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 1.29 2004/06/06 05:38:58 hsu 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 #include <machine/stdarg.h>
  60
  61 #include <net/if.h>
  62 #include <net/route.h>
  63
  64 #include <netinet/in.h>
  65 #include <netinet/in_systm.h>
  66 #include <netinet/ip.h>
  67 #include <netinet/ip_icmp.h>    /* for ICMP_BANDLIM             */
  68 #include <netinet/in_var.h>
  69 #include <netinet/icmp_var.h>   /* for ICMP_BANDLIM             */
  70 #include <netinet/in_pcb.h>
  71 #include <netinet/ip_var.h>
  72 #include <netinet/ip6.h>
  73 #include <netinet/icmp6.h>
  74 #include <netinet6/nd6.h>
  75 #include <netinet6/ip6_var.h>
  76 #include <netinet6/in6_pcb.h>
  77 #include <netinet/tcp.h>
  78 #include <netinet/tcp_fsm.h>
  79 #include <netinet/tcp_seq.h>
  80 #include <netinet/tcp_timer.h>
  81 #include <netinet/tcp_var.h>
  82 #include <netinet6/tcp6_var.h>
  83 #include <netinet/tcpip.h>
  84 #ifdef TCPDEBUG
  85 #include <netinet/tcp_debug.h>
  86
  87 u_char tcp_saveipgen[40]; /* the size must be of max ip header, now IPv6 */
  88 struct tcphdr tcp_savetcp;
  89 #endif /* TCPDEBUG */
  90
  91 #ifdef FAST_IPSEC
  92 #include <netipsec/ipsec.h>
  93 #include <netipsec/ipsec6.h>
  94 #endif
  95
  96 #ifdef IPSEC
  97 #include <netinet6/ipsec.h>
  98 #include <netinet6/ipsec6.h>
  99 #include <netproto/key/key.h>
 100 #endif /*IPSEC*/
 101
 102 MALLOC_DEFINE(M_TSEGQ, "tseg_qent", "TCP segment queue entry");
 103
 104 static const int tcprexmtthresh = 3;
 105 tcp_cc  tcp_ccgen;
 106 static int log_in_vain = 0;
 107 SYSCTL_INT(_net_inet_tcp, OID_AUTO, log_in_vain, CTLFLAG_RW,
 108     &log_in_vain, 0, "Log all incoming TCP connections");
 109
 110 static int blackhole = 0;
 111 SYSCTL_INT(_net_inet_tcp, OID_AUTO, blackhole, CTLFLAG_RW,
 112     &blackhole, 0, "Do not send RST when dropping refused connections");
 113
 114 int tcp_delack_enabled = 1;
 115 SYSCTL_INT(_net_inet_tcp, OID_AUTO, delayed_ack, CTLFLAG_RW,
 116     &tcp_delack_enabled, 0,
 117     "Delay ACK to try and piggyback it onto a data packet");
 118
 119 #ifdef TCP_DROP_SYNFIN
 120 static int drop_synfin = 0;
 121 SYSCTL_INT(_net_inet_tcp, OID_AUTO, drop_synfin, CTLFLAG_RW,
 122     &drop_synfin, 0, "Drop TCP packets with SYN+FIN set");
 123 #endif
 124
 125 static int tcp_do_limitedtransmit = 1;
 126 SYSCTL_INT(_net_inet_tcp, OID_AUTO, limitedtransmit, CTLFLAG_RW,
 127     &tcp_do_limitedtransmit, 0, "Enable RFC 3042 (Limited Transmit)");
 128
 129 static int tcp_do_early_retransmit = 0;
 130 SYSCTL_INT(_net_inet_tcp, OID_AUTO, earlyretransmit, CTLFLAG_RW,
 131     &tcp_do_early_retransmit, 0, "Early retransmit");
 132
 133 static int tcp_do_rfc3390 = 1;
 134 SYSCTL_INT(_net_inet_tcp, OID_AUTO, rfc3390, CTLFLAG_RW,
 135     &tcp_do_rfc3390, 0,
 136     "Enable RFC 3390 (Increasing TCP's Initial Congestion Window)");
 137
 138 static int tcp_do_eifel_detect = 1;
 139 SYSCTL_INT(_net_inet_tcp, OID_AUTO, eifel, CTLFLAG_RW,
 140     &tcp_do_eifel_detect, 0, "Eifel detection algorithm (RFC 3522)");
 141
 142 SYSCTL_NODE(_net_inet_tcp, OID_AUTO, reass, CTLFLAG_RW, 0,
 143     "TCP Segment Reassembly Queue");
 144
 145 int tcp_reass_maxseg = 0;
 146 SYSCTL_INT(_net_inet_tcp_reass, OID_AUTO, maxsegments, CTLFLAG_RD,
 147     &tcp_reass_maxseg, 0,
 148     "Global maximum number of TCP Segments in Reassembly Queue");
 149
 150 int tcp_reass_qsize = 0;
 151 SYSCTL_INT(_net_inet_tcp_reass, OID_AUTO, cursegments, CTLFLAG_RD,
 152     &tcp_reass_qsize, 0,
 153     "Global number of TCP Segments currently in Reassembly Queue");
 154
 155 static int tcp_reass_overflows = 0;
 156 SYSCTL_INT(_net_inet_tcp_reass, OID_AUTO, overflows, CTLFLAG_RD,
 157     &tcp_reass_overflows, 0,
 158     "Global number of TCP Segment Reassembly Queue Overflows");
 159
 160 struct inpcbinfo tcbinfo[MAXCPU];
 161
 162 static void      tcp_dooptions(struct tcpopt *, u_char *, int, int);
 163 static void      tcp_pulloutofband(struct socket *,
 164                      struct tcphdr *, struct mbuf *, int);
 165 static int       tcp_reass(struct tcpcb *, struct tcphdr *, int *,
 166                      struct mbuf *);
 167 static void      tcp_xmit_timer(struct tcpcb *, int);
 168 static void      tcp_newreno_partial_ack(struct tcpcb *, struct tcphdr *);
 169
 170 /* Neighbor Discovery, Neighbor Unreachability Detection Upper layer hint. */
 171 #ifdef INET6
 172 #define ND6_HINT(tp) \
 173 do { \
 174         if ((tp) && (tp)->t_inpcb && \
 175             ((tp)->t_inpcb->inp_vflag & INP_IPV6) != 0 && \
 176             (tp)->t_inpcb->in6p_route.ro_rt) \
 177                 nd6_nud_hint((tp)->t_inpcb->in6p_route.ro_rt, NULL, 0); \
 178 } while (0)
 179 #else
 180 #define ND6_HINT(tp)
 181 #endif
 182
 183 /*
 184  * Indicate whether this ack should be delayed.  We can delay the ack if
 185  *      - delayed acks are enabled and
 186  *      - there is no delayed ack timer in progress and
 187  *      - our last ack wasn't a 0-sized window.  We never want to delay
 188  *        the ack that opens up a 0-sized window.
 189  */
 190 #define DELAY_ACK(tp) \
 191         (tcp_delack_enabled && !callout_pending(tp->tt_delack) && \
 192         (tp->t_flags & TF_RXWIN0SENT) == 0)
 193
 194 static int
 195 tcp_reass(tp, th, tlenp, m)
 196         struct tcpcb *tp;
 197         struct tcphdr *th;
 198         int *tlenp;
 199         struct mbuf *m;
 200 {
 201         struct tseg_qent *q;
 202         struct tseg_qent *p = NULL;
 203         struct tseg_qent *nq;
 204         struct tseg_qent *te;
 205         struct socket *so = tp->t_inpcb->inp_socket;
 206         int flags;
 207
 208         /*
 209          * Call with th==0 after become established to
 210          * force pre-ESTABLISHED data up to user socket.
 211          */
 212         if (th == 0)
 213                 goto present;
 214
 215         /*
 216          * Limit the number of segments in the reassembly queue to prevent
 217          * holding on to too many segments (and thus running out of mbufs).
 218          * Make sure to let the missing segment through which caused this
 219          * queue.  Always keep one global queue entry spare to be able to
 220          * process the missing segment.
 221          */
 222         if (th->th_seq != tp->rcv_nxt &&
 223             tcp_reass_qsize + 1 >= tcp_reass_maxseg) {
 224                 tcp_reass_overflows++;
 225                 tcpstat.tcps_rcvmemdrop++;
 226                 m_freem(m);
 227                 return (0);
 228         }
 229
 230         /* Allocate a new queue entry. */
 231         MALLOC(te, struct tseg_qent *, sizeof(struct tseg_qent), M_TSEGQ,
 232                M_INTWAIT | M_NULLOK);
 233         if (te == NULL) {
 234                 tcpstat.tcps_rcvmemdrop++;
 235                 m_freem(m);
 236                 return (0);
 237         }
 238         tcp_reass_qsize++;
 239
 240         /*
 241          * Find a segment which begins after this one does.
 242          */
 243         LIST_FOREACH(q, &tp->t_segq, tqe_q) {
 244                 if (SEQ_GT(q->tqe_th->th_seq, th->th_seq))
 245                         break;
 246                 p = q;
 247         }
 248
 249         /*
 250          * If there is a preceding segment, it may provide some of
 251          * our data already.  If so, drop the data from the incoming
 252          * segment.  If it provides all of our data, drop us.
 253          */
 254         if (p != NULL) {
 255                 int i;
 256                 /* conversion to int (in i) handles seq wraparound */
 257                 i = p->tqe_th->th_seq + p->tqe_len - th->th_seq;
 258                 if (i > 0) {
 259                         if (i >= *tlenp) {
 260                                 tcpstat.tcps_rcvduppack++;
 261                                 tcpstat.tcps_rcvdupbyte += *tlenp;
 262                                 m_freem(m);
 263                                 free(te, M_TSEGQ);
 264                                 tcp_reass_qsize--;
 265                                 /*
 266                                  * Try to present any queued data
 267                                  * at the left window edge to the user.
 268                                  * This is needed after the 3-WHS
 269                                  * completes.
 270                                  */
 271                                 goto present;   /* ??? */
 272                         }
 273                         m_adj(m, i);
 274                         *tlenp -= i;
 275                         th->th_seq += i;
 276                 }
 277         }
 278         tcpstat.tcps_rcvoopack++;
 279         tcpstat.tcps_rcvoobyte += *tlenp;
 280
 281         /*
 282          * While we overlap succeeding segments trim them or,
 283          * if they are completely covered, dequeue them.
 284          */
 285         while (q) {
 286                 int i = (th->th_seq + *tlenp) - q->tqe_th->th_seq;
 287                 if (i <= 0)
 288                         break;
 289                 if (i < q->tqe_len) {
 290                         q->tqe_th->th_seq += i;
 291                         q->tqe_len -= i;
 292                         m_adj(q->tqe_m, i);
 293                         break;
 294                 }
 295
 296                 nq = LIST_NEXT(q, tqe_q);
 297                 LIST_REMOVE(q, tqe_q);
 298                 m_freem(q->tqe_m);
 299                 free(q, M_TSEGQ);
 300                 tcp_reass_qsize--;
 301                 q = nq;
 302         }
 303
 304         /* Insert the new segment queue entry into place. */
 305         te->tqe_m = m;
 306         te->tqe_th = th;
 307         te->tqe_len = *tlenp;
 308
 309         if (p == NULL) {
 310                 LIST_INSERT_HEAD(&tp->t_segq, te, tqe_q);
 311         } else {
 312                 LIST_INSERT_AFTER(p, te, tqe_q);
 313         }
 314
 315 present:
 316         /*
 317          * Present data to user, advancing rcv_nxt through
 318          * completed sequence space.
 319          */
 320         if (!TCPS_HAVEESTABLISHED(tp->t_state))
 321                 return (0);
 322         q = LIST_FIRST(&tp->t_segq);
 323         if (!q || q->tqe_th->th_seq != tp->rcv_nxt)
 324                 return (0);
 325         do {
 326                 tp->rcv_nxt += q->tqe_len;
 327                 flags = q->tqe_th->th_flags & TH_FIN;
 328                 nq = LIST_NEXT(q, tqe_q);
 329                 LIST_REMOVE(q, tqe_q);
 330                 if (so->so_state & SS_CANTRCVMORE)
 331                         m_freem(q->tqe_m);
 332                 else
 333                         sbappend(&so->so_rcv, q->tqe_m);
 334                 free(q, M_TSEGQ);
 335                 tcp_reass_qsize--;
 336                 q = nq;
 337         } while (q && q->tqe_th->th_seq == tp->rcv_nxt);
 338         ND6_HINT(tp);
 339         sorwakeup(so);
 340         return (flags);
 341 }
 342
 343 /*
 344  * TCP input routine, follows pages 65-76 of the
 345  * protocol specification dated September, 1981 very closely.
 346  */
 347 #ifdef INET6
 348 int
 349 tcp6_input(mp, offp, proto)
 350         struct mbuf **mp;
 351         int *offp, proto;
 352 {
 353         struct mbuf *m = *mp;
 354         struct in6_ifaddr *ia6;
 355
 356         IP6_EXTHDR_CHECK(m, *offp, sizeof(struct tcphdr), IPPROTO_DONE);
 357
 358         /*
 359          * draft-itojun-ipv6-tcp-to-anycast
 360          * better place to put this in?
 361          */
 362         ia6 = ip6_getdstifaddr(m);
 363         if (ia6 && (ia6->ia6_flags & IN6_IFF_ANYCAST)) {
 364                 struct ip6_hdr *ip6;
 365
 366                 ip6 = mtod(m, struct ip6_hdr *);
 367                 icmp6_error(m, ICMP6_DST_UNREACH, ICMP6_DST_UNREACH_ADDR,
 368                             (caddr_t)&ip6->ip6_dst - (caddr_t)ip6);
 369                 return (IPPROTO_DONE);
 370         }
 371
 372         tcp_input(m, *offp, proto);
 373         return (IPPROTO_DONE);
 374 }
 375 #endif
 376
 377 void
 378 tcp_input(struct mbuf *m, ...)
 379 {
 380         __va_list ap;
 381         int off0, proto;
 382         struct tcphdr *th;
 383         struct ip *ip = NULL;
 384         struct ipovly *ipov;
 385         struct inpcb *inp = NULL;
 386         u_char *optp = NULL;
 387         int optlen = 0;
 388         int len, tlen, off;
 389         int drop_hdrlen;
 390         struct tcpcb *tp = NULL;
 391         int thflags;
 392         struct socket *so = 0;
 393         int todrop, acked, ourfinisacked, needoutput = 0;
 394         u_long tiwin;
 395         struct tcpopt to;               /* options in this segment */
 396         struct rmxp_tao *taop;          /* pointer to our TAO cache entry */
 397         struct rmxp_tao tao_noncached;  /* in case there's no cached entry */
 398         struct sockaddr_in *next_hop = NULL;
 399         int rstreason; /* For badport_bandlim accounting purposes */
 400         int cpu;
 401         struct ip6_hdr *ip6 = NULL;
 402 #ifdef INET6
 403         boolean_t isipv6;
 404 #else
 405         const boolean_t isipv6 = FALSE;
 406 #endif
 407 #ifdef TCPDEBUG
 408         short ostate = 0;
 409 #endif
 410
 411         __va_start(ap, m);
 412         off0 = __va_arg(ap, int);
 413         proto = __va_arg(ap, int);
 414         __va_end(ap);
 415
 416         tcpstat.tcps_rcvtotal++;
 417
 418         /* Grab info from and strip MT_TAG mbufs prepended to the chain. */
 419         while  (m->m_type == MT_TAG) {
 420                 if (m->_m_tag_id == PACKET_TAG_IPFORWARD)
 421                         next_hop = (struct sockaddr_in *)m->m_hdr.mh_data;
 422                 m = m->m_next;
 423         }
 424
 425 #ifdef INET6
 426         isipv6 = (mtod(m, struct ip *)->ip_v == 6) ? TRUE : FALSE;
 427 #endif
 428
 429         if (isipv6) {
 430                 /* IP6_EXTHDR_CHECK() is already done at tcp6_input() */
 431                 ip6 = mtod(m, struct ip6_hdr *);
 432                 tlen = sizeof(*ip6) + ntohs(ip6->ip6_plen) - off0;
 433                 if (in6_cksum(m, IPPROTO_TCP, off0, tlen)) {
 434                         tcpstat.tcps_rcvbadsum++;
 435                         goto drop;
 436                 }
 437                 th = (struct tcphdr *)((caddr_t)ip6 + off0);
 438
 439                 /*
 440                  * Be proactive about unspecified IPv6 address in source.
