The MASSIVE-1 and MASSIVE-2 distributed virtual environment systems are based on an explicit model of user communication requirements known as the spatial model of interaction; this model addresses the general issue of managing communication in large space-based systems such as DVEs. The goals of the spatial model are to guide the distribution and presentation of information in a way which facilitates collaboration between participants (based on observed patterns of interaction in real-world environments) and which is also efficient and appropriate. The MASSIVE-1 system emphasized the spatial model's `aura' concept as the basis for a distinct shared service in an otherwise wholly unicast peer-to-peer system. This approach was effective for relatively small systems but could not be extended to make general use of multicast communication (to reduce total bandwidth requirements). The MASSIVE-2 system added the concept of `third-party objects' which support the definition and management of multiple multicast groups for updates and continuous media distribution.