The TCP/IP model or Internet reference model, sometimes called the DoD model (DoD, Department of Defense), ARPANET reference model, is a layered abstract description for communications and computer network protocol design. It was created in the 1970s by DARPA for use in developing the Internet’s protocols, and the structure of the Internet is still closely reflected by the TCP/IP model. This model was developed before the OSI Reference Model, and the Internet Engineering Task Force (IETF), which is responsible for the model and protocols developed under it, has never felt obligated to be compliant with OSI. While the basic OSI model is widely used in teaching, OSI, as presented as a seven-layer model, does not reflect real-world protocol architecture.
Key Architectural Principles
An early architectural document, RFC 1122, emphasizes architectural principles over layering.
1. End-to-End Principle: This principle has evolved over time. Its original expression put the maintenance of state and overall intelligence at the edges, and assumed the Internet that connected the edges retained no state and concentrated on speed and simplicitly. Real-world needs for firewalls, network address translators, web content caches and the like have forced changes in this Principle 
2. Robustness Principle: “Be liberal in what you accept, and conservative in what you send. software on other hosts may contain deficiencies that make it unwise to exploit legal but obscure protocol features”.
Even when layer is examined, the assorted architectural documents—there is no single architectural model such as ISO 7498, the OSI Reference Model—has fewer, less rigidly defined layers than the commonly referenced OSI model, and thus provides an easier fit for real-world protocols. In point of fact, one frequently referenced document does not contain a stack of layers. The lack of emphasis on layering is a strong difference between the IETF and OSI approaches. It... [continues]
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