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

     /*-
      * Copyright (c) 1982, 1986, 1988, 1990, 1993, 1994, 1995
      *      The Regents of the University of California.  All rights reserved.
      * 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: releng/11.2/sys/netinet/tcp_input.c 332829 2018-04-20 15:44:29Z jtl $");
    
    #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/sdt.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/if_var.h>
    #include <net/route.h>
    #include <net/vnet.h>
    
    #define TCPSTATES               /* for logging */
    
    #include <netinet/in.h>
    #include <netinet/in_kdtrace.h>
    #include <netinet/in_pcb.h>
    #include <netinet/in_systm.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/in6_var.h>
    #include <netinet6/ip6_var.h>
    #include <netinet6/nd6.h>
    #ifdef TCP_RFC7413
   #include <netinet/tcp_fastopen.h>
   #endif
   #include <netinet/tcp.h>
   #include <netinet/tcp_fsm.h>
   #include <netinet/tcp_seq.h>
   #include <netinet/tcp_timer.h>
   #include <netinet/tcp_var.h>
   #include <netinet6/tcp6_var.h>
   #include <netinet/tcpip.h>
   #include <netinet/cc/cc.h>
   #ifdef TCPPCAP
   #include <netinet/tcp_pcap.h>
   #endif
   #include <netinet/tcp_syncache.h>
   #ifdef TCPDEBUG
   #include <netinet/tcp_debug.h>
   #endif /* TCPDEBUG */
   #ifdef TCP_OFFLOAD
   #include <netinet/tcp_offload.h>
   #endif
   
   #include <netipsec/ipsec_support.h>
   
   #include <machine/in_cksum.h>
   
   #include <security/mac/mac_framework.h>
   
   const int tcprexmtthresh = 3;
   
   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_INT(_net_inet_tcp, OID_AUTO, blackhole, CTLFLAG_VNET | CTLFLAG_RW,
       &VNET_NAME(blackhole), 0,
       "Do not send RST on segments to closed ports");
   
   VNET_DEFINE(int, tcp_delack_enabled) = 1;
   SYSCTL_INT(_net_inet_tcp, OID_AUTO, delayed_ack, CTLFLAG_VNET | 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_INT(_net_inet_tcp, OID_AUTO, drop_synfin, CTLFLAG_VNET | CTLFLAG_RW,
       &VNET_NAME(drop_synfin), 0,
       "Drop TCP packets with SYN+FIN set");
   
   VNET_DEFINE(int, tcp_do_rfc6675_pipe) = 0;
   SYSCTL_INT(_net_inet_tcp, OID_AUTO, rfc6675_pipe, CTLFLAG_VNET | CTLFLAG_RW,
       &VNET_NAME(tcp_do_rfc6675_pipe), 0,
       "Use calculated pipe/in-flight bytes per RFC 6675");
   
   VNET_DEFINE(int, tcp_do_rfc3042) = 1;
   #define V_tcp_do_rfc3042        VNET(tcp_do_rfc3042)
   SYSCTL_INT(_net_inet_tcp, OID_AUTO, rfc3042, CTLFLAG_VNET | CTLFLAG_RW,
       &VNET_NAME(tcp_do_rfc3042), 0,
       "Enable RFC 3042 (Limited Transmit)");
   
   VNET_DEFINE(int, tcp_do_rfc3390) = 1;
   SYSCTL_INT(_net_inet_tcp, OID_AUTO, rfc3390, CTLFLAG_VNET | CTLFLAG_RW,
       &VNET_NAME(tcp_do_rfc3390), 0,
       "Enable RFC 3390 (Increasing TCP's Initial Congestion Window)");
   
   VNET_DEFINE(int, tcp_initcwnd_segments) = 10;
   SYSCTL_INT(_net_inet_tcp, OID_AUTO, initcwnd_segments,
       CTLFLAG_VNET | CTLFLAG_RW, &VNET_NAME(tcp_initcwnd_segments), 0,
       "Slow-start flight size (initial congestion window) in number of segments");
   
   VNET_DEFINE(int, tcp_do_rfc3465) = 1;
   SYSCTL_INT(_net_inet_tcp, OID_AUTO, rfc3465, CTLFLAG_VNET | CTLFLAG_RW,
       &VNET_NAME(tcp_do_rfc3465), 0,
       "Enable RFC 3465 (Appropriate Byte Counting)");
   
   VNET_DEFINE(int, tcp_abc_l_var) = 2;
   SYSCTL_INT(_net_inet_tcp, OID_AUTO, abc_l_var, CTLFLAG_VNET | CTLFLAG_RW,
       &VNET_NAME(tcp_abc_l_var), 2,
       "Cap the max cwnd increment during slow-start to this number of segments");
   
   static SYSCTL_NODE(_net_inet_tcp, OID_AUTO, ecn, CTLFLAG_RW, 0, "TCP ECN");
   
   VNET_DEFINE(int, tcp_do_ecn) = 2;
   SYSCTL_INT(_net_inet_tcp_ecn, OID_AUTO, enable, CTLFLAG_VNET | CTLFLAG_RW,
       &VNET_NAME(tcp_do_ecn), 0,
       "TCP ECN support");
   
   VNET_DEFINE(int, tcp_ecn_maxretries) = 1;
   SYSCTL_INT(_net_inet_tcp_ecn, OID_AUTO, maxretries, CTLFLAG_VNET | CTLFLAG_RW,
       &VNET_NAME(tcp_ecn_maxretries), 0,
       "Max retries before giving up on ECN");
   
   VNET_DEFINE(int, tcp_insecure_syn) = 0;
   #define V_tcp_insecure_syn      VNET(tcp_insecure_syn)
   SYSCTL_INT(_net_inet_tcp, OID_AUTO, insecure_syn, CTLFLAG_VNET | CTLFLAG_RW,
       &VNET_NAME(tcp_insecure_syn), 0,
       "Follow RFC793 instead of RFC5961 criteria for accepting SYN packets");
   
   VNET_DEFINE(int, tcp_insecure_rst) = 0;
   #define V_tcp_insecure_rst      VNET(tcp_insecure_rst)
   SYSCTL_INT(_net_inet_tcp, OID_AUTO, insecure_rst, CTLFLAG_VNET | CTLFLAG_RW,
       &VNET_NAME(tcp_insecure_rst), 0,
       "Follow RFC793 instead of RFC5961 criteria for accepting RST packets");
   
   VNET_DEFINE(int, tcp_recvspace) = 1024*64;
   #define V_tcp_recvspace VNET(tcp_recvspace)
   SYSCTL_INT(_net_inet_tcp, TCPCTL_RECVSPACE, recvspace, CTLFLAG_VNET | CTLFLAG_RW,
       &VNET_NAME(tcp_recvspace), 0, "Initial receive socket buffer size");
   
   VNET_DEFINE(int, tcp_do_autorcvbuf) = 1;
   #define V_tcp_do_autorcvbuf     VNET(tcp_do_autorcvbuf)
   SYSCTL_INT(_net_inet_tcp, OID_AUTO, recvbuf_auto, CTLFLAG_VNET | 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_INT(_net_inet_tcp, OID_AUTO, recvbuf_inc, CTLFLAG_VNET | 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_INT(_net_inet_tcp, OID_AUTO, recvbuf_max, CTLFLAG_VNET | 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);
   
