sys/netinet/tcp_input.c

   1 /*
   2  * Copyright (c) 2002-2004 Jeffrey Hsu.  All rights reserved.
   3  * Copyright (c) 1982, 1986, 1988, 1990, 1993, 1994, 1995
   4  *      The Regents of the University of California.  All rights reserved.
   5  *
   6  * Redistribution and use in source and binary forms, with or without
   7  * modification, are permitted provided that the following conditions
   8  * are met:
   9  * 1. Redistributions of source code must retain the above copyright
  10  *    notice, this list of conditions and the following disclaimer.
  11  * 2. Redistributions in binary form must reproduce the above copyright
  12  *    notice, this list of conditions and the following disclaimer in the
  13  *    documentation and/or other materials provided with the distribution.
  14  * 3. All advertising materials mentioning features or use of this software
  15  *    must display the following acknowledgement:
  16  *      This product includes software developed by the University of
  17  *      California, Berkeley and its contributors.
  18  * 4. Neither the name of the University nor the names of its contributors
  19  *    may be used to endorse or promote products derived from this software
  20  *    without specific prior written permission.
  21  *
  22  * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
  23  * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
  24  * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
  25  * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
  26  * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
  27  * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
  28  * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
  29  * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
  30  * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
  31  * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
  32  * SUCH DAMAGE.
  33  *
  34  *      @(#)tcp_input.c 8.12 (Berkeley) 5/24/95
  35  * $FreeBSD: src/sys/netinet/tcp_input.c,v 1.107.2.38 2003/05/21 04:46:41 cjc Exp $
  36  * $DragonFly: src/sys/netinet/tcp_input.c,v 1.25 2004/04/13 05:23:13 dillon Exp $
  37  */
  38
  39 #include "opt_ipfw.h"           /* for ipfw_fwd         */
  40 #include "opt_inet6.h"
  41 #include "opt_ipsec.h"
  42 #include "opt_tcpdebug.h"
  43 #include "opt_tcp_input.h"
  44
  45 #include <sys/param.h>
  46 #include <sys/systm.h>
  47 #include <sys/kernel.h>
  48 #include <sys/sysctl.h>
  49 #include <sys/malloc.h>
  50 #include <sys/mbuf.h>
  51 #include <sys/proc.h>           /* for proc0 declaration */
  52 #include <sys/protosw.h>
  53 #include <sys/socket.h>
  54 #include <sys/socketvar.h>
  55 #include <sys/syslog.h>
  56 #include <sys/in_cksum.h>
  57
  58 #include <machine/cpu.h>        /* before tcp_seq.h, for tcp_random18() */
  59
  60 #include <net/if.h>
  61 #include <net/route.h>
  62
  63 #include <netinet/in.h>
  64 #include <netinet/in_systm.h>
  65 #include <netinet/ip.h>
  66 #include <netinet/ip_icmp.h>    /* for ICMP_BANDLIM             */
  67 #include <netinet/in_var.h>
  68 #include <netinet/icmp_var.h>   /* for ICMP_BANDLIM             */
  69 #include <netinet/in_pcb.h>
  70 #include <netinet/ip_var.h>
  71 #include <netinet/ip6.h>
  72 #include <netinet/icmp6.h>
  73 #include <netinet6/nd6.h>
  74 #include <netinet6/ip6_var.h>
  75 #include <netinet6/in6_pcb.h>
  76 #include <netinet/tcp.h>
  77 #include <netinet/tcp_fsm.h>
  78 #include <netinet/tcp_seq.h>
  79 #include <netinet/tcp_timer.h>
  80 #include <netinet/tcp_var.h>
  81 #include <netinet6/tcp6_var.h>
  82 #include <netinet/tcpip.h>
  83 #ifdef TCPDEBUG
  84 #include <netinet/tcp_debug.h>
  85
  86 u_char tcp_saveipgen[40]; /* the size must be of max ip header, now IPv6 */
  87 struct tcphdr tcp_savetcp;
  88 #endif /* TCPDEBUG */
  89
  90 #ifdef FAST_IPSEC
  91 #include <netipsec/ipsec.h>
  92 #include <netipsec/ipsec6.h>
  93 #endif
  94
  95 #ifdef IPSEC
  96 #include <netinet6/ipsec.h>
  97 #include <netinet6/ipsec6.h>
  98 #include <netproto/key/key.h>
  99 #endif /*IPSEC*/
 100
 101 MALLOC_DEFINE(M_TSEGQ, "tseg_qent", "TCP segment queue entry");
 102
 103 static const int tcprexmtthresh = 3;
 104 tcp_cc  tcp_ccgen;
 105 static int log_in_vain = 0;
 106 SYSCTL_INT(_net_inet_tcp, OID_AUTO, log_in_vain, CTLFLAG_RW,
 107     &log_in_vain, 0, "Log all incoming TCP connections");
 108
 109 static int blackhole = 0;
 110 SYSCTL_INT(_net_inet_tcp, OID_AUTO, blackhole, CTLFLAG_RW,
 111     &blackhole, 0, "Do not send RST when dropping refused connections");
 112
 113 int tcp_delack_enabled = 1;
 114 SYSCTL_INT(_net_inet_tcp, OID_AUTO, delayed_ack, CTLFLAG_RW,
 115     &tcp_delack_enabled, 0,
 116     "Delay ACK to try and piggyback it onto a data packet");
 117
 118 #ifdef TCP_DROP_SYNFIN
 119 static int drop_synfin = 0;
 120 SYSCTL_INT(_net_inet_tcp, OID_AUTO, drop_synfin, CTLFLAG_RW,
 121     &drop_synfin, 0, "Drop TCP packets with SYN+FIN set");
 122 #endif
 123
 124 static int tcp_do_limitedtransmit = 1;
 125 SYSCTL_INT(_net_inet_tcp, OID_AUTO, limitedtransmit, CTLFLAG_RW,
 126     &tcp_do_limitedtransmit, 0, "Enable RFC 3042 (Limited Transmit)");
 127
 128 static int tcp_do_early_retransmit = 0;
 129 SYSCTL_INT(_net_inet_tcp, OID_AUTO, earlyretransmit, CTLFLAG_RW,
 130     &tcp_do_early_retransmit, 0, "Early retransmit");
 131
 132 static int tcp_do_rfc3390 = 1;
 133 SYSCTL_INT(_net_inet_tcp, OID_AUTO, rfc3390, CTLFLAG_RW,
 134     &tcp_do_rfc3390, 0,
 135     "Enable RFC 3390 (Increasing TCP's Initial Congestion Window)");
 136
 137 static int tcp_do_eifel_detect = 1;
 138 SYSCTL_INT(_net_inet_tcp, OID_AUTO, eifel, CTLFLAG_RW,
 139     &tcp_do_eifel_detect, 0, "Eifel detection algorithm (RFC 3522)");
 140
 141 SYSCTL_NODE(_net_inet_tcp, OID_AUTO, reass, CTLFLAG_RW, 0,
 142     "TCP Segment Reassembly Queue");
 143
 144 int tcp_reass_maxseg = 0;
 145 SYSCTL_INT(_net_inet_tcp_reass, OID_AUTO, maxsegments, CTLFLAG_RD,
 146     &tcp_reass_maxseg, 0,
 147     "Global maximum number of TCP Segments in Reassembly Queue");
 148
 149 int tcp_reass_qsize = 0;
 150 SYSCTL_INT(_net_inet_tcp_reass, OID_AUTO, cursegments, CTLFLAG_RD,
 151     &tcp_reass_qsize, 0,
 152     "Global number of TCP Segments currently in Reassembly Queue");
 153
 154 static int tcp_reass_overflows = 0;
 155 SYSCTL_INT(_net_inet_tcp_reass, OID_AUTO, overflows, CTLFLAG_RD,
 156     &tcp_reass_overflows, 0,
 157     "Global number of TCP Segment Reassembly Queue Overflows");
 158
 159 struct inpcbinfo tcbinfo[MAXCPU];
 160
 161 static void      tcp_dooptions(struct tcpopt *, u_char *, int, int);
 162 static void      tcp_pulloutofband(struct socket *,
 163                      struct tcphdr *, struct mbuf *, int);
 164 static int       tcp_reass(struct tcpcb *, struct tcphdr *, int *,
 165                      struct mbuf *);
 166 static void      tcp_xmit_timer(struct tcpcb *, int);
 167 static void      tcp_newreno_partial_ack(struct tcpcb *, struct tcphdr *);
 168
 169 /* Neighbor Discovery, Neighbor Unreachability Detection Upper layer hint. */
 170 #ifdef INET6
 171 #define ND6_HINT(tp) \
 172 do { \
 173         if ((tp) && (tp)->t_inpcb && \
 174             ((tp)->t_inpcb->inp_vflag & INP_IPV6) != 0 && \
 175             (tp)->t_inpcb->in6p_route.ro_rt) \
 176                 nd6_nud_hint((tp)->t_inpcb->in6p_route.ro_rt, NULL, 0); \
 177 } while (0)
 178 #else
 179 #define ND6_HINT(tp)
 180 #endif
 181
 182 /*
 183  * Indicate whether this ack should be delayed.  We can delay the ack if
 184  *      - delayed acks are enabled and
 185  *      - there is no delayed ack timer in progress and
 186  *      - our last ack wasn't a 0-sized window.  We never want to delay
 187  *        the ack that opens up a 0-sized window.
 188  */
 189 #define DELAY_ACK(tp) \
 190         (tcp_delack_enabled && !callout_pending(tp->tt_delack) && \
 191         (tp->t_flags & TF_RXWIN0SENT) == 0)
 192
 193 static int
 194 tcp_reass(tp, th, tlenp, m)
 195         struct tcpcb *tp;
 196         struct tcphdr *th;
 197         int *tlenp;
 198         struct mbuf *m;
 199 {
 200         struct tseg_qent *q;
 201         struct tseg_qent *p = NULL;
 202         struct tseg_qent *nq;
 203         struct tseg_qent *te;
 204         struct socket *so = tp->t_inpcb->inp_socket;
 205         int flags;
 206
 207         /*
 208          * Call with th==0 after become established to
 209          * force pre-ESTABLISHED data up to user socket.
 210          */
 211         if (th == 0)
 212                 goto present;
 213
 214         /*
 215          * Limit the number of segments in the reassembly queue to prevent
 216          * holding on to too many segments (and thus running out of mbufs).
 217          * Make sure to let the missing segment through which caused this
 218          * queue.  Always keep one global queue entry spare to be able to
 219          * process the missing segment.
 220          */
 221         if (th->th_seq != tp->rcv_nxt &&
 222             tcp_reass_qsize + 1 >= tcp_reass_maxseg) {
 223                 tcp_reass_overflows++;
 224                 tcpstat.tcps_rcvmemdrop++;
 225                 m_freem(m);
 226                 return (0);
 227         }
 228
 229         /* Allocate a new queue entry. If we can't, just drop the pkt. XXX */
 230         MALLOC(te, struct tseg_qent *, sizeof(struct tseg_qent), M_TSEGQ,
 231                M_NOWAIT);
 232         if (te == NULL) {
 233                 tcpstat.tcps_rcvmemdrop++;
 234                 m_freem(m);
 235                 return (0);
 236         }
 237         tcp_reass_qsize++;
 238
 239         /*
 240          * Find a segment which begins after this one does.
 241          */
 242         LIST_FOREACH(q, &tp->t_segq, tqe_q) {
 243                 if (SEQ_GT(q->tqe_th->th_seq, th->th_seq))
 244                         break;
 245                 p = q;
 246         }
 247
 248         /*
 249          * If there is a preceding segment, it may provide some of
 250          * our data already.  If so, drop the data from the incoming
 251          * segment.  If it provides all of our data, drop us.
 252          */
 253         if (p != NULL) {
 254                 int i;
 255                 /* conversion to int (in i) handles seq wraparound */
 256                 i = p->tqe_th->th_seq + p->tqe_len - th->th_seq;
 257                 if (i > 0) {
 258                         if (i >= *tlenp) {
 259                                 tcpstat.tcps_rcvduppack++;
 260                                 tcpstat.tcps_rcvdupbyte += *tlenp;
 261                                 m_freem(m);
 262                                 free(te, M_TSEGQ);
 263                                 tcp_reass_qsize--;
 264                                 /*
 265                                  * Try to present any queued data
 266                                  * at the left window edge to the user.
 267                                  * This is needed after the 3-WHS
 268                                  * completes.
 269                                  */
 270                                 goto present;   /* ??? */
 271                         }
 272                         m_adj(m, i);
 273                         *tlenp -= i;
 274                         th->th_seq += i;
 275                 }
 276         }
 277         tcpstat.tcps_rcvoopack++;
 278         tcpstat.tcps_rcvoobyte += *tlenp;
 279
 280         /*
 281          * While we overlap succeeding segments trim them or,
 282          * if they are completely covered, dequeue them.
 283          */
 284         while (q) {
 285                 int i = (th->th_seq + *tlenp) - q->tqe_th->th_seq;
 286                 if (i <= 0)
 287                         break;
 288                 if (i < q->tqe_len) {
 289                         q->tqe_th->th_seq += i;
 290                         q->tqe_len -= i;
 291                         m_adj(q->tqe_m, i);
 292                         break;
 293                 }
 294
 295                 nq = LIST_NEXT(q, tqe_q);
 296                 LIST_REMOVE(q, tqe_q);
 297                 m_freem(q->tqe_m);
 298                 free(q, M_TSEGQ);
 299                 tcp_reass_qsize--;
 300                 q = nq;
 301         }
 302
 303         /* Insert the new segment queue entry into place. */
 304         te->tqe_m = m;
 305         te->tqe_th = th;
 306         te->tqe_len = *tlenp;
 307
 308         if (p == NULL) {
 309                 LIST_INSERT_HEAD(&tp->t_segq, te, tqe_q);
 310         } else {
 311                 LIST_INSERT_AFTER(p, te, tqe_q);
 312         }
 313
 314 present:
 315         /*
 316          * Present data to user, advancing rcv_nxt through
 317          * completed sequence space.
 318          */
 319         if (!TCPS_HAVEESTABLISHED(tp->t_state))
 320                 return (0);
 321         q = LIST_FIRST(&tp->t_segq);
 322         if (!q || q->tqe_th->th_seq != tp->rcv_nxt)
 323                 return (0);
 324         do {
 325                 tp->rcv_nxt += q->tqe_len;
 326                 flags = q->tqe_th->th_flags & TH_FIN;
 327                 nq = LIST_NEXT(q, tqe_q);
 328                 LIST_REMOVE(q, tqe_q);
 329                 if (so->so_state & SS_CANTRCVMORE)
 330                         m_freem(q->tqe_m);
 331                 else
 332                         sbappend(&so->so_rcv, q->tqe_m);
 333                 free(q, M_TSEGQ);
 334                 tcp_reass_qsize--;
 335                 q = nq;
 336         } while (q && q->tqe_th->th_seq == tp->rcv_nxt);
 337         ND6_HINT(tp);
 338         sorwakeup(so);
 339         return (flags);
 340 }
 341
 342 /*
 343  * TCP input routine, follows pages 65-76 of the
 344  * protocol specification dated September, 1981 very closely.
 345  */
 346 #ifdef INET6
 347 int
 348 tcp6_input(mp, offp, proto)
 349         struct mbuf **mp;
 350         int *offp, proto;
 351 {
 352         struct mbuf *m = *mp;
 353         struct in6_ifaddr *ia6;
 354
 355         IP6_EXTHDR_CHECK(m, *offp, sizeof(struct tcphdr), IPPROTO_DONE);
 356
 357         /*
 358          * draft-itojun-ipv6-tcp-to-anycast
 359          * better place to put this in?
