Extend critical section protection around portions of selection processing

[dragonfly/port-amd64.git] / sys / netinet / tcp_input.c

/*
 * Copyright (c) 2002, 2003, 2004 Jeffrey M. Hsu.  All rights reserved.
 * Copyright (c) 2002, 2003, 2004 The DragonFly Project.  All rights reserved.
 *
 * This code is derived from software contributed to The DragonFly Project
 * by Jeffrey M. Hsu.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 * 3. Neither the name of The DragonFly Project nor the names of its
 *    contributors may be used to endorse or promote products derived
 *    from this software without specific, prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS 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
 * COPYRIGHT HOLDERS OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
 * INCIDENTAL, SPECIAL, EXEMPLARY OR CONSEQUENTIAL DAMAGES (INCLUDING,
 * BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
 * LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED
 * AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
 * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
 * OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 * SUCH DAMAGE.
 */

/*
 * Copyright (c) 1982, 1986, 1988, 1990, 1993, 1994, 1995
 *      The Regents of the University of California.  All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 * 3. All advertising materials mentioning features or use of this software
 *    must display the following acknowledgement:
 *      This product includes software developed by the University of
 *      California, Berkeley and its contributors.
 * 4. Neither the name of the University nor the names of its contributors
 *    may be used to endorse or promote products derived from this software
 *    without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 * SUCH DAMAGE.
 *
 *      @(#)tcp_input.c 8.12 (Berkeley) 5/24/95
 * $FreeBSD: src/sys/netinet/tcp_input.c,v 1.107.2.38 2003/05/21 04:46:41 cjc Exp $
 * $DragonFly: src/sys/netinet/tcp_input.c,v 1.67 2007/04/22 01:13:14 dillon Exp $
 */

#include "opt_ipfw.h"           /* for ipfw_fwd         */
#include "opt_inet6.h"
#include "opt_ipsec.h"
#include "opt_tcpdebug.h"
#include "opt_tcp_input.h"

#include <sys/param.h>
#include <sys/systm.h>
#include <sys/kernel.h>
#include <sys/sysctl.h>
#include <sys/malloc.h>
#include <sys/mbuf.h>
#include <sys/proc.h>           /* for proc0 declaration */
#include <sys/protosw.h>
#include <sys/socket.h>
#include <sys/socketvar.h>
#include <sys/syslog.h>
#include <sys/in_cksum.h>

#include <machine/cpu.h>        /* before tcp_seq.h, for tcp_random18() */
#include <machine/stdarg.h>

#include <net/if.h>
#include <net/route.h>

#include <netinet/in.h>
#include <netinet/in_systm.h>
#include <netinet/ip.h>
#include <netinet/ip_icmp.h>    /* for ICMP_BANDLIM */
#include <netinet/in_var.h>
#include <netinet/icmp_var.h>   /* for ICMP_BANDLIM */
#include <netinet/in_pcb.h>
#include <netinet/ip_var.h>
#include <netinet/ip6.h>
#include <netinet/icmp6.h>
#include <netinet6/nd6.h>
#include <netinet6/ip6_var.h>
#include <netinet6/in6_pcb.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>

u_char tcp_saveipgen[40];    /* the size must be of max ip header, now IPv6 */
struct tcphdr tcp_savetcp;
#endif

#ifdef FAST_IPSEC
#include <netproto/ipsec/ipsec.h>
#include <netproto/ipsec/ipsec6.h>
#endif

#ifdef IPSEC
#include <netinet6/ipsec.h>
#include <netinet6/ipsec6.h>
#include <netproto/key/key.h>
#endif

MALLOC_DEFINE(M_TSEGQ, "tseg_qent", "TCP segment queue entry");

tcp_cc  tcp_ccgen;
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_limitedtransmit = 1;
SYSCTL_INT(_net_inet_tcp, OID_AUTO, limitedtransmit, CTLFLAG_RW,
    &tcp_do_limitedtransmit, 0, "Enable RFC 3042 (Limited Transmit)");

static int tcp_do_early_retransmit = 1;
SYSCTL_INT(_net_inet_tcp, OID_AUTO, earlyretransmit, CTLFLAG_RW,
    &tcp_do_early_retransmit, 0, "Early retransmit");

int tcp_aggregate_acks = 1;
SYSCTL_INT(_net_inet_tcp, OID_AUTO, aggregate_acks, CTLFLAG_RW,
    &tcp_aggregate_acks, 0, "Aggregate built-up acks into one ack");

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_do_eifel_detect = 1;
SYSCTL_INT(_net_inet_tcp, OID_AUTO, eifel, CTLFLAG_RW,
    &tcp_do_eifel_detect, 0, "Eifel detection algorithm (RFC 3522)");

static int tcp_do_abc = 1;
SYSCTL_INT(_net_inet_tcp, OID_AUTO, abc, CTLFLAG_RW,
    &tcp_do_abc, 0,
    "TCP Appropriate Byte Counting (RFC 3465)");

/*
 * Define as tunable for easy testing with SACK on and off.
 * Warning:  do not change setting in the middle of an existing active TCP flow,
 *   else strange things might happen to that flow.
 */
int tcp_do_sack = 1;
SYSCTL_INT(_net_inet_tcp, OID_AUTO, sack, CTLFLAG_RW,
    &tcp_do_sack, 0, "Enable SACK Algorithms");

int tcp_do_smartsack = 1;
SYSCTL_INT(_net_inet_tcp, OID_AUTO, smartsack, CTLFLAG_RW,
    &tcp_do_smartsack, 0, "Enable Smart SACK Algorithms");

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

int tcp_reass_maxseg = 0;
SYSCTL_INT(_net_inet_tcp_reass, OID_AUTO, maxsegments, CTLFLAG_RD,
    &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_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");

static void      tcp_dooptions(struct tcpopt *, u_char *, int, boolean_t);
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 *, int);
static void      tcp_sack_rexmt(struct tcpcb *, struct tcphdr *);

/* 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) && \
            (tp)->t_inpcb->in6p_route.ro_rt) \
                nd6_nud_hint((tp)->t_inpcb->in6p_route.ro_rt, NULL, 0); \
} while (0)
#else
#define ND6_HINT(tp)
#endif

/*
 * Indicate whether this ack should be delayed.  We can delay the ack if
 *      - delayed acks are enabled and
 *      - 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.
 */
#define DELAY_ACK(tp) \
        (tcp_delack_enabled && !callout_pending(tp->tt_delack) && \
        !(tp->t_flags & TF_RXWIN0SENT))

#define acceptable_window_update(tp, th, tiwin)                         \
    (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))))

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

        /*
         * 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) {
                tcp_reass_overflows++;
                tcpstat.tcps_rcvmemdrop++;
                m_freem(m);
                /* no SACK block to report */
                tp->reportblk.rblk_start = tp->reportblk.rblk_end;
                return (0);
        }

        /* Allocate a new queue entry. */
        MALLOC(te, struct tseg_qent *, sizeof(struct tseg_qent), M_TSEGQ,
               M_INTWAIT | M_NULLOK);
        if (te == NULL) {
                tcpstat.tcps_rcvmemdrop++;
                m_freem(m);
                /* no SACK block to report */
                tp->reportblk.rblk_start = tp->reportblk.rblk_end;
                return (0);
        }
        tcp_reass_qsize++;

        /*
         * Find a segment which begins after this one does.
         */
        LIST_FOREACH(q, &tp->t_segq, tqe_q) {
                if (SEQ_GT(q->tqe_th->th_seq, th->th_seq))
                        break;
                p = q;
        }

        /*
         * If there is a preceding segment, it may provide some of
         * our data already.  If so, drop the data from the incoming
         * segment.  If it provides all of our data, drop us.
         */
        if (p != NULL) {
                tcp_seq_diff_t i;

                /* conversion to int (in i) handles seq wraparound */
                i = p->tqe_th->th_seq + p->tqe_len - th->th_seq;
                if (i > 0) {            /* overlaps preceding segment */
                        tp->t_flags |= (TF_DUPSEG | TF_ENCLOSESEG);
                        /* enclosing block starts w/ preceding segment */
                        tp->encloseblk.rblk_start = p->tqe_th->th_seq;
                        if (i >= *tlenp) {
                                /* preceding encloses incoming segment */
                                tp->encloseblk.rblk_end = p->tqe_th->th_seq +
                                    p->tqe_len;
                                tcpstat.tcps_rcvduppack++;
                                tcpstat.tcps_rcvdupbyte += *tlenp;
                                m_freem(m);
                                kfree(te, M_TSEGQ);
                                tcp_reass_qsize--;
                                /*
                                 * Try to present any queued data
                                 * at the left window edge to the user.
                                 * This is needed after the 3-WHS
                                 * completes.
                                 */
                                goto present;   /* ??? */
                        }
                        m_adj(m, i);
                        *tlenp -= i;
                        th->th_seq += i;
                        /* incoming segment end is enclosing block end */
                        tp->encloseblk.rblk_end = th->th_seq + *tlenp +
                            ((th->th_flags & TH_FIN) != 0);
                        /* trim end of reported D-SACK block */
                        tp->reportblk.rblk_end = th->th_seq;
                }
        }
        tcpstat.tcps_rcvoopack++;
        tcpstat.tcps_rcvoobyte += *tlenp;

        /*
         * While we overlap succeeding segments trim them or,
         * if they are completely covered, dequeue them.
         */
        while (q) {
                tcp_seq_diff_t i = (th->th_seq + *tlenp) - q->tqe_th->th_seq;
                tcp_seq qend = q->tqe_th->th_seq + q->tqe_len;
                struct tseg_qent *nq;

                if (i <= 0)
                        break;
                if (!(tp->t_flags & TF_DUPSEG)) {    /* first time through */
                        tp->t_flags |= (TF_DUPSEG | TF_ENCLOSESEG);
                        tp->encloseblk = tp->reportblk;
                        /* report trailing duplicate D-SACK segment */
                        tp->reportblk.rblk_start = q->tqe_th->th_seq;
                }
                if ((tp->t_flags & TF_ENCLOSESEG) &&
                    SEQ_GT(qend, tp->encloseblk.rblk_end)) {
                        /* extend enclosing block if one exists */
                        tp->encloseblk.rblk_end = qend;
                }
                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);
                kfree(q, M_TSEGQ);
                tcp_reass_qsize--;
                q = nq;
        }

