TransNexus hereUare GRIC The Open Settlement Protocol (OSP)[1], an open standard from the European Telecommunications Standards Institute, specifies a means by which IP telephony equipment in one administrative domain may request access to IP telephony equipment (including, but not limited to: Gateways, Proxy Servers, Gatekeepers, etc.) in another administrative domain. OSP grants such access by returning authorization tokens, which must then be passed to the destination IP telephony device during call signaling. In order to support access control via OSP, IP telephony signaling protocols must be capable of carrying these authorization tokens in an interoperable way. This memo defines just such a method for protocols, such as the Session Initiation Protocol[2], that can support carriage of MIME types during call signaling. This memo conforms to the requirements for MIME type registration defined in RFC 2048[3].