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

/*
 * Copyright (c) 2000-2016 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 <mach/boolean.h>

#include <net/route.h>
#include <net/if_var.h>
#include <net/ntstat.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_cache.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>
#include <netinet/mptcp_var.h>

/* Max number of times a stretch ack can be delayed on a connection */
#define TCP_STRETCHACK_DELAY_THRESHOLD  5

/*
 * If the host processor has been sleeping for too long, this is the threshold
 * used to avoid sending stale retransmissions.
 */
#define TCP_SLEEP_TOO_LONG      (10 * 60 * 1000) /* 10 minutes in ms */

/* tcp timer list */
struct tcptimerlist tcp_timer_list;

/* List of pcbs in timewait state, protected by tcbinfo's ipi_lock */
struct tcptailq tcp_tw_tailq;

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_keepcnt;
SYSCTL_INT(_net_inet_tcp, OID_AUTO, keepcnt,
    CTLTYPE_INT | CTLFLAG_RW | CTLFLAG_LOCKED,
    &tcp_keepcnt, 0, "number of times to repeat keepalive");

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 timeout 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 or
 * quick mode even though all connections are idle. In this state, the
 * timer will run more frequently anticipating new data.
 */
int timer_fastmode_idlemax = TCP_FASTMODE_IDLERUN_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 = 10;
SYSCTL_INT(_net_inet_tcp, OID_AUTO, broken_peer_syn_rexmit_thres,
    CTLFLAG_RW | CTLFLAG_LOCKED, &tcp_broken_peer_syn_rxmit_thres, 0,
    "Number of retransmitted SYNs before disabling RFC 1323 "
    "options on local connections");

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 u_int32_t tcp_mss_rec_medium = 1200;
static u_int32_t tcp_mss_rec_low = 512;

#define TCP_REPORT_STATS_INTERVAL       43200 /* 12 hours, in seconds */
int tcp_report_stats_interval = TCP_REPORT_STATS_INTERVAL;

/* performed garbage collection of "used" sockets */
static boolean_t tcp_gc_done = FALSE;

/* max idle probes */
int     tcp_maxpersistidle;

/*
 * 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;

#if MPTCP
/*
 * MP_JOIN retransmission of 3rd ACK will be every 500 msecs without backoff
 */
int     tcp_jack_rxmt = TCP_RETRANSHZ / 2;
#endif /* MPTCP */

static boolean_t tcp_itimer_done = FALSE;

static void tcp_remove_timer(struct tcpcb *tp);
static void tcp_sched_timerlist(uint32_t offset);
static u_int32_t tcp_run_conn_timer(struct tcpcb *tp, u_int16_t *mode,
    u_int16_t probe_if_index);
static void tcp_sched_timers(struct tcpcb *tp);
static inline void tcp_set_lotimer_index(struct tcpcb *);
__private_extern__ void tcp_remove_from_time_wait(struct inpcb *inp);
static inline void tcp_update_mss_core(struct tcpcb *tp, struct ifnet *ifp);
__private_extern__ void tcp_report_stats(void);

static  u_int64_t tcp_last_report_time;

/*
 * Structure to store previously reported stats so that we can send
 * incremental changes in each report interval.
 */
struct tcp_last_report_stats {
        u_int32_t       tcps_connattempt;
        u_int32_t       tcps_accepts;
        u_int32_t       tcps_ecn_client_setup;
        u_int32_t       tcps_ecn_server_setup;
        u_int32_t       tcps_ecn_client_success;
        u_int32_t       tcps_ecn_server_success;
        u_int32_t       tcps_ecn_not_supported;
        u_int32_t       tcps_ecn_lost_syn;
        u_int32_t       tcps_ecn_lost_synack;
        u_int32_t       tcps_ecn_recv_ce;
        u_int32_t       tcps_ecn_recv_ece;
        u_int32_t       tcps_ecn_sent_ece;
        u_int32_t       tcps_ecn_conn_recv_ce;
        u_int32_t       tcps_ecn_conn_recv_ece;
        u_int32_t       tcps_ecn_conn_plnoce;
        u_int32_t       tcps_ecn_conn_pl_ce;
        u_int32_t       tcps_ecn_conn_nopl_ce;
        u_int32_t       tcps_ecn_fallback_synloss;
        u_int32_t       tcps_ecn_fallback_reorder;
        u_int32_t       tcps_ecn_fallback_ce;

        /* TFO-related statistics */
        u_int32_t       tcps_tfo_syn_data_rcv;
        u_int32_t       tcps_tfo_cookie_req_rcv;
        u_int32_t       tcps_tfo_cookie_sent;
        u_int32_t       tcps_tfo_cookie_invalid;
        u_int32_t       tcps_tfo_cookie_req;
        u_int32_t       tcps_tfo_cookie_rcv;
        u_int32_t       tcps_tfo_syn_data_sent;
        u_int32_t       tcps_tfo_syn_data_acked;
        u_int32_t       tcps_tfo_syn_loss;
        u_int32_t       tcps_tfo_blackhole;
        u_int32_t       tcps_tfo_cookie_wrong;
        u_int32_t       tcps_tfo_no_cookie_rcv;
        u_int32_t       tcps_tfo_heuristics_disable;
        u_int32_t       tcps_tfo_sndblackhole;
};


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

/* Run the TCP timerlist atleast once every hour */
#define TCP_TIMERLIST_MAX_OFFSET (60 * 60 * TCP_RETRANSHZ)


static void add_to_time_wait_locked(struct tcpcb *tp, uint32_t delay);
static boolean_t tcp_garbage_collect(struct inpcb *, int);

#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)

#define TCP_SET_TIMER_MODE(mode, i) do { \
        if (IS_TIMER_HZ_10MS(i)) \
                (mode) |= TCP_TIMERLIST_10MS_MODE; \
        else if (IS_TIMER_HZ_100MS(i)) \
                (mode) |= TCP_TIMERLIST_100MS_MODE; \
        else \
                (mode) |= TCP_TIMERLIST_500MS_MODE; \
} while(0)

#if (DEVELOPMENT || DEBUG)
SYSCTL_UINT(_net_inet_tcp, OID_AUTO, mss_rec_medium,
    CTLFLAG_RW | CTLFLAG_LOCKED, &tcp_mss_rec_medium, 0,
    "Medium MSS based on recommendation in link status report");
SYSCTL_UINT(_net_inet_tcp, OID_AUTO, mss_rec_low,
    CTLFLAG_RW | CTLFLAG_LOCKED, &tcp_mss_rec_low, 0,
    "Low MSS based on recommendation in link status report");

static int32_t tcp_change_mss_recommended = 0;
static int
sysctl_change_mss_recommended SYSCTL_HANDLER_ARGS
{
#pragma unused(oidp, arg1, arg2)
        int i, err = 0, changed = 0;
        struct ifnet *ifp;
        struct if_link_status ifsr;
        struct if_cellular_status_v1 *new_cell_sr;
        err = sysctl_io_number(req, tcp_change_mss_recommended,
            sizeof (int32_t), &i, &changed);
        if (changed) {
                ifnet_head_lock_shared();
                TAILQ_FOREACH(ifp, &ifnet_head, if_link) {
                        if (IFNET_IS_CELLULAR(ifp)) {
                                bzero(&ifsr, sizeof (ifsr));
                                new_cell_sr = &ifsr.ifsr_u.ifsr_cell.if_cell_u.if_status_v1;
                                ifsr.ifsr_version = IF_CELLULAR_STATUS_REPORT_CURRENT_VERSION;
                                ifsr.ifsr_len = sizeof(*new_cell_sr);

                                /* Set MSS recommended */
                                new_cell_sr->valid_bitmask |= IF_CELL_UL_MSS_RECOMMENDED_VALID;
                                new_cell_sr->mss_recommended = i;
                                err = ifnet_link_status_report(ifp, new_cell_sr, sizeof (new_cell_sr));
                                if (err == 0) {
                                        tcp_change_mss_recommended = i;
                                } else {
                                        break;
                                }
                        }
                }
                ifnet_head_done();
        }
        return (err);
}

SYSCTL_PROC(_net_inet_tcp, OID_AUTO, change_mss_recommended,
    CTLTYPE_INT | CTLFLAG_RW | CTLFLAG_LOCKED, &tcp_change_mss_recommended,
    0, sysctl_change_mss_recommended, "IU", "Change MSS recommended");

SYSCTL_INT(_net_inet_tcp, OID_AUTO, report_stats_interval,
    CTLFLAG_RW | CTLFLAG_LOCKED, &tcp_report_stats_interval, 0,
    "Report stats interval");
#endif /* (DEVELOPMENT || DEBUG) */