        /* Insert the new segment queue entry into place. */
        te->tqe_m = m;
        te->tqe_th = th;
        te->tqe_len = *tlenp;

        /* check if can coalesce with following segment */
        if (q != NULL && (th->th_seq + *tlenp == q->tqe_th->th_seq)) {
                tcp_seq tend = te->tqe_th->th_seq + te->tqe_len;

                te->tqe_len += q->tqe_len;
                if (q->tqe_th->th_flags & TH_FIN)
                        te->tqe_th->th_flags |= TH_FIN;
                m_cat(te->tqe_m, q->tqe_m);
                tp->encloseblk.rblk_end = tend;
                /*
                 * When not reporting a duplicate segment, use
                 * the larger enclosing block as the SACK block.
                 */
                if (!(tp->t_flags & TF_DUPSEG))
                        tp->reportblk.rblk_end = tend;
                LIST_REMOVE(q, tqe_q);
                kfree(q, M_TSEGQ);
                tcp_reass_qsize--;
        }

        if (p == NULL) {
                LIST_INSERT_HEAD(&tp->t_segq, te, tqe_q);
        } else {
                /* check if can coalesce with preceding segment */
                if (p->tqe_th->th_seq + p->tqe_len == th->th_seq) {
                        p->tqe_len += te->tqe_len;
                        m_cat(p->tqe_m, te->tqe_m);
                        tp->encloseblk.rblk_start = p->tqe_th->th_seq;
                        /*
                         * When not reporting a duplicate segment, use
                         * the larger enclosing block as the SACK block.
                         */
                        if (!(tp->t_flags & TF_DUPSEG))
                                tp->reportblk.rblk_start = p->tqe_th->th_seq;
                        kfree(te, M_TSEGQ);
                        tcp_reass_qsize--;
                } 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 == NULL || q->tqe_th->th_seq != tp->rcv_nxt)
                return (0);
        tp->rcv_nxt += q->tqe_len;
        if (!(tp->t_flags & TF_DUPSEG)) {
                /* no SACK block to report since ACK advanced */
                tp->reportblk.rblk_start = tp->reportblk.rblk_end;
        }
        /* no enclosing block to report since ACK advanced */
        tp->t_flags &= ~TF_ENCLOSESEG;
        flags = q->tqe_th->th_flags & TH_FIN;
        LIST_REMOVE(q, tqe_q);
        KASSERT(LIST_EMPTY(&tp->t_segq) ||
                LIST_FIRST(&tp->t_segq)->tqe_th->th_seq != tp->rcv_nxt,
                ("segment not coalesced"));
        if (so->so_state & SS_CANTRCVMORE)
                m_freem(q->tqe_m);
        else
                ssb_appendstream(&so->so_rcv, q->tqe_m);
        kfree(q, M_TSEGQ);
        tcp_reass_qsize--;
        ND6_HINT(tp);
        sorwakeup(so);
        return (flags);
}

/*
 * TCP input routine, follows pages 65-76 of the
 * protocol specification dated September, 1981 very closely.
 */
#ifdef INET6
int
tcp6_input(struct mbuf **mp, int *offp, int proto)
{
        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,
                            offsetof(struct ip6_hdr, ip6_dst));
                return (IPPROTO_DONE);
        }

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

void
tcp_input(struct mbuf *m, ...)
{
        __va_list ap;
        int off0, proto;
        struct tcphdr *th;
        struct ip *ip = NULL;
        struct ipovly *ipov;
        struct inpcb *inp = NULL;
        u_char *optp = NULL;
        int optlen = 0;
        int len, tlen, off;
        int drop_hdrlen;
        struct tcpcb *tp = NULL;
        int thflags;
        struct socket *so = 0;
        int todrop, acked;
        boolean_t ourfinisacked, needoutput = FALSE;
        u_long tiwin;
        int recvwin;
        struct tcpopt to;               /* options in this segment */
        struct rmxp_tao *taop;          /* pointer to our TAO cache entry */
        struct rmxp_tao tao_noncached;  /* in case there's no cached entry */
        struct sockaddr_in *next_hop = NULL;
        int rstreason; /* For badport_bandlim accounting purposes */
        int cpu;
        struct ip6_hdr *ip6 = NULL;
#ifdef INET6
        boolean_t isipv6;
#else
        const boolean_t isipv6 = FALSE;
#endif
#ifdef TCPDEBUG
        short ostate = 0;
#endif

        __va_start(ap, m);
        off0 = __va_arg(ap, int);
        proto = __va_arg(ap, int);
        __va_end(ap);

        tcpstat.tcps_rcvtotal++;

        /* Grab info from and strip MT_TAG mbufs prepended to the chain. */
        while  (m->m_type == MT_TAG) {
                if (m->_m_tag_id == PACKET_TAG_IPFORWARD)
                        next_hop = (struct sockaddr_in *)m->m_hdr.mh_data;
                m = m->m_next;
        }

#ifdef INET6
        isipv6 = (mtod(m, struct ip *)->ip_v == 6) ? TRUE : FALSE;
#endif

        if (isipv6) {
                /* IP6_EXTHDR_CHECK() is already done at tcp6_input() */
                ip6 = mtod(m, struct ip6_hdr *);
                tlen = (sizeof *ip6) + ntohs(ip6->ip6_plen) - off0;
                if (in6_cksum(m, IPPROTO_TCP, off0, tlen)) {
                        tcpstat.tcps_rcvbadsum++;
                        goto 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 {
                /*
                 * 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);
                        off0 = sizeof(struct ip);
                }
                /* already checked and pulled up in ip_demux() */
                KASSERT(m->m_len >= sizeof(struct tcpiphdr),
                    ("TCP header not in one mbuf: m->m_len %d", m->m_len));
                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;
                } 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;
        /* already checked and pulled up in ip_demux() */
        KASSERT(off >= sizeof(struct tcphdr) && off <= tlen,
            ("bad TCP data offset %d (tlen %d)", off, tlen));
        tlen -= off;    /* tlen is used instead of ti->ti_len */
        if (off > sizeof(struct tcphdr)) {
                if (isipv6) {
                        IP6_EXTHDR_CHECK(m, off0, off, );
                        ip6 = mtod(m, struct ip6_hdr *);
                        th = (struct tcphdr *)((caddr_t)ip6 + off0);
                } else {
                        /* already pulled up in ip_demux() */
                        KASSERT(m->m_len >= sizeof(struct ip) + off,
                            ("TCP header and options not in one mbuf: "
                             "m_len %d, off %d", m->m_len, off));
                }
                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.
         */
findpcb:
        /* IPFIREWALL_FORWARD section */
        if (next_hop != NULL && !isipv6) {  /* IPv6 support is not there yet */
                /*
                 * Transparently forwarded. Pretend to be the destination.
                 * already got one like this?
                 */
                cpu = mycpu->gd_cpuid;
                inp = in_pcblookup_hash(&tcbinfo[cpu],
                                        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.
                         */

                        /*
                         * The rest of the ipfw code stores the port in
                         * host order.  XXX
                         * (The IP address is still in network order.)
                         */
                        in_port_t dport = next_hop->sin_port ?
                                                htons(next_hop->sin_port) :
                                                th->th_dport;

                        cpu = tcp_addrcpu(ip->ip_src.s_addr, th->th_sport,
                                          next_hop->sin_addr.s_addr, dport);
                        inp = in_pcblookup_hash(&tcbinfo[cpu],
                                                ip->ip_src, th->th_sport,
                                                next_hop->sin_addr, dport,
                                                1, m->m_pkthdr.rcvif);
                }
        } else {
                if (isipv6) {
                        inp = in6_pcblookup_hash(&tcbinfo[0],
                                                 &ip6->ip6_src, th->th_sport,
                                                 &ip6->ip6_dst, th->th_dport,
                                                 1, m->m_pkthdr.rcvif);
                } else {
                        cpu = mycpu->gd_cpuid;
                        inp = in_pcblookup_hash(&tcbinfo[cpu],
                                                ip->ip_src, th->th_sport,
                                                ip->ip_dst, th->th_dport,
                                                1, m->m_pkthdr.rcvif);
                }
        }

        /*
         * 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[sizeof "aaa.bbb.ccc.ddd"];
                        char sbuf[sizeof "aaa.bbb.ccc.ddd"];
#endif
                        if (isipv6) {
                                strcpy(dbuf, "[");
                                strcat(dbuf, ip6_sprintf(&ip6->ip6_dst));
                                strcat(dbuf, "]");
                                strcpy(sbuf, "[");
                                strcat(sbuf, ip6_sprintf(&ip6->ip6_src));
                                strcat(sbuf, "]");
                        } 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))
                                        break;
                        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;
        }

#ifdef IPSEC
        if (isipv6) {
                if (ipsec6_in_reject_so(m, inp->inp_socket)) {
                        ipsec6stat.in_polvio++;
                        goto drop;
                }
        } else {
                if (ipsec4_in_reject_so(m, inp->inp_socket)) {
                        ipsecstat.in_polvio++;
                        goto drop;
                }
        }
#endif
#ifdef FAST_IPSEC
        if (isipv6) {
                if (ipsec6_in_reject(m, inp))
                        goto drop;
        } else {
                if (ipsec4_in_reject(m, inp))
                        goto drop;
        }
#endif
        /* Check the minimum TTL for socket. */
#ifdef INET6
        if ((isipv6 ? ip6->ip6_hlim : ip->ip_ttl) < inp->inp_ip_minttl)
                goto drop;
#endif

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

        /* Unscale the window into a 32-bit value. */
        if (!(thflags & TH_SYN))
                tiwin = th->th_win << tp->snd_scale;
        else
                tiwin = th->th_win;

        so = inp->inp_socket;

#ifdef TCPDEBUG
        if (so->so_options & SO_DEBUG) {
                ostate = tp->t_state;
                if (isipv6)
                        bcopy(ip6, tcp_saveipgen, sizeof(*ip6));
                else
                        bcopy(ip, tcp_saveipgen, sizeof(*ip));
                tcp_savetcp = *th;
        }
#endif

        bzero(&to, sizeof to);

        if (so->so_options & SO_ACCEPTCONN) {
                struct in_conninfo inc;

