Windows® Internals, Sixth Edition, Part 1

Windows provides an extensive array of security functions that meet the key requirements of both government agencies and commercial installations. In this chapter, we’ve taken a brief tour of the internal components that are the basis of these security features. In the next chapter, we’ll look at the I/O system.

Chapter 7. Networking

Microsoft Windows was designed with networking in mind, and it includes broad networking support that is integrated with the I/O system and the Windows APIs. The four basic types of network software components are services, APIs, protocols, and drivers for network adapters—with each component layered on top of the next to form a network stack. Windows has well-defined interfaces for each layer, so in addition to using the wide variety of APIs, protocols, and network adapter device drivers that ship with Windows, third parties can extend the operating system’s networking capabilities by developing their own components.

In this chapter, we take you from the top of the Windows networking stack to the bottom. First, we present the mapping between the Windows networking software components and the Open Systems Interconnection (OSI) reference model. Then we briefly describe the networking APIs available on Windows and explain how they are implemented. You’ll learn how multiple redirector support and name resolution work, see how to access and cache remote files, and learn how a multitude of drivers interact to form a network protocol stack. After looking at the implementation of network adapter device drivers, we examine binding, which is the glue that connects services, protocol stacks, and network adapters.

Windows Networking Architecture

The goal of network software is to take a request (in the form of an I/O request) from an application on one machine, pass it to another machine, execute the request on the remote machine, and return the results to the first machine. In the course of this process, the request must be transformed several times. A high-level request, such as “read x number of bytes from file y on machine z,” requires software that can determine how to get to machine z and what communication software that machine understands. Then the request must be altered for transmission across a network—for example, divided into short packets of information. When the request reaches the other side, it must be checked for completeness, decoded, and sent to the correct operating system component for execution. Finally, the reply must be encoded for sending back across the network.

The OSI Reference Model

To help different computer manufacturers standardize and integrate their networking software, in 1984 the International Organization for Standardization (ISO) defined a software model for sending messages between machines. The result was the Open Systems Interconnection (OSI) reference model. The model defines six layers of software and one physical layer of hardware, as shown in Figure 7-1.

Figure 7-1. OSI reference model

The OSI reference model is an idealized scheme that few systems implement precisely, but it’s often used to frame discussions of networking principles. Each layer on one machine assumes that it is “talking to” the same layer on the other machine. Both machines “speak” the same language, or protocol, at the same level. In reality, however, a network transmission must pass down each layer on the client machine, be transmitted across the network, and then pass up the layers on the destination machine until it reaches a layer that can understand and implement the request.

The purpose of each layer in the OSI model is to provide services to higher layers and to abstract how the services are implemented at lower layers. Describing the details of each layer is beyond the scope of this book, but following is a brief description of each layer in the OSI model.

Note

Most network descriptions start with the top-most layer and work down to the lowest layer; however, here the description of the layers will start at the bottom and work toward the top, to demonstrate how each layer builds upon the services provided by the layer beneath it.

Physical. This is the lowest layer in the OSI model, and it exchanges signals between cooperating network entities over some physical medium (wire, radio, fiber, or other type). The physical layer specifies the mechanical, electrical, functional, and procedural standards for accessing the medium, such as connectors, cabling, signaling, and so on. Common examples are Ethernet (IEEE 802.3) and Wi-Fi (IEEE 802.11).