The document discusses DCERPC and the Endpoint Mapper. It begins with an overview of how RPC works, including the roles of client and server stubs, runtime libraries, and transports. It then covers the concepts and functions of the Endpoint Mapper, which maintains information about RPC service endpoints and listens on port 135. The document outlines some key Endpoint Mapper functions like epmInsert, epmDelete, and epmLookup. It provides context on DCERPC, RPC, and the role of the Endpoint Mapper in facilitating communication between clients and servers.