System Administration Guide: IP Services

Protocol Layers and the OSI Model

Most network protocol suites are structured as a series of layers, sometimes referred to collectively as a protocol stack. Each layer is designed for a specific purpose. Each layer exists on both the sending and receiving hosts. A specific layer on one machine sends or receives exactly the same object that another machine's peer process sends or receives. These activities occur independently from activities in layers above or below the layer under consideration. Effectively, each layer on a host acts independently of other layers on the same machine. Each layer acts in parallel with the same layer on other hosts.

OSI Reference Model

Most network protocol suites are viewed as structured in layers. The International Organization for Standardization (ISO) designed the Open Systems Interconnection (OSI) Reference Model that uses structured layers. The OSI model describes a structure with seven layers for network activities. Each layer associates one or more protocols with the layer. The layers represent data transfer operations common to all types of data transfers among cooperating networks.

The OSI Reference Model lists the protocol layers from the top (layer 7) to the bottom (layer 1). The following table shows the model.

Table 2–1 Open Systems Interconnection Reference Model

Layer No. 

Layer Name 



Consists of standard communication services and applications that everyone can use. 


Ensures that information is delivered to the receiving machine in a form that the machine can understand. 


Manages the connections and terminations between cooperating computers. 


Manages the transfer of data. Also assures that the received data are identical to the transmitted data. 


Manages data addressing and delivery between networks. 

Data Link

Handles the transfer of data across the network media. 


Defines the characteristics of the network hardware. 

The OSI model defines conceptual operations that are not unique to any particular network protocol suite. For example, the OSI network protocol suite implements all seven layers of the OSI Reference Model. TCP/IP uses some of OSI model layers. TCP/IP also combines other layers. Other network protocols, such as SNA, add an eighth layer.