FreeBSD/Linux Kernel Cross Reference
sys/netinet/tcp_input.c

     /*      $NetBSD: tcp_input.c,v 1.438 2022/11/04 09:01:53 ozaki-r Exp $  */
     
     /*
      * Copyright (C) 1995, 1996, 1997, and 1998 WIDE Project.
      * 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. Neither the name of the project 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 PROJECT 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 PROJECT 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.
     */
    
    /*
     *      @(#)COPYRIGHT   1.1 (NRL) 17 January 1995
     *
     * NRL grants permission for redistribution and use in source and binary
     * forms, with or without modification, of the software and documentation
     * created at NRL 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 acknowledgements:
     *      This product includes software developed by the University of
     *      California, Berkeley and its contributors.
     *      This product includes software developed at the Information
     *      Technology Division, US Naval Research Laboratory.
     * 4. Neither the name of the NRL nor the names of its contributors
     *    may be used to endorse or promote products derived from this software
     *    without specific prior written permission.
     *
     * THE SOFTWARE PROVIDED BY NRL IS PROVIDED BY NRL 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 NRL 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.
     *
     * The views and conclusions contained in the software and documentation
     * are those of the authors and should not be interpreted as representing
     * official policies, either expressed or implied, of the US Naval
     * Research Laboratory (NRL).
     */
    
    /*-
     * Copyright (c) 1997, 1998, 1999, 2001, 2005, 2006,
     * 2011 The NetBSD Foundation, Inc.
     * All rights reserved.
     *
     * This code is derived from software contributed to The NetBSD Foundation
     * by Coyote Point Systems, Inc.
     * This code is derived from software contributed to The NetBSD Foundation
     * by Jason R. Thorpe and Kevin M. Lahey of the Numerical Aerospace Simulation
     * Facility, NASA Ames Research Center.
     * This code is derived from software contributed to The NetBSD Foundation
     * by Charles M. Hannum.
     * This code is derived from software contributed to The NetBSD Foundation
     * by Rui Paulo.
     *
     * 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.
     *
     * THIS SOFTWARE IS PROVIDED BY THE NETBSD FOUNDATION, INC. 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 FOUNDATION 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.
    */
   
   /*
    * 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. 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
    */
   
   #include <sys/cdefs.h>
   __KERNEL_RCSID(0, "$NetBSD: tcp_input.c,v 1.438 2022/11/04 09:01:53 ozaki-r Exp $");
   
   #ifdef _KERNEL_OPT
   #include "opt_inet.h"
   #include "opt_ipsec.h"
   #include "opt_inet_csum.h"
   #include "opt_tcp_debug.h"
   #endif
   
   #include <sys/param.h>
   #include <sys/systm.h>
   #include <sys/malloc.h>
   #include <sys/mbuf.h>
   #include <sys/protosw.h>
   #include <sys/socket.h>
   #include <sys/socketvar.h>
   #include <sys/errno.h>
   #include <sys/syslog.h>
   #include <sys/pool.h>
   #include <sys/domain.h>
   #include <sys/kernel.h>
   #ifdef TCP_SIGNATURE
   #include <sys/md5.h>
   #endif
   #include <sys/lwp.h> /* for lwp0 */
   #include <sys/cprng.h>
   
   #include <net/if.h>
   #include <net/if_types.h>
   
   #include <netinet/in.h>
   #include <netinet/in_systm.h>
   #include <netinet/ip.h>
   #include <netinet/in_pcb.h>
   #include <netinet/in_var.h>
   #include <netinet/ip_var.h>
   #include <netinet/in_offload.h>
   
   #if NARP > 0
   #include <netinet/if_inarp.h>
   #endif
   #ifdef INET6
   #include <netinet/ip6.h>
   #include <netinet6/ip6_var.h>
   #include <netinet6/in6_pcb.h>
   #include <netinet6/ip6_var.h>
   #include <netinet6/in6_var.h>
   #include <netinet/icmp6.h>
   #include <netinet6/nd6.h>
   #ifdef TCP_SIGNATURE
   #include <netinet6/scope6_var.h>
   #endif
   #endif
   
   #ifndef INET6
   #include <netinet/ip6.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/tcp_private.h>
   #include <netinet/tcp_congctl.h>
   #include <netinet/tcp_debug.h>
   #include <netinet/tcp_syncache.h>
   
   #ifdef INET6
   #include "faith.h"
   #if defined(NFAITH) && NFAITH > 0
   #include <net/if_faith.h>
   #endif
   #endif
   
   #ifdef IPSEC
   #include <netipsec/ipsec.h>
   #include <netipsec/key.h>
   #ifdef INET6
   #include <netipsec/ipsec6.h>
   #endif
   #endif  /* IPSEC*/
   
   #include <netinet/tcp_vtw.h>
   
   int     tcprexmtthresh = 3;
   int     tcp_log_refused;
   
   int     tcp_do_autorcvbuf = 1;
   int     tcp_autorcvbuf_inc = 16 * 1024;
   int     tcp_autorcvbuf_max = 256 * 1024;
   int     tcp_msl = (TCPTV_MSL / PR_SLOWHZ);
   
   static int tcp_rst_ppslim_count = 0;
   static struct timeval tcp_rst_ppslim_last;
   static int tcp_ackdrop_ppslim_count = 0;
   static struct timeval tcp_ackdrop_ppslim_last;
   
   #define TCP_PAWS_IDLE   (24U * 24 * 60 * 60 * PR_SLOWHZ)
   
   /* for modulo comparisons of timestamps */
   #define TSTMP_LT(a,b)   ((int)((a)-(b)) < 0)
   #define TSTMP_GEQ(a,b)  ((int)((a)-(b)) >= 0)
   
   /*
    * Neighbor Discovery, Neighbor Unreachability Detection Upper layer hint.
    */
   static void
   nd_hint(struct tcpcb *tp)
   {
           struct route *ro = NULL;
           struct rtentry *rt;
   
           if (tp == NULL)
                   return;
   
           ro = &tp->t_inpcb->inp_route;
           if (ro == NULL)
                   return;
   
           rt = rtcache_validate(ro);
           if (rt == NULL)
                   return;
   
           switch (tp->t_family) {
   #if NARP > 0
           case AF_INET:
                   arp_nud_hint(rt);
                   break;
   #endif
   #ifdef INET6
           case AF_INET6:
                   nd6_nud_hint(rt);
                   break;
   #endif
           }
   
           rtcache_unref(rt, ro);
   }
   
   /*
    * Compute ACK transmission behavior.  Delay the ACK unless
    * we have already delayed an ACK (must send an ACK every two segments).
    * We also ACK immediately if we received a PUSH and the ACK-on-PUSH
    * option is enabled.
    */
   static void
   tcp_setup_ack(struct tcpcb *tp, const struct tcphdr *th)
   {
   
           if (tp->t_flags & TF_DELACK ||
               (tcp_ack_on_push && th->th_flags & TH_PUSH))
                   tp->t_flags |= TF_ACKNOW;
           else
                   TCP_SET_DELACK(tp);
   }
   
   static void
   icmp_check(struct tcpcb *tp, const struct tcphdr *th, int acked)
   {
   
           /*
            * If we had a pending ICMP message that refers to data that have
            * just been acknowledged, disregard the recorded ICMP message.
            */
           if ((tp->t_flags & TF_PMTUD_PEND) &&
               SEQ_GT(th->th_ack, tp->t_pmtud_th_seq))
                   tp->t_flags &= ~TF_PMTUD_PEND;
   
           /*
            * Keep track of the largest chunk of data
            * acknowledged since last PMTU update
            */
           if (tp->t_pmtud_mss_acked < acked)
                   tp->t_pmtud_mss_acked = acked;
   }
   
   /*
    * Convert TCP protocol fields to host order for easier processing.
    */
   static void
   tcp_fields_to_host(struct tcphdr *th)
   {
   
           NTOHL(th->th_seq);
           NTOHL(th->th_ack);
           NTOHS(th->th_win);
           NTOHS(th->th_urp);
   }
   
   /*
    * ... and reverse the above.
    */
   static void
   tcp_fields_to_net(struct tcphdr *th)
   {
   
           HTONL(th->th_seq);
           HTONL(th->th_ack);
           HTONS(th->th_win);
           HTONS(th->th_urp);
   }
   
   static void
   tcp_urp_drop(struct tcphdr *th, int todrop, int *tiflags)
   {
           if (th->th_urp > todrop) {
                   th->th_urp -= todrop;
           } else {
                   *tiflags &= ~TH_URG;
                   th->th_urp = 0;
           }
   }
   
   #ifdef TCP_CSUM_COUNTERS
   #include <sys/device.h>
   
   extern struct evcnt tcp_hwcsum_ok;
   extern struct evcnt tcp_hwcsum_bad;
   extern struct evcnt tcp_hwcsum_data;
   extern struct evcnt tcp_swcsum;
   #if defined(INET6)
   extern struct evcnt tcp6_hwcsum_ok;
   extern struct evcnt tcp6_hwcsum_bad;
   extern struct evcnt tcp6_hwcsum_data;
   extern struct evcnt tcp6_swcsum;
   #endif /* defined(INET6) */
   
   #define TCP_CSUM_COUNTER_INCR(ev)       (ev)->ev_count++
   
   #else
   
   #define TCP_CSUM_COUNTER_INCR(ev)       /* nothing */
   
   #endif /* TCP_CSUM_COUNTERS */
   
   #ifdef TCP_REASS_COUNTERS
   #include <sys/device.h>
   
   extern struct evcnt tcp_reass_;
   extern struct evcnt tcp_reass_empty;
   extern struct evcnt tcp_reass_iteration[8];
   extern struct evcnt tcp_reass_prependfirst;
   extern struct evcnt tcp_reass_prepend;
   extern struct evcnt tcp_reass_insert;
   extern struct evcnt tcp_reass_inserttail;
   extern struct evcnt tcp_reass_append;
   extern struct evcnt tcp_reass_appendtail;
   extern struct evcnt tcp_reass_overlaptail;
   extern struct evcnt tcp_reass_overlapfront;
   extern struct evcnt tcp_reass_segdup;
   extern struct evcnt tcp_reass_fragdup;
   
   #define TCP_REASS_COUNTER_INCR(ev)      (ev)->ev_count++
   
   #else
   
   #define TCP_REASS_COUNTER_INCR(ev)      /* nothing */
   
   #endif /* TCP_REASS_COUNTERS */
   
   static int tcp_reass(struct tcpcb *, const struct tcphdr *, struct mbuf *,
       int);
   
   static void tcp4_log_refused(const struct ip *, const struct tcphdr *);
   #ifdef INET6
   static void tcp6_log_refused(const struct ip6_hdr *, const struct tcphdr *);
   #endif
   
   #if defined(MBUFTRACE)
   struct mowner tcp_reass_mowner = MOWNER_INIT("tcp", "reass");
   #endif /* defined(MBUFTRACE) */
   
   static struct pool tcpipqent_pool;
   
   void
   tcpipqent_init(void)
   {
   
           pool_init(&tcpipqent_pool, sizeof(struct ipqent), 0, 0, 0, "tcpipqepl",
               NULL, IPL_VM);
   }
   
   struct ipqent *
   tcpipqent_alloc(void)
   {
           struct ipqent *ipqe;
           int s;
   
           s = splvm();
           ipqe = pool_get(&tcpipqent_pool, PR_NOWAIT);
           splx(s);
   
           return ipqe;
   }
   
   void
   tcpipqent_free(struct ipqent *ipqe)
   {
           int s;
   
           s = splvm();
           pool_put(&tcpipqent_pool, ipqe);
           splx(s);
   }
   
   /*
    * Insert segment ti into reassembly queue of tcp with
    * control block tp.  Return TH_FIN if reassembly now includes
    * a segment with FIN.
    */
   static int
   tcp_reass(struct tcpcb *tp, const struct tcphdr *th, struct mbuf *m, int tlen)
   {
           struct ipqent *p, *q, *nq, *tiqe = NULL;
           struct socket *so = NULL;
           int pkt_flags;
           tcp_seq pkt_seq;
           unsigned pkt_len;
           u_long rcvpartdupbyte = 0;
           u_long rcvoobyte;
   #ifdef TCP_REASS_COUNTERS
           u_int count = 0;
   #endif
           uint64_t *tcps;
   
           so = tp->t_inpcb->inp_socket;
   
           TCP_REASS_LOCK_CHECK(tp);
   
           /*
            * Call with th==NULL after become established to
            * force pre-ESTABLISHED data up to user socket.
            */
           if (th == NULL)
                   goto present;
   
           m_claimm(m, &tcp_reass_mowner);
   
           rcvoobyte = tlen;
           /*
            * Copy these to local variables because the TCP header gets munged
            * while we are collapsing mbufs.
            */
           pkt_seq = th->th_seq;
           pkt_len = tlen;
           pkt_flags = th->th_flags;
   
           TCP_REASS_COUNTER_INCR(&tcp_reass_);
   
           if ((p = TAILQ_LAST(&tp->segq, ipqehead)) != NULL) {
                   /*
                    * When we miss a packet, the vast majority of time we get
                    * packets that follow it in order.  So optimize for that.
                    */
                   if (pkt_seq == p->ipqe_seq + p->ipqe_len) {
                           p->ipqe_len += pkt_len;
                           p->ipqe_flags |= pkt_flags;
                           m_cat(p->ipqe_m, m);
                           m = NULL;
                           tiqe = p;
                           TAILQ_REMOVE(&tp->timeq, p, ipqe_timeq);
                           TCP_REASS_COUNTER_INCR(&tcp_reass_appendtail);
                           goto skip_replacement;
                   }
                   /*
                    * While we're here, if the pkt is completely beyond
                    * anything we have, just insert it at the tail.
                    */
                   if (SEQ_GT(pkt_seq, p->ipqe_seq + p->ipqe_len)) {
                           TCP_REASS_COUNTER_INCR(&tcp_reass_inserttail);
                           goto insert_it;
                   }
           }
   
           q = TAILQ_FIRST(&tp->segq);
   
           if (q != NULL) {
                   /*
                    * If this segment immediately precedes the first out-of-order
                    * block, simply slap the segment in front of it and (mostly)
                    * skip the complicated logic.
                    */
                   if (pkt_seq + pkt_len == q->ipqe_seq) {
                           q->ipqe_seq = pkt_seq;
                           q->ipqe_len += pkt_len;
                           q->ipqe_flags |= pkt_flags;
                           m_cat(m, q->ipqe_m);
                           q->ipqe_m = m;
                           tiqe = q;
                           TAILQ_REMOVE(&tp->timeq, q, ipqe_timeq);
                           TCP_REASS_COUNTER_INCR(&tcp_reass_prependfirst);
                           goto skip_replacement;
                   }
           } else {
                   TCP_REASS_COUNTER_INCR(&tcp_reass_empty);
           }
   
