The Design and Implementation of the FreeBSD Operating System, Second Edition
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FreeBSD/Linux Kernel Cross Reference
sys/netinet/tcp_input.c

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    1 /*-
    2  * Copyright (c) 1982, 1986, 1988, 1990, 1993, 1994, 1995
    3  *      The Regents of the University of California.  All rights reserved.
    4  * Copyright (c) 2007-2008,2010
    5  *      Swinburne University of Technology, Melbourne, Australia.
    6  * Copyright (c) 2009-2010 Lawrence Stewart <lstewart@freebsd.org>
    7  * Copyright (c) 2010 The FreeBSD Foundation
    8  * Copyright (c) 2010-2011 Juniper Networks, Inc.
    9  * All rights reserved.
   10  *
   11  * Portions of this software were developed at the Centre for Advanced Internet
   12  * Architectures, Swinburne University of Technology, by Lawrence Stewart,
   13  * James Healy and David Hayes, made possible in part by a grant from the Cisco
   14  * University Research Program Fund at Community Foundation Silicon Valley.
   15  *
   16  * Portions of this software were developed at the Centre for Advanced
   17  * Internet Architectures, Swinburne University of Technology, Melbourne,
   18  * Australia by David Hayes under sponsorship from the FreeBSD Foundation.
   19  *
   20  * Portions of this software were developed by Robert N. M. Watson under
   21  * contract to Juniper Networks, Inc.
   22  *
   23  * Redistribution and use in source and binary forms, with or without
   24  * modification, are permitted provided that the following conditions
   25  * are met:
   26  * 1. Redistributions of source code must retain the above copyright
   27  *    notice, this list of conditions and the following disclaimer.
   28  * 2. Redistributions in binary form must reproduce the above copyright
   29  *    notice, this list of conditions and the following disclaimer in the
   30  *    documentation and/or other materials provided with the distribution.
   31  * 4. Neither the name of the University nor the names of its contributors
   32  *    may be used to endorse or promote products derived from this software
   33  *    without specific prior written permission.
   34  *
   35  * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
   36  * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
   37  * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
   38  * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
   39  * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
   40  * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
   41  * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
   42  * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
   43  * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
   44  * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
   45  * SUCH DAMAGE.
   46  *
   47  *      @(#)tcp_input.c 8.12 (Berkeley) 5/24/95
   48  */
   49 
   50 #include <sys/cdefs.h>
   51 __FBSDID("$FreeBSD: head/sys/netinet/tcp_input.c 275358 2014-12-01 11:45:24Z hselasky $");
   52 
   53 #include "opt_ipfw.h"           /* for ipfw_fwd */
   54 #include "opt_inet.h"
   55 #include "opt_inet6.h"
   56 #include "opt_ipsec.h"
   57 #include "opt_tcpdebug.h"
   58 
   59 #include <sys/param.h>
   60 #include <sys/kernel.h>
   61 #include <sys/hhook.h>
   62 #include <sys/malloc.h>
   63 #include <sys/mbuf.h>
   64 #include <sys/proc.h>           /* for proc0 declaration */
   65 #include <sys/protosw.h>
   66 #include <sys/sdt.h>
   67 #include <sys/signalvar.h>
   68 #include <sys/socket.h>
   69 #include <sys/socketvar.h>
   70 #include <sys/sysctl.h>
   71 #include <sys/syslog.h>
   72 #include <sys/systm.h>
   73 
   74 #include <machine/cpu.h>        /* before tcp_seq.h, for tcp_random18() */
   75 
   76 #include <vm/uma.h>
   77 
   78 #include <net/if.h>
   79 #include <net/if_var.h>
   80 #include <net/route.h>
   81 #include <net/vnet.h>
   82 
   83 #define TCPSTATES               /* for logging */
   84 
   85 #include <netinet/cc.h>
   86 #include <netinet/in.h>
   87 #include <netinet/in_kdtrace.h>
   88 #include <netinet/in_pcb.h>
   89 #include <netinet/in_systm.h>
   90 #include <netinet/in_var.h>
   91 #include <netinet/ip.h>
   92 #include <netinet/ip_icmp.h>    /* required for icmp_var.h */
   93 #include <netinet/icmp_var.h>   /* for ICMP_BANDLIM */
   94 #include <netinet/ip_var.h>
   95 #include <netinet/ip_options.h>
   96 #include <netinet/ip6.h>
   97 #include <netinet/icmp6.h>
   98 #include <netinet6/in6_pcb.h>
   99 #include <netinet6/ip6_var.h>
  100 #include <netinet6/nd6.h>
  101 #include <netinet/tcp_fsm.h>
  102 #include <netinet/tcp_seq.h>
  103 #include <netinet/tcp_timer.h>
  104 #include <netinet/tcp_var.h>
  105 #include <netinet6/tcp6_var.h>
  106 #include <netinet/tcpip.h>
  107 #include <netinet/tcp_syncache.h>
  108 #ifdef TCPDEBUG
  109 #include <netinet/tcp_debug.h>
  110 #endif /* TCPDEBUG */
  111 #ifdef TCP_OFFLOAD
  112 #include <netinet/tcp_offload.h>
  113 #endif
  114 
  115 #ifdef IPSEC
  116 #include <netipsec/ipsec.h>
  117 #include <netipsec/ipsec6.h>
  118 #endif /*IPSEC*/
  119 
  120 #include <machine/in_cksum.h>
  121 
  122 #include <security/mac/mac_framework.h>
  123 
  124 const int tcprexmtthresh = 3;
  125 
  126 int tcp_log_in_vain = 0;
  127 SYSCTL_INT(_net_inet_tcp, OID_AUTO, log_in_vain, CTLFLAG_RW,
  128     &tcp_log_in_vain, 0,
  129     "Log all incoming TCP segments to closed ports");
  130 
  131 VNET_DEFINE(int, blackhole) = 0;
  132 #define V_blackhole             VNET(blackhole)
  133 SYSCTL_INT(_net_inet_tcp, OID_AUTO, blackhole, CTLFLAG_VNET | CTLFLAG_RW,
  134     &VNET_NAME(blackhole), 0,
  135     "Do not send RST on segments to closed ports");
  136 
  137 VNET_DEFINE(int, tcp_delack_enabled) = 1;
  138 SYSCTL_INT(_net_inet_tcp, OID_AUTO, delayed_ack, CTLFLAG_VNET | CTLFLAG_RW,
  139     &VNET_NAME(tcp_delack_enabled), 0,
  140     "Delay ACK to try and piggyback it onto a data packet");
  141 
  142 VNET_DEFINE(int, drop_synfin) = 0;
  143 #define V_drop_synfin           VNET(drop_synfin)
  144 SYSCTL_INT(_net_inet_tcp, OID_AUTO, drop_synfin, CTLFLAG_VNET | CTLFLAG_RW,
  145     &VNET_NAME(drop_synfin), 0,
  146     "Drop TCP packets with SYN+FIN set");
  147 
  148 VNET_DEFINE(int, tcp_do_rfc3042) = 1;
  149 #define V_tcp_do_rfc3042        VNET(tcp_do_rfc3042)
  150 SYSCTL_INT(_net_inet_tcp, OID_AUTO, rfc3042, CTLFLAG_VNET | CTLFLAG_RW,
  151     &VNET_NAME(tcp_do_rfc3042), 0,
  152     "Enable RFC 3042 (Limited Transmit)");
  153 
  154 VNET_DEFINE(int, tcp_do_rfc3390) = 1;
  155 SYSCTL_INT(_net_inet_tcp, OID_AUTO, rfc3390, CTLFLAG_VNET | CTLFLAG_RW,
  156     &VNET_NAME(tcp_do_rfc3390), 0,
  157     "Enable RFC 3390 (Increasing TCP's Initial Congestion Window)");
  158 
  159 SYSCTL_NODE(_net_inet_tcp, OID_AUTO, experimental, CTLFLAG_RW, 0,
  160     "Experimental TCP extensions");
  161 
  162 VNET_DEFINE(int, tcp_do_initcwnd10) = 1;
  163 SYSCTL_INT(_net_inet_tcp_experimental, OID_AUTO, initcwnd10, CTLFLAG_VNET | CTLFLAG_RW,
  164     &VNET_NAME(tcp_do_initcwnd10), 0,
  165     "Enable RFC 6928 (Increasing initial CWND to 10)");
  166 
  167 VNET_DEFINE(int, tcp_do_rfc3465) = 1;
  168 SYSCTL_INT(_net_inet_tcp, OID_AUTO, rfc3465, CTLFLAG_VNET | CTLFLAG_RW,
  169     &VNET_NAME(tcp_do_rfc3465), 0,
  170     "Enable RFC 3465 (Appropriate Byte Counting)");
  171 
  172 VNET_DEFINE(int, tcp_abc_l_var) = 2;
  173 SYSCTL_INT(_net_inet_tcp, OID_AUTO, abc_l_var, CTLFLAG_VNET | CTLFLAG_RW,
  174     &VNET_NAME(tcp_abc_l_var), 2,
  175     "Cap the max cwnd increment during slow-start to this number of segments");
  176 
  177 static SYSCTL_NODE(_net_inet_tcp, OID_AUTO, ecn, CTLFLAG_RW, 0, "TCP ECN");
  178 
  179 VNET_DEFINE(int, tcp_do_ecn) = 0;
  180 SYSCTL_INT(_net_inet_tcp_ecn, OID_AUTO, enable, CTLFLAG_VNET | CTLFLAG_RW,
  181     &VNET_NAME(tcp_do_ecn), 0,
  182     "TCP ECN support");
  183 
  184 VNET_DEFINE(int, tcp_ecn_maxretries) = 1;
  185 SYSCTL_INT(_net_inet_tcp_ecn, OID_AUTO, maxretries, CTLFLAG_VNET | CTLFLAG_RW,
  186     &VNET_NAME(tcp_ecn_maxretries), 0,
  187     "Max retries before giving up on ECN");
  188 
  189 VNET_DEFINE(int, tcp_insecure_syn) = 0;
  190 #define V_tcp_insecure_syn      VNET(tcp_insecure_syn)
  191 SYSCTL_INT(_net_inet_tcp, OID_AUTO, insecure_syn, CTLFLAG_VNET | CTLFLAG_RW,
  192     &VNET_NAME(tcp_insecure_syn), 0,
  193     "Follow RFC793 instead of RFC5961 criteria for accepting SYN packets");
  194 
  195 VNET_DEFINE(int, tcp_insecure_rst) = 0;
  196 #define V_tcp_insecure_rst      VNET(tcp_insecure_rst)
  197 SYSCTL_INT(_net_inet_tcp, OID_AUTO, insecure_rst, CTLFLAG_VNET | CTLFLAG_RW,
  198     &VNET_NAME(tcp_insecure_rst), 0,
  199     "Follow RFC793 instead of RFC5961 criteria for accepting RST packets");
  200 
  201 VNET_DEFINE(int, tcp_recvspace) = 1024*64;
  202 #define V_tcp_recvspace VNET(tcp_recvspace)
  203 SYSCTL_INT(_net_inet_tcp, TCPCTL_RECVSPACE, recvspace, CTLFLAG_VNET | CTLFLAG_RW,
  204     &VNET_NAME(tcp_recvspace), 0, "Initial receive socket buffer size");
  205 
  206 VNET_DEFINE(int, tcp_do_autorcvbuf) = 1;
  207 #define V_tcp_do_autorcvbuf     VNET(tcp_do_autorcvbuf)
  208 SYSCTL_INT(_net_inet_tcp, OID_AUTO, recvbuf_auto, CTLFLAG_VNET | CTLFLAG_RW,
  209     &VNET_NAME(tcp_do_autorcvbuf), 0,
  210     "Enable automatic receive buffer sizing");
  211 
  212 VNET_DEFINE(int, tcp_autorcvbuf_inc) = 16*1024;
  213 #define V_tcp_autorcvbuf_inc    VNET(tcp_autorcvbuf_inc)
  214 SYSCTL_INT(_net_inet_tcp, OID_AUTO, recvbuf_inc, CTLFLAG_VNET | CTLFLAG_RW,
  215     &VNET_NAME(tcp_autorcvbuf_inc), 0,
  216     "Incrementor step size of automatic receive buffer");
  217 
  218 VNET_DEFINE(int, tcp_autorcvbuf_max) = 2*1024*1024;
  219 #define V_tcp_autorcvbuf_max    VNET(tcp_autorcvbuf_max)
  220 SYSCTL_INT(_net_inet_tcp, OID_AUTO, recvbuf_max, CTLFLAG_VNET | CTLFLAG_RW,
  221     &VNET_NAME(tcp_autorcvbuf_max), 0,
  222     "Max size of automatic receive buffer");
  223 
  224 VNET_DEFINE(struct inpcbhead, tcb);
  225 #define tcb6    tcb  /* for KAME src sync over BSD*'s */
  226 VNET_DEFINE(struct inpcbinfo, tcbinfo);
  227 
  228 static void      tcp_dooptions(struct tcpopt *, u_char *, int, int);
  229 static void      tcp_do_segment(struct mbuf *, struct tcphdr *,
  230                      struct socket *, struct tcpcb *, int, int, uint8_t,
  231                      int);
  232 static void      tcp_dropwithreset(struct mbuf *, struct tcphdr *,
  233                      struct tcpcb *, int, int);
  234 static void      tcp_pulloutofband(struct socket *,
  235                      struct tcphdr *, struct mbuf *, int);
  236 static void      tcp_xmit_timer(struct tcpcb *, int);
  237 static void      tcp_newreno_partial_ack(struct tcpcb *, struct tcphdr *);
  238 static void inline      cc_ack_received(struct tcpcb *tp, struct tcphdr *th,
  239                             uint16_t type);
  240 static void inline      cc_conn_init(struct tcpcb *tp);
  241 static void inline      cc_post_recovery(struct tcpcb *tp, struct tcphdr *th);
  242 static void inline      hhook_run_tcp_est_in(struct tcpcb *tp,
  243                             struct tcphdr *th, struct tcpopt *to);
  244 
  245 /*
  246  * TCP statistics are stored in an "array" of counter(9)s.
  247  */
  248 VNET_PCPUSTAT_DEFINE(struct tcpstat, tcpstat);
  249 VNET_PCPUSTAT_SYSINIT(tcpstat);
  250 SYSCTL_VNET_PCPUSTAT(_net_inet_tcp, TCPCTL_STATS, stats, struct tcpstat,
  251     tcpstat, "TCP statistics (struct tcpstat, netinet/tcp_var.h)");
  252 
  253 #ifdef VIMAGE
  254 VNET_PCPUSTAT_SYSUNINIT(tcpstat);
  255 #endif /* VIMAGE */
  256 /*
  257  * Kernel module interface for updating tcpstat.  The argument is an index
  258  * into tcpstat treated as an array.
  259  */
  260 void
  261 kmod_tcpstat_inc(int statnum)
  262 {
  263 
  264         counter_u64_add(VNET(tcpstat)[statnum], 1);
  265 }
  266 
  267 /*
  268  * Wrapper for the TCP established input helper hook.
  269  */
  270 static void inline
  271 hhook_run_tcp_est_in(struct tcpcb *tp, struct tcphdr *th, struct tcpopt *to)
  272 {
  273         struct tcp_hhook_data hhook_data;
  274 
  275         if (V_tcp_hhh[HHOOK_TCP_EST_IN]->hhh_nhooks > 0) {
  276                 hhook_data.tp = tp;
  277                 hhook_data.th = th;
  278                 hhook_data.to = to;
  279 
  280                 hhook_run_hooks(V_tcp_hhh[HHOOK_TCP_EST_IN], &hhook_data,
  281                     tp->osd);
  282         }
  283 }
  284 
  285 /*
  286  * CC wrapper hook functions
  287  */
  288 static void inline
  289 cc_ack_received(struct tcpcb *tp, struct tcphdr *th, uint16_t type)
  290 {
  291         INP_WLOCK_ASSERT(tp->t_inpcb);
  292 
  293         tp->ccv->bytes_this_ack = BYTES_THIS_ACK(tp, th);
  294         if (tp->snd_cwnd <= tp->snd_wnd)
  295                 tp->ccv->flags |= CCF_CWND_LIMITED;
  296         else
  297                 tp->ccv->flags &= ~CCF_CWND_LIMITED;
  298 
  299         if (type == CC_ACK) {
  300                 if (tp->snd_cwnd > tp->snd_ssthresh) {
  301                         tp->t_bytes_acked += min(tp->ccv->bytes_this_ack,
  302                              V_tcp_abc_l_var * tp->t_maxseg);
  303                         if (tp->t_bytes_acked >= tp->snd_cwnd) {
  304                                 tp->t_bytes_acked -= tp->snd_cwnd;
  305                                 tp->ccv->flags |= CCF_ABC_SENTAWND;
  306                         }
  307                 } else {
  308                                 tp->ccv->flags &= ~CCF_ABC_SENTAWND;
  309                                 tp->t_bytes_acked = 0;
  310                 }
  311         }
  312 
  313         if (CC_ALGO(tp)->ack_received != NULL) {
  314                 /* XXXLAS: Find a way to live without this */
  315                 tp->ccv->curack = th->th_ack;
  316                 CC_ALGO(tp)->ack_received(tp->ccv, type);
  317         }
  318 }
  319 
  320 static void inline
  321 cc_conn_init(struct tcpcb *tp)
  322 {
  323         struct hc_metrics_lite metrics;
  324         struct inpcb *inp = tp->t_inpcb;
  325         int rtt;
  326 
  327         INP_WLOCK_ASSERT(tp->t_inpcb);
  328 
  329         tcp_hc_get(&inp->inp_inc, &metrics);
  330 
  331         if (tp->t_srtt == 0 && (rtt = metrics.rmx_rtt)) {
  332                 tp->t_srtt = rtt;
  333                 tp->t_rttbest = tp->t_srtt + TCP_RTT_SCALE;
  334                 TCPSTAT_INC(tcps_usedrtt);
  335                 if (metrics.rmx_rttvar) {
  336                         tp->t_rttvar = metrics.rmx_rttvar;
  337                         TCPSTAT_INC(tcps_usedrttvar);
  338                 } else {
  339                         /* default variation is +- 1 rtt */
  340                         tp->t_rttvar =
  341                             tp->t_srtt * TCP_RTTVAR_SCALE / TCP_RTT_SCALE;
  342                 }
  343                 TCPT_RANGESET(tp->t_rxtcur,
  344                     ((tp->t_srtt >> 2) + tp->t_rttvar) >> 1,
  345                     tp->t_rttmin, TCPTV_REXMTMAX);
  346         }
  347         if (metrics.rmx_ssthresh) {
  348                 /*
  349                  * There's some sort of gateway or interface
  350                  * buffer limit on the path.  Use this to set
  351                  * the slow start threshhold, but set the
  352                  * threshold to no less than 2*mss.
  353                  */
  354                 tp->snd_ssthresh = max(2 * tp->t_maxseg, metrics.rmx_ssthresh);
  355                 TCPSTAT_INC(tcps_usedssthresh);
  356         }
  357 
  358         /*
  359          * Set the initial slow-start flight size.
  360          *
  361          * RFC5681 Section 3.1 specifies the default conservative values.
  362          * RFC3390 specifies slightly more aggressive values.
  363          * RFC6928 increases it to ten segments.
  364          *
  365          * If a SYN or SYN/ACK was lost and retransmitted, we have to
  366          * reduce the initial CWND to one segment as congestion is likely
  367          * requiring us to be cautious.
