Copy stable/9 to releng/9.0 as part of the FreeBSD 9.0-RELEASE release

[FreeBSD/releng/9.0.git] / sys / netinet / tcp_input.c

/*-
 * Copyright (c) 1982, 1986, 1988, 1990, 1993, 1994, 1995
 *      The Regents of the University of California.  All rights reserved.
 * Copyright (c) 2007-2008,2010
 *      Swinburne University of Technology, Melbourne, Australia.
 * Copyright (c) 2009-2010 Lawrence Stewart <lstewart@freebsd.org>
 * Copyright (c) 2010 The FreeBSD Foundation
 * Copyright (c) 2010-2011 Juniper Networks, Inc.
 * All rights reserved.
 *
 * Portions of this software were developed at the Centre for Advanced Internet
 * Architectures, Swinburne University of Technology, by Lawrence Stewart,
 * James Healy and David Hayes, made possible in part by a grant from the Cisco
 * University Research Program Fund at Community Foundation Silicon Valley.
 *
 * Portions of this software were developed at the Centre for Advanced
 * Internet Architectures, Swinburne University of Technology, Melbourne,
 * Australia by David Hayes under sponsorship from the FreeBSD Foundation.
 *
 * Portions of this software were developed by Robert N. M. Watson under
 * contract to Juniper Networks, Inc.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 * 4. Neither the name of the University nor the names of its contributors
 *    may be used to endorse or promote products derived from this software
 *    without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 * SUCH DAMAGE.
 *
 *      @(#)tcp_input.c 8.12 (Berkeley) 5/24/95
 */

#include <sys/cdefs.h>
__FBSDID("$FreeBSD$");

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

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

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

#include <vm/uma.h>

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

#define TCPSTATES               /* for logging */

#include <netinet/cc.h>
#include <netinet/in.h>
#include <netinet/in_pcb.h>
#include <netinet/in_systm.h>
#include <netinet/in_var.h>
#include <netinet/ip.h>
#include <netinet/ip_icmp.h>    /* required for icmp_var.h */
#include <netinet/icmp_var.h>   /* for ICMP_BANDLIM */
#include <netinet/ip_var.h>
#include <netinet/ip_options.h>
#include <netinet/ip6.h>
#include <netinet/icmp6.h>
#include <netinet6/in6_pcb.h>
#include <netinet6/ip6_var.h>
#include <netinet6/nd6.h>
#include <netinet/tcp_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>
#include <netinet/tcp_syncache.h>
#ifdef TCPDEBUG
#include <netinet/tcp_debug.h>
#endif /* TCPDEBUG */

#ifdef IPSEC
#include <netipsec/ipsec.h>
#include <netipsec/ipsec6.h>
#endif /*IPSEC*/

#include <machine/in_cksum.h>

#include <security/mac/mac_framework.h>

const int tcprexmtthresh = 3;

VNET_DEFINE(struct tcpstat, tcpstat);
SYSCTL_VNET_STRUCT(_net_inet_tcp, TCPCTL_STATS, stats, CTLFLAG_RW,
    &VNET_NAME(tcpstat), tcpstat,
    "TCP statistics (struct tcpstat, netinet/tcp_var.h)");

int tcp_log_in_vain = 0;
SYSCTL_INT(_net_inet_tcp, OID_AUTO, log_in_vain, CTLFLAG_RW,
    &tcp_log_in_vain, 0,
    "Log all incoming TCP segments to closed ports");

VNET_DEFINE(int, blackhole) = 0;
#define V_blackhole             VNET(blackhole)
SYSCTL_VNET_INT(_net_inet_tcp, OID_AUTO, blackhole, CTLFLAG_RW,
    &VNET_NAME(blackhole), 0,
    "Do not send RST on segments to closed ports");

VNET_DEFINE(int, tcp_delack_enabled) = 1;
SYSCTL_VNET_INT(_net_inet_tcp, OID_AUTO, delayed_ack, CTLFLAG_RW,
    &VNET_NAME(tcp_delack_enabled), 0,
    "Delay ACK to try and piggyback it onto a data packet");

VNET_DEFINE(int, drop_synfin) = 0;
#define V_drop_synfin           VNET(drop_synfin)
SYSCTL_VNET_INT(_net_inet_tcp, OID_AUTO, drop_synfin, CTLFLAG_RW,
    &VNET_NAME(drop_synfin), 0,
    "Drop TCP packets with SYN+FIN set");

VNET_DEFINE(int, tcp_do_rfc3042) = 1;
#define V_tcp_do_rfc3042        VNET(tcp_do_rfc3042)
SYSCTL_VNET_INT(_net_inet_tcp, OID_AUTO, rfc3042, CTLFLAG_RW,
    &VNET_NAME(tcp_do_rfc3042), 0,
    "Enable RFC 3042 (Limited Transmit)");

VNET_DEFINE(int, tcp_do_rfc3390) = 1;
SYSCTL_VNET_INT(_net_inet_tcp, OID_AUTO, rfc3390, CTLFLAG_RW,
    &VNET_NAME(tcp_do_rfc3390), 0,
    "Enable RFC 3390 (Increasing TCP's Initial Congestion Window)");

VNET_DEFINE(int, tcp_do_rfc3465) = 1;
SYSCTL_VNET_INT(_net_inet_tcp, OID_AUTO, rfc3465, CTLFLAG_RW,
    &VNET_NAME(tcp_do_rfc3465), 0,
    "Enable RFC 3465 (Appropriate Byte Counting)");

VNET_DEFINE(int, tcp_abc_l_var) = 2;
SYSCTL_VNET_INT(_net_inet_tcp, OID_AUTO, abc_l_var, CTLFLAG_RW,
    &VNET_NAME(tcp_abc_l_var), 2,
    "Cap the max cwnd increment during slow-start to this number of segments");

SYSCTL_NODE(_net_inet_tcp, OID_AUTO, ecn, CTLFLAG_RW, 0, "TCP ECN");

VNET_DEFINE(int, tcp_do_ecn) = 0;
SYSCTL_VNET_INT(_net_inet_tcp_ecn, OID_AUTO, enable, CTLFLAG_RW,
    &VNET_NAME(tcp_do_ecn), 0,
    "TCP ECN support");

VNET_DEFINE(int, tcp_ecn_maxretries) = 1;
SYSCTL_VNET_INT(_net_inet_tcp_ecn, OID_AUTO, maxretries, CTLFLAG_RW,
    &VNET_NAME(tcp_ecn_maxretries), 0,
    "Max retries before giving up on ECN");

VNET_DEFINE(int, tcp_insecure_rst) = 0;
#define V_tcp_insecure_rst      VNET(tcp_insecure_rst)
SYSCTL_VNET_INT(_net_inet_tcp, OID_AUTO, insecure_rst, CTLFLAG_RW,
    &VNET_NAME(tcp_insecure_rst), 0,
    "Follow the old (insecure) criteria for accepting RST packets");

VNET_DEFINE(int, tcp_do_autorcvbuf) = 1;
#define V_tcp_do_autorcvbuf     VNET(tcp_do_autorcvbuf)
SYSCTL_VNET_INT(_net_inet_tcp, OID_AUTO, recvbuf_auto, CTLFLAG_RW,
    &VNET_NAME(tcp_do_autorcvbuf), 0,
    "Enable automatic receive buffer sizing");

VNET_DEFINE(int, tcp_autorcvbuf_inc) = 16*1024;
#define V_tcp_autorcvbuf_inc    VNET(tcp_autorcvbuf_inc)
SYSCTL_VNET_INT(_net_inet_tcp, OID_AUTO, recvbuf_inc, CTLFLAG_RW,
    &VNET_NAME(tcp_autorcvbuf_inc), 0,
    "Incrementor step size of automatic receive buffer");

VNET_DEFINE(int, tcp_autorcvbuf_max) = 2*1024*1024;
#define V_tcp_autorcvbuf_max    VNET(tcp_autorcvbuf_max)
SYSCTL_VNET_INT(_net_inet_tcp, OID_AUTO, recvbuf_max, CTLFLAG_RW,
    &VNET_NAME(tcp_autorcvbuf_max), 0,
    "Max size of automatic receive buffer");

VNET_DEFINE(struct inpcbhead, tcb);
#define tcb6    tcb  /* for KAME src sync over BSD*'s */
VNET_DEFINE(struct inpcbinfo, tcbinfo);

static void      tcp_dooptions(struct tcpopt *, u_char *, int, int);
static void      tcp_do_segment(struct mbuf *, struct tcphdr *,
                     struct socket *, struct tcpcb *, int, int, uint8_t,
                     int);
static void      tcp_dropwithreset(struct mbuf *, struct tcphdr *,
                     struct tcpcb *, int, int);
static void      tcp_pulloutofband(struct socket *,
                     struct tcphdr *, struct mbuf *, int);
static void      tcp_xmit_timer(struct tcpcb *, int);
static void      tcp_newreno_partial_ack(struct tcpcb *, struct tcphdr *);
static void inline      tcp_fields_to_host(struct tcphdr *);
#ifdef TCP_SIGNATURE
static void inline      tcp_fields_to_net(struct tcphdr *);
static int inline       tcp_signature_verify_input(struct mbuf *, int, int,
                            int, struct tcpopt *, struct tcphdr *, u_int);
#endif
static void inline      cc_ack_received(struct tcpcb *tp, struct tcphdr *th,
                            uint16_t type);
static void inline      cc_conn_init(struct tcpcb *tp);
static void inline      cc_post_recovery(struct tcpcb *tp, struct tcphdr *th);
static void inline      hhook_run_tcp_est_in(struct tcpcb *tp,
                            struct tcphdr *th, struct tcpopt *to);

/*
 * Kernel module interface for updating tcpstat.  The argument is an index
 * into tcpstat treated as an array of u_long.  While this encodes the
 * general layout of tcpstat into the caller, it doesn't encode its location,
 * so that future changes to add, for example, per-CPU stats support won't
 * cause binary compatibility problems for kernel modules.
 */
void
kmod_tcpstat_inc(int statnum)
{

        (*((u_long *)&V_tcpstat + statnum))++;
}

/*
 * Wrapper for the TCP established input helper hook.
 */
static void inline
hhook_run_tcp_est_in(struct tcpcb *tp, struct tcphdr *th, struct tcpopt *to)
{
        struct tcp_hhook_data hhook_data;

        if (V_tcp_hhh[HHOOK_TCP_EST_IN]->hhh_nhooks > 0) {
                hhook_data.tp = tp;
                hhook_data.th = th;
                hhook_data.to = to;

                hhook_run_hooks(V_tcp_hhh[HHOOK_TCP_EST_IN], &hhook_data,
                    tp->osd);
        }
}

/*
 * CC wrapper hook functions
 */
static void inline
cc_ack_received(struct tcpcb *tp, struct tcphdr *th, uint16_t type)
{
        INP_WLOCK_ASSERT(tp->t_inpcb);

        tp->ccv->bytes_this_ack = BYTES_THIS_ACK(tp, th);
        if (tp->snd_cwnd == min(tp->snd_cwnd, tp->snd_wnd))
                tp->ccv->flags |= CCF_CWND_LIMITED;
        else
                tp->ccv->flags &= ~CCF_CWND_LIMITED;

        if (type == CC_ACK) {
                if (tp->snd_cwnd > tp->snd_ssthresh) {
                        tp->t_bytes_acked += min(tp->ccv->bytes_this_ack,
                             V_tcp_abc_l_var * tp->t_maxseg);
                        if (tp->t_bytes_acked >= tp->snd_cwnd) {
                                tp->t_bytes_acked -= tp->snd_cwnd;
                                tp->ccv->flags |= CCF_ABC_SENTAWND;
                        }
                } else {
                                tp->ccv->flags &= ~CCF_ABC_SENTAWND;
                                tp->t_bytes_acked = 0;
                }
        }

        if (CC_ALGO(tp)->ack_received != NULL) {
                /* XXXLAS: Find a way to live without this */
                tp->ccv->curack = th->th_ack;
                CC_ALGO(tp)->ack_received(tp->ccv, type);
        }
}

static void inline
cc_conn_init(struct tcpcb *tp)
{
        struct hc_metrics_lite metrics;
        struct inpcb *inp = tp->t_inpcb;
        int rtt;
#ifdef INET6
        int isipv6 = ((inp->inp_vflag & INP_IPV6) != 0) ? 1 : 0;
#endif

        INP_WLOCK_ASSERT(tp->t_inpcb);

        tcp_hc_get(&inp->inp_inc, &metrics);

        if (tp->t_srtt == 0 && (rtt = metrics.rmx_rtt)) {
                tp->t_srtt = rtt;
                tp->t_rttbest = tp->t_srtt + TCP_RTT_SCALE;
                TCPSTAT_INC(tcps_usedrtt);
                if (metrics.rmx_rttvar) {
                        tp->t_rttvar = metrics.rmx_rttvar;
                        TCPSTAT_INC(tcps_usedrttvar);
                } else {
                        /* default variation is +- 1 rtt */
                        tp->t_rttvar =
                            tp->t_srtt * TCP_RTTVAR_SCALE / TCP_RTT_SCALE;
                }
                TCPT_RANGESET(tp->t_rxtcur,
                    ((tp->t_srtt >> 2) + tp->t_rttvar) >> 1,
                    tp->t_rttmin, TCPTV_REXMTMAX);
        }
        if (metrics.rmx_ssthresh) {
                /*
                 * There's some sort of gateway or interface
                 * buffer limit on the path.  Use this to set
                 * the slow start threshhold, but set the
                 * threshold to no less than 2*mss.
                 */
                tp->snd_ssthresh = max(2 * tp->t_maxseg, metrics.rmx_ssthresh);
                TCPSTAT_INC(tcps_usedssthresh);
        }

