slirp/tcp_output.c

   1 /*
   2  * Copyright (c) 1982, 1986, 1988, 1990, 1993
   3  *      The Regents of the University of California.  All rights reserved.
   4  *
   5  * Redistribution and use in source and binary forms, with or without
   6  * modification, are permitted provided that the following conditions
   7  * are met:
   8  * 1. Redistributions of source code must retain the above copyright
   9  *    notice, this list of conditions and the following disclaimer.
  10  * 2. Redistributions in binary form must reproduce the above copyright
  11  *    notice, this list of conditions and the following disclaimer in the
  12  *    documentation and/or other materials provided with the distribution.
  13  * 3. All advertising materials mentioning features or use of this software
  14  *    must display the following acknowledgement:
  15  *      This product includes software developed by the University of
  16  *      California, Berkeley and its contributors.
  17  * 4. Neither the name of the University nor the names of its contributors
  18  *    may be used to endorse or promote products derived from this software
  19  *    without specific prior written permission.
  20  *
  21  * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
  22  * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
  23  * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
  24  * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
  25  * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
  26  * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
  27  * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
  28  * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
  29  * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
  30  * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
  31  * SUCH DAMAGE.
  32  *
  33  *      @(#)tcp_output.c        8.3 (Berkeley) 12/30/93
  34  * tcp_output.c,v 1.3 1994/09/15 10:36:55 davidg Exp
  35  */
  36
  37 /*
  38  * Changes and additions relating to SLiRP
  39  * Copyright (c) 1995 Danny Gasparovski.
  40  *
  41  * Please read the file COPYRIGHT for the
  42  * terms and conditions of the copyright.
  43  */
  44
  45 #include <slirp.h>
  46
  47 /*
  48  * Since this is only used in "stats socket", we give meaning
  49  * names instead of the REAL names
  50  */
  51 char *tcpstates[] = {
  52 /*      "CLOSED",       "LISTEN",       "SYN_SENT",     "SYN_RCVD", */
  53         "REDIRECT",     "LISTEN",       "SYN_SENT",     "SYN_RCVD",
  54         "ESTABLISHED",  "CLOSE_WAIT",   "FIN_WAIT_1",   "CLOSING",
  55         "LAST_ACK",     "FIN_WAIT_2",   "TIME_WAIT",
  56 };
  57
  58 u_char  tcp_outflags[TCP_NSTATES] = {
  59         TH_RST|TH_ACK, 0,      TH_SYN,        TH_SYN|TH_ACK,
  60         TH_ACK,        TH_ACK, TH_FIN|TH_ACK, TH_FIN|TH_ACK,
  61         TH_FIN|TH_ACK, TH_ACK, TH_ACK,
  62 };
  63
  64
  65 #define MAX_TCPOPTLEN   32      /* max # bytes that go in options */
  66
  67 /*
  68  * Tcp output routine: figure out what should be sent and send it.
  69  */
  70 int
  71 tcp_output(tp)
  72         register struct tcpcb *tp;
  73 {
  74         register struct socket *so = tp->t_socket;
  75         register long len, win;
  76         int off, flags, error;
  77         register struct mbuf *m;
  78         register struct tcpiphdr *ti;
  79         u_char opt[MAX_TCPOPTLEN];
  80         unsigned optlen, hdrlen;
  81         int idle, sendalot;
  82
  83         DEBUG_CALL("tcp_output");
  84         DEBUG_ARG("tp = %lx", (long )tp);
  85
  86         /*
  87          * Determine length of data that should be transmitted,
  88          * and flags that will be used.
  89          * If there is some data or critical controls (SYN, RST)
  90          * to send, then transmit; otherwise, investigate further.
  91          */
  92         idle = (tp->snd_max == tp->snd_una);
  93         if (idle && tp->t_idle >= tp->t_rxtcur)
  94                 /*
  95                  * We have been idle for "a while" and no acks are
  96                  * expected to clock out any data we send --
  97                  * slow start to get ack "clock" running again.
  98                  */
  99                 tp->snd_cwnd = tp->t_maxseg;
 100 again:
 101         sendalot = 0;
 102         off = tp->snd_nxt - tp->snd_una;
 103         win = min(tp->snd_wnd, tp->snd_cwnd);
 104
 105         flags = tcp_outflags[tp->t_state];
 106
 107         DEBUG_MISC((dfd, " --- tcp_output flags = 0x%x\n",flags));
 108
 109         /*
 110          * If in persist timeout with window of 0, send 1 byte.
