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

/*
 * Copyright (c) 2000-2011 Apple Inc. All rights reserved.
 *
 * @APPLE_OSREFERENCE_LICENSE_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_OSREFERENCE_LICENSE_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 <sys/domain.h>
#include <sys/mcache.h>
#include <sys/queue.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/tcp_cc.h>
#if INET6
#include <netinet6/tcp6_var.h>
#endif
#include <netinet/tcpip.h>
#if TCPDEBUG
#include <netinet/tcp_debug.h>
#endif
#include <sys/kdebug.h>
#include <mach/sdt.h>

extern void postevent(struct socket *, struct sockbuf *,
                                               int);
#define DBG_FNC_TCP_FAST        NETDBG_CODE(DBG_NETTCP, (5 << 8))
#define DBG_FNC_TCP_SLOW        NETDBG_CODE(DBG_NETTCP, (5 << 8) | 1)

#define TIMERENTRY_TO_TP(te) ((struct tcpcb *)((uintptr_t)te - offsetof(struct tcpcb, tentry.le.le_next))) 

#define VERIFY_NEXT_LINK(elm,field) do {        \
        if (LIST_NEXT((elm),field) != NULL &&   \
            LIST_NEXT((elm),field)->field.le_prev !=    \
                &((elm)->field.le_next))        \
                panic("Bad link elm %p next->prev != elm", (elm));      \
} while(0)

#define VERIFY_PREV_LINK(elm,field) do {        \
        if (*(elm)->field.le_prev != (elm))     \
                panic("Bad link elm %p prev->next != elm", (elm));      \
} while(0)

static int      background_io_trigger = 5;
SYSCTL_INT(_net_inet_tcp, OID_AUTO, background_io_trigger, CTLFLAG_RW | CTLFLAG_LOCKED,
    &background_io_trigger, 0, "Background IO Trigger Setting");

static int
sysctl_msec_to_ticks SYSCTL_HANDLER_ARGS
{
#pragma unused(arg1, arg2)
        int error, s, tt;

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

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

        tt = s * TCP_RETRANSHZ / 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 | CTLFLAG_LOCKED,
    &tcp_keepinit, 0, sysctl_msec_to_ticks, "I", "");

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

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

int     tcp_msl;
SYSCTL_PROC(_net_inet_tcp, OID_AUTO, msl, CTLTYPE_INT | CTLFLAG_RW | CTLFLAG_LOCKED,
    &tcp_msl, 0, sysctl_msec_to_ticks, "I", "Maximum segment lifetime");

/* 
 * Avoid DoS via TCP Robustness in Persist Condition (see http://www.ietf.org/id/draft-ananth-tcpm-persist-02.txt)
 * by allowing a system wide maximum persistence timeout value when in Zero Window Probe mode.
 * Expressed in milliseconds to be consistent without timeout related values, the TCP socket option is in seconds.
 */
u_int32_t tcp_max_persist_timeout = 0;
SYSCTL_PROC(_net_inet_tcp, OID_AUTO, max_persist_timeout, CTLTYPE_INT | CTLFLAG_RW | CTLFLAG_LOCKED,
    &tcp_max_persist_timeout, 0, sysctl_msec_to_ticks, "I", "Maximum persistence timout for ZWP");

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

/* This parameter determines how long the timer list will stay in fast mode even
 * though all connections are idle. In fast mode, the timer will fire more frequently
 * anticipating new data.
 */
int timer_fastmode_idlemax = TCP_FASTMODE_IDLEGEN_MAX;
SYSCTL_INT(_net_inet_tcp, OID_AUTO, timer_fastmode_idlemax, CTLFLAG_RW | CTLFLAG_LOCKED,
        &timer_fastmode_idlemax, 0, "Maximum idle generations in fast mode");

/*
 * See tcp_syn_backoff[] for interval values between SYN retransmits;
 * the value set below defines the number of retransmits, before we
 * disable the timestamp and window scaling options during subsequent
 * SYN retransmits.  Setting it to 0 disables the dropping off of those
 * two options.
 */
static int tcp_broken_peer_syn_rxmit_thres = 7;
SYSCTL_INT(_net_inet_tcp, OID_AUTO, broken_peer_syn_rxmit_thres, CTLFLAG_RW | CTLFLAG_LOCKED,
    &tcp_broken_peer_syn_rxmit_thres, 0, "Number of retransmitted SYNs before "
    "TCP disables rfc1323 and rfc1644 during the rest of attempts");

static int tcp_timer_advanced = 0;
SYSCTL_INT(_net_inet_tcp, OID_AUTO, tcp_timer_advanced, CTLFLAG_RD | CTLFLAG_LOCKED,
    &tcp_timer_advanced, 0, "Number of times one of the timers was advanced");

