1 /*-
2 * SPDX-License-Identifier: BSD-3-Clause
3 *
4 * Copyright (c) 1982, 1986, 1988, 1990, 1993, 1994, 1995
5 * The Regents of the University of California. All rights reserved.
6 * Copyright (c) 2007-2008,2010
7 * Swinburne University of Technology, Melbourne, Australia.
8 * Copyright (c) 2009-2010 Lawrence Stewart <lstewart@freebsd.org>
9 * Copyright (c) 2010 The FreeBSD Foundation
10 * Copyright (c) 2010-2011 Juniper Networks, Inc.
11 * Copyright (c) 2019 Richard Scheffenegger <srichard@netapp.com>
12 * All rights reserved.
13 *
14 * Portions of this software were developed at the Centre for Advanced Internet
15 * Architectures, Swinburne University of Technology, by Lawrence Stewart,
16 * James Healy and David Hayes, made possible in part by a grant from the Cisco
17 * University Research Program Fund at Community Foundation Silicon Valley.
18 *
19 * Portions of this software were developed at the Centre for Advanced
20 * Internet Architectures, Swinburne University of Technology, Melbourne,
21 * Australia by David Hayes under sponsorship from the FreeBSD Foundation.
22 *
23 * Portions of this software were developed by Robert N. M. Watson under
24 * contract to Juniper Networks, Inc.
25 *
26 * Redistribution and use in source and binary forms, with or without
27 * modification, are permitted provided that the following conditions
28 * are met:
29 * 1. Redistributions of source code must retain the above copyright
30 * notice, this list of conditions and the following disclaimer.
31 * 2. Redistributions in binary form must reproduce the above copyright
32 * notice, this list of conditions and the following disclaimer in the
33 * documentation and/or other materials provided with the distribution.
34 * 3. Neither the name of the University nor the names of its contributors
35 * may be used to endorse or promote products derived from this software
36 * without specific prior written permission.
37 *
38 * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
39 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
40 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
41 * ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
42 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
43 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
44 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
45 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
46 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
47 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
48 * SUCH DAMAGE.
49 */
50
51 /*
52 * Utility functions to deal with Explicit Congestion Notification in TCP
53 * implementing the essential parts of the Accurate ECN extension
54 * https://tools.ietf.org/html/draft-ietf-tcpm-accurate-ecn-09
55 */
56
57 #include "opt_inet.h"
58 #include "opt_inet6.h"
59
60 #include <sys/param.h>
61 #include <sys/systm.h>
62 #include <sys/kernel.h>
