xnu-124.13.tar.gz

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

/*
 * Copyright (c) 2000 Apple Computer, Inc. All rights reserved.
 *
 * @APPLE_LICENSE_HEADER_START@
 * 
 * The contents of this file constitute Original Code as defined in and
 * are subject to the Apple Public Source License Version 1.1 (the
 * "License").  You may not use this file except in compliance with the
 * License.  Please obtain a copy of the License at
 * http://www.apple.com/publicsource and read it before using this file.
 * 
 * This 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 OR NON-INFRINGEMENT.  Please see the
 * License for the specific language governing rights and limitations
 * under the License.
 * 
 * @APPLE_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
 */

#if ISFB31
#include "opt_ipfw.h"           /* for ipfw_fwd         */
#include "opt_tcpdebug.h"
#endif

#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 <net/if.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/ip_var.h>
#include <netinet/icmp_var.h>   /* for ICMP_BANDLIM             */
#include <netinet/in_pcb.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>
#include <netinet/tcpip.h>
#if TCPDEBUG
#include <netinet/tcp_debug.h>
#if INET6
union {
        struct ip _tcp_si4;
        struct ip6_hdr _tcp_si6;
} tcp_saveip;
#else
struct ip tcp_saveip;
#endif /* INET6 */
struct tcphdr tcp_savetcp;
#endif /* TCPDEBUG */

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

#include <sys/kdebug.h>

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

static int      tcprexmtthresh = 3;
tcp_seq tcp_iss;
tcp_cc  tcp_ccgen;

struct  tcpstat tcpstat;
SYSCTL_STRUCT(_net_inet_tcp, TCPCTL_STATS, stats,
        CTLFLAG_RD, &tcpstat , tcpstat, "");

int log_in_vain = 0;
SYSCTL_INT(_net_inet_tcp, OID_AUTO, log_in_vain, CTLFLAG_RW, 
        &log_in_vain, 0, "");

int tcp_delack_enabled = 1;
SYSCTL_INT(_net_inet_tcp, OID_AUTO, delayed_ack, CTLFLAG_RW, 
        &tcp_delack_enabled, 0, "");

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

static void      tcp_dooptions __P((struct tcpcb *,
            u_char *, int, struct tcphdr *, struct tcpopt *));
static void      tcp_pulloutofband __P((struct socket *,
            struct tcphdr *, struct mbuf *));
static void      tcp_xmit_timer __P((struct tcpcb *, int));

/*
 * Neighbor Discovery, Neighbor Unreachability Detection
 * Upper layer hint.
 */
#define ND6_HINT(tp) { \
        if ((tp) && (tp)->t_inpcb && (tp)->t_inpcb->in6p_route.ro_rt) \
                nd6_nud_hint((tp)->t_inpcb->in6p_route.ro_rt, NULL); \
}


extern u_long           current_active_connections;
extern u_long           last_active_conn_count;

extern u_long           *delack_bitmask;




/*
 * Insert segment ti into reassembly queue of tcp with
 * control block tp.  Return TH_FIN if reassembly now includes
 * a segment with FIN.  The macro form does 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 INET6
#define _ONLY_IF_INET6_(x)      x
#else
#define _ONLY_IF_INET6_(x)
#endif
#define TCP_REASS(tp, th, tilen, m, so, flags, isipv6, needwakeup) { \
        if ((th)->th_seq == (tp)->rcv_nxt && \
            (tp)->segq.lh_first == NULL && \
            (tp)->t_state == TCPS_ESTABLISHED) { \
                if (tcp_delack_enabled) {\
                    if (last_active_conn_count > DELACK_BITMASK_THRESH) \
                       TCP_DELACK_BITSET(tp->t_inpcb->hash_element); \
                    tp->t_flags |= TF_DELACK; \
                    } \
                else \
                        tp->t_flags |= TF_ACKNOW; \
                (tp)->rcv_nxt += (tilen); \
                flags = (th)->th_flags & TH_FIN; \
                tcpstat.tcps_rcvpack++;\
                tcpstat.tcps_rcvbyte += (tilen);\
                _ONLY_IF_INET6_(ND6_HINT(tp);) \
                sbappend(&(so)->so_rcv, (m)); \
                needwakeup++; \
        } else { \
                (flags) = tcp_reass((tp), (th), (tilen), (m), (isipv6)); \
                tp->t_flags |= TF_ACKNOW; \
        } \
}

/*
 * Note:
 * in the ip header part of the ipqe_tcp structure only the length is used.
 */
int
tcp_reass(tp, th, tilen, m, isipv6)
        register struct tcpcb *tp;
        register struct tcphdr *th;
        u_int16_t tilen;
        struct mbuf *m;
#if INET6
        int isipv6;
#endif
{
        register struct ipqent *p, *q, *nq, *tiqe;
        struct socket *so = tp->t_inpcb->inp_socket;
        int flags;

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

#if 0 /* Not using GETTCP(m) macro */
        m->m_pkthdr.header = ti;
#endif

        /*
         * Allocate a new queue entry, before we throw away any data.
         * If we can't, just drop the packet.  XXX
         */
        MALLOC(tiqe, struct ipqent *, sizeof (struct ipqent), M_SONAME, M_NOWAIT);
        if (tiqe == NULL) {
                tcpstat.tcps_rcvmemdrop++;
                m_freem(m);
                return (0);
        }

        /*
         * Find a segment which begins after this one does.
         */
        for (p = NULL, q = tp->segq.lh_first; q != NULL;
             p = q, q = q->ipqe_q.le_next)
                if (SEQ_GT(q->ipqe_tcp->ti_seq, th->th_seq))
                        break;

        /*
         * 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 struct tcpiphdr *phdr = p->ipqe_tcp;
                register int i;

                /* conversion to int (in i) handles seq wraparound */
                i = phdr->ti_seq + phdr->ti_len - th->th_seq;
                if (i > 0) {
                        if (i >= tilen) {
                                tcpstat.tcps_rcvduppack++;
                                tcpstat.tcps_rcvdupbyte += tilen;
                                m_freem(m);
                                FREE(tiqe, M_SONAME);

#if 1 /* XXX: NetBSD just return 0 here */
                                /*
                                 * Try to present any queued data
                                 * at the left window edge to the user.
                                 * This is needed after the 3-WHS
                                 * completes.
                                 */
                                goto present;   /* ??? */
#endif
                        }
                        m_adj(m, i);
                        tilen -= i;
                        th->th_seq += i;
                }
        }
        tcpstat.tcps_rcvoopack++;
        tcpstat.tcps_rcvoobyte += tilen;

        /*
         * While we overlap succeeding segments trim them or,
         * if they are completely covered, dequeue them.
         */
        while (q) {
                register struct tcpiphdr *qhdr = q->ipqe_tcp;
                register int i = (th->th_seq + tilen) - qhdr->ti_seq;

                if (i <= 0)
                        break;
                if (i < qhdr->ti_len) {
                        qhdr->ti_seq += i;
                        qhdr->ti_len -= i;
                        m_adj(q->ipqe_m, i);
                        break;
                }
                nq = q->ipqe_q.le_next;
                m_freem(q->ipqe_m);
                LIST_REMOVE(q, ipqe_q);
                FREE(q, M_SONAME);
                q = nq;
        }

