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

/*
 * Copyright (c) 2000-2020 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, 1994, 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_input.c 8.12 (Berkeley) 5/24/95
 * $FreeBSD: src/sys/netinet/tcp_input.c,v 1.107.2.16 2001/08/22 00:59:12 silby Exp $
 */
/*
 * NOTICE: This file was modified by SPARTA, Inc. in 2005 to introduce
 * support for mandatory and extensible security protections.  This notice
 * is included in support of clause 2.2 (b) of the Apple Public License,
 * Version 2.0.
 */

#include <sys/param.h>
#include <sys/systm.h>
#include <sys/kernel.h>
#include <sys/sysctl.h>
#include <sys/malloc.h>
#include <sys/mbuf.h>
#include <sys/proc.h>           /* for proc0 declaration */
#include <sys/protosw.h>
#include <sys/socket.h>
#include <sys/socketvar.h>
#include <sys/syslog.h>
#include <sys/mcache.h>
#if XNU_TARGET_OS_OSX
#include <sys/kasl.h>
#endif /* XNU_TARGET_OS_OSX */
#include <sys/kauth.h>
#include <kern/cpu_number.h>    /* before tcp_seq.h, for tcp_random18() */

#include <machine/endian.h>

#include <net/if.h>
#include <net/if_types.h>
#include <net/route.h>
#include <net/ntstat.h>
#include <net/content_filter.h>
#include <net/dlil.h>
#include <net/multi_layer_pkt_log.h>

#include <netinet/in.h>
#include <netinet/in_systm.h>
#include <netinet/ip.h>
#include <netinet/ip_icmp.h>    /* for ICMP_BANDLIM             */
#include <netinet/in_var.h>
#include <netinet/icmp_var.h>   /* for ICMP_BANDLIM     */
#include <netinet/in_pcb.h>
#include <netinet/ip_var.h>
#include <mach/sdt.h>
#include <netinet/ip6.h>
#include <netinet/icmp6.h>
#include <netinet6/nd6.h>
#include <netinet6/ip6_var.h>
#include <netinet6/in6_pcb.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>
#include <dev/random/randomdev.h>
#include <kern/zalloc.h>
#include <netinet6/tcp6_var.h>
#include <netinet/tcpip.h>
#if TCPDEBUG
#include <netinet/tcp_debug.h>
u_char tcp_saveipgen[40]; /* the size must be of max ip header, now IPv6 */
struct tcphdr tcp_savetcp;
#endif /* TCPDEBUG */
#include <netinet/tcp_log.h>

#if IPSEC
#include <netinet6/ipsec.h>
#include <netinet6/ipsec6.h>
#include <netkey/key.h>
#endif /*IPSEC*/

#include <sys/kdebug.h>
#if MPTCP
#include <netinet/mptcp_var.h>
#include <netinet/mptcp.h>
#include <netinet/mptcp_opt.h>
#endif /* MPTCP */

#include <corecrypto/ccaes.h>

#define DBG_LAYER_BEG           NETDBG_CODE(DBG_NETTCP, 0)
#define DBG_LAYER_END           NETDBG_CODE(DBG_NETTCP, 2)
#define DBG_FNC_TCP_INPUT       NETDBG_CODE(DBG_NETTCP, (3 << 8))
#define DBG_FNC_TCP_NEWCONN     NETDBG_CODE(DBG_NETTCP, (7 << 8))

#define TCP_RTT_HISTORY_EXPIRE_TIME     (60 * TCP_RETRANSHZ)
#define TCP_RECV_THROTTLE_WIN   (5 * TCP_RETRANSHZ)
#define TCP_STRETCHACK_ENABLE_PKTCNT    2000

struct  tcpstat tcpstat;

SYSCTL_SKMEM_TCP_INT(OID_AUTO, flow_control_response,
    CTLFLAG_RW | CTLFLAG_LOCKED, int, tcp_flow_control_response, 1,
    "Improved response to Flow-control events");

static int log_in_vain = 0;
SYSCTL_INT(_net_inet_tcp, OID_AUTO, log_in_vain,
    CTLFLAG_RW | CTLFLAG_LOCKED, &log_in_vain, 0,
    "Log all incoming TCP connections");

SYSCTL_SKMEM_TCP_INT(OID_AUTO, ack_strategy,
    CTLFLAG_RW | CTLFLAG_LOCKED, int, tcp_ack_strategy, TCP_ACK_STRATEGY_MODERN,
    "Revised TCP ACK-strategy, avoiding stretch-ACK implementation");

static int blackhole = 0;
SYSCTL_INT(_net_inet_tcp, OID_AUTO, blackhole,
    CTLFLAG_RW | CTLFLAG_LOCKED, &blackhole, 0,
    "Do not send RST when dropping refused connections");

SYSCTL_SKMEM_TCP_INT(OID_AUTO, aggressive_rcvwnd_inc,
    CTLFLAG_RW | CTLFLAG_LOCKED, int, tcp_aggressive_rcvwnd_inc, 1,
    "Be more aggressive about increasing the receive-window.");

SYSCTL_SKMEM_TCP_INT(OID_AUTO, delayed_ack,
    CTLFLAG_RW | CTLFLAG_LOCKED, int, tcp_delack_enabled, 3,
    "Delay ACK to try and piggyback it onto a data packet");

SYSCTL_SKMEM_TCP_INT(OID_AUTO, recvbg, CTLFLAG_RW | CTLFLAG_LOCKED,
    int, tcp_recv_bg, 0, "Receive background");

SYSCTL_SKMEM_TCP_INT(OID_AUTO, drop_synfin,
    CTLFLAG_RW | CTLFLAG_LOCKED, static int, drop_synfin, 1,
    "Drop TCP packets with SYN+FIN set");

SYSCTL_NODE(_net_inet_tcp, OID_AUTO, reass, CTLFLAG_RW | CTLFLAG_LOCKED, 0,
    "TCP Segment Reassembly Queue");

static int tcp_reass_overflows = 0;
SYSCTL_INT(_net_inet_tcp_reass, OID_AUTO, overflows,
    CTLFLAG_RD | CTLFLAG_LOCKED, &tcp_reass_overflows, 0,
    "Global number of TCP Segment Reassembly Queue Overflows");


SYSCTL_SKMEM_TCP_INT(OID_AUTO, slowlink_wsize, CTLFLAG_RW | CTLFLAG_LOCKED,
    __private_extern__ int, slowlink_wsize, 8192,
    "Maximum advertised window size for slowlink");

SYSCTL_SKMEM_TCP_INT(OID_AUTO, maxseg_unacked,
    CTLFLAG_RW | CTLFLAG_LOCKED, int, maxseg_unacked, 8,
    "Maximum number of outstanding segments left unacked");

SYSCTL_SKMEM_TCP_INT(OID_AUTO, rfc3465, CTLFLAG_RW | CTLFLAG_LOCKED,
    int, tcp_do_rfc3465, 1, "");

SYSCTL_SKMEM_TCP_INT(OID_AUTO, rfc3465_lim2,
    CTLFLAG_RW | CTLFLAG_LOCKED, int, tcp_do_rfc3465_lim2, 1,
    "Appropriate bytes counting w/ L=2*SMSS");

int rtt_samples_per_slot = 20;

int tcp_acc_iaj_high_thresh = ACC_IAJ_HIGH_THRESH;
u_int32_t tcp_autorcvbuf_inc_shift = 3;
SYSCTL_SKMEM_TCP_INT(OID_AUTO, recv_allowed_iaj,
    CTLFLAG_RW | CTLFLAG_LOCKED, int, tcp_allowed_iaj, ALLOWED_IAJ,
    "Allowed inter-packet arrival jiter");

SYSCTL_SKMEM_TCP_INT(OID_AUTO, doautorcvbuf,
    CTLFLAG_RW | CTLFLAG_LOCKED, u_int32_t, tcp_do_autorcvbuf, 1,
    "Enable automatic socket buffer tuning");

SYSCTL_SKMEM_TCP_INT(OID_AUTO, autotunereorder,
    CTLFLAG_RW | CTLFLAG_LOCKED, u_int32_t, tcp_autotune_reorder, 1,
    "Enable automatic socket buffer tuning even when reordering is present");

SYSCTL_SKMEM_TCP_INT(OID_AUTO, autorcvbufmax,
    CTLFLAG_RW | CTLFLAG_LOCKED, u_int32_t, tcp_autorcvbuf_max, 2 * 1024 * 1024,
    "Maximum receive socket buffer size");

int tcp_disable_access_to_stats = 1;
SYSCTL_INT(_net_inet_tcp, OID_AUTO, disable_access_to_stats,
    CTLFLAG_RW | CTLFLAG_LOCKED, &tcp_disable_access_to_stats, 0,
    "Disable access to tcpstat");

SYSCTL_SKMEM_TCP_INT(OID_AUTO, challengeack_limit,
    CTLFLAG_RW | CTLFLAG_LOCKED, uint32_t, tcp_challengeack_limit, 10,
    "Maximum number of challenge ACKs per connection per second");

/* TO BE REMOVED */
SYSCTL_SKMEM_TCP_INT(OID_AUTO, do_rfc5961,
    CTLFLAG_RW | CTLFLAG_LOCKED, static int, tcp_do_rfc5961, 1,
    "Enable/Disable full RFC 5961 compliance");

SYSCTL_SKMEM_TCP_INT(OID_AUTO, do_better_lr,
    CTLFLAG_RW | CTLFLAG_LOCKED, int, tcp_do_better_lr, 1,
    "Improved TCP Loss Recovery");

extern int tcp_acc_iaj_high;
extern int tcp_acc_iaj_react_limit;

int tcprexmtthresh = 3;

u_int32_t tcp_now;
struct timeval tcp_uptime;      /* uptime when tcp_now was last updated */
lck_spin_t *tcp_uptime_lock;    /* Used to sychronize updates to tcp_now */

struct inpcbhead tcb;
#define tcb6    tcb  /* for KAME src sync over BSD*'s */
struct inpcbinfo tcbinfo;

static void tcp_dooptions(struct tcpcb *, u_char *, int, struct tcphdr *,
    struct tcpopt *);
static void tcp_finalize_options(struct tcpcb *, struct tcpopt *, unsigned int);
static void tcp_pulloutofband(struct socket *,
    struct tcphdr *, struct mbuf *, int);
static void tcp_xmit_timer(struct tcpcb *, int, u_int32_t, tcp_seq);
static inline unsigned int tcp_maxmtu(struct rtentry *);
static inline int tcp_stretch_ack_enable(struct tcpcb *tp, int thflags);
static inline void tcp_adaptive_rwtimo_check(struct tcpcb *, int);

#if TRAFFIC_MGT
static inline void update_iaj_state(struct tcpcb *tp, uint32_t tlen,
    int reset_size);
static inline void compute_iaj(struct tcpcb *tp);
static inline void compute_iaj_meat(struct tcpcb *tp, uint32_t cur_iaj);
#endif /* TRAFFIC_MGT */

static inline unsigned int tcp_maxmtu6(struct rtentry *);
unsigned int get_maxmtu(struct rtentry *);

static void tcp_sbrcv_grow(struct tcpcb *tp, struct sockbuf *sb,
    struct tcpopt *to, uint32_t tlen);
void tcp_sbrcv_trim(struct tcpcb *tp, struct sockbuf *sb);
static void tcp_sbsnd_trim(struct sockbuf *sbsnd);
static inline void tcp_sbrcv_tstmp_check(struct tcpcb *tp);
static inline void tcp_sbrcv_reserve(struct tcpcb *tp, struct sockbuf *sb,
    u_int32_t newsize, u_int32_t idealsize, u_int32_t rcvbuf_max);
static void tcp_bad_rexmt_restore_state(struct tcpcb *tp, struct tcphdr *th);
static void tcp_compute_rtt(struct tcpcb *tp, struct tcpopt *to,
    struct tcphdr *th);
static void tcp_early_rexmt_check(struct tcpcb *tp, struct tcphdr *th);
static void tcp_bad_rexmt_check(struct tcpcb *tp, struct tcphdr *th,
    struct tcpopt *to);
/*
 * Constants used for resizing receive socket buffer
 * when timestamps are not supported
 */
#define TCPTV_RCVNOTS_QUANTUM 100
#define TCP_RCVNOTS_BYTELEVEL 204800

/*
 * Constants used for limiting early retransmits
 * to 10 per minute.
 */
#define TCP_EARLY_REXMT_WIN (60 * TCP_RETRANSHZ) /* 60 seconds */
#define TCP_EARLY_REXMT_LIMIT 10

#define log_in_vain_log( a ) { log a; }

int tcp_rcvunackwin = TCPTV_UNACKWIN;
int tcp_maxrcvidle = TCPTV_MAXRCVIDLE;
SYSCTL_SKMEM_TCP_INT(OID_AUTO, rcvsspktcnt, CTLFLAG_RW | CTLFLAG_LOCKED,
    int, tcp_rcvsspktcnt, TCP_RCV_SS_PKTCOUNT, "packets to be seen before receiver stretches acks");

#define DELAY_ACK(tp, th) \
        (CC_ALGO(tp)->delay_ack != NULL && CC_ALGO(tp)->delay_ack(tp, th))

static int tcp_dropdropablreq(struct socket *head);
static void tcp_newreno_partial_ack(struct tcpcb *tp, struct tcphdr *th);
static void update_base_rtt(struct tcpcb *tp, uint32_t rtt);
void tcp_set_background_cc(struct socket *so);
void tcp_set_foreground_cc(struct socket *so);
static void tcp_set_new_cc(struct socket *so, uint16_t cc_index);
static void tcp_bwmeas_check(struct tcpcb *tp);

#if TRAFFIC_MGT
void
reset_acc_iaj(struct tcpcb *tp)
{
        tp->acc_iaj = 0;
        CLEAR_IAJ_STATE(tp);
}

static inline void
update_iaj_state(struct tcpcb *tp, uint32_t size, int rst_size)
{
        if (rst_size > 0) {
                tp->iaj_size = 0;
        }
        if (tp->iaj_size == 0 || size >= tp->iaj_size) {
                tp->iaj_size = size;
                tp->iaj_rcv_ts = tcp_now;
                tp->iaj_small_pkt = 0;
        }
}

/* For every 32 bit unsigned integer(v), this function will find the
 * largest integer n such that (n*n <= v). This takes at most 16 iterations
 * irrespective of the value of v and does not involve multiplications.
 */
static inline int
isqrt(unsigned int val)
{
        unsigned int sqrt_cache[11] = {0, 1, 4, 9, 16, 25, 36, 49, 64, 81, 100};
        unsigned int temp, g = 0, b = 0x8000, bshft = 15;
        if (val <= 100) {
                for (g = 0; g <= 10; ++g) {
                        if (sqrt_cache[g] > val) {
                                g--;
                                break;
                        } else if (sqrt_cache[g] == val) {
                                break;
                        }
                }
        } else {
                do {
                        temp = (((g << 1) + b) << (bshft--));
                        if (val >= temp) {
                                g += b;
                                val -= temp;
                        }
                        b >>= 1;
                } while (b > 0 && val > 0);
        }
        return g;
}

static inline void
compute_iaj(struct tcpcb *tp)
{
        compute_iaj_meat(tp, (tcp_now - tp->iaj_rcv_ts));
}

static inline void
compute_iaj_meat(struct tcpcb *tp, uint32_t cur_iaj)
{
        /* When accumulated IAJ reaches MAX_ACC_IAJ in milliseconds,
         * throttle the receive window to a minimum of MIN_IAJ_WIN packets
         */
#define MAX_ACC_IAJ (tcp_acc_iaj_high_thresh + tcp_acc_iaj_react_limit)
#define IAJ_DIV_SHIFT 4
#define IAJ_ROUNDUP_CONST (1 << (IAJ_DIV_SHIFT - 1))

        uint32_t allowed_iaj, acc_iaj = 0;

        uint32_t mean, temp;
        int32_t cur_iaj_dev;

        cur_iaj_dev = (cur_iaj - tp->avg_iaj);

        /* Allow a jitter of "allowed_iaj" milliseconds. Some connections
         * may have a constant jitter more than that. We detect this by
         * using standard deviation.
         */
        allowed_iaj = tp->avg_iaj + tp->std_dev_iaj;
        if (allowed_iaj < tcp_allowed_iaj) {
                allowed_iaj = tcp_allowed_iaj;
        }

        /* Initially when the connection starts, the senders congestion
         * window is small. During this period we avoid throttling a
         * connection because we do not have a good starting point for
         * allowed_iaj. IAJ_IGNORE_PKTCNT is used to quietly gloss over
         * the first few packets.
         */
        if (tp->iaj_pktcnt > IAJ_IGNORE_PKTCNT) {
                if (cur_iaj <= allowed_iaj) {
                        if (tp->acc_iaj >= 2) {
                                acc_iaj = tp->acc_iaj - 2;
                        } else {
                                acc_iaj = 0;
                        }
                } else {
                        acc_iaj = tp->acc_iaj + (cur_iaj - allowed_iaj);
                }

                if (acc_iaj > MAX_ACC_IAJ) {
                        acc_iaj = MAX_ACC_IAJ;
                }
                tp->acc_iaj = acc_iaj;
        }

        /* Compute weighted average where the history has a weight of
         * 15 out of 16 and the current value has a weight of 1 out of 16.
         * This will make the short-term measurements have more weight.
         *
         * The addition of 8 will help to round-up the value
         * instead of round-down
         */
        tp->avg_iaj = (((tp->avg_iaj << IAJ_DIV_SHIFT) - tp->avg_iaj)
            + cur_iaj + IAJ_ROUNDUP_CONST) >> IAJ_DIV_SHIFT;

        /* Compute Root-mean-square of deviation where mean is a weighted
         * average as described above.
         */
        temp = tp->std_dev_iaj * tp->std_dev_iaj;
        mean = (((temp << IAJ_DIV_SHIFT) - temp)
            + (cur_iaj_dev * cur_iaj_dev)
            + IAJ_ROUNDUP_CONST) >> IAJ_DIV_SHIFT;

        tp->std_dev_iaj = isqrt(mean);

        DTRACE_TCP3(iaj, struct tcpcb *, tp, uint32_t, cur_iaj,
            uint32_t, allowed_iaj);

        return;
}
#endif /* TRAFFIC_MGT */

/*
 * Perform rate limit check per connection per second
 * tp->t_challengeack_last is the last_time diff was greater than 1sec
 * tp->t_challengeack_count is the number of ACKs sent (within 1sec)
 * Return TRUE if we shouldn't send the ACK due to rate limitation
 * Return FALSE if it is still ok to send challenge ACK
 */
static boolean_t
tcp_is_ack_ratelimited(struct tcpcb *tp)
{
        boolean_t ret = TRUE;
        uint32_t now = tcp_now;
        int32_t diff = 0;

        diff = timer_diff(now, 0, tp->t_challengeack_last, 0);
        /* If it is first time or diff > 1000ms,
         * update the challengeack_last and reset the
         * current count of ACKs
         */
        if (tp->t_challengeack_last == 0 || diff >= 1000) {
                tp->t_challengeack_last = now;
                tp->t_challengeack_count = 0;
                ret = FALSE;
        } else if (tp->t_challengeack_count < tcp_challengeack_limit) {
                ret = FALSE;
        }

        /* Careful about wrap-around */
        if (ret == FALSE && (tp->t_challengeack_count + 1 > 0)) {
                tp->t_challengeack_count++;
        }

        return ret;
}

/* Check if enough amount of data has been acknowledged since
 * bw measurement was started
 */
static void
tcp_bwmeas_check(struct tcpcb *tp)
{
        int32_t bw_meas_bytes;
        uint32_t bw, bytes, elapsed_time;

        if (SEQ_LEQ(tp->snd_una, tp->t_bwmeas->bw_start)) {
                return;
        }

        bw_meas_bytes = tp->snd_una - tp->t_bwmeas->bw_start;
        if ((tp->t_flagsext & TF_BWMEAS_INPROGRESS) &&
            bw_meas_bytes >= (int32_t)(tp->t_bwmeas->bw_size)) {
                bytes = bw_meas_bytes;
                elapsed_time = tcp_now - tp->t_bwmeas->bw_ts;
                if (elapsed_time > 0) {
                        bw = bytes / elapsed_time;
                        if (bw > 0) {
                                if (tp->t_bwmeas->bw_sndbw > 0) {
                                        tp->t_bwmeas->bw_sndbw =
                                            (((tp->t_bwmeas->bw_sndbw << 3)
                                            - tp->t_bwmeas->bw_sndbw)
                                            + bw) >> 3;
                                } else {
                                        tp->t_bwmeas->bw_sndbw = bw;
                                }

                                /* Store the maximum value */
                                if (tp->t_bwmeas->bw_sndbw_max == 0) {
                                        tp->t_bwmeas->bw_sndbw_max =
                                            tp->t_bwmeas->bw_sndbw;
                                } else {
                                        tp->t_bwmeas->bw_sndbw_max =
                                            max(tp->t_bwmeas->bw_sndbw,
                                            tp->t_bwmeas->bw_sndbw_max);
                                }
                        }
                }
                tp->t_flagsext &= ~(TF_BWMEAS_INPROGRESS);
        }
}

static int
tcp_reass(struct tcpcb *tp, struct tcphdr *th, int *tlenp, struct mbuf *m,
    struct ifnet *ifp, int *dowakeup)
{
        struct tseg_qent *q;
        struct tseg_qent *p = NULL;
        struct tseg_qent *nq;
        struct tseg_qent *te = NULL;
        struct inpcb *inp = tp->t_inpcb;
        struct socket *so = inp->inp_socket;
        int flags = 0;
        u_int16_t qlimit;
        boolean_t cell = IFNET_IS_CELLULAR(ifp);
        boolean_t wifi = (!cell && IFNET_IS_WIFI(ifp));
        boolean_t wired = (!wifi && IFNET_IS_WIRED(ifp));
        boolean_t dsack_set = FALSE;

        /*
         * Call with th==0 after become established to
         * force pre-ESTABLISHED data up to user socket.
         */
        if (th == NULL) {
                goto present;
        }

        /*
         * If the reassembly queue already has entries or if we are going
         * to add a new one, then the connection has reached a loss state.
         * Reset the stretch-ack algorithm at this point.
         */
        tcp_reset_stretch_ack(tp);
        tp->t_forced_acks = TCP_FORCED_ACKS_COUNT;

#if TRAFFIC_MGT
        if (tp->acc_iaj > 0) {
                reset_acc_iaj(tp);
        }
#endif /* TRAFFIC_MGT */

        /*
         * Limit the number of segments in the reassembly queue to prevent
         * holding on to too many segments (and thus running out of mbufs).
         * Make sure to let the missing segment through which caused this
         * queue.  Always keep one global queue entry spare to be able to
         * process the missing segment.
         */
        qlimit = min(max(100, so->so_rcv.sb_hiwat >> 10),
            (tcp_autorcvbuf_max >> 10));
        if (th->th_seq != tp->rcv_nxt &&
            (tp->t_reassqlen + 1) >= qlimit) {
                tcp_reass_overflows++;
                tcpstat.tcps_rcvmemdrop++;
                m_freem(m);
                *tlenp = 0;
                return 0;
        }

        /* Allocate a new queue entry. If we can't, just drop the pkt. XXX */
        te = (struct tseg_qent *) zalloc(tcp_reass_zone);
        if (te == NULL) {
                tcpstat.tcps_rcvmemdrop++;
                m_freem(m);
                return 0;
        }
        tp->t_reassqlen++;

        /*
         * Find a segment which begins after this one does.
         */
        LIST_FOREACH(q, &tp->t_segq, tqe_q) {
                if (SEQ_GT(q->tqe_th->th_seq, th->th_seq)) {
                        break;
                }
                p = q;
        }

        /*
         * If there is a preceding segment, it may provide some of
         * our data already.  If so, drop the data from the incoming
         * segment.  If it provides all of our data, drop us.
         */
        if (p != NULL) {
                int i;
                /* conversion to int (in i) handles seq wraparound */
                i = p->tqe_th->th_seq + p->tqe_len - th->th_seq;
                if (i > 0) {
                        if (i > 1) {
                                /*
                                 * Note duplicate data sequnce numbers
                                 * to report in DSACK option
                                 */
                                tp->t_dsack_lseq = th->th_seq;
                                tp->t_dsack_rseq = th->th_seq +
                                    min(i, *tlenp);

                                /*
                                 * Report only the first part of partial/
                                 * non-contiguous duplicate sequence space
                                 */
                                dsack_set = TRUE;
                        }
                        if (i >= *tlenp) {
                                tcpstat.tcps_rcvduppack++;
                                tcpstat.tcps_rcvdupbyte += *tlenp;
                                if (nstat_collect) {
                                        nstat_route_rx(inp->inp_route.ro_rt,
                                            1, *tlenp,
                                            NSTAT_RX_FLAG_DUPLICATE);
                                        INP_ADD_STAT(inp, cell, wifi, wired,
                                            rxpackets, 1);
                                        INP_ADD_STAT(inp, cell, wifi, wired,
                                            rxbytes, *tlenp);
                                        tp->t_stat.rxduplicatebytes += *tlenp;
                                        inp_set_activity_bitmap(inp);
                                }
                                m_freem(m);
                                zfree(tcp_reass_zone, te);
                                te = NULL;
                                tp->t_reassqlen--;
                                /*
                                 * Try to present any queued data
                                 * at the left window edge to the user.
                                 * This is needed after the 3-WHS
                                 * completes.
                                 */
                                goto present;
                        }
                        m_adj(m, i);
                        *tlenp -= i;
                        th->th_seq += i;
                }
        }
        tp->t_rcvoopack++;
        tcpstat.tcps_rcvoopack++;
        tcpstat.tcps_rcvoobyte += *tlenp;
        if (nstat_collect) {
                nstat_route_rx(inp->inp_route.ro_rt, 1, *tlenp,
                    NSTAT_RX_FLAG_OUT_OF_ORDER);
                INP_ADD_STAT(inp, cell, wifi, wired, rxpackets, 1);
                INP_ADD_STAT(inp, cell, wifi, wired, rxbytes, *tlenp);
                tp->t_stat.rxoutoforderbytes += *tlenp;
                inp_set_activity_bitmap(inp);
        }

        /*
         * While we overlap succeeding segments trim them or,
         * if they are completely covered, dequeue them.
         */
        while (q) {
                int i = (th->th_seq + *tlenp) - q->tqe_th->th_seq;
                if (i <= 0) {
                        break;
                }

                /*
                 * Report only the first part of partial/non-contiguous
                 * duplicate segment in dsack option. The variable
                 * dsack_set will be true if a previous entry has some of
                 * the duplicate sequence space.
                 */
                if (i > 1 && !dsack_set) {
                        if (tp->t_dsack_lseq == 0) {
                                tp->t_dsack_lseq = q->tqe_th->th_seq;
                                tp->t_dsack_rseq =
                                    tp->t_dsack_lseq + min(i, q->tqe_len);
                        } else {
                                /*
                                 * this segment overlaps data in multple
                                 * entries in the reassembly queue, move
                                 * the right sequence number further.
                                 */
                                tp->t_dsack_rseq =
                                    tp->t_dsack_rseq + min(i, q->tqe_len);
                        }
                }
                if (i < q->tqe_len) {
                        q->tqe_th->th_seq += i;
                        q->tqe_len -= i;
                        m_adj(q->tqe_m, i);
                        break;
                }

                nq = LIST_NEXT(q, tqe_q);
                LIST_REMOVE(q, tqe_q);
                m_freem(q->tqe_m);
                zfree(tcp_reass_zone, q);
                tp->t_reassqlen--;
                q = nq;
        }

        /* Insert the new segment queue entry into place. */
        te->tqe_m = m;
        te->tqe_th = th;
        te->tqe_len = *tlenp;

        if (p == NULL) {
                LIST_INSERT_HEAD(&tp->t_segq, te, tqe_q);
        } else {
                LIST_INSERT_AFTER(p, te, tqe_q);
        }

present:
        /*
         * Present data to user, advancing rcv_nxt through
         * completed sequence space.
         */
        if (!TCPS_HAVEESTABLISHED(tp->t_state)) {
                return 0;
        }
        q = LIST_FIRST(&tp->t_segq);
        if (!q || q->tqe_th->th_seq != tp->rcv_nxt) {
                return 0;
        }

        /*
         * If there is already another thread doing reassembly for this
         * connection, it is better to let it finish the job --
         * (radar 16316196)
         */
        if (tp->t_flagsext & TF_REASS_INPROG) {
                return 0;
        }

        tp->t_flagsext |= TF_REASS_INPROG;
        /* lost packet was recovered, so ooo data can be returned */
        tcpstat.tcps_recovered_pkts++;

        do {
                tp->rcv_nxt += q->tqe_len;
                flags = q->tqe_th->th_flags & TH_FIN;
                LIST_REMOVE(q, tqe_q);
                if (so->so_state & SS_CANTRCVMORE) {
                        m_freem(q->tqe_m);
                } else {
                        /*
                         * The mbuf may be freed after it has been added to the
                         * receive socket buffer so we reinitialize th to point
                         * to a safe copy of the TCP header
                         */
                        struct tcphdr saved_tcphdr = {};

                        so_recv_data_stat(so, q->tqe_m, 0); /* XXXX */
                        memcpy(&saved_tcphdr, th, sizeof(struct tcphdr));

                        if (q->tqe_th->th_flags & TH_PUSH) {
                                tp->t_flagsext |= TF_LAST_IS_PSH;
                        } else {
                                tp->t_flagsext &= ~TF_LAST_IS_PSH;
                        }

                        if (sbappendstream_rcvdemux(so, q->tqe_m)) {
                                *dowakeup = 1;
                        }
                        th = &saved_tcphdr;
                }
                zfree(tcp_reass_zone, q);
                tp->t_reassqlen--;
                q = LIST_FIRST(&tp->t_segq);
        } while (q && q->tqe_th->th_seq == tp->rcv_nxt);
        tp->t_flagsext &= ~TF_REASS_INPROG;

        if ((inp->inp_vflag & INP_IPV6) != 0) {
                KERNEL_DEBUG(DBG_LAYER_BEG,
                    ((inp->inp_fport << 16) | inp->inp_lport),
                    (((inp->in6p_laddr.s6_addr16[0] & 0xffff) << 16) |
                    (inp->in6p_faddr.s6_addr16[0] & 0xffff)),
                    0, 0, 0);
        } else {
                KERNEL_DEBUG(DBG_LAYER_BEG,
                    ((inp->inp_fport << 16) | inp->inp_lport),
                    (((inp->inp_laddr.s_addr & 0xffff) << 16) |
                    (inp->inp_faddr.s_addr & 0xffff)),
                    0, 0, 0);
        }

        return flags;
}

/*
 * Reduce congestion window -- used when ECN is seen or when a tail loss
 * probe recovers the last packet.
 */
static void
tcp_reduce_congestion_window(struct tcpcb *tp)
{
        /*
         * If the current tcp cc module has
         * defined a hook for tasks to run
         * before entering FR, call it
         */
        if (CC_ALGO(tp)->pre_fr != NULL) {
                CC_ALGO(tp)->pre_fr(tp);
        }
        ENTER_FASTRECOVERY(tp);
        if (tp->t_flags & TF_SENTFIN) {
                tp->snd_recover = tp->snd_max - 1;
        } else {
                tp->snd_recover = tp->snd_max;
        }
        tp->t_timer[TCPT_REXMT] = 0;
        tp->t_timer[TCPT_PTO] = 0;
        tp->t_rtttime = 0;
        if (tp->t_flagsext & TF_CWND_NONVALIDATED) {
                tcp_cc_adjust_nonvalidated_cwnd(tp);
        } else {
                tp->snd_cwnd = tp->snd_ssthresh +
                    tp->t_maxseg * tcprexmtthresh;
        }
}

/*
 * This function is called upon reception of data on a socket. It's purpose is
 * to handle the adaptive keepalive timers that monitor whether the connection
 * is making progress. First the adaptive read-timer, second the TFO probe-timer.
 *
 * The application wants to get an event if there is a stall during read.
 * Set the initial keepalive timeout to be equal to twice RTO.
 *
 * If the outgoing interface is in marginal conditions, we need to
 * enable read probes for that too.
 */
static inline void
tcp_adaptive_rwtimo_check(struct tcpcb *tp, int tlen)
{
        struct ifnet *outifp = tp->t_inpcb->inp_last_outifp;

        if ((tp->t_adaptive_rtimo > 0 ||
            (outifp != NULL &&
            (outifp->if_eflags & IFEF_PROBE_CONNECTIVITY)))
            && tlen > 0 &&
            tp->t_state == TCPS_ESTABLISHED) {
                tp->t_timer[TCPT_KEEP] = OFFSET_FROM_START(tp,
                    (TCP_REXMTVAL(tp) << 1));
                tp->t_flagsext |= TF_DETECT_READSTALL;
                tp->t_rtimo_probes = 0;
        }
}

inline void
tcp_keepalive_reset(struct tcpcb *tp)
{
        tp->t_timer[TCPT_KEEP] = OFFSET_FROM_START(tp,
            TCP_CONN_KEEPIDLE(tp));
        tp->t_flagsext &= ~(TF_DETECT_READSTALL);
        tp->t_rtimo_probes = 0;
}