   /*
    * TCP statistics are stored in an array of counter(9)s, which size matches
    * size of struct tcpstat.  TCP running connection count is a regular array.
    */
   VNET_PCPUSTAT_DEFINE(struct tcpstat, tcpstat);
   SYSCTL_VNET_PCPUSTAT(_net_inet_tcp, TCPCTL_STATS, stats, struct tcpstat,
       tcpstat, "TCP statistics (struct tcpstat, netinet/tcp_var.h)");
   VNET_DEFINE(counter_u64_t, tcps_states[TCP_NSTATES]);
   SYSCTL_COUNTER_U64_ARRAY(_net_inet_tcp, TCPCTL_STATES, states, CTLFLAG_RD |
       CTLFLAG_VNET, &VNET_NAME(tcps_states)[0], TCP_NSTATES,
       "TCP connection counts by TCP state");
   
   static void
   tcp_vnet_init(const void *unused)
   {
   
           COUNTER_ARRAY_ALLOC(V_tcps_states, TCP_NSTATES, M_WAITOK);
           VNET_PCPUSTAT_ALLOC(tcpstat, M_WAITOK);
   }
   VNET_SYSINIT(tcp_vnet_init, SI_SUB_PROTO_IFATTACHDOMAIN, SI_ORDER_ANY,
       tcp_vnet_init, NULL);
   
   #ifdef VIMAGE
   static void
   tcp_vnet_uninit(const void *unused)
   {
   
           COUNTER_ARRAY_FREE(V_tcps_states, TCP_NSTATES);
           VNET_PCPUSTAT_FREE(tcpstat);
   }
   VNET_SYSUNINIT(tcp_vnet_uninit, SI_SUB_PROTO_IFATTACHDOMAIN, SI_ORDER_ANY,
       tcp_vnet_uninit, NULL);
   #endif /* VIMAGE */
   
   /*
    * Kernel module interface for updating tcpstat.  The argument is an index
    * into tcpstat treated as an array.
    */
   void
   kmod_tcpstat_inc(int statnum)
   {
   
           counter_u64_add(VNET(tcpstat)[statnum], 1);
   }
   
   /*
    * Wrapper for the TCP established input helper hook.
    */
   void
   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
    */
   void
   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 <= 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 * tcp_maxseg(tp));
                           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);
           }
   }
   
   void 
   cc_conn_init(struct tcpcb *tp)
   {
           struct hc_metrics_lite metrics;
           struct inpcb *inp = tp->t_inpcb;
           u_int maxseg;
           int rtt;
   
           INP_WLOCK_ASSERT(tp->t_inpcb);
   
           tcp_hc_get(&inp->inp_inc, &metrics);
           maxseg = tcp_maxseg(tp);
   
           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 threshold, but set the
                    * threshold to no less than 2*mss.
                    */
                   tp->snd_ssthresh = max(2 * maxseg, metrics.rmx_ssthresh);
                   TCPSTAT_INC(tcps_usedssthresh);
           }
   
           /*
            * Set the initial slow-start flight size.
            *
            * RFC5681 Section 3.1 specifies the default conservative values.
            * RFC3390 specifies slightly more aggressive values.
            * RFC6928 increases it to ten segments.
            * Support for user specified value for initial flight size.
            *
            * If a SYN or SYN/ACK was lost and retransmitted, we have to
            * reduce the initial CWND to one segment as congestion is likely
            * requiring us to be cautious.
            */
           if (tp->snd_cwnd == 1)
                   tp->snd_cwnd = maxseg;          /* SYN(-ACK) lost */
           else if (V_tcp_initcwnd_segments)
                   tp->snd_cwnd = min(V_tcp_initcwnd_segments * maxseg,
                       max(2 * maxseg, V_tcp_initcwnd_segments * 1460));
           else if (V_tcp_do_rfc3390)
                   tp->snd_cwnd = min(4 * maxseg, max(2 * maxseg, 4380));
           else {
                   /* Per RFC5681 Section 3.1 */
                   if (maxseg > 2190)
                           tp->snd_cwnd = 2 * maxseg;
                   else if (maxseg > 1095)
                           tp->snd_cwnd = 3 * maxseg;
                   else
                           tp->snd_cwnd = 4 * maxseg;
           }
   
           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)
   {
           u_int maxseg;
   
           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:
                   maxseg = tcp_maxseg(tp);
                   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 /
                       maxseg) * maxseg;
                   tp->snd_cwnd = 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);
           }
   }
   
   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;
   }
   
   /*
    * Indicate whether this ack should be delayed.  We can delay the ack if
    * following conditions are met:
    *      - There is no delayed ack timer in progress.
    *      - Our last ack wasn't a 0-sized window. We never want to delay
    *        the ack that opens up a 0-sized window.
    *      - LRO wasn't used for this segment. We make sure by checking that the
    *        segment size is not larger than the MSS.
    */
   #define DELAY_ACK(tp, tlen)                                             \
           ((!tcp_timer_active(tp, TT_DELACK) &&                           \
               (tp->t_flags & TF_RXWIN0SENT) == 0) &&                      \
               (tlen <= tp->t_maxseg) &&                                   \
               (V_tcp_delack_enabled || (tp->t_flags & TF_NEEDSYN)))
   
   static void inline
   cc_ecnpkt_handler(struct tcpcb *tp, struct tcphdr *th, uint8_t iptos)
   {
           INP_WLOCK_ASSERT(tp->t_inpcb);
   
           if (CC_ALGO(tp)->ecnpkt_handler != NULL) {
                   switch (iptos & IPTOS_ECN_MASK) {
                   case IPTOS_ECN_CE:
                       tp->ccv->flags |= CCF_IPHDR_CE;
                       break;
                   case IPTOS_ECN_ECT0:
                       tp->ccv->flags &= ~CCF_IPHDR_CE;
                       break;
                   case IPTOS_ECN_ECT1:
                       tp->ccv->flags &= ~CCF_IPHDR_CE;
                       break;
                   }
   
                   if (th->th_flags & TH_CWR)
                           tp->ccv->flags |= CCF_TCPHDR_CWR;
                   else
                           tp->ccv->flags &= ~CCF_TCPHDR_CWR;
   
                   if (tp->t_flags & TF_DELACK)
                           tp->ccv->flags |= CCF_DELACK;
                   else
                           tp->ccv->flags &= ~CCF_DELACK;
   
                   CC_ALGO(tp)->ecnpkt_handler(tp->ccv);
   
                   if (tp->ccv->flags & CCF_ACKNOW)
                           tcp_timer_activate(tp, TT_DELACK, tcp_delacktime);
           }
   }
   
   /*
    * 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;
           struct ip6_hdr *ip6;
   
           IP6_EXTHDR_CHECK(m, *offp, sizeof(struct tcphdr), IPPROTO_DONE);
   
           /*
            * draft-itojun-ipv6-tcp-to-anycast
            * better place to put this in?
            */
           ip6 = mtod(m, struct ip6_hdr *);
           ia6 = in6ifa_ifwithaddr(&ip6->ip6_dst, 0 /* XXX */);
           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);
           }
           if (ia6)
                   ifa_free(&ia6->ia_ifa);
   
           return (tcp_input(mp, offp, proto));
   }
   #endif /* INET6 */
   
   int
   tcp_input(struct mbuf **mp, int *offp, int proto)
   {
           struct mbuf *m = *mp;
           struct tcphdr *th = NULL;
           struct ip *ip = NULL;
           struct inpcb *inp = NULL;
           struct tcpcb *tp = NULL;
           struct socket *so = NULL;
           u_char *optp = NULL;
           int off0;
           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 */
           uint8_t iptos;
           struct m_tag *fwd_tag = NULL;
   #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;
   #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
   
           off0 = *offp;
           m = *mp;
           *mp = NULL;
           to.to_flags = 0;
           TCPSTAT_INC(tcps_rcvtotal);
   
