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 274421 2014-11-12 09:57:15Z glebius $");
   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_flags & INP_HW_FLOWID)
  888             && (m->m_flags & M_FLOWID)
  889             && ((inp->inp_socket == NULL)
  890                 || !(inp->inp_socket->so_options & SO_ACCEPTCONN))) {
  891                 inp->inp_flags |= INP_HW_FLOWID;
  892                 inp->inp_flags &= ~INP_SW_FLOWID;
  893                 inp->inp_flowid = m->m_pkthdr.flowid;
  894                 inp->inp_flowtype = M_HASHTYPE_GET(m);
  895         }
  896 #ifdef IPSEC
  897 #ifdef INET6
  898         if (isipv6 && ipsec6_in_reject(m, inp)) {
  899                 IPSEC6STAT_INC(ips_in_polvio);
  900                 goto dropunlock;
  901         } else
  902 #endif /* INET6 */
  903         if (ipsec4_in_reject(m, inp) != 0) {
  904                 IPSECSTAT_INC(ips_in_polvio);
  905                 goto dropunlock;
  906         }
  907 #endif /* IPSEC */
  908 
  909         /*
  910          * Check the minimum TTL for socket.
  911          */
  912         if (inp->inp_ip_minttl != 0) {
  913 #ifdef INET6
  914                 if (isipv6 && inp->inp_ip_minttl > ip6->ip6_hlim)
  915                         goto dropunlock;
  916                 else
  917 #endif
  918                 if (inp->inp_ip_minttl > ip->ip_ttl)
  919                         goto dropunlock;
  920         }
  921 
  922         /*
  923          * A previous connection in TIMEWAIT state is supposed to catch stray
  924          * or duplicate segments arriving late.  If this segment was a
  925          * legitimate new connection attempt, the old INPCB gets removed and
  926          * we can try again to find a listening socket.
  927          *
  928          * At this point, due to earlier optimism, we may hold only an inpcb
  929          * lock, and not the inpcbinfo write lock.  If so, we need to try to
  930          * acquire it, or if that fails, acquire a reference on the inpcb,
  931          * drop all locks, acquire a global write lock, and then re-acquire
  932          * the inpcb lock.  We may at that point discover that another thread
  933          * has tried to free the inpcb, in which case we need to loop back
  934          * and try to find a new inpcb to deliver to.
  935          *
  936          * XXXRW: It may be time to rethink timewait locking.
  937          */
  938 relocked:
  939         if (inp->inp_flags & INP_TIMEWAIT) {
  940                 if (ti_locked == TI_UNLOCKED) {
  941                         if (INP_INFO_TRY_WLOCK(&V_tcbinfo) == 0) {
  942                                 in_pcbref(inp);
  943                                 INP_WUNLOCK(inp);
  944                                 INP_INFO_WLOCK(&V_tcbinfo);
  945                                 ti_locked = TI_WLOCKED;
  946                                 INP_WLOCK(inp);
  947                                 if (in_pcbrele_wlocked(inp)) {
  948                                         inp = NULL;
  949                                         goto findpcb;
  950                                 }
  951                         } else
  952                                 ti_locked = TI_WLOCKED;
  953                 }
  954                 INP_INFO_WLOCK_ASSERT(&V_tcbinfo);
  955 
  956                 if (thflags & TH_SYN)
  957                         tcp_dooptions(&to, optp, optlen, TO_SYN);
  958                 /*
  959                  * NB: tcp_twcheck unlocks the INP and frees the mbuf.
  960                  */
  961                 if (tcp_twcheck(inp, &to, th, m, tlen))
  962                         goto findpcb;
  963                 INP_INFO_WUNLOCK(&V_tcbinfo);
  964                 return (IPPROTO_DONE);
  965         }
  966         /*
  967          * The TCPCB may no longer exist if the connection is winding
  968          * down or it is in the CLOSED state.  Either way we drop the
  969          * segment and send an appropriate response.
  970          */
  971         tp = intotcpcb(inp);
  972         if (tp == NULL || tp->t_state == TCPS_CLOSED) {
  973                 rstreason = BANDLIM_RST_CLOSEDPORT;
  974                 goto dropwithreset;
  975         }
  976 
  977 #ifdef TCP_OFFLOAD
  978         if (tp->t_flags & TF_TOE) {
  979                 tcp_offload_input(tp, m);
  980                 m = NULL;       /* consumed by the TOE driver */
  981                 goto dropunlock;
  982         }
  983 #endif
  984 
  985         /*
  986          * We've identified a valid inpcb, but it could be that we need an
  987          * inpcbinfo write lock but don't hold it.  In this case, attempt to
  988          * acquire using the same strategy as the TIMEWAIT case above.  If we
  989          * relock, we have to jump back to 'relocked' as the connection might
  990          * now be in TIMEWAIT.
  991          */
  992 #ifdef INVARIANTS
  993         if ((thflags & (TH_FIN | TH_RST)) != 0)
  994                 INP_INFO_WLOCK_ASSERT(&V_tcbinfo);
  995 #endif
  996         if (!((tp->t_state == TCPS_ESTABLISHED && (thflags & TH_SYN) == 0) ||
  997             (tp->t_state == TCPS_LISTEN && (thflags & TH_SYN)))) {
  998                 if (ti_locked == TI_UNLOCKED) {
  999                         if (INP_INFO_TRY_WLOCK(&V_tcbinfo) == 0) {
 1000                                 in_pcbref(inp);
 1001                                 INP_WUNLOCK(inp);
 1002                                 INP_INFO_WLOCK(&V_tcbinfo);
 1003                                 ti_locked = TI_WLOCKED;
 1004                                 INP_WLOCK(inp);
 1005                                 if (in_pcbrele_wlocked(inp)) {
 1006                                         inp = NULL;
 1007                                         goto findpcb;
 1008                                 }
 1009                                 goto relocked;
 1010                         } else
 1011                                 ti_locked = TI_WLOCKED;
 1012                 }
 1013                 INP_INFO_WLOCK_ASSERT(&V_tcbinfo);
 1014         }
 1015 
 1016 #ifdef MAC
 1017         INP_WLOCK_ASSERT(inp);
 1018         if (mac_inpcb_check_deliver(inp, m))
 1019                 goto dropunlock;
 1020 #endif
 1021         so = inp->inp_socket;
 1022         KASSERT(so != NULL, ("%s: so == NULL", __func__));
 1023 #ifdef TCPDEBUG
 1024         if (so->so_options & SO_DEBUG) {
 1025                 ostate = tp->t_state;
 1026 #ifdef INET6
 1027                 if (isipv6) {
 1028                         bcopy((char *)ip6, (char *)tcp_saveipgen, sizeof(*ip6));
 1029                 } else
 1030 #endif
 1031                         bcopy((char *)ip, (char *)tcp_saveipgen, sizeof(*ip));
 1032                 tcp_savetcp = *th;
 1033         }
 1034 #endif /* TCPDEBUG */
 1035         /*
 1036          * When the socket is accepting connections (the INPCB is in LISTEN
 1037          * state) we look into the SYN cache if this is a new connection
 1038          * attempt or the completion of a previous one.
 1039          */
 1040         if (so->so_options & SO_ACCEPTCONN) {
 1041                 struct in_conninfo inc;
 1042 
 1043                 KASSERT(tp->t_state == TCPS_LISTEN, ("%s: so accepting but "
 1044                     "tp not listening", __func__));
 1045                 bzero(&inc, sizeof(inc));
 1046 #ifdef INET6
 1047                 if (isipv6) {
 1048                         inc.inc_flags |= INC_ISIPV6;
 1049                         inc.inc6_faddr = ip6->ip6_src;
 1050                         inc.inc6_laddr = ip6->ip6_dst;
 1051                 } else
 1052 #endif
 1053                 {
 1054                         inc.inc_faddr = ip->ip_src;
 1055                         inc.inc_laddr = ip->ip_dst;
 1056                 }
 1057                 inc.inc_fport = th->th_sport;
 1058                 inc.inc_lport = th->th_dport;
 1059                 inc.inc_fibnum = so->so_fibnum;
 1060 
 1061                 /*
 1062                  * Check for an existing connection attempt in syncache if
 1063                  * the flag is only ACK.  A successful lookup creates a new
 1064                  * socket appended to the listen queue in SYN_RECEIVED state.
 1065                  */
 1066                 if ((thflags & (TH_RST|TH_ACK|TH_SYN)) == TH_ACK) {
 1067 
 1068                         INP_INFO_WLOCK_ASSERT(&V_tcbinfo);
 1069                         /*
 1070                          * Parse the TCP options here because
 1071                          * syncookies need access to the reflected
 1072                          * timestamp.
 1073                          */
 1074                         tcp_dooptions(&to, optp, optlen, 0);
 1075                         /*
 1076                          * NB: syncache_expand() doesn't unlock
 1077                          * inp and tcpinfo locks.
 1078                          */
 1079                         if (!syncache_expand(&inc, &to, th, &so, m)) {
 1080                                 /*
 1081                                  * No syncache entry or ACK was not
 1082                                  * for our SYN/ACK.  Send a RST.
 1083                                  * NB: syncache did its own logging
 1084                                  * of the failure cause.
 1085                                  */
 1086                                 rstreason = BANDLIM_RST_OPENPORT;
 1087                                 goto dropwithreset;
 1088                         }
 1089                         if (so == NULL) {
 1090                                 /*
 1091                                  * We completed the 3-way handshake
 1092                                  * but could not allocate a socket
 1093                                  * either due to memory shortage,
 1094                                  * listen queue length limits or
 1095                                  * global socket limits.  Send RST
 1096                                  * or wait and have the remote end
 1097                                  * retransmit the ACK for another
 1098                                  * try.
 1099                                  */
 1100                                 if ((s = tcp_log_addrs(&inc, th, NULL, NULL)))
 1101                                         log(LOG_DEBUG, "%s; %s: Listen socket: "
 1102                                             "Socket allocation failed due to "
 1103                                             "limits or memory shortage, %s\n",
 1104                                             s, __func__,
 1105                                             V_tcp_sc_rst_sock_fail ?
 1106                                             "sending RST" : "try again");
 1107                                 if (V_tcp_sc_rst_sock_fail) {
 1108                                         rstreason = BANDLIM_UNLIMITED;
 1109                                         goto dropwithreset;
 1110                                 } else
 1111                                         goto dropunlock;
 1112                         }
 1113                         /*
 1114                          * Socket is created in state SYN_RECEIVED.
 1115                          * Unlock the listen socket, lock the newly
 1116                          * created socket and update the tp variable.
 1117                          */
 1118                         INP_WUNLOCK(inp);       /* listen socket */
 1119                         inp = sotoinpcb(so);
 1120                         INP_WLOCK(inp);         /* new connection */
 1121                         tp = intotcpcb(inp);
 1122                         KASSERT(tp->t_state == TCPS_SYN_RECEIVED,
 1123                             ("%s: ", __func__));
 1124 #ifdef TCP_SIGNATURE
 1125                         if (sig_checked == 0)  {
 1126                                 tcp_dooptions(&to, optp, optlen,
 1127                                     (thflags & TH_SYN) ? TO_SYN : 0);
 1128                                 if (!tcp_signature_verify_input(m, off0, tlen,
 1129                                     optlen, &to, th, tp->t_flags)) {
 1130 
 1131                                         /*
 1132                                          * In SYN_SENT state if it receives an
 1133                                          * RST, it is allowed for further
 1134                                          * processing.
 1135                                          */
 1136                                         if ((thflags & TH_RST) == 0 ||
 1137                                             (tp->t_state == TCPS_SYN_SENT) == 0)
 1138                                                 goto dropunlock;
 1139                                 }
 1140                                 sig_checked = 1;
 1141                         }
 1142 #endif
 1143 
 1144                         /*
 1145                          * Process the segment and the data it
 1146                          * contains.  tcp_do_segment() consumes
 1147                          * the mbuf chain and unlocks the inpcb.
 1148                          */
 1149                         tcp_do_segment(m, th, so, tp, drop_hdrlen, tlen,
 1150                             iptos, ti_locked);
 1151                         INP_INFO_UNLOCK_ASSERT(&V_tcbinfo);
 1152                         return (IPPROTO_DONE);
 1153                 }
 1154                 /*
 1155                  * Segment flag validation for new connection attempts:
 1156                  *
 1157                  * Our (SYN|ACK) response was rejected.
 1158                  * Check with syncache and remove entry to prevent
 1159                  * retransmits.
 1160                  *
 1161                  * NB: syncache_chkrst does its own logging of failure
 1162                  * causes.
 1163                  */
 1164                 if (thflags & TH_RST) {
 1165                         syncache_chkrst(&inc, th);
 1166                         goto dropunlock;
 1167                 }
 1168                 /*
 1169                  * We can't do anything without SYN.
 1170                  */
 1171                 if ((thflags & TH_SYN) == 0) {
 1172                         if ((s = tcp_log_addrs(&inc, th, NULL, NULL)))
 1173                                 log(LOG_DEBUG, "%s; %s: Listen socket: "
 1174                                     "SYN is missing, segment ignored\n",
 1175                                     s, __func__);
 1176                         TCPSTAT_INC(tcps_badsyn);
 1177                         goto dropunlock;
 1178                 }
 1179                 /*
 1180                  * (SYN|ACK) is bogus on a listen socket.
 1181                  */
 1182                 if (thflags & TH_ACK) {
 1183                         if ((s = tcp_log_addrs(&inc, th, NULL, NULL)))
 1184                                 log(LOG_DEBUG, "%s; %s: Listen socket: "
 1185                                     "SYN|ACK invalid, segment rejected\n",
 1186                                     s, __func__);
 1187                         syncache_badack(&inc);  /* XXX: Not needed! */
 1188                         TCPSTAT_INC(tcps_badsyn);
 1189                         rstreason = BANDLIM_RST_OPENPORT;
 1190                         goto dropwithreset;
 1191                 }
 1192                 /*
 1193                  * If the drop_synfin option is enabled, drop all
 1194                  * segments with both the SYN and FIN bits set.
 1195                  * This prevents e.g. nmap from identifying the
 1196                  * TCP/IP stack.
 1197                  * XXX: Poor reasoning.  nmap has other methods
 1198                  * and is constantly refining its stack detection
 1199                  * strategies.
 1200                  * XXX: This is a violation of the TCP specification
 1201                  * and was used by RFC1644.
 1202                  */
 1203                 if ((thflags & TH_FIN) && V_drop_synfin) {
 1204                         if ((s = tcp_log_addrs(&inc, th, NULL, NULL)))
 1205                                 log(LOG_DEBUG, "%s; %s: Listen socket: "
 1206                                     "SYN|FIN segment ignored (based on "
 1207                                     "sysctl setting)\n", s, __func__);
 1208                         TCPSTAT_INC(tcps_badsyn);
 1209                         goto dropunlock;
 1210                 }
 1211                 /*
 1212                  * Segment's flags are (SYN) or (SYN|FIN).
 1213                  *
 1214                  * TH_PUSH, TH_URG, TH_ECE, TH_CWR are ignored
 1215                  * as they do not affect the state of the TCP FSM.
 1216                  * The data pointed to by TH_URG and th_urp is ignored.
 1217                  */
 1218                 KASSERT((thflags & (TH_RST|TH_ACK)) == 0,
 1219                     ("%s: Listen socket: TH_RST or TH_ACK set", __func__));
 1220                 KASSERT(thflags & (TH_SYN),
 1221                     ("%s: Listen socket: TH_SYN not set", __func__));
 1222 #ifdef INET6
 1223                 /*
 1224                  * If deprecated address is forbidden,
 1225                  * we do not accept SYN to deprecated interface
 1226                  * address to prevent any new inbound connection from
 1227                  * getting established.
 1228                  * When we do not accept SYN, we send a TCP RST,
 1229                  * with deprecated source address (instead of dropping
 1230                  * it).  We compromise it as it is much better for peer
 1231                  * to send a RST, and RST will be the final packet
 1232                  * for the exchange.
 1233                  *
 1234                  * If we do not forbid deprecated addresses, we accept
 1235                  * the SYN packet.  RFC2462 does not suggest dropping
 1236                  * SYN in this case.
 1237                  * If we decipher RFC2462 5.5.4, it says like this:
 1238                  * 1. use of deprecated addr with existing
 1239                  *    communication is okay - "SHOULD continue to be
 1240                  *    used"
 1241                  * 2. use of it with new communication:
 1242                  *   (2a) "SHOULD NOT be used if alternate address
 1243                  *        with sufficient scope is available"
 1244                  *   (2b) nothing mentioned otherwise.