  368          */
  369         if (tp->snd_cwnd == 1)
  370                 tp->snd_cwnd = tp->t_maxseg;            /* SYN(-ACK) lost */
  371         else if (V_tcp_do_initcwnd10)
  372                 tp->snd_cwnd = min(10 * tp->t_maxseg,
  373                     max(2 * tp->t_maxseg, 14600));
  374         else if (V_tcp_do_rfc3390)
  375                 tp->snd_cwnd = min(4 * tp->t_maxseg,
  376                     max(2 * tp->t_maxseg, 4380));
  377         else {
  378                 /* Per RFC5681 Section 3.1 */
  379                 if (tp->t_maxseg > 2190)
  380                         tp->snd_cwnd = 2 * tp->t_maxseg;
  381                 else if (tp->t_maxseg > 1095)
  382                         tp->snd_cwnd = 3 * tp->t_maxseg;
  383                 else
  384                         tp->snd_cwnd = 4 * tp->t_maxseg;
  385         }
  386 
  387         if (CC_ALGO(tp)->conn_init != NULL)
  388                 CC_ALGO(tp)->conn_init(tp->ccv);
  389 }
  390 
  391 void inline
  392 cc_cong_signal(struct tcpcb *tp, struct tcphdr *th, uint32_t type)
  393 {
  394         INP_WLOCK_ASSERT(tp->t_inpcb);
  395 
  396         switch(type) {
  397         case CC_NDUPACK:
  398                 if (!IN_FASTRECOVERY(tp->t_flags)) {
  399                         tp->snd_recover = tp->snd_max;
  400                         if (tp->t_flags & TF_ECN_PERMIT)
  401                                 tp->t_flags |= TF_ECN_SND_CWR;
  402                 }
  403                 break;
  404         case CC_ECN:
  405                 if (!IN_CONGRECOVERY(tp->t_flags)) {
  406                         TCPSTAT_INC(tcps_ecn_rcwnd);
  407                         tp->snd_recover = tp->snd_max;
  408                         if (tp->t_flags & TF_ECN_PERMIT)
  409                                 tp->t_flags |= TF_ECN_SND_CWR;
  410                 }
  411                 break;
  412         case CC_RTO:
  413                 tp->t_dupacks = 0;
  414                 tp->t_bytes_acked = 0;
  415                 EXIT_RECOVERY(tp->t_flags);
  416                 tp->snd_ssthresh = max(2, min(tp->snd_wnd, tp->snd_cwnd) / 2 /
  417                     tp->t_maxseg) * tp->t_maxseg;
  418                 tp->snd_cwnd = tp->t_maxseg;
  419                 break;
  420         case CC_RTO_ERR:
  421                 TCPSTAT_INC(tcps_sndrexmitbad);
  422                 /* RTO was unnecessary, so reset everything. */
  423                 tp->snd_cwnd = tp->snd_cwnd_prev;
  424                 tp->snd_ssthresh = tp->snd_ssthresh_prev;
  425                 tp->snd_recover = tp->snd_recover_prev;
  426                 if (tp->t_flags & TF_WASFRECOVERY)
  427                         ENTER_FASTRECOVERY(tp->t_flags);
  428                 if (tp->t_flags & TF_WASCRECOVERY)
  429                         ENTER_CONGRECOVERY(tp->t_flags);
  430                 tp->snd_nxt = tp->snd_max;
  431                 tp->t_flags &= ~TF_PREVVALID;
  432                 tp->t_badrxtwin = 0;
  433                 break;
  434         }
  435 
  436         if (CC_ALGO(tp)->cong_signal != NULL) {
  437                 if (th != NULL)
  438                         tp->ccv->curack = th->th_ack;
  439                 CC_ALGO(tp)->cong_signal(tp->ccv, type);
  440         }
  441 }
  442 
  443 static void inline
  444 cc_post_recovery(struct tcpcb *tp, struct tcphdr *th)
  445 {
  446         INP_WLOCK_ASSERT(tp->t_inpcb);
  447 
  448         /* XXXLAS: KASSERT that we're in recovery? */
  449 
  450         if (CC_ALGO(tp)->post_recovery != NULL) {
  451                 tp->ccv->curack = th->th_ack;
  452                 CC_ALGO(tp)->post_recovery(tp->ccv);
  453         }
  454         /* XXXLAS: EXIT_RECOVERY ? */
  455         tp->t_bytes_acked = 0;
  456 }
  457 
  458 #ifdef TCP_SIGNATURE
  459 static inline int
  460 tcp_signature_verify_input(struct mbuf *m, int off0, int tlen, int optlen,
  461     struct tcpopt *to, struct tcphdr *th, u_int tcpbflag)
  462 {
  463         int ret;
  464 
  465         tcp_fields_to_net(th);
  466         ret = tcp_signature_verify(m, off0, tlen, optlen, to, th, tcpbflag);
  467         tcp_fields_to_host(th);
  468         return (ret);
  469 }
  470 #endif
  471 
  472 /* Neighbor Discovery, Neighbor Unreachability Detection Upper layer hint. */
  473 #ifdef INET6
  474 #define ND6_HINT(tp) \
  475 do { \
  476         if ((tp) && (tp)->t_inpcb && \
  477             ((tp)->t_inpcb->inp_vflag & INP_IPV6) != 0) \
  478                 nd6_nud_hint(NULL, NULL, 0); \
  479 } while (0)
  480 #else
  481 #define ND6_HINT(tp)
  482 #endif
  483 
  484 /*
  485  * Indicate whether this ack should be delayed.  We can delay the ack if
  486  * following conditions are met:
  487  *      - There is no delayed ack timer in progress.
  488  *      - Our last ack wasn't a 0-sized window. We never want to delay
  489  *        the ack that opens up a 0-sized window.
  490  *      - LRO wasn't used for this segment. We make sure by checking that the
  491  *        segment size is not larger than the MSS.
  492  *      - Delayed acks are enabled or this is a half-synchronized T/TCP
  493  *        connection.
  494  */
  495 #define DELAY_ACK(tp, tlen)                                             \
  496         ((!tcp_timer_active(tp, TT_DELACK) &&                           \
  497             (tp->t_flags & TF_RXWIN0SENT) == 0) &&                      \
  498             (tlen <= tp->t_maxopd) &&                                   \
  499             (V_tcp_delack_enabled || (tp->t_flags & TF_NEEDSYN)))
  500 
  501 /*
  502  * TCP input handling is split into multiple parts:
  503  *   tcp6_input is a thin wrapper around tcp_input for the extended
  504  *      ip6_protox[] call format in ip6_input
  505  *   tcp_input handles primary segment validation, inpcb lookup and
  506  *      SYN processing on listen sockets
  507  *   tcp_do_segment processes the ACK and text of the segment for
  508  *      establishing, established and closing connections
  509  */
  510 #ifdef INET6
  511 int
  512 tcp6_input(struct mbuf **mp, int *offp, int proto)
  513 {
  514         struct mbuf *m = *mp;
  515         struct in6_ifaddr *ia6;
  516         struct ip6_hdr *ip6;
  517 
  518         IP6_EXTHDR_CHECK(m, *offp, sizeof(struct tcphdr), IPPROTO_DONE);
  519 
  520         /*
  521          * draft-itojun-ipv6-tcp-to-anycast
  522          * better place to put this in?
  523          */
  524         ip6 = mtod(m, struct ip6_hdr *);
  525         ia6 = in6ifa_ifwithaddr(&ip6->ip6_dst, 0 /* XXX */);
  526         if (ia6 && (ia6->ia6_flags & IN6_IFF_ANYCAST)) {
  527                 struct ip6_hdr *ip6;
  528 
  529                 ifa_free(&ia6->ia_ifa);
  530                 ip6 = mtod(m, struct ip6_hdr *);
  531                 icmp6_error(m, ICMP6_DST_UNREACH, ICMP6_DST_UNREACH_ADDR,
  532                             (caddr_t)&ip6->ip6_dst - (caddr_t)ip6);
  533                 return (IPPROTO_DONE);
  534         }
  535         if (ia6)
  536                 ifa_free(&ia6->ia_ifa);
  537 
  538         return (tcp_input(mp, offp, proto));
  539 }
  540 #endif /* INET6 */
  541 
  542 int
  543 tcp_input(struct mbuf **mp, int *offp, int proto)
  544 {
  545         struct mbuf *m = *mp;
  546         struct tcphdr *th = NULL;
  547         struct ip *ip = NULL;
  548         struct inpcb *inp = NULL;
  549         struct tcpcb *tp = NULL;
  550         struct socket *so = NULL;
  551         u_char *optp = NULL;
  552         int off0;
  553         int optlen = 0;
  554 #ifdef INET
  555         int len;
  556 #endif
  557         int tlen = 0, off;
  558         int drop_hdrlen;
  559         int thflags;
  560         int rstreason = 0;      /* For badport_bandlim accounting purposes */
  561 #ifdef TCP_SIGNATURE
  562         uint8_t sig_checked = 0;
  563 #endif
  564         uint8_t iptos = 0;
  565         struct m_tag *fwd_tag = NULL;
  566 #ifdef INET6
  567         struct ip6_hdr *ip6 = NULL;
  568         int isipv6;
  569 #else
  570         const void *ip6 = NULL;
  571 #endif /* INET6 */
  572         struct tcpopt to;               /* options in this segment */
  573         char *s = NULL;                 /* address and port logging */
  574         int ti_locked;
  575 #define TI_UNLOCKED     1
  576 #define TI_WLOCKED      2
  577 
  578 #ifdef TCPDEBUG
  579         /*
  580          * The size of tcp_saveipgen must be the size of the max ip header,
  581          * now IPv6.
  582          */
  583         u_char tcp_saveipgen[IP6_HDR_LEN];
  584         struct tcphdr tcp_savetcp;
  585         short ostate = 0;
  586 #endif
  587 
  588 #ifdef INET6
  589         isipv6 = (mtod(m, struct ip *)->ip_v == 6) ? 1 : 0;
  590 #endif
  591 
  592         off0 = *offp;
  593         m = *mp;
  594         *mp = NULL;
  595         to.to_flags = 0;
  596         TCPSTAT_INC(tcps_rcvtotal);
  597 
  598 #ifdef INET6
  599         if (isipv6) {
  600                 /* IP6_EXTHDR_CHECK() is already done at tcp6_input(). */
  601 
  602                 if (m->m_len < (sizeof(*ip6) + sizeof(*th))) {
  603                         m = m_pullup(m, sizeof(*ip6) + sizeof(*th));
  604                         if (m == NULL) {
  605                                 TCPSTAT_INC(tcps_rcvshort);
  606                                 return (IPPROTO_DONE);
  607                         }
  608                 }
  609 
  610                 ip6 = mtod(m, struct ip6_hdr *);
  611                 th = (struct tcphdr *)((caddr_t)ip6 + off0);
  612                 tlen = sizeof(*ip6) + ntohs(ip6->ip6_plen) - off0;
  613                 if (m->m_pkthdr.csum_flags & CSUM_DATA_VALID_IPV6) {
  614                         if (m->m_pkthdr.csum_flags & CSUM_PSEUDO_HDR)
  615                                 th->th_sum = m->m_pkthdr.csum_data;
  616                         else
  617                                 th->th_sum = in6_cksum_pseudo(ip6, tlen,
  618                                     IPPROTO_TCP, m->m_pkthdr.csum_data);
  619                         th->th_sum ^= 0xffff;
  620                 } else
  621                         th->th_sum = in6_cksum(m, IPPROTO_TCP, off0, tlen);
  622                 if (th->th_sum) {
  623                         TCPSTAT_INC(tcps_rcvbadsum);
  624                         goto drop;
  625                 }
  626 
  627                 /*
  628                  * Be proactive about unspecified IPv6 address in source.
  629                  * As we use all-zero to indicate unbounded/unconnected pcb,
  630                  * unspecified IPv6 address can be used to confuse us.
  631                  *
  632                  * Note that packets with unspecified IPv6 destination is
  633                  * already dropped in ip6_input.
  634                  */
  635                 if (IN6_IS_ADDR_UNSPECIFIED(&ip6->ip6_src)) {
  636                         /* XXX stat */
  637                         goto drop;
  638                 }
  639         }
  640 #endif
  641 #if defined(INET) && defined(INET6)
  642         else
  643 #endif
  644 #ifdef INET
  645         {
  646                 /*
  647                  * Get IP and TCP header together in first mbuf.
  648                  * Note: IP leaves IP header in first mbuf.
  649                  */
  650                 if (off0 > sizeof (struct ip)) {
  651                         ip_stripoptions(m);
  652                         off0 = sizeof(struct ip);
  653                 }
  654                 if (m->m_len < sizeof (struct tcpiphdr)) {
  655                         if ((m = m_pullup(m, sizeof (struct tcpiphdr)))
  656                             == NULL) {
  657                                 TCPSTAT_INC(tcps_rcvshort);
  658                                 return (IPPROTO_DONE);
  659                         }
  660                 }
  661                 ip = mtod(m, struct ip *);
  662                 th = (struct tcphdr *)((caddr_t)ip + off0);
  663                 tlen = ntohs(ip->ip_len) - off0;
  664 
  665                 if (m->m_pkthdr.csum_flags & CSUM_DATA_VALID) {
  666                         if (m->m_pkthdr.csum_flags & CSUM_PSEUDO_HDR)
  667                                 th->th_sum = m->m_pkthdr.csum_data;
  668                         else
  669                                 th->th_sum = in_pseudo(ip->ip_src.s_addr,
  670                                     ip->ip_dst.s_addr,
  671                                     htonl(m->m_pkthdr.csum_data + tlen +
  672                                     IPPROTO_TCP));
  673                         th->th_sum ^= 0xffff;
  674                 } else {
  675                         struct ipovly *ipov = (struct ipovly *)ip;
  676 
  677                         /*
  678                          * Checksum extended TCP header and data.
  679                          */
  680                         len = off0 + tlen;
  681                         bzero(ipov->ih_x1, sizeof(ipov->ih_x1));
  682                         ipov->ih_len = htons(tlen);
  683                         th->th_sum = in_cksum(m, len);
  684                         /* Reset length for SDT probes. */
  685                         ip->ip_len = htons(tlen + off0);
  686                 }
  687 
  688                 if (th->th_sum) {
  689                         TCPSTAT_INC(tcps_rcvbadsum);
  690                         goto drop;
  691                 }
  692                 /* Re-initialization for later version check */
  693                 ip->ip_v = IPVERSION;
  694         }
  695 #endif /* INET */
  696 
  697 #ifdef INET6
  698         if (isipv6)
  699                 iptos = (ntohl(ip6->ip6_flow) >> 20) & 0xff;
  700 #endif
  701 #if defined(INET) && defined(INET6)
  702         else
  703 #endif
  704 #ifdef INET
  705                 iptos = ip->ip_tos;
  706 #endif
  707 
  708         /*
  709          * Check that TCP offset makes sense,
  710          * pull out TCP options and adjust length.              XXX
  711          */
  712         off = th->th_off << 2;
  713         if (off < sizeof (struct tcphdr) || off > tlen) {
  714                 TCPSTAT_INC(tcps_rcvbadoff);
  715                 goto drop;
  716         }
  717         tlen -= off;    /* tlen is used instead of ti->ti_len */
  718         if (off > sizeof (struct tcphdr)) {
  719 #ifdef INET6
  720                 if (isipv6) {
  721                         IP6_EXTHDR_CHECK(m, off0, off, IPPROTO_DONE);
  722                         ip6 = mtod(m, struct ip6_hdr *);
  723                         th = (struct tcphdr *)((caddr_t)ip6 + off0);
  724                 }
  725 #endif
  726 #if defined(INET) && defined(INET6)
  727                 else
  728 #endif
  729 #ifdef INET
  730                 {
  731                         if (m->m_len < sizeof(struct ip) + off) {
  732                                 if ((m = m_pullup(m, sizeof (struct ip) + off))
  733                                     == NULL) {
  734                                         TCPSTAT_INC(tcps_rcvshort);
  735                                         return (IPPROTO_DONE);
  736                                 }
  737                                 ip = mtod(m, struct ip *);
  738                                 th = (struct tcphdr *)((caddr_t)ip + off0);
  739                         }
  740                 }
  741 #endif
  742                 optlen = off - sizeof (struct tcphdr);
  743                 optp = (u_char *)(th + 1);
  744         }
  745         thflags = th->th_flags;
  746 
  747         /*
  748          * Convert TCP protocol specific fields to host format.
  749          */
  750         tcp_fields_to_host(th);
  751 
  752         /*
  753          * Delay dropping TCP, IP headers, IPv6 ext headers, and TCP options.
  754          */
  755         drop_hdrlen = off0 + off;
  756 
  757         /*
  758          * Locate pcb for segment; if we're likely to add or remove a
  759          * connection then first acquire pcbinfo lock.  There are three cases
  760          * where we might discover later we need a write lock despite the
  761          * flags: ACKs moving a connection out of the syncache, ACKs for a
  762          * connection in TIMEWAIT and SYNs not targeting a listening socket.
  763          */
  764         if ((thflags & (TH_FIN | TH_RST)) != 0) {
  765                 INP_INFO_WLOCK(&V_tcbinfo);
  766                 ti_locked = TI_WLOCKED;
  767         } else
  768                 ti_locked = TI_UNLOCKED;
  769 
  770         /*
  771          * Grab info from PACKET_TAG_IPFORWARD tag prepended to the chain.
  772          */
  773         if (
  774 #ifdef INET6
  775             (isipv6 && (m->m_flags & M_IP6_NEXTHOP))
  776 #ifdef INET
  777             || (!isipv6 && (m->m_flags & M_IP_NEXTHOP))
  778 #endif
  779 #endif
  780 #if defined(INET) && !defined(INET6)
  781             (m->m_flags & M_IP_NEXTHOP)
  782 #endif
  783             )
  784                 fwd_tag = m_tag_find(m, PACKET_TAG_IPFORWARD, NULL);
  785 
  786 findpcb:
  787 #ifdef INVARIANTS
  788         if (ti_locked == TI_WLOCKED) {
  789                 INP_INFO_WLOCK_ASSERT(&V_tcbinfo);
  790         } else {
  791                 INP_INFO_UNLOCK_ASSERT(&V_tcbinfo);
  792         }
  793 #endif
  794 #ifdef INET6
  795         if (isipv6 && fwd_tag != NULL) {
  796                 struct sockaddr_in6 *next_hop6;
  797 
  798                 next_hop6 = (struct sockaddr_in6 *)(fwd_tag + 1);
  799                 /*
  800                  * Transparently forwarded. Pretend to be the destination.
  801                  * Already got one like this?
  802                  */
  803                 inp = in6_pcblookup_mbuf(&V_tcbinfo,
  804                     &ip6->ip6_src, th->th_sport, &ip6->ip6_dst, th->th_dport,
  805                     INPLOOKUP_WLOCKPCB, m->m_pkthdr.rcvif, m);
  806                 if (!inp) {
  807                         /*
  808                          * It's new.  Try to find the ambushing socket.
  809                          * Because we've rewritten the destination address,
  810                          * any hardware-generated hash is ignored.
  811                          */
  812                         inp = in6_pcblookup(&V_tcbinfo, &ip6->ip6_src,
  813                             th->th_sport, &next_hop6->sin6_addr,
  814                             next_hop6->sin6_port ? ntohs(next_hop6->sin6_port) :
  815                             th->th_dport, INPLOOKUP_WILDCARD |
  816                             INPLOOKUP_WLOCKPCB, m->m_pkthdr.rcvif);
  817                 }
  818         } else if (isipv6) {
  819                 inp = in6_pcblookup_mbuf(&V_tcbinfo, &ip6->ip6_src,
  820                     th->th_sport, &ip6->ip6_dst, th->th_dport,
  821                     INPLOOKUP_WILDCARD | INPLOOKUP_WLOCKPCB,
  822                     m->m_pkthdr.rcvif, m);
  823         }
  824 #endif /* INET6 */
  825 #if defined(INET6) && defined(INET)
  826         else
  827 #endif
  828 #ifdef INET
  829         if (fwd_tag != NULL) {
  830                 struct sockaddr_in *next_hop;
  831 
  832                 next_hop = (struct sockaddr_in *)(fwd_tag+1);
  833                 /*
  834                  * Transparently forwarded. Pretend to be the destination.
  835                  * already got one like this?
  836                  */
  837                 inp = in_pcblookup_mbuf(&V_tcbinfo, ip->ip_src, th->th_sport,
  838                     ip->ip_dst, th->th_dport, INPLOOKUP_WLOCKPCB,
  839                     m->m_pkthdr.rcvif, m);
  840                 if (!inp) {
  841                         /*
  842                          * It's new.  Try to find the ambushing socket.
  843                          * Because we've rewritten the destination address,
  844                          * any hardware-generated hash is ignored.
  845                          */
  846                         inp = in_pcblookup(&V_tcbinfo, ip->ip_src,
  847                             th->th_sport, next_hop->sin_addr,
  848                             next_hop->sin_port ? ntohs(next_hop->sin_port) :
  849                             th->th_dport, INPLOOKUP_WILDCARD |
  850                             INPLOOKUP_WLOCKPCB, m->m_pkthdr.rcvif);
  851                 }
  852         } else
  853                 inp = in_pcblookup_mbuf(&V_tcbinfo, ip->ip_src,
  854                     th->th_sport, ip->ip_dst, th->th_dport,
  855                     INPLOOKUP_WILDCARD | INPLOOKUP_WLOCKPCB,
  856                     m->m_pkthdr.rcvif, m);
  857 #endif /* INET */
  858 
  859         /*
  860          * If the INPCB does not exist then all data in the incoming
  861          * segment is discarded and an appropriate RST is sent back.
  862          * XXX MRT Send RST using which routing table?
  863          */
  864         if (inp == NULL) {
  865                 /*
  866                  * Log communication attempts to ports that are not
  867                  * in use.
  868                  */
  869                 if ((tcp_log_in_vain == 1 && (thflags & TH_SYN)) ||
  870                     tcp_log_in_vain == 2) {
  871                         if ((s = tcp_log_vain(NULL, th, (void *)ip, ip6)))
  872                                 log(LOG_INFO, "%s; %s: Connection attempt "
  873                                     "to closed port\n", s, __func__);
  874                 }
  875                 /*
  876                  * When blackholing do not respond with a RST but
  877                  * completely ignore the segment and drop it.
  878                  */
  879                 if ((V_blackhole == 1 && (thflags & TH_SYN)) ||
  880                     V_blackhole == 2)
  881                         goto dropunlock;
  882 
  883                 rstreason = BANDLIM_RST_CLOSEDPORT;
  884                 goto dropwithreset;
  885         }
  886         INP_WLOCK_ASSERT(inp);
  887         if ((inp->inp_flowtype == M_HASHTYPE_NONE) &&
  888             (M_HASHTYPE_GET(m) != M_HASHTYPE_NONE) &&
  889             ((inp->inp_socket == NULL) ||
  890             (inp->inp_socket->so_options & SO_ACCEPTCONN) == 0)) {
  891                 inp->inp_flowid = m->m_pkthdr.flowid;
  892                 inp->inp_flowtype = M_HASHTYPE_GET(m);
  893         }
  894 #ifdef IPSEC
  895 #ifdef INET6
  896         if (isipv6 && ipsec6_in_reject(m, inp)) {
  897                 IPSEC6STAT_INC(ips_in_polvio);
  898                 goto dropunlock;
  899         } else
  900 #endif /* INET6 */
  901         if (ipsec4_in_reject(m, inp) != 0) {
  902                 IPSECSTAT_INC(ips_in_polvio);
  903                 goto dropunlock;
  904         }
  905 #endif /* IPSEC */
  906 
  907         /*
  908          * Check the minimum TTL for socket.