        /*
         * Set the slow-start flight size depending on whether this
         * is a local network or not.
         *
         * Extend this so we cache the cwnd too and retrieve it here.
         * Make cwnd even bigger than RFC3390 suggests but only if we
         * have previous experience with the remote host. Be careful
         * not make cwnd bigger than remote receive window or our own
         * send socket buffer. Maybe put some additional upper bound
         * on the retrieved cwnd. Should do incremental updates to
         * hostcache when cwnd collapses so next connection doesn't
         * overloads the path again.
         *
         * XXXAO: Initializing the CWND from the hostcache is broken
         * and in its current form not RFC conformant.  It is disabled
         * until fixed or removed entirely.
         *
         * RFC3390 says only do this if SYN or SYN/ACK didn't got lost.
         * We currently check only in syncache_socket for that.
         */
/* #define TCP_METRICS_CWND */
#ifdef TCP_METRICS_CWND
        if (metrics.rmx_cwnd)
                tp->snd_cwnd = max(tp->t_maxseg, min(metrics.rmx_cwnd / 2,
                    min(tp->snd_wnd, so->so_snd.sb_hiwat)));
        else
#endif
        if (V_tcp_do_rfc3390)
                tp->snd_cwnd = min(4 * tp->t_maxseg,
                    max(2 * tp->t_maxseg, 4380));
#ifdef INET6
        else if (isipv6 && in6_localaddr(&inp->in6p_faddr))
                tp->snd_cwnd = tp->t_maxseg * V_ss_fltsz_local;
#endif
#if defined(INET) && defined(INET6)
        else if (!isipv6 && in_localaddr(inp->inp_faddr))
                tp->snd_cwnd = tp->t_maxseg * V_ss_fltsz_local;
#endif
#ifdef INET
        else if (in_localaddr(inp->inp_faddr))
                tp->snd_cwnd = tp->t_maxseg * V_ss_fltsz_local;
#endif
        else
                tp->snd_cwnd = tp->t_maxseg * V_ss_fltsz;

        if (CC_ALGO(tp)->conn_init != NULL)
                CC_ALGO(tp)->conn_init(tp->ccv);
}

void inline
cc_cong_signal(struct tcpcb *tp, struct tcphdr *th, uint32_t type)
{
        INP_WLOCK_ASSERT(tp->t_inpcb);

        switch(type) {
        case CC_NDUPACK:
                if (!IN_FASTRECOVERY(tp->t_flags)) {
                        tp->snd_recover = tp->snd_max;
                        if (tp->t_flags & TF_ECN_PERMIT)
                                tp->t_flags |= TF_ECN_SND_CWR;
                }
                break;
        case CC_ECN:
                if (!IN_CONGRECOVERY(tp->t_flags)) {
                        TCPSTAT_INC(tcps_ecn_rcwnd);
                        tp->snd_recover = tp->snd_max;
                        if (tp->t_flags & TF_ECN_PERMIT)
                                tp->t_flags |= TF_ECN_SND_CWR;
                }
                break;
        case CC_RTO:
                tp->t_dupacks = 0;
                tp->t_bytes_acked = 0;
                EXIT_RECOVERY(tp->t_flags);
                tp->snd_ssthresh = max(2, min(tp->snd_wnd, tp->snd_cwnd) / 2 /
                    tp->t_maxseg) * tp->t_maxseg;
                tp->snd_cwnd = tp->t_maxseg;
                break;
        case CC_RTO_ERR:
                TCPSTAT_INC(tcps_sndrexmitbad);
                /* RTO was unnecessary, so reset everything. */
                tp->snd_cwnd = tp->snd_cwnd_prev;
                tp->snd_ssthresh = tp->snd_ssthresh_prev;
                tp->snd_recover = tp->snd_recover_prev;
                if (tp->t_flags & TF_WASFRECOVERY)
                        ENTER_FASTRECOVERY(tp->t_flags);
                if (tp->t_flags & TF_WASCRECOVERY)
                        ENTER_CONGRECOVERY(tp->t_flags);
                tp->snd_nxt = tp->snd_max;
                tp->t_flags &= ~TF_PREVVALID;
                tp->t_badrxtwin = 0;
                break;
        }

        if (CC_ALGO(tp)->cong_signal != NULL) {
                if (th != NULL)
                        tp->ccv->curack = th->th_ack;
                CC_ALGO(tp)->cong_signal(tp->ccv, type);
        }
}

static void inline
cc_post_recovery(struct tcpcb *tp, struct tcphdr *th)
{
        INP_WLOCK_ASSERT(tp->t_inpcb);

        /* XXXLAS: KASSERT that we're in recovery? */

        if (CC_ALGO(tp)->post_recovery != NULL) {
                tp->ccv->curack = th->th_ack;
                CC_ALGO(tp)->post_recovery(tp->ccv);
        }
        /* XXXLAS: EXIT_RECOVERY ? */
        tp->t_bytes_acked = 0;
}

static inline void
tcp_fields_to_host(struct tcphdr *th)
{

        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);
}

#ifdef TCP_SIGNATURE
static inline void
tcp_fields_to_net(struct tcphdr *th)
{

        th->th_seq = htonl(th->th_seq);
        th->th_ack = htonl(th->th_ack);
        th->th_win = htons(th->th_win);
        th->th_urp = htons(th->th_urp);
}

static inline int
tcp_signature_verify_input(struct mbuf *m, int off0, int tlen, int optlen,
    struct tcpopt *to, struct tcphdr *th, u_int tcpbflag)
{
        int ret;

        tcp_fields_to_net(th);
        ret = tcp_signature_verify(m, off0, tlen, optlen, to, th, tcpbflag);
        tcp_fields_to_host(th);
        return (ret);
}
#endif

/* Neighbor Discovery, Neighbor Unreachability Detection Upper layer hint. */
#ifdef INET6
#define ND6_HINT(tp) \
do { \
        if ((tp) && (tp)->t_inpcb && \
            ((tp)->t_inpcb->inp_vflag & INP_IPV6) != 0) \
                nd6_nud_hint(NULL, NULL, 0); \
} while (0)
#else
#define ND6_HINT(tp)
#endif

/*
 * Indicate whether this ack should be delayed.  We can delay the ack if
 *      - there is no delayed ack timer in progress and
 *      - our last ack wasn't a 0-sized window.  We never want to delay
 *        the ack that opens up a 0-sized window and
 *              - delayed acks are enabled or
 *              - this is a half-synchronized T/TCP connection.
 */
#define DELAY_ACK(tp)                                                   \
        ((!tcp_timer_active(tp, TT_DELACK) &&                           \
            (tp->t_flags & TF_RXWIN0SENT) == 0) &&                      \
            (V_tcp_delack_enabled || (tp->t_flags & TF_NEEDSYN)))

/*
 * TCP input handling is split into multiple parts:
 *   tcp6_input is a thin wrapper around tcp_input for the extended
 *      ip6_protox[] call format in ip6_input
 *   tcp_input handles primary segment validation, inpcb lookup and
 *      SYN processing on listen sockets
 *   tcp_do_segment processes the ACK and text of the segment for
 *      establishing, established and closing connections
 */
#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;

                ifa_free(&ia6->ia_ifa);
                ip6 = mtod(m, struct ip6_hdr *);
                icmp6_error(m, ICMP6_DST_UNREACH, ICMP6_DST_UNREACH_ADDR,
                            (caddr_t)&ip6->ip6_dst - (caddr_t)ip6);
                return IPPROTO_DONE;
        }

        tcp_input(m, *offp);
        return IPPROTO_DONE;
}
#endif /* INET6 */

void
tcp_input(struct mbuf *m, int off0)
{
        struct tcphdr *th = NULL;
        struct ip *ip = NULL;
#ifdef INET
        struct ipovly *ipov;
#endif
        struct inpcb *inp = NULL;
        struct tcpcb *tp = NULL;
        struct socket *so = NULL;
        u_char *optp = NULL;
        int optlen = 0;
#ifdef INET
        int len;
#endif
        int tlen = 0, off;
        int drop_hdrlen;
        int thflags;
        int rstreason = 0;      /* For badport_bandlim accounting purposes */
#ifdef TCP_SIGNATURE
        uint8_t sig_checked = 0;
#endif
        uint8_t iptos = 0;
#ifdef IPFIREWALL_FORWARD
        struct m_tag *fwd_tag;
#endif
#ifdef INET6
        struct ip6_hdr *ip6 = NULL;
        int isipv6;
#else
        const void *ip6 = NULL;
#endif /* INET6 */
        struct tcpopt to;               /* options in this segment */
        char *s = NULL;                 /* address and port logging */
        int ti_locked;
#define TI_UNLOCKED     1
#define TI_WLOCKED      2

#ifdef TCPDEBUG
        /*
         * The size of tcp_saveipgen must be the size of the max ip header,
         * now IPv6.
         */
        u_char tcp_saveipgen[IP6_HDR_LEN];
        struct tcphdr tcp_savetcp;
        short ostate = 0;
#endif

#ifdef INET6
        isipv6 = (mtod(m, struct ip *)->ip_v == 6) ? 1 : 0;
#endif

        to.to_flags = 0;
        TCPSTAT_INC(tcps_rcvtotal);

#ifdef INET6
        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_INC(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;
                }
        }
#endif
#if defined(INET) && defined(INET6)
        else
#endif
#ifdef INET
        {
                /*
                 * Get IP and TCP header together in first mbuf.
                 * Note: IP leaves IP header in first mbuf.
                 */
                if (off0 > sizeof (struct ip)) {
                        ip_stripoptions(m, (struct mbuf *)0);
                        off0 = sizeof(struct ip);
                }
                if (m->m_len < sizeof (struct tcpiphdr)) {
                        if ((m = m_pullup(m, sizeof (struct tcpiphdr)))
                            == NULL) {
                                TCPSTAT_INC(tcps_rcvshort);
                                return;
                        }
                }
                ip = mtod(m, struct ip *);
                ipov = (struct ipovly *)ip;
                th = (struct tcphdr *)((caddr_t)ip + off0);
                tlen = ip->ip_len;

                if (m->m_pkthdr.csum_flags & CSUM_DATA_VALID) {
                        if (m->m_pkthdr.csum_flags & CSUM_PSEUDO_HDR)
                                th->th_sum = m->m_pkthdr.csum_data;
                        else
                                th->th_sum = in_pseudo(ip->ip_src.s_addr,
                                                ip->ip_dst.s_addr,
                                                htonl(m->m_pkthdr.csum_data +
                                                        ip->ip_len +
                                                        IPPROTO_TCP));
                        th->th_sum ^= 0xffff;
#ifdef TCPDEBUG
                        ipov->ih_len = (u_short)tlen;
                        ipov->ih_len = htons(ipov->ih_len);
#endif
                } else {
                        /*
                         * Checksum extended TCP header and data.
                         */
                        len = sizeof (struct ip) + tlen;
                        bzero(ipov->ih_x1, sizeof(ipov->ih_x1));
                        ipov->ih_len = (u_short)tlen;
                        ipov->ih_len = htons(ipov->ih_len);
                        th->th_sum = in_cksum(m, len);
                }
                if (th->th_sum) {
                        TCPSTAT_INC(tcps_rcvbadsum);
                        goto drop;
                }
                /* Re-initialization for later version check */
                ip->ip_v = IPVERSION;
        }
#endif /* INET */

#ifdef INET6
        if (isipv6)
                iptos = (ntohl(ip6->ip6_flow) >> 20) & 0xff;
#endif
#if defined(INET) && defined(INET6)
        else
#endif
#ifdef INET
                iptos = ip->ip_tos;
#endif

        /*
         * Check that TCP offset makes sense,
         * pull out TCP options and adjust length.              XXX
         */
        off = th->th_off << 2;
        if (off < sizeof (struct tcphdr) || off > tlen) {
                TCPSTAT_INC(tcps_rcvbadoff);
                goto drop;
        }
        tlen -= off;    /* tlen is used instead of ti->ti_len */
        if (off > sizeof (struct tcphdr)) {
#ifdef INET6
                if (isipv6) {
                        IP6_EXTHDR_CHECK(m, off0, off, );
                        ip6 = mtod(m, struct ip6_hdr *);
                        th = (struct tcphdr *)((caddr_t)ip6 + off0);
                }
#endif
#if defined(INET) && defined(INET6)
                else
#endif
#ifdef INET
                {
                        if (m->m_len < sizeof(struct ip) + off) {
                                if ((m = m_pullup(m, sizeof (struct ip) + off))
                                    == NULL) {
                                        TCPSTAT_INC(tcps_rcvshort);
                                        return;
                                }
                                ip = mtod(m, struct ip *);
                                ipov = (struct ipovly *)ip;
                                th = (struct tcphdr *)((caddr_t)ip + off0);
                        }
                }
#endif
                optlen = off - sizeof (struct tcphdr);
                optp = (u_char *)(th + 1);
        }
        thflags = th->th_flags;

        /*
         * Convert TCP protocol specific fields to host format.
         */
        tcp_fields_to_host(th);

        /*
         * Delay dropping TCP, IP headers, IPv6 ext headers, and TCP options.
         */
        drop_hdrlen = off0 + off;

        /*
         * Locate pcb for segment; if we're likely to add or remove a
         * connection then first acquire pcbinfo lock.  There are two cases
         * where we might discover later we need a write lock despite the
         * flags: ACKs moving a connection out of the syncache, and ACKs for
         * a connection in TIMEWAIT.
         */
        if ((thflags & (TH_SYN | TH_FIN | TH_RST)) != 0) {
                INP_INFO_WLOCK(&V_tcbinfo);
                ti_locked = TI_WLOCKED;
        } else
                ti_locked = TI_UNLOCKED;

findpcb:
#ifdef INVARIANTS
        if (ti_locked == TI_WLOCKED) {
                INP_INFO_WLOCK_ASSERT(&V_tcbinfo);
        } else {
                INP_INFO_UNLOCK_ASSERT(&V_tcbinfo);
        }
#endif

#ifdef IPFIREWALL_FORWARD
        /*
         * Grab info from PACKET_TAG_IPFORWARD tag prepended to the chain.
         */
        fwd_tag = m_tag_find(m, PACKET_TAG_IPFORWARD, NULL);
#endif /* IPFIREWALL_FORWARD */

#ifdef INET6
#ifdef IPFIREWALL_FORWARD
        if (isipv6 && fwd_tag != NULL) {
                struct sockaddr_in6 *next_hop6;

                next_hop6 = (struct sockaddr_in6 *)(fwd_tag + 1);
                /*
                 * Transparently forwarded. Pretend to be the destination.
                 * Already got one like this?
                 */
                inp = in6_pcblookup_mbuf(&V_tcbinfo,
                    &ip6->ip6_src, th->th_sport, &ip6->ip6_dst, th->th_dport,
                    INPLOOKUP_WLOCKPCB, m->m_pkthdr.rcvif, m);
                if (!inp) {
                        /*
                         * It's new.  Try to find the ambushing socket.
                         * Because we've rewritten the destination address,
                         * any hardware-generated hash is ignored.
                         */
                        inp = in6_pcblookup(&V_tcbinfo, &ip6->ip6_src,
                            th->th_sport, &next_hop6->sin6_addr,
                            next_hop6->sin6_port ? ntohs(next_hop6->sin6_port) :
                            th->th_dport, INPLOOKUP_WILDCARD |
                            INPLOOKUP_WLOCKPCB, m->m_pkthdr.rcvif);
                }
                /* Remove the tag from the packet.  We don't need it anymore. */
                m_tag_delete(m, fwd_tag);
        } else
#endif /* IPFIREWALL_FORWARD */
        if (isipv6) {
                inp = in6_pcblookup_mbuf(&V_tcbinfo, &ip6->ip6_src,
                    th->th_sport, &ip6->ip6_dst, th->th_dport,
                    INPLOOKUP_WILDCARD | INPLOOKUP_WLOCKPCB,
                    m->m_pkthdr.rcvif, m);
        }
#endif /* INET6 */
#if defined(INET6) && defined(INET)
        else
#endif
#ifdef INET
#ifdef IPFIREWALL_FORWARD
        if (fwd_tag != NULL) {
                struct sockaddr_in *next_hop;

                next_hop = (struct sockaddr_in *)(fwd_tag+1);
                /*
                 * Transparently forwarded. Pretend to be the destination.
                 * already got one like this?
                 */
                inp = in_pcblookup_mbuf(&V_tcbinfo, ip->ip_src, th->th_sport,
                    ip->ip_dst, th->th_dport, INPLOOKUP_WLOCKPCB,
                    m->m_pkthdr.rcvif, m);
                if (!inp) {
                        /*
                         * It's new.  Try to find the ambushing socket.
                         * Because we've rewritten the destination address,
                         * any hardware-generated hash is ignored.
                         */
                        inp = in_pcblookup(&V_tcbinfo, ip->ip_src,
                            th->th_sport, next_hop->sin_addr,
                            next_hop->sin_port ? ntohs(next_hop->sin_port) :
                            th->th_dport, INPLOOKUP_WILDCARD |
                            INPLOOKUP_WLOCKPCB, m->m_pkthdr.rcvif);
                }
                /* Remove the tag from the packet.  We don't need it anymore. */
                m_tag_delete(m, fwd_tag);
        } else
#endif /* IPFIREWALL_FORWARD */
                inp = in_pcblookup_mbuf(&V_tcbinfo, ip->ip_src,
                    th->th_sport, ip->ip_dst, th->th_dport,
                    INPLOOKUP_WILDCARD | INPLOOKUP_WLOCKPCB,
                    m->m_pkthdr.rcvif, m);
#endif /* INET */

