Hook up interrupt last. Use ti_detach as common error path.

[dragonfly/netmp.git] / sys / netinet / tcp_sack.c

/*
 * Copyright (c) 2003, 2004 Jeffrey M. Hsu.  All rights reserved.
 * Copyright (c) 2003, 2004 The DragonFly Project.  All rights reserved.
 *
 * This code is derived from software contributed to The DragonFly Project
 * by Jeffrey M. Hsu.
 *
 * 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 DragonFly Project 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 COPYRIGHT HOLDERS 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
 * COPYRIGHT HOLDERS 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.
 *
 * $DragonFly: src/sys/netinet/tcp_sack.c,v 1.2 2005/03/09 06:54:34 hsu Exp $
 */

/*
 * Copyright (c) 2003, 2004 Jeffrey M. Hsu.  All rights reserved.
 *
 * License terms: all terms for the DragonFly license above plus the following:
 *
 * 4. All advertising materials mentioning features or use of this software
 *    must display the following acknowledgement:
 *
 *      This product includes software developed by Jeffrey M. Hsu
 *      for the DragonFly Project.
 *
 *    This requirement may be waived with permission from Jeffrey Hsu.
 *    This requirement will sunset and may be removed on November 13, 2005,
 *    after which the standard DragonFly license (as shown above) will
 *    apply.
 */

#include <sys/param.h>
#include <sys/systm.h>
#include <sys/kernel.h>
#include <sys/malloc.h>
#include <sys/queue.h>
#include <sys/thread.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <sys/socketvar.h>

#include <net/if.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/tcp.h>
#include <netinet/tcp_seq.h>
#include <netinet/tcp_var.h>

#include <vm/vm_zone.h>

struct sackblock {
        tcp_seq                 sblk_start;
        tcp_seq                 sblk_end;
        TAILQ_ENTRY(sackblock)  sblk_list;
};

#define MAXSAVEDBLOCKS  8                       /* per connection limit */

static void insert_block(struct scoreboard *scb, struct sackblock *newblock);
static void update_lostseq(struct scoreboard *scb, tcp_seq snd_una,
                           u_int maxseg);

static vm_zone_t sackblock_zone;

/*
 * Initialize SACK module.
 */
void
tcp_sack_init()
{
        sackblock_zone = zinit("sack blocks", sizeof(struct sackblock),
            maxsockets, ZONE_INTERRUPT, 0);
}

/*
 * Per-tcpcb initialization.
 */
void
tcp_sack_tcpcb_init(struct tcpcb *tp)
{
        struct scoreboard *scb = &tp->scb;

        scb->nblocks = 0;
        TAILQ_INIT(&scb->sackblocks);
        scb->lastfound = NULL;
}

/*
 * Find the SACK block containing or immediately preceding "seq".
 * The boolean result indicates whether the sequence is actually
 * contained in the SACK block.
 */
static boolean_t
sack_block_lookup(struct scoreboard *scb, tcp_seq seq, struct sackblock **sb)
{
        struct sackblock *hint = scb->lastfound;
        struct sackblock *cur, *last, *prev;

        if (TAILQ_EMPTY(&scb->sackblocks)) {
                *sb = NULL;
                return FALSE;
        }

        if (hint == NULL) {
                /* No hint.  Search from start to end. */
                cur = TAILQ_FIRST(&scb->sackblocks);
                last = NULL;
                prev = TAILQ_LAST(&scb->sackblocks, sackblock_list);
        } else  {
                if (SEQ_GEQ(seq, hint->sblk_start)) {
                        /* Search from hint to end of list. */
                        cur = hint;
                        last = NULL;
                        prev = TAILQ_LAST(&scb->sackblocks, sackblock_list);
                } else {
                        /* Search from front of list to hint. */
                        cur = TAILQ_FIRST(&scb->sackblocks);
                        last = hint;
                        prev = TAILQ_PREV(hint, sackblock_list, sblk_list);
                }
        }

        do {
                if (SEQ_GT(cur->sblk_end, seq)) {
                        if (SEQ_GEQ(seq, cur->sblk_start)) {
                                *sb = scb->lastfound = cur;
                                return TRUE;
                        } else {
                                *sb = scb->lastfound =
                                    TAILQ_PREV(cur, sackblock_list, sblk_list);
                                return FALSE;
                        }
                }
                cur = TAILQ_NEXT(cur, sblk_list);
        } while (cur != last);

