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32 * @(#)tcp_sack.c 8.12 (Berkeley) 5/24/95
36 * @@(#)COPYRIGHT 1.1 (NRL) 17 January 1995
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64 * PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF
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72 * Research Laboratory (NRL).
75 #include <sys/cdefs.h>
76 __FBSDID("$FreeBSD$");
79 #include "opt_inet6.h"
80 #include "opt_tcpdebug.h"
82 #include <sys/param.h>
83 #include <sys/systm.h>
84 #include <sys/kernel.h>
85 #include <sys/sysctl.h>
86 #include <sys/malloc.h>
88 #include <sys/proc.h> /* for proc0 declaration */
89 #include <sys/protosw.h>
90 #include <sys/socket.h>
91 #include <sys/socketvar.h>
92 #include <sys/syslog.h>
93 #include <sys/systm.h>
95 #include <machine/cpu.h> /* before tcp_seq.h, for tcp_random18() */
100 #include <net/if_var.h>
101 #include <net/route.h>
102 #include <net/vnet.h>
104 #include <netinet/in.h>
105 #include <netinet/in_systm.h>
106 #include <netinet/ip.h>
107 #include <netinet/in_var.h>
108 #include <netinet/in_pcb.h>
109 #include <netinet/ip_var.h>
110 #include <netinet/ip6.h>
111 #include <netinet/icmp6.h>
112 #include <netinet6/nd6.h>
113 #include <netinet6/ip6_var.h>
114 #include <netinet6/in6_pcb.h>
115 #include <netinet/tcp.h>
116 #include <netinet/tcp_fsm.h>
117 #include <netinet/tcp_seq.h>
118 #include <netinet/tcp_timer.h>
119 #include <netinet/tcp_var.h>
120 #include <netinet6/tcp6_var.h>
121 #include <netinet/tcpip.h>
123 #include <netinet/tcp_debug.h>
124 #endif /* TCPDEBUG */
126 #include <machine/in_cksum.h>
128 VNET_DECLARE(struct uma_zone *, sack_hole_zone);
129 #define V_sack_hole_zone VNET(sack_hole_zone)
131 SYSCTL_NODE(_net_inet_tcp, OID_AUTO, sack, CTLFLAG_RW, 0, "TCP SACK");
132 VNET_DEFINE(int, tcp_do_sack) = 1;
133 #define V_tcp_do_sack VNET(tcp_do_sack)
134 SYSCTL_INT(_net_inet_tcp_sack, OID_AUTO, enable, CTLFLAG_VNET | CTLFLAG_RW,
135 &VNET_NAME(tcp_do_sack), 0, "Enable/Disable TCP SACK support");
137 VNET_DEFINE(int, tcp_sack_maxholes) = 128;
138 SYSCTL_INT(_net_inet_tcp_sack, OID_AUTO, maxholes, CTLFLAG_VNET | CTLFLAG_RW,
139 &VNET_NAME(tcp_sack_maxholes), 0,
140 "Maximum number of TCP SACK holes allowed per connection");
142 VNET_DEFINE(int, tcp_sack_globalmaxholes) = 65536;
143 SYSCTL_INT(_net_inet_tcp_sack, OID_AUTO, globalmaxholes, CTLFLAG_VNET | CTLFLAG_RW,
144 &VNET_NAME(tcp_sack_globalmaxholes), 0,
145 "Global maximum number of TCP SACK holes");
147 VNET_DEFINE(int, tcp_sack_globalholes) = 0;
148 SYSCTL_INT(_net_inet_tcp_sack, OID_AUTO, globalholes, CTLFLAG_VNET | CTLFLAG_RD,
149 &VNET_NAME(tcp_sack_globalholes), 0,
150 "Global number of TCP SACK holes currently allocated");
153 * This function is called upon receipt of new valid data (while not in
154 * header prediction mode), and it updates the ordered list of sacks.
157 tcp_update_sack_list(struct tcpcb *tp, tcp_seq rcv_start, tcp_seq rcv_end)
160 * First reported block MUST be the most recent one. Subsequent
161 * blocks SHOULD be in the order in which they arrived at the
162 * receiver. These two conditions make the implementation fully
163 * compliant with RFC 2018.
