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From: Noritoshi Demizu <demizu@acm.org>
To: freebsd-net@FreeBSD.ORG
Subject: question on tcp_xmit_timer() of FreeBSD 4.0R
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I have a question about the 2nd argument "rtt" of tcp_xmit_timer()
of FreeBSD 4.0R.

Why are the 2nd arguments in following code calculated differently ?
i.e. for timestamp option, +1.

  l.1857, netinet/tcp_input.c, tcp_input()
		if (to.to_flag & TOF_TS)
			tcp_xmit_timer(tp, ticks - to.to_tsecr + 1);
		else if (tp->t_rtttime && SEQ_GT(th->th_ack, tp->t_rtseq))
			tcp_xmit_timer(tp, ticks - tp->t_rtttime);

Why is `1' subtracted from the 2nd argument "rtt" here ?

  l.2448, netinet/tcp_input.c, tcp_xmit_timer()
		delta = ((rtt - 1) << TCP_DELTA_SHIFT)
			- (tp->t_srtt >> (TCP_RTT_SHIFT - TCP_DELTA_SHIFT));

These codes works for 3.x and earlier versions.  But I am not sure
they are still OK for 4.0R.  I could not explain to students, so,,,
could somebody help us?

Thank you very much.

Noritoshi Demizu


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