 360          */
 361         ia6 = ip6_getdstifaddr(m);
 362         if (ia6 && (ia6->ia6_flags & IN6_IFF_ANYCAST)) {
 363                 struct ip6_hdr *ip6;
 364
 365                 ip6 = mtod(m, struct ip6_hdr *);
 366                 icmp6_error(m, ICMP6_DST_UNREACH, ICMP6_DST_UNREACH_ADDR,
 367                             (caddr_t)&ip6->ip6_dst - (caddr_t)ip6);
 368                 return (IPPROTO_DONE);
 369         }
 370
 371         tcp_input(m, *offp, proto);
 372         return (IPPROTO_DONE);
 373 }
 374 #endif
 375
 376 void
 377 tcp_input(m, off0, proto)
 378         struct mbuf *m;
 379         int off0, proto;
 380 {
 381         struct tcphdr *th;
 382         struct ip *ip = NULL;
 383         struct ipovly *ipov;
 384         struct inpcb *inp = NULL;
 385         u_char *optp = NULL;
 386         int optlen = 0;
 387         int len, tlen, off;
 388         int drop_hdrlen;
 389         struct tcpcb *tp = NULL;
 390         int thflags;
 391         struct socket *so = 0;
 392         int todrop, acked, ourfinisacked, needoutput = 0;
 393         u_long tiwin;
 394         struct tcpopt to;               /* options in this segment */
 395         struct rmxp_tao *taop;          /* pointer to our TAO cache entry */
 396         struct rmxp_tao tao_noncached;  /* in case there's no cached entry */
 397         struct sockaddr_in *next_hop = NULL;
 398         int rstreason; /* For badport_bandlim accounting purposes */
 399         int cpu;
 400         struct ip6_hdr *ip6 = NULL;
 401 #ifdef INET6
 402         boolean_t isipv6;
 403 #else
 404         const boolean_t isipv6 = FALSE;
 405 #endif
 406 #ifdef TCPDEBUG
 407         short ostate = 0;
 408 #endif
 409
 410         tcpstat.tcps_rcvtotal++;
 411
 412         /* Grab info from and strip MT_TAG mbufs prepended to the chain. */
 413         while  (m->m_type == MT_TAG) {
 414                 if (m->_m_tag_id == PACKET_TAG_IPFORWARD)
 415                         next_hop = (struct sockaddr_in *)m->m_hdr.mh_data;
 416                 m = m->m_next;
 417         }
 418
 419 #ifdef INET6
 420         isipv6 = (mtod(m, struct ip *)->ip_v == 6) ? TRUE : FALSE;
 421 #endif
 422
 423         if (isipv6) {
 424                 /* IP6_EXTHDR_CHECK() is already done at tcp6_input() */
 425                 ip6 = mtod(m, struct ip6_hdr *);
 426                 tlen = sizeof(*ip6) + ntohs(ip6->ip6_plen) - off0;
 427                 if (in6_cksum(m, IPPROTO_TCP, off0, tlen)) {
 428                         tcpstat.tcps_rcvbadsum++;
 429                         goto drop;
 430                 }
 431                 th = (struct tcphdr *)((caddr_t)ip6 + off0);
 432
 433                 /*
 434                  * Be proactive about unspecified IPv6 address in source.
 435                  * As we use all-zero to indicate unbounded/unconnected pcb,
 436                  * unspecified IPv6 address can be used to confuse us.
 437                  *
 438                  * Note that packets with unspecified IPv6 destination is
 439                  * already dropped in ip6_input.
 440                  */
 441                 if (IN6_IS_ADDR_UNSPECIFIED(&ip6->ip6_src)) {
 442                         /* XXX stat */
 443                         goto drop;
 444                 }
 445         } else {
 446                 /*
 447                  * Get IP and TCP header together in first mbuf.
 448                  * Note: IP leaves IP header in first mbuf.
 449                  */
 450                 if (off0 > sizeof(struct ip)) {
 451                         ip_stripoptions(m);
 452                         off0 = sizeof(struct ip);
 453                 }
 454                 /* already checked and pulled up in ip_demux() */
 455                 KASSERT(m->m_len >= sizeof(struct tcpiphdr),
 456                     ("TCP header not in one mbuf"));
 457                 ip = mtod(m, struct ip *);
 458                 ipov = (struct ipovly *)ip;
 459                 th = (struct tcphdr *)((caddr_t)ip + off0);
 460                 tlen = ip->ip_len;
 461
 462                 if (m->m_pkthdr.csum_flags & CSUM_DATA_VALID) {
 463                         if (m->m_pkthdr.csum_flags & CSUM_PSEUDO_HDR)
 464                                 th->th_sum = m->m_pkthdr.csum_data;
 465                         else
 466                                 th->th_sum = in_pseudo(ip->ip_src.s_addr,
 467                                                 ip->ip_dst.s_addr,
 468                                                 htonl(m->m_pkthdr.csum_data +
 469                                                         ip->ip_len +
 470                                                         IPPROTO_TCP));
 471                         th->th_sum ^= 0xffff;
 472                 } else {
 473                         /*
 474                          * Checksum extended TCP header and data.
 475                          */
 476                         len = sizeof(struct ip) + tlen;
 477                         bzero(ipov->ih_x1, sizeof(ipov->ih_x1));
 478                         ipov->ih_len = (u_short)tlen;
 479                         ipov->ih_len = htons(ipov->ih_len);
 480                         th->th_sum = in_cksum(m, len);
 481                 }
 482                 if (th->th_sum) {
 483                         tcpstat.tcps_rcvbadsum++;
 484                         goto drop;
 485                 }
 486 #ifdef INET6
 487                 /* Re-initialization for later version check */
 488                 ip->ip_v = IPVERSION;
 489 #endif
 490         }
 491
 492         /*
 493          * Check that TCP offset makes sense,
 494          * pull out TCP options and adjust length.              XXX
 495          */
 496         off = th->th_off << 2;
 497         /* already checked and pulled up in ip_demux() */
 498         KASSERT(off >= sizeof(struct tcphdr) && off <= tlen,
 499             ("bad TCP data offset"));
 500         tlen -= off;    /* tlen is used instead of ti->ti_len */
 501         if (off > sizeof(struct tcphdr)) {
 502                 if (isipv6) {
 503                         IP6_EXTHDR_CHECK(m, off0, off, );
 504                         ip6 = mtod(m, struct ip6_hdr *);
 505                         th = (struct tcphdr *)((caddr_t)ip6 + off0);
 506                 } else {
 507                         /* already pulled up in ip_demux() */
 508                         KASSERT(m->m_len >= sizeof(struct ip) + off,
 509                             ("TCP header and options not in one mbuf"));
 510                 }
 511                 optlen = off - sizeof(struct tcphdr);
 512                 optp = (u_char *)(th + 1);
 513         }
 514         thflags = th->th_flags;
 515
 516 #ifdef TCP_DROP_SYNFIN
 517         /*
 518          * If the drop_synfin option is enabled, drop all packets with
 519          * both the SYN and FIN bits set. This prevents e.g. nmap from
 520          * identifying the TCP/IP stack.
 521          *
 522          * This is a violation of the TCP specification.
 523          */
 524         if (drop_synfin && (thflags & (TH_SYN|TH_FIN)) == (TH_SYN|TH_FIN))
 525                 goto drop;
 526 #endif
 527
 528         /*
 529          * Convert TCP protocol specific fields to host format.
 530          */
 531         th->th_seq = ntohl(th->th_seq);
 532         th->th_ack = ntohl(th->th_ack);
 533         th->th_win = ntohs(th->th_win);
 534         th->th_urp = ntohs(th->th_urp);
 535
 536         /*
 537          * Delay dropping TCP, IP headers, IPv6 ext headers, and TCP options,
 538          * until after ip6_savecontrol() is called and before other functions
 539          * which don't want those proto headers.
 540          * Because ip6_savecontrol() is going to parse the mbuf to
 541          * search for data to be passed up to user-land, it wants mbuf
 542          * parameters to be unchanged.
 543          * XXX: the call of ip6_savecontrol() has been obsoleted based on
 544          * latest version of the advanced API (20020110).
 545          */
 546         drop_hdrlen = off0 + off;
 547
 548         /*
 549          * Locate pcb for segment.
 550          */
 551 findpcb:
 552         /* IPFIREWALL_FORWARD section */
 553         if (next_hop != NULL && !isipv6) {  /* IPv6 support is not there yet */
 554                 /*
 555                  * Transparently forwarded. Pretend to be the destination.
 556                  * already got one like this?
 557                  */
 558                 cpu = mycpu->gd_cpuid;
 559                 inp = in_pcblookup_hash(&tcbinfo[cpu],
 560                                         ip->ip_src, th->th_sport,
 561                                         ip->ip_dst, th->th_dport,
 562                                         0, m->m_pkthdr.rcvif);
 563                 if (!inp) {
 564                         /*
 565                          * It's new.  Try to find the ambushing socket.
 566                          */
 567
 568                         /*
 569                          * The rest of the ipfw code stores the port in
 570                          * host order.  XXX
 571                          * (The IP address is still in network order.)
 572                          */
 573                         in_port_t dport = next_hop->sin_port ?
 574                                                 htons(next_hop->sin_port) :
 575                                                 th->th_dport;
 576
 577                         cpu = tcp_addrcpu(ip->ip_src.s_addr, th->th_sport,
 578                                           next_hop->sin_addr.s_addr, dport);
 579                         inp = in_pcblookup_hash(&tcbinfo[cpu],
 580                                                 ip->ip_src, th->th_sport,
 581                                                 next_hop->sin_addr, dport,
 582                                                 1, m->m_pkthdr.rcvif);
 583                 }
 584         } else {
 585                 if (isipv6) {
 586                         inp = in6_pcblookup_hash(&tcbinfo[0],
 587                                                  &ip6->ip6_src, th->th_sport,
 588                                                  &ip6->ip6_dst, th->th_dport,
 589                                                  1, m->m_pkthdr.rcvif);
 590                 } else {
 591                         cpu = mycpu->gd_cpuid;
 592                         inp = in_pcblookup_hash(&tcbinfo[cpu],
 593                                                 ip->ip_src, th->th_sport,
 594                                                 ip->ip_dst, th->th_dport,
 595                                                 1, m->m_pkthdr.rcvif);
 596                 }
 597       }
 598
 599 #ifdef IPSEC
 600         if (isipv6) {
 601                 if (inp != NULL && ipsec6_in_reject_so(m, inp->inp_socket)) {
 602                         ipsec6stat.in_polvio++;
 603                         goto drop;
 604                 }
 605         } else {
 606                 if (inp != NULL && ipsec4_in_reject_so(m, inp->inp_socket)) {
 607                         ipsecstat.in_polvio++;
 608                         goto drop;
 609                 }
 610         }
 611 #endif
 612 #ifdef FAST_IPSEC
 613         if (isipv6) {
 614                 if (inp != NULL && ipsec6_in_reject(m, inp)) {
 615                         goto drop;
 616                 }
 617         } else {
 618                 if (inp != NULL && ipsec4_in_reject(m, inp)) {
 619                         goto drop;
 620                 }
 621         }
 622 #endif
 623
 624         /*
 625          * If the state is CLOSED (i.e., TCB does not exist) then
 626          * all data in the incoming segment is discarded.
 627          * If the TCB exists but is in CLOSED state, it is embryonic,
 628          * but should either do a listen or a connect soon.
 629          */
 630         if (inp == NULL) {
 631                 if (log_in_vain) {
 632 #ifdef INET6
 633                         char dbuf[INET6_ADDRSTRLEN+2], sbuf[INET6_ADDRSTRLEN+2];
 634 #else
 635                         char dbuf[4 * sizeof "123"], sbuf[4 * sizeof "123"];
 636 #endif
 637                         if (isipv6) {
 638                                 strcpy(dbuf, "[");
 639                                 strcpy(sbuf, "[");
 640                                 strcat(dbuf, ip6_sprintf(&ip6->ip6_dst));
 641                                 strcat(sbuf, ip6_sprintf(&ip6->ip6_src));
 642                                 strcat(dbuf, "]");
 643                                 strcat(sbuf, "]");
 644                         } else {
 645                                 strcpy(dbuf, inet_ntoa(ip->ip_dst));
 646                                 strcpy(sbuf, inet_ntoa(ip->ip_src));
 647                         }
 648                         switch (log_in_vain) {
 649                         case 1:
 650                                 if ((thflags & TH_SYN) == 0)
 651                                         break;
 652                         case 2:
 653                                 log(LOG_INFO,
 654                                     "Connection attempt to TCP %s:%d "
 655                                     "from %s:%d flags:0x%02x\n",
 656                                     dbuf, ntohs(th->th_dport), sbuf,
 657                                     ntohs(th->th_sport), thflags);
 658                                 break;
 659                         default:
 660                                 break;
 661                         }
 662                 }
 663                 if (blackhole) {
 664                         switch (blackhole) {
 665                         case 1:
 666                                 if (thflags & TH_SYN)
 667                                         goto drop;
 668                                 break;
 669                         case 2:
 670                                 goto drop;
 671                         default:
 672                                 goto drop;
 673                         }
 674                 }
 675                 rstreason = BANDLIM_RST_CLOSEDPORT;
 676                 goto dropwithreset;
 677         }
 678         tp = intotcpcb(inp);
 679         if (tp == NULL) {
 680                 rstreason = BANDLIM_RST_CLOSEDPORT;
 681                 goto dropwithreset;
 682         }
 683         if (tp->t_state == TCPS_CLOSED)
 684                 goto drop;
 685
 686         /* Unscale the window into a 32-bit value. */
 687         if ((thflags & TH_SYN) == 0)
 688                 tiwin = th->th_win << tp->snd_scale;
 689         else
 690                 tiwin = th->th_win;
 691
 692         so = inp->inp_socket;
 693
 694 #ifdef TCPDEBUG
 695         if (so->so_options & SO_DEBUG) {
 696                 ostate = tp->t_state;
 697                 if (isipv6)
 698                         bcopy((char *)ip6, (char *)tcp_saveipgen, sizeof(*ip6));
 699                 else
 700                         bcopy((char *)ip, (char *)tcp_saveipgen, sizeof(*ip));
 701                 tcp_savetcp = *th;
 702         }
 703 #endif
 704
 705         bzero((char *)&to, sizeof(to));
 706
 707         if (so->so_options & SO_ACCEPTCONN) {
 708                 struct in_conninfo inc;
 709
 710 #ifdef INET6
 711                 inc.inc_isipv6 = (isipv6 == TRUE);
 712 #endif
 713                 if (isipv6) {
 714                         inc.inc6_faddr = ip6->ip6_src;
 715                         inc.inc6_laddr = ip6->ip6_dst;
 716                         inc.inc6_route.ro_rt = NULL;            /* XXX */
 717                 } else {
 718                         inc.inc_faddr = ip->ip_src;
 719                         inc.inc_laddr = ip->ip_dst;
 720                         inc.inc_route.ro_rt = NULL;             /* XXX */
 721                 }
 722                 inc.inc_fport = th->th_sport;
 723                 inc.inc_lport = th->th_dport;
 724
 725                 /*
 726                  * If the state is LISTEN then ignore segment if it contains
 727                  * a RST.  If the segment contains an ACK then it is bad and
 728                  * send a RST.  If it does not contain a SYN then it is not
 729                  * interesting; drop it.
 730                  *
 731                  * If the state is SYN_RECEIVED (syncache) and seg contains
 732                  * an ACK, but not for our SYN/ACK, send a RST.  If the seg
 733                  * contains a RST, check the sequence number to see if it
 734                  * is a valid reset segment.
 735                  */
 736                 if ((thflags & (TH_RST|TH_ACK|TH_SYN)) != TH_SYN) {
 737                         if ((thflags & (TH_RST|TH_ACK|TH_SYN)) == TH_ACK) {
 738                                 if (!syncache_expand(&inc, th, &so, m)) {
 739                                         /*
 740                                          * No syncache entry, or ACK was not
 741                                          * for our SYN/ACK.  Send a RST.
