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

/*
 * Copyright (c) 2000-2008 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 <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 <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>
#if INET6
#include <netinet/ip6.h>
#include <netinet/icmp6.h>
#include <netinet6/nd6.h>
#include <netinet6/ip6_var.h>
#include <netinet6/in6_pcb.h>
#endif
#include <netinet/tcp.h>
#include <netinet/tcp_fsm.h>
#include <netinet/tcp_seq.h>
#include <netinet/tcp_timer.h>
#include <netinet/tcp_var.h>
#if INET6
#include <netinet6/tcp6_var.h>
#endif
#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 */

#if IPSEC
#include <netinet6/ipsec.h>
#if INET6
#include <netinet6/ipsec6.h>
#endif
#include <netkey/key.h>
#endif /*IPSEC*/

#if CONFIG_MACF_NET || CONFIG_MACF_SOCKET
#include <security/mac_framework.h>
#endif /* CONFIG_MACF_NET || CONFIG_MACF_SOCKET */

#include <sys/kdebug.h>

#ifndef __APPLE__
MALLOC_DEFINE(M_TSEGQ, "tseg_qent", "TCP segment queue entry");
#endif

#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))

static int      tcprexmtthresh = 2;
tcp_cc  tcp_ccgen;

#if IPSEC
extern int ipsec_bypass;
#endif

struct  tcpstat tcpstat;

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

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

int tcp_delack_enabled = 3;
SYSCTL_INT(_net_inet_tcp, OID_AUTO, delayed_ack, CTLFLAG_RW, 
    &tcp_delack_enabled, 0, 
    "Delay ACK to try and piggyback it onto a data packet");

int tcp_lq_overflow = 1;
SYSCTL_INT(_net_inet_tcp, OID_AUTO, tcp_lq_overflow, CTLFLAG_RW,
    &tcp_lq_overflow, 0, 
    "Listen Queue Overflow");

#if TCP_DROP_SYNFIN
static int drop_synfin = 1;
SYSCTL_INT(_net_inet_tcp, OID_AUTO, drop_synfin, CTLFLAG_RW,
    &drop_synfin, 0, "Drop TCP packets with SYN+FIN set");
#endif

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

__private_extern__ int tcp_reass_maxseg = 0;
SYSCTL_INT(_net_inet_tcp_reass, OID_AUTO, maxsegments, CTLFLAG_RW,
    &tcp_reass_maxseg, 0,
    "Global maximum number of TCP Segments in Reassembly Queue");

__private_extern__ int tcp_reass_qsize = 0;
SYSCTL_INT(_net_inet_tcp_reass, OID_AUTO, cursegments, CTLFLAG_RD,
    &tcp_reass_qsize, 0,
    "Global number of TCP Segments currently in Reassembly Queue");

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


__private_extern__ int slowlink_wsize = 8192;
SYSCTL_INT(_net_inet_tcp, OID_AUTO, slowlink_wsize, CTLFLAG_RW,
        &slowlink_wsize, 0, "Maximum advertised window size for slowlink");

static int maxseg_unacked = 8;
SYSCTL_INT(_net_inet_tcp, OID_AUTO, maxseg_unacked, CTLFLAG_RW,
        &maxseg_unacked, 0, "Maximum number of outstanding segments left unacked");

static int      tcp_do_rfc3465 = 1;
SYSCTL_INT(_net_inet_tcp, OID_AUTO, rfc3465, CTLFLAG_RW,
        &tcp_do_rfc3465, 0, "");

static int      tcp_do_rfc3465_lim2 = 1;
SYSCTL_INT(_net_inet_tcp, OID_AUTO, rfc3465_lim2, CTLFLAG_RW,
        &tcp_do_rfc3465_lim2, 0, "Appropriate bytes counting w/ L=2*SMSS");

#if CONFIG_IFEF_NOWINDOWSCALE
int tcp_obey_ifef_nowindowscale = 0;
SYSCTL_INT(_net_inet_tcp, OID_AUTO, obey_ifef_nowindowscale, CTLFLAG_RW,
        &tcp_obey_ifef_nowindowscale, 0, "");
#endif

extern int tcp_TCPTV_MIN;

u_int32_t 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 *, unsigned int);
static void      tcp_pulloutofband(struct socket *,
            struct tcphdr *, struct mbuf *, int);
static int       tcp_reass(struct tcpcb *, struct tcphdr *, int *,
                                struct mbuf *);
static void     tcp_xmit_timer(struct tcpcb *, int);
static inline unsigned int tcp_maxmtu(struct rtentry *);
#if INET6
static inline unsigned int tcp_maxmtu6(struct rtentry *);
#endif

/* Neighbor Discovery, Neighbor Unreachability Detection Upper layer hint. */
#if INET6
#define ND6_HINT(tp) \
do { \
        if ((tp) && (tp)->t_inpcb && \
            ((tp)->t_inpcb->inp_vflag & INP_IPV6) != 0 && \
            (tp)->t_inpcb->in6p_route.ro_rt) \
                nd6_nud_hint((tp)->t_inpcb->in6p_route.ro_rt, NULL, 0); \
} while (0)
#else
#define ND6_HINT(tp)
#endif

extern u_int32_t        *delack_bitmask;

extern void     add_to_time_wait(struct tcpcb *);
extern void postevent(struct socket *, struct sockbuf *, int);

extern  void    ipfwsyslog( int level, const char *format,...);
extern int ChkAddressOK( __uint32_t dstaddr, __uint32_t srcaddr );
extern int fw_verbose;
__private_extern__ int tcp_sockthreshold;
__private_extern__ int tcp_win_scale;

#if IPFIREWALL
#define log_in_vain_log( a ) {            \
        if ( (log_in_vain == 3 ) && (fw_verbose == 2)) {        /* Apple logging, log to ipfw.log */ \
                ipfwsyslog a ;  \
        }                       \
        else log a ;            \
}
#else
#define log_in_vain_log( a ) { log a; }
#endif


/*
 * Indicate whether this ack should be delayed.  
 * We can delay the ack if:
 *  - delayed acks are enabled (set to 1) and
 *      - our last ack wasn't a 0-sized window.  We never want to delay
 *        the ack that opens up a 0-sized window.
 *  - delayed acks are enabled (set to 2, "more compatible") and
 *      - our last ack wasn't a 0-sized window.
 *      - if the peer hasn't sent us a TH_PUSH data packet (this solves 3649245)
 *      - the peer hasn't sent us a TH_PUSH data packet, if he did, take this as a clue that we
 *        need to ACK with no delay. This helps higher level protocols who won't send
 *        us more data even if the window is open because their last "segment" hasn't been ACKed
 *  - delayed acks are enabled (set to 3, "streaming detection") and
 *      - if we receive more than "maxseg_unacked"  full packets per second on this socket
 *              - if we don't have more than "maxseg_unacked" delayed so far
 *      - if those criteria aren't met, acts like "2". Allowing faster acking while browsing for example.
 *
 */
#define DELAY_ACK(tp) \
        (((tcp_delack_enabled == 1) && ((tp->t_flags & TF_RXWIN0SENT) == 0)) || \
         (((tcp_delack_enabled == 2) && (tp->t_flags & TF_RXWIN0SENT) == 0) && \
           ((thflags & TH_PUSH) == 0) && ((tp->t_flags & TF_DELACK) == 0)) || \
         (((tcp_delack_enabled == 3) && (tp->t_flags & TF_RXWIN0SENT) == 0) && \
           (tp->t_rcvtime == 0) && ((thflags & TH_PUSH) == 0) && \
           (((tp->t_unacksegs == 0)) || \
           ((tp->rcv_byps > (maxseg_unacked * tp->t_maxseg))  && (tp->t_unacksegs < maxseg_unacked)))))

static int tcp_dropdropablreq(struct socket *head);
static void tcp_newreno_partial_ack(struct tcpcb *tp, struct tcphdr *th);


static int
tcp_reass(tp, th, tlenp, m)
        register struct tcpcb *tp;
        register struct tcphdr *th;
        int *tlenp;
        struct mbuf *m;
{
        struct tseg_qent *q;
        struct tseg_qent *p = NULL;
        struct tseg_qent *nq;
        struct tseg_qent *te = NULL;
        struct socket *so = tp->t_inpcb->inp_socket;
        int flags;
        int dowakeup = 0;

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

        /*
         * 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.
         */
        if (th->th_seq != tp->rcv_nxt &&
            tcp_reass_qsize + 1 >= tcp_reass_maxseg) {
                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 */
        MALLOC(te, struct tseg_qent *, sizeof (struct tseg_qent), M_TSEGQ,
               M_NOWAIT);
        if (te == NULL) {
                tcpstat.tcps_rcvmemdrop++;
                m_freem(m);
                return (0);
        }
        tcp_reass_qsize++;

        /*
         * 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) {
                register 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 >= *tlenp) {
                                tcpstat.tcps_rcvduppack++;
                                tcpstat.tcps_rcvdupbyte += *tlenp;
                                m_freem(m);
                                FREE(te, M_TSEGQ);
                                tcp_reass_qsize--;
                                /*
                                 * 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;
                }
        }
        tcpstat.tcps_rcvoopack++;
        tcpstat.tcps_rcvoobyte += *tlenp;

        /*
         * While we overlap succeeding segments trim them or,
         * if they are completely covered, dequeue them.
         */
        while (q) {
                register int i = (th->th_seq + *tlenp) - q->tqe_th->th_seq;
                if (i <= 0)
                        break;
                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);
                FREE(q, M_TSEGQ);
                tcp_reass_qsize--;
                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);
        do {
                tp->rcv_nxt += q->tqe_len;
                flags = q->tqe_th->th_flags & TH_FIN;
                nq = LIST_NEXT(q, tqe_q);
                LIST_REMOVE(q, tqe_q);
                if (so->so_state & SS_CANTRCVMORE)
                        m_freem(q->tqe_m);
                else {
                        if (sbappendstream(&so->so_rcv, q->tqe_m))
                                dowakeup = 1;
                }
                FREE(q, M_TSEGQ);
                tcp_reass_qsize--;
                q = nq;
        } while (q && q->tqe_th->th_seq == tp->rcv_nxt);
        ND6_HINT(tp);

