xref: /freebsd/sys/netinet/tcp_output.c (revision 4d293dd8dcde59fc9842a0ce1125fef8fcf83a8c)
1 /*-
2  * Copyright (c) 1982, 1986, 1988, 1990, 1993, 1995
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  * 4. 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	8.4 (Berkeley) 5/24/95
30  */
31 
32 #include <sys/cdefs.h>
33 __FBSDID("$FreeBSD$");
34 
35 #include "opt_inet.h"
36 #include "opt_inet6.h"
37 #include "opt_ipsec.h"
38 #include "opt_tcpdebug.h"
39 
40 #include <sys/param.h>
41 #include <sys/systm.h>
42 #include <sys/domain.h>
43 #include <sys/hhook.h>
44 #include <sys/kernel.h>
45 #include <sys/lock.h>
46 #include <sys/mbuf.h>
47 #include <sys/mutex.h>
48 #include <sys/protosw.h>
49 #include <sys/sdt.h>
50 #include <sys/socket.h>
51 #include <sys/socketvar.h>
52 #include <sys/sysctl.h>
53 
54 #include <net/if.h>
55 #include <net/route.h>
56 #include <net/vnet.h>
57 
58 #include <netinet/cc.h>
59 #include <netinet/in.h>
60 #include <netinet/in_kdtrace.h>
61 #include <netinet/in_systm.h>
62 #include <netinet/ip.h>
63 #include <netinet/in_pcb.h>
64 #include <netinet/ip_var.h>
65 #include <netinet/ip_options.h>
66 #ifdef INET6
67 #include <netinet6/in6_pcb.h>
68 #include <netinet/ip6.h>
69 #include <netinet6/ip6_var.h>
70 #endif
71 #define	TCPOUTFLAGS
72 #include <netinet/tcp_fsm.h>
73 #include <netinet/tcp_seq.h>
74 #include <netinet/tcp_timer.h>
75 #include <netinet/tcp_var.h>
76 #include <netinet/tcpip.h>
77 #ifdef TCPPCAP
78 #include <netinet/tcp_pcap.h>
79 #endif
80 #ifdef TCPDEBUG
81 #include <netinet/tcp_debug.h>
82 #endif
83 #ifdef TCP_OFFLOAD
84 #include <netinet/tcp_offload.h>
85 #endif
86 
87 #ifdef IPSEC
88 #include <netipsec/ipsec.h>
89 #endif /*IPSEC*/
90 
91 #include <machine/in_cksum.h>
92 
93 #include <security/mac/mac_framework.h>
94 
95 VNET_DEFINE(int, path_mtu_discovery) = 1;
96 SYSCTL_INT(_net_inet_tcp, OID_AUTO, path_mtu_discovery, CTLFLAG_VNET | CTLFLAG_RW,
97 	&VNET_NAME(path_mtu_discovery), 1,
98 	"Enable Path MTU Discovery");
99 
100 VNET_DEFINE(int, tcp_do_tso) = 1;
101 #define	V_tcp_do_tso		VNET(tcp_do_tso)
102 SYSCTL_INT(_net_inet_tcp, OID_AUTO, tso, CTLFLAG_VNET | CTLFLAG_RW,
103 	&VNET_NAME(tcp_do_tso), 0,
104 	"Enable TCP Segmentation Offload");
105 
106 VNET_DEFINE(int, tcp_sendspace) = 1024*32;
107 #define	V_tcp_sendspace	VNET(tcp_sendspace)
108 SYSCTL_INT(_net_inet_tcp, TCPCTL_SENDSPACE, sendspace, CTLFLAG_VNET | CTLFLAG_RW,
109 	&VNET_NAME(tcp_sendspace), 0, "Initial send socket buffer size");
110 
111 VNET_DEFINE(int, tcp_do_autosndbuf) = 1;
112 #define	V_tcp_do_autosndbuf	VNET(tcp_do_autosndbuf)
113 SYSCTL_INT(_net_inet_tcp, OID_AUTO, sendbuf_auto, CTLFLAG_VNET | CTLFLAG_RW,
114 	&VNET_NAME(tcp_do_autosndbuf), 0,
115 	"Enable automatic send buffer sizing");
116 
117 VNET_DEFINE(int, tcp_autosndbuf_inc) = 8*1024;
118 #define	V_tcp_autosndbuf_inc	VNET(tcp_autosndbuf_inc)
119 SYSCTL_INT(_net_inet_tcp, OID_AUTO, sendbuf_inc, CTLFLAG_VNET | CTLFLAG_RW,
120 	&VNET_NAME(tcp_autosndbuf_inc), 0,
121 	"Incrementor step size of automatic send buffer");
122 
123 VNET_DEFINE(int, tcp_autosndbuf_max) = 2*1024*1024;
124 #define	V_tcp_autosndbuf_max	VNET(tcp_autosndbuf_max)
125 SYSCTL_INT(_net_inet_tcp, OID_AUTO, sendbuf_max, CTLFLAG_VNET | CTLFLAG_RW,
126 	&VNET_NAME(tcp_autosndbuf_max), 0,
127 	"Max size of automatic send buffer");
128 
129 static void inline	hhook_run_tcp_est_out(struct tcpcb *tp,
130 			    struct tcphdr *th, struct tcpopt *to,
131 			    long len, int tso);
132 static void inline	cc_after_idle(struct tcpcb *tp);
133 
134 /*
135  * Wrapper for the TCP established output helper hook.
136  */
137 static void inline
138 hhook_run_tcp_est_out(struct tcpcb *tp, struct tcphdr *th,
139     struct tcpopt *to, long len, int tso)
140 {
141 	struct tcp_hhook_data hhook_data;
142 
143 	if (V_tcp_hhh[HHOOK_TCP_EST_OUT]->hhh_nhooks > 0) {
144 		hhook_data.tp = tp;
145 		hhook_data.th = th;
146 		hhook_data.to = to;
147 		hhook_data.len = len;
148 		hhook_data.tso = tso;
149 
150 		hhook_run_hooks(V_tcp_hhh[HHOOK_TCP_EST_OUT], &hhook_data,
151 		    tp->osd);
152 	}
153 }
154 
155 /*
156  * CC wrapper hook functions
157  */
158 static void inline
159 cc_after_idle(struct tcpcb *tp)
160 {
161 	INP_WLOCK_ASSERT(tp->t_inpcb);
162 
163 	if (CC_ALGO(tp)->after_idle != NULL)
164 		CC_ALGO(tp)->after_idle(tp->ccv);
165 }
166 
167 /*
168  * Tcp output routine: figure out what should be sent and send it.
169  */
170 int
171 tcp_output(struct tcpcb *tp)
172 {
173 	struct socket *so = tp->t_inpcb->inp_socket;
174 	long len, recwin, sendwin;
175 	int off, flags, error = 0;	/* Keep compiler happy */
176 	struct mbuf *m;
177 	struct ip *ip = NULL;
178 	struct ipovly *ipov = NULL;
179 	struct tcphdr *th;
180 	u_char opt[TCP_MAXOLEN];
181 	unsigned ipoptlen, optlen, hdrlen;
182 #ifdef IPSEC
183 	unsigned ipsec_optlen = 0;
184 #endif
185 	int idle, sendalot;
186 	int sack_rxmit, sack_bytes_rxmt;
187 	struct sackhole *p;
188 	int tso, mtu;
189 	struct tcpopt to;
190 #if 0
191 	int maxburst = TCP_MAXBURST;
192 #endif
193 #ifdef INET6
194 	struct ip6_hdr *ip6 = NULL;
195 	int isipv6;
196 
197 	isipv6 = (tp->t_inpcb->inp_vflag & INP_IPV6) != 0;
198 #endif
199 
200 	INP_WLOCK_ASSERT(tp->t_inpcb);
201 
202 #ifdef TCP_OFFLOAD
203 	if (tp->t_flags & TF_TOE)
204 		return (tcp_offload_output(tp));
205 #endif
206 
207 	/*
208 	 * Determine length of data that should be transmitted,
209 	 * and flags that will be used.
210 	 * If there is some data or critical controls (SYN, RST)
211 	 * to send, then transmit; otherwise, investigate further.
212 	 */
213 	idle = (tp->t_flags & TF_LASTIDLE) || (tp->snd_max == tp->snd_una);
214 	if (idle && ticks - tp->t_rcvtime >= tp->t_rxtcur)
215 		cc_after_idle(tp);
216 	tp->t_flags &= ~TF_LASTIDLE;
217 	if (idle) {
218 		if (tp->t_flags & TF_MORETOCOME) {
219 			tp->t_flags |= TF_LASTIDLE;
220 			idle = 0;
221 		}
222 	}
223 again:
224 	/*
225 	 * If we've recently taken a timeout, snd_max will be greater than
226 	 * snd_nxt.  There may be SACK information that allows us to avoid
227 	 * resending already delivered data.  Adjust snd_nxt accordingly.
228 	 */
229 	if ((tp->t_flags & TF_SACK_PERMIT) &&
230 	    SEQ_LT(tp->snd_nxt, tp->snd_max))
231 		tcp_sack_adjust(tp);
232 	sendalot = 0;
233 	tso = 0;
234 	mtu = 0;
235 	off = tp->snd_nxt - tp->snd_una;
236 	sendwin = min(tp->snd_wnd, tp->snd_cwnd);
237 
238 	flags = tcp_outflags[tp->t_state];
239 	/*
240 	 * Send any SACK-generated retransmissions.  If we're explicitly trying
241 	 * to send out new data (when sendalot is 1), bypass this function.
