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: releng/6.0/sys/netinet/tcp_timer.c 149330 2005-08-20 13:34:13Z rwatson $
   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         struct inpcb *inp;
  309 #ifdef TCPDEBUG
  310         int ostate;
  311 
  312         ostate = tp->t_state;
  313 #endif
  314         INP_INFO_WLOCK(&tcbinfo);
  315         inp = tp->t_inpcb;
  316         if (!inp) {
  317                 INP_INFO_WUNLOCK(&tcbinfo);
  318                 return;
  319         }
  320         INP_LOCK(inp);
  321         if (callout_pending(tp->tt_keep) || !callout_active(tp->tt_keep)) {
  322                 INP_UNLOCK(inp);
  323                 INP_INFO_WUNLOCK(&tcbinfo);
  324                 return;
  325         }
  326         callout_deactivate(tp->tt_keep);
  327         /*
  328          * Keep-alive timer went off; send something
  329          * or drop connection if idle for too long.
  330          */
  331         tcpstat.tcps_keeptimeo++;
  332         if (tp->t_state < TCPS_ESTABLISHED)
  333                 goto dropit;
  334         if ((always_keepalive || inp->inp_socket->so_options & SO_KEEPALIVE) &&
  335             tp->t_state <= TCPS_CLOSING) {
  336                 if ((ticks - tp->t_rcvtime) >= tcp_keepidle + tcp_maxidle)
  337                         goto dropit;
  338                 /*
  339                  * Send a packet designed to force a response
  340                  * if the peer is up and reachable:
  341                  * either an ACK if the connection is still alive,
  342                  * or an RST if the peer has closed the connection
  343                  * due to timeout or reboot.
  344                  * Using sequence number tp->snd_una-1
  345                  * causes the transmitted zero-length segment
  346                  * to lie outside the receive window;
  347                  * by the protocol spec, this requires the
  348                  * correspondent TCP to respond.
  349                  */
  350                 tcpstat.tcps_keepprobe++;
  351                 t_template = tcpip_maketemplate(inp);
  352                 if (t_template) {
  353                         tcp_respond(tp, t_template->tt_ipgen,
  354                                     &t_template->tt_t, (struct mbuf *)NULL,
  355                                     tp->rcv_nxt, tp->snd_una - 1, 0);
  356                         (void) m_free(dtom(t_template));
  357                 }
  358                 callout_reset(tp->tt_keep, tcp_keepintvl, tcp_timer_keep, tp);
  359         } else
  360                 callout_reset(tp->tt_keep, tcp_keepidle, tcp_timer_keep, tp);
  361 
  362 #ifdef TCPDEBUG
  363         if (inp->inp_socket->so_options & SO_DEBUG)
  364                 tcp_trace(TA_USER, ostate, tp, (void *)0, (struct tcphdr *)0,
  365                           PRU_SLOWTIMO);
  366 #endif
  367         INP_UNLOCK(inp);
  368         INP_INFO_WUNLOCK(&tcbinfo);
  369         return;
  370 
  371 dropit:
  372         tcpstat.tcps_keepdrops++;
  373         tp = tcp_drop(tp, ETIMEDOUT);
  374 
  375 #ifdef TCPDEBUG
  376         if (tp && (tp->t_inpcb->inp_socket->so_options & SO_DEBUG))
  377                 tcp_trace(TA_USER, ostate, tp, (void *)0, (struct tcphdr *)0,
  378                           PRU_SLOWTIMO);
  379 #endif
  380         if (tp)
  381                 INP_UNLOCK(tp->t_inpcb);
  382         INP_INFO_WUNLOCK(&tcbinfo);
  383 }
  384 
  385 void
  386 tcp_timer_persist(xtp)
  387         void *xtp;
  388 {
  389         struct tcpcb *tp = xtp;
  390         struct inpcb *inp;
  391 #ifdef TCPDEBUG
  392         int ostate;
  393 
  394         ostate = tp->t_state;
  395 #endif
  396         INP_INFO_WLOCK(&tcbinfo);
  397         inp = tp->t_inpcb;
  398         if (!inp) {
  399                 INP_INFO_WUNLOCK(&tcbinfo);
  400                 return;
  401         }
  402         INP_LOCK(inp);
  403         if (callout_pending(tp->tt_persist) || !callout_active(tp->tt_persist)){
  404                 INP_UNLOCK(inp);
  405                 INP_INFO_WUNLOCK(&tcbinfo);
  406                 return;
  407         }
  408         callout_deactivate(tp->tt_persist);
  409         /*
  410          * Persistance timer into zero window.