#if INET6
        if ((tp->t_inpcb->inp_vflag & INP_IPV6) != 0) {
        
                KERNEL_DEBUG(DBG_LAYER_BEG,
                     ((tp->t_inpcb->inp_fport << 16) | tp->t_inpcb->inp_lport),
                     (((tp->t_inpcb->in6p_laddr.s6_addr16[0] & 0xffff) << 16) |
                      (tp->t_inpcb->in6p_faddr.s6_addr16[0] & 0xffff)),
                     0,0,0);
        }
        else
#endif
        {
                KERNEL_DEBUG(DBG_LAYER_BEG,
                     ((tp->t_inpcb->inp_fport << 16) | tp->t_inpcb->inp_lport),
                     (((tp->t_inpcb->inp_laddr.s_addr & 0xffff) << 16) |
                      (tp->t_inpcb->inp_faddr.s_addr & 0xffff)),
                     0,0,0);
        }
        if (dowakeup)
                sorwakeup(so); /* done with socket lock held */
        return (flags);

}

/*
 * Reduce congestion window.
 */
static void
tcp_reduce_congestion_window(
        struct tcpcb    *tp)
{
        u_int win;
        
        win = min(tp->snd_wnd, tp->snd_cwnd) /
                2 / tp->t_maxseg;
        if (win < 2)
                win = 2;
        tp->snd_ssthresh = win * tp->t_maxseg;
        ENTER_FASTRECOVERY(tp);
        tp->snd_recover = tp->snd_max;
        tp->t_timer[TCPT_REXMT] = 0;
        tp->t_rtttime = 0;
        tp->ecn_flags |= TE_SENDCWR;
        tp->snd_cwnd = tp->snd_ssthresh +
                 tp->t_maxseg * tcprexmtthresh;
}


/*
 * TCP input routine, follows pages 65-76 of the
 * protocol specification dated September, 1981 very closely.
 */
#if INET6
int
tcp6_input(mp, offp)
        struct mbuf **mp;
        int *offp;
{
        register struct mbuf *m = *mp;
        struct in6_ifaddr *ia6;

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

        /*
         * draft-itojun-ipv6-tcp-to-anycast
         * better place to put this in?
         */
        ia6 = ip6_getdstifaddr(m);
        if (ia6 && (ia6->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);
                return IPPROTO_DONE;
        }

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

void
tcp_input(m, off0)
        struct mbuf *m;
        int off0;
{
        register struct tcphdr *th;
        register struct ip *ip = NULL;
        register struct ipovly *ipov;
        register struct inpcb *inp;
        u_char *optp = NULL;
        int optlen = 0;
        int len, tlen, off;
        int drop_hdrlen;
        register struct tcpcb *tp = 0;
        register int thflags;
        struct socket *so = 0;
        int todrop, acked, ourfinisacked, needoutput = 0;
        struct in_addr laddr;
#if INET6
        struct in6_addr laddr6;
#endif
        int dropsocket = 0;
        int iss = 0;
        int nosock = 0;
        u_int32_t tiwin;
        struct tcpopt to;               /* options in this segment */
        struct sockaddr_in *next_hop = NULL;
#if TCPDEBUG
        short ostate = 0;
#endif
        struct m_tag *fwd_tag;
        u_char ip_ecn = IPTOS_ECN_NOTECT;
        unsigned int ifscope;

        /*
         * 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_flags & M_PKTHDR) && m->m_pkthdr.rcvif != NULL)
                ifscope = m->m_pkthdr.rcvif->if_index;
        else
                ifscope = IFSCOPE_NONE;

        /* Grab info from PACKET_TAG_IPFORWARD tag prepended to the chain. */
        if (!SLIST_EMPTY(&m->m_pkthdr.tags)) {
                fwd_tag = m_tag_locate(m, KERNEL_MODULE_TAG_ID,
                    KERNEL_TAG_TYPE_IPFORWARD, NULL);
        } else {
                fwd_tag = NULL;
        }
        if (fwd_tag != NULL) {
                struct ip_fwd_tag *ipfwd_tag = (struct ip_fwd_tag *)(fwd_tag+1);
                
                next_hop = ipfwd_tag->next_hop;
                m_tag_delete(m, fwd_tag);
        }
        
#if INET6
        struct ip6_hdr *ip6 = NULL;
        int isipv6;
#endif /* INET6 */
        int rstreason; /* For badport_bandlim accounting purposes */
        struct proc *proc0=current_proc();
        
        KERNEL_DEBUG(DBG_FNC_TCP_INPUT | DBG_FUNC_START,0,0,0,0,0);

#if INET6
        isipv6 = (mtod(m, struct ip *)->ip_v == 6) ? 1 : 0;
#endif
        bzero((char *)&to, sizeof(to));

        tcpstat.tcps_rcvtotal++;



#if INET6
        if (isipv6) {
                /* IP6_EXTHDR_CHECK() is already done at tcp6_input() */
                ip6 = mtod(m, struct ip6_hdr *);
                tlen = sizeof(*ip6) + ntohs(ip6->ip6_plen) - off0;
                if (in6_cksum(m, IPPROTO_TCP, off0, tlen)) {
                        tcpstat.tcps_rcvbadsum++;
                        goto dropnosock;
                }
                th = (struct tcphdr *)((caddr_t)ip6 + off0);

                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 */
                        goto dropnosock;
                }
        } else
#endif /* INET6 */
        {
        /*
         * 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, (struct mbuf *)0);
                off0 = sizeof(struct ip);
                if (m->m_pkthdr.csum_flags & CSUM_TCP_SUM16)
                        m->m_pkthdr.csum_flags = 0; /* invalidate hwcksuming */

        }
        if (m->m_len < sizeof (struct tcpiphdr)) {
                if ((m = m_pullup(m, sizeof (struct tcpiphdr))) == 0) {
                        tcpstat.tcps_rcvshort++;
                        return;
                }
        }
        ip = mtod(m, struct ip *);
        ipov = (struct ipovly *)ip;
        th = (struct tcphdr *)((caddr_t)ip + off0);
        tlen = ip->ip_len;

        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);

        if (m->m_pkthdr.csum_flags & CSUM_DATA_VALID) {
                if (m->m_pkthdr.csum_flags & CSUM_TCP_SUM16) {
                        u_short pseudo;
                        char b[9];
                        *(uint32_t*)&b[0] = *(uint32_t*)&ipov->ih_x1[0];
                        *(uint32_t*)&b[4] = *(uint32_t*)&ipov->ih_x1[4];
                        *(uint8_t*)&b[8] = *(uint8_t*)&ipov->ih_x1[8];
                        
                        bzero(ipov->ih_x1, sizeof(ipov->ih_x1));
                        ipov->ih_len = (u_short)tlen;

#if BYTE_ORDER != BIG_ENDIAN
                        HTONS(ipov->ih_len);
#endif

                        pseudo = in_cksum(m, sizeof (struct ip));
                        
                        *(uint32_t*)&ipov->ih_x1[0] = *(uint32_t*)&b[0];
                        *(uint32_t*)&ipov->ih_x1[4] = *(uint32_t*)&b[4];
                        *(uint8_t*)&ipov->ih_x1[8] = *(uint8_t*)&b[8];
                        
                        th->th_sum = in_addword(pseudo, (m->m_pkthdr.csum_data & 0xFFFF));
                } else {
                        if (m->m_pkthdr.csum_flags & CSUM_PSEUDO_HDR)
                                th->th_sum = m->m_pkthdr.csum_data;
                        else
                                th->th_sum = in_pseudo(ip->ip_src.s_addr,
                                        ip->ip_dst.s_addr, htonl(m->m_pkthdr.csum_data +
                                        ip->ip_len + IPPROTO_TCP));
                }
                th->th_sum ^= 0xffff;
        } else {
                char b[9];
                /*
                 * Checksum extended TCP header and data.
                 */
                *(uint32_t*)&b[0] = *(uint32_t*)&ipov->ih_x1[0];
                *(uint32_t*)&b[4] = *(uint32_t*)&ipov->ih_x1[4];
                *(uint8_t*)&b[8] = *(uint8_t*)&ipov->ih_x1[8];
                
                len = sizeof (struct ip) + tlen;
                bzero(ipov->ih_x1, sizeof(ipov->ih_x1));
                ipov->ih_len = (u_short)tlen;

#if BYTE_ORDER != BIG_ENDIAN
                HTONS(ipov->ih_len);
#endif

                th->th_sum = in_cksum(m, len);
                
                *(uint32_t*)&ipov->ih_x1[0] = *(uint32_t*)&b[0];
                *(uint32_t*)&ipov->ih_x1[4] = *(uint32_t*)&b[4];
                *(uint8_t*)&ipov->ih_x1[8] = *(uint8_t*)&b[8];

                tcp_in_cksum_stats(len);
        }
        if (th->th_sum) {
                tcpstat.tcps_rcvbadsum++;
                goto dropnosock;
        }
#if INET6
        /* Re-initialization for later version check */
        ip->ip_v = IPVERSION;
#endif
        ip_ecn = (ip->ip_tos & IPTOS_ECN_MASK);
        }

        /*
         * Check that TCP offset makes sense,
         * pull out TCP options and adjust length.              XXX
         */
        off = th->th_off << 2;
        if (off < sizeof (struct tcphdr) || off > tlen) {
                tcpstat.tcps_rcvbadoff++;
                goto dropnosock;
        }
        tlen -= off;    /* tlen is used instead of ti->ti_len */
        if (off > sizeof (struct tcphdr)) {
#if INET6
                if (isipv6) {
                        IP6_EXTHDR_CHECK(m, off0, off, return);
                        ip6 = mtod(m, struct ip6_hdr *);
                        th = (struct tcphdr *)((caddr_t)ip6 + off0);
                } else
#endif /* INET6 */
                {
                        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 *);
                                ipov = (struct ipovly *)ip;
                                th = (struct tcphdr *)((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 *)optp == htonl(TCPOPT_TSTAMP_HDR) &&
                        (th->th_flags & TH_SYN) == 0) {
                        to.to_flags |= TOF_TS;
                        to.to_tsval = ntohl(*(u_int32_t *)(optp + 4));
                        to.to_tsecr = ntohl(*(u_int32_t *)(optp + 8));
                        optp = NULL;    /* we've parsed the options */
                }
        }
        thflags = th->th_flags;

#if TCP_DROP_SYNFIN
        /*
         * If the drop_synfin option is enabled, 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 (drop_synfin && (thflags & (TH_SYN|TH_FIN)) == (TH_SYN|TH_FIN))
                goto dropnosock;
#endif

        /*
         * 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

        /*
         * 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;

