A method and apparatus for interworking between a mobile network operator and an application provider are disclosed. A network application function (NAF) may be co-located with an OpenID provider such that an application server may communicate with the NAF to access a home subscriber server (HSS) via a bootstrapping server function (BSF). The interfaces between BSF and HSS, and between BSF and NAF may be enhanced to carry information that is available through Sh interface between the application server and the HSS. When the WTRU is roaming in a visited network, the application server may communicate with the visited network for charging and policing for serving the service request from the WTRU. The application server may be co-located with an NAF, and may authenticate the WTRU using Generic Bootstrapping Architecture, and may communicate with a BSF in a home network via an eZn-proxy function to access an HSS.