#ifdef INET6
                inc.inc_isipv6 = (isipv6 == TRUE);
#endif
                if (isipv6) {
                        inc.inc6_faddr = ip6->ip6_src;
                        inc.inc6_laddr = ip6->ip6_dst;
                        inc.inc6_route.ro_rt = NULL;            /* XXX */
                } else {
                        inc.inc_faddr = ip->ip_src;
                        inc.inc_laddr = ip->ip_dst;
                        inc.inc_route.ro_rt = NULL;             /* XXX */
                }
                inc.inc_fport = th->th_sport;
                inc.inc_lport = th->th_dport;

                /*
                 * If the state is LISTEN then ignore segment if it contains
                 * a RST.  If the segment contains an ACK then it is bad and
                 * send a RST.  If it does not contain a SYN then it is not
                 * interesting; drop it.
                 *
                 * If the state is SYN_RECEIVED (syncache) and seg contains
                 * an ACK, but not for our SYN/ACK, send a RST.  If the seg
                 * contains a RST, check the sequence number to see if it
                 * is a valid reset segment.
                 */
                if ((thflags & (TH_RST | TH_ACK | TH_SYN)) != TH_SYN) {
                        if ((thflags & (TH_RST | TH_ACK | TH_SYN)) == TH_ACK) {
                                if (!syncache_expand(&inc, th, &so, m)) {
                                        /*
                                         * No syncache entry, or ACK was not
                                         * for our SYN/ACK.  Send a RST.
                                         */
                                        tcpstat.tcps_badsyn++;
                                        rstreason = BANDLIM_RST_OPENPORT;
                                        goto dropwithreset;
                                }
                                if (so == NULL)
                                        /*
                                         * Could not complete 3-way handshake,
                                         * connection is being closed down, and
                                         * syncache will free mbuf.
                                         */
                                        return;
                                /*
                                 * Socket is created in state SYN_RECEIVED.
                                 * Continue processing segment.
                                 */
                                inp = so->so_pcb;
                                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;
/*
 * XXX possible bug - it doesn't appear that tp->snd_wnd is unscaled
 * until the _second_ ACK is received:
 *    rcv SYN (set wscale opts)  --> send SYN/ACK, set snd_wnd = window.
 *    rcv ACK, calculate tiwin --> process SYN_RECEIVED, determine wscale,
 *        move to ESTAB, set snd_wnd to tiwin.
 */
                                tp->snd_wnd = tiwin;    /* unscaled */
                                goto after_listen;
                        }
                        if (thflags & TH_RST) {
                                syncache_chkrst(&inc, th);
                                goto drop;
                        }
                        if (thflags & TH_ACK) {
                                syncache_badack(&inc);
                                tcpstat.tcps_badsyn++;
                                rstreason = BANDLIM_RST_OPENPORT;
                                goto dropwithreset;
                        }
                        goto drop;
                }

                /*
                 * Segment's flags are (SYN) or (SYN | FIN).
                 */
#ifdef INET6
                /*
                 * If deprecated address is forbidden,
                 * we do not accept SYN to deprecated interface
                 * address to prevent any new inbound connection from
                 * getting established.
                 * When we do not accept SYN, we send a TCP RST,
                 * with deprecated source address (instead of dropping
                 * it).  We compromise it as it is much better for peer
                 * to send a RST, and RST will be the final packet
                 * for the exchange.
                 *
                 * If we do not forbid deprecated addresses, we accept
                 * the SYN packet.  RFC2462 does not suggest dropping
                 * SYN in this case.
                 * If we decipher RFC2462 5.5.4, it says like this:
                 * 1. use of deprecated addr with existing
                 *    communication is okay - "SHOULD continue to be
                 *    used"
                 * 2. use of it with new communication:
                 *   (2a) "SHOULD NOT be used if alternate address
                 *        with sufficient scope is available"
                 *   (2b) nothing mentioned otherwise.
                 * Here we fall into (2b) case as we have no choice in
                 * our source address selection - we must obey the peer.
                 *
                 * The wording in RFC2462 is confusing, and there are
                 * multiple description text for deprecated address
                 * handling - worse, they are not exactly the same.
                 * I believe 5.5.4 is the best one, so we follow 5.5.4.
                 */
                if (isipv6 && !ip6_use_deprecated) {
                        struct in6_ifaddr *ia6;

                        if ((ia6 = ip6_getdstifaddr(m)) &&
                            (ia6->ia6_flags & IN6_IFF_DEPRECATED)) {
                                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) {
                        tcp_dooptions(&to, optp, optlen, TRUE);
                        if (!syncache_add(&inc, &to, th, &so, m))
                                goto drop;
                        if (so == NULL)
                                /*
                                 * Entry added to syncache, mbuf used to
                                 * send SYN,ACK packet.
                                 */
                                return;
                        /*
                         * Segment passed TAO tests.
                         */
                        inp = so->so_pcb;
                        tp = intotcpcb(inp);
                        tp->snd_wnd = tiwin;
                        tp->t_starttime = ticks;
                        tp->t_state = TCPS_ESTABLISHED;

                        /*
                         * If there is a FIN, or if there is data and the
                         * connection is local, then delay SYN,ACK(SYN) in
                         * the hope of piggy-backing it on a response
                         * segment.  Otherwise must send ACK now in case
                         * the other side is slow starting.
                         */
                        if (DELAY_ACK(tp) &&
                            ((thflags & TH_FIN) ||
                             (tlen != 0 &&
                              ((isipv6 && in6_localaddr(&inp->in6p_faddr)) ||
                               (!isipv6 && in_localaddr(inp->inp_faddr)))))) {
                                callout_reset(tp->tt_delack, tcp_delacktime,
                                                tcp_timer_delack, tp);
                                tp->t_flags |= TF_NEEDSYN;
                        } else
                                tp->t_flags |= (TF_ACKNOW | TF_NEEDSYN);

                        tcpstat.tcps_connects++;
                        soisconnected(so);
                        goto trimthenstep6;
                }
                goto drop;
        }
after_listen:

        /* should not happen - syncache should pick up these connections */
        KASSERT(tp->t_state != TCPS_LISTEN, ("tcp_input: TCPS_LISTEN state"));

        /*
         * Segment received on connection.
         * Reset idle time and keep-alive timer.
         */
        tp->t_rcvtime = ticks;
        if (TCPS_HAVEESTABLISHED(tp->t_state))
                callout_reset(tp->tt_keep, tcp_keepidle, tcp_timer_keep, tp);

        /*
         * Process options.
         * XXX this is tradtitional behavior, may need to be cleaned up.
         */
        tcp_dooptions(&to, optp, optlen, (thflags & TH_SYN) != 0);
        if (tp->t_state == TCPS_SYN_SENT && (thflags & TH_SYN)) {
                if (to.to_flags & TOF_SCALE) {
                        tp->t_flags |= TF_RCVD_SCALE;
                        tp->requested_s_scale = to.to_requested_s_scale;
                }
                if (to.to_flags & TOF_TS) {
                        tp->t_flags |= TF_RCVD_TSTMP;
                        tp->ts_recent = to.to_tsval;
                        tp->ts_recent_age = ticks;
                }
                if (to.to_flags & (TOF_CC | TOF_CCNEW))
                        tp->t_flags |= TF_RCVD_CC;
                if (to.to_flags & TOF_MSS)
                        tcp_mss(tp, to.to_mss);
                /*
                 * Only set the TF_SACK_PERMITTED per-connection flag
                 * if we got a SACK_PERMITTED option from the other side
                 * and the global tcp_do_sack variable is true.
                 */
                if (tcp_do_sack && (to.to_flags & TOF_SACK_PERMITTED))
                        tp->t_flags |= TF_SACK_PERMITTED;
        }