        /* Insert the new fragment queue entry into place. */
        tiqe->ipqe_m = m;
        /*
         * There is a IP or IPv6 header in the mbuf before th
         * so there is space for an ip header (for the length field)
         */
#define thtoti(x) \
        ((struct tcpiphdr *)(((char *)(x)) - (sizeof (struct ip))))

        tiqe->ipqe_tcp = thtoti(th);
        tiqe->ipqe_tcp->ti_len = tilen;
        if (p == NULL) {
                LIST_INSERT_HEAD(&tp->segq, tiqe, ipqe_q);
        } else {
                LIST_INSERT_AFTER(p, tiqe, ipqe_q);
        }

present:
        /*
         * Present data to user, advancing rcv_nxt through
         * completed sequence space.
         */
        if (!TCPS_HAVEESTABLISHED(tp->t_state))
                return (0);
        q = tp->segq.lh_first;
        if (!q || q->ipqe_tcp->ti_seq != tp->rcv_nxt)
                return (0);
#if 0
      /*
       * XXX from INRIA for NetBSD, but should not happen because
       * TCPS_HAVEESTABLISHED(tp->t_state) should be true here.
       */
        if (tp->t_state == TCPS_SYN_RECEIVED && q->ipqe_tcp->ti_len)
                return (0);
#endif
        do {
                tp->rcv_nxt += q->ipqe_tcp->ti_len;
                flags = q->ipqe_tcp->ti_flags & TH_FIN;
                nq = q->ipqe_q.le_next;
                LIST_REMOVE(q, ipqe_q);
                if (so->so_state & SS_CANTRCVMORE)
                        m_freem(q->ipqe_m);
                else
                        sbappend(&so->so_rcv, q->ipqe_m);
                FREE(q, M_SONAME);
                q = nq;
        } while (q && q->ipqe_tcp->ti_seq == tp->rcv_nxt);
#if INET6
        if (isipv6)
                ND6_HINT(tp);
#endif

        KERNEL_DEBUG(DBG_LAYER_END, ((ti->ti_dport << 16) | ti->ti_sport),
                     (((ti->ti_src.s_addr & 0xffff) << 16) | (ti->ti_dst.s_addr & 0xffff)),
                     ti->ti_seq, ti->ti_ack, ti->ti_win);

        sorwakeup(so);
        return (flags);
}

/*
 * TCP input routine, follows pages 65-76 of the
 * protocol specification dated September, 1981 very closely.
 */
#if INET6
int
tcp6_input(mp, offp, proto)
        struct mbuf **mp;
        int *offp, proto;
{
        tcp_input(*mp, *offp);
        return IPPROTO_DONE;
}
#endif

void
tcp_input(m, off)
        struct mbuf *m;
        int off;
{
        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, toff;
        int hdroptlen;
        u_int16_t tilen;
        register struct tcpcb *tp = 0;
        register int thflags;
        struct socket *so = 0;
        int todrop, acked, ourfinisacked, needoutput = 0;
        struct in_addr laddr;
#if 0
        struct in6_addr laddr6;
#endif
        int dropsocket = 0;
        int iss = 0;
        u_long tiwin;
        struct tcpopt to;               /* options in this segment */
        struct rmxp_tao *taop;          /* pointer to our TAO cache entry */
        struct rmxp_tao tao_noncached;  /* in case there's no cached entry */
        int need_sowwakeup = 0;
        int need_sorwakeup = 0;
#if TCPDEBUG
        short ostate = 0;
#endif
#if INET6
        struct ip6_hdr *ip6 = NULL;
        int lgminh;
#else /* INET6 */
#define lgminh  (sizeof (struct tcpiphdr))
#endif /* INET6 */
        int isipv6 = (mtod(m, struct ip *)->ip_v == 6) ? 1 : 0;

        struct proc *proc0=current_proc();

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

        bzero((char *)&to, sizeof(to));

        tcpstat.tcps_rcvtotal++;
        /*
         * Get IP and TCP header together in first mbuf.
         * Note: IP leaves IP header in first mbuf.
         */
        th = mtod(m, struct tcpiphdr *);

        KERNEL_DEBUG(DBG_LAYER_BEG, ((ti->ti_dport << 16) | ti->ti_sport),
                     (((ti->ti_src.s_addr & 0xffff) << 16) | (ti->ti_dst.s_addr & 0xffff)),
                     ti->ti_seq, ti->ti_ack, ti->ti_win);

#if INET6
        if (isipv6) {
                ip6 = mtod(m, struct ip6_hdr *);
                lgminh = sizeof(struct tcpip6hdr);
        } else {
                lgminh = sizeof(struct tcpiphdr);
#endif /* INET6 */
                ip = mtod(m, struct ip *);
                ipov = (struct ipovly *)ip;
#if INET6
        }
#endif /* INET6 */

#if INET6
        /* XXX not a good place to put this into... */
        if (isipv6 &&
            m && (m->m_flags & M_ANYCAST6)) {
                icmp6_error(m, ICMP6_DST_UNREACH, ICMP6_DST_UNREACH_ADDR,
                        (caddr_t)&ip6->ip6_dst - (caddr_t)ip6);
                return;
        }
#endif /* INET6 */

#if INET6
        if (isipv6) {
                IP6_EXTHDR_CHECK(m, off, sizeof(struct tcphdr), );
                ip6 = mtod(m, struct ip6_hdr *);
                tilen = ntohs(ip6->ip6_plen) - off + sizeof(*ip6);

                if (in6_cksum(m, IPPROTO_TCP, off, tilen)) {
                        tcpstat.tcps_rcvbadsum++;
                        goto drop;
                }
                th = (struct tcphdr *)((caddr_t)ip6 + off);
        } else
#endif /* INET6 */
        {
                /*
                 * Get IP and TCP header together in first mbuf.
                 * Note: IP leaves IP header in first mbuf.
                 */
                /* XXX: should we still require this for IPv4? */
                if (off > sizeof (struct ip)) {
                        ip_stripoptions(m, (struct mbuf *)0);
                        off = sizeof(struct ip);
                }
                if (m->m_len < lgminh) {
                        if ((m = m_pullup(m, lgminh)) == 0) {
                                tcpstat.tcps_rcvshort++;
                                return;
                        }
                }
                ip = mtod(m, struct ip *);
                ipov = (struct ipovly *)ip;

                /*
                 * Checksum extended TCP header and data.
                 */
                tilen = ip->ip_len;
                len = sizeof (struct ip) + tilen;
                bzero(ipov->ih_x1, sizeof(ipov->ih_x1));
                ipov->ih_len = (u_short)tilen;
                HTONS(ipov->ih_len);
                th = (struct tcphdr *)((caddr_t)ip + off);
                th->th_sum = in_cksum(m, len);
                if (th->th_sum) {
                        tcpstat.tcps_rcvbadsum++;
                        goto drop;
                }
        }

        /*
         * Check that TCP offset makes sense,
         * pull out TCP options and adjust length.              XXX
         */
        toff = th->th_off << 2;
        if (toff < sizeof (struct tcphdr) || toff > tilen) {
                tcpstat.tcps_rcvbadoff++;
                goto drop;
        }
        tilen -= toff;
        if (toff > sizeof (struct tcphdr)) {
#if INET6
                if (isipv6) {
                        IP6_EXTHDR_CHECK(m, off, toff, );
                        ip6 = mtod(m, struct ip6_hdr *);
                        th = (struct tcphdr *)((caddr_t)ip6 + off);
                } else
#endif /* INET6 */
                {
                        if (m->m_len < sizeof(struct ip) + toff) {
                                if ((m = m_pullup(m, sizeof (struct ip) + toff)) == 0) {
                                        tcpstat.tcps_rcvshort++;
                                        return;
                                }
                                ip = mtod(m, struct ip *);
                                ipov = (struct ipovly *)ip;
                                th = (struct tcphdr *)((caddr_t)ip + off);
                        }
                }
                optlen = toff - 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_flag |= 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;

        /*
         * Convert TCP protocol specific fields to host format.
         */
        NTOHL(th->th_seq);
        NTOHL(th->th_ack);
        NTOHS(th->th_win);
        NTOHS(th->th_urp);

