinit, 0, sysctl_msec_to_ticks, "I", "");
   89 
   90 int     tcp_keepidle;
   91 SYSCTL_PROC(_net_inet_tcp, TCPCTL_KEEPIDLE, keepidle, CTLTYPE_INT|CTLFLAG_RW,
   92     &tcp_keepidle, 0, sysctl_msec_to_ticks, "I", "");
   93 
   94 int     tcp_keepintvl;
   95 SYSCTL_PROC(_net_inet_tcp, TCPCTL_KEEPINTVL, keepintvl, CTLTYPE_INT|CTLFLAG_RW,
   96     &tcp_keepintvl, 0, sysctl_msec_to_ticks, "I", "");
   97 
   98 int     tcp_delacktime;
   99 SYSCTL_PROC(_net_inet_tcp, TCPCTL_DELACKTIME, delacktime,
  100     CTLTYPE_INT|CTLFLAG_RW, &tcp_delacktime, 0, sysctl_msec_to_ticks, "I",
  101     "Time before a delayed ACK is sent");
  102 
  103 int     tcp_msl;
  104 SYSCTL_PROC(_net_inet_tcp, OID_AUTO, msl, CTLTYPE_INT|CTLFLAG_RW,
  105     &tcp_msl, 0, sysctl_msec_to_ticks, "I", "Maximum segment lifetime");
  106 
  107 int     tcp_rexmit_min;
  108 SYSCTL_PROC(_net_inet_tcp, OID_AUTO, rexmit_min, CTLTYPE_INT|CTLFLAG_RW,
  109     &tcp_rexmit_min, 0, sysctl_msec_to_ticks, "I", "Minimum Retransmission Timeout");
  110 
  111 int     tcp_rexmit_slop;
  112 SYSCTL_PROC(_net_inet_tcp, OID_AUTO, rexmit_slop, CTLTYPE_INT|CTLFLAG_RW,
  113     &tcp_rexmit_slop, 0, sysctl_msec_to_ticks, "I", "Retransmission Timer Slop");
  114 
  115 static int      always_keepalive = 1;
  116 SYSCTL_INT(_net_inet_tcp, OID_AUTO, always_keepalive, CTLFLAG_RW,
  117     &always_keepalive , 0, "Assume SO_KEEPALIVE on all TCP connections");
  118 
  119 static int      tcp_keepcnt = TCPTV_KEEPCNT;
  120         /* max idle probes */
  121 int     tcp_maxpersistidle;
  122         /* max idle time in persist */
  123 int     tcp_maxidle;
  124 
  125 /*
  126  * Tcp protocol timeout routine called every 500 ms.
  127  * Updates timestamps used for TCP
  128  * causes finite state machine actions if timers expire.
  129  */
  130 void
  131 tcp_slowtimo()
  132 {
  133 
  134         tcp_maxidle = tcp_keepcnt * tcp_keepintvl;
  135         INP_INFO_WLOCK(&tcbinfo);
  136         (void) tcp_timer_2msl_tw(0);
  137         INP_INFO_WUNLOCK(&tcbinfo);
  138 }
  139 
  140 int     tcp_syn_backoff[TCP_MAXRXTSHIFT + 1] =
  141     { 1, 1, 1, 1, 1, 2, 4, 8, 16, 32, 64, 64, 64 };
  142 
  143 int     tcp_backoff[TCP_MAXRXTSHIFT + 1] =
  144     { 1, 2, 4, 8, 16, 32, 64, 128, 256, 512, 512, 512, 512 };
  145 
  146 static int tcp_totbackoff = 2559;       /* sum of tcp_backoff[] */
  147 
  148 /*
  149  * TCP timer processing.
