FreeBSD/Linux Kernel Cross Reference
sys/netinet/tcp_sack.c

     /*-
      * Copyright (c) 1982, 1986, 1988, 1990, 1993, 1994, 1995
      *      The Regents of the University of California.
      * All rights reserved.
      *
      * Redistribution and use in source and binary forms, with or without
      * modification, are permitted provided that the following conditions
      * are met:
      * 1. Redistributions of source code must retain the above copyright
     *    notice, this list of conditions and the following disclaimer.
     * 2. Redistributions in binary form must reproduce the above copyright
     *    notice, this list of conditions and the following disclaimer in the
     *    documentation and/or other materials provided with the distribution.
     * 4. Neither the name of the University nor the names of its contributors
     *    may be used to endorse or promote products derived from this software
     *    without specific prior written permission.
     *
     * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
     * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
     * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
     * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
     * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
     * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
     * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
     * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
     * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
     * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
     * SUCH DAMAGE.
     *
     *      @(#)tcp_sack.c  8.12 (Berkeley) 5/24/95
     */
    
    /*-
     *      @@(#)COPYRIGHT  1.1 (NRL) 17 January 1995
     *
     * NRL grants permission for redistribution and use in source and binary
     * forms, with or without modification, of the software and documentation
     * created at NRL provided that the following conditions are met:
     *
     * 1. Redistributions of source code must retain the above copyright
     *    notice, this list of conditions and the following disclaimer.
     * 2. Redistributions in binary form must reproduce the above copyright
     *    notice, this list of conditions and the following disclaimer in the
     *    documentation and/or other materials provided with the distribution.
     * 3. All advertising materials mentioning features or use of this software
     *    must display the following acknowledgements:
     *      This product includes software developed by the University of
     *      California, Berkeley and its contributors.
     *      This product includes software developed at the Information
     *      Technology Division, US Naval Research Laboratory.
     * 4. Neither the name of the NRL nor the names of its contributors
     *    may be used to endorse or promote products derived from this software
     *    without specific prior written permission.
     *
     * THE SOFTWARE PROVIDED BY NRL IS PROVIDED BY NRL AND CONTRIBUTORS ``AS
     * IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
     * TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A
     * PARTICULAR PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL NRL OR
     * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
     * EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
     * PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
     * PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF
     * LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING
     * NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
     * SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
     *
     * The views and conclusions contained in the software and documentation
     * are those of the authors and should not be interpreted as representing
     * official policies, either expressed or implied, of the US Naval
     * Research Laboratory (NRL).
     */
    
    #include <sys/cdefs.h>
    __FBSDID("$FreeBSD: releng/11.0/sys/netinet/tcp_sack.c 298995 2016-05-03 18:05:43Z pfg $");
    
    #include "opt_inet.h"
    #include "opt_inet6.h"
    #include "opt_tcpdebug.h"
    
    #include <sys/param.h>
    #include <sys/systm.h>
    #include <sys/kernel.h>
    #include <sys/sysctl.h>
    #include <sys/malloc.h>
    #include <sys/mbuf.h>
    #include <sys/proc.h>           /* for proc0 declaration */
    #include <sys/protosw.h>
    #include <sys/socket.h>
    #include <sys/socketvar.h>
    #include <sys/syslog.h>
    #include <sys/systm.h>
    
    #include <machine/cpu.h>        /* before tcp_seq.h, for tcp_random18() */
    
    #include <vm/uma.h>
    
    #include <net/if.h>
    #include <net/if_var.h>
    #include <net/route.h>
   #include <net/vnet.h>
   
