Multimedia group communication emerges to focal interest at mobile devices, enriching voice or video conferencing and complex collaborative environments. The Internet uniquely provides the benefit of scalable, dynamic group communication services, vitally aiding commonly limited mobile terminals. The traditional Internet approach of Any Source Multicast (ASM) routing, though, remains hesitant to spread beyond limited, controlled environments. It is widely believed that simpler and more selective mechanisms for group distribution in Source Specific Multicast (SSM) will globally disseminate to many users of multicast infrastructure and services. Mobility management for the recent SSM standard is under debate -- SSM group session initiation up until now remains unsupported by the Session Initiation Protocol (SIP). In this paper we present straightforward extensions to SIP for negotiating SSM sessions. SIP protocol spec- ifications and semantic are compatibly extended without adding new SIP methods. We will introduce a multimedia communication software with distributed architecture as implementation reference. Furthermore mobility management of the underlying routing layer is discussed and evaluated on grounds of real-world Internet topologies.