static int tcp_resched_timerlist = 0;
SYSCTL_INT(_net_inet_tcp, OID_AUTO, tcp_resched_timerlist, CTLFLAG_RD | CTLFLAG_LOCKED,
    &tcp_resched_timerlist, 0, 
    "Number of times timer list was rescheduled as part of processing a packet");

int     tcp_pmtud_black_hole_detect = 1 ;
SYSCTL_INT(_net_inet_tcp, OID_AUTO, pmtud_blackhole_detection, CTLFLAG_RW | CTLFLAG_LOCKED,
    &tcp_pmtud_black_hole_detect, 0, "Path MTU Discovery Black Hole Detection");

int     tcp_pmtud_black_hole_mss = 1200 ;
SYSCTL_INT(_net_inet_tcp, OID_AUTO, pmtud_blackhole_mss, CTLFLAG_RW | CTLFLAG_LOCKED,
    &tcp_pmtud_black_hole_mss, 0, "Path MTU Discovery Black Hole Detection lowered MSS");

static int      tcp_keepcnt = TCPTV_KEEPCNT;
static int      tcp_gc_done = FALSE;    /* perfromed garbage collection of "used" sockets */
        /* max idle probes */
int     tcp_maxpersistidle;
        /* max idle time in persist */
int     tcp_maxidle;

/* TCP delack timer is set to 100 ms. Since the processing of timer list in fast
 * mode will happen no faster than 100 ms, the delayed ack timer will fire some where 
 * between 100 and 200 ms.
 */
int     tcp_delack = TCP_RETRANSHZ / 10;

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

/* tcp timer list */
struct tcptimerlist tcp_timer_list;

/* The frequency of running through the TCP timer list in 
 * fast and slow mode can be configured.
 */
SYSCTL_UINT(_net_inet_tcp, OID_AUTO, timer_fastquantum, CTLFLAG_RW | CTLFLAG_LOCKED,
        &tcp_timer_list.fast_quantum, TCP_FASTTIMER_QUANTUM, 
        "Frequency of running timer list in fast mode");

SYSCTL_UINT(_net_inet_tcp, OID_AUTO, timer_slowquantum, CTLFLAG_RW | CTLFLAG_LOCKED,
        &tcp_timer_list.slow_quantum, TCP_SLOWTIMER_QUANTUM, 
        "Frequency of running timer list in slow mode");

static void tcp_remove_timer(struct tcpcb *tp);
static void tcp_sched_timerlist(uint32_t offset);
static uint32_t tcp_run_conn_timer(struct tcpcb *tp, uint16_t *next_index);
static void tcp_sched_timers(struct tcpcb *tp);
static inline void tcp_set_lotimer_index(struct tcpcb *);

/* Macro to compare two timers. If there is a reset of the sign bit, it is 
 * safe to assume that the timer has wrapped around. By doing signed comparision, 
 * we take care of wrap around such that the value with the sign bit reset is 
 * actually ahead of the other.
 */

static inline int32_t
timer_diff(uint32_t t1, uint32_t toff1, uint32_t t2, uint32_t toff2) { 
        return (int32_t)((t1 + toff1) - (t2 + toff2));
};

/* Returns true if the timer is on the timer list */
#define TIMER_IS_ON_LIST(tp) ((tp)->t_flags & TF_TIMER_ONLIST)


void    add_to_time_wait_locked(struct tcpcb *tp, uint32_t delay);
void    add_to_time_wait(struct tcpcb *tp, uint32_t delay) ;

static void tcp_garbage_collect(struct inpcb *, int);

void    add_to_time_wait_locked(struct tcpcb *tp, uint32_t delay) 
{
        int             tw_slot;
        struct inpcbinfo *pcbinfo       = &tcbinfo;
        uint32_t timer;

        /* pcb list should be locked when we get here */        
        lck_rw_assert(pcbinfo->mtx, LCK_RW_ASSERT_EXCLUSIVE);

        LIST_REMOVE(tp->t_inpcb, inp_list);

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

        /*
         * Because we're pulling this pcb out of the main TCP pcb list,
         * we need to recalculate the TCPT_2MSL timer value for tcp_slowtimo
         * higher timer granularity.
         */

        timer = (delay / TCP_RETRANSHZ) * PR_SLOWHZ;
        tp->t_rcvtime = (tp->t_rcvtime / TCP_RETRANSHZ) * PR_SLOWHZ;

        tp->t_rcvtime += timer & (N_TIME_WAIT_SLOTS - 1); 

        tw_slot = (timer & (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(struct tcpcb *tp, uint32_t delay) 
{
        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, delay);
        lck_rw_done(pcbinfo->mtx);
}

static void
tcp_garbage_collect(struct inpcb *inp, int istimewait)
{
        struct socket *so;
        struct tcpcb *tp;

        so = inp->inp_socket;
        tp = intotcpcb(inp);