   #include <netinet/in.h>
   #include <netinet/in_systm.h>
   #include <netinet/ip.h>
   #include <netinet/in_var.h>
   #include <netinet/in_pcb.h>
   #include <netinet/ip_var.h>
   #include <netinet/ip6.h>
   #include <netinet/icmp6.h>
   #include <netinet6/nd6.h>
   #include <netinet6/ip6_var.h>
   #include <netinet6/in6_pcb.h>
   #include <netinet/tcp.h>
   #include <netinet/tcp_fsm.h>
   #include <netinet/tcp_seq.h>
   #include <netinet/tcp_timer.h>
   #include <netinet/tcp_var.h>
   #include <netinet6/tcp6_var.h>
   #include <netinet/tcpip.h>
   #ifdef TCPDEBUG
   #include <netinet/tcp_debug.h>
   #endif /* TCPDEBUG */
   
   #include <machine/in_cksum.h>
   
   VNET_DECLARE(struct uma_zone *, sack_hole_zone);
   #define V_sack_hole_zone                VNET(sack_hole_zone)
   
   SYSCTL_NODE(_net_inet_tcp, OID_AUTO, sack, CTLFLAG_RW, 0, "TCP SACK");
   VNET_DEFINE(int, tcp_do_sack) = 1;
   #define V_tcp_do_sack                   VNET(tcp_do_sack)
   SYSCTL_INT(_net_inet_tcp_sack, OID_AUTO, enable, CTLFLAG_VNET | CTLFLAG_RW,
       &VNET_NAME(tcp_do_sack), 0, "Enable/Disable TCP SACK support");
   
   VNET_DEFINE(int, tcp_sack_maxholes) = 128;
   #define V_tcp_sack_maxholes             VNET(tcp_sack_maxholes)
   SYSCTL_INT(_net_inet_tcp_sack, OID_AUTO, maxholes, CTLFLAG_VNET | CTLFLAG_RW,
       &VNET_NAME(tcp_sack_maxholes), 0,
       "Maximum number of TCP SACK holes allowed per connection");
   
   VNET_DEFINE(int, tcp_sack_globalmaxholes) = 65536;
   #define V_tcp_sack_globalmaxholes       VNET(tcp_sack_globalmaxholes)
   SYSCTL_INT(_net_inet_tcp_sack, OID_AUTO, globalmaxholes, CTLFLAG_VNET | CTLFLAG_RW,
       &VNET_NAME(tcp_sack_globalmaxholes), 0, 
       "Global maximum number of TCP SACK holes");
   
   VNET_DEFINE(int, tcp_sack_globalholes) = 0;
   #define V_tcp_sack_globalholes          VNET(tcp_sack_globalholes)
   SYSCTL_INT(_net_inet_tcp_sack, OID_AUTO, globalholes, CTLFLAG_VNET | CTLFLAG_RD,
       &VNET_NAME(tcp_sack_globalholes), 0,
       "Global number of TCP SACK holes currently allocated");
   
   /*
    * This function is called upon receipt of new valid data (while not in
    * header prediction mode), and it updates the ordered list of sacks.
    */
   void
   tcp_update_sack_list(struct tcpcb *tp, tcp_seq rcv_start, tcp_seq rcv_end)
   {
           /*
            * First reported block MUST be the most recent one.  Subsequent
            * blocks SHOULD be in the order in which they arrived at the
            * receiver.  These two conditions make the implementation fully
            * compliant with RFC 2018.
            */
           struct sackblk head_blk, saved_blks[MAX_SACK_BLKS];
           int num_head, num_saved, i;
   
           INP_WLOCK_ASSERT(tp->t_inpcb);
   
           /* Check arguments. */
           KASSERT(SEQ_LT(rcv_start, rcv_end), ("rcv_start < rcv_end"));
   
           /* SACK block for the received segment. */
           head_blk.start = rcv_start;
           head_blk.end = rcv_end;
   