 1245                  * Here we fall into (2b) case as we have no choice in
 1246                  * our source address selection - we must obey the peer.
 1247                  *
 1248                  * The wording in RFC2462 is confusing, and there are
 1249                  * multiple description text for deprecated address
 1250                  * handling - worse, they are not exactly the same.
 1251                  * I believe 5.5.4 is the best one, so we follow 5.5.4.
 1252                  */
 1253                 if (isipv6 && !V_ip6_use_deprecated) {
 1254                         struct in6_ifaddr *ia6;
 1255 
 1256                         ia6 = in6ifa_ifwithaddr(&ip6->ip6_dst, 0 /* XXX */);
 1257                         if (ia6 != NULL &&
 1258                             (ia6->ia6_flags & IN6_IFF_DEPRECATED)) {
 1259                                 ifa_free(&ia6->ia_ifa);
 1260                                 if ((s = tcp_log_addrs(&inc, th, NULL, NULL)))
 1261                                     log(LOG_DEBUG, "%s; %s: Listen socket: "
 1262                                         "Connection attempt to deprecated "
 1263                                         "IPv6 address rejected\n",
 1264                                         s, __func__);
 1265                                 rstreason = BANDLIM_RST_OPENPORT;
 1266                                 goto dropwithreset;
 1267                         }
 1268                         if (ia6)
 1269                                 ifa_free(&ia6->ia_ifa);
 1270                 }
 1271 #endif /* INET6 */
 1272                 /*
 1273                  * Basic sanity checks on incoming SYN requests:
 1274                  *   Don't respond if the destination is a link layer
 1275                  *      broadcast according to RFC1122 4.2.3.10, p. 104.
 1276                  *   If it is from this socket it must be forged.
 1277                  *   Don't respond if the source or destination is a
 1278                  *      global or subnet broad- or multicast address.
 1279                  *   Note that it is quite possible to receive unicast
 1280                  *      link-layer packets with a broadcast IP address. Use
 1281                  *      in_broadcast() to find them.
 1282                  */
 1283                 if (m->m_flags & (M_BCAST|M_MCAST)) {
 1284                         if ((s = tcp_log_addrs(&inc, th, NULL, NULL)))
 1285                             log(LOG_DEBUG, "%s; %s: Listen socket: "
 1286                                 "Connection attempt from broad- or multicast "
 1287                                 "link layer address ignored\n", s, __func__);
 1288                         goto dropunlock;
 1289                 }
 1290 #ifdef INET6
 1291                 if (isipv6) {
 1292                         if (th->th_dport == th->th_sport &&
 1293                             IN6_ARE_ADDR_EQUAL(&ip6->ip6_dst, &ip6->ip6_src)) {
 1294                                 if ((s = tcp_log_addrs(&inc, th, NULL, NULL)))
 1295                                     log(LOG_DEBUG, "%s; %s: Listen socket: "
 1296                                         "Connection attempt to/from self "
 1297                                         "ignored\n", s, __func__);
 1298                                 goto dropunlock;
 1299                         }
 1300                         if (IN6_IS_ADDR_MULTICAST(&ip6->ip6_dst) ||
 1301                             IN6_IS_ADDR_MULTICAST(&ip6->ip6_src)) {
 1302                                 if ((s = tcp_log_addrs(&inc, th, NULL, NULL)))
 1303                                     log(LOG_DEBUG, "%s; %s: Listen socket: "
 1304                                         "Connection attempt from/to multicast "
 1305                                         "address ignored\n", s, __func__);
 1306                                 goto dropunlock;
 1307                         }
 1308                 }
 1309 #endif
 1310 #if defined(INET) && defined(INET6)
 1311                 else
 1312 #endif
 1313 #ifdef INET
 1314                 {
 1315                         if (th->th_dport == th->th_sport &&
 1316                             ip->ip_dst.s_addr == ip->ip_src.s_addr) {
 1317                                 if ((s = tcp_log_addrs(&inc, th, NULL, NULL)))
 1318                                     log(LOG_DEBUG, "%s; %s: Listen socket: "
 1319                                         "Connection attempt from/to self "
 1320                                         "ignored\n", s, __func__);
 1321                                 goto dropunlock;
 1322                         }
 1323                         if (IN_MULTICAST(ntohl(ip->ip_dst.s_addr)) ||
 1324                             IN_MULTICAST(ntohl(ip->ip_src.s_addr)) ||
 1325                             ip->ip_src.s_addr == htonl(INADDR_BROADCAST) ||
 1326                             in_broadcast(ip->ip_dst, m->m_pkthdr.rcvif)) {
 1327                                 if ((s = tcp_log_addrs(&inc, th, NULL, NULL)))
 1328                                     log(LOG_DEBUG, "%s; %s: Listen socket: "
 1329                                         "Connection attempt from/to broad- "
 1330                                         "or multicast address ignored\n",
 1331                                         s, __func__);
 1332                                 goto dropunlock;
 1333                         }
 1334                 }
 1335 #endif
 1336                 /*
 1337                  * SYN appears to be valid.  Create compressed TCP state
 1338                  * for syncache.
 1339                  */
 1340 #ifdef TCPDEBUG
 1341                 if (so->so_options & SO_DEBUG)
 1342                         tcp_trace(TA_INPUT, ostate, tp,
 1343                             (void *)tcp_saveipgen, &tcp_savetcp, 0);
 1344 #endif
 1345                 tcp_dooptions(&to, optp, optlen, TO_SYN);
 1346                 syncache_add(&inc, &to, th, inp, &so, m, NULL, NULL);
 1347                 /*
 1348                  * Entry added to syncache and mbuf consumed.
 1349                  * Only the listen socket is unlocked by syncache_add().
 1350                  */
 1351                 if (ti_locked == TI_WLOCKED) {
 1352                         INP_INFO_WUNLOCK(&V_tcbinfo);
 1353                         ti_locked = TI_UNLOCKED;
 1354                 }
 1355                 INP_INFO_UNLOCK_ASSERT(&V_tcbinfo);
 1356                 return (IPPROTO_DONE);
 1357         } else if (tp->t_state == TCPS_LISTEN) {
 1358                 /*
 1359                  * When a listen socket is torn down the SO_ACCEPTCONN
 1360                  * flag is removed first while connections are drained
 1361                  * from the accept queue in a unlock/lock cycle of the
 1362                  * ACCEPT_LOCK, opening a race condition allowing a SYN
 1363                  * attempt go through unhandled.
 1364                  */
 1365                 goto dropunlock;
 1366         }
 1367 
 1368 #ifdef TCP_SIGNATURE
 1369         if (sig_checked == 0)  {
 1370                 tcp_dooptions(&to, optp, optlen,
 1371                     (thflags & TH_SYN) ? TO_SYN : 0);
 1372                 if (!tcp_signature_verify_input(m, off0, tlen, optlen, &to,
 1373                     th, tp->t_flags)) {
 1374 
 1375                         /*
 1376                          * In SYN_SENT state if it receives an RST, it is
 1377                          * allowed for further processing.
 1378                          */
 1379                         if ((thflags & TH_RST) == 0 ||
 1380                             (tp->t_state == TCPS_SYN_SENT) == 0)
 1381                                 goto dropunlock;
 1382                 }
 1383                 sig_checked = 1;
 1384         }
 1385 #endif
 1386 
 1387         TCP_PROBE5(receive, NULL, tp, mtod(m, const char *), tp, th);
 1388 
 1389         /*
 1390          * Segment belongs to a connection in SYN_SENT, ESTABLISHED or later
 1391          * state.  tcp_do_segment() always consumes the mbuf chain, unlocks
 1392          * the inpcb, and unlocks pcbinfo.
 1393          */
 1394         tcp_do_segment(m, th, so, tp, drop_hdrlen, tlen, iptos, ti_locked);
 1395         INP_INFO_UNLOCK_ASSERT(&V_tcbinfo);
 1396         return (IPPROTO_DONE);
 1397 
 1398 dropwithreset:
 1399         TCP_PROBE5(receive, NULL, tp, mtod(m, const char *), tp, th);
 1400 
 1401         if (ti_locked == TI_WLOCKED) {
 1402                 INP_INFO_WUNLOCK(&V_tcbinfo);
 1403                 ti_locked = TI_UNLOCKED;
 1404         }
 1405 #ifdef INVARIANTS
 1406         else {
 1407                 KASSERT(ti_locked == TI_UNLOCKED, ("%s: dropwithreset "
 1408                     "ti_locked: %d", __func__, ti_locked));
 1409                 INP_INFO_UNLOCK_ASSERT(&V_tcbinfo);
 1410         }
 1411 #endif
 1412 
 1413         if (inp != NULL) {
 1414                 tcp_dropwithreset(m, th, tp, tlen, rstreason);
 1415                 INP_WUNLOCK(inp);
 1416         } else
 1417                 tcp_dropwithreset(m, th, NULL, tlen, rstreason);
 1418         m = NULL;       /* mbuf chain got consumed. */
 1419         goto drop;
 1420 
 1421 dropunlock:
 1422         if (m != NULL)
 1423                 TCP_PROBE5(receive, NULL, tp, mtod(m, const char *), tp, th);
 1424 
 1425         if (ti_locked == TI_WLOCKED) {
 1426                 INP_INFO_WUNLOCK(&V_tcbinfo);
 1427                 ti_locked = TI_UNLOCKED;
 1428         }
 1429 #ifdef INVARIANTS
 1430         else {
 1431                 KASSERT(ti_locked == TI_UNLOCKED, ("%s: dropunlock "
 1432                     "ti_locked: %d", __func__, ti_locked));
 1433                 INP_INFO_UNLOCK_ASSERT(&V_tcbinfo);
 1434         }
 1435 #endif
 1436 
 1437         if (inp != NULL)
 1438                 INP_WUNLOCK(inp);
 1439 
 1440 drop:
 1441         INP_INFO_UNLOCK_ASSERT(&V_tcbinfo);
 1442         if (s != NULL)
 1443                 free(s, M_TCPLOG);
 1444         if (m != NULL)
 1445                 m_freem(m);
 1446         return (IPPROTO_DONE);
 1447 }
 1448 
 1449 static void
 1450 tcp_do_segment(struct mbuf *m, struct tcphdr *th, struct socket *so,
 1451     struct tcpcb *tp, int drop_hdrlen, int tlen, uint8_t iptos,
 1452     int ti_locked)
 1453 {
 1454         int thflags, acked, ourfinisacked, needoutput = 0;
 1455         int rstreason, todrop, win;
 1456         u_long tiwin;
 1457         char *s;
 1458         struct in_conninfo *inc;
 1459         struct mbuf *mfree;
 1460         struct tcpopt to;
 1461 
 1462 #ifdef TCPDEBUG
 1463         /*
 1464          * The size of tcp_saveipgen must be the size of the max ip header,
 1465          * now IPv6.
 1466          */
 1467         u_char tcp_saveipgen[IP6_HDR_LEN];
 1468         struct tcphdr tcp_savetcp;
 1469         short ostate = 0;
 1470 #endif
 1471         thflags = th->th_flags;
 1472         inc = &tp->t_inpcb->inp_inc;
 1473         tp->sackhint.last_sack_ack = 0;
 1474 
 1475         /*
 1476          * If this is either a state-changing packet or current state isn't
 1477          * established, we require a write lock on tcbinfo.  Otherwise, we
 1478          * allow the tcbinfo to be in either alocked or unlocked, as the
 1479          * caller may have unnecessarily acquired a write lock due to a race.
 1480          */
 1481         if ((thflags & (TH_SYN | TH_FIN | TH_RST)) != 0 ||
 1482             tp->t_state != TCPS_ESTABLISHED) {
 1483                 KASSERT(ti_locked == TI_WLOCKED, ("%s ti_locked %d for "
 1484                     "SYN/FIN/RST/!EST", __func__, ti_locked));
 1485                 INP_INFO_WLOCK_ASSERT(&V_tcbinfo);
 1486         } else {
 1487 #ifdef INVARIANTS
 1488                 if (ti_locked == TI_WLOCKED)
 1489                         INP_INFO_WLOCK_ASSERT(&V_tcbinfo);
 1490                 else {
 1491                         KASSERT(ti_locked == TI_UNLOCKED, ("%s: EST "
 1492                             "ti_locked: %d", __func__, ti_locked));
 1493                         INP_INFO_UNLOCK_ASSERT(&V_tcbinfo);
 1494                 }
 1495 #endif
 1496         }
 1497         INP_WLOCK_ASSERT(tp->t_inpcb);
 1498         KASSERT(tp->t_state > TCPS_LISTEN, ("%s: TCPS_LISTEN",
 1499             __func__));
 1500         KASSERT(tp->t_state != TCPS_TIME_WAIT, ("%s: TCPS_TIME_WAIT",
 1501             __func__));
 1502 
 1503         /*
 1504          * Segment received on connection.
 1505          * Reset idle time and keep-alive timer.
 1506          * XXX: This should be done after segment
 1507          * validation to ignore broken/spoofed segs.
 1508          */
 1509         tp->t_rcvtime = ticks;
 1510         if (TCPS_HAVEESTABLISHED(tp->t_state))
 1511                 tcp_timer_activate(tp, TT_KEEP, TP_KEEPIDLE(tp));
 1512 
 1513         /*
 1514          * Unscale the window into a 32-bit value.
 1515          * For the SYN_SENT state the scale is zero.
 1516          */
 1517         tiwin = th->th_win << tp->snd_scale;
 1518 
 1519         /*
 1520          * TCP ECN processing.
 1521          */
 1522         if (tp->t_flags & TF_ECN_PERMIT) {
 1523                 if (thflags & TH_CWR)
 1524                         tp->t_flags &= ~TF_ECN_SND_ECE;
 1525                 switch (iptos & IPTOS_ECN_MASK) {
 1526                 case IPTOS_ECN_CE:
 1527                         tp->t_flags |= TF_ECN_SND_ECE;
 1528                         TCPSTAT_INC(tcps_ecn_ce);
 1529                         break;
 1530                 case IPTOS_ECN_ECT0:
 1531                         TCPSTAT_INC(tcps_ecn_ect0);
 1532                         break;
 1533                 case IPTOS_ECN_ECT1:
 1534                         TCPSTAT_INC(tcps_ecn_ect1);
 1535                         break;
 1536                 }
 1537                 /* Congestion experienced. */
 1538                 if (thflags & TH_ECE) {
 1539                         cc_cong_signal(tp, th, CC_ECN);
 1540                 }
 1541         }
 1542 
 1543         /*
 1544          * Parse options on any incoming segment.
 1545          */
 1546         tcp_dooptions(&to, (u_char *)(th + 1),
 1547             (th->th_off << 2) - sizeof(struct tcphdr),
 1548             (thflags & TH_SYN) ? TO_SYN : 0);
 1549 
 1550         /*
 1551          * If echoed timestamp is later than the current time,
 1552          * fall back to non RFC1323 RTT calculation.  Normalize
 1553          * timestamp if syncookies were used when this connection
 1554          * was established.
 1555          */
 1556         if ((to.to_flags & TOF_TS) && (to.to_tsecr != 0)) {
 1557                 to.to_tsecr -= tp->ts_offset;
 1558                 if (TSTMP_GT(to.to_tsecr, tcp_ts_getticks()))
 1559                         to.to_tsecr = 0;
 1560         }
 1561         /*
 1562          * If timestamps were negotiated during SYN/ACK they should
 1563          * appear on every segment during this session and vice versa.
 1564          */
 1565         if ((tp->t_flags & TF_RCVD_TSTMP) && !(to.to_flags & TOF_TS)) {
 1566                 if ((s = tcp_log_addrs(inc, th, NULL, NULL))) {
 1567                         log(LOG_DEBUG, "%s; %s: Timestamp missing, "
 1568                             "no action\n", s, __func__);
 1569                         free(s, M_TCPLOG);
 1570                 }
 1571         }
 1572         if (!(tp->t_flags & TF_RCVD_TSTMP) && (to.to_flags & TOF_TS)) {
 1573                 if ((s = tcp_log_addrs(inc, th, NULL, NULL))) {
 1574                         log(LOG_DEBUG, "%s; %s: Timestamp not expected, "
 1575                             "no action\n", s, __func__);
 1576                         free(s, M_TCPLOG);
 1577                 }
 1578         }
 1579 
 1580         /*
 1581          * Process options only when we get SYN/ACK back. The SYN case
 1582          * for incoming connections is handled in tcp_syncache.
 1583          * According to RFC1323 the window field in a SYN (i.e., a <SYN>
 1584          * or <SYN,ACK>) segment itself is never scaled.
