The Design and Implementation of the FreeBSD Operating System, Second Edition
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sys/netinet/tcp_sack.c

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    1 /*-
    2  * Copyright (c) 1982, 1986, 1988, 1990, 1993, 1994, 1995
    3  *      The Regents of the University of California.
    4  * All rights reserved.
    5  *
    6  * Redistribution and use in source and binary forms, with or without
    7  * modification, are permitted provided that the following conditions
    8  * are met:
    9  * 1. Redistributions of source code must retain the above copyright
   10  *    notice, this list of conditions and the following disclaimer.
   11  * 2. Redistributions in binary form must reproduce the above copyright
   12  *    notice, this list of conditions and the following disclaimer in the
   13  *    documentation and/or other materials provided with the distribution.
   14  * 4. Neither the name of the University nor the names of its contributors
   15  *    may be used to endorse or promote products derived from this software
   16  *    without specific prior written permission.
   17  *
   18  * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
   19  * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
   20  * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
   21  * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
   22  * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
   23  * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
   24  * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
   25  * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
   26  * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
   27  * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
   28  * SUCH DAMAGE.
   29  *
   30  *      @(#)tcp_sack.c  8.12 (Berkeley) 5/24/95
   31  */
   32 
   33 /*-
   34  *      @@(#)COPYRIGHT  1.1 (NRL) 17 January 1995
   35  *
   36  * NRL grants permission for redistribution and use in source and binary
   37  * forms, with or without modification, of the software and documentation
   38  * created at NRL provided that the following conditions are met:
   39  *
   40  * 1. Redistributions of source code must retain the above copyright
   41  *    notice, this list of conditions and the following disclaimer.
   42  * 2. Redistributions in binary form must reproduce the above copyright
   43  *    notice, this list of conditions and the following disclaimer in the
   44  *    documentation and/or other materials provided with the distribution.
   45  * 3. All advertising materials mentioning features or use of this software
   46  *    must display the following acknowledgements:
   47  *      This product includes software developed by the University of
   48  *      California, Berkeley and its contributors.
   49  *      This product includes software developed at the Information
   50  *      Technology Division, US Naval Research Laboratory.
   51  * 4. Neither the name of the NRL nor the names of its contributors
   52  *    may be used to endorse or promote products derived from this software
   53  *    without specific prior written permission.
   54  *
   55  * THE SOFTWARE PROVIDED BY NRL IS PROVIDED BY NRL AND CONTRIBUTORS ``AS
   56  * IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
   57  * TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A
   58  * PARTICULAR PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL NRL OR
   59  * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
   60  * EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
   61  * PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
   62  * PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF
   63  * LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING
   64  * NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
   65  * SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
   66  *
   67  * The views and conclusions contained in the software and documentation
   68  * are those of the authors and should not be interpreted as representing
   69  * official policies, either expressed or implied, of the US Naval
   70  * Research Laboratory (NRL).
   71  */
   72 
   73 #include <sys/cdefs.h>
   74 __FBSDID("$FreeBSD$");
   75 
   76 #include "opt_inet.h"
   77 #include "opt_inet6.h"