        *sb = scb->lastfound = prev;
        return FALSE;
}

/*
 * Allocate a SACK block.
 */
static __inline struct sackblock *
alloc_sackblock(void)
{
        return zalloc(sackblock_zone);
}

/*
 * Free a SACK block.
 */
static __inline void
free_sackblock(struct sackblock *s)
{
        zfree(sackblock_zone, s);
}

/*
 * Free up SACK blocks for data that's been acked.
 */
static void
tcp_sack_ack_blocks(struct scoreboard *scb, tcp_seq th_ack)
{
        struct sackblock *sb, *nb;

        sb = TAILQ_FIRST(&scb->sackblocks);
        while (sb && SEQ_LEQ(sb->sblk_end, th_ack)) {
                nb = TAILQ_NEXT(sb, sblk_list);
                if (sb == scb->lastfound)
                        scb->lastfound = NULL;
                TAILQ_REMOVE(&scb->sackblocks, sb, sblk_list);
                free_sackblock(sb);
                --scb->nblocks;
                KASSERT(scb->nblocks >= 0,
                    ("SACK block count underflow: %d < 0", scb->nblocks));
                sb = nb;
        }
        if (sb && SEQ_GT(th_ack, sb->sblk_start))
                sb->sblk_start = th_ack;        /* other side reneged? XXX */
}

/*
 * Delete and free SACK blocks saved in scoreboard.
 */
void
tcp_sack_cleanup(struct scoreboard *scb)
{
        struct sackblock *sb, *nb;

        TAILQ_FOREACH_MUTABLE(sb, &scb->sackblocks, sblk_list, nb) {
                free_sackblock(sb);
                --scb->nblocks;
        }
        KASSERT(scb->nblocks == 0,
            ("SACK block %d count not zero", scb->nblocks));
        TAILQ_INIT(&scb->sackblocks);
        scb->lastfound = NULL;
}

/*
 * Returns      0 if not D-SACK block,
 *              1 if D-SACK,
 *              2 if duplicate of out-of-order D-SACK block.
 */
int
tcp_sack_ndsack_blocks(struct raw_sackblock *blocks, const int numblocks,
                       tcp_seq snd_una)
{
        if (numblocks == 0)
                return 0;

        if (SEQ_LT(blocks[0].rblk_start, snd_una))
                return 1;

        /* block 0 inside block 1 */
        if (numblocks > 1 &&
            SEQ_GEQ(blocks[0].rblk_start, blocks[1].rblk_start) &&
            SEQ_LEQ(blocks[0].rblk_end, blocks[1].rblk_end))
                return 2;

        return 0;
}

/*
 * Update scoreboard on new incoming ACK.
 */
static void
tcp_sack_add_blocks(struct tcpcb *tp, struct tcpopt *to)
{
        const int numblocks = to->to_nsackblocks;
        struct raw_sackblock *blocks = to->to_sackblocks;
        struct scoreboard *scb = &tp->scb;
        struct sackblock *sb;
        int startblock;
        int i;

        if (tcp_sack_ndsack_blocks(blocks, numblocks, tp->snd_una) > 0)
                startblock = 1;
        else
                startblock = 0;

        for (i = startblock; i < numblocks; i++) {
                struct raw_sackblock *newsackblock = &blocks[i];

                /* don't accept bad SACK blocks */
                if (SEQ_GT(newsackblock->rblk_end, tp->snd_max))
                        break;          /* skip all other blocks */

                sb = alloc_sackblock();
                if (sb == NULL)         /* do some sort of cleanup? XXX */
                        break;          /* just skip rest of blocks */
                sb->sblk_start = newsackblock->rblk_start;
                sb->sblk_end = newsackblock->rblk_end;
                if (TAILQ_EMPTY(&scb->sackblocks)) {
                        KASSERT(scb->nblocks == 0, ("emply scb w/ blocks"));
                        scb->nblocks = 1;
                        TAILQ_INSERT_HEAD(&scb->sackblocks, sb, sblk_list);
                } else {
                        insert_block(scb, sb);
                }
        }
}

void
tcp_sack_update_scoreboard(struct tcpcb *tp, struct tcpopt *to)
{
        struct scoreboard *scb = &tp->scb;

        tcp_sack_ack_blocks(scb, tp->snd_una);
        tcp_sack_add_blocks(tp, to);
        update_lostseq(scb, tp->snd_una, tp->t_maxseg);
        if (SEQ_LT(tp->rexmt_high, tp->snd_una))
                tp->rexmt_high = tp->snd_una;
}

