[apple/xnu.git] / bsd / netinet / tcp_timer.c

/*
 * Copyright (c) 2000 Apple Computer, Inc. All rights reserved.
 *
 * @APPLE_LICENSE_OSREFERENCE_HEADER_START@
 * 
 * This file contains Original Code and/or Modifications of Original Code 
 * as defined in and that are subject to the Apple Public Source License 
 * Version 2.0 (the 'License'). You may not use this file except in 
 * compliance with the License.  The rights granted to you under the 
 * License may not be used to create, or enable the creation or 
 * redistribution of, unlawful or unlicensed copies of an Apple operating 
 * system, or to circumvent, violate, or enable the circumvention or 
 * violation of, any terms of an Apple operating system software license 
 * agreement.
 *
 * Please obtain a copy of the License at 
 * http://www.opensource.apple.com/apsl/ and read it before using this 
 * file.
 *
 * The Original Code and all software distributed under the License are 
 * distributed on an 'AS IS' basis, WITHOUT WARRANTY OF ANY KIND, EITHER 
 * EXPRESS OR IMPLIED, AND APPLE HEREBY DISCLAIMS ALL SUCH WARRANTIES, 
 * INCLUDING WITHOUT LIMITATION, ANY WARRANTIES OF MERCHANTABILITY, 
 * FITNESS FOR A PARTICULAR PURPOSE, QUIET ENJOYMENT OR NON-INFRINGEMENT. 
 * Please see the License for the specific language governing rights and 
 * limitations under the License.
 *
 * @APPLE_LICENSE_OSREFERENCE_HEADER_END@
 */
/*
 * 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. All advertising materials mentioning features or use of this software
 *    must display the following acknowledgement:
 *      This product includes software developed by the University of
 *      California, Berkeley and its contributors.
 * 4. 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 1.34.2.11 2001/08/22 00:59:12 silby Exp $
 */


#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 <kern/locks.h>

#include <kern/cpu_number.h>    /* before tcp_seq.h, for tcp_random18() */

#include <net/route.h>

#include <netinet/in.h>
#include <netinet/in_systm.h>
#include <netinet/in_pcb.h>
#if 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_var.h>
#include <netinet/tcpip.h>
#if TCPDEBUG
#include <netinet/tcp_debug.h>
#endif
#include <sys/kdebug.h>

#define DBG_FNC_TCP_FAST        NETDBG_CODE(DBG_NETTCP, (5 << 8))
#define DBG_FNC_TCP_SLOW        NETDBG_CODE(DBG_NETTCP, (5 << 8) | 1)

/*
 * NOTE - WARNING
 *
 *
 * 
 *
 */
static int
sysctl_msec_to_ticks SYSCTL_HANDLER_ARGS
{
        int error, s, tt;

        tt = *(int *)oidp->oid_arg1;
        s = tt * 1000 / hz;

        error = sysctl_handle_int(oidp, &s, 0, req);
        if (error || !req->newptr)
                return (error);

        tt = 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", "");

int     tcp_keepidle;
SYSCTL_PROC(_net_inet_tcp, TCPCTL_KEEPIDLE, keepidle, CTLTYPE_INT|CTLFLAG_RW,
    &tcp_keepidle, 0, sysctl_msec_to_ticks, "I", "");

int     tcp_keepintvl;
SYSCTL_PROC(_net_inet_tcp, TCPCTL_KEEPINTVL, keepintvl, CTLTYPE_INT|CTLFLAG_RW,
    &tcp_keepintvl, 0, sysctl_msec_to_ticks, "I", "");

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");

static int      always_keepalive = 0;
SYSCTL_INT(_net_inet_tcp, OID_AUTO, always_keepalive, CTLFLAG_RW, 
    &always_keepalive , 0, "Assume SO_KEEPALIVE on all TCP connections");

static int      tcp_keepcnt = TCPTV_KEEPCNT;
        /* max idle probes */
int     tcp_maxpersistidle;
        /* max idle time in persist */
int     tcp_maxidle;

struct  inpcbhead       time_wait_slots[N_TIME_WAIT_SLOTS];
int             cur_tw_slot = 0;

u_long          *delack_bitmask;


void    add_to_time_wait_locked(tp) 
        struct tcpcb    *tp;
{
        int             tw_slot;

