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Concluded WG Call Control UUI Service for SIP (cuss)

Note: The data for concluded WGs is occasionally incorrect.

WG Name Call Control UUI Service for SIP
Acronym cuss
Area Real-time Applications and Infrastructure Area (rai)
State Concluded
Charter charter-ietf-cuss-01 Approved
Document dependencies
Additional resources Issue tracker, Wiki
Personnel Chairs Enrico Marocco, Vijay K. Gurbani
Area Director Alissa Cooper
Mailing list Address
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Final Charter for Working Group

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

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

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

  • An application usage specification for the ISDN UUI Service.


Date Milestone Associated documents
Nov 2011 ISDN UUI Service application usage specification to IESG (PS)
Oct 2011 SIP call control UUI specification to IESG (PS)
Aug 2011 Problem statement and requirements document to IESG (Informational)