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

     /*
      * Copyright (c) 2003, 2004 Jeffrey M. Hsu.  All rights reserved.
      * Copyright (c) 2003, 2004 The DragonFly Project.  All rights reserved.
      *
      * This code is derived from software contributed to The DragonFly Project
      * by Jeffrey M. Hsu.
      *
      * Redistribution and use in source and binary forms, with or without
      * modification, are permitted provided that the following conditions
     * are met:
     * 1. Redistributions of source code must retain the above copyright
     *    notice, this list of conditions and the following disclaimer.
     * 2. Redistributions in binary form must reproduce the above copyright
     *    notice, this list of conditions and the following disclaimer in the
     *    documentation and/or other materials provided with the distribution.
     * 3. Neither the name of The DragonFly Project nor the names of its
     *    contributors may be used to endorse or promote products derived
     *    from this software without specific, prior written permission.
     *
     * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
     * ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
     * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
     * FOR A PARTICULAR PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE
     * COPYRIGHT HOLDERS OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
     * INCIDENTAL, SPECIAL, EXEMPLARY OR CONSEQUENTIAL DAMAGES (INCLUDING,
     * BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
     * LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED
     * AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
     * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
     * OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
     * SUCH DAMAGE.
     */
    
    /*
     * Copyright (c) 1982, 1986, 1988, 1990, 1993, 1995
     *      The Regents of the University of California.  All rights reserved.
     *
     * Redistribution and use in source and binary forms, with or without
     * modification, are permitted provided that the following conditions
     * are met:
     * 1. Redistributions of source code must retain the above copyright
     *    notice, this list of conditions and the following disclaimer.
     * 2. Redistributions in binary form must reproduce the above copyright
     *    notice, this list of conditions and the following disclaimer in the
     *    documentation and/or other materials provided with the distribution.
     * 3. Neither the name of the University nor the names of its contributors
     *    may be used to endorse or promote products derived from this software
     *    without specific prior written permission.
     *
     * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
     * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
     * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
     * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
     * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
     * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
     * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
     * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
     * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
     * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
     * SUCH DAMAGE.
     *
     *      @(#)tcp_timer.c 8.2 (Berkeley) 5/24/95
     * $FreeBSD: src/sys/netinet/tcp_timer.c,v 1.34.2.14 2003/02/03 02:33:41 hsu Exp $
     * $DragonFly: src/sys/netinet/tcp_timer.c,v 1.17 2008/03/30 20:39:01 dillon Exp $
     */
    
    #include "opt_compat.h"
    #include "opt_inet6.h"
    #include "opt_tcpdebug.h"
    
    #include <sys/param.h>
    #include <sys/systm.h>
    #include <sys/kernel.h>
    #include <sys/mbuf.h>
    #include <sys/sysctl.h>
    #include <sys/socket.h>
    #include <sys/socketvar.h>
    #include <sys/protosw.h>
    #include <sys/thread.h>
    #include <sys/globaldata.h>
    #include <sys/thread2.h>
    #include <sys/msgport2.h>
    
    #include <machine/cpu.h>        /* before tcp_seq.h, for tcp_random18() */
    
    #include <net/route.h>
    #include <net/netmsg2.h>
    
    #include <netinet/in.h>
    #include <netinet/in_systm.h>
    #include <netinet/in_pcb.h>
    #ifdef INET6
    #include <netinet6/in6_pcb.h>
    #endif
    #include <netinet/ip_var.h>
    #include <netinet/tcp.h>
    #include <netinet/tcp_fsm.h>
    #include <netinet/tcp_seq.h>
    #include <netinet/tcp_timer.h>
   #include <netinet/tcp_timer2.h>
   #include <netinet/tcp_var.h>
   #include <netinet/tcpip.h>
   #ifdef TCPDEBUG
   #include <netinet/tcp_debug.h>
   #endif
   
