The TCP/IP Protocol System

Before looking at the elements of TCP/IP, it is best to begin with a brief review of the responsibilities of a protocol system.

A protocol system such as TCP/IP must be responsible for the following tasks:

• Dividing messages into manageable chunks of data that will pass efficiently through the transmission medium.

• Interfacing with the network adapter hardware.

• Addressing: The sending computer must be capable of targeting data to a receiving computer. The receiving computer must be capable of recognizing a message that it is supposed to receive.

• Routing data to the subnet of the destination computer, even if the source subnet and the destination subnet are dissimilar physical networks.

• Performing error control, flow control, and acknowledgment: For reliable communication, the sending and receiving computers must be able to identify and correct faulty transmissions and control the flow of data.

• Accepting data from an application and passing it to the network.

• Receiving data from the network and passing it to an application.

To accomplish the preceding tasks, the creators of TCP/IP settled on a modular design. The TCP/IP protocol system is divided into separate components that theoretically function independently from one another. Each component is responsible for a piece of the communication process.

The advantage of this modular design is that it lets vendors easily adapt the protocol software to specific hardware and operating systems. For instance, the Network Access layer (as you'll learn in Hour 3, "The Network Access Layer") includes functions relating to a specific LAN architecture, such as token ring or ethernet. Because of TCP/IP's modular design, a vendor such as Microsoft does not have to build a completely different software package for token ring TCP/IP (as opposed to ethernet TCP/IP) networks. The upper layers are not affected; only the Network Access layer must change.

The TCP/IP protocol system is subdivided into layered components, each of which performs specific duties (see Figure 2.1). This model, or stack, comes from the early days of TCP/IP, and it is sometimes called the TCP/IP model. The official TCP/IP protocol layers and their functions are described in the following list.

Compare the functions in the list with the responsibilities listed earlier in this section, and you'll see how the responsibilities of the protocol system are distributed among the layers.

By the Way

The four-layer model shown in Figure 2.1 is a common model for describing TCP/IP networking, but it isn't the only model. The ARPAnet model, for instance, as described in RFC 871, describes three layers: the Network Interface layer, the Host-to-Host layer, and the Process-Level/Applications layer. Other descriptions of TCP/IP call for a five-layer model, with Physical and Data Link layers in place of the Network Access layer (to match OSI). Still other models might exclude either the Network Access or the Application layer, which are less uniform and harder to define than the intermediate layers.

The names of the layers also vary. The ARPAnet layer names still appear in some discussions of TCP/IP, and the Internet layer is sometimes called the Internetwork layer or the Network layer.

This book uses the four-layer model, with names shown in Figure 2.1.

• Network Access layer— Provides an interface with the physical network. Formats the data for the transmission medium and addresses data for the subnet based on physical hardware addresses. Provides error control for data delivered on the physical network.

• Internet layer— Provides logical, hardware-independent addressing so that data can pass among subnets with different physical architectures. Provides routing to reduce traffic and support delivery across the internetwork. (The term internetwork refers to an interconnected, greater network of LANs, such as what you find in a large company or on the Internet.) Relates physical addresses (used at the Network Access layer) to logical addresses.

• Transport layer— Provides flow control, error control, and acknowledgment services for the internetwork. Serves as an interface for network applications.

• Application layer— Provides applications for network troubleshooting, file transfer, remote control, and Internet activities. Also supports the network Application Programming Interfaces (APIs) that enable programs written for a particular operating environment to access the network.

Part II, "The TCP/IP Protocol System," provides more detailed descriptions of the activities at each of these TCP/IP protocol layers.

When the TCP/IP protocol software prepares a piece of data for transmission across the network, each layer on the sending machine adds a layer of information to the data that will be relevant to the corresponding layer on the receiving machine. For instance, the Internet layer of the computer sending the data adds a header with some information that is significant to the Internet layer of the computer receiving the message. This process is sometimes referred to as encapsulation. At the receiving end these headers are removed as the data is passed up the protocol stack.

By the Way

The term layer is used throughout the computer industry for protocol component levels such as the ones shown in Figure 2.1. Header information is applied in layers to the data as it passes through the components of the protocol stack. (You'll learn more about this later in this hour.) When it comes to the components themselves, however, the term layer is somewhat metaphorical.

Diagrams such as Figure 2.1 are meant to show that the data passes across a series of interfaces. As long as the interfaces are maintained, the processes within one component are not affected by the processes in other components. If you turned Figure 2.1 sideways, it would look more like an assembly line, and this is also a useful analogy for the relationship of the protocol components. The data stops at each point in the line and, as long as it arrives at each point as specified, the components can operate independently.