TCP/IP and the OSI Model
The networking industry has a standard seven-layer model for network protocol architecture called the Open Systems Interconnection (OSI) model. The OSI model represents an effort by ISO, an international standards organization, to standardize the design of network protocol systems to promote interconnectivity and open access to protocol standards for software developers.
TCP/IP was already on the path of development when the OSI standard architecture appeared and, strictly speaking, TCP/IP does not conform to the OSI model. However, the two models did have similar goals, and there was enough interaction among the designers of these standards that they emerged with a certain compatibility. The OSI model has been very influential in the growth and development of protocol implementations, and it is quite common to see the OSI terminology applied to TCP/IP.
Figure 2.2 shows the relationship between the four-layer TCP/IP standard and the seven-layer OSI model. Note that the OSI model divides the duties of the Application layer into three layers: Application, Presentation, and Session. OSI splits the activities of the Network Access layer into a Data Link layer and a Physical layer. This increased subdivision adds some complexity, but it also adds flexibility for developers by targeting the protocol layers to more specific services. In particular, the division at the lower level into the Data Link and Physical layers separates the functions related to organizing communication from the functions related to accessing the communication medium. The three upper OSI layers offer a greater variety of alternatives for an application to interface with the protocol stack.
Figure 2.2. The seven-layer OSI model.
The seven layers of the OSI model are as follows:
Physical layer— Converts the data into the stream of electric or analog pulses that will actually cross the transmission medium and oversees the transmission of the data
Data Link layer— Provides an interface with the network adapter; maintains logical links for the subnet
Network layer— Supports logical addressing and routing
Transport layer— Provides error control and flow control for the internetwork
Session layer— Establishes sessions between communicating applications on the communicating computers
Presentation layer— Translates data to a standard format; manages encryption and data compression
Application layer— Provides a network interface for applications; supports network applications for file transfer, communications, and so forth
It is important to remember that the TCP/IP model and the OSI model are standards, not implementations. Real-world implementations of TCP/IP do not always map cleanly to the models shown in Figures 2.1 and 2.2, and the perfect correspondence depicted in Figure 2.2 is also a matter of some discussion within the industry.
Notice that the OSI and TCP/IP models are most similar at the important Transport and Internet (called Network in OSI) layers. These layers include the most identifiable and distinguishing components of the protocol system, and it is no coincidence that protocol systems are sometimes named for their Transport and Network layer protocols. As you’ll learn later in this book, the TCP/IP protocol suite is named for TCP, a Transport layer protocol, and IP, an Internet/Network layer protocol.