   78 #include "opt_tcpdebug.h"
   79 
   80 #include <sys/param.h>
   81 #include <sys/systm.h>
   82 #include <sys/kernel.h>
   83 #include <sys/sysctl.h>
   84 #include <sys/malloc.h>
   85 #include <sys/mbuf.h>
   86 #include <sys/proc.h>           /* for proc0 declaration */
   87 #include <sys/protosw.h>
   88 #include <sys/socket.h>
   89 #include <sys/socketvar.h>
   90 #include <sys/syslog.h>
   91 #include <sys/systm.h>
   92 
   93 #include <machine/cpu.h>        /* before tcp_seq.h, for tcp_random18() */
   94 
   95 #include <vm/uma.h>
   96 
   97 #include <net/if.h>
   98 #include <net/route.h>
   99 
  100 #include <netinet/in.h>
  101 #include <netinet/in_systm.h>
  102 #include <netinet/ip.h>
  103 #include <netinet/in_var.h>
  104 #include <netinet/in_pcb.h>
  105 #include <netinet/ip_var.h>
  106 #include <netinet/ip6.h>
  107 #include <netinet/icmp6.h>
  108 #include <netinet6/nd6.h>
  109 #include <netinet6/ip6_var.h>
  110 #include <netinet6/in6_pcb.h>
  111 #include <netinet/tcp.h>
  112 #include <netinet/tcp_fsm.h>
  113 #include <netinet/tcp_seq.h>
  114 #include <netinet/tcp_timer.h>
  115 #include <netinet/tcp_var.h>
  116 #include <netinet6/tcp6_var.h>
  117 #include <netinet/tcpip.h>
  118 #ifdef TCPDEBUG
  119 #include <netinet/tcp_debug.h>
  120 #endif /* TCPDEBUG */
  121 
  122 #include <machine/in_cksum.h>
  123 
  124 extern struct uma_zone *sack_hole_zone;
  125 
  126 SYSCTL_NODE(_net_inet_tcp, OID_AUTO, sack, CTLFLAG_RW, 0, "TCP SACK");
  127 int tcp_do_sack = 1;
  128 SYSCTL_INT(_net_inet_tcp_sack, OID_AUTO, enable, CTLFLAG_RW,
  129     &tcp_do_sack, 0, "Enable/Disable TCP SACK support");
  130 TUNABLE_INT("net.inet.tcp.sack.enable", &tcp_do_sack);
  131 
  132 static int tcp_sack_maxholes = 128;
  133 SYSCTL_INT(_net_inet_tcp_sack, OID_AUTO, maxholes, CTLFLAG_RW,
  134     &tcp_sack_maxholes, 0, 
  135     "Maximum number of TCP SACK holes allowed per connection");
  136 
  137 static int tcp_sack_globalmaxholes = 65536;
  138 SYSCTL_INT(_net_inet_tcp_sack, OID_AUTO, globalmaxholes, CTLFLAG_RW,
  139     &tcp_sack_globalmaxholes, 0, 
  140     "Global maximum number of TCP SACK holes");
  141 
  142 static int tcp_sack_globalholes = 0;
  143 SYSCTL_INT(_net_inet_tcp_sack, OID_AUTO, globalholes, CTLFLAG_RD,
  144     &tcp_sack_globalholes, 0,
  145     "Global number of TCP SACK holes currently allocated");
  146 
  147 /*
  148  * This function is called upon receipt of new valid data (while not in
  149  * header prediction mode), and it updates the ordered list of sacks.
  150  */
  151 void
  152 tcp_update_sack_list(struct tcpcb *tp, tcp_seq rcv_start, tcp_seq rcv_end)
  153 {
  154         /*
  155          * First reported block MUST be the most recent one.  Subsequent
  156          * blocks SHOULD be in the order in which they arrived at the
  157          * receiver.  These two conditions make the implementation fully
  158          * compliant with RFC 2018.
  159          */
  160         struct sackblk head_blk, saved_blks[MAX_SACK_BLKS];
  161         int num_head, num_saved, i;
  162 
  163         INP_WLOCK_ASSERT(tp->t_inpcb);
  164 
  165         /* Check arguments. */
  166         KASSERT(SEQ_LT(rcv_start, rcv_end), ("rcv_start < rcv_end"));
  167 
  168         /* SACK block for the received segment. */
  169         head_blk.start = rcv_start;
  170         head_blk.end = rcv_end;
  171 
  172         /*
  173          * Merge updated SACK blocks into head_blk, and save unchanged SACK
  174          * blocks into saved_blks[].  num_saved will have the number of the
  175          * saved SACK blocks.
  176          */
  177         num_saved = 0;
  178         for (i = 0; i < tp->rcv_numsacks; i++) {
  179                 tcp_seq start = tp->sackblks[i].start;
  180                 tcp_seq end = tp->sackblks[i].end;
  181                 if (SEQ_GEQ(start, end) || SEQ_LEQ(start, tp->rcv_nxt)) {
  182                         /*
  183                          * Discard this SACK block.
  184                          */
  185                 } else if (SEQ_LEQ(head_blk.start, end) &&
  186                            SEQ_GEQ(head_blk.end, start)) {
  187                         /*
  188                          * Merge this SACK block into head_blk.  This SACK
  189                          * block itself will be discarded.