 1585          * XXX this is traditional behavior, may need to be cleaned up.
 1586          */
 1587         if (tp->t_state == TCPS_SYN_SENT && (thflags & TH_SYN)) {
 1588                 if ((to.to_flags & TOF_SCALE) &&
 1589                     (tp->t_flags & TF_REQ_SCALE)) {
 1590                         tp->t_flags |= TF_RCVD_SCALE;
 1591                         tp->snd_scale = to.to_wscale;
 1592                 }
 1593                 /*
 1594                  * Initial send window.  It will be updated with
 1595                  * the next incoming segment to the scaled value.
 1596                  */
 1597                 tp->snd_wnd = th->th_win;
 1598                 if (to.to_flags & TOF_TS) {
 1599                         tp->t_flags |= TF_RCVD_TSTMP;
 1600                         tp->ts_recent = to.to_tsval;
 1601                         tp->ts_recent_age = tcp_ts_getticks();
 1602                 }
 1603                 if (to.to_flags & TOF_MSS)
 1604                         tcp_mss(tp, to.to_mss);
 1605                 if ((tp->t_flags & TF_SACK_PERMIT) &&
 1606                     (to.to_flags & TOF_SACKPERM) == 0)
 1607                         tp->t_flags &= ~TF_SACK_PERMIT;
 1608         }
 1609 
 1610         /*
 1611          * Header prediction: check for the two common cases
 1612          * of a uni-directional data xfer.  If the packet has
 1613          * no control flags, is in-sequence, the window didn't
 1614          * change and we're not retransmitting, it's a
 1615          * candidate.  If the length is zero and the ack moved
 1616          * forward, we're the sender side of the xfer.  Just
 1617          * free the data acked & wake any higher level process
 1618          * that was blocked waiting for space.  If the length
 1619          * is non-zero and the ack didn't move, we're the
 1620          * receiver side.  If we're getting packets in-order
 1621          * (the reassembly queue is empty), add the data to
 1622          * the socket buffer and note that we need a delayed ack.
 1623          * Make sure that the hidden state-flags are also off.
 1624          * Since we check for TCPS_ESTABLISHED first, it can only
 1625          * be TH_NEEDSYN.
 1626          */
 1627         if (tp->t_state == TCPS_ESTABLISHED &&
 1628             th->th_seq == tp->rcv_nxt &&
 1629             (thflags & (TH_SYN|TH_FIN|TH_RST|TH_URG|TH_ACK)) == TH_ACK &&
 1630             tp->snd_nxt == tp->snd_max &&
 1631             tiwin && tiwin == tp->snd_wnd && 
 1632             ((tp->t_flags & (TF_NEEDSYN|TF_NEEDFIN)) == 0) &&
 1633             tp->t_segq == NULL && ((to.to_flags & TOF_TS) == 0 ||
 1634              TSTMP_GEQ(to.to_tsval, tp->ts_recent)) ) {
 1635 
 1636                 /*
 1637                  * If last ACK falls within this segment's sequence numbers,
 1638                  * record the timestamp.
 1639                  * NOTE that the test is modified according to the latest
 1640                  * proposal of the tcplw@cray.com list (Braden 1993/04/26).
 1641                  */
 1642                 if ((to.to_flags & TOF_TS) != 0 &&
 1643                     SEQ_LEQ(th->th_seq, tp->last_ack_sent)) {
 1644                         tp->ts_recent_age = tcp_ts_getticks();
 1645                         tp->ts_recent = to.to_tsval;
 1646                 }
 1647 
 1648                 if (tlen == 0) {
 1649                         if (SEQ_GT(th->th_ack, tp->snd_una) &&
 1650                             SEQ_LEQ(th->th_ack, tp->snd_max) &&
 1651                             !IN_RECOVERY(tp->t_flags) &&
 1652                             (to.to_flags & TOF_SACK) == 0 &&
 1653                             TAILQ_EMPTY(&tp->snd_holes)) {
 1654                                 /*
 1655                                  * This is a pure ack for outstanding data.
 1656                                  */
 1657                                 if (ti_locked == TI_WLOCKED)
 1658                                         INP_INFO_WUNLOCK(&V_tcbinfo);
 1659                                 ti_locked = TI_UNLOCKED;
 1660 
 1661                                 TCPSTAT_INC(tcps_predack);
 1662 
 1663                                 /*
 1664                                  * "bad retransmit" recovery.
 1665                                  */
 1666                                 if (tp->t_rxtshift == 1 &&
 1667                                     tp->t_flags & TF_PREVVALID &&
 1668                                     (int)(ticks - tp->t_badrxtwin) < 0) {
 1669                                         cc_cong_signal(tp, th, CC_RTO_ERR);
 1670                                 }
 1671 
 1672                                 /*
 1673                                  * Recalculate the transmit timer / rtt.
 1674                                  *
 1675                                  * Some boxes send broken timestamp replies
 1676                                  * during the SYN+ACK phase, ignore
 1677                                  * timestamps of 0 or we could calculate a
 1678                                  * huge RTT and blow up the retransmit timer.
 1679                                  */
 1680                                 if ((to.to_flags & TOF_TS) != 0 &&
 1681                                     to.to_tsecr) {
 1682                                         u_int t;
 1683 
 1684                                         t = tcp_ts_getticks() - to.to_tsecr;
 1685                                         if (!tp->t_rttlow || tp->t_rttlow > t)
 1686                                                 tp->t_rttlow = t;
 1687                                         tcp_xmit_timer(tp,
 1688                                             TCP_TS_TO_TICKS(t) + 1);
 1689                                 } else if (tp->t_rtttime &&
 1690                                     SEQ_GT(th->th_ack, tp->t_rtseq)) {
 1691                                         if (!tp->t_rttlow ||
 1692                                             tp->t_rttlow > ticks - tp->t_rtttime)
 1693                                                 tp->t_rttlow = ticks - tp->t_rtttime;
 1694                                         tcp_xmit_timer(tp,
 1695                                                         ticks - tp->t_rtttime);
 1696                                 }
 1697                                 acked = BYTES_THIS_ACK(tp, th);
 1698 
 1699                                 /* Run HHOOK_TCP_ESTABLISHED_IN helper hooks. */
 1700                                 hhook_run_tcp_est_in(tp, th, &to);
 1701 
 1702                                 TCPSTAT_INC(tcps_rcvackpack);
 1703                                 TCPSTAT_ADD(tcps_rcvackbyte, acked);
 1704                                 sbdrop(&so->so_snd, acked);
 1705                                 if (SEQ_GT(tp->snd_una, tp->snd_recover) &&
 1706                                     SEQ_LEQ(th->th_ack, tp->snd_recover))
 1707                                         tp->snd_recover = th->th_ack - 1;
 1708                                 
 1709                                 /*
 1710                                  * Let the congestion control algorithm update
 1711                                  * congestion control related information. This
 1712                                  * typically means increasing the congestion
 1713                                  * window.
 1714                                  */
 1715                                 cc_ack_received(tp, th, CC_ACK);
 1716 
 1717                                 tp->snd_una = th->th_ack;
 1718                                 /*
 1719                                  * Pull snd_wl2 up to prevent seq wrap relative
 1720                                  * to th_ack.
 1721                                  */
 1722                                 tp->snd_wl2 = th->th_ack;
 1723                                 tp->t_dupacks = 0;
 1724                                 m_freem(m);
 1725                                 ND6_HINT(tp); /* Some progress has been made. */
 1726 
 1727                                 /*
 1728                                  * If all outstanding data are acked, stop
 1729                                  * retransmit timer, otherwise restart timer
 1730                                  * using current (possibly backed-off) value.
 1731                                  * If process is waiting for space,
 1732                                  * wakeup/selwakeup/signal.  If data
 1733                                  * are ready to send, let tcp_output
 1734                                  * decide between more output or persist.
 1735                                  */
 1736 #ifdef TCPDEBUG
 1737                                 if (so->so_options & SO_DEBUG)
 1738                                         tcp_trace(TA_INPUT, ostate, tp,
 1739                                             (void *)tcp_saveipgen,
 1740                                             &tcp_savetcp, 0);
 1741 #endif
 1742                                 if (tp->snd_una == tp->snd_max)
 1743                                         tcp_timer_activate(tp, TT_REXMT, 0);
 1744                                 else if (!tcp_timer_active(tp, TT_PERSIST))
 1745                                         tcp_timer_activate(tp, TT_REXMT,
 1746                                                       tp->t_rxtcur);
 1747                                 sowwakeup(so);
 1748                                 if (sbavail(&so->so_snd))
 1749                                         (void) tcp_output(tp);
 1750                                 goto check_delack;
 1751                         }
 1752                 } else if (th->th_ack == tp->snd_una &&
 1753                     tlen <= sbspace(&so->so_rcv)) {
 1754                         int newsize = 0;        /* automatic sockbuf scaling */
 1755 
 1756                         /*
 1757                          * This is a pure, in-sequence data packet with
 1758                          * nothing on the reassembly queue and we have enough
 1759                          * buffer space to take it.
 1760                          */
 1761                         if (ti_locked == TI_WLOCKED)
 1762                                 INP_INFO_WUNLOCK(&V_tcbinfo);
 1763                         ti_locked = TI_UNLOCKED;
 1764 
 1765                         /* Clean receiver SACK report if present */
 1766                         if ((tp->t_flags & TF_SACK_PERMIT) && tp->rcv_numsacks)
 1767                                 tcp_clean_sackreport(tp);
 1768                         TCPSTAT_INC(tcps_preddat);
 1769                         tp->rcv_nxt += tlen;
 1770                         /*
 1771                          * Pull snd_wl1 up to prevent seq wrap relative to
 1772                          * th_seq.
 1773                          */
 1774                         tp->snd_wl1 = th->th_seq;
 1775                         /*
 1776                          * Pull rcv_up up to prevent seq wrap relative to
 1777                          * rcv_nxt.
 1778                          */
 1779                         tp->rcv_up = tp->rcv_nxt;
 1780                         TCPSTAT_INC(tcps_rcvpack);
 1781                         TCPSTAT_ADD(tcps_rcvbyte, tlen);
 1782                         ND6_HINT(tp);   /* Some progress has been made */
 1783 #ifdef TCPDEBUG
 1784                         if (so->so_options & SO_DEBUG)
 1785                                 tcp_trace(TA_INPUT, ostate, tp,
 1786                                     (void *)tcp_saveipgen, &tcp_savetcp, 0);
 1787 #endif
 1788                 /*
 1789                  * Automatic sizing of receive socket buffer.  Often the send
 1790                  * buffer size is not optimally adjusted to the actual network
 1791                  * conditions at hand (delay bandwidth product).  Setting the
 1792                  * buffer size too small limits throughput on links with high
 1793                  * bandwidth and high delay (eg. trans-continental/oceanic links).
 1794                  *
 1795                  * On the receive side the socket buffer memory is only rarely
 1796                  * used to any significant extent.  This allows us to be much
 1797                  * more aggressive in scaling the receive socket buffer.  For
 1798                  * the case that the buffer space is actually used to a large
 1799                  * extent and we run out of kernel memory we can simply drop
 1800                  * the new segments; TCP on the sender will just retransmit it
 1801                  * later.  Setting the buffer size too big may only consume too
 1802                  * much kernel memory if the application doesn't read() from
 1803                  * the socket or packet loss or reordering makes use of the
 1804                  * reassembly queue.
 1805                  *
 1806                  * The criteria to step up the receive buffer one notch are:
 1807                  *  1. Application has not set receive buffer size with
 1808                  *     SO_RCVBUF. Setting SO_RCVBUF clears SB_AUTOSIZE.
 1809                  *  2. the number of bytes received during the time it takes
 1810                  *     one timestamp to be reflected back to us (the RTT);
 1811                  *  3. received bytes per RTT is within seven eighth of the
 1812                  *     current socket buffer size;
 1813                  *  4. receive buffer size has not hit maximal automatic size;
 1814                  *
 1815                  * This algorithm does one step per RTT at most and only if
 1816                  * we receive a bulk stream w/o packet losses or reorderings.
 1817                  * Shrinking the buffer during idle times is not necessary as
 1818                  * it doesn't consume any memory when idle.
 1819                  *
 1820                  * TODO: Only step up if the application is actually serving
 1821                  * the buffer to better manage the socket buffer resources.
 1822                  */
 1823                         if (V_tcp_do_autorcvbuf &&
 1824                             to.to_tsecr &&
 1825                             (so->so_rcv.sb_flags & SB_AUTOSIZE)) {
 1826                                 if (TSTMP_GT(to.to_tsecr, tp->rfbuf_ts) &&
 1827                                     to.to_tsecr - tp->rfbuf_ts < hz) {
 1828                                         if (tp->rfbuf_cnt >
 1829                                             (so->so_rcv.sb_hiwat / 8 * 7) &&
 1830                                             so->so_rcv.sb_hiwat <
 1831                                             V_tcp_autorcvbuf_max) {
 1832                                                 newsize =
 1833                                                     min(so->so_rcv.sb_hiwat +
 1834                                                     V_tcp_autorcvbuf_inc,
 1835                                                     V_tcp_autorcvbuf_max);
 1836                                         }
 1837                                         /* Start over with next RTT. */
 1838                                         tp->rfbuf_ts = 0;
 1839                                         tp->rfbuf_cnt = 0;
 1840                                 } else
 1841                                         tp->rfbuf_cnt += tlen;  /* add up */
 1842                         }
 1843 
 1844                         /* Add data to socket buffer. */
 1845                         SOCKBUF_LOCK(&so->so_rcv);
 1846                         if (so->so_rcv.sb_state & SBS_CANTRCVMORE) {
 1847                                 m_freem(m);
 1848                         } else {
 1849                                 /*
 1850                                  * Set new socket buffer size.
 1851                                  * Give up when limit is reached.
 1852                                  */
 1853                                 if (newsize)
 1854                                         if (!sbreserve_locked(&so->so_rcv,
 1855                                             newsize, so, NULL))
 1856                                                 so->so_rcv.sb_flags &= ~SB_AUTOSIZE;
 1857                                 m_adj(m, drop_hdrlen);  /* delayed header drop */
 1858                                 sbappendstream_locked(&so->so_rcv, m);
 1859                         }
 1860                         /* NB: sorwakeup_locked() does an implicit unlock. */
 1861                         sorwakeup_locked(so);
 1862                         if (DELAY_ACK(tp, tlen)) {
 1863                                 tp->t_flags |= TF_DELACK;
 1864                         } else {
 1865                                 tp->t_flags |= TF_ACKNOW;
 1866                                 tcp_output(tp);
 1867                         }
 1868                         goto check_delack;
 1869                 }
 1870         }
 1871 
 1872         /*
 1873          * Calculate amount of space in receive window,
 1874          * and then do TCP input processing.
 1875          * Receive window is amount of space in rcv queue,
 1876          * but not less than advertised window.
 1877          */
 1878         win = sbspace(&so->so_rcv);
 1879         if (win < 0)
 1880                 win = 0;
 1881         tp->rcv_wnd = imax(win, (int)(tp->rcv_adv - tp->rcv_nxt));
 1882 
 1883         /* Reset receive buffer auto scaling when not in bulk receive mode. */
 1884         tp->rfbuf_ts = 0;
 1885         tp->rfbuf_cnt = 0;
 1886 
 1887         switch (tp->t_state) {
 1888 
 1889         /*
 1890          * If the state is SYN_RECEIVED:
 1891          *      if seg contains an ACK, but not for our SYN/ACK, send a RST.
 1892          */
 1893         case TCPS_SYN_RECEIVED:
 1894                 if ((thflags & TH_ACK) &&
 1895                     (SEQ_LEQ(th->th_ack, tp->snd_una) ||
 1896                      SEQ_GT(th->th_ack, tp->snd_max))) {
 1897                                 rstreason = BANDLIM_RST_OPENPORT;
 1898                                 goto dropwithreset;
 1899                 }
 1900                 break;
 1901 
 1902         /*
 1903          * If the state is SYN_SENT:
 1904          *      if seg contains an ACK, but not for our SYN, drop the input.
 1905          *      if seg contains a RST, then drop the connection.
 1906          *      if seg does not contain SYN, then drop it.
 1907          * Otherwise this is an acceptable SYN segment
 1908          *      initialize tp->rcv_nxt and tp->irs
 1909          *      if seg contains ack then advance tp->snd_una
 1910          *      if seg contains an ECE and ECN support is enabled, the stream
 1911          *          is ECN capable.