        /* pcb list should be locked when we get here */        
#if 0
        lck_mtx_assert(tp->t_inpcb->inpcb_mtx, LCK_MTX_ASSERT_OWNED);
#endif

        LIST_REMOVE(tp->t_inpcb, inp_list);

        if (tp->t_timer[TCPT_2MSL] == 0) 
            tp->t_timer[TCPT_2MSL] = 1;

        tp->t_rcvtime += tp->t_timer[TCPT_2MSL] & (N_TIME_WAIT_SLOTS - 1);
        tw_slot = (tp->t_timer[TCPT_2MSL] & (N_TIME_WAIT_SLOTS - 1)) + cur_tw_slot; 
        if (tw_slot >= N_TIME_WAIT_SLOTS)
            tw_slot -= N_TIME_WAIT_SLOTS;

        LIST_INSERT_HEAD(&time_wait_slots[tw_slot], tp->t_inpcb, inp_list);
}

void    add_to_time_wait(tp) 
        struct tcpcb    *tp;
{
        struct inpcbinfo *pcbinfo               = &tcbinfo;
        
        if (!lck_rw_try_lock_exclusive(pcbinfo->mtx)) {
                tcp_unlock(tp->t_inpcb->inp_socket, 0, 0);
                lck_rw_lock_exclusive(pcbinfo->mtx);
                tcp_lock(tp->t_inpcb->inp_socket, 0, 0);
        }
        add_to_time_wait_locked(tp);
        lck_rw_done(pcbinfo->mtx);
}




/*
 * Fast timeout routine for processing delayed acks
 */
void
tcp_fasttimo()
{
    struct inpcb *inp, *inpnxt;
    register struct tcpcb *tp;


    struct inpcbinfo *pcbinfo   = &tcbinfo;

    int delack_checked = 0, delack_done = 0;

    KERNEL_DEBUG(DBG_FNC_TCP_FAST | DBG_FUNC_START, 0,0,0,0,0);

    if (tcp_delack_enabled == 0) 
        return;

    lck_rw_lock_shared(pcbinfo->mtx);

    /* Walk the list of valid tcpcbs and send ACKS on the ones with DELACK bit set */

    for (inp = tcb.lh_first; inp != NULL; inp = inpnxt) {
        inpnxt = inp->inp_list.le_next;
        /* NOTE: it's OK to check the tp because the pcb can't be removed while we hold pcbinfo->mtx) */
        if ((tp = (struct tcpcb *)inp->inp_ppcb) && (tp->t_flags & TF_DELACK)) {
                if (in_pcb_checkstate(inp, WNT_ACQUIRE, 0) == WNT_STOPUSING) 
                        continue;
                tcp_lock(inp->inp_socket, 1, 0);
                if (in_pcb_checkstate(inp, WNT_RELEASE, 1) == WNT_STOPUSING) {
                        tcp_unlock(inp->inp_socket, 1, 0);
                        continue;
                }
                if (tp->t_flags & TF_DELACK) {
                        delack_done++;
                        tp->t_flags &= ~TF_DELACK;
                        tp->t_flags |= TF_ACKNOW;
                        tcpstat.tcps_delack++;
                        (void) tcp_output(tp);
                }
                tcp_unlock(inp->inp_socket, 1, 0);
        }
    }
    KERNEL_DEBUG(DBG_FNC_TCP_FAST | DBG_FUNC_END, delack_checked, delack_done, tcpstat.tcps_delack,0,0);
    lck_rw_done(pcbinfo->mtx);
}

/*
 * 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.
 */
void
tcp_slowtimo()
{
        struct inpcb *inp, *inpnxt;
        struct tcpcb *tp;
        struct socket *so;
        int i;
#if TCPDEBUG
        int ostate;
#endif
#if KDEBUG
        static int tws_checked;
#endif
        struct inpcbinfo *pcbinfo               = &tcbinfo;

        KERNEL_DEBUG(DBG_FNC_TCP_SLOW | DBG_FUNC_START, 0,0,0,0,0);

        tcp_maxidle = tcp_keepcnt * tcp_keepintvl;

        lck_rw_lock_shared(pcbinfo->mtx);

