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

     /*      $NetBSD: tcp_timer.c,v 1.99 2022/11/04 09:00:58 ozaki-r Exp $   */
     
     /*
      * Copyright (C) 1995, 1996, 1997, and 1998 WIDE Project.
      * 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 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 PROJECT 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 PROJECT 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) 1997, 1998, 2001, 2005 The NetBSD Foundation, Inc.
     * All rights reserved.
     *
     * This code is derived from software contributed to The NetBSD Foundation
     * by Jason R. Thorpe and Kevin M. Lahey of the Numerical Aerospace Simulation
     * Facility, NASA Ames Research Center.
     * This code is derived from software contributed to The NetBSD Foundation
     * by Charles M. Hannum.
     *
     * 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.
     *
     * THIS SOFTWARE IS PROVIDED BY THE NETBSD FOUNDATION, INC. 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 FOUNDATION 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
     */
    
    #include <sys/cdefs.h>
    __KERNEL_RCSID(0, "$NetBSD: tcp_timer.c,v 1.99 2022/11/04 09:00:58 ozaki-r Exp $");
    
    #ifdef _KERNEL_OPT
    #include "opt_inet.h"
   #include "opt_tcp_debug.h"
   #include "opt_net_mpsafe.h"
   #endif
   
   #include <sys/param.h>
   #include <sys/systm.h>
   #include <sys/mbuf.h>
   #include <sys/socket.h>
   #include <sys/socketvar.h>
   #include <sys/protosw.h>
   #include <sys/errno.h>
   #include <sys/kernel.h>
   #include <sys/callout.h>
   #include <sys/workqueue.h>
   #include <sys/cprng.h>
   
   #include <net/if.h>
   
   #include <netinet/in.h>
   #include <netinet/in_systm.h>
   #include <netinet/ip.h>
   #include <netinet/in_pcb.h>
   #include <netinet/ip_var.h>
   #include <netinet/ip_icmp.h>
   
   #ifdef INET6
   #include <netinet/ip6.h>
   #include <netinet6/in6_pcb.h>
   #endif
   
   #include <netinet/tcp.h>
   #include <netinet/tcp_fsm.h>
   #include <netinet/tcp_seq.h>
   #include <netinet/tcp_timer.h>
   #include <netinet/tcp_var.h>
   #include <netinet/tcp_private.h>
   #include <netinet/tcp_congctl.h>
   #ifdef TCP_DEBUG
   #include <netinet/tcp_debug.h>
   #endif
   
   /*
    * Various tunable timer parameters.  These are initialized in tcp_init(),
    * unless they are patched.
    */
   u_int   tcp_keepinit = 0;
   u_int   tcp_keepidle = 0;
   u_int   tcp_keepintvl = 0;
   u_int   tcp_keepcnt = 0;                /* max idle probes */
   
   int     tcp_maxpersistidle = 0;         /* max idle time in persist */
   
   static callout_t        tcp_slowtimo_ch;
   #ifdef NET_MPSAFE
   static struct workqueue *tcp_slowtimo_wq;
   static struct work      tcp_slowtimo_wk;
   #endif
   
   static void tcp_slowtimo_work(struct work *, void *);
   static void tcp_slowtimo(void *);
   
   /*
    * Time to delay the ACK.  This is initialized in tcp_init(), unless
    * its patched.
    */
   int     tcp_delack_ticks = 0;
   
   void    tcp_timer_rexmt(void *);
   void    tcp_timer_persist(void *);
   void    tcp_timer_keep(void *);
   void    tcp_timer_2msl(void *);
   
   const tcp_timer_func_t tcp_timer_funcs[TCPT_NTIMERS] = {
           tcp_timer_rexmt,
           tcp_timer_persist,
           tcp_timer_keep,
           tcp_timer_2msl,
   };
   
   /*
    * Timer state initialization, called from tcp_init().
    */
   void
   tcp_timer_init(void)
   {
   
           if (tcp_keepinit == 0)
                   tcp_keepinit = TCPTV_KEEP_INIT;
   
           if (tcp_keepidle == 0)
                   tcp_keepidle = TCPTV_KEEP_IDLE;
   
           if (tcp_keepintvl == 0)
                   tcp_keepintvl = TCPTV_KEEPINTVL;
   
           if (tcp_keepcnt == 0)
                   tcp_keepcnt = TCPTV_KEEPCNT;
   
           if (tcp_maxpersistidle == 0)
                   tcp_maxpersistidle = TCPTV_KEEP_IDLE;
   
           if (tcp_delack_ticks == 0)
                   tcp_delack_ticks = TCP_DELACK_TICKS;
   }
   
   void
   tcp_slowtimo_init(void)
   {
   #ifdef NET_MPSAFE
           int error;
   
           error = workqueue_create(&tcp_slowtimo_wq, "tcp_slowtimo",
               tcp_slowtimo_work, NULL, PRI_SOFTNET, IPL_SOFTNET, WQ_MPSAFE);
           if (error != 0)
                   panic("%s: workqueue_create failed (%d)\n", __func__, error);
   #endif
           callout_init(&tcp_slowtimo_ch, CALLOUT_MPSAFE);
           callout_reset(&tcp_slowtimo_ch, 1, tcp_slowtimo, NULL);
   }
   