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

/*
 * Add to tcp timewait list, delay is given in milliseconds.
 */
static void
add_to_time_wait_locked(struct tcpcb *tp, uint32_t delay)
{
        struct inpcbinfo *pcbinfo = &tcbinfo;
        struct inpcb *inp = tp->t_inpcb;
        uint32_t timer;

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

        /* We may get here multiple times, so check */
        if (!(inp->inp_flags2 & INP2_TIMEWAIT)) {
                pcbinfo->ipi_twcount++;
                inp->inp_flags2 |= INP2_TIMEWAIT;

                /* Remove from global inp list */
                LIST_REMOVE(inp, inp_list);
        } else {
                TAILQ_REMOVE(&tcp_tw_tailq, tp, t_twentry);
        }

        /* Compute the time at which this socket can be closed */
        timer = tcp_now + delay;

        /* We will use the TCPT_2MSL timer for tracking this delay */

        if (TIMER_IS_ON_LIST(tp))
                tcp_remove_timer(tp);
        tp->t_timer[TCPT_2MSL] = timer;

        TAILQ_INSERT_TAIL(&tcp_tw_tailq, tp, t_twentry);
}

void
add_to_time_wait(struct tcpcb *tp, uint32_t delay)
{
        struct inpcbinfo *pcbinfo = &tcbinfo;
        if (tp->t_inpcb->inp_socket->so_options & SO_NOWAKEFROMSLEEP)
                socket_post_kev_msg_closed(tp->t_inpcb->inp_socket);

        /* 19182803: Notify nstat that connection is closing before waiting. */
        nstat_pcb_detach(tp->t_inpcb);

        if (!lck_rw_try_lock_exclusive(pcbinfo->ipi_lock)) {
                tcp_unlock(tp->t_inpcb->inp_socket, 0, 0);
                lck_rw_lock_exclusive(pcbinfo->ipi_lock);
                tcp_lock(tp->t_inpcb->inp_socket, 0, 0);
        }
        add_to_time_wait_locked(tp, delay);
        lck_rw_done(pcbinfo->ipi_lock);

        inpcb_gc_sched(pcbinfo, INPCB_TIMER_LAZY);
}

/* If this is on time wait queue, remove it. */
void
tcp_remove_from_time_wait(struct inpcb *inp)
{
        struct tcpcb *tp = intotcpcb(inp);
        if (inp->inp_flags2 & INP2_TIMEWAIT)
                TAILQ_REMOVE(&tcp_tw_tailq, tp, t_twentry);
}

static boolean_t
tcp_garbage_collect(struct inpcb *inp, int istimewait)
{
        boolean_t active = FALSE;
        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 (!lck_mtx_try_lock_spin(&inp->inpcb_mtx))
                return (TRUE);

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

        if (istimewait &&
                TSTMP_GEQ(tcp_now, tp->t_timer[TCPT_2MSL]) &&
                tp->t_state != TCPS_CLOSED) {
                /* Become a regular mutex */
                lck_mtx_convert_spin(&inp->inpcb_mtx);
                tcp_close(tp);
        }

        /*
         * 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 (SOCK_CHECK_DOM(so, PF_INET6))
                                in6_pcbdetach(inp);
                        else
#endif /* INET6 */
                                in_pcbdetach(inp);
                }
                so->so_usecount--;
                if (inp->inp_wantcnt == WNT_STOPUSING)
                        active = TRUE;
                lck_mtx_unlock(&inp->inpcb_mtx);
                return (active);
        } else if (inp->inp_wantcnt != WNT_STOPUSING) {
                lck_mtx_unlock(&inp->inpcb_mtx);
                return (FALSE);
        }

        /*
         * 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 (SOCK_CHECK_DOM(so, PF_INET6))
                                in6_pcbdetach(inp);
                        else
#endif /* INET6 */
                                in_pcbdetach(inp);
                }
                in_pcbdispose(inp);
                return (FALSE);
        }

        lck_mtx_unlock(&inp->inpcb_mtx);
        return (TRUE);
}

/*
 * TCP garbage collector callback (inpcb_timer_func_t).
 *
 * Returns the number of pcbs that will need to be gc-ed soon,
 * returnining > 0 will keep timer active.
 */
void
tcp_gc(struct inpcbinfo *ipi)
{
        struct inpcb *inp, *nxt;
        struct tcpcb *tw_tp, *tw_ntp;
#if TCPDEBUG
        int ostate;
#endif
#if  KDEBUG
        static int tws_checked = 0;
#endif

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

        /*
         * 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(ipi->ipi_lock) == FALSE) {
                /* don't sweat it this time; cleanup was done last time */
                if (tcp_gc_done == TRUE) {
                        tcp_gc_done = FALSE;
                        KERNEL_DEBUG(DBG_FNC_TCP_SLOW | DBG_FUNC_END,
                            tws_checked, cur_tw_slot, 0, 0, 0);
                        /* Lock upgrade failed, give up this round */
                        atomic_add_32(&ipi->ipi_gc_req.intimer_fast, 1);
                        return;
                }
                /* Upgrade failed, lost lock now take it again exclusive */
                lck_rw_lock_exclusive(ipi->ipi_lock);
        }
        tcp_gc_done = TRUE;

        LIST_FOREACH_SAFE(inp, &tcb, inp_list, nxt) {
                if (tcp_garbage_collect(inp, 0))
                        atomic_add_32(&ipi->ipi_gc_req.intimer_fast, 1);
        }

        /* Now cleanup the time wait ones */
        TAILQ_FOREACH_SAFE(tw_tp, &tcp_tw_tailq, t_twentry, tw_ntp) {
                /*
                 * We check the timestamp here without holding the
                 * socket lock for better performance. If there are
                 * any pcbs in time-wait, the timer will get rescheduled.
                 * Hence some error in this check can be tolerated.
                 *
                 * Sometimes a socket on time-wait queue can be closed if
                 * 2MSL timer expired but the application still has a
                 * usecount on it.
                 */
                if (tw_tp->t_state == TCPS_CLOSED ||
                    TSTMP_GEQ(tcp_now, tw_tp->t_timer[TCPT_2MSL])) {
                        if (tcp_garbage_collect(tw_tp->t_inpcb, 1))
                                atomic_add_32(&ipi->ipi_gc_req.intimer_lazy, 1);
                }
        }

        /* take into account pcbs that are still in time_wait_slots */
        atomic_add_32(&ipi->ipi_gc_req.intimer_lazy, ipi->ipi_twcount);

        lck_rw_done(ipi->ipi_lock);

        /* Clean up the socache while we are here */
        if (so_cache_timer())
                atomic_add_32(&ipi->ipi_gc_req.intimer_lazy, 1);

        KERNEL_DEBUG(DBG_FNC_TCP_SLOW | DBG_FUNC_END, tws_checked,
            cur_tw_slot, 0, 0, 0);

        return;
}

/*
 * Cancel all timers for TCP tp.
 */
void
tcp_canceltimers(struct tcpcb *tp)
{
        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[] */

void
tcp_rexmt_save_state(struct tcpcb *tp)
{
        u_int32_t fsize;
        if (TSTMP_SUPPORTED(tp)) {
                /*
                 * Since timestamps are supported on the connection,
                 * we can do recovery as described in rfc 4015.
                 */
                fsize = tp->snd_max - tp->snd_una;
                tp->snd_ssthresh_prev = max(fsize, tp->snd_ssthresh);
                tp->snd_recover_prev = tp->snd_recover;
        } else {
                /*
                 * Timestamp option is not supported on this connection.
                 * 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_srtt_prev = (tp->t_srtt >> TCP_RTT_SHIFT) + 2;
        tp->t_rttvar_prev = (tp->t_rttvar >> TCP_RTTVAR_SHIFT);
        tp->t_flagsext &= ~(TF_RECOMPUTE_RTT);
}

/*
 * Revert to the older segment size if there is an indication that PMTU
 * blackhole detection was not needed.
 */
void
tcp_pmtud_revert_segment_size(struct tcpcb *tp)
{
        int32_t optlen;

        VERIFY(tp->t_pmtud_saved_maxopd > 0);
        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 depend on the new MSS
         */
        if (CC_ALGO(tp)->cwnd_init != NULL)
                CC_ALGO(tp)->cwnd_init(tp);
        tp->t_pmtud_start_ts = 0;
        tcpstat.tcps_pmtudbh_reverted++;
}