  150  */
  151 
  152 void
  153 tcp_timer_delack(xtp)
  154         void *xtp;
  155 {
  156         struct tcpcb *tp = xtp;
  157         struct inpcb *inp;
  158 
  159         INP_INFO_RLOCK(&tcbinfo);
  160         inp = tp->t_inpcb;
  161         if (inp == NULL) {
  162                 INP_INFO_RUNLOCK(&tcbinfo);
  163                 return;
  164         }
  165         INP_LOCK(inp);
  166         INP_INFO_RUNLOCK(&tcbinfo);
  167         if (callout_pending(tp->tt_delack) || !callout_active(tp->tt_delack)) {
  168                 INP_UNLOCK(inp);
  169                 return;
  170         }
  171         callout_deactivate(tp->tt_delack);
  172 
  173         tp->t_flags |= TF_ACKNOW;
  174         tcpstat.tcps_delack++;
  175         (void) tcp_output(tp);
  176         INP_UNLOCK(inp);
  177 }
  178 
  179 void
  180 tcp_timer_2msl(xtp)
  181         void *xtp;
  182 {
  183         struct tcpcb *tp = xtp;
  184         struct inpcb *inp;
  185 #ifdef TCPDEBUG
  186         int ostate;
  187 
  188         ostate = tp->t_state;
  189 #endif
  190         INP_INFO_WLOCK(&tcbinfo);
  191         inp = tp->t_inpcb;
  192         if (inp == NULL) {
  193                 INP_INFO_WUNLOCK(&tcbinfo);
  194                 return;
  195         }
  196         INP_LOCK(inp);
  197         tcp_free_sackholes(tp);
  198         if (callout_pending(tp->tt_2msl) || !callout_active(tp->tt_2msl)) {
  199                 INP_UNLOCK(tp->t_inpcb);
  200                 INP_INFO_WUNLOCK(&tcbinfo);
  201                 return;
  202         }
  203         callout_deactivate(tp->tt_2msl);
  204         /*
  205          * 2 MSL timeout in shutdown went off.  If we're closed but
  206          * still waiting for peer to close and connection has been idle
  207          * too long, or if 2MSL time is up from TIME_WAIT, delete connection
  208          * control block.  Otherwise, check again in a bit.
  209          */
  210         if (tp->t_state != TCPS_TIME_WAIT &&
  211             (ticks - tp->t_rcvtime) <= tcp_maxidle)
  212                 callout_reset(tp->tt_2msl, tcp_keepintvl,
  213                               tcp_timer_2msl, tp);
  214         else
  215                 tp = tcp_close(tp);
  216 
  217 #ifdef TCPDEBUG
  218         if (tp && (tp->t_inpcb->inp_socket->so_options & SO_DEBUG))
  219                 tcp_trace(TA_USER, ostate, tp, (void *)0, (struct tcphdr *)0,
  220                           PRU_SLOWTIMO);
  221 #endif
  222         if (tp)
  223                 INP_UNLOCK(inp);
  224         INP_INFO_WUNLOCK(&tcbinfo);
  225 }
  226 
  227 /*
  228  * The timed wait lists contain references to each of the TCP sessions
  229  * currently TIME_WAIT state.  The list pointers, including the list pointers
  230  * in each tcptw structure, are protected using the global tcbinfo lock,
  231  * which must be held over list iteration and modification.