        /*
         * 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)) &&
            (!(to.to_flags & TOF_TS) ||
             TSTMP_GEQ(to.to_tsval, tp->ts_recent)) &&
            /*
             * Using the CC option is compulsory if once started:
             *   the segment is OK if no T/TCP was negotiated or
             *   if the segment has a CC option equal to CCrecv
             */
            ((tp->t_flags & (TF_REQ_CC|TF_RCVD_CC)) != (TF_REQ_CC|TF_RCVD_CC) ||
             ((to.to_flags & TOF_CC) && to.to_cc == tp->cc_recv)) &&
            th->th_seq == tp->rcv_nxt &&
            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) &&
                    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 &&
                            !IN_FASTRECOVERY(tp)) {
                                /*
                                 * This is a pure ack for outstanding data.
                                 */
                                ++tcpstat.tcps_predack;
                                /*
                                 * "bad retransmit" recovery
                                 *
                                 * If Eifel detection applies, then
                                 * it is deterministic, so use it
                                 * unconditionally over the old heuristic.
                                 * Otherwise, fall back to the old heuristic.
                                 */
                                if (tcp_do_eifel_detect &&
                                    (to.to_flags & TOF_TS) && to.to_tsecr &&
                                    (tp->t_flags & TF_FIRSTACCACK)) {
                                        /* Eifel detection applicable. */
                                        if (to.to_tsecr < tp->t_rexmtTS) {
                                                tcp_revert_congestion_state(tp);
                                                ++tcpstat.tcps_eifeldetected;
                                        }
                                } else if (tp->t_rxtshift == 1 &&
                                           ticks < tp->t_badrxtwin) {
                                        tcp_revert_congestion_state(tp);
                                        ++tcpstat.tcps_rttdetected;
                                }
                                tp->t_flags &= ~(TF_FIRSTACCACK |
                                                 TF_FASTREXMT | TF_EARLYREXMT);
                                /*
                                 * Recalculate the retransmit timer / rtt.
                                 *
                                 * Some machines (certain windows boxes)
                                 * send broken timestamp replies during the
                                 * SYN+ACK phase, ignore timestamps of 0.
                                 */
                                if ((to.to_flags & TOF_TS) && 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)) {
                                        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.sb, acked);
                                tp->snd_recover = th->th_ack - 1;
                                tp->snd_una = th->th_ack;
                                tp->t_dupacks = 0;
                                /*
                                 * Update window information.
                                 */
                                if (tiwin != tp->snd_wnd &&
                                    acceptable_window_update(tp, th, tiwin)) {
                                        /* keep track of pure window updates */
                                        if (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;
                                }
                                m_freem(m);
                                ND6_HINT(tp); /* some progress has been done */
                                /*
                                 * If all outstanding data are acked, stop
                                 * retransmit timer, otherwise restart timer
                                 * using current (possibly backed-off) value.
                                 * If process is waiting for space,
                                 * wakeup/selwakeup/signal.  If data
                                 * are ready to send, let tcp_output
                                 * decide between more output or persist.
                                 */
                                if (tp->snd_una == tp->snd_max)
                                        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.ssb_cc > 0)
                                        tcp_output(tp);
                                return;
                        }
                } else if (tiwin == tp->snd_wnd &&
                    th->th_ack == tp->snd_una &&
                    LIST_EMPTY(&tp->t_segq) &&
                    tlen <= ssb_space(&so->so_rcv)) {
                        /*
                         * This is a pure, in-sequence data packet
                         * with nothing on the reassembly queue and
                         * we have enough buffer space to take it.
                         */
                        ++tcpstat.tcps_preddat;
                        tp->rcv_nxt += tlen;
                        tcpstat.tcps_rcvpack++;
                        tcpstat.tcps_rcvbyte += tlen;
                        ND6_HINT(tp);   /* some progress has been done */
                        /*
                         * Add data to socket buffer.
                         */
                        if (so->so_state & SS_CANTRCVMORE) {
                                m_freem(m);
                        } else {
                                m_adj(m, drop_hdrlen); /* delayed header drop */
                                ssb_appendstream(&so->so_rcv, m);
                        }
                        sorwakeup(so);
                        /*
                         * This code is responsible for most of the ACKs
                         * the TCP stack sends back after receiving a data
                         * packet.  Note that the DELAY_ACK check fails if
                         * the delack timer is already running, which results
                         * in an ack being sent every other packet (which is
                         * what we want).
                         *
                         * We then further aggregate acks by not actually
                         * sending one until the protocol thread has completed
                         * processing the current backlog of packets.  This
                         * does not delay the ack any further, but allows us
                         * to take advantage of the packet aggregation that
                         * high speed NICs do (usually blocks of 8-10 packets)
                         * to send a single ack rather then four or five acks,
                         * greatly reducing the ack rate, the return channel
                         * bandwidth, and the protocol overhead on both ends.
                         *
                         * Since this also has the effect of slowing down
                         * the exponential slow-start ramp-up, systems with 
                         * very large bandwidth-delay products might want
                         * to turn the feature off.
                         */
                        if (DELAY_ACK(tp)) {
                                callout_reset(tp->tt_delack, tcp_delacktime,
                                    tcp_timer_delack, tp);
                        } else if (tcp_aggregate_acks) {
                                tp->t_flags |= TF_ACKNOW;
                                if (!(tp->t_flags & TF_ONOUTPUTQ)) {
                                        tp->t_flags |= TF_ONOUTPUTQ;
                                        tp->tt_cpu = mycpu->gd_cpuid;
                                        TAILQ_INSERT_TAIL(
                                            &tcpcbackq[tp->tt_cpu],
                                            tp, t_outputq);
                                }
                        } else {
                                tp->t_flags |= TF_ACKNOW;
                                tcp_output(tp);
                        }
                        return;
                }
        }

        /*
         * Calculate amount of space in receive window,
         * and then do TCP input processing.
         * Receive window is amount of space in rcv queue,
         * but not less than advertised window.
         */
        recvwin = ssb_space(&so->so_rcv);
        if (recvwin < 0)
                recvwin = 0;
        tp->rcv_wnd = imax(recvwin, (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 ((taop = tcp_gettaocache(&inp->inp_inc)) == NULL) {
                        taop = &tao_noncached;
                        bzero(taop, sizeof *taop);
                }

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

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

                        tp->rcv_adv += tp->rcv_wnd;
                        tp->snd_una++;          /* SYN is acked */
                        callout_stop(tp->tt_rexmt);
                        /*
                         * If there's data, delay ACK; if there's also a FIN
                         * ACKNOW will be turned on later.
                         */
                        if (DELAY_ACK(tp) && tlen != 0)
                                callout_reset(tp->tt_delack, tcp_delacktime,
                                    tcp_timer_delack, tp);
                        else
                                tp->t_flags |= TF_ACKNOW;
                        /*
                         * Received <SYN,ACK> in SYN_SENT[*] state.
                         * Transitions:
                         *      SYN_SENT  --> ESTABLISHED
                         *      SYN_SENT* --> FIN_WAIT_1
                         */
                        tp->t_starttime = ticks;
                        if (tp->t_flags & TF_NEEDFIN) {
                                tp->t_state = TCPS_FIN_WAIT_1;
                                tp->t_flags &= ~TF_NEEDFIN;
                                thflags &= ~TH_SYN;
                        } else {
                                tp->t_state = TCPS_ESTABLISHED;
                                callout_reset(tp->tt_keep, tcp_keepidle,
                                              tcp_timer_keep, tp);
                        }
                } else {
                        /*
                         * Received initial SYN in SYN-SENT[*] state =>
                         * simultaneous open.  If segment contains CC option
                         * and there is a cached CC, apply TAO test.
                         * If it succeeds, connection is * half-synchronized.
                         * Otherwise, do 3-way handshake:
                         *        SYN-SENT -> SYN-RECEIVED
                         *        SYN-SENT* -> SYN-RECEIVED*
                         * If there was no CC option, clear cached CC value.
                         */
                        tp->t_flags |= TF_ACKNOW;
                        callout_stop(tp->tt_rexmt);
                        if (to.to_flags & TOF_CC) {
                                if (taop->tao_cc != 0 &&
                                    CC_GT(to.to_cc, taop->tao_cc)) {
                                        /*
                                         * update cache and make transition:
                                         *        SYN-SENT -> ESTABLISHED*
                                         *        SYN-SENT* -> FIN-WAIT-1*
                                         */
                                        taop->tao_cc = to.to_cc;
                                        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);
                                        }
                                        tp->t_flags |= TF_NEEDSYN;
                                } else
                                        tp->t_state = TCPS_SYN_RECEIVED;
                        } else {
                                /* CC.NEW or no option => invalidate cache */
                                taop->tao_cc = 0;
                                tp->t_state = TCPS_SYN_RECEIVED;
                        }
                }

trimthenstep6:
                /*
                 * Advance th->th_seq to correspond to first data byte.
                 * If data, trim to stay within window,
                 * dropping FIN if necessary.
                 */
                th->th_seq++;
                if (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:
         *      if segment contains a SYN and CC [not CC.NEW] option:
         *              if state == TIME_WAIT and connection duration > MSL,
         *                  drop packet and send RST;
         *
         *              if SEG.CC > CCrecv then is new SYN, and can implicitly
         *                  ack the FIN (and data) in retransmission queue.
         *                  Complete close and delete TCPCB.  Then reprocess
         *                  segment, hoping to find new TCPCB in LISTEN state;
         *
         *              else must be old SYN; drop it.
         *      else do normal processing.
         */
        case TCPS_LAST_ACK:
        case TCPS_CLOSING:
        case TCPS_TIME_WAIT:
                if ((thflags & TH_SYN) &&
                    (to.to_flags & TOF_CC) && tp->cc_recv != 0) {
                        if (tp->t_state == TCPS_TIME_WAIT &&
                                        (ticks - tp->t_starttime) > tcp_msl) {
                                rstreason = BANDLIM_UNLIMITED;
                                goto dropwithreset;
                        }
                        if (CC_GT(to.to_cc, tp->cc_recv)) {
                                tp = tcp_close(tp);
                                goto findpcb;
                        }
                        else
                                goto drop;
                }
                break;  /* continue normal processing */
        }

        /*
         * States other than LISTEN or SYN_SENT.
         * First check the RST flag and sequence number since reset segments
         * are exempt from the timestamp and connection count tests.  This
         * fixes a bug introduced by the Stevens, vol. 2, p. 960 bugfix
         * below which allowed reset segments in half the sequence space
         * to fall though and be processed (which gives forged reset
         * segments with a random sequence number a 50 percent chance of
         * killing a connection).
         * Then check timestamp, if present.
         * Then check the connection count, if present.
         * Then check that at least some bytes of segment are within
         * receive window.  If segment begins before rcv_nxt,
         * drop leading data (and SYN); if nothing left, just ack.
         *
         *
         * If the RST bit is set, check the sequence number to see
         * if this is a valid reset segment.
         * RFC 793 page 37:
         *   In all states except SYN-SENT, all reset (RST) segments
         *   are validated by checking their SEQ-fields.  A reset is
         *   valid if its sequence number is in the window.
         * Note: this does not take into account delayed ACKs, so
         *   we should test against last_ack_sent instead of rcv_nxt.
         *   The sequence number in the reset segment is normally an
         *   echo of our outgoing acknowledgement 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.
         * 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_LEQ(th->th_seq, tp->last_ack_sent + tp->rcv_wnd)) {
                        switch (tp->t_state) {

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

                        case TCPS_ESTABLISHED:
                        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++;
                                tp = tcp_close(tp);
                                break;

                        case TCPS_CLOSING:
                        case TCPS_LAST_ACK:
                                tp = tcp_close(tp);
                                break;

                        case TCPS_TIME_WAIT:
                                break;
                        }
                }
                goto drop;
        }

        /*
         * RFC 1323 PAWS: If we have a timestamp reply on this segment
         * and it's less than ts_recent, drop it.
         */
        if ((to.to_flags & TOF_TS) && tp->ts_recent != 0 &&
            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;
                }
        }

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

        /*
         * In the SYN-RECEIVED state, validate that the packet belongs to
         * this connection before trimming the data to fit the receive
         * window.  Check the sequence number versus IRS since we know
         * the sequence numbers haven't wrapped.  This is a partial fix
         * for the "LAND" DoS attack.
         */
        if (tp->t_state == TCPS_SYN_RECEIVED && SEQ_LT(th->th_seq, tp->irs)) {
                rstreason = BANDLIM_RST_OPENPORT;
                goto dropwithreset;
        }

        todrop = tp->rcv_nxt - th->th_seq;
        if (todrop > 0) {
                if (TCP_DO_SACK(tp)) {
                        /* Report duplicate segment at head of packet. */
                        tp->reportblk.rblk_start = th->th_seq;
                        tp->reportblk.rblk_end = th->th_seq + tlen;
                        if (thflags & TH_FIN)
                                ++tp->reportblk.rblk_end;
                        if (SEQ_GT(tp->reportblk.rblk_end, tp->rcv_nxt))
                                tp->reportblk.rblk_end = tp->rcv_nxt;
                        tp->t_flags |= (TF_DUPSEG | TF_SACKLEFT | TF_ACKNOW);
                }
                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))) {
                        /*
                         * 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) {
                tp = tcp_close(tp);
                tcpstat.tcps_rcvafterclose++;
                rstreason = BANDLIM_UNLIMITED;
                goto dropwithreset;
        }