        /*
         * Search through tcb's and update active timers.
         */
        for (inp = tcb.lh_first; inp != NULL; inp = inpnxt) {
                inpnxt = inp->inp_list.le_next;

                so = inp->inp_socket;

                if (so == &tcbinfo.nat_dummy_socket)
                                continue;

                if (in_pcb_checkstate(inp, WNT_ACQUIRE,0) == WNT_STOPUSING) 
                        continue;

                tcp_lock(so, 1, 0);

                if ((in_pcb_checkstate(inp, WNT_RELEASE,1) == WNT_STOPUSING)  && so->so_usecount == 1) {
                        tcp_unlock(so, 1, 0);
                        continue;
                }
                tp = intotcpcb(inp);
                if (tp == 0 || tp->t_state == TCPS_LISTEN) {
                        tcp_unlock(so, 1, 0);
                        continue; 
                }

                for (i = 0; i < TCPT_NTIMERS; i++) {
                        if (tp->t_timer[i] && --tp->t_timer[i] == 0) {
#if TCPDEBUG
                                ostate = tp->t_state;
#endif
                                tp = tcp_timers(tp, i);
                                if (tp == NULL)
                                        goto tpgone;
#if TCPDEBUG
                                if (tp->t_inpcb->inp_socket->so_options
                                    & SO_DEBUG)
                                        tcp_trace(TA_USER, ostate, tp,
                                                  (void *)0,
                                                  (struct tcphdr *)0,
                                                  PRU_SLOWTIMO);
#endif
                        }
                }
                tp->t_rcvtime++;
                tp->t_starttime++;
                if (tp->t_rtttime)
                        tp->t_rtttime++;        
tpgone:
                tcp_unlock(so, 1, 0);
        }

#if KDEBUG
        tws_checked = 0;
#endif
        KERNEL_DEBUG(DBG_FNC_TCP_SLOW | DBG_FUNC_NONE, tws_checked,0,0,0,0);

        /*
         * Process the items in the current time-wait slot
         */

        for (inp = time_wait_slots[cur_tw_slot].lh_first; inp; inp = inpnxt)
        {
                inpnxt = inp->inp_list.le_next;
#if KDEBUG
                tws_checked++;
#endif

                if (in_pcb_checkstate(inp, WNT_ACQUIRE, 0) == WNT_STOPUSING) 
                        continue;

                tcp_lock(inp->inp_socket, 1, 0);

                if (in_pcb_checkstate(inp, WNT_RELEASE, 1) == WNT_STOPUSING) 
                        goto twunlock;

                tp = intotcpcb(inp);
                if (tp == NULL) { /* tp already closed, remove from list */
#if TEMPDEBUG
                        printf("tcp_slowtimo: tp is null in time-wait slot!\n");
#endif
                        goto twunlock;
                }
                if (tp->t_timer[TCPT_2MSL] >= N_TIME_WAIT_SLOTS) {
                    tp->t_timer[TCPT_2MSL] -= N_TIME_WAIT_SLOTS;
                    tp->t_rcvtime += N_TIME_WAIT_SLOTS;
                }
                else
                    tp->t_timer[TCPT_2MSL] = 0;

                if (tp->t_timer[TCPT_2MSL] == 0)  
                    tp = tcp_timers(tp, TCPT_2MSL);     /* tp can be returned null if tcp_close is called */
twunlock:
                tcp_unlock(inp->inp_socket, 1, 0);
        }

        if (lck_rw_lock_shared_to_exclusive(pcbinfo->mtx) != 0)
                lck_rw_lock_exclusive(pcbinfo->mtx);    /* Upgrade failed, lost lock no take it again exclusive */


        for (inp = tcb.lh_first; inp != NULL; inp = inpnxt) {
                inpnxt = inp->inp_list.le_next;
                /* Ignore nat/SharedIP dummy pcbs */
                if (inp->inp_socket == &tcbinfo.nat_dummy_socket)
                                continue;

                if (inp->inp_wantcnt != WNT_STOPUSING) 
                        continue;