 742                                          */
 743                                         tcpstat.tcps_badsyn++;
 744                                         rstreason = BANDLIM_RST_OPENPORT;
 745                                         goto dropwithreset;
 746                                 }
 747                                 if (so == NULL)
 748                                         /*
 749                                          * Could not complete 3-way handshake,
 750                                          * connection is being closed down, and
 751                                          * syncache will free mbuf.
 752                                          */
 753                                         return;
 754                                 /*
 755                                  * Socket is created in state SYN_RECEIVED.
 756                                  * Continue processing segment.
 757                                  */
 758                                 inp = sotoinpcb(so);
 759                                 tp = intotcpcb(inp);
 760                                 /*
 761                                  * This is what would have happened in
 762                                  * tcp_output() when the SYN,ACK was sent.
 763                                  */
 764                                 tp->snd_up = tp->snd_una;
 765                                 tp->snd_max = tp->snd_nxt = tp->iss + 1;
 766                                 tp->last_ack_sent = tp->rcv_nxt;
 767 /*
 768  * XXX possible bug - it doesn't appear that tp->snd_wnd is unscaled
 769  * until the _second_ ACK is received:
 770  *    rcv SYN (set wscale opts)  --> send SYN/ACK, set snd_wnd = window.
 771  *    rcv ACK, calculate tiwin --> process SYN_RECEIVED, determine wscale,
 772  *        move to ESTAB, set snd_wnd to tiwin.
 773  */
 774                                 tp->snd_wnd = tiwin;    /* unscaled */
 775                                 goto after_listen;
 776                         }
 777                         if (thflags & TH_RST) {
 778                                 syncache_chkrst(&inc, th);
 779                                 goto drop;
 780                         }
 781                         if (thflags & TH_ACK) {
 782                                 syncache_badack(&inc);
 783                                 tcpstat.tcps_badsyn++;
 784                                 rstreason = BANDLIM_RST_OPENPORT;
 785                                 goto dropwithreset;
 786                         }
 787                         goto drop;
 788                 }
 789
 790                 /*
 791                  * Segment's flags are (SYN) or (SYN|FIN).
 792                  */
 793 #ifdef INET6
 794                 /*
 795                  * If deprecated address is forbidden,
 796                  * we do not accept SYN to deprecated interface
 797                  * address to prevent any new inbound connection from
 798                  * getting established.
 799                  * When we do not accept SYN, we send a TCP RST,
 800                  * with deprecated source address (instead of dropping
 801                  * it).  We compromise it as it is much better for peer
 802                  * to send a RST, and RST will be the final packet
 803                  * for the exchange.
 804                  *
 805                  * If we do not forbid deprecated addresses, we accept
 806                  * the SYN packet.  RFC2462 does not suggest dropping
 807                  * SYN in this case.
 808                  * If we decipher RFC2462 5.5.4, it says like this:
 809                  * 1. use of deprecated addr with existing
 810                  *    communication is okay - "SHOULD continue to be
 811                  *    used"
 812                  * 2. use of it with new communication:
 813                  *   (2a) "SHOULD NOT be used if alternate address
 814                  *        with sufficient scope is available"
 815                  *   (2b) nothing mentioned otherwise.
 816                  * Here we fall into (2b) case as we have no choice in
 817                  * our source address selection - we must obey the peer.
 818                  *
 819                  * The wording in RFC2462 is confusing, and there are
 820                  * multiple description text for deprecated address
 821                  * handling - worse, they are not exactly the same.
 822                  * I believe 5.5.4 is the best one, so we follow 5.5.4.
 823                  */
 824                 if (isipv6 && !ip6_use_deprecated) {
 825                         struct in6_ifaddr *ia6;
 826
 827                         if ((ia6 = ip6_getdstifaddr(m)) &&
 828                             (ia6->ia6_flags & IN6_IFF_DEPRECATED)) {
 829                                 tp = NULL;
 830                                 rstreason = BANDLIM_RST_OPENPORT;
 831                                 goto dropwithreset;
 832                         }
 833                 }
 834 #endif
 835                 /*
 836                  * If it is from this socket, drop it, it must be forged.
 837                  * Don't bother responding if the destination was a broadcast.
 838                  */
 839                 if (th->th_dport == th->th_sport) {
 840                         if (isipv6) {
 841                                 if (IN6_ARE_ADDR_EQUAL(&ip6->ip6_dst,
 842                                                        &ip6->ip6_src))
 843                                         goto drop;
 844                         } else {
 845                                 if (ip->ip_dst.s_addr == ip->ip_src.s_addr)
 846                                         goto drop;
 847                         }
 848                 }
 849                 /*
 850                  * RFC1122 4.2.3.10, p. 104: discard bcast/mcast SYN
 851                  *
 852                  * Note that it is quite possible to receive unicast
 853                  * link-layer packets with a broadcast IP address. Use
 854                  * in_broadcast() to find them.
 855                  */
 856                 if (m->m_flags & (M_BCAST|M_MCAST))
 857                         goto drop;
 858                 if (isipv6) {
 859                         if (IN6_IS_ADDR_MULTICAST(&ip6->ip6_dst) ||
 860                             IN6_IS_ADDR_MULTICAST(&ip6->ip6_src))
 861                                 goto drop;
 862                 } else {
 863                         if (IN_MULTICAST(ntohl(ip->ip_dst.s_addr)) ||
 864                             IN_MULTICAST(ntohl(ip->ip_src.s_addr)) ||
 865                             ip->ip_src.s_addr == htonl(INADDR_BROADCAST) ||
 866                             in_broadcast(ip->ip_dst, m->m_pkthdr.rcvif))
 867                                 goto drop;
 868                 }
 869                 /*
 870                  * SYN appears to be valid; create compressed TCP state
 871                  * for syncache, or perform t/tcp connection.
 872                  */
 873                 if (so->so_qlen <= so->so_qlimit) {
 874                         tcp_dooptions(&to, optp, optlen, 1);
 875                         if (!syncache_add(&inc, &to, th, &so, m))
 876                                 goto drop;
 877                         if (so == NULL)
 878                                 /*
 879                                  * Entry added to syncache, mbuf used to
 880                                  * send SYN,ACK packet.
 881                                  */
 882                                 return;
 883                         /*
 884                          * Segment passed TAO tests.
 885                          */
 886                         inp = sotoinpcb(so);
 887                         tp = intotcpcb(inp);
 888                         tp->snd_wnd = tiwin;
 889                         tp->t_starttime = ticks;
 890                         tp->t_state = TCPS_ESTABLISHED;
 891
 892                         /*
 893                          * If there is a FIN, or if there is data and the
 894                          * connection is local, then delay SYN,ACK(SYN) in
 895                          * the hope of piggy-backing it on a response
 896                          * segment.  Otherwise must send ACK now in case
 897                          * the other side is slow starting.
 898                          */
 899                         if (DELAY_ACK(tp) &&
 900                             ((thflags & TH_FIN) ||
 901                              (tlen != 0 &&
 902                               ((isipv6 && in6_localaddr(&inp->in6p_faddr)) ||
 903                                (!isipv6 && in_localaddr(inp->inp_faddr)))))) {
 904                                 callout_reset(tp->tt_delack, tcp_delacktime,
 905                                                 tcp_timer_delack, tp);
 906                                 tp->t_flags |= TF_NEEDSYN;
 907                         } else
 908                                 tp->t_flags |= (TF_ACKNOW | TF_NEEDSYN);
 909
 910                         tcpstat.tcps_connects++;
 911                         soisconnected(so);
 912                         goto trimthenstep6;
 913                 }
 914                 goto drop;
 915         }
 916 after_listen:
 917
 918 /* XXX temp debugging */
 919         /* should not happen - syncache should pick up these connections */
 920         if (tp->t_state == TCPS_LISTEN)
 921                 panic("tcp_input: TCPS_LISTEN");
 922
 923         /*
 924          * Segment received on connection.
 925          * Reset idle time and keep-alive timer.
 926          */
 927         tp->t_rcvtime = ticks;
 928         if (TCPS_HAVEESTABLISHED(tp->t_state))
 929                 callout_reset(tp->tt_keep, tcp_keepidle, tcp_timer_keep, tp);
 930
 931         /*
 932          * Process options.
 933          * XXX this is tradtitional behavior, may need to be cleaned up.
 934          */
 935         tcp_dooptions(&to, optp, optlen, thflags & TH_SYN);
 936         if (thflags & TH_SYN) {
 937                 if (to.to_flags & TOF_SCALE) {
 938                         tp->t_flags |= TF_RCVD_SCALE;
 939                         tp->requested_s_scale = to.to_requested_s_scale;
 940                 }
 941                 if (to.to_flags & TOF_TS) {
 942                         tp->t_flags |= TF_RCVD_TSTMP;
 943                         tp->ts_recent = to.to_tsval;
 944                         tp->ts_recent_age = ticks;
 945                 }
 946                 if (to.to_flags & (TOF_CC|TOF_CCNEW))
 947                         tp->t_flags |= TF_RCVD_CC;
 948                 if (to.to_flags & TOF_MSS)
 949                         tcp_mss(tp, to.to_mss);
 950         }
 951
 952         /*
 953          * Header prediction: check for the two common cases
 954          * of a uni-directional data xfer.  If the packet has
 955          * no control flags, is in-sequence, the window didn't
 956          * change and we're not retransmitting, it's a
 957          * candidate.  If the length is zero and the ack moved
 958          * forward, we're the sender side of the xfer.  Just
 959          * free the data acked & wake any higher level process
 960          * that was blocked waiting for space.  If the length
 961          * is non-zero and the ack didn't move, we're the
 962          * receiver side.  If we're getting packets in-order
 963          * (the reassembly queue is empty), add the data to
 964          * the socket buffer and note that we need a delayed ack.
 965          * Make sure that the hidden state-flags are also off.
 966          * Since we check for TCPS_ESTABLISHED above, it can only
 967          * be TH_NEEDSYN.
 968          */
 969         if (tp->t_state == TCPS_ESTABLISHED &&
 970             (thflags & (TH_SYN|TH_FIN|TH_RST|TH_URG|TH_ACK)) == TH_ACK &&
 971             ((tp->t_flags & (TF_NEEDSYN|TF_NEEDFIN)) == 0) &&
 972             ((to.to_flags & TOF_TS) == 0 ||
 973              TSTMP_GEQ(to.to_tsval, tp->ts_recent)) &&
 974             /*
 975              * Using the CC option is compulsory if once started:
 976              *   the segment is OK if no T/TCP was negotiated or
 977              *   if the segment has a CC option equal to CCrecv
 978              */
 979             ((tp->t_flags & (TF_REQ_CC|TF_RCVD_CC)) != (TF_REQ_CC|TF_RCVD_CC) ||
 980              ((to.to_flags & TOF_CC) != 0 && to.to_cc == tp->cc_recv)) &&
 981             th->th_seq == tp->rcv_nxt &&
 982             tiwin && tiwin == tp->snd_wnd &&
 983             tp->snd_nxt == tp->snd_max) {
 984
 985                 /*
 986                  * If last ACK falls within this segment's sequence numbers,
 987                  * record the timestamp.
 988                  * NOTE that the test is modified according to the latest
 989                  * proposal of the tcplw@cray.com list (Braden 1993/04/26).
 990                  */
 991                 if ((to.to_flags & TOF_TS) != 0 &&
 992                     SEQ_LEQ(th->th_seq, tp->last_ack_sent)) {
 993                         tp->ts_recent_age = ticks;
 994                         tp->ts_recent = to.to_tsval;
 995                 }
 996
 997                 if (tlen == 0) {
 998                         if (SEQ_GT(th->th_ack, tp->snd_una) &&
 999                             SEQ_LEQ(th->th_ack, tp->snd_max) &&
1000                             tp->snd_cwnd >= tp->snd_wnd &&
1001                             ((!tcp_do_newreno &&
1002                               tp->t_dupacks < tcprexmtthresh) ||
1003                              (tcp_do_newreno && !IN_FASTRECOVERY(tp)))) {
1004                                 /*
1005                                  * this is a pure ack for outstanding data.
1006                                  */
1007                                 ++tcpstat.tcps_predack;
1008                                 /*
1009                                  * "bad retransmit" recovery
1010                                  *
1011                                  * If Eifel detection applies, then
1012                                  * it is deterministic, so use it
1013                                  * unconditionally over the old heuristic.
1014                                  * Otherwise, fall back to the old heuristic.
1015                                  */
1016                                 if (tcp_do_eifel_detect &&
1017                                     (to.to_flags & TOF_TS) && to.to_tsecr &&
1018                                     (tp->t_flags & TF_FIRSTACCACK)) {
1019                                         /* Eifel detection applicable. */
1020                                         if (to.to_tsecr < tp->t_rexmtTS) {
1021                                                 tcp_revert_congestion_state(tp);
1022                                                 ++tcpstat.tcps_eifeldetected;
1023                                         }
1024                                 } else if (tp->t_rxtshift == 1 &&
1025                                            ticks < tp->t_badrxtwin) {
1026                                         tcp_revert_congestion_state(tp);
1027                                         ++tcpstat.tcps_rttdetected;
1028                                 }
1029                                 tp->t_flags &= ~(TF_FIRSTACCACK |
1030                                                  TF_FASTREXMT | TF_EARLYREXMT);
1031                                 /*
1032                                  * Recalculate the retransmit timer / rtt.
1033                                  *
1034                                  * Some machines (certain windows boxes)
1035                                  * send broken timestamp replies during the
1036                                  * SYN+ACK phase, ignore timestamps of 0.
1037                                  */
1038                                 if ((to.to_flags & TOF_TS) != 0 &&
1039                                     to.to_tsecr) {
1040                                         tcp_xmit_timer(tp,
1041                                             ticks - to.to_tsecr + 1);
1042                                 } else if (tp->t_rtttime &&
1043                                             SEQ_GT(th->th_ack, tp->t_rtseq)) {
1044                                         tcp_xmit_timer(tp,
1045                                                        ticks - tp->t_rtttime);
1046                                 }
1047                                 tcp_xmit_bandwidth_limit(tp, th->th_ack);
1048                                 acked = th->th_ack - tp->snd_una;
1049                                 tcpstat.tcps_rcvackpack++;
1050                                 tcpstat.tcps_rcvackbyte += acked;
1051                                 sbdrop(&so->so_snd, acked);
1052                                 if (SEQ_GT(tp->snd_una, tp->snd_recover) &&
1053                                     SEQ_LEQ(th->th_ack, tp->snd_recover))
1054                                         tp->snd_recover = th->th_ack - 1;
1055                                 tp->snd_una = th->th_ack;
1056                                 tp->t_dupacks = 0;
1057                                 m_freem(m);
1058                                 ND6_HINT(tp); /* some progress has been done */
1059
1060                                 /*
1061                                  * If all outstanding data are acked, stop
1062                                  * retransmit timer, otherwise restart timer
1063                                  * using current (possibly backed-off) value.
1064                                  * If process is waiting for space,
1065                                  * wakeup/selwakeup/signal.  If data
1066                                  * are ready to send, let tcp_output
1067                                  * decide between more output or persist.
1068                                  */
1069                                 if (tp->snd_una == tp->snd_max)
1070                                         callout_stop(tp->tt_rexmt);
1071                                 else if (!callout_active(tp->tt_persist))
1072                                         callout_reset(tp->tt_rexmt,
1073                                                       tp->t_rxtcur,
1074                                                       tcp_timer_rexmt, tp);
1075
1076                                 sowwakeup(so);
1077                                 if (so->so_snd.sb_cc)
1078                                         (void) tcp_output(tp);
1079                                 return;
1080                         }
1081                 } else if (th->th_ack == tp->snd_una &&
1082                     LIST_EMPTY(&tp->t_segq) &&
1083                     tlen <= sbspace(&so->so_rcv)) {
1084                         /*
1085                          * this is a pure, in-sequence data packet
1086                          * with nothing on the reassembly queue and
1087                          * we have enough buffer space to take it.