        /*
         * Locate pcb for segment.
         */
findpcb:
#if IPFIREWALL_FORWARD
        if (next_hop != NULL
#if INET6
            && isipv6 == 0 /* IPv6 support is not yet */
#endif /* INET6 */
            ) {
                /*
                 * Diverted. Pretend to be the destination.
                 * already got one like this? 
                 */
                inp = in_pcblookup_hash(&tcbinfo, ip->ip_src, th->th_sport,
                        ip->ip_dst, th->th_dport, 0, m->m_pkthdr.rcvif);
                if (!inp) {
                        /* 
                         * No, then it's new. Try find the ambushing socket
                         */
                        if (!next_hop->sin_port) {
                                inp = in_pcblookup_hash(&tcbinfo, ip->ip_src,
                                    th->th_sport, next_hop->sin_addr,
                                    th->th_dport, 1, m->m_pkthdr.rcvif);
                        } else {
                                inp = in_pcblookup_hash(&tcbinfo,
                                    ip->ip_src, th->th_sport,
                                    next_hop->sin_addr,
                                    ntohs(next_hop->sin_port), 1,
                                    m->m_pkthdr.rcvif);
                        }
                }
        } else
#endif  /* IPFIREWALL_FORWARD */
      {
#if INET6
        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
#endif /* INET6 */
        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_boundif;
#if IPSEC
        if (ipsec_bypass == 0)  {
#if INET6
                if (isipv6) {
                        if (inp != NULL && ipsec6_in_reject_so(m, inp->inp_socket)) {
                                IPSEC_STAT_INCREMENT(ipsec6stat.in_polvio);
                                if (in_pcb_checkstate(inp, WNT_RELEASE, 0) == WNT_STOPUSING) 
                                        inp = NULL;     // pretend we didn't find it 
                                goto dropnosock;
                        }
                } else
#endif /* INET6 */
                        if (inp != NULL && ipsec4_in_reject_so(m, inp->inp_socket)) {
                                IPSEC_STAT_INCREMENT(ipsecstat.in_polvio);
                                if (in_pcb_checkstate(inp, WNT_RELEASE, 0) == WNT_STOPUSING) 
                                        inp = NULL;     // pretend we didn't find it 
                                goto dropnosock;
                        }
        }
#endif /*IPSEC*/

        /*
         * 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) {
#if INET6
                        char dbuf[MAX_IPv6_STR_LEN], sbuf[MAX_IPv6_STR_LEN];
#else /* INET6 */
                        char dbuf[MAX_IPv4_STR_LEN], sbuf[MAX_IPv4_STR_LEN];
#endif /* INET6 */

#if INET6
                        if (isipv6) {
                                inet_ntop(AF_INET6, &ip6->ip6_dst, dbuf, sizeof(dbuf));
                                inet_ntop(AF_INET6, &ip6->ip6_src, sbuf, sizeof(sbuf));
                        } else
#endif
                        {
                                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:
                                if ((thflags & TH_SYN) &&
                                        !(m->m_flags & (M_BCAST | M_MCAST)) &&
#if INET6
                                        ((isipv6 && !IN6_ARE_ADDR_EQUAL(&ip6->ip6_dst, &ip6->ip6_src)) ||
                                         (!isipv6 && ip->ip_dst.s_addr != ip->ip_src.s_addr))
#else
                                        ip->ip_dst.s_addr != ip->ip_src.s_addr
#endif
                                         )
                                        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)
                                                goto dropnosock;
                                        break;
                                case 2:
                                        goto dropnosock;
                                default:
                                        goto dropnosock;
                                }
                }
                rstreason = BANDLIM_RST_CLOSEDPORT;
                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
                goto dropnosock;
        }

        tcp_lock(so, 1, (void *)2);
        if (in_pcb_checkstate(inp, WNT_RELEASE, 1) == WNT_STOPUSING) {
                tcp_unlock(so, 1, (void *)2);
                inp = NULL;     // pretend we didn't find it 
                goto dropnosock;
        }

        tp = intotcpcb(inp);
        if (tp == 0) {
                rstreason = BANDLIM_RST_CLOSEDPORT;
                goto dropwithreset;
        }
        if (tp->t_state == TCPS_CLOSED)
                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;

#if CONFIG_MACF_NET
        if (mac_inpcb_check_deliver(inp, m, AF_INET, SOCK_STREAM))
                goto drop;
#endif

        if (so->so_options & (SO_DEBUG|SO_ACCEPTCONN)) {
#if TCPDEBUG
                if (so->so_options & SO_DEBUG) {
                        ostate = tp->t_state;
#if INET6
                        if (isipv6)
                                bcopy((char *)ip6, (char *)tcp_saveipgen,
                                      sizeof(*ip6));
                        else
#endif /* INET6 */
                        bcopy((char *)ip, (char *)tcp_saveipgen, sizeof(*ip));
                        tcp_savetcp = *th;
                }
#endif
                if (so->so_options & SO_ACCEPTCONN) {
                    register struct tcpcb *tp0 = tp;
                        struct socket *so2;
                        struct socket *oso;
                        struct sockaddr_storage from;
#if INET6
                        struct inpcb *oinp = sotoinpcb(so);
#endif /* INET6 */
                        unsigned int head_ifscope;

                        /* Get listener's bound-to-interface, if any */
                        head_ifscope = (inp->inp_flags & INP_BOUND_IF) ?
                            inp->inp_boundif : IFSCOPE_NONE;

#if !IPSEC
                        /*
                         * Current IPsec implementation makes incorrect IPsec
                         * cache if this check is done here.
                         * So delay this until duplicated socket is created.
                         */
                        if ((thflags & (TH_RST|TH_ACK|TH_SYN)) != TH_SYN) {
                                /*
                                 * Note: dropwithreset makes sure we don't
                                 * send a RST in response to a RST.
                                 */
                                if (thflags & TH_ACK) {
                                        tcpstat.tcps_badsyn++;
                                        rstreason = BANDLIM_RST_OPENPORT;
                                        goto dropwithreset;
                                }
                                goto drop;
                        }
#endif
                        KERNEL_DEBUG(DBG_FNC_TCP_NEWCONN | DBG_FUNC_START,0,0,0,0,0);

#if INET6
                        /*
                         * 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.  RFC2462 does not suggest dropping
                         * SYN in this case.
                         * If we decipher RFC2462 5.5.4, it says like this:
                         * 1. use of deprecated addr with existing
                         *    communication is okay - "SHOULD continue to be
                         *    used"
                         * 2. use of it with new communication:
                         *   (2a) "SHOULD NOT be used if alternate address
                         *        with sufficient scope is available"
                         *   (2b) nothing mentioned otherwise.
                         * Here we fall into (2b) case as we have no choice in
                         * our source address selection - we must obey the peer.
                         *
                         * The wording in RFC2462 is confusing, and there are
                         * multiple description text for deprecated address
                         * handling - worse, they are not exactly the same.
                         * I believe 5.5.4 is the best one, so we follow 5.5.4.
                         */
                        if (isipv6 && !ip6_use_deprecated) {
                                struct in6_ifaddr *ia6;

                                if ((ia6 = ip6_getdstifaddr(m)) &&
                                    (ia6->ia6_flags & IN6_IFF_DEPRECATED)) {
                                        tp = NULL;
                                        rstreason = BANDLIM_RST_OPENPORT;
                                        goto dropwithreset;
                                }
                        }
#endif
                        if (so->so_filt) {
#if INET6
                                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;
                                }
                                else
#endif
                                {
                                        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;
                                }
                                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) 
                                        goto drop;
                        }

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

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

                        so = so2;
                        tcp_lock(so, 1, 0);
                        /*
                         * This is ugly, but ....
                         *
                         * 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.
                         */
                        dropsocket++;

                        /*
                         * Inherit INP_BOUND_IF from listener; testing if
                         * head_ifscope is non-zero is sufficient, since it
                         * can only be set to a non-zero value earlier if
                         * the listener has such a flag set.
                         */
#if INET6
                        if (head_ifscope != IFSCOPE_NONE && !isipv6) {
#else
                        if (head_ifscope != IFSCOPE_NONE) {
#endif /* INET6 */
                                inp->inp_flags |= INP_BOUND_IF;
                                inp->inp_boundif = head_ifscope;
                        }
#if INET6
                        if (isipv6)
                                inp->in6p_laddr = ip6->ip6_dst;
                        else {
                                inp->inp_vflag &= ~INP_IPV6;
                                inp->inp_vflag |= INP_IPV4;
#endif /* INET6 */
                        inp->inp_laddr = ip->ip_dst;
#if INET6
                        }
#endif /* INET6 */
                        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 INET6
                                if (isipv6)
                                        inp->in6p_laddr = in6addr_any;
                                else
#endif /* INET6 */
                                inp->inp_laddr.s_addr = INADDR_ANY;
                                inp->inp_lport = 0;
                                tcp_lock(oso, 0, 0);    /* release ref on parent */
                                tcp_unlock(oso, 1, 0);
                                goto drop;
                        }
#if IPSEC
                        /*
                         * To avoid creating incorrectly cached IPsec
                         * association, this is need to be done here.
                         *
                         * Subject: (KAME-snap 748)
                         * From: Wayne Knowles <w.knowles@niwa.cri.nz>
                         * ftp://ftp.kame.net/pub/mail-list/snap-users/748
                         */
                        if ((thflags & (TH_RST|TH_ACK|TH_SYN)) != TH_SYN) {
                                /*
                                 * Note: dropwithreset makes sure we don't
                                 * send a RST in response to a RST.
                                 */
                                tcp_lock(oso, 0, 0);    /* release ref on parent */
                                tcp_unlock(oso, 1, 0);
                                if (thflags & TH_ACK) {
                                        tcpstat.tcps_badsyn++;
                                        rstreason = BANDLIM_RST_OPENPORT;
                                        goto dropwithreset;
                                }
                                goto drop;
                        }
#endif
#if INET6
                        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
#endif /* INET6 */
                        inp->inp_options = ip_srcroute();
                        tcp_lock(oso, 0, 0);
#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 */
                        tp->t_state = TCPS_LISTEN;
                        tp->t_flags |= tp0->t_flags & (TF_NOPUSH|TF_NOOPT|TF_NODELAY);
                        tp->t_keepinit = tp0->t_keepinit;
                        tp->t_inpcb->inp_ip_ttl = tp0->t_inpcb->inp_ip_ttl;

                        /* now drop the reference on the listener */
                        tcp_unlock(oso, 1, 0);

