Subject: Re: [sipcore] Requesting assist with expert review of media feature tag

From: [email-masked] (Dale R. Worley)
Date: Tue, 10 Nov 2015 15:33:34 -0500

Paul Kyzivat <[email-masked]> writes:
> I have a recent request that concerns me. I am inclined to reject it, 
> but the decision is subjective, so I would like to have some discussion 
> on it first.
> 
> The request comes from 3gpp - it has had no action in ietf. It is for 
> the media feature tag g.3gpp.nw-init-ussi, with semantics defined in 
> 3GPP TS 24.390 ("Unstructured Supplementary Service Data (USSD) using IP 
> Multimedia (IM) Core Network (CN) subsystem IMS; Stage 3")

> - a new mechanism is introduced for passing alert indications, rather 
> than reusing the defined mechanism of Alert-Info. (RFC3261 and RFC7462)

I think you're right here.  The alertPattern values that 24.390 uses
(which are defined in 29.002) could be straightforwardly mapped into a
set of 8 alert URNs (in two categories).  Section 4.5.5.1 of 24.390
warns:

    The USSI AS should not include an Alert-Info header field into the
    initial INVITE request.  ...  Information provided in the Alert-Info
    header field can be in conflict with the content of the
    <alertingPattern> element of the application/vnd.3gpp.ussd+xml MIME
    body of the initial INVITE request.

But that sort of problem already exists; In an INVITE the <ussd-string>
element in the body must match the user part of the request-URI.

> - it establishes a media-less INVITE dialog, for the purpose of 
> exchanging messages in the signaling, primarily using INFO. IMO this is 
> inappropriate, for the same reason session-mode messaging via MESSAGE 
> was rejected, in favor of MSRP. As I understand the service being 
> provided, it is a specialized application of session mode instant messaging.

This is a messier question, IMHO.  As a matter of principle, you're
right, of course.  But the additional protocol machinery needed to set
up an MSRP connection is sort of burdensome, and the IMS infrastructure
might not have facilities for end-to-end TCP connections.  It's possible
that using MSRP would require many of the B2BUAs in the IMS architecture
to implement MSRP relays.

Dale