           /*
            * Find a segment which begins after this one does.
            */
           for (p = NULL; q != NULL; q = nq) {
                   nq = TAILQ_NEXT(q, ipqe_q);
   #ifdef TCP_REASS_COUNTERS
                   count++;
   #endif
   
                   /*
                    * If the received segment is just right after this
                    * fragment, merge the two together and then check
                    * for further overlaps.
                    */
                   if (q->ipqe_seq + q->ipqe_len == pkt_seq) {
                           pkt_len += q->ipqe_len;
                           pkt_flags |= q->ipqe_flags;
                           pkt_seq = q->ipqe_seq;
                           m_cat(q->ipqe_m, m);
                           m = q->ipqe_m;
                           TCP_REASS_COUNTER_INCR(&tcp_reass_append);
                           goto free_ipqe;
                   }
   
                   /*
                    * If the received segment is completely past this
                    * fragment, we need to go to the next fragment.
                    */
                   if (SEQ_LT(q->ipqe_seq + q->ipqe_len, pkt_seq)) {
                           p = q;
                           continue;
                   }
   
                   /*
                    * If the fragment is past the received segment,
                    * it (or any following) can't be concatenated.
                    */
                   if (SEQ_GT(q->ipqe_seq, pkt_seq + pkt_len)) {
                           TCP_REASS_COUNTER_INCR(&tcp_reass_insert);
                           break;
                   }
   
                   /*
                    * We've received all the data in this segment before.
                    * Mark it as a duplicate and return.
                    */
                   if (SEQ_LEQ(q->ipqe_seq, pkt_seq) &&
                       SEQ_GEQ(q->ipqe_seq + q->ipqe_len, pkt_seq + pkt_len)) {
                           tcps = TCP_STAT_GETREF();
                           tcps[TCP_STAT_RCVDUPPACK]++;
                           tcps[TCP_STAT_RCVDUPBYTE] += pkt_len;
                           TCP_STAT_PUTREF();
                           tcp_new_dsack(tp, pkt_seq, pkt_len);
                           m_freem(m);
                           if (tiqe != NULL) {
                                   tcpipqent_free(tiqe);
                           }
                           TCP_REASS_COUNTER_INCR(&tcp_reass_segdup);
                           goto out;
                   }
   
                   /*
                    * Received segment completely overlaps this fragment
                    * so we drop the fragment (this keeps the temporal
                    * ordering of segments correct).
                    */
                   if (SEQ_GEQ(q->ipqe_seq, pkt_seq) &&
                       SEQ_LEQ(q->ipqe_seq + q->ipqe_len, pkt_seq + pkt_len)) {
                           rcvpartdupbyte += q->ipqe_len;
                           m_freem(q->ipqe_m);
                           TCP_REASS_COUNTER_INCR(&tcp_reass_fragdup);
                           goto free_ipqe;
                   }
   
                   /*
                    * Received segment extends past the end of the fragment.
                    * Drop the overlapping bytes, merge the fragment and
                    * segment, and treat as a longer received packet.
                    */
                   if (SEQ_LT(q->ipqe_seq, pkt_seq) &&
                       SEQ_GT(q->ipqe_seq + q->ipqe_len, pkt_seq))  {
                           int overlap = q->ipqe_seq + q->ipqe_len - pkt_seq;
                           m_adj(m, overlap);
                           rcvpartdupbyte += overlap;
                           m_cat(q->ipqe_m, m);
                           m = q->ipqe_m;
                           pkt_seq = q->ipqe_seq;
                           pkt_len += q->ipqe_len - overlap;
                           rcvoobyte -= overlap;
                           TCP_REASS_COUNTER_INCR(&tcp_reass_overlaptail);
                           goto free_ipqe;
                   }
   
                   /*
                    * Received segment extends past the front of the fragment.
                    * Drop the overlapping bytes on the received packet. The
                    * packet will then be concatenated with this fragment a
                    * bit later.
                    */
                   if (SEQ_GT(q->ipqe_seq, pkt_seq) &&
                       SEQ_LT(q->ipqe_seq, pkt_seq + pkt_len))  {
                           int overlap = pkt_seq + pkt_len - q->ipqe_seq;
                           m_adj(m, -overlap);
                           pkt_len -= overlap;
                           rcvpartdupbyte += overlap;
                           TCP_REASS_COUNTER_INCR(&tcp_reass_overlapfront);
                           rcvoobyte -= overlap;
                   }
   
                   /*
                    * If the received segment immediately precedes this
                    * fragment then tack the fragment onto this segment
                    * and reinsert the data.
                    */
                   if (q->ipqe_seq == pkt_seq + pkt_len) {
                           pkt_len += q->ipqe_len;
                           pkt_flags |= q->ipqe_flags;
                           m_cat(m, q->ipqe_m);
                           TAILQ_REMOVE(&tp->segq, q, ipqe_q);
                           TAILQ_REMOVE(&tp->timeq, q, ipqe_timeq);
                           tp->t_segqlen--;
                           KASSERT(tp->t_segqlen >= 0);
                           KASSERT(tp->t_segqlen != 0 ||
                               (TAILQ_EMPTY(&tp->segq) &&
                               TAILQ_EMPTY(&tp->timeq)));
                           if (tiqe == NULL) {
                                   tiqe = q;
                           } else {
                                   tcpipqent_free(q);
                           }
                           TCP_REASS_COUNTER_INCR(&tcp_reass_prepend);
                           break;
                   }
   
                   /*
                    * If the fragment is before the segment, remember it.
                    * When this loop is terminated, p will contain the
                    * pointer to the fragment that is right before the
                    * received segment.
                    */
                   if (SEQ_LEQ(q->ipqe_seq, pkt_seq))
                           p = q;
   
                   continue;
   
                   /*
                    * This is a common operation.  It also will allow
                    * to save doing a malloc/free in most instances.
                    */
             free_ipqe:
                   TAILQ_REMOVE(&tp->segq, q, ipqe_q);
                   TAILQ_REMOVE(&tp->timeq, q, ipqe_timeq);
                   tp->t_segqlen--;
                   KASSERT(tp->t_segqlen >= 0);
                   KASSERT(tp->t_segqlen != 0 ||
                       (TAILQ_EMPTY(&tp->segq) && TAILQ_EMPTY(&tp->timeq)));
                   if (tiqe == NULL) {
                           tiqe = q;
                   } else {
                           tcpipqent_free(q);
                   }
           }
   
   #ifdef TCP_REASS_COUNTERS
           if (count > 7)
                   TCP_REASS_COUNTER_INCR(&tcp_reass_iteration[0]);
           else if (count > 0)
                   TCP_REASS_COUNTER_INCR(&tcp_reass_iteration[count]);
   #endif
   
   insert_it:
           /*
            * Allocate a new queue entry (block) since the received segment
            * did not collapse onto any other out-of-order block. If it had
            * collapsed, tiqe would not be NULL and we would be reusing it.
            *
            * If the allocation fails, drop the packet.
            */
           if (tiqe == NULL) {
                   tiqe = tcpipqent_alloc();
                   if (tiqe == NULL) {
                           TCP_STATINC(TCP_STAT_RCVMEMDROP);
                           m_freem(m);
                           goto out;
                   }
           }
   
           /*
            * Update the counters.
            */
           tp->t_rcvoopack++;
           tcps = TCP_STAT_GETREF();
           tcps[TCP_STAT_RCVOOPACK]++;
           tcps[TCP_STAT_RCVOOBYTE] += rcvoobyte;
           if (rcvpartdupbyte) {
               tcps[TCP_STAT_RCVPARTDUPPACK]++;
               tcps[TCP_STAT_RCVPARTDUPBYTE] += rcvpartdupbyte;
           }
           TCP_STAT_PUTREF();
   
           /*
            * Insert the new fragment queue entry into both queues.
            */
           tiqe->ipqe_m = m;
           tiqe->ipqe_seq = pkt_seq;
           tiqe->ipqe_len = pkt_len;
           tiqe->ipqe_flags = pkt_flags;
           if (p == NULL) {
                   TAILQ_INSERT_HEAD(&tp->segq, tiqe, ipqe_q);
           } else {
                   TAILQ_INSERT_AFTER(&tp->segq, p, tiqe, ipqe_q);
           }
           tp->t_segqlen++;
   
   skip_replacement:
           TAILQ_INSERT_HEAD(&tp->timeq, tiqe, ipqe_timeq);
   
   present:
           /*
            * Present data to user, advancing rcv_nxt through
            * completed sequence space.
            */
           if (TCPS_HAVEESTABLISHED(tp->t_state) == 0)
                   goto out;
           q = TAILQ_FIRST(&tp->segq);
           if (q == NULL || q->ipqe_seq != tp->rcv_nxt)
                   goto out;
           if (tp->t_state == TCPS_SYN_RECEIVED && q->ipqe_len)
                   goto out;
   
           tp->rcv_nxt += q->ipqe_len;
           pkt_flags = q->ipqe_flags & TH_FIN;
           nd_hint(tp);
   
           TAILQ_REMOVE(&tp->segq, q, ipqe_q);
           TAILQ_REMOVE(&tp->timeq, q, ipqe_timeq);
           tp->t_segqlen--;
           KASSERT(tp->t_segqlen >= 0);
           KASSERT(tp->t_segqlen != 0 ||
               (TAILQ_EMPTY(&tp->segq) && TAILQ_EMPTY(&tp->timeq)));
           if (so->so_state & SS_CANTRCVMORE)
                   m_freem(q->ipqe_m);
           else
                   sbappendstream(&so->so_rcv, q->ipqe_m);
           tcpipqent_free(q);
           TCP_REASS_UNLOCK(tp);
           sorwakeup(so);
           return pkt_flags;
   
   out:
           TCP_REASS_UNLOCK(tp);
           return 0;
   }
   
   #ifdef INET6
   int
   tcp6_input(struct mbuf **mp, int *offp, int proto)
   {
           struct mbuf *m = *mp;
   
           /*
            * draft-itojun-ipv6-tcp-to-anycast
            * better place to put this in?
            */
           if (m->m_flags & M_ANYCAST6) {
                   struct ip6_hdr *ip6;
                   if (m->m_len < sizeof(struct ip6_hdr)) {
                           if ((m = m_pullup(m, sizeof(struct ip6_hdr))) == NULL) {
                                   TCP_STATINC(TCP_STAT_RCVSHORT);
                                   return IPPROTO_DONE;
                           }
                   }
                   ip6 = mtod(m, struct ip6_hdr *);
                   icmp6_error(m, ICMP6_DST_UNREACH, ICMP6_DST_UNREACH_ADDR,
                       (char *)&ip6->ip6_dst - (char *)ip6);
                   return IPPROTO_DONE;
           }
   
           tcp_input(m, *offp, proto);
           return IPPROTO_DONE;
   }
   #endif
   
   static void
   tcp4_log_refused(const struct ip *ip, const struct tcphdr *th)
   {
           char src[INET_ADDRSTRLEN];
           char dst[INET_ADDRSTRLEN];
   
           if (ip) {
                   in_print(src, sizeof(src), &ip->ip_src);
                   in_print(dst, sizeof(dst), &ip->ip_dst);
           } else {
                   strlcpy(src, "(unknown)", sizeof(src));
                   strlcpy(dst, "(unknown)", sizeof(dst));
           }
           log(LOG_INFO,
               "Connection attempt to TCP %s:%d from %s:%d\n",
               dst, ntohs(th->th_dport),
               src, ntohs(th->th_sport));
   }
   
   #ifdef INET6
   static void
   tcp6_log_refused(const struct ip6_hdr *ip6, const struct tcphdr *th)
   {
           char src[INET6_ADDRSTRLEN];
           char dst[INET6_ADDRSTRLEN];
   
           if (ip6) {
                   in6_print(src, sizeof(src), &ip6->ip6_src);
                   in6_print(dst, sizeof(dst), &ip6->ip6_dst);
           } else {
                   strlcpy(src, "(unknown v6)", sizeof(src));
                   strlcpy(dst, "(unknown v6)", sizeof(dst));
           }
           log(LOG_INFO,
               "Connection attempt to TCP [%s]:%d from [%s]:%d\n",
               dst, ntohs(th->th_dport),
               src, ntohs(th->th_sport));
   }
   #endif
   
   /*
    * Checksum extended TCP header and data.
    */
   int
   tcp_input_checksum(int af, struct mbuf *m, const struct tcphdr *th,
       int toff, int off, int tlen)
   {
           struct ifnet *rcvif;
           int s;
   
           /*
            * XXX it's better to record and check if this mbuf is
            * already checked.
            */
   
           rcvif = m_get_rcvif(m, &s);
           if (__predict_false(rcvif == NULL))
                   goto badcsum; /* XXX */
   
           switch (af) {
           case AF_INET:
                   switch (m->m_pkthdr.csum_flags &
                           ((rcvif->if_csum_flags_rx & M_CSUM_TCPv4) |
                            M_CSUM_TCP_UDP_BAD | M_CSUM_DATA)) {
                   case M_CSUM_TCPv4|M_CSUM_TCP_UDP_BAD:
                           TCP_CSUM_COUNTER_INCR(&tcp_hwcsum_bad);
                           goto badcsum;
   
                   case M_CSUM_TCPv4|M_CSUM_DATA: {
                           u_int32_t hw_csum = m->m_pkthdr.csum_data;
   