/*
 * TCP input routine, follows pages 65-76 of the
 * protocol specification dated September, 1981 very closely.
 */
int
tcp6_input(struct mbuf **mp, int *offp, int proto)
{
#pragma unused(proto)
        struct mbuf *m = *mp;
        uint32_t ia6_flags;
        struct ifnet *ifp = m->m_pkthdr.rcvif;

        IP6_EXTHDR_CHECK(m, *offp, sizeof(struct tcphdr), return IPPROTO_DONE);

        /* Expect 32-bit aligned data pointer on strict-align platforms */
        MBUF_STRICT_DATA_ALIGNMENT_CHECK_32(m);

        /*
         * draft-itojun-ipv6-tcp-to-anycast
         * better place to put this in?
         */
        if (ip6_getdstifaddr_info(m, NULL, &ia6_flags) == 0) {
                if (ia6_flags & IN6_IFF_ANYCAST) {
                        struct ip6_hdr *ip6;

                        ip6 = mtod(m, struct ip6_hdr *);
                        icmp6_error(m, ICMP6_DST_UNREACH,
                            ICMP6_DST_UNREACH_ADDR,
                            (caddr_t)&ip6->ip6_dst - (caddr_t)ip6);

                        IF_TCP_STATINC(ifp, icmp6unreach);

                        return IPPROTO_DONE;
                }
        }

        tcp_input(m, *offp);
        return IPPROTO_DONE;
}

/* Depending on the usage of mbuf space in the system, this function
 * will return true or false. This is used to determine if a socket
 * buffer can take more memory from the system for auto-tuning or not.
 */
u_int8_t
tcp_cansbgrow(struct sockbuf *sb)
{
        /* Calculate the host level space limit in terms of MSIZE buffers.
         * We can use a maximum of half of the available mbuf space for
         * socket buffers.
         */
        u_int32_t mblim = ((nmbclusters >> 1) << (MCLSHIFT - MSIZESHIFT));

        /* Calculate per sb limit in terms of bytes. We optimize this limit
         * for upto 16 socket buffers.
         */

        u_int32_t sbspacelim = ((nmbclusters >> 4) << MCLSHIFT);

        if ((total_sbmb_cnt < mblim) &&
            (sb->sb_hiwat < sbspacelim)) {
                return 1;
        } else {
                OSIncrementAtomic64(&sbmb_limreached);
        }
        return 0;
}

static void
tcp_sbrcv_reserve(struct tcpcb *tp, struct sockbuf *sbrcv,
    u_int32_t newsize, u_int32_t idealsize, u_int32_t rcvbuf_max)
{
        /* newsize should not exceed max */
        newsize = min(newsize, rcvbuf_max);

        /* The receive window scale negotiated at the
         * beginning of the connection will also set a
         * limit on the socket buffer size
         */
        newsize = min(newsize, TCP_MAXWIN << tp->rcv_scale);

        /* Set new socket buffer size */
        if (newsize > sbrcv->sb_hiwat &&
            (sbreserve(sbrcv, newsize) == 1)) {
                sbrcv->sb_idealsize = min(max(sbrcv->sb_idealsize,
                    (idealsize != 0) ? idealsize : newsize), rcvbuf_max);

                /* Again check the limit set by the advertised
                 * window scale
                 */
                sbrcv->sb_idealsize = min(sbrcv->sb_idealsize,
                    TCP_MAXWIN << tp->rcv_scale);
        }
}

/*
 * This function is used to grow  a receive socket buffer. It
 * will take into account system-level memory usage and the
 * bandwidth available on the link to make a decision.
 */
static void
tcp_sbrcv_grow(struct tcpcb *tp, struct sockbuf *sbrcv,
    struct tcpopt *to, uint32_t pktlen)
{
        struct socket *so = sbrcv->sb_so;

        /*
         * Do not grow the receive socket buffer if
         * - auto resizing is disabled, globally or on this socket
         * - the high water mark already reached the maximum
         * - the stream is in background and receive side is being
         * throttled
         */
        if (tcp_do_autorcvbuf == 0 ||
            (sbrcv->sb_flags & SB_AUTOSIZE) == 0 ||
            tcp_cansbgrow(sbrcv) == 0 ||
            sbrcv->sb_hiwat >= tcp_autorcvbuf_max ||
            (tp->t_flagsext & TF_RECV_THROTTLE) ||
            (so->so_flags1 & SOF1_EXTEND_BK_IDLE_WANTED) ||
            (!tcp_autotune_reorder && !LIST_EMPTY(&tp->t_segq))) {
                /* Can not resize the socket buffer, just return */
                goto out;
        }

        if (!TSTMP_SUPPORTED(tp)) {
                /*
                 * Timestamp option is not supported on this connection.
                 * If the connection reached a state to indicate that
                 * the receive socket buffer needs to grow, increase
                 * the high water mark.
                 */
                if (TSTMP_GEQ(tcp_now,
                    tp->rfbuf_ts + TCPTV_RCVNOTS_QUANTUM)) {
                        if (tp->rfbuf_cnt + pktlen >= TCP_RCVNOTS_BYTELEVEL) {
                                tcp_sbrcv_reserve(tp, sbrcv,
                                    tcp_autorcvbuf_max, 0,
                                    tcp_autorcvbuf_max);
                        }
                        goto out;
                } else {
                        tp->rfbuf_cnt += pktlen;
                        return;
                }
        } else if (to->to_tsecr != 0) {
                /*
                 * If the timestamp shows that one RTT has
                 * completed, we can stop counting the
                 * bytes. Here we consider increasing
                 * the socket buffer if the bandwidth measured in
                 * last rtt, is more than half of sb_hiwat, this will
                 * help to scale the buffer according to the bandwidth
                 * on the link.
                 */
                if (TSTMP_GEQ(to->to_tsecr, tp->rfbuf_ts)) {
                        if (tcp_aggressive_rcvwnd_inc) {
                                tp->rfbuf_cnt += pktlen;
                        }

                        if ((tcp_aggressive_rcvwnd_inc == 0 &&
                            tp->rfbuf_cnt + pktlen > (sbrcv->sb_hiwat -
                            (sbrcv->sb_hiwat >> 1))) ||
                            (tcp_aggressive_rcvwnd_inc &&
                            tp->rfbuf_cnt > tp->rfbuf_space)) {
                                int32_t rcvbuf_inc;
                                uint32_t idealsize;

                                if (tcp_aggressive_rcvwnd_inc == 0) {
                                        int32_t min_incr;

                                        tp->rfbuf_cnt += pktlen;
                                        /*
                                         * Increment the receive window by a
                                         * multiple of maximum sized segments.
                                         * This will prevent a connection from
                                         * sending smaller segments on wire if it
                                         * is limited by the receive window.
                                         *
                                         * Set the ideal size based on current
                                         * bandwidth measurements. We set the
                                         * ideal size on receive socket buffer to
                                         * be twice the bandwidth delay product.
                                         */
                                        rcvbuf_inc = (tp->rfbuf_cnt << 1)
                                            - sbrcv->sb_hiwat;

                                        /*
                                         * Make the increment equal to 8 segments
                                         * at least
                                         */
                                        min_incr = tp->t_maxseg << tcp_autorcvbuf_inc_shift;
                                        if (rcvbuf_inc < min_incr) {
                                                rcvbuf_inc = min_incr;
                                        }

                                        idealsize = (tp->rfbuf_cnt << 1);
                                } else {
                                        if (tp->rfbuf_cnt > tp->rfbuf_space + (tp->rfbuf_space >> 1)) {
                                                rcvbuf_inc = (tp->rfbuf_cnt << 2) - sbrcv->sb_hiwat;
                                                idealsize = (tp->rfbuf_cnt << 2);
                                        } else {
                                                rcvbuf_inc = (tp->rfbuf_cnt << 1) - sbrcv->sb_hiwat;
                                                idealsize = (tp->rfbuf_cnt << 1);
                                        }
                                }

                                tp->rfbuf_space = tp->rfbuf_cnt;

                                if (rcvbuf_inc > 0) {
                                        rcvbuf_inc =
                                            (rcvbuf_inc / tp->t_maxseg) * tp->t_maxseg;

                                        tcp_sbrcv_reserve(tp, sbrcv,
                                            sbrcv->sb_hiwat + rcvbuf_inc,
                                            idealsize, tcp_autorcvbuf_max);
                                }
                        }
                        /* Measure instantaneous receive bandwidth */
                        if (tp->t_bwmeas != NULL && tp->rfbuf_cnt > 0 &&
                            TSTMP_GT(tcp_now, tp->rfbuf_ts)) {
                                u_int32_t rcv_bw;
                                rcv_bw = tp->rfbuf_cnt /
                                    (int)(tcp_now - tp->rfbuf_ts);
                                if (tp->t_bwmeas->bw_rcvbw_max == 0) {
                                        tp->t_bwmeas->bw_rcvbw_max = rcv_bw;
                                } else {
                                        tp->t_bwmeas->bw_rcvbw_max = max(
                                                tp->t_bwmeas->bw_rcvbw_max, rcv_bw);
                                }
                        }
                        goto out;
                } else {
                        tp->rfbuf_cnt += pktlen;
                        return;
                }
        }
out:
        /* Restart the measurement */
        tp->rfbuf_ts = tcp_now;
        tp->rfbuf_cnt = 0;
        return;
}

/* This function will trim the excess space added to the socket buffer
 * to help a slow-reading app. The ideal-size of a socket buffer depends
 * on the link bandwidth or it is set by an application and we aim to
 * reach that size.
 */
void
tcp_sbrcv_trim(struct tcpcb *tp, struct sockbuf *sbrcv)
{
        if (tcp_do_autorcvbuf == 1 && sbrcv->sb_idealsize > 0 &&
            sbrcv->sb_hiwat > sbrcv->sb_idealsize) {
                int32_t trim;
                /* compute the difference between ideal and current sizes */
                u_int32_t diff = sbrcv->sb_hiwat - sbrcv->sb_idealsize;

                /* Compute the maximum advertised window for
                 * this connection.
                 */
                u_int32_t advwin = tp->rcv_adv - tp->rcv_nxt;

                /* How much can we trim the receive socket buffer?
                 * 1. it can not be trimmed beyond the max rcv win advertised
                 * 2. if possible, leave 1/16 of bandwidth*delay to
                 * avoid closing the win completely
                 */
                u_int32_t leave = max(advwin, (sbrcv->sb_idealsize >> 4));

                /* Sometimes leave can be zero, in that case leave at least
                 * a few segments worth of space.
                 */
                if (leave == 0) {
                        leave = tp->t_maxseg << tcp_autorcvbuf_inc_shift;
                }

                trim = sbrcv->sb_hiwat - (sbrcv->sb_cc + leave);
                trim = imin(trim, (int32_t)diff);

                if (trim > 0) {
                        sbreserve(sbrcv, (sbrcv->sb_hiwat - trim));
                }
        }
}

/* We may need to trim the send socket buffer size for two reasons:
 * 1. if the rtt seen on the connection is climbing up, we do not
 * want to fill the buffers any more.
 * 2. if the congestion win on the socket backed off, there is no need
 * to hold more mbufs for that connection than what the cwnd will allow.
 */
void
tcp_sbsnd_trim(struct sockbuf *sbsnd)
{
        if (((sbsnd->sb_flags & (SB_AUTOSIZE | SB_TRIM)) ==
            (SB_AUTOSIZE | SB_TRIM)) &&
            (sbsnd->sb_idealsize > 0) &&
            (sbsnd->sb_hiwat > sbsnd->sb_idealsize)) {
                u_int32_t trim = 0;
                if (sbsnd->sb_cc <= sbsnd->sb_idealsize) {
                        trim = sbsnd->sb_hiwat - sbsnd->sb_idealsize;
                } else {
                        trim = sbsnd->sb_hiwat - sbsnd->sb_cc;
                }
                sbreserve(sbsnd, (sbsnd->sb_hiwat - trim));
        }
        if (sbsnd->sb_hiwat <= sbsnd->sb_idealsize) {
                sbsnd->sb_flags &= ~(SB_TRIM);
        }
}

/*
 * If timestamp option was not negotiated on this connection
 * and this connection is on the receiving side of a stream
 * then we can not measure the delay on the link accurately.
 * Instead of enabling automatic receive socket buffer
 * resizing, just give more space to the receive socket buffer.
 */
static inline void
tcp_sbrcv_tstmp_check(struct tcpcb *tp)
{
        struct socket *so = tp->t_inpcb->inp_socket;
        u_int32_t newsize = 2 * tcp_recvspace;
        struct sockbuf *sbrcv = &so->so_rcv;

        if ((tp->t_flags & (TF_REQ_TSTMP | TF_RCVD_TSTMP)) !=
            (TF_REQ_TSTMP | TF_RCVD_TSTMP) &&
            (sbrcv->sb_flags & SB_AUTOSIZE) != 0) {
                tcp_sbrcv_reserve(tp, sbrcv, newsize, 0, newsize);
        }
}

/* A receiver will evaluate the flow of packets on a connection
 * to see if it can reduce ack traffic. The receiver will start
 * stretching acks if all of the following conditions are met:
 * 1. tcp_delack_enabled is set to 3
 * 2. If the bytes received in the last 100ms is greater than a threshold
 *      defined by maxseg_unacked
 * 3. If the connection has not been idle for tcp_maxrcvidle period.
 * 4. If the connection has seen enough packets to let the slow-start
 *      finish after connection establishment or after some packet loss.
 *
 * The receiver will stop stretching acks if there is congestion/reordering
 * as indicated by packets on reassembly queue or an ECN. If the delayed-ack
 * timer fires while stretching acks, it means that the packet flow has gone
 * below the threshold defined by maxseg_unacked and the receiver will stop
 * stretching acks. The receiver gets no indication when slow-start is completed
 * or when the connection reaches an idle state. That is why we use
 * tcp_rcvsspktcnt to cover slow-start and tcp_maxrcvidle to identify idle
 * state.
 */
static inline int
tcp_stretch_ack_enable(struct tcpcb *tp, int thflags)
{
        if (tp->rcv_by_unackwin >= (maxseg_unacked * tp->t_maxseg) &&
            TSTMP_GEQ(tp->rcv_unackwin, tcp_now)) {
                tp->t_flags |= TF_STREAMING_ON;
        } else {
                tp->t_flags &= ~TF_STREAMING_ON;
        }

        /* If there has been an idle time, reset streaming detection */
        if (TSTMP_GT(tcp_now, tp->rcv_unackwin + tcp_maxrcvidle)) {
                tp->t_flags &= ~TF_STREAMING_ON;
        }

        /*
         * If there are flags other than TH_ACK set, reset streaming
         * detection
         */
        if (thflags & ~TH_ACK) {
                tp->t_flags &= ~TF_STREAMING_ON;
        }

        if (tp->t_flagsext & TF_DISABLE_STRETCHACK) {
                if (tp->rcv_nostrack_pkts >= TCP_STRETCHACK_ENABLE_PKTCNT) {
                        tp->t_flagsext &= ~TF_DISABLE_STRETCHACK;
                        tp->rcv_nostrack_pkts = 0;
                        tp->rcv_nostrack_ts = 0;
                } else {
                        tp->rcv_nostrack_pkts++;
                }
        }

        if (!(tp->t_flagsext & (TF_NOSTRETCHACK | TF_DISABLE_STRETCHACK)) &&
            (tp->t_flags & TF_STREAMING_ON) &&
            (!(tp->t_flagsext & TF_RCVUNACK_WAITSS) ||
            (tp->rcv_waitforss >= tcp_rcvsspktcnt))) {
                return 1;
        }

        return 0;
}

/*
 * Reset the state related to stretch-ack algorithm. This will make
 * the receiver generate an ack every other packet. The receiver
 * will start re-evaluating the rate at which packets come to decide
 * if it can benefit by lowering the ack traffic.
 */
void
tcp_reset_stretch_ack(struct tcpcb *tp)
{
        tp->t_flags &= ~(TF_STRETCHACK | TF_STREAMING_ON);
        tp->rcv_by_unackwin = 0;
        tp->rcv_by_unackhalfwin = 0;
        tp->rcv_unackwin = tcp_now + tcp_rcvunackwin;

        /*
         * When there is packet loss or packet re-ordering or CWR due to
         * ECN, the sender's congestion window is reduced. In these states,
         * generate an ack for every other packet for some time to allow
         * the sender's congestion window to grow.
         */
        tp->t_flagsext |= TF_RCVUNACK_WAITSS;
        tp->rcv_waitforss = 0;
}

/*
 * The last packet was a retransmission, check if this ack
 * indicates that the retransmission was spurious.
 *
 * If the connection supports timestamps, we could use it to
 * detect if the last retransmit was not needed. Otherwise,
 * we check if the ACK arrived within RTT/2 window, then it
 * was a mistake to do the retransmit in the first place.
 *
 * This function will return 1 if it is a spurious retransmit,
 * 0 otherwise.
 */
int
tcp_detect_bad_rexmt(struct tcpcb *tp, struct tcphdr *th,
    struct tcpopt *to, u_int32_t rxtime)
{
        int32_t tdiff, bad_rexmt_win;
        bad_rexmt_win = (tp->t_srtt >> (TCP_RTT_SHIFT + 1));

        /* If the ack has ECN CE bit, then cwnd has to be adjusted */
        if (TCP_ECN_ENABLED(tp) && (th->th_flags & TH_ECE)) {
                return 0;
        }
        if (TSTMP_SUPPORTED(tp)) {
                if (rxtime > 0 && (to->to_flags & TOF_TS) && to->to_tsecr != 0 &&
                    TSTMP_LT(to->to_tsecr, rxtime)) {
                        return 1;
                }
        } else {
                if ((tp->t_rxtshift == 1 || (tp->t_flagsext & TF_SENT_TLPROBE)) &&
                    rxtime > 0) {
                        tdiff = (int32_t)(tcp_now - rxtime);
                        if (tdiff < bad_rexmt_win) {
                                return 1;
                        }
                }
        }
        return 0;
}


/*
 * Restore congestion window state if a spurious timeout
 * was detected.
 */
static void
tcp_bad_rexmt_restore_state(struct tcpcb *tp, struct tcphdr *th)
{
        if (TSTMP_SUPPORTED(tp)) {
                u_int32_t fsize, acked;
                fsize = tp->snd_max - th->th_ack;
                acked = BYTES_ACKED(th, tp);

                /*
                 * Implement bad retransmit recovery as
                 * described in RFC 4015.
                 */
                tp->snd_ssthresh = tp->snd_ssthresh_prev;

                /* Initialize cwnd to the initial window */
                if (CC_ALGO(tp)->cwnd_init != NULL) {
                        CC_ALGO(tp)->cwnd_init(tp);
                }

                tp->snd_cwnd = fsize + min(acked, tp->snd_cwnd);
        } else {
                tp->snd_cwnd = tp->snd_cwnd_prev;
                tp->snd_ssthresh = tp->snd_ssthresh_prev;
                if (tp->t_flags & TF_WASFRECOVERY) {
                        ENTER_FASTRECOVERY(tp);
                }

                /* Do not use the loss flight size in this case */
                tp->t_lossflightsize = 0;
        }
        tp->snd_cwnd = max(tp->snd_cwnd, tcp_initial_cwnd(tp));
        tp->snd_recover = tp->snd_recover_prev;
        tp->snd_nxt = tp->snd_max;

        /* Fix send socket buffer to reflect the change in cwnd */
        tcp_bad_rexmt_fix_sndbuf(tp);

        /*
         * This RTT might reflect the extra delay induced
         * by the network. Skip using this sample for RTO
         * calculation and mark the connection so we can
         * recompute RTT when the next eligible sample is
         * found.
         */
        tp->t_flagsext |= TF_RECOMPUTE_RTT;
        tp->t_badrexmt_time = tcp_now;
        tp->t_rtttime = 0;
}

/*
 * If the previous packet was sent in retransmission timer, and it was
 * not needed, then restore the congestion window to the state before that
 * transmission.
 *
 * If the last packet was sent in tail loss probe timeout, check if that
 * recovered the last packet. If so, that will indicate a real loss and
 * the congestion window needs to be lowered.
 */
static void
tcp_bad_rexmt_check(struct tcpcb *tp, struct tcphdr *th, struct tcpopt *to)
{
        if (tp->t_rxtshift > 0 &&
            tcp_detect_bad_rexmt(tp, th, to, tp->t_rxtstart)) {
                ++tcpstat.tcps_sndrexmitbad;
                tcp_bad_rexmt_restore_state(tp, th);
                tcp_ccdbg_trace(tp, th, TCP_CC_BAD_REXMT_RECOVERY);
        } else if ((tp->t_flagsext & TF_SENT_TLPROBE) && tp->t_tlphighrxt > 0 &&
            SEQ_GEQ(th->th_ack, tp->t_tlphighrxt) &&
            !tcp_detect_bad_rexmt(tp, th, to, tp->t_tlpstart)) {
                /*
                 * check DSACK information also to make sure that
                 * the TLP was indeed needed
                 */
                if (tcp_rxtseg_dsack_for_tlp(tp)) {
                        /*
                         * received a DSACK to indicate that TLP was
                         * not needed
                         */
                        tcp_rxtseg_clean(tp);
                        goto out;
                }

                /*
                 * The tail loss probe recovered the last packet and
                 * we need to adjust the congestion window to take
                 * this loss into account.
                 */
                ++tcpstat.tcps_tlp_recoverlastpkt;
                if (!IN_FASTRECOVERY(tp)) {
                        tcp_reduce_congestion_window(tp);
                        EXIT_FASTRECOVERY(tp);
                }
                tcp_ccdbg_trace(tp, th, TCP_CC_TLP_RECOVER_LASTPACKET);
        } else if (tcp_rxtseg_detect_bad_rexmt(tp, th->th_ack)) {
                /*
                 * All of the retransmitted segments were duplicated, this
                 * can be an indication of bad fast retransmit.
                 */
                tcpstat.tcps_dsack_badrexmt++;
                tcp_bad_rexmt_restore_state(tp, th);
                tcp_ccdbg_trace(tp, th, TCP_CC_DSACK_BAD_REXMT);
                tcp_rxtseg_clean(tp);
        }
out:
        tp->t_flagsext &= ~(TF_SENT_TLPROBE);
        tp->t_tlphighrxt = 0;
        tp->t_tlpstart = 0;

        /*
         * check if the latest ack was for a segment sent during PMTU
         * blackhole detection. If the timestamp on the ack is before
         * PMTU blackhole detection, then revert the size of the max
         * segment to previous size.
         */
        if (tp->t_rxtshift > 0 && (tp->t_flags & TF_BLACKHOLE) &&
            tp->t_pmtud_start_ts > 0 && TSTMP_SUPPORTED(tp)) {
                if ((to->to_flags & TOF_TS) && to->to_tsecr != 0
                    && TSTMP_LT(to->to_tsecr, tp->t_pmtud_start_ts)) {
                        tcp_pmtud_revert_segment_size(tp);
                }
        }
        if (tp->t_pmtud_start_ts > 0) {
                tp->t_pmtud_start_ts = 0;
        }

        tp->t_pmtud_lastseg_size = 0;
}

/*
 * Check if early retransmit can be attempted according to RFC 5827.
 *
 * If packet reordering is detected on a connection, fast recovery will
 * be delayed until it is clear that the packet was lost and not reordered.
 * But reordering detection is done only when SACK is enabled.
 *
 * On connections that do not support SACK, there is a limit on the number
 * of early retransmits that can be done per minute. This limit is needed
 * to make sure that too many packets are not retransmitted when there is
 * packet reordering.
 */
static void
tcp_early_rexmt_check(struct tcpcb *tp, struct tcphdr *th)
{
        u_int32_t obytes, snd_off;
        int32_t snd_len;
        struct socket *so = tp->t_inpcb->inp_socket;

        if ((SACK_ENABLED(tp) || tp->t_early_rexmt_count < TCP_EARLY_REXMT_LIMIT) &&
            SEQ_GT(tp->snd_max, tp->snd_una) &&
            (tp->t_dupacks == 1 || (SACK_ENABLED(tp) && !TAILQ_EMPTY(&tp->snd_holes)))) {
                /*
                 * If there are only a few outstanding
                 * segments on the connection, we might need
                 * to lower the retransmit threshold. This
                 * will allow us to do Early Retransmit as
                 * described in RFC 5827.
                 */
                if (SACK_ENABLED(tp) &&
                    !TAILQ_EMPTY(&tp->snd_holes)) {
                        obytes = (tp->snd_max - tp->snd_fack) +
                            tp->sackhint.sack_bytes_rexmit;
                } else {
                        obytes = (tp->snd_max - tp->snd_una);
                }

                /*
                 * In order to lower retransmit threshold the
                 * following two conditions must be met.
                 * 1. the amount of outstanding data is less
                 * than 4*SMSS bytes
                 * 2. there is no unsent data ready for
                 * transmission or the advertised window
                 * will limit sending new segments.
                 */
                snd_off = tp->snd_max - tp->snd_una;
                snd_len = min(so->so_snd.sb_cc, tp->snd_wnd) - snd_off;
                if (obytes < (tp->t_maxseg << 2) &&
                    snd_len <= 0) {
                        u_int32_t osegs;

                        osegs = obytes / tp->t_maxseg;
                        if ((osegs * tp->t_maxseg) < obytes) {
                                osegs++;
                        }

                        /*
                         * Since the connection might have already
                         * received some dupacks, we add them to
                         * to the outstanding segments count to get
                         * the correct retransmit threshold.
                         *
                         * By checking for early retransmit after
                         * receiving some duplicate acks when SACK
                         * is supported, the connection will
                         * enter fast recovery even if multiple
                         * segments are lost in the same window.
                         */
                        osegs += tp->t_dupacks;
                        if (osegs < 4) {
                                tp->t_rexmtthresh =
                                    ((osegs - 1) > 1) ? (osegs - 1) : 1;
                                tp->t_rexmtthresh =
                                    min(tp->t_rexmtthresh, tcprexmtthresh);
                                tp->t_rexmtthresh =
                                    max(tp->t_rexmtthresh, tp->t_dupacks);

                                if (tp->t_early_rexmt_count == 0) {
                                        tp->t_early_rexmt_win = tcp_now;
                                }

                                if (tp->t_flagsext & TF_SENT_TLPROBE) {
                                        tcpstat.tcps_tlp_recovery++;
                                        tcp_ccdbg_trace(tp, th,
                                            TCP_CC_TLP_RECOVERY);
                                } else {
                                        tcpstat.tcps_early_rexmt++;
                                        tp->t_early_rexmt_count++;
                                        tcp_ccdbg_trace(tp, th,
                                            TCP_CC_EARLY_RETRANSMIT);
                                }
                        }
                }
        }

        /*
         * If we ever sent a TLP probe, the acknowledgement will trigger
         * early retransmit because the value of snd_fack will be close
         * to snd_max. This will take care of adjustments to the
         * congestion window. So we can reset TF_SENT_PROBE flag.
         */
        tp->t_flagsext &= ~(TF_SENT_TLPROBE);
        tp->t_tlphighrxt = 0;
        tp->t_tlpstart = 0;
}

static boolean_t
tcp_tfo_syn(struct tcpcb *tp, struct tcpopt *to)
{
        u_char out[CCAES_BLOCK_SIZE];
        unsigned char len;

        if (!(to->to_flags & (TOF_TFO | TOF_TFOREQ)) ||
            !(tcp_fastopen & TCP_FASTOPEN_SERVER)) {
                return FALSE;
        }

        if ((to->to_flags & TOF_TFOREQ)) {
                tp->t_tfo_flags |= TFO_F_OFFER_COOKIE;

                tp->t_tfo_stats |= TFO_S_COOKIEREQ_RECV;
                tcpstat.tcps_tfo_cookie_req_rcv++;
                return FALSE;
        }

        /* Ok, then it must be an offered cookie. We need to check that ... */
        tcp_tfo_gen_cookie(tp->t_inpcb, out, sizeof(out));

        len = *to->to_tfo - TCPOLEN_FASTOPEN_REQ;
        to->to_tfo++;
        if (memcmp(out, to->to_tfo, len)) {
                /* Cookies are different! Let's return and offer a new cookie */
                tp->t_tfo_flags |= TFO_F_OFFER_COOKIE;

                tp->t_tfo_stats |= TFO_S_COOKIE_INVALID;
                tcpstat.tcps_tfo_cookie_invalid++;
                return FALSE;
        }

        if (OSIncrementAtomic(&tcp_tfo_halfcnt) >= tcp_tfo_backlog) {
                /* Need to decrement again as we just increased it... */
                OSDecrementAtomic(&tcp_tfo_halfcnt);
                return FALSE;
        }

        tp->t_tfo_flags |= TFO_F_COOKIE_VALID;

        tp->t_tfo_stats |= TFO_S_SYNDATA_RCV;
        tcpstat.tcps_tfo_syn_data_rcv++;

        return TRUE;
}

static void
tcp_tfo_synack(struct tcpcb *tp, struct tcpopt *to)
{
        if (to->to_flags & TOF_TFO) {
                unsigned char len = *to->to_tfo - TCPOLEN_FASTOPEN_REQ;

                /*
                 * If this happens, things have gone terribly wrong. len should
                 * have been checked in tcp_dooptions.
                 */
                VERIFY(len <= TFO_COOKIE_LEN_MAX);

                to->to_tfo++;

                tcp_cache_set_cookie(tp, to->to_tfo, len);
                tcp_heuristic_tfo_success(tp);

                tp->t_tfo_stats |= TFO_S_COOKIE_RCV;
                tcpstat.tcps_tfo_cookie_rcv++;
                if (tp->t_tfo_flags & TFO_F_COOKIE_SENT) {
                        tcpstat.tcps_tfo_cookie_wrong++;
                        tp->t_tfo_stats |= TFO_S_COOKIE_WRONG;
                }
        } else {
                /*
                 * Thus, no cookie in the response, but we either asked for one
                 * or sent SYN+DATA. Now, we need to check whether we had to
                 * rexmit the SYN. If that's the case, it's better to start
                 * backing of TFO-cookie requests.
                 */
                if (!(tp->t_flagsext & TF_FASTOPEN_FORCE_ENABLE) &&
                    tp->t_tfo_flags & TFO_F_SYN_LOSS) {
                        tp->t_tfo_stats |= TFO_S_SYN_LOSS;
                        tcpstat.tcps_tfo_syn_loss++;

                        tcp_heuristic_tfo_loss(tp);
                } else {
                        if (tp->t_tfo_flags & TFO_F_COOKIE_REQ) {
                                tp->t_tfo_stats |= TFO_S_NO_COOKIE_RCV;
                                tcpstat.tcps_tfo_no_cookie_rcv++;
                        }

                        tcp_heuristic_tfo_success(tp);
                }
        }
}

static void
tcp_tfo_rcv_probe(struct tcpcb *tp, int tlen)
{
        if (tlen != 0) {
                return;
        }

        tp->t_tfo_probe_state = TFO_PROBE_PROBING;

        /*
         * We send the probe out rather quickly (after one RTO). It does not
         * really hurt that much, it's only one additional segment on the wire.
         */
        tp->t_timer[TCPT_KEEP] = OFFSET_FROM_START(tp, (TCP_REXMTVAL(tp)));
}

static void
tcp_tfo_rcv_data(struct tcpcb *tp)
{
        /* Transition from PROBING to NONE as data has been received */
        if (tp->t_tfo_probe_state >= TFO_PROBE_PROBING) {
                tp->t_tfo_probe_state = TFO_PROBE_NONE;
        }
}

static void
tcp_tfo_rcv_ack(struct tcpcb *tp, struct tcphdr *th)
{
        if (tp->t_tfo_probe_state == TFO_PROBE_PROBING &&
            tp->t_tfo_probes > 0) {
                if (th->th_seq == tp->rcv_nxt) {
                        /* No hole, so stop probing */
                        tp->t_tfo_probe_state = TFO_PROBE_NONE;
                } else if (SEQ_GT(th->th_seq, tp->rcv_nxt)) {
                        /* There is a hole! Wait a bit for data... */
                        tp->t_tfo_probe_state = TFO_PROBE_WAIT_DATA;
                        tp->t_timer[TCPT_KEEP] = OFFSET_FROM_START(tp,
                            TCP_REXMTVAL(tp));
                }
        }
}