  232  */
  233 struct twlist {
  234         LIST_HEAD(, tcptw)      tw_list;
  235         struct tcptw    tw_tail;
  236 };
  237 #define TWLIST_NLISTS   2
  238 static struct twlist twl_2msl[TWLIST_NLISTS];
  239 static struct twlist *tw_2msl_list[] = { &twl_2msl[0], &twl_2msl[1], NULL };
  240 
  241 void
  242 tcp_timer_init(void)
  243 {
  244         int i;
  245         struct twlist *twl;
  246 
  247         for (i = 0; i < TWLIST_NLISTS; i++) {
  248                 twl = &twl_2msl[i];
  249                 LIST_INIT(&twl->tw_list);
  250                 LIST_INSERT_HEAD(&twl->tw_list, &twl->tw_tail, tw_2msl);
  251         }
  252 }
  253 
  254 void
  255 tcp_timer_2msl_reset(struct tcptw *tw, int timeo)
  256 {
  257         int i;
  258         struct tcptw *tw_tail;
  259 
  260         INP_INFO_WLOCK_ASSERT(&tcbinfo);
  261         INP_LOCK_ASSERT(tw->tw_inpcb);
  262         if (tw->tw_time != 0)
  263                 LIST_REMOVE(tw, tw_2msl);
  264         tw->tw_time = timeo + ticks;
  265         i = timeo > tcp_msl ? 1 : 0;
  266         tw_tail = &twl_2msl[i].tw_tail;
  267         LIST_INSERT_BEFORE(tw_tail, tw, tw_2msl);
  268 }
  269 
  270 void
  271 tcp_timer_2msl_stop(struct tcptw *tw)
  272 {
  273 
  274         INP_INFO_WLOCK_ASSERT(&tcbinfo);
  275         if (tw->tw_time != 0)
  276                 LIST_REMOVE(tw, tw_2msl);
  277 }
  278 
  279 struct tcptw *
  280 tcp_timer_2msl_tw(int reuse)
  281 {
  282         struct tcptw *tw, *tw_tail;
  283         struct twlist *twl;
  284         int i;
  285 
  286         INP_INFO_WLOCK_ASSERT(&tcbinfo);
  287         for (i = 0; i < 2; i++) {
  288                 twl = tw_2msl_list[i];
  289                 tw_tail = &twl->tw_tail;
  290                 for (;;) {
  291                         tw = LIST_FIRST(&twl->tw_list);
  292                         if (tw == tw_tail || (!reuse && tw->tw_time > ticks))
  293                                 break;
  294                         INP_LOCK(tw->tw_inpcb);
  295                         if (tcp_twclose(tw, reuse) != NULL)
  296                                 return (tw);
  297                 }
  298         }
  299         return (NULL);
  300 }
  301 
  302 void
  303 tcp_timer_keep(xtp)
  304         void *xtp;
  305 {
  306         struct tcpcb *tp = xtp;
  307         struct tcptemp *t_template;
  308         int s;
  309         struct inpcb *inp;
  310 #ifdef TCPDEBUG
  311         int ostate;
  312 
  313         ostate = tp->t_state;
  314 #endif
  315         s = splnet();
  316         INP_INFO_WLOCK(&tcbinfo);
  317         inp = tp->t_inpcb;
  318         if (!inp) {
  319                 INP_INFO_WUNLOCK(&tcbinfo);
  320                 splx(s);
  321                 return;
  322         }
  323         INP_LOCK(inp);
  324         if (callout_pending(tp->tt_keep) || !callout_active(tp->tt_keep)) {
  325                 INP_UNLOCK(inp);
  326                 INP_INFO_WUNLOCK(&tcbinfo);
  327                 splx(s);
  328                 return;
  329         }
  330         callout_deactivate(tp->tt_keep);
  331         /*
  332          * Keep-alive timer went off; send something
  333          * or drop connection if idle for too long.
  334          */
  335         tcpstat.tcps_keeptimeo++;
  336         if (tp->t_state < TCPS_ESTABLISHED)
  337                 goto dropit;
  338         if ((always_keepalive || inp->inp_socket->so_options & SO_KEEPALIVE) &&
  339             tp->t_state <= TCPS_CLOSING) {
  340                 if ((ticks - tp->t_rcvtime) >= tcp_keepidle + tcp_maxidle)
  341                         goto dropit;
  342                 /*
  343                  * Send a packet designed to force a response
  344                  * if the peer is up and reachable:
  345                  * either an ACK if the connection is still alive,
  346                  * or an RST if the peer has closed the connection
  347                  * due to timeout or reboot.