242 	 * If we retransmit in fast recovery mode, decrement snd_cwnd, since
243 	 * we're replacing a (future) new transmission with a retransmission
244 	 * now, and we previously incremented snd_cwnd in tcp_input().
245 	 */
246 	/*
247 	 * Still in sack recovery , reset rxmit flag to zero.
248 	 */
249 	sack_rxmit = 0;
250 	sack_bytes_rxmt = 0;
251 	len = 0;
252 	p = NULL;
253 	if ((tp->t_flags & TF_SACK_PERMIT) && IN_FASTRECOVERY(tp->t_flags) &&
254 	    (p = tcp_sack_output(tp, &sack_bytes_rxmt))) {
255 		long cwin;
256 
257 		cwin = min(tp->snd_wnd, tp->snd_cwnd) - sack_bytes_rxmt;
258 		if (cwin < 0)
259 			cwin = 0;
260 		/* Do not retransmit SACK segments beyond snd_recover */
261 		if (SEQ_GT(p->end, tp->snd_recover)) {
262 			/*
263 			 * (At least) part of sack hole extends beyond
264 			 * snd_recover. Check to see if we can rexmit data
265 			 * for this hole.
266 			 */
267 			if (SEQ_GEQ(p->rxmit, tp->snd_recover)) {
268 				/*
269 				 * Can't rexmit any more data for this hole.
270 				 * That data will be rexmitted in the next
271 				 * sack recovery episode, when snd_recover
272 				 * moves past p->rxmit.
273 				 */
274 				p = NULL;
275 				goto after_sack_rexmit;
276 			} else
277 				/* Can rexmit part of the current hole */
278 				len = ((long)ulmin(cwin,
279 						   tp->snd_recover - p->rxmit));
280 		} else
281 			len = ((long)ulmin(cwin, p->end - p->rxmit));
282 		off = p->rxmit - tp->snd_una;
283 		KASSERT(off >= 0,("%s: sack block to the left of una : %d",
284 		    __func__, off));
285 		if (len > 0) {
286 			sack_rxmit = 1;
287 			sendalot = 1;
288 			TCPSTAT_INC(tcps_sack_rexmits);
289 			TCPSTAT_ADD(tcps_sack_rexmit_bytes,
290 			    min(len, tp->t_maxseg));
291 		}
292 	}
293 after_sack_rexmit:
294 	/*
295 	 * Get standard flags, and add SYN or FIN if requested by 'hidden'
296 	 * state flags.
297 	 */
298 	if (tp->t_flags & TF_NEEDFIN)
299 		flags |= TH_FIN;
300 	if (tp->t_flags & TF_NEEDSYN)
301 		flags |= TH_SYN;
302 
303 	SOCKBUF_LOCK(&so->so_snd);
304 	/*
305 	 * If in persist timeout with window of 0, send 1 byte.
306 	 * Otherwise, if window is small but nonzero
307 	 * and timer expired, we will send what we can
308 	 * and go to transmit state.
309 	 */
310 	if (tp->t_flags & TF_FORCEDATA) {
311 		if (sendwin == 0) {
312 			/*
313 			 * If we still have some data to send, then
314 			 * clear the FIN bit.  Usually this would
315 			 * happen below when it realizes that we
316 			 * aren't sending all the data.  However,
317 			 * if we have exactly 1 byte of unsent data,
318 			 * then it won't clear the FIN bit below,
319 			 * and if we are in persist state, we wind
320 			 * up sending the packet without recording
321 			 * that we sent the FIN bit.
322 			 *
323 			 * We can't just blindly clear the FIN bit,
324 			 * because if we don't have any more data
325 			 * to send then the probe will be the FIN
326 			 * itself.
327 			 */
328 			if (off < sbused(&so->so_snd))
329 				flags &= ~TH_FIN;
330 			sendwin = 1;
331 		} else {
332 			tcp_timer_activate(tp, TT_PERSIST, 0);
333 			tp->t_rxtshift = 0;
334 		}
335 	}
336 
337 	/*
338 	 * If snd_nxt == snd_max and we have transmitted a FIN, the
339 	 * offset will be > 0 even if so_snd.sb_cc is 0, resulting in
340 	 * a negative length.  This can also occur when TCP opens up
341 	 * its congestion window while receiving additional duplicate
342 	 * acks after fast-retransmit because TCP will reset snd_nxt
343 	 * to snd_max after the fast-retransmit.
344 	 *
345 	 * In the normal retransmit-FIN-only case, however, snd_nxt will
346 	 * be set to snd_una, the offset will be 0, and the length may
347 	 * wind up 0.
348 	 *
349 	 * If sack_rxmit is true we are retransmitting from the scoreboard
350 	 * in which case len is already set.
351 	 */
352 	if (sack_rxmit == 0) {
353 		if (sack_bytes_rxmt == 0)
354 			len = ((long)ulmin(sbavail(&so->so_snd), sendwin) -
355 			    off);
356 		else {
357 			long cwin;
358 
359                         /*
360 			 * We are inside of a SACK recovery episode and are
361 			 * sending new data, having retransmitted all the
362 			 * data possible in the scoreboard.
363 			 */
364 			len = ((long)ulmin(sbavail(&so->so_snd), tp->snd_wnd) -
365 			    off);
366 			/*
367 			 * Don't remove this (len > 0) check !
368 			 * We explicitly check for len > 0 here (although it
369 			 * isn't really necessary), to work around a gcc
370 			 * optimization issue - to force gcc to compute
371 			 * len above. Without this check, the computation
372 			 * of len is bungled by the optimizer.
373 			 */
374 			if (len > 0) {
375 				cwin = tp->snd_cwnd -
376 					(tp->snd_nxt - tp->sack_newdata) -
377 					sack_bytes_rxmt;
378 				if (cwin < 0)
379 					cwin = 0;
380 				len = lmin(len, cwin);
381 			}
382 		}
383 	}
384 
385 	/*
386 	 * Lop off SYN bit if it has already been sent.  However, if this
387 	 * is SYN-SENT state and if segment contains data and if we don't
388 	 * know that foreign host supports TAO, suppress sending segment.
389 	 */
390 	if ((flags & TH_SYN) && SEQ_GT(tp->snd_nxt, tp->snd_una)) {
391 		if (tp->t_state != TCPS_SYN_RECEIVED)
392 			flags &= ~TH_SYN;
393 		off--, len++;
394 	}
395 
396 	/*
397 	 * Be careful not to send data and/or FIN on SYN segments.
398 	 * This measure is needed to prevent interoperability problems
399 	 * with not fully conformant TCP implementations.
400 	 */
401 	if ((flags & TH_SYN) && (tp->t_flags & TF_NOOPT)) {
402 		len = 0;
403 		flags &= ~TH_FIN;
404 	}
405 
406 	if (len <= 0) {
407 		/*
408 		 * If FIN has been sent but not acked,
409 		 * but we haven't been called to retransmit,
410 		 * len will be < 0.  Otherwise, window shrank
411 		 * after we sent into it.  If window shrank to 0,
412 		 * cancel pending retransmit, pull snd_nxt back
413 		 * to (closed) window, and set the persist timer
414 		 * if it isn't already going.  If the window didn't
415 		 * close completely, just wait for an ACK.
416 		 *
417 		 * We also do a general check here to ensure that
418 		 * we will set the persist timer when we have data
419 		 * to send, but a 0-byte window. This makes sure
420 		 * the persist timer is set even if the packet
421 		 * hits one of the "goto send" lines below.
422 		 */
423 		len = 0;
424 		if ((sendwin == 0) && (TCPS_HAVEESTABLISHED(tp->t_state)) &&
425 			(off < (int) sbavail(&so->so_snd))) {
426 			tcp_timer_activate(tp, TT_REXMT, 0);
427 			tp->t_rxtshift = 0;
428 			tp->snd_nxt = tp->snd_una;
429 			if (!tcp_timer_active(tp, TT_PERSIST))
430 				tcp_setpersist(tp);
431 		}
432 	}
433 
434 	/* len will be >= 0 after this point. */
435 	KASSERT(len >= 0, ("[%s:%d]: len < 0", __func__, __LINE__));
436 
437 	/*
438 	 * Automatic sizing of send socket buffer.  Often the send buffer
439 	 * size is not optimally adjusted to the actual network conditions
440 	 * at hand (delay bandwidth product).  Setting the buffer size too
441 	 * small limits throughput on links with high bandwidth and high
442 	 * delay (eg. trans-continental/oceanic links).  Setting the
443 	 * buffer size too big consumes too much real kernel memory,
444 	 * especially with many connections on busy servers.
445 	 *
446 	 * The criteria to step up the send buffer one notch are:
447 	 *  1. receive window of remote host is larger than send buffer
448 	 *     (with a fudge factor of 5/4th);
449 	 *  2. send buffer is filled to 7/8th with data (so we actually
450 	 *     have data to make use of it);
451 	 *  3. send buffer fill has not hit maximal automatic size;
452 	 *  4. our send window (slow start and cogestion controlled) is
453 	 *     larger than sent but unacknowledged data in send buffer.
454 	 *
455 	 * The remote host receive window scaling factor may limit the
456 	 * growing of the send buffer before it reaches its allowed
457 	 * maximum.