        /*
         * If segment ends after window, drop trailing data
         * (and PUSH and FIN); if nothing left, just ACK.
         */
        todrop = (th->th_seq + tlen) - (tp->rcv_nxt + tp->rcv_wnd);
        if (todrop > 0) {
                tcpstat.tcps_rcvpackafterwin++;
                if (todrop >= tlen) {
                        tcpstat.tcps_rcvbyteafterwin += tlen;
                        /*
                         * If a new connection request is received
                         * while in TIME_WAIT, drop the old connection
                         * and start over if the sequence numbers
                         * are above the previous ones.
                         */
                        if (thflags & TH_SYN &&
                            tp->t_state == TCPS_TIME_WAIT &&
                            SEQ_GT(th->th_seq, tp->rcv_nxt)) {
                                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) && SEQ_LEQ(th->th_seq, tp->last_ack_sent) &&
            SEQ_LEQ(tp->last_ack_sent, (th->th_seq + tlen
                                        + ((thflags & TH_SYN) != 0)
                                        + ((thflags & 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) {
                tp = tcp_drop(tp, ECONNRESET);
                rstreason = BANDLIM_UNLIMITED;
                goto dropwithreset;
        }

        /*
         * If the ACK bit is off:  if in SYN-RECEIVED state or SENDSYN
         * flag is on (half-synchronized state), then queue data for
         * later processing; else drop segment and return.
         */
        if (!(thflags & TH_ACK)) {
                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 acknowledges our SYN, so enter
         * ESTABLISHED state and continue processing.
         * The ACK was checked above.
         */
        case TCPS_SYN_RECEIVED:

                tcpstat.tcps_connects++;
                soisconnected(so);
                /* Do window scaling? */
                if ((tp->t_flags & (TF_RCVD_SCALE | TF_REQ_SCALE)) ==
                    (TF_RCVD_SCALE | TF_REQ_SCALE)) {
                        tp->snd_scale = tp->requested_s_scale;
                        tp->rcv_scale = tp->request_r_scale;
                }
                /*
                 * Upon successful completion of 3-way handshake,
                 * update cache.CC if it was undefined, pass any queued
                 * data to the user, and advance state appropriately.
                 */
                if ((taop = tcp_gettaocache(&inp->inp_inc)) != NULL &&
                    taop->tao_cc == 0)
                        taop->tao_cc = tp->cc_recv;

                /*
                 * 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))
                        tcp_reass(tp, NULL, NULL, NULL);
                /* fall into ... */

        /*
         * In ESTABLISHED state: drop duplicate ACKs; ACK out of range
         * ACKs.  If the ack is in the range
         *      tp->snd_una < th->th_ack <= tp->snd_max
         * then advance tp->snd_una to th->th_ack and drop
         * data from the retransmission queue.  If this ACK reflects
         * more up to date window information we update our window information.
         */
        case TCPS_ESTABLISHED:
        case TCPS_FIN_WAIT_1:
        case TCPS_FIN_WAIT_2:
        case TCPS_CLOSE_WAIT:
        case TCPS_CLOSING:
        case TCPS_LAST_ACK:
        case TCPS_TIME_WAIT:

                if (SEQ_LEQ(th->th_ack, tp->snd_una)) {
                        if (TCP_DO_SACK(tp))
                                tcp_sack_update_scoreboard(tp, &to);
                        if (tlen != 0 || tiwin != tp->snd_wnd) {
                                tp->t_dupacks = 0;
                                break;
                        }
                        tcpstat.tcps_rcvdupack++;
                        if (!callout_active(tp->tt_rexmt) ||
                            th->th_ack != tp->snd_una) {
                                tp->t_dupacks = 0;
                                break;
                        }
                        /*
                         * 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, so assume a packet
                         * has been dropped and retransmit it.
                         * Kludge snd_nxt & the congestion
                         * window so we send only this one
                         * packet.
                         */
                        if (IN_FASTRECOVERY(tp)) {
                                if (TCP_DO_SACK(tp)) {
                                        /* No artifical cwnd inflation. */
                                        tcp_sack_rexmt(tp, th);
                                } else {
                                        /*
                                         * 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.
                                         */
                                        tp->snd_cwnd += tp->t_maxseg;
                                        tcp_output(tp);
                                }
                        } else if (SEQ_LT(th->th_ack, tp->snd_recover)) {
                                tp->t_dupacks = 0;
                                break;
                        } else if (++tp->t_dupacks == tcprexmtthresh) {
                                tcp_seq old_snd_nxt;
                                u_int win;

fastretransmit:
                                if (tcp_do_eifel_detect &&
                                    (tp->t_flags & TF_RCVD_TSTMP)) {
                                        tcp_save_congestion_state(tp);
                                        tp->t_flags |= TF_FASTREXMT;
                                }
                                /*
                                 * 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.
                                 */
                                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;
                                old_snd_nxt = tp->snd_nxt;
                                tp->snd_nxt = th->th_ack;
                                tp->snd_cwnd = tp->t_maxseg;
                                tcp_output(tp);
                                ++tcpstat.tcps_sndfastrexmit;
                                tp->snd_cwnd = tp->snd_ssthresh;
                                tp->rexmt_high = tp->snd_nxt;
                                if (SEQ_GT(old_snd_nxt, tp->snd_nxt))
                                        tp->snd_nxt = old_snd_nxt;
                                KASSERT(tp->snd_limited <= 2,
                                    ("tp->snd_limited too big"));
                                if (TCP_DO_SACK(tp))
                                        tcp_sack_rexmt(tp, th);
                                else
                                        tp->snd_cwnd += tp->t_maxseg *
                                            (tp->t_dupacks - tp->snd_limited);
                        } else if (tcp_do_limitedtransmit) {
                                u_long oldcwnd = tp->snd_cwnd;
                                tcp_seq oldsndmax = tp->snd_max;
                                tcp_seq oldsndnxt = tp->snd_nxt;
                                /* outstanding data */
                                uint32_t ownd = tp->snd_max - tp->snd_una;
                                u_int sent;

#define iceildiv(n, d)          (((n)+(d)-1) / (d))

                                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_nxt = tp->snd_max;
                                tp->snd_cwnd = ownd +
                                    (tp->t_dupacks - tp->snd_limited) *
                                    tp->t_maxseg;
                                tcp_output(tp);

                                /*
                                 * Other acks may have been processed,
                                 * snd_nxt cannot be reset to a value less
                                 * then snd_una.
                                 */
                                if (SEQ_LT(oldsndnxt, oldsndmax)) {
                                    if (SEQ_GT(oldsndnxt, tp->snd_una))
                                        tp->snd_nxt = oldsndnxt;
                                    else
                                        tp->snd_nxt = tp->snd_una;
                                }
                                tp->snd_cwnd = oldcwnd;
                                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"));
                                        KASSERT(sent <= tp->t_maxseg * 2,
                                            ("sent too many segments"));
                                        tp->snd_limited = 2;
                                        tcpstat.tcps_sndlimited += 2;
                                } else if (sent > 0) {
                                        ++tp->snd_limited;
                                        ++tcpstat.tcps_sndlimited;
                                } else if (tcp_do_early_retransmit &&
                                    (tcp_do_eifel_detect &&
                                     (tp->t_flags & TF_RCVD_TSTMP)) &&
                                    ownd < 4 * tp->t_maxseg &&
                                    tp->t_dupacks + 1 >=
                                      iceildiv(ownd, tp->t_maxseg) &&
                                    (!TCP_DO_SACK(tp) ||
                                     ownd <= tp->t_maxseg ||
                                     tcp_sack_has_sacked(&tp->scb,
                                                        ownd - tp->t_maxseg))) {
                                        ++tcpstat.tcps_sndearlyrexmit;
                                        tp->t_flags |= TF_EARLYREXMT;
                                        goto fastretransmit;
                                }
                        }
                        goto drop;
                }

                KASSERT(SEQ_GT(th->th_ack, tp->snd_una), ("th_ack <= snd_una"));
                tp->t_dupacks = 0;
                if (SEQ_GT(th->th_ack, tp->snd_max)) {
                        /*
                         * Detected optimistic ACK attack.
                         * Force slow-start to de-synchronize attack.
                         */
                        tp->snd_cwnd = tp->t_maxseg;
                        tp->snd_wacked = 0;

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

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

                if (tcp_do_eifel_detect && acked > 0 &&
                    (to.to_flags & TOF_TS) && (to.to_tsecr != 0) &&
                    (tp->t_flags & TF_FIRSTACCACK)) {
                        /* Eifel detection applicable. */
                        if (to.to_tsecr < tp->t_rexmtTS) {
                                ++tcpstat.tcps_eifeldetected;
                                tcp_revert_congestion_state(tp);
                                if (tp->t_rxtshift == 1 &&
                                    ticks >= tp->t_badrxtwin)
                                        ++tcpstat.tcps_rttcantdetect;
                        }
                } else if (tp->t_rxtshift == 1 && ticks < tp->t_badrxtwin) {
                        /*
                         * 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.
                         */
                        tcp_revert_congestion_state(tp);
                        ++tcpstat.tcps_rttdetected;
                }

                /*
                 * 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 machines (certain windows boxes) send broken
                 * timestamp replies during the SYN+ACK phase, ignore
                 * timestamps of 0.
                 */
                if ((to.to_flags & TOF_TS) && (to.to_tsecr != 0))
                        tcp_xmit_timer(tp, ticks - to.to_tsecr + 1);
                else if (tp->t_rtttime && SEQ_GT(th->th_ack, tp->t_rtseq))
                        tcp_xmit_timer(tp, ticks - tp->t_rtttime);
                tcp_xmit_bandwidth_limit(tp, th->th_ack);