/*
 * Insert SACK block into sender's scoreboard.
 */
static void
insert_block(struct scoreboard *scb, struct sackblock *newblock)
{
        struct sackblock *sb, *workingblock;
        boolean_t overlap_front;

        KASSERT(scb->nblocks > 0, ("insert_block() called w/ no blocks"));

        if (scb->nblocks == MAXSAVEDBLOCKS) {
                /*
                 * Should try to kick out older blocks XXX JH
                 * May be able to coalesce with existing block.
                 * Or, go other way and free all blocks if we hit this limit.
                 */
                free_sackblock(newblock);
                return;
        }
        KASSERT(scb->nblocks < MAXSAVEDBLOCKS,
            ("too many SACK blocks %d", scb->nblocks));

        overlap_front = sack_block_lookup(scb, newblock->sblk_start,  &sb);

        if (sb == NULL) {
                workingblock = newblock;
                TAILQ_INSERT_HEAD(&scb->sackblocks, newblock, sblk_list);
                ++scb->nblocks;
        } else {
                if (overlap_front || sb->sblk_end == newblock->sblk_start) {
                        /* extend old block and discard new one */
                        workingblock = sb;
                        if (SEQ_GT(newblock->sblk_end, sb->sblk_end))
                                sb->sblk_end = newblock->sblk_end;
                        free_sackblock(newblock);
                } else {
                        workingblock = newblock;
                        TAILQ_INSERT_AFTER(&scb->sackblocks, sb, newblock,
                                           sblk_list);
                        ++scb->nblocks;
                }
        }

        /* Consolidate right-hand side. */
        sb = TAILQ_NEXT(workingblock, sblk_list);
        while (sb != NULL &&
            SEQ_GEQ(workingblock->sblk_end, sb->sblk_end)) {
                struct sackblock *nextblock;

                nextblock = TAILQ_NEXT(sb, sblk_list);
                /* Remove completely overlapped block */
                TAILQ_REMOVE(&scb->sackblocks, sb, sblk_list);
                free_sackblock(sb);
                --scb->nblocks;
                KASSERT(scb->nblocks > 0,
                    ("removed overlapped block: %d blocks left", scb->nblocks));
                sb = nextblock;
        }
        if (sb != NULL &&
            SEQ_GEQ(workingblock->sblk_end, sb->sblk_start)) {
                /* Extend new block to cover partially overlapped old block. */
                workingblock->sblk_end = sb->sblk_end;
                TAILQ_REMOVE(&scb->sackblocks, sb, sblk_list);
                free_sackblock(sb);
                --scb->nblocks;
                KASSERT(scb->nblocks > 0,
                    ("removed partial right: %d blocks left", scb->nblocks));
        }
}

#ifdef DEBUG_SACK_BLOCKS
static void
tcp_sack_dump_blocks(struct scoreboard *scb)
{
        struct sackblock *sb;

        printf("%d blocks:", scb->nblocks);
        TAILQ_FOREACH(sb, &scb->sackblocks, sblk_list)
                printf(" [%u, %u)", sb->sblk_start, sb->sblk_end);
        printf("\n");
}
#else
static __inline void
tcp_sack_dump_blocks(struct scoreboard *scb)
{
}
#endif

/*
 * Optimization to quickly determine which packets are lost.
 */
static void
update_lostseq(struct scoreboard *scb, tcp_seq snd_una, u_int maxseg)
{
        struct sackblock *sb;
        int nsackblocks = 0;
        int bytes_sacked = 0;

        sb = TAILQ_LAST(&scb->sackblocks, sackblock_list);
        while (sb != NULL) {
                ++nsackblocks;
                bytes_sacked += sb->sblk_end - sb->sblk_start;
                if (nsackblocks == tcprexmtthresh ||
                    bytes_sacked >= tcprexmtthresh * maxseg) {
                        scb->lostseq = sb->sblk_start;
                        return;
                }
                sb = TAILQ_PREV(sb, sackblock_list, sblk_list);
        }
        scb->lostseq = snd_una;
}