                        /* Compute proper scaling value from buffer space */
                        if (inp->inp_pcbinfo->ipi_count < tcp_sockthreshold) {
                                tp->request_r_scale = max(tcp_win_scale, tp->request_r_scale);
                                so->so_rcv.sb_hiwat = imin(TCP_MAXWIN << tp->request_r_scale, (sb_max / (MSIZE+MCLBYTES)) * MCLBYTES);  
                        }
                        else {
                                while (tp->request_r_scale < TCP_MAX_WINSHIFT &&
                                TCP_MAXWIN << tp->request_r_scale <
                                so->so_rcv.sb_hiwat)
                                        tp->request_r_scale++;
                        }

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

#if 1
        lck_mtx_assert(((struct inpcb *)so->so_pcb)->inpcb_mtx, LCK_MTX_ASSERT_OWNED);
#endif
        /*
         * Radar 3529618
         * This is the second part of the MSS DoS prevention code (after
         * minmss on the sending side) and it deals with too many too small
         * tcp packets in a too short timeframe (1 second).
         *
         * For every full second we count the number of received packets
         * and bytes. If we get a lot of packets per second for this connection
         * (tcp_minmssoverload) we take a closer look at it and compute the
         * average packet size for the past second. If that is less than
         * tcp_minmss we get too many packets with very small payload which
         * is not good and burdens our system (and every packet generates
         * a wakeup to the process connected to our socket). We can reasonable
         * expect this to be small packet DoS attack to exhaust our CPU
         * cycles.
         *
         * Care has to be taken for the minimum packet overload value. This
         * value defines the minimum number of packets per second before we
         * start to worry. This must not be too low to avoid killing for
         * example interactive connections with many small packets like
         * telnet or SSH.
         *
         * Setting either tcp_minmssoverload or tcp_minmss to "0" disables
         * this check.
         *
         * Account for packet if payload packet, skip over ACK, etc.
         *
         * The packet per second count is done all the time and is also used
         * by "DELAY_ACK" to detect streaming situations.
         *
         */
        if (tp->t_state == TCPS_ESTABLISHED && tlen > 0) {
                if (tp->rcv_reset > tcp_now) {
                        tp->rcv_pps++;
                        tp->rcv_byps += tlen + off;
                        if (tp->rcv_byps > tp->rcv_maxbyps)
                                tp->rcv_maxbyps = tp->rcv_byps;
                /*
                 * Setting either tcp_minmssoverload or tcp_minmss to "0" disables
                 * the check. 
                 */
                        if (tcp_minmss && tcp_minmssoverload && tp->rcv_pps > tcp_minmssoverload) {
                                if ((tp->rcv_byps / tp->rcv_pps) < tcp_minmss) {
                                        char    ipstrbuf[MAX_IPv6_STR_LEN];
                                        printf("too many small tcp packets from "
                                               "%s:%u, av. %ubyte/packet, "
                                               "dropping connection\n",
#if INET6
                                                isipv6 ?
                                                inet_ntop(AF_INET6, &inp->in6p_faddr, ipstrbuf,
                                                                  sizeof(ipstrbuf)) :
#endif
                                                inet_ntop(AF_INET, &inp->inp_faddr, ipstrbuf,
                                                                  sizeof(ipstrbuf)),
                                                inp->inp_fport,
                                                tp->rcv_byps / tp->rcv_pps);
                                        tp = tcp_drop(tp, ECONNRESET);
/*                                      tcpstat.tcps_minmssdrops++; */
                                        goto drop;
                                }
                        }
                } else {
                        tp->rcv_reset = tcp_now + TCP_RETRANSHZ;
                        tp->rcv_pps = 1;
                        tp->rcv_byps = tlen + off;
                }
        }
        
#if TRAFFIC_MGT
        if (so->so_traffic_mgt_flags & TRAFFIC_MGT_SO_BACKGROUND) {
                tcpstat.tcps_bg_rcvtotal++;

                 /* Take snapshots of pkts recv;
                  * tcpcb should have been initialized to 0 when allocated, 
                  * so if 0 then this is the first time we're doing this
                  */
                if (!tp->tot_recv_snapshot) {
                         tp->tot_recv_snapshot = tcpstat.tcps_rcvtotal;
                 }
                if (!tp->bg_recv_snapshot) {
                         tp->bg_recv_snapshot = tcpstat.tcps_bg_rcvtotal;
                 }
        }
#endif /* TRAFFIC_MGT */

        /*
           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 &&
                (tp->ecn_flags & (TE_SETUPSENT | TE_SETUPRECEIVED)) ==
                 (TE_SETUPSENT | TE_SETUPRECEIVED) && tlen > 0 &&
                SEQ_GEQ(th->th_seq, tp->last_ack_sent) &&
                SEQ_LT(th->th_seq, tp->last_ack_sent + tp->rcv_wnd)) {
                tp->ecn_flags |= TE_SENDECE;
        }
        
        /*
           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) == TH_CWR) {
                tp->ecn_flags &= ~TE_SENDECE;
        }
        
        /*
         * Segment received on connection.
         * Reset idle time and keep-alive timer.
         */
        tp->t_rcvtime = 0;
        if (TCPS_HAVEESTABLISHED(tp->t_state))
                tp->t_timer[TCPT_KEEP] = TCP_KEEPIDLE(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, ifscope);

        if (tp->t_state == TCPS_SYN_SENT && (thflags & TH_SYN)) {
                if (to.to_flags & TOF_SCALE) {
                        tp->t_flags |= TF_RCVD_SCALE;
                        tp->requested_s_scale = to.to_requested_s_scale;
                        tp->snd_wnd = th->th_win << tp->snd_scale;
                        tiwin = tp->snd_wnd;
                }
                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 (tp->sack_enable) {
                        if (!(to.to_flags & TOF_SACK))
                                tp->sack_enable = 0;
                        else
                                tp->t_flags |= TF_SACK_PERMIT;
                }
        }

        /*
         * 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_ACK &&
            ((tp->t_flags & (TF_NEEDSYN|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;
                }

                /* Force acknowledgment if we received a FIN */

                if (thflags & TH_FIN)
                        tp->t_flags |= TF_ACKNOW;

                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 &&
                            ((!tcp_do_newreno && !tp->sack_enable &&
                              tp->t_dupacks < tcprexmtthresh) ||
                             ((tcp_do_newreno || tp->sack_enable) &&
                              !IN_FASTRECOVERY(tp) && to.to_nsacks == 0 &&
                              TAILQ_EMPTY(&tp->snd_holes)))) {
                                /*
                                 * this is a pure ack for outstanding data.
                                 */
                                ++tcpstat.tcps_predack;
                                /*
                                 * "bad retransmit" recovery
                                 */
                                if (tp->t_rxtshift == 1 &&
                                    tcp_now < tp->t_badrxtwin) {
                                        ++tcpstat.tcps_sndrexmitbad;
                                        tp->snd_cwnd = tp->snd_cwnd_prev;
                                        tp->snd_ssthresh =
                                            tp->snd_ssthresh_prev;
                                        tp->snd_recover = tp->snd_recover_prev;
                                        if (tp->t_flags & TF_WASFRECOVERY)
                                            ENTER_FASTRECOVERY(tp);
                                        tp->snd_nxt = tp->snd_max;
                                        tp->t_badrxtwin = 0;
                                }
                                /*
                                 * Recalculate the transmit timer / rtt.
                                 *
                                 * Some boxes send broken timestamp replies
                                 * during the SYN+ACK phase, ignore
                                 * timestamps of 0 or we could calculate a
                                 * huge RTT and blow up the retransmit timer.
                                 */
                                if (((to.to_flags & TOF_TS) != 0) && (to.to_tsecr != 0)) { /* Makes sure we already have a TS */
                                        if (!tp->t_rttlow ||
                                            tp->t_rttlow > tcp_now - to.to_tsecr)
                                                tp->t_rttlow = tcp_now - to.to_tsecr;
                                        tcp_xmit_timer(tp,
                                            tcp_now - to.to_tsecr);
                                } else if (tp->t_rtttime &&
                                            SEQ_GT(th->th_ack, tp->t_rtseq)) {
                                        if (!tp->t_rttlow ||
                                            tp->t_rttlow > tcp_now - tp->t_rtttime)
                                                tp->t_rttlow = tcp_now - tp->t_rtttime;
                                        tcp_xmit_timer(tp, tp->t_rtttime);
                                }
                                acked = th->th_ack - tp->snd_una;
                                tcpstat.tcps_rcvackpack++;
                                tcpstat.tcps_rcvackbyte += acked;
                                /*
                                 * Grow the congestion window, if the
                                 * connection is cwnd bound.
                                 */
                                if (tp->snd_cwnd < tp->snd_wnd) {
                                        tp->t_bytes_acked += acked;
                                        if (tp->t_bytes_acked > tp->snd_cwnd) {
                                                tp->t_bytes_acked -= tp->snd_cwnd;
                                                tp->snd_cwnd += tp->t_maxseg;
                                        }
                                }
                                sbdrop(&so->so_snd, acked);
                                if (SEQ_GT(tp->snd_una, tp->snd_recover) &&
                                    SEQ_LEQ(th->th_ack, tp->snd_recover))
                                        tp->snd_recover = th->th_ack - 1;
                                tp->snd_una = th->th_ack;
                                /*
                                 * pull snd_wl2 up to prevent seq wrap relative
                                 * to th_ack.
                                 */
                                tp->snd_wl2 = th->th_ack;
                                tp->t_dupacks = 0;
                                m_freem(m);
                                ND6_HINT(tp); /* some progress has been done */

                                /*
                                 * 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;
                                else if (tp->t_timer[TCPT_PERSIST] == 0)
                                        tp->t_timer[TCPT_REXMT] = tp->t_rxtcur;

                                sowwakeup(so); /* has to be done with socket lock held */
                                if ((so->so_snd.sb_cc) || (tp->t_flags & TF_ACKNOW)) {
                                        tp->t_unacksegs = 0;
                                        (void) tcp_output(tp);
                                }
                                tcp_unlock(so, 1, 0);
                                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 (tp->sack_enable && 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;
                        tcpstat.tcps_rcvpack++;
                        tcpstat.tcps_rcvbyte += tlen;
                        ND6_HINT(tp);   /* some progress has been done */
                        /*
                         * Add data to socket buffer.
                         */
                        m_adj(m, drop_hdrlen);  /* delayed header drop */
                        if (sbappendstream(&so->so_rcv, m))
                                sorwakeup(so);
#if INET6
                        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
#endif 
                        {
                                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); 
                        }
                        if (DELAY_ACK(tp))  {
                                tp->t_flags |= TF_DELACK;
                                tp->t_unacksegs++;
                        } else {
                                tp->t_unacksegs = 0;
                                tp->t_flags |= TF_ACKNOW;
                                tcp_output(tp);
                        }
                        tcp_unlock(so, 1, 0);
                        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.
         */
#if 1
        lck_mtx_assert(((struct inpcb *)so->so_pcb)->inpcb_mtx, LCK_MTX_ASSERT_OWNED);
#endif
        { int win;