   #ifdef INET6
           if (isipv6) {
                   /* IP6_EXTHDR_CHECK() is already done at tcp6_input(). */
   
                   if (m->m_len < (sizeof(*ip6) + sizeof(*th))) {
                           m = m_pullup(m, sizeof(*ip6) + sizeof(*th));
                           if (m == NULL) {
                                   TCPSTAT_INC(tcps_rcvshort);
                                   return (IPPROTO_DONE);
                           }
                   }
   
                   ip6 = mtod(m, struct ip6_hdr *);
                   th = (struct tcphdr *)((caddr_t)ip6 + off0);
                   tlen = sizeof(*ip6) + ntohs(ip6->ip6_plen) - off0;
                   if (m->m_pkthdr.csum_flags & CSUM_DATA_VALID_IPV6) {
                           if (m->m_pkthdr.csum_flags & CSUM_PSEUDO_HDR)
                                   th->th_sum = m->m_pkthdr.csum_data;
                           else
                                   th->th_sum = in6_cksum_pseudo(ip6, tlen,
                                       IPPROTO_TCP, m->m_pkthdr.csum_data);
                           th->th_sum ^= 0xffff;
                   } else
                           th->th_sum = in6_cksum(m, IPPROTO_TCP, off0, tlen);
                   if (th->th_sum) {
                           TCPSTAT_INC(tcps_rcvbadsum);
                           goto drop;
                   }
   
                   /*
                    * 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;
                   }
                   iptos = (ntohl(ip6->ip6_flow) >> 20) & 0xff;
           }
   #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);
                           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 (IPPROTO_DONE);
                           }
                   }
                   ip = mtod(m, struct ip *);
                   th = (struct tcphdr *)((caddr_t)ip + off0);
                   tlen = ntohs(ip->ip_len) - off0;
   
                   iptos = ip->ip_tos;
                   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 + tlen +
                                       IPPROTO_TCP));
                           th->th_sum ^= 0xffff;
                   } else {
                           struct ipovly *ipov = (struct ipovly *)ip;
   
                           /*
                            * Checksum extended TCP header and data.
                            */
                           len = off0 + tlen;
                           bzero(ipov->ih_x1, sizeof(ipov->ih_x1));
                           ipov->ih_len = htons(tlen);
                           th->th_sum = in_cksum(m, len);
                           /* Reset length for SDT probes. */
                           ip->ip_len = htons(len);
                           /* Reset TOS bits */
                           ip->ip_tos = iptos;
                           /* Re-initialization for later version check */
                           ip->ip_v = IPVERSION;
                           ip->ip_hl = off0 >> 2;
                   }
   
                   if (th->th_sum) {
                           TCPSTAT_INC(tcps_rcvbadsum);
                           goto drop;
                   }
           }
   #endif /* INET */
   
           /*
            * 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, IPPROTO_DONE);
                           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 (IPPROTO_DONE);
                                   }
                                   ip = mtod(m, struct 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 three cases
            * where we might discover later we need a write lock despite the
            * flags: ACKs moving a connection out of the syncache, ACKs for a
            * connection in TIMEWAIT and SYNs not targeting a listening socket.
            */
           if ((thflags & (TH_FIN | TH_RST)) != 0) {
                   INP_INFO_RLOCK(&V_tcbinfo);
                   ti_locked = TI_RLOCKED;
           } else
                   ti_locked = TI_UNLOCKED;
   
           /*
            * Grab info from PACKET_TAG_IPFORWARD tag prepended to the chain.
            */
           if (
   #ifdef INET6
               (isipv6 && (m->m_flags & M_IP6_NEXTHOP))
   #ifdef INET
               || (!isipv6 && (m->m_flags & M_IP_NEXTHOP))
   #endif
   #endif
   #if defined(INET) && !defined(INET6)
               (m->m_flags & M_IP_NEXTHOP)
   #endif
               )
                   fwd_tag = m_tag_find(m, PACKET_TAG_IPFORWARD, NULL);
   
   findpcb:
   #ifdef INVARIANTS
           if (ti_locked == TI_RLOCKED) {
                   INP_INFO_RLOCK_ASSERT(&V_tcbinfo);
           } else {
                   INP_INFO_UNLOCK_ASSERT(&V_tcbinfo);
           }
   #endif
   #ifdef INET6
           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);
                   }
           } else 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
           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);
                   }
           } else
                   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);
           /*
            * While waiting for inp lock during the lookup, another thread
            * can have dropped the inpcb, in which case we need to loop back
            * and try to find a new inpcb to deliver to.
            */
           if (inp->inp_flags & INP_DROPPED) {
                   INP_WUNLOCK(inp);
                   inp = NULL;
                   goto findpcb;
           }
           if ((inp->inp_flowtype == M_HASHTYPE_NONE) &&
               (M_HASHTYPE_GET(m) != M_HASHTYPE_NONE) &&
               ((inp->inp_socket == NULL) ||
               (inp->inp_socket->so_options & SO_ACCEPTCONN) == 0)) {
                   inp->inp_flowid = m->m_pkthdr.flowid;
                   inp->inp_flowtype = M_HASHTYPE_GET(m);
           }
   #if defined(IPSEC) || defined(IPSEC_SUPPORT)
   #ifdef INET6
           if (isipv6 && IPSEC_ENABLED(ipv6) &&
               IPSEC_CHECK_POLICY(ipv6, m, inp) != 0) {
                   goto dropunlock;
           }
   #ifdef INET
           else
   #endif
   #endif /* INET6 */
   #ifdef INET
           if (IPSEC_ENABLED(ipv4) &&
               IPSEC_CHECK_POLICY(ipv4, m, inp) != 0) {
                   goto dropunlock;
           }
   #endif /* INET */
   #endif /* IPSEC */
   
           /*
            * Check the minimum TTL for socket.
            */
           if (inp->inp_ip_minttl != 0) {
   #ifdef INET6
                   if (isipv6) {
                           if (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_RLOCK(&V_tcbinfo) == 0) {
                                   in_pcbref(inp);
                                   INP_WUNLOCK(inp);
                                   INP_INFO_RLOCK(&V_tcbinfo);
                                   ti_locked = TI_RLOCKED;
                                   INP_WLOCK(inp);
                                   if (in_pcbrele_wlocked(inp)) {
                                           inp = NULL;
                                           goto findpcb;
                                   } else if (inp->inp_flags & INP_DROPPED) {
                                           INP_WUNLOCK(inp);
                                           inp = NULL;
                                           goto findpcb;
                                   }
                           } else
                                   ti_locked = TI_RLOCKED;
                   }
                   INP_INFO_RLOCK_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_RUNLOCK(&V_tcbinfo);
                   return (IPPROTO_DONE);
           }
           /*
            * 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;
          }
  
  #ifdef TCP_OFFLOAD
          if (tp->t_flags & TF_TOE) {
                  tcp_offload_input(tp, m);
                  m = NULL;       /* consumed by the TOE driver */
                  goto dropunlock;
          }
  #endif
  