/*
 * Update snd_wnd information.
 */
static inline bool
tcp_update_window(struct tcpcb *tp, int thflags, struct tcphdr * th,
    u_int32_t tiwin, int tlen)
{
        /* Don't look at the window if there is no ACK flag */
        if ((thflags & TH_ACK) &&
            (SEQ_LT(tp->snd_wl1, th->th_seq) ||
            (tp->snd_wl1 == th->th_seq && (SEQ_LT(tp->snd_wl2, th->th_ack) ||
            (tp->snd_wl2 == th->th_ack && tiwin > tp->snd_wnd))))) {
                /* keep track of pure window updates */
                if (tlen == 0 &&
                    tp->snd_wl2 == th->th_ack && tiwin > tp->snd_wnd) {
                        tcpstat.tcps_rcvwinupd++;
                }
                tp->snd_wnd = tiwin;
                tp->snd_wl1 = th->th_seq;
                tp->snd_wl2 = th->th_ack;
                if (tp->snd_wnd > tp->max_sndwnd) {
                        tp->max_sndwnd = tp->snd_wnd;
                }

                if (tp->t_inpcb->inp_socket->so_flags & SOF_MP_SUBFLOW) {
                        mptcp_update_window_wakeup(tp);
                }
                return true;
        }
        return false;
}

static void
tcp_handle_wakeup(struct socket *so, int read_wakeup, int write_wakeup)
{
        if (read_wakeup != 0) {
                sorwakeup(so);
        }
        if (write_wakeup != 0) {
                sowwakeup(so);
        }
}

static void
tcp_update_snd_una(struct tcpcb *tp, uint32_t ack)
{
        tp->snd_una = ack;
        if (SACK_ENABLED(tp) && SEQ_LT(tp->send_highest_sack, tp->snd_una)) {
                tp->send_highest_sack = tp->snd_una;

                /* If we move our marker, we need to start fresh */
                tp->t_new_dupacks = 0;
        }
}

static bool
tcp_syn_data_valid(struct tcpcb *tp, struct tcphdr *tcp_hdr, int tlen)
{
        /* No data? */
        if (tlen <= 0) {
                return false;
        }

        /* Not the right sequence-number? */
        if (tcp_hdr->th_seq != tp->irs) {
                return false;
        }

        /* We could have wrapped around, check that */
        if (tp->t_inpcb->inp_stat->rxbytes > INT32_MAX) {
                return false;
        }

        return true;
}

void
tcp_input(struct mbuf *m, int off0)
{
        int exiting_fr = 0;
        struct tcphdr *th;
        struct ip *ip = NULL;
        struct inpcb *inp;
        u_char *optp = NULL;
        int optlen = 0;
        int tlen, off;
        int drop_hdrlen;
        struct tcpcb *tp = 0;
        int thflags;
        struct socket *so = 0;
        int todrop, acked, ourfinisacked, needoutput = 0;
        int read_wakeup = 0;
        int write_wakeup = 0;
        struct in_addr laddr;
        struct in6_addr laddr6;
        int dropsocket = 0;
        int iss = 0, nosock = 0;
        u_int32_t tiwin, sack_bytes_acked = 0, sack_bytes_newly_acked = 0;
        struct tcpopt to;               /* options in this segment */
#if TCPDEBUG
        short ostate = 0;
#endif
        u_char ip_ecn = IPTOS_ECN_NOTECT;
        unsigned int ifscope;
        uint8_t isconnected, isdisconnected;
        struct ifnet *ifp = m->m_pkthdr.rcvif;
        int segment_count = m->m_pkthdr.seg_cnt ? : 1;
        int win;
        u_int16_t pf_tag = 0;
#if MPTCP
        struct mptcb *mp_tp = NULL;
#endif /* MPTCP */
        boolean_t cell = IFNET_IS_CELLULAR(ifp);
        boolean_t wifi = (!cell && IFNET_IS_WIFI(ifp));
        boolean_t wired = (!wifi && IFNET_IS_WIRED(ifp));
        boolean_t recvd_dsack = FALSE;
        struct tcp_respond_args tra;
        int prev_t_state;
        boolean_t check_cfil = cfil_filter_present();
        bool findpcb_iterated = false;
        /*
         * The mbuf may be freed after it has been added to the receive socket
         * buffer or the reassembly queue, so we reinitialize th to point to a
         * safe copy of the TCP header
         */
        struct tcphdr saved_tcphdr = {};
        /*
         * Save copy of the IPv4/IPv6 header.
         * Note: use array of uint32_t to silence compiler warning when casting
         * to a struct ip6_hdr pointer.
         */
#define MAX_IPWORDS ((sizeof(struct ip) + MAX_IPOPTLEN) / sizeof(uint32_t))
        uint32_t saved_hdr[MAX_IPWORDS];

#define TCP_INC_VAR(stat, npkts) do {                   \
                stat += npkts;                          \
} while (0)

        if (tcp_ack_strategy == TCP_ACK_STRATEGY_LEGACY) {
                segment_count = 1;
        }
        TCP_INC_VAR(tcpstat.tcps_rcvtotal, segment_count);

        struct ip6_hdr *ip6 = NULL;
        int isipv6;
        struct proc *kernel_proc = current_proc();

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

        isipv6 = (mtod(m, struct ip *)->ip_v == 6) ? 1 : 0;
        bzero((char *)&to, sizeof(to));

        if (m->m_flags & M_PKTHDR) {
                pf_tag = m_pftag(m)->pftag_tag;
        }

        if (isipv6) {
                /*
                 * Expect 32-bit aligned data pointer on
                 * strict-align platforms
                 */
                MBUF_STRICT_DATA_ALIGNMENT_CHECK_32(m);

                /* IP6_EXTHDR_CHECK() is already done at tcp6_input() */
                ip6 = mtod(m, struct ip6_hdr *);
                tlen = sizeof(*ip6) + ntohs(ip6->ip6_plen) - off0;
                th = (struct tcphdr *)(void *)((caddr_t)ip6 + off0);

                if (tcp_input_checksum(AF_INET6, m, th, off0, tlen)) {
                        TCP_LOG_DROP_PKT(ip6, th, ifp, "IPv6 bad tcp checksum");
                        goto dropnosock;
                }

                KERNEL_DEBUG(DBG_LAYER_BEG, ((th->th_dport << 16) | th->th_sport),
                    (((ip6->ip6_src.s6_addr16[0]) << 16) | (ip6->ip6_dst.s6_addr16[0])),
                    th->th_seq, th->th_ack, th->th_win);
                /*
                 * Be proactive about unspecified IPv6 address in source.
                 * As we use all-zero to indicate unbounded/unconnected pcb,
                 * unspecified IPv6 address can be used to confuse us.
                 *
                 * Note that packets with unspecified IPv6 destination is
                 * already dropped in ip6_input.
                 */
                if (IN6_IS_ADDR_UNSPECIFIED(&ip6->ip6_src)) {
                        /* XXX stat */
                        IF_TCP_STATINC(ifp, unspecv6);
                        TCP_LOG_DROP_PKT(ip6, th, ifp, "src IPv6 address unspecified");
                        goto dropnosock;
                }
                DTRACE_TCP5(receive, struct mbuf *, m, struct inpcb *, NULL,
                    struct ip6_hdr *, ip6, struct tcpcb *, NULL,
                    struct tcphdr *, th);

                ip_ecn = (ntohl(ip6->ip6_flow) >> 20) & IPTOS_ECN_MASK;
        } else {
                /*
                 * Get IP and TCP header together in first mbuf.
                 * Note: IP leaves IP header in first mbuf.
                 */
                if (off0 > sizeof(struct ip)) {
                        ip_stripoptions(m);
                        off0 = sizeof(struct ip);
                }
                if (m->m_len < sizeof(struct tcpiphdr)) {
                        if ((m = m_pullup(m, sizeof(struct tcpiphdr))) == 0) {
                                tcpstat.tcps_rcvshort++;
                                return;
                        }
                }

                /* Expect 32-bit aligned data pointer on strict-align platforms */
                MBUF_STRICT_DATA_ALIGNMENT_CHECK_32(m);

                ip = mtod(m, struct ip *);
                th = (struct tcphdr *)(void *)((caddr_t)ip + off0);
                tlen = ip->ip_len;

                if (tcp_input_checksum(AF_INET, m, th, off0, tlen)) {
                        TCP_LOG_DROP_PKT(ip, th, ifp, "IPv4 bad tcp checksum");
                        goto dropnosock;
                }

                /* Re-initialization for later version check */
                ip->ip_v = IPVERSION;
                ip_ecn = (ip->ip_tos & IPTOS_ECN_MASK);

                DTRACE_TCP5(receive, struct mbuf *, m, struct inpcb *, NULL,
                    struct ip *, ip, struct tcpcb *, NULL, struct tcphdr *, th);

                KERNEL_DEBUG(DBG_LAYER_BEG, ((th->th_dport << 16) | th->th_sport),
                    (((ip->ip_src.s_addr & 0xffff) << 16) | (ip->ip_dst.s_addr & 0xffff)),
                    th->th_seq, th->th_ack, th->th_win);
        }

#define TCP_LOG_HDR (isipv6 ? (void *)ip6 : (void *)ip)

        /*
         * Check that TCP offset makes sense,
         * pull out TCP options and adjust length.
         */
        off = th->th_off << 2;
        if (off < sizeof(struct tcphdr) || off > tlen) {
                tcpstat.tcps_rcvbadoff++;
                IF_TCP_STATINC(ifp, badformat);
                TCP_LOG_DROP_PKT(TCP_LOG_HDR, th, ifp, "bad tcp offset");
                goto dropnosock;
        }
        tlen -= off;    /* tlen is used instead of ti->ti_len */
        if (off > sizeof(struct tcphdr)) {
                if (isipv6) {
                        IP6_EXTHDR_CHECK(m, off0, off, return );
                        ip6 = mtod(m, struct ip6_hdr *);
                        th = (struct tcphdr *)(void *)((caddr_t)ip6 + off0);
                } else {
                        if (m->m_len < sizeof(struct ip) + off) {
                                if ((m = m_pullup(m, sizeof(struct ip) + off)) == 0) {
                                        tcpstat.tcps_rcvshort++;
                                        return;
                                }
                                ip = mtod(m, struct ip *);
                                th = (struct tcphdr *)(void *)((caddr_t)ip + off0);
                        }
                }
                optlen = off - sizeof(struct tcphdr);
                optp = (u_char *)(th + 1);
                /*
                 * Do quick retrieval of timestamp options ("options
                 * prediction?").  If timestamp is the only option and it's
                 * formatted as recommended in RFC 1323 appendix A, we
                 * quickly get the values now and not bother calling
                 * tcp_dooptions(), etc.
                 */
                if ((optlen == TCPOLEN_TSTAMP_APPA ||
                    (optlen > TCPOLEN_TSTAMP_APPA &&
                    optp[TCPOLEN_TSTAMP_APPA] == TCPOPT_EOL)) &&
                    *(u_int32_t *)(void *)optp == htonl(TCPOPT_TSTAMP_HDR) &&
                    (th->th_flags & TH_SYN) == 0) {
                        to.to_flags |= TOF_TS;
                        to.to_tsval = ntohl(*(u_int32_t *)(void *)(optp + 4));
                        to.to_tsecr = ntohl(*(u_int32_t *)(void *)(optp + 8));
                        optp = NULL;    /* we've parsed the options */
                }
        }
        thflags = th->th_flags;

        /*
         * Drop all packets with both the SYN and FIN bits set.
         * This prevents e.g. nmap from identifying the TCP/IP stack.
         *
         * This is a violation of the TCP specification.
         */
        if ((thflags & (TH_SYN | TH_FIN)) == (TH_SYN | TH_FIN)) {
                IF_TCP_STATINC(ifp, synfin);
                TCP_LOG_DROP_PKT(TCP_LOG_HDR, th, ifp, "drop SYN FIN");
                goto dropnosock;
        }

        /*
         * Delay dropping TCP, IP headers, IPv6 ext headers, and TCP options,
         * until after ip6_savecontrol() is called and before other functions
         * which don't want those proto headers.
         * Because ip6_savecontrol() is going to parse the mbuf to
         * search for data to be passed up to user-land, it wants mbuf
         * parameters to be unchanged.
         */
        drop_hdrlen = off0 + off;

        /* Since this is an entry point for input processing of tcp packets, we
         * can update the tcp clock here.
         */
        calculate_tcp_clock();

        /*
         * Record the interface where this segment arrived on; this does not
         * affect normal data output (for non-detached TCP) as it provides a
         * hint about which route and interface to use for sending in the
         * absence of a PCB, when scoped routing (and thus source interface
         * selection) are enabled.
         */
        if ((m->m_pkthdr.pkt_flags & PKTF_LOOP) || m->m_pkthdr.rcvif == NULL) {
                ifscope = IFSCOPE_NONE;
        } else {
                ifscope = m->m_pkthdr.rcvif->if_index;
        }

        /*
         * Convert TCP protocol specific fields to host format.
         */

#if BYTE_ORDER != BIG_ENDIAN
        NTOHL(th->th_seq);
        NTOHL(th->th_ack);
        NTOHS(th->th_win);
        NTOHS(th->th_urp);
#endif

        /*
         * Locate pcb for segment.
         */
findpcb:

        isconnected = FALSE;
        isdisconnected = FALSE;

        if (isipv6) {
                inp = in6_pcblookup_hash(&tcbinfo, &ip6->ip6_src, th->th_sport,
                    &ip6->ip6_dst, th->th_dport, 1,
                    m->m_pkthdr.rcvif);
        } else {
                inp = in_pcblookup_hash(&tcbinfo, ip->ip_src, th->th_sport,
                    ip->ip_dst, th->th_dport, 1, m->m_pkthdr.rcvif);
        }

        /*
         * Use the interface scope information from the PCB for outbound
         * segments.  If the PCB isn't present and if scoped routing is
         * enabled, tcp_respond will use the scope of the interface where
         * the segment arrived on.
         */
        if (inp != NULL && (inp->inp_flags & INP_BOUND_IF)) {
                ifscope = inp->inp_boundifp->if_index;
        }

        /*
         * If the state is CLOSED (i.e., TCB does not exist) then
         * all data in the incoming segment is discarded.
         * If the TCB exists but is in CLOSED state, it is embryonic,
         * but should either do a listen or a connect soon.
         */
        if (inp == NULL) {
                if (log_in_vain) {
                        char dbuf[MAX_IPv6_STR_LEN], sbuf[MAX_IPv6_STR_LEN];

                        if (isipv6) {
                                inet_ntop(AF_INET6, &ip6->ip6_dst, dbuf, sizeof(dbuf));
                                inet_ntop(AF_INET6, &ip6->ip6_src, sbuf, sizeof(sbuf));
                        } else {
                                inet_ntop(AF_INET, &ip->ip_dst, dbuf, sizeof(dbuf));
                                inet_ntop(AF_INET, &ip->ip_src, sbuf, sizeof(sbuf));
                        }
                        switch (log_in_vain) {
                        case 1:
                                if (thflags & TH_SYN) {
                                        log(LOG_INFO,
                                            "Connection attempt to TCP %s:%d from %s:%d\n",
                                            dbuf, ntohs(th->th_dport),
                                            sbuf,
                                            ntohs(th->th_sport));
                                }
                                break;
                        case 2:
                                log(LOG_INFO,
                                    "Connection attempt to TCP %s:%d from %s:%d flags:0x%x\n",
                                    dbuf, ntohs(th->th_dport), sbuf,
                                    ntohs(th->th_sport), thflags);
                                break;
                        case 3:
                        case 4:
                                if ((thflags & TH_SYN) && !(thflags & TH_ACK) &&
                                    !(m->m_flags & (M_BCAST | M_MCAST)) &&
                                    ((isipv6 && !IN6_ARE_ADDR_EQUAL(&ip6->ip6_dst, &ip6->ip6_src)) ||
                                    (!isipv6 && ip->ip_dst.s_addr != ip->ip_src.s_addr))) {
                                        log_in_vain_log((LOG_INFO,
                                            "Stealth Mode connection attempt to TCP %s:%d from %s:%d\n",
                                            dbuf, ntohs(th->th_dport),
                                            sbuf,
                                            ntohs(th->th_sport)));
                                }
                                break;
                        default:
                                break;
                        }
                }
                if (blackhole) {
                        if (m->m_pkthdr.rcvif && m->m_pkthdr.rcvif->if_type != IFT_LOOP) {
                                switch (blackhole) {
                                case 1:
                                        if (thflags & TH_SYN) {
                                                TCP_LOG_DROP_PKT(TCP_LOG_HDR, th, ifp, "blackhole 1 syn for closed port");
                                                goto dropnosock;
                                        }
                                        break;
                                case 2:
                                        TCP_LOG_DROP_PKT(TCP_LOG_HDR, th, ifp, "blackhole 2 closed port");
                                        goto dropnosock;
                                default:
                                        TCP_LOG_DROP_PKT(TCP_LOG_HDR, th, ifp, "blackhole closed port");
                                        goto dropnosock;
                                }
                        }
                }
                IF_TCP_STATINC(ifp, noconnnolist);
                TCP_LOG_DROP_PKT(TCP_LOG_HDR, th, ifp, "closed port");
                goto dropwithresetnosock;
        }
        so = inp->inp_socket;
        if (so == NULL) {
                /* This case shouldn't happen  as the socket shouldn't be null
                 * if inp_state isn't set to INPCB_STATE_DEAD
                 * But just in case, we pretend we didn't find the socket if we hit this case
                 * as this isn't cause for a panic (the socket might be leaked however)...
                 */
                inp = NULL;
#if TEMPDEBUG
                printf("tcp_input: no more socket for inp=%x. This shouldn't happen\n", inp);
#endif
                TCP_LOG_DROP_PKT(TCP_LOG_HDR, th, ifp, "inp_socket NULL");
                goto dropnosock;
        }

        socket_lock(so, 1);
        if (in_pcb_checkstate(inp, WNT_RELEASE, 1) == WNT_STOPUSING) {
                socket_unlock(so, 1);
                inp = NULL;     // pretend we didn't find it
                TCP_LOG_DROP_PKT(TCP_LOG_HDR, th, ifp, "inp state WNT_STOPUSING");
                goto dropnosock;
        }

        if (!isipv6 && inp->inp_faddr.s_addr != INADDR_ANY) {
                if (inp->inp_faddr.s_addr != ip->ip_src.s_addr ||
                    inp->inp_laddr.s_addr != ip->ip_dst.s_addr ||
                    inp->inp_fport != th->th_sport ||
                    inp->inp_lport != th->th_dport) {
                        os_log_error(OS_LOG_DEFAULT, "%s 5-tuple does not match: %u:%u %u:%u\n",
                            __func__,
                            ntohs(inp->inp_fport), ntohs(th->th_sport),
                            ntohs(inp->inp_lport), ntohs(th->th_dport));
                        if (findpcb_iterated) {
                                goto drop;
                        }
                        findpcb_iterated = true;
                        socket_unlock(so, 1);
                        inp = NULL;
                        goto findpcb;
                }
        } else if (isipv6 && !IN6_IS_ADDR_UNSPECIFIED(&inp->in6p_faddr)) {
                if (!IN6_ARE_ADDR_EQUAL(&inp->in6p_faddr, &ip6->ip6_src) ||
                    !IN6_ARE_ADDR_EQUAL(&inp->in6p_laddr, &ip6->ip6_dst) ||
                    inp->inp_fport != th->th_sport ||
                    inp->inp_lport != th->th_dport) {
                        os_log_error(OS_LOG_DEFAULT, "%s 5-tuple does not match: %u:%u %u:%u\n",
                            __func__,
                            ntohs(inp->inp_fport), ntohs(th->th_sport),
                            ntohs(inp->inp_lport), ntohs(th->th_dport));
                        if (findpcb_iterated) {
                                goto drop;
                        }
                        findpcb_iterated = true;
                        socket_unlock(so, 1);
                        inp = NULL;
                        goto findpcb;
                }
        }

        tp = intotcpcb(inp);
        if (tp == NULL) {
                IF_TCP_STATINC(ifp, noconnlist);
                TCP_LOG_DROP_PKT(TCP_LOG_HDR, th, ifp, "tp is NULL");
                goto dropwithreset;
        }

        TCP_LOG_TH_FLAGS(TCP_LOG_HDR, th, tp, false, ifp);

        if (tp->t_state == TCPS_CLOSED) {
                TCP_LOG_DROP_PCB(TCP_LOG_HDR, th, tp, false, "tp state TCPS_CLOSED");
                goto drop;
        }

#if NECP
        if (so->so_state & SS_ISCONNECTED) {
                // Connected TCP sockets have a fully-bound local and remote,
                // so the policy check doesn't need to override addresses
                if (!necp_socket_is_allowed_to_send_recv(inp, ifp, pf_tag, NULL, NULL, NULL, NULL)) {
                        TCP_LOG_DROP_NECP(TCP_LOG_HDR, th, intotcpcb(inp), false);
                        IF_TCP_STATINC(ifp, badformat);
                        goto drop;
                }
        } else {
                /*
                 * If the proc_uuid_policy table has been updated since the last use
                 * of the listening socket (i.e., the proc_uuid_policy_table_gencount
                 * has been updated), the flags in the socket may be out of date.
                 * If INP2_WANT_APP_POLICY is stale, inbound packets may
                 * be dropped by NECP if the socket should now match a per-app
                 * exception policy.
                 * In order to avoid this refresh the proc_uuid_policy state to
                 * potentially recalculate the socket's flags before checking
                 * with NECP.
                 */
                (void) inp_update_policy(inp);

                if (isipv6) {
                        if (!necp_socket_is_allowed_to_send_recv_v6(inp,
                            th->th_dport, th->th_sport, &ip6->ip6_dst,
                            &ip6->ip6_src, ifp, pf_tag, NULL, NULL, NULL, NULL)) {
                                TCP_LOG_DROP_NECP(TCP_LOG_HDR, th, intotcpcb(inp), false);
                                IF_TCP_STATINC(ifp, badformat);
                                goto drop;
                        }
                } else {
                        if (!necp_socket_is_allowed_to_send_recv_v4(inp,
                            th->th_dport, th->th_sport, &ip->ip_dst, &ip->ip_src,
                            ifp, pf_tag, NULL, NULL, NULL, NULL)) {
                                TCP_LOG_DROP_NECP(TCP_LOG_HDR, th, intotcpcb(inp), false);
                                IF_TCP_STATINC(ifp, badformat);
                                goto drop;
                        }
                }
        }
#endif /* NECP */

        prev_t_state = tp->t_state;

        /* If none of the FIN|SYN|RST|ACK flag is set, drop */
        if ((thflags & TH_ACCEPT) == 0) {
                TCP_LOG_DROP_PCB(TCP_LOG_HDR, th, tp, false, "rfc5961 TH_ACCEPT == 0");
                goto drop;
        }

        /* Unscale the window into a 32-bit value. */
        if ((thflags & TH_SYN) == 0) {
                tiwin = th->th_win << tp->snd_scale;
        } else {
                tiwin = th->th_win;
        }

        /* Avoid processing packets while closing a listen socket */
        if (tp->t_state == TCPS_LISTEN &&
            (so->so_options & SO_ACCEPTCONN) == 0) {
                TCP_LOG_DROP_PCB(TCP_LOG_HDR, th, tp, false, "closing a listening socket");
                goto drop;
        }

        if (so->so_options & (SO_DEBUG | SO_ACCEPTCONN)) {
#if TCPDEBUG
                if (so->so_options & SO_DEBUG) {
                        ostate = tp->t_state;
                        if (isipv6) {
                                bcopy((char *)ip6, (char *)tcp_saveipgen,
                                    sizeof(*ip6));
                        } else {
                                bcopy((char *)ip, (char *)tcp_saveipgen, sizeof(*ip));
                        }
                        tcp_savetcp = *th;
                }
#endif
                if (so->so_options & SO_ACCEPTCONN) {
                        struct tcpcb *tp0 = tp;
                        struct socket *so2;
                        struct socket *oso;
                        struct sockaddr_storage from;
                        struct sockaddr_storage to2;
                        struct inpcb *oinp = sotoinpcb(so);
                        struct ifnet *head_ifscope;
                        unsigned int head_nocell, head_recvanyif,
                            head_noexpensive, head_awdl_unrestricted,
                            head_intcoproc_allowed, head_external_port,
                            head_noconstrained;

                        /* Get listener's bound-to-interface, if any */
                        head_ifscope = (inp->inp_flags & INP_BOUND_IF) ?
                            inp->inp_boundifp : NULL;
                        /* Get listener's no-cellular information, if any */
                        head_nocell = INP_NO_CELLULAR(inp);
                        /* Get listener's recv-any-interface, if any */
                        head_recvanyif = (inp->inp_flags & INP_RECV_ANYIF);
                        /* Get listener's no-expensive information, if any */
                        head_noexpensive = INP_NO_EXPENSIVE(inp);
                        head_noconstrained = INP_NO_CONSTRAINED(inp);
                        head_awdl_unrestricted = INP_AWDL_UNRESTRICTED(inp);
                        head_intcoproc_allowed = INP_INTCOPROC_ALLOWED(inp);
                        head_external_port = (inp->inp_flags2 & INP2_EXTERNAL_PORT);

                        /*
                         * If the state is LISTEN then ignore segment if it contains an RST.
                         * If the segment contains an ACK then it is bad and send a RST.
                         * If it does not contain a SYN then it is not interesting; drop it.
                         * If it is from this socket, drop it, it must be forged.
                         */
                        if ((thflags & (TH_RST | TH_ACK | TH_SYN)) != TH_SYN) {
                                IF_TCP_STATINC(ifp, listbadsyn);

                                if (thflags & TH_RST) {
                                        TCP_LOG_DROP_PCB(TCP_LOG_HDR, th, tp, false, "SYN with RST");
                                        goto drop;
                                }
                                if (thflags & TH_ACK) {
                                        TCP_LOG_DROP_PCB(TCP_LOG_HDR, th, tp, false, "SYN with ACK");
                                        tp = NULL;
                                        tcpstat.tcps_badsyn++;
                                        goto dropwithreset;
                                }

                                /* We come here if there is no SYN set */
                                tcpstat.tcps_badsyn++;
                                TCP_LOG_DROP_PCB(TCP_LOG_HDR, th, tp, false, "bad SYN");
                                goto drop;
                        }
                        KERNEL_DEBUG(DBG_FNC_TCP_NEWCONN | DBG_FUNC_START, 0, 0, 0, 0, 0);
                        if (th->th_dport == th->th_sport) {
                                if (isipv6) {
                                        if (IN6_ARE_ADDR_EQUAL(&ip6->ip6_dst,
                                            &ip6->ip6_src)) {
                                                TCP_LOG_DROP_PCB(TCP_LOG_HDR, th, tp, false, "bad tuple same port");
                                                goto drop;
                                        }
                                } else if (ip->ip_dst.s_addr == ip->ip_src.s_addr) {
                                        TCP_LOG_DROP_PCB(TCP_LOG_HDR, th, tp, false, "bad tuple same IPv4 address");
                                        goto drop;
                                }
                        }
                        /*
                         * RFC1122 4.2.3.10, p. 104: discard bcast/mcast SYN
                         * in_broadcast() should never return true on a received
                         * packet with M_BCAST not set.
                         *
                         * Packets with a multicast source address should also
                         * be discarded.
                         */
                        if (m->m_flags & (M_BCAST | M_MCAST)) {
                                TCP_LOG_DROP_PCB(TCP_LOG_HDR, th, tp, false, "mbuf M_BCAST | M_MCAST");
                                goto drop;
                        }
                        if (isipv6) {
                                if (IN6_IS_ADDR_MULTICAST(&ip6->ip6_dst) ||
                                    IN6_IS_ADDR_MULTICAST(&ip6->ip6_src)) {
                                        TCP_LOG_DROP_PCB(TCP_LOG_HDR, th, tp, false, "IN6_IS_ADDR_MULTICAST");
                                        goto drop;
                                }
                        } else if (IN_MULTICAST(ntohl(ip->ip_dst.s_addr)) ||
                            IN_MULTICAST(ntohl(ip->ip_src.s_addr)) ||
                            ip->ip_src.s_addr == htonl(INADDR_BROADCAST) ||
                            in_broadcast(ip->ip_dst, m->m_pkthdr.rcvif)) {
                                TCP_LOG_DROP_PCB(TCP_LOG_HDR, th, tp, false, "multicast or broadcast address");
                                goto drop;
                        }


                        /*
                         * If deprecated address is forbidden,
                         * we do not accept SYN to deprecated interface
                         * address to prevent any new inbound connection from
                         * getting established.
                         * When we do not accept SYN, we send a TCP RST,
                         * with deprecated source address (instead of dropping
                         * it).  We compromise it as it is much better for peer
                         * to send a RST, and RST will be the final packet
                         * for the exchange.
                         *
                         * If we do not forbid deprecated addresses, we accept
                         * the SYN packet.  RFC 4862 forbids dropping SYN in
                         * this case.
                         */
                        if (isipv6 && !ip6_use_deprecated) {
                                uint32_t ia6_flags;

                                if (ip6_getdstifaddr_info(m, NULL,
                                    &ia6_flags) == 0) {
                                        if (ia6_flags & IN6_IFF_DEPRECATED) {
                                                tp = NULL;
                                                IF_TCP_STATINC(ifp, deprecate6);
                                                TCP_LOG_DROP_PCB(TCP_LOG_HDR, th, tp, false, "deprecated IPv6 address");
                                                goto dropwithreset;
                                        }
                                }
                        }
                        if (so->so_filt || check_cfil) {
                                if (isipv6) {
                                        struct sockaddr_in6     *sin6 = (struct sockaddr_in6*)&from;

                                        sin6->sin6_len = sizeof(*sin6);
                                        sin6->sin6_family = AF_INET6;
                                        sin6->sin6_port = th->th_sport;
                                        sin6->sin6_flowinfo = 0;
                                        sin6->sin6_addr = ip6->ip6_src;
                                        sin6->sin6_scope_id = 0;

                                        sin6 = (struct sockaddr_in6*)&to2;

                                        sin6->sin6_len = sizeof(struct sockaddr_in6);
                                        sin6->sin6_family = AF_INET6;
                                        sin6->sin6_port = th->th_dport;
                                        sin6->sin6_flowinfo = 0;
                                        sin6->sin6_addr = ip6->ip6_dst;
                                        sin6->sin6_scope_id = 0;
                                } else {
                                        struct sockaddr_in *sin = (struct sockaddr_in*)&from;

                                        sin->sin_len = sizeof(*sin);
                                        sin->sin_family = AF_INET;
                                        sin->sin_port = th->th_sport;
                                        sin->sin_addr = ip->ip_src;

                                        sin = (struct sockaddr_in*)&to2;

                                        sin->sin_len = sizeof(struct sockaddr_in);
                                        sin->sin_family = AF_INET;
                                        sin->sin_port = th->th_dport;
                                        sin->sin_addr = ip->ip_dst;
                                }
                        }

                        if (so->so_filt) {
                                so2 = sonewconn(so, 0, (struct sockaddr*)&from);
                        } else {
                                so2 = sonewconn(so, 0, NULL);
                        }
                        if (so2 == 0) {
                                tcpstat.tcps_listendrop++;
                                if (tcp_dropdropablreq(so)) {
                                        if (so->so_filt) {
                                                so2 = sonewconn(so, 0, (struct sockaddr*)&from);
                                        } else {
                                                so2 = sonewconn(so, 0, NULL);
                                        }
                                }
                                if (!so2) {
                                        TCP_LOG_DROP_PCB(TCP_LOG_HDR, th, tp, false, " listen drop");
                                        goto drop;
                                }
                        }

                        /* Point "inp" and "tp" in tandem to new socket */
                        inp = (struct inpcb *)so2->so_pcb;
                        tp = intotcpcb(inp);

                        oso = so;
                        socket_unlock(so, 0); /* Unlock but keep a reference on listener for now */

                        so = so2;
                        socket_lock(so, 1);
                        /*
                         * Mark socket as temporary until we're
                         * committed to keeping it.  The code at
                         * ``drop'' and ``dropwithreset'' check the
                         * flag dropsocket to see if the temporary
                         * socket created here should be discarded.
                         * We mark the socket as discardable until
                         * we're committed to it below in TCPS_LISTEN.
                         * There are some error conditions in which we
                         * have to drop the temporary socket.
                         */
                        dropsocket++;
                        /*
                         * Inherit INP_BOUND_IF from listener; testing if
                         * head_ifscope is non-NULL is sufficient, since it
                         * can only be set to a non-zero value earlier if
                         * the listener has such a flag set.
                         */
                        if (head_ifscope != NULL) {
                                inp->inp_flags |= INP_BOUND_IF;
                                inp->inp_boundifp = head_ifscope;
                        } else {
                                inp->inp_flags &= ~INP_BOUND_IF;
                        }
                        /*
                         * Inherit restrictions from listener.
                         */
                        if (head_nocell) {
                                inp_set_nocellular(inp);
                        }
                        if (head_noexpensive) {
                                inp_set_noexpensive(inp);
                        }
                        if (head_noconstrained) {
                                inp_set_noconstrained(inp);
                        }
                        if (head_awdl_unrestricted) {
                                inp_set_awdl_unrestricted(inp);
                        }
                        if (head_intcoproc_allowed) {
                                inp_set_intcoproc_allowed(inp);
                        }
                        /*
                         * Inherit {IN,IN6}_RECV_ANYIF from listener.
                         */
                        if (head_recvanyif) {
                                inp->inp_flags |= INP_RECV_ANYIF;
                        } else {
                                inp->inp_flags &= ~INP_RECV_ANYIF;
                        }