63 #include <sys/sysctl.h>
64 #include <sys/malloc.h>
65 #include <sys/mbuf.h>
66 #include <sys/socket.h>
67 #include <sys/socketvar.h>
68
69 #include <machine/cpu.h>
70
71 #include <vm/uma.h>
72
73 #include <net/if.h>
74 #include <net/if_var.h>
75 #include <net/route.h>
76 #include <net/vnet.h>
77
78 #include <netinet/in.h>
79 #include <netinet/in_systm.h>
80 #include <netinet/ip.h>
81 #include <netinet/in_var.h>
82 #include <netinet/in_pcb.h>
83 #include <netinet/ip_var.h>
84 #include <netinet/ip6.h>
85 #include <netinet/icmp6.h>
86 #include <netinet6/nd6.h>
87 #include <netinet6/ip6_var.h>
88 #include <netinet6/in6_pcb.h>
89 #include <netinet/tcp.h>
90 #include <netinet/tcp_fsm.h>
91 #include <netinet/tcp_seq.h>
92 #include <netinet/tcp_var.h>
93 #include <netinet/tcp_syncache.h>
94 #include <netinet/tcp_timer.h>
95 #include <netinet/tcpip.h>
96 #include <netinet/tcp_ecn.h>
97
98 static inline int tcp_ecn_get_ace(uint16_t);
99 static inline void tcp_ecn_set_ace(uint16_t *, uint32_t);
100
101 static SYSCTL_NODE(_net_inet_tcp, OID_AUTO, ecn,
102 CTLFLAG_RW | CTLFLAG_MPSAFE, 0,
103 "TCP ECN");
104
105 VNET_DEFINE(int, tcp_do_ecn) = 2;
106 SYSCTL_INT(_net_inet_tcp_ecn, OID_AUTO, enable,
107 CTLFLAG_VNET | CTLFLAG_RW, &VNET_NAME(tcp_do_ecn), 0,
108 "TCP ECN support");
109
110 VNET_DEFINE(int, tcp_ecn_maxretries) = 1;
111 SYSCTL_INT(_net_inet_tcp_ecn, OID_AUTO, maxretries,
112 CTLFLAG_VNET | CTLFLAG_RW, &VNET_NAME(tcp_ecn_maxretries), 0,
113 "Max retries before giving up on ECN");
114
115 /*
116 * Process incoming SYN,ACK packet
117 */
118 void
tcp_ecn_input_syn_sent(struct tcpcb * tp,uint16_t thflags,int iptos)119 tcp_ecn_input_syn_sent(struct tcpcb *tp, uint16_t thflags, int iptos)
120 {
121 switch (V_tcp_do_ecn) {
122 case 0:
123 return;
124 case 1:
125 /* FALLTHROUGH */
126 case 2:
127 /* RFC3168 ECN handling */
128 if ((thflags & (TH_CWR | TH_ECE)) == (0 | TH_ECE)) {
129 tp->t_flags2 |= TF2_ECN_PERMIT;
130 tp->t_flags2 &= ~TF2_ACE_PERMIT;
131 TCPSTAT_INC(tcps_ecn_shs);
132 }
133 break;
134 case 3:
135 /* FALLTHROUGH */
136 case 4:
137 /*
138 * Decoding Accurate ECN according to
139 * table in section 3.1.1
140 *
141 * On the SYN,ACK, process the AccECN
142 * flags indicating the state the SYN
143 * was delivered.
144 * Reactions to Path ECN mangling can
145 * come here.
146 */
147 switch (thflags & (TH_AE | TH_CWR | TH_ECE)) {
148 /* RFC3168 SYN */
149 case (0|0|TH_ECE):
150 tp->t_flags2 |= TF2_ECN_PERMIT;
151 tp->t_flags2 &= ~TF2_ACE_PERMIT;
152 TCPSTAT_INC(tcps_ecn_shs);
153 break;
154 /* non-ECT SYN */
155 case (0|TH_CWR|0):
156 tp->t_flags2 |= TF2_ACE_PERMIT;
157 tp->t_flags2 &= ~TF2_ECN_PERMIT;
158 tp->t_scep = 5;
159 TCPSTAT_INC(tcps_ecn_shs);
160 TCPSTAT_INC(tcps_ace_nect);
161 break;
162 /* ECT0 SYN */
163 case (TH_AE|0|0):
164 tp->t_flags2 |= TF2_ACE_PERMIT;
165 tp->t_flags2 &= ~TF2_ECN_PERMIT;
166 tp->t_scep = 5;
