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

     /*-
      * 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.
     * 4. Neither the name of the University nor the names of its contributors
     *    may be used to endorse or promote products derived from this software
     *    without specific prior written permission.
     *
     * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
     * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
     * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
     * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
     * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
     * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
     * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
     * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
     * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
     * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
     * SUCH DAMAGE.
     *
     *      @(#)tcp_input.c 8.12 (Berkeley) 5/24/95
     * $FreeBSD$
     */
    
    #include "opt_ipfw.h"           /* for ipfw_fwd         */
    #include "opt_inet.h"
    #include "opt_inet6.h"
    #include "opt_ipsec.h"
    #include "opt_mac.h"
    #include "opt_tcpdebug.h"
    #include "opt_tcp_input.h"
    #include "opt_tcp_sack.h"
    
    #include <sys/param.h>
    #include <sys/kernel.h>
    #include <sys/mac.h>
    #include <sys/malloc.h>
    #include <sys/mbuf.h>
    #include <sys/proc.h>           /* for proc0 declaration */
    #include <sys/protosw.h>
    #include <sys/signalvar.h>
    #include <sys/socket.h>
    #include <sys/socketvar.h>
    #include <sys/sysctl.h>
    #include <sys/syslog.h>
    #include <sys/systm.h>
    
    #include <machine/cpu.h>        /* before tcp_seq.h, for tcp_random18() */
    
    #include <vm/uma.h>
    
    #include <net/if.h>
    #include <net/route.h>
    
    #include <netinet/in.h>
    #include <netinet/in_pcb.h>
    #include <netinet/in_systm.h>
    #include <netinet/in_var.h>
    #include <netinet/ip.h>
    #include <netinet/ip_icmp.h>    /* for ICMP_BANDLIM             */
    #include <netinet/icmp_var.h>   /* for ICMP_BANDLIM             */
    #include <netinet/ip_var.h>
    #include <netinet/ip6.h>
    #include <netinet/icmp6.h>
    #include <netinet6/in6_pcb.h>
    #include <netinet6/ip6_var.h>
    #include <netinet6/nd6.h>
    #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 <netinet6/tcp6_var.h>
    #include <netinet/tcpip.h>
    #ifdef TCPDEBUG
    #include <netinet/tcp_debug.h>
    #endif /* TCPDEBUG */
    
    #ifdef FAST_IPSEC
    #include <netipsec/ipsec.h>
    #include <netipsec/ipsec6.h>
    #endif /*FAST_IPSEC*/
    
    #ifdef IPSEC
    #include <netinet6/ipsec.h>
    #include <netinet6/ipsec6.h>
    #include <netkey/key.h>
    #endif /*IPSEC*/
    
    #include <machine/in_cksum.h>
    
   static const int tcprexmtthresh = 3;
   
   struct  tcpstat tcpstat;
   SYSCTL_STRUCT(_net_inet_tcp, TCPCTL_STATS, stats, CTLFLAG_RW,
       &tcpstat , tcpstat, "TCP statistics (struct tcpstat, netinet/tcp_var.h)");
   
   static int log_in_vain = 0;
   SYSCTL_INT(_net_inet_tcp, OID_AUTO, log_in_vain, CTLFLAG_RW,
       &log_in_vain, 0, "Log all incoming TCP connections");
   
   static int blackhole = 0;
   SYSCTL_INT(_net_inet_tcp, OID_AUTO, blackhole, CTLFLAG_RW,
           &blackhole, 0, "Do not send RST when dropping refused connections");
   
   int tcp_delack_enabled = 1;
   SYSCTL_INT(_net_inet_tcp, OID_AUTO, delayed_ack, CTLFLAG_RW,
       &tcp_delack_enabled, 0,
       "Delay ACK to try and piggyback it onto a data packet");
   
   #ifdef TCP_DROP_SYNFIN
   static int drop_synfin = 0;
   SYSCTL_INT(_net_inet_tcp, OID_AUTO, drop_synfin, CTLFLAG_RW,
       &drop_synfin, 0, "Drop TCP packets with SYN+FIN set");
   #endif
   
   static int tcp_do_rfc3042 = 1;
   SYSCTL_INT(_net_inet_tcp, OID_AUTO, rfc3042, CTLFLAG_RW,
       &tcp_do_rfc3042, 0, "Enable RFC 3042 (Limited Transmit)");
   
   static int tcp_do_rfc3390 = 1;
   SYSCTL_INT(_net_inet_tcp, OID_AUTO, rfc3390, CTLFLAG_RW,
       &tcp_do_rfc3390, 0,
       "Enable RFC 3390 (Increasing TCP's Initial Congestion Window)");
   
   static int tcp_insecure_rst = 0;
   SYSCTL_INT(_net_inet_tcp, OID_AUTO, insecure_rst, CTLFLAG_RW,
       &tcp_insecure_rst, 0,
       "Follow the old (insecure) criteria for accepting RST packets.");
   
   SYSCTL_NODE(_net_inet_tcp, OID_AUTO, reass, CTLFLAG_RW, 0,
               "TCP Segment Reassembly Queue");
   
   static int tcp_reass_maxseg = 0;
   SYSCTL_INT(_net_inet_tcp_reass, OID_AUTO, maxsegments, CTLFLAG_RDTUN,
              &tcp_reass_maxseg, 0,
              "Global maximum number of TCP Segments in Reassembly Queue");
   
   int tcp_reass_qsize = 0;
   SYSCTL_INT(_net_inet_tcp_reass, OID_AUTO, cursegments, CTLFLAG_RD,
              &tcp_reass_qsize, 0,
              "Global number of TCP Segments currently in Reassembly Queue");
   
   static int tcp_reass_maxqlen = 48;
   SYSCTL_INT(_net_inet_tcp_reass, OID_AUTO, maxqlen, CTLFLAG_RW,
              &tcp_reass_maxqlen, 0,
              "Maximum number of TCP Segments per individual Reassembly Queue");
   
   static int tcp_reass_overflows = 0;
   SYSCTL_INT(_net_inet_tcp_reass, OID_AUTO, overflows, CTLFLAG_RD,
              &tcp_reass_overflows, 0,
              "Global number of TCP Segment Reassembly Queue Overflows");
   
   struct inpcbhead tcb;
   #define tcb6    tcb  /* for KAME src sync over BSD*'s */
   struct inpcbinfo tcbinfo;
   struct mtx      *tcbinfo_mtx;
   
   static void      tcp_dooptions(struct tcpopt *, u_char *, int, int);
   
   static void      tcp_pulloutofband(struct socket *,
                        struct tcphdr *, struct mbuf *, int);
   static int       tcp_reass(struct tcpcb *, struct tcphdr *, int *,
                        struct mbuf *);
   static void      tcp_xmit_timer(struct tcpcb *, int);
   static void      tcp_newreno_partial_ack(struct tcpcb *, struct tcphdr *);
   static int       tcp_timewait(struct tcptw *, struct tcpopt *,
                        struct tcphdr *, struct mbuf *, int);
   
   /* Neighbor Discovery, Neighbor Unreachability Detection Upper layer hint. */
   #ifdef INET6
   #define ND6_HINT(tp) \
   do { \
           if ((tp) && (tp)->t_inpcb && \
               ((tp)->t_inpcb->inp_vflag & INP_IPV6) != 0) \
                   nd6_nud_hint(NULL, NULL, 0); \
   } while (0)
   #else
   #define ND6_HINT(tp)
   #endif
   
   /*
    * Indicate whether this ack should be delayed.  We can delay the ack if
    *      - there is no delayed ack timer in progress and
    *      - our last ack wasn't a 0-sized window.  We never want to delay
    *        the ack that opens up a 0-sized window and
    *              - delayed acks are enabled or
    *              - this is a half-synchronized T/TCP connection.
    */
   #define DELAY_ACK(tp)                                                   \
           ((!callout_active(tp->tt_delack) &&                             \
               (tp->t_flags & TF_RXWIN0SENT) == 0) &&                      \
               (tcp_delack_enabled || (tp->t_flags & TF_NEEDSYN)))
   
   /* Initialize TCP reassembly queue */
   static void
   tcp_reass_zone_change(void *tag)
   {
   
           tcp_reass_maxseg = nmbclusters / 16;
           uma_zone_set_max(tcp_reass_zone, tcp_reass_maxseg);
   }
   
   uma_zone_t      tcp_reass_zone;
   void
   tcp_reass_init()
   {
           tcp_reass_maxseg = nmbclusters / 16;
           TUNABLE_INT_FETCH("net.inet.tcp.reass.maxsegments",
               &tcp_reass_maxseg);
           tcp_reass_zone = uma_zcreate("tcpreass", sizeof (struct tseg_qent),
               NULL, NULL, NULL, NULL, UMA_ALIGN_PTR, UMA_ZONE_NOFREE);
           uma_zone_set_max(tcp_reass_zone, tcp_reass_maxseg);
           EVENTHANDLER_REGISTER(nmbclusters_change,
               tcp_reass_zone_change, NULL, EVENTHANDLER_PRI_ANY);
   }
   
   static int
   tcp_reass(tp, th, tlenp, m)
           register struct tcpcb *tp;
           register struct tcphdr *th;
           int *tlenp;
           struct mbuf *m;
   {
           struct tseg_qent *q;
           struct tseg_qent *p = NULL;
           struct tseg_qent *nq;
           struct tseg_qent *te = NULL;
           struct socket *so = tp->t_inpcb->inp_socket;
           int flags;
   
           INP_LOCK_ASSERT(tp->t_inpcb);
   
           /*
            * XXX: tcp_reass() is rather inefficient with its data structures
            * and should be rewritten (see NetBSD for optimizations).  While
            * doing that it should move to its own file tcp_reass.c.
            */
   
           /*
            * Call with th==NULL after become established to
            * force pre-ESTABLISHED data up to user socket.
            */
           if (th == NULL)
                   goto present;
   
           /*
            * Limit the number of segments in the reassembly queue to prevent
            * holding on to too many segments (and thus running out of mbufs).
            * Make sure to let the missing segment through which caused this
            * queue.  Always keep one global queue entry spare to be able to
            * process the missing segment.
            */
           if (th->th_seq != tp->rcv_nxt &&
               (tcp_reass_qsize + 1 >= tcp_reass_maxseg ||
                tp->t_segqlen >= tcp_reass_maxqlen)) {
                   tcp_reass_overflows++;
                   tcpstat.tcps_rcvmemdrop++;
                   m_freem(m);
                   *tlenp = 0;
                   return (0);
           }
   
           /*
            * Allocate a new queue entry. If we can't, or hit the zone limit
            * just drop the pkt.
            */
           te = uma_zalloc(tcp_reass_zone, M_NOWAIT);
           if (te == NULL) {
                   tcpstat.tcps_rcvmemdrop++;
                   m_freem(m);
                   *tlenp = 0;
                   return (0);
           }
           tp->t_segqlen++;
           tcp_reass_qsize++;
   
           /*
            * Find a segment which begins after this one does.
            */
           LIST_FOREACH(q, &tp->t_segq, tqe_q) {
                   if (SEQ_GT(q->tqe_th->th_seq, th->th_seq))
                           break;
                   p = q;
           }
   
           /*
            * If there is a preceding segment, it may provide some of
            * our data already.  If so, drop the data from the incoming
            * segment.  If it provides all of our data, drop us.
            */
           if (p != NULL) {
                   register int i;
                   /* conversion to int (in i) handles seq wraparound */
                   i = p->tqe_th->th_seq + p->tqe_len - th->th_seq;
                   if (i > 0) {
                           if (i >= *tlenp) {
                                   tcpstat.tcps_rcvduppack++;
                                   tcpstat.tcps_rcvdupbyte += *tlenp;
                                   m_freem(m);
                                   uma_zfree(tcp_reass_zone, te);
                                   tp->t_segqlen--;
                                   tcp_reass_qsize--;
                                   /*
                                    * Try to present any queued data
                                    * at the left window edge to the user.
                                    * This is needed after the 3-WHS
                                    * completes.
                                    */
                                   goto present;   /* ??? */
                           }
                           m_adj(m, i);
                           *tlenp -= i;
                           th->th_seq += i;
                   }
           }
           tcpstat.tcps_rcvoopack++;
           tcpstat.tcps_rcvoobyte += *tlenp;
   
           /*
            * While we overlap succeeding segments trim them or,
            * if they are completely covered, dequeue them.
            */
           while (q) {
                   register int i = (th->th_seq + *tlenp) - q->tqe_th->th_seq;
                   if (i <= 0)
                           break;
                   if (i < q->tqe_len) {
                           q->tqe_th->th_seq += i;
                           q->tqe_len -= i;
                           m_adj(q->tqe_m, i);
                           break;
                   }
   
                   nq = LIST_NEXT(q, tqe_q);
                   LIST_REMOVE(q, tqe_q);
                   m_freem(q->tqe_m);
                   uma_zfree(tcp_reass_zone, q);
                   tp->t_segqlen--;
                   tcp_reass_qsize--;
                   q = nq;
           }
   
           /* Insert the new segment queue entry into place. */
           te->tqe_m = m;
           te->tqe_th = th;
           te->tqe_len = *tlenp;
   
           if (p == NULL) {
                   LIST_INSERT_HEAD(&tp->t_segq, te, tqe_q);
           } else {
                   LIST_INSERT_AFTER(p, te, tqe_q);
           }
   
   present:
           /*
            * Present data to user, advancing rcv_nxt through
            * completed sequence space.
            */
           if (!TCPS_HAVEESTABLISHED(tp->t_state))
                   return (0);
           q = LIST_FIRST(&tp->t_segq);
           if (!q || q->tqe_th->th_seq != tp->rcv_nxt)
                   return (0);
           SOCKBUF_LOCK(&so->so_rcv);
           do {
                   tp->rcv_nxt += q->tqe_len;
                   flags = q->tqe_th->th_flags & TH_FIN;
                   nq = LIST_NEXT(q, tqe_q);
                   LIST_REMOVE(q, tqe_q);
                   if (so->so_rcv.sb_state & SBS_CANTRCVMORE)
                           m_freem(q->tqe_m);
                   else
                           sbappendstream_locked(&so->so_rcv, q->tqe_m);
                   uma_zfree(tcp_reass_zone, q);
                   tp->t_segqlen--;
                   tcp_reass_qsize--;
                   q = nq;
           } while (q && q->tqe_th->th_seq == tp->rcv_nxt);
           ND6_HINT(tp);
           sorwakeup_locked(so);
           return (flags);
   }
   
   /*
    * TCP input routine, follows pages 65-76 of the
    * protocol specification dated September, 1981 very closely.
    */
   #ifdef INET6
   int
   tcp6_input(mp, offp, proto)
           struct mbuf **mp;
           int *offp, proto;
   {
           register struct mbuf *m = *mp;
           struct in6_ifaddr *ia6;
   
           IP6_EXTHDR_CHECK(m, *offp, sizeof(struct tcphdr), IPPROTO_DONE);
   