458 	 *
459 	 * It scales directly with slow start or congestion window
460 	 * and does at most one step per received ACK.  This fast
461 	 * scaling has the drawback of growing the send buffer beyond
462 	 * what is strictly necessary to make full use of a given
463 	 * delay*bandwith product.  However testing has shown this not
464 	 * to be much of an problem.  At worst we are trading wasting
465 	 * of available bandwith (the non-use of it) for wasting some
466 	 * socket buffer memory.
467 	 *
468 	 * TODO: Shrink send buffer during idle periods together
469 	 * with congestion window.  Requires another timer.  Has to
470 	 * wait for upcoming tcp timer rewrite.
471 	 *
472 	 * XXXGL: should there be used sbused() or sbavail()?
473 	 */
474 	if (V_tcp_do_autosndbuf && so->so_snd.sb_flags & SB_AUTOSIZE) {
475 		if ((tp->snd_wnd / 4 * 5) >= so->so_snd.sb_hiwat &&
476 		    sbused(&so->so_snd) >= (so->so_snd.sb_hiwat / 8 * 7) &&
477 		    sbused(&so->so_snd) < V_tcp_autosndbuf_max &&
478 		    sendwin >= (sbused(&so->so_snd) -
479 		    (tp->snd_nxt - tp->snd_una))) {
480 			if (!sbreserve_locked(&so->so_snd,
481 			    min(so->so_snd.sb_hiwat + V_tcp_autosndbuf_inc,
482 			     V_tcp_autosndbuf_max), so, curthread))
483 				so->so_snd.sb_flags &= ~SB_AUTOSIZE;
484 		}
485 	}
486 
487 	/*
488 	 * Decide if we can use TCP Segmentation Offloading (if supported by
489 	 * hardware).
490 	 *
491 	 * TSO may only be used if we are in a pure bulk sending state.  The
492 	 * presence of TCP-MD5, SACK retransmits, SACK advertizements and
493 	 * IP options prevent using TSO.  With TSO the TCP header is the same
494 	 * (except for the sequence number) for all generated packets.  This
495 	 * makes it impossible to transmit any options which vary per generated
496 	 * segment or packet.
497 	 */
498 #ifdef IPSEC
499 	/*
500 	 * Pre-calculate here as we save another lookup into the darknesses
501 	 * of IPsec that way and can actually decide if TSO is ok.
502 	 */
503 	ipsec_optlen = ipsec_hdrsiz_tcp(tp);
504 #endif
505 	if ((tp->t_flags & TF_TSO) && V_tcp_do_tso && len > tp->t_maxseg &&
506 	    ((tp->t_flags & TF_SIGNATURE) == 0) &&
507 	    tp->rcv_numsacks == 0 && sack_rxmit == 0 &&
508 #ifdef IPSEC
509 	    ipsec_optlen == 0 &&
510 #endif
511 	    tp->t_inpcb->inp_options == NULL &&
512 	    tp->t_inpcb->in6p_options == NULL)
513 		tso = 1;
514 
515 	if (sack_rxmit) {
516 		if (SEQ_LT(p->rxmit + len, tp->snd_una + sbused(&so->so_snd)))
517 			flags &= ~TH_FIN;
518 	} else {
519 		if (SEQ_LT(tp->snd_nxt + len, tp->snd_una +
520 		    sbused(&so->so_snd)))
521 			flags &= ~TH_FIN;
522 	}
523 
524 	recwin = sbspace(&so->so_rcv);
525 
526 	/*
527 	 * Sender silly window avoidance.   We transmit under the following
528 	 * conditions when len is non-zero:
529 	 *
530 	 *	- We have a full segment (or more with TSO)
531 	 *	- This is the last buffer in a write()/send() and we are
532 	 *	  either idle or running NODELAY
533 	 *	- we've timed out (e.g. persist timer)
534 	 *	- we have more then 1/2 the maximum send window's worth of
535 	 *	  data (receiver may be limited the window size)
536 	 *	- we need to retransmit
537 	 */
538 	if (len) {
539 		if (len >= tp->t_maxseg)
540 			goto send;
541 		/*
542 		 * NOTE! on localhost connections an 'ack' from the remote
543 		 * end may occur synchronously with the output and cause
544 		 * us to flush a buffer queued with moretocome.  XXX
545 		 *
546 		 * note: the len + off check is almost certainly unnecessary.
547 		 */
548 		if (!(tp->t_flags & TF_MORETOCOME) &&	/* normal case */
549 		    (idle || (tp->t_flags & TF_NODELAY)) &&
550 		    len + off >= sbavail(&so->so_snd) &&
551 		    (tp->t_flags & TF_NOPUSH) == 0) {
552 			goto send;
553 		}
554 		if (tp->t_flags & TF_FORCEDATA)		/* typ. timeout case */
555 			goto send;
556 		if (len >= tp->max_sndwnd / 2 && tp->max_sndwnd > 0)
557 			goto send;
558 		if (SEQ_LT(tp->snd_nxt, tp->snd_max))	/* retransmit case */
559 			goto send;
560 		if (sack_rxmit)
561 			goto send;
562 	}
563 
564 	/*
565 	 * Sending of standalone window updates.
566 	 *
567 	 * Window updates are important when we close our window due to a
568 	 * full socket buffer and are opening it again after the application
569 	 * reads data from it.  Once the window has opened again and the
570 	 * remote end starts to send again the ACK clock takes over and
571 	 * provides the most current window information.
572 	 *
573 	 * We must avoid the silly window syndrome whereas every read
574 	 * from the receive buffer, no matter how small, causes a window
575 	 * update to be sent.  We also should avoid sending a flurry of
576 	 * window updates when the socket buffer had queued a lot of data
577 	 * and the application is doing small reads.
578 	 *
579 	 * Prevent a flurry of pointless window updates by only sending
580 	 * an update when we can increase the advertized window by more
581 	 * than 1/4th of the socket buffer capacity.  When the buffer is
582 	 * getting full or is very small be more aggressive and send an
583 	 * update whenever we can increase by two mss sized segments.
584 	 * In all other situations the ACK's to new incoming data will
585 	 * carry further window increases.
586 	 *
587 	 * Don't send an independent window update if a delayed
588 	 * ACK is pending (it will get piggy-backed on it) or the
589 	 * remote side already has done a half-close and won't send
590 	 * more data.  Skip this if the connection is in T/TCP
591 	 * half-open state.
592 	 */
593 	if (recwin > 0 && !(tp->t_flags & TF_NEEDSYN) &&
594 	    !(tp->t_flags & TF_DELACK) &&
595 	    !TCPS_HAVERCVDFIN(tp->t_state)) {
596 		/*
597 		 * "adv" is the amount we could increase the window,
598 		 * taking into account that we are limited by
599 		 * TCP_MAXWIN << tp->rcv_scale.
600 		 */
601 		long adv;
602 		int oldwin;
603 
604 		adv = min(recwin, (long)TCP_MAXWIN << tp->rcv_scale);
605 		if (SEQ_GT(tp->rcv_adv, tp->rcv_nxt)) {
606 			oldwin = (tp->rcv_adv - tp->rcv_nxt);
607 			adv -= oldwin;
608 		} else
609 			oldwin = 0;
610 
611 		/*
612 		 * If the new window size ends up being the same as the old
613 		 * size when it is scaled, then don't force a window update.
614 		 */
615 		if (oldwin >> tp->rcv_scale == (adv + oldwin) >> tp->rcv_scale)
616 			goto dontupdate;
617 
618 		if (adv >= (long)(2 * tp->t_maxseg) &&
619 		    (adv >= (long)(so->so_rcv.sb_hiwat / 4) ||
620 		     recwin <= (long)(so->so_rcv.sb_hiwat / 8) ||
621 		     so->so_rcv.sb_hiwat <= 8 * tp->t_maxseg))
622 			goto send;
623 	}
624 dontupdate:
625 
626 	/*
627 	 * Send if we owe the peer an ACK, RST, SYN, or urgent data.  ACKNOW
628 	 * is also a catch-all for the retransmit timer timeout case.
629 	 */
630 	if (tp->t_flags & TF_ACKNOW)
631 		goto send;
632 	if ((flags & TH_RST) ||
633 	    ((flags & TH_SYN) && (tp->t_flags & TF_NEEDSYN) == 0))
634 		goto send;
635 	if (SEQ_GT(tp->snd_up, tp->snd_una))
636 		goto send;
637 	/*
638 	 * If our state indicates that FIN should be sent
639 	 * and we have not yet done so, then we need to send.
640 	 */
641 	if (flags & TH_FIN &&
642 	    ((tp->t_flags & TF_SENTFIN) == 0 || tp->snd_nxt == tp->snd_una))
643 		goto send;
644 	/*
645 	 * In SACK, it is possible for tcp_output to fail to send a segment
646 	 * after the retransmission timer has been turned off.  Make sure
647 	 * that the retransmission timer is set.
648 	 */
649 	if ((tp->t_flags & TF_SACK_PERMIT) &&
650 	    SEQ_GT(tp->snd_max, tp->snd_una) &&
651 	    !tcp_timer_active(tp, TT_REXMT) &&
652 	    !tcp_timer_active(tp, TT_PERSIST)) {
653 		tcp_timer_activate(tp, TT_REXMT, tp->t_rxtcur);
654 		goto just_return;
655 	}
656 	/*
657 	 * TCP window updates are not reliable, rather a polling protocol
658 	 * using ``persist'' packets is used to insure receipt of window
659 	 * updates.  The three ``states'' for the output side are:
660 	 *	idle			not doing retransmits or persists
661 	 *	persisting		to move a small or zero window
662 	 *	(re)transmitting	and thereby not persisting
663 	 *
664 	 * tcp_timer_active(tp, TT_PERSIST)
665 	 *	is true when we are in persist state.