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

                /* Stop looking for an acceptable ACK since one was received. */
                tp->t_flags &= ~(TF_FIRSTACCACK | TF_FASTREXMT | TF_EARLYREXMT);

                if (acked > so->so_snd.ssb_cc) {
                        tp->snd_wnd -= so->so_snd.ssb_cc;
                        sbdrop(&so->so_snd.sb, (int)so->so_snd.ssb_cc);
                        ourfinisacked = TRUE;
                } else {
                        sbdrop(&so->so_snd.sb, acked);
                        tp->snd_wnd -= acked;
                        ourfinisacked = FALSE;
                }
                sowwakeup(so);

                /*
                 * Update window information.
                 * Don't look at window if no ACK:
                 * TAC's send garbage on first SYN.
                 */
                if (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 = TRUE;
                }

                tp->snd_una = th->th_ack;
                if (TCP_DO_SACK(tp))
                        tcp_sack_update_scoreboard(tp, &to);
                if (IN_FASTRECOVERY(tp)) {
                        if (SEQ_GEQ(th->th_ack, tp->snd_recover)) {
                                EXIT_FASTRECOVERY(tp);
                                needoutput = TRUE;
                                /*
                                 * If the congestion window was inflated
                                 * to account for the other side's
                                 * cached packets, retract it.
                                 */
                                if (!TCP_DO_SACK(tp))
                                        tp->snd_cwnd = tp->snd_ssthresh;

                                /*
                                 * Window inflation should have left us
                                 * with approximately snd_ssthresh outstanding
                                 * data.  But, in case we would be inclined
                                 * to send a burst, better do it using
                                 * slow start.
                                 */
                                if (SEQ_GT(th->th_ack + tp->snd_cwnd,
                                           tp->snd_max + 2 * tp->t_maxseg))
                                        tp->snd_cwnd =
                                            (tp->snd_max - tp->snd_una) +
                                            2 * tp->t_maxseg;

                                tp->snd_wacked = 0;
                        } else {
                                if (TCP_DO_SACK(tp)) {
                                        tp->snd_max_rexmt = tp->snd_max;
                                        tcp_sack_rexmt(tp, th);
                                } else {
                                        tcp_newreno_partial_ack(tp, th, acked);
                                }
                                needoutput = FALSE;
                        }
                } else {
                        /*
                         * Open the congestion window.  When in slow-start,
                         * open exponentially: maxseg per packet.  Otherwise,
                         * open linearly: maxseg per window.
                         */
                        if (tp->snd_cwnd <= tp->snd_ssthresh) {
                                u_int abc_sslimit =
                                    (SEQ_LT(tp->snd_nxt, tp->snd_max) ?
                                     tp->t_maxseg : 2 * tp->t_maxseg);

                                /* slow-start */
                                tp->snd_cwnd += tcp_do_abc ?
                                    min(acked, abc_sslimit) : tp->t_maxseg;
                        } else {
                                /* linear increase */
                                tp->snd_wacked += tcp_do_abc ? acked :
                                    tp->t_maxseg;
                                if (tp->snd_wacked >= tp->snd_cwnd) {
                                        tp->snd_wacked -= tp->snd_cwnd;
                                        tp->snd_cwnd += tp->t_maxseg;
                                }
                        }
                        tp->snd_cwnd = min(tp->snd_cwnd,
                                           TCP_MAXWIN << tp->snd_scale);
                        tp->snd_recover = th->th_ack - 1;
                }
                if (SEQ_LT(tp->snd_nxt, tp->snd_una))
                        tp->snd_nxt = tp->snd_una;

                /*
                 * 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 = TRUE;
                } else if (!callout_active(tp->tt_persist))
                        callout_reset(tp->tt_rexmt, tp->t_rxtcur,
                                      tcp_timer_rexmt, tp);

                switch (tp->t_state) {
                /*
                 * In FIN_WAIT_1 STATE in addition to the processing
                 * for the ESTABLISHED state if our FIN is now acknowledged
                 * then enter FIN_WAIT_2.
                 */
                case TCPS_FIN_WAIT_1:
                        if (ourfinisacked) {
                                /*
                                 * If we can't receive any more
                                 * data, then closing user can proceed.
                                 * Starting the timer is contrary to the
                                 * specification, but if we don't get a FIN
                                 * we'll hang forever.
                                 */
                                if (so->so_state & SS_CANTRCVMORE) {
                                        soisdisconnected(so);
                                        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) {
                                tp->t_state = TCPS_TIME_WAIT;
                                tcp_canceltimers(tp);
                                /* Shorten TIME_WAIT [RFC-1644, p.28] */
                                if (tp->cc_recv != 0 &&
                                    (ticks - tp->t_starttime) < tcp_msl)
                                        callout_reset(tp->tt_2msl,
                                            tp->t_rxtcur * TCPTV_TWTRUNC,
                                            tcp_timer_2msl, tp);
                                else
                                        callout_reset(tp->tt_2msl, 2 * tcp_msl,
                                            tcp_timer_2msl, tp);
                                soisdisconnected(so);
                        }
                        break;

                /*
                 * In LAST_ACK, we may still be waiting for data to drain
                 * and/or to be acked, as well as for the ack of our FIN.
                 * If our FIN is now acknowledged, delete the TCB,
                 * enter the closed state and return.
                 */
                case TCPS_LAST_ACK:
                        if (ourfinisacked) {
                                tp = tcp_close(tp);
                                goto drop;
                        }
                        break;

                /*
                 * In TIME_WAIT state the only thing that should arrive
                 * is a retransmission of the remote FIN.  Acknowledge
                 * it and restart the finack timer.
                 */
                case TCPS_TIME_WAIT:
                        callout_reset(tp->tt_2msl, 2 * tcp_msl,
                                      tcp_timer_2msl, tp);
                        goto dropafterack;
                }
        }

step6:
        /*
         * Update window information.
         * Don't look at window if no ACK: TAC's send garbage on first SYN.
         */
        if ((thflags & TH_ACK) &&
            acceptable_window_update(tp, th, tiwin)) {
                /* 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 = TRUE;
        }

        /*
         * Process segments with URG.
         */
        if ((thflags & TH_URG) && th->th_urp &&
            !TCPS_HAVERCVDFIN(tp->t_state)) {
                /*
                 * This is a kludge, but if we receive and accept
                 * random urgent pointers, we'll crash in
                 * soreceive.  It's hard to imagine someone
                 * actually wanting to send this much urgent data.
                 */
                if (th->th_urp + so->so_rcv.ssb_cc > sb_max) {
                        th->th_urp = 0;                 /* XXX */
                        thflags &= ~TH_URG;             /* XXX */
                        goto dodata;                    /* XXX */
                }
                /*
                 * If this segment advances the known urgent pointer,
                 * then mark the data stream.  This should not happen
                 * in CLOSE_WAIT, CLOSING, LAST_ACK or TIME_WAIT STATES since
                 * a FIN has been received from the remote side.
                 * In these states we ignore the URG.
                 *
                 * According to RFC961 (Assigned Protocols),
                 * the urgent pointer points to the last octet
                 * of urgent data.  We continue, however,
                 * to consider it to indicate the first octet
                 * of data past the urgent section as the original
                 * spec states (in one of two places).
                 */
                if (SEQ_GT(th->th_seq + th->th_urp, tp->rcv_up)) {
                        tp->rcv_up = th->th_seq + th->th_urp;
                        so->so_oobmark = so->so_rcv.ssb_cc +
                            (tp->rcv_up - tp->rcv_nxt) - 1;
                        if (so->so_oobmark == 0)
                                so->so_state |= SS_RCVATMARK;
                        sohasoutofband(so);
                        tp->t_oobflags &= ~(TCPOOB_HAVEDATA | TCPOOB_HADDATA);
                }
                /*
                 * Remove out of band data so doesn't get presented to user.
                 * This can happen independent of advancing the URG pointer,
                 * but if two URG's are pending at once, some out-of-band
                 * data may creep in... ick.
                 */
                if (th->th_urp <= (u_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 */
        /*
         * 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)) {
                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))
                                callout_reset(tp->tt_delack, tcp_delacktime,
                                              tcp_timer_delack, tp);
                        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);
                        if (so->so_state & SS_CANTRCVMORE)
                                m_freem(m);
                        else
                                ssb_appendstream(&so->so_rcv, m);
                        sorwakeup(so);
                } else {
                        if (!(tp->t_flags & TF_DUPSEG)) {
                                /* Initialize SACK report block. */
                                tp->reportblk.rblk_start = th->th_seq;
                                tp->reportblk.rblk_end = th->th_seq + tlen +
                                    ((thflags & TH_FIN) != 0);
                        }
                        thflags = tcp_reass(tp, th, &tlen, m);
                        tp->t_flags |= TF_ACKNOW;
                }

                /*
                 * Note the amount of data that peer has sent into
                 * our window, in order to estimate the sender's
                 * buffer size.
                 */
                len = so->so_rcv.ssb_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)) {
                        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 (DELAY_ACK(tp) && (tp->t_flags & TF_NEEDSYN))
                                callout_reset(tp->tt_delack, tcp_delacktime,
                                    tcp_timer_delack, tp);
                        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:
                        tp->t_state = TCPS_TIME_WAIT;
                        tcp_canceltimers(tp);
                        /* Shorten TIME_WAIT [RFC-1644, p.28] */
                        if (tp->cc_recv != 0 &&
                            (ticks - tp->t_starttime) < tcp_msl) {
                                callout_reset(tp->tt_2msl,
                                              tp->t_rxtcur * TCPTV_TWTRUNC,
                                              tcp_timer_2msl, tp);
                                /* For transaction client, force ACK now. */
                                tp->t_flags |= TF_ACKNOW;
                        }
                        else
                                callout_reset(tp->tt_2msl, 2 * tcp_msl,
                                              tcp_timer_2msl, tp);
                        soisdisconnected(so);
                        break;

                /*
                 * In TIME_WAIT state restart the 2 MSL time_wait timer.
                 */
                case TCPS_TIME_WAIT:
                        callout_reset(tp->tt_2msl, 2 * tcp_msl,
                                      tcp_timer_2msl, tp);
                        break;
                }
        }