 111          * Otherwise, if window is small but nonzero
 112          * and timer expired, we will send what we can
 113          * and go to transmit state.
 114          */
 115         if (tp->t_force) {
 116                 if (win == 0) {
 117                         /*
 118                          * If we still have some data to send, then
 119                          * clear the FIN bit.  Usually this would
 120                          * happen below when it realizes that we
 121                          * aren't sending all the data.  However,
 122                          * if we have exactly 1 byte of unset data,
 123                          * then it won't clear the FIN bit below,
 124                          * and if we are in persist state, we wind
 125                          * up sending the packet without recording
 126                          * that we sent the FIN bit.
 127                          *
 128                          * We can't just blindly clear the FIN bit,
 129                          * because if we don't have any more data
 130                          * to send then the probe will be the FIN
 131                          * itself.
 132                          */
 133                         if (off < so->so_snd.sb_cc)
 134                                 flags &= ~TH_FIN;
 135                         win = 1;
 136                 } else {
 137                         tp->t_timer[TCPT_PERSIST] = 0;
 138                         tp->t_rxtshift = 0;
 139                 }
 140         }
 141
 142         len = min(so->so_snd.sb_cc, win) - off;
 143
 144         if (len < 0) {
 145                 /*
 146                  * If FIN has been sent but not acked,
 147                  * but we haven't been called to retransmit,
 148                  * len will be -1.  Otherwise, window shrank
 149                  * after we sent into it.  If window shrank to 0,
 150                  * cancel pending retransmit and pull snd_nxt
 151                  * back to (closed) window.  We will enter persist
 152                  * state below.  If the window didn't close completely,
 153                  * just wait for an ACK.
 154                  */
 155                 len = 0;
 156                 if (win == 0) {
 157                         tp->t_timer[TCPT_REXMT] = 0;
 158                         tp->snd_nxt = tp->snd_una;
 159                 }
 160         }
 161
 162         if (len > tp->t_maxseg) {
 163                 len = tp->t_maxseg;
 164                 sendalot = 1;
 165         }
 166         if (SEQ_LT(tp->snd_nxt + len, tp->snd_una + so->so_snd.sb_cc))
 167                 flags &= ~TH_FIN;
 168
 169         win = sbspace(&so->so_rcv);
 170
 171         /*
 172          * Sender silly window avoidance.  If connection is idle
 173          * and can send all data, a maximum segment,
 174          * at least a maximum default-size segment do it,
 175          * or are forced, do it; otherwise don't bother.
 176          * If peer's buffer is tiny, then send
 177          * when window is at least half open.
 178          * If retransmitting (possibly after persist timer forced us
 179          * to send into a small window), then must resend.
 180          */
 181         if (len) {
 182                 if (len == tp->t_maxseg)
 183                         goto send;
 184                 if ((1 || idle || tp->t_flags & TF_NODELAY) &&
 185                     len + off >= so->so_snd.sb_cc)
 186                         goto send;
 187                 if (tp->t_force)
 188                         goto send;
 189                 if (len >= tp->max_sndwnd / 2 && tp->max_sndwnd > 0)
 190                         goto send;
 191                 if (SEQ_LT(tp->snd_nxt, tp->snd_max))
 192                         goto send;
 193         }
 194
 195         /*
 196          * Compare available window to amount of window
 197          * known to peer (as advertised window less
 198          * next expected input).  If the difference is at least two
 199          * max size segments, or at least 50% of the maximum possible
 200          * window, then want to send a window update to peer.
 201          */
 202         if (win > 0) {
 203                 /*
 204                  * "adv" is the amount we can increase the window,
 205                  * taking into account that we are limited by
 206                  * TCP_MAXWIN << tp->rcv_scale.
 207                  */
 208                 long adv = min(win, (long)TCP_MAXWIN << tp->rcv_scale) -
 209                         (tp->rcv_adv - tp->rcv_nxt);
 210
 211                 if (adv >= (long) (2 * tp->t_maxseg))
 212                         goto send;
 213                 if (2 * adv >= (long) so->so_rcv.sb_datalen)
 214                         goto send;
 215         }
 216
 217         /*
 218          * Send if we owe peer an ACK.
