xnu-3248.30.4.tar.gz

[apple/xnu.git] / bsd / netinet / tcp_sack.c

/*
 * Copyright (c) 2004-2015 Apple Inc. All rights reserved.
 *
 * @APPLE_OSREFERENCE_LICENSE_HEADER_START@
 * 
 * This file contains Original Code and/or Modifications of Original Code
 * as defined in and that are subject to the Apple Public Source License
 * Version 2.0 (the 'License'). You may not use this file except in
 * compliance with the License. The rights granted to you under the License
 * may not be used to create, or enable the creation or redistribution of,
 * unlawful or unlicensed copies of an Apple operating system, or to
 * circumvent, violate, or enable the circumvention or violation of, any
 * terms of an Apple operating system software license agreement.
 * 
 * Please obtain a copy of the License at
 * http://www.opensource.apple.com/apsl/ and read it before using this file.
 * 
 * The Original Code and all software distributed under the License are
 * distributed on an 'AS IS' basis, WITHOUT WARRANTY OF ANY KIND, EITHER
 * EXPRESS OR IMPLIED, AND APPLE HEREBY DISCLAIMS ALL SUCH WARRANTIES,
 * INCLUDING WITHOUT LIMITATION, ANY WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE, QUIET ENJOYMENT OR NON-INFRINGEMENT.
 * Please see the License for the specific language governing rights and
 * limitations under the License.
 * 
 * @APPLE_OSREFERENCE_LICENSE_HEADER_END@
 */
/*
 * Copyright (c) 1982, 1986, 1988, 1990, 1993, 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. All advertising materials mentioning features or use of this software
 *    must display the following acknowledgement:
 *      This product includes software developed by the University of
 *      California, Berkeley and its contributors.
 * 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.
 *
 */

#define _IP_VHL


#include <sys/param.h>
#include <sys/systm.h>
#include <sys/kernel.h>
#include <sys/sysctl.h>
#include <sys/mbuf.h>
#include <sys/domain.h>
#include <sys/protosw.h>
#include <sys/socket.h>
#include <sys/socketvar.h>

#include <kern/zalloc.h>

#include <net/route.h>

#include <netinet/in.h>
#include <netinet/in_systm.h>
#include <netinet/ip.h>
#include <netinet/in_pcb.h>
#include <netinet/ip_var.h>
#if INET6
#include <netinet6/in6_pcb.h>
#include <netinet/ip6.h>
#include <netinet6/ip6_var.h>
#endif
#include <netinet/tcp.h>
//#define       TCPOUTFLAGS
#include <netinet/tcp_fsm.h>
#include <netinet/tcp_seq.h>
#include <netinet/tcp_timer.h>
#include <netinet/tcp_var.h>
#include <netinet/tcpip.h>
#if TCPDEBUG
#include <netinet/tcp_debug.h>
#endif
#include <sys/kdebug.h>

#if IPSEC
#include <netinet6/ipsec.h>
#endif /*IPSEC*/

#include <libkern/OSAtomic.h>

int     tcp_do_sack = 1;
SYSCTL_INT(_net_inet_tcp, OID_AUTO, sack, CTLFLAG_RW | CTLFLAG_LOCKED, &tcp_do_sack, 0,
        "Enable/Disable TCP SACK support");
static int tcp_sack_maxholes = 128;
SYSCTL_INT(_net_inet_tcp, OID_AUTO, sack_maxholes, CTLFLAG_RW | CTLFLAG_LOCKED,
        &tcp_sack_maxholes, 0, 
    "Maximum number of TCP SACK holes allowed per connection");

static int tcp_sack_globalmaxholes = 65536;
SYSCTL_INT(_net_inet_tcp, OID_AUTO, sack_globalmaxholes, CTLFLAG_RW | CTLFLAG_LOCKED,
        &tcp_sack_globalmaxholes, 0, 
    "Global maximum number of TCP SACK holes");

static SInt32 tcp_sack_globalholes = 0;
SYSCTL_INT(_net_inet_tcp, OID_AUTO, sack_globalholes, CTLFLAG_RD | CTLFLAG_LOCKED,
    &tcp_sack_globalholes, 0,
    "Global number of TCP SACK holes currently allocated");

static int tcp_detect_reordering = 1;
static int tcp_dsack_ignore_hw_duplicates = 0;