1088                          */
1089                         ++tcpstat.tcps_preddat;
1090                         tp->rcv_nxt += tlen;
1091                         tcpstat.tcps_rcvpack++;
1092                         tcpstat.tcps_rcvbyte += tlen;
1093                         ND6_HINT(tp);   /* some progress has been done */
1094                         /*
1095                          * Add data to socket buffer.
1096                          */
1097                         if (so->so_state & SS_CANTRCVMORE) {
1098                                 m_freem(m);
1099                         } else {
1100                                 m_adj(m, drop_hdrlen);  /* delayed header drop */
1101                                 sbappend(&so->so_rcv, m);
1102                         }
1103                         sorwakeup(so);
1104                         if (DELAY_ACK(tp)) {
1105                                 callout_reset(tp->tt_delack, tcp_delacktime,
1106                                     tcp_timer_delack, tp);
1107                         } else {
1108                                 tp->t_flags |= TF_ACKNOW;
1109                                 tcp_output(tp);
1110                         }
1111                         return;
1112                 }
1113         }
1114
1115         /*
1116          * Calculate amount of space in receive window,
1117          * and then do TCP input processing.
1118          * Receive window is amount of space in rcv queue,
1119          * but not less than advertised window.
1120          */
1121         { int win;
1122
1123         win = sbspace(&so->so_rcv);
1124         if (win < 0)
1125                 win = 0;
1126         tp->rcv_wnd = imax(win, (int)(tp->rcv_adv - tp->rcv_nxt));
1127         }
1128
1129         switch (tp->t_state) {
1130
1131         /*
1132          * If the state is SYN_RECEIVED:
1133          *      if seg contains an ACK, but not for our SYN/ACK, send a RST.
1134          */
1135         case TCPS_SYN_RECEIVED:
1136                 if ((thflags & TH_ACK) &&
1137                     (SEQ_LEQ(th->th_ack, tp->snd_una) ||
1138                      SEQ_GT(th->th_ack, tp->snd_max))) {
1139                                 rstreason = BANDLIM_RST_OPENPORT;
1140                                 goto dropwithreset;
1141                 }
1142                 break;
1143
1144         /*
1145          * If the state is SYN_SENT:
1146          *      if seg contains an ACK, but not for our SYN, drop the input.
1147          *      if seg contains a RST, then drop the connection.
1148          *      if seg does not contain SYN, then drop it.
1149          * Otherwise this is an acceptable SYN segment
1150          *      initialize tp->rcv_nxt and tp->irs
1151          *      if seg contains ack then advance tp->snd_una
1152          *      if SYN has been acked change to ESTABLISHED else SYN_RCVD state
1153          *      arrange for segment to be acked (eventually)
1154          *      continue processing rest of data/controls, beginning with URG
1155          */
1156         case TCPS_SYN_SENT:
1157                 if ((taop = tcp_gettaocache(&inp->inp_inc)) == NULL) {
1158                         taop = &tao_noncached;
1159                         bzero(taop, sizeof(*taop));
1160                 }
1161
1162                 if ((thflags & TH_ACK) &&
1163                     (SEQ_LEQ(th->th_ack, tp->iss) ||
1164                      SEQ_GT(th->th_ack, tp->snd_max))) {
1165                         /*
1166                          * If we have a cached CCsent for the remote host,
1167                          * hence we haven't just crashed and restarted,
1168                          * do not send a RST.  This may be a retransmission
1169                          * from the other side after our earlier ACK was lost.
1170                          * Our new SYN, when it arrives, will serve as the
1171                          * needed ACK.
1172                          */
1173                         if (taop->tao_ccsent != 0)
1174                                 goto drop;
1175                         else {
1176                                 rstreason = BANDLIM_UNLIMITED;
1177                                 goto dropwithreset;
1178                         }
1179                 }
1180                 if (thflags & TH_RST) {
1181                         if (thflags & TH_ACK)
1182                                 tp = tcp_drop(tp, ECONNREFUSED);
1183                         goto drop;
1184                 }
1185                 if ((thflags & TH_SYN) == 0)
1186                         goto drop;
1187                 tp->snd_wnd = th->th_win;       /* initial send window */
1188                 tp->cc_recv = to.to_cc;         /* foreign CC */
1189
1190                 tp->irs = th->th_seq;
1191                 tcp_rcvseqinit(tp);
1192                 if (thflags & TH_ACK) {
1193                         /*
1194                          * Our SYN was acked.  If segment contains CC.ECHO
1195                          * option, check it to make sure this segment really
1196                          * matches our SYN.  If not, just drop it as old
1197                          * duplicate, but send an RST if we're still playing
1198                          * by the old rules.  If no CC.ECHO option, make sure
1199                          * we don't get fooled into using T/TCP.
1200                          */
1201                         if (to.to_flags & TOF_CCECHO) {
1202                                 if (tp->cc_send != to.to_ccecho) {
1203                                         if (taop->tao_ccsent != 0)
1204                                                 goto drop;
1205                                         else {
1206                                                 rstreason = BANDLIM_UNLIMITED;
1207                                                 goto dropwithreset;
1208                                         }
1209                                 }
1210                         } else
1211                                 tp->t_flags &= ~TF_RCVD_CC;
1212                         tcpstat.tcps_connects++;
1213                         soisconnected(so);
1214                         /* Do window scaling on this connection? */
1215                         if ((tp->t_flags & (TF_RCVD_SCALE|TF_REQ_SCALE)) ==
1216                                 (TF_RCVD_SCALE|TF_REQ_SCALE)) {
1217                                 tp->snd_scale = tp->requested_s_scale;
1218                                 tp->rcv_scale = tp->request_r_scale;
1219                         }
1220                         /* Segment is acceptable, update cache if undefined. */
1221                         if (taop->tao_ccsent == 0)
1222                                 taop->tao_ccsent = to.to_ccecho;
1223
1224                         tp->rcv_adv += tp->rcv_wnd;
1225                         tp->snd_una++;          /* SYN is acked */
1226                         /*
1227                          * If there's data, delay ACK; if there's also a FIN
1228                          * ACKNOW will be turned on later.
1229                          */
1230                         if (DELAY_ACK(tp) && tlen != 0)
1231                                 callout_reset(tp->tt_delack, tcp_delacktime,
1232                                     tcp_timer_delack, tp);
1233                         else
1234                                 tp->t_flags |= TF_ACKNOW;
1235                         /*
1236                          * Received <SYN,ACK> in SYN_SENT[*] state.
1237                          * Transitions:
1238                          *      SYN_SENT  --> ESTABLISHED
1239                          *      SYN_SENT* --> FIN_WAIT_1
1240                          */
1241                         tp->t_starttime = ticks;
1242                         if (tp->t_flags & TF_NEEDFIN) {
1243                                 tp->t_state = TCPS_FIN_WAIT_1;
1244                                 tp->t_flags &= ~TF_NEEDFIN;
1245                                 thflags &= ~TH_SYN;
1246                         } else {
1247                                 tp->t_state = TCPS_ESTABLISHED;
1248                                 callout_reset(tp->tt_keep, tcp_keepidle,
1249                                               tcp_timer_keep, tp);
1250                         }
1251                 } else {
1252                         /*
1253                          * Received initial SYN in SYN-SENT[*] state =>
1254                          * simultaneous open.  If segment contains CC option
1255                          * and there is a cached CC, apply TAO test.
1256                          * If it succeeds, connection is * half-synchronized.
1257                          * Otherwise, do 3-way handshake:
1258                          *        SYN-SENT -> SYN-RECEIVED
1259                          *        SYN-SENT* -> SYN-RECEIVED*
1260                          * If there was no CC option, clear cached CC value.
1261                          */
1262                         tp->t_flags |= TF_ACKNOW;
1263                         callout_stop(tp->tt_rexmt);
1264                         if (to.to_flags & TOF_CC) {
1265                                 if (taop->tao_cc != 0 &&
1266                                     CC_GT(to.to_cc, taop->tao_cc)) {
1267                                         /*
1268                                          * update cache and make transition:
1269                                          *        SYN-SENT -> ESTABLISHED*
1270                                          *        SYN-SENT* -> FIN-WAIT-1*
1271                                          */
1272                                         taop->tao_cc = to.to_cc;
1273                                         tp->t_starttime = ticks;
1274                                         if (tp->t_flags & TF_NEEDFIN) {
1275                                                 tp->t_state = TCPS_FIN_WAIT_1;
1276                                                 tp->t_flags &= ~TF_NEEDFIN;
1277                                         } else {
1278                                                 tp->t_state = TCPS_ESTABLISHED;
1279                                                 callout_reset(tp->tt_keep,
1280                                                               tcp_keepidle,
1281                                                               tcp_timer_keep,
1282                                                               tp);
1283                                         }
1284                                         tp->t_flags |= TF_NEEDSYN;
1285                                 } else
1286                                         tp->t_state = TCPS_SYN_RECEIVED;
1287                         } else {
1288                                 /* CC.NEW or no option => invalidate cache */
1289                                 taop->tao_cc = 0;
1290                                 tp->t_state = TCPS_SYN_RECEIVED;
1291                         }
1292                 }
1293
1294 trimthenstep6:
1295                 /*
1296                  * Advance th->th_seq to correspond to first data byte.
1297                  * If data, trim to stay within window,
1298                  * dropping FIN if necessary.
1299                  */
1300                 th->th_seq++;
1301                 if (tlen > tp->rcv_wnd) {
1302                         todrop = tlen - tp->rcv_wnd;
1303                         m_adj(m, -todrop);
1304                         tlen = tp->rcv_wnd;
1305                         thflags &= ~TH_FIN;
1306                         tcpstat.tcps_rcvpackafterwin++;
1307                         tcpstat.tcps_rcvbyteafterwin += todrop;
1308                 }
1309                 tp->snd_wl1 = th->th_seq - 1;
1310                 tp->rcv_up = th->th_seq;
1311                 /*
1312                  * Client side of transaction: already sent SYN and data.
1313                  * If the remote host used T/TCP to validate the SYN,
1314                  * our data will be ACK'd; if so, enter normal data segment
1315                  * processing in the middle of step 5, ack processing.
1316                  * Otherwise, goto step 6.
1317                  */
1318                 if (thflags & TH_ACK)
1319                         goto process_ACK;
1320
1321                 goto step6;
1322
1323         /*
1324          * If the state is LAST_ACK or CLOSING or TIME_WAIT:
1325          *      if segment contains a SYN and CC [not CC.NEW] option:
1326          *              if state == TIME_WAIT and connection duration > MSL,
1327          *                  drop packet and send RST;
1328          *
1329          *              if SEG.CC > CCrecv then is new SYN, and can implicitly
1330          *                  ack the FIN (and data) in retransmission queue.
1331          *                  Complete close and delete TCPCB.  Then reprocess
1332          *                  segment, hoping to find new TCPCB in LISTEN state;
1333          *
1334          *              else must be old SYN; drop it.
1335          *      else do normal processing.
1336          */
1337         case TCPS_LAST_ACK:
1338         case TCPS_CLOSING:
1339         case TCPS_TIME_WAIT:
1340                 if ((thflags & TH_SYN) &&
1341                     (to.to_flags & TOF_CC) && tp->cc_recv != 0) {
1342                         if (tp->t_state == TCPS_TIME_WAIT &&
1343                                         (ticks - tp->t_starttime) > tcp_msl) {
1344                                 rstreason = BANDLIM_UNLIMITED;
1345                                 goto dropwithreset;
1346                         }
1347                         if (CC_GT(to.to_cc, tp->cc_recv)) {
1348                                 tp = tcp_close(tp);
1349                                 goto findpcb;
1350                         }
1351                         else
1352                                 goto drop;
1353                 }
1354                 break;  /* continue normal processing */
1355         }
1356
1357         /*
1358          * States other than LISTEN or SYN_SENT.
1359          * First check the RST flag and sequence number since reset segments
1360          * are exempt from the timestamp and connection count tests.  This
1361          * fixes a bug introduced by the Stevens, vol. 2, p. 960 bugfix
1362          * below which allowed reset segments in half the sequence space
1363          * to fall though and be processed (which gives forged reset
1364          * segments with a random sequence number a 50 percent chance of
1365          * killing a connection).
1366          * Then check timestamp, if present.
1367          * Then check the connection count, if present.
1368          * Then check that at least some bytes of segment are within
1369          * receive window.  If segment begins before rcv_nxt,
1370          * drop leading data (and SYN); if nothing left, just ack.
1371          *
1372          *
1373          * If the RST bit is set, check the sequence number to see
1374          * if this is a valid reset segment.
1375          * RFC 793 page 37:
1376          *   In all states except SYN-SENT, all reset (RST) segments
1377          *   are validated by checking their SEQ-fields.  A reset is
1378          *   valid if its sequence number is in the window.
1379          * Note: this does not take into account delayed ACKs, so
1380          *   we should test against last_ack_sent instead of rcv_nxt.
1381          *   The sequence number in the reset segment is normally an
1382          *   echo of our outgoing acknowlegement numbers, but some hosts
1383          *   send a reset with the sequence number at the rightmost edge
1384          *   of our receive window, and we have to handle this case.
1385          * If we have multiple segments in flight, the intial reset
1386          * segment sequence numbers will be to the left of last_ack_sent,
1387          * but they will eventually catch up.
1388          * In any case, it never made sense to trim reset segments to
1389          * fit the receive window since RFC 1122 says:
1390          *   4.2.2.12  RST Segment: RFC-793 Section 3.4
1391          *
1392          *    A TCP SHOULD allow a received RST segment to include data.
1393          *
1394          *    DISCUSSION
1395          *         It has been suggested that a RST segment could contain
1396          *         ASCII text that encoded and explained the cause of the
1397          *         RST.  No standard has yet been established for such
1398          *         data.
1399          *
1400          * If the reset segment passes the sequence number test examine
1401          * the state:
1402          *    SYN_RECEIVED STATE:
1403          *      If passive open, return to LISTEN state.
1404          *      If active open, inform user that connection was refused.
1405          *    ESTABLISHED, FIN_WAIT_1, FIN_WAIT_2, CLOSE_WAIT STATES:
1406          *      Inform user that connection was reset, and close tcb.
1407          *    CLOSING, LAST_ACK STATES:
1408          *      Close the tcb.
1409          *    TIME_WAIT STATE:
1410          *      Drop the segment - see Stevens, vol. 2, p. 964 and
1411          *      RFC 1337.
1412          */
1413         if (thflags & TH_RST) {
1414                 if (SEQ_GEQ(th->th_seq, tp->last_ack_sent) &&
1415                     SEQ_LT(th->th_seq, tp->last_ack_sent + tp->rcv_wnd)) {
1416                         switch (tp->t_state) {
1417
1418                         case TCPS_SYN_RECEIVED:
1419                                 so->so_error = ECONNREFUSED;
1420                                 goto close;
1421
1422                         case TCPS_ESTABLISHED:
1423                         case TCPS_FIN_WAIT_1:
1424                         case TCPS_FIN_WAIT_2:
1425                         case TCPS_CLOSE_WAIT:
1426                                 so->so_error = ECONNRESET;
1427                         close:
1428                                 tp->t_state = TCPS_CLOSED;
1429                                 tcpstat.tcps_drops++;
1430                                 tp = tcp_close(tp);
1431                                 break;
1432
1433                         case TCPS_CLOSING:
1434                         case TCPS_LAST_ACK:
1435                                 tp = tcp_close(tp);
1436                                 break;
1437
1438                         case TCPS_TIME_WAIT:
1439                                 break;
1440                         }
1441                 }
1442                 goto drop;
1443         }
1444
1445         /*
1446          * RFC 1323 PAWS: If we have a timestamp reply on this segment
1447          * and it's less than ts_recent, drop it.