165 struct sackblk head_blk, saved_blks[MAX_SACK_BLKS];
166 int num_head, num_saved, i;
168 INP_WLOCK_ASSERT(tp->t_inpcb);
170 /* Check arguments. */
171 KASSERT(SEQ_LEQ(rcv_start, rcv_end), ("rcv_start <= rcv_end"));
173 /* SACK block for the received segment. */
174 head_blk.start = rcv_start;
175 head_blk.end = rcv_end;
178 * Merge updated SACK blocks into head_blk, and save unchanged SACK
179 * blocks into saved_blks[]. num_saved will have the number of the
183 for (i = 0; i < tp->rcv_numsacks; i++) {
184 tcp_seq start = tp->sackblks[i].start;
185 tcp_seq end = tp->sackblks[i].end;
186 if (SEQ_GEQ(start, end) || SEQ_LEQ(start, tp->rcv_nxt)) {
188 * Discard this SACK block.
190 } else if (SEQ_LEQ(head_blk.start, end) &&
191 SEQ_GEQ(head_blk.end, start)) {
193 * Merge this SACK block into head_blk. This SACK
194 * block itself will be discarded.
207 * |-----| DSACK smaller
209 if (head_blk.start == end)
210 head_blk.start = start;
211 else if (head_blk.end == start)
214 if (SEQ_LT(head_blk.start, start)) {
215 tcp_seq temp = start;
216 start = head_blk.start;
217 head_blk.start = temp;
219 if (SEQ_GT(head_blk.end, end)) {
224 if ((head_blk.start != start) ||
225 (head_blk.end != end)) {
226 if ((num_saved >= 1) &&
227 SEQ_GEQ(saved_blks[num_saved-1].start, start) &&
228 SEQ_LEQ(saved_blks[num_saved-1].end, end))
230 saved_blks[num_saved].start = start;
231 saved_blks[num_saved].end = end;
237 * This block supercedes the prior block
239 if ((num_saved >= 1) &&
240 SEQ_GEQ(saved_blks[num_saved-1].start, start) &&
241 SEQ_LEQ(saved_blks[num_saved-1].end, end))
244 * Save this SACK block.
246 saved_blks[num_saved].start = start;
247 saved_blks[num_saved].end = end;
253 * Update SACK list in tp->sackblks[].
256 if (SEQ_LT(rcv_start, rcv_end)) {
258 * The received data segment is an out-of-order segment. Put
259 * head_blk at the top of SACK list.
261 tp->sackblks[0] = head_blk;
264 * If the number of saved SACK blocks exceeds its limit,
265 * discard the last SACK block.
267 if (num_saved >= MAX_SACK_BLKS)
272 * Copy the saved SACK blocks back.
274 bcopy(saved_blks, &tp->sackblks[num_head],
275 sizeof(struct sackblk) * num_saved);
278 /* Save the number of SACK blocks. */
279 tp->rcv_numsacks = num_head + num_saved;
283 * Delete all receiver-side SACK information.
286 tcp_clean_sackreport(struct tcpcb *tp)
290 INP_WLOCK_ASSERT(tp->t_inpcb);
291 tp->rcv_numsacks = 0;
292 for (i = 0; i < MAX_SACK_BLKS; i++)
293 tp->sackblks[i].start = tp->sackblks[i].end=0;
297 * Allocate struct sackhole.
299 static struct sackhole *
300 tcp_sackhole_alloc(struct tcpcb *tp, tcp_seq start, tcp_seq end)
302 struct sackhole *hole;
304 if (tp->snd_numholes >= V_tcp_sack_maxholes ||
305 V_tcp_sack_globalholes >= V_tcp_sack_globalmaxholes) {
306 TCPSTAT_INC(tcps_sack_sboverflow);
310 hole = (struct sackhole *)uma_zalloc(V_sack_hole_zone, M_NOWAIT);
319 atomic_add_int(&V_tcp_sack_globalholes, 1);
325 * Free struct sackhole.
328 tcp_sackhole_free(struct tcpcb *tp, struct sackhole *hole)
331 uma_zfree(V_sack_hole_zone, hole);
334 atomic_subtract_int(&V_tcp_sack_globalholes, 1);
336 KASSERT(tp->snd_numholes >= 0, ("tp->snd_numholes >= 0"));
337 KASSERT(V_tcp_sack_globalholes >= 0, ("tcp_sack_globalholes >= 0"));
341 * Insert new SACK hole into scoreboard.
343 static struct sackhole *
344 tcp_sackhole_insert(struct tcpcb *tp, tcp_seq start, tcp_seq end,
345 struct sackhole *after)
347 struct sackhole *hole;
349 /* Allocate a new SACK hole. */
350 hole = tcp_sackhole_alloc(tp, start, end);
354 /* Insert the new SACK hole into scoreboard. */
356 TAILQ_INSERT_AFTER(&tp->snd_holes, after, hole, scblink);
358 TAILQ_INSERT_TAIL(&tp->snd_holes, hole, scblink);
360 /* Update SACK hint. */
361 if (tp->sackhint.nexthole == NULL)
362 tp->sackhint.nexthole = hole;
368 * Remove SACK hole from scoreboard.