 441                  * As we use all-zero to indicate unbounded/unconnected pcb,
 442                  * unspecified IPv6 address can be used to confuse us.
 443                  *
 444                  * Note that packets with unspecified IPv6 destination is
 445                  * already dropped in ip6_input.
 446                  */
 447                 if (IN6_IS_ADDR_UNSPECIFIED(&ip6->ip6_src)) {
 448                         /* XXX stat */
 449                         goto drop;
 450                 }
 451         } else {
 452                 /*
 453                  * Get IP and TCP header together in first mbuf.
 454                  * Note: IP leaves IP header in first mbuf.
 455                  */
 456                 if (off0 > sizeof(struct ip)) {
 457                         ip_stripoptions(m);
 458                         off0 = sizeof(struct ip);
 459                 }
 460                 /* already checked and pulled up in ip_demux() */
 461                 KASSERT(m->m_len >= sizeof(struct tcpiphdr),
 462                     ("TCP header not in one mbuf"));
 463                 ip = mtod(m, struct ip *);
 464                 ipov = (struct ipovly *)ip;
 465                 th = (struct tcphdr *)((caddr_t)ip + off0);
 466                 tlen = ip->ip_len;
 467
 468                 if (m->m_pkthdr.csum_flags & CSUM_DATA_VALID) {
 469                         if (m->m_pkthdr.csum_flags & CSUM_PSEUDO_HDR)
 470                                 th->th_sum = m->m_pkthdr.csum_data;
 471                         else
 472                                 th->th_sum = in_pseudo(ip->ip_src.s_addr,
 473                                                 ip->ip_dst.s_addr,
 474                                                 htonl(m->m_pkthdr.csum_data +
 475                                                         ip->ip_len +
 476                                                         IPPROTO_TCP));
 477                         th->th_sum ^= 0xffff;
 478                 } else {
 479                         /*
 480                          * Checksum extended TCP header and data.
 481                          */
 482                         len = sizeof(struct ip) + tlen;
 483                         bzero(ipov->ih_x1, sizeof(ipov->ih_x1));
 484                         ipov->ih_len = (u_short)tlen;
 485                         ipov->ih_len = htons(ipov->ih_len);
 486                         th->th_sum = in_cksum(m, len);
 487                 }
 488                 if (th->th_sum) {
 489                         tcpstat.tcps_rcvbadsum++;
 490                         goto drop;
 491                 }
 492 #ifdef INET6
 493                 /* Re-initialization for later version check */
 494                 ip->ip_v = IPVERSION;
 495 #endif
 496         }
 497
 498         /*
 499          * Check that TCP offset makes sense,
 500          * pull out TCP options and adjust length.              XXX
 501          */
 502         off = th->th_off << 2;
 503         /* already checked and pulled up in ip_demux() */
 504         KASSERT(off >= sizeof(struct tcphdr) && off <= tlen,
 505             ("bad TCP data offset"));
 506         tlen -= off;    /* tlen is used instead of ti->ti_len */
 507         if (off > sizeof(struct tcphdr)) {
 508                 if (isipv6) {
 509                         IP6_EXTHDR_CHECK(m, off0, off, );
 510                         ip6 = mtod(m, struct ip6_hdr *);
 511                         th = (struct tcphdr *)((caddr_t)ip6 + off0);
 512                 } else {
 513                         /* already pulled up in ip_demux() */
 514                         KASSERT(m->m_len >= sizeof(struct ip) + off,
 515                             ("TCP header and options not in one mbuf"));
 516                 }
 517                 optlen = off - sizeof(struct tcphdr);
 518                 optp = (u_char *)(th + 1);
 519         }
 520         thflags = th->th_flags;
 521
 522 #ifdef TCP_DROP_SYNFIN
 523         /*
 524          * If the drop_synfin option is enabled, drop all packets with
 525          * both the SYN and FIN bits set. This prevents e.g. nmap from
 526          * identifying the TCP/IP stack.
 527          *
 528          * This is a violation of the TCP specification.
 529          */
 530         if (drop_synfin && (thflags & (TH_SYN|TH_FIN)) == (TH_SYN|TH_FIN))
 531                 goto drop;
 532 #endif
 533
 534         /*
 535          * Convert TCP protocol specific fields to host format.
 536          */
 537         th->th_seq = ntohl(th->th_seq);
 538         th->th_ack = ntohl(th->th_ack);
 539         th->th_win = ntohs(th->th_win);
 540         th->th_urp = ntohs(th->th_urp);
 541
 542         /*
 543          * Delay dropping TCP, IP headers, IPv6 ext headers, and TCP options,
 544          * until after ip6_savecontrol() is called and before other functions
 545          * which don't want those proto headers.
 546          * Because ip6_savecontrol() is going to parse the mbuf to
 547          * search for data to be passed up to user-land, it wants mbuf
 548          * parameters to be unchanged.
 549          * XXX: the call of ip6_savecontrol() has been obsoleted based on
 550          * latest version of the advanced API (20020110).
 551          */
 552         drop_hdrlen = off0 + off;
 553
 554         /*
 555          * Locate pcb for segment.
 556          */
 557 findpcb:
 558         /* IPFIREWALL_FORWARD section */
 559         if (next_hop != NULL && !isipv6) {  /* IPv6 support is not there yet */
 560                 /*
 561                  * Transparently forwarded. Pretend to be the destination.
 562                  * already got one like this?
 563                  */
 564                 cpu = mycpu->gd_cpuid;
 565                 inp = in_pcblookup_hash(&tcbinfo[cpu],
 566                                         ip->ip_src, th->th_sport,
 567                                         ip->ip_dst, th->th_dport,
 568                                         0, m->m_pkthdr.rcvif);
 569                 if (!inp) {
 570                         /*
 571                          * It's new.  Try to find the ambushing socket.
 572                          */
 573
 574                         /*
 575                          * The rest of the ipfw code stores the port in
 576                          * host order.  XXX
 577                          * (The IP address is still in network order.)
 578                          */
 579                         in_port_t dport = next_hop->sin_port ?
 580                                                 htons(next_hop->sin_port) :
 581                                                 th->th_dport;
 582
 583                         cpu = tcp_addrcpu(ip->ip_src.s_addr, th->th_sport,
 584                                           next_hop->sin_addr.s_addr, dport);
 585                         inp = in_pcblookup_hash(&tcbinfo[cpu],
 586                                                 ip->ip_src, th->th_sport,
 587                                                 next_hop->sin_addr, dport,
 588                                                 1, m->m_pkthdr.rcvif);
 589                 }
 590         } else {
 591                 if (isipv6) {
 592                         inp = in6_pcblookup_hash(&tcbinfo[0],
 593                                                  &ip6->ip6_src, th->th_sport,
 594                                                  &ip6->ip6_dst, th->th_dport,
 595                                                  1, m->m_pkthdr.rcvif);
 596                 } else {
 597                         cpu = mycpu->gd_cpuid;
 598                         inp = in_pcblookup_hash(&tcbinfo[cpu],
 599                                                 ip->ip_src, th->th_sport,
 600                                                 ip->ip_dst, th->th_dport,
 601                                                 1, m->m_pkthdr.rcvif);
 602                 }
 603       }
 604
 605 #ifdef IPSEC
 606         if (isipv6) {
 607                 if (inp != NULL && ipsec6_in_reject_so(m, inp->inp_socket)) {
 608                         ipsec6stat.in_polvio++;
 609                         goto drop;
 610                 }
 611         } else {
 612                 if (inp != NULL && ipsec4_in_reject_so(m, inp->inp_socket)) {
 613                         ipsecstat.in_polvio++;
 614                         goto drop;
 615                 }
 616         }
 617 #endif
 618 #ifdef FAST_IPSEC
 619         if (isipv6) {
 620                 if (inp != NULL && ipsec6_in_reject(m, inp)) {
 621                         goto drop;
 622                 }
 623         } else {
 624                 if (inp != NULL && ipsec4_in_reject(m, inp)) {
 625                         goto drop;
 626                 }
 627         }
 628 #endif
 629
 630         /*
 631          * If the state is CLOSED (i.e., TCB does not exist) then
 632          * all data in the incoming segment is discarded.
 633          * If the TCB exists but is in CLOSED state, it is embryonic,
 634          * but should either do a listen or a connect soon.
 635          */
 636         if (inp == NULL) {
 637                 if (log_in_vain) {
 638 #ifdef INET6
 639                         char dbuf[INET6_ADDRSTRLEN+2], sbuf[INET6_ADDRSTRLEN+2];
 640 #else
 641                         char dbuf[4 * sizeof "123"], sbuf[4 * sizeof "123"];
 642 #endif
 643                         if (isipv6) {
 644                                 strcpy(dbuf, "[");
 645                                 strcpy(sbuf, "[");
 646                                 strcat(dbuf, ip6_sprintf(&ip6->ip6_dst));
 647                                 strcat(sbuf, ip6_sprintf(&ip6->ip6_src));
 648                                 strcat(dbuf, "]");
 649                                 strcat(sbuf, "]");
 650                         } else {
 651                                 strcpy(dbuf, inet_ntoa(ip->ip_dst));
 652                                 strcpy(sbuf, inet_ntoa(ip->ip_src));
 653                         }
 654                         switch (log_in_vain) {
 655                         case 1:
 656                                 if ((thflags & TH_SYN) == 0)
 657                                         break;
 658                         case 2:
 659                                 log(LOG_INFO,
 660                                     "Connection attempt to TCP %s:%d "
 661                                     "from %s:%d flags:0x%02x\n",
 662                                     dbuf, ntohs(th->th_dport), sbuf,
 663                                     ntohs(th->th_sport), thflags);
 664                                 break;
 665                         default:
 666                                 break;
 667                         }
 668                 }
 669                 if (blackhole) {
 670                         switch (blackhole) {
 671                         case 1:
 672                                 if (thflags & TH_SYN)
 673                                         goto drop;
 674                                 break;
 675                         case 2:
 676                                 goto drop;
 677                         default:
 678                                 goto drop;
 679                         }
 680                 }
 681                 rstreason = BANDLIM_RST_CLOSEDPORT;
 682                 goto dropwithreset;
 683         }
 684         tp = intotcpcb(inp);
 685         if (tp == NULL) {
 686                 rstreason = BANDLIM_RST_CLOSEDPORT;
 687                 goto dropwithreset;
 688         }
 689         if (tp->t_state == TCPS_CLOSED)
 690                 goto drop;
 691
 692         /* Unscale the window into a 32-bit value. */
 693         if ((thflags & TH_SYN) == 0)
 694                 tiwin = th->th_win << tp->snd_scale;
 695         else
 696                 tiwin = th->th_win;
 697
 698         so = inp->inp_socket;
 699
 700 #ifdef TCPDEBUG
 701         if (so->so_options & SO_DEBUG) {
 702                 ostate = tp->t_state;
 703                 if (isipv6)
 704                         bcopy((char *)ip6, (char *)tcp_saveipgen, sizeof(*ip6));
 705                 else
 706                         bcopy((char *)ip, (char *)tcp_saveipgen, sizeof(*ip));
 707                 tcp_savetcp = *th;
 708         }
 709 #endif
 710
 711         bzero((char *)&to, sizeof(to));
 712
 713         if (so->so_options & SO_ACCEPTCONN) {
 714                 struct in_conninfo inc;
 715
 716 #ifdef INET6
 717                 inc.inc_isipv6 = (isipv6 == TRUE);
 718 #endif
 719                 if (isipv6) {
 720                         inc.inc6_faddr = ip6->ip6_src;
 721                         inc.inc6_laddr = ip6->ip6_dst;
 722                         inc.inc6_route.ro_rt = NULL;            /* XXX */
 723                 } else {
 724                         inc.inc_faddr = ip->ip_src;
 725                         inc.inc_laddr = ip->ip_dst;
 726                         inc.inc_route.ro_rt = NULL;             /* XXX */
 727                 }
 728                 inc.inc_fport = th->th_sport;
 729                 inc.inc_lport = th->th_dport;
 730
 731                 /*
 732                  * If the state is LISTEN then ignore segment if it contains
 733                  * a RST.  If the segment contains an ACK then it is bad and
 734                  * send a RST.  If it does not contain a SYN then it is not
 735                  * interesting; drop it.
 736                  *
 737                  * If the state is SYN_RECEIVED (syncache) and seg contains
 738                  * an ACK, but not for our SYN/ACK, send a RST.  If the seg
 739                  * contains a RST, check the sequence number to see if it
 740                  * is a valid reset segment.
 741                  */
 742                 if ((thflags & (TH_RST|TH_ACK|TH_SYN)) != TH_SYN) {
 743                         if ((thflags & (TH_RST|TH_ACK|TH_SYN)) == TH_ACK) {
 744                                 if (!syncache_expand(&inc, th, &so, m)) {
 745                                         /*
 746                                          * No syncache entry, or ACK was not
 747                                          * for our SYN/ACK.  Send a RST.
 748                                          */
 749                                         tcpstat.tcps_badsyn++;
 750                                         rstreason = BANDLIM_RST_OPENPORT;
 751                                         goto dropwithreset;
 752                                 }
 753                                 if (so == NULL)
 754                                         /*
 755                                          * Could not complete 3-way handshake,
 756                                          * connection is being closed down, and
 757                                          * syncache will free mbuf.
 758                                          */
 759                                         return;
 760                                 /*
 761                                  * Socket is created in state SYN_RECEIVED.
 762                                  * Continue processing segment.
 763                                  */
 764                                 inp = sotoinpcb(so);
 765                                 tp = intotcpcb(inp);
 766                                 /*
 767                                  * This is what would have happened in
 768                                  * tcp_output() when the SYN,ACK was sent.
 769                                  */
 770                                 tp->snd_up = tp->snd_una;
 771                                 tp->snd_max = tp->snd_nxt = tp->iss + 1;
 772                                 tp->last_ack_sent = tp->rcv_nxt;
 773 /*
 774  * XXX possible bug - it doesn't appear that tp->snd_wnd is unscaled
 775  * until the _second_ ACK is received:
 776  *    rcv SYN (set wscale opts)  --> send SYN/ACK, set snd_wnd = window.
 777  *    rcv ACK, calculate tiwin --> process SYN_RECEIVED, determine wscale,
 778  *        move to ESTAB, set snd_wnd to tiwin.
