tcp_sack.c

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/*-
 * SPDX-License-Identifier: BSD-3-Clause
 *
 * 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.
 * 3. 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("$FreeBSD$");
 
#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>
#include <netinet/cc/cc.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 | CTLFLAG_MPSAFE, 0,
    "TCP SACK");
 
VNET_DEFINE(int, tcp_do_sack) = 1;
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_do_newsack) = 1;
SYSCTL_INT(_net_inet_tcp_sack, OID_AUTO, revised, CTLFLAG_VNET | CTLFLAG_RW,
    &VNET_NAME(tcp_do_newsack), 0,
    "Use revised SACK loss recovery per RFC 6675");
 
VNET_DEFINE(int, tcp_sack_maxholes) = 128;
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;
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;
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");
 
int
tcp_dsack_block_exists(struct tcpcb *tp)
{
        /* Return true if a DSACK block exists */
        if (tp->rcv_numsacks == 0)
                return (0);
        if (SEQ_LEQ(tp->sackblks[0].end, tp->rcv_nxt))
                return(1);
        return (0);
}
 
/*
 * This function will find overlaps with the currently stored sackblocks
 * and add any overlap as a dsack block upfront
 */
void
tcp_update_dsack_list(struct tcpcb *tp, tcp_seq rcv_start, tcp_seq rcv_end)
{
        struct sackblk head_blk,mid_blk,saved_blks[MAX_SACK_BLKS];
        int i, j, n, identical;
        tcp_seq start, end;
 
        INP_WLOCK_ASSERT(tp->t_inpcb);
 
        KASSERT(SEQ_LT(rcv_start, rcv_end), ("rcv_start < rcv_end"));
 
        if (SEQ_LT(rcv_end, tp->rcv_nxt) ||
            ((rcv_end == tp->rcv_nxt) &&
             (tp->rcv_numsacks > 0 ) &&
             (tp->sackblks[0].end == tp->rcv_nxt))) {
                saved_blks[0].start = rcv_start;
                saved_blks[0].end = rcv_end;
        } else {
                saved_blks[0].start = saved_blks[0].end = 0;
        }
 
        head_blk.start = head_blk.end = 0;
        mid_blk.start = rcv_start;
        mid_blk.end = rcv_end;
        identical = 0;
 
        for (i = 0; i < tp->rcv_numsacks; i++) {
                start = tp->sackblks[i].start;
                end = tp->sackblks[i].end;
                if (SEQ_LT(rcv_end, start)) {
                        /* pkt left to sack blk */
                        continue;
                }
                if (SEQ_GT(rcv_start, end)) {
                        /* pkt right to sack blk */
                        continue;
                }
                if (SEQ_GT(tp->rcv_nxt, end)) {
                        if ((SEQ_MAX(rcv_start, start) != SEQ_MIN(rcv_end, end)) &&
                            (SEQ_GT(head_blk.start, SEQ_MAX(rcv_start, start)) ||
                            (head_blk.start == head_blk.end))) {
                                head_blk.start = SEQ_MAX(rcv_start, start);
                                head_blk.end = SEQ_MIN(rcv_end, end);
                        }
                        continue;
                }
                if (((head_blk.start == head_blk.end) ||
                     SEQ_LT(start, head_blk.start)) &&
                     (SEQ_GT(end, rcv_start) &&
                      SEQ_LEQ(start, rcv_end))) {
                        head_blk.start = start;
                        head_blk.end = end;
                }
                mid_blk.start = SEQ_MIN(mid_blk.start, start);
                mid_blk.end = SEQ_MAX(mid_blk.end, end);
                if ((mid_blk.start == start) &&
                    (mid_blk.end == end))
                        identical = 1;
        }
        if (SEQ_LT(head_blk.start, head_blk.end)) {
                /* store overlapping range */
                saved_blks[0].start = SEQ_MAX(rcv_start, head_blk.start);
                saved_blks[0].end   = SEQ_MIN(rcv_end, head_blk.end);
        }
        n = 1;
        /*
         * Second, if not ACKed, store the SACK block that
         * overlaps with the DSACK block unless it is identical
         */
        if ((SEQ_LT(tp->rcv_nxt, mid_blk.end) &&
            !((mid_blk.start == saved_blks[0].start) &&
            (mid_blk.end == saved_blks[0].end))) ||
            identical == 1) {
                saved_blks[n].start = mid_blk.start;
                saved_blks[n++].end = mid_blk.end;
        }
        for (j = 0; (j < tp->rcv_numsacks) && (n < MAX_SACK_BLKS); j++) {
                if (((SEQ_LT(tp->sackblks[j].end, mid_blk.start) ||
                      SEQ_GT(tp->sackblks[j].start, mid_blk.end)) &&
                    (SEQ_GT(tp->sackblks[j].start, tp->rcv_nxt))))
                saved_blks[n++] = tp->sackblks[j];
        }
        j = 0;
        for (i = 0; i < n; i++) {
                /* we can end up with a stale initial entry */
                if (SEQ_LT(saved_blks[i].start, saved_blks[i].end)) {
                        tp->sackblks[j++] = saved_blks[i];
                }
        }
        tp->rcv_numsacks = j;
}
 