  190                          */
  191                         if (SEQ_GT(head_blk.start, start))
  192                                 head_blk.start = start;
  193                         if (SEQ_LT(head_blk.end, end))
  194                                 head_blk.end = end;
  195                 } else {
  196                         /*
  197                          * Save this SACK block.
  198                          */
  199                         saved_blks[num_saved].start = start;
  200                         saved_blks[num_saved].end = end;
  201                         num_saved++;
  202                 }
  203         }
  204 
  205         /*
  206          * Update SACK list in tp->sackblks[].
  207          */
  208         num_head = 0;
  209         if (SEQ_GT(head_blk.start, tp->rcv_nxt)) {
  210                 /*
  211                  * The received data segment is an out-of-order segment.  Put
  212                  * head_blk at the top of SACK list.
  213                  */
  214                 tp->sackblks[0] = head_blk;
  215                 num_head = 1;
  216                 /*
  217                  * If the number of saved SACK blocks exceeds its limit,
  218                  * discard the last SACK block.
  219                  */
  220                 if (num_saved >= MAX_SACK_BLKS)
  221                         num_saved--;
  222         }
  223         if (num_saved > 0) {
  224                 /*
  225                  * Copy the saved SACK blocks back.
  226                  */
  227                 bcopy(saved_blks, &tp->sackblks[num_head],
  228                       sizeof(struct sackblk) * num_saved);
  229         }
  230 
  231         /* Save the number of SACK blocks. */
  232         tp->rcv_numsacks = num_head + num_saved;
  233 }
  234 
  235 /*
  236  * Delete all receiver-side SACK information.
  237  */
  238 void
  239 tcp_clean_sackreport(struct tcpcb *tp)
  240 {
  241         int i;
  242 
  243         INP_WLOCK_ASSERT(tp->t_inpcb);
  244         tp->rcv_numsacks = 0;
  245         for (i = 0; i < MAX_SACK_BLKS; i++)
  246                 tp->sackblks[i].start = tp->sackblks[i].end=0;
  247 }
  248 
  249 /*
  250  * Allocate struct sackhole.
  251  */
  252 static struct sackhole *
  253 tcp_sackhole_alloc(struct tcpcb *tp, tcp_seq start, tcp_seq end)
  254 {
  255         struct sackhole *hole;
  256 
  257         if (tp->snd_numholes >= tcp_sack_maxholes ||
  258             tcp_sack_globalholes >= tcp_sack_globalmaxholes) {
  259                 tcpstat.tcps_sack_sboverflow++;
  260                 return NULL;
  261         }
  262 
  263         hole = (struct sackhole *)uma_zalloc(sack_hole_zone, M_NOWAIT);
  264         if (hole == NULL)
  265                 return NULL;
  266 
  267         hole->start = start;
  268         hole->end = end;
  269         hole->rxmit = start;
  270 
  271         tp->snd_numholes++;
  272         tcp_sack_globalholes++;
  273 
  274         return hole;
  275 }
  276 
  277 /*
  278  * Free struct sackhole.
  279  */
  280 static void
  281 tcp_sackhole_free(struct tcpcb *tp, struct sackhole *hole)
  282 {
  283 
  284         uma_zfree(sack_hole_zone, hole);
  285 
  286         tp->snd_numholes--;
  287         tcp_sack_globalholes--;
  288 
  289         KASSERT(tp->snd_numholes >= 0, ("tp->snd_numholes >= 0"));
  290         KASSERT(tcp_sack_globalholes >= 0, ("tcp_sack_globalholes >= 0"));
  291 }
  292 
  293 /*
  294  * Insert new SACK hole into scoreboard.
  295  */
  296 static struct sackhole *
  297 tcp_sackhole_insert(struct tcpcb *tp, tcp_seq start, tcp_seq end,
  298     struct sackhole *after)
  299 {
  300         struct sackhole *hole;
  301 
  302         /* Allocate a new SACK hole. */
  303         hole = tcp_sackhole_alloc(tp, start, end);
  304         if (hole == NULL)
  305                 return NULL;
  306 
  307         /* Insert the new SACK hole into scoreboard. */
  308         if (after != NULL)
  309                 TAILQ_INSERT_AFTER(&tp->snd_holes, after, hole, scblink);
  310         else
  311                 TAILQ_INSERT_TAIL(&tp->snd_holes, hole, scblink);
  312 
  313         /* Update SACK hint. */
  314         if (tp->sackhint.nexthole == NULL)
  315                 tp->sackhint.nexthole = hole;
  316 
  317         return hole;
  318 }
  319 
  320 /*
  321  * Remove SACK hole from scoreboard.