           /*
            * Merge updated SACK blocks into head_blk, and save unchanged SACK
            * blocks into saved_blks[].  num_saved will have the number of the
            * saved SACK blocks.
            */
           num_saved = 0;
           for (i = 0; i < tp->rcv_numsacks; i++) {
                   tcp_seq start = tp->sackblks[i].start;
                   tcp_seq end = tp->sackblks[i].end;
                   if (SEQ_GEQ(start, end) || SEQ_LEQ(start, tp->rcv_nxt)) {
                           /*
                            * Discard this SACK block.
                            */
                   } else if (SEQ_LEQ(head_blk.start, end) &&
                              SEQ_GEQ(head_blk.end, start)) {
                           /*
                            * Merge this SACK block into head_blk.  This SACK
                            * block itself will be discarded.
                            */
                           if (SEQ_GT(head_blk.start, start))
                                   head_blk.start = start;
                           if (SEQ_LT(head_blk.end, end))
                                   head_blk.end = end;
                   } else {
                           /*
                            * Save this SACK block.
                            */
                           saved_blks[num_saved].start = start;
                           saved_blks[num_saved].end = end;
                           num_saved++;
                   }
           }
   
           /*
            * Update SACK list in tp->sackblks[].
            */
           num_head = 0;
           if (SEQ_GT(head_blk.start, tp->rcv_nxt)) {
                   /*
                    * The received data segment is an out-of-order segment.  Put
                    * head_blk at the top of SACK list.
                    */
                   tp->sackblks[0] = head_blk;
                   num_head = 1;
                   /*
                    * If the number of saved SACK blocks exceeds its limit,
                    * discard the last SACK block.
                    */
                   if (num_saved >= MAX_SACK_BLKS)
                           num_saved--;
           }
           if (num_saved > 0) {
                   /*
                    * Copy the saved SACK blocks back.
                    */
                   bcopy(saved_blks, &tp->sackblks[num_head],
                         sizeof(struct sackblk) * num_saved);
           }
   
           /* Save the number of SACK blocks. */
           tp->rcv_numsacks = num_head + num_saved;
   }
   
   /*
    * Delete all receiver-side SACK information.
    */
   void
   tcp_clean_sackreport(struct tcpcb *tp)
   {
           int i;
   
           INP_WLOCK_ASSERT(tp->t_inpcb);
           tp->rcv_numsacks = 0;
           for (i = 0; i < MAX_SACK_BLKS; i++)
                   tp->sackblks[i].start = tp->sackblks[i].end=0;
   }
   
   /*
    * Allocate struct sackhole.
    */
   static struct sackhole *
   tcp_sackhole_alloc(struct tcpcb *tp, tcp_seq start, tcp_seq end)
   {
           struct sackhole *hole;
   
           if (tp->snd_numholes >= V_tcp_sack_maxholes ||
               V_tcp_sack_globalholes >= V_tcp_sack_globalmaxholes) {
                   TCPSTAT_INC(tcps_sack_sboverflow);
                   return NULL;
           }
   
           hole = (struct sackhole *)uma_zalloc(V_sack_hole_zone, M_NOWAIT);
           if (hole == NULL)
                   return NULL;
   
           hole->start = start;
           hole->end = end;
           hole->rxmit = start;
   
           tp->snd_numholes++;
           atomic_add_int(&V_tcp_sack_globalholes, 1);
   
           return hole;
   }
   
   /*
    * Free struct sackhole.
    */
   static void
   tcp_sackhole_free(struct tcpcb *tp, struct sackhole *hole)
   {
   
           uma_zfree(V_sack_hole_zone, hole);
   
           tp->snd_numholes--;
           atomic_subtract_int(&V_tcp_sack_globalholes, 1);
   
           KASSERT(tp->snd_numholes >= 0, ("tp->snd_numholes >= 0"));
           KASSERT(V_tcp_sack_globalholes >= 0, ("tcp_sack_globalholes >= 0"));
   }
   
   /*
    * Insert new SACK hole into scoreboard.
    */
   static struct sackhole *
   tcp_sackhole_insert(struct tcpcb *tp, tcp_seq start, tcp_seq end,
       struct sackhole *after)
   {
           struct sackhole *hole;
   
           /* Allocate a new SACK hole. */
           hole = tcp_sackhole_alloc(tp, start, end);
           if (hole == NULL)
                   return NULL;
   
