Subject: [sipcore] Summary of issues around registering feature tags

From: Cullen Jennings <[email-masked]>
Date: Wed, 29 Apr 2009 11:13:12 -0600

3GPP has requested that IANA register two new feature tags
(g.3gpp.icsi-ref and g.3gpp.iari-ref) in the global tree. Per
RFC 2506 (Section 3.1.2), the policy for registrations in the
global tree is expert review.

This is the description of the feature tags to be registered:

"Each value of the Application Reference media feature-tag indicates the
software applications supported by the agent. The values for this tag
equal the IMS communication Service Identifier (ICSI) and IMS
Application Reference Identifier (IARI) values supported by
the agent. The Application Reference media feature tag is
defined to fulfill the requirements for forking to an appropriate UE  
when
multiple UEs are registered and dispatch to an appropriate application
within the UE based upon the IMS communication Service Identifier
(ICSI) and IMS Application Reference Identifier (IARI) values as
stated in 3GPP TS 23.228.  Multiple tag-values can be included in the
Application Reference media feature-tag."

Section 12.1 of RFC 3840 states that feature tags that are applicable to
SIP and whose meaning is only defined within that usage are to be
registered in the SIP tree. Since these feature tags are to be used only
within the context of SIP, they would need to be registered in the SIP
tree and not in the global tree. The policy for registrations in the SIP
tree is IETF Consensus. That is, registrations need to be documented in
an RFC approved by the IESG.

Regardless of the tree these feature tags would need to be registered
in, the expert reviewer indicated that this tags will not contain simple
features but rather more complex services or feature sets. Therefore,
the expert reviewer did not find it appropriate to register them because
Section 2.1 of RFC 2506 states that "media feature tags represent
individual and simple characteristics" and that "each media feature tag
identifies a single characteristic".

Additionally, Section 6 of
draft-ietf-sipping-service-identification-03.txt describes the perils of
performing declarative service identifications. The way these feature
tags are described make them look as if they were going to be used to
perform such declarative service identifications.

Consequently, 3GPP is encouraged to submit an Internet Draft describing
the use of these feature tags so that they can be registered in the SIP
tree. Such Internet Draft needs to address the issues discussed in
draft-ietf-sipping-service-identification-03.txt clarifying why these
tags will not be used to perform declarative service identifications.

Cullen <RAI AD>

Thank you to the people that helped prepare this summary.