1 /* SPDX-License-Identifier: BSD-3-Clause */
2 /*
3 * Copyright (c) 1982, 1986, 1988, 1990, 1993
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. Neither the name of the University nor the names of its contributors
15 * may be used to endorse or promote products derived from this software
16 * without specific prior written permission.
17 *
18 * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
19 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
20 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
21 * ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
22 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
23 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
24 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
25 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
26 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
27 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
28 * SUCH DAMAGE.
29 *
30 * @(#)tcp_output.c 8.3 (Berkeley) 12/30/93
31 * tcp_output.c,v 1.3 1994/09/15 10:36:55 davidg Exp
32 */
33
34 /*
35 * Changes and additions relating to SLiRP
36 * Copyright (c) 1995 Danny Gasparovski.
37 */
38
39 #include "slirp.h"
40
41 static const uint8_t tcp_outflags[TCP_NSTATES] = {
42 TH_RST | TH_ACK, 0, TH_SYN, TH_SYN | TH_ACK,
43 TH_ACK, TH_ACK, TH_FIN | TH_ACK, TH_FIN | TH_ACK,
44 TH_FIN | TH_ACK, TH_ACK, TH_ACK,
45 };
46
47
48 #undef MAX_TCPOPTLEN
49 #define MAX_TCPOPTLEN 32 /* max # bytes that go in options */
50
51 /*
52 * Tcp output routine: figure out what should be sent and send it.
53 */
tcp_output(struct tcpcb * tp)54 int tcp_output(struct tcpcb *tp)
55 {
56 register struct socket *so = tp->t_socket;
57 register long len, win;
58 int off, flags, error;
59 register struct mbuf *m;
60 register struct tcpiphdr *ti, tcpiph_save;
61 struct ip *ip;
62 struct ip6 *ip6;
63 uint8_t opt[MAX_TCPOPTLEN];
64 unsigned optlen, hdrlen;
65 int idle, sendalot;
66
67 DEBUG_CALL("tcp_output");
68 DEBUG_ARG("tp = %p", tp);
69
70 /*
71 * Determine length of data that should be transmitted,
72 * and flags that will be used.
73 * If there is some data or critical controls (SYN, RST)
74 * to send, then transmit; otherwise, investigate further.
75 */
76 idle = (tp->snd_max == tp->snd_una);
77 if (idle && tp->t_idle >= tp->t_rxtcur)
78 /*
79 * We have been idle for "a while" and no acks are
80 * expected to clock out any data we send --
81 * slow start to get ack "clock" running again.
82 */
83 tp->snd_cwnd = tp->t_maxseg;
84 again:
85 sendalot = 0;
86 off = tp->snd_nxt - tp->snd_una;
87 win = MIN(tp->snd_wnd, tp->snd_cwnd);
88
89 flags = tcp_outflags[tp->t_state];
90
91 DEBUG_MISC(" --- tcp_output flags = 0x%x", flags);
92
93 /*
94 * If in persist timeout with window of 0, send 1 byte.
95 * Otherwise, if window is small but nonzero
96 * and timer expired, we will send what we can
97 * and go to transmit state.
98 */
99 if (tp->t_force) {
100 if (win == 0) {
101 /*
102 * If we still have some data to send, then
103 * clear the FIN bit. Usually this would
104 * happen below when it realizes that we
105 * aren't sending all the data. However,
106 * if we have exactly 1 byte of unset data,
107 * then it won't clear the FIN bit below,
108 * and if we are in persist state, we wind
109 * up sending the packet without recording
110 * that we sent the FIN bit.
111 *
112 * We can't just blindly clear the FIN bit,
113 * because if we don't have any more data
114 * to send then the probe will be the FIN
115 * itself.
116 */
117 if (off < so->so_snd.sb_cc)
118 flags &= ~TH_FIN;
119 win = 1;
120 } else {
121 tp->t_timer[TCPT_PERSIST] = 0;
122 tp->t_rxtshift = 0;
123 }
124 }
125
126 len = MIN(so->so_snd.sb_cc, win) - off;
127
128 if (len < 0) {
129 /*
130 * If FIN has been sent but not acked,
131 * but we haven't been called to retransmit,
132 * len will be -1. Otherwise, window shrank
133 * after we sent into it. If window shrank to 0,
134 * cancel pending retransmit and pull snd_nxt
135 * back to (closed) window. We will enter persist
136 * state below. If the window didn't close completely,
137 * just wait for an ACK.
