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The TCP/IP Protocol Architecture

Placing or overlaying the TCP/IP suite on the OSI model helps demonstrate TCP/IP’s operation and its relationship to other protocols. Figure 1 shows a pictorial view of where TCP/IP fits into the OSI model.



Figure 1: TCP/IP and the OSI networking model

As shown in Figure 1, the TCP/IP model consists of four layers. Each layer maps to one or more of the OSI layers, which include

1) Process
2) Host-to-host
3) Internet
4) Network access

Except for the network access layer, the other three components are software-based and consist of programmed modules that give the required functionality. Usually, these components are incorporated into operating systems to provide generalized access so that network applications can invoke basic network services without concern for implementation details.

Process/Application Layer

This layer offers user applications and interfaces with the host-to-host layer. Additional protocols and services are also found on this layer. The process layer maps to the presentation and application layers, which are defined within the OSI model. Applications on this layer include Telnet, FTP, sendmail, and many others.

Host-to-Host Layer

This layer is responsible for ensuring that data is reliable and that each higher-level service gets the correct information from the sending entity. The protocol supported on this layer is TCP. The layer maps to the OSI transport layer. The term used to describe information (units of data) on the host-to-host layer is segment.

Internet Layer

This layer offers an unreliable flow of information from one network to another. From an OSI standpoint, this layer is defined as the network layer. The Internet layer (or network) is responsible for routing between different IP networks. The protocol supported on this layer is IP. The term used to describe the information processed on this layer is packet.

Network Access Layer

The network access layer involves the physical attachment to a network, which traditionally needs a hardware interface from the network to a computer’s internals. This layer contains both physical and data link layers from the OSI model. The network access component defines the network architecture and topology. Some examples include Ethernet, FDDI, and Token Ring. The term used to describe the information on this layer is frame. A small driver program, which is provided by the network interface manufacturers, is also needed to connect the hardware to the operating system.

The TCP/IP and associated protocols can also be placed on the OSI seven-layer model, as shown in Figure 2. Here, we can see how the protocols and services relate to the model. The next few pages further describe the many additional services provided by the TCP/IP protocol suite.



Figure 2: TCP/IP suite and model