        /*
         * If the INPCB does not exist then all data in the incoming
         * segment is discarded and an appropriate RST is sent back.
         * XXX MRT Send RST using which routing table?
         */
        if (inp == NULL) {
                /*
                 * Log communication attempts to ports that are not
                 * in use.
                 */
                if ((tcp_log_in_vain == 1 && (thflags & TH_SYN)) ||
                    tcp_log_in_vain == 2) {
                        if ((s = tcp_log_vain(NULL, th, (void *)ip, ip6)))
                                log(LOG_INFO, "%s; %s: Connection attempt "
                                    "to closed port\n", s, __func__);
                }
                /*
                 * When blackholing do not respond with a RST but
                 * completely ignore the segment and drop it.
                 */
                if ((V_blackhole == 1 && (thflags & TH_SYN)) ||
                    V_blackhole == 2)
                        goto dropunlock;

                rstreason = BANDLIM_RST_CLOSEDPORT;
                goto dropwithreset;
        }
        INP_WLOCK_ASSERT(inp);
        if (!(inp->inp_flags & INP_HW_FLOWID)
            && (m->m_flags & M_FLOWID)
            && ((inp->inp_socket == NULL)
                || !(inp->inp_socket->so_options & SO_ACCEPTCONN))) {
                inp->inp_flags |= INP_HW_FLOWID;
                inp->inp_flags &= ~INP_SW_FLOWID;
                inp->inp_flowid = m->m_pkthdr.flowid;
        }
#ifdef IPSEC
#ifdef INET6
        if (isipv6 && ipsec6_in_reject(m, inp)) {
                V_ipsec6stat.in_polvio++;
                goto dropunlock;
        } else
#endif /* INET6 */
        if (ipsec4_in_reject(m, inp) != 0) {
                V_ipsec4stat.in_polvio++;
                goto dropunlock;
        }
#endif /* IPSEC */

        /*
         * Check the minimum TTL for socket.
         */
        if (inp->inp_ip_minttl != 0) {
#ifdef INET6
                if (isipv6 && inp->inp_ip_minttl > ip6->ip6_hlim)
                        goto dropunlock;
                else
#endif
                if (inp->inp_ip_minttl > ip->ip_ttl)
                        goto dropunlock;
        }

        /*
         * A previous connection in TIMEWAIT state is supposed to catch stray
         * or duplicate segments arriving late.  If this segment was a
         * legitimate new connection attempt the old INPCB gets removed and
         * we can try again to find a listening socket.
         *
         * At this point, due to earlier optimism, we may hold only an inpcb
         * lock, and not the inpcbinfo write lock.  If so, we need to try to
         * acquire it, or if that fails, acquire a reference on the inpcb,
         * drop all locks, acquire a global write lock, and then re-acquire
         * the inpcb lock.  We may at that point discover that another thread
         * has tried to free the inpcb, in which case we need to loop back
         * and try to find a new inpcb to deliver to.
         *
         * XXXRW: It may be time to rethink timewait locking.
         */
relocked:
        if (inp->inp_flags & INP_TIMEWAIT) {
                if (ti_locked == TI_UNLOCKED) {
                        if (INP_INFO_TRY_WLOCK(&V_tcbinfo) == 0) {
                                in_pcbref(inp);
                                INP_WUNLOCK(inp);
                                INP_INFO_WLOCK(&V_tcbinfo);
                                ti_locked = TI_WLOCKED;
                                INP_WLOCK(inp);
                                if (in_pcbrele_wlocked(inp)) {
                                        inp = NULL;
                                        goto findpcb;
                                }
                        } else
                                ti_locked = TI_WLOCKED;
                }
                INP_INFO_WLOCK_ASSERT(&V_tcbinfo);

                if (thflags & TH_SYN)
                        tcp_dooptions(&to, optp, optlen, TO_SYN);
                /*
                 * NB: tcp_twcheck unlocks the INP and frees the mbuf.
                 */
                if (tcp_twcheck(inp, &to, th, m, tlen))
                        goto findpcb;
                INP_INFO_WUNLOCK(&V_tcbinfo);
                return;
        }
        /*
         * The TCPCB may no longer exist if the connection is winding
         * down or it is in the CLOSED state.  Either way we drop the
         * segment and send an appropriate response.
         */
        tp = intotcpcb(inp);
        if (tp == NULL || tp->t_state == TCPS_CLOSED) {
                rstreason = BANDLIM_RST_CLOSEDPORT;
                goto dropwithreset;
        }

        /*
         * We've identified a valid inpcb, but it could be that we need an
         * inpcbinfo write lock but don't hold it.  In this case, attempt to
         * acquire using the same strategy as the TIMEWAIT case above.  If we
         * relock, we have to jump back to 'relocked' as the connection might
         * now be in TIMEWAIT.
         */
#ifdef INVARIANTS
        if ((thflags & (TH_SYN | TH_FIN | TH_RST)) != 0)
                INP_INFO_WLOCK_ASSERT(&V_tcbinfo);
#endif
        if (tp->t_state != TCPS_ESTABLISHED) {
                if (ti_locked == TI_UNLOCKED) {
                        if (INP_INFO_TRY_WLOCK(&V_tcbinfo) == 0) {
                                in_pcbref(inp);
                                INP_WUNLOCK(inp);
                                INP_INFO_WLOCK(&V_tcbinfo);
                                ti_locked = TI_WLOCKED;
                                INP_WLOCK(inp);
                                if (in_pcbrele_wlocked(inp)) {
                                        inp = NULL;
                                        goto findpcb;
                                }
                                goto relocked;
                        } else
                                ti_locked = TI_WLOCKED;
                }
                INP_INFO_WLOCK_ASSERT(&V_tcbinfo);
        }

#ifdef MAC
        INP_WLOCK_ASSERT(inp);
        if (mac_inpcb_check_deliver(inp, m))
                goto dropunlock;
#endif
        so = inp->inp_socket;
        KASSERT(so != NULL, ("%s: so == NULL", __func__));
#ifdef TCPDEBUG
        if (so->so_options & SO_DEBUG) {
                ostate = tp->t_state;
#ifdef INET6
                if (isipv6) {
                        bcopy((char *)ip6, (char *)tcp_saveipgen, sizeof(*ip6));
                } else
#endif
                        bcopy((char *)ip, (char *)tcp_saveipgen, sizeof(*ip));
                tcp_savetcp = *th;
        }
#endif /* TCPDEBUG */
        /*
         * When the socket is accepting connections (the INPCB is in LISTEN
         * state) we look into the SYN cache if this is a new connection
         * attempt or the completion of a previous one.  Because listen
         * sockets are never in TCPS_ESTABLISHED, the V_tcbinfo lock will be
         * held in this case.
         */
        if (so->so_options & SO_ACCEPTCONN) {
                struct in_conninfo inc;

                KASSERT(tp->t_state == TCPS_LISTEN, ("%s: so accepting but "
                    "tp not listening", __func__));
                INP_INFO_WLOCK_ASSERT(&V_tcbinfo);

                bzero(&inc, sizeof(inc));
#ifdef INET6
                if (isipv6) {
                        inc.inc_flags |= INC_ISIPV6;
                        inc.inc6_faddr = ip6->ip6_src;
                        inc.inc6_laddr = ip6->ip6_dst;
                } else
#endif
                {
                        inc.inc_faddr = ip->ip_src;
                        inc.inc_laddr = ip->ip_dst;
                }
                inc.inc_fport = th->th_sport;
                inc.inc_lport = th->th_dport;
                inc.inc_fibnum = so->so_fibnum;

                /*
                 * Check for an existing connection attempt in syncache if
                 * the flag is only ACK.  A successful lookup creates a new
                 * socket appended to the listen queue in SYN_RECEIVED state.
                 */
                if ((thflags & (TH_RST|TH_ACK|TH_SYN)) == TH_ACK) {
                        /*
                         * Parse the TCP options here because
                         * syncookies need access to the reflected
                         * timestamp.
                         */
                        tcp_dooptions(&to, optp, optlen, 0);
                        /*
                         * NB: syncache_expand() doesn't unlock
                         * inp and tcpinfo locks.
                         */
                        if (!syncache_expand(&inc, &to, th, &so, m)) {
                                /*
                                 * No syncache entry or ACK was not
                                 * for our SYN/ACK.  Send a RST.
                                 * NB: syncache did its own logging
                                 * of the failure cause.
                                 */
                                rstreason = BANDLIM_RST_OPENPORT;
                                goto dropwithreset;
                        }
                        if (so == NULL) {
                                /*
                                 * We completed the 3-way handshake
                                 * but could not allocate a socket
                                 * either due to memory shortage,
                                 * listen queue length limits or
                                 * global socket limits.  Send RST
                                 * or wait and have the remote end
                                 * retransmit the ACK for another
                                 * try.
                                 */
                                if ((s = tcp_log_addrs(&inc, th, NULL, NULL)))
                                        log(LOG_DEBUG, "%s; %s: Listen socket: "
                                            "Socket allocation failed due to "
                                            "limits or memory shortage, %s\n",
                                            s, __func__,
                                            V_tcp_sc_rst_sock_fail ?
                                            "sending RST" : "try again");
                                if (V_tcp_sc_rst_sock_fail) {
                                        rstreason = BANDLIM_UNLIMITED;
                                        goto dropwithreset;
                                } else
                                        goto dropunlock;
                        }
                        /*
                         * Socket is created in state SYN_RECEIVED.
                         * Unlock the listen socket, lock the newly
                         * created socket and update the tp variable.
                         */
                        INP_WUNLOCK(inp);       /* listen socket */
                        inp = sotoinpcb(so);
                        INP_WLOCK(inp);         /* new connection */
                        tp = intotcpcb(inp);
                        KASSERT(tp->t_state == TCPS_SYN_RECEIVED,
                            ("%s: ", __func__));
#ifdef TCP_SIGNATURE
                        if (sig_checked == 0)  {
                                tcp_dooptions(&to, optp, optlen,
                                    (thflags & TH_SYN) ? TO_SYN : 0);
                                if (!tcp_signature_verify_input(m, off0, tlen,
                                    optlen, &to, th, tp->t_flags)) {

                                        /*
                                         * In SYN_SENT state if it receives an
                                         * RST, it is allowed for further
                                         * processing.
                                         */
                                        if ((thflags & TH_RST) == 0 ||
                                            (tp->t_state == TCPS_SYN_SENT) == 0)
                                                goto dropunlock;
                                }
                                sig_checked = 1;
                        }
#endif

                        /*
                         * Process the segment and the data it
                         * contains.  tcp_do_segment() consumes
                         * the mbuf chain and unlocks the inpcb.
                         */
                        tcp_do_segment(m, th, so, tp, drop_hdrlen, tlen,
                            iptos, ti_locked);
                        INP_INFO_UNLOCK_ASSERT(&V_tcbinfo);
                        return;
                }
                /*
                 * Segment flag validation for new connection attempts:
                 *
                 * Our (SYN|ACK) response was rejected.
                 * Check with syncache and remove entry to prevent
                 * retransmits.
                 *
                 * NB: syncache_chkrst does its own logging of failure
                 * causes.
                 */
                if (thflags & TH_RST) {
                        syncache_chkrst(&inc, th);
                        goto dropunlock;
                }
                /*
                 * We can't do anything without SYN.
                 */
                if ((thflags & TH_SYN) == 0) {
                        if ((s = tcp_log_addrs(&inc, th, NULL, NULL)))
                                log(LOG_DEBUG, "%s; %s: Listen socket: "
                                    "SYN is missing, segment ignored\n",
                                    s, __func__);
                        TCPSTAT_INC(tcps_badsyn);
                        goto dropunlock;
                }
                /*
                 * (SYN|ACK) is bogus on a listen socket.
                 */
                if (thflags & TH_ACK) {
                        if ((s = tcp_log_addrs(&inc, th, NULL, NULL)))
                                log(LOG_DEBUG, "%s; %s: Listen socket: "
                                    "SYN|ACK invalid, segment rejected\n",
                                    s, __func__);
                        syncache_badack(&inc);  /* XXX: Not needed! */
                        TCPSTAT_INC(tcps_badsyn);
                        rstreason = BANDLIM_RST_OPENPORT;
                        goto dropwithreset;
                }
                /*
                 * If the drop_synfin option is enabled, drop all
                 * segments with both the SYN and FIN bits set.
                 * This prevents e.g. nmap from identifying the
                 * TCP/IP stack.
                 * XXX: Poor reasoning.  nmap has other methods
                 * and is constantly refining its stack detection
                 * strategies.
                 * XXX: This is a violation of the TCP specification
                 * and was used by RFC1644.
                 */
                if ((thflags & TH_FIN) && V_drop_synfin) {
                        if ((s = tcp_log_addrs(&inc, th, NULL, NULL)))
                                log(LOG_DEBUG, "%s; %s: Listen socket: "
                                    "SYN|FIN segment ignored (based on "
                                    "sysctl setting)\n", s, __func__);
                        TCPSTAT_INC(tcps_badsyn);
                        goto dropunlock;
                }
                /*
                 * Segment's flags are (SYN) or (SYN|FIN).
                 *
                 * TH_PUSH, TH_URG, TH_ECE, TH_CWR are ignored
                 * as they do not affect the state of the TCP FSM.
                 * The data pointed to by TH_URG and th_urp is ignored.
                 */
                KASSERT((thflags & (TH_RST|TH_ACK)) == 0,
                    ("%s: Listen socket: TH_RST or TH_ACK set", __func__));
                KASSERT(thflags & (TH_SYN),
                    ("%s: Listen socket: TH_SYN not set", __func__));
#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 && !V_ip6_use_deprecated) {
                        struct in6_ifaddr *ia6;

                        ia6 = ip6_getdstifaddr(m);
                        if (ia6 != NULL &&
                            (ia6->ia6_flags & IN6_IFF_DEPRECATED)) {
                                ifa_free(&ia6->ia_ifa);
                                if ((s = tcp_log_addrs(&inc, th, NULL, NULL)))
                                    log(LOG_DEBUG, "%s; %s: Listen socket: "
                                        "Connection attempt to deprecated "
                                        "IPv6 address rejected\n",
                                        s, __func__);
                                rstreason = BANDLIM_RST_OPENPORT;
                                goto dropwithreset;
                        }
                        ifa_free(&ia6->ia_ifa);
                }
#endif /* INET6 */
                /*
                 * Basic sanity checks on incoming SYN requests:
                 *   Don't respond if the destination is a link layer
                 *      broadcast according to RFC1122 4.2.3.10, p. 104.
                 *   If it is from this socket it must be forged.
                 *   Don't respond if the source or destination is a
                 *      global or subnet broad- or multicast address.
                 *   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)) {
                        if ((s = tcp_log_addrs(&inc, th, NULL, NULL)))
                            log(LOG_DEBUG, "%s; %s: Listen socket: "
                                "Connection attempt from broad- or multicast "
                                "link layer address ignored\n", s, __func__);
                        goto dropunlock;
                }
#ifdef INET6
                if (isipv6) {
                        if (th->th_dport == th->th_sport &&
                            IN6_ARE_ADDR_EQUAL(&ip6->ip6_dst, &ip6->ip6_src)) {
                                if ((s = tcp_log_addrs(&inc, th, NULL, NULL)))
                                    log(LOG_DEBUG, "%s; %s: Listen socket: "
                                        "Connection attempt to/from self "
                                        "ignored\n", s, __func__);
                                goto dropunlock;
                        }
                        if (IN6_IS_ADDR_MULTICAST(&ip6->ip6_dst) ||
                            IN6_IS_ADDR_MULTICAST(&ip6->ip6_src)) {
                                if ((s = tcp_log_addrs(&inc, th, NULL, NULL)))
                                    log(LOG_DEBUG, "%s; %s: Listen socket: "
                                        "Connection attempt from/to multicast "
                                        "address ignored\n", s, __func__);
                                goto dropunlock;
                        }
                }
#endif
#if defined(INET) && defined(INET6)
                else
#endif
#ifdef INET
                {
                        if (th->th_dport == th->th_sport &&
                            ip->ip_dst.s_addr == ip->ip_src.s_addr) {
                                if ((s = tcp_log_addrs(&inc, th, NULL, NULL)))
                                    log(LOG_DEBUG, "%s; %s: Listen socket: "
                                        "Connection attempt from/to self "
                                        "ignored\n", s, __func__);
                                goto dropunlock;
                        }
                        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)) {
                                if ((s = tcp_log_addrs(&inc, th, NULL, NULL)))
                                    log(LOG_DEBUG, "%s; %s: Listen socket: "
                                        "Connection attempt from/to broad- "
                                        "or multicast address ignored\n",
                                        s, __func__);
                                goto dropunlock;
                        }
                }
#endif
                /*
                 * SYN appears to be valid.  Create compressed TCP state
                 * for syncache.
                 */
#ifdef TCPDEBUG
                if (so->so_options & SO_DEBUG)
                        tcp_trace(TA_INPUT, ostate, tp,
                            (void *)tcp_saveipgen, &tcp_savetcp, 0);
#endif
                tcp_dooptions(&to, optp, optlen, TO_SYN);
                syncache_add(&inc, &to, th, inp, &so, m);
                /*
                 * Entry added to syncache and mbuf consumed.
                 * Everything already unlocked by syncache_add().
                 */
                INP_INFO_UNLOCK_ASSERT(&V_tcbinfo);
                return;
        }