/*
 * 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_LEQ(rcv_start, rcv_end), ("rcv_start <= rcv_end"));
 
        if ((rcv_start == rcv_end) &&
            (tp->rcv_numsacks >= 1) &&
            (rcv_end == tp->sackblks[0].end)) {
                /* retaining DSACK block below rcv_nxt (todrop) */
                head_blk = tp->sackblks[0];
        } else {
                /* 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.
                         */
                        /*
                         * |-|
                         *   |---|  merge
                         *
                         *     |-|
                         * |---|    merge
                         *
                         * |-----|
                         *   |-|    DSACK smaller
                         *
                         *   |-|
                         * |-----|  DSACK smaller
                         */
                        if (head_blk.start == end)
                                head_blk.start = start;
                        else if (head_blk.end == start)
                                head_blk.end = end;
                        else {
                                if (SEQ_LT(head_blk.start, start)) {
                                        tcp_seq temp = start;
                                        start = head_blk.start;
                                        head_blk.start = temp;
                                }
                                if (SEQ_GT(head_blk.end, end)) {
                                        tcp_seq temp = end;
                                        end = head_blk.end;
                                        head_blk.end = temp;
                                }
                                if ((head_blk.start != start) ||
                                    (head_blk.end != end)) {
                                        if ((num_saved >= 1) &&
                                           SEQ_GEQ(saved_blks[num_saved-1].start, start) &&
                                           SEQ_LEQ(saved_blks[num_saved-1].end, end))
                                                num_saved--;
                                        saved_blks[num_saved].start = start;
                                        saved_blks[num_saved].end = end;
                                        num_saved++;
                                }
                        }
                } else {
                        /*
                         * This block supercedes the prior block
                         */
                        if ((num_saved >= 1) &&
                           SEQ_GEQ(saved_blks[num_saved-1].start, start) &&
                           SEQ_LEQ(saved_blks[num_saved-1].end, end))
                                num_saved--;
                        /*
                         * 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_LT(rcv_start, rcv_end)) {
                /*
                 * 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 ((rcv_start == rcv_end) &&
            (rcv_start == tp->sackblks[0].end)) {
                num_head = 1;
        }
        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;
}
 
void
tcp_clean_dsack_blocks(struct tcpcb *tp)
{
        struct sackblk saved_blks[MAX_SACK_BLKS];
        int num_saved, i;
 
        INP_WLOCK_ASSERT(tp->t_inpcb);
        /*
         * Clean up any DSACK blocks that
         * are in our queue of 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 D-SACK block.
                         */
                        continue;
                }
                /*
                 * Save this SACK block.
                 */
                saved_blks[num_saved].start = start;
                saved_blks[num_saved].end = end;
                num_saved++;
        }
        if (num_saved > 0) {
                /*
                 * Copy the saved SACK blocks back.
                 */
                bcopy(saved_blks, &tp->sackblks[0],
                      sizeof(struct sackblk) * num_saved);
        }
        tp->rcv_numsacks = 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.
 */
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;
        int delivered_data, left_edge_delta;
 