 1912          *      if SYN has been acked change to ESTABLISHED else SYN_RCVD state
 1913          *      arrange for segment to be acked (eventually)
 1914          *      continue processing rest of data/controls, beginning with URG
 1915          */
 1916         case TCPS_SYN_SENT:
 1917                 if ((thflags & TH_ACK) &&
 1918                     (SEQ_LEQ(th->th_ack, tp->iss) ||
 1919                      SEQ_GT(th->th_ack, tp->snd_max))) {
 1920                         rstreason = BANDLIM_UNLIMITED;
 1921                         goto dropwithreset;
 1922                 }
 1923                 if ((thflags & (TH_ACK|TH_RST)) == (TH_ACK|TH_RST)) {
 1924                         TCP_PROBE5(connect__refused, NULL, tp,
 1925                             mtod(m, const char *), tp, th);
 1926                         tp = tcp_drop(tp, ECONNREFUSED);
 1927                 }
 1928                 if (thflags & TH_RST)
 1929                         goto drop;
 1930                 if (!(thflags & TH_SYN))
 1931                         goto drop;
 1932 
 1933                 tp->irs = th->th_seq;
 1934                 tcp_rcvseqinit(tp);
 1935                 if (thflags & TH_ACK) {
 1936                         TCPSTAT_INC(tcps_connects);
 1937                         soisconnected(so);
 1938 #ifdef MAC
 1939                         mac_socketpeer_set_from_mbuf(m, so);
 1940 #endif
 1941                         /* Do window scaling on this connection? */
 1942                         if ((tp->t_flags & (TF_RCVD_SCALE|TF_REQ_SCALE)) ==
 1943                                 (TF_RCVD_SCALE|TF_REQ_SCALE)) {
 1944                                 tp->rcv_scale = tp->request_r_scale;
 1945                         }
 1946                         tp->rcv_adv += imin(tp->rcv_wnd,
 1947                             TCP_MAXWIN << tp->rcv_scale);
 1948                         tp->snd_una++;          /* SYN is acked */
 1949                         /*
 1950                          * If there's data, delay ACK; if there's also a FIN
 1951                          * ACKNOW will be turned on later.
 1952                          */
 1953                         if (DELAY_ACK(tp, tlen) && tlen != 0)
 1954                                 tcp_timer_activate(tp, TT_DELACK,
 1955                                     tcp_delacktime);
 1956                         else
 1957                                 tp->t_flags |= TF_ACKNOW;
 1958 
 1959                         if ((thflags & TH_ECE) && V_tcp_do_ecn) {
 1960                                 tp->t_flags |= TF_ECN_PERMIT;
 1961                                 TCPSTAT_INC(tcps_ecn_shs);
 1962                         }
 1963                         
 1964                         /*
 1965                          * Received <SYN,ACK> in SYN_SENT[*] state.
 1966                          * Transitions:
 1967                          *      SYN_SENT  --> ESTABLISHED
 1968                          *      SYN_SENT* --> FIN_WAIT_1
 1969                          */
 1970                         tp->t_starttime = ticks;
 1971                         if (tp->t_flags & TF_NEEDFIN) {
 1972                                 tcp_state_change(tp, TCPS_FIN_WAIT_1);
 1973                                 tp->t_flags &= ~TF_NEEDFIN;
 1974                                 thflags &= ~TH_SYN;
 1975                         } else {
 1976                                 tcp_state_change(tp, TCPS_ESTABLISHED);
 1977                                 TCP_PROBE5(connect__established, NULL, tp,
 1978                                     mtod(m, const char *), tp, th);
 1979                                 cc_conn_init(tp);
 1980                                 tcp_timer_activate(tp, TT_KEEP,
 1981                                     TP_KEEPIDLE(tp));
 1982                         }
 1983                 } else {
 1984                         /*
 1985                          * Received initial SYN in SYN-SENT[*] state =>
 1986                          * simultaneous open.
 1987                          * If it succeeds, connection is * half-synchronized.
 1988                          * Otherwise, do 3-way handshake:
 1989                          *        SYN-SENT -> SYN-RECEIVED
 1990                          *        SYN-SENT* -> SYN-RECEIVED*
 1991                          */
 1992                         tp->t_flags |= (TF_ACKNOW | TF_NEEDSYN);
 1993                         tcp_timer_activate(tp, TT_REXMT, 0);
 1994                         tcp_state_change(tp, TCPS_SYN_RECEIVED);
 1995                 }
 1996 
 1997                 KASSERT(ti_locked == TI_WLOCKED, ("%s: trimthenstep6: "
 1998                     "ti_locked %d", __func__, ti_locked));
 1999                 INP_INFO_WLOCK_ASSERT(&V_tcbinfo);
 2000                 INP_WLOCK_ASSERT(tp->t_inpcb);
 2001 
 2002                 /*
 2003                  * Advance th->th_seq to correspond to first data byte.
 2004                  * If data, trim to stay within window,
 2005                  * dropping FIN if necessary.
 2006                  */
 2007                 th->th_seq++;
 2008                 if (tlen > tp->rcv_wnd) {
 2009                         todrop = tlen - tp->rcv_wnd;
 2010                         m_adj(m, -todrop);
 2011                         tlen = tp->rcv_wnd;
 2012                         thflags &= ~TH_FIN;
 2013                         TCPSTAT_INC(tcps_rcvpackafterwin);
 2014                         TCPSTAT_ADD(tcps_rcvbyteafterwin, todrop);
 2015                 }
 2016                 tp->snd_wl1 = th->th_seq - 1;
 2017                 tp->rcv_up = th->th_seq;
 2018                 /*
 2019                  * Client side of transaction: already sent SYN and data.
 2020                  * If the remote host used T/TCP to validate the SYN,
 2021                  * our data will be ACK'd; if so, enter normal data segment
 2022                  * processing in the middle of step 5, ack processing.
 2023                  * Otherwise, goto step 6.
 2024                  */
 2025                 if (thflags & TH_ACK)
 2026                         goto process_ACK;
 2027 
 2028                 goto step6;
 2029 
 2030         /*
 2031          * If the state is LAST_ACK or CLOSING or TIME_WAIT:
 2032          *      do normal processing.
 2033          *
 2034          * NB: Leftover from RFC1644 T/TCP.  Cases to be reused later.
 2035          */
 2036         case TCPS_LAST_ACK:
 2037         case TCPS_CLOSING:
 2038                 break;  /* continue normal processing */
 2039         }
 2040 
 2041         /*
 2042          * States other than LISTEN or SYN_SENT.
 2043          * First check the RST flag and sequence number since reset segments
 2044          * are exempt from the timestamp and connection count tests.  This
 2045          * fixes a bug introduced by the Stevens, vol. 2, p. 960 bugfix
 2046          * below which allowed reset segments in half the sequence space
 2047          * to fall though and be processed (which gives forged reset
 2048          * segments with a random sequence number a 50 percent chance of
 2049          * killing a connection).
 2050          * Then check timestamp, if present.
 2051          * Then check the connection count, if present.
 2052          * Then check that at least some bytes of segment are within
 2053          * receive window.  If segment begins before rcv_nxt,
 2054          * drop leading data (and SYN); if nothing left, just ack.
 2055          */
 2056         if (thflags & TH_RST) {
 2057                 /*
 2058                  * RFC5961 Section 3.2
 2059                  *
 2060                  * - RST drops connection only if SEG.SEQ == RCV.NXT.
 2061                  * - If RST is in window, we send challenge ACK.
 2062                  *
 2063                  * Note: to take into account delayed ACKs, we should
 2064                  *   test against last_ack_sent instead of rcv_nxt.
 2065                  * Note 2: we handle special case of closed window, not
 2066                  *   covered by the RFC.
 2067                  */
 2068                 if ((SEQ_GEQ(th->th_seq, tp->last_ack_sent) &&
 2069                     SEQ_LT(th->th_seq, tp->last_ack_sent + tp->rcv_wnd)) ||
 2070                     (tp->rcv_wnd == 0 && tp->last_ack_sent == th->th_seq)) {
 2071 
 2072                         INP_INFO_WLOCK_ASSERT(&V_tcbinfo);
 2073                         KASSERT(ti_locked == TI_WLOCKED,
 2074                             ("%s: TH_RST ti_locked %d, th %p tp %p",
 2075                             __func__, ti_locked, th, tp));
 2076                         KASSERT(tp->t_state != TCPS_SYN_SENT,
 2077                             ("%s: TH_RST for TCPS_SYN_SENT th %p tp %p",
 2078                             __func__, th, tp));
 2079 
 2080                         if (V_tcp_insecure_rst ||
 2081                             tp->last_ack_sent == th->th_seq) {
 2082                                 TCPSTAT_INC(tcps_drops);
 2083                                 /* Drop the connection. */
 2084                                 switch (tp->t_state) {
 2085                                 case TCPS_SYN_RECEIVED:
 2086                                         so->so_error = ECONNREFUSED;
 2087                                         goto close;
 2088                                 case TCPS_ESTABLISHED:
 2089                                 case TCPS_FIN_WAIT_1:
 2090                                 case TCPS_FIN_WAIT_2:
 2091                                 case TCPS_CLOSE_WAIT:
 2092                                         so->so_error = ECONNRESET;
 2093                                 close:
 2094                                         tcp_state_change(tp, TCPS_CLOSED);
 2095                                         /* FALLTHROUGH */
 2096                                 default:
 2097                                         tp = tcp_close(tp);
 2098                                 }
 2099                         } else {
 2100                                 TCPSTAT_INC(tcps_badrst);
 2101                                 /* Send challenge ACK. */
 2102                                 tcp_respond(tp, mtod(m, void *), th, m,
 2103                                     tp->rcv_nxt, tp->snd_nxt, TH_ACK);
 2104                                 tp->last_ack_sent = tp->rcv_nxt;
 2105                                 m = NULL;
 2106                         }
 2107                 }
 2108                 goto drop;
 2109         }
 2110 
 2111         /*
 2112          * RFC5961 Section 4.2
 2113          * Send challenge ACK for any SYN in synchronized state.
 2114          */
 2115         if ((thflags & TH_SYN) && tp->t_state != TCPS_SYN_SENT) {
 2116                 KASSERT(ti_locked == TI_WLOCKED,
 2117                     ("tcp_do_segment: TH_SYN ti_locked %d", ti_locked));
 2118                 INP_INFO_WLOCK_ASSERT(&V_tcbinfo);
 2119 
 2120                 TCPSTAT_INC(tcps_badsyn);
 2121                 if (V_tcp_insecure_syn &&
 2122                     SEQ_GEQ(th->th_seq, tp->last_ack_sent) &&
 2123                     SEQ_LT(th->th_seq, tp->last_ack_sent + tp->rcv_wnd)) {
 2124                         tp = tcp_drop(tp, ECONNRESET);
 2125                         rstreason = BANDLIM_UNLIMITED;
 2126                 } else {
 2127                         /* Send challenge ACK. */
 2128                         tcp_respond(tp, mtod(m, void *), th, m, tp->rcv_nxt,
 2129                             tp->snd_nxt, TH_ACK);
 2130                         tp->last_ack_sent = tp->rcv_nxt;
 2131                         m = NULL;
 2132                 }
 2133                 goto drop;
 2134         }
 2135 
 2136         /*
 2137          * RFC 1323 PAWS: If we have a timestamp reply on this segment
 2138          * and it's less than ts_recent, drop it.
 2139          */
 2140         if ((to.to_flags & TOF_TS) != 0 && tp->ts_recent &&
 2141             TSTMP_LT(to.to_tsval, tp->ts_recent)) {
 2142 
 2143                 /* Check to see if ts_recent is over 24 days old.  */
 2144                 if (tcp_ts_getticks() - tp->ts_recent_age > TCP_PAWS_IDLE) {
 2145                         /*
 2146                          * Invalidate ts_recent.  If this segment updates
 2147                          * ts_recent, the age will be reset later and ts_recent
 2148                          * will get a valid value.  If it does not, setting
 2149                          * ts_recent to zero will at least satisfy the
 2150                          * requirement that zero be placed in the timestamp
 2151                          * echo reply when ts_recent isn't valid.  The
 2152                          * age isn't reset until we get a valid ts_recent
 2153                          * because we don't want out-of-order segments to be
 2154                          * dropped when ts_recent is old.
 2155                          */
 2156                         tp->ts_recent = 0;
 2157                 } else {
 2158                         TCPSTAT_INC(tcps_rcvduppack);
 2159                         TCPSTAT_ADD(tcps_rcvdupbyte, tlen);
 2160                         TCPSTAT_INC(tcps_pawsdrop);
 2161                         if (tlen)
 2162                                 goto dropafterack;
 2163                         goto drop;
 2164                 }
 2165         }
 2166 
 2167         /*
 2168          * In the SYN-RECEIVED state, validate that the packet belongs to
 2169          * this connection before trimming the data to fit the receive
 2170          * window.  Check the sequence number versus IRS since we know
 2171          * the sequence numbers haven't wrapped.  This is a partial fix
 2172          * for the "LAND" DoS attack.
 2173          */
 2174         if (tp->t_state == TCPS_SYN_RECEIVED && SEQ_LT(th->th_seq, tp->irs)) {
 2175                 rstreason = BANDLIM_RST_OPENPORT;
 2176                 goto dropwithreset;
 2177         }
 2178 
 2179         todrop = tp->rcv_nxt - th->th_seq;
 2180         if (todrop > 0) {
 2181                 if (thflags & TH_SYN) {
 2182                         thflags &= ~TH_SYN;
 2183                         th->th_seq++;
 2184                         if (th->th_urp > 1)
 2185                                 th->th_urp--;
 2186                         else
 2187                                 thflags &= ~TH_URG;
 2188                         todrop--;
 2189                 }
 2190                 /*
 2191                  * Following if statement from Stevens, vol. 2, p. 960.
 2192                  */
 2193                 if (todrop > tlen
 2194                     || (todrop == tlen && (thflags & TH_FIN) == 0)) {
 2195                         /*
 2196                          * Any valid FIN must be to the left of the window.
 2197                          * At this point the FIN must be a duplicate or out
 2198                          * of sequence; drop it.
 2199                          */
 2200                         thflags &= ~TH_FIN;
 2201 
 2202                         /*
 2203                          * Send an ACK to resynchronize and drop any data.
 2204                          * But keep on processing for RST or ACK.
 2205                          */
 2206                         tp->t_flags |= TF_ACKNOW;
 2207                         todrop = tlen;
 2208                         TCPSTAT_INC(tcps_rcvduppack);
 2209                         TCPSTAT_ADD(tcps_rcvdupbyte, todrop);
 2210                 } else {
 2211                         TCPSTAT_INC(tcps_rcvpartduppack);
 2212                         TCPSTAT_ADD(tcps_rcvpartdupbyte, todrop);
 2213                 }
 2214                 drop_hdrlen += todrop;  /* drop from the top afterwards */
 2215                 th->th_seq += todrop;
 2216                 tlen -= todrop;
 2217                 if (th->th_urp > todrop)
 2218                         th->th_urp -= todrop;
 2219                 else {
 2220                         thflags &= ~TH_URG;
 2221                         th->th_urp = 0;
 2222                 }
 2223         }
 2224 
 2225         /*
 2226          * If new data are received on a connection after the
 2227          * user processes are gone, then RST the other end.
 2228          */
 2229         if ((so->so_state & SS_NOFDREF) &&
 2230             tp->t_state > TCPS_CLOSE_WAIT && tlen) {
 2231                 KASSERT(ti_locked == TI_WLOCKED, ("%s: SS_NOFDEREF && "
 2232                     "CLOSE_WAIT && tlen ti_locked %d", __func__, ti_locked));
 2233                 INP_INFO_WLOCK_ASSERT(&V_tcbinfo);
 2234 
 2235                 if ((s = tcp_log_addrs(inc, th, NULL, NULL))) {
 2236                         log(LOG_DEBUG, "%s; %s: %s: Received %d bytes of data "
 2237                             "after socket was closed, "
 2238                             "sending RST and removing tcpcb\n",
 2239                             s, __func__, tcpstates[tp->t_state], tlen);
 2240                         free(s, M_TCPLOG);
 2241                 }
 2242                 tp = tcp_close(tp);
 2243                 TCPSTAT_INC(tcps_rcvafterclose);
 2244                 rstreason = BANDLIM_UNLIMITED;
 2245                 goto dropwithreset;
 2246         }
 2247 
 2248         /*
 2249          * If segment ends after window, drop trailing data
 2250          * (and PUSH and FIN); if nothing left, just ACK.