1448          */
1449         if ((to.to_flags & TOF_TS) != 0 && tp->ts_recent &&
1450             TSTMP_LT(to.to_tsval, tp->ts_recent)) {
1451
1452                 /* Check to see if ts_recent is over 24 days old.  */
1453                 if ((int)(ticks - tp->ts_recent_age) > TCP_PAWS_IDLE) {
1454                         /*
1455                          * Invalidate ts_recent.  If this segment updates
1456                          * ts_recent, the age will be reset later and ts_recent
1457                          * will get a valid value.  If it does not, setting
1458                          * ts_recent to zero will at least satisfy the
1459                          * requirement that zero be placed in the timestamp
1460                          * echo reply when ts_recent isn't valid.  The
1461                          * age isn't reset until we get a valid ts_recent
1462                          * because we don't want out-of-order segments to be
1463                          * dropped when ts_recent is old.
1464                          */
1465                         tp->ts_recent = 0;
1466                 } else {
1467                         tcpstat.tcps_rcvduppack++;
1468                         tcpstat.tcps_rcvdupbyte += tlen;
1469                         tcpstat.tcps_pawsdrop++;
1470                         if (tlen)
1471                                 goto dropafterack;
1472                         goto drop;
1473                 }
1474         }
1475
1476         /*
1477          * T/TCP mechanism
1478          *   If T/TCP was negotiated and the segment doesn't have CC,
1479          *   or if its CC is wrong then drop the segment.
1480          *   RST segments do not have to comply with this.
1481          */
1482         if ((tp->t_flags & (TF_REQ_CC|TF_RCVD_CC)) == (TF_REQ_CC|TF_RCVD_CC) &&
1483             ((to.to_flags & TOF_CC) == 0 || tp->cc_recv != to.to_cc))
1484                 goto dropafterack;
1485
1486         /*
1487          * In the SYN-RECEIVED state, validate that the packet belongs to
1488          * this connection before trimming the data to fit the receive
1489          * window.  Check the sequence number versus IRS since we know
1490          * the sequence numbers haven't wrapped.  This is a partial fix
1491          * for the "LAND" DoS attack.
1492          */
1493         if (tp->t_state == TCPS_SYN_RECEIVED && SEQ_LT(th->th_seq, tp->irs)) {
1494                 rstreason = BANDLIM_RST_OPENPORT;
1495                 goto dropwithreset;
1496         }
1497
1498         todrop = tp->rcv_nxt - th->th_seq;
1499         if (todrop > 0) {
1500                 if (thflags & TH_SYN) {
1501                         thflags &= ~TH_SYN;
1502                         th->th_seq++;
1503                         if (th->th_urp > 1)
1504                                 th->th_urp--;
1505                         else
1506                                 thflags &= ~TH_URG;
1507                         todrop--;
1508                 }
1509                 /*
1510                  * Following if statement from Stevens, vol. 2, p. 960.
1511                  */
1512                 if (todrop > tlen
1513                     || (todrop == tlen && (thflags & TH_FIN) == 0)) {
1514                         /*
1515                          * Any valid FIN must be to the left of the window.
1516                          * At this point the FIN must be a duplicate or out
1517                          * of sequence; drop it.
1518                          */
1519                         thflags &= ~TH_FIN;
1520
1521                         /*
1522                          * Send an ACK to resynchronize and drop any data.
1523                          * But keep on processing for RST or ACK.
1524                          */
1525                         tp->t_flags |= TF_ACKNOW;
1526                         todrop = tlen;
1527                         tcpstat.tcps_rcvduppack++;
1528                         tcpstat.tcps_rcvdupbyte += todrop;
1529                 } else {
1530                         tcpstat.tcps_rcvpartduppack++;
1531                         tcpstat.tcps_rcvpartdupbyte += todrop;
1532                 }
1533                 drop_hdrlen += todrop;  /* drop from the top afterwards */
1534                 th->th_seq += todrop;
1535                 tlen -= todrop;
1536                 if (th->th_urp > todrop)
1537                         th->th_urp -= todrop;
1538                 else {
1539                         thflags &= ~TH_URG;
1540                         th->th_urp = 0;
1541                 }
1542         }
1543
1544         /*
1545          * If new data are received on a connection after the
1546          * user processes are gone, then RST the other end.
1547          */
1548         if ((so->so_state & SS_NOFDREF) &&
1549             tp->t_state > TCPS_CLOSE_WAIT && tlen) {
1550                 tp = tcp_close(tp);
1551                 tcpstat.tcps_rcvafterclose++;
1552                 rstreason = BANDLIM_UNLIMITED;
1553                 goto dropwithreset;
1554         }
1555
1556         /*
1557          * If segment ends after window, drop trailing data
1558          * (and PUSH and FIN); if nothing left, just ACK.
1559          */
1560         todrop = (th->th_seq+tlen) - (tp->rcv_nxt+tp->rcv_wnd);
1561         if (todrop > 0) {
1562                 tcpstat.tcps_rcvpackafterwin++;
1563                 if (todrop >= tlen) {
1564                         tcpstat.tcps_rcvbyteafterwin += tlen;
1565                         /*
1566                          * If a new connection request is received
1567                          * while in TIME_WAIT, drop the old connection
1568                          * and start over if the sequence numbers
1569                          * are above the previous ones.
1570                          */
1571                         if (thflags & TH_SYN &&
1572                             tp->t_state == TCPS_TIME_WAIT &&
1573                             SEQ_GT(th->th_seq, tp->rcv_nxt)) {
1574                                 tp = tcp_close(tp);
1575                                 goto findpcb;
1576                         }
1577                         /*
1578                          * If window is closed can only take segments at
1579                          * window edge, and have to drop data and PUSH from
1580                          * incoming segments.  Continue processing, but
1581                          * remember to ack.  Otherwise, drop segment
1582                          * and ack.
1583                          */
1584                         if (tp->rcv_wnd == 0 && th->th_seq == tp->rcv_nxt) {
1585                                 tp->t_flags |= TF_ACKNOW;
1586                                 tcpstat.tcps_rcvwinprobe++;
1587                         } else
1588                                 goto dropafterack;
1589                 } else
1590                         tcpstat.tcps_rcvbyteafterwin += todrop;
1591                 m_adj(m, -todrop);
1592                 tlen -= todrop;
1593                 thflags &= ~(TH_PUSH|TH_FIN);
1594         }
1595
1596         /*
1597          * If last ACK falls within this segment's sequence numbers,
1598          * record its timestamp.
1599          * NOTE that the test is modified according to the latest
1600          * proposal of the tcplw@cray.com list (Braden 1993/04/26).
1601          */
1602         if ((to.to_flags & TOF_TS) != 0 &&
1603             SEQ_LEQ(th->th_seq, tp->last_ack_sent)) {
1604                 tp->ts_recent_age = ticks;
1605                 tp->ts_recent = to.to_tsval;
1606         }
1607
1608         /*
1609          * If a SYN is in the window, then this is an
1610          * error and we send an RST and drop the connection.
1611          */
1612         if (thflags & TH_SYN) {
1613                 tp = tcp_drop(tp, ECONNRESET);
1614                 rstreason = BANDLIM_UNLIMITED;
1615                 goto dropwithreset;
1616         }
1617
1618         /*
1619          * If the ACK bit is off:  if in SYN-RECEIVED state or SENDSYN
1620          * flag is on (half-synchronized state), then queue data for
1621          * later processing; else drop segment and return.
1622          */
1623         if ((thflags & TH_ACK) == 0) {
1624                 if (tp->t_state == TCPS_SYN_RECEIVED ||
1625                     (tp->t_flags & TF_NEEDSYN))
1626                         goto step6;
1627                 else
1628                         goto drop;
1629         }
1630
1631         /*
1632          * Ack processing.
1633          */
1634         switch (tp->t_state) {
1635
1636         /*
1637          * In SYN_RECEIVED state, the ack ACKs our SYN, so enter
1638          * ESTABLISHED state and continue processing.
1639          * The ACK was checked above.
1640          */
1641         case TCPS_SYN_RECEIVED:
1642
1643                 tcpstat.tcps_connects++;
1644                 soisconnected(so);
1645                 /* Do window scaling? */
1646                 if ((tp->t_flags & (TF_RCVD_SCALE|TF_REQ_SCALE)) ==
1647                         (TF_RCVD_SCALE|TF_REQ_SCALE)) {
1648                         tp->snd_scale = tp->requested_s_scale;
1649                         tp->rcv_scale = tp->request_r_scale;
1650                 }
1651                 /*
1652                  * Upon successful completion of 3-way handshake,
1653                  * update cache.CC if it was undefined, pass any queued
1654                  * data to the user, and advance state appropriately.
1655                  */
1656                 if ((taop = tcp_gettaocache(&inp->inp_inc)) != NULL &&
1657                     taop->tao_cc == 0)
1658                         taop->tao_cc = tp->cc_recv;
1659
1660                 /*
1661                  * Make transitions:
1662                  *      SYN-RECEIVED  -> ESTABLISHED
1663                  *      SYN-RECEIVED* -> FIN-WAIT-1
1664                  */
1665                 tp->t_starttime = ticks;
1666                 if (tp->t_flags & TF_NEEDFIN) {
1667                         tp->t_state = TCPS_FIN_WAIT_1;
1668                         tp->t_flags &= ~TF_NEEDFIN;
1669                 } else {
1670                         tp->t_state = TCPS_ESTABLISHED;
1671                         callout_reset(tp->tt_keep, tcp_keepidle,
1672                                       tcp_timer_keep, tp);
1673                 }
1674                 /*
1675                  * If segment contains data or ACK, will call tcp_reass()
1676                  * later; if not, do so now to pass queued data to user.
1677                  */
1678                 if (tlen == 0 && (thflags & TH_FIN) == 0)
1679                         (void) tcp_reass(tp, (struct tcphdr *)0, 0,
1680                             (struct mbuf *)0);
1681                 tp->snd_wl1 = th->th_seq - 1;
1682                 /* fall into ... */
1683
1684         /*
1685          * In ESTABLISHED state: drop duplicate ACKs; ACK out of range
1686          * ACKs.  If the ack is in the range
1687          *      tp->snd_una < th->th_ack <= tp->snd_max
1688          * then advance tp->snd_una to th->th_ack and drop
1689          * data from the retransmission queue.  If this ACK reflects
1690          * more up to date window information we update our window information.
1691          */
1692         case TCPS_ESTABLISHED:
1693         case TCPS_FIN_WAIT_1:
1694         case TCPS_FIN_WAIT_2:
1695         case TCPS_CLOSE_WAIT:
1696         case TCPS_CLOSING:
1697         case TCPS_LAST_ACK:
1698         case TCPS_TIME_WAIT:
1699
1700                 if (SEQ_LEQ(th->th_ack, tp->snd_una)) {
1701                         if (tlen == 0 && tiwin == tp->snd_wnd) {
1702                                 tcpstat.tcps_rcvdupack++;
1703                                 /*
1704                                  * If we have outstanding data (other than
1705                                  * a window probe), this is a completely
1706                                  * duplicate ack (ie, window info didn't
1707                                  * change), the ack is the biggest we've
1708                                  * seen and we've seen exactly our rexmt
1709                                  * threshhold of them, assume a packet
1710                                  * has been dropped and retransmit it.
1711                                  * Kludge snd_nxt & the congestion
1712                                  * window so we send only this one
1713                                  * packet.
1714                                  *
1715                                  * We know we're losing at the current
1716                                  * window size so do congestion avoidance
1717                                  * (set ssthresh to half the current window
1718                                  * and pull our congestion window back to
1719                                  * the new ssthresh).
1720                                  *
1721                                  * Dup acks mean that packets have left the
1722                                  * network (they're now cached at the receiver)
1723                                  * so bump cwnd by the amount in the receiver
1724                                  * to keep a constant cwnd packets in the
1725                                  * network.
1726                                  */
1727                                 if (!callout_active(tp->tt_rexmt) ||
1728                                     th->th_ack != tp->snd_una)
1729                                         tp->t_dupacks = 0;
1730                                 else if (++tp->t_dupacks > tcprexmtthresh ||
1731                                          (tcp_do_newreno &&
1732                                           IN_FASTRECOVERY(tp))) {
1733                                         tp->snd_cwnd += tp->t_maxseg;
1734                                         (void) tcp_output(tp);
1735                                         goto drop;
1736                                 } else if (tp->t_dupacks == tcprexmtthresh) {
1737                                         tcp_seq onxt;
1738                                         u_int win;
1739
1740                                         if (tcp_do_newreno &&
1741                                             SEQ_LEQ(th->th_ack,
1742                                                     tp->snd_recover)) {
1743                                                 tp->t_dupacks = 0;
1744                                                 break;
1745                                         }
1746 fastretransmit:
1747                                         if (tcp_do_eifel_detect &&
1748                                             (tp->t_flags & TF_RCVD_TSTMP)) {
1749                                                 tcp_save_congestion_state(tp);
1750                                                 tp->t_flags |= TF_FASTREXMT;
1751                                         }
1752                                         win = min(tp->snd_wnd, tp->snd_cwnd) /
1753                                             2 / tp->t_maxseg;
1754                                         if (win < 2)
1755                                                 win = 2;
1756                                         tp->snd_ssthresh = win * tp->t_maxseg;
1757                                         ENTER_FASTRECOVERY(tp);
1758                                         tp->snd_recover = tp->snd_max;
1759                                         callout_stop(tp->tt_rexmt);
1760                                         tp->t_rtttime = 0;
1761                                         onxt = tp->snd_nxt;
1762                                         tp->snd_nxt = th->th_ack;
1763                                         tp->snd_cwnd = tp->t_maxseg;
1764                                         (void) tcp_output(tp);
1765                                         ++tcpstat.tcps_sndfastrexmit;
1766                                         KASSERT(tp->snd_limited <= 2,
1767                                             ("tp->snd_limited too big"));
1768                                         tp->snd_cwnd = tp->snd_ssthresh +
1769                                             (tp->t_maxseg *
1770                                              (tp->t_dupacks - tp->snd_limited));
1771                                         if (SEQ_GT(onxt, tp->snd_nxt))
1772                                                 tp->snd_nxt = onxt;
1773                                         goto drop;
1774                                 } else if (tcp_do_limitedtransmit) {
1775                                         u_long oldcwnd = tp->snd_cwnd;
1776                                         tcp_seq oldsndmax = tp->snd_max;
1777                                         /* outstanding data */
1778                                         uint32_t ownd =
1779                                             tp->snd_max - tp->snd_una;
1780                                         u_int sent;
1781
1782 #define iceildiv(n, d)          (((n)+(d)-1) / (d))
1783
1784                                         KASSERT(tp->t_dupacks == 1 ||
1785                                             tp->t_dupacks == 2,
1786                                             ("dupacks not 1 or 2"));
1787                                         if (tp->t_dupacks == 1)
1788                                                 tp->snd_limited = 0;
1789                                         tp->snd_cwnd = ownd +
1790                                             (tp->t_dupacks - tp->snd_limited) *
1791                                             tp->t_maxseg;
1792                                         (void) tcp_output(tp);
1793                                         tp->snd_cwnd = oldcwnd;
1794                                         sent = tp->snd_max - oldsndmax;
1795                                         if (sent > tp->t_maxseg) {
1796                                                 KASSERT((tp->t_dupacks == 2 &&
1797                                                     tp->snd_limited == 0) ||
1798                                                    (sent == tp->t_maxseg + 1 &&
1799                                                     tp->t_flags & TF_SENTFIN),
1800                                                     ("sent too much"));
1801                                                 KASSERT(sent <=
1802                                                         tp->t_maxseg * 2,
1803                                                     ("sent too many segments"));
1804                                                 tp->snd_limited = 2;
1805                                                 tcpstat.tcps_sndlimited += 2;
1806                                         } else if (sent > 0) {
1807                                                 ++tp->snd_limited;
1808                                                 ++tcpstat.tcps_sndlimited;
1809                                         } else if (tcp_do_early_retransmit &&
1810                                             (tcp_do_eifel_detect &&
1811                                              (tp->t_flags & TF_RCVD_TSTMP)) &&
1812                                             tcp_do_newreno &&
1813                                             tp->t_dupacks + 1 >=
1814                                               iceildiv(ownd, tp->t_maxseg)) {
1815                                                 ++tcpstat.tcps_sndearlyrexmit;
1816                                                 tp->t_flags |= TF_EARLYREXMT;
1817                                                 goto fastretransmit;
1818                                         }
1819                                         goto drop;
1820                                 }
1821                         } else
1822                                 tp->t_dupacks = 0;
1823                         break;
1824                 }
1825
1826                 KASSERT(SEQ_GT(th->th_ack, tp->snd_una), ("th_ack <= snd_una"));
1827
1828                 /*
1829                  * If the congestion window was inflated to account
1830                  * for the other side's cached packets, retract it.