/*
 * Return whether the given sequence number is considered lost.
 */
static boolean_t
scb_islost(struct scoreboard *scb, tcp_seq seqnum)
{
        return SEQ_LT(seqnum, scb->lostseq);
}

/*
 * True if at least "amount" has been SACKed.  Used by Early Retransmit.
 */
boolean_t
tcp_sack_has_sacked(struct scoreboard *scb, u_int amount)
{
        struct sackblock *sb;
        int bytes_sacked = 0;

        TAILQ_FOREACH(sb, &scb->sackblocks, sblk_list) {
                bytes_sacked += sb->sblk_end - sb->sblk_start;
                if (bytes_sacked >= amount)
                        return TRUE;
        }
        return FALSE;
}

/*
 * Number of bytes SACKed below seq.
 */
int
tcp_sack_bytes_below(struct scoreboard *scb, tcp_seq seq)
{
        struct sackblock *sb;
        int bytes_sacked = 0;

        sb = TAILQ_FIRST(&scb->sackblocks);
        while (sb && SEQ_GT(seq, sb->sblk_start)) {
                bytes_sacked += seq_min(seq, sb->sblk_end) - sb->sblk_start;
                sb = TAILQ_NEXT(sb, sblk_list);
        }
        return bytes_sacked;
}

/*
 * Return estimate of the number of bytes outstanding in the network.
 */
uint32_t
tcp_sack_compute_pipe(struct tcpcb *tp)
{
        struct scoreboard *scb = &tp->scb;
        struct sackblock *sb;
        int nlost, nretransmitted;
        tcp_seq end;

        nlost = tp->snd_max - scb->lostseq;
        nretransmitted = tp->rexmt_high - tp->snd_una;

        TAILQ_FOREACH(sb, &scb->sackblocks, sblk_list) {
                if (SEQ_LT(sb->sblk_start, tp->rexmt_high)) {
                        end = seq_min(sb->sblk_end, tp->rexmt_high);
                        nretransmitted -= end - sb->sblk_start;
                }
                if (SEQ_GEQ(sb->sblk_start, scb->lostseq))
                        nlost -= sb->sblk_end - sb->sblk_start;
        }

        return (nlost + nretransmitted);
}

/*
 * Return the sequence number and length of the next segment to transmit
 * when in Fast Recovery.
 */
boolean_t
tcp_sack_nextseg(struct tcpcb *tp, tcp_seq *nextrexmt, uint32_t *plen,
                 boolean_t *lostdup)
{
        struct scoreboard *scb = &tp->scb;
        struct socket *so = tp->t_inpcb->inp_socket;
        struct sackblock *sb;
        const struct sackblock *lastblock =
            TAILQ_LAST(&scb->sackblocks, sackblock_list);
        tcp_seq torexmt;
        long len, off;

        /* skip SACKed data */
        tcp_sack_skip_sacked(scb, &tp->rexmt_high);

        /* Look for lost data. */
        torexmt = tp->rexmt_high;
        *lostdup = FALSE;
        if (lastblock != NULL) {
                if (SEQ_LT(torexmt, lastblock->sblk_end) &&
                    scb_islost(scb, torexmt)) {
sendunsacked:
                        *nextrexmt = torexmt;
                        /* If the left-hand edge has been SACKed, pull it in. */
                        if (sack_block_lookup(scb, torexmt + tp->t_maxseg, &sb))
                                *plen = sb->sblk_start - torexmt;
                        else
                                *plen = tp->t_maxseg;
                        return TRUE;
                }
        }

        /* See if unsent data available within send window. */
        off = tp->snd_max - tp->snd_una;
        len = (long) ulmin(so->so_snd.sb_cc, tp->snd_wnd) - off;
        if (len > 0) {
                *nextrexmt = tp->snd_max;       /* Send new data. */
                *plen = tp->t_maxseg;
                return TRUE;
        }

        /* We're less certain this data has been lost. */
        if (lastblock == NULL || SEQ_LT(torexmt, lastblock->sblk_end))
                goto sendunsacked;

        return FALSE;
}

/*
 * Return the next sequence number higher than "*prexmt" that has
 * not been SACKed.
 */
void
tcp_sack_skip_sacked(struct scoreboard *scb, tcp_seq *prexmt)
{
        struct sackblock *sb;

        /* skip SACKed data */
        if (sack_block_lookup(scb, *prexmt, &sb))
                *prexmt = sb->sblk_end;
}