                        if (head_external_port) {
                                inp->inp_flags2 |= INP2_EXTERNAL_PORT;
                        }
                        if (isipv6) {
                                inp->in6p_laddr = ip6->ip6_dst;
                        } else {
                                inp->inp_vflag &= ~INP_IPV6;
                                inp->inp_vflag |= INP_IPV4;
                                inp->inp_laddr = ip->ip_dst;
                        }
                        inp->inp_lport = th->th_dport;
                        if (in_pcbinshash(inp, 0) != 0) {
                                /*
                                 * Undo the assignments above if we failed to
                                 * put the PCB on the hash lists.
                                 */
                                if (isipv6) {
                                        inp->in6p_laddr = in6addr_any;
                                } else {
                                        inp->inp_laddr.s_addr = INADDR_ANY;
                                }
                                inp->inp_lport = 0;
                                socket_lock(oso, 0);    /* release ref on parent */
                                socket_unlock(oso, 1);
                                TCP_LOG_DROP_PCB(TCP_LOG_HDR, th, tp, false, " in_pcbinshash failed");
                                goto drop;
                        }
                        socket_lock(oso, 0);
                        if (isipv6) {
                                /*
                                 * Inherit socket options from the listening
                                 * socket.
                                 * Note that in6p_inputopts are not (even
                                 * should not be) copied, since it stores
                                 * previously received options and is used to
                                 * detect if each new option is different than
                                 * the previous one and hence should be passed
                                 * to a user.
                                 * If we copied in6p_inputopts, a user would
                                 * not be able to receive options just after
                                 * calling the accept system call.
                                 */
                                inp->inp_flags |=
                                    oinp->inp_flags & INP_CONTROLOPTS;
                                if (oinp->in6p_outputopts) {
                                        inp->in6p_outputopts =
                                            ip6_copypktopts(oinp->in6p_outputopts,
                                            M_NOWAIT);
                                }
                        } else {
                                inp->inp_options = ip_srcroute();
                                inp->inp_ip_tos = oinp->inp_ip_tos;
                        }
#if IPSEC
                        /* copy old policy into new socket's */
                        if (sotoinpcb(oso)->inp_sp) {
                                int error = 0;
                                /* Is it a security hole here to silently fail to copy the policy? */
                                if (inp->inp_sp != NULL) {
                                        error = ipsec_init_policy(so, &inp->inp_sp);
                                }
                                if (error != 0 || ipsec_copy_policy(sotoinpcb(oso)->inp_sp, inp->inp_sp)) {
                                        printf("tcp_input: could not copy policy\n");
                                }
                        }
#endif
                        /* inherit states from the listener */
                        DTRACE_TCP4(state__change, void, NULL, struct inpcb *, inp,
                            struct tcpcb *, tp, int32_t, TCPS_LISTEN);
                        tp->t_state = TCPS_LISTEN;
                        tp->t_flags |= tp0->t_flags & (TF_NOPUSH | TF_NOOPT | TF_NODELAY);
                        tp->t_flagsext |= (tp0->t_flagsext & (TF_RXTFINDROP | TF_NOTIMEWAIT | TF_FASTOPEN));
                        tp->t_keepinit = tp0->t_keepinit;
                        tp->t_keepcnt = tp0->t_keepcnt;
                        tp->t_keepintvl = tp0->t_keepintvl;
                        tp->t_adaptive_wtimo = tp0->t_adaptive_wtimo;
                        tp->t_adaptive_rtimo = tp0->t_adaptive_rtimo;
                        tp->t_inpcb->inp_ip_ttl = tp0->t_inpcb->inp_ip_ttl;
                        if ((so->so_flags & SOF_NOTSENT_LOWAT) != 0) {
                                tp->t_notsent_lowat = tp0->t_notsent_lowat;
                        }
                        tp->t_inpcb->inp_flags2 |=
                            tp0->t_inpcb->inp_flags2 & INP2_KEEPALIVE_OFFLOAD;

                        /* now drop the reference on the listener */
                        socket_unlock(oso, 1);

                        tcp_set_max_rwinscale(tp, so);

#if CONTENT_FILTER
                        if (check_cfil) {
                                int error = cfil_sock_attach(so2, (struct sockaddr*)&to2, (struct sockaddr*)&from,
                                    CFS_CONNECTION_DIR_IN);
                                if (error != 0) {
                                        TCP_LOG_DROP_PCB(TCP_LOG_HDR, th, tp, false, " cfil_sock_attach failed");
                                        goto drop;
                                }
                        }
#endif /* CONTENT_FILTER */

                        KERNEL_DEBUG(DBG_FNC_TCP_NEWCONN | DBG_FUNC_END, 0, 0, 0, 0, 0);
                }
        }
        socket_lock_assert_owned(so);

        if (net_mpklog_enabled && (m->m_pkthdr.rcvif->if_xflags & IFXF_MPK_LOG)) {
                MPKL_TCP_INPUT(tcp_mpkl_log_object,
                    ntohs(tp->t_inpcb->inp_lport), ntohs(tp->t_inpcb->inp_fport),
                    th->th_seq, th->th_ack, tlen, thflags,
                    so->last_pid, so->so_log_seqn++);
        }

        if (tp->t_state == TCPS_ESTABLISHED && tlen > 0) {
                /*
                 * Evaluate the rate of arrival of packets to see if the
                 * receiver can reduce the ack traffic. The algorithm to
                 * stretch acks will be enabled if the connection meets
                 * certain criteria defined in tcp_stretch_ack_enable function.
                 */
                if ((tp->t_flagsext & TF_RCVUNACK_WAITSS) != 0) {
                        TCP_INC_VAR(tp->rcv_waitforss, segment_count);
                }
                if (tcp_stretch_ack_enable(tp, thflags)) {
                        tp->t_flags |= TF_STRETCHACK;
                        tp->t_flagsext &= ~(TF_RCVUNACK_WAITSS);
                        tp->rcv_waitforss = 0;
                } else {
                        tp->t_flags &= ~(TF_STRETCHACK);
                }
                if (TSTMP_GT(tp->rcv_unackwin - (tcp_rcvunackwin >> 1), tcp_now)) {
                        tp->rcv_by_unackhalfwin += (tlen + off);
                        tp->rcv_by_unackwin += (tlen + off);
                } else {
                        tp->rcv_unackwin = tcp_now + tcp_rcvunackwin;
                        tp->rcv_by_unackwin = tp->rcv_by_unackhalfwin + tlen + off;
                        tp->rcv_by_unackhalfwin = tlen + off;
                }
        }

        /*
         * Clear TE_SENDECE if TH_CWR is set. This is harmless, so we don't
         * bother doing extensive checks for state and whatnot.
         */
        if (thflags & TH_CWR) {
                tp->ecn_flags &= ~TE_SENDECE;
                tp->t_ecn_recv_cwr++;
        }

        /*
         * Explicit Congestion Notification - Flag that we need to send ECT if
         *      + The IP Congestion experienced flag was set.
         *      + Socket is in established state
         *      + We negotiated ECN in the TCP setup
         *      + This isn't a pure ack (tlen > 0)
         *      + The data is in the valid window
         *
         *      TE_SENDECE will be cleared when we receive a packet with TH_CWR set.
         */
        if (ip_ecn == IPTOS_ECN_CE && tp->t_state == TCPS_ESTABLISHED &&
            TCP_ECN_ENABLED(tp) && tlen > 0 &&
            SEQ_GEQ(th->th_seq, tp->last_ack_sent) &&
            SEQ_LT(th->th_seq, tp->last_ack_sent + tp->rcv_wnd)) {
                tp->t_ecn_recv_ce++;
                tcpstat.tcps_ecn_recv_ce++;
                INP_INC_IFNET_STAT(inp, ecn_recv_ce);
                /* Mark this connection as it received CE from network */
                tp->ecn_flags |= TE_RECV_ECN_CE;
                tp->ecn_flags |= TE_SENDECE;
        }

        /*
         * If we received an  explicit notification of congestion in
         * ip tos ecn bits or by the CWR bit in TCP header flags, reset
         * the ack-stretching state. We need to handle ECN notification if
         * an ECN setup SYN was sent even once.
         */
        if (tp->t_state == TCPS_ESTABLISHED &&
            (tp->ecn_flags & TE_SETUPSENT) &&
            (ip_ecn == IPTOS_ECN_CE || (thflags & TH_CWR))) {
                tcp_reset_stretch_ack(tp);
                tp->t_forced_acks = TCP_FORCED_ACKS_COUNT;
                CLEAR_IAJ_STATE(tp);
        }

        if (ip_ecn == IPTOS_ECN_CE && tp->t_state == TCPS_ESTABLISHED &&
            !TCP_ECN_ENABLED(tp) && !(tp->ecn_flags & TE_CEHEURI_SET)) {
                tcpstat.tcps_ecn_fallback_ce++;
                tcp_heuristic_ecn_aggressive(tp);
                tp->ecn_flags |= TE_CEHEURI_SET;
        }

        if (tp->t_state == TCPS_ESTABLISHED && TCP_ECN_ENABLED(tp) &&
            ip_ecn == IPTOS_ECN_CE && !(tp->ecn_flags & TE_CEHEURI_SET)) {
                if (inp->inp_stat->rxpackets < ECN_MIN_CE_PROBES) {
                        tp->t_ecn_recv_ce_pkt++;
                } else if (tp->t_ecn_recv_ce_pkt > ECN_MAX_CE_RATIO) {
                        tcpstat.tcps_ecn_fallback_ce++;
                        tcp_heuristic_ecn_aggressive(tp);
                        tp->ecn_flags |= TE_CEHEURI_SET;
                        INP_INC_IFNET_STAT(inp, ecn_fallback_ce);
                } else {
                        /* We tracked the first ECN_MIN_CE_PROBES segments, we
                         * now know that the path is good.
                         */
                        tp->ecn_flags |= TE_CEHEURI_SET;
                }
        }

        /* Update rcvtime as a new segment was received on the connection */
        tp->t_rcvtime = tcp_now;

        /*
         * Segment received on connection.
         * Reset idle time and keep-alive timer.
         */
        if (TCPS_HAVEESTABLISHED(tp->t_state)) {
                tcp_keepalive_reset(tp);

                if (tp->t_mpsub) {
                        mptcp_reset_keepalive(tp);
                }
        }

        /*
         * Process options if not in LISTEN state,
         * else do it below (after getting remote address).
         */
        if (tp->t_state != TCPS_LISTEN && optp) {
                tcp_dooptions(tp, optp, optlen, th, &to);
        }
#if MPTCP
        if (tp->t_state != TCPS_LISTEN && (so->so_flags & SOF_MP_SUBFLOW) &&
            mptcp_input_preproc(tp, m, th, drop_hdrlen) != 0) {
                tp->t_flags |= TF_ACKNOW;
                (void) tcp_output(tp);
                tcp_check_timer_state(tp);
                socket_unlock(so, 1);
                return;
        }
#endif /* MPTCP */
        if (tp->t_state == TCPS_SYN_SENT && (thflags & TH_SYN)) {
                if (!(thflags & TH_ACK) ||
                    (SEQ_GT(th->th_ack, tp->iss) &&
                    SEQ_LEQ(th->th_ack, tp->snd_max))) {
                        tcp_finalize_options(tp, &to, ifscope);
                }
        }

#if TRAFFIC_MGT
        /*
         * Compute inter-packet arrival jitter. According to RFC 3550,
         * inter-packet arrival jitter is defined as the difference in
         * packet spacing at the receiver compared to the sender for a
         * pair of packets. When two packets of maximum segment size come
         * one after the other with consecutive sequence numbers, we
         * consider them as packets sent together at the sender and use
         * them as a pair to compute inter-packet arrival jitter. This
         * metric indicates the delay induced by the network components due
         * to queuing in edge/access routers.
         */
        if (tp->t_state == TCPS_ESTABLISHED &&
            (thflags & (TH_SYN | TH_FIN | TH_RST | TH_URG | TH_ACK | TH_ECE | TH_PUSH)) == TH_ACK &&
            ((tp->t_flags & TF_NEEDFIN) == 0) &&
            ((to.to_flags & TOF_TS) == 0 ||
            TSTMP_GEQ(to.to_tsval, tp->ts_recent)) &&
            th->th_seq == tp->rcv_nxt && LIST_EMPTY(&tp->t_segq)) {
                int seg_size = tlen;
                if (tp->iaj_pktcnt <= IAJ_IGNORE_PKTCNT) {
                        TCP_INC_VAR(tp->iaj_pktcnt, segment_count);
                }

                if (tp->iaj_size == 0 || seg_size > tp->iaj_size ||
                    (seg_size == tp->iaj_size && tp->iaj_rcv_ts == 0)) {
                        /*
                         * State related to inter-arrival jitter is
                         * uninitialized or we are trying to find a good
                         * first packet to start computing the metric
                         */
                        update_iaj_state(tp, seg_size, 0);
                } else {
                        if (seg_size == tp->iaj_size) {
                                /*
                                 * Compute inter-arrival jitter taking
                                 * this packet as the second packet
                                 */
                                compute_iaj(tp);
                        }
                        if (seg_size < tp->iaj_size) {
                                /*
                                 * There is a smaller packet in the stream.
                                 * Some times the maximum size supported
                                 * on a path can change if there is a new
                                 * link with smaller MTU. The receiver will
                                 * not know about this change. If there
                                 * are too many packets smaller than
                                 * iaj_size, we try to learn the iaj_size
                                 * again.
                                 */
                                TCP_INC_VAR(tp->iaj_small_pkt, segment_count);
                                if (tp->iaj_small_pkt > RESET_IAJ_SIZE_THRESH) {
                                        update_iaj_state(tp, seg_size, 1);
                                } else {
                                        CLEAR_IAJ_STATE(tp);
                                }
                        } else {
                                update_iaj_state(tp, seg_size, 0);
                        }
                }
        } else {
                CLEAR_IAJ_STATE(tp);
        }
#endif /* TRAFFIC_MGT */

        /*
         * Header prediction: check for the two common cases
         * of a uni-directional data xfer.  If the packet has
         * no control flags, is in-sequence, the window didn't
         * change and we're not retransmitting, it's a
         * candidate.  If the length is zero and the ack moved
         * forward, we're the sender side of the xfer.  Just
         * free the data acked & wake any higher level process
         * that was blocked waiting for space.  If the length
         * is non-zero and the ack didn't move, we're the
         * receiver side.  If we're getting packets in-order
         * (the reassembly queue is empty), add the data to
         * the socket buffer and note that we need a delayed ack.
         * Make sure that the hidden state-flags are also off.
         * Since we check for TCPS_ESTABLISHED above, it can only
         * be TH_NEEDSYN.
         */
        if (tp->t_state == TCPS_ESTABLISHED &&
            (thflags & (TH_SYN | TH_FIN | TH_RST | TH_URG | TH_ACK | TH_ECE | TH_CWR)) == TH_ACK &&
            ((tp->t_flags & TF_NEEDFIN) == 0) &&
            ((to.to_flags & TOF_TS) == 0 ||
            TSTMP_GEQ(to.to_tsval, tp->ts_recent)) &&
            th->th_seq == tp->rcv_nxt &&
            tiwin && tiwin == tp->snd_wnd &&
            tp->snd_nxt == tp->snd_max) {
                /*
                 * If last ACK falls within this segment's sequence numbers,
                 * record the timestamp.
                 * NOTE that the test is modified according to the latest
                 * proposal of the tcplw@cray.com list (Braden 1993/04/26).
                 */
                if ((to.to_flags & TOF_TS) != 0 &&
                    SEQ_LEQ(th->th_seq, tp->last_ack_sent)) {
                        tp->ts_recent_age = tcp_now;
                        tp->ts_recent = to.to_tsval;
                }

                if (tlen == 0) {
                        if (SEQ_GT(th->th_ack, tp->snd_una) &&
                            SEQ_LEQ(th->th_ack, tp->snd_max) &&
                            tp->snd_cwnd >= tp->snd_ssthresh &&
                            (!IN_FASTRECOVERY(tp) &&
                            ((!(SACK_ENABLED(tp)) &&
                            tp->t_dupacks < tp->t_rexmtthresh) ||
                            (SACK_ENABLED(tp) && to.to_nsacks == 0 &&
                            TAILQ_EMPTY(&tp->snd_holes))))) {
                                /*
                                 * this is a pure ack for outstanding data.
                                 */
                                ++tcpstat.tcps_predack;

                                tcp_bad_rexmt_check(tp, th, &to);

                                /* Recalculate the RTT */
                                tcp_compute_rtt(tp, &to, th);

                                VERIFY(SEQ_GEQ(th->th_ack, tp->snd_una));
                                acked = BYTES_ACKED(th, tp);
                                tcpstat.tcps_rcvackpack++;
                                tcpstat.tcps_rcvackbyte += acked;

                                /*
                                 * Handle an ack that is in sequence during
                                 * congestion avoidance phase. The
                                 * calculations in this function
                                 * assume that snd_una is not updated yet.
                                 */
                                if (CC_ALGO(tp)->congestion_avd != NULL) {
                                        CC_ALGO(tp)->congestion_avd(tp, th);
                                }
                                tcp_ccdbg_trace(tp, th, TCP_CC_INSEQ_ACK_RCVD);
                                sbdrop(&so->so_snd, acked);
                                tcp_sbsnd_trim(&so->so_snd);

                                if (SEQ_GT(tp->snd_una, tp->snd_recover) &&
                                    SEQ_LEQ(th->th_ack, tp->snd_recover)) {
                                        tp->snd_recover = th->th_ack - 1;
                                }

                                tcp_update_snd_una(tp, th->th_ack);

                                TCP_RESET_REXMT_STATE(tp);

                                /*
                                 * pull snd_wl2 up to prevent seq wrap relative
                                 * to th_ack.
                                 */
                                tp->snd_wl2 = th->th_ack;

                                if (tp->t_dupacks > 0) {
                                        tp->t_dupacks = 0;
                                        tp->t_rexmtthresh = tcprexmtthresh;
                                        tp->t_new_dupacks = 0;
                                }

                                tp->sackhint.sack_bytes_acked = 0;

                                /*
                                 * If all outstanding data are acked, stop
                                 * retransmit timer, otherwise restart timer
                                 * using current (possibly backed-off) value.
                                 * If process is waiting for space,
                                 * wakeup/selwakeup/signal.  If data
                                 * are ready to send, let tcp_output
                                 * decide between more output or persist.
                                 */
                                if (tp->snd_una == tp->snd_max) {
                                        tp->t_timer[TCPT_REXMT] = 0;
                                        tp->t_timer[TCPT_PTO] = 0;
                                } else if (tp->t_timer[TCPT_PERSIST] == 0) {
                                        tp->t_timer[TCPT_REXMT] = OFFSET_FROM_START(tp, tp->t_rxtcur);
                                }
                                if (!SLIST_EMPTY(&tp->t_rxt_segments) &&
                                    !TCP_DSACK_SEQ_IN_WINDOW(tp,
                                    tp->t_dsack_lastuna, tp->snd_una)) {
                                        tcp_rxtseg_clean(tp);
                                }

                                if ((tp->t_flagsext & TF_MEASURESNDBW) != 0 &&
                                    tp->t_bwmeas != NULL) {
                                        tcp_bwmeas_check(tp);
                                }

                                write_wakeup = 1;
                                if (!SLIST_EMPTY(&tp->t_notify_ack)) {
                                        tcp_notify_acknowledgement(tp, so);
                                }

                                if ((so->so_snd.sb_cc) || (tp->t_flags & TF_ACKNOW)) {
                                        (void) tcp_output(tp);
                                }

                                tcp_tfo_rcv_ack(tp, th);

                                m_freem(m);

                                tcp_check_timer_state(tp);

                                tcp_handle_wakeup(so, read_wakeup, write_wakeup);

                                socket_unlock(so, 1);
                                KERNEL_DEBUG(DBG_FNC_TCP_INPUT | DBG_FUNC_END, 0, 0, 0, 0, 0);
                                return;
                        }
                } else if (th->th_ack == tp->snd_una && LIST_EMPTY(&tp->t_segq) &&
                    tlen <= tcp_sbspace(tp)) {
                        /*
                         * this is a pure, in-sequence data packet
                         * with nothing on the reassembly queue and
                         * we have enough buffer space to take it.
                         */

                        /* Clean receiver SACK report if present */
                        if (SACK_ENABLED(tp) && tp->rcv_numsacks) {
                                tcp_clean_sackreport(tp);
                        }
                        ++tcpstat.tcps_preddat;
                        tp->rcv_nxt += tlen;
                        /*
                         * Pull snd_wl1 up to prevent seq wrap relative to
                         * th_seq.
                         */
                        tp->snd_wl1 = th->th_seq;
                        /*
                         * Pull rcv_up up to prevent seq wrap relative to
                         * rcv_nxt.
                         */
                        tp->rcv_up = tp->rcv_nxt;
                        TCP_INC_VAR(tcpstat.tcps_rcvpack, segment_count);
                        tcpstat.tcps_rcvbyte += tlen;
                        if (nstat_collect) {
                                INP_ADD_STAT(inp, cell, wifi, wired,
                                    rxpackets, 1);
                                INP_ADD_STAT(inp, cell, wifi, wired, rxbytes,
                                    tlen);
                                inp_set_activity_bitmap(inp);
                        }

                        /*
                         * Calculate the RTT on the receiver only if the
                         * connection is in streaming mode and the last
                         * packet was not an end-of-write
                         */
                        if (tp->t_flags & TF_STREAMING_ON) {
                                tcp_compute_rtt(tp, &to, th);
                        }

                        tcp_sbrcv_grow(tp, &so->so_rcv, &to, tlen);

                        /*
                         * Add data to socket buffer.
                         */
                        so_recv_data_stat(so, m, 0);
                        m_adj(m, drop_hdrlen);  /* delayed header drop */

                        /*
                         * If message delivery (SOF_ENABLE_MSGS) is enabled on
                         * this socket, deliver the packet received as an
                         * in-order message with sequence number attached to it.
                         */
                        if (isipv6) {
                                memcpy(&saved_hdr, ip6, sizeof(struct ip6_hdr));
                                ip6 = (struct ip6_hdr *)&saved_hdr[0];
                        } else {
                                memcpy(&saved_hdr, ip, ip->ip_hl << 2);
                                ip = (struct ip *)&saved_hdr[0];
                        }
                        memcpy(&saved_tcphdr, th, sizeof(struct tcphdr));

                        if (th->th_flags & TH_PUSH) {
                                tp->t_flagsext |= TF_LAST_IS_PSH;
                        } else {
                                tp->t_flagsext &= ~TF_LAST_IS_PSH;
                        }

                        if (sbappendstream_rcvdemux(so, m)) {
                                mptcp_handle_input(so);
                                read_wakeup = 1;
                        }
                        th = &saved_tcphdr;

                        if (isipv6) {
                                KERNEL_DEBUG(DBG_LAYER_END, ((th->th_dport << 16) | th->th_sport),
                                    (((ip6->ip6_src.s6_addr16[0]) << 16) | (ip6->ip6_dst.s6_addr16[0])),
                                    th->th_seq, th->th_ack, th->th_win);
                        } else {
                                KERNEL_DEBUG(DBG_LAYER_END, ((th->th_dport << 16) | th->th_sport),
                                    (((ip->ip_src.s_addr & 0xffff) << 16) | (ip->ip_dst.s_addr & 0xffff)),
                                    th->th_seq, th->th_ack, th->th_win);
                        }
                        TCP_INC_VAR(tp->t_unacksegs, segment_count);
                        if (DELAY_ACK(tp, th)) {
                                if ((tp->t_flags & TF_DELACK) == 0) {
                                        tp->t_flags |= TF_DELACK;
                                        tp->t_timer[TCPT_DELACK] = OFFSET_FROM_START(tp, tcp_delack);
                                }
                        } else {
                                tp->t_flags |= TF_ACKNOW;
                                tcp_output(tp);
                        }

                        tcp_adaptive_rwtimo_check(tp, tlen);

                        if (tlen > 0) {
                                tcp_tfo_rcv_data(tp);
                        }

                        tcp_check_timer_state(tp);

                        tcp_handle_wakeup(so, read_wakeup, write_wakeup);

                        socket_unlock(so, 1);
                        KERNEL_DEBUG(DBG_FNC_TCP_INPUT | DBG_FUNC_END, 0, 0, 0, 0, 0);
                        return;
                }
        }

        /*
         * Calculate amount of space in receive window,
         * and then do TCP input processing.
         * Receive window is amount of space in rcv queue,
         * but not less than advertised window.
         */
        socket_lock_assert_owned(so);
        win = tcp_sbspace(tp);
        if (win < 0) {
                win = 0;
        } else { /* clip rcv window to 4K for modems */
                if (tp->t_flags & TF_SLOWLINK && slowlink_wsize > 0) {
                        win = min(win, slowlink_wsize);
                }
        }
        tp->rcv_wnd = imax(win, (int)(tp->rcv_adv - tp->rcv_nxt));
#if MPTCP
        /*
         * Ensure that the subflow receive window isn't greater
         * than the connection level receive window.
         */
        if ((tp->t_mpflags & TMPF_MPTCP_TRUE) && (mp_tp = tptomptp(tp))) {
                socket_lock_assert_owned(mptetoso(mp_tp->mpt_mpte));
                int64_t recwin_conn = (int64_t)(mp_tp->mpt_rcvadv - mp_tp->mpt_rcvnxt);

                VERIFY(recwin_conn < INT32_MAX && recwin_conn > INT32_MIN);
                if (recwin_conn > 0 && tp->rcv_wnd > (uint32_t)recwin_conn) {
                        tp->rcv_wnd = (uint32_t)recwin_conn;
                        tcpstat.tcps_mp_reducedwin++;
                }
        }
#endif /* MPTCP */

        switch (tp->t_state) {
        /*
         * Initialize tp->rcv_nxt, and tp->irs, select an initial
         * tp->iss, and send a segment:
         *              <SEQ=ISS><ACK=RCV_NXT><CTL=SYN,ACK>
         * Also initialize tp->snd_nxt to tp->iss+1 and tp->snd_una to tp->iss.
         * Fill in remote peer address fields if not previously specified.
         * Enter SYN_RECEIVED state, and process any other fields of this
         * segment in this state.
         */
        case TCPS_LISTEN: {
                struct sockaddr_in *sin;
                struct sockaddr_in6 *sin6;

                socket_lock_assert_owned(so);

                /* Clear the logging flags inherited from the listening socket */
                tp->t_log_flags = 0;
                tp->t_flagsext &= ~TF_LOGGED_CONN_SUMMARY;

                if (isipv6) {
                        MALLOC(sin6, struct sockaddr_in6 *, sizeof *sin6,
                            M_SONAME, M_NOWAIT);
                        if (sin6 == NULL) {
                                TCP_LOG_DROP_PCB(TCP_LOG_HDR, th, tp, false, "LISTEN malloc M_SONAME failed");
                                goto drop;
                        }
                        bzero(sin6, sizeof(*sin6));
                        sin6->sin6_family = AF_INET6;
                        sin6->sin6_len = sizeof(*sin6);
                        sin6->sin6_addr = ip6->ip6_src;
                        sin6->sin6_port = th->th_sport;
                        laddr6 = inp->in6p_laddr;
                        if (IN6_IS_ADDR_UNSPECIFIED(&inp->in6p_laddr)) {
                                inp->in6p_laddr = ip6->ip6_dst;
                        }
                        if (in6_pcbconnect(inp, (struct sockaddr *)sin6,
                            kernel_proc)) {
                                inp->in6p_laddr = laddr6;
                                FREE(sin6, M_SONAME);
                                TCP_LOG_DROP_PCB(TCP_LOG_HDR, th, tp, false, " LISTEN in6_pcbconnect failed");
                                goto drop;
                        }
                        FREE(sin6, M_SONAME);
                } else {
                        socket_lock_assert_owned(so);
                        MALLOC(sin, struct sockaddr_in *, sizeof *sin, M_SONAME,
                            M_NOWAIT);
                        if (sin == NULL) {
                                TCP_LOG_DROP_PCB(TCP_LOG_HDR, th, tp, false, "LISTEN malloc M_SONAME failed");
                                goto drop;
                        }
                        sin->sin_family = AF_INET;
                        sin->sin_len = sizeof(*sin);
                        sin->sin_addr = ip->ip_src;
                        sin->sin_port = th->th_sport;
                        bzero((caddr_t)sin->sin_zero, sizeof(sin->sin_zero));
                        laddr = inp->inp_laddr;
                        if (inp->inp_laddr.s_addr == INADDR_ANY) {
                                inp->inp_laddr = ip->ip_dst;
                        }
                        if (in_pcbconnect(inp, (struct sockaddr *)sin, kernel_proc,
                            IFSCOPE_NONE, NULL)) {
                                inp->inp_laddr = laddr;
                                FREE(sin, M_SONAME);
                                TCP_LOG_DROP_PCB(TCP_LOG_HDR, th, tp, false, " LISTEN in_pcbconnect failed");
                                goto drop;
                        }
                        FREE(sin, M_SONAME);
                }

                tcp_dooptions(tp, optp, optlen, th, &to);
                tcp_finalize_options(tp, &to, ifscope);

                if (tfo_enabled(tp) && tcp_tfo_syn(tp, &to)) {
                        isconnected = TRUE;
                }

                if (iss) {
                        tp->iss = iss;
                } else {
                        tp->iss = tcp_new_isn(tp);
                }
                tp->irs = th->th_seq;
                tcp_sendseqinit(tp);
                tcp_rcvseqinit(tp);
                tp->snd_recover = tp->snd_una;
                /*
                 * Initialization of the tcpcb for transaction;
                 *   set SND.WND = SEG.WND,
                 *   initialize CCsend and CCrecv.
                 */
                tp->snd_wnd = tiwin;    /* initial send-window */
                tp->max_sndwnd = tp->snd_wnd;
                tp->t_flags |= TF_ACKNOW;
                tp->t_unacksegs = 0;
                DTRACE_TCP4(state__change, void, NULL, struct inpcb *, inp,
                    struct tcpcb *, tp, int32_t, TCPS_SYN_RECEIVED);
                tp->t_state = TCPS_SYN_RECEIVED;
                tp->t_timer[TCPT_KEEP] = OFFSET_FROM_START(tp,
                    TCP_CONN_KEEPINIT(tp));
                tp->t_connect_time = tcp_now;
                dropsocket = 0;         /* committed to socket */

                if (inp->inp_flowhash == 0) {
                        inp->inp_flowhash = inp_calc_flowhash(inp);
                }
                /* update flowinfo - RFC 6437 */
                if (inp->inp_flow == 0 &&
                    inp->in6p_flags & IN6P_AUTOFLOWLABEL) {
                        inp->inp_flow &= ~IPV6_FLOWLABEL_MASK;
                        inp->inp_flow |=
                            (htonl(inp->inp_flowhash) & IPV6_FLOWLABEL_MASK);
                }

                /* reset the incomp processing flag */
                so->so_flags &= ~(SOF_INCOMP_INPROGRESS);
                tcpstat.tcps_accepts++;
                if ((thflags & (TH_ECE | TH_CWR)) == (TH_ECE | TH_CWR)) {
                        /* ECN-setup SYN */
                        tp->ecn_flags |= (TE_SETUPRECEIVED | TE_SENDIPECT);
                }

                /*
                 * The address and connection state are finalized
                 */
                TCP_LOG_CONNECT(tp, false, 0);

                tcp_add_fsw_flow(tp, ifp);

                goto trimthenstep6;
        }

        /*
         * If the state is SYN_RECEIVED and the seg contains an ACK,
         * but not for our SYN/ACK, send a RST.
         */
        case TCPS_SYN_RECEIVED:
                if ((thflags & TH_ACK) &&
                    (SEQ_LEQ(th->th_ack, tp->snd_una) ||
                    SEQ_GT(th->th_ack, tp->snd_max))) {
                        IF_TCP_STATINC(ifp, ooopacket);
                        TCP_LOG_DROP_PCB(TCP_LOG_HDR, th, tp, false, "SYN_RECEIVED bad ACK");
                        goto dropwithreset;
                }

                /*
                 * In SYN_RECEIVED state, if we recv some SYNS with
                 * window scale and others without, window scaling should
                 * be disabled. Otherwise the window advertised will be
                 * lower if we assume scaling and the other end does not.
                 */
                if ((thflags & TH_SYN) &&
                    (tp->irs == th->th_seq) &&
                    !(to.to_flags & TOF_SCALE)) {
                        tp->t_flags &= ~TF_RCVD_SCALE;
                }
                break;