#ifdef TCP_SIGNATURE
        if (sig_checked == 0)  {
                tcp_dooptions(&to, optp, optlen,
                    (thflags & TH_SYN) ? TO_SYN : 0);
                if (!tcp_signature_verify_input(m, off0, tlen, optlen, &to,
                    th, tp->t_flags)) {

                        /*
                         * In SYN_SENT state if it receives an RST, it is
                         * allowed for further processing.
                         */
                        if ((thflags & TH_RST) == 0 ||
                            (tp->t_state == TCPS_SYN_SENT) == 0)
                                goto dropunlock;
                }
                sig_checked = 1;
        }
#endif

        /*
         * Segment belongs to a connection in SYN_SENT, ESTABLISHED or later
         * state.  tcp_do_segment() always consumes the mbuf chain, unlocks
         * the inpcb, and unlocks pcbinfo.
         */
        tcp_do_segment(m, th, so, tp, drop_hdrlen, tlen, iptos, ti_locked);
        INP_INFO_UNLOCK_ASSERT(&V_tcbinfo);
        return;

dropwithreset:
        if (ti_locked == TI_WLOCKED) {
                INP_INFO_WUNLOCK(&V_tcbinfo);
                ti_locked = TI_UNLOCKED;
        }
#ifdef INVARIANTS
        else {
                KASSERT(ti_locked == TI_UNLOCKED, ("%s: dropwithreset "
                    "ti_locked: %d", __func__, ti_locked));
                INP_INFO_UNLOCK_ASSERT(&V_tcbinfo);
        }
#endif

        if (inp != NULL) {
                tcp_dropwithreset(m, th, tp, tlen, rstreason);
                INP_WUNLOCK(inp);
        } else
                tcp_dropwithreset(m, th, NULL, tlen, rstreason);
        m = NULL;       /* mbuf chain got consumed. */
        goto drop;

dropunlock:
        if (ti_locked == TI_WLOCKED) {
                INP_INFO_WUNLOCK(&V_tcbinfo);
                ti_locked = TI_UNLOCKED;
        }
#ifdef INVARIANTS
        else {
                KASSERT(ti_locked == TI_UNLOCKED, ("%s: dropunlock "
                    "ti_locked: %d", __func__, ti_locked));
                INP_INFO_UNLOCK_ASSERT(&V_tcbinfo);
        }
#endif

        if (inp != NULL)
                INP_WUNLOCK(inp);

drop:
        INP_INFO_UNLOCK_ASSERT(&V_tcbinfo);
        if (s != NULL)
                free(s, M_TCPLOG);
        if (m != NULL)
                m_freem(m);
}

static void
tcp_do_segment(struct mbuf *m, struct tcphdr *th, struct socket *so,
    struct tcpcb *tp, int drop_hdrlen, int tlen, uint8_t iptos,
    int ti_locked)
{
        int thflags, acked, ourfinisacked, needoutput = 0;
        int rstreason, todrop, win;
        u_long tiwin;
        struct tcpopt to;

#ifdef TCPDEBUG
        /*
         * The size of tcp_saveipgen must be the size of the max ip header,
         * now IPv6.
         */
        u_char tcp_saveipgen[IP6_HDR_LEN];
        struct tcphdr tcp_savetcp;
        short ostate = 0;
#endif
        thflags = th->th_flags;
        tp->sackhint.last_sack_ack = 0;

        /*
         * If this is either a state-changing packet or current state isn't
         * established, we require a write lock on tcbinfo.  Otherwise, we
         * allow either a read lock or a write lock, as we may have acquired
         * a write lock due to a race.
         *
         * Require a global write lock for SYN/FIN/RST segments or
         * non-established connections; otherwise accept either a read or
         * write lock, as we may have conservatively acquired a write lock in
         * certain cases in tcp_input() (is this still true?).  Currently we
         * will never enter with no lock, so we try to drop it quickly in the
         * common pure ack/pure data cases.
         */
        if ((thflags & (TH_SYN | TH_FIN | TH_RST)) != 0 ||
            tp->t_state != TCPS_ESTABLISHED) {
                KASSERT(ti_locked == TI_WLOCKED, ("%s ti_locked %d for "
                    "SYN/FIN/RST/!EST", __func__, ti_locked));
                INP_INFO_WLOCK_ASSERT(&V_tcbinfo);
        } else {
#ifdef INVARIANTS
                if (ti_locked == TI_WLOCKED)
                        INP_INFO_WLOCK_ASSERT(&V_tcbinfo);
                else {
                        KASSERT(ti_locked == TI_UNLOCKED, ("%s: EST "
                            "ti_locked: %d", __func__, ti_locked));
                        INP_INFO_UNLOCK_ASSERT(&V_tcbinfo);
                }
#endif
        }
        INP_WLOCK_ASSERT(tp->t_inpcb);
        KASSERT(tp->t_state > TCPS_LISTEN, ("%s: TCPS_LISTEN",
            __func__));
        KASSERT(tp->t_state != TCPS_TIME_WAIT, ("%s: TCPS_TIME_WAIT",
            __func__));

        /*
         * Segment received on connection.
         * Reset idle time and keep-alive timer.
         * XXX: This should be done after segment
         * validation to ignore broken/spoofed segs.
         */
        tp->t_rcvtime = ticks;
        if (TCPS_HAVEESTABLISHED(tp->t_state))
                tcp_timer_activate(tp, TT_KEEP, tcp_keepidle);

        /*
         * Unscale the window into a 32-bit value.
         * For the SYN_SENT state the scale is zero.
         */
        tiwin = th->th_win << tp->snd_scale;

        /*
         * TCP ECN processing.
         */
        if (tp->t_flags & TF_ECN_PERMIT) {
                if (thflags & TH_CWR)
                        tp->t_flags &= ~TF_ECN_SND_ECE;
                switch (iptos & IPTOS_ECN_MASK) {
                case IPTOS_ECN_CE:
                        tp->t_flags |= TF_ECN_SND_ECE;
                        TCPSTAT_INC(tcps_ecn_ce);
                        break;
                case IPTOS_ECN_ECT0:
                        TCPSTAT_INC(tcps_ecn_ect0);
                        break;
                case IPTOS_ECN_ECT1:
                        TCPSTAT_INC(tcps_ecn_ect1);
                        break;
                }
                /* Congestion experienced. */
                if (thflags & TH_ECE) {
                        cc_cong_signal(tp, th, CC_ECN);
                }
        }

        /*
         * Parse options on any incoming segment.
         */
        tcp_dooptions(&to, (u_char *)(th + 1),
            (th->th_off << 2) - sizeof(struct tcphdr),
            (thflags & TH_SYN) ? TO_SYN : 0);

        /*
         * If echoed timestamp is later than the current time,
         * fall back to non RFC1323 RTT calculation.  Normalize
         * timestamp if syncookies were used when this connection
         * was established.
         */
        if ((to.to_flags & TOF_TS) && (to.to_tsecr != 0)) {
                to.to_tsecr -= tp->ts_offset;
                if (TSTMP_GT(to.to_tsecr, ticks))
                        to.to_tsecr = 0;
        }

        /*
         * Process options only when we get SYN/ACK back. The SYN case
         * for incoming connections is handled in tcp_syncache.
         * According to RFC1323 the window field in a SYN (i.e., a <SYN>
         * or <SYN,ACK>) segment itself is never scaled.
         * XXX this is traditional behavior, may need to be cleaned up.
         */
        if (tp->t_state == TCPS_SYN_SENT && (thflags & TH_SYN)) {
                if ((to.to_flags & TOF_SCALE) &&
                    (tp->t_flags & TF_REQ_SCALE)) {
                        tp->t_flags |= TF_RCVD_SCALE;
                        tp->snd_scale = to.to_wscale;
                }
                /*
                 * Initial send window.  It will be updated with
                 * the next incoming segment to the scaled value.
                 */
                tp->snd_wnd = th->th_win;
                if (to.to_flags & TOF_TS) {
                        tp->t_flags |= TF_RCVD_TSTMP;
                        tp->ts_recent = to.to_tsval;
                        tp->ts_recent_age = ticks;
                }
                if (to.to_flags & TOF_MSS)
                        tcp_mss(tp, to.to_mss);
                if ((tp->t_flags & TF_SACK_PERMIT) &&
                    (to.to_flags & TOF_SACKPERM) == 0)
                        tp->t_flags &= ~TF_SACK_PERMIT;
        }

        /*
         * Header prediction: check for the two common cases
         * of a uni-directional data xfer.  If the packet has
         * no control flags, is in-sequence, the window didn't
         * change and we're not retransmitting, it's a
         * candidate.  If the length is zero and the ack moved
         * forward, we're the sender side of the xfer.  Just
         * free the data acked & wake any higher level process
         * that was blocked waiting for space.  If the length
         * is non-zero and the ack didn't move, we're the
         * receiver side.  If we're getting packets in-order
         * (the reassembly queue is empty), add the data to
         * the socket buffer and note that we need a delayed ack.
         * Make sure that the hidden state-flags are also off.
         * Since we check for TCPS_ESTABLISHED first, it can only
         * be TH_NEEDSYN.
         */
        if (tp->t_state == TCPS_ESTABLISHED &&
            th->th_seq == tp->rcv_nxt &&
            (thflags & (TH_SYN|TH_FIN|TH_RST|TH_URG|TH_ACK)) == TH_ACK &&
            tp->snd_nxt == tp->snd_max &&
            tiwin && tiwin == tp->snd_wnd && 
            ((tp->t_flags & (TF_NEEDSYN|TF_NEEDFIN)) == 0) &&
            LIST_EMPTY(&tp->t_segq) &&
            ((to.to_flags & TOF_TS) == 0 ||
             TSTMP_GEQ(to.to_tsval, tp->ts_recent)) ) {

                /*
                 * If last ACK falls within this segment's sequence numbers,
                 * record the timestamp.
                 * NOTE that the test is modified according to the latest
                 * proposal of the tcplw@cray.com list (Braden 1993/04/26).
                 */
                if ((to.to_flags & TOF_TS) != 0 &&
                    SEQ_LEQ(th->th_seq, tp->last_ack_sent)) {
                        tp->ts_recent_age = ticks;
                        tp->ts_recent = to.to_tsval;
                }

                if (tlen == 0) {
                        if (SEQ_GT(th->th_ack, tp->snd_una) &&
                            SEQ_LEQ(th->th_ack, tp->snd_max) &&
                            !IN_RECOVERY(tp->t_flags) &&
                            (to.to_flags & TOF_SACK) == 0 &&
                            TAILQ_EMPTY(&tp->snd_holes)) {
                                /*
                                 * This is a pure ack for outstanding data.
                                 */
                                if (ti_locked == TI_WLOCKED)
                                        INP_INFO_WUNLOCK(&V_tcbinfo);
                                ti_locked = TI_UNLOCKED;

                                TCPSTAT_INC(tcps_predack);

                                /*
                                 * "bad retransmit" recovery.
                                 */
                                if (tp->t_rxtshift == 1 &&
                                    tp->t_flags & TF_PREVVALID &&
                                    (int)(ticks - tp->t_badrxtwin) < 0) {
                                        cc_cong_signal(tp, th, CC_RTO_ERR);
                                }

                                /*
                                 * Recalculate the transmit timer / rtt.
                                 *
                                 * Some boxes send broken timestamp replies
                                 * during the SYN+ACK phase, ignore
                                 * timestamps of 0 or we could calculate a
                                 * huge RTT and blow up the retransmit timer.
                                 */
                                if ((to.to_flags & TOF_TS) != 0 &&
                                    to.to_tsecr) {
                                        if (!tp->t_rttlow ||
                                            tp->t_rttlow > ticks - to.to_tsecr)
                                                tp->t_rttlow = ticks - to.to_tsecr;
                                        tcp_xmit_timer(tp,
                                            ticks - to.to_tsecr + 1);
                                } else if (tp->t_rtttime &&
                                    SEQ_GT(th->th_ack, tp->t_rtseq)) {
                                        if (!tp->t_rttlow ||
                                            tp->t_rttlow > ticks - tp->t_rtttime)
                                                tp->t_rttlow = ticks - tp->t_rtttime;
                                        tcp_xmit_timer(tp,
                                                        ticks - tp->t_rtttime);
                                }
                                acked = BYTES_THIS_ACK(tp, th);

                                /* Run HHOOK_TCP_ESTABLISHED_IN helper hooks. */
                                hhook_run_tcp_est_in(tp, th, &to);

                                TCPSTAT_INC(tcps_rcvackpack);
                                TCPSTAT_ADD(tcps_rcvackbyte, acked);
                                sbdrop(&so->so_snd, acked);
                                if (SEQ_GT(tp->snd_una, tp->snd_recover) &&
                                    SEQ_LEQ(th->th_ack, tp->snd_recover))
                                        tp->snd_recover = th->th_ack - 1;
                                
                                /*
                                 * Let the congestion control algorithm update
                                 * congestion control related information. This
                                 * typically means increasing the congestion
                                 * window.
                                 */
                                cc_ack_received(tp, th, CC_ACK);

                                tp->snd_una = th->th_ack;
                                /*
                                 * Pull snd_wl2 up to prevent seq wrap relative
                                 * to th_ack.
                                 */
                                tp->snd_wl2 = th->th_ack;
                                tp->t_dupacks = 0;
                                m_freem(m);
                                ND6_HINT(tp); /* Some progress has been made. */

                                /*
                                 * If all outstanding data are acked, stop
                                 * retransmit timer, otherwise restart timer
                                 * using current (possibly backed-off) value.
                                 * If process is waiting for space,
                                 * wakeup/selwakeup/signal.  If data
                                 * are ready to send, let tcp_output
                                 * decide between more output or persist.
                                 */
#ifdef TCPDEBUG
                                if (so->so_options & SO_DEBUG)
                                        tcp_trace(TA_INPUT, ostate, tp,
                                            (void *)tcp_saveipgen,
                                            &tcp_savetcp, 0);
#endif
                                if (tp->snd_una == tp->snd_max)
                                        tcp_timer_activate(tp, TT_REXMT, 0);
                                else if (!tcp_timer_active(tp, TT_PERSIST))
                                        tcp_timer_activate(tp, TT_REXMT,
                                                      tp->t_rxtcur);
                                sowwakeup(so);
                                if (so->so_snd.sb_cc)
                                        (void) tcp_output(tp);
                                goto check_delack;
                        }
                } else if (th->th_ack == tp->snd_una &&
                    tlen <= sbspace(&so->so_rcv)) {
                        int newsize = 0;        /* automatic sockbuf scaling */