   #define TCP_TIMER_REXMT         0x01
   #define TCP_TIMER_PERSIST       0x02
   #define TCP_TIMER_KEEP          0x04
   #define TCP_TIMER_2MSL          0x08
   #define TCP_TIMER_DELACK        0x10
   
   static struct tcpcb     *tcp_timer_rexmt_handler(struct tcpcb *);
   static struct tcpcb     *tcp_timer_persist_handler(struct tcpcb *);
   static struct tcpcb     *tcp_timer_keep_handler(struct tcpcb *);
   static struct tcpcb     *tcp_timer_2msl_handler(struct tcpcb *);
   static struct tcpcb     *tcp_timer_delack_handler(struct tcpcb *);
   
   static const struct tcp_timer {
           uint32_t        tt_task;
           struct tcpcb    *(*tt_handler)(struct tcpcb *);
   } tcp_timer_handlers[] = {
           { TCP_TIMER_DELACK,     tcp_timer_delack_handler },
           { TCP_TIMER_REXMT,      tcp_timer_rexmt_handler },
           { TCP_TIMER_PERSIST,    tcp_timer_persist_handler },
           { TCP_TIMER_KEEP,       tcp_timer_keep_handler },
           { TCP_TIMER_2MSL,       tcp_timer_2msl_handler },
           { 0, NULL }
   };
   
   static int
   sysctl_msec_to_ticks(SYSCTL_HANDLER_ARGS)
   {
           int error, s, tt;
   
           tt = *(int *)oidp->oid_arg1;
           s = (int)((int64_t)tt * 1000 / hz);
   
           error = sysctl_handle_int(oidp, &s, 0, req);
           if (error || !req->newptr)
                   return (error);
   
           tt = (int)((int64_t)s * hz / 1000);
           if (tt < 1)
                   return (EINVAL);
   
           *(int *)oidp->oid_arg1 = tt;
           return (0);
   }
   
   int     tcp_keepinit;
   SYSCTL_PROC(_net_inet_tcp, TCPCTL_KEEPINIT, keepinit, CTLTYPE_INT|CTLFLAG_RW,
       &tcp_keepinit, 0, sysctl_msec_to_ticks, "I", "Time to establish TCP connection");
   
   int     tcp_keepidle;
   SYSCTL_PROC(_net_inet_tcp, TCPCTL_KEEPIDLE, keepidle, CTLTYPE_INT|CTLFLAG_RW,
       &tcp_keepidle, 0, sysctl_msec_to_ticks, "I", "Time before TCP keepalive probes begin");
   
   int     tcp_keepintvl;
   SYSCTL_PROC(_net_inet_tcp, TCPCTL_KEEPINTVL, keepintvl, CTLTYPE_INT|CTLFLAG_RW,
       &tcp_keepintvl, 0, sysctl_msec_to_ticks, "I", "Time between TCP keepalive probes");
   
   int     tcp_delacktime;
   SYSCTL_PROC(_net_inet_tcp, TCPCTL_DELACKTIME, delacktime,
       CTLTYPE_INT|CTLFLAG_RW, &tcp_delacktime, 0, sysctl_msec_to_ticks, "I",
       "Time before a delayed ACK is sent");
   
   int     tcp_msl;
   SYSCTL_PROC(_net_inet_tcp, OID_AUTO, msl, CTLTYPE_INT|CTLFLAG_RW,
       &tcp_msl, 0, sysctl_msec_to_ticks, "I", "Maximum segment lifetime");
   
   int     tcp_rexmit_min;
   SYSCTL_PROC(_net_inet_tcp, OID_AUTO, rexmit_min, CTLTYPE_INT|CTLFLAG_RW,
       &tcp_rexmit_min, 0, sysctl_msec_to_ticks, "I", "Minimum Retransmission Timeout");
   
   int     tcp_rexmit_slop;
   SYSCTL_PROC(_net_inet_tcp, OID_AUTO, rexmit_slop, CTLTYPE_INT|CTLFLAG_RW,
       &tcp_rexmit_slop, 0, sysctl_msec_to_ticks, "I",
       "Retransmission Timer Slop");
   
   static int      always_keepalive = 1;
   SYSCTL_INT(_net_inet_tcp, OID_AUTO, always_keepalive, CTLFLAG_RW,
       &always_keepalive , 0, "Assume SO_KEEPALIVE on all TCP connections");
   
   /* max idle probes */
   int     tcp_keepcnt = TCPTV_KEEPCNT;
   SYSCTL_INT(_net_inet_tcp, OID_AUTO, keepcnt, CTLFLAG_RW,
       &tcp_keepcnt, 0, "Maximum number of keepalive probes to be sent");
   
   static int tcp_do_eifel_response = 1;
   SYSCTL_INT(_net_inet_tcp, OID_AUTO, eifel_response, CTLFLAG_RW,
       &tcp_do_eifel_response, 0, "Eifel response algorithm (RFC 4015)");
   
   int tcp_eifel_rtoinc = 2;
   SYSCTL_PROC(_net_inet_tcp, OID_AUTO, eifel_rtoinc, CTLTYPE_INT|CTLFLAG_RW,
       &tcp_eifel_rtoinc, 0, sysctl_msec_to_ticks, "I",
       "Eifel response RTO increment");
   