#ifdef later
void
tcp_sack_save_scoreboard(struct scoreboard *scb)
{
        struct scoreboard *scb = &tp->scb;

        scb->sackblocks_prev = scb->sackblocks;
        TAILQ_INIT(&scb->sackblocks);
}

void
tcp_sack_revert_scoreboard(struct scoreboard *scb, tcp_seq snd_una,
                           u_int maxseg)
{
        struct sackblock *sb;

        scb->sackblocks = scb->sackblocks_prev;
        scb->nblocks = 0;
        TAILQ_FOREACH(sb, &scb->sackblocks, sblk_list)
                ++scb->nblocks;
        tcp_sack_ack_blocks(scb, snd_una);
        scb->lastfound = NULL;
}
#endif

#ifdef DEBUG_SACK_HISTORY
static void
tcp_sack_dump_history(char *msg, struct tcpcb *tp)
{
        int i;
        static int ndumped;

        /* only need a couple of these to debug most problems */
        if (++ndumped > 900)
                return;

        printf("%s:\tnsackhistory %d: ", msg, tp->nsackhistory);
        for (i = 0; i < tp->nsackhistory; ++i)
                printf("[%u, %u) ", tp->sackhistory[i].rblk_start,
                    tp->sackhistory[i].rblk_end);
        printf("\n");
}
#else
static __inline void
tcp_sack_dump_history(char *msg, struct tcpcb *tp)
{
}
#endif

/*
 * Remove old SACK blocks from the SACK history that have already been ACKed.
 */
static void
tcp_sack_ack_history(struct tcpcb *tp)
{
        int i, nblocks, openslot;

        tcp_sack_dump_history("before tcp_sack_ack_history", tp);
        nblocks = tp->nsackhistory;
        for (i = openslot = 0; i < nblocks; ++i) {
                if (SEQ_LEQ(tp->sackhistory[i].rblk_end, tp->rcv_nxt)) {
                        --tp->nsackhistory;
                        continue;
                }
                if (SEQ_LT(tp->sackhistory[i].rblk_start, tp->rcv_nxt))
                        tp->sackhistory[i].rblk_start = tp->rcv_nxt;
                if (i == openslot)
                        ++openslot;
                else
                        tp->sackhistory[openslot++] = tp->sackhistory[i];
        }
        tcp_sack_dump_history("after tcp_sack_ack_history", tp);
        KASSERT(openslot == tp->nsackhistory,
            ("tcp_sack_ack_history miscounted: %d != %d",
            openslot, tp->nsackhistory));
}

/*
 * Add or merge newblock into reported history.
 * Also remove or update SACK blocks that will be acked.
 */
static void
tcp_sack_update_reported_history(struct tcpcb *tp, tcp_seq start, tcp_seq end)
{
        struct raw_sackblock copy[MAX_SACK_REPORT_BLOCKS];
        int i, cindex;

        tcp_sack_dump_history("before tcp_sack_update_reported_history", tp);
        /*
         * Six cases:
         *      0) no overlap
         *      1) newblock == oldblock
         *      2) oldblock contains newblock
         *      3) newblock contains oldblock
         *      4) tail of oldblock overlaps or abuts start of newblock
         *      5) tail of newblock overlaps or abuts head of oldblock
         */
        for (i = cindex = 0; i < tp->nsackhistory; ++i) {
                struct raw_sackblock *oldblock = &tp->sackhistory[i];
                tcp_seq old_start = oldblock->rblk_start;
                tcp_seq old_end = oldblock->rblk_end;

                if (SEQ_LT(end, old_start) || SEQ_GT(start, old_end)) {
                        /* Case 0:  no overlap.  Copy old block. */
                        copy[cindex++] = *oldblock;
                        continue;
                }

                if (SEQ_GEQ(start, old_start) && SEQ_LEQ(end, old_end)) {
                        /* Cases 1 & 2.  Move block to front of history. */
                        int j;

                        start = old_start;
                        end = old_end;
                        /* no need to check rest of blocks */
                        for (j = i + 1; j < tp->nsackhistory; ++j)
                                copy[cindex++] = tp->sackhistory[j];
                        break;
                }

                if (SEQ_GEQ(old_end, start) && SEQ_LT(old_start, start)) {
                        /* Case 4:  extend start of new block. */
                        start = old_start;
                } else if (SEQ_GEQ(end, old_start) && SEQ_GT(old_end, end)) {
                        /* Case 5: extend end of new block */
                        end = old_end;
                } else {
                        /* Case 3.  Delete old block by not copying it. */
                        KASSERT(SEQ_LEQ(start, old_start) &&
                                SEQ_GEQ(end, old_end),
                            ("bad logic: old [%u, %u), new [%u, %u)",
                             old_start, old_end, start, end));
                }
        }