        /*
         * Skip if still in use or busy; it would have been more efficient
         * if we were to test so_usecount against 0, but this isn't possible
         * due to the current implementation of tcp_dropdropablreq() where
         * overflow sockets that are eligible for garbage collection have
         * their usecounts set to 1.
         */
        if (so->so_usecount > 1 || !lck_mtx_try_lock_spin(&inp->inpcb_mtx))
                return;

        /* Check again under the lock */
        if (so->so_usecount > 1) {
                lck_mtx_unlock(&inp->inpcb_mtx);
                return;
        }

        /*
         * Overflowed socket dropped from the listening queue?  Do this
         * only if we are called to clean up the time wait slots, since
         * tcp_dropdropablreq() considers a socket to have been fully
         * dropped after add_to_time_wait() is finished.
         * Also handle the case of connections getting closed by the peer while in the queue as
         * seen with rdar://6422317
         * 
         */
        if (so->so_usecount == 1 && 
            ((istimewait && (so->so_flags & SOF_OVERFLOW)) ||
            ((tp != NULL) && (tp->t_state == TCPS_CLOSED) && (so->so_head != NULL)
                 && ((so->so_state & (SS_INCOMP|SS_CANTSENDMORE|SS_CANTRCVMORE)) ==
                         (SS_INCOMP|SS_CANTSENDMORE|SS_CANTRCVMORE))))) {

                if (inp->inp_state != INPCB_STATE_DEAD) {
                        /* Become a regular mutex */
                        lck_mtx_convert_spin(&inp->inpcb_mtx);
#if INET6
                        if (INP_CHECK_SOCKAF(so, AF_INET6))
                                in6_pcbdetach(inp);
                        else
#endif /* INET6 */
                        in_pcbdetach(inp);
                }
                so->so_usecount--;
                lck_mtx_unlock(&inp->inpcb_mtx);
                return;
        } else if (inp->inp_wantcnt != WNT_STOPUSING) {
                lck_mtx_unlock(&inp->inpcb_mtx);
                return;
        }

        /*
         * We get here because the PCB is no longer searchable (WNT_STOPUSING);
         * detach (if needed) and dispose if it is dead (usecount is 0).  This
         * covers all cases, including overflow sockets and those that are
         * considered as "embryonic", i.e. created by sonewconn() in TCP input
         * path, and have not yet been committed.  For the former, we reduce
         * the usecount to 0 as done by the code above.  For the latter, the
         * usecount would have reduced to 0 as part calling soabort() when the
         * socket is dropped at the end of tcp_input().
         */
        if (so->so_usecount == 0) {
                DTRACE_TCP4(state__change, void, NULL, struct inpcb *, inp,
                        struct tcpcb *, tp, int32_t, TCPS_CLOSED);
                /* Become a regular mutex */
                lck_mtx_convert_spin(&inp->inpcb_mtx);
                
                /* If this tp still happens to be on the timer list, 
                 * take it out
                 */
                if (TIMER_IS_ON_LIST(tp)) {
                        tcp_remove_timer(tp);
                }

                if (inp->inp_state != INPCB_STATE_DEAD) {
#if INET6
                        if (INP_CHECK_SOCKAF(so, AF_INET6))
                                in6_pcbdetach(inp);
                        else
#endif /* INET6 */
                        in_pcbdetach(inp);
                }
                in_pcbdispose(inp);
        } else {
                lck_mtx_unlock(&inp->inpcb_mtx);
        }
}

void
tcp_slowtimo(void)
{
        struct inpcb *inp, *nxt;
        struct tcpcb *tp;
#if TCPDEBUG
        int ostate;
#endif

#if  KDEBUG
        static int tws_checked = 0;
#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;

        /* Update tcp_now here as it may get used while processing the slow timer */
        calculate_tcp_clock();

        /* Garbage collect socket/tcpcb: We need to acquire the list lock 
         * exclusively to do this
         */

        if (lck_rw_try_lock_exclusive(pcbinfo->mtx) == FALSE) {
                if (tcp_gc_done == TRUE) {      /* don't sweat it this time. cleanup was done last time */
                        tcp_gc_done = FALSE;
                        KERNEL_DEBUG(DBG_FNC_TCP_SLOW | DBG_FUNC_END, tws_checked, cur_tw_slot,0,0,0);
                        return; /* Upgrade failed and lost lock - give up this time. */
                }
                lck_rw_lock_exclusive(pcbinfo->mtx);    /* Upgrade failed, lost lock now take it again exclusive */
        }
        tcp_gc_done = TRUE;