                        /*
                         * This is a pure, in-sequence data packet with
                         * nothing on the reassembly queue and we have enough
                         * buffer space to take it.
                         */
                        if (ti_locked == TI_WLOCKED)
                                INP_INFO_WUNLOCK(&V_tcbinfo);
                        ti_locked = TI_UNLOCKED;

                        /* Clean receiver SACK report if present */
                        if ((tp->t_flags & TF_SACK_PERMIT) && tp->rcv_numsacks)
                                tcp_clean_sackreport(tp);
                        TCPSTAT_INC(tcps_preddat);
                        tp->rcv_nxt += tlen;
                        /*
                         * Pull snd_wl1 up to prevent seq wrap relative to
                         * th_seq.
                         */
                        tp->snd_wl1 = th->th_seq;
                        /*
                         * Pull rcv_up up to prevent seq wrap relative to
                         * rcv_nxt.
                         */
                        tp->rcv_up = tp->rcv_nxt;
                        TCPSTAT_INC(tcps_rcvpack);
                        TCPSTAT_ADD(tcps_rcvbyte, tlen);
                        ND6_HINT(tp);   /* Some progress has been made */
#ifdef TCPDEBUG
                        if (so->so_options & SO_DEBUG)
                                tcp_trace(TA_INPUT, ostate, tp,
                                    (void *)tcp_saveipgen, &tcp_savetcp, 0);
#endif
                /*
                 * Automatic sizing of receive socket buffer.  Often the send
                 * buffer size is not optimally adjusted to the actual network
                 * conditions at hand (delay bandwidth product).  Setting the
                 * buffer size too small limits throughput on links with high
                 * bandwidth and high delay (eg. trans-continental/oceanic links).
                 *
                 * On the receive side the socket buffer memory is only rarely
                 * used to any significant extent.  This allows us to be much
                 * more aggressive in scaling the receive socket buffer.  For
                 * the case that the buffer space is actually used to a large
                 * extent and we run out of kernel memory we can simply drop
                 * the new segments; TCP on the sender will just retransmit it
                 * later.  Setting the buffer size too big may only consume too
                 * much kernel memory if the application doesn't read() from
                 * the socket or packet loss or reordering makes use of the
                 * reassembly queue.
                 *
                 * The criteria to step up the receive buffer one notch are:
                 *  1. the number of bytes received during the time it takes
                 *     one timestamp to be reflected back to us (the RTT);
                 *  2. received bytes per RTT is within seven eighth of the
                 *     current socket buffer size;
                 *  3. receive buffer size has not hit maximal automatic size;
                 *
                 * This algorithm does one step per RTT at most and only if
                 * we receive a bulk stream w/o packet losses or reorderings.
                 * Shrinking the buffer during idle times is not necessary as
                 * it doesn't consume any memory when idle.
                 *
                 * TODO: Only step up if the application is actually serving
                 * the buffer to better manage the socket buffer resources.
                 */
                        if (V_tcp_do_autorcvbuf &&
                            to.to_tsecr &&
                            (so->so_rcv.sb_flags & SB_AUTOSIZE)) {
                                if (TSTMP_GT(to.to_tsecr, tp->rfbuf_ts) &&
                                    to.to_tsecr - tp->rfbuf_ts < hz) {
                                        if (tp->rfbuf_cnt >
                                            (so->so_rcv.sb_hiwat / 8 * 7) &&
                                            so->so_rcv.sb_hiwat <
                                            V_tcp_autorcvbuf_max) {
                                                newsize =
                                                    min(so->so_rcv.sb_hiwat +
                                                    V_tcp_autorcvbuf_inc,
                                                    V_tcp_autorcvbuf_max);
                                        }
                                        /* Start over with next RTT. */
                                        tp->rfbuf_ts = 0;
                                        tp->rfbuf_cnt = 0;
                                } else
                                        tp->rfbuf_cnt += tlen;  /* add up */
                        }

                        /* Add data to socket buffer. */
                        SOCKBUF_LOCK(&so->so_rcv);
                        if (so->so_rcv.sb_state & SBS_CANTRCVMORE) {
                                m_freem(m);
                        } else {
                                /*
                                 * Set new socket buffer size.
                                 * Give up when limit is reached.
                                 */
                                if (newsize)
                                        if (!sbreserve_locked(&so->so_rcv,
                                            newsize, so, NULL))
                                                so->so_rcv.sb_flags &= ~SB_AUTOSIZE;
                                m_adj(m, drop_hdrlen);  /* delayed header drop */
                                sbappendstream_locked(&so->so_rcv, m);
                        }
                        /* NB: sorwakeup_locked() does an implicit unlock. */
                        sorwakeup_locked(so);
                        if (DELAY_ACK(tp)) {
                                tp->t_flags |= TF_DELACK;
                        } else {
                                tp->t_flags |= TF_ACKNOW;
                                tcp_output(tp);
                        }
                        goto check_delack;
                }
        }

        /*
         * Calculate amount of space in receive window,
         * and then do TCP input processing.
         * Receive window is amount of space in rcv queue,
         * but not less than advertised window.
         */
        win = sbspace(&so->so_rcv);
        if (win < 0)
                win = 0;
        KASSERT(SEQ_GEQ(tp->rcv_adv, tp->rcv_nxt),
            ("tcp_input negative window: tp %p rcv_nxt %u rcv_adv %u", tp,
            tp->rcv_nxt, tp->rcv_adv));
        tp->rcv_wnd = imax(win, (int)(tp->rcv_adv - tp->rcv_nxt));

        /* Reset receive buffer auto scaling when not in bulk receive mode. */
        tp->rfbuf_ts = 0;
        tp->rfbuf_cnt = 0;

        switch (tp->t_state) {

        /*
         * If the state is SYN_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 seg contains an ECE and ECN support is enabled, the stream
         *          is ECN capable.
         *      if SYN has been acked change to ESTABLISHED else SYN_RCVD state
         *      arrange for segment to be acked (eventually)
         *      continue processing rest of data/controls, beginning with URG
         */
        case TCPS_SYN_SENT:
                if ((thflags & TH_ACK) &&
                    (SEQ_LEQ(th->th_ack, tp->iss) ||
                     SEQ_GT(th->th_ack, tp->snd_max))) {
                        rstreason = BANDLIM_UNLIMITED;
                        goto dropwithreset;
                }
                if ((thflags & (TH_ACK|TH_RST)) == (TH_ACK|TH_RST))
                        tp = tcp_drop(tp, ECONNREFUSED);
                if (thflags & TH_RST)
                        goto drop;
                if (!(thflags & TH_SYN))
                        goto drop;

                tp->irs = th->th_seq;
                tcp_rcvseqinit(tp);
                if (thflags & TH_ACK) {
                        TCPSTAT_INC(tcps_connects);
                        soisconnected(so);
#ifdef MAC
                        mac_socketpeer_set_from_mbuf(m, so);
#endif
                        /* Do window scaling on this connection? */
                        if ((tp->t_flags & (TF_RCVD_SCALE|TF_REQ_SCALE)) ==
                                (TF_RCVD_SCALE|TF_REQ_SCALE)) {
                                tp->rcv_scale = tp->request_r_scale;
                        }
                        tp->rcv_adv += imin(tp->rcv_wnd,
                            TCP_MAXWIN << tp->rcv_scale);
                        tp->snd_una++;          /* SYN is acked */
                        /*
                         * If there's data, delay ACK; if there's also a FIN
                         * ACKNOW will be turned on later.
                         */
                        if (DELAY_ACK(tp) && tlen != 0)
                                tcp_timer_activate(tp, TT_DELACK,
                                    tcp_delacktime);
                        else
                                tp->t_flags |= TF_ACKNOW;

                        if ((thflags & TH_ECE) && V_tcp_do_ecn) {
                                tp->t_flags |= TF_ECN_PERMIT;
                                TCPSTAT_INC(tcps_ecn_shs);
                        }
                        
                        /*
                         * 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;
                                cc_conn_init(tp);
                                tcp_timer_activate(tp, TT_KEEP, tcp_keepidle);
                        }
                } else {
                        /*
                         * Received initial SYN in SYN-SENT[*] state =>
                         * simultaneous open.  If segment contains CC option
                         * and there is a cached CC, apply TAO test.
                         * If it succeeds, connection is * half-synchronized.
                         * Otherwise, do 3-way handshake:
                         *        SYN-SENT -> SYN-RECEIVED
                         *        SYN-SENT* -> SYN-RECEIVED*
                         * If there was no CC option, clear cached CC value.
                         */
                        tp->t_flags |= (TF_ACKNOW | TF_NEEDSYN);
                        tcp_timer_activate(tp, TT_REXMT, 0);
                        tp->t_state = TCPS_SYN_RECEIVED;
                }

                KASSERT(ti_locked == TI_WLOCKED, ("%s: trimthenstep6: "
                    "ti_locked %d", __func__, ti_locked));
                INP_INFO_WLOCK_ASSERT(&V_tcbinfo);
                INP_WLOCK_ASSERT(tp->t_inpcb);

                /*
                 * 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_INC(tcps_rcvpackafterwin);
                        TCPSTAT_ADD(tcps_rcvbyteafterwin, todrop);
                }
                tp->snd_wl1 = th->th_seq - 1;
                tp->rcv_up = th->th_seq;
                /*
                 * Client side of transaction: already sent SYN and data.
                 * If the remote host used T/TCP to validate the SYN,
                 * our data will be ACK'd; if so, enter normal data segment
                 * processing in the middle of step 5, ack processing.
                 * Otherwise, goto step 6.
                 */
                if (thflags & TH_ACK)
                        goto process_ACK;

                goto step6;

        /*
         * If the state is LAST_ACK or CLOSING or TIME_WAIT:
         *      do normal processing.
         *
         * NB: Leftover from RFC1644 T/TCP.  Cases to be reused later.
         */
        case TCPS_LAST_ACK:
        case TCPS_CLOSING:
                break;  /* continue normal processing */
        }

        /*
         * States other than LISTEN or SYN_SENT.
         * First check the RST flag and sequence number since reset segments
         * are exempt from the timestamp and connection count tests.  This
         * fixes a bug introduced by the Stevens, vol. 2, p. 960 bugfix
         * below which allowed reset segments in half the sequence space
         * to fall though and be processed (which gives forged reset
         * segments with a random sequence number a 50 percent chance of
         * killing a connection).
         * Then check timestamp, if present.
         * Then check the connection count, if present.
         * Then check that at least some bytes of segment are within
         * receive window.  If segment begins before rcv_nxt,
         * drop leading data (and SYN); if nothing left, just ack.
         *
         *
         * If the RST bit is set, check the sequence number to see
         * if this is a valid reset segment.
         * RFC 793 page 37:
         *   In all states except SYN-SENT, all reset (RST) segments
         *   are validated by checking their SEQ-fields.  A reset is
         *   valid if its sequence number is in the window.
         * Note: this does not take into account delayed ACKs, so
         *   we should test against last_ack_sent instead of rcv_nxt.
         *   The sequence number in the reset segment is normally an
         *   echo of our outgoing acknowlegement numbers, but some hosts
         *   send a reset with the sequence number at the rightmost edge
         *   of our receive window, and we have to handle this case.
         * Note 2: Paul Watson's paper "Slipping in the Window" has shown
         *   that brute force RST attacks are possible.  To combat this,
         *   we use a much stricter check while in the ESTABLISHED state,
         *   only accepting RSTs where the sequence number is equal to
         *   last_ack_sent.  In all other states (the states in which a
         *   RST is more likely), the more permissive check is used.
         * If we have multiple segments in flight, the initial reset
         * segment sequence numbers will be to the left of last_ack_sent,
         * but they will eventually catch up.
         * In any case, it never made sense to trim reset segments to
         * fit the receive window since RFC 1122 says:
         *   4.2.2.12  RST Segment: RFC-793 Section 3.4
         *
         *    A TCP SHOULD allow a received RST segment to include data.
         *
         *    DISCUSSION
         *         It has been suggested that a RST segment could contain
         *         ASCII text that encoded and explained the cause of the
         *         RST.  No standard has yet been established for such
         *         data.
         *
         * If the reset segment passes the sequence number test examine
         * the state:
         *    SYN_RECEIVED STATE:
         *      If passive open, return to LISTEN state.
         *      If active open, inform user that connection was refused.
         *    ESTABLISHED, FIN_WAIT_1, FIN_WAIT_2, CLOSE_WAIT STATES:
         *      Inform user that connection was reset, and close tcb.
         *    CLOSING, LAST_ACK STATES:
         *      Close the tcb.
         *    TIME_WAIT STATE:
         *      Drop the segment - see Stevens, vol. 2, p. 964 and
         *      RFC 1337.
         */
        if (thflags & TH_RST) {
                if (SEQ_GEQ(th->th_seq, tp->last_ack_sent - 1) &&
                    SEQ_LEQ(th->th_seq, tp->last_ack_sent + tp->rcv_wnd)) {
                        switch (tp->t_state) {

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

                        case TCPS_ESTABLISHED:
                                if (V_tcp_insecure_rst == 0 &&
                                    !(SEQ_GEQ(th->th_seq, tp->rcv_nxt - 1) &&
                                    SEQ_LEQ(th->th_seq, tp->rcv_nxt + 1)) &&
                                    !(SEQ_GEQ(th->th_seq, tp->last_ack_sent - 1) &&
                                    SEQ_LEQ(th->th_seq, tp->last_ack_sent + 1))) {
                                        TCPSTAT_INC(tcps_badrst);
                                        goto drop;
                                }
                                /* FALLTHROUGH */
                        case TCPS_FIN_WAIT_1:
                        case TCPS_FIN_WAIT_2:
                        case TCPS_CLOSE_WAIT:
                                so->so_error = ECONNRESET;
                        close:
                                KASSERT(ti_locked == TI_WLOCKED,
                                    ("tcp_do_segment: TH_RST 1 ti_locked %d",
                                    ti_locked));
                                INP_INFO_WLOCK_ASSERT(&V_tcbinfo);

                                tp->t_state = TCPS_CLOSED;
                                TCPSTAT_INC(tcps_drops);
                                tp = tcp_close(tp);
                                break;

                        case TCPS_CLOSING:
                        case TCPS_LAST_ACK:
                                KASSERT(ti_locked == TI_WLOCKED,
                                    ("tcp_do_segment: TH_RST 2 ti_locked %d",
                                    ti_locked));
                                INP_INFO_WLOCK_ASSERT(&V_tcbinfo);

                                tp = tcp_close(tp);
                                break;
                        }
                }
                goto drop;
        }

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

                /* Check to see if ts_recent is over 24 days old.  */
                if (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_INC(tcps_rcvduppack);
                        TCPSTAT_ADD(tcps_rcvdupbyte, tlen);
                        TCPSTAT_INC(tcps_pawsdrop);
                        if (tlen)
                                goto dropafterack;
                        goto drop;
                }
        }

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

        todrop = tp->rcv_nxt - th->th_seq;
        if (todrop > 0) {
                /*
                 * If this is a duplicate SYN for our current connection,
                 * advance over it and pretend and it's not a SYN.
                 */
                if (thflags & TH_SYN && th->th_seq == tp->irs) {
                        thflags &= ~TH_SYN;
                        th->th_seq++;
                        if (th->th_urp > 1)
                                th->th_urp--;
                        else
                                thflags &= ~TH_URG;
                        todrop--;
                }
                /*
                 * Following if statement from Stevens, vol. 2, p. 960.
                 */
                if (todrop > tlen
                    || (todrop == tlen && (thflags & TH_FIN) == 0)) {
                        /*
                         * Any valid FIN must be to the left of the window.
                         * At this point the FIN must be a duplicate or out
                         * of sequence; drop it.
                         */
                        thflags &= ~TH_FIN;