           /* Insert the new SACK hole into scoreboard. */
           if (after != NULL)
                   TAILQ_INSERT_AFTER(&tp->snd_holes, after, hole, scblink);
           else
                   TAILQ_INSERT_TAIL(&tp->snd_holes, hole, scblink);
   
           /* Update SACK hint. */
           if (tp->sackhint.nexthole == NULL)
                   tp->sackhint.nexthole = hole;
   
           return hole;
   }
   
   /*
    * Remove SACK hole from scoreboard.
    */
   static void
   tcp_sackhole_remove(struct tcpcb *tp, struct sackhole *hole)
   {
   
           /* Update SACK hint. */
           if (tp->sackhint.nexthole == hole)
                   tp->sackhint.nexthole = TAILQ_NEXT(hole, scblink);
   
           /* Remove this SACK hole. */
           TAILQ_REMOVE(&tp->snd_holes, hole, scblink);
   
           /* Free this SACK hole. */
           tcp_sackhole_free(tp, hole);
   }
   
   /*
    * Process cumulative ACK and the TCP SACK option to update the scoreboard.
    * tp->snd_holes is an ordered list of holes (oldest to newest, in terms of
    * the sequence space).
    * Returns 1 if incoming ACK has previously unknown SACK information,
    * 0 otherwise. Note: We treat (snd_una, th_ack) as a sack block so any changes
    * to that (i.e. left edge moving) would also be considered a change in SACK
    * information which is slightly different than rfc6675.
    */
   int
   tcp_sack_doack(struct tcpcb *tp, struct tcpopt *to, tcp_seq th_ack)
   {
           struct sackhole *cur, *temp;
           struct sackblk sack, sack_blocks[TCP_MAX_SACK + 1], *sblkp;
           int i, j, num_sack_blks, sack_changed;
   
           INP_WLOCK_ASSERT(tp->t_inpcb);
   
           num_sack_blks = 0;
           sack_changed = 0;
           /*
            * If SND.UNA will be advanced by SEG.ACK, and if SACK holes exist,
            * treat [SND.UNA, SEG.ACK) as if it is a SACK block.
            */
           if (SEQ_LT(tp->snd_una, th_ack) && !TAILQ_EMPTY(&tp->snd_holes)) {
                   sack_blocks[num_sack_blks].start = tp->snd_una;
                   sack_blocks[num_sack_blks++].end = th_ack;
           }
           /*
            * Append received valid SACK blocks to sack_blocks[], but only if we
            * received new blocks from the other side.
            */
           if (to->to_flags & TOF_SACK) {
                   tp->sackhint.sacked_bytes = 0;  /* reset */
                   for (i = 0; i < to->to_nsacks; i++) {
                           bcopy((to->to_sacks + i * TCPOLEN_SACK),
                               &sack, sizeof(sack));
                           sack.start = ntohl(sack.start);
                           sack.end = ntohl(sack.end);
                           if (SEQ_GT(sack.end, sack.start) &&
                               SEQ_GT(sack.start, tp->snd_una) &&
                               SEQ_GT(sack.start, th_ack) &&
                               SEQ_LT(sack.start, tp->snd_max) &&
                               SEQ_GT(sack.end, tp->snd_una) &&
                               SEQ_LEQ(sack.end, tp->snd_max)) {
                                   sack_blocks[num_sack_blks++] = sack;
                                   tp->sackhint.sacked_bytes +=
                                       (sack.end-sack.start);
                           }
                   }
           }
           /*
            * Return if SND.UNA is not advanced and no valid SACK block is
            * received.
            */
           if (num_sack_blks == 0)
                   return (sack_changed);
   