 219          */
 220         if (tp->t_flags & TF_ACKNOW)
 221                 goto send;
 222         if (flags & (TH_SYN|TH_RST))
 223                 goto send;
 224         if (SEQ_GT(tp->snd_up, tp->snd_una))
 225                 goto send;
 226         /*
 227          * If our state indicates that FIN should be sent
 228          * and we have not yet done so, or we're retransmitting the FIN,
 229          * then we need to send.
 230          */
 231         if (flags & TH_FIN &&
 232             ((tp->t_flags & TF_SENTFIN) == 0 || tp->snd_nxt == tp->snd_una))
 233                 goto send;
 234
 235         /*
 236          * TCP window updates are not reliable, rather a polling protocol
 237          * using ``persist'' packets is used to insure receipt of window
 238          * updates.  The three ``states'' for the output side are:
 239          *      idle                    not doing retransmits or persists
 240          *      persisting              to move a small or zero window
 241          *      (re)transmitting        and thereby not persisting
 242          *
 243          * tp->t_timer[TCPT_PERSIST]
 244          *      is set when we are in persist state.
 245          * tp->t_force
 246          *      is set when we are called to send a persist packet.
 247          * tp->t_timer[TCPT_REXMT]
 248          *      is set when we are retransmitting
 249          * The output side is idle when both timers are zero.
 250          *
 251          * If send window is too small, there is data to transmit, and no
 252          * retransmit or persist is pending, then go to persist state.
 253          * If nothing happens soon, send when timer expires:
 254          * if window is nonzero, transmit what we can,
 255          * otherwise force out a byte.
 256          */
 257         if (so->so_snd.sb_cc && tp->t_timer[TCPT_REXMT] == 0 &&
 258             tp->t_timer[TCPT_PERSIST] == 0) {
 259                 tp->t_rxtshift = 0;
 260                 tcp_setpersist(tp);
 261         }
 262
 263         /*
 264          * No reason to send a segment, just return.
 265          */
 266         tcpstat.tcps_didnuttin++;
 267
 268         return (0);
 269
 270 send:
 271         /*
 272          * Before ESTABLISHED, force sending of initial options
 273          * unless TCP set not to do any options.
 274          * NOTE: we assume that the IP/TCP header plus TCP options
 275          * always fit in a single mbuf, leaving room for a maximum
 276          * link header, i.e.
 277          *      max_linkhdr + sizeof (struct tcpiphdr) + optlen <= MHLEN
 278          */
 279         optlen = 0;
 280         hdrlen = sizeof (struct tcpiphdr);
 281         if (flags & TH_SYN) {
 282                 tp->snd_nxt = tp->iss;
 283                 if ((tp->t_flags & TF_NOOPT) == 0) {
 284                         u_int16_t mss;
 285
 286                         opt[0] = TCPOPT_MAXSEG;
 287                         opt[1] = 4;
 288                         mss = htons((u_int16_t) tcp_mss(tp, 0));
 289                         memcpy((caddr_t)(opt + 2), (caddr_t)&mss, sizeof(mss));
 290                         optlen = 4;
 291
 292 /*                      if ((tp->t_flags & TF_REQ_SCALE) &&
 293  *                          ((flags & TH_ACK) == 0 ||
 294  *                          (tp->t_flags & TF_RCVD_SCALE))) {
 295  *                              *((u_int32_t *) (opt + optlen)) = htonl(
 296  *                                      TCPOPT_NOP << 24 |
 297  *                                      TCPOPT_WINDOW << 16 |
 298  *                                      TCPOLEN_WINDOW << 8 |
 299  *                                      tp->request_r_scale);
 300  *                              optlen += 4;
 301  *                      }
 302  */
 303                 }
 304         }
 305
 306         /*
 307          * Send a timestamp and echo-reply if this is a SYN and our side
 308          * wants to use timestamps (TF_REQ_TSTMP is set) or both our side
 309          * and our peer have sent timestamps in our SYN's.