  909          */
  910         if (inp->inp_ip_minttl != 0) {
  911 #ifdef INET6
  912                 if (isipv6 && inp->inp_ip_minttl > ip6->ip6_hlim)
  913                         goto dropunlock;
  914                 else
  915 #endif
  916                 if (inp->inp_ip_minttl > ip->ip_ttl)
  917                         goto dropunlock;
  918         }
  919 
  920         /*
  921          * A previous connection in TIMEWAIT state is supposed to catch stray
  922          * or duplicate segments arriving late.  If this segment was a
  923          * legitimate new connection attempt, the old INPCB gets removed and
  924          * we can try again to find a listening socket.
  925          *
  926          * At this point, due to earlier optimism, we may hold only an inpcb
  927          * lock, and not the inpcbinfo write lock.  If so, we need to try to
  928          * acquire it, or if that fails, acquire a reference on the inpcb,
  929          * drop all locks, acquire a global write lock, and then re-acquire
  930          * the inpcb lock.  We may at that point discover that another thread
  931          * has tried to free the inpcb, in which case we need to loop back
  932          * and try to find a new inpcb to deliver to.
  933          *
  934          * XXXRW: It may be time to rethink timewait locking.
  935          */
  936 relocked:
  937         if (inp->inp_flags & INP_TIMEWAIT) {
  938                 if (ti_locked == TI_UNLOCKED) {
  939                         if (INP_INFO_TRY_WLOCK(&V_tcbinfo) == 0) {
  940                                 in_pcbref(inp);
  941                                 INP_WUNLOCK(inp);
  942                                 INP_INFO_WLOCK(&V_tcbinfo);
  943                                 ti_locked = TI_WLOCKED;
  944                                 INP_WLOCK(inp);
  945                                 if (in_pcbrele_wlocked(inp)) {
  946                                         inp = NULL;
  947                                         goto findpcb;
  948                                 }
  949                         } else
  950                                 ti_locked = TI_WLOCKED;
  951                 }
  952                 INP_INFO_WLOCK_ASSERT(&V_tcbinfo);
  953 
  954                 if (thflags & TH_SYN)
  955                         tcp_dooptions(&to, optp, optlen, TO_SYN);
  956                 /*
  957                  * NB: tcp_twcheck unlocks the INP and frees the mbuf.
  958                  */
  959                 if (tcp_twcheck(inp, &to, th, m, tlen))
  960                         goto findpcb;
  961                 INP_INFO_WUNLOCK(&V_tcbinfo);
  962                 return (IPPROTO_DONE);
  963         }
  964         /*
  965          * The TCPCB may no longer exist if the connection is winding
  966          * down or it is in the CLOSED state.  Either way we drop the
  967          * segment and send an appropriate response.
  968          */
  969         tp = intotcpcb(inp);
  970         if (tp == NULL || tp->t_state == TCPS_CLOSED) {
  971                 rstreason = BANDLIM_RST_CLOSEDPORT;
  972                 goto dropwithreset;
  973         }
  974 
  975 #ifdef TCP_OFFLOAD
  976         if (tp->t_flags & TF_TOE) {
  977                 tcp_offload_input(tp, m);
  978                 m = NULL;       /* consumed by the TOE driver */
  979                 goto dropunlock;
  980         }
  981 #endif
  982 
  983         /*
  984          * We've identified a valid inpcb, but it could be that we need an
  985          * inpcbinfo write lock but don't hold it.  In this case, attempt to
  986          * acquire using the same strategy as the TIMEWAIT case above.  If we
  987          * relock, we have to jump back to 'relocked' as the connection might
  988          * now be in TIMEWAIT.
  989          */
  990 #ifdef INVARIANTS
  991         if ((thflags & (TH_FIN | TH_RST)) != 0)
  992                 INP_INFO_WLOCK_ASSERT(&V_tcbinfo);
  993 #endif
  994         if (!((tp->t_state == TCPS_ESTABLISHED && (thflags & TH_SYN) == 0) ||
  995             (tp->t_state == TCPS_LISTEN && (thflags & TH_SYN)))) {
  996                 if (ti_locked == TI_UNLOCKED) {
  997                         if (INP_INFO_TRY_WLOCK(&V_tcbinfo) == 0) {
  998                                 in_pcbref(inp);
  999                                 INP_WUNLOCK(inp);
 1000                                 INP_INFO_WLOCK(&V_tcbinfo);
 1001                                 ti_locked = TI_WLOCKED;
 1002                                 INP_WLOCK(inp);
 1003                                 if (in_pcbrele_wlocked(inp)) {
 1004                                         inp = NULL;
 1005                                         goto findpcb;
 1006                                 }
 1007                                 goto relocked;
 1008                         } else
 1009                                 ti_locked = TI_WLOCKED;
 1010                 }
 1011                 INP_INFO_WLOCK_ASSERT(&V_tcbinfo);
 1012         }
 1013 
 1014 #ifdef MAC
 1015         INP_WLOCK_ASSERT(inp);
 1016         if (mac_inpcb_check_deliver(inp, m))
 1017                 goto dropunlock;
 1018 #endif
 1019         so = inp->inp_socket;
 1020         KASSERT(so != NULL, ("%s: so == NULL", __func__));
 1021 #ifdef TCPDEBUG
 1022         if (so->so_options & SO_DEBUG) {
 1023                 ostate = tp->t_state;
 1024 #ifdef INET6
 1025                 if (isipv6) {
 1026                         bcopy((char *)ip6, (char *)tcp_saveipgen, sizeof(*ip6));
 1027                 } else
 1028 #endif
 1029                         bcopy((char *)ip, (char *)tcp_saveipgen, sizeof(*ip));
 1030                 tcp_savetcp = *th;
 1031         }
 1032 #endif /* TCPDEBUG */
 1033         /*
 1034          * When the socket is accepting connections (the INPCB is in LISTEN
 1035          * state) we look into the SYN cache if this is a new connection
 1036          * attempt or the completion of a previous one.
 1037          */
 1038         if (so->so_options & SO_ACCEPTCONN) {
 1039                 struct in_conninfo inc;
 1040 
 1041                 KASSERT(tp->t_state == TCPS_LISTEN, ("%s: so accepting but "
 1042                     "tp not listening", __func__));
 1043                 bzero(&inc, sizeof(inc));
 1044 #ifdef INET6
 1045                 if (isipv6) {
 1046                         inc.inc_flags |= INC_ISIPV6;
 1047                         inc.inc6_faddr = ip6->ip6_src;
 1048                         inc.inc6_laddr = ip6->ip6_dst;
 1049                 } else
 1050 #endif
 1051                 {
 1052                         inc.inc_faddr = ip->ip_src;
 1053                         inc.inc_laddr = ip->ip_dst;
 1054                 }
 1055                 inc.inc_fport = th->th_sport;
 1056                 inc.inc_lport = th->th_dport;
 1057                 inc.inc_fibnum = so->so_fibnum;
 1058 
 1059                 /*
 1060                  * Check for an existing connection attempt in syncache if
 1061                  * the flag is only ACK.  A successful lookup creates a new
 1062                  * socket appended to the listen queue in SYN_RECEIVED state.
 1063                  */
 1064                 if ((thflags & (TH_RST|TH_ACK|TH_SYN)) == TH_ACK) {
 1065 
 1066                         INP_INFO_WLOCK_ASSERT(&V_tcbinfo);
 1067                         /*
 1068                          * Parse the TCP options here because
 1069                          * syncookies need access to the reflected
 1070                          * timestamp.
 1071                          */
 1072                         tcp_dooptions(&to, optp, optlen, 0);
 1073                         /*
 1074                          * NB: syncache_expand() doesn't unlock
 1075                          * inp and tcpinfo locks.
 1076                          */
 1077                         if (!syncache_expand(&inc, &to, th, &so, m)) {
 1078                                 /*
 1079                                  * No syncache entry or ACK was not
 1080                                  * for our SYN/ACK.  Send a RST.
 1081                                  * NB: syncache did its own logging
 1082                                  * of the failure cause.
 1083                                  */
 1084                                 rstreason = BANDLIM_RST_OPENPORT;
 1085                                 goto dropwithreset;
 1086                         }
 1087                         if (so == NULL) {
 1088                                 /*
 1089                                  * We completed the 3-way handshake
 1090                                  * but could not allocate a socket
 1091                                  * either due to memory shortage,
 1092                                  * listen queue length limits or
 1093                                  * global socket limits.  Send RST
 1094                                  * or wait and have the remote end
 1095                                  * retransmit the ACK for another
 1096                                  * try.
 1097                                  */
 1098                                 if ((s = tcp_log_addrs(&inc, th, NULL, NULL)))
 1099                                         log(LOG_DEBUG, "%s; %s: Listen socket: "
 1100                                             "Socket allocation failed due to "
 1101                                             "limits or memory shortage, %s\n",
 1102                                             s, __func__,
 1103                                             V_tcp_sc_rst_sock_fail ?
 1104                                             "sending RST" : "try again");
 1105                                 if (V_tcp_sc_rst_sock_fail) {
 1106                                         rstreason = BANDLIM_UNLIMITED;
 1107                                         goto dropwithreset;
 1108                                 } else
 1109                                         goto dropunlock;
 1110                         }
 1111                         /*
 1112                          * Socket is created in state SYN_RECEIVED.
 1113                          * Unlock the listen socket, lock the newly
 1114                          * created socket and update the tp variable.
 1115                          */
 1116                         INP_WUNLOCK(inp);       /* listen socket */
 1117                         inp = sotoinpcb(so);
 1118                         INP_WLOCK(inp);         /* new connection */
 1119                         tp = intotcpcb(inp);
 1120                         KASSERT(tp->t_state == TCPS_SYN_RECEIVED,
 1121                             ("%s: ", __func__));
 1122 #ifdef TCP_SIGNATURE
 1123                         if (sig_checked == 0)  {
 1124                                 tcp_dooptions(&to, optp, optlen,
 1125                                     (thflags & TH_SYN) ? TO_SYN : 0);
 1126                                 if (!tcp_signature_verify_input(m, off0, tlen,
 1127                                     optlen, &to, th, tp->t_flags)) {
 1128 
 1129                                         /*
 1130                                          * In SYN_SENT state if it receives an
 1131                                          * RST, it is allowed for further
 1132                                          * processing.
 1133                                          */
 1134                                         if ((thflags & TH_RST) == 0 ||
 1135                                             (tp->t_state == TCPS_SYN_SENT) == 0)
 1136                                                 goto dropunlock;
 1137                                 }
 1138                                 sig_checked = 1;
 1139                         }
 1140 #endif
 1141 
 1142                         /*
 1143                          * Process the segment and the data it
 1144                          * contains.  tcp_do_segment() consumes
 1145                          * the mbuf chain and unlocks the inpcb.
 1146                          */
 1147                         tcp_do_segment(m, th, so, tp, drop_hdrlen, tlen,
 1148                             iptos, ti_locked);
 1149                         INP_INFO_UNLOCK_ASSERT(&V_tcbinfo);
 1150                         return (IPPROTO_DONE);
 1151                 }
 1152                 /*
 1153                  * Segment flag validation for new connection attempts:
 1154                  *
 1155                  * Our (SYN|ACK) response was rejected.
 1156                  * Check with syncache and remove entry to prevent
 1157                  * retransmits.
 1158                  *
 1159                  * NB: syncache_chkrst does its own logging of failure
 1160                  * causes.
 1161                  */
 1162                 if (thflags & TH_RST) {
 1163                         syncache_chkrst(&inc, th);
 1164                         goto dropunlock;
 1165                 }
 1166                 /*
 1167                  * We can't do anything without SYN.
 1168                  */
 1169                 if ((thflags & TH_SYN) == 0) {
 1170                         if ((s = tcp_log_addrs(&inc, th, NULL, NULL)))
 1171                                 log(LOG_DEBUG, "%s; %s: Listen socket: "
 1172                                     "SYN is missing, segment ignored\n",
 1173                                     s, __func__);
 1174                         TCPSTAT_INC(tcps_badsyn);
 1175                         goto dropunlock;
 1176                 }
 1177                 /*
 1178                  * (SYN|ACK) is bogus on a listen socket.
 1179                  */
 1180                 if (thflags & TH_ACK) {
 1181                         if ((s = tcp_log_addrs(&inc, th, NULL, NULL)))
 1182                                 log(LOG_DEBUG, "%s; %s: Listen socket: "
 1183                                     "SYN|ACK invalid, segment rejected\n",
 1184                                     s, __func__);
 1185                         syncache_badack(&inc);  /* XXX: Not needed! */
 1186                         TCPSTAT_INC(tcps_badsyn);
 1187                         rstreason = BANDLIM_RST_OPENPORT;
 1188                         goto dropwithreset;
 1189                 }
 1190                 /*
 1191                  * If the drop_synfin option is enabled, drop all
 1192                  * segments with both the SYN and FIN bits set.
 1193                  * This prevents e.g. nmap from identifying the
 1194                  * TCP/IP stack.
 1195                  * XXX: Poor reasoning.  nmap has other methods
 1196                  * and is constantly refining its stack detection
 1197                  * strategies.
 1198                  * XXX: This is a violation of the TCP specification
 1199                  * and was used by RFC1644.
 1200                  */
 1201                 if ((thflags & TH_FIN) && V_drop_synfin) {
 1202                         if ((s = tcp_log_addrs(&inc, th, NULL, NULL)))
 1203                                 log(LOG_DEBUG, "%s; %s: Listen socket: "
 1204                                     "SYN|FIN segment ignored (based on "
 1205                                     "sysctl setting)\n", s, __func__);
 1206                         TCPSTAT_INC(tcps_badsyn);
 1207                         goto dropunlock;
 1208                 }
 1209                 /*
 1210                  * Segment's flags are (SYN) or (SYN|FIN).
 1211                  *
 1212                  * TH_PUSH, TH_URG, TH_ECE, TH_CWR are ignored
 1213                  * as they do not affect the state of the TCP FSM.
 1214                  * The data pointed to by TH_URG and th_urp is ignored.
 1215                  */
 1216                 KASSERT((thflags & (TH_RST|TH_ACK)) == 0,
 1217                     ("%s: Listen socket: TH_RST or TH_ACK set", __func__));
 1218                 KASSERT(thflags & (TH_SYN),
 1219                     ("%s: Listen socket: TH_SYN not set", __func__));
 1220 #ifdef INET6
 1221                 /*
 1222                  * If deprecated address is forbidden,
 1223                  * we do not accept SYN to deprecated interface
 1224                  * address to prevent any new inbound connection from
 1225                  * getting established.
 1226                  * When we do not accept SYN, we send a TCP RST,
 1227                  * with deprecated source address (instead of dropping
 1228                  * it).  We compromise it as it is much better for peer
 1229                  * to send a RST, and RST will be the final packet
 1230                  * for the exchange.
 1231                  *
 1232                  * If we do not forbid deprecated addresses, we accept
 1233                  * the SYN packet.  RFC2462 does not suggest dropping
 1234                  * SYN in this case.
 1235                  * If we decipher RFC2462 5.5.4, it says like this:
 1236                  * 1. use of deprecated addr with existing
 1237                  *    communication is okay - "SHOULD continue to be
 1238                  *    used"
 1239                  * 2. use of it with new communication:
 1240                  *   (2a) "SHOULD NOT be used if alternate address
 1241                  *        with sufficient scope is available"
 1242                  *   (2b) nothing mentioned otherwise.
 1243                  * Here we fall into (2b) case as we have no choice in
 1244                  * our source address selection - we must obey the peer.
 1245                  *
 1246                  * The wording in RFC2462 is confusing, and there are
 1247                  * multiple description text for deprecated address
 1248                  * handling - worse, they are not exactly the same.
 1249                  * I believe 5.5.4 is the best one, so we follow 5.5.4.
 1250                  */
 1251                 if (isipv6 && !V_ip6_use_deprecated) {
 1252                         struct in6_ifaddr *ia6;
 1253 
 1254                         ia6 = in6ifa_ifwithaddr(&ip6->ip6_dst, 0 /* XXX */);
 1255                         if (ia6 != NULL &&
 1256                             (ia6->ia6_flags & IN6_IFF_DEPRECATED)) {
 1257                                 ifa_free(&ia6->ia_ifa);
 1258                                 if ((s = tcp_log_addrs(&inc, th, NULL, NULL)))
 1259                                     log(LOG_DEBUG, "%s; %s: Listen socket: "
 1260                                         "Connection attempt to deprecated "
 1261                                         "IPv6 address rejected\n",
 1262                                         s, __func__);
 1263                                 rstreason = BANDLIM_RST_OPENPORT;
 1264                                 goto dropwithreset;
 1265                         }
 1266                         if (ia6)
 1267                                 ifa_free(&ia6->ia_ifa);
 1268                 }
 1269 #endif /* INET6 */
 1270                 /*
 1271                  * Basic sanity checks on incoming SYN requests:
 1272                  *   Don't respond if the destination is a link layer
 1273                  *      broadcast according to RFC1122 4.2.3.10, p. 104.
 1274                  *   If it is from this socket it must be forged.
 1275                  *   Don't respond if the source or destination is a
 1276                  *      global or subnet broad- or multicast address.
 1277                  *   Note that it is quite possible to receive unicast
 1278                  *      link-layer packets with a broadcast IP address. Use
 1279                  *      in_broadcast() to find them.
 1280                  */
 1281                 if (m->m_flags & (M_BCAST|M_MCAST)) {
 1282                         if ((s = tcp_log_addrs(&inc, th, NULL, NULL)))
 1283                             log(LOG_DEBUG, "%s; %s: Listen socket: "
 1284                                 "Connection attempt from broad- or multicast "
 1285                                 "link layer address ignored\n", s, __func__);
 1286                         goto dropunlock;
 1287                 }
 1288 #ifdef INET6
 1289                 if (isipv6) {
 1290                         if (th->th_dport == th->th_sport &&
 1291                             IN6_ARE_ADDR_EQUAL(&ip6->ip6_dst, &ip6->ip6_src)) {
 1292                                 if ((s = tcp_log_addrs(&inc, th, NULL, NULL)))
 1293                                     log(LOG_DEBUG, "%s; %s: Listen socket: "
 1294                                         "Connection attempt to/from self "
 1295                                         "ignored\n", s, __func__);
 1296                                 goto dropunlock;
 1297                         }
 1298                         if (IN6_IS_ADDR_MULTICAST(&ip6->ip6_dst) ||
 1299                             IN6_IS_ADDR_MULTICAST(&ip6->ip6_src)) {
 1300                                 if ((s = tcp_log_addrs(&inc, th, NULL, NULL)))
 1301                                     log(LOG_DEBUG, "%s; %s: Listen socket: "
 1302                                         "Connection attempt from/to multicast "
 1303                                         "address ignored\n", s, __func__);
 1304                                 goto dropunlock;
 1305                         }
 1306                 }
 1307 #endif
 1308 #if defined(INET) && defined(INET6)
 1309                 else
 1310 #endif
 1311 #ifdef INET
 1312                 {
 1313                         if (th->th_dport == th->th_sport &&
 1314                             ip->ip_dst.s_addr == ip->ip_src.s_addr) {
 1315                                 if ((s = tcp_log_addrs(&inc, th, NULL, NULL)))
 1316                                     log(LOG_DEBUG, "%s; %s: Listen socket: "
 1317                                         "Connection attempt from/to self "
 1318                                         "ignored\n", s, __func__);
 1319                                 goto dropunlock;
 1320                         }
 1321                         if (IN_MULTICAST(ntohl(ip->ip_dst.s_addr)) ||
 1322                             IN_MULTICAST(ntohl(ip->ip_src.s_addr)) ||
 1323                             ip->ip_src.s_addr == htonl(INADDR_BROADCAST) ||
 1324                             in_broadcast(ip->ip_dst, m->m_pkthdr.rcvif)) {
 1325                                 if ((s = tcp_log_addrs(&inc, th, NULL, NULL)))
 1326                                     log(LOG_DEBUG, "%s; %s: Listen socket: "
 1327                                         "Connection attempt from/to broad- "
 1328                                         "or multicast address ignored\n",
 1329                                         s, __func__);
 1330                                 goto dropunlock;
 1331                         }
 1332                 }
 1333 #endif
 1334                 /*
 1335                  * SYN appears to be valid.  Create compressed TCP state
 1336                  * for syncache.
 1337                  */
 1338 #ifdef TCPDEBUG
 1339                 if (so->so_options & SO_DEBUG)
 1340                         tcp_trace(TA_INPUT, ostate, tp,
 1341                             (void *)tcp_saveipgen, &tcp_savetcp, 0);
 1342 #endif
 1343                 tcp_dooptions(&to, optp, optlen, TO_SYN);
 1344                 syncache_add(&inc, &to, th, inp, &so, m, NULL, NULL);
 1345                 /*
 1346                  * Entry added to syncache and mbuf consumed.
 1347                  * Only the listen socket is unlocked by syncache_add().
 1348                  */
 1349                 if (ti_locked == TI_WLOCKED) {
 1350                         INP_INFO_WUNLOCK(&V_tcbinfo);
 1351                         ti_locked = TI_UNLOCKED;
 1352                 }
 1353                 INP_INFO_UNLOCK_ASSERT(&V_tcbinfo);
 1354                 return (IPPROTO_DONE);
 1355         } else if (tp->t_state == TCPS_LISTEN) {
 1356                 /*
 1357                  * When a listen socket is torn down the SO_ACCEPTCONN
 1358                  * flag is removed first while connections are drained
 1359                  * from the accept queue in a unlock/lock cycle of the
 1360                  * ACCEPT_LOCK, opening a race condition allowing a SYN
 1361                  * attempt go through unhandled.
 1362                  */
 1363                 goto dropunlock;
 1364         }
 1365 
 1366 #ifdef TCP_SIGNATURE
 1367         if (sig_checked == 0)  {
 1368                 tcp_dooptions(&to, optp, optlen,
 1369                     (thflags & TH_SYN) ? TO_SYN : 0);
 1370                 if (!tcp_signature_verify_input(m, off0, tlen, optlen, &to,
 1371                     th, tp->t_flags)) {
 1372 
 1373                         /*
 1374                          * In SYN_SENT state if it receives an RST, it is
 1375                          * allowed for further processing.