167 TCPSTAT_INC(tcps_ecn_shs);
168 TCPSTAT_INC(tcps_ace_ect0);
169 break;
170 /* ECT1 SYN */
171 case (0|TH_CWR|TH_ECE):
172 tp->t_flags2 |= TF2_ACE_PERMIT;
173 tp->t_flags2 &= ~TF2_ECN_PERMIT;
174 tp->t_scep = 5;
175 TCPSTAT_INC(tcps_ecn_shs);
176 TCPSTAT_INC(tcps_ace_ect1);
177 break;
178 /* CE SYN */
179 case (TH_AE|TH_CWR|0):
180 tp->t_flags2 |= TF2_ACE_PERMIT;
181 tp->t_flags2 &= ~TF2_ECN_PERMIT;
182 tp->t_scep = 6;
183 /*
184 * reduce the IW to 2 MSS (to
185 * account for delayed acks) if
186 * the SYN,ACK was CE marked
187 */
188 tp->snd_cwnd = 2 * tcp_maxseg(tp);
189 TCPSTAT_INC(tcps_ecn_shs);
190 TCPSTAT_INC(tcps_ace_nect);
191 break;
192 default:
193 tp->t_flags2 &= ~(TF2_ECN_PERMIT | TF2_ACE_PERMIT);
194 break;
195 }
196 /*
197 * Set the AccECN Codepoints on
198 * the outgoing <ACK> to the ECN
199 * state of the <SYN,ACK>
200 * according to table 3 in the
201 * AccECN draft
202 */
203 switch (iptos & IPTOS_ECN_MASK) {
204 case (IPTOS_ECN_NOTECT):
205 tp->t_rcep = 0b010;
206 break;
207 case (IPTOS_ECN_ECT0):
208 tp->t_rcep = 0b100;
209 break;
210 case (IPTOS_ECN_ECT1):
211 tp->t_rcep = 0b011;
212 break;
213 case (IPTOS_ECN_CE):
214 tp->t_rcep = 0b110;
215 break;
216 }
217 break;
218 }
219 }
220
221 /*
222 * Handle parallel SYN for ECN
223 */
224 void
tcp_ecn_input_parallel_syn(struct tcpcb * tp,uint16_t thflags,int iptos)225 tcp_ecn_input_parallel_syn(struct tcpcb *tp, uint16_t thflags, int iptos)
226 {
227 if (thflags & TH_ACK)
228 return;
229 switch (V_tcp_do_ecn) {
230 case 0:
231 return;
232 case 1:
233 /* FALLTHROUGH */
234 case 2:
235 /* RFC3168 ECN handling */
236 if ((thflags & (TH_CWR | TH_ECE)) == (TH_CWR | TH_ECE)) {
237 tp->t_flags2 |= TF2_ECN_PERMIT;
238 tp->t_flags2 &= ~TF2_ACE_PERMIT;
239 tp->t_flags2 |= TF2_ECN_SND_ECE;
240 TCPSTAT_INC(tcps_ecn_shs);
241 }
242 break;
243 case 3:
244 /* FALLTHROUGH */
245 case 4:
246 /* AccECN handling */
247 switch (thflags & (TH_AE | TH_CWR | TH_ECE)) {
248 default:
249 case (0|0|0):
250 tp->t_flags2 &= ~(TF2_ECN_PERMIT | TF2_ACE_PERMIT);
251 break;
252 case (0|TH_CWR|TH_ECE):
253 tp->t_flags2 |= TF2_ECN_PERMIT;
254 tp->t_flags2 &= ~TF2_ACE_PERMIT;
255 tp->t_flags2 |= TF2_ECN_SND_ECE;
256 TCPSTAT_INC(tcps_ecn_shs);
257 break;
258 case (TH_AE|TH_CWR|TH_ECE):
259 tp->t_flags2 |= TF2_ACE_PERMIT;
260 tp->t_flags2 &= ~TF2_ECN_PERMIT;
261 TCPSTAT_INC(tcps_ecn_shs);
262 /*
263 * Set the AccECN Codepoints on
264 * the outgoing <ACK> to the ECN
265 * state of the <SYN,ACK>
266 * according to table 3 in the
267 * AccECN draft
268 */
269 switch (iptos & IPTOS_ECN_MASK) {
270 case (IPTOS_ECN_NOTECT):
271 tp->t_rcep = 0b010;
272 break;
273 case (IPTOS_ECN_ECT0):
274 tp->t_rcep = 0b100;
275 break;
276 case (IPTOS_ECN_ECT1):
277 tp->t_rcep = 0b011;
278 break;
279 case (IPTOS_ECN_CE):
280 tp->t_rcep = 0b110;
281 break;
282 }
283 break;
284 }
285 break;
286 }
287 }
288
289 /*
290 * TCP ECN processing.
