%% You should probably cite rfc7433 instead of this I-D.
@techreport{ietf-cuss-sip-uui-15,
    number =    {draft-ietf-cuss-sip-uui-15},
    type =      {Internet-Draft},
    institution =   {Internet Engineering Task Force},
    publisher = {Internet Engineering Task Force},
    note =      {Work in Progress},
    url =       {https://datatracker.ietf.org/doc/draft-ietf-cuss-sip-uui/15/},
    author =    {Alan Johnston and James Rafferty},
    title =     {{A Mechanism for Transporting User to User Call Control Information in SIP}},
    pagetotal = 19,
    year =      2014,
    month =     apr,
    day =       2,
    abstract =  {There is a class of applications which 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. 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.},
}