                           TCP_CSUM_COUNTER_INCR(&tcp_hwcsum_data);
                           if (m->m_pkthdr.csum_flags & M_CSUM_NO_PSEUDOHDR) {
                                   const struct ip *ip =
                                       mtod(m, const struct ip *);
   
                                   hw_csum = in_cksum_phdr(ip->ip_src.s_addr,
                                       ip->ip_dst.s_addr,
                                       htons(hw_csum + tlen + off + IPPROTO_TCP));
                           }
                           if ((hw_csum ^ 0xffff) != 0)
                                   goto badcsum;
                           break;
                   }
   
                   case M_CSUM_TCPv4:
                           /* Checksum was okay. */
                           TCP_CSUM_COUNTER_INCR(&tcp_hwcsum_ok);
                           break;
   
                   default:
                           /*
                            * Must compute it ourselves.  Maybe skip checksum
                            * on loopback interfaces.
                            */
                           if (__predict_true(!(rcvif->if_flags & IFF_LOOPBACK) ||
                                              tcp_do_loopback_cksum)) {
                                   TCP_CSUM_COUNTER_INCR(&tcp_swcsum);
                                   if (in4_cksum(m, IPPROTO_TCP, toff,
                                                 tlen + off) != 0)
                                           goto badcsum;
                           }
                           break;
                   }
                   break;
   
   #ifdef INET6
           case AF_INET6:
                   switch (m->m_pkthdr.csum_flags &
                           ((rcvif->if_csum_flags_rx & M_CSUM_TCPv6) |
                            M_CSUM_TCP_UDP_BAD | M_CSUM_DATA)) {
                   case M_CSUM_TCPv6|M_CSUM_TCP_UDP_BAD:
                           TCP_CSUM_COUNTER_INCR(&tcp6_hwcsum_bad);
                           goto badcsum;
   
   #if 0 /* notyet */
                   case M_CSUM_TCPv6|M_CSUM_DATA:
   #endif
   
                   case M_CSUM_TCPv6:
                           /* Checksum was okay. */
                           TCP_CSUM_COUNTER_INCR(&tcp6_hwcsum_ok);
                           break;
   
                   default:
                           /*
                            * Must compute it ourselves.  Maybe skip checksum
                            * on loopback interfaces.
                            */
                           if (__predict_true((m->m_flags & M_LOOP) == 0 ||
                               tcp_do_loopback_cksum)) {
                                   TCP_CSUM_COUNTER_INCR(&tcp6_swcsum);
                                   if (in6_cksum(m, IPPROTO_TCP, toff,
                                       tlen + off) != 0)
                                           goto badcsum;
                           }
                   }
                   break;
   #endif /* INET6 */
           }
           m_put_rcvif(rcvif, &s);
   
           return 0;
   
   badcsum:
           m_put_rcvif(rcvif, &s);
           TCP_STATINC(TCP_STAT_RCVBADSUM);
           return -1;
   }
   
   /*
    * When a packet arrives addressed to a vestigial tcpbp, we
    * nevertheless have to respond to it per the spec.
    *
    * This code is duplicated from the one in tcp_input().
    */
   static void tcp_vtw_input(struct tcphdr *th, vestigial_inpcb_t *vp,
       struct mbuf *m, int tlen)
   {
           int tiflags;
           int todrop;
           uint32_t t_flags = 0;
           uint64_t *tcps;
   
           tiflags = th->th_flags;
           todrop  = vp->rcv_nxt - th->th_seq;
   
           if (todrop > 0) {
                   if (tiflags & TH_SYN) {
                           tiflags &= ~TH_SYN;
                           th->th_seq++;
                           tcp_urp_drop(th, 1, &tiflags);
                           todrop--;
                   }
                   if (todrop > tlen ||
                       (todrop == tlen && (tiflags & TH_FIN) == 0)) {
                           /*
                            * Any valid FIN or RST must be to the left of the
                           * window.  At this point the FIN or RST must be a
                           * duplicate or out of sequence; drop it.
                           */
                          if (tiflags & TH_RST)
                                  goto drop;
                          tiflags &= ~(TH_FIN|TH_RST);
  
                          /*
                           * Send an ACK to resynchronize and drop any data.
                           * But keep on processing for RST or ACK.
                           */
                          t_flags |= TF_ACKNOW;
                          todrop = tlen;
                          tcps = TCP_STAT_GETREF();
                          tcps[TCP_STAT_RCVDUPPACK] += 1;
                          tcps[TCP_STAT_RCVDUPBYTE] += todrop;
                          TCP_STAT_PUTREF();
                  } else if ((tiflags & TH_RST) &&
                      th->th_seq != vp->rcv_nxt) {
                          /*
                           * Test for reset before adjusting the sequence
                           * number for overlapping data.
                           */
                          goto dropafterack_ratelim;
                  } else {
                          tcps = TCP_STAT_GETREF();
                          tcps[TCP_STAT_RCVPARTDUPPACK] += 1;
                          tcps[TCP_STAT_RCVPARTDUPBYTE] += todrop;
                          TCP_STAT_PUTREF();
                  }
  
  //              tcp_new_dsack(tp, th->th_seq, todrop);
  //              hdroptlen += todrop;    /*drop from head afterwards*/
  
                  th->th_seq += todrop;
                  tlen -= todrop;
                  tcp_urp_drop(th, todrop, &tiflags);
          }
  
          /*
           * If new data are received on a connection after the
           * user processes are gone, then RST the other end.
           */
          if (tlen) {
                  TCP_STATINC(TCP_STAT_RCVAFTERCLOSE);
                  goto dropwithreset;
          }
  
          /*
           * If segment ends after window, drop trailing data
           * (and PUSH and FIN); if nothing left, just ACK.
           */
          todrop = (th->th_seq + tlen) - (vp->rcv_nxt + vp->rcv_wnd);
  
          if (todrop > 0) {
                  TCP_STATINC(TCP_STAT_RCVPACKAFTERWIN);
                  if (todrop >= tlen) {
                          /*
                           * The segment actually starts after the window.
                           * th->th_seq + tlen - vp->rcv_nxt - vp->rcv_wnd >= tlen
                           * th->th_seq - vp->rcv_nxt - vp->rcv_wnd >= 0
                           * th->th_seq >= vp->rcv_nxt + vp->rcv_wnd
                           */
                          TCP_STATADD(TCP_STAT_RCVBYTEAFTERWIN, tlen);
  
                          /*
                           * If a new connection request is received
                           * while in TIME_WAIT, drop the old connection
                           * and start over if the sequence numbers
                           * are above the previous ones.
                           */
                          if ((tiflags & TH_SYN) &&
                              SEQ_GT(th->th_seq, vp->rcv_nxt)) {
                                  /*
                                   * We only support this in the !NOFDREF case, which
                                   * is to say: not here.
                                   */
                                  goto dropwithreset;
                          }
  
                          /*
                           * 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 (if not RST) ack.
                           */
                          if (vp->rcv_wnd == 0 && th->th_seq == vp->rcv_nxt) {
                                  t_flags |= TF_ACKNOW;
                                  TCP_STATINC(TCP_STAT_RCVWINPROBE);
                          } else {
                                  goto dropafterack;
                          }
                  } else {
                          TCP_STATADD(TCP_STAT_RCVBYTEAFTERWIN, todrop);
                  }
                  m_adj(m, -todrop);
                  tlen -= todrop;
                  tiflags &= ~(TH_PUSH|TH_FIN);
          }
  
          if (tiflags & TH_RST) {
                  if (th->th_seq != vp->rcv_nxt)
                          goto dropafterack_ratelim;
  
                  vtw_del(vp->ctl, vp->vtw);
                  goto drop;
          }
  
          /*
           * If the ACK bit is off we drop the segment and return.
           */
          if ((tiflags & TH_ACK) == 0) {
                  if (t_flags & TF_ACKNOW)
                          goto dropafterack;
                  goto drop;
          }
  
          /*
           * In TIME_WAIT state the only thing that should arrive
           * is a retransmission of the remote FIN.  Acknowledge
           * it and restart the finack timer.
           */
          vtw_restart(vp);
          goto dropafterack;
  
  dropafterack:
          /*
           * Generate an ACK dropping incoming segment if it occupies
           * sequence space, where the ACK reflects our state.
           */
          if (tiflags & TH_RST)
                  goto drop;
          goto dropafterack2;
  
  dropafterack_ratelim:
          /*
           * We may want to rate-limit ACKs against SYN/RST attack.
           */
          if (ppsratecheck(&tcp_ackdrop_ppslim_last, &tcp_ackdrop_ppslim_count,
              tcp_ackdrop_ppslim) == 0) {
                  /* XXX stat */
                  goto drop;
          }
          /* ...fall into dropafterack2... */
  
  dropafterack2:
          (void)tcp_respond(0, m, m, th, th->th_seq + tlen, th->th_ack, TH_ACK);
          return;
  
  dropwithreset:
          /*
           * Generate a RST, dropping incoming segment.
           * Make ACK acceptable to originator of segment.
           */
          if (tiflags & TH_RST)
                  goto drop;
  
          if (tiflags & TH_ACK) {
                  tcp_respond(0, m, m, th, (tcp_seq)0, th->th_ack, TH_RST);
          } else {
                  if (tiflags & TH_SYN)
                          ++tlen;
                  (void)tcp_respond(0, m, m, th, th->th_seq + tlen, (tcp_seq)0,
                      TH_RST|TH_ACK);
          }
          return;
  drop:
          m_freem(m);
  }
  
  /*
   * TCP input routine, follows pages 65-76 of RFC 793 very closely.
   */
  void
  tcp_input(struct mbuf *m, int off, int proto)
  {
          struct tcphdr *th;
          struct ip *ip;
          struct inpcb *inp;
  #ifdef INET6
          struct ip6_hdr *ip6;
  #endif
          u_int8_t *optp = NULL;
          int optlen = 0;
          int len, tlen, hdroptlen = 0;
          struct tcpcb *tp = NULL;
          int tiflags;
          struct socket *so = NULL;
          int todrop, acked, ourfinisacked, needoutput = 0;
          bool dupseg;
  #ifdef TCP_DEBUG
          short ostate = 0;
  #endif
          u_long tiwin;
          struct tcp_opt_info opti;
          int thlen, iphlen;
          int af;         /* af on the wire */
          struct mbuf *tcp_saveti = NULL;
          uint32_t ts_rtt;
          uint8_t iptos;
          uint64_t *tcps;
          vestigial_inpcb_t vestige;
  
          vestige.valid = 0;
  
          MCLAIM(m, &tcp_rx_mowner);
  
          TCP_STATINC(TCP_STAT_RCVTOTAL);
  
          memset(&opti, 0, sizeof(opti));
          opti.ts_present = 0;
          opti.maxseg = 0;
  
          /*
           * RFC1122 4.2.3.10, p. 104: discard bcast/mcast SYN.
           *
           * TCP is, by definition, unicast, so we reject all
           * multicast outright.
           *
           * Note, there are additional src/dst address checks in
           * the AF-specific code below.
           */
          if (m->m_flags & (M_BCAST|M_MCAST)) {
                  /* XXX stat */
                  goto drop;
          }
  #ifdef INET6
          if (m->m_flags & M_ANYCAST6) {
                  /* XXX stat */
                  goto drop;
          }
  #endif
  
          M_REGION_GET(th, struct tcphdr *, m, off, sizeof(struct tcphdr));
          if (th == NULL) {
                  TCP_STATINC(TCP_STAT_RCVSHORT);
                  return;
          }
  
          /*
           * Enforce alignment requirements that are violated in
           * some cases, see kern/50766 for details.
           */
          if (ACCESSIBLE_POINTER(th, struct tcphdr) == 0) {
                  m = m_copyup(m, off + sizeof(struct tcphdr), 0);
                  if (m == NULL) {
                          TCP_STATINC(TCP_STAT_RCVSHORT);
                          return;
                  }
                  th = (struct tcphdr *)(mtod(m, char *) + off);
          }
          KASSERT(ACCESSIBLE_POINTER(th, struct tcphdr));
  
          /*
           * Get IP and TCP header.
           * Note: IP leaves IP header in first mbuf.
           */
          ip = mtod(m, struct ip *);
  #ifdef INET6
          ip6 = mtod(m, struct ip6_hdr *);
  #endif
          switch (ip->ip_v) {
          case 4:
                  af = AF_INET;
                  iphlen = sizeof(struct ip);
  
                  if (IN_MULTICAST(ip->ip_dst.s_addr) ||
                      in_broadcast(ip->ip_dst, m_get_rcvif_NOMPSAFE(m)))
                          goto drop;
  
                  /* We do the checksum after PCB lookup... */
                  len = ntohs(ip->ip_len);
                  tlen = len - off;
                  iptos = ip->ip_tos;
                  break;
  #ifdef INET6
          case 6:
                  iphlen = sizeof(struct ip6_hdr);
                  af = AF_INET6;
  
                  /*
                   * Be proactive about unspecified IPv6 address in source.
                   * As we use all-zero to indicate unbounded/unconnected pcb,
                   * unspecified IPv6 address can be used to confuse us.
                   *
                   * Note that packets with unspecified IPv6 destination is
                   * already dropped in ip6_input.
                   */
                  if (IN6_IS_ADDR_UNSPECIFIED(&ip6->ip6_src)) {
                          /* XXX stat */
                          goto drop;
                  }
  
                  /*
                   * Make sure destination address is not multicast.
                   * Source address checked in ip6_input().
                   */
                  if (IN6_IS_ADDR_MULTICAST(&ip6->ip6_dst)) {
                          /* XXX stat */
                          goto drop;
                  }
  
