tcp_notify() and the connection establishment timer

[Fernando Gont]

Folks,

I was trying to figure out what the BSD policy for aborting 
connection-establishment attempts was.

According to Stevens' TCPv2, when TCP sends the first SYN for establishing 
a connection, a 75-seconds timer is initialized. If the connection cannot 
be established before that 75-second period is over, the conenction will be 
aborted.

However, looking at the tcp_notify() function, I see the following code:

static struct inpcb *
tcp_notify(inp, error)
	struct inpcb *inp;
	int error;
{
	struct tcpcb *tp = (struct tcpcb *)inp->inp_ppcb;

	/*
	 * Ignore some errors if we are hooked up.
	 * If connection hasn't completed, has retransmitted several times,
	 * and receives a second error, give up now.  This is better
	 * than waiting a long time to establish a connection that
	 * can never complete.
	 */
	if (tp->t_state == TCPS_ESTABLISHED &&
	    (error == EHOSTUNREACH || error == ENETUNREACH ||
	     error == EHOSTDOWN)) {
		return inp;
	} else if (tp->t_state < TCPS_ESTABLISHED && tp->t_rxtshift > 3 &&
	    tp->t_softerror) {
		tcp_drop(tp, error);
		return (struct inpcb *)0;
	} else {
		tp->t_softerror = error;
		return inp;
	}
#if 0
	wakeup( &so->so_timeo);
	sorwakeup(so);
	sowwakeup(so);
#endif
}

Maybe I'm missing something, but I get the impression that, considering the 
value (six seconds) to which the RTO is initialized, that part that says

	} else if (tp->t_state < TCPS_ESTABLISHED && tp->t_rxtshift > 3 &&
	    tp->t_softerror) {
		tcp_drop(tp, error);
		return (struct inpcb *)0;

will never be executed, as the connection-establishment timer will always 
timeout before the evaluated condition becomes true.

Am I missing something? Or is it that this code is there just in case the 
initial RTO is reduced to such a value that, in that case, this code would 
kick in before the 75-seconds tconnection-establishment timer?

Thanks!

--
Fernando Gont
e-mail: fernando at gont.com.ar || fgont at acm.org