        INP_WLOCK_ASSERT(tp->t_inpcb);
 
        num_sack_blks = 0;
        sack_changed = 0;
        delivered_data = 0;
        left_edge_delta = 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.
         * Account changes to SND.UNA always in delivered data.
         */
        if (SEQ_LT(tp->snd_una, th_ack) && !TAILQ_EMPTY(&tp->snd_holes)) {
                left_edge_delta = th_ack - tp->snd_una;
                sack_blocks[num_sack_blks].start = tp->snd_una;
                sack_blocks[num_sack_blks++].end = th_ack;
                /*
                 * Pulling snd_fack forward if we got here
                 * due to DSACK blocks
                 */
                if (SEQ_LT(tp->snd_fack, th_ack)) {
                        delivered_data += th_ack - tp->snd_una;
                        tp->snd_fack = th_ack;
                        sack_changed = 1;
                }
        }
        /*
         * 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) {
                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;
                        } else if (SEQ_LEQ(sack.start, th_ack) &&
                            SEQ_LEQ(sack.end, th_ack)) {
                                /*
                                 * Its a D-SACK block.
                                 */
                                tcp_record_dsack(tp, sack.start, sack.end, 0);
                        }
                }
        }
        /*
         * 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);
                tp->sackhint.sacked_bytes = 0;  /* reset */
        }
        /*
         * 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.  First,
                 * check if there was a successful Rescue Retransmission,
                 * and move this hole left. With normal holes, snd_fack
                 * is always to the right of the end.
                 */
                if (((temp = TAILQ_LAST(&tp->snd_holes, sackhole_head)) != NULL) &&
                    SEQ_LEQ(tp->snd_fack,temp->end)) {
                        temp->start = SEQ_MAX(tp->snd_fack, SEQ_MAX(tp->snd_una, th_ack));
                        temp->end = sblkp->start;
                        temp->rxmit = temp->start;
                        delivered_data += sblkp->end - sblkp->start;
                        tp->snd_fack = sblkp->end;
                        sblkp--;
                        sack_changed = 1;
                } else {
                        /*
                         * Append a 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) {
                                delivered_data += sblkp->end - sblkp->start;
                                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)) {
                                        delivered_data += sblkp->end - tp->snd_fack;
                                        tp->snd_fack = sblkp->end;
                                        sack_changed = 1;
                                }
                        }
                }
        } else if (SEQ_LT(tp->snd_fack, sblkp->end)) {
                /* fack is advanced. */
                delivered_data += sblkp->end - tp->snd_fack;
                tp->snd_fack = sblkp->end;
                sack_changed = 1;
        }
        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 -=
                    (SEQ_MIN(cur->rxmit, cur->end) - 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. */
                                delivered_data += (cur->end - cur->start);
                                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. */
                                delivered_data += (sblkp->end - cur->start);
                                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. */
                                delivered_data += (cur->end - sblkp->start);
                                cur->end = sblkp->start;
                                cur->rxmit = SEQ_MIN(cur->rxmit, cur->end);
                                if ((tp->t_flags & TF_LRD) && SEQ_GEQ(cur->rxmit, cur->end))
                                        cur->rxmit = tp->snd_recover;
                        } 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 +=
                                                    (SEQ_MIN(temp->rxmit,
                                                    temp->end) - temp->start);
                                        }
                                        cur->end = sblkp->start;
                                        cur->rxmit = SEQ_MIN(cur->rxmit,
                                            cur->end);
                                        if ((tp->t_flags & TF_LRD) && SEQ_GEQ(cur->rxmit, cur->end))
                                                cur->rxmit = tp->snd_recover;
                                        delivered_data += (sblkp->end - sblkp->start);
                                }
                        }
                }
                tp->sackhint.sack_bytes_rexmit +=
                    (SEQ_MIN(cur->rxmit, cur->end) - 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--;
        }
        if (!(to->to_flags & TOF_SACK))
                /*
                 * If this ACK did not contain any
                 * SACK blocks, any only moved the
                 * left edge right, it is a pure
                 * cumulative ACK. Do not count
                 * DupAck for this. Also required
                 * for RFC6675 rescue retransmission.
                 */
                sack_changed = 0;
        tp->sackhint.delivered_data = delivered_data;
        tp->sackhint.sacked_bytes += delivered_data - left_edge_delta;
        KASSERT((delivered_data >= 0), ("delivered_data < 0"));
        KASSERT((tp->sackhint.sacked_bytes >= 0), ("sacked_bytes < 0"));
        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;
        u_int maxseg = tcp_maxseg(tp);
 