 2251          */
 2252         todrop = (th->th_seq + tlen) - (tp->rcv_nxt + tp->rcv_wnd);
 2253         if (todrop > 0) {
 2254                 TCPSTAT_INC(tcps_rcvpackafterwin);
 2255                 if (todrop >= tlen) {
 2256                         TCPSTAT_ADD(tcps_rcvbyteafterwin, tlen);
 2257                         /*
 2258                          * If window is closed can only take segments at
 2259                          * window edge, and have to drop data and PUSH from
 2260                          * incoming segments.  Continue processing, but
 2261                          * remember to ack.  Otherwise, drop segment
 2262                          * and ack.
 2263                          */
 2264                         if (tp->rcv_wnd == 0 && th->th_seq == tp->rcv_nxt) {
 2265                                 tp->t_flags |= TF_ACKNOW;
 2266                                 TCPSTAT_INC(tcps_rcvwinprobe);
 2267                         } else
 2268                                 goto dropafterack;
 2269                 } else
 2270                         TCPSTAT_ADD(tcps_rcvbyteafterwin, todrop);
 2271                 m_adj(m, -todrop);
 2272                 tlen -= todrop;
 2273                 thflags &= ~(TH_PUSH|TH_FIN);
 2274         }
 2275 
 2276         /*
 2277          * If last ACK falls within this segment's sequence numbers,
 2278          * record its timestamp.
 2279          * NOTE: 
 2280          * 1) That the test incorporates suggestions from the latest
 2281          *    proposal of the tcplw@cray.com list (Braden 1993/04/26).
 2282          * 2) That updating only on newer timestamps interferes with
 2283          *    our earlier PAWS tests, so this check should be solely
 2284          *    predicated on the sequence space of this segment.
 2285          * 3) That we modify the segment boundary check to be 
 2286          *        Last.ACK.Sent <= SEG.SEQ + SEG.Len  
 2287          *    instead of RFC1323's
 2288          *        Last.ACK.Sent < SEG.SEQ + SEG.Len,
 2289          *    This modified check allows us to overcome RFC1323's
 2290          *    limitations as described in Stevens TCP/IP Illustrated
 2291          *    Vol. 2 p.869. In such cases, we can still calculate the
 2292          *    RTT correctly when RCV.NXT == Last.ACK.Sent.
 2293          */
 2294         if ((to.to_flags & TOF_TS) != 0 &&
 2295             SEQ_LEQ(th->th_seq, tp->last_ack_sent) &&
 2296             SEQ_LEQ(tp->last_ack_sent, th->th_seq + tlen +
 2297                 ((thflags & (TH_SYN|TH_FIN)) != 0))) {
 2298                 tp->ts_recent_age = tcp_ts_getticks();
 2299                 tp->ts_recent = to.to_tsval;
 2300         }
 2301 
 2302         /*
 2303          * If the ACK bit is off:  if in SYN-RECEIVED state or SENDSYN
 2304          * flag is on (half-synchronized state), then queue data for
 2305          * later processing; else drop segment and return.
 2306          */
 2307         if ((thflags & TH_ACK) == 0) {
 2308                 if (tp->t_state == TCPS_SYN_RECEIVED ||
 2309                     (tp->t_flags & TF_NEEDSYN))
 2310                         goto step6;
 2311                 else if (tp->t_flags & TF_ACKNOW)
 2312                         goto dropafterack;
 2313                 else
 2314                         goto drop;
 2315         }
 2316 
 2317         /*
 2318          * Ack processing.
 2319          */
 2320         switch (tp->t_state) {
 2321 
 2322         /*
 2323          * In SYN_RECEIVED state, the ack ACKs our SYN, so enter
 2324          * ESTABLISHED state and continue processing.
 2325          * The ACK was checked above.
 2326          */
 2327         case TCPS_SYN_RECEIVED:
 2328 
 2329                 TCPSTAT_INC(tcps_connects);
 2330                 soisconnected(so);
 2331                 /* Do window scaling? */
 2332                 if ((tp->t_flags & (TF_RCVD_SCALE|TF_REQ_SCALE)) ==
 2333                         (TF_RCVD_SCALE|TF_REQ_SCALE)) {
 2334                         tp->rcv_scale = tp->request_r_scale;
 2335                         tp->snd_wnd = tiwin;
 2336                 }
 2337                 /*
 2338                  * Make transitions:
 2339                  *      SYN-RECEIVED  -> ESTABLISHED
 2340                  *      SYN-RECEIVED* -> FIN-WAIT-1
 2341                  */
 2342                 tp->t_starttime = ticks;
 2343                 if (tp->t_flags & TF_NEEDFIN) {
 2344                         tcp_state_change(tp, TCPS_FIN_WAIT_1);
 2345                         tp->t_flags &= ~TF_NEEDFIN;
 2346                 } else {
 2347                         tcp_state_change(tp, TCPS_ESTABLISHED);
 2348                         TCP_PROBE5(accept__established, NULL, tp,
 2349                             mtod(m, const char *), tp, th);
 2350                         cc_conn_init(tp);
 2351                         tcp_timer_activate(tp, TT_KEEP, TP_KEEPIDLE(tp));
 2352                 }
 2353                 /*
 2354                  * If segment contains data or ACK, will call tcp_reass()
 2355                  * later; if not, do so now to pass queued data to user.
 2356                  */
 2357                 if (tlen == 0 && (thflags & TH_FIN) == 0)
 2358                         (void) tcp_reass(tp, (struct tcphdr *)0, 0,
 2359                             (struct mbuf *)0);
 2360                 tp->snd_wl1 = th->th_seq - 1;
 2361                 /* FALLTHROUGH */
 2362 
 2363         /*
 2364          * In ESTABLISHED state: drop duplicate ACKs; ACK out of range
 2365          * ACKs.  If the ack is in the range
 2366          *      tp->snd_una < th->th_ack <= tp->snd_max
 2367          * then advance tp->snd_una to th->th_ack and drop
 2368          * data from the retransmission queue.  If this ACK reflects
 2369          * more up to date window information we update our window information.
 2370          */
 2371         case TCPS_ESTABLISHED:
 2372         case TCPS_FIN_WAIT_1:
 2373         case TCPS_FIN_WAIT_2:
 2374         case TCPS_CLOSE_WAIT:
 2375         case TCPS_CLOSING:
 2376         case TCPS_LAST_ACK:
 2377                 if (SEQ_GT(th->th_ack, tp->snd_max)) {
 2378                         TCPSTAT_INC(tcps_rcvacktoomuch);
 2379                         goto dropafterack;
 2380                 }
 2381                 if ((tp->t_flags & TF_SACK_PERMIT) &&
 2382                     ((to.to_flags & TOF_SACK) ||
 2383                      !TAILQ_EMPTY(&tp->snd_holes)))
 2384                         tcp_sack_doack(tp, &to, th->th_ack);
 2385 
 2386                 /* Run HHOOK_TCP_ESTABLISHED_IN helper hooks. */
 2387                 hhook_run_tcp_est_in(tp, th, &to);
 2388 
 2389                 if (SEQ_LEQ(th->th_ack, tp->snd_una)) {
 2390                         if (tlen == 0 && tiwin == tp->snd_wnd) {
 2391                                 /*
 2392                                  * If this is the first time we've seen a
 2393                                  * FIN from the remote, this is not a
 2394                                  * duplicate and it needs to be processed
 2395                                  * normally.  This happens during a
 2396                                  * simultaneous close.
 2397                                  */
 2398                                 if ((thflags & TH_FIN) &&
 2399                                     (TCPS_HAVERCVDFIN(tp->t_state) == 0)) {
 2400                                         tp->t_dupacks = 0;
 2401                                         break;
 2402                                 }
 2403                                 TCPSTAT_INC(tcps_rcvdupack);
 2404                                 /*
 2405                                  * If we have outstanding data (other than
 2406                                  * a window probe), this is a completely
 2407                                  * duplicate ack (ie, window info didn't
 2408                                  * change and FIN isn't set),
 2409                                  * the ack is the biggest we've
 2410                                  * seen and we've seen exactly our rexmt
 2411                                  * threshhold of them, assume a packet
 2412                                  * has been dropped and retransmit it.
 2413                                  * Kludge snd_nxt & the congestion
 2414                                  * window so we send only this one
 2415                                  * packet.
 2416                                  *
 2417                                  * We know we're losing at the current
 2418                                  * window size so do congestion avoidance
 2419                                  * (set ssthresh to half the current window
 2420                                  * and pull our congestion window back to
 2421                                  * the new ssthresh).
 2422                                  *
 2423                                  * Dup acks mean that packets have left the
 2424                                  * network (they're now cached at the receiver)
 2425                                  * so bump cwnd by the amount in the receiver
 2426                                  * to keep a constant cwnd packets in the
 2427                                  * network.
 2428                                  *
 2429                                  * When using TCP ECN, notify the peer that
 2430                                  * we reduced the cwnd.
 2431                                  */
 2432                                 if (!tcp_timer_active(tp, TT_REXMT) ||
 2433                                     th->th_ack != tp->snd_una)
 2434                                         tp->t_dupacks = 0;
 2435                                 else if (++tp->t_dupacks > tcprexmtthresh ||
 2436                                      IN_FASTRECOVERY(tp->t_flags)) {
 2437                                         cc_ack_received(tp, th, CC_DUPACK);
 2438                                         if ((tp->t_flags & TF_SACK_PERMIT) &&
 2439                                             IN_FASTRECOVERY(tp->t_flags)) {
 2440                                                 int awnd;
 2441                                                 
 2442                                                 /*
 2443                                                  * Compute the amount of data in flight first.
 2444                                                  * We can inject new data into the pipe iff 
 2445                                                  * we have less than 1/2 the original window's
 2446                                                  * worth of data in flight.
 2447                                                  */
 2448                                                 awnd = (tp->snd_nxt - tp->snd_fack) +
 2449                                                         tp->sackhint.sack_bytes_rexmit;
 2450                                                 if (awnd < tp->snd_ssthresh) {
 2451                                                         tp->snd_cwnd += tp->t_maxseg;
 2452                                                         if (tp->snd_cwnd > tp->snd_ssthresh)
 2453                                                                 tp->snd_cwnd = tp->snd_ssthresh;
 2454                                                 }
 2455                                         } else
 2456                                                 tp->snd_cwnd += tp->t_maxseg;
 2457                                         (void) tcp_output(tp);
 2458                                         goto drop;
 2459                                 } else if (tp->t_dupacks == tcprexmtthresh) {
 2460                                         tcp_seq onxt = tp->snd_nxt;
 2461 
 2462                                         /*
 2463                                          * If we're doing sack, check to
 2464                                          * see if we're already in sack
 2465                                          * recovery. If we're not doing sack,
 2466                                          * check to see if we're in newreno
 2467                                          * recovery.
 2468                                          */
 2469                                         if (tp->t_flags & TF_SACK_PERMIT) {
 2470                                                 if (IN_FASTRECOVERY(tp->t_flags)) {
 2471                                                         tp->t_dupacks = 0;
 2472                                                         break;
 2473                                                 }
 2474                                         } else {
 2475                                                 if (SEQ_LEQ(th->th_ack,
 2476                                                     tp->snd_recover)) {
 2477                                                         tp->t_dupacks = 0;
 2478                                                         break;
 2479                                                 }
 2480                                         }
 2481                                         /* Congestion signal before ack. */
 2482                                         cc_cong_signal(tp, th, CC_NDUPACK);
 2483                                         cc_ack_received(tp, th, CC_DUPACK);
 2484                                         tcp_timer_activate(tp, TT_REXMT, 0);
 2485                                         tp->t_rtttime = 0;
 2486                                         if (tp->t_flags & TF_SACK_PERMIT) {
 2487                                                 TCPSTAT_INC(
 2488                                                     tcps_sack_recovery_episode);
 2489                                                 tp->sack_newdata = tp->snd_nxt;
 2490                                                 tp->snd_cwnd = tp->t_maxseg;
 2491                                                 (void) tcp_output(tp);
 2492                                                 goto drop;
 2493                                         }
 2494                                         tp->snd_nxt = th->th_ack;
 2495                                         tp->snd_cwnd = tp->t_maxseg;
 2496                                         (void) tcp_output(tp);
 2497                                         KASSERT(tp->snd_limited <= 2,
 2498                                             ("%s: tp->snd_limited too big",
 2499                                             __func__));
 2500                                         tp->snd_cwnd = tp->snd_ssthresh +
 2501                                              tp->t_maxseg *
 2502                                              (tp->t_dupacks - tp->snd_limited);
 2503                                         if (SEQ_GT(onxt, tp->snd_nxt))
 2504                                                 tp->snd_nxt = onxt;
 2505                                         goto drop;
 2506                                 } else if (V_tcp_do_rfc3042) {
 2507                                         cc_ack_received(tp, th, CC_DUPACK);
 2508                                         u_long oldcwnd = tp->snd_cwnd;
 2509                                         tcp_seq oldsndmax = tp->snd_max;
 2510                                         u_int sent;
 2511                                         int avail;
 2512 
 2513                                         KASSERT(tp->t_dupacks == 1 ||
 2514                                             tp->t_dupacks == 2,
 2515                                             ("%s: dupacks not 1 or 2",
 2516                                             __func__));
 2517                                         if (tp->t_dupacks == 1)
 2518                                                 tp->snd_limited = 0;
 2519                                         tp->snd_cwnd =
 2520                                             (tp->snd_nxt - tp->snd_una) +
 2521                                             (tp->t_dupacks - tp->snd_limited) *
 2522                                             tp->t_maxseg;
 2523                                         /*
 2524                                          * Only call tcp_output when there
 2525                                          * is new data available to be sent.
 2526                                          * Otherwise we would send pure ACKs.
 2527                                          */
 2528                                         SOCKBUF_LOCK(&so->so_snd);
 2529                                         avail = sbavail(&so->so_snd) -
 2530                                             (tp->snd_nxt - tp->snd_una);
 2531                                         SOCKBUF_UNLOCK(&so->so_snd);
 2532                                         if (avail > 0)
 2533                                                 (void) tcp_output(tp);
 2534                                         sent = tp->snd_max - oldsndmax;
 2535                                         if (sent > tp->t_maxseg) {
 2536                                                 KASSERT((tp->t_dupacks == 2 &&
 2537                                                     tp->snd_limited == 0) ||
 2538                                                    (sent == tp->t_maxseg + 1 &&
 2539                                                     tp->t_flags & TF_SENTFIN),
 2540                                                     ("%s: sent too much",
 2541                                                     __func__));
 2542                                                 tp->snd_limited = 2;
 2543                                         } else if (sent > 0)
 2544                                                 ++tp->snd_limited;
 2545                                         tp->snd_cwnd = oldcwnd;
 2546                                         goto drop;
 2547                                 }
 2548                         } else
 2549                                 tp->t_dupacks = 0;
 2550                         break;
 2551                 }
 2552 
 2553                 KASSERT(SEQ_GT(th->th_ack, tp->snd_una),
 2554                     ("%s: th_ack <= snd_una", __func__));
 2555 
 2556                 /*
 2557                  * If the congestion window was inflated to account
 2558                  * for the other side's cached packets, retract it.
 2559                  */
 2560                 if (IN_FASTRECOVERY(tp->t_flags)) {
 2561                         if (SEQ_LT(th->th_ack, tp->snd_recover)) {
 2562                                 if (tp->t_flags & TF_SACK_PERMIT)
 2563                                         tcp_sack_partialack(tp, th);
 2564                                 else
 2565                                         tcp_newreno_partial_ack(tp, th);
 2566                         } else
 2567                                 cc_post_recovery(tp, th);
 2568                 }
 2569                 tp->t_dupacks = 0;
 2570                 /*
 2571                  * If we reach this point, ACK is not a duplicate,
 2572                  *     i.e., it ACKs something we sent.
 2573                  */
 2574                 if (tp->t_flags & TF_NEEDSYN) {
 2575                         /*
 2576                          * T/TCP: Connection was half-synchronized, and our
 2577                          * SYN has been ACK'd (so connection is now fully
 2578                          * synchronized).  Go to non-starred state,
 2579                          * increment snd_una for ACK of SYN, and check if
 2580                          * we can do window scaling.