371 tcp_sackhole_remove(struct tcpcb *tp, struct sackhole *hole)
374 /* Update SACK hint. */
375 if (tp->sackhint.nexthole == hole)
376 tp->sackhint.nexthole = TAILQ_NEXT(hole, scblink);
378 /* Remove this SACK hole. */
379 TAILQ_REMOVE(&tp->snd_holes, hole, scblink);
381 /* Free this SACK hole. */
382 tcp_sackhole_free(tp, hole);
386 * Process cumulative ACK and the TCP SACK option to update the scoreboard.
387 * tp->snd_holes is an ordered list of holes (oldest to newest, in terms of
388 * the sequence space).
389 * Returns 1 if incoming ACK has previously unknown SACK information,
390 * 0 otherwise. Note: We treat (snd_una, th_ack) as a sack block so any changes
391 * to that (i.e. left edge moving) would also be considered a change in SACK
392 * information which is slightly different than rfc6675.
395 tcp_sack_doack(struct tcpcb *tp, struct tcpopt *to, tcp_seq th_ack)
397 struct sackhole *cur, *temp;
398 struct sackblk sack, sack_blocks[TCP_MAX_SACK + 1], *sblkp;
399 int i, j, num_sack_blks, sack_changed;
401 INP_WLOCK_ASSERT(tp->t_inpcb);
406 * If SND.UNA will be advanced by SEG.ACK, and if SACK holes exist,
407 * treat [SND.UNA, SEG.ACK) as if it is a SACK block.
409 if (SEQ_LT(tp->snd_una, th_ack) && !TAILQ_EMPTY(&tp->snd_holes)) {
410 sack_blocks[num_sack_blks].start = tp->snd_una;
411 sack_blocks[num_sack_blks++].end = th_ack;
414 * Append received valid SACK blocks to sack_blocks[], but only if we
415 * received new blocks from the other side.
417 if (to->to_flags & TOF_SACK) {
418 tp->sackhint.sacked_bytes = 0; /* reset */
419 for (i = 0; i < to->to_nsacks; i++) {
420 bcopy((to->to_sacks + i * TCPOLEN_SACK),
421 &sack, sizeof(sack));
422 sack.start = ntohl(sack.start);
423 sack.end = ntohl(sack.end);
424 if (SEQ_GT(sack.end, sack.start) &&
425 SEQ_GT(sack.start, tp->snd_una) &&
426 SEQ_GT(sack.start, th_ack) &&
427 SEQ_LT(sack.start, tp->snd_max) &&
428 SEQ_GT(sack.end, tp->snd_una) &&
429 SEQ_LEQ(sack.end, tp->snd_max)) {
430 sack_blocks[num_sack_blks++] = sack;
431 tp->sackhint.sacked_bytes +=
432 (sack.end-sack.start);
437 * Return if SND.UNA is not advanced and no valid SACK block is
440 if (num_sack_blks == 0)
441 return (sack_changed);
444 * Sort the SACK blocks so we can update the scoreboard with just one
445 * pass. The overhead of sorting up to 4+1 elements is less than
446 * making up to 4+1 passes over the scoreboard.
448 for (i = 0; i < num_sack_blks; i++) {
449 for (j = i + 1; j < num_sack_blks; j++) {
450 if (SEQ_GT(sack_blocks[i].end, sack_blocks[j].end)) {
451 sack = sack_blocks[i];
452 sack_blocks[i] = sack_blocks[j];
453 sack_blocks[j] = sack;
457 if (TAILQ_EMPTY(&tp->snd_holes))
459 * Empty scoreboard. Need to initialize snd_fack (it may be
460 * uninitialized or have a bogus value). Scoreboard holes
461 * (from the sack blocks received) are created later below
462 * (in the logic that adds holes to the tail of the
465 tp->snd_fack = SEQ_MAX(tp->snd_una, th_ack);
467 * In the while-loop below, incoming SACK blocks (sack_blocks[]) and
468 * SACK holes (snd_holes) are traversed from their tails with just
469 * one pass in order to reduce the number of compares especially when
470 * the bandwidth-delay product is large.
472 * Note: Typically, in the first RTT of SACK recovery, the highest
473 * three or four SACK blocks with the same ack number are received.
