From the application viewpoint, there are two sets of commands that cause network traffic—any I/O command, such as Open, which contains a UNC name—and WNet commands. UNC commands are sent to the MUP, which is in the Microsoft Windows NT kernel, where it finds a UNC provider or redirector that makes a connection to the specified UNC name. WNet commands are passed to the MPR, which passes the request to each redirector in turn until one is found that can satisfy the request. One computer gains access to another computer by a redirector. Microsoft Windows NT ships with a redirector that allows connection to Microsoft LAN Manager, LAN Server, and Microsoft Networks servers. This redirector communicates to the protocol stacks to which it is bound by the TDI layer. The TDI layer is a boundary layer between the file system modules and the network protocol stacks. Boundary layers are used to provide a unified platform for others to develop "plug-and-play" components. Microsoft Windows NT ships with four protocol stacks: TCP/IP, NetBEUI, NWLink and DLC. NDIS 3.0 provides another boundary layer that makes interoperability between components at different layers easier. In addition to providing file- and print-sharing capabilities, Microsoft Windows NT provides five mechanisms for building distributed applications. Named pipes, mailslots, NetBIOS, Sockets, and RPC can all be used. RPC is the most portable mechanism. RPCs use other IPC mechanisms to transfer functions and data between client and server computers.