          /*
           * 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_FIN | TH_RST)) != 0)
                  INP_INFO_RLOCK_ASSERT(&V_tcbinfo);
  #endif
          if (!((tp->t_state == TCPS_ESTABLISHED && (thflags & TH_SYN) == 0) ||
                (tp->t_state == TCPS_LISTEN && (thflags & TH_SYN) &&
                 !(tp->t_flags & TF_FASTOPEN)))) {
                  if (ti_locked == TI_UNLOCKED) {
                          if (INP_INFO_TRY_RLOCK(&V_tcbinfo) == 0) {
                                  in_pcbref(inp);
                                  INP_WUNLOCK(inp);
                                  INP_INFO_RLOCK(&V_tcbinfo);
                                  ti_locked = TI_RLOCKED;
                                  INP_WLOCK(inp);
                                  if (in_pcbrele_wlocked(inp)) {
                                          inp = NULL;
                                          goto findpcb;
                                  } else if (inp->inp_flags & INP_DROPPED) {
                                          INP_WUNLOCK(inp);
                                          inp = NULL;
                                          goto findpcb;
                                  }
                                  goto relocked;
                          } else
                                  ti_locked = TI_RLOCKED;
                  }
                  INP_INFO_RLOCK_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.
           */
          KASSERT(tp->t_state == TCPS_LISTEN || !(so->so_options & SO_ACCEPTCONN),
              ("%s: so accepting but tp %p not listening", __func__, tp));
          if (tp->t_state == TCPS_LISTEN && (so->so_options & SO_ACCEPTCONN)) {
                  struct in_conninfo inc;
  
                  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) {
  
                          INP_INFO_RLOCK_ASSERT(&V_tcbinfo);
                          /*
                           * 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.
                           */
                          rstreason = syncache_expand(&inc, &to, th, &so, m);
                          if (rstreason < 0) {
                                  /*
                                   * A failing TCP MD5 signature comparison
                                   * must result in the segment being dropped
                                   * and must not produce any response back
                                   * to the sender.
                                   */
                                  goto dropunlock;
                          } else if (rstreason == 0) {
                                  /*
                                   * 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;
                          }
  #ifdef TCP_RFC7413
  new_tfo_socket:
  #endif
                          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);
                          /*
                           * New connection inpcb is already locked by
                           * syncache_expand().
                           */
                          INP_WLOCK_ASSERT(inp);
                          tp = intotcpcb(inp);
                          KASSERT(tp->t_state == TCPS_SYN_RECEIVED,
                              ("%s: ", __func__));
                          /*
                           * Process the segment and the data it
                           * contains.  tcp_do_segment() consumes
                           * the mbuf chain and unlocks the inpcb.
                           */
                          tp->t_fb->tfb_tcp_do_segment(m, th, so, tp, drop_hdrlen, tlen,
                              iptos, ti_locked);
                          INP_INFO_UNLOCK_ASSERT(&V_tcbinfo);
                          return (IPPROTO_DONE);
                  }
                  /*
                   * 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 = in6ifa_ifwithaddr(&ip6->ip6_dst, 0 /* XXX */);
                          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;
                          }
                          if (ia6)
                                  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_PROBE3(debug__input, tp, th, m);
                  tcp_dooptions(&to, optp, optlen, TO_SYN);
  #ifdef TCP_RFC7413
                  if (syncache_add(&inc, &to, th, inp, &so, m, NULL, NULL))
                          goto new_tfo_socket;
  #else
                  syncache_add(&inc, &to, th, inp, &so, m, NULL, NULL);
  #endif
                  /*
                   * Entry added to syncache and mbuf consumed.
                   * Only the listen socket is unlocked by syncache_add().
                   */
                  if (ti_locked == TI_RLOCKED) {
                          INP_INFO_RUNLOCK(&V_tcbinfo);
                          ti_locked = TI_UNLOCKED;
                  }
                  INP_INFO_UNLOCK_ASSERT(&V_tcbinfo);
                  return (IPPROTO_DONE);
          } else if (tp->t_state == TCPS_LISTEN) {
                  /*
                   * When a listen socket is torn down the SO_ACCEPTCONN
                   * flag is removed first while connections are drained
                   * from the accept queue in a unlock/lock cycle of the
                   * ACCEPT_LOCK, opening a race condition allowing a SYN
                   * attempt go through unhandled.
                   */
                  goto dropunlock;
          }
  #if defined(IPSEC_SUPPORT) || defined(TCP_SIGNATURE)
          if (tp->t_flags & TF_SIGNATURE) {
                  tcp_dooptions(&to, optp, optlen, thflags);
                  if ((to.to_flags & TOF_SIGNATURE) == 0) {
                          TCPSTAT_INC(tcps_sig_err_nosigopt);
                          goto dropunlock;
                  }
                  if (!TCPMD5_ENABLED() ||
                      TCPMD5_INPUT(m, th, to.to_signature) != 0)
                          goto dropunlock;
          }
  #endif
          TCP_PROBE5(receive, NULL, tp, m, tp, th);
  
          /*
           * 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.
           */
          tp->t_fb->tfb_tcp_do_segment(m, th, so, tp, drop_hdrlen, tlen, iptos, ti_locked);
          INP_INFO_UNLOCK_ASSERT(&V_tcbinfo);
          return (IPPROTO_DONE);
  
  dropwithreset:
          TCP_PROBE5(receive, NULL, tp, m, tp, th);
  
          if (ti_locked == TI_RLOCKED) {
                  INP_INFO_RUNLOCK(&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 (m != NULL)
                  TCP_PROBE5(receive, NULL, tp, m, tp, th);
  
          if (ti_locked == TI_RLOCKED) {
                  INP_INFO_RUNLOCK(&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);
          return (IPPROTO_DONE);
  }
  
  /*
   * 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. Application has not set receive buffer size with
   *     SO_RCVBUF. Setting SO_RCVBUF clears SB_AUTOSIZE.
   *  2. the number of bytes received during the time it takes
   *     one timestamp to be reflected back to us (the RTT);
   *  3. received bytes per RTT is within seven eighth of the
   *     current socket buffer size;
   *  4. 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.
   */
  int
  tcp_autorcvbuf(struct mbuf *m, struct tcphdr *th, struct socket *so,
      struct tcpcb *tp, int tlen)
  {
          int newsize = 0;
  
          if (V_tcp_do_autorcvbuf && (so->so_rcv.sb_flags & SB_AUTOSIZE) &&
              tp->t_srtt != 0 && tp->rfbuf_ts != 0 &&
              TCP_TS_TO_TICKS(tcp_ts_getticks() - tp->rfbuf_ts) >
              (tp->t_srtt >> TCP_RTT_SHIFT)) {
                  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);
                  }
                  TCP_PROBE6(receive__autoresize, NULL, tp, m, tp, th, newsize);
  
                  /* Start over with next RTT. */
                  tp->rfbuf_ts = 0;
                  tp->rfbuf_cnt = 0;
          } else {
                  tp->rfbuf_cnt += tlen;  /* add up */
          }
  
          return (newsize);
  }
  
  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, sack_changed;
          int rstreason, todrop, win;
          u_long tiwin;
          char *s;
          struct in_conninfo *inc;
          struct mbuf *mfree;
          struct tcpopt to;
          int tfo_syn;
          
