Extension of SIP for group-based registration

Proposed is an extension of the Session Initiation Protocol (SIP) to provide group-based registration (GBR). In the GBR scheme, to reduce the load on the registrar, a single registration is performed by the access point, not the multiple mobile nodes. For this purpose, a new `gbr` header is defined in the SIP REGISTER message. The proposed scheme is analysed using the Markov chain model. The GBR can alleviate the load on the registrar, compared to the existing SIP registration.