The Windows NT networking model was designed to provide a platform on which other vendors can develop distributed applications. The NDIS boundary helps to do this by providing a unified interface at a significant break point in the model. At another significant breakpoint, namely the Session Layer of the OSI model, Windows NT includes another boundary layer. The TDI provides a common interface for networking components that communicate at the Session Layer. These boundaries allow software components above and below a level to be mixed and matched without reprogramming.
Figure 1.7 The Transport Driver Interface
The TDI is not a single program but a protocol specification to which the upper bounds of transport protocol device drivers are written. (Windows NT also includes a TDI driver that handles IRQ packet traffic from multiple TDI providers.) At this layer, networking software provides a virtual connection between the local redirector and each local or remote destination with which the redirector communicates. Similar connections are made between the server and the sources of the requests it receives.