   /* max idle time in persist */
   int     tcp_maxpersistidle;
   
   /*
    * Cancel all timers for TCP tp.
    */
   void
   tcp_canceltimers(struct tcpcb *tp)
   {
           tcp_callout_stop(tp, tp->tt_2msl);
           tcp_callout_stop(tp, tp->tt_persist);
           tcp_callout_stop(tp, tp->tt_keep);
           tcp_callout_stop(tp, tp->tt_rexmt);
   }
   
   /*
    * Caller should be in critical section
    */
   static void
   tcp_send_timermsg(struct tcpcb *tp, uint32_t task)
   {
           struct netmsg_tcp_timer *tmsg = tp->tt_msg;
   
           KKASSERT(tmsg != NULL && tmsg->tt_cpuid == mycpuid &&
                    tmsg->tt_tcb != NULL);
   
           tmsg->tt_tasks |= task;
           if (tmsg->tt_msg.lmsg.ms_flags & MSGF_DONE)
                   lwkt_sendmsg_oncpu(tmsg->tt_msgport, &tmsg->tt_msg.lmsg);
   }
   
   int     tcp_syn_backoff[TCP_MAXRXTSHIFT + 1] =
       { 1, 1, 1, 1, 1, 2, 4, 8, 16, 32, 64, 64, 64 };
   
   int     tcp_syn_backoff_low[TCP_MAXRXTSHIFT + 1] =
       { 1, 1, 2, 4, 8, 8, 16, 16, 32, 64, 64, 64, 64 };
   
   int     tcp_backoff[TCP_MAXRXTSHIFT + 1] =
       { 1, 2, 4, 8, 16, 32, 64, 64, 64, 64, 64, 64, 64 };
   
   static int tcp_totbackoff = 511;        /* sum of tcp_backoff[] */
   
   /* Caller should be in critical section */
   static struct tcpcb *
   tcp_timer_delack_handler(struct tcpcb *tp)
   {
           tp->t_flags |= TF_ACKNOW;
           tcpstat.tcps_delack++;
           tcp_output(tp);
           return tp;
   }
   
   /*
    * TCP timer processing.
    */
   void
   tcp_timer_delack(void *xtp)
   {
           struct tcpcb *tp = xtp;
           struct callout *co = &tp->tt_delack->tc_callout;
   
           crit_enter();
           if (callout_pending(co) || !callout_active(co)) {
                   crit_exit();
                   return;
           }
           callout_deactivate(co);
           tcp_send_timermsg(tp, TCP_TIMER_DELACK);
           crit_exit();
   }
   
   /* Caller should be in critical section */
   static struct tcpcb *
   tcp_timer_2msl_handler(struct tcpcb *tp)
   {
   #ifdef TCPDEBUG
           int ostate;
   #endif
   
   #ifdef TCPDEBUG
           ostate = tp->t_state;
   #endif
           /*
            * 2 MSL timeout in shutdown went off.  If we're closed but
            * still waiting for peer to close and connection has been idle
            * too long, or if 2MSL time is up from TIME_WAIT, delete connection
            * control block.  Otherwise, check again in a bit.
            */
           if (tp->t_state != TCPS_TIME_WAIT &&
               (ticks - tp->t_rcvtime) <= tp->t_maxidle) {
                   tcp_callout_reset(tp, tp->tt_2msl, tp->t_keepintvl,
                                     tcp_timer_2msl);
           } else {
                   tp = tcp_close(tp);
           }
   
   #ifdef TCPDEBUG
           if (tp && (tp->t_inpcb->inp_socket->so_options & SO_DEBUG))
                   tcp_trace(TA_USER, ostate, tp, NULL, NULL, PRU_SLOWTIMO);
   #endif
           return tp;
   }
   
   void
   tcp_timer_2msl(void *xtp)
   {
           struct tcpcb *tp = xtp;
           struct callout *co = &tp->tt_2msl->tc_callout;
   
           crit_enter();
           if (callout_pending(co) || !callout_active(co)) {
                   crit_exit();
                   return;
           }
           callout_deactivate(co);
           tcp_send_timermsg(tp, TCP_TIMER_2MSL);
           crit_exit();
   }
   