        /*
         * Process the items in the current time-wait slot
         */
#if  KDEBUG
        tws_checked = 0;
#endif
        KERNEL_DEBUG(DBG_FNC_TCP_SLOW | DBG_FUNC_NONE, tws_checked,0,0,0,0);

        LIST_FOREACH(inp, &time_wait_slots[cur_tw_slot], inp_list) {
#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 */
                        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)  {

                        /* That pcb is ready for a close */     
                        tcp_free_sackholes(tp);
                        tp = tcp_close(tp);
                }
twunlock:
                tcp_unlock(inp->inp_socket, 1, 0);
        }


        LIST_FOREACH_SAFE(inp, &tcb, inp_list, nxt) {
                tcp_garbage_collect(inp, 0);
        }

        /* Now cleanup the time wait ones */
        LIST_FOREACH_SAFE(inp, &time_wait_slots[cur_tw_slot], inp_list, nxt) {
                tcp_garbage_collect(inp, 1);
        }

        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;

        tcp_remove_timer(tp);
        for (i = 0; i < TCPT_NTIMERS; i++)
                tp->t_timer[i] = 0;
        tp->tentry.timer_start = tcp_now;
        tp->tentry.index = TCPT_NONE;
}

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;
        int optlen = 0;
        int idle_time = 0;

#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;
        idle_time = tcp_now - tp->t_rcvtime;

        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 or FIN_WAIT_2,
         * delete connection control block.
         * Otherwise, (this case shouldn't happen) check again in a bit
         * we keep the socket in the main list in that case.
         */
        case TCPT_2MSL:
                tcp_free_sackholes(tp);
                if (tp->t_state != TCPS_TIME_WAIT &&
                    tp->t_state != TCPS_FIN_WAIT_2 &&
                    ((idle_time > 0) && (idle_time < tcp_maxidle))) {
                        tp->t_timer[TCPT_2MSL] = OFFSET_FROM_START(tp, (u_int32_t)tcp_keepintvl);
                }
                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);
                /* Drop a connection in the retransmit timer
                 * 1. If we have retransmitted more than TCP_MAXRXTSHIFT times
                 * 2. If the time spent in this retransmission episode is more than
                 *    the time limit set with TCP_RXT_CONNDROPTIME socket option
                 * 3. If TCP_RXT_FINDROP socket option was set and we have already
                 *    retransmitted the FIN 3 times without receiving an ack
                 */
                if (++tp->t_rxtshift > TCP_MAXRXTSHIFT ||
                        (tp->rxt_conndroptime > 0 && tp->rxt_start > 0 && 
                        (tcp_now - tp->rxt_start) >= tp->rxt_conndroptime) ||
                        ((tp->t_flagsext & TF_RXTFINDROP) != 0 &&
                        (tp->t_flags & TF_SENTFIN) != 0 &&
                        tp->t_rxtshift >= 4)) {

                        if ((tp->t_flagsext & TF_RXTFINDROP) != 0) {
                                tcpstat.tcps_rxtfindrop++;
                        } else {
                                tcpstat.tcps_timeoutdrop++;
                        }
                        tp->t_rxtshift = TCP_MAXRXTSHIFT;
                        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)); 

                        /* Set the time at which retransmission on this 
                         * connection started
                         */
                        tp->rxt_start = tcp_now;
                }
                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, 
                        TCP_ADD_REXMTSLOP(tp));
                tp->t_timer[TCPT_REXMT] = OFFSET_FROM_START(tp, tp->t_rxtcur);

                /*
                 * Check for potential Path MTU Discovery Black Hole 
                 */

                if (tcp_pmtud_black_hole_detect && (tp->t_state == TCPS_ESTABLISHED)) {
                        if (((tp->t_flags & (TF_PMTUD|TF_MAXSEGSNT)) == (TF_PMTUD|TF_MAXSEGSNT)) && (tp->t_rxtshift == 2)) {
                                /* 
                                 * Enter Path MTU Black-hole Detection mechanism:
                                 * - Disable Path MTU Discovery (IP "DF" bit).
                                 * - Reduce MTU to lower value than what we negociated with peer.
                                 */

                                tp->t_flags &= ~TF_PMTUD; /* Disable Path MTU Discovery for now */
                                tp->t_flags |= TF_BLACKHOLE; /* Record that we may have found a black hole */
                                optlen = tp->t_maxopd - tp->t_maxseg;
                                tp->t_pmtud_saved_maxopd = tp->t_maxopd; /* Keep track of previous MSS */
                                if (tp->t_maxopd > tcp_pmtud_black_hole_mss)
                                        tp->t_maxopd = tcp_pmtud_black_hole_mss; /* Reduce the MSS to intermediary value */
                                else {
                                        tp->t_maxopd =  /* use the default MSS */
#if INET6
                                                isipv6 ? tcp_v6mssdflt :
#endif /* INET6 */
                                                        tcp_mssdflt;
                                }
                                tp->t_maxseg = tp->t_maxopd - optlen;

