The High Level Architecture (HLA) is a popular standard for distributed simulation interoperability. The specification defines a distributed runtime infrastructure (RTI) but not its implementation. Consequently, heterogeneous RTIs do not interoperate. Furthermore, the specification does not include an explicit session layer or security. The SIP-RTI is a new model for RTI interoperability. A distributed conferencing mechanism, called the Conferencing Infrastructure (CI), is used below the RTI to provide generic communication services. RTI semantics are given in terms of the CI. The way in which the RTI uses the CI is known, therefore, other applications can interoperate with the RTI at the CI level. The CI uses the Session Initiation Protocol (SIP) to establish communication between CI nodes, which has the potential to solve other RTI issues as well. Ownership Management services (IEEE 1516) demonstrate the viability of the conferencing model to support HLA semantics. Interoperability between heterogeneous applications is demonstrated.