A Mechanism for Transporting User-to-User Call Control Information in SIP, January 2015
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There is a class of applications that benefit from using SIP to exchange User-to-User Information (UUI) data during session establishment. This information, known as call control UUI data, is a small piece of data inserted by an application initiating the session and utilized by an application accepting the session. The syntax and semantics for the UUI data used by a specific application are defined by a UUI package. This UUI data is opaque to SIP and its function is unrelated to any basic SIP function. This document defines a new SIP header field, User-to-User, to transport UUI data, along with an extension mechanism.
For the definition of Status, see RFC 2026.
For the definition of Stream, see RFC 8729.