IETF RFC 7433 PDF

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.