                  /* We do the checksum after PCB lookup... */
                  len = m->m_pkthdr.len;
                  tlen = len - off;
                  iptos = (ntohl(ip6->ip6_flow) >> 20) & 0xff;
                  break;
  #endif
          default:
                  m_freem(m);
                  return;
          }
  
  
          /*
           * Check that TCP offset makes sense, pull out TCP options and
           * adjust length.
           */
          thlen = th->th_off << 2;
          if (thlen < sizeof(struct tcphdr) || thlen > tlen) {
                  TCP_STATINC(TCP_STAT_RCVBADOFF);
                  goto drop;
          }
          tlen -= thlen;
  
          if (thlen > sizeof(struct tcphdr)) {
                  M_REGION_GET(th, struct tcphdr *, m, off, thlen);
                  if (th == NULL) {
                          TCP_STATINC(TCP_STAT_RCVSHORT);
                          return;
                  }
                  KASSERT(ACCESSIBLE_POINTER(th, struct tcphdr));
                  optlen = thlen - sizeof(struct tcphdr);
                  optp = ((u_int8_t *)th) + sizeof(struct tcphdr);
  
                  /*
                   * Do quick retrieval of timestamp options.
                   *
                   * If timestamp is the only option and it's formatted as
                   * recommended in RFC 1323 appendix A, we quickly get the
                   * values now and don't bother calling tcp_dooptions(),
                   * etc.
                   */
                  if ((optlen == TCPOLEN_TSTAMP_APPA ||
                       (optlen > TCPOLEN_TSTAMP_APPA &&
                        optp[TCPOLEN_TSTAMP_APPA] == TCPOPT_EOL)) &&
                      be32dec(optp) == TCPOPT_TSTAMP_HDR &&
                      (th->th_flags & TH_SYN) == 0) {
                          opti.ts_present = 1;
                          opti.ts_val = be32dec(optp + 4);
                          opti.ts_ecr = be32dec(optp + 8);
                          optp = NULL;    /* we've parsed the options */
                  }
          }
          tiflags = th->th_flags;
  
          /*
           * Checksum extended TCP header and data
           */
          if (tcp_input_checksum(af, m, th, off, thlen, tlen))
                  goto badcsum;
  
          /*
           * Locate pcb for segment.
           */
  findpcb:
          inp = NULL;
          switch (af) {
          case AF_INET:
                  inp = inpcb_lookup(&tcbtable, ip->ip_src, th->th_sport,
                      ip->ip_dst, th->th_dport, &vestige);
                  if (inp == NULL && !vestige.valid) {
                          TCP_STATINC(TCP_STAT_PCBHASHMISS);
                          inp = inpcb_lookup_bound(&tcbtable, ip->ip_dst,
                              th->th_dport);
                  }
  #ifdef INET6
                  if (inp == NULL && !vestige.valid) {
                          struct in6_addr s, d;
  
                          /* mapped addr case */
                          in6_in_2_v4mapin6(&ip->ip_src, &s);
                          in6_in_2_v4mapin6(&ip->ip_dst, &d);
                          inp = in6pcb_lookup(&tcbtable, &s,
                              th->th_sport, &d, th->th_dport, 0, &vestige);
                          if (inp == NULL && !vestige.valid) {
                                  TCP_STATINC(TCP_STAT_PCBHASHMISS);
                                  inp = in6pcb_lookup_bound(&tcbtable, &d,
                                      th->th_dport, 0);
                          }
                  }
  #endif
                  if (inp == NULL && !vestige.valid) {
                          TCP_STATINC(TCP_STAT_NOPORT);
                          if (tcp_log_refused &&
                              (tiflags & (TH_RST|TH_ACK|TH_SYN)) == TH_SYN) {
                                  tcp4_log_refused(ip, th);
                          }
                          tcp_fields_to_host(th);
                          goto dropwithreset_ratelim;
                  }
  #if defined(IPSEC)
                  if (ipsec_used) {
                          if (inp && ipsec_in_reject(m, inp))
                                  goto drop;
                  }
  #endif /*IPSEC*/
                  break;
  #ifdef INET6
          case AF_INET6:
              {
                  int faith;
  
  #if defined(NFAITH) && NFAITH > 0
                  faith = faithprefix(&ip6->ip6_dst);
  #else
                  faith = 0;
  #endif
                  inp = in6pcb_lookup(&tcbtable, &ip6->ip6_src,
                      th->th_sport, &ip6->ip6_dst, th->th_dport, faith, &vestige);
                  if (inp == NULL && !vestige.valid) {
                          TCP_STATINC(TCP_STAT_PCBHASHMISS);
                          inp = in6pcb_lookup_bound(&tcbtable, &ip6->ip6_dst,
                              th->th_dport, faith);
                  }
                  if (inp == NULL && !vestige.valid) {
                          TCP_STATINC(TCP_STAT_NOPORT);
                          if (tcp_log_refused &&
                              (tiflags & (TH_RST|TH_ACK|TH_SYN)) == TH_SYN) {
                                  tcp6_log_refused(ip6, th);
                          }
                          tcp_fields_to_host(th);
                          goto dropwithreset_ratelim;
                  }
  #if defined(IPSEC)
                  if (ipsec_used && inp && ipsec_in_reject(m, inp))
                          goto drop;
  #endif
                  break;
              }
  #endif
          }
  
          tcp_fields_to_host(th);
  
          /*
           * 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.
           */
          tp = NULL;
          so = NULL;
          if (inp) {
                  /* Check the minimum TTL for socket. */
                  if (inp->inp_af == AF_INET && ip->ip_ttl < in4p_ip_minttl(inp))
                          goto drop;
  
                  tp = intotcpcb(inp);
                  so = inp->inp_socket;
          } else if (vestige.valid) {
                  /* We do not support the resurrection of vtw tcpcps. */
                  tcp_vtw_input(th, &vestige, m, tlen);
                  m = NULL;
                  goto drop;
          }
  
          if (tp == NULL)
                  goto dropwithreset_ratelim;
          if (tp->t_state == TCPS_CLOSED)
                  goto drop;
  
          KASSERT(so->so_lock == softnet_lock);
          KASSERT(solocked(so));
  
          /* Unscale the window into a 32-bit value. */
          if ((tiflags & TH_SYN) == 0)
                  tiwin = th->th_win << tp->snd_scale;
          else
                  tiwin = th->th_win;
  
  #ifdef INET6
          /* save packet options if user wanted */
          if (inp->inp_af == AF_INET6 && (inp->inp_flags & IN6P_CONTROLOPTS)) {
                  if (inp->inp_options) {
                          m_freem(inp->inp_options);
                          inp->inp_options = NULL;
                  }
                  ip6_savecontrol(inp, &inp->inp_options, ip6, m);
          }
  #endif
  
          if (so->so_options & SO_DEBUG) {
  #ifdef TCP_DEBUG
                  ostate = tp->t_state;
  #endif
  
                  tcp_saveti = NULL;
                  if (iphlen + sizeof(struct tcphdr) > MHLEN)
                          goto nosave;
  
                  if (m->m_len > iphlen && (m->m_flags & M_EXT) == 0) {
                          tcp_saveti = m_copym(m, 0, iphlen, M_DONTWAIT);
                          if (tcp_saveti == NULL)
                                  goto nosave;
                  } else {
                          MGETHDR(tcp_saveti, M_DONTWAIT, MT_HEADER);
                          if (tcp_saveti == NULL)
                                  goto nosave;
                          MCLAIM(m, &tcp_mowner);
                          tcp_saveti->m_len = iphlen;
                          m_copydata(m, 0, iphlen,
                              mtod(tcp_saveti, void *));
                  }
  
                  if (M_TRAILINGSPACE(tcp_saveti) < sizeof(struct tcphdr)) {
                          m_freem(tcp_saveti);
                          tcp_saveti = NULL;
                  } else {
                          tcp_saveti->m_len += sizeof(struct tcphdr);
                          memcpy(mtod(tcp_saveti, char *) + iphlen, th,
                              sizeof(struct tcphdr));
                  }
  nosave:;
          }
  
          if (so->so_options & SO_ACCEPTCONN) {
                  union syn_cache_sa src;
                  union syn_cache_sa dst;
  
                  KASSERT(tp->t_state == TCPS_LISTEN);
  
                  memset(&src, 0, sizeof(src));
                  memset(&dst, 0, sizeof(dst));
                  switch (af) {
                  case AF_INET:
                          src.sin.sin_len = sizeof(struct sockaddr_in);
                          src.sin.sin_family = AF_INET;
                          src.sin.sin_addr = ip->ip_src;
                          src.sin.sin_port = th->th_sport;
  
                          dst.sin.sin_len = sizeof(struct sockaddr_in);
                          dst.sin.sin_family = AF_INET;
                          dst.sin.sin_addr = ip->ip_dst;
                          dst.sin.sin_port = th->th_dport;
                          break;
  #ifdef INET6
                  case AF_INET6:
                          src.sin6.sin6_len = sizeof(struct sockaddr_in6);
                          src.sin6.sin6_family = AF_INET6;
                          src.sin6.sin6_addr = ip6->ip6_src;
                          src.sin6.sin6_port = th->th_sport;
  
                          dst.sin6.sin6_len = sizeof(struct sockaddr_in6);
                          dst.sin6.sin6_family = AF_INET6;
                          dst.sin6.sin6_addr = ip6->ip6_dst;
                          dst.sin6.sin6_port = th->th_dport;
                          break;
  #endif
                  }
  
                  if ((tiflags & (TH_RST|TH_ACK|TH_SYN)) != TH_SYN) {
                          if (tiflags & TH_RST) {
                                  syn_cache_reset(&src.sa, &dst.sa, th);
                          } else if ((tiflags & (TH_ACK|TH_SYN)) ==
                              (TH_ACK|TH_SYN)) {
                                  /*
                                   * Received a SYN,ACK. This should never
                                   * happen while we are in LISTEN. Send an RST.
                                   */
                                  goto badsyn;
                          } else if (tiflags & TH_ACK) {
                                  so = syn_cache_get(&src.sa, &dst.sa, th, so, m);
                                  if (so == NULL) {
                                          /*
                                           * We don't have a SYN for this ACK;
                                           * send an RST.
                                           */
                                          goto badsyn;
                                  } else if (so == (struct socket *)(-1)) {
                                          /*
                                           * We were unable to create the
                                           * connection. If the 3-way handshake
                                           * was completed, and RST has been
                                           * sent to the peer. Since the mbuf
                                           * might be in use for the reply, do
                                           * not free it.
                                           */
                                          m = NULL;
                                  } else {
                                          /*
                                           * We have created a full-blown
                                           * connection.
                                           */
                                          inp = sotoinpcb(so);
                                          tp = intotcpcb(inp);
                                          if (tp == NULL)
                                                  goto badsyn;    /*XXX*/
                                          tiwin <<= tp->snd_scale;
                                          goto after_listen;
                                  }
                          } else {
                                  /*
                                   * None of RST, SYN or ACK was set.
                                   * This is an invalid packet for a
                                   * TCB in LISTEN state.  Send a RST.
                                   */
                                  goto badsyn;
                          }
                  } else {
                          /*
                           * Received a SYN.
                           */
  
  #ifdef INET6
                          /*
                           * If deprecated address is forbidden, we do
                           * not accept SYN to deprecated interface
                           * address to prevent any new inbound
                           * connection from getting established.
                           * When we do not accept SYN, we send a TCP
                           * RST, with deprecated source address (instead
                           * of dropping it).  We compromise it as it is
                           * much better for peer to send a RST, and
                           * RST will be the final packet for the
                           * exchange.
                           *
                           * If we do not forbid deprecated addresses, we
                           * accept the SYN packet.  RFC2462 does not
                           * suggest dropping SYN in this case.
                           * If we decipher RFC2462 5.5.4, it says like
                           * this:
                           * 1. use of deprecated addr with existing
                           *    communication is okay - "SHOULD continue
                           *    to be used"
                           * 2. use of it with new communication:
                           *   (2a) "SHOULD NOT be used if alternate
                           *        address with sufficient scope is
                           *        available"
                           *   (2b) nothing mentioned otherwise.
                           * Here we fall into (2b) case as we have no
                           * choice in our source address selection - we
                           * must obey the peer.
                           *
                           * The wording in RFC2462 is confusing, and
                           * there are multiple description text for
                           * deprecated address handling - worse, they
                           * are not exactly the same.  I believe 5.5.4
                           * is the best one, so we follow 5.5.4.
                           */
                          if (af == AF_INET6 && !ip6_use_deprecated) {
                                  struct in6_ifaddr *ia6;
                                  int s;
                                  struct ifnet *rcvif = m_get_rcvif(m, &s);
                                  if (rcvif == NULL)
                                          goto dropwithreset; /* XXX */
                                  if ((ia6 = in6ifa_ifpwithaddr(rcvif,
                                      &ip6->ip6_dst)) &&
                                      (ia6->ia6_flags & IN6_IFF_DEPRECATED)) {
                                          tp = NULL;
                                          m_put_rcvif(rcvif, &s);
                                          goto dropwithreset;
                                  }
                                  m_put_rcvif(rcvif, &s);
                          }
  #endif
  
                          /*
                           * LISTEN socket received a SYN from itself? This
                           * can't possibly be valid; drop the packet.
                           */
                          if (th->th_sport == th->th_dport) {
                                  int eq = 0;
  
                                  switch (af) {
                                  case AF_INET:
                                          eq = in_hosteq(ip->ip_src, ip->ip_dst);
                                          break;
  #ifdef INET6
                                  case AF_INET6:
                                          eq = IN6_ARE_ADDR_EQUAL(&ip6->ip6_src,
                                              &ip6->ip6_dst);
                                          break;
  #endif
                                  }
                                  if (eq) {
                                          TCP_STATINC(TCP_STAT_BADSYN);
                                          goto drop;
                                  }
                          }
  