#if (DEVELOPMENT || DEBUG)
SYSCTL_INT(_net_inet_tcp, OID_AUTO, detect_reordering,
    CTLFLAG_RW | CTLFLAG_LOCKED,
    &tcp_detect_reordering, 0, "");

SYSCTL_INT(_net_inet_tcp, OID_AUTO, ignore_hw_duplicates,
    CTLFLAG_RW | CTLFLAG_LOCKED,
    &tcp_dsack_ignore_hw_duplicates, 0, "");
#endif /* (DEVELOPMENT || DEBUG) */

extern struct zone *sack_hole_zone;

#define TCP_VALIDATE_SACK_SEQ_NUMBERS(_tp_, _sb_, _ack_) \
    (SEQ_GT((_sb_)->end, (_sb_)->start) && \
    SEQ_GT((_sb_)->start, (_tp_)->snd_una) && \
    SEQ_GT((_sb_)->start, (_ack_)) && \
    SEQ_LT((_sb_)->start, (_tp_)->snd_max) && \
    SEQ_GT((_sb_)->end, (_tp_)->snd_una) && \
    SEQ_LEQ((_sb_)->end, (_tp_)->snd_max))

/*
 * 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;

        /* 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;

        /* If we are requesting SACK recovery, reset the stretch-ack state
         * so that connection will generate more acks after recovery and
         * sender's cwnd will open.
         */
        if ((tp->t_flags & TF_STRETCHACK) != 0 && tp->rcv_numsacks > 0)
                tcp_reset_stretch_ack(tp);

#if TRAFFIC_MGT
        if (tp->acc_iaj > 0 && tp->rcv_numsacks > 0) 
                reset_acc_iaj(tp);
#endif /* TRAFFIC_MGT */
}

/*
 * Delete all receiver-side SACK information.
 */
void
tcp_clean_sackreport( struct tcpcb *tp)
{

        tp->rcv_numsacks = 0;
        bzero(&tp->sackblks[0], sizeof (struct sackblk) * MAX_SACK_BLKS);
}

/*
 * 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 >= tcp_sack_maxholes ||
            tcp_sack_globalholes >= tcp_sack_globalmaxholes) {
                tcpstat.tcps_sack_sboverflow++;
                return NULL;
        }

        hole = (struct sackhole *)zalloc(sack_hole_zone);
        if (hole == NULL)
                return NULL;

        hole->start = start;
        hole->end = end;
        hole->rxmit = start;

        tp->snd_numholes++;
        OSIncrementAtomic(&tcp_sack_globalholes);

        return hole;
}

/*
 * Free struct sackhole.
 */
static void
tcp_sackhole_free(struct tcpcb *tp, struct sackhole *hole)
{
        zfree(sack_hole_zone, hole);

        tp->snd_numholes--;
        OSDecrementAtomic(&tcp_sack_globalholes);
}

/*
 * 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;
        hole->rxmit_start = tcp_now;
        /* 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);
}
/*
 * When a new ack with SACK is received, check if it indicates packet
 * reordering. If there is packet reordering, the socket is marked and
 * the late time offset by which the packet was reordered with
 * respect to its closest neighboring packets is computed.
 */
static void
tcp_sack_detect_reordering(struct tcpcb *tp, struct sackhole *s,
    tcp_seq sacked_seq, tcp_seq snd_fack)
{
        int32_t rext = 0, reordered = 0;

        /*
         * If the SACK hole is past snd_fack, this is from new SACK
         * information, so we can ignore it.
         */
        if (SEQ_GT(s->end, snd_fack))
                return;
        /*
         * If there has been a retransmit timeout, then the timestamp on 
         * the SACK segment will be newer. This might lead to a
         * false-positive. Avoid re-ordering detection in this case.
         */
        if (tp->t_rxtshift > 0)
                return;

        /*
         * Detect reordering from SACK information by checking
         * if recently sacked data was never retransmitted from this hole.
         */
        if (SEQ_LT(s->rxmit, sacked_seq)) {
                reordered = 1;
                tcpstat.tcps_avoid_rxmt++;
        }

        if (reordered) {
                if (tcp_detect_reordering == 1 &&
                    !(tp->t_flagsext & TF_PKTS_REORDERED)) {
                        tp->t_flagsext |= TF_PKTS_REORDERED;
                        tcpstat.tcps_detect_reordering++;
                }

                tcpstat.tcps_reordered_pkts++;
                tp->t_reordered_pkts++;