666 	 * (tp->t_flags & TF_FORCEDATA)
667 	 *	is set when we are called to send a persist packet.
668 	 * tcp_timer_active(tp, TT_REXMT)
669 	 *	is set when we are retransmitting
670 	 * The output side is idle when both timers are zero.
671 	 *
672 	 * If send window is too small, there is data to transmit, and no
673 	 * retransmit or persist is pending, then go to persist state.
674 	 * If nothing happens soon, send when timer expires:
675 	 * if window is nonzero, transmit what we can,
676 	 * otherwise force out a byte.
677 	 */
678 	if (sbavail(&so->so_snd) && !tcp_timer_active(tp, TT_REXMT) &&
679 	    !tcp_timer_active(tp, TT_PERSIST)) {
680 		tp->t_rxtshift = 0;
681 		tcp_setpersist(tp);
682 	}
683 
684 	/*
685 	 * No reason to send a segment, just return.
686 	 */
687 just_return:
688 	SOCKBUF_UNLOCK(&so->so_snd);
689 	return (0);
690 
691 send:
692 	SOCKBUF_LOCK_ASSERT(&so->so_snd);
693 	if (len > 0) {
694 		if (len >= tp->t_maxseg)
695 			tp->t_flags2 |= TF2_PLPMTU_MAXSEGSNT;
696 		else
697 			tp->t_flags2 &= ~TF2_PLPMTU_MAXSEGSNT;
698 	}
699 	/*
700 	 * Before ESTABLISHED, force sending of initial options
701 	 * unless TCP set not to do any options.
702 	 * NOTE: we assume that the IP/TCP header plus TCP options
703 	 * always fit in a single mbuf, leaving room for a maximum
704 	 * link header, i.e.
705 	 *	max_linkhdr + sizeof (struct tcpiphdr) + optlen <= MCLBYTES
706 	 */
707 	optlen = 0;
708 #ifdef INET6
709 	if (isipv6)
710 		hdrlen = sizeof (struct ip6_hdr) + sizeof (struct tcphdr);
711 	else
712 #endif
713 		hdrlen = sizeof (struct tcpiphdr);
714 
715 	/*
716 	 * Compute options for segment.
717 	 * We only have to care about SYN and established connection
718 	 * segments.  Options for SYN-ACK segments are handled in TCP
719 	 * syncache.
720 	 */
721 	if ((tp->t_flags & TF_NOOPT) == 0) {
722 		to.to_flags = 0;
723 		/* Maximum segment size. */
724 		if (flags & TH_SYN) {
725 			tp->snd_nxt = tp->iss;
726 			to.to_mss = tcp_mssopt(&tp->t_inpcb->inp_inc);
727 			to.to_flags |= TOF_MSS;
728 		}
729 		/* Window scaling. */
730 		if ((flags & TH_SYN) && (tp->t_flags & TF_REQ_SCALE)) {
731 			to.to_wscale = tp->request_r_scale;
732 			to.to_flags |= TOF_SCALE;
733 		}
734 		/* Timestamps. */
735 		if ((tp->t_flags & TF_RCVD_TSTMP) ||
736 		    ((flags & TH_SYN) && (tp->t_flags & TF_REQ_TSTMP))) {
737 			to.to_tsval = tcp_ts_getticks() + tp->ts_offset;
738 			to.to_tsecr = tp->ts_recent;
739 			to.to_flags |= TOF_TS;
740 			/* Set receive buffer autosizing timestamp. */
741 			if (tp->rfbuf_ts == 0 &&
742 			    (so->so_rcv.sb_flags & SB_AUTOSIZE))
743 				tp->rfbuf_ts = tcp_ts_getticks();
744 		}
745 		/* Selective ACK's. */
746 		if (tp->t_flags & TF_SACK_PERMIT) {
747 			if (flags & TH_SYN)
748 				to.to_flags |= TOF_SACKPERM;
749 			else if (TCPS_HAVEESTABLISHED(tp->t_state) &&
750 			    (tp->t_flags & TF_SACK_PERMIT) &&
751 			    tp->rcv_numsacks > 0) {
752 				to.to_flags |= TOF_SACK;
753 				to.to_nsacks = tp->rcv_numsacks;
754 				to.to_sacks = (u_char *)tp->sackblks;
755 			}
756 		}
757 #ifdef TCP_SIGNATURE
758 		/* TCP-MD5 (RFC2385). */
759 		if (tp->t_flags & TF_SIGNATURE)
760 			to.to_flags |= TOF_SIGNATURE;
761 #endif /* TCP_SIGNATURE */
762 
763 		/* Processing the options. */
764 		hdrlen += optlen = tcp_addoptions(&to, opt);
765 	}
766 
767 #ifdef INET6
768 	if (isipv6)
769 		ipoptlen = ip6_optlen(tp->t_inpcb);
770 	else
771 #endif
772 	if (tp->t_inpcb->inp_options)
773 		ipoptlen = tp->t_inpcb->inp_options->m_len -
774 				offsetof(struct ipoption, ipopt_list);
775 	else
776 		ipoptlen = 0;
777 #ifdef IPSEC
778 	ipoptlen += ipsec_optlen;
779 #endif
780 
781 	/*
782 	 * Adjust data length if insertion of options will
783 	 * bump the packet length beyond the t_maxopd length.
784 	 * Clear the FIN bit because we cut off the tail of
785 	 * the segment.
786 	 */
787 	if (len + optlen + ipoptlen > tp->t_maxopd) {
788 		flags &= ~TH_FIN;
789 
790 		if (tso) {
791 			u_int if_hw_tsomax;
792 			u_int if_hw_tsomaxsegcount;
793 			u_int if_hw_tsomaxsegsize;
794 			struct mbuf *mb;
795 			u_int moff;
796 			int max_len;
797 
798 			/* extract TSO information */
799 			if_hw_tsomax = tp->t_tsomax;
800 			if_hw_tsomaxsegcount = tp->t_tsomaxsegcount;
801 			if_hw_tsomaxsegsize = tp->t_tsomaxsegsize;
802 
803 			/*
804 			 * Limit a TSO burst to prevent it from
805 			 * overflowing or exceeding the maximum length
806 			 * allowed by the network interface:
807 			 */
808 			KASSERT(ipoptlen == 0,
809 			    ("%s: TSO can't do IP options", __func__));
810 
811 			/*
812 			 * Check if we should limit by maximum payload
813 			 * length:
814 			 */
815 			if (if_hw_tsomax != 0) {
816 				/* compute maximum TSO length */
817 				max_len = (if_hw_tsomax - hdrlen -
818 				    max_linkhdr);
819 				if (max_len <= 0) {
820 					len = 0;
821 				} else if (len > max_len) {
822 					sendalot = 1;
823 					len = max_len;
824 				}
825 			}
826 
827 			/*
828 			 * Check if we should limit by maximum segment
829 			 * size and count:
830 			 */
831 			if (if_hw_tsomaxsegcount != 0 &&
832 			    if_hw_tsomaxsegsize != 0) {
833 				/*
834 				 * Subtract one segment for the LINK
835 				 * and TCP/IP headers mbuf that will
836 				 * be prepended to this mbuf chain
837 				 * after the code in this section
838 				 * limits the number of mbufs in the
839 				 * chain to if_hw_tsomaxsegcount.
840 				 */
841 				if_hw_tsomaxsegcount -= 1;
842 				max_len = 0;
843 				mb = sbsndmbuf(&so->so_snd, off, &moff);
844 
845 				while (mb != NULL && max_len < len) {
846 					u_int mlen;
847 					u_int frags;
848 
849 					/*
850 					 * Get length of mbuf fragment
851 					 * and how many hardware frags,
852 					 * rounded up, it would use:
853 					 */
854 					mlen = (mb->m_len - moff);
855 					frags = howmany(mlen,
856 					    if_hw_tsomaxsegsize);
857 
858 					/* Handle special case: Zero Length Mbuf */
859 					if (frags == 0)
860 						frags = 1;
861 
862 					/*
863 					 * Check if the fragment limit
864 					 * will be reached or exceeded:
865 					 */
866 					if (frags >= if_hw_tsomaxsegcount) {
867 						max_len += min(mlen,
868 						    if_hw_tsomaxsegcount *
869 						    if_hw_tsomaxsegsize);
870 						break;
871 					}
872 					max_len += mlen;
873 					if_hw_tsomaxsegcount -= frags;
874 					moff = 0;
875 					mb = mb->m_next;
876 				}
877 				if (max_len <= 0) {
878 					len = 0;
879 				} else if (len > max_len) {
880 					sendalot = 1;
881 					len = max_len;
882 				}
883 			}
884 
885 			/*
886 			 * Prevent the last segment from being
887 			 * fractional unless the send sockbuf can be
888 			 * emptied:
889 			 */
890 			max_len = (tp->t_maxopd - optlen);
891 			if ((off + len) < sbavail(&so->so_snd)) {
892 				moff = len % max_len;
893 				if (moff != 0) {
894 					len -= moff;
895 					sendalot = 1;
896 				}
897 			}
898 
899 			/*
900 			 * In case there are too many small fragments
901 			 * don't use TSO:
902 			 */
903 			if (len <= max_len) {
904 				len = max_len;
905 				sendalot = 1;
906 				tso = 0;
907 			}
908 
909 			/*
910 			 * Send the FIN in a separate segment
911 			 * after the bulk sending is done.