 2581                          */
 2582                         tp->t_flags &= ~TF_NEEDSYN;
 2583                         tp->snd_una++;
 2584                         /* Do window scaling? */
 2585                         if ((tp->t_flags & (TF_RCVD_SCALE|TF_REQ_SCALE)) ==
 2586                                 (TF_RCVD_SCALE|TF_REQ_SCALE)) {
 2587                                 tp->rcv_scale = tp->request_r_scale;
 2588                                 /* Send window already scaled. */
 2589                         }
 2590                 }
 2591 
 2592 process_ACK:
 2593                 INP_WLOCK_ASSERT(tp->t_inpcb);
 2594 
 2595                 acked = BYTES_THIS_ACK(tp, th);
 2596                 TCPSTAT_INC(tcps_rcvackpack);
 2597                 TCPSTAT_ADD(tcps_rcvackbyte, acked);
 2598 
 2599                 /*
 2600                  * If we just performed our first retransmit, and the ACK
 2601                  * arrives within our recovery window, then it was a mistake
 2602                  * to do the retransmit in the first place.  Recover our
 2603                  * original cwnd and ssthresh, and proceed to transmit where
 2604                  * we left off.
 2605                  */
 2606                 if (tp->t_rxtshift == 1 && tp->t_flags & TF_PREVVALID &&
 2607                     (int)(ticks - tp->t_badrxtwin) < 0)
 2608                         cc_cong_signal(tp, th, CC_RTO_ERR);
 2609 
 2610                 /*
 2611                  * If we have a timestamp reply, update smoothed
 2612                  * round trip time.  If no timestamp is present but
 2613                  * transmit timer is running and timed sequence
 2614                  * number was acked, update smoothed round trip time.
 2615                  * Since we now have an rtt measurement, cancel the
 2616                  * timer backoff (cf., Phil Karn's retransmit alg.).
 2617                  * Recompute the initial retransmit timer.
 2618                  *
 2619                  * Some boxes send broken timestamp replies
 2620                  * during the SYN+ACK phase, ignore
 2621                  * timestamps of 0 or we could calculate a
 2622                  * huge RTT and blow up the retransmit timer.
 2623                  */
 2624                 if ((to.to_flags & TOF_TS) != 0 && to.to_tsecr) {
 2625                         u_int t;
 2626 
 2627                         t = tcp_ts_getticks() - to.to_tsecr;
 2628                         if (!tp->t_rttlow || tp->t_rttlow > t)
 2629                                 tp->t_rttlow = t;
 2630                         tcp_xmit_timer(tp, TCP_TS_TO_TICKS(t) + 1);
 2631                 } else if (tp->t_rtttime && SEQ_GT(th->th_ack, tp->t_rtseq)) {
 2632                         if (!tp->t_rttlow || tp->t_rttlow > ticks - tp->t_rtttime)
 2633                                 tp->t_rttlow = ticks - tp->t_rtttime;
 2634                         tcp_xmit_timer(tp, ticks - tp->t_rtttime);
 2635                 }
 2636 
 2637                 /*
 2638                  * If all outstanding data is acked, stop retransmit
 2639                  * timer and remember to restart (more output or persist).
 2640                  * If there is more data to be acked, restart retransmit
 2641                  * timer, using current (possibly backed-off) value.
 2642                  */
 2643                 if (th->th_ack == tp->snd_max) {
 2644                         tcp_timer_activate(tp, TT_REXMT, 0);
 2645                         needoutput = 1;
 2646                 } else if (!tcp_timer_active(tp, TT_PERSIST))
 2647                         tcp_timer_activate(tp, TT_REXMT, tp->t_rxtcur);
 2648 
 2649                 /*
 2650                  * If no data (only SYN) was ACK'd,
 2651                  *    skip rest of ACK processing.
 2652                  */
 2653                 if (acked == 0)
 2654                         goto step6;
 2655 
 2656                 /*
 2657                  * Let the congestion control algorithm update congestion
 2658                  * control related information. This typically means increasing
 2659                  * the congestion window.
 2660                  */
 2661                 cc_ack_received(tp, th, CC_ACK);
 2662 
 2663                 SOCKBUF_LOCK(&so->so_snd);
 2664                 if (acked > sbavail(&so->so_snd)) {
 2665                         tp->snd_wnd -= sbavail(&so->so_snd);
 2666                         mfree = sbcut_locked(&so->so_snd,
 2667                             (int)sbavail(&so->so_snd));
 2668                         ourfinisacked = 1;
 2669                 } else {
 2670                         mfree = sbcut_locked(&so->so_snd, acked);
 2671                         tp->snd_wnd -= acked;
 2672                         ourfinisacked = 0;
 2673                 }
 2674                 /* NB: sowwakeup_locked() does an implicit unlock. */
 2675                 sowwakeup_locked(so);
 2676                 m_freem(mfree);
 2677                 /* Detect una wraparound. */
 2678                 if (!IN_RECOVERY(tp->t_flags) &&
 2679                     SEQ_GT(tp->snd_una, tp->snd_recover) &&
 2680                     SEQ_LEQ(th->th_ack, tp->snd_recover))
 2681                         tp->snd_recover = th->th_ack - 1;
 2682                 /* XXXLAS: Can this be moved up into cc_post_recovery? */
 2683                 if (IN_RECOVERY(tp->t_flags) &&
 2684                     SEQ_GEQ(th->th_ack, tp->snd_recover)) {
 2685                         EXIT_RECOVERY(tp->t_flags);
 2686                 }
 2687                 tp->snd_una = th->th_ack;
 2688                 if (tp->t_flags & TF_SACK_PERMIT) {
 2689                         if (SEQ_GT(tp->snd_una, tp->snd_recover))
 2690                                 tp->snd_recover = tp->snd_una;
 2691                 }
 2692                 if (SEQ_LT(tp->snd_nxt, tp->snd_una))
 2693                         tp->snd_nxt = tp->snd_una;
 2694 
 2695                 switch (tp->t_state) {
 2696 
 2697                 /*
 2698                  * In FIN_WAIT_1 STATE in addition to the processing
 2699                  * for the ESTABLISHED state if our FIN is now acknowledged
 2700                  * then enter FIN_WAIT_2.
 2701                  */
 2702                 case TCPS_FIN_WAIT_1:
 2703                         if (ourfinisacked) {
 2704                                 /*
 2705                                  * If we can't receive any more
 2706                                  * data, then closing user can proceed.
 2707                                  * Starting the timer is contrary to the
 2708                                  * specification, but if we don't get a FIN
 2709                                  * we'll hang forever.
 2710                                  *
 2711                                  * XXXjl:
 2712                                  * we should release the tp also, and use a
 2713                                  * compressed state.
 2714                                  */
 2715                                 if (so->so_rcv.sb_state & SBS_CANTRCVMORE) {
 2716                                         soisdisconnected(so);
 2717                                         tcp_timer_activate(tp, TT_2MSL,
 2718                                             (tcp_fast_finwait2_recycle ?
 2719                                             tcp_finwait2_timeout :
 2720                                             TP_MAXIDLE(tp)));
 2721                                 }
 2722                                 tcp_state_change(tp, TCPS_FIN_WAIT_2);
 2723                         }
 2724                         break;
 2725 
 2726                 /*
 2727                  * In CLOSING STATE in addition to the processing for
 2728                  * the ESTABLISHED state if the ACK acknowledges our FIN
 2729                  * then enter the TIME-WAIT state, otherwise ignore
 2730                  * the segment.
 2731                  */
 2732                 case TCPS_CLOSING:
 2733                         if (ourfinisacked) {
 2734                                 INP_INFO_WLOCK_ASSERT(&V_tcbinfo);
 2735                                 tcp_twstart(tp);
 2736                                 INP_INFO_WUNLOCK(&V_tcbinfo);
 2737                                 m_freem(m);
 2738                                 return;
 2739                         }
 2740                         break;
 2741 
 2742                 /*
 2743                  * In LAST_ACK, we may still be waiting for data to drain
 2744                  * and/or to be acked, as well as for the ack of our FIN.
 2745                  * If our FIN is now acknowledged, delete the TCB,
 2746                  * enter the closed state and return.
 2747                  */
 2748                 case TCPS_LAST_ACK:
 2749                         if (ourfinisacked) {
 2750                                 INP_INFO_WLOCK_ASSERT(&V_tcbinfo);
 2751                                 tp = tcp_close(tp);
 2752                                 goto drop;
 2753                         }
 2754                         break;
 2755                 }
 2756         }
 2757 
 2758 step6:
 2759         INP_WLOCK_ASSERT(tp->t_inpcb);
 2760 
 2761         /*
 2762          * Update window information.
 2763          * Don't look at window if no ACK: TAC's send garbage on first SYN.
 2764          */
 2765         if ((thflags & TH_ACK) &&
 2766             (SEQ_LT(tp->snd_wl1, th->th_seq) ||
 2767             (tp->snd_wl1 == th->th_seq && (SEQ_LT(tp->snd_wl2, th->th_ack) ||
 2768              (tp->snd_wl2 == th->th_ack && tiwin > tp->snd_wnd))))) {
 2769                 /* keep track of pure window updates */
 2770                 if (tlen == 0 &&
 2771                     tp->snd_wl2 == th->th_ack && tiwin > tp->snd_wnd)
 2772                         TCPSTAT_INC(tcps_rcvwinupd);
 2773                 tp->snd_wnd = tiwin;
 2774                 tp->snd_wl1 = th->th_seq;
 2775                 tp->snd_wl2 = th->th_ack;
 2776                 if (tp->snd_wnd > tp->max_sndwnd)
 2777                         tp->max_sndwnd = tp->snd_wnd;
 2778                 needoutput = 1;
 2779         }
 2780 
 2781         /*
 2782          * Process segments with URG.
 2783          */
 2784         if ((thflags & TH_URG) && th->th_urp &&
 2785             TCPS_HAVERCVDFIN(tp->t_state) == 0) {
 2786                 /*
 2787                  * This is a kludge, but if we receive and accept
 2788                  * random urgent pointers, we'll crash in
 2789                  * soreceive.  It's hard to imagine someone
 2790                  * actually wanting to send this much urgent data.
 2791                  */
 2792                 SOCKBUF_LOCK(&so->so_rcv);
 2793                 if (th->th_urp + sbavail(&so->so_rcv) > sb_max) {
 2794                         th->th_urp = 0;                 /* XXX */
 2795                         thflags &= ~TH_URG;             /* XXX */
 2796                         SOCKBUF_UNLOCK(&so->so_rcv);    /* XXX */
 2797                         goto dodata;                    /* XXX */
 2798                 }
 2799                 /*
 2800                  * If this segment advances the known urgent pointer,
 2801                  * then mark the data stream.  This should not happen
 2802                  * in CLOSE_WAIT, CLOSING, LAST_ACK or TIME_WAIT STATES since
 2803                  * a FIN has been received from the remote side.
 2804                  * In these states we ignore the URG.
 2805                  *
 2806                  * According to RFC961 (Assigned Protocols),
 2807                  * the urgent pointer points to the last octet
 2808                  * of urgent data.  We continue, however,
 2809                  * to consider it to indicate the first octet
 2810                  * of data past the urgent section as the original
 2811                  * spec states (in one of two places).
 2812                  */
 2813                 if (SEQ_GT(th->th_seq+th->th_urp, tp->rcv_up)) {
 2814                         tp->rcv_up = th->th_seq + th->th_urp;
 2815                         so->so_oobmark = sbavail(&so->so_rcv) +
 2816                             (tp->rcv_up - tp->rcv_nxt) - 1;
 2817                         if (so->so_oobmark == 0)
 2818                                 so->so_rcv.sb_state |= SBS_RCVATMARK;
 2819                         sohasoutofband(so);
 2820                         tp->t_oobflags &= ~(TCPOOB_HAVEDATA | TCPOOB_HADDATA);
 2821                 }
 2822                 SOCKBUF_UNLOCK(&so->so_rcv);
 2823                 /*
 2824                  * Remove out of band data so doesn't get presented to user.
 2825                  * This can happen independent of advancing the URG pointer,
 2826                  * but if two URG's are pending at once, some out-of-band
 2827                  * data may creep in... ick.
 2828                  */
 2829                 if (th->th_urp <= (u_long)tlen &&
 2830                     !(so->so_options & SO_OOBINLINE)) {
 2831                         /* hdr drop is delayed */
 2832                         tcp_pulloutofband(so, th, m, drop_hdrlen);
 2833                 }
 2834         } else {
 2835                 /*
 2836                  * If no out of band data is expected,
 2837                  * pull receive urgent pointer along
 2838                  * with the receive window.
 2839                  */
 2840                 if (SEQ_GT(tp->rcv_nxt, tp->rcv_up))
 2841                         tp->rcv_up = tp->rcv_nxt;
 2842         }
 2843 dodata:                                                 /* XXX */
 2844         INP_WLOCK_ASSERT(tp->t_inpcb);
 2845 
 2846         /*
 2847          * Process the segment text, merging it into the TCP sequencing queue,
 2848          * and arranging for acknowledgment of receipt if necessary.
 2849          * This process logically involves adjusting tp->rcv_wnd as data
 2850          * is presented to the user (this happens in tcp_usrreq.c,
 2851          * case PRU_RCVD).  If a FIN has already been received on this
 2852          * connection then we just ignore the text.
 2853          */
 2854         if ((tlen || (thflags & TH_FIN)) &&
 2855             TCPS_HAVERCVDFIN(tp->t_state) == 0) {
 2856                 tcp_seq save_start = th->th_seq;
 2857                 m_adj(m, drop_hdrlen);  /* delayed header drop */
 2858                 /*
 2859                  * Insert segment which includes th into TCP reassembly queue
 2860                  * with control block tp.  Set thflags to whether reassembly now
 2861                  * includes a segment with FIN.  This handles the common case
 2862                  * inline (segment is the next to be received on an established
 2863                  * connection, and the queue is empty), avoiding linkage into
 2864                  * and removal from the queue and repetition of various
 2865                  * conversions.
 2866                  * Set DELACK for segments received in order, but ack
 2867                  * immediately when segments are out of order (so
 2868                  * fast retransmit can work).
 2869                  */
 2870                 if (th->th_seq == tp->rcv_nxt && tp->t_segq == NULL &&
 2871                     TCPS_HAVEESTABLISHED(tp->t_state)) {
 2872                         if (DELAY_ACK(tp, tlen))
 2873                                 tp->t_flags |= TF_DELACK;
 2874                         else
 2875                                 tp->t_flags |= TF_ACKNOW;
 2876                         tp->rcv_nxt += tlen;
 2877                         thflags = th->th_flags & TH_FIN;
 2878                         TCPSTAT_INC(tcps_rcvpack);
 2879                         TCPSTAT_ADD(tcps_rcvbyte, tlen);
 2880                         ND6_HINT(tp);
 2881                         SOCKBUF_LOCK(&so->so_rcv);
 2882                         if (so->so_rcv.sb_state & SBS_CANTRCVMORE)
 2883                                 m_freem(m);
 2884                         else
 2885                                 sbappendstream_locked(&so->so_rcv, m);
 2886                         /* NB: sorwakeup_locked() does an implicit unlock. */
 2887                         sorwakeup_locked(so);
 2888                 } else {
 2889                         /*
 2890                          * XXX: Due to the header drop above "th" is
 2891                          * theoretically invalid by now.  Fortunately
 2892                          * m_adj() doesn't actually frees any mbufs
 2893                          * when trimming from the head.
 2894                          */
 2895                         thflags = tcp_reass(tp, th, &tlen, m);
 2896                         tp->t_flags |= TF_ACKNOW;
 2897                 }
 2898                 if (tlen > 0 && (tp->t_flags & TF_SACK_PERMIT))
 2899                         tcp_update_sack_list(tp, save_start, save_start + tlen);
 2900 #if 0
 2901                 /*
 2902                  * Note the amount of data that peer has sent into
 2903                  * our window, in order to estimate the sender's
 2904                  * buffer size.
 2905                  * XXX: Unused.
 2906                  */
 2907                 if (SEQ_GT(tp->rcv_adv, tp->rcv_nxt))
 2908                         len = so->so_rcv.sb_hiwat - (tp->rcv_adv - tp->rcv_nxt);
 2909                 else
 2910                         len = so->so_rcv.sb_hiwat;
 2911 #endif
 2912         } else {
 2913                 m_freem(m);
 2914                 thflags &= ~TH_FIN;
 2915         }
 2916 
 2917         /*
 2918          * If FIN is received ACK the FIN and let the user know
 2919          * that the connection is closing.
 2920          */
 2921         if (thflags & TH_FIN) {
 2922                 if (TCPS_HAVERCVDFIN(tp->t_state) == 0) {
 2923                         socantrcvmore(so);
 2924                         /*
 2925                          * If connection is half-synchronized
 2926                          * (ie NEEDSYN flag on) then delay ACK,
 2927                          * so it may be piggybacked when SYN is sent.
