rexmit.c

/*
 * Copyright (c) 1994, 1995, 1996, 1997, 1998, 1999, 2000, 2001,
 *               2002, 2003, 2004
 *	Ohio University.
 *
 * ---
 * 
 * Starting with the release of tcptrace version 6 in 2001, tcptrace
 * is licensed under the GNU General Public License (GPL).  We believe
 * that, among the available licenses, the GPL will do the best job of
 * allowing tcptrace to continue to be a valuable, freely-available
 * and well-maintained tool for the networking community.
 *
 * Previous versions of tcptrace were released under a license that
 * was much less restrictive with respect to how tcptrace could be
 * used in commercial products.  Because of this, I am willing to
 * consider alternate license arrangements as allowed in Section 10 of
 * the GNU GPL.  Before I would consider licensing tcptrace under an
 * alternate agreement with a particular individual or company,
 * however, I would have to be convinced that such an alternative
 * would be to the greater benefit of the networking community.
 * 
 * ---
 *
 * This file is part of Tcptrace.
 *
 * Tcptrace was originally written and continues to be maintained by
 * Shawn Ostermann with the help of a group of devoted students and
 * users (see the file 'THANKS').  The work on tcptrace has been made
 * possible over the years through the generous support of NASA GRC,
 * the National Science Foundation, and Sun Microsystems.
 *
 * Tcptrace is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License as published by
 * the Free Software Foundation; either version 2 of the License, or
 * (at your option) any later version.
 *
 * Tcptrace is distributed in the hope that it will be useful, but
 * WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
 * General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with Tcptrace (in the file 'COPYING'); if not, write to the
 * Free Software Foundation, Inc., 59 Temple Place, Suite 330, Boston,
 * MA 02111-1307 USA
 * 
 * Author:	Shawn Ostermann
 * 		School of Electrical Engineering and Computer Science
 * 		Ohio University
 * 		Athens, OH
 *		ostermann@cs.ohiou.edu
 *		http:https://www.tcptrace.org/
 */
#include "tcptrace.h"
static char const GCC_UNUSED copyright[] =
    "@(#)Copyright (c) 2004 -- Ohio University.\n";
static char const GCC_UNUSED rcsid[] =
    "@(#)$Header$";


/* 
 * rexmit.c -- Determine if a segment is a retransmit and perform RTT stats
 * 
 * Author:	Sita Menon
 * 		Computer Science Department
 * 		Ohio University
 * Date:	Tue Nov  1, 1994
 */




/*
This function rexmit() checks to see if a particular packet
is a retransmit. It returns 0 if it isn't a retransmit and 
returns the number of bytes retransmitted if it is a retransmit - 
considering the fact that it might be a partial retransmit. 
It can also keep track of packets that come out of order.
*/



/* locally global variables*/


/* local routine definitions*/
static void insert_seg_between (quadrant *,
				segment *,
				segment *,
				segment *);
static void collapse_quad (quadrant *);
static segment *create_seg (seqnum,
			    seglen);
static quadrant *whichquad (seqspace *,
			    seqnum);
static quadrant *create_quadrant (void);
static int addseg (tcb *,
		   quadrant *,
		   seqnum,
		   seglen,
		   Bool *);
static void rtt_retrans (tcb *,
			 segment *);
static enum t_ack rtt_ackin (tcb *,
			     segment *,
			     Bool rexmit);
static void dump_rtt_sample (tcb *,
			     segment *,
			     double);
static void graph_rtt_sample (tcb *,
			      segment *,
			      unsigned long);



/*
 * rexmit: is the specified segment a retransmit?
 *   returns: number of retransmitted bytes in segment, 0 if not a rexmit
 *            *pout_order to to TRUE if segment is out of order
 */
int
rexmit (tcb * ptcb,
	seqnum seq,
	seglen len,
	Bool * pout_order)
{
    seqspace *sspace = ptcb->ss;
    seqnum seq_last = seq + len - 1;
    quadrant *pquad;
    int rexlen = 0;

    /* unless told otherwise, it's IN order */
    *pout_order = FALSE;

    /* see which quadrant it starts in */
    pquad = whichquad (sspace, seq);

    /* add the new segment into the segment database */
    if (BOUNDARY (seq, seq_last)) {
	/* lives in two different quadrants (can't be > 2) */
	seqnum seq1, seq2;
	u_long len1, len2;