                VERIFY(SEQ_GEQ(snd_fack, s->rxmit));

                if (s->rxmit_start > 0) {
                        rext = timer_diff(tcp_now, 0, s->rxmit_start, 0);
                        if (rext < 0)
                                return;

                        /*
                         * We take the maximum reorder window to schedule
                         * DELAYFR timer as that will take care of jitter
                         * on the network path.
                         *
                         * Computing average and standard deviation seems
                         * to cause unnecessary retransmissions when there
                         * is high jitter.
                         *
                         * We set a maximum of SRTT/2 and a minimum of
                         * 10 ms on the reorder window.
                         */
                        tp->t_reorderwin = max(tp->t_reorderwin, rext);
                        tp->t_reorderwin = min(tp->t_reorderwin,
                            (tp->t_srtt >> (TCP_RTT_SHIFT - 1)));
                        tp->t_reorderwin = max(tp->t_reorderwin, 10);
                }
        }
}

/*
 * 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).
 */
void
tcp_sack_doack(struct tcpcb *tp, struct tcpopt *to, struct tcphdr *th, 
        u_int32_t *newbytes_acked)
{
        struct sackhole *cur, *temp;
        struct sackblk sack, sack_blocks[TCP_MAX_SACK + 1], *sblkp;
        int i, j, num_sack_blks;
        tcp_seq old_snd_fack = 0, th_ack = th->th_ack;

        num_sack_blks = 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[].
         * Check that the SACK block range is valid.
         */
        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 (TCP_VALIDATE_SACK_SEQ_NUMBERS(tp, &sack, th_ack))
                        sack_blocks[num_sack_blks++] = sack;
        }

        /*
         * Return if SND.UNA is not advanced and no valid SACK block
         * is received.
         */
        if (num_sack_blks == 0)
                return;

        VERIFY(num_sack_blks <= (TCP_MAX_SACK + 1));
        /*
         * Sort the SACK blocks so we can update the scoreboard
         * with just one pass. The overhead of sorting upto 4+1 elements
         * is less than making upto 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);
                *newbytes_acked += (tp->snd_fack - tp->snd_una);
        }

        old_snd_fack = tp->snd_fack;
        /*
         * 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 */
        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;
                        *newbytes_acked += (sblkp->end - sblkp->start);

                        /* Go to the previous sack block. */
                        sblkp--;
                } 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)) {
                                *newbytes_acked += (sblkp->end - tp->snd_fack);
                                tp->snd_fack = sblkp->end;
                        }
                }
        } else if (SEQ_LT(tp->snd_fack, sblkp->end)) {
                /* fack is advanced. */
                *newbytes_acked += (sblkp->end - tp->snd_fack);
                tp->snd_fack = sblkp->end;
        }
        /* We must have at least one SACK hole in scoreboard */
        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);
                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 */
                                *newbytes_acked += (cur->end - cur->start);

                                tcp_sack_detect_reordering(tp, cur,
                                    cur->end, old_snd_fack);
                                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 */
                                *newbytes_acked += (sblkp->end - cur->start);
                                tcp_sack_detect_reordering(tp, cur,
                                    sblkp->end, old_snd_fack);
                                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 */
                                *newbytes_acked += (cur->end - sblkp->start);
                                tcp_sack_detect_reordering(tp, cur,
                                    cur->end, old_snd_fack);
                                cur->end = sblkp->start;
                                cur->rxmit = SEQ_MIN(cur->rxmit, cur->end);
                        } else {
                                /*
                                 * ACKs some data in the middle of a hole;
                                 * need to split current hole
                                 */
                                *newbytes_acked += (sblkp->end - sblkp->start);
                                tcp_sack_detect_reordering(tp, cur,
                                    sblkp->end, old_snd_fack);
                                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);
                                        /*
                                         * Reset the rxmit_start to that of
                                         * the current hole as that will
                                         * help to compute the reorder
                                         * window correctly
                                         */
                                        temp->rxmit_start = cur->rxmit_start;
                                }
                        }
                }
                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--;
        }
}