        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));
        }

        switch (tp->t_state) {

        /*
         * 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.
         * Don't bother responding if the destination was a broadcast.
         * Otherwise 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: {
                register struct sockaddr_in *sin;
#if INET6
                register struct sockaddr_in6 *sin6;
#endif

#if 1
                lck_mtx_assert(((struct inpcb *)so->so_pcb)->inpcb_mtx, LCK_MTX_ASSERT_OWNED);
#endif
                if (thflags & TH_RST) 
                        goto drop;
                if (thflags & TH_ACK) {
                        rstreason = BANDLIM_RST_OPENPORT;
                        goto dropwithreset;
                }
                if ((thflags & TH_SYN) == 0)
                        goto drop;
                if (th->th_dport == th->th_sport) {
#if INET6
                        if (isipv6) {
                                if (IN6_ARE_ADDR_EQUAL(&ip6->ip6_dst,
                                                       &ip6->ip6_src))
                                        goto drop;
                        } else
#endif /* INET6 */
                        if (ip->ip_dst.s_addr == ip->ip_src.s_addr)
                                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))
                        goto drop;
#if INET6
                if (isipv6) {
                        if (IN6_IS_ADDR_MULTICAST(&ip6->ip6_dst) ||
                            IN6_IS_ADDR_MULTICAST(&ip6->ip6_src))
                                goto drop;
                } else
#endif
                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;
#if INET6
                if (isipv6) {
                        MALLOC(sin6, struct sockaddr_in6 *, sizeof *sin6,
                               M_SONAME, M_NOWAIT);
                        if (sin6 == NULL)
                                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,
                                           proc0)) {
                                inp->in6p_laddr = laddr6;
                                FREE(sin6, M_SONAME);
                                goto drop;
                        }
                        FREE(sin6, M_SONAME);
                } else
#endif
            {
#if 0
                        lck_mtx_assert(((struct inpcb *)so->so_pcb)->inpcb_mtx, LCK_MTX_ASSERT_OWNED);
#endif
                        MALLOC(sin, struct sockaddr_in *, sizeof *sin, M_SONAME,
                       M_NOWAIT);
                        if (sin == NULL)
                                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, proc0)) {
                                inp->inp_laddr = laddr;
                                FREE(sin, M_SONAME);
                                goto drop;
                        }
                        FREE(sin, M_SONAME);
                }

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

                if (tp->sack_enable) {
                        if (!(to.to_flags & TOF_SACK))
                                tp->sack_enable = 0;
                        else
                                tp->t_flags |= TF_SACK_PERMIT;
                }
                
                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->t_flags |= TF_ACKNOW;
                tp->t_unacksegs = 0;
                tp->t_state = TCPS_SYN_RECEIVED;
                tp->t_timer[TCPT_KEEP] = tp->t_keepinit ? tp->t_keepinit : tcp_keepinit;
                dropsocket = 0;         /* committed to socket */
                tcpstat.tcps_accepts++;
                if ((thflags & (TH_ECE | TH_CWR)) == (TH_ECE | TH_CWR)) {
                        /* ECN-setup SYN */
                        tp->ecn_flags |= (TE_SETUPRECEIVED | TE_SENDIPECT);
                }
#if CONFIG_IFEF_NOWINDOWSCALE
                if (tcp_obey_ifef_nowindowscale && m->m_pkthdr.rcvif != NULL &&
                    (m->m_pkthdr.rcvif->if_eflags & IFEF_NOWINDOWSCALE)) {
                        /* Window scaling is not enabled on this interface */
                        tp->t_flags &= ~TF_REQ_SCALE;
                }
#endif
                goto trimthenstep6;
                }

        /*
         * If the state is SYN_RECEIVED:
         *      if 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))) {
                                rstreason = BANDLIM_RST_OPENPORT;
                                goto dropwithreset;
                }
                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))) {
                        rstreason = BANDLIM_UNLIMITED;
                        goto dropwithreset;
                }
                if (thflags & TH_RST) {
                        if ((thflags & TH_ACK) != 0) {
                                tp = tcp_drop(tp, ECONNREFUSED);
                                postevent(so, 0, EV_RESET);
                        }
                        goto drop;
                }
                if ((thflags & TH_SYN) == 0)
                        goto drop;
                tp->snd_wnd = th->th_win;       /* initial send window */

                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;
                        }
                        else {
                                /* non-ECN-setup SYN-ACK */
                                tp->ecn_flags &= ~TE_SENDIPECT;
                        }
                        
#if CONFIG_MACF_NET && CONFIG_MACF_SOCKET
                        /* XXXMAC: recursive lock: SOCK_LOCK(so); */
                        mac_socketpeer_label_associate_mbuf(m, so);
                        /* XXXMAC: SOCK_UNLOCK(so); */
#endif
                        /* Do window scaling on this connection? */
                        if ((tp->t_flags & (TF_RCVD_SCALE|TF_REQ_SCALE)) ==
                                (TF_RCVD_SCALE|TF_REQ_SCALE)) {
                                tp->snd_scale = tp->requested_s_scale;
                                tp->rcv_scale = tp->request_r_scale;
                        }
                        tp->rcv_adv += tp->rcv_wnd;
                        tp->snd_una++;          /* SYN is acked */
                        /*
                         * If there's data, delay ACK; if there's also a FIN
                         * ACKNOW will be turned on later.
                         */
                        if (DELAY_ACK(tp) && tlen != 0) {
                                tp->t_flags |= TF_DELACK;
                                tp->t_unacksegs++;
                        }
                        else {
                                tp->t_flags |= TF_ACKNOW;
                                tp->t_unacksegs = 0;
                        }
                        /*
                         * Received <SYN,ACK> in SYN_SENT[*] state.
                         * Transitions:
                         *      SYN_SENT  --> ESTABLISHED
                         *      SYN_SENT* --> FIN_WAIT_1
                         */
                        tp->t_starttime = 0;
                        if (tp->t_flags & TF_NEEDFIN) {
                                tp->t_state = TCPS_FIN_WAIT_1;
                                tp->t_flags &= ~TF_NEEDFIN;
                                thflags &= ~TH_SYN;
                        } else {
                                tp->t_state = TCPS_ESTABLISHED;
                                tp->t_timer[TCPT_KEEP] = TCP_KEEPIDLE(tp);
                        }
                        /* soisconnected may lead to socket_unlock in case of upcalls,
                         * make sure this is done when everything is setup.
                         */
                        soisconnected(so);
                } else {
                /*
                 *  Received initial SYN in SYN-SENT[*] state => simul-
                 *  taneous open.  If segment contains CC option and there is
                 *  a cached CC, apply TAO test; if it succeeds, connection is
                 *  half-synchronized.  Otherwise, do 3-way handshake:
                 *        SYN-SENT -> SYN-RECEIVED
                 *        SYN-SENT* -> SYN-RECEIVED*
                 */
                        tp->t_flags |= TF_ACKNOW;
                        tp->t_timer[TCPT_REXMT] = 0;
                        tp->t_state = TCPS_SYN_RECEIVED;

                }

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 numberering 
         */
        case TCPS_ESTABLISHED:
                if (thflags & TH_SYN)  
                        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.
         * 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)))) {
                        switch (tp->t_state) {

                        case TCPS_SYN_RECEIVED:
                                so->so_error = ECONNREFUSED;
                                goto close;

                        case TCPS_ESTABLISHED:
                                if (tp->last_ack_sent != th->th_seq) {
                                        tcpstat.tcps_badrst++;
                                        goto drop;
                                }
                        case TCPS_FIN_WAIT_1:
                        case TCPS_CLOSE_WAIT:
                                /*
                                  Drop through ...
                                */
                        case TCPS_FIN_WAIT_2:
                                so->so_error = ECONNRESET;
                        close:
                                postevent(so, 0, EV_RESET);
                                tp->t_state = TCPS_CLOSED;
                                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;
                        }
                }
                goto drop;
        }

#if 0
        lck_mtx_assert(((struct inpcb *)so->so_pcb)->inpcb_mtx, LCK_MTX_ASSERT_OWNED);
#endif

        /*
         * 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;
                        tcpstat.tcps_pawsdrop++;
                        if (tlen)
                                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)) {
                rstreason = BANDLIM_RST_OPENPORT;
                goto dropwithreset;
        }

        todrop = tp->rcv_nxt - th->th_seq;
        if (todrop > 0) {
                if (thflags & TH_SYN) {
                        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.
                 */
                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.
                         */
                        tp->t_flags |= TF_ACKNOW;
                        tp->t_unacksegs = 0;
                        todrop = tlen;
                        tcpstat.tcps_rcvduppack++;
                        tcpstat.tcps_rcvdupbyte += todrop;
                } else {
                        tcpstat.tcps_rcvpartduppack++;
                        tcpstat.tcps_rcvpartdupbyte += todrop;
                }
                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.
         */
        if ((so->so_state & SS_NOFDREF) &&
            tp->t_state > TCPS_CLOSE_WAIT && tlen) {
                tp = tcp_close(tp);
                tcpstat.tcps_rcvafterclose++;
                rstreason = BANDLIM_UNLIMITED;
                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);
                                tcp_unlock(so, 1, 0);
                                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;
                                tp->t_unacksegs = 0;
                                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;
        }

        /*
         * If a SYN is in the window, then this is an
         * error and we send an RST and drop the connection.
         */
        if (thflags & TH_SYN) {
                tp = tcp_drop(tp, ECONNRESET);
                rstreason = BANDLIM_UNLIMITED;
                postevent(so, 0, EV_RESET);
                goto dropwithreset;
        }