        /*
         * If the state is SYN_SENT:
         *      if seg contains an ACK, but not for our SYN, drop the input.
         *      if seg contains a RST, then drop the connection.
         *      if seg does not contain SYN, then drop it.
         * Otherwise this is an acceptable SYN segment
         *      initialize tp->rcv_nxt and tp->irs
         *      if seg contains ack then advance tp->snd_una
         *      if SYN has been acked change to ESTABLISHED else SYN_RCVD state
         *      arrange for segment to be acked (eventually)
         *      continue processing rest of data/controls, beginning with URG
         */
        case TCPS_SYN_SENT:
                if ((thflags & TH_ACK) &&
                    (SEQ_LEQ(th->th_ack, tp->iss) ||
                    SEQ_GT(th->th_ack, tp->snd_max))) {
                        IF_TCP_STATINC(ifp, ooopacket);
                        TCP_LOG_DROP_PCB(TCP_LOG_HDR, th, tp, false, "SYN_SENT bad ACK");
                        goto dropwithreset;
                }
                if (thflags & TH_RST) {
                        if ((thflags & TH_ACK) != 0) {
                                if (tfo_enabled(tp) &&
                                    !(tp->t_flagsext & TF_FASTOPEN_FORCE_ENABLE)) {
                                        tcp_heuristic_tfo_rst(tp);
                                }
                                if ((tp->ecn_flags & (TE_SETUPSENT | TE_RCVD_SYN_RST)) == TE_SETUPSENT) {
                                        /*
                                         * On local connections, send
                                         * non-ECN syn one time before
                                         * dropping the connection
                                         */
                                        if (tp->t_flags & TF_LOCAL) {
                                                tp->ecn_flags |= TE_RCVD_SYN_RST;
                                                goto drop;
                                        } else {
                                                tcp_heuristic_ecn_synrst(tp);
                                        }
                                }
                                soevent(so,
                                    (SO_FILT_HINT_LOCKED |
                                    SO_FILT_HINT_CONNRESET));
                                tp = tcp_drop(tp, ECONNREFUSED);
                        }
                        TCP_LOG_DROP_PCB(TCP_LOG_HDR, th, tp, false, "SYN_SENT got RST");
                        goto drop;
                }
                if ((thflags & TH_SYN) == 0) {
                        TCP_LOG_DROP_PCB(TCP_LOG_HDR, th, tp, false, "SYN_SENT no SYN");
                        goto drop;
                }
                tp->snd_wnd = th->th_win;       /* initial send window */
                tp->max_sndwnd = tp->snd_wnd;

                tp->irs = th->th_seq;
                tcp_rcvseqinit(tp);
                if (thflags & TH_ACK) {
                        tcpstat.tcps_connects++;

                        if ((thflags & (TH_ECE | TH_CWR)) == (TH_ECE)) {
                                /* ECN-setup SYN-ACK */
                                tp->ecn_flags |= TE_SETUPRECEIVED;
                                if (TCP_ECN_ENABLED(tp)) {
                                        tcp_heuristic_ecn_success(tp);
                                        tcpstat.tcps_ecn_client_success++;
                                }
                        } else {
                                if (tp->ecn_flags & TE_SETUPSENT &&
                                    tp->t_rxtshift == 0) {
                                        tcp_heuristic_ecn_success(tp);
                                        tcpstat.tcps_ecn_not_supported++;
                                }
                                if (tp->ecn_flags & TE_SETUPSENT &&
                                    tp->t_rxtshift > 0) {
                                        tcp_heuristic_ecn_loss(tp);
                                }

                                /* non-ECN-setup SYN-ACK */
                                tp->ecn_flags &= ~TE_SENDIPECT;
                        }

                        /* Do window scaling on this connection? */
                        if (TCP_WINDOW_SCALE_ENABLED(tp)) {
                                tp->snd_scale = tp->requested_s_scale;
                                tp->rcv_scale = tp->request_r_scale;
                        }

                        tp->rcv_adv += min(tp->rcv_wnd, TCP_MAXWIN << tp->rcv_scale);
                        tp->snd_una++;          /* SYN is acked */
                        if (SEQ_LT(tp->snd_nxt, tp->snd_una)) {
                                tp->snd_nxt = tp->snd_una;
                        }

                        /*
                         * We have sent more in the SYN than what is being
                         * acked. (e.g., TFO)
                         * We should restart the sending from what the receiver
                         * has acknowledged immediately.
                         */
                        if (SEQ_GT(tp->snd_nxt, th->th_ack)) {
                                /*
                                 * rdar://problem/33214601
                                 * There is a middlebox that acks all but one
                                 * byte and still drops the data.
                                 */
                                if (!(tp->t_flagsext & TF_FASTOPEN_FORCE_ENABLE) &&
                                    (tp->t_tfo_stats & TFO_S_SYN_DATA_SENT) &&
                                    tp->snd_max == th->th_ack + 1 &&
                                    tp->snd_max > tp->snd_una + 1) {
                                        tcp_heuristic_tfo_middlebox(tp);

                                        so->so_error = ENODATA;
                                        soevent(so,
                                            (SO_FILT_HINT_LOCKED | SO_FILT_HINT_MP_SUB_ERROR));

                                        tp->t_tfo_stats |= TFO_S_ONE_BYTE_PROXY;
                                }

                                tp->snd_max = tp->snd_nxt = th->th_ack;
                        }

                        /*
                         * If there's data, delay ACK; if there's also a FIN
                         * ACKNOW will be turned on later.
                         */
                        TCP_INC_VAR(tp->t_unacksegs, segment_count);
                        if (DELAY_ACK(tp, th) && tlen != 0) {
                                if ((tp->t_flags & TF_DELACK) == 0) {
                                        tp->t_flags |= TF_DELACK;
                                        tp->t_timer[TCPT_DELACK] = OFFSET_FROM_START(tp, tcp_delack);
                                }
                        } else {
                                tp->t_flags |= TF_ACKNOW;
                        }
                        /*
                         * Received <SYN,ACK> in SYN_SENT[*] state.
                         * Transitions:
                         *      SYN_SENT  --> ESTABLISHED
                         *      SYN_SENT* --> FIN_WAIT_1
                         */
                        tp->t_starttime = tcp_now;
                        tcp_sbrcv_tstmp_check(tp);
                        if (tp->t_flags & TF_NEEDFIN) {
                                DTRACE_TCP4(state__change, void, NULL,
                                    struct inpcb *, inp,
                                    struct tcpcb *, tp, int32_t,
                                    TCPS_FIN_WAIT_1);
                                tp->t_state = TCPS_FIN_WAIT_1;
                                tp->t_flags &= ~TF_NEEDFIN;
                                thflags &= ~TH_SYN;

                                TCP_LOG_CONNECTION_SUMMARY(tp);
                        } else {
                                DTRACE_TCP4(state__change, void, NULL,
                                    struct inpcb *, inp, struct tcpcb *,
                                    tp, int32_t, TCPS_ESTABLISHED);
                                tp->t_state = TCPS_ESTABLISHED;
                                tp->t_timer[TCPT_KEEP] =
                                    OFFSET_FROM_START(tp,
                                    TCP_CONN_KEEPIDLE(tp));
                                if (nstat_collect) {
                                        nstat_route_connect_success(
                                                inp->inp_route.ro_rt);
                                }
                                /*
                                 * The SYN is acknowledged but una is not
                                 * updated yet. So pass the value of
                                 * ack to compute sndbytes correctly
                                 */
                                inp_count_sndbytes(inp, th->th_ack);
                        }
                        tp->t_forced_acks = TCP_FORCED_ACKS_COUNT;
#if MPTCP
                        /*
                         * Do not send the connect notification for additional
                         * subflows until ACK for 3-way handshake arrives.
                         */
                        if ((!(tp->t_mpflags & TMPF_MPTCP_TRUE)) &&
                            (tp->t_mpflags & TMPF_SENT_JOIN)) {
                                isconnected = FALSE;
                        } else
#endif /* MPTCP */
                        isconnected = TRUE;

                        if ((tp->t_tfo_flags & (TFO_F_COOKIE_REQ | TFO_F_COOKIE_SENT)) ||
                            (tp->t_tfo_stats & TFO_S_SYN_DATA_SENT)) {
                                tcp_tfo_synack(tp, &to);

                                if ((tp->t_tfo_stats & TFO_S_SYN_DATA_SENT) &&
                                    SEQ_LT(tp->snd_una, th->th_ack)) {
                                        tp->t_tfo_stats |= TFO_S_SYN_DATA_ACKED;
                                        tcpstat.tcps_tfo_syn_data_acked++;
#if MPTCP
                                        if (so->so_flags & SOF_MP_SUBFLOW) {
                                                so->so_flags1 |= SOF1_TFO_REWIND;
                                        }
#endif
                                        tcp_tfo_rcv_probe(tp, tlen);
                                }
                        }
                } else {
                        /*
                         *  Received initial SYN in SYN-SENT[*] state => simul-
                         *  taneous open.
                         *  Do 3-way handshake:
                         *        SYN-SENT -> SYN-RECEIVED
                         *        SYN-SENT* -> SYN-RECEIVED*
                         */
                        tp->t_flags |= TF_ACKNOW;
                        tp->t_timer[TCPT_REXMT] = 0;
                        DTRACE_TCP4(state__change, void, NULL, struct inpcb *, inp,
                            struct tcpcb *, tp, int32_t, TCPS_SYN_RECEIVED);
                        tp->t_state = TCPS_SYN_RECEIVED;

                        /*
                         * During simultaneous open, TFO should not be used.
                         * So, we disable it here, to prevent that data gets
                         * sent on the SYN/ACK.
                         */
                        tcp_disable_tfo(tp);
                }

trimthenstep6:
                /*
                 * Advance th->th_seq to correspond to first data byte.
                 * If data, trim to stay within window,
                 * dropping FIN if necessary.
                 */
                th->th_seq++;
                if (tlen > tp->rcv_wnd) {
                        todrop = tlen - tp->rcv_wnd;
                        m_adj(m, -todrop);
                        tlen = tp->rcv_wnd;
                        thflags &= ~TH_FIN;
                        tcpstat.tcps_rcvpackafterwin++;
                        tcpstat.tcps_rcvbyteafterwin += todrop;
                }
                tp->snd_wl1 = th->th_seq - 1;
                tp->rcv_up = th->th_seq;
                /*
                 *  Client side of transaction: already sent SYN and data.
                 *  If the remote host used T/TCP to validate the SYN,
                 *  our data will be ACK'd; if so, enter normal data segment
                 *  processing in the middle of step 5, ack processing.
                 *  Otherwise, goto step 6.
                 */
                if (thflags & TH_ACK) {
                        goto process_ACK;
                }
                goto step6;
        /*
         * If the state is LAST_ACK or CLOSING or TIME_WAIT:
         *      do normal processing.
         *
         * NB: Leftover from RFC1644 T/TCP.  Cases to be reused later.
         */
        case TCPS_LAST_ACK:
        case TCPS_CLOSING:
        case TCPS_TIME_WAIT:
                break;  /* continue normal processing */

        /* Received a SYN while connection is already established.
         * This is a "half open connection and other anomalies" described
         * in RFC793 page 34, send an ACK so the remote reset the connection
         * or recovers by adjusting its sequence numbering. Sending an ACK is
         * in accordance with RFC 5961 Section 4.2
         */
        case TCPS_ESTABLISHED:
                if (thflags & TH_SYN && tlen <= 0) {
                        /* Drop the packet silently if we have reached the limit */
                        if (tcp_is_ack_ratelimited(tp)) {
                                TCP_LOG_DROP_PCB(TCP_LOG_HDR, th, tp, false, "ESTABLISHED rfc5961 rate limited");
                                goto drop;
                        } else {
                                /* Send challenge ACK */
                                tcpstat.tcps_synchallenge++;
                                TCP_LOG_DROP_PCB(TCP_LOG_HDR, th, tp, false, "ESTABLISHED rfc5961 challenge ACK");
                                goto dropafterack;
                        }
                }
                break;
        }

        /*
         * States other than LISTEN or SYN_SENT.
         * First check the RST flag and sequence number since reset segments
         * are exempt from the timestamp and connection count tests.  This
         * fixes a bug introduced by the Stevens, vol. 2, p. 960 bugfix
         * below which allowed reset segments in half the sequence space
         * to fall though and be processed (which gives forged reset
         * segments with a random sequence number a 50 percent chance of
         * killing a connection).
         * Then check timestamp, if present.
         * Then check the connection count, if present.
         * Then check that at least some bytes of segment are within
         * receive window.  If segment begins before rcv_nxt,
         * drop leading data (and SYN); if nothing left, just ack.
         *
         *
         * If the RST bit is set, check the sequence number to see
         * if this is a valid reset segment.
         * RFC 793 page 37:
         *   In all states except SYN-SENT, all reset (RST) segments
         *   are validated by checking their SEQ-fields.  A reset is
         *   valid if its sequence number is in the window.
         * Note: this does not take into account delayed ACKs, so
         *   we should test against last_ack_sent instead of rcv_nxt.
         *   The sequence number in the reset segment is normally an
         *   echo of our outgoing acknowlegement numbers, but some hosts
         *   send a reset with the sequence number at the rightmost edge
         *   of our receive window, and we have to handle this case.
         * Note 2: Paul Watson's paper "Slipping in the Window" has shown
         *   that brute force RST attacks are possible.  To combat this,
         *   we use a much stricter check while in the ESTABLISHED state,
         *   only accepting RSTs where the sequence number is equal to
         *   last_ack_sent.  In all other states (the states in which a
         *   RST is more likely), the more permissive check is used.
         * RFC 5961 Section 3.2: if the RST bit is set, sequence # is
         *    within the receive window and last_ack_sent == seq,
         *    then reset the connection. Otherwise if the seq doesn't
         *    match last_ack_sent, TCP must send challenge ACK. Perform
         *    rate limitation when sending the challenge ACK.
         * If we have multiple segments in flight, the intial reset
         * segment sequence numbers will be to the left of last_ack_sent,
         * but they will eventually catch up.
         * In any case, it never made sense to trim reset segments to
         * fit the receive window since RFC 1122 says:
         *   4.2.2.12  RST Segment: RFC-793 Section 3.4
         *
         *    A TCP SHOULD allow a received RST segment to include data.
         *
         *    DISCUSSION
         *         It has been suggested that a RST segment could contain
         *         ASCII text that encoded and explained the cause of the
         *         RST.  No standard has yet been established for such
         *         data.
         *
         * If the reset segment passes the sequence number test examine
         * the state:
         *    SYN_RECEIVED STATE:
         *      If passive open, return to LISTEN state.
         *      If active open, inform user that connection was refused.
         *    ESTABLISHED, FIN_WAIT_1, FIN_WAIT_2, CLOSE_WAIT STATES:
         *      Inform user that connection was reset, and close tcb.
         *    CLOSING, LAST_ACK STATES:
         *      Close the tcb.
         *    TIME_WAIT STATE:
         *      Drop the segment - see Stevens, vol. 2, p. 964 and
         *      RFC 1337.
         *
         *      Radar 4803931: Allows for the case where we ACKed the FIN but
         *                     there is already a RST in flight from the peer.
         *                     In that case, accept the RST for non-established
         *                     state if it's one off from last_ack_sent.
         *
         */
        if (thflags & TH_RST) {
                if ((SEQ_GEQ(th->th_seq, tp->last_ack_sent) &&
                    SEQ_LT(th->th_seq, tp->last_ack_sent + tp->rcv_wnd)) ||
                    (tp->rcv_wnd == 0 &&
                    ((tp->last_ack_sent == th->th_seq) ||
                    ((tp->last_ack_sent - 1) == th->th_seq)))) {
                        if (tp->last_ack_sent == th->th_seq) {
                                switch (tp->t_state) {
                                case TCPS_SYN_RECEIVED:
                                        IF_TCP_STATINC(ifp, rstinsynrcv);
                                        so->so_error = ECONNREFUSED;
                                        goto close;

                                case TCPS_ESTABLISHED:
                                        if (TCP_ECN_ENABLED(tp) &&
                                            tp->snd_una == tp->iss + 1 &&
                                            SEQ_GT(tp->snd_max, tp->snd_una)) {
                                                /*
                                                 * If the first data packet on an
                                                 * ECN connection, receives a RST
                                                 * increment the heuristic
                                                 */
                                                tcp_heuristic_ecn_droprst(tp);
                                        }
                                        OS_FALLTHROUGH;
                                case TCPS_FIN_WAIT_1:
                                case TCPS_CLOSE_WAIT:
                                case TCPS_FIN_WAIT_2:
                                        so->so_error = ECONNRESET;
close:
                                        soevent(so,
                                            (SO_FILT_HINT_LOCKED |
                                            SO_FILT_HINT_CONNRESET));

                                        tcpstat.tcps_drops++;
                                        tp = tcp_close(tp);
                                        break;

                                case TCPS_CLOSING:
                                case TCPS_LAST_ACK:
                                        tp = tcp_close(tp);
                                        break;

                                case TCPS_TIME_WAIT:
                                        break;
                                }
                        } else {
                                tcpstat.tcps_badrst++;
                                /* Drop if we have reached the ACK limit */
                                if (tcp_is_ack_ratelimited(tp)) {
                                        TCP_LOG_DROP_PCB(TCP_LOG_HDR, th, tp, false, "ESTABLISHED rfc5961 rate limited");
                                        goto drop;
                                } else {
                                        /* Send challenge ACK */
                                        tcpstat.tcps_rstchallenge++;
                                        TCP_LOG_DROP_PCB(TCP_LOG_HDR, th, tp, false, "ESTABLISHED rfc5961 challenge ACK");
                                        goto dropafterack;
                                }
                        }
                }
                goto drop;
        }

        /*
         * RFC 1323 PAWS: If we have a timestamp reply on this segment
         * and it's less than ts_recent, drop it.
         */
        if ((to.to_flags & TOF_TS) != 0 && tp->ts_recent &&
            TSTMP_LT(to.to_tsval, tp->ts_recent)) {
                /* Check to see if ts_recent is over 24 days old.  */
                if ((int)(tcp_now - tp->ts_recent_age) > TCP_PAWS_IDLE) {
                        /*
                         * Invalidate ts_recent.  If this segment updates
                         * ts_recent, the age will be reset later and ts_recent
                         * will get a valid value.  If it does not, setting
                         * ts_recent to zero will at least satisfy the
                         * requirement that zero be placed in the timestamp
                         * echo reply when ts_recent isn't valid.  The
                         * age isn't reset until we get a valid ts_recent
                         * because we don't want out-of-order segments to be
                         * dropped when ts_recent is old.
                         */
                        tp->ts_recent = 0;
                } else {
                        tcpstat.tcps_rcvduppack++;
                        tcpstat.tcps_rcvdupbyte += tlen;
                        tp->t_pawsdrop++;
                        tcpstat.tcps_pawsdrop++;

                        /*
                         * PAWS-drop when ECN is being used? That indicates
                         * that ECT-marked packets take a different path, with
                         * different congestion-characteristics.
                         *
                         * Only fallback when we did send less than 2GB as PAWS
                         * really has no reason to kick in earlier.
                         */
                        if (TCP_ECN_ENABLED(tp) &&
                            inp->inp_stat->rxbytes < 2147483648) {
                                INP_INC_IFNET_STAT(inp, ecn_fallback_reorder);
                                tcpstat.tcps_ecn_fallback_reorder++;
                                tcp_heuristic_ecn_aggressive(tp);
                        }

                        if (nstat_collect) {
                                nstat_route_rx(tp->t_inpcb->inp_route.ro_rt,
                                    1, tlen, NSTAT_RX_FLAG_DUPLICATE);
                                INP_ADD_STAT(inp, cell, wifi, wired,
                                    rxpackets, 1);
                                INP_ADD_STAT(inp, cell, wifi, wired,
                                    rxbytes, tlen);
                                tp->t_stat.rxduplicatebytes += tlen;
                                inp_set_activity_bitmap(inp);
                        }
                        if (tlen > 0) {
                                goto dropafterack;
                        }
                        goto drop;
                }
        }

        /*
         * In the SYN-RECEIVED state, validate that the packet belongs to
         * this connection before trimming the data to fit the receive
         * window.  Check the sequence number versus IRS since we know
         * the sequence numbers haven't wrapped.  This is a partial fix
         * for the "LAND" DoS attack.
         */
        if (tp->t_state == TCPS_SYN_RECEIVED && SEQ_LT(th->th_seq, tp->irs)) {
                IF_TCP_STATINC(ifp, dospacket);
                TCP_LOG_DROP_PCB(TCP_LOG_HDR, th, tp, false, "SYN_RECEIVED bad SEQ");
                goto dropwithreset;
        }

        /*
         * Check if there is old data at the beginning of the window
         * i.e. the sequence number is before rcv_nxt
         */
        todrop = tp->rcv_nxt - th->th_seq;
        if (todrop > 0) {
                boolean_t is_syn_set = FALSE;

                if (thflags & TH_SYN) {
                        is_syn_set = TRUE;
                        thflags &= ~TH_SYN;
                        th->th_seq++;
                        if (th->th_urp > 1) {
                                th->th_urp--;
                        } else {
                                thflags &= ~TH_URG;
                        }
                        todrop--;
                }
                /*
                 * Following if statement from Stevens, vol. 2, p. 960.
                 * The amount of duplicate data is greater than or equal
                 * to the size of the segment - entire segment is duplicate
                 */
                if (todrop > tlen
                    || (todrop == tlen && (thflags & TH_FIN) == 0)) {
                        /*
                         * Any valid FIN must be to the left of the window.
                         * At this point the FIN must be a duplicate or out
                         * of sequence; drop it.
                         */
                        thflags &= ~TH_FIN;

                        /*
                         * Send an ACK to resynchronize and drop any data.
                         * But keep on processing for RST or ACK.
                         *
                         * If the SYN bit was originally set, then only send
                         * an ACK if we are not rate-limiting this connection.
                         */
                        if (is_syn_set) {
                                if (!tcp_is_ack_ratelimited(tp)) {
                                        tcpstat.tcps_synchallenge++;
                                        tp->t_flags |= TF_ACKNOW;
                                }
                        } else {
                                tp->t_flags |= TF_ACKNOW;
                        }

                        if (todrop == 1) {
                                /* This could be a keepalive */
                                soevent(so, SO_FILT_HINT_LOCKED |
                                    SO_FILT_HINT_KEEPALIVE);
                        }
                        todrop = tlen;
                        tcpstat.tcps_rcvduppack++;
                        tcpstat.tcps_rcvdupbyte += todrop;
                } else {
                        tcpstat.tcps_rcvpartduppack++;
                        tcpstat.tcps_rcvpartdupbyte += todrop;
                }

                if (todrop > 1) {
                        /*
                         * Note the duplicate data sequence space so that
                         * it can be reported in DSACK option.
                         */
                        tp->t_dsack_lseq = th->th_seq;
                        tp->t_dsack_rseq = th->th_seq + todrop;
                        tp->t_flags |= TF_ACKNOW;
                }
                if (nstat_collect) {
                        nstat_route_rx(tp->t_inpcb->inp_route.ro_rt, 1,
                            todrop, NSTAT_RX_FLAG_DUPLICATE);
                        INP_ADD_STAT(inp, cell, wifi, wired, rxpackets, 1);
                        INP_ADD_STAT(inp, cell, wifi, wired, rxbytes, todrop);
                        tp->t_stat.rxduplicatebytes += todrop;
                        inp_set_activity_bitmap(inp);
                }
                drop_hdrlen += todrop;  /* drop from the top afterwards */
                th->th_seq += todrop;
                tlen -= todrop;
                if (th->th_urp > todrop) {
                        th->th_urp -= todrop;
                } else {
                        thflags &= ~TH_URG;
                        th->th_urp = 0;
                }
        }

        /*
         * If new data are received on a connection after the user
         * processes are gone, then RST the other end.
         * Send also a RST when we received a data segment after we've
         * sent our FIN when the socket is defunct.
         * Note that an MPTCP subflow socket would have SS_NOFDREF set
         * by default. So, if it's an MPTCP-subflow we rather check the
         * MPTCP-level's socket state for SS_NOFDREF.
         */
        if (tlen) {
                boolean_t close_it = FALSE;

                if (!(so->so_flags & SOF_MP_SUBFLOW) && (so->so_state & SS_NOFDREF) &&
                    tp->t_state > TCPS_CLOSE_WAIT) {
                        TCP_LOG_DROP_PCB(TCP_LOG_HDR, th, tp, false, "SS_NOFDREF");
                        close_it = TRUE;
                }

                if ((so->so_flags & SOF_MP_SUBFLOW) && (mptetoso(tptomptp(tp)->mpt_mpte)->so_state & SS_NOFDREF) &&
                    tp->t_state > TCPS_CLOSE_WAIT) {
                        TCP_LOG_DROP_PCB(TCP_LOG_HDR, th, tp, false, "SOF_MP_SUBFLOW SS_NOFDREF");
                        close_it = TRUE;
                }

                if ((so->so_flags & SOF_DEFUNCT) && tp->t_state > TCPS_FIN_WAIT_1) {
                        TCP_LOG_DROP_PCB(TCP_LOG_HDR, th, tp, false, "SOF_DEFUNCT");
                        close_it = TRUE;
                }

                if (close_it) {
                        tp = tcp_close(tp);
                        tcpstat.tcps_rcvafterclose++;
                        IF_TCP_STATINC(ifp, cleanup);
                        goto dropwithreset;
                }
        }

        /*
         * If segment ends after window, drop trailing data
         * (and PUSH and FIN); if nothing left, just ACK.
         */
        todrop = (th->th_seq + tlen) - (tp->rcv_nxt + tp->rcv_wnd);
        if (todrop > 0) {
                tcpstat.tcps_rcvpackafterwin++;
                if (todrop >= tlen) {
                        tcpstat.tcps_rcvbyteafterwin += tlen;
                        /*
                         * If a new connection request is received
                         * while in TIME_WAIT, drop the old connection
                         * and start over if the sequence numbers
                         * are above the previous ones.
                         */
                        if (thflags & TH_SYN &&
                            tp->t_state == TCPS_TIME_WAIT &&
                            SEQ_GT(th->th_seq, tp->rcv_nxt)) {
                                iss = tcp_new_isn(tp);
                                tp = tcp_close(tp);
                                socket_unlock(so, 1);
                                goto findpcb;
                        }
                        /*
                         * If window is closed can only take segments at
                         * window edge, and have to drop data and PUSH from
                         * incoming segments.  Continue processing, but
                         * remember to ack.  Otherwise, drop segment
                         * and ack.
                         */
                        if (tp->rcv_wnd == 0 && th->th_seq == tp->rcv_nxt) {
                                tp->t_flags |= TF_ACKNOW;
                                tcpstat.tcps_rcvwinprobe++;
                        } else {
                                goto dropafterack;
                        }
                } else {
                        tcpstat.tcps_rcvbyteafterwin += todrop;
                }
                m_adj(m, -todrop);
                tlen -= todrop;
                thflags &= ~(TH_PUSH | TH_FIN);
        }

        /*
         * If last ACK falls within this segment's sequence numbers,
         * record its timestamp.
         * NOTE:
         * 1) That the test incorporates suggestions from the latest
         *    proposal of the tcplw@cray.com list (Braden 1993/04/26).
         * 2) That updating only on newer timestamps interferes with
         *    our earlier PAWS tests, so this check should be solely
         *    predicated on the sequence space of this segment.
         * 3) That we modify the segment boundary check to be
         *        Last.ACK.Sent <= SEG.SEQ + SEG.Len
         *    instead of RFC1323's
         *        Last.ACK.Sent < SEG.SEQ + SEG.Len,
         *    This modified check allows us to overcome RFC1323's
         *    limitations as described in Stevens TCP/IP Illustrated
         *    Vol. 2 p.869. In such cases, we can still calculate the
         *    RTT correctly when RCV.NXT == Last.ACK.Sent.
         */
        if ((to.to_flags & TOF_TS) != 0 &&
            SEQ_LEQ(th->th_seq, tp->last_ack_sent) &&
            SEQ_LEQ(tp->last_ack_sent, th->th_seq + tlen +
            ((thflags & (TH_SYN | TH_FIN)) != 0))) {
                tp->ts_recent_age = tcp_now;
                tp->ts_recent = to.to_tsval;
        }

        /*
         * Stevens: If a SYN is in the window, then this is an
         * error and we send an RST and drop the connection.
         *
         * RFC 5961 Section 4.2
         * Send challenge ACK for any SYN in synchronized state
         * Perform rate limitation in doing so.
         */
        if (thflags & TH_SYN) {
                if (!tcp_syn_data_valid(tp, th, tlen)) {
                        tcpstat.tcps_badsyn++;
                        /* Drop if we have reached ACK limit */
                        if (tcp_is_ack_ratelimited(tp)) {
                                TCP_LOG_DROP_PCB(TCP_LOG_HDR, th, tp, false, "rfc5961 bad SYN rate limited");
                                goto drop;
                        } else {
                                /* Send challenge ACK */
                                tcpstat.tcps_synchallenge++;
                                TCP_LOG_DROP_PCB(TCP_LOG_HDR, th, tp, false, "rfc5961 bad SYN challenge ack");
                                goto dropafterack;
                        }
                } else {
                        /*
                         * Received SYN (/ACK) with data.
                         * Move sequence number along to process the data.
                         */
                        th->th_seq++;
                        thflags &= ~TH_SYN;
                }
        }

        /*
         * If the ACK bit is off:  if in SYN-RECEIVED state or SENDSYN
         * flag is on (half-synchronized state), then queue data for
         * later processing; else drop segment and return.
         */
        if ((thflags & TH_ACK) == 0) {
                if (tp->t_state == TCPS_SYN_RECEIVED) {
                        if ((tfo_enabled(tp))) {
                                /*
                                 * So, we received a valid segment while in
                                 * SYN-RECEIVED.
                                 * As this cannot be an RST (see that if a bit
                                 * higher), and it does not have the ACK-flag
                                 * set, we want to retransmit the SYN/ACK.
                                 * Thus, we have to reset snd_nxt to snd_una to
                                 * trigger the going back to sending of the
                                 * SYN/ACK. This is more consistent with the
                                 * behavior of tcp_output(), which expects
                                 * to send the segment that is pointed to by
                                 * snd_nxt.
                                 */
                                tp->snd_nxt = tp->snd_una;

                                /*
                                 * We need to make absolutely sure that we are
                                 * going to reply upon a duplicate SYN-segment.
                                 */
                                if (th->th_flags & TH_SYN) {
                                        needoutput = 1;
                                }
                        }

                        goto step6;
                } else if (tp->t_flags & TF_ACKNOW) {
                        TCP_LOG_DROP_PCB(TCP_LOG_HDR, th, tp, false, "bad ACK");
                        goto dropafterack;
                } else {
                        TCP_LOG_DROP_PCB(TCP_LOG_HDR, th, tp, false, "bad ACK");
                        goto drop;
                }
        }

        /*
         * Ack processing.
         */

        switch (tp->t_state) {
        /*
         * In SYN_RECEIVED state, the ack ACKs our SYN, so enter
         * ESTABLISHED state and continue processing.
         * The ACK was checked above.
         */
        case TCPS_SYN_RECEIVED:

                tcpstat.tcps_connects++;

                /* Do window scaling? */
                if (TCP_WINDOW_SCALE_ENABLED(tp)) {
                        tp->snd_scale = tp->requested_s_scale;
                        tp->rcv_scale = tp->request_r_scale;
                        tp->snd_wnd = th->th_win << tp->snd_scale;
                        tp->max_sndwnd = tp->snd_wnd;
                        tiwin = tp->snd_wnd;
                }
                /*
                 * Make transitions:
                 *      SYN-RECEIVED  -> ESTABLISHED
                 *      SYN-RECEIVED* -> FIN-WAIT-1
                 */
                tp->t_starttime = tcp_now;
                tcp_sbrcv_tstmp_check(tp);
                if (tp->t_flags & TF_NEEDFIN) {
                        DTRACE_TCP4(state__change, void, NULL,
                            struct inpcb *, inp,
                            struct tcpcb *, tp, int32_t, TCPS_FIN_WAIT_1);
                        tp->t_state = TCPS_FIN_WAIT_1;
                        tp->t_flags &= ~TF_NEEDFIN;

                        TCP_LOG_CONNECTION_SUMMARY(tp);
                } else {
                        DTRACE_TCP4(state__change, void, NULL,
                            struct inpcb *, inp,
                            struct tcpcb *, tp, int32_t, TCPS_ESTABLISHED);
                        tp->t_state = TCPS_ESTABLISHED;
                        tp->t_timer[TCPT_KEEP] = OFFSET_FROM_START(tp,
                            TCP_CONN_KEEPIDLE(tp));
                        if (nstat_collect) {
                                nstat_route_connect_success(
                                        tp->t_inpcb->inp_route.ro_rt);
                        }