  411          * Force a byte to be output, if possible.
  412          */
  413         tcpstat.tcps_persisttimeo++;
  414         /*
  415          * Hack: if the peer is dead/unreachable, we do not
  416          * time out if the window is closed.  After a full
  417          * backoff, drop the connection if the idle time
  418          * (no responses to probes) reaches the maximum
  419          * backoff that we would use if retransmitting.
  420          */
  421         if (tp->t_rxtshift == TCP_MAXRXTSHIFT &&
  422             ((ticks - tp->t_rcvtime) >= tcp_maxpersistidle ||
  423              (ticks - tp->t_rcvtime) >= TCP_REXMTVAL(tp) * tcp_totbackoff)) {
  424                 tcpstat.tcps_persistdrop++;
  425                 tp = tcp_drop(tp, ETIMEDOUT);
  426                 goto out;
  427         }
  428         tcp_setpersist(tp);
  429         tp->t_flags |= TF_FORCEDATA;
  430         (void) tcp_output(tp);
  431         tp->t_flags &= ~TF_FORCEDATA;
  432 
  433 out:
  434 #ifdef TCPDEBUG
  435         if (tp && tp->t_inpcb->inp_socket->so_options & SO_DEBUG)
  436                 tcp_trace(TA_USER, ostate, tp, (void *)0, (struct tcphdr *)0,
  437                           PRU_SLOWTIMO);
  438 #endif
  439         if (tp)
  440                 INP_UNLOCK(inp);
  441         INP_INFO_WUNLOCK(&tcbinfo);
  442 }
  443 
  444 void
  445 tcp_timer_rexmt(xtp)
  446         void *xtp;
  447 {
  448         struct tcpcb *tp = xtp;
  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(&tcbinfo);
  458         headlocked = 1;
  459         inp = tp->t_inpcb;
  460         if (!inp) {
  461                 INP_INFO_WUNLOCK(&tcbinfo);
  462                 return;
  463         }
  464         INP_LOCK(inp);
  465         if (callout_pending(tp->tt_rexmt) || !callout_active(tp->tt_rexmt)) {
  466                 INP_UNLOCK(inp);
  467                 INP_INFO_WUNLOCK(&tcbinfo);
  468                 return;
  469         }
  470         callout_deactivate(tp->tt_rexmt);
  471         tcp_free_sackholes(tp);
  472         /*
  473          * Retransmission timer went off.  Message has not
  474          * been acked within retransmit interval.  Back off
  475          * to a longer retransmit interval and retransmit one segment.
  476          */
  477         if (++tp->t_rxtshift > TCP_MAXRXTSHIFT) {
  478                 tp->t_rxtshift = TCP_MAXRXTSHIFT;
  479                 tcpstat.tcps_timeoutdrop++;
  480                 tp = tcp_drop(tp, tp->t_softerror ?
  481                               tp->t_softerror : ETIMEDOUT);
  482                 goto out;
  483         }
  484         INP_INFO_WUNLOCK(&tcbinfo);
  485         headlocked = 0;
  486         if (tp->t_rxtshift == 1) {
  487                 /*
  488                  * first retransmit; record ssthresh and cwnd so they can
  489                  * be recovered if this turns out to be a "bad" retransmit.
  490                  * A retransmit is considered "bad" if an ACK for this
  491                  * segment is received within RTT/2 interval; the assumption
  492                  * here is that the ACK was already in flight.  See
  493                  * "On Estimating End-to-End Network Path Properties" by
  494                  * Allman and Paxson for more details.