  322  */
  323 static void
  324 tcp_sackhole_remove(struct tcpcb *tp, struct sackhole *hole)
  325 {
  326 
  327         /* Update SACK hint. */
  328         if (tp->sackhint.nexthole == hole)
  329                 tp->sackhint.nexthole = TAILQ_NEXT(hole, scblink);
  330 
  331         /* Remove this SACK hole. */
  332         TAILQ_REMOVE(&tp->snd_holes, hole, scblink);
  333 
  334         /* Free this SACK hole. */
  335         tcp_sackhole_free(tp, hole);
  336 }
  337 
  338 /*
  339  * Process cumulative ACK and the TCP SACK option to update the scoreboard.
  340  * tp->snd_holes is an ordered list of holes (oldest to newest, in terms of
  341  * the sequence space).
  342  */
  343 void
  344 tcp_sack_doack(struct tcpcb *tp, struct tcpopt *to, tcp_seq th_ack)
  345 {
  346         struct sackhole *cur, *temp;
  347         struct sackblk sack, sack_blocks[TCP_MAX_SACK + 1], *sblkp;
  348         int i, j, num_sack_blks;
  349 
  350         INP_WLOCK_ASSERT(tp->t_inpcb);
  351 
  352         num_sack_blks = 0;
  353         /*
  354          * If SND.UNA will be advanced by SEG.ACK, and if SACK holes exist,
  355          * treat [SND.UNA, SEG.ACK) as if it is a SACK block.
  356          */
  357         if (SEQ_LT(tp->snd_una, th_ack) && !TAILQ_EMPTY(&tp->snd_holes)) {
  358                 sack_blocks[num_sack_blks].start = tp->snd_una;
  359                 sack_blocks[num_sack_blks++].end = th_ack;
  360         }
  361         /*
  362          * Append received valid SACK blocks to sack_blocks[], but only if we
  363          * received new blocks from the other side.
  364          */
  365         if (to->to_flags & TOF_SACK) {
  366                 for (i = 0; i < to->to_nsacks; i++) {
  367                         bcopy((to->to_sacks + i * TCPOLEN_SACK),
  368                             &sack, sizeof(sack));
  369                         sack.start = ntohl(sack.start);
  370                         sack.end = ntohl(sack.end);
  371                         if (SEQ_GT(sack.end, sack.start) &&
  372                             SEQ_GT(sack.start, tp->snd_una) &&
  373                             SEQ_GT(sack.start, th_ack) &&
  374                             SEQ_LT(sack.start, tp->snd_max) &&
  375                             SEQ_GT(sack.end, tp->snd_una) &&
  376                             SEQ_LEQ(sack.end, tp->snd_max))
  377                                 sack_blocks[num_sack_blks++] = sack;
  378                 }
  379         }
  380         /*
  381          * Return if SND.UNA is not advanced and no valid SACK block is
  382          * received.
  383          */
  384         if (num_sack_blks == 0)
  385                 return;
  386 
  387         /*
  388          * Sort the SACK blocks so we can update the scoreboard with just one
  389          * pass. The overhead of sorting upto 4+1 elements is less than
  390          * making upto 4+1 passes over the scoreboard.
  391          */
  392         for (i = 0; i < num_sack_blks; i++) {
  393                 for (j = i + 1; j < num_sack_blks; j++) {
  394                         if (SEQ_GT(sack_blocks[i].end, sack_blocks[j].end)) {
  395                                 sack = sack_blocks[i];
  396                                 sack_blocks[i] = sack_blocks[j];
  397                                 sack_blocks[j] = sack;
  398                         }
  399                 }
  400         }
  401         if (TAILQ_EMPTY(&tp->snd_holes))
  402                 /*
  403                  * Empty scoreboard. Need to initialize snd_fack (it may be
  404                  * uninitialized or have a bogus value). Scoreboard holes
  405                  * (from the sack blocks received) are created later below
  406                  * (in the logic that adds holes to the tail of the
  407                  * scoreboard).