 779  */
 780                                 tp->snd_wnd = tiwin;    /* unscaled */
 781                                 goto after_listen;
 782                         }
 783                         if (thflags & TH_RST) {
 784                                 syncache_chkrst(&inc, th);
 785                                 goto drop;
 786                         }
 787                         if (thflags & TH_ACK) {
 788                                 syncache_badack(&inc);
 789                                 tcpstat.tcps_badsyn++;
 790                                 rstreason = BANDLIM_RST_OPENPORT;
 791                                 goto dropwithreset;
 792                         }
 793                         goto drop;
 794                 }
 795
 796                 /*
 797                  * Segment's flags are (SYN) or (SYN|FIN).
 798                  */
 799 #ifdef INET6
 800                 /*
 801                  * If deprecated address is forbidden,
 802                  * we do not accept SYN to deprecated interface
 803                  * address to prevent any new inbound connection from
 804                  * getting established.
 805                  * When we do not accept SYN, we send a TCP RST,
 806                  * with deprecated source address (instead of dropping
 807                  * it).  We compromise it as it is much better for peer
 808                  * to send a RST, and RST will be the final packet
 809                  * for the exchange.
 810                  *
 811                  * If we do not forbid deprecated addresses, we accept
 812                  * the SYN packet.  RFC2462 does not suggest dropping
 813                  * SYN in this case.
 814                  * If we decipher RFC2462 5.5.4, it says like this:
 815                  * 1. use of deprecated addr with existing
 816                  *    communication is okay - "SHOULD continue to be
 817                  *    used"
 818                  * 2. use of it with new communication:
 819                  *   (2a) "SHOULD NOT be used if alternate address
 820                  *        with sufficient scope is available"
 821                  *   (2b) nothing mentioned otherwise.
 822                  * Here we fall into (2b) case as we have no choice in
 823                  * our source address selection - we must obey the peer.
 824                  *
 825                  * The wording in RFC2462 is confusing, and there are
 826                  * multiple description text for deprecated address
 827                  * handling - worse, they are not exactly the same.
 828                  * I believe 5.5.4 is the best one, so we follow 5.5.4.
 829                  */
 830                 if (isipv6 && !ip6_use_deprecated) {
 831                         struct in6_ifaddr *ia6;
 832
 833                         if ((ia6 = ip6_getdstifaddr(m)) &&
 834                             (ia6->ia6_flags & IN6_IFF_DEPRECATED)) {
 835                                 tp = NULL;
 836                                 rstreason = BANDLIM_RST_OPENPORT;
 837                                 goto dropwithreset;
 838                         }
 839                 }
 840 #endif
 841                 /*
 842                  * If it is from this socket, drop it, it must be forged.
 843                  * Don't bother responding if the destination was a broadcast.
 844                  */
 845                 if (th->th_dport == th->th_sport) {
 846                         if (isipv6) {
 847                                 if (IN6_ARE_ADDR_EQUAL(&ip6->ip6_dst,
 848                                                        &ip6->ip6_src))
 849                                         goto drop;
 850                         } else {
 851                                 if (ip->ip_dst.s_addr == ip->ip_src.s_addr)
 852                                         goto drop;
 853                         }
 854                 }
 855                 /*
 856                  * RFC1122 4.2.3.10, p. 104: discard bcast/mcast SYN
 857                  *
 858                  * Note that it is quite possible to receive unicast
 859                  * link-layer packets with a broadcast IP address. Use
 860                  * in_broadcast() to find them.
 861                  */
 862                 if (m->m_flags & (M_BCAST|M_MCAST))
 863                         goto drop;
 864                 if (isipv6) {
 865                         if (IN6_IS_ADDR_MULTICAST(&ip6->ip6_dst) ||
 866                             IN6_IS_ADDR_MULTICAST(&ip6->ip6_src))
 867                                 goto drop;
 868                 } else {
 869                         if (IN_MULTICAST(ntohl(ip->ip_dst.s_addr)) ||
 870                             IN_MULTICAST(ntohl(ip->ip_src.s_addr)) ||
 871                             ip->ip_src.s_addr == htonl(INADDR_BROADCAST) ||
 872                             in_broadcast(ip->ip_dst, m->m_pkthdr.rcvif))
 873                                 goto drop;
 874                 }
 875                 /*
 876                  * SYN appears to be valid; create compressed TCP state
 877                  * for syncache, or perform t/tcp connection.
 878                  */
 879                 if (so->so_qlen <= so->so_qlimit) {
 880                         tcp_dooptions(&to, optp, optlen, 1);
 881                         if (!syncache_add(&inc, &to, th, &so, m))
 882                                 goto drop;
 883                         if (so == NULL)
 884                                 /*
 885                                  * Entry added to syncache, mbuf used to
 886                                  * send SYN,ACK packet.
 887                                  */
 888                                 return;
 889                         /*
 890                          * Segment passed TAO tests.
 891                          */
 892                         inp = sotoinpcb(so);
 893                         tp = intotcpcb(inp);
 894                         tp->snd_wnd = tiwin;
 895                         tp->t_starttime = ticks;
 896                         tp->t_state = TCPS_ESTABLISHED;
 897
 898                         /*
 899                          * If there is a FIN, or if there is data and the
 900                          * connection is local, then delay SYN,ACK(SYN) in
 901                          * the hope of piggy-backing it on a response
 902                          * segment.  Otherwise must send ACK now in case
 903                          * the other side is slow starting.
 904                          */
 905                         if (DELAY_ACK(tp) &&
 906                             ((thflags & TH_FIN) ||
 907                              (tlen != 0 &&
 908                               ((isipv6 && in6_localaddr(&inp->in6p_faddr)) ||
 909                                (!isipv6 && in_localaddr(inp->inp_faddr)))))) {
 910                                 callout_reset(tp->tt_delack, tcp_delacktime,
 911                                                 tcp_timer_delack, tp);
 912                                 tp->t_flags |= TF_NEEDSYN;
 913                         } else
 914                                 tp->t_flags |= (TF_ACKNOW | TF_NEEDSYN);
 915
 916                         tcpstat.tcps_connects++;
 917                         soisconnected(so);
 918                         goto trimthenstep6;
 919                 }
 920                 goto drop;
 921         }
 922 after_listen:
 923
 924 /* XXX temp debugging */
 925         /* should not happen - syncache should pick up these connections */
 926         if (tp->t_state == TCPS_LISTEN)
 927                 panic("tcp_input: TCPS_LISTEN");
 928
 929         /*
 930          * Segment received on connection.
 931          * Reset idle time and keep-alive timer.
 932          */
 933         tp->t_rcvtime = ticks;
 934         if (TCPS_HAVEESTABLISHED(tp->t_state))
 935                 callout_reset(tp->tt_keep, tcp_keepidle, tcp_timer_keep, tp);
 936
 937         /*
 938          * Process options.
 939          * XXX this is tradtitional behavior, may need to be cleaned up.
 940          */
 941         tcp_dooptions(&to, optp, optlen, thflags & TH_SYN);
 942         if (thflags & TH_SYN) {
 943                 if (to.to_flags & TOF_SCALE) {
 944                         tp->t_flags |= TF_RCVD_SCALE;
 945                         tp->requested_s_scale = to.to_requested_s_scale;
 946                 }
 947                 if (to.to_flags & TOF_TS) {
 948                         tp->t_flags |= TF_RCVD_TSTMP;
 949                         tp->ts_recent = to.to_tsval;
 950                         tp->ts_recent_age = ticks;
 951                 }
 952                 if (to.to_flags & (TOF_CC|TOF_CCNEW))
 953                         tp->t_flags |= TF_RCVD_CC;
 954                 if (to.to_flags & TOF_MSS)
 955                         tcp_mss(tp, to.to_mss);
 956         }
 957
 958         /*
 959          * Header prediction: check for the two common cases
 960          * of a uni-directional data xfer.  If the packet has
 961          * no control flags, is in-sequence, the window didn't
 962          * change and we're not retransmitting, it's a
 963          * candidate.  If the length is zero and the ack moved
 964          * forward, we're the sender side of the xfer.  Just
 965          * free the data acked & wake any higher level process
 966          * that was blocked waiting for space.  If the length
 967          * is non-zero and the ack didn't move, we're the
 968          * receiver side.  If we're getting packets in-order
 969          * (the reassembly queue is empty), add the data to
 970          * the socket buffer and note that we need a delayed ack.
 971          * Make sure that the hidden state-flags are also off.
 972          * Since we check for TCPS_ESTABLISHED above, it can only
 973          * be TH_NEEDSYN.
 974          */
 975         if (tp->t_state == TCPS_ESTABLISHED &&
 976             (thflags & (TH_SYN|TH_FIN|TH_RST|TH_URG|TH_ACK)) == TH_ACK &&
 977             ((tp->t_flags & (TF_NEEDSYN|TF_NEEDFIN)) == 0) &&
 978             ((to.to_flags & TOF_TS) == 0 ||
 979              TSTMP_GEQ(to.to_tsval, tp->ts_recent)) &&
 980             /*
 981              * Using the CC option is compulsory if once started:
 982              *   the segment is OK if no T/TCP was negotiated or
 983              *   if the segment has a CC option equal to CCrecv
 984              */
 985             ((tp->t_flags & (TF_REQ_CC|TF_RCVD_CC)) != (TF_REQ_CC|TF_RCVD_CC) ||
 986              ((to.to_flags & TOF_CC) != 0 && to.to_cc == tp->cc_recv)) &&
 987             th->th_seq == tp->rcv_nxt &&
 988             tiwin && tiwin == tp->snd_wnd &&
 989             tp->snd_nxt == tp->snd_max) {
 990
 991                 /*
 992                  * If last ACK falls within this segment's sequence numbers,
 993                  * record the timestamp.
 994                  * NOTE that the test is modified according to the latest
 995                  * proposal of the tcplw@cray.com list (Braden 1993/04/26).
 996                  */
 997                 if ((to.to_flags & TOF_TS) != 0 &&
 998                     SEQ_LEQ(th->th_seq, tp->last_ack_sent)) {
 999                         tp->ts_recent_age = ticks;
1000                         tp->ts_recent = to.to_tsval;
1001                 }
1002
1003                 if (tlen == 0) {
1004                         if (SEQ_GT(th->th_ack, tp->snd_una) &&
1005                             SEQ_LEQ(th->th_ack, tp->snd_max) &&
1006                             tp->snd_cwnd >= tp->snd_wnd &&
1007                             ((!tcp_do_newreno &&
1008                               tp->t_dupacks < tcprexmtthresh) ||
1009                              (tcp_do_newreno && !IN_FASTRECOVERY(tp)))) {
1010                                 /*
1011                                  * this is a pure ack for outstanding data.
1012                                  */
1013                                 ++tcpstat.tcps_predack;
1014                                 /*
1015                                  * "bad retransmit" recovery
1016                                  *
1017                                  * If Eifel detection applies, then
1018                                  * it is deterministic, so use it
1019                                  * unconditionally over the old heuristic.
1020                                  * Otherwise, fall back to the old heuristic.
1021                                  */
1022                                 if (tcp_do_eifel_detect &&
1023                                     (to.to_flags & TOF_TS) && to.to_tsecr &&
1024                                     (tp->t_flags & TF_FIRSTACCACK)) {
1025                                         /* Eifel detection applicable. */
1026                                         if (to.to_tsecr < tp->t_rexmtTS) {
1027                                                 tcp_revert_congestion_state(tp);
1028                                                 ++tcpstat.tcps_eifeldetected;
1029                                         }
1030                                 } else if (tp->t_rxtshift == 1 &&
1031                                            ticks < tp->t_badrxtwin) {
1032                                         tcp_revert_congestion_state(tp);
1033                                         ++tcpstat.tcps_rttdetected;
1034                                 }
1035                                 tp->t_flags &= ~(TF_FIRSTACCACK |
1036                                                  TF_FASTREXMT | TF_EARLYREXMT);
1037                                 /*
1038                                  * Recalculate the retransmit timer / rtt.
1039                                  *
1040                                  * Some machines (certain windows boxes)
1041                                  * send broken timestamp replies during the
1042                                  * SYN+ACK phase, ignore timestamps of 0.
1043                                  */
1044                                 if ((to.to_flags & TOF_TS) != 0 &&
1045                                     to.to_tsecr) {
1046                                         tcp_xmit_timer(tp,
1047                                             ticks - to.to_tsecr + 1);
1048                                 } else if (tp->t_rtttime &&
1049                                             SEQ_GT(th->th_ack, tp->t_rtseq)) {
1050                                         tcp_xmit_timer(tp,
1051                                                        ticks - tp->t_rtttime);
1052                                 }
1053                                 tcp_xmit_bandwidth_limit(tp, th->th_ack);
1054                                 acked = th->th_ack - tp->snd_una;
1055                                 tcpstat.tcps_rcvackpack++;
1056                                 tcpstat.tcps_rcvackbyte += acked;
1057                                 sbdrop(&so->so_snd, acked);
1058                                 tp->snd_recover = th->th_ack - 1;
1059                                 tp->snd_una = th->th_ack;
1060                                 tp->t_dupacks = 0;
1061                                 m_freem(m);
1062                                 ND6_HINT(tp); /* some progress has been done */
1063
1064                                 /*
1065                                  * If all outstanding data are acked, stop
1066                                  * retransmit timer, otherwise restart timer
1067                                  * using current (possibly backed-off) value.
1068                                  * If process is waiting for space,
1069                                  * wakeup/selwakeup/signal.  If data
1070                                  * are ready to send, let tcp_output
1071                                  * decide between more output or persist.
1072                                  */
1073                                 if (tp->snd_una == tp->snd_max)
1074                                         callout_stop(tp->tt_rexmt);
1075                                 else if (!callout_active(tp->tt_persist))
1076                                         callout_reset(tp->tt_rexmt,
1077                                                       tp->t_rxtcur,
1078                                                       tcp_timer_rexmt, tp);
1079
1080                                 sowwakeup(so);
1081                                 if (so->so_snd.sb_cc)
1082                                         (void) tcp_output(tp);
1083                                 return;
1084                         }
1085                 } else if (th->th_ack == tp->snd_una &&
1086                     LIST_EMPTY(&tp->t_segq) &&
1087                     tlen <= sbspace(&so->so_rcv)) {
1088                         /*
1089                          * this is a pure, in-sequence data packet
1090                          * with nothing on the reassembly queue and
1091                          * we have enough buffer space to take it.
1092                          */
1093                         ++tcpstat.tcps_preddat;
1094                         tp->rcv_nxt += tlen;
1095                         tcpstat.tcps_rcvpack++;
1096                         tcpstat.tcps_rcvbyte += tlen;
1097                         ND6_HINT(tp);   /* some progress has been done */
1098                         /*
1099                          * Add data to socket buffer.
1100                          */
1101                         if (so->so_state & SS_CANTRCVMORE) {
1102                                 m_freem(m);
1103                         } else {
1104                                 m_adj(m, drop_hdrlen);  /* delayed header drop */
1105                                 sbappend(&so->so_rcv, m);
1106                         }
1107                         sorwakeup(so);
1108                         if (DELAY_ACK(tp)) {
1109                                 callout_reset(tp->tt_delack, tcp_delacktime,
1110                                     tcp_timer_delack, tp);
1111                         } else {
1112                                 tp->t_flags |= TF_ACKNOW;
1113                                 tcp_output(tp);
1114                         }
1115                         return;
1116                 }
1117         }
1118
1119         /*
1120          * Calculate amount of space in receive window,
1121          * and then do TCP input processing.
1122          * Receive window is amount of space in rcv queue,
1123          * but not less than advertised window.