 1376                          */
 1377                         if ((thflags & TH_RST) == 0 ||
 1378                             (tp->t_state == TCPS_SYN_SENT) == 0)
 1379                                 goto dropunlock;
 1380                 }
 1381                 sig_checked = 1;
 1382         }
 1383 #endif
 1384 
 1385         TCP_PROBE5(receive, NULL, tp, mtod(m, const char *), tp, th);
 1386 
 1387         /*
 1388          * Segment belongs to a connection in SYN_SENT, ESTABLISHED or later
 1389          * state.  tcp_do_segment() always consumes the mbuf chain, unlocks
 1390          * the inpcb, and unlocks pcbinfo.
 1391          */
 1392         tcp_do_segment(m, th, so, tp, drop_hdrlen, tlen, iptos, ti_locked);
 1393         INP_INFO_UNLOCK_ASSERT(&V_tcbinfo);
 1394         return (IPPROTO_DONE);
 1395 
 1396 dropwithreset:
 1397         TCP_PROBE5(receive, NULL, tp, mtod(m, const char *), tp, th);
 1398 
 1399         if (ti_locked == TI_WLOCKED) {
 1400                 INP_INFO_WUNLOCK(&V_tcbinfo);
 1401                 ti_locked = TI_UNLOCKED;
 1402         }
 1403 #ifdef INVARIANTS
 1404         else {
 1405                 KASSERT(ti_locked == TI_UNLOCKED, ("%s: dropwithreset "
 1406                     "ti_locked: %d", __func__, ti_locked));
 1407                 INP_INFO_UNLOCK_ASSERT(&V_tcbinfo);
 1408         }
 1409 #endif
 1410 
 1411         if (inp != NULL) {
 1412                 tcp_dropwithreset(m, th, tp, tlen, rstreason);
 1413                 INP_WUNLOCK(inp);
 1414         } else
 1415                 tcp_dropwithreset(m, th, NULL, tlen, rstreason);
 1416         m = NULL;       /* mbuf chain got consumed. */
 1417         goto drop;
 1418 
 1419 dropunlock:
 1420         if (m != NULL)
 1421                 TCP_PROBE5(receive, NULL, tp, mtod(m, const char *), tp, th);
 1422 
 1423         if (ti_locked == TI_WLOCKED) {
 1424                 INP_INFO_WUNLOCK(&V_tcbinfo);
 1425                 ti_locked = TI_UNLOCKED;
 1426         }
 1427 #ifdef INVARIANTS
 1428         else {
 1429                 KASSERT(ti_locked == TI_UNLOCKED, ("%s: dropunlock "
 1430                     "ti_locked: %d", __func__, ti_locked));
 1431                 INP_INFO_UNLOCK_ASSERT(&V_tcbinfo);
 1432         }
 1433 #endif
 1434 
 1435         if (inp != NULL)
 1436                 INP_WUNLOCK(inp);
 1437 
 1438 drop:
 1439         INP_INFO_UNLOCK_ASSERT(&V_tcbinfo);
 1440         if (s != NULL)
 1441                 free(s, M_TCPLOG);
 1442         if (m != NULL)
 1443                 m_freem(m);
 1444         return (IPPROTO_DONE);
 1445 }
 1446 
 1447 static void
 1448 tcp_do_segment(struct mbuf *m, struct tcphdr *th, struct socket *so,
 1449     struct tcpcb *tp, int drop_hdrlen, int tlen, uint8_t iptos,
 1450     int ti_locked)
 1451 {
 1452         int thflags, acked, ourfinisacked, needoutput = 0;
 1453         int rstreason, todrop, win;
 1454         u_long tiwin;
 1455         char *s;
 1456         struct in_conninfo *inc;
 1457         struct mbuf *mfree;
 1458         struct tcpopt to;
 1459 
 1460 #ifdef TCPDEBUG
 1461         /*
 1462          * The size of tcp_saveipgen must be the size of the max ip header,
 1463          * now IPv6.
 1464          */
 1465         u_char tcp_saveipgen[IP6_HDR_LEN];
 1466         struct tcphdr tcp_savetcp;
 1467         short ostate = 0;
 1468 #endif
 1469         thflags = th->th_flags;
 1470         inc = &tp->t_inpcb->inp_inc;
 1471         tp->sackhint.last_sack_ack = 0;
 1472 
 1473         /*
 1474          * If this is either a state-changing packet or current state isn't
 1475          * established, we require a write lock on tcbinfo.  Otherwise, we
 1476          * allow the tcbinfo to be in either alocked or unlocked, as the
 1477          * caller may have unnecessarily acquired a write lock due to a race.
 1478          */
 1479         if ((thflags & (TH_SYN | TH_FIN | TH_RST)) != 0 ||
 1480             tp->t_state != TCPS_ESTABLISHED) {
 1481                 KASSERT(ti_locked == TI_WLOCKED, ("%s ti_locked %d for "
 1482                     "SYN/FIN/RST/!EST", __func__, ti_locked));
 1483                 INP_INFO_WLOCK_ASSERT(&V_tcbinfo);
 1484         } else {
 1485 #ifdef INVARIANTS
 1486                 if (ti_locked == TI_WLOCKED)
 1487                         INP_INFO_WLOCK_ASSERT(&V_tcbinfo);
 1488                 else {
 1489                         KASSERT(ti_locked == TI_UNLOCKED, ("%s: EST "
 1490                             "ti_locked: %d", __func__, ti_locked));
 1491                         INP_INFO_UNLOCK_ASSERT(&V_tcbinfo);
 1492                 }
 1493 #endif
 1494         }
 1495         INP_WLOCK_ASSERT(tp->t_inpcb);
 1496         KASSERT(tp->t_state > TCPS_LISTEN, ("%s: TCPS_LISTEN",
 1497             __func__));
 1498         KASSERT(tp->t_state != TCPS_TIME_WAIT, ("%s: TCPS_TIME_WAIT",
 1499             __func__));
 1500 
 1501         /*
 1502          * Segment received on connection.
 1503          * Reset idle time and keep-alive timer.
 1504          * XXX: This should be done after segment
 1505          * validation to ignore broken/spoofed segs.
 1506          */
 1507         tp->t_rcvtime = ticks;
 1508         if (TCPS_HAVEESTABLISHED(tp->t_state))
 1509                 tcp_timer_activate(tp, TT_KEEP, TP_KEEPIDLE(tp));
 1510 
 1511         /*
 1512          * Unscale the window into a 32-bit value.
 1513          * For the SYN_SENT state the scale is zero.
 1514          */
 1515         tiwin = th->th_win << tp->snd_scale;
 1516 
 1517         /*
 1518          * TCP ECN processing.
 1519          */
 1520         if (tp->t_flags & TF_ECN_PERMIT) {
 1521                 if (thflags & TH_CWR)
 1522                         tp->t_flags &= ~TF_ECN_SND_ECE;
 1523                 switch (iptos & IPTOS_ECN_MASK) {
 1524                 case IPTOS_ECN_CE:
 1525                         tp->t_flags |= TF_ECN_SND_ECE;
 1526                         TCPSTAT_INC(tcps_ecn_ce);
 1527                         break;
 1528                 case IPTOS_ECN_ECT0:
 1529                         TCPSTAT_INC(tcps_ecn_ect0);
 1530                         break;
 1531                 case IPTOS_ECN_ECT1:
 1532                         TCPSTAT_INC(tcps_ecn_ect1);
 1533                         break;
 1534                 }
 1535                 /* Congestion experienced. */
 1536                 if (thflags & TH_ECE) {
 1537                         cc_cong_signal(tp, th, CC_ECN);
 1538                 }
 1539         }
 1540 
 1541         /*
 1542          * Parse options on any incoming segment.
 1543          */
 1544         tcp_dooptions(&to, (u_char *)(th + 1),
 1545             (th->th_off << 2) - sizeof(struct tcphdr),
 1546             (thflags & TH_SYN) ? TO_SYN : 0);
 1547 
 1548         /*
 1549          * If echoed timestamp is later than the current time,
 1550          * fall back to non RFC1323 RTT calculation.  Normalize
 1551          * timestamp if syncookies were used when this connection
 1552          * was established.
 1553          */
 1554         if ((to.to_flags & TOF_TS) && (to.to_tsecr != 0)) {
 1555                 to.to_tsecr -= tp->ts_offset;
 1556                 if (TSTMP_GT(to.to_tsecr, tcp_ts_getticks()))
 1557                         to.to_tsecr = 0;
 1558         }
 1559         /*
 1560          * If timestamps were negotiated during SYN/ACK they should
 1561          * appear on every segment during this session and vice versa.
 1562          */
 1563         if ((tp->t_flags & TF_RCVD_TSTMP) && !(to.to_flags & TOF_TS)) {
 1564                 if ((s = tcp_log_addrs(inc, th, NULL, NULL))) {
 1565                         log(LOG_DEBUG, "%s; %s: Timestamp missing, "
 1566                             "no action\n", s, __func__);
 1567                         free(s, M_TCPLOG);
 1568                 }
 1569         }
 1570         if (!(tp->t_flags & TF_RCVD_TSTMP) && (to.to_flags & TOF_TS)) {
 1571                 if ((s = tcp_log_addrs(inc, th, NULL, NULL))) {
 1572                         log(LOG_DEBUG, "%s; %s: Timestamp not expected, "
 1573                             "no action\n", s, __func__);
 1574                         free(s, M_TCPLOG);
 1575                 }
 1576         }
 1577 
 1578         /*
 1579          * Process options only when we get SYN/ACK back. The SYN case
 1580          * for incoming connections is handled in tcp_syncache.
 1581          * According to RFC1323 the window field in a SYN (i.e., a <SYN>
 1582          * or <SYN,ACK>) segment itself is never scaled.
 1583          * XXX this is traditional behavior, may need to be cleaned up.
 1584          */
 1585         if (tp->t_state == TCPS_SYN_SENT && (thflags & TH_SYN)) {
 1586                 if ((to.to_flags & TOF_SCALE) &&
 1587                     (tp->t_flags & TF_REQ_SCALE)) {
 1588                         tp->t_flags |= TF_RCVD_SCALE;
 1589                         tp->snd_scale = to.to_wscale;
 1590                 }
 1591                 /*
 1592                  * Initial send window.  It will be updated with
 1593                  * the next incoming segment to the scaled value.
 1594                  */
 1595                 tp->snd_wnd = th->th_win;
 1596                 if (to.to_flags & TOF_TS) {
 1597                         tp->t_flags |= TF_RCVD_TSTMP;
 1598                         tp->ts_recent = to.to_tsval;
 1599                         tp->ts_recent_age = tcp_ts_getticks();
 1600                 }
 1601                 if (to.to_flags & TOF_MSS)
 1602                         tcp_mss(tp, to.to_mss);
 1603                 if ((tp->t_flags & TF_SACK_PERMIT) &&
 1604                     (to.to_flags & TOF_SACKPERM) == 0)
 1605                         tp->t_flags &= ~TF_SACK_PERMIT;
 1606         }
 1607 
 1608         /*
 1609          * Header prediction: check for the two common cases
 1610          * of a uni-directional data xfer.  If the packet has
 1611          * no control flags, is in-sequence, the window didn't
 1612          * change and we're not retransmitting, it's a
 1613          * candidate.  If the length is zero and the ack moved
 1614          * forward, we're the sender side of the xfer.  Just
 1615          * free the data acked & wake any higher level process
 1616          * that was blocked waiting for space.  If the length
 1617          * is non-zero and the ack didn't move, we're the
 1618          * receiver side.  If we're getting packets in-order
 1619          * (the reassembly queue is empty), add the data to
 1620          * the socket buffer and note that we need a delayed ack.
 1621          * Make sure that the hidden state-flags are also off.
 1622          * Since we check for TCPS_ESTABLISHED first, it can only
 1623          * be TH_NEEDSYN.
 1624          */
 1625         if (tp->t_state == TCPS_ESTABLISHED &&
 1626             th->th_seq == tp->rcv_nxt &&
 1627             (thflags & (TH_SYN|TH_FIN|TH_RST|TH_URG|TH_ACK)) == TH_ACK &&
 1628             tp->snd_nxt == tp->snd_max &&
 1629             tiwin && tiwin == tp->snd_wnd && 
 1630             ((tp->t_flags & (TF_NEEDSYN|TF_NEEDFIN)) == 0) &&
 1631             tp->t_segq == NULL && ((to.to_flags & TOF_TS) == 0 ||
 1632              TSTMP_GEQ(to.to_tsval, tp->ts_recent)) ) {
 1633 
 1634                 /*
 1635                  * If last ACK falls within this segment's sequence numbers,
 1636                  * record the timestamp.
 1637                  * NOTE that the test is modified according to the latest
 1638                  * proposal of the tcplw@cray.com list (Braden 1993/04/26).
 1639                  */
 1640                 if ((to.to_flags & TOF_TS) != 0 &&
 1641                     SEQ_LEQ(th->th_seq, tp->last_ack_sent)) {
 1642                         tp->ts_recent_age = tcp_ts_getticks();
 1643                         tp->ts_recent = to.to_tsval;
 1644                 }
 1645 
 1646                 if (tlen == 0) {
 1647                         if (SEQ_GT(th->th_ack, tp->snd_una) &&
 1648                             SEQ_LEQ(th->th_ack, tp->snd_max) &&
 1649                             !IN_RECOVERY(tp->t_flags) &&
 1650                             (to.to_flags & TOF_SACK) == 0 &&
 1651                             TAILQ_EMPTY(&tp->snd_holes)) {
 1652                                 /*
 1653                                  * This is a pure ack for outstanding data.
 1654                                  */
 1655                                 if (ti_locked == TI_WLOCKED)
 1656                                         INP_INFO_WUNLOCK(&V_tcbinfo);
 1657                                 ti_locked = TI_UNLOCKED;
 1658 
 1659                                 TCPSTAT_INC(tcps_predack);
 1660 
 1661                                 /*
 1662                                  * "bad retransmit" recovery.
 1663                                  */
 1664                                 if (tp->t_rxtshift == 1 &&
 1665                                     tp->t_flags & TF_PREVVALID &&
 1666                                     (int)(ticks - tp->t_badrxtwin) < 0) {
 1667                                         cc_cong_signal(tp, th, CC_RTO_ERR);
 1668                                 }
 1669 
 1670                                 /*
 1671                                  * Recalculate the transmit timer / rtt.
 1672                                  *
 1673                                  * Some boxes send broken timestamp replies
 1674                                  * during the SYN+ACK phase, ignore
 1675                                  * timestamps of 0 or we could calculate a
 1676                                  * huge RTT and blow up the retransmit timer.
 1677                                  */
 1678                                 if ((to.to_flags & TOF_TS) != 0 &&
 1679                                     to.to_tsecr) {
 1680                                         u_int t;
 1681 
 1682                                         t = tcp_ts_getticks() - to.to_tsecr;
 1683                                         if (!tp->t_rttlow || tp->t_rttlow > t)
 1684                                                 tp->t_rttlow = t;
 1685                                         tcp_xmit_timer(tp,
 1686                                             TCP_TS_TO_TICKS(t) + 1);
 1687                                 } else if (tp->t_rtttime &&
 1688                                     SEQ_GT(th->th_ack, tp->t_rtseq)) {
 1689                                         if (!tp->t_rttlow ||
 1690                                             tp->t_rttlow > ticks - tp->t_rtttime)
 1691                                                 tp->t_rttlow = ticks - tp->t_rtttime;
 1692                                         tcp_xmit_timer(tp,
 1693                                                         ticks - tp->t_rtttime);
 1694                                 }
 1695                                 acked = BYTES_THIS_ACK(tp, th);
 1696 
 1697                                 /* Run HHOOK_TCP_ESTABLISHED_IN helper hooks. */
 1698                                 hhook_run_tcp_est_in(tp, th, &to);
 1699 
 1700                                 TCPSTAT_INC(tcps_rcvackpack);
 1701                                 TCPSTAT_ADD(tcps_rcvackbyte, acked);
 1702                                 sbdrop(&so->so_snd, acked);
 1703                                 if (SEQ_GT(tp->snd_una, tp->snd_recover) &&
 1704                                     SEQ_LEQ(th->th_ack, tp->snd_recover))
 1705                                         tp->snd_recover = th->th_ack - 1;
 1706                                 
 1707                                 /*
 1708                                  * Let the congestion control algorithm update
 1709                                  * congestion control related information. This
 1710                                  * typically means increasing the congestion
 1711                                  * window.
 1712                                  */
 1713                                 cc_ack_received(tp, th, CC_ACK);
 1714 
 1715                                 tp->snd_una = th->th_ack;
 1716                                 /*
 1717                                  * Pull snd_wl2 up to prevent seq wrap relative
 1718                                  * to th_ack.
 1719                                  */
 1720                                 tp->snd_wl2 = th->th_ack;
 1721                                 tp->t_dupacks = 0;
 1722                                 m_freem(m);
 1723                                 ND6_HINT(tp); /* Some progress has been made. */
 1724 
 1725                                 /*
 1726                                  * If all outstanding data are acked, stop
 1727                                  * retransmit timer, otherwise restart timer
 1728                                  * using current (possibly backed-off) value.
 1729                                  * If process is waiting for space,
 1730                                  * wakeup/selwakeup/signal.  If data
 1731                                  * are ready to send, let tcp_output
 1732                                  * decide between more output or persist.
 1733                                  */
 1734 #ifdef TCPDEBUG
 1735                                 if (so->so_options & SO_DEBUG)
 1736                                         tcp_trace(TA_INPUT, ostate, tp,
 1737                                             (void *)tcp_saveipgen,
 1738                                             &tcp_savetcp, 0);
 1739 #endif
 1740                                 if (tp->snd_una == tp->snd_max)
 1741                                         tcp_timer_activate(tp, TT_REXMT, 0);
 1742                                 else if (!tcp_timer_active(tp, TT_PERSIST))
 1743                                         tcp_timer_activate(tp, TT_REXMT,
 1744                                                       tp->t_rxtcur);
 1745                                 sowwakeup(so);
 1746                                 if (sbavail(&so->so_snd))
 1747                                         (void) tcp_output(tp);
 1748                                 goto check_delack;
 1749                         }
 1750                 } else if (th->th_ack == tp->snd_una &&
 1751                     tlen <= sbspace(&so->so_rcv)) {
 1752                         int newsize = 0;        /* automatic sockbuf scaling */
 1753 
 1754                         /*
 1755                          * This is a pure, in-sequence data packet with
 1756                          * nothing on the reassembly queue and we have enough
 1757                          * buffer space to take it.
 1758                          */
 1759                         if (ti_locked == TI_WLOCKED)
 1760                                 INP_INFO_WUNLOCK(&V_tcbinfo);
 1761                         ti_locked = TI_UNLOCKED;
 1762 
 1763                         /* Clean receiver SACK report if present */
 1764                         if ((tp->t_flags & TF_SACK_PERMIT) && tp->rcv_numsacks)
 1765                                 tcp_clean_sackreport(tp);
 1766                         TCPSTAT_INC(tcps_preddat);
 1767                         tp->rcv_nxt += tlen;
 1768                         /*
 1769                          * Pull snd_wl1 up to prevent seq wrap relative to
 1770                          * th_seq.
 1771                          */
 1772                         tp->snd_wl1 = th->th_seq;
 1773                         /*
 1774                          * Pull rcv_up up to prevent seq wrap relative to
 1775                          * rcv_nxt.
 1776                          */
 1777                         tp->rcv_up = tp->rcv_nxt;
 1778                         TCPSTAT_INC(tcps_rcvpack);
 1779                         TCPSTAT_ADD(tcps_rcvbyte, tlen);
 1780                         ND6_HINT(tp);   /* Some progress has been made */
 1781 #ifdef TCPDEBUG
 1782                         if (so->so_options & SO_DEBUG)
 1783                                 tcp_trace(TA_INPUT, ostate, tp,
 1784                                     (void *)tcp_saveipgen, &tcp_savetcp, 0);
 1785 #endif
 1786                 /*
 1787                  * Automatic sizing of receive socket buffer.  Often the send
 1788                  * buffer size is not optimally adjusted to the actual network
 1789                  * conditions at hand (delay bandwidth product).  Setting the
 1790                  * buffer size too small limits throughput on links with high
 1791                  * bandwidth and high delay (eg. trans-continental/oceanic links).
 1792                  *
 1793                  * On the receive side the socket buffer memory is only rarely
 1794                  * used to any significant extent.  This allows us to be much
 1795                  * more aggressive in scaling the receive socket buffer.  For
 1796                  * the case that the buffer space is actually used to a large
 1797                  * extent and we run out of kernel memory we can simply drop
 1798                  * the new segments; TCP on the sender will just retransmit it
 1799                  * later.  Setting the buffer size too big may only consume too
 1800                  * much kernel memory if the application doesn't read() from
 1801                  * the socket or packet loss or reordering makes use of the
 1802                  * reassembly queue.
 1803                  *
 1804                  * The criteria to step up the receive buffer one notch are:
 1805                  *  1. Application has not set receive buffer size with
 1806                  *     SO_RCVBUF. Setting SO_RCVBUF clears SB_AUTOSIZE.
 1807                  *  2. the number of bytes received during the time it takes
 1808                  *     one timestamp to be reflected back to us (the RTT);
 1809                  *  3. received bytes per RTT is within seven eighth of the
 1810                  *     current socket buffer size;
 1811                  *  4. receive buffer size has not hit maximal automatic size;
 1812                  *
 1813                  * This algorithm does one step per RTT at most and only if
 1814                  * we receive a bulk stream w/o packet losses or reorderings.