138 */
139 len = 0;
140 if (win == 0) {
141 tp->t_timer[TCPT_REXMT] = 0;
142 tp->snd_nxt = tp->snd_una;
143 }
144 }
145
146 if (len > tp->t_maxseg) {
147 len = tp->t_maxseg;
148 sendalot = 1;
149 }
150 if (SEQ_LT(tp->snd_nxt + len, tp->snd_una + so->so_snd.sb_cc))
151 flags &= ~TH_FIN;
152
153 win = sbspace(&so->so_rcv);
154
155 /*
156 * Sender silly window avoidance. If connection is idle
157 * and can send all data, a maximum segment,
158 * at least a maximum default-size segment do it,
159 * or are forced, do it; otherwise don't bother.
160 * If peer's buffer is tiny, then send
161 * when window is at least half open.
162 * If retransmitting (possibly after persist timer forced us
163 * to send into a small window), then must resend.
164 */
165 if (len) {
166 if (len == tp->t_maxseg)
167 goto send;
168 if ((1 || idle || tp->t_flags & TF_NODELAY) &&
169 len + off >= so->so_snd.sb_cc)
170 goto send;
171 if (tp->t_force)
172 goto send;
173 if (len >= tp->max_sndwnd / 2 && tp->max_sndwnd > 0)
174 goto send;
175 if (SEQ_LT(tp->snd_nxt, tp->snd_max))
176 goto send;
177 }
178
179 /*
180 * Compare available window to amount of window
181 * known to peer (as advertised window less
182 * next expected input). If the difference is at least two
183 * max size segments, or at least 50% of the maximum possible
184 * window, then want to send a window update to peer.
185 */
186 if (win > 0) {
187 /*
188 * "adv" is the amount we can increase the window,
189 * taking into account that we are limited by
190 * TCP_MAXWIN << tp->rcv_scale.
191 */
192 long adv = MIN(win, (long)TCP_MAXWIN << tp->rcv_scale) -
193 (tp->rcv_adv - tp->rcv_nxt);
194
195 if (adv >= (long)(2 * tp->t_maxseg))
196 goto send;
197 if (2 * adv >= (long)so->so_rcv.sb_datalen)
198 goto send;
199 }
200
201 /*
202 * Send if we owe peer an ACK.
203 */
204 if (tp->t_flags & TF_ACKNOW)
205 goto send;
206 if (flags & (TH_SYN | TH_RST))
207 goto send;
208 if (SEQ_GT(tp->snd_up, tp->snd_una))
209 goto send;
210 /*
211 * If our state indicates that FIN should be sent
212 * and we have not yet done so, or we're retransmitting the FIN,
213 * then we need to send.
214 */
215 if (flags & TH_FIN &&
216 ((tp->t_flags & TF_SENTFIN) == 0 || tp->snd_nxt == tp->snd_una))
217 goto send;
218
219 /*
220 * TCP window updates are not reliable, rather a polling protocol
221 * using ``persist'' packets is used to insure receipt of window
222 * updates. The three ``states'' for the output side are:
223 * idle not doing retransmits or persists
224 * persisting to move a small or zero window
225 * (re)transmitting and thereby not persisting
226 *
227 * tp->t_timer[TCPT_PERSIST]
228 * is set when we are in persist state.
229 * tp->t_force
230 * is set when we are called to send a persist packet.
231 * tp->t_timer[TCPT_REXMT]
232 * is set when we are retransmitting
233 * The output side is idle when both timers are zero.
234 *
235 * If send window is too small, there is data to transmit, and no
236 * retransmit or persist is pending, then go to persist state.
237 * If nothing happens soon, send when timer expires:
238 * if window is nonzero, transmit what we can,
239 * otherwise force out a byte.
240 */
241 if (so->so_snd.sb_cc && tp->t_timer[TCPT_REXMT] == 0 &&
242 tp->t_timer[TCPT_PERSIST] == 0) {
243 tp->t_rxtshift = 0;
244 tcp_setpersist(tp);
245 }
246
247 /*
248 * No reason to send a segment, just return.
249 */
250 return (0);
251
252 send:
253 /*
254 * Before ESTABLISHED, force sending of initial options
255 * unless TCP set not to do any options.
256 * NOTE: we assume that the IP/TCP header plus TCP options
257 * always fit in a single mbuf, leaving room for a maximum
258 * link header, i.e.
259 * max_linkhdr + sizeof (struct tcpiphdr) + optlen <= MHLEN
260 */
261 optlen = 0;
262 hdrlen = sizeof(struct tcpiphdr);
263 if (flags & TH_SYN) {
264 tp->snd_nxt = tp->iss;
265 if ((tp->t_flags & TF_NOOPT) == 0) {
266 uint16_t mss;
267
268 opt[0] = TCPOPT_MAXSEG;
269 opt[1] = 4;
270 mss = htons((uint16_t)tcp_mss(tp, 0));
271 memcpy((char *)(opt + 2), (char *)&mss, sizeof(mss));
272 optlen = 4;
273 }
274 }
275
276 hdrlen += optlen;
277
278 /*
279 * Adjust data length if insertion of options will
280 * bump the packet length beyond the t_maxseg length.