/*
 * TCP timer processing.
 */
struct tcpcb *
tcp_timers(struct tcpcb *tp, int timer)
{
        int32_t rexmt, optlen = 0, idle_time = 0;
        struct socket *so;
        struct tcptemp *t_template;
#if TCPDEBUG
        int ostate;
#endif

#if INET6
        int isipv6 = (tp->t_inpcb->inp_vflag & INP_IPV4) == 0;
#endif /* INET6 */
        u_int64_t accsleep_ms;
        u_int32_t last_sleep_ms = 0;

        so = 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_CONN_MAXIDLE(tp)))) {
                        tp->t_timer[TCPT_2MSL] = OFFSET_FROM_START(tp,
                                (u_int32_t)TCP_CONN_KEEPINTVL(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:
                absolutetime_to_nanoseconds(mach_absolutetime_asleep,
                    &accsleep_ms);
                accsleep_ms = accsleep_ms / 1000000UL;
                if (accsleep_ms > tp->t_accsleep_ms)
                        last_sleep_ms = accsleep_ms - tp->t_accsleep_ms;
                /*
                 * 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->t_rxt_conndroptime > 0 && tp->t_rxtstart > 0 &&
                    (tcp_now - tp->t_rxtstart) >= tp->t_rxt_conndroptime) ||
                    ((tp->t_flagsext & TF_RXTFINDROP) != 0 &&
                    (tp->t_flags & TF_SENTFIN) != 0 && tp->t_rxtshift >= 4) ||
                    (tp->t_rxtshift > 4 && last_sleep_ms >= TCP_SLEEP_TOO_LONG)) {
                        if ((tp->t_flagsext & TF_RXTFINDROP) != 0) {
                                tcpstat.tcps_rxtfindrop++;
                        } else if (last_sleep_ms >= TCP_SLEEP_TOO_LONG) {
                                tcpstat.tcps_drop_after_sleep++;
                        } else {
                                tcpstat.tcps_timeoutdrop++;
                        }
                        if (tp->t_rxtshift >= TCP_MAXRXTSHIFT) {
                                if (TCP_ECN_ENABLED(tp)) {
                                        INP_INC_IFNET_STAT(tp->t_inpcb,
                                            ecn_on.rxmit_drop);
                                } else {
                                        INP_INC_IFNET_STAT(tp->t_inpcb,
                                            ecn_off.rxmit_drop);
                                }
                        }
                        tp->t_rxtshift = TCP_MAXRXTSHIFT;
                        postevent(so, 0, EV_TIMEOUT);
                        soevent(so,
                            (SO_FILT_HINT_LOCKED|SO_FILT_HINT_TIMEOUT));

                        if (TCP_ECN_ENABLED(tp) &&
                            tp->t_state == TCPS_ESTABLISHED)
                                tcp_heuristic_ecn_droprxmt(tp);

                        tp = tcp_drop(tp, tp->t_softerror ?
                            tp->t_softerror : ETIMEDOUT);

                        break;
                }

                tcpstat.tcps_rexmttimeo++;
                tp->t_accsleep_ms = accsleep_ms;

                if (tp->t_rxtshift == 1 &&
                        tp->t_state == TCPS_ESTABLISHED) {
                        /* Set the time at which retransmission started. */
                        tp->t_rxtstart = tcp_now;

                        /*
                         * if this is the first retransmit timeout, save
                         * the state so that we can recover if the timeout
                         * is spurious.
                         */
                        tcp_rexmt_save_state(tp);
                }
#if MPTCP
                if ((tp->t_rxtshift >= mptcp_fail_thresh) &&
                    (tp->t_state == TCPS_ESTABLISHED) &&
                    (tp->t_mpflags & TMPF_MPTCP_TRUE)) {
                        mptcp_act_on_txfail(so);

                }
#endif /* MPTCP */

                if (tp->t_adaptive_wtimo > 0 &&
                        tp->t_rxtshift > tp->t_adaptive_wtimo &&
                        TCPS_HAVEESTABLISHED(tp->t_state)) {
                        /* Send an event to the application */
                        soevent(so,
                                (SO_FILT_HINT_LOCKED|
                                SO_FILT_HINT_ADAPTIVE_WTIMO));
                }

                /*
                 * If this is a retransmit timeout after PTO, the PTO
                 * was not effective
                 */
                if (tp->t_flagsext & TF_SENT_TLPROBE) {
                        tp->t_flagsext &= ~(TF_SENT_TLPROBE);
                        tcpstat.tcps_rto_after_pto++;
                }

                if (tp->t_flagsext & TF_DELAY_RECOVERY) {
                        /*
                         * Retransmit timer fired before entering recovery
                         * on a connection with packet re-ordering. This
                         * suggests that the reordering metrics computed
                         * are not accurate.
                         */
                        tp->t_reorderwin = 0;
                        tp->t_timer[TCPT_DELAYFR] = 0;
                        tp->t_flagsext &= ~(TF_DELAY_RECOVERY);
                }

                if (tp->t_state == TCPS_SYN_RECEIVED)
                        tcp_disable_tfo(tp);

                if ((tp->t_tfo_stats & TFO_S_SYN_DATA_SENT) &&
                    !(tp->t_tfo_flags & TFO_F_NO_SNDPROBING) &&
                    ((tp->t_state != TCPS_SYN_SENT && tp->t_rxtshift > 1) ||
                     tp->t_rxtshift > 2)) {
                        /*
                         * For regular retransmissions, a first one is being
                         * done for tail-loss probe.
                         * Thus, if rxtshift > 1, this means we have sent the segment
                         * a total of 3 times.
                         *
                         * If we are in SYN-SENT state, then there is no tail-loss
                         * probe thus we have to let rxtshift go up to 3.
                         */
                        tcp_heuristic_tfo_middlebox(tp);

                        so->so_error = ENODATA;
                        sorwakeup(so);
                        sowwakeup(so);

                        tp->t_tfo_stats |= TFO_S_SEND_BLACKHOLE;
                        tcpstat.tcps_tfo_sndblackhole++;
                }

                if (tp->t_state == TCPS_SYN_SENT) {
                        rexmt = TCP_REXMTVAL(tp) * tcp_syn_backoff[tp->t_rxtshift];
                        tp->t_stat.synrxtshift = tp->t_rxtshift;

                        /* When retransmitting, disable TFO */
                        if (tfo_enabled(tp)) {
                                tp->t_flagsext &= ~TF_FASTOPEN;
                                tp->t_tfo_flags |= TFO_F_SYN_LOSS;
                        }
                } 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);

                if (INP_WAIT_FOR_IF_FEEDBACK(tp->t_inpcb))
                        goto fc_output;

                tcp_free_sackholes(tp);
                /*
                 * Check for potential Path MTU Discovery Black Hole
                 */
                if (tcp_pmtud_black_hole_detect &&
                        !(tp->t_flagsext & TF_NOBLACKHOLE_DETECTION) &&
                        (tp->t_state == TCPS_ESTABLISHED)) {
                        if ((tp->t_flags & TF_PMTUD) &&
                            ((tp->t_flags & TF_MAXSEGSNT)
                            || tp->t_pmtud_lastseg_size > tcp_pmtud_black_hole_mss) &&
                            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
                                 * negotiated with the peer.
                                 */
                                /* Disable Path MTU Discovery for now */
                                tp->t_flags &= ~TF_PMTUD;
                                /* Record that we may have found a black hole */
                                tp->t_flags |= TF_BLACKHOLE;
                                optlen = tp->t_maxopd - tp->t_maxseg;
                                /* Keep track of previous MSS */
                                tp->t_pmtud_saved_maxopd = tp->t_maxopd;
                                tp->t_pmtud_start_ts = tcp_now;
                                if (tp->t_pmtud_start_ts == 0)
                                        tp->t_pmtud_start_ts++;
                                /* Reduce the MSS to intermediary value */
                                if (tp->t_maxopd > tcp_pmtud_black_hole_mss) {
                                        tp->t_maxopd = tcp_pmtud_black_hole_mss;
                                } 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 depend on the new MSS
                                 */
                                if (CC_ALGO(tp)->cwnd_init != NULL)
                                        CC_ALGO(tp)->cwnd_init(tp);
                                tp->snd_cwnd = tp->t_maxseg;
                        }
                        /*
                         * 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)) {
                                        tcp_pmtud_revert_segment_size(tp);
                                        tp->snd_cwnd = tp->t_maxseg;
                                }
                        }
                }


                /*
                 * 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.
                 * Do this only on non-local connections.
                 */
                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 (!IN_FASTRECOVERY(tp) && tp->t_rxtshift == 1)
                        tcpstat.tcps_tailloss_rto++;


