http://www.rfc-editor.org/rfc/rfc7433.txt
RFC 7433: A Mechanism for Transporting User-to-User Call Control Information in SIP, A. Johnston, J. Rafferty2015-01-01T23:00:00-00:00There 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.