Physical Network Medium

The Windows OS is divided into three primary areas: the User, the Executive, and the Kernel. The Kernel is the core of the Microsoft operating system architecture and it manages the most basic operations including interacting with the hardware abstraction layer that interacts with the hardware (CPU, memory, etc.).The Kernel also synchronizes activities with the Executive level, which includes the Input/Output (I/O) Manager and the Process Manager. The User level interacts with the Executive level; this is the level at which most applications and user interfaces reside.

The Network Driver Interface Specification Boundary Layer

The Network Driver Interface Specification (NDIS) works at the bottom of the networking architecture and maps to the Data Link layer of the OSI model and the Network Interface layer of the DARPA model. The NDIS layer is the boundary between the physical network (Physical layer of the OSI model) 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.The earliest versions of NDIS were developed by a Microsoft and 3Com joint effort. Current NDIS versions are proprietary to Microsoft operating systems.

The Transport Driver Interface Boundary Layer

The Transport Driver Interface (TDI) provides a portal into the transport protocols for kernel mode components such as servers and redirectors. In essence, it is the gateway between the Transport layer and the Session layer in the OSI model, providing a common interface developers can use to access both Transport and Session layer functionality.

The Application Program Interface Boundary Layer

The Application Program Interface (API) is the interface through which developers can access network infrastructure services such as various Application layer protocols. Dynamic Host Configuration Protocol (DHCP), Domain Name Service (DNS), and Windows Internet Name Service (WINS) all work at this level and connect to the lower layers through APIs. There are also Windows Sockets (WinSock), NetBIOS, telephony, and messaging APIs used to assist in carrying out lower level network functions.

Understanding Component Layers

Within each layer are component layers that provide very specific functionality.

The NDIS Wrapper

The NDIS wrapper is a library of common NDIS functions that can be used both by the MAC protocols beneath it and by TCP/IP above it.The NDIS wrapper is implemented by a file called ndis.sys, which is software code that surrounds all NDIS device drivers. It provides a common interface for device drivers and protocol drivers.The NDIS wrapper is used to reduce platform dependencies during development of network interface devices.

Network Transport Protocols

Network transport protocols allow applications or clients to send and receive data over the network. Although we're discussing TCP/IP specifically in this chapter, other network transport protocols include Internet Packet Exchange/Sequenced Packet Exchange (IPX/SPX), ATM, NetBEUI, Infrared Data Association (IrDA),AppleTalk, and SNA.These protocols are used on a variety of non-Microsoft operating systems including Novell, Apple, and IBM.

File System Drivers

The file system drivers are the Redirector and the Server service. When there is a request to open a shared file, the I/O Manager sends a request to the Redirector, which selects the appropriate Transport layer protocol via the TDI layer. When there is a request to access a local file, the Server service responds to requests from the remote Redirector and provides access to the requested file. Named pipes, mailslots, server service, and redirector are file system drivers that work at both the Presentation and Session layers of the OSI model.

Applications and User Mode Services

Applications must interface with the lower layer protocols and must interact in some manner with the user.These services are implemented in a number of ways, but there are four commonly used APIs implemented at this point that provide access to lower transport protocols.

The WinSock API allows Windows-based applications to communicate with the lower layers. Winsock is a protocol-independent networking API that provides standardized access to datagram and session services over TCP/IP, IPX/SPX, AppleTalk, and others.

Telephony integrates computers with telephone technology and utilizes the Telephony API (TAPI) to provide a standardized interface to networking protocols for various telephony applications. The NetBIOS API has been used for developing client/server applications and is supported in Windows Server 2003 for backward compatibility.The Messaging API (MAPI) is an industry standard that assists applications in interfacing with messaging services via a single interface. Microsoft Exchange uses MAPI.

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