the sip - based multimedia communication system for providing mobility using lifelong numbers provides mobility through sip network service domains and a global domain . the sip network service domain comprises a user agent and an sip network server . the user agent transmits request / response messages between users to set up , correct , and cancel calls , and when contact information of the user agent is registered and registration of the contact information is cancelled , requests to be informed of state changes in registration information . the sip network server performs message routing between user agents and informs the contact information of the user agents during message routing . the global domain manages the sip network service domains and allocates global sip identifiers , that is , lifelong numbers to users to provide user - centered mobility .