                                /*
                                 * Reset the slow-start flight size as it may depends on the new MSS
                                 */
                                if (CC_ALGO(tp)->cwnd_init != NULL)
                                        CC_ALGO(tp)->cwnd_init(tp);
                        }
                        /*
                         * If further retransmissions are still unsuccessful with a lowered MTU,
                         * maybe this isn't a Black Hole and we restore the previous MSS and
                         * blackhole detection flags.
                         */
                        else {
        
                                if ((tp->t_flags & TF_BLACKHOLE) && (tp->t_rxtshift > 4)) {
                                        tp->t_flags |= TF_PMTUD; 
                                        tp->t_flags &= ~TF_BLACKHOLE; 
                                        optlen = tp->t_maxopd - tp->t_maxseg;
                                        tp->t_maxopd = tp->t_pmtud_saved_maxopd;
                                        tp->t_maxseg = tp->t_maxopd - optlen;
                                        /*
                                        * Reset the slow-start flight size as it may depends on the new MSS
                                        */
                                        if (CC_ALGO(tp)->cwnd_init != NULL)
                                                CC_ALGO(tp)->cwnd_init(tp);
                                }
                        }
                }


                /*
                 * Disable rfc1323 and rfc1644 if we haven't got any response to
                 * our SYN (after we reach the threshold) 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 == tcp_broken_peer_syn_rxmit_thres))
                        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;

                if (CC_ALGO(tp)->after_timeout != NULL)
                        CC_ALGO(tp)->after_timeout(tp);

                tp->t_dupacks = 0;
                EXIT_FASTRECOVERY(tp);

                DTRACE_TCP5(cc, void, NULL, struct inpcb *, tp->t_inpcb,
                        struct tcpcb *, tp, struct tcphdr *, NULL,
                        int32_t, TCP_CC_REXMT_TIMEOUT);

                (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.
                 * 
                 * Drop the connection if we reached the maximum allowed time for 
                 * Zero Window Probes without a non-zero update from the peer. 
                 * See rdar://5805356
                 */
                if ((tp->t_rxtshift == TCP_MAXRXTSHIFT &&
                    (idle_time >= tcp_maxpersistidle ||
                    idle_time >= TCP_REXMTVAL(tp) * tcp_totbackoff)) || 
                    ((tp->t_persist_stop != 0) && (tp->t_persist_stop <= tcp_now))) {
                        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 (idle_time >= TCP_KEEPIDLE(tp) + (u_int32_t)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) {
                                unsigned int ifscope, nocell = 0;

                                if (tp->t_inpcb->inp_flags & INP_BOUND_IF)
                                        ifscope = tp->t_inpcb->inp_boundif;
                                else
                                        ifscope = IFSCOPE_NONE;

                                /*
                                 * If the socket isn't allowed to use the
                                 * cellular interface, indicate it as such.
                                 */
                                if (tp->t_inpcb->inp_flags & INP_NO_IFT_CELLULAR)
                                        nocell = 1;

                                tcp_respond(tp, t_template->tt_ipgen,
                                    &t_template->tt_t, (struct mbuf *)NULL,
                                    tp->rcv_nxt, tp->snd_una - 1, 0, ifscope,
                                    nocell);
                                (void) m_free(dtom(t_template));
                        }
                        tp->t_timer[TCPT_KEEP] = OFFSET_FROM_START(tp, tcp_keepintvl);
                } else
                        tp->t_timer[TCPT_KEEP] = OFFSET_FROM_START(tp, TCP_KEEPIDLE(tp));
                break;
        case TCPT_DELACK:
                if (tcp_delack_enabled && (tp->t_flags & TF_DELACK)) {
                        tp->t_flags &= ~TF_DELACK;
                        tp->t_timer[TCPT_DELACK] = 0;
                        tp->t_flags |= TF_ACKNOW;

                        /* If delayed ack timer fired while we are stretching acks, 
                         * go back to acking every other packet
                         */
                        if ((tp->t_flags & TF_STRETCHACK) != 0)
                                tcp_reset_stretch_ack(tp);

                        tcpstat.tcps_delack++;
                        (void) tcp_output(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);
}

/* Remove a timer entry from timer list */
void
tcp_remove_timer(struct tcpcb *tp)
{
        struct tcptimerlist *listp = &tcp_timer_list;

        lck_mtx_assert(&tp->t_inpcb->inpcb_mtx, LCK_MTX_ASSERT_OWNED);
        if (!(TIMER_IS_ON_LIST(tp))) {
                return;
        }
        lck_mtx_lock(listp->mtx);
        