           /*
            * Sort the SACK blocks so we can update the scoreboard with just one
            * pass. The overhead of sorting up to 4+1 elements is less than
            * making up to 4+1 passes over the scoreboard.
            */
           for (i = 0; i < num_sack_blks; i++) {
                   for (j = i + 1; j < num_sack_blks; j++) {
                           if (SEQ_GT(sack_blocks[i].end, sack_blocks[j].end)) {
                                   sack = sack_blocks[i];
                                   sack_blocks[i] = sack_blocks[j];
                                   sack_blocks[j] = sack;
                           }
                   }
           }
           if (TAILQ_EMPTY(&tp->snd_holes))
                   /*
                    * Empty scoreboard. Need to initialize snd_fack (it may be
                    * uninitialized or have a bogus value). Scoreboard holes
                    * (from the sack blocks received) are created later below
                    * (in the logic that adds holes to the tail of the
                    * scoreboard).
                    */
                   tp->snd_fack = SEQ_MAX(tp->snd_una, th_ack);
           /*
            * In the while-loop below, incoming SACK blocks (sack_blocks[]) and
            * SACK holes (snd_holes) are traversed from their tails with just
            * one pass in order to reduce the number of compares especially when
            * the bandwidth-delay product is large.
            *
            * Note: Typically, in the first RTT of SACK recovery, the highest
            * three or four SACK blocks with the same ack number are received.
            * In the second RTT, if retransmitted data segments are not lost,
            * the highest three or four SACK blocks with ack number advancing
            * are received.
            */
           sblkp = &sack_blocks[num_sack_blks - 1];        /* Last SACK block */
           tp->sackhint.last_sack_ack = sblkp->end;
           if (SEQ_LT(tp->snd_fack, sblkp->start)) {
                   /*
                    * The highest SACK block is beyond fack.  Append new SACK
                    * hole at the tail.  If the second or later highest SACK
                    * blocks are also beyond the current fack, they will be
                    * inserted by way of hole splitting in the while-loop below.
                    */
                   temp = tcp_sackhole_insert(tp, tp->snd_fack,sblkp->start,NULL);
                   if (temp != NULL) {
                           tp->snd_fack = sblkp->end;
                           /* Go to the previous sack block. */
                           sblkp--;
                           sack_changed = 1;
                   } else {
                           /* 
                            * We failed to add a new hole based on the current 
                            * sack block.  Skip over all the sack blocks that 
                            * fall completely to the right of snd_fack and
                            * proceed to trim the scoreboard based on the
                            * remaining sack blocks.  This also trims the
                            * scoreboard for th_ack (which is sack_blocks[0]).
                            */
                           while (sblkp >= sack_blocks && 
                                  SEQ_LT(tp->snd_fack, sblkp->start))
                                   sblkp--;
                           if (sblkp >= sack_blocks && 
                               SEQ_LT(tp->snd_fack, sblkp->end))
                                   tp->snd_fack = sblkp->end;
                   }
           } else if (SEQ_LT(tp->snd_fack, sblkp->end)) {
                   /* fack is advanced. */
                   tp->snd_fack = sblkp->end;
                   sack_changed = 1;
           }
           /* We must have at least one SACK hole in scoreboard. */
           KASSERT(!TAILQ_EMPTY(&tp->snd_holes),
               ("SACK scoreboard must not be empty"));
           cur = TAILQ_LAST(&tp->snd_holes, sackhole_head); /* Last SACK hole. */
           /*
            * Since the incoming sack blocks are sorted, we can process them
            * making one sweep of the scoreboard.
            */
           while (sblkp >= sack_blocks  && cur != NULL) {
                   if (SEQ_GEQ(sblkp->start, cur->end)) {
                           /*
                            * SACKs data beyond the current hole.  Go to the
                            * previous sack block.
                            */
                           sblkp--;
                           continue;
                   }
                   if (SEQ_LEQ(sblkp->end, cur->start)) {
                           /*
                            * SACKs data before the current hole.  Go to the
                            * previous hole.
                            */
                           cur = TAILQ_PREV(cur, sackhole_head, scblink);
                           continue;
                   }
                   tp->sackhint.sack_bytes_rexmit -= (cur->rxmit - cur->start);
                   KASSERT(tp->sackhint.sack_bytes_rexmit >= 0,
                       ("sackhint bytes rtx >= 0"));
                   sack_changed = 1;
                   if (SEQ_LEQ(sblkp->start, cur->start)) {
                           /* Data acks at least the beginning of hole. */
                           if (SEQ_GEQ(sblkp->end, cur->end)) {
                                   /* Acks entire hole, so delete hole. */
                                   temp = cur;
                                   cur = TAILQ_PREV(cur, sackhole_head, scblink);
                                   tcp_sackhole_remove(tp, temp);
                                   /*
                                    * The sack block may ack all or part of the
                                    * next hole too, so continue onto the next
                                    * hole.
                                    */
                                   continue;
                           } else {
                                   /* Move start of hole forward. */
                                   cur->start = sblkp->end;
                                   cur->rxmit = SEQ_MAX(cur->rxmit, cur->start);
                           }
                   } else {
                           /* Data acks at least the end of hole. */
                           if (SEQ_GEQ(sblkp->end, cur->end)) {
                                   /* Move end of hole backward. */
                                   cur->end = sblkp->start;
                                   cur->rxmit = SEQ_MIN(cur->rxmit, cur->end);
                           } else {
                                   /*
                                    * ACKs some data in middle of a hole; need
                                    * to split current hole
                                    */
                                   temp = tcp_sackhole_insert(tp, sblkp->end,
                                       cur->end, cur);
                                   if (temp != NULL) {
                                           if (SEQ_GT(cur->rxmit, temp->rxmit)) {
                                                   temp->rxmit = cur->rxmit;
                                                   tp->sackhint.sack_bytes_rexmit
                                                       += (temp->rxmit
                                                       - temp->start);
                                           }
                                           cur->end = sblkp->start;
                                           cur->rxmit = SEQ_MIN(cur->rxmit,
                                               cur->end);
                                   }
                           }
                   }
                   tp->sackhint.sack_bytes_rexmit += (cur->rxmit - cur->start);
                   /*
                    * Testing sblkp->start against cur->start tells us whether
                    * we're done with the sack block or the sack hole.
                    * Accordingly, we advance one or the other.
                    */
                   if (SEQ_LEQ(sblkp->start, cur->start))
                           cur = TAILQ_PREV(cur, sackhole_head, scblink);
                   else
                           sblkp--;
           }
           return (sack_changed);
   }
   
