The Department of Defense DoD Model

Although OSI is a protocol independent framework for defining communications, and thus is portable and applicable to almost all network communications, it does not always map directly to a particular communications process. For example, just because the OSI model defines seven distinct layers does not mean that there must be seven distinct communications processes or protocols in use. In many cases, a protocol may implement functions that span multiple layers (for example, TCP which has some functionality that bleeds into the session layer of the OSI model).

The TCP/IP protocol suite in particular does not map directly to the OSI model, in no small part because most of the protocols that make up the TCP/IP protocol suite were actually based upon a four-layer model known as the DoD model. Figure 3-4 shows a comparison of the different layers in both the DoD and OSI models.

Figure 3-4. Comparison of the DoD and OSI Models

Figure 3-4. Comparison of the DoD and OSI Models

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The four layers of the DoD Model are as follows:

• Application layer (Layer 4) The application layer is where higher-layer protocols, services, and applications such as HTTP, DNS, SMTP, and FTP function and reside. The application layer roughly overlays the application, presentation, and session layers of the OSI model.

• Host-to-host or transport layer (Layer 3) The host-to-host layer is where protocols such as TCP and UDP reside. It handles flow control, connection and session establishment, maintenance, and teardown. The host-to-host layer roughly overlays the transport layer of the OSI model.

• The Internet layer (Layer 2) The Internet layer is where protocols such as IP reside and handles the logical addressing and routing of data across the network. The Internet layer roughly overlays the network layer of the OSI model.

• The network access layer (Layer 1) The network access layer handles the physical addressing and delivery of data across the network and is where protocols such as 802.2, 802.3, and Ethernet reside. The network access layer roughly overlays the datalink and physical layer of the OSI model.

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