281 */
282 if (len > tp->t_maxseg - optlen) {
283 len = tp->t_maxseg - optlen;
284 sendalot = 1;
285 }
286
287 /*
288 * Grab a header mbuf, attaching a copy of data to
289 * be transmitted, and initialize the header from
290 * the template for sends on this connection.
291 */
292 if (len) {
293 m = m_get(so->slirp);
294 if (m == NULL) {
295 error = 1;
296 goto out;
297 }
298 m->m_data += IF_MAXLINKHDR;
299 m->m_len = hdrlen;
300
301 sbcopy(&so->so_snd, off, (int)len, mtod(m, char *) + hdrlen);
302 m->m_len += len;
303
304 /*
305 * If we're sending everything we've got, set PUSH.
306 * (This will keep happy those implementations which only
307 * give data to the user when a buffer fills or
308 * a PUSH comes in.)
309 */
310 if (off + len == so->so_snd.sb_cc)
311 flags |= TH_PUSH;
312 } else {
313 m = m_get(so->slirp);
314 if (m == NULL) {
315 error = 1;
316 goto out;
317 }
318 m->m_data += IF_MAXLINKHDR;
319 m->m_len = hdrlen;
320 }
321
322 ti = mtod(m, struct tcpiphdr *);
323
324 memcpy((char *)ti, &tp->t_template, sizeof(struct tcpiphdr));
325
326 /*
327 * Fill in fields, remembering maximum advertised
328 * window for use in delaying messages about window sizes.
329 * If resending a FIN, be sure not to use a new sequence number.
330 */
331 if (flags & TH_FIN && tp->t_flags & TF_SENTFIN &&
332 tp->snd_nxt == tp->snd_max)
333 tp->snd_nxt--;
334 /*
335 * If we are doing retransmissions, then snd_nxt will
336 * not reflect the first unsent octet. For ACK only
337 * packets, we do not want the sequence number of the
338 * retransmitted packet, we want the sequence number
339 * of the next unsent octet. So, if there is no data
340 * (and no SYN or FIN), use snd_max instead of snd_nxt
341 * when filling in ti_seq. But if we are in persist
342 * state, snd_max might reflect one byte beyond the
343 * right edge of the window, so use snd_nxt in that
344 * case, since we know we aren't doing a retransmission.
345 * (retransmit and persist are mutually exclusive...)
346 */
347 if (len || (flags & (TH_SYN | TH_FIN)) || tp->t_timer[TCPT_PERSIST])
348 ti->ti_seq = htonl(tp->snd_nxt);
349 else
350 ti->ti_seq = htonl(tp->snd_max);
351 ti->ti_ack = htonl(tp->rcv_nxt);
352 if (optlen) {
353 memcpy((char *)(ti + 1), (char *)opt, optlen);
354 ti->ti_off = (sizeof(struct tcphdr) + optlen) >> 2;
355 }
356 ti->ti_flags = flags;
357 /*
358 * Calculate receive window. Don't shrink window,
359 * but avoid silly window syndrome.
360 */
361 if (win < (long)(so->so_rcv.sb_datalen / 4) && win < (long)tp->t_maxseg)
362 win = 0;
363 if (win > (long)TCP_MAXWIN << tp->rcv_scale)
364 win = (long)TCP_MAXWIN << tp->rcv_scale;
365 if (win < (long)(tp->rcv_adv - tp->rcv_nxt))
366 win = (long)(tp->rcv_adv - tp->rcv_nxt);
367 ti->ti_win = htons((uint16_t)(win >> tp->rcv_scale));
368
369 if (SEQ_GT(tp->snd_up, tp->snd_una)) {
370 ti->ti_urp = htons((uint16_t)(tp->snd_up - ntohl(ti->ti_seq)));
371 ti->ti_flags |= TH_URG;
372 } else
373 /*
374 * If no urgent pointer to send, then we pull
375 * the urgent pointer to the left edge of the send window
376 * so that it doesn't drift into the send window on sequence
377 * number wraparound.
378 */
379 tp->snd_up = tp->snd_una; /* drag it along */
380
381 /*
382 * Put TCP length in extended header, and then
383 * checksum extended header and data.
384 */
385 if (len + optlen)
386 ti->ti_len = htons((uint16_t)(sizeof(struct tcphdr) + optlen + len));
387 ti->ti_sum = cksum(m, (int)(hdrlen + len));
388
389 /*
390 * In transmit state, time the transmission and arrange for
391 * the retransmit. In persist state, just set snd_max.
392 */
393 if (tp->t_force == 0 || tp->t_timer[TCPT_PERSIST] == 0) {
394 tcp_seq startseq = tp->snd_nxt;
395
396 /*
397 * Advance snd_nxt over sequence space of this segment.