1831                  */
1832                 if (tcp_do_newreno) {
1833                         if (IN_FASTRECOVERY(tp)) {
1834                                 if (SEQ_LT(th->th_ack, tp->snd_recover)) {
1835                                         tcp_newreno_partial_ack(tp, th);
1836                                 } else {
1837                                         /*
1838                                          * Window inflation should have left us
1839                                          * with approximately snd_ssthresh
1840                                          * outstanding data.
1841                                          * But in case we would be inclined to
1842                                          * send a burst, better to do it via
1843                                          * the slow start mechanism.
1844                                          */
1845                                         if (SEQ_GT(th->th_ack +
1846                                                         tp->snd_ssthresh,
1847                                                    tp->snd_max))
1848                                                 tp->snd_cwnd = tp->snd_max -
1849                                                                 th->th_ack +
1850                                                                 tp->t_maxseg;
1851                                         else
1852                                                 tp->snd_cwnd = tp->snd_ssthresh;
1853                                 }
1854                         }
1855                 } else {
1856                         if (tp->t_dupacks >= tcprexmtthresh &&
1857                             tp->snd_cwnd > tp->snd_ssthresh)
1858                                 tp->snd_cwnd = tp->snd_ssthresh;
1859                 }
1860                 tp->t_dupacks = 0;
1861                 if (SEQ_GT(th->th_ack, tp->snd_max)) {
1862                         tcpstat.tcps_rcvacktoomuch++;
1863                         goto dropafterack;
1864                 }
1865                 /*
1866                  * If we reach this point, ACK is not a duplicate,
1867                  *     i.e., it ACKs something we sent.
1868                  */
1869                 if (tp->t_flags & TF_NEEDSYN) {
1870                         /*
1871                          * T/TCP: Connection was half-synchronized, and our
1872                          * SYN has been ACK'd (so connection is now fully
1873                          * synchronized).  Go to non-starred state,
1874                          * increment snd_una for ACK of SYN, and check if
1875                          * we can do window scaling.
1876                          */
1877                         tp->t_flags &= ~TF_NEEDSYN;
1878                         tp->snd_una++;
1879                         /* Do window scaling? */
1880                         if ((tp->t_flags & (TF_RCVD_SCALE|TF_REQ_SCALE)) ==
1881                                 (TF_RCVD_SCALE|TF_REQ_SCALE)) {
1882                                 tp->snd_scale = tp->requested_s_scale;
1883                                 tp->rcv_scale = tp->request_r_scale;
1884                         }
1885                 }
1886
1887 process_ACK:
1888                 acked = th->th_ack - tp->snd_una;
1889                 tcpstat.tcps_rcvackpack++;
1890                 tcpstat.tcps_rcvackbyte += acked;
1891
1892                 /*
1893                  * If we just performed our first retransmit, and the ACK
1894                  * arrives within our recovery window, then it was a mistake
1895                  * to do the retransmit in the first place.  Recover our
1896                  * original cwnd and ssthresh, and proceed to transmit where
1897                  * we left off.
1898                  */
1899                 if (tcp_do_eifel_detect && acked &&
1900                     (to.to_flags & TOF_TS) && to.to_tsecr &&
1901                     (tp->t_flags & TF_FIRSTACCACK)) {
1902                         /* Eifel detection applicable. */
1903                         if (to.to_tsecr < tp->t_rexmtTS) {
1904                                 ++tcpstat.tcps_eifeldetected;
1905                                 tcp_revert_congestion_state(tp);
1906                                 if (tp->t_rxtshift == 1 &&
1907                                     ticks >= tp->t_badrxtwin)
1908                                         ++tcpstat.tcps_rttcantdetect;
1909                         }
1910                 } else if (tp->t_rxtshift == 1 && ticks < tp->t_badrxtwin) {
1911                         tcp_revert_congestion_state(tp);
1912                         ++tcpstat.tcps_rttdetected;
1913                 }
1914
1915                 /*
1916                  * If we have a timestamp reply, update smoothed
1917                  * round trip time.  If no timestamp is present but
1918                  * transmit timer is running and timed sequence
1919                  * number was acked, update smoothed round trip time.
1920                  * Since we now have an rtt measurement, cancel the
1921                  * timer backoff (cf., Phil Karn's retransmit alg.).
1922                  * Recompute the initial retransmit timer.
1923                  *
1924                  * Some machines (certain windows boxes) send broken
1925                  * timestamp replies during the SYN+ACK phase, ignore
1926                  * timestamps of 0.
1927                  */
1928                 if ((to.to_flags & TOF_TS) != 0 &&
1929                     to.to_tsecr) {
1930                         tcp_xmit_timer(tp, ticks - to.to_tsecr + 1);
1931                 } else if (tp->t_rtttime && SEQ_GT(th->th_ack, tp->t_rtseq)) {
1932                         tcp_xmit_timer(tp, ticks - tp->t_rtttime);
1933                 }
1934                 tcp_xmit_bandwidth_limit(tp, th->th_ack);
1935
1936                 /*
1937                  * If all outstanding data is acked, stop retransmit
1938                  * timer and remember to restart (more output or persist).
1939                  * If there is more data to be acked, restart retransmit
1940                  * timer, using current (possibly backed-off) value.
1941                  */
1942                 if (th->th_ack == tp->snd_max) {
1943                         callout_stop(tp->tt_rexmt);
1944                         needoutput = 1;
1945                 } else if (!callout_active(tp->tt_persist))
1946                         callout_reset(tp->tt_rexmt, tp->t_rxtcur,
1947                                       tcp_timer_rexmt, tp);
1948
1949                 /*
1950                  * If no data (only SYN) was ACK'd,
1951                  *    skip rest of ACK processing.
1952                  */
1953                 if (acked == 0)
1954                         goto step6;
1955
1956                 /* Stop looking for an acceptable ACK since one was received. */
1957                 tp->t_flags &= ~(TF_FIRSTACCACK | TF_FASTREXMT | TF_EARLYREXMT);
1958
1959                 /*
1960                  * When new data is acked, open the congestion window.
1961                  * If the window gives us less than ssthresh packets
1962                  * in flight, open exponentially (maxseg per packet).
1963                  * Otherwise open linearly: maxseg per window
1964                  * (maxseg^2 / cwnd per packet).
1965                  */
1966                 if (!tcp_do_newreno || !IN_FASTRECOVERY(tp)) {
1967                         u_int cw = tp->snd_cwnd;
1968                         u_int incr = tp->t_maxseg;
1969                         if (cw > tp->snd_ssthresh)
1970                                 incr = incr * incr / cw;
1971                         tp->snd_cwnd = min(cw+incr, TCP_MAXWIN<<tp->snd_scale);
1972                 }
1973                 if (acked > so->so_snd.sb_cc) {
1974                         tp->snd_wnd -= so->so_snd.sb_cc;
1975                         sbdrop(&so->so_snd, (int)so->so_snd.sb_cc);
1976                         ourfinisacked = 1;
1977                 } else {
1978                         sbdrop(&so->so_snd, acked);
1979                         tp->snd_wnd -= acked;
1980                         ourfinisacked = 0;
1981                 }
1982                 sowwakeup(so);
1983                 /* detect una wraparound */
1984                 if (tcp_do_newreno && !IN_FASTRECOVERY(tp) &&
1985                     SEQ_GT(tp->snd_una, tp->snd_recover) &&
1986                     SEQ_LEQ(th->th_ack, tp->snd_recover))
1987                         tp->snd_recover = th->th_ack - 1;
1988                 if (tcp_do_newreno && IN_FASTRECOVERY(tp) &&
1989                     SEQ_GEQ(th->th_ack, tp->snd_recover))
1990                         EXIT_FASTRECOVERY(tp);
1991                 tp->snd_una = th->th_ack;
1992                 if (SEQ_LT(tp->snd_nxt, tp->snd_una))
1993                         tp->snd_nxt = tp->snd_una;
1994
1995                 switch (tp->t_state) {
1996
1997                 /*
1998                  * In FIN_WAIT_1 STATE in addition to the processing
1999                  * for the ESTABLISHED state if our FIN is now acknowledged
2000                  * then enter FIN_WAIT_2.
2001                  */
2002                 case TCPS_FIN_WAIT_1:
2003                         if (ourfinisacked) {
2004                                 /*
2005                                  * If we can't receive any more
2006                                  * data, then closing user can proceed.
2007                                  * Starting the timer is contrary to the
2008                                  * specification, but if we don't get a FIN
2009                                  * we'll hang forever.
2010                                  */
2011                                 if (so->so_state & SS_CANTRCVMORE) {
2012                                         soisdisconnected(so);
2013                                         callout_reset(tp->tt_2msl, tcp_maxidle,
2014                                                       tcp_timer_2msl, tp);
2015                                 }
2016                                 tp->t_state = TCPS_FIN_WAIT_2;
2017                         }
2018                         break;
2019
2020                 /*
2021                  * In CLOSING STATE in addition to the processing for
2022                  * the ESTABLISHED state if the ACK acknowledges our FIN
2023                  * then enter the TIME-WAIT state, otherwise ignore
2024                  * the segment.
2025                  */
2026                 case TCPS_CLOSING:
2027                         if (ourfinisacked) {
2028                                 tp->t_state = TCPS_TIME_WAIT;
2029                                 tcp_canceltimers(tp);
2030                                 /* Shorten TIME_WAIT [RFC-1644, p.28] */
2031                                 if (tp->cc_recv != 0 &&
2032                                     (ticks - tp->t_starttime) < tcp_msl)
2033                                         callout_reset(tp->tt_2msl,
2034                                                       tp->t_rxtcur *
2035                                                       TCPTV_TWTRUNC,
2036                                                       tcp_timer_2msl, tp);
2037                                 else
2038                                         callout_reset(tp->tt_2msl, 2 * tcp_msl,
2039                                                       tcp_timer_2msl, tp);
2040                                 soisdisconnected(so);
2041                         }
2042                         break;
2043
2044                 /*
2045                  * In LAST_ACK, we may still be waiting for data to drain
2046                  * and/or to be acked, as well as for the ack of our FIN.
2047                  * If our FIN is now acknowledged, delete the TCB,
2048                  * enter the closed state and return.
2049                  */
2050                 case TCPS_LAST_ACK:
2051                         if (ourfinisacked) {
2052                                 tp = tcp_close(tp);
2053                                 goto drop;
2054                         }
2055                         break;
2056
2057                 /*
2058                  * In TIME_WAIT state the only thing that should arrive
2059                  * is a retransmission of the remote FIN.  Acknowledge
2060                  * it and restart the finack timer.
2061                  */
2062                 case TCPS_TIME_WAIT:
2063                         callout_reset(tp->tt_2msl, 2 * tcp_msl,
2064                                       tcp_timer_2msl, tp);
2065                         goto dropafterack;
2066                 }
2067         }
2068
2069 step6:
2070         /*
2071          * Update window information.
2072          * Don't look at window if no ACK: TAC's send garbage on first SYN.
2073          */
2074         if ((thflags & TH_ACK) &&
2075             (SEQ_LT(tp->snd_wl1, th->th_seq) ||
2076             (tp->snd_wl1 == th->th_seq && (SEQ_LT(tp->snd_wl2, th->th_ack) ||
2077              (tp->snd_wl2 == th->th_ack && tiwin > tp->snd_wnd))))) {
2078                 /* keep track of pure window updates */
2079                 if (tlen == 0 &&
2080                     tp->snd_wl2 == th->th_ack && tiwin > tp->snd_wnd)
2081                         tcpstat.tcps_rcvwinupd++;
2082                 tp->snd_wnd = tiwin;
2083                 tp->snd_wl1 = th->th_seq;
2084                 tp->snd_wl2 = th->th_ack;
2085                 if (tp->snd_wnd > tp->max_sndwnd)
2086                         tp->max_sndwnd = tp->snd_wnd;
2087                 needoutput = 1;
2088         }
2089
2090         /*
2091          * Process segments with URG.
2092          */
2093         if ((thflags & TH_URG) && th->th_urp &&
2094             TCPS_HAVERCVDFIN(tp->t_state) == 0) {
2095                 /*
2096                  * This is a kludge, but if we receive and accept
2097                  * random urgent pointers, we'll crash in
2098                  * soreceive.  It's hard to imagine someone
2099                  * actually wanting to send this much urgent data.
2100                  */
2101                 if (th->th_urp + so->so_rcv.sb_cc > sb_max) {
2102                         th->th_urp = 0;                 /* XXX */
2103                         thflags &= ~TH_URG;             /* XXX */
2104                         goto dodata;                    /* XXX */
2105                 }
2106                 /*
2107                  * If this segment advances the known urgent pointer,
2108                  * then mark the data stream.  This should not happen
2109                  * in CLOSE_WAIT, CLOSING, LAST_ACK or TIME_WAIT STATES since
2110                  * a FIN has been received from the remote side.
2111                  * In these states we ignore the URG.
2112                  *
2113                  * According to RFC961 (Assigned Protocols),
2114                  * the urgent pointer points to the last octet
2115                  * of urgent data.  We continue, however,
2116                  * to consider it to indicate the first octet
2117                  * of data past the urgent section as the original
2118                  * spec states (in one of two places).
2119                  */
2120                 if (SEQ_GT(th->th_seq+th->th_urp, tp->rcv_up)) {
2121                         tp->rcv_up = th->th_seq + th->th_urp;
2122                         so->so_oobmark = so->so_rcv.sb_cc +
2123                             (tp->rcv_up - tp->rcv_nxt) - 1;
2124                         if (so->so_oobmark == 0)
2125                                 so->so_state |= SS_RCVATMARK;
2126                         sohasoutofband(so);
2127                         tp->t_oobflags &= ~(TCPOOB_HAVEDATA | TCPOOB_HADDATA);
2128                 }
2129                 /*
2130                  * Remove out of band data so doesn't get presented to user.
2131                  * This can happen independent of advancing the URG pointer,
2132                  * but if two URG's are pending at once, some out-of-band
2133                  * data may creep in... ick.
2134                  */
2135                 if (th->th_urp <= (u_long)tlen
2136 #ifdef SO_OOBINLINE
2137                      && (so->so_options & SO_OOBINLINE) == 0
2138 #endif
2139                      )
2140                         tcp_pulloutofband(so, th, m,
2141                                 drop_hdrlen);   /* hdr drop is delayed */
2142         } else {
2143                 /*
2144                  * If no out of band data is expected,
2145                  * pull receive urgent pointer along
2146                  * with the receive window.
