The Call control UUI Service for SIP (CUSS) working group is chartered to define a Session Initiation Protocol (SIP) mechanism for transporting call-control related user-to-user information (UUI) for applications using SIP to establish media sessions. The information transported is essentially opaque to SIP, but is tagged with the application that generated it and the encoding method.
One important application of this mechanism is interworking with the ISDN User to User Information Service. This application, defined by ITU-T Q.931, is extensively deployed in the PSTN today supporting such applications as contact centers, call centers, and automatic call distributors (ACDs). A major barrier to the movement of these applications to SIP is the lack of a standard mechanism to transport this information in SIP. For interworking with ISDN, minimal information about the content of the UUI is available to the PSTN-SIP gateways. In this special case, the application tag will indicate ISDN UUI Service 1 interworking.
Call control UUI is a small piece of application information conveyed between user agents during call control operations. As a result, the information must be conveyed with the INVITE transaction. A goal is to avoid a dependency on multipart MIME support. The mechanism must interwork with the existing ISDN service but should also be extensible for use by other applications and non-ISDN protocols.
Even though interworking with the PSTN is an important requirement, call control UUI can be exchanged between native SIP clients that do not have any ISUP support. Therefore, existing SIP-T encapsulation-based approaches defined in RFC3372 do not meet the requirements to transport this type of information.
Mechanisms based on the SIP INFO method, RFC2976, will not be considered by the working group since the UUI contents carry information that must be conveyed during session setup at the user agent - the information must be conveyed with the INVITE transaction. The information must be passed with the session setup request (INVITE), responses to that INVITE, or session termination requests. As a result, it is not possible to use INFO in these cases.
The working group will define guidelines for other applications to utilize the mechanism and the information that each application must specify to utilize the mechanism. New applications of the mechanism will require a standards-track document.
The mechanism developed in this working group is applicable in the following situations:
1. The information is generated and consumed by an application during session setup using SIP, but the application is not necessarily even SIP aware.
2. The behavior of SIP entities that support it is not significantly changed (as discussed in Section 4 of RFC 5727).
3. User Agent Clients (UAC) and User Agent Servers (UAS) are the generator and consumer of the UUI data. Proxies may route based on the application tag.
4. Intermediary elements or proxies can remove or insert UUI information
This mechanism is not applicable in the following situations:
1. The behavior of SIP entities that support it is significantly changed (as discussed in Section 4 of RFC 5727).
2. The information is generated and consumed at the SIP layer by SIP elements.
3. SIP elements besides the UAC and UAS might be interested in consuming (beyond adding or removing) the information.
4. There are specific privacy issues involved with the information being transported (e.g., geolocation or emergency-related information).
The group will produce:
- A problem statement and requirements document for implementing a SIP call control UUI mechanism.
- A specification of the SIP extension to best meet those requirements.
- An application usage specification for the ISDN UUI Service.
Problem statement and requirements document to IESG (Informational)
SIP call control UUI specification to IESG (PS)
ISDN UUI Service application usage specification to IESG (PS)