   /* Caller should be in critical section */
   static struct tcpcb *
   tcp_timer_keep_handler(struct tcpcb *tp)
   {
           struct tcptemp *t_template;
   #ifdef TCPDEBUG
           int ostate = tp->t_state;
   #endif
   
           /*
            * Keep-alive timer went off; send something
            * or drop connection if idle for too long.
            */
           tcpstat.tcps_keeptimeo++;
           if (tp->t_state < TCPS_ESTABLISHED)
                   goto dropit;
           if ((always_keepalive || (tp->t_flags & TF_KEEPALIVE) ||
                (tp->t_inpcb->inp_socket->so_options & SO_KEEPALIVE)) &&
               tp->t_state <= TCPS_CLOSING) {
                   if ((ticks - tp->t_rcvtime) >= tp->t_keepidle + tp->t_maxidle)
                           goto dropit;
                   /*
                    * Send a packet designed to force a response
                    * if the peer is up and reachable:
                    * either an ACK if the connection is still alive,
                    * or an RST if the peer has closed the connection
                    * due to timeout or reboot.
                    * Using sequence number tp->snd_una-1
                    * causes the transmitted zero-length segment
                    * to lie outside the receive window;
                    * by the protocol spec, this requires the
                    * correspondent TCP to respond.
                    */
                   tcpstat.tcps_keepprobe++;
                   t_template = tcp_maketemplate(tp);
                   if (t_template) {
                           tcp_respond(tp, t_template->tt_ipgen,
                                       &t_template->tt_t, NULL,
                                       tp->rcv_nxt, tp->snd_una - 1, 0);
                           tcp_freetemplate(t_template);
                   }
                   tcp_callout_reset(tp, tp->tt_keep, tp->t_keepintvl,
                                     tcp_timer_keep);
           } else {
                   tcp_callout_reset(tp, tp->tt_keep, tp->t_keepidle,
                                     tcp_timer_keep);
           }
   
   #ifdef TCPDEBUG
           if (tp->t_inpcb->inp_socket->so_options & SO_DEBUG)
                   tcp_trace(TA_USER, ostate, tp, NULL, NULL, PRU_SLOWTIMO);
   #endif
           return tp;
   
   dropit:
           tcpstat.tcps_keepdrops++;
           tp = tcp_drop(tp, ETIMEDOUT);
   
   #ifdef TCPDEBUG
           if (tp && (tp->t_inpcb->inp_socket->so_options & SO_DEBUG))
                   tcp_trace(TA_USER, ostate, tp, NULL, NULL, PRU_SLOWTIMO);
   #endif
           return tp;
   }
   
   void
   tcp_timer_keep(void *xtp)
   {
           struct tcpcb *tp = xtp;
           struct callout *co = &tp->tt_keep->tc_callout;
   
           crit_enter();
           if (callout_pending(co) || !callout_active(co)) {
                   crit_exit();
                   return;
           }
           callout_deactivate(co);
           tcp_send_timermsg(tp, TCP_TIMER_KEEP);
           crit_exit();
   }
   
   /* Caller should be in critical section */
   static struct tcpcb *
   tcp_timer_persist_handler(struct tcpcb *tp)
   {
   #ifdef TCPDEBUG
           int ostate;
   #endif
   
   #ifdef TCPDEBUG
           ostate = tp->t_state;
   #endif
           /*
            * Persistance timer into zero window.
            * Force a byte to be output, if possible.
            */
           tcpstat.tcps_persisttimeo++;
           /*
            * Hack: if the peer is dead/unreachable, we do not
            * time out if the window is closed.  After a full
            * backoff, drop the connection if the idle time
            * (no responses to probes) reaches the maximum
            * backoff that we would use if retransmitting.
            */
           if (tp->t_rxtshift == TCP_MAXRXTSHIFT &&
               ((ticks - tp->t_rcvtime) >= tcp_maxpersistidle ||
                (ticks - tp->t_rcvtime) >= TCP_REXMTVAL(tp) * tcp_totbackoff)) {
                   tcpstat.tcps_persistdrop++;
                   tp = tcp_drop(tp, ETIMEDOUT);
                   goto out;
           }
           tcp_setpersist(tp);
           tp->t_flags |= TF_FORCE;
           tcp_output(tp);
           tp->t_flags &= ~TF_FORCE;
   