/*
 * Free all SACK holes to clear the scoreboard.
 */
void
tcp_free_sackholes(struct tcpcb *tp)
{
        struct sackhole *q;

        while ((q = TAILQ_FIRST(&tp->snd_holes)) != NULL)
                tcp_sackhole_remove(tp, q);
        tp->sackhint.sack_bytes_rexmit = 0;
        tp->sackhint.nexthole = NULL;
        tp->sack_newdata = 0;

}

/*
 * 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(tp, th)
        struct tcpcb *tp;
        struct tcphdr *th;
{
        int num_segs = 1;

        tp->t_timer[TCPT_REXMT] = 0;
        tp->t_rtttime = 0;
        /* send one or 2 segments based on how much new data was acked */
        if (((BYTES_ACKED(th, tp)) / 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;
        if (SEQ_LT(tp->snd_fack, tp->snd_recover) &&
            tp->snd_fack == th->th_ack && TAILQ_EMPTY(&tp->snd_holes)) {
                struct sackhole *temp;
                /*
                 * we received a partial ack but there is no sack_hole
                 * that will cover the remaining seq space. In this case,
                 * create a hole from snd_fack to snd_recover so that
                 * the sack recovery will continue.
                 */
                temp = tcp_sackhole_insert(tp, tp->snd_fack,
                    tp->snd_recover, NULL);
                if (temp != NULL)
                        tp->snd_fack = tp->snd_recover;
        }
        (void) tcp_output(tp);
}

/*
 * 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;

        *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);
}

/*
 * 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, *dbg_hole = NULL;
        int dbg_bytes_rexmt;

        dbg_hole = tcp_sack_output_debug(tp, &dbg_bytes_rexmt);
        *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:
        if (dbg_hole != hole) {
                printf("%s: Computed sack hole not the same as cached value\n", __func__);
                hole = dbg_hole;
        }
        if (*sack_bytes_rexmt != dbg_bytes_rexmt) {
                printf("%s: Computed sack_bytes_retransmitted (%d) not "
                       "the same as cached value (%d)\n",
                       __func__, dbg_bytes_rexmt, *sack_bytes_rexmt);
                *sack_bytes_rexmt = dbg_bytes_rexmt;
        }
        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);

        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;
        return;
}

/*
 * This function returns TRUE if more than (tcprexmtthresh - 1) * SMSS
 * bytes with sequence numbers greater than snd_una have been SACKed. 
 */
boolean_t
tcp_sack_byte_islost(struct tcpcb *tp)
{
        u_int32_t unacked_bytes, sndhole_bytes = 0;
        struct sackhole *sndhole;
        if (!SACK_ENABLED(tp) || IN_FASTRECOVERY(tp) ||
            TAILQ_EMPTY(&tp->snd_holes) ||
            (tp->t_flagsext & TF_PKTS_REORDERED))
                return (FALSE);

        unacked_bytes = tp->snd_max - tp->snd_una;

        TAILQ_FOREACH(sndhole, &tp->snd_holes, scblink) {
                sndhole_bytes += (sndhole->end - sndhole->start);
        }

        VERIFY(unacked_bytes >= sndhole_bytes);
        return ((unacked_bytes - sndhole_bytes) >
            ((tcprexmtthresh - 1) * tp->t_maxseg));
}

/*
 * Process any DSACK options that might be present on an input packet
 */

boolean_t
tcp_sack_process_dsack(struct tcpcb *tp, struct tcpopt *to,
    struct tcphdr *th)
{
        struct sackblk first_sack, second_sack;
        struct tcp_rxt_seg *rxseg;

        bcopy(to->to_sacks, &first_sack, sizeof(first_sack));
        first_sack.start = ntohl(first_sack.start);
        first_sack.end = ntohl(first_sack.end);

        if (to->to_nsacks > 1) {
                bcopy((to->to_sacks + TCPOLEN_SACK), &second_sack,
                    sizeof(second_sack));
                second_sack.start = ntohl(second_sack.start);
                second_sack.end = ntohl(second_sack.end);
        }