                        /*
                         * Send an ACK to resynchronize and drop any data.
                         * But keep on processing for RST or ACK.
                         */
                        tp->t_flags |= TF_ACKNOW;
                        todrop = tlen;
                        TCPSTAT_INC(tcps_rcvduppack);
                        TCPSTAT_ADD(tcps_rcvdupbyte, todrop);
                } else {
                        TCPSTAT_INC(tcps_rcvpartduppack);
                        TCPSTAT_ADD(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) {
                char *s;

                KASSERT(ti_locked == TI_WLOCKED, ("%s: SS_NOFDEREF && "
                    "CLOSE_WAIT && tlen ti_locked %d", __func__, ti_locked));
                INP_INFO_WLOCK_ASSERT(&V_tcbinfo);

                if ((s = tcp_log_addrs(&tp->t_inpcb->inp_inc, th, NULL, NULL))) {
                        log(LOG_DEBUG, "%s; %s: %s: Received %d bytes of data after socket "
                            "was closed, sending RST and removing tcpcb\n",
                            s, __func__, tcpstates[tp->t_state], tlen);
                        free(s, M_TCPLOG);
                }
                tp = tcp_close(tp);
                TCPSTAT_INC(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_INC(tcps_rcvpackafterwin);
                if (todrop >= tlen) {
                        TCPSTAT_ADD(tcps_rcvbyteafterwin, tlen);
                        /*
                         * 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_INC(tcps_rcvwinprobe);
                        } else
                                goto dropafterack;
                } else
                        TCPSTAT_ADD(tcps_rcvbyteafterwin, todrop);
                m_adj(m, -todrop);
                tlen -= todrop;
                thflags &= ~(TH_PUSH|TH_FIN);
        }

        /*
         * If last ACK falls within this segment's sequence numbers,
         * record its timestamp.
         * NOTE: 
         * 1) That the test incorporates suggestions from the latest
         *    proposal of the tcplw@cray.com list (Braden 1993/04/26).
         * 2) That updating only on newer timestamps interferes with
         *    our earlier PAWS tests, so this check should be solely
         *    predicated on the sequence space of this segment.
         * 3) That we modify the segment boundary check to be 
         *        Last.ACK.Sent <= SEG.SEQ + SEG.Len  
         *    instead of RFC1323's
         *        Last.ACK.Sent < SEG.SEQ + SEG.Len,
         *    This modified check allows us to overcome RFC1323's
         *    limitations as described in Stevens TCP/IP Illustrated
         *    Vol. 2 p.869. In such cases, we can still calculate the
         *    RTT correctly when RCV.NXT == Last.ACK.Sent.
         */
        if ((to.to_flags & TOF_TS) != 0 &&
            SEQ_LEQ(th->th_seq, tp->last_ack_sent) &&
            SEQ_LEQ(tp->last_ack_sent, th->th_seq + tlen +
                ((thflags & (TH_SYN|TH_FIN)) != 0))) {
                tp->ts_recent_age = ticks;
                tp->ts_recent = to.to_tsval;
        }

        /*
         * If a SYN is in the window, then this is an
         * error and we send an RST and drop the connection.
         */
        if (thflags & TH_SYN) {
                KASSERT(ti_locked == TI_WLOCKED,
                    ("tcp_do_segment: TH_SYN ti_locked %d", ti_locked));
                INP_INFO_WLOCK_ASSERT(&V_tcbinfo);

                tp = tcp_drop(tp, ECONNRESET);
                rstreason = BANDLIM_UNLIMITED;
                goto drop;
        }

        /*
         * If the ACK bit is off:  if in SYN-RECEIVED state or SENDSYN
         * flag is on (half-synchronized state), then queue data for
         * later processing; else drop segment and return.
         */
        if ((thflags & TH_ACK) == 0) {
                if (tp->t_state == TCPS_SYN_RECEIVED ||
                    (tp->t_flags & TF_NEEDSYN))
                        goto step6;
                else if (tp->t_flags & TF_ACKNOW)
                        goto dropafterack;
                else
                        goto drop;
        }

        /*
         * Ack processing.
         */
        switch (tp->t_state) {

        /*
         * In SYN_RECEIVED state, the ack ACKs our SYN, so enter
         * ESTABLISHED state and continue processing.
         * The ACK was checked above.
         */
        case TCPS_SYN_RECEIVED:

                TCPSTAT_INC(tcps_connects);
                soisconnected(so);
                /* Do window scaling? */
                if ((tp->t_flags & (TF_RCVD_SCALE|TF_REQ_SCALE)) ==
                        (TF_RCVD_SCALE|TF_REQ_SCALE)) {
                        tp->rcv_scale = tp->request_r_scale;
                        tp->snd_wnd = tiwin;
                }
                /*
                 * Make transitions:
                 *      SYN-RECEIVED  -> ESTABLISHED
                 *      SYN-RECEIVED* -> FIN-WAIT-1
                 */
                tp->t_starttime = ticks;
                if (tp->t_flags & TF_NEEDFIN) {
                        tp->t_state = TCPS_FIN_WAIT_1;
                        tp->t_flags &= ~TF_NEEDFIN;
                } else {
                        tp->t_state = TCPS_ESTABLISHED;
                        cc_conn_init(tp);
                        tcp_timer_activate(tp, TT_KEEP, tcp_keepidle);
                }
                /*
                 * If segment contains data or ACK, will call tcp_reass()
                 * later; if not, do so now to pass queued data to user.
                 */
                if (tlen == 0 && (thflags & TH_FIN) == 0)
                        (void) tcp_reass(tp, (struct tcphdr *)0, 0,
                            (struct mbuf *)0);
                tp->snd_wl1 = th->th_seq - 1;
                /* FALLTHROUGH */

        /*
         * In ESTABLISHED state: drop duplicate ACKs; ACK out of range
         * ACKs.  If the ack is in the range
         *      tp->snd_una < th->th_ack <= tp->snd_max
         * then advance tp->snd_una to th->th_ack and drop
         * data from the retransmission queue.  If this ACK reflects
         * more up to date window information we update our window information.
         */
        case TCPS_ESTABLISHED:
        case TCPS_FIN_WAIT_1:
        case TCPS_FIN_WAIT_2:
        case TCPS_CLOSE_WAIT:
        case TCPS_CLOSING:
        case TCPS_LAST_ACK:
                if (SEQ_GT(th->th_ack, tp->snd_max)) {
                        TCPSTAT_INC(tcps_rcvacktoomuch);
                        goto dropafterack;
                }
                if ((tp->t_flags & TF_SACK_PERMIT) &&
                    ((to.to_flags & TOF_SACK) ||
                     !TAILQ_EMPTY(&tp->snd_holes)))
                        tcp_sack_doack(tp, &to, th->th_ack);

                /* Run HHOOK_TCP_ESTABLISHED_IN helper hooks. */
                hhook_run_tcp_est_in(tp, th, &to);

                if (SEQ_LEQ(th->th_ack, tp->snd_una)) {
                        if (tlen == 0 && tiwin == tp->snd_wnd) {
                                TCPSTAT_INC(tcps_rcvdupack);
                                /*
                                 * If we have outstanding data (other than
                                 * a window probe), this is a completely
                                 * duplicate ack (ie, window info didn't
                                 * change), the ack is the biggest we've
                                 * seen and we've seen exactly our rexmt
                                 * threshhold of them, assume a packet
                                 * has been dropped and retransmit it.
                                 * Kludge snd_nxt & the congestion
                                 * window so we send only this one
                                 * packet.
                                 *
                                 * We know we're losing at the current
                                 * window size so do congestion avoidance
                                 * (set ssthresh to half the current window
                                 * and pull our congestion window back to
                                 * the new ssthresh).
                                 *
                                 * Dup acks mean that packets have left the
                                 * network (they're now cached at the receiver)
                                 * so bump cwnd by the amount in the receiver
                                 * to keep a constant cwnd packets in the
                                 * network.
                                 *
                                 * When using TCP ECN, notify the peer that
                                 * we reduced the cwnd.
                                 */
                                if (!tcp_timer_active(tp, TT_REXMT) ||
                                    th->th_ack != tp->snd_una)
                                        tp->t_dupacks = 0;
                                else if (++tp->t_dupacks > tcprexmtthresh ||
                                     IN_FASTRECOVERY(tp->t_flags)) {
                                        cc_ack_received(tp, th, CC_DUPACK);
                                        if ((tp->t_flags & TF_SACK_PERMIT) &&
                                            IN_FASTRECOVERY(tp->t_flags)) {
                                                int awnd;
                                                
                                                /*
                                                 * Compute the amount of data in flight first.
                                                 * We can inject new data into the pipe iff 
                                                 * we have less than 1/2 the original window's
                                                 * worth of data in flight.
                                                 */
                                                awnd = (tp->snd_nxt - tp->snd_fack) +
                                                        tp->sackhint.sack_bytes_rexmit;
                                                if (awnd < tp->snd_ssthresh) {
                                                        tp->snd_cwnd += tp->t_maxseg;
                                                        if (tp->snd_cwnd > tp->snd_ssthresh)
                                                                tp->snd_cwnd = tp->snd_ssthresh;
                                                }
                                        } else
                                                tp->snd_cwnd += tp->t_maxseg;
                                        (void) tcp_output(tp);
                                        goto drop;
                                } else if (tp->t_dupacks == tcprexmtthresh) {
                                        tcp_seq onxt = tp->snd_nxt;

                                        /*
                                         * If we're doing sack, check to
                                         * see if we're already in sack
                                         * recovery. If we're not doing sack,
                                         * check to see if we're in newreno
                                         * recovery.
                                         */
                                        if (tp->t_flags & TF_SACK_PERMIT) {
                                                if (IN_FASTRECOVERY(tp->t_flags)) {
                                                        tp->t_dupacks = 0;
                                                        break;
                                                }
                                        } else {
                                                if (SEQ_LEQ(th->th_ack,
                                                    tp->snd_recover)) {
                                                        tp->t_dupacks = 0;
                                                        break;
                                                }
                                        }
                                        /* Congestion signal before ack. */
                                        cc_cong_signal(tp, th, CC_NDUPACK);
                                        cc_ack_received(tp, th, CC_DUPACK);
                                        tcp_timer_activate(tp, TT_REXMT, 0);
                                        tp->t_rtttime = 0;
                                        if (tp->t_flags & TF_SACK_PERMIT) {
                                                TCPSTAT_INC(
                                                    tcps_sack_recovery_episode);
                                                tp->sack_newdata = tp->snd_nxt;
                                                tp->snd_cwnd = tp->t_maxseg;
                                                (void) tcp_output(tp);
                                                goto drop;
                                        }
                                        tp->snd_nxt = th->th_ack;
                                        tp->snd_cwnd = tp->t_maxseg;
                                        (void) tcp_output(tp);
                                        KASSERT(tp->snd_limited <= 2,
                                            ("%s: tp->snd_limited too big",
                                            __func__));
                                        tp->snd_cwnd = tp->snd_ssthresh +
                                             tp->t_maxseg *
                                             (tp->t_dupacks - tp->snd_limited);
                                        if (SEQ_GT(onxt, tp->snd_nxt))
                                                tp->snd_nxt = onxt;
                                        goto drop;
                                } else if (V_tcp_do_rfc3042) {
                                        cc_ack_received(tp, th, CC_DUPACK);
                                        u_long oldcwnd = tp->snd_cwnd;
                                        tcp_seq oldsndmax = tp->snd_max;
                                        u_int sent;

                                        KASSERT(tp->t_dupacks == 1 ||
                                            tp->t_dupacks == 2,
                                            ("%s: dupacks not 1 or 2",
                                            __func__));
                                        if (tp->t_dupacks == 1)
                                                tp->snd_limited = 0;
                                        tp->snd_cwnd =
                                            (tp->snd_nxt - tp->snd_una) +
                                            (tp->t_dupacks - tp->snd_limited) *
                                            tp->t_maxseg;
                                        (void) tcp_output(tp);
                                        sent = tp->snd_max - oldsndmax;
                                        if (sent > tp->t_maxseg) {
                                                KASSERT((tp->t_dupacks == 2 &&
                                                    tp->snd_limited == 0) ||
                                                   (sent == tp->t_maxseg + 1 &&
                                                    tp->t_flags & TF_SENTFIN),
                                                    ("%s: sent too much",
                                                    __func__));
                                                tp->snd_limited = 2;
                                        } else if (sent > 0)
                                                ++tp->snd_limited;
                                        tp->snd_cwnd = oldcwnd;
                                        goto drop;
                                }
                        } else
                                tp->t_dupacks = 0;
                        break;
                }

                KASSERT(SEQ_GT(th->th_ack, tp->snd_una),
                    ("%s: th_ack <= snd_una", __func__));

                /*
                 * If the congestion window was inflated to account
                 * for the other side's cached packets, retract it.
                 */
                if (IN_FASTRECOVERY(tp->t_flags)) {
                        if (SEQ_LT(th->th_ack, tp->snd_recover)) {
                                if (tp->t_flags & TF_SACK_PERMIT)
                                        tcp_sack_partialack(tp, th);
                                else
                                        tcp_newreno_partial_ack(tp, th);
                        } else
                                cc_post_recovery(tp, th);
                }
                tp->t_dupacks = 0;
                /*
                 * If we reach this point, ACK is not a duplicate,
                 *     i.e., it ACKs something we sent.
                 */
                if (tp->t_flags & TF_NEEDSYN) {
                        /*
                         * T/TCP: Connection was half-synchronized, and our
                         * SYN has been ACK'd (so connection is now fully
                         * synchronized).  Go to non-starred state,
                         * increment snd_una for ACK of SYN, and check if
                         * we can do window scaling.
                         */
                        tp->t_flags &= ~TF_NEEDSYN;
                        tp->snd_una++;
                        /* Do window scaling? */
                        if ((tp->t_flags & (TF_RCVD_SCALE|TF_REQ_SCALE)) ==
                                (TF_RCVD_SCALE|TF_REQ_SCALE)) {
                                tp->rcv_scale = tp->request_r_scale;
                                /* Send window already scaled. */
                        }
                }

process_ACK:
                INP_WLOCK_ASSERT(tp->t_inpcb);

                acked = BYTES_THIS_ACK(tp, th);
                TCPSTAT_INC(tcps_rcvackpack);
                TCPSTAT_ADD(tcps_rcvackbyte, acked);

                /*
                 * If we just performed our first retransmit, and the ACK
                 * arrives within our recovery window, then it was a mistake
                 * to do the retransmit in the first place.  Recover our
                 * original cwnd and ssthresh, and proceed to transmit where
                 * we left off.
                 */
                if (tp->t_rxtshift == 1 && tp->t_flags & TF_PREVVALID &&
                    (int)(ticks - tp->t_badrxtwin) < 0)
                        cc_cong_signal(tp, th, CC_RTO_ERR);

                /*
                 * If we have a timestamp reply, update smoothed
                 * round trip time.  If no timestamp is present but
                 * transmit timer is running and timed sequence
                 * number was acked, update smoothed round trip time.
                 * Since we now have an rtt measurement, cancel the
                 * timer backoff (cf., Phil Karn's retransmit alg.).
                 * Recompute the initial retransmit timer.
                 *
                 * Some boxes send broken timestamp replies
                 * during the SYN+ACK phase, ignore
                 * timestamps of 0 or we could calculate a
                 * huge RTT and blow up the retransmit timer.
                 */
                if ((to.to_flags & TOF_TS) != 0 &&
                    to.to_tsecr) {
                        if (!tp->t_rttlow || tp->t_rttlow > ticks - to.to_tsecr)
                                tp->t_rttlow = ticks - to.to_tsecr;
                        tcp_xmit_timer(tp, ticks - to.to_tsecr + 1);
                } else if (tp->t_rtttime && SEQ_GT(th->th_ack, tp->t_rtseq)) {
                        if (!tp->t_rttlow || tp->t_rttlow > ticks - tp->t_rtttime)
                                tp->t_rttlow = ticks - tp->t_rtttime;
                        tcp_xmit_timer(tp, ticks - tp->t_rtttime);
                }