   out:
   #ifdef TCPDEBUG
           if (tp && tp->t_inpcb->inp_socket->so_options & SO_DEBUG)
                   tcp_trace(TA_USER, ostate, tp, NULL, NULL, PRU_SLOWTIMO);
   #endif
           return tp;
   }
   
   void
   tcp_timer_persist(void *xtp)
   {
           struct tcpcb *tp = xtp;
           struct callout *co = &tp->tt_persist->tc_callout;
   
           crit_enter();
           if (callout_pending(co) || !callout_active(co)){
                   crit_exit();
                   return;
           }
           callout_deactivate(co);
           tcp_send_timermsg(tp, TCP_TIMER_PERSIST);
           crit_exit();
   }
   
   void
   tcp_save_congestion_state(struct tcpcb *tp)
   {
           /*
            * Record connection's current states so that they could be
            * recovered, if this turns out to be a spurious retransmit.
            */
           tp->snd_cwnd_prev = tp->snd_cwnd;
           tp->snd_wacked_prev = tp->snd_wacked;
           tp->snd_ssthresh_prev = tp->snd_ssthresh;
           tp->snd_recover_prev = tp->snd_recover;
   
           /*
            * State for Eifel response after spurious timeout retransmit
            * is detected.  We save the current value of snd_max even if
            * we are called from fast retransmit code, so if RTO needs
            * rebase, it will be rebased using the RTT of segment that
            * is not sent during possible congestion.
            */
           tp->snd_max_prev = tp->snd_max;
   
           if (IN_FASTRECOVERY(tp))
                   tp->rxt_flags |= TRXT_F_WASFRECOVERY;
           else
                   tp->rxt_flags &= ~TRXT_F_WASFRECOVERY;
           if (tp->t_flags & TF_RCVD_TSTMP) {
                   /* States for Eifel detection */
                   tp->t_rexmtTS = ticks;
                   tp->rxt_flags |= TRXT_F_FIRSTACCACK;
           }
   #ifdef later
           tcp_sack_save_scoreboard(&tp->scb);
   #endif
   }
   
   void
   tcp_revert_congestion_state(struct tcpcb *tp)
   {
           tp->snd_cwnd = tp->snd_cwnd_prev;
           tp->snd_wacked = tp->snd_wacked_prev;
           tp->snd_ssthresh = tp->snd_ssthresh_prev;
           tp->snd_recover = tp->snd_recover_prev;
           if (tp->rxt_flags & TRXT_F_WASFRECOVERY)
                   ENTER_FASTRECOVERY(tp);
           if (tp->rxt_flags & TRXT_F_FASTREXMT) {
                   ++tcpstat.tcps_sndfastrexmitbad;
                   if (tp->rxt_flags & TRXT_F_EARLYREXMT)
                           ++tcpstat.tcps_sndearlyrexmitbad;
           } else {
                   ++tcpstat.tcps_sndrtobad;
                   tp->snd_last = ticks;
                   if (tcp_do_eifel_response)
                           tp->rxt_flags |= TRXT_F_REBASERTO;
           }
           tp->t_badrxtwin = 0;
           tp->t_rxtshift = 0;
           tp->snd_nxt = tp->snd_max;
   #ifdef later
           tcp_sack_revert_scoreboard(&tp->scb, tp->snd_una);
   #endif
   }
   
   /* Caller should be in critical section */
   static struct tcpcb *
   tcp_timer_rexmt_handler(struct tcpcb *tp)
   {
           int rexmt;
   #ifdef TCPDEBUG
           int ostate;
   #endif
   
   #ifdef TCPDEBUG
           ostate = tp->t_state;
   #endif
           /*
            * Retransmission timer went off.  Message has not
            * been acked within retransmit interval.  Back off
            * to a longer retransmit interval and retransmit one segment.
            */
           if (++tp->t_rxtshift > TCP_MAXRXTSHIFT) {
                   tp->t_rxtshift = TCP_MAXRXTSHIFT;
                   tcpstat.tcps_timeoutdrop++;
                   tp = tcp_drop(tp, tp->t_softerror ?
                                 tp->t_softerror : ETIMEDOUT);
                   goto out;
           }
           if (tp->t_rxtshift == 1) {
                   /*
                    * First retransmit.
                    */
   