        /*
         * 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 ||
                    (tp->t_flags & TF_NEEDSYN))
                        goto step6;
                else if (tp->t_flags & TF_ACKNOW)
                        goto dropafterack;
                else
                        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 ((tp->t_flags & (TF_RCVD_SCALE|TF_REQ_SCALE)) ==
                        (TF_RCVD_SCALE|TF_REQ_SCALE)) {
                        tp->snd_scale = tp->requested_s_scale;
                        tp->rcv_scale = tp->request_r_scale;
                }
                /*
                 * Make transitions:
                 *      SYN-RECEIVED  -> ESTABLISHED
                 *      SYN-RECEIVED* -> FIN-WAIT-1
                 */
                tp->t_starttime = 0;
                if (tp->t_flags & TF_NEEDFIN) {
                        tp->t_state = TCPS_FIN_WAIT_1;
                        tp->t_flags &= ~TF_NEEDFIN;
                } else {
                        tp->t_state = TCPS_ESTABLISHED;
                        tp->t_timer[TCPT_KEEP] = TCP_KEEPIDLE(tp);
                }
                /*
                 * 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)
                        (void) tcp_reass(tp, (struct tcphdr *)0, &tlen,
                            (struct mbuf *)0);
                tp->snd_wl1 = th->th_seq - 1;

                /* FALLTHROUGH */

                /* soisconnected may lead to socket_unlock in case of upcalls,
                 * make sure this is done when everything is setup.
                 */
                soisconnected(so);

        /*
         * 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++;
                        goto dropafterack;
                }
                if (tp->sack_enable &&
                    (to.to_nsacks > 0 || !TAILQ_EMPTY(&tp->snd_holes)))
                        tcp_sack_doack(tp, &to, th->th_ack);
                if (SEQ_LEQ(th->th_ack, tp->snd_una)) {
                        if (tlen == 0 && tiwin == tp->snd_wnd) {
                                tcpstat.tcps_rcvdupack++;
                                /*
                                 * If we have outstanding data (other than
                                 * a window probe), this is a completely
                                 * duplicate ack (ie, window info didn't
                                 * change), the ack is the biggest we've
                                 * seen and we've seen exactly our rexmt
                                 * threshhold of them, 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)
                                        tp->t_dupacks = 0;
                                else if (++tp->t_dupacks > tcprexmtthresh ||
                                         ((tcp_do_newreno || tp->sack_enable) &&
                                          IN_FASTRECOVERY(tp))) {
                                        if (tp->sack_enable && 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 1/2 the original window's  
                                                 * 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;
                                        tp->t_unacksegs = 0;
                                        (void) tcp_output(tp);
                                        goto drop;
                                } else if (tp->t_dupacks == tcprexmtthresh) {
                                        tcp_seq onxt = tp->snd_nxt;
                                        u_int win;

                                        /*
                                         * 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 (tp->sack_enable) {
                                                if (IN_FASTRECOVERY(tp)) {
                                                        tp->t_dupacks = 0;
                                                        break;
                                                }
                                        } else if (tcp_do_newreno) {
                                                if (SEQ_LEQ(th->th_ack,
                                                    tp->snd_recover)) {
                                                        tp->t_dupacks = 0;
                                                        break;
                                                }
                                        }
                                        win = min(tp->snd_wnd, tp->snd_cwnd) /
                                            2 / tp->t_maxseg;
                                        if (win < 2)
                                                win = 2;
                                        tp->snd_ssthresh = win * tp->t_maxseg;
                                        ENTER_FASTRECOVERY(tp);
                                        tp->snd_recover = tp->snd_max;
                                        tp->t_timer[TCPT_REXMT] = 0;
                                        tp->t_rtttime = 0;
                                        tp->ecn_flags |= TE_SENDCWR;
                                        if (tp->sack_enable) {
                                                tcpstat.tcps_sack_recovery_episode++;
                                                tp->sack_newdata = tp->snd_nxt;
                                                tp->snd_cwnd = tp->t_maxseg;
                                                tp->t_unacksegs = 0;
                                                (void) tcp_output(tp);
                                                goto drop;
                                        }
                                        tp->snd_nxt = th->th_ack;
                                        tp->snd_cwnd = tp->t_maxseg;
                                        tp->t_unacksegs = 0;
                                        (void) tcp_output(tp);
                                        tp->snd_cwnd = tp->snd_ssthresh +
                                             tp->t_maxseg * tp->t_dupacks;
                                        if (SEQ_GT(onxt, tp->snd_nxt))
                                                tp->snd_nxt = onxt;
                                        goto drop;
                                }
                        } else
                                tp->t_dupacks = 0;
                        break;
                }
                /*
                 * If the congestion window was inflated to account
                 * for the other side's cached packets, retract it.
                 */
                if (!IN_FASTRECOVERY(tp)) {
                        /*
                         * We were not in fast recovery.  Reset the duplicate ack
                         * counter.
                         */
                        tp->t_dupacks = 0;
                }
                /*
                 * If the congestion window was inflated to account
                 * for the other side's cached packets, retract it.
                 */
                else {
                        if (tcp_do_newreno || tp->sack_enable) {
                                if (SEQ_LT(th->th_ack, tp->snd_recover)) {
                                        if (tp->sack_enable)
                                                tcp_sack_partialack(tp, th);
                                        else
                                                tcp_newreno_partial_ack(tp, th);                        
                                }
                                else {
                                        if (tcp_do_newreno) {
                                                int32_t ss = tp->snd_max - th->th_ack;
        
                                                /*
                                                 * Complete ack.  Inflate the congestion window to
                                                 * ssthresh and exit fast recovery.
                                                 *
                                                 * Window inflation should have left us with approx.
                                                 * snd_ssthresh outstanding data.  But in case we
                                                 * would be inclined to send a burst, better to do
                                                 * it via the slow start mechanism.
                                                 */
                                                if (ss < tp->snd_ssthresh)
                                                        tp->snd_cwnd = ss + tp->t_maxseg;
                                                else
                                                        tp->snd_cwnd = tp->snd_ssthresh;
                                        }
                                        else {
                                                /*
                                                 * Clamp the congestion window to the crossover point
                                                 * and exit fast recovery.
                                                 */
                                                if (tp->snd_cwnd > tp->snd_ssthresh)
                                                        tp->snd_cwnd = tp->snd_ssthresh;                                        
                                        }
        
                                        EXIT_FASTRECOVERY(tp);
                                        tp->t_dupacks = 0;
                                        tp->t_bytes_acked = 0;
                                }
                        }
                        else {
                                /*
                                 * Clamp the congestion window to the crossover point
                                 * and exit fast recovery in non-newreno and non-SACK case.
                                 */
                                if (tp->snd_cwnd > tp->snd_ssthresh)
                                        tp->snd_cwnd = tp->snd_ssthresh;                                        
                                EXIT_FASTRECOVERY(tp);
                                tp->t_dupacks = 0;
                                tp->t_bytes_acked = 0;
                        }
                }


                /*
                 * If we reach this point, ACK is not a duplicate,
                 *     i.e., it ACKs something we sent.
                 */
                if (tp->t_flags & TF_NEEDSYN) {
                        /*
                         * T/TCP: Connection was half-synchronized, and our
                         * SYN has been ACK'd (so connection is now fully
                         * synchronized).  Go to non-starred state,
                         * increment snd_una for ACK of SYN, and check if
                         * we can do window scaling.
                         */
                        tp->t_flags &= ~TF_NEEDSYN;
                        tp->snd_una++;
                        /* Do window scaling? */
                        if ((tp->t_flags & (TF_RCVD_SCALE|TF_REQ_SCALE)) ==
                                (TF_RCVD_SCALE|TF_REQ_SCALE)) {
                                tp->snd_scale = tp->requested_s_scale;
                                tp->rcv_scale = tp->request_r_scale;
                        }
                }

process_ACK:
                acked = th->th_ack - tp->snd_una;
                tcpstat.tcps_rcvackpack++;
                tcpstat.tcps_rcvackbyte += acked;

                /*
                 * If we just performed our first retransmit, and the ACK
                 * arrives within our recovery window, then it was a mistake
                 * to do the retransmit in the first place.  Recover our
                 * original cwnd and ssthresh, and proceed to transmit where
                 * we left off.
                 */
                if (tp->t_rxtshift == 1 && tcp_now < tp->t_badrxtwin) {
                        ++tcpstat.tcps_sndrexmitbad;
                        tp->snd_cwnd = tp->snd_cwnd_prev;
                        tp->snd_ssthresh = tp->snd_ssthresh_prev;
                        tp->snd_recover = tp->snd_recover_prev;
                        if (tp->t_flags & TF_WASFRECOVERY)
                                ENTER_FASTRECOVERY(tp);
                        tp->snd_nxt = tp->snd_max;
                        tp->t_badrxtwin = 0;    /* XXX probably not required */ 
                }

                /*
                 * If we have a timestamp reply, update smoothed
                 * round trip time.  If no timestamp is present but
                 * transmit timer is running and timed sequence
                 * number was acked, update smoothed round trip time.
                 * Since we now have an rtt measurement, cancel the
                 * timer backoff (cf., Phil Karn's retransmit alg.).
                 * Recompute the initial retransmit timer.
                 * Also makes sure we have a valid time stamp in hand
                 *
                 * Some boxes send broken timestamp replies
                 * during the SYN+ACK phase, ignore
                 * timestamps of 0 or we could calculate a
                 * huge RTT and blow up the retransmit timer.
                 */
                if (((to.to_flags & TOF_TS) != 0) && (to.to_tsecr != 0)) {
                        if (!tp->t_rttlow || tp->t_rttlow > tcp_now - to.to_tsecr)
                                tp->t_rttlow = tcp_now - to.to_tsecr;
                        tcp_xmit_timer(tp, tcp_now - to.to_tsecr);
                } else if (tp->t_rtttime && SEQ_GT(th->th_ack, tp->t_rtseq)) {
                        if (!tp->t_rttlow || tp->t_rttlow > tcp_now - tp->t_rtttime)
                                tp->t_rttlow = tcp_now - tp->t_rtttime;
                        tcp_xmit_timer(tp, tp->t_rtttime);
                }

                /*
                 * 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.
                 */
                if (th->th_ack == tp->snd_max) {
                        tp->t_timer[TCPT_REXMT] = 0;
                        needoutput = 1;
                } else if (tp->t_timer[TCPT_PERSIST] == 0)
                        tp->t_timer[TCPT_REXMT] = tp->t_rxtcur;

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

                /*
                 * When new data is acked, open the congestion window.
                 */
                if ((thflags & TH_ECE) != 0 &&
                        (tp->ecn_flags & TE_SETUPSENT) != 0) {
                        /*
                         * Reduce the congestion window if we haven't done so.
                         */
                        if (!(tp->sack_enable && IN_FASTRECOVERY(tp)) &&
                                !(tcp_do_newreno && SEQ_LEQ(th->th_ack, tp->snd_recover))) {
                                tcp_reduce_congestion_window(tp);
                        }
                } else if ((!tcp_do_newreno && !tp->sack_enable) ||
                    !IN_FASTRECOVERY(tp)) {
                        /*
                         * RFC 3465 - Appropriate Byte Counting.
                         *
                         * If the window is currently less than ssthresh,
                         * open the window by the number of bytes ACKed by
                         * the last ACK, however clamp the window increase
                         * to an upper limit "L".
                         *
                         * In congestion avoidance phase, open the window by
                         * one segment each time "bytes_acked" grows to be
                         * greater than or equal to the congestion window.
                         */