                /*
                 * RFC 5681 says: when a TCP sender detects segment loss
                 * using retransmit timer and the given segment has already
                 * been retransmitted by way of the retransmission timer at
                 * least once, the value of ssthresh is held constant
                 */
                if (tp->t_rxtshift == 1 &&
                    CC_ALGO(tp)->after_timeout != NULL) {
                        CC_ALGO(tp)->after_timeout(tp);
                        /*
                         * CWR notifications are to be sent on new data
                         * right after Fast Retransmits and ECE
                         * notification receipts.
                         */
                        if (TCP_ECN_ENABLED(tp))
                                tp->ecn_flags |= TE_SENDCWR;
                }

                EXIT_FASTRECOVERY(tp);

                /* Exit cwnd non validated phase */
                tp->t_flagsext &= ~TF_CWND_NONVALIDATED;


fc_output:
                tcp_ccdbg_trace(tp, NULL, 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) &&
                        TSTMP_LEQ(tp->t_persist_stop, tcp_now))) {
                        tcpstat.tcps_persistdrop++;
                        postevent(so, 0, EV_TIMEOUT);
                        soevent(so,
                            (SO_FILT_HINT_LOCKED|SO_FILT_HINT_TIMEOUT));
                        tp = tcp_drop(tp, ETIMEDOUT);
                        break;
                }
                tcp_setpersist(tp);
                tp->t_flagsext |= TF_FORCE;
                (void) tcp_output(tp);
                tp->t_flagsext &= ~TF_FORCE;
                break;

        /*
         * Keep-alive timer went off; send something
         * or drop connection if idle for too long.
         */
        case TCPT_KEEP:
                tcpstat.tcps_keeptimeo++;
#if MPTCP
                /*
                 * Regular TCP connections do not send keepalives after closing
                 * MPTCP must not also, after sending Data FINs.
                 */
                struct mptcb *mp_tp = tp->t_mptcb;
                if ((tp->t_mpflags & TMPF_MPTCP_TRUE) &&
                    (tp->t_state > TCPS_ESTABLISHED)) {
                        goto dropit;
                } else if (mp_tp != NULL) {
                        if ((mptcp_ok_to_keepalive(mp_tp) == 0))
                                goto dropit;
                }
#endif /* MPTCP */
                if (tp->t_state < TCPS_ESTABLISHED)
                        goto dropit;
                if ((always_keepalive ||
                    (tp->t_inpcb->inp_socket->so_options & SO_KEEPALIVE) ||
                    (tp->t_flagsext & TF_DETECT_READSTALL) ||
                    (tp->t_tfo_probe_state == TFO_PROBE_PROBING)) &&
                    (tp->t_state <= TCPS_CLOSING || tp->t_state == TCPS_FIN_WAIT_2)) {
                        if (idle_time >= TCP_CONN_KEEPIDLE(tp) + TCP_CONN_MAXIDLE(tp))
                                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) {
                                struct inpcb *inp = tp->t_inpcb;
                                struct tcp_respond_args tra;

                                bzero(&tra, sizeof(tra));
                                tra.nocell = INP_NO_CELLULAR(inp);
                                tra.noexpensive = INP_NO_EXPENSIVE(inp);
                                tra.awdl_unrestricted = INP_AWDL_UNRESTRICTED(inp);
                                tra.intcoproc_allowed = INP_INTCOPROC_ALLOWED(inp);
                                if (tp->t_inpcb->inp_flags & INP_BOUND_IF)
                                        tra.ifscope = tp->t_inpcb->inp_boundifp->if_index;
                                else
                                        tra.ifscope = IFSCOPE_NONE;
                                tcp_respond(tp, t_template->tt_ipgen,
                                    &t_template->tt_t, (struct mbuf *)NULL,
                                    tp->rcv_nxt, tp->snd_una - 1, 0, &tra);
                                (void) m_free(dtom(t_template));
                                if (tp->t_flagsext & TF_DETECT_READSTALL)
                                        tp->t_rtimo_probes++;
                        }
                        tp->t_timer[TCPT_KEEP] = OFFSET_FROM_START(tp,
                            TCP_CONN_KEEPINTVL(tp));
                } else {
                        tp->t_timer[TCPT_KEEP] = OFFSET_FROM_START(tp,
                            TCP_CONN_KEEPIDLE(tp));
                }
                if (tp->t_flagsext & TF_DETECT_READSTALL) {
                        struct ifnet *outifp = tp->t_inpcb->inp_last_outifp;
                        bool reenable_probe = false;
                        /*
                         * The keep alive packets sent to detect a read
                         * stall did not get a response from the
                         * peer. Generate more keep-alives to confirm this.
                         * If the number of probes sent reaches the limit,
                         * generate an event.
                         */
                        if (tp->t_adaptive_rtimo > 0) {
                                if (tp->t_rtimo_probes > tp->t_adaptive_rtimo) {
                                        /* Generate an event */
                                        soevent(so,
                                            (SO_FILT_HINT_LOCKED |
                                            SO_FILT_HINT_ADAPTIVE_RTIMO));
                                        tcp_keepalive_reset(tp);
                                } else {
                                        reenable_probe = true;
                                }
                        } else if (outifp != NULL &&
                            (outifp->if_eflags & IFEF_PROBE_CONNECTIVITY) &&
                            tp->t_rtimo_probes <= TCP_CONNECTIVITY_PROBES_MAX) {
                                reenable_probe = true;
                        } else {
                                tp->t_flagsext &= ~TF_DETECT_READSTALL;
                        }
                        if (reenable_probe) {
                                int ind = min(tp->t_rtimo_probes,
                                    TCP_MAXRXTSHIFT);
                                tp->t_timer[TCPT_KEEP] = OFFSET_FROM_START(
                                    tp, tcp_backoff[ind] * TCP_REXMTVAL(tp));
                        }
                }
                if (tp->t_tfo_probe_state == TFO_PROBE_PROBING) {
                        int ind;

                        tp->t_tfo_probes++;
                        ind = min(tp->t_tfo_probes, TCP_MAXRXTSHIFT);

                        /*
                         * We take the minimum among the time set by true
                         * keepalive (see above) and the backoff'd RTO. That
                         * way we backoff in case of packet-loss but will never
                         * timeout slower than regular keepalive due to the
                         * backing off.
                         */
                        tp->t_timer[TCPT_KEEP] = min(OFFSET_FROM_START(
                            tp, tcp_backoff[ind] * TCP_REXMTVAL(tp)),
                            tp->t_timer[TCPT_KEEP]);
                } else if (tp->t_tfo_probe_state == TFO_PROBE_WAIT_DATA) {
                        /* Still no data! Let's assume a TFO-error and err out... */
                        tcp_heuristic_tfo_middlebox(tp);

                        so->so_error = ENODATA;
                        sorwakeup(so);
                        tp->t_tfo_stats |= TFO_S_RECV_BLACKHOLE;
                        tcpstat.tcps_tfo_blackhole++;
                }
                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 stretching
                         * acks, count the number of times the streaming
                         * detection was not correct. If this exceeds a
                         * threshold, disable strech ack on this
                         * connection
                         *
                         * Also, go back to acking every other packet.
                         */
                        if ((tp->t_flags & TF_STRETCHACK)) {
                                if (tp->t_unacksegs > 1 &&
                                    tp->t_unacksegs < maxseg_unacked)
                                        tp->t_stretchack_delayed++;

                                if (tp->t_stretchack_delayed >
                                        TCP_STRETCHACK_DELAY_THRESHOLD) {
                                        tp->t_flagsext |= TF_DISABLE_STRETCHACK;
                                        /*
                                         * Note the time at which stretch
                                         * ack was disabled automatically
                                         */
                                        tp->rcv_nostrack_ts = tcp_now;
                                        tcpstat.tcps_nostretchack++;
                                        tp->t_stretchack_delayed = 0;
                                        tp->rcv_nostrack_pkts = 0;
                                }
                                tcp_reset_stretch_ack(tp);
                        }

                        /*
                         * If we are measuring inter packet arrival jitter
                         * for throttling a connection, this delayed ack
                         * might be the reason for accumulating some
                         * jitter. So let's restart the measurement.
                         */
                        CLEAR_IAJ_STATE(tp);