 310          */
 311 /*      if ((tp->t_flags & (TF_REQ_TSTMP|TF_NOOPT)) == TF_REQ_TSTMP &&
 312  *           (flags & TH_RST) == 0 &&
 313  *          ((flags & (TH_SYN|TH_ACK)) == TH_SYN ||
 314  *           (tp->t_flags & TF_RCVD_TSTMP))) {
 315  *              u_int32_t *lp = (u_int32_t *)(opt + optlen);
 316  *
 317  *              / * Form timestamp option as shown in appendix A of RFC 1323. *  /
 318  *              *lp++ = htonl(TCPOPT_TSTAMP_HDR);
 319  *              *lp++ = htonl(tcp_now);
 320  *              *lp   = htonl(tp->ts_recent);
 321  *              optlen += TCPOLEN_TSTAMP_APPA;
 322  *      }
 323  */
 324         hdrlen += optlen;
 325
 326         /*
 327          * Adjust data length if insertion of options will
 328          * bump the packet length beyond the t_maxseg length.
 329          */
 330          if (len > tp->t_maxseg - optlen) {
 331                 len = tp->t_maxseg - optlen;
 332                 sendalot = 1;
 333          }
 334
 335         /*
 336          * Grab a header mbuf, attaching a copy of data to
 337          * be transmitted, and initialize the header from
 338          * the template for sends on this connection.
 339          */
 340         if (len) {
 341                 if (tp->t_force && len == 1)
 342                         tcpstat.tcps_sndprobe++;
 343                 else if (SEQ_LT(tp->snd_nxt, tp->snd_max)) {
 344                         tcpstat.tcps_sndrexmitpack++;
 345                         tcpstat.tcps_sndrexmitbyte += len;
 346                 } else {
 347                         tcpstat.tcps_sndpack++;
 348                         tcpstat.tcps_sndbyte += len;
 349                 }
 350
 351                 m = m_get();
 352                 if (m == NULL) {
 353 /*                      error = ENOBUFS; */
 354                         error = 1;
 355                         goto out;
 356                 }
 357                 m->m_data += if_maxlinkhdr;
 358                 m->m_len = hdrlen;
 359
 360                 /*
 361                  * This will always succeed, since we make sure our mbufs
 362                  * are big enough to hold one MSS packet + header + ... etc.
 363                  */
 364 /*              if (len <= MHLEN - hdrlen - max_linkhdr) { */
 365
 366                         sbcopy(&so->so_snd, off, (int) len, mtod(m, caddr_t) + hdrlen);
 367                         m->m_len += len;
 368
 369 /*              } else {
 370  *                      m->m_next = m_copy(so->so_snd.sb_mb, off, (int) len);
 371  *                      if (m->m_next == 0)
 372  *                              len = 0;
 373  *              }
 374  */
 375                 /*
 376                  * If we're sending everything we've got, set PUSH.
 377                  * (This will keep happy those implementations which only
 378                  * give data to the user when a buffer fills or
 379                  * a PUSH comes in.)
 380                  */
 381                 if (off + len == so->so_snd.sb_cc)
 382                         flags |= TH_PUSH;
 383         } else {
 384                 if (tp->t_flags & TF_ACKNOW)
 385                         tcpstat.tcps_sndacks++;
 386                 else if (flags & (TH_SYN|TH_FIN|TH_RST))
 387                         tcpstat.tcps_sndctrl++;
 388                 else if (SEQ_GT(tp->snd_up, tp->snd_una))
 389                         tcpstat.tcps_sndurg++;
 390                 else
 391                         tcpstat.tcps_sndwinup++;
 392
 393                 m = m_get();
 394                 if (m == NULL) {
 395 /*                      error = ENOBUFS; */
 396                         error = 1;
 397                         goto out;
 398                 }
 399                 m->m_data += if_maxlinkhdr;
 400                 m->m_len = hdrlen;
 401         }
 402
 403         ti = mtod(m, struct tcpiphdr *);
 404
 405         memcpy((caddr_t)ti, &tp->t_template, sizeof (struct tcpiphdr));
 406
 407         /*
 408          * Fill in fields, remembering maximum advertised
 409          * window for use in delaying messages about window sizes.
 410          * If resending a FIN, be sure not to use a new sequence number.