1124          */
1125         { int win;
1126
1127         win = sbspace(&so->so_rcv);
1128         if (win < 0)
1129                 win = 0;
1130         tp->rcv_wnd = imax(win, (int)(tp->rcv_adv - tp->rcv_nxt));
1131         }
1132
1133         switch (tp->t_state) {
1134
1135         /*
1136          * If the state is SYN_RECEIVED:
1137          *      if seg contains an ACK, but not for our SYN/ACK, send a RST.
1138          */
1139         case TCPS_SYN_RECEIVED:
1140                 if ((thflags & TH_ACK) &&
1141                     (SEQ_LEQ(th->th_ack, tp->snd_una) ||
1142                      SEQ_GT(th->th_ack, tp->snd_max))) {
1143                                 rstreason = BANDLIM_RST_OPENPORT;
1144                                 goto dropwithreset;
1145                 }
1146                 break;
1147
1148         /*
1149          * If the state is SYN_SENT:
1150          *      if seg contains an ACK, but not for our SYN, drop the input.
1151          *      if seg contains a RST, then drop the connection.
1152          *      if seg does not contain SYN, then drop it.
1153          * Otherwise this is an acceptable SYN segment
1154          *      initialize tp->rcv_nxt and tp->irs
1155          *      if seg contains ack then advance tp->snd_una
1156          *      if SYN has been acked change to ESTABLISHED else SYN_RCVD state
1157          *      arrange for segment to be acked (eventually)
1158          *      continue processing rest of data/controls, beginning with URG
1159          */
1160         case TCPS_SYN_SENT:
1161                 if ((taop = tcp_gettaocache(&inp->inp_inc)) == NULL) {
1162                         taop = &tao_noncached;
1163                         bzero(taop, sizeof(*taop));
1164                 }
1165
1166                 if ((thflags & TH_ACK) &&
1167                     (SEQ_LEQ(th->th_ack, tp->iss) ||
1168                      SEQ_GT(th->th_ack, tp->snd_max))) {
1169                         /*
1170                          * If we have a cached CCsent for the remote host,
1171                          * hence we haven't just crashed and restarted,
1172                          * do not send a RST.  This may be a retransmission
1173                          * from the other side after our earlier ACK was lost.
1174                          * Our new SYN, when it arrives, will serve as the
1175                          * needed ACK.
1176                          */
1177                         if (taop->tao_ccsent != 0)
1178                                 goto drop;
1179                         else {
1180                                 rstreason = BANDLIM_UNLIMITED;
1181                                 goto dropwithreset;
1182                         }
1183                 }
1184                 if (thflags & TH_RST) {
1185                         if (thflags & TH_ACK)
1186                                 tp = tcp_drop(tp, ECONNREFUSED);
1187                         goto drop;
1188                 }
1189                 if ((thflags & TH_SYN) == 0)
1190                         goto drop;
1191                 tp->snd_wnd = th->th_win;       /* initial send window */
1192                 tp->cc_recv = to.to_cc;         /* foreign CC */
1193
1194                 tp->irs = th->th_seq;
1195                 tcp_rcvseqinit(tp);
1196                 if (thflags & TH_ACK) {
1197                         /*
1198                          * Our SYN was acked.  If segment contains CC.ECHO
1199                          * option, check it to make sure this segment really
1200                          * matches our SYN.  If not, just drop it as old
1201                          * duplicate, but send an RST if we're still playing
1202                          * by the old rules.  If no CC.ECHO option, make sure
1203                          * we don't get fooled into using T/TCP.
1204                          */
1205                         if (to.to_flags & TOF_CCECHO) {
1206                                 if (tp->cc_send != to.to_ccecho) {
1207                                         if (taop->tao_ccsent != 0)
1208                                                 goto drop;
1209                                         else {
1210                                                 rstreason = BANDLIM_UNLIMITED;
1211                                                 goto dropwithreset;
1212                                         }
1213                                 }
1214                         } else
1215                                 tp->t_flags &= ~TF_RCVD_CC;
1216                         tcpstat.tcps_connects++;
1217                         soisconnected(so);
1218                         /* Do window scaling on this connection? */
1219                         if ((tp->t_flags & (TF_RCVD_SCALE|TF_REQ_SCALE)) ==
1220                                 (TF_RCVD_SCALE|TF_REQ_SCALE)) {
1221                                 tp->snd_scale = tp->requested_s_scale;
1222                                 tp->rcv_scale = tp->request_r_scale;
1223                         }
1224                         /* Segment is acceptable, update cache if undefined. */
1225                         if (taop->tao_ccsent == 0)
1226                                 taop->tao_ccsent = to.to_ccecho;
1227
1228                         tp->rcv_adv += tp->rcv_wnd;
1229                         tp->snd_una++;          /* SYN is acked */
1230                         /*
1231                          * If there's data, delay ACK; if there's also a FIN
1232                          * ACKNOW will be turned on later.
1233                          */
1234                         if (DELAY_ACK(tp) && tlen != 0)
1235                                 callout_reset(tp->tt_delack, tcp_delacktime,
1236                                     tcp_timer_delack, tp);
1237                         else
1238                                 tp->t_flags |= TF_ACKNOW;
1239                         /*
1240                          * Received <SYN,ACK> in SYN_SENT[*] state.
1241                          * Transitions:
1242                          *      SYN_SENT  --> ESTABLISHED
1243                          *      SYN_SENT* --> FIN_WAIT_1
1244                          */
1245                         tp->t_starttime = ticks;
1246                         if (tp->t_flags & TF_NEEDFIN) {
1247                                 tp->t_state = TCPS_FIN_WAIT_1;
1248                                 tp->t_flags &= ~TF_NEEDFIN;
1249                                 thflags &= ~TH_SYN;
1250                         } else {
1251                                 tp->t_state = TCPS_ESTABLISHED;
1252                                 callout_reset(tp->tt_keep, tcp_keepidle,
1253                                               tcp_timer_keep, tp);
1254                         }
1255                 } else {
1256                         /*
1257                          * Received initial SYN in SYN-SENT[*] state =>
1258                          * simultaneous open.  If segment contains CC option
1259                          * and there is a cached CC, apply TAO test.
1260                          * If it succeeds, connection is * half-synchronized.
1261                          * Otherwise, do 3-way handshake:
1262                          *        SYN-SENT -> SYN-RECEIVED
1263                          *        SYN-SENT* -> SYN-RECEIVED*
1264                          * If there was no CC option, clear cached CC value.
1265                          */
1266                         tp->t_flags |= TF_ACKNOW;
1267                         callout_stop(tp->tt_rexmt);
1268                         if (to.to_flags & TOF_CC) {
1269                                 if (taop->tao_cc != 0 &&
1270                                     CC_GT(to.to_cc, taop->tao_cc)) {
1271                                         /*
1272                                          * update cache and make transition:
1273                                          *        SYN-SENT -> ESTABLISHED*
1274                                          *        SYN-SENT* -> FIN-WAIT-1*
1275                                          */
1276                                         taop->tao_cc = to.to_cc;
1277                                         tp->t_starttime = ticks;
1278                                         if (tp->t_flags & TF_NEEDFIN) {
1279                                                 tp->t_state = TCPS_FIN_WAIT_1;
1280                                                 tp->t_flags &= ~TF_NEEDFIN;
1281                                         } else {
1282                                                 tp->t_state = TCPS_ESTABLISHED;
1283                                                 callout_reset(tp->tt_keep,
1284                                                               tcp_keepidle,
1285                                                               tcp_timer_keep,
1286                                                               tp);
1287                                         }
1288                                         tp->t_flags |= TF_NEEDSYN;
1289                                 } else
1290                                         tp->t_state = TCPS_SYN_RECEIVED;
1291                         } else {
1292                                 /* CC.NEW or no option => invalidate cache */
1293                                 taop->tao_cc = 0;
1294                                 tp->t_state = TCPS_SYN_RECEIVED;
1295                         }
1296                 }
1297
1298 trimthenstep6:
1299                 /*
1300                  * Advance th->th_seq to correspond to first data byte.
1301                  * If data, trim to stay within window,
1302                  * dropping FIN if necessary.
1303                  */
1304                 th->th_seq++;
1305                 if (tlen > tp->rcv_wnd) {
1306                         todrop = tlen - tp->rcv_wnd;
1307                         m_adj(m, -todrop);
1308                         tlen = tp->rcv_wnd;
1309                         thflags &= ~TH_FIN;
1310                         tcpstat.tcps_rcvpackafterwin++;
1311                         tcpstat.tcps_rcvbyteafterwin += todrop;
1312                 }
1313                 tp->snd_wl1 = th->th_seq - 1;
1314                 tp->rcv_up = th->th_seq;
1315                 /*
1316                  * Client side of transaction: already sent SYN and data.
1317                  * If the remote host used T/TCP to validate the SYN,
1318                  * our data will be ACK'd; if so, enter normal data segment
1319                  * processing in the middle of step 5, ack processing.
1320                  * Otherwise, goto step 6.
1321                  */
1322                 if (thflags & TH_ACK)
1323                         goto process_ACK;
1324
1325                 goto step6;
1326
1327         /*
1328          * If the state is LAST_ACK or CLOSING or TIME_WAIT:
1329          *      if segment contains a SYN and CC [not CC.NEW] option:
1330          *              if state == TIME_WAIT and connection duration > MSL,
1331          *                  drop packet and send RST;
1332          *
1333          *              if SEG.CC > CCrecv then is new SYN, and can implicitly
1334          *                  ack the FIN (and data) in retransmission queue.
1335          *                  Complete close and delete TCPCB.  Then reprocess
1336          *                  segment, hoping to find new TCPCB in LISTEN state;
1337          *
1338          *              else must be old SYN; drop it.
1339          *      else do normal processing.
1340          */
1341         case TCPS_LAST_ACK:
1342         case TCPS_CLOSING:
1343         case TCPS_TIME_WAIT:
1344                 if ((thflags & TH_SYN) &&
1345                     (to.to_flags & TOF_CC) && tp->cc_recv != 0) {
1346                         if (tp->t_state == TCPS_TIME_WAIT &&
1347                                         (ticks - tp->t_starttime) > tcp_msl) {
1348                                 rstreason = BANDLIM_UNLIMITED;
1349                                 goto dropwithreset;
1350                         }
1351                         if (CC_GT(to.to_cc, tp->cc_recv)) {
1352                                 tp = tcp_close(tp);
1353                                 goto findpcb;
1354                         }
1355                         else
1356                                 goto drop;
1357                 }
1358                 break;  /* continue normal processing */
1359         }
1360
1361         /*
1362          * States other than LISTEN or SYN_SENT.
1363          * First check the RST flag and sequence number since reset segments
1364          * are exempt from the timestamp and connection count tests.  This
1365          * fixes a bug introduced by the Stevens, vol. 2, p. 960 bugfix
1366          * below which allowed reset segments in half the sequence space
1367          * to fall though and be processed (which gives forged reset
1368          * segments with a random sequence number a 50 percent chance of
1369          * killing a connection).
1370          * Then check timestamp, if present.
1371          * Then check the connection count, if present.
1372          * Then check that at least some bytes of segment are within
1373          * receive window.  If segment begins before rcv_nxt,
1374          * drop leading data (and SYN); if nothing left, just ack.
1375          *
1376          *
1377          * If the RST bit is set, check the sequence number to see
1378          * if this is a valid reset segment.
1379          * RFC 793 page 37:
1380          *   In all states except SYN-SENT, all reset (RST) segments
1381          *   are validated by checking their SEQ-fields.  A reset is
1382          *   valid if its sequence number is in the window.
1383          * Note: this does not take into account delayed ACKs, so
1384          *   we should test against last_ack_sent instead of rcv_nxt.
1385          *   The sequence number in the reset segment is normally an
1386          *   echo of our outgoing acknowlegement numbers, but some hosts
1387          *   send a reset with the sequence number at the rightmost edge
1388          *   of our receive window, and we have to handle this case.
1389          * If we have multiple segments in flight, the intial reset
1390          * segment sequence numbers will be to the left of last_ack_sent,
1391          * but they will eventually catch up.
1392          * In any case, it never made sense to trim reset segments to
1393          * fit the receive window since RFC 1122 says:
1394          *   4.2.2.12  RST Segment: RFC-793 Section 3.4
1395          *
1396          *    A TCP SHOULD allow a received RST segment to include data.
1397          *
1398          *    DISCUSSION
1399          *         It has been suggested that a RST segment could contain
1400          *         ASCII text that encoded and explained the cause of the
1401          *         RST.  No standard has yet been established for such
1402          *         data.
1403          *
1404          * If the reset segment passes the sequence number test examine
1405          * the state:
1406          *    SYN_RECEIVED STATE:
1407          *      If passive open, return to LISTEN state.
1408          *      If active open, inform user that connection was refused.
1409          *    ESTABLISHED, FIN_WAIT_1, FIN_WAIT_2, CLOSE_WAIT STATES:
1410          *      Inform user that connection was reset, and close tcb.
1411          *    CLOSING, LAST_ACK STATES:
1412          *      Close the tcb.
1413          *    TIME_WAIT STATE:
1414          *      Drop the segment - see Stevens, vol. 2, p. 964 and
1415          *      RFC 1337.
1416          */
1417         if (thflags & TH_RST) {
1418                 if (SEQ_GEQ(th->th_seq, tp->last_ack_sent) &&
1419                     SEQ_LT(th->th_seq, tp->last_ack_sent + tp->rcv_wnd)) {
1420                         switch (tp->t_state) {
1421
1422                         case TCPS_SYN_RECEIVED:
1423                                 so->so_error = ECONNREFUSED;
1424                                 goto close;
1425
1426                         case TCPS_ESTABLISHED:
1427                         case TCPS_FIN_WAIT_1:
1428                         case TCPS_FIN_WAIT_2:
1429                         case TCPS_CLOSE_WAIT:
1430                                 so->so_error = ECONNRESET;
1431                         close:
1432                                 tp->t_state = TCPS_CLOSED;
1433                                 tcpstat.tcps_drops++;
1434                                 tp = tcp_close(tp);
1435                                 break;
1436
1437                         case TCPS_CLOSING:
1438                         case TCPS_LAST_ACK:
1439                                 tp = tcp_close(tp);
1440                                 break;
1441
1442                         case TCPS_TIME_WAIT:
1443                                 break;
1444                         }
1445                 }
1446                 goto drop;
1447         }
1448
1449         /*
1450          * RFC 1323 PAWS: If we have a timestamp reply on this segment
1451          * and it's less than ts_recent, drop it.
1452          */
1453         if ((to.to_flags & TOF_TS) != 0 && tp->ts_recent &&
1454             TSTMP_LT(to.to_tsval, tp->ts_recent)) {
1455
1456                 /* Check to see if ts_recent is over 24 days old.  */
1457                 if ((int)(ticks - tp->ts_recent_age) > TCP_PAWS_IDLE) {
1458                         /*
1459                          * Invalidate ts_recent.  If this segment updates
1460                          * ts_recent, the age will be reset later and ts_recent
1461                          * will get a valid value.  If it does not, setting
1462                          * ts_recent to zero will at least satisfy the
1463                          * requirement that zero be placed in the timestamp
1464                          * echo reply when ts_recent isn't valid.  The
1465                          * age isn't reset until we get a valid ts_recent
1466                          * because we don't want out-of-order segments to be
1467                          * dropped when ts_recent is old.