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

#if MPTCP
        case TCPT_JACK_RXMT:
                if ((tp->t_state == TCPS_ESTABLISHED) &&
                    (tp->t_mpflags & TMPF_PREESTABLISHED) &&
                    (tp->t_mpflags & TMPF_JOINED_FLOW)) {
                        if (++tp->t_mprxtshift > TCP_MAXRXTSHIFT) {
                                tcpstat.tcps_timeoutdrop++;
                                postevent(so, 0, EV_TIMEOUT);
                                soevent(so,
                                    (SO_FILT_HINT_LOCKED|
                                    SO_FILT_HINT_TIMEOUT));
                                tp = tcp_drop(tp, tp->t_softerror ?
                                    tp->t_softerror : ETIMEDOUT);
                                break;
                        }
                        tcpstat.tcps_join_rxmts++;
                        tp->t_flags |= TF_ACKNOW;

                        /*
                         * No backoff is implemented for simplicity for this
                         * corner case.
                         */
                        (void) tcp_output(tp);
                }
                break;
#endif /* MPTCP */

        case TCPT_PTO:
        {
                int32_t snd_len;
                tp->t_flagsext &= ~(TF_SENT_TLPROBE);

                /*
                 * Check if the connection is in the right state to
                 * send a probe
                 */
                if (tp->t_state != TCPS_ESTABLISHED ||
                    (tp->t_rxtshift > 0 && !(tp->t_flagsext & TF_PROBING))
                    || tp->snd_max == tp->snd_una ||
                    !SACK_ENABLED(tp) || !TAILQ_EMPTY(&tp->snd_holes) ||
                    IN_FASTRECOVERY(tp))
                        break;

                /*
                 * If there is no new data to send or if the
                 * connection is limited by receive window then
                 * retransmit the last segment, otherwise send
                 * new data.
                 */
                snd_len = min(so->so_snd.sb_cc, tp->snd_wnd)
                    - (tp->snd_max - tp->snd_una);
                if (snd_len > 0) {
                        tp->snd_nxt = tp->snd_max;
                } else {
                        snd_len = min((tp->snd_max - tp->snd_una),
                            tp->t_maxseg);
                        tp->snd_nxt = tp->snd_max - snd_len;
                }

                tcpstat.tcps_pto++;
                if (tp->t_flagsext & TF_PROBING)
                        tcpstat.tcps_probe_if++;

                /* If timing a segment in this window, stop the timer */
                tp->t_rtttime = 0;
                /* Note that tail loss probe is being sent */
                tp->t_flagsext |= TF_SENT_TLPROBE;
                tp->t_tlpstart = tcp_now;

                tp->snd_cwnd += tp->t_maxseg;
                (void )tcp_output(tp);
                tp->snd_cwnd -= tp->t_maxseg;

                tp->t_tlphighrxt = tp->snd_nxt;
                break;
        }
        case TCPT_DELAYFR:
                tp->t_flagsext &= ~TF_DELAY_RECOVERY;

                /*
                 * Don't do anything if one of the following is true:
                 * - the connection is already in recovery
                 * - sequence until snd_recover has been acknowledged.
                 * - retransmit timeout has fired
                 */
                if (IN_FASTRECOVERY(tp) ||
                    SEQ_GEQ(tp->snd_una, tp->snd_recover) ||
                    tp->t_rxtshift > 0)
                        break;

                VERIFY(SACK_ENABLED(tp));
                tcp_rexmt_save_state(tp);
                if (CC_ALGO(tp)->pre_fr != NULL) {
                        CC_ALGO(tp)->pre_fr(tp);
                        if (TCP_ECN_ENABLED(tp))
                                tp->ecn_flags |= TE_SENDCWR;
                }
                ENTER_FASTRECOVERY(tp);

                tp->t_timer[TCPT_REXMT] = 0;
                tcpstat.tcps_sack_recovery_episode++;
                tp->t_sack_recovery_episode++;
                tp->sack_newdata = tp->snd_nxt;
                tp->snd_cwnd = tp->t_maxseg;
                tcp_ccdbg_trace(tp, NULL, TCP_CC_ENTER_FASTRECOVERY);
                (void) tcp_output(tp);
                break;
        dropit:
                tcpstat.tcps_keepdrops++;
                postevent(so, 0, EV_TIMEOUT);
                soevent(so,
                    (SO_FILT_HINT_LOCKED|SO_FILT_HINT_TIMEOUT));
                tp = tcp_drop(tp, ETIMEDOUT);
                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
        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(u_int32_t runtime, u_int16_t mode)
{
        struct tcptimerlist *listp = &tcp_timer_list;
        int32_t diff;

        /*
         * 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);

        if (!listp->scheduled)
                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 (mode & TCP_TIMERLIST_10MS_MODE) {
                        if (diff <= TCP_TIMER_10MS_QUANTUM)
                                return (FALSE);
                } else if (mode & TCP_TIMERLIST_100MS_MODE) {
                        if (diff <= TCP_TIMER_100MS_QUANTUM)
                                return (FALSE);
                } else {
                        if (diff <= TCP_TIMER_500MS_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);

        offset = min(offset, TCP_TIMERLIST_MAX_OFFSET);
        listp->runtime = tcp_now + offset;
        if (listp->runtime == 0) {
                listp->runtime++;
                offset++;
        }

        clock_interval_to_deadline(offset, USEC_PER_SEC, &deadline);

        thread_call_enter_delayed(listp->call, deadline);
        listp->scheduled = TRUE;
}

/*
 * 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.
 *
 * te_mode is an out parameter that indicates the modes of active
 * timers for this connection.
 */
u_int32_t
tcp_run_conn_timer(struct tcpcb *tp, u_int16_t *te_mode,
        u_int16_t probe_if_index)
{
        struct socket *so;
        u_int16_t i = 0, index = TCPT_NONE, lo_index = TCPT_NONE;
        u_int32_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));
        *te_mode = 0;

        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;
        }

        /*
         * If this connection is over an interface that needs to
         * be probed, send probe packets to reinitiate communication.
         */
        if (probe_if_index > 0 && tp->t_inpcb->inp_last_outifp != NULL &&
            tp->t_inpcb->inp_last_outifp->if_index == probe_if_index) {
                tp->t_flagsext |= TF_PROBING;
                tcp_timers(tp, TCPT_PTO);
                tp->t_timer[TCPT_PTO] = 0;
                tp->t_flagsext &= ~TF_PROBING;
        }

        /*
         * 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.
         */
        if ((index = tp->tentry.index) == TCPT_NONE)
                goto done;

        timer_val = tp->t_timer[index];

        diff = timer_diff(tp->tentry.runtime, 0, tcp_now, 0);
        if (diff > 0) {
                if (tp->tentry.index != TCPT_NONE) {
                        offset = diff;
                        *(te_mode) = tp->tentry.mode;
                }
                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.
         */
        tp->tentry.mode = 0;
        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) {
                                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;
                                }
                                TCP_SET_TIMER_MODE(tp->tentry.mode, i);
                        }
                }
        }

        tp->tentry.timer_start = tcp_now;
        tp->tentry.index = lo_index;
        VERIFY(tp->tentry.index == TCPT_NONE || tp->tentry.mode > 0);

        if (tp->tentry.index != TCPT_NONE) {
                tp->tentry.runtime = tp->tentry.timer_start +
                        tp->t_timer[tp->tentry.index];
                if (tp->tentry.runtime == 0)
                        tp->tentry.runtime++;
        }

        if (count > 0) {
                /* run any other timers 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) {
                                        offset = 0;
                                        *(te_mode) = 0;
                                        goto done;
                                }
                        }
                }
                tcp_set_lotimer_index(tp);
        }

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

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

        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; /* offset of the next timer on the list */
        u_int16_t te_mode = 0;  /* modes of all active timers in a tcpcb */
        u_int16_t list_mode = 0; /* cumulative of modes of all tcpcbs */
        uint32_t active_count = 0;

        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 (te->index < TCPT_NONE && TSTMP_GT(runtime, tcp_now)) {
                        offset = timer_diff(runtime, 0, tcp_now, 0);
                        if (next_timer == 0 || offset < next_timer) {
                                next_timer = offset;
                        }
                        list_mode |= te->mode;
                        continue;
                }

                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 without the socket lock.
                         */
                        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;
                }
                active_count++;

                /*
                 * 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);

                offset = tcp_run_conn_timer(tp, &te_mode,
                    listp->probe_if_index);

                lck_mtx_lock(listp->mtx);

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

                if (offset > 0 && te_mode != 0) {
                        list_mode |= te_mode;