           /*
            * draft-itojun-ipv6-tcp-to-anycast
            * better place to put this in?
            */
           ia6 = ip6_getdstifaddr(m);
           if (ia6 && (ia6->ia6_flags & IN6_IFF_ANYCAST)) {
                   struct ip6_hdr *ip6;
   
                   ip6 = mtod(m, struct ip6_hdr *);
                   icmp6_error(m, ICMP6_DST_UNREACH, ICMP6_DST_UNREACH_ADDR,
                               (caddr_t)&ip6->ip6_dst - (caddr_t)ip6);
                   return IPPROTO_DONE;
           }
   
           tcp_input(m, *offp);
           return IPPROTO_DONE;
   }
   #endif
   
   void
   tcp_input(m, off0)
           register struct mbuf *m;
           int off0;
   {
           register struct tcphdr *th;
           register struct ip *ip = NULL;
           register struct ipovly *ipov;
           register struct inpcb *inp = NULL;
           u_char *optp = NULL;
           int optlen = 0;
           int len, tlen, off;
           int drop_hdrlen;
           register struct tcpcb *tp = 0;
           register int thflags;
           struct socket *so = 0;
           int todrop, acked, ourfinisacked, needoutput = 0;
           u_long tiwin;
           struct tcpopt to;               /* options in this segment */
           int headlocked = 0;
   #ifdef IPFIREWALL_FORWARD
           struct m_tag *fwd_tag;
   #endif
           int rstreason; /* For badport_bandlim accounting purposes */
   
           struct ip6_hdr *ip6 = NULL;
   #ifdef INET6
           int isipv6;
   #else
           const int isipv6 = 0;
   #endif
   
   #ifdef TCPDEBUG
           /*
            * The size of tcp_saveipgen must be the size of the max ip header,
            * now IPv6.
            */
           u_char tcp_saveipgen[40];
           struct tcphdr tcp_savetcp;
           short ostate = 0;
   #endif
   
   #ifdef INET6
           isipv6 = (mtod(m, struct ip *)->ip_v == 6) ? 1 : 0;
   #endif
           bzero((char *)&to, sizeof(to));
   
           tcpstat.tcps_rcvtotal++;
   
           if (isipv6) {
   #ifdef INET6
                   /* IP6_EXTHDR_CHECK() is already done at tcp6_input() */
                   ip6 = mtod(m, struct ip6_hdr *);
                   tlen = sizeof(*ip6) + ntohs(ip6->ip6_plen) - off0;
                   if (in6_cksum(m, IPPROTO_TCP, off0, tlen)) {
                           tcpstat.tcps_rcvbadsum++;
                           goto drop;
                   }
                   th = (struct tcphdr *)((caddr_t)ip6 + off0);
   
                   /*
                    * 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;
                   }
   #else
                   th = NULL;              /* XXX: avoid compiler warning */
   #endif
           } else {
                   /*
                    * Get IP and TCP header together in first mbuf.
                    * Note: IP leaves IP header in first mbuf.
                    */
                   if (off0 > sizeof (struct ip)) {
                           ip_stripoptions(m, (struct mbuf *)0);
                           off0 = sizeof(struct ip);
                   }
                   if (m->m_len < sizeof (struct tcpiphdr)) {
                           if ((m = m_pullup(m, sizeof (struct tcpiphdr))) == 0) {
                                   tcpstat.tcps_rcvshort++;
                                   return;
                           }
                   }
                   ip = mtod(m, struct ip *);
                   ipov = (struct ipovly *)ip;
                   th = (struct tcphdr *)((caddr_t)ip + off0);
                   tlen = ip->ip_len;
   
                   if (m->m_pkthdr.csum_flags & CSUM_DATA_VALID) {
                           if (m->m_pkthdr.csum_flags & CSUM_PSEUDO_HDR)
                                   th->th_sum = m->m_pkthdr.csum_data;
                           else
                                   th->th_sum = in_pseudo(ip->ip_src.s_addr,
                                                   ip->ip_dst.s_addr,
                                                   htonl(m->m_pkthdr.csum_data +
                                                           ip->ip_len +
                                                           IPPROTO_TCP));
                           th->th_sum ^= 0xffff;
   #ifdef TCPDEBUG
                           ipov->ih_len = (u_short)tlen;
                           ipov->ih_len = htons(ipov->ih_len);
   #endif
                   } else {
                           /*
                            * Checksum extended TCP header and data.
                            */
                           len = sizeof (struct ip) + tlen;
                           bzero(ipov->ih_x1, sizeof(ipov->ih_x1));
                           ipov->ih_len = (u_short)tlen;
                           ipov->ih_len = htons(ipov->ih_len);
                           th->th_sum = in_cksum(m, len);
                   }
                   if (th->th_sum) {
                           tcpstat.tcps_rcvbadsum++;
                           goto drop;
                   }
   #ifdef INET6
                   /* Re-initialization for later version check */
                   ip->ip_v = IPVERSION;
   #endif
           }
   
           /*
            * Check that TCP offset makes sense,
            * pull out TCP options and adjust length.              XXX
            */
           off = th->th_off << 2;
           if (off < sizeof (struct tcphdr) || off > tlen) {
                   tcpstat.tcps_rcvbadoff++;
                   goto drop;
           }
           tlen -= off;    /* tlen is used instead of ti->ti_len */
           if (off > sizeof (struct tcphdr)) {
                   if (isipv6) {
   #ifdef INET6
                           IP6_EXTHDR_CHECK(m, off0, off, );
                           ip6 = mtod(m, struct ip6_hdr *);
                           th = (struct tcphdr *)((caddr_t)ip6 + off0);
   #endif
                   } else {
                           if (m->m_len < sizeof(struct ip) + off) {
                                   if ((m = m_pullup(m, sizeof (struct ip) + off))
                                                   == 0) {
                                           tcpstat.tcps_rcvshort++;
                                           return;
                                   }
                                   ip = mtod(m, struct ip *);
                                   ipov = (struct ipovly *)ip;
                                   th = (struct tcphdr *)((caddr_t)ip + off0);
                           }
                   }
                   optlen = off - sizeof (struct tcphdr);
                   optp = (u_char *)(th + 1);
           }
           thflags = th->th_flags;
   
   #ifdef TCP_DROP_SYNFIN
           /*
            * If the drop_synfin option is enabled, drop all packets with
            * both the SYN and FIN bits set. This prevents e.g. nmap from
            * identifying the TCP/IP stack.
            *
            * This is a violation of the TCP specification.
            */
           if (drop_synfin && (thflags & (TH_SYN|TH_FIN)) == (TH_SYN|TH_FIN))
                   goto drop;
   #endif
   
           /*
            * Convert TCP protocol specific fields to host format.
            */
           th->th_seq = ntohl(th->th_seq);
           th->th_ack = ntohl(th->th_ack);
           th->th_win = ntohs(th->th_win);
           th->th_urp = ntohs(th->th_urp);
   
           /*
            * Delay dropping TCP, IP headers, IPv6 ext headers, and TCP options,
            * until after ip6_savecontrol() is called and before other functions
            * which don't want those proto headers.
            * Because ip6_savecontrol() is going to parse the mbuf to
            * search for data to be passed up to user-land, it wants mbuf
            * parameters to be unchanged.
            * XXX: the call of ip6_savecontrol() has been obsoleted based on
            * latest version of the advanced API (20020110).
            */
           drop_hdrlen = off0 + off;
   
           /*
            * Locate pcb for segment.
            */
           INP_INFO_WLOCK(&tcbinfo);
           headlocked = 1;
   findpcb:
           KASSERT(headlocked, ("tcp_input: findpcb: head not locked"));
   #ifdef IPFIREWALL_FORWARD
           /* Grab info from PACKET_TAG_IPFORWARD tag prepended to the chain. */
           fwd_tag = m_tag_find(m, PACKET_TAG_IPFORWARD, NULL);
   
           if (fwd_tag != NULL && isipv6 == 0) {   /* IPv6 support is not yet */
                   struct sockaddr_in *next_hop;
   
                   next_hop = (struct sockaddr_in *)(fwd_tag+1);
                   /*
                    * Transparently forwarded. Pretend to be the destination.
                    * already got one like this?
                    */
                   inp = in_pcblookup_hash(&tcbinfo,
                                           ip->ip_src, th->th_sport,
                                           ip->ip_dst, th->th_dport,
                                           0, m->m_pkthdr.rcvif);
                   if (!inp) {
                           /* It's new.  Try to find the ambushing socket. */
                           inp = in_pcblookup_hash(&tcbinfo,
                                                   ip->ip_src, th->th_sport,
                                                   next_hop->sin_addr,
                                                   next_hop->sin_port ?
                                                       ntohs(next_hop->sin_port) :
                                                       th->th_dport,
                                                   1, m->m_pkthdr.rcvif);
                   }
                   /* Remove the tag from the packet.  We don't need it anymore. */
                   m_tag_delete(m, fwd_tag);
           } else {
   #endif /* IPFIREWALL_FORWARD */
                   if (isipv6) {
   #ifdef INET6
                           inp = in6_pcblookup_hash(&tcbinfo,
                                                    &ip6->ip6_src, th->th_sport,
                                                    &ip6->ip6_dst, th->th_dport,
                                                    1, m->m_pkthdr.rcvif);
   #endif
                   } else
                           inp = in_pcblookup_hash(&tcbinfo,
                                                   ip->ip_src, th->th_sport,
                                                   ip->ip_dst, th->th_dport,
                                                   1, m->m_pkthdr.rcvif);
   #ifdef IPFIREWALL_FORWARD
           }
   #endif /* IPFIREWALL_FORWARD */
   
   #if defined(IPSEC) || defined(FAST_IPSEC)
   #ifdef INET6
           if (isipv6) {
                   if (inp != NULL && ipsec6_in_reject(m, inp)) {
   #ifdef IPSEC
                           ipsec6stat.in_polvio++;
   #endif
                           goto drop;
                   }
           } else
   #endif /* INET6 */
           if (inp != NULL && ipsec4_in_reject(m, inp)) {
   #ifdef IPSEC
                   ipsecstat.in_polvio++;
   #endif
                   goto drop;
           }
   #endif /*IPSEC || FAST_IPSEC*/
   
           /*
            * If the state is CLOSED (i.e., TCB does not exist) then
            * all data in the incoming segment is discarded.
            * If the TCB exists but is in CLOSED state, it is embryonic,
            * but should either do a listen or a connect soon.
            */
           if (inp == NULL) {
                   if (log_in_vain) {
   #ifdef INET6
                           char dbuf[INET6_ADDRSTRLEN+2], sbuf[INET6_ADDRSTRLEN+2];
   #else
                           char dbuf[4*sizeof "123"], sbuf[4*sizeof "123"];
   #endif
   
                           if (isipv6) {
   #ifdef INET6
                                   strcpy(dbuf, "[");
                                   strcpy(sbuf, "[");
                                   strcat(dbuf, ip6_sprintf(&ip6->ip6_dst));
                                   strcat(sbuf, ip6_sprintf(&ip6->ip6_src));
                                   strcat(dbuf, "]");
                                   strcat(sbuf, "]");
   #endif
                           } else {
                                   strcpy(dbuf, inet_ntoa(ip->ip_dst));
                                   strcpy(sbuf, inet_ntoa(ip->ip_src));
                           }
                           switch (log_in_vain) {
                           case 1:
                                   if ((thflags & TH_SYN) == 0)
                                           break;
                                   /* FALLTHROUGH */
                           case 2:
                                   log(LOG_INFO,
                                       "Connection attempt to TCP %s:%d "
                                       "from %s:%d flags:0x%02x\n",
                                       dbuf, ntohs(th->th_dport), sbuf,
                                       ntohs(th->th_sport), thflags);
                                   break;
                           default:
                                   break;
                           }
                   }
                   if (blackhole) {
                           switch (blackhole) {
                           case 1:
                                   if (thflags & TH_SYN)
                                           goto drop;
                                   break;
                           case 2:
                                   goto drop;
                           default:
                                   goto drop;
                           }
                   }
                   rstreason = BANDLIM_RST_CLOSEDPORT;
                   goto dropwithreset;
           }
           INP_LOCK(inp);
   
           /* Check the minimum TTL for socket. */
           if (inp->inp_ip_minttl != 0) {
   #ifdef INET6
                   if (isipv6 && inp->inp_ip_minttl > ip6->ip6_hlim)
                           goto drop;
                   else
   #endif
                   if (inp->inp_ip_minttl > ip->ip_ttl)
                           goto drop;
           }
   
           if (inp->inp_vflag & INP_TIMEWAIT) {
                   /*
                    * The only option of relevance is TOF_CC, and only if
                    * present in a SYN segment.  See tcp_timewait().
                    */
                   if (thflags & TH_SYN)
                           tcp_dooptions(&to, optp, optlen, 1);
                   if (tcp_timewait((struct tcptw *)inp->inp_ppcb,
                       &to, th, m, tlen))
                           goto findpcb;
                   /*
                    * tcp_timewait unlocks inp.
                    */
                   INP_INFO_WUNLOCK(&tcbinfo);
                   return;
           }
           tp = intotcpcb(inp);
           if (tp == 0) {
                   INP_UNLOCK(inp);
                   rstreason = BANDLIM_RST_CLOSEDPORT;
                   goto dropwithreset;
           }
           if (tp->t_state == TCPS_CLOSED)
                   goto drop;
   
           /* Unscale the window into a 32-bit value. */
           if ((thflags & TH_SYN) == 0)
                   tiwin = th->th_win << tp->snd_scale;
           else
                   tiwin = th->th_win;
   
   #ifdef MAC
           INP_LOCK_ASSERT(inp);
           if (mac_check_inpcb_deliver(inp, m))
                   goto drop;
   #endif
           so = inp->inp_socket;
           KASSERT(so != NULL, ("tcp_input: so == NULL"));
   #ifdef TCPDEBUG
           if (so->so_options & SO_DEBUG) {
                   ostate = tp->t_state;
                   if (isipv6)
                           bcopy((char *)ip6, (char *)tcp_saveipgen, sizeof(*ip6));
                   else
                           bcopy((char *)ip, (char *)tcp_saveipgen, sizeof(*ip));
                   tcp_savetcp = *th;
           }
   #endif
           if (so->so_options & SO_ACCEPTCONN) {
                   struct in_conninfo inc;
   
   #ifdef INET6
                   inc.inc_isipv6 = isipv6;
   #endif
                   if (isipv6) {
                           inc.inc6_faddr = ip6->ip6_src;
                           inc.inc6_laddr = ip6->ip6_dst;
                   } else {
                           inc.inc_faddr = ip->ip_src;
                           inc.inc_laddr = ip->ip_dst;
                   }
                   inc.inc_fport = th->th_sport;
                   inc.inc_lport = th->th_dport;
   