                        /*
                         * The SYN is acknowledged but una is not updated
                         * yet. So pass the value of ack to compute
                         * sndbytes correctly
                         */
                        inp_count_sndbytes(inp, th->th_ack);
                }
                tp->t_forced_acks = TCP_FORCED_ACKS_COUNT;
                /*
                 * If segment contains data or ACK, will call tcp_reass()
                 * later; if not, do so now to pass queued data to user.
                 */
                if (tlen == 0 && (thflags & TH_FIN) == 0) {
                        if (isipv6) {
                                memcpy(&saved_hdr, ip6, sizeof(struct ip6_hdr));
                                ip6 = (struct ip6_hdr *)&saved_hdr[0];
                        } else {
                                memcpy(&saved_hdr, ip, ip->ip_hl << 2);
                                ip = (struct ip *)&saved_hdr[0];
                        }
                        memcpy(&saved_tcphdr, th, sizeof(struct tcphdr));
                        (void) tcp_reass(tp, (struct tcphdr *)0, &tlen,
                            NULL, ifp, &read_wakeup);
                        th = &saved_tcphdr;
                }
                tp->snd_wl1 = th->th_seq - 1;

#if MPTCP
                /*
                 * Do not send the connect notification for additional subflows
                 * until ACK for 3-way handshake arrives.
                 */
                if ((!(tp->t_mpflags & TMPF_MPTCP_TRUE)) &&
                    (tp->t_mpflags & TMPF_SENT_JOIN)) {
                        isconnected = FALSE;
                } else
#endif /* MPTCP */
                isconnected = TRUE;
                if ((tp->t_tfo_flags & TFO_F_COOKIE_VALID)) {
                        /* Done this when receiving the SYN */
                        isconnected = FALSE;

                        OSDecrementAtomic(&tcp_tfo_halfcnt);

                        /* Panic if something has gone terribly wrong. */
                        VERIFY(tcp_tfo_halfcnt >= 0);

                        tp->t_tfo_flags &= ~TFO_F_COOKIE_VALID;
                }

                /*
                 * In case there is data in the send-queue (e.g., TFO is being
                 * used, or connectx+data has been done), then if we would
                 * "FALLTHROUGH", we would handle this ACK as if data has been
                 * acknowledged. But, we have to prevent this. And this
                 * can be prevented by increasing snd_una by 1, so that the
                 * SYN is not considered as data (snd_una++ is actually also
                 * done in SYN_SENT-state as part of the regular TCP stack).
                 *
                 * In case there is data on this ack as well, the data will be
                 * handled by the label "dodata" right after step6.
                 */
                if (so->so_snd.sb_cc) {
                        tp->snd_una++;  /* SYN is acked */
                        if (SEQ_LT(tp->snd_nxt, tp->snd_una)) {
                                tp->snd_nxt = tp->snd_una;
                        }

                        /*
                         * No duplicate-ACK handling is needed. So, we
                         * directly advance to processing the ACK (aka,
                         * updating the RTT estimation,...)
                         *
                         * But, we first need to handle eventual SACKs,
                         * because TFO will start sending data with the
                         * SYN/ACK, so it might be that the client
                         * includes a SACK with its ACK.
                         */
                        if (SACK_ENABLED(tp) &&
                            (to.to_nsacks > 0 || !TAILQ_EMPTY(&tp->snd_holes))) {
                                tcp_sack_doack(tp, &to, th, &sack_bytes_acked, &sack_bytes_newly_acked);
                        }

                        goto process_ACK;
                }

                OS_FALLTHROUGH;

        /*
         * In ESTABLISHED state: drop duplicate ACKs; ACK out of range
         * ACKs.  If the ack is in the range
         *      tp->snd_una < th->th_ack <= tp->snd_max
         * then advance tp->snd_una to th->th_ack and drop
         * data from the retransmission queue.  If this ACK reflects
         * more up to date window information we update our window information.
         */
        case TCPS_ESTABLISHED:
        case TCPS_FIN_WAIT_1:
        case TCPS_FIN_WAIT_2:
        case TCPS_CLOSE_WAIT:
        case TCPS_CLOSING:
        case TCPS_LAST_ACK:
        case TCPS_TIME_WAIT:
                if (SEQ_GT(th->th_ack, tp->snd_max)) {
                        tcpstat.tcps_rcvacktoomuch++;
                        if (tcp_is_ack_ratelimited(tp)) {
                                TCP_LOG_DROP_PCB(TCP_LOG_HDR, th, tp, false, "rfc5961 rcvacktoomuch");
                                goto drop;
                        } else {
                                goto dropafterack;
                        }
                }
                if (SEQ_LT(th->th_ack, tp->snd_una - tp->max_sndwnd)) {
                        if (tcp_is_ack_ratelimited(tp)) {
                                TCP_LOG_DROP_PCB(TCP_LOG_HDR, th, tp, false, "rfc5961 bad ACK");
                                goto drop;
                        } else {
                                goto dropafterack;
                        }
                }
                if (SACK_ENABLED(tp) && to.to_nsacks > 0) {
                        recvd_dsack = tcp_sack_process_dsack(tp, &to, th);
                        /*
                         * If DSACK is received and this packet has no
                         * other SACK information, it can be dropped.
                         * We do not want to treat it as a duplicate ack.
                         */
                        if (recvd_dsack &&
                            SEQ_LEQ(th->th_ack, tp->snd_una) &&
                            to.to_nsacks == 0) {
                                tcp_bad_rexmt_check(tp, th, &to);
                                goto drop;
                        }
                }

                if (SACK_ENABLED(tp) &&
                    (to.to_nsacks > 0 || !TAILQ_EMPTY(&tp->snd_holes))) {
                        tcp_sack_doack(tp, &to, th, &sack_bytes_acked, &sack_bytes_newly_acked);
                }

#if MPTCP
                if (tp->t_mpuna && SEQ_GEQ(th->th_ack, tp->t_mpuna)) {
                        if (tp->t_mpflags & TMPF_PREESTABLISHED) {
                                /* MP TCP establishment succeeded */
                                tp->t_mpuna = 0;
                                if (tp->t_mpflags & TMPF_JOINED_FLOW) {
                                        if (tp->t_mpflags & TMPF_SENT_JOIN) {
                                                tp->t_mpflags &=
                                                    ~TMPF_PREESTABLISHED;
                                                tp->t_mpflags |=
                                                    TMPF_MPTCP_TRUE;

                                                tp->t_timer[TCPT_JACK_RXMT] = 0;
                                                tp->t_mprxtshift = 0;
                                                isconnected = TRUE;
                                        } else {
                                                isconnected = FALSE;
                                        }
                                } else {
                                        isconnected = TRUE;
                                }
                        }
                }
#endif /* MPTCP */

                tcp_tfo_rcv_ack(tp, th);

                /*
                 * If we have outstanding data (other than
                 * a window probe), this is a completely
                 * duplicate ack and the ack is the biggest we've seen.
                 *
                 * Need to accommodate a change in window on duplicate acks
                 * to allow operating systems that update window during
                 * recovery with SACK
                 */
                if (SEQ_LEQ(th->th_ack, tp->snd_una)) {
                        if (tlen == 0 && (tiwin == tp->snd_wnd ||
                            (to.to_nsacks > 0 && sack_bytes_acked > 0))) {
                                uint32_t old_dupacks;
                                /*
                                 * If both ends send FIN at the same time,
                                 * then the ack will be a duplicate ack
                                 * but we have to process the FIN. Check
                                 * for this condition and process the FIN
                                 * instead of the dupack
                                 */
                                if ((thflags & TH_FIN) &&
                                    !TCPS_HAVERCVDFIN(tp->t_state)) {
                                        break;
                                }
process_dupack:
                                old_dupacks = tp->t_dupacks;
#if MPTCP
                                /*
                                 * MPTCP options that are ignored must
                                 * not be treated as duplicate ACKs.
                                 */
                                if (to.to_flags & TOF_MPTCP) {
                                        goto drop;
                                }

                                if ((isconnected) && (tp->t_mpflags & TMPF_JOINED_FLOW)) {
                                        mptcplog((LOG_DEBUG, "MPTCP "
                                            "Sockets: bypass ack recovery\n"),
                                            MPTCP_SOCKET_DBG,
                                            MPTCP_LOGLVL_VERBOSE);
                                        break;
                                }
#endif /* MPTCP */
                                /*
                                 * If a duplicate acknowledgement was seen
                                 * after ECN, it indicates packet loss in
                                 * addition to ECN. Reset INRECOVERY flag
                                 * so that we can process partial acks
                                 * correctly
                                 */
                                if (tp->ecn_flags & TE_INRECOVERY) {
                                        tp->ecn_flags &= ~TE_INRECOVERY;
                                }

                                tcpstat.tcps_rcvdupack++;
                                if (SACK_ENABLED(tp) && tcp_do_better_lr) {
                                        tp->t_dupacks += max(1, sack_bytes_acked / tp->t_maxseg);
                                } else {
                                        ++tp->t_dupacks;
                                }

                                tp->sackhint.sack_bytes_acked += sack_bytes_acked;

                                if (SACK_ENABLED(tp) && tcp_do_better_lr) {
                                        tp->t_new_dupacks += (sack_bytes_newly_acked / tp->t_maxseg);

                                        if (tp->t_new_dupacks >= tp->t_rexmtthresh && IN_FASTRECOVERY(tp)) {
                                                /* Let's restart the retransmission */
                                                tcp_sack_lost_rexmit(tp);

                                                /*
                                                 * If the current tcp cc module has
                                                 * defined a hook for tasks to run
                                                 * before entering FR, call it
                                                 */
                                                if (CC_ALGO(tp)->pre_fr != NULL) {
                                                        CC_ALGO(tp)->pre_fr(tp);
                                                }

                                                ENTER_FASTRECOVERY(tp);

                                                if (tp->t_flags & TF_SENTFIN) {
                                                        tp->snd_recover = tp->snd_max - 1;
                                                } else {
                                                        tp->snd_recover = tp->snd_max;
                                                }
                                                tp->t_rtttime = 0;

                                                if (TCP_ECN_ENABLED(tp)) {
                                                        tp->ecn_flags |= TE_SENDCWR;
                                                }

                                                if (tp->t_flagsext & TF_CWND_NONVALIDATED) {
                                                        tcp_cc_adjust_nonvalidated_cwnd(tp);
                                                } else {
                                                        tp->snd_cwnd = tp->snd_ssthresh;
                                                }
                                        }
                                }

                                /*
                                 * Check if we need to reset the limit on
                                 * early retransmit
                                 */
                                if (tp->t_early_rexmt_count > 0 &&
                                    TSTMP_GEQ(tcp_now,
                                    (tp->t_early_rexmt_win +
                                    TCP_EARLY_REXMT_WIN))) {
                                        tp->t_early_rexmt_count = 0;
                                }

                                /*
                                 * Is early retransmit needed? We check for
                                 * this when the connection is waiting for
                                 * duplicate acks to enter fast recovery.
                                 */
                                if (!IN_FASTRECOVERY(tp)) {
                                        tcp_early_rexmt_check(tp, th);
                                }

                                /*
                                 * If we've seen exactly rexmt threshold
                                 * of duplicate acks, assume a packet
                                 * has been dropped and retransmit it.
                                 * Kludge snd_nxt & the congestion
                                 * window so we send only this one
                                 * packet.
                                 *
                                 * We know we're losing at the current
                                 * window size so do congestion avoidance
                                 * (set ssthresh to half the current window
                                 * and pull our congestion window back to
                                 * the new ssthresh).
                                 *
                                 * Dup acks mean that packets have left the
                                 * network (they're now cached at the receiver)
                                 * so bump cwnd by the amount in the receiver
                                 * to keep a constant cwnd packets in the
                                 * network.
                                 */
                                if (tp->t_timer[TCPT_REXMT] == 0 ||
                                    (th->th_ack != tp->snd_una && sack_bytes_acked == 0)) {
                                        tp->t_dupacks = 0;
                                        tp->t_rexmtthresh = tcprexmtthresh;
                                        tp->t_new_dupacks = 0;
                                } else if ((tp->t_dupacks > tp->t_rexmtthresh && (!tcp_do_better_lr || old_dupacks >= tp->t_rexmtthresh)) ||
                                    IN_FASTRECOVERY(tp)) {
                                        /*
                                         * If this connection was seeing packet
                                         * reordering, then recovery might be
                                         * delayed to disambiguate between
                                         * reordering and loss
                                         */
                                        if (SACK_ENABLED(tp) && !IN_FASTRECOVERY(tp) &&
                                            (tp->t_flagsext &
                                            (TF_PKTS_REORDERED | TF_DELAY_RECOVERY)) ==
                                            (TF_PKTS_REORDERED | TF_DELAY_RECOVERY)) {
                                                /*
                                                 * Since the SACK information is already
                                                 * updated, this ACK will be dropped
                                                 */
                                                break;
                                        }

                                        /*
                                         * Dup acks mean that packets have left the
                                         * network (they're now cached at the receiver)
                                         * so bump cwnd by the amount in the receiver
                                         * to keep a constant cwnd packets in the
                                         * network.
                                         */
                                        if (SACK_ENABLED(tp) && IN_FASTRECOVERY(tp)) {
                                                int awnd;

                                                /*
                                                 * Compute the amount of data in flight first.
                                                 * We can inject new data into the pipe iff
                                                 * we have less than snd_ssthres worth of data in
                                                 * flight.
                                                 */
                                                awnd = (tp->snd_nxt - tp->snd_fack) + tp->sackhint.sack_bytes_rexmit;
                                                if (awnd < tp->snd_ssthresh) {
                                                        tp->snd_cwnd += tp->t_maxseg;
                                                        if (tp->snd_cwnd > tp->snd_ssthresh) {
                                                                tp->snd_cwnd = tp->snd_ssthresh;
                                                        }
                                                }
                                        } else {
                                                tp->snd_cwnd += tp->t_maxseg;
                                        }

                                        /* Process any window updates */
                                        if (tiwin > tp->snd_wnd) {
                                                tcp_update_window(tp, thflags,
                                                    th, tiwin, tlen);
                                        }
                                        tcp_ccdbg_trace(tp, th,
                                            TCP_CC_IN_FASTRECOVERY);

                                        (void) tcp_output(tp);

                                        goto drop;
                                } else if ((!tcp_do_better_lr && tp->t_dupacks == tp->t_rexmtthresh) ||
                                    (tcp_do_better_lr && tp->t_dupacks >= tp->t_rexmtthresh)) {
                                        tcp_seq onxt = tp->snd_nxt;

                                        /*
                                         * If we're doing sack, check to
                                         * see if we're already in sack
                                         * recovery. If we're not doing sack,
                                         * check to see if we're in newreno
                                         * recovery.
                                         */
                                        if (SACK_ENABLED(tp)) {
                                                if (IN_FASTRECOVERY(tp)) {
                                                        tp->t_dupacks = 0;
                                                        break;
                                                } else if (tp->t_flagsext & TF_DELAY_RECOVERY) {
                                                        break;
                                                }
                                        } else {
                                                if (SEQ_LEQ(th->th_ack, tp->snd_recover)) {
                                                        tp->t_dupacks = 0;
                                                        break;
                                                }
                                        }
                                        if (tp->t_flags & TF_SENTFIN) {
                                                tp->snd_recover = tp->snd_max - 1;
                                        } else {
                                                tp->snd_recover = tp->snd_max;
                                        }
                                        tp->t_timer[TCPT_PTO] = 0;
                                        tp->t_rtttime = 0;

                                        /*
                                         * If the connection has seen pkt
                                         * reordering, delay recovery until
                                         * it is clear that the packet
                                         * was lost.
                                         */
                                        if (SACK_ENABLED(tp) &&
                                            (tp->t_flagsext &
                                            (TF_PKTS_REORDERED | TF_DELAY_RECOVERY))
                                            == TF_PKTS_REORDERED &&
                                            !IN_FASTRECOVERY(tp) &&
                                            tp->t_reorderwin > 0 &&
                                            (tp->t_state == TCPS_ESTABLISHED ||
                                            tp->t_state == TCPS_FIN_WAIT_1)) {
                                                tp->t_timer[TCPT_DELAYFR] =
                                                    OFFSET_FROM_START(tp,
                                                    tp->t_reorderwin);
                                                tp->t_flagsext |= TF_DELAY_RECOVERY;
                                                tcpstat.tcps_delay_recovery++;
                                                tcp_ccdbg_trace(tp, th,
                                                    TCP_CC_DELAY_FASTRECOVERY);
                                                break;
                                        }

                                        tcp_rexmt_save_state(tp);
                                        /*
                                         * If the current tcp cc module has
                                         * defined a hook for tasks to run
                                         * before entering FR, call it
                                         */
                                        if (CC_ALGO(tp)->pre_fr != NULL) {
                                                CC_ALGO(tp)->pre_fr(tp);
                                        }
                                        ENTER_FASTRECOVERY(tp);
                                        tp->t_timer[TCPT_REXMT] = 0;
                                        if (TCP_ECN_ENABLED(tp)) {
                                                tp->ecn_flags |= TE_SENDCWR;
                                        }

                                        if (SACK_ENABLED(tp)) {
                                                tcpstat.tcps_sack_recovery_episode++;
                                                tp->t_sack_recovery_episode++;
                                                tp->sack_newdata = tp->snd_nxt;
                                                if (tcp_do_better_lr) {
                                                        tp->snd_cwnd = tp->snd_ssthresh;
                                                } else {
                                                        tp->snd_cwnd = tp->t_maxseg;
                                                }
                                                tp->t_flagsext &= ~TF_CWND_NONVALIDATED;

                                                /* Process any window updates */
                                                if (tiwin > tp->snd_wnd) {
                                                        tcp_update_window(tp, thflags, th, tiwin, tlen);
                                                }

                                                tcp_ccdbg_trace(tp, th, TCP_CC_ENTER_FASTRECOVERY);
                                                (void) tcp_output(tp);
                                                goto drop;
                                        }
                                        tp->snd_nxt = th->th_ack;
                                        tp->snd_cwnd = tp->t_maxseg;

                                        /* Process any window updates */
                                        if (tiwin > tp->snd_wnd) {
                                                tcp_update_window(tp, thflags, th, tiwin, tlen);
                                        }

                                        (void) tcp_output(tp);
                                        if (tp->t_flagsext & TF_CWND_NONVALIDATED) {
                                                tcp_cc_adjust_nonvalidated_cwnd(tp);
                                        } else {
                                                tp->snd_cwnd = tp->snd_ssthresh + tp->t_maxseg * tp->t_dupacks;
                                        }
                                        if (SEQ_GT(onxt, tp->snd_nxt)) {
                                                tp->snd_nxt = onxt;
                                        }

                                        tcp_ccdbg_trace(tp, th, TCP_CC_ENTER_FASTRECOVERY);
                                        goto drop;
                                } else if (ALLOW_LIMITED_TRANSMIT(tp) &&
                                    (!(SACK_ENABLED(tp)) || sack_bytes_acked > 0) &&
                                    (so->so_snd.sb_cc - (tp->snd_max - tp->snd_una)) > 0) {
                                        u_int32_t incr = (tp->t_maxseg * tp->t_dupacks);

                                        /* Use Limited Transmit algorithm on the first two
                                         * duplicate acks when there is new data to transmit
                                         */
                                        tp->snd_cwnd += incr;
                                        tcpstat.tcps_limited_txt++;
                                        (void) tcp_output(tp);

                                        tcp_ccdbg_trace(tp, th, TCP_CC_LIMITED_TRANSMIT);

                                        /* Reset snd_cwnd back to normal */
                                        tp->snd_cwnd -= incr;
                                }
                        }
                        break;
                }
                /*
                 * If the congestion window was inflated to account
                 * for the other side's cached packets, retract it.
                 */
                if (IN_FASTRECOVERY(tp)) {
                        if (SEQ_LT(th->th_ack, tp->snd_recover)) {
                                /*
                                 * If we received an ECE and entered
                                 * recovery, the subsequent ACKs should
                                 * not be treated as partial acks.
                                 */
                                if (tp->ecn_flags & TE_INRECOVERY) {
                                        goto process_ACK;
                                }

                                if (SACK_ENABLED(tp)) {
                                        tcp_sack_partialack(tp, th);
                                } else {
                                        tcp_newreno_partial_ack(tp, th);
                                }
                                tcp_ccdbg_trace(tp, th, TCP_CC_PARTIAL_ACK);
                        } else {
                                if (tcp_cubic_minor_fixes) {
                                        exiting_fr = 1;
                                }
                                EXIT_FASTRECOVERY(tp);
                                if (CC_ALGO(tp)->post_fr != NULL) {
                                        CC_ALGO(tp)->post_fr(tp, th);
                                }
                                tp->t_pipeack = 0;
                                tcp_clear_pipeack_state(tp);
                                tcp_ccdbg_trace(tp, th,
                                    TCP_CC_EXIT_FASTRECOVERY);
                        }
                } else if ((tp->t_flagsext &
                    (TF_PKTS_REORDERED | TF_DELAY_RECOVERY))
                    == (TF_PKTS_REORDERED | TF_DELAY_RECOVERY)) {
                        /*
                         * If the ack acknowledges upto snd_recover or if
                         * it acknowledges all the snd holes, exit
                         * recovery and cancel the timer. Otherwise,
                         * this is a partial ack. Wait for recovery timer
                         * to enter recovery. The snd_holes have already
                         * been updated.
                         */
                        if (SEQ_GEQ(th->th_ack, tp->snd_recover) ||
                            TAILQ_EMPTY(&tp->snd_holes)) {
                                tp->t_timer[TCPT_DELAYFR] = 0;
                                tp->t_flagsext &= ~TF_DELAY_RECOVERY;
                                EXIT_FASTRECOVERY(tp);
                                tcp_ccdbg_trace(tp, th,
                                    TCP_CC_EXIT_FASTRECOVERY);
                        }
                } else {
                        /*
                         * We were not in fast recovery. Reset the
                         * duplicate ack counter.
                         */
                        tp->t_dupacks = 0;
                        tp->t_rexmtthresh = tcprexmtthresh;
                        tp->t_new_dupacks = 0;
                }

process_ACK:
                VERIFY(SEQ_GEQ(th->th_ack, tp->snd_una));
                acked = BYTES_ACKED(th, tp);
                tcpstat.tcps_rcvackpack++;
                tcpstat.tcps_rcvackbyte += acked;

                /*
                 * If the last packet was a retransmit, make sure
                 * it was not spurious.
                 *
                 * This will also take care of congestion window
                 * adjustment if a last packet was recovered due to a
                 * tail loss probe.
                 */
                tcp_bad_rexmt_check(tp, th, &to);

                /* Recalculate the RTT */
                tcp_compute_rtt(tp, &to, th);

                /*
                 * If all outstanding data is acked, stop retransmit
                 * timer and remember to restart (more output or persist).
                 * If there is more data to be acked, restart retransmit
                 * timer, using current (possibly backed-off) value.
                 */
                TCP_RESET_REXMT_STATE(tp);
                TCPT_RANGESET(tp->t_rxtcur, TCP_REXMTVAL(tp),
                    tp->t_rttmin, TCPTV_REXMTMAX,
                    TCP_ADD_REXMTSLOP(tp));
                if (th->th_ack == tp->snd_max) {
                        tp->t_timer[TCPT_REXMT] = 0;
                        tp->t_timer[TCPT_PTO] = 0;
                        needoutput = 1;
                } else if (tp->t_timer[TCPT_PERSIST] == 0) {
                        tp->t_timer[TCPT_REXMT] = OFFSET_FROM_START(tp,
                            tp->t_rxtcur);
                }

                if ((prev_t_state == TCPS_SYN_SENT ||
                    prev_t_state == TCPS_SYN_RECEIVED) &&
                    tp->t_state == TCPS_ESTABLISHED) {
                        TCP_LOG_RTT_INFO(tp);
                }

                /*
                 * If no data (only SYN) was ACK'd, skip rest of ACK
                 * processing.
                 */
                if (acked == 0) {
                        goto step6;
                }

                /*
                 * When outgoing data has been acked (except the SYN+data), we
                 * mark this connection as "sending good" for TFO.
                 */
                if ((tp->t_tfo_stats & TFO_S_SYN_DATA_SENT) &&
                    !(tp->t_tfo_flags & TFO_F_NO_SNDPROBING) &&
                    !(th->th_flags & TH_SYN)) {
                        tp->t_tfo_flags |= TFO_F_NO_SNDPROBING;
                }

                /*
                 * If TH_ECE is received, make sure that ECN is enabled
                 * on that connection and we have sent ECT on data packets.
                 */
                if ((thflags & TH_ECE) != 0 && TCP_ECN_ENABLED(tp) &&
                    (tp->ecn_flags & TE_SENDIPECT)) {
                        /*
                         * Reduce the congestion window if we haven't
                         * done so.
                         */
                        if (!IN_FASTRECOVERY(tp)) {
                                tcp_reduce_congestion_window(tp);
                                tp->ecn_flags |= (TE_INRECOVERY | TE_SENDCWR);
                                /*
                                 * Also note that the connection received
                                 * ECE atleast once
                                 */
                                tp->ecn_flags |= TE_RECV_ECN_ECE;
                                INP_INC_IFNET_STAT(inp, ecn_recv_ece);
                                tcpstat.tcps_ecn_recv_ece++;
                                tcp_ccdbg_trace(tp, th, TCP_CC_ECN_RCVD);
                        }
                }

                /*
                 * When new data is acked, open the congestion window.
                 * The specifics of how this is achieved are up to the
                 * congestion control algorithm in use for this connection.
                 *
                 * The calculations in this function assume that snd_una is
                 * not updated yet.
                 */
                if (!IN_FASTRECOVERY(tp) && !exiting_fr) {
                        if (CC_ALGO(tp)->ack_rcvd != NULL) {
                                CC_ALGO(tp)->ack_rcvd(tp, th);
                        }
                        tcp_ccdbg_trace(tp, th, TCP_CC_ACK_RCVD);
                }
                if (acked > so->so_snd.sb_cc) {
                        tp->snd_wnd -= so->so_snd.sb_cc;
                        sbdrop(&so->so_snd, (int)so->so_snd.sb_cc);
                        ourfinisacked = 1;
                } else {
                        sbdrop(&so->so_snd, acked);
                        tcp_sbsnd_trim(&so->so_snd);
                        tp->snd_wnd -= acked;
                        ourfinisacked = 0;
                }
                /* detect una wraparound */
                if (!IN_FASTRECOVERY(tp) &&
                    SEQ_GT(tp->snd_una, tp->snd_recover) &&
                    SEQ_LEQ(th->th_ack, tp->snd_recover)) {
                        tp->snd_recover = th->th_ack - 1;
                }

                if (IN_FASTRECOVERY(tp) &&
                    SEQ_GEQ(th->th_ack, tp->snd_recover)) {
                        EXIT_FASTRECOVERY(tp);
                }

                tcp_update_snd_una(tp, th->th_ack);

                if (SACK_ENABLED(tp)) {
                        if (SEQ_GT(tp->snd_una, tp->snd_recover)) {
                                tp->snd_recover = tp->snd_una;
                        }
                }
                if (SEQ_LT(tp->snd_nxt, tp->snd_una)) {
                        tp->snd_nxt = tp->snd_una;
                }
                if (!SLIST_EMPTY(&tp->t_rxt_segments) &&
                    !TCP_DSACK_SEQ_IN_WINDOW(tp, tp->t_dsack_lastuna,
                    tp->snd_una)) {
                        tcp_rxtseg_clean(tp);
                }
                if ((tp->t_flagsext & TF_MEASURESNDBW) != 0 &&
                    tp->t_bwmeas != NULL) {
                        tcp_bwmeas_check(tp);
                }

                write_wakeup = 1;

                if (!SLIST_EMPTY(&tp->t_notify_ack)) {
                        tcp_notify_acknowledgement(tp, so);
                }

                switch (tp->t_state) {
                /*
                 * In FIN_WAIT_1 STATE in addition to the processing
                 * for the ESTABLISHED state if our FIN is now acknowledged
                 * then enter FIN_WAIT_2.
                 */
                case TCPS_FIN_WAIT_1:
                        if (ourfinisacked) {
                                /*
                                 * If we can't receive any more
                                 * data, then closing user can proceed.
                                 * Starting the TCPT_2MSL timer is contrary to the
                                 * specification, but if we don't get a FIN
                                 * we'll hang forever.
                                 */
                                if (so->so_state & SS_CANTRCVMORE) {
                                        tp->t_timer[TCPT_2MSL] = OFFSET_FROM_START(tp,
                                            TCP_CONN_MAXIDLE(tp));
                                        isconnected = FALSE;
                                        isdisconnected = TRUE;
                                }
                                DTRACE_TCP4(state__change, void, NULL,
                                    struct inpcb *, inp,
                                    struct tcpcb *, tp,
                                    int32_t, TCPS_FIN_WAIT_2);
                                tp->t_state = TCPS_FIN_WAIT_2;
                                /* fall through and make sure we also recognize
                                 * data ACKed with the FIN
                                 */
                        }
                        break;

                /*
                 * In CLOSING STATE in addition to the processing for
                 * the ESTABLISHED state if the ACK acknowledges our FIN
                 * then enter the TIME-WAIT state, otherwise ignore
                 * the segment.
                 */
                case TCPS_CLOSING:
                        if (ourfinisacked) {
                                DTRACE_TCP4(state__change, void, NULL,
                                    struct inpcb *, inp,
                                    struct tcpcb *, tp,
                                    int32_t, TCPS_TIME_WAIT);
                                tp->t_state = TCPS_TIME_WAIT;
                                tcp_canceltimers(tp);
                                if (tp->t_flagsext & TF_NOTIMEWAIT) {
                                        tp->t_flags |= TF_CLOSING;
                                } else {
                                        add_to_time_wait(tp, 2 * tcp_msl);
                                }
                                isconnected = FALSE;
                                isdisconnected = TRUE;
                        }
                        break;

                /*
                 * In LAST_ACK, we may still be waiting for data to drain
                 * and/or to be acked, as well as for the ack of our FIN.
                 * If our FIN is now acknowledged, delete the TCB,
                 * enter the closed state and return.
                 */
                case TCPS_LAST_ACK:
                        if (ourfinisacked) {
                                tp = tcp_close(tp);
                                goto drop;
                        }
                        break;

                /*
                 * In TIME_WAIT state the only thing that should arrive
                 * is a retransmission of the remote FIN.  Acknowledge
                 * it and restart the finack timer.
                 */
                case TCPS_TIME_WAIT:
                        add_to_time_wait(tp, 2 * tcp_msl);
                        goto dropafterack;
                }

                /*
                 * If there is a SACK option on the ACK and we
                 * haven't seen any duplicate acks before, count
                 * it as a duplicate ack even if the cumulative
                 * ack is advanced. If the receiver delayed an
                 * ack and detected loss afterwards, then the ack
                 * will advance cumulative ack and will also have
                 * a SACK option. So counting it as one duplicate
                 * ack is ok.
                 */
                if (tp->t_state == TCPS_ESTABLISHED &&
                    SACK_ENABLED(tp) && sack_bytes_acked > 0 &&
                    to.to_nsacks > 0 && tp->t_dupacks == 0 &&
                    SEQ_LEQ(th->th_ack, tp->snd_una) && tlen == 0 &&
                    !(tp->t_flagsext & TF_PKTS_REORDERED)) {
                        tcpstat.tcps_sack_ackadv++;
                        goto process_dupack;
                }
        }

step6:
        /*
         * Update window information.
         */
        if (tcp_update_window(tp, thflags, th, tiwin, tlen)) {
                needoutput = 1;
        }

        /*
         * Process segments with URG.
         */
        if ((thflags & TH_URG) && th->th_urp &&
            TCPS_HAVERCVDFIN(tp->t_state) == 0) {
                /*
                 * This is a kludge, but if we receive and accept
                 * random urgent pointers, we'll crash in
                 * soreceive.  It's hard to imagine someone
                 * actually wanting to send this much urgent data.
                 */
                if (th->th_urp + so->so_rcv.sb_cc > sb_max) {
                        th->th_urp = 0;                 /* XXX */
                        thflags &= ~TH_URG;             /* XXX */
                        goto dodata;                    /* XXX */
                }
                /*
                 * If this segment advances the known urgent pointer,
                 * then mark the data stream.  This should not happen
                 * in CLOSE_WAIT, CLOSING, LAST_ACK or TIME_WAIT STATES since
                 * a FIN has been received from the remote side.
                 * In these states we ignore the URG.
                 *
                 * According to RFC961 (Assigned Protocols),
                 * the urgent pointer points to the last octet
                 * of urgent data.  We continue, however,
                 * to consider it to indicate the first octet
                 * of data past the urgent section as the original
                 * spec states (in one of two places).
                 */
                if (SEQ_GT(th->th_seq + th->th_urp, tp->rcv_up)) {
                        tp->rcv_up = th->th_seq + th->th_urp;
                        so->so_oobmark = so->so_rcv.sb_cc +
                            (tp->rcv_up - tp->rcv_nxt) - 1;
                        if (so->so_oobmark == 0) {
                                so->so_state |= SS_RCVATMARK;
                        }
                        sohasoutofband(so);
                        tp->t_oobflags &= ~(TCPOOB_HAVEDATA | TCPOOB_HADDATA);
                }
                /*
                 * Remove out of band data so doesn't get presented to user.
                 * This can happen independent of advancing the URG pointer,
                 * but if two URG's are pending at once, some out-of-band
                 * data may creep in... ick.
                 */
                if (th->th_urp <= (u_int32_t)tlen
#if SO_OOBINLINE
                    && (so->so_options & SO_OOBINLINE) == 0
#endif
                    ) {
                        tcp_pulloutofband(so, th, m,
                            drop_hdrlen);       /* hdr drop is delayed */
                }
        } else {
                /*
                 * If no out of band data is expected,
                 * pull receive urgent pointer along
                 * with the receive window.
                 */
                if (SEQ_GT(tp->rcv_nxt, tp->rcv_up)) {
                        tp->rcv_up = tp->rcv_nxt;
                }
        }
dodata:

        /* Set socket's connect or disconnect state correcly before doing data.
         * The following might unlock the socket if there is an upcall or a socket
         * filter.
         */
        if (isconnected) {
                soisconnected(so);
        } else if (isdisconnected) {
                soisdisconnected(so);
        }

        /* Let's check the state of pcb just to make sure that it did not get closed
         * when we unlocked above
         */
        if (inp->inp_state == INPCB_STATE_DEAD) {
                /* Just drop the packet that we are processing and return */
                TCP_LOG_DROP_PCB(TCP_LOG_HDR, th, tp, false, "INPCB_STATE_DEAD");
                goto drop;
        }

        /*
         * Process the segment text, merging it into the TCP sequencing queue,
         * and arranging for acknowledgment of receipt if necessary.
         * This process logically involves adjusting tp->rcv_wnd as data
         * is presented to the user (this happens in tcp_usrreq.c,
         * case PRU_RCVD).  If a FIN has already been received on this
         * connection then we just ignore the text.
         *
         * If we are in SYN-received state and got a valid TFO cookie, we want
         * to process the data.
         */
        if ((tlen || (thflags & TH_FIN)) &&
            TCPS_HAVERCVDFIN(tp->t_state) == 0 &&
            (TCPS_HAVEESTABLISHED(tp->t_state) ||
            (tp->t_state == TCPS_SYN_RECEIVED &&
            (tp->t_tfo_flags & TFO_F_COOKIE_VALID)))) {
                tcp_seq save_start = th->th_seq;
                tcp_seq save_end = th->th_seq + tlen;
                m_adj(m, drop_hdrlen);  /* delayed header drop */
                /*
                 * Insert segment which includes th into TCP reassembly queue
                 * with control block tp.  Set thflags to whether reassembly now
                 * includes a segment with FIN.  This handles the common case
                 * inline (segment is the next to be received on an established
                 * connection, and the queue is empty), avoiding linkage into
                 * and removal from the queue and repetition of various
                 * conversions.
                 * Set DELACK for segments received in order, but ack
                 * immediately when segments are out of order (so
                 * fast retransmit can work).
                 */
                if (th->th_seq == tp->rcv_nxt && LIST_EMPTY(&tp->t_segq)) {
                        TCP_INC_VAR(tp->t_unacksegs, segment_count);
                        /*
                         * Calculate the RTT on the receiver only if the
                         * connection is in streaming mode and the last
                         * packet was not an end-of-write
                         */
                        if (tp->t_flags & TF_STREAMING_ON) {
                                tcp_compute_rtt(tp, &to, th);
                        }

                        if (DELAY_ACK(tp, th) &&
                            ((tp->t_flags & TF_ACKNOW) == 0)) {
                                if ((tp->t_flags & TF_DELACK) == 0) {
                                        tp->t_flags |= TF_DELACK;
                                        tp->t_timer[TCPT_DELACK] =
                                            OFFSET_FROM_START(tp, tcp_delack);
                                }
                        } else {
                                tp->t_flags |= TF_ACKNOW;
                        }
                        tp->rcv_nxt += tlen;
                        thflags = th->th_flags & TH_FIN;
                        TCP_INC_VAR(tcpstat.tcps_rcvpack, segment_count);
                        tcpstat.tcps_rcvbyte += tlen;
                        if (nstat_collect) {
                                INP_ADD_STAT(inp, cell, wifi, wired,
                                    rxpackets, 1);
                                INP_ADD_STAT(inp, cell, wifi, wired,
                                    rxbytes, tlen);
                                inp_set_activity_bitmap(inp);
                        }
                        tcp_sbrcv_grow(tp, &so->so_rcv, &to, tlen);
                        so_recv_data_stat(so, m, drop_hdrlen);

                        if (isipv6) {
                                memcpy(&saved_hdr, ip6, sizeof(struct ip6_hdr));
                                ip6 = (struct ip6_hdr *)&saved_hdr[0];
                        } else {
                                memcpy(&saved_hdr, ip, ip->ip_hl << 2);
                                ip = (struct ip *)&saved_hdr[0];
                        }
                        memcpy(&saved_tcphdr, th, sizeof(struct tcphdr));

                        if (th->th_flags & TH_PUSH) {
                                tp->t_flagsext |= TF_LAST_IS_PSH;
                        } else {
                                tp->t_flagsext &= ~TF_LAST_IS_PSH;
                        }

                        if (sbappendstream_rcvdemux(so, m)) {
                                read_wakeup = 1;
                        }
                        th = &saved_tcphdr;
                } else {
                        if (isipv6) {
                                memcpy(&saved_hdr, ip6, sizeof(struct ip6_hdr));
                                ip6 = (struct ip6_hdr *)&saved_hdr[0];
                        } else {
                                memcpy(&saved_hdr, ip, ip->ip_hl << 2);
                                ip = (struct ip *)&saved_hdr[0];
                        }

                        if (tcp_autotune_reorder) {
                                tcp_sbrcv_grow(tp, &so->so_rcv, &to, tlen);
                        }

                        memcpy(&saved_tcphdr, th, sizeof(struct tcphdr));
                        thflags = tcp_reass(tp, th, &tlen, m, ifp, &read_wakeup);
                        th = &saved_tcphdr;
                        tp->t_flags |= TF_ACKNOW;
                }

                if ((tlen > 0 || (th->th_flags & TH_FIN)) && SACK_ENABLED(tp)) {
                        if (th->th_flags & TH_FIN) {
                                save_end++;
                        }
                        tcp_update_sack_list(tp, save_start, save_end);
                }

                tcp_adaptive_rwtimo_check(tp, tlen);

                if (tlen > 0) {
                        tcp_tfo_rcv_data(tp);
                }

                if (tp->t_flags & TF_DELACK) {
                        if (isipv6) {
                                KERNEL_DEBUG(DBG_LAYER_END, ((th->th_dport << 16) | th->th_sport),
                                    (((ip6->ip6_src.s6_addr16[0]) << 16) | (ip6->ip6_dst.s6_addr16[0])),
                                    th->th_seq, th->th_ack, th->th_win);
                        } else {
                                KERNEL_DEBUG(DBG_LAYER_END, ((th->th_dport << 16) | th->th_sport),
                                    (((ip->ip_src.s_addr & 0xffff) << 16) | (ip->ip_dst.s_addr & 0xffff)),
                                    th->th_seq, th->th_ack, th->th_win);
                        }
                }
        } else {
                if ((so->so_flags & SOF_MP_SUBFLOW) && tlen == 0 &&
                    (m->m_pkthdr.pkt_flags & PKTF_MPTCP_DFIN) &&
                    (m->m_pkthdr.pkt_flags & PKTF_MPTCP)) {
                        m_adj(m, drop_hdrlen);  /* delayed header drop */
                        mptcp_input(tptomptp(tp)->mpt_mpte, m);
                        tp->t_flags |= TF_ACKNOW;
                } else {
                        m_freem(m);
                }
                thflags &= ~TH_FIN;
        }

        /*
         * If FIN is received ACK the FIN and let the user know
         * that the connection is closing.
         */
        if (thflags & TH_FIN) {
                if (TCPS_HAVERCVDFIN(tp->t_state) == 0) {
                        socantrcvmore(so);
                        /*
                         * If connection is half-synchronized
                         * (ie NEEDSYN flag on) then delay ACK,
                         * so it may be piggybacked when SYN is sent.
                         * Otherwise, since we received a FIN then no
                         * more input can be expected, send ACK now.
                         */
                        TCP_INC_VAR(tp->t_unacksegs, segment_count);
                        tp->t_flags |= TF_ACKNOW;
                        tp->rcv_nxt++;
                }
                switch (tp->t_state) {
                /*
                 * In SYN_RECEIVED and ESTABLISHED STATES
                 * enter the CLOSE_WAIT state.
                 */
                case TCPS_SYN_RECEIVED:
                        tp->t_starttime = tcp_now;
                        OS_FALLTHROUGH;
                case TCPS_ESTABLISHED:
                        DTRACE_TCP4(state__change, void, NULL, struct inpcb *, inp,
                            struct tcpcb *, tp, int32_t, TCPS_CLOSE_WAIT);
                        tp->t_state = TCPS_CLOSE_WAIT;
                        break;

                /*
                 * If still in FIN_WAIT_1 STATE FIN has not been acked so
                 * enter the CLOSING state.
                 */
                case TCPS_FIN_WAIT_1:
                        DTRACE_TCP4(state__change, void, NULL, struct inpcb *, inp,
                            struct tcpcb *, tp, int32_t, TCPS_CLOSING);
                        tp->t_state = TCPS_CLOSING;
                        break;

                /*
                 * In FIN_WAIT_2 state enter the TIME_WAIT state,
                 * starting the time-wait timer, turning off the other
                 * standard timers.
                 */
                case TCPS_FIN_WAIT_2:
                        DTRACE_TCP4(state__change, void, NULL,
                            struct inpcb *, inp,
                            struct tcpcb *, tp,
                            int32_t, TCPS_TIME_WAIT);
                        tp->t_state = TCPS_TIME_WAIT;
                        tcp_canceltimers(tp);
                        tp->t_flags |= TF_ACKNOW;
                        if (tp->t_flagsext & TF_NOTIMEWAIT) {
                                tp->t_flags |= TF_CLOSING;
                        } else {
                                add_to_time_wait(tp, 2 * tcp_msl);
                        }
                        soisdisconnected(so);
                        break;

                /*
                 * In TIME_WAIT state restart the 2 MSL time_wait timer.
                 */
                case TCPS_TIME_WAIT:
                        add_to_time_wait(tp, 2 * tcp_msl);
                        break;
                }
        }
#if TCPDEBUG
        if (so->so_options & SO_DEBUG) {
                tcp_trace(TA_INPUT, ostate, tp, (void *)tcp_saveipgen,
                    &tcp_savetcp, 0);
        }
#endif

        if (read_wakeup) {
                mptcp_handle_input(so);
        }

        /*
         * Return any desired output.
         */
        if (needoutput || (tp->t_flags & TF_ACKNOW)) {
                (void) tcp_output(tp);
        }

        tcp_check_timer_state(tp);

        tcp_handle_wakeup(so, read_wakeup, write_wakeup);

        socket_unlock(so, 1);
        KERNEL_DEBUG(DBG_FNC_TCP_INPUT | DBG_FUNC_END, 0, 0, 0, 0, 0);
        return;

dropafterack:
        /*
         * Generate an ACK dropping incoming segment if it occupies
         * sequence space, where the ACK reflects our state.
         *
         * We can now skip the test for the RST flag since all
         * paths to this code happen after packets containing
         * RST have been dropped.
         *
         * In the SYN-RECEIVED state, don't send an ACK unless the
         * segment we received passes the SYN-RECEIVED ACK test.
         * If it fails send a RST.  This breaks the loop in the
         * "LAND" DoS attack, and also prevents an ACK storm
         * between two listening ports that have been sent forged
         * SYN segments, each with the source address of the other.
         */
        if (tp->t_state == TCPS_SYN_RECEIVED && (thflags & TH_ACK) &&
            (SEQ_GT(tp->snd_una, th->th_ack) ||
            SEQ_GT(th->th_ack, tp->snd_max))) {
                IF_TCP_STATINC(ifp, dospacket);
                goto dropwithreset;
        }
#if TCPDEBUG
        if (so->so_options & SO_DEBUG) {
                tcp_trace(TA_DROP, ostate, tp, (void *)tcp_saveipgen,
                    &tcp_savetcp, 0);
        }
#endif
        m_freem(m);
        tp->t_flags |= TF_ACKNOW;

        (void) tcp_output(tp);

        tcp_handle_wakeup(so, read_wakeup, write_wakeup);

        /* Don't need to check timer state as we should have done it during tcp_output */
        socket_unlock(so, 1);
        KERNEL_DEBUG(DBG_FNC_TCP_INPUT | DBG_FUNC_END, 0, 0, 0, 0, 0);
        return;
dropwithresetnosock:
        nosock = 1;
dropwithreset:
        /*
         * Generate a RST, dropping incoming segment.
         * Make ACK acceptable to originator of segment.
         * Don't bother to respond if destination was broadcast/multicast.
         */
        if ((thflags & TH_RST) || m->m_flags & (M_BCAST | M_MCAST)) {
                goto drop;
        }
        if (isipv6) {
                if (IN6_IS_ADDR_MULTICAST(&ip6->ip6_dst) ||
                    IN6_IS_ADDR_MULTICAST(&ip6->ip6_src)) {
                        goto drop;
                }
        } else if (IN_MULTICAST(ntohl(ip->ip_dst.s_addr)) ||
            IN_MULTICAST(ntohl(ip->ip_src.s_addr)) ||
            ip->ip_src.s_addr == htonl(INADDR_BROADCAST) ||
            in_broadcast(ip->ip_dst, m->m_pkthdr.rcvif)) {
                goto drop;
        }
        /* IPv6 anycast check is done at tcp6_input() */

#if TCPDEBUG
        if (tp == 0 || (tp->t_inpcb->inp_socket->so_options & SO_DEBUG)) {
                tcp_trace(TA_DROP, ostate, tp, (void *)tcp_saveipgen,
                    &tcp_savetcp, 0);
        }
#endif
        bzero(&tra, sizeof(tra));
        tra.ifscope = ifscope;
        tra.awdl_unrestricted = 1;
        tra.intcoproc_allowed = 1;
        if (thflags & TH_ACK) {
                /* mtod() below is safe as long as hdr dropping is delayed */
                tcp_respond(tp, mtod(m, void *), th, m, (tcp_seq)0, th->th_ack,
                    TH_RST, &tra);
        } else {
                if (thflags & TH_SYN) {
                        tlen++;
                }
                /* mtod() below is safe as long as hdr dropping is delayed */
                tcp_respond(tp, mtod(m, void *), th, m, th->th_seq + tlen,
                    (tcp_seq)0, TH_RST | TH_ACK, &tra);
        }
        /* destroy temporarily created socket */
        if (dropsocket) {
                (void) soabort(so);
                socket_unlock(so, 1);
        } else if ((inp != NULL) && (nosock == 0)) {
                tcp_handle_wakeup(so, read_wakeup, write_wakeup);

                socket_unlock(so, 1);
        }
        KERNEL_DEBUG(DBG_FNC_TCP_INPUT | DBG_FUNC_END, 0, 0, 0, 0, 0);
        return;
dropnosock:
        nosock = 1;
drop:
        /*
         * Drop space held by incoming segment and return.
         */
#if TCPDEBUG
        if (tp == 0 || (tp->t_inpcb->inp_socket->so_options & SO_DEBUG)) {
                tcp_trace(TA_DROP, ostate, tp, (void *)tcp_saveipgen,
                    &tcp_savetcp, 0);
        }
#endif
        m_freem(m);
        /* destroy temporarily created socket */
        if (dropsocket) {
                (void) soabort(so);
                socket_unlock(so, 1);
        } else if (nosock == 0) {
                tcp_handle_wakeup(so, read_wakeup, write_wakeup);

                socket_unlock(so, 1);
        }
        KERNEL_DEBUG(DBG_FNC_TCP_INPUT | DBG_FUNC_END, 0, 0, 0, 0, 0);
        return;
}

/*
 * Parse TCP options and place in tcpopt.
 */
static void
tcp_dooptions(struct tcpcb *tp, u_char *cp, int cnt, struct tcphdr *th,
    struct tcpopt *to)
{
        u_short mss = 0;
        int opt, optlen;

        for (; cnt > 0; cnt -= optlen, cp += optlen) {
                opt = cp[0];
                if (opt == TCPOPT_EOL) {
                        break;
                }
                if (opt == TCPOPT_NOP) {
                        optlen = 1;
                } else {
                        if (cnt < 2) {
                                break;
                        }
                        optlen = cp[1];
                        if (optlen < 2 || optlen > cnt) {
                                break;
                        }
                }
                switch (opt) {
                default:
                        continue;

                case TCPOPT_MAXSEG:
                        if (optlen != TCPOLEN_MAXSEG) {
                                continue;
                        }
                        if (!(th->th_flags & TH_SYN)) {
                                continue;
                        }
                        bcopy((char *) cp + 2, (char *) &mss, sizeof(mss));
                        NTOHS(mss);
                        to->to_mss = mss;
                        to->to_flags |= TOF_MSS;
                        break;

                case TCPOPT_WINDOW:
                        if (optlen != TCPOLEN_WINDOW) {
                                continue;
                        }
                        if (!(th->th_flags & TH_SYN)) {
                                continue;
                        }
                        to->to_flags |= TOF_SCALE;
                        to->to_requested_s_scale = min(cp[2], TCP_MAX_WINSHIFT);
                        break;

                case TCPOPT_TIMESTAMP:
                        if (optlen != TCPOLEN_TIMESTAMP) {
                                continue;
                        }
                        to->to_flags |= TOF_TS;
                        bcopy((char *)cp + 2,
                            (char *)&to->to_tsval, sizeof(to->to_tsval));
                        NTOHL(to->to_tsval);
                        bcopy((char *)cp + 6,
                            (char *)&to->to_tsecr, sizeof(to->to_tsecr));
                        NTOHL(to->to_tsecr);
                        /* Re-enable sending Timestamps if we received them */
                        if (!(tp->t_flags & TF_REQ_TSTMP)) {
                                tp->t_flags |= TF_REQ_TSTMP;
                        }
                        break;
                case TCPOPT_SACK_PERMITTED:
                        if (optlen != TCPOLEN_SACK_PERMITTED) {
                                continue;
                        }
                        if (th->th_flags & TH_SYN) {
                                to->to_flags |= TOF_SACK;
                        }
                        break;
                case TCPOPT_SACK:
                        if (optlen <= 2 || (optlen - 2) % TCPOLEN_SACK != 0) {
                                continue;
                        }
                        to->to_nsacks = (optlen - 2) / TCPOLEN_SACK;
                        to->to_sacks = cp + 2;
                        tcpstat.tcps_sack_rcv_blocks++;

                        break;
                case TCPOPT_FASTOPEN:
                        if (optlen == TCPOLEN_FASTOPEN_REQ) {
                                if (tp->t_state != TCPS_LISTEN) {
                                        continue;
                                }

                                to->to_flags |= TOF_TFOREQ;
                        } else {
                                if (optlen < TCPOLEN_FASTOPEN_REQ ||
                                    (optlen - TCPOLEN_FASTOPEN_REQ) > TFO_COOKIE_LEN_MAX ||
                                    (optlen - TCPOLEN_FASTOPEN_REQ) < TFO_COOKIE_LEN_MIN) {
                                        continue;
                                }
                                if (tp->t_state != TCPS_LISTEN &&
                                    tp->t_state != TCPS_SYN_SENT) {
                                        continue;
                                }

                                to->to_flags |= TOF_TFO;
                                to->to_tfo = cp + 1;
                        }

                        break;
#if MPTCP
                case TCPOPT_MULTIPATH:
                        tcp_do_mptcp_options(tp, cp, th, to, optlen);
                        break;
#endif /* MPTCP */
                }
        }
}

static void
tcp_finalize_options(struct tcpcb *tp, struct tcpopt *to, unsigned int ifscope)
{
        if (to->to_flags & TOF_TS) {
                tp->t_flags |= TF_RCVD_TSTMP;
                tp->ts_recent = to->to_tsval;
                tp->ts_recent_age = tcp_now;
        }
        if (to->to_flags & TOF_MSS) {
                tcp_mss(tp, to->to_mss, ifscope);
        }
        if (SACK_ENABLED(tp)) {
                if (!(to->to_flags & TOF_SACK)) {
                        tp->t_flagsext &= ~(TF_SACK_ENABLE);
                } else {
                        tp->t_flags |= TF_SACK_PERMIT;
                }
        }
        if (to->to_flags & TOF_SCALE) {
                tp->t_flags |= TF_RCVD_SCALE;
                tp->requested_s_scale = to->to_requested_s_scale;

                /* Re-enable window scaling, if the option is received */
                if (tp->request_r_scale > 0) {
                        tp->t_flags |= TF_REQ_SCALE;
                }
        }
}

/*
 * Pull out of band byte out of a segment so
 * it doesn't appear in the user's data queue.
 * It is still reflected in the segment length for
 * sequencing purposes.
 *
 * @param off delayed to be droped hdrlen
 */
static void
tcp_pulloutofband(struct socket *so, struct tcphdr *th, struct mbuf *m, int off)
{
        int cnt = off + th->th_urp - 1;

        while (cnt >= 0) {
                if (m->m_len > cnt) {
                        char *cp = mtod(m, caddr_t) + cnt;
                        struct tcpcb *tp = sototcpcb(so);

                        tp->t_iobc = *cp;
                        tp->t_oobflags |= TCPOOB_HAVEDATA;
                        bcopy(cp + 1, cp, (unsigned)(m->m_len - cnt - 1));
                        m->m_len--;
                        if (m->m_flags & M_PKTHDR) {
                                m->m_pkthdr.len--;
                        }
                        return;
                }
                cnt -= m->m_len;
                m = m->m_next;
                if (m == 0) {
                        break;
                }
        }
        panic("tcp_pulloutofband");
}

uint32_t
get_base_rtt(struct tcpcb *tp)
{
        struct rtentry *rt = tp->t_inpcb->inp_route.ro_rt;
        return (rt == NULL) ? 0 : rt->rtt_min;
}

/* Each value of RTT base represents the minimum RTT seen in a minute.
 * We keep upto N_RTT_BASE minutes worth of history.
 */
void
update_base_rtt(struct tcpcb *tp, uint32_t rtt)
{
        u_int32_t base_rtt, i;
        struct rtentry *rt;

        if ((rt = tp->t_inpcb->inp_route.ro_rt) == NULL) {
                return;
        }
        if (rt->rtt_expire_ts == 0) {
                RT_LOCK_SPIN(rt);
                if (rt->rtt_expire_ts != 0) {
                        RT_UNLOCK(rt);
                        goto update;
                }
                rt->rtt_expire_ts = tcp_now;
                rt->rtt_index = 0;
                rt->rtt_hist[0] = rtt;
                rt->rtt_min = rtt;
                RT_UNLOCK(rt);
                return;
        }
update:
#if TRAFFIC_MGT
        /*
         * If the recv side is being throttled, check if the
         * current RTT is closer to the base RTT seen in
         * first (recent) two slots. If so, unthrottle the stream.
         */
        if ((tp->t_flagsext & TF_RECV_THROTTLE) &&
            (int)(tcp_now - tp->t_recv_throttle_ts) >= TCP_RECV_THROTTLE_WIN) {
                base_rtt = rt->rtt_min;
                if (tp->t_rttcur <= (base_rtt + target_qdelay)) {
                        tp->t_flagsext &= ~TF_RECV_THROTTLE;
                        tp->t_recv_throttle_ts = 0;
                }
        }
#endif /* TRAFFIC_MGT */
        if ((int)(tcp_now - rt->rtt_expire_ts) >=
            TCP_RTT_HISTORY_EXPIRE_TIME) {
                RT_LOCK_SPIN(rt);
                /* check the condition again to avoid race */
                if ((int)(tcp_now - rt->rtt_expire_ts) >=
                    TCP_RTT_HISTORY_EXPIRE_TIME) {
                        rt->rtt_index++;
                        if (rt->rtt_index >= NRTT_HIST) {
                                rt->rtt_index = 0;
                        }
                        rt->rtt_hist[rt->rtt_index] = rtt;
                        rt->rtt_expire_ts = tcp_now;
                } else {
                        rt->rtt_hist[rt->rtt_index] =
                            min(rt->rtt_hist[rt->rtt_index], rtt);
                }
                /* forget the old value and update minimum */
                rt->rtt_min = 0;
                for (i = 0; i < NRTT_HIST; ++i) {
                        if (rt->rtt_hist[i] != 0 &&
                            (rt->rtt_min == 0 ||
                            rt->rtt_hist[i] < rt->rtt_min)) {
                                rt->rtt_min = rt->rtt_hist[i];
                        }
                }
                RT_UNLOCK(rt);
        } else {
                rt->rtt_hist[rt->rtt_index] =
                    min(rt->rtt_hist[rt->rtt_index], rtt);
                if (rt->rtt_min == 0) {
                        rt->rtt_min = rtt;
                } else {
                        rt->rtt_min = min(rt->rtt_min, rtt);
                }
        }
}

/*
 * If we have a timestamp reply, update smoothed RTT. If no timestamp is
 * present but transmit timer is running and timed sequence number was
 * acked, update smoothed RTT.
 *
 * If timestamps are supported, a receiver can update RTT even if
 * there is no outstanding data.
 *
 * Some boxes send broken timestamp replies during the SYN+ACK phase,
 * ignore timestamps of 0or we could calculate a huge RTT and blow up
 * the retransmit timer.
 */
static void
tcp_compute_rtt(struct tcpcb *tp, struct tcpopt *to, struct tcphdr *th)
{
        int rtt = 0;
        VERIFY(to != NULL && th != NULL);
        if (tp->t_rtttime != 0 && SEQ_GT(th->th_ack, tp->t_rtseq)) {
                u_int32_t pipe_ack_val;
                rtt = tcp_now - tp->t_rtttime;
                /*
                 * Compute pipe ack -- the amount of data acknowledged
                 * in the last RTT
                 */
                if (SEQ_GT(th->th_ack, tp->t_pipeack_lastuna)) {
                        pipe_ack_val = th->th_ack - tp->t_pipeack_lastuna;
                        /* Update the sample */
                        tp->t_pipeack_sample[tp->t_pipeack_ind++] =
                            pipe_ack_val;
                        tp->t_pipeack_ind %= TCP_PIPEACK_SAMPLE_COUNT;

                        /* Compute the max of the pipeack samples */
                        pipe_ack_val = tcp_get_max_pipeack(tp);
                        tp->t_pipeack = (pipe_ack_val >
                            tcp_initial_cwnd(tp)) ?
                            pipe_ack_val : 0;
                }
                /* start another measurement */
                tp->t_rtttime = 0;
        }
        if (((to->to_flags & TOF_TS) != 0) &&
            (to->to_tsecr != 0) &&
            TSTMP_GEQ(tcp_now, to->to_tsecr)) {
                tcp_xmit_timer(tp, (tcp_now - to->to_tsecr),
                    to->to_tsecr, th->th_ack);
        } else if (rtt > 0) {
                tcp_xmit_timer(tp, rtt, 0, th->th_ack);
        }
}

/*
 * Collect new round-trip time estimate and update averages and
 * current timeout.
 */
static void
tcp_xmit_timer(struct tcpcb *tp, int rtt,
    u_int32_t tsecr, tcp_seq th_ack)
{
        int delta;
        int old_srtt = tp->t_srtt;
        int old_rttvar = tp->t_rttvar;
        bool log_rtt = false;

        /*
         * On AWDL interface, the initial RTT measurement on SYN
         * can be wrong due to peer caching. Avoid the first RTT
         * measurement as it might skew up the RTO.
         * <rdar://problem/28739046>
         */
        if (tp->t_inpcb->inp_last_outifp != NULL &&
            (tp->t_inpcb->inp_last_outifp->if_eflags & IFEF_AWDL) &&
            th_ack == tp->iss + 1) {
                return;
        }

        if (tp->t_flagsext & TF_RECOMPUTE_RTT) {
                if (SEQ_GT(th_ack, tp->snd_una) &&
                    SEQ_LEQ(th_ack, tp->snd_max) &&
                    (tsecr == 0 ||
                    TSTMP_GEQ(tsecr, tp->t_badrexmt_time))) {
                        /*
                         * We received a new ACK after a
                         * spurious timeout. Adapt retransmission
                         * timer as described in rfc 4015.
                         */
                        tp->t_flagsext &= ~(TF_RECOMPUTE_RTT);
                        tp->t_badrexmt_time = 0;
                        tp->t_srtt = max(tp->t_srtt_prev, rtt);
                        tp->t_srtt = tp->t_srtt << TCP_RTT_SHIFT;
                        tp->t_rttvar = max(tp->t_rttvar_prev, (rtt >> 1));
                        tp->t_rttvar = tp->t_rttvar << TCP_RTTVAR_SHIFT;

                        if (tp->t_rttbest > (tp->t_srtt + tp->t_rttvar)) {
                                tp->t_rttbest = tp->t_srtt + tp->t_rttvar;
                        }

                        goto compute_rto;
                } else {
                        return;
                }
        }

        tcpstat.tcps_rttupdated++;
        tp->t_rttupdated++;

        if (rtt > 0) {
                tp->t_rttcur = rtt;
                update_base_rtt(tp, rtt);
        }

        if (tp->t_srtt != 0) {
                /*
                 * srtt is stored as fixed point with 5 bits after the
                 * binary point (i.e., scaled by 32).  The following magic
                 * is equivalent to the smoothing algorithm in rfc793 with
                 * an alpha of .875 (srtt = rtt/8 + srtt*7/8 in fixed
                 * point).
                 *
                 * Freebsd adjusts rtt to origin 0 by subtracting 1
                 * from the provided rtt value. This was required because
                 * of the way t_rtttime was initiailised to 1 before.
                 * Since we changed t_rtttime to be based on
                 * tcp_now, this extra adjustment is not needed.
                 */
                delta = (rtt << TCP_DELTA_SHIFT)
                    - (tp->t_srtt >> (TCP_RTT_SHIFT - TCP_DELTA_SHIFT));

                if ((tp->t_srtt += delta) <= 0) {
                        tp->t_srtt = 1;
                }

                /*
                 * We accumulate a smoothed rtt variance (actually, a
                 * smoothed mean difference), then set the retransmit
                 * timer to smoothed rtt + 4 times the smoothed variance.
                 * rttvar is stored as fixed point with 4 bits after the
                 * binary point (scaled by 16).  The following is
                 * equivalent to rfc793 smoothing with an alpha of .75
                 * (rttvar = rttvar*3/4 + |delta| / 4).  This replaces
                 * rfc793's wired-in beta.
                 */
                if (delta < 0) {
                        delta = -delta;
                }
                delta -= tp->t_rttvar >> (TCP_RTTVAR_SHIFT - TCP_DELTA_SHIFT);
                if ((tp->t_rttvar += delta) <= 0) {
                        tp->t_rttvar = 1;
                }
                if (tp->t_rttbest == 0 ||
                    tp->t_rttbest > (tp->t_srtt + tp->t_rttvar)) {
                        tp->t_rttbest = tp->t_srtt + tp->t_rttvar;
                }
        } else {
                /*
                 * No rtt measurement yet - use the unsmoothed rtt.
                 * Set the variance to half the rtt (so our first
                 * retransmit happens at 3*rtt).
                 */
                tp->t_srtt = rtt << TCP_RTT_SHIFT;
                tp->t_rttvar = rtt << (TCP_RTTVAR_SHIFT - 1);
                tp->t_rttbest = tp->t_srtt + tp->t_rttvar;
        }

compute_rto:
        nstat_route_rtt(tp->t_inpcb->inp_route.ro_rt, tp->t_srtt,
            tp->t_rttvar);

        /*
         * the retransmit should happen at rtt + 4 * rttvar.
         * Because of the way we do the smoothing, srtt and rttvar
         * will each average +1/2 tick of bias.  When we compute
         * the retransmit timer, we want 1/2 tick of rounding and
         * 1 extra tick because of +-1/2 tick uncertainty in the
         * firing of the timer.  The bias will give us exactly the
         * 1.5 tick we need.  But, because the bias is
         * statistical, we have to test that we don't drop below
         * the minimum feasible timer (which is 2 ticks).
         */
        TCPT_RANGESET(tp->t_rxtcur, TCP_REXMTVAL(tp),
            max(tp->t_rttmin, rtt + 2), TCPTV_REXMTMAX,
            TCP_ADD_REXMTSLOP(tp));

        /*
         * We received an ack for a packet that wasn't retransmitted;
         * it is probably safe to discard any error indications we've
         * received recently.  This isn't quite right, but close enough
         * for now (a route might have failed after we sent a segment,
         * and the return path might not be symmetrical).
         */
        tp->t_softerror = 0;

        if (log_rtt) {
                TCP_LOG_RTT_INFO(tp);
        }

        TCP_LOG_RTT_CHANGE(tp, old_srtt, old_rttvar);
}

static inline unsigned int
tcp_maxmtu(struct rtentry *rt)
{
        unsigned int maxmtu;
        int interface_mtu = 0;