  #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;
          inc = &tp->t_inpcb->inp_inc;
          tp->sackhint.last_sack_ack = 0;
          sack_changed = 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 the tcbinfo to be in either alocked or unlocked, as the
           * caller may have unnecessarily acquired a write lock due to a race.
           */
          if ((thflags & (TH_SYN | TH_FIN | TH_RST)) != 0 ||
              tp->t_state != TCPS_ESTABLISHED) {
                  KASSERT(ti_locked == TI_RLOCKED, ("%s ti_locked %d for "
                      "SYN/FIN/RST/!EST", __func__, ti_locked));
                  INP_INFO_RLOCK_ASSERT(&V_tcbinfo);
          } else {
  #ifdef INVARIANTS
                  if (ti_locked == TI_RLOCKED)
                          INP_INFO_RLOCK_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__));
  
  #ifdef TCPPCAP
          /* Save segment, if requested. */
          tcp_pcap_add(th, m, &(tp->t_inpkts));
  #endif
  
          if ((thflags & TH_SYN) && (thflags & TH_FIN) && V_drop_synfin) {
                  if ((s = tcp_log_addrs(inc, th, NULL, NULL))) {
                          log(LOG_DEBUG, "%s; %s: "
                              "SYN|FIN segment ignored (based on "
                              "sysctl setting)\n", s, __func__);
                          free(s, M_TCPLOG);
                  }
                  goto drop;
          }
  
          /*
           * If a segment with the ACK-bit set arrives in the SYN-SENT state
           * check SEQ.ACK first.
           */
          if ((tp->t_state == TCPS_SYN_SENT) && (thflags & TH_ACK) &&
              (SEQ_LEQ(th->th_ack, tp->iss) || SEQ_GT(th->th_ack, tp->snd_max))) {
                  rstreason = BANDLIM_UNLIMITED;
                  goto dropwithreset;
          }
  
          /*
           * 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, TP_KEEPIDLE(tp));
  
          /*
           * Scale up 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;
                  }
  
                  /* Process a packet differently from RFC3168. */
                  cc_ecnpkt_handler(tp, th, iptos);
  
                  /* 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 defined(IPSEC_SUPPORT) || defined(TCP_SIGNATURE)
          if ((tp->t_flags & TF_SIGNATURE) != 0 &&
              (to.to_flags & TOF_SIGNATURE) == 0) {
                  TCPSTAT_INC(tcps_sig_err_sigopt);
                  /* XXX: should drop? */
          }
  #endif
          /*
           * 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, tcp_ts_getticks()))
                          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 = tcp_ts_getticks();
                  }
                  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;
          }
  
          /*
           * If timestamps were negotiated during SYN/ACK they should
           * appear on every segment during this session and vice versa.
           */
          if ((tp->t_flags & TF_RCVD_TSTMP) && !(to.to_flags & TOF_TS)) {
                  if ((s = tcp_log_addrs(inc, th, NULL, NULL))) {
                          log(LOG_DEBUG, "%s; %s: Timestamp missing, "
                              "no action\n", s, __func__);
                          free(s, M_TCPLOG);
                  }
          }
          if (!(tp->t_flags & TF_RCVD_TSTMP) && (to.to_flags & TOF_TS)) {
                  if ((s = tcp_log_addrs(inc, th, NULL, NULL))) {
                          log(LOG_DEBUG, "%s; %s: Timestamp not expected, "
                              "no action\n", s, __func__);
                          free(s, M_TCPLOG);
                  }
          }
  
          /*
           * 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 = tcp_ts_getticks();
                          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_RLOCKED)
                                          INP_INFO_RUNLOCK(&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) {
                                          u_int t;
  
                                          t = tcp_ts_getticks() - to.to_tsecr;
                                          if (!tp->t_rttlow || tp->t_rttlow > t)
                                                  tp->t_rttlow = t;
                                          tcp_xmit_timer(tp,
                                              TCP_TS_TO_TICKS(t) + 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);
  
                                  /*
                                   * 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
                                  TCP_PROBE3(debug__input, tp, th, m);
                                  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 (sbavail(&so->so_snd))
                                          (void) tp->t_fb->tfb_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_RLOCKED)
                                  INP_INFO_RUNLOCK(&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);
  #ifdef TCPDEBUG
                          if (so->so_options & SO_DEBUG)
                                  tcp_trace(TA_INPUT, ostate, tp,
                                      (void *)tcp_saveipgen, &tcp_savetcp, 0);
  #endif
                          TCP_PROBE3(debug__input, tp, th, m);
  
                          newsize = tcp_autorcvbuf(m, th, so, tp, tlen);
  
                          /* 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, 0);
                          }
                          /* NB: sorwakeup_locked() does an implicit unlock. */
                          sorwakeup_locked(so);
                          if (DELAY_ACK(tp, tlen)) {
                                  tp->t_flags |= TF_DELACK;
                          } else {
                                  tp->t_flags |= TF_ACKNOW;
                                  tp->t_fb->tfb_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;
          tp->rcv_wnd = imax(win, (int)(tp->rcv_adv - tp->rcv_nxt));
  
          switch (tp->t_state) {
  
          /*
           * If the state is SYN_RECEIVED:
           *      if seg contains an ACK, but not for our SYN/ACK, send a RST.
           */
          case TCPS_SYN_RECEIVED:
                  if ((thflags & TH_ACK) &&
                      (SEQ_LEQ(th->th_ack, tp->snd_una) ||
                       SEQ_GT(th->th_ack, tp->snd_max))) {
                                  rstreason = BANDLIM_RST_OPENPORT;
                                  goto dropwithreset;
                  }
  #ifdef TCP_RFC7413
                  if (tp->t_flags & TF_FASTOPEN) {
                          /*
                           * When a TFO connection is in SYN_RECEIVED, the
                           * only valid packets are the initial SYN, a
                           * retransmit/copy of the initial SYN (possibly with
                           * a subset of the original data), a valid ACK, a
                           * FIN, or a RST.
                           */
                          if ((thflags & (TH_SYN|TH_ACK)) == (TH_SYN|TH_ACK)) {
                                  rstreason = BANDLIM_RST_OPENPORT;
                                  goto dropwithreset;
                          } else if (thflags & TH_SYN) {
                                  /* non-initial SYN is ignored */
                                  if ((tcp_timer_active(tp, TT_DELACK) || 
                                       tcp_timer_active(tp, TT_REXMT)))
                                          goto drop;
                          } else if (!(thflags & (TH_ACK|TH_FIN|TH_RST))) {
                                  goto drop;
                          }
                  }
  #endif
                  break;
  
          /*
           * If the state is SYN_SENT:
           *      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|TH_RST)) == (TH_ACK|TH_RST)) {
                          TCP_PROBE5(connect__refused, NULL, tp,
                              m, tp, th);
                          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) && 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) {
                                  tcp_state_change(tp, TCPS_FIN_WAIT_1);
                                  tp->t_flags &= ~TF_NEEDFIN;
                                  thflags &= ~TH_SYN;
                          } else {
                                  tcp_state_change(tp, TCPS_ESTABLISHED);
                                  TCP_PROBE5(connect__established, NULL, tp,
                                      m, tp, th);
                                  cc_conn_init(tp);
                                  tcp_timer_activate(tp, TT_KEEP,
                                      TP_KEEPIDLE(tp));
                          }
                  } else {
                          /*
                           * Received initial SYN in SYN-SENT[*] state =>
                           * simultaneous open.
                           * If it succeeds, connection is * half-synchronized.
                           * Otherwise, do 3-way handshake:
                           *        SYN-SENT -> SYN-RECEIVED
                           *        SYN-SENT* -> SYN-RECEIVED*
                           */
                          tp->t_flags |= (TF_ACKNOW | TF_NEEDSYN);
                          tcp_timer_activate(tp, TT_REXMT, 0);
                          tcp_state_change(tp, TCPS_SYN_RECEIVED);
                  }
  