 1815                  * Shrinking the buffer during idle times is not necessary as
 1816                  * it doesn't consume any memory when idle.
 1817                  *
 1818                  * TODO: Only step up if the application is actually serving
 1819                  * the buffer to better manage the socket buffer resources.
 1820                  */
 1821                         if (V_tcp_do_autorcvbuf &&
 1822                             to.to_tsecr &&
 1823                             (so->so_rcv.sb_flags & SB_AUTOSIZE)) {
 1824                                 if (TSTMP_GT(to.to_tsecr, tp->rfbuf_ts) &&
 1825                                     to.to_tsecr - tp->rfbuf_ts < hz) {
 1826                                         if (tp->rfbuf_cnt >
 1827                                             (so->so_rcv.sb_hiwat / 8 * 7) &&
 1828                                             so->so_rcv.sb_hiwat <
 1829                                             V_tcp_autorcvbuf_max) {
 1830                                                 newsize =
 1831                                                     min(so->so_rcv.sb_hiwat +
 1832                                                     V_tcp_autorcvbuf_inc,
 1833                                                     V_tcp_autorcvbuf_max);
 1834                                         }
 1835                                         /* Start over with next RTT. */
 1836                                         tp->rfbuf_ts = 0;
 1837                                         tp->rfbuf_cnt = 0;
 1838                                 } else
 1839                                         tp->rfbuf_cnt += tlen;  /* add up */
 1840                         }
 1841 
 1842                         /* Add data to socket buffer. */
 1843                         SOCKBUF_LOCK(&so->so_rcv);
 1844                         if (so->so_rcv.sb_state & SBS_CANTRCVMORE) {
 1845                                 m_freem(m);
 1846                         } else {
 1847                                 /*
 1848                                  * Set new socket buffer size.
 1849                                  * Give up when limit is reached.
 1850                                  */
 1851                                 if (newsize)
 1852                                         if (!sbreserve_locked(&so->so_rcv,
 1853                                             newsize, so, NULL))
 1854                                                 so->so_rcv.sb_flags &= ~SB_AUTOSIZE;
 1855                                 m_adj(m, drop_hdrlen);  /* delayed header drop */
 1856                                 sbappendstream_locked(&so->so_rcv, m, 0);
 1857                         }
 1858                         /* NB: sorwakeup_locked() does an implicit unlock. */
 1859                         sorwakeup_locked(so);
 1860                         if (DELAY_ACK(tp, tlen)) {
 1861                                 tp->t_flags |= TF_DELACK;
 1862                         } else {
 1863                                 tp->t_flags |= TF_ACKNOW;
 1864                                 tcp_output(tp);
 1865                         }
 1866                         goto check_delack;
 1867                 }
 1868         }
 1869 
 1870         /*
 1871          * Calculate amount of space in receive window,
 1872          * and then do TCP input processing.
 1873          * Receive window is amount of space in rcv queue,
 1874          * but not less than advertised window.
 1875          */
 1876         win = sbspace(&so->so_rcv);
 1877         if (win < 0)
 1878                 win = 0;
 1879         tp->rcv_wnd = imax(win, (int)(tp->rcv_adv - tp->rcv_nxt));
 1880 
 1881         /* Reset receive buffer auto scaling when not in bulk receive mode. */
 1882         tp->rfbuf_ts = 0;
 1883         tp->rfbuf_cnt = 0;
 1884 
 1885         switch (tp->t_state) {
 1886 
 1887         /*
 1888          * If the state is SYN_RECEIVED:
 1889          *      if seg contains an ACK, but not for our SYN/ACK, send a RST.
 1890          */
 1891         case TCPS_SYN_RECEIVED:
 1892                 if ((thflags & TH_ACK) &&
 1893                     (SEQ_LEQ(th->th_ack, tp->snd_una) ||
 1894                      SEQ_GT(th->th_ack, tp->snd_max))) {
 1895                                 rstreason = BANDLIM_RST_OPENPORT;
 1896                                 goto dropwithreset;
 1897                 }
 1898                 break;
 1899 
 1900         /*
 1901          * If the state is SYN_SENT:
 1902          *      if seg contains an ACK, but not for our SYN, drop the input.
 1903          *      if seg contains a RST, then drop the connection.
 1904          *      if seg does not contain SYN, then drop it.
 1905          * Otherwise this is an acceptable SYN segment
 1906          *      initialize tp->rcv_nxt and tp->irs
 1907          *      if seg contains ack then advance tp->snd_una
 1908          *      if seg contains an ECE and ECN support is enabled, the stream
 1909          *          is ECN capable.
 1910          *      if SYN has been acked change to ESTABLISHED else SYN_RCVD state
 1911          *      arrange for segment to be acked (eventually)
 1912          *      continue processing rest of data/controls, beginning with URG
 1913          */
 1914         case TCPS_SYN_SENT:
 1915                 if ((thflags & TH_ACK) &&
 1916                     (SEQ_LEQ(th->th_ack, tp->iss) ||
 1917                      SEQ_GT(th->th_ack, tp->snd_max))) {
 1918                         rstreason = BANDLIM_UNLIMITED;
 1919                         goto dropwithreset;
 1920                 }
 1921                 if ((thflags & (TH_ACK|TH_RST)) == (TH_ACK|TH_RST)) {
 1922                         TCP_PROBE5(connect__refused, NULL, tp,
 1923                             mtod(m, const char *), tp, th);
 1924                         tp = tcp_drop(tp, ECONNREFUSED);
 1925                 }
 1926                 if (thflags & TH_RST)
 1927                         goto drop;
 1928                 if (!(thflags & TH_SYN))
 1929                         goto drop;
 1930 
 1931                 tp->irs = th->th_seq;
 1932                 tcp_rcvseqinit(tp);
 1933                 if (thflags & TH_ACK) {
 1934                         TCPSTAT_INC(tcps_connects);
 1935                         soisconnected(so);
 1936 #ifdef MAC
 1937                         mac_socketpeer_set_from_mbuf(m, so);
 1938 #endif
 1939                         /* Do window scaling on this connection? */
 1940                         if ((tp->t_flags & (TF_RCVD_SCALE|TF_REQ_SCALE)) ==
 1941                                 (TF_RCVD_SCALE|TF_REQ_SCALE)) {
 1942                                 tp->rcv_scale = tp->request_r_scale;
 1943                         }
 1944                         tp->rcv_adv += imin(tp->rcv_wnd,
 1945                             TCP_MAXWIN << tp->rcv_scale);
 1946                         tp->snd_una++;          /* SYN is acked */
 1947                         /*
 1948                          * If there's data, delay ACK; if there's also a FIN
 1949                          * ACKNOW will be turned on later.
 1950                          */
 1951                         if (DELAY_ACK(tp, tlen) && tlen != 0)
 1952                                 tcp_timer_activate(tp, TT_DELACK,
 1953                                     tcp_delacktime);
 1954                         else
 1955                                 tp->t_flags |= TF_ACKNOW;
 1956 
 1957                         if ((thflags & TH_ECE) && V_tcp_do_ecn) {
 1958                                 tp->t_flags |= TF_ECN_PERMIT;
 1959                                 TCPSTAT_INC(tcps_ecn_shs);
 1960                         }
 1961                         
 1962                         /*
 1963                          * Received <SYN,ACK> in SYN_SENT[*] state.
 1964                          * Transitions:
 1965                          *      SYN_SENT  --> ESTABLISHED
 1966                          *      SYN_SENT* --> FIN_WAIT_1
 1967                          */
 1968                         tp->t_starttime = ticks;
 1969                         if (tp->t_flags & TF_NEEDFIN) {
 1970                                 tcp_state_change(tp, TCPS_FIN_WAIT_1);
 1971                                 tp->t_flags &= ~TF_NEEDFIN;
 1972                                 thflags &= ~TH_SYN;
 1973                         } else {
 1974                                 tcp_state_change(tp, TCPS_ESTABLISHED);
 1975                                 TCP_PROBE5(connect__established, NULL, tp,
 1976                                     mtod(m, const char *), tp, th);
 1977                                 cc_conn_init(tp);
 1978                                 tcp_timer_activate(tp, TT_KEEP,
 1979                                     TP_KEEPIDLE(tp));
 1980                         }
 1981                 } else {
 1982                         /*
 1983                          * Received initial SYN in SYN-SENT[*] state =>
 1984                          * simultaneous open.
 1985                          * If it succeeds, connection is * half-synchronized.
 1986                          * Otherwise, do 3-way handshake:
 1987                          *        SYN-SENT -> SYN-RECEIVED
 1988                          *        SYN-SENT* -> SYN-RECEIVED*
 1989                          */
 1990                         tp->t_flags |= (TF_ACKNOW | TF_NEEDSYN);
 1991                         tcp_timer_activate(tp, TT_REXMT, 0);
 1992                         tcp_state_change(tp, TCPS_SYN_RECEIVED);
 1993                 }
 1994 
 1995                 KASSERT(ti_locked == TI_WLOCKED, ("%s: trimthenstep6: "
 1996                     "ti_locked %d", __func__, ti_locked));
 1997                 INP_INFO_WLOCK_ASSERT(&V_tcbinfo);
 1998                 INP_WLOCK_ASSERT(tp->t_inpcb);
 1999 
 2000                 /*
 2001                  * Advance th->th_seq to correspond to first data byte.
 2002                  * If data, trim to stay within window,
 2003                  * dropping FIN if necessary.
 2004                  */
 2005                 th->th_seq++;
 2006                 if (tlen > tp->rcv_wnd) {
 2007                         todrop = tlen - tp->rcv_wnd;
 2008                         m_adj(m, -todrop);
 2009                         tlen = tp->rcv_wnd;
 2010                         thflags &= ~TH_FIN;
 2011                         TCPSTAT_INC(tcps_rcvpackafterwin);
 2012                         TCPSTAT_ADD(tcps_rcvbyteafterwin, todrop);
 2013                 }
 2014                 tp->snd_wl1 = th->th_seq - 1;
 2015                 tp->rcv_up = th->th_seq;
 2016                 /*
 2017                  * Client side of transaction: already sent SYN and data.
 2018                  * If the remote host used T/TCP to validate the SYN,
 2019                  * our data will be ACK'd; if so, enter normal data segment
 2020                  * processing in the middle of step 5, ack processing.
 2021                  * Otherwise, goto step 6.
 2022                  */
 2023                 if (thflags & TH_ACK)
 2024                         goto process_ACK;
 2025 
 2026                 goto step6;
 2027 
 2028         /*
 2029          * If the state is LAST_ACK or CLOSING or TIME_WAIT:
 2030          *      do normal processing.
 2031          *
 2032          * NB: Leftover from RFC1644 T/TCP.  Cases to be reused later.
 2033          */
 2034         case TCPS_LAST_ACK:
 2035         case TCPS_CLOSING:
 2036                 break;  /* continue normal processing */
 2037         }
 2038 
 2039         /*
 2040          * States other than LISTEN or SYN_SENT.
 2041          * First check the RST flag and sequence number since reset segments
 2042          * are exempt from the timestamp and connection count tests.  This
 2043          * fixes a bug introduced by the Stevens, vol. 2, p. 960 bugfix
 2044          * below which allowed reset segments in half the sequence space
 2045          * to fall though and be processed (which gives forged reset
 2046          * segments with a random sequence number a 50 percent chance of
 2047          * killing a connection).
 2048          * Then check timestamp, if present.
 2049          * Then check the connection count, if present.
 2050          * Then check that at least some bytes of segment are within
 2051          * receive window.  If segment begins before rcv_nxt,
 2052          * drop leading data (and SYN); if nothing left, just ack.
 2053          */
 2054         if (thflags & TH_RST) {
 2055                 /*
 2056                  * RFC5961 Section 3.2
 2057                  *
 2058                  * - RST drops connection only if SEG.SEQ == RCV.NXT.
 2059                  * - If RST is in window, we send challenge ACK.
 2060                  *
 2061                  * Note: to take into account delayed ACKs, we should
 2062                  *   test against last_ack_sent instead of rcv_nxt.
 2063                  * Note 2: we handle special case of closed window, not
 2064                  *   covered by the RFC.
 2065                  */
 2066                 if ((SEQ_GEQ(th->th_seq, tp->last_ack_sent) &&
 2067                     SEQ_LT(th->th_seq, tp->last_ack_sent + tp->rcv_wnd)) ||
 2068                     (tp->rcv_wnd == 0 && tp->last_ack_sent == th->th_seq)) {
 2069 
 2070                         INP_INFO_WLOCK_ASSERT(&V_tcbinfo);
 2071                         KASSERT(ti_locked == TI_WLOCKED,
 2072                             ("%s: TH_RST ti_locked %d, th %p tp %p",
 2073                             __func__, ti_locked, th, tp));
 2074                         KASSERT(tp->t_state != TCPS_SYN_SENT,
 2075                             ("%s: TH_RST for TCPS_SYN_SENT th %p tp %p",
 2076                             __func__, th, tp));
 2077 
 2078                         if (V_tcp_insecure_rst ||
 2079                             tp->last_ack_sent == th->th_seq) {
 2080                                 TCPSTAT_INC(tcps_drops);
 2081                                 /* Drop the connection. */
 2082                                 switch (tp->t_state) {
 2083                                 case TCPS_SYN_RECEIVED:
 2084                                         so->so_error = ECONNREFUSED;
 2085                                         goto close;
 2086                                 case TCPS_ESTABLISHED:
 2087                                 case TCPS_FIN_WAIT_1:
 2088                                 case TCPS_FIN_WAIT_2:
 2089                                 case TCPS_CLOSE_WAIT:
 2090                                         so->so_error = ECONNRESET;
 2091                                 close:
 2092                                         tcp_state_change(tp, TCPS_CLOSED);
 2093                                         /* FALLTHROUGH */
 2094                                 default:
 2095                                         tp = tcp_close(tp);
 2096                                 }
 2097                         } else {
 2098                                 TCPSTAT_INC(tcps_badrst);
 2099                                 /* Send challenge ACK. */
 2100                                 tcp_respond(tp, mtod(m, void *), th, m,
 2101                                     tp->rcv_nxt, tp->snd_nxt, TH_ACK);
 2102                                 tp->last_ack_sent = tp->rcv_nxt;
 2103                                 m = NULL;
 2104                         }
 2105                 }
 2106                 goto drop;
 2107         }
 2108 
 2109         /*
 2110          * RFC5961 Section 4.2
 2111          * Send challenge ACK for any SYN in synchronized state.
 2112          */
 2113         if ((thflags & TH_SYN) && tp->t_state != TCPS_SYN_SENT) {
 2114                 KASSERT(ti_locked == TI_WLOCKED,
 2115                     ("tcp_do_segment: TH_SYN ti_locked %d", ti_locked));
 2116                 INP_INFO_WLOCK_ASSERT(&V_tcbinfo);
 2117 
 2118                 TCPSTAT_INC(tcps_badsyn);
 2119                 if (V_tcp_insecure_syn &&
 2120                     SEQ_GEQ(th->th_seq, tp->last_ack_sent) &&
 2121                     SEQ_LT(th->th_seq, tp->last_ack_sent + tp->rcv_wnd)) {
 2122                         tp = tcp_drop(tp, ECONNRESET);
 2123                         rstreason = BANDLIM_UNLIMITED;
 2124                 } else {
 2125                         /* Send challenge ACK. */
 2126                         tcp_respond(tp, mtod(m, void *), th, m, tp->rcv_nxt,
 2127                             tp->snd_nxt, TH_ACK);
 2128                         tp->last_ack_sent = tp->rcv_nxt;
 2129                         m = NULL;
 2130                 }
 2131                 goto drop;
 2132         }
 2133 
 2134         /*
 2135          * RFC 1323 PAWS: If we have a timestamp reply on this segment
 2136          * and it's less than ts_recent, drop it.
 2137          */
 2138         if ((to.to_flags & TOF_TS) != 0 && tp->ts_recent &&
 2139             TSTMP_LT(to.to_tsval, tp->ts_recent)) {
 2140 
 2141                 /* Check to see if ts_recent is over 24 days old.  */
 2142                 if (tcp_ts_getticks() - tp->ts_recent_age > TCP_PAWS_IDLE) {
 2143                         /*
 2144                          * Invalidate ts_recent.  If this segment updates
 2145                          * ts_recent, the age will be reset later and ts_recent
 2146                          * will get a valid value.  If it does not, setting
 2147                          * ts_recent to zero will at least satisfy the
 2148                          * requirement that zero be placed in the timestamp
 2149                          * echo reply when ts_recent isn't valid.  The
 2150                          * age isn't reset until we get a valid ts_recent
 2151                          * because we don't want out-of-order segments to be
 2152                          * dropped when ts_recent is old.
 2153                          */
 2154                         tp->ts_recent = 0;
 2155                 } else {
 2156                         TCPSTAT_INC(tcps_rcvduppack);
 2157                         TCPSTAT_ADD(tcps_rcvdupbyte, tlen);
 2158                         TCPSTAT_INC(tcps_pawsdrop);
 2159                         if (tlen)
 2160                                 goto dropafterack;
 2161                         goto drop;
 2162                 }
 2163         }
 2164 
 2165         /*
 2166          * In the SYN-RECEIVED state, validate that the packet belongs to
 2167          * this connection before trimming the data to fit the receive
 2168          * window.  Check the sequence number versus IRS since we know
 2169          * the sequence numbers haven't wrapped.  This is a partial fix
 2170          * for the "LAND" DoS attack.
 2171          */
 2172         if (tp->t_state == TCPS_SYN_RECEIVED && SEQ_LT(th->th_seq, tp->irs)) {
 2173                 rstreason = BANDLIM_RST_OPENPORT;
 2174                 goto dropwithreset;
 2175         }
 2176 
 2177         todrop = tp->rcv_nxt - th->th_seq;
 2178         if (todrop > 0) {
 2179                 if (thflags & TH_SYN) {
 2180                         thflags &= ~TH_SYN;
 2181                         th->th_seq++;
 2182                         if (th->th_urp > 1)
 2183                                 th->th_urp--;
 2184                         else
 2185                                 thflags &= ~TH_URG;
 2186                         todrop--;
 2187                 }
 2188                 /*
 2189                  * Following if statement from Stevens, vol. 2, p. 960.
 2190                  */
 2191                 if (todrop > tlen
 2192                     || (todrop == tlen && (thflags & TH_FIN) == 0)) {
 2193                         /*
 2194                          * Any valid FIN must be to the left of the window.
 2195                          * At this point the FIN must be a duplicate or out
 2196                          * of sequence; drop it.
 2197                          */
 2198                         thflags &= ~TH_FIN;
 2199 
 2200                         /*
 2201                          * Send an ACK to resynchronize and drop any data.
 2202                          * But keep on processing for RST or ACK.
 2203                          */
 2204                         tp->t_flags |= TF_ACKNOW;
 2205                         todrop = tlen;
 2206                         TCPSTAT_INC(tcps_rcvduppack);
 2207                         TCPSTAT_ADD(tcps_rcvdupbyte, todrop);
 2208                 } else {
 2209                         TCPSTAT_INC(tcps_rcvpartduppack);
 2210                         TCPSTAT_ADD(tcps_rcvpartdupbyte, todrop);
 2211                 }
 2212                 drop_hdrlen += todrop;  /* drop from the top afterwards */
 2213                 th->th_seq += todrop;
 2214                 tlen -= todrop;
 2215                 if (th->th_urp > todrop)
 2216                         th->th_urp -= todrop;
 2217                 else {
 2218                         thflags &= ~TH_URG;
 2219                         th->th_urp = 0;
 2220                 }
 2221         }
 2222 
 2223         /*
 2224          * If new data are received on a connection after the
 2225          * user processes are gone, then RST the other end.
 2226          */
 2227         if ((so->so_state & SS_NOFDREF) &&
 2228             tp->t_state > TCPS_CLOSE_WAIT && tlen) {
 2229                 KASSERT(ti_locked == TI_WLOCKED, ("%s: SS_NOFDEREF && "
 2230                     "CLOSE_WAIT && tlen ti_locked %d", __func__, ti_locked));
 2231                 INP_INFO_WLOCK_ASSERT(&V_tcbinfo);
 2232 
 2233                 if ((s = tcp_log_addrs(inc, th, NULL, NULL))) {
 2234                         log(LOG_DEBUG, "%s; %s: %s: Received %d bytes of data "
 2235                             "after socket was closed, "
 2236                             "sending RST and removing tcpcb\n",
 2237                             s, __func__, tcpstates[tp->t_state], tlen);
 2238                         free(s, M_TCPLOG);
 2239                 }
 2240                 tp = tcp_close(tp);
 2241                 TCPSTAT_INC(tcps_rcvafterclose);
 2242                 rstreason = BANDLIM_UNLIMITED;
 2243                 goto dropwithreset;
 2244         }
 2245 
 2246         /*
 2247          * If segment ends after window, drop trailing data
 2248          * (and PUSH and FIN); if nothing left, just ACK.
 2249          */
 2250         todrop = (th->th_seq + tlen) - (tp->rcv_nxt + tp->rcv_wnd);
 2251         if (todrop > 0) {
 2252                 TCPSTAT_INC(tcps_rcvpackafterwin);
 2253                 if (todrop >= tlen) {
 2254                         TCPSTAT_ADD(tcps_rcvbyteafterwin, tlen);
 2255                         /*
 2256                          * If window is closed can only take segments at
 2257                          * window edge, and have to drop data and PUSH from
 2258                          * incoming segments.  Continue processing, but
 2259                          * remember to ack.  Otherwise, drop segment
 2260                          * and ack.