	/* in first quadrant */
	seq1 = seq;
	len1 = LAST_SEQ (QUADNUM (seq1)) - seq1 + 1;
	rexlen = addseg (ptcb, pquad, seq1, len1, pout_order);

	/* in second quadrant */
	seq2 = FIRST_SEQ (QUADNUM (seq_last));
	len2 = len - len1;
	rexlen += addseg (ptcb, pquad->next, seq2, len2, pout_order);
    } else {
	rexlen = addseg (ptcb, pquad, seq, len, pout_order);
    }

    return (rexlen);
}


/********************************************************************/
static int
addseg (tcb * ptcb,
	quadrant * pquad,
	seqnum thisseg_firstbyte,
	seglen len,
	Bool * pout_order)
{
    seqnum thisseg_lastbyte = thisseg_firstbyte + len - 1;
    segment *pseg;
    segment *pseg_new;
    int rexlen = 0;
    Bool split = FALSE;

    /* check each segment in the segment list */
    pseg = pquad->seglist_head;

    /* (optimize expected case, it just goes at the end) */
    if (pquad->seglist_tail &&
	(thisseg_firstbyte > pquad->seglist_tail->seq_lastbyte))
	pseg = NULL;
    for (; pseg != NULL; pseg = pseg->next) {
	if (thisseg_firstbyte > pseg->seq_lastbyte) {
	    /* goes beyond this one */
	    continue;
	}

	if (thisseg_firstbyte < pseg->seq_firstbyte) {
	    /* starts BEFORE this recorded segment */

	    /* if it also FINISHES before this segment, then it's */
	    /* out of order (otherwise it's a resend the collapsed */
	    /* multiple segments into one */
	    if (thisseg_lastbyte < pseg->seq_lastbyte)
		*pout_order = TRUE;

	    /* make a new segment record for it */
	    pseg_new = create_seg (thisseg_firstbyte, len);
	    insert_seg_between (pquad, pseg_new, pseg->prev, pseg);

	    /* see if we overlap the next segment in the list */
	    if (thisseg_lastbyte < pseg->seq_firstbyte) {
		/* we don't overlap, so we're done */
		return (rexlen);
	    } else {
		/* overlap him, split myself in 2 */

		/* adjust new piece to mate with old piece */
		pseg_new->seq_lastbyte = pseg->seq_firstbyte - 1;

		/* pretend to be just the second half of this segment */
		pseg_new->seq_lastbyte = pseg->seq_firstbyte - 1;
		thisseg_firstbyte = pseg->seq_firstbyte;
		len = thisseg_lastbyte - thisseg_firstbyte + 1;

		/* fall through */
	    }
	}

	/* no ELSE, we might have fallen through */
	if (thisseg_firstbyte >= pseg->seq_firstbyte) {
	    /* starts within this recorded sequence */
	    ++pseg->retrans;
	    if (!split)
		rtt_retrans (ptcb, pseg);	/* must be a retransmission */

	    if (thisseg_lastbyte <= pseg->seq_lastbyte) {
		/* entirely contained within this sequence */
		rexlen += len;
		return (rexlen);
	    }
	    /* else */
	    /* we extend beyond this sequence, split ourself in 2 */
	    /* (pretend to be just the second half of this segment) */
	    split = TRUE;
	    rexlen += pseg->seq_lastbyte - thisseg_firstbyte + 1;
	    thisseg_firstbyte = pseg->seq_lastbyte + 1;
	    len = thisseg_lastbyte - thisseg_firstbyte + 1;
	}
    }


    /* if we got to the end, then it doesn't go BEFORE anybody, */
    /* tack it onto the end */
    pseg_new = create_seg (thisseg_firstbyte, len);
    insert_seg_between (pquad, pseg_new, pquad->seglist_tail, NULL);

    return (rexlen);
}



/**********************************************************************/
static segment *
create_seg (seqnum seq,
	    seglen len)
{
    segment *pseg;

    pseg = (segment *) MallocZ (sizeof (segment));

    pseg->time = current_time;
    pseg->seq_firstbyte = seq;
    pseg->seq_lastbyte = seq + len - 1;

    return (pseg);
}

/**********************************************************************/
static quadrant *
create_quadrant (void)
{
    quadrant *pquad;

    pquad = (quadrant *) MallocZ (sizeof (quadrant));

    return (pquad);
}

/********************************************************************/

static quadrant *
whichquad (seqspace * sspace,
	   seqnum seq)
{
    quadnum qid = QUADNUM (seq);
    quadrant *pquad;
    int qix;
    int qix_next;
    int qix_opposite;
    int qix_prev;