   /*
    * Free all SACK holes to clear the scoreboard.
    */
   void
   tcp_free_sackholes(struct tcpcb *tp)
   {
           struct sackhole *q;
   
           INP_WLOCK_ASSERT(tp->t_inpcb);
           while ((q = TAILQ_FIRST(&tp->snd_holes)) != NULL)
                   tcp_sackhole_remove(tp, q);
           tp->sackhint.sack_bytes_rexmit = 0;
   
           KASSERT(tp->snd_numholes == 0, ("tp->snd_numholes == 0"));
           KASSERT(tp->sackhint.nexthole == NULL,
                   ("tp->sackhint.nexthole == NULL"));
   }
   
   /*
    * Partial ack handling within a sack recovery episode.  Keeping this very
    * simple for now.  When a partial ack is received, force snd_cwnd to a value
    * that will allow the sender to transmit no more than 2 segments.  If
    * necessary, a better scheme can be adopted at a later point, but for now,
    * the goal is to prevent the sender from bursting a large amount of data in
    * the midst of sack recovery.
    */
   void
   tcp_sack_partialack(struct tcpcb *tp, struct tcphdr *th)
   {
           int num_segs = 1;
   
           INP_WLOCK_ASSERT(tp->t_inpcb);
           tcp_timer_activate(tp, TT_REXMT, 0);
           tp->t_rtttime = 0;
           /* Send one or 2 segments based on how much new data was acked. */
           if ((BYTES_THIS_ACK(tp, th) / tp->t_maxseg) >= 2)
                   num_segs = 2;
           tp->snd_cwnd = (tp->sackhint.sack_bytes_rexmit +
               (tp->snd_nxt - tp->sack_newdata) + num_segs * tp->t_maxseg);
           if (tp->snd_cwnd > tp->snd_ssthresh)
                   tp->snd_cwnd = tp->snd_ssthresh;
           tp->t_flags |= TF_ACKNOW;
           (void) tp->t_fb->tfb_tcp_output(tp);
   }
   