 2261                          */
 2262                         if (tp->rcv_wnd == 0 && th->th_seq == tp->rcv_nxt) {
 2263                                 tp->t_flags |= TF_ACKNOW;
 2264                                 TCPSTAT_INC(tcps_rcvwinprobe);
 2265                         } else
 2266                                 goto dropafterack;
 2267                 } else
 2268                         TCPSTAT_ADD(tcps_rcvbyteafterwin, todrop);
 2269                 m_adj(m, -todrop);
 2270                 tlen -= todrop;
 2271                 thflags &= ~(TH_PUSH|TH_FIN);
 2272         }
 2273 
 2274         /*
 2275          * If last ACK falls within this segment's sequence numbers,
 2276          * record its timestamp.
 2277          * NOTE: 
 2278          * 1) That the test incorporates suggestions from the latest
 2279          *    proposal of the tcplw@cray.com list (Braden 1993/04/26).
 2280          * 2) That updating only on newer timestamps interferes with
 2281          *    our earlier PAWS tests, so this check should be solely
 2282          *    predicated on the sequence space of this segment.
 2283          * 3) That we modify the segment boundary check to be 
 2284          *        Last.ACK.Sent <= SEG.SEQ + SEG.Len  
 2285          *    instead of RFC1323's
 2286          *        Last.ACK.Sent < SEG.SEQ + SEG.Len,
 2287          *    This modified check allows us to overcome RFC1323's
 2288          *    limitations as described in Stevens TCP/IP Illustrated
 2289          *    Vol. 2 p.869. In such cases, we can still calculate the
 2290          *    RTT correctly when RCV.NXT == Last.ACK.Sent.
 2291          */
 2292         if ((to.to_flags & TOF_TS) != 0 &&
 2293             SEQ_LEQ(th->th_seq, tp->last_ack_sent) &&
 2294             SEQ_LEQ(tp->last_ack_sent, th->th_seq + tlen +
 2295                 ((thflags & (TH_SYN|TH_FIN)) != 0))) {
 2296                 tp->ts_recent_age = tcp_ts_getticks();
 2297                 tp->ts_recent = to.to_tsval;
 2298         }
 2299 
 2300         /*
 2301          * If the ACK bit is off:  if in SYN-RECEIVED state or SENDSYN
 2302          * flag is on (half-synchronized state), then queue data for
 2303          * later processing; else drop segment and return.
 2304          */
 2305         if ((thflags & TH_ACK) == 0) {
 2306                 if (tp->t_state == TCPS_SYN_RECEIVED ||
 2307                     (tp->t_flags & TF_NEEDSYN))
 2308                         goto step6;
 2309                 else if (tp->t_flags & TF_ACKNOW)
 2310                         goto dropafterack;
 2311                 else
 2312                         goto drop;
 2313         }
 2314 
 2315         /*
 2316          * Ack processing.
 2317          */
 2318         switch (tp->t_state) {
 2319 
 2320         /*
 2321          * In SYN_RECEIVED state, the ack ACKs our SYN, so enter
 2322          * ESTABLISHED state and continue processing.
 2323          * The ACK was checked above.
 2324          */
 2325         case TCPS_SYN_RECEIVED:
 2326 
 2327                 TCPSTAT_INC(tcps_connects);
 2328                 soisconnected(so);
 2329                 /* Do window scaling? */
 2330                 if ((tp->t_flags & (TF_RCVD_SCALE|TF_REQ_SCALE)) ==
 2331                         (TF_RCVD_SCALE|TF_REQ_SCALE)) {
 2332                         tp->rcv_scale = tp->request_r_scale;
 2333                         tp->snd_wnd = tiwin;
 2334                 }
 2335                 /*
 2336                  * Make transitions:
 2337                  *      SYN-RECEIVED  -> ESTABLISHED
 2338                  *      SYN-RECEIVED* -> FIN-WAIT-1
 2339                  */
 2340                 tp->t_starttime = ticks;
 2341                 if (tp->t_flags & TF_NEEDFIN) {
 2342                         tcp_state_change(tp, TCPS_FIN_WAIT_1);
 2343                         tp->t_flags &= ~TF_NEEDFIN;
 2344                 } else {
 2345                         tcp_state_change(tp, TCPS_ESTABLISHED);
 2346                         TCP_PROBE5(accept__established, NULL, tp,
 2347                             mtod(m, const char *), tp, th);
 2348                         cc_conn_init(tp);
 2349                         tcp_timer_activate(tp, TT_KEEP, TP_KEEPIDLE(tp));
 2350                 }
 2351                 /*
 2352                  * If segment contains data or ACK, will call tcp_reass()
 2353                  * later; if not, do so now to pass queued data to user.
 2354                  */
 2355                 if (tlen == 0 && (thflags & TH_FIN) == 0)
 2356                         (void) tcp_reass(tp, (struct tcphdr *)0, 0,
 2357                             (struct mbuf *)0);
 2358                 tp->snd_wl1 = th->th_seq - 1;
 2359                 /* FALLTHROUGH */
 2360 
 2361         /*
 2362          * In ESTABLISHED state: drop duplicate ACKs; ACK out of range
 2363          * ACKs.  If the ack is in the range
 2364          *      tp->snd_una < th->th_ack <= tp->snd_max
 2365          * then advance tp->snd_una to th->th_ack and drop
 2366          * data from the retransmission queue.  If this ACK reflects
 2367          * more up to date window information we update our window information.
 2368          */
 2369         case TCPS_ESTABLISHED:
 2370         case TCPS_FIN_WAIT_1:
 2371         case TCPS_FIN_WAIT_2:
 2372         case TCPS_CLOSE_WAIT:
 2373         case TCPS_CLOSING:
 2374         case TCPS_LAST_ACK:
 2375                 if (SEQ_GT(th->th_ack, tp->snd_max)) {
 2376                         TCPSTAT_INC(tcps_rcvacktoomuch);
 2377                         goto dropafterack;
 2378                 }
 2379                 if ((tp->t_flags & TF_SACK_PERMIT) &&
 2380                     ((to.to_flags & TOF_SACK) ||
 2381                      !TAILQ_EMPTY(&tp->snd_holes)))
 2382                         tcp_sack_doack(tp, &to, th->th_ack);
 2383 
 2384                 /* Run HHOOK_TCP_ESTABLISHED_IN helper hooks. */
 2385                 hhook_run_tcp_est_in(tp, th, &to);
 2386 
 2387                 if (SEQ_LEQ(th->th_ack, tp->snd_una)) {
 2388                         if (tlen == 0 && tiwin == tp->snd_wnd) {
 2389                                 /*
 2390                                  * If this is the first time we've seen a
 2391                                  * FIN from the remote, this is not a
 2392                                  * duplicate and it needs to be processed
 2393                                  * normally.  This happens during a
 2394                                  * simultaneous close.
 2395                                  */
 2396                                 if ((thflags & TH_FIN) &&
 2397                                     (TCPS_HAVERCVDFIN(tp->t_state) == 0)) {
 2398                                         tp->t_dupacks = 0;
 2399                                         break;
 2400                                 }
 2401                                 TCPSTAT_INC(tcps_rcvdupack);
 2402                                 /*
 2403                                  * If we have outstanding data (other than
 2404                                  * a window probe), this is a completely
 2405                                  * duplicate ack (ie, window info didn't
 2406                                  * change and FIN isn't set),
 2407                                  * the ack is the biggest we've
 2408                                  * seen and we've seen exactly our rexmt
 2409                                  * threshhold of them, assume a packet
 2410                                  * has been dropped and retransmit it.
 2411                                  * Kludge snd_nxt & the congestion
 2412                                  * window so we send only this one
 2413                                  * packet.
 2414                                  *
 2415                                  * We know we're losing at the current
 2416                                  * window size so do congestion avoidance
 2417                                  * (set ssthresh to half the current window
 2418                                  * and pull our congestion window back to
 2419                                  * the new ssthresh).
 2420                                  *
 2421                                  * Dup acks mean that packets have left the
 2422                                  * network (they're now cached at the receiver)
 2423                                  * so bump cwnd by the amount in the receiver
 2424                                  * to keep a constant cwnd packets in the
 2425                                  * network.
 2426                                  *
 2427                                  * When using TCP ECN, notify the peer that
 2428                                  * we reduced the cwnd.
 2429                                  */
 2430                                 if (!tcp_timer_active(tp, TT_REXMT) ||
 2431                                     th->th_ack != tp->snd_una)
 2432                                         tp->t_dupacks = 0;
 2433                                 else if (++tp->t_dupacks > tcprexmtthresh ||
 2434                                      IN_FASTRECOVERY(tp->t_flags)) {
 2435                                         cc_ack_received(tp, th, CC_DUPACK);
 2436                                         if ((tp->t_flags & TF_SACK_PERMIT) &&
 2437                                             IN_FASTRECOVERY(tp->t_flags)) {
 2438                                                 int awnd;
 2439                                                 
 2440                                                 /*
 2441                                                  * Compute the amount of data in flight first.
 2442                                                  * We can inject new data into the pipe iff 
 2443                                                  * we have less than 1/2 the original window's
 2444                                                  * worth of data in flight.
 2445                                                  */
 2446                                                 awnd = (tp->snd_nxt - tp->snd_fack) +
 2447                                                         tp->sackhint.sack_bytes_rexmit;
 2448                                                 if (awnd < tp->snd_ssthresh) {
 2449                                                         tp->snd_cwnd += tp->t_maxseg;
 2450                                                         if (tp->snd_cwnd > tp->snd_ssthresh)
 2451                                                                 tp->snd_cwnd = tp->snd_ssthresh;
 2452                                                 }
 2453                                         } else
 2454                                                 tp->snd_cwnd += tp->t_maxseg;
 2455                                         (void) tcp_output(tp);
 2456                                         goto drop;
 2457                                 } else if (tp->t_dupacks == tcprexmtthresh) {
 2458                                         tcp_seq onxt = tp->snd_nxt;
 2459 
 2460                                         /*
 2461                                          * If we're doing sack, check to
 2462                                          * see if we're already in sack
 2463                                          * recovery. If we're not doing sack,
 2464                                          * check to see if we're in newreno
 2465                                          * recovery.
 2466                                          */
 2467                                         if (tp->t_flags & TF_SACK_PERMIT) {
 2468                                                 if (IN_FASTRECOVERY(tp->t_flags)) {
 2469                                                         tp->t_dupacks = 0;
 2470                                                         break;
 2471                                                 }
 2472                                         } else {
 2473                                                 if (SEQ_LEQ(th->th_ack,
 2474                                                     tp->snd_recover)) {
 2475                                                         tp->t_dupacks = 0;
 2476                                                         break;
 2477                                                 }
 2478                                         }
 2479                                         /* Congestion signal before ack. */
 2480                                         cc_cong_signal(tp, th, CC_NDUPACK);
 2481                                         cc_ack_received(tp, th, CC_DUPACK);
 2482                                         tcp_timer_activate(tp, TT_REXMT, 0);
 2483                                         tp->t_rtttime = 0;
 2484                                         if (tp->t_flags & TF_SACK_PERMIT) {
 2485                                                 TCPSTAT_INC(
 2486                                                     tcps_sack_recovery_episode);
 2487                                                 tp->sack_newdata = tp->snd_nxt;
 2488                                                 tp->snd_cwnd = tp->t_maxseg;
 2489                                                 (void) tcp_output(tp);
 2490                                                 goto drop;
 2491                                         }
 2492                                         tp->snd_nxt = th->th_ack;
 2493                                         tp->snd_cwnd = tp->t_maxseg;
 2494                                         (void) tcp_output(tp);
 2495                                         KASSERT(tp->snd_limited <= 2,
 2496                                             ("%s: tp->snd_limited too big",
 2497                                             __func__));
 2498                                         tp->snd_cwnd = tp->snd_ssthresh +
 2499                                              tp->t_maxseg *
 2500                                              (tp->t_dupacks - tp->snd_limited);
 2501                                         if (SEQ_GT(onxt, tp->snd_nxt))
 2502                                                 tp->snd_nxt = onxt;
 2503                                         goto drop;
 2504                                 } else if (V_tcp_do_rfc3042) {
 2505                                         cc_ack_received(tp, th, CC_DUPACK);
 2506                                         u_long oldcwnd = tp->snd_cwnd;
 2507                                         tcp_seq oldsndmax = tp->snd_max;
 2508                                         u_int sent;
 2509                                         int avail;
 2510 
 2511                                         KASSERT(tp->t_dupacks == 1 ||
 2512                                             tp->t_dupacks == 2,
 2513                                             ("%s: dupacks not 1 or 2",
 2514                                             __func__));
 2515                                         if (tp->t_dupacks == 1)
 2516                                                 tp->snd_limited = 0;
 2517                                         tp->snd_cwnd =
 2518                                             (tp->snd_nxt - tp->snd_una) +
 2519                                             (tp->t_dupacks - tp->snd_limited) *
 2520                                             tp->t_maxseg;
 2521                                         /*
 2522                                          * Only call tcp_output when there
 2523                                          * is new data available to be sent.
 2524                                          * Otherwise we would send pure ACKs.
 2525                                          */
 2526                                         SOCKBUF_LOCK(&so->so_snd);
 2527                                         avail = sbavail(&so->so_snd) -
 2528                                             (tp->snd_nxt - tp->snd_una);
 2529                                         SOCKBUF_UNLOCK(&so->so_snd);
 2530                                         if (avail > 0)
 2531                                                 (void) tcp_output(tp);
 2532                                         sent = tp->snd_max - oldsndmax;
 2533                                         if (sent > tp->t_maxseg) {
 2534                                                 KASSERT((tp->t_dupacks == 2 &&
 2535                                                     tp->snd_limited == 0) ||
 2536                                                    (sent == tp->t_maxseg + 1 &&
 2537                                                     tp->t_flags & TF_SENTFIN),
 2538                                                     ("%s: sent too much",
 2539                                                     __func__));
 2540                                                 tp->snd_limited = 2;
 2541                                         } else if (sent > 0)
 2542                                                 ++tp->snd_limited;
 2543                                         tp->snd_cwnd = oldcwnd;
 2544                                         goto drop;
 2545                                 }
 2546                         } else
 2547                                 tp->t_dupacks = 0;
 2548                         break;
 2549                 }
 2550 
 2551                 KASSERT(SEQ_GT(th->th_ack, tp->snd_una),
 2552                     ("%s: th_ack <= snd_una", __func__));
 2553 
 2554                 /*
 2555                  * If the congestion window was inflated to account
 2556                  * for the other side's cached packets, retract it.
 2557                  */
 2558                 if (IN_FASTRECOVERY(tp->t_flags)) {
 2559                         if (SEQ_LT(th->th_ack, tp->snd_recover)) {
 2560                                 if (tp->t_flags & TF_SACK_PERMIT)
 2561                                         tcp_sack_partialack(tp, th);
 2562                                 else
 2563                                         tcp_newreno_partial_ack(tp, th);
 2564                         } else
 2565                                 cc_post_recovery(tp, th);
 2566                 }
 2567                 tp->t_dupacks = 0;
 2568                 /*
 2569                  * If we reach this point, ACK is not a duplicate,
 2570                  *     i.e., it ACKs something we sent.
 2571                  */
 2572                 if (tp->t_flags & TF_NEEDSYN) {
 2573                         /*
 2574                          * T/TCP: Connection was half-synchronized, and our
 2575                          * SYN has been ACK'd (so connection is now fully
 2576                          * synchronized).  Go to non-starred state,
 2577                          * increment snd_una for ACK of SYN, and check if
 2578                          * we can do window scaling.
 2579                          */
 2580                         tp->t_flags &= ~TF_NEEDSYN;
 2581                         tp->snd_una++;
 2582                         /* Do window scaling? */
 2583                         if ((tp->t_flags & (TF_RCVD_SCALE|TF_REQ_SCALE)) ==
 2584                                 (TF_RCVD_SCALE|TF_REQ_SCALE)) {
 2585                                 tp->rcv_scale = tp->request_r_scale;
 2586                                 /* Send window already scaled. */
 2587                         }
 2588                 }
 2589 
 2590 process_ACK:
 2591                 INP_WLOCK_ASSERT(tp->t_inpcb);
 2592 
 2593                 acked = BYTES_THIS_ACK(tp, th);
 2594                 TCPSTAT_INC(tcps_rcvackpack);
 2595                 TCPSTAT_ADD(tcps_rcvackbyte, acked);
 2596 
 2597                 /*
 2598                  * If we just performed our first retransmit, and the ACK
 2599                  * arrives within our recovery window, then it was a mistake
 2600                  * to do the retransmit in the first place.  Recover our
 2601                  * original cwnd and ssthresh, and proceed to transmit where
 2602                  * we left off.
 2603                  */
 2604                 if (tp->t_rxtshift == 1 && tp->t_flags & TF_PREVVALID &&
 2605                     (int)(ticks - tp->t_badrxtwin) < 0)
 2606                         cc_cong_signal(tp, th, CC_RTO_ERR);
 2607 
 2608                 /*
 2609                  * If we have a timestamp reply, update smoothed
 2610                  * round trip time.  If no timestamp is present but
 2611                  * transmit timer is running and timed sequence
 2612                  * number was acked, update smoothed round trip time.
 2613                  * Since we now have an rtt measurement, cancel the
 2614                  * timer backoff (cf., Phil Karn's retransmit alg.).
 2615                  * Recompute the initial retransmit timer.
 2616                  *
 2617                  * Some boxes send broken timestamp replies
 2618                  * during the SYN+ACK phase, ignore
 2619                  * timestamps of 0 or we could calculate a
 2620                  * huge RTT and blow up the retransmit timer.
 2621                  */
 2622                 if ((to.to_flags & TOF_TS) != 0 && to.to_tsecr) {
 2623                         u_int t;
 2624 
 2625                         t = tcp_ts_getticks() - to.to_tsecr;
 2626                         if (!tp->t_rttlow || tp->t_rttlow > t)
 2627                                 tp->t_rttlow = t;
 2628                         tcp_xmit_timer(tp, TCP_TS_TO_TICKS(t) + 1);
 2629                 } else if (tp->t_rtttime && SEQ_GT(th->th_ack, tp->t_rtseq)) {
 2630                         if (!tp->t_rttlow || tp->t_rttlow > ticks - tp->t_rtttime)
 2631                                 tp->t_rttlow = ticks - tp->t_rtttime;
 2632                         tcp_xmit_timer(tp, ticks - tp->t_rtttime);
 2633                 }
 2634 
 2635                 /*
 2636                  * If all outstanding data is acked, stop retransmit
 2637                  * timer and remember to restart (more output or persist).
 2638                  * If there is more data to be acked, restart retransmit
 2639                  * timer, using current (possibly backed-off) value.
 2640                  */
 2641                 if (th->th_ack == tp->snd_max) {
 2642                         tcp_timer_activate(tp, TT_REXMT, 0);
 2643                         needoutput = 1;
 2644                 } else if (!tcp_timer_active(tp, TT_PERSIST))
 2645                         tcp_timer_activate(tp, TT_REXMT, tp->t_rxtcur);
 2646 
 2647                 /*
 2648                  * If no data (only SYN) was ACK'd,
 2649                  *    skip rest of ACK processing.
 2650                  */
 2651                 if (acked == 0)
 2652                         goto step6;
 2653 
 2654                 /*
 2655                  * Let the congestion control algorithm update congestion
 2656                  * control related information. This typically means increasing
 2657                  * the congestion window.
 2658                  */
 2659                 cc_ack_received(tp, th, CC_ACK);
 2660 
 2661                 SOCKBUF_LOCK(&so->so_snd);
 2662                 if (acked > sbavail(&so->so_snd)) {
 2663                         tp->snd_wnd -= sbavail(&so->so_snd);
 2664                         mfree = sbcut_locked(&so->so_snd,
 2665                             (int)sbavail(&so->so_snd));
 2666                         ourfinisacked = 1;
 2667                 } else {
 2668                         mfree = sbcut_locked(&so->so_snd, acked);
 2669                         tp->snd_wnd -= acked;
 2670                         ourfinisacked = 0;
 2671                 }
 2672                 /* NB: sowwakeup_locked() does an implicit unlock. */
 2673                 sowwakeup_locked(so);
 2674                 m_freem(mfree);
 2675                 /* Detect una wraparound. */
 2676                 if (!IN_RECOVERY(tp->t_flags) &&
 2677                     SEQ_GT(tp->snd_una, tp->snd_recover) &&
 2678                     SEQ_LEQ(th->th_ack, tp->snd_recover))
 2679                         tp->snd_recover = th->th_ack - 1;
 2680                 /* XXXLAS: Can this be moved up into cc_post_recovery? */
 2681                 if (IN_RECOVERY(tp->t_flags) &&
 2682                     SEQ_GEQ(th->th_ack, tp->snd_recover)) {
 2683                         EXIT_RECOVERY(tp->t_flags);
 2684                 }
 2685                 tp->snd_una = th->th_ack;
 2686                 if (tp->t_flags & TF_SACK_PERMIT) {
 2687                         if (SEQ_GT(tp->snd_una, tp->snd_recover))
 2688                                 tp->snd_recover = tp->snd_una;
 2689                 }
 2690                 if (SEQ_LT(tp->snd_nxt, tp->snd_una))
 2691                         tp->snd_nxt = tp->snd_una;
 2692 
 2693                 switch (tp->t_state) {
 2694 
 2695                 /*
 2696                  * In FIN_WAIT_1 STATE in addition to the processing
 2697                  * for the ESTABLISHED state if our FIN is now acknowledged
 2698                  * then enter FIN_WAIT_2.
 2699                  */
 2700                 case TCPS_FIN_WAIT_1:
 2701                         if (ourfinisacked) {
 2702                                 /*
 2703                                  * If we can't receive any more
 2704                                  * data, then closing user can proceed.
 2705                                  * Starting the timer is contrary to the
 2706                                  * specification, but if we don't get a FIN
 2707                                  * we'll hang forever.