        /*
         * Drop TCP, IP headers and TCP options.
         */
        hdroptlen = off+toff;
        m->m_data += hdroptlen;
        m->m_len  -= hdroptlen;

        /*
         * Locate pcb for segment.
         */
findpcb:
#if IPFIREWALL_FORWARD
        if (ip_fw_fwd_addr != NULL
#if INET6
            && isipv6 == NULL
#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 (!ip_fw_fwd_addr->sin_port) {
                                inp = in_pcblookup_hash(&tcbinfo, ip->ip_src,
                                    th->th_sport, ip_fw_fwd_addr->sin_addr,
                                    th->th_dport, 1, m->m_pkthdr.rcvif);
                        } else {
                                inp = in_pcblookup_hash(&tcbinfo,
                                    ip->ip_src, th->th_sport,
                                    ip_fw_fwd_addr->sin_addr,
                                    ntohs(ip_fw_fwd_addr->sin_port), 1,
                                    m->m_pkthdr.rcvif);
                        }
                }
                ip_fw_fwd_addr = NULL;
        } 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);

#if IPSEC
        /* due to difference from other BSD stacks */
        m->m_data -= hdroptlen;
        m->m_len  += hdroptlen;
#if INET6
        if (isipv6) {
                if (inp != NULL && ipsec6_in_reject_so(m, inp->inp_socket)) {
                        ipsec6stat.in_polvio++;
                        goto drop;
                }
        } else
#endif /* INET6 */
        if (inp != NULL && ipsec4_in_reject_so(m, inp->inp_socket)) {
                ipsecstat.in_polvio++;
                goto drop;
        }
        m->m_data += hdroptlen;
        m->m_len  -= hdroptlen;
#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 && thflags & TH_SYN) {
#if INET6
                        char buf[INET6_ADDRSTRLEN];
#else /* INET6 */
                        char buf[4*sizeof "123"];
#endif /* INET6 */

#if INET6
                        if (isipv6) {
                                strcpy(buf, ip6_sprintf(&ip6->ip6_dst));
                                log(LOG_INFO,
                                    "Connection attempt to TCP %s:%d from %s:%d\n",
                                    buf, ntohs(th->th_dport),
                                    ip6_sprintf(&ip6->ip6_src),
                                    ntohs(th->th_sport));
                        } else {
#endif
                        strcpy(buf, inet_ntoa(ip->ip_dst));
                        log(LOG_INFO,
                            "Connection attempt to TCP %s:%d from %s:%d\n",
                            buf, ntohs(th->th_dport), inet_ntoa(ip->ip_src),
                            ntohs(th->th_sport));
#if INET6
                        }
#endif /* INET6 */
                }
#if ICMP_BANDLIM
                if (badport_bandlim(1) < 0)
                        goto drop;
#endif
                goto dropwithreset;
        }
        tp = intotcpcb(inp);
        if (tp == 0)
                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;

        so = inp->inp_socket;
        if (so->so_options & (SO_DEBUG|SO_ACCEPTCONN)) {
#if TCPDEBUG
                if (so->so_options & SO_DEBUG) {
                        ostate = tp->t_state;
#if INET6
                        if (isipv6)
                                tcp_saveip._tcp_si6 = *ip6;
                        else
                                tcp_saveip._tcp_si4 = *ip;
#else /* INET6 */
                        tcp_saveip = *ip;
#endif /* INET6 */

                        tcp_savetcp = *th;
                }
#endif
                if (so->so_options & SO_ACCEPTCONN) {
                        register struct tcpcb *tp0 = tp;
                        struct socket *so2;
#if IPSEC
                        struct socket *oso;
#endif
#if INET6
                        struct inpcb *oinp = sotoinpcb(so);
#endif /* INET6 */

#if !IPSEC
                        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++;
                                        goto dropwithreset;
                                }
                                goto drop;
                        }
#endif
                        KERNEL_DEBUG(DBG_FNC_TCP_NEWCONN | DBG_FUNC_START,0,0,0,0,0);
                        so2 = sonewconn(so, 0);


                        if (so2 == 0) {
                                tcpstat.tcps_listendrop++;
                                so2 = sodropablereq(so);
                                if (so2) {
                                        tcp_drop(sototcpcb(so2), ETIMEDOUT);
                                        so2 = sonewconn(so, 0);
                                }
                                if (!so2)
                                        goto drop;
                        }
#if IPSEC
                        oso = so;
#endif
                        so = so2;
                        /*
                         * 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++;
                        inp = (struct inpcb *)so->so_pcb;
#if INET6
                        if (isipv6)
                                inp->in6p_laddr = ip6->ip6_dst;
                        else {
                                if (ip6_mapped_addr_on) {
                                        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) {
                                /*
                                 * 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;
                                goto drop;
                        }
#if IPSEC
                        /*
                         * from IPsec perspective, it is important to do it
                         * after making actual listening socket.
                         * otherwise, cached security association will bark.
                         *
                         * Subject: (KAME-snap 748)
                         * From: Wayne Knowles <w.knowles@niwa.cri.nz>
                         */
                        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++;
                                        goto dropwithreset;
                                }
                                goto drop;
                        }
#endif
#if INET6
                        if (isipv6) {
                                struct ip6_recvpktopts newopts;

                                /*
                                 * 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();
#if IPSEC
                        /* copy old policy into new socket's */
                        if (ipsec_copy_policy(sotoinpcb(oso)->inp_sp,
                                              inp->inp_sp))
                                printf("tcp_input: could not copy policy\n");
#endif

                        tp = intotcpcb(inp);
                        tp->t_state = TCPS_LISTEN;
                        tp->t_flags |= tp0->t_flags & (TF_NOPUSH|TF_NOOPT);

                        /* Compute proper scaling value from buffer space */
                        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 INET6
        /* save packet options if user wanted */
        if (isipv6 && (inp->in6p_flags & INP_CONTROLOPTS) != 0) {
                struct ip6_recvpktopts opts6;

                /*
                 * Temporarily re-adjusting the mbuf before ip6_savecontrol(),
                 * which is necessary for FreeBSD only due to difference from
                 * other BSD stacks.
                 * XXX: we'll soon make a more natural fix after getting a
                 *      consensus.
                 */
#ifndef DEFER_MADJ
                m->m_data -= hdroptlen;
                m->m_len  += hdroptlen;
#endif
                ip6_savecontrol(inp, ip6, m, &opts6, &inp->in6p_inputopts);
                if (inp->in6p_inputopts)
                        ip6_update_recvpcbopt(inp->in6p_inputopts, &opts6);
                if (opts6.head) {
                        if (sbappendcontrol(&inp->in6p_socket->so_rcv,
                                            NULL, opts6.head)
                            == 0)
                                m_freem(opts6.head);
                }
#ifndef DEFER_MADJ
                m->m_data += hdroptlen; /* XXX */
                m->m_len  -= hdroptlen; /* XXX */
#endif
        }
#endif /* INET6 */

        /*
         * Segment received on connection.
         * Reset idle time and keep-alive timer.
         */
        tp->t_idle = 0;
        if (TCPS_HAVEESTABLISHED(tp->t_state))
                tp->t_timer[TCPT_KEEP] = tcp_keepidle;

        /*
         * Process options if not in LISTEN state,
         * else do it below (after getting remote address).
         */
        if (tp->t_state != TCPS_LISTEN && optp)
                tcp_dooptions(tp, optp, optlen, th, &to);
        if (th->th_flags & TH_SYN)
                tcp_mss(tp, to.to_maxseg, isipv6);      /* sets t_maxseg */

