The present document provides the stage 3 specification of the Gq interface. The functional requirements and the stage 2 specifications of the Gq interface are contained in 3GPP TS 23.002 [2] and 3GPP TS 23.207 [3]. The Gq interface is used for session based policy set-up information exchange between the Policy Decision Function (PDF) and the Application Function (AF). Whenever it is possible the present document specifies the requirements for the protocol by reference to specifications produced by the IETF within the scope of Diameter. Where this is not possible@ extensions to Diameter are defined within the present document.