474 * In the second RTT, if retransmitted data segments are not lost,
475 * the highest three or four SACK blocks with ack number advancing
478 sblkp = &sack_blocks[num_sack_blks - 1]; /* Last SACK block */
479 tp->sackhint.last_sack_ack = sblkp->end;
480 if (SEQ_LT(tp->snd_fack, sblkp->start)) {
482 * The highest SACK block is beyond fack. Append new SACK
483 * hole at the tail. If the second or later highest SACK
484 * blocks are also beyond the current fack, they will be
485 * inserted by way of hole splitting in the while-loop below.
487 temp = tcp_sackhole_insert(tp, tp->snd_fack,sblkp->start,NULL);
489 tp->snd_fack = sblkp->end;
490 /* Go to the previous sack block. */
495 * We failed to add a new hole based on the current
496 * sack block. Skip over all the sack blocks that
497 * fall completely to the right of snd_fack and
498 * proceed to trim the scoreboard based on the
499 * remaining sack blocks. This also trims the
500 * scoreboard for th_ack (which is sack_blocks[0]).
502 while (sblkp >= sack_blocks &&
503 SEQ_LT(tp->snd_fack, sblkp->start))
505 if (sblkp >= sack_blocks &&
506 SEQ_LT(tp->snd_fack, sblkp->end))
507 tp->snd_fack = sblkp->end;
509 } else if (SEQ_LT(tp->snd_fack, sblkp->end)) {
510 /* fack is advanced. */
511 tp->snd_fack = sblkp->end;
514 cur = TAILQ_LAST(&tp->snd_holes, sackhole_head); /* Last SACK hole. */
516 * Since the incoming sack blocks are sorted, we can process them
517 * making one sweep of the scoreboard.
519 while (sblkp >= sack_blocks && cur != NULL) {
520 if (SEQ_GEQ(sblkp->start, cur->end)) {
522 * SACKs data beyond the current hole. Go to the
523 * previous sack block.
528 if (SEQ_LEQ(sblkp->end, cur->start)) {
530 * SACKs data before the current hole. Go to the
533 cur = TAILQ_PREV(cur, sackhole_head, scblink);
536 tp->sackhint.sack_bytes_rexmit -= (cur->rxmit - cur->start);
537 KASSERT(tp->sackhint.sack_bytes_rexmit >= 0,
538 ("sackhint bytes rtx >= 0"));
540 if (SEQ_LEQ(sblkp->start, cur->start)) {
541 /* Data acks at least the beginning of hole. */
542 if (SEQ_GEQ(sblkp->end, cur->end)) {
543 /* Acks entire hole, so delete hole. */
545 cur = TAILQ_PREV(cur, sackhole_head, scblink);
546 tcp_sackhole_remove(tp, temp);
548 * The sack block may ack all or part of the
549 * next hole too, so continue onto the next
554 /* Move start of hole forward. */
555 cur->start = sblkp->end;
556 cur->rxmit = SEQ_MAX(cur->rxmit, cur->start);
559 /* Data acks at least the end of hole. */
560 if (SEQ_GEQ(sblkp->end, cur->end)) {
561 /* Move end of hole backward. */
562 cur->end = sblkp->start;
563 cur->rxmit = SEQ_MIN(cur->rxmit, cur->end);
566 * ACKs some data in middle of a hole; need
567 * to split current hole
569 temp = tcp_sackhole_insert(tp, sblkp->end,
572 if (SEQ_GT(cur->rxmit, temp->rxmit)) {
573 temp->rxmit = cur->rxmit;
574 tp->sackhint.sack_bytes_rexmit
578 cur->end = sblkp->start;
579 cur->rxmit = SEQ_MIN(cur->rxmit,
584 tp->sackhint.sack_bytes_rexmit += (cur->rxmit - cur->start);
586 * Testing sblkp->start against cur->start tells us whether
587 * we're done with the sack block or the sack hole.
588 * Accordingly, we advance one or the other.
590 if (SEQ_LEQ(sblkp->start, cur->start))
591 cur = TAILQ_PREV(cur, sackhole_head, scblink);
595 return (sack_changed);
599 * Free all SACK holes to clear the scoreboard.
602 tcp_free_sackholes(struct tcpcb *tp)
606 INP_WLOCK_ASSERT(tp->t_inpcb);
607 while ((q = TAILQ_FIRST(&tp->snd_holes)) != NULL)
608 tcp_sackhole_remove(tp, q);
609 tp->sackhint.sack_bytes_rexmit = 0;
611 KASSERT(tp->snd_numholes == 0, ("tp->snd_numholes == 0"));
612 KASSERT(tp->sackhint.nexthole == NULL,
613 ("tp->sackhint.nexthole == NULL"));
617 * Partial ack handling within a sack recovery episode. Keeping this very
618 * simple for now. When a partial ack is received, force snd_cwnd to a value
619 * that will allow the sender to transmit no more than 2 segments. If
620 * necessary, a better scheme can be adopted at a later point, but for now,
621 * the goal is to prevent the sender from bursting a large amount of data in
622 * the midst of sack recovery.