                          /*
                           * SYN looks ok; create compressed TCP
                           * state for it.
                           */
                          if (so->so_qlen <= so->so_qlimit &&
                              syn_cache_add(&src.sa, &dst.sa, th, off,
                              so, m, optp, optlen, &opti))
                                  m = NULL;
                  }
  
                  goto drop;
          }
  
  after_listen:
          /*
           * From here on, we're dealing with !LISTEN.
           */
          KASSERT(tp->t_state != TCPS_LISTEN);
  
          /*
           * Segment received on connection.
           * Reset idle time and keep-alive timer.
           */
          tp->t_rcvtime = tcp_now;
          if (TCPS_HAVEESTABLISHED(tp->t_state))
                  TCP_TIMER_ARM(tp, TCPT_KEEP, tp->t_keepidle);
  
          /*
           * Process options.
           */
  #ifdef TCP_SIGNATURE
          if (optp || (tp->t_flags & TF_SIGNATURE))
  #else
          if (optp)
  #endif
                  if (tcp_dooptions(tp, optp, optlen, th, m, off, &opti) < 0)
                          goto drop;
  
          if (TCP_SACK_ENABLED(tp)) {
                  tcp_del_sackholes(tp, th);
          }
  
          if (TCP_ECN_ALLOWED(tp)) {
                  if (tiflags & TH_CWR) {
                          tp->t_flags &= ~TF_ECN_SND_ECE;
                  }
                  switch (iptos & IPTOS_ECN_MASK) {
                  case IPTOS_ECN_CE:
                          tp->t_flags |= TF_ECN_SND_ECE;
                          TCP_STATINC(TCP_STAT_ECN_CE);
                          break;
                  case IPTOS_ECN_ECT0:
                          TCP_STATINC(TCP_STAT_ECN_ECT);
                          break;
                  case IPTOS_ECN_ECT1:
                          /* XXX: ignore for now -- rpaulo */
                          break;
                  }
                  /*
                   * Congestion experienced.
                   * Ignore if we are already trying to recover.
                   */
                  if ((tiflags & TH_ECE) && SEQ_GEQ(tp->snd_una, tp->snd_recover))
                          tp->t_congctl->cong_exp(tp);
          }
  
          if (opti.ts_present && opti.ts_ecr) {
                  /*
                   * Calculate the RTT from the returned time stamp and the
                   * connection's time base.  If the time stamp is later than
                   * the current time, or is extremely old, fall back to non-1323
                   * RTT calculation.  Since ts_rtt is unsigned, we can test both
                   * at the same time.
                   *
                   * Note that ts_rtt is in units of slow ticks (500
                   * ms).  Since most earthbound RTTs are < 500 ms,
                   * observed values will have large quantization noise.
                   * Our smoothed RTT is then the fraction of observed
                   * samples that are 1 tick instead of 0 (times 500
                   * ms).
                   *
                   * ts_rtt is increased by 1 to denote a valid sample,
                   * with 0 indicating an invalid measurement.  This
                   * extra 1 must be removed when ts_rtt is used, or
                   * else an erroneous extra 500 ms will result.
                   */
                  ts_rtt = TCP_TIMESTAMP(tp) - opti.ts_ecr + 1;
                  if (ts_rtt > TCP_PAWS_IDLE)
                          ts_rtt = 0;
          } else {
                  ts_rtt = 0;
          }
  
          /*
           * Fast path: check for the two common cases of a uni-directional
           * data transfer. If:
           *    o We are in the ESTABLISHED state, and
           *    o The packet has no control flags, and
           *    o The packet is in-sequence, and
           *    o The window didn't change, and
           *    o We are not retransmitting
           * It's a candidate.
           *
           * If the length (tlen) is zero and the ack moved forward, we're
           * the sender side of the transfer. Just free the data acked and
           * 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.
           */
          if (tp->t_state == TCPS_ESTABLISHED &&
              (tiflags & (TH_SYN|TH_FIN|TH_RST|TH_URG|TH_ECE|TH_CWR|TH_ACK))
                  == TH_ACK &&
              (!opti.ts_present || TSTMP_GEQ(opti.ts_val, tp->ts_recent)) &&
              th->th_seq == tp->rcv_nxt &&
              tiwin && tiwin == tp->snd_wnd &&
              tp->snd_nxt == tp->snd_max) {
  
                  /*
                   * If last ACK falls within this segment's sequence numbers,
                   * record the timestamp.
                   * NOTE that the test is modified according to the latest
                   * proposal of the tcplw@cray.com list (Braden 1993/04/26).
                   *
                   * note that we already know
                   *      TSTMP_GEQ(opti.ts_val, tp->ts_recent)
                   */
                  if (opti.ts_present && SEQ_LEQ(th->th_seq, tp->last_ack_sent)) {
                          tp->ts_recent_age = tcp_now;
                          tp->ts_recent = opti.ts_val;
                  }
  
                  if (tlen == 0) {
                          /* Ack prediction. */
                          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_partialacks < 0) {
                                  /*
                                   * this is a pure ack for outstanding data.
                                   */
                                  if (ts_rtt)
                                          tcp_xmit_timer(tp, ts_rtt - 1);
                                  else if (tp->t_rtttime &&
                                      SEQ_GT(th->th_ack, tp->t_rtseq))
                                          tcp_xmit_timer(tp,
                                            tcp_now - tp->t_rtttime);
                                  acked = th->th_ack - tp->snd_una;
                                  tcps = TCP_STAT_GETREF();
                                  tcps[TCP_STAT_PREDACK]++;
                                  tcps[TCP_STAT_RCVACKPACK]++;
                                  tcps[TCP_STAT_RCVACKBYTE] += acked;
                                  TCP_STAT_PUTREF();
                                  nd_hint(tp);
  
                                  if (acked > (tp->t_lastoff - tp->t_inoff))
                                          tp->t_lastm = NULL;
                                  sbdrop(&so->so_snd, acked);
                                  tp->t_lastoff -= acked;
  
                                  icmp_check(tp, th, acked);
  
                                  tp->snd_una = th->th_ack;
                                  tp->snd_fack = tp->snd_una;
                                  if (SEQ_LT(tp->snd_high, tp->snd_una))
                                          tp->snd_high = tp->snd_una;
                                  /*
                                   * drag snd_wl2 along so only newer
                                   * ACKs can update the window size.
                                   * also avoids the state where snd_wl2
                                   * is eventually larger than th_ack and thus
                                   * blocking the window update mechanism and
                                   * the connection gets stuck for a loooong
                                   * time in the zero sized send window state.
                                   *
                                   * see PR/kern 55567
                                   */
                                  tp->snd_wl2 = tp->snd_una;
  
                                  m_freem(m);
  
                                  /*
                                   * 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/selnotify/signal.  If data
                                   * are ready to send, let tcp_output
                                   * decide between more output or persist.
                                   */
                                  if (tp->snd_una == tp->snd_max)
                                          TCP_TIMER_DISARM(tp, TCPT_REXMT);
                                  else if (TCP_TIMER_ISARMED(tp,
                                      TCPT_PERSIST) == 0)
                                          TCP_TIMER_ARM(tp, TCPT_REXMT,
                                              tp->t_rxtcur);
  
                                  sowwakeup(so);
                                  if (so->so_snd.sb_cc) {
                                          KERNEL_LOCK(1, NULL);
                                          (void)tcp_output(tp);
                                          KERNEL_UNLOCK_ONE(NULL);
                                  }
                                  if (tcp_saveti)
                                          m_freem(tcp_saveti);
                                  return;
                          }
                  } else if (th->th_ack == tp->snd_una &&
                      TAILQ_FIRST(&tp->segq) == NULL &&
                      tlen <= sbspace(&so->so_rcv)) {
                          int newsize = 0;
  
                          /*
                           * this is a pure, in-sequence data packet
                           * with nothing on the reassembly queue and
                           * we have enough buffer space to take it.
                           */
                          tp->rcv_nxt += tlen;
  
                          /*
                           * Pull rcv_up up to prevent seq wrap relative to
                           * rcv_nxt.
                           */
                          tp->rcv_up = tp->rcv_nxt;
  
                          /*
                           * Pull snd_wl1 up to prevent seq wrap relative to
                           * th_seq.
                           */
                          tp->snd_wl1 = th->th_seq;
  
                          tcps = TCP_STAT_GETREF();
                          tcps[TCP_STAT_PREDDAT]++;
                          tcps[TCP_STAT_RCVPACK]++;
                          tcps[TCP_STAT_RCVBYTE] += tlen;
                          TCP_STAT_PUTREF();
                          nd_hint(tp);
                  /*
                   * Automatic sizing enables the performance of large buffers
                   * and most of the efficiency of small ones by only allocating
                   * space when it is needed.
                   *
                   * On the receive side the socket buffer memory is only rarely
                   * used to any significant extent.  This allows us to be much
                   * more aggressive in scaling the receive socket buffer.  For
                   * the case that the buffer space is actually used to a large
                   * extent and we run out of kernel memory we can simply drop
                   * the new segments; TCP on the sender will just retransmit it
                   * later.  Setting the buffer size too big may only consume too
                   * much kernel memory if the application doesn't read() from
                   * the socket or packet loss or reordering makes use of the
                   * reassembly queue.
                   *
                   * The criteria to step up the receive buffer one notch are:
                   *  1. the number of bytes received during the time it takes
                   *     one timestamp to be reflected back to us (the RTT);
                   *  2. received bytes per RTT is within seven eighth of the
                   *     current socket buffer size;
                   *  3. receive buffer size has not hit maximal automatic size;
                   *
                   * This algorithm does one step per RTT at most and only if
                   * we receive a bulk stream w/o packet losses or reorderings.
                   * Shrinking the buffer during idle times is not necessary as
                   * it doesn't consume any memory when idle.
                   *
                   * TODO: Only step up if the application is actually serving
                   * the buffer to better manage the socket buffer resources.
                   */
                          if (tcp_do_autorcvbuf &&
                              opti.ts_ecr &&
                              (so->so_rcv.sb_flags & SB_AUTOSIZE)) {
                                  if (opti.ts_ecr > tp->rfbuf_ts &&
                                      opti.ts_ecr - tp->rfbuf_ts < PR_SLOWHZ) {
                                          if (tp->rfbuf_cnt >
                                              (so->so_rcv.sb_hiwat / 8 * 7) &&
                                              so->so_rcv.sb_hiwat <
                                              tcp_autorcvbuf_max) {
                                                  newsize =
                                                      uimin(so->so_rcv.sb_hiwat +
                                                      tcp_autorcvbuf_inc,
                                                      tcp_autorcvbuf_max);
                                          }
                                          /* Start over with next RTT. */
                                          tp->rfbuf_ts = 0;
                                          tp->rfbuf_cnt = 0;
                                  } else
                                          tp->rfbuf_cnt += tlen;  /* add up */
                          }
  
                          /*
                           * Drop TCP, IP headers and TCP options then add data
                           * to socket buffer.
                           */
                          if (so->so_state & SS_CANTRCVMORE) {
                                  m_freem(m);
                          } else {
                                  /*
                                   * Set new socket buffer size.
                                   * Give up when limit is reached.
                                   */
                                  if (newsize)
                                          if (!sbreserve(&so->so_rcv,
                                              newsize, so))
                                                  so->so_rcv.sb_flags &= ~SB_AUTOSIZE;
                                  m_adj(m, off + thlen);
                                  sbappendstream(&so->so_rcv, m);
                          }
                          sorwakeup(so);
                          tcp_setup_ack(tp, th);
                          if (tp->t_flags & TF_ACKNOW) {
                                  KERNEL_LOCK(1, NULL);
                                  (void)tcp_output(tp);
                                  KERNEL_UNLOCK_ONE(NULL);
                          }
                          if (tcp_saveti)
                                  m_freem(tcp_saveti);
                          return;
                  }
          }
  
          /*
           * Compute mbuf offset to TCP data segment.
           */
          hdroptlen = off + thlen;
  
          /*
           * Calculate amount of space in receive window. 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));
          }
  
          /* Reset receive buffer auto scaling when not in bulk receive mode. */
          tp->rfbuf_ts = 0;
          tp->rfbuf_cnt = 0;
  
          switch (tp->t_state) {
          /*
           * 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 seg contains a ECE and ECN support is enabled, the stream
           *          is ECN capable.
           *      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 ((tiflags & TH_ACK) &&
                      (SEQ_LEQ(th->th_ack, tp->iss) ||
                       SEQ_GT(th->th_ack, tp->snd_max)))
                          goto dropwithreset;
                  if (tiflags & TH_RST) {
                          if (tiflags & TH_ACK)
                                  tp = tcp_drop(tp, ECONNREFUSED);
                          goto drop;
                  }
                  if ((tiflags & TH_SYN) == 0)
                          goto drop;
                  if (tiflags & TH_ACK) {
                          tp->snd_una = th->th_ack;
                          if (SEQ_LT(tp->snd_nxt, tp->snd_una))
                                  tp->snd_nxt = tp->snd_una;
                          if (SEQ_LT(tp->snd_high, tp->snd_una))
                                  tp->snd_high = tp->snd_una;
                          TCP_TIMER_DISARM(tp, TCPT_REXMT);
  
                          if ((tiflags & TH_ECE) && tcp_do_ecn) {
                                  tp->t_flags |= TF_ECN_PERMIT;
                                  TCP_STATINC(TCP_STAT_ECN_SHS);
                          }
                  }
                  tp->irs = th->th_seq;
                  tcp_rcvseqinit(tp);
                  tp->t_flags |= TF_ACKNOW;
                  tcp_mss_from_peer(tp, opti.maxseg);
  
