xref: /macosx-10.9.5/xnu-2422.115.4/bsd/netinet/tcp_sack.c

/*
 * Copyright (c) 2004-2012 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*/

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 int 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");

extern struct zone *sack_hole_zone;

/*
 * 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_noblock(sack_hole_zone);
	if (hole == NULL)
		return NULL;

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

	tp->snd_numholes++;
	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--;
	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;

	/* 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).
 */
void
tcp_sack_doack(struct tcpcb *tp, struct tcpopt *to, tcp_seq th_ack,
	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;

	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 (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;
	}

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

	/*
	 * 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);
	}

	/*
	 * 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);
				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);
				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);
				cur->end = sblkp->start;
				cur->rxmit = SEQ_MIN(cur->rxmit, cur->end);
			} else {
				/*
				 * ACKs some data in middle of a hole; need to
				 * split current hole
				 */
				*newbytes_acked += (sblkp->end - sblkp->start);
				temp = tcp_sackhole_insert(tp, sblkp->end,
							   cur->end, cur);
				if (temp != NULL) {
					if (SEQ_GT(cur->rxmit, temp->rxmit)) {
						temp->rxmit = cur->rxmit;
						tp->sackhint.sack_bytes_rexmit
							+= (temp->rxmit
							    - temp->start);
					}
					cur->end = sblkp->start;
					cur->rxmit = SEQ_MIN(cur->rxmit,
							     cur->end);
				}
			}
		}
		tp->sackhint.sack_bytes_rexmit += (cur->rxmit - cur->start);
		/*
		 * Testing sblkp->start against cur->start tells us whether
		 * we're done with the sack block or the sack hole.
		 * Accordingly, we advance one or the other.
		 */
		if (SEQ_LEQ(sblkp->start, cur->start))
			cur = TAILQ_PREV(cur, sackhole_head, scblink);
		else
			sblkp--;
	}
}

/*
 * 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;
	tp->t_flags |= TF_ACKNOW;
	(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;
}