The Department of Defense Networking Model 79 The interfaces defined by Microsoft are as follows: Network Device Interface Specification (NDIS) This layer maps to the data link layer of the OSI model and the network interface layer of the Defense Advanced Research Projects Agency (DARPA) model, which we will cover next. The NDIS layer is the boundary between the physical network and the higher level transport protocols. This layer provides the standardized functions that allow various transport protocols to use any network device driver that is compatible with the specifications of this layer, providing both flexibility and reliability to developers. Transport Driver Interface (TDI) This layer provides a portal into the trans- port protocols for kernel mode components such as servers and redirectors. It acts as the gateway between the transport layer and the session layer in the OSI model, providing a common interface, which developers can use to access both transport and session layer functionalities. Application Program Interface (API) This layer is the interface through which developers can access network infrastructure services such as various application layer protocols. The Windows OS is divided into three primary areas: the User, the Executive, and