2147                  */
2148                 if (SEQ_GT(tp->rcv_nxt, tp->rcv_up))
2149                         tp->rcv_up = tp->rcv_nxt;
2150         }
2151 dodata:                                                 /* XXX */
2152
2153         /*
2154          * Process the segment text, merging it into the TCP sequencing queue,
2155          * and arranging for acknowledgment of receipt if necessary.
2156          * This process logically involves adjusting tp->rcv_wnd as data
2157          * is presented to the user (this happens in tcp_usrreq.c,
2158          * case PRU_RCVD).  If a FIN has already been received on this
2159          * connection then we just ignore the text.
2160          */
2161         if ((tlen || (thflags & TH_FIN)) &&
2162             TCPS_HAVERCVDFIN(tp->t_state) == 0) {
2163                 m_adj(m, drop_hdrlen);  /* delayed header drop */
2164                 /*
2165                  * Insert segment which includes th into TCP reassembly queue
2166                  * with control block tp.  Set thflags to whether reassembly now
2167                  * includes a segment with FIN.  This handles the common case
2168                  * inline (segment is the next to be received on an established
2169                  * connection, and the queue is empty), avoiding linkage into
2170                  * and removal from the queue and repetition of various
2171                  * conversions.
2172                  * Set DELACK for segments received in order, but ack
2173                  * immediately when segments are out of order (so
2174                  * fast retransmit can work).
2175                  */
2176                 if (th->th_seq == tp->rcv_nxt &&
2177                     LIST_EMPTY(&tp->t_segq) &&
2178                     TCPS_HAVEESTABLISHED(tp->t_state)) {
2179                         if (DELAY_ACK(tp))
2180                                 callout_reset(tp->tt_delack, tcp_delacktime,
2181                                               tcp_timer_delack, tp);
2182                         else
2183                                 tp->t_flags |= TF_ACKNOW;
2184                         tp->rcv_nxt += tlen;
2185                         thflags = th->th_flags & TH_FIN;
2186                         tcpstat.tcps_rcvpack++;
2187                         tcpstat.tcps_rcvbyte += tlen;
2188                         ND6_HINT(tp);
2189                         if (so->so_state & SS_CANTRCVMORE)
2190                                 m_freem(m);
2191                         else
2192                                 sbappend(&so->so_rcv, m);
2193                         sorwakeup(so);
2194                 } else {
2195                         thflags = tcp_reass(tp, th, &tlen, m);
2196                         tp->t_flags |= TF_ACKNOW;
2197                 }
2198
2199                 /*
2200                  * Note the amount of data that peer has sent into
2201                  * our window, in order to estimate the sender's
2202                  * buffer size.
2203                  */
2204                 len = so->so_rcv.sb_hiwat - (tp->rcv_adv - tp->rcv_nxt);
2205         } else {
2206                 m_freem(m);
2207                 thflags &= ~TH_FIN;
2208         }
2209
2210         /*
2211          * If FIN is received ACK the FIN and let the user know
2212          * that the connection is closing.
2213          */
2214         if (thflags & TH_FIN) {
2215                 if (TCPS_HAVERCVDFIN(tp->t_state) == 0) {
2216                         socantrcvmore(so);
2217                         /*
2218                          * If connection is half-synchronized
2219                          * (ie NEEDSYN flag on) then delay ACK,
2220                          * so it may be piggybacked when SYN is sent.
2221                          * Otherwise, since we received a FIN then no
2222                          * more input can be expected, send ACK now.
2223                          */
2224                         if (DELAY_ACK(tp) && (tp->t_flags & TF_NEEDSYN))
2225                                 callout_reset(tp->tt_delack, tcp_delacktime,
2226                                     tcp_timer_delack, tp);
2227                         else
2228                                 tp->t_flags |= TF_ACKNOW;
2229                         tp->rcv_nxt++;
2230                 }
2231                 switch (tp->t_state) {
2232
2233                 /*
2234                  * In SYN_RECEIVED and ESTABLISHED STATES
2235                  * enter the CLOSE_WAIT state.
2236                  */
2237                 case TCPS_SYN_RECEIVED:
2238                         tp->t_starttime = ticks;
2239                         /*FALLTHROUGH*/
2240                 case TCPS_ESTABLISHED:
2241                         tp->t_state = TCPS_CLOSE_WAIT;
2242                         break;
2243
2244                 /*
2245                  * If still in FIN_WAIT_1 STATE FIN has not been acked so
2246                  * enter the CLOSING state.
2247                  */
2248                 case TCPS_FIN_WAIT_1:
2249                         tp->t_state = TCPS_CLOSING;
2250                         break;
2251
2252                 /*
2253                  * In FIN_WAIT_2 state enter the TIME_WAIT state,
2254                  * starting the time-wait timer, turning off the other
2255                  * standard timers.
2256                  */
2257                 case TCPS_FIN_WAIT_2:
2258                         tp->t_state = TCPS_TIME_WAIT;
2259                         tcp_canceltimers(tp);
2260                         /* Shorten TIME_WAIT [RFC-1644, p.28] */
2261                         if (tp->cc_recv != 0 &&
2262                             (ticks - tp->t_starttime) < tcp_msl) {
2263                                 callout_reset(tp->tt_2msl,
2264                                               tp->t_rxtcur * TCPTV_TWTRUNC,
2265                                               tcp_timer_2msl, tp);
2266                                 /* For transaction client, force ACK now. */
2267                                 tp->t_flags |= TF_ACKNOW;
2268                         }
2269                         else
2270                                 callout_reset(tp->tt_2msl, 2 * tcp_msl,
2271                                               tcp_timer_2msl, tp);
2272                         soisdisconnected(so);
2273                         break;
2274
2275                 /*
2276                  * In TIME_WAIT state restart the 2 MSL time_wait timer.
2277                  */
2278                 case TCPS_TIME_WAIT:
2279                         callout_reset(tp->tt_2msl, 2 * tcp_msl,
2280                                       tcp_timer_2msl, tp);
2281                         break;
2282                 }
2283         }
2284 #ifdef TCPDEBUG
2285         if (so->so_options & SO_DEBUG)
2286                 tcp_trace(TA_INPUT, ostate, tp, (void *)tcp_saveipgen,
2287                           &tcp_savetcp, 0);
2288 #endif
2289
2290         /*
2291          * Return any desired output.
2292          */
2293         if (needoutput || (tp->t_flags & TF_ACKNOW))
2294                 (void) tcp_output(tp);
2295         return;
2296
2297 dropafterack:
2298         /*
2299          * Generate an ACK dropping incoming segment if it occupies
2300          * sequence space, where the ACK reflects our state.
2301          *
2302          * We can now skip the test for the RST flag since all
2303          * paths to this code happen after packets containing
2304          * RST have been dropped.
2305          *
2306          * In the SYN-RECEIVED state, don't send an ACK unless the
2307          * segment we received passes the SYN-RECEIVED ACK test.
2308          * If it fails send a RST.  This breaks the loop in the
2309          * "LAND" DoS attack, and also prevents an ACK storm
2310          * between two listening ports that have been sent forged
2311          * SYN segments, each with the source address of the other.
2312          */
2313         if (tp->t_state == TCPS_SYN_RECEIVED && (thflags & TH_ACK) &&
2314             (SEQ_GT(tp->snd_una, th->th_ack) ||
2315              SEQ_GT(th->th_ack, tp->snd_max)) ) {
2316                 rstreason = BANDLIM_RST_OPENPORT;
2317                 goto dropwithreset;
2318         }
2319 #ifdef TCPDEBUG
2320         if (so->so_options & SO_DEBUG)
2321                 tcp_trace(TA_DROP, ostate, tp, (void *)tcp_saveipgen,
2322                           &tcp_savetcp, 0);
2323 #endif
2324         m_freem(m);
2325         tp->t_flags |= TF_ACKNOW;
2326         (void) tcp_output(tp);
2327         return;
2328
2329 dropwithreset:
2330         /*
2331          * Generate a RST, dropping incoming segment.
2332          * Make ACK acceptable to originator of segment.
2333          * Don't bother to respond if destination was broadcast/multicast.
2334          */
2335         if ((thflags & TH_RST) || m->m_flags & (M_BCAST|M_MCAST))
2336                 goto drop;
2337         if (isipv6) {
2338                 if (IN6_IS_ADDR_MULTICAST(&ip6->ip6_dst) ||
2339                     IN6_IS_ADDR_MULTICAST(&ip6->ip6_src))
2340                         goto drop;
2341         } else {
2342                 if (IN_MULTICAST(ntohl(ip->ip_dst.s_addr)) ||
2343                     IN_MULTICAST(ntohl(ip->ip_src.s_addr)) ||
2344                     ip->ip_src.s_addr == htonl(INADDR_BROADCAST) ||
2345                     in_broadcast(ip->ip_dst, m->m_pkthdr.rcvif))
2346                         goto drop;
2347         }
2348         /* IPv6 anycast check is done at tcp6_input() */
2349
2350         /*
2351          * Perform bandwidth limiting.
2352          */
2353 #ifdef ICMP_BANDLIM
2354         if (badport_bandlim(rstreason) < 0)
2355                 goto drop;
2356 #endif
2357
2358 #ifdef TCPDEBUG
2359         if (tp == NULL || (tp->t_inpcb->inp_socket->so_options & SO_DEBUG))
2360                 tcp_trace(TA_DROP, ostate, tp, (void *)tcp_saveipgen,
2361                           &tcp_savetcp, 0);
2362 #endif
2363         if (thflags & TH_ACK)
2364                 /* mtod() below is safe as long as hdr dropping is delayed */
2365                 tcp_respond(tp, mtod(m, void *), th, m, (tcp_seq)0, th->th_ack,
2366                             TH_RST);
2367         else {
2368                 if (thflags & TH_SYN)
2369                         tlen++;
2370                 /* mtod() below is safe as long as hdr dropping is delayed */
2371                 tcp_respond(tp, mtod(m, void *), th, m, th->th_seq+tlen,
2372                             (tcp_seq)0, TH_RST|TH_ACK);
2373         }
2374         return;
2375
2376 drop:
2377         /*
2378          * Drop space held by incoming segment and return.
2379          */
2380 #ifdef TCPDEBUG
2381         if (tp == NULL || (tp->t_inpcb->inp_socket->so_options & SO_DEBUG))
2382                 tcp_trace(TA_DROP, ostate, tp, (void *)tcp_saveipgen,
2383                           &tcp_savetcp, 0);
2384 #endif
2385         m_freem(m);
2386         return;
2387 }
2388
2389 /*
2390  * Parse TCP options and place in tcpopt.
2391  */
2392 static void
2393 tcp_dooptions(to, cp, cnt, is_syn)
2394         struct tcpopt *to;
2395         u_char *cp;
2396         int cnt;
2397 {
2398         int opt, optlen;
2399
2400         to->to_flags = 0;
2401         for (; cnt > 0; cnt -= optlen, cp += optlen) {
2402                 opt = cp[0];
2403                 if (opt == TCPOPT_EOL)
2404                         break;
2405                 if (opt == TCPOPT_NOP)
2406                         optlen = 1;
2407                 else {
2408                         if (cnt < 2)
2409                                 break;
2410                         optlen = cp[1];
2411                         if (optlen < 2 || optlen > cnt)
2412                                 break;
2413                 }
2414                 switch (opt) {
2415                 case TCPOPT_MAXSEG:
2416                         if (optlen != TCPOLEN_MAXSEG)
2417                                 continue;
2418                         if (!is_syn)
2419                                 continue;
2420                         to->to_flags |= TOF_MSS;
2421                         bcopy((char *)cp + 2,
2422                             (char *)&to->to_mss, sizeof(to->to_mss));
2423                         to->to_mss = ntohs(to->to_mss);
2424                         break;
2425                 case TCPOPT_WINDOW:
2426                         if (optlen != TCPOLEN_WINDOW)
2427                                 continue;
2428                         if (! is_syn)
2429                                 continue;
2430                         to->to_flags |= TOF_SCALE;
2431                         to->to_requested_s_scale = min(cp[2], TCP_MAX_WINSHIFT);
2432                         break;
2433                 case TCPOPT_TIMESTAMP:
2434                         if (optlen != TCPOLEN_TIMESTAMP)
2435                                 continue;
2436                         to->to_flags |= TOF_TS;
2437                         bcopy((char *)cp + 2,
2438                             (char *)&to->to_tsval, sizeof(to->to_tsval));
2439                         to->to_tsval = ntohl(to->to_tsval);
2440                         bcopy((char *)cp + 6,
2441                             (char *)&to->to_tsecr, sizeof(to->to_tsecr));
2442                         to->to_tsecr = ntohl(to->to_tsecr);
2443                         break;
2444                 case TCPOPT_CC:
2445                         if (optlen != TCPOLEN_CC)
2446                                 continue;
2447                         to->to_flags |= TOF_CC;
2448                         bcopy((char *)cp + 2,
2449                             (char *)&to->to_cc, sizeof(to->to_cc));
2450                         to->to_cc = ntohl(to->to_cc);
2451                         break;
2452                 case TCPOPT_CCNEW:
2453                         if (optlen != TCPOLEN_CC)
2454                                 continue;
2455                         if (!is_syn)
2456                                 continue;
2457                         to->to_flags |= TOF_CCNEW;
2458                         bcopy((char *)cp + 2,
2459                             (char *)&to->to_cc, sizeof(to->to_cc));
2460                         to->to_cc = ntohl(to->to_cc);
2461                         break;
2462                 case TCPOPT_CCECHO:
2463                         if (optlen != TCPOLEN_CC)
2464                                 continue;
2465                         if (!is_syn)
2466                                 continue;
2467                         to->to_flags |= TOF_CCECHO;
2468                         bcopy((char *)cp + 2,
2469                             (char *)&to->to_ccecho, sizeof(to->to_ccecho));
2470                         to->to_ccecho = ntohl(to->to_ccecho);
2471                         break;
2472                 default:
2473                         continue;
2474                 }
2475         }
2476 }
2477
2478 /*
2479  * Pull out of band byte out of a segment so
2480  * it doesn't appear in the user's data queue.
2481  * It is still reflected in the segment length for
2482  * sequencing purposes.
2483  */
2484 static void
2485 tcp_pulloutofband(so, th, m, off)
2486         struct socket *so;
2487         struct tcphdr *th;
2488         struct mbuf *m;
2489         int off;                /* delayed to be droped hdrlen */
2490 {
2491         int cnt = off + th->th_urp - 1;
2492
2493         while (cnt >= 0) {
2494                 if (m->m_len > cnt) {
2495                         char *cp = mtod(m, caddr_t) + cnt;
2496                         struct tcpcb *tp = sototcpcb(so);
2497
2498                         tp->t_iobc = *cp;
2499                         tp->t_oobflags |= TCPOOB_HAVEDATA;
2500                         bcopy(cp+1, cp, (unsigned)(m->m_len - cnt - 1));
2501                         m->m_len--;
2502                         if (m->m_flags & M_PKTHDR)
2503                                 m->m_pkthdr.len--;
2504                         return;
2505                 }
2506                 cnt -= m->m_len;
2507                 m = m->m_next;
2508                 if (m == 0)
2509                         break;
2510         }
2511         panic("tcp_pulloutofband");
2512 }
2513
2514 /*
2515  * Collect new round-trip time estimate
2516  * and update averages and current timeout.
2517  */
2518 static void
2519 tcp_xmit_timer(tp, rtt)
2520         struct tcpcb *tp;
2521         int rtt;
2522 {
2523         int delta;
2524
2525         tcpstat.tcps_rttupdated++;
2526         tp->t_rttupdated++;
2527         if (tp->t_srtt != 0) {
2528                 /*
2529                  * srtt is stored as fixed point with 5 bits after the
2530                  * binary point (i.e., scaled by 8).  The following magic
2531                  * is equivalent to the smoothing algorithm in rfc793 with
2532                  * an alpha of .875 (srtt = rtt/8 + srtt*7/8 in fixed
2533                  * point).  Adjust rtt to origin 0.