   /*
    * Callout to process delayed ACKs for a TCPCB.
    */
   void
   tcp_delack(void *arg)
   {
           struct tcpcb *tp = arg;
   
           /*
            * If tcp_output() wasn't able to transmit the ACK
            * for whatever reason, it will restart the delayed
            * ACK callout.
            */
   
           mutex_enter(softnet_lock);
           if ((tp->t_flags & (TF_DEAD | TF_DELACK)) != TF_DELACK) {
                   mutex_exit(softnet_lock);
                   return;
           }
           if (!callout_expired(&tp->t_delack_ch)) {
                   mutex_exit(softnet_lock);
                   return;
           }
   
           tp->t_flags |= TF_ACKNOW;
           KERNEL_LOCK(1, NULL);
           (void) tcp_output(tp);
           KERNEL_UNLOCK_ONE(NULL);
           mutex_exit(softnet_lock);
   }
   
   /*
    * Tcp protocol timeout routine called every 500 ms.
    * Updates the timers in all active tcb's and
    * causes finite state machine actions if timers expire.
    */
   static void
   tcp_slowtimo_work(struct work *wk, void *arg)
   {
   
           mutex_enter(softnet_lock);
           tcp_iss_seq += TCP_ISSINCR + (TCP_ISS_RANDOM_MASK & cprng_fast32());
           tcp_now++;                                      /* for timestamps */
           mutex_exit(softnet_lock);
   
           callout_schedule(&tcp_slowtimo_ch, hz / PR_SLOWHZ);
   }
   
   static void
   tcp_slowtimo(void *arg)
   {
   
   #ifdef NET_MPSAFE
           workqueue_enqueue(tcp_slowtimo_wq, &tcp_slowtimo_wk, NULL);
   #else
           tcp_slowtimo_work(NULL, NULL);
   #endif
   }
   
   /*
    * Cancel all timers for TCP tp.
    */
   void
   tcp_canceltimers(struct tcpcb *tp)
   {
           int i;
   
           for (i = 0; i < TCPT_NTIMERS; i++)
                   TCP_TIMER_DISARM(tp, i);
   }
   
   const int       tcp_backoff[TCP_MAXRXTSHIFT + 1] =
       { 1, 2, 4, 8, 16, 32, 64, 64, 64, 64, 64, 64, 64 };
   
   const int       tcp_totbackoff = 511;   /* sum of tcp_backoff[] */
   
   /*
    * TCP timer processing.
    */
   
   void
   tcp_timer_rexmt(void *arg)
   {
           struct tcpcb *tp = arg;
           uint32_t rto;
   #ifdef TCP_DEBUG
           struct socket *so = NULL;
           short ostate;
   #endif
   
           mutex_enter(softnet_lock);
           if ((tp->t_flags & TF_DEAD) != 0) {
                   mutex_exit(softnet_lock);
                   return;
           }
           if (!callout_expired(&tp->t_timer[TCPT_REXMT])) {
                   mutex_exit(softnet_lock);
                   return;
           }
   
           KERNEL_LOCK(1, NULL);
           if ((tp->t_flags & TF_PMTUD_PEND) && tp->t_inpcb &&
               SEQ_GEQ(tp->t_pmtud_th_seq, tp->snd_una) &&
               SEQ_LT(tp->t_pmtud_th_seq, (int)(tp->snd_una + tp->t_ourmss))) {
                   extern struct sockaddr_in icmpsrc;
                   struct icmp icmp;
   
                   tp->t_flags &= ~TF_PMTUD_PEND;
   
                   /* XXX create fake icmp message with relevant entries */
                   icmp.icmp_nextmtu = tp->t_pmtud_nextmtu;
                   icmp.icmp_ip.ip_len = tp->t_pmtud_ip_len;
                   icmp.icmp_ip.ip_hl = tp->t_pmtud_ip_hl;
                   icmpsrc.sin_addr = in4p_faddr(tp->t_inpcb);
                   icmp_mtudisc(&icmp, icmpsrc.sin_addr);
   