1468                          */
1469                         tp->ts_recent = 0;
1470                 } else {
1471                         tcpstat.tcps_rcvduppack++;
1472                         tcpstat.tcps_rcvdupbyte += tlen;
1473                         tcpstat.tcps_pawsdrop++;
1474                         if (tlen)
1475                                 goto dropafterack;
1476                         goto drop;
1477                 }
1478         }
1479
1480         /*
1481          * T/TCP mechanism
1482          *   If T/TCP was negotiated and the segment doesn't have CC,
1483          *   or if its CC is wrong then drop the segment.
1484          *   RST segments do not have to comply with this.
1485          */
1486         if ((tp->t_flags & (TF_REQ_CC|TF_RCVD_CC)) == (TF_REQ_CC|TF_RCVD_CC) &&
1487             ((to.to_flags & TOF_CC) == 0 || tp->cc_recv != to.to_cc))
1488                 goto dropafterack;
1489
1490         /*
1491          * In the SYN-RECEIVED state, validate that the packet belongs to
1492          * this connection before trimming the data to fit the receive
1493          * window.  Check the sequence number versus IRS since we know
1494          * the sequence numbers haven't wrapped.  This is a partial fix
1495          * for the "LAND" DoS attack.
1496          */
1497         if (tp->t_state == TCPS_SYN_RECEIVED && SEQ_LT(th->th_seq, tp->irs)) {
1498                 rstreason = BANDLIM_RST_OPENPORT;
1499                 goto dropwithreset;
1500         }
1501
1502         todrop = tp->rcv_nxt - th->th_seq;
1503         if (todrop > 0) {
1504                 if (thflags & TH_SYN) {
1505                         thflags &= ~TH_SYN;
1506                         th->th_seq++;
1507                         if (th->th_urp > 1)
1508                                 th->th_urp--;
1509                         else
1510                                 thflags &= ~TH_URG;
1511                         todrop--;
1512                 }
1513                 /*
1514                  * Following if statement from Stevens, vol. 2, p. 960.
1515                  */
1516                 if (todrop > tlen
1517                     || (todrop == tlen && (thflags & TH_FIN) == 0)) {
1518                         /*
1519                          * Any valid FIN must be to the left of the window.
1520                          * At this point the FIN must be a duplicate or out
1521                          * of sequence; drop it.
1522                          */
1523                         thflags &= ~TH_FIN;
1524
1525                         /*
1526                          * Send an ACK to resynchronize and drop any data.
1527                          * But keep on processing for RST or ACK.
1528                          */
1529                         tp->t_flags |= TF_ACKNOW;
1530                         todrop = tlen;
1531                         tcpstat.tcps_rcvduppack++;
1532                         tcpstat.tcps_rcvdupbyte += todrop;
1533                 } else {
1534                         tcpstat.tcps_rcvpartduppack++;
1535                         tcpstat.tcps_rcvpartdupbyte += todrop;
1536                 }
1537                 drop_hdrlen += todrop;  /* drop from the top afterwards */
1538                 th->th_seq += todrop;
1539                 tlen -= todrop;
1540                 if (th->th_urp > todrop)
1541                         th->th_urp -= todrop;
1542                 else {
1543                         thflags &= ~TH_URG;
1544                         th->th_urp = 0;
1545                 }
1546         }
1547
1548         /*
1549          * If new data are received on a connection after the
1550          * user processes are gone, then RST the other end.
1551          */
1552         if ((so->so_state & SS_NOFDREF) &&
1553             tp->t_state > TCPS_CLOSE_WAIT && tlen) {
1554                 tp = tcp_close(tp);
1555                 tcpstat.tcps_rcvafterclose++;
1556                 rstreason = BANDLIM_UNLIMITED;
1557                 goto dropwithreset;
1558         }
1559
1560         /*
1561          * If segment ends after window, drop trailing data
1562          * (and PUSH and FIN); if nothing left, just ACK.
1563          */
1564         todrop = (th->th_seq+tlen) - (tp->rcv_nxt+tp->rcv_wnd);
1565         if (todrop > 0) {
1566                 tcpstat.tcps_rcvpackafterwin++;
1567                 if (todrop >= tlen) {
1568                         tcpstat.tcps_rcvbyteafterwin += tlen;
1569                         /*
1570                          * If a new connection request is received
1571                          * while in TIME_WAIT, drop the old connection
1572                          * and start over if the sequence numbers
1573                          * are above the previous ones.
1574                          */
1575                         if (thflags & TH_SYN &&
1576                             tp->t_state == TCPS_TIME_WAIT &&
1577                             SEQ_GT(th->th_seq, tp->rcv_nxt)) {
1578                                 tp = tcp_close(tp);
1579                                 goto findpcb;
1580                         }
1581                         /*
1582                          * If window is closed can only take segments at
1583                          * window edge, and have to drop data and PUSH from
1584                          * incoming segments.  Continue processing, but
1585                          * remember to ack.  Otherwise, drop segment
1586                          * and ack.
1587                          */
1588                         if (tp->rcv_wnd == 0 && th->th_seq == tp->rcv_nxt) {
1589                                 tp->t_flags |= TF_ACKNOW;
1590                                 tcpstat.tcps_rcvwinprobe++;
1591                         } else
1592                                 goto dropafterack;
1593                 } else
1594                         tcpstat.tcps_rcvbyteafterwin += todrop;
1595                 m_adj(m, -todrop);
1596                 tlen -= todrop;
1597                 thflags &= ~(TH_PUSH|TH_FIN);
1598         }
1599
1600         /*
1601          * If last ACK falls within this segment's sequence numbers,
1602          * record its timestamp.
1603          * NOTE that the test is modified according to the latest
1604          * proposal of the tcplw@cray.com list (Braden 1993/04/26).
1605          */
1606         if ((to.to_flags & TOF_TS) != 0 &&
1607             SEQ_LEQ(th->th_seq, tp->last_ack_sent)) {
1608                 tp->ts_recent_age = ticks;
1609                 tp->ts_recent = to.to_tsval;
1610         }
1611
1612         /*
1613          * If a SYN is in the window, then this is an
1614          * error and we send an RST and drop the connection.
1615          */
1616         if (thflags & TH_SYN) {
1617                 tp = tcp_drop(tp, ECONNRESET);
1618                 rstreason = BANDLIM_UNLIMITED;
1619                 goto dropwithreset;
1620         }
1621
1622         /*
1623          * If the ACK bit is off:  if in SYN-RECEIVED state or SENDSYN
1624          * flag is on (half-synchronized state), then queue data for
1625          * later processing; else drop segment and return.
1626          */
1627         if ((thflags & TH_ACK) == 0) {
1628                 if (tp->t_state == TCPS_SYN_RECEIVED ||
1629                     (tp->t_flags & TF_NEEDSYN))
1630                         goto step6;
1631                 else
1632                         goto drop;
1633         }
1634
1635         /*
1636          * Ack processing.
1637          */
1638         switch (tp->t_state) {
1639
1640         /*
1641          * In SYN_RECEIVED state, the ack ACKs our SYN, so enter
1642          * ESTABLISHED state and continue processing.
1643          * The ACK was checked above.
1644          */
1645         case TCPS_SYN_RECEIVED:
1646
1647                 tcpstat.tcps_connects++;
1648                 soisconnected(so);
1649                 /* Do window scaling? */
1650                 if ((tp->t_flags & (TF_RCVD_SCALE|TF_REQ_SCALE)) ==
1651                         (TF_RCVD_SCALE|TF_REQ_SCALE)) {
1652                         tp->snd_scale = tp->requested_s_scale;
1653                         tp->rcv_scale = tp->request_r_scale;
1654                 }
1655                 /*
1656                  * Upon successful completion of 3-way handshake,
1657                  * update cache.CC if it was undefined, pass any queued
1658                  * data to the user, and advance state appropriately.
1659                  */
1660                 if ((taop = tcp_gettaocache(&inp->inp_inc)) != NULL &&
1661                     taop->tao_cc == 0)
1662                         taop->tao_cc = tp->cc_recv;
1663
1664                 /*
1665                  * Make transitions:
1666                  *      SYN-RECEIVED  -> ESTABLISHED
1667                  *      SYN-RECEIVED* -> FIN-WAIT-1
1668                  */
1669                 tp->t_starttime = ticks;
1670                 if (tp->t_flags & TF_NEEDFIN) {
1671                         tp->t_state = TCPS_FIN_WAIT_1;
1672                         tp->t_flags &= ~TF_NEEDFIN;
1673                 } else {
1674                         tp->t_state = TCPS_ESTABLISHED;
1675                         callout_reset(tp->tt_keep, tcp_keepidle,
1676                                       tcp_timer_keep, tp);
1677                 }
1678                 /*
1679                  * If segment contains data or ACK, will call tcp_reass()
1680                  * later; if not, do so now to pass queued data to user.
1681                  */
1682                 if (tlen == 0 && (thflags & TH_FIN) == 0)
1683                         (void) tcp_reass(tp, (struct tcphdr *)0, 0,
1684                             (struct mbuf *)0);
1685                 tp->snd_wl1 = th->th_seq - 1;
1686                 /* fall into ... */
1687
1688         /*
1689          * In ESTABLISHED state: drop duplicate ACKs; ACK out of range
1690          * ACKs.  If the ack is in the range
1691          *      tp->snd_una < th->th_ack <= tp->snd_max
1692          * then advance tp->snd_una to th->th_ack and drop
1693          * data from the retransmission queue.  If this ACK reflects
1694          * more up to date window information we update our window information.
1695          */
1696         case TCPS_ESTABLISHED:
1697         case TCPS_FIN_WAIT_1:
1698         case TCPS_FIN_WAIT_2:
1699         case TCPS_CLOSE_WAIT:
1700         case TCPS_CLOSING:
1701         case TCPS_LAST_ACK:
1702         case TCPS_TIME_WAIT:
1703
1704                 if (SEQ_LEQ(th->th_ack, tp->snd_una)) {
1705                         if (tlen == 0 && tiwin == tp->snd_wnd) {
1706                                 tcpstat.tcps_rcvdupack++;
1707                                 /*
1708                                  * If we have outstanding data (other than
1709                                  * a window probe), this is a completely
1710                                  * duplicate ack (ie, window info didn't
1711                                  * change), the ack is the biggest we've
1712                                  * seen and we've seen exactly our rexmt
1713                                  * threshhold of them, assume a packet
1714                                  * has been dropped and retransmit it.
1715                                  * Kludge snd_nxt & the congestion
1716                                  * window so we send only this one
1717                                  * packet.
1718                                  *
1719                                  * We know we're losing at the current
1720                                  * window size so do congestion avoidance
1721                                  * (set ssthresh to half the current window
1722                                  * and pull our congestion window back to
1723                                  * the new ssthresh).
1724                                  *
1725                                  * Dup acks mean that packets have left the
1726                                  * network (they're now cached at the receiver)
1727                                  * so bump cwnd by the amount in the receiver
1728                                  * to keep a constant cwnd packets in the
1729                                  * network.
1730                                  */
1731                                 if (!callout_active(tp->tt_rexmt) ||
1732                                     th->th_ack != tp->snd_una)
1733                                         tp->t_dupacks = 0;
1734                                 else if (++tp->t_dupacks > tcprexmtthresh ||
1735                                          (tcp_do_newreno &&
1736                                           IN_FASTRECOVERY(tp))) {
1737                                         tp->snd_cwnd += tp->t_maxseg;
1738                                         (void) tcp_output(tp);
1739                                         goto drop;
1740                                 } else if (tp->t_dupacks == tcprexmtthresh) {
1741                                         tcp_seq onxt;
1742                                         u_int win;
1743
1744                                         if (tcp_do_newreno &&
1745                                             SEQ_LEQ(th->th_ack,
1746                                                     tp->snd_recover)) {
1747                                                 tp->t_dupacks = 0;
1748                                                 break;
1749                                         }
1750 fastretransmit:
1751                                         if (tcp_do_eifel_detect &&
1752                                             (tp->t_flags & TF_RCVD_TSTMP)) {
1753                                                 tcp_save_congestion_state(tp);
1754                                                 tp->t_flags |= TF_FASTREXMT;
1755                                         }
1756                                         win = min(tp->snd_wnd, tp->snd_cwnd) /
1757                                             2 / tp->t_maxseg;
1758                                         if (win < 2)
1759                                                 win = 2;
1760                                         tp->snd_ssthresh = win * tp->t_maxseg;
1761                                         ENTER_FASTRECOVERY(tp);
1762                                         tp->snd_recover = tp->snd_max;
1763                                         callout_stop(tp->tt_rexmt);
1764                                         tp->t_rtttime = 0;
1765                                         onxt = tp->snd_nxt;
1766                                         tp->snd_nxt = th->th_ack;
1767                                         tp->snd_cwnd = tp->t_maxseg;
1768                                         (void) tcp_output(tp);
1769                                         ++tcpstat.tcps_sndfastrexmit;
1770                                         KASSERT(tp->snd_limited <= 2,
1771                                             ("tp->snd_limited too big"));
1772                                         tp->snd_cwnd = tp->snd_ssthresh +
1773                                             (tp->t_maxseg *
1774                                              (tp->t_dupacks - tp->snd_limited));
1775                                         if (SEQ_GT(onxt, tp->snd_nxt))
1776                                                 tp->snd_nxt = onxt;
1777                                         goto drop;
1778                                 } else if (tcp_do_limitedtransmit) {
1779                                         u_long oldcwnd = tp->snd_cwnd;
1780                                         tcp_seq oldsndmax = tp->snd_max;
1781                                         /* outstanding data */
1782                                         uint32_t ownd =
1783                                             tp->snd_max - tp->snd_una;
1784                                         u_int sent;
1785
1786 #define iceildiv(n, d)          (((n)+(d)-1) / (d))
1787
1788                                         KASSERT(tp->t_dupacks == 1 ||
1789                                             tp->t_dupacks == 2,
1790                                             ("dupacks not 1 or 2"));
1791                                         if (tp->t_dupacks == 1)
1792                                                 tp->snd_limited = 0;
1793                                         tp->snd_cwnd = ownd +
1794                                             (tp->t_dupacks - tp->snd_limited) *
1795                                             tp->t_maxseg;
1796                                         (void) tcp_output(tp);
1797                                         tp->snd_cwnd = oldcwnd;
1798                                         sent = tp->snd_max - oldsndmax;
1799                                         if (sent > tp->t_maxseg) {
1800                                                 KASSERT((tp->t_dupacks == 2 &&
1801                                                     tp->snd_limited == 0) ||
1802                                                    (sent == tp->t_maxseg + 1 &&
1803                                                     tp->t_flags & TF_SENTFIN),
1804                                                     ("sent too much"));
1805                                                 KASSERT(sent <=
1806                                                         tp->t_maxseg * 2,
1807                                                     ("sent too many segments"));
1808                                                 tp->snd_limited = 2;
1809                                                 tcpstat.tcps_sndlimited += 2;
1810                                         } else if (sent > 0) {
1811                                                 ++tp->snd_limited;
1812                                                 ++tcpstat.tcps_sndlimited;
1813                                         } else if (tcp_do_early_retransmit &&
1814                                             (tcp_do_eifel_detect &&
1815                                              (tp->t_flags & TF_RCVD_TSTMP)) &&
1816                                             tcp_do_newreno &&
1817                                             tp->t_dupacks + 1 >=
1818                                               iceildiv(ownd, tp->t_maxseg)) {
1819                                                 ++tcpstat.tcps_sndearlyrexmit;
1820                                                 tp->t_flags |= TF_EARLYREXMT;
1821                                                 goto fastretransmit;
1822                                         }
1823                                         goto drop;
1824                                 }
1825                         } else
1826                                 tp->t_dupacks = 0;
1827                         break;
1828                 }
1829
1830                 KASSERT(SEQ_GT(th->th_ack, tp->snd_una), ("th_ack <= snd_una"));
1831
1832                 /*
1833                  * If the congestion window was inflated to account
1834                  * for the other side's cached packets, retract it.
