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