        /*
         * 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_ACK &&
            ((tp->t_flags & (TF_NEEDSYN|TF_NEEDFIN)) == 0) &&
            ((to.to_flag & TOF_TS) == 0 ||
             TSTMP_GEQ(to.to_tsval, tp->ts_recent)) &&
            /*
             * Using the CC option is compulsory if once started:
             *   the segment is OK if no T/TCP was negotiated or
             *   if the segment has a CC option equal to CCrecv
             */
            ((tp->t_flags & (TF_REQ_CC|TF_RCVD_CC)) != (TF_REQ_CC|TF_RCVD_CC) ||
             ((to.to_flag & TOF_CC) != 0 && to.to_cc == tp->cc_recv)) &&
            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_flag & TOF_TS) != 0 &&
                   SEQ_LEQ(th->th_seq, tp->last_ack_sent)) {
                        tp->ts_recent_age = tcp_now;
                        tp->ts_recent = to.to_tsval;
                }

                if (tilen == 0) {
                        if (SEQ_GT(th->th_ack, tp->snd_una) &&
                            SEQ_LEQ(th->th_ack, tp->snd_max) &&
                            tp->snd_cwnd >= tp->snd_wnd &&
                            tp->t_dupacks < tcprexmtthresh) {
                                /*
                                 * this is a pure ack for outstanding data.
                                 */
                                ++tcpstat.tcps_predack;
                                if ((to.to_flag & TOF_TS) != 0)
                                        tcp_xmit_timer(tp,
                                            tcp_now - to.to_tsecr + 1);
                                else if (tp->t_rtt &&
                                            SEQ_GT(th->th_ack, tp->t_rtseq))
                                        tcp_xmit_timer(tp, tp->t_rtt);
                                acked = th->th_ack - tp->snd_una;
                                tcpstat.tcps_rcvackpack++;
                                tcpstat.tcps_rcvackbyte += acked;
                                sbdrop(&so->so_snd, acked);
                                tp->snd_una = th->th_ack;
                                m_freem(m);
#if INET6
                                /* some progress has been done */
                                if (isipv6)
                                        ND6_HINT(tp);
#endif

                                /*
                                 * 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;

                                if (so->so_snd.sb_cc)
                                        (void) tcp_output(tp);
                                sowwakeup(so);
                                KERNEL_DEBUG(DBG_FNC_TCP_INPUT | DBG_FUNC_END,0,0,0,0,0);
                                return;
                        }
                } else if (th->th_ack == tp->snd_una &&
                    tp->segq.lh_first == NULL &&
                    tilen <= sbspace(&so->so_rcv)) {
                        /*
                         * this is a pure, in-sequence data packet
                         * with nothing on the reassembly queue and
                         * we have enough buffer space to take it.
                         */
                        ++tcpstat.tcps_preddat;
                        tp->rcv_nxt += tilen;
                        tcpstat.tcps_rcvpack++;
                        tcpstat.tcps_rcvbyte += tilen;
#if INET6
                        /* some progress has been done */
                        if (isipv6)
                                ND6_HINT(tp);
#endif
                        sbappend(&so->so_rcv, m);
                        KERNEL_DEBUG(DBG_LAYER_END, ((th->th_dport << 16) | th->th_sport),
                             (((th->th_src.s_addr & 0xffff) << 16) | (th->th_dst.s_addr & 0xffff)),
                             th->th_seq, th->th_ack, th->th_win); 
                        if (tcp_delack_enabled) {
                            if (last_active_conn_count > DELACK_BITMASK_THRESH)
                                TCP_DELACK_BITSET(tp->t_inpcb->hash_element); 
                            tp->t_flags |= TF_DELACK;
                        } else {
                                tp->t_flags |= TF_ACKNOW;
                                tcp_output(tp);
                        }
                        sorwakeup(so);
                        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.
         */
        { int win;

        win = sbspace(&so->so_rcv);
        if (win < 0)
                win = 0;
        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 0
                register struct sockaddr_in6 *sin6;
#endif

                if (thflags & TH_RST)
                        goto drop;
                if (thflags & TH_ACK)
                        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;
                }

#if INET6
                if (isipv6) {
                        if (m->m_flags & (M_BCAST|M_MCAST) ||
                            IN6_IS_ADDR_MULTICAST(&ip6->ip6_dst))
                                goto drop;
#if 1
                        /*
                         * Perhaps this should be a call/macro
                         * to a function like in6_pcbconnect(), but almost
                         * all of the checks have been done: we know
                         * that the association is unique, and the
                         * local address is always set here.
                         */
                        if (IN6_IS_ADDR_UNSPECIFIED(&inp->in6p_laddr))
                                inp->in6p_laddr = ip6->ip6_dst;
                        inp->in6p_faddr = ip6->ip6_src;
                        inp->inp_fport = th->th_sport;

                        /* TODO: flowinfo initialization */

                        in_pcbrehash(inp);
#else
                        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);
#endif
                }
                else {
#endif /* INET6 */
                        /*
                         * 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.
                         */
                        if (m->m_flags & (M_BCAST|M_MCAST) ||
                            IN_MULTICAST(ntohl(ip->ip_dst.s_addr)))
                                goto drop;
                        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);
#if INET6
                }
#endif /* INET6 */

                tp->t_template = tcp_template(tp);
                if (tp->t_template == 0) {
                        tp = tcp_drop(tp, ENOBUFS);
                        dropsocket = 0;         /* socket is already gone */
                        goto drop;
                }
                if ((taop = tcp_gettaocache(inp)) == NULL) {
                        taop = &tao_noncached;
                        bzero(taop, sizeof(*taop));
                }
                tcp_dooptions(tp, optp, optlen, th, &to);
                if (th->th_flags & TH_SYN)
                        tcp_mss(tp, to.to_maxseg, isipv6);      /* sets t_maxseg */
                if (iss)
                        tp->iss = iss;
                else
                        tp->iss = tcp_iss;
                tcp_iss += TCP_ISSINCR/4;
                tp->irs = th->th_seq;
                tcp_sendseqinit(tp);
                tcp_rcvseqinit(tp);
                /*
                 * Initialization of the tcpcb for transaction;
                 *   set SND.WND = SEG.WND,
                 *   initialize CCsend and CCrecv.
                 */
                tp->snd_wnd = tiwin;    /* initial send-window */
                tp->cc_send = CC_INC(tcp_ccgen);
                tp->cc_recv = to.to_cc;
                /*
                 * Perform TAO test on incoming CC (SEG.CC) option, if any.
                 * - compare SEG.CC against cached CC from the same host,
                 *      if any.
                 * - if SEG.CC > chached value, SYN must be new and is accepted
                 *      immediately: save new CC in the cache, mark the socket
                 *      connected, enter ESTABLISHED state, turn on flag to
                 *      send a SYN in the next segment.
                 *      A virtual advertised window is set in rcv_adv to
                 *      initialize SWS prevention.  Then enter normal segment
                 *      processing: drop SYN, process data and FIN.
                 * - otherwise do a normal 3-way handshake.
                 */
                if ((to.to_flag & TOF_CC) != 0) {
                    if (((tp->t_flags & TF_NOPUSH) != 0) &&
                        taop->tao_cc != 0 && CC_GT(to.to_cc, taop->tao_cc)) {

                        taop->tao_cc = to.to_cc;
                        if (tp->t_state != TCPS_ESTABLISHED)
                                current_active_connections++;

                        tp->t_state = TCPS_ESTABLISHED;

