[asterisk-dev] session-refresher ignored if UAC supports but does not request timers.

[Dave WOOLLEY]

The same rule in the RFC covers the case where Session-Expires is sent, but no refresher is sent as covers the case where no Session-Expires is sent at all, as long as session timers are supported:

       UAC supports?  refresher parameter  refresher parameter
                           in request           in response
       -------------------------------------------------------
             Y                none             uas or uac

The definition of the refresher parameter in the request being none is: "there was no  refresher parameter in the request".  There is no mention of Session-Expires headers, and the table also uses "none" in this column for the timers unsupported case, in which case Session-Expires would be invalid.

Asterisk does not allow uac (SESSION_TIMER_REFRESHER_THEM) to be set unless the peer included a Session-Expires header.  Whilst it is a local policy choice which is sent, to me, this breaches the least surprise principle, and prevents the use of a legal combination.    Note, I don't actually have a production environment use for this; I was using it to test a back port of the fix to the bug that resulted in refresher being interpreted relative to the direction of the original INVITE.

This is the relevant code in today's branch 13:

	} else {
		/* The UAC did not request session-timers.  Asterisk (UAS), will now decide
		(based on session-timer-mode in sip.conf) whether to run session-timers for
		this session or not. */
		switch (st_get_mode(p, 1)) {
		case SESSION_TIMER_MODE_ORIGINATE:
			st_active = TRUE;
			st_interval = st_get_se(p, TRUE);
			tmp_st_ref = SESSION_TIMER_REFRESHER_US;
			p->stimer->st_active_peer_ua = (p->sipoptions & SIP_OPT_TIMER) ? TRUE : FALSE;
			break;

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