  408                  */
  409                 tp->snd_fack = SEQ_MAX(tp->snd_una, th_ack);
  410         /*
  411          * In the while-loop below, incoming SACK blocks (sack_blocks[]) and
  412          * SACK holes (snd_holes) are traversed from their tails with just
  413          * one pass in order to reduce the number of compares especially when
  414          * the bandwidth-delay product is large.
  415          *
  416          * Note: Typically, in the first RTT of SACK recovery, the highest
  417          * three or four SACK blocks with the same ack number are received.
  418          * In the second RTT, if retransmitted data segments are not lost,
  419          * the highest three or four SACK blocks with ack number advancing
  420          * are received.
  421          */
  422         sblkp = &sack_blocks[num_sack_blks - 1];        /* Last SACK block */
  423         if (SEQ_LT(tp->snd_fack, sblkp->start)) {
  424                 /*
  425                  * The highest SACK block is beyond fack.  Append new SACK
  426                  * hole at the tail.  If the second or later highest SACK
  427                  * blocks are also beyond the current fack, they will be
  428                  * inserted by way of hole splitting in the while-loop below.
  429                  */
  430                 temp = tcp_sackhole_insert(tp, tp->snd_fack,sblkp->start,NULL);
  431                 if (temp != NULL) {
  432                         tp->snd_fack = sblkp->end;
  433                         /* Go to the previous sack block. */
  434                         sblkp--;
  435                 } else {
  436                         /* 
  437                          * We failed to add a new hole based on the current 
  438                          * sack block.  Skip over all the sack blocks that 
  439                          * fall completely to the right of snd_fack and
  440                          * proceed to trim the scoreboard based on the
  441                          * remaining sack blocks.  This also trims the
  442                          * scoreboard for th_ack (which is sack_blocks[0]).
  443                          */
  444                         while (sblkp >= sack_blocks && 
  445                                SEQ_LT(tp->snd_fack, sblkp->start))
  446                                 sblkp--;
  447                         if (sblkp >= sack_blocks && 
  448                             SEQ_LT(tp->snd_fack, sblkp->end))
  449                                 tp->snd_fack = sblkp->end;
  450                 }
  451         } else if (SEQ_LT(tp->snd_fack, sblkp->end))
  452                 /* fack is advanced. */
  453                 tp->snd_fack = sblkp->end;
  454         /* We must have at least one SACK hole in scoreboard. */
  455         KASSERT(!TAILQ_EMPTY(&tp->snd_holes),
  456             ("SACK scoreboard must not be empty"));
  457         cur = TAILQ_LAST(&tp->snd_holes, sackhole_head); /* Last SACK hole. */
  458         /*
  459          * Since the incoming sack blocks are sorted, we can process them
  460          * making one sweep of the scoreboard.
  461          */
  462         while (sblkp >= sack_blocks  && cur != NULL) {
  463                 if (SEQ_GEQ(sblkp->start, cur->end)) {
  464                         /*
  465                          * SACKs data beyond the current hole.  Go to the
  466                          * previous sack block.
  467                          */
  468                         sblkp--;
  469                         continue;
  470                 }
  471                 if (SEQ_LEQ(sblkp->end, cur->start)) {
  472                         /*
  473                          * SACKs data before the current hole.  Go to the
  474                          * previous hole.
  475                          */
  476                         cur = TAILQ_PREV(cur, sackhole_head, scblink);
  477                         continue;
  478                 }
  479                 tp->sackhint.sack_bytes_rexmit -= (cur->rxmit - cur->start);
  480                 KASSERT(tp->sackhint.sack_bytes_rexmit >= 0,
  481                     ("sackhint bytes rtx >= 0"));
  482                 if (SEQ_LEQ(sblkp->start, cur->start)) {
  483                         /* Data acks at least the beginning of hole. */
  484                         if (SEQ_GEQ(sblkp->end, cur->end)) {
  485                                 /* Acks entire hole, so delete hole. */
  486                                 temp = cur;
  487                                 cur = TAILQ_PREV(cur, sackhole_head, scblink);
  488                                 tcp_sackhole_remove(tp, temp);
  489                                 /*
  490                                  * The sack block may ack all or part of the
  491                                  * next hole too, so continue onto the next
  492                                  * hole.
