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4. Network Protocol Layers

Computers on a network communicate in agreed upon ways called protocols. The complexity of networking protocol software calls for the problem to be divided into smaller pieces. A layering model aids this division and provides the conceptual basis for understanding how software protocols together with hardware devices provide a powerful communication system.

4.1 Layering Models

In the early days of networking, before the rise of the ubiquitous Internet, the International Organization for Standardization (ISO) developed a layering model whose terminology persists today.

Table 4-1. ISO 7-Layer Reference Model

	Name of Layer	Purpose of Layer
Layer 7	Application	Specifies how a particular application uses a network.
Layer 6	Presentation	Specifies how to represent data.
Layer 5	Session	Specifies how to establish communication with a remote system.
Layer 4	Transport	Specifies how to reliably handle data transfer.
Layer 3	Network	Specifies addressing assignments and how packets are forwarded.
Layer 2	Data Link	Specifies the organization of data into frames and how to send frames over a network.
Layer 1	Physical	Specifies the basic network hardware.

The 7-layer model has been revised to the 5-layer TCP/IP reference model to meet the current needs of protocol designers.

Table 4-2. TCP/IP 5-Layer Reference Model

	Name of Layer	Purpose of Layer
Layer 5	Application	Specifies how a particular application uses a network.
Layer 4	Transport	Specifies how to ensure reliable transport of data.
Layer 3	Internet	Specifies packet format and routing.
Layer 2	Network	Specifies frame organization and transmittal.
Layer 1	Physical	Specifies the basic network hardware.

4.2 TCP/IP Protocol Stack

TCP/IP is the protocol suite upon which all Internet communication is based. Different vendors have developed other networking protocols, but even most network operating systems with their own protocols, such as Netware, support TCP/IP. It has become the de facto standard.

Protocols are sometimes referred to as protocol stacks or protocol suites. A protocol stack is an appropriate term because it indicates the layered approach used to design the networking software

Figure 4.1 Flow of Data Between Two Computers Using TCP/IP Stacks

Each host or router in the internet must run a protocol stack. The details of the underlying physical connections are hidden by the software. The sending software at each layer communicates with the corresponding layer at the receiving side through information stored in headers. Each layer adds its header to the front of the message from the next higher layer. The header is removed by the corresponding layer on the receiving side.