                   /*
                    * State for "RTT based spurious timeout retransmit detection"
                    *
                    * RTT based spurious timeout retransmit detection:
                    * A retransmit is considered spurious if an ACK for this
                    * segment is received within RTT/2 interval; the assumption
                    * here is that the ACK was already in flight.  See
                    * "On Estimating End-to-End Network Path Properties" by
                    * Allman and Paxson for more details.
                    */
                   tp->t_badrxtwin = ticks + (tp->t_srtt >> (TCP_RTT_SHIFT + 1));
   
                   /*
                    * States for Eifel response after spurious timeout retransmit
                    * is detected.
                    */
                   tp->t_rxtcur_prev = tp->t_rxtcur;
                   tp->t_srtt_prev = tp->t_srtt +
                       (tcp_eifel_rtoinc << TCP_RTT_SHIFT);
                   tp->t_rttvar_prev = tp->t_rttvar;
   
                   tcp_save_congestion_state(tp);
                   tp->rxt_flags &= ~(TRXT_F_FASTREXMT | TRXT_F_EARLYREXMT |
                       TRXT_F_REBASERTO);
           }
           if (tp->t_state == TCPS_SYN_SENT || tp->t_state == TCPS_SYN_RECEIVED) {
                   /*
                    * Record the time that we spent in SYN or SYN|ACK
                    * retransmition.
                    *
                    * Needed by RFC3390 and RFC6298.
                    */
                   tp->t_rxtsyn += tp->t_rxtcur;
           }
           /* Throw away SACK blocks on a RTO, as specified by RFC2018. */
           tcp_sack_discard(tp);
           tcpstat.tcps_rexmttimeo++;
           if (tp->t_state == TCPS_SYN_SENT) {
                   if (tcp_low_rtobase) {
                           rexmt = TCP_REXMTVAL(tp) *
                                   tcp_syn_backoff_low[tp->t_rxtshift];
                   } else {
                           rexmt = TCP_REXMTVAL(tp) *
                                   tcp_syn_backoff[tp->t_rxtshift];
                   }
           } else {
                   rexmt = TCP_REXMTVAL(tp) * tcp_backoff[tp->t_rxtshift];
           }
           TCPT_RANGESET(tp->t_rxtcur, rexmt,
                         tp->t_rttmin, TCPTV_REXMTMAX);
           /*
            * If losing, let the lower level know and try for
            * a better route.  Also, if we backed off this far,
            * our srtt estimate is probably bogus.  Clobber it
            * so we'll take the next rtt measurement as our srtt;
            * move the current srtt into rttvar to keep the current
            * retransmit times until then.
            */
           if (tp->t_rxtshift > TCP_MAXRXTSHIFT / 4) {
   #ifdef INET6
                   if ((tp->t_inpcb->inp_vflag & INP_IPV6) != 0)
                           in6_losing(tp->t_inpcb);
                   else
   #endif
                   in_losing(tp->t_inpcb);
                   tp->t_rttvar += (tp->t_srtt >> TCP_RTT_SHIFT);
                   tp->t_srtt = 0;
           }
           tp->snd_nxt = tp->snd_una;
           tp->snd_recover = tp->snd_max;
           /*
            * Force a segment to be sent.
            */
           tp->t_flags |= TF_ACKNOW;
           /*
            * If timing a segment in this window, stop the timer.
            */
           tp->t_rtttime = 0;
           /*
            * Close the congestion window down to one segment
            * (we'll open it by one segment for each ack we get).
            * Since we probably have a window's worth of unacked
            * data accumulated, this "slow start" keeps us from
            * dumping all that data as back-to-back packets (which
            * might overwhelm an intermediate gateway).
            *
            * There are two phases to the opening: Initially we
            * open by one mss on each ack.  This makes the window
            * size increase exponentially with time.  If the
            * window is larger than the path can handle, this
            * exponential growth results in dropped packet(s)
            * almost immediately.  To get more time between
            * drops but still "push" the network to take advantage
            * of improving conditions, we switch from exponential
            * to linear window opening at some threshhold size.
            * For a threshhold, we use half the current window
            * size, truncated to a multiple of the mss.
            *
            * (the minimum cwnd that will give us exponential
            * growth is 2 mss.  We don't allow the threshhold
            * to go below this.)
            */
           {
                   u_int win = min(tp->snd_wnd, tp->snd_cwnd) / 2 / tp->t_maxseg;
   