                  KASSERT(ti_locked == TI_RLOCKED, ("%s: trimthenstep6: "
                      "ti_locked %d", __func__, ti_locked));
                  INP_INFO_RLOCK_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 (thflags & TH_RST) {
                  /*
                   * RFC5961 Section 3.2
                   *
                   * - RST drops connection only if SEG.SEQ == RCV.NXT.
                   * - If RST is in window, we send challenge ACK.
                   *
                   * Note: to take into account delayed ACKs, we should
                   *   test against last_ack_sent instead of rcv_nxt.
                   * Note 2: we handle special case of closed window, not
                   *   covered by the RFC.
                   */
                  if ((SEQ_GEQ(th->th_seq, tp->last_ack_sent) &&
                      SEQ_LT(th->th_seq, tp->last_ack_sent + tp->rcv_wnd)) ||
                      (tp->rcv_wnd == 0 && tp->last_ack_sent == th->th_seq)) {
  
                          INP_INFO_RLOCK_ASSERT(&V_tcbinfo);
                          KASSERT(ti_locked == TI_RLOCKED,
                              ("%s: TH_RST ti_locked %d, th %p tp %p",
                              __func__, ti_locked, th, tp));
                          KASSERT(tp->t_state != TCPS_SYN_SENT,
                              ("%s: TH_RST for TCPS_SYN_SENT th %p tp %p",
                              __func__, th, tp));
  
                          if (V_tcp_insecure_rst ||
                              tp->last_ack_sent == th->th_seq) {
                                  TCPSTAT_INC(tcps_drops);
                                  /* Drop the connection. */
                                  switch (tp->t_state) {
                                  case TCPS_SYN_RECEIVED:
                                          so->so_error = ECONNREFUSED;
                                          goto close;
                                  case TCPS_ESTABLISHED:
                                  case TCPS_FIN_WAIT_1:
                                  case TCPS_FIN_WAIT_2:
                                  case TCPS_CLOSE_WAIT:
                                  case TCPS_CLOSING:
                                  case TCPS_LAST_ACK:
                                          so->so_error = ECONNRESET;
                                  close:
                                          /* FALLTHROUGH */
                                  default:
                                          tp = tcp_close(tp);
                                  }
                          } else {
                                  TCPSTAT_INC(tcps_badrst);
                                  /* Send challenge ACK. */
                                  tcp_respond(tp, mtod(m, void *), th, m,
                                      tp->rcv_nxt, tp->snd_nxt, TH_ACK);
                                  tp->last_ack_sent = tp->rcv_nxt;
                                  m = NULL;
                          }
                  }
                  goto drop;
          }
  
          /*
           * RFC5961 Section 4.2
           * Send challenge ACK for any SYN in synchronized state.
           */
          if ((thflags & TH_SYN) && tp->t_state != TCPS_SYN_SENT &&
              tp->t_state != TCPS_SYN_RECEIVED) {
                  KASSERT(ti_locked == TI_RLOCKED,
                      ("tcp_do_segment: TH_SYN ti_locked %d", ti_locked));
                  INP_INFO_RLOCK_ASSERT(&V_tcbinfo);
  
                  TCPSTAT_INC(tcps_badsyn);
                  if (V_tcp_insecure_syn &&
                      SEQ_GEQ(th->th_seq, tp->last_ack_sent) &&
                      SEQ_LT(th->th_seq, tp->last_ack_sent + tp->rcv_wnd)) {
                          tp = tcp_drop(tp, ECONNRESET);
                          rstreason = BANDLIM_UNLIMITED;
                  } else {
                          /* Send challenge ACK. */
                          tcp_respond(tp, mtod(m, void *), th, m, tp->rcv_nxt,
                              tp->snd_nxt, TH_ACK);
                          tp->last_ack_sent = tp->rcv_nxt;
                          m = NULL;
                  }
                  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 (tcp_ts_getticks() - 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 (thflags & TH_SYN) {
                          thflags &= ~TH_SYN;
                          th->th_seq++;
                          if (th->th_urp > 1)
                                  th->th_urp--;
                          else
                                  thflags &= ~TH_URG;
                          todrop--;
                  }
                  /*
                   * Following if statement from Stevens, vol. 2, p. 960.
                   */
                  if (todrop > tlen
                      || (todrop == tlen && (thflags & TH_FIN) == 0)) {
                          /*
                           * Any valid FIN must be to the left of the window.
                           * At this point the FIN must be a duplicate or out
                           * of sequence; drop it.
                           */
                          thflags &= ~TH_FIN;
  
                          /*
                           * Send an ACK to resynchronize and drop any data.
                           * But keep on processing for RST or ACK.
                           */
                          tp->t_flags |= TF_ACKNOW;
                          todrop = tlen;
                          TCPSTAT_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) {
                  KASSERT(ti_locked == TI_RLOCKED, ("%s: SS_NOFDEREF && "
                      "CLOSE_WAIT && tlen ti_locked %d", __func__, ti_locked));
                  INP_INFO_RLOCK_ASSERT(&V_tcbinfo);
  
                  if ((s = tcp_log_addrs(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 = tcp_ts_getticks();
                  tp->ts_recent = to.to_tsval;
          }
  
          /*
           * 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)) {
  #ifdef TCP_RFC7413
                          if (tp->t_state == TCPS_SYN_RECEIVED &&
                              tp->t_flags & TF_FASTOPEN) {
                                  tp->snd_wnd = tiwin;
                                  cc_conn_init(tp);
                          }
  #endif
                          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) {
                          tcp_state_change(tp, TCPS_FIN_WAIT_1);
                          tp->t_flags &= ~TF_NEEDFIN;
                  } else {
                          tcp_state_change(tp, TCPS_ESTABLISHED);
                          TCP_PROBE5(accept__established, NULL, tp,
                              m, tp, th);
  #ifdef TCP_RFC7413
                          if (tp->t_tfo_pending) {
                                  tcp_fastopen_decrement_counter(tp->t_tfo_pending);
                                  tp->t_tfo_pending = NULL;
  
                                  /*
                                   * Account for the ACK of our SYN prior to
                                   * regular ACK processing below.
                                   */ 
                                  tp->snd_una++;
                          }
                          /*
                           * TFO connections call cc_conn_init() during SYN
                           * processing.  Calling it again here for such
                           * connections is not harmless as it would undo the
                           * snd_cwnd reduction that occurs when a TFO SYN|ACK
                           * is retransmitted.
                           */
                          if (!(tp->t_flags & TF_FASTOPEN))
  #endif
                                  cc_conn_init(tp);
                          tcp_timer_activate(tp, TT_KEEP, TP_KEEPIDLE(tp));
                  }
                  /*
                   * If segment contains data or ACK, will call tcp_reass()
                   * later; if not, do so now to pass queued data to user.
                   */
                  if (tlen == 0 && (thflags & TH_FIN) == 0)
                          (void) tcp_reass(tp, (struct tcphdr *)0, 0,
                              (struct mbuf *)0);
                  tp->snd_wl1 = th->th_seq - 1;
                  /* FALLTHROUGH */
  