                        /*
                         * If there is a FIN, or if there is data and the
                         * connection is local, then delay SYN,ACK(SYN) in
                         * the hope of piggy-backing it on a response
                         * segment.  Otherwise must send ACK now in case
                         * the other side is slow starting.
                         */
                        if (tcp_delack_enabled &&
                            ((thflags & TH_FIN) ||
                             (tilen != 0 &&
#if INET6
                              (isipv6 && in6_localaddr(&inp->in6p_faddr))
                              ||
                              (!isipv6 &&
#endif /* INET6 */
                               in_localaddr(inp->inp_faddr)
#if INET6
                               )
#endif /* INET6 */
                              ))) {
                            if (last_active_conn_count > DELACK_BITMASK_THRESH)
                                TCP_DELACK_BITSET(tp->t_inpcb->hash_element); 

                                tp->t_flags |= (TF_DELACK | TF_NEEDSYN);
                        }
                        else
                                tp->t_flags |= (TF_ACKNOW | TF_NEEDSYN);

                        /*
                         * Limit the `virtual advertised window' to TCP_MAXWIN
                         * here.  Even if we requested window scaling, it will
                         * become effective only later when our SYN is acked.
                         */
                        tp->rcv_adv += min(tp->rcv_wnd, TCP_MAXWIN);
                        tcpstat.tcps_connects++;
                        soisconnected(so);
                        tp->t_timer[TCPT_KEEP] = tcp_keepinit;
                        dropsocket = 0;         /* committed to socket */
                        tcpstat.tcps_accepts++;
                        goto trimthenstep6;
                    }
                /* else do standard 3-way handshake */
                } else {
                    /*
                     * No CC option, but maybe CC.NEW:
                     *   invalidate cached value.
                     */
                     taop->tao_cc = 0;
                }
                /*
                 * TAO test failed or there was no CC option,
                 *    do a standard 3-way handshake.
                 */
                tp->t_flags |= TF_ACKNOW;
                tp->t_state = TCPS_SYN_RECEIVED;
                tp->t_timer[TCPT_KEEP] = tcp_keepinit;
                dropsocket = 0;         /* committed to socket */
                tcpstat.tcps_accepts++;
                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)))
                                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 ((taop = tcp_gettaocache(inp)) == NULL) {
                        taop = &tao_noncached;
                        bzero(taop, sizeof(*taop));
                }

                if ((thflags & TH_ACK) &&
                    (SEQ_LEQ(th->th_ack, tp->iss) ||
                     SEQ_GT(th->th_ack, tp->snd_max))) {
                        /*
                         * If we have a cached CCsent for the remote host,
                         * hence we haven't just crashed and restarted,
                         * do not send a RST.  This may be a retransmission
                         * from the other side after our earlier ACK was lost.
                         * Our new SYN, when it arrives, will serve as the
                         * needed ACK.
                         */
                        if (taop->tao_ccsent != 0)
                                goto drop;
                        else
                                goto dropwithreset;
                }
                if (thflags & TH_RST) {
                        if (thflags & TH_ACK) {
                                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->cc_recv = to.to_cc;         /* foreign CC */

                tp->irs = th->th_seq;
                tcp_rcvseqinit(tp);
                if (thflags & TH_ACK) {
                        /*
                         * Our SYN was acked.  If segment contains CC.ECHO
                         * option, check it to make sure this segment really
                         * matches our SYN.  If not, just drop it as old
                         * duplicate, but send an RST if we're still playing
                         * by the old rules.  If no CC.ECHO option, make sure
                         * we don't get fooled into using T/TCP.
                         */
                        if (to.to_flag & TOF_CCECHO) {
                                if (tp->cc_send != to.to_ccecho)
                                        if (taop->tao_ccsent != 0)
                                                goto drop;
                                        else
                                                goto dropwithreset;
                        } else
                                tp->t_flags &= ~TF_RCVD_CC;
                        tcpstat.tcps_connects++;
                        soisconnected(so);
                        /* 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;
                        }
                        /* Segment is acceptable, update cache if undefined. */
                        if (taop->tao_ccsent == 0)
                                taop->tao_ccsent = to.to_ccecho;

                        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 (tcp_delack_enabled && tilen != 0) {
                            if (last_active_conn_count > DELACK_BITMASK_THRESH)
                                TCP_DELACK_BITSET(tp->t_inpcb->hash_element); 
                                tp->t_flags |= TF_DELACK;
                        }
                        else
                                tp->t_flags |= TF_ACKNOW;
                        /*
                         * Received <SYN,ACK> in SYN_SENT[*] state.
                         * Transitions:
                         *      SYN_SENT  --> ESTABLISHED
                         *      SYN_SENT* --> FIN_WAIT_1
                         */
                        if (tp->t_flags & TF_NEEDFIN) {
                                tp->t_state = TCPS_FIN_WAIT_1;
                                tp->t_flags &= ~TF_NEEDFIN;
                                thflags &= ~TH_SYN;
                        } else {
                                if (tp->t_state != TCPS_ESTABLISHED)
                                        current_active_connections++;
                                tp->t_state = TCPS_ESTABLISHED;
                                tp->t_timer[TCPT_KEEP] = tcp_keepidle;
                        }
                } 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*
                 *  If there was no CC option, clear cached CC value.
                 */
                        tp->t_flags |= TF_ACKNOW;
                        tp->t_timer[TCPT_REXMT] = 0;
                        if (to.to_flag & TOF_CC) {
                                if (taop->tao_cc != 0 &&
                                    CC_GT(to.to_cc, taop->tao_cc)) {
                                        /*
                                         * update cache and make transition:
                                         *        SYN-SENT -> ESTABLISHED*
                                         *        SYN-SENT* -> FIN-WAIT-1*
                                         */
                                        taop->tao_cc = to.to_cc;
                                        if (tp->t_flags & TF_NEEDFIN) {
                                                tp->t_state = TCPS_FIN_WAIT_1;
                                                tp->t_flags &= ~TF_NEEDFIN;
                                        } else {
                                                if (tp->t_state != TCPS_ESTABLISHED)
                                                        current_active_connections++;
                                                tp->t_state = TCPS_ESTABLISHED;
                                                tp->t_timer[TCPT_KEEP] = tcp_keepidle;
                                        }
                                        tp->t_flags |= TF_NEEDSYN;
                                } else
                                        tp->t_state = TCPS_SYN_RECEIVED;
                        } else {
                                /* CC.NEW or no option => invalidate cache */
                                taop->tao_cc = 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 (tilen > tp->rcv_wnd) {
                        todrop = tilen - tp->rcv_wnd;
                        m_adj(m, -todrop);
                        tilen = 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:
         *      if segment contains a SYN and CC [not CC.NEW] option:
         *              if state == TIME_WAIT and connection duration > MSL,
         *                  drop packet and send RST;
         *
         *              if SEG.CC > CCrecv then is new SYN, and can implicitly
         *                  ack the FIN (and data) in retransmission queue.
         *                  Complete close and delete TCPCB.  Then reprocess
         *                  segment, hoping to find new TCPCB in LISTEN state;
         *
         *              else must be old SYN; drop it.
         *      else do normal processing.
         */
        case TCPS_LAST_ACK:
        case TCPS_CLOSING:
        case TCPS_TIME_WAIT:
                if ((thflags & TH_SYN) &&
                    (to.to_flag & TOF_CC) && tp->cc_recv != 0) {
                        if (tp->t_state == TCPS_TIME_WAIT &&
                                        tp->t_duration > TCPTV_MSL)
                                goto dropwithreset;
                        if (CC_GT(to.to_cc, tp->cc_recv)) {
                                tp = tcp_close(tp);
                                goto findpcb;
                        }
                        else
                                goto drop;
                }
                break;  /* continue normal processing */
        }