                   /*
                    * Notify all connections to the same peer about
                    * new mss and trigger retransmit.
                    */
                   inpcb_notifyall(&tcbtable, icmpsrc.sin_addr, EMSGSIZE,
                       tcp_mtudisc);
                   KERNEL_UNLOCK_ONE(NULL);
                   mutex_exit(softnet_lock);
                   return;
           }
   #ifdef TCP_DEBUG
           so = tp->t_inpcb->inp_socket;
           ostate = tp->t_state;
   #endif /* TCP_DEBUG */
   
           /*
            * Clear the SACK scoreboard, reset FACK estimate.
            */
           tcp_free_sackholes(tp);
           tp->snd_fack = tp->snd_una;
   
           /*
            * 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;
                   TCP_STATINC(TCP_STAT_TIMEOUTDROP);
                   tp = tcp_drop(tp, tp->t_softerror ?
                       tp->t_softerror : ETIMEDOUT);
                   goto out;
           }
           TCP_STATINC(TCP_STAT_REXMTTIMEO);
           rto = TCP_REXMTVAL(tp);
           if (rto < tp->t_rttmin)
                   rto = tp->t_rttmin;
           TCPT_RANGESET(tp->t_rxtcur, rto * tcp_backoff[tp->t_rxtshift],
               tp->t_rttmin, TCPTV_REXMTMAX);
           TCP_TIMER_ARM(tp, TCPT_REXMT, tp->t_rxtcur);
   
           /*
            * If we are losing and we are trying path MTU discovery,
            * try turning it off.  This will avoid black holes in
            * the network which suppress or fail to send "packet
            * too big" ICMP messages.  We should ideally do
            * lots more sophisticated searching to find the right
            * value here...
            */
           if (tp->t_mtudisc && tp->t_rxtshift > TCP_MAXRXTSHIFT / 6) {
                   TCP_STATINC(TCP_STAT_PMTUBLACKHOLE);
   
                   /* try turning PMTUD off or try using IPv6 minimum MTU */
                   tp->t_mtudisc = 0;
   
                   /* XXX: more sophisticated Black hole recovery code? */
           }
   
           /*
            * 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) {
                   inpcb_losing(tp->t_inpcb);
                   /*
                    * This operation is not described in RFC2988.  The
                    * point is to keep srtt+4*rttvar constant, so we
                    * should shift right 2 bits to divide by 4, and then
                    * shift right one bit because the storage
                    * representation of rttvar is 1/16s vs 1/32s for
                    * srtt.
                    */
                   tp->t_rttvar += (tp->t_srtt >> TCP_RTT_SHIFT);
                   tp->t_srtt = 0;
           }
           tp->snd_nxt = tp->snd_una;
           tp->snd_high = tp->snd_max;
           /*
            * If timing a segment in this window, stop the timer.
            */
           tp->t_rtttime = 0;
           /*
            * Remember if we are retransmitting a SYN, because if
            * we do, set the initial congestion window must be set
            * to 1 segment.
            */
           if (tp->t_state == TCPS_SYN_SENT)
                   tp->t_flags |= TF_SYN_REXMT;
   
           /*
            * Adjust congestion control parameters.
            */
           tp->t_congctl->slow_retransmit(tp);
   
           (void) tcp_output(tp);
   
    out:
   #ifdef TCP_DEBUG
           if (tp && so->so_options & SO_DEBUG)
                   tcp_trace(TA_USER, ostate, tp, NULL,
                       PRU_SLOWTIMO | (TCPT_REXMT << 8));
   #endif
           KERNEL_UNLOCK_ONE(NULL);
           mutex_exit(softnet_lock);
   }
   
   void
   tcp_timer_persist(void *arg)
   {
           struct tcpcb *tp = arg;
           uint32_t rto;
   #ifdef TCP_DEBUG
           struct socket *so = NULL;
           short ostate;
   #endif
   
           mutex_enter(softnet_lock);
           if ((tp->t_flags & TF_DEAD) != 0) {
                   mutex_exit(softnet_lock);
                   return;
           }
           if (!callout_expired(&tp->t_timer[TCPT_PERSIST])) {
                   mutex_exit(softnet_lock);
                   return;
           }
   
           KERNEL_LOCK(1, NULL);
   #ifdef TCP_DEBUG
           so = tp->t_inpcb->inp_socket;
           ostate = tp->t_state;
   #endif /* TCP_DEBUG */
   
           /*
            * Persistance timer into zero window.
            * Force a byte to be output, if possible.
            */
   