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

        if (!LIST_EMPTY(&listp->lhead)) {
                u_int16_t next_mode = 0;
                if ((list_mode & TCP_TIMERLIST_10MS_MODE) ||
                        (listp->pref_mode & TCP_TIMERLIST_10MS_MODE))
                        next_mode = TCP_TIMERLIST_10MS_MODE;
                else if ((list_mode & TCP_TIMERLIST_100MS_MODE) ||
                        (listp->pref_mode & TCP_TIMERLIST_100MS_MODE))
                        next_mode = TCP_TIMERLIST_100MS_MODE;
                else
                        next_mode = TCP_TIMERLIST_500MS_MODE;

                if (next_mode != TCP_TIMERLIST_500MS_MODE) {
                        listp->idleruns = 0;
                } else {
                        /*
                         * the next required mode is slow mode, but if
                         * the last one was a faster mode and we did not
                         * have enough idle runs, repeat the last mode.
                         *
                         * We try to keep the timer list in fast mode for
                         * some idle time in expectation of new data.
                         */
                        if (listp->mode != next_mode &&
                            listp->idleruns < timer_fastmode_idlemax) {
                                listp->idleruns++;
                                next_mode = listp->mode;
                                next_timer = TCP_TIMER_100MS_QUANTUM;
                        } else {
                                listp->idleruns = 0;
                        }
                }
                listp->mode = next_mode;
                if (listp->pref_offset != 0)
                        next_timer = min(listp->pref_offset, next_timer);

                if (listp->mode == TCP_TIMERLIST_500MS_MODE)
                        next_timer = max(next_timer,
                                TCP_TIMER_500MS_QUANTUM);

                tcp_sched_timerlist(next_timer);
        } else {
                /*
                 * No need to reschedule this timer, but always run
                 * periodically at a much higher granularity.
                 */
                tcp_sched_timerlist(TCP_TIMERLIST_MAX_OFFSET);
        }

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

        lck_mtx_unlock(listp->mtx);
}

/*
 * Function to check if the timerlist needs to be rescheduled to run this
 * connection's timers correctly.
 */
void
tcp_sched_timers(struct tcpcb *tp)
{
        struct tcptimerentry *te = &tp->tentry;
        u_int16_t index = te->index;
        u_int16_t mode = te->mode;
        struct tcptimerlist *listp = &tcp_timer_list;
        int32_t offset = 0;
        boolean_t list_locked = FALSE;

        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) {
                /* Nothing to run */
                tcp_remove_timer(tp);
                return;
        }

        /*
         * compute the offset at which the next timer for this connection
         * has to run.
         */
        offset = timer_diff(te->runtime, 0, tcp_now, 0);
        if (offset <= 0) {
                offset = 1;
                tcp_timer_advanced++;
        }

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

                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->scheduled)
                        goto schedule;
        }


        /*
         * Timer entry is currently on the list, check if the list needs
         * to be rescheduled.
         */
        if (need_to_resched_timerlist(te->runtime, mode)) {
                tcp_resched_timerlist++;

                if (!list_locked) {
                        lck_mtx_lock(listp->mtx);
                        list_locked = TRUE;
                }

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

                if (listp->running) {
                        listp->pref_mode |= mode;
                        if (listp->pref_offset == 0 ||
                                offset < listp->pref_offset) {
                                listp->pref_offset = offset;
                        }
                } else {
                        /*
                         * The list could have got rescheduled while
                         * this thread was waiting for the lock
                         */
                        if (listp->scheduled) {
                                int32_t diff;
                                diff = timer_diff(listp->runtime, 0,
                                    tcp_now, offset);
                                if (diff <= 0)
                                        goto done;
                                else
                                        goto schedule;
                        } else {
                                goto schedule;
                        }
                }
        }
        goto done;

schedule:
        /*
         * Since a connection with timers is getting scheduled, the timer
         * list moves from idle to active state and that is why idlegen is
         * reset
         */
        if (mode & TCP_TIMERLIST_10MS_MODE) {
                listp->mode = TCP_TIMERLIST_10MS_MODE;
                listp->idleruns = 0;
                offset = min(offset, TCP_TIMER_10MS_QUANTUM);
        } else if (mode & TCP_TIMERLIST_100MS_MODE) {
                if (listp->mode > TCP_TIMERLIST_100MS_MODE)
                        listp->mode = TCP_TIMERLIST_100MS_MODE;
                listp->idleruns = 0;
                offset = min(offset, TCP_TIMER_100MS_QUANTUM);
        }
        tcp_sched_timerlist(offset);

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

        return;
}

static inline void
tcp_set_lotimer_index(struct tcpcb *tp)
{
        uint16_t i, lo_index = TCPT_NONE, mode = 0;
        uint32_t lo_timer = 0;
        for (i = 0; i < TCPT_NTIMERS; ++i) {
                if (tp->t_timer[i] != 0) {
                        TCP_SET_TIMER_MODE(mode, i);
                        if (lo_timer == 0 || tp->t_timer[i] < lo_timer) {
                                lo_timer = tp->t_timer[i];
                                lo_index = i;
                        }
                }
        }
        tp->tentry.index = lo_index;
        tp->tentry.mode = mode;
        VERIFY(tp->tentry.index == TCPT_NONE || tp->tentry.mode > 0);

        if (tp->tentry.index != TCPT_NONE) {
                tp->tentry.runtime = tp->tentry.timer_start
                    + tp->t_timer[tp->tentry.index];
                if (tp->tentry.runtime == 0)
                        tp->tentry.runtime++;
        }
}

void
tcp_check_timer_state(struct tcpcb *tp)
{
        lck_mtx_assert(&tp->t_inpcb->inpcb_mtx, LCK_MTX_ASSERT_OWNED);

        if (tp->t_inpcb->inp_flags2 & INP2_TIMEWAIT)
                return;

        tcp_set_lotimer_index(tp);

        tcp_sched_timers(tp);
        return;
}

static inline void
tcp_cumulative_stat(u_int32_t cur, u_int32_t *prev, u_int32_t *dest)
{
        /* handle wrap around */
        int32_t diff = (int32_t) (cur - *prev);
        if (diff > 0)
                *dest = diff;
        else
                *dest = 0;
        *prev = cur;
        return;
}

__private_extern__ void
tcp_report_stats(void)
{
        struct nstat_sysinfo_data data;
        struct sockaddr_in dst;
        struct sockaddr_in6 dst6;
        struct rtentry *rt = NULL;
        static struct tcp_last_report_stats prev;
        u_int64_t var, uptime;

#define stat    data.u.tcp_stats
        if (((uptime = net_uptime()) - tcp_last_report_time) <
                tcp_report_stats_interval)
                return;

        tcp_last_report_time = uptime;

        bzero(&data, sizeof(data));
        data.flags = NSTAT_SYSINFO_TCP_STATS;

        bzero(&dst, sizeof(dst));
        dst.sin_len = sizeof(dst);
        dst.sin_family = AF_INET;

        /* ipv4 avg rtt */
        lck_mtx_lock(rnh_lock);
        rt =  rt_lookup(TRUE, (struct sockaddr *)&dst, NULL,
                rt_tables[AF_INET], IFSCOPE_NONE);
        lck_mtx_unlock(rnh_lock);
        if (rt != NULL) {
                RT_LOCK(rt);
                if (rt_primary_default(rt, rt_key(rt)) &&
                        rt->rt_stats != NULL) {
                        stat.ipv4_avgrtt = rt->rt_stats->nstat_avg_rtt;
                }
                RT_UNLOCK(rt);
                rtfree(rt);
                rt = NULL;
        }

        /* ipv6 avg rtt */
        bzero(&dst6, sizeof(dst6));
        dst6.sin6_len = sizeof(dst6);
        dst6.sin6_family = AF_INET6;

        lck_mtx_lock(rnh_lock);
        rt = rt_lookup(TRUE,(struct sockaddr *)&dst6, NULL,
                rt_tables[AF_INET6], IFSCOPE_NONE);
        lck_mtx_unlock(rnh_lock);
        if (rt != NULL) {
                RT_LOCK(rt);
                if (rt_primary_default(rt, rt_key(rt)) &&
                        rt->rt_stats != NULL) {
                        stat.ipv6_avgrtt = rt->rt_stats->nstat_avg_rtt;
                }
                RT_UNLOCK(rt);
                rtfree(rt);
                rt = NULL;
        }

        /* send packet loss rate, shift by 10 for precision */
        if (tcpstat.tcps_sndpack > 0 && tcpstat.tcps_sndrexmitpack > 0) {
                var = tcpstat.tcps_sndrexmitpack << 10;
                stat.send_plr = (var * 100) / tcpstat.tcps_sndpack;
        }