    /* optimize expected case, it's all set up correctly already */
    qix = qid - 1;
    if ((pquad = sspace->pquad[qix]) && pquad->next && pquad->prev)
	return (pquad);

    /* determine indices of "neighbor" quadrants */
    qix_next = (qix + 1) % 4;
    qix_opposite = (qix + 2) % 4;
    qix_prev = (qix + 3) % 4;

    /* make sure that THIS quadrant exists */
    if (sspace->pquad[qix] == NULL) {
	sspace->pquad[qix] = create_quadrant ();
    }

    /* make sure that the quadrant AFTER this one exists */
    if (sspace->pquad[qix_next] == NULL) {
	sspace->pquad[qix_next] = create_quadrant ();
    }

    /* make sure that the quadrant BEFORE this one exists */
    if (sspace->pquad[qix_prev] == NULL) {
	sspace->pquad[qix_prev] = create_quadrant ();
    }

    /* clear out the opposite side, we don't need it anymore */
    if (sspace->pquad[qix_opposite] != NULL) {
	freequad (&sspace->pquad[qix_opposite]);

	sspace->pquad[qix_opposite] = NULL;
    }

    /* set all the pointers */
    sspace->pquad[qix]->prev = sspace->pquad[qix_prev];
    sspace->pquad[qix]->next = sspace->pquad[qix_next];
    sspace->pquad[qix_next]->prev = sspace->pquad[qix];
    sspace->pquad[qix_prev]->next = sspace->pquad[qix];
    sspace->pquad[qix_next]->next = NULL;
    sspace->pquad[qix_prev]->prev = NULL;

    return (sspace->pquad[qix]);
}



/*********************************************************************/
static void
collapse_quad (quadrant * pquad)
{
    Bool freed;
    segment *pseg;
    segment *tmpseg;

    if ((pquad == NULL) || (pquad->seglist_head == NULL))
	return;

    pseg = pquad->seglist_head;
    while (pseg != NULL) {
	freed = FALSE;
	if (pseg->next == NULL)
	    break;

	/* if this segment has not been ACKed, then neither have the */
	/* ones that follow, so no need to continue */
	if (!pseg->acked)
	    break;

	/* if this segment and the next one have both been ACKed and they */
	/* "fit together", then collapse them into one (larger) segment   */
	if (pseg->acked && pseg->next->acked &&
	    (pseg->seq_lastbyte + 1 == pseg->next->seq_firstbyte)) {
	    pseg->seq_lastbyte = pseg->next->seq_lastbyte;

	    /* the new ACK count is the ACK count of the later segment */
	    pseg->acked = pseg->next->acked;

	    /* the new "transmit time" is the greater of the two */
	    if (tv_gt (pseg->next->time, pseg->time))
		pseg->time = pseg->next->time;

	    tmpseg = pseg->next;
	    pseg->next = pseg->next->next;
	    if (pseg->next != NULL)
		pseg->next->prev = pseg;
	    if (tmpseg == pquad->seglist_tail)
		pquad->seglist_tail = pseg;
	    free (tmpseg);
	    freed = TRUE;
	}

	if (!freed)
	    pseg = pseg->next;
	/* else, see if the next one also can be collapsed into me */
    }

    /* see if the quadrant is now "full" */
    if ((pquad->seglist_head->seq_lastbyte -
	 pquad->seglist_head->seq_firstbyte + 1) == QUADSIZE) {
	pquad->full = TRUE;
    }
}


static void
insert_seg_between (quadrant * pquad,
		    segment * pseg_new,
		    segment * pseg_before,
		    segment * pseg_after)
{
    /* fix forward pointers */
    pseg_new->next = pseg_after;
    if (pseg_after != NULL) {
	pseg_after->prev = pseg_new;
    } else {
	/* I'm the tail of the list */
	pquad->seglist_tail = pseg_new;
    }