912 			 * We don't trust the TSO implementations
913 			 * to clear the FIN flag on all but the
914 			 * last segment.
915 			 */
916 			if (tp->t_flags & TF_NEEDFIN)
917 				sendalot = 1;
918 
919 		} else {
920 			len = tp->t_maxopd - optlen - ipoptlen;
921 			sendalot = 1;
922 		}
923 	} else
924 		tso = 0;
925 
926 	KASSERT(len + hdrlen + ipoptlen <= IP_MAXPACKET,
927 	    ("%s: len > IP_MAXPACKET", __func__));
928 
929 /*#ifdef DIAGNOSTIC*/
930 #ifdef INET6
931 	if (max_linkhdr + hdrlen > MCLBYTES)
932 #else
933 	if (max_linkhdr + hdrlen > MHLEN)
934 #endif
935 		panic("tcphdr too big");
936 /*#endif*/
937 
938 	/*
939 	 * This KASSERT is here to catch edge cases at a well defined place.
940 	 * Before, those had triggered (random) panic conditions further down.
941 	 */
942 	KASSERT(len >= 0, ("[%s:%d]: len < 0", __func__, __LINE__));
943 
944 	/*
945 	 * Grab a header mbuf, attaching a copy of data to
946 	 * be transmitted, and initialize the header from
947 	 * the template for sends on this connection.
948 	 */
949 	if (len) {
950 		struct mbuf *mb;
951 		u_int moff;
952 
953 		if ((tp->t_flags & TF_FORCEDATA) && len == 1)
954 			TCPSTAT_INC(tcps_sndprobe);
955 		else if (SEQ_LT(tp->snd_nxt, tp->snd_max) || sack_rxmit) {
956 			tp->t_sndrexmitpack++;
957 			TCPSTAT_INC(tcps_sndrexmitpack);
958 			TCPSTAT_ADD(tcps_sndrexmitbyte, len);
959 		} else {
960 			TCPSTAT_INC(tcps_sndpack);
961 			TCPSTAT_ADD(tcps_sndbyte, len);
962 		}
963 #ifdef INET6
964 		if (MHLEN < hdrlen + max_linkhdr)
965 			m = m_getcl(M_NOWAIT, MT_DATA, M_PKTHDR);
966 		else
967 #endif
968 			m = m_gethdr(M_NOWAIT, MT_DATA);
969 
970 		if (m == NULL) {
971 			SOCKBUF_UNLOCK(&so->so_snd);
972 			error = ENOBUFS;
973 			sack_rxmit = 0;
974 			goto out;
975 		}
976 
977 		m->m_data += max_linkhdr;
978 		m->m_len = hdrlen;
979 
980 		/*
981 		 * Start the m_copy functions from the closest mbuf
982 		 * to the offset in the socket buffer chain.
983 		 */
984 		mb = sbsndptr(&so->so_snd, off, len, &moff);
985 
986 		if (len <= MHLEN - hdrlen - max_linkhdr) {
987 			m_copydata(mb, moff, (int)len,
988 			    mtod(m, caddr_t) + hdrlen);
989 			m->m_len += len;
990 		} else {
991 			m->m_next = m_copy(mb, moff, (int)len);
992 			if (m->m_next == NULL) {
993 				SOCKBUF_UNLOCK(&so->so_snd);
994 				(void) m_free(m);
995 				error = ENOBUFS;
996 				sack_rxmit = 0;
997 				goto out;
998 			}
999 		}
1000 
1001 		/*
1002 		 * If we're sending everything we've got, set PUSH.
1003 		 * (This will keep happy those implementations which only
1004 		 * give data to the user when a buffer fills or
1005 		 * a PUSH comes in.)
1006 		 */
1007 		if (off + len == sbused(&so->so_snd))
1008 			flags |= TH_PUSH;
1009 		SOCKBUF_UNLOCK(&so->so_snd);
1010 	} else {
1011 		SOCKBUF_UNLOCK(&so->so_snd);
1012 		if (tp->t_flags & TF_ACKNOW)
1013 			TCPSTAT_INC(tcps_sndacks);
1014 		else if (flags & (TH_SYN|TH_FIN|TH_RST))
1015 			TCPSTAT_INC(tcps_sndctrl);
1016 		else if (SEQ_GT(tp->snd_up, tp->snd_una))
1017 			TCPSTAT_INC(tcps_sndurg);
1018 		else
1019 			TCPSTAT_INC(tcps_sndwinup);
1020 
1021 		m = m_gethdr(M_NOWAIT, MT_DATA);
1022 		if (m == NULL) {
1023 			error = ENOBUFS;
1024 			sack_rxmit = 0;
1025 			goto out;
1026 		}
1027 #ifdef INET6
1028 		if (isipv6 && (MHLEN < hdrlen + max_linkhdr) &&
1029 		    MHLEN >= hdrlen) {
1030 			M_ALIGN(m, hdrlen);
1031 		} else
1032 #endif
1033 		m->m_data += max_linkhdr;
1034 		m->m_len = hdrlen;
1035 	}
1036 	SOCKBUF_UNLOCK_ASSERT(&so->so_snd);
1037 	m->m_pkthdr.rcvif = (struct ifnet *)0;
1038 #ifdef MAC
1039 	mac_inpcb_create_mbuf(tp->t_inpcb, m);
1040 #endif
1041 #ifdef INET6
1042 	if (isipv6) {
1043 		ip6 = mtod(m, struct ip6_hdr *);
1044 		th = (struct tcphdr *)(ip6 + 1);
1045 		tcpip_fillheaders(tp->t_inpcb, ip6, th);
1046 	} else
1047 #endif /* INET6 */
1048 	{
1049 		ip = mtod(m, struct ip *);
1050 		ipov = (struct ipovly *)ip;
1051 		th = (struct tcphdr *)(ip + 1);
1052 		tcpip_fillheaders(tp->t_inpcb, ip, th);
1053 	}
1054 
1055 	/*
1056 	 * Fill in fields, remembering maximum advertised
1057 	 * window for use in delaying messages about window sizes.
1058 	 * If resending a FIN, be sure not to use a new sequence number.
1059 	 */
1060 	if (flags & TH_FIN && tp->t_flags & TF_SENTFIN &&
1061 	    tp->snd_nxt == tp->snd_max)
1062 		tp->snd_nxt--;
1063 	/*
1064 	 * If we are starting a connection, send ECN setup
1065 	 * SYN packet. If we are on a retransmit, we may
1066 	 * resend those bits a number of times as per
1067 	 * RFC 3168.
1068 	 */
1069 	if (tp->t_state == TCPS_SYN_SENT && V_tcp_do_ecn) {
1070 		if (tp->t_rxtshift >= 1) {
1071 			if (tp->t_rxtshift <= V_tcp_ecn_maxretries)
1072 				flags |= TH_ECE|TH_CWR;
1073 		} else
1074 			flags |= TH_ECE|TH_CWR;
1075 	}
1076 
1077 	if (tp->t_state == TCPS_ESTABLISHED &&
1078 	    (tp->t_flags & TF_ECN_PERMIT)) {
1079 		/*
1080 		 * If the peer has ECN, mark data packets with
1081 		 * ECN capable transmission (ECT).
1082 		 * Ignore pure ack packets, retransmissions and window probes.
1083 		 */
1084 		if (len > 0 && SEQ_GEQ(tp->snd_nxt, tp->snd_max) &&
1085 		    !((tp->t_flags & TF_FORCEDATA) && len == 1)) {
1086 #ifdef INET6
1087 			if (isipv6)
1088 				ip6->ip6_flow |= htonl(IPTOS_ECN_ECT0 << 20);
1089 			else
1090 #endif
1091 				ip->ip_tos |= IPTOS_ECN_ECT0;
1092 			TCPSTAT_INC(tcps_ecn_ect0);
1093 		}
1094 
1095 		/*
1096 		 * Reply with proper ECN notifications.
1097 		 */
1098 		if (tp->t_flags & TF_ECN_SND_CWR) {
1099 			flags |= TH_CWR;
1100 			tp->t_flags &= ~TF_ECN_SND_CWR;
1101 		}
1102 		if (tp->t_flags & TF_ECN_SND_ECE)
1103 			flags |= TH_ECE;
1104 	}
1105 
1106 	/*
1107 	 * If we are doing retransmissions, then snd_nxt will
1108 	 * not reflect the first unsent octet.  For ACK only
1109 	 * packets, we do not want the sequence number of the
1110 	 * retransmitted packet, we want the sequence number
1111 	 * of the next unsent octet.  So, if there is no data
1112 	 * (and no SYN or FIN), use snd_max instead of snd_nxt
1113 	 * when filling in ti_seq.  But if we are in persist
1114 	 * state, snd_max might reflect one byte beyond the
1115 	 * right edge of the window, so use snd_nxt in that
1116 	 * case, since we know we aren't doing a retransmission.
1117 	 * (retransmit and persist are mutually exclusive...)