291 */
292 int
tcp_ecn_input_segment(struct tcpcb * tp,uint16_t thflags,int tlen,int pkts,int iptos)293 tcp_ecn_input_segment(struct tcpcb *tp, uint16_t thflags, int tlen, int pkts, int iptos)
294 {
295 int delta_cep = 0;
296
297 switch (iptos & IPTOS_ECN_MASK) {
298 case IPTOS_ECN_CE:
299 TCPSTAT_INC(tcps_ecn_rcvce);
300 break;
301 case IPTOS_ECN_ECT0:
302 TCPSTAT_INC(tcps_ecn_rcvect0);
303 break;
304 case IPTOS_ECN_ECT1:
305 TCPSTAT_INC(tcps_ecn_rcvect1);
306 break;
307 }
308
309 if (tp->t_flags2 & (TF2_ECN_PERMIT | TF2_ACE_PERMIT)) {
310 if (tp->t_flags2 & TF2_ACE_PERMIT) {
311 if ((iptos & IPTOS_ECN_MASK) == IPTOS_ECN_CE)
312 tp->t_rcep += 1;
313 if (tp->t_flags2 & TF2_ECN_PERMIT) {
314 delta_cep = (tcp_ecn_get_ace(thflags) + 8 -
315 (tp->t_scep & 7)) & 7;
316 if (delta_cep < pkts)
317 delta_cep = pkts -
318 ((pkts - delta_cep) & 7);
319 tp->t_scep += delta_cep;
320 } else {
321 /*
322 * process the final ACK of the 3WHS
323 * see table 3 in draft-ietf-tcpm-accurate-ecn
324 */
325 switch (tcp_ecn_get_ace(thflags)) {
326 case 0b010:
327 /* nonECT SYN or SYN,ACK */
328 /* FALLTHROUGH */
329 case 0b011:
330 /* ECT1 SYN or SYN,ACK */
331 /* FALLTHROUGH */
332 case 0b100:
333 /* ECT0 SYN or SYN,ACK */
334 tp->t_scep = 5;
335 break;
336 case 0b110:
337 /* CE SYN or SYN,ACK */
338 tp->t_scep = 6;
339 tp->snd_cwnd = 2 * tcp_maxseg(tp);
340 break;
341 default:
342 /* mangled AccECN handshake */
343 tp->t_scep = 5;
344 break;
345 }
346 tp->t_flags2 |= TF2_ECN_PERMIT;
347 }
348 } else {
349 /* RFC3168 ECN handling */
350 if ((thflags & (TH_SYN | TH_ECE)) == TH_ECE) {
351 delta_cep = 1;
352 tp->t_scep++;
353 }
354 if (thflags & TH_CWR) {
355 tp->t_flags2 &= ~TF2_ECN_SND_ECE;
356 if ((tp->t_state == TCPS_ESTABLISHED) ||
357 (tp->t_state == TCPS_FIN_WAIT_1) ||
358 (tp->t_state == TCPS_FIN_WAIT_2))
359 tp->t_flags |= TF_ACKNOW;
360 }
361 if ((iptos & IPTOS_ECN_MASK) == IPTOS_ECN_CE)
362 tp->t_flags2 |= TF2_ECN_SND_ECE;
363 }
364
365 /* Process a packet differently from RFC3168. */
366 cc_ecnpkt_handler_flags(tp, thflags, iptos);
367 }
368
369 return delta_cep;
370 }
371
372 /*
373 * Send ECN setup <SYN> packet header flags
374 */
375 uint16_t
tcp_ecn_output_syn_sent(struct tcpcb * tp)376 tcp_ecn_output_syn_sent(struct tcpcb *tp)
377 {
378 uint16_t thflags = 0;
379
380 if (V_tcp_do_ecn == 0)
381 return thflags;
382 if (V_tcp_do_ecn == 1) {
383 /* Send a RFC3168 ECN setup <SYN> packet */
384 if (tp->t_rxtshift >= 1) {
385 if (tp->t_rxtshift <= V_tcp_ecn_maxretries)
386 thflags = TH_ECE|TH_CWR;
387 } else
388 thflags = TH_ECE|TH_CWR;
389 } else if (V_tcp_do_ecn == 3) {
390 /* Send an Accurate ECN setup <SYN> packet */
391 if (tp->t_rxtshift >= 1) {
392 if (tp->t_rxtshift <= V_tcp_ecn_maxretries)
393 thflags = TH_ECE|TH_CWR|TH_AE;
394 } else
395 thflags = TH_ECE|TH_CWR|TH_AE;
396 }
397
398 return thflags;
399 }
400
401 /*
402 * output processing of ECN feature
403 * returning IP ECN header codepoint
404 */
405 int
tcp_ecn_output_established(struct tcpcb * tp,uint16_t * thflags,int len,bool rxmit)406 tcp_ecn_output_established(struct tcpcb *tp, uint16_t *thflags, int len, bool rxmit)
407 {
408 int ipecn = IPTOS_ECN_NOTECT;
409 bool newdata;
410
411 /*
412 * If the peer has ECN, mark data packets with
413 * ECN capable transmission (ECT).
