|  | 2.3 | Comparison of the OSI Model and the TCP/IP Model
| 2.3.2 | The layers of the TCP/IP reference model

The U.S. Department of Defense (DoD) created the TCP/IP reference model because it wanted a network that could survive any conditions, even a nuclear war. To illustrate further, imagine a world at war, criss-crossed by different kinds of connections - wires, microwaves, optical fibers, and satellite links. Then imagine that you need information/data (in the form of packets) to flow, regardless of the condition of any particular node or network on the internetwork (which in this case may have been destroyed by the war). The DoD wants its packets to get through every time, under any conditions, from any one point to any other point. It was this very difficult design problem that brought about the creation of the TCP/IP model, and which has since become the standard on which the Internet has grown.

As you read about the TCP/IP model layers, keep in mind the original intent of the Internet; it will help explain why certain things are as they are. The TCP/IP model has four layers: the application layer, the transport layer, the Internet layer, and the network access layer. It is important to note that some of the layers in the TCP/IP model have the same name as layers in the OSI model. Do not confuse the layers of the two models, because the application layer has different functions in each model.

Application Layer
The designers of TCP/IP felt that the higher level protocols should include the session and presentation layer details. They simply created an application layer that handles high-level protocols, issues of representation, encoding, and dialog control. The TCP/IP combines all application-related issues into one layer, and assures this data is properly packaged for the next layer.

Transport Layer
The transport layer deals with the quality-of-service issues of reliability, flow control, and error correction. One of its protocols, the transmission control protocol (TCP), provides excellent and flexible ways to create reliable, well-flowing, low-error network communications. TCP is a connection-oriented protocol. It dialogues between source and destination while packaging application layer information into units called segments. Connection-oriented does not mean that a circuit exists between the communicating computers (that would be circuit switching). It does mean that Layer 4 segments travel back and forth between two hosts to acknowledge the connection exists logically for some period. This is known as packet switching.

Internet Layer
The purpose of the Internet layer is to send source packets from any network on the internetwork and have them arrive at the destination independent of the path and networks they took to get there. The specific protocol that governs this layer is called the Internet protocol (IP). Best path determination and packet switching occur at this layer. Think of it in terms of the postal system. When you mail a letter, you do not know how it gets there (there are various possible routes), but you do care that it arrives.

Network Access Layer
The name of this layer is very broad and somewhat confusing. It is also called the host-to-network layer. It is the layer that is concerned with all of the issues that an IP packet requires to actually make a physical link, and then to make another physical link. It includes the LAN and WAN technology details, and all the details in the OSI physical and data link layers.

For more TCP/IP information, visit the following Web sites:

| Web Links
| Introduction to the Internet Protocols