                   /*
                    * If the state is LISTEN then ignore segment if it contains
                    * a RST.  If the segment contains an ACK then it is bad and
                    * send a RST.  If it does not contain a SYN then it is not
                    * interesting; drop it.
                    *
                    * If the state is SYN_RECEIVED (syncache) and seg contains
                    * an ACK, but not for our SYN/ACK, send a RST.  If the seg
                    * contains a RST, check the sequence number to see if it
                    * is a valid reset segment.
                    */
                   if ((thflags & (TH_RST|TH_ACK|TH_SYN)) != TH_SYN) {
                           if ((thflags & (TH_RST|TH_ACK|TH_SYN)) == TH_ACK) {
                                   if (!syncache_expand(&inc, th, &so, m)) {
                                           /*
                                            * No syncache entry, or ACK was not
                                            * for our SYN/ACK.  Send a RST.
                                            */
                                           tcpstat.tcps_badsyn++;
                                           rstreason = BANDLIM_RST_OPENPORT;
                                           goto dropwithreset;
                                   }
                                   if (so == NULL) {
                                           /*
                                            * Could not complete 3-way handshake,
                                            * connection is being closed down, and
                                            * syncache will free mbuf.
                                            */
                                           INP_UNLOCK(inp);
                                           INP_INFO_WUNLOCK(&tcbinfo);
                                           return;
                                   }
                                   /*
                                    * Socket is created in state SYN_RECEIVED.
                                    * Continue processing segment.
                                    */
                                   INP_UNLOCK(inp);
                                   inp = sotoinpcb(so);
                                   INP_LOCK(inp);
                                   tp = intotcpcb(inp);
                                   /*
                                    * This is what would have happened in
                                    * tcp_output() when the SYN,ACK was sent.
                                    */
                                   tp->snd_up = tp->snd_una;
                                   tp->snd_max = tp->snd_nxt = tp->iss + 1;
                                   tp->last_ack_sent = tp->rcv_nxt;
                                   /*
                                    * RFC1323: The window in SYN & SYN/ACK
                                    * segments is never scaled.
                                    */
                                   tp->snd_wnd = tiwin;    /* unscaled */
                                   goto after_listen;
                           }
                           if (thflags & TH_RST) {
                                   syncache_chkrst(&inc, th);
                                   goto drop;
                           }
                           if (thflags & TH_ACK) {
                                   syncache_badack(&inc);
                                   tcpstat.tcps_badsyn++;
                                   rstreason = BANDLIM_RST_OPENPORT;
                                   goto dropwithreset;
                           }
                           goto drop;
                   }
   
                   /*
                    * Segment's flags are (SYN) or (SYN|FIN).
                    */
   #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 (isipv6 && !ip6_use_deprecated) {
                           struct in6_ifaddr *ia6;
   
                           if ((ia6 = ip6_getdstifaddr(m)) &&
                               (ia6->ia6_flags & IN6_IFF_DEPRECATED)) {
                                   INP_UNLOCK(inp);
                                   tp = NULL;
                                   rstreason = BANDLIM_RST_OPENPORT;
                                   goto dropwithreset;
                           }
                   }
   #endif
                   /*
                    * If it is from this socket, drop it, it must be forged.
                    * Don't bother responding if the destination was a broadcast.
                    */
                   if (th->th_dport == th->th_sport) {
                           if (isipv6) {
                                   if (IN6_ARE_ADDR_EQUAL(&ip6->ip6_dst,
                                                          &ip6->ip6_src))
                                           goto drop;
                           } else {
                                   if (ip->ip_dst.s_addr == ip->ip_src.s_addr)
                                           goto drop;
                           }
                   }
                   /*
                    * RFC1122 4.2.3.10, p. 104: discard bcast/mcast SYN
                    *
                    * Note that it is quite possible to receive unicast
                    * link-layer packets with a broadcast IP address. Use
                    * in_broadcast() to find them.
                    */
                   if (m->m_flags & (M_BCAST|M_MCAST))
                           goto drop;
                   if (isipv6) {
                           if (IN6_IS_ADDR_MULTICAST(&ip6->ip6_dst) ||
                               IN6_IS_ADDR_MULTICAST(&ip6->ip6_src))
                                   goto drop;
                   } else {
                           if (IN_MULTICAST(ntohl(ip->ip_dst.s_addr)) ||
                               IN_MULTICAST(ntohl(ip->ip_src.s_addr)) ||
                               ip->ip_src.s_addr == htonl(INADDR_BROADCAST) ||
                               in_broadcast(ip->ip_dst, m->m_pkthdr.rcvif))
                                   goto drop;
                   }
                   /*
                    * SYN appears to be valid; create compressed TCP state
                    * for syncache, or perform t/tcp connection.
                    */
                   if (so->so_qlen <= so->so_qlimit) {
   #ifdef TCPDEBUG
                           if (so->so_options & SO_DEBUG)
                                   tcp_trace(TA_INPUT, ostate, tp,
                                       (void *)tcp_saveipgen, &tcp_savetcp, 0);
   #endif
                           tcp_dooptions(&to, optp, optlen, 1);
                           if (!syncache_add(&inc, &to, th, &so, m))
                                   goto drop;
                           if (so == NULL) {
                                   /*
                                    * Entry added to syncache, mbuf used to
                                    * send SYN,ACK packet.
                                    */
                                   KASSERT(headlocked, ("headlocked"));
                                   INP_UNLOCK(inp);
                                   INP_INFO_WUNLOCK(&tcbinfo);
                                   return;
                           }
                           /*
                           * Segment passed TAO tests.
                           */
                          INP_UNLOCK(inp);
                          inp = sotoinpcb(so);
                          INP_LOCK(inp);
                          tp = intotcpcb(inp);
                          tp->snd_wnd = tiwin;
                          tp->t_starttime = ticks;
                          tp->t_state = TCPS_ESTABLISHED;
  
                          /*
                           * T/TCP logic:
                           * If there is a FIN or if there is data, then
                           * delay SYN,ACK(SYN) in the hope of piggy-backing
                           * it on a response segment.  Otherwise must send
                           * ACK now in case the other side is slow starting.
                           */
                          if (thflags & TH_FIN || tlen != 0)
                                  tp->t_flags |= (TF_DELACK | TF_NEEDSYN);
                          else
                                  tp->t_flags |= (TF_ACKNOW | TF_NEEDSYN);
                          tcpstat.tcps_connects++;
                          soisconnected(so);
                          goto trimthenstep6;
                  }
                  goto drop;
          }
  after_listen:
          KASSERT(headlocked, ("tcp_input: after_listen: head not locked"));
          INP_LOCK_ASSERT(inp);
  
          /* Syncache takes care of sockets in the listen state. */
          KASSERT(tp->t_state != TCPS_LISTEN, ("tcp_input: TCPS_LISTEN"));
  
          /*
           * This is the second part of the MSS DoS prevention code (after
           * minmss on the sending side) and it deals with too many too small
           * tcp packets in a too short timeframe (1 second).
           *
           * For every full second we count the number of received packets
           * and bytes. If we get a lot of packets per second for this connection
           * (tcp_minmssoverload) we take a closer look at it and compute the
           * average packet size for the past second. If that is less than
           * tcp_minmss we get too many packets with very small payload which
           * is not good and burdens our system (and every packet generates
           * a wakeup to the process connected to our socket). We can reasonable
           * expect this to be small packet DoS attack to exhaust our CPU
           * cycles.
           *
           * Care has to be taken for the minimum packet overload value. This
           * value defines the minimum number of packets per second before we
           * start to worry. This must not be too low to avoid killing for
           * example interactive connections with many small packets like
           * telnet or SSH.
           *
           * Setting either tcp_minmssoverload or tcp_minmss to "" disables
           * this check.
           *
           * Account for packet if payload packet, skip over ACK, etc.
           */
          if (tcp_minmss && tcp_minmssoverload &&
              tp->t_state == TCPS_ESTABLISHED && tlen > 0) {
                  if ((unsigned int)(tp->rcv_second - ticks) < hz) {
                          tp->rcv_pps++;
                          tp->rcv_byps += tlen + off;
                          if (tp->rcv_pps > tcp_minmssoverload) {
                                  if ((tp->rcv_byps / tp->rcv_pps) < tcp_minmss) {
                                          printf("too many small tcp packets from "
                                                 "%s:%u, av. %lubyte/packet, "
                                                 "dropping connection\n",
  #ifdef INET6
                                                  isipv6 ?
                                                  ip6_sprintf(&inp->inp_inc.inc6_faddr) :
  #endif
                                                  inet_ntoa(inp->inp_inc.inc_faddr),
                                                  inp->inp_inc.inc_fport,
                                                  tp->rcv_byps / tp->rcv_pps);
                                          KASSERT(headlocked, ("tcp_input: "
                                              "after_listen: tcp_drop: head "
                                              "not locked"));
                                          tp = tcp_drop(tp, ECONNRESET);
                                          tcpstat.tcps_minmssdrops++;
                                          goto drop;
                                  }
                          }
                  } else {
                          tp->rcv_second = ticks + hz;
                          tp->rcv_pps = 1;
                          tp->rcv_byps = tlen + off;
                  }
          }
  
          /*
           * Segment received on connection.
           * Reset idle time and keep-alive timer.
           */
          tp->t_rcvtime = ticks;
          if (TCPS_HAVEESTABLISHED(tp->t_state))
                  callout_reset(tp->tt_keep, tcp_keepidle, tcp_timer_keep, tp);
  
          /*
           * Process options only when we get SYN/ACK back. The SYN case
           * for incoming connections is handled in tcp_syncache.
           * XXX this is traditional behavior, may need to be cleaned up.
           */
          tcp_dooptions(&to, optp, optlen, thflags & TH_SYN);
          if (tp->t_state == TCPS_SYN_SENT && (thflags & TH_SYN)) {
                  if (to.to_flags & TOF_SCALE) {
                          tp->t_flags |= TF_RCVD_SCALE;
                          tp->requested_s_scale = to.to_requested_s_scale;
                  }
                  if (to.to_flags & TOF_TS) {
                          tp->t_flags |= TF_RCVD_TSTMP;
                          tp->ts_recent = to.to_tsval;
                          tp->ts_recent_age = ticks;
                  }
                  tp->snd_wnd = th->th_win;       /* initial send window */
                  if (to.to_flags & TOF_MSS)
                          tcp_mss(tp, to.to_mss);
                  if (tp->sack_enable) {
                          if (!(to.to_flags & TOF_SACK))
                                  tp->sack_enable = 0;
                          else
                                  tp->t_flags |= TF_SACK_PERMIT;
                  }
  
          }
  
          /*
           * Header prediction: check for the two common cases
           * of a uni-directional data xfer.  If the packet has
           * no control flags, is in-sequence, the window didn't
           * change and we're not retransmitting, it's a
           * candidate.  If the length is zero and the ack moved
           * forward, we're the sender side of the xfer.  Just
           * free the data acked & wake any higher level process
           * that was blocked waiting for space.  If the length
           * is non-zero and the ack didn't move, we're the
           * receiver side.  If we're getting packets in-order
           * (the reassembly queue is empty), add the data to
           * the socket buffer and note that we need a delayed ack.
           * Make sure that the hidden state-flags are also off.
           * Since we check for TCPS_ESTABLISHED above, it can only
           * be TH_NEEDSYN.
           */
          if (tp->t_state == TCPS_ESTABLISHED &&
              (thflags & (TH_SYN|TH_FIN|TH_RST|TH_URG|TH_ACK)) == TH_ACK &&
              ((tp->t_flags & (TF_NEEDSYN|TF_NEEDFIN)) == 0) &&
              ((to.to_flags & TOF_TS) == 0 ||
               TSTMP_GEQ(to.to_tsval, tp->ts_recent)) &&
               th->th_seq == tp->rcv_nxt && tiwin && tiwin == tp->snd_wnd &&
               tp->snd_nxt == tp->snd_max) {
  
                  /*
                   * If last ACK falls within this segment's sequence numbers,
                   * record the timestamp.
                   * NOTE that the test is modified according to the latest
                   * proposal of the tcplw@cray.com list (Braden 1993/04/26).
                   */
                  if ((to.to_flags & TOF_TS) != 0 &&
                      SEQ_LEQ(th->th_seq, tp->last_ack_sent)) {
                          tp->ts_recent_age = ticks;
                          tp->ts_recent = to.to_tsval;
                  }
  
                  if (tlen == 0) {
                          if (SEQ_GT(th->th_ack, tp->snd_una) &&
                              SEQ_LEQ(th->th_ack, tp->snd_max) &&
                              tp->snd_cwnd >= tp->snd_wnd &&
                              ((!tcp_do_newreno && !tp->sack_enable &&
                                tp->t_dupacks < tcprexmtthresh) ||
                               ((tcp_do_newreno || tp->sack_enable) &&
                                !IN_FASTRECOVERY(tp) && to.to_nsacks == 0 &&
                                TAILQ_EMPTY(&tp->snd_holes)))) {
                                  KASSERT(headlocked, ("headlocked"));
                                  INP_INFO_WUNLOCK(&tcbinfo);
                                  headlocked = 0;
                                  /*
                                   * this is a pure ack for outstanding data.
                                   */
                                  ++tcpstat.tcps_predack;
                                  /*
                                   * "bad retransmit" recovery
                                   */
                                  if (tp->t_rxtshift == 1 &&
                                      (int)(ticks - tp->t_badrxtwin) < 0) {
                                          ++tcpstat.tcps_sndrexmitbad;
                                          tp->snd_cwnd = tp->snd_cwnd_prev;
                                          tp->snd_ssthresh =
                                              tp->snd_ssthresh_prev;
                                          tp->snd_recover = tp->snd_recover_prev;
                                          if (tp->t_flags & TF_WASFRECOVERY)
                                              ENTER_FASTRECOVERY(tp);
                                          tp->snd_nxt = tp->snd_max;
                                          tp->t_badrxtwin = 0;
                                  }
  
                                  /*
                                   * Recalculate the transmit timer / rtt.
                                   *
                                   * Some boxes send broken timestamp replies
                                   * during the SYN+ACK phase, ignore
                                   * timestamps of 0 or we could calculate a
                                   * huge RTT and blow up the retransmit timer.
                                   */
                                  if ((to.to_flags & TOF_TS) != 0 &&
                                      to.to_tsecr) {
                                          if (!tp->t_rttlow ||
                                              tp->t_rttlow > ticks - to.to_tsecr)
                                                  tp->t_rttlow = ticks - to.to_tsecr;
                                          tcp_xmit_timer(tp,
                                              ticks - to.to_tsecr + 1);
                                  } else if (tp->t_rtttime &&
                                              SEQ_GT(th->th_ack, tp->t_rtseq)) {
                                          if (!tp->t_rttlow ||
                                              tp->t_rttlow > ticks - tp->t_rtttime)
                                                  tp->t_rttlow = ticks - tp->t_rtttime;
                                          tcp_xmit_timer(tp,
                                                          ticks - tp->t_rtttime);
                                  }
                                  tcp_xmit_bandwidth_limit(tp, th->th_ack);
                                  acked = th->th_ack - tp->snd_una;
                                  tcpstat.tcps_rcvackpack++;
                                  tcpstat.tcps_rcvackbyte += acked;
                                  sbdrop(&so->so_snd, acked);
                                  if (SEQ_GT(tp->snd_una, tp->snd_recover) &&
                                      SEQ_LEQ(th->th_ack, tp->snd_recover))
                                          tp->snd_recover = th->th_ack - 1;
                                  tp->snd_una = th->th_ack;
                                  /*
                                   * pull snd_wl2 up to prevent seq wrap relative
                                   * to th_ack.
                                   */
                                  tp->snd_wl2 = th->th_ack;
                                  tp->t_dupacks = 0;
                                  m_freem(m);
                                  ND6_HINT(tp); /* some progress has been done */
  
                                  /*
                                   * If all outstanding data are acked, stop
                                   * retransmit timer, otherwise restart timer
                                   * using current (possibly backed-off) value.
                                   * If process is waiting for space,
                                   * wakeup/selwakeup/signal.  If data
                                   * are ready to send, let tcp_output
                                   * decide between more output or persist.
  