414 * Ignore pure control packets, retransmissions
415 * and window probes.
416 */
417 newdata = (len > 0 && SEQ_GEQ(tp->snd_nxt, tp->snd_max) &&
418 !rxmit &&
419 !((tp->t_flags & TF_FORCEDATA) && len == 1));
420 /* RFC3168 ECN marking, only new data segments */
421 if (newdata) {
422 if (tp->t_flags2 & TF2_ECN_USE_ECT1) {
423 ipecn = IPTOS_ECN_ECT1;
424 TCPSTAT_INC(tcps_ecn_sndect1);
425 } else {
426 ipecn = IPTOS_ECN_ECT0;
427 TCPSTAT_INC(tcps_ecn_sndect0);
428 }
429 }
430 /*
431 * Reply with proper ECN notifications.
432 */
433 if (tp->t_flags2 & TF2_ACE_PERMIT) {
434 tcp_ecn_set_ace(thflags, tp->t_rcep);
435 if (!(tp->t_flags2 & TF2_ECN_PERMIT)) {
436 /*
437 * here we process the final
438 * ACK of the 3WHS
439 */
440 if (tp->t_rcep == 0b110) {
441 tp->t_rcep = 6;
442 } else {
443 tp->t_rcep = 5;
444 }
445 tp->t_flags2 |= TF2_ECN_PERMIT;
446 }
447 } else {
448 if (newdata &&
449 (tp->t_flags2 & TF2_ECN_SND_CWR)) {
450 *thflags |= TH_CWR;
451 tp->t_flags2 &= ~TF2_ECN_SND_CWR;
452 }
453 if (tp->t_flags2 & TF2_ECN_SND_ECE)
454 *thflags |= TH_ECE;
455 }
456
457 return ipecn;
458 }
459
460 /*
461 * Set up the ECN related tcpcb fields from
462 * a syncache entry
463 */
464 void
tcp_ecn_syncache_socket(struct tcpcb * tp,struct syncache * sc)465 tcp_ecn_syncache_socket(struct tcpcb *tp, struct syncache *sc)
466 {
467 if (sc->sc_flags & SCF_ECN_MASK) {
468 switch (sc->sc_flags & SCF_ECN_MASK) {
469 case SCF_ECN:
470 tp->t_flags2 |= TF2_ECN_PERMIT;
471 break;
472 case SCF_ACE_N:
473 /* FALLTHROUGH */
474 case SCF_ACE_0:
475 /* FALLTHROUGH */
476 case SCF_ACE_1:
477 tp->t_flags2 |= TF2_ACE_PERMIT;
478 tp->t_scep = 5;
479 tp->t_rcep = 5;
480 break;
481 case SCF_ACE_CE:
482 tp->t_flags2 |= TF2_ACE_PERMIT;
483 tp->t_scep = 6;
484 tp->t_rcep = 6;
485 break;
486 }
487 }
488 }
489
490 /*
491 * Process a <SYN> packets ECN information, and provide the
492 * syncache with the relevant information.