        /* Check if pcb is on timer list again after acquiring the lock */
        if (!(TIMER_IS_ON_LIST(tp))) {
                lck_mtx_unlock(listp->mtx);
                return;
        }
        
        if (listp->next_te != NULL && listp->next_te == &tp->tentry)
                listp->next_te = LIST_NEXT(&tp->tentry, le);

        LIST_REMOVE(&tp->tentry, le);
        tp->t_flags &= ~(TF_TIMER_ONLIST);

        listp->entries--;

        tp->tentry.le.le_next = NULL;
        tp->tentry.le.le_prev = NULL;
        lck_mtx_unlock(listp->mtx);
}

/* Function to check if the timerlist needs to be rescheduled to run
 * the timer entry correctly. Basically, this is to check if we can avoid
 * taking the list lock.
 */

static boolean_t
need_to_resched_timerlist(uint32_t runtime, uint16_t index) {
        struct tcptimerlist *listp = &tcp_timer_list;
        int32_t diff;
        boolean_t is_fast;

        if (runtime == 0 || index == TCPT_NONE)
                return FALSE;
        is_fast = !(IS_TIMER_SLOW(index));

        /* If the list is being processed then the state of the list is in flux.
         * In this case always acquire the lock and set the state correctly.
         */
        if (listp->running) {
                return TRUE;
        }

        diff = timer_diff(listp->runtime, 0, runtime, 0);
        if (diff <= 0) {
                /* The list is going to run before this timer */
                return FALSE;
        } else {
                if (is_fast) {
                        if (diff <= listp->fast_quantum)
                                return FALSE;
                } else {
                        if (diff <= listp->slow_quantum)
                                return FALSE;
                }
        }
        return TRUE;
}

void
tcp_sched_timerlist(uint32_t offset) 
{

        uint64_t deadline = 0;
        struct tcptimerlist *listp = &tcp_timer_list;

        lck_mtx_assert(listp->mtx, LCK_MTX_ASSERT_OWNED);

        listp->runtime = tcp_now + offset;

        clock_interval_to_deadline(offset, NSEC_PER_SEC / TCP_RETRANSHZ,
                &deadline);

        thread_call_enter_delayed(listp->call, deadline);
}

/* Function to run the timers for a connection.
 *
 * Returns the offset of next timer to be run for this connection which 
 * can be used to reschedule the timerlist.
 */
uint32_t
tcp_run_conn_timer(struct tcpcb *tp, uint16_t *next_index) {

        struct socket *so;
        uint16_t i = 0, index = TCPT_NONE, lo_index = TCPT_NONE;
        uint32_t timer_val, offset = 0, lo_timer = 0;
        int32_t diff;
        boolean_t needtorun[TCPT_NTIMERS];
        int count = 0;

        VERIFY(tp != NULL);
        bzero(needtorun, sizeof(needtorun));

        tcp_lock(tp->t_inpcb->inp_socket, 1, 0);

        so = tp->t_inpcb->inp_socket;
        /* Release the want count on inp */ 
        if (in_pcb_checkstate(tp->t_inpcb, WNT_RELEASE, 1) == WNT_STOPUSING) {
                if (TIMER_IS_ON_LIST(tp)) {
                        tcp_remove_timer(tp);
                }

                /* Looks like the TCP connection got closed while we 
                 * were waiting for the lock.. Done
                 */
                goto done;
        }

        /* Since the timer thread needs to wait for tcp lock, it may race
         * with another thread that can cancel or reschedule the timer that is
         * about to run. Check if we need to run anything.
         */
        index = tp->tentry.index;
        timer_val = tp->t_timer[index];

        if (index == TCPT_NONE || tp->tentry.runtime == 0) 
                goto done;

        diff = timer_diff(tp->tentry.runtime, 0, tcp_now, 0);
        if (diff > 0) {
                if (tp->tentry.index != TCPT_NONE) {
                        offset = diff;
                        *(next_index) = tp->tentry.index;
                }
                goto done;
        }

        tp->t_timer[index] = 0;
        if (timer_val > 0) {
                tp = tcp_timers(tp, index);
                if (tp == NULL) 
                        goto done;
        }
        