  #ifdef TCPDEBUG
                                  if (so->so_options & SO_DEBUG)
                                          tcp_trace(TA_INPUT, ostate, tp,
                                              (void *)tcp_saveipgen,
                                              &tcp_savetcp, 0);
  #endif
                                   */
                                  if (tp->snd_una == tp->snd_max)
                                          callout_stop(tp->tt_rexmt);
                                  else if (!callout_active(tp->tt_persist))
                                          callout_reset(tp->tt_rexmt,
                                                        tp->t_rxtcur,
                                                        tcp_timer_rexmt, tp);
  
                                  sowwakeup(so);
                                  if (so->so_snd.sb_cc)
                                          (void) tcp_output(tp);
                                  goto check_delack;
                          }
                  } else if (th->th_ack == tp->snd_una &&
                      LIST_EMPTY(&tp->t_segq) &&
                      tlen <= sbspace(&so->so_rcv)) {
                          KASSERT(headlocked, ("headlocked"));
                          INP_INFO_WUNLOCK(&tcbinfo);
                          headlocked = 0;
                          /*
                           * this is a pure, in-sequence data packet
                           * with nothing on the reassembly queue and
                           * we have enough buffer space to take it.
                           */
                          /* Clean receiver SACK report if present */
                          if (tp->sack_enable && tp->rcv_numsacks)
                                  tcp_clean_sackreport(tp);
                          ++tcpstat.tcps_preddat;
                          tp->rcv_nxt += tlen;
                          /*
                           * Pull snd_wl1 up to prevent seq wrap relative to
                           * th_seq.
                           */
                          tp->snd_wl1 = th->th_seq;
                          /*
                           * Pull rcv_up up to prevent seq wrap relative to
                           * rcv_nxt.
                           */
                          tp->rcv_up = tp->rcv_nxt;
                          tcpstat.tcps_rcvpack++;
                          tcpstat.tcps_rcvbyte += tlen;
                          ND6_HINT(tp);   /* some progress has been done */
                          /*
  #ifdef TCPDEBUG
                          if (so->so_options & SO_DEBUG)
                                  tcp_trace(TA_INPUT, ostate, tp,
                                      (void *)tcp_saveipgen, &tcp_savetcp, 0);
  #endif
                           * Add data to socket buffer.
                           */
                          SOCKBUF_LOCK(&so->so_rcv);
                          if (so->so_rcv.sb_state & SBS_CANTRCVMORE) {
                                  m_freem(m);
                          } else {
                                  m_adj(m, drop_hdrlen);  /* delayed header drop */
                                  sbappendstream_locked(&so->so_rcv, m);
                          }
                          sorwakeup_locked(so);
                          if (DELAY_ACK(tp)) {
                                  tp->t_flags |= TF_DELACK;
                          } else {
                                  tp->t_flags |= TF_ACKNOW;
                                  tcp_output(tp);
                          }
                          goto check_delack;
                  }
          }
  
          /*
           * Calculate amount of space in receive window,
           * and then do TCP input processing.
           * Receive window is amount of space in rcv queue,
           * but not less than advertised window.
           */
          { int win;
  
          win = sbspace(&so->so_rcv);
          if (win < 0)
                  win = 0;
          tp->rcv_wnd = imax(win, (int)(tp->rcv_adv - tp->rcv_nxt));
          }
  
          switch (tp->t_state) {
  
          /*
           * If the state is SYN_RECEIVED:
           *      if seg contains an ACK, but not for our SYN/ACK, send a RST.
           */
          case TCPS_SYN_RECEIVED:
                  if ((thflags & TH_ACK) &&
                      (SEQ_LEQ(th->th_ack, tp->snd_una) ||
                       SEQ_GT(th->th_ack, tp->snd_max))) {
                                  rstreason = BANDLIM_RST_OPENPORT;
                                  goto dropwithreset;
                  }
                  break;
  
          /*
           * If the state is SYN_SENT:
           *      if seg contains an ACK, but not for our SYN, drop the input.
           *      if seg contains a RST, then drop the connection.
           *      if seg does not contain SYN, then drop it.
           * Otherwise this is an acceptable SYN segment
           *      initialize tp->rcv_nxt and tp->irs
           *      if seg contains ack then advance tp->snd_una
           *      if SYN has been acked change to ESTABLISHED else SYN_RCVD state
           *      arrange for segment to be acked (eventually)
           *      continue processing rest of data/controls, beginning with URG
           */
          case TCPS_SYN_SENT:
                  if ((thflags & TH_ACK) &&
                      (SEQ_LEQ(th->th_ack, tp->iss) ||
                       SEQ_GT(th->th_ack, tp->snd_max))) {
                          rstreason = BANDLIM_UNLIMITED;
                          goto dropwithreset;
                  }
                  if (thflags & TH_RST) {
                          if (thflags & TH_ACK) {
                                  KASSERT(headlocked, ("tcp_input: after_listen"
                                      ": tcp_drop.2: head not locked"));
                                  tp = tcp_drop(tp, ECONNREFUSED);
                          }
                          goto drop;
                  }
                  if ((thflags & TH_SYN) == 0)
                          goto drop;
  
                  tp->irs = th->th_seq;
                  tcp_rcvseqinit(tp);
                  if (thflags & TH_ACK) {
                          tcpstat.tcps_connects++;
                          soisconnected(so);
  #ifdef MAC
                          SOCK_LOCK(so);
                          mac_set_socket_peer_from_mbuf(m, so);
                          SOCK_UNLOCK(so);
  #endif
                          /* Do window scaling on this connection? */
                          if ((tp->t_flags & (TF_RCVD_SCALE|TF_REQ_SCALE)) ==
                                  (TF_RCVD_SCALE|TF_REQ_SCALE)) {
                                  tp->snd_scale = tp->requested_s_scale;
                                  tp->rcv_scale = tp->request_r_scale;
                          }
                          tp->rcv_adv += tp->rcv_wnd;
                          tp->snd_una++;          /* SYN is acked */
                          /*
                           * If there's data, delay ACK; if there's also a FIN
                           * ACKNOW will be turned on later.
                           */
                          if (DELAY_ACK(tp) && tlen != 0)
                                  callout_reset(tp->tt_delack, tcp_delacktime,
                                      tcp_timer_delack, tp);
                          else
                                  tp->t_flags |= TF_ACKNOW;
                          /*
                           * Received <SYN,ACK> in SYN_SENT[*] state.
                           * Transitions:
                           *      SYN_SENT  --> ESTABLISHED
                           *      SYN_SENT* --> FIN_WAIT_1
                           */
                          tp->t_starttime = ticks;
                          if (tp->t_flags & TF_NEEDFIN) {
                                  tp->t_state = TCPS_FIN_WAIT_1;
                                  tp->t_flags &= ~TF_NEEDFIN;
                                  thflags &= ~TH_SYN;
                          } else {
                                  tp->t_state = TCPS_ESTABLISHED;
                                  callout_reset(tp->tt_keep, tcp_keepidle,
                                                tcp_timer_keep, tp);
                          }
                  } else {
                          /*
                           * Received initial SYN in SYN-SENT[*] state =>
                           * simultaneous open.  If segment contains CC option
                           * and there is a cached CC, apply TAO test.
                           * If it succeeds, connection is * half-synchronized.
                           * Otherwise, do 3-way handshake:
                           *        SYN-SENT -> SYN-RECEIVED
                           *        SYN-SENT* -> SYN-RECEIVED*
                           * If there was no CC option, clear cached CC value.
                           */
                          tp->t_flags |= TF_ACKNOW;
                          callout_stop(tp->tt_rexmt);
                          tp->t_state = TCPS_SYN_RECEIVED;
                  }
  
  trimthenstep6:
                  KASSERT(headlocked, ("tcp_input: trimthenstep6: head not "
                      "locked"));
                  INP_LOCK_ASSERT(inp);
  
                  /*
                   * Advance th->th_seq to correspond to first data byte.
                   * If data, trim to stay within window,
                   * dropping FIN if necessary.
                   */
                  th->th_seq++;
                  if (tlen > tp->rcv_wnd) {
                          todrop = tlen - tp->rcv_wnd;
                          m_adj(m, -todrop);
                          tlen = tp->rcv_wnd;
                          thflags &= ~TH_FIN;
                          tcpstat.tcps_rcvpackafterwin++;
                          tcpstat.tcps_rcvbyteafterwin += todrop;
                  }
                  tp->snd_wl1 = th->th_seq - 1;
                  tp->rcv_up = th->th_seq;
                  /*
                   * Client side of transaction: already sent SYN and data.
                   * If the remote host used T/TCP to validate the SYN,
                   * our data will be ACK'd; if so, enter normal data segment
                   * processing in the middle of step 5, ack processing.
                   * Otherwise, goto step 6.
                   */
                  if (thflags & TH_ACK)
                          goto process_ACK;
  
                  goto step6;
  
          /*
           * If the state is LAST_ACK or CLOSING or TIME_WAIT:
           *      do normal processing.
           *
           * NB: Leftover from RFC1644 T/TCP.  Cases to be reused later.
           */
          case TCPS_LAST_ACK:
          case TCPS_CLOSING:
          case TCPS_TIME_WAIT:
                  KASSERT(tp->t_state != TCPS_TIME_WAIT, ("timewait"));
                  break;  /* continue normal processing */
          }
  
          /*
           * States other than LISTEN or SYN_SENT.
           * First check the RST flag and sequence number since reset segments
           * are exempt from the timestamp and connection count tests.  This
           * fixes a bug introduced by the Stevens, vol. 2, p. 960 bugfix
           * below which allowed reset segments in half the sequence space
           * to fall though and be processed (which gives forged reset
           * segments with a random sequence number a 50 percent chance of
           * killing a connection).
           * Then check timestamp, if present.
           * Then check the connection count, if present.
           * Then check that at least some bytes of segment are within
           * receive window.  If segment begins before rcv_nxt,
           * drop leading data (and SYN); if nothing left, just ack.
           *
           *
           * If the RST bit is set, check the sequence number to see
           * if this is a valid reset segment.
           * RFC 793 page 37:
           *   In all states except SYN-SENT, all reset (RST) segments
           *   are validated by checking their SEQ-fields.  A reset is
           *   valid if its sequence number is in the window.
           * Note: this does not take into account delayed ACKs, so
           *   we should test against last_ack_sent instead of rcv_nxt.
           *   The sequence number in the reset segment is normally an
           *   echo of our outgoing acknowlegement numbers, but some hosts
           *   send a reset with the sequence number at the rightmost edge
           *   of our receive window, and we have to handle this case.
           * Note 2: Paul Watson's paper "Slipping in the Window" has shown
           *   that brute force RST attacks are possible.  To combat this,
           *   we use a much stricter check while in the ESTABLISHED state,
           *   only accepting RSTs where the sequence number is equal to
           *   last_ack_sent.  In all other states (the states in which a
           *   RST is more likely), the more permissive check is used.
           * If we have multiple segments in flight, the intial reset
           * segment sequence numbers will be to the left of last_ack_sent,
           * but they will eventually catch up.
           * In any case, it never made sense to trim reset segments to
           * fit the receive window since RFC 1122 says:
           *   4.2.2.12  RST Segment: RFC-793 Section 3.4
           *
           *    A TCP SHOULD allow a received RST segment to include data.
           *
           *    DISCUSSION
           *         It has been suggested that a RST segment could contain
           *         ASCII text that encoded and explained the cause of the
           *         RST.  No standard has yet been established for such
           *         data.
           *
           * If the reset segment passes the sequence number test examine
           * the state:
           *    SYN_RECEIVED STATE:
           *      If passive open, return to LISTEN state.
           *      If active open, inform user that connection was refused.
           *    ESTABLISHED, FIN_WAIT_1, FIN_WAIT_2, CLOSE_WAIT STATES:
           *      Inform user that connection was reset, and close tcb.
           *    CLOSING, LAST_ACK STATES:
           *      Close the tcb.
           *    TIME_WAIT STATE:
           *      Drop the segment - see Stevens, vol. 2, p. 964 and
           *      RFC 1337.
           */
          if (thflags & TH_RST) {
                  if (SEQ_GEQ(th->th_seq, tp->last_ack_sent - 1) &&
                      SEQ_LEQ(th->th_seq, tp->last_ack_sent + tp->rcv_wnd)) {
                          switch (tp->t_state) {
  
                          case TCPS_SYN_RECEIVED:
                                  so->so_error = ECONNREFUSED;
                                  goto close;
  
                          case TCPS_ESTABLISHED:
                                  if (tcp_insecure_rst == 0 &&
                                      !(SEQ_GEQ(th->th_seq, tp->rcv_nxt - 1) &&
                                      SEQ_LEQ(th->th_seq, tp->rcv_nxt + 1)) &&
                                      !(SEQ_GEQ(th->th_seq, tp->last_ack_sent - 1) &&
                                      SEQ_LEQ(th->th_seq, tp->last_ack_sent + 1))) {
                                          tcpstat.tcps_badrst++;
                                          goto drop;
                                  }
                          case TCPS_FIN_WAIT_1:
                          case TCPS_FIN_WAIT_2:
                          case TCPS_CLOSE_WAIT:
                                  so->so_error = ECONNRESET;
                          close:
                                  tp->t_state = TCPS_CLOSED;
                                  tcpstat.tcps_drops++;
                                  KASSERT(headlocked, ("tcp_input: "
                                      "trimthenstep6: tcp_close: head not "
                                      "locked"));
                                  tp = tcp_close(tp);
                                  break;
  
                          case TCPS_CLOSING:
                          case TCPS_LAST_ACK:
                                  KASSERT(headlocked, ("trimthenstep6: "
                                      "tcp_close.2: head not locked"));
                                  tp = tcp_close(tp);
                                  break;
  
                          case TCPS_TIME_WAIT:
                                  KASSERT(tp->t_state != TCPS_TIME_WAIT,
                                      ("timewait"));
                                  break;
                          }
                  }
                  goto drop;
          }
  
          /*
           * RFC 1323 PAWS: If we have a timestamp reply on this segment
           * and it's less than ts_recent, drop it.
           */
          if ((to.to_flags & TOF_TS) != 0 && tp->ts_recent &&
              TSTMP_LT(to.to_tsval, tp->ts_recent)) {
  
                  /* Check to see if ts_recent is over 24 days old.  */
                  if ((int)(ticks - tp->ts_recent_age) > TCP_PAWS_IDLE) {
                          /*
                           * Invalidate ts_recent.  If this segment updates
                           * ts_recent, the age will be reset later and ts_recent
                           * will get a valid value.  If it does not, setting
                           * ts_recent to zero will at least satisfy the
                           * requirement that zero be placed in the timestamp
                           * echo reply when ts_recent isn't valid.  The
                           * age isn't reset until we get a valid ts_recent
                           * because we don't want out-of-order segments to be
                           * dropped when ts_recent is old.
                           */
                          tp->ts_recent = 0;
                  } else {
                          tcpstat.tcps_rcvduppack++;
                          tcpstat.tcps_rcvdupbyte += tlen;
                          tcpstat.tcps_pawsdrop++;
                          if (tlen)
                                  goto dropafterack;
                          goto drop;
                  }
          }
  
