text: /*-
   * 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: src/sys/netinet/tcp_input.c,v 1.291 2006/02/28 23:05:59 andre Exp $
   */
  
  #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>	/* required for icmp_var.h */
  #include <netinet/icmp_var.h>	/* for ICMP_BANDLIM */
  #include <netinet/ip_var.h>
  #include <netinet/ip_options.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 */
  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);
  }
  
  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;
  
  #ifdef MAC
  	INP_LOCK_ASSERT(inp);
  	if (mac_check_inpcb_deliver(inp, m))
  		goto drop;
  #endif
  	so = inp->inp_socket;
  #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 has free'd 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;
  				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.
  			 * XXX: Can't happen at the moment.
  			 */
  			INP_UNLOCK(inp);
  			inp = sotoinpcb(so);
  			INP_LOCK(inp);
  			tp = intotcpcb(inp);
  			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);
  			tiwin = th->th_win << tp->snd_scale;
  			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 "0" 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);
  
  	/*
  	 * Unscale the window into a 32-bit value.
  	 * This value is bogus for the TCPS_SYN_SENT state
  	 * and is overwritten later.
  	 */
  	tiwin = th->th_win << tp->snd_scale;
  
  	/*
  	 * 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_REQ_SCALE)) {
  			tp->t_flags |= TF_RCVD_SCALE;
  			tp->snd_scale = to.to_requested_s_scale;
  			tp->snd_wnd = th->th_win << tp->snd_scale;
  			tiwin = tp->snd_wnd;
  		}
  		if (to.to_flags & TOF_TS) {
  			tp->t_flags |= TF_RCVD_TSTMP;
  			tp->ts_recent = to.to_tsval;
  			tp->ts_recent_age = ticks;
  		}
  		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 &&
  				    ticks < tp->t_badrxtwin) {
  					++tcpstat.tcps_sndrexmitbad;
  					tp->snd_cwnd = tp->snd_cwnd_prev;
  					tp->snd_ssthresh =
  					    tp->snd_ssthresh_prev;
  					tp->snd_recover = tp->snd_recover_prev;
  					if (tp->t_flags & TF_WASFRECOVERY)
  					    ENTER_FASTRECOVERY(tp);
  					tp->snd_nxt = tp->snd_max;
  					tp->t_badrxtwin = 0;
  				}
  
  				/*
  				 * Recalculate the transmit timer / rtt.
  				 *
  				 * Some boxes send broken timestamp replies
  				 * during the SYN+ACK phase, ignore
  				 * timestamps of 0 or we could calculate a
  				 * huge RTT and blow up the retransmit timer.
  				 */
  				if ((to.to_flags & TOF_TS) != 0 &&
  				    to.to_tsecr) {
  					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;
  
  		/* Initial send window, already scaled. */
  		tp->snd_wnd = th->th_win;
  
  		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->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 | TF_NEEDSYN);
  			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) &&
  		    SEQ_LT(th->th_seq, tp->last_ack_sent + tp->rcv_wnd)) ||
  		    (tp->rcv_wnd == 0 && tp->last_ack_sent == th->th_seq)) {
  			switch (tp->t_state) {
  
  			case TCPS_SYN_RECEIVED:
  				so->so_error = ECONNREFUSED;
  				goto close;
  
  			case TCPS_ESTABLISHED:
  				if (tp->last_ack_sent != th->th_seq &&
  			 	    tcp_insecure_rst == 0) {
  					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
  			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->rcv_scale = tp->request_r_scale;
  			tp->snd_wnd = tiwin;
  		}
  		/*
  		 * 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;
  					(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;
  					(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;
  					(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->rcv_scale = tp->request_r_scale;
  				/* Send window already scaled. */
  			}
  		}
  
  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 && ticks < tp->t_badrxtwin) {
  			++tcpstat.tcps_sndrexmitbad;
  			tp->snd_cwnd = tp->snd_cwnd_prev;
  			tp->snd_ssthresh = tp->snd_ssthresh_prev;
  			tp->snd_recover = tp->snd_recover_prev;
  			if (tp->t_flags & TF_WASFRECOVERY)
  				ENTER_FASTRECOVERY(tp);
  			tp->snd_nxt = tp->snd_max;
  			tp->t_badrxtwin = 0;	/* XXX probably not required */
  		}
  
  		/*
  		 * If we have a timestamp reply, update smoothed
  		 * round trip time.  If no timestamp is present but
  		 * transmit timer is running and timed sequence
  		 * number was acked, update smoothed round trip time.
  		 * Since we now have an rtt measurement, cancel the
  		 * timer backoff (cf., Phil Karn's retransmit alg.).
  		 * Recompute the initial retransmit timer.
  		 *
  		 * 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 ((!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 = incr * incr / cw;
  			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)
  		INP_UNLOCK(inp);
  	if (headlocked)
  		INP_INFO_WUNLOCK(&tcbinfo);
  	return;
  
  drop:
  	/*
  	 * Drop space held by incoming segment and return.
  	 */
  #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 (tp)
  		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_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)) {
  		(void) 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, 2 * tcp_msl);
  	}
  
  	/*
  	 * 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);
  }
  