                so = inp->inp_socket;
                if (!lck_mtx_try_lock(inp->inpcb_mtx)) {/* skip if in use */
#if TEMPDEBUG
                        printf("tcp_slowtimo so=%x STOPUSING but locked...\n", so);
#endif
                        continue;
                }

                if (so->so_usecount == 0) 
                        in_pcbdispose(inp);
                else {
                        tp = intotcpcb(inp);
                        /* Check for embryonic socket stuck on listener queue (4023660) */
                        if ((so->so_usecount == 1) && (tp->t_state == TCPS_CLOSED) &&
                            (so->so_head != NULL) && (so->so_state & SS_INCOMP)) {
                                so->so_usecount--; 
                                in_pcbdispose(inp);
                        } else
                                lck_mtx_unlock(inp->inpcb_mtx);
                }
        }

        /* Now cleanup the time wait ones */
        for (inp = time_wait_slots[cur_tw_slot].lh_first; inp; inp = inpnxt)
        {
                inpnxt = inp->inp_list.le_next;

                if (inp->inp_wantcnt != WNT_STOPUSING) 
                        continue;

                so = inp->inp_socket;
                if (!lck_mtx_try_lock(inp->inpcb_mtx)) /* skip if in use */
                        continue;
                if (so->so_usecount == 0)  
                        in_pcbdispose(inp);
                else  {
                        tp = intotcpcb(inp);
                        /* Check for embryonic socket stuck on listener queue (4023660) */
                        if ((so->so_usecount == 1) && (tp->t_state == TCPS_CLOSED) &&
                            (so->so_head != NULL) && (so->so_state & SS_INCOMP)) {
                                so->so_usecount--; 
                                in_pcbdispose(inp);
                        } else
                                lck_mtx_unlock(inp->inpcb_mtx);
                }
        }

        tcp_now++;
        if (++cur_tw_slot >= N_TIME_WAIT_SLOTS)
                cur_tw_slot = 0;
        
        lck_rw_done(pcbinfo->mtx);
        KERNEL_DEBUG(DBG_FNC_TCP_SLOW | DBG_FUNC_END, tws_checked, cur_tw_slot,0,0,0);
}

/*
 * Cancel all timers for TCP tp.
 */
void
tcp_canceltimers(tp)
        struct tcpcb *tp;
{
        register int i;

        for (i = 0; i < TCPT_NTIMERS; i++)
                tp->t_timer[i] = 0;
}

int     tcp_syn_backoff[TCP_MAXRXTSHIFT + 1] =
    { 1, 1, 1, 1, 1, 2, 4, 8, 16, 32, 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[] */

/*
 * TCP timer processing.
 */
struct tcpcb *
tcp_timers(tp, timer)
        register struct tcpcb *tp;
        int timer;
{
        register int rexmt;
        struct socket *so_tmp;
        struct tcptemp *t_template;

#if TCPDEBUG
        int ostate;
#endif

#if INET6
        int isipv6 = (tp->t_inpcb->inp_vflag & INP_IPV4) == 0;
#endif /* INET6 */

        so_tmp = tp->t_inpcb->inp_socket;

        switch (timer) {

        /*
         * 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.
         */
        case TCPT_2MSL:
                tcp_free_sackholes(tp);
                if (tp->t_state != TCPS_TIME_WAIT &&
                    tp->t_rcvtime <= tcp_maxidle) {
                        tp->t_timer[TCPT_2MSL] = (unsigned long)tcp_keepintvl;
                        add_to_time_wait_locked(tp);
                }
                else {
                        tp = tcp_close(tp);
                        return(tp);
                }
                break;