        /* Check if there are any other timers that need to be run. While doing it,
         * adjust the timer values wrt tcp_now.
         */
        for (i = 0; i < TCPT_NTIMERS; ++i) {
                if (tp->t_timer[i] != 0) {
                        diff = timer_diff(tp->tentry.timer_start, tp->t_timer[i], tcp_now, 0);
                        if (diff <= 0) {
                                tp->t_timer[i] = 0;
                                needtorun[i] = TRUE;
                                count++;
                        } else {
                                tp->t_timer[i] = diff;
                                needtorun[i] = FALSE;
                                if (lo_timer == 0 || diff < lo_timer) {
                                        lo_timer = diff;
                                        lo_index = i;
                                }
                        }
                }
        }
        
        tp->tentry.timer_start = tcp_now;
        tp->tentry.index = lo_index;
        if (lo_index != TCPT_NONE) {
                tp->tentry.runtime = tp->tentry.timer_start + tp->t_timer[lo_index];
        } else {
                tp->tentry.runtime = 0;
        }

        if (count > 0) {
                /* run any other timers that are also outstanding at this time. */
                for (i = 0; i < TCPT_NTIMERS; ++i) {
                        if (needtorun[i]) {
                                tp->t_timer[i] = 0;
                                tp = tcp_timers(tp, i);
                                if (tp == NULL) 
                                        goto done;
                        }
                }
                tcp_set_lotimer_index(tp);
        }

        if (tp->tentry.index < TCPT_NONE) {
                offset = tp->t_timer[tp->tentry.index];
                *(next_index) = tp->tentry.index;
        }

done:
        if (tp != NULL && tp->tentry.index == TCPT_NONE) {
                tcp_remove_timer(tp);
        }
        tcp_unlock(so, 1, 0);
        return offset;
}

void
tcp_run_timerlist(void * arg1, void * arg2) {

#pragma unused(arg1, arg2)
        
        struct tcptimerentry *te, *next_te;
        struct tcptimerlist *listp = &tcp_timer_list;
        struct tcpcb *tp;
        uint32_t next_timer = 0;
        uint16_t index = TCPT_NONE;
        boolean_t need_fast = FALSE;
        uint32_t active_count = 0;
        uint32_t mode = TCP_TIMERLIST_FASTMODE;

        calculate_tcp_clock();

        lck_mtx_lock(listp->mtx);

        listp->running = TRUE;
        
        LIST_FOREACH_SAFE(te, &listp->lhead, le, next_te) {
                uint32_t offset = 0;
                uint32_t runtime = te->runtime;
                if (TSTMP_GT(runtime, tcp_now)) {
                        offset = timer_diff(runtime, 0, tcp_now, 0);
                        if (next_timer == 0 || offset < next_timer) {
                                next_timer = offset;
                        }
                        continue;
                }
                active_count++;

                tp = TIMERENTRY_TO_TP(te);

                /* Acquire an inp wantcnt on the inpcb so that the socket won't get
                 * detached even if tcp_close is called
                 */
                if (in_pcb_checkstate(tp->t_inpcb, WNT_ACQUIRE, 0) == WNT_STOPUSING) {
                        /* Some how this pcb went into dead state while on the timer list,
                         * just take it off the list. Since the timer list entry pointers 
                         * are protected by the timer list lock, we can do it here
                         */
                        if (TIMER_IS_ON_LIST(tp)) {
                                tp->t_flags &= ~(TF_TIMER_ONLIST);
                                LIST_REMOVE(&tp->tentry, le);
                                listp->entries--;

                                tp->tentry.le.le_next = NULL;
                                tp->tentry.le.le_prev = NULL;
                        }
                        continue;
                }

                /* Store the next timerentry pointer before releasing the list lock.
                 * If that entry has to be removed when we release the lock, this
                 * pointer will be updated to the element after that.
                 */
                listp->next_te = next_te; 

                VERIFY_NEXT_LINK(&tp->tentry, le);
                VERIFY_PREV_LINK(&tp->tentry, le);

                lck_mtx_unlock(listp->mtx);

                index = TCPT_NONE;
                offset = tcp_run_conn_timer(tp, &index);
                
                lck_mtx_lock(listp->mtx);

                next_te = listp->next_te;
                listp->next_te = NULL;

                if (offset > 0) {
                        if (index < TCPT_NONE) {
                                /* Check if this is a fast_timer. */
                                if (!need_fast && !(IS_TIMER_SLOW(index))) {
                                        need_fast = TRUE;
                                }

