TCP/IP and the OSI Model

TCP/IP and the OSI Model
As you learned in Chapter 2, “The OSI Model,” the OSI model divides computer-to-computer communications into seven connected layers; TCP/IP uses the Department of Defense (DoD)
model, which describes communications in only four layers, as Figure 3.1 shows. Each successively higher layer builds on the functions provided by the layers below.

Note:

The DoD model has fewer layers than the OSI model has, but that does not mean that it has less functionality. We draw the models to the same height because all data communications functionality is there. The DoD model simply combines the functionality of those layers into “larger” layers whose protocols perform all related functions of the equivalent OSI layers. Remember, that’s part of the OSI reference model’s success. Even though the original protocols never really caught on, the model itself is at once generic in its description of
protocol functionality and specific in its separation of communications tasks into more layers than just about any other model.

FIGURE 3 . 1 A comparison of the seven-layer OSI model, the four-layer DoD model, and how TCP/IP maps to each model 

As you may remember from Chapter 2’s discussion of the OSI model, the layers are as follows:

Application Layer The highest layer; defines the manner in which applications interact with the network—including databases, e-mail, and terminal-emulation programs using Application layer protocols similar to Lightweight Directory Access Protocol (LDAP), Simple Mail Transfer Protocol (SMTP), and Telnet.

Presentation Layer Defines the way in which data is formatted, presented, converted, and encoded.

Session Layer Coordinates communications and maintains the session for as long as it is needed—performing security, logging, and administrative functions.

Transport Layer Defines protocols for structuring messages and supervises the validity of the transmission by performing error checking.

Network Layer Defines data-routing protocols to increase the likelihood that the information arrives at the correct destination node.

Data Link Layer Validates the integrity of the flow of the data from one node to another by synchronizing blocks of data and controlling the flow.

Physical Layer Defines the mechanism for communicating with the transmission medium and the interface hardware.

Note:

Although no commercially available networking protocol suite follows the OSI model exactly, most perform all the same functions.

In the DoD model, the four layers are as follows:

Process/Application Layer The highest layer; applications such as FTP, Telnet, and others interact through this layer. Corresponds to the top three layers of the OSI model.

Host-to-Host Layer TCP and UDP add transport control information to the user data. Corresponds to the Transport layer of the OSI model.

Internet Layer Adds IP information to form a packet. Corresponds to the Network layer of the OSI model.

Network Access Layer Defines the mechanism for communicating with the transmission medium and the interface hardware. Corresponds to the bottom two layers of the OSI model.

Each layer adds its own header and, in the case of Data Link protocols, trailer control information to the basic data structure and encapsulates the protocol data unit (PDU) from the layer

above. On the receiving end, this header and trailer information is stripped, one layer at a time, until the equivalent of the original data arrives at its final destination.

Note:

PDU is a generic term used to describe the end product of a protocol. It can be thought of as the entire data structure handed down by that protocol to the protocol at the next lowest layer, or the information placed on the network media by the Physical layer. A PDU will consist of the original user data and any upper-layer control information (headers and trailers) imposed by upper-layer protocols encapsulated by the control information of the protocol creating the PDU.

Now let’s look at how TCP and IP work together.