1118 	 */
1119 	if (sack_rxmit == 0) {
1120 		if (len || (flags & (TH_SYN|TH_FIN)) ||
1121 		    tcp_timer_active(tp, TT_PERSIST))
1122 			th->th_seq = htonl(tp->snd_nxt);
1123 		else
1124 			th->th_seq = htonl(tp->snd_max);
1125 	} else {
1126 		th->th_seq = htonl(p->rxmit);
1127 		p->rxmit += len;
1128 		tp->sackhint.sack_bytes_rexmit += len;
1129 	}
1130 	th->th_ack = htonl(tp->rcv_nxt);
1131 	if (optlen) {
1132 		bcopy(opt, th + 1, optlen);
1133 		th->th_off = (sizeof (struct tcphdr) + optlen) >> 2;
1134 	}
1135 	th->th_flags = flags;
1136 	/*
1137 	 * Calculate receive window.  Don't shrink window,
1138 	 * but avoid silly window syndrome.
1139 	 */
1140 	if (recwin < (long)(so->so_rcv.sb_hiwat / 4) &&
1141 	    recwin < (long)tp->t_maxseg)
1142 		recwin = 0;
1143 	if (SEQ_GT(tp->rcv_adv, tp->rcv_nxt) &&
1144 	    recwin < (long)(tp->rcv_adv - tp->rcv_nxt))
1145 		recwin = (long)(tp->rcv_adv - tp->rcv_nxt);
1146 	if (recwin > (long)TCP_MAXWIN << tp->rcv_scale)
1147 		recwin = (long)TCP_MAXWIN << tp->rcv_scale;
1148 
1149 	/*
1150 	 * According to RFC1323 the window field in a SYN (i.e., a <SYN>
1151 	 * or <SYN,ACK>) segment itself is never scaled.  The <SYN,ACK>
1152 	 * case is handled in syncache.
1153 	 */
1154 	if (flags & TH_SYN)
1155 		th->th_win = htons((u_short)
1156 				(min(sbspace(&so->so_rcv), TCP_MAXWIN)));
1157 	else
1158 		th->th_win = htons((u_short)(recwin >> tp->rcv_scale));
1159 
1160 	/*
1161 	 * Adjust the RXWIN0SENT flag - indicate that we have advertised
1162 	 * a 0 window.  This may cause the remote transmitter to stall.  This
1163 	 * flag tells soreceive() to disable delayed acknowledgements when
1164 	 * draining the buffer.  This can occur if the receiver is attempting
1165 	 * to read more data than can be buffered prior to transmitting on
1166 	 * the connection.
1167 	 */
1168 	if (th->th_win == 0) {
1169 		tp->t_sndzerowin++;
1170 		tp->t_flags |= TF_RXWIN0SENT;
1171 	} else
1172 		tp->t_flags &= ~TF_RXWIN0SENT;
1173 	if (SEQ_GT(tp->snd_up, tp->snd_nxt)) {
1174 		th->th_urp = htons((u_short)(tp->snd_up - tp->snd_nxt));
1175 		th->th_flags |= TH_URG;
1176 	} else
1177 		/*
1178 		 * If no urgent pointer to send, then we pull
1179 		 * the urgent pointer to the left edge of the send window
1180 		 * so that it doesn't drift into the send window on sequence
1181 		 * number wraparound.
1182 		 */
1183 		tp->snd_up = tp->snd_una;		/* drag it along */
1184 
1185 #ifdef TCP_SIGNATURE
1186 	if (tp->t_flags & TF_SIGNATURE) {
1187 		int sigoff = to.to_signature - opt;
1188 		tcp_signature_compute(m, 0, len, optlen,
1189 		    (u_char *)(th + 1) + sigoff, IPSEC_DIR_OUTBOUND);
1190 	}
1191 #endif
1192 
1193 	/*
1194 	 * Put TCP length in extended header, and then
1195 	 * checksum extended header and data.
1196 	 */
1197 	m->m_pkthdr.len = hdrlen + len; /* in6_cksum() need this */
1198 	m->m_pkthdr.csum_data = offsetof(struct tcphdr, th_sum);
1199 #ifdef INET6
1200 	if (isipv6) {
1201 		/*
1202 		 * ip6_plen is not need to be filled now, and will be filled
1203 		 * in ip6_output.
1204 		 */
1205 		m->m_pkthdr.csum_flags = CSUM_TCP_IPV6;
1206 		th->th_sum = in6_cksum_pseudo(ip6, sizeof(struct tcphdr) +
1207 		    optlen + len, IPPROTO_TCP, 0);
1208 	}
1209 #endif
1210 #if defined(INET6) && defined(INET)
1211 	else
1212 #endif
1213 #ifdef INET
1214 	{
1215 		m->m_pkthdr.csum_flags = CSUM_TCP;
1216 		th->th_sum = in_pseudo(ip->ip_src.s_addr, ip->ip_dst.s_addr,
1217 		    htons(sizeof(struct tcphdr) + IPPROTO_TCP + len + optlen));
1218 
1219 		/* IP version must be set here for ipv4/ipv6 checking later */
1220 		KASSERT(ip->ip_v == IPVERSION,
1221 		    ("%s: IP version incorrect: %d", __func__, ip->ip_v));
1222 	}
1223 #endif
1224 
1225 	/*
1226 	 * Enable TSO and specify the size of the segments.
1227 	 * The TCP pseudo header checksum is always provided.
1228 	 */
1229 	if (tso) {
1230 		KASSERT(len > tp->t_maxopd - optlen,
1231 		    ("%s: len <= tso_segsz", __func__));
1232 		m->m_pkthdr.csum_flags |= CSUM_TSO;
1233 		m->m_pkthdr.tso_segsz = tp->t_maxopd - optlen;
1234 	}
1235 
1236 #ifdef IPSEC
1237 	KASSERT(len + hdrlen + ipoptlen - ipsec_optlen == m_length(m, NULL),
1238 	    ("%s: mbuf chain shorter than expected: %ld + %u + %u - %u != %u",
1239 	    __func__, len, hdrlen, ipoptlen, ipsec_optlen, m_length(m, NULL)));
1240 #else
1241 	KASSERT(len + hdrlen + ipoptlen == m_length(m, NULL),
1242 	    ("%s: mbuf chain shorter than expected: %ld + %u + %u != %u",
1243 	    __func__, len, hdrlen, ipoptlen, m_length(m, NULL)));
1244 #endif
1245 
1246 	/* Run HHOOK_TCP_ESTABLISHED_OUT helper hooks. */
1247 	hhook_run_tcp_est_out(tp, th, &to, len, tso);
1248 
1249 #ifdef TCPDEBUG
1250 	/*
1251 	 * Trace.
1252 	 */
1253 	if (so->so_options & SO_DEBUG) {
1254 		u_short save = 0;
1255 #ifdef INET6
1256 		if (!isipv6)
1257 #endif
1258 		{
1259 			save = ipov->ih_len;
1260 			ipov->ih_len = htons(m->m_pkthdr.len /* - hdrlen + (th->th_off << 2) */);
1261 		}
1262 		tcp_trace(TA_OUTPUT, tp->t_state, tp, mtod(m, void *), th, 0);
1263 #ifdef INET6
1264 		if (!isipv6)
1265 #endif
1266 		ipov->ih_len = save;
1267 	}
1268 #endif /* TCPDEBUG */
1269 	TCP_PROBE3(debug__input, tp, th, mtod(m, const char *));
1270 
1271 	/*
1272 	 * Fill in IP length and desired time to live and
1273 	 * send to IP level.  There should be a better way
1274 	 * to handle ttl and tos; we could keep them in
1275 	 * the template, but need a way to checksum without them.
1276 	 */
1277 	/*
1278 	 * m->m_pkthdr.len should have been set before checksum calculation,
1279 	 * because in6_cksum() need it.
1280 	 */
1281 #ifdef INET6
1282 	if (isipv6) {
1283 		struct route_in6 ro;
1284 
1285 		bzero(&ro, sizeof(ro));
1286 		/*
1287 		 * we separately set hoplimit for every segment, since the
1288 		 * user might want to change the value via setsockopt.
1289 		 * Also, desired default hop limit might be changed via
1290 		 * Neighbor Discovery.
1291 		 */
1292 		ip6->ip6_hlim = in6_selecthlim(tp->t_inpcb, NULL);
1293 
1294 		/*
1295 		 * Set the packet size here for the benefit of DTrace probes.
1296 		 * ip6_output() will set it properly; it's supposed to include
1297 		 * the option header lengths as well.
1298 		 */
1299 		ip6->ip6_plen = htons(m->m_pkthdr.len - sizeof(*ip6));
1300 
1301 		if (V_path_mtu_discovery && tp->t_maxopd > V_tcp_minmss)
1302 			tp->t_flags2 |= TF2_PLPMTU_PMTUD;
1303 		else
1304 			tp->t_flags2 &= ~TF2_PLPMTU_PMTUD;
1305 
1306 		if (tp->t_state == TCPS_SYN_SENT)
1307 			TCP_PROBE5(connect__request, NULL, tp, ip6, tp, th);
1308 
1309 		TCP_PROBE5(send, NULL, tp, ip6, tp, th);
1310 
1311 #ifdef TCPPCAP
1312 		/* Save packet, if requested. */
1313 		tcp_pcap_add(th, m, &(tp->t_outpkts));
1314 #endif
1315 
1316 		/* TODO: IPv6 IP6TOS_ECT bit on */
1317 		error = ip6_output(m, tp->t_inpcb->in6p_outputopts, &ro,
1318 		    ((so->so_options & SO_DONTROUTE) ?  IP_ROUTETOIF : 0),
1319 		    NULL, NULL, tp->t_inpcb);
1320 
1321 		if (error == EMSGSIZE && ro.ro_rt != NULL)
1322 			mtu = ro.ro_rt->rt_mtu;
1323 		RO_RTFREE(&ro);
1324 	}
1325 #endif /* INET6 */
1326 #if defined(INET) && defined(INET6)
1327 	else
1328 #endif
1329 #ifdef INET
1330     {
1331 	struct route ro;
1332 
1333 	bzero(&ro, sizeof(ro));
1334 	ip->ip_len = htons(m->m_pkthdr.len);
1335 #ifdef INET6
1336 	if (tp->t_inpcb->inp_vflag & INP_IPV6PROTO)
1337 		ip->ip_ttl = in6_selecthlim(tp->t_inpcb, NULL);
1338 #endif /* INET6 */
1339 	/*
1340 	 * If we do path MTU discovery, then we set DF on every packet.