                                if (next_timer == 0 || offset < next_timer) {
                                        next_timer = offset;
                                }
                        }
                }
        }

        if (!LIST_EMPTY(&listp->lhead)) {
                if (listp->mode == TCP_TIMERLIST_FASTMODE) {
                        if (need_fast || active_count > 0 || 
                                listp->pref_mode == TCP_TIMERLIST_FASTMODE) {
                                listp->idlegen = 0;
                        } else {
                                listp->idlegen++;
                                if (listp->idlegen > timer_fastmode_idlemax) {
                                        mode = TCP_TIMERLIST_SLOWMODE;
                                        listp->idlegen = 0;
                                }
                        }
                } else {
                        if (!need_fast) {
                                mode = TCP_TIMERLIST_SLOWMODE;
                        }
                }

                if (mode == TCP_TIMERLIST_FASTMODE || 
                        listp->pref_mode == TCP_TIMERLIST_FASTMODE) {
                        next_timer = listp->fast_quantum;
                } else {
                        if (listp->pref_offset != 0 && 
                                listp->pref_offset < next_timer)
                                next_timer = listp->pref_offset;
                        if (next_timer < listp->slow_quantum)
                                next_timer = listp->slow_quantum;
                }

                listp->mode = mode;

                tcp_sched_timerlist(next_timer);
        } else {
                /* No need to reschedule this timer */
                listp->runtime = 0;
        }

        listp->running = FALSE;
        listp->pref_mode = 0;
        listp->pref_offset = 0;

        lck_mtx_unlock(listp->mtx);
}

/* Function to verify if a change in timer state is required for a connection */
void 
tcp_sched_timers(struct tcpcb *tp) 
{
        struct tcptimerentry *te = &tp->tentry;
        uint16_t index = te->index;
        struct tcptimerlist *listp = &tcp_timer_list;
        uint32_t offset = 0;
        boolean_t is_fast;
        int list_locked = 0;

        if (tp->t_inpcb->inp_state == INPCB_STATE_DEAD) {
                /* Just return without adding the dead pcb to the list */
                if (TIMER_IS_ON_LIST(tp)) {
                        tcp_remove_timer(tp);
                }
                return;
        }

        if (index == TCPT_NONE) {
                tcp_remove_timer(tp);
                return;
        }

        is_fast = !(IS_TIMER_SLOW(index));
        offset = te->runtime - tcp_now;
        if (offset == 0) {
                offset = 1;
                tcp_timer_advanced++;
        }
        if (is_fast)
                offset = listp->fast_quantum;

        if (!TIMER_IS_ON_LIST(tp)) {
                if (!list_locked) {
                        lck_mtx_lock(listp->mtx);
                        list_locked = 1;
                }

                LIST_INSERT_HEAD(&listp->lhead, te, le);
                tp->t_flags |= TF_TIMER_ONLIST;

                listp->entries++;
                if (listp->entries > listp->maxentries)
                        listp->maxentries = listp->entries;

                /* if the list is not scheduled, just schedule it */
                if (listp->runtime == 0)
                        goto schedule;

        }


        /* timer entry is currently on the list */
        if (need_to_resched_timerlist(te->runtime, index)) {
                tcp_resched_timerlist++;
        
                if (!list_locked) {
                        lck_mtx_lock(listp->mtx);
                        list_locked = 1;
                }

                VERIFY_NEXT_LINK(te, le);
                VERIFY_PREV_LINK(te, le);

                if (listp->running) {
                        if (is_fast) {
                                listp->pref_mode = TCP_TIMERLIST_FASTMODE;
                        } else if (listp->pref_offset == 0 ||
                                ((int)offset) < listp->pref_offset) {
                                listp->pref_offset = offset;
                        }
                } else {
                        int32_t diff;
                        diff = timer_diff(listp->runtime, 0, tcp_now, offset);
                        if (diff <= 0) {
                                /* The list is going to run before this timer */
                                goto done;
                        } else {
                                goto schedule;
                        }
                }
        }
        goto done;

schedule:
        if (is_fast) {
                listp->mode = TCP_TIMERLIST_FASTMODE;
                listp->idlegen = 0;
        }
        tcp_sched_timerlist(offset);

done:
        if (list_locked)
                lck_mtx_unlock(listp->mtx);

        return;
}
                
void
tcp_set_lotimer_index(struct tcpcb *tp) {
        uint16_t i, lo_index = TCPT_NONE;
        uint32_t lo_timer = 0;
        for (i = 0; i < TCPT_NTIMERS; ++i) {
                if (tp->t_timer[i] != 0 &&
                        (lo_timer == 0 || tp->t_timer[i] < lo_timer)) {
                        lo_timer = tp->t_timer[i];
                        lo_index = i;
                }
        }
        tp->tentry.index = lo_index;
        if (lo_index != TCPT_NONE) {
                tp->tentry.runtime = tp->tentry.timer_start + tp->t_timer[lo_index];
        } else {
                tp->tentry.runtime = 0;
        }
}

void
tcp_check_timer_state(struct tcpcb *tp) {

        lck_mtx_assert(&tp->t_inpcb->inpcb_mtx, LCK_MTX_ASSERT_OWNED);

        tcp_set_lotimer_index(tp);

        tcp_sched_timers(tp);
        return;
}