OSI is a standard "reference model" that describes how protocols should interact with one another. Invented by the Department of Defense, TCP/IP became one of the "standards" that enabled the Internet to become what it is today. TCP/IP doesn't map cleanly into the OSI model, but it is convenient to think in terms of the OSI model when describing protocols.
The major differences between the OSI and TCP/IP are:
The application layer in TCP/IP handles the responsibilities of layers 5, 6 and 7 in the OSI model.
- The transport layer in TCP/IP does not always guarantee reliable delivery of packets at the transport layer, while the OSI model does. TCP/IP also offers an option called UDP that does not guarantee reliable packet delivery.
Rather then reinventing the wheel here, there is a great description of the OSI model on SearchNetworking.com.
This was first published in April 2005