#ifdef TCPDEBUG
        if (so->so_options & SO_DEBUG)
                tcp_trace(TA_INPUT, ostate, tp, tcp_saveipgen, &tcp_savetcp, 0);
#endif

        /*
         * Return any desired output.
         */
        if (needoutput || (tp->t_flags & TF_ACKNOW))
                tcp_output(tp);
        return;

dropafterack:
        /*
         * Generate an ACK dropping incoming segment if it occupies
         * sequence space, where the ACK reflects our state.
         *
         * We can now skip the test for the RST flag since all
         * paths to this code happen after packets containing
         * RST have been dropped.
         *
         * In the SYN-RECEIVED state, don't send an ACK unless the
         * segment we received passes the SYN-RECEIVED ACK test.
         * If it fails send a RST.  This breaks the loop in the
         * "LAND" DoS attack, and also prevents an ACK storm
         * between two listening ports that have been sent forged
         * SYN segments, each with the source address of the other.
         */
        if (tp->t_state == TCPS_SYN_RECEIVED && (thflags & TH_ACK) &&
            (SEQ_GT(tp->snd_una, th->th_ack) ||
             SEQ_GT(th->th_ack, tp->snd_max)) ) {
                rstreason = BANDLIM_RST_OPENPORT;
                goto dropwithreset;
        }
#ifdef TCPDEBUG
        if (so->so_options & SO_DEBUG)
                tcp_trace(TA_DROP, ostate, tp, tcp_saveipgen, &tcp_savetcp, 0);
#endif
        m_freem(m);
        tp->t_flags |= TF_ACKNOW;
        tcp_output(tp);
        return;

dropwithreset:
        /*
         * Generate a RST, dropping incoming segment.
         * Make ACK acceptable to originator of segment.
         * Don't bother to respond if destination was broadcast/multicast.
         */
        if ((thflags & TH_RST) || m->m_flags & (M_BCAST | M_MCAST))
                goto drop;
        if (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.
         */
#ifdef ICMP_BANDLIM
        if (badport_bandlim(rstreason) < 0)
                goto drop;
#endif

#ifdef TCPDEBUG
        if (tp == NULL || (tp->t_inpcb->inp_socket->so_options & SO_DEBUG))
                tcp_trace(TA_DROP, ostate, tp, 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);
        }
        return;

drop:
        /*
         * Drop space held by incoming segment and return.
         */
#ifdef TCPDEBUG
        if (tp == NULL || (tp->t_inpcb->inp_socket->so_options & SO_DEBUG))
                tcp_trace(TA_DROP, ostate, tp, tcp_saveipgen, &tcp_savetcp, 0);
#endif
        m_freem(m);
        return;
}

/*
 * Parse TCP options and place in tcpopt.
 */
static void
tcp_dooptions(struct tcpopt *to, u_char *cp, int cnt, boolean_t is_syn)
{
        int opt, optlen, i;

        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(cp + 2, &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(cp + 2, &to->to_tsval, sizeof to->to_tsval);
                        to->to_tsval = ntohl(to->to_tsval);
                        bcopy(cp + 6, &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;
                case TCPOPT_CC:
                        if (optlen != TCPOLEN_CC)
                                continue;
                        to->to_flags |= TOF_CC;
                        bcopy(cp + 2, &to->to_cc, sizeof to->to_cc);
                        to->to_cc = ntohl(to->to_cc);
                        break;
                case TCPOPT_CCNEW:
                        if (optlen != TCPOLEN_CC)
                                continue;
                        if (!is_syn)
                                continue;
                        to->to_flags |= TOF_CCNEW;
                        bcopy(cp + 2, &to->to_cc, sizeof to->to_cc);
                        to->to_cc = ntohl(to->to_cc);
                        break;
                case TCPOPT_CCECHO:
                        if (optlen != TCPOLEN_CC)
                                continue;
                        if (!is_syn)
                                continue;
                        to->to_flags |= TOF_CCECHO;
                        bcopy(cp + 2, &to->to_ccecho, sizeof to->to_ccecho);
                        to->to_ccecho = ntohl(to->to_ccecho);
                        break;
                case TCPOPT_SACK_PERMITTED:
                        if (optlen != TCPOLEN_SACK_PERMITTED)
                                continue;
                        if (!is_syn)
                                continue;
                        to->to_flags |= TOF_SACK_PERMITTED;
                        break;
                case TCPOPT_SACK:
                        if ((optlen - 2) & 0x07)        /* not multiple of 8 */
                                continue;
                        to->to_nsackblocks = (optlen - 2) / 8;
                        to->to_sackblocks = (struct raw_sackblock *) (cp + 2);
                        to->to_flags |= TOF_SACK;
                        for (i = 0; i < to->to_nsackblocks; i++) {
                                struct raw_sackblock *r = &to->to_sackblocks[i];

                                r->rblk_start = ntohl(r->rblk_start);
                                r->rblk_end = ntohl(r->rblk_end);
                        }
                        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.
 * "off" is the delayed to be dropped hdrlen.
 */
static void
tcp_pulloutofband(struct socket *so, struct tcphdr *th, struct mbuf *m, int off)
{
        int cnt = off + th->th_urp - 1;

        while (cnt >= 0) {
                if (m->m_len > cnt) {
                        char *cp = mtod(m, caddr_t) + cnt;
                        struct tcpcb *tp = sototcpcb(so);

                        tp->t_iobc = *cp;
                        tp->t_oobflags |= TCPOOB_HAVEDATA;
                        bcopy(cp + 1, cp, 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(struct tcpcb *tp, int rtt)
{
        int delta;

        tcpstat.tcps_rttupdated++;
        tp->t_rttupdated++;
        if (tp->t_srtt != 0) {
                /*
                 * srtt is stored as fixed point with 5 bits after the
                 * binary point (i.e., scaled by 8).  The following magic
                 * is equivalent to the smoothing algorithm in rfc793 with
                 * an alpha of .875 (srtt = rtt/8 + srtt*7/8 in fixed
                 * point).  Adjust rtt to origin 0.
                 */
                delta = ((rtt - 1) << TCP_DELTA_SHIFT)
                        - (tp->t_srtt >> (TCP_RTT_SHIFT - TCP_DELTA_SHIFT));

                if ((tp->t_srtt += delta) <= 0)
                        tp->t_srtt = 1;

                /*
                 * We accumulate a smoothed rtt variance (actually, a
                 * smoothed mean difference), then set the retransmit
                 * timer to smoothed rtt + 4 times the smoothed variance.
                 * rttvar is stored as fixed point with 4 bits after the
                 * binary point (scaled by 16).  The following is
                 * equivalent to rfc793 smoothing with an alpha of .75
                 * (rttvar = rttvar*3/4 + |delta| / 4).  This replaces
                 * rfc793's wired-in beta.
                 */
                if (delta < 0)
                        delta = -delta;
                delta -= tp->t_rttvar >> (TCP_RTTVAR_SHIFT - TCP_DELTA_SHIFT);
                if ((tp->t_rttvar += delta) <= 0)
                        tp->t_rttvar = 1;
                if (tp->t_rttbest > tp->t_srtt + tp->t_rttvar)
                        tp->t_rttbest = tp->t_srtt + tp->t_rttvar;
        } else {
                /*
                 * No rtt measurement yet - use the unsmoothed rtt.
                 * Set the variance to half the rtt (so our first
                 * retransmit happens at 3*rtt).
                 */
                tp->t_srtt = rtt << TCP_RTT_SHIFT;
                tp->t_rttvar = rtt << (TCP_RTTVAR_SHIFT - 1);
                tp->t_rttbest = tp->t_srtt + tp->t_rttvar;
        }
        tp->t_rtttime = 0;
        tp->t_rxtshift = 0;

        /*
         * the retransmit should happen at rtt + 4 * rttvar.
         * Because of the way we do the smoothing, srtt and rttvar
         * will each average +1/2 tick of bias.  When we compute
         * the retransmit timer, we want 1/2 tick of rounding and
         * 1 extra tick because of +-1/2 tick uncertainty in the
         * firing of the timer.  The bias will give us exactly the
         * 1.5 tick we need.  But, because the bias is
         * statistical, we have to test that we don't drop below
         * the minimum feasible timer (which is 2 ticks).
         */
        TCPT_RANGESET(tp->t_rxtcur, TCP_REXMTVAL(tp),
                      max(tp->t_rttmin, rtt + 2), TCPTV_REXMTMAX);

        /*
         * We received an ack for a packet that wasn't retransmitted;
         * it is probably safe to discard any error indications we've
         * received recently.  This isn't quite right, but close enough
         * for now (a route might have failed after we sent a segment,
         * and the return path might not be symmetrical).
         */
        tp->t_softerror = 0;
}

/*
 * Determine a reasonable value for maxseg size.
 * If the route is known, check route for mtu.
 * If none, use an mss that can be handled on the outgoing
 * interface without forcing IP to fragment; if bigger than
 * an mbuf cluster (MCLBYTES), round down to nearest multiple of MCLBYTES
 * to utilize large mbufs.  If no route is found, route has no mtu,
 * or the destination isn't local, use a default, hopefully conservative
 * size (usually 512 or the default IP max size, but no more than the mtu
 * of the interface), as we can't discover anything about intervening
 * gateways or networks.  We also initialize the congestion/slow start
 * window to be a single segment if the destination isn't local.
 * While looking at the routing entry, we also initialize other path-dependent
 * parameters from pre-set or cached values in the routing entry.
 *
 * Also take into account the space needed for options that we
 * send regularly.  Make maxseg shorter by that amount to assure
 * that we can send maxseg amount of data even when the options
 * are present.  Store the upper limit of the length of options plus
 * data in maxopd.
 *
 * NOTE that this routine is only called when we process an incoming
 * segment, for outgoing segments only tcp_mssopt is called.
 *
 * In case of T/TCP, we call this routine during implicit connection
 * setup as well (offer = -1), to initialize maxseg from the cached
 * MSS of our peer.
 */
void
tcp_mss(struct tcpcb *tp, int offer)
{
        struct rtentry *rt;
        struct ifnet *ifp;
        int rtt, mss;
        u_long bufsize;
        struct inpcb *inp = tp->t_inpcb;
        struct socket *so;
        struct rmxp_tao *taop;
        int origoffer = offer;
#ifdef INET6
        boolean_t isipv6 = ((inp->inp_vflag & INP_IPV6) ? TRUE : FALSE);
        size_t min_protoh = isipv6 ?
                            sizeof(struct ip6_hdr) + sizeof(struct tcphdr) :
                            sizeof(struct tcpiphdr);
#else
        const boolean_t isipv6 = FALSE;
        const size_t min_protoh = sizeof(struct tcpiphdr);
#endif