 2928                          * Otherwise, since we received a FIN then no
 2929                          * more input can be expected, send ACK now.
 2930                          */
 2931                         if (tp->t_flags & TF_NEEDSYN)
 2932                                 tp->t_flags |= TF_DELACK;
 2933                         else
 2934                                 tp->t_flags |= TF_ACKNOW;
 2935                         tp->rcv_nxt++;
 2936                 }
 2937                 switch (tp->t_state) {
 2938 
 2939                 /*
 2940                  * In SYN_RECEIVED and ESTABLISHED STATES
 2941                  * enter the CLOSE_WAIT state.
 2942                  */
 2943                 case TCPS_SYN_RECEIVED:
 2944                         tp->t_starttime = ticks;
 2945                         /* FALLTHROUGH */
 2946                 case TCPS_ESTABLISHED:
 2947                         tcp_state_change(tp, TCPS_CLOSE_WAIT);
 2948                         break;
 2949 
 2950                 /*
 2951                  * If still in FIN_WAIT_1 STATE FIN has not been acked so
 2952                  * enter the CLOSING state.
 2953                  */
 2954                 case TCPS_FIN_WAIT_1:
 2955                         tcp_state_change(tp, TCPS_CLOSING);
 2956                         break;
 2957 
 2958                 /*
 2959                  * In FIN_WAIT_2 state enter the TIME_WAIT state,
 2960                  * starting the time-wait timer, turning off the other
 2961                  * standard timers.
 2962                  */
 2963                 case TCPS_FIN_WAIT_2:
 2964                         INP_INFO_WLOCK_ASSERT(&V_tcbinfo);
 2965                         KASSERT(ti_locked == TI_WLOCKED, ("%s: dodata "
 2966                             "TCP_FIN_WAIT_2 ti_locked: %d", __func__,
 2967                             ti_locked));
 2968 
 2969                         tcp_twstart(tp);
 2970                         INP_INFO_WUNLOCK(&V_tcbinfo);
 2971                         return;
 2972                 }
 2973         }
 2974         if (ti_locked == TI_WLOCKED)
 2975                 INP_INFO_WUNLOCK(&V_tcbinfo);
 2976         ti_locked = TI_UNLOCKED;
 2977 
 2978 #ifdef TCPDEBUG
 2979         if (so->so_options & SO_DEBUG)
 2980                 tcp_trace(TA_INPUT, ostate, tp, (void *)tcp_saveipgen,
 2981                           &tcp_savetcp, 0);
 2982 #endif
 2983 
 2984         /*
 2985          * Return any desired output.
 2986          */
 2987         if (needoutput || (tp->t_flags & TF_ACKNOW))
 2988                 (void) tcp_output(tp);
 2989 
 2990 check_delack:
 2991         KASSERT(ti_locked == TI_UNLOCKED, ("%s: check_delack ti_locked %d",
 2992             __func__, ti_locked));
 2993         INP_INFO_UNLOCK_ASSERT(&V_tcbinfo);
 2994         INP_WLOCK_ASSERT(tp->t_inpcb);
 2995 
 2996         if (tp->t_flags & TF_DELACK) {
 2997                 tp->t_flags &= ~TF_DELACK;
 2998                 tcp_timer_activate(tp, TT_DELACK, tcp_delacktime);
 2999         }
 3000         INP_WUNLOCK(tp->t_inpcb);
 3001         return;
 3002 
 3003 dropafterack:
 3004         /*
 3005          * Generate an ACK dropping incoming segment if it occupies
 3006          * sequence space, where the ACK reflects our state.
 3007          *
 3008          * We can now skip the test for the RST flag since all
 3009          * paths to this code happen after packets containing
 3010          * RST have been dropped.
 3011          *
 3012          * In the SYN-RECEIVED state, don't send an ACK unless the
 3013          * segment we received passes the SYN-RECEIVED ACK test.
 3014          * If it fails send a RST.  This breaks the loop in the
 3015          * "LAND" DoS attack, and also prevents an ACK storm
 3016          * between two listening ports that have been sent forged
 3017          * SYN segments, each with the source address of the other.
 3018          */
 3019         if (tp->t_state == TCPS_SYN_RECEIVED && (thflags & TH_ACK) &&
 3020             (SEQ_GT(tp->snd_una, th->th_ack) ||
 3021              SEQ_GT(th->th_ack, tp->snd_max)) ) {
 3022                 rstreason = BANDLIM_RST_OPENPORT;
 3023                 goto dropwithreset;
 3024         }
 3025 #ifdef TCPDEBUG
 3026         if (so->so_options & SO_DEBUG)
 3027                 tcp_trace(TA_DROP, ostate, tp, (void *)tcp_saveipgen,
 3028                           &tcp_savetcp, 0);
 3029 #endif
 3030         if (ti_locked == TI_WLOCKED)
 3031                 INP_INFO_WUNLOCK(&V_tcbinfo);
 3032         ti_locked = TI_UNLOCKED;
 3033 
 3034         tp->t_flags |= TF_ACKNOW;
 3035         (void) tcp_output(tp);
 3036         INP_WUNLOCK(tp->t_inpcb);
 3037         m_freem(m);
 3038         return;
 3039 
 3040 dropwithreset:
 3041         if (ti_locked == TI_WLOCKED)
 3042                 INP_INFO_WUNLOCK(&V_tcbinfo);
 3043         ti_locked = TI_UNLOCKED;
 3044 
 3045         if (tp != NULL) {
 3046                 tcp_dropwithreset(m, th, tp, tlen, rstreason);
 3047                 INP_WUNLOCK(tp->t_inpcb);
 3048         } else
 3049                 tcp_dropwithreset(m, th, NULL, tlen, rstreason);
 3050         return;
 3051 
 3052 drop:
 3053         if (ti_locked == TI_WLOCKED) {
 3054                 INP_INFO_WUNLOCK(&V_tcbinfo);
 3055                 ti_locked = TI_UNLOCKED;
 3056         }
 3057 #ifdef INVARIANTS
 3058         else
 3059                 INP_INFO_UNLOCK_ASSERT(&V_tcbinfo);
 3060 #endif
 3061 
 3062         /*
 3063          * Drop space held by incoming segment and return.
 3064          */
 3065 #ifdef TCPDEBUG
 3066         if (tp == NULL || (tp->t_inpcb->inp_socket->so_options & SO_DEBUG))
 3067                 tcp_trace(TA_DROP, ostate, tp, (void *)tcp_saveipgen,
 3068                           &tcp_savetcp, 0);
 3069 #endif
 3070         if (tp != NULL)
 3071                 INP_WUNLOCK(tp->t_inpcb);
 3072         m_freem(m);
 3073 }
 3074 
 3075 /*
 3076  * Issue RST and make ACK acceptable to originator of segment.
 3077  * The mbuf must still include the original packet header.
 3078  * tp may be NULL.
 3079  */
 3080 static void
 3081 tcp_dropwithreset(struct mbuf *m, struct tcphdr *th, struct tcpcb *tp,
 3082     int tlen, int rstreason)
 3083 {
 3084 #ifdef INET
 3085         struct ip *ip;
 3086 #endif
 3087 #ifdef INET6
 3088         struct ip6_hdr *ip6;
 3089 #endif
 3090 
 3091         if (tp != NULL) {
 3092                 INP_WLOCK_ASSERT(tp->t_inpcb);
 3093         }
 3094 
 3095         /* Don't bother if destination was broadcast/multicast. */
 3096         if ((th->th_flags & TH_RST) || m->m_flags & (M_BCAST|M_MCAST))
 3097                 goto drop;
 3098 #ifdef INET6
 3099         if (mtod(m, struct ip *)->ip_v == 6) {
 3100                 ip6 = mtod(m, struct ip6_hdr *);
 3101                 if (IN6_IS_ADDR_MULTICAST(&ip6->ip6_dst) ||
 3102                     IN6_IS_ADDR_MULTICAST(&ip6->ip6_src))
 3103                         goto drop;
 3104                 /* IPv6 anycast check is done at tcp6_input() */
 3105         }
 3106 #endif
 3107 #if defined(INET) && defined(INET6)
 3108         else
 3109 #endif
 3110 #ifdef INET
 3111         {
 3112                 ip = mtod(m, struct ip *);
 3113                 if (IN_MULTICAST(ntohl(ip->ip_dst.s_addr)) ||
 3114                     IN_MULTICAST(ntohl(ip->ip_src.s_addr)) ||
 3115                     ip->ip_src.s_addr == htonl(INADDR_BROADCAST) ||
 3116                     in_broadcast(ip->ip_dst, m->m_pkthdr.rcvif))
 3117                         goto drop;
 3118         }
 3119 #endif
 3120 
 3121         /* Perform bandwidth limiting. */
 3122         if (badport_bandlim(rstreason) < 0)
 3123                 goto drop;
 3124 
 3125         /* tcp_respond consumes the mbuf chain. */
 3126         if (th->th_flags & TH_ACK) {
 3127                 tcp_respond(tp, mtod(m, void *), th, m, (tcp_seq)0,
 3128                     th->th_ack, TH_RST);
 3129         } else {
 3130                 if (th->th_flags & TH_SYN)
 3131                         tlen++;
 3132                 tcp_respond(tp, mtod(m, void *), th, m, th->th_seq+tlen,
 3133                     (tcp_seq)0, TH_RST|TH_ACK);
 3134         }
 3135         return;
 3136 drop:
 3137         m_freem(m);
 3138 }
 3139 
 3140 /*
 3141  * Parse TCP options and place in tcpopt.
 3142  */
 3143 static void
 3144 tcp_dooptions(struct tcpopt *to, u_char *cp, int cnt, int flags)
 3145 {
 3146         int opt, optlen;
 3147 
 3148         to->to_flags = 0;
 3149         for (; cnt > 0; cnt -= optlen, cp += optlen) {
 3150                 opt = cp[0];
 3151                 if (opt == TCPOPT_EOL)
 3152                         break;
 3153                 if (opt == TCPOPT_NOP)
 3154                         optlen = 1;
 3155                 else {
 3156                         if (cnt < 2)
 3157                                 break;
 3158                         optlen = cp[1];
 3159                         if (optlen < 2 || optlen > cnt)
 3160                                 break;
 3161                 }
 3162                 switch (opt) {
 3163                 case TCPOPT_MAXSEG:
 3164                         if (optlen != TCPOLEN_MAXSEG)
 3165                                 continue;
 3166                         if (!(flags & TO_SYN))
 3167                                 continue;
 3168                         to->to_flags |= TOF_MSS;
 3169                         bcopy((char *)cp + 2,
 3170                             (char *)&to->to_mss, sizeof(to->to_mss));
 3171                         to->to_mss = ntohs(to->to_mss);
 3172                         break;
 3173                 case TCPOPT_WINDOW:
 3174                         if (optlen != TCPOLEN_WINDOW)
 3175                                 continue;
 3176                         if (!(flags & TO_SYN))
 3177                                 continue;
 3178                         to->to_flags |= TOF_SCALE;
 3179                         to->to_wscale = min(cp[2], TCP_MAX_WINSHIFT);
 3180                         break;
 3181                 case TCPOPT_TIMESTAMP:
 3182                         if (optlen != TCPOLEN_TIMESTAMP)
 3183                                 continue;
 3184                         to->to_flags |= TOF_TS;
 3185                         bcopy((char *)cp + 2,
 3186                             (char *)&to->to_tsval, sizeof(to->to_tsval));
 3187                         to->to_tsval = ntohl(to->to_tsval);
 3188                         bcopy((char *)cp + 6,
 3189                             (char *)&to->to_tsecr, sizeof(to->to_tsecr));
 3190                         to->to_tsecr = ntohl(to->to_tsecr);
 3191                         break;
 3192 #ifdef TCP_SIGNATURE
 3193                 /*
 3194                  * XXX In order to reply to a host which has set the
 3195                  * TCP_SIGNATURE option in its initial SYN, we have to
 3196                  * record the fact that the option was observed here
 3197                  * for the syncache code to perform the correct response.
 3198                  */
 3199                 case TCPOPT_SIGNATURE:
 3200                         if (optlen != TCPOLEN_SIGNATURE)
 3201                                 continue;
 3202                         to->to_flags |= TOF_SIGNATURE;
 3203                         to->to_signature = cp + 2;
 3204                         break;
 3205 #endif
 3206                 case TCPOPT_SACK_PERMITTED:
 3207                         if (optlen != TCPOLEN_SACK_PERMITTED)
 3208                                 continue;
 3209                         if (!(flags & TO_SYN))
 3210                                 continue;
 3211                         if (!V_tcp_do_sack)
 3212                                 continue;
 3213                         to->to_flags |= TOF_SACKPERM;
 3214                         break;
 3215                 case TCPOPT_SACK:
 3216                         if (optlen <= 2 || (optlen - 2) % TCPOLEN_SACK != 0)
 3217                                 continue;
 3218                         if (flags & TO_SYN)
 3219                                 continue;
 3220                         to->to_flags |= TOF_SACK;
 3221                         to->to_nsacks = (optlen - 2) / TCPOLEN_SACK;
 3222                         to->to_sacks = cp + 2;
 3223                         TCPSTAT_INC(tcps_sack_rcv_blocks);
 3224                         break;
 3225                 default:
 3226                         continue;
 3227                 }
 3228         }
 3229 }
 3230 
 3231 /*
 3232  * Pull out of band byte out of a segment so
 3233  * it doesn't appear in the user's data queue.
 3234  * It is still reflected in the segment length for
 3235  * sequencing purposes.
 3236  */
 3237 static void
 3238 tcp_pulloutofband(struct socket *so, struct tcphdr *th, struct mbuf *m,
 3239     int off)
 3240 {
 3241         int cnt = off + th->th_urp - 1;
 3242 
 3243         while (cnt >= 0) {
 3244                 if (m->m_len > cnt) {
 3245                         char *cp = mtod(m, caddr_t) + cnt;
 3246                         struct tcpcb *tp = sototcpcb(so);
 3247 
 3248                         INP_WLOCK_ASSERT(tp->t_inpcb);
 3249 
 3250                         tp->t_iobc = *cp;
 3251                         tp->t_oobflags |= TCPOOB_HAVEDATA;
 3252                         bcopy(cp+1, cp, (unsigned)(m->m_len - cnt - 1));
 3253                         m->m_len--;
 3254                         if (m->m_flags & M_PKTHDR)
 3255                                 m->m_pkthdr.len--;
 3256                         return;
 3257                 }
 3258                 cnt -= m->m_len;
 3259                 m = m->m_next;
 3260                 if (m == NULL)
 3261                         break;
 3262         }
 3263         panic("tcp_pulloutofband");
 3264 }
 3265 
 3266 /*
 3267  * Collect new round-trip time estimate
 3268  * and update averages and current timeout.
 3269  */
 3270 static void
 3271 tcp_xmit_timer(struct tcpcb *tp, int rtt)
 3272 {
 3273         int delta;
 3274 
 3275         INP_WLOCK_ASSERT(tp->t_inpcb);
 3276 
 3277         TCPSTAT_INC(tcps_rttupdated);
 3278         tp->t_rttupdated++;
 3279         if (tp->t_srtt != 0) {
 3280                 /*
 3281                  * srtt is stored as fixed point with 5 bits after the
 3282                  * binary point (i.e., scaled by 8).  The following magic
 3283                  * is equivalent to the smoothing algorithm in rfc793 with
 3284                  * an alpha of .875 (srtt = rtt/8 + srtt*7/8 in fixed
 3285                  * point).  Adjust rtt to origin 0.
 3286                  */
 3287                 delta = ((rtt - 1) << TCP_DELTA_SHIFT)
 3288                         - (tp->t_srtt >> (TCP_RTT_SHIFT - TCP_DELTA_SHIFT));
 3289 
 3290                 if ((tp->t_srtt += delta) <= 0)
 3291                         tp->t_srtt = 1;
 3292 
 3293                 /*
 3294                  * We accumulate a smoothed rtt variance (actually, a
 3295                  * smoothed mean difference), then set the retransmit
 3296                  * timer to smoothed rtt + 4 times the smoothed variance.
 3297                  * rttvar is stored as fixed point with 4 bits after the
 3298                  * binary point (scaled by 16).  The following is
 3299                  * equivalent to rfc793 smoothing with an alpha of .75
 3300                  * (rttvar = rttvar*3/4 + |delta| / 4).  This replaces
 3301                  * rfc793's wired-in beta.
