Comparison of OSI and TCP/IP Models

Comparison of TCP/IP and OSI Reference Models

- Similarities and Differences in Layering

Following are some similarities between OSI Reference Model and TCP/IP Reference Model: Both have layered architecture. Layers provide similar functionalities. Both are protocol stack. Both are reference models. In both the models, protocols are defined in a layer-wise manner. In both models, data is divided into packets and each packet may take the individual route from the source to the destination.

The three top layers in the OSI model, i.e. the application layer, the presentation layer and the session layer, are not distinguished separately in the TCP/IP model which only has an application layer above the transport layer. 

For example, the session and presentation layers of the OSI suite are considered to be included to the application layer of the TCP/IP suite. The functionality of the session layer can be found in protocols like HTTP and SMTP and is more evident in protocols like Telnet and the Session Initiation Protocol (SIP). Session layer functionality is also realized with the port numbering of the TCP and UDP protocols, which cover the transport layer in the TCP/IP suite. Functions of the presentation layer are realized in the TCP/IP applications with the MIME standard in data exchange.

The Internet Protocol suite is unique in that it is made up of non-proprietary protocols. This means that they do not belong to any one company or organization and that the technology is available for anyone to use them. 

- Internet Middle-Layer Protocols

The TCP/IP model does not cover the two layers of the OSI model. This means that TCP/IP is hardware independent. The OSI model's network and transport layers are concerned with transporting packets across the internetwork. TCP/IP and other Internet protocols use three types of addresses for network addressing:

• Hardware or physical addresses used the data link and physical layers.
• IP addresses provide logical node IDs. IP addresses are unique addresses assigned by an administrator according to certain guidelines.  They are expressed in four-part dotted-decimal notation.
• Logical node names, which an administrator can assign, such as google.com

- Major Differences 

Following are some major differences between OSI Reference Model and TCP/IP Reference Model.

• OSI is a generic, protocol independent standard, acting as a communication gateway between the network and end user. TCP/IP model is based on standard protocols around which the Internet has developed. It is a communication protocol, which allows connection of hosts over a network.
• In OSI model the transport layer guarantees the delivery of packets. In TCP/IP model the transport layer does not guarantees delivery of packets. Still the TCP/IP model is more reliable.
• OSI model follows vertical approach. TCP/IP model follows horizontal approach.
• OSI model has a separate Presentation layer and Session layer.4. TCP/IP does not have a separate Presentation layer or Session layer.
• OSI Transport Layer is Connection Oriented. TCP/IP Transport Layer is both Connection Oriented and Connectionless.
• OSI Network Layer is both Connection Oriented and Connection less. TCP/IP Network Layer is Connection less.
• OSI is a reference model around which the networks are built. Generally it is used as a guidance tool. TCP/IP model is, in a way implementation of the OSI model.
• Network layer of OSI model provides both connection oriented and connectionless service. The Network layer in TCP/IP model provides connectionless service.
• OSI model has a problem of fitting the protocols into the model. TCP/IP model does not fit any protocol
• Protocols are hidden in OSI model and are easily replaced as the technology changes. In TCP/IP replacing protocol is not easy.
• OSI model defines services, interfaces and protocols very clearly and makes clear distinction between them. It is protocol independent. In TCP/IP, services, interfaces and protocols are not clearly separated. It is also protocol dependent.
• OSI model has 7 layers. TCP/IP model has 4 layers

[More to come ...]