  348                  * Using sequence number tp->snd_una-1
  349                  * causes the transmitted zero-length segment
  350                  * to lie outside the receive window;
  351                  * by the protocol spec, this requires the
  352                  * correspondent TCP to respond.
  353                  */
  354                 tcpstat.tcps_keepprobe++;
  355                 t_template = tcpip_maketemplate(inp);
  356                 if (t_template) {
  357                         tcp_respond(tp, t_template->tt_ipgen,
  358                                     &t_template->tt_t, (struct mbuf *)NULL,
  359                                     tp->rcv_nxt, tp->snd_una - 1, 0);
  360                         (void) m_free(dtom(t_template));
  361                 }
  362                 callout_reset(tp->tt_keep, tcp_keepintvl, tcp_timer_keep, tp);
  363         } else
  364                 callout_reset(tp->tt_keep, tcp_keepidle, tcp_timer_keep, tp);
  365 
  366 #ifdef TCPDEBUG
  367         if (inp->inp_socket->so_options & SO_DEBUG)
  368                 tcp_trace(TA_USER, ostate, tp, (void *)0, (struct tcphdr *)0,
  369                           PRU_SLOWTIMO);
  370 #endif
  371         INP_UNLOCK(inp);
  372         INP_INFO_WUNLOCK(&tcbinfo);
  373         splx(s);
  374         return;
  375 
  376 dropit:
  377         tcpstat.tcps_keepdrops++;
  378         tp = tcp_drop(tp, ETIMEDOUT);
  379 
  380 #ifdef TCPDEBUG
  381         if (tp && (tp->t_inpcb->inp_socket->so_options & SO_DEBUG))
  382                 tcp_trace(TA_USER, ostate, tp, (void *)0, (struct tcphdr *)0,
  383                           PRU_SLOWTIMO);
  384 #endif
  385         if (tp)
  386                 INP_UNLOCK(tp->t_inpcb);
  387         INP_INFO_WUNLOCK(&tcbinfo);
  388         splx(s);
  389 }
  390 
  391 void
  392 tcp_timer_persist(xtp)
  393         void *xtp;
  394 {
  395         struct tcpcb *tp = xtp;
  396         int s;
  397         struct inpcb *inp;
  398 #ifdef TCPDEBUG
  399         int ostate;
  400 
  401         ostate = tp->t_state;
  402 #endif
  403         s = splnet();
  404         INP_INFO_WLOCK(&tcbinfo);
  405         inp = tp->t_inpcb;
  406         if (!inp) {
  407                 INP_INFO_WUNLOCK(&tcbinfo);
  408                 splx(s);
  409                 return;
  410         }
  411         INP_LOCK(inp);
  412         if (callout_pending(tp->tt_persist) || !callout_active(tp->tt_persist)){
  413                 INP_UNLOCK(inp);
  414                 INP_INFO_WUNLOCK(&tcbinfo);
  415                 splx(s);
  416                 return;
  417         }
  418         callout_deactivate(tp->tt_persist);
  419         /*
  420          * Persistance timer into zero window.
  421          * Force a byte to be output, if possible.
  422          */
  423         tcpstat.tcps_persisttimeo++;
  424         /*
  425          * Hack: if the peer is dead/unreachable, we do not
  426          * time out if the window is closed.  After a full
  427          * backoff, drop the connection if the idle time
  428          * (no responses to probes) reaches the maximum
  429          * backoff that we would use if retransmitting.