  493                                  */
  494                                 continue;
  495                         } else {
  496                                 /* Move start of hole forward. */
  497                                 cur->start = sblkp->end;
  498                                 cur->rxmit = SEQ_MAX(cur->rxmit, cur->start);
  499                         }
  500                 } else {
  501                         /* Data acks at least the end of hole. */
  502                         if (SEQ_GEQ(sblkp->end, cur->end)) {
  503                                 /* Move end of hole backward. */
  504                                 cur->end = sblkp->start;
  505                                 cur->rxmit = SEQ_MIN(cur->rxmit, cur->end);
  506                         } else {
  507                                 /*
  508                                  * ACKs some data in middle of a hole; need
  509                                  * to split current hole
  510                                  */
  511                                 temp = tcp_sackhole_insert(tp, sblkp->end,
  512                                     cur->end, cur);
  513                                 if (temp != NULL) {
  514                                         if (SEQ_GT(cur->rxmit, temp->rxmit)) {
  515                                                 temp->rxmit = cur->rxmit;
  516                                                 tp->sackhint.sack_bytes_rexmit
  517                                                     += (temp->rxmit
  518                                                     - temp->start);
  519                                         }
  520                                         cur->end = sblkp->start;
  521                                         cur->rxmit = SEQ_MIN(cur->rxmit,
  522                                             cur->end);
  523                                 }
  524                         }
  525                 }
  526                 tp->sackhint.sack_bytes_rexmit += (cur->rxmit - cur->start);
  527                 /*
  528                  * Testing sblkp->start against cur->start tells us whether
  529                  * we're done with the sack block or the sack hole.
  530                  * Accordingly, we advance one or the other.
  531                  */
  532                 if (SEQ_LEQ(sblkp->start, cur->start))
  533                         cur = TAILQ_PREV(cur, sackhole_head, scblink);
  534                 else
  535                         sblkp--;
  536         }
  537 }
  538 
  539 /*
  540  * Free all SACK holes to clear the scoreboard.
  541  */
  542 void
  543 tcp_free_sackholes(struct tcpcb *tp)
  544 {
  545         struct sackhole *q;
  546 
  547         INP_WLOCK_ASSERT(tp->t_inpcb);
  548         while ((q = TAILQ_FIRST(&tp->snd_holes)) != NULL)
  549                 tcp_sackhole_remove(tp, q);
  550         tp->sackhint.sack_bytes_rexmit = 0;
  551 
  552         KASSERT(tp->snd_numholes == 0, ("tp->snd_numholes == 0"));
  553         KASSERT(tp->sackhint.nexthole == NULL,
  554                 ("tp->sackhint.nexthole == NULL"));
  555 }
  556 
  557 /*
  558  * Partial ack handling within a sack recovery episode.  Keeping this very
  559  * simple for now.  When a partial ack is received, force snd_cwnd to a value
  560  * that will allow the sender to transmit no more than 2 segments.  If
  561  * necessary, a better scheme can be adopted at a later point, but for now,
  562  * the goal is to prevent the sender from bursting a large amount of data in
  563  * the midst of sack recovery.
  564  */
  565 void
  566 tcp_sack_partialack(struct tcpcb *tp, struct tcphdr *th)
  567 {
  568         int num_segs = 1;
  569 
  570         INP_WLOCK_ASSERT(tp->t_inpcb);
  571         tcp_timer_activate(tp, TT_REXMT, 0);
  572         tp->t_rtttime = 0;
  573         /* Send one or 2 segments based on how much new data was acked. */
  574         if (((th->th_ack - tp->snd_una) / tp->t_maxseg) > 2)
  575                 num_segs = 2;
  576         tp->snd_cwnd = (tp->sackhint.sack_bytes_rexmit +
  577             (tp->snd_nxt - tp->sack_newdata) + num_segs * tp->t_maxseg);
  578         if (tp->snd_cwnd > tp->snd_ssthresh)
  579                 tp->snd_cwnd = tp->snd_ssthresh;
  580         tp->t_flags |= TF_ACKNOW;
  581         (void) tcp_output(tp);
  582 }
  583 
  584 #if 0
  585 /*
  586  * Debug version of tcp_sack_output() that walks the scoreboard.  Used for
  587  * now to sanity check the hint.
