| #### Network Services In general, DCE is layered over a transport level service, such as UDP (User Datagram Protocol), TCP (Transmission Control Protocol), or ISO TP0-TP4 transport protocols, which is accessed through a transport interface, such as sockets or XTI (X/Open Transport Interface). DCE assumes that all nodes participating in the DCE environment are physically connected by a highly available network. The network can be a Local Area Network (LAN), a Wide Area Network (WAN), or a combination of both. The DCE architecture supports different types of network protocol families. For example, DCE could be ported to run over Open Systems Interconnection (OSI) protocols. (The OSF DCE reference implementation runs over the Internet Protocol (IP) family.) However, in order for DCE systems to communicate with one another they must have at least one set of network protocols in common. For example, DCE is not designed to enable a node running only IP protocols to communicate with a node running only OSI protocols. Finally, DCE assumes the ability to identify a node with a unique network address, and the ability to identify a process with a network endpoint address (for example, a port or T-selector). ####