        /*
         * 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.
         *   Also, it does not make sense to allow reset segments with
         *   sequence numbers greater than last_ack_sent to be processed
         *   since these sequence numbers are just the acknowledgement
         *   numbers in our outgoing packets being echoed back at us,
         *   and these acknowledgement numbers are monotonically
         *   increasing.
         * 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_WAIT2, CLOSE_WAIT STATES:
         *      Inform user that connection was reset, and close tcb.
         *    CLOSING, LAST_ACK, TIME_WAIT STATES
         *      Close the tcb.
         *    TIME_WAIT state:
         *      Drop the segment - see Stevens, vol. 2, p. 964 and
         *      RFC 1337.
         */
        if (thflags & TH_RST) {
                if (tp->last_ack_sent == th->th_seq) {
                        switch (tp->t_state) {

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

                        case TCPS_ESTABLISHED:
                        case TCPS_FIN_WAIT_1:
                        case TCPS_CLOSE_WAIT:
                                current_active_connections--;
                                /*
                                  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:
                                current_active_connections--;
                                tp = tcp_close(tp);
                                break;

                        case TCPS_TIME_WAIT:
                                break;
                        }
                }
                goto drop;
        }

        /*
         * RFC 1323 PAWS: If we have a timestamp reply on this segment
         * and it's less than ts_recent, drop it.
         */
        if ((to.to_flag & 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 += tilen;
                        tcpstat.tcps_pawsdrop++;
                        goto dropafterack;
                }
        }

        /*
         * T/TCP mechanism
         *   If T/TCP was negotiated and the segment doesn't have CC,
         *   or if its CC is wrong then drop the segment.
         *   RST segments do not have to comply with this.
         */
        if ((tp->t_flags & (TF_REQ_CC|TF_RCVD_CC)) == (TF_REQ_CC|TF_RCVD_CC) &&
            ((to.to_flag & TOF_CC) == 0 || tp->cc_recv != to.to_cc))
                goto dropafterack;

        /*
         * 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))
                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 > tilen
                    || (todrop == tilen && (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;
                        todrop = tilen;
                        tcpstat.tcps_rcvduppack++;
                        tcpstat.tcps_rcvdupbyte += todrop;
                } else {
                        tcpstat.tcps_rcvpartduppack++;
                        tcpstat.tcps_rcvpartdupbyte += todrop;
                }
                m_adj(m, todrop);
                th->th_seq += todrop;
                tilen -= 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 && tilen) {
                tp = tcp_close(tp);
                tcpstat.tcps_rcvafterclose++;
                goto dropwithreset;
        }

        /*
         * If segment ends after window, drop trailing data
         * (and PUSH and FIN); if nothing left, just ACK.
         */
        todrop = (th->th_seq+tilen) - (tp->rcv_nxt+tp->rcv_wnd);
        if (todrop > 0) {
                tcpstat.tcps_rcvpackafterwin++;
                if (todrop >= tilen) {
                        tcpstat.tcps_rcvbyteafterwin += tilen;
                        /*
                         * 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 = tp->rcv_nxt + TCP_ISSINCR;
                                tp = tcp_close(tp);
                                goto findpcb;
                        }
                        /*
                         * If window is closed can only take segments at
                         * window edge, and have to drop data and PUSH from
                         * incoming segments.  Continue processing, but
                         * remember to ack.  Otherwise, drop segment
                         * and ack.
                         */
                        if (tp->rcv_wnd == 0 && th->th_seq == tp->rcv_nxt) {
                                tp->t_flags |= TF_ACKNOW;
                                tcpstat.tcps_rcvwinprobe++;
                        } else
                                goto dropafterack;
                } else
                        tcpstat.tcps_rcvbyteafterwin += todrop;
                m_adj(m, -todrop);
                tilen -= todrop;
                thflags &= ~(TH_PUSH|TH_FIN);
        }

        /*
         * If last ACK falls within this segment's sequence numbers,
         * record its 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_flag & TOF_TS) != 0 &&
            SEQ_LEQ(th->th_seq, tp->last_ack_sent)) {
                tp->ts_recent_age = tcp_now;
                tp->ts_recent = to.to_tsval;
        }

        /*
         * If 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);
                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
                        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++;
                soisconnected(so);
                current_active_connections++;

                /* 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;
                }
                /*
                 * Upon successful completion of 3-way handshake,
                 * update cache.CC if it was undefined, pass any queued
                 * data to the user, and advance state appropriately.
                 */
                if ((taop = tcp_gettaocache(inp)) != NULL &&
                    taop->tao_cc == 0)
                        taop->tao_cc = tp->cc_recv;

                /*
                 * Make transitions:
                 *      SYN-RECEIVED  -> ESTABLISHED
                 *      SYN-RECEIVED* -> FIN-WAIT-1
                 */
                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;
                }
                /*
                 * If segment contains data or ACK, will call tcp_reass()
                 * later; if not, do so now to pass queued data to user.
                 */
                if (tilen == 0 && (thflags & TH_FIN) == 0)
                        (void) tcp_reass(tp, (struct tcphdr *)0, 0,
                            (struct mbuf *)0, isipv6);
                tp->snd_wl1 = th->th_seq - 1;
                /* fall into ... */

        /*
         * 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_LEQ(th->th_ack, tp->snd_una)) {
                        if (tilen == 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_seq onxt = tp->snd_nxt;
                                        u_int win =
                                            min(tp->snd_wnd, tp->snd_cwnd) / 2 /
                                                tp->t_maxseg;

                                        if (win < 2)
                                                win = 2;
                                        tp->snd_ssthresh = win * tp->t_maxseg;
                                        tp->t_timer[TCPT_REXMT] = 0;
                                        tp->t_rtt = 0;
                                        tp->snd_nxt = th->th_ack;
                                        tp->snd_cwnd = tp->t_maxseg;
                                        (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 if (tp->t_dupacks > tcprexmtthresh) {
                                        tp->snd_cwnd += tp->t_maxseg;
                                        (void) tcp_output(tp);
                                        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 (tp->t_dupacks >= tcprexmtthresh &&
                    tp->snd_cwnd > tp->snd_ssthresh)
                        tp->snd_cwnd = tp->snd_ssthresh;
                tp->t_dupacks = 0;
                if (SEQ_GT(th->th_ack, tp->snd_max)) {
                        tcpstat.tcps_rcvacktoomuch++;
                        goto dropafterack;
                }
                /*
                 *  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 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.
                 */
                if (to.to_flag & TOF_TS)
                        tcp_xmit_timer(tp, tcp_now - to.to_tsecr + 1);
                else if (tp->t_rtt && SEQ_GT(th->th_ack, tp->t_rtseq))
                        tcp_xmit_timer(tp,tp->t_rtt);

                /*
                 * 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 the window gives us less than ssthresh packets
                 * in flight, open exponentially (maxseg per packet).
                 * Otherwise open linearly: maxseg per window
                 * (maxseg^2 / cwnd per packet).
                 */
                {
                register u_int cw = tp->snd_cwnd;
                register u_int incr = tp->t_maxseg;

                if (cw > tp->snd_ssthresh)
                        incr = incr * incr / cw;
                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;
                }
                need_sowwakeup++;
                tp->snd_una = th->th_ack;
                if (SEQ_LT(tp->snd_nxt, tp->snd_una))
                        tp->snd_nxt = tp->snd_una;