 2708                                  *
 2709                                  * XXXjl:
 2710                                  * we should release the tp also, and use a
 2711                                  * compressed state.
 2712                                  */
 2713                                 if (so->so_rcv.sb_state & SBS_CANTRCVMORE) {
 2714                                         soisdisconnected(so);
 2715                                         tcp_timer_activate(tp, TT_2MSL,
 2716                                             (tcp_fast_finwait2_recycle ?
 2717                                             tcp_finwait2_timeout :
 2718                                             TP_MAXIDLE(tp)));
 2719                                 }
 2720                                 tcp_state_change(tp, TCPS_FIN_WAIT_2);
 2721                         }
 2722                         break;
 2723 
 2724                 /*
 2725                  * In CLOSING STATE in addition to the processing for
 2726                  * the ESTABLISHED state if the ACK acknowledges our FIN
 2727                  * then enter the TIME-WAIT state, otherwise ignore
 2728                  * the segment.
 2729                  */
 2730                 case TCPS_CLOSING:
 2731                         if (ourfinisacked) {
 2732                                 INP_INFO_WLOCK_ASSERT(&V_tcbinfo);
 2733                                 tcp_twstart(tp);
 2734                                 INP_INFO_WUNLOCK(&V_tcbinfo);
 2735                                 m_freem(m);
 2736                                 return;
 2737                         }
 2738                         break;
 2739 
 2740                 /*
 2741                  * In LAST_ACK, we may still be waiting for data to drain
 2742                  * and/or to be acked, as well as for the ack of our FIN.
 2743                  * If our FIN is now acknowledged, delete the TCB,
 2744                  * enter the closed state and return.
 2745                  */
 2746                 case TCPS_LAST_ACK:
 2747                         if (ourfinisacked) {
 2748                                 INP_INFO_WLOCK_ASSERT(&V_tcbinfo);
 2749                                 tp = tcp_close(tp);
 2750                                 goto drop;
 2751                         }
 2752                         break;
 2753                 }
 2754         }
 2755 
 2756 step6:
 2757         INP_WLOCK_ASSERT(tp->t_inpcb);
 2758 
 2759         /*
 2760          * Update window information.
 2761          * Don't look at window if no ACK: TAC's send garbage on first SYN.
 2762          */
 2763         if ((thflags & TH_ACK) &&
 2764             (SEQ_LT(tp->snd_wl1, th->th_seq) ||
 2765             (tp->snd_wl1 == th->th_seq && (SEQ_LT(tp->snd_wl2, th->th_ack) ||
 2766              (tp->snd_wl2 == th->th_ack && tiwin > tp->snd_wnd))))) {
 2767                 /* keep track of pure window updates */
 2768                 if (tlen == 0 &&
 2769                     tp->snd_wl2 == th->th_ack && tiwin > tp->snd_wnd)
 2770                         TCPSTAT_INC(tcps_rcvwinupd);
 2771                 tp->snd_wnd = tiwin;
 2772                 tp->snd_wl1 = th->th_seq;
 2773                 tp->snd_wl2 = th->th_ack;
 2774                 if (tp->snd_wnd > tp->max_sndwnd)
 2775                         tp->max_sndwnd = tp->snd_wnd;
 2776                 needoutput = 1;
 2777         }
 2778 
 2779         /*
 2780          * Process segments with URG.
 2781          */
 2782         if ((thflags & TH_URG) && th->th_urp &&
 2783             TCPS_HAVERCVDFIN(tp->t_state) == 0) {
 2784                 /*
 2785                  * This is a kludge, but if we receive and accept
 2786                  * random urgent pointers, we'll crash in
 2787                  * soreceive.  It's hard to imagine someone
 2788                  * actually wanting to send this much urgent data.
 2789                  */
 2790                 SOCKBUF_LOCK(&so->so_rcv);
 2791                 if (th->th_urp + sbavail(&so->so_rcv) > sb_max) {
 2792                         th->th_urp = 0;                 /* XXX */
 2793                         thflags &= ~TH_URG;             /* XXX */
 2794                         SOCKBUF_UNLOCK(&so->so_rcv);    /* XXX */
 2795                         goto dodata;                    /* XXX */
 2796                 }
 2797                 /*
 2798                  * If this segment advances the known urgent pointer,
 2799                  * then mark the data stream.  This should not happen
 2800                  * in CLOSE_WAIT, CLOSING, LAST_ACK or TIME_WAIT STATES since
 2801                  * a FIN has been received from the remote side.
 2802                  * In these states we ignore the URG.
 2803                  *
 2804                  * According to RFC961 (Assigned Protocols),
 2805                  * the urgent pointer points to the last octet
 2806                  * of urgent data.  We continue, however,
 2807                  * to consider it to indicate the first octet
 2808                  * of data past the urgent section as the original
 2809                  * spec states (in one of two places).
 2810                  */
 2811                 if (SEQ_GT(th->th_seq+th->th_urp, tp->rcv_up)) {
 2812                         tp->rcv_up = th->th_seq + th->th_urp;
 2813                         so->so_oobmark = sbavail(&so->so_rcv) +
 2814                             (tp->rcv_up - tp->rcv_nxt) - 1;
 2815                         if (so->so_oobmark == 0)
 2816                                 so->so_rcv.sb_state |= SBS_RCVATMARK;
 2817                         sohasoutofband(so);
 2818                         tp->t_oobflags &= ~(TCPOOB_HAVEDATA | TCPOOB_HADDATA);
 2819                 }
 2820                 SOCKBUF_UNLOCK(&so->so_rcv);
 2821                 /*
 2822                  * Remove out of band data so doesn't get presented to user.
 2823                  * This can happen independent of advancing the URG pointer,
 2824                  * but if two URG's are pending at once, some out-of-band
 2825                  * data may creep in... ick.
 2826                  */
 2827                 if (th->th_urp <= (u_long)tlen &&
 2828                     !(so->so_options & SO_OOBINLINE)) {
 2829                         /* hdr drop is delayed */
 2830                         tcp_pulloutofband(so, th, m, drop_hdrlen);
 2831                 }
 2832         } else {
 2833                 /*
 2834                  * If no out of band data is expected,
 2835                  * pull receive urgent pointer along
 2836                  * with the receive window.
 2837                  */
 2838                 if (SEQ_GT(tp->rcv_nxt, tp->rcv_up))
 2839                         tp->rcv_up = tp->rcv_nxt;
 2840         }
 2841 dodata:                                                 /* XXX */
 2842         INP_WLOCK_ASSERT(tp->t_inpcb);
 2843 
 2844         /*
 2845          * Process the segment text, merging it into the TCP sequencing queue,
 2846          * and arranging for acknowledgment of receipt if necessary.
 2847          * This process logically involves adjusting tp->rcv_wnd as data
 2848          * is presented to the user (this happens in tcp_usrreq.c,
 2849          * case PRU_RCVD).  If a FIN has already been received on this
 2850          * connection then we just ignore the text.
 2851          */
 2852         if ((tlen || (thflags & TH_FIN)) &&
 2853             TCPS_HAVERCVDFIN(tp->t_state) == 0) {
 2854                 tcp_seq save_start = th->th_seq;
 2855                 m_adj(m, drop_hdrlen);  /* delayed header drop */
 2856                 /*
 2857                  * Insert segment which includes th into TCP reassembly queue
 2858                  * with control block tp.  Set thflags to whether reassembly now
 2859                  * includes a segment with FIN.  This handles the common case
 2860                  * inline (segment is the next to be received on an established
 2861                  * connection, and the queue is empty), avoiding linkage into
 2862                  * and removal from the queue and repetition of various
 2863                  * conversions.
 2864                  * Set DELACK for segments received in order, but ack
 2865                  * immediately when segments are out of order (so
 2866                  * fast retransmit can work).
 2867                  */
 2868                 if (th->th_seq == tp->rcv_nxt && tp->t_segq == NULL &&
 2869                     TCPS_HAVEESTABLISHED(tp->t_state)) {
 2870                         if (DELAY_ACK(tp, tlen))
 2871                                 tp->t_flags |= TF_DELACK;
 2872                         else
 2873                                 tp->t_flags |= TF_ACKNOW;
 2874                         tp->rcv_nxt += tlen;
 2875                         thflags = th->th_flags & TH_FIN;
 2876                         TCPSTAT_INC(tcps_rcvpack);
 2877                         TCPSTAT_ADD(tcps_rcvbyte, tlen);
 2878                         ND6_HINT(tp);
 2879                         SOCKBUF_LOCK(&so->so_rcv);
 2880                         if (so->so_rcv.sb_state & SBS_CANTRCVMORE)
 2881                                 m_freem(m);
 2882                         else
 2883                                 sbappendstream_locked(&so->so_rcv, m, 0);
 2884                         /* NB: sorwakeup_locked() does an implicit unlock. */
 2885                         sorwakeup_locked(so);
 2886                 } else {
 2887                         /*
 2888                          * XXX: Due to the header drop above "th" is
 2889                          * theoretically invalid by now.  Fortunately
 2890                          * m_adj() doesn't actually frees any mbufs
 2891                          * when trimming from the head.
 2892                          */
 2893                         thflags = tcp_reass(tp, th, &tlen, m);
 2894                         tp->t_flags |= TF_ACKNOW;
 2895                 }
 2896                 if (tlen > 0 && (tp->t_flags & TF_SACK_PERMIT))
 2897                         tcp_update_sack_list(tp, save_start, save_start + tlen);
 2898 #if 0
 2899                 /*
 2900                  * Note the amount of data that peer has sent into
 2901                  * our window, in order to estimate the sender's
 2902                  * buffer size.
 2903                  * XXX: Unused.
 2904                  */
 2905                 if (SEQ_GT(tp->rcv_adv, tp->rcv_nxt))
 2906                         len = so->so_rcv.sb_hiwat - (tp->rcv_adv - tp->rcv_nxt);
 2907                 else
 2908                         len = so->so_rcv.sb_hiwat;
 2909 #endif
 2910         } else {
 2911                 m_freem(m);
 2912                 thflags &= ~TH_FIN;
 2913         }
 2914 
 2915         /*
 2916          * If FIN is received ACK the FIN and let the user know
 2917          * that the connection is closing.
 2918          */
 2919         if (thflags & TH_FIN) {
 2920                 if (TCPS_HAVERCVDFIN(tp->t_state) == 0) {
 2921                         socantrcvmore(so);
 2922                         /*
 2923                          * If connection is half-synchronized
 2924                          * (ie NEEDSYN flag on) then delay ACK,
 2925                          * so it may be piggybacked when SYN is sent.
 2926                          * Otherwise, since we received a FIN then no
 2927                          * more input can be expected, send ACK now.
 2928                          */
 2929                         if (tp->t_flags & TF_NEEDSYN)
 2930                                 tp->t_flags |= TF_DELACK;
 2931                         else
 2932                                 tp->t_flags |= TF_ACKNOW;
 2933                         tp->rcv_nxt++;
 2934                 }
 2935                 switch (tp->t_state) {
 2936 
 2937                 /*
 2938                  * In SYN_RECEIVED and ESTABLISHED STATES
 2939                  * enter the CLOSE_WAIT state.
 2940                  */
 2941                 case TCPS_SYN_RECEIVED:
 2942                         tp->t_starttime = ticks;
 2943                         /* FALLTHROUGH */
 2944                 case TCPS_ESTABLISHED:
 2945                         tcp_state_change(tp, TCPS_CLOSE_WAIT);
 2946                         break;
 2947 
 2948                 /*
 2949                  * If still in FIN_WAIT_1 STATE FIN has not been acked so
 2950                  * enter the CLOSING state.
 2951                  */
 2952                 case TCPS_FIN_WAIT_1:
 2953                         tcp_state_change(tp, TCPS_CLOSING);
 2954                         break;
 2955 
 2956                 /*
 2957                  * In FIN_WAIT_2 state enter the TIME_WAIT state,
 2958                  * starting the time-wait timer, turning off the other
 2959                  * standard timers.
 2960                  */
 2961                 case TCPS_FIN_WAIT_2:
 2962                         INP_INFO_WLOCK_ASSERT(&V_tcbinfo);
 2963                         KASSERT(ti_locked == TI_WLOCKED, ("%s: dodata "
 2964                             "TCP_FIN_WAIT_2 ti_locked: %d", __func__,
 2965                             ti_locked));
 2966 
 2967                         tcp_twstart(tp);
 2968                         INP_INFO_WUNLOCK(&V_tcbinfo);
 2969                         return;
 2970                 }
 2971         }
 2972         if (ti_locked == TI_WLOCKED)
 2973                 INP_INFO_WUNLOCK(&V_tcbinfo);
 2974         ti_locked = TI_UNLOCKED;
 2975 
 2976 #ifdef TCPDEBUG
 2977         if (so->so_options & SO_DEBUG)
 2978                 tcp_trace(TA_INPUT, ostate, tp, (void *)tcp_saveipgen,
 2979                           &tcp_savetcp, 0);
 2980 #endif
 2981 
 2982         /*
 2983          * Return any desired output.
 2984          */
 2985         if (needoutput || (tp->t_flags & TF_ACKNOW))
 2986                 (void) tcp_output(tp);
 2987 
 2988 check_delack:
 2989         KASSERT(ti_locked == TI_UNLOCKED, ("%s: check_delack ti_locked %d",
 2990             __func__, ti_locked));
 2991         INP_INFO_UNLOCK_ASSERT(&V_tcbinfo);
 2992         INP_WLOCK_ASSERT(tp->t_inpcb);
 2993 
 2994         if (tp->t_flags & TF_DELACK) {
 2995                 tp->t_flags &= ~TF_DELACK;
 2996                 tcp_timer_activate(tp, TT_DELACK, tcp_delacktime);
 2997         }
 2998         INP_WUNLOCK(tp->t_inpcb);
 2999         return;
 3000 
 3001 dropafterack:
 3002         /*
 3003          * Generate an ACK dropping incoming segment if it occupies
 3004          * sequence space, where the ACK reflects our state.
 3005          *
 3006          * We can now skip the test for the RST flag since all
 3007          * paths to this code happen after packets containing
 3008          * RST have been dropped.
 3009          *
 3010          * In the SYN-RECEIVED state, don't send an ACK unless the
 3011          * segment we received passes the SYN-RECEIVED ACK test.
 3012          * If it fails send a RST.  This breaks the loop in the
 3013          * "LAND" DoS attack, and also prevents an ACK storm
 3014          * between two listening ports that have been sent forged
 3015          * SYN segments, each with the source address of the other.
 3016          */
 3017         if (tp->t_state == TCPS_SYN_RECEIVED && (thflags & TH_ACK) &&
 3018             (SEQ_GT(tp->snd_una, th->th_ack) ||
 3019              SEQ_GT(th->th_ack, tp->snd_max)) ) {
 3020                 rstreason = BANDLIM_RST_OPENPORT;
 3021                 goto dropwithreset;
 3022         }
 3023 #ifdef TCPDEBUG
 3024         if (so->so_options & SO_DEBUG)
 3025                 tcp_trace(TA_DROP, ostate, tp, (void *)tcp_saveipgen,
 3026                           &tcp_savetcp, 0);
 3027 #endif
 3028         if (ti_locked == TI_WLOCKED)
 3029                 INP_INFO_WUNLOCK(&V_tcbinfo);
 3030         ti_locked = TI_UNLOCKED;
 3031 
 3032         tp->t_flags |= TF_ACKNOW;
 3033         (void) tcp_output(tp);
 3034         INP_WUNLOCK(tp->t_inpcb);
 3035         m_freem(m);
 3036         return;
 3037 
 3038 dropwithreset:
 3039         if (ti_locked == TI_WLOCKED)
 3040                 INP_INFO_WUNLOCK(&V_tcbinfo);
 3041         ti_locked = TI_UNLOCKED;
 3042 
 3043         if (tp != NULL) {
 3044                 tcp_dropwithreset(m, th, tp, tlen, rstreason);
 3045                 INP_WUNLOCK(tp->t_inpcb);
 3046         } else
 3047                 tcp_dropwithreset(m, th, NULL, tlen, rstreason);
 3048         return;
 3049 
 3050 drop:
 3051         if (ti_locked == TI_WLOCKED) {
 3052                 INP_INFO_WUNLOCK(&V_tcbinfo);
 3053                 ti_locked = TI_UNLOCKED;
 3054         }
 3055 #ifdef INVARIANTS
 3056         else
 3057                 INP_INFO_UNLOCK_ASSERT(&V_tcbinfo);
 3058 #endif
 3059 
 3060         /*
 3061          * Drop space held by incoming segment and return.
 3062          */
 3063 #ifdef TCPDEBUG
 3064         if (tp == NULL || (tp->t_inpcb->inp_socket->so_options & SO_DEBUG))
 3065                 tcp_trace(TA_DROP, ostate, tp, (void *)tcp_saveipgen,
 3066                           &tcp_savetcp, 0);
 3067 #endif
 3068         if (tp != NULL)
 3069                 INP_WUNLOCK(tp->t_inpcb);
 3070         m_freem(m);
 3071 }
 3072 
 3073 /*
 3074  * Issue RST and make ACK acceptable to originator of segment.
 3075  * The mbuf must still include the original packet header.
 3076  * tp may be NULL.
 3077  */
 3078 static void
 3079 tcp_dropwithreset(struct mbuf *m, struct tcphdr *th, struct tcpcb *tp,
 3080     int tlen, int rstreason)
 3081 {
 3082 #ifdef INET
 3083         struct ip *ip;
 3084 #endif
 3085 #ifdef INET6
 3086         struct ip6_hdr *ip6;
 3087 #endif
 3088 
 3089         if (tp != NULL) {
 3090                 INP_WLOCK_ASSERT(tp->t_inpcb);
 3091         }
 3092 
 3093         /* Don't bother if destination was broadcast/multicast. */
 3094         if ((th->th_flags & TH_RST) || m->m_flags & (M_BCAST|M_MCAST))
 3095                 goto drop;
 3096 #ifdef INET6
 3097         if (mtod(m, struct ip *)->ip_v == 6) {
 3098                 ip6 = mtod(m, struct ip6_hdr *);
 3099                 if (IN6_IS_ADDR_MULTICAST(&ip6->ip6_dst) ||
 3100                     IN6_IS_ADDR_MULTICAST(&ip6->ip6_src))
 3101                         goto drop;
 3102                 /* IPv6 anycast check is done at tcp6_input() */
 3103         }
 3104 #endif
 3105 #if defined(INET) && defined(INET6)
 3106         else
 3107 #endif
 3108 #ifdef INET
 3109         {
 3110                 ip = mtod(m, struct ip *);
 3111                 if (IN_MULTICAST(ntohl(ip->ip_dst.s_addr)) ||
 3112                     IN_MULTICAST(ntohl(ip->ip_src.s_addr)) ||
 3113                     ip->ip_src.s_addr == htonl(INADDR_BROADCAST) ||
 3114                     in_broadcast(ip->ip_dst, m->m_pkthdr.rcvif))
 3115                         goto drop;
 3116         }
 3117 #endif
 3118 
 3119         /* Perform bandwidth limiting. */
 3120         if (badport_bandlim(rstreason) < 0)
 3121                 goto drop;
 3122 
 3123         /* tcp_respond consumes the mbuf chain. */
 3124         if (th->th_flags & TH_ACK) {
 3125                 tcp_respond(tp, mtod(m, void *), th, m, (tcp_seq)0,
 3126                     th->th_ack, TH_RST);
 3127         } else {
 3128                 if (th->th_flags & TH_SYN)
 3129                         tlen++;
 3130                 tcp_respond(tp, mtod(m, void *), th, m, th->th_seq+tlen,
 3131                     (tcp_seq)0, TH_RST|TH_ACK);
 3132         }
 3133         return;
 3134 drop:
 3135         m_freem(m);
 3136 }
 3137 
 3138 /*
 3139  * Parse TCP options and place in tcpopt.
 3140  */
 3141 static void
 3142 tcp_dooptions(struct tcpopt *to, u_char *cp, int cnt, int flags)
 3143 {
 3144         int opt, optlen;
 3145 
 3146         to->to_flags = 0;
 3147         for (; cnt > 0; cnt -= optlen, cp += optlen) {
 3148                 opt = cp[0];
 3149                 if (opt == TCPOPT_EOL)
 3150                         break;
 3151                 if (opt == TCPOPT_NOP)
 3152                         optlen = 1;
 3153                 else {
 3154                         if (cnt < 2)
 3155                                 break;
 3156                         optlen = cp[1];
 3157                         if (optlen < 2 || optlen > cnt)
 3158                                 break;
 3159                 }
 3160                 switch (opt) {
 3161                 case TCPOPT_MAXSEG:
 3162                         if (optlen != TCPOLEN_MAXSEG)
 3163                                 continue;
 3164                         if (!(flags & TO_SYN))
 3165                                 continue;
 3166                         to->to_flags |= TOF_MSS;
 3167                         bcopy((char *)cp + 2,
 3168                             (char *)&to->to_mss, sizeof(to->to_mss));
 3169                         to->to_mss = ntohs(to->to_mss);
 3170                         break;
 3171                 case TCPOPT_WINDOW:
 3172                         if (optlen != TCPOLEN_WINDOW)
 3173                                 continue;
 3174                         if (!(flags & TO_SYN))
 3175                                 continue;
 3176                         to->to_flags |= TOF_SCALE;
 3177                         to->to_wscale = min(cp[2], TCP_MAX_WINSHIFT);
 3178                         break;
 3179                 case TCPOPT_TIMESTAMP:
 3180                         if (optlen != TCPOLEN_TIMESTAMP)
 3181                                 continue;
 3182                         to->to_flags |= TOF_TS;
 3183                         bcopy((char *)cp + 2,
 3184                             (char *)&to->to_tsval, sizeof(to->to_tsval));
 3185                         to->to_tsval = ntohl(to->to_tsval);
 3186                         bcopy((char *)cp + 6,
 3187                             (char *)&to->to_tsecr, sizeof(to->to_tsecr));
 3188                         to->to_tsecr = ntohl(to->to_tsecr);
 3189                         break;
 3190 #ifdef TCP_SIGNATURE
 3191                 /*
 3192                  * XXX In order to reply to a host which has set the
 3193                  * TCP_SIGNATURE option in its initial SYN, we have to
 3194                  * record the fact that the option was observed here
 3195                  * for the syncache code to perform the correct response.