  588  */
  589 static struct sackhole *
  590 tcp_sack_output_debug(struct tcpcb *tp, int *sack_bytes_rexmt)
  591 {
  592         struct sackhole *p;
  593 
  594         INP_WLOCK_ASSERT(tp->t_inpcb);
  595         *sack_bytes_rexmt = 0;
  596         TAILQ_FOREACH(p, &tp->snd_holes, scblink) {
  597                 if (SEQ_LT(p->rxmit, p->end)) {
  598                         if (SEQ_LT(p->rxmit, tp->snd_una)) {/* old SACK hole */
  599                                 continue;
  600                         }
  601                         *sack_bytes_rexmt += (p->rxmit - p->start);
  602                         break;
  603                 }
  604                 *sack_bytes_rexmt += (p->rxmit - p->start);
  605         }
  606         return (p);
  607 }
  608 #endif
  609 
  610 /*
  611  * Returns the next hole to retransmit and the number of retransmitted bytes
  612  * from the scoreboard.  We store both the next hole and the number of
  613  * retransmitted bytes as hints (and recompute these on the fly upon SACK/ACK
  614  * reception).  This avoids scoreboard traversals completely.
  615  *
  616  * The loop here will traverse *at most* one link.  Here's the argument.  For
  617  * the loop to traverse more than 1 link before finding the next hole to
  618  * retransmit, we would need to have at least 1 node following the current
  619  * hint with (rxmit == end).  But, for all holes following the current hint,
  620  * (start == rxmit), since we have not yet retransmitted from them.
  621  * Therefore, in order to traverse more 1 link in the loop below, we need to
  622  * have at least one node following the current hint with (start == rxmit ==
  623  * end).  But that can't happen, (start == end) means that all the data in
  624  * that hole has been sacked, in which case, the hole would have been removed
  625  * from the scoreboard.
  626  */
  627 struct sackhole *
  628 tcp_sack_output(struct tcpcb *tp, int *sack_bytes_rexmt)
  629 {
  630         struct sackhole *hole = NULL;
  631 
  632         INP_WLOCK_ASSERT(tp->t_inpcb);
  633         *sack_bytes_rexmt = tp->sackhint.sack_bytes_rexmit;
  634         hole = tp->sackhint.nexthole;
  635         if (hole == NULL || SEQ_LT(hole->rxmit, hole->end))
  636                 goto out;
  637         while ((hole = TAILQ_NEXT(hole, scblink)) != NULL) {
  638                 if (SEQ_LT(hole->rxmit, hole->end)) {
  639                         tp->sackhint.nexthole = hole;
  640                         break;
  641                 }
  642         }
  643 out:
  644         return (hole);
  645 }
  646 
  647 /*
  648  * After a timeout, the SACK list may be rebuilt.  This SACK information
  649  * should be used to avoid retransmitting SACKed data.  This function
  650  * traverses the SACK list to see if snd_nxt should be moved forward.
  651  */
  652 void
  653 tcp_sack_adjust(struct tcpcb *tp)
  654 {
  655         struct sackhole *p, *cur = TAILQ_FIRST(&tp->snd_holes);
  656 
  657         INP_WLOCK_ASSERT(tp->t_inpcb);
  658         if (cur == NULL)
  659                 return; /* No holes */
  660         if (SEQ_GEQ(tp->snd_nxt, tp->snd_fack))
  661                 return; /* We're already beyond any SACKed blocks */
  662         /*-
  663          * Two cases for which we want to advance snd_nxt:
  664          * i) snd_nxt lies between end of one hole and beginning of another
  665          * ii) snd_nxt lies between end of last hole and snd_fack
  666          */
  667         while ((p = TAILQ_NEXT(cur, scblink)) != NULL) {
  668                 if (SEQ_LT(tp->snd_nxt, cur->end))
  669                         return;
  670                 if (SEQ_GEQ(tp->snd_nxt, p->start))
  671                         cur = p;
  672                 else {
  673                         tp->snd_nxt = p->start;
  674                         return;
  675                 }
  676         }
  677         if (SEQ_LT(tp->snd_nxt, cur->end))
  678                 return;
  679         tp->snd_nxt = tp->snd_fack;
  680 }

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