2534                  */
2535                 delta = ((rtt - 1) << TCP_DELTA_SHIFT)
2536                         - (tp->t_srtt >> (TCP_RTT_SHIFT - TCP_DELTA_SHIFT));
2537
2538                 if ((tp->t_srtt += delta) <= 0)
2539                         tp->t_srtt = 1;
2540
2541                 /*
2542                  * We accumulate a smoothed rtt variance (actually, a
2543                  * smoothed mean difference), then set the retransmit
2544                  * timer to smoothed rtt + 4 times the smoothed variance.
2545                  * rttvar is stored as fixed point with 4 bits after the
2546                  * binary point (scaled by 16).  The following is
2547                  * equivalent to rfc793 smoothing with an alpha of .75
2548                  * (rttvar = rttvar*3/4 + |delta| / 4).  This replaces
2549                  * rfc793's wired-in beta.
2550                  */
2551                 if (delta < 0)
2552                         delta = -delta;
2553                 delta -= tp->t_rttvar >> (TCP_RTTVAR_SHIFT - TCP_DELTA_SHIFT);
2554                 if ((tp->t_rttvar += delta) <= 0)
2555                         tp->t_rttvar = 1;
2556                 if (tp->t_rttbest > tp->t_srtt + tp->t_rttvar)
2557                         tp->t_rttbest = tp->t_srtt + tp->t_rttvar;
2558         } else {
2559                 /*
2560                  * No rtt measurement yet - use the unsmoothed rtt.
2561                  * Set the variance to half the rtt (so our first
2562                  * retransmit happens at 3*rtt).
2563                  */
2564                 tp->t_srtt = rtt << TCP_RTT_SHIFT;
2565                 tp->t_rttvar = rtt << (TCP_RTTVAR_SHIFT - 1);
2566                 tp->t_rttbest = tp->t_srtt + tp->t_rttvar;
2567         }
2568         tp->t_rtttime = 0;
2569         tp->t_rxtshift = 0;
2570
2571         /*
2572          * the retransmit should happen at rtt + 4 * rttvar.
2573          * Because of the way we do the smoothing, srtt and rttvar
2574          * will each average +1/2 tick of bias.  When we compute
2575          * the retransmit timer, we want 1/2 tick of rounding and
2576          * 1 extra tick because of +-1/2 tick uncertainty in the
2577          * firing of the timer.  The bias will give us exactly the
2578          * 1.5 tick we need.  But, because the bias is
2579          * statistical, we have to test that we don't drop below
2580          * the minimum feasible timer (which is 2 ticks).
2581          */
2582         TCPT_RANGESET(tp->t_rxtcur, TCP_REXMTVAL(tp),
2583                       max(tp->t_rttmin, rtt + 2), TCPTV_REXMTMAX);
2584
2585         /*
2586          * We received an ack for a packet that wasn't retransmitted;
2587          * it is probably safe to discard any error indications we've
2588          * received recently.  This isn't quite right, but close enough
2589          * for now (a route might have failed after we sent a segment,
2590          * and the return path might not be symmetrical).
2591          */
2592         tp->t_softerror = 0;
2593 }
2594
2595 /*
2596  * Determine a reasonable value for maxseg size.
2597  * If the route is known, check route for mtu.
2598  * If none, use an mss that can be handled on the outgoing
2599  * interface without forcing IP to fragment; if bigger than
2600  * an mbuf cluster (MCLBYTES), round down to nearest multiple of MCLBYTES
2601  * to utilize large mbufs.  If no route is found, route has no mtu,
2602  * or the destination isn't local, use a default, hopefully conservative
2603  * size (usually 512 or the default IP max size, but no more than the mtu
2604  * of the interface), as we can't discover anything about intervening
2605  * gateways or networks.  We also initialize the congestion/slow start
2606  * window to be a single segment if the destination isn't local.
2607  * While looking at the routing entry, we also initialize other path-dependent
2608  * parameters from pre-set or cached values in the routing entry.
2609  *
2610  * Also take into account the space needed for options that we
2611  * send regularly.  Make maxseg shorter by that amount to assure
2612  * that we can send maxseg amount of data even when the options
2613  * are present.  Store the upper limit of the length of options plus
2614  * data in maxopd.
2615  *
2616  * NOTE that this routine is only called when we process an incoming
2617  * segment, for outgoing segments only tcp_mssopt is called.
2618  *
2619  * In case of T/TCP, we call this routine during implicit connection
2620  * setup as well (offer = -1), to initialize maxseg from the cached
2621  * MSS of our peer.
2622  */
2623 void
2624 tcp_mss(tp, offer)
2625         struct tcpcb *tp;
2626         int offer;
2627 {
2628         struct rtentry *rt;
2629         struct ifnet *ifp;
2630         int rtt, mss;
2631         u_long bufsize;
2632         struct inpcb *inp = tp->t_inpcb;
2633         struct socket *so;
2634         struct rmxp_tao *taop;
2635         int origoffer = offer;
2636 #ifdef INET6
2637         boolean_t isipv6 = ((inp->inp_vflag & INP_IPV6) ? TRUE : FALSE);
2638         size_t min_protoh = isipv6 ?
2639                             sizeof(struct ip6_hdr) + sizeof(struct tcphdr) :
2640                             sizeof(struct tcpiphdr);
2641 #else
2642         const boolean_t isipv6 = FALSE;
2643         const size_t min_protoh = sizeof(struct tcpiphdr);
2644 #endif
2645
2646         if (isipv6)
2647                 rt = tcp_rtlookup6(&inp->inp_inc);
2648         else
2649                 rt = tcp_rtlookup(&inp->inp_inc);
2650         if (rt == NULL) {
2651                 tp->t_maxopd = tp->t_maxseg =
2652                     (isipv6 ? tcp_v6mssdflt : tcp_mssdflt);
2653                 return;
2654         }
2655         ifp = rt->rt_ifp;
2656         so = inp->inp_socket;
2657
2658         taop = rmx_taop(rt->rt_rmx);
2659         /*
2660          * Offer == -1 means that we didn't receive SYN yet,
2661          * use cached value in that case;
2662          */
2663         if (offer == -1)
2664                 offer = taop->tao_mssopt;
2665         /*
2666          * Offer == 0 means that there was no MSS on the SYN segment,
2667          * in this case we use tcp_mssdflt.
2668          */
2669         if (offer == 0)
2670                 offer = (isipv6 ? tcp_v6mssdflt : tcp_mssdflt);
2671         else
2672                 /*
2673                  * Sanity check: make sure that maxopd will be large
2674                  * enough to allow some data on segments even is the
2675                  * all the option space is used (40bytes).  Otherwise
2676                  * funny things may happen in tcp_output.
2677                  */
2678                 offer = max(offer, 64);
2679         taop->tao_mssopt = offer;
2680
2681         /*
2682          * While we're here, check if there's an initial rtt
2683          * or rttvar.  Convert from the route-table units
2684          * to scaled multiples of the slow timeout timer.
2685          */
2686         if (tp->t_srtt == 0 && (rtt = rt->rt_rmx.rmx_rtt)) {
2687                 /*
2688                  * XXX the lock bit for RTT indicates that the value
2689                  * is also a minimum value; this is subject to time.
2690                  */
2691                 if (rt->rt_rmx.rmx_locks & RTV_RTT)
2692                         tp->t_rttmin = rtt / (RTM_RTTUNIT / hz);
2693                 tp->t_srtt = rtt / (RTM_RTTUNIT / (hz * TCP_RTT_SCALE));
2694                 tp->t_rttbest = tp->t_srtt + TCP_RTT_SCALE;
2695                 tcpstat.tcps_usedrtt++;
2696                 if (rt->rt_rmx.rmx_rttvar) {
2697                         tp->t_rttvar = rt->rt_rmx.rmx_rttvar /
2698                             (RTM_RTTUNIT / (hz * TCP_RTTVAR_SCALE));
2699                         tcpstat.tcps_usedrttvar++;
2700                 } else {
2701                         /* default variation is +- 1 rtt */
2702                         tp->t_rttvar =
2703                             tp->t_srtt * TCP_RTTVAR_SCALE / TCP_RTT_SCALE;
2704                 }
2705                 TCPT_RANGESET(tp->t_rxtcur,
2706                               ((tp->t_srtt >> 2) + tp->t_rttvar) >> 1,
2707                               tp->t_rttmin, TCPTV_REXMTMAX);
2708         }
2709         /*
2710          * if there's an mtu associated with the route, use it
2711          * else, use the link mtu.
2712          */
2713         if (rt->rt_rmx.rmx_mtu)
2714                 mss = rt->rt_rmx.rmx_mtu - min_protoh;
2715         else {
2716                 if (isipv6) {
2717                         mss = nd_ifinfo[rt->rt_ifp->if_index].linkmtu -
2718                                 min_protoh;
2719                         if (!in6_localaddr(&inp->in6p_faddr))
2720                                 mss = min(mss, tcp_v6mssdflt);
2721                 } else {
2722                         mss = ifp->if_mtu - min_protoh;
2723                         if (!in_localaddr(inp->inp_faddr))
2724                                 mss = min(mss, tcp_mssdflt);
2725                 }
2726         }
2727         mss = min(mss, offer);
2728         /*
2729          * maxopd stores the maximum length of data AND options
2730          * in a segment; maxseg is the amount of data in a normal
2731          * segment.  We need to store this value (maxopd) apart
2732          * from maxseg, because now every segment carries options
2733          * and thus we normally have somewhat less data in segments.
2734          */
2735         tp->t_maxopd = mss;
2736
2737         /*
2738          * In case of T/TCP, origoffer==-1 indicates, that no segments
2739          * were received yet.  In this case we just guess, otherwise
2740          * we do the same as before T/TCP.
2741          */
2742         if ((tp->t_flags & (TF_REQ_TSTMP|TF_NOOPT)) == TF_REQ_TSTMP &&
2743             (origoffer == -1 ||
2744              (tp->t_flags & TF_RCVD_TSTMP) == TF_RCVD_TSTMP))
2745                 mss -= TCPOLEN_TSTAMP_APPA;
2746         if ((tp->t_flags & (TF_REQ_CC|TF_NOOPT)) == TF_REQ_CC &&
2747             (origoffer == -1 ||
2748              (tp->t_flags & TF_RCVD_CC) == TF_RCVD_CC))
2749                 mss -= TCPOLEN_CC_APPA;
2750
2751 #if     (MCLBYTES & (MCLBYTES - 1)) == 0
2752                 if (mss > MCLBYTES)
2753                         mss &= ~(MCLBYTES-1);
2754 #else
2755                 if (mss > MCLBYTES)
2756                         mss = mss / MCLBYTES * MCLBYTES;
2757 #endif
2758         /*
2759          * If there's a pipesize, change the socket buffer
2760          * to that size.  Make the socket buffers an integral
2761          * number of mss units; if the mss is larger than
2762          * the socket buffer, decrease the mss.
2763          */
2764 #ifdef RTV_SPIPE
2765         if ((bufsize = rt->rt_rmx.rmx_sendpipe) == 0)
2766 #endif
2767                 bufsize = so->so_snd.sb_hiwat;
2768         if (bufsize < mss)
2769                 mss = bufsize;
2770         else {
2771                 bufsize = roundup(bufsize, mss);
2772                 if (bufsize > sb_max)
2773                         bufsize = sb_max;
2774                 if (bufsize > so->so_snd.sb_hiwat)
2775                         (void)sbreserve(&so->so_snd, bufsize, so, NULL);
2776         }
2777         tp->t_maxseg = mss;
2778
2779 #ifdef RTV_RPIPE
2780         if ((bufsize = rt->rt_rmx.rmx_recvpipe) == 0)
2781 #endif
2782                 bufsize = so->so_rcv.sb_hiwat;
2783         if (bufsize > mss) {
2784                 bufsize = roundup(bufsize, mss);
2785                 if (bufsize > sb_max)
2786                         bufsize = sb_max;
2787                 if (bufsize > so->so_rcv.sb_hiwat)
2788                         (void)sbreserve(&so->so_rcv, bufsize, so, NULL);
2789         }
2790
2791         /*
2792          * Set the slow-start flight size depending on whether this
2793          * is a local network or not.
2794          */
2795         if (tcp_do_rfc3390)
2796                 tp->snd_cwnd = min(4 * mss, max(2 * mss, 4380));
2797         else if ((isipv6 && in6_localaddr(&inp->in6p_faddr)) ||
2798                  (!isipv6 && in_localaddr(inp->inp_faddr)))
2799                 tp->snd_cwnd = mss * ss_fltsz_local;
2800         else
2801                 tp->snd_cwnd = mss * ss_fltsz;
2802
2803         if (rt->rt_rmx.rmx_ssthresh) {
2804                 /*
2805                  * There's some sort of gateway or interface
2806                  * buffer limit on the path.  Use this to set
2807                  * the slow start threshhold, but set the
2808                  * threshold to no less than 2*mss.
2809                  */
2810                 tp->snd_ssthresh = max(2 * mss, rt->rt_rmx.rmx_ssthresh);
2811                 tcpstat.tcps_usedssthresh++;
2812         }
2813 }
2814
2815 /*
2816  * Determine the MSS option to send on an outgoing SYN.
2817  */
2818 int
2819 tcp_mssopt(tp)
2820         struct tcpcb *tp;
2821 {
2822         struct rtentry *rt;
2823 #ifdef INET6
2824         boolean_t isipv6 =
2825             ((tp->t_inpcb->inp_vflag & INP_IPV6) ? TRUE : FALSE);
2826         int min_protoh = isipv6 ?
2827                              sizeof(struct ip6_hdr) + sizeof(struct tcphdr) :
2828                              sizeof(struct tcpiphdr);
2829 #else
2830         const boolean_t isipv6 = FALSE;
2831         const size_t min_protoh = sizeof(struct tcpiphdr);
2832 #endif
2833
2834         if (isipv6)
2835                 rt = tcp_rtlookup6(&tp->t_inpcb->inp_inc);
2836         else
2837                 rt = tcp_rtlookup(&tp->t_inpcb->inp_inc);
2838         if (rt == NULL)
2839                 return (isipv6 ? tcp_v6mssdflt : tcp_mssdflt);
2840
2841         return (rt->rt_ifp->if_mtu - min_protoh);
2842 }
2843
2844
2845 /*
2846  * When a partial ack arrives, force the retransmission of the
2847  * next unacknowledged segment.  Do not clear tp->t_dupacks.
2848  * By setting snd_nxt to ti_ack, this forces retransmission timer to
2849  * be started again.
2850  */
2851 static void
2852 tcp_newreno_partial_ack(tp, th)
2853         struct tcpcb *tp;
2854         struct tcphdr *th;
2855 {
2856         tcp_seq onxt = tp->snd_nxt;
2857         u_long  ocwnd = tp->snd_cwnd;
2858
2859         callout_stop(tp->tt_rexmt);
2860         tp->t_rtttime = 0;
2861         tp->snd_nxt = th->th_ack;
2862         /*
2863          * Set snd_cwnd to one segment beyond acknowledged offset
2864          * (tp->snd_una has not yet been updated when this function is called.)
2865          */
2866         tp->snd_cwnd = tp->t_maxseg + (th->th_ack - tp->snd_una);
2867         tp->t_flags |= TF_ACKNOW;
2868         (void) tcp_output(tp);
2869         tp->snd_cwnd = ocwnd;
2870         if (SEQ_GT(onxt, tp->snd_nxt))
2871                 tp->snd_nxt = onxt;
2872         /*
2873          * Partial window deflation.  Relies on fact that tp->snd_una
2874          * not updated yet.
2875          */
2876         tp->snd_cwnd -= (th->th_ack - tp->snd_una - tp->t_maxseg);
2877 }