          /*
           * In the SYN-RECEIVED state, validate that the packet belongs to
           * this connection before trimming the data to fit the receive
           * window.  Check the sequence number versus IRS since we know
           * the sequence numbers haven't wrapped.  This is a partial fix
           * for the "LAND" DoS attack.
           */
          if (tp->t_state == TCPS_SYN_RECEIVED && SEQ_LT(th->th_seq, tp->irs)) {
                  rstreason = BANDLIM_RST_OPENPORT;
                  goto dropwithreset;
          }
  
          todrop = tp->rcv_nxt - th->th_seq;
          if (todrop > 0) {
                  if (thflags & TH_SYN) {
                          thflags &= ~TH_SYN;
                          th->th_seq++;
                          if (th->th_urp > 1)
                                  th->th_urp--;
                          else
                                  thflags &= ~TH_URG;
                          todrop--;
                  }
                  /*
                   * Following if statement from Stevens, vol. 2, p. 960.
                   */
                  if (todrop > tlen
                      || (todrop == tlen && (thflags & TH_FIN) == 0)) {
                          /*
                           * Any valid FIN must be to the left of the window.
                           * At this point the FIN must be a duplicate or out
                           * of sequence; drop it.
                           */
                          thflags &= ~TH_FIN;
  
                          /*
                           * Send an ACK to resynchronize and drop any data.
                           * But keep on processing for RST or ACK.
                           */
                          tp->t_flags |= TF_ACKNOW;
                          todrop = tlen;
                          tcpstat.tcps_rcvduppack++;
                          tcpstat.tcps_rcvdupbyte += todrop;
                  } else {
                          tcpstat.tcps_rcvpartduppack++;
                          tcpstat.tcps_rcvpartdupbyte += todrop;
                  }
                  drop_hdrlen += todrop;  /* drop from the top afterwards */
                  th->th_seq += todrop;
                  tlen -= todrop;
                  if (th->th_urp > todrop)
                          th->th_urp -= todrop;
                  else {
                          thflags &= ~TH_URG;
                          th->th_urp = 0;
                  }
          }
  
          /*
           * If new data are received on a connection after the
           * user processes are gone, then RST the other end.
           */
          if ((so->so_state & SS_NOFDREF) &&
              tp->t_state > TCPS_CLOSE_WAIT && tlen) {
                  KASSERT(headlocked, ("trimthenstep6: tcp_close.3: head not "
                      "locked"));
                  tp = tcp_close(tp);
                  tcpstat.tcps_rcvafterclose++;
                  rstreason = BANDLIM_UNLIMITED;
                  goto dropwithreset;
          }
  
          /*
           * If segment ends after window, drop trailing data
           * (and PUSH and FIN); if nothing left, just ACK.
           */
          todrop = (th->th_seq+tlen) - (tp->rcv_nxt+tp->rcv_wnd);
          if (todrop > 0) {
                  tcpstat.tcps_rcvpackafterwin++;
                  if (todrop >= tlen) {
                          tcpstat.tcps_rcvbyteafterwin += tlen;
                          /*
                           * If a new connection request is received
                           * while in TIME_WAIT, drop the old connection
                           * and start over if the sequence numbers
                           * are above the previous ones.
                           */
                          KASSERT(tp->t_state != TCPS_TIME_WAIT, ("timewait"));
                          if (thflags & TH_SYN &&
                              tp->t_state == TCPS_TIME_WAIT &&
                              SEQ_GT(th->th_seq, tp->rcv_nxt)) {
                                  KASSERT(headlocked, ("trimthenstep6: "
                                      "tcp_close.4: head not locked"));
                                  tp = tcp_close(tp);
                                  goto findpcb;
                          }
                          /*
                           * If window is closed can only take segments at
                           * window edge, and have to drop data and PUSH from
                           * incoming segments.  Continue processing, but
                           * remember to ack.  Otherwise, drop segment
                           * and ack.
                           */
                          if (tp->rcv_wnd == 0 && th->th_seq == tp->rcv_nxt) {
                                  tp->t_flags |= TF_ACKNOW;
                                  tcpstat.tcps_rcvwinprobe++;
                          } else
                                  goto dropafterack;
                  } else
                          tcpstat.tcps_rcvbyteafterwin += todrop;
                  m_adj(m, -todrop);
                  tlen -= todrop;
                  thflags &= ~(TH_PUSH|TH_FIN);
          }
  
          /*
           * If last ACK falls within this segment's sequence numbers,
           * record its timestamp.
           * NOTE: 
           * 1) That the test incorporates suggestions from the latest
           *    proposal of the tcplw@cray.com list (Braden 1993/04/26).
           * 2) That updating only on newer timestamps interferes with
           *    our earlier PAWS tests, so this check should be solely
           *    predicated on the sequence space of this segment.
           * 3) That we modify the segment boundary check to be 
           *        Last.ACK.Sent <= SEG.SEQ + SEG.Len  
           *    instead of RFC1323's
           *        Last.ACK.Sent < SEG.SEQ + SEG.Len,
           *    This modified check allows us to overcome RFC1323's
           *    limitations as described in Stevens TCP/IP Illustrated
           *    Vol. 2 p.869. In such cases, we can still calculate the
           *    RTT correctly when RCV.NXT == Last.ACK.Sent.
           */
          if ((to.to_flags & TOF_TS) != 0 &&
              SEQ_LEQ(th->th_seq, tp->last_ack_sent) &&
              SEQ_LEQ(tp->last_ack_sent, th->th_seq + tlen +
                  ((thflags & (TH_SYN|TH_FIN)) != 0))) {
                  tp->ts_recent_age = ticks;
                  tp->ts_recent = to.to_tsval;
          }
  
          /*
           * If a SYN is in the window, then this is an
           * error and we send an RST and drop the connection.
           */
          if (thflags & TH_SYN) {
                  KASSERT(headlocked, ("tcp_input: tcp_drop: trimthenstep6: "
                      "head not locked"));
                  tp = tcp_drop(tp, ECONNRESET);
                  rstreason = BANDLIM_UNLIMITED;
                  goto drop;
          }
  
          /*
           * If the ACK bit is off:  if in SYN-RECEIVED state or SENDSYN
           * flag is on (half-synchronized state), then queue data for
           * later processing; else drop segment and return.
           */
          if ((thflags & TH_ACK) == 0) {
                  if (tp->t_state == TCPS_SYN_RECEIVED ||
                      (tp->t_flags & TF_NEEDSYN))
                          goto step6;
                  else if (tp->t_flags & TF_ACKNOW)
                          goto dropafterack;
                  else
                          goto drop;
          }
  
          /*
           * Ack processing.
           */
          switch (tp->t_state) {
  
          /*
           * In SYN_RECEIVED state, the ack ACKs our SYN, so enter
           * ESTABLISHED state and continue processing.
           * The ACK was checked above.
           */
          case TCPS_SYN_RECEIVED:
  
                  tcpstat.tcps_connects++;
                  soisconnected(so);
                  /* Do window scaling? */
                  if ((tp->t_flags & (TF_RCVD_SCALE|TF_REQ_SCALE)) ==
                          (TF_RCVD_SCALE|TF_REQ_SCALE)) {
                          tp->snd_scale = tp->requested_s_scale;
                          tp->rcv_scale = tp->request_r_scale;
                  }
                  /*
                   * Make transitions:
                   *      SYN-RECEIVED  -> ESTABLISHED
                   *      SYN-RECEIVED* -> FIN-WAIT-1
                   */
                  tp->t_starttime = ticks;
                  if (tp->t_flags & TF_NEEDFIN) {
                          tp->t_state = TCPS_FIN_WAIT_1;
                          tp->t_flags &= ~TF_NEEDFIN;
                  } else {
                          tp->t_state = TCPS_ESTABLISHED;
                          callout_reset(tp->tt_keep, tcp_keepidle,
                                        tcp_timer_keep, tp);
                  }
                  /*
                   * If segment contains data or ACK, will call tcp_reass()
                   * later; if not, do so now to pass queued data to user.
                   */
                  if (tlen == 0 && (thflags & TH_FIN) == 0)
                          (void) tcp_reass(tp, (struct tcphdr *)0, 0,
                              (struct mbuf *)0);
                  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:
                  KASSERT(tp->t_state != TCPS_TIME_WAIT, ("timewait"));
                  if (SEQ_GT(th->th_ack, tp->snd_max)) {
                          tcpstat.tcps_rcvacktoomuch++;
                          goto dropafterack;
                  }
                  if (tp->sack_enable &&
                      (to.to_nsacks > 0 || !TAILQ_EMPTY(&tp->snd_holes)))
                          tcp_sack_doack(tp, &to, th->th_ack);
                  if (SEQ_LEQ(th->th_ack, tp->snd_una)) {
                          if (tlen == 0 && tiwin == tp->snd_wnd) {
                                  tcpstat.tcps_rcvdupack++;
                                  /*
                                   * If we have outstanding data (other than
                                   * a window probe), this is a completely
                                   * duplicate ack (ie, window info didn't
                                   * change), the ack is the biggest we've
                                   * seen and we've seen exactly our rexmt
                                   * threshhold of them, assume a packet
                                   * has been dropped and retransmit it.
                                   * Kludge snd_nxt & the congestion
                                   * window so we send only this one
                                   * packet.
                                   *
                                   * We know we're losing at the current
                                   * window size so do congestion avoidance
                                   * (set ssthresh to half the current window
                                   * and pull our congestion window back to
                                   * the new ssthresh).
                                   *
                                   * Dup acks mean that packets have left the
                                   * network (they're now cached at the receiver)
                                   * so bump cwnd by the amount in the receiver
                                   * to keep a constant cwnd packets in the
                                   * network.
                                   */
                                  if (!callout_active(tp->tt_rexmt) ||
                                      th->th_ack != tp->snd_una)
                                          tp->t_dupacks = 0;
                                  else if (++tp->t_dupacks > tcprexmtthresh ||
                                           ((tcp_do_newreno || tp->sack_enable) &&
                                            IN_FASTRECOVERY(tp))) {
                                          if (tp->sack_enable && IN_FASTRECOVERY(tp)) {
                                                  int awnd;
                                                  
                                                  /*
                                                   * Compute the amount of data in flight first.
                                                   * We can inject new data into the pipe iff 
                                                   * we have less than 1/2 the original window's  
                                                   * worth of data in flight.
                                                   */
                                                  awnd = (tp->snd_nxt - tp->snd_fack) +
                                                          tp->sackhint.sack_bytes_rexmit;
                                                  if (awnd < tp->snd_ssthresh) {
                                                          tp->snd_cwnd += tp->t_maxseg;
                                                          if (tp->snd_cwnd > tp->snd_ssthresh)
                                                                  tp->snd_cwnd = tp->snd_ssthresh;
                                                  }
                                          } else
                                                  tp->snd_cwnd += tp->t_maxseg;
                                          if ((thflags & TH_FIN) &&
                                              (TCPS_HAVERCVDFIN(tp->t_state) == 0)) {
                                                  /* 
                                                   * If its a fin we need to process
                                                   * it to avoid a race where both
                                                   * sides enter FIN-WAIT and send FIN|ACK
                                                   * at the same time.
                                                   */
                                                  break;
                                          }
                                          (void) tcp_output(tp);
                                          goto drop;
                                  } else if (tp->t_dupacks == tcprexmtthresh) {
                                          tcp_seq onxt = tp->snd_nxt;
                                          u_int win;
  
                                          /*
                                           * If we're doing sack, check to
                                           * see if we're already in sack
                                           * recovery. If we're not doing sack,
                                           * check to see if we're in newreno
                                           * recovery.
                                           */
                                          if (tp->sack_enable) {
                                                  if (IN_FASTRECOVERY(tp)) {
                                                          tp->t_dupacks = 0;
                                                          break;
                                                  }
                                          } else if (tcp_do_newreno) {
                                                  if (SEQ_LEQ(th->th_ack,
                                                      tp->snd_recover)) {
                                                          tp->t_dupacks = 0;
                                                          break;
                                                  }
                                          }
                                          win = min(tp->snd_wnd, tp->snd_cwnd) /
                                              2 / tp->t_maxseg;
                                          if (win < 2)
                                                  win = 2;
                                          tp->snd_ssthresh = win * tp->t_maxseg;
                                          ENTER_FASTRECOVERY(tp);
                                          tp->snd_recover = tp->snd_max;
                                          callout_stop(tp->tt_rexmt);
                                          tp->t_rtttime = 0;
                                          if (tp->sack_enable) {
                                                  tcpstat.tcps_sack_recovery_episode++;
                                                  tp->sack_newdata = tp->snd_nxt;
                                                  tp->snd_cwnd = tp->t_maxseg;
                                                  (void) tcp_output(tp);
                                                  goto drop;
                                          }
                                          tp->snd_nxt = th->th_ack;
                                          tp->snd_cwnd = tp->t_maxseg;
                                          if ((thflags & TH_FIN) &&
                                              (TCPS_HAVERCVDFIN(tp->t_state) == 0)) {
                                                  /* 
                                                   * If its a fin we need to process
                                                   * it to avoid a race where both
                                                   * sides enter FIN-WAIT and send FIN|ACK
                                                   * at the same time.
                                                   */
                                                  break;
                                          }
                                          (void) tcp_output(tp);
                                          KASSERT(tp->snd_limited <= 2,
                                              ("tp->snd_limited too big"));
                                          tp->snd_cwnd = tp->snd_ssthresh +
                                               tp->t_maxseg *
                                               (tp->t_dupacks - tp->snd_limited);
                                          if (SEQ_GT(onxt, tp->snd_nxt))
                                                  tp->snd_nxt = onxt;
                                          goto drop;
                                  } else if (tcp_do_rfc3042) {
                                          u_long oldcwnd = tp->snd_cwnd;
                                          tcp_seq oldsndmax = tp->snd_max;
                                          u_int sent;
  
                                          KASSERT(tp->t_dupacks == 1 ||
                                              tp->t_dupacks == 2,
                                              ("dupacks not 1 or 2"));
                                          if (tp->t_dupacks == 1)
                                                  tp->snd_limited = 0;
                                          tp->snd_cwnd =
                                              (tp->snd_nxt - tp->snd_una) +
                                              (tp->t_dupacks - tp->snd_limited) *
                                              tp->t_maxseg;
                                          if ((thflags & TH_FIN) &&
                                              (TCPS_HAVERCVDFIN(tp->t_state) == 0)) {
                                                  /* 
                                                   * If its a fin we need to process
                                                   * it to avoid a race where both
                                                   * sides enter FIN-WAIT and send FIN|ACK
                                                   * at the same time.
                                                   */
                                                  break;
                                          }
                                          (void) tcp_output(tp);
                                          sent = tp->snd_max - oldsndmax;
                                          if (sent > tp->t_maxseg) {
                                                  KASSERT((tp->t_dupacks == 2 &&
                                                      tp->snd_limited == 0) ||
                                                     (sent == tp->t_maxseg + 1 &&
                                                      tp->t_flags & TF_SENTFIN),
                                                      ("sent too much"));
                                                  tp->snd_limited = 2;
                                          } else if (sent > 0)
                                                  ++tp->snd_limited;
                                          tp->snd_cwnd = oldcwnd;
                                          goto drop;
                                  }
                          } else
                                  tp->t_dupacks = 0;
                          break;
                  }
  
                  KASSERT(SEQ_GT(th->th_ack, tp->snd_una), ("th_ack <= snd_una"));
  