                   if (win < 2)
                           win = 2;
                   tp->snd_cwnd = tp->t_maxseg;
                   tp->snd_wacked = 0;
                   tp->snd_ssthresh = win * tp->t_maxseg;
                   tp->t_dupacks = 0;
           }
           EXIT_FASTRECOVERY(tp);
           tcp_output(tp);
   
   out:
   #ifdef TCPDEBUG
           if (tp && (tp->t_inpcb->inp_socket->so_options & SO_DEBUG))
                   tcp_trace(TA_USER, ostate, tp, NULL, NULL, PRU_SLOWTIMO);
   #endif
           return tp;
   }
   
   void
   tcp_timer_rexmt(void *xtp)
   {
           struct tcpcb *tp = xtp;
           struct callout *co = &tp->tt_rexmt->tc_callout;
   
           crit_enter();
           if (callout_pending(co) || !callout_active(co)) {
                   crit_exit();
                   return;
           }
           callout_deactivate(co);
           tcp_send_timermsg(tp, TCP_TIMER_REXMT);
           crit_exit();
   }
   
   static void
   tcp_timer_handler(netmsg_t msg)
   {
           struct netmsg_tcp_timer *tmsg = (struct netmsg_tcp_timer *)msg;
           const struct tcp_timer *tt;
           struct tcpcb *tp;
   
           crit_enter();
   
           KKASSERT(tmsg->tt_cpuid == mycpuid && tmsg->tt_tcb != NULL);
           tp = tmsg->tt_tcb;
   
           /* Save pending tasks and reset the tasks in message */
           tmsg->tt_running_tasks = tmsg->tt_tasks;
           tmsg->tt_prev_tasks = tmsg->tt_tasks;
           tmsg->tt_tasks = 0;
   
           /* Reply ASAP */
           lwkt_replymsg(&tmsg->tt_msg.lmsg, 0);
   
           if (tmsg->tt_running_tasks == 0) {
                   /*
                    * All of the timers are cancelled when the message
                    * is pending; bail out.
                    */
                   crit_exit();
                   return;
           }
   
           for (tt = tcp_timer_handlers; tt->tt_handler != NULL; ++tt) {
                   if ((tmsg->tt_running_tasks & tt->tt_task) == 0)
                           continue;
   
                   tmsg->tt_running_tasks &= ~tt->tt_task;
                   tp = tt->tt_handler(tp);
                   if (tp == NULL)
                           break;
   
                   if (tmsg->tt_running_tasks == 0) /* nothing left to do */
                           break;
           }
   
           crit_exit();
   }
   
   void
   tcp_create_timermsg(struct tcpcb *tp, struct lwkt_port *msgport)
   {
           struct netmsg_tcp_timer *tmsg = tp->tt_msg;
   
           netmsg_init(&tmsg->tt_msg, NULL, &netisr_adone_rport,
                       MSGF_DROPABLE | MSGF_PRIORITY, tcp_timer_handler);
           tmsg->tt_cpuid = mycpuid;
           tmsg->tt_msgport = msgport;
           tmsg->tt_tcb = tp;
           tmsg->tt_tasks = 0;
   }
   
   void
   tcp_destroy_timermsg(struct tcpcb *tp)
   {
           struct netmsg_tcp_timer *tmsg = tp->tt_msg;
   
           if (tmsg == NULL ||             /* listen socket */
               tmsg->tt_tcb == NULL)       /* only tcp_attach() is called */
                   return;
   
           KKASSERT(tmsg->tt_cpuid == mycpuid);
   
           /*
            * This message is still pending to be processed;
            * drop it.  Optimized.
            */
           crit_enter();
           if ((tmsg->tt_msg.lmsg.ms_flags & MSGF_DONE) == 0) {
                   lwkt_dropmsg(&tmsg->tt_msg.lmsg);
           }
           crit_exit();
   }
   
   static __inline void
   tcp_callout_init(struct tcp_callout *tc, uint32_t task)
   {
           callout_init_mp(&tc->tc_callout);
           tc->tc_task = task;
   }
   
   void
   tcp_inittimers(struct tcpcb *tp)
   {
           tcp_callout_init(tp->tt_rexmt, TCP_TIMER_REXMT);
           tcp_callout_init(tp->tt_persist, TCP_TIMER_PERSIST);
           tcp_callout_init(tp->tt_keep, TCP_TIMER_KEEP);
           tcp_callout_init(tp->tt_2msl, TCP_TIMER_2MSL);
           tcp_callout_init(tp->tt_delack, TCP_TIMER_DELACK);
   }

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