                  /*
                   * Initialize the initial congestion window.  If we
                   * had to retransmit the SYN, we must initialize cwnd
                   * to 1 segment (i.e. the Loss Window).
                   */
                  if (tp->t_flags & TF_SYN_REXMT)
                          tp->snd_cwnd = tp->t_peermss;
                  else {
                          int ss = tcp_init_win;
                          if (inp->inp_af == AF_INET && in_localaddr(in4p_faddr(inp)))
                                  ss = tcp_init_win_local;
  #ifdef INET6
                          else if (inp->inp_af == AF_INET6 && in6_localaddr(&in6p_faddr(inp)))
                                  ss = tcp_init_win_local;
  #endif
                          tp->snd_cwnd = TCP_INITIAL_WINDOW(ss, tp->t_peermss);
                  }
  
                  tcp_rmx_rtt(tp);
                  if (tiflags & TH_ACK) {
                          TCP_STATINC(TCP_STAT_CONNECTS);
                          /*
                           * move tcp_established before soisconnected
                           * because upcall handler can drive tcp_output
                           * functionality.
                           * XXX we might call soisconnected at the end of
                           * all processing
                           */
                          tcp_established(tp);
                          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;
                          }
                          TCP_REASS_LOCK(tp);
                          (void)tcp_reass(tp, NULL, NULL, tlen);
                          /*
                           * if we didn't have to retransmit the SYN,
                           * use its rtt as our initial srtt & rtt var.
                           */
                          if (tp->t_rtttime)
                                  tcp_xmit_timer(tp, tcp_now - tp->t_rtttime);
                  } else {
                          tp->t_state = TCPS_SYN_RECEIVED;
                  }
  
                  /*
                   * Advance th->th_seq to correspond to first data byte.
                   * If data, trim to stay within window,
                   * dropping FIN if necessary.
                   */
                  th->th_seq++;
                  if (tlen > tp->rcv_wnd) {
                          todrop = tlen - tp->rcv_wnd;
                          m_adj(m, -todrop);
                          tlen = tp->rcv_wnd;
                          tiflags &= ~TH_FIN;
                          tcps = TCP_STAT_GETREF();
                          tcps[TCP_STAT_RCVPACKAFTERWIN]++;
                          tcps[TCP_STAT_RCVBYTEAFTERWIN] += todrop;
                          TCP_STAT_PUTREF();
                  }
                  tp->snd_wl1 = th->th_seq - 1;
                  tp->rcv_up = th->th_seq;
                  goto step6;
  
          /*
           * If the state is SYN_RECEIVED:
           *      If seg contains an ACK, but not for our SYN, drop the input
           *      and generate an RST.  See page 36, rfc793
           */
          case TCPS_SYN_RECEIVED:
                  if ((tiflags & TH_ACK) &&
                      (SEQ_LEQ(th->th_ack, tp->iss) ||
                       SEQ_GT(th->th_ack, tp->snd_max)))
                          goto dropwithreset;
                  break;
          }
  
          /*
           * From here on, we're dealing with !LISTEN and !SYN_SENT.
           */
          KASSERT(tp->t_state != TCPS_LISTEN &&
              tp->t_state != TCPS_SYN_SENT);
  
          /*
           * RFC1323 PAWS: if we have a timestamp reply on this segment and
           * it's less than ts_recent, drop it.
           */
          if (opti.ts_present && (tiflags & TH_RST) == 0 && tp->ts_recent &&
              TSTMP_LT(opti.ts_val, tp->ts_recent)) {
                  /* Check to see if ts_recent is over 24 days old.  */
                  if (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 {
                          tcps = TCP_STAT_GETREF();
                          tcps[TCP_STAT_RCVDUPPACK]++;
                          tcps[TCP_STAT_RCVDUPBYTE] += tlen;
                          tcps[TCP_STAT_PAWSDROP]++;
                          TCP_STAT_PUTREF();
                          tcp_new_dsack(tp, th->th_seq, tlen);
                          goto dropafterack;
                  }
          }
  
          /*
           * Check that at least some bytes of the segment are within the
           * receive window. If segment begins before rcv_nxt, drop leading
           * data (and SYN); if nothing left, just ack.
           */
          todrop = tp->rcv_nxt - th->th_seq;
          dupseg = false;
          if (todrop > 0) {
                  if (tiflags & TH_SYN) {
                          tiflags &= ~TH_SYN;
                          th->th_seq++;
                          tcp_urp_drop(th, 1, &tiflags);
                          todrop--;
                  }
                  if (todrop > tlen ||
                      (todrop == tlen && (tiflags & TH_FIN) == 0)) {
                          /*
                           * Any valid FIN or RST must be to the left of the
                           * window.  At this point the FIN or RST must be a
                           * duplicate or out of sequence; drop it.
                           */
                          if (tiflags & TH_RST)
                                  goto drop;
                          tiflags &= ~(TH_FIN|TH_RST);
  
                          /*
                           * Send an ACK to resynchronize and drop any data.
                           * But keep on processing for RST or ACK.
                           */
                          tp->t_flags |= TF_ACKNOW;
                          todrop = tlen;
                          dupseg = true;
                          tcps = TCP_STAT_GETREF();
                          tcps[TCP_STAT_RCVDUPPACK]++;
                          tcps[TCP_STAT_RCVDUPBYTE] += todrop;
                          TCP_STAT_PUTREF();
                  } else if ((tiflags & TH_RST) && th->th_seq != tp->rcv_nxt) {
                          /*
                           * Test for reset before adjusting the sequence
                           * number for overlapping data.
                           */
                          goto dropafterack_ratelim;
                  } else {
                          tcps = TCP_STAT_GETREF();
                          tcps[TCP_STAT_RCVPARTDUPPACK]++;
                          tcps[TCP_STAT_RCVPARTDUPBYTE] += todrop;
                          TCP_STAT_PUTREF();
                  }
                  tcp_new_dsack(tp, th->th_seq, todrop);
                  hdroptlen += todrop;    /* drop from head afterwards (m_adj) */
                  th->th_seq += todrop;
                  tlen -= todrop;
                  tcp_urp_drop(th, todrop, &tiflags);
          }
  
          /*
           * If new data is received on a connection after the user processes
           * are gone, then RST the other end.
           */
          if ((so->so_state & SS_NOFDREF) &&
              tp->t_state > TCPS_CLOSE_WAIT && tlen) {
                  tp = tcp_close(tp);
                  TCP_STATINC(TCP_STAT_RCVAFTERCLOSE);
                  goto dropwithreset;
          }
  
          /*
           * If the segment ends after the window, drop trailing data (and
           * PUSH and FIN); if nothing left, just ACK.
           */
          todrop = (th->th_seq + tlen) - (tp->rcv_nxt + tp->rcv_wnd);
          if (todrop > 0) {
                  TCP_STATINC(TCP_STAT_RCVPACKAFTERWIN);
                  if (todrop >= tlen) {
                          /*
                           * The segment actually starts after the window.
                           * th->th_seq + tlen - tp->rcv_nxt - tp->rcv_wnd >= tlen
                           * th->th_seq - tp->rcv_nxt - tp->rcv_wnd >= 0
                           * th->th_seq >= tp->rcv_nxt + tp->rcv_wnd
                           */
                          TCP_STATADD(TCP_STAT_RCVBYTEAFTERWIN, tlen);
  
                          /*
                           * If a new connection request is received while in
                           * TIME_WAIT, drop the old connection and start over
                           * if the sequence numbers are above the previous
                           * ones.
                           *
                           * NOTE: We need to put the header fields back into
                           * network order.
                           */
                          if ((tiflags & TH_SYN) &&
                              tp->t_state == TCPS_TIME_WAIT &&
                              SEQ_GT(th->th_seq, tp->rcv_nxt)) {
                                  tp = tcp_close(tp);
                                  tcp_fields_to_net(th);
                                  m_freem(tcp_saveti);
                                  tcp_saveti = NULL;
                                  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 (if not RST) ack.
                           */
                          if (tp->rcv_wnd == 0 && th->th_seq == tp->rcv_nxt) {
                                  KASSERT(todrop == tlen);
                                  tp->t_flags |= TF_ACKNOW;
                                  TCP_STATINC(TCP_STAT_RCVWINPROBE);
                          } else {
                                  goto dropafterack;
                          }
                  } else {
                          TCP_STATADD(TCP_STAT_RCVBYTEAFTERWIN, todrop);
                  }
                  m_adj(m, -todrop);
                  tlen -= todrop;
                  tiflags &= ~(TH_PUSH|TH_FIN);
          }
  
          /*
           * If last ACK falls within this segment's sequence numbers,
           *  record the timestamp.
           * NOTE: 
           * 1) That the test incorporates suggestions from the latest
           *    proposal of the tcplw@cray.com list (Braden 1993/04/26).
           * 2) That updating only on newer timestamps interferes with
           *    our earlier PAWS tests, so this check should be solely
           *    predicated on the sequence space of this segment.
           * 3) That we modify the segment boundary check to be 
           *        Last.ACK.Sent <= SEG.SEQ + SEG.Len  
           *    instead of RFC1323's
           *        Last.ACK.Sent < SEG.SEQ + SEG.Len,
           *    This modified check allows us to overcome RFC1323's
           *    limitations as described in Stevens TCP/IP Illustrated
           *    Vol. 2 p.869. In such cases, we can still calculate the
           *    RTT correctly when RCV.NXT == Last.ACK.Sent.
           */
          if (opti.ts_present &&
              SEQ_LEQ(th->th_seq, tp->last_ack_sent) &&
              SEQ_LEQ(tp->last_ack_sent, th->th_seq + tlen +
                   ((tiflags & (TH_SYN|TH_FIN)) != 0))) {
                  tp->ts_recent_age = tcp_now;
                  tp->ts_recent = opti.ts_val;
          }
  
          /*
           * If the RST bit is set examine the state:
           *    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.
           */
          if (tiflags & TH_RST) {
                  if (th->th_seq != tp->rcv_nxt)
                          goto dropafterack_ratelim;
  
                  switch (tp->t_state) {
                  case TCPS_SYN_RECEIVED:
                          so->so_error = ECONNREFUSED;
                          goto close;
  
                  case TCPS_ESTABLISHED:
                  case TCPS_FIN_WAIT_1:
                  case TCPS_FIN_WAIT_2:
                  case TCPS_CLOSE_WAIT:
                          so->so_error = ECONNRESET;
                  close:
                          tp->t_state = TCPS_CLOSED;
                          TCP_STATINC(TCP_STAT_DROPS);
                          tp = tcp_close(tp);
                          goto drop;
  
                  case TCPS_CLOSING:
                  case TCPS_LAST_ACK:
                  case TCPS_TIME_WAIT:
                          tp = tcp_close(tp);
                          goto drop;
                  }
          }
  
          /*
           * Since we've covered the SYN-SENT and SYN-RECEIVED states above
           * we must be in a synchronized state.  RFC793 states (under Reset
           * Generation) that any unacceptable segment (an out-of-order SYN
           * qualifies) received in a synchronized state must elicit only an
           * empty acknowledgment segment ... and the connection remains in
           * the same state.
           */
          if (tiflags & TH_SYN) {
                  if (tp->rcv_nxt == th->th_seq) {
                          tcp_respond(tp, m, m, th, (tcp_seq)0, th->th_ack - 1,
                              TH_ACK);
                          if (tcp_saveti)
                                  m_freem(tcp_saveti);
                          return;
                  }
  
                  goto dropafterack_ratelim;
          }
  
          /*
           * If the ACK bit is off we drop the segment and return.
           */
          if ((tiflags & TH_ACK) == 0) {
                  if (tp->t_flags & TF_ACKNOW)
                          goto dropafterack;
                  goto drop;
          }
  
          /*
           * From here on, we're doing ACK processing.
           */
  
          switch (tp->t_state) {
          /*
           * In SYN_RECEIVED state if the ack ACKs our SYN then enter
           * ESTABLISHED state and continue processing, otherwise
           * send an RST.
           */
          case TCPS_SYN_RECEIVED:
                  if (SEQ_GT(tp->snd_una, th->th_ack) ||
                      SEQ_GT(th->th_ack, tp->snd_max))
                          goto dropwithreset;
                  TCP_STATINC(TCP_STAT_CONNECTS);
                  soisconnected(so);
                  tcp_established(tp);
                  /* 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;
                  }
                  TCP_REASS_LOCK(tp);
                  (void)tcp_reass(tp, NULL, NULL, tlen);
                  tp->snd_wl1 = th->th_seq - 1;
                  /* FALLTHROUGH */
  
          /*
           * In ESTABLISHED state: drop duplicate ACKs; ACK out of range
           * ACKs.  If the ack is in the range
           *      tp->snd_una < th->th_ack <= tp->snd_max
           * then advance tp->snd_una to th->th_ack and drop
           * data from the retransmission queue.  If this ACK reflects
           * more up to date window information we update our window information.
           */
          case TCPS_ESTABLISHED:
          case TCPS_FIN_WAIT_1:
          case TCPS_FIN_WAIT_2:
          case TCPS_CLOSE_WAIT:
          case TCPS_CLOSING:
          case TCPS_LAST_ACK:
          case TCPS_TIME_WAIT:
                  if (SEQ_LEQ(th->th_ack, tp->snd_una)) {
                          if (tlen == 0 && !dupseg && tiwin == tp->snd_wnd) {
                                  TCP_STATINC(TCP_STAT_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
                                   * threshold of them, assume a packet
                                   * has been dropped and retransmit it.
                                   * Kludge snd_nxt & the congestion
                                   * window so we send only this one
                                   * packet.
                                   */
                                  if (TCP_TIMER_ISARMED(tp, TCPT_REXMT) == 0 ||
                                      th->th_ack != tp->snd_una)
                                          tp->t_dupacks = 0;
                                  else if (tp->t_partialacks < 0 &&
                                      (++tp->t_dupacks == tcprexmtthresh ||
                                       TCP_FACK_FASTRECOV(tp))) {
                                          /*
                                           * Do the fast retransmit, and adjust
                                           * congestion control parameters.
                                           */
                                          if (tp->t_congctl->fast_retransmit(tp, th)) {
                                                  /* False fast retransmit */
                                                  break;
                                          }
                                          goto drop;
                                  } else if (tp->t_dupacks > tcprexmtthresh) {
                                          tp->snd_cwnd += tp->t_segsz;
                                          KERNEL_LOCK(1, NULL);
                                          (void)tcp_output(tp);
                                          KERNEL_UNLOCK_ONE(NULL);
                                          goto drop;
                                  }
                          } else {
                                  /*
                                   * If the ack appears to be very old, only
                                   * allow data that is in-sequence.  This
                                   * makes it somewhat more difficult to insert
                                   * forged data by guessing sequence numbers.
                                   * Sent an ack to try to update the send
                                   * sequence number on the other side.
                                   */
                                  if (tlen && th->th_seq != tp->rcv_nxt &&
                                      SEQ_LT(th->th_ack,
                                      tp->snd_una - tp->max_sndwnd))
                                          goto dropafterack;
                          }
                          break;
                  }
                  /*
                   * If the congestion window was inflated to account
                   * for the other side's cached packets, retract it.
                   */
                  tp->t_congctl->fast_retransmit_newack(tp, th);
  
                  if (SEQ_GT(th->th_ack, tp->snd_max)) {
                          TCP_STATINC(TCP_STAT_RCVACKTOOMUCH);
                          goto dropafterack;
                  }
                  acked = th->th_ack - tp->snd_una;
                  tcps = TCP_STAT_GETREF();
                  tcps[TCP_STAT_RCVACKPACK]++;
                  tcps[TCP_STAT_RCVACKBYTE] += acked;
                  TCP_STAT_PUTREF();
  
