DOD model

This model is sometimes called the DOD model since it was designed for the department of defense It is also called the TCP/IP four layer protocol, or the internet protocol. It has the following layers:

• Link - Device driver and interface card which maps to the data link and physical layer of the OSI model.
• Network - Corresponds to the network layer of the OSI model and includes the IP, ICMP, and IGMP protocols.
• Transport - Corresponds to the transport layer and includes the TCP and UDP protocols.
• Application - Corresponds to the OSI Session, Presentation and Application layers and includes FTP, Telnet, ping, Rlogin, rsh, TFTP, SMTP, SNMP, DNS, your program, etc.
• The four layer TCP/IP protocol. Each layer has a set of data that it generates.
• The Link layer corresponds to the hardware, including the device driver and interface card. The link layer has data packets associated with it depending on the type of network being used such as ARCnet, Token ring or ethernet. In our case, we will be talking about ethernet.
• The network layer manages the movement of packets around the network and includes IP, ICMP, and IGMP. It is responsible for making sure that packages reach their destinations, and if they don't, reporting errors.

The transport layer is the mechanism used for two computers to exchange data with regards to software. The two types of protocols that are the transport mechanisms are TCP and UDP.

The application layer refers to networking protocols that are used to support various services such as FTP, Telnet, BOOTP, etc. Note here to avoid confusion, that the application layer is generally referring to protocols such as FTP, telnet, ping, and other programs designed for specific purposes which are governed by a specific set of protocols defined with RFC's (request for comments). However a program that you may write can define its own data structure to send between your client and server program so long as the program you run on both the client and server machine understand your protocol. For example when your program opens a socket to another machine, it is using TCP protocol, but the data you send depends on how you structure it.