                        register u_int cw = tp->snd_cwnd;
                        register u_int incr = tp->t_maxseg;

                        if (tcp_do_rfc3465) {

                                if (cw >= tp->snd_ssthresh) {
                                        tp->t_bytes_acked += acked;
                                        if (tp->t_bytes_acked >= cw) {
                                        /* Time to increase the window. */
                                                tp->t_bytes_acked -= cw;
                                        } else {
                                        /* No need to increase yet. */
                                                incr = 0;
                                        }
                                } else {
                                        /*
                                         * If the user explicitly enables RFC3465
                                         * use 2*SMSS for the "L" param.  Otherwise
                                         * use the more conservative 1*SMSS.
                                         *
                                         * (See RFC 3465 2.3 Choosing the Limit)
                                         */
                                        u_int abc_lim;

                                        abc_lim = (tcp_do_rfc3465_lim2 &&
                                                tp->snd_nxt == tp->snd_max) ? incr * 2 : incr;

                                        incr = lmin(acked, abc_lim);
                                }
                        }
                        else {
                                /*
                                 * If the window gives us less than ssthresh packets
                                 * in flight, open exponentially (segsz per packet).
                                 * Otherwise open linearly: segsz per window
                                 * (segsz^2 / cwnd per packet).
                                 */
                
                                        if (cw >= tp->snd_ssthresh) {
                                                incr = max((incr * incr / cw), 1);
                                        }
                        }


                        tp->snd_cwnd = min(cw+incr, TCP_MAXWIN<<tp->snd_scale);
                }
                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);
                        tp->snd_wnd -= acked;
                        ourfinisacked = 0;
                }
                /* detect una wraparound */
                if ((tcp_do_newreno || tp->sack_enable) &&
                    !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 ((tcp_do_newreno || tp->sack_enable) &&
                    IN_FASTRECOVERY(tp) &&
                    SEQ_GEQ(th->th_ack, tp->snd_recover))
                        EXIT_FASTRECOVERY(tp);
                tp->snd_una = th->th_ack;
                if (tp->sack_enable) {
                        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;
                        
                /*
                 * sowwakeup must happen after snd_una, et al. are updated so that
                 * the sequence numbers are in sync with so_snd
                 */
                sowwakeup(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 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] = tcp_maxidle;
                                        add_to_time_wait(tp);
                                        soisdisconnected(so);
                                }
                                tp->t_state = TCPS_FIN_WAIT_2;
                                /* fall through and make sure we also recognize data ACKed with the FIN */
                        }
                        tp->t_flags |= TF_ACKNOW;
                        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) {
                                tp->t_state = TCPS_TIME_WAIT;
                                tcp_canceltimers(tp);
                                /* Shorten TIME_WAIT [RFC-1644, p.28] */
                                if (tp->cc_recv != 0 &&
                                    tp->t_starttime < (u_int32_t)tcp_msl)
                                        tp->t_timer[TCPT_2MSL] =
                                            tp->t_rxtcur * TCPTV_TWTRUNC;
                                else
                                        tp->t_timer[TCPT_2MSL] = 2 * tcp_msl;
                                add_to_time_wait(tp);
                                soisdisconnected(so);
                        }
                        tp->t_flags |= TF_ACKNOW;
                        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:
                        tp->t_timer[TCPT_2MSL] = 2 * tcp_msl;
                        add_to_time_wait(tp);
                        goto dropafterack;
                }
        }

step6:
        /*
         * Update window information.
         * Don't look at window if no ACK: TAC's send garbage on first SYN.
         */
        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;
                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;
                                postevent(so, 0, EV_OOB);
                        }
                        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:                                                 /* XXX */

        /*
         * 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 ((tlen || (thflags & TH_FIN)) &&
            TCPS_HAVERCVDFIN(tp->t_state) == 0) {
                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) &&
                    TCPS_HAVEESTABLISHED(tp->t_state)) {
                        if (DELAY_ACK(tp) && ((tp->t_flags & TF_ACKNOW) == 0)) {
                                tp->t_flags |= TF_DELACK;
                                tp->t_unacksegs++;
                        }         
                        else {
                                tp->t_unacksegs = 0;
                                tp->t_flags |= TF_ACKNOW;
                        }
                        tp->rcv_nxt += tlen;
                        thflags = th->th_flags & TH_FIN;
                        tcpstat.tcps_rcvpack++;
                        tcpstat.tcps_rcvbyte += tlen;
                        ND6_HINT(tp);
                        if (sbappendstream(&so->so_rcv, m))
                                sorwakeup(so);
                } else {
                        thflags = tcp_reass(tp, th, &tlen, m);
                        tp->t_flags |= TF_ACKNOW;
                        tp->t_unacksegs = 0;
                }

                if (tlen > 0 && tp->sack_enable)
                        tcp_update_sack_list(tp, save_start, save_end);

                if (tp->t_flags & TF_DELACK) 
                {
#if INET6
                        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
#endif
                        {
                                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); 
                        }
                                
                }
                /*
                 * Note the amount of data that peer has sent into
                 * our window, in order to estimate the sender's
                 * buffer size.
                 */
                len = (u_int)(so->so_rcv.sb_hiwat - (tp->rcv_adv - tp->rcv_nxt));
                if (len > so->so_rcv.sb_maxused)
                        so->so_rcv.sb_maxused = len;
        } 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);
                        postevent(so, 0, EV_FIN);
                        /*
                         *  If connection is half-synchronized
                         *  (ie NEEDSYN flag on) then delay ACK,
                         * 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.
                         */
                        if (DELAY_ACK(tp) && (tp->t_flags & TF_NEEDSYN)) {
                                tp->t_flags |= TF_DELACK;
                                tp->t_unacksegs++;
                        }
                        else {
                                tp->t_flags |= TF_ACKNOW;
                                tp->t_unacksegs = 0;
                        }
                        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 = 0;
                case TCPS_ESTABLISHED:
                        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:
                        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:
                        tp->t_state = TCPS_TIME_WAIT;
                        tcp_canceltimers(tp);
                        /* Shorten TIME_WAIT [RFC-1644, p.28] */
                        if (tp->cc_recv != 0 &&
                            tp->t_starttime < (u_int32_t)tcp_msl) {
                                tp->t_timer[TCPT_2MSL] =
                                    tp->t_rxtcur * TCPTV_TWTRUNC;
                                /* For transaction client, force ACK now. */
                                tp->t_flags |= TF_ACKNOW;
                                tp->t_unacksegs = 0;
                        }
                        else
                                tp->t_timer[TCPT_2MSL] = 2 * tcp_msl;

                        add_to_time_wait(tp);
                        soisdisconnected(so);
                        break;

                /*
                 * In TIME_WAIT state restart the 2 MSL time_wait timer.
                 */
                case TCPS_TIME_WAIT:
                        tp->t_timer[TCPT_2MSL] = 2 * tcp_msl;
                        add_to_time_wait(tp);
                        break;
                }
        }
#if TCPDEBUG
        if (so->so_options & SO_DEBUG)
                tcp_trace(TA_INPUT, ostate, tp, (void *)tcp_saveipgen,
                          &tcp_savetcp, 0);
#endif

        /*
         * Return any desired output.
         */
        if (needoutput || (tp->t_flags & TF_ACKNOW)) {
                tp->t_unacksegs = 0;
                (void) tcp_output(tp);
        }
        tcp_unlock(so, 1, 0);
        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)) ) {
                rstreason = BANDLIM_RST_OPENPORT;
                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;
        tp->t_unacksegs = 0;
        (void) tcp_output(tp);
        tcp_unlock(so, 1, 0);
        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 INET6
        if (isipv6) {
                if (IN6_IS_ADDR_MULTICAST(&ip6->ip6_dst) ||
                    IN6_IS_ADDR_MULTICAST(&ip6->ip6_src))
                        goto drop;
        } else
#endif /* INET6 */
        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() */

        /* 
         * Perform bandwidth limiting.
         */
#if ICMP_BANDLIM
        if (badport_bandlim(rstreason) < 0)
                goto drop;
#endif

#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
        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, ifscope);
        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, ifscope);
        }
        /* destroy temporarily created socket */
        if (dropsocket) {
                (void) soabort(so); 
                tcp_unlock(so, 1, 0);
        }
        else
                if ((inp != NULL) && (nosock == 0))
                        tcp_unlock(so, 1, 0);
        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); 
                tcp_unlock(so, 1, 0);
        }
        else
                if (nosock == 0)
                        tcp_unlock(so, 1, 0);
        KERNEL_DEBUG(DBG_FNC_TCP_INPUT | DBG_FUNC_END,0,0,0,0,0);
        return;
}

static void
tcp_dooptions(tp, cp, cnt, th, to, input_ifscope)
/*
 * Parse TCP options and place in tcpopt.
 */
        struct tcpcb *tp;
        u_char *cp;
        int cnt;
        struct tcphdr *th;
        struct tcpopt *to;
        unsigned int input_ifscope;
{
        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));

#if BYTE_ORDER != BIG_ENDIAN
                        NTOHS(mss);
#endif

                        break;

                case TCPOPT_WINDOW:
                        if (optlen != TCPOLEN_WINDOW)
                                continue;
                        if (!(th->th_flags & TH_SYN))
                                continue;
                        tp->t_flags |= TF_RCVD_SCALE;
                        tp->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));

#if BYTE_ORDER != BIG_ENDIAN
                        NTOHL(to->to_tsval);
#endif

                        bcopy((char *)cp + 6,
                            (char *)&to->to_tsecr, sizeof(to->to_tsecr));

#if BYTE_ORDER != BIG_ENDIAN
                        NTOHL(to->to_tsecr);
#endif

                        /*
                         * A timestamp received in a SYN makes
                         * it ok to send timestamp requests and replies.
                         */
                        if (th->th_flags & TH_SYN) {
                                tp->t_flags |= TF_RCVD_TSTMP;
                                tp->ts_recent = to->to_tsval;
                                tp->ts_recent_age = tcp_now;
                        }
                        break;
                case TCPOPT_SACK_PERMITTED:
                        if (!tcp_do_sack ||
                            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;
                }
        }
        if (th->th_flags & TH_SYN)
                tcp_mss(tp, mss, input_ifscope);        /* sets t_maxseg */
}