                  /*
                   * 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 (ts_rtt)
                          tcp_xmit_timer(tp, ts_rtt - 1);
                  else if (tp->t_rtttime && SEQ_GT(th->th_ack, tp->t_rtseq))
                          tcp_xmit_timer(tp, tcp_now - tp->t_rtttime);
  
                  /*
                   * If all outstanding data is acked, stop retransmit
                   * timer and remember to restart (more output or persist).
                   * If there is more data to be acked, restart retransmit
                   * timer, using current (possibly backed-off) value.
                   */
                  if (th->th_ack == tp->snd_max) {
                          TCP_TIMER_DISARM(tp, TCPT_REXMT);
                          needoutput = 1;
                  } else if (TCP_TIMER_ISARMED(tp, TCPT_PERSIST) == 0)
                          TCP_TIMER_ARM(tp, TCPT_REXMT, tp->t_rxtcur);
  
                  /*
                   * New data has been acked, adjust the congestion window.
                   */
                  tp->t_congctl->newack(tp, th);
  
                  nd_hint(tp);
                  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 {
                          if (acked > (tp->t_lastoff - tp->t_inoff))
                                  tp->t_lastm = NULL;
                          sbdrop(&so->so_snd, acked);
                          tp->t_lastoff -= acked;
                          if (tp->snd_wnd > acked)
                                  tp->snd_wnd -= acked;
                          else
                                  tp->snd_wnd = 0;
                          ourfinisacked = 0;
                  }
                  sowwakeup(so);
  
                  icmp_check(tp, th, acked);
  
                  tp->snd_una = th->th_ack;
                  if (SEQ_GT(tp->snd_una, tp->snd_fack))
                          tp->snd_fack = tp->snd_una;
                  if (SEQ_LT(tp->snd_nxt, tp->snd_una))
                          tp->snd_nxt = tp->snd_una;
                  if (SEQ_LT(tp->snd_high, tp->snd_una))
                          tp->snd_high = 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);
                                          if (tp->t_maxidle > 0)
                                                  TCP_TIMER_ARM(tp, TCPT_2MSL,
                                                      tp->t_maxidle);
                                  }
                                  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);
                                  TCP_TIMER_ARM(tp, TCPT_2MSL, 2 * tp->t_msl);
                                  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:
                          TCP_TIMER_ARM(tp, TCPT_2MSL, 2 * tp->t_msl);
                          goto dropafterack;
                  }
          }
  
  step6:
          /*
           * Update window information.
           * Don't look at window if no ACK: TAC's send garbage on first SYN.
           */
          if ((tiflags & TH_ACK) && (SEQ_LT(tp->snd_wl1, th->th_seq) ||
              (tp->snd_wl1 == th->th_seq && (SEQ_LT(tp->snd_wl2, th->th_ack) ||
              (tp->snd_wl2 == th->th_ack && tiwin > tp->snd_wnd))))) {
                  /* keep track of pure window updates */
                  if (tlen == 0 &&
                      tp->snd_wl2 == th->th_ack && tiwin > tp->snd_wnd)
                          TCP_STATINC(TCP_STAT_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 ((tiflags & 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 */
                          tiflags &= ~TH_URG;             /* XXX */
                          goto dodata;                    /* XXX */
                  }
  
                  /*
                   * If this segment advances the known urgent pointer,
                   * then mark the data stream.  This should not happen
                   * in CLOSE_WAIT, CLOSING, LAST_ACK or TIME_WAIT STATES since
                   * a FIN has been received from the remote side.
                   * In these states we ignore the URG.
                   *
                   * According to RFC961 (Assigned Protocols),
                   * the urgent pointer points to the last octet
                   * of urgent data.  We continue, however,
                   * to consider it to indicate the first octet
                   * of data past the urgent section as the original
                   * spec states (in one of two places).
                   */
                  if (SEQ_GT(th->th_seq+th->th_urp, tp->rcv_up)) {
                          tp->rcv_up = th->th_seq + th->th_urp;
                          so->so_oobmark = so->so_rcv.sb_cc +
                              (tp->rcv_up - tp->rcv_nxt) - 1;
                          if (so->so_oobmark == 0)
                                  so->so_state |= SS_RCVATMARK;
                          sohasoutofband(so);
                          tp->t_oobflags &= ~(TCPOOB_HAVEDATA | TCPOOB_HADDATA);
                  }
  
                  /*
                   * Remove out of band data so doesn't get presented to user.
                   * This can happen independent of advancing the URG pointer,
                   * but if two URG's are pending at once, some out-of-band
                   * data may creep in... ick.
                   */
                  if (th->th_urp <= (u_int16_t)tlen &&
                      (so->so_options & SO_OOBINLINE) == 0)
                          tcp_pulloutofband(so, th, m, hdroptlen);
          } 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:
  
          /*
           * Process the segment text, merging it into the TCP sequencing queue,
           * and arranging for acknowledgement 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,
           * tcp_rcvd()).  If a FIN has already been received on this
           * connection then we just ignore the text.
           */
          if ((tlen || (tiflags & TH_FIN)) &&
              TCPS_HAVERCVDFIN(tp->t_state) == 0) {
                  /*
                   * Handle the common case:
                   *  o Segment is the next to be received, and
                   *  o The queue is empty, and
                   *  o The connection is established
                   * In this case, we avoid calling tcp_reass.
                   *
                   * tcp_setup_ack: set DELACK for segments received in order,
                   * but ack immediately when segments are out of order (so that
                   * fast retransmit can work).
                   */
                  TCP_REASS_LOCK(tp);
                  if (th->th_seq == tp->rcv_nxt &&
                      TAILQ_FIRST(&tp->segq) == NULL &&
                      tp->t_state == TCPS_ESTABLISHED) {
                          tcp_setup_ack(tp, th);
                          tp->rcv_nxt += tlen;
                          tiflags = th->th_flags & TH_FIN;
                          tcps = TCP_STAT_GETREF();
                          tcps[TCP_STAT_RCVPACK]++;
                          tcps[TCP_STAT_RCVBYTE] += tlen;
                          TCP_STAT_PUTREF();
                          nd_hint(tp);
                          if (so->so_state & SS_CANTRCVMORE) {
                                  m_freem(m);
                          } else {
                                  m_adj(m, hdroptlen);
                                  sbappendstream(&(so)->so_rcv, m);
                          }
                          TCP_REASS_UNLOCK(tp);
                          sorwakeup(so);
                  } else {
                          m_adj(m, hdroptlen);
                          tiflags = tcp_reass(tp, th, m, tlen);
                          tp->t_flags |= TF_ACKNOW;
                  }
  
                  /*
                   * 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);
                  m = NULL;
                  tiflags &= ~TH_FIN;
          }
  
          /*
           * If FIN is received ACK the FIN and let the user know
           * that the connection is closing.  Ignore a FIN received before
           * the connection is fully established.
           */
          if ((tiflags & TH_FIN) && TCPS_HAVEESTABLISHED(tp->t_state)) {
                  if (TCPS_HAVERCVDFIN(tp->t_state) == 0) {
                          socantrcvmore(so);
                          tp->t_flags |= TF_ACKNOW;
                          tp->rcv_nxt++;
                  }
                  switch (tp->t_state) {
  
                  /*
                   * In ESTABLISHED STATE enter the CLOSE_WAIT state.
                   */
                  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);
                          TCP_TIMER_ARM(tp, TCPT_2MSL, 2 * tp->t_msl);
                          soisdisconnected(so);
                          break;
  
                  /*
                   * In TIME_WAIT state restart the 2 MSL time_wait timer.
                   */
                  case TCPS_TIME_WAIT:
                          TCP_TIMER_ARM(tp, TCPT_2MSL, 2 * tp->t_msl);
                          break;
                  }
          }
  #ifdef TCP_DEBUG
          if (so->so_options & SO_DEBUG)
                  tcp_trace(TA_INPUT, ostate, tp, tcp_saveti, 0);
  #endif
  
          /*
           * Return any desired output.
           */
          if (needoutput || (tp->t_flags & TF_ACKNOW)) {
                  KERNEL_LOCK(1, NULL);
                  (void)tcp_output(tp);
                  KERNEL_UNLOCK_ONE(NULL);
          }
          if (tcp_saveti)
                  m_freem(tcp_saveti);
  
          if (tp->t_state == TCPS_TIME_WAIT
              && (so->so_state & SS_NOFDREF)
              && (tp->t_inpcb || af != AF_INET || af != AF_INET6)
              && ((af == AF_INET ? tcp4_vtw_enable : tcp6_vtw_enable) & 1) != 0
              && TAILQ_EMPTY(&tp->segq)
              && vtw_add(af, tp)) {
                  ;
          }
          return;
  
  badsyn:
          /*
           * Received a bad SYN.  Increment counters and dropwithreset.
           */
          TCP_STATINC(TCP_STAT_BADSYN);
          tp = NULL;
          goto dropwithreset;
  
  dropafterack:
          /*
           * Generate an ACK dropping incoming segment if it occupies
           * sequence space, where the ACK reflects our state.
           */
          if (tiflags & TH_RST)
                  goto drop;
          goto dropafterack2;
  
  dropafterack_ratelim:
          /*
           * We may want to rate-limit ACKs against SYN/RST attack.
           */
          if (ppsratecheck(&tcp_ackdrop_ppslim_last, &tcp_ackdrop_ppslim_count,
              tcp_ackdrop_ppslim) == 0) {
                  /* XXX stat */
                  goto drop;
          }
  
  dropafterack2:
          m_freem(m);
          tp->t_flags |= TF_ACKNOW;
          KERNEL_LOCK(1, NULL);
          (void)tcp_output(tp);
          KERNEL_UNLOCK_ONE(NULL);
          if (tcp_saveti)
                  m_freem(tcp_saveti);
          return;
  
  dropwithreset_ratelim:
          /*
           * We may want to rate-limit RSTs in certain situations,
           * particularly if we are sending an RST in response to
           * an attempt to connect to or otherwise communicate with
           * a port for which we have no socket.
           */
          if (ppsratecheck(&tcp_rst_ppslim_last, &tcp_rst_ppslim_count,
              tcp_rst_ppslim) == 0) {
                  /* XXX stat */
                  goto drop;
          }
  
  dropwithreset:
          /*
           * Generate a RST, dropping incoming segment.
           * Make ACK acceptable to originator of segment.
           */
          if (tiflags & TH_RST)
                  goto drop;
          if (tiflags & TH_ACK) {
                  (void)tcp_respond(tp, m, m, th, (tcp_seq)0, th->th_ack, TH_RST);
          } else {
                  if (tiflags & TH_SYN)
                          tlen++;
                  (void)tcp_respond(tp, m, m, th, th->th_seq + tlen, (tcp_seq)0,
                      TH_RST|TH_ACK);
          }
          if (tcp_saveti)
                  m_freem(tcp_saveti);
          return;
  
  badcsum:
  drop:
          /*
           * Drop space held by incoming segment and return.
           */
          if (tp) {
                  so = tp->t_inpcb->inp_socket;
  #ifdef TCP_DEBUG
                  if (so && (so->so_options & SO_DEBUG) != 0)
                          tcp_trace(TA_DROP, ostate, tp, tcp_saveti, 0);
  #endif
          }
          if (tcp_saveti)
                  m_freem(tcp_saveti);
          m_freem(m);
          return;
  }
  
  #ifdef TCP_SIGNATURE
  int
  tcp_signature_apply(void *fstate, void *data, u_int len)
  {
  
          MD5Update(fstate, (u_char *)data, len);
          return (0);
  }
  
  struct secasvar *
  tcp_signature_getsav(struct mbuf *m)
  {
          struct ip *ip;
          struct ip6_hdr *ip6;
  
          ip = mtod(m, struct ip *);
          switch (ip->ip_v) {
          case 4:
                  ip = mtod(m, struct ip *);
                  ip6 = NULL;
                  break;
          case 6:
                  ip = NULL;
                  ip6 = mtod(m, struct ip6_hdr *);
                  break;
          default:
                  return (NULL);
          }
  
  #ifdef IPSEC
          union sockaddr_union dst;
  
          /* Extract the destination from the IP header in the mbuf. */
          memset(&dst, 0, sizeof(union sockaddr_union));
          if (ip != NULL) {
                  dst.sa.sa_len = sizeof(struct sockaddr_in);
                  dst.sa.sa_family = AF_INET;
                  dst.sin.sin_addr = ip->ip_dst;
          } else {
                  dst.sa.sa_len = sizeof(struct sockaddr_in6);
                  dst.sa.sa_family = AF_INET6;
                  dst.sin6.sin6_addr = ip6->ip6_dst;
          }
  