    /* fix backward pointers */
    pseg_new->prev = pseg_before;
    if (pseg_before != NULL) {
	pseg_before->next = pseg_new;
    } else {
	/* I'm the head of the list */
	pquad->seglist_head = pseg_new;
    }
}


static enum t_ack
rtt_ackin (tcb * ptcb,
	   segment * pseg,
	   Bool rexmit_prev)
{
    double etime_rtt;
    enum t_ack ret;

    u_long current_size = 0;

    /* how long did it take */
    etime_rtt = elapsed (pseg->time, current_time);

    if (rexmit_prev) {
	/* first, check for the situation in which the segment being ACKed */
	/* was sent a while ago, and we've been piddling around */
	/* retransmitting lost segments that came before it */
	ptcb->rtt_last = 0.0;	/* don't use this sample, it's very long */
	etime_rtt = 0.0;

	++ptcb->rtt_nosample;	/* no sample, even though not ambig */
	ret = NOSAMP;
    } else if (pseg->retrans == 0) {
	ptcb->rtt_last = etime_rtt;

	if ((ptcb->rtt_min == 0) || (ptcb->rtt_min > etime_rtt))
	    ptcb->rtt_min = etime_rtt;

	if (ptcb->rtt_max < etime_rtt)
	    ptcb->rtt_max = etime_rtt;

	ptcb->rtt_sum += etime_rtt;
	ptcb->rtt_sum2 += etime_rtt * etime_rtt;
	++ptcb->rtt_count;

	/* Collecting stats for full size segments */
	/* Calculate the current_size of the segment,
	   taking care of possible sequence space wrap around */

	if (pseg->seq_lastbyte > pseg->seq_firstbyte)
	    current_size = pseg->seq_lastbyte - pseg->seq_firstbyte + 1;
	else
	    /* MAX_32 is 0x1,0000,0000
	       So we don't need the "+ 1" while calculating the size here */
	    current_size =
		(MAX_32 - pseg->seq_firstbyte) + pseg->seq_lastbyte;

	if (!ptcb->rtt_full_size || (ptcb->rtt_full_size < current_size)) {
	    /* Found a bigger segment.. Reset all stats. */
	    ptcb->rtt_full_size = current_size;

	    ptcb->rtt_full_min = etime_rtt;
	    ptcb->rtt_full_max = etime_rtt;
	    ptcb->rtt_full_sum = etime_rtt;
	    ptcb->rtt_full_sum2 = (etime_rtt * etime_rtt);
	    ptcb->rtt_full_count = 1;
	} else if (ptcb->rtt_full_size == current_size) {
	    ++ptcb->rtt_full_count;

	    if ((ptcb->rtt_full_min == 0)
		|| (ptcb->rtt_full_min > etime_rtt))
		ptcb->rtt_full_min = etime_rtt;

	    if (ptcb->rtt_full_max < etime_rtt)
		ptcb->rtt_full_max = etime_rtt;

	    ptcb->rtt_full_sum += etime_rtt;
	    ptcb->rtt_full_sum2 += (etime_rtt * etime_rtt);
	}
	ret = NORMAL;
    } else {
	/* retrans, can't use it */
	if ((ptcb->rtt_min_last == 0) || (ptcb->rtt_min_last > etime_rtt))
	    ptcb->rtt_min_last = etime_rtt;

	if (ptcb->rtt_max_last < etime_rtt)
	    ptcb->rtt_max_last = etime_rtt;

	ptcb->rtt_sum_last += etime_rtt;
	ptcb->rtt_sum2_last += etime_rtt * etime_rtt;
	++ptcb->rtt_count_last;

	++ptcb->rtt_amback;	/* ambiguous ACK */

	/* numbers not useful for plotting/dumping */
	ptcb->rtt_last = 0.0;
	etime_rtt = 0.0;

	ret = AMBIG;
    }

    /* dump RTT samples, if asked */
    if (dump_rtt && (etime_rtt != 0.0)) {
	dump_rtt_sample (ptcb, pseg, etime_rtt);
    }