/*
 * 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.
 */
static void
tcp_pulloutofband(so, th, m, off)
        struct socket *so;
        struct tcphdr *th;
        register struct mbuf *m;
        int off;                /* delayed to be droped hdrlen */
{
        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");
}

/*
 * Collect new round-trip time estimate
 * and update averages and current timeout.
 */
static void
tcp_xmit_timer(tp, rtt)
        register struct tcpcb *tp;
        int rtt;
{
        register int delta;

        tcpstat.tcps_rttupdated++;
        tp->t_rttupdated++;
        if (tp->t_srtt != 0) {
                /*
                 * srtt is stored as fixed point with 5 bits after the
                 * binary point (i.e., scaled by 8).  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).  Adjust rtt to origin 0.
                 */
                delta = ((rtt - 1) << 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 > 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;
        }
        tp->t_rtttime = 0;
        tp->t_rxtshift = 0;

        /*
         * 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);

        /*
         * 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;
}

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

        RT_LOCK_ASSERT_HELD(rt);
        if (rt->rt_rmx.rmx_mtu == 0)
                maxmtu = rt->rt_ifp->if_mtu;
        else
                maxmtu = MIN(rt->rt_rmx.rmx_mtu, rt->rt_ifp->if_mtu);

        return (maxmtu);
}

#if INET6
static inline unsigned int
tcp_maxmtu6(struct rtentry *rt)
{
        unsigned int maxmtu;

        RT_LOCK_ASSERT_HELD(rt);
        lck_rw_lock_shared(nd_if_rwlock);
        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));
        lck_rw_done(nd_if_rwlock);

        return (maxmtu);
}
#endif

/*
 * 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 to be a single segment if the destination isn't local.
 * 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(tp, offer, input_ifscope)
        struct tcpcb *tp;
        int offer;
        unsigned int input_ifscope;
{
        register struct rtentry *rt;
        struct ifnet *ifp;
        register int rtt, mss;
        u_int32_t bufsize;
        struct inpcb *inp;
        struct socket *so;
        struct rmxp_tao *taop;
        int origoffer = offer;
        u_int32_t sb_max_corrected;
        int isnetlocal = 0;
#if INET6
        int isipv6;
        int min_protoh;
#endif

        inp = tp->t_inpcb;
#if INET6
        isipv6 = ((inp->inp_vflag & INP_IPV6) != 0) ? 1 : 0;
        min_protoh = isipv6 ? sizeof (struct ip6_hdr) + sizeof (struct tcphdr)
                            : sizeof (struct tcpiphdr);
#else
#define min_protoh  (sizeof (struct tcpiphdr))
#endif

#if INET6
        if (isipv6) {
                rt = tcp_rtlookup6(inp);
                if (rt != NULL &&
                    (IN6_IS_ADDR_LOOPBACK(&inp->in6p_faddr) ||
                    IN6_IS_ADDR_LINKLOCAL(&inp->in6p_faddr) ||
                    rt->rt_gateway->sa_family == AF_LINK))
                         isnetlocal = TRUE;
        }
        else
#endif /* INET6 */
        {
                rt = tcp_rtlookup(inp, input_ifscope);
                if (rt != NULL &&
                    (rt->rt_gateway->sa_family == AF_LINK ||
                    rt->rt_ifp->if_flags & IFF_LOOPBACK))
                         isnetlocal = TRUE;
        }
        if (rt == NULL) {
                tp->t_maxopd = tp->t_maxseg =
#if INET6
                isipv6 ? tcp_v6mssdflt :
#endif /* INET6 */
                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;
        }
        so = inp->inp_socket;

        taop = rmx_taop(rt->rt_rmx);
        /*
         * Offer == -1 means that we didn't receive SYN yet,
         * use cached value in that case;
         */
        if (offer == -1)
                offer = taop->tao_mssopt;
        /*
         * Offer == 0 means that there was no MSS on the SYN segment,
         * in this case we use tcp_mssdflt.
         */
        if (offer == 0)
                offer =
#if INET6
                        isipv6 ? tcp_v6mssdflt :
#endif /* INET6 */
                        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);
        }
        taop->tao_mssopt = 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)) {
                /*
                 * XXX the lock bit for RTT indicates that the value
                 * is also a minimum value; this is subject to time.
                 */
                if (rt->rt_rmx.rmx_locks & RTV_RTT)
                        tp->t_rttmin = rtt / (RTM_RTTUNIT / TCP_RETRANSHZ);
                else
                        tp->t_rttmin = isnetlocal ? tcp_TCPTV_MIN : TCP_RETRANSHZ;
                tp->t_srtt = rtt / (RTM_RTTUNIT / (TCP_RETRANSHZ * TCP_RTT_SCALE));
                tcpstat.tcps_usedrtt++;
                if (rt->rt_rmx.rmx_rttvar) {
                        tp->t_rttvar = rt->rt_rmx.rmx_rttvar /
                            (RTM_RTTUNIT / (TCP_RETRANSHZ * TCP_RTTVAR_SCALE));
                        tcpstat.tcps_usedrttvar++;
                } else {
                        /* default variation is +- 1 rtt */
                        tp->t_rttvar =
                            tp->t_srtt * TCP_RTTVAR_SCALE / TCP_RTT_SCALE;
                }
                TCPT_RANGESET(tp->t_rxtcur,
                              ((tp->t_srtt >> 2) + tp->t_rttvar) >> 1,
                              tp->t_rttmin, TCPTV_REXMTMAX);
        }
        else
                tp->t_rttmin = isnetlocal ? tcp_TCPTV_MIN : TCP_RETRANSHZ;

#if INET6
        mss = (isipv6 ? tcp_maxmtu6(rt) : tcp_maxmtu(rt));
#else
        mss = tcp_maxmtu(rt);
#endif
        mss -= min_protoh;

        if (rt->rt_rmx.rmx_mtu == 0) {
#if INET6
                if (isipv6) {
                        if (!isnetlocal)
                                mss = min(mss, tcp_v6mssdflt);
                } else
#endif /* INET6 */
                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;
        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;

#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 the slow-start flight size depending on whether this
         * is a local network or not.
         */
        if (isnetlocal)
                tp->snd_cwnd = mss * ss_fltsz_local;
        else 
                tp->snd_cwnd = mss * ss_fltsz;

        if (rt->rt_rmx.rmx_ssthresh) {
                /*
                 * There's some sort of gateway or interface
                 * buffer limit on the path.  Use this to set
                 * the slow start threshhold, 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;
        }

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

/*
 * Determine the MSS option to send on an outgoing SYN.
 */
int
tcp_mssopt(tp)
        struct tcpcb *tp;
{
        struct rtentry *rt;
        int mss;
#if INET6
        int isipv6;
        int min_protoh;
#endif

#if INET6
        isipv6 = ((tp->t_inpcb->inp_vflag & INP_IPV6) != 0) ? 1 : 0;
        min_protoh = isipv6 ? sizeof (struct ip6_hdr) + sizeof (struct tcphdr)
                            : sizeof (struct tcpiphdr);
#else
#define min_protoh  (sizeof (struct tcpiphdr))
#endif

#if INET6
        if (isipv6)
                rt = tcp_rtlookup6(tp->t_inpcb);
        else
#endif /* INET6 */
        rt = tcp_rtlookup(tp->t_inpcb, IFSCOPE_NONE);
        if (rt == NULL) {
                return (
#if INET6
                        isipv6 ? tcp_v6mssdflt :
#endif /* INET6 */
                        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;
        }

#if INET6
        mss = (isipv6 ? tcp_maxmtu6(rt) : tcp_maxmtu(rt));
#else
        mss = tcp_maxmtu(rt);
#endif
        /* Route locked during lookup above */
        RT_UNLOCK(rt);
        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 ti_ack, this forces retransmission timer to
 * be started again.
 */
static void
tcp_newreno_partial_ack(tp, th)
        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_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 + (th->th_ack - tp->snd_una);
                tp->t_flags |= TF_ACKNOW;
                tp->t_unacksegs = 0;
                (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 > th->th_ack - tp->snd_una)
                        tp->snd_cwnd -= th->th_ack - tp->snd_una;
                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 int rnd;
        static struct timeval old_runtime;
        static unsigned int cur_cnt, old_cnt;
        struct timeval tv;
        struct inpcb *inp = NULL;
        struct tcpcb *tp;

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

        so = TAILQ_FIRST(&head->so_incomp);
        if (!so)
                return 0;

        microtime(&tv);
        if ((i = (tv.tv_sec - old_runtime.tv_sec)) != 0) {
                old_runtime = tv;
                old_cnt = cur_cnt / i;
                cur_cnt = 0;
        }
        
        
        qlen = head->so_incqlen;
        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 (lck_mtx_try_lock(inp->inpcb_mtx)) {
                                so->so_usecount++;
                                if ((so->so_usecount == 2) && so->so_state & SS_INCOMP) 
                                        break;
                                else {/* don't use if beeing accepted or used in any other way */
                                        in_pcb_checkstate(inp, WNT_RELEASE, 1);
                                        tcp_unlock(so, 1, 0);
                                }
                        }
                        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)
                return 0;

        TAILQ_REMOVE(&head->so_incomp, so, so_list);
        tcp_unlock(head, 0, 0);

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

        if (in_pcb_checkstate(inp, WNT_RELEASE, 1) == WNT_STOPUSING) {
                tcp_unlock(so, 1, 0);
                tcp_lock(head, 0, 0);
                return 0;
        }

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

        so->so_head = NULL;

        /* 
         * We do not want to lose track of the PCB right away in case we receive 
         * more segments from the peer
         */
        tp = sototcpcb(so);
        so->so_flags |= SOF_OVERFLOW;
        tp->t_state = TCPS_TIME_WAIT;
        (void) tcp_close(tp);
        tp->t_unacksegs = 0;
        tcpstat.tcps_drops++;
        tcp_canceltimers(tp);
        add_to_time_wait(tp);
        
        tcp_unlock(so, 1, 0);
        tcp_lock(head, 0, 0);
        head->so_incqlen--;
        head->so_qlen--;
        return 1;
}

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

        int error;

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

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

        return (error);

}

SYSCTL_PROC(_net_inet_tcp, TCPCTL_STATS, stats, CTLFLAG_RD, 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,
        &tcprexmtthresh, 0, &sysctl_rexmtthresh, "I", "Duplicate ACK Threshold for Fast Retransmit");