  430          */
  431         if (tp->t_rxtshift == TCP_MAXRXTSHIFT &&
  432             ((ticks - tp->t_rcvtime) >= tcp_maxpersistidle ||
  433              (ticks - tp->t_rcvtime) >= TCP_REXMTVAL(tp) * tcp_totbackoff)) {
  434                 tcpstat.tcps_persistdrop++;
  435                 tp = tcp_drop(tp, ETIMEDOUT);
  436                 goto out;
  437         }
  438         tcp_setpersist(tp);
  439         tp->t_force = 1;
  440         (void) tcp_output(tp);
  441         tp->t_force = 0;
  442 
  443 out:
  444 #ifdef TCPDEBUG
  445         if (tp && tp->t_inpcb->inp_socket->so_options & SO_DEBUG)
  446                 tcp_trace(TA_USER, ostate, tp, (void *)0, (struct tcphdr *)0,
  447                           PRU_SLOWTIMO);
  448 #endif
  449         if (tp)
  450                 INP_UNLOCK(inp);
  451         INP_INFO_WUNLOCK(&tcbinfo);
  452         splx(s);
  453 }
  454 
  455 void
  456 tcp_timer_rexmt(xtp)
  457         void *xtp;
  458 {
  459         struct tcpcb *tp = xtp;
  460         int s;
  461         int rexmt;
  462         int headlocked;
  463         struct inpcb *inp;
  464 #ifdef TCPDEBUG
  465         int ostate;
  466 
  467         ostate = tp->t_state;
  468 #endif
  469         s = splnet();
  470         INP_INFO_WLOCK(&tcbinfo);
  471         headlocked = 1;
  472         inp = tp->t_inpcb;
  473         if (!inp) {
  474                 INP_INFO_WUNLOCK(&tcbinfo);
  475                 splx(s);
  476                 return;
  477         }
  478         INP_LOCK(inp);
  479         if (callout_pending(tp->tt_rexmt) || !callout_active(tp->tt_rexmt)) {
  480                 INP_UNLOCK(inp);
  481                 INP_INFO_WUNLOCK(&tcbinfo);
  482                 splx(s);
  483                 return;
  484         }
  485         callout_deactivate(tp->tt_rexmt);
  486         tcp_free_sackholes(tp);
  487         /*
  488          * Retransmission timer went off.  Message has not
  489          * been acked within retransmit interval.  Back off
  490          * to a longer retransmit interval and retransmit one segment.
  491          */
  492         if (++tp->t_rxtshift > TCP_MAXRXTSHIFT) {
  493                 tp->t_rxtshift = TCP_MAXRXTSHIFT;
  494                 tcpstat.tcps_timeoutdrop++;
  495                 tp = tcp_drop(tp, tp->t_softerror ?
  496                               tp->t_softerror : ETIMEDOUT);
  497                 goto out;
  498         }
  499         INP_INFO_WUNLOCK(&tcbinfo);
  500         headlocked = 0;
  501         if (tp->t_rxtshift == 1) {
  502                 /*
  503                  * first retransmit; record ssthresh and cwnd so they can
  504                  * be recovered if this turns out to be a "bad" retransmit.
  505                  * A retransmit is considered "bad" if an ACK for this
  506                  * segment is received within RTT/2 interval; the assumption
  507                  * here is that the ACK was already in flight.  See
  508                  * "On Estimating End-to-End Network Path Properties" by
  509                  * Allman and Paxson for more details.
  510                  */
  511                 tp->snd_cwnd_prev = tp->snd_cwnd;
  512                 tp->snd_ssthresh_prev = tp->snd_ssthresh;
  513                 tp->snd_recover_prev = tp->snd_recover;
  514                 if (IN_FASTRECOVERY(tp))
  515                   tp->t_flags |= TF_WASFRECOVERY;
  516                 else
  517                   tp->t_flags &= ~TF_WASFRECOVERY;
  518                 tp->t_badrxtwin = ticks + (tp->t_srtt >> (TCP_RTT_SHIFT + 1));
  519         }
  520         tcpstat.tcps_rexmttimeo++;
  521         if (tp->t_state == TCPS_SYN_SENT)
  522                 rexmt = TCP_REXMTVAL(tp) * tcp_syn_backoff[tp->t_rxtshift];
  523         else
  524                 rexmt = TCP_REXMTVAL(tp) * tcp_backoff[tp->t_rxtshift];
  525         TCPT_RANGESET(tp->t_rxtcur, rexmt,
  526                       tp->t_rttmin, TCPTV_REXMTMAX);
  527         /*
  528          * Disable rfc1323 and rfc1644 if we havn't got any response to
  529          * our third SYN to work-around some broken terminal servers
  530          * (most of which have hopefully been retired) that have bad VJ
  531          * header compression code which trashes TCP segments containing
  532          * unknown-to-them TCP options.