           /*
            * 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.
            */
           rto = TCP_REXMTVAL(tp);
           if (rto < tp->t_rttmin)
                   rto = tp->t_rttmin;
           if (tp->t_rxtshift == TCP_MAXRXTSHIFT &&
               ((tcp_now - tp->t_rcvtime) >= tcp_maxpersistidle ||
               (tcp_now - tp->t_rcvtime) >= rto * tcp_totbackoff)) {
                   TCP_STATINC(TCP_STAT_PERSISTDROPS);
                   tp = tcp_drop(tp, ETIMEDOUT);
                   goto out;
           }
           TCP_STATINC(TCP_STAT_PERSISTTIMEO);
           tcp_setpersist(tp);
           tp->t_force = 1;
           (void) tcp_output(tp);
           tp->t_force = 0;
   
    out:
   #ifdef TCP_DEBUG
           if (tp && so->so_options & SO_DEBUG)
                   tcp_trace(TA_USER, ostate, tp, NULL,
                       PRU_SLOWTIMO | (TCPT_PERSIST << 8));
   #endif
           KERNEL_UNLOCK_ONE(NULL);
           mutex_exit(softnet_lock);
   }
   
   void
   tcp_timer_keep(void *arg)
   {
           struct tcpcb *tp = arg;
           struct socket *so = NULL;       /* Quell compiler warning */
   #ifdef TCP_DEBUG
           short ostate;
   #endif
   
           mutex_enter(softnet_lock);
           if ((tp->t_flags & TF_DEAD) != 0) {
                   mutex_exit(softnet_lock);
                   return;
           }
           if (!callout_expired(&tp->t_timer[TCPT_KEEP])) {
                   mutex_exit(softnet_lock);
                   return;
           }
   
           KERNEL_LOCK(1, NULL);
   
   #ifdef TCP_DEBUG
           ostate = tp->t_state;
   #endif /* TCP_DEBUG */
   
           /*
            * Keep-alive timer went off; send something
            * or drop connection if idle for too long.
            */
   
           TCP_STATINC(TCP_STAT_KEEPTIMEO);
           if (TCPS_HAVEESTABLISHED(tp->t_state) == 0)
                   goto dropit;
           so = tp->t_inpcb->inp_socket;
           KASSERT(so != NULL);
           if (so->so_options & SO_KEEPALIVE &&
               tp->t_state <= TCPS_CLOSE_WAIT) {
                   if ((tp->t_maxidle > 0) &&
                       ((tcp_now - 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.
                    */
                   TCP_STATINC(TCP_STAT_KEEPPROBE);
   
                   (void)tcp_respond(tp, tp->t_template,
                       NULL, NULL, tp->rcv_nxt,
                       tp->snd_una - 1, 0);
   
                   TCP_TIMER_ARM(tp, TCPT_KEEP, tp->t_keepintvl);
           } else
                   TCP_TIMER_ARM(tp, TCPT_KEEP, tp->t_keepidle);
   
   #ifdef TCP_DEBUG
           if (tp && so->so_options & SO_DEBUG)
                   tcp_trace(TA_USER, ostate, tp, NULL,
                       PRU_SLOWTIMO | (TCPT_KEEP << 8));
   #endif
           KERNEL_UNLOCK_ONE(NULL);
           mutex_exit(softnet_lock);
           return;
   
    dropit:
           TCP_STATINC(TCP_STAT_KEEPDROPS);
           (void) tcp_drop(tp, ETIMEDOUT);
           KERNEL_UNLOCK_ONE(NULL);
           mutex_exit(softnet_lock);
   }
   
   void
   tcp_timer_2msl(void *arg)
   {
           struct tcpcb *tp = arg;
   #ifdef TCP_DEBUG
           struct socket *so = NULL;
           short ostate;
   #endif
   
           mutex_enter(softnet_lock);
           if ((tp->t_flags & TF_DEAD) != 0) {
                   mutex_exit(softnet_lock);
                   return;
           }
           if (!callout_expired(&tp->t_timer[TCPT_2MSL])) {
                   mutex_exit(softnet_lock);
                   return;
           }
   
           /*
            * 2 MSL timeout went off, clear the SACK scoreboard, reset
            * the FACK estimate.
            */
           KERNEL_LOCK(1, NULL);
           tcp_free_sackholes(tp);
           tp->snd_fack = tp->snd_una;
   
   #ifdef TCP_DEBUG
           so = tp->t_inpcb->inp_socket;
           ostate = tp->t_state;
   #endif /* TCP_DEBUG */
   
           /*
            * 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 &&
               ((tp->t_maxidle == 0) ||
               ((tcp_now - tp->t_rcvtime) <= tp->t_maxidle)))
               TCP_TIMER_ARM(tp, TCPT_2MSL, tp->t_keepintvl);
           else
                   tp = tcp_close(tp);
   
   #ifdef TCP_DEBUG
           if (tp && so->so_options & SO_DEBUG)
                   tcp_trace(TA_USER, ostate, tp, NULL,
                       PRU_SLOWTIMO | (TCPT_2MSL << 8));
   #endif
           KERNEL_UNLOCK_ONE(NULL);
           mutex_exit(softnet_lock);
   }

Cache object: ca839e20de5908b2e1a919b9e5565d51