        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) / maxseg) >= 2)
                num_segs = 2;
        tp->snd_cwnd = (tp->sackhint.sack_bytes_rexmit +
            (tp->snd_nxt - tp->snd_recover) + num_segs * maxseg);
        if (tp->snd_cwnd > tp->snd_ssthresh)
                tp->snd_cwnd = tp->snd_ssthresh;
        tp->t_flags |= TF_ACKNOW;
        /*
         * RFC6675 rescue retransmission
         * Add a hole between th_ack (snd_una is not yet set) and snd_max,
         * if this was a pure cumulative ACK and no data was send beyond
         * recovery point. Since the data in the socket has not been freed
         * at this point, we check if the scoreboard is empty, and the ACK
         * delivered some new data, indicating a full ACK. Also, if the
         * recovery point is still at snd_max, we are probably application
         * limited. However, this inference might not always be true. The
         * rescue retransmission may rarely be slightly premature
         * compared to RFC6675.
         * The corresponding ACK+SACK will cause any further outstanding
         * segments to be retransmitted. This addresses a corner case, when
         * the trailing packets of a window are lost and no further data
         * is available for sending.
         */
        if ((V_tcp_do_newsack) &&
            SEQ_LT(th->th_ack, tp->snd_recover) &&
            (tp->snd_recover == tp->snd_max) &&
            TAILQ_EMPTY(&tp->snd_holes) &&
            (tp->sackhint.delivered_data > 0)) {
                /*
                 * Exclude FIN sequence space in
                 * the hole for the rescue retransmission,
                 * and also don't create a hole, if only
                 * the ACK for a FIN is outstanding.
                 */
                tcp_seq highdata = tp->snd_max;
                if (tp->t_flags & TF_SENTFIN)
                        highdata--;
                if (th->th_ack != highdata) {
                        tp->snd_fack = th->th_ack;
                        (void)tcp_sackhole_insert(tp, SEQ_MAX(th->th_ack,
                            highdata - maxseg), highdata, NULL);
                }
        }
        (void) 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 += (SEQ_MIN(p->rxmit, p->end) - 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;
}
 
/*
 * Lost Retransmission Detection
 * Check is FACK is beyond the rexmit of the leftmost hole.
 * If yes, we restart sending from still existing holes,
 * and adjust cwnd via the congestion control module.
 */
void
tcp_sack_lost_retransmission(struct tcpcb *tp, struct tcphdr *th)
{
        struct sackhole *temp;
 
        if (IN_RECOVERY(tp->t_flags) &&
            SEQ_GT(tp->snd_fack, tp->snd_recover) &&
            ((temp = TAILQ_FIRST(&tp->snd_holes)) != NULL) &&
            SEQ_GEQ(temp->rxmit, temp->end) &&
            SEQ_GEQ(tp->snd_fack, temp->rxmit)) {
                TCPSTAT_INC(tcps_sack_lostrexmt);
                /*
                 * Start retransmissions from the first hole, and
                 * subsequently all other remaining holes, including
                 * those, which had been sent completely before.
                 */
                tp->sackhint.nexthole = temp;
                TAILQ_FOREACH(temp, &tp->snd_holes, scblink) {
                        if (SEQ_GEQ(tp->snd_fack, temp->rxmit) &&
                            SEQ_GEQ(temp->rxmit, temp->end))
                                temp->rxmit = temp->start;
                }
                /*
                 * Remember the old ssthresh, to deduct the beta factor used
                 * by the CC module. Finally, set cwnd to ssthresh just
                 * prior to invoking another cwnd reduction by the CC
                 * module, to not shrink it excessively.
                 */
                tp->snd_cwnd = tp->snd_ssthresh;
                /*
                 * Formally exit recovery, and let the CC module adjust
                 * ssthresh as intended.
                 */
                EXIT_RECOVERY(tp->t_flags);
                cc_cong_signal(tp, th, CC_NDUPACK);
                /*
                 * For PRR, adjust recover_fs as if this new reduction
                 * initialized this variable.
                 * cwnd will be adjusted by SACK or PRR processing
                 * subsequently, only set it to a safe value here.
                 */
                tp->snd_cwnd = tcp_maxseg(tp);
                tp->sackhint.recover_fs = (tp->snd_max - tp->snd_una) -
                                            tp->sackhint.recover_fs;
        }
}