                  /*
                   * If the congestion window was inflated to account
                   * for the other side's cached packets, retract it.
                   */
                  if (tcp_do_newreno || tp->sack_enable) {
                          if (IN_FASTRECOVERY(tp)) {
                                  if (SEQ_LT(th->th_ack, tp->snd_recover)) {
                                          if (tp->sack_enable)
                                                  tcp_sack_partialack(tp, th);
                                          else
                                                  tcp_newreno_partial_ack(tp, th);
                                  } else {
                                          /*
                                           * Out of fast recovery.
                                           * Window inflation should have left us
                                           * with approximately snd_ssthresh
                                           * outstanding data.
                                           * But in case we would be inclined to
                                           * send a burst, better to do it via
                                           * the slow start mechanism.
                                           */
                                          if (SEQ_GT(th->th_ack +
                                                          tp->snd_ssthresh,
                                                     tp->snd_max))
                                                  tp->snd_cwnd = tp->snd_max -
                                                                  th->th_ack +
                                                                  tp->t_maxseg;
                                          else
                                                  tp->snd_cwnd = tp->snd_ssthresh;
                                  }
                          }
                  } else {
                          if (tp->t_dupacks >= tcprexmtthresh &&
                              tp->snd_cwnd > tp->snd_ssthresh)
                                  tp->snd_cwnd = tp->snd_ssthresh;
                  }
                  tp->t_dupacks = 0;
                  /*
                   * If we reach this point, ACK is not a duplicate,
                   *     i.e., it ACKs something we sent.
                   */
                  if (tp->t_flags & TF_NEEDSYN) {
                          /*
                           * T/TCP: Connection was half-synchronized, and our
                           * SYN has been ACK'd (so connection is now fully
                           * synchronized).  Go to non-starred state,
                           * increment snd_una for ACK of SYN, and check if
                           * we can do window scaling.
                           */
                          tp->t_flags &= ~TF_NEEDSYN;
                          tp->snd_una++;
                          /* Do window scaling? */
                          if ((tp->t_flags & (TF_RCVD_SCALE|TF_REQ_SCALE)) ==
                                  (TF_RCVD_SCALE|TF_REQ_SCALE)) {
                                  tp->snd_scale = tp->requested_s_scale;
                                  tp->rcv_scale = tp->request_r_scale;
                          }
                  }
  
  process_ACK:
                  KASSERT(headlocked, ("tcp_input: process_ACK: head not "
                      "locked"));
                  INP_LOCK_ASSERT(inp);
  
                  acked = th->th_ack - tp->snd_una;
                  tcpstat.tcps_rcvackpack++;
                  tcpstat.tcps_rcvackbyte += acked;
  
                  /*
                   * If we just performed our first retransmit, and the ACK
                   * arrives within our recovery window, then it was a mistake
                   * to do the retransmit in the first place.  Recover our
                   * original cwnd and ssthresh, and proceed to transmit where
                   * we left off.
                   */
                  if (tp->t_rxtshift == 1 && (int)(ticks - tp->t_badrxtwin) < 0) {
                          ++tcpstat.tcps_sndrexmitbad;
                          tp->snd_cwnd = tp->snd_cwnd_prev;
                          tp->snd_ssthresh = tp->snd_ssthresh_prev;
                          tp->snd_recover = tp->snd_recover_prev;
                          if (tp->t_flags & TF_WASFRECOVERY)
                                  ENTER_FASTRECOVERY(tp);
                          tp->snd_nxt = tp->snd_max;
                          tp->t_badrxtwin = 0;    /* XXX probably not required */
                  }
  
                  /*
                   * If we have a timestamp reply, update smoothed
                   * round trip time.  If no timestamp is present but
                   * transmit timer is running and timed sequence
                   * number was acked, update smoothed round trip time.
                   * Since we now have an rtt measurement, cancel the
                   * timer backoff (cf., Phil Karn's retransmit alg.).
                   * Recompute the initial retransmit timer.
                   *
                   * Some boxes send broken timestamp replies
                   * during the SYN+ACK phase, ignore
                   * timestamps of 0 or we could calculate a
                   * huge RTT and blow up the retransmit timer.
                   */
                  if ((to.to_flags & TOF_TS) != 0 &&
                      to.to_tsecr) {
                          if (!tp->t_rttlow || tp->t_rttlow > ticks - to.to_tsecr)
                                  tp->t_rttlow = ticks - to.to_tsecr;
                          tcp_xmit_timer(tp, ticks - to.to_tsecr + 1);
                  } else if (tp->t_rtttime && SEQ_GT(th->th_ack, tp->t_rtseq)) {
                          if (!tp->t_rttlow || tp->t_rttlow > ticks - tp->t_rtttime)
                                  tp->t_rttlow = ticks - tp->t_rtttime;
                          tcp_xmit_timer(tp, ticks - tp->t_rtttime);
                  }
                  tcp_xmit_bandwidth_limit(tp, th->th_ack);
  
                  /*
                   * 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) {
                          callout_stop(tp->tt_rexmt);
                          needoutput = 1;
                  } else if (!callout_active(tp->tt_persist))
                          callout_reset(tp->tt_rexmt, tp->t_rxtcur,
                                        tcp_timer_rexmt, tp);
  
                  /*
                   * If no data (only SYN) was ACK'd,
                   *    skip rest of ACK processing.
                   */
                  if (acked == 0)
                          goto step6;
  
                  /*
                   * When new data is acked, open the congestion window.
                   * If the window gives us less than ssthresh packets
                   * in flight, open exponentially (maxseg per packet).
                   * Otherwise open linearly: maxseg per window
                   * (maxseg^2 / cwnd per packet).
                   * If cwnd > maxseg^2, fix the cwnd increment at 1 byte
                   * to avoid capping cwnd (as suggested in RFC2581).
                   */
                  if ((!tcp_do_newreno && !tp->sack_enable) ||
                      !IN_FASTRECOVERY(tp)) {
                          register u_int cw = tp->snd_cwnd;
                          register u_int incr = tp->t_maxseg;
                          if (cw > tp->snd_ssthresh)
                                  incr = max((incr * incr / cw), 1);
                          tp->snd_cwnd = min(cw+incr, TCP_MAXWIN<<tp->snd_scale);
                  }
                  SOCKBUF_LOCK(&so->so_snd);
                  if (acked > so->so_snd.sb_cc) {
                          tp->snd_wnd -= so->so_snd.sb_cc;
                          sbdrop_locked(&so->so_snd, (int)so->so_snd.sb_cc);
                          ourfinisacked = 1;
                  } else {
                          sbdrop_locked(&so->so_snd, acked);
                          tp->snd_wnd -= acked;
                          ourfinisacked = 0;
                  }
                  sowwakeup_locked(so);
                  /* detect una wraparound */
                  if ((tcp_do_newreno || tp->sack_enable) &&
                      !IN_FASTRECOVERY(tp) &&
                      SEQ_GT(tp->snd_una, tp->snd_recover) &&
                      SEQ_LEQ(th->th_ack, tp->snd_recover))
                          tp->snd_recover = th->th_ack - 1;
                  if ((tcp_do_newreno || tp->sack_enable) &&
                      IN_FASTRECOVERY(tp) &&
                      SEQ_GEQ(th->th_ack, tp->snd_recover))
                          EXIT_FASTRECOVERY(tp);
                  tp->snd_una = th->th_ack;
                  if (tp->sack_enable) {
                          if (SEQ_GT(tp->snd_una, tp->snd_recover))
                                  tp->snd_recover = tp->snd_una;
                  }
                  if (SEQ_LT(tp->snd_nxt, tp->snd_una))
                          tp->snd_nxt = tp->snd_una;
  
                  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.
                                   */
                  /* XXXjl
                   * we should release the tp also, and use a
                   * compressed state.
                   */
                                  if (so->so_rcv.sb_state & SBS_CANTRCVMORE) {
                                          soisdisconnected(so);
                                          callout_reset(tp->tt_2msl, tcp_maxidle,
                                                        tcp_timer_2msl, tp);
                                  }
                                  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) {
                                  KASSERT(headlocked, ("tcp_input: process_ACK: "
                                      "head not locked"));
                                  tcp_twstart(tp);
                                  INP_INFO_WUNLOCK(&tcbinfo);
                                  m_freem(m);
                                  return;
                          }
                          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) {
                                  KASSERT(headlocked, ("tcp_input: process_ACK:"
                                      " tcp_close: head not locked"));
                                  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:
                          KASSERT(tp->t_state != TCPS_TIME_WAIT, ("timewait"));
                          callout_reset(tp->tt_2msl, 2 * tcp_msl,
                                        tcp_timer_2msl, tp);
                          goto dropafterack;
                  }
          }
  
  step6:
          KASSERT(headlocked, ("tcp_input: step6: head not locked"));
          INP_LOCK_ASSERT(inp);
  
          /*
           * Update window information.
           * Don't look at window if no ACK: TAC's send garbage on first SYN.
           */
          if ((thflags & TH_ACK) &&
              (SEQ_LT(tp->snd_wl1, th->th_seq) ||
              (tp->snd_wl1 == th->th_seq && (SEQ_LT(tp->snd_wl2, th->th_ack) ||
               (tp->snd_wl2 == th->th_ack && tiwin > tp->snd_wnd))))) {
                  /* keep track of pure window updates */
                  if (tlen == 0 &&
                      tp->snd_wl2 == th->th_ack && tiwin > tp->snd_wnd)
                          tcpstat.tcps_rcvwinupd++;
                  tp->snd_wnd = tiwin;
                  tp->snd_wl1 = th->th_seq;
                  tp->snd_wl2 = th->th_ack;
                  if (tp->snd_wnd > tp->max_sndwnd)
                          tp->max_sndwnd = tp->snd_wnd;
                  needoutput = 1;
          }
  
          /*
           * Process segments with URG.
           */
          if ((thflags & TH_URG) && th->th_urp &&
              TCPS_HAVERCVDFIN(tp->t_state) == 0) {
                  /*
                   * This is a kludge, but if we receive and accept
                   * random urgent pointers, we'll crash in
                   * soreceive.  It's hard to imagine someone
                   * actually wanting to send this much urgent data.
                   */
                  SOCKBUF_LOCK(&so->so_rcv);
                  if (th->th_urp + so->so_rcv.sb_cc > sb_max) {
                          th->th_urp = 0;                 /* XXX */
                          thflags &= ~TH_URG;             /* XXX */
                          SOCKBUF_UNLOCK(&so->so_rcv);    /* 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_rcv.sb_state |= SBS_RCVATMARK;
                          sohasoutofband(so);
                          tp->t_oobflags &= ~(TCPOOB_HAVEDATA | TCPOOB_HADDATA);
                  }
                  SOCKBUF_UNLOCK(&so->so_rcv);
                  /*
                   * Remove out of band data so doesn't get presented to user.
                   * This can happen independent of advancing the URG pointer,
                   * but if two URG's are pending at once, some out-of-band
                   * data may creep in... ick.
                   */
                  if (th->th_urp <= (u_long)tlen &&
                      !(so->so_options & SO_OOBINLINE)) {
                          /* hdr drop is delayed */
                          tcp_pulloutofband(so, th, m, drop_hdrlen);
                  }
          } else {
                  /*
                   * If no out of band data is expected,
                   * pull receive urgent pointer along
                   * with the receive window.
                   */
                  if (SEQ_GT(tp->rcv_nxt, tp->rcv_up))
                          tp->rcv_up = tp->rcv_nxt;
          }
  dodata:                                                 /* XXX */
          KASSERT(headlocked, ("tcp_input: dodata: head not locked"));
          INP_LOCK_ASSERT(inp);
  
          /*
           * Process the segment text, merging it into the TCP sequencing queue,
           * and arranging for acknowledgment of receipt if necessary.
           * This process logically involves adjusting tp->rcv_wnd as data
           * is presented to the user (this happens in tcp_usrreq.c,
           * case PRU_RCVD).  If a FIN has already been received on this
           * connection then we just ignore the text.
           */
          if ((tlen || (thflags & TH_FIN)) &&
              TCPS_HAVERCVDFIN(tp->t_state) == 0) {
                  tcp_seq save_start = th->th_seq;
                  tcp_seq save_end = th->th_seq + tlen;
                  m_adj(m, drop_hdrlen);  /* delayed header drop */
                  /*
                   * Insert segment which includes th into TCP reassembly queue
                   * with control block tp.  Set thflags to whether reassembly now
                   * includes a segment with FIN.  This handles the common case
                   * inline (segment is the next to be received on an established
                   * connection, and the queue is empty), avoiding linkage into
                   * and removal from the queue and repetition of various
                   * conversions.
                   * Set DELACK for segments received in order, but ack
                   * immediately when segments are out of order (so
                   * fast retransmit can work).
                   */
                  if (th->th_seq == tp->rcv_nxt &&
                      LIST_EMPTY(&tp->t_segq) &&
                      TCPS_HAVEESTABLISHED(tp->t_state)) {
                          if (DELAY_ACK(tp))
                                  tp->t_flags |= TF_DELACK;
                          else
                                  tp->t_flags |= TF_ACKNOW;
                          tp->rcv_nxt += tlen;
                          thflags = th->th_flags & TH_FIN;
                          tcpstat.tcps_rcvpack++;
                          tcpstat.tcps_rcvbyte += tlen;
                          ND6_HINT(tp);
                          SOCKBUF_LOCK(&so->so_rcv);
                          if (so->so_rcv.sb_state & SBS_CANTRCVMORE)
                                  m_freem(m);
                          else
                                  sbappendstream_locked(&so->so_rcv, m);
                          sorwakeup_locked(so);
                  } else {
                          thflags = tcp_reass(tp, th, &tlen, m);
                          tp->t_flags |= TF_ACKNOW;
                  }
                  if (tlen > 0 && tp->sack_enable)
                          tcp_update_sack_list(tp, save_start, save_end);
                  /*
                   * Note the amount of data that peer has sent into
                   * our window, in order to estimate the sender's
                   * buffer size.
                   */
                  len = so->so_rcv.sb_hiwat - (tp->rcv_adv - tp->rcv_nxt);
          } else {
                  m_freem(m);
                  thflags &= ~TH_FIN;
          }
  
          /*
           * If FIN is received ACK the FIN and let the user know
           * that the connection is closing.
           */
          if (thflags & TH_FIN) {
                  if (TCPS_HAVERCVDFIN(tp->t_state) == 0) {
                          socantrcvmore(so);
                          /*
                           * If connection is half-synchronized
                           * (ie NEEDSYN flag on) then delay ACK,
                           * so it may be piggybacked when SYN is sent.
                           * Otherwise, since we received a FIN then no
                           * more input can be expected, send ACK now.
                           */
                          if (tp->t_flags & TF_NEEDSYN)
                                  tp->t_flags |= TF_DELACK;
                          else
                                  tp->t_flags |= TF_ACKNOW;
                          tp->rcv_nxt++;
                  }
                  switch (tp->t_state) {
  
                  /*
                   * In SYN_RECEIVED and ESTABLISHED STATES
                   * enter the CLOSE_WAIT state.
                   */
                  case TCPS_SYN_RECEIVED:
                          tp->t_starttime = ticks;
                          /*FALLTHROUGH*/
                  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:
                          KASSERT(headlocked == 1, ("tcp_input: dodata: "
                              "TCP_FIN_WAIT_2: head not locked"));
                          tcp_twstart(tp);
                          INP_INFO_WUNLOCK(&tcbinfo);
                          return;
  
