Subject: RE: [Sipping] new draft-ietf-sipping-isup-05

From: William Marshall <[email-masked]>
Date: Fri, 9 Aug 2002 09:22:00 -0400 (EDT)

The latest version of this draft has gone a long way toward
consistency with the offer/answer model of rfc3261, thereby
allowing integration with the additional procedures for preconditions,
and resolving the in-band vs. local ringback issue.  However, I 
don't think it is there yet.

With in-band ringback there is a race condition with the 180-Ringing
provisional response; the UAC may receive the 180 response and
start a local ringback, then start receiving media and preempt it
with the in-band ringback.  

This is not an acceptible user experience.

The UAS in this case knows that it will be generating a media
stream as soon as it sends the 180, so it could clearly indicate
this in the signaling message.  I believe use of 183 instead of 180
was discussed in Yokohama, but isup-05 stayed with 180.

I'd like to propose a convention on use of the Alert-Info header
in a 180-Ringing response, where one particular value of the header
parameter indicates to the UAC that in-band media will be coming
and that it should not start a local ringback.  This approach seems
completely backwards compatible with all existing systems, and
those that understand the convention can do the proper thing.
Other solutions solicited.

Bill Marshall
[email-masked]

_______________________________________________
Sipping mailing list  https://www1.ietf.org/mailman/listinfo/sipping
This list is for NEW development of the application of SIP
Use [email-masked] for questions on current sip
Use [email-masked] for new developments of core SIP

---

From: Michael Hammer <[email-masked]>
Date: Fri, 09 Aug 2002 10:14:15 -0400

Bill,

In general, I agree that the indication of alerting and how the alerting is 
to be provided needs to be explicitly signaled for both the encapsulated 
ISUP case and the no-ISUP case.  That is, it requires an explicit element 
in both the SIP headers as it would be in the encapsulated ISUP.  Becasue 
the SDP may be part of the independent Offer/Answer and not related to 
providing tones and announcements, a call signaling element indicating 
local generation versus inband would avoid making likely false assumptions.

Related to this, section 8.2.3 "18x received":

"Upon receipt of a 18x response without any SDP, the gateway SHOULD 
generates [sic] the ringback tone to be heard by the caller on the PSTN 
side (unless the gateway knows that ringback will be provided by some other 
device on the PSTN side)."

It is possible that an ACM or CPG with "no indication" triggered this 18x.

Mike