The Internet, TCP/IP, 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:
1. Link - Device driver and interface card which maps to the data link and physical layer of the OSI model.
2. Network - Corresponds to the network layer of the OSI model and includes the IP, ICMP, and IGMP
protocols.
3. Transport - Corresponds to the transport layer and includes the TCP and UDP protocols.
4. Application - Corresponds to the OSI Session, Presentation and Application layers and includes FTP,
Telnet, ping, Rlogin, rsh, TFTP, SMTP, SNMP, DNS, your program, etc.
Please note the four layer TCP/IP protocol. Each layer has a set of data that it generates.
1. 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.
2. 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.
3. 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. There are also other types of protocols for systems other than TCP/IP but we will talk about TCP and UDP in this document.
4. 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.