          /*
           * Look up an SADB entry which matches the address of the peer.
           */
          return KEY_LOOKUP_SA(&dst, IPPROTO_TCP, htonl(TCP_SIG_SPI), 0, 0);
  #else
          return NULL;
  #endif
  }
  
  int
  tcp_signature(struct mbuf *m, struct tcphdr *th, int thoff,
      struct secasvar *sav, char *sig)
  {
          MD5_CTX ctx;
          struct ip *ip;
          struct ipovly *ipovly;
  #ifdef INET6
          struct ip6_hdr *ip6;
          struct ip6_hdr_pseudo ip6pseudo;
  #endif
          struct ippseudo ippseudo;
          struct tcphdr th0;
          int l, tcphdrlen;
  
          if (sav == NULL)
                  return (-1);
  
          tcphdrlen = th->th_off * 4;
  
          switch (mtod(m, struct ip *)->ip_v) {
          case 4:
                  MD5Init(&ctx);
                  ip = mtod(m, struct ip *);
                  memset(&ippseudo, 0, sizeof(ippseudo));
                  ipovly = (struct ipovly *)ip;
                  ippseudo.ippseudo_src = ipovly->ih_src;
                  ippseudo.ippseudo_dst = ipovly->ih_dst;
                  ippseudo.ippseudo_pad = 0;
                  ippseudo.ippseudo_p = IPPROTO_TCP;
                  ippseudo.ippseudo_len = htons(m->m_pkthdr.len - thoff);
                  MD5Update(&ctx, (char *)&ippseudo, sizeof(ippseudo));
                  break;
  #if INET6
          case 6:
                  MD5Init(&ctx);
                  ip6 = mtod(m, struct ip6_hdr *);
                  memset(&ip6pseudo, 0, sizeof(ip6pseudo));
                  ip6pseudo.ip6ph_src = ip6->ip6_src;
                  in6_clearscope(&ip6pseudo.ip6ph_src);
                  ip6pseudo.ip6ph_dst = ip6->ip6_dst;
                  in6_clearscope(&ip6pseudo.ip6ph_dst);
                  ip6pseudo.ip6ph_len = htons(m->m_pkthdr.len - thoff);
                  ip6pseudo.ip6ph_nxt = IPPROTO_TCP;
                  MD5Update(&ctx, (char *)&ip6pseudo, sizeof(ip6pseudo));
                  break;
  #endif
          default:
                  return (-1);
          }
  
          th0 = *th;
          th0.th_sum = 0;
          MD5Update(&ctx, (char *)&th0, sizeof(th0));
  
          l = m->m_pkthdr.len - thoff - tcphdrlen;
          if (l > 0)
                  m_apply(m, thoff + tcphdrlen,
                      m->m_pkthdr.len - thoff - tcphdrlen,
                      tcp_signature_apply, &ctx);
  
          MD5Update(&ctx, _KEYBUF(sav->key_auth), _KEYLEN(sav->key_auth));
          MD5Final(sig, &ctx);
  
          return (0);
  }
  #endif
  
  /*
   * Parse and process tcp options.
   *
   * Returns -1 if this segment should be dropped.  (eg. wrong signature)
   * Otherwise returns 0.
   */
  int
  tcp_dooptions(struct tcpcb *tp, const u_char *cp, int cnt, struct tcphdr *th,
      struct mbuf *m, int toff, struct tcp_opt_info *oi)
  {
          u_int16_t mss;
          int opt, optlen = 0;
  #ifdef TCP_SIGNATURE
          void *sigp = NULL;
          char sigbuf[TCP_SIGLEN];
          struct secasvar *sav = NULL;
  #endif
  
          for (; cp && cnt > 0; cnt -= optlen, cp += optlen) {
                  opt = cp[0];
                  if (opt == TCPOPT_EOL)
                          break;
                  if (opt == TCPOPT_NOP)
                          optlen = 1;
                  else {
                          if (cnt < 2)
                                  break;
                          optlen = cp[1];
                          if (optlen < 2 || optlen > cnt)
                                  break;
                  }
                  switch (opt) {
  
                  default:
                          continue;
  
                  case TCPOPT_MAXSEG:
                          if (optlen != TCPOLEN_MAXSEG)
                                  continue;
                          if (!(th->th_flags & TH_SYN))
                                  continue;
                          if (TCPS_HAVERCVDSYN(tp->t_state))
                                  continue;
                          memcpy(&mss, cp + 2, sizeof(mss));
                          oi->maxseg = ntohs(mss);
                          break;
  
                  case TCPOPT_WINDOW:
                          if (optlen != TCPOLEN_WINDOW)
                                  continue;
                          if (!(th->th_flags & TH_SYN))
                                  continue;
                          if (TCPS_HAVERCVDSYN(tp->t_state))
                                  continue;
                          tp->t_flags |= TF_RCVD_SCALE;
                          tp->requested_s_scale = cp[2];
                          if (tp->requested_s_scale > TCP_MAX_WINSHIFT) {
                                  char buf[INET6_ADDRSTRLEN];
                                  struct ip *ip = mtod(m, struct ip *);
  #ifdef INET6
                                  struct ip6_hdr *ip6 = mtod(m, struct ip6_hdr *);
  #endif
  
                                  switch (ip->ip_v) {
                                  case 4:
                                          in_print(buf, sizeof(buf),
                                              &ip->ip_src);
                                          break;
  #ifdef INET6
                                  case 6:
                                          in6_print(buf, sizeof(buf),
                                              &ip6->ip6_src);
                                          break;
  #endif
                                  default:
                                          strlcpy(buf, "(unknown)", sizeof(buf));
                                          break;
                                  }
  
                                  log(LOG_ERR, "TCP: invalid wscale %d from %s, "
                                      "assuming %d\n",
                                      tp->requested_s_scale, buf,
                                      TCP_MAX_WINSHIFT);
                                  tp->requested_s_scale = TCP_MAX_WINSHIFT;
                          }
                          break;
  
                  case TCPOPT_TIMESTAMP:
                          if (optlen != TCPOLEN_TIMESTAMP)
                                  continue;
                          oi->ts_present = 1;
                          memcpy(&oi->ts_val, cp + 2, sizeof(oi->ts_val));
                          NTOHL(oi->ts_val);
                          memcpy(&oi->ts_ecr, cp + 6, sizeof(oi->ts_ecr));
                          NTOHL(oi->ts_ecr);
  
                          if (!(th->th_flags & TH_SYN))
                                  continue;
                          if (TCPS_HAVERCVDSYN(tp->t_state))
                                  continue;
                          /*
                           * A timestamp received in a SYN makes
                           * it ok to send timestamp requests and replies.
                           */
                          tp->t_flags |= TF_RCVD_TSTMP;
                          tp->ts_recent = oi->ts_val;
                          tp->ts_recent_age = tcp_now;
                          break;
  
                  case TCPOPT_SACK_PERMITTED:
                          if (optlen != TCPOLEN_SACK_PERMITTED)
                                  continue;
                          if (!(th->th_flags & TH_SYN))
                                  continue;
                          if (TCPS_HAVERCVDSYN(tp->t_state))
                                  continue;
                          if (tcp_do_sack) {
                                  tp->t_flags |= TF_SACK_PERMIT;
                                  tp->t_flags |= TF_WILL_SACK;
                          }
                          break;
  
                  case TCPOPT_SACK:
                          tcp_sack_option(tp, th, cp, optlen);
                          break;
  #ifdef TCP_SIGNATURE
                  case TCPOPT_SIGNATURE:
                          if (optlen != TCPOLEN_SIGNATURE)
                                  continue;
                          if (sigp &&
                              !consttime_memequal(sigp, cp + 2, TCP_SIGLEN))
                                  return (-1);
  
                          sigp = sigbuf;
                          memcpy(sigbuf, cp + 2, TCP_SIGLEN);
                          tp->t_flags |= TF_SIGNATURE;
                          break;
  #endif
                  }
          }
  
  #ifndef TCP_SIGNATURE
          return 0;
  #else
          if (tp->t_flags & TF_SIGNATURE) {
                  sav = tcp_signature_getsav(m);
                  if (sav == NULL && tp->t_state == TCPS_LISTEN)
                          return (-1);
          }
  
          if ((sigp ? TF_SIGNATURE : 0) ^ (tp->t_flags & TF_SIGNATURE))
                  goto out;
  
          if (sigp) {
                  char sig[TCP_SIGLEN];
  
                  tcp_fields_to_net(th);
                  if (tcp_signature(m, th, toff, sav, sig) < 0) {
                          tcp_fields_to_host(th);
                          goto out;
                  }
                  tcp_fields_to_host(th);
  
                  if (!consttime_memequal(sig, sigp, TCP_SIGLEN)) {
                          TCP_STATINC(TCP_STAT_BADSIG);
                          goto out;
                  } else
                          TCP_STATINC(TCP_STAT_GOODSIG);
  
                  key_sa_recordxfer(sav, m);
                  KEY_SA_UNREF(&sav);
          }
          return 0;
  out:
          if (sav != NULL)
                  KEY_SA_UNREF(&sav);
          return -1;
  #endif
  }
  
  /*
   * 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.
   */
  void
  tcp_pulloutofband(struct socket *so, struct tcphdr *th,
      struct mbuf *m, int off)
  {
          int cnt = off + th->th_urp - 1;
  
          while (cnt >= 0) {
                  if (m->m_len > cnt) {
                          char *cp = mtod(m, char *) + cnt;
                          struct tcpcb *tp = sototcpcb(so);
  
                          tp->t_iobc = *cp;
                          tp->t_oobflags |= TCPOOB_HAVEDATA;
                          memmove(cp, cp + 1, (unsigned)(m->m_len - cnt - 1));
                          m->m_len--;
                          return;
                  }
                  cnt -= m->m_len;
                  m = m->m_next;
                  if (m == NULL)
                          break;
          }
          panic("tcp_pulloutofband");
  }
  
  /*
   * Collect new round-trip time estimate
   * and update averages and current timeout.
   *
   * rtt is in units of slow ticks (typically 500 ms) -- essentially the
   * difference of two timestamps.
   */
  void
  tcp_xmit_timer(struct tcpcb *tp, uint32_t rtt)
  {
          int32_t delta;
  
          TCP_STATINC(TCP_STAT_RTTUPDATED);
          if (tp->t_srtt != 0) {
                  /*
                   * Compute the amount to add to srtt for smoothing,
                   * *alpha, or 2^(-TCP_RTT_SHIFT).  Because
                   * srtt is stored in 1/32 slow ticks, we conceptually
                   * shift left 5 bits, subtract srtt to get the
                   * difference, and then shift right by TCP_RTT_SHIFT
                   * (3) to obtain 1/8 of the difference.
                   */
                  delta = (rtt << 2) - (tp->t_srtt >> TCP_RTT_SHIFT);
                  /* 
                   * This can never happen, because delta's lowest
                   * possible value is 1/8 of t_srtt.  But if it does,
                   * set srtt to some reasonable value, here chosen
                   * as 1/8 tick.
                   */
                  if ((tp->t_srtt += delta) <= 0)
                          tp->t_srtt = 1 << 2;
                  /*
                   * RFC2988 requires that rttvar be updated first.
                   * This code is compliant because "delta" is the old
                   * srtt minus the new observation (scaled).
                   *
                   * RFC2988 says:
                   *   rttvar = (1-beta) * rttvar + beta * |srtt-observed|
                   *
                   * delta is in units of 1/32 ticks, and has then been
                   * divided by 8.  This is equivalent to being in 1/16s
                   * units and divided by 4.  Subtract from it 1/4 of
                   * the existing rttvar to form the (signed) amount to
                   * adjust.
                   */
                  if (delta < 0)
                          delta = -delta;
                  delta -= (tp->t_rttvar >> TCP_RTTVAR_SHIFT);
                  /*
                   * As with srtt, this should never happen.  There is
                   * no support in RFC2988 for this operation.  But 1/4s
                   * as rttvar when faced with something arguably wrong
                   * is ok.
                   */
                  if ((tp->t_rttvar += delta) <= 0)
                          tp->t_rttvar = 1 << 2;
  
                  /*
                   * If srtt exceeds .01 second, ensure we use the 'remote' MSL
                   * Problem is: it doesn't work.  Disabled by defaulting
                   * tcp_rttlocal to 0; see corresponding code in
                   * tcp_subr that selects local vs remote in a different way.
                   *
                   * The static branch prediction hint here should be removed
                   * when the rtt estimator is fixed and the rtt_enable code
                   * is turned back on.
                   */
                  if (__predict_false(tcp_rttlocal) && tcp_msl_enable
                      && tp->t_srtt > tcp_msl_remote_threshold
                      && tp->t_msl  < tcp_msl_remote) {
                          tp->t_msl = MIN(tcp_msl_remote, TCP_MAXMSL);
                  }
          } else {
                  /*
                   * This is the first measurement.  Per RFC2988, 2.2,
                   * set rtt=R and srtt=R/2.
                   * For srtt, storage representation is 1/32 ticks,
                   * so shift left by 5.
                   * For rttvar, storage representation is 1/16 ticks,
                   * So shift left by 4, but then right by 1 to halve.
                   */
                  tp->t_srtt = rtt << (TCP_RTT_SHIFT + 2);
                  tp->t_rttvar = rtt << (TCP_RTTVAR_SHIFT + 2 - 1);
          }
          tp->t_rtttime = 0;
          tp->t_rxtshift = 0;
  
          /*
           * the retransmit should happen at rtt + 4 * rttvar.
           * Because of the way we do the smoothing, srtt and rttvar
           * will each average +1/2 tick of bias.  When we compute
           * the retransmit timer, we want 1/2 tick of rounding and
           * 1 extra tick because of +-1/2 tick uncertainty in the
           * firing of the timer.  The bias will give us exactly the
           * 1.5 tick we need.  But, because the bias is
           * statistical, we have to test that we don't drop below
           * the minimum feasible timer (which is 2 ticks).
           */
          TCPT_RANGESET(tp->t_rxtcur, TCP_REXMTVAL(tp),
              uimax(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;
  }