In this paper we propose a software architecture for achieving completely transparent optimistic synchronization in the High-Level-Architecture (HLA), namely the emerging standard for the interoperability and the integration of heterogeneous simulators. The software architecture is based on a layer called MAgic State Manager (MASM), which we have designed and implemented for transparently handling state related operations (i.e. checkpointing and recovery), and on an additional software layer, we propose in this paper, acting as Rollback Controller (RC) and taking care of all the other operations (e.g. event retractions) required to support optimistic synchronization. Compared to existing proposals, which only partially move synchronization related operations outside the application level code, our proposal has the advantage of completely removing the need for modules in support of optimistic synchronization at the application level, thus strongly simplifying the application level programmer job.