398 */
399 if (flags & (TH_SYN | TH_FIN)) {
400 if (flags & TH_SYN)
401 tp->snd_nxt++;
402 if (flags & TH_FIN) {
403 tp->snd_nxt++;
404 tp->t_flags |= TF_SENTFIN;
405 }
406 }
407 tp->snd_nxt += len;
408 if (SEQ_GT(tp->snd_nxt, tp->snd_max)) {
409 tp->snd_max = tp->snd_nxt;
410 /*
411 * Time this transmission if not a retransmission and
412 * not currently timing anything.
413 */
414 if (tp->t_rtt == 0) {
415 tp->t_rtt = 1;
416 tp->t_rtseq = startseq;
417 }
418 }
419
420 /*
421 * Set retransmit timer if not currently set,
422 * and not doing an ack or a keep-alive probe.
423 * Initial value for retransmit timer is smoothed
424 * round-trip time + 2 * round-trip time variance.
425 * Initialize shift counter which is used for backoff
426 * of retransmit time.
427 */
428 if (tp->t_timer[TCPT_REXMT] == 0 && tp->snd_nxt != tp->snd_una) {
429 tp->t_timer[TCPT_REXMT] = tp->t_rxtcur;
430 if (tp->t_timer[TCPT_PERSIST]) {
431 tp->t_timer[TCPT_PERSIST] = 0;
432 tp->t_rxtshift = 0;
433 }
434 }
435 } else if (SEQ_GT(tp->snd_nxt + len, tp->snd_max))
436 tp->snd_max = tp->snd_nxt + len;
437
438 /*
439 * Fill in IP length and desired time to live and
440 * send to IP level. There should be a better way
441 * to handle ttl and tos; we could keep them in
442 * the template, but need a way to checksum without them.
443 */
444 m->m_len = hdrlen + len; /* XXX Needed? m_len should be correct */
445 tcpiph_save = *mtod(m, struct tcpiphdr *);
446
447 switch (so->so_ffamily) {
448 case AF_INET:
449 m->m_data +=
450 sizeof(struct tcpiphdr) - sizeof(struct tcphdr) - sizeof(struct ip);
451 m->m_len -=
452 sizeof(struct tcpiphdr) - sizeof(struct tcphdr) - sizeof(struct ip);
453 ip = mtod(m, struct ip *);
454
455 ip->ip_len = m->m_len;
456 ip->ip_dst = tcpiph_save.ti_dst;
457 ip->ip_src = tcpiph_save.ti_src;
458 ip->ip_p = tcpiph_save.ti_pr;
459
460 ip->ip_ttl = IPDEFTTL;
461 ip->ip_tos = so->so_iptos;
462 error = ip_output(so, m);
463 break;
464
465 case AF_INET6:
466 m->m_data += sizeof(struct tcpiphdr) - sizeof(struct tcphdr) -
467 sizeof(struct ip6);
468 m->m_len -= sizeof(struct tcpiphdr) - sizeof(struct tcphdr) -
469 sizeof(struct ip6);
470 ip6 = mtod(m, struct ip6 *);
471
472 ip6->ip_pl = tcpiph_save.ti_len;
473 ip6->ip_dst = tcpiph_save.ti_dst6;
474 ip6->ip_src = tcpiph_save.ti_src6;
475 ip6->ip_nh = tcpiph_save.ti_nh6;
476
477 error = ip6_output(so, m, 0);
478 break;
479
480 default:
481 g_assert_not_reached();
482 }
483
484 if (error) {
485 out:
486 return (error);
487 }
488
489 /*
490 * Data sent (as far as we can tell).
491 * If this advertises a larger window than any other segment,
492 * then remember the size of the advertised window.
493 * Any pending ACK has now been sent.
494 */
495 if (win > 0 && SEQ_GT(tp->rcv_nxt + win, tp->rcv_adv))
496 tp->rcv_adv = tp->rcv_nxt + win;
497 tp->last_ack_sent = tp->rcv_nxt;
498 tp->t_flags &= ~(TF_ACKNOW | TF_DELACK);
499 if (sendalot)
500 goto again;
501
502 return (0);
503 }
504
tcp_setpersist(struct tcpcb * tp)505 void tcp_setpersist(struct tcpcb *tp)
506 {
507 int t = ((tp->t_srtt >> 2) + tp->t_rttvar) >> 1;
508
509 /*
510 * Start/restart persistence timer.
511 */
512 TCPT_RANGESET(tp->t_timer[TCPT_PERSIST], t * tcp_backoff[tp->t_rxtshift],
513 TCPTV_PERSMIN, TCPTV_PERSMAX);
514 if (tp->t_rxtshift < TCP_MAXRXTSHIFT)
515 tp->t_rxtshift++;
516 }
517