Every IDL-compiled interface specification has its own UUID associated with it, and the IDL-generated stub routines include this interface UUID with every operation request or return sent over the network by clients and servers. In this way receiving stubs ensure that they and the sending stubs are sharing exactly the same interface. If the interface UUIDs are different, or are not present, then the remote call will not be completed. But interface UUIDs, although they are required, play only a secondary role in a client's finding the interface (that is, finding a server that offers the interface); the main tool for this is NSI, which makes use of the DCE Directory Service, as explained later in this part of the topic.