                  /*
                   * In TIME_WAIT state restart the 2 MSL time_wait timer.
                   */
                  case TCPS_TIME_WAIT:
                          KASSERT(tp->t_state != TCPS_TIME_WAIT, ("timewait"));
                          callout_reset(tp->tt_2msl, 2 * tcp_msl,
                                        tcp_timer_2msl, tp);
                          break;
                  }
          }
          INP_INFO_WUNLOCK(&tcbinfo);
          headlocked = 0;
  #ifdef TCPDEBUG
          if (so->so_options & SO_DEBUG)
                  tcp_trace(TA_INPUT, ostate, tp, (void *)tcp_saveipgen,
                            &tcp_savetcp, 0);
  #endif
  
          /*
           * Return any desired output.
           */
          if (needoutput || (tp->t_flags & TF_ACKNOW))
                  (void) tcp_output(tp);
  
  check_delack:
          KASSERT(headlocked == 0, ("tcp_input: check_delack: head locked"));
          INP_LOCK_ASSERT(inp);
          if (tp->t_flags & TF_DELACK) {
                  tp->t_flags &= ~TF_DELACK;
                  callout_reset(tp->tt_delack, tcp_delacktime,
                      tcp_timer_delack, tp);
          }
          INP_UNLOCK(inp);
          return;
  
  dropafterack:
          KASSERT(headlocked, ("tcp_input: dropafterack: head not locked"));
          /*
           * Generate an ACK dropping incoming segment if it occupies
           * sequence space, where the ACK reflects our state.
           *
           * We can now skip the test for the RST flag since all
           * paths to this code happen after packets containing
           * RST have been dropped.
           *
           * In the SYN-RECEIVED state, don't send an ACK unless the
           * segment we received passes the SYN-RECEIVED ACK test.
           * If it fails send a RST.  This breaks the loop in the
           * "LAND" DoS attack, and also prevents an ACK storm
           * between two listening ports that have been sent forged
           * SYN segments, each with the source address of the other.
           */
          if (tp->t_state == TCPS_SYN_RECEIVED && (thflags & TH_ACK) &&
              (SEQ_GT(tp->snd_una, th->th_ack) ||
               SEQ_GT(th->th_ack, tp->snd_max)) ) {
                  rstreason = BANDLIM_RST_OPENPORT;
                  goto dropwithreset;
          }
  #ifdef TCPDEBUG
          if (so->so_options & SO_DEBUG)
                  tcp_trace(TA_DROP, ostate, tp, (void *)tcp_saveipgen,
                            &tcp_savetcp, 0);
  #endif
          KASSERT(headlocked, ("headlocked should be 1"));
          INP_INFO_WUNLOCK(&tcbinfo);
          tp->t_flags |= TF_ACKNOW;
          (void) tcp_output(tp);
          INP_UNLOCK(inp);
          m_freem(m);
          return;
  
  dropwithreset:
          KASSERT(headlocked, ("tcp_input: dropwithreset: head not locked"));
          /*
           * Generate a RST, dropping incoming segment.
           * Make ACK acceptable to originator of segment.
           * Don't bother to respond if destination was broadcast/multicast.
           */
          if ((thflags & TH_RST) || m->m_flags & (M_BCAST|M_MCAST))
                  goto drop;
          if (isipv6) {
                  if (IN6_IS_ADDR_MULTICAST(&ip6->ip6_dst) ||
                      IN6_IS_ADDR_MULTICAST(&ip6->ip6_src))
                          goto drop;
          } else {
                  if (IN_MULTICAST(ntohl(ip->ip_dst.s_addr)) ||
                      IN_MULTICAST(ntohl(ip->ip_src.s_addr)) ||
                      ip->ip_src.s_addr == htonl(INADDR_BROADCAST) ||
                      in_broadcast(ip->ip_dst, m->m_pkthdr.rcvif))
                          goto drop;
          }
          /* IPv6 anycast check is done at tcp6_input() */
  
          /*
           * Perform bandwidth limiting.
           */
          if (badport_bandlim(rstreason) < 0)
                  goto drop;
  
  #ifdef TCPDEBUG
          if (tp == 0 || (tp->t_inpcb->inp_socket->so_options & SO_DEBUG))
                  tcp_trace(TA_DROP, ostate, tp, (void *)tcp_saveipgen,
                            &tcp_savetcp, 0);
  #endif
  
          if (thflags & TH_ACK)
                  /* mtod() below is safe as long as hdr dropping is delayed */
                  tcp_respond(tp, mtod(m, void *), th, m, (tcp_seq)0, th->th_ack,
                              TH_RST);
          else {
                  if (thflags & TH_SYN)
                          tlen++;
                  /* mtod() below is safe as long as hdr dropping is delayed */
                  tcp_respond(tp, mtod(m, void *), th, m, th->th_seq+tlen,
                              (tcp_seq)0, TH_RST|TH_ACK);
          }
  
          if (tp != NULL)
                  INP_UNLOCK(inp);
          if (headlocked)
                  INP_INFO_WUNLOCK(&tcbinfo);
          return;
  
  drop:
          /*
           * Drop space held by incoming segment and return.
           */
  #ifdef TCPDEBUG
          if (tp == NULL || (tp->t_inpcb->inp_socket->so_options & SO_DEBUG))
                  tcp_trace(TA_DROP, ostate, tp, (void *)tcp_saveipgen,
                            &tcp_savetcp, 0);
  #endif
          if (tp != NULL)
                  INP_UNLOCK(inp);
          if (headlocked)
                  INP_INFO_WUNLOCK(&tcbinfo);
          m_freem(m);
          return;
  }
  
  /*
   * Parse TCP options and place in tcpopt.
   */
  static void
  tcp_dooptions(to, cp, cnt, is_syn)
          struct tcpopt *to;
          u_char *cp;
          int cnt;
          int is_syn;
  {
          int opt, optlen;
  
          to->to_flags = 0;
          for (; cnt > 0; cnt -= optlen, cp += optlen) {
                  opt = cp[0];
                  if (opt == TCPOPT_EOL)
                          break;
                  if (opt == TCPOPT_NOP)
                          optlen = 1;
                  else {
                          if (cnt < 2)
                                  break;
                          optlen = cp[1];
                          if (optlen < 2 || optlen > cnt)
                                  break;
                  }
                  switch (opt) {
                  case TCPOPT_MAXSEG:
                          if (optlen != TCPOLEN_MAXSEG)
                                  continue;
                          if (!is_syn)
                                  continue;
                          to->to_flags |= TOF_MSS;
                          bcopy((char *)cp + 2,
                              (char *)&to->to_mss, sizeof(to->to_mss));
                          to->to_mss = ntohs(to->to_mss);
                          break;
                  case TCPOPT_WINDOW:
                          if (optlen != TCPOLEN_WINDOW)
                                  continue;
                          if (! is_syn)
                                  continue;
                          to->to_flags |= TOF_SCALE;
                          to->to_requested_s_scale = min(cp[2], TCP_MAX_WINSHIFT);
                          break;
                  case TCPOPT_TIMESTAMP:
                          if (optlen != TCPOLEN_TIMESTAMP)
                                  continue;
                          to->to_flags |= TOF_TS;
                          bcopy((char *)cp + 2,
                              (char *)&to->to_tsval, sizeof(to->to_tsval));
                          to->to_tsval = ntohl(to->to_tsval);
                          bcopy((char *)cp + 6,
                              (char *)&to->to_tsecr, sizeof(to->to_tsecr));
                          to->to_tsecr = ntohl(to->to_tsecr);
                          /*
                           * If echoed timestamp is later than the current time,
                           * fall back to non RFC1323 RTT calculation.
                           */
                          if ((to->to_tsecr != 0) && TSTMP_GT(to->to_tsecr, ticks))
                                  to->to_tsecr = 0;
                          break;
  #ifdef TCP_SIGNATURE
                  /*
                   * XXX In order to reply to a host which has set the
                   * TCP_SIGNATURE option in its initial SYN, we have to
                   * record the fact that the option was observed here
                   * for the syncache code to perform the correct response.
                   */
                  case TCPOPT_SIGNATURE:
                          if (optlen != TCPOLEN_SIGNATURE)
                                  continue;
                          to->to_flags |= (TOF_SIGNATURE | TOF_SIGLEN);
                          break;
  #endif
                  case TCPOPT_SACK_PERMITTED:
                          if (!tcp_do_sack ||
                              optlen != TCPOLEN_SACK_PERMITTED)
                                  continue;
                          if (is_syn) {
                                  /* MUST only be set on SYN */
                                  to->to_flags |= TOF_SACK;
                          }
                          break;
                  case TCPOPT_SACK:
                          if (optlen <= 2 || (optlen - 2) % TCPOLEN_SACK != 0)
                                  continue;
                          to->to_nsacks = (optlen - 2) / TCPOLEN_SACK;
                          to->to_sacks = cp + 2;
                          tcpstat.tcps_sack_rcv_blocks++;
                          break;
                  default:
                          continue;
                  }
          }
  }
  
  /*
   * Pull out of band byte out of a segment so
   * it doesn't appear in the user's data queue.
   * It is still reflected in the segment length for
   * sequencing purposes.
   */
  static void
  tcp_pulloutofband(so, th, m, off)
          struct socket *so;
          struct tcphdr *th;
          register struct mbuf *m;
          int off;                /* delayed to be droped hdrlen */
  {
          int cnt = off + th->th_urp - 1;
  
          while (cnt >= 0) {
                  if (m->m_len > cnt) {
                          char *cp = mtod(m, caddr_t) + cnt;
                          struct tcpcb *tp = sototcpcb(so);
  
                          tp->t_iobc = *cp;
                          tp->t_oobflags |= TCPOOB_HAVEDATA;
                          bcopy(cp+1, cp, (unsigned)(m->m_len - cnt - 1));
                          m->m_len--;
                          if (m->m_flags & M_PKTHDR)
                                  m->m_pkthdr.len--;
                          return;
                  }
                  cnt -= m->m_len;
                  m = m->m_next;
                  if (m == 0)
                          break;
          }
          panic("tcp_pulloutofband");
  }
  
  /*
   * Collect new round-trip time estimate
   * and update averages and current timeout.
   */
  static void
  tcp_xmit_timer(tp, rtt)
          register struct tcpcb *tp;
          int rtt;
  {
          register int delta;
  
          INP_LOCK_ASSERT(tp->t_inpcb);
  
          tcpstat.tcps_rttupdated++;
          tp->t_rttupdated++;
          if (tp->t_srtt != 0) {
                  /*
                   * srtt is stored as fixed point with 5 bits after the
                   * binary point (i.e., scaled by 8).  The following magic
                   * is equivalent to the smoothing algorithm in rfc793 with
                   * an alpha of .875 (srtt = rtt/8 + srtt*7/8 in fixed
                   * point).  Adjust rtt to origin 0.
                   */
                  delta = ((rtt - 1) << TCP_DELTA_SHIFT)
                          - (tp->t_srtt >> (TCP_RTT_SHIFT - TCP_DELTA_SHIFT));
  
                  if ((tp->t_srtt += delta) <= 0)
                          tp->t_srtt = 1;
  
                  /*
                   * We accumulate a smoothed rtt variance (actually, a
                   * smoothed mean difference), then set the retransmit
                   * timer to smoothed rtt + 4 times the smoothed variance.
                   * rttvar is stored as fixed point with 4 bits after the
                   * binary point (scaled by 16).  The following is
                   * equivalent to rfc793 smoothing with an alpha of .75
                   * (rttvar = rttvar*3/4 + |delta| / 4).  This replaces
                   * rfc793's wired-in beta.
                   */
                  if (delta < 0)
                          delta = -delta;
                  delta -= tp->t_rttvar >> (TCP_RTTVAR_SHIFT - TCP_DELTA_SHIFT);
                  if ((tp->t_rttvar += delta) <= 0)
                          tp->t_rttvar = 1;
                  if (tp->t_rttbest > tp->t_srtt + tp->t_rttvar)
                      tp->t_rttbest = tp->t_srtt + tp->t_rttvar;
          } else {
                  /*
                   * No rtt measurement yet - use the unsmoothed rtt.
                   * Set the variance to half the rtt (so our first
                   * retransmit happens at 3*rtt).
                   */
                  tp->t_srtt = rtt << TCP_RTT_SHIFT;
                  tp->t_rttvar = rtt << (TCP_RTTVAR_SHIFT - 1);
                  tp->t_rttbest = tp->t_srtt + tp->t_rttvar;
          }
          tp->t_rtttime = 0;
          tp->t_rxtshift = 0;
  
          /*
           * the retransmit should happen at rtt + 4 * rttvar.
           * Because of the way we do the smoothing, srtt and rttvar
           * will each average +1/2 tick of bias.  When we compute
           * the retransmit timer, we want 1/2 tick of rounding and
           * 1 extra tick because of +-1/2 tick uncertainty in the
           * firing of the timer.  The bias will give us exactly the
           * 1.5 tick we need.  But, because the bias is
           * statistical, we have to test that we don't drop below
           * the minimum feasible timer (which is 2 ticks).
           */
          TCPT_RANGESET(tp->t_rxtcur, TCP_REXMTVAL(tp),
                        max(tp->t_rttmin, rtt + 2), TCPTV_REXMTMAX);
  
          /*
           * We received an ack for a packet that wasn't retransmitted;
           * it is probably safe to discard any error indications we've
           * received recently.  This isn't quite right, but close enough
           * for now (a route might have failed after we sent a segment,
           * and the return path might not be symmetrical).
           */
          tp->t_softerror = 0;
  }
  
  /*
   * Determine a reasonable value for maxseg size.
   * If the route is known, check route for mtu.
   * If none, use an mss that can be handled on the outgoing
   * interface without forcing IP to fragment; if bigger than
   * an mbuf cluster (MCLBYTES), round down to nearest multiple of MCLBYTES
   * to utilize large mbufs.  If no route is found, route has no mtu,
   * or the destination isn't local, use a default, hopefully conservative
   * size (usually 512 or the default IP max size, but no more than the mtu
   * of the interface), as we can't discover anything about intervening
   * gateways or networks.  We also initialize the congestion/slow start
   * window to be a single segment if the destination isn't local.
   * While looking at the routing entry, we also initialize other path-dependent
   * parameters from pre-set or cached values in the routing entry.
   *
   * Also take into account the space needed for options that we
   * send regularly.  Make maxseg shorter by that amount to assure
   * that we can send maxseg amount of data even when the options
   * are present.  Store the upper limit of the length of options plus
   * data in maxopd.
   *
   *
   * In case of T/TCP, we call this routine during implicit connection
   * setup as well (offer = -1), to initialize maxseg from the cached
   * MSS of our peer.
   *
   * NOTE that this routine is only called when we process an incoming
   * segment. Outgoing SYN/ACK MSS settings are handled in tcp_mssopt().
   */
  void
  tcp_mss(tp, offer)
          struct tcpcb *tp;
          int offer;
  {
          int rtt, mss;
          u_long bufsize;
          u_long maxmtu;
          struct inpcb *inp = tp->t_inpcb;
          struct socket *so;
          struct hc_metrics_lite metrics;
          int origoffer = offer;
  #ifdef INET6
          int isipv6 = ((inp->inp_vflag & INP_IPV6) != 0) ? 1 : 0;
          size_t min_protoh = isipv6 ?
                              sizeof (struct ip6_hdr) + sizeof (struct tcphdr) :
                              sizeof (struct tcpiphdr);
  #else
          const size_t min_protoh = sizeof(struct tcpiphdr);
  #endif
  