1835                  */
1836                 if (tcp_do_newreno) {
1837                         if (IN_FASTRECOVERY(tp)) {
1838                                 if (SEQ_LT(th->th_ack, tp->snd_recover)) {
1839                                         tcp_newreno_partial_ack(tp, th);
1840                                 } else {
1841                                         /*
1842                                          * Window inflation should have left us
1843                                          * with approximately snd_ssthresh
1844                                          * outstanding data.
1845                                          * But in case we would be inclined to
1846                                          * send a burst, better to do it via
1847                                          * the slow start mechanism.
1848                                          */
1849                                         if (SEQ_GT(th->th_ack +
1850                                                         tp->snd_ssthresh,
1851                                                    tp->snd_max))
1852                                                 tp->snd_cwnd = tp->snd_max -
1853                                                                 th->th_ack +
1854                                                                 tp->t_maxseg;
1855                                         else
1856                                                 tp->snd_cwnd = tp->snd_ssthresh;
1857                                 }
1858                         }
1859                 } else {
1860                         if (tp->t_dupacks >= tcprexmtthresh &&
1861                             tp->snd_cwnd > tp->snd_ssthresh)
1862                                 tp->snd_cwnd = tp->snd_ssthresh;
1863                 }
1864                 tp->t_dupacks = 0;
1865                 if (SEQ_GT(th->th_ack, tp->snd_max)) {
1866                         /*
1867                          * Detected optimistic ACK attack.
1868                          * Force slow-start to de-synchronize attack.
1869                          */
1870                         tp->snd_cwnd = tp->t_maxseg;
1871
1872                         tcpstat.tcps_rcvacktoomuch++;
1873                         goto dropafterack;
1874                 }
1875                 /*
1876                  * If we reach this point, ACK is not a duplicate,
1877                  *     i.e., it ACKs something we sent.
1878                  */
1879                 if (tp->t_flags & TF_NEEDSYN) {
1880                         /*
1881                          * T/TCP: Connection was half-synchronized, and our
1882                          * SYN has been ACK'd (so connection is now fully
1883                          * synchronized).  Go to non-starred state,
1884                          * increment snd_una for ACK of SYN, and check if
1885                          * we can do window scaling.
1886                          */
1887                         tp->t_flags &= ~TF_NEEDSYN;
1888                         tp->snd_una++;
1889                         /* Do window scaling? */
1890                         if ((tp->t_flags & (TF_RCVD_SCALE|TF_REQ_SCALE)) ==
1891                                 (TF_RCVD_SCALE|TF_REQ_SCALE)) {
1892                                 tp->snd_scale = tp->requested_s_scale;
1893                                 tp->rcv_scale = tp->request_r_scale;
1894                         }
1895                 }
1896
1897 process_ACK:
1898                 acked = th->th_ack - tp->snd_una;
1899                 tcpstat.tcps_rcvackpack++;
1900                 tcpstat.tcps_rcvackbyte += acked;
1901
1902                 /*
1903                  * If we just performed our first retransmit, and the ACK
1904                  * arrives within our recovery window, then it was a mistake
1905                  * to do the retransmit in the first place.  Recover our
1906                  * original cwnd and ssthresh, and proceed to transmit where
1907                  * we left off.
1908                  */
1909                 if (tcp_do_eifel_detect && acked &&
1910                     (to.to_flags & TOF_TS) && to.to_tsecr &&
1911                     (tp->t_flags & TF_FIRSTACCACK)) {
1912                         /* Eifel detection applicable. */
1913                         if (to.to_tsecr < tp->t_rexmtTS) {
1914                                 ++tcpstat.tcps_eifeldetected;
1915                                 tcp_revert_congestion_state(tp);
1916                                 if (tp->t_rxtshift == 1 &&
1917                                     ticks >= tp->t_badrxtwin)
1918                                         ++tcpstat.tcps_rttcantdetect;
1919                         }
1920                 } else if (tp->t_rxtshift == 1 && ticks < tp->t_badrxtwin) {
1921                         tcp_revert_congestion_state(tp);
1922                         ++tcpstat.tcps_rttdetected;
1923                 }
1924
1925                 /*
1926                  * If we have a timestamp reply, update smoothed
1927                  * round trip time.  If no timestamp is present but
1928                  * transmit timer is running and timed sequence
1929                  * number was acked, update smoothed round trip time.
1930                  * Since we now have an rtt measurement, cancel the
1931                  * timer backoff (cf., Phil Karn's retransmit alg.).
1932                  * Recompute the initial retransmit timer.
1933                  *
1934                  * Some machines (certain windows boxes) send broken
1935                  * timestamp replies during the SYN+ACK phase, ignore
1936                  * timestamps of 0.
1937                  */
1938                 if ((to.to_flags & TOF_TS) != 0 &&
1939                     to.to_tsecr) {
1940                         tcp_xmit_timer(tp, ticks - to.to_tsecr + 1);
1941                 } else if (tp->t_rtttime && SEQ_GT(th->th_ack, tp->t_rtseq)) {
1942                         tcp_xmit_timer(tp, ticks - tp->t_rtttime);
1943                 }
1944                 tcp_xmit_bandwidth_limit(tp, th->th_ack);
1945
1946                 /*
1947                  * If all outstanding data is acked, stop retransmit
1948                  * timer and remember to restart (more output or persist).
1949                  * If there is more data to be acked, restart retransmit
1950                  * timer, using current (possibly backed-off) value.
1951                  */
1952                 if (th->th_ack == tp->snd_max) {
1953                         callout_stop(tp->tt_rexmt);
1954                         needoutput = 1;
1955                 } else if (!callout_active(tp->tt_persist))
1956                         callout_reset(tp->tt_rexmt, tp->t_rxtcur,
1957                                       tcp_timer_rexmt, tp);
1958
1959                 /*
1960                  * If no data (only SYN) was ACK'd,
1961                  *    skip rest of ACK processing.
1962                  */
1963                 if (acked == 0)
1964                         goto step6;
1965
1966                 /* Stop looking for an acceptable ACK since one was received. */
1967                 tp->t_flags &= ~(TF_FIRSTACCACK | TF_FASTREXMT | TF_EARLYREXMT);
1968
1969                 /*
1970                  * When new data is acked, open the congestion window.
1971                  * If the window gives us less than ssthresh packets
1972                  * in flight, open exponentially (maxseg per packet).
1973                  * Otherwise open linearly: maxseg per window
1974                  * (maxseg^2 / cwnd per packet).
1975                  */
1976                 if (!tcp_do_newreno || !IN_FASTRECOVERY(tp)) {
1977                         u_int cw = tp->snd_cwnd;
1978                         u_int incr = tp->t_maxseg;
1979                         if (cw > tp->snd_ssthresh)
1980                                 incr = incr * incr / cw;
1981                         tp->snd_cwnd = min(cw+incr, TCP_MAXWIN<<tp->snd_scale);
1982                 }
1983                 if (acked > so->so_snd.sb_cc) {
1984                         tp->snd_wnd -= so->so_snd.sb_cc;
1985                         sbdrop(&so->so_snd, (int)so->so_snd.sb_cc);
1986                         ourfinisacked = 1;
1987                 } else {
1988                         sbdrop(&so->so_snd, acked);
1989                         tp->snd_wnd -= acked;
1990                         ourfinisacked = 0;
1991                 }
1992                 sowwakeup(so);
1993                 if (tcp_do_newreno) {
1994                         if (IN_FASTRECOVERY(tp)) {
1995                                 if (SEQ_GEQ(th->th_ack, tp->snd_recover))
1996                                         EXIT_FASTRECOVERY(tp);
1997                         } else {
1998                                 tp->snd_recover = th->th_ack - 1;
1999                         }
2000                 }
2001                 tp->snd_una = th->th_ack;
2002                 if (SEQ_LT(tp->snd_nxt, tp->snd_una))
2003                         tp->snd_nxt = tp->snd_una;
2004
2005                 switch (tp->t_state) {
2006
2007                 /*
2008                  * In FIN_WAIT_1 STATE in addition to the processing
2009                  * for the ESTABLISHED state if our FIN is now acknowledged
2010                  * then enter FIN_WAIT_2.
2011                  */
2012                 case TCPS_FIN_WAIT_1:
2013                         if (ourfinisacked) {
2014                                 /*
2015                                  * If we can't receive any more
2016                                  * data, then closing user can proceed.
2017                                  * Starting the timer is contrary to the
2018                                  * specification, but if we don't get a FIN
2019                                  * we'll hang forever.
2020                                  */
2021                                 if (so->so_state & SS_CANTRCVMORE) {
2022                                         soisdisconnected(so);
2023                                         callout_reset(tp->tt_2msl, tcp_maxidle,
2024                                                       tcp_timer_2msl, tp);
2025                                 }
2026                                 tp->t_state = TCPS_FIN_WAIT_2;
2027                         }
2028                         break;
2029
2030                 /*
2031                  * In CLOSING STATE in addition to the processing for
2032                  * the ESTABLISHED state if the ACK acknowledges our FIN
2033                  * then enter the TIME-WAIT state, otherwise ignore
2034                  * the segment.
2035                  */
2036                 case TCPS_CLOSING:
2037                         if (ourfinisacked) {
2038                                 tp->t_state = TCPS_TIME_WAIT;
2039                                 tcp_canceltimers(tp);
2040                                 /* Shorten TIME_WAIT [RFC-1644, p.28] */
2041                                 if (tp->cc_recv != 0 &&
2042                                     (ticks - tp->t_starttime) < tcp_msl)
2043                                         callout_reset(tp->tt_2msl,
2044                                                       tp->t_rxtcur *
2045                                                       TCPTV_TWTRUNC,
2046                                                       tcp_timer_2msl, tp);
2047                                 else
2048                                         callout_reset(tp->tt_2msl, 2 * tcp_msl,
2049                                                       tcp_timer_2msl, tp);
2050                                 soisdisconnected(so);
2051                         }
2052                         break;
2053
2054                 /*
2055                  * In LAST_ACK, we may still be waiting for data to drain
2056                  * and/or to be acked, as well as for the ack of our FIN.
2057                  * If our FIN is now acknowledged, delete the TCB,
2058                  * enter the closed state and return.
2059                  */
2060                 case TCPS_LAST_ACK:
2061                         if (ourfinisacked) {
2062                                 tp = tcp_close(tp);
2063                                 goto drop;
2064                         }
2065                         break;
2066
2067                 /*
2068                  * In TIME_WAIT state the only thing that should arrive
2069                  * is a retransmission of the remote FIN.  Acknowledge
2070                  * it and restart the finack timer.
2071                  */
2072                 case TCPS_TIME_WAIT:
2073                         callout_reset(tp->tt_2msl, 2 * tcp_msl,
2074                                       tcp_timer_2msl, tp);
2075                         goto dropafterack;
2076                 }
2077         }
2078
2079 step6:
2080         /*
2081          * Update window information.
2082          * Don't look at window if no ACK: TAC's send garbage on first SYN.
2083          */
2084         if ((thflags & TH_ACK) &&
2085             (SEQ_LT(tp->snd_wl1, th->th_seq) ||
2086             (tp->snd_wl1 == th->th_seq && (SEQ_LT(tp->snd_wl2, th->th_ack) ||
2087              (tp->snd_wl2 == th->th_ack && tiwin > tp->snd_wnd))))) {
2088                 /* keep track of pure window updates */
2089                 if (tlen == 0 &&
2090                     tp->snd_wl2 == th->th_ack && tiwin > tp->snd_wnd)
2091                         tcpstat.tcps_rcvwinupd++;
2092                 tp->snd_wnd = tiwin;
2093                 tp->snd_wl1 = th->th_seq;
2094                 tp->snd_wl2 = th->th_ack;
2095                 if (tp->snd_wnd > tp->max_sndwnd)
2096                         tp->max_sndwnd = tp->snd_wnd;
2097                 needoutput = 1;
2098         }
2099
2100         /*
2101          * Process segments with URG.
2102          */
2103         if ((thflags & TH_URG) && th->th_urp &&
2104             TCPS_HAVERCVDFIN(tp->t_state) == 0) {
2105                 /*
2106                  * This is a kludge, but if we receive and accept
2107                  * random urgent pointers, we'll crash in
2108                  * soreceive.  It's hard to imagine someone
2109                  * actually wanting to send this much urgent data.
2110                  */
2111                 if (th->th_urp + so->so_rcv.sb_cc > sb_max) {
2112                         th->th_urp = 0;                 /* XXX */
2113                         thflags &= ~TH_URG;             /* XXX */
2114                         goto dodata;                    /* XXX */
2115                 }
2116                 /*
2117                  * If this segment advances the known urgent pointer,
2118                  * then mark the data stream.  This should not happen
2119                  * in CLOSE_WAIT, CLOSING, LAST_ACK or TIME_WAIT STATES since
2120                  * a FIN has been received from the remote side.
2121                  * In these states we ignore the URG.
2122                  *
2123                  * According to RFC961 (Assigned Protocols),
2124                  * the urgent pointer points to the last octet
2125                  * of urgent data.  We continue, however,
2126                  * to consider it to indicate the first octet
2127                  * of data past the urgent section as the original
2128                  * spec states (in one of two places).
2129                  */
2130                 if (SEQ_GT(th->th_seq+th->th_urp, tp->rcv_up)) {
2131                         tp->rcv_up = th->th_seq + th->th_urp;
2132                         so->so_oobmark = so->so_rcv.sb_cc +
2133                             (tp->rcv_up - tp->rcv_nxt) - 1;
2134                         if (so->so_oobmark == 0)
2135                                 so->so_state |= SS_RCVATMARK;
2136                         sohasoutofband(so);
2137                         tp->t_oobflags &= ~(TCPOOB_HAVEDATA | TCPOOB_HADDATA);
2138                 }
2139                 /*
2140                  * Remove out of band data so doesn't get presented to user.
2141                  * This can happen independent of advancing the URG pointer,
2142                  * but if two URG's are pending at once, some out-of-band
2143                  * data may creep in... ick.
2144                  */
2145                 if (th->th_urp <= (u_long)tlen
2146 #ifdef SO_OOBINLINE
2147                      && (so->so_options & SO_OOBINLINE) == 0
2148 #endif
2149                      )
2150                         tcp_pulloutofband(so, th, m,
2151                                 drop_hdrlen);   /* hdr drop is delayed */
2152         } else {
2153                 /*
2154                  * If no out of band data is expected,
2155                  * pull receive urgent pointer along
2156                  * with the receive window.
2157                  */
2158                 if (SEQ_GT(tp->rcv_nxt, tp->rcv_up))
2159                         tp->rcv_up = tp->rcv_nxt;
2160         }
2161 dodata:                                                 /* XXX */
2162
2163         /*
2164          * Process the segment text, merging it into the TCP sequencing queue,
2165          * and arranging for acknowledgment of receipt if necessary.
2166          * This process logically involves adjusting tp->rcv_wnd as data
2167          * is presented to the user (this happens in tcp_usrreq.c,
2168          * case PRU_RCVD).  If a FIN has already been received on this
2169          * connection then we just ignore the text.
