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