                /*
                 * If all outstanding data is acked, stop retransmit
                 * timer and remember to restart (more output or persist).
                 * If there is more data to be acked, restart retransmit
                 * timer, using current (possibly backed-off) value.
                 */
                if (th->th_ack == tp->snd_max) {
                        tcp_timer_activate(tp, TT_REXMT, 0);
                        needoutput = 1;
                } else if (!tcp_timer_active(tp, TT_PERSIST))
                        tcp_timer_activate(tp, TT_REXMT, tp->t_rxtcur);

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

                /*
                 * Let the congestion control algorithm update congestion
                 * control related information. This typically means increasing
                 * the congestion window.
                 */
                cc_ack_received(tp, th, CC_ACK);

                SOCKBUF_LOCK(&so->so_snd);
                if (acked > so->so_snd.sb_cc) {
                        tp->snd_wnd -= so->so_snd.sb_cc;
                        sbdrop_locked(&so->so_snd, (int)so->so_snd.sb_cc);
                        ourfinisacked = 1;
                } else {
                        sbdrop_locked(&so->so_snd, acked);
                        tp->snd_wnd -= acked;
                        ourfinisacked = 0;
                }
                /* NB: sowwakeup_locked() does an implicit unlock. */
                sowwakeup_locked(so);
                /* Detect una wraparound. */
                if (!IN_RECOVERY(tp->t_flags) &&
                    SEQ_GT(tp->snd_una, tp->snd_recover) &&
                    SEQ_LEQ(th->th_ack, tp->snd_recover))
                        tp->snd_recover = th->th_ack - 1;
                /* XXXLAS: Can this be moved up into cc_post_recovery? */
                if (IN_RECOVERY(tp->t_flags) &&
                    SEQ_GEQ(th->th_ack, tp->snd_recover)) {
                        EXIT_RECOVERY(tp->t_flags);
                }
                tp->snd_una = th->th_ack;
                if (tp->t_flags & TF_SACK_PERMIT) {
                        if (SEQ_GT(tp->snd_una, tp->snd_recover))
                                tp->snd_recover = tp->snd_una;
                }
                if (SEQ_LT(tp->snd_nxt, tp->snd_una))
                        tp->snd_nxt = tp->snd_una;

                switch (tp->t_state) {

                /*
                 * In FIN_WAIT_1 STATE in addition to the processing
                 * for the ESTABLISHED state if our FIN is now acknowledged
                 * then enter FIN_WAIT_2.
                 */
                case TCPS_FIN_WAIT_1:
                        if (ourfinisacked) {
                                /*
                                 * If we can't receive any more
                                 * data, then closing user can proceed.
                                 * Starting the timer is contrary to the
                                 * specification, but if we don't get a FIN
                                 * we'll hang forever.
                                 *
                                 * XXXjl:
                                 * we should release the tp also, and use a
                                 * compressed state.
                                 */
                                if (so->so_rcv.sb_state & SBS_CANTRCVMORE) {
                                        int timeout;

                                        soisdisconnected(so);
                                        timeout = (tcp_fast_finwait2_recycle) ? 
                                                tcp_finwait2_timeout : tcp_maxidle;
                                        tcp_timer_activate(tp, TT_2MSL, timeout);
                                }
                                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) {
                                INP_INFO_WLOCK_ASSERT(&V_tcbinfo);
                                tcp_twstart(tp);
                                INP_INFO_WUNLOCK(&V_tcbinfo);
                                m_freem(m);
                                return;
                        }
                        break;

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

step6:
        INP_WLOCK_ASSERT(tp->t_inpcb);

        /*
         * Update window information.
         * Don't look at window if no ACK: TAC's send garbage on first SYN.
         */
        if ((thflags & TH_ACK) &&
            (SEQ_LT(tp->snd_wl1, th->th_seq) ||
            (tp->snd_wl1 == th->th_seq && (SEQ_LT(tp->snd_wl2, th->th_ack) ||
             (tp->snd_wl2 == th->th_ack && tiwin > tp->snd_wnd))))) {
                /* keep track of pure window updates */
                if (tlen == 0 &&
                    tp->snd_wl2 == th->th_ack && tiwin > tp->snd_wnd)
                        TCPSTAT_INC(tcps_rcvwinupd);
                tp->snd_wnd = tiwin;
                tp->snd_wl1 = th->th_seq;
                tp->snd_wl2 = th->th_ack;
                if (tp->snd_wnd > tp->max_sndwnd)
                        tp->max_sndwnd = tp->snd_wnd;
                needoutput = 1;
        }

        /*
         * Process segments with URG.
         */
        if ((thflags & TH_URG) && th->th_urp &&
            TCPS_HAVERCVDFIN(tp->t_state) == 0) {
                /*
                 * This is a kludge, but if we receive and accept
                 * random urgent pointers, we'll crash in
                 * soreceive.  It's hard to imagine someone
                 * actually wanting to send this much urgent data.
                 */
                SOCKBUF_LOCK(&so->so_rcv);
                if (th->th_urp + so->so_rcv.sb_cc > sb_max) {
                        th->th_urp = 0;                 /* XXX */
                        thflags &= ~TH_URG;             /* XXX */
                        SOCKBUF_UNLOCK(&so->so_rcv);    /* XXX */
                        goto dodata;                    /* XXX */
                }
                /*
                 * If this segment advances the known urgent pointer,
                 * then mark the data stream.  This should not happen
                 * in CLOSE_WAIT, CLOSING, LAST_ACK or TIME_WAIT STATES since
                 * a FIN has been received from the remote side.
                 * In these states we ignore the URG.
                 *
                 * According to RFC961 (Assigned Protocols),
                 * the urgent pointer points to the last octet
                 * of urgent data.  We continue, however,
                 * to consider it to indicate the first octet
                 * of data past the urgent section as the original
                 * spec states (in one of two places).
                 */
                if (SEQ_GT(th->th_seq+th->th_urp, tp->rcv_up)) {
                        tp->rcv_up = th->th_seq + th->th_urp;
                        so->so_oobmark = so->so_rcv.sb_cc +
                            (tp->rcv_up - tp->rcv_nxt) - 1;
                        if (so->so_oobmark == 0)
                                so->so_rcv.sb_state |= SBS_RCVATMARK;
                        sohasoutofband(so);
                        tp->t_oobflags &= ~(TCPOOB_HAVEDATA | TCPOOB_HADDATA);
                }
                SOCKBUF_UNLOCK(&so->so_rcv);
                /*
                 * Remove out of band data so doesn't get presented to user.
                 * This can happen independent of advancing the URG pointer,
                 * but if two URG's are pending at once, some out-of-band
                 * data may creep in... ick.
                 */
                if (th->th_urp <= (u_long)tlen &&
                    !(so->so_options & SO_OOBINLINE)) {
                        /* hdr drop is delayed */
                        tcp_pulloutofband(so, th, m, drop_hdrlen);
                }
        } else {
                /*
                 * If no out of band data is expected,
                 * pull receive urgent pointer along
                 * with the receive window.
                 */
                if (SEQ_GT(tp->rcv_nxt, tp->rcv_up))
                        tp->rcv_up = tp->rcv_nxt;
        }
dodata:                                                 /* XXX */
        INP_WLOCK_ASSERT(tp->t_inpcb);

        /*
         * Process the segment text, merging it into the TCP sequencing queue,
         * and arranging for acknowledgment of receipt if necessary.
         * This process logically involves adjusting tp->rcv_wnd as data
         * is presented to the user (this happens in tcp_usrreq.c,
         * case PRU_RCVD).  If a FIN has already been received on this
         * connection then we just ignore the text.
         */
        if ((tlen || (thflags & TH_FIN)) &&
            TCPS_HAVERCVDFIN(tp->t_state) == 0) {
                tcp_seq save_start = th->th_seq;
                m_adj(m, drop_hdrlen);  /* delayed header drop */
                /*
                 * Insert segment which includes th into TCP reassembly queue
                 * with control block tp.  Set thflags to whether reassembly now
                 * includes a segment with FIN.  This handles the common case
                 * inline (segment is the next to be received on an established
                 * connection, and the queue is empty), avoiding linkage into
                 * and removal from the queue and repetition of various
                 * conversions.
                 * Set DELACK for segments received in order, but ack
                 * immediately when segments are out of order (so
                 * fast retransmit can work).
                 */
                if (th->th_seq == tp->rcv_nxt &&
                    LIST_EMPTY(&tp->t_segq) &&
                    TCPS_HAVEESTABLISHED(tp->t_state)) {
                        if (DELAY_ACK(tp))
                                tp->t_flags |= TF_DELACK;
                        else
                                tp->t_flags |= TF_ACKNOW;
                        tp->rcv_nxt += tlen;
                        thflags = th->th_flags & TH_FIN;
                        TCPSTAT_INC(tcps_rcvpack);
                        TCPSTAT_ADD(tcps_rcvbyte, tlen);
                        ND6_HINT(tp);
                        SOCKBUF_LOCK(&so->so_rcv);
                        if (so->so_rcv.sb_state & SBS_CANTRCVMORE)
                                m_freem(m);
                        else
                                sbappendstream_locked(&so->so_rcv, m);
                        /* NB: sorwakeup_locked() does an implicit unlock. */
                        sorwakeup_locked(so);
                } else {
                        /*
                         * XXX: Due to the header drop above "th" is
                         * theoretically invalid by now.  Fortunately
                         * m_adj() doesn't actually frees any mbufs
                         * when trimming from the head.
                         */
                        thflags = tcp_reass(tp, th, &tlen, m);
                        tp->t_flags |= TF_ACKNOW;
                }
                if (tlen > 0 && (tp->t_flags & TF_SACK_PERMIT))
                        tcp_update_sack_list(tp, save_start, save_start + tlen);
#if 0
                /*
                 * Note the amount of data that peer has sent into
                 * our window, in order to estimate the sender's
                 * buffer size.
                 * XXX: Unused.
                 */
                if (SEQ_GT(tp->rcv_adv, tp->rcv_nxt))
                        len = so->so_rcv.sb_hiwat - (tp->rcv_adv - tp->rcv_nxt);
                else
                        len = so->so_rcv.sb_hiwat;
#endif
        } else {
                m_freem(m);
                thflags &= ~TH_FIN;
        }

        /*
         * If FIN is received ACK the FIN and let the user know
         * that the connection is closing.
         */
        if (thflags & TH_FIN) {
                if (TCPS_HAVERCVDFIN(tp->t_state) == 0) {
                        socantrcvmore(so);
                        /*
                         * If connection is half-synchronized
                         * (ie NEEDSYN flag on) then delay ACK,
                         * so it may be piggybacked when SYN is sent.
                         * Otherwise, since we received a FIN then no
                         * more input can be expected, send ACK now.
                         */
                        if (tp->t_flags & TF_NEEDSYN)
                                tp->t_flags |= TF_DELACK;
                        else
                                tp->t_flags |= TF_ACKNOW;
                        tp->rcv_nxt++;
                }
                switch (tp->t_state) {

                /*
                 * In SYN_RECEIVED and ESTABLISHED STATES
                 * enter the CLOSE_WAIT state.
                 */
                case TCPS_SYN_RECEIVED:
                        tp->t_starttime = ticks;
                        /* FALLTHROUGH */
                case TCPS_ESTABLISHED:
                        tp->t_state = TCPS_CLOSE_WAIT;
                        break;

                /*
                 * If still in FIN_WAIT_1 STATE FIN has not been acked so
                 * enter the CLOSING state.
                 */
                case TCPS_FIN_WAIT_1:
                        tp->t_state = TCPS_CLOSING;
                        break;

                /*
                 * In FIN_WAIT_2 state enter the TIME_WAIT state,
                 * starting the time-wait timer, turning off the other
                 * standard timers.
                 */
                case TCPS_FIN_WAIT_2:
                        INP_INFO_WLOCK_ASSERT(&V_tcbinfo);
                        KASSERT(ti_locked == TI_WLOCKED, ("%s: dodata "
                            "TCP_FIN_WAIT_2 ti_locked: %d", __func__,
                            ti_locked));

                        tcp_twstart(tp);
                        INP_INFO_WUNLOCK(&V_tcbinfo);
                        return;
                }
        }
        if (ti_locked == TI_WLOCKED)
                INP_INFO_WUNLOCK(&V_tcbinfo);
        ti_locked = TI_UNLOCKED;

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

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

check_delack:
        KASSERT(ti_locked == TI_UNLOCKED, ("%s: check_delack ti_locked %d",
            __func__, ti_locked));
        INP_INFO_UNLOCK_ASSERT(&V_tcbinfo);
        INP_WLOCK_ASSERT(tp->t_inpcb);

        if (tp->t_flags & TF_DELACK) {
                tp->t_flags &= ~TF_DELACK;
                tcp_timer_activate(tp, TT_DELACK, tcp_delacktime);
        }
        INP_WUNLOCK(tp->t_inpcb);
        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, (void *)tcp_saveipgen,
                          &tcp_savetcp, 0);
#endif
        if (ti_locked == TI_WLOCKED)
                INP_INFO_WUNLOCK(&V_tcbinfo);
        ti_locked = TI_UNLOCKED;

        tp->t_flags |= TF_ACKNOW;
        (void) tcp_output(tp);
        INP_WUNLOCK(tp->t_inpcb);
        m_freem(m);
        return;

dropwithreset:
        if (ti_locked == TI_WLOCKED)
                INP_INFO_WUNLOCK(&V_tcbinfo);
        ti_locked = TI_UNLOCKED;

        if (tp != NULL) {
                tcp_dropwithreset(m, th, tp, tlen, rstreason);
                INP_WUNLOCK(tp->t_inpcb);
        } else
                tcp_dropwithreset(m, th, NULL, tlen, rstreason);
        return;

drop:
        if (ti_locked == TI_WLOCKED) {
                INP_INFO_WUNLOCK(&V_tcbinfo);
                ti_locked = TI_UNLOCKED;
        }
#ifdef INVARIANTS
        else
                INP_INFO_UNLOCK_ASSERT(&V_tcbinfo);
#endif

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

/*
 * Issue RST and make ACK acceptable to originator of segment.
 * The mbuf must still include the original packet header.
 * tp may be NULL.
 */
static void
tcp_dropwithreset(struct mbuf *m, struct tcphdr *th, struct tcpcb *tp,
    int tlen, int rstreason)
{
#ifdef INET
        struct ip *ip;
#endif
#ifdef INET6
        struct ip6_hdr *ip6;
#endif

        if (tp != NULL) {
                INP_WLOCK_ASSERT(tp->t_inpcb);
        }

        /* Don't bother if destination was broadcast/multicast. */
        if ((th->th_flags & TH_RST) || m->m_flags & (M_BCAST|M_MCAST))
                goto drop;
#ifdef INET6
        if (mtod(m, struct ip *)->ip_v == 6) {
                ip6 = mtod(m, struct ip6_hdr *);
                if (IN6_IS_ADDR_MULTICAST(&ip6->ip6_dst) ||
                    IN6_IS_ADDR_MULTICAST(&ip6->ip6_src))
                        goto drop;
                /* IPv6 anycast check is done at tcp6_input() */
        }
#endif
#if defined(INET) && defined(INET6)
        else
#endif
#ifdef INET
        {
                ip = mtod(m, struct ip *);
                if (IN_MULTICAST(ntohl(ip->ip_dst.s_addr)) ||
                    IN_MULTICAST(ntohl(ip->ip_src.s_addr)) ||
                    ip->ip_src.s_addr == htonl(INADDR_BROADCAST) ||
                    in_broadcast(ip->ip_dst, m->m_pkthdr.rcvif))
                        goto drop;
        }
#endif

        /* Perform bandwidth limiting. */
        if (badport_bandlim(rstreason) < 0)
                goto drop;