    /* plot RTT samples, if asked */
    if (graph_rtt && (pseg->retrans == 0)) {
	graph_rtt_sample (ptcb, pseg, etime_rtt);
    }

    return (ret);
}



static void
rtt_retrans (tcb * ptcb,
	     segment * pseg)
{
    double etime;

    if (!pseg->acked) {
	/* if it was acked, then it's been collapsed and these */
	/* are no longer meaningful */
	etime = elapsed (pseg->time, current_time);
	if (pseg->retrans > ptcb->retr_max)
	    ptcb->retr_max = pseg->retrans;

	if (etime > ptcb->retr_max_tm)
	    ptcb->retr_max_tm = etime;
	if ((ptcb->retr_min_tm == 0) || (etime < ptcb->retr_min_tm))
	    ptcb->retr_min_tm = etime;

	ptcb->retr_tm_sum += etime;
	ptcb->retr_tm_sum2 += etime * etime;
	++ptcb->retr_tm_count;
    }

    pseg->time = current_time;
}


enum t_ack
ack_in (tcb * ptcb,
	seqnum ack,
	unsigned tcp_data_length,
	u_long eff_win,
        int ignore)
{
    quadrant *pquad;
    quadrant *pquad_prev;
    segment *pseg;
    Bool changed_one = FALSE;
    Bool intervening_xmits = FALSE;
    timeval last_xmit = { 0, 0 };
    enum t_ack ret = 0;

    enum dup_ack_handling {
	BSD_VERSION = 1,	/* Handling of duplicate ack's
				   based on the specifications of
				   BSD code */
	LEGACY_VERSION = 2	/* Handling of duplicate ack's according to the 
				   old versions of "tcptrace" */
    };
    enum dup_ack_handling dup_ack_type;	/* default type is the code based on
					   BSD specifications */


    /* check each segment in the segment list for the PREVIOUS quadrant */
    pquad = whichquad (ptcb->ss, ack);
    pquad_prev = pquad->prev;
    for (pseg = pquad_prev->seglist_head; pseg != NULL; pseg = pseg->next) {
	if (!pseg->acked) {
	    ++pseg->acked;
	    changed_one = TRUE;
	    ++ptcb->rtt_cumack;

	    /* keep track of the newest transmission */
	    if (tv_gt (pseg->time, last_xmit))
		last_xmit = pseg->time;
	}
    }
    if (changed_one)
	collapse_quad (pquad_prev);

    /* check each segment in the segment list for the CURRENT quadrant */
    changed_one = FALSE;
    for (pseg = pquad->seglist_head; pseg != NULL; pseg = pseg->next) {
	if (ack <= pseg->seq_firstbyte) {
	    /* doesn't cover anything else on the list */
	    break;
	}

	/* keep track of the newest transmission */
	if (tv_gt (pseg->time, last_xmit))
	    last_xmit = pseg->time;

	/* (ELSE) ACK covers this sequence */
	if (pseg->acked) {
	    /* default will be the BSD version, it can be changed by giving 
	       '--turn_off_BSD_dupack' switch */
	    dup_ack_type = (dup_ack_handling) ? BSD_VERSION : LEGACY_VERSION;

	    /* default type is the specifications based on BSD code */
	    switch (dup_ack_type) {
	    case LEGACY_VERSION:
		if (ack == (pseg->seq_lastbyte + 1)) {
		    ++pseg->acked;	/* already acked this one */
		    ++ptcb->rtt_dupack;	/* one more duplicate ack */
		    ret = CUMUL;
		    if (pseg->acked == 4) {
			/* some people say these CAN'T have data */
			if ((tcp_data_length == 0)
			    || triple_dupack_allows_data) {
			    ++ptcb->rtt_triple_dupack;
			    ret = TRIPLE;
			}
		    }
		}
		break;
	    case BSD_VERSION:
		/* For an acknowledgement to be considered as duplicate ACK in 
		   BSD version, following rules must be followed:
		   1) the received segment should contain the biggest ACK TCP 
		   has seen,
		   2) the length of the segment containing dup ack should be 0,
		   3) advertised window in this segment should not change,
		   4) and there must be some outstanding data */

		if ((ack == (pseg->seq_lastbyte + 1)) &&
		      (ack == ptcb->ptwin->ack) &&
		      (tcp_data_length == 0) &&
		      (eff_win == ptcb->ptwin->win_last) &&
		      (ptcb->owin_tot > 0)) {	
		    ++ptcb->rtt_dupack;
		    ret = CUMUL;

		    /* already acked this one */
		    ++pseg->acked;
		    if (pseg->acked == 4) {
			++ptcb->rtt_triple_dupack;
			ret = TRIPLE;
		    }
		} else
		    pseg->acked = 1;	/* received segment is not pure
					   duplicate acknowledgement */
	    }
	    continue;
	}
	/* ELSE !acked */

	++pseg->acked;
	changed_one = TRUE;

	if (ack == (pseg->seq_lastbyte + 1)) {
	    /* if ANY preceding segment was xmitted after this one,
	       the the RTT sample is invalid */
	    intervening_xmits = (tv_gt (last_xmit, pseg->time));

	    ret = rtt_ackin (ptcb, pseg, intervening_xmits);
	} else {
	    /* cumulatively ACKed */
	    ++ptcb->rtt_cumack;
	    ret = CUMUL;
	}
    }
    if (changed_one)
	collapse_quad (pquad);
    return (ret);
}


void
freequad (quadrant ** ppquad)
{
    segment *pseg;
    segment *pseg_next;

    pseg = (*ppquad)->seglist_head;
    while (pseg && pseg->next) {
	pseg_next = pseg->next;
	free (pseg);
	pseg = pseg_next;
    }
    if (pseg)
	free (pseg);
    free (*ppquad);
    *ppquad = NULL;
}


/* dump RTT samples in milliseconds */
static void
dump_rtt_sample (tcb * ptcb,
		 segment * pseg,
		 double etime_rtt)
{
    /* if the FILE is "-1", couldn't open file */
    if (ptcb->rtt_dump_file == (MFILE *) - 1) {
	return;
    }

    /* if the FILE is NULL, open file */
    if (ptcb->rtt_dump_file == (MFILE *) NULL) {
	MFILE *f;
	static char filename[15];

	snprintf (filename, sizeof (filename), "%s2%s%s",
		  ptcb->host_letter, ptcb->ptwin->host_letter,
		  RTT_DUMP_FILE_EXTENSION);

	if ((f = Mfopen (filename, "w")) == NULL) {
	    perror (filename);
	    ptcb->rtt_dump_file = (MFILE *) - 1;
	}

	if (debug)
	    fprintf (stderr, "RTT Sample file is '%s'\n", filename);

	ptcb->rtt_dump_file = f;
    }

    Mfprintf (ptcb->rtt_dump_file, "%lu %lu\n",
	      pseg->seq_firstbyte,
	      (int) (etime_rtt / 1000) /* convert from us to ms */ );
}



/* graph RTT samples in milliseconds */
static void
graph_rtt_sample (tcb * ptcb,
		  segment * pseg,
		  unsigned long etime_rtt)
{
    char title[210];

    /* if the FILE is NULL, open file */
    if (ptcb->rtt_plotter == NO_PLOTTER) {
	char *name_from, *name_to;
	if (ptcb == &ptcb->ptp->a2b) {
	    name_from = ptcb->ptp->a_endpoint;
	    name_to = ptcb->ptp->b_endpoint;
	} else {
	    name_from = ptcb->ptp->b_endpoint;
	    name_to = ptcb->ptp->a_endpoint;
	}
	snprintf (title, sizeof (title), "%s_==>_%s (rtt samples)",
		  name_from, name_to);
	ptcb->rtt_plotter =
	    new_plotter (ptcb, NULL, title, "time", "rtt (ms)",
			 RTT_GRAPH_FILE_EXTENSION);
	plotter_perm_color (ptcb->rtt_plotter, "red");

	if (graph_time_zero) {
	    /* set graph zero points */
	    plotter_nothing (ptcb->rtt_plotter, current_time);
	}
	ptcb->rtt_line = new_line (ptcb->rtt_plotter, "rtt", "red");
    }

    if (etime_rtt <= 1)
	return;

    extend_line (ptcb->rtt_line, current_time, (int) (etime_rtt / 1000));
}

Bool IsRTO(tcb *ptcb, seqnum s) {
  quadrant *pquad = whichquad(ptcb->ss,s);
  segment *pseg;

  for (pseg = pquad->seglist_head; pseg != NULL; pseg = pseg->next) {
    if (s == (pseg->seq_lastbyte+1)) {
      if (pseg->acked < 4) return TRUE;
      else return FALSE;
    }
  }

  return TRUE;
}