          /* initialize */
  #ifdef INET6
          if (isipv6) {
                  maxmtu = tcp_maxmtu6(&inp->inp_inc);
                  tp->t_maxopd = tp->t_maxseg = tcp_v6mssdflt;
          } else
  #endif
          {
                  maxmtu = tcp_maxmtu(&inp->inp_inc);
                  tp->t_maxopd = tp->t_maxseg = tcp_mssdflt;
          }
          so = inp->inp_socket;
  
          /*
           * no route to sender, stay with default mss and return
           */
          if (maxmtu == 0)
                  return;
  
          /* what have we got? */
          switch (offer) {
                  case 0:
                          /*
                           * Offer == 0 means that there was no MSS on the SYN
                           * segment, in this case we use tcp_mssdflt.
                           */
                          offer =
  #ifdef INET6
                                  isipv6 ? tcp_v6mssdflt :
  #endif
                                  tcp_mssdflt;
                          break;
  
                  case -1:
                          /*
                           * Offer == -1 means that we didn't receive SYN yet.
                           */
                          /* FALLTHROUGH */
  
                  default:
                          /*
                           * Prevent DoS attack with too small MSS. Round up
                           * to at least minmss.
                           */
                          offer = max(offer, tcp_minmss);
                          /*
                           * Sanity check: make sure that maxopd will be large
                           * enough to allow some data on segments even if the
                           * all the option space is used (40bytes).  Otherwise
                           * funny things may happen in tcp_output.
                           */
                          offer = max(offer, 64);
          }
  
          /*
           * rmx information is now retrieved from tcp_hostcache
           */
          tcp_hc_get(&inp->inp_inc, &metrics);
  
          /*
           * if there's a discovered mtu int tcp hostcache, use it
           * else, use the link mtu.
           */
          if (metrics.rmx_mtu)
                  mss = min(metrics.rmx_mtu, maxmtu) - min_protoh;
          else {
  #ifdef INET6
                  if (isipv6) {
                          mss = maxmtu - min_protoh;
                          if (!path_mtu_discovery &&
                              !in6_localaddr(&inp->in6p_faddr))
                                  mss = min(mss, tcp_v6mssdflt);
                  } else
  #endif
                  {
                          mss = maxmtu - min_protoh;
                          if (!path_mtu_discovery &&
                              !in_localaddr(inp->inp_faddr))
                                  mss = min(mss, tcp_mssdflt);
                  }
          }
          mss = min(mss, offer);
  
          /*
           * maxopd stores the maximum length of data AND options
           * in a segment; maxseg is the amount of data in a normal
           * segment.  We need to store this value (maxopd) apart
           * from maxseg, because now every segment carries options
           * and thus we normally have somewhat less data in segments.
           */
          tp->t_maxopd = mss;
  
          /*
           * origoffer==-1 indicates, that no segments were received yet.
           * In this case we just guess.
           */
          if ((tp->t_flags & (TF_REQ_TSTMP|TF_NOOPT)) == TF_REQ_TSTMP &&
              (origoffer == -1 ||
               (tp->t_flags & TF_RCVD_TSTMP) == TF_RCVD_TSTMP))
                  mss -= TCPOLEN_TSTAMP_APPA;
          tp->t_maxseg = mss;
  
  #if     (MCLBYTES & (MCLBYTES - 1)) == 0
                  if (mss > MCLBYTES)
                          mss &= ~(MCLBYTES-1);
  #else
                  if (mss > MCLBYTES)
                          mss = mss / MCLBYTES * MCLBYTES;
  #endif
          tp->t_maxseg = mss;
  
          /*
           * If there's a pipesize, change the socket buffer to that size,
           * don't change if sb_hiwat is different than default (then it
           * has been changed on purpose with setsockopt).
           * Make the socket buffers an integral number of mss units;
           * if the mss is larger than the socket buffer, decrease the mss.
           */
          SOCKBUF_LOCK(&so->so_snd);
          if ((so->so_snd.sb_hiwat == tcp_sendspace) && metrics.rmx_sendpipe)
                  bufsize = metrics.rmx_sendpipe;
          else
                  bufsize = so->so_snd.sb_hiwat;
          if (bufsize < mss)
                  mss = bufsize;
          else {
                  bufsize = roundup(bufsize, mss);
                  if (bufsize > sb_max)
                          bufsize = sb_max;
                  if (bufsize > so->so_snd.sb_hiwat)
                          (void)sbreserve_locked(&so->so_snd, bufsize, so, NULL);
          }
          SOCKBUF_UNLOCK(&so->so_snd);
          tp->t_maxseg = mss;
  
          SOCKBUF_LOCK(&so->so_rcv);
          if ((so->so_rcv.sb_hiwat == tcp_recvspace) && metrics.rmx_recvpipe)
                  bufsize = metrics.rmx_recvpipe;
          else
                  bufsize = so->so_rcv.sb_hiwat;
          if (bufsize > mss) {
                  bufsize = roundup(bufsize, mss);
                  if (bufsize > sb_max)
                          bufsize = sb_max;
                  if (bufsize > so->so_rcv.sb_hiwat)
                          (void)sbreserve_locked(&so->so_rcv, bufsize, so, NULL);
          }
          SOCKBUF_UNLOCK(&so->so_rcv);
          /*
           * While we're here, check the others too
           */
          if (tp->t_srtt == 0 && (rtt = metrics.rmx_rtt)) {
                  tp->t_srtt = rtt;
                  tp->t_rttbest = tp->t_srtt + TCP_RTT_SCALE;
                  tcpstat.tcps_usedrtt++;
                  if (metrics.rmx_rttvar) {
                          tp->t_rttvar = metrics.rmx_rttvar;
                          tcpstat.tcps_usedrttvar++;
                  } else {
                          /* default variation is +- 1 rtt */
                          tp->t_rttvar =
                              tp->t_srtt * TCP_RTTVAR_SCALE / TCP_RTT_SCALE;
                  }
                  TCPT_RANGESET(tp->t_rxtcur,
                                ((tp->t_srtt >> 2) + tp->t_rttvar) >> 1,
                                tp->t_rttmin, TCPTV_REXMTMAX);
          }
          if (metrics.rmx_ssthresh) {
                  /*
                   * There's some sort of gateway or interface
                   * buffer limit on the path.  Use this to set
                   * the slow start threshhold, but set the
                   * threshold to no less than 2*mss.
                   */
                  tp->snd_ssthresh = max(2 * mss, metrics.rmx_ssthresh);
                  tcpstat.tcps_usedssthresh++;
          }
          if (metrics.rmx_bandwidth)
                  tp->snd_bandwidth = metrics.rmx_bandwidth;
  
          /*
           * Set the slow-start flight size depending on whether this
           * is a local network or not.
           *
           * Extend this so we cache the cwnd too and retrieve it here.
           * Make cwnd even bigger than RFC3390 suggests but only if we
           * have previous experience with the remote host. Be careful
           * not make cwnd bigger than remote receive window or our own
           * send socket buffer. Maybe put some additional upper bound
           * on the retrieved cwnd. Should do incremental updates to
           * hostcache when cwnd collapses so next connection doesn't
           * overloads the path again.
           *
           * RFC3390 says only do this if SYN or SYN/ACK didn't got lost.
           * We currently check only in syncache_socket for that.
           */
  #define TCP_METRICS_CWND
  #ifdef TCP_METRICS_CWND
          if (metrics.rmx_cwnd)
                  tp->snd_cwnd = max(mss,
                                  min(metrics.rmx_cwnd / 2,
                                   min(tp->snd_wnd, so->so_snd.sb_hiwat)));
          else
  #endif
          if (tcp_do_rfc3390)
                  tp->snd_cwnd = min(4 * mss, max(2 * mss, 4380));
  #ifdef INET6
          else if ((isipv6 && in6_localaddr(&inp->in6p_faddr)) ||
                   (!isipv6 && in_localaddr(inp->inp_faddr)))
  #else
          else if (in_localaddr(inp->inp_faddr))
  #endif
                  tp->snd_cwnd = mss * ss_fltsz_local;
          else
                  tp->snd_cwnd = mss * ss_fltsz;
  }
  
  /*
   * Determine the MSS option to send on an outgoing SYN.
   */
  int
  tcp_mssopt(inc)
          struct in_conninfo *inc;
  {
          int mss = 0;
          u_long maxmtu = 0;
          u_long thcmtu = 0;
          size_t min_protoh;
  #ifdef INET6
          int isipv6 = inc->inc_isipv6 ? 1 : 0;
  #endif
  
          KASSERT(inc != NULL, ("tcp_mssopt with NULL in_conninfo pointer"));
  
  #ifdef INET6
          if (isipv6) {
                  mss = tcp_v6mssdflt;
                  maxmtu = tcp_maxmtu6(inc);
                  thcmtu = tcp_hc_getmtu(inc); /* IPv4 and IPv6 */
                  min_protoh = sizeof(struct ip6_hdr) + sizeof(struct tcphdr);
          } else
  #endif
          {
                  mss = tcp_mssdflt;
                  maxmtu = tcp_maxmtu(inc);
                  thcmtu = tcp_hc_getmtu(inc); /* IPv4 and IPv6 */
                  min_protoh = sizeof(struct tcpiphdr);
          }
          if (maxmtu && thcmtu)
                  mss = min(maxmtu, thcmtu) - min_protoh;
          else if (maxmtu || thcmtu)
                  mss = max(maxmtu, thcmtu) - min_protoh;
  
          return (mss);
  }
  
  
  /*
   * On a partial ack arrives, force the retransmission of the
   * next unacknowledged segment.  Do not clear tp->t_dupacks.
   * By setting snd_nxt to ti_ack, this forces retransmission timer to
   * be started again.
   */
  static void
  tcp_newreno_partial_ack(tp, th)
          struct tcpcb *tp;
          struct tcphdr *th;
  {
          tcp_seq onxt = tp->snd_nxt;
          u_long  ocwnd = tp->snd_cwnd;
  
          callout_stop(tp->tt_rexmt);
          tp->t_rtttime = 0;
          tp->snd_nxt = th->th_ack;
          /*
           * Set snd_cwnd to one segment beyond acknowledged offset.
           * (tp->snd_una has not yet been updated when this function is called.)
           */
          tp->snd_cwnd = tp->t_maxseg + (th->th_ack - tp->snd_una);
          tp->t_flags |= TF_ACKNOW;
          (void) tcp_output(tp);
          tp->snd_cwnd = ocwnd;
          if (SEQ_GT(onxt, tp->snd_nxt))
                  tp->snd_nxt = onxt;
          /*
           * Partial window deflation.  Relies on fact that tp->snd_una
           * not updated yet.
           */
          if (tp->snd_cwnd > th->th_ack - tp->snd_una)
                  tp->snd_cwnd -= th->th_ack - tp->snd_una;
          else
                  tp->snd_cwnd = 0;
          tp->snd_cwnd += tp->t_maxseg;
  }
  
  /*
   * Returns 1 if the TIME_WAIT state was killed and we should start over,
   * looking for a pcb in the listen state.  Returns 0 otherwise.
   */
  static int
  tcp_timewait(tw, to, th, m, tlen)
          struct tcptw *tw;
          struct tcpopt *to;
          struct tcphdr *th;
          struct mbuf *m;
          int tlen;
  {
          int thflags;
          tcp_seq seq;
  #ifdef INET6
          int isipv6 = (mtod(m, struct ip *)->ip_v == 6) ? 1 : 0;
  #else
          const int isipv6 = 0;
  #endif
  
          /* tcbinfo lock required for tcp_twclose(), tcp_timer_2msl_reset(). */
          INP_INFO_WLOCK_ASSERT(&tcbinfo);
          INP_LOCK_ASSERT(tw->tw_inpcb);
  
          thflags = th->th_flags;
  
          /*
           * NOTE: for FIN_WAIT_2 (to be added later),
           * must validate sequence number before accepting RST
           */
  
          /*
           * If the segment contains RST:
           *      Drop the segment - see Stevens, vol. 2, p. 964 and
           *      RFC 1337.
           */
          if (thflags & TH_RST)
                  goto drop;
  
  #if 0
  /* PAWS not needed at the moment */
          /*
           * RFC 1323 PAWS: If we have a timestamp reply on this segment
           * and it's less than ts_recent, drop it.
           */
          if ((to.to_flags & TOF_TS) != 0 && tp->ts_recent &&
              TSTMP_LT(to.to_tsval, tp->ts_recent)) {
                  if ((thflags & TH_ACK) == 0)
                          goto drop;
                  goto ack;
          }
          /*
           * ts_recent is never updated because we never accept new segments.
           */
  #endif
  
          /*
           * If a new connection request is received
           * while in TIME_WAIT, drop the old connection
           * and start over if the sequence numbers
           * are above the previous ones.
           */
          if ((thflags & TH_SYN) && SEQ_GT(th->th_seq, tw->rcv_nxt)) {
                  tcp_twclose(tw, 0);
                  return (1);
          }
  
          /*
           * Drop the the segment if it does not contain an ACK.
           */
          if ((thflags & TH_ACK) == 0)
                  goto drop;
  
          /*
           * Reset the 2MSL timer if this is a duplicate FIN.
           */
          if (thflags & TH_FIN) {
                  seq = th->th_seq + tlen + (thflags & TH_SYN ? 1 : 0);
                  if (seq + 1 == tw->rcv_nxt)
                          tcp_timer_2msl_reset(tw, 1);
          }
  
          /*
           * Acknowledge the segment if it has data or is not a duplicate ACK.
           */
          if (thflags != TH_ACK || tlen != 0 ||
              th->th_seq != tw->rcv_nxt || th->th_ack != tw->snd_nxt)
                  tcp_twrespond(tw, TH_ACK);
          goto drop;
  
          /*
           * Generate a RST, dropping incoming segment.
           * Make ACK acceptable to originator of segment.
           * Don't bother to respond if destination was broadcast/multicast.
           */
          if (m->m_flags & (M_BCAST|M_MCAST))
                  goto drop;
          if (isipv6) {
                  struct ip6_hdr *ip6;
  
                  /* IPv6 anycast check is done at tcp6_input() */
                  ip6 = mtod(m, struct ip6_hdr *);
                  if (IN6_IS_ADDR_MULTICAST(&ip6->ip6_dst) ||
                      IN6_IS_ADDR_MULTICAST(&ip6->ip6_src))
                          goto drop;
          } else {
                  struct ip *ip;
  
                  ip = mtod(m, struct ip *);
                  if (IN_MULTICAST(ntohl(ip->ip_dst.s_addr)) ||
                      IN_MULTICAST(ntohl(ip->ip_src.s_addr)) ||
                      ip->ip_src.s_addr == htonl(INADDR_BROADCAST) ||
                      in_broadcast(ip->ip_dst, m->m_pkthdr.rcvif))
                          goto drop;
          }
          if (thflags & TH_ACK) {
                  tcp_respond(NULL,
                      mtod(m, void *), th, m, 0, th->th_ack, TH_RST);
          } else {
                  seq = th->th_seq + (thflags & TH_SYN ? 1 : 0);
                  tcp_respond(NULL,
                      mtod(m, void *), th, m, seq, 0, TH_RST|TH_ACK);
          }
          INP_UNLOCK(tw->tw_inpcb);
          return (0);
  
  drop:
          INP_UNLOCK(tw->tw_inpcb);
          m_freem(m);
          return (0);
  }