In this paper, we propose a service architecture that enables end users to establish multipoint multimedia communications without being conscious of one another's networking environment. The proposed architecture is composed of three conceptual models based on RM-ODP(Reference Model of Open Distributed Processing), those are concerned with enterprise, information and computational viewpoint, respectively. From the enterprise viewpoint, the Session Coordinator Role is introduced, which manages connections among end users in consideration of their terminal devices' capabilities. Furthermore, the Content Provider and Content Consumer Role, which respectively play the role of providing and consuming a multimedia content, are also introduced to represent these capabilities. These roles are embodied as service components from the computational viewpoint. From the information viewpoint, two concepts, session and content flow, are introduced to facilitate unified management of various multimedia communications. The concept, continuous session mobility, is also the key idea of the architecture. This enables end users to move around participating in a multimedia conferencing independently of their networking environments. We show the concrete model of realizing it by implementing a prototype system based on the architecture.