   #if 0
   /*
    * Debug version of tcp_sack_output() that walks the scoreboard.  Used for
    * now to sanity check the hint.
    */
   static struct sackhole *
   tcp_sack_output_debug(struct tcpcb *tp, int *sack_bytes_rexmt)
   {
           struct sackhole *p;
   
           INP_WLOCK_ASSERT(tp->t_inpcb);
           *sack_bytes_rexmt = 0;
           TAILQ_FOREACH(p, &tp->snd_holes, scblink) {
                   if (SEQ_LT(p->rxmit, p->end)) {
                           if (SEQ_LT(p->rxmit, tp->snd_una)) {/* old SACK hole */
                                   continue;
                           }
                           *sack_bytes_rexmt += (p->rxmit - p->start);
                           break;
                   }
                   *sack_bytes_rexmt += (p->rxmit - p->start);
           }
           return (p);
   }
   #endif
   
   /*
    * Returns the next hole to retransmit and the number of retransmitted bytes
    * from the scoreboard.  We store both the next hole and the number of
    * retransmitted bytes as hints (and recompute these on the fly upon SACK/ACK
    * reception).  This avoids scoreboard traversals completely.
    *
    * The loop here will traverse *at most* one link.  Here's the argument.  For
    * the loop to traverse more than 1 link before finding the next hole to
    * retransmit, we would need to have at least 1 node following the current
    * hint with (rxmit == end).  But, for all holes following the current hint,
    * (start == rxmit), since we have not yet retransmitted from them.
    * Therefore, in order to traverse more 1 link in the loop below, we need to
    * have at least one node following the current hint with (start == rxmit ==
    * end).  But that can't happen, (start == end) means that all the data in
    * that hole has been sacked, in which case, the hole would have been removed
    * from the scoreboard.
    */
   struct sackhole *
   tcp_sack_output(struct tcpcb *tp, int *sack_bytes_rexmt)
   {
           struct sackhole *hole = NULL;
   
           INP_WLOCK_ASSERT(tp->t_inpcb);
           *sack_bytes_rexmt = tp->sackhint.sack_bytes_rexmit;
           hole = tp->sackhint.nexthole;
           if (hole == NULL || SEQ_LT(hole->rxmit, hole->end))
                   goto out;
           while ((hole = TAILQ_NEXT(hole, scblink)) != NULL) {
                   if (SEQ_LT(hole->rxmit, hole->end)) {
                           tp->sackhint.nexthole = hole;
                           break;
                   }
           }
   out:
           return (hole);
   }
   
   /*
    * After a timeout, the SACK list may be rebuilt.  This SACK information
    * should be used to avoid retransmitting SACKed data.  This function
    * traverses the SACK list to see if snd_nxt should be moved forward.
    */
   void
   tcp_sack_adjust(struct tcpcb *tp)
   {
           struct sackhole *p, *cur = TAILQ_FIRST(&tp->snd_holes);
   
           INP_WLOCK_ASSERT(tp->t_inpcb);
           if (cur == NULL)
                   return; /* No holes */
           if (SEQ_GEQ(tp->snd_nxt, tp->snd_fack))
                   return; /* We're already beyond any SACKed blocks */
           /*-
            * Two cases for which we want to advance snd_nxt:
            * i) snd_nxt lies between end of one hole and beginning of another
            * ii) snd_nxt lies between end of last hole and snd_fack
            */
           while ((p = TAILQ_NEXT(cur, scblink)) != NULL) {
                   if (SEQ_LT(tp->snd_nxt, cur->end))
                           return;
                   if (SEQ_GEQ(tp->snd_nxt, p->start))
                           cur = p;
                   else {
                           tp->snd_nxt = p->start;
                           return;
                   }
           }
           if (SEQ_LT(tp->snd_nxt, cur->end))
                   return;
           tp->snd_nxt = tp->snd_fack;
   }

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