1341 	 * This might not be the best thing to do according to RFC3390
1342 	 * Section 2. However the tcp hostcache migitates the problem
1343 	 * so it affects only the first tcp connection with a host.
1344 	 *
1345 	 * NB: Don't set DF on small MTU/MSS to have a safe fallback.
1346 	 */
1347 	if (V_path_mtu_discovery && tp->t_maxopd > V_tcp_minmss) {
1348 		ip->ip_off |= htons(IP_DF);
1349 		tp->t_flags2 |= TF2_PLPMTU_PMTUD;
1350 	} else {
1351 		tp->t_flags2 &= ~TF2_PLPMTU_PMTUD;
1352 	}
1353 
1354 	if (tp->t_state == TCPS_SYN_SENT)
1355 		TCP_PROBE5(connect__request, NULL, tp, ip, tp, th);
1356 
1357 	TCP_PROBE5(send, NULL, tp, ip, tp, th);
1358 
1359 #ifdef TCPPCAP
1360 	/* Save packet, if requested. */
1361 	tcp_pcap_add(th, m, &(tp->t_outpkts));
1362 #endif
1363 
1364 	error = ip_output(m, tp->t_inpcb->inp_options, &ro,
1365 	    ((so->so_options & SO_DONTROUTE) ? IP_ROUTETOIF : 0), 0,
1366 	    tp->t_inpcb);
1367 
1368 	if (error == EMSGSIZE && ro.ro_rt != NULL)
1369 		mtu = ro.ro_rt->rt_mtu;
1370 	RO_RTFREE(&ro);
1371     }
1372 #endif /* INET */
1373 
1374 out:
1375 	/*
1376 	 * In transmit state, time the transmission and arrange for
1377 	 * the retransmit.  In persist state, just set snd_max.
1378 	 */
1379 	if ((tp->t_flags & TF_FORCEDATA) == 0 ||
1380 	    !tcp_timer_active(tp, TT_PERSIST)) {
1381 		tcp_seq startseq = tp->snd_nxt;
1382 
1383 		/*
1384 		 * Advance snd_nxt over sequence space of this segment.
1385 		 */
1386 		if (flags & (TH_SYN|TH_FIN)) {
1387 			if (flags & TH_SYN)
1388 				tp->snd_nxt++;
1389 			if (flags & TH_FIN) {
1390 				tp->snd_nxt++;
1391 				tp->t_flags |= TF_SENTFIN;
1392 			}
1393 		}
1394 		if (sack_rxmit)
1395 			goto timer;
1396 		tp->snd_nxt += len;
1397 		if (SEQ_GT(tp->snd_nxt, tp->snd_max)) {
1398 			tp->snd_max = tp->snd_nxt;
1399 			/*
1400 			 * Time this transmission if not a retransmission and
1401 			 * not currently timing anything.
1402 			 */
1403 			if (tp->t_rtttime == 0) {
1404 				tp->t_rtttime = ticks;
1405 				tp->t_rtseq = startseq;
1406 				TCPSTAT_INC(tcps_segstimed);
1407 			}
1408 		}
1409 
1410 		/*
1411 		 * Set retransmit timer if not currently set,
1412 		 * and not doing a pure ack or a keep-alive probe.
1413 		 * Initial value for retransmit timer is smoothed
1414 		 * round-trip time + 2 * round-trip time variance.
1415 		 * Initialize shift counter which is used for backoff
1416 		 * of retransmit time.
1417 		 */
1418 timer:
1419 		if (!tcp_timer_active(tp, TT_REXMT) &&
1420 		    ((sack_rxmit && tp->snd_nxt != tp->snd_max) ||
1421 		     (tp->snd_nxt != tp->snd_una))) {
1422 			if (tcp_timer_active(tp, TT_PERSIST)) {
1423 				tcp_timer_activate(tp, TT_PERSIST, 0);
1424 				tp->t_rxtshift = 0;
1425 			}
1426 			tcp_timer_activate(tp, TT_REXMT, tp->t_rxtcur);
1427 		} else if (len == 0 && sbavail(&so->so_snd) &&
1428 		    !tcp_timer_active(tp, TT_REXMT) &&
1429 		    !tcp_timer_active(tp, TT_PERSIST)) {
1430 			/*
1431 			 * Avoid a situation where we do not set persist timer
1432 			 * after a zero window condition. For example:
1433 			 * 1) A -> B: packet with enough data to fill the window
1434 			 * 2) B -> A: ACK for #1 + new data (0 window
1435 			 *    advertisement)
1436 			 * 3) A -> B: ACK for #2, 0 len packet
1437 			 *
1438 			 * In this case, A will not activate the persist timer,
1439 			 * because it chose to send a packet. Unless tcp_output
1440 			 * is called for some other reason (delayed ack timer,
1441 			 * another input packet from B, socket syscall), A will
1442 			 * not send zero window probes.
1443 			 *
1444 			 * So, if you send a 0-length packet, but there is data
1445 			 * in the socket buffer, and neither the rexmt or
1446 			 * persist timer is already set, then activate the
1447 			 * persist timer.
1448 			 */
1449 			tp->t_rxtshift = 0;
1450 			tcp_setpersist(tp);
1451 		}
1452 	} else {
1453 		/*
1454 		 * Persist case, update snd_max but since we are in
1455 		 * persist mode (no window) we do not update snd_nxt.
1456 		 */
1457 		int xlen = len;
1458 		if (flags & TH_SYN)
1459 			++xlen;
1460 		if (flags & TH_FIN) {
1461 			++xlen;
1462 			tp->t_flags |= TF_SENTFIN;
1463 		}
1464 		if (SEQ_GT(tp->snd_nxt + xlen, tp->snd_max))
1465 			tp->snd_max = tp->snd_nxt + len;
1466 	}
1467 
1468 	if (error) {
1469 
1470 		/*
1471 		 * We know that the packet was lost, so back out the
1472 		 * sequence number advance, if any.
1473 		 *
1474 		 * If the error is EPERM the packet got blocked by the
1475 		 * local firewall.  Normally we should terminate the
1476 		 * connection but the blocking may have been spurious
1477 		 * due to a firewall reconfiguration cycle.  So we treat
1478 		 * it like a packet loss and let the retransmit timer and
1479 		 * timeouts do their work over time.
1480 		 * XXX: It is a POLA question whether calling tcp_drop right
1481 		 * away would be the really correct behavior instead.
1482 		 */
1483 		if (((tp->t_flags & TF_FORCEDATA) == 0 ||
1484 		    !tcp_timer_active(tp, TT_PERSIST)) &&
1485 		    ((flags & TH_SYN) == 0) &&
1486 		    (error != EPERM)) {
1487 			if (sack_rxmit) {
1488 				p->rxmit -= len;
1489 				tp->sackhint.sack_bytes_rexmit -= len;
1490 				KASSERT(tp->sackhint.sack_bytes_rexmit >= 0,
1491 				    ("sackhint bytes rtx >= 0"));
1492 			} else
1493 				tp->snd_nxt -= len;
1494 		}
1495 		SOCKBUF_UNLOCK_ASSERT(&so->so_snd);	/* Check gotos. */
1496 		switch (error) {
1497 		case EPERM:
1498 			tp->t_softerror = error;
1499 			return (error);
1500 		case ENOBUFS:
1501 	                if (!tcp_timer_active(tp, TT_REXMT) &&
1502 			    !tcp_timer_active(tp, TT_PERSIST))
1503 	                        tcp_timer_activate(tp, TT_REXMT, tp->t_rxtcur);
1504 			tp->snd_cwnd = tp->t_maxseg;
1505 			return (0);
1506 		case EMSGSIZE:
1507 			/*
1508 			 * For some reason the interface we used initially
1509 			 * to send segments changed to another or lowered
1510 			 * its MTU.
1511 			 * If TSO was active we either got an interface
1512 			 * without TSO capabilits or TSO was turned off.
1513 			 * If we obtained mtu from ip_output() then update
1514 			 * it and try again.
1515 			 */
1516 			if (tso)
1517 				tp->t_flags &= ~TF_TSO;
1518 			if (mtu != 0) {
1519 				tcp_mss_update(tp, -1, mtu, NULL, NULL);
1520 				goto again;
1521 			}
1522 			return (error);
1523 		case EHOSTDOWN:
1524 		case EHOSTUNREACH:
1525 		case ENETDOWN:
1526 		case ENETUNREACH:
1527 			if (TCPS_HAVERCVDSYN(tp->t_state)) {
1528 				tp->t_softerror = error;
1529 				return (0);
1530 			}
1531 			/* FALLTHROUGH */
1532 		default:
1533 			return (error);
1534 		}
1535 	}
1536 	TCPSTAT_INC(tcps_sndtotal);
1537 
1538 	/*
1539 	 * Data sent (as far as we can tell).