        /* recv packet loss rate, shift by 10 for precision */
        if (tcpstat.tcps_rcvpack > 0 && tcpstat.tcps_recovered_pkts > 0) {
                var = tcpstat.tcps_recovered_pkts << 10;
                stat.recv_plr = (var * 100) / tcpstat.tcps_rcvpack;
        }

        /* RTO after tail loss, shift by 10 for precision */
        if (tcpstat.tcps_sndrexmitpack > 0
            && tcpstat.tcps_tailloss_rto > 0) {
                var = tcpstat.tcps_tailloss_rto << 10;
                stat.send_tlrto_rate =
                        (var * 100) / tcpstat.tcps_sndrexmitpack;
        }

        /* packet reordering */
        if (tcpstat.tcps_sndpack > 0 && tcpstat.tcps_reordered_pkts > 0) {
                var = tcpstat.tcps_reordered_pkts << 10;
                stat.send_reorder_rate =
                        (var * 100) / tcpstat.tcps_sndpack;
        }

        if (tcp_ecn_outbound == 1)
                stat.ecn_client_enabled = 1;
        if (tcp_ecn_inbound == 1)
                stat.ecn_server_enabled = 1;
        tcp_cumulative_stat(tcpstat.tcps_connattempt,
            &prev.tcps_connattempt, &stat.connection_attempts);
        tcp_cumulative_stat(tcpstat.tcps_accepts,
            &prev.tcps_accepts, &stat.connection_accepts);
        tcp_cumulative_stat(tcpstat.tcps_ecn_client_setup,
            &prev.tcps_ecn_client_setup, &stat.ecn_client_setup);
        tcp_cumulative_stat(tcpstat.tcps_ecn_server_setup,
            &prev.tcps_ecn_server_setup, &stat.ecn_server_setup);
        tcp_cumulative_stat(tcpstat.tcps_ecn_client_success,
            &prev.tcps_ecn_client_success, &stat.ecn_client_success);
        tcp_cumulative_stat(tcpstat.tcps_ecn_server_success,
            &prev.tcps_ecn_server_success, &stat.ecn_server_success);
        tcp_cumulative_stat(tcpstat.tcps_ecn_not_supported,
            &prev.tcps_ecn_not_supported, &stat.ecn_not_supported);
        tcp_cumulative_stat(tcpstat.tcps_ecn_lost_syn,
            &prev.tcps_ecn_lost_syn, &stat.ecn_lost_syn);
        tcp_cumulative_stat(tcpstat.tcps_ecn_lost_synack,
            &prev.tcps_ecn_lost_synack, &stat.ecn_lost_synack);
        tcp_cumulative_stat(tcpstat.tcps_ecn_recv_ce,
            &prev.tcps_ecn_recv_ce, &stat.ecn_recv_ce);
        tcp_cumulative_stat(tcpstat.tcps_ecn_recv_ece,
            &prev.tcps_ecn_recv_ece, &stat.ecn_recv_ece);
        tcp_cumulative_stat(tcpstat.tcps_ecn_recv_ece,
            &prev.tcps_ecn_recv_ece, &stat.ecn_recv_ece);
        tcp_cumulative_stat(tcpstat.tcps_ecn_sent_ece,
            &prev.tcps_ecn_sent_ece, &stat.ecn_sent_ece);
        tcp_cumulative_stat(tcpstat.tcps_ecn_sent_ece,
            &prev.tcps_ecn_sent_ece, &stat.ecn_sent_ece);
        tcp_cumulative_stat(tcpstat.tcps_ecn_conn_recv_ce,
            &prev.tcps_ecn_conn_recv_ce, &stat.ecn_conn_recv_ce);
        tcp_cumulative_stat(tcpstat.tcps_ecn_conn_recv_ece,
            &prev.tcps_ecn_conn_recv_ece, &stat.ecn_conn_recv_ece);
        tcp_cumulative_stat(tcpstat.tcps_ecn_conn_plnoce,
            &prev.tcps_ecn_conn_plnoce, &stat.ecn_conn_plnoce);
        tcp_cumulative_stat(tcpstat.tcps_ecn_conn_pl_ce,
            &prev.tcps_ecn_conn_pl_ce, &stat.ecn_conn_pl_ce);
        tcp_cumulative_stat(tcpstat.tcps_ecn_conn_nopl_ce,
            &prev.tcps_ecn_conn_nopl_ce, &stat.ecn_conn_nopl_ce);
        tcp_cumulative_stat(tcpstat.tcps_ecn_fallback_synloss,
            &prev.tcps_ecn_fallback_synloss, &stat.ecn_fallback_synloss);
        tcp_cumulative_stat(tcpstat.tcps_ecn_fallback_reorder,
            &prev.tcps_ecn_fallback_reorder, &stat.ecn_fallback_reorder);
        tcp_cumulative_stat(tcpstat.tcps_ecn_fallback_ce,
            &prev.tcps_ecn_fallback_ce, &stat.ecn_fallback_ce);
        tcp_cumulative_stat(tcpstat.tcps_tfo_syn_data_rcv,
            &prev.tcps_tfo_syn_data_rcv, &stat.tfo_syn_data_rcv);
        tcp_cumulative_stat(tcpstat.tcps_tfo_cookie_req_rcv,
            &prev.tcps_tfo_cookie_req_rcv, &stat.tfo_cookie_req_rcv);
        tcp_cumulative_stat(tcpstat.tcps_tfo_cookie_sent,
            &prev.tcps_tfo_cookie_sent, &stat.tfo_cookie_sent);
        tcp_cumulative_stat(tcpstat.tcps_tfo_cookie_invalid,
            &prev.tcps_tfo_cookie_invalid, &stat.tfo_cookie_invalid);
        tcp_cumulative_stat(tcpstat.tcps_tfo_cookie_req,
            &prev.tcps_tfo_cookie_req, &stat.tfo_cookie_req);
        tcp_cumulative_stat(tcpstat.tcps_tfo_cookie_rcv,
            &prev.tcps_tfo_cookie_rcv, &stat.tfo_cookie_rcv);
        tcp_cumulative_stat(tcpstat.tcps_tfo_syn_data_sent,
            &prev.tcps_tfo_syn_data_sent, &stat.tfo_syn_data_sent);
        tcp_cumulative_stat(tcpstat.tcps_tfo_syn_data_acked,
            &prev.tcps_tfo_syn_data_acked, &stat.tfo_syn_data_acked);
        tcp_cumulative_stat(tcpstat.tcps_tfo_syn_loss,
            &prev.tcps_tfo_syn_loss, &stat.tfo_syn_loss);
        tcp_cumulative_stat(tcpstat.tcps_tfo_blackhole,
            &prev.tcps_tfo_blackhole, &stat.tfo_blackhole);
        tcp_cumulative_stat(tcpstat.tcps_tfo_cookie_wrong,
            &prev.tcps_tfo_cookie_wrong, &stat.tfo_cookie_wrong);
        tcp_cumulative_stat(tcpstat.tcps_tfo_no_cookie_rcv,
            &prev.tcps_tfo_no_cookie_rcv, &stat.tfo_no_cookie_rcv);
        tcp_cumulative_stat(tcpstat.tcps_tfo_heuristics_disable,
            &prev.tcps_tfo_heuristics_disable, &stat.tfo_heuristics_disable);
        tcp_cumulative_stat(tcpstat.tcps_tfo_sndblackhole,
            &prev.tcps_tfo_sndblackhole, &stat.tfo_sndblackhole);




        nstat_sysinfo_send_data(&data);

#undef  stat
}

void
tcp_interface_send_probe(u_int16_t probe_if_index)
{
        int32_t offset = 0;
        struct tcptimerlist *listp = &tcp_timer_list;

        /* Make sure TCP clock is up to date */
        calculate_tcp_clock();

        lck_mtx_lock(listp->mtx);
        if (listp->probe_if_index > 0) {
                tcpstat.tcps_probe_if_conflict++;
                goto done;
        }

        listp->probe_if_index = probe_if_index;
        if (listp->running)
                goto done;