  495                  */
  496                 tp->snd_cwnd_prev = tp->snd_cwnd;
  497                 tp->snd_ssthresh_prev = tp->snd_ssthresh;
  498                 tp->snd_recover_prev = tp->snd_recover;
  499                 if (IN_FASTRECOVERY(tp))
  500                   tp->t_flags |= TF_WASFRECOVERY;
  501                 else
  502                   tp->t_flags &= ~TF_WASFRECOVERY;
  503                 tp->t_badrxtwin = ticks + (tp->t_srtt >> (TCP_RTT_SHIFT + 1));
  504         }
  505         tcpstat.tcps_rexmttimeo++;
  506         if (tp->t_state == TCPS_SYN_SENT)
  507                 rexmt = TCP_REXMTVAL(tp) * tcp_syn_backoff[tp->t_rxtshift];
  508         else
  509                 rexmt = TCP_REXMTVAL(tp) * tcp_backoff[tp->t_rxtshift];
  510         TCPT_RANGESET(tp->t_rxtcur, rexmt,
  511                       tp->t_rttmin, TCPTV_REXMTMAX);
  512         /*
  513          * Disable rfc1323 if we havn't got any response to
  514          * our third SYN to work-around some broken terminal servers
  515          * (most of which have hopefully been retired) that have bad VJ
  516          * header compression code which trashes TCP segments containing
  517          * unknown-to-them TCP options.
  518          */
  519         if ((tp->t_state == TCPS_SYN_SENT) && (tp->t_rxtshift == 3))
  520                 tp->t_flags &= ~(TF_REQ_SCALE|TF_REQ_TSTMP);
  521         /*
  522          * If we backed off this far, our srtt estimate is probably bogus.
  523          * Clobber it so we'll take the next rtt measurement as our srtt;
  524          * move the current srtt into rttvar to keep the current
  525          * retransmit times until then.
  526          */
  527         if (tp->t_rxtshift > TCP_MAXRXTSHIFT / 4) {
  528 #ifdef INET6
  529                 if ((tp->t_inpcb->inp_vflag & INP_IPV6) != 0)
  530                         in6_losing(tp->t_inpcb);
  531                 else
  532 #endif
  533                 tp->t_rttvar += (tp->t_srtt >> TCP_RTT_SHIFT);
  534                 tp->t_srtt = 0;
  535         }
  536         tp->snd_nxt = tp->snd_una;
  537         tp->snd_recover = tp->snd_max;
  538         /*
  539          * Force a segment to be sent.
  540          */
  541         tp->t_flags |= TF_ACKNOW;
  542         /*
  543          * If timing a segment in this window, stop the timer.
  544          */
  545         tp->t_rtttime = 0;
  546         /*
  547          * Close the congestion window down to one segment
  548          * (we'll open it by one segment for each ack we get).
  549          * Since we probably have a window's worth of unacked
  550          * data accumulated, this "slow start" keeps us from
  551          * dumping all that data as back-to-back packets (which
  552          * might overwhelm an intermediate gateway).
  553          *
  554          * There are two phases to the opening: Initially we
  555          * open by one mss on each ack.  This makes the window
  556          * size increase exponentially with time.  If the
  557          * window is larger than the path can handle, this
  558          * exponential growth results in dropped packet(s)
  559          * almost immediately.  To get more time between
  560          * drops but still "push" the network to take advantage
  561          * of improving conditions, we switch from exponential
  562          * to linear window opening at some threshhold size.
  563          * For a threshhold, we use half the current window
  564          * size, truncated to a multiple of the mss.
  565          *
  566          * (the minimum cwnd that will give us exponential
  567          * growth is 2 mss.  We don't allow the threshhold
  568          * to go below this.)
  569          */
  570         {
  571                 u_int win = min(tp->snd_wnd, tp->snd_cwnd) / 2 / tp->t_maxseg;
  572                 if (win < 2)
  573                         win = 2;
  574                 tp->snd_cwnd = tp->t_maxseg;
  575                 tp->snd_ssthresh = win * tp->t_maxseg;
  576                 tp->t_dupacks = 0;
  577         }
  578         EXIT_FASTRECOVERY(tp);
  579         (void) tcp_output(tp);
  580 
  581 out:
  582 #ifdef TCPDEBUG
  583         if (tp && (tp->t_inpcb->inp_socket->so_options & SO_DEBUG))
  584                 tcp_trace(TA_USER, ostate, tp, (void *)0, (struct tcphdr *)0,
  585                           PRU_SLOWTIMO);
  586 #endif
  587         if (tp)
  588                 INP_UNLOCK(inp);
  589         if (headlocked)
  590                 INP_INFO_WUNLOCK(&tcbinfo);
  591 }
Cache object: c58fcde73f9f923c5aeaaf78ecc652e5
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FreeBSD/Linux Kernel Cross Reference sys/netinet/tcp_timer.c

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