        RT_LOCK_ASSERT_HELD(rt);
        interface_mtu = rt->rt_ifp->if_mtu;

        if (rt_key(rt)->sa_family == AF_INET &&
            INTF_ADJUST_MTU_FOR_CLAT46(rt->rt_ifp)) {
                interface_mtu = IN6_LINKMTU(rt->rt_ifp);
                /* Further adjust the size for CLAT46 expansion */
                interface_mtu -= CLAT46_HDR_EXPANSION_OVERHD;
        }

        if (rt->rt_rmx.rmx_mtu == 0) {
                maxmtu = interface_mtu;
        } else {
                maxmtu = MIN(rt->rt_rmx.rmx_mtu, interface_mtu);
        }

        return maxmtu;
}

static inline unsigned int
tcp_maxmtu6(struct rtentry *rt)
{
        unsigned int maxmtu;
        struct nd_ifinfo *ndi = NULL;

        RT_LOCK_ASSERT_HELD(rt);
        if ((ndi = ND_IFINFO(rt->rt_ifp)) != NULL && !ndi->initialized) {
                ndi = NULL;
        }
        if (ndi != NULL) {
                lck_mtx_lock(&ndi->lock);
        }
        if (rt->rt_rmx.rmx_mtu == 0) {
                maxmtu = IN6_LINKMTU(rt->rt_ifp);
        } else {
                maxmtu = MIN(rt->rt_rmx.rmx_mtu, IN6_LINKMTU(rt->rt_ifp));
        }
        if (ndi != NULL) {
                lck_mtx_unlock(&ndi->lock);
        }

        return maxmtu;
}

unsigned int
get_maxmtu(struct rtentry *rt)
{
        unsigned int maxmtu = 0;

        RT_LOCK_ASSERT_NOTHELD(rt);

        RT_LOCK(rt);

        if (rt_key(rt)->sa_family == AF_INET6) {
                maxmtu = tcp_maxmtu6(rt);
        } else {
                maxmtu = tcp_maxmtu(rt);
        }

        RT_UNLOCK(rt);

        return maxmtu;
}

/*
 * Determine a reasonable value for maxseg size.
 * If the route is known, check route for mtu.
 * If none, use an mss that can be handled on the outgoing
 * interface without forcing IP to fragment; if bigger than
 * an mbuf cluster (MCLBYTES), round down to nearest multiple of MCLBYTES
 * to utilize large mbufs.  If no route is found, route has no mtu,
 * or the destination isn't local, use a default, hopefully conservative
 * size (usually 512 or the default IP max size, but no more than the mtu
 * of the interface), as we can't discover anything about intervening
 * gateways or networks.  We also initialize the congestion/slow start
 * window. While looking at the routing entry, we also initialize
 * other path-dependent parameters from pre-set or cached values
 * in the routing entry.
 *
 * Also take into account the space needed for options that we
 * send regularly.  Make maxseg shorter by that amount to assure
 * that we can send maxseg amount of data even when the options
 * are present.  Store the upper limit of the length of options plus
 * data in maxopd.
 *
 * NOTE that this routine is only called when we process an incoming
 * segment, for outgoing segments only tcp_mssopt is called.
 *
 */
void
tcp_mss(struct tcpcb *tp, int offer, unsigned int input_ifscope)
{
        struct rtentry *rt;
        struct ifnet *ifp;
        int rtt, mss;
        u_int32_t bufsize;
        struct inpcb *inp;
        struct socket *so;
        int origoffer = offer;
        u_int32_t sb_max_corrected;
        int isnetlocal = 0;
        int isipv6;
        int min_protoh;

        inp = tp->t_inpcb;

        so = inp->inp_socket;
        /*
         * Nothing left to send after the socket is defunct or TCP is in the closed state
         */
        if ((so->so_state & SS_DEFUNCT) || tp->t_state == TCPS_CLOSED) {
                return;
        }

        isipv6 = ((inp->inp_vflag & INP_IPV6) != 0) ? 1 : 0;
        min_protoh = isipv6 ? sizeof(struct ip6_hdr) + sizeof(struct tcphdr)
            : sizeof(struct tcpiphdr);

        if (isipv6) {
                rt = tcp_rtlookup6(inp, input_ifscope);
        } else {
                rt = tcp_rtlookup(inp, input_ifscope);
        }
        isnetlocal = (tp->t_flags & TF_LOCAL);

        if (rt == NULL) {
                tp->t_maxopd = tp->t_maxseg = isipv6 ? tcp_v6mssdflt : tcp_mssdflt;
                return;
        }
        ifp = rt->rt_ifp;
        /*
         * Slower link window correction:
         * If a value is specificied for slowlink_wsize use it for
         * PPP links believed to be on a serial modem (speed <128Kbps).
         * Excludes 9600bps as it is the default value adversized
         * by pseudo-devices over ppp.
         */
        if (ifp->if_type == IFT_PPP && slowlink_wsize > 0 &&
            ifp->if_baudrate > 9600 && ifp->if_baudrate <= 128000) {
                tp->t_flags |= TF_SLOWLINK;
        }

        /*
         * Offer == -1 means that we didn't receive SYN yet. Use 0 then.
         */
        if (offer == -1) {
                offer = rt->rt_rmx.rmx_filler[0];
        }
        /*
         * Offer == 0 means that there was no MSS on the SYN segment,
         * in this case we use tcp_mssdflt.
         */
        if (offer == 0) {
                offer = isipv6 ? tcp_v6mssdflt : tcp_mssdflt;
        } else {
                /*
                 * Prevent DoS attack with too small MSS. Round up
                 * to at least minmss.
                 */
                offer = max(offer, tcp_minmss);
                /*
                 * Sanity check: make sure that maxopd will be large
                 * enough to allow some data on segments even is the
                 * all the option space is used (40bytes).  Otherwise
                 * funny things may happen in tcp_output.
                 */
                offer = max(offer, 64);
        }
        rt->rt_rmx.rmx_filler[0] = offer;

        /*
         * While we're here, check if there's an initial rtt
         * or rttvar.  Convert from the route-table units
         * to scaled multiples of the slow timeout timer.
         */
        if (tp->t_srtt == 0 && (rtt = rt->rt_rmx.rmx_rtt) != 0) {
                tcp_getrt_rtt(tp, rt);
        } else {
                tp->t_rttmin = isnetlocal ? tcp_TCPTV_MIN : TCPTV_REXMTMIN;
        }

        mss = (isipv6 ? tcp_maxmtu6(rt) : tcp_maxmtu(rt));

#if NECP
        // At this point, the mss is just the MTU. Adjust if necessary.
        mss = necp_socket_get_effective_mtu(inp, mss);
#endif /* NECP */

        mss -= min_protoh;

        if (rt->rt_rmx.rmx_mtu == 0) {
                if (isipv6) {
                        if (!isnetlocal) {
                                mss = min(mss, tcp_v6mssdflt);
                        }
                } else if (!isnetlocal) {
                        mss = min(mss, tcp_mssdflt);
                }
        }

        mss = min(mss, offer);
        /*
         * maxopd stores the maximum length of data AND options
         * in a segment; maxseg is the amount of data in a normal
         * segment.  We need to store this value (maxopd) apart
         * from maxseg, because now every segment carries options
         * and thus we normally have somewhat less data in segments.
         */
        tp->t_maxopd = mss;

        /*
         * origoffer==-1 indicates, that no segments were received yet.
         * In this case we just guess.
         */
        if ((tp->t_flags & (TF_REQ_TSTMP | TF_NOOPT)) == TF_REQ_TSTMP &&
            (origoffer == -1 ||
            (tp->t_flags & TF_RCVD_TSTMP) == TF_RCVD_TSTMP)) {
                mss -= TCPOLEN_TSTAMP_APPA;
        }

#if MPTCP
        mss -= mptcp_adj_mss(tp, FALSE);
#endif /* MPTCP */
        tp->t_maxseg = mss;

        /*
         * Calculate corrected value for sb_max; ensure to upgrade the
         * numerator for large sb_max values else it will overflow.
         */
        sb_max_corrected = (sb_max * (u_int64_t)MCLBYTES) / (MSIZE + MCLBYTES);

        /*
         * If there's a pipesize (ie loopback), change the socket
         * buffer to that size only if it's bigger than the current
         * sockbuf size.  Make the socket buffers an integral
         * number of mss units; if the mss is larger than
         * the socket buffer, decrease the mss.
         */
#if RTV_SPIPE
        bufsize = rt->rt_rmx.rmx_sendpipe;
        if (bufsize < so->so_snd.sb_hiwat)
#endif
        bufsize = so->so_snd.sb_hiwat;
        if (bufsize < mss) {
                mss = bufsize;
        } else {
                bufsize = (((bufsize + (u_int64_t)mss - 1) / (u_int64_t)mss) * (u_int64_t)mss);
                if (bufsize > sb_max_corrected) {
                        bufsize = sb_max_corrected;
                }
                (void)sbreserve(&so->so_snd, bufsize);
        }
        tp->t_maxseg = mss;

        ASSERT(tp->t_maxseg);

        /*
         * Update MSS using recommendation from link status report. This is
         * temporary
         */
        tcp_update_mss_locked(so, ifp);

#if RTV_RPIPE
        bufsize = rt->rt_rmx.rmx_recvpipe;
        if (bufsize < so->so_rcv.sb_hiwat)
#endif
        bufsize = so->so_rcv.sb_hiwat;
        if (bufsize > mss) {
                bufsize = (((bufsize + (u_int64_t)mss - 1) / (u_int64_t)mss) * (u_int64_t)mss);
                if (bufsize > sb_max_corrected) {
                        bufsize = sb_max_corrected;
                }
                (void)sbreserve(&so->so_rcv, bufsize);
        }

        set_tcp_stream_priority(so);

        if (rt->rt_rmx.rmx_ssthresh) {
                /*
                 * There's some sort of gateway or interface
                 * buffer limit on the path.  Use this to set
                 * slow-start threshold, but set the threshold to
                 * no less than 2*mss.
                 */
                tp->snd_ssthresh = max(2 * mss, rt->rt_rmx.rmx_ssthresh);
                tcpstat.tcps_usedssthresh++;
        } else {
                tp->snd_ssthresh = TCP_MAXWIN << TCP_MAX_WINSHIFT;
        }

        /*
         * Set the slow-start flight size depending on whether this
         * is a local network or not.
         */
        if (CC_ALGO(tp)->cwnd_init != NULL) {
                CC_ALGO(tp)->cwnd_init(tp);
        }

        tcp_ccdbg_trace(tp, NULL, TCP_CC_CWND_INIT);

        /* Route locked during lookup above */
        RT_UNLOCK(rt);
}

/*
 * Determine the MSS option to send on an outgoing SYN.
 */
int
tcp_mssopt(struct tcpcb *tp)
{
        struct rtentry *rt;
        int mss;
        int isipv6;
        int min_protoh;

        isipv6 = ((tp->t_inpcb->inp_vflag & INP_IPV6) != 0) ? 1 : 0;
        min_protoh = isipv6 ? sizeof(struct ip6_hdr) + sizeof(struct tcphdr)
            : sizeof(struct tcpiphdr);

        if (isipv6) {
                rt = tcp_rtlookup6(tp->t_inpcb, IFSCOPE_NONE);
        } else {
                rt = tcp_rtlookup(tp->t_inpcb, IFSCOPE_NONE);
        }
        if (rt == NULL) {
                return isipv6 ? tcp_v6mssdflt : tcp_mssdflt;
        }
        /*
         * Slower link window correction:
         * If a value is specificied for slowlink_wsize use it for PPP links
         * believed to be on a serial modem (speed <128Kbps). Excludes 9600bps as
         * it is the default value adversized by pseudo-devices over ppp.
         */
        if (rt->rt_ifp->if_type == IFT_PPP && slowlink_wsize > 0 &&
            rt->rt_ifp->if_baudrate > 9600 && rt->rt_ifp->if_baudrate <= 128000) {
                tp->t_flags |= TF_SLOWLINK;
        }

        mss = (isipv6 ? tcp_maxmtu6(rt) : tcp_maxmtu(rt));
        /* Route locked during lookup above */
        RT_UNLOCK(rt);

#if NECP
        // At this point, the mss is just the MTU. Adjust if necessary.
        mss = necp_socket_get_effective_mtu(tp->t_inpcb, mss);
#endif /* NECP */

        return mss - min_protoh;
}

/*
 * On a partial ack arrives, force the retransmission of the
 * next unacknowledged segment.  Do not clear tp->t_dupacks.
 * By setting snd_nxt to th_ack, this forces retransmission timer to
 * be started again.
 */
static void
tcp_newreno_partial_ack(struct tcpcb *tp, struct tcphdr *th)
{
        tcp_seq onxt = tp->snd_nxt;
        u_int32_t  ocwnd = tp->snd_cwnd;
        tp->t_timer[TCPT_REXMT] = 0;
        tp->t_timer[TCPT_PTO] = 0;
        tp->t_rtttime = 0;
        tp->snd_nxt = th->th_ack;
        /*
         * Set snd_cwnd to one segment beyond acknowledged offset
         * (tp->snd_una has not yet been updated when this function
         *  is called)
         */
        tp->snd_cwnd = tp->t_maxseg + BYTES_ACKED(th, tp);
        (void) tcp_output(tp);
        tp->snd_cwnd = ocwnd;
        if (SEQ_GT(onxt, tp->snd_nxt)) {
                tp->snd_nxt = onxt;
        }
        /*
         * Partial window deflation.  Relies on fact that tp->snd_una
         * not updated yet.
         */
        if (tp->snd_cwnd > BYTES_ACKED(th, tp)) {
                tp->snd_cwnd -= BYTES_ACKED(th, tp);
        } else {
                tp->snd_cwnd = 0;
        }
        tp->snd_cwnd += tp->t_maxseg;
}

/*
 * Drop a random TCP connection that hasn't been serviced yet and
 * is eligible for discard.  There is a one in qlen chance that
 * we will return a null, saying that there are no dropable
 * requests.  In this case, the protocol specific code should drop
 * the new request.  This insures fairness.
 *
 * The listening TCP socket "head" must be locked
 */
static int
tcp_dropdropablreq(struct socket *head)
{
        struct socket *so, *sonext;
        unsigned int i, j, qlen;
        static u_int32_t rnd = 0;
        static u_int64_t old_runtime;
        static unsigned int cur_cnt, old_cnt;
        u_int64_t now_sec;
        struct inpcb *inp = NULL;
        struct tcpcb *tp;

        if ((head->so_options & SO_ACCEPTCONN) == 0) {
                return 0;
        }

        if (TAILQ_EMPTY(&head->so_incomp)) {
                return 0;
        }

        so_acquire_accept_list(head, NULL);
        socket_unlock(head, 0);

        /*
         * Check if there is any socket in the incomp queue
         * that is closed because of a reset from the peer and is
         * waiting to be garbage collected. If so, pick that as
         * the victim
         */
        TAILQ_FOREACH_SAFE(so, &head->so_incomp, so_list, sonext) {
                inp = sotoinpcb(so);
                tp = intotcpcb(inp);
                if (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)) {
                        /*
                         * The listen socket is already locked but we
                         * can lock this socket here without lock ordering
                         * issues because it is in the incomp queue and
                         * is not visible to others.
                         */
                        if (socket_try_lock(so)) {
                                so->so_usecount++;
                                goto found_victim;
                        } else {
                                continue;
                        }
                }
        }

        so = TAILQ_FIRST(&head->so_incomp);

        now_sec = net_uptime();
        if ((i = (now_sec - old_runtime)) != 0) {
                old_runtime = now_sec;
                old_cnt = cur_cnt / i;
                cur_cnt = 0;
        }

        qlen = head->so_incqlen;
        if (rnd == 0) {
                rnd = RandomULong();
        }

        if (++cur_cnt > qlen || old_cnt > qlen) {
                rnd = (314159 * rnd + 66329) & 0xffff;
                j = ((qlen + 1) * rnd) >> 16;

                while (j-- && so) {
                        so = TAILQ_NEXT(so, so_list);
                }
        }
        /* Find a connection that is not already closing (or being served) */
        while (so) {
                inp = (struct inpcb *)so->so_pcb;

                sonext = TAILQ_NEXT(so, so_list);

                if (in_pcb_checkstate(inp, WNT_ACQUIRE, 0) != WNT_STOPUSING) {
                        /*
                         * Avoid the issue of a socket being accepted
                         * by one input thread and being dropped by
                         * another input thread. If we can't get a hold
                         * on this mutex, then grab the next socket in
                         * line.
                         */
                        if (socket_try_lock(so)) {
                                so->so_usecount++;
                                if ((so->so_usecount == 2) &&
                                    (so->so_state & SS_INCOMP) &&
                                    !(so->so_flags & SOF_INCOMP_INPROGRESS)) {
                                        break;
                                } else {
                                        /*
                                         * don't use if being accepted or
                                         * used in any other way
                                         */
                                        in_pcb_checkstate(inp, WNT_RELEASE, 1);
                                        socket_unlock(so, 1);
                                }
                        } else {
                                /*
                                 * do not try to lock the inp in
                                 * in_pcb_checkstate because the lock
                                 * is already held in some other thread.
                                 * Only drop the inp_wntcnt reference.
                                 */
                                in_pcb_checkstate(inp, WNT_RELEASE, 1);
                        }
                }
                so = sonext;
        }
        if (so == NULL) {
                socket_lock(head, 0);
                so_release_accept_list(head);
                return 0;
        }

        /* Makes sure socket is still in the right state to be discarded */

        if (in_pcb_checkstate(inp, WNT_RELEASE, 1) == WNT_STOPUSING) {
                socket_unlock(so, 1);
                socket_lock(head, 0);
                so_release_accept_list(head);
                return 0;
        }

found_victim:
        if (so->so_usecount != 2 || !(so->so_state & SS_INCOMP)) {
                /* do not discard: that socket is being accepted */
                socket_unlock(so, 1);
                socket_lock(head, 0);
                so_release_accept_list(head);
                return 0;
        }

        socket_lock(head, 0);
        TAILQ_REMOVE(&head->so_incomp, so, so_list);
        head->so_incqlen--;
        head->so_qlen--;
        so->so_state &= ~SS_INCOMP;
        so->so_flags |= SOF_OVERFLOW;
        so->so_head = NULL;
        so_release_accept_list(head);
        socket_unlock(head, 0);

        socket_lock_assert_owned(so);
        tp = sototcpcb(so);

        tcp_close(tp);
        if (inp->inp_wantcnt > 0 && inp->inp_wantcnt != WNT_STOPUSING) {
                /*
                 * Some one has a wantcnt on this pcb. Since WNT_ACQUIRE
                 * doesn't require a lock, it could have happened while
                 * we are holding the lock. This pcb will have to
                 * be garbage collected later.
                 * Release the reference held for so_incomp queue
                 */
                VERIFY(so->so_usecount > 0);
                so->so_usecount--;
                socket_unlock(so, 1);
        } else {
                /*
                 * Unlock this socket and leave the reference on.
                 * We need to acquire the pcbinfo lock in order to
                 * fully dispose it off
                 */
                socket_unlock(so, 0);

                lck_rw_lock_exclusive(tcbinfo.ipi_lock);

                socket_lock(so, 0);
                /* Release the reference held for so_incomp queue */
                VERIFY(so->so_usecount > 0);
                so->so_usecount--;

                if (so->so_usecount != 1 ||
                    (inp->inp_wantcnt > 0 &&
                    inp->inp_wantcnt != WNT_STOPUSING)) {
                        /*
                         * There is an extra wantcount or usecount
                         * that must have been added when the socket
                         * was unlocked. This socket will have to be
                         * garbage collected later
                         */
                        socket_unlock(so, 1);
                } else {
                        /* Drop the reference held for this function */
                        VERIFY(so->so_usecount > 0);
                        so->so_usecount--;

                        in_pcbdispose(inp);
                }
                lck_rw_done(tcbinfo.ipi_lock);
        }
        tcpstat.tcps_drops++;

        socket_lock(head, 0);
        return 1;
}

/* Set background congestion control on a socket */
void
tcp_set_background_cc(struct socket *so)
{
        tcp_set_new_cc(so, TCP_CC_ALGO_BACKGROUND_INDEX);
}

/* Set foreground congestion control on a socket */
void
tcp_set_foreground_cc(struct socket *so)
{
        if (tcp_use_newreno) {
                tcp_set_new_cc(so, TCP_CC_ALGO_NEWRENO_INDEX);
        } else {
                tcp_set_new_cc(so, TCP_CC_ALGO_CUBIC_INDEX);
        }
}

static void
tcp_set_new_cc(struct socket *so, uint16_t cc_index)
{
        struct inpcb *inp = sotoinpcb(so);
        struct tcpcb *tp = intotcpcb(inp);
        u_char old_cc_index = 0;
        if (tp->tcp_cc_index != cc_index) {
                old_cc_index = tp->tcp_cc_index;

                if (CC_ALGO(tp)->cleanup != NULL) {
                        CC_ALGO(tp)->cleanup(tp);
                }
                tp->tcp_cc_index = cc_index;

                tcp_cc_allocate_state(tp);

                if (CC_ALGO(tp)->switch_to != NULL) {
                        CC_ALGO(tp)->switch_to(tp, old_cc_index);
                }

                tcp_ccdbg_trace(tp, NULL, TCP_CC_CHANGE_ALGO);
        }
}

void
tcp_set_recv_bg(struct socket *so)
{
        if (!IS_TCP_RECV_BG(so)) {
                so->so_flags1 |= SOF1_TRAFFIC_MGT_TCP_RECVBG;
        }
}

void
tcp_clear_recv_bg(struct socket *so)
{
        if (IS_TCP_RECV_BG(so)) {
                so->so_flags1 &= ~(SOF1_TRAFFIC_MGT_TCP_RECVBG);
        }
}

void
inp_fc_throttle_tcp(struct inpcb *inp)
{
        struct tcpcb *tp = inp->inp_ppcb;

        if (!tcp_flow_control_response) {
                return;
        }

        /*
         * Back off the slow-start threshold and enter
         * congestion avoidance phase
         */
        if (CC_ALGO(tp)->pre_fr != NULL) {
                CC_ALGO(tp)->pre_fr(tp);
        }
}

void
inp_fc_unthrottle_tcp(struct inpcb *inp)
{
        struct tcpcb *tp = inp->inp_ppcb;

        if (tcp_flow_control_response) {
                if (CC_ALGO(tp)->post_fr != NULL) {
                        CC_ALGO(tp)->post_fr(tp, NULL);
                }

                tp->t_bytes_acked = 0;

                /*
                 * Reset retransmit shift as we know that the reason
                 * for delay in sending a packet is due to flow
                 * control on the outgoing interface. There is no need
                 * to backoff retransmit timer.
                 */
                TCP_RESET_REXMT_STATE(tp);

                tp->t_flagsext &= ~TF_CWND_NONVALIDATED;

                /*
                 * Start the output stream again. Since we are
                 * not retransmitting data, do not reset the
                 * retransmit timer or rtt calculation.
                 */
                tcp_output(tp);
                return;
        }

        /*
         * Back off the slow-start threshold and enter
         * congestion avoidance phase
         */
        if (CC_ALGO(tp)->pre_fr != NULL) {
                CC_ALGO(tp)->pre_fr(tp);
        }

        tp->snd_cwnd = tp->snd_ssthresh;
        tp->t_flagsext &= ~TF_CWND_NONVALIDATED;
        /*
         * Restart counting for ABC as we changed the
         * congestion window just now.
         */
        tp->t_bytes_acked = 0;

        /* Reset retransmit shift as we know that the reason
         * for delay in sending a packet is due to flow
         * control on the outgoing interface. There is no need
         * to backoff retransmit timer.
         */
        TCP_RESET_REXMT_STATE(tp);

        /*
         * Start the output stream again. Since we are
         * not retransmitting data, do not reset the
         * retransmit timer or rtt calculation.
         */
        tcp_output(tp);
}

static int
tcp_getstat SYSCTL_HANDLER_ARGS
{
#pragma unused(oidp, arg1, arg2)

        int error;
        struct tcpstat *stat;
        stat = &tcpstat;
#if XNU_TARGET_OS_OSX
        struct tcpstat zero_stat;

        if (tcp_disable_access_to_stats &&
            !kauth_cred_issuser(kauth_cred_get())) {
                bzero(&zero_stat, sizeof(zero_stat));
                stat = &zero_stat;
        }

#endif /* XNU_TARGET_OS_OSX */

        if (req->oldptr == 0) {
                req->oldlen = (size_t)sizeof(struct tcpstat);
        }

        error = SYSCTL_OUT(req, stat, MIN(sizeof(tcpstat), req->oldlen));

        return error;
}

/*
 * Checksum extended TCP header and data.
 */
int
tcp_input_checksum(int af, struct mbuf *m, struct tcphdr *th, int off, int tlen)
{
        struct ifnet *ifp = m->m_pkthdr.rcvif;

        switch (af) {
        case AF_INET: {
                struct ip *ip = mtod(m, struct ip *);
                struct ipovly *ipov = (struct ipovly *)ip;

                /* ip_stripoptions() must have been called before we get here */
                ASSERT((ip->ip_hl << 2) == sizeof(*ip));

                if ((hwcksum_rx || (ifp->if_flags & IFF_LOOPBACK) ||
                    (m->m_pkthdr.pkt_flags & PKTF_LOOP)) &&
                    (m->m_pkthdr.csum_flags & CSUM_DATA_VALID)) {
                        if (m->m_pkthdr.csum_flags & CSUM_PSEUDO_HDR) {
                                th->th_sum = m->m_pkthdr.csum_rx_val;
                        } else {
                                uint32_t sum = m->m_pkthdr.csum_rx_val;
                                uint32_t start = m->m_pkthdr.csum_rx_start;
                                int32_t trailer = (m_pktlen(m) - (off + tlen));

                                /*
                                 * Perform 1's complement adjustment of octets
                                 * that got included/excluded in the hardware-
                                 * calculated checksum value.  Ignore cases
                                 * where the value already includes the entire
                                 * IP header span, as the sum for those octets
                                 * would already be 0 by the time we get here;
                                 * IP has already performed its header checksum
                                 * checks.  If we do need to adjust, restore
                                 * the original fields in the IP header when
                                 * computing the adjustment value.  Also take
                                 * care of any trailing bytes and subtract out
                                 * their partial sum.
                                 */
                                ASSERT(trailer >= 0);
                                if ((m->m_pkthdr.csum_flags & CSUM_PARTIAL) &&
                                    ((start != 0 && start != off) || trailer)) {
                                        uint32_t swbytes = (uint32_t)trailer;

                                        if (start < off) {
                                                ip->ip_len += sizeof(*ip);
#if BYTE_ORDER != BIG_ENDIAN
                                                HTONS(ip->ip_len);
                                                HTONS(ip->ip_off);
#endif /* BYTE_ORDER != BIG_ENDIAN */
                                        }
                                        /* callee folds in sum */
                                        sum = m_adj_sum16(m, start, off,
                                            tlen, sum);
                                        if (off > start) {
                                                swbytes += (off - start);
                                        } else {
                                                swbytes += (start - off);
                                        }

                                        if (start < off) {
#if BYTE_ORDER != BIG_ENDIAN
                                                NTOHS(ip->ip_off);
                                                NTOHS(ip->ip_len);
#endif /* BYTE_ORDER != BIG_ENDIAN */
                                                ip->ip_len -= sizeof(*ip);
                                        }

                                        if (swbytes != 0) {
                                                tcp_in_cksum_stats(swbytes);
                                        }
                                        if (trailer != 0) {
                                                m_adj(m, -trailer);
                                        }
                                }

                                /* callee folds in sum */
                                th->th_sum = in_pseudo(ip->ip_src.s_addr,
                                    ip->ip_dst.s_addr,
                                    sum + htonl(tlen + IPPROTO_TCP));
                        }
                        th->th_sum ^= 0xffff;
                } else {
                        uint16_t ip_sum;
                        int len;
                        char b[9];

                        bcopy(ipov->ih_x1, b, sizeof(ipov->ih_x1));
                        bzero(ipov->ih_x1, sizeof(ipov->ih_x1));
                        ip_sum = ipov->ih_len;
                        ipov->ih_len = (u_short)tlen;
#if BYTE_ORDER != BIG_ENDIAN
                        HTONS(ipov->ih_len);
#endif
                        len = sizeof(struct ip) + tlen;
                        th->th_sum = in_cksum(m, len);
                        bcopy(b, ipov->ih_x1, sizeof(ipov->ih_x1));
                        ipov->ih_len = ip_sum;

                        tcp_in_cksum_stats(len);
                }
                break;
        }
        case AF_INET6: {
                struct ip6_hdr *ip6 = mtod(m, struct ip6_hdr *);

                if ((hwcksum_rx || (ifp->if_flags & IFF_LOOPBACK) ||
                    (m->m_pkthdr.pkt_flags & PKTF_LOOP)) &&
                    (m->m_pkthdr.csum_flags & CSUM_DATA_VALID)) {
                        if (m->m_pkthdr.csum_flags & CSUM_PSEUDO_HDR) {
                                th->th_sum = m->m_pkthdr.csum_rx_val;
                        } else {
                                uint32_t sum = m->m_pkthdr.csum_rx_val;
                                uint32_t start = m->m_pkthdr.csum_rx_start;
                                int32_t trailer = (m_pktlen(m) - (off + tlen));

                                /*
                                 * Perform 1's complement adjustment of octets
                                 * that got included/excluded in the hardware-
                                 * calculated checksum value.  Also take care
                                 * of any trailing bytes and subtract out their
                                 * partial sum.
                                 */
                                ASSERT(trailer >= 0);
                                if ((m->m_pkthdr.csum_flags & CSUM_PARTIAL) &&
                                    (start != off || trailer != 0)) {
                                        uint16_t s = 0, d = 0;
                                        uint32_t swbytes = (uint32_t)trailer;

                                        if (IN6_IS_SCOPE_EMBED(&ip6->ip6_src)) {
                                                s = ip6->ip6_src.s6_addr16[1];
                                                ip6->ip6_src.s6_addr16[1] = 0;
                                        }
                                        if (IN6_IS_SCOPE_EMBED(&ip6->ip6_dst)) {
                                                d = ip6->ip6_dst.s6_addr16[1];
                                                ip6->ip6_dst.s6_addr16[1] = 0;
                                        }

                                        /* callee folds in sum */
                                        sum = m_adj_sum16(m, start, off,
                                            tlen, sum);
                                        if (off > start) {
                                                swbytes += (off - start);
                                        } else {
                                                swbytes += (start - off);
                                        }

                                        if (IN6_IS_SCOPE_EMBED(&ip6->ip6_src)) {
                                                ip6->ip6_src.s6_addr16[1] = s;
                                        }
                                        if (IN6_IS_SCOPE_EMBED(&ip6->ip6_dst)) {
                                                ip6->ip6_dst.s6_addr16[1] = d;
                                        }

                                        if (swbytes != 0) {
                                                tcp_in6_cksum_stats(swbytes);
                                        }
                                        if (trailer != 0) {
                                                m_adj(m, -trailer);
                                        }
                                }

                                th->th_sum = in6_pseudo(
                                        &ip6->ip6_src, &ip6->ip6_dst,
                                        sum + htonl(tlen + IPPROTO_TCP));
                        }
                        th->th_sum ^= 0xffff;
                } else {
                        tcp_in6_cksum_stats(tlen);
                        th->th_sum = in6_cksum(m, IPPROTO_TCP, off, tlen);
                }
                break;
        }
        default:
                VERIFY(0);
                /* NOTREACHED */
        }

        if (th->th_sum != 0) {
                tcpstat.tcps_rcvbadsum++;
                IF_TCP_STATINC(ifp, badformat);
                return -1;
        }

        return 0;
}


SYSCTL_PROC(_net_inet_tcp, TCPCTL_STATS, stats,
    CTLTYPE_STRUCT | CTLFLAG_RD | CTLFLAG_LOCKED, 0, 0, tcp_getstat,
    "S,tcpstat", "TCP statistics (struct tcpstat, netinet/tcp_var.h)");

static int
sysctl_rexmtthresh SYSCTL_HANDLER_ARGS
{
#pragma unused(arg1, arg2)

        int error, val = tcprexmtthresh;

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

        /*
         * Constrain the number of duplicate ACKs
         * to consider for TCP fast retransmit
         * to either 2 or 3
         */

        if (val < 2 || val > 3) {
                return EINVAL;
        }

        tcprexmtthresh = val;

        return 0;
}

SYSCTL_PROC(_net_inet_tcp, OID_AUTO, rexmt_thresh, CTLTYPE_INT | CTLFLAG_RW |
    CTLFLAG_LOCKED, &tcprexmtthresh, 0, &sysctl_rexmtthresh, "I",
    "Duplicate ACK Threshold for Fast Retransmit");