        if (SEQ_LT(first_sack.start, th->th_ack) &&
            SEQ_LEQ(first_sack.end, th->th_ack)) {
                /*
                 * There is a dsack option reporting a duplicate segment
                 * also covered by cumulative acknowledgement.
                 *
                 * Validate the sequence numbers before looking at dsack
                 * option. The duplicate notification can come after
                 * snd_una moves forward. In order to set a window of valid
                 * sequence numbers to look for, we set a maximum send
                 * window within which the DSACK option will be processed.
                 */
                if (!(TCP_DSACK_SEQ_IN_WINDOW(tp, first_sack.start, th->th_ack) &&
                    TCP_DSACK_SEQ_IN_WINDOW(tp, first_sack.end, th->th_ack))) {
                        to->to_nsacks--;
                        to->to_sacks += TCPOLEN_SACK;
                        tcpstat.tcps_dsack_recvd_old++;

                        /*
                         * returning true here so that the ack will not be
                         * treated as duplicate ack.
                         */
                        return (TRUE);
                }
        } else if (to->to_nsacks > 1 &&
            SEQ_LEQ(second_sack.start, first_sack.start) &&
            SEQ_GEQ(second_sack.end, first_sack.end)) {
                /*
                 * there is a dsack option in the first block not
                 * covered by the cumulative acknowledgement but covered
                 * by the second sack block.
                 *
                 * verify the sequence numbes on the second sack block
                 * before processing the DSACK option. Returning false
                 * here will treat the ack as a duplicate ack.
                 */
                if (!TCP_VALIDATE_SACK_SEQ_NUMBERS(tp, &second_sack,
                    th->th_ack)) {
                        to->to_nsacks--;
                        to->to_sacks += TCPOLEN_SACK;
                        tcpstat.tcps_dsack_recvd_old++;
                        return (TRUE);
                }
        } else {
                /* no dsack options, proceed with processing the sack */
                return (FALSE);
        }

        /* Update the tcpopt pointer to exclude dsack block */
        to->to_nsacks--;
        to->to_sacks += TCPOLEN_SACK;
        tcpstat.tcps_dsack_recvd++;
        tp->t_dsack_recvd++;

        /* ignore DSACK option, if DSACK is disabled */
        if (tp->t_flagsext & TF_DISABLE_DSACK)
                return (TRUE);

        /* If the DSACK is for TLP mark it as such */
        if ((tp->t_flagsext & TF_SENT_TLPROBE) &&
            first_sack.end == tp->t_tlphighrxt) {
                if ((rxseg = tcp_rxtseg_find(tp, first_sack.start,
                    (first_sack.end - 1))) != NULL)
                        rxseg->rx_flags |= TCP_RXT_DSACK_FOR_TLP;
        }
        /* Update the sender's retransmit segment state */
        if (((tp->t_rxtshift == 1 && first_sack.start == tp->snd_una) ||
            ((tp->t_flagsext & TF_SENT_TLPROBE) &&
            first_sack.end == tp->t_tlphighrxt)) &&
            TAILQ_EMPTY(&tp->snd_holes) &&
            SEQ_GT(th->th_ack, tp->snd_una)) {
                /*
                 * If the dsack is for a retransmitted packet and one of
                 * the two cases is true, it indicates ack loss:
                 * - retransmit timeout and first_sack.start == snd_una
                 * - TLP probe and first_sack.end == tlphighrxt
                 *
                 * Ignore dsack and do not update state when there is
                 * ack loss
                 */
                tcpstat.tcps_dsack_ackloss++;

                return (TRUE);
        } else if ((rxseg = tcp_rxtseg_find(tp, first_sack.start,
            (first_sack.end - 1))) == NULL) {
                /*
                 * Duplicate notification was not triggered by a
                 * retransmission. This might be due to network duplication,
                 * disable further DSACK processing.
                 */
                if (!tcp_dsack_ignore_hw_duplicates) {
                        tp->t_flagsext |= TF_DISABLE_DSACK;
                        tcpstat.tcps_dsack_disable++;
                }
        } else {
                /*
                 * If the segment was retransmitted only once, mark it as
                 * spurious. Otherwise ignore the duplicate notification.
                 */
                if (rxseg->rx_count == 1)
                        rxseg->rx_flags |= TCP_RXT_SPURIOUS;
                else
                        rxseg->rx_flags &= ~TCP_RXT_SPURIOUS;
        }
        return (TRUE);
}