        /*
         * Retransmission timer went off.  Message has not
         * been acked within retransmit interval.  Back off
         * to a longer retransmit interval and retransmit one segment.
         */
        case TCPT_REXMT:
                tcp_free_sackholes(tp);
                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);
                        postevent(so_tmp, 0, EV_TIMEOUT);                       
                        break;
                }

                if (tp->t_rxtshift == 1) {
                        /*
                         * first retransmit; record ssthresh and cwnd so they can
                         * be recovered if this turns out to be a "bad" retransmit.
                         * A retransmit is considered "bad" 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->snd_cwnd_prev = tp->snd_cwnd;
                        tp->snd_ssthresh_prev = tp->snd_ssthresh;
                        tp->snd_recover_prev = tp->snd_recover;
                        if (IN_FASTRECOVERY(tp))
                                  tp->t_flags |= TF_WASFRECOVERY;
                        else
                                  tp->t_flags &= ~TF_WASFRECOVERY;
                        tp->t_badrxtwin = tcp_now + (tp->t_srtt >> (TCP_RTT_SHIFT + 1));
                }
                tcpstat.tcps_rexmttimeo++;
                if (tp->t_state == TCPS_SYN_SENT)
                        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);
                tp->t_timer[TCPT_REXMT] = tp->t_rxtcur;

                /*
                 * Disable rfc1323 and rfc1644 if we havn't got any response to
                 * our third SYN to work-around some broken terminal servers 
                 * (most of which have hopefully been retired) that have bad VJ 
                 * header compression code which trashes TCP segments containing 
                 * unknown-to-them TCP options.
                 */
                if ((tp->t_state == TCPS_SYN_SENT) && (tp->t_rxtshift == 3))
                                tp->t_flags &= ~(TF_REQ_SCALE|TF_REQ_TSTMP|TF_REQ_CC);
                /*
                 * 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) {
#if INET6
                        if (isipv6)
                                in6_losing(tp->t_inpcb);
                        else
#endif /* INET6 */
                        in_losing(tp->t_inpcb);
                        tp->t_rttvar += (tp->t_srtt >> TCP_RTT_SHIFT);
                        tp->t_srtt = 0;
                }
                tp->snd_nxt = tp->snd_una;
                /*
                 * Note:  We overload snd_recover to function also as the
                 * snd_last variable described in RFC 2582
                 */
                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_ssthresh = win * tp->t_maxseg;
                tp->t_dupacks = 0;
                }
                EXIT_FASTRECOVERY(tp);
                (void) tcp_output(tp);
                break;

        /*
         * Persistance timer into zero window.
         * Force a byte to be output, if possible.
         */
        case TCPT_PERSIST:
                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 &&
                    (tp->t_rcvtime >= tcp_maxpersistidle ||
                    tp->t_rcvtime >= TCP_REXMTVAL(tp) * tcp_totbackoff)) {
                        tcpstat.tcps_persistdrop++;
                        so_tmp = tp->t_inpcb->inp_socket;
                        tp = tcp_drop(tp, ETIMEDOUT);
                        postevent(so_tmp, 0, EV_TIMEOUT);
                        break;
                }
                tcp_setpersist(tp);
                tp->t_force = 1;
                (void) tcp_output(tp);
                tp->t_force = 0;
                break;

        /*
         * Keep-alive timer went off; send something
         * or drop connection if idle for too long.
         */
        case TCPT_KEEP:
                tcpstat.tcps_keeptimeo++;
                if (tp->t_state < TCPS_ESTABLISHED)
                        goto dropit;
                if ((always_keepalive ||
                    tp->t_inpcb->inp_socket->so_options & SO_KEEPALIVE) &&
                    tp->t_state <= TCPS_CLOSING || tp->t_state == TCPS_FIN_WAIT_2) {
                        if (tp->t_rcvtime >= TCP_KEEPIDLE(tp) + (unsigned long)tcp_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, (struct mbuf *)NULL,
                                    tp->rcv_nxt, tp->snd_una - 1, 0);
                                (void) m_free(dtom(t_template));
                        }
                        tp->t_timer[TCPT_KEEP] = tcp_keepintvl;
                } else
                        tp->t_timer[TCPT_KEEP] = TCP_KEEPIDLE(tp);
                break;

#if TCPDEBUG
        if (tp->t_inpcb->inp_socket->so_options & SO_DEBUG)
                tcp_trace(TA_USER, ostate, tp, (void *)0, (struct tcphdr *)0,
                          PRU_SLOWTIMO);
#endif
        dropit:
                tcpstat.tcps_keepdrops++;
                tp = tcp_drop(tp, ETIMEDOUT);
                postevent(so_tmp, 0, EV_TIMEOUT);
                break;
        }
        return (tp);
}