        /*
         * Reschedule the timerlist to run within the next 10ms, which is
         * the fastest that we can do.
         */
        offset = TCP_TIMER_10MS_QUANTUM;
        if (listp->scheduled) {
                int32_t diff;
                diff = timer_diff(listp->runtime, 0, tcp_now, offset);
                if (diff <= 0) {
                        /* The timer will fire sooner than what's needed */
                        goto done;
                }
        }
        listp->mode = TCP_TIMERLIST_10MS_MODE;
        listp->idleruns = 0;

        tcp_sched_timerlist(offset);

done:
        lck_mtx_unlock(listp->mtx);
        return;
}

/*
 * Enable read probes on this connection, if:
 * - it is in established state
 * - doesn't have any data outstanding
 * - the outgoing ifp matches
 * - we have not already sent any read probes
 */
static void
tcp_enable_read_probe(struct tcpcb *tp, struct ifnet *ifp)
{
        if (tp->t_state == TCPS_ESTABLISHED &&
            tp->snd_max == tp->snd_una &&
            tp->t_inpcb->inp_last_outifp == ifp &&
            !(tp->t_flagsext & TF_DETECT_READSTALL) &&
            tp->t_rtimo_probes == 0) {
                tp->t_flagsext |= TF_DETECT_READSTALL;
                tp->t_rtimo_probes = 0;
                tp->t_timer[TCPT_KEEP] = OFFSET_FROM_START(tp,
                    TCP_TIMER_10MS_QUANTUM);
                if (tp->tentry.index == TCPT_NONE) {
                        tp->tentry.index = TCPT_KEEP;
                        tp->tentry.runtime = tcp_now +
                            TCP_TIMER_10MS_QUANTUM;
                } else {
                        int32_t diff = 0;

                        /* Reset runtime to be in next 10ms */
                        diff = timer_diff(tp->tentry.runtime, 0,
                            tcp_now, TCP_TIMER_10MS_QUANTUM);
                        if (diff > 0) {
                                tp->tentry.index = TCPT_KEEP;
                                tp->tentry.runtime = tcp_now +
                                    TCP_TIMER_10MS_QUANTUM;
                                if (tp->tentry.runtime == 0)
                                        tp->tentry.runtime++;
                        }
                }
        }
}

/*
 * Disable read probe and reset the keep alive timer
 */
static void
tcp_disable_read_probe(struct tcpcb *tp)
{
        if (tp->t_adaptive_rtimo == 0 &&
            ((tp->t_flagsext & TF_DETECT_READSTALL) ||
            tp->t_rtimo_probes > 0)) {
                tcp_keepalive_reset(tp);
        }
}

/*
 * Reschedule the tcp timerlist in the next 10ms to re-enable read/write
 * probes on connections going over a particular interface.
 */
void
tcp_probe_connectivity(struct ifnet *ifp, u_int32_t enable)
{
        int32_t offset;
        struct tcptimerlist *listp = &tcp_timer_list;
        struct inpcbinfo *pcbinfo = &tcbinfo;
        struct inpcb *inp, *nxt;

        if (ifp == NULL)
                return;

        /* update clock */
        calculate_tcp_clock();

        /*
         * Enable keep alive timer on all connections that are
         * active/established on this interface.
         */
        lck_rw_lock_shared(pcbinfo->ipi_lock);

        LIST_FOREACH_SAFE(inp, pcbinfo->ipi_listhead, inp_list, nxt) {
                struct tcpcb *tp = NULL;
                if (in_pcb_checkstate(inp, WNT_ACQUIRE, 0) ==
                    WNT_STOPUSING)
                        continue;

                /* Acquire lock to look at the state of the connection */
                tcp_lock(inp->inp_socket, 1, 0);

                /* Release the want count */
                if (inp->inp_ppcb == NULL ||
                    (in_pcb_checkstate(inp, WNT_RELEASE, 1) == WNT_STOPUSING)) {
                        tcp_unlock(inp->inp_socket, 1, 0);
                        continue;
                }
                tp = intotcpcb(inp);
                if (enable)
                        tcp_enable_read_probe(tp, ifp);
                else
                        tcp_disable_read_probe(tp);

                tcp_unlock(inp->inp_socket, 1, 0);
        }
        lck_rw_done(pcbinfo->ipi_lock);

        lck_mtx_lock(listp->mtx);
        if (listp->running) {
                listp->pref_mode |= TCP_TIMERLIST_10MS_MODE;
                goto done;
        }

        /* Reschedule within the next 10ms */
        offset = TCP_TIMER_10MS_QUANTUM;
        if (listp->scheduled) {
                int32_t diff;
                diff = timer_diff(listp->runtime, 0, tcp_now, offset);
                if (diff <= 0) {
                        /* The timer will fire sooner than what's needed */
                        goto done;
                }
        }
        listp->mode = TCP_TIMERLIST_10MS_MODE;
        listp->idleruns = 0;

        tcp_sched_timerlist(offset);
done:
        lck_mtx_unlock(listp->mtx);
        return;
}

inline void
tcp_update_mss_core(struct tcpcb *tp, struct ifnet *ifp)
{
        struct if_cellular_status_v1 *ifsr;
        u_int32_t optlen;
        ifsr = &ifp->if_link_status->ifsr_u.ifsr_cell.if_cell_u.if_status_v1;
        if (ifsr->valid_bitmask & IF_CELL_UL_MSS_RECOMMENDED_VALID) {
                optlen = tp->t_maxopd - tp->t_maxseg;

                if (ifsr->mss_recommended ==
                    IF_CELL_UL_MSS_RECOMMENDED_NONE &&
                    tp->t_cached_maxopd > 0 &&
                    tp->t_maxopd < tp->t_cached_maxopd) {
                        tp->t_maxopd = tp->t_cached_maxopd;
                        tcpstat.tcps_mss_to_default++;
                } else if (ifsr->mss_recommended ==
                    IF_CELL_UL_MSS_RECOMMENDED_MEDIUM &&
                    tp->t_maxopd > tcp_mss_rec_medium) {
                        tp->t_cached_maxopd = tp->t_maxopd;
                        tp->t_maxopd = tcp_mss_rec_medium;
                        tcpstat.tcps_mss_to_medium++;
                } else if (ifsr->mss_recommended ==
                    IF_CELL_UL_MSS_RECOMMENDED_LOW &&
                    tp->t_maxopd > tcp_mss_rec_low) {
                        tp->t_cached_maxopd = tp->t_maxopd;
                        tp->t_maxopd = tcp_mss_rec_low;
                        tcpstat.tcps_mss_to_low++;
                }
                tp->t_maxseg = tp->t_maxopd - optlen;

                /*
                 * clear the cached value if it is same as the current
                 */
                if (tp->t_maxopd == tp->t_cached_maxopd)
                        tp->t_cached_maxopd = 0;
        }
}

void
tcp_update_mss_locked(struct socket *so, struct ifnet *ifp)
{
        struct inpcb *inp = sotoinpcb(so);
        struct tcpcb *tp = intotcpcb(inp);

        if (ifp == NULL && inp->inp_last_outifp == NULL)
                return;

        if (ifp == NULL)
                ifp = inp->inp_last_outifp;

        if (!IFNET_IS_CELLULAR(ifp)) {
                /*
                 * This optimization is implemented for cellular
                 * networks only
                 */
                return;
        }
        if ( tp->t_state <= TCPS_CLOSE_WAIT) {
                /*
                 * If the connection is currently doing or has done PMTU
                 * blackhole detection, do not change the MSS
                 */
                if (tp->t_flags & TF_BLACKHOLE)
                        return;
                if (ifp->if_link_status == NULL)
                        return;
                tcp_update_mss_core(tp, ifp);
        }
}

void
tcp_itimer(struct inpcbinfo *ipi)
{
        struct inpcb *inp, *nxt;

        if (lck_rw_try_lock_exclusive(ipi->ipi_lock) == FALSE) {
                if (tcp_itimer_done == TRUE) {
                        tcp_itimer_done = FALSE;
                        atomic_add_32(&ipi->ipi_timer_req.intimer_fast, 1);
                        return;
                }
                /* Upgrade failed, lost lock now take it again exclusive */
                lck_rw_lock_exclusive(ipi->ipi_lock);
        }
        tcp_itimer_done = TRUE;

        LIST_FOREACH_SAFE(inp, &tcb, inp_list, nxt) {
                struct socket *so;

                if (inp->inp_ppcb == NULL ||
                    in_pcb_checkstate(inp, WNT_ACQUIRE, 0) == WNT_STOPUSING)
                        continue;
                so = inp->inp_socket;
                tcp_lock(so, 1, 0);
                if (in_pcb_checkstate(inp, WNT_RELEASE, 1) == WNT_STOPUSING) {
                        tcp_unlock(so, 1, 0);
                        continue;
                }
                so_check_extended_bk_idle_time(so);
                if (ipi->ipi_flags & INPCBINFO_UPDATE_MSS) {
                        tcp_update_mss_locked(so, NULL);
                }
                tcp_unlock(so, 1, 0);
        }

        ipi->ipi_flags &= ~INPCBINFO_UPDATE_MSS;
        lck_rw_done(ipi->ipi_lock);
}