                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) {
                                        soisdisconnected(so);
                                        tp->t_timer[TCPT_2MSL] = tcp_maxidle;
                                }
                                add_to_time_wait(tp);
                                current_active_connections--;
                                tp->t_state = TCPS_FIN_WAIT_2;
                        }
                        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_duration < TCPTV_MSL)
                                        tp->t_timer[TCPT_2MSL] =
                                            tp->t_rxtcur * TCPTV_TWTRUNC;
                                else
                                        tp->t_timer[TCPT_2MSL] = 2 * TCPTV_MSL;
                                add_to_time_wait(tp);
                                current_active_connections--;
                                soisdisconnected(so);
                        }
                        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 * TCPTV_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 (tilen == 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_long)tilen
#if SO_OOBINLINE
                     && (so->so_options & SO_OOBINLINE) == 0
#endif
                     )
                        tcp_pulloutofband(so, th, m);
        } 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 ((tilen || (thflags&TH_FIN)) &&
            TCPS_HAVERCVDFIN(tp->t_state) == 0) {
                TCP_REASS(tp, th, tilen, m, so, thflags, isipv6, need_sorwakeup);

                if (tp->t_flags & TF_DELACK) 
                {
                KERNEL_DEBUG(DBG_LAYER_END, ((th->th_dport << 16) | th->th_sport),
                             (((th->th_src.s_addr & 0xffff) << 16) | (th->th_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 = so->so_rcv.sb_hiwat - (tp->rcv_adv - tp->rcv_nxt);
        } 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,
                         *  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 (tcp_delack_enabled && (tp->t_flags & TF_NEEDSYN)) {
                            if (last_active_conn_count > DELACK_BITMASK_THRESH)
                                TCP_DELACK_BITSET(tp->t_inpcb->hash_element); 

                                tp->t_flags |= TF_DELACK;
                        }
                        else
                                tp->t_flags |= TF_ACKNOW;
                        tp->rcv_nxt++;
                }
                switch (tp->t_state) {

                /*
                 * In SYN_RECEIVED and ESTABLISHED STATES
                 * enter the CLOSE_WAIT state.
                 */
                case TCPS_SYN_RECEIVED:
                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_duration < TCPTV_MSL) {
                                tp->t_timer[TCPT_2MSL] =
                                    tp->t_rxtcur * TCPTV_TWTRUNC;
                                /* For transaction client, force ACK now. */
                                tp->t_flags |= TF_ACKNOW;
                        }
                        else
                                tp->t_timer[TCPT_2MSL] = 2 * TCPTV_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 * TCPTV_MSL;
                        add_to_time_wait(tp);
                        break;
                }
        }
#if TCPDEBUG
        if (so->so_options & SO_DEBUG) {
#if INET6
                if (isipv6)
                        tcp_saveip._tcp_si6.ip6_plen = tilen;
                else
                        tcp_saveip._tcp_si4.ip_len = tilen;
#else /* INET6 */
                tcp_saveip.ip_len = tilen;
#endif /* INET6 */

                tcp_trace(TA_INPUT, ostate, tp, (void *)&tcp_saveip,
                          &tcp_savetcp, 0);
        }
#endif

        /*
         * Return any desired output.
         */
        if (needoutput || (tp->t_flags & TF_ACKNOW))
                (void) tcp_output(tp);
        if (need_sorwakeup)
                sorwakeup(so);
        if (need_sowwakeup)
                sowwakeup(so);
        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)) )
                goto dropwithreset;
#if TCPDEBUG
        if (so->so_options & SO_DEBUG) {
#if INET6
                if (isipv6)
                        tcp_saveip._tcp_si6.ip6_plen = tilen;
                else
                        tcp_saveip._tcp_si4.ip_len = tilen;
#else /* INET6 */
                tcp_saveip.ip_len = tilen;
#endif /* INET6 */
                tcp_trace(TA_DROP, ostate, tp, (void *)&tcp_saveip,
                          &tcp_savetcp, 0);
        }
#endif
        m_freem(m);
        tp->t_flags |= TF_ACKNOW;
        (void) tcp_output(tp);
        if (need_sorwakeup)
                sorwakeup(so);
        if (need_sowwakeup)
                sowwakeup(so);
        KERNEL_DEBUG(DBG_FNC_TCP_INPUT | DBG_FUNC_END,0,0,0,0,0);
        return;

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))
                        goto drop; /* anycast check is done at the top */
        } else
#endif /* INET6 */
        if (IN_MULTICAST(ntohl(ip->ip_dst.s_addr)))
                goto drop;
#if TCPDEBUG
        if (tp == 0 || (tp->t_inpcb->inp_socket->so_options & SO_DEBUG)) {
                if (tp == 0) {
#if INET6
                        if (isipv6)
                                tcp_saveip._tcp_si6 = *ip6;
                        else
                                tcp_saveip._tcp_si4 = *ip;
#else /* INET6 */
                        tcp_saveip = *ip;
#endif /* INET6 */
                }
#if INET6
                if (isipv6)
                        tcp_saveip._tcp_si6.ip6_plen = tilen;
                else
                        tcp_saveip._tcp_si4.ip_len = tilen;
#else /* INET6 */
                tcp_saveip.ip_len = tilen;
#endif /* INET6 */
                tcp_trace(TA_DROP, ostate, tp, (void *)&tcp_saveip,
                          &tcp_savetcp, 0);
        }
#endif
        if (thflags & TH_ACK)
#if INET6
                tcp_respond(tp, isipv6 ? (void *)ip6 : (void *)ip, th, m,
                            (tcp_seq)0, th->th_ack, TH_RST, isipv6);
#else /* INET6 */
                tcp_respond(tp, (void *)ip, th, m,
                            (tcp_seq)0, th->th_ack, TH_RST, isipv6);
#endif /* INET6 */
        else {
                if (thflags & TH_SYN)
                        tilen++;
#if INET6
                tcp_respond(tp, isipv6 ? (void *)ip6 : (void *)ip, th, m,
                            th->th_seq+tilen, (tcp_seq)0, TH_RST|TH_ACK,
                            isipv6);
#else /* INET6 */
                tcp_respond(tp, (void *)ip, th, m,
                            th->th_seq+tilen, (tcp_seq)0, TH_RST|TH_ACK,
                            isipv6);
#endif /* INET6 */
        }
        /* destroy temporarily created socket */
        if (need_sorwakeup)
                sorwakeup(so);
        if (need_sowwakeup)
                sowwakeup(so);
        if (dropsocket)
                (void) soabort(so);
        KERNEL_DEBUG(DBG_FNC_TCP_INPUT | DBG_FUNC_END,0,0,0,0,0);
        return;

drop:
        /*
         * Drop space held by incoming segment and return.
         */
#if TCPDEBUG
        if (tp == 0 || (tp->t_inpcb->inp_socket->so_options & SO_DEBUG)) {
                if (tp == 0) {
#if INET6
                        if (isipv6)
                                tcp_saveip._tcp_si6 = *ip6;
                        else
                                tcp_saveip._tcp_si4 = *ip;
#else /* INET6 */
                        tcp_saveip = *ip;
#endif /* INET6 */
                }
#if INET6
                if (isipv6)
                        tcp_saveip._tcp_si6.ip6_plen = tilen;
                else
                        tcp_saveip._tcp_si4.ip_len = tilen;
#else /* INET6 */
                tcp_saveip.ip_len = tilen;
#endif /* INET6 */
                tcp_trace(TA_DROP, ostate, tp, (void *)&tcp_saveip,
                          &tcp_savetcp, 0);
        }
#endif
        m_freem(m);
        if (need_sorwakeup)
                sorwakeup(so);
        if (need_sowwakeup)
                sowwakeup(so);
        /* destroy temporarily created socket */
        if (dropsocket)
                (void) soabort(so);
        KERNEL_DEBUG(DBG_FNC_TCP_INPUT | DBG_FUNC_END,0,0,0,0,0);
        return;
}

static void
tcp_dooptions(tp, cp, cnt, th, to)
        struct tcpcb *tp;
        u_char *cp;
        int cnt;
        struct tcphdr *th;
        struct tcpopt *to;
{
        u_short mss = 0;
        int opt, optlen;

        for (; cnt > 0; cnt -= optlen, cp += optlen) {
                opt = cp[0];
                if (opt == TCPOPT_EOL)
                        break;
                if (opt == TCPOPT_NOP)
                        optlen = 1;
                else {
                        optlen = cp[1];
                        if (optlen <= 0)
                                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));
                        to->to_maxseg = ntohs(mss);
                        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_flag |= TOF_TS;
                        bcopy((char *)cp + 2,
                            (char *)&to->to_tsval, sizeof(to->to_tsval));
                        NTOHL(to->to_tsval);
                        bcopy((char *)cp + 6,
                            (char *)&to->to_tsecr, sizeof(to->to_tsecr));
                        NTOHL(to->to_tsecr);