2170          */
2171         if ((tlen || (thflags & TH_FIN)) &&
2172             TCPS_HAVERCVDFIN(tp->t_state) == 0) {
2173                 m_adj(m, drop_hdrlen);  /* delayed header drop */
2174                 /*
2175                  * Insert segment which includes th into TCP reassembly queue
2176                  * with control block tp.  Set thflags to whether reassembly now
2177                  * includes a segment with FIN.  This handles the common case
2178                  * inline (segment is the next to be received on an established
2179                  * connection, and the queue is empty), avoiding linkage into
2180                  * and removal from the queue and repetition of various
2181                  * conversions.
2182                  * Set DELACK for segments received in order, but ack
2183                  * immediately when segments are out of order (so
2184                  * fast retransmit can work).
2185                  */
2186                 if (th->th_seq == tp->rcv_nxt &&
2187                     LIST_EMPTY(&tp->t_segq) &&
2188                     TCPS_HAVEESTABLISHED(tp->t_state)) {
2189                         if (DELAY_ACK(tp))
2190                                 callout_reset(tp->tt_delack, tcp_delacktime,
2191                                               tcp_timer_delack, tp);
2192                         else
2193                                 tp->t_flags |= TF_ACKNOW;
2194                         tp->rcv_nxt += tlen;
2195                         thflags = th->th_flags & TH_FIN;
2196                         tcpstat.tcps_rcvpack++;
2197                         tcpstat.tcps_rcvbyte += tlen;
2198                         ND6_HINT(tp);
2199                         if (so->so_state & SS_CANTRCVMORE)
2200                                 m_freem(m);
2201                         else
2202                                 sbappend(&so->so_rcv, m);
2203                         sorwakeup(so);
2204                 } else {
2205                         thflags = tcp_reass(tp, th, &tlen, m);
2206                         tp->t_flags |= TF_ACKNOW;
2207                 }
2208
2209                 /*
2210                  * Note the amount of data that peer has sent into
2211                  * our window, in order to estimate the sender's
2212                  * buffer size.
2213                  */
2214                 len = so->so_rcv.sb_hiwat - (tp->rcv_adv - tp->rcv_nxt);
2215         } else {
2216                 m_freem(m);
2217                 thflags &= ~TH_FIN;
2218         }
2219
2220         /*
2221          * If FIN is received ACK the FIN and let the user know
2222          * that the connection is closing.
2223          */
2224         if (thflags & TH_FIN) {
2225                 if (TCPS_HAVERCVDFIN(tp->t_state) == 0) {
2226                         socantrcvmore(so);
2227                         /*
2228                          * If connection is half-synchronized
2229                          * (ie NEEDSYN flag on) then delay ACK,
2230                          * so it may be piggybacked when SYN is sent.
2231                          * Otherwise, since we received a FIN then no
2232                          * more input can be expected, send ACK now.
2233                          */
2234                         if (DELAY_ACK(tp) && (tp->t_flags & TF_NEEDSYN))
2235                                 callout_reset(tp->tt_delack, tcp_delacktime,
2236                                     tcp_timer_delack, tp);
2237                         else
2238                                 tp->t_flags |= TF_ACKNOW;
2239                         tp->rcv_nxt++;
2240                 }
2241                 switch (tp->t_state) {
2242
2243                 /*
2244                  * In SYN_RECEIVED and ESTABLISHED STATES
2245                  * enter the CLOSE_WAIT state.
2246                  */
2247                 case TCPS_SYN_RECEIVED:
2248                         tp->t_starttime = ticks;
2249                         /*FALLTHROUGH*/
2250                 case TCPS_ESTABLISHED:
2251                         tp->t_state = TCPS_CLOSE_WAIT;
2252                         break;
2253
2254                 /*
2255                  * If still in FIN_WAIT_1 STATE FIN has not been acked so
2256                  * enter the CLOSING state.
2257                  */
2258                 case TCPS_FIN_WAIT_1:
2259                         tp->t_state = TCPS_CLOSING;
2260                         break;
2261
2262                 /*
2263                  * In FIN_WAIT_2 state enter the TIME_WAIT state,
2264                  * starting the time-wait timer, turning off the other
2265                  * standard timers.
2266                  */
2267                 case TCPS_FIN_WAIT_2:
2268                         tp->t_state = TCPS_TIME_WAIT;
2269                         tcp_canceltimers(tp);
2270                         /* Shorten TIME_WAIT [RFC-1644, p.28] */
2271                         if (tp->cc_recv != 0 &&
2272                             (ticks - tp->t_starttime) < tcp_msl) {
2273                                 callout_reset(tp->tt_2msl,
2274                                               tp->t_rxtcur * TCPTV_TWTRUNC,
2275                                               tcp_timer_2msl, tp);
2276                                 /* For transaction client, force ACK now. */
2277                                 tp->t_flags |= TF_ACKNOW;
2278                         }
2279                         else
2280                                 callout_reset(tp->tt_2msl, 2 * tcp_msl,
2281                                               tcp_timer_2msl, tp);
2282                         soisdisconnected(so);
2283                         break;
2284
2285                 /*
2286                  * In TIME_WAIT state restart the 2 MSL time_wait timer.
2287                  */
2288                 case TCPS_TIME_WAIT:
2289                         callout_reset(tp->tt_2msl, 2 * tcp_msl,
2290                                       tcp_timer_2msl, tp);
2291                         break;
2292                 }
2293         }
2294 #ifdef TCPDEBUG
2295         if (so->so_options & SO_DEBUG)
2296                 tcp_trace(TA_INPUT, ostate, tp, (void *)tcp_saveipgen,
2297                           &tcp_savetcp, 0);
2298 #endif
2299
2300         /*
2301          * Return any desired output.
2302          */
2303         if (needoutput || (tp->t_flags & TF_ACKNOW))
2304                 (void) tcp_output(tp);
2305         return;
2306
2307 dropafterack:
2308         /*
2309          * Generate an ACK dropping incoming segment if it occupies
2310          * sequence space, where the ACK reflects our state.
2311          *
2312          * We can now skip the test for the RST flag since all
2313          * paths to this code happen after packets containing
2314          * RST have been dropped.
2315          *
2316          * In the SYN-RECEIVED state, don't send an ACK unless the
2317          * segment we received passes the SYN-RECEIVED ACK test.
2318          * If it fails send a RST.  This breaks the loop in the
2319          * "LAND" DoS attack, and also prevents an ACK storm
2320          * between two listening ports that have been sent forged
2321          * SYN segments, each with the source address of the other.
2322          */
2323         if (tp->t_state == TCPS_SYN_RECEIVED && (thflags & TH_ACK) &&
2324             (SEQ_GT(tp->snd_una, th->th_ack) ||
2325              SEQ_GT(th->th_ack, tp->snd_max)) ) {
2326                 rstreason = BANDLIM_RST_OPENPORT;
2327                 goto dropwithreset;
2328         }
2329 #ifdef TCPDEBUG
2330         if (so->so_options & SO_DEBUG)
2331                 tcp_trace(TA_DROP, ostate, tp, (void *)tcp_saveipgen,
2332                           &tcp_savetcp, 0);
2333 #endif
2334         m_freem(m);
2335         tp->t_flags |= TF_ACKNOW;
2336         (void) tcp_output(tp);
2337         return;
2338
2339 dropwithreset:
2340         /*
2341          * Generate a RST, dropping incoming segment.
2342          * Make ACK acceptable to originator of segment.
2343          * Don't bother to respond if destination was broadcast/multicast.
2344          */
2345         if ((thflags & TH_RST) || m->m_flags & (M_BCAST|M_MCAST))
2346                 goto drop;
2347         if (isipv6) {
2348                 if (IN6_IS_ADDR_MULTICAST(&ip6->ip6_dst) ||
2349                     IN6_IS_ADDR_MULTICAST(&ip6->ip6_src))
2350                         goto drop;
2351         } else {
2352                 if (IN_MULTICAST(ntohl(ip->ip_dst.s_addr)) ||
2353                     IN_MULTICAST(ntohl(ip->ip_src.s_addr)) ||
2354                     ip->ip_src.s_addr == htonl(INADDR_BROADCAST) ||
2355                     in_broadcast(ip->ip_dst, m->m_pkthdr.rcvif))
2356                         goto drop;
2357         }
2358         /* IPv6 anycast check is done at tcp6_input() */
2359
2360         /*
2361          * Perform bandwidth limiting.
2362          */
2363 #ifdef ICMP_BANDLIM
2364         if (badport_bandlim(rstreason) < 0)
2365                 goto drop;
2366 #endif
2367
2368 #ifdef TCPDEBUG
2369         if (tp == NULL || (tp->t_inpcb->inp_socket->so_options & SO_DEBUG))
2370                 tcp_trace(TA_DROP, ostate, tp, (void *)tcp_saveipgen,
2371                           &tcp_savetcp, 0);
2372 #endif
2373         if (thflags & TH_ACK)
2374                 /* mtod() below is safe as long as hdr dropping is delayed */
2375                 tcp_respond(tp, mtod(m, void *), th, m, (tcp_seq)0, th->th_ack,
2376                             TH_RST);
2377         else {
2378                 if (thflags & TH_SYN)
2379                         tlen++;
2380                 /* mtod() below is safe as long as hdr dropping is delayed */
2381                 tcp_respond(tp, mtod(m, void *), th, m, th->th_seq+tlen,
2382                             (tcp_seq)0, TH_RST|TH_ACK);
2383         }
2384         return;
2385
2386 drop:
2387         /*
2388          * Drop space held by incoming segment and return.
2389          */
2390 #ifdef TCPDEBUG
2391         if (tp == NULL || (tp->t_inpcb->inp_socket->so_options & SO_DEBUG))
2392                 tcp_trace(TA_DROP, ostate, tp, (void *)tcp_saveipgen,
2393                           &tcp_savetcp, 0);
2394 #endif
2395         m_freem(m);
2396         return;
2397 }
2398
2399 /*
2400  * Parse TCP options and place in tcpopt.
2401  */
2402 static void
2403 tcp_dooptions(to, cp, cnt, is_syn)
2404         struct tcpopt *to;
2405         u_char *cp;
2406         int cnt;
2407 {
2408         int opt, optlen;
2409
2410         to->to_flags = 0;
2411         for (; cnt > 0; cnt -= optlen, cp += optlen) {
2412                 opt = cp[0];
2413                 if (opt == TCPOPT_EOL)
2414                         break;
2415                 if (opt == TCPOPT_NOP)
2416                         optlen = 1;
2417                 else {
2418                         if (cnt < 2)
2419                                 break;
2420                         optlen = cp[1];
2421                         if (optlen < 2 || optlen > cnt)
2422                                 break;
2423                 }
2424                 switch (opt) {
2425                 case TCPOPT_MAXSEG:
2426                         if (optlen != TCPOLEN_MAXSEG)
2427                                 continue;
2428                         if (!is_syn)
2429                                 continue;
2430                         to->to_flags |= TOF_MSS;
2431                         bcopy((char *)cp + 2,
2432                             (char *)&to->to_mss, sizeof(to->to_mss));
2433                         to->to_mss = ntohs(to->to_mss);
2434                         break;
2435                 case TCPOPT_WINDOW:
2436                         if (optlen != TCPOLEN_WINDOW)
2437                                 continue;
2438                         if (! is_syn)
2439                                 continue;
2440                         to->to_flags |= TOF_SCALE;
2441                         to->to_requested_s_scale = min(cp[2], TCP_MAX_WINSHIFT);
2442                         break;
2443                 case TCPOPT_TIMESTAMP:
2444                         if (optlen != TCPOLEN_TIMESTAMP)
2445                                 continue;
2446                         to->to_flags |= TOF_TS;
2447                         bcopy((char *)cp + 2,
2448                             (char *)&to->to_tsval, sizeof(to->to_tsval));
2449                         to->to_tsval = ntohl(to->to_tsval);
2450                         bcopy((char *)cp + 6,
2451                             (char *)&to->to_tsecr, sizeof(to->to_tsecr));
2452                         to->to_tsecr = ntohl(to->to_tsecr);
2453                         break;
2454                 case TCPOPT_CC:
2455                         if (optlen != TCPOLEN_CC)
2456                                 continue;
2457                         to->to_flags |= TOF_CC;
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_CCNEW:
2463                         if (optlen != TCPOLEN_CC)
2464                                 continue;
2465                         if (!is_syn)
2466                                 continue;
2467                         to->to_flags |= TOF_CCNEW;
2468                         bcopy((char *)cp + 2,
2469                             (char *)&to->to_cc, sizeof(to->to_cc));
2470                         to->to_cc = ntohl(to->to_cc);
2471                         break;
2472                 case TCPOPT_CCECHO:
2473                         if (optlen != TCPOLEN_CC)
2474                                 continue;
2475                         if (!is_syn)
2476                                 continue;
2477                         to->to_flags |= TOF_CCECHO;
2478                         bcopy((char *)cp + 2,
2479                             (char *)&to->to_ccecho, sizeof(to->to_ccecho));
2480                         to->to_ccecho = ntohl(to->to_ccecho);
2481                         break;
2482                 default:
2483                         continue;
2484                 }
2485         }
2486 }
2487
2488 /*
2489  * Pull out of band byte out of a segment so
2490  * it doesn't appear in the user's data queue.
2491  * It is still reflected in the segment length for
2492  * sequencing purposes.
2493  */
2494 static void
2495 tcp_pulloutofband(so, th, m, off)
2496         struct socket *so;
2497         struct tcphdr *th;
2498         struct mbuf *m;
2499         int off;                /* delayed to be droped hdrlen */
2500 {
2501         int cnt = off + th->th_urp - 1;
2502
2503         while (cnt >= 0) {
2504                 if (m->m_len > cnt) {
2505                         char *cp = mtod(m, caddr_t) + cnt;
2506                         struct tcpcb *tp = sototcpcb(so);
2507
2508                         tp->t_iobc = *cp;
2509                         tp->t_oobflags |= TCPOOB_HAVEDATA;
2510                         bcopy(cp+1, cp, (unsigned)(m->m_len - cnt - 1));
2511                         m->m_len--;
2512                         if (m->m_flags & M_PKTHDR)
2513                                 m->m_pkthdr.len--;
2514                         return;
2515                 }
2516                 cnt -= m->m_len;
2517                 m = m->m_next;
2518                 if (m == 0)
2519                         break;
2520         }
2521         panic("tcp_pulloutofband");
2522 }
2523
2524 /*
2525  * Collect new round-trip time estimate
2526  * and update averages and current timeout.
2527  */
2528 static void
2529 tcp_xmit_timer(tp, rtt)
2530         struct tcpcb *tp;
2531         int rtt;
2532 {
2533         int delta;
2534
2535         tcpstat.tcps_rttupdated++;
2536         tp->t_rttupdated++;
2537         if (tp->t_srtt != 0) {
2538                 /*
2539                  * srtt is stored as fixed point with 5 bits after the
2540                  * binary point (i.e., scaled by 8).  The following magic
2541                  * is equivalent to the smoothing algorithm in rfc793 with
2542                  * an alpha of .875 (srtt = rtt/8 + srtt*7/8 in fixed
2543                  * point).  Adjust rtt to origin 0.