625 tcp_sack_partialack(struct tcpcb *tp, struct tcphdr *th)
629 INP_WLOCK_ASSERT(tp->t_inpcb);
630 tcp_timer_activate(tp, TT_REXMT, 0);
632 /* Send one or 2 segments based on how much new data was acked. */
633 if ((BYTES_THIS_ACK(tp, th) / tp->t_maxseg) >= 2)
635 tp->snd_cwnd = (tp->sackhint.sack_bytes_rexmit +
636 (tp->snd_nxt - tp->sack_newdata) + num_segs * tp->t_maxseg);
637 if (tp->snd_cwnd > tp->snd_ssthresh)
638 tp->snd_cwnd = tp->snd_ssthresh;
639 tp->t_flags |= TF_ACKNOW;
640 (void) tp->t_fb->tfb_tcp_output(tp);
645 * Debug version of tcp_sack_output() that walks the scoreboard. Used for
646 * now to sanity check the hint.
648 static struct sackhole *
649 tcp_sack_output_debug(struct tcpcb *tp, int *sack_bytes_rexmt)
653 INP_WLOCK_ASSERT(tp->t_inpcb);
654 *sack_bytes_rexmt = 0;
655 TAILQ_FOREACH(p, &tp->snd_holes, scblink) {
656 if (SEQ_LT(p->rxmit, p->end)) {
657 if (SEQ_LT(p->rxmit, tp->snd_una)) {/* old SACK hole */
660 *sack_bytes_rexmt += (p->rxmit - p->start);
663 *sack_bytes_rexmt += (p->rxmit - p->start);
670 * Returns the next hole to retransmit and the number of retransmitted bytes
671 * from the scoreboard. We store both the next hole and the number of
672 * retransmitted bytes as hints (and recompute these on the fly upon SACK/ACK
673 * reception). This avoids scoreboard traversals completely.
675 * The loop here will traverse *at most* one link. Here's the argument. For
676 * the loop to traverse more than 1 link before finding the next hole to
677 * retransmit, we would need to have at least 1 node following the current
678 * hint with (rxmit == end). But, for all holes following the current hint,
679 * (start == rxmit), since we have not yet retransmitted from them.
680 * Therefore, in order to traverse more 1 link in the loop below, we need to
681 * have at least one node following the current hint with (start == rxmit ==
682 * end). But that can't happen, (start == end) means that all the data in
683 * that hole has been sacked, in which case, the hole would have been removed
684 * from the scoreboard.
687 tcp_sack_output(struct tcpcb *tp, int *sack_bytes_rexmt)
689 struct sackhole *hole = NULL;
691 INP_WLOCK_ASSERT(tp->t_inpcb);
692 *sack_bytes_rexmt = tp->sackhint.sack_bytes_rexmit;
693 hole = tp->sackhint.nexthole;
694 if (hole == NULL || SEQ_LT(hole->rxmit, hole->end))
696 while ((hole = TAILQ_NEXT(hole, scblink)) != NULL) {
697 if (SEQ_LT(hole->rxmit, hole->end)) {
698 tp->sackhint.nexthole = hole;
707 * After a timeout, the SACK list may be rebuilt. This SACK information
708 * should be used to avoid retransmitting SACKed data. This function
709 * traverses the SACK list to see if snd_nxt should be moved forward.
712 tcp_sack_adjust(struct tcpcb *tp)
714 struct sackhole *p, *cur = TAILQ_FIRST(&tp->snd_holes);
716 INP_WLOCK_ASSERT(tp->t_inpcb);
718 return; /* No holes */
719 if (SEQ_GEQ(tp->snd_nxt, tp->snd_fack))
720 return; /* We're already beyond any SACKed blocks */
722 * Two cases for which we want to advance snd_nxt:
723 * i) snd_nxt lies between end of one hole and beginning of another
724 * ii) snd_nxt lies between end of last hole and snd_fack
726 while ((p = TAILQ_NEXT(cur, scblink)) != NULL) {
727 if (SEQ_LT(tp->snd_nxt, cur->end))
729 if (SEQ_GEQ(tp->snd_nxt, p->start))
732 tp->snd_nxt = p->start;
736 if (SEQ_LT(tp->snd_nxt, cur->end))
738 tp->snd_nxt = tp->snd_fack;