 3196                  */
 3197                 case TCPOPT_SIGNATURE:
 3198                         if (optlen != TCPOLEN_SIGNATURE)
 3199                                 continue;
 3200                         to->to_flags |= TOF_SIGNATURE;
 3201                         to->to_signature = cp + 2;
 3202                         break;
 3203 #endif
 3204                 case TCPOPT_SACK_PERMITTED:
 3205                         if (optlen != TCPOLEN_SACK_PERMITTED)
 3206                                 continue;
 3207                         if (!(flags & TO_SYN))
 3208                                 continue;
 3209                         if (!V_tcp_do_sack)
 3210                                 continue;
 3211                         to->to_flags |= TOF_SACKPERM;
 3212                         break;
 3213                 case TCPOPT_SACK:
 3214                         if (optlen <= 2 || (optlen - 2) % TCPOLEN_SACK != 0)
 3215                                 continue;
 3216                         if (flags & TO_SYN)
 3217                                 continue;
 3218                         to->to_flags |= TOF_SACK;
 3219                         to->to_nsacks = (optlen - 2) / TCPOLEN_SACK;
 3220                         to->to_sacks = cp + 2;
 3221                         TCPSTAT_INC(tcps_sack_rcv_blocks);
 3222                         break;
 3223                 default:
 3224                         continue;
 3225                 }
 3226         }
 3227 }
 3228 
 3229 /*
 3230  * Pull out of band byte out of a segment so
 3231  * it doesn't appear in the user's data queue.
 3232  * It is still reflected in the segment length for
 3233  * sequencing purposes.
 3234  */
 3235 static void
 3236 tcp_pulloutofband(struct socket *so, struct tcphdr *th, struct mbuf *m,
 3237     int off)
 3238 {
 3239         int cnt = off + th->th_urp - 1;
 3240 
 3241         while (cnt >= 0) {
 3242                 if (m->m_len > cnt) {
 3243                         char *cp = mtod(m, caddr_t) + cnt;
 3244                         struct tcpcb *tp = sototcpcb(so);
 3245 
 3246                         INP_WLOCK_ASSERT(tp->t_inpcb);
 3247 
 3248                         tp->t_iobc = *cp;
 3249                         tp->t_oobflags |= TCPOOB_HAVEDATA;
 3250                         bcopy(cp+1, cp, (unsigned)(m->m_len - cnt - 1));
 3251                         m->m_len--;
 3252                         if (m->m_flags & M_PKTHDR)
 3253                                 m->m_pkthdr.len--;
 3254                         return;
 3255                 }
 3256                 cnt -= m->m_len;
 3257                 m = m->m_next;
 3258                 if (m == NULL)
 3259                         break;
 3260         }
 3261         panic("tcp_pulloutofband");
 3262 }
 3263 
 3264 /*
 3265  * Collect new round-trip time estimate
 3266  * and update averages and current timeout.
 3267  */
 3268 static void
 3269 tcp_xmit_timer(struct tcpcb *tp, int rtt)
 3270 {
 3271         int delta;
 3272 
 3273         INP_WLOCK_ASSERT(tp->t_inpcb);
 3274 
 3275         TCPSTAT_INC(tcps_rttupdated);
 3276         tp->t_rttupdated++;
 3277         if (tp->t_srtt != 0) {
 3278                 /*
 3279                  * srtt is stored as fixed point with 5 bits after the
 3280                  * binary point (i.e., scaled by 8).  The following magic
 3281                  * is equivalent to the smoothing algorithm in rfc793 with
 3282                  * an alpha of .875 (srtt = rtt/8 + srtt*7/8 in fixed
 3283                  * point).  Adjust rtt to origin 0.
 3284                  */
 3285                 delta = ((rtt - 1) << TCP_DELTA_SHIFT)
 3286                         - (tp->t_srtt >> (TCP_RTT_SHIFT - TCP_DELTA_SHIFT));
 3287 
 3288                 if ((tp->t_srtt += delta) <= 0)
 3289                         tp->t_srtt = 1;
 3290 
 3291                 /*
 3292                  * We accumulate a smoothed rtt variance (actually, a
 3293                  * smoothed mean difference), then set the retransmit
 3294                  * timer to smoothed rtt + 4 times the smoothed variance.
 3295                  * rttvar is stored as fixed point with 4 bits after the
 3296                  * binary point (scaled by 16).  The following is
 3297                  * equivalent to rfc793 smoothing with an alpha of .75
 3298                  * (rttvar = rttvar*3/4 + |delta| / 4).  This replaces
 3299                  * rfc793's wired-in beta.
 3300                  */
 3301                 if (delta < 0)
 3302                         delta = -delta;
 3303                 delta -= tp->t_rttvar >> (TCP_RTTVAR_SHIFT - TCP_DELTA_SHIFT);
 3304                 if ((tp->t_rttvar += delta) <= 0)
 3305                         tp->t_rttvar = 1;
 3306                 if (tp->t_rttbest > tp->t_srtt + tp->t_rttvar)
 3307                     tp->t_rttbest = tp->t_srtt + tp->t_rttvar;
 3308         } else {
 3309                 /*
 3310                  * No rtt measurement yet - use the unsmoothed rtt.
 3311                  * Set the variance to half the rtt (so our first
 3312                  * retransmit happens at 3*rtt).
 3313                  */
 3314                 tp->t_srtt = rtt << TCP_RTT_SHIFT;
 3315                 tp->t_rttvar = rtt << (TCP_RTTVAR_SHIFT - 1);
 3316                 tp->t_rttbest = tp->t_srtt + tp->t_rttvar;
 3317         }
 3318         tp->t_rtttime = 0;
 3319         tp->t_rxtshift = 0;
 3320 
 3321         /*
 3322          * the retransmit should happen at rtt + 4 * rttvar.
 3323          * Because of the way we do the smoothing, srtt and rttvar
 3324          * will each average +1/2 tick of bias.  When we compute
 3325          * the retransmit timer, we want 1/2 tick of rounding and
 3326          * 1 extra tick because of +-1/2 tick uncertainty in the
 3327          * firing of the timer.  The bias will give us exactly the
 3328          * 1.5 tick we need.  But, because the bias is
 3329          * statistical, we have to test that we don't drop below
 3330          * the minimum feasible timer (which is 2 ticks).
 3331          */
 3332         TCPT_RANGESET(tp->t_rxtcur, TCP_REXMTVAL(tp),
 3333                       max(tp->t_rttmin, rtt + 2), TCPTV_REXMTMAX);
 3334 
 3335         /*
 3336          * We received an ack for a packet that wasn't retransmitted;
 3337          * it is probably safe to discard any error indications we've
 3338          * received recently.  This isn't quite right, but close enough
 3339          * for now (a route might have failed after we sent a segment,
 3340          * and the return path might not be symmetrical).
 3341          */
 3342         tp->t_softerror = 0;
 3343 }
 3344 
 3345 /*
 3346  * Determine a reasonable value for maxseg size.
 3347  * If the route is known, check route for mtu.
 3348  * If none, use an mss that can be handled on the outgoing interface
 3349  * without forcing IP to fragment.  If no route is found, route has no mtu,
 3350  * or the destination isn't local, use a default, hopefully conservative
 3351  * size (usually 512 or the default IP max size, but no more than the mtu
 3352  * of the interface), as we can't discover anything about intervening
 3353  * gateways or networks.  We also initialize the congestion/slow start
 3354  * window to be a single segment if the destination isn't local.
 3355  * While looking at the routing entry, we also initialize other path-dependent
 3356  * parameters from pre-set or cached values in the routing entry.
 3357  *
 3358  * Also take into account the space needed for options that we
 3359  * send regularly.  Make maxseg shorter by that amount to assure
 3360  * that we can send maxseg amount of data even when the options
 3361  * are present.  Store the upper limit of the length of options plus
 3362  * data in maxopd.
 3363  *
 3364  * NOTE that this routine is only called when we process an incoming
 3365  * segment, or an ICMP need fragmentation datagram. Outgoing SYN/ACK MSS
 3366  * settings are handled in tcp_mssopt().
 3367  */
 3368 void
 3369 tcp_mss_update(struct tcpcb *tp, int offer, int mtuoffer,
 3370     struct hc_metrics_lite *metricptr, struct tcp_ifcap *cap)
 3371 {
 3372         int mss = 0;
 3373         u_long maxmtu = 0;
 3374         struct inpcb *inp = tp->t_inpcb;
 3375         struct hc_metrics_lite metrics;
 3376         int origoffer;
 3377 #ifdef INET6
 3378         int isipv6 = ((inp->inp_vflag & INP_IPV6) != 0) ? 1 : 0;
 3379         size_t min_protoh = isipv6 ?
 3380                             sizeof (struct ip6_hdr) + sizeof (struct tcphdr) :
 3381                             sizeof (struct tcpiphdr);
 3382 #else
 3383         const size_t min_protoh = sizeof(struct tcpiphdr);
 3384 #endif
 3385 
 3386         INP_WLOCK_ASSERT(tp->t_inpcb);
 3387 
 3388         if (mtuoffer != -1) {
 3389                 KASSERT(offer == -1, ("%s: conflict", __func__));
 3390                 offer = mtuoffer - min_protoh;
 3391         }
 3392         origoffer = offer;
 3393 
 3394         /* Initialize. */
 3395 #ifdef INET6
 3396         if (isipv6) {
 3397                 maxmtu = tcp_maxmtu6(&inp->inp_inc, cap);
 3398                 tp->t_maxopd = tp->t_maxseg = V_tcp_v6mssdflt;
 3399         }
 3400 #endif
 3401 #if defined(INET) && defined(INET6)
 3402         else
 3403 #endif
 3404 #ifdef INET
 3405         {
 3406                 maxmtu = tcp_maxmtu(&inp->inp_inc, cap);
 3407                 tp->t_maxopd = tp->t_maxseg = V_tcp_mssdflt;
 3408         }
 3409 #endif
 3410 
 3411         /*
 3412          * No route to sender, stay with default mss and return.
 3413          */
 3414         if (maxmtu == 0) {
 3415                 /*
 3416                  * In case we return early we need to initialize metrics
 3417                  * to a defined state as tcp_hc_get() would do for us
 3418                  * if there was no cache hit.
 3419                  */
 3420                 if (metricptr != NULL)
 3421                         bzero(metricptr, sizeof(struct hc_metrics_lite));
 3422                 return;
 3423         }
 3424 
 3425         /* What have we got? */
 3426         switch (offer) {
 3427                 case 0:
 3428                         /*
 3429                          * Offer == 0 means that there was no MSS on the SYN
 3430                          * segment, in this case we use tcp_mssdflt as
 3431                          * already assigned to t_maxopd above.
 3432                          */
 3433                         offer = tp->t_maxopd;
 3434                         break;
 3435 
 3436                 case -1:
 3437                         /*
 3438                          * Offer == -1 means that we didn't receive SYN yet.
 3439                          */
 3440                         /* FALLTHROUGH */
 3441 
 3442                 default:
 3443                         /*
 3444                          * Prevent DoS attack with too small MSS. Round up
 3445                          * to at least minmss.
 3446                          */
 3447                         offer = max(offer, V_tcp_minmss);
 3448         }
 3449 
 3450         /*
 3451          * rmx information is now retrieved from tcp_hostcache.
 3452          */
 3453         tcp_hc_get(&inp->inp_inc, &metrics);
 3454         if (metricptr != NULL)
 3455                 bcopy(&metrics, metricptr, sizeof(struct hc_metrics_lite));
 3456 
 3457         /*
 3458          * If there's a discovered mtu in tcp hostcache, use it.
 3459          * Else, use the link mtu.
 3460          */
 3461         if (metrics.rmx_mtu)
 3462                 mss = min(metrics.rmx_mtu, maxmtu) - min_protoh;
 3463         else {
 3464 #ifdef INET6
 3465                 if (isipv6) {
 3466                         mss = maxmtu - min_protoh;
 3467                         if (!V_path_mtu_discovery &&
 3468                             !in6_localaddr(&inp->in6p_faddr))
 3469                                 mss = min(mss, V_tcp_v6mssdflt);
 3470                 }
 3471 #endif
 3472 #if defined(INET) && defined(INET6)
 3473                 else
 3474 #endif
 3475 #ifdef INET
 3476                 {
 3477                         mss = maxmtu - min_protoh;
 3478                         if (!V_path_mtu_discovery &&
 3479                             !in_localaddr(inp->inp_faddr))
 3480                                 mss = min(mss, V_tcp_mssdflt);
 3481                 }
 3482 #endif
 3483                 /*
 3484                  * XXX - The above conditional (mss = maxmtu - min_protoh)
 3485                  * probably violates the TCP spec.
 3486                  * The problem is that, since we don't know the
 3487                  * other end's MSS, we are supposed to use a conservative
 3488                  * default.  But, if we do that, then MTU discovery will
 3489                  * never actually take place, because the conservative
 3490                  * default is much less than the MTUs typically seen
 3491                  * on the Internet today.  For the moment, we'll sweep
 3492                  * this under the carpet.
 3493                  *
 3494                  * The conservative default might not actually be a problem
 3495                  * if the only case this occurs is when sending an initial
 3496                  * SYN with options and data to a host we've never talked
 3497                  * to before.  Then, they will reply with an MSS value which
 3498                  * will get recorded and the new parameters should get
 3499                  * recomputed.  For Further Study.
 3500                  */
 3501         }
 3502         mss = min(mss, offer);
 3503 
 3504         /*
 3505          * Sanity check: make sure that maxopd will be large
 3506          * enough to allow some data on segments even if the
 3507          * all the option space is used (40bytes).  Otherwise
 3508          * funny things may happen in tcp_output.
 3509          */
 3510         mss = max(mss, 64);
 3511 
 3512         /*
 3513          * maxopd stores the maximum length of data AND options
 3514          * in a segment; maxseg is the amount of data in a normal
 3515          * segment.  We need to store this value (maxopd) apart
 3516          * from maxseg, because now every segment carries options
 3517          * and thus we normally have somewhat less data in segments.
 3518          */
 3519         tp->t_maxopd = mss;
 3520 
 3521         /*
 3522          * origoffer==-1 indicates that no segments were received yet.
 3523          * In this case we just guess.
 3524          */
 3525         if ((tp->t_flags & (TF_REQ_TSTMP|TF_NOOPT)) == TF_REQ_TSTMP &&
 3526             (origoffer == -1 ||
 3527              (tp->t_flags & TF_RCVD_TSTMP) == TF_RCVD_TSTMP))
 3528                 mss -= TCPOLEN_TSTAMP_APPA;
 3529 
 3530         tp->t_maxseg = mss;
 3531 }
 3532 
 3533 void
 3534 tcp_mss(struct tcpcb *tp, int offer)
 3535 {
 3536         int mss;
 3537         u_long bufsize;
 3538         struct inpcb *inp;
 3539         struct socket *so;
 3540         struct hc_metrics_lite metrics;
 3541         struct tcp_ifcap cap;
 3542 
 3543         KASSERT(tp != NULL, ("%s: tp == NULL", __func__));
 3544 
 3545         bzero(&cap, sizeof(cap));
 3546         tcp_mss_update(tp, offer, -1, &metrics, &cap);
 3547 
 3548         mss = tp->t_maxseg;
 3549         inp = tp->t_inpcb;
 3550 
 3551         /*
 3552          * If there's a pipesize, change the socket buffer to that size,
 3553          * don't change if sb_hiwat is different than default (then it
 3554          * has been changed on purpose with setsockopt).
 3555          * Make the socket buffers an integral number of mss units;
 3556          * if the mss is larger than the socket buffer, decrease the mss.
 3557          */
 3558         so = inp->inp_socket;
 3559         SOCKBUF_LOCK(&so->so_snd);
 3560         if ((so->so_snd.sb_hiwat == V_tcp_sendspace) && metrics.rmx_sendpipe)
 3561                 bufsize = metrics.rmx_sendpipe;
 3562         else
 3563                 bufsize = so->so_snd.sb_hiwat;
 3564         if (bufsize < mss)
 3565                 mss = bufsize;
 3566         else {
 3567                 bufsize = roundup(bufsize, mss);
 3568                 if (bufsize > sb_max)
 3569                         bufsize = sb_max;
 3570                 if (bufsize > so->so_snd.sb_hiwat)
 3571                         (void)sbreserve_locked(&so->so_snd, bufsize, so, NULL);
 3572         }
 3573         SOCKBUF_UNLOCK(&so->so_snd);
 3574         tp->t_maxseg = mss;
 3575 
 3576         SOCKBUF_LOCK(&so->so_rcv);
 3577         if ((so->so_rcv.sb_hiwat == V_tcp_recvspace) && metrics.rmx_recvpipe)
 3578                 bufsize = metrics.rmx_recvpipe;
 3579         else
 3580                 bufsize = so->so_rcv.sb_hiwat;
 3581         if (bufsize > mss) {
 3582                 bufsize = roundup(bufsize, mss);
 3583                 if (bufsize > sb_max)
 3584                         bufsize = sb_max;
 3585                 if (bufsize > so->so_rcv.sb_hiwat)
 3586                         (void)sbreserve_locked(&so->so_rcv, bufsize, so, NULL);
 3587         }
 3588         SOCKBUF_UNLOCK(&so->so_rcv);
 3589 
 3590         /* Check the interface for TSO capabilities. */
 3591         if (cap.ifcap & CSUM_TSO) {
 3592                 tp->t_flags |= TF_TSO;
 3593                 tp->t_tsomax = cap.tsomax;
 3594                 tp->t_tsomaxsegcount = cap.tsomaxsegcount;
 3595                 tp->t_tsomaxsegsize = cap.tsomaxsegsize;
 3596         }
 3597 }
 3598 
 3599 /*
 3600  * Determine the MSS option to send on an outgoing SYN.
 3601  */
 3602 int
 3603 tcp_mssopt(struct in_conninfo *inc)
 3604 {
 3605         int mss = 0;
 3606         u_long maxmtu = 0;
 3607         u_long thcmtu = 0;
 3608         size_t min_protoh;
 3609 
 3610         KASSERT(inc != NULL, ("tcp_mssopt with NULL in_conninfo pointer"));
 3611 
 3612 #ifdef INET6
 3613         if (inc->inc_flags & INC_ISIPV6) {
 3614                 mss = V_tcp_v6mssdflt;
 3615                 maxmtu = tcp_maxmtu6(inc, NULL);
 3616                 min_protoh = sizeof(struct ip6_hdr) + sizeof(struct tcphdr);
 3617         }
 3618 #endif
 3619 #if defined(INET) && defined(INET6)
 3620         else
 3621 #endif
 3622 #ifdef INET
 3623         {
 3624                 mss = V_tcp_mssdflt;
 3625                 maxmtu = tcp_maxmtu(inc, NULL);
 3626                 min_protoh = sizeof(struct tcpiphdr);
 3627         }
 3628 #endif
 3629 #if defined(INET6) || defined(INET)
 3630         thcmtu = tcp_hc_getmtu(inc); /* IPv4 and IPv6 */
 3631 #endif
 3632 
 3633         if (maxmtu && thcmtu)
 3634                 mss = min(maxmtu, thcmtu) - min_protoh;
 3635         else if (maxmtu || thcmtu)
 3636                 mss = max(maxmtu, thcmtu) - min_protoh;
 3637 
 3638         return (mss);
 3639 }
 3640 
 3641 
 3642 /*
 3643  * On a partial ack arrives, force the retransmission of the
 3644  * next unacknowledged segment.  Do not clear tp->t_dupacks.
 3645  * By setting snd_nxt to ti_ack, this forces retransmission timer to
 3646  * be started again.
 3647  */
 3648 static void
 3649 tcp_newreno_partial_ack(struct tcpcb *tp, struct tcphdr *th)
 3650 {
 3651         tcp_seq onxt = tp->snd_nxt;
 3652         u_long  ocwnd = tp->snd_cwnd;
 3653 
 3654         INP_WLOCK_ASSERT(tp->t_inpcb);
 3655 
 3656         tcp_timer_activate(tp, TT_REXMT, 0);
 3657         tp->t_rtttime = 0;
 3658         tp->snd_nxt = th->th_ack;
 3659         /*
 3660          * Set snd_cwnd to one segment beyond acknowledged offset.
 3661          * (tp->snd_una has not yet been updated when this function is called.)
 3662          */
 3663         tp->snd_cwnd = tp->t_maxseg + BYTES_THIS_ACK(tp, th);
 3664         tp->t_flags |= TF_ACKNOW;
 3665         (void) tcp_output(tp);
 3666         tp->snd_cwnd = ocwnd;
 3667         if (SEQ_GT(onxt, tp->snd_nxt))
 3668                 tp->snd_nxt = onxt;
 3669         /*
 3670          * Partial window deflation.  Relies on fact that tp->snd_una
 3671          * not updated yet.
 3672          */
 3673         if (tp->snd_cwnd > BYTES_THIS_ACK(tp, th))
 3674                 tp->snd_cwnd -= BYTES_THIS_ACK(tp, th);
 3675         else
 3676                 tp->snd_cwnd = 0;
 3677         tp->snd_cwnd += tp->t_maxseg;
 3678 }

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