          /*
           * In ESTABLISHED state: drop duplicate ACKs; ACK out of range
           * ACKs.  If the ack is in the range
           *      tp->snd_una < th->th_ack <= tp->snd_max
           * then advance tp->snd_una to th->th_ack and drop
           * data from the retransmission queue.  If this ACK reflects
           * more up to date window information we update our window information.
           */
          case TCPS_ESTABLISHED:
          case TCPS_FIN_WAIT_1:
          case TCPS_FIN_WAIT_2:
          case TCPS_CLOSE_WAIT:
          case TCPS_CLOSING:
          case TCPS_LAST_ACK:
                  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)))
                          sack_changed = tcp_sack_doack(tp, &to, th->th_ack);
                  else
                          /*
                           * Reset the value so that previous (valid) value
                           * from the last ack with SACK doesn't get used.
                           */
                          tp->sackhint.sacked_bytes = 0;
  
                  /* Run HHOOK_TCP_ESTABLISHED_IN helper hooks. */
                  hhook_run_tcp_est_in(tp, th, &to);
  
                  if (SEQ_LEQ(th->th_ack, tp->snd_una)) {
                          u_int maxseg;
  
                          maxseg = tcp_maxseg(tp);
                          if (tlen == 0 &&
                              (tiwin == tp->snd_wnd ||
                              (tp->t_flags & TF_SACK_PERMIT))) {
                                  /*
                                   * If this is the first time we've seen a
                                   * FIN from the remote, this is not a
                                   * duplicate and it needs to be processed
                                   * normally.  This happens during a
                                   * simultaneous close.
                                   */
                                  if ((thflags & TH_FIN) &&
                                      (TCPS_HAVERCVDFIN(tp->t_state) == 0)) {
                                          tp->t_dupacks = 0;
                                          break;
                                  }
                                  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 and FIN isn't set),
                                   * the ack is the biggest we've
                                   * seen and we've seen exactly our rexmt
                                   * threshold of them, assume a packet
                                   * has been dropped and retransmit it.
                                   * Kludge snd_nxt & the congestion
                                   * window so we send only this one
                                   * packet.
                                   *
                                   * 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.
                                   */
                                  /*
                                   * Following 2 kinds of acks should not affect
                                   * dupack counting:
                                   * 1) Old acks
                                   * 2) Acks with SACK but without any new SACK
                                   * information in them. These could result from
                                   * any anomaly in the network like a switch
                                   * duplicating packets or a possible DoS attack.
                                   */
                                  if (th->th_ack != tp->snd_una ||
                                      ((tp->t_flags & TF_SACK_PERMIT) &&
                                      !sack_changed))
                                          break;
                                  else if (!tcp_timer_active(tp, TT_REXMT))
                                          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.
                                                   */
                                                  if (V_tcp_do_rfc6675_pipe)
                                                          awnd = tcp_compute_pipe(tp);
                                                  else
                                                          awnd = (tp->snd_nxt - tp->snd_fack) +
                                                                  tp->sackhint.sack_bytes_rexmit;
  
                                                  if (awnd < tp->snd_ssthresh) {
                                                          tp->snd_cwnd += maxseg;
                                                          if (tp->snd_cwnd > tp->snd_ssthresh)
                                                                  tp->snd_cwnd = tp->snd_ssthresh;
                                                  }
                                          } else
                                                  tp->snd_cwnd += maxseg;
                                          (void) tp->t_fb->tfb_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 = maxseg;
                                                  (void) tp->t_fb->tfb_tcp_output(tp);
                                                  goto drop;
                                          }
                                          tp->snd_nxt = th->th_ack;
                                          tp->snd_cwnd = maxseg;
                                          (void) tp->t_fb->tfb_tcp_output(tp);
                                          KASSERT(tp->snd_limited <= 2,
                                              ("%s: tp->snd_limited too big",
                                              __func__));
                                          tp->snd_cwnd = tp->snd_ssthresh +
                                               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) {
                                          /*
                                           * Process first and second duplicate
                                           * ACKs. Each indicates a segment
                                           * leaving the network, creating room
                                           * for more. Make sure we can send a
                                           * packet on reception of each duplicate
                                           * ACK by increasing snd_cwnd by one
                                           * segment. Restore the original
                                           * snd_cwnd after packet transmission.
                                           */
                                          cc_ack_received(tp, th, CC_DUPACK);
                                          u_long oldcwnd = tp->snd_cwnd;
                                          tcp_seq oldsndmax = tp->snd_max;
                                          u_int sent;
                                          int avail;
  
                                          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) *
                                              maxseg;
                                          /*
                                           * Only call tcp_output when there
                                           * is new data available to be sent.
                                           * Otherwise we would send pure ACKs.
                                           */
                                          SOCKBUF_LOCK(&so->so_snd);
                                          avail = sbavail(&so->so_snd) -
                                              (tp->snd_nxt - tp->snd_una);
                                          SOCKBUF_UNLOCK(&so->so_snd);
                                          if (avail > 0)
                                                  (void) tp->t_fb->tfb_tcp_output(tp);
                                          sent = tp->snd_max - oldsndmax;
                                          if (sent > maxseg) {
                                                  KASSERT((tp->t_dupacks == 2 &&
                                                      tp->snd_limited == 0) ||
                                                     (sent == 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;
                                  }
                          }
                          break;
                  } else {
                          /*
                           * This ack is advancing the left edge, reset the
                           * counter.
                           */
                          tp->t_dupacks = 0;
                          /*
                           * If this ack also has new SACK info, increment the
                           * counter as per rfc6675.
                           */
                          if ((tp->t_flags & TF_SACK_PERMIT) && sack_changed)
                                  tp->t_dupacks++;
                  }
  
                  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);
                  }
                  /*
                   * 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);
                  KASSERT(acked >= 0, ("%s: acked unexepectedly negative "
                      "(tp->snd_una=%u, th->th_ack=%u, tp=%p, m=%p)", __func__,
                      tp->snd_una, th->th_ack, tp, m));
                  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) {
                          u_int t;
  