 3302                  */
 3303                 if (delta < 0)
 3304                         delta = -delta;
 3305                 delta -= tp->t_rttvar >> (TCP_RTTVAR_SHIFT - TCP_DELTA_SHIFT);
 3306                 if ((tp->t_rttvar += delta) <= 0)
 3307                         tp->t_rttvar = 1;
 3308                 if (tp->t_rttbest > tp->t_srtt + tp->t_rttvar)
 3309                     tp->t_rttbest = tp->t_srtt + tp->t_rttvar;
 3310         } else {
 3311                 /*
 3312                  * No rtt measurement yet - use the unsmoothed rtt.
 3313                  * Set the variance to half the rtt (so our first
 3314                  * retransmit happens at 3*rtt).
 3315                  */
 3316                 tp->t_srtt = rtt << TCP_RTT_SHIFT;
 3317                 tp->t_rttvar = rtt << (TCP_RTTVAR_SHIFT - 1);
 3318                 tp->t_rttbest = tp->t_srtt + tp->t_rttvar;
 3319         }
 3320         tp->t_rtttime = 0;
 3321         tp->t_rxtshift = 0;
 3322 
 3323         /*
 3324          * the retransmit should happen at rtt + 4 * rttvar.
 3325          * Because of the way we do the smoothing, srtt and rttvar
 3326          * will each average +1/2 tick of bias.  When we compute
 3327          * the retransmit timer, we want 1/2 tick of rounding and
 3328          * 1 extra tick because of +-1/2 tick uncertainty in the
 3329          * firing of the timer.  The bias will give us exactly the
 3330          * 1.5 tick we need.  But, because the bias is
 3331          * statistical, we have to test that we don't drop below
 3332          * the minimum feasible timer (which is 2 ticks).
 3333          */
 3334         TCPT_RANGESET(tp->t_rxtcur, TCP_REXMTVAL(tp),
 3335                       max(tp->t_rttmin, rtt + 2), TCPTV_REXMTMAX);
 3336 
 3337         /*
 3338          * We received an ack for a packet that wasn't retransmitted;
 3339          * it is probably safe to discard any error indications we've
 3340          * received recently.  This isn't quite right, but close enough
 3341          * for now (a route might have failed after we sent a segment,
 3342          * and the return path might not be symmetrical).
 3343          */
 3344         tp->t_softerror = 0;
 3345 }
 3346 
 3347 /*
 3348  * Determine a reasonable value for maxseg size.
 3349  * If the route is known, check route for mtu.
 3350  * If none, use an mss that can be handled on the outgoing interface
 3351  * without forcing IP to fragment.  If no route is found, route has no mtu,
 3352  * or the destination isn't local, use a default, hopefully conservative
 3353  * size (usually 512 or the default IP max size, but no more than the mtu
 3354  * of the interface), as we can't discover anything about intervening
 3355  * gateways or networks.  We also initialize the congestion/slow start
 3356  * window to be a single segment if the destination isn't local.
 3357  * While looking at the routing entry, we also initialize other path-dependent
 3358  * parameters from pre-set or cached values in the routing entry.
 3359  *
 3360  * Also take into account the space needed for options that we
 3361  * send regularly.  Make maxseg shorter by that amount to assure
 3362  * that we can send maxseg amount of data even when the options
 3363  * are present.  Store the upper limit of the length of options plus
 3364  * data in maxopd.
 3365  *
 3366  * NOTE that this routine is only called when we process an incoming
 3367  * segment, or an ICMP need fragmentation datagram. Outgoing SYN/ACK MSS
 3368  * settings are handled in tcp_mssopt().
 3369  */
 3370 void
 3371 tcp_mss_update(struct tcpcb *tp, int offer, int mtuoffer,
 3372     struct hc_metrics_lite *metricptr, struct tcp_ifcap *cap)
 3373 {
 3374         int mss = 0;
 3375         u_long maxmtu = 0;
 3376         struct inpcb *inp = tp->t_inpcb;
 3377         struct hc_metrics_lite metrics;
 3378         int origoffer;
 3379 #ifdef INET6
 3380         int isipv6 = ((inp->inp_vflag & INP_IPV6) != 0) ? 1 : 0;
 3381         size_t min_protoh = isipv6 ?
 3382                             sizeof (struct ip6_hdr) + sizeof (struct tcphdr) :
 3383                             sizeof (struct tcpiphdr);
 3384 #else
 3385         const size_t min_protoh = sizeof(struct tcpiphdr);
 3386 #endif
 3387 
 3388         INP_WLOCK_ASSERT(tp->t_inpcb);
 3389 
 3390         if (mtuoffer != -1) {
 3391                 KASSERT(offer == -1, ("%s: conflict", __func__));
 3392                 offer = mtuoffer - min_protoh;
 3393         }
 3394         origoffer = offer;
 3395 
 3396         /* Initialize. */
 3397 #ifdef INET6
 3398         if (isipv6) {
 3399                 maxmtu = tcp_maxmtu6(&inp->inp_inc, cap);
 3400                 tp->t_maxopd = tp->t_maxseg = V_tcp_v6mssdflt;
 3401         }
 3402 #endif
 3403 #if defined(INET) && defined(INET6)
 3404         else
 3405 #endif
 3406 #ifdef INET
 3407         {
 3408                 maxmtu = tcp_maxmtu(&inp->inp_inc, cap);
 3409                 tp->t_maxopd = tp->t_maxseg = V_tcp_mssdflt;
 3410         }
 3411 #endif
 3412 
 3413         /*
 3414          * No route to sender, stay with default mss and return.
 3415          */
 3416         if (maxmtu == 0) {
 3417                 /*
 3418                  * In case we return early we need to initialize metrics
 3419                  * to a defined state as tcp_hc_get() would do for us
 3420                  * if there was no cache hit.
 3421                  */
 3422                 if (metricptr != NULL)
 3423                         bzero(metricptr, sizeof(struct hc_metrics_lite));
 3424                 return;
 3425         }
 3426 
 3427         /* What have we got? */
 3428         switch (offer) {
 3429                 case 0:
 3430                         /*
 3431                          * Offer == 0 means that there was no MSS on the SYN
 3432                          * segment, in this case we use tcp_mssdflt as
 3433                          * already assigned to t_maxopd above.
 3434                          */
 3435                         offer = tp->t_maxopd;
 3436                         break;
 3437 
 3438                 case -1:
 3439                         /*
 3440                          * Offer == -1 means that we didn't receive SYN yet.
 3441                          */
 3442                         /* FALLTHROUGH */
 3443 
 3444                 default:
 3445                         /*
 3446                          * Prevent DoS attack with too small MSS. Round up
 3447                          * to at least minmss.
 3448                          */
 3449                         offer = max(offer, V_tcp_minmss);
 3450         }
 3451 
 3452         /*
 3453          * rmx information is now retrieved from tcp_hostcache.
 3454          */
 3455         tcp_hc_get(&inp->inp_inc, &metrics);
 3456         if (metricptr != NULL)
 3457                 bcopy(&metrics, metricptr, sizeof(struct hc_metrics_lite));
 3458 
 3459         /*
 3460          * If there's a discovered mtu in tcp hostcache, use it.
 3461          * Else, use the link mtu.
 3462          */
 3463         if (metrics.rmx_mtu)
 3464                 mss = min(metrics.rmx_mtu, maxmtu) - min_protoh;
 3465         else {
 3466 #ifdef INET6
 3467                 if (isipv6) {
 3468                         mss = maxmtu - min_protoh;
 3469                         if (!V_path_mtu_discovery &&
 3470                             !in6_localaddr(&inp->in6p_faddr))
 3471                                 mss = min(mss, V_tcp_v6mssdflt);
 3472                 }
 3473 #endif
 3474 #if defined(INET) && defined(INET6)
 3475                 else
 3476 #endif
 3477 #ifdef INET
 3478                 {
 3479                         mss = maxmtu - min_protoh;
 3480                         if (!V_path_mtu_discovery &&
 3481                             !in_localaddr(inp->inp_faddr))
 3482                                 mss = min(mss, V_tcp_mssdflt);
 3483                 }
 3484 #endif
 3485                 /*
 3486                  * XXX - The above conditional (mss = maxmtu - min_protoh)
 3487                  * probably violates the TCP spec.
 3488                  * The problem is that, since we don't know the
 3489                  * other end's MSS, we are supposed to use a conservative
 3490                  * default.  But, if we do that, then MTU discovery will
 3491                  * never actually take place, because the conservative
 3492                  * default is much less than the MTUs typically seen
 3493                  * on the Internet today.  For the moment, we'll sweep
 3494                  * this under the carpet.
 3495                  *
 3496                  * The conservative default might not actually be a problem
 3497                  * if the only case this occurs is when sending an initial
 3498                  * SYN with options and data to a host we've never talked
 3499                  * to before.  Then, they will reply with an MSS value which
 3500                  * will get recorded and the new parameters should get
 3501                  * recomputed.  For Further Study.
 3502                  */
 3503         }
 3504         mss = min(mss, offer);
 3505 
 3506         /*
 3507          * Sanity check: make sure that maxopd will be large
 3508          * enough to allow some data on segments even if the
 3509          * all the option space is used (40bytes).  Otherwise
 3510          * funny things may happen in tcp_output.
 3511          */
 3512         mss = max(mss, 64);
 3513 
 3514         /*
 3515          * maxopd stores the maximum length of data AND options
 3516          * in a segment; maxseg is the amount of data in a normal
 3517          * segment.  We need to store this value (maxopd) apart
 3518          * from maxseg, because now every segment carries options
 3519          * and thus we normally have somewhat less data in segments.
 3520          */
 3521         tp->t_maxopd = mss;
 3522 
 3523         /*
 3524          * origoffer==-1 indicates that no segments were received yet.
 3525          * In this case we just guess.
 3526          */
 3527         if ((tp->t_flags & (TF_REQ_TSTMP|TF_NOOPT)) == TF_REQ_TSTMP &&
 3528             (origoffer == -1 ||
 3529              (tp->t_flags & TF_RCVD_TSTMP) == TF_RCVD_TSTMP))
 3530                 mss -= TCPOLEN_TSTAMP_APPA;
 3531 
 3532         tp->t_maxseg = mss;
 3533 }
 3534 
 3535 void
 3536 tcp_mss(struct tcpcb *tp, int offer)
 3537 {
 3538         int mss;
 3539         u_long bufsize;
 3540         struct inpcb *inp;
 3541         struct socket *so;
 3542         struct hc_metrics_lite metrics;
 3543         struct tcp_ifcap cap;
 3544 
 3545         KASSERT(tp != NULL, ("%s: tp == NULL", __func__));
 3546 
 3547         bzero(&cap, sizeof(cap));
 3548         tcp_mss_update(tp, offer, -1, &metrics, &cap);
 3549 
 3550         mss = tp->t_maxseg;
 3551         inp = tp->t_inpcb;
 3552 
 3553         /*
 3554          * If there's a pipesize, change the socket buffer to that size,
 3555          * don't change if sb_hiwat is different than default (then it
 3556          * has been changed on purpose with setsockopt).
 3557          * Make the socket buffers an integral number of mss units;
 3558          * if the mss is larger than the socket buffer, decrease the mss.
 3559          */
 3560         so = inp->inp_socket;
 3561         SOCKBUF_LOCK(&so->so_snd);
 3562         if ((so->so_snd.sb_hiwat == V_tcp_sendspace) && metrics.rmx_sendpipe)
 3563                 bufsize = metrics.rmx_sendpipe;
 3564         else
 3565                 bufsize = so->so_snd.sb_hiwat;
 3566         if (bufsize < mss)
 3567                 mss = bufsize;
 3568         else {
 3569                 bufsize = roundup(bufsize, mss);
 3570                 if (bufsize > sb_max)
 3571                         bufsize = sb_max;
 3572                 if (bufsize > so->so_snd.sb_hiwat)
 3573                         (void)sbreserve_locked(&so->so_snd, bufsize, so, NULL);
 3574         }
 3575         SOCKBUF_UNLOCK(&so->so_snd);
 3576         tp->t_maxseg = mss;
 3577 
 3578         SOCKBUF_LOCK(&so->so_rcv);
 3579         if ((so->so_rcv.sb_hiwat == V_tcp_recvspace) && metrics.rmx_recvpipe)
 3580                 bufsize = metrics.rmx_recvpipe;
 3581         else
 3582                 bufsize = so->so_rcv.sb_hiwat;
 3583         if (bufsize > mss) {
 3584                 bufsize = roundup(bufsize, mss);
 3585                 if (bufsize > sb_max)
 3586                         bufsize = sb_max;
 3587                 if (bufsize > so->so_rcv.sb_hiwat)
 3588                         (void)sbreserve_locked(&so->so_rcv, bufsize, so, NULL);
 3589         }
 3590         SOCKBUF_UNLOCK(&so->so_rcv);
 3591 
 3592         /* Check the interface for TSO capabilities. */
 3593         if (cap.ifcap & CSUM_TSO) {
 3594                 tp->t_flags |= TF_TSO;
 3595                 tp->t_tsomax = cap.tsomax;
 3596                 tp->t_tsomaxsegcount = cap.tsomaxsegcount;
 3597                 tp->t_tsomaxsegsize = cap.tsomaxsegsize;
 3598         }
 3599 }
 3600 
 3601 /*
 3602  * Determine the MSS option to send on an outgoing SYN.
 3603  */
 3604 int
 3605 tcp_mssopt(struct in_conninfo *inc)
 3606 {
 3607         int mss = 0;
 3608         u_long maxmtu = 0;
 3609         u_long thcmtu = 0;
 3610         size_t min_protoh;
 3611 
 3612         KASSERT(inc != NULL, ("tcp_mssopt with NULL in_conninfo pointer"));
 3613 
 3614 #ifdef INET6
 3615         if (inc->inc_flags & INC_ISIPV6) {
 3616                 mss = V_tcp_v6mssdflt;
 3617                 maxmtu = tcp_maxmtu6(inc, NULL);
 3618                 min_protoh = sizeof(struct ip6_hdr) + sizeof(struct tcphdr);
 3619         }
 3620 #endif
 3621 #if defined(INET) && defined(INET6)
 3622         else
 3623 #endif
 3624 #ifdef INET
 3625         {
 3626                 mss = V_tcp_mssdflt;
 3627                 maxmtu = tcp_maxmtu(inc, NULL);
 3628                 min_protoh = sizeof(struct tcpiphdr);
 3629         }
 3630 #endif
 3631 #if defined(INET6) || defined(INET)
 3632         thcmtu = tcp_hc_getmtu(inc); /* IPv4 and IPv6 */
 3633 #endif
 3634 
 3635         if (maxmtu && thcmtu)
 3636                 mss = min(maxmtu, thcmtu) - min_protoh;
 3637         else if (maxmtu || thcmtu)
 3638                 mss = max(maxmtu, thcmtu) - min_protoh;
 3639 
 3640         return (mss);
 3641 }
 3642 
 3643 
 3644 /*
 3645  * On a partial ack arrives, force the retransmission of the
 3646  * next unacknowledged segment.  Do not clear tp->t_dupacks.
 3647  * By setting snd_nxt to ti_ack, this forces retransmission timer to
 3648  * be started again.
 3649  */
 3650 static void
 3651 tcp_newreno_partial_ack(struct tcpcb *tp, struct tcphdr *th)
 3652 {
 3653         tcp_seq onxt = tp->snd_nxt;
 3654         u_long  ocwnd = tp->snd_cwnd;
 3655 
 3656         INP_WLOCK_ASSERT(tp->t_inpcb);
 3657 
 3658         tcp_timer_activate(tp, TT_REXMT, 0);
 3659         tp->t_rtttime = 0;
 3660         tp->snd_nxt = th->th_ack;
 3661         /*
 3662          * Set snd_cwnd to one segment beyond acknowledged offset.
 3663          * (tp->snd_una has not yet been updated when this function is called.)
 3664          */
 3665         tp->snd_cwnd = tp->t_maxseg + BYTES_THIS_ACK(tp, th);
 3666         tp->t_flags |= TF_ACKNOW;
 3667         (void) tcp_output(tp);
 3668         tp->snd_cwnd = ocwnd;
 3669         if (SEQ_GT(onxt, tp->snd_nxt))
 3670                 tp->snd_nxt = onxt;
 3671         /*
 3672          * Partial window deflation.  Relies on fact that tp->snd_una
 3673          * not updated yet.
 3674          */
 3675         if (tp->snd_cwnd > BYTES_THIS_ACK(tp, th))
 3676                 tp->snd_cwnd -= BYTES_THIS_ACK(tp, th);
 3677         else
 3678                 tp->snd_cwnd = 0;
 3679         tp->snd_cwnd += tp->t_maxseg;
 3680 }

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