                        /*
                         * 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_CC:
                        if (optlen != TCPOLEN_CC)
                                continue;
                        to->to_flag |= TOF_CC;
                        bcopy((char *)cp + 2,
                            (char *)&to->to_cc, sizeof(to->to_cc));
                        NTOHL(to->to_cc);
                        /*
                         * A CC or CC.new option received in a SYN makes
                         * it ok to send CC in subsequent segments.
                         */
                        if (th->th_flags & TH_SYN)
                                tp->t_flags |= TF_RCVD_CC;
                        break;
                case TCPOPT_CCNEW:
                        if (optlen != TCPOLEN_CC)
                                continue;
                        if (!(th->th_flags & TH_SYN))
                                continue;
                        to->to_flag |= TOF_CCNEW;
                        bcopy((char *)cp + 2,
                            (char *)&to->to_cc, sizeof(to->to_cc));
                        NTOHL(to->to_cc);
                        /*
                         * A CC or CC.new option received in a SYN makes
                         * it ok to send CC in subsequent segments.
                         */
                        tp->t_flags |= TF_RCVD_CC;
                        break;
                case TCPOPT_CCECHO:
                        if (optlen != TCPOLEN_CC)
                                continue;
                        if (!(th->th_flags & TH_SYN))
                                continue;
                        to->to_flag |= TOF_CCECHO;
                        bcopy((char *)cp + 2,
                            (char *)&to->to_ccecho, sizeof(to->to_ccecho));
                        NTOHL(to->to_ccecho);
                        break;
                }
        }
}

/*
 * 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)
        struct socket *so;
        struct tcphdr *th;
        register struct mbuf *m;
{
        int cnt = 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--;
                        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;
        short 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;
        } 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_rtt = 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;
}

/*
 * 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.
 *
 * In case of T/TCP, we call this routine during implicit connection
 * setup as well (offer = -1), to initialize maxseg from the cached
 * MSS of our peer.
 */
void
tcp_mss(tp, offer, isipv6)
        struct tcpcb *tp;
        int offer;
#if INET6
        int isipv6;
#endif
{
        register struct rtentry *rt;
        struct ifnet *ifp;
        register int rtt, mss;
        u_long bufsize;
        struct inpcb *inp;
        struct socket *so;
        struct rmxp_tao *taop;
        int origoffer = offer;
#if INET6
        int lgminh = isipv6 ? sizeof (struct tcpip6hdr) :
                              sizeof (struct tcpiphdr);
#else /* INET6 */
#define lgminh  (sizeof (struct tcpiphdr))
#endif /* INET6 */

        inp = tp->t_inpcb;
#if INET6
        if (isipv6)
                rt = tcp_rtlookup6(inp);
        else
#endif /* INET6 */
        rt = tcp_rtlookup(inp);
        if (rt == NULL) {
                tp->t_maxopd = tp->t_maxseg =
#if INET6
                isipv6 ? tcp_v6mssdflt :
#endif /* INET6 */
                tcp_mssdflt;
                return;
        }
        ifp = rt->rt_ifp;
        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
                /*
                 * 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 / PR_SLOWHZ);
                tp->t_srtt = rtt / (RTM_RTTUNIT / (PR_SLOWHZ * TCP_RTT_SCALE));
                tcpstat.tcps_usedrtt++;
                if (rt->rt_rmx.rmx_rttvar) {
                        tp->t_rttvar = rt->rt_rmx.rmx_rttvar /
                            (RTM_RTTUNIT / (PR_SLOWHZ * 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);
        }
        /*
         * if there's an mtu associated with the route, use it
         * else, use the link mtu.
         */
        if (rt->rt_rmx.rmx_mtu)
                mss = rt->rt_rmx.rmx_mtu - lgminh;
        else
                mss =
#if INET6
                        isipv6 ? nd_ifinfo[rt->rt_ifp->if_index].linkmtu :
#endif
                        ifp->if_mtu - lgminh;

        if (rt->rt_rmx.rmx_mtu == 0) {
#if INET6
                if (isipv6) {
                        if (!in6_localaddr(&inp->in6p_faddr))
                                mss = min(mss, tcp_v6mssdflt);
                } else
#endif /* INET6 */
                if (!in_localaddr(inp->inp_faddr))
                        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;

        /*
         * In case of T/TCP, origoffer==-1 indicates, that no segments
         * were received yet.  In this case we just guess, otherwise
         * we do the same as before T/TCP.
         */
        if ((tp->t_flags & (TF_REQ_TSTMP|TF_NOOPT)) == TF_REQ_TSTMP &&
            (origoffer == -1 ||
             (tp->t_flags & TF_RCVD_TSTMP) == TF_RCVD_TSTMP))
                mss -= TCPOLEN_TSTAMP_APPA;
        if ((tp->t_flags & (TF_REQ_CC|TF_NOOPT)) == TF_REQ_CC &&
            (origoffer == -1 ||
             (tp->t_flags & TF_RCVD_CC) == TF_RCVD_CC))
                mss -= TCPOLEN_CC_APPA;

#if     (MCLBYTES & (MCLBYTES - 1)) == 0
                if (mss > MCLBYTES)
                        mss &= ~(MCLBYTES-1);
#else
                if (mss > MCLBYTES)
                        mss = mss / MCLBYTES * MCLBYTES;
#endif
        /*
         * If there's a pipesize, change the socket buffer
         * to that 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
        if ((bufsize = rt->rt_rmx.rmx_sendpipe) == 0)
#endif
                bufsize = so->so_snd.sb_hiwat;
        if (bufsize < mss)
                mss = bufsize;
        else {
                bufsize = roundup(bufsize, mss);
                if (bufsize > sb_max)
                        bufsize = sb_max;
                (void)sbreserve(&so->so_snd, bufsize);
        }
        tp->t_maxseg = mss;

#if RTV_RPIPE
        if ((bufsize = rt->rt_rmx.rmx_recvpipe) == 0)
#endif
                bufsize = so->so_rcv.sb_hiwat;
        if (bufsize > mss) {
                bufsize = roundup(bufsize, mss);
                if (bufsize > sb_max)
                        bufsize = sb_max;
                (void)sbreserve(&so->so_rcv, bufsize);
        }
        /*
         * Don't force slow-start on local network.
         */
#if INET6
        if (isipv6) {
                if (!in6_localaddr(&inp->in6p_faddr))
                        tp->snd_cwnd = mss;
        } else
#endif /* INET6 */
        if (!in_localaddr(inp->inp_faddr))
                tp->snd_cwnd = mss;

        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++;
        }
}

/*
 * Determine the MSS option to send on an outgoing SYN.
 */
int
tcp_mssopt(tp, isipv6)
        struct tcpcb *tp;
#if INET6
        int isipv6;
#endif
{
        struct rtentry *rt;
        int mss;
#if INET6
        int lgminh = isipv6 ? sizeof (struct tcpip6hdr) :
                              sizeof (struct tcpiphdr);
#else /* INET6 */
#define lgminh  (sizeof (struct tcpiphdr))
#endif /* INET6 */

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

        mss = rt->rt_ifp->if_mtu - lgminh;

        return mss;
}