493 */
494 int
tcp_ecn_syncache_add(uint16_t thflags,int iptos)495 tcp_ecn_syncache_add(uint16_t thflags, int iptos)
496 {
497 int scflags = 0;
498
499 switch (iptos & IPTOS_ECN_MASK) {
500 case IPTOS_ECN_CE:
501 TCPSTAT_INC(tcps_ecn_rcvce);
502 break;
503 case IPTOS_ECN_ECT0:
504 TCPSTAT_INC(tcps_ecn_rcvect0);
505 break;
506 case IPTOS_ECN_ECT1:
507 TCPSTAT_INC(tcps_ecn_rcvect1);
508 break;
509 }
510
511 switch (thflags & (TH_AE|TH_CWR|TH_ECE)) {
512 /* no ECN */
513 case (0|0|0):
514 break;
515 /* legacy ECN */
516 case (0|TH_CWR|TH_ECE):
517 scflags = SCF_ECN;
518 break;
519 /* Accurate ECN */
520 case (TH_AE|TH_CWR|TH_ECE):
521 if ((V_tcp_do_ecn == 3) ||
522 (V_tcp_do_ecn == 4)) {
523 switch (iptos & IPTOS_ECN_MASK) {
524 case IPTOS_ECN_CE:
525 scflags = SCF_ACE_CE;
526 break;
527 case IPTOS_ECN_ECT0:
528 scflags = SCF_ACE_0;
529 break;
530 case IPTOS_ECN_ECT1:
531 scflags = SCF_ACE_1;
532 break;
533 case IPTOS_ECN_NOTECT:
534 scflags = SCF_ACE_N;
535 break;
536 }
537 } else
538 scflags = SCF_ECN;
539 break;
540 /* Default Case (section 3.1.2) */
541 default:
542 if ((V_tcp_do_ecn == 3) ||
543 (V_tcp_do_ecn == 4)) {
544 switch (iptos & IPTOS_ECN_MASK) {
545 case IPTOS_ECN_CE:
546 scflags = SCF_ACE_CE;
547 break;
548 case IPTOS_ECN_ECT0:
549 scflags = SCF_ACE_0;
550 break;
551 case IPTOS_ECN_ECT1:
552 scflags = SCF_ACE_1;
553 break;
554 case IPTOS_ECN_NOTECT:
555 scflags = SCF_ACE_N;
556 break;
557 }
558 }
559 break;
560 }
561 return scflags;
562 }
563
564 /*
565 * Set up the ECN information for the <SYN,ACK> from
566 * syncache information.
567 */
568 uint16_t
tcp_ecn_syncache_respond(uint16_t thflags,struct syncache * sc)569 tcp_ecn_syncache_respond(uint16_t thflags, struct syncache *sc)
570 {
571 if ((thflags & TH_SYN) &&
572 (sc->sc_flags & SCF_ECN_MASK)) {
573 switch (sc->sc_flags & SCF_ECN_MASK) {
574 case SCF_ECN:
575 thflags |= (0 | 0 | TH_ECE);
576 TCPSTAT_INC(tcps_ecn_shs);
577 break;
578 case SCF_ACE_N:
579 thflags |= (0 | TH_CWR | 0);
580 TCPSTAT_INC(tcps_ecn_shs);
581 TCPSTAT_INC(tcps_ace_nect);
582 break;
583 case SCF_ACE_0:
584 thflags |= (TH_AE | 0 | 0);
585 TCPSTAT_INC(tcps_ecn_shs);
586 TCPSTAT_INC(tcps_ace_ect0);
587 break;
588 case SCF_ACE_1:
589 thflags |= (0 | TH_ECE | TH_CWR);
590 TCPSTAT_INC(tcps_ecn_shs);
591 TCPSTAT_INC(tcps_ace_ect1);
592 break;
593 case SCF_ACE_CE:
594 thflags |= (TH_AE | TH_CWR | 0);
595 TCPSTAT_INC(tcps_ecn_shs);
596 TCPSTAT_INC(tcps_ace_ce);
597 break;
598 }
599 }
600 return thflags;
601 }
602
603 static inline int
tcp_ecn_get_ace(uint16_t thflags)604 tcp_ecn_get_ace(uint16_t thflags)
605 {
606 return ((thflags & (TH_AE|TH_CWR|TH_ECE)) >> TH_ACE_SHIFT);
607 }
608
609 static inline void
tcp_ecn_set_ace(uint16_t * thflags,uint32_t t_rcep)610 tcp_ecn_set_ace(uint16_t *thflags, uint32_t t_rcep)
611 {
612 *thflags &= ~(TH_AE|TH_CWR|TH_ECE);
613 *thflags |= ((t_rcep << TH_ACE_SHIFT) & (TH_AE|TH_CWR|TH_ECE));
614 }
615