 411          */
 412         if (flags & TH_FIN && tp->t_flags & TF_SENTFIN &&
 413             tp->snd_nxt == tp->snd_max)
 414                 tp->snd_nxt--;
 415         /*
 416          * If we are doing retransmissions, then snd_nxt will
 417          * not reflect the first unsent octet.  For ACK only
 418          * packets, we do not want the sequence number of the
 419          * retransmitted packet, we want the sequence number
 420          * of the next unsent octet.  So, if there is no data
 421          * (and no SYN or FIN), use snd_max instead of snd_nxt
 422          * when filling in ti_seq.  But if we are in persist
 423          * state, snd_max might reflect one byte beyond the
 424          * right edge of the window, so use snd_nxt in that
 425          * case, since we know we aren't doing a retransmission.
 426          * (retransmit and persist are mutually exclusive...)
 427          */
 428         if (len || (flags & (TH_SYN|TH_FIN)) || tp->t_timer[TCPT_PERSIST])
 429                 ti->ti_seq = htonl(tp->snd_nxt);
 430         else
 431                 ti->ti_seq = htonl(tp->snd_max);
 432         ti->ti_ack = htonl(tp->rcv_nxt);
 433         if (optlen) {
 434                 memcpy((caddr_t)(ti + 1), (caddr_t)opt, optlen);
 435                 ti->ti_off = (sizeof (struct tcphdr) + optlen) >> 2;
 436         }
 437         ti->ti_flags = flags;
 438         /*
 439          * Calculate receive window.  Don't shrink window,
 440          * but avoid silly window syndrome.
 441          */
 442         if (win < (long)(so->so_rcv.sb_datalen / 4) && win < (long)tp->t_maxseg)
 443                 win = 0;
 444         if (win > (long)TCP_MAXWIN << tp->rcv_scale)
 445                 win = (long)TCP_MAXWIN << tp->rcv_scale;
 446         if (win < (long)(tp->rcv_adv - tp->rcv_nxt))
 447                 win = (long)(tp->rcv_adv - tp->rcv_nxt);
 448         ti->ti_win = htons((u_int16_t) (win>>tp->rcv_scale));
 449
 450         if (SEQ_GT(tp->snd_up, tp->snd_una)) {
 451                 ti->ti_urp = htons((u_int16_t)(tp->snd_up - ntohl(ti->ti_seq)));
 452 #ifdef notdef
 453         if (SEQ_GT(tp->snd_up, tp->snd_nxt)) {
 454                 ti->ti_urp = htons((u_int16_t)(tp->snd_up - tp->snd_nxt));
 455 #endif
 456                 ti->ti_flags |= TH_URG;
 457         } else
 458                 /*
 459                  * If no urgent pointer to send, then we pull
 460                  * the urgent pointer to the left edge of the send window
 461                  * so that it doesn't drift into the send window on sequence
 462                  * number wraparound.
 463                  */
 464                 tp->snd_up = tp->snd_una;               /* drag it along */
 465
 466         /*
 467          * Put TCP length in extended header, and then
 468          * checksum extended header and data.
 469          */
 470         if (len + optlen)
 471                 ti->ti_len = htons((u_int16_t)(sizeof (struct tcphdr) +
 472                     optlen + len));
 473         ti->ti_sum = cksum(m, (int)(hdrlen + len));
 474
 475         /*
 476          * In transmit state, time the transmission and arrange for
 477          * the retransmit.  In persist state, just set snd_max.
 478          */
 479         if (tp->t_force == 0 || tp->t_timer[TCPT_PERSIST] == 0) {
 480                 tcp_seq startseq = tp->snd_nxt;
 481
 482                 /*
 483                  * Advance snd_nxt over sequence space of this segment.
 484                  */
 485                 if (flags & (TH_SYN|TH_FIN)) {
 486                         if (flags & TH_SYN)
 487                                 tp->snd_nxt++;
 488                         if (flags & TH_FIN) {
 489                                 tp->snd_nxt++;
 490                                 tp->t_flags |= TF_SENTFIN;
 491                         }
 492                 }
 493                 tp->snd_nxt += len;
 494                 if (SEQ_GT(tp->snd_nxt, tp->snd_max)) {
 495                         tp->snd_max = tp->snd_nxt;
 496                         /*
 497                          * Time this transmission if not a retransmission and
 498                          * not currently timing anything.