2544                  */
2545                 delta = ((rtt - 1) << TCP_DELTA_SHIFT)
2546                         - (tp->t_srtt >> (TCP_RTT_SHIFT - TCP_DELTA_SHIFT));
2547
2548                 if ((tp->t_srtt += delta) <= 0)
2549                         tp->t_srtt = 1;
2550
2551                 /*
2552                  * We accumulate a smoothed rtt variance (actually, a
2553                  * smoothed mean difference), then set the retransmit
2554                  * timer to smoothed rtt + 4 times the smoothed variance.
2555                  * rttvar is stored as fixed point with 4 bits after the
2556                  * binary point (scaled by 16).  The following is
2557                  * equivalent to rfc793 smoothing with an alpha of .75
2558                  * (rttvar = rttvar*3/4 + |delta| / 4).  This replaces
2559                  * rfc793's wired-in beta.
2560                  */
2561                 if (delta < 0)
2562                         delta = -delta;
2563                 delta -= tp->t_rttvar >> (TCP_RTTVAR_SHIFT - TCP_DELTA_SHIFT);
2564                 if ((tp->t_rttvar += delta) <= 0)
2565                         tp->t_rttvar = 1;
2566                 if (tp->t_rttbest > tp->t_srtt + tp->t_rttvar)
2567                         tp->t_rttbest = tp->t_srtt + tp->t_rttvar;
2568         } else {
2569                 /*
2570                  * No rtt measurement yet - use the unsmoothed rtt.
2571                  * Set the variance to half the rtt (so our first
2572                  * retransmit happens at 3*rtt).
2573                  */
2574                 tp->t_srtt = rtt << TCP_RTT_SHIFT;
2575                 tp->t_rttvar = rtt << (TCP_RTTVAR_SHIFT - 1);
2576                 tp->t_rttbest = tp->t_srtt + tp->t_rttvar;
2577         }
2578         tp->t_rtttime = 0;
2579         tp->t_rxtshift = 0;
2580
2581         /*
2582          * the retransmit should happen at rtt + 4 * rttvar.
2583          * Because of the way we do the smoothing, srtt and rttvar
2584          * will each average +1/2 tick of bias.  When we compute
2585          * the retransmit timer, we want 1/2 tick of rounding and
2586          * 1 extra tick because of +-1/2 tick uncertainty in the
2587          * firing of the timer.  The bias will give us exactly the
2588          * 1.5 tick we need.  But, because the bias is
2589          * statistical, we have to test that we don't drop below
2590          * the minimum feasible timer (which is 2 ticks).
2591          */
2592         TCPT_RANGESET(tp->t_rxtcur, TCP_REXMTVAL(tp),
2593                       max(tp->t_rttmin, rtt + 2), TCPTV_REXMTMAX);
2594
2595         /*
2596          * We received an ack for a packet that wasn't retransmitted;
2597          * it is probably safe to discard any error indications we've
2598          * received recently.  This isn't quite right, but close enough
2599          * for now (a route might have failed after we sent a segment,
2600          * and the return path might not be symmetrical).
2601          */
2602         tp->t_softerror = 0;
2603 }
2604
2605 /*
2606  * Determine a reasonable value for maxseg size.
2607  * If the route is known, check route for mtu.
2608  * If none, use an mss that can be handled on the outgoing
2609  * interface without forcing IP to fragment; if bigger than
2610  * an mbuf cluster (MCLBYTES), round down to nearest multiple of MCLBYTES
2611  * to utilize large mbufs.  If no route is found, route has no mtu,
2612  * or the destination isn't local, use a default, hopefully conservative
2613  * size (usually 512 or the default IP max size, but no more than the mtu
2614  * of the interface), as we can't discover anything about intervening
2615  * gateways or networks.  We also initialize the congestion/slow start
2616  * window to be a single segment if the destination isn't local.
2617  * While looking at the routing entry, we also initialize other path-dependent
2618  * parameters from pre-set or cached values in the routing entry.
2619  *
2620  * Also take into account the space needed for options that we
2621  * send regularly.  Make maxseg shorter by that amount to assure
2622  * that we can send maxseg amount of data even when the options
2623  * are present.  Store the upper limit of the length of options plus
2624  * data in maxopd.
2625  *
2626  * NOTE that this routine is only called when we process an incoming
2627  * segment, for outgoing segments only tcp_mssopt is called.
2628  *
2629  * In case of T/TCP, we call this routine during implicit connection
2630  * setup as well (offer = -1), to initialize maxseg from the cached
2631  * MSS of our peer.
2632  */
2633 void
2634 tcp_mss(tp, offer)
2635         struct tcpcb *tp;
2636         int offer;
2637 {
2638         struct rtentry *rt;
2639         struct ifnet *ifp;
2640         int rtt, mss;
2641         u_long bufsize;
2642         struct inpcb *inp = tp->t_inpcb;
2643         struct socket *so;
2644         struct rmxp_tao *taop;
2645         int origoffer = offer;
2646 #ifdef INET6
2647         boolean_t isipv6 = ((inp->inp_vflag & INP_IPV6) ? TRUE : FALSE);
2648         size_t min_protoh = isipv6 ?
2649                             sizeof(struct ip6_hdr) + sizeof(struct tcphdr) :
2650                             sizeof(struct tcpiphdr);
2651 #else
2652         const boolean_t isipv6 = FALSE;
2653         const size_t min_protoh = sizeof(struct tcpiphdr);
2654 #endif
2655
2656         if (isipv6)
2657                 rt = tcp_rtlookup6(&inp->inp_inc);
2658         else
2659                 rt = tcp_rtlookup(&inp->inp_inc);
2660         if (rt == NULL) {
2661                 tp->t_maxopd = tp->t_maxseg =
2662                     (isipv6 ? tcp_v6mssdflt : tcp_mssdflt);
2663                 return;
2664         }
2665         ifp = rt->rt_ifp;
2666         so = inp->inp_socket;
2667
2668         taop = rmx_taop(rt->rt_rmx);
2669         /*
2670          * Offer == -1 means that we didn't receive SYN yet,
2671          * use cached value in that case;
2672          */
2673         if (offer == -1)
2674                 offer = taop->tao_mssopt;
2675         /*
2676          * Offer == 0 means that there was no MSS on the SYN segment,
2677          * in this case we use tcp_mssdflt.
2678          */
2679         if (offer == 0)
2680                 offer = (isipv6 ? tcp_v6mssdflt : tcp_mssdflt);
2681         else
2682                 /*
2683                  * Sanity check: make sure that maxopd will be large
2684                  * enough to allow some data on segments even is the
2685                  * all the option space is used (40bytes).  Otherwise
2686                  * funny things may happen in tcp_output.
2687                  */
2688                 offer = max(offer, 64);
2689         taop->tao_mssopt = offer;
2690
2691         /*
2692          * While we're here, check if there's an initial rtt
2693          * or rttvar.  Convert from the route-table units
2694          * to scaled multiples of the slow timeout timer.
2695          */
2696         if (tp->t_srtt == 0 && (rtt = rt->rt_rmx.rmx_rtt)) {
2697                 /*
2698                  * XXX the lock bit for RTT indicates that the value
2699                  * is also a minimum value; this is subject to time.
2700                  */
2701                 if (rt->rt_rmx.rmx_locks & RTV_RTT)
2702                         tp->t_rttmin = rtt / (RTM_RTTUNIT / hz);
2703                 tp->t_srtt = rtt / (RTM_RTTUNIT / (hz * TCP_RTT_SCALE));
2704                 tp->t_rttbest = tp->t_srtt + TCP_RTT_SCALE;
2705                 tcpstat.tcps_usedrtt++;
2706                 if (rt->rt_rmx.rmx_rttvar) {
2707                         tp->t_rttvar = rt->rt_rmx.rmx_rttvar /
2708                             (RTM_RTTUNIT / (hz * TCP_RTTVAR_SCALE));
2709                         tcpstat.tcps_usedrttvar++;
2710                 } else {
2711                         /* default variation is +- 1 rtt */
2712                         tp->t_rttvar =
2713                             tp->t_srtt * TCP_RTTVAR_SCALE / TCP_RTT_SCALE;
2714                 }
2715                 TCPT_RANGESET(tp->t_rxtcur,
2716                               ((tp->t_srtt >> 2) + tp->t_rttvar) >> 1,
2717                               tp->t_rttmin, TCPTV_REXMTMAX);
2718         }
2719         /*
2720          * if there's an mtu associated with the route, use it
2721          * else, use the link mtu.
2722          */
2723         if (rt->rt_rmx.rmx_mtu)
2724                 mss = rt->rt_rmx.rmx_mtu - min_protoh;
2725         else {
2726                 if (isipv6) {
2727                         mss = nd_ifinfo[rt->rt_ifp->if_index].linkmtu -
2728                                 min_protoh;
2729                         if (!in6_localaddr(&inp->in6p_faddr))
2730                                 mss = min(mss, tcp_v6mssdflt);
2731                 } else {
2732                         mss = ifp->if_mtu - min_protoh;
2733                         if (!in_localaddr(inp->inp_faddr))
2734                                 mss = min(mss, tcp_mssdflt);
2735                 }
2736         }
2737         mss = min(mss, offer);
2738         /*
2739          * maxopd stores the maximum length of data AND options
2740          * in a segment; maxseg is the amount of data in a normal
2741          * segment.  We need to store this value (maxopd) apart
2742          * from maxseg, because now every segment carries options
2743          * and thus we normally have somewhat less data in segments.
2744          */
2745         tp->t_maxopd = mss;
2746
2747         /*
2748          * In case of T/TCP, origoffer==-1 indicates, that no segments
2749          * were received yet.  In this case we just guess, otherwise
2750          * we do the same as before T/TCP.
2751          */
2752         if ((tp->t_flags & (TF_REQ_TSTMP|TF_NOOPT)) == TF_REQ_TSTMP &&
2753             (origoffer == -1 ||
2754              (tp->t_flags & TF_RCVD_TSTMP) == TF_RCVD_TSTMP))
2755                 mss -= TCPOLEN_TSTAMP_APPA;
2756         if ((tp->t_flags & (TF_REQ_CC|TF_NOOPT)) == TF_REQ_CC &&
2757             (origoffer == -1 ||
2758              (tp->t_flags & TF_RCVD_CC) == TF_RCVD_CC))
2759                 mss -= TCPOLEN_CC_APPA;
2760
2761 #if     (MCLBYTES & (MCLBYTES - 1)) == 0
2762                 if (mss > MCLBYTES)
2763                         mss &= ~(MCLBYTES-1);
2764 #else
2765                 if (mss > MCLBYTES)
2766                         mss = mss / MCLBYTES * MCLBYTES;
2767 #endif
2768         /*
2769          * If there's a pipesize, change the socket buffer
2770          * to that size.  Make the socket buffers an integral
2771          * number of mss units; if the mss is larger than
2772          * the socket buffer, decrease the mss.
2773          */
2774 #ifdef RTV_SPIPE
2775         if ((bufsize = rt->rt_rmx.rmx_sendpipe) == 0)
2776 #endif
2777                 bufsize = so->so_snd.sb_hiwat;
2778         if (bufsize < mss)
2779                 mss = bufsize;
2780         else {
2781                 bufsize = roundup(bufsize, mss);
2782                 if (bufsize > sb_max)
2783                         bufsize = sb_max;
2784                 if (bufsize > so->so_snd.sb_hiwat)
2785                         (void)sbreserve(&so->so_snd, bufsize, so, NULL);
2786         }
2787         tp->t_maxseg = mss;
2788
2789 #ifdef RTV_RPIPE
2790         if ((bufsize = rt->rt_rmx.rmx_recvpipe) == 0)
2791 #endif
2792                 bufsize = so->so_rcv.sb_hiwat;
2793         if (bufsize > mss) {
2794                 bufsize = roundup(bufsize, mss);
2795                 if (bufsize > sb_max)
2796                         bufsize = sb_max;
2797                 if (bufsize > so->so_rcv.sb_hiwat)
2798                         (void)sbreserve(&so->so_rcv, bufsize, so, NULL);
2799         }
2800
2801         /*
2802          * Set the slow-start flight size depending on whether this
2803          * is a local network or not.
2804          */
2805         if (tcp_do_rfc3390)
2806                 tp->snd_cwnd = min(4 * mss, max(2 * mss, 4380));
2807         else if ((isipv6 && in6_localaddr(&inp->in6p_faddr)) ||
2808                  (!isipv6 && in_localaddr(inp->inp_faddr)))
2809                 tp->snd_cwnd = mss * ss_fltsz_local;
2810         else
2811                 tp->snd_cwnd = mss * ss_fltsz;
2812
2813         if (rt->rt_rmx.rmx_ssthresh) {
2814                 /*
2815                  * There's some sort of gateway or interface
2816                  * buffer limit on the path.  Use this to set
2817                  * the slow start threshhold, but set the
2818                  * threshold to no less than 2*mss.
2819                  */
2820                 tp->snd_ssthresh = max(2 * mss, rt->rt_rmx.rmx_ssthresh);
2821                 tcpstat.tcps_usedssthresh++;
2822         }
2823 }
2824
2825 /*
2826  * Determine the MSS option to send on an outgoing SYN.
2827  */
2828 int
2829 tcp_mssopt(tp)
2830         struct tcpcb *tp;
2831 {
2832         struct rtentry *rt;
2833 #ifdef INET6
2834         boolean_t isipv6 =
2835             ((tp->t_inpcb->inp_vflag & INP_IPV6) ? TRUE : FALSE);
2836         int min_protoh = isipv6 ?
2837                              sizeof(struct ip6_hdr) + sizeof(struct tcphdr) :
2838                              sizeof(struct tcpiphdr);
2839 #else
2840         const boolean_t isipv6 = FALSE;
2841         const size_t min_protoh = sizeof(struct tcpiphdr);
2842 #endif
2843
2844         if (isipv6)
2845                 rt = tcp_rtlookup6(&tp->t_inpcb->inp_inc);
2846         else
2847                 rt = tcp_rtlookup(&tp->t_inpcb->inp_inc);
2848         if (rt == NULL)
2849                 return (isipv6 ? tcp_v6mssdflt : tcp_mssdflt);
2850
2851         return (rt->rt_ifp->if_mtu - min_protoh);
2852 }
2853
2854
2855 /*
2856  * When a partial ack arrives, force the retransmission of the
2857  * next unacknowledged segment.  Do not clear tp->t_dupacks.
2858  * By setting snd_nxt to ti_ack, this forces retransmission timer to
2859  * be started again.
2860  */
2861 static void
2862 tcp_newreno_partial_ack(tp, th)
2863         struct tcpcb *tp;
2864         struct tcphdr *th;
2865 {
2866         tcp_seq onxt = tp->snd_nxt;
2867         u_long  ocwnd = tp->snd_cwnd;
2868
2869         callout_stop(tp->tt_rexmt);
2870         tp->t_rtttime = 0;
2871         tp->snd_nxt = th->th_ack;
2872         /*
2873          * Set snd_cwnd to one segment beyond acknowledged offset
2874          * (tp->snd_una has not yet been updated when this function is called.)
2875          */
2876         tp->snd_cwnd = tp->t_maxseg + (th->th_ack - tp->snd_una);
2877         tp->t_flags |= TF_ACKNOW;
2878         (void) tcp_output(tp);
2879         tp->snd_cwnd = ocwnd;
2880         if (SEQ_GT(onxt, tp->snd_nxt))
2881                 tp->snd_nxt = onxt;
2882         /*
2883          * Partial window deflation.  Relies on fact that tp->snd_una
2884          * not updated yet.
2885          */
2886         tp->snd_cwnd -= (th->th_ack - tp->snd_una - tp->t_maxseg);
2887 }