In practice, OSI is a de-jure (according to law) standard and, TCP/IP is a de-facto (in reality) standard. The focus in the TCP/IP world is on agreeing on a protocol standard which can be made to work in diverse heterogeneous networks. The focus in the OSI world has always been more on the standard than the implementation of the standard.

The OSI reference model was devised before the protocols were implemented. This ordering means that the model was not biased toward one particular set of protocols, which made it quite general. The down side of this generalization was standard became quite complicated to implement and handle. Because of the complications OSI never gained the popularity though it was implemented in several organizations.

Nevertheless, since as a standard it was a much generalized one, it has been used as a reference model against which one can make comparison. With the TCP/IP the reverse was true: the protocols came first, and the model was just a description of the existing protocols. There was no problem with the protocols fitting the model, but it is hardly possible to be use to describe other models.

Most of the applications that are exchanging data, there is usually a requirement that the data be exchanged reliably. That is, we would like to be assured that all of the data arrive at the destination application and that the data arrive in the same order in which they were sent. And also the mechanisms for providing reliability should be essentially independent of the nature of the applications. The TCP is the most commonly used protocol to provide this functionality. Apart from these this also provides the mechanism by which the data can be delivered to the destination process within the destination system.

The TCP/IP protocol suite has always had an applied, get the job done orientation. Over the years it has handled most challenges by growing to meet the needs, and it is now the de-facto standard for internetworking for several reasons, including:

1) It s relatively simple and robust compared to alternatives such as OSI;

2) It is available on virtually every hardware and operating system platform, often free;

3) It is the protocol suite on which the Internet depends.

Although, OSI standardized by the ISO committee. This model is a very good reference model though it is not the widely used one. Another model is the TCP/IP model which is the outcome of longstanding research activities at the universities and research laboratories. TCP/IP model is the most widely used architecture and is the technology behind the today’s Internet. OSI has seven layers where as TCP/IP defines only five. The seven layers of the OSI are the physical, data link, network, transport, session, and presentation and application layers. TCP/IP does not have session and presentation layers but if their functionalities are required by any application then it is implemented in that particular application. There are various protocols specified at each layers and their interface is well defined so that if required one can just change one implementation of a particular protocol with the other implementation.