  533          */
  534         if ((tp->t_state == TCPS_SYN_SENT) && (tp->t_rxtshift == 3))
  535                 tp->t_flags &= ~(TF_REQ_SCALE|TF_REQ_TSTMP|TF_REQ_CC);
  536         /*
  537          * If we backed off this far, our srtt estimate is probably bogus.
  538          * Clobber it so we'll take the next rtt measurement as our srtt;
  539          * move the current srtt into rttvar to keep the current
  540          * retransmit times until then.
  541          */
  542         if (tp->t_rxtshift > TCP_MAXRXTSHIFT / 4) {
  543 #ifdef INET6
  544                 if ((tp->t_inpcb->inp_vflag & INP_IPV6) != 0)
  545                         in6_losing(tp->t_inpcb);
  546                 else
  547 #endif
  548                 tp->t_rttvar += (tp->t_srtt >> TCP_RTT_SHIFT);
  549                 tp->t_srtt = 0;
  550         }
  551         tp->snd_nxt = tp->snd_una;
  552         tp->snd_recover = tp->snd_max;
  553         /*
  554          * Force a segment to be sent.
  555          */
  556         tp->t_flags |= TF_ACKNOW;
  557         /*
  558          * If timing a segment in this window, stop the timer.
  559          */
  560         tp->t_rtttime = 0;
  561         /*
  562          * Close the congestion window down to one segment
  563          * (we'll open it by one segment for each ack we get).
  564          * Since we probably have a window's worth of unacked
  565          * data accumulated, this "slow start" keeps us from
  566          * dumping all that data as back-to-back packets (which
  567          * might overwhelm an intermediate gateway).
  568          *
  569          * There are two phases to the opening: Initially we
  570          * open by one mss on each ack.  This makes the window
  571          * size increase exponentially with time.  If the
  572          * window is larger than the path can handle, this
  573          * exponential growth results in dropped packet(s)
  574          * almost immediately.  To get more time between
  575          * drops but still "push" the network to take advantage
  576          * of improving conditions, we switch from exponential
  577          * to linear window opening at some threshhold size.
  578          * For a threshhold, we use half the current window
  579          * size, truncated to a multiple of the mss.
  580          *
  581          * (the minimum cwnd that will give us exponential
  582          * growth is 2 mss.  We don't allow the threshhold
  583          * to go below this.)
  584          */
  585         {
  586                 u_int win = min(tp->snd_wnd, tp->snd_cwnd) / 2 / tp->t_maxseg;
  587                 if (win < 2)
  588                         win = 2;
  589                 tp->snd_cwnd = tp->t_maxseg;
  590                 tp->snd_ssthresh = win * tp->t_maxseg;
  591                 tp->t_dupacks = 0;
  592         }
  593         EXIT_FASTRECOVERY(tp);
  594         (void) tcp_output(tp);
  595 
  596 out:
  597 #ifdef TCPDEBUG
  598         if (tp && (tp->t_inpcb->inp_socket->so_options & SO_DEBUG))
  599                 tcp_trace(TA_USER, ostate, tp, (void *)0, (struct tcphdr *)0,
  600                           PRU_SLOWTIMO);
  601 #endif
  602         if (tp)
  603                 INP_UNLOCK(inp);
  604         if (headlocked)
  605                 INP_INFO_WUNLOCK(&tcbinfo);
  606         splx(s);
  607 }

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