                          t = tcp_ts_getticks() - to.to_tsecr;
                          if (!tp->t_rttlow || tp->t_rttlow > t)
                                  tp->t_rttlow = t;
                          tcp_xmit_timer(tp, TCP_TS_TO_TICKS(t) + 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 > sbavail(&so->so_snd)) {
                          if (tp->snd_wnd >= sbavail(&so->so_snd))
                                  tp->snd_wnd -= sbavail(&so->so_snd);
                          else
                                  tp->snd_wnd = 0;
                          mfree = sbcut_locked(&so->so_snd,
                              (int)sbavail(&so->so_snd));
                          ourfinisacked = 1;
                  } else {
                          mfree = sbcut_locked(&so->so_snd, acked);
                          if (tp->snd_wnd >= (u_long) acked)
                                  tp->snd_wnd -= acked;
                          else
                                  tp->snd_wnd = 0;
                          ourfinisacked = 0;
                  }
                  /* NB: sowwakeup_locked() does an implicit unlock. */
                  sowwakeup_locked(so);
                  m_freem(mfree);
                  /* 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) {
                                          soisdisconnected(so);
                                          tcp_timer_activate(tp, TT_2MSL,
                                              (tcp_fast_finwait2_recycle ?
                                              tcp_finwait2_timeout :
                                              TP_MAXIDLE(tp)));
                                  }
                                  tcp_state_change(tp, 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_RLOCK_ASSERT(&V_tcbinfo);
                                  tcp_twstart(tp);
                                  INP_INFO_RUNLOCK(&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_RLOCK_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 + sbavail(&so->so_rcv) > 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 = sbavail(&so->so_rcv) +
                              (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.
           */
          tfo_syn = ((tp->t_state == TCPS_SYN_RECEIVED) &&
                     (tp->t_flags & TF_FASTOPEN));
          if ((tlen || (thflags & TH_FIN) || tfo_syn) &&
              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) ||
                       tfo_syn)) {
                          if (DELAY_ACK(tp, tlen) || tfo_syn)
                                  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);
                          SOCKBUF_LOCK(&so->so_rcv);
                          if (so->so_rcv.sb_state & SBS_CANTRCVMORE)
                                  m_freem(m);
                          else
                                  sbappendstream_locked(&so->so_rcv, m, 0);
                          /* 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:
                          tcp_state_change(tp, 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:
                          tcp_state_change(tp, 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_RLOCK_ASSERT(&V_tcbinfo);
                          KASSERT(ti_locked == TI_RLOCKED, ("%s: dodata "
                              "TCP_FIN_WAIT_2 ti_locked: %d", __func__,
                              ti_locked));
  
                          tcp_twstart(tp);
                          INP_INFO_RUNLOCK(&V_tcbinfo);
                          return;
                  }
          }
          if (ti_locked == TI_RLOCKED)
                  INP_INFO_RUNLOCK(&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
          TCP_PROBE3(debug__input, tp, th, m);
  
          /*
           * Return any desired output.
           */
          if (needoutput || (tp->t_flags & TF_ACKNOW))
                  (void) tp->t_fb->tfb_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
          TCP_PROBE3(debug__input, tp, th, m);
          if (ti_locked == TI_RLOCKED)
                  INP_INFO_RUNLOCK(&V_tcbinfo);
          ti_locked = TI_UNLOCKED;
  
          tp->t_flags |= TF_ACKNOW;
          (void) tp->t_fb->tfb_tcp_output(tp);
          INP_WUNLOCK(tp->t_inpcb);
          m_freem(m);
          return;
  
  dropwithreset:
          if (ti_locked == TI_RLOCKED)
                  INP_INFO_RUNLOCK(&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_RLOCKED) {
                  INP_INFO_RUNLOCK(&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
          TCP_PROBE3(debug__input, tp, th, m);
          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.
   */
  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++;
                  if (th->th_flags & TH_FIN)
                          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.
   */
  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;
                  case TCPOPT_SIGNATURE:
                          /*
                           * 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.
                           */
                          if (optlen != TCPOLEN_SIGNATURE)
                                  continue;
                          to->to_flags |= TOF_SIGNATURE;
                          to->to_signature = cp + 2;
                          break;
                  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;
  #ifdef TCP_RFC7413
                  case TCPOPT_FAST_OPEN:
                          if ((optlen != TCPOLEN_FAST_OPEN_EMPTY) &&
                              (optlen < TCPOLEN_FAST_OPEN_MIN) &&
                              (optlen > TCPOLEN_FAST_OPEN_MAX))
                                  continue;
                          if (!(flags & TO_SYN))
                                  continue;
                          if (!V_tcp_fastopen_enabled)
                                  continue;
                          to->to_flags |= TOF_FASTOPEN;
                          to->to_tfo_len = optlen - 2;
                          to->to_tfo_cookie = to->to_tfo_len ? cp + 2 : NULL;
                          break;
  #endif
                  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.
   */
  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.
   */
  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 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.
   *
   * NOTE that resulting t_maxseg doesn't include space for TCP options or
   * IP options, e.g. IPSEC data, since length of this data may vary, and
   * thus it is calculated for every segment separately in tcp_output().
   *
   * NOTE that this routine is only called when we process an incoming
   * segment, or an ICMP need fragmentation datagram. Outgoing SYN/ACK MSS
   * settings are handled in tcp_mssopt().
   */
  void
  tcp_mss_update(struct tcpcb *tp, int offer, int mtuoffer,
      struct hc_metrics_lite *metricptr, struct tcp_ifcap *cap)
  {
          int mss = 0;
          u_long maxmtu = 0;
          struct inpcb *inp = tp->t_inpcb;
          struct hc_metrics_lite metrics;
  #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);
  
          if (mtuoffer != -1) {
                  KASSERT(offer == -1, ("%s: conflict", __func__));
                  offer = mtuoffer - min_protoh;
          }
  
          /* Initialize. */
  #ifdef INET6
          if (isipv6) {
                  maxmtu = tcp_maxmtu6(&inp->inp_inc, cap);
                  tp->t_maxseg = V_tcp_v6mssdflt;
          }
  #endif
  #if defined(INET) && defined(INET6)
          else
  #endif
  #ifdef INET
          {
                  maxmtu = tcp_maxmtu(&inp->inp_inc, cap);
                  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_maxseg above.
                           */
                          offer = tp->t_maxseg;
                          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 in 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 maxseg 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.
           *
           * XXXGL: shouldn't we reserve space for IP/IPv6 options?
           */
          mss = max(mss, 64);
  
          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;
          struct tcp_ifcap cap;
  
          KASSERT(tp != NULL, ("%s: tp == NULL", __func__));
  
          bzero(&cap, sizeof(cap));
          tcp_mss_update(tp, offer, -1, &metrics, &cap);
  
          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 == V_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);
          /*
           * Sanity check: make sure that maxseg 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.
           *
           * XXXGL: shouldn't we reserve space for IP/IPv6 options?
           */
          tp->t_maxseg = max(mss, 64);
  
          SOCKBUF_LOCK(&so->so_rcv);
          if ((so->so_rcv.sb_hiwat == V_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 (cap.ifcap & CSUM_TSO) {
                  tp->t_flags |= TF_TSO;
                  tp->t_tsomax = cap.tsomax;
                  tp->t_tsomaxsegcount = cap.tsomaxsegcount;
                  tp->t_tsomaxsegsize = cap.tsomaxsegsize;
          }
  }
  
  /*
   * 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);
                  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);
                  min_protoh = sizeof(struct tcpiphdr);
          }
  #endif
  #if defined(INET6) || defined(INET)
          thcmtu = tcp_hc_getmtu(inc); /* IPv4 and IPv6 */
  #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.
   */
  void
  tcp_newreno_partial_ack(struct tcpcb *tp, struct tcphdr *th)
  {
          tcp_seq onxt = tp->snd_nxt;
          u_long ocwnd = tp->snd_cwnd;
          u_int maxseg = tcp_maxseg(tp);
  
          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 = maxseg + BYTES_THIS_ACK(tp, th);
          tp->t_flags |= TF_ACKNOW;
          (void) tp->t_fb->tfb_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 += maxseg;
  }
  
  int
  tcp_compute_pipe(struct tcpcb *tp)
  {
          return (tp->snd_max - tp->snd_una +
                  tp->sackhint.sack_bytes_rexmit -
                  tp->sackhint.sacked_bytes);
  }