        /* insert new block */
        tp->sackhistory[0].rblk_start = start;
        tp->sackhistory[0].rblk_end = end;
        cindex = min(cindex, MAX_SACK_REPORT_BLOCKS - 1);
        for (i = 0; i < cindex; ++i)
                tp->sackhistory[i + 1] = copy[i];
        tp->nsackhistory = cindex + 1;
        tcp_sack_dump_history("after tcp_sack_update_reported_history", tp);
}

/*
 * Fill in SACK report to return to data sender.
 */
void
tcp_sack_fill_report(struct tcpcb *tp, u_char *opt, u_int *plen)
{
        u_int optlen = *plen;
        uint32_t *lp = (uint32_t *)(opt + optlen);
        uint32_t *olp;
        tcp_seq hstart = tp->rcv_nxt, hend;
        int nblocks;

        KASSERT(TCP_MAXOLEN - optlen >=
            TCPOLEN_SACK_ALIGNED + TCPOLEN_SACK_BLOCK,
            ("no room for SACK header and one block: optlen %d", optlen));

        olp = lp++;
        optlen += TCPOLEN_SACK_ALIGNED;

        tcp_sack_ack_history(tp);
        if (tp->reportblk.rblk_start != tp->reportblk.rblk_end) {
                *lp++ = htonl(tp->reportblk.rblk_start);
                *lp++ = htonl(tp->reportblk.rblk_end);
                optlen += TCPOLEN_SACK_BLOCK;
                hstart = tp->reportblk.rblk_start;
                hend = tp->reportblk.rblk_end;
                if (tp->t_flags & TF_ENCLOSESEG) {
                        KASSERT(TCP_MAXOLEN - optlen >= TCPOLEN_SACK_BLOCK,
                            ("no room for enclosing SACK block: oplen %d",
                            optlen));
                        *lp++ = htonl(tp->encloseblk.rblk_start);
                        *lp++ = htonl(tp->encloseblk.rblk_end);
                        optlen += TCPOLEN_SACK_BLOCK;
                        hstart = tp->encloseblk.rblk_start;
                        hend = tp->encloseblk.rblk_end;
                }
                if (SEQ_GT(hstart, tp->rcv_nxt))
                        tcp_sack_update_reported_history(tp, hstart, hend);
        }
        if (tcp_do_smartsack && (tp->t_flags & TF_SACKLEFT)) {
                /* Fill in from left!  Walk re-assembly queue. */
                struct tseg_qent *q;

                q = LIST_FIRST(&tp->t_segq);
                while (q != NULL &&
                    TCP_MAXOLEN - optlen >= TCPOLEN_SACK_BLOCK) {
                        *lp++ = htonl(q->tqe_th->th_seq);
                        *lp++ = htonl(q->tqe_th->th_seq + q->tqe_len);
                        optlen += TCPOLEN_SACK_BLOCK;
                        q = LIST_NEXT(q, tqe_q);
                }
        } else {
                int n = 0;

                /* Fill in SACK blocks from right side. */
                while (n < tp->nsackhistory &&
                    TCP_MAXOLEN - optlen >= TCPOLEN_SACK_BLOCK) {
                        if (tp->sackhistory[n].rblk_start != hstart) {
                                *lp++ = htonl(tp->sackhistory[n].rblk_start);
                                *lp++ = htonl(tp->sackhistory[n].rblk_end);
                                optlen += TCPOLEN_SACK_BLOCK;
                        }
                        ++n;
                }
        }
        tp->reportblk.rblk_start = tp->reportblk.rblk_end;
        tp->t_flags &= ~(TF_DUPSEG | TF_ENCLOSESEG | TF_SACKLEFT);
        nblocks = (lp - olp - 1) / 2;
        *olp = htonl(TCPOPT_SACK_ALIGNED |
                     (TCPOLEN_SACK + nblocks * TCPOLEN_SACK_BLOCK));
        *plen = optlen;
}