        /* tcp_respond consumes the mbuf chain. */
        if (th->th_flags & TH_ACK) {
                tcp_respond(tp, mtod(m, void *), th, m, (tcp_seq)0,
                    th->th_ack, TH_RST);
        } else {
                if (th->th_flags & TH_SYN)
                        tlen++;
                tcp_respond(tp, mtod(m, void *), th, m, th->th_seq+tlen,
                    (tcp_seq)0, TH_RST|TH_ACK);
        }
        return;
drop:
        m_freem(m);
}

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

        to->to_flags = 0;
        for (; cnt > 0; cnt -= optlen, cp += optlen) {
                opt = cp[0];
                if (opt == TCPOPT_EOL)
                        break;
                if (opt == TCPOPT_NOP)
                        optlen = 1;
                else {
                        if (cnt < 2)
                                break;
                        optlen = cp[1];
                        if (optlen < 2 || optlen > cnt)
                                break;
                }
                switch (opt) {
                case TCPOPT_MAXSEG:
                        if (optlen != TCPOLEN_MAXSEG)
                                continue;
                        if (!(flags & TO_SYN))
                                continue;
                        to->to_flags |= TOF_MSS;
                        bcopy((char *)cp + 2,
                            (char *)&to->to_mss, sizeof(to->to_mss));
                        to->to_mss = ntohs(to->to_mss);
                        break;
                case TCPOPT_WINDOW:
                        if (optlen != TCPOLEN_WINDOW)
                                continue;
                        if (!(flags & TO_SYN))
                                continue;
                        to->to_flags |= TOF_SCALE;
                        to->to_wscale = min(cp[2], TCP_MAX_WINSHIFT);
                        break;
                case TCPOPT_TIMESTAMP:
                        if (optlen != TCPOLEN_TIMESTAMP)
                                continue;
                        to->to_flags |= TOF_TS;
                        bcopy((char *)cp + 2,
                            (char *)&to->to_tsval, sizeof(to->to_tsval));
                        to->to_tsval = ntohl(to->to_tsval);
                        bcopy((char *)cp + 6,
                            (char *)&to->to_tsecr, sizeof(to->to_tsecr));
                        to->to_tsecr = ntohl(to->to_tsecr);
                        break;
#ifdef TCP_SIGNATURE
                /*
                 * XXX In order to reply to a host which has set the
                 * TCP_SIGNATURE option in its initial SYN, we have to
                 * record the fact that the option was observed here
                 * for the syncache code to perform the correct response.
                 */
                case TCPOPT_SIGNATURE:
                        if (optlen != TCPOLEN_SIGNATURE)
                                continue;
                        to->to_flags |= TOF_SIGNATURE;
                        to->to_signature = cp + 2;
                        break;
#endif
                case TCPOPT_SACK_PERMITTED:
                        if (optlen != TCPOLEN_SACK_PERMITTED)
                                continue;
                        if (!(flags & TO_SYN))
                                continue;
                        if (!V_tcp_do_sack)
                                continue;
                        to->to_flags |= TOF_SACKPERM;
                        break;
                case TCPOPT_SACK:
                        if (optlen <= 2 || (optlen - 2) % TCPOLEN_SACK != 0)
                                continue;
                        if (flags & TO_SYN)
                                continue;
                        to->to_flags |= TOF_SACK;
                        to->to_nsacks = (optlen - 2) / TCPOLEN_SACK;
                        to->to_sacks = cp + 2;
                        TCPSTAT_INC(tcps_sack_rcv_blocks);
                        break;
                default:
                        continue;
                }
        }
}

/*
 * Pull out of band byte out of a segment so
 * it doesn't appear in the user's data queue.
 * It is still reflected in the segment length for
 * sequencing purposes.
 */
static void
tcp_pulloutofband(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);

                        INP_WLOCK_ASSERT(tp->t_inpcb);

                        tp->t_iobc = *cp;
                        tp->t_oobflags |= TCPOOB_HAVEDATA;
                        bcopy(cp+1, cp, (unsigned)(m->m_len - cnt - 1));
                        m->m_len--;
                        if (m->m_flags & M_PKTHDR)
                                m->m_pkthdr.len--;
                        return;
                }
                cnt -= m->m_len;
                m = m->m_next;
                if (m == NULL)
                        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;

        INP_WLOCK_ASSERT(tp->t_inpcb);

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

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

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

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

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

/*
 * Determine a reasonable value for maxseg size.
 * If the route is known, check route for mtu.
 * If none, use an mss that can be handled on the outgoing
 * interface without forcing IP to fragment; if bigger than
 * an mbuf cluster (MCLBYTES), round down to nearest multiple of MCLBYTES
 * to utilize large mbufs.  If no route is found, route has no mtu,
 * or the destination isn't local, use a default, hopefully conservative
 * size (usually 512 or the default IP max size, but no more than the mtu
 * of the interface), as we can't discover anything about intervening
 * gateways or networks.  We also initialize the congestion/slow start
 * window to be a single segment if the destination isn't local.
 * While looking at the routing entry, we also initialize other path-dependent
 * parameters from pre-set or cached values in the routing entry.
 *
 * Also take into account the space needed for options that we
 * send regularly.  Make maxseg shorter by that amount to assure
 * that we can send maxseg amount of data even when the options
 * are present.  Store the upper limit of the length of options plus
 * data in maxopd.
 *
 * In case of T/TCP, we call this routine during implicit connection
 * setup as well (offer = -1), to initialize maxseg from the cached
 * MSS of our peer.
 *
 * NOTE that this routine is only called when we process an incoming
 * segment. Outgoing SYN/ACK MSS settings are handled in tcp_mssopt().
 */
void
tcp_mss_update(struct tcpcb *tp, int offer,
    struct hc_metrics_lite *metricptr, int *mtuflags)
{
        int mss = 0;
        u_long maxmtu = 0;
        struct inpcb *inp = tp->t_inpcb;
        struct hc_metrics_lite metrics;
        int origoffer = offer;
#ifdef INET6
        int isipv6 = ((inp->inp_vflag & INP_IPV6) != 0) ? 1 : 0;
        size_t min_protoh = isipv6 ?
                            sizeof (struct ip6_hdr) + sizeof (struct tcphdr) :
                            sizeof (struct tcpiphdr);
#else
        const size_t min_protoh = sizeof(struct tcpiphdr);
#endif

        INP_WLOCK_ASSERT(tp->t_inpcb);

        /* Initialize. */
#ifdef INET6
        if (isipv6) {
                maxmtu = tcp_maxmtu6(&inp->inp_inc, mtuflags);
                tp->t_maxopd = tp->t_maxseg = V_tcp_v6mssdflt;
        }
#endif
#if defined(INET) && defined(INET6)
        else
#endif
#ifdef INET
        {
                maxmtu = tcp_maxmtu(&inp->inp_inc, mtuflags);
                tp->t_maxopd = tp->t_maxseg = V_tcp_mssdflt;
        }
#endif

        /*
         * No route to sender, stay with default mss and return.
         */
        if (maxmtu == 0) {
                /*
                 * In case we return early we need to initialize metrics
                 * to a defined state as tcp_hc_get() would do for us
                 * if there was no cache hit.
                 */
                if (metricptr != NULL)
                        bzero(metricptr, sizeof(struct hc_metrics_lite));
                return;
        }

        /* What have we got? */
        switch (offer) {
                case 0:
                        /*
                         * Offer == 0 means that there was no MSS on the SYN
                         * segment, in this case we use tcp_mssdflt as
                         * already assigned to t_maxopd above.
                         */
                        offer = tp->t_maxopd;
                        break;

                case -1:
                        /*
                         * Offer == -1 means that we didn't receive SYN yet.
                         */
                        /* FALLTHROUGH */

                default:
                        /*
                         * Prevent DoS attack with too small MSS. Round up
                         * to at least minmss.
                         */
                        offer = max(offer, V_tcp_minmss);
        }

        /*
         * rmx information is now retrieved from tcp_hostcache.
         */
        tcp_hc_get(&inp->inp_inc, &metrics);
        if (metricptr != NULL)
                bcopy(&metrics, metricptr, sizeof(struct hc_metrics_lite));

        /*
         * If there's a discovered mtu int tcp hostcache, use it
         * else, use the link mtu.
         */
        if (metrics.rmx_mtu)
                mss = min(metrics.rmx_mtu, maxmtu) - min_protoh;
        else {
#ifdef INET6
                if (isipv6) {
                        mss = maxmtu - min_protoh;
                        if (!V_path_mtu_discovery &&
                            !in6_localaddr(&inp->in6p_faddr))
                                mss = min(mss, V_tcp_v6mssdflt);
                }
#endif
#if defined(INET) && defined(INET6)
                else
#endif
#ifdef INET
                {
                        mss = maxmtu - min_protoh;
                        if (!V_path_mtu_discovery &&
                            !in_localaddr(inp->inp_faddr))
                                mss = min(mss, V_tcp_mssdflt);
                }
#endif
                /*
                 * XXX - The above conditional (mss = maxmtu - min_protoh)
                 * probably violates the TCP spec.
                 * The problem is that, since we don't know the
                 * other end's MSS, we are supposed to use a conservative
                 * default.  But, if we do that, then MTU discovery will
                 * never actually take place, because the conservative
                 * default is much less than the MTUs typically seen
                 * on the Internet today.  For the moment, we'll sweep
                 * this under the carpet.
                 *
                 * The conservative default might not actually be a problem
                 * if the only case this occurs is when sending an initial
                 * SYN with options and data to a host we've never talked
                 * to before.  Then, they will reply with an MSS value which
                 * will get recorded and the new parameters should get
                 * recomputed.  For Further Study.
                 */
        }
        mss = min(mss, offer);

        /*
         * Sanity check: make sure that maxopd will be large
         * enough to allow some data on segments even if the
         * all the option space is used (40bytes).  Otherwise
         * funny things may happen in tcp_output.
         */
        mss = max(mss, 64);

        /*
         * maxopd stores the maximum length of data AND options
         * in a segment; maxseg is the amount of data in a normal
         * segment.  We need to store this value (maxopd) apart
         * from maxseg, because now every segment carries options
         * and thus we normally have somewhat less data in segments.
         */
        tp->t_maxopd = mss;

        /*
         * origoffer==-1 indicates that no segments were received yet.
         * In this case we just guess.
         */
        if ((tp->t_flags & (TF_REQ_TSTMP|TF_NOOPT)) == TF_REQ_TSTMP &&
            (origoffer == -1 ||
             (tp->t_flags & TF_RCVD_TSTMP) == TF_RCVD_TSTMP))
                mss -= TCPOLEN_TSTAMP_APPA;

#if     (MCLBYTES & (MCLBYTES - 1)) == 0
        if (mss > MCLBYTES)
                mss &= ~(MCLBYTES-1);
#else
        if (mss > MCLBYTES)
                mss = mss / MCLBYTES * MCLBYTES;
#endif
        tp->t_maxseg = mss;
}

void
tcp_mss(struct tcpcb *tp, int offer)
{
        int mss;
        u_long bufsize;
        struct inpcb *inp;
        struct socket *so;
        struct hc_metrics_lite metrics;
        int mtuflags = 0;

        KASSERT(tp != NULL, ("%s: tp == NULL", __func__));
        
        tcp_mss_update(tp, offer, &metrics, &mtuflags);

        mss = tp->t_maxseg;
        inp = tp->t_inpcb;

        /*
         * If there's a pipesize, change the socket buffer to that size,
         * don't change if sb_hiwat is different than default (then it
         * has been changed on purpose with setsockopt).
         * Make the socket buffers an integral number of mss units;
         * if the mss is larger than the socket buffer, decrease the mss.
         */
        so = inp->inp_socket;
        SOCKBUF_LOCK(&so->so_snd);
        if ((so->so_snd.sb_hiwat == tcp_sendspace) && metrics.rmx_sendpipe)
                bufsize = metrics.rmx_sendpipe;
        else
                bufsize = so->so_snd.sb_hiwat;
        if (bufsize < mss)
                mss = bufsize;
        else {
                bufsize = roundup(bufsize, mss);
                if (bufsize > sb_max)
                        bufsize = sb_max;
                if (bufsize > so->so_snd.sb_hiwat)
                        (void)sbreserve_locked(&so->so_snd, bufsize, so, NULL);
        }
        SOCKBUF_UNLOCK(&so->so_snd);
        tp->t_maxseg = mss;

        SOCKBUF_LOCK(&so->so_rcv);
        if ((so->so_rcv.sb_hiwat == tcp_recvspace) && metrics.rmx_recvpipe)
                bufsize = metrics.rmx_recvpipe;
        else
                bufsize = so->so_rcv.sb_hiwat;
        if (bufsize > mss) {
                bufsize = roundup(bufsize, mss);
                if (bufsize > sb_max)
                        bufsize = sb_max;
                if (bufsize > so->so_rcv.sb_hiwat)
                        (void)sbreserve_locked(&so->so_rcv, bufsize, so, NULL);
        }
        SOCKBUF_UNLOCK(&so->so_rcv);

        /* Check the interface for TSO capabilities. */
        if (mtuflags & CSUM_TSO)
                tp->t_flags |= TF_TSO;
}

/*
 * Determine the MSS option to send on an outgoing SYN.
 */
int
tcp_mssopt(struct in_conninfo *inc)
{
        int mss = 0;
        u_long maxmtu = 0;
        u_long thcmtu = 0;
        size_t min_protoh;

        KASSERT(inc != NULL, ("tcp_mssopt with NULL in_conninfo pointer"));

#ifdef INET6
        if (inc->inc_flags & INC_ISIPV6) {
                mss = V_tcp_v6mssdflt;
                maxmtu = tcp_maxmtu6(inc, NULL);
                thcmtu = tcp_hc_getmtu(inc); /* IPv4 and IPv6 */
                min_protoh = sizeof(struct ip6_hdr) + sizeof(struct tcphdr);
        }
#endif
#if defined(INET) && defined(INET6)
        else
#endif
#ifdef INET
        {
                mss = V_tcp_mssdflt;
                maxmtu = tcp_maxmtu(inc, NULL);
                thcmtu = tcp_hc_getmtu(inc); /* IPv4 and IPv6 */
                min_protoh = sizeof(struct tcpiphdr);
        }
#endif
        if (maxmtu && thcmtu)
                mss = min(maxmtu, thcmtu) - min_protoh;
        else if (maxmtu || thcmtu)
                mss = max(maxmtu, thcmtu) - min_protoh;

        return (mss);
}


/*
 * On a partial ack arrives, force the retransmission of the
 * next unacknowledged segment.  Do not clear tp->t_dupacks.
 * By setting snd_nxt to ti_ack, this forces retransmission timer to
 * be started again.
 */
static void
tcp_newreno_partial_ack(struct tcpcb *tp, struct tcphdr *th)
{
        tcp_seq onxt = tp->snd_nxt;
        u_long  ocwnd = tp->snd_cwnd;

        INP_WLOCK_ASSERT(tp->t_inpcb);

        tcp_timer_activate(tp, TT_REXMT, 0);
        tp->t_rtttime = 0;
        tp->snd_nxt = th->th_ack;
        /*
         * Set snd_cwnd to one segment beyond acknowledged offset.
         * (tp->snd_una has not yet been updated when this function is called.)
         */
        tp->snd_cwnd = tp->t_maxseg + BYTES_THIS_ACK(tp, th);
        tp->t_flags |= TF_ACKNOW;
        (void) tcp_output(tp);
        tp->snd_cwnd = ocwnd;
        if (SEQ_GT(onxt, tp->snd_nxt))
                tp->snd_nxt = onxt;
        /*
         * Partial window deflation.  Relies on fact that tp->snd_una
         * not updated yet.
         */
        if (tp->snd_cwnd > BYTES_THIS_ACK(tp, th))
                tp->snd_cwnd -= BYTES_THIS_ACK(tp, th);
        else
                tp->snd_cwnd = 0;
        tp->snd_cwnd += tp->t_maxseg;
}