        if (isipv6)
                rt = tcp_rtlookup6(&inp->inp_inc);
        else
                rt = tcp_rtlookup(&inp->inp_inc);
        if (rt == NULL) {
                tp->t_maxopd = tp->t_maxseg =
                    (isipv6 ? tcp_v6mssdflt : tcp_mssdflt);
                return;
        }
        ifp = rt->rt_ifp;
        so = inp->inp_socket;

        taop = rmx_taop(rt->rt_rmx);
        /*
         * Offer == -1 means that we didn't receive SYN yet,
         * use cached value in that case;
         */
        if (offer == -1)
                offer = taop->tao_mssopt;
        /*
         * Offer == 0 means that there was no MSS on the SYN segment,
         * in this case we use tcp_mssdflt.
         */
        if (offer == 0)
                offer = (isipv6 ? tcp_v6mssdflt : tcp_mssdflt);
        else
                /*
                 * Sanity check: make sure that maxopd will be large
                 * enough to allow some data on segments even is the
                 * all the option space is used (40bytes).  Otherwise
                 * funny things may happen in tcp_output.
                 */
                offer = max(offer, 64);
        taop->tao_mssopt = offer;

        /*
         * While we're here, check if there's an initial rtt
         * or rttvar.  Convert from the route-table units
         * to scaled multiples of the slow timeout timer.
         */
        if (tp->t_srtt == 0 && (rtt = rt->rt_rmx.rmx_rtt)) {
                /*
                 * XXX the lock bit for RTT indicates that the value
                 * is also a minimum value; this is subject to time.
                 */
                if (rt->rt_rmx.rmx_locks & RTV_RTT)
                        tp->t_rttmin = rtt / (RTM_RTTUNIT / hz);
                tp->t_srtt = rtt / (RTM_RTTUNIT / (hz * TCP_RTT_SCALE));
                tp->t_rttbest = tp->t_srtt + TCP_RTT_SCALE;
                tcpstat.tcps_usedrtt++;
                if (rt->rt_rmx.rmx_rttvar) {
                        tp->t_rttvar = rt->rt_rmx.rmx_rttvar /
                            (RTM_RTTUNIT / (hz * TCP_RTTVAR_SCALE));
                        tcpstat.tcps_usedrttvar++;
                } else {
                        /* default variation is +- 1 rtt */
                        tp->t_rttvar =
                            tp->t_srtt * TCP_RTTVAR_SCALE / TCP_RTT_SCALE;
                }
                TCPT_RANGESET(tp->t_rxtcur,
                              ((tp->t_srtt >> 2) + tp->t_rttvar) >> 1,
                              tp->t_rttmin, TCPTV_REXMTMAX);
        }
        /*
         * if there's an mtu associated with the route, use it
         * else, use the link mtu.
         */
        if (rt->rt_rmx.rmx_mtu)
                mss = rt->rt_rmx.rmx_mtu - min_protoh;
        else {
                if (isipv6) {
                        mss = ND_IFINFO(rt->rt_ifp)->linkmtu - min_protoh;
                        if (!in6_localaddr(&inp->in6p_faddr))
                                mss = min(mss, tcp_v6mssdflt);
                } else {
                        mss = ifp->if_mtu - min_protoh;
                        if (!in_localaddr(inp->inp_faddr))
                                mss = min(mss, tcp_mssdflt);
                }
        }
        mss = min(mss, offer);
        /*
         * maxopd stores the maximum length of data AND options
         * in a segment; maxseg is the amount of data in a normal
         * segment.  We need to store this value (maxopd) apart
         * from maxseg, because now every segment carries options
         * and thus we normally have somewhat less data in segments.
         */
        tp->t_maxopd = mss;

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

#if     (MCLBYTES & (MCLBYTES - 1)) == 0
                if (mss > MCLBYTES)
                        mss &= ~(MCLBYTES-1);
#else
                if (mss > MCLBYTES)
                        mss = mss / MCLBYTES * MCLBYTES;
#endif
        /*
         * If there's a pipesize, change the socket buffer
         * to that size.  Make the socket buffers an integral
         * number of mss units; if the mss is larger than
         * the socket buffer, decrease the mss.
         */
#ifdef RTV_SPIPE
        if ((bufsize = rt->rt_rmx.rmx_sendpipe) == 0)
#endif
                bufsize = so->so_snd.ssb_hiwat;
        if (bufsize < mss)
                mss = bufsize;
        else {
                bufsize = roundup(bufsize, mss);
                if (bufsize > sb_max)
                        bufsize = sb_max;
                if (bufsize > so->so_snd.ssb_hiwat)
                        ssb_reserve(&so->so_snd, bufsize, so, NULL);
        }
        tp->t_maxseg = mss;

#ifdef RTV_RPIPE
        if ((bufsize = rt->rt_rmx.rmx_recvpipe) == 0)
#endif
                bufsize = so->so_rcv.ssb_hiwat;
        if (bufsize > mss) {
                bufsize = roundup(bufsize, mss);
                if (bufsize > sb_max)
                        bufsize = sb_max;
                if (bufsize > so->so_rcv.ssb_hiwat)
                        ssb_reserve(&so->so_rcv, bufsize, so, NULL);
        }

        /*
         * Set the slow-start flight size depending on whether this
         * is a local network or not.
         */
        if (tcp_do_rfc3390)
                tp->snd_cwnd = min(4 * mss, max(2 * mss, 4380));
        else
                tp->snd_cwnd = mss;

        if (rt->rt_rmx.rmx_ssthresh) {
                /*
                 * There's some sort of gateway or interface
                 * buffer limit on the path.  Use this to set
                 * the slow start threshhold, but set the
                 * threshold to no less than 2*mss.
                 */
                tp->snd_ssthresh = max(2 * mss, rt->rt_rmx.rmx_ssthresh);
                tcpstat.tcps_usedssthresh++;
        }
}

/*
 * Determine the MSS option to send on an outgoing SYN.
 */
int
tcp_mssopt(struct tcpcb *tp)
{
        struct rtentry *rt;
#ifdef INET6
        boolean_t isipv6 =
            ((tp->t_inpcb->inp_vflag & INP_IPV6) ? TRUE : FALSE);
        int min_protoh = isipv6 ?
                             sizeof(struct ip6_hdr) + sizeof(struct tcphdr) :
                             sizeof(struct tcpiphdr);
#else
        const boolean_t isipv6 = FALSE;
        const size_t min_protoh = sizeof(struct tcpiphdr);
#endif

        if (isipv6)
                rt = tcp_rtlookup6(&tp->t_inpcb->inp_inc);
        else
                rt = tcp_rtlookup(&tp->t_inpcb->inp_inc);
        if (rt == NULL)
                return (isipv6 ? tcp_v6mssdflt : tcp_mssdflt);

        return (rt->rt_ifp->if_mtu - min_protoh);
}

/*
 * When a partial ack arrives, force the retransmission of the
 * next unacknowledged segment.  Do not exit Fast Recovery.
 *
 * Implement the Slow-but-Steady variant of NewReno by restarting the
 * the retransmission timer.  Turn it off here so it can be restarted
 * later in tcp_output().
 */
static void
tcp_newreno_partial_ack(struct tcpcb *tp, struct tcphdr *th, int acked)
{
        tcp_seq old_snd_nxt = 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_cwnd = tp->t_maxseg;
        tp->t_flags |= TF_ACKNOW;
        tcp_output(tp);
        if (SEQ_GT(old_snd_nxt, tp->snd_nxt))
                tp->snd_nxt = old_snd_nxt;
        /* partial window deflation */
        if (ocwnd > acked)
                tp->snd_cwnd = ocwnd - acked + tp->t_maxseg;
        else
                tp->snd_cwnd = tp->t_maxseg;
}

/*
 * In contrast to the Slow-but-Steady NewReno variant,
 * we do not reset the retransmission timer for SACK retransmissions,
 * except when retransmitting snd_una.
 */
static void
tcp_sack_rexmt(struct tcpcb *tp, struct tcphdr *th)
{
        uint32_t pipe, seglen;
        tcp_seq nextrexmt;
        boolean_t lostdup;
        tcp_seq old_snd_nxt = tp->snd_nxt;
        u_long ocwnd = tp->snd_cwnd;
        int nseg = 0;           /* consecutive new segments */
#define MAXBURST 4              /* limit burst of new packets on partial ack */

        tp->t_rtttime = 0;
        pipe = tcp_sack_compute_pipe(tp);
        while ((tcp_seq_diff_t)(ocwnd - pipe) >= (tcp_seq_diff_t)tp->t_maxseg &&
            (!tcp_do_smartsack || nseg < MAXBURST) &&
            tcp_sack_nextseg(tp, &nextrexmt, &seglen, &lostdup)) {
                uint32_t sent;
                tcp_seq old_snd_max;
                int error;

                if (nextrexmt == tp->snd_max)
                        ++nseg;
                tp->snd_nxt = nextrexmt;
                tp->snd_cwnd = nextrexmt - tp->snd_una + seglen;
                old_snd_max = tp->snd_max;
                if (nextrexmt == tp->snd_una)
                        callout_stop(tp->tt_rexmt);
                error = tcp_output(tp);
                if (error != 0)
                        break;
                sent = tp->snd_nxt - nextrexmt;
                if (sent <= 0)
                        break;
                if (!lostdup)
                        pipe += sent;
                tcpstat.tcps_sndsackpack++;
                tcpstat.tcps_sndsackbyte += sent;
                if (SEQ_LT(nextrexmt, old_snd_max) &&
                    SEQ_LT(tp->rexmt_high, tp->snd_nxt))
                        tp->rexmt_high = seq_min(tp->snd_nxt, old_snd_max);
        }
        if (SEQ_GT(old_snd_nxt, tp->snd_nxt))
                tp->snd_nxt = old_snd_nxt;
        tp->snd_cwnd = ocwnd;
}