 499                          */
 500                         if (tp->t_rtt == 0) {
 501                                 tp->t_rtt = 1;
 502                                 tp->t_rtseq = startseq;
 503                                 tcpstat.tcps_segstimed++;
 504                         }
 505                 }
 506
 507                 /*
 508                  * Set retransmit timer if not currently set,
 509                  * and not doing an ack or a keep-alive probe.
 510                  * Initial value for retransmit timer is smoothed
 511                  * round-trip time + 2 * round-trip time variance.
 512                  * Initialize shift counter which is used for backoff
 513                  * of retransmit time.
 514                  */
 515                 if (tp->t_timer[TCPT_REXMT] == 0 &&
 516                     tp->snd_nxt != tp->snd_una) {
 517                         tp->t_timer[TCPT_REXMT] = tp->t_rxtcur;
 518                         if (tp->t_timer[TCPT_PERSIST]) {
 519                                 tp->t_timer[TCPT_PERSIST] = 0;
 520                                 tp->t_rxtshift = 0;
 521                         }
 522                 }
 523         } else
 524                 if (SEQ_GT(tp->snd_nxt + len, tp->snd_max))
 525                         tp->snd_max = tp->snd_nxt + len;
 526
 527         /*
 528          * Fill in IP length and desired time to live and
 529          * send to IP level.  There should be a better way
 530          * to handle ttl and tos; we could keep them in
 531          * the template, but need a way to checksum without them.
 532          */
 533         m->m_len = hdrlen + len; /* XXX Needed? m_len should be correct */
 534
 535     {
 536
 537         ((struct ip *)ti)->ip_len = m->m_len;
 538
 539         ((struct ip *)ti)->ip_ttl = ip_defttl;
 540         ((struct ip *)ti)->ip_tos = so->so_iptos;
 541
 542 /* #if BSD >= 43 */
 543         /* Don't do IP options... */
 544 /*      error = ip_output(m, tp->t_inpcb->inp_options, &tp->t_inpcb->inp_route,
 545  *          so->so_options & SO_DONTROUTE, 0);
 546  */
 547         error = ip_output(so, m);
 548
 549 /* #else
 550  *      error = ip_output(m, (struct mbuf *)0, &tp->t_inpcb->inp_route,
 551  *          so->so_options & SO_DONTROUTE);
 552  * #endif
 553  */
 554     }
 555         if (error) {
 556 out:
 557 /*              if (error == ENOBUFS) {
 558  *                      tcp_quench(tp->t_inpcb, 0);
 559  *                      return (0);
 560  *              }
 561  */
 562 /*              if ((error == EHOSTUNREACH || error == ENETDOWN)
 563  *                  && TCPS_HAVERCVDSYN(tp->t_state)) {
 564  *                      tp->t_softerror = error;
 565  *                      return (0);
 566  *              }
 567  */
 568                 return (error);
 569         }
 570         tcpstat.tcps_sndtotal++;
 571
 572         /*
 573          * Data sent (as far as we can tell).
 574          * If this advertises a larger window than any other segment,
 575          * then remember the size of the advertised window.
 576          * Any pending ACK has now been sent.
 577          */
 578         if (win > 0 && SEQ_GT(tp->rcv_nxt+win, tp->rcv_adv))
 579                 tp->rcv_adv = tp->rcv_nxt + win;
 580         tp->last_ack_sent = tp->rcv_nxt;
 581         tp->t_flags &= ~(TF_ACKNOW|TF_DELACK);
 582         if (sendalot)
 583                 goto again;
 584
 585         return (0);
 586 }
 587
 588 void
 589 tcp_setpersist(tp)
 590         register struct tcpcb *tp;
 591 {
 592     int t = ((tp->t_srtt >> 2) + tp->t_rttvar) >> 1;
 593
 594 /*      if (tp->t_timer[TCPT_REXMT])
 595  *              panic("tcp_output REXMT");
 596  */
 597         /*
 598          * Start/restart persistence timer.
 599          */
 600         TCPT_RANGESET(tp->t_timer[TCPT_PERSIST],
 601             t * tcp_backoff[tp->t_rxtshift],
 602             TCPTV_PERSMIN, TCPTV_PERSMAX);
 603         if (tp->t_rxtshift < TCP_MAXRXTSHIFT)
 604                 tp->t_rxtshift++;
 605 }