1540 	 * If this advertises a larger window than any other segment,
1541 	 * then remember the size of the advertised window.
1542 	 * Any pending ACK has now been sent.
1543 	 */
1544 	if (recwin >= 0 && SEQ_GT(tp->rcv_nxt + recwin, tp->rcv_adv))
1545 		tp->rcv_adv = tp->rcv_nxt + recwin;
1546 	tp->last_ack_sent = tp->rcv_nxt;
1547 	tp->t_flags &= ~(TF_ACKNOW | TF_DELACK);
1548 	if (tcp_timer_active(tp, TT_DELACK))
1549 		tcp_timer_activate(tp, TT_DELACK, 0);
1550 #if 0
1551 	/*
1552 	 * This completely breaks TCP if newreno is turned on.  What happens
1553 	 * is that if delayed-acks are turned on on the receiver, this code
1554 	 * on the transmitter effectively destroys the TCP window, forcing
1555 	 * it to four packets (1.5Kx4 = 6K window).
1556 	 */
1557 	if (sendalot && --maxburst)
1558 		goto again;
1559 #endif
1560 	if (sendalot)
1561 		goto again;
1562 	return (0);
1563 }
1564 
1565 void
1566 tcp_setpersist(struct tcpcb *tp)
1567 {
1568 	int t = ((tp->t_srtt >> 2) + tp->t_rttvar) >> 1;
1569 	int tt;
1570 
1571 	tp->t_flags &= ~TF_PREVVALID;
1572 	if (tcp_timer_active(tp, TT_REXMT))
1573 		panic("tcp_setpersist: retransmit pending");
1574 	/*
1575 	 * Start/restart persistance timer.
1576 	 */
1577 	TCPT_RANGESET(tt, t * tcp_backoff[tp->t_rxtshift],
1578 		      TCPTV_PERSMIN, TCPTV_PERSMAX);
1579 	tcp_timer_activate(tp, TT_PERSIST, tt);
1580 	if (tp->t_rxtshift < TCP_MAXRXTSHIFT)
1581 		tp->t_rxtshift++;
1582 }
1583 
1584 /*
1585  * Insert TCP options according to the supplied parameters to the place
1586  * optp in a consistent way.  Can handle unaligned destinations.
1587  *
1588  * The order of the option processing is crucial for optimal packing and
1589  * alignment for the scarce option space.
1590  *
1591  * The optimal order for a SYN/SYN-ACK segment is:
1592  *   MSS (4) + NOP (1) + Window scale (3) + SACK permitted (2) +
1593  *   Timestamp (10) + Signature (18) = 38 bytes out of a maximum of 40.
1594  *
1595  * The SACK options should be last.  SACK blocks consume 8*n+2 bytes.
1596  * So a full size SACK blocks option is 34 bytes (with 4 SACK blocks).
1597  * At minimum we need 10 bytes (to generate 1 SACK block).  If both
1598  * TCP Timestamps (12 bytes) and TCP Signatures (18 bytes) are present,
1599  * we only have 10 bytes for SACK options (40 - (12 + 18)).
1600  */
1601 int
1602 tcp_addoptions(struct tcpopt *to, u_char *optp)
1603 {
1604 	u_int mask, optlen = 0;
1605 
1606 	for (mask = 1; mask < TOF_MAXOPT; mask <<= 1) {
1607 		if ((to->to_flags & mask) != mask)
1608 			continue;
1609 		if (optlen == TCP_MAXOLEN)
1610 			break;
1611 		switch (to->to_flags & mask) {
1612 		case TOF_MSS:
1613 			while (optlen % 4) {
1614 				optlen += TCPOLEN_NOP;
1615 				*optp++ = TCPOPT_NOP;
1616 			}
1617 			if (TCP_MAXOLEN - optlen < TCPOLEN_MAXSEG)
1618 				continue;
1619 			optlen += TCPOLEN_MAXSEG;
1620 			*optp++ = TCPOPT_MAXSEG;
1621 			*optp++ = TCPOLEN_MAXSEG;
1622 			to->to_mss = htons(to->to_mss);
1623 			bcopy((u_char *)&to->to_mss, optp, sizeof(to->to_mss));
1624 			optp += sizeof(to->to_mss);
1625 			break;
1626 		case TOF_SCALE:
1627 			while (!optlen || optlen % 2 != 1) {
1628 				optlen += TCPOLEN_NOP;
1629 				*optp++ = TCPOPT_NOP;
1630 			}
1631 			if (TCP_MAXOLEN - optlen < TCPOLEN_WINDOW)
1632 				continue;
1633 			optlen += TCPOLEN_WINDOW;
1634 			*optp++ = TCPOPT_WINDOW;
1635 			*optp++ = TCPOLEN_WINDOW;
1636 			*optp++ = to->to_wscale;
1637 			break;
1638 		case TOF_SACKPERM:
1639 			while (optlen % 2) {
1640 				optlen += TCPOLEN_NOP;
1641 				*optp++ = TCPOPT_NOP;
1642 			}
1643 			if (TCP_MAXOLEN - optlen < TCPOLEN_SACK_PERMITTED)
1644 				continue;
1645 			optlen += TCPOLEN_SACK_PERMITTED;
1646 			*optp++ = TCPOPT_SACK_PERMITTED;
1647 			*optp++ = TCPOLEN_SACK_PERMITTED;
1648 			break;
1649 		case TOF_TS:
1650 			while (!optlen || optlen % 4 != 2) {
1651 				optlen += TCPOLEN_NOP;
1652 				*optp++ = TCPOPT_NOP;
1653 			}
1654 			if (TCP_MAXOLEN - optlen < TCPOLEN_TIMESTAMP)
1655 				continue;
1656 			optlen += TCPOLEN_TIMESTAMP;
1657 			*optp++ = TCPOPT_TIMESTAMP;
1658 			*optp++ = TCPOLEN_TIMESTAMP;
1659 			to->to_tsval = htonl(to->to_tsval);
1660 			to->to_tsecr = htonl(to->to_tsecr);
1661 			bcopy((u_char *)&to->to_tsval, optp, sizeof(to->to_tsval));
1662 			optp += sizeof(to->to_tsval);
1663 			bcopy((u_char *)&to->to_tsecr, optp, sizeof(to->to_tsecr));
1664 			optp += sizeof(to->to_tsecr);
1665 			break;
1666 		case TOF_SIGNATURE:
1667 			{
1668 			int siglen = TCPOLEN_SIGNATURE - 2;
1669 
1670 			while (!optlen || optlen % 4 != 2) {
1671 				optlen += TCPOLEN_NOP;
1672 				*optp++ = TCPOPT_NOP;
1673 			}
1674 			if (TCP_MAXOLEN - optlen < TCPOLEN_SIGNATURE)
1675 				continue;
1676 			optlen += TCPOLEN_SIGNATURE;
1677 			*optp++ = TCPOPT_SIGNATURE;
1678 			*optp++ = TCPOLEN_SIGNATURE;
1679 			to->to_signature = optp;
1680 			while (siglen--)
1681 				 *optp++ = 0;
1682 			break;
1683 			}
1684 		case TOF_SACK:
1685 			{
1686 			int sackblks = 0;
1687 			struct sackblk *sack = (struct sackblk *)to->to_sacks;
1688 			tcp_seq sack_seq;
1689 
1690 			while (!optlen || optlen % 4 != 2) {
1691 				optlen += TCPOLEN_NOP;
1692 				*optp++ = TCPOPT_NOP;
1693 			}
1694 			if (TCP_MAXOLEN - optlen < TCPOLEN_SACKHDR + TCPOLEN_SACK)
1695 				continue;
1696 			optlen += TCPOLEN_SACKHDR;
1697 			*optp++ = TCPOPT_SACK;
1698 			sackblks = min(to->to_nsacks,
1699 					(TCP_MAXOLEN - optlen) / TCPOLEN_SACK);
1700 			*optp++ = TCPOLEN_SACKHDR + sackblks * TCPOLEN_SACK;
1701 			while (sackblks--) {
1702 				sack_seq = htonl(sack->start);
1703 				bcopy((u_char *)&sack_seq, optp, sizeof(sack_seq));
1704 				optp += sizeof(sack_seq);
1705 				sack_seq = htonl(sack->end);
1706 				bcopy((u_char *)&sack_seq, optp, sizeof(sack_seq));
1707 				optp += sizeof(sack_seq);
1708 				optlen += TCPOLEN_SACK;
1709 				sack++;
1710 			}
1711 			TCPSTAT_INC(tcps_sack_send_blocks);
1712 			break;
1713 			}
1714 		default:
1715 			panic("%s: unknown TCP option type", __func__);
1716 			break;
1717 		}
1718 	}
1719 
1720 	/* Terminate and pad TCP options to a 4 byte boundary. */
1721 	if (optlen % 4) {
1722 		optlen += TCPOLEN_EOL;
1723 		*optp++ = TCPOPT_EOL;
1724 	}
1725 	/*
1726 	 * According to RFC 793 (STD0007):
1727 	 *   "The content of the header beyond the End-of-Option option
1728 	 *    must be header padding (i.e., zero)."
1729 	 *   and later: "The padding is composed of zeros."
1730 	 */
1731 	while (optlen % 4) {
1732 		optlen += TCPOLEN_PAD;
1733 		*optp++ = TCPOPT_PAD;
1734 	}
1735 
1736 	KASSERT(optlen <= TCP_MAXOLEN, ("%s: TCP options too long", __func__));
1737 	return (optlen);
1738 }
1739