[ns] sack1 tcp bug: pipe_ being set incorrectly

[Mark Allman <mallman@grc.nasa.gov>]

Ethan Blanton and I believe we have identified a bug in the one-way
ns SACK code.  Specifically, we are looking at this portion of
Sack1TcpAgent::dupack_action():

sack_action:
	recover_ = maxseq_;
	last_cwnd_action_ = CWND_ACTION_DUPACK;
	pipe_ = int(cwnd_) - NUMDUPACKS;
	slowdown(CLOSE_SSTHRESH_HALF|CLOSE_CWND_HALF);
	reset_rtx_timer(1,0);
	fastrecov_ = TRUE;
	scb_.MarkRetran(highest_ack_+1);
	output(last_ack_ + 1, TCP_REASON_DUPACK);	// from top
	/*
	 * If dynamically adjusting NUMDUPACKS, record information
	 *  at this point.
	 */
	return;

We believe the "pipe_ = ..." line above should be changed to:

	pipe_ = maxseq_ - highest_ack_ - NUMDUPACKS;

as it appears elsewhere in the same function.  Making the pipe_
depend on cwnd_ does not account for the fact that cwnd_ can grow
much larger than the maximum (advertised) TCP window.  In this case